Author(s): Rahul Roy

Recent numerical simulations of flat-band models with interactions which show clear evidence of fractionalized topological phases in the absence of a net magnetic field have generated a great deal of interest. We provide an explanation for these observations by showing that the physics of these syst…

[Phys. Rev. B 90, 165139] Published Thu Oct 30, 2014

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Author(s):

We define and study parafermion stabilizer codes which can be viewed as
generalizations of Kitaev’s one dimensional model of unpaired Majorana
fermions. Parafermion stabilizer codes can protect against low-weight errors
acting on a small subset of parafermion modes in analogy to qudit stabilizer
codes. Examples of several smallest parafermion stabilizer codes are given. A
locality preserving embedding of qudit operators into parafermion operators is
established which allows one to map known qudit stabilizer codes to parafermion
codes. We also present a local 2D parafermion construction that combines
topological protection of Kitaev’s toric code with additional protection
relying on parity conservation.

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Author(s):

Semiconductor nanowires with strong spin-orbit coupling and proximity-induced
s-wave superconductivity in an external magnetic field have been the most
promising settings for approaches towards experimental evidence of topological
Majorana zero-modes. We investigate the effect of tilting the magnetic field
relative to the spin-orbit coupling direction in a simple continuum model and
provide an analytical derivation of the critical angle, at which the
topological states disappear. We also obtain the differential conductance
characteristic of a junction with a normal wire for different tilting angles
and propose the qualitative change of the dependence of the zero-energy
differential conductance on the tunnel barrier strength at the critical angle
as a new criterion for establishing the topological nature of the observed
signal.

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Author(s):

We report directional point-contact spectroscopy data on the novel
Bi2Te3/Fe1+yTe interfacial superconductor for a Bi2Te3 thickness of 9 quintuple
layers, bonded by van der Waals epitaxy to a Fe1+yTe film at an atomically
sharp interface. Our data show a very large superconducting twin-gap structure
with an energy scale exceeding that of bulk FeSe or FeSe1-xTex by a factor of
4. While the larger gap is isotropic and attributed to a thin FeTe layer in
proximity of the interface, the smaller gap has a pronounced anisotropy and is
associated with proximity-induced superconductivity in the topological
insulator Bi2Te3. Zero resistance is lost above 8 K, but superconducting
fluctuations are visible up to at least 12 K and the large gap is replaced by a
pseudogap that persists up to 40 K. The spectra show a pronounced zero-bias
conductance peak in the superconducting state, which may be a signature of an
unconventional pairing mechanism.

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Author(s):

We investigate electromagnetic properties of a topological crystalline
insulator (TCI) thin film under external electromagnetic fields. The TCI thin
film is a topological insulator indexed by the mirror-Chern number. It is
demonstrated that the gap closes together with the emergence of a pair of
gapless cones carrying opposite chirarities by applying in-plane magnetic
field. A pair of gapless points have opposite vortex numbers. This is a
reminiscence of a pair of Weyl cones in 3D Weyl semimetal. We thus present an a
magnetic-field induced semimetal-semiconductor transition in 2D material. This
is a giant-magnetoresistance, where resistivity is controlled by magnetic
field. Perpendicular electric field is found to shift the gapless points and
also renormalize the Fermi velocity in the direction of the in-plane magnetic
field.

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Author(s):

Topological insulators (TIs) are a new class of quantum materials that
exhibit spin momentum locking (SML) of massless Dirac fermions in the surface
states. Usually optical methods, such as angle and spin-resolved photoemission
spectroscopy, have been employed to observe the helical spin polarization in
the surface states of three-dimensional (3D) TIs up to room temperatures.
Recently, spin polarized surface currents in 3D TIs were detected by electrical
methods using ferromagnetic (FM) contacts in a lateral spin-valve measurement
geometry. However, probing the spin texture with such electrical approaches is
so far limited to temperatures below 125K, which restricts its application
potential. Here we demonstrate the room temperature electrical detection of the
spin polarization on the surface of Bi$_2$Se$_3$ due to SML by employing spin
sensitive FM tunnel contacts. The current-induced spin polarization on the
Bi$_2$Se$_3$ surface is probed at room temperature by measuring a spin-valve
signal while switching the magnetization direction of the FM detector. The spin
signal increases linearly with current bias, reverses sign with current
direction, exhibits a weak temperature dependence and decreases with higher TI
thickness, as predicted theoretically. Our results demonstrate the electrical
detection of the spin polarization on the surface of 3D TIs, which could lead
to innovative spin-based quantum information technology at ambient
temperatures.

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Author(s):

Authors: Utkan Güngördü, Rabindra Nepal, Alexey A. Kovalev

We define and study parafermion stabilizer codes which can be viewed as
generalizations of Kitaev’s one dimensional model of unpaired Majorana
fermions. Parafermion stabilizer codes can protect against low-weight errors
acting on a small subset of parafermion modes in analogy to qudit stabilizer
codes. Examples of several smallest parafermion stabilizer codes are given. A
locality preserving embedding of qudit operators into parafermion operators is
established which allows one to map known qudit stabilizer codes to parafermion
codes. We also present a local 2D parafermion construction that combines
topological protection of Kitaev’s toric code with additional protection
relying on parity conservation.

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Author(s):

Authors: Stefan Rex, Asle Sudbø

Semiconductor nanowires with strong spin-orbit coupling and proximity-induced
s-wave superconductivity in an external magnetic field have been the most
promising settings for approaches towards experimental evidence of topological
Majorana zero-modes. We investigate the effect of tilting the magnetic field
relative to the spin-orbit coupling direction in a simple continuum model and
provide an analytical derivation of the critical angle, at which the
topological states disappear. We also obtain the differential conductance
characteristic of a junction with a normal wire for different tilting angles
and propose the qualitative change of the dependence of the zero-energy
differential conductance on the tunnel barrier strength at the critical angle
as a new criterion for establishing the topological nature of the observed
signal.

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Author(s):

Authors: M. Q. He, Q. L. He, J. Y. Shen, H. C. Liu, Y. Zheng, C. H. Wong, Q. H. Chen, J. N. Wang, K. T. Law, I. K. Sou, A. P. Petrovic, R. Lortz

We report directional point-contact spectroscopy data on the novel
Bi2Te3/Fe1+yTe interfacial superconductor for a Bi2Te3 thickness of 9 quintuple
layers, bonded by van der Waals epitaxy to a Fe1+yTe film at an atomically
sharp interface. Our data show a very large superconducting twin-gap structure
with an energy scale exceeding that of bulk FeSe or FeSe1-xTex by a factor of
4. While the larger gap is isotropic and attributed to a thin FeTe layer in
proximity of the interface, the smaller gap has a pronounced anisotropy and is
associated with proximity-induced superconductivity in the topological
insulator Bi2Te3. Zero resistance is lost above 8 K, but superconducting
fluctuations are visible up to at least 12 K and the large gap is replaced by a
pseudogap that persists up to 40 K. The spectra show a pronounced zero-bias
conductance peak in the superconducting state, which may be a signature of an
unconventional pairing mechanism.

link to article (opens in new tab)

 

Author(s):

Authors: Motohiko Ezawa

We investigate electromagnetic properties of a topological crystalline
insulator (TCI) thin film under external electromagnetic fields. The TCI thin
film is a topological insulator indexed by the mirror-Chern number. It is
demonstrated that the gap closes together with the emergence of a pair of
gapless cones carrying opposite chirarities by applying in-plane magnetic
field. A pair of gapless points have opposite vortex numbers. This is a
reminiscence of a pair of Weyl cones in 3D Weyl semimetal. We thus present an a
magnetic-field induced semimetal-semiconductor transition in 2D material. This
is a giant-magnetoresistance, where resistivity is controlled by magnetic
field. Perpendicular electric field is found to shift the gapless points and
also renormalize the Fermi velocity in the direction of the in-plane magnetic
field.

link to article (opens in new tab)

 

Author(s):

Authors: André Dankert, Johannes Geurs, M. Venkata Kamalakar, Saroj P. Dash

Topological insulators (TIs) are a new class of quantum materials that
exhibit spin momentum locking (SML) of massless Dirac fermions in the surface
states. Usually optical methods, such as angle and spin-resolved photoemission
spectroscopy, have been employed to observe the helical spin polarization in
the surface states of three-dimensional (3D) TIs up to room temperatures.
Recently, spin polarized surface currents in 3D TIs were detected by electrical
methods using ferromagnetic (FM) contacts in a lateral spin-valve measurement
geometry. However, probing the spin texture with such electrical approaches is
so far limited to temperatures below 125K, which restricts its application
potential. Here we demonstrate the room temperature electrical detection of the
spin polarization on the surface of Bi$_2$Se$_3$ due to SML by employing spin
sensitive FM tunnel contacts. The current-induced spin polarization on the
Bi$_2$Se$_3$ surface is probed at room temperature by measuring a spin-valve
signal while switching the magnetization direction of the FM detector. The spin
signal increases linearly with current bias, reverses sign with current
direction, exhibits a weak temperature dependence and decreases with higher TI
thickness, as predicted theoretically. Our results demonstrate the electrical
detection of the spin polarization on the surface of 3D TIs, which could lead
to innovative spin-based quantum information technology at ambient
temperatures.

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Author(s):Gaurav Gupta

Three-dimensional (3D) topological insulator (TI) has been conjectured as an emerging material to replace copper (Cu) as an interconnect material because of the suppression of elastic scattering from doping and charge impurities for carrier transport on TI surface. We, therefore via full real-space simulation, examine the feasibility of using thin 3D-TI (Bi2Se3) wire for the local electrical interconnects in the presence of edge roughness, vacancies, acoustic phonons and charge impurities across temperature and Fermi-level by simulating quantum transport through Non-Equilibrium Green Function algorithm. We found that because of the scattering induced by the acoustic phonons, the mobility reduces considerably at the room temperature which complemented with the low density of states near Dirac-point does not position Bi2Se3 3D-TI as a promising material to replace Cu for local interconnects. Properties required in suitable TI material for this application have also been discussed.

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Author(s): Aaron Farrell and T. Pereg-Barnea

The hope to realize Majorana fermions at the vortex core of a two-dimensional topological superconductor has led to a variety of proposals for devices which exhibit topological superconductivity. Many of these include superconductivity through the proximity effect and therefore require a layer of a …

[Phys. Rev. B 90, 144518] Published Wed Oct 29, 2014

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Author(s): A. A. Burkov

We present a theory of the anomalous Hall effect (AHE) in a doped Weyl semimetal, or Weyl metal, including both intrinsic and extrinsic (impurity scattering) contributions. We demonstrate that a Weyl metal is distinguished from an ordinary ferromagnetic metal by the absence of the extrinsic and the …

[Phys. Rev. Lett. 113, 187202] Published Wed Oct 29, 2014

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Author(s):

Authors: Daisuke A. Takahashi

An inequality for spin-$F$ Bose-Einstein condensates (BECs) $
F^2(\rho^2-|\Theta|^2)-\boldsymbol{M}^2\ge0 $ is reported, where $\rho$,
$\Theta$, and $\boldsymbol{M}$ represent the density, singlet pair amplitude,
and magnetization vector, respectively. The distribution of high-symmetry
spinors in the allowed region by the inequality is elucidated with using the
Majorana representation. The result is illustrated by the example of spin-2
BECs.

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Author(s):

Authors: Timothy J. Atherton, Celia A. M. Butler, Melita C. Taylor, Ian R. Hooper, Alastair P. Hibbins, J. Roy Sambles, Harsh Mathur

It is shown theoretically that a one-dimensional crystal with time reversal
symmetry is characterized by a Z_{2} topological invariant that predicts the
existence or otherwise of edge states. This is confirmed experimentally through
the construction and simulation of a photonic crystal analogue in the microwave
regime.

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Author(s):

Authors: Isaac H. Kim, Benjamin J. Brown

For a general quantum many-body system, we show that its ground state
entanglement imposes a fundamental constraint on the low-energy excitations.
For two-dimensional systems, our result implies that any system that supports
anyons must have a nonvanishing topological entanglement entropy. We
demonstrate the generality of this argument by applying it to three-dimensional
quantum many-body systems, and showing that there is a pair of ground state
topological invariants that are associated to their physical boundaries. From
the pair, one can determine whether the given boundary can or cannot absorb
point-like or line-like excitations.

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Author(s):

Authors: G. Eguchi, K. Kuroda, K. Shirai, A. Kimura, M. Shiraishi

We report the observation of two-dimensional Shubnikov-de Hass (SdH)
oscillations in the topological insulator Tl$_{1-x}$Bi$_{1+x}$Se$_2$. Hall
effect measurements exhibited electron-hole inversion in samples with bulk
insulating properties. The SdH oscillations accompanying the hole conduction
yielded a large surface carrier density of $n_{\rm{s}}=5.1
\times10^{12}$/cm$^2$, with the Landau-level fan diagram exhibiting the $\pi$
Berry phase. These results showed the electron-hole reversibility around the
in-gap Dirac point and the hole conduction on the surface Dirac cone without
involving the bulk metallic conduction.

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Author(s):

Authors: Manuel Laubach, Johannes Reuther, Ronny Thomale, Stephan Rachel

Spin-orbit (SO) coupling is the crucial parameter to drive topological
insulating phases in electronic band models. In particular, the generic
emergence of SO coupling involves the Rashba term which fully breaks the SU(2)
spin symmetry. As soon as interactions are taken into account, however, many
theoretical studies have to content themselves with the analysis of a
simplified U(1) conserving SO term without Rashba coupling. We intend to fill
this gap by studying the Kane-Mele-Hubbard (KMH) model in the presence of
Rashba SO coupling and present the first systematic analysis of the effect of
Rashba SO coupling in a correlated two-dimensional topological insulator. We
apply the variational cluster approach (VCA) to determine the interacting phase
diagram by computing local density of states, magnetization, single particle
spectral function, and edge states. Preceded by a detailed VCA analysis of the
KMH model in the presence of U(1) conserving SO coupling, we find that the
additional Rashba SO coupling drives new electronic phases such as a metallic
regime and a direct-gap only topological insulating phase which persist in the
presence of interactions.

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Author(s):

Authors: Vincent Sacksteder, Tomi Ohtsuki, Koji Kobayashi

We numerically demonstrate a practical means of systematically controlling
topological transport on the surface of a three dimensional topological
insulator, by introducing strong disorder in a layer of depth $d$ extending
inward from the surface of the topological insulator. The dependence on $d$ of
the density of states, conductance, scattering time, scattering length,
diffusion constant, and mean Fermi velocity are investigated. The proposed
mechanism requires that the disorder strength be near the large value which is
necessary to drive the TI into the non-topological phase. If the disorder depth
$d$ is patterned using masks, gates, ion implantation, etc., then integrated
circuits may be fabricated. This technique will be useful for experiments and
for device engineering.

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