arXiv subjects

Geoffroy Kremer

Publications and source records attributed to Geoffroy Kremer.

14 recordsLinked to original sources

arXiv2026

Ultrafast dynamics of excitons in black phosphorus

Excitons are key quasiparticles determining the optical properties of solids. As such, they can be utilized to coherently control the electronic structure of materials using optical femtosecond pulses. Identifying the decoherence mechanism during the early non-equilibrium dynamics is crucial to achieve light-induced band-structure engineering in semiconductors. Here, we generate excitons in the direct band gap semiconductor black phosphorus with a resonant mid-infrared photoexcitation. Using time- and angle-resolved photoemission spectroscopy, we track their complex ultrafast dynamics on the few-picosecond time scale. We develop a quantum-kinetic theoretical framework to model the decoherence of excitons into dark excitons via phonon scattering. By combining simulation and experiment, we quantify key parameters describing the early dynamics of the excitons. Our work highlights phonon-mediated intravalley scattering as a fundamental limitation for coherent exciton phenomena in single-valley semiconductors.

cond-mat.mtrl-sci↗

arXiv2025

Mexican hat-like valence band dispersion and quantum confinement in rhombohedral ferroelectric alpha-In2Se3

Two-dimensional (2D) ferroelectric (FE) materials offer a large variety of electronic properties depending on chemical composition, number of layers and stacking-order. Among them, alpha-In2Se3 has attracted much attention due to the promise of outstanding electronic properties, attractive quantum physics, in- and out-of-plane ferroelectricity and high photo-response. Precise experimental determination of the electronic structure of rhombohedral (3R) alpha-In2Se3 is needed for a better understanding of potential properties and device applications. Here, combining angle resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations, we demonstrate that 3R alpha-In2Se3 phase exhibits a robust inversion of the valence band parabolicity at the Gamma point forming a bow-shaped dispersion with a depth of 140 +- 10 meV between the valence band maximum (VBM) along the GammaK direction of the Brillouin zone (BZ). Moreover, we unveil an indirect band gap of about 1.25 eV, as well as a highly electron doping of approximatively 5.1012 electrons per cmsquare at the surface. This leads to surface band bending and the formation of a prominent electron accumulation layer. These findings allow a deeper understanding of the rhombohedral alpha-In2Se3 electronic properties underlying the potential of III/VI semiconductors for electronic and photonic technologies.

cond-mat.mtrl-sci↗

arXiv2025

Conduction Band Structure and Ultrafast Dynamics of Ferroelectric $α$-GeTe(111)

$α$-GeTe(111) is a non-centrosymmetric ferroelectric (FE) material for which a significative lattice distortion combined with a strong spin-orbit interaction gives rise to giant Rashba split states in the bulk and at the surface, which have been intensively probed in the occupied valence states using static angle-resolved photoemission spectroscopy (ARPES). Nevertheless, its unoccupied conduction band structure remains unexplored, in particular the experimental determination of its electronic band gap across momentum space. Using time-resolved ARPES based on high-repetition rate and extreme ultraviolet femtosecond (fs) laser, we unveil the band structure of $α$-GeTe(111) in the full Brillouin zone, both in the valence and conduction states, as well as the exploration of its out-of-equilibrium dynamics. Our work confirms the semiconducting nature of $α$-GeTe(111) with a 0.85 eV indirect band gap, which provides an upper limit for comparison to density functional theory calculations. We finally reveal the dominant scattering mechanisms of photoexcited carriers during the out-of-equilibrium dynamics under fs light pulses.

cond-mat.mtrl-sci↗

arXiv2025

Revealing electronic correlations in YNi$_2$B$_2$C using photoemission spectroscopy

We present a combined density functional theory (DFT), one-step model of photoemission, and soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) study of the electronic structure of the quaternary borocarbide superconductor YNi$_2$B$_2$C. Our analysis reveals the presence of moderate electronic correlations beyond the semilocal DFT within the generalized gradient approximation. We show that DFT and the full potential Korringa-Kohn-Rostoker method combined with the dynamical mean field theory (DFT+DMFT) with average Coulomb interaction U = 3.0 eV and the exchange energy J = 0.9 eV applied to the Ni d-states are necessary for reproducing the experimentally observed SX-ARPES spectra.

cond-mat.str-el↗

arXiv2024

Van der Waals epitaxy of Weyl-semimetal Td-WTe$_2$

Epitaxial growth of WTe$_2$ offers significant advantages, including the production of high-qualityfilms, possible long range in-plane ordering and precise control over layer thicknesses. However,the mean island size of WTe$_2$ grown by molecular beam epitaxy (MBE) in litterature is only a fewtens of nanometers, which is not suitable for an implementation of devices at large lateral scales.Here we report the growth of Td-WTe$_2$ ultrathin films by MBE on monolayer (ML) graphenereaching a mean flake size of $\cong$110nm, which is, on overage, more than three time larger thanprevious results. WTe$_2$ films thicker than 5nm have been successfully synthesized and exhibit theexpected Td-phase atomic structure. We rationalize epitaxial growth of Td-WTe$_2$ and propose asimple model to estimate the mean flake size as a function of growth parameters that can be appliedto other transition metal dichalcogenides (TMDCs). Based on nucleation theory and Kolmogorov-Johnson-Meh-Avrami (KJMA) equation, our analytical model supports experimental data showinga critical coverage of 0.13ML above which WTe$_2$ nucleation becomes negligible. The quality ofmonolayer WTe$_2$ films is demonstrated from electronic band structure analysis using angle-resolved photoemission spectroscopy (ARPES) in agreement with first-principle calculationsperformed on free-standing WTe$_2$ and previous reports.

cond-mat.mtrl-sci↗

arXiv2023

Quasi van der Waals Epitaxy of Rhombohedral-stacked Bilayer WSe2 on GaP(111) Heterostructure

The growth of bilayers of two-dimensional (2D) materials on conventional 3D semiconductors results in 2D/3D hybrid heterostructures, which can provide additional advantages over more established 3D semiconductors while retaining some specificities of 2D materials. Understanding and exploiting these phenomena hinge on knowing the electronic properties and the hybridization of these structures. Here, we demonstrate that rhombohedral-stacked bilayer (AB stacking) can be obtained by molecular beam epitaxy growth of tungsten diselenide (WSe2) on gallium phosphide (GaP) substrate. We confirm the presence of 3R-stacking of the WSe2 bilayer structure using scanning transmission electron microscopy (STEM) and micro-Raman spectroscopy. Also, we report high-resolution angle-resolved photoemission spectroscopy (ARPES) on our rhombohedral-stacked WSe2 bilayer grown on GaP(111)B substrate. Our ARPES measurements confirm the expected valence band structure of WSe2 with the band maximum located at the gamma point of the Brillouin zone. The epitaxial growth of WSe2 on GaP(111)B heterostructures paves the way for further studies of the fundamental properties of these complex materials, as well as prospects for their implementation in devices to exploit their promising electronic and optical properties.

cond-mat.mes-hall↗

arXiv2023

Persistence of structural distortion and bulk band Rashba splitting in SnTe above its ferroelectric critical temperature

The ferroelectric semiconductor $α$-SnTe has been regarded as a topological crystalline insulator and the dispersion of its surface states has been intensively measured with angle-resolved photoemission spectroscopy (ARPES) over the last decade. However, much less attention has been given to the impact of the ferroelectric transition on its electronic structure, and in particular on its bulk states. Here, we investigate the low-energy electronic structure of $α$-SnTe with ARPES and follow the evolution of the bulk-state Rashba splitting as a function of temperature, across its ferroelectric critical temperature of about $T_c\sim 110$ K. Unexpectedly, we observe a persistent band splitting up to room temperature, which is consistent with an order-disorder contribution to the phase transition that requires the presence of fluctuating local dipoles above $T_c$. We conclude that no topological surface state can occur at the (111) surface of SnTe, at odds with recent literature.

cond-mat.mtrl-sci↗

arXiv2023

Intrinsic defects and mid-gap states in quasi-one-dimensional Indium Telluride

Recently, intriguing physical properties have been unraveled in anisotropic semiconductors, in which the in-plane electronic band structure anisotropy often originates from the low crystallographic symmetry. The atomic chain is the ultimate limit in material downscaling for electronics, a frontier for establishing an entirely new field of one-dimensional quantum materials. Electronic and structural properties of chain-like InTe are essential for better understanding of device applications such as thermoelectrics. Here, we use scanning tunneling microscopy/spectroscopy (STM/STS) measurements and density functional theory (DFT) calculations to directly image the in-plane structural anisotropy in tetragonal Indium Telluride (InTe). As results, we report the direct observation of one-dimensional In1+ chains in InTe. We demonstrate that InTe exhibits a band gap of about 0.40 +-0.02 eV located at the M point of the Brillouin zone. Additionally, line defects are observed in our sample, were attributed to In1+ chain vacancy along the c-axis, a general feature in many other TlSe-like compounds. Our STS and DFT results prove that the presence of In1+ induces localized gap state, located near the valence band maximum (VBM). This acceptor state is responsible for the high intrinsic p-type doping of InTe that we also confirm using angle-resolved photoemission spectroscopy.

cond-mat.mtrl-sci↗

arXiv2023

Quantum Confinement and Electronic Structure at the Surface of van der Waals Ferroelectric α-In$_{2}$Se$_{3}$

Two-dimensional (2D) ferroelectric (FE) materials are promising compounds for next-generation nonvolatile memories, due to their low energy consumption and high endurance. Among them, α-In$_{2}$Se$_{3}$ has drawn particular attention due to its in- and out-of-plane ferroelectricity, whose robustness has been demonstrated down to the monolayer limit. This is a relatively uncommon behavior since most bulk FE materials lose their ferroelectric character at the 2D limit due to depolarization field. Using angle resolved photoemission spectroscopy (ARPES), we unveil another unusual 2D phenomena appearing in 2H α-In$_{2}$Se$_{3}$ single crystals, the occurrence of a highly metallic two-dimensional electron gas (2DEG) at the surface of vacuum-cleaved crystals. This 2DEG exhibits two confined states which correspond to an electron density of approximatively 10$^{13}$ electrons/cm$^{3}$, also confirmed by thermoelectric measurements. Combination of ARPES and density functional theory (DFT) calculations reveals a direct band gap of energy equal to 1.3 +/- 0.1 eV, with the bottom of the conduction band localized at the center of the Brillouin zone, just below the Fermi level. Such strong n-type doping further supports the quantum confinement of electrons and the formation of the 2DEG.

cond-mat.mtrl-sci↗

arXiv2023

Protection of Ising spin-orbit coupling in bulk misfit superconductors

Low-dimensional materials have remarkable properties that are distinct from their bulk counterparts. A paradigmatic example is Ising superconductivity that occurs in monolayer materials such as NbSe2 which show a strong violation of the Pauli limit. In monolayers, this occurs due to a combination of broken inversion symmetry and spin-orbit coupling that locks the spins of the electrons out-of-plane. Bulk NbSe2 is centrosymmetric and is therefore not an Ising superconductor. We show that bulk misfit compound superconductors, (LaSe)1.14(NbSe2) and (LaSe)1.14(NbSe2)2, comprised of monolayers and bilayers of NbSe2, exhibit unexpected Ising protection with a Pauli-limit violation comparable to monolayer NbSe2, despite formally having inversion symmetry. We study these misfit compounds using complementary experimental methods in combination with first-principles calculations. We propose theoretical mechanisms of how the Ising protection can survive in bulk materials. We show how some of these mechanisms operate in these bulk compounds due to a concerted effect of charge-transfer, defects, reduction of interlayer hopping, and stacking. This highlights how Ising superconductivity can, unexpectedly, arise in bulk materials, and possibly enable the design of bulk superconductors that are resilient to magnetic fields.

cond-mat.supr-con↗

arXiv2022

Field-induced ultrafast modulation of Rashba coupling at room temperature in ferroelectric $α$-GeTe(111)

Rashba materials have appeared as an ideal playground for spin-to-charge conversion in prototype spintronics devices. Among them, $α$-GeTe(111) is a non-centrosymmetric ferroelectric (FE) semiconductor for which a strong spin-orbit interaction gives rise to giant Rashba coupling. Its room temperature ferroelectricity was recently demonstrated as a route towards a new type of highly energy-efficient non-volatile memory device based on switchable polarization. Currently based on the application of an electric field, the writing and reading processes could be outperformed by the use of femtosecond (fs) light pulses requiring exploration of the possible control of ferroelectricity on this timescale. Here, we probe the room temperature transient dynamics of the electronic band structure of $α$-GeTe(111) using time and angle-resolved photoemission spectroscopy (tr-ARPES). Our experiments reveal an ultrafast modulation of the Rashba coupling mediated on the fs timescale by a surface photovoltage (SPV), namely an increase corresponding to a 13 % enhancement of the lattice distortion. This opens the route for the control of the FE polarization in $α$-GeTe(111) and FE semiconducting materials in quantum heterostructures.

cond-mat.mtrl-sci↗

arXiv2022

Modified interlayer stacking and insulator to correlated-metal transition driven by uniaxial strain in 1$T$-TaS$_{2}$

Interlayer coupling is strongly implicated in the complex electronic properties of 1$T$-TaS$_2$ , but the interplay between this and electronic correlations remains unresolved. Here, we employ angle-resolved photoemission spectroscopy (ARPES) to reveal the effect of uniaxial strain engineering on the electronic structure and interlayer coupling in 1$T$-TaS$_2$ . The normally insulating ground state is transformed into a correlated-metal phase under strain, as evidenced by the emergence of a narrow band at the Fermi level. Temperature dependent ARPES measurements reveal that the metallic behaviour only develops below the commensurate charge density wave (CCDW) transition, where interlayer dimerization produces a band-insulator in unstrained samples. Electronic structure calculations demonstrate that the correlated metallic behaviour is stabilized by a previously predicted but unobserved bulk stacking structure with a modified interlayer coupling of the Ta d$_{z^{2}}$ electrons. Our combined approach lays bare the role of correlations and interlayer coupling in 1$T$-TaS$_2$ , providing critical input for understanding superconductivity under pressure and the metastable hidden phase induced using non-equilibrium protocols in this platform material for correlated physics.

cond-mat.str-el↗

arXiv2022

Electron-momentum dependence of electron-phonon coupling underlies dramatic phonon renormalization in YNi$_2$B$_2$C

Electron-phonon coupling, i.e., the scattering of lattice vibrations by electrons and vice versa, is ubiquitous in solids and can lead to emergent ground states such as superconductivity and charge-density wave order. Strong coupling of phonons to electrons near the Fermi surface, which reduces the phonon lifetimes and broadens the phonon peaks in scattering experiments, is often associated with Fermi surface nesting. Here, we show that strong phonon broadening can occur in the absence of both Fermi surface nesting and lattice anharmonicity, if electron-phonon coupling is strongly enhanced for specific values of electron-momentum, k. We use inelastic neutron scattering, soft x-ray angle-resolved photoemission spectroscopy measurements and ab-initio lattice dynamical and electronic band structure calculations to demonstrate this scenario in the highly anisotropic tetragonal electron-phonon superconductor YNi$_2$B$_2$C. This new scenario likely applies to a wide range of compounds.

cond-mat.supr-con↗

arXiv2020

Misfit layer compounds: a platform for heavily-doped two-dimensional transition metal dichalcogenides

Transition metal dichalcogenides (TMDs) display a rich variety of instabilities such as spin and charge orders, Ising superconductivity and topological properties. Their physical properties can be controlled by doping in electric double-layer field-effect transistors (FET). However, for the case of single layer NbSe$_2$, FET doping is limited to $\approx 1\times 10^{14}$ cm$^{-2}$, while a somewhat larger charge injection can be obtained via deposition of K atoms. Here, by performing ARPES, STM, quasiparticle interference measurements, and first principles calculations we show that a misfit compound formed by sandwiching NbSe$_2$ and LaSe layers behaves as a NbSe$_2$ single layer with a rigid doping of $0.55-0.6$ electrons per Nb atom or $\approx 6\times 10^{14}$ cm$^{-2}$. Due to this huge doping, the $3\times3$ charge density wave is replaced by a $2\times2$ order with very short coherence length. As a tremendous number of different misfit compounds can be obtained by sandwiching TMDs layers with rock salt or other layers, our work paves the way to the exploration of heavily doped 2D TMDs over an unprecedented wide range of doping.

cond-mat.mtrl-sci↗