Author(s):

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May 212015

Author(s):

The stability of the topological order phase induced by the $Z_3$ Kitaev

model, which is a candidate for fault-tolerant quantum computation, against the

local order phase induced by the 3-State Potts model is studied. We show that

the low energy sector of the Kitaev-Potts model is mapped to the Potts model in

the presence of transverse magnetic field. Our study relies on two high-order

series expansion based on continuous unitary transformations in the limits of

small- and large-Potts couplings as well as mean-field approximation. Our

analysis reveals that the topological phase of the $Z_3$ Kitaev model breaks

down to the Potts model through a first order phase transition. We capture the

phase transition by analysis of the ground state energy, one-quasiparticle gap

and geometric measure of entanglement.

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May 212015

Author(s):

Recently, the search for Majorana fermions has become one of the most

prominent subjects in condensed matter physics. This search involves

explorations of new materials and hence offers interesting opportunities for

chemistry. Theoretically, Majorana fermions may reside in various types of

topological superconductor materials, and superconducting Sn_{1-x}In_{x}Te,

which is a doped topological crystalline insulator, is one of the promising

candidates to harbor Majorana fermions. Here, we report the first successful

growth of superconducting Sn_{1-x}In_{x}Te nanoplates on Si substrates by a

simple vapor transport method without employing any catalyst. We observed

robust superconducting transitions in those nanoplates after device fabrication

and found that the relation between the critical temperature and the carrier

density is consistent with that of bulk single crystals, suggesting that the

superconducting properties of the nanoplate devices are essentially the same as

those of bulk crystals. With the help of nanofabrication, those nanoplates

would prove useful for elucidating the potentially topological nature of

superconductivity in Sn_{1-x}In_{x}Te to harbor Majorana fermions and thereby

contribute to the future quantum technologies.

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Here we provide a picture of transport in quantum well heterostructures with

a periodic driving field in terms of a probabilistic occupation of the

topologically protected edge states in the system. This is done by generalizing

methods from the field of photon assisted tunneling. We show that the time

dependent field {\it dresses} the underlying Hamiltonian of the heterostructure

and splits the system into side-bands. Each of these sidebands is occupied with

a certain probability which depends on the drive frequency and strength. This

leads to a reduction in the topological transport signatures of the system

because of the probability to absorb/emit a photon. Therefore when the voltage

is tuned to the bulk gap the conductance is smaller then the expected $2e^2/h$.

We refer to this as photon inhibited topological transport. Nevertheless, the

edge modes reveal their topological origin in the robustness of the edge

conductance to disorder and changes in model parameters. In this work the

analogy with photon assisted tunneling allows us to interpret the calculated

conductivity and explain the sum rule observed by previous authors

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Chains of magnetic atoms placed on the surface of an s-wave superconductor

with large spin-orbit coupling provide a promising platform for the realization

of topological superconducting states characterized by the presence of Majorana

zero-energy modes. In this work we study the properties of the one-dimensional

chain of Yu-Shiba-Rusinov states induced by magnetic impurities using a

realistic model for the magnetic atoms that include the presence of multiple

scattering channels. These channels are mixed by the spin-orbit coupling and,

via the hybridization of the Yu-Shiba-Rusinov states at different sites of the

chain, result in a multi-band structure for the chain. We obtain the

topological phase diagram for such band structure. We identify the parameter

regimes for which the different bands lead to a topological phase and show that

the inclusion of higher bands can greatly enlarge the phase space for the

realization of topological states.

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May 212015

Author(s):

Recently, the search for Majorana fermions (MFs) has become one of the most

important and exciting issues in condensed matter physics since such an exotic

quasiparticle is expected to potentially give rise to unprecedented quantum

phenomena whose functional properties will be used to develop future quantum

technology. Theoretically, the MFs may reside in various types of topological

superconductor materials that is characterized by the topologically protected

gapless surface state which are essentially an Andreev bound state.

Superconducting doped topological insulators and topological crystalline

insulators are promising candidates to harbor the MFs. In this review, we

discuss recent progress and understanding on the research of MFs based on

time-reversal-invariant superconducting topological materials to deepen our

understanding and have a better outlook on both the search for and realization

of MFs in these systems. We also discuss some advantages of these bulk systems

to realize MFs including remarkable superconducting robustness against

nonmagnetic impurities.

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May 212015

Author(s):

We have synthesized a new ferromagnetic topological insulator by doping Cr to

the ternary topological-insulator material TlSbTe2. Single crystals of

Tl_{1-x}Cr_{x}SbTe2 were grown by a melting method and it was found that Cr can

be incorporated into the TlSbTe2 matrix only within the solubility limit of

about 1%. The Curie temperature \theta_c was found to increase with the Cr

content but remained relatively low, with the maximum value of about 4 K. The

easy axis was identified to be the c-axis and the saturation moment was 2.8

\mu_B (Bohr magneton) at 1.8 K. The in-plane resistivity of all the samples

studied showed metallic behavior with p-type carriers. Shubnikov-de Hass (SdH)

oscillations were observed in samples with the Cr-doping level of up to 0.76%.

We also tried to induce ferromagnetism in TlBiTe2 by doping Cr, but no

ferromagnetism was observed in Cr-doped TlBiTe2 crystals within the solubility

limit of Cr which turned out to be also about 1%.

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Author(s):

We report the electric transport study of the three-dimensional topological

insulator TlBiSe$_2$. We applied a newly developed analysis procedure and

precisely determined two-carrier transport properties. Magnetotransport

properties revealed a multicarrier conduction of high- and low-mobility

electrons in the bulk, which was in qualitative agreement with angle-resolved

photoemission results~[K. Kuroda $et~al.$, Phys. Rev. Lett. $\bm{105}$, 146801

(2010)]. The temperature dependence of the Hall mobility was explained well

with the conventional Bloch-Gr{\”u}neisen formula and yielded the Debye

temperature $\varTheta_{\rm{D}}=113 \pm 14$~K. The results indicate that the

scattering of bulk electrons is dominated by acoustic phonons.

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May 212015

Author(s):

Floquet topological insulators are systems in which the topology emerges out

of equilibrium when a time periodic perturbation is applied. In these systems

one can define quasi-energy states which replace the quilibrium stationary

states. The system exhibits its non-trivial topology by developing edge

localized quasi-energy states which lie in a gap of the quasi energy spectrum.

These states represent a non-equilibrium analogue of the topologically

protected edge-states in equilibrium topological insulators. In equilibrium

these edge-states lead to very specific transport properties, in particular the

two-terminal conductivity of these systems is $2e^2/h$. Here we explore the

transport properties of the edge-states in a Floquet topological insulator. In

stark contrast to the equilibrium result, we find that the two terminal

conductivity of these edge states is significantly different from $2e^2/h$.

This fact notwithstanding, we find that for certain external potential

strengths the conductivity is smaller than $2e^2/h$ and robust to the effects

of disorder and smooth changes to the Hamiltonian’s parameters. This robustness

is reminiscent of the robustness found in equilibrium topological insulators.

We provide an intuitive understanding of the reduction of the conductivity in

terms of scattering by photons. This leads us to consider a previously proposed

Floquet sum rule which recovers the equilibrium value of $2e^2/h$ for the

conductivity when edge states are present. We show that this sum rule holds in

our system using both numerical and analytic techniques.

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May 212015

Author(s):

Authors: Razieh Mohseninia, Saeed S. Jahromi, Laleh Memarzadeh, Vahid Karimipour

The stability of the topological order phase induced by the $Z_3$ Kitaev

model, which is a candidate for fault-tolerant quantum computation, against the

local order phase induced by the 3-State Potts model is studied. We show that

the low energy sector of the Kitaev-Potts model is mapped to the Potts model in

the presence of transverse magnetic field. Our study relies on two high-order

series expansion based on continuous unitary transformations in the limits of

small- and large-Potts couplings as well as mean-field approximation. Our

analysis reveals that the topological phase of the $Z_3$ Kitaev model breaks

down to the Potts model through a first order phase transition. We capture the

phase transition by analysis of the ground state energy, one-quasiparticle gap

and geometric measure of entanglement.

link to article (opens in new tab)

Uncategorized
No Responses »

May 212015

Author(s):

Authors: Junhua Zhang, Younghyun Kim, E. Rossi, Roman M. Lutchyn

Chains of magnetic atoms placed on the surface of an s-wave superconductor

with large spin-orbit coupling provide a promising platform for the realization

of topological superconducting states characterized by the presence of Majorana

zero-energy modes. In this work we study the properties of the one-dimensional

chain of Yu-Shiba-Rusinov states induced by magnetic impurities using a

realistic model for the magnetic atoms that include the presence of multiple

scattering channels. These channels are mixed by the spin-orbit coupling and,

via the hybridization of the Yu-Shiba-Rusinov states at different sites of the

chain, result in a multi-band structure for the chain. We obtain the

topological phase diagram for such band structure. We identify the parameter

regimes for which the different bands lead to a topological phase and show that

the inclusion of higher bands can greatly enlarge the phase space for the

realization of topological states.

link to article (opens in new tab)

Uncategorized
No Responses »

May 212015

Author(s):

Authors: Zhiwei Wang, Kouji Segawa, Satoshi Sasaki, A. A. Taskin, Yoichi Ando

We have synthesized a new ferromagnetic topological insulator by doping Cr to

the ternary topological-insulator material TlSbTe2. Single crystals of

Tl_{1-x}Cr_{x}SbTe2 were grown by a melting method and it was found that Cr can

be incorporated into the TlSbTe2 matrix only within the solubility limit of

about 1%. The Curie temperature \theta_c was found to increase with the Cr

content but remained relatively low, with the maximum value of about 4 K. The

easy axis was identified to be the c-axis and the saturation moment was 2.8

\mu_B (Bohr magneton) at 1.8 K. The in-plane resistivity of all the samples

studied showed metallic behavior with p-type carriers. Shubnikov-de Hass (SdH)

oscillations were observed in samples with the Cr-doping level of up to 0.76%.

We also tried to induce ferromagnetism in TlBiTe2 by doping Cr, but no

ferromagnetism was observed in Cr-doped TlBiTe2 crystals within the solubility

limit of Cr which turned out to be also about 1%.

link to article (opens in new tab)

Uncategorized
No Responses »

May 212015

Author(s):

Authors: G. Eguchi, K. Kuroda, K. Shirai, Y. Ando, T. Shinjo, A. Kimura, M. Shiraishi

We report the electric transport study of the three-dimensional topological

insulator TlBiSe$_2$. We applied a newly developed analysis procedure and

precisely determined two-carrier transport properties. Magnetotransport

properties revealed a multicarrier conduction of high- and low-mobility

electrons in the bulk, which was in qualitative agreement with angle-resolved

photoemission results~[K. Kuroda $et~al.$, Phys. Rev. Lett. $\bm{105}$, 146801

(2010)]. The temperature dependence of the Hall mobility was explained well

with the conventional Bloch-Gr{\”u}neisen formula and yielded the Debye

temperature $\varTheta_{\rm{D}}=113 \pm 14$~K. The results indicate that the

scattering of bulk electrons is dominated by acoustic phonons.

link to article (opens in new tab)

May 212015

Author(s):S E Harrison, L J Collins-McIntyre, S-L Zhang, A A Baker, A I Figueroa, A J Kellock, A Pushp, S S P Parkin, J S Harris, G van der Laan and T Hesjedal

Breaking the time-reversal symmetry (TRS) in topological insulators (TIs) through ferromagnetic

doping is an essential prerequisite for unlocking novel physical phenomena and exploring potential

device applications. Here, we report the successful growth of high-quality (Dy x Bi 1− x ) 2 Te 3

thin films with Dy concentrations up to x = 0.355 by molecular beam epitaxy. Bulk-sensitive

magnetisation studies using superconducting quantum interference device magnetometry find

paramagnetic behaviour down to 2 K for the entire doping series. The effective magnetic moment, μ

eff , is strongly doping concentration-dependent and reduces from∼12.6 μ B Dy −1 for x = 0.023 to

∼4.3 μ B Dy −1 for x = 0.355. X-ray absorption spectra and x-ray magnetic circular dichroism (XMCD)

at the Dy M 4,5 edge are employed to provide a deeper insight in…

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Author(s):Andreas P Schnyder and Philip M R Brydon

Topological superconductors have become a subject of intense research due to their potential use for

technical applications in device fabrication and quantum information. Besides fully gapped

superconductors, unconventional superconductors with point or line nodes in their order parameter

can also exhibit nontrivial topological characteristics. This article reviews recent progress in the

theoretical understanding of nodal topological superconductors, with a focus on Weyl and

noncentrosymmetric superconductors and their protected surface states. Using selected examples, we

review the bulk topological properties of these systems, study different types of topological

surface states, and examine their unusual properties. Furthermore, we survey some candidate

materials for topological superconductivity and discuss different experimental signatures of

topological surface states.

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Author(s):

ExtremeTech |
Quantum signatures of electronic transport in graphene discoveredExtremeTechIn contrast to the unidirectional current flow of electrons in a regular metal, a material that behaves as a ' topological insulator' would be useful in several spintronic applications. If all that terminology isn't enough physics for you, there's more … |

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