787 research outputs found
Response functions in multicomponent Luttinger liquids
We derive an analytic expression for the zero temperature Fourier transform
of the density-density correlation function of a multicomponent Luttinger
liquid with different velocities. By employing Schwinger identity and a
generalized Feynman identity exact integral expressions are derived, and
approximate analytical forms are given for frequencies close to each component
singularity. We find power-like singularities and compute the corresponding
exponents. Numerical results are shown for the case of three components.Comment: 16 pages, two EPS figure
Raman Scattering cross section of Spin Ladders
The Raman scattering spectra from magnetic excitations in an
antiferromagnetic spin-1/2 two leg ladder is investigated for weak and strong
interladder coupling. In the first case, a cusp in the Raman intensityis
obtained at a frequency twice the gap. In the second case, a peak at twice the
gap replaces the cusp. We discuss the relevance of our calculation to recent
experiments on CaV2O5 and Sr14Cu24O41.Comment: RevTeX, 4 pages, 2 EPS Figure
Minimal model of point contact Andreev reflection spectroscopy of multiband superconductors
We formulate a minimal model of point contact Andreev reflection spectroscopy
of a normal- metal/multiband superconductor interface. The theory generalizes
the Blonder-Tinkham-Klapwijk (BTK) formulation to a multiband superconductor
and it is based on the quantum waveguides theory. The proposed approach allows
an analytic evaluation of the Andreev and normal reflection coefficients and
thus is suitable for a data fitting of point contact experiments. The obtained
differential conductance curves present distinctive features similar to the
ones measured in the experiments on multiband systems, like the iron-based
pnictides and the MgB2.Comment: 5 pages, 4 figure
Magnetostriction in an array of spin chains under magnetic field
We consider an array of XX spin-1/2 chains coupled to acoustic phonons and
placed in a magnetic field. Treating the phonons in the mean field
approximation, we show that this system presents a first order transition as a
function of the magnetic field between a partially magnetized distorted state
and the fully polarized undistorted state at low temperature. This behavior
results from the magnetostriction of the coupled chain system. A dip in the
elastic constant of the material near the saturation field along with an
anomaly in the magnetic susceptibility is predicted. We also predict the
contraction of the material as the magnetic field is reduced (positive
magnetostriction) and the reciprocal effect i.e. a decrease of magnetization
under applied pressure. At higher temperature, the first order transition is
replaced by a crossover. However, the anomalies in the susceptibilities in the
system near the saturation field are still present. We discuss the relevance of
our analysis in relation to recent experiments on spin-1/2 chain and ladder
materials in strong magnetic fields.Comment: 23 pages, Revtex 4, more detailed discussion of the connection with
Bose-Einstein condensation. Discussion of long range ordering by interladder
exchange, comparison with mean field theory. Some extra references adde
Modification of the Bloch law in ferromagnetic nanostructures
The temperature dependence of magnetization in ferromagnetic nanostructures
(e.g., nanoparticles or nanoclusters) is usually analyzed by means of an
empirical extension of the Bloch law sufficiently flexible for a good fitting
to the observed data and indicates a strong softening of magnetic coupling
compared to the bulk material. We analytically derive a microscopic
generalization of the Bloch law for the Heisenberg spin model which takes into
account the effects of size, shape and various surface boundary conditions. The
result establishes explicit connection to the microscopic parameters and
differs significantly from the existing description. In particular, we show
with a specific example that the latter may be misleading and grossly
overestimates magnetic softening in nanoparticles. It becomes clear why the
usual dependence appears to be valid in some nanostructures, while
large deviations are a general rule. We demonstrate that combination of
geometrical characteristics and coupling to environment can be used to
efficiently control magnetization and, in particular, to reach a magnetization
higher than in the bulk material.Comment: 7 pages, 4 figure
Electrically Controlled Pumping of Spin Currents in Topological Insulators
Pure spin currents are shown to be generated by an electrically controlled
quantum pump applied at the edges of a topological insulator. The electric
rather than the more conventional magnetic control offers several advantages
and avoids, in particular, the necessity of delicate control of magnetization
dynamics over tiny regions. The pump is implemented by pinching the sample at
two quantum point contacts and phase modulating two external gate voltages
between them. The spin current is generated for the full range of parameters.
On the other hand, pumping via amplitude modulation of the inter-boundary
couplings generates both charge and spin currents, with a pure charge current
appearing only for special values of the parameters for which the Bohm-Aharonov
flux takes integer values. Our setup can therefore serve to fingerprint the
helical nature of the edges states with the zeros of the pumped spin and charge
currents occurring at distinct universal locations where the Fabry-Perot or the
Aharonov-Bohm phases take integer values.Comment: 5 pages, 5figure
Impurity effects on Fabry-Perot physics of ballistic carbon nanotubes
We present a theoretical model accounting for the anomalous Fabry-Perot
pattern observed in the ballistic conductance of a single-wall carbon
nanotubes. Using the scattering field theory, it is shown that the presence of
a limited number of impurities along the nanotube can be identified by a
measurement of the conductance and their position determined. Impurities can be
made active or silent depending on the interaction with the substrate via the
back-gate. The conceptual steps for designing a bio-molecules detector are
briefly discussed.Comment: 4 pages, 4 figure
Quantum Bose Josephson Junction with binary mixtures of BECs
We study the quantum behaviour of a binary mixture of Bose-Einstein
condensates (BEC) in a double-well potential starting from a two-mode
Bose-Hubbard Hamiltonian. We focus on the small tunneling amplitude regime and
apply perturbation theory up to second order. Analytical expressions for the
energy eigenvalues and eigenstates are obtained. Then the quantum evolution of
the number difference of bosons between the two potential wells is fully
investigated for two different initial conditions: completely localized states
and coherent spin states. In the first case both the short and the long time
dynamics is studied and a rich behaviour is found, ranging from small amplitude
oscillations and collapses and revivals to coherent tunneling. In the second
case the short-time scale evolution of number difference is determined and a
more irregular dynamics is evidenced. Finally, the formation of Schroedinger
cat states is considered and shown to affect the momentum distribution.Comment: 14 pages, 4 figure
Spin-torque generation by dc or ac voltages in magnetic layered structures
A general expression of the current induced spin torque in a magnetic layered
structure in the presence of external dc or ac voltages is derived in the
framework of the scattering matrix approach. A detailed analysis is performed
for a magnetic-nonmagnetic-magnetic trilayer connected to external leads in the
presence of dc voltage bias in the ballistic regime. Alternatively, the
possibility of producing spin torque by means of the adiabatic ac modulation of
external gate voltages (quantum pumping) is proposed and discussed
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