Author(s):

This data comes from pipes.yahoo.com but the Pipe does not exist or has been deleted.

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Author(s):

The Humble Supermaterial: There's More To Tin Than Cans
Gizmodo Australia
Scientists from the SLAC National Accelerator Laboratory and Stanford University have long been thinking about topological insulators, which should conduct electricity just through their outside edges or surfaces, but not through their interiors. Make

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Author(s):

The Humble Supermaterial: There's More To Tin Than Cans
Gizmodo Australia
Scientists from the SLAC National Accelerator Laboratory and Stanford University have long been thinking about topological insulators, which should conduct electricity just through their outside edges or surfaces, but not through their interiors. Make

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Author(s):

Networks in the real world do not exist as isolated entities, but they are
often part of more complicated structures composed of many interconnected
network layers. Recent studies have shown that such mutual dependence makes
real networked systems potentially exposed to atypical structural and dynamical
behaviors, and thus there is a urgent necessity to better understand the
mechanisms at the basis of these anomalies. Previous research has mainly
focused on the emergence of atypical properties in relation with the moments of
the intra- and inter-layer degree distributions. In this paper, we show that an
additional ingredient plays a fundamental role for the possible scenario that
an interconnected network can face: the correlation between intra- and
inter-layer degrees. For sufficiently high amounts of correlation, an
interconnected network can be tuned, by varying the moments of the intra- and
inter-layer degree distributions, in distinct topological and dynamical
regimes. When instead the correlation between intra- and inter-layer degrees is
lower than a critical value, the system enters in a supercricritical regime
where dynamical and topological phases are not longer distinguishable.

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Author(s):

We consider the temporal evolution of a zero energy edge Majorana of a
spinless $p$-wave superconducting chain following a sudden change of a
parameter of the Hamiltonian. Starting from one of the topological phases that
has an edge Majorana, the system is suddenly driven to the other topological
phase or to the (topologically) trivial phases and also to the quantum critical
points (QCPs) separating these phases. The survival probability of the initial
edge Majorana as a function of time is studied following the quench.
Interestingly when the chain is quenched to the QCP, we find a nearly perfect
oscillations of the survival probability, indicating that the Majorana travels
back and forth between two ends, with a time period that scales with the system
size. We also generalize to the situation when there is a next-nearest-neighbor
hopping in superconducting chain and there resulting in a pair of edge Majorana
at the each end of the chain in the topological phase. We show that the
frequency of oscillation of the survival probability gets doubled in this case.
We also perform an instantaneous quenching the Hamiltonian (with two Majorana
modes at each end of the chain) to an another Hamiltonian which has only one
Majorana mode in equilibrium; the MSP shows oscillations as a function of time
with a noticeable decay in the amplitude. On the other hand for a quenching
which is reverse to the previous one, the MSP decays rapidly and stays close to
zero with fluctuations in amplitude.

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Author(s):

We investigate the admittance of a metallic quantum RC circuit with a spinful
single-channel lead or equally with two conducting spin-polarized channels, in
which Majorana fermions play a crucial role in the charge dynamics. We address
how the two-channel Kondo physics and its emergent Majoranas arise. The
existence of a single unscreened Majorana mode results in non-Fermi-liquid
features and we determine the universal crossover function describing the
Fermi-liquid to non-Fermi-liquid region. Remarkably, the same universal form
emerges both at weak transmission and large transmission. We find that the
charge relaxation resistance strongly increases in the non-Fermi-liquid realm.
Our findings can be measured using current technology assuming a large cavity.

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Author(s):

We propose and study a setup realizing a stable manifold of non-Fermi liquid
states. The device consists of a mesoscopic superconducting island hosting $N
\ge 3$ Majorana bound states tunnel-coupled to normal leads, with a Josephson
contact to a bulk superconductor. We find a nontrivial interplay between
multi-channel Kondo and resonant Andreev reflection processes, which results in
the fixed point manifold. The scaling dimension of the leading irrelevant
perturbation changes continuously within the manifold and determines the
power-law scaling of the temperature dependent conductance.

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Author(s):

Migdal’s theorem plays a central role in the physics of electron-phonon
interactions in metals and semiconductors, and has been extensively studied
theoretically for parabolic band electronic systems in three-, two-, and
one-dimensional systems over the last fifty years. In the current work, we
theoretically study the relevance of Migdal’s theorem in graphene and Weyl
semimetals which are examples of 2D and 3D Dirac materials, respectively, with
linear and chiral band dispersion. Our work also applies to 2D and 3D
topological insulator systems. In Fermi liquids, the renormalization of the
electron-phonon vertex scales as the ratio of sound ($v_s$) to Fermi ($v_F$)
velocity, which is typically a small quantity. In two- and three-dimensional
quasirelativistic systems, such as undoped graphene and Weyl semimetals, the
one loop electron-phonon vertex renormalization, which also scales as
$\eta=v_s/v_F$ as $\eta \rightarrow 0$, is, however, enhanced by an ultraviolet
\emph{logarithmic divergent correction}, arising from the linear, chiral Dirac
band dispersion. Such enhancement of the electron-phonon vertex can be
significantly softened due to the logarithmic increment of the Fermi velocity,
arising from the long range Coulomb interaction, and therefore, the
electron-phonon vertex correction does not have a logarithmic divergence at low
energy. Otherwise, the Coulomb interaction does not lead to any additional
renormalization of the electron-phonon vertex. Therefore, electron-phonon
vertex corrections in two- and three-dimensional Dirac fermionic systems scale
as $v_s/v^0_F$, where $v^0_F$ is the bare Fermi velocity, and small when $v_s
\ll v^0_F$. These results, although explicitly derived for the intrinsic
undoped systems, should hold even when the chemical potential is tuned away
from the Dirac points.

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Author(s):

We demonstrate how to control the spectra and current flow of Dirac electrons
in both a graphene sheet and a topological insulator by applying either two
linearly polarized laser fields with frequencies $\omega$ and $2\omega$ or a
monochromatic (one-frequency) laser field together with a spatially periodic
static potential(graphene/TI superlattice). Using the Floquet theory and the
resonance approximation, we show that a Dirac point in the electron spectrum
can be split into several Dirac points whose relative location in momentum
space can be efficiently manipulated by changing the characteristics of the
laser fields. In addition, the laser-field controlled Dirac fermion band
structure — Dirac fermion time-Floquet crystal — allows the manipulation of
the electron currents in graphene and topological insulators. Furthermore, the
generation of dc currents of desirable intensity in a chosen direction occurs
when applying the bi-harmonic laser field which can provide a straightforward
experimental test of the predicted phenomena.

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Author(s):

A two-dimensional spin-directed $\mathbb{Z}^{\,}_{2}$ network model is
constructed that describes the combined effects of dimerization and disorder
for the surface states of a weak three-dimensional $\mathbb{Z}^{\,}_{2}$
topological insulator. The network model consists of helical edge states of
two-dimensional layers of $\mathbb{Z}^{\,}_{2}$ topological insulators which
are coupled by time-reversal symmetric interlayer tunneling. It is argued that,
without dimerization of interlayer couplings, the network model has no
insulating phase for any disorder strength. However, a sufficiently strong
dimerization induces a transition from a metallic phase to an insulating phase.
The critical exponent $\nu$ for the diverging localization length at
metal-insulator transition points is obtained by finite-size scaling analysis
of numerical data from simulations of this network model. It is shown that the
phase transition belongs to the two-dimensional symplectic universality class
of Anderson transition.

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Author(s):

We define a class of insulators with gapless surface states protected from
localization due to the statistical properties of a disordered ensemble, namely
due to the ensemble’s invariance under a certain symmetry. We show that these
insulators are topological, and are protected by a $\mathbb{Z}_2$ invariant.
Finally, we prove that every topological insulator gives rise to an infinite
number of classes of statistical topological insulators in higher dimensions.
Our conclusions are confirmed by numerical simulations.

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Author(s):

We report a reproducible technique for the fabrication of sharp
superconducting Nb tips for scanning tunneling microscopy (STM) and scanning
tunneling spectroscopy. Sections of Nb wire with 250 $\mu$m diameter are dry
etched in an SF$_6$ plasma in a Reactive Ion Etcher. The gas pressure, etching
time and applied power are chosen to produce a self-sharpening effect to obtain
the desired tip shape. The resulting tips are atomically sharp, with radii of
less than 100 nm, and generate good STM images and spectroscopy on single
crystal samples of Au(111), Au(100), and Nb(100), as well as a doped
topological insulator Bi$_2$Se$_3$ at temperatures ranging from 30 mK to 9 K.

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Author(s):

Authors: Filippo Radicchi

Networks in the real world do not exist as isolated entities, but they are
often part of more complicated structures composed of many interconnected
network layers. Recent studies have shown that such mutual dependence makes
real networked systems potentially exposed to atypical structural and dynamical
behaviors, and thus there is a urgent necessity to better understand the
mechanisms at the basis of these anomalies. Previous research has mainly
focused on the emergence of atypical properties in relation with the moments of
the intra- and inter-layer degree distributions. In this paper, we show that an
additional ingredient plays a fundamental role for the possible scenario that
an interconnected network can face: the correlation between intra- and
inter-layer degrees. For sufficiently high amounts of correlation, an
interconnected network can be tuned, by varying the moments of the intra- and
inter-layer degree distributions, in distinct topological and dynamical
regimes. When instead the correlation between intra- and inter-layer degrees is
lower than a critical value, the system enters in a supercricritical regime
where dynamical and topological phases are not longer distinguishable.

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Author(s):

Authors: Atanu Rajak, Amit Dutta

We consider the temporal evolution of a zero energy edge Majorana of a
spinless $p$-wave superconducting chain following a sudden change of a
parameter of the Hamiltonian. Starting from one of the topological phases that
has an edge Majorana, the system is suddenly driven to the other topological
phase or to the (topologically) trivial phases and also to the quantum critical
points (QCPs) separating these phases. The survival probability of the initial
edge Majorana as a function of time is studied following the quench.
Interestingly when the chain is quenched to the QCP, we find a nearly perfect
oscillations of the survival probability, indicating that the Majorana travels
back and forth between two ends, with a time period that scales with the system
size. We also generalize to the situation when there is a next-nearest-neighbor
hopping in superconducting chain and there resulting in a pair of edge Majorana
at the each end of the chain in the topological phase. We show that the
frequency of oscillation of the survival probability gets doubled in this case.
We also perform an instantaneous quenching the Hamiltonian (with two Majorana
modes at each end of the chain) to an another Hamiltonian which has only one
Majorana mode in equilibrium; the MSP shows oscillations as a function of time
with a noticeable decay in the amplitude. On the other hand for a quenching
which is reverse to the previous one, the MSP decays rapidly and stays close to
zero with fluctuations in amplitude.

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Author(s):

Authors: Christophe Mora, Karyn Le Hur

We investigate the admittance of a metallic quantum RC circuit with a spinful
single-channel lead or equally with two conducting spin-polarized channels, in
which Majorana fermions play a crucial role in the charge dynamics. We address
how the two-channel Kondo physics and its emergent Majoranas arise. The
existence of a single unscreened Majorana mode results in non-Fermi-liquid
features and we determine the universal crossover function describing the
Fermi-liquid to non-Fermi-liquid region. Remarkably, the same universal form
emerges both at weak transmission and large transmission. We find that the
charge relaxation resistance strongly increases in the non-Fermi-liquid realm.
Our findings can be measured using current technology assuming a large cavity.

link to article (opens in new tab)

 

Author(s):

Authors: Erik Eriksson, Christophe Mora, Alex Zazunov, Reinhold Egger

We propose and study a setup realizing a stable manifold of non-Fermi liquid
states. The device consists of a mesoscopic superconducting island hosting $N
\ge 3$ Majorana bound states tunnel-coupled to normal leads, with a Josephson
contact to a bulk superconductor. We find a nontrivial interplay between
multi-channel Kondo and resonant Andreev reflection processes, which results in
the fixed point manifold. The scaling dimension of the leading irrelevant
perturbation changes continuously within the manifold and determines the
power-law scaling of the temperature dependent conductance.

link to article (opens in new tab)

 

Author(s):

Authors: Bitan Roy, Jay Deep Sau, S. Das Sarma

Migdal’s theorem plays a central role in the physics of electron-phonon
interactions in metals and semiconductors, and has been extensively studied
theoretically for parabolic band electronic systems in three-, two-, and
one-dimensional systems over the last fifty years. In the current work, we
theoretically study the relevance of Migdal’s theorem in graphene and Weyl
semimetals which are examples of 2D and 3D Dirac materials, respectively, with
linear and chiral band dispersion. Our work also applies to 2D and 3D
topological insulator systems. In Fermi liquids, the renormalization of the
electron-phonon vertex scales as the ratio of sound ($v_s$) to Fermi ($v_F$)
velocity, which is typically a small quantity. In two- and three-dimensional
quasirelativistic systems, such as undoped graphene and Weyl semimetals, the
one loop electron-phonon vertex renormalization, which also scales as
$\eta=v_s/v_F$ as $\eta \rightarrow 0$, is, however, enhanced by an ultraviolet
\emph{logarithmic divergent correction}, arising from the linear, chiral Dirac
band dispersion. Such enhancement of the electron-phonon vertex can be
significantly softened due to the logarithmic increment of the Fermi velocity,
arising from the long range Coulomb interaction, and therefore, the
electron-phonon vertex correction does not have a logarithmic divergence at low
energy. Otherwise, the Coulomb interaction does not lead to any additional
renormalization of the electron-phonon vertex. Therefore, electron-phonon
vertex corrections in two- and three-dimensional Dirac fermionic systems scale
as $v_s/v^0_F$, where $v^0_F$ is the bare Fermi velocity, and small when $v_s
\ll v^0_F$. These results, although explicitly derived for the intrinsic
undoped systems, should hold even when the chemical potential is tuned away
from the Dirac points.

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Author(s):

Authors: Pablo Rodriguez-Lopez, Joseph J.Betouras, Sergey E. Savel'ev

We demonstrate how to control the spectra and current flow of Dirac electrons
in both a graphene sheet and a topological insulator by applying either two
linearly polarized laser fields with frequencies $\omega$ and $2\omega$ or a
monochromatic (one-frequency) laser field together with a spatially periodic
static potential(graphene/TI superlattice). Using the Floquet theory and the
resonance approximation, we show that a Dirac point in the electron spectrum
can be split into several Dirac points whose relative location in momentum
space can be efficiently manipulated by changing the characteristics of the
laser fields. In addition, the laser-field controlled Dirac fermion band
structure — Dirac fermion time-Floquet crystal — allows the manipulation of
the electron currents in graphene and topological insulators. Furthermore, the
generation of dc currents of desirable intensity in a chosen direction occurs
when applying the bi-harmonic laser field which can provide a straightforward
experimental test of the predicted phenomena.

link to article (opens in new tab)

 

Author(s):

Authors: Hideaki Obuse, Shinsei Ryu, Akira Furusaki, Christopher Mudry

A two-dimensional spin-directed $\mathbb{Z}^{\,}_{2}$ network model is
constructed that describes the combined effects of dimerization and disorder
for the surface states of a weak three-dimensional $\mathbb{Z}^{\,}_{2}$
topological insulator. The network model consists of helical edge states of
two-dimensional layers of $\mathbb{Z}^{\,}_{2}$ topological insulators which
are coupled by time-reversal symmetric interlayer tunneling. It is argued that,
without dimerization of interlayer couplings, the network model has no
insulating phase for any disorder strength. However, a sufficiently strong
dimerization induces a transition from a metallic phase to an insulating phase.
The critical exponent $\nu$ for the diverging localization length at
metal-insulator transition points is obtained by finite-size scaling analysis
of numerical data from simulations of this network model. It is shown that the
phase transition belongs to the two-dimensional symplectic universality class
of Anderson transition.

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Author(s):

Authors: I. C. Fulga, B. van Heck, J. M. Edge, A. R. Akhmerov

We define a class of insulators with gapless surface states protected from
localization due to the statistical properties of a disordered ensemble, namely
due to the ensemble’s invariance under a certain symmetry. We show that these
insulators are topological, and are protected by a $\mathbb{Z}_2$ invariant.
Finally, we prove that every topological insulator gives rise to an infinite
number of classes of statistical topological insulators in higher dimensions.
Our conclusions are confirmed by numerical simulations.

link to article (opens in new tab)

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