Author(s):

Introduction of a Josephson field effect transistor (JoFET) concept sparked
active research on proximity effects in semiconductors. Induced
superconductivity and electrostatic control of critical current has been
demonstrated in two-dimensional gases in InAs, graphene and topological
insulators, and in one-dimensional systems including quantum spin Hall edges.
Recently, interest in superconductor-semiconductor interfaces was renewed by
the search for Majorana fermions, which were predicted to reside at the
interface. More exotic non-Abelian excitations, such as parafermions
(fractional Majorana fermions) or Fibonacci fermions may be formed when
fractional quantum Hall edge states interface with superconductivity. In this
paper we develop transparent superconducting contacts to high mobility
two-dimensional electron gas (2DEG) in GaAs and demonstrate induced
superconductivity across several microns. Supercurrent in a ballistic junction
has been observed across 0.6 $\mu$m of 2DEG, a regime previously achieved only
in point contacts but essential to the formation of well separated non-Abelian
states. High critical fields ($>16$ Tesla) in NbN contacts enables
investigation of a long-sought regime of an interplay between superconductivity
and strongly correlated states in a 2DEG at high magnetic fields.

link to article (opens in new tab)

 

Author(s):

Authors: Zhong Wan, Aleksandr Kazakov, Michael J. Manfra, Loren N. Pfeiffer, Ken W. West, Leonid P. Rokhinson

Introduction of a Josephson field effect transistor (JoFET) concept sparked
active research on proximity effects in semiconductors. Induced
superconductivity and electrostatic control of critical current has been
demonstrated in two-dimensional gases in InAs, graphene and topological
insulators, and in one-dimensional systems including quantum spin Hall edges.
Recently, interest in superconductor-semiconductor interfaces was renewed by
the search for Majorana fermions, which were predicted to reside at the
interface. More exotic non-Abelian excitations, such as parafermions
(fractional Majorana fermions) or Fibonacci fermions may be formed when
fractional quantum Hall edge states interface with superconductivity. In this
paper we develop transparent superconducting contacts to high mobility
two-dimensional electron gas (2DEG) in GaAs and demonstrate induced
superconductivity across several microns. Supercurrent in a ballistic junction
has been observed across 0.6 $\mu$m of 2DEG, a regime previously achieved only
in point contacts but essential to the formation of well separated non-Abelian
states. High critical fields ($>16$ Tesla) in NbN contacts enables
investigation of a long-sought regime of an interplay between superconductivity
and strongly correlated states in a 2DEG at high magnetic fields.

link to article (opens in new tab)

 

Author(s):

Authors: Gian Paolo Vacca, Luca Zambelli

The critical behavior of a relativistic $\mathbb{Z}_2$-symmetric Yukawa model
at zero temperature and density is discussed for a continuous number of fermion
degrees of freedom and of spacetime dimensions, with emphasis on the role
played by multi-meson exchange in the Yukawa sector. We argue that this should
be generically taken into account in studies based on the functional
renormalization group, either in four-dimensional high-energy models or in
lower-dimensional condensed-matter systems. By means of the latter method, we
describe the generation of multi-critical models in less then three dimensions,
both at infinite and finite number of flavors. We also provide different
estimates of the critical exponents of the chiral Ising universality class in
three dimensions for various field contents, from a couple of massless Dirac
fermions down to the supersymmetric theory with a single Majorana spinor.

link to article (opens in new tab)

 

Author(s):

Authors: Andrea Coser, Erik Tonni, Pasquale Calabrese

We consider the partial transpose of the spin reduced density matrix of two
disjoint blocks in spin chains admitting a representation in terms of free
fermions, such as XY chains. We exploit the solution of the model in terms of
Majorana fermions and show that such partial transpose in the spin variables is
a linear combination of four Gaussian fermionic operators. This representation
allows to explicitly construct and evaluate the integer moments of the partial
transpose. We numerically study critical XX and Ising chains and we show that
the asymptotic results for large blocks agree with conformal field theory
predictions if corrections to the scaling are properly taken into account.

link to article (opens in new tab)

 

Author(s):

Authors: Akinori Tanaka

We show that the one-dimensional extended Hubbard model has saturated
ferromagnetic ground states with the spin-triplet electron pair condensation in
a certain range of parameters. The ground state wave functions with fixed
electron numbers are explicitly obtained. We also construct two ground states
in which both the spin-rotation and the gauge symmetries are broken, and show
that these states transfer from one to the other by acting the edge operators.
The edge operators are reduced to the Majorana fermions in a special case.
These symmetry breaking ground states are shown to be stabilized by a
superconducting mean field Hamiltonian which is related to the Kitaev chain
with the charge-charge interaction.

link to article (opens in new tab)

 

Author(s):

Authors: Luca D'Alessio, Marcos Rigol

Realizing topological insulators is of great current interest because of
their remarkable properties and possible future applications. There are recent
proposals, based on Floquet analyses, that one can generate topologically
nontrivial insulators by periodically diving topologically trivial ones. Here
we address what happens if one follows the dynamics in such systems.
Specifically, we present an exact study of the time evolution of a
graphene-like system subjected to a circularly polarized electric field. We
prove that, for infinite (translationally invariant) systems, the Chern number
is conserved under unitary evolution, i.e., one cannot change the topological
character of the initial wavefunction. For systems with boundaries, on the
other hand, we show that the properly defined topological invariant, the Bott
index, can change. Hence, it should be possible to experimentally prepare
topological states starting from non-topological ones. We show that the
chirality of the edge current in such systems can be controlled by adjusting
the filling.

link to article (opens in new tab)

 

Author(s):

Authors: B. Q. Lv, N. Xu, H. M. Weng, J. Z. Ma, P. Richard, X. C. Huang, L. X. Zhao, G. F. Chen, C. Matt, F. Bisti, V. Strokov, J. Mesot, Z. Fang, X. Dai, T. Qian, M. Shi, H. Ding

In 1929, H. Weyl proposed that the massless solution of Dirac equation
represents a pair of new type particles, the so-called Weyl fermions [1].
However the existence of them in particle physics remains elusive for more than
eight decades. Recently, significant advances in both topological insulators
and topological semimetals have provided an alternative way to realize Weyl
fermions in condensed matter as an emergent phenomenon: when two non-degenerate
bands in the three-dimensional momentum space cross in the vicinity of Fermi
energy (called as Weyl nodes), the low energy excitation behaves exactly the
same as Weyl fermions. Here, by performing soft x-ray angle-resolved
photoemission spectroscopy measurements which mainly probe bulk band structure,
we directly observe the long-sought-after Weyl nodes for the first time in
TaAs, whose projected locations on the (001) surface match well to the Fermi
arcs, providing undisputable experimental evidence of existence of Weyl fermion
quasiparticles in TaAs.

link to article (opens in new tab)

 

Author(s):

Authors: S. M. Nie, Zhida Song, Hongming Weng, Zhong Fang

By using first-principles calculation, we have found that a family of 2D
transition metal dichalcogenide haeckelites with square-octagonal lattice
$MX_2$-4-8 ($M$=Mo, W and $X$=S, Se and Te) can host quantum spin hall effect.
The phonon spectra indicate that they are dynamically stable and the largest
band gap is predicted to be around 54 meV, higher than room temperature. These
will pave the way to potential applications of topological insulators. We have
also established a simple tight-binding model on a square-like lattice to
achieve topological nontrivial quantum states, which extends the study from
honeycomb lattice to square-like lattice and broads the potential topological
material system greatly.

link to article (opens in new tab)

 

Author(s):

Authors: E. B. Olshanetsky, Z. D. Kvon, G. M. Gusev, A. D. Levin, O. E. Raichev, N. N. Mikhailov, S. A. Dvoretsky

Our experimental studies of electron transport in wide (14 nm) HgTe quantum
wells confirm persistence of a two-dimensional topological insulator state
reported previously for narrower wells, where it was justified theoretically.
Comparison of local and nonlocal resistance measurements indicate edge state
transport in the samples of about 1 mm size at temperatures below 1 K.
Temperature dependence of the resistances suggests an insulating gap of the
order of a few meV. In samples with sizes smaller than 10 $\mu$m a
quasiballistic transport via the edge states is observed.

link to article (opens in new tab)

 

Author(s):

Authors: Martin R. Zirnbauer

Quantum mechanical systems with some degree of complexity due to multiple
scattering behave as if their Hamiltonians were random matrices. Such behavior,
while originally surmised for the interacting many-body system of highly
excited atomic nuclei, was later discovered in a variety of situations
including single-particle systems with disorder or chaos. A fascinating theme
in this context is the emergence of universal laws for the fluctuations of
energy spectra and transport observables. After an introduction to the basic
phenomenology, the talk highlights the role of symmetries for universality, in
particular the correspondence between symmetry classes and symmetric spaces
that led to a classification scheme dubbed the ‘Tenfold Way’. Perhaps
surprisingly, the same scheme has turned out to organize also the world of
topological insulators.

link to article (opens in new tab)

 

Author(s):

Authors: V.Ya. Demikhovskii, R.V. Turkevich

The quasiclassical dynamics is studied for charge carriers moving on the
surface of 3D topological insulator of Bi2Te3 type and subjected to static
magnetic field. The effects connected to the symmetry changes of electron
isoenergetic surfaces (contours) and to the nonzero Berry curvature are taken
into account. It is shown that in contrast to the standard dynamics of the
electrons moving in constant and uniform magnetic field along the trajectories
defined by the equations E(k)=const and pz=const, here some new effects are
arising, being related to both the appearance of the anomalous velocity term
proportional to the Berry curvature, and to the trajectory bending related to
the additional term for the energy proportional to the orbital momentum of the
wavepacket. This should lead to the changes in cyclotron resonance conditions
of the surface electrons. Although the time reversal invariance and the
topological order are broken in the magnetic field, the investigation of
cyclotron resonance allows determining whether this insulator was trivial or
nontrivial at zero magnetic field.

link to article (opens in new tab)

 

Author(s):

Authors: Wei Luo, Hongjun Xiang

Two-dimensional (2D) topological insulators (TIs), also known as quantum spin
Hall (QSH) insulators, are excellent candidates for coherent spin transport
related applications because the edge states of 2D TIs are robust against
nonmagnetic impurities since the only available backscattering channel is
forbidden. Currently, most known 2D TIs are based on a hexagonal (specifically,
honeycomb) lattice. Here, we propose that there exists the quantum spin Hall
effect (QSHE) in a buckled square lattice. Through performing global structure
optimization, we predict a new three-layer quasi-2D (Q2D) structure which has
the lowest energy among all structures with the thickness less than 6.0 {\AA}
for the BiF system. It is identified to be a Q2D TI with a large band gap (0.69
eV). The electronic states of the Q2D BiF system near the Fermi level are
mainly contributed by the middle Bi square lattice, which are sandwiched by two
inert BiF2 layers. This is beneficial since the interaction between a substrate
and the Q2D material may not change the topological properties of the system,
as we demonstrate in the case of the NaF substrate. Finally, we come up with a
new tight-binding model for a two-orbital system with the buckled square
lattice to explain the low-energy physics of the Q2D BiF material. Our study
not only predicts a QSH insulator for realistic room temperature applications,
but also provides a new lattice system for engineering topological states such
as quantum anomalous Hall effect.

link to article (opens in new tab)

 

Author(s):

Authors: Motohiko Ezawa

We report the recent progress on the theoretical aspects of monolayer
topological insulators including silicene, germanene and stanene, which are
monolayer honeycomb structures of silicon, germanium and tin, respectively.
They show quantum spin Hall effects in nature due to the spin orbit
interaction. The band gap can be tuned by applying perpendicular electric
field, which induces a topological phase transition. We also analyze the
topological properties of generic honeycomb systems together with the
classification of topological insulators. Phase diagram of topological
insulators and superconductors in honeycomb systems are explicitly determined.
We also investigate topological electronics including a topological
field-effect transistor, the topological Kirchhoff’s law and the topological
spin-valleytronics.

link to article (opens in new tab)

 

Author(s):

The critical behavior of a relativistic $\mathbb{Z}_2$-symmetric Yukawa model
at zero temperature and density is discussed for a continuous number of fermion
degrees of freedom and of spacetime dimensions, with emphasis on the role
played by multi-meson exchange in the Yukawa sector. We argue that this should
be generically taken into account in studies based on the functional
renormalization group, either in four-dimensional high-energy models or in
lower-dimensional condensed-matter systems. By means of the latter method, we
describe the generation of multi-critical models in less then three dimensions,
both at infinite and finite number of flavors. We also provide different
estimates of the critical exponents of the chiral Ising universality class in
three dimensions for various field contents, from a couple of massless Dirac
fermions down to the supersymmetric theory with a single Majorana spinor.

link to article (opens in new tab)

 

Author(s):

We consider the partial transpose of the spin reduced density matrix of two
disjoint blocks in spin chains admitting a representation in terms of free
fermions, such as XY chains. We exploit the solution of the model in terms of
Majorana fermions and show that such partial transpose in the spin variables is
a linear combination of four Gaussian fermionic operators. This representation
allows to explicitly construct and evaluate the integer moments of the partial
transpose. We numerically study critical XX and Ising chains and we show that
the asymptotic results for large blocks agree with conformal field theory
predictions if corrections to the scaling are properly taken into account.

link to article (opens in new tab)

 

Author(s):

We show that the one-dimensional extended Hubbard model has saturated
ferromagnetic ground states with the spin-triplet electron pair condensation in
a certain range of parameters. The ground state wave functions with fixed
electron numbers are explicitly obtained. We also construct two ground states
in which both the spin-rotation and the gauge symmetries are broken, and show
that these states transfer from one to the other by acting the edge operators.
The edge operators are reduced to the Majorana fermions in a special case.
These symmetry breaking ground states are shown to be stabilized by a
superconducting mean field Hamiltonian which is related to the Kitaev chain
with the charge-charge interaction.

link to article (opens in new tab)

 

Author(s):

Realizing topological insulators is of great current interest because of
their remarkable properties and possible future applications. There are recent
proposals, based on Floquet analyses, that one can generate topologically
nontrivial insulators by periodically diving topologically trivial ones. Here
we address what happens if one follows the dynamics in such systems.
Specifically, we present an exact study of the time evolution of a
graphene-like system subjected to a circularly polarized electric field. We
prove that, for infinite (translationally invariant) systems, the Chern number
is conserved under unitary evolution, i.e., one cannot change the topological
character of the initial wavefunction. For systems with boundaries, on the
other hand, we show that the properly defined topological invariant, the Bott
index, can change. Hence, it should be possible to experimentally prepare
topological states starting from non-topological ones. We show that the
chirality of the edge current in such systems can be controlled by adjusting
the filling.

link to article (opens in new tab)

 

Author(s):

In 1929, H. Weyl proposed that the massless solution of Dirac equation
represents a pair of new type particles, the so-called Weyl fermions [1].
However the existence of them in particle physics remains elusive for more than
eight decades. Recently, significant advances in both topological insulators
and topological semimetals have provided an alternative way to realize Weyl
fermions in condensed matter as an emergent phenomenon: when two non-degenerate
bands in the three-dimensional momentum space cross in the vicinity of Fermi
energy (called as Weyl nodes), the low energy excitation behaves exactly the
same as Weyl fermions. Here, by performing soft x-ray angle-resolved
photoemission spectroscopy measurements which mainly probe bulk band structure,
we directly observe the long-sought-after Weyl nodes for the first time in
TaAs, whose projected locations on the (001) surface match well to the Fermi
arcs, providing undisputable experimental evidence of existence of Weyl fermion
quasiparticles in TaAs.

link to article (opens in new tab)

 

Author(s):

By using first-principles calculation, we have found that a family of 2D
transition metal dichalcogenide haeckelites with square-octagonal lattice
$MX_2$-4-8 ($M$=Mo, W and $X$=S, Se and Te) can host quantum spin hall effect.
The phonon spectra indicate that they are dynamically stable and the largest
band gap is predicted to be around 54 meV, higher than room temperature. These
will pave the way to potential applications of topological insulators. We have
also established a simple tight-binding model on a square-like lattice to
achieve topological nontrivial quantum states, which extends the study from
honeycomb lattice to square-like lattice and broads the potential topological
material system greatly.

link to article (opens in new tab)

 

Author(s):

Our experimental studies of electron transport in wide (14 nm) HgTe quantum
wells confirm persistence of a two-dimensional topological insulator state
reported previously for narrower wells, where it was justified theoretically.
Comparison of local and nonlocal resistance measurements indicate edge state
transport in the samples of about 1 mm size at temperatures below 1 K.
Temperature dependence of the resistances suggests an insulating gap of the
order of a few meV. In samples with sizes smaller than 10 $\mu$m a
quasiballistic transport via the edge states is observed.

link to article (opens in new tab)

© 2012 Topological Insulator Today Powered by WordPress Suffusion theme by Sayontan Sinha