arXiv subjects

J. Ebad-Allah

Publications and source records attributed to J. Ebad-Allah.

At least 19 recordsLinked to original sources

arXiv2026

Charge dynamics in the Weyl semimetals NbIrTe$_4$ and TaIrTe$_4$ under pressure: Signatures of an electronic phase transition

A high-pressure investigation of the Weyl semimetals NbIrTe$_4$ and TaIrTe$_4$ is presented, using infrared spectroscopy supplemented by density functional theory calculations. The experimental optical conductivity spectra as a function of pressure suggest the occurrence of a pressure-induced phase transition at a critical pressure $P_\text{c}=7\text{--}8$ GPa. This transition is most likely electronic in nature, as Raman scattering measurements provide no evidence of a significant structural phase transition. Above $P_\text{c}$ a significant redistribution of spectral weight occurs in the optical conductivity spectrum for both materials. A Drude-Lorentz analysis of the optical data indicates a sharp reduction in the free carrier concentration at $P_\text{c}$, concomitant with the appearance of a low-energy phonon, which was initially screened by free charge carriers. A predominantly electronic origin of the phase transition is supported by the calculated electronic band structure, Fermi surface, and interband optical conductivity as a function of pressure. Our findings provide collective evidence for a pressure-induced, most likely electronic phase transition in both van der Waals materials at $P_\text{c}=7\text{--}8$ GPa, highlighting the tunability of their electronic band structure by hydrostatic pressure.

cond-mat.mtrl-sci↗

arXiv2025

Suppression of magnetism in Co$_3$Sn$_2$S$_2$ under external pressure

The ability to control the magnetic state provides a powerful means to tune the underlying band topology, enabling transitions between distinct electronic phases and the emergence of novel quantum phenomena. In this work, we address the evolution of ferromagnetic state upon applying external pressures up to 10.8~GPa using a combined experimental and theoretical study. The standard \emph{ab initio} Density Functional Theory computation including ionic relaxations grossly overestimates the unit cell magnetization as a function of pressure. In our theoretical analysis we identify two possible mechanisms to remedy this shortcoming. Matching the experimental observations is achieved by a symmetry-preserving adjustment of the sulfur atoms position within the unit cell. Alternatively, we explore various combinations of the exchange and correlation parts of the effective potential which reproduce the experimental magnetization, the structural parameters and the measured optical conductivity spectra. Thus, the pressure-dependent behavior of magnetization demands a careful theoretical treatment and analysis of theoretical and experimental data.

cond-mat.mtrl-sci↗

arXiv2024

Optical anisotropy of the kagome magnet FeSn: Dominant role of excitations between kagome and Sn layers

Antiferromagnetic FeSn is considered to be a close realization of the ideal two-dimensional (2D) kagome lattice, hosting Dirac cones, van Hove singularities, and flat bands, as it comprises Fe$_3$Sn kagome layers well separated by Sn buffer layers. We observe a pronounced optical anisotropy, with the low-energy optical conductivity being surprisingly higher perpendicular to the kagome planes than along the layers. This finding contradicts the prevalent picture of dominantly 2D electronic structure for FeSn. Our material-specific theory reproduces the measured conductivity spectra remarkarbly well. A site-specific decomposition of the optical response to individual excitation channels shows that the optical conductivity for polarizations both parallel and perpendicular to the kagome plane is dominated by interlayer transitions between kagome layers and adjacent Sn-based layers. Moreover, the matrix elements corresponding to these transitions are highly anisotropic, leading to larger out-of-plane conductivity. Our results evidence the crucial role of interstitial layers in charge dynamics even in seemingly 2D systems.

cond-mat.mtrl-sci↗

arXiv2023

Signatures of van Hove singularities in the anisotropic in-plane optical conductivity of the topological semimetal Nb$_3$SiTe$_6$

We present a temperature-dependent infrared spectroscopy study on the layered topological semimetal Nb$_3$SiTe$_6$ combined with density-functional theory (DFT) calculations of the electronic band structure and optical conductivity. Our results reveal an anisotropic behavior of the in-plane ($ac$-plane) optical conductivity, with three pronounced excitations located at around 0.15, 0.28, and 0.41~eV for the polarization of the incident radiation along the $c$ axis. These excitations are well reproduced in the theoretical spectra. Based on the \textit{ab initio} results, the excitations around 0.15 eV and 0.28 eV are interpreted as fingerprints of van Hove singularities in the electronic band structure and compared to the findings for other topological semimetals.

cond-mat.mtrl-sci↗

arXiv2022

In-plane and out-of-plane optical response of the nodal-line semimetals ZrGeS and ZrGeSe

Polarization-dependent reflectivity measurements were carried out over a broad frequency range on single-crystalline ZrGeSe and ZrGeS compounds, which are closely related to the prototype nodal-line semimetal ZrSiS. These measurements revealed the strongly anisotropic character of both ZrGeSe and ZrGeS, with a reduced plasma frequency for the out-of-plane direction {\bf E}$\| c$ as compared to the in-plane direction {\bf E}$\| ab$. For {\bf E}$\| ab$ the optical conductivity spectrum consists of two Drude terms followed by a shoulder or plateau-like behavior and a distinct U shape at higher energies, while for {\bf E}$\| c$ one Drude term is followed by a peak-like behavior and the U shape of the profile is less developed. Under external pressure, two prominent excitations appear in the out-of-plane optical conductivity spectrum of ZrGeSe, whose frequency position and oscillator strength show a weak anomaly at $\sim$3~GPa. Overall, the pressure-induced changes in the profile of the {\bf E}$\| c$ conductivity spectrum are much enhanced above $\sim$3~GPa. We compare our results to those recently reported for ZrSiS in a quantitative manner.

cond-mat.mtrl-sci↗

arXiv2021

Pressure-induced excitations in the out-of-plane optical response of the nodal-line semimetal ZrSiS

The anisotropic optical response of the layered, nodal-line semimetal ZrSiS at ambient and high pressure is investigated by frequency-dependent reflectivity measurements for the polarization along and perpendicular to the layers. The highly anisotropic optical conductivity is in very good agreement with results from density functional theory calculations and confirms the anisotropic character of ZrSiS. Whereas the in-plane optical conductivity shows only modest pressure-induced changes, we found strong effects on the out-of-plane optical conductivity spectrum of ZrSiS, with the appearance of two prominent excitations. These pronounced pressure-induced effects can neither be attributed to a structural phase transition according to our single-crystal x-ray diffraction measurements, nor can they be explained by electronic correlation and electron-hole pairing effects, as revealed by theoretical calculations. Our findings are discussed in the context of the recently proposed excitonic insulator phase in ZrSiS.

cond-mat.mtrl-sci↗

arXiv2020

Optical conductivity of the type-II Weyl semimetal WTe$_2$ under pressure

Tungsten ditelluride WTe$_2$ is a type-II Weyl semimetal with electronic properties highly sensitive to external pressure, as demonstrated by the superconductivity emerging under pressure. Here, we study the optical conductivity of the type-II Weyl semimetal WTe$_2$ under external pressure at room temperature. With increasing pressure, a pronounced spectral weight transfer occurs from the high-energy to the low-energy interband transitions, with drastic changes in the profile of the optical conductivity spectrum indicating a high sensitivity of the electronic band structure to external pressure. The detailed analysis of the pressure-dependent optical conductivity furthermore reveals anomalies at the pressures $\sim$2 and $\sim$4.5~GPa, where an electronic and a structural phase transition, respectively, were reported in the literature.

cond-mat.mtrl-sci↗

arXiv2020

Influence of magnetic ordering on the optical response of the antiferromagnetic topological insulator MnBi$_2$Te$_4$

The layered topological insulator MnBi$_2$Te$_4$ has attracted great interest recently due to its intrinsic antiferromagnetic order, potentially hosting various topological phases. By temperature-dependent infrared spectroscopy over a broad frequency range, we studied the changes in the optical conductivity of MnBi$_2$Te$_4$ at the magnetic ordering temperature. The temperature dependence of several optical parameters reveals an anomaly at the magnetic phase transition, which suggests the correlation between the bulk electronic band structure and the magnetism. We relate our findings to recent reports on the temperature dependence of the electronic band structure of MnBi$_2$Te$_4$.

cond-mat.mtrl-sci↗

arXiv2020

Indications for Lifshitz transitions in the nodal-line semimetal ZrSiTe induced by interlayer interaction

The layered material ZrSiTe is currently extensively investigated as a nodal-line semimetal with Dirac-like band crossings protected by nonsymmorphic symmetry close to the Fermi energy. A recent infrared spectroscopy study on ZrSiTe under external pressure found anomalies in the optical response, providing hints for pressure-induced phase transitions at $\approx$4.1 and $\approx$6.5 GPa. By pressure-dependent Raman spectroscopy and x-ray diffraction measurements combined with electronic band structure calculations we find indications for two pressure-induced Lifshitz transitions with major changes in the Fermi surface topology in the absence of lattice symmetry changes. These electronic phase transitions can be attributed to the enhanced interlayer interaction induced by external pressure. Our findings demonstrate the crucial role of the interlayer distance for the electronic properties of layered van der Waals topological materials.

cond-mat.mtrl-sci↗

arXiv2019

Optical spectroscopy on the photo-response in multiferroic BiFeO$_3$ at high pressure

The pressure dependence of light-induced effects in single-crystalline BiFeO$_3$ is studied by optical spectroscopy. At low pressures, we observe three light-induced absorption features with energies just below the two crystal field excitations and the absorption onset, respectively. These absorption features were previously ascribed to excitons, possibly connected with the ultra-fast photostriction effect in BiFeO$_3$. The pressure-induced redshift of the absorption features follows the pressure dependence of the corresponding crystal field excitations and absorption onset, suggesting the link between them. Above the structural phase transition at $P_{\mathrm{c1}}\approx{}3.5$ GPa the three absorption features disappear, suggesting their connection to the polar phase in BiFeO$_3$. The pressure-induced disappearance of the photo-induced features is irreversible upon pressure release.

cond-mat.mtrl-sci↗

arXiv2019

Infrared spectroscopy study of the nodal-line semimetal candidate ZrSiTe under pressure: Hints for pressure-induced phase transitions

We studied the effect of external pressure on the optical response of the nodal-line semimetal candidate ZrSiTe by reflectivity measurements. At pressures of a few GPa, the reflectivity, optical conductivity, and loss function are strongly affected in the whole measured frequency range (200-16500 cm$^{-1}$), indicating drastic changes in the electronic band structure. The pressure-induced shift of the electronic bands affects both the intraband and interband transitions. We find anomalies in the pressure dependence of several optical parameters at the pressures $P_{c1}$$\approx$4.1 GPa and $P_{c2}$$\approx$6.5 GPa, suggesting the occurrence of two phase transitions of either structural or electronic type.

cond-mat.mtrl-sci↗

arXiv2019

Optical signature of the pressure-induced dimerization in the honeycomb iridate $α$-Li$_2$IrO$_3$

We studied the effect of external pressure on the electrodynamic properties of $α$-Li$_2$IrO$_3$ single crystals in the frequency range of the phonon modes and the Ir $d$-$d$ transitions. The abrupt hardening of several phonon modes under pressure supports the onset of the dimerized phase at the critical pressure $P_c$=3.8 GPa. With increasing pressure an overall decrease in spectral weight of the Ir $d$-$d$ transitions is found up to $P_c$. Above $P_c$, the local (on-site) $d$-$d$ excitations gain spectral weight with increasing pressure, which hints at a pressure-induced increase in the octahedral distortions. The non-local (intersite) Ir $d$-$d$ transitions show a monotonic blue-shift and decrease in spectral weight. The changes observed for the non-local excitations are most prominent well above $P_c$, namely for pressures $\geq$12 GPa, and only small changes occur for pressures close to $P_c$. The profile of the optical conductivity at high pressures ($\sim$20 GPa) appears to be indicative for the dimerized state in iridates.

cond-mat.str-el↗

arXiv2019

Pressure-induced formation of rhodium zigzag chains in the honeycomb rhodate Li$_2$RhO$_3$

We use powder x-ray diffraction to study the effect of pressure on the crystal structure of the honeycomb rhodate Li$_2$RhO$_3$. We observe low-pressure ($P$$<$$P_{c1}$ = 6.5 GPa) and high-pressure ($P$$>$$P_{c2}$ = 14 GPa) regions corresponding to the monoclinic $C2/m$ symmetry, while a phase mixture is observed at intermediate pressures. At $P$$>$$P_{c2}$, the honeycomb structure becomes distorted and features short Rh--Rh bonds forming zigzag chains stretched along the crystallographic $a$ direction. This is in contrast to dimerized patterns observed in triclinic high-pressure polymorphs of $α$-Li$_2$IrO$_3$ and $α$-RuCl$_3$. Density-functional theory calculations at various pressure conditions reveal that the observed rhodium zigzag-chain pattern is not expected under hydrostatic pressure but can be reproduced by assuming anisotropic pressure conditions.

cond-mat.str-el↗

arXiv2019

Chemical pressure effect on the optical conductivity of the nodal-line semimetals ZrSi$Y$ ($Y$=S, Se, Te) and ZrGe$Y$ ($Y$=S, Te)

ZrSiS is a nodal-line semimetal, whose electronic band structure contains a diamond-shaped line of Dirac nodes. We carried out a comparative study on the optical conductivity of ZrSiS and related compounds ZrSiSe, ZrSiTe, ZrGeS, and ZrGeTe by reflectivity measurements over a broad frequency range combined with density functional theory calculations. The optical conductivity exhibits a distinct U shape, ending at a sharp peak at around 10000~cm$^{-1}$ for all studied compounds, except for ZrSiTe. The U shape of the optical conductivity is due to transitions between the linearly dispersing bands crossing each other along the nodal line. The sharp high-energy peak is related to transitions between almost parallel bands, and its energy position depends on the interlayer bonding correlated with the $c$/$a$ ratio, which can be tuned by either chemical or external pressure. For ZrSiTe, another pair of crossing bands appears in the vicinity of the Fermi level, corrugating the nodal-line electronic structure and leading to the observed difference in optical conductivity. The findings suggest that the Dirac physics in Zr$XY$ compounds with $X$=Si, Ge and $Y$=S, Se, Te is closely connected to the interlayer bonding.

cond-mat.mtrl-sci↗

arXiv2017

Competition between spin-orbit coupling, magnetism, and dimerization in the honeycomb iridates: $α$-Li$_{2}$IrO$_{3}$ under pressure

Single-crystal x-ray diffraction studies with synchrotron radiation on the honeycomb iridate $α$-Li$_{2}$IrO$_{3}$ reveal a pressure-induced structural phase transition with symmetry lowering from monoclinic to triclinic at a critical pressure of $P_{c}$ = 3.8 GPa. According to the evolution of the lattice parameters with pressure, the transition mainly affects the $ab$ plane and thereby the Ir hexagon network, leading to the formation of Ir--Ir dimers. These observations are independently predicted and corroborated by our \textit{ab initio} density functional theory calculations where we find that the appearance of Ir--Ir dimers at finite pressure is a consequence of a subtle interplay between magnetism, correlation, spin-orbit coupling, and covalent bonding. Our results further suggest that at $P_{c}$ the system undergoes a magnetic collapse. Finally we provide a general picture of competing interactions for the honeycomb lattices $A_{2}$$M$O$_{3}$ with $A$= Li, Na and $M$ = Ir, Ru.

cond-mat.str-el↗

arXiv2017

High-pressure versus isoelectronic doping effect on the honeycomb iridate Na$_2$IrO$_3$

We study the effect of isoelectronic doping and external pressure in tuning the ground state of the honeycomb iridate Na$_2$IrO$_3$ by combining optical spectroscopy with synchrotron x-ray diffraction measurements on single crystals. The obtained optical conductivity of Na$_2$IrO$_3$ is discussed in terms of a Mott insulating picture versus the formation of quasimolecular orbitals and in terms of Kitaev-interactions. With increasing Li content $x$, (Na$_{1-x}$Li$_x$)$_2$IrO$_3$ moves deeper into the Mott insulating regime and there are indications that up to a doping level of 24\% the compound comes closer to the Kitaev-limit. The optical conductivity spectrum of single crystalline $α$-Li$_2$IrO$_3$ does not follow the trends observed for the series up to $x=0.24$. There are strong indications that $α$-Li$_2$IrO$_3$ is less close to the Kitaev-limit compared to Na$_2$IrO$_3$ and closer to the quasimolecular orbital picture. Except for the pressure-induced hardening of the phonon modes, the optical properties of Na$_2$IrO$_3$ seem to be robust against external pressure. Possible explanations of the unexpected evolution of the optical conductivity with isolectronic doping and the drastic change between $x=0.24$ and $x=1$ are given by comparing the pressure-induced changes of lattice parameters and the optical conductivity with the corresponding changes induced by doping.

cond-mat.str-el↗

arXiv2012

Pressure dependence of the Verwey transition in magnetite: an infrared spectroscopic point of view

We investigated the electronic and vibrational properties of magnetite at temperatures from 300 K down to 10 K and for pressures up to 10 GPa by far-infrared reflectivity measurements. The Verwey transition is manifested by a drastic decrease of the overall reflectance and the splitting of the phonon modes as well as the activation of additional phonon modes. In the whole studied pressure range the down-shift of the overall reflectance spectrum saturates and the maximum number of phonon modes is reached at a critical temperature, which sets a lower bound for the Verwey transition temperature T$_{\mathrm{v}}$. Based on these optical results a pressure-temperature phase diagram for magnetite is proposed.

cond-mat.str-el↗

arXiv2012

Polaron physics and crossover transition in magnetite probed by pressure-dependent infrared spectroscopy

The optical properties of magnetite at room temperature were studied by infrared reflectivity measurements as a function of pressure up to 8 GPa. The optical conductivity spectrum consists of a Drude term, two sharp phonon modes, a far-infrared band at around 600 cm$^{-1}$, and a pronounced mid-infrared absorption band. With increasing pressure both absorption bands shift to lower frequencies and the phonon modes harden in a linear fashion. Based on the shape of the MIR band, the temperature dependence of the dc transport data, and the occurrence of the far-infrared band in the optical conductivity spectrum the polaronic coupling strength in magnetite at room temperature should be classified as intermediate. For the lower-energy phonon mode an abrupt increase of the linear pressure coefficient occurs at around 6 GPa, which could be attributed to minor alterations of the charge distribution among the different Fe sites.

cond-mat.str-el↗