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The Humble Supermaterial: There's More To Tin Than CansGizmodo AustraliaScientists 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 …and more » |

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Author(s):

The Humble Supermaterial: There's More To Tin Than CansGizmodo AustraliaScientists 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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Apr 222014

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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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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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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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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Apr 222014

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.

link to article (opens in new tab)

Uncategorized
No Responses »

Apr 222014

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)

Uncategorized
No Responses »

Apr 222014

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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