19 research outputs found
The electron g factor for one-band and two-band extended models of the electron energy spectrum
At present, explicit expressions for the electron g factor in crystals are known only for the following
two cases: either the Fermi energy εF of the electrons lies at the edge of the electron energy
band, ε (kex), or the electron energy spectrum of a crystal can be approximated by the two-band
model. Here we obtain explicit formulas for the g factor in situations when the Fermi level ε F is
close to but does not coincide with the band edge and when the two-band model of the spectrum
includes small corrections from other electron energy bands. In particular, we derive the expressions
that describe the dependences of the g factor on ε F - ε (kex) and on the direction of the magnetic
field for doped semiconductors. The results are applied to III–V semiconductors and to bismuth
The phase of the de Haas–van Alphen oscillations, the Berry phase, and band-contact lines in metals
We point out that measurements of a phase of the de Haas–van Alphen oscillations can give information
on a degeneracy of electron-energy bands in a metal even though this degeneracy occurs far away from its
Fermi level. As an illustration of this statement, the published experimental data on the de Haas–van Alphen
effect in LaRhIn₅, graphite, and zinc are discussed
The Berry phase in graphene and graphite multilayers
We discuss the electron energy spectra and the Berry phases for graphene, a graphite bilayer, and bulk
graphite allowing for a small spin-orbit interaction. If an electron orbit in the Brillouin zone surrounds several
Dirac points (band-contact lines in graphite), one can find relative signs of the Berry phases generated
by these points (lines) by taking into account this interaction
Secondary peak on asymmetric magnetization loop of type-II superconductors
Asymmetric magnetization loops with a second peak effect were parameterized
by the extended critical state model. The magnetic field distribution in a
sample is considered. Expression is suggested for a peak of the critical
current density and corresponding depression on field dependence of the depth
of surface layer with equilibrium magnetization. These functions determine the
width and the asymmetry of a magnetization loop. Asymmetry of the secondary
peak height on magnetization branches for increasing and decreasing field is
reproduced on the computed magnetization curves.Comment: 6 pages, 2 figures, Equation 6 is modified to be f=0 at B=
Exact Solution for the Critical State in Thin Superconductor Strips with Field Dependent or Anisotropic Pinning
An exact analytical solution is given for the critical state problem in long
thin superconductor strips in a perpendicular magnetic field, when the critical
current density j_c(B) depends on the local induction B according to a simple
three-parameter model. This model describes both isotropic superconductors with
this j_c(B) dependence, but also superconductors with anisotropic pinning
described by a dependence j_c(theta) where theta is the tilt angle of the flux
lines away from the normal to the specimen plane
Unified order-disorder vortex phase transition in high-Tc superconductors
The diversity of vortex melting and solid-solid transition lines measured in
different high-T superconductors is explained, postulating a unified
order-disorder phase transition driven by both thermally- and disorder-induced
fluctuations. The temperature dependence of the transition line and the nature
of the disordered phase (solid, liquid, or pinned liquid) are determined by the
relative contributions of these fluctuations and by the pinning mechanism. By
varying the pinning mechanism and the pinning strength one obtains a spectrum
of monotonic and non-monotonic transition lines similar to those measured in
BiSrCaCuO, YBaCuO,
NdCeCuO,
BiPbSrCaCuO and (LaSr)CuOComment: To be published in Phys. Rev. B Rapid Com
A universal Hamiltonian for the motion and the merging of Dirac cones in a two-dimensional crystal
We propose a simple Hamiltonian to describe the motion and the merging of
Dirac points in the electronic spectrum of two-dimensional electrons. This
merging is a topological transition which separates a semi-metallic phase with
two Dirac cones from an insulating phase with a gap. We calculate the density
of states and the specific heat. The spectrum in a magnetic field B is related
to the resolution of a Schrodinger equation in a double well potential. They
obey the general scaling law e_n \propto B^{2/3} f_n(Delta /B^{2/3}. They
evolve continuously from a sqrt{n B} to a linear (n+1/2)B dependence, with a
[(n+1/2)B]^{2/3} dependence at the transition. The spectrum in the vicinity of
the topological transition is very well described by a semiclassical
quantization rule. This model describes continuously the coupling between
valleys associated with the two Dirac points, when approaching the transition.
It is applied to the tight-binding model of graphene and its generalization
when one hopping parameter is varied. It remarkably reproduces the low field
part of the Rammal-Hofstadter spectrum for the honeycomb lattice.Comment: 18 pages, 15 figure
Berry Curvature in Graphene: A New Approach
In the present paper we have directly computed the Berry curvature terms
relevant for Graphene in the presence of an \textit{inhomogeneous} lattice
distortion. We have employed the generalized Foldy Wouthuysen framework,
developed by some of us \cite{ber0,ber1,ber2}. We show that a non-constant
lattice distortion leads to a valley-orbit coupling which is responsible to a
valley-Hall effect. This is similar to the valley-Hall effect induced by an
electric field proposed in \cite{niu2} and is the analogue of the spin-Hall
effect in semiconductors \cite{MURAKAMI, SINOVA}. Our general expressions for
Berry curvature, for the special case of homogeneous distortion, reduce to the
previously obtained results \cite{niu2}. We also discuss the Berry phase in the
quantization of cyclotron motion.Comment: Slightly modified version, to appear in EPJ
Dependence of the vortex configuration on the geometry of mesoscopic flat samples
The influence of the geometry of a thin superconducting sample on the
penetration of the magnetic field lines and the arrangement of vortices are
investigated theoretically. We compare superconducting disks, squares and
triangles with the same surface area having nonzero thickness. The coupled
nonlinear Ginzburg-Landau equations are solved self-consistently and the
important demagnetization effects are taken into account. We calculate and
compare quantities like the free energy, the magnetization, the Cooper-pair
density, the magnetic field distribution and the superconducting current
density for the three geometries. For given vorticity the vortex lattice is
different for the three geometries, i.e. it tries to adapt to the geometry of
the sample. This also influences the stability range of the different vortex
states. For certain magnetic field ranges we found a coexistence of a giant
vortex placed in the center and single vortices toward the corners of the
sample. Also the H-T phase diagram is obtained.Comment: 9 pages, 17 figures (submitted to Phys. Rev. B
Hysteretic behavior of the vortex lattice at the onset of the second peak for HgBaCuO superconductor
By means of local Hall probe ac and dc permeability measurements we
investigated the phase diagram of vortex matter for the HgBaCuO superconductor in the regime near the critical temperature. The second peak
line, , in contrast to what is usually assumed, doesn't terminate
at the critical temperature. Our local ac permeability measurements revealed
pronounced hysteretic behavior and thermomagnetic history effects near the
onset of the second peak, giving evidence for a phase transition of vortex
matter from an ordered qausilattice state to a disordered glass