Magneto-Optics of a Weyl Semimetal beyond the Conical Band Approximation: Case Study of TaP.

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.

Landau Velocity for Collective Quantum Hall Breakdown in Bilayer Graphene.

Breakdown of the quantum Hall effect (QHE) is commonly associated with an electric field approaching the inter-Landau-level (LL) Zener field, the ratio of the Landau gap and the 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 super-Poissonian 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. In addition, we show that breakdown is a precursor of an electric-field induced QHE-metal transition.

Magneto-Optical Signature of Massless Kane Electrons in Cd_{3}As_{2}.

We report on optical reflectivity experiments performed on Cd_{3}As_{2} 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.

SU(4) Skyrmions in the ν=±1 Quantum Hall State of Graphene.

We explore different Skyrmion types in the lowest Landau level of graphene at a filling factor ν=±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.

Magnetoplasmons in quasineutral epitaxial graphene nanoribbons.

We present an infrared transmission spectroscopy study of the inter-Landau-level excitations in quasineutral epitaxial graphene nanoribbon arrays. We observed a substantial deviation in energy of the L(0(-1)) → L(1(0)) transition from the characteristic square root magnetic-field dependence of two-dimensional graphene. This deviation arises from the formation of an upper-hybrid mode between the Landau-level transition and the plasmon resonance. In the quantum regime, the hybrid mode exhibits a distinct dispersion relation, markedly different from that expected for conventional two-dimensional systems and highly doped graphene.

Atypical fractional quantum Hall effect in graphene at filling factor 1/3.

We study, with the help of exact-diagonalization calculations, a four-component trial wave function that may be relevant for the recently observed graphene fractional quantum Hall state at a filling factor ν(G) = 1/3. Although it is adiabatically connected to a 1/3 Laughlin state in the upper spin branch, with SU(2) valley-isospin ferromagnetic ordering and a completely filled lower spin branch, it reveals physical properties beyond such a state that is the natural ground state for a large Zeeman effect. Most saliently, it possesses at experimentally relevant values of the Zeeman gap low-energy spin-flip excitations that may be unveiled in inelastic light-scattering experiments.

Bridge between Abelian and non-Abelian fractional quantum Hall states.

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 K-component Halperin wave functions may be a common basis for both Abelian and non-Abelian trial wave functions in the study of one-component quantum Hall systems.

High-energy limit of massless Dirac fermions in multilayer graphene using magneto-optical transmission spectroscopy.

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 approximately 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.

Filling-factor-dependent magnetophonon resonance in graphene.

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} approximately 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 vibrations.

Possible reentrance of the fractional quantum Hall effect in the lowest Landau level.

In the framework of a recently developed model of interacting composite fermions, we calculate the energy of different solid and Laughlin-type liquid phases of spin-polarized composite fermions. The liquid phases have a lower energy than the competing solids around the electronic filling factors nu = 4/11,6/17, and 4/19 and may thus be responsible for the fractional quantum Hall effect at nu = 4/11. The alternation between solid and liquid phases when varying the magnetic field may lead to reentrance phenomena in analogy with the observed reentrant integral quantum Hall effect.