arXiv subjects

T. Ivek

Publications and source records attributed to T. Ivek.

17 recordsLinked to original sources

arXiv2019

Elementary excitation in the spin-stripe phase in quantum chains

Elementary excitations in condensed matter capture the complex many-body dynamics of interacting basic entities in a simple quasiparticle picture. In magnetic systems the most established quasiparticles are magnons, collective excitations that reside in ordered spin structures, and spinons, their fractional counterparts that emerge in disordered, yet correlated spin states. Here we report on the discovery of elementary excitation inherent to spin-stripe order that represents a bound state of two phason quasiparticles, resulting in a wiggling-like motion of the magnetic moments. We observe these excitations, which we dub "wigglons", in the frustrated zigzag spin-1/2 chain compound $β$-TeVO$_4$, where they give rise to unusual low-frequency spin dynamics in the spin-stripe phase. This result provides insights into the stripe physics of strongly-correlated electron systems.

cond-mat.str-el↗

arXiv2017

Importance of van der Waals interactions and cation-anion coupling in an organic quantum spin liquid

The Mott insulator $β'$-EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ belongs to a class of charge transfer solids with highly-frustrated triangular lattice of $S=1/2$ molecular dimers and a quantum-spin-liquid ground state. Our experimental and ab initio theoretical studies show the fingerprints of strong correlations and disorder, important role of cation-dimer bonding in charge redistribution, no sign of intra- and inter-dimer dipoles, and the decisive van der Waals contribution to inter-dimer interactions and the ground state structure. The latter consists of quasi-degenerate electronic states related to the different configurations of cation moieties which permit two different equally probable orientations. Upon reducing the temperature, the low-energy excitations slow down, indicating glassy signatures as the cation motion freezes out.

cond-mat.str-el↗

arXiv2016

Anions effects on the electronic structure and electrodynamic properties of the Mott insulator $κ$-(BEDT-TTF)$_2$Ag$_2$(CN)$_{3}$

The Mott insulator $κ$-(BEDT-TTF)$_2$Ag$_2$(CN)$_3$ forms a highly-frustrated triangular lattice of $S=1/2$ dimers with a possible quantum-spin-liquid state. Our experimental and numerical studies reveal the emergence of a slight charge imbalance between crystallographically inequivalent sites, relaxor dielectric response and hopping dc transport. In a broader perspective we conclude that the universal properties of strongly-correlated charge-transfer salts with spin liquid state are an anion-supported valence band and cyanide-induced quasi-degenerate electronic configurations in the relaxed state. The generic low-energy excitations are caused by charged domain walls rather than by fluctuating electric dipoles. They give rise to glassy dynamics characteristic of dimerized Mott insulators, including the sibling compound $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$.

cond-mat.str-el↗

arXiv2016

Pressure-dependent optical investigations of $α$-(BEDT-TTF)$_2$I$_3$: tuning charge order and narrow gap towards a Dirac semimetal

Infrared optical investigations of $α$-(BEDT-TTF)$_2$I$_3$ have been performed in the spectral range from 80 to 8000~cm$^{-1}$ down to temperatures as low as 10~K by applying hydrostatic pressure. In the metallic state, $T > 135$~K, we observe a 50\% increase in the Drude contribution as well as the mid-infrared band due to the growing intermolecular orbital overlap with pressure up to 11~kbar. In the ordered state, $T<T_{\rm CO}$, we extract how the electronic charge per molecule varies with temperature and pressure: Transport and optical studies demonstrate that charge order and metal-insulator transition coincide and consistently yield a linear decrease of the transition temperature $T_{\rm CO}$ by $8-9$~K/kbar. The charge disproportionation $Δρ$ diminishes by $0.017~e$/kbar and the optical gap $Δ$ between the bands decreases with pressure by -47~cm$^{-1}$/kbar. In our high-pressure and low-temperature experiments, we do observe contributions from the massive charge carriers as well as from massless Dirac electrons to the low-frequency optical conductivity, however, without being able to disentangle them unambiguously.

cond-mat.str-el↗

arXiv2014

Anisotropic charge dynamics in the quantum spin-liquid candidate $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$

We have in detail characterized the anisotropic charge response of the dimer Mott insulator $κ$-(BEDT-TTF)$_2$\-Cu$_2$(CN)$_3$ by dc conductivity, Hall effect and dielectric spectroscopy. At room temperature the Hall coefficient is positive and close to the value expected from stoichiometry; the temperature behavior follows the dc resistivity $ρ(T)$. Within the planes the dc conductivity is well described by variable-range hopping in two dimensions; this model, however, fails for the out-of-plane direction. An unusually broad in-plane dielectric relaxation is detected below about 60 K; it slows down much faster than the dc conductivity following an Arrhenius law. At around 17 K we can identify a pronounced dielectric anomaly concomitantly with anomalous features in the mean relaxation time and spectral broadening. The out-of-plane relaxation, on the other hand, shows a much weaker dielectric anomaly; it closely follows the temperature behavior of the respective dc resistivity. At lower temperatures, the dielectric constant becomes smaller both within and perpendicular to the planes; also the relaxation levels off. The observed behavior bears features of relaxor-like ferroelectricity. Because heterogeneities impede its long-range development, only a weak tunneling-like dynamics persists at low temperatures. We suggest that the random potential and domain structure gradually emerge due to the coupling to the anion network.

cond-mat.str-el↗

arXiv2014

ESR studies on the spin-liquid candidate $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_{3}$: anomalous response below $T=8$ K

The organic conductor $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_{3}$ seems to form a quantum spin liquid, although at low temperatures unusual properties are seen in the charge, spin and lattice degrees of freedom. Here we report results of X-band ESR studies of $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_{3}$ single crystals as a function of temperature and angle. We find indications of two anisotropic relaxation mechanisms at low temperatures and compare them to the spin-liquid behavior observed in other strongly correlated systems. In addition, we can recognize charge inhomogeneities in the copper ions of the anion layer. This disorder might be linked to the dielectric response measured in this compound.

cond-mat.str-el↗

arXiv2014

Evolution of ferroelectricity in tetrathiafulvalene-p-chloranil as a function of pressure and temperature

The neutral-to-ionic phase transition in the mixed-stack charge-transfer complex tetrathiafulvalene-p-chloranil (TTF-CA) has been studied by pressure-dependent infrared spectroscopy up to p=11 kbar and down to low temperatures, T = 10 K. By tracking the C=O antisymmetric stretching mode of CA molecules, we accurately determine the ionicity of TTF-CA in the pressure-temperature phase diagram. At any point the TTF-CA crystal bears only a single ionicity; there is no coexistence region or an exotic high-pressure phase. Our findings shed new light on the role of electron-phonon interaction in the neutral-ionic transition.

cond-mat.str-el↗

arXiv2013

EuFe2(As1-xPx)2: reentrant spin glass and superconductivity

By systematic investigations of the magnetic, transport and thermodynamic properties of single crystals of EuFe2(As1-xPx)2 we explore the complex interplay of superconductivity and Eu2+ magnetism. Below 30K, two magnetic transitions are observed for all P substituted crystals suggesting a revision of the phase diagram. In addition to the canted A-type antiferromagnetic order of Eu2+ at approximately 20K, a spin glass transition is discovered at lower temperatures. Most remarkably, the reentrant spin glass state of EuFe2(As1-xPx)2 coexists with superconductivity around x = 0.2.

cond-mat.supr-con↗

arXiv2012

Cooperative dynamics in charge-ordered state of alpha-(BEDT-TTF)2I3

Electric-field-dependent pulse measurements are reported in the charge-ordered state of alpha-(BEDT-TTF)2I3. At low electric fields up to about 50 V/cm only negligible deviations from Ohmic behavior can be identified with no threshold field. At larger electric fields and up to about 100 V/cm a reproducible negative differential resistance is observed with a significant change in shape of the measured resistivity in time. These changes critically depend whether constant voltage or constant current is applied to the single crystal. At high enough electric fields the resistance displays a dramatic drop down to metallic values and relaxes subsequently in a single-exponential manner to its low-field steady-state value. We argue that such an electric-field induced negative differential resistance and switching to transient states are fingerprints of cooperative domain-wall dynamics inherent to two-dimensional bond-charge density wave with ferroelectric-like nature.

cond-mat.str-el↗

arXiv2012

Two-step transition in a magnetoelectric ferrimagnet Cu2OSeO3

We report a detailed single crystal investigation of a magnetoelectric ferrimagnet Cu2OSeO3 using dc magnetization and ac susceptibility along the three principal directions [100], [110] and [111]. We have observed that in small magnetic fields two magnetic transitions occur, one at Tc = 57 K and the second one at TN = 58 K. At Tc the non-linear susceptibility reveals the emergence of the ferromagnetic component and below Tc the magnetization measurements show the splitting between field-cooled and zero-field-cooled regimes. Above 1000 Oe the magnetization saturates and the system is in a single domain state. The temperature dependence of the saturation below Tc can be well described by m(T) = m(0)[1 - (T/Tc)^2]^β, with m(0) = 0.56 (mu)B/Cu, corresponding to the 3-up-1-down configuration. The dielectric constant measured on a thin single crystal shows a systematic deviation below the transition, indicating an intrinsic magnetoelectric effect.

cond-mat.mtrl-sci↗

arXiv2011

Electrodynamic Response of the Charge Ordering Phase: Dielectric and Optical Studies of alpha-(BEDT-TTF)2I3

We report on the anisotropic response, the charge and lattice dynamics of normal and charge-ordered phases with horizontal stripes in single crystals of the organic conductor alpha-(BEDT-TTF)2I3 determined by dc resistivity, dielectric and optical spectroscopy. An overdamped Drude response and a small conductivity anisotropy observed in optics is consistent with a weakly temperature dependent dc conductivity and anisotropy at high temperatures. The splitting of the molecular vibrations nu27(Bu) evidences the abrupt onset of static charge order below TCO=136 K. The drop of optical conductivity measured within the ab plane of the crystal is characterized by an isotropic gap that opens of approximately 75 meV with several phonons becoming pronounced below. Conversely, the dc conductivity anisotropy rises steeply, attaining at 50 K a value 25 times larger than at high temperatures. The dielectric response within this plane reveals two broad relaxation modes of strength Deltaepsilon_LD ~= 5000 and Deltaepsilon_SD ~= 400, centered at 1 kHz < f_LD < 100 MHz and f_SD ~= 1 MHz. The anisotropy of the large-mode (LD) mean relaxation time closely follows the temperature behavior of the respective dc conductivity ratio. We argue that this phason-like excitation is best described as a long-wavelength excitation of a 2kF bond-charge density wave expected theoretically for layered quarter-filled electronic systems with horizontal stripes. Conversely, based on the theoretically expected ferroelectric-like nature of the charge-ordered phase, we associate the small-mode (SD) relaxation with the motion of domain-wall pairs, created at the interface between two types of domains, along the a and b axes. We also consider other possible theoretical interpretations and discuss their limitations.

cond-mat.str-el↗

arXiv2010

Structure and dynamics of hyaluronic acid semidilute solutions: a dielectric spectroscopy study

Dielectric spectroscopy is used to investigate fundamental length scales describing the structure of hyaluronic acid sodium salt (Na-HA) semidilute aqueous solutions. In salt-free regime, the length scale of the relaxation mode detected in MHz range scales with HA concentration as $c_\mathrm{HA}^{-0.5}$ and corresponds to the de Gennes-Pfeuty-Dobrynin correlation length of polyelectrolytes in semidilute solution. The same scaling was observed for the case of long, genomic DNA. Conversely, the length scale of the mode detected in kHz range also varies with HA concentration as $c_\mathrm{HA}^{-0.5}$ which differs from the case of DNA ($c_\mathrm{DNA}^{-0.25}$). The observed behavior suggests that the relaxation in the kHz range reveals the de Gennes-Dobrynin renormalized Debye screening length, and not the average size of the chain, as the pertinent length scale. Similarly, with increasing added salt the electrostatic contribution to the HA persistence length is observed to scale as the Debye length, contrary to scaling pertinent to the Odijk-Skolnick-Fixman electrostatic persistence length observed in the case of DNA. We argue that the observed features of the kHz range relaxation are due to much weaker electrostatic interactions that lead to the absence of Manning condensation as well as a rather high flexibility of HA as compared to DNA.

cond-mat.soft↗

arXiv2010

Collective Excitations in the Charge-Ordered Phase of alpha-(BEDT-TTF)2I3

The charge response of charge-ordered state in the organic conductor alpha-(BEDT-TTF)2I3 is characterized by dc resistivity, dielectric and optical spectroscopy in different crystallographic directions within the two-dimensional conduction layer. Two dielectric modes are detected. The large mode is related to the phason-like excitation of the 2kF bond-charge density wave which forms in the ab plane. The small dielectric mode is associated with the motion of domain-wall pairs along the a- and b-axes between two types of domains which are created due to inversion symmetry breaking.

cond-mat.str-el↗

arXiv2008

Crossover from one-dimensional copper-oxygen chains to two-dimensional ladders charge transport in (La,Y)y(Sr,Ca)14-yCu24O41

The charge transport in the copper-oxygen chain/ladder layers of (La,Y)y(Sr,Ca)14-yCu24O41 is investigated along two crystallographic directions in the temperature range from 50 K to 700 K and for doping levels from y ~= 6 (number of holes nh < 1) to y = 0 (number of holes nh = 6). A crossover from a one-dimensional hopping transport along the chains for y >= 3 to a quasi-two-dimensional charge conduction in the ladder planes for y <~ 2 is observed. This is attributed to a partial hole transfer from chains to ladders when the hole doping exceeds nh ~= 4 and approaches fully doped value n_h = 6. For y <~ 2 a weak dielectric relaxation at radio-frequencies and a microwave mode are detected, which might be recognized as signatures of a charge-density wave phase developed at short length scales in the ladders planes.

cond-mat.str-el↗

arXiv2008

Collective Charge Excitations below the Metal-to-Insulator Transition in BaVS3

The charge response in the barium vanadium sulfide (BaVS3) single crystals is characterized by dc resistivity and low frequency dielectric spectroscopy. A broad relaxation mode in MHz range with huge dielectric constant ~= 10^6 emerges at the metal-to-insulator phase transition TMI ~= 67 K, weakens with lowering temperature and eventually levels off below the magnetic transition Tchi ~= 30 K. The mean relaxation time is thermally activated in a manner similar to the dc resistivity. These features are interpreted as signatures of the collective charge excitations characteristic for the orbital ordering that gradually develops below TMI and stabilizes at long-range scale below Tchi.

cond-mat.str-el↗

arXiv2008

Short-fragment Na-DNA dilute aqueous solutions: fundamental length scales and screening

Dielectric spectroscopy is used to investigate fundamental length scales of 146 bp short-fragment (nucleosomal) dilute Na-DNA solutions. Two relaxation modes are detected: the high- and the low-frequency mode. Dependence of the corresponding length scales on the DNA and on the (uni-valent) salt concentration is studied in detail, being different from the case of long, genomic DNA, investigated before. In low added salt regime, the length scale of the high-frequency mode scales as the average separation between DNAs, though it is smaller in absolute magnitude, whereas the length scale of the low-frequency mode is equal to the contour length of DNA. These fundamental length scales in low added salt regime do not depend on whether DNA is in a double stranded or single stranded form. On the other hand, with increasing added salt, the characteristic length scale of low-frequency mode diminishes at low DNA concentrations probably due to dynamical formation of denaturation bubbles and/or fraying in the vicinity of DNA denaturation threshold.

cond-mat.soft↗

arXiv2004

Anisotropic Charge Modulation in Ladder Planes of Sr_14-xCa_xCu_24O_41

The charge response of the ladders in Sr_14-xCa_xCu_24O_41 is characterized by dc resistivity, low frequency dielectric and optical spectroscopy in all three crystallographic directions. The collective charge-density wave screened mode is observed in the direction of the rungs for x=0, 3 and 6, in addition to the mode along the legs. For x=8 and 9, the charge-density-wave response along the rungs fully vanishes, while the one along the legs persists. The transport perpendicular to the planes is always dominated by hopping.

cond-mat.str-el↗