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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Apr 172014

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.

link to article (opens in new tab)

Apr 162014

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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Apr 162014

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.

link to article (opens in new tab)

Uncategorized
No Responses »

Apr 162014

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)

Uncategorized
No Responses »

Apr 162014

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)

Uncategorized
No Responses »

Apr 162014

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)

Uncategorized
No Responses »

Apr 162014

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)

Uncategorized
No Responses »

Apr 162014

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)

Uncategorized
No Responses »

Apr 162014

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)

Uncategorized
No Responses »

Apr 162014

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)

Uncategorized
No Responses »

Apr 162014

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.

link to article (opens in new tab)

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