Eldorado Community:
http://hdl.handle.net/2003/24986
2024-03-28T17:06:09ZDiluting the hydrogen bonds in viscous solutions of n-butanol with n-bromobutane
http://hdl.handle.net/2003/25794
Title: Diluting the hydrogen bonds in viscous solutions of n-butanol with n-bromobutane
Authors: Böhmer, Roland; El Goresy, T.
Abstract: Glass-forming monohydroxy alcohols exhibit not only a structural relaxation but also a slower, single-exponential Debye-type relaxation process which already freezes in the liquid phase. By using dielectric spectroscopy, we study how these relaxations evolve when the aprotic alkyl halide n-bromobutane is added to n-butanol, thereby diluting the hydrogen-bond network. The structural relaxation times smoothly vary over the concentration range of this completely miscible binary system. The Debye process remains unaffected by the dilution of the OH groups up to n-bromobutane mole fractions of about 50%. For larger hydroxy dilutions, it turns rather abruptly into a feature which develops significant spectral broadening and it becomes faster. In the dilute limit, the decoupling between the time scale of the Debye process and that of the structural relaxation amounts to almost 6 decades when extrapolated to the glass transition temperature. This relatively large, strongly concentration dependent decoupling is interpreted in analogy to normal modes in polymers. The present results suggest that the structural and the Debye-like responses of monohydroxy alcohols are unrelated.2008-04-21T00:00:00ZDielectric properties of fluoromethanes and fluoromethane mixtures
http://hdl.handle.net/2003/25721
Title: Dielectric properties of fluoromethanes and fluoromethane mixtures
Authors: Böhmer, Roland; Loidl, A.1988-10-15T00:00:00ZDipolar relaxations in solid (CF4)1-x(CClF3)x
http://hdl.handle.net/2003/25720
Title: Dipolar relaxations in solid (CF4)1-x(CClF3)x
Authors: Böhmer, Roland1989-09-01T00:00:00ZRelaxation dynamics in molecular alloys
http://hdl.handle.net/2003/25719
Title: Relaxation dynamics in molecular alloys
Authors: Böhmer, Roland; Loidl, A.1991-02-01T00:00:00ZSpin fluctuations in the heavy fermion compound CeCu2Ge2 probed by nuclear magnetic resonance
http://hdl.handle.net/2003/25718
Title: Spin fluctuations in the heavy fermion compound CeCu2Ge2 probed by nuclear magnetic resonance
Authors: Böhmer, Roland; Büttgen, N.; Loidl, A.1995-01-01T00:00:00ZNMR studies of the heavy fermion alloy Ce(Cu1-xNix)2Ge2
http://hdl.handle.net/2003/25717
Title: NMR studies of the heavy fermion alloy Ce(Cu1-xNix)2Ge2
Authors: Böhmer, Roland; Büttgen, N.; Krimmel, A.; Loidl, A.
Abstract: A detailed 63Cu NMR investigation is presented for the heavy-fermion system Ce(Cu1-xNix)2Ge2, focusing especially on the alloying induced transition from magnetic order to pure Kondo-like behavior. The broadening of the resonance line and the temperature dependence of the spin-lattice relaxation rate 1/T1 are studied in full detail. We provide experimental evidence for the appearance of heavy-fermion band magnetism for concentrations x=0.6 and x=0.7. For x=0.8 the spin-lattice relaxation rate 1/T1 follows a logarithmic temperature dependence which we interpret as a fingerprint of a non-Fermi-liquid behavior. Finally, 1/T1 is compared to the dynamic structure factor S(Q,ω) as determined in inelastic-neutron-scattering experiments.1996-01-01T00:00:00ZSurface-induced order and diffusion of 5CB liquid crystal confined to porous glass
http://hdl.handle.net/2003/25716
Title: Surface-induced order and diffusion of 5CB liquid crystal confined to porous glass
Authors: Böhmer, Roland; Vilfan, M.; Apih, T.; Gregorovic, A.; Zalar, B.; Lahajnar, G.; Zumer, S.; Hinze, G.; Althoff, G.2001-01-01T00:00:00ZRadio-frequency dielectric measurements at temperatures from 10 to 450 K
http://hdl.handle.net/2003/25715
Title: Radio-frequency dielectric measurements at temperatures from 10 to 450 K
Authors: Böhmer, Roland; Maglione, M.; Lunkenheimer, P.; Loidl, A.
Abstract: A coaxial air line was constructed to connect a radio-frequency impedance analyzer and a temperature-stabilized sample holder. It is suitable for dielectric measurements in the frequency range 1 MHz–1 GHz and at temperatures between 10 and 450 K. The dielectric dispersion of Fe-doped BaTiO3 and Na-doped KCN is presented. The results demonstrate the capability of this setup when investigating materials with high as well as with low dielectric constants.1989-02-01T00:00:00ZMechanical stress relaxation in inorganic glasses studied by a step-strain technique
http://hdl.handle.net/2003/25714
Title: Mechanical stress relaxation in inorganic glasses studied by a step-strain technique
Authors: Böhmer, Roland; Senapati, H.; Angell, C. A.
Abstract: A computer-controlled transient viscoelastometer, which is an adaption of the Rheovibron, has been developed to study the mechanical stress relaxation of amorphous materials over a range of more than five decades in time. The instrument was used to investigate the degree of non-exponentiality of the tensile stress autocorrelation function of AgI---Ag2SO4---Ag2WO4 and Ge---As---Se glasses. Upon variation of the composition, both ternary systems show large variations of the smearing of the calorimetric glass-transition anomaly. For most cases low-noise decay functions could be observed which are well fitted by stretched exponentials. For 50AgI·25Ag2SO4·25Ag2WO4 an exceptionally broad distribution of relaxation times is found. This is consistent with the sub-Tg specific heat peak previously observed in these mixed oxyanion glasses. For pure amorphous selenium two distinct relaxation processes are observed.1991-06-11T00:00:00ZCondensed (Ar)1-x(N2)x in a four rod capacitor
http://hdl.handle.net/2003/25713
Title: Condensed (Ar)1-x(N2)x in a four rod capacitor
Authors: Böhmer, Roland
Abstract: A modification of the conventional capacitive dilatometer, a four-rod assembly, was used to measure the thermal density variations in (Ar)1-x(N2)x alloys with x=0.22, 0.71 and 1. The results are in favour of complete solid state miscibility, contrary to the recent findings of Yantsevich et al. (1986). The potential use of the four-rod capacitor as a device to detect molecular electric quadrupole relaxations at audio frequencies is discussed.1991-08-14T00:00:00ZPulsed dielectric spectroscopy of supercooled liquids
http://hdl.handle.net/2003/25712
Title: Pulsed dielectric spectroscopy of supercooled liquids
Authors: Böhmer, Roland; Schiener, B.; Hemberger, J.; Chamberlin, R. V.
Abstract: Pulsed dielectric spectroscopy is introduced as a technique for selectively emphasizing specific components of the non-exponential dielectric response of matter. Samples studied include supercooled liquid propanol, propylene carbonate, and poly(lauryl-methacrylate). It is shown that particular sequences of pulses can be used to emphasize the fast response regime, to produce a crossover or memory effect, or to eliminate the response of selected components. Furthermore, for materials characterized by broad distributions of relaxation times, the technique facilitates the investigation of a relatively narrow band from that distribution. It is also shown that the time domain spectroscopy can be combined with conventional frequency domain techniques to provide the characterization of dielectric response over an extraordinarily broad spectral range.1995-11-01T00:00:00ZFar-off-resonance averaging of dipolar interactions in solids
http://hdl.handle.net/2003/25711
Title: Far-off-resonance averaging of dipolar interactions in solids
Authors: Böhmer, Roland; Chang, I.; Diezemann, G.; Hinze, G.; Sillescu, H.
Abstract: The far-off-resonance performance of several line-narrowing sequences is investigated. Both theoretically and experimentally, it is found that transverse relaxation times, dominated by dipole–dipole interactions, are most effectively prolonged not only on-resonance but also for certain, generally large, resonance offsets. These correspond to a situation when, during the basic pulse separation, the frequency offset rotates the toggling-frame Hamiltonian by multiples of 180°. The implications of these results for the study of macroscopic translational diffusion using static-field-gradient NMR are discussed.1997-01-01T00:00:00ZDielectric hole burning spectroscopy of supercooled liquids
http://hdl.handle.net/2003/25710
Title: Dielectric hole burning spectroscopy of supercooled liquids
Authors: Böhmer, Roland; Schiener, B.; Chamberlin, R. V.; Diezemann, G.
Abstract: The nonexponential response of propylene carbonate and glycerol near their glass transitions could be selectively altered using nonresonant spectral hole burning (NSHB) experiments. This observation provides evidence of the existence of a distribution of relaxation times in these supercooled liquids. NSHB is based on a pump, wait, and probe scheme and uses low-frequency large amplitude electrical fields to modify the dielectric relaxation. The temporal evolution of the polarization of the sample is then measured subsequent to a small voltage step. By variation of a recovery time inserted between pump and probe, the refilling of the spectral features could be monitored and was found to take place on the time scale set by the peak in the distribution. The recovery time and pump frequency dependences of the spectral modifications were successfully simulated using a set of coupled rate equations.1997-11-15T00:00:00ZDecay of single and double quantum coherences in deuterated glassy ortho-terphenyl
http://hdl.handle.net/2003/25709
Title: Decay of single and double quantum coherences in deuterated glassy ortho-terphenyl
Authors: Böhmer, Roland; Kircher, O.; Diezemann, G.; Hinze, G.; Schug, K. U.; Sillescu, H.; Zimmermann, H.
Abstract: The decay of single and double quantum coherences in amorphous ortho-terphenyl is studied by means of deuteron NMR echo experiments near and below the calorimetric glass transition temperature Tg. In perdeuterated samples a Gaussian decay for these coherences is found at low temperatures, in accord with theoretical predictions for random local dipolar fields. A method is proposed to extract information about slow motional processes from ordinary solid echo experiments below Tg. Partly deuterated ortho-terphenyl does not show a Gaussian decay. Simple model calculations show that this observation indicates the importance of correlations of the local dipolar fields in the presence of protons1998-05-22T00:00:00ZExperimental determination of four-time stimulated echoes in liquids, colloidal suspensions, and crystals
http://hdl.handle.net/2003/25708
Title: Experimental determination of four-time stimulated echoes in liquids, colloidal suspensions, and crystals
Authors: Böhmer, Roland; Hinze, G.; Diezemann, G.; Sillescu, H.
Abstract: Four-time stimulated echo experiments have been used to monitor the temporal evolution of the reorientation rates of deuterated molecules or colloidal suspensions. We present extended phase cycles for this seven-pulse experiment. In order to test its performance three vastly different materials are chosen. These include a crystal in which the molecules carry out well-defined 180° flips and a supercooled liquid characterized by a distribution of jump angles centered around 10°. As an example for rotational diffusion, data on a concentrated suspension of polystyrene spheres in a viscous medium are presented.1998-01-01T00:00:00ZCapacitive scanning dilatometry and frequency dependent thermal expansion of polymer films
http://hdl.handle.net/2003/25707
Title: Capacitive scanning dilatometry and frequency dependent thermal expansion of polymer films
Authors: Böhmer, Roland; Bauer, C.; Moreno-Flores, S.; Richert, R.; Sillescu, H.; Neher, D.
Abstract: The dilatometric properties of polymer films near and above their glass-transition temperatures were explored using capacitive high-frequency detection in temperature ramping as well as in harmonic temperature cycling experiments. The broad applicability of capacitive scanning dilatometry is demonstrated by the investigation of macromolecular systems of vastly different polarity such as polystyrene, polybutadiene, and polyvinylacetate. From temperature cycling experiments the real and imaginary parts of the frequency-dependent thermal-expansion coefficient are determined in the sub-Hz regime.2000-01-01T00:00:00ZMultiple-time correlation functions from spin-3/2 solid-state NMR spectroscopy
http://hdl.handle.net/2003/25592
Title: Multiple-time correlation functions from spin-3/2 solid-state NMR spectroscopy
Authors: Böhmer, Roland
Abstract: Stimulated echo spectroscopy of nonselectively excitable I = 3/2 nuclei offers new perspectives for the investigation of ultraslow motions predominantly in inorganic solids and solid-like materials. Conditions for the generation of pure, quadrupole modulated multipolar spin orders and for the detection of two- and four-time correlation functions are discussed. The case of spins I > 3/2 is also briefly considered.2000-11-01T00:00:00ZStimulated-echo NMR spectroscopy of 9Be and 7Li in solids
http://hdl.handle.net/2003/25591
Title: Stimulated-echo NMR spectroscopy of 9Be and 7Li in solids
Authors: Böhmer, Roland; Qi, F.; Jörg, T.2002-12-01T00:00:00ZDipolar and quadrupolar freezing in (KBr)1-x(KCN)x
http://hdl.handle.net/2003/25590
Title: Dipolar and quadrupolar freezing in (KBr)1-x(KCN)x
Authors: Böhmer, Roland; Volkmann, U. G.; Loidl, A.; Knorr, K.; Höchli, U. T.; Haussühl, S.1986-01-01T00:00:00ZDipolar and quadrupolar freezing in (NaCN)1-x(KCN)x
http://hdl.handle.net/2003/25589
Title: Dipolar and quadrupolar freezing in (NaCN)1-x(KCN)x
Authors: Böhmer, Roland; Loidl, A.; Schräder, T.; Knorr, K.; Kjems, J. K.; Born, R.1986-01-01T00:00:00ZElastic and quasielastic neutron scattering studies in KBr:KCN mixed crystals
http://hdl.handle.net/2003/25588
Title: Elastic and quasielastic neutron scattering studies in KBr:KCN mixed crystals
Authors: Böhmer, Roland; Loidl, A.; Schräder, T.; Knorr, K.; Mertz, B.; McIntyre, G. J.; Vogt, T.; Mutka, H.; Müllner, M.; Jex, H.; Haussühl, S.
Abstract: Neutron scattering studies in (KBr)1–x(KCN)x mixed crystals are presented utilizing powder diffraction, single crystal diffraction and time-of-flight techniques. Forx>0.6 (KBr)1–x(KCN)x crystals exhibit ferroelastic and ferroelectric low-temperature phases. Crystals withx<0.6 undergo transitions into an orientational glass state. Here we present a detailed phase diagram including new results for x=0.85 and x=0.65. For the latter system a stable rhombohedral low-temperature phase has been detected where the orientational disorder of the plastic phase is only partly removed and quadrupolar relaxations between three body diagonals are still possible. From the powder diffraction experiments we determined further the concentration dependence of the static Debye-Waller factors which can be explained by an interplay of the rotation-translation and the rotation-random strain coupling. With single crystal diffraction techniques we studied the diffuse scattered intensities which are directly related to the order parameter of the glass state. The temperature dependence of the quasielastic intensities near the critical concentration shows a strong increase forT<110K indicative for a freezing-in of shear fluctuations which is a characteristic feature of a non-ergodic instability. This phenomenon appears for ordering (x=0.65) and for non-ordering, glassy compounds. A further anomaly in (KBr)0.43(KCN)0.57 at 75 K is interpreted in terms of a residual elastic ordering process. With high-resolution time-of-flight techniques we analysed the dynamic structure factor for x=0.57. We demonstrate that the central peak consists of a static and a dynamic component. The results are compared with mode coupling theories which describe the glass transition in supercooled liquids.1989-03-01T00:00:00ZReorientations and phase transitions in (Kr)1-x(CH4-nDn)x
http://hdl.handle.net/2003/25587
Title: Reorientations and phase transitions in (Kr)1-x(CH4-nDn)x
Authors: Böhmer, Roland; Loidl, A.
Abstract: The condensed isotopic modifications of methane, CH4, CH2D2, and CD4 and their solutions with Kr were investigated using dielectric techniques. The polarizabilities were computed from the permittivities of the pure liquids. Phase diagrams of methane-krypton alloys were deduced in the liquid-solid coexistence region and for the low temperature phases (T<30 K). From the Curielaws, which described the permittivities of the (Kr)1-x(CH2D2)x mixed crystals, the permanent dioole moment of CH2D2 is calculated to be 4 mD. In the octopolar ordered phases of the solid solutions the effective dipole moments are enhanced significantly.1990-01-01T00:00:00Zß-Relaxation and low temperature specific heat in (KBr)1-x(KCN)x
http://hdl.handle.net/2003/25586
Title: ß-Relaxation and low temperature specific heat in (KBr)1-x(KCN)x
Authors: Böhmer, Roland; Mertz, B.; Eisele, B.; Loidl, A.1990-01-01T00:00:00ZKI)1-x(NH4I)x
http://hdl.handle.net/2003/25585
Title: KI)1-x(NH4I)x
Authors: Böhmer, Roland; Fehst, I.; Ott, W.; Loidl, A.; Haussühl, S.; Bostoen, C.1990-01-01T00:00:00ZDielectric study of orientational disorder in (CO2)1-x(N2O)x mixed crystals
http://hdl.handle.net/2003/25584
Title: Dielectric study of orientational disorder in (CO2)1-x(N2O)x mixed crystals
Authors: Böhmer, Roland; Loidl, A.
Abstract: The dipolar relaxation dynamics of solid solutions of carbon dioxide and nitrous oxide has been investigated using dielectric spectroscopy. The temperature dependence of the relaxation rate reveals an ideal Arrhenius behavior for all concentrations, with energy barriers which scale linearly between the two pure compounds. The attempt frequencies are anomalously high. The distribution of relaxation times due to the static random fields as introduced by the substitutional impurity molecules is almost negligible. The melting temperature is determined by a critical relaxation rate of 40 kHz.1990-01-01T00:00:00ZCalorimetric investigations of (NaCN)1-x(KCN)x orientational glasses
http://hdl.handle.net/2003/25583
Title: Calorimetric investigations of (NaCN)1-x(KCN)x orientational glasses
Authors: Böhmer, Roland; Mertz, B.; Berret, J. F.; Loidl, A.; Meissner, M.; Knaak, W.1990-01-01T00:00:00ZRelaxation dynamics in molecular alloys
http://hdl.handle.net/2003/25582
Title: Relaxation dynamics in molecular alloys
Authors: Böhmer, Roland; Loidl, A.1991-01-01T00:00:00ZThe glass transition in (KI)0.5(ND4I)0.5 mixed crystals as studied by deuteron spin-lattice relaxation
http://hdl.handle.net/2003/25581
Title: The glass transition in (KI)0.5(ND4I)0.5 mixed crystals as studied by deuteron spin-lattice relaxation
Authors: Böhmer, Roland; Fujara, F.; Hinze, G.1993-01-01T00:00:00ZRelaxation dynamics in C60 and C70 as studied by dielectric spectroscopy
http://hdl.handle.net/2003/25580
Title: Relaxation dynamics in C60 and C70 as studied by dielectric spectroscopy
Authors: Böhmer, Roland; Mondal, P.; Lunkenheimer, P.; Loidl, A.; Gugenberger, F.; Adelmann, P.; Meingast, C.1994-01-01T00:00:00ZDielectric study of partially isotope-substituted solid and liquid hydrogen and methane
http://hdl.handle.net/2003/25579
Title: Dielectric study of partially isotope-substituted solid and liquid hydrogen and methane
Authors: Böhmer, Roland; Giebenhain, U.; Loidl, A.1994-01-01T00:00:00ZThe freezing transition in (NH4I)x(KI)1-x
http://hdl.handle.net/2003/25578
Title: The freezing transition in (NH4I)x(KI)1-x
Authors: Böhmer, Roland; Winterlich, M.; Loidl, A.1995-01-01T00:00:00ZOrder parameters and dipole relaxation in betaine proton glasses
http://hdl.handle.net/2003/25577
Title: Order parameters and dipole relaxation in betaine proton glasses
Authors: Böhmer, Roland; Ries, H.; Fehst, I.; Loidl, A.1996-02-01T00:00:00ZThe phase diagram of (NH4I)x(KI)1-x
http://hdl.handle.net/2003/25573
Title: The phase diagram of (NH4I)x(KI)1-x
Authors: Böhmer, Roland; Paasch, M.; Winterlich, M.; Sonntag, R.; McIntyre, G. C.; Loidl, A.1996-02-01T00:00:00ZLinear and non-linear dielectric spectroscopy in dipolar glasses
http://hdl.handle.net/2003/25571
Title: Linear and non-linear dielectric spectroscopy in dipolar glasses
Authors: Böhmer, Roland; Loidl, A.; Hemberger, J.; Winterlich, M.; Ries, H.1996-01-01T00:00:00ZStatic freezing transition at a finite temperature in a quasi-one dimensional deuteron glass
http://hdl.handle.net/2003/25570
Title: Static freezing transition at a finite temperature in a quasi-one dimensional deuteron glass
Authors: Böhmer, Roland; Hemberger, J.; Ries, H.; Loidl, A.1996-01-01T00:00:00ZReply to Comment on "Static freezing transition at a finite temperature in a quasi-one dimensional deuteron glass"
http://hdl.handle.net/2003/25569
Title: Reply to Comment on "Static freezing transition at a finite temperature in a quasi-one dimensional deuteron glass"
Authors: Böhmer, Roland; Hemberger, J.; Ries, H.; Loidl, A.1997-01-01T00:00:00ZThe orientational glass (KI)0.56(NH4I)0.44 studied by 127I nuclear magnetic resonance
http://hdl.handle.net/2003/25568
Title: The orientational glass (KI)0.56(NH4I)0.44 studied by 127I nuclear magnetic resonance
Authors: Böhmer, Roland; Hinze, G.; Zalar, B.; Blinc, R.1997-01-06T00:00:00ZDeuteron NMR investigations of glass and phase transitions in (KI)1-x(ND4I)x mixed crystals
http://hdl.handle.net/2003/25567
Title: Deuteron NMR investigations of glass and phase transitions in (KI)1-x(ND4I)x mixed crystals
Authors: Böhmer, Roland; Winterlich, M.; Titze, A.; Hinze, G.
Abstract: Powdered potassium ammonium iodide mixed crystals with ammonium concentrations of 20% and 70% have been studied using deuteron NMR techniques. For the less doped sample the deuteron line starts to broaden and the spin-lattice relaxation time T1 becomes nonexponential below about 20 K. These observations indicate the breakdown of ergodicity and the onset of orientational glass freezing. At the lowest temperatures T1 reaches a plateau value. For the 70% sample a relatively abrupt onset of two-phase behavior at T<35 K is inferred from measurements of the spin-spin and spin-lattice relaxation times. These observations are taken to indicate an order-disorder transition to a crystalline phase which exhibits dynamically distinguishable lattice sites.1998-01-01T00:00:00ZHysteresis effects and mesoscopic domains
http://hdl.handle.net/2003/25566
Title: Hysteresis effects and mesoscopic domains
Authors: Böhmer, Roland; Winterlich, M.; Diezemann, G.; Hinze, G.1998-10-01T00:00:00ZPhase transition kinetics and reorientational dynamics of the plastic crystal meta-carborane studied by deuteron NMR
http://hdl.handle.net/2003/25565
Title: Phase transition kinetics and reorientational dynamics of the plastic crystal meta-carborane studied by deuteron NMR
Authors: Böhmer, Roland; Winterlich, M.; Zimmermann, H.
Abstract: Meta-carborane shows an orientationally disordered, orthorhombic phase which transforms into an ordered monoclinic phase near 170 K and into a quasi-isotropic cubic phase near 280 K. From deuteron magnetization measurements both transitions are found to exhibit a thermal hysteresis and slow transformation kinetics. Furthermore, solid-echo NMR spectra and spin-lattice relaxation times are reported. These results indicate that in the orthorhombic phase the meta-carborane molecules perform a strongly anisotropic motion which is characterized by a temperature dependent amplitude.2002-09-01T00:00:00ZMicroscopic origin of the nonexponential dynamics in a glassy crystal
http://hdl.handle.net/2003/25564
Title: Microscopic origin of the nonexponential dynamics in a glassy crystal
Authors: Böhmer, Roland; Winterlich, M.; Diezemann, G.; Zimmermann, H.
Abstract: The origin of the slow relaxation and of the dynamic heterogeneity is studied for an orientationally disordered crystal, orthocarborane, composed of quasi-icosahedrally shaped molecules. Multidimensional deuteron magnetic resonance reveals that large jump angles dominate their complex, anisotropic reorientational motion. It involves a sequence of small-angle tilts about locally preferred axes as well as symmetry adapted threefold jumps. The intrinsic dynamics of this glassy crystal is nonexponential and can be fully accounted for in terms of the tilt and jump motion.2003-12-03T00:00:00ZRotational motion in the molecular crystals meta- and ortho-carborane studied by deuteron nuclear magnetic resonance
http://hdl.handle.net/2003/25563
Title: Rotational motion in the molecular crystals meta- and ortho-carborane studied by deuteron nuclear magnetic resonance
Authors: Böhmer, Roland; Winterlich, M.; Diezemann, G.; Zimmermann, H.
Abstract: Spin-lattice and spin-spin-relaxation times, one- and two-dimensional spectra as well as two- and four-time correlation functions were measured for the molecular crystals ortho- and meta-carborane using deuteron nuclear magnetic resonance. It is found that in their noncubic phases these crystals exhibit highly anisotropic motions. In order to allow for a quantitative description of the motional geometry of the carboranes several stochastic models are formulated. By comparison of the model calculations with the experimental results it is found that the dynamics of these quasi-icosahedrally shaped molecules is governed by a composite reorientation process. Here the molecules perform threefold jumps around a molecule-fixed axis which itself can be tilted in four different directions with respect to a crystal-fixed axis. The tilt angle increases significantly with increasing temperature. On the basis of measurements of four-time stimulated-echo functions, implications for dynamic heterogeneity also in comparison with that of supercooled liquids are discussed.2005-09-07T00:00:00ZPolar relaxation mode in pure and iron-doped barium titanate
http://hdl.handle.net/2003/25562
Title: Polar relaxation mode in pure and iron-doped barium titanate
Authors: Böhmer, Roland; Maglione, M.; Loidl, A.; Höchli, U. T.1989-01-01T00:00:00ZCorrelated reorientations in NH4Al(SO4) · 12 H2O single crystals
http://hdl.handle.net/2003/25561
Title: Correlated reorientations in NH4Al(SO4) · 12 H2O single crystals
Authors: Böhmer, Roland; Lunkenheimer, P.; Vij, J. K.; Svare, I.
Abstract: Single crystals of aluminium ammonium alum NH4Al(SO4)2.12H2O were investigated using dielectric spectroscopy in the frequency range 1 kHz to 1 GHz. On approaching the ferroelectric transition at 58 K from above, a monodispersive polar mode is observed to slow down. The relaxation data can be described in terms of a pure Arrhenius law. The analysis of the dielectric loss reveals that in the paraelectric phase the static susceptibility cannot be described by a single Curie-Weiss law. The thermal variations of the Curie constant indicate that the dipolar correlations of the ammonium ions increase as the temperature is lowered.1990-06-18T00:00:00ZParaelectric and ferroelectric phases of betaine phosphite
http://hdl.handle.net/2003/25560
Title: Paraelectric and ferroelectric phases of betaine phosphite
Authors: Böhmer, Roland; Fehst, I.; Paasch, M.; Hutton, S. L.; Braune, M.; Loidl, A.; Dörffel, M.; Narz, T.; Haussühl, S.; McIntyre, G. J.
Abstract: The paraelectric and ferroelectric phases of betaine phosphite [BPI: (CH3)3 NCH2COO H3PO3] single crystals have been studied. The structure of BPI was determined at room temperature using x-ray and elastic neutron diffraction. Details of the ferroelectric transition and various low temperature properties were investigated using broad-band dielectric spectroscopy and calorimetric experiments. We compare the results obtained for single crystals of different origin which exhibit different ferroelectric phase transition temperatures.1993-01-01T00:00:00ZDielectric spectroscopy of SrTiO3
http://hdl.handle.net/2003/25559
Title: Dielectric spectroscopy of SrTiO3
Authors: Böhmer, Roland; Viana, R.; Lunkenheimer, P.; Hemberger, J.; Loidl, A.1994-01-01T00:00:00ZElectric field dependent dielectric constant and non-linear susceptibility in SrTiO3
http://hdl.handle.net/2003/25558
Title: Electric field dependent dielectric constant and non-linear susceptibility in SrTiO3
Authors: Böhmer, Roland; Hemberger, J.; Lunkenheimer, P.; Viana, R.; Loidl, A.
Abstract: SrTiO3 belongs to the class of incipient ferroelectrics in which an electrically ordered state is suppressed by quantum fluctuations. To investigate the nature of these excitations, we have measured the linear and nonlinear susceptibilities of SrTiO3 as a function of temperature T and field E. The application of large fields E counteracts the fluctuations and forces the system into an ordered state. The (E,T) diagram is presented. The field dependence of the dielectric constant can be well described within an Ising model including quantum tunneling. Minima in the third harmonic susceptibility, which is a measure of ferroelectric correlations, are taken as evidence for the onset of coherent tunneling phenomena near T=33 K.1995-01-01T00:00:00ZQuantum paraelectric and induced ferroelectric states in SrTiO3
http://hdl.handle.net/2003/25557
Title: Quantum paraelectric and induced ferroelectric states in SrTiO3
Authors: Böhmer, Roland; Hemberger, J.; Nicklas, M.; Viana, R.; Lunkenheimer, P.; Loidl, A.1996-01-01T00:00:00ZThe antiferroelectric phase transition in Rb3D(SO4)2 single crystals studied by 87Rb NMR
http://hdl.handle.net/2003/25556
Title: The antiferroelectric phase transition in Rb3D(SO4)2 single crystals studied by 87Rb NMR
Authors: Böhmer, Roland; Titze, A.; Hinze, G.
Abstract: The quadrupole perturbed central transition of the magnetic 87Rb resonance was used to study Rb3D(SO4)2 single crystals. Three resonance lines were found at room temperature. This result is in accord with the monoclinic space group reported for these crystals and does not require the previously invoked self-trapped proton states. The number of resonance lines quadruples below the antiferroelectric phase transition near 82 K. From the line splitting the order-parameter exponent of the low-temperature phase was found to be β=0.2. Spin-lattice relaxation times have been determined in the fully protonated and deuterated disulfates and were used to deduce the temperature dependence of the order-parameter fluctuations.1998-01-01T00:00:00ZLong-lived dynamic heterogeneity in a relaxor ferroelectric
http://hdl.handle.net/2003/25555
Title: Long-lived dynamic heterogeneity in a relaxor ferroelectric
Authors: Böhmer, Roland; Kircher, O.; Schiener, B.
Abstract: The polydispersive dielectric response of the relaxor ferroelectric lead magnesium niobate was investigated using nonresonant spectral hole-burning experiments. Using large alternating electric fields, specific parts of the relaxation-time distribution could be addressed selectively. This provides evidence for the dynamic heterogeneity of the dipolar reorientation process. The refilling of single as well as of double holes was much slower than the time scale set by the pump frequencies. Upon refilling, the holes exhibited no signs of spectral broadening. The present findings suggest a speedup of polarization response associated with a domain-wall depinning that is induced by the hole-burning event.1998-01-01T00:00:00ZInvestigations of the ferroelectric to paraelectric phase transition of vinyliden-fluoride trifluoroethylene copolymer thin films by electromechanical interferometry
http://hdl.handle.net/2003/25554
Title: Investigations of the ferroelectric to paraelectric phase transition of vinyliden-fluoride trifluoroethylene copolymer thin films by electromechanical interferometry
Authors: Böhmer, Roland; Urayama, K.; Kircher, O.; Neher, D.
Abstract: Electrostrictive and piezoelectric properties for a 1.3 µm thick film of a vinylidenefluoride/trifluoroethylene copolymer exhibiting a ferroelectric-to-paraelectric phase transition have been investigated as a function of temperature by means of electromechanical interferometry. The electrostriction remarkably increases in the vicinity of the phase transition temperature according to a sharp increase in the dielectric constant. For the unpoled sample in the ferroelectric phase with a polydomain structure in which the local spontaneous polarization is macroscopically cancelled out, the dependence of the electrostriction on the square of the applied electric field is nonlinear in the ferroelectric phase, while it is linear in the paraelectric phase. A theoretical model which takes into account the nonlinear dielectric constant can quantitatively explain the nonlinear contribution to the electrostriction in the vicinity of the ferroelectric–paraelectric phase transition, but it underestimates the contribution in the ferroelectric phase. Interactions within and/or between the ferroelectric domains in the polydomain structure are expected to contribute significantly to the nonlinear electrostriction. For poled samples a pronounced inverse-piezoelectric effect is measured. The achieved polarization and its temperature dependence are almost the same as for thicker films reported in earlier studies, obtained by different experimental techniques.1999-12-01T00:00:00Z87Rb-NMR spectra of the hydrogen-bonded crystal Rb3D(SO4)2
http://hdl.handle.net/2003/25553
Title: 87Rb-NMR spectra of the hydrogen-bonded crystal Rb3D(SO4)2
Authors: Böhmer, Roland; Titze, A.; Maiazza, A.; Hinze, G.
Abstract: The paraelectric and antiferroelectric phases of Rb3D(SO4)2 and Rb3H(SO4)2 were investigated using rubidium NMR. The angle dependence of the second order quadrupole shifted central 87Rb transitions was recorded at 300 K. It was found to be in full agreement with the symmetry elements of a monoclinic A2/a structure. The temperature dependence of the magnitude and asymmetry of the electrical field gradient (EFG) tensors at the Rb sites was determined using powder samples and revealed no differences between protonated and deuterated specimens. The antiferroelectric transition of Rb3D(SO4)2 (TN=82 K) is associated with a quadrupling of the unit cell. Below TN the magnitudes of the EFG at the Rb sites remain constant and smoothly tilt away from their high temperature orientations. The temperature dependence of the NMR line splittings associated with this tilt follows a power law implying an order parameter critical exponent of 0.21.1999-01-01T00:00:00ZNonresonant dielectric hole-burning on a titanium-modified lead magnesium niobate ceramic
http://hdl.handle.net/2003/25552
Title: Nonresonant dielectric hole-burning on a titanium-modified lead magnesium niobate ceramic
Authors: Böhmer, Roland; Kircher, O.; Diezemann, G.
Abstract: Nonresonant dielectric hole-burning experiments were performed on the titanium-modified relaxor ferroelectric lead magnesium niobate around the diffuse maximum in the dielectric permittivity. After applying large alternating electric pump fields we monitored the polarization response to small field steps for times between 0.3 ms and 100 s. Depending on the frequency of the pump oscillation a speedup of the polarization response was observed with a maximum located around times corresponding to the inverse pump frequency. The refilling of the dielectric holes was investigated for several temperatures, pump frequencies, and pump field amplitudes. It proceeded always slower than the time scale set by the pump frequencies. Additionally, we observe a significant increase of the refilling times for increasing pump field amplitudes. This finding can be interpreted to indicate that increasingly large pump fields enable the domain walls to cross larger and larger pinning barriers. The subsequent recovery process, which leads back to the equilibrium domain size distribution, proceeds in the absence of an external electrical field. This rationalizes that recovery is slowed down significantly by application of large pump field amplitudes since then the pinning barriers that have to be traversed back are larger.2001-07-02T00:00:00ZX-ray diffraction, optical birefringence, and 87Rb nuclear magnetic resonance spectroscopy of the paraelectric and antiferroelectric phases of Rb3DxH1-x(SO4)2
http://hdl.handle.net/2003/25551
Title: X-ray diffraction, optical birefringence, and 87Rb nuclear magnetic resonance spectroscopy of the paraelectric and antiferroelectric phases of Rb3DxH1-x(SO4)2
Authors: Böhmer, Roland; Titze, A.; Kusz, J.; Böhm, H.; Weber, H. J.2002-02-04T00:00:00ZNonresonant hole burning spectroscopy of the relaxor ferroelectric PLZT
http://hdl.handle.net/2003/25550
Title: Nonresonant hole burning spectroscopy of the relaxor ferroelectric PLZT
Authors: Böhmer, Roland; El Goresy, T.; Kircher, O.2002-01-01T00:00:00ZAging, rejuvenation, and memory phenomena in a lead-based relaxor ferroelectric
http://hdl.handle.net/2003/25549
Title: Aging, rejuvenation, and memory phenomena in a lead-based relaxor ferroelectric
Authors: Böhmer, Roland; Kircher, O.
Abstract: Isothermal aging and temperature cycle experiments were done on the relaxor ferroelectric lead magnesium niobate mixed with 10% lead titanate (PMN-10PT) around and below the diffuse maximum of the dielectric loss. With increasing aging time tw the isothermal evolution of the linear susceptibility follows a power law and does not show frequency scaling. The non-linear susceptibility, however, obeys nearly perfect ωt w-scaling. After aging the sample at a single temperature we observed both rejuvenation and memory effects in temperature cycle experiments. This observation indicates symmetric behavior in the sense that it shows up irrespective of whether cooling with subsequent re-heating or heating with subsequent re-cooling was performed. The memory effect is absent if subsequent to aging the temperature is increased significantly above that corresponding to the maximum in the dielectric loss. The symmetric behavior within negative and positive temperature cycles under these conditions can be rationalized by the notion of movable domain walls. These become fixed in their configuration on a large spatial scale while more flexible wall segments still show re-conformation processes when cooling or heating the sample after aging.2002-04-01T00:00:00ZPseudo-stochastic multiple-pulse excitation in dielectric spectroscopy
http://hdl.handle.net/2003/25548
Title: Pseudo-stochastic multiple-pulse excitation in dielectric spectroscopy
Authors: Böhmer, Roland; Kircher, O.; Hinze, G.
Abstract: The nonlinear dielectric response of a lead titanate-doped lead magnesium niobate relaxor ferroelectric was studied using pseudo-stochastic binary electrical field excitation. The polarization was recorded for various pulse spacings and electrical field amplitudes. The decay of the field-polarization cross-correlation function could be accelerated by increasing the amplitude of the pulse fields. The extension of these experiments in order to record multidimensional dielectric spectra is illustrated.2003-03-26T00:00:00ZGlass transitions in supercooled fluorocarbon mixtures
http://hdl.handle.net/2003/25547
Title: Glass transitions in supercooled fluorocarbon mixtures
Authors: Böhmer, Roland; Loidl, A.1992-01-01T00:00:00ZCovalent bond connectivity, medium range order, and physical properties in TeX and TeXAs glasses
http://hdl.handle.net/2003/25546
Title: Covalent bond connectivity, medium range order, and physical properties in TeX and TeXAs glasses
Authors: Böhmer, Roland; Ma, H.-L.; Zhang, X. H.; Lucas, J.; Senapati, H.; Angell, C. A.1992-01-01T00:00:00ZCorrelations of the nonexponentiality and state dependence of mechanical relaxations with bond connectivity in Ge-As-Se supercooled liquids
http://hdl.handle.net/2003/25545
Title: Correlations of the nonexponentiality and state dependence of mechanical relaxations with bond connectivity in Ge-As-Se supercooled liquids
Authors: Böhmer, Roland; Angell, C. A.1992-01-01T00:00:00ZSignature of ergodicity in the dynamic response of amorphous systems
http://hdl.handle.net/2003/25544
Title: Signature of ergodicity in the dynamic response of amorphous systems
Authors: Böhmer, Roland; Chamberlin, R. V.; Sanchez, E.; Angell, C. A.
Abstract: We have analyzed dynamical measurements from materials near their liquid-glass transition. The primary response is accurately characterized by a model for dispersive excitations on specific distributions of independently relaxing domains. Liquids have an equilibrium (Gaussian) distribution of domain sizes, which changes abruptly to a locally random (Poisson-like) distribution in the glass. When quenched to a temperature initially in the glass phase, after a definite anneal time, the Poisson-like distribution changes abruptly back to the Gaussian distribution characteristic of a liquid.1992-01-01T00:00:00ZElastic and viscoelastic properties of amorphous selenium and identification of the phase transition between ring and chain structures
http://hdl.handle.net/2003/25543
Title: Elastic and viscoelastic properties of amorphous selenium and identification of the phase transition between ring and chain structures
Authors: Böhmer, Roland; Angell, C. A.1992-01-01T00:00:00ZConnectivity, fragility, and non-exponential relaxation in covalently bonded glassformers
http://hdl.handle.net/2003/25542
Title: Connectivity, fragility, and non-exponential relaxation in covalently bonded glassformers
Authors: Böhmer, Roland; Angell, C. A.1993-01-01T00:00:00ZNon-exponential relaxations in strong and fragile glass-formers
http://hdl.handle.net/2003/25541
Title: Non-exponential relaxations in strong and fragile glass-formers
Authors: Böhmer, Roland; Ngai, K. L.; Angell, C. A.; Plazek, D. J.
Abstract: Deviations from thermally activated and from exponential response are typical features of the vitrification phenomenon and previously have been studied using viscoelastic, dielectric, calorimetric, optical, and other techniques. Linear response data from literature on about 70 covalent glass formers, ionic melts, supercooled liquids, amorphous polymers, and glassy crystals are surveyed. Except for orientational glasses and monohydric aliphatic alcohols a distinct but broad correlation of non-Debye behavior with non-Arrhenius relaxations is found. Within the broad trend several groups of materials, distinguished by their respective molecular complexity, can be identified and are shown to exhibit narrow correlations. At a given degree of deviation from Arrhenius behavior externally imposed stresses are relaxed with a departure from exponential behavior which is stronger the more the molecular or atomic subunits of the glassforming material are interconnected with each other.1993-09-01T00:00:00ZNon-linearity and non-exponentiality of primary relaxations
http://hdl.handle.net/2003/25540
Title: Non-linearity and non-exponentiality of primary relaxations
Authors: Böhmer, Roland
Abstract: The glass transformation range of supercooled liquids and amorphous polymers is characterized by a number of unusual properties such as structural state dependence as well as non-Debye and non-Arrhenius behaviors. It is shown how the latter phenomenon, quantified in terms of the fragility index m, is related to the associated spectral width in the linear response regime and to the non-linearity of the enthalpy relaxation for a diverse variety of glass-forming materials.1994-09-01T00:00:00ZDielectric relaxation in supercooled 1-propanol
http://hdl.handle.net/2003/25539
Title: Dielectric relaxation in supercooled 1-propanol
Authors: Böhmer, Roland; Schiener, B.
Abstract: By combining time- and frequency-domain techniques, the dielectric response of 1-propanol has been measured in a spectral range covering eleven decades. In particular it has been demonstrated that the dielectric loss due to the main relaxation in 1-propanol exhibits the Debye width up to timescales of 1000 s. In order to extend the range in which the dynamics of propanol can be exploited to high temperatures, conductivity relaxation due to ionic impurities has been used. The occurrence of high-frequency relaxation processes has been studied in both isomers of propanol. It is shown that the 1-propanol data yield a master curve in a previously suggested scaling representation. However, owing to the presence of the high-frequency excitations, no agreement with the universal scaling form is obtained.1995-01-01T00:00:00ZHigh-frequency dielectric spectroscopy of glycerol
http://hdl.handle.net/2003/25538
Title: High-frequency dielectric spectroscopy of glycerol
Authors: Böhmer, Roland; Lunkenheimer, P.; Pimenov, A.; Schiener, B.; Loidl, A.1996-03-10T00:00:00ZDielectric study of supercooled triphenylphosphite and butyronitrile
http://hdl.handle.net/2003/25537
Title: Dielectric study of supercooled triphenylphosphite and butyronitrile
Authors: Böhmer, Roland; Schiener, B.; Loidl, A.; Chamberlin, R. V.
Abstract: Dielectric relaxation has been studied in the supercooled liquids triphenylphosphite (TPP) and butyrontrile (BN). BN is relatively strong according to Angell's classification and can be characterized by a fragility index m = 47. TPP, on the other hand, appears to be the most fragile non-polymeric liquid studied so far (m = 160). The dielectric response of the two glass-formers exhibits different degrees of non-exponentiality which is analyzed in terms of a mesoscopic model of dynamically correlated domains. The relation of this model to the strong versus fragile liquid classification scheme is discussed.1996-01-01T00:00:00ZFast dynamics of glass-forming glycerol studied by dielectric spectroscopy
http://hdl.handle.net/2003/25536
Title: Fast dynamics of glass-forming glycerol studied by dielectric spectroscopy
Authors: Böhmer, Roland; Lunkenheimer, P.; Pimenov, A.; Dressel, M.; Goncharov, Yu G.; Loidl, A.1996-01-01T00:00:00ZDynamic heterogeneity in supercooled ortho-terphenyl studied by multidimensional deuteron NMR
http://hdl.handle.net/2003/25535
Title: Dynamic heterogeneity in supercooled ortho-terphenyl studied by multidimensional deuteron NMR
Authors: Böhmer, Roland; Hinze, G.; Diezemann, G.; Geil, B.; Sillescu, H.
Abstract: Using deuteron NMR, we have studied molecular reorientation rates and rate exchange processes in supercooled ortho-terphenyl. We monitor the re-equilibration of differently selected subensembles through four-time stimulated echo experiments. A comparison of the two-time with the four-time echoes suggests that the characteristic time scales for reorientation and dynamical exchange are relatively similar. The four-time correlation functions were described using various multi-state rate exchange models.1996-10-01T00:00:00ZNonresonant spectral hole burning in the slow dielectric response of supercooled liquids
http://hdl.handle.net/2003/25534
Title: Nonresonant spectral hole burning in the slow dielectric response of supercooled liquids
Authors: Böhmer, Roland; Schiener, B.; Chamberlin, R. V.
Abstract: Large-amplitude, low-frequency electric fields can be used to burn spectral holes in the dielectric response of supercooled propylene carbonate and glycerol. This ability to selectively modify the dielectric response establishes that the non-Debye behavior results from a distribution of relaxation times. Refilling of the spectral hole was consistent with a single recovery time that coincided with the peak in the distribution. Moreover, refilling occurred without significant broadening, which indicates negligible direct exchange between the degrees of freedom that responded to the field. Nonresonant spectral hole burning facilitates direct investigation of the intrinsic response of systems that exhibit nonexponential relaxation.1996-11-01T00:00:00ZDielectric relaxation in the fragile viscous liquid state of toluene
http://hdl.handle.net/2003/25533
Title: Dielectric relaxation in the fragile viscous liquid state of toluene
Authors: Böhmer, Roland; Döß, A.; Hinze, G.; Schiener, B.; Hemberger, J.
Abstract: Dielectric measurements were carried out on viscous toluene covering a frequency range from 0.1 Hz to 1 MHz. In order to suppress the pronounced crystallization tendency of this supercooled liquid it was contained in thin walled capillaries with outer diameters of 300 µm. From the temperature dependence of the characteristic dielectric relaxation times it was found that toluene is one of the most fragile low molecular weight glass-forming liquids, with a fragility index m=105. By comparison with time constants available from other experimental techniques it appears that near the glass transition the dielectric relaxation mode is not the slowest one.1997-08-08T00:00:00ZReorientations in supercooled glycerol studied by two-dimensional time-domain deuteron nuclear magnetic resonance spectroscopy
http://hdl.handle.net/2003/25532
Title: Reorientations in supercooled glycerol studied by two-dimensional time-domain deuteron nuclear magnetic resonance spectroscopy
Authors: Böhmer, Roland; Hinze, G.1998-07-01T00:00:00ZAmorphous polymorphis in ice investigated by inelastic neutron scattering
http://hdl.handle.net/2003/25531
Title: Amorphous polymorphis in ice investigated by inelastic neutron scattering
Authors: Böhmer, Roland; Schober, H.; Tölle, A.; Koza, M.; Fujara, F.; Angell, C. A.1998-01-01T00:00:00ZReorientational dynamics in simple supercooled liquids
http://hdl.handle.net/2003/25530
Title: Reorientational dynamics in simple supercooled liquids
Authors: Böhmer, Roland; Diezemann, G.; Hinze, G.; Sillescu, H.1998-08-02T00:00:00ZFragility under pressure
http://hdl.handle.net/2003/25529
Title: Fragility under pressure
Authors: Böhmer, Roland; Schug, K. U.; King, H. E.
Abstract: Viscosities of the fragile glass-forming liquids ortho-terphenyl and salol have been measured in a diamond anvil cell for pressures up to 1 GPa and for temperatures 300 K < T < 425 K. Utilizing a rolling ball technique viscosities up to 10^8 mPas could be determined. From these data the pressure dependence of the fragility index, corresponding to a reference viscosity of about 10^8 mPas, has been calculated. For ortho-terphenyl it is found that this "low-viscosity" fragility index is pressure invariant within experimental error. Salol exhibits a more complicated behavior1998-07-22T00:00:00ZRotational correlation functions and apparently enhanced translational diffusion in a free energy landscape model for the primary relaxation in glass-forming liquids
http://hdl.handle.net/2003/25528
Title: Rotational correlation functions and apparently enhanced translational diffusion in a free energy landscape model for the primary relaxation in glass-forming liquids
Authors: Böhmer, Roland; Hinze, G.; Diezemann, G.; Sillescu, H.
Abstract: We explore a simple model for supercooled liquids in which reorientational and translational motions are inherently coupled to the structural relaxation. Several controversial aspects of the molecular motion in glass-forming materials are naturally resolved within the framework of our model. These include the geometry of the molecular reorientations as well as the apparent enhancement of translational diffusion. The breakdown of the Stokes-Einstein relation is thus explained without assuming the existence of local diffusion coefficients.1998-01-01T00:00:00ZA nuclear magnetic resonance study of higher-order correlation functions in supercooled ortho-terphenyl
http://hdl.handle.net/2003/25527
Title: A nuclear magnetic resonance study of higher-order correlation functions in supercooled ortho-terphenyl
Authors: Böhmer, Roland; Diezemann, G.; Sillescu, H.; Hinze, G.
Abstract: Using deuteron NMR techniques two-, effective three-, and various four-time correlation functions were recorded for supercooled ortho-terphenyl at 10–15 K above the calorimetric glass transition in order to characterize the heterogeneous nature of its primary response. The experimental results could successfully be described within various energy landscape models as well as via continuous time random walk simulations. These theoretical considerations provide a suitable basis for a definition of the term dynamic heterogeneity. We discuss the power but also some limitations of the present multidimensional NMR techniques when applied to amorphous materials1998-01-15T00:00:00ZSpatial heterogeneity in glassy polystyrene detected by deuteron NMR relaxation
http://hdl.handle.net/2003/25526
Title: Spatial heterogeneity in glassy polystyrene detected by deuteron NMR relaxation
Authors: Böhmer, Roland; Döß, A.; Diezemann, G.; Hinze, G.; Sillescu, H.
Abstract: Using deuteron NMR, the dynamics of supercooled polystyrene-d3 was investigated near the calorimetric glass transition. At these temperatures non-exponential spin lattice relaxation is found, indicating the presence of spatial heterogeneity. With increasing temperature, structural relaxation becomes fast enough to average efficiently over different spatial environments, leading to exponential magnetization decays. A qualitative comparison with toluene as a representative of a low molecular weight glass former is carried out. Indications are found that in polystyrene the observed averaging process is more effective at Tg than it is in toluene.1999-02-27T00:00:00ZReorientations and translations in a fragile glass-former
http://hdl.handle.net/2003/25525
Title: Reorientations and translations in a fragile glass-former
Authors: Böhmer, Roland; Kircher, O.; Alba-Simionesco, C.
Abstract: The rotational dynamics in supercooled liquid and glassy meta-fluoroaniline was studied using proton and fluorine spin-lattice relaxation times. It is shown that while proton relaxation is dominated by homonuclear relaxation, for fluorine heteronuclear relaxation prevails. The results could be well described using a distribution of correlation times. The mean correlation times show pronounced deviations from the simple Arrhenius law. In addition translational self-diffusion coefficients were measured for T>200 K using a static magnetic field gradient technique.2000-02-23T00:00:00ZStructural relaxation of the fragile glass-former propylene carbonate studied by nuclear magnetic resonance
http://hdl.handle.net/2003/25524
Title: Structural relaxation of the fragile glass-former propylene carbonate studied by nuclear magnetic resonance
Authors: Böhmer, Roland; Qi, F.; Schug, K . U.; Döß, A.; Dupont, S.; Sillescu, H.; Kolshorn, H.; Zimmermann, H.
Abstract: Proton, deuteron, and carbon NMR methods were applied to study various isotopic species of supercooled and glassy propylene carbonate. The molecular diffusion was investigated via static field gradient diffusometry and yielded evidence for a particularly pronounced enhancement of the translational over the reorientational dynamics. The latter was monitored using measurements of spin-lattice relaxation times and of two-time stimulated echoes. The sin–sin correlation functions exhibited a pronounced dependence on the evolution times. From these results it is concluded that the geometry of molecular reorientation can be described as a superposition of moderately small jump angles (about 30°) and a quasi-diffusive component which is responsible for about one-third of the total angular displacement. Finally, four-time stimulated echoes confirmed that the nonexponential relaxation in propylene carbonate is dynamically heterogeneous in nature.2000-06-01T00:00:00ZDynamic thermal expansivity near the glass transition
http://hdl.handle.net/2003/25523
Title: Dynamic thermal expansivity near the glass transition
Authors: Böhmer, Roland; Bauer, C.; Richert, R.; Christensen, T.
Abstract: Dielectric techniques were used to investigate the thermal expansivity of polystyrene films. Capacitive scanning dilatometry (CSD) employs temperature ramping in order to monitor the non-linear structural relaxation in the glass transformation range and to quantify liquid fragility. In the linear response regime, the complex thermal expansivity is obtained as a function of the temperature cycling frequency and is observed to reflect the structural relaxation.2000-02-01T00:00:00ZDeuteron and carbon magnetic resonance studies of liquid and glassy salol
http://hdl.handle.net/2003/25522
Title: Deuteron and carbon magnetic resonance studies of liquid and glassy salol
Authors: Böhmer, Roland; Döß, A.; Hinze, G.; Sillescu, H.; Kolshorn, H.; Vogel, M.; Zimmermann, H.
Abstract: The glass-former salol was studied using carbon and deuteron NMR. For the 13C measurements protonated samples were used. For the deuteron experiments salol was selectively deuterated at the phenyl ring. The measurement of the spin-lattice relaxation times provides clear evidence for an anisotropic reorientation of the molecules in the supercooled liquid. Close to the glass transition a bimodal longitudinal magnetization recovery is observed which indicates the presence of relatively slow intramolecular degrees of freedom. From deuteron solid echo spectra this process is identified as a uniaxial large amplitude motion of the phenyl group which exhibits a broad distribution of correlation times.2000-04-01T00:00:00ZIsotope effects on the dynamics of a supercooled van der Waals liquid
http://hdl.handle.net/2003/25521
Title: Isotope effects on the dynamics of a supercooled van der Waals liquid
Authors: Böhmer, Roland; Jörg, T.; Sillescu, H.; Zimmermann, H.
Abstract: Deuteron magnetic resonance was used to study three differently isotope-labeled species of the fragile glass-forming liquid ortho-terphenyl. The calorimetric glass transition of the isotope deuterated only at the central phenyl ring is significantly lower than that of the perdeuterated one. It is shown that while the ortho-terphenyl molecule is not as rigid as previously often assumed, its overall reorientation geometry is independent of deuteration. The characteristic jump angles are found to increase with temperature, thus resolving an apparent discrepancy previously noted when comparing typical jump sizes from NMR with other data.2000-03-15T00:00:00ZDynamical heterogeneity in a- and ß-relaxations of glass-forming liquids as seen by deuteron NMR
http://hdl.handle.net/2003/25520
Title: Dynamical heterogeneity in a- and ß-relaxations of glass-forming liquids as seen by deuteron NMR
Authors: Böhmer, Roland; Hinze, G.; Jörg, T.; Qi, F.; Sillescu, H.
Abstract: 2H NMR experiments in deuterated supercooled liquids are discussed in relation with α- and β-relaxation heterogeneity with particular emphasis on the latter. The results are exemplified using data from deuterated ortho-terphenyl, propylene carbonate and toluene where partial deuteration provides additional information on intramolecular motion.2000-02-28T00:00:00ZSlow and fast methyl group rotations in fragile glass-formers studied by NMR
http://hdl.handle.net/2003/25519
Title: Slow and fast methyl group rotations in fragile glass-formers studied by NMR
Authors: Böhmer, Roland; Qi, F.; Hinze, G.; Sillescu, H.; Zimmermann, H.
Abstract: The spin-lattice relaxation times of the selectively ring deuterated, fragile glass-formers propylene carbonate and toluene were compared with those measured for species which were specifically labeled at the methyl groups. It was found that the dynamics of the CD3 group is strongly decoupled from that associated with the primary response of toluene, while for propylene carbonate the degree of decoupling is relatively weak. The experimental results could be described successfully using a model which takes into account the ring dynamics as well as those of the methyl group.2000-10-05T00:00:00ZThe methyl group as built-in probe of the glassy dynamics in propylene carbonate
http://hdl.handle.net/2003/25518
Title: The methyl group as built-in probe of the glassy dynamics in propylene carbonate
Authors: Böhmer, Roland; Qi, F.; Sillescu, H.
Abstract: Deuteron nuclear magnetic resonance was employed to study primarily the glassy phase of the fragile glass-former propylene carbonate, selectively deuterated at the methyl group. The analysis of temperature dependent line shapes allowed us to map out the energy barrier distribution below the glass transition temperature. The results were compared to measurements of spin-relaxation times. We determined the relative width of the barrier distribution and the extent of the decoupling between the methyl group rotation time and the structural relaxation time near the glass transition. These findings are discussed in the context of what has been reported for secondary relaxation processes in other glass-formers. Stimulated echo experiments on propylene carbonate demonstrate that, while in the supercooled liquid small angle reorientations dominate, in the glass the methyl group performs 120° (or 60°) jumps.2001-01-01T00:00:00ZComment on "Hole-Burning Experiments within Glassy Models with Infinite Range Interactions"
http://hdl.handle.net/2003/25517
Title: Comment on "Hole-Burning Experiments within Glassy Models with Infinite Range Interactions"
Authors: Böhmer, Roland; Diezemann, G.
Abstract: A Comment on the Letter by Leticia F. Cugliandolo and José Luis Iguain, Phys. Rev. Lett. 85, 3448 (2000). The authors of the Letter offer a Reply.2001-08-31T00:00:00ZIntramolecular motions in simple glass-forming liquids studied by deuteron NMR
http://hdl.handle.net/2003/25516
Title: Intramolecular motions in simple glass-forming liquids studied by deuteron NMR
Authors: Böhmer, Roland; Sillescu, H.; Döß, A.; Hinze, G.; Jörg, T.; Qi, F.2002-01-01T00:00:00ZHeterogeneity at the glass transition
http://hdl.handle.net/2003/25515
Title: Heterogeneity at the glass transition
Authors: Böhmer, Roland; Sillescu, H.; Diezemann, G.; Hinze, G.
Abstract: We critically discuss the information that can be obtained from experiments with respect to the existence, the life time, and the length scale of dynamical heterogeneity in glass-forming liquids. The ability to select a dynamically distinguishable subensemble and observe its return to the full equilibrium ensemble is illustrated by examples from multi-dimensional NMR. We also discuss non-resonant hole burning spectroscopy as an example for which two separate time scales are involved.2002-05-30T00:00:00ZNuclear magnetic resonance and dielectric spectroscopy of a simple supercooled liquid
http://hdl.handle.net/2003/25514
Title: Nuclear magnetic resonance and dielectric spectroscopy of a simple supercooled liquid
Authors: Böhmer, Roland; Qi, F.; El Goresy, T.; Döß, A.; Diezemann, G.; Hinze, G.; Sillescu, H.; Blochowicz, T.; Gainaru, C.; Rössler, E.; Zimmermann, H.
Abstract: The small-molecule glass former methyl tetrahydrofuran (MTHF) was investigated using dielectric spectroscopy, spin-lattice relaxometry, multidimensional stimulated-echo nuclear magnetic resonance techniques, and field gradient diffusometry. We show experimentally that MTHF nicely fits into the pattern of related small-molecule glass-forming liquids, including the existence of a high-frequency contribution to the dielectric loss, the appearance of a pronounced translational enhancement, the dominance of small average rotational jump angles, and the existence of short-lived dynamical heterogeneity.2003-04-22T00:00:00ZDielectric study of the viscous and glassy states of a binary, nifedipine-based pharmaceutical alloy
http://hdl.handle.net/2003/25513
Title: Dielectric study of the viscous and glassy states of a binary, nifedipine-based pharmaceutical alloy
Authors: Böhmer, Roland; El Goresy, T.
Abstract: An equimolar mixture of nifedipine and acetaminophen was studied in its supercooled liquid and in its glassy state using dielectric spectroscopy. It is found that this alloy is characterized by a relatively large steepness index m = 83. The low-temperature relaxation of pure nifedipine is studied and compared with that of the mixture.2006-08-08T00:00:00ZAging in a free-energy landscape model for glassy relaxation
http://hdl.handle.net/2003/25512
Title: Aging in a free-energy landscape model for glassy relaxation
Authors: Böhmer, Roland; Diezemann, G.
Abstract: Several fluctuation-dissipation relations are investigated for a simple free-energy landscape model designed to describe the primary relaxation in supercooled liquids. The calculations of the response and of the correlation functions are performed for a quench from a high temperature to a low temperature. In the model, all dynamical quantities reach equilibrium after long times, but for times shorter than the re-equilibration time they do not exhibit time-translational invariance and the fluctuation-dissipation theorem is violated. Two measures for these violations are considered. One such measure is given by the slope in a plot of the integrated response versus the correlation function and another one by the so-called fluctuation-dissipation ratio. It is found that these measures do not coincide and furthermore are not independent of the dynamical variable considered in the calculation. We propose to determine the fluctuation-dissipation ratio experimentally via measurements of the deuteron spin-lattice relaxation rate and the dielectric loss2006-01-01T00:00:00ZRelaxation in the glass-former acetyl salicylic acid studied by deuteron magnetic resonance and dielectric spectroscopy
http://hdl.handle.net/2003/25511
Title: Relaxation in the glass-former acetyl salicylic acid studied by deuteron magnetic resonance and dielectric spectroscopy
Authors: Böhmer, Roland; Nath, R.; El Goresy, T.; Geil, B.; Zimmermann, H.
Abstract: Supercooled liquid and glassy acetylsalicylic acid was studied using dielectric spectroscopy and deuteron relaxometry in a wide temperature range. The supercooled liquid is characterized by major deviations from thermally activated behavior. In the glass the secondary relaxation exhibits the typical features of a Johari-Goldstein process. Via measurements of spin-lattice relaxation times the selectively deuterated methyl group was used as a sensitive probe of its local environments. There is a large difference in the mean activation energy in the glass with respect to that in crystalline acetylsalicylic acid. This can be understood by taking into account the broad energy barrier distribution in the glass.2006-01-01T00:00:00ZCorrelation of primary relaxations and high-frequency modes in supercooled liquids
http://hdl.handle.net/2003/25510
Title: Correlation of primary relaxations and high-frequency modes in supercooled liquids
Authors: Böhmer, Roland; Diezemann, G.; Geil, B.
Abstract: The question regarding a possible correlation of the time scales of primary and secondary relaxations in supercooled liquids is formulated quantitatively. It is shown how this question can be answered using spin-lattice relaxation weighted stimulated-echo experiments, which are presented in an accompanying paper [A. Nowaczyk, B. Geil, G. Hinze, and R. Böhmer, Phys. Rev. E 74, 041505 (2006)]. General theoretical expressions relevant for the description of such experiments in the presence of correlation effects are derived. These expressions are analyzed by Monte Carlo integration for various correlation scenarios also including exchange processes, which are the hallmark of dynamical heterogeneity. The results of these numerical simulations provide clear signatures that allow one to distinguish uncorrelated from differently correlated cases. Since modified spin-lattice relaxation effects occur in the presence of nonexponential magnetization recovery, it is shown how to correct for them to a good approximation.2006-01-01T00:00:00ZCorrelation of primary relaxations and high-frequency modes in supercooled liquids
http://hdl.handle.net/2003/25509
Title: Correlation of primary relaxations and high-frequency modes in supercooled liquids
Authors: Böhmer, Roland; Nowaczyk, A.; Geil, B.; Hinze, G.
Abstract: Using spin-lattice relaxation weighted stimulated-echo spectroscopy, we report evidence for a correlation of the primary and secondary relaxation times. The experiments are performed using deuteron nuclear magnetic resonance somewhat above the calorimetric glass-transition of ortho-terphenyl, D-sorbitol, and cresolphthalein-dimethylether. The data analysis is based on the procedure outlined in the accompanying theoretical paper [B. Geil, G. Diezemann, and R. Böhmer, Phys. Rev. E 74, 041504 (2006)]. Direct experimental evidence for a modified spin-lattice relaxation is obtained from measurements on a methyl deuterated acetyl salicylic acid glass. The limitations of the present experimental method are discussed.2006-01-01T00:00:00ZCorrelation of primary and secondary relaxations in a supercooled liquid
http://hdl.handle.net/2003/25508
Title: Correlation of primary and secondary relaxations in a supercooled liquid
Authors: Böhmer, Roland; Diezemann, G.; Geil, B.; Hinze, G.; Nowaczyk, A.; Winterlich, M.
Abstract: The widespread assumption that primary and secondary relaxations in glass-forming materials are independent processes is scrutinized using spin-lattice relaxation weighted stimulated-echo spectroscopy. This nuclear magnetic resonance (NMR) technique is simultaneously sensitive to the dynamics on well-separated time scales. For the deeply supercooled liquid sorbitol, which exhibits a strong secondary relaxation, the primary relaxation (that is observable using NMR) can be modified by suppressing the contributions of those subensembles which are characterized by relatively slow secondary relaxations. This is clear evidence for a correlation between primary and secondary relaxation times. In the disordered crystal orthocarborane high-frequency processes are absent and consequently no such modifications could be achieved.2007-01-01T00:00:00ZDielectric relaxation processes in solid and supercooled liquid solutions of acetaminophen and nifedipine
http://hdl.handle.net/2003/25507
Title: Dielectric relaxation processes in solid and supercooled liquid solutions of acetaminophen and nifedipine
Authors: Böhmer, Roland; El Goresy, Tarek2007-01-01T00:00:00Z2H-NMR studies of supercooled and glassy aspirin
http://hdl.handle.net/2003/25506
Title: 2H-NMR studies of supercooled and glassy aspirin
Authors: Böhmer, Roland; Nath, R.; Geil, B.; Nowaczyk, A.
Abstract: Acetyl salicylic acid, deuterated at the methyl group, was investigated using 2H NMR in its supercooled and glassy states. Just above the glass transition temperature the molecular reorientations, studied using stimulated-echo spectroscopy, demonstrated a large degree of similarity with other glass formers. Deep in the glassy phase the NMR spectra look similar to those reported for the crystal and below 20 K they are indicating that rotational tunneling plays a role. Measurements of the spin-lattice relaxation times for temperatures below 150 K reveal a broad distribution of correlation times in the glass. The dominant energy barrier characterizing the slow-down of the methyl group motion is significantly lower than the well-defined barrier in the crystal.2007-01-01T00:00:00ZA.c. technique for simultaneous study of local and global linear responses near the glass transition
http://hdl.handle.net/2003/25505
Title: A.c. technique for simultaneous study of local and global linear responses near the glass transition
Authors: Böhmer, Roland; Sanchez, E.; Angell, C. A.1992-01-01T00:00:00Z