arXiv subjects

P. Lunkenheimer

Publications and source records attributed to P. Lunkenheimer.

At least 19 recordsLinked to original sources

arXiv2024

Crystal melting influenced by particle cooperativity of the liquid

Recently, a universal relation between the thermal expansion coefficient of glasses $\alpha_g$, their glass-transition temperature Tg, and the so-called fragility index m of the corresponding supercooled liquid state was found to be valid for more than 200 glass formers, namely $\alpha_g/m\propto 1/T_g$ [P. Lunkenheimer et al., Nat. Phys. 19, 694 (2023)]. Here we show that this could also have far-reaching consequences for our understanding of crystal melting. Namely, when considering the empirically founded 2/3-rule, stating that the ratio of Tg and the melting temperature Tm is about 2/3 for almost all materials, for crystals a similar relation, $\alpha_c/m\propto 1/T_m$, should apply. Indeed, we find that the available experimental data are well consistent with such a relation. This implies that the melting of a crystal into an ordinary (non-supercooled) liquid is influenced by the fragility, a property quantifying the non-Arrhenius dynamics in the supercooled-liquid state of the material. We argue that this can be explained by a significant enhancement of the "ideal" (non-cooperative) melting temperature arising from the cooperativity of the particle motion in the liquid state above Tm. Therefore, a reassessment of the currently widely accepted microscopic understanding of crystal melting, still founded on the general ideas that lead to the time-honored Lindemann melting criterion, may be necessary.

cond-mat.soft↗

arXiv2024

Ionic conductivity of a lithium-doped deep eutectic solvent: Glass formation and rotation-translation coupling

Deep eutectic solvents with admixed lithium salts are considered as electrolytes in electrochemical devices like batteries or supercapacitors. Compared to eutectic mixtures of hydrogen-bond donors and lithium salts, their raw-material costs are significantly lower. Not much is known about the glassy freezing and rotation-translation coupling of such systems. Here we investigate these phenomena by applying dielectric spectroscopy to the widely studied deep eutectic solvent glyceline, to which 1 and 5 mol% LiCl were added. Our studies cover a wide temperature range, including the deeply supercooled state. The temperature dependences of the detected dipolar reorientation dynamics and of the ionic dc conductivity reveal the signatures of glassy freezing. In comparison to pure glyceline, lithium admixture leads to a reduction of ionic conductivity, which is accompanied by a slowing down of the rotational dipolar motions. However, this reduction is much smaller than for DESs, where one main component is a lithium salt, which we trace back to the smaller glass-transition temperatures of the lithium-doped DESs. In contrast to pure glyceline, the ionic and dipolar dynamics become increasingly decoupled at low temperatures and obey a fractional Debye-Stokes-Einstein relation as previously found in other glass-forming liquids. The obtained results demonstrate the relevance of decoupling effects and of the glass transition for the enhancement of the technically relevant ionic conductivity in such lithium-doped DESs.

cond-mat.soft↗

arXiv2024

Post-synthesis tuning of dielectric constant via ferroelectric domain wall engineering

A promising mechanism for achieving colossal dielectric constants is to use insulating internal barrier layers, which typically form during synthesis and then remain in the material. It has recently been shown that insulating domain walls in ferroelectrics can act as such barriers. One advantage domain walls have, in comparison to stationary interfaces, is that they can be moved, offering the potential of post-synthesis control of the dielectric constant. However, to date, direct imaging of how changes in domain wall pattern cause a change in dielectric constant within a single sample has not been realized. In this work, we demonstrate that changing the domain wall density allows the engineering of the dielectric constant in hexagonal-ErMnO3 single crystals. The changes of the domain wall density are quantified via microscopy techniques, while the dielectric constant is determined via macroscopic dielectric spectroscopy measurements. The observed changes in the dielectric constant are quantitatively consistent with the observed variation in domain wall density, implying that the insulating domain walls behave as 'ideal' capacitors connected in series. Our approach to engineer the domain wall density can be readily extended to other control methods, e.g., electric fields or mechanical stresses, providing a novel degree of flexibility to in-situ tune the dielectric constant.

cond-mat.mtrl-sci↗

arXiv2023

Translational and reorientational dynamics in carboxylic acid-based deep eutectic solvents

The glass formation and the dipolar reorientational motions in deep eutectic solvents (DESs) are frequently overlooked, despite their crucial role in defining the room-temperature physiochemical properties. To understand the effects of these dynamics on the ionic conductivity and their relation to the mechanical properties of the DES, we conducted broadband dielectric and rheological spectroscopy over a wide temperature range on three well-established carboxylic-acid-based natural DESs: oxaline, maline, and the eutectic mixture of choline chloride with phenylacetic acid (phenylaceline). In all three DESs, we observe signs of glassy freezing in the temperature dependence of their dipolar reorientational and structural dynamics, as well as varying degrees of motional decoupling between the different observed dynamics: Maline and oxaline display a breaking of the Walden rule near the glass-transition temperature, while the relation between the dc conductivity and dipolar relaxation time in both maline and phenylaceline is best described by a power law. The glass-forming properties of the investigated systems not only govern the orientational dipolar motions and rheological properties, which are of interest from a fundamental point of view, but they also affect the dc conductivity, even at room temperature, which is of high technical relevance.

cond-mat.soft↗

arXiv2023

Rotational dynamics, ionic conductivity, and glass formation in a ZnCl2-based deep eutectic solvent

Glass formation and reorientational motions are widespread, but often-neglected features of deep eutectic solvents, although both can be relevant for the technically important ionic conductivity at room temperature. Here we investigate these properties for two mixtures of ethylene glycol and ZnCl2, which were recently considered as superior electrolyte materials for application in zinc-ion batteries. For this purpose, we employed dielectric spectroscopy performed in a broad temperature range, extending from the supercooled state at low temperatures up to the liquid phase around room temperature and beyond. We find evidence for a relaxation process arising from dipolar reorientation dynamics, which reveals the clear signatures of glassy freezing. This freezing also governs the temperature dependence of the ionic dc conductivity. We compare the obtained results with those for deep eutectic solvents that are formed by the same hydrogen-bond donor, ethylene glycol, but by two different salts, choline chloride and lithium triflate. The four materials reveal significantly different ionic and reorientational dynamics. Moreover, we find varying degrees of decoupling of rotational dipolar and translational ionic motions, which partly can be described by a fractional Debye-Stokes-Einstein relation. The typical glass-forming properties of these solvents strongly affect their room-temperature conductivity.

cond-mat.soft↗

arXiv2023

Dipolar relaxation, conductivity, and polar order in AgCN

By using dielectric spectroscopy in a broad range of temperatures and frequencies, we have investigated dipolar relaxations, the dc conductivity, and the possible occurrence of polar order in AgCN. Conductivity contributions dominate the dielectric response at elevated temperatures and low frequencies, most likely arising from the mobility of the small silver ions. In addition, we observe dipolar relaxation dynamics of the dumbbell-shaped CN- ions, whose temperature dependence follows Arrhenius behavior with a hindering barrier of 0.59 eV. It correlates well with a systematic development of the relaxation dynamics with the cation radius, previously observed in various alkali cyanides. By comparison with the latter, we conclude that AgCN does not exhibit a plastic high-temperature phase with a free rotation of the cyanide ions. Instead, our results indicate that a phase with quadrupolar order, revealing dipolar head-to-tail disorder of the CN- ions, exists at elevated temperatures up to the decomposition temperature, which crosses over to long-range polar order of the CN dipole moments below about 475 K. Dipole ordering was also reported for NaCN and KCN and a comparison with these systems suggests a critical relaxation rate of 10^5 - 10^7 Hz marking the onset of dipolar order in the cyanides. The detected relaxation dynamics in this order-disorder type polar state points to glasslike freezing below about 195 K of a fraction of non-ordered CN dipoles.

cond-mat.dis-nn↗

arXiv2022

Thermal expansion and the glass transition

Melting is well understood in terms of the Lindemann criterion, essentially stating that crystalline materials melt when the thermal vibrations of their atoms become such vigorous that they shake themselves free of the binding forces. However, how about another common type of solids: glasses, where the nature of the solid-liquid crossover is highly controversial? The Lindemann criterion implies that the thermal expansion coefficients alpha of crystals are inversely proportional to their melting temperatures. Here we find that, unexpectedly, alpha of glasses decreases much stronger with increasing glass-transition temperature Tg marking the liquid-solid crossover in this material class. However, scaling alpha by the fragility m, a measure of particle cooperativity, restores the proportionality, i.e., alpha/m ~ 1/Tg. Obviously, for a glass to become liquid, it is not sufficient to simply overcome the interparticle binding energies. Instead, more energy has to be invested to break up the typical cooperative particle network which is considered a hallmark feature of glassy materials. Surprisingly, alpha of the liquid phase reveals similar anomalous behaviour and is universally enhanced by a constant factor of ~3. The found universalities allow estimating glass-transition temperatures from thermal expansion and vice versa.

cond-mat.dis-nn↗

arXiv2022

Antipolar transitions in GaNb$_4$Se$_8$ and GaTa$_4$Se$_8$

We present dielectric, polarization, resistivity, specific heat, and magnetic susceptibility data on single crystals of the lacunar spinels GaNb4Se8 and GaTa4Se8, tetrahedral cluster-based materials with substantial spin-orbit coupling. We concentrate on the possible occurrence of antipolar order in these compounds, as previously reported for the isoelectronic GaNb4S8, where spin-orbit coupling plays a less important role. Our broadband dielectric-spectroscopy investigations reveal clear anomalies of the intrinsic dielectric constant at the magneto-structural transitions in both systems that are in accord with the expectations for antipolar transitions. A similar anomaly is also observed at the cubic-cubic transition of the Nb compound leading to an intermediate phase. Similar to other polar and antipolar lacunar spinels, we find indications for dipolar relaxation dynamics at low temperatures. Polarization measurements on GaNb4Se8 reveal weak ferroelectric ordering below the magneto-structural transition, either superimposed to antipolar order or emerging at structural domain walls. The temperature-dependent dc resistivity evidences essentially thermally-activated charge transport with different activation energies in the different phases. A huge step-like increase of the resistivity at the magneto-structural transition of the Ta compound points to a fundamental change in the electronic structure or the mechanism of the charge transport. At low temperatures, charge transport is governed by in-gap impurity states, as also invoked to explain the resistive switching in these compounds.

cond-mat.str-el↗

arXiv2021

Lithium-salt-based deep eutectic solvents: Importance of glass formation and rotation-translation coupling for the ionic charge transport

Lithium-salt-based deep eutectic solvents, where the only cation is Li+, are promising candidates as electrolytes in electrochemical energy-storage devices like batteries. We have performed broadband dielectric spectroscopy on three such systems, covering a broad temperature and dynamic range that extends from the low-viscosity liquid around room temperature down to the glassy state approaching the glass-transition temperature. We detect a relaxational process that can be ascribed to dipolar reorientational dynamics and exhibits the clear signatures of glassy freezing. We find that the temperature dependence of the ionic dc conductivity and its room-temperature value also are governed by the glassy dynamics of these systems, depending, e.g., on the glass-transition temperature and fragility. Compared to the previously investigated corresponding systems, containing choline chloride instead of a lithium salt, both the reorientational and ionic dynamics are significantly reduced due to variations of the glass-transition temperature and the higher ionic potential of the lithium ions. These lithium-based deep eutectic solvents partly exhibit significant decoupling of the dipolar reorientational and the ionic translational dynamics and approximately follow a fractional Debye-Stokes-Einstein relation, leading to an enhancement of the dc conductivity, especially at low temperatures. The presented results clearly reveal the importance of decoupling effects and of the typical glass-forming properties of these systems for the technically relevant room-temperature conductivity.

cond-mat.soft↗

arXiv2021

Translational and reorientational dynamics in deep eutectic solvents

We performed rheological measurements of the typical deep eutectic solvents (DESs) glyceline, ethaline, and reline in a very broad temperature and dynamic range, extending from the low-viscosity to the high-viscosity supercooled-liquid regime. We find that the mechanical compliance spectra can be well described by the random free-energy barrier hopping model, while the dielectric spectra on the same materials involve significant contributions arising from reorientational dynamics. The temperature-dependent viscosity and structural relaxation time, revealing non-Arrhenius behavior typical for glassy freezing, are compared to the ionic dc conductivity and relaxation times determined by broadband dielectric spectroscopy. For glyceline and ethaline we find essentially identical temperature dependences for all dynamic quantities. These findings point to a close coupling of the ionic and molecular translational and reorientational motions in these systems. However, for reline the ionic charge transport appears decoupled from the structural and reorientational dynamics, following a fractional Walden rule. Especially, at low temperatures the ionic conductivity in this DES is enhanced by about one decade compared to expectations based on the temperature dependence of the viscosity. The results for all three DESs can be understood without invoking a revolving-door mechanism previously considered as a possible charge-transport mechanism in DESs.

cond-mat.soft↗

arXiv2021

Optical, dielectric, and magnetoelectric properties of ferroelectric and antiferroelectric lacunar spinels

Lacunar spinels with a chemical formula of $AM_4X_8$ form a populous family of narrow-gap semiconductors, which offer a fertile ground to explore correlation and quantum phenomena, including transition between Mott and spin-orbit insulator states, ferro/antiferroelectricity driven by cluster Jahn-Teller effect, and magnetoelectric response of magnetic skyrmions with polar dressing. The electronic and magnetic properties of lacunar spinels are determined to a large extent by their molecular-crystal-like structure. The interplay of electronic correlations with spin-orbit and vibronic couplings leads to a complex electronic structure already on the single-cluster level, which -- together with weaker inter-cluster interactions -- gives rise to a plethora of unconventional correlated states. This review primarily focuses on recent progresses in the field of optical, dielectric, and magnetoelectric properties on lacunar spinels. After introducing the main structural aspects, lattice dynamics and electronic structure of these compounds are discussed on the basis of optical spectroscopy measurements. Dielectric and polarization studies reveal the main characteristics of their low-temperature ferro- or antiferroelectric phases as well as orbital fluctuations in their high-temperature cubic state. Strong couplings between spin, lattice, and orbital degrees of freedom are manifested in singlet formation upon magnetostructural transitions, the emergence of various multiferroic phases, and exotic domain-wall functionalities.

cond-mat.str-el↗

arXiv2021

On the complexity of spinels: Magnetic, electronic, and polar ground states

This review summarizes more than 100 years of research on spinel compounds, mainly focusing on the progress in understanding their magnetic, electronic, and polar properties during the last two decades. Many spinel compounds are magnetic insulators or semiconductors; however, a number of spinel-type metals exists including superconductors and some rare examples of d-derived heavy-fermion compounds. In the early days, they gained importance as ferrimagnetic or even ferromagnetic insulators with relatively high saturation magnetization and high ordering temperatures, with magnetite being the first magnetic mineral known to mankind. However, spinels played an outstanding role in the development of concepts of magnetism, in testing and verifying the fundamentals of magnetic exchange, in understanding orbital-ordering and charge-ordering phenomena. In addition, the A- site as well as the B-site cations in the spinel structure form lattices prone to strong frustration effects resulting in exotic ground-state properties. In case the A-site cation is Jahn-Teller active, additional entanglements of spin and orbital degrees of freedom appear, which can give rise to a spin-orbital liquid or an orbital glass state. The B-site cations form a pyrochlore lattice, one of the strongest contenders of frustration in three dimensions. In addition, in spinels with both cation lattices carrying magnetic moments, competing magnetic exchange interactions become important, yielding ground states like the time-honoured triangular Yafet-Kittel structure. Finally, yet importantly, there exists a long-standing dispute about the possibility of a polar ground state in spinels, despite their reported overall cubic symmetry. Indeed, over the years number of multiferroic spinels were identified.

cond-mat.str-el↗

arXiv2021

Cooperative cluster-Jahn-Teller effect as a possible route to antiferroelectricity

We report the observation of an antipolar phase in cubic GaNb$_4$S$_8$ driven by an unconventional microscopic mechanism, the cooperative Jahn-Teller effect of Nb$_4$S$_4$ molecular clusters. The assignment of the antipolar nature is based on sudden changes in the crystal structure and a strong drop of the dielectric constant at $T_\mathrm{JT}=31$\,K, also indicating the first-order nature of the transition. In addition, we found that local symmetry lowering precedes long-range orbital ordering, implying the presence of a dynamic Jahn-Teller effect in the cubic phase above $T_\mathrm{JT}$. Based on the variety of structural polymorphs reported in lacunar spinels, also including ferroelectric phases, we argue that GaNb$_4$S$_8$ may be transformable to a ferroelectric state, which would further classify the observed antipolar phase as antiferrolectric.

cond-mat.str-el↗

arXiv2020

Intriguing relaxor ferroelectricity in a polar CT crystal at the neutral-ionic interface

We investigated the mixed-stack charge-transfer crystal, N,N'-dimethylphenazine-TCNQ (M$_2$P-TCNQ), which is polar at room temperature and just at the neutral-to-ionic interface (ionicity $ρ\approx 0.5$). We detect the typical dielectric signature of a relaxor ferroelectric and an asymmetric positive-up-negative-down behavior. While relaxor ferroelectricity is usually ascribed to disorder in the crystal, we find no evidence for structural disorder in the investigated crystals. To elucidate the origin of M$_2$P-TCNQ's dielectric properties we perform parallel structural and spectroscopic measurements, associated with theoretical modeling and quantum-mechanical calculations. Our combined effort points to a highly polarizable electronic system that is strongly coupled to lattice vibrations. The found indications for polarization reversal imply flipping of the bent conformation of the M$_2$P molecule with an associated energy barrier of a few tens of an eV, broadly consistent with an Arrhenius fit of the dielectric relaxation times. While the polarization is mostly of electronic origin, its possible reversal implies slow collective motions that are affected by solid-state intermolecular interactions.

cond-mat.str-el↗

arXiv2020

Ionic conductivity and relaxation dynamics in plastic-crystals with nearly globular molecules

We have performed a dielectric investigation of the ionic charge transport and the relaxation dynamics in plastic-crystalline 1-cyano-adamantane (CNA) and in two mixtures of CNA with the related plastic crystals adamantane or 2-adamantanon. Ionic charge carriers were provided by adding 1% of Li salt. The molecules of these compounds have nearly globular shape and, thus, the so-called revolving-door mechanism assumed to promote ionic charge transport via molecular reorientations in other PC electrolytes, should not be active here. Indeed, a comparison of the dc resistivity and the reorientational alpha-relaxation times in the investigated PCs, reveals complete decoupling of both dynamics. Similar to other PCs, we find a significant mixing-induced enhancement of the ionic conductivity. Finally, these solid-state electrolytes reveal a second relaxation process, slower than the alpha-relaxation, which is related to ionic hopping. Due to the mentioned decoupling, it can be unequivocally detected and is not superimposed by the reorientational contributions as found for most other ionic conductors.

cond-mat.mtrl-sci↗

arXiv2020

Quantum Paraelectricity in the Kitaev Quantum-Spin-Liquid Candidates H$_{3}$LiIr$_{2}$O$_{6}$ and D$_{3}$LiIr$_{2}$O$_{6}$

H3LiIr2O6 is the first honeycomb-lattice system without any signs of long-range magnetic order down to the lowest temperatures, raising the hope for the realization of an ideal Kitaev quantum spin liquid. Its honeycomb layers are coupled by interlayer hydrogen bonds. Static or dynamic disorder of these hydrogen bonds was proposed to strongly affect the magnetic exchange and to make Kitaev-type interactions dominant. Using dielectric spectroscopy, here we provide experimental evidence for dipolar relaxations in H3LiIr2O6 and deuterated D3LiIr2O6, which mirror the dynamics of protons and deuterons within the double-well potentials of the hydrogen bonds. The detected hydrogen dynamics reveals glassy freezing, characterized by a strong slowing down under cooling, with a crossover from thermally-activated hopping to quantum-mechanical tunneling towards low temperatures. Thus, besides being Kitaev quantum-spin-liquid candidates, these materials also are quantum paraelectrics. However, the small relaxation rates in the mHz range, found at low temperatures, practically realize quasi-static hydrogen disorder, as assumed in recent theoretical works to explain the quantum-spin-liquid ground state of both compounds.

cond-mat.str-el↗

arXiv2020

Macroscopic Manifestation of Domain-wall Magnetism and Magnetoelectric Effect in a Néel-type Skyrmion Host

We report a magnetic state in GaV$_4$Se$_8$ which emerges exclusively in samples with mesoscale polar domains and not in polar mono-domain crystals. Its onset is accompanied with a sharp anomaly in the magnetic susceptibility and the magnetic torque, distinct from other anomalies observed also in polar mono-domain samples upon transitions between the cycloidal, the Néel-type skyrmion lattice and the ferromagnetic states. We ascribe this additional transition to the formation of magnetic textures localized at structural domain walls, where the magnetic interactions change stepwise and spin textures with different spiral planes, hosted by neighbouring domains, need to be matched. A clear anomaly in the magneto-current indicates that the domain-wall-confined magnetic states also have strong contributions to the magnetoelectric response. We expect polar domain walls to commonly host such confined magnetic edge states, especially in materials with long wavelength magnetic order.

cond-mat.str-el↗

arXiv2019

Johari-Goldstein relaxation in glass-electrets

We investigate the dielectric response in the glass-electret state of two dipolar glass-forming materials. This unusual polar glassy state of matter is produced when a dipolar liquid is supercooled under the influence of a high electric dc field, which leads to partial orientational order of the molecules carrying a dipole moment. Investigation of the prepared glass-electrets by using low-field dielectric spectroscopy reveals a clear modification of their dielectric response in the regime of the Johari-Goldstein beta-relaxation, pointing to a small but significant increase of its relaxation strength compared to the normal glass. We discuss the implications of this finding for the still controversial microscopic interpretation of the Johari-Goldstein relaxation, an inherent property of glassy matter.

cond-mat.dis-nn↗