arXiv subjects

A. Akrap

Publications and source records attributed to A. Akrap.

At least 19 recordsLinked to original sources

arXiv2026

Strain-tunable charge localization coupled to complex magnetic orders in EuAl$_4$

Charge localization is particularly interesting when coupled to antiferromagnetic spin structures. Coupled spin-charge orders are well established in elemental chromium and correlated oxide superconductors, yet the interplay between charge order and more complex magnetic textures -- such as skyrmion lattices and chiral spin structures -- remains largely unexplored. Here we report a comprehensive study of how charge localization couples to the unusually rich sequence of magnetic phases in EuAl$_4$. Using x-ray diffraction under applied magnetic field and uniaxial pressure, we demonstrate a direct coupling between the charge and spin order parameters. In the absence of external stimuli, charge localization is markedly enhanced upon entering the magnetically ordered phases. Strikingly, this effect is highly susceptible to strain: uniaxial pressure applied along the charge-order propagation direction further enhances localization, whereas pressure applied perpendicular to it weakens it. Application of magnetic field reveals both competitive and possible collaborative interactions between spin and charge ordering.This flexible coupling between spin and charge ordering opens a new route to designing symmetry-breaking states. Chiral charge order may for example be patterned from spin structures with that symmetry.

cond-mat.str-el↗

arXiv2025

Dynamics of surface electrons in a topological insulator: cyclotron resonance at room temperature

The ability to manipulate the surface states of topological insulators using electric or magnetic fields under ambient conditions is a key step toward their integration into future electronic and optoelectronic devices. Here, we demonstrate - using cyclotron resonance measurements on a tin-doped BiSbTe$_2$S topological insulator - that moderate magnetic fields can quantize massless surface electrons into Landau levels even at room temperature. This finding suggests that surface-state electrons can behave as long-lived quasiparticles at unexpectedly high temperatures.

cond-mat.mes-hall↗

arXiv2025

Report on reproducibility in condensed matter physics

We present recommendations to improve reproducibility and replicability in condensed matter physics. This area of physics has consistently produced both fundamental insights into the workings of matter and transformative inventions. Our recommendations result from a collaboration that includes researchers from academia and government laboratories, scientific journalists, legal professionals, representatives of publishers, professional societies, and other experts. The group met in person in May 2024 at a conference at the University of Pittsburgh to discuss the growing challenges related to research reproducibility and replicability in condensed matter physics. In this report, we discuss best practices and policies at all stages of the scientific process to safeguard the value of condensed matter. We hope this report will lay the groundwork for a broader conversation to develop subfield-specific recommendations.

cond-mat.other↗

arXiv2024

Revealing the KH2PO4 soft-mode coupling mechanism with infrared spectroscopy under pressure

We measured the far-infrared reflectivity of a KH2PO4 single crystal up to pressures of 2 GPa in the ferroelectric and paraelectric phases. We find that the nu4 vibrational mode of the PO4 tetrahedron is strongly affected by the applied pressure. At ambient pressure this phonon is destabilized by the presence of the H ions and hence shows a highly damped character, beyond the phonon propagation threshold. Applying a pressure close to 0.6 GPa makes this phonon clearly underdamped. Its behavior closely follows the soft-mode behavior observed in Raman spectroscopy. Our results solve a long standing open problem, demonstrating that the nu4 mode is the excitation mediating the coupling of the hydrogen network to the lattice modes that create the ferroelectic polarization in KH2PO4.

cond-mat.mtrl-sci↗

arXiv2023

Evidence of 3D Dirac conical bands in TlBiSSe by optical and magneto-optical spectroscopy

TlBiSSe is a rare realization of a 3D semimetal with a conically dispersing band that has an optical response which is well isolated from other contributions in a broad range of photon eneries. We report optical and magneto-optical spectroscopy on this material. When the compound is chemically tuned into a state of the lowest carrier concentration, we find a nearly linear frequency dependence of the optical conductivity below 0.5~eV. Landau level spectroscopy allows us to describe the system with a massive Dirac model, giving a gap $2\Delta =32$~meV and an in-plane velocity parameter $v= 4.0\times 10^5$~m/s. % Finally, we provide a theoretical recipe to extract all parameters of the anisotropic Dirac band, including the Fermi energy and band degeneracy.

cond-mat.mtrl-sci↗

arXiv2022

Temperature dependence of the energy band gap in ZrTe$_5$: implications for the topological phase

Using Landau level spectroscopy, we determine the temperature dependence of the energy band gap in zirconium pentatelluride (ZrTe$_5$). We find that the band gap reaches $E_g=(5 \pm 1)$ meV at low temperatures and increases monotonously when the temperature is raised. This implies that ZrTe$_5$ is a weak topological insulator, with non-inverted ordering of electronic bands in the center of the Brillouin zone. Our magneto-transport experiments performed in parallel show that the resistivity anomaly in ZrTe$_5$ is not connected with the temperature dependence of the band gap.

cond-mat.mes-hall↗

arXiv2021

Lorentz-boost-driven magneto-optics in a Dirac nodal-line semimetal

Optical response of crystalline solids is to a large extent driven by excitations that promote electrons among individual bands. This allows one to apply optical and magneto-optical methods to determine experimentally the energy band gap - a fundamental property crucial to our understanding of any solid - with a great precision. Here we show that such conventional methods, applied with great success to many materials in the past, do not work in topological Dirac semimetals with a dispersive nodal line. There, the optically deduced band gap depends on how the magnetic field is oriented with respect to the crystal axes. Such highly unusual behaviour is explained in terms of band-gap renormalization driven by Lorentz boosts.

cond-mat.mes-hall↗

arXiv2021

Pressure-induced structural transitions triggering dimensional crossover in lithium purple bronze Li0.9M6O17

At ambient pressure, lithium molybdenum purple bronze (Li0.9Mo6O17) is a quasi-one dimensional solid in which the anisotropic crystal structure and the linear dispersion of the underlying bands produced by electronic correlations possibly bring about a rare experimental realization of Tomomaga-Luttinger liquid physics. It is also the sole member of the broader purple molybdenum bronzes family where a Peierls instability has not been identified at low temperatures. The present study reports a pressure-induced series of phase transitions between 0 and 12 GPa. These transitions are strongly reflected in infrared spectroscopy, Raman spectroscopy, and x-ray diffraction. The most dramatic effect seen in optical conductivity is the metallization of the c-axis, concomitant to the decrease of conductivity along the b-axis. This indicates that high pressure drives the material away from its quasi-one dimensional behavior at ambient pressure. While the first pressure-induced structure of the series is resolved, the identification of the underlying mechanisms driving the dimensional change in the physics remains a challenge.

cond-mat.str-el↗

arXiv2021

Optical conductivity signatures of open Dirac nodal lines

We investigate the optical conductivity and far-infrared magneto-optical response of BaNiS$_2$, a simple square-lattice semimetal characterized by Dirac nodal lines that disperse exclusively along the out-of-plane direction. With the magnetic field aligned along the nodal line the in-plane Landau level spectra show a nearly $\sqrt{B}$ behavior, the hallmark of a conical-band dispersion with a small spin-orbit coupling gap. The optical conductivity exhibits an unusual temperature-independent isosbestic line, ending at a Van Hove singularity. First-principles calculations unambiguously assign the isosbestic line to transitions across Dirac nodal states. Our work suggests a universal topology of the electronic structure of Dirac nodal lines.

cond-mat.mtrl-sci↗

arXiv2020

Nature of native atomic defects in ZrTe$_5$ and their impact on the low-energy electronic structure

Over the past decades, investigations of the anomalous low-energy electronic properties of ZrTe$_5$ have reached a wide array of conclusions. An open question is the growth method's impact on the stoichiometry of ZrTe$_5$ samples, especially given the very small density of states near its chemical potential. Here we report on high resolution scanning tunneling microscopy and spectroscopy measurements performed on samples grown via different methods. Using density functional theory calculations, we identify the most prevalent types of atomic defects on the surface of ZrTe$_5$, namely Te vacancies and intercalated Zr atoms. Finally, we precisely quantify their density and outline their role as ionized defects in the anomalous resistivity of this material.

cond-mat.mtrl-sci↗

arXiv2019

Magneto-optics of a Weyl semimetal beyond the conical band approximation: the case study of TaP

Landau-level spectroscopy, the optical analysis of electrons in materials subject to a strong magnetic field, is a versatile probe of the electronic band structure and has been successfully used in the identification of novel states of matter such as Dirac electrons, topological materials or Weyl semimetals. The latter arise from a complex interplay between crystal symmetry, spin-orbit interaction and inverse ordering of electronic bands. Here, we report on unusual Landau-level transitions in the monopnictide TaP that decrease in energy with increasing magnetic field. We show that these transitions arise naturally at intermediate energies in time-reversal-invariant Weyl semimetals where the Weyl nodes are formed by a partially gapped nodal-loop in the band structure. We propose a simple theoretical model for electronic bands in these Weyl materials that captures the collected magneto-optical data to great extent.

cond-mat.mes-hall↗

arXiv2018

3D Dirac semimetal Cd3As2: A review of material properties

Cadmium arsenide (Cd3As2) - a time-honored and widely explored material in solid-state physics - has recently attracted considerable attention. This was triggered by a theoretical prediction concerning the presence of 3D symmetry-protected massless Dirac electrons, which could turn Cd3As2 into a 3D analogue of graphene. Subsequent extended experimental studies have provided us with compelling experimental evidence of conical bands in this system, and revealed a number of interesting properties and phenomena. At the same time, some of the material properties remain the subject of vast discussions despite recent intensive experimental and theoretical efforts, which may hinder the progress in understanding and applications of this appealing material. In this review, we focus on the basic material parameters and properties of Cd3As2, in particular those which are directly related to the conical features in the electronic band structure of this material. The outcome of experimental investigations, performed on Cd3As2 using various spectroscopic and transport techniques within the past sixty years, is compared with theoretical studies. These theoretical works gave us not only simplified effective models, but more recently, also the electronic band structure calculated numerically using ab initio methods.

cond-mat.mes-hall↗

arXiv2018

The energy scale of Dirac electrons in Cd3As2

Cadmium arsenide (Cd3As2) has recently became conspicuous in solid-state physics due to several reports proposing that it hosts a pair of symmetry-protected 3D Dirac cones. Despite vast investigations, a solid experimental insight into the band structure of this material is still missing. Here we fill one of the existing gaps in our understanding of Cd3As2, and based on our Landau level spectroscopy study, we provide an estimate for the energy scale of 3D Dirac electrons in this system. We find that the appearance of such charge carriers is limited - contrary to a widespread belief in the solid-state community - to a relatively small energy scale (below 40 meV).

cond-mat.mes-hall↗

arXiv2016

BiTeCl and BiTeBr: a comparative high-pressure optical study

We here report a detailed high-pressure infrared transmission study of BiTeCl and BiTeBr. We follow the evolution of two band transitions: the optical excitation $\beta$ between two Rashba-split conduction bands, and the absorption $\gamma$ across the band gap. In the low pressure range, $p< 4$~GPa, for both compounds $\beta$ is approximately constant with pressure and $\gamma$ decreases, in agreement with band structure calculations. In BiTeCl, a clear pressure-induced phase transition at 6~GPa leads to a different ground state. For BiTeBr, the pressure evolution is more subtle, and we discuss the possibility of closing and reopening of the band gap. Our data is consistent with a Weyl phase in BiTeBr at 5$-$6~GPa, followed by the onset of a structural phase transition at 7~GPa.

cond-mat.mtrl-sci↗

arXiv2016

Optical properties of $A$Fe$_\mathbf{2}$As$_\mathbf{2}$ ($A=\,$Ca, Sr, and Ba) single crystals

The detailed optical properties have been determined for the iron-based materials $A$Fe$_2$As$_2$, where $A=\,$Ca, Sr, and Ba, for light polarized in the iron-arsenic ($a-b$) planes over a wide frequency range, above and below the magnetic and structural transitions at $T_N =$ 172, 195, and 138 K, respectively. The real and imaginary parts of the complex conductivity are fit simultaneously using two Drude terms in combination with a series of oscillators. Above $T_N$, the free-carrier response consists of a weak, narrow Drude term, and a strong, broad Drude term, both of which show only a weak temperature dependence. Below $T_N$ there is a slight decrease of the plasma frequency but a dramatic drop in the scattering rate for the narrow Drude term, and for the broad Drude term there is a significant decrease in the plasma frequency, while the decrease in the scattering rate, albeit significant, is not as severe. The small values observed for the scattering rates for the narrow Drude term for $T\ll{T_N}$ may be related to the Dirac cone-like dispersion of the electronic bands. Below $T_N$ new features emerge in the optical conductivity that are associated with the reconstruction Fermi surface and the gapping of bands at $\Delta_1 \simeq$ 45 $-$ 80 meV, and $\Delta_2 \simeq$ 110 $-$ 210 meV. The reduction in the spectral weight associated with the free carriers is captured by the gap structure, specifically, the spectral weight from the narrow Drude term appears to be transferred into the low-energy gap feature, while the missing weight from the broad term shifts to the high-energy gap.

cond-mat.str-el↗

arXiv2016

Magneto-optical signature of massless Kane electrons in Cd3As2

We report on optical reflectivity experiments performed on Cd3As2 over a broad range of photon energies and magnetic fields. The observed response clearly indicates the presence of 3D massless charge carriers. The specific cyclotron resonance absorption in the quantum limit implies that we are probing massless Kane electrons rather than symmetry-protected 3D Dirac particles. The latter may appear at a smaller energy scale and are not directly observed in our infrared experiments.

cond-mat.mes-hall↗

arXiv2014

Spectral weight transfer in strongly correlated Fe$_{1.03}$Te

The temperature dependence of the in-plane optical conductivity has been determined for Fe$_{1.03}$Te above and below the magnetic and structural transition at $T_N\simeq 68$ K. The electron and hole pockets are treated as two separate electronic subsystems; a strong, broad Drude response that is largely temperature independent, and a much weaker, narrow Drude response with a strong temperature dependence. Spectral weight is transferred from high to low frequency below $T_N$, resulting in the dramatic increase of both the low-frequency conductivity and the related plasma frequency. The change in the plasma frequency is due to an increase in the carrier concentration resulting from the closing of the pseudogap on the electron pocket, as well as the likely decrease of the effective mass in the antiferromagnetic state.

cond-mat.str-el↗

arXiv2014

Optical properties of BiTeBr and BiTeCl

We present a comparative study of the optical properties - reflectance, transmission and optical conductivity - and Raman spectra of two layered bismuth-tellurohalides BiTeBr and BiTeCl at 300 K and 5 K, for light polarized in the a-b planes. Despite different space groups, the optical properties of the two compounds are very similar. Both materials are doped semiconductors, with the absorption edge above the optical gap which is lower in BiTeBr (0.62 eV) than in BiTeCl (0.77 eV). The same Rashba splitting is observed in the two materials. A non-Drude free carrier contribution in the optical conductivity, as well as three Raman and two infrared phonon modes, are observed in each compound. There is a dramatic difference in the highest infrared phonon intensity for the two compounds, and a difference in the doping levels. Aspects of the strong electron-phonon interaction are identified. Several interband transitions are assigned, among them the low-lying absorption $β$ which has the same value 0.25 eV in both compounds, and is caused by the Rashba spin splitting of the conduction band. An additional weak transition is found in BiTeCl, caused by the lower crystal symmetry.

cond-mat.mtrl-sci↗