3,112 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
Disentangling Majorana fermions from topologically trivial low-energy states in semiconductor Majorana wires
Coherent control of injection currents in high-quality films of Bi2Se3
Films of the topological insulator Bi2Se3 are grown by molecular beam epitaxy
with in-situ reflection high-energy electron diffraction. The films are shown
to be high-quality by X-ray reflectivity and diffraction and atomic-force
microscopy. Quantum interference control of photocurrents is observed by
excitation with harmonically related pulses and detected by terahertz
radiation. The injection current obeys the expected excitation irradiance
dependence, showing linear dependence on the fundamental pulse irradiance and
square-root irradiance dependence of the frequency-doubled optical pulses. The
injection current also follows a sinusoidal relative-phase dependence between
the two excitation pulses. These results confirm the third-order nonlinear
optical origins of the coherently controlled injection current. Experiments are
compared to a tight-binding band structure to illustrate the possible optical
transitions that occur in creating the injection current.Comment: 11 pages, 3 figure, journal articl
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
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
Local Dynamics and Strong Correlation Physics I: 1D and 2D Half-filled Hubbard Models
We report on a non-perturbative approach to the 1D and 2D Hubbard models that
is capable of recovering both strong and weak-coupling limits. We first show
that even when the on-site Coulomb repulsion, U, is much smaller than the
bandwith, the Mott-Hubbard gap never closes at half-filling in both 1D and 2D.
Consequently, the Hubbard model at half-filling is always in the
strong-coupling non-perturbative regime. For both large and small U, we find
that the population of nearest-neighbour singlet states approaches a value of
order unity as as would be expected for antiferromagnetic order. We
also find that the double occupancy is a smooth monotonic function of U and
approaches the anticipated non-interacting limit and large U limits. Finally,
in our results for the heat capacity in 1D differ by no more than 1% from the
Bethe ansatz predictions. In addition, we find that in 2D, the heat capacity vs
T for different values of U exhibits a universal crossing point at two
characteristic temperatures as is seen experimentally in a wide range of
strongly-correlated systems such as , , and . The
success of this method in recovering well-established results that stem
fundamentally from the Coulomb interaction suggests that local dynamics are at
the heart of the physics of strongly correlated systems.Comment: 10 pages, 16 figures included in text, Final version for publication
with a reference added and minor corrections. Phys. Rev. B, in pres
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