arXiv subjects

A. Loidl

Publications and source records attributed to A. Loidl.

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↗

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↗

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

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

Element-specific field-induced spin reorientation and an unusual tetracritical point in MnCr2S4

The ferrimagnetic spinel MnCr2S4 shows a variety of magnetic-field-induced phase transitions owing to bond frustration and strong spin-lattice coupling. However, the site-resolved magnetic properties at the respective field-induced phases in high magnetic fields remain elusive. Our soft x-ray magnetic circular dichroism studies up to 40 T directly evidence element-selective magnetic-moment reorientations in the field-induced phases. The complex magnetic structures are further supported by entropy changes extracted from magnetocaloric-effect measurements. Moreover, thermodynamic experiments reveal an unusual tetracritical point in the H-T phase diagram of MnCr2S4 due to strong spin-lattice coupling.

cond-mat.str-el↗

arXiv2021

Angle-dependent thermodynamics of $α$-RuCl$_3$

Thermodynamics of the Kitaev honeycomb magnet $α$-RuCl$_3$ is studied for different directions of in-plane magnetic field using measurements of the magnetic Grüneisen parameter $Γ_B$ and specific heat $C$. We identify two critical fields $B_c^{\rm AF1}$ and $B_c^{\rm AF2}$ corresponding, respectively, to a transition between two magnetically ordered states and the loss of magnetic order toward a quantum paramagnetic state. The $B_c^{AF2}$ phase boundary reveals a narrow region of magnetic fields where inverse melting of the ordered phase may occur. No additional transitions are detected above $B_c^{\rm AF2}$ for any direction of the in-plane field, although a shoulder anomaly in $Γ_B$ is observed systematically at $8-10$ T. Large field-induced entropy effects imply additional low-energy excitations at low fields and/or strongly field-dependent phonon entropies. Our results establish universal features of $α$-RuCl$_3$ in high magnetic fields and challenge the presence of a field-induced Kitaev spin liquid in this material.

cond-mat.str-el↗

arXiv2021

Magnetic surface reconstruction in the van-der-Waals antiferromagnet Fe$_{1+x}$Te

Fe$_{1+x}$Te is a two dimensional van der Waals antiferromagnet that becomes superconducting on anion substitution on the Te site. The parent phase of Fe$_{1+x}$Te is sensitive to the amount of interstitial iron situated between the iron-tellurium layers displaying collinear magnetic order coexisting with low temperature metallic resistivity for small concentrations of interstitial iron $x$ and helical magnetic order for large values of $x$. While this phase diagram has been established through scattering [see for example E. E. Rodriguez $\textit{et al.}$ Phys. Rev. B ${\bf{84}}$, 064403 (2011) and S. Rössler $\textit{et al.}$ Phys. Rev. B ${\bf{84}}$, 174506 (2011)], recent scanning tunnelling microscopy measurements [C. Trainer $\textit{et al.}$ Sci. Adv. ${\bf{5}}$, eaav3478 (2019)] have observed a different magnetic structure for small interstitial iron concentrations $x$ with a significant canting of the magnetic moments along the crystallographic $c$ axis of $θ$=28 $\pm$ 3$^{\circ}$. In this paper, we revisit the magnetic structure of Fe$_{1.09}$Te using spherical neutron polarimetry and scanning tunnelling microscopy to search for this canting in the bulk phase and compare surface and bulk magnetism. The results show that the bulk magnetic structure of Fe$_{1.09}$Te is consistent with collinear in-plane order ($θ=0$ with an error of $\sim$ 5$^{\circ}$). Comparison with scanning tunnelling microscopy on a series of Fe$_{1+x}$Te samples reveals that the surface exhibits a magnetic surface reconstruction with a canting angle of the spins of $θ=29.8^{\circ}$. We suggest that this is a consequence of structural relaxation of the surface layer resulting in an out-of-plane magnetocrystalline anisotropy. The magnetism in Fe$_{1+x}$Te displays different properties at the surface when the symmetry constraints of the bulk are removed.

cond-mat.str-el↗

arXiv2020

Magnetic-field dependence of low-energy magnons, anisotropic heat conduction, and spontaneous relaxation of magnetic domains in the cubic helimagnet ZnCr2Se4

Anisotropic low-temperature properties of the cubic spinel helimagnet ZnCr2Se4 in the single-domain spin-spiral state are investigated by a combination of neutron scattering, thermal conductivity, ultrasound velocity, and dilatometry measurements. In an applied magnetic field, neutron spectroscopy shows a complex and nonmonotonic evolution of the spin-wave spectrum across the quantum-critical point that separates the spin-spiral phase from the field-polarized ferromagnetic phase at high fields. A tiny spin gap of the pseudo-Goldstone magnon mode, observed at wave vectors that are structurally equivalent but orthogonal to the propagation vector of the spin helix, vanishes at this quantum critical point, restoring the cubic symmetry in the magnetic subsystem. The anisotropy imposed by the spin helix has only a minor influence on the lattice structure and sound velocity but has a much stronger effect on the heat conductivities measured parallel and perpendicular to the magnetic propagation vector. The thermal transport is anisotropic at T < 2 K, highly sensitive to an external magnetic field, and likely results directly from magnonic heat conduction. We also report long-time thermal relaxation phenomena, revealed by capacitive dilatometry, which are due to magnetic domain motion related to the destruction of the single-domain magnetic state, initially stabilized in the sample by the application and removal of magnetic field. Our results can be generalized to a broad class of helimagnetic materials in which a discrete lattice symmetry is spontaneously broken by the magnetic order.

cond-mat.str-el↗

arXiv2020

Magneto-electric properties and low-energy excitations of multiferroic FeCr2S4

We report on the low-frequency optical excitations in the multiferroic ground state of polycrystalline FeCr2S4 in the frequency range 0.3-3~THz and their changes upon applying external magnetic fields up to 7~T. In the ground state below the orbital-ordering temperature T_OO=9K we observe the appearance of several new modes. By applying the external magnetic field parallel and perpendicular to the propagation direction of the THz radiation, we can identify the strongest absorptions to be of predominantly electric-dipole origin. We discuss these modes as the low-energy electronic excitations of the Fe^{2+} ions (3d^6, S=2) in an tetrahedral environment. The eigenfrequencies and relative intensities of these absorption lines are satisfactorily reproduced by our calculation assuming an effective exchange field of 12.8 cm-1 at the Fe^{2+}-ions sites. The direction of the exchange field is found to be slightly tilted out of the ab-plane. With our approach we can also describe previously reported results from Mössbauer studies and the order of magnitude of the electric polarisation induced by orbital and non-collinear spin ordering.

cond-mat.str-el↗

arXiv2020

Thermodynamic perspective on the field-induced behavior of $α$-RuCl$_3$

Measurements of the magnetic Grüneisen parameter ($Γ_B$) and specific heat on the Kitaev material candidate $α$-RuCl$_3$ are used to access in-plane field- and temperature-dependence of the entropy up to 12 T and down to 1 K. No signatures corresponding to phase transitions are detected beyond the boundary of the magnetically ordered region, but only a shoulder-like anomaly in $Γ_B$, involving an entropy increment as small as $10^{-5} R\log 2$. These observations put into question the presence of a thermodynamic phase transition between the purported quantum spin liquid and the field-polarized state of $α$-RuCl$_3$. We show theoretically that at low temperatures $Γ_B$ is sensitive to crossings in the lowest excitations within gapped phases, and identify the measured shoulder-like anomaly as being of such origin. Exact diagonalization calculations demonstrate that the shoulder-like anomaly can be reproduced in extended Kitaev models that gain proximity to an additional phase at finite field without entering it. We discuss manifestations of this proximity in other measurements.

cond-mat.str-el↗

arXiv2020

Nonequilibrium quasistationary spin disordered state in the Kitaev-Heisenberg magnet $α$-RuCl$_3$

Excitation by light pulses enables the manipulation of phases of quantum condensed matter. Here, we photoexcite high-energy holon-doublon pairs as a way to alter the magnetic free energy landscape of the Kitaev-Heisenberg magnet $α$-RuCl$_3$, with the aim to dynamically stabilize a proximate spin liquid phase. The holon-doublon pair recombination through multimagnon emission is tracked through the time-evolution of the magnetic linear dichroism originating from the competing zigzag spin ordered ground state. A small holon-doublon density suffices to reach a spin disordered state. The phase transition is described within a dynamic Ginzburg-Landau framework, corroborating the quasistationary nature of the transient spin disordered phase. Our work provides insight into the coupling between the electronic and magnetic degrees of freedom in $α$-RuCl$_3$ and suggests a new route to reach a proximate spin liquid phase in Kitaev-Heisenberg magnets.

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

Nanoscale electronic inhomogeneity in FeSe$_{0.4}$Te$_{0.6}$ revealed through unsupervised machine learning

We report on an apparent low-energy nanoscale electronic inhomogeneity in FeSe$_{0.4}$Te$_{0.6}$ due to the distribution of selenium and tellurium atoms revealed through unsupervised machine learning. Through an unsupervised clustering algorithm, characteristic spectra of selenium- and tellurium-rich regions are identified. The inhomogeneity linked to these spectra can clearly be traced in the differential conductance and is detected both at energy scales of a few electron volts as well as within a few millielectronvolts of the Fermi energy. By comparison with ARPES, this inhomogeneity can be linked to an electron-like band just above the Fermi energy. It is directly correlated with the local distribution of selenium and tellurium. There is no clear correlation with the magnitude of the superconducting gap, however the height of the coherence peaks shows significant correlation with the intensity with which this band is detected, and hence with the local chemical composition.

cond-mat.str-el↗

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↗