12,332 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
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
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
A Unified Algebraic Approach to Classical Yang-Baxter Equation
In this paper, the different operator forms of classical Yang-Baxter equation
are given in the tensor expression through a unified algebraic method. It is
closely related to left-symmetric algebras which play an important role in many
fields in mathematics and mathematical physics. By studying the relations
between left-symmetric algebras and classical Yang-Baxter equation, we can
construct left-symmetric algebras from certain classical r-matrices and
conversely, there is a natural classical r-matrix constructed from a
left-symmetric algebra which corresponds to a parak\"ahler structure in
geometry. Moreover, the former in a special case gives an algebraic
interpretation of the ``left-symmetry'' as a Lie bracket ``left-twisted'' by a
classical r-matrix.Comment: To appear in Journal of Physics A: Mathematical and Theoretica
Efeito da irrigação no crescimento de plantas de seringueira em viveiro.
Durante tres anos (1981-1983) foi conduzido, em viveiro, na Estacao Experimental do CNPSD, em Manaus-AM, um estudo objetivando avaliar os efeitos da irrigacao e da interacao irrigacao x fertilizantes sobre a altura e diametro de caule da seringueira (Hevea spp.). O delineamento experimental utilizado foi o de blocos ao acaso em esquema fatorial, contendo os seguintes tratamentos: irrigado/adubado (IA), irrigado/nao adubado (INA), nao irrigado/adubado (NIA) e nao irrigado/nao adubado (NINA). Nos tratamentos irrigados, o solo foi abastecido ate a capacidade de campo, a profundidade de 25 cm, toda vez que o potencial matricial por metro atingia -0,5 bar, ao passo que, nos adubados, uma mistura de NPKMg foi parcelada em cinco aplicacoes a cada 30 dias, sendo a primeira efetuada aos 60 dias apos o plantio. Nos dois primeiros anos analisados, verificou-se que os tratamentos irrigados foram significativamente superiores aos nao irrigados, particularmente quando a irrigacao esteve associada a adubacao. Em geral, o comportamenteo das diferentes praticas testadas apresentaram a seguinte ordem decrescente: IA>INA>NIA>NINA. Nao foi observada interacao dos efeitos principais (irrigacao x adubacao)
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