Search

Smooth interfaces of topological systems are known to host massive surface states along with the topologically protected chiral one. We show that in Weyl semimetals these massive states, along with the chiral Fermi arc, strongly alter the form of the Fermi-arc plasmon, Most saliently, they yield further collective plasmonic modes that are absent in a conventional interfaces. The plasmon modes are completely anisotropic as a consequence of the underlying anisotropy in the surface model and expected to have a clear-cut experimental signature, e.g. in electron-energy loss spectroscopy., Maintext: 5 pages + 4 figures; Supp.: 9 pages + 1 figure

International audience; As a consequence of the approximate spin-valley symmetry in graphene, the ground state of electrons in graphene at charge neutrality is a particular SU(4) quantum Hall ferromagnet to minimize their exchange energy. If only the Coulomb interaction is taken into account, this ferromagnet can appeal either to the spin degree of freedom or equivalently to the valley pseudo-spin degree of freedom. This freedom in choice is then limited by subleading energy scales that explicitly break the SU(4) symmetry, the simplest of which is given by the Zeeman effect that orients the spin in the direction of the magnetic field. In addition, there are also valley symmetry-breaking terms that can arise from short-range interactions or electron-phonon couplings. Here, we build upon the phase diagram, which has been obtained by Kharitonov [M. Kharitonov, Phys. Rev. B 85, 155439 (2012)PRBMDO1098-012110.1103/PhysRevB.85.155439], in order to identify the different skyrmions that are compatible with these types of quantum Hall ferromagnets. Similarly to the ferromagnets, the skyrmions at charge neutrality are described by the Gr(2,4) Grassmannian at the center, which allows us to construct the skyrmion spinors. The different skyrmion types are then obtained by minimizing their energy within a variational approach, with respect to the remaining free parameters that are not fixed by the requirement that the skyrmion at large distances from their center must be compatible with the ferromagnetic background. We show that the different skyrmion types have a clear signature in the local, sublattice-resolved, spin magnetization, which is in principle accessible in scanning-tunneling microscopy and spectroscopy.

We study generalized spin waves in graphene under a strong magnetic field when the Landau-level filling factor is $\nu=\pm 1$. In this case, the ground state is a particular SU(4) quantum Hall ferromagnet, in which not only the physical spin is fully polarized but also the pseudo-spin associated with the valley degree of freedom. The nature of the ground state and the spin-valley polarization depend on explicit symmetry breaking terms that are also reflected in the generalised spin-wave spectrum. In addition to pure spin waves, one encounters valley-pseudo-spin waves as well as more exotic entanglement waves that have a mixed spin-valley character. Most saliently, the SU(4) symmetry-breaking terms do not only yield gaps in the spectra, but under certain circumstances, namely in the case of residual ground-state symmetries, render the originally quadratic (in the wave vector) spin-wave dispersion linear.

Landau-level spectroscopy, the optical analysis of electrons in materials subject to a strong magnetic field, is a versatile probe of the electronic band structure and has been successfully used in the identification of novel states of matter such as Dirac electrons, topological materials or Weyl semimetals. The latter arise from a complex interplay between crystal symmetry, spin-orbit interaction and inverse ordering of electronic bands. Here, we report on unusual Landau-level transitions in the monopnictide TaP that decrease in energy with increasing magnetic field. We show that these transitions arise naturally at intermediate energies in time-reversal-invariant Weyl semimetals where the Weyl nodes are formed by a partially gapped nodal-loop in the band structure. We propose a simple theoretical model for electronic bands in these Weyl materials that captures the collected magneto-optical data to great extent., Comment: 6 pages, 3 figures + supplementary materials, accepted in Phys. Rev. Lett

International audience; We study the neutral excitations of fractional quantum Hall states in electronic systems of finite width where an external anisotropy is introduced by tilting the magnetic field. As in the isotropic case, the neutral collective excitation can be worked out through the conserving method of composite fermions in the Hamiltonian theory because the interaction potential has a natural cutoff due to the quantum well width. We show how such a computation can be carried out perturbatively for an anisotropic interaction. We find that unlike the charge gap, the neutral collective gap is much more sensitive to the tilt and can thus, for certain fractional quantum Hall states, be easily destroyed by the parallel component of the magnetic field. We also discuss the convergence of the collective spectrum to the activation gap in the large-momentum limit.

A flat energy dispersion of electrons at the Fermi level of a material leads to instabilities in the electronic system and can drive phase transitions. Here we introduce a method to induce a flat band in two-dimensional (2D) materials. We show that the flat band can be achieved by sandwiching the 2D material by two cesium (Cs) layers. We apply this method to monolayer graphene and investigate the flat band by a combination of angle-resolved photoemission spectroscopy experiment and the calculation. Our work highlights that charge transfer, zone folding of graphene bands and the covalent bonding between C and Cs atoms are at the origin of the flat energy band formation. The presented approach is an alternative route for obtaining flat band materials to twisting bilayer graphene which yields thermodynamically stable flat band materials in large areas., 22 pages, 4 Figures

We investigate a quantum well that consists of a thin topological insulator sandwiched between two trivial insulators. More specifically, we consider smooth interfaces between these different types of materials such that the interfaces host not only the chiral interface states, whose existence is dictated by the bulk-edge correspondence, but also massive Volkov-Pankratov states. We investigate possible hybridization between these interface states as a function of the width of the topological material and of the characteristic interface size. Most saliently, we find a strong qualitative difference between an extremely weak effect on the chiral interface states and a more common hybridization of the massive Volkov-Pankratov states that can be easily understood in terms of quantum tunneling in the framework of the model of a (Dirac) quantum well we introduce here., accepted for publication in PRB (13 pages with appendix + 7 figures)

Weyl semimetals are known to host massless surface states called Fermi arcs. These Fermi arcs are the manifestation of the bulk-edge correspondence in topological matter and thus are analogous to the topological chiral surface states of topological insulators. It has been shown that the latter, depending on the smoothness of the surface, host massive Volkov-Pankratov(VP) states that coexist with the chiral ones. Here, we investigate these VP states in the framework of Weyl semimetals, namely their density of states and magneto-optical response. We find the selection rules corresponding to optical transitions which lead to anisotropic responses to external fields. In the presence of a magnetic field parallel to the interface, the selection rules and hence the poles of the response functions are mixed., 9 pages, 9 figures

In addition to the usual chiral surface states, massive surface states can arise at a smooth interface between a topological and a trivial bulk insulator. While not subject to topological protection as the chiral states, these massive states, theorized by Volkov and Pankratov in the 1980s, reflect nevertheless emergent Dirac physics at the interface. We study theoretically the magneto-optical response of these surface states, which is strikingly different from that of the bulk states. Most saliently, we show that these states can be identified clearly in the presence of a magnetic field and its orientation with respect to the interface., 5 pages with 2 figures, added references, fig.1a replaced

International audience; We report experimental observation of the reentrant integer quantum Hall effect in graphene, appearing in the N=2 Landau level. Similar to high-mobility GaAs/AlGaAs heterostructures, the effect is due to a competition between incompressible fractional quantum Hall states, and electron solid phases. The tunability of graphene allows us to measure the B-T phase diagram of the electron solid phase. The hierarchy of reentrant states suggests spin and valley degrees of freedom play a role in determining the ground state energy. We find that the melting temperature scales with magnetic field, and construct a phase diagram of the electron liquid-solid transition.

We study the conductivity of two-dimensional graphene-type materials with tilted cones as well as their three-dimensional Weyl counterparts and show that a covariant quantum Boltzmann equation is capable of providing an accurate description of these materials' transport properties. The validity of the covariant Boltzmann approach is corroborated by calculations within the Kubo formula. We find a strong anisotropy in the conductivities parallel and perpendicular to the tilt direction upon increase of the tilt parameter $\eta$, which can be interpreted as the boost parameter of a Lorentz transformation. While the ratio between the two conductivities is $\sqrt{1-\eta^2}$ in the two-dimensional case where only the conductivity perpendicular to the tilt direction diverges for $\eta\rightarrow 1$, both conductivities diverge in three-dimensional Weyl semimetals, where $\eta=1$ separates a type-I (for $\eta1$)., Comment: 14 pages, 9 figures

Breakdown of the quantum Hall effect (QHE) is commonly associated with an electric field approaching the inter Landau-level (LL) Zener field, ratio of the Landau gap and cyclotron radius. Eluded in semiconducting heterostructures, in spite of extensive investigation, the intrinsic Zener limit is reported here using high-mobility bilayer graphene and high-frequency current noise. We show that collective excitations arising from electron-electron interactions are essential. Beyond a noiseless ballistic QHE regime a large superpoissonian shot noise signals the breakdown via inter-LL scattering. The breakdown is ultimately limited by collective excitations in a regime where phonon and impurity scattering are quenched. The breakdown mechanism can be described by a Landau critical velocity as it bears strong similarities with the roton mechanism of superfluids., 13 pages, 3 figures

International audience; We use the Hamiltonian theory developed by Shankar and Murthy to study a quantum Hall system in a tilted magnetic field. With a finite width of the system in the z direction, the parallel component of the magnetic field introduces anisotropy into the effective two-dimensional interactions. The effects of such anisotropy can be effectively captured by the recently proposed generalized pseudopotentials. We find that the off-diagonal components of the pseudopotentials lead to mixing of composite fermions Landau levels, which is a perturbation to the picture of p filled Landau levels in composite-fermion theory. By changing the internal geometry of the composite fermions, such a perturbation can be minimized and one can find the corresponding activation gaps for different tilting angles, and we calculate the associated optimal metric. Our results show that the activation gap is remarkably robust against the in-plane magnetic field in the lowest and first Landau levels.

It is well established that topological insulators sustain Dirac fermion surface states as a consequence of band inversion in the bulk. These states have a helical spin polarization and a linear dispersion with large Fermi velocity. In this article we report on a set of experimental observations indicating the existence of massive surface states. These states are confined at the interface and dominate equilibrium and transport properties at high energy and/or high electric field. By monitoring the AC admittance of HgTe topological insulator field-effect capacitors, we access the compressibility and conductivity of surface states in a broad range of energy and electric fields. The Dirac surface states are characterized by a compressibility minimum, a linear energy dependence and a high mobility persisting up to energies much larger than the transport bandgap of the bulk. New features are revealed at high energies with signatures such as conductance peaks, compressibility bumps, a strong charge metastability and a Hall resistance anomaly. These features point to the existence of excited massive surface states, responsible for a strong intersubband scattering with the Dirac states and the nucleation of metastable bulk carriers. The spectrum of excited states agrees with predictions of a phenomenological model of the topological-trivial semiconductor interface. The model accounts for the finite interface depth and the effect of electric fields. The existence of excited topological states is essential for the understanding of topological phases and opens a route for engineering and exploiting topological resources in quantum technology., Comment: 29 pages, 8 figures

We show that a smooth interface between two insulators of opposite topological Z2 indices possesses multiple surface states, both massless and massive. While the massless surface state is non-degenerate, chiral and insensitive to the interface potential, the massive surface states only appear for a sufficiently smooth heterojunction. The surface states are particle-hole symmetric and a voltage drop reveals their intrinsic relativistic nature, similarly to Landau bands of Dirac electrons in a magnetic field. We discuss the relevance of the massive Dirac surface states in recent ARPES and transport experiments., Comment: 5 pages, 3 figures; 6 pages of supplementary material incorporated; accepted for publication in Physical Review B as a Rapid Communication

Defect-free SrTiO3 (STO) is a band insulator but Angle Resolved Photoemission Spectroscopy (ARPES) experiments have demonstrated the existence of a nanometer thin two-dimensional electron liquid (2DEG) at the (001) oriented surface of this compound. The bulk is a trivial insulator, but our theoretical study reveals that the parity of electronic wavefunctions in this 2DEG is inverted in the vicinity of special points in reciprocal space where the low-energy dispersion consists of four gapped Dirac cones with a tilted and anisotropic shape. This gives rise to linearly dispersing topological edge states at the one-dimensional boundary. We propose to probe these modes by measuring the Josephson radiation from gapless bound Andreev states in STO based junctions, as it is predicted that they display distinctive signatures of topology., 10 pages, 6 figures

International audience; We report experimental measurements and theoretical analysis of Shubnikov-de Haas (SdH) oscillations in a Dirac cone system: the α-(BEDT-TTF)2I3 organic metal under hydrostatic pressure. The measured SdH oscillations reveal anomalies at high magnetic fields B where the 1/B oscillations periodicity is lost above 7 T. We interpret these unusual results within a theoretical model that takes into account intrinsic distortions of the α-(BEDT-TTF)2I3 Dirac cones such as a parabolic particle-hole asymmetric correction. Others possible causes, such as a cone tilting or a Zeeman effect, are carefully ruled out. The observations are consistent among α-(BEDT-TTF)2I3 samples with different Fermi levels.

We model quantum Hall skyrmions in graphene monolayer at quarter filling by a theory of CP3 fields and study the energy minimizing skyrmions in presence of valley pseudospin anisotropy and Zeeman coupling. We present a diagram of all types of skyrmions in a wide range of the anisotropy parameters. For each type of skyrmion, we visualize it on three Bloch spheres, and present the profiles of its texture on the graphene honeycomb lattice, thus providing references for the STM/STS imaging of spin-pseudospin textures in graphene monolayer in quantum Hall regime. Besides the spin and pseudospin skyrmions for the corresponding degrees of freedom of an electron in the N=0 Landau level, we discuss two unusual types -- the "entanglement skyrmion" whose texture lies in the space of the entanglement of spin and pseudospin, as well as the "deflated pseudospin skyrmion" with partial entanglement. For all skyrmion types, we study the dependence of the energy and the size of a skyrmion on the anisotropy parameters and perpendicular magnetic field. We also propose three ways to modify the anisotropy energy, namely the sample tilting, the substrate anisotropy and the valley pseudospin analogue of Zeeman coupling., Comment: 28 pages, 25 figures

We investigate Weyl semimetals with tilted conical bands in a magnetic field. Even when the cones are overtilted (type-II Weyl semimetal), Landau-level quantization can be possible as long as the magnetic field is oriented close to the tilt direction. Most saliently, the tilt can be described within the relativistic framework of Lorentz transformations that give rise to a rich spectrum, displaying new transitions beyond the usual dipolar ones in the optical conductivity. We identify particular features in the latter that allow one to distinguish between semimetals of different types., 5 pages, 3 figures; 3 pages of supplementary material incorporated; published version

We present a theory of quantum oscillations in insulators that are particle-hole symmetric and nontopological but with arbitrary band dispersion, at both zero and nonzero temperature. At temperatures $T$ less than or comparable to the gap, the dependence of oscillations on $T$ is markedly different from that in metals and depends crucially on the position of the chemical potential $\ensuremath{\mu}$ in the gap. If $\ensuremath{\mu}$ is in the middle of the gap, oscillations do not change with $T$; however, if $\ensuremath{\mu}$ is asymmetrically positioned in the gap, surprisingly, oscillations go to zero at a critical value of the inverse field determined by $T$ and $\ensuremath{\mu}$ and then change their phase by $\ensuremath{\pi}$ and grow again. Additionally, the temperature dependence is different for quantities derived from the grand canonical potential, such as magnetization and susceptibility, and those derived from the density of states, such as resistivity. However, the nontrivial features arising from asymmetric $\ensuremath{\mu}$ are present in both.

We explore different skyrmion types in the lowest Landau level of graphene at a filling factor $\nu=\pm 1$. In addition to the formation of spin and valley pseudospin skyrmions, we show that another type of spin-valley entangled skyrmions can be stabilized in graphene due to an approximate SU(4) spin-valley symmetry that is affected by sublattice symmetry-breaking terms. These skyrmions have a clear signature in spin-resolved density measurements on the lattice scale, and we discuss the expected patterns for the different skyrmion types., Comment: 5 pages, 3 figures ; 6 pages of supplementary material included ; version accepted for publication in PRL

We study the fractional quantum Hall effect in a bilayer with charge-distribution imbalance induced, for instance, by a bias gate voltage. The bilayer can either be intrinsic or it can be formed spontaneously in wide quantum wells, due to the Coulomb repulsion between electrons. We focus on fractional quantum Hall effect in asymmetric bilayer systems at filling factor nu=4/11 and show that an asymmetric Halperin-like trial wavefunction gives a valid description of the ground state of the system., 6 pages, 5 figures, HMF-20 proceedings

El pdf del artículo es la versión pre-print: arXiv:1305.1448, The electrodynamics of a two-dimensional gas of massless fermions in graphene is studied by a collisionless hydrodynamic approach. A low-energy dispersion relation for the collective modes (plasmons) is derived both in the absence and in the presence of a perpendicular magnetic field. The results for graphene are compared to those for a standard two-dimensional gas of massive electrons. We further compare the results within the classical hydrodynamic approach to the full quantum mechanical calculation in the random phase approximation. The low-energy dispersion relation is shown to be a good approximation at small wave vectors. The limitations of this approach at higher order are also discussed. © 2013 Elsevier Ltd., R.R. acknowledges financial support from the Juan de la Cierva Program (MINECO,Spain). This work was partially funded by Triangle de la Physique.

We study the density of states in graphene at high magnetic field, when the physics is dominated by strong correlations between electrons. In particular we use the method of Haldane pseudopotentials to focus on almost empty or almost filled Landau levels. We find that, besides the usual Landau level peaks, additional peaks ("sashes") appear in the spectrum. The energies of these peaks are determined by the strength of Haldane's pseudopotentials, but as opposed to the usual two-dimensional gas, when there is a one-to-one correspondence between a Haldane pseudopotential and a peak in the spectrum, the energy of each peak is determined in general by a combination of more than one pseudopotential values. An eventual measure of these peak in the density of states spectrum of graphene would allow one to determine the value of the pseudopotentials in graphene, and thus test the strength of the interactions in this system., 9 pages, 1 table

This short theoretical review deals with some essential ingredients for the understanding of the quantum Hall effect in graphene in comparison with the effect in conventional two-dimensional electron systems with a parabolic band dispersion. The main difference between the two systems stems from the "ultra-relativistic" character of the low-energy carriers in graphene, which are described in terms of a Dirac equation, as compared to the non-relativistic Schr\"odinger equation used for electrons with a parabolic band dispersion. In spite of this fundamental difference, the Hall resistance quantisation is universal in the sense that it is given in terms of the universal constant h/e^2 and an integer number, regardless of whether the charge carriers are characterised by Galilean or Lorentz invariance, for non-relativistic or relativistic carriers, respectively., Comment: 9 pages, 4 figures; brief review article for Comptes Rendus de l'Academie des Sciences; references added with respect to previous version

We show that the twisted graphene bilayer can reveal unusual topological properties at low energies, as a consequence of a Dirac-point splitting. These features rely on a symmetry analysis of the electron hopping between the two layers of graphene and we derive a simplified effective low-energy Hamiltonian which captures the essential topological properties of twisted bilayer graphene. The corresponding Landau levels peculiarly reveal a degenerate zero-energy mode which cannot be lifted by strong magnetic fields., 5 pages, 3 figures; published version

We present a theoretical description of Bernstein modes that arise as a result of the coupling between plasmon-like collective excitations (upper-hybrid mode) and inter-Landau-level excitations, in graphene in a perpendicular magnetic field. These modes, which are apparent as avoided level crossings in the spectral function obtained in the random-phase approximation, are described to great accuracy in a phenomenological model. Bernstein modes, which may be measured in inelastic light-scattering experiments or in photo-conductivity spectroscopy, are a manifestation of the Coulomb interaction between the electrons and may be used for a high-precision measurement of the upper-hybrid mode at small non-zero wave vectors., 5 pages. 2 figures; slightly extended version, title changed; accepted for publication in Phys. Rev. B

The semiclassical quantization of cyclotron orbits for two-dimensional Bloch electrons in a coupled two band model with a particle-hole symmetric spectrum is considered. As concrete examples, we study graphene (both mono and bilayer) and boron nitride. The main focus is on wave effects -- such as Berry phase and Maslov index -- occurring at order $\hbar$ in the semiclassical quantization and producing non-trivial shifts in the resulting Landau levels. Specifically, we show that the index shift appearing in the Landau levels is related to a topological part of the Berry phase -- which is basically a winding number of the direction of the pseudo-spin 1/2 associated to the coupled bands -- acquired by an electron during a cyclotron orbit and not to the complete Berry phase, as commonly stated. As a consequence, the Landau levels of a coupled band insulator are shifted as compared to a usual band insulator. We also study in detail the Berry curvature in the whole Brillouin zone on a specific example (boron nitride) and show that its computation requires care in defining the "k-dependent Hamiltonian" H(k), where k is the Bloch wavevector., 15 pages, 6 figures, submitted to EPJB

We review the basic aspects of electrons in graphene (two-dimensional graphite) exposed to a strong perpendicular magnetic field. One of its most salient features is the relativistic quantum Hall effect the observation of which has been the experimental breakthrough in identifying pseudo-relativistic massless charge carriers as the low-energy excitations in graphene. The effect may be understood in terms of Landau quantization for massless Dirac fermions, which is also the theoretical basis for the understanding of more involved phenomena due to electronic interactions. We present the role of electron-electron interactions both in the weak-coupling limit, where the electron-hole excitations are determined by collective modes, and in the strong-coupling regime of partially filled relativistic Landau levels. In the latter limit, exotic ferromagnetic phases and incompressible quantum liquids are expected to be at the origin of recently observed (fractional) quantum Hall states. Furthermore, we discuss briefly the electron-phonon coupling in a strong magnetic field. Although the present review has a dominating theoretical character, a close connection with available experimental observation is intended., 56 pages, 27 figures; published version with minor corrections and updated references

We examine the possibility of creating the Moore-Read Pfaffian in the lowest Landau level when the multicomponent Halperin 331 state (believed to describe quantum Hall bilayers and wide quantum wells at the filling factor $\nu=1/2$) is destroyed by the increase of tunneling. Using exact diagonalization of the bilayer Hamiltonian with short-range and long-range (Coulomb) interactions in spherical and periodic rectangular geometries, we establish that tunneling is a perturbation that drives the 331 state into a compressible composite Fermi liquid, with the possibility for an intermediate critical state that possesses some properties of the Moore-Read Pfaffian. These results are interpreted in the two-component BCS model for Cauchy pairing with a tunneling constraint. We comment on the conditions to be imposed on a system with fluctuating density in order to achieve the stable Pfaffian phase., Comment: 10 pages, 7 figures

Due to its fourfold spin-valley degeneracy, graphene in a strong magnetic field may be viewed as a four-component quantum Hall system. We investigate the consequences of this particular structure on a possible, yet unobserved, fractional quantum Hall effect in graphene within a trial-wavefunction approach and exact-diagonalisation calculations. This trial-wavefunction approach generalises an original idea by Halperin to account for the SU(2) spin in semiconductor heterostructures with a relatively weak Zeeman effect. Whereas the four-component structure at a filling factor nu=1/3 adds simply a SU(4)-ferromagnetic spinor ordering to the otherwise unaltered Laughlin state, the system favours a valley-unpolarised state at nu=2/5 and a completely unpolarised state at nu=4/9. Due to the similar behaviour of the interaction potential in the zero-energy graphene Landau level and the first excited one, we expect these states to be present in both levels., 6 pages, 1 figure; review article for Solid State Comm. (Graphene Week 2008)

We discuss charged topological spin textures in quantum Hall ferromagnets in which the electrons carry a pseudospin as well as the usual spin degree of freedom, as is the case in bilayer GaAs or monolayer graphene samples. We develop a theory which treats spin and pseudospin on a manifestly equal footing, which may also be of help in visualizing the relevant spin textures. We in particular consider the entanglement of spin and pseudospin in the presence of realistic anisotropies. An entanglement operator is introduced which generates families of degenerate Skyrmions with differing entanglement properties. We propose a local characterization of the latter, and touch on the role entangled Skyrmions play in the nuclear relaxation time of quantum Hall ferromagnets., Comment: 13 pages, 3 figures; discussion of collective skyrmion modes added; version accepted for publication in Phys. Rev. B

We propose a scheme to construct the most prominent Abelian and non-Abelian fractional quantum Hall states from K-component Halperin wave functions. In order to account for a one-component quantum Hall system, these SU(K) colors are distributed over all particles by an appropriate symmetrization. Numerical calculations corroborate the picture that the proposed scheme allows for a unification of both Abelian and non-Abelian trial wave functions in the study of one-component quantum Hall systems., 4 pages, 2 figures; revised version, published in Phys. Rev. Lett

We have investigated the absorption spectrum of multilayer graphene in high magnetic fields. The low energy part of the spectrum of electrons in graphene is well described by the relativistic Dirac equation with a linear dispersion relation. However, at higher energies (>500 meV) a deviation from the ideal behavior of Dirac particles is observed. At an energy of 1.25 eV, the deviation from linearity is 40 meV. This result is in good agreement with the theoretical model, which includes trigonal warping of the Fermi surface and higher-order band corrections. Polarization-resolved measurements show no observable electron-hole asymmetry., 4 pages,3 figures

We describe a peculiar fine structure acquired by the in-plane optical phonon at the Gamma-point in graphene when it is brought into resonance with one of the inter-Landau-level transitions in this material. The effect is most pronounced when this lattice mode (associated with the G-band in graphene Raman spectrum) is in resonance with inter-Landau-level transitions 0 -> (+,1) and (-,1) -> 0, at a magnetic field B_0 ~ 30 T. It can be used to measure the strength of the electron-phonon coupling directly, and its filling-factor dependence can be used experimentally to detect circularly polarized lattice modes., 4 pages, 2 figures; mistakes due to an erroneous electron-phonon coupling constant have been corrected; mode splitting is larger than originally expected

Graphene in the quantum Hall regime exhibits a multi-component structure due to the electronic spin and chirality degrees of freedom. While the applied field breaks the spin symmetry explicitly, we show that the fate of the chirality SU(2) symmetry is more involved: the leading symmetry-breaking terms differ in origin when the Hamiltonian is projected onto the central (n=0) rather than any of the other Landau levels. Our description at the lattice level leads to a Harper equation; in its continuum limit, the ratio of lattice constant a and magnetic length l_B assumes the role of a small control parameter in different guises. The leading symmetry-breaking terms are direct (n=0) and exchange (n different from 0) terms, which are algebraically small in a/l_B. We comment on the Haldane pseudopotentials for graphene, and evaluate the easy-plane anisotropy of the graphene ferromagnet., 4 pages, 1 figure; revised version contains a more detailed comparison with experimental results; accepted for publication in PRB

The Dirac equation and its variants are ubiquitous in the low-energy description of topological materials ranging from topological insulators to semimetals as well as their topologically protected surface states. Their spectroscopic properties can be studied theoretically with the help of the corresponding Dirac Hamiltonian. On the one hand, thanks to the underlying Lorentz symmetry of the Dirac Hamiltonians, the relativistic renormalization manifests itself as the renormalization of the band gap resulting in a clear-cut experimental signature in magneto-optics. Most saliently, the renormalization of the band gap is experimentally observed in the magneto-optical measurements of the nodal-line Dirac semimetal niobium diarsenide by our collaborators in LNCMI, Grenoble. Not only are several typical Dirac-type Hamiltonians systematically discussed in order to unveil their relativistic properties, but their interpretation in the language of condensed matter physics is also provided. On the other hand, the magneto-optical signature of additional massive surface states of topological materials is also predicted using linear response theory and the corresponding Dirac Hamiltonians. These massive states called Volkov-Pankratov states arise generically on a smooth surface of topological materials. Their emergence can be equivalently seen as a consequence of a pseudo-magnetic field and/or of a quantum confinement effect. While ARPES cannot unambiguously substantiate the presence of Volkov-Pankratov states, their magneto-optical signature offers a smoking gun thanks to unusual selection rules. Furthermore, the presence of the additional Volkov-Pankratov states gives birth to non-reciprocal surface plasmon modes which provide possibilities in surface plasmonics of topological materials., L'équation de Dirac et ses variantes sont omniprésentes dans la description à basse énergie des matériaux topologiques incluant les isolants topologiques et les semi-métaux topologiques mais aussi leurs états de surface protégés. Leurs propriétés spectroscopiques pourraient être étudiées théoriquement en se basant sur le Hamiltonien de Dirac correspondant. D'une part, grâce à la symétrie de Lorentz sous-jacente des Hamiltonians de Dirac, la renormalisation relativiste se manifeste sous la forme de la renormalisation de gap qui offre une signature expérimentale claire et directe dans la spectroscopie magnéto-optique. En particulier, la renormalisation de gap est observée expérimentalement dans les mesures magnéto-optiques du semi-métal de lignes nodales de Dirac, diarséniure de niobium, par nos collaborateurs du LNCMI à Grenoble. Non seulement plusieurs Hamiltonians de Dirac sont systématiquement discutés afin de dévoiler leurs propriétés relativistes, mais aussi leurs interprétations dans le langage de la physique des solides. D'autre part, la théorie de la réponse linéaire appliquée au Hamiltonien de Dirac correspondant prédit une signature magnéto-optique des états de surface massifs au-delà des états de surface chiraux et topologiquement protégés. Ces états de surface massifs, qui sont aussi appelés états de Volkov-Pankratov, émergent génériquement aux surfaces douces des matériaux topologiques. L'origine de leur apparition peut être considérée de façon équivalente comme une conséquence soit d'un pseudo-champs magnétique soit d'un effet du confinement quantique. En comparant APRES qui ne peut pas établir sans ambiguïté la présence des états de Volkov-Pankratov, leurs signatures magnéto-optiques associées offrent des preuves claires pour leur présence grâce aux règles de sélection exceptionnelles. En plus, la présence des états de Volkov-Pankratov donne lieu aux plasmons de surface non-réciproques qui fournissent les possibilités dans la plasmonique de surface des matériaux topologiques.

The topological properties in quantum Hall systems are thoroughly studied in the past thirty years. In constrast, the geometric aspects of quantum Hall systems are far from being fully understood. In this thesis, I am going to investigate the geometric aspects from the view of the composite fermion Hamiltonian theory and test the response of quantum Hall states under anisotropic perturbation. I find in the presence of anisotropy, composite fermions receive mixing effects between different composite fermion Landau levels. A variational metric can be combined to the composite fermions in order to minimize such an effect. The activation gaps and neutral collective gaps are calculated for a quantum Hall system with tilted magnetic field. The former exhibits a robustness while the latter is susceptible to anisotropic perturbation. The charge density wave states under mass anisotropy are also studied. The bubble phase is found to be strongly suppressed by the mass anisotropy. All the first-order phase transitions present in the isotropic case are replaced by continuous phase transitions in the anisotropic case.; Les propriétés topologiques des systèmes quantiques de Hall ont bien été étudiées au cours des trente dernières années. En contraste, les aspects géométriques des systèmes quantiques de Hall sont loin d’être pleinement compris. Dans cette thèse, je vais étudier les aspects géométriques de la vue de la théorie Hamiltonienne de fermions composites et tester la réponse de les états quantiques de Hall sous perturbation anisotrope. Je trouve dans le présence d'anisotropie, les fermions composites reçoivent des effets de mélange entre les différents niveaux de Landau de fermions composites. Une métrique variationnelle peut être définié aux fermions composites afin de minimiser un tel effet. Les gaps d'activation et les gaps collectifs neutres sont calculé pour un système quantique à effet Hall avec champ magnétique incliné. Le premier se présente une robustesse tandis que le dernier est susceptible de perturbation anisotrope. Les états d'onde de densité de charge sous masse anisotropes sont également étudiés. La phase de bulle se révèle être fortement supprimé par l'anisotropie de masse. Toute les transitions de phase du premier ordre dans le cas isotrope sont remplacées par des transitions de phase continues dans le cas anisotrope.

The topological properties in quantum Hall systems are thoroughly studied in the past thirty years. In constrast, the geometric aspects of quantum Hall systems are far from being fully understood. In this thesis, I am going to investigate the geometric aspects from the view of the composite fermion Hamiltonian theory and test the response of quantum Hall states under anisotropic perturbation. I find in the presence of anisotropy, composite fermions receive mixing effects between different composite fermion Landau levels. A variational metric can be combined to the composite fermions in order to minimize such an effect. The activation gaps and neutral collective gaps are calculated for a quantum Hall system with tilted magnetic field. The former exhibits a robustness while the latter is susceptible to anisotropic perturbation. The charge density wave states under mass anisotropy are also studied. The bubble phase is found to be strongly suppressed by the mass anisotropy. All the first-order phase transitions present in the isotropic case are replaced by continuous phase transitions in the anisotropic case., Les propriétés topologiques des systèmes quantiques de Hall ont bien été étudiées au cours des trente dernières années. En contraste, les aspects géométriques des systèmes quantiques de Hall sont loin d’être pleinement compris. Dans cette thèse, je vais étudier les aspects géométriques de la vue de la théorie Hamiltonienne de fermions composites et tester la réponse de les états quantiques de Hall sous perturbation anisotrope. Je trouve dans le présence d'anisotropie, les fermions composites reçoivent des effets de mélange entre les différents niveaux de Landau de fermions composites. Une métrique variationnelle peut être définié aux fermions composites afin de minimiser un tel effet. Les gaps d'activation et les gaps collectifs neutres sont calculé pour un système quantique à effet Hall avec champ magnétique incliné. Le premier se présente une robustesse tandis que le dernier est susceptible de perturbation anisotrope. Les états d'onde de densité de charge sous masse anisotropes sont également étudiés. La phase de bulle se révèle être fortement supprimé par l'anisotropie de masse. Toute les transitions de phase du premier ordre dans le cas isotrope sont remplacées par des transitions de phase continues dans le cas anisotrope.

This thesis studies skyrmions in the SU(4) quantum Hall ferromagnet. Skyrmions are localized textures in ferromagnetic systems. The graphene monolayer in a strong magnetic field can be viewed as a ferromagnet with electron spin and Dirac-valley pseudospin – Landau levels with different spin and valley are close in energy and form well-separated groups. Within one group, the Coulomb interaction has a manifest SU(4)-invariant form. The model of skyrmions used in this thesis is a classical, static field theory obtained from the variational principle. The model has phenomenological parameters, which depend on substrates and other experimental settings. Based on symmetry analysis, I propose the ansatz for skyrmions at quarter-filling and halffilling of the N = 0 Landau level in graphene monolayer. Energy minimization of single skyrmions is then performed to determine the parameters in the skyrmion ansatz, resulting in different types of spin-valley skyrmions at both filling factors. Large skyrmions are identified in certain ranges of the phenomenological parameters, where the ferromagnetic background of the skyrmion undergoes a phase transition. Single-mode spin-valley waves are also analyzed to characterize the SU(4) quantum Hall ferromagnet. A particular example shows instability of the ferromagnetic ground state., Dans cette thèse, j’étudie les skyrmions dans le ferromagnétique SU(4) d’effet Hall quantique. Les skyrmions sont des textures localisées dans les systèmes ferromagnétiques. La monocouche de graphène dans un fort champ magnétique peut être considérée comme un ferromagnétique avec le spin électronique et le pseudospin de vallée de Dirac. Les niveaux de Landau associés à des spins et des vallées différentes sont proches en l’énergie et forment des groupes bien séparés. Dans un groupe, l’interaction de Coulomb montre forme invariance de SU(4). Le modèle de skyrmions utilisé dans cette thèse est une théorie de champ classique et statique obtenue à partir du principe variationnel. Le modèle comporte des paramètres phénoménologiques, qui dépendent des substrats et d’autres paramètres expérimentaux. Sur la base de l’analyse de symétrie, nous proposons un ansatz pour les skyrmions au quart de remplissage et à la moitié du remplissage du niveau de Landau N = 0 de la monocouche de graphène. La minimisation de l’énergie du skyrmion unique est ensuite effectuée pour déterminer les paramètres dans l’ansatz de skyrmion ansatz, ce qui entraîne différents types de skyrmions spin-valley aux deux facteurs de remplissage. Des grands skyrmions sont identifiés dans certaines gammes des paramètres phénoménologiques, où l’arrière-plan ferromagnétique du skyrmion subit une transition de phase. Les ondes de spin-vallée monomode sont également analysées pour caractériser le ferromagnétique SU(4) d’effet Hall quantique. Un exemple particulier montre l’instabilité de l’état fondamental ferromagnétique.