arXiv subjects

M. Dressel

Publications and source records attributed to M. Dressel.

At least 55 records · Page 3Linked to original sources

arXiv2018

Low-Energy Excitations in Quantum Spin-Liquids Identified by Optical Spectroscopy

The electrodynamic response of organic spin liquids with highly-frustrated triangular lattices has been measured in a wide energy range. While the overall optical spectra of these Mott insulators are governed by transitions between the Hubbard bands, distinct in-gap excitations can be identified at low temperatures and frequencies which we attribute to the quantum spin liquid state. For the strongly correlated $β^{\prime}$-EtMe$_3$\-Sb\-[Pd(dmit)$_2$]$_2$, we discover enhanced conductivity below $175~{\rm cm}^{-1}$, comparable to the energy of the magnetic coupling $J\approx 250$ K. For $ω\rightarrow 0$ these low-frequency excitations vanish faster than the charge-carrier response subject to Mott-Hubbard correlations, resulting in a dome-shape band peaked at 100~\cm. Possible relations to spinons, magnons and disorder are discussed.

cond-mat.str-el↗

arXiv2018

Unique interplay between superconducting and ferromagnetic orders in EuRbFe$_4$As$_4$

Transport, magnetic and optical investigations on EuRbFe$_4$As$_4$ single crystals evidence that the ferromagnetic ordering of the Eu$^{2+}$ magnetic moments at $T_N=15$ K, below the superconducting transition ($T_c=36$ K), affects superconductivity in a weak but intriguing way. Upon cooling below $T_N$, the zero resistance state is preserved and the superconductivity is affected by the in-plane ferromagnetism mainly at domain boundaries; a perfect diamagnetism is recovered at low temperatures. The infrared conductivity is strongly suppressed in the far-infrared region below $T_c$, associated with the opening of a complete superconducting gap at $2Δ= 10$ meV. A gap smaller than the weak coupling limit suggests the strong orbital effects or, within a multiband superconductivity scenario, the existence of a larger yet unrevealed gap.

cond-mat.supr-con↗

arXiv2018

Optical conductivity of the Weyl semimetal NbP

The optical properties of (001)-oriented NbP single crystals have been studied in a wide spectral range from 6 meV to 3 eV from room temperature down to 10 K. The itinerant carriers lead to a Drude-like contribution to the optical response; we can further identify two pronounced phonon modes and interband transitions starting already at rather low frequencies. By comparing our experimental findings to the calculated interband optical conductivity, we can assign the features observed in the measured conductivity to certain interband transitions. In particular, we find that transitions between the electronic bands spilt by spin-orbit coupling dominate the interband conductivity of NbP below 100 meV. At low temperatures, the momentum-relaxing scattering rate of the itinerant carriers in NbP is very small, leading to macroscopic characteristic length scales of the momentum relaxation of approximately 0.5 $μ$m.

cond-mat.mes-hall↗

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↗

arXiv2017

Quantum Spin Liquids Unveil the Genuine Mott State

The Widom line identifies the locus in the phase diagram where a supercritical gas crosses over from gas-like to a more liquid-like behavior. A similar transition exists in correlated electron liquids, where the interplay of Coulomb repulsion, bandwidth and temperature triggers between the Mott insulating state and an incoherent conduction regime. Here we explore the electrodynamic response of three organic quantum spin liquids with different degrees of effective correlation, where the absence of magnetic order enables unique insight into the nature of the genuine Mott state down to the most relevant low-temperature region. Combining optical spectroscopy with pressure-dependent dc transport and theoretical calculations, we succeeded to construct a phase diagram valid for all Mott insulators on a quantitative scale. In the vicinity of the low-temperature phase boundary, we discover metallic fluctuations within the Mott gap, exhibiting enhanced absorption upon cooling that is not present in antiferromagnetic Mott insulators. Our findings reveal the phase coexistence region and Pomeranchuk-like anomaly of the Mott transition, previously predicted but never observed.

cond-mat.str-el↗

arXiv2017

Flat optical conductivity in ZrSiS due to two-dimensional Dirac bands

ZrSiS exhibits a frequency-independent interband conductivity $σ(ω) = \rm{const}(ω) \equiv σ_{\rm{flat}}$ in a broad range from 250 to 2500 cm$^{-1}$ (30 - 300 meV). This makes ZrSiS similar to (quasi)two-dimensional Dirac electron systems, such as graphite and graphene. We assign the flat optical conductivity to the transitions between quasi-two-dimensional Dirac bands near the Fermi level. In contrast to graphene, $σ_{\rm{flat}}$ is not supposed to be universal but related to the length of the nodal line in the reciprocal space, $k_{0}$. When $σ_{\rm{flat}}$ and $k_{0}$ are connected by a simple model, we find good agreement between experiment and theory. Due to the spin-orbit coupling, the discussed Dirac bands in ZrSiS possess a small gap $Δ$, for which we determine an upper bound max($Δ$) = 30 meV from our optical measurements. At low temperatures the momentum-relaxation rate collapses, and the characteristic length scale of momentum relaxation is of the order of microns below 50 K.

cond-mat.mes-hall↗

arXiv2017

Effects of pressure and magnetic field on the re-entrant superconductor Eu(Fe$_{0.93}$Rh$_{0.07}$)$_2$As$_2$

Electron-doped Eu(Fe$_{0.93}$Rh$_{0.07}$)$_2$As$_2$ has been systematically studied by high pressure investigations of the magnetic and electrical transport properties, in order to unravel the complex interplay of superconductivity and magnetism. The compound reveals an exceedingly broad re-entrant transition to the superconducting state between $T_{\rm{c,on}} = 19.8$ K and $T_{\rm{c,0}} = 5.2$ K due to a canted A-type antiferromagnetic ordering of the Eu$^{2+}$ moments at $T_{\rm{N}} = 16.6$ K and a re-entrant spin glass transition at $T_{\rm{SG}} = 14.1$ K. At ambient pressure evidences for the coexistence of superconductivity and ferromagnetism could be observed, as well as a magnetic-field-induced enhancement of the zero-resistance temperature $T_{\rm{c,0}}$ up to $7.2$ K with small magnetic fields applied parallel to the \textit{ab}-plane of the crystal. We attribute the field-induced-enhancement of superconductivity to the suppression of the ferromagnetic component of the Eu$^{2+}$ moments along the \textit{c}-axis, which leads to a reduction of the orbital pair breaking effect. Application of hydrostatic pressure suppresses the superconducting state around $14$ kbar along with a linear temperature dependence of the resistivity, implying that a non-Fermi liquid region is located at the boundary of the superconducting phase. At intermediate pressure, an additional feature in the resistivity curves is identified, which can be suppressed by external magnetic fields and competes with the superconducting phase. We suggest that the effect of negative pressure by the chemical Rh substitution in Eu(Fe$_{0.93}$Rh$_{0.07}$)$_2$As$_2$ is partially reversed, leading to a re-activation of the spin density wave.

cond-mat.supr-con↗

arXiv2017

Two-channel conduction in YbPtBi

We investigated transport, magnetotransport, and broadband optical properties of the half-Heusler compound YbPtBi. Hall measurements evidence two types of charge carriers: highly mobile electrons with a temperature-dependent concentration and low-mobile holes; their concentration stays almost constant within the investigated temperature range from 2.5 to 300 K. The optical spectra (10 meV - 2.7 eV) can be naturally decomposed into contributions from intra- and interband absorption processes, the former manifesting themselves as two Drude bands with very different scattering rates, corresponding to the charges with different mobilities. These results of the optical measurements allow us to separate the contributions from electrons and holes to the total conductivity and to implement a two-channel-conduction model for description of the magnetotransport data. In this approach, the electron and hole mobilities are found to be around 50000 and 10 cm$^{2}$/Vs at the lowest temperatures (2.5 K), respectively.

cond-mat.str-el↗

arXiv2017

Reentrant phases in electron-doped $\text{EuFe}_2\text{As}_2$: spin glass and superconductivity

We report evidence for a reentrant spin glass phase in electron-doped $\text{EuFe}_2\text{As}_2$ single crystals and first traces of the superconductivity re-entrance in optics. In the close-to-optimal doped $\text{Eu}(\text{Fe}_{0.91}\text{Ir}_{0.09})_2\text{As}_2$ and $\text{Eu}(\text{Fe}_{0.93}\text{Rh}_{0.07})_2\text{As}_2$ samples two magnetic transitions are observed below the superconducting critical temperature $T_c \approx 21$~K: the canted $A$-type antiferromagnetic order of the $\text{Eu}^{2+}$ ions sets in around $17\,\text{K}$; the spin glass behavior occurs another $2\,\text{K}$ lower in temperature. In addition, strong evidence for an additional transition is found far below the spin glass temperature. Our extensive optical and magnetic investigations provide new insight into the interplay of local magnetism and superconductivity in these systems and elucidate the effect of the spin-glass phase on the reentrant superconducting state.

cond-mat.supr-con↗

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

Interband optical conductivity of the [001]-oriented Dirac semimetal Cd3As2

We measured the optical reflectivity of [001]-oriented $n$-doped Cd$_{3}$As$_{2}$ in a broad frequency range (50 - 22000 cm$^{-1}$) for temperatures from 10 to 300 K. The optical conductivity, $σ(ω) = σ_{1}(ω) + {\rm i}σ_{2}(ω)$, is isotropic within the (001) plane; its real part follows a power law, $σ_{1}(ω) \propto ω^{1.65}$, in a large interval from 2000 to 8000 cm$^{-1}$. This behavior is caused by interband transitions between two Dirac bands, which are effectively described by a sublinear dispersion relation, $E(k) \propto \lvert k \rvert ^{0.6}$. The momentum-averaged Fermi velocity of the carriers in these bands is energy dependent and ranges from $1.2 \times 10^{5}$ to $3 \times 10^{5}$ m/s, depending on the distance from the Dirac points. We detect a gaplike feature in $σ_{1}(ω)$ and associate it with the Fermi level positioned around $100$ meV above the Dirac points.

cond-mat.mes-hall↗

arXiv2016

Ultrafast response of plasma-like reflectivity edge in (TMTTF)2AsF6 driven by 7-fs, 1.5-cycle strong-light field

The strong light-field effect of (TMTTF)2AsF6 was investigated utilizing 1.5-cycle, 7-fs infrared pulses. The ultarfast (20 fs) and large (40%) response of the plasma-like reflectivity edge (0.7 eV) was analyzed by the changes in omega_p=sqrt(ne2/(epsilon_0*epsilon(infty)*m)} (n: number of charges in the 1/4 filled-band, m: mass of charge, epsilon(infty): dielectric constants for high-frequency and vacuum, e: elementary charge). The 3% reduction in omega_p is attributed to the 6% increase in m. Furthermore, 20 fs oscillation of omega_p in the time domain indicates that the plasma-like edge is affected by the charge gap (0.2 eV) nature. Theoretical calculations suggest that the Coulomb repulsion plays an important role in the increase in m.

cond-mat.str-el↗

arXiv2016

Effect of electronic correlations on the metal-insulator transition of $α$-(BEDT-TTF)$_2$I$_3$: theoretical and experimental investigations of its optical properties

The organic salt $α$-(BEDT-TTF)$_2$I$_3$ is considered a model system for metal-insulator transition due to electronic charge ordering at $T_{\rm CO}=135$~K. The optical properties obtained from polarized reflection measurements above and below $T_{\rm CO}$ can be well described by calculations based on first-principle density-functional theory (DFT). We discuss the effect of electronic correlations on the metal-insulator transition.

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↗

arXiv2016

Electronic Scattering Effects in Europium-Based Iron Pnictides

In a comprehensive study, we investigate the electronic scattering effects in EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ by using Fourier-transform infrared spectroscopy. In spite of the fact that Eu$^{2+}$ local moments order around $T_\text{Eu} \approx 20$\,K, the overall optical response is strikingly similar to the one of the well-known Ba-122 pnictides. The main difference lies within the suppression of the lower spin-density-wave gap feature. By analysing our spectra with a multi-component model, we find that the high-energy feature around 0.7\,eV -- often associated with Hund's rule coupling -- is highly sensitive to the spin-density-wave ordering, this further confirms its direct relationship to the dynamics of itinerant carriers. The same model is also used to investigate the in-plane anisotropy of magnetically detwinned EuFe$_{2}$As$_{2}$ in the antiferromagnetically ordered state, yielding a higher Drude weight and lower scattering rate along the crystallographic $a$-axis. Finally, we analyse the development of the room temperature spectra with isovalent phosphor substitution and highlight changes in the scattering rate of hole-like carriers induced by a Lifshitz transition.

cond-mat.supr-con↗

arXiv2016

Lattice vibrations of the charge-transfer salt $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_{3}$: novel interpretation of the electrodynamic response in a spin-liquid compound

The dimer Mott insulator $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ exhibits unusual electrodynamic properties. Numerical investigations of the electronic ground state and the molecular and lattice vibrations reveal the importance of the Cu$_2$(CN)$_3^-$ anion network coupled to the BEDT-TTF molecules: The threefold cyanide coordination of copper and linkage isomerism in the anion structure cause a loss of symmetry, frustration, disorder, and domain formation. Our findings consistently explain the temperature and polarization-dependent THz and infrared measurements, reinforce the understanding of dielectric properties and have important implications for the quantum spin-liquid state, which should be treated beyond two-dimensional, purely electronic models.

cond-mat.str-el↗

arXiv2015

Pressure dependent relaxation in the photo-excited Mott insulator ETF2TCNQ: Influence of hopping and correlations on quasiparticle recombination rates

Femtosecond relaxation of photo-excited quasiparticles in the one dimensional Mott insulator ET-F2TCNQ are measured as a function of external pressure, which is used to tune the electronic structure. By fitting the static optical properties and measuring femtosecond decay times at each pressure value, we correlate the relaxation rates with the electronic bandwidth t and on the intersite correlation energy V. The scaling of relaxation times with microscopic parameters is different than for metals and semiconductors. The competition between localization and delocalization of the Mott-Hubbard exciton dictates the efficiency of the decay, as exposed by a fit based on the solution of the time-dependent extended Hubbard Hamiltonian.

cond-mat.str-el↗

arXiv2014

Low-Energy Excitations in the Quantum Spin-Liquid $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_{3}$

The electrodynamic response of the organic spin-liquid candidate $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ has been measured in an extremely wide energy range ($10^{-13}$ to 2 eV) as a function of temperature (5 to 300 K). Below the Mott gap, excitations from the un-gapped spinon continuum cause a considerable contribution to the infrared conductivity, as suggested by the U(1) gauge theory. At THz frequencies we can identify a power-law behavior $σ(ω) \propto ω^β$ with two distinct exponents $β$ that change from 0.9 to 1.3 at low temperatures. The corresponding crossover scales with temperature: $\hbarω_c \approx k_B T$. The observed exponents differ by more than a factor of 2 from the theoretically predicted ones. The findings are compared with those obtained on Herbertsmithites.

cond-mat.str-el↗