arXiv subjects

P. Lunkenheimer

Publications and source records attributed to P. Lunkenheimer.

At least 73 records · Page 4Linked to original sources

arXiv2014

Dynamics of Protein Hydration Water

We present the frequency- and temperature-dependent dielectric properties of lysozyme solutions in a broad concentration regime, measured at subzero temperatures and compare the results with measurements above the freezing point of water and on hydrated lysozyme powder. Our experiments allow examining the dynamics of unfreezable hydration water in a broad temperature range including the so-called No Man's Land (160 - 235 K). The obtained results prove the bimodality of the hydration shell dynamics and are discussed in the context of the highly-debated fragile-to-strong transition of water.

cond-mat.soft↗

arXiv2014

Low temperature dielectric relaxation study of aqueous solutions of diethylsulfoxide

In the present work, dielectric spectra of mixtures of diethylsulfoxide (DESO) and water are presented, covering a concentration range of 0.2 - 0.3 molar fraction of DESO. The measurements were performed at frequencies between 1 Hz and 10 MHz and for temperatures between 150 and 300 K. It is shown that DESO/water mixtures have strong glass-forming abilities. The permittivity spectra in these mixtures reveal a single relaxation process. It can be described by the Havriliak-Negami relaxation function and its relaxation times follow the Vogel-Fulcher-Tammann law, thus showing the typical signatures of glassy dynamics. The concentration dependence of the relaxation parameters, like fragility, broadening, and glass temperature, are discussed in detail.

cond-mat.dis-nn↗

arXiv2014

Nonlinear dielectric response of Debye, alpha, and beta relaxation in 1-propanol

We present nonlinear dielectric measurements of glass-forming 1-propanol, a prototypical example for the monohydroxy alcohols that are known to exhibit unusual relaxation dynamics, namely an additional Debye relaxation, slower than the structural alpha relaxation. Applying high ac fields of 468 kV/cm allows for a detailed investigation of the nonlinear properties of all three relaxation processes occurring in 1-propanol, namely the Debye, alpha, and beta relaxation. Both the field-induced variations of dielectric constant and loss are reported. Polarization saturation and the absorption of field energy govern the findings in the Debye-relaxation regime, well consistent with the suggested cluster-like nature of the relaxing entities. The behavior of the alpha relaxation is in good accord with the expectations for a heterogeneous relaxation scenario. Finally, the Johari-Goldstein beta-relaxation in 1-propanol seems to exhibit no or only weak field dependence, in agreement with recent findings for the excess wing of canonical glass formers.

cond-mat.soft↗

arXiv2014

Excess wing in glass-forming glycerol and LiCl-glycerol mixtures detected by neutron scattering

The relaxational dynamics in glass-forming glycerol and glycerol mixed with LiCl is in-vestigated using different neutron scattering techniques. The performed neutron spin-echo experiments, which extend up to relatively long relaxation-time scales of the order of 10 ns, should allow for the detection of contributions from the so-called excess wing. This phenomenon, whose microscopic origin is controversially discussed, arises in a variety of glass formers and, until now, was almost exclusively investigated by dielectric spectros-copy and light scattering. Here we show that the relaxational process causing the excess wing also can be detected by neutron scattering, which directly couples to density fluctua-tions.

cond-mat.dis-nn↗

arXiv2013

Cooperativity and the freezing of molecular motion at the glass transition

The slowing down of molecular dynamics when approaching the glass transition generally proceeds much stronger than expected for thermally activated motions. This strange phenomenon can be formally ascribed to a temperature-dependent activation energy E(T). In the present work, via measurements of the third-order nonlinear dielectric susceptibility, we deduce the increase of the number of correlated molecules Ncorr when approaching the glass transition and find a surprisingly simple correlation of E(T) and Ncorr(T). This provides strong evidence that the non-canonical temperature development of glassy dynamics is caused by a temperature-dependent energy barrier arising from the cooperative motion of ever larger numbers of molecules at low temperatures.

cond-mat.soft↗

arXiv2013

Multiferroicity in the Mott Insulating Charge-Transfer Salt κ-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl

The recently proposed multiferroic state of the charge-transfer salt κ-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl [P. Lunkenheimer et al., Nature Mater., vol. 11, pp. 755-758, Sept. 2012] has been studied by dc-conductivity, magnetic susceptibility and measurements of the dielectric constant on various, differently prepared single crystals. In the majority of crystals we confirm the existence of an order-disorder-type ferroelectric state which coincides with antiferromagnetic order. This phenomenology rules out scenarios which consider an inhomogeneous, short-range-ordered ferroelectric state. Measurements of the dielectric constant and the magnetic susceptibility on the same crystals reveal that both transitions lie very close to each other or even collapse, indicating that both types of order are intimately coupled to each other. We address issues of the frequency dependence of the dielectric constant ε' and the dielectric loss ε'' and discuss sample-to-sample variations.

cond-mat.str-el↗

arXiv2013

Supercooled-liquid and plastic-crystalline state in succinonitrile-glutaronitrile mixtures

We report a thorough characterization of the glassy phases of mixtures of succinonitrile and glutaronitrile via dielectric spectroscopy and differential scanning calorimetry. This system is revealed to be one of the rare examples where both glassy states of matter, a structurally disordered supercooled liquid and an orientationally disordered plastic crystal, can be prepared in the same material. Both disordered states can be easily supercooled, finally arriving at a structural-glass or a glassy-crystal state. Detailed investigations using broadband dielectric spectroscopy enable a comparison of the glassy dynamics in both phases. Just as previously demonstrated for supercooled-liquid and plastic-crystalline ethanol, our experiments reveal very similar relaxational behavior and glass temperatures of both disordered states. Thus the prominent role of orientational degrees of freedom in the glass transition, suggested on the basis of the findings for ethanol, is fully corroborated by the present work. Moreover, the fragilities of both phases are determined and compared for different mixtures. The findings can be well understood within an energy-landscape based explanation of fragility.

cond-mat.dis-nn↗

arXiv2013

Magnetic-field induced multiferroicity in a quantum critical frustrated spin liquid

Dielectric spectroscopy is used to check for the onset of polar order in the quasi one-dimensional quantum spin system Sul-Cu2Cl4 when passing from the spin-liquid state into the ordered spiral phase in an external magnetic field. We find clear evidence for multiferroicity in this material and treat in detail its H-T phase diagram close to the quantum-critical regime.

cond-mat.str-el↗

arXiv2013

Debye relaxation and 250 K anomaly in glass forming monohydroxy alcohols

A previous dielectric, near-infrared (NIR), and nuclear magnetic resonance study on the hydrogen-bonded liquid 2-ethyl-1-hexanol [C. Gainaru et al., Phys. Rev. Lett. 107, 118304 (2011)] revealed anomalous behavior in various static quantities near 250 K. To check whether corresponding observations can be made for other monohydroxy alcohols as well, these experimental methods were applied to such substances with 5, 6, 7, 8, and 10 carbon atoms in their molecular backbone. All studied liquids exhibit a change of behavior near 250 K which is tentatively ascribed to effects of hydrogen bond cooperativity. By analyzing the NIR band intensities, a linear cluster size is derived that agrees with estimates from dielectric spectroscopy. All studied alcohols, except 4-methyl-3-heptanol, display a dominant Debye-like peak. Furthermore, neat 2-ethyl-1-butanol exhibits a well resolved structural relaxation in its dielectric loss spectrum which so far has only been observed for diluted monohydroxy alcohols.

physics.chem-ph↗

arXiv2013

Nonlinear dielectric response at the excess wing of glass-forming liquids

We present nonlinear dielectric measurements of glass-forming glycerol and propylene carbonate applying electrical fields up to 671 kV/cm. The measurements extend to sufficiently high frequencies to allow for the investigation of the nonlinear behavior in the regime of the so-far mysterious excess wing, showing up in the loss spectra of many glass formers as a second power law at high frequencies. Surprisingly, we find a complete lack of nonlinear behavior in the excess wing, in marked contrast to the alpha-relaxation where, in agreement with previous reports, a strong increase of dielectric constant and loss is found.

cond-mat.soft↗

arXiv2012

Ions in glass forming glycerol: Close correlation of alpha and fast beta relaxation

We provide broadband dielectric loss spectra of glass-forming glycerol with varying additions of LiCl. The measurements covering frequencies up to 10 THz extend well into the region of the fast beta process, commonly ascribed to caged molecule dynamics. Aside of the known variation of the structural alpha relaxation time and a modification of the excess wing with ion content, we also find a clear influence on the shallow loss minimum arising from the fast beta relaxation. Within the framework of mode-coupling theory, the detected significant broadening of this minimum is in reasonable accord with the found variation of the alpha-relaxation dynamics. A correlation between alpha-relaxation rate and minimum position holds for all ion concentrations and temperatures, even below the critical temperature defined by mode-coupling theory.

cond-mat.soft↗

arXiv2012

Absence of polar order in LuFe2O4

LuFe2O4 often is considered as a prototypical multiferroic with polar order arising from the electronic degrees of freedom only ("electronic ferroelectricity"). In the present work, we check the intrinsic nature of the dielectric response of this material by performing dielectric measurements of polycrystalline samples with different types of contact materials and with different grain sizes. In addition, frequency-dependent measurements of the electric-field dependent polarization are provided. The obtained results unequivocally prove that the reported colossal dielectric constants in LuFe2O4, which were interpreted in terms of electronic ferroelectricity, are of non-intrinsic surface-related origin. The intrinsic dielectric properties of this material show no indications of any ferroelectric order and, thus, LuFe2O4 is not multiferroic. Its intrinsic dielectric constant is close to 20 and its dielectric loss is dominated by charge transport via variable range hopping.

cond-mat.str-el↗

arXiv2012

Wüstite: Electric, thermodynamic and optical properties of FeO

We report on a systematic optical investigation of wüstite. In addition, the sample under consideration, Fe0.93O, has been characterized in detail by electrical transport, dielectric, magnetic and thermodynamic measurements. From infrared reflectivity experiments, phonon properties, Drude-like conductivity contributions and electronic transitions have been systematically investigated. The phonon modes reveal a clear splitting below the antiferromagnetic ordering temperature, similar to observations in other transition-metal monoxides and in spinel compounds which have been explained in terms of a spin-driven Jahn-Teller effect. The electronic transitions can best be described assuming a crystal-field parameter Dq = 750 cm-1 and a spin-orbit coupling constant λ= 95 cm-1. A well defined crystal field excitation at low temperatures reveals significant broadening on increasing temperature with an overall transfer of optical weight into dc conductivity contributions. This fact seems to indicate a melting of the on-site excitation into a Drude behavior of delocalized charge carriers. The optical band gap in wüstite is close to 1.0 eV at room temperature. With decreasing temperatures and passing the magnetic phase transition we have detected a strong blue shift of the correlation-induced band edge, which amounts more than 15% and has been rarely observed in antiferromagnets.

cond-mat.str-el↗

arXiv2012

Dielectric signature of charge order in lanthanum nickelates

Three charge-ordering lanthanum nickelates La2-xAxNiO4, substituted with specific amounts of A = Sr, Ca, and Ba to achieve commensurate charge order, are investigated using broadband dielectric spectroscopy up to GHz frequencies. The transition temperatures of the samples are characterized by additional specific heat and magnetic susceptibility measurements. We find colossal magnitudes of the dielectric constant for all three compounds and strong relaxation features, which partly are of Maxwell-Wagner type arising from electrode polarization. Quite unexpectedly, the temperature-dependent colossal dielectric constants of these materials exhibit distinct anomalies at the charge-order transitions.

cond-mat.str-el↗

arXiv2012

Relaxation dynamics of a protein solution investigated by dielectric spectroscopy

In the present work, we provide a dielectric study on two differently concentrated aqueous lysozyme solutions in the frequency range from 1 MHz to 40 GHz and for temperatures from 275 to 330 K. We analyze the three dispersion regions, commonly found in protein solutions, usually termed beta-, gamma-, and delta-relaxation. The beta-relaxation, occurring in the frequency range around 10 MHz and the gamma-relaxation around 20 GHz (at room temperature) can be attributed to the rotation of the polar protein molecules in their aqueous medium and the reorientational motion of the free water molecules, respectively. The nature of the delta-relaxation, which often is ascribed to the motion of bound water molecules, is not yet fully understood. Here we provide data on the temperature dependence of the relaxation times and relaxation strengths of all three detected processes and on the dc conductivity arising from ionic charge transport. The temperature dependences of the beta- and gamma-relaxations are closely correlated. We found a significant temperature dependence of the dipole moment of the protein, indicating conformational changes. Moreover we find a breakdown of the Debye-Stokes-Einstein relation in this protein solution, i.e., the dc conductivity is not completely governed by the mobility of the solvent molecules. Instead it seems that the dc conductivity is closely connected to the hydration shell dynamics.

cond-mat.soft↗

arXiv2011

Multiferroicity in an organic charge-transfer salt: Electric-dipole-driven magnetism

Multiferroics, showing simultaneous ordering of electrical and magnetic degrees of freedom, are remarkable materials as seen from both the academic and technological points of view. A prominent mechanism of multiferroicity is the spin-driven ferroelectricity, often found in frustrated antiferromagnets with helical spin order. There, similar to conventional ferroelectrics, the electrical dipoles arise from an off-centre displacement of ions. However, recently a different mechanism, namely purely electronic ferroelectricity, where charge order breaks inversion symmetry, has attracted considerable interest. Here we provide evidence for this exotic type of ferroelectricity, accompanied by antiferromagnetic spin order, in a two-dimensional organic charge-transfer salt, thus representing a new class of multiferroics. Quite unexpectedly for electronic ferroelectrics, dipolar and spin order arise nearly simultaneously. This can be ascribed to the loss of spin frustration induced by the ferroelectric ordering. Hence, here the spin order is driven by the ferroelectricity, in marked contrast to the spin-driven ferroelectricity in helical magnets.

cond-mat.str-el↗

arXiv2011

Hydrogen-bond equilibria and life times in a supercooled monohydroxy alcohol

Dielectric loss spectra covering 13 decades in frequency were collected for 2-ethyl-1-hexanol, a monohydroxy alcohol that exhibits a prominent Debye-like relaxation, typical for several classes of hydrogen-bonded liquids. The thermal variation of the dielectric absorption amplitude agrees well with that of the hydrogen-bond equilibrium population, experimentally mapped out using near infrared (NIR) and nuclear magnetic resonance (NMR) measurements. Despite this agreement, temperature-jump NIR spectroscopy reveals that the hydrogen-bond switching rate does not define the frequency position of the prominent absorption peak. This contrasts with widespread notions and models based thereon, but is consistent with a recent approach.

cond-mat.soft↗

arXiv2011

Relaxor ferroelectricity and the freezing of short-range polar order in magnetite

A thorough investigation of single crystalline magnetite using broadband dielectric spectroscopy and other methods provides evidence for relaxor-like polar order in Fe3O4. We find long-range ferroelectric order to be im-peded by the continuous freezing of polar degrees of freedom and the formation of a tunneling-dominated glasslike state at low temperatures. This also explains the lack of clear evidence for a non-centrosymmetric crystal structure below the Verwey transition. Within the framework of recent models assuming an intimate relation of charge and polar order, the charge order, too, can be speculated to be of short-range type only and to be dominated by tunneling at low temperatures.

cond-mat.str-el↗