Author(s): A. E. Koshelev and K. A. Matveev

The authors describe anomalous superconducting properties in a shallow band in which Cooper pairing is induced by deep bands. They demonstrate that the thermodynamic topological phase transition is eliminated by superconductivity, but nevertheless the density of states shows likely detectable qualitative changes with variation of the chemical potential. The authors argue that the proposed physical scenario provides the most natural interpretation for the gap structure recently observed in some iron pnictides and chalcogenides with shallow bands.

[Phys. Rev. B 90, 140505] Published Fri Oct 24, 2014

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Author(s):

Authors: Nasser Alidoust, Chang Liu, Su-Yang Xu, Ilya Belopolski, Tongfei Qi, Minggang Zeng, Madhab Neupane, Guang Bian, Yu-Tzu Liu, Stephen D. Wilson, Hsin Lin, Arun Bansil, Gang Cao, M. Zahid Hasan

We report high-resolution angle-resolved photoemission spectroscopy
measurements on the honeycomb iridate Na2IrO3. Our measurements reveal the
existence of a metallic surface band feature crossing the Fermi level with
nearly linear dispersion and an estimated surface carrier density of 3.2
$\times$ 10$^{13}$ cm$^{-2}$, which has not been theoretically predicted or
experimentally observed, and provides the first evidence for metallic behavior
on the boundary of this material, whereas the bulk bands exhibit a robust
insulating gap. We further show the lack of theoretically predicted Dirac cones
at the $\overline{M}$ points of the surface Brillouin zone, which confirms the
absence of a stacked quantum spin Hall phase in this material. Our data
indicates that the surface ground state of this material is exotic and
metallic, unlike as predicted in theory, and establishes Na2IrO3 as a rare
example of a strongly correlated spin-orbit insulator with surface metallicity.

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Author(s):

Authors: Hiromi Ebisu, Keiji Yada, Hideaki Kasai, Yukio Tanaka

A chain of magnetic atoms with non-collinear spin configuration on a
superconductor is a promising new system that can host Majorana Fermions (MFs).
In this study, we clarify that in the presence of MFs, an odd-frequency Cooper
pair is generated at the edge of the chain. Furthermore, it is revealed that
this feature is robust against the distance between magnetic atoms. We also
elucidate the close relationship between the pair amplitude of the
odd-frequency pair, the parity of the wave function, and the direction of the
MF spin. If Rashba-type spin-orbit coupling is included, MFs can be realized
even in a collinear alignment of magnetic atoms, $i.e.$, in a ferromagnetic or
anti-ferromagnetic chain on a superconductor. Then, the odd-frequency pairing
is generated at the edge, similar to the non-collinear case. Based on our
results, it can be concluded that the detection of the zero energy peak of the
local density of states by scanning tunneling microscopy at the edge of the
magnetic chain is strong evidence for the generation of odd-frequency pairing.

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Author(s):

Authors: Juven Wang, Zheng-Cheng Gu, Xiao-Gang Wen

The challenge of identifying symmetry-protected topological states (SPTs) is
due to their lack of symmetry-breaking order parameters and intrinsic
topological orders. For this reason, it is impossible to formulate SPTs under
Ginzburg-Landau theory or probe SPTs via fractionalized bulk excitations and
topology-dependent ground state degeneracy. However, the partition functions
from path integrals with various symmetry twists are universal SPT invariants,
fully characterizing SPTs. In this work, we use gauge fields to represent those
symmetry twists in closed spacetimes of any dimensionality and arbitrary
topology. This allows us to express the SPT invariants in terms of continuum
field theory. We show that SPT invariants of pure gauge actions describe the
SPTs predicted by group cohomology, while the mixed gauge-gravity actions
describe the beyond-group-cohomology SPTs, recently observed by Kapustin. We
find new examples of mixed gauge-gravity actions for U(1) SPTs in 3+1D and 4+1D
via the Stiefel-Whitney class and the gravitational Chern-Simons term. Field
theory representations of SPT invariants not only serve as tools for
classifying SPTs, but also guide us in designing physical probes for them. In
addition, our field theory representations are independently powerful for
studying group cohomology within the mathematical context.

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Author(s):

Authors: Andreas Heimes, Daniel Mendler, Panagiotis Kotetes

We investigate the topological properties and the accessible Majorana fermion
(MF) phases ari\-sing in a hybrid device consisting of a chain of magnetic
adatoms placed on the surface of a conventional superconductor with Rashba
spin-orbit coupling (SOC). By identifying the favored classical magnetic ground
state of the adatom chain, we extract the corresponding phase diagram which
exhibits an interplay of ferromagnetic (FM), antiferromagnetic (AFM) and spiral
orders. We determine the parameter regime for which the FM or AFM phases
dominate over the spiral and additionally become stable against thermal and
quantum fluctuations. For the topological analysis we focus on the FM and AFM
cases and employ a low-energy effective model relying on Shiba bound states. We
find that for both magnetic patterns the hybrid system behaves as a topological
superconductor which can harbor one or even two MFs per edge, due to chiral
symmetry. As we show, the two magnetic orderings lead to qualitatively and
quantitatively distinct topological features that are reflected in the spatial
profile of the MF wavefunctions. Finally, we propose directions on how to
experimentally access the diverse MF phases by varying the adatom spacing, the
SOC strength, or the magnetic moment of the adatoms in consideration.

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Author(s):

Authors: Yuri D. Glinka, Sercan Babakiray, Trent A. Johnson, Mikel B. Holcomb, David Lederman

Transient reflectivity (TR) from thin films (6 – 40 nm thick) of the
topological insulator Bi2Se3 reveal ultrafast carrier dynamics, which suggest
the existence of both radiative and non-radiative recombination between
electrons residing in the upper cone of initially unoccupied high energy Dirac
surface states (SS) and holes residing in the lower cone of occupied low energy
Dirac SS. The modeling of measured TR traces allowed us to conclude that
recombination is induced by the depletion of bulk electrons in films below ~20
nm thick due to the charge captured on the surface defects. We predict that
such recombination processes can be observed using time-resolved
photoluminescence techniques.

link to article (opens in new tab)

 

Author(s):

We report high-resolution angle-resolved photoemission spectroscopy
measurements on the honeycomb iridate Na2IrO3. Our measurements reveal the
existence of a metallic surface band feature crossing the Fermi level with
nearly linear dispersion and an estimated surface carrier density of 3.2
$\times$ 10$^{13}$ cm$^{-2}$, which has not been theoretically predicted or
experimentally observed, and provides the first evidence for metallic behavior
on the boundary of this material, whereas the bulk bands exhibit a robust
insulating gap. We further show the lack of theoretically predicted Dirac cones
at the $\overline{M}$ points of the surface Brillouin zone, which confirms the
absence of a stacked quantum spin Hall phase in this material. Our data
indicates that the surface ground state of this material is exotic and
metallic, unlike as predicted in theory, and establishes Na2IrO3 as a rare
example of a strongly correlated spin-orbit insulator with surface metallicity.

link to article (opens in new tab)

 

Author(s):

A chain of magnetic atoms with non-collinear spin configuration on a
superconductor is a promising new system that can host Majorana Fermions (MFs).
In this study, we clarify that in the presence of MFs, an odd-frequency Cooper
pair is generated at the edge of the chain. Furthermore, it is revealed that
this feature is robust against the distance between magnetic atoms. We also
elucidate the close relationship between the pair amplitude of the
odd-frequency pair, the parity of the wave function, and the direction of the
MF spin. If Rashba-type spin-orbit coupling is included, MFs can be realized
even in a collinear alignment of magnetic atoms, $i.e.$, in a ferromagnetic or
anti-ferromagnetic chain on a superconductor. Then, the odd-frequency pairing
is generated at the edge, similar to the non-collinear case. Based on our
results, it can be concluded that the detection of the zero energy peak of the
local density of states by scanning tunneling microscopy at the edge of the
magnetic chain is strong evidence for the generation of odd-frequency pairing.

link to article (opens in new tab)

 

Author(s):

The challenge of identifying symmetry-protected topological states (SPTs) is
due to their lack of symmetry-breaking order parameters and intrinsic
topological orders. For this reason, it is impossible to formulate SPTs under
Ginzburg-Landau theory or probe SPTs via fractionalized bulk excitations and
topology-dependent ground state degeneracy. However, the partition functions
from path integrals with various symmetry twists are universal SPT invariants,
fully characterizing SPTs. In this work, we use gauge fields to represent those
symmetry twists in closed spacetimes of any dimensionality and arbitrary
topology. This allows us to express the SPT invariants in terms of continuum
field theory. We show that SPT invariants of pure gauge actions describe the
SPTs predicted by group cohomology, while the mixed gauge-gravity actions
describe the beyond-group-cohomology SPTs, recently observed by Kapustin. We
find new examples of mixed gauge-gravity actions for U(1) SPTs in 3+1D and 4+1D
via the Stiefel-Whitney class and the gravitational Chern-Simons term. Field
theory representations of SPT invariants not only serve as tools for
classifying SPTs, but also guide us in designing physical probes for them. In
addition, our field theory representations are independently powerful for
studying group cohomology within the mathematical context.

link to article (opens in new tab)

 

Author(s):

We investigate the topological properties and the accessible Majorana fermion
(MF) phases ari\-sing in a hybrid device consisting of a chain of magnetic
adatoms placed on the surface of a conventional superconductor with Rashba
spin-orbit coupling (SOC). By identifying the favored classical magnetic ground
state of the adatom chain, we extract the corresponding phase diagram which
exhibits an interplay of ferromagnetic (FM), antiferromagnetic (AFM) and spiral
orders. We determine the parameter regime for which the FM or AFM phases
dominate over the spiral and additionally become stable against thermal and
quantum fluctuations. For the topological analysis we focus on the FM and AFM
cases and employ a low-energy effective model relying on Shiba bound states. We
find that for both magnetic patterns the hybrid system behaves as a topological
superconductor which can harbor one or even two MFs per edge, due to chiral
symmetry. As we show, the two magnetic orderings lead to qualitatively and
quantitatively distinct topological features that are reflected in the spatial
profile of the MF wavefunctions. Finally, we propose directions on how to
experimentally access the diverse MF phases by varying the adatom spacing, the
SOC strength, or the magnetic moment of the adatoms in consideration.

link to article (opens in new tab)

 

Author(s):

Transient reflectivity (TR) from thin films (6 – 40 nm thick) of the
topological insulator Bi2Se3 reveal ultrafast carrier dynamics, which suggest
the existence of both radiative and non-radiative recombination between
electrons residing in the upper cone of initially unoccupied high energy Dirac
surface states (SS) and holes residing in the lower cone of occupied low energy
Dirac SS. The modeling of measured TR traces allowed us to conclude that
recombination is induced by the depletion of bulk electrons in films below ~20
nm thick due to the charge captured on the surface defects. We predict that
such recombination processes can be observed using time-resolved
photoluminescence techniques.

link to article (opens in new tab)

 

Author(s):Yuri D. Glinka

Transient reflectivity (TR) from thin films (6 – 40 nm thick) of the
topological insulator Bi2Se3 reveal ultrafast carrier dynamics, which suggest
the existence of both radiative and non-radiative recombination between
electrons residing in the upper cone of initially unoccupied high energy Dirac
surface states (SS) and holes residing in the lower cone of occupied low energy
Dirac SS. The modeling of measured TR traces allowed us to conclude that
recombination is induced by the depletion of bulk electrons in films below ~20
nm thick due to the charge captured on the surface defects. We predict that
such recombination processes can be observed using time-resolved
photoluminescence techniques.

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Author(s): Xiaoxiang Xi, Xu-Gang He, Fen Guan, Zhenxian Liu, R. D. Zhong, J. A. Schneeloch, T. S. Liu, G. D. Gu, X. Du, Z. Chen, X. G. Hong, Wei Ku, and G. L. Carr

[Phys. Rev. Lett. 113, 179902] Published Thu Oct 23, 2014

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Author(s): Yukinori Yoshimura, Ken-Ichiro Imura, Takahiro Fukui, and Yasuhiro Hatsugai

We propose classification schemes for characterizing two-dimensional topological phases with nontrivial weak indices. Here, weak implies that the Chern number in the corresponding phase is trivial, while the system shows edge states along specific boundaries. As concrete examples, we analyze differe…

[Phys. Rev. B 90, 155443] Published Thu Oct 23, 2014

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Author(s):

Authors: Eugene B. Kolomeisky

In 1952 Dyson put forward a simple and powerful argument indicating that the
perturbative expansions of QED are asymptotic. His argument can be related to
Chandrasekhar’s limit on the mass of a star for stability against gravitational
collapse. Combining these two arguments we estimate the optimal number of terms
of the QED series to be $3.1(137)^{3/2}\approx5000$. For condensed matter
manifestations of QED in narrow band-gap semiconductors and Weyl semimetals the
optimal number of terms is around $80$ while in graphene the utility of the
perturbation theory is severely limited.

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Author(s):

Authors: Jorge Cayao, Elsa Prada, Pablo San-Jose, Ramón Aguado

We study normal transport and the sub-gap spectrum of
superconductor-normal-superconductor (SNS) junctions made of semiconducting
nanowires with strong Rashba spin-orbit coupling. We focus, in particular, on
the role of confinement effects in long ballistic junctions. In the normal
regime, scattering at the two contacts gives rise to two distinct features in
conductance, Fabry-Perot resonances and Fano dips. The latter arise in the
presence of a strong Zeeman field $B$ that removes a spin sector in the leads
(\emph{helical} leads), but not in the central region. Conversely, a helical
central region between non-helical leads exhibits helical gaps of half-quantum
conductance, with superimposed helical Fabry-Perot oscillations. These normal
features translate into distinct subgap states when the leads become
superconducting. In particular, Fabry-Perot resonances within the helical gap
become parity-protected zero-energy states (parity crossings), well below the
critical field $B_c$ at which the superconducting leads become topological. As
a function of Zeeman field or Fermi energy, these zero-modes oscillate around
zero energy, forming characteristic loops, which evolve continuously into
Majorana bound states as $B$ exceeds $B_c$. The relation with the physics of
parity crossings of Yu-Shiba-Rusinov bound states is discussed.

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Author(s):

Authors: Zohar Nussinov, Gerardo Ortiz

We prove sufficient conditions for Topological Quantum Order at both zero and
finite temperatures. The crux of the proof hinges on the existence of
low-dimensional Gauge-Like Symmetries (that notably extend and differ from
standard local gauge symmetries) and their associated defects, thus providing a
unifying framework based on a symmetry principle. These symmetries may be
actual invariances of the system, or may emerge in the low-energy sector.
Prominent examples of Topological Quantum Order display Gauge-Like Symmetries.
New systems exhibiting such symmetries include Hamiltonians depicting
orbital-dependent spin exchange and Jahn-Teller effects in transition metal
orbital compounds, short-range frustrated Klein spin models, and p+ip
superconducting arrays. We analyze the physical consequences of Gauge-Like
Symmetries (including topological terms and charges), discuss associated
braiding, and show the insufficiency of the energy spectrum, topological
entanglement entropy, maximal string correlators, and fractionalization in
establishing Topological Quantum Order. General symmetry considerations
illustrate that not withstanding spectral gaps, thermal fluctuations may impose
restrictions on certain suggested quantum computing schemes and lead to
“thermal fragility”. Our results allow us to go beyond standard topological
field theories and engineer systems with Topological Quantum Order.

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Author(s):

Authors: E Dagrosa, A L Owczarek, T Prellberg

We consider a simple lattice model of a topological phase transition in open
polymers. To be precise, we study a model of self-avoiding walks on the simple
cubic lattice tethered to a surface and weighted by an appropriately defined
writhe. We also consider the effect of pulling the untethered end of the
polymer from the surface.

Regardless of the force we find a first-order phase transition which we argue
is a consequence of increased knotting in the lattice polymer, rather than due
to other effects such as the formation of plectonemes.

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Author(s):

Authors: Emilio Cobanera

I investigate the ansatz $(C_\alpha=\sqrt[m]{f_\alpha},\ C_\alpha^\dagger =
\sqrt[m]{f_\alpha^\dagger})$ for the second quantization operators of
quasiparticles of charge (e^*=e/m) that can emerge in systems of fermions
$(f_\alpha,f_\alpha^\dagger)$ of charge (e). After introducing a lattice
regularization, I solve the ansatz in order to obtain a set of concrete
quasiparticle operators. In less than three space dimensions, they display the
precise anyonic features of the low-lying excitations of the fractional quantum
Hall fluids at general filling fractions with odd denominator. If the
single-particle labels (\alpha) include non-Abelian quantum numbers, then the
ansatz fractionalizes the electric charge of the fermions, but not their
non-Abelian charges. Charge breakup processes at the interface between a normal
and a fractionalizing medium must be conserving processes for all the
symmetries of the system. The lattice-regularized ansatz can be modified in a
natural way to enforce this requirement. There always exists a local
transformation that maps the (C) quasiparticles into a new set of
quasiparticles carrying definite values of both fractionalized electric and
non-fractionalized non-Abelian charges. To provide an application I study
numerically the zero-energy modes that emerge at the interface between a
one-dimensional electronic and a fractional topological superconductor.

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Author(s):

Authors: Claudio Torres, Maria Emelianenko, Dmitry Golovaty, David Kinderlehrer, Shlomo Ta'asan

Polycrystalline materials undergoing coarsening can be represented as
evolving networks of grain boundaries, whose statistical characteristics
determine macroscopic materials properties. The process of formation of various
statistical distributions is extremely complex and is strongly influenced by
topological changes in the network. This work is an attempt to elucidate the
role of these changes by conducting a thorough numerical investigation of one
of the simplest types of grain growth simulation models, called vertex models.
While having obvious limitations in terms of its ability to represent realistic
systems, the vertex model enables full control over topological transitions and
retains essential geometric features of the network. We formulate a
self-consistent vertex model and investigate the role of microscopic parameters
on the mesoscale network behavior. This study sheds light onto several
important questions, such as how statistics are affected by the choice of
temporal and spatial resolution and rules governing topological changes.
Statistical analysis of the data produced by the simulation is performed for
both isotropic and anisotropic grain boundary energy.

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