arXiv subjects

W. Desrat

Publications and source records attributed to W. Desrat.

At least 19 recordsLinked to original sources

arXiv2026

Spontaneous emission of light by non-equilibrium phonons

When a system is brought out of equilibrium by an external excitation, its relaxation to thermodynamic equilibrium generates phonons. These non-equilibrium phonons degrade via cascaded anharmonic decay processes, progressively leading to a thermal population of phonons following a Bose-Einstein distribution at the system temperature. Preceding heat dissipation by convection, conduction and incandescence, this early phase of the relaxation dynamics is commonly assumed to be exclusively non-radiative. Here, we demonstrate that the radiative emission by phonons can be an efficient relaxation pathway competing with the intrinsic anharmonic decay. Optical spectroscopy under femtosecond two-photon excitation in boron nitride unveils a photoluminescence signal in the mid-infrared spectral range, stemming from the spontaneous emission of light by non-equilibrium phonons. This observation of non-thermal radiation from phonons introduces a new paradigm for out-of-equilibrium physics, mid-infrared optics, and thermal management.

cond-mat.other↗

arXiv2025

Growth of rhombohedral boron nitride crystals using an iron flux

We report the growth of high quality rhombohedral boron nitride (rBN) crystals by the iron flux method at atmospheric pressure. In contrast to the lamellar structure of standard hexagonal boron nitride (hBN) covering the metal ingot, the current synthetized BN shell is composed of many triangular bulk crystallites, of submillimeter size, predominantly in the rhombohedral phase, with only weak traces of hBN detected by high resolution X-ray diffraction. The low-temperature photoluminescence emission confirms the excellent quality of the rhombohedral stacking with unprecedented width for all phonon replicas at the band edge. The Raman spectra are characterized by the absence of any low-energy modes at $\sim50$ cm$^{-1}$, the presence of an extended band around $800$ cm$^{-1}$ and an asymmetric high energy mode at $1366$ cm$^{-1}$. These observations are in very good agreement with the phonon dispersion calculated for the rhombohedral primitive cell with $C_{3v}$ symmetry.

cond-mat.mtrl-sci↗

arXiv2024

Isotope substitution and polytype control for point defects identification: the case of the ultraviolet color center in hexagonal boron nitride

Defects in crystals can have a transformative effect on the properties and functionalities of solid-state systems. Dopants in semiconductors are core components in electronic and optoelectronic devices. The control of single color centers is at the basis of advanced applications for quantum technologies. Unintentional defects can also be detrimental to the crystalline structure and hinder the development of novel materials. Whatever the research perspective, the identification of defects is a key but complicated, and often long-standing issue. Here, we present a general methodology to identify point defects by combining isotope substitution and polytype control, with a systematic comparison between experiments and first-principles calculations. We apply this methodology to hexagonal boron nitride (hBN) and its ubiquitous color center emitting in the ultraviolet spectral range. From isotopic purification of the host hBN matrix, a local vibrational mode of the defect is uncovered, and isotope-selective carbon doping proves that this mode belongs to a carbon-based center. Then, by varying the stacking sequence of the host hBN matrix, we unveil different optical responses to hydrostatic pressure for the non-equivalent configurations of this ultraviolet color center. We conclude that this defect is a carbon dimer in the honeycomb lattice of hBN. Our results show that tuning the stacking sequence in different polytypes of a given crystal provides unique fingerprints contributing to the identification of defects in 2D materials.

cond-mat.mtrl-sci↗

arXiv2023

Boron and nitrogen isotope effects on hexagonal boron nitride properties

The unique physical, mechanical, chemical, optical, and electronic properties of hexagonal boron nitride (hBN) make it a promising two-dimensional material for electronic, optoelectronic, nanophotonic, and quantum devices. Here we report on the changes in hBN's properties induced by isotopic purification in both boron and nitrogen. Previous studies on isotopically pure hBN have focused on purifying the boron isotope concentration in hBN from its natural concentration (approximately 20 at$\%$ $^{10}$B, 80 at$\%$ $^{11}$B) while using naturally abundant nitrogen (99.6 at$\%$ $^{14}$N, 0.4 at$\%$ $^{15}$N), i.e. almost pure $^{14}$N. In this study, we extend the class of isotopically-purified hBN crystals to $^{15}$N. Crystals in the four configurations, namely h$^{10}$B$^{14}$N, h$^{11}$B$^{14}$N, h$^{10}$B$^{15}$N, and h$^{11}$B$^{15}$N, were grown by the metal flux method using boron and nitrogen single isotope ($>99\%$) enriched sources, with nickel plus chromium as the solvent. In-depth Raman and photoluminescence spectroscopies demonstrate the high quality of the monoisotopic hBN crystals with vibrational and optical properties of the $^{15}$N-purified crystals at the state of the art of currently available $^{14}$N-purified hBN. The growth of high-quality h$^{10}$B$^{14}$N, h$^{11}$B$^{14}$N, h$^{10}$B$^{15}$N, and h$^{11}$B$^{15}$N opens exciting perspectives for thermal conductivity control in heat management, as well as for advanced functionalities in quantum technologies.

cond-mat.mtrl-sci↗

arXiv2019

InSe as a case between 3D and 2D layered crystals for excitons

We demonstrate the successive appearance of the exciton, biexciton, and P band of the exciton-exciton scattering with increasing excitation power in the photoluminescence of indium selenide layered crystals. The strict energy and momentum conservation rules of the P band are used to reexamine the exciton binding energy. The new value $\geq 20$ meV is markedly higher than the currently accepted 14 meV, being however well consistent with the robustness of excitons up to room temperature. A peak controlled by the Sommerfeld factor is found near the bandgap ($\sim 1.36$ eV), which puts the question on the pure three-dimensional character of the exciton in InSe, which has been assumed up to now. Our findings are of paramount importance for the successful application of InSe in nanophotonics.

cond-mat.mtrl-sci↗

arXiv2019

Massless Dirac fermions in III-V semiconductor quantum wells

We report on the clear evidence of massless Dirac fermions in two-dimensional system based on III-V semiconductors. Using a gated Hall bar made on a three-layer InAs/GaSb/InAs quantum well, we restore the Landau levels fan chart by magnetotransport and unequivocally demonstrate a gapless state in our sample. Measurements of cyclotron resonance at different electron concentrations directly indicate a linear band crossing at the $Γ$ point of Brillouin zone. Analysis of experimental data within analytical Dirac-like Hamiltonian allows us not only determing velocity $v_F=1.8\cdot10^5$ m/s of massless Dirac fermions but also demonstrating significant non-linear dispersion at high energies.

cond-mat.mes-hall↗

arXiv2018

Unconventional Reentrant Quantum Hall Effect in a HgTe/CdHgTe Double Quantum Well

We report on observation of an unconventional structure of the quantum Hall effect (QHE) in a $ p$-type HgTe/Cd$_x$Hg$_{1-x}$Te double quantum well (DQW) consisting of two HgTe layers of critical width. The observed QHE is a reentrant function of magnetic field between two $i=2$ states (plateaus at $ρ_{xy}=h/ie^2$) separated by an intermediate $i=1$ state, which looks like some anomalous peak on the extra-long $i=2$ plateau when weakly expressed. The anomalous peak apparently separates two different regimes: a traditional QHE at relatively weak fields for a small density of mobile holes $p_s$ and a high-field QH structure with a $2-1$ plateau--plateau transition corresponding to much larger $p_s$. We show that only a part of holes, residing in an additional light hole subband in the DQW, participate in QHE at weak fields while the rest of holes is excluded into the reservoir formed in the lateral maximum of the valence subband. All the holes come into play at high fields due to a peculiar behavior of the zero-mode levels.

cond-mat.mes-hall↗

arXiv2018

Temperature-induced topological phase transition in HgTe quantum wells

We report a direct observation of temperature-induced topological phase transition between trivial and topological insulator in HgTe quantum well. By using a gated Hall bar device, we measure and represent Landau levels in fan charts at different temperatures and we follow the temperature evolution of a peculiar pair of "zero-mode" Landau levels, which split from the edge of electron-like and hole-like subbands. Their crossing at critical magnetic field $B_c$ is a characteristic of inverted band structure in the quantum well. By measuring the temperature dependence of $B_c$, we directly extract the critical temperature $T_c$, at which the bulk band-gap vanishes and the topological phase transition occurs. Above this critical temperature, the opening of a trivial gap is clearly observed.

cond-mat.mtrl-sci↗

arXiv2017

Magnetic-field driven ambipolar quantum Hall effect in epitaxial graphene close to the charge neutrality point

We have investigated the disorder of epitaxial graphene close to the charge neutrality point (CNP) by various methods: i) at room temperature, by analyzing the dependence of the resistivity on the Hall coefficient ; ii) by fitting the temperature dependence of the Hall coefficient down to liquid helium temperature; iii) by fitting the magnetoresistances at low temperature. All methods converge to give a disorder amplitude of $(20 \pm 10)$ meV. Because of this relatively low disorder, close to the CNP, at low temperature, the sample resistivity does not exhibit the standard value $\simeq h/4e^2$ but diverges. Moreover, the magnetoresistance curves have a unique ambipolar behavior, which has been systematically observed for all studied samples. This is a signature of both asymmetry in the density of states and in-plane charge transfer. The microscopic origin of this behavior cannot be unambiguously determined. However, we propose a model in which the SiC substrate steps qualitatively explain the ambipolar behavior.

cond-mat.mes-hall↗

arXiv2017

Temperature-driven single-valley Dirac fermions in HgTe quantum wells

We report on temperature-dependent magnetospectroscopy of two HgTe/CdHgTe quantum wells below and above the critical well thickness $d_c$. Our results, obtained in magnetic fields up to 16 T and temperature range from 2 K to 150 K, clearly indicate a change of the band-gap energy with temperature. The quantum well wider than $d_c$ evidences a temperature-driven transition from topological insulator to semiconductor phases. At the critical temperature of 90 K, the merging of inter- and intra-band transitions in weak magnetic fields clearly specifies the formation of gapless state, revealing the appearance of single-valley massless Dirac fermions with velocity of $5.6\times10^5$ m$\times$s$^{-1}$. For both quantum wells, the energies extracted from experimental data are in good agreement with calculations on the basis of the 8-band Kane Hamiltonian with temperature-dependent parameters.

cond-mat.mtrl-sci↗

arXiv2016

Puddle-induced resistance oscillations in the breakdown of the graphene quantum Hall effect

We report on the stability of the quantum Hall plateau in wide Hall bars made from a chemically gated graphene film grown on SiC. The $ν=2$ quantized plateau appears from fields $B \simeq 5$ T and persists up to $B \simeq 80$ T. At high current density, in the breakdown regime, the longitudinal resistance oscillates with a $1/B$ periodicity and an anomalous phase, which we relate to the presence of additional electron reservoirs. The high field experimental data suggest that these reservoirs induce a continuous increase of the carrier density up to the highest available magnetic field, thus enlarging the quantum plateaus. These in-plane inhomogeneities, in the form of high carrier density graphene pockets, modulate the quantum Hall effect breakdown and decrease the breakdown current.

cond-mat.mes-hall↗

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↗

arXiv2016

The hole Fermi surface in Bi$_{2}$Se$_{3}$ probed by quantum oscillations

Transport and torque magnetometry measurements are performed at high magnetic fields and low temperatures in a series of p-type (Ca-doped) Bi$_{2}$Se$_{3}$ crystals. The angular dependence of the Shubnikov-de Haas and de Haas-van Alphen quantum oscillations enables us to determine the Fermi surface of the bulk valence band states as a function of the carrier density. At low density, the angular dependence exhibits a downturn in the oscillations frequency between $0^\circ$ and $90^\circ$, reflecting a bag-shaped hole Fermi surface. The detection of a single frequency for all tilt angles rules out the existence of a Fermi surface with different extremal cross-sections down to $24$~meV. There is therefore no signature of a camel-back in the valence band of our bulk samples, in accordance with the direct band gap predicted by $GW$ calculations.

cond-mat.mes-hall↗

arXiv2016

Temperature-driven massless Kane fermions in HgCdTe crystals: verification of universal velocity and rest-mass description

It has recently been shown that the electronic states in bulk gapless HgCdTe offer another realization of pseudo-relativistic three-dimensional particles in a condensed matter system. These single valley relativistic states, referred to as massless Kane fermions, cannot be described by any other well-known relativistic massless particles. Furthermore, the HgCdTe band structure can be continuously tailored by modifying either the cadmium content or temperature. At the critical concentration or temperature, the bandgap, Eg, collapses as the system undergoes a semimetal-to-semiconductor topological phase transition between the inverted and normal alignments. Here, using far-infrared magneto-spectroscopy we explore the continuous evolution of band structure of bulk HgCdTe as temperature is tuned across the topological phase transition. We demonstrate that the rest-mass of the Dirac-like Kane fermions, m changes sign at the critical temperature, while their velocity, c remains constant. The relation Eg = 2mc2 with the universal value of c = (1.07 +- 0.05)10x6 m/s remains valid in a broad range of temperatures and Cd concentrations, indicating a striking universality of the pseudo-relativistic description of the Dirac-like Kane fermions in HgCdTe.

cond-mat.mtrl-sci↗

arXiv2015

Disorder-induced stabilization of the quantum Hall ferromagnet

We report on an absolute measurement of the electronic spin polarization of the $ν=1$ integer quantum Hall state. The spin polarization is extracted in the vicinity of $ν=1$ (including at exactly $ν=1$) via resistive NMR experiments performed at different magnetic fields (electron densities), and Zeeman energy configurations. At the lowest magnetic fields, the polarization is found to be complete in a narrow region around $ν=1$. Increasing the magnetic field (electron density) induces a significant depolarization of the system, which we attribute to a transition between the quantum Hall ferromagnet and the Skyrmion glass phase theoretically expected as the ratio between Coulomb interactions and disorder is increased. These observations account for the fragility of the polarization previously observed in high mobility 2D electron gas, and experimentally demonstrate the existence of an optimal amount of disorder to stabilize the ferromagnetic state.

cond-mat.mes-hall↗

arXiv2014

Magnetoresistance of disordered graphene: from low to high temperatures

We present the magnetoresistance (MR) of highly doped monolayer graphene layers grown by chemical vapor deposition on 6H-SiC. The magnetotransport studies are performed on a large temperature range, from $T$ = 1.7 K up to room temperature. The MR exhibits a maximum in the temperature range $120-240$ K. The maximum is observed at intermediate magnetic fields ($B=2-6$ T), in between the weak localization and the Shubnikov-de Haas regimes. It results from the competition of two mechanisms. First, the low field magnetoresistance increases continuously with $T$ and has a purely classical origin. This positive MR is induced by thermal averaging and finds its physical origin in the energy dependence of the mobility around the Fermi energy. Second, the high field negative MR originates from the electron-electron interaction (EEI). The transition from the diffusive to the ballistic regime is observed. The amplitude of the EEI correction points towards the coexistence of both long and short range disorder in these samples.

cond-mat.mes-hall↗

arXiv2013

Dispersive line shape in the vicinity of the ν = 1 quantum Hall state: Coexistence of Knight shifted and unshifted resistively detected NMR responses

The frequency splitting between the dip and the peak of the resistively detected nuclear magnetic resonance (RDNMR) dispersive line shape (DLS) has been measured in the quantum Hall effect regime as a function of filling factor, carrier density and nuclear isotope. The splitting increases as the filling factor tends to ν = 1 and is proportional to the hyperfine coupling, similar to the usual Knight shift versus ν-dependence. The peak frequency shifts linearly with magnetic field throughout the studied filling factor range and matches the unshifted substrate signal, detected by classical NMR. Thus, the evolution of the splitting is entirely due to the changing Knight shift of the dip feature. The nuclear spin relaxation time, T1, is extremely long (hours) at precisely the peak frequency. These results are consistent with the local formation of a ν = 2 phase due to the existence of spin singlet D$^-$ complexes.

cond-mat.mes-hall↗

arXiv2011

Interplay between interferences and electron-electron interactions in epitaxial graphene

We separate localization and interaction effects in epitaxial graphene devices grown on the C-face of a 4H-SiC substrate by analyzing the low temperature conductivities. Weak localization and antilocalization are extracted at low magnetic fields, after elimination of a geometric magnetoresistance and subtraction of the magnetic field dependent Drude conductivity. The electron electron interaction correction is extracted at higher magnetic fields, where localization effects disappear. Both phenomena are weak but sizable and of the same order of magnitude. If compared to graphene on silicon dioxide, electron electron interaction on epitaxial graphene are not significantly reduced by the larger dielectric constant of the SiC substrate.

cond-mat.mes-hall↗