Author(s):Tarun Grover

An elusive symmetry is predicted to emerge at the boundary of an exotic condensed matter system.
Authors: Tarun Grover, D. N. Sheng, Ashvin Vishwanath

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Author(s): Chris Mann, Damien West, Ireneusz Miotkowski, Yong P. Chen, Shengbai Zhang, and Chih-Kang Shih

Using scanning tunneling microscopy and ab initio simulations, we have identified several configurations for Cu dopants in CuxBi2Se3, with Cu intercalants being the most abundant. Through statistical analysis, we show strong short-range repulsive interactions between Cu intercalants. At intermediate…

[Phys. Rev. B 89, 155312] Published Thu Apr 17, 2014

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Author(s):

Non-trivial topology of phase is crucial for many important physics phenomena
such as, for example, the Aharonov-Bohm effect 1 and the Berry phase 2. Light
phase allows one to create “twisted” photons 3, 4 , vortex knots 5,
dislocations 6 which has led to an emerging field of singular optics relying on
abrupt phase changes 7. Here we demonstrate the feasibility of singular
visible-light nanooptics which exploits the benefits of both plasmonic field
enhancement and non-trivial topology of light phase. We show that properly
designed plasmonic nanomaterials exhibit topologically protected singular phase
behaviour which can be employed to radically improve sensitivity of detectors
based on plasmon resonances. By using reversible hydrogenation of graphene 8
and a streptavidin-biotin test 9, we demonstrate areal mass sensitivity at a
level of femto-grams per mm2 and detection of individual biomolecules,
respectively. Our proof-of-concept results offer a way towards simple and
scalable single-molecular label-free biosensing technologies.

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Author(s):

Fermi liquid theory provides a remarkably powerful framework for the
description of the conduction electrons in metals and their ordering phenomena,
such as superconductivity, ferromagnetism, and spin- and charge-density-wave
order. A different class of ordering phenomena of great interest concerns spin
configurations that are topologically protected, that is, their topology can be
destroyed only by forcing the average magnetization locally to zero. Examples
of such configurations are hedgehogs (points at which all spins are either
pointing inwards or outwards) or vortices. A central question concerns the
nature of the metallic state in the presence of such topologically distinct
spin textures. Here we report a high-pressure study of the metallic state at
the border of the skyrmion lattice in MnSi, which represents a new form of
magnetic order composed of topologically non-trivial vortices. When long-range
magnetic order is suppressed under pressure, the key characteristic of the
skyrmion lattice – that is, the topological Hall signal due to the emergent
magnetic flux associated with their topological winding – is unaffected in sign
or magnitude and becomes an important characteristic of the metallic state. The
regime of the topological Hall signal in temperature, pressure and magnetic
field coincides thereby with the exceptionally extended regime of a pronounced
non-Fermi-liquid resistivity. The observation of this topological Hall signal
in the regime of the NFL resistivity suggests empirically that spin
correlations with non-trivial topological character may drive a breakdown of
Fermi liquid theory in pure metals.

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Author(s):

We study the effects of the scaling of the electron charge in the Quantum
Electrodynamics of 3D Dirac semimetals, which arise as electron-hole
excitations become less efficient to screen the bare charge at short-distance
scales. We show that these 3D electron systems have a critical point in the
regime where the bare coupling gets large at the high-energy cutoff. The
critical behavior is characterized by the suppression of the quasiparticle
weight at low energies, making the system to fall into the class of marginal
Fermi liquids. We also investigate the phase beyond the critical point, finding
that it can be formally characterized in terms of electron quasiparticles, but
with parameters that have large imaginary parts implying an increasing
deviation from the Fermi liquid picture.

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Author(s):

The stability of the Majorana modes in the presence of a repulsive
interaction is studied in the standard semiconductor wire – metallic
superconductor configuration. The effects of short-range Coulomb interaction,
which is incorporated using a purely repulsive $\delta$-function to model the
strong screening effect due to the presence of the superconductor, are
determined within a Hartree-Fock approximation of the effective Bogoliubov-De
Gennes Hamiltonian that describes the low-energy physics of the wire. Through a
numerical diagonalization procedure we obtain interaction corrections to the
single particle eigenstates and calculate the extended topological phase
diagram in terms of the chemical potential and the Zeeman energy. We find that,
for a fixed Zeeman energy, the interaction shifts the phase boundaries to a
higher chemical potential, whereas for a fixed chemical potential this shift
can occur either to lower or to higher Zeeman energies. This effects can be
interpreted as a renormalization of the g-factor due to the interaction. The
minimum Zeeman energy needed to realize Majorana fermions decreases with
increasing the strength of the Coulomb repulsion. Furthermore, we find that in
wires with multi-band occupancy this effect can be enhanced by increasing the
chemical potential, i. e. by occupying higher energy bands.

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Author(s):

Non-Abelian quasiparticles have been predicted to exist in a variety of
condensed matter systems. Their defining property is that an adiabatic braid
between two of them results in a nontrivial change of the quantum state of the
system. To date, no experimental platform has reached the desired control over
non-Abelian quasiparticles to demonstrate this remarkable property. The
simplest non-Abelian quasiparticles — the Majorana bound states (MBS) — can
occur in one-dimensional (1D) electronic nano-structures proximity-coupled to a
bulk superconductor — a platform that is being currently explored in great
depth both theoretically and experimentally. When tuned appropriately, such
nano-wires can localize MBS at their ends, a pair of which forms a two-level
system that is robust to local perturbations. This constitutes a topologically
protected qubit that can serve as the building block for a topological quantum
computer. To implement braiding operations among the MBS, schemes that allow to
move MBS across wire networks have been explored theoretically. Here, we
propose a simpler alternative setup, based on chains of magnetic adatoms on the
surface of a thin-film superconductor, in which the control over an externally
applied magnetic field suffices to create and manipulate MBS. We consider
specific 1D patterns of adatoms, which can be engineered with
scanning-tunneling-microscope based lateral atomic manipulation techniques, and
show that they allow for the creation, annihilation, adiabatic motion, and
braiding of pairs of MBS by varying the magnitude and orientation of the
external magnetic field.

link to article (opens in new tab)

 

Author(s):

We investigate the effect of quantum phase slips on a helical quantum wire
coupled to a superconductor by proximity. The effective low-energy description
of the wire is that of a Majorana chain minimally coupled to a dynamical
$\mathbb{Z}_2$ gauge field. Hence the wire emulates a matter-coupled gauge
theory, with fermion parity playing the role of the gauged global symmetry.
Quantum phase slips lift the ground state degeneracy associated with unpaired
Majorana edge modes at the ends of the chain, a change that can be understood
as a transition between the confined and the Higgs-mechanism regimes of the
gauge theory. We identify the quantization of thermal conductance at the
transition as a robust experimental feature separating the two regimes. We
explain this result by establishing a relation between thermal conductance and
the Fredenhagen-Marcu string order-parameter for confinement in gauge theories.
Our work indicates that thermal transport could serve as a measure of non-local
order parameters for emergent or simulated topological quantum order.

link to article (opens in new tab)

 

Author(s):

Since the discovery of topological insulators (TIs)1,2, the peculiar nature
of their chiral surface states has been experimentally demonstrated both in
bulk and in film materials with open boundaries3,4. Closed boundary on a TI
surface may intrigue more interesting phenomena such as quantum confinement of
massless Dirac fermions (DFs), which is analogous to the quantum corral (QC)
for massive free electrons on a metal surface5-10. To date, it keeps a highly
stringent challenge to realize a true Dirac QC due to the unusual transmitting
power of a massless fermion. Through heteroepitaxially growing a Bi bilayer on
the Bi2Te3 surface with appropriate coverage, here we demonstrate the
realization of a true Dirac QC. Specifically, spectacular maps of quantum
interference in equilateral triangle-shaped QCs surrounded by Bi bilayers are
directly visualized by using a low-temperature scanning tunneling microscope.
The present success is ascribed to a perfect orientation matching between the
QC boundary and the stationary-phase scattering of massless DFs. In addition,
the quasiparticle lifetime of the confined DFs is also systematically measured
and analyzed.

link to article (opens in new tab)

 

Author(s):

Since the discovery of topological insulators (TIs)1,2, the peculiar nature
of their chiral surface states has been experimentally demonstrated both in
bulk and in film materials with open boundaries3,4. Closed boundary on a TI
surface may intrigue more interesting phenomena such as quantum confinement of
massless Dirac fermions (DFs), which is analogous to the quantum corral (QC)
for massive free electrons on a metal surface5-10. To date, it keeps a highly
stringent challenge to realize a true Dirac QC due to the unusual transmitting
power of a massless fermion. Through heteroepitaxially growing a Bi bilayer on
the Bi2Te3 surface with appropriate coverage, here we demonstrate the
realization of a true Dirac QC. Specifically, spectacular maps of quantum
interference in equilateral triangle-shaped QCs surrounded by Bi bilayers are
directly visualized by using a low-temperature scanning tunneling microscope.
The present success is ascribed to a perfect orientation matching between the
QC boundary and the stationary-phase scattering of massless DFs. In addition,
the quasiparticle lifetime of the confined DFs is also systematically measured
and analyzed.

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Author(s):

Authors: V. G. Kravets, F. Schedin, R. Jalil, L. Britnell, R. V. Gorbachev, D. Ansell, B. Thackray, K. S. Novoselov, A. K. Geim, A. V. Kabashin, A. N. Grigorenko

Non-trivial topology of phase is crucial for many important physics phenomena
such as, for example, the Aharonov-Bohm effect 1 and the Berry phase 2. Light
phase allows one to create “twisted” photons 3, 4 , vortex knots 5,
dislocations 6 which has led to an emerging field of singular optics relying on
abrupt phase changes 7. Here we demonstrate the feasibility of singular
visible-light nanooptics which exploits the benefits of both plasmonic field
enhancement and non-trivial topology of light phase. We show that properly
designed plasmonic nanomaterials exhibit topologically protected singular phase
behaviour which can be employed to radically improve sensitivity of detectors
based on plasmon resonances. By using reversible hydrogenation of graphene 8
and a streptavidin-biotin test 9, we demonstrate areal mass sensitivity at a
level of femto-grams per mm2 and detection of individual biomolecules,
respectively. Our proof-of-concept results offer a way towards simple and
scalable single-molecular label-free biosensing technologies.

link to article (opens in new tab)

 

Author(s):

Authors: R. Ritz, M. Halder, M. Wagner, C. Franz, A. Bauer, C. Pfleiderer

Fermi liquid theory provides a remarkably powerful framework for the
description of the conduction electrons in metals and their ordering phenomena,
such as superconductivity, ferromagnetism, and spin- and charge-density-wave
order. A different class of ordering phenomena of great interest concerns spin
configurations that are topologically protected, that is, their topology can be
destroyed only by forcing the average magnetization locally to zero. Examples
of such configurations are hedgehogs (points at which all spins are either
pointing inwards or outwards) or vortices. A central question concerns the
nature of the metallic state in the presence of such topologically distinct
spin textures. Here we report a high-pressure study of the metallic state at
the border of the skyrmion lattice in MnSi, which represents a new form of
magnetic order composed of topologically non-trivial vortices. When long-range
magnetic order is suppressed under pressure, the key characteristic of the
skyrmion lattice – that is, the topological Hall signal due to the emergent
magnetic flux associated with their topological winding – is unaffected in sign
or magnitude and becomes an important characteristic of the metallic state. The
regime of the topological Hall signal in temperature, pressure and magnetic
field coincides thereby with the exceptionally extended regime of a pronounced
non-Fermi-liquid resistivity. The observation of this topological Hall signal
in the regime of the NFL resistivity suggests empirically that spin
correlations with non-trivial topological character may drive a breakdown of
Fermi liquid theory in pure metals.

link to article (opens in new tab)

 

Author(s):

Authors: J. Gonzalez

We study the effects of the scaling of the electron charge in the Quantum
Electrodynamics of 3D Dirac semimetals, which arise as electron-hole
excitations become less efficient to screen the bare charge at short-distance
scales. We show that these 3D electron systems have a critical point in the
regime where the bare coupling gets large at the high-energy cutoff. The
critical behavior is characterized by the suppression of the quasiparticle
weight at low energies, making the system to fall into the class of marginal
Fermi liquids. We also investigate the phase beyond the critical point, finding
that it can be formally characterized in terms of electron quasiparticles, but
with parameters that have large imaginary parts implying an increasing
deviation from the Fermi liquid picture.

link to article (opens in new tab)

 

Author(s):

Authors: Andrei Manolescu, D. C. Marinescu, Tudor D. Stanescu

The stability of the Majorana modes in the presence of a repulsive
interaction is studied in the standard semiconductor wire – metallic
superconductor configuration. The effects of short-range Coulomb interaction,
which is incorporated using a purely repulsive $\delta$-function to model the
strong screening effect due to the presence of the superconductor, are
determined within a Hartree-Fock approximation of the effective Bogoliubov-De
Gennes Hamiltonian that describes the low-energy physics of the wire. Through a
numerical diagonalization procedure we obtain interaction corrections to the
single particle eigenstates and calculate the extended topological phase
diagram in terms of the chemical potential and the Zeeman energy. We find that,
for a fixed Zeeman energy, the interaction shifts the phase boundaries to a
higher chemical potential, whereas for a fixed chemical potential this shift
can occur either to lower or to higher Zeeman energies. This effects can be
interpreted as a renormalization of the g-factor due to the interaction. The
minimum Zeeman energy needed to realize Majorana fermions decreases with
increasing the strength of the Coulomb repulsion. Furthermore, we find that in
wires with multi-band occupancy this effect can be enhanced by increasing the
chemical potential, i. e. by occupying higher energy bands.

link to article (opens in new tab)

 

Author(s):

Authors: Jian Li, Titus Neupert, B. Andrei Bernevig, Ali Yazdani

Non-Abelian quasiparticles have been predicted to exist in a variety of
condensed matter systems. Their defining property is that an adiabatic braid
between two of them results in a nontrivial change of the quantum state of the
system. To date, no experimental platform has reached the desired control over
non-Abelian quasiparticles to demonstrate this remarkable property. The
simplest non-Abelian quasiparticles — the Majorana bound states (MBS) — can
occur in one-dimensional (1D) electronic nano-structures proximity-coupled to a
bulk superconductor — a platform that is being currently explored in great
depth both theoretically and experimentally. When tuned appropriately, such
nano-wires can localize MBS at their ends, a pair of which forms a two-level
system that is robust to local perturbations. This constitutes a topologically
protected qubit that can serve as the building block for a topological quantum
computer. To implement braiding operations among the MBS, schemes that allow to
move MBS across wire networks have been explored theoretically. Here, we
propose a simpler alternative setup, based on chains of magnetic adatoms on the
surface of a thin-film superconductor, in which the control over an externally
applied magnetic field suffices to create and manipulate MBS. We consider
specific 1D patterns of adatoms, which can be engineered with
scanning-tunneling-microscope based lateral atomic manipulation techniques, and
show that they allow for the creation, annihilation, adiabatic motion, and
braiding of pairs of MBS by varying the magnitude and orientation of the
external magnetic field.

link to article (opens in new tab)

 

Author(s):

Authors: Jian Li, Titus Neupert, B. Andrei Bernevig, Ali Yazdani

Non-Abelian quasiparticles have been predicted to exist in a variety of
condensed matter systems. Their defining property is that an adiabatic braid
between two of them results in a nontrivial change of the quantum state of the
system. To date, no experimental platform has reached the desired control over
non-Abelian quasiparticles to demonstrate this remarkable property. The
simplest non-Abelian quasiparticles — the Majorana bound states (MBS) — can
occur in one-dimensional (1D) electronic nano-structures proximity-coupled to a
bulk superconductor — a platform that is being currently explored in great
depth both theoretically and experimentally. When tuned appropriately, such
nano-wires can localize MBS at their ends, a pair of which forms a two-level
system that is robust to local perturbations. This constitutes a topologically
protected qubit that can serve as the building block for a topological quantum
computer. To implement braiding operations among the MBS, schemes that allow to
move MBS across wire networks have been explored theoretically. Here, we
propose a simpler alternative setup, based on chains of magnetic adatoms on the
surface of a thin-film superconductor, in which the control over an externally
applied magnetic field suffices to create and manipulate MBS. We consider
specific 1D patterns of adatoms, which can be engineered with
scanning-tunneling-microscope based lateral atomic manipulation techniques, and
show that they allow for the creation, annihilation, adiabatic motion, and
braiding of pairs of MBS by varying the magnitude and orientation of the
external magnetic field.

link to article (opens in new tab)

 

Author(s):

Authors: B. van Heck, E. Cobanera, J. Ulrich, F. Hassler

We investigate the effect of quantum phase slips on a helical quantum wire
coupled to a superconductor by proximity. The effective low-energy description
of the wire is that of a Majorana chain minimally coupled to a dynamical
$\mathbb{Z}_2$ gauge field. Hence the wire emulates a matter-coupled gauge
theory, with fermion parity playing the role of the gauged global symmetry.
Quantum phase slips lift the ground state degeneracy associated with unpaired
Majorana edge modes at the ends of the chain, a change that can be understood
as a transition between the confined and the Higgs-mechanism regimes of the
gauge theory. We identify the quantization of thermal conductance at the
transition as a robust experimental feature separating the two regimes. We
explain this result by establishing a relation between thermal conductance and
the Fredenhagen-Marcu string order-parameter for confinement in gauge theories.
Our work indicates that thermal transport could serve as a measure of non-local
order parameters for emergent or simulated topological quantum order.

link to article (opens in new tab)

 

Author(s):

Authors: B. van Heck, E. Cobanera, J. Ulrich, F. Hassler

We investigate the effect of quantum phase slips on a helical quantum wire
coupled to a superconductor by proximity. The effective low-energy description
of the wire is that of a Majorana chain minimally coupled to a dynamical
$\mathbb{Z}_2$ gauge field. Hence the wire emulates a matter-coupled gauge
theory, with fermion parity playing the role of the gauged global symmetry.
Quantum phase slips lift the ground state degeneracy associated with unpaired
Majorana edge modes at the ends of the chain, a change that can be understood
as a transition between the confined and the Higgs-mechanism regimes of the
gauge theory. We identify the quantization of thermal conductance at the
transition as a robust experimental feature separating the two regimes. We
explain this result by establishing a relation between thermal conductance and
the Fredenhagen-Marcu string order-parameter for confinement in gauge theories.
Our work indicates that thermal transport could serve as a measure of non-local
order parameters for emergent or simulated topological quantum order.

link to article (opens in new tab)

 

Author(s):

Authors: Mu Chen, Zhen-Guo Fu, Jun-Ping Peng, Fawei Zheng, Hui-Min Zhang, Xiao Feng, Cui-Zu Chang, Ke He, Lili Wang, Ping Zhang, Xucun Ma, Qi-Kun Xue

Since the discovery of topological insulators (TIs)1,2, the peculiar nature
of their chiral surface states has been experimentally demonstrated both in
bulk and in film materials with open boundaries3,4. Closed boundary on a TI
surface may intrigue more interesting phenomena such as quantum confinement of
massless Dirac fermions (DFs), which is analogous to the quantum corral (QC)
for massive free electrons on a metal surface5-10. To date, it keeps a highly
stringent challenge to realize a true Dirac QC due to the unusual transmitting
power of a massless fermion. Through heteroepitaxially growing a Bi bilayer on
the Bi2Te3 surface with appropriate coverage, here we demonstrate the
realization of a true Dirac QC. Specifically, spectacular maps of quantum
interference in equilateral triangle-shaped QCs surrounded by Bi bilayers are
directly visualized by using a low-temperature scanning tunneling microscope.
The present success is ascribed to a perfect orientation matching between the
QC boundary and the stationary-phase scattering of massless DFs. In addition,
the quasiparticle lifetime of the confined DFs is also systematically measured
and analyzed.

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Author(s):

Authors: R. Nourafkan, G. Kotliar, A.-M.S. Tremblay

Spin-orbit coupling introduces chirality into electronic structure. This can
have profound effects on the magnetization induced by orbital motion of
electrons. Here we derive a formula for the orbital magnetization of
interacting electrons in terms of the full Green’s function and vertex
functions. The formula is applied within dynamical mean-field theory to the
Kane-Mele-Hubbard model that allows both topological and trivial insulating
phases. We study the insulating and metallic phases in the presence of an
exchange magnetic field. In the presence of interactions, the orbital
magnetization of the quantum spin Hall insulating phase with inversion symmetry
is renormalized by the bulk quasi-particle weight. The orbital magnetization
vanishes for the in-plane antiferromagnetic phase with trivial topology. In the
metallic phase, the enhanced effective spin-orbit coupling due to the
interaction sometimes leads to an enhancement of the orbital magnetization.
However, at low doping, magnetization is suppressed at large interaction
strengths.

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