arXiv subjects

P. Lunkenheimer

Publications and source records attributed to P. Lunkenheimer.

At least 55 records · Page 3Linked to original sources

arXiv2017

Conductivity contrast and tunneling charge transport in the vortex-like ferroelectric domain patterns of multiferroic hexagonal YMnO3

We deduce the intrinsic conductivity properties of the ferroelectric domain walls around the topologically protected domain vortex cores in multiferroic YMnO3. This is achieved by performing a careful equivalent-circuit analysis of dielectric spectra measured in single-crystalline samples with different vortex densities. The conductivity contrast between the bulk domains and the less conducting domain boundaries is revealed to reach up to a factor 500 at room temperature, depending on sample preparation. Tunneling of localized defect charge carriers is the dominant charge-transport process in the domain walls that are depleted of mo-bile charge carriers. This work demonstrates that via equivalent-circuit analysis, dielectric spectroscopy can provide valuable information on the intrinsic charge-transport properties of ferroelectric domain walls, which is of high relevance for the design of new domain-wall-based microelectronic devices.

cond-mat.str-el↗

arXiv2016

Structural, magnetic, electric, dielectric, and thermodynamic properties of multiferroic GeV4S8

The lacunar spinel GeV4S8 undergoes orbital and ferroelectric ordering at the Jahn-Teller transition around 30 K and exhibits antiferromagnetic order below about 14 K. In addition to this orbitally driven ferroelectricity, lacunar spinels are an interesting material class, as the vanadium ions form V4 clusters representing stable molecular entities with a common electron distribution and a well-defined level scheme of molecular states resulting in a unique spin state per V4 molecule. Here we report detailed x-ray, magnetic susceptibility, electrical resistivity, heat capacity, thermal expansion, and dielectric results to characterize the structural, electric, dielectric, magnetic, and thermodynamic properties of this interesting material, which also exhibits strong electronic correlations. From the magnetic susceptibility, we determine a negative Curie-Weiss temperature, indicative for antiferromagnetic exchange and a paramagnetic moment close to a spin S = 1 of the V4 molecular clusters. The low-temperature heat capacity provides experimental evidence for gapped magnon excitations. From the entropy release, we conclude about strong correlations between magnetic order and lattice distortions. In addition, the observed anomalies at the phase transitions also indicate strong coupling between structural and electronic degrees of freedom. Utilizing dielectric spectroscopy, we find the onset of significant dispersion effects at the polar Jahn-Teller transition. The dispersion becomes fully suppressed again with the onset of spin order. In addition, the temperature dependencies of dielectric constant and specific heat possibly indicate a sequential appearance of orbital and polar order.

cond-mat.str-el↗

arXiv2016

Effect of adding nanometre-sized heterogeneities on the structural dynamics and the excess wing of a molecular glass former

We present the relaxation dynamics of glass-forming glycerol mixed with 1.1 nm sized polyhedral oligomeric silsesquioxane (POSS) molecules using dielectric spectroscopy (DS) and two different neutron scattering (NS) techniques. Both, the reorientational dynamics as measured by DS and the density fluctuations detected by NS reveal a broadening of the alpha relaxation when POSS molecules are added. Moreover, we find a significant slowing down of the alpha-relaxation time. These effects are in accord with the heterogeneity scenario considered for the dynamics of glasses and supercooled liquids. The addition of POSS also affects the excess wing in glycerol arising from a secondary relaxation process, which seems to exhibit a dramatic increase in relative strength compared to the alpha-relaxation.

cond-mat.dis-nn↗

arXiv2016

On the multiferroic skyrmion-host GaV4S8

The lacunar spinel GaV4S8 exhibits orbital ordering at 44 K and shows a complex magnetic phase diagram below 12.7 K, which includes ferromagnetic and cycloidal spin order. At low but finite external magnetic fields, Néel-type skyrmions are formed in this material. Skyrmions are whirl-like spin vortices that have received great theoretical interest because of their non-trivial spin topology and that are also considered as basic entities for new data-storage technologies. Interestingly, we found that the orbitally ordered phase shows sizable ferroelectric polarization and that excess spin-driven polarizations appear in all magnetic phases, including the skyrmion-lattice phase. Hence, GaV4S8 shows simultaneous magnetic and polar order and belongs to the class of multiferroics, materials that attracted enormous attention in recent years. Here, we summarize the existing experimental information on the magnetic, electronic, and dielectric properties of GaV4S8. By performing detailed magnetic susceptibility, resistivity, specific heat, and dielectric experiments, we complement the low-temperature phase diagram. Specifically, we show that the low-temperature and low-field ground state of GaV4S8 seems to have a more complex spin configuration than purely collinear ferromagnetic spin order. In addition, at the structural Jahn-Teller transition the magnetic exchange interaction changes from antiferromagnetic to ferromagnetic. We also provide experimental evidence that the vanadium V4 clusters in GaV4S8 can be regarded as molecular units with spin 1/2. However, at high temperatures deviations in the susceptibility show up, indicating that either the magnetic moments of the vanadium atoms fluctuate independently or excited states of the V4 molecule become relevant.

cond-mat.str-el↗

arXiv2016

Fifth-order susceptibility unveils growth of thermodynamic amorphous order in glass-formers

Glasses are ubiquitous in daily life and technology. However the microscopic mechanisms generating this state of matter remain subject to debate: Glasses are considered either as merely hyper-viscous liquids or as resulting from a genuine thermodynamic phase transition towards a rigid state. We show that third- and fifth-order susceptibilities provide a definite answer to this longstanding controversy. Performing the corresponding high-precision nonlinear dielectric experiments for supercooled glycerol and propylene carbonate, we find strong support for theories based upon thermodynamic amorphous order. Moreover, when lowering temperature, we find that the growing transient domains are compact - that is their fractal dimension d_f = 3. The glass transition may thus represent a class of critical phenomena different from canonical second-order phase transitions for which d_f < 3.

cond-mat.dis-nn↗

arXiv2016

Impact of water on the charge transport of a glass-forming ionic liquid

Using dielectric spectroscopy and differential scanning calorimetry, we have performed a detailed investigation of the influence of water uptake on the translational and reorientational glassy dynamics in the typical ionic liquid 1-Butyl-3-methyl-imidazolium chloride. From a careful analysis of the measured dielectric permittivity and conductivity spectra, we find a significant acceleration of cation reorientation and a marked increase of the ionic conductivity for higher water contents. The latter effect mainly arises due to a strong impact of water content on the glass temperature, which for the well-dried material is found to be larger than any values reported in literature for this system. The fragility, characterizing the non-Arrhenius glassy dynamics of the ionic subsystem, also changes with varying water content. Decoupling of the ionic motion from the structural dynamics has to be considered to explain the results.

cond-mat.mtrl-sci↗

arXiv2015

Nonlinear dielectric spectroscopy in a fragile plastic crystal

In this work we provide a thorough examination of the nonlinear dielectric properties of a succinonitrile-glutaronitrile mixture, representing one of the rare example of a plastic crystal with fragile glassy dynamics. The detected alteration of the complex dielectric permittivity under high fields can be explained considering the heterogeneous nature of glassy dynamics and a field-induced variation of entropy. While the first mechanism was also found in structural glass formers, the latter effect seems to be typical for plastic crystals. Morover, the third harmonic component of the dielectric susceptibility is reported, revealing a spectral shape as predicted for cooperative molecular dynamics. In accord with the fragile nature of the glass transition in this plastic crystal, we deduce a relatively strong temperature dependence of the number of correlated molecules, in contrast to the much weaker temperature dependence in plastic-crystalline cyclo-octanol, whose glass transition is of strong nature.

cond-mat.dis-nn↗

arXiv2015

Multiferroicity and skyrmions carrying electric polarization in GaV4S8

Skyrmions are whirl like topological spin objects with high potential for future magnetic data storage. It is a fundamental question, relevant for both basic research and application, if a ferroelectric (FE) polarization can be associated with their magnetic texture and if these objects can be manipulated by electric fields. Here, we study the interplay between magnetism and electric polarization in the lacunar spinel GaV4S8, which undergoes a structural transition associated with orbital ordering at 44 K and reveals a complex magnetic phase diagram below 13 K, including a ferromagnetic (FM), cycloidal, and Néel-type skyrmion lattice (SkL) phase. We found that the orbitally ordered phase of GaV4S8 is FE with a sizable polarization of ~1 μC/cm2. Moreover, we observed spin-driven excess polarizations in all magnetic phases and, hence, GaV4S8 hosts three different multiferroic phases with coexisting polar and magnetic order. These include the SkL phase where we predict a strong spatial modulation of the FE polarization close to the skyrmion cores. By taking into account the crystal symmetry and spin patterns of the magnetically ordered phases, we identify the exchange striction as the main microscopic mechanism behind the spin-driven FE polarization in each multiferroic phase. Since GaV4S8 is unique among the known SkL host materials due to its polar crystal structure and the observed strong magnetoelectric effect, this study is an important step towards the non-dissipative electric-field control of skyrmions.

cond-mat.str-el↗

arXiv2015

Polar Dynamics at the Jahn-Teller Transition in Ferroelectric GaV4S8

We present a dielectric spectroscopy study of the polar dynamics linked to the orbitally driven ferroelectric transition in the skyrmion host GaV4S8. By combining THz and MHz-GHz spectroscopy techniques, we succeed in detecting the relaxational dynamics arising from coupled orbital and polar fluctuations in this material and traced its temperature dependence in the paraelectric as well as in the ferroelectric phase. The relaxation time significantly increases when approaching the critical temperature from both sides of the transition. It is natural to assume that these polar fluctuations map the orbital dynamics at the Jahn-Teller transition. Due to the first-order character of the orbital-ordering transition, the relaxation time shows an enormous jump of about five orders of magnitude at the polar and structural phase transition.

cond-mat.str-el↗

arXiv2015

MFU-type metal-organic frameworks as host materials of confined supercooled liquids

In this work we examine the use of metal-organic framework (MOF) systems as host materials for the investigation of glassy dynamics in confined geometry. We investigate the confinement of the molecular glass former glycerol in three MFU-type MOFs with different pore sizes and study the dynamics of the confined liquid via dielectric spectroscopy. In accord with previous reports on confined glass formers, we find different degrees of deviations from bulk behavior depending on pore size, demonstrating that MOFs are well-suited host systems for confinement investigations.

cond-mat.soft↗

arXiv2015

Dielectric Spectroscopy on Organic Charge-Transfer Salts

This Topical Review provides an overview of the dielectric properties of a variety of organic charge-transfer salts, based on both, data reported in literature and our own experimental results. Moreover, we discuss in detail the different processes that can contribute to the dielectric response of these materials. We concentrate on the family of the one-dimensional (TMTTF)2X systems and the two-dimensional BEDT-TTF-based charge-transfer salts, which in recent years have attracted considerable interest due to their often intriguing dielectric properties. We will mainly focus on the occurrence of electronic ferroelectricity in these systems, which also includes examples of multiferroicity.

cond-mat.str-el↗

arXiv2015

Conductivity enhancement in plastic-crystalline solid-state electrolytes

Finding new ionic conductors that enable significant advancements in the development of energy-storage devices is a challenging goal of current material science. Aside of material classes as ionic liquids or amorphous ion conductors, the so-called plastic crystals (PCs) have been shown to be good candidates combining high conductivity and favourable mechanical properties. PCs are formed by molecules whose orientational degrees of freedom still fluctuate despite the material exhibits a well-defined crystalline lattice. Here we show that the conductivity of Li+ ions in succinonitrile, the most prominent molecular PC electrolyte, can be enhanced by several decades when replacing part of the molecules in the crystalline lattice by larger ones. Dielectric spectroscopy reveals that this is accompanied by a stronger coupling of ionic and reorientational motions. These findings, which can be understood in terms of an optimised "revolving door" mechanism, open a new path towards the development of better solid-state electrolytes.

cond-mat.mtrl-sci↗

arXiv2015

Ferroelectric properties of charge-ordered alpha-(BEDT-TTF)2I3

A detailed investigation of the out-of-plane electrical properties of charge-ordered alpha-(BEDT-TTF)2I3 provides clear evidence for ferroelectricity. Similar to multiferroic alpha-(BEDT-TTF)2Cu[N(CN)2]Cl, the polar order in this material is ascribed to the occurrence of bond- and site-centered charge order. Dielectric response typical for relaxor ferroelectricity is found deep in the charge-ordered state. We suggest an explanation in terms of the existence of polar and nonpolar stacks of the organic molecules in this material, preventing long-range ferroelectricity. The results are discussed in relation to the formation or absence of electronic polar order in related charge-transfer salts.

cond-mat.str-el↗

arXiv2015

Crystal structure, incommensurate magnetic order and ferroelectricity in mn$_{1-x}$cu$_{x}$wo${_4}$ (x=0-0.19)

We have carried out a systematic study on the effect of Cu doping on nuclear, magnetic, and dielectric properties in Mn$_{1-x}$Cu$_{x}$WO$_4$ for ${0}\leq{x}\leq{0.19}$ by a synergic use of different techniques, viz, heat capacity, magnetization, dielectric, and neutron powder diffraction measurements. Via heat capacity and magnetization measurements we show that with increasing Cu concentration magnetic frustration decreases, which leads to the stabilization of commensurate magnetic ordering. This was further verified by temperature-dependent unit cell volume changes derived from neutron diffraction measurements which was modeled by the Grüneisen approximation. Dielectric measurements show a low temperature phase transition below about 9-10 K. Further more, magnetic refinements reveal no changes below this transition indicating a possible spin-flop transition which is unique to the Cu doped system. From these combined studies we have constructed a magnetoelectric phase diagram of this compound.

cond-mat.str-el↗

arXiv2015

Magnetoelectric effects in the skyrmion host material Cu2OSeO3

Insulating helimagnetic Cu2OSeO3 shows sizeable magnetoelectric effects in its skyrmion phase. Using magnetization measurements, magneto-current analysis and dielectric spectroscopy, we provide a thorough investigation of magnetoelectric coupling, polarization and dielectric constants of the ordered magnetic and polar phases of single-crystalline Cu2OSeO3 in external magnetic fields up to 150 mT and at temperatures below 60 K. From these measurements we construct a detailed phase diagram. Especially, the skyrmion phase and the metamagnetic transition of helical to conical spin order are characterized in detail. Finally we address the question if there is any signature of polar order that can be switched by an external electric field, which would imply multiferroic behaviour of Cu2OSeO3.

cond-mat.str-el↗

arXiv2015

Importance of glassy fragility for energy applications of ionic liquids

Ionic liquids (ILs) are salts that are liquid close to room temperature. Their possible applications are numerous, e.g., as solvents for green chemistry in various electrochemical devices, and even for such "exotic" purposes as spinning-liquid mirrors for lunar telescopes. Here we concentrate on their use for new advancements in energy-storage and -conversion devices: Batteries, supercapacitors or fuel cells using ILs as electrolytes could be important building blocks for the sustainable energy supply of tomorrow. Interestingly, ILs show glassy freezing and the universal, but until now only poorly understood dynamic properties of glassy matter, dominate many of their physical properties. We show that the conductivity of ILs, an essential figure of merit for any electrochemical application, depends in a systematic way not only on their glass temperature but also on the so-called fragility, characterizing the non-canonical super-Arrhenius temperature dependence of their ionic mobility.

cond-mat.mtrl-sci↗

arXiv2015

Cooperativity and Heterogeneity in Plastic Crystals Studied by Nonlinear Dielectric Spectroscopy

The glassy dynamics of plastic-crystalline cyclo-octanol and ortho-carborane, where only the molecular reorientational degrees of freedom freeze without long-range order, is investigated by nonlinear dielectric spectroscopy. Marked differences to canonical glass formers show up: While molecular cooperativity governs the glassy freezing, it leads to a much weaker slowing down of molecular dynamics than in supercooled liquids. Moreover, the observed nonlinear effects cannot be explained with the same heterogeneity scenario recently applied to canonical glass formers. This supports ideas that molecular relaxation in plastic crystals may be intrinsically non-exponential. Finally, no nonlinear effects were detected for the secondary processes in cyclo-octanol.

cond-mat.dis-nn↗

arXiv2015

Spin-orbiton and quantum criticality in FeSc2S4

In FeSc2S4 spin-orbital exchange competes with strong spin-orbit coupling, suppressing long-range spin and orbital order and, hence, this material represents one of the rare examples of a spin-orbital liquid ground state. Moreover, it is close to a quantum-critical point separating the ordered and disordered regimes. Using THz and FIR spectroscopy we study low-lying excitations in FeSc2S4 and provide clear evidence for a spin-orbiton, an excitation of strongly entangled spins and orbitals. It becomes particularly well pronounced upon cooling, when advancing deep into the quantum-critical regime. Moreover, indications of an underlying structureless excitation continuum are found, a possible signature of quantum criticality.

cond-mat.str-el↗