arXiv subjects

P. Lunkenheimer

Publications and source records attributed to P. Lunkenheimer.

At least 127 records · Page 7Linked to original sources

arXiv2005

Charge-Density-Wave like Behavior in the One-Dimensional Charge-Ordered Semiconductor (NbSe4)3I

We report on broadband dielectric spectroscopy on the one-dimensional semiconductor (NbSe4)3I. Below the structural phase transition close to 270 K we observe colossal dielectric constants with a frequency and temperature dependence very similar to what is observed in canonical charge-density wave systems. Guided by structural details we interpret this structural phase transition as driven by complex charge-order processes.

cond-mat.str-el↗

arXiv2005

Glassy freezing of orbital dynamics in FeCr2S4 and FeSc2S4

We report on a thorough dielectric investigation of the glass-like freezing of the orbital reorientation-dynamics, recently found for the crystalline sulpho-spinels FeCr2S4 and FeSc2S4. As the orbital reorientations are coupled to a rearrangement of the surrounding ionic lattice via the Jahn-Teller effect, the freezing of the orbital moments is revealed by a relaxational behaviour of the complex dielectric permittivity. Additional conductivity (both dc and ac) and contact contributions showing up in the spectra are taken into account by an equivalent circuit description. The orbital relaxation dynamics continuously slows down over six decades in time, before at the lowest temperatures the glass transition becomes suppressed by quantum tunnelling.

cond-mat.dis-nn↗

arXiv2005

Relaxor ferroelectricity and colossal magnetocapacitive coupling in ferromagnetic CdCr2S4

Multiferroic materials, which reveal magnetic and electric order, are in the focus of recent solid state research. Especially the simultaneous occurrence of ferroelectricity and ferromagnetism, combined with an intimate coupling of magnetization and polarization via magneto-capacitive effects, could pave the way for a new generation of electronic devices. Here we present measurements on a simple cubic spinel with unusual properties: It shows ferromagnetic order and simultaneously relaxor ferroelectricity, i.e. a ferroelectric cluster state, reached by a smeared-out phase transition, both with sizable ordering temperatures and moments. Close to the ferromagnetic ordering temperature the magneto-capacitive coupling, characterized by a variation of the dielectric constant in an external magnetic field, reaches colossal values of nearly 500%. We attribute the relaxor properties to geometric frustration, which is well known for magnetic moments, but here is found to impede long-range order of the structural degrees of freedom.

cond-mat.str-el↗

arXiv2005

Glassy Aging Dynamics

We present time-dependent dielectric loss data at different frequencies for a variety of glass formers after cooling below the glass temperature. The observed aging dynamics is described using a modified Kohlrausch-Williams-Watts law, which takes into account the time-dependent variation of the relaxation time during aging. It leads to values for the aging relaxation time and stretching exponent that are fully consistent with the results from equilibrium measurements performed at higher temperatures. Irrespective of the dynamic process prevailing in the investigated frequency region, the aging dynamics is always determined by the structural relaxation process.

cond-mat.dis-nn↗

arXiv2004

Orbital Freezing in FeCr2S4 Studied by Dielectric Spectroscopy

Broadband dielectric spectroscopy has been performed on single-crystalline FeCr2S4 revealing a transition into a low-temperature orbital glass phase and on polycrystalline FeCr2S4 where long-range orbital order is established via a cooperative Jahn-Teller transition. The freezing of the orbital moments is revealed by a clear relaxational behavior of the dielectric permittivity, which allows a unique characterization of the orbital glass transition. The orbital relaxation dynamics continuously slows down over six decades in time, before at the lowest temperatures the glass transition becomes suppressed by quantum tunneling.

cond-mat.dis-nn↗

arXiv2004

'Boson peak' and high-frequency excitations in glassy crystals

The high-frequency excitations in glasses and supercooled liquids belong to the great mysteries of the physics of condensed matter. While the fast process, located at GHz-THz, can be interpreted as encaged molecular motion, the occurrence of the boson peak (BP), dominating at THz frequencies, still is controversially debated. Two scenarios have evolved during recent years: the BP is truly collective in nature or, alternatively, it results from localized modes. To help solving this controversy, we have investigated the high-frequency dielectric loss of plastic crystals (PCs), having long-range translational but no orientational order. Having in mind the well-defined phonon modes in PCs compared to canonical glasses, and the infrared silence of acoustic phonon modes, millimeter-wave and far-infrared spectroscopy should provide valuable hints on the true origin of the BP. Here we show that PCs exhibit a BP-like feature with markedly different spectral shape compared to canonical glasses, evidencing that mixing of collective phonon with local relaxational modes causes the BP. In addition, we find clear evidence for a fast process also in PCs.

cond-mat.soft↗

arXiv2004

Non-intrinsic origin of the Colossal Dielectric Constants in CaCu3Ti4O12

The dielectric properties of CaCu3Ti4O12, a material showing colossal values of the dielectric constant, were investigated in a broad temperature and frequency range extending up to 1.3 GHz. A detailed equivalent circuit analysis of the results and two crucial experiments, employing different types of contacts and varying sample thickness, provide clear evidence that the apparently high values of the dielectric constant in CaCu3Ti4O12 are non-intrinsic and due to electrode polarization effects. The intrinsic properties of CaCu3Ti4O12 are characterized by charge transport via hopping of localized charge carriers and a relatively high dielectric constant of the order of 100.

cond-mat.mtrl-sci↗

arXiv2003

Dielectric behavior of Copper Tantalum Oxide

A thorough investigation of the dielectric properties of Cu2Ta4O12, a material crystallizing in a pseudo-cubic, perovskite-derived structure is presented. We measured the dielectric constant and conductivity of single crystals in an exceptionally broad frequency range up to GHz frequencies and at temperatures from 25 - 500 K. The detected dielectric constant is unusually high (reaching values up to 105) and almost constant in a broad frequency and temperature range. Cu2Ta4O12 possesses a crystal structure similar to CaCu3Ti4O12, the compound for which such an unusually high dielectric constant was first observed. An analysis of the results using a simple equivalent circuit and measurements with different types of contact revealed that extrinsic interfacial polarization effects, derived from surface barrier capacitors are the origin of the observed giant dielectric constants. The intrinsic properties of Cu2Ta4O12 are characterized by a (still relatively high) dielectric constant in the order of 100 and by charge transport via hopping conduction of Anderson-localized charge carriers.

cond-mat.mtrl-sci↗

arXiv2003

Evidence for Jahn-Teller distortions at the antiferromagnetic transition in LaTiO$_3$

LaTiO$_3$ is known as Mott-insulator which orders antiferromagnetically at $T_{\rm N}=146$ K. We report on results of thermal expansion and temperature dependent x-ray diffraction together with measurements of the heat capacity, electrical transport measurements, and optical spectroscopy in untwinned single crystals. At $T_{\rm N}$ significant structural changes appear, which are volume conserving. Concomitant anomalies are also observed in the dc-resistivity, in bulk modulus, and optical reflectivity spectra. We interpret these experimental observations as evidence of orbital order.

cond-mat.str-el↗

arXiv2003

Dielectric properties and dynamical conductivity of LaTiO3: From dc to optical frequencies

We provide a complete and detailed characterization of the temperature-dependent response to ac electrical fields of LaTiO3, a Mott-Hubbard insulator close to the metal-insulator transition. We present combined dc, broadband dielectric, mm-wave, and infrared spectra of ac conductivity and dielectric constant, covering an overall frequency range of 17 decades. The dc and dielectric measurements reveal information on the semiconducting charge-transport properties of LaTiO3, indicating the importance of Anderson localization, and on the dielectric response due to ionic polarization. In the infrared region, the temperature dependence of the phonon modes gives strong hints for a structural phase transition at the magnetic ordering temperature. In addition, a gap-like electronic excitation following the phonon region is analyzed in detail. We compare the results to the soft-edge behavior of the optical spectra characteristic for Mott-Hubbard insulators. Overall a consistent picture of the charge-transport mechanisms in LaTiO3 emerges.

cond-mat.str-el↗

arXiv2003

The response of disordered matter to electromagnetic fields

We have studied a variety of different disordered materials, including molecular and ionic liquids, supercooled liquids and glasses, ionic conductors, and doped semiconductors, in ac electromagnetic fields over an exceptional broad dynamic range including the rarely investigated GHz to THz region. We show that all classes of disordered matter exhibit an astonishingly similar response: In addition to Jonscher's time-honored "Universal Dielectric Response", a superlinear power-law increase of the frequency-dependent conductivity shows up bridging the gap between the classical dielectric and the infrared region. We demonstrate that the universal dielectric behavior of disordered matter extends up to much higher frequencies than thought until now and covers the range from ultra-low (mHz) to phonon (THz) frequencies.

cond-mat↗

arXiv2002

Electronic and optical properties of LiBC

LiBC, a semiconducting ternary borocarbide constituted of the lightest elements only, has been synthesized and characterized by x-ray powder diffraction, dielectric spectroscopy, and conductivity measurements. Utilizing an infrared microscope the phonon spectrum has been investigated in single crystals. The in-plane B-C stretching mode has been detected at 150 meV, noticeably higher than in AlB2, a non-superconducting isostructural analog of MgB2. It is this stretching mode, which reveals a strong electron-phonon coupling in MgB2, driving it into a superconducting state below 40 K, and is believed to mediate predicted high-temperature superconductivity in hole-doped LiBC [H. Rosner, A. Kitaigorodsky, and W. E. Pickett, Phys. Rev. Lett. 88, 127001 (2002)].

cond-mat.mtrl-sci↗

arXiv2002

Materials with Colossal Dielectric Constant: Do They Exist?

Experimental evidence is provided that colossal dielectric constants, epsilon >= 1000, sometimes reported to exist in a broad temperature range, can often be explained by Maxwell-Wagner type contributions of depletion layers at the interface between sample and contacts, or at grain boundaries. We demonstrate this on a variety of different materials. We speculate that the largest intrinsic dielectric constant observed so far in non-ferroelectric materials is of order 100.

cond-mat↗

arXiv2001

Determination of the parameters of semiconducting CdF2:In with Schottky barriers from radio-frequency measurements

Physical properties of semiconducting CdF_2 crystals doped with In are determined from measurements of the radio-frequency response of a sample with Schottky barriers at frequencies 10 - 10^6 Hz. The dc conductivity, the activation energy of the amphoteric impurity, and the total concentration of the active In ions in CdF_2 are found through an equivalent-circuit analysis of the frequency dependencies of the sample complex impedance at temperatures from 20 K to 300 K. Kinetic coefficients determining the thermally induced transitions between the deep and the shallow states of the In impurity and the barrier height between these states are obtained from the time-dependent radio-frequency response after illumination of the material. The results on the low-frequency conductivity in CdF_2:In are compared with submillimeter (10^{11} - 10^{12} Hz) measurements and with room-temperature infrared measurements of undoped CdF_2. The low-frequency impedance measurements of semiconductor samples with Schottky barriers are shown to be a good tool for investigation of the physical properties of semiconductors.

cond-mat.mtrl-sci↗

arXiv2001

Calcium Rubidium Nitrate: Mode-Coupling $β$ Scaling without Factorization

The fast dynamics of viscous calcium rubidium nitrate is investigated by depolarized light scattering, neutron scattering and dielectric loss. Fast $β$ relaxation evolves as in calcium potassium nitrate. The dynamic susceptibilities can be described by the asymptotic scaling law of mode-coupling theory with a shape parameter $λ= 0.79$; the temperature dependence of the amplitudes extrapolates to $T_c\simeq 378$ K. However, the frequencies of the minima of the three different spectroscopies never coincide, in conflict with the factorization prediction, indicating that the true asymptotic regime is unreachable.

cond-mat.soft↗

arXiv2000

The Excess Wing in the Dielectric Loss of Glass-Formers: A Johari-Goldstein beta-Relaxation?

Dielectric loss spectra of glass-forming propylene carbonate and glycerol at temperatures above and below T_g are presented. By performing aging experiments lasting up to five weeks, equilibrium spectra below T_g have been obtained. During aging, the excess wing, showing up as a second power law at high frequencies, develops into a shoulder. The results strongly suggest that the excess wing, observed in a variety of glass formers, is the high-frequency flank of a beta-relaxation.

cond-mat.soft↗

arXiv2000

The excess wing in the dielectric loss of glass-forming ethanol: A relaxation process

A detailed dielectric investigation of liquid, supercooled liquid, and glassy ethanol reveals a third relaxation process, in addition to the two processes already known. The relaxation time of the newly detected process exhibits strong deviations from thermally activated behavior. Most important, this process is the cause of the apparent excess wing, which was claimed to be present in the dielectric loss spectra of glass-forming ethanol. In addition, marked deviations of the spectra of ethanol from the scaling proposed by Dixon and Nagel have been detected.

cond-mat.dis-nn↗

arXiv1999

Scaling of broadband dielectric data of glass-forming liquids and plastic crystals

The Nagel-scaling and the modified scaling procedure proposed recently by Dendzik et al. have been applied to broadband dielectric data on two glass- forming liquids (glycerol and propylene carbonate) and three plastic crystals (ortho-carborane, meta-carborane, and 1-cyano-adamantane). Our data extend the upper limit of the abscissa range to considerably higher values than in previously published analyses. At the highest frequencies investigated, deviations from a single master curve show up which are most pronounced in the Dendzik-scaling plot. The loss curves of the plastic crystals do not scale in the Nagel-plot, but they fall onto a separate master curve in the Dendzik-plot. In addition, we address the question of a possible divergence of the static susceptibility near the Vogel-Fulcher temperature. For this purpose, the low-temperature evolution of the high-frequency wing of the dielectric loss peaks is investigated in detail. No convincing proof for such a divergence can be deduced from the present broadband data.

cond-mat.dis-nn↗