Landau levels of scalar QED in time-dependent magnetic fields

The Landau levels of scalar QED undergo continuous transitions under a homogeneous, time-dependent magnetic field. We analytically formulate the Klein–Gordon equation for a charged spinless scalar as a Cauchy initial value problem in the two-component first order formalism and then put forth a measure that classifies the quantum motions into the adiabatic change, the nonadiabatic change, and the sudden change. We find the exact quantum motion and calculate the pair-production rate when the magnetic field suddenly changes as a step function.     

Landau levels in deformed bilayer graphene at low magnetic fields

We review the effect of uniaxial strain on the low-energy electronic dispersion and Landau level structure of bilayer graphene. Based on the tight-binding approach, we derive a strain-induced term in the low-energy Hamiltonian and show how strain affects the low-energy electronic band structure. Depending on the magnitude and direction of applied strain, we identify three regimes of qualitatively different electronic dispersions. We also show that in a weak magnetic field, sufficient strain results in the filling factor ν=±4 being the most stable in the quantum Hall effect measurement, instead of ν=±8 in unperturbed bilayer at a weak magnetic field. To mention, in one of the strain regimes, the activation gap at ν=±4 is, down to very low fields, weakly dependent on the strength of the magnetic field.     

Landau levels in graphite intercalation compounds

The first calculation of the magnetic energy level structure of a graphite intercalation compound is presented. The calculational technique exploits the staging symmetry through the k_z-axis zone folding of the magnetic energy levels of the graphite π-bands. The results are applicable to the interpretation of the magnetoreflection and de Haas-van Alphen type experiments in intercalated graphite.     

Quantum rings in tilted magnetic fields

The electronic states of semiconductor quantum rings (QRs) under tilted magnetic fields are studied in the framework of the effective mass and envelope function approximations. For an axial field, the orbital Zeeman contribution prevails leading to the well-known Aharanov–Bohm spectrum, but it slowly decreases as the magnetic field direction declines. For an in-plane field, only the diamagnetic shift survives and it leads to the formation of double quantum well solutions, this result being relevant for experimental techniques which use in-plane magnetic fields to determine the spin of QR ground states. We also investigate the magnetic response of partially overlapped QRs, which are characteristic of high-density samples of self-assembled rings, and find that the spectrum is quite sensitive to ring coupling.     

Exact diagonalization of Landau levels in Bernal graphite

The magneto-electronic structure of Bernal graphite is derived by the Peierls tight-binding model. Seven important atomic interactions are taken into account and the corresponding Hamiltonian matrix is rearranged as a band-like form for efficient calculation. The results derived from this exact diagonalization method demonstrate that interlayer interactions play remarkable roles in Landau level structures.     

Landau level spectrum for bilayer graphene in a tilted magnetic field

We consider a graphene bilayer in a constant magnetic field of arbitrary orientation, i.e., tilted with respect to the graphene plane. In the low energy approximation to the tight-binding model with Peierls substitution, we find the Landau level spectrum analytically in terms of spheroidal functions and the respective eigenvalues. We compare our result to the perpendicular and purely in-plane field cases. In the limit of perpendicular field we reproduce the known equidistant spectrum for Landau levels. In the opposite limit of large in-plane field this spectrum becomes two-fold degenerate, which is a consequence of Dirac point splitting induced by the in-plane field.     

STS observations of Landau levels at graphite surfaces

Scanning tunneling spectroscopy (STS) measurements were made on surfaces of two different kinds of graphite samples, Kish graphite and highly oriented pyrolytic graphite (HOPG), at very low temperatures and in high magnetic fields. We observed a series of peaks in the tunnel spectra associated with Landau quantization of the quasi-two-dimensional electrons and holes. A comparison with the calculated local density of states at the surface layers allows us to identify Kish graphite as bulk graphite and HOPG as graphite with a finite thickness of 40 layers. This explains the qualitative difference between the two graphites reported in the recent transport measurements which suggested the quantum-Hall effect in HOPG. This work demonstrates how powerful the combined approach between the high quality STS measurement and the first-principles calculation is in material science.     

Angular magnetoresistance oscillations in bilayers in tilted magnetic fields

Angular magnetoresistance oscillations (AMRO) were originally discovered in organic conductors and then found in many other layered metals. It should be possible to observe AMRO to semiconducting bilayers as well. Here we present an intuitive geometrical interpretation of AMRO as the Aharonov–Bohm interference effect, both in real and momentum spaces, for balanced and imbalanced bilayers. Applications to the experiments with bilayers in tilted magnetic fields in the metallic state are discussed. We speculate that AMRO may be also observed when each layer of the bilayer is in the composite-fermion state.     

Quenching of non-local electron transport in tilted magnetic fields

A variety of transport phenomena observed at laterally confined two- dimensional electron systems (2DES) prove the occurrence of non-local contributions to the electronic conductance in these systems. However, this non-local regime accompanied by a non-equilibrium population of the edge states with respect to the 2D bulk state is quenched at rather low values of current-driving electric fields.We analyse the non-Ohmic behaviour of SdH oscillations at GaAs/GaAlAs Quantum Hall conductors on the basis of a model including edge and bulk conduction and deduce the non-equilibrium population of edge and bulk states quantitatively.The spatial separation between edge and bulk states was changed by tilting the samples with respect to the magnetic field. The resulting angular dependences of equilibration parameters could be quantitatively explained by the change of the ratio of spin splitting to cyclotron energy being present in 2DES in tilted magnetic fields.PACS index numbers: 73.20.Dx; 73.40.Hm     

Shell effects in quantum dots in tilted magnetic fields

The motion of a few electrons in a three-dimensional harmonic oscillator potential under the influence of a homogeneous magnetic field of arbitrary direction is studied. The ground state of the Fermi system is obtained by minimizing the total energy with regard to the confining frequencies. From this a dependence of the equilibrium shape on the electron number, the magnetic field parameters and the slab thickness is found.     

Correlations of two-dimensional super-paramagnetic colloids in tilted external magnetic fields

We outline the formalism of liquid integral equation theory for anisotropic interactions in two dimensions and subsequently apply this theory to one-component super-paramagnetic particles exposed to a tilted magnetic field. Inhomogeneous local ordering of the particles is observed for different in-plane directions. The anisotropy of the interaction as well as of the liquid structure is increased by increasing the tilt angle. Furthermore, the particles favour an alignment in the direction of the in-plane component of the magnetic field. For increasing tilt angle, the anisotropy of the structural correlations is qualitatively similar to that of the corresponding solid lattice which is stable at lower temperatures. However, the mean-square displacements behave qualitatively different in the solid and fluid phases as a function of the tilt angle.     

Cyclotron resonance of inversion electrons on InSb in tilted magnetic fields

Cyclotron resonance (CR) of inversion electrons on InSb is studied in magnetic fields tilted away from the surface normal. Particularly, a pronounced splitting of the CR signals into two distinct resonances is observed. When the magnetic field is parallel to the inversion layer one of the two resonances vanishes and the other evolves into a bulk like CR at sufficiently low electron densities and in sufficiently high resonance magnetic fields. The different absorption modes are explained by a strong coupling of the electric and magnetic quantization on InSb in tilted magnetic fields.     

Interlayer Aharonov-Bohm interference in tilted magnetic fields in quasi-one-dimensional organic conductors

Different types of angular magnetoresistance oscillations in quasi-one-dimensional layered materials, such as organic conductors (TMTSF)2X, are explained in terms of Aharonov-Bohm interference in interlayer electron tunneling. A two-parameter pattern of oscillations for generic orientations of a magnetic field is visualized and compared to the experimental data. Connections with angular magnetoresistance oscillations in other layered materials are discussed.     

Low temperature magnetic susceptibility of pure graphite in strong magnetic fields

In this paper we present a calculation of the magnetic susceptibility of pure graphite for fields higher than 65kG so that at low temperatures, the only contribution to the susceptibility comes from the two ‘near’ n = 0 levels which intersect the Fermi surface. Under these conditions the susceptibility can be very different from its zero-field value and our results show that graphite which is strongly diamagnetic for low fields, becomes strongly paramagnetic at high fields.