2,787 research outputs found
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
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
Effect of Subband Landau Level Coupling to the Linearly Dispersing Collective Mode in a Quantum Hall Ferromagnet
In a recent experiment (Phys. Rev. Lett. {\bf 87}, 036903 (2001)), Spielman
et al observed a linearly dispersing collective mode in quantum Hall
ferromagnet. While it qualitatively agrees with the Goldstone mode dispersion
at small wave vector, the experimental mode velocity is slower than that
calculated by previous theories by a factor about 0.55. A better agreement with
the experimental data may possibly be achieved by taking the subband Landau
level coupling into account due to the finiteness of the layer thickness. A
novel coupling of quantum fluctuation to the tunneling is briefly discussed.Comment: 4 pages; published versio
Impurity Effects on Superconductivity on Surfaces of Topological Insulators
A two-dimensional superconductor (SC) on surfaces of topological insulators
(TIs) is a mixture of s-wave and helical p-wave components when induced by
s-wave interactions, since spin and momentum are correlated. On the basis of
the Abrikosov-Gor'kov theory, we reveal that unconventional SCs on the surfaces
of TIs are stable against time-reversal symmetric (TRS) impurities within a
region of small impurity concentration. Moreover, we analyze the stability of
the SC on the surfaces of TIs against impurities beyond the perturbation theory
by solving the real-space Bogoliubov-de Gennes equation for an effective
tight-binding model of a TI. We find that the SC is stable against strong TRS
impurities. The behaviors of bound states around an impurity suggest that the
SC on the surfaces of TIs is not a topological SC.Comment: 17 pages, 14 figures, to appear in J. Phys. Soc. Jp
Stability of Unconventional Superconductivity on Surfaces of Topological Insulators
Superconductivity on the surface of topological insulators is known to be
anisotropic and unconventional in that the symmetry is the mixture of s-wave
and nodeless p-wave component. In contrast to Anderson's theorem for the
insensitivity of the s-wave superconducting critical temperature to the
nonmagnetic (time-reversal symmetric (TRS)) impurities, anisotropic
superconductors including nodeless p-wave one are in general fragile even with
small concentration of the TRS impurities. By employing the Abrikosov-Gor'kov
theory, we clarify that this type of unconventional superconductivity emergent
on the surface state of the strong topological insulators robustly survive
against TRS impurities
How to realize a robust practical Majorana chain in a quantum dot-superconductor linear array
Semiconducting nanowires in proximity to superconductors are promising
experimental systems for Majorana fermions, which may ultimately be used as
building blocks for topological quantum computers. A serious challenge in the
experimental realization of the Majorana fermions is the supression of
topological superconductivity by disorder. We show that Majorana fermions
protected by a robust topological gap can occur at the ends of a chain of
quantum dots connected by s-wave superconductors. In the appropriate parameter
regime, we establish that the quantum dot/superconductor system is equivalent
to a 1D Kitaev chain, which can be tuned to be in a robust topological phase
with Majorana end modes even in the case where the quantum dots and
superconductors are both strongly disordered. Such a spin-orbit coupled quantum
dot - s-wave superconductor array provides an ideal experimental platform for
the observation of non-Abelian Majorana modes.Comment: 8 pages; 3 figures; version 2: Supplementary material updated to
include more general proof for localized Majorana fermion
Dimensional crossover in topological matter: Evolution of the multiple Dirac point in the layered system to the flat band on the surface
We consider the dimensional crossover in the topological matter, which
involves the transformation of different types of topologically protected
zeroes in the fermionic spectrum. In the considered case, the multiple Dirac
(Fermi) point in quasi 2-dimensional system evolves into the flat band on the
surface of the 3-dimensional system when the number of atomic layers increases.
This is accompanied by formation of the spiral nodal lines in the bulk. We also
discuss the topological quantum phase transition at which the surface flat band
shrinks and changes its chirality, while the nodal spiral changes its helicity.Comment: 13 pages, 7 figure
Density Induced Interchange of Anisotropy Axes at Half-Filled High Landau Levels
We observe density induced 90 rotations of the anisotropy axes in
transport measurements at half-filled high Landau levels in the two dimensional
electron system, where stripe states are proposed (=9/2, 11/2, etc). Using
a field effect transistor, we find the transition density to be
cm at =9/2. Hysteresis is observed in the
vicinity of the transition. We construct a phase boundary in the filling
factor-magnetic field plane in the regime . An in-plane magnetic
field applied along either anisotropy axis always stabilizes the low density
orientation of the stripes.Comment: 1 revtex file, 3 eps figure
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