290 research outputs found
Gauge field theory approach to spin transport in a 2D electron gas
We discuss the Pauli Hamiltonian including the spin-orbit interaction within
an U(1) x SU(2) gauge theory interpretation, where the gauge symmetry appears
to be broken. This interpretation offers new insight into the problem of spin
currents in the condensed matter environment, and can be extended to Rashba and
Dresselhaus spin-orbit interactions. We present a few outcomes of the present
formulation: i) it automatically leads to zero spin conductivity, in contrast
to predictions of Gauge symmetric treatments, ii) a topological quantization
condition leading to voltage quantization follows, and iii) spin
interferometers can be conceived in which, starting from a arbitrary incoming
unpolarized spinor, it is always possible to construct a perfect spin filtering
condition.Comment: Invited contribution to Statphys conference, June 2009, Lviv
(Ukraine
Equilibrium currents in a Corbino graphene ring
We address the description of a graphene Corbino disk in the context of a
tight binding approach that includes both kinetic and Rashba spin-orbit
coupling due to an external out-of-plane electric field. Persistent equilibrium
currents are induced by an external magnetic field breaking time reversal
symmetry. By direct diagonalization, we compute the spectrum and focus on the
dispersion near the points at the Fermi level. The dispersion keenly
reproduces that of a continuum model in spite of the complexity of the boundary
conditions. We validate the assumptions of the continuum model in terms of
predominant zig-zag boundaries conditions and weak sub-band coupling. The wave
functions displaying the lowest transverse modes are obtained, showing the
predominance of edge states with charge density at the zig-zag edges. The
persistent charge currents, nevertheless, do not follow the traditional
argument of current cancellation from levels below the Fermi level, and thus
they depart in the tight-binding from those found in the continuum model.Comment: 8 pages, 6 figure
Influence of quenched dilution on the quasi-long-range ordered phase of the 2d XY model
The influence of non magnetic impurities in the 2d XY model is investigated
through Monte Carlo (MC) simulations. The general picture of the transition is
fully understood from the Harris criterion which predicts that the universality
class is unchanged, and the Berezinskii-Kosterlitz-Thouless description of the
topological transition remains valid. We nevertheless address here the question
about the influence of dilution on the quasi-long-range order at low
temperatures. In particular, we study the asymptotic of the pair correlation
function and report the MC estimates for the critical exponent at
different dilutions. In the weak dilution region, our MC calculations are
further supported by simple spin-wave-like calculations.Comment: 8 pages, 7 eps figure
Finite temperature behavior of strongly disordered quantum magnets coupled to a dissipative bath
We study the effect of dissipation on the infinite randomness fixed point and
the Griffiths-McCoy singularities of random transverse Ising systems in chains,
ladders and in two-dimensions. A strong disorder renormalization group scheme
is presented that allows the computation of the finite temperature behavior of
the magnetic susceptibility and the spin specific heat. In the case of Ohmic
dissipation the susceptibility displays a crossover from Griffiths-McCoy
behavior (with a continuously varying dynamical exponent) to classical Curie
behavior at some temperature . The specific heat displays Griffiths-McCoy
singularities over the whole temperature range. For super-Ohmic dissipation we
find an infinite randomness fixed point within the same universality class as
the transverse Ising system without dissipation. In this case the phase diagram
and the parameter dependence of the dynamical exponent in the Griffiths-McCoy
phase can be determined analytically.Comment: 23 pages, 12 figure
Using torsion to manipulate spin currents
We address the problem of quantum particles moving on a manifold
characterised by the presence of torsion along a preferential axis. In fact,
such a torsion may be taylored by the presence of a single screw dislocation,
whose Burgers vector measures the torsion amplitude. The problem, first treated
in the relativistic limit describing fermions that couple minimally to torsion,
is then analysed in the Pauli limit We show that torsion induces a geometric
potential and also that it couples generically to the phase of the wave
function, giving rise to the possibility of using torsion to manipulate spin
currents in the case of spinor wave functions. These results emerge as an
alternative strategy for using screw dislocations in the design of
spintronic-based devices
Continuum model for chiral induced spin selectivity in helical molecules
A minimal model is exactly solved for electron spin transport on a helix.
Electron transport is assumed to be supported by well oriented type
orbitals on base molecules forming a staircase of definite chirality. In a
tight binding interpretation, the SOC opens up an effective
coupling via interbase hopping, introducing spin coupled
transport. The resulting continuum model spectrum shows two Kramers doublet
transport channels with a gap proportional to the SOC. Each doubly degenerate
channel satisfies time reversal symmetry, nevertheless, a bias chooses a
transport direction and thus selects for spin orientation. The model predicts
which spin orientation is selected depending on chirality and bias, changes in
spin preference as a function of input Fermi level and scattering suppression
protected by the SO gap. We compute the spin current with a definite helicity
and find it to be proportional to the torsion of the chiral structure and the
non-adiabatic Aharonov- Anandan phase. To describe room temperature transport
we assume that the total transmission is the result of a product of coherent
steps limited by the coherence length
Quasi-long-range ordering in a finite-size 2D Heisenberg model
We analyse the low-temperature behaviour of the Heisenberg model on a
two-dimensional lattice of finite size. Presence of a residual magnetisation in
a finite-size system enables us to use the spin wave approximation, which is
known to give reliable results for the XY model at low temperatures T. For the
system considered, we find that the spin-spin correlation function decays as
1/r^eta(T) for large separations r bringing about presence of a
quasi-long-range ordering. We give analytic estimates for the exponent eta(T)
in different regimes and support our findings by Monte Carlo simulations of the
model on lattices of different sizes at different temperatures.Comment: 9 pages, 3 postscript figs, style files include
Aperiodic Ising Quantum Chains
Some years ago, Luck proposed a relevance criterion for the effect of
aperiodic disorder on the critical behaviour of ferromagnetic Ising systems. In
this article, we show how Luck's criterion can be derived within an exact
renormalisation scheme for Ising quantum chains with coupling constants
modulated according to substitution rules. Luck's conjectures for this case are
confirmed and refined. Among other outcomes, we give an exact formula for the
correlation length critical exponent for arbitrary two-letter substitution
sequences with marginal fluctuations of the coupling constants.Comment: 27 pages, LaTeX, 1 Postscript figure included, using epsf.sty and
amssymb.sty (one error corrected, some minor changes
Topological transition in a two-dimensional model of liquid crystal
Simulations of nematic-isotropic transition of liquid crystals in two
dimensions are performed using an O(2) vector model characterised by non linear
nearest neighbour spin interaction governed by the fourth Legendre polynomial
. The system is studied through standard Finite-Size Scaling and
conformal rescaling of density profiles of correlation functions. A topological
transition between a paramagnetic phase at high temperature and a critical
phase at low temperature is observed. The low temperature limit is discussed in
the spin wave approximation and confirms the numerical results
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