3,348 research outputs found
Topological minigap in quasi-one-dimensional spin-orbit-coupled semiconductor Majorana wires
The excitation gap above the Majorana fermion (MF) modes at the ends of 1D
topological superconducting (TS) semiconductor wires scales with the bulk
quasiparticle gap E_{qp}. This gap, also called minigap, facilitates
experimental detection of the pristine TS state and MFs at experimentally
accessible temperatures T << E_{qp}. Here we show that the linear scaling of
minigap with E_{qp} can fail in quasi-1D wires with multiple confinement bands
when the applied Zeeman field is greater than or equal to about half of the
confinement-induced bandgap. TS states in such wires have an approximate chiral
symmetry supporting multiple near zero energy modes at each end leading to a
minigap which can effectively vanish. We show that the problem of small minigap
in such wires can be resolved by forcing the system to break the approximate
chirality symmetry externally with a second Zeeman field. Although experimental
signatures such as zero bias peak from the wire ends is suppressed by the
second Zeeman field above a critical value, such a field is required in some
important parameter regimes of quasi-1D wires to isolate the topological
physics of end state MFs. We also discuss the crucial difference of our minigap
calculations from the previously reported minigap results appropriate for
idealized spinless p-wave superconductors and explain why the clustering of
fermionic subgap states around the zero energy Majorana end state with
increasing chemical potential seen in the latter system does not apply to the
experimental TS states in spin-orbit coupled nanowires.Comment: Crucial difference of the present results with previously reported
results for idealized spinless p-wave wires discussed (see conclusion); new
references added; Title changed in response to Editor comment; new version as
accepted in PR
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
Topological Insulators and Metals in Atomic Optical Lattices
We propose the realization of topological quantum states with cold atoms
trapped in an optical lattice. We discuss an experimental setup that generates
a two-dimensional hexagonal lattice in the presence of a light-induced periodic
vector potential, which represents a realization of the Haldane model with cold
atoms. We determine theoretically the conditions necessary for observing the
topological states and show that two of the key conditions are: 1) the
realization of sharp boundaries and 2) the minimization of any smoothly varying
component of the confining potential. We argue that, unlike their condensed
matter counterparts, cold atom topological quantum states can be i) "seen", by
mapping out the characteristic chiral edge states, and ii) controlled, by
controlling the periodic vector potential and the properties of the confining
potential.Comment: 4+ pages, 5 color figure
Intrinsic photoanode band engineering: enhanced solar water splitting efficiency mediated by surface segregation in Ti-doped hematite nanorods
Band engineering is thoroughly employed nowadays targeting technologically
scalable photoanodes for solar water splitting applications. Most often complex
and costly recipes are necessary, for average performances. Here we report very
simple photoanode growth and thermal annealing, with effective band engineering
results. Strongly enhanced photocurrent, of more than 200 %, is measured for
Ti-doped hematite nanorods grown from aqueous solutions and annealed under
Nitrogen atmosphere, compared to air annealed ones. Oxidized surface states and
increased density of charge carriers are found responsible for the enhanced
photoelectrochemical activity, as shown by electrochemical impedance
spectroscopy and synchrotron X-rays spectromicroscopies. They are found related
to oxygen vacancies, acting as n-dopants, and the formation of pseudo- brookite
clusters by surface Ti segregation. Spectro-ptychography is used for the first
time at Ti L3 absorption edge to isolate Ti chemical coordination arising from
pseudo-brookite clusters contribution. Correlated with electron microscopy
investigation and Density Functional Theory (DFT) calculations, our data
unambiguously prove the origin of the enhanced photoelectrochemical activity of
N2-annealed Ti-doped hematite nanorods. Finally, we present here a handy and
cheap surface engineering method beyond the known oxygen vacancy doping,
allowing a net gain in the photoelectrochemical activity for the hematite-based
photoanodes.Comment: 2 parts: first main manuscript with 39 pages, second supplementary
informations with 14 page
Breakdown of Strong-Coupling Perturbation Theory in Doped Mott Insulators
We show that doped Mott insulators, such as the copper-oxide superconductors,
are asymptotically slaved in that the quasiparticle weight, , near
half-filling depends critically on the existence of the high energy scale set
by the upper Hubbard band. In particular, near half filling, the following
dichotomy arises: when the high energy scale is integrated out but Z=0
in the thermodynamic limit when it is retained. Slavery to the high energy
scale arises from quantum interference between electronic excitations across
the Mott gap. Broad spectral features seen in photoemission in the normal state
of the cuprates are argued to arise from high energy slavery.Comment: Published versio
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
An improved method for pricing and hedging long dated American options
The majority of quasi-analytic pricing methods for American options are efficient near maturity but are prone to larger errors when time-to-maturity increases. We introduce a new methodology to increase the accuracy of almost any existing quasi-analytic approach in pricing long-maturity American options. The new methodology, called the “extension-method”, relies on an approximation of the optimal exercise price near the beginning of the contract combined with existing pricing approaches so that the maturity range for which small errors are attainable is extended. Our method retains the quasi-analytic nature of the methods it improves. Generic quasi-analytic formulae for the price of an American put as well as for its hedging parameter are derived. Our scenarios-based numerical study indicates that our method considerably improves both the pricing and the hedging performance of a number of established approaches for a wide range of maturities. The superiority of this approach is illustrated with real financial data by considering S&P 100TM LEAPS® options traded from January 2008 to May 2015
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
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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