Böhmer, Roland Prof. Dr.
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Item Diluting the hydrogen bonds in viscous solutions of n-butanol with n-bromobutane(American Institute of Physics, 2008-04-21) Böhmer, Roland; El Goresy, T.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.Item Dielectric properties of fluoromethanes and fluoromethane mixtures(The American Institute of Physics, 1988-10-15) Böhmer, Roland; Loidl, A.Item Dipolar relaxations in solid (CF4)1-x(CClF3)x(The American Institute of Physics, 1989-09-01) Böhmer, RolandItem Relaxation dynamics in molecular alloys(The American Institute of Physics, 1991-02-01) Böhmer, Roland; Loidl, A.Item Spin fluctuations in the heavy fermion compound CeCu2Ge2 probed by nuclear magnetic resonance(Elsevier Science, 1995) Böhmer, Roland; Büttgen, N.; Loidl, A.Item NMR studies of the heavy fermion alloy Ce(Cu1-xNix)2Ge2(The American Physical Society, 1996) Böhmer, Roland; Büttgen, N.; Krimmel, A.; Loidl, A.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.Item Surface-induced order and diffusion of 5CB liquid crystal confined to porous glass(Elsevier Scienc, 2001) Böhmer, Roland; Vilfan, M.; Apih, T.; Gregorovic, A.; Zalar, B.; Lahajnar, G.; Zumer, S.; Hinze, G.; Althoff, G.Item Radio-frequency dielectric measurements at temperatures from 10 to 450 K(The American Institute of Physics., 1989-02-01) Böhmer, Roland; Maglione, M.; Lunkenheimer, P.; Loidl, A.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.Item Mechanical stress relaxation in inorganic glasses studied by a step-strain technique(Elsevier Science, 1991-06-11) Böhmer, Roland; Senapati, H.; Angell, C. A.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.Item Condensed (Ar)1-x(N2)x in a four rod capacitor(IOP Publishing, 1991-08-14) Böhmer, RolandA 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.Item Pulsed dielectric spectroscopy of supercooled liquids(Springer-Verlag, 1995-11) Böhmer, Roland; Schiener, B.; Hemberger, J.; Chamberlin, R. V.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.Item Far-off-resonance averaging of dipolar interactions in solids(Academic Press, 1997) Böhmer, Roland; Chang, I.; Diezemann, G.; Hinze, G.; Sillescu, H.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.Item Dielectric hole burning spectroscopy of supercooled liquids(American Institute of Physics, 1997-11-15) Böhmer, Roland; Schiener, B.; Chamberlin, R. V.; Diezemann, G.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.Item Decay of single and double quantum coherences in deuterated glassy ortho-terphenyl(American Institute of Physics, 1998-05-22) Böhmer, Roland; Kircher, O.; Diezemann, G.; Hinze, G.; Schug, K. U.; Sillescu, H.; Zimmermann, H.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 protonsItem Experimental determination of four-time stimulated echoes in liquids, colloidal suspensions, and crystals(Academic Press, 1998) Böhmer, Roland; Hinze, G.; Diezemann, G.; Sillescu, H.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.Item Capacitive scanning dilatometry and frequency dependent thermal expansion of polymer films(The American Physical Society, 2000) Böhmer, Roland; Bauer, C.; Moreno-Flores, S.; Richert, R.; Sillescu, H.; Neher, D.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.Item Multiple-time correlation functions from spin-3/2 solid-state NMR spectroscopy(Academic Press, 2000-11) Böhmer, RolandStimulated 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.Item Stimulated-echo NMR spectroscopy of 9Be and 7Li in solids(Elsevier Science, 2002-12) Böhmer, Roland; Qi, F.; Jörg, T.Item Dipolar and quadrupolar freezing in (KBr)1-x(KCN)x(The American Physical Society, 1986) Böhmer, Roland; Volkmann, U. G.; Loidl, A.; Knorr, K.; Höchli, U. T.; Haussühl, S.Item Dipolar and quadrupolar freezing in (NaCN)1-x(KCN)x(The American Physical Society, 1986) Böhmer, Roland; Loidl, A.; Schräder, T.; Knorr, K.; Kjems, J. K.; Born, R.