3,185 research outputs found
Strong Coupling Theory for Interacting Lattice Models
We develop a strong coupling approach for a general lattice problem. We argue
that this strong coupling perspective represents the natural framework for a
generalization of the dynamical mean field theory (DMFT). The main result of
this analysis is twofold: 1) It provides the tools for a unified treatment of
any non-local contribution to the Hamiltonian. Within our scheme, non-local
terms such as hopping terms, spin-spin interactions, or non-local Coulomb
interactions are treated on equal footing. 2) By performing a detailed
strong-coupling analysis of a generalized lattice problem, we establish the
basis for possible clean and systematic extensions beyond DMFT. To this end, we
study the problem using three different perspectives. First, we develop a
generalized expansion around the atomic limit in terms of the coupling
constants for the non-local contributions to the Hamiltonian. By analyzing the
diagrammatics associated with this expansion, we establish the equations for a
generalized dynamical mean-field theory (G-DMFT). Second, we formulate the
theory in terms of a generalized strong coupling version of the Baym-Kadanoff
functional. Third, following Pairault, Senechal, and Tremblay, we present our
scheme in the language of a perturbation theory for canonical fermionic and
bosonic fields and we establish the interpretation of various strong coupling
quantities within a standard perturbative picture.Comment: Revised Version, 17 pages, 5 figure
Nodal/Antinodal Dichotomy and the Two Gaps of a Superconducting Doped Mott Insulator
We study the superconducting state of the hole-doped two-dimensional Hubbard
model using Cellular Dynamical Mean Field Theory, with the Lanczos method as
impurity solver. In the under-doped regime, we find a natural decomposition of
the one-particle (photoemission) energy-gap into two components. The gap in the
nodal regions, stemming from the anomalous self-energy, decreases with
decreasing doping. The antinodal gap has an additional contribution from the
normal component of the self-energy, inherited from the normal-state pseudogap,
and it increases as the Mott insulating phase is approached.Comment: Corrected typos, 4.5 pages, 4 figure
Spin relaxation in a generic two-dimensional spin-orbit coupled system
We study the relaxation of a spin density injected into a two-dimensional
electron system with generic spin-orbit interactions. Our model includes the
Rashba as well as linear and cubic Dresselhaus terms. We explicitly derive a
general spin-charge coupled diffusion equation. Spin diffusion is characterized
by just two independent dimensionless parameters which control the interplay
between different spin-orbit couplings. The real-time representation of the
diffuson matrix (Green's function of the diffusion equation) is evaluated
analytically. The diffuson describes space-time dynamics of the injected spin
distribution. We explicitly study two regimes: The first regime corresponds to
negligible spin-charge coupling and is characterized by standard charge
diffusion decoupled from the spin dynamics. It is shown that there exist
several qualitatively different dynamic behaviors of the spin density, which
correspond to various domains in the spin-orbit coupling parameter space. We
discuss in detail a few interesting phenomena such as an enhancement of the
spin relaxation times, real space oscillatory dynamics, and anisotropic
transport. In the second regime, we include the effects of spin-charge
coupling. It is shown that the spin-charge coupling leads to an enhancement of
the effective charge diffusion coefficient. We also find that in the case of
strong spin-charge coupling, the relaxation rates formally become complex and
the spin/charge dynamics is characterized by real time oscillations. These
effects are qualitatively similar to those observed in spin-grating experiments
[Weber et al., Nature 437, 1330 (2005)].Comment: 18 pages, 7 figure
Electronic and magnetic properties of metallic phases under coexisting short-range interaction and diagonal disorder
We study a three-dimensional Anderson-Hubbard model under the coexistence of
short-range interaction and diagonal disorder within the Hartree-Fock
approximation. We show that the density of states at the Fermi energy is
suppressed in the metallic phases near the metal-insulator transition as a
proximity effect of the soft Hubbard gap in the insulating phases. The
transition to the insulator is characterized by a vanishing DOS in contrast to
formation of a quasiparticle peak at the Fermi energy obtained by the dynamical
mean field theory in pure systems. Furthermore, we show that there exist frozen
spin moments in the paramagnetic metal.Comment: 4 pages, 2 figures, published versio
Surface states, Friedel oscillations, and spin accumulation in p-doped semiconductors
We consider a hole-doped semiconductor with a sharp boundary and study the
boundary spin accumulation in response to a charge current. First, we solve
exactly a single-particle quantum mechanics problem described by the isotropic
Luttinger model in half-space and construct an orthonormal basis for the
corresponding Hamiltonian. It is shown that the complete basis includes two
types of eigenstates. The first class of states contains conventional incident
and reflected waves, while the other class includes localized surface states.
Second, we consider a many-body system in the presence of a charge current
flowing parallel to the boundary. It is shown that the localized states
contribute to spin accumulation near the surface. We also show that the spin
density exhibits current-induced Friedel oscillations with three different
periods determined by the Fermi momenta of the light and heavy holes. We find
an exact asymptotic expression for the Friedel oscillations far from the
boundary. We also calculate numerically the spin density profile and compute
the total spin accumulation, which is defined as the integral of the spin
density in the direction perpendicular to the boundary. The total spin
accumulation is shown to fit very well the simple formula S ~(1 - m_L/m_H)^2,
where m_L and m_H are the light- and heavy-hole masses. The effects of disorder
are discussed. We estimate the spin relaxation time in the Luttinger model and
argue that spin physics cannot be described within the diffusion approximation.Comment: 22 pages, 8 color figure
Surface composition of BaTiO3/SrTiO3(001) films grown by atomic oxygen plasma assisted molecular beam epitaxy
We have investigated the growth of BaTiO3 thin films deposited on pure and 1%
Nb-doped SrTiO3(001) single crystals using atomic oxygen assisted molecular
beam epitaxy (AO-MBE) and dedicated Ba and Ti Knudsen cells. Thicknesses up to
30 nm were investigated for various layer compositions. We demonstrate 2D
growth and epitaxial single crystalline BaTiO3 layers up to 10 nm before
additional 3D features appear; lattice parameter relaxation occurs during the
first few nanometers and is completed at {\guillemotright}10 nm. The presence
of a Ba oxide rich top layer that probably favors 2D growth is evidenced for
well crystallized layers. We show that the Ba oxide rich top layer can be
removed by chemical etching. The present work stresses the importance of
stoichiometry and surface composition of BaTiO3 layers, especially in view of
their integration in devices.Comment: In press in J. Appl. Phy
Pheochromocytoma – clinical manifestations, diagnosis and current perioperative management
Pheochromocytoma is a neuroendocrine tumor characterized by the excessive production of catecholamines (epinephrine, norepinephrine, and dopamine). The diagnosis is suspected due to hypertensive paroxysms, associated with vegetative phenomena, due to the catecholaminergic hypersecretion. Diagnosis involves biochemical tests that reveal elevated levels of catecholamine metabolites (metanephrine and normetanephrine). Functional imaging, such as 123I-metaiodobenzylguanidine scintigraphy (123I-MIBG), has increased specificity in identifying the catecholamine-producing tumor and its metastases. The gold-standard treatment for patients with pheochromocytoma is represented by the surgical removal of the tumor. Before surgical resection, it is important to optimize blood pressure and intravascular volume in order to avoid negative hemodynamic events
Majorana Fermions in Semiconductor Nanowires
We study multiband semiconducting nanowires proximity-coupled with an s-wave
superconductor and calculate the topological phase diagram as a function of the
chemical potential and magnetic field. The non-trivial topological state
corresponds to a superconducting phase supporting an odd number of pairs of
Majorana modes localized at the ends of the wire, whereas the non-topological
state corresponds to a superconducting phase with no Majoranas or with an even
number of pairs of Majorana modes. Our key finding is that multiband occupancy
not only lifts the stringent constraint of one-dimensionality, but also allows
having higher carrier density in the nanowire. Consequently, multiband
nanowires are better-suited for stabilizing the topological superconducting
phase and for observing the Majorana physics. We present a detailed study of
the parameter space for multiband semiconductor nanowires focusing on
understanding the key experimental conditions required for the realization and
detection of Majorana fermions in solid-state systems. We include various
sources of disorder and characterize their effects on the stability of the
topological phase. Finally, we calculate the local density of states as well as
the differential tunneling conductance as functions of external parameters and
predict the experimental signatures that would establish the existence of
emergent Majorana zero-energy modes in solid-state systems.Comment: 29 pages, 30 figure
Structural and magnetic properties of CoPt mixed clusters
In this present work, we report a structural and magnetic study of mixed
Co58Pt42 clusters. MgO, Nb and Si matrix can be used to embed clusters,
avoiding any magnetic interactions between particles. Transmission Electron
Microscopy (TEM) observations show that Co58Pt42 supported isolated clusters
are about 2nm in diameter and crystallized in the A1 fcc chemically disordered
phase. Grazing Incidence Small Angle X-ray Scattering (GISAXS) and Grazing
Incidence Wide Angle X-ray Scattering (GIWAXS) reveal that buried clusters
conserve these properties, interaction with matrix atoms being limited to their
first atomic layers. Considering that 60% of particle atoms are located at
surface, this interactions leads to a drastic change in magnetic properties
which were investigated with conventional magnetometry and X-Ray Magnetic
Circular Dichro\"{i}sm (XMCD). Magnetization and blocking temperature are
weaker for clusters embedded in Nb than in MgO, and totally vanish in silicon
as silicides are formed. Magnetic volume of clusters embedded in MgO is close
to the crystallized volume determined by GIWAXS experiments. Cluster can be
seen as a pure ferromagnetic CoPt crystallized core surrounded by a
cluster-matrix mixed shell. The outer shell plays a predominant role in
magnetic properties, especially for clusters embedded in niobium which have a
blocking temperature 3 times smaller than clusters embedded in MgO
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