24 research outputs found
Topological minigap in quasi-one-dimensional spin-orbit-coupled semiconductor Majorana wires
The excitation gap above the Majorana fermion (MF) modes at the ends of 1D
topological superconducting (TS) semiconductor wires scales with the bulk
quasiparticle gap E_{qp}. This gap, also called minigap, facilitates
experimental detection of the pristine TS state and MFs at experimentally
accessible temperatures T << E_{qp}. Here we show that the linear scaling of
minigap with E_{qp} can fail in quasi-1D wires with multiple confinement bands
when the applied Zeeman field is greater than or equal to about half of the
confinement-induced bandgap. TS states in such wires have an approximate chiral
symmetry supporting multiple near zero energy modes at each end leading to a
minigap which can effectively vanish. We show that the problem of small minigap
in such wires can be resolved by forcing the system to break the approximate
chirality symmetry externally with a second Zeeman field. Although experimental
signatures such as zero bias peak from the wire ends is suppressed by the
second Zeeman field above a critical value, such a field is required in some
important parameter regimes of quasi-1D wires to isolate the topological
physics of end state MFs. We also discuss the crucial difference of our minigap
calculations from the previously reported minigap results appropriate for
idealized spinless p-wave superconductors and explain why the clustering of
fermionic subgap states around the zero energy Majorana end state with
increasing chemical potential seen in the latter system does not apply to the
experimental TS states in spin-orbit coupled nanowires.Comment: Crucial difference of the present results with previously reported
results for idealized spinless p-wave wires discussed (see conclusion); new
references added; Title changed in response to Editor comment; new version as
accepted in PR
Probing a topological quantum critical point in semiconductor-superconductor heterostructures
Quantum ground states on the non-trivial side of a topological quantum
critical point (TQCP) have unique properties that make them attractive
candidates for quantum information applications. A recent example is provided
by s-wave superconductivity on a semiconductor platform, which is tuned through
a TQCP to a topological superconducting (TS) state by an external Zeeman field.
Despite many attractive features of TS states, TQCPs themselves do not break
any symmetries, making it impossible to distinguish the TS state from a regular
superconductor in conventional bulk measurements. Here we show that for the
semiconductor TQCP this problem can be overcome by tracking suitable bulk
transport properties across the topological quantum critical regime itself. The
universal low-energy effective theory and the scaling form of the relevant
susceptibilities also provide a useful theoretical framework in which to
understand the topological transitions in semiconductor heterostructures. Based
on our theory, specific bulk measurements are proposed here in order to
characterize the novel TQCP in semiconductor heterostructures.Comment: 8+ pages, 5 figures, Revised version as accepted in PR
Topological superfluids on a lattice with non-Abelian gauge fields
Two-component fermionic superfluids on a lattice with an external non-Abelian
gauge field give access to a variety of topological phases in presence of a
sufficiently large spin imbalance. We address here the important issue of
superfluidity breakdown induced by spin imbalance by a self-consistent
calculation of the pairing gap, showing which of the predicted phases will be
experimentally accessible. We present the full topological phase diagram, and
we analyze the connection between Chern numbers and the existence of
topologically protected and non-protected edge modes. The Chern numbers are
calculated via a very efficient and simple method.Comment: 6 pages, 5 figures to be published in Europhysics Letter
Topologically non-trivial superconductivity in spin-orbit coupled systems: Bulk phases and quantum phase transitions
Topologically non-trivial superconductivity has been predicted to occur in
superconductors with a sizable spin-orbit coupling in the presence of an
external Zeeman splitting. Two such systems have been proposed: (a) s-wave
superconductor pair potential is proximity induced on a semiconductor, and (b)
pair potential naturally arises from an intrinsic s-wave pairing interaction.
As is now well known, such systems in the form of a 2D film or 1D nano-wires in
a wire-network can be used for topological quantum computation. When the
external Zeeman splitting crosses a critical value , the
system passes from a regular superconducting phase to a non-Abelian topological
superconducting phase. In both cases (a) and (b) we consider in this paper the
pair potential is strictly s-wave in both the ordinary and the
topological superconducting phases, which are separated by a topological
quantum critical point at , where is the chemical potential. On the other hand, since , the Zeeman splitting required for the topological phase () far exceeds the value () above which an s-wave
pair potential is expected to vanish (and the system to become
non-superconducting) in the absence of spin-orbit coupling. We are thus led to
a situation that the topological superconducting phase appears to set in a
parameter regime at which the system actually is non-superconducting in the
absence of spin-orbit coupling. In this paper we address the question of how a
pure s-wave pair potential can survive a strong Zeeman field to give rise to a
topological superconducting phase. We show that the spin-orbit coupling is the
crucial parameter for the quantum transition into and the robustness of the
topologically non-trivial superconducting phase realized for .Comment: as published in the focus issue on Topological Quantum Computation,
New J. Phys. 13 (2011
Engineering and manipulating topological qubits in 1D quantum wires
We investigate the Josephson effect in TNT and NTN junctions, consisting of
topological (T) and normal (N) phases of semiconductor-superconductor 1D
heterostructures in the presence of a Zeeman field. A key feature of our setup
is that, in addition to the variation of the phase of the superconducting order
parameter, we allow the orientation of the magnetic field to change along the
junction. We find a novel magnetic contribution to the Majorana Josephson
coupling that permits the Josephson current to be tuned by changing the
orientation of the magnetic field along the junction. We also predict that a
spin current can be generated by a finite superconducting phase difference,
rendering these materials potential candidates for spintronic applications.
Finally, this new type of coupling not only constitutes a unique fingerprint
for the existence of Majorana bound states but also provides an alternative
pathway for manipulating and braiding topological qubits in networks of wires.Comment: references and a note were added in v2; 6 pages, 2 figures; v1 had
been submitted for the ICM2012 proceedings on the 31st of May 201
Top-transmon: hybrid superconducting qubit for parity-protected quantum computation
Qubits constructed from uncoupled Majorana fermions are protected from
decoherence, but to perform a quantum computation this topological protection
needs to be broken. Parity-protected quantum computation breaks the protection
in a minimally invasive way, by coupling directly to the fermion parity of the
system --- irrespective of any quasiparticle excitations. Here we propose to
use a superconducting charge qubit in a transmission line resonator (a socalled
transmon) to perform parity-protected rotations and read-out of a topological
(top) qubit. The advantage over an earlier proposal using a flux qubit is that
the coupling can be switched on and off with exponential accuracy, promising a
reduced sensitivity to charge noise.Comment: 7 pages, 5 figure
Introduction to topological superconductivity and Majorana fermions
This short review article provides a pedagogical introduction to the rapidly
growing research field of Majorana fermions in topological superconductors. We
first discuss in some details the simplest "toy model" in which Majoranas
appear, namely a one-dimensional tight-binding representation of a p-wave
superconductor, introduced more than ten years ago by Kitaev. We then give a
general introduction to the remarkable properties of Majorana fermions in
condensed matter systems, such as their intrinsically non-local nature and
exotic exchange statistics, and explain why these quasiparticles are suspected
to be especially well suited for low-decoherence quantum information
processing. We also discuss the experimentally promising (and perhaps already
successfully realized) possibility of creating topological superconductors
using semiconductors with strong spin-orbit coupling, proximity-coupled to
standard s-wave superconductors and exposed to a magnetic field. The goal is to
provide an introduction to the subject for experimentalists or theorists who
are new to the field, focusing on the aspects which are most important for
understanding the basic physics. The text should be accessible for readers with
a basic understanding of quantum mechanics and second quantization, and does
not require knowledge of quantum field theory or topological states of matter.Comment: 21 pages, 5 figure
Symmetry and Topology in Superconductors - Odd-frequency pairing and edge states -
Superconductivity is a phenomenon where the macroscopic quantum coherence
appears due to the pairing of electrons. This offers a fascinating arena to
study the physics of broken gauge symmetry. However, the important symmetries
in superconductors are not only the gauge invariance. Especially, the symmetry
properties of the pairing, i.e., the parity and spin-singlet/spin-triplet,
determine the physical properties of the superconducting state. Recently it has
been recognized that there is the important third symmetry of the pair
amplitude, i.e., even or odd parity with respect to the frequency. The
conventional uniform superconducting states correspond to the even-frequency
pairing, but the recent finding is that the odd-frequency pair amplitude arises
in the spatially non-uniform situation quite ubiquitously. Especially, this is
the case in the Andreev bound state (ABS) appearing at the surface/interface of
the sample. The other important recent development is on the nontrivial
topological aspects of superconductors. As the band insulators are classified
by topological indices into (i) conventional insulator, (ii) quantum Hall
insulator, and (iii) topological insulator, also are the gapped
superconductors. The influence of the nontrivial topology of the bulk states
appears as the edge or surface of the sample. In the superconductors, this
leads to the formation of zero energy ABS (ZEABS). Therefore, the ABSs of the
superconductors are the place where the symmetry and topology meet each other
which offer the stage of rich physics. In this review, we discuss the physics
of ABS from the viewpoint of the odd-frequency pairing, the topological
bulk-edge correspondence, and the interplay of these two issues. It is
described how the symmetry of the pairing and topological indices determines
the absence/presence of the ZEABS, its energy dispersion, and properties as the
Majorana fermions.Comment: 91 pages, 38 figures, Review article, references adde
Pooled analysis of WHO Surgical Safety Checklist use and mortality after emergency laparotomy
Background The World Health Organization (WHO) Surgical Safety Checklist has fostered safe practice for 10 years, yet its place in emergency surgery has not been assessed on a global scale. The aim of this study was to evaluate reported checklist use in emergency settings and examine the relationship with perioperative mortality in patients who had emergency laparotomy. Methods In two multinational cohort studies, adults undergoing emergency laparotomy were compared with those having elective gastrointestinal surgery. Relationships between reported checklist use and mortality were determined using multivariable logistic regression and bootstrapped simulation. Results Of 12 296 patients included from 76 countries, 4843 underwent emergency laparotomy. After adjusting for patient and disease factors, checklist use before emergency laparotomy was more common in countries with a high Human Development Index (HDI) (2455 of 2741, 89.6 per cent) compared with that in countries with a middle (753 of 1242, 60.6 per cent; odds ratio (OR) 0.17, 95 per cent c.i. 0.14 to 0.21, P <0001) or low (363 of 860, 422 per cent; OR 008, 007 to 010, P <0.001) HDI. Checklist use was less common in elective surgery than for emergency laparotomy in high-HDI countries (risk difference -94 (95 per cent c.i. -11.9 to -6.9) per cent; P <0001), but the relationship was reversed in low-HDI countries (+121 (+7.0 to +173) per cent; P <0001). In multivariable models, checklist use was associated with a lower 30-day perioperative mortality (OR 0.60, 0.50 to 073; P <0.001). The greatest absolute benefit was seen for emergency surgery in low- and middle-HDI countries. Conclusion Checklist use in emergency laparotomy was associated with a significantly lower perioperative mortality rate. Checklist use in low-HDI countries was half that in high-HDI countries.Peer reviewe