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We report on magneto-optical studies of the quasi-two-dimensional van der Waals antiferromagnet FePS

We investigate the origin of emission lines apparent in the low-temperature photoluminescence spectra of $n$-doped WS$_2$ monolayer embedded in hexagonal BN layers using external magnetic fields and first-principles calculations. Apart from the neutral A exciton line, all observed emission lines are related to the negatively charged excitons. Consequently, we identify emissions due to both the bright (singlet and triplet) and dark (spin- and momentum-forbidden) negative trions as well as the phonon replicas of the latter optically-inactive complexes. The semi-dark trions and negative biexcitons are distinguished. Based on their experimentally extracted and theoretically calculated $g$-factors, we identify three distinct families of emissions due to exciton complexes in WS$_2$: bright, intravalley and intervalley dark. The $g$-factors of the spin-split subbands in both the conduction and valence bands are also determined., Manuscript: 7 pages, 5 figures; SI: 5 pages, 3 figures

Low temperature and polarization resolved magneto-photoluminescence experiments are used to investigate the properties of dark excitons and dark trions in a monolayer of WS$_2$ encapsulated in hexagonal BN (hBN). We find that this system is an $n$-type doped semiconductor and that dark trions dominate the emission spectrum. In line with previous studies on WSe$_2$, we identify the Coulomb exchange interaction coupled neutral dark and grey excitons through their polarization properties, while an analogous effect is not observed for dark trions. Applying the magnetic field in both perpendicular and parallel configurations with respect to the monolayer plane, we determine the g-factor of dark trions to be $g\sim$-8.6. Their decay rate is close to 0.5 ns, more than 2 orders of magnitude longer than that of bright excitons., Comment: 6 pages, 6 figures, supplemental material

The spectral signatures associated with different negatively charged exciton complexes (trions) in a WS$_2$ monolayer encapsulated in hBN, are analyzed from low temperature and polarization resolved reflectance contrast (RC) and photoluminescence (PL) experiments, with an applied magnetic field. Based on results obtained from the RC experiment, we show that the valley Zeeman effect affects the optical response of both the singlet and the triplet trion species through the evolution of their energy and of their relative intensity, when applying an external magnetic field. Our analysis allows us to estimate a free electron concentration of $\sim 1.3 \cdot 10^{11}$ cm$^{-2}$. The observed evolutions based on PL experiments on the same sample are different and can hardly be understood within the same simple frame highlighting the complexity of relaxation processes involved in the PL response., Comment: 7 pages, 4 figures; source file corrected

International audience; Absolute far-infrared magnetotransmission experiments have been performed in magnetic fields up to 33.5 T on a series of single GaAs quantum wells doped with different electron concentrations. The transmission spectra have been simulated with a multilayer dielectric model. The imaginary part of the optical response function, which reveals singular features related to the electron-phonon interactions, has been extracted. In addition to the expected polaronic effects due to the longitudinal-optical phonon of GaAs, additional interactions with interface phonons are observed. The main interaction is analyzed quantitatively with the Feynman-Hellwarth-Iddings-Platzman model, which is shown to predict correctly the concentration of carriers beyond which the Fröhlich interaction is completely screened.

Strong Coulomb correlations together with multi-valley electronic bands in the presence of spin-orbit interaction and possible new optoelectronic applications are at the heart of studies of the rich physics of excitons in semiconductor structures made of monolayers of transition metal dichalcogenides (TMD). In intrinsic TMD monolayers the basic, intravalley excitons are formed by a hole from the top of the valence band and an electron either from the lower or upper spin-orbit-split conduction band subbands: one of these excitons is optically active, the second one is "dark", although possibly observed under special conditions. Here we demonstrate the s-series of Rydberg dark exciton states in monolayer WSe$_2$, which appears in addition to a conventional bright exciton series in photoluminescence spectra measured in high in-plane magnetic fields. The comparison of energy ladders of bright and dark Rydberg excitons is shown to be a method to experimentally evaluate one of the missing band parameters in TMD monolayers: the amplitude of the spin-orbit splitting of the conduction band., Comment: Manuscript: 9 pages, 4 figures; SM: 3 pages, 2 figures

The hybridization of magnons (spin waves) with phonons, if sufficiently strong and comprising long wavelength excitations, may offer a new playground when manipulating the magnetically ordered systems with light. Applying a magnetic field to a quasi-2D antiferromagnet, FePS3, we tune the magnon-gap excitation towards coincidence with the initially lower-in-energy phonon modes. Hybrid magnon-phonon modes, the magnon polarons are unveiled with demonstration of a pronounced avoided crossing between the otherwise bare magnon and phonon excitations. The magnon polarons in FePS3 are primary traced with Raman scattering experiments, but, as we show, they also couple directly to terahertz photons, what evokes their further explorations in the domain of antiferromagnetic optospintronics., Comment: 10 pages, 4 figures and Supplementary Materials, to be published in Phys. Rev. B

The spectral signatures associated with different negatively charged exciton complexes (trions) in a WS2 monolayer encapsulated in hBN are analyzed from low temperature and polarization resolved reflectance contrast (RC) and photoluminescence (PL) experiments, with an applied magnetic field. Based on results obtained from the RC experiment, we show that the valley Zeeman effect affects the optical response of both the singlet and the triplet trion species through the evolution of their energy and of their relative intensity, when applying an external magnetic field. Our analysis allows us to estimate a free electron concentration of ∼1.3 × 1011 cm-2. The observed evolutions based on PL experiments on the same sample are different and can hardly be understood within the same simple frame, highlighting the complexity of relaxation processes involved in the PL response.

We review the different results obtained in the last decade in the field of Raman scattering of graphene based systems, with an applied magnetic field. Electronic properties of graphene based systems with an applied magnetic field will first be described. The phonon response in magnetic field, the magneto-phono resonance, will then be introduced and described in different systems, including graphene, multilayer graphene and bulk graphite. Electronic Raman scattering is then be discussed in the context of Landau level spectroscopy, of electron phonon interaction and of electron-electron interaction.

We report magneto-optical spectroscopy of gated monolayer MoS$_2$ in high magnetic fields up to 28T and obtain new insights on the many-body interaction of neutral and charged excitons with the resident charges of distinct spin and valley texture. For neutral excitons at low electron doping, we observe a nonlinear valley Zeeman shift due to dipolar spin-interactions that depends sensitively on the local carrier concentration. As the Fermi energy increases to dominate over the other relevant energy scales in the system, the magneto-optical response depends on the occupation of the fully spin-polarized Landau levels in both $K/K^{\prime}$ valleys. This manifests itself in a many-body state. Our experiments demonstrate that the exciton in monolayer semiconductors is only a single particle boson close to charge neutrality. We find that away from charge neutrality it smoothly transitions into polaronic states with a distinct spin-valley flavour that is defined by the Landau level quantized spin and valley texture., Comment: Main manuscript: 7 pages, 4 figures ; Supplemental material: 20 pages, 8 figures

Magnetotransmission (MT) at magnetic fields up to 29 T was used to study the electronic structure of $\mathrm{CuInS}{\mathrm{e}}_{2}$ in thin polycrystalline films. The zero field absorption spectra exhibited resolved A, B, and C free excitons. Quantum oscillations, due to diamagnetic excitons comprising electrons and holes from Landau levels quantized in the conduction and valence band, respectively, appeared in the MT spectra at fields over 5 T. Spectral energies of Landau levels and binding energies of the corresponding diamagnetic excitons, theoretically calculated assuming a quasicubic approximation of the $\mathrm{CuInS}{\mathrm{e}}_{2}$ tetragonal lattice structure, helped to identify the character of the experimentally observed diamagnetic excitons. Spectral energies of diamagnetic excitons in the MT spectra with different circular polarizations were used to determine the electron and light hole effective masses, whereas heavy hole masses as well as the \ensuremath{\gamma} and ${\ensuremath{\gamma}}_{1}$ Luttinger parameters, ${E}_{p}$ Kane energy, and F parameter of the influence of remote bands, as well as their polaron values, were calculated using the Luttinger theory.

A time-resolved observation of coherent interlayer longitudinal acoustic phonons in thin layers of 2H-MoSe2 is reported. A femtosecond pump-probe technique is used to investigate the evolution of the energy loss of these vibrational modes in a wide selection of MoSe2 flakes with different thicknesses ranging from bilayer up to the bulk limit. By directly analysing the temporal decay of the modes, we can clearly distinguish an abrupt crossover related to the acoustic mean free path of the phonons in a layered system, and the constraints imposed on the acoustic decay channels when reducing the dimensionality. For thicker samples, the main acoustic attenuation mechanism is attributed to the scattering of the acoustic modes with thermal phonons. For samples thinner than ∼20 molecular layers, the predominant damping mechanism is ascribed to the effects of surface asperity. Losses intrinsic to the low dimensionality of single or few layer materials impose critical limitations for their use in optomechanical and optoelectronic devices.

Graphene layers are known to stack in two stable configurations, namely, ABA or ABC stacking, with drastically distinct electronic properties. Unlike the ABA stacking, little has been done to experimentally investigate the electronic properties of ABC graphene multilayers. Here, we report on the first magneto optical study of a large ABC domain in a graphene multilayer flake, with ABC sequences exceeding 17 graphene sheets. ABC-stacked multilayers can be fingerprinted with a characteristic electronic Raman scattering response, which persists even at room temperatures. Tracing the magnetic field evolution of the inter Landau level excitations from this domain gives strong evidence for the existence of a dispersionless electronic band near the Fermi level, characteristic of such stacking. Our findings present a simple yet powerful approach to probe ABC stacking in graphene multilayer flakes, where this highly degenerated band appears as an appealing candidate to host strongly correlated states. Fil: Henni, Younes. Centre National de la Recherche Scientifique; Francia Fil: Ojeda Collado, Hector Pablo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina Fil: Nogajewski, Karol. Centre National de la Recherche Scientifique; Francia Fil: Molas, MacIej R.. Centre National de la Recherche Scientifique; Francia Fil: Usaj, Gonzalo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina Fil: Balseiro, Carlos Antonio. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina Fil: Orlita, Milan. Centre National de la Recherche Scientifique; Francia Fil: Potemski, Marek. Centre National de la Recherche Scientifique; Francia Fil: Faugeras, Clement. Centre National de la Recherche Scientifique; Francia

We demonstrate that, in monolayers (MLs) of semiconducting transition metal dichalcogenides, the $s$-type Rydberg series of excitonic states follows a simple energy ladder: $\epsilon_n=-Ry^*/(n+\delta)^2$, $n$=1,2,\ldots, in which $Ry^*$ is very close to the Rydberg energy scaled by the dielectric constant of the medium surrounding the ML and by the reduced effective electron-hole mass, whereas the ML polarizability is only accounted for by $\delta$. This is justified by the analysis of experimental data on excitonic resonances, as extracted from magneto-optical measurements of a high-quality WSe$_2$ ML encapsulated in hexagonal boron nitride (hBN), and well reproduced with an analytically solvable Schr\"odinger equation when approximating the electron-hole potential in the form of a modified Kratzer potential. Applying our convention to other, MoSe$_2$, WS$_2$, MoS$_2$ MLs encapsulated in hBN, we estimate an apparent magnitude of $\delta$ for each of the studied structures. Intriguingly, $\delta$ is found to be close to zero for WSe$_2$ as well as for MoS$_2$ monolayers, what implies that the energy ladder of excitonic states in these two-dimensional structures resembles that of Rydberg states of a three-dimensional hydrogen atom., Comment: Manuscript: 6 pages, 4 figures; SM: 11 pages, 12 figures

We study the relaxation dynamics of the electron system in graphene flakes under Landau quantization regime using a novel approach of time-resolved Raman scattering. The non-resonant character of the experiment allows us to analyze the field dependence of the relaxation rate. Our results clearly evidence sharp increase in the relaxation rate upon the resonance between the energy of the Landau transition and the G-band and shed new light on relaxation mechanism of the Landau-quantized electrons in graphene beyond the previously studied Auger scattering.

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)., Comment: to be published in Phys. Rev. B

We report on infrared transmission and reflectivity experiments performed on Cd 3 As 2 in a wide range of the photon energies and magnetic fields. The observed magneto-optical response unambiguously indicates the presence of 3D massless charge carriers. The detailed analysis of cyclotron resonances implies the presence of massless Kane electrons at a large energy scale, while the symmetry-protected 3D Dirac cones may appear at a small scale.

Recent experimental and theoretical work on hydrogen-like absorption spectra of excitons in external magnetic fields revealed new effects when the Coulomb interaction becomes comparable to the magnetic perturbation. We present a theoretical approach that allows for calculation of absorption spectra for any value of magnetic field. This approach opens the possibility to compute the optical functions i.e. reflectivity, transmission and absorption including the excitonic effects for various strength of external magnetic field.

CuInTe2 single crystals are studied using optical magneto-reflectance (MR) in magnetic fields B up to 20 T at 4.2 K. The spectra exhibit the A and B free excitons' blue shifting at increasing magnetic fields. Fitting quadratic functions to the experimental dependencies of the exciton spectral energy on B assuming a low field limit allow the determination of diamagnetic shift rates of 8.2 × 10−5 and 8.5 × 10−5 eV T−2 for the A and B free excitons, respectively. The excitons' reduced masses of 0.0575m0 and 0.0568m0 (m0 is the free electron mass), Rydbergs of 6.2 and 6.1 meV, and Bohr radii of 10.4 and 10.5 nm are then estimated. An electron effective mass of 0.062m0 and B sub-band effective hole mass of 0.70m0 are determined using a literature value of the A valence sub-band hole of 0.78m0. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Russian Science Foundation, RSF: 17-12-01500 The study was supported by the Russian Science Foundation (grant 17-12-01500) and LNCMI-CNRS (EMFL).

A recently discovered class of two-dimensional semiconductors exhibits a novel degree of freedom known as valley pseudospin. New results show that a weak magnetic field can significantly extend the depolarization time of this pseudospin.

Embedding a WS$_2$ monolayer in flakes of hexagonal boron nitride allowed us to resolve and study the photoluminescence response due to both singlet and triplet states of negatively charged excitons (trions) in this atomically thin semiconductor. The energy separation between the singlet and triplet states has been found to be relatively small reflecting rather weak effects of the electron-electron exchange interaction for the trion triplet in a WS$_2$ monolayer, which involves two electrons with the same spin but from different valleys. Polarization-resolved experiments demonstrate that the helicity of the excitation light is better preserved in the emission spectrum of the triplet trion than in that of the singlet trion. Finally, the singlet (intravalley) trions are found to be observable even at ambient conditions whereas the emission due to the triplet (intervalley) trions is only efficient at low temperatures., Comment: 11 pages, 4 figures

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.

International audience; The quantum efficiency of an electroluminescent intersubband emitter based on InAs/AlSb has been measured as a function of the magnetic field up to 20T. Two series of oscillations periodic in 1/B are observed, corresponding to the elastic and inelastic scattering of electrons of the upper state of the radiative transitions. Experimental results are accurately reproduced by a calculation of the excited state lifetime as a function of the applied magnetic field. The interpretation of these data gives an exact measure of the relative weight of the scattering mechanisms and allows the extraction of material parameters such as the energy dependent electron effective mass and the optical phonon energy

We report on magneto-optical studies of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, a representative member of the 3D topological insulator family. Its electronic states in bulk are shown to be well described by a simple Dirac-type Hamiltonian for massive particles with only two parameters: the fundamental band gap and the band velocity. In a magnetic field, this model implies a unique property---spin splitting equal to twice the cyclotron energy: ${E}_{s}=2{E}_{c}$. This explains the extensive magnetotransport studies concluding a fortuitous degeneracy of the spin and orbital split Landau levels in this material. The ${E}_{s}=2{E}_{c}$ match differentiates the massive Dirac electrons in bulk ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ from those in quantum electrodynamics, for which ${E}_{s}={E}_{c}$ always holds.

The quantum cascade laser is a semiconductor light source based on resonant tunnelling and optical transitions between quantised conduction band states. In these devices the principles of operation are not re lated to the physical properties of the constituent materials, but arise from an artificial potential designed using a sequence of very thin layers of different semiconductor materials. The quantum design implemented by highly sophisticated epitaxial growth, allows one to ascribe in semiconductor crystals this artificial potential with the desired electronic energy levels and wavefunctions. In recent years the performance of these devices has improved markedly and this semiconductor technology is now an attractive choice for the fabrication of mid-far infrared lasers in a very wide spectral range (3.5–160 µm). At present, the best performances are obtained at wavelengths between 5–12 µm, where continuous-wave room temperature operation is routinely achieved and record average power in the order of 1 W has been demonstrated. The long wavelength region has been only recently explored and lasers operating temperatures do not currently exceed 150 K. Interesting nonlinear mixing of diode lasers with a THz laser has been recently demonstrated. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present two different techniques for processing InP-based /spl lambda/=9 /spl mu/m quantum cascade lasers which improve the thermal dissipation in the device. The first process is based on hydrogen implantation creating an insulating layer to inject current selectively in one part of the active region. The second process uses a thick electroplated gold layer on the laser ridge to efficiently remove the heat produced in the active region. Each process is designed to improve heat evacuation leading to higher performances of the lasers and will be compared to a standard ridge structure from the same wafer. We give evidence that the process of proton implantation, efficient in GaAs based structures, is not directly applicable to InP based devices and we present a detailed analysis of the thermal properties of devices with an electroplated gold thick layer. With these lasers, an average power of 174 mW at a duty cycle of 40% has been measured at 10/spl deg/C.

We have investigated the infrared transmission of a two-dimensional (2DEG) electron gas confined in a $Al_{0.25}In_{0.75}As/Ga_{0.25}In_{0.75}As$ single quantum well in order to study the electron optical phonon interaction in a two phonon mode system. Infrared transmission experiments have been performed in both the perpendicular Faraday (PF) and tilted Faraday (TF) configurations for which the growth axis of the sample is tilted with respect to the incident light propagation direction and to the magnetic field direction. The experimental results lead to question the validity of the concept of polaron mass in a real material.

We investigate the cyclotron resonance (CR) on a single GaAs quantum well containing a two-dimensional electron gas (7.0×10 11 cm −2 ) , in magnetic fields up to 28 T in order to study the resonant polaron coupling. When the normal to the surface of the sample is in the same direction as that of light propagation, we do not observe an anomaly of the CR absorption in the optical phonon range of energy. When the normal of the sample is tilted with respect to the light propagation direction (and with the magnetic field direction) we observe a pinning of the CR peak at an energy lower than that of the longitudinal optical (LO) phonon energy. This mode is identified as the coupled intersubband plasmon–LO phonon of the quantum well.

We present optical spectroscopy (photoluminescence and reflectance) studies of thin layers of the transition metal dichalcogenide WSe2, with thickness ranging from mono- to tetra-layer and in the bulk limit. The investigated spectra show the evolution of excitonic resonances as a function of layer thickness, due to changes in the band structure and, importantly, due to modifications of the strength of Coulomb interaction as well. The observed temperature-activated energy shift and broadening of the fundamental direct exciton are well accounted for by standard formalisms used for conventional semiconductors. A large increase of the photoluminescence yield with temperature is observed in WSe2 monolayer, indicating the existence of competing radiative channels. The observation of absorption-type resonances due to both neutral and charged excitons in WSe2 monolayer is reported and the effect of the transfer of oscillator strength from charged to neutral exciton upon increase of temperature is demonstrated., Comment: 12 pages, 5 figures

Optical orientation of localized/bound excitons is shown to be effectively enhanced by the application of magnetic fields as low as 20 mT in monolayer WS2. At low temperatures, the evolution of the polarization degree of different emission lines of monolayer WS2 with increasing magnetic fields is analyzed and compared to similar results obtained on a WSe2 monolayer. We study the temperature dependence of this effect up to K for both materials, focusing on the dynamics of the valley pseudospin relaxation. A rate equation model is used to analyze our data and from the analysis of the width of the polarization dip in magnetic field we conclude that the competition between the dark exciton pseudospin relaxation and the decay of the dark exciton population into the localized states are rather different in these two materials which are representative of the two extreme cases for the ratio of relaxation rate and depolarization rate.

We investigated the cyclotron resonance (CR) on a series of samples containing high-density 2DEG (7.4– 12×10 11 cm −2 ) in GaAs/AlAs single quantum wells (in high magnetic fields up to 28 T ) in order to study the resonant polaron coupling. We did not observe any anomalies of the CR peak's evolution around the longitudinal optical (LO) phonon energy of GaAs (ℏω LO =36.3 meV ) . Instead, a pinning of the CR peak occurs near the transverse optical (TO) phonon energy and is accompanied by an increase of the width and of the intensity of the peak. We have simulated the transmission of the whole structure using a multilayer dielectric model, which allows us to determine the correct values of the relaxation frequency and its variation with magnetic field. We discuss the different possibilities of the coupling observed around the TO energy.

This paper presents a flexible numerical technique which is especially suited to analyze lateral modulation of quantum effects in short channel MOS transistors. We discuss boundary conditions for the Schrodinger equation and the impact of the finite element meshing. We show how channel length shortening alters the sub-band structure, thus giving an evaluation of the limits of a 1D quantum approach.

The results of the study of cyclotron resonance on a series of high electronic density 2DEG in GaAs/AlAs single quantum well are presented. The samples contain different amount of acceptors in the well. We observe clear oscillations of the CR line width with the filling factor. This effect is accompanied by a periodical modulation of the CR peak energy. We simulated the transmission of the whole structure using a multi-layer dielectric model. This model allows us to determine the correct values of the relaxation frequency and its variation with magnetic field. The oscillations of the CR line width and of the CR peak energy are explained within the theory of the memory function.

We probe electronic excitations between Landau levels in freestanding $N-$layer graphene over a broad energy range, with unprecedented spectral and spatial resolution, using micro-magneto Raman scattering spectroscopy. A characteristic evolution of electronic bands in up to five Bernal-stacked graphene layers is evidenced and shown to remarkably follow a simple theoretical approach, based on an effective bilayer model. $(N>3)$-layer graphene appear as appealing candidates in the quest for novel phenomena, particularly in the quantum Hall effect regime. Our work paves the way towards minimally-invasive investigations of magneto-excitons in other emerging low-dimensional systems, with a spatial resolution down to 1$~��$m., to appear in Nano Letters

We report on a magneto-Raman scattering study of graphene flakes located on the surface of a bulk graphite substrate. By spatially mapping the Raman scattering response of the surface of bulk graphite with an applied magnetic field, we pinpoint specific locations which show the electronic excitation spectrum of graphene. We present the characteristic Raman scattering signatures of these specific locations. We show that such flakes can be superimposed with another flake and still exhibit a graphene-like excitation spectrum. Two different excitation laser energies (514.5 and 720 nm) are used to investigate the excitation wavelength dependence of the electronic Raman scattering signal., Comment: 6 pages, 5 figures

Recent results of magneto-absorption and Raman scattering studies of different graphene based systems are reviewed. The potential of these techniques to derive the band structure, scattering efficiency and effects of interactions is discussed in reference to studies of two representative allotropes of sp2-bonded carbon: graphene and graphite.

The electronic structure of Bernal-stacked graphite subject to tilted magnetic fields has been investigated using infrared magneto-transmission experiments. With the increasing in-plane component of the magnetic field B, we observe significant broadening and partially also splitting of interband inter-Landau level transitions, which originate at the H point of the graphite Brillouin zone, where the charge carriers behave as massless Dirac fermions. The observed behavior is attributed to the lifting of the twofold degeneracy of Landau levels at the H point - a degeneracy which in graphite complements the standard spin and valley degeneracies typical of graphene., Comment: 6 pages, 3 figures

The polarization-resolved Raman-scattering response due to E-2g phonons in monolayer graphene has been investigated in magnetic fields up to 29 T. The hybridization of the E-2g phonon is only observed with the fundamental inter-Landau-level excitation (involving the n = 0 Landau level) and in just one of the two configurations of the circularly cross-polarized excitation and scattered light. This polarization anisotropy of the magnetophonon resonance is shown to be inherent to relatively strongly doped graphene samples with carrier concentrations typical for graphene deposited on Si/SiO2 substrates.

We use polarized magneto-Raman scattering to study purely electronic excitations and the electron-phonon coupling in bulk graphite. At a temperature of 4.2 K and in magnetic fields up to 28 T we observe $K$-point electronic excitations involving Landau bands with $\Delta |n|=0$ and with $\Delta |n|=\pm2$ that can be selected by controlling the angular momentum of the excitation laser and of the scattered light. The magneto-phonon effect involving the $E_{2g}$ optical phonon and $K$-point inter Landau bands electronic excitations with $\Delta |n|=\pm1$ is revealed and analyzed within a model taking into account the full $k_z$ dispersion. These polarization resolved results are explained in the frame of the Slonczewski-Weiss-McClure (SWM) model which directly allows to quantify the electron-hole asymmetry., Comment: 13 pages, 10 figures

The energy-dependence of the electronic scattering time is probed by Landau level spectroscopy in quasi neutral multilayer epitaxial graphene. From the Landau levels broadening we find that the scattering rate increases linearly with energy. This implies a surprising property of the Landau level spectrum in graphene - the number of the resolved Landau levels remains constant with the applied magnetic field. Insights are given about possible scattering mechanism and carrier mobilities in the graphene system investigated., 5 pages, 2 figures, to appear in Phys. Rev. Lett

The Integer Quantum Hall Effect (IQHE) is a distinctive phase of two-dimensional electronic systems subjected to a perpendicular magnetic field. Thus far, the IQHE has been observed in semiconductor heterostructures and in mono- and bi-layer graphene. Here we report on the IQHE in a new system: trilayer graphene. Experimental data are compared with self-consistent Hartree calculations of the Landau levels for the gated trilayer. The plateau structure in the Hall resistivity determines the stacking order (ABA versus ABC). We find that the IQHE in ABC trilayer graphene is similar to that in the monolayer, except for the absence of a plateau at filling factor v=2. At very low filling factor, the Hall resistance vanishes due to the presence of mixed electron and hole carriers induced by disorder., 5 pages, 4 figures

HgTe/Hg${}_{x}$Cd${}_{1\ensuremath{-}x}$Te quantum wells with the inverted band structure have been probed using far-infrared magnetospectroscopy. Realistic calculations of Landau level diagrams have been performed to identify the observed transitions. Investigations have been greatly focused on the magnetic field dependence of the peculiar pair of ``zero-mode'' Landau levels, which characteristically split from the upper conduction and bottom valence bands, and merge under the applied magnetic field. The observed avoided crossing of these levels is tentatively attributed to the bulk inversion asymmetry of zinc-blende compounds.

We have studied, both experimentally and theoretically, the change of the so-called 2D band of the Raman scattering spectrum of graphene (the two-phonon peak near 2700 cm-1) in an external magnetic field applied perpendicular to the graphene crystal plane at liquid helium temperature. A shift to lower frequency and broadening of this band is observed as the magnetic field is increased from 0 to 33 T. At fields up to 5--10 T the changes are quadratic in the field while they become linear at higher magnetic fields. This effect is explained by the curving of the quasiclassical trajectories of the photo-excited electrons and holes in the magnetic field, which enables us (i) to extract the electron inelastic scattering rate, and (ii) to conclude that electronic scattering accounts for about half of the measured width of the 2D peak., Comment: 11 pages, 7 figures

Precise absolute far-infra-red magneto-transmission experiments have been performed in magnetic fields up to 33 T on a series of single GaAs quantum wells doped at different levels. The transmission spectra have been simulated with a multilayer dielectric model. The imaginary part of the optical response function which reveals new singular features related to the electron-phonon interactions has been extracted. In addition to the expected polaronic effects due to the longitudinal optical (LO) phonon of GaAs, a new kind of carrier concentration dependent interaction with interface phonons is observed. A simple physical model is used to try to quantify these interactions and explore their origin., 4 pages, 2 Figures

Fabrication of graphene structures has triggered vast research efforts focused on the properties of two-dimensional systems with massless Dirac fermions. Nevertheless, further progress in exploring this quantum electrodynamics system in solid-state laboratories seems to be limited by insufficient electronic quality of manmade structures and the crucial question arises whether existing technologies have reached their limits or major advances are in principle possible. Here we show that graphene in a significantly purer state can be found in nature on the surface of bulk graphite, in form of flakes decoupled from the substrate material. Probing such flakes with Landau level spectroscopy in the THz range at very low magnetic fields, we demonstrate a superior electronic quality of these ultra low density layers (n~3x10^9 cm^-2) expressed by the carrier mobility in excess of 10^7 cm^2/(V.s). This finding represents an important challenge for further improvements of current graphene technologies., Comment: 5 pages, 4 figures, to appear in PRL

Multi-layer epitaxial graphene (MEG) is investigated using far infrared (FIR) transmission experiments in the different limits of low magnetic fields and high temperatures. The cyclotron-resonance like absorption is observed at low temperature in magnetic fields below 50 mT, allowing thus to probe the nearest vicinity of the Dirac point and to estimate the conductivity in nearly undoped graphene. The carrier mobility is found to exceed 250,000 cm$^2$/(V.s). In the limit of high temperatures, the well-defined Landau level (LL) quantization is observed up to room temperature at magnetic fields below 1 T, a phenomenon unique in solid state systems. A negligible increase in the width of the cyclotron resonance lines with increasing temperature indicates that no important scattering mechanism is thermally activated, supporting recent expectations of high room-temperature mobilities in graphene., Comment: 5 pages, 3 figures

Our low-temperature magneto-Raman scattering measurements performed on graphene-like locations on the surface of bulk graphite, carries the energyite reveal a new series of magneto-phonon resonances involving both K point and Γ point phonons. These are resonances between a purely electronic excitation, an electronic excitation accompanied by one phonon, and a two-phonon excitation. In particular, we observe the resonant splitting of three crossing excitation branches. We give a detailed theoretical analysis of these multi-excitation resonances. Our results highlight the role of combined excitations and the importance of multi-phonon processes (from both K and Γ points) for the relaxation of hot carriers in graphene.

We have studied the Raman and infrared spectral response of TbMn2O5 under an applied magnetic field parallel to the easy magnetic a-axis at 4.2 K. Strong spin-lattice coupling in TbMn2O5 is evidenced by a frequency shift of Raman and infrared phonons as a function of magnetic field compared to the phonon response of BiMn2O5 that remains unaffected. The magnetic field behavior of the highest frequency phonons retraces the polarization switching in TbMn2O5 and shows an important frequency softening below 3 T that is modulated by the J 3 and J 4 exchange parameters. The role of the Tb3+ spin alignment with H is interpreted in terms of a local lattice striction and the contribution of the charge transfer mechanism to the magnetoelectric process is evaluated.