14,870 research outputs found
Rashba spin-orbit interaction enhanced by graphene in-plane deformations
Graphene consists in a single-layer carbon crystal where 2 electrons
display a linear dispersion relation in the vicinity of the Fermi level,
conveniently described by a massless Dirac equation in spacetime.
Spin-orbit effects open a gap in the band structure and offer perspectives for
the manipulation of the conducting electrons spin. Ways to manipulate
spin-orbit couplings in graphene have been generally assessed by proximity
effects to metals that do not compromise the mobility of the unperturbed system
and are likely to induce strain in the graphene layer. In this work we explore
the gauge fields that result from the uniform
stretching of a graphene sheet under a perpendicular electric field.
Considering such deformations is particularly relevant due to the
counter-intuitive enhancement of the Rashba coupling between 30-50% for small
bond deformations well known from tight-binding and DFT calculations. We report
the accessible changes that can be operated in the band structure in the
vicinity of the K points as a function of the deformation strength and
direction.Comment: 10 pages, 7 figure
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
Observation of the Quantum Zeno and Anti-Zeno effects in an unstable system
We report the first observation of the Quantum Zeno and Anti-Zeno effects in
an unstable system. Cold sodium atoms are trapped in a far-detuned standing
wave of light that is accelerated for a controlled duration. For a large
acceleration the atoms can escape the trapping potential via tunneling.
Initially the number of trapped atoms shows strong non-exponential decay
features, evolving into the characteristic exponential decay behavior. We
repeatedly measure the number of atoms remaining trapped during the initial
period of non-exponential decay. Depending on the frequency of measurements we
observe a decay that is suppressed or enhanced as compared to the unperturbed
system.Comment: 4 pages, 5 figures, submitted to PR
Magneto-Conductance Anisotropy and Interference Effects in Variable Range Hopping
We investigate the magneto-conductance (MC) anisotropy in the variable range
hopping regime, caused by quantum interference effects in three dimensions.
When no spin-orbit scattering is included, there is an increase in the
localization length (as in two dimensions), producing a large positive MC. By
contrast, with spin-orbit scattering present, there is no change in the
localization length, and only a small increase in the overall tunneling
amplitude. The numerical data for small magnetic fields , and hopping
lengths , can be collapsed by using scaling variables , and
in the perpendicular and parallel field orientations
respectively. This is in agreement with the flux through a `cigar'--shaped
region with a diffusive transverse dimension proportional to . If a
single hop dominates the conductivity of the sample, this leads to a
characteristic orientational `finger print' for the MC anisotropy. However, we
estimate that many hops contribute to conductivity of typical samples, and thus
averaging over critical hop orientations renders the bulk sample isotropic, as
seen experimentally. Anisotropy appears for thin films, when the length of the
hop is comparable to the thickness. The hops are then restricted to align with
the sample plane, leading to different MC behaviors parallel and perpendicular
to it, even after averaging over many hops. We predict the variations of such
anisotropy with both the hop size and the magnetic field strength. An
orientational bias produced by strong electric fields will also lead to MC
anisotropy.Comment: 24 pages, RevTex, 9 postscript figures uuencoded Submitted to PR
String equations in Whitham hierarchies: tau-functions and Virasoro constraints
A scheme for solving Whitham hierarchies satisfying a special class of string
equations is presented. The tau-function of the corresponding solutions is
obtained and the differential expressions of the underlying Virasoro
constraints are characterized. Illustrative examples of exact solutions of
Whitham hierarchies are derived and applications to conformal maps dynamics are
indicated.Comment: 26 pages, 2 figure
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