arXiv subjects

J. Teyssier

Publications and source records attributed to J. Teyssier.

At least 19 recordsLinked to original sources

arXiv2025

Non-Fermi liquid quasiparticles in strain-tuned Sr2RuO4

Interacting electrons can form metallic states beyond the Fermi liquid paradigm, a conceptual frontier of many-body physics mainly explored via bulk thermodynamics and transport. In contrast, the microscopics of anomalous single-particle excitations underlying non-Fermi liquid properties have largely remained in the dark. Here we spectroscopically map such quantum-critical excitations in Sr$_2$RuO$_4$ under uniaxial pressure, an experimental challenge overcome by technical advances combining focused ion beam micro-milling with laser angle resolved photoemission. We show that quasiparticle excitations acquire a non-Fermi liquid scattering rate near the critical point but remain remarkably robust throughout the transition. These experiments serve as a benchmark for the theory of anomalous metals and settle the long-standing question if quantum-critical systems host quasiparticle excitations.

cond-mat.str-el↗

arXiv2021

Raman spectroscopic evidence for multiferroicity in rare earth nickelate single crystals

The rare earth nickelates RNiO3 are metallic at high temperatures and insulating and magnetically ordered at low temperatures. The low temperature phase has been predicted to be type II multiferroic, i.e. ferroelectric and magnetic order are coupled and occur simultaneously. Confirmation of those ideas has been inhibited by the absence of experimental data on single crystals. Here we report on Raman spectroscopic data of RNiO3 single crystals (R = Y, Er, Ho, Dy, Sm, Nd) for temperatures between 10 K and 1000 K. Entering the magnetically ordered phase we observe the appearance of a large number of additional vibrational modes, implying a breaking of inversion symmetry expected for multiferroic order.

cond-mat.str-el↗

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↗

arXiv2020

Optical properties of LaNiO3 films tuned from compressive to tensile strain

Materials with strong electronic correlations host remarkable -- and technologically relevant -- phenomena such as magnetism, superconductivity and metal-insulator transitions. Harnessing and controlling these effects is a major challenge, on which key advances are being made through lattice and strain engineering in thin films and heterostructures, leveraging the complex interplay between electronic and structural degrees of freedom. Here we show that the electronic structure of LaNiO3 can be tuned by means of lattice engineering. We use different substrates to induce compressive and tensile biaxial epitaxial strain in LaNiO3 thin films. Our measurements reveal systematic changes of the optical spectrum as a function of strain and, notably, an increase of the low-frequency free carrier weight as tensile strain is applied. Using density functional theory (DFT) calculations, we show that this apparently counter-intuitive effect is due to a change of orientation of the oxygen octahedra.The calculations also reveal drastic changes of the electronic structure under strain, associated with a Fermi surface Lifshitz transition. We provide an online applet to explore these effects. The experimental value of integrated spectral weight below 2 eV is significantly (up to a factor of 3) smaller than the DFT results, indicating a transfer of spectral weight from the infrared to energies above 2 eV. The suppression of the free carrier weight and the transfer of spectral weight to high energies together indicate a correlation-induced band narrowing and free carrier mass enhancement due to electronic correlations. Our findings provide a promising avenue for the tuning and control of quantum materials employing lattice engineering.

cond-mat.str-el↗

arXiv2020

Unconventional free charge in the correlated semimetal Nd2Ir2O7

Nd2Ir2O7 is a correlated semimetal with the pyrochlore structure, in which competing spin-orbit coupling and electron-electron interactions are believed to induce a time-reversal symmetry broken Weyl semimetal phase characterized by pairs of topologically protected Dirac points at the Fermi energy. However, the emergent properties in these materials are far from clear, and exotic new states of matter have been conjectured. Here we demonstrate optically that at low temperatures the free carrier spectral weight is proportional to T^2 where T is the temperature, as expected for massless Dirac electrons. However, we do {\em not} observe the corresponding T^3 term in the specific heat. That the system is not in a Fermi liquid state is further corroborated by the "Planckian" T-linear temperature dependence of the momentum relaxation rate and the progressive opening of a correlation-induced gap at low temperatures. These observations can not be reconciled within the framework of band theory of electron-like quasiparticles and point toward the effective decoupling of the charge transport from the single particle sector.

cond-mat.str-el↗

arXiv2020

Manifestion of structural Higgs and Goldstone modes in the hexagonal manganites

Structural phase transitions described by Mexican hat potentials should in principle exhibit aspects of Higgs and Goldstone physics. Here, we investigate the relationship between the phonons that soften at such structural phase transitions and the Higgs- and Goldstone-boson analogues associated with the crystallographic Mexican hat potential. We show that, with the exception of systems containing only one atom type, the usual Higgs and Goldstone modes are represented by a combination of several phonon modes, with the lowest energy phonons of the relevant symmetry having substantial contribution. Taking the hexagonal manganites as a model system, we identify these modes using Landau theory, and predict the temperature dependence of their frequencies using parameters obtained from density functional theory. Separately, we calculate the additional temperature dependence of all phonon mode frequencies arising from thermal expansion within the quasi-harmonic approximation. We predict that Higgs-mode softening will dominate the low-frequency vibrational spectrum of InMnO$_3$ between zero kelvin and room-temperature, whereas the behavior of ErMnO$_3$ will be dominated by lattice expansion effects. We present temperature-dependent Raman scattering data that support our predictions, in particular confirming the existence of the Higgs mode in InMnO$_3$.

cond-mat.mtrl-sci↗

arXiv2018

Cooling a polaronic liquid: Phase mixture and pseudogap-like spectra in superconducting $Ba_{1-x}K_{x}BiO_{3}$

Many complex electronic systems exhibit so-called pseudogaps, which are poorly-understood suppression of low-energy spectral intensity in the absence of an obvious gap-inducing symmetry. Here we investigate the superconductor $Ba_{1-x}K_{x}BiO_{3}$ near optimal doping, where unconventional transport behavior and evidence of pseudogap(s) have been observed above the superconducting transition temperature $T_{c}$, and near an insulating phase with long-range lattice distortions. Angle-resolved photoemission spectroscopy (ARPES) reveals a dispersive band with vanishing quasiparticle weight and "tails" of deep-energy intensity that strongly decay approaching the Fermi level. Upon cooling below a transition temperature $T_{p} > T_{c}$, which correlates with a change in the slope of the resistivity vs. temperature, a partial transfer of spectral weight near $E_{F}$ into the deep-binding energy tails is found to result from metal-insulator phase separation. Combined with simulations and Raman scattering, our results signal that insulating islands of ordered bipolarons precipitate out of a disordered polaronic liquid and provide evidence that this process is regulated by a crossover in the electronic mean free path.

cond-mat.supr-con↗

arXiv2018

Mott transition and collective charge pinning in electron doped Sr2IrO4

We studied the in-plane dynamic and static charge conductivity of electron doped Sr2IrO4 using optical spectroscopy and DC transport measurements. The optical conductivity indicates that the pristine material is an indirect semiconductor with a direct Mott-gap of 0.55 eV. Upon substitution of 2% La per formula unit the Mott-gap is suppressed except in a small fraction of the material (15%) where the gap survives, and overall the material remains insulating. Instead of a zero energy mode (or Drude peak) we observe a soft collective mode (SCM) with a broad maximum at 40 meV. Doping to 10% increases the strength of the SCM, and a zero-energy mode occurs together with metallic DC conductivity. Further increase of the La substitution doesn't change the spectral weight integral up to 3 eV. It does however result in a transfer of the SCM spectral weight to the zero-energy mode, with a corresponding reduction of the DC resistivity for all temperatures from 4 to 300 K. The presence of a zero-energy mode signals that at least part of the Fermi surface remains ungapped at low temperatures, whereas the SCM appears to be caused by pinning a collective frozen state involving part of the doped electrons.

cond-mat.str-el↗

arXiv2017

Impact of antiferromagnetism on the optical properties of rare earth nickelates

We study the temperature dependence of the optical conductivity of rare-earth nickelate films of varying composition and strain close to the antiferromagnetic ordering temperature, TN. Two prominent peaks at 0.6 and 1.3 eV, which are characteristic of the insulating phase, display a small but significant increase in intensity when the material passes from para- to antiferromagnetic. This observation indicates the presence of a positive feedback between antiferromagnetic (AF) and bond disproportionation (BD) order. By analyzing the temperature dependence near TN, and using a Landau-type free-energy expression for BD and AF order, we infer that BD order is a necessary condition for the AF phase to appear, and that the antiferromagnetism contributes to stabilization of the bond disproportionation. This model also explains why hysteresis is particularly strong when the transition into the insulating state occurs simultaneously with antiferromagnetic order.

cond-mat.str-el↗

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 $β$ between two Rashba-split conduction bands, and the absorption $γ$ across the band gap. In the low pressure range, $p< 4$~GPa, for both compounds $β$ is approximately constant with pressure and $γ$ 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

Jahn-Teller induced nematic orbital order in tetragonal Sr$_2$VO$_4$

Using high resolution X-Ray diffraction (XRD) on high purity powders, we resolved the structure and $ab$ symmetry of the intriguing compound \svo$ $ from room temperature down to 20 K to an unprecedented level of accuracy. Upon cooling, this new set of data unambiguously reveals a second order phase transition lowering the symmetry from tetragonal to orthorhombic at a temperature $T_{c2}=136$ K. The observation of an orthorhombic distortion of the $ab$-plane is attributed to nematic phase formation supported by local Jahn-Teller (JT) dynamical instability. At $T_{N}=105$ K, spins order and at $T_{c1}=100$ K the tetragonal structure is recovered with an elongated c-axis.

cond-mat.str-el↗

arXiv2015

Optical spectroscopy and the nature of the insulating state of rare-earth nickelates

Using a combination of spectroscopic ellipsometry and DC transport measurements, we determine the temperature dependence of the optical conductivity of NdNiO$_3$ and SmNiO$_{3}$ films. The optical spectra show the appearance of a characteristic two-peak structure in the near-infrared when the material passes from the metal to the insulator phase. Dynamical mean-field theory calculations confirm this two-peak structure, and allow to identify these spectral changes and the associated changes in the electronic structure. We demonstrate that the insulating phase in these compounds and the associated characteristic two-peak structure are due to the combined effect of bond-disproportionation and Mott physics associated with half of the disproportionated sites. We also provide insights into the structure of excited states above the gap.

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↗

arXiv2013

Infrared and Raman spectroscopy measurements of a transition in the crystal structure and a closing of the energy gap of BiTeI under pressure

BiTeI is a giant Rashba spin splitting system, in which a non-centro symmetric topological phase has recently been suggested to appear under high pressure. We investigated the optical properties of this compound, reflectivity and transmission, under pressures up to $15$ GPa. The gap feature in the optical conductivity vanishes above $p \sim 9$ GPa and does not reappear up to at least $15$ GPa. The plasma edge, associated with intrinsically doped charge carriers, is smeared out through a phase transition at $9$ GPa. Using high pressure Raman spectroscopy, we follow the vibrational modes of BiTeI, providing additional clear evidence that the transition at 9 GPa involves a change of crystal structure. This change of crystal structure possibly inhibits the high-pressure topological phase from occurring.

cond-mat.mtrl-sci↗

arXiv2012

Optical properties of Bi2Te2Se at ambient and high pressure

The temperature dependence of the complex optical properties of the three-dimensional topological insulator Bi2Te2Se is reported for light polarized in the a-b planes at ambient pressure, as well as the effects of pressure at room temperature. This material displays a semiconducting character with a bulk optical gap of 300 meV at 295 K. In addition to the two expected infrared-active vibrations observed in the planes, there is additional fine structure that is attributed to either the removal of degeneracy or the activation of Raman modes due to disorder. A strong impurity band located at 200 cm^{-1} is also observed. At and just above the optical gap, several interband absorptions are found to show a strong temperature and pressure dependence. As the temperature is lowered these features increase in strength and harden. The application of pressure leads to a very abrupt closing of the gap above 8 GPa, and strongly modifies the interband absorptions in the mid-infrared spectral range. While ab initio calculations fail to predict the collapse of the gap, they do successfully describe the size of the band gap at ambient pressure, and the magnitude and shape of the optical conductivity.

cond-mat.mtrl-sci↗

arXiv2012

Electron spin resonance and exchange paths in the orthorhombic dimer system Sr2VO4

We report on magnetization and electron spin resonance (ESR) measurements of Sr$_{2}$VO$_4$ with orthorhombic symmetry. In this dimer system the $V^{4+}$ ions are in tetrahedral environment and are coupled by an antiferromagnetic intra-dimer exchange constant $J/k_B \approx$ 100 K to form a singlet ground state without any phase transitions between room temperature and 2 K. Based on an extended-Hückel-Tight-Binding analysis we identify the strongest exchange interaction to occur between two inequivalent vanadium sites via two intermediate oxygen ions. The ESR absorption spectra can be well described by a single Lorentzian line with an effective g-factor $g$ = 1.89. The temperature dependence of the ESR intensity is well described by a dimer model in agreement with the magnetization data. The temperature dependence of the ESR linewidth can be modeled by a superposition of a linear increase with temperature with a slope $α$ = 1.35 Oe/K and a thermally activated behavior with an activation energy $Δ/k_B$ = 1418 K, both of which point to spin-phonon coupling as the dominant relaxation mechanism in this compound.

cond-mat.str-el↗

arXiv2011

Alternating spin-orbital order in tetragonal Sr2VO4

Considering spin-orbit coupling, the tetragonal crystal-field, and all relevant superexchange processes including quantum interference, we derive expressions for the energy levels of the vanadium ions in tetragonal Sr2VO4. The used parameters of the model Hamiltonian allow to describe well the excitation spectra observed in neutron scattering and optical experiments at low temperatures. The free energy exhibits a minimum which corresponds to a novel alternating spin-orbital order with strong thermal fluctuation of the orbital mixing parameter.

cond-mat.str-el↗

arXiv2011

Optical study of phonons and electronic excitations in tetragonal Sr$_2$VO$_4$

We report on the optical excitation spectra in Sr$_2$VO$_4$. The phonon modes are assigned and their evolution with temperature is discussed in the frame of the different phase transitions crossed upon cooling. Besides the expected infrared-active phonons we observe two additional excitations at about 290 cm$^{-1}$ and 840 cm$^{-1}$ which could correspond to electronic transitions of the V$^{4+}$ ions. Our experimental results are discussed in the context of recent experimental and theoretical studies of this material with a unique spin-orbital ground state.

cond-mat.str-el↗