4,689 research outputs found
Towards searching for Majorana fermions in topological insulator nanowires
Developing a gate-tunable, scalable, and topologically-protectable
supercurrent qubit and integrating it into a quantum circuit are crucial for
applications in the fields of quantum information technology and topological
phenomena. Here we propose that the nano-hybrid supercurrent transistors, a
superconducting quantum analogue of a transistor, made of topological insulator
nanowire would be a promising platform for unprecedented control of both the
supercurrent magnitude and the current-phase relation by applying a voltage on
a gate electrode. We believe that our experimental design will help probing
Majorana state in topological insulator nanowire and establishing a solid-state
platform for topological supercurrent qubit.Comment: 11 pages, 2 figure
Dilute magnetic topological semiconductors: What's new beyond the physics of dilute magnetic semiconductors?
Role of localized magnetic moments in metal-insulator transitions lies at the
heart of modern condensed matter physics, for example, the mechanism of high
T superconductivity, the nature of non-Fermi liquid physics near heavy
fermion quantum criticality, the problem of metal-insulator transitions in
doped semiconductors, and etc. Dilute magnetic semiconductors have been studied
for more than twenty years, achieving spin polarized electric currents in spite
of low Curie temperatures. Replacing semiconductors with topological
insulators, we propose the problem of dilute magnetic topological
semiconductors. Increasing disorder strength which corresponds to the size
distribution of ferromagnetic clusters, we suggest a novel disordered metallic
state, where Weyl metallic islands appear to form inhomogeneous mixtures with
topological insulating phases. Performing the renormalization group analysis
combined with experimental results, we propose a phase diagram in
, where the spin-orbit coupling
controls a topological phase transition from a topological semiconductor to a
semiconductor with temperature and the distribution for ferromagnetic
clusters gives rise to a novel insulator-metal transition from either
a topological insulating or band insulating phase to an inhomogeneously
distributed Weyl metallic state with such insulating islands. Since
electromagnetic properties in Weyl metal are described by axion
electrodynamics, the role of random axion electrodynamics in transport
phenomena casts an interesting problem beyond the physics of percolation in
conventional disorder-driven metal-insulator transitions. We also discuss how
to verify such inhomogeneous mixtures based on atomic force microscopy
Controlling transport properties of graphene nanoribbons by codoping-induced edge distortions
One notable manifestation of the peculiar edge-localized states in zigzag
graphene nanoribbons (zGNRs) is the p-type (n-type) characteristics of nitrogen
(boron) edge-doped GNRs, and such behavior was so far considered to be
exclusive for zGNRs. Carrying out first-principles electronic structure and
quantum transport calculations, we herein show that the donor-acceptor
transition behavior can also arise in the B/N edge-doped armchair GNRs (aGNRs)
by introducing a bipolar P codopant atom into the energetically most favorable
nearest neighbor edge sites. The n-type (p-type) transport properties of B,P
(N,P) co-doped aGNRs are also shown to be superior to those of reference single
N (B) doped aGNRs in that the valence (conduction) band edge conductance
spectra are better preserved. Disentangling the chemical doping and structural
distortion effects, we will demonstrate that the latter plays an important role
in determining the transport type and explains the donor-acceptor transition
feature as well as the bipolar character of P-doped aGNRs. We thus propose the
systematic modification of GNR edge atomic structures via co-doping as a novel
approach to control charge transport characteristics of aGNRs.Comment: 11 pages, 5 figures, 1 tabl
Conductance recovery and spin polarization in boron and nitrogen codoped graphene nanoribbons
We present an ab initio study of the structural, electronic, and quantum
transport properties of B-N-complex edge-doped graphene nanoribbons (GNRs). We
find that the B-N edge codop-ing is energetically a very favorable process and
furthermore can achieve novel doping effects that are absent for the single B
or N doping. The compensation effect between B and N is predicted to generally
recover the excellent electronic transport properties of pristine GNRs. For the
zigzag GNRs, however, the spatially localized B-N defect states selectively
destroy the doped-side spin-polarized GNR edge currents at the valence and
conduction band edges. We show that the energetically and spatially
spin-polarized currents survive even in the fully ferromagnetic metallic state
and heterojunction configurations. This suggests a simple yet ef-ficient scheme
to achieve effectively smooth GNR edges and graphene-based spintronic de-vices.Comment: 17 pages, 5 figure
A topological Fermi-liquid theory for interacting Weyl metals with time reversal symmetry breaking
Introducing both Berry curvature and chiral anomaly into Landau's
Fermi-liquid theory, we construct a topological Fermi-liquid theory, applicable
to interacting Weyl metals in the absence of time reversal symmetry. Following
the Landau's Fermi-liquid theory, we obtain an effective free-energy functional
in terms of the density field of chiral fermions. The density field of chiral
fermions is determined by a self-consistent equation, minimizing the effective
free-energy functional with respect to the order-parameter field. Beyond these
thermodynamic properties, we construct Boltzmann transport theory to encode
both the Berry curvature and the chiral anomaly in the presence of forward
scattering of a Fermi-liquid state, essential for understanding dynamic
correlations in interacting Weyl metals. This generalizes the Boltzmann
transport theory for the Landau's Fermi-liquid state in the respect of
incorporating the topological structure and extends that for noninteracting
Weyl metals in the sense of introducing the forward scattering. Finally, we
justify this topological Fermi-liquid theory, generalizing the
first-quantization description for noninteracting Weyl metals into the
second-quantization representation for interacting Weyl metals. First, we
derive a topological Fermi-gas theory, integrating over high-energy electronic
degrees of freedom deep inside a pair of chiral Fermi surfaces. As a result, we
reproduce a topological Drude model with both the Berry curvature and the
chiral anomaly. Second, we take into account interactions between such
low-energy chiral fermions on the pair of chiral Fermi surfaces. We perform the
renormalization group analysis, and find that only forward scattering turns out
to be marginal above possible superconducting transition temperatures,
justifying the topological Fermi-liquid theory of interacting Weyl metals with
time reversal symmetry breaking
Representation theory of symmetric groups and the strong Lefschetz property
We investigate the structure and properties of an Artinian monomial complete
intersection quotient . We construct explicit homogeneous bases of
that are compatible with the -module structure for , all
exponents and all homogeneous degrees . Moreover, we derive
the multiplicity formulas, both in recursive form and in closed form, for each
irreducible component appearing in the -module decomposition of
homogeneous subspaces. 4, 5$
Spin-liquid Mott quantum criticality in two dimensions: Destabilization of a spinon Fermi surface and emergence of one-dimensional spin dynamics
Resorting to a recently developed theoretical device called dimensional
regularization for quantum criticality with a Fermi surface, we examine a
metal-insulator quantum phase transition from a Landau's Fermi-liquid state to
a U(1) spin-liquid phase with a spinon Fermi surface in two dimensions.
Unfortunately, we fail to approach the spin-liquid Mott quantum critical point
from the U(1) spin-liquid state within the dimensional regularization
technique. Self-interactions between charge fluctuations called holons are not
screened, which shows a run-away renormalization group flow, interpreted as
holons remain gapped. This leads us to consider another fixed point, where the
spinon Fermi surface can be destabilized across the Mott transition. Based on
this conjecture, we reveal the nature of the spin-liquid Mott quantum critical
point: Dimensional reduction to one dimension occurs for spin dynamics
described by spinons. As a result, Landau damping for both spin and charge
dynamics disappear in the vicinity of the Mott quantum critical point. When the
flavor number of holons is over its critical value, an interacting fixed point
appears to be identified with an inverted XY universality class, controlled
within the dimensional regularization technique. On the other hand, a
fluctuation-driven first order metal-insulator transition results when it is
below the critical number. We propose that the destabilization of a spinon
Fermi surface and the emergence of one-dimensional spin dynamics near the
spin-liquid Mott quantum critical point can be checked out by spin
susceptibility with a transfer momentum, where is a Fermi
momentum in the U(1) spin-liquid state: The absence of Landau damping in U(1)
gauge fluctuations gives rise to a divergent behavior at zero temperature while
it vanishes in the presence of a spinon Fermi surface.Comment: Sign mistakes in previous RG equations were corrected. Physical
aspects were rewritte
Temperature dependences of the surface resistance and the diamagnetic shielding susceptibility at T_c-T<<T_c for high-T_c superconductors
At T_c-T<<T_c(i.e., near T_c), in order to demonstrate the condudction
mechanism and temperature dependencies of the diamagnetic-shielding
susceptibility and the penetration depth, we fabricated Ba_1_xBiO_3(BKBO) thin
films and measured the energy gap by tunnel effect and shielding
susceptibilities which are compared with those measureed for BKBO and YBCO
single crystals. The shielding susceptibilities for BKBO and YBCO single
crystals well-fit \chi(T)/\chi(0)=1-exp(-2\triangle(T)/k_BT), while that for
the BKBO film follows \chi(T)/\chi(0)=1-T/T_c) which may not be intrinsic. The
exponential decrease of the susceptibilities near T_c indicates that the
conduction mechanism is hopping. The energy gaps are observed as
2\triangle(0)=(3.5+-0.1)k_BT_c for the BKBO film by the tunnel effect,
2\triangle(0)=(3.9+-0.1)k_BT_c for the BKBO single crystal, and
2\triangle(0)=(8+-0.2)k_BT_c for the YBCO single crystal. Furthermore, for
microwave device applications of superconductors, at T_c-T<<T_c, the surface
resistance R_s(T) is derived from the surface impedance at \omega\tau_(tr)<<1,
where \sigma_s(0) and \sigma_n are the conductivities of the superconducting
state and the normal state, respectively, and
f(T)=\chi(T)/\chi(0)=1-exp(-2\tringle(T)/k_BT).Comment: RevTex, 5 pages, 5 eps figure
Macroscopic Quantum Tunneling in Superconducting Junctions of \beta-AgSe Topological Insulator Nanowire
We report on the fabrication and electrical transport properties of
superconducting junctions made of \beta-AgSe topological insulator (TI)
nanowires in contact with Al superconducting electrodes. The temperature
dependence of the critical current indicates that the superconducting junction
belongs to a short and diffusive junction regime. As a characteristic feature
of the narrow junction, the critical current decreases monotonously with
increasing magnetic field. The stochastic distribution of the switching current
exhibits the macroscopic quantum tunneling behavior, which is robust up to T =
0.8 K. Our observations indicate that the TI nanowire-based Josephson junctions
can be a promising building block for the development of nanohybrid
superconducting quantum bits.Comment: 27 pages, 8 figure
An ultrahigh-Q microsphere laser based on the evanescent-wave-coupled gain
We have demonstrated an ultrahigh-Q whispering-gallery-mode (WGM) microsphere
laser based on the evanescent-wave-coupled gain. Dye molecules outside the
sphere near the equator were excited, resulting in WGM lasing in the lowest
radial mode order. The loaded quality factor of the lasing WGM was 8(2)\times
10^9, the highest ever achieved in the microlaser.Comment: 4 pages, 5 figure
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