Kam Tuen Law

Majorana Fermion Induced Resonant Andreev Reflection

We describe experimental signatures of Majorana fermion edge states, which form at the interface between a superconductor and the surface of a topological insulator. If a lead couples to the Majorana fermions through electron tunneling, the Majorana fermions induce resonant Andreev reflections from the lead to the grounded superconductor. The linear tunneling conductance is 0 (2e(2)/h) if there is an even (odd) number of vortices in the superconductor. Similar resonance occurs for tunneling into the zero mode in the vortex core. We also study the current and noise of a two-lead device.

Ising pairing in superconducting NbSe2 atomic layers

The superconducting properties of NbSe2 as it approaches the monolayer limit are investigated by means of magnetotransport measurements, uncovering evidence of spin–momentum locking.

Zero-Bias Peaks in the Tunneling Conductance of Spin-Orbit-Coupled Superconducting Wires with and without Majorana End-States

One of the simplest proposed experimental probes of a Majorana bound state is a quantized (2e(2)/h) value of zero-bias tunneling conductance. When temperature is somewhat larger than the intrinsic width of the Majorana peak, conductance is no longer quantized, but a zero-bias peak can remain. Such a nonquantized zero-bias peak has been recently reported for semiconducting nanowires with proximity induced superconductivity. In this Letter we analyze the relation of the zero-bias peak to the presence of Majorana end states, by simulating the tunneling conductance for multiband wires with realistic amounts of disorder. We show that this system generically exhibits a (nonquantized) zero-bias peak even when the wire is topologically trivial and does not possess Majorana end states. We make comparisons to recent experiments, and discuss the necessary requirements for confirming the existence of a Majorana state.

Evidence for two-dimensional Ising superconductivity in gated MoS2

Locking the spins in a superconductor In Cooper pairs—pairs of electrons responsible for the exotic properties of superconductors—the two electrons spins typically point in opposite directions. A strong-enough external magnetic field will destroy superconductivity by making the spins point in the same direction. Lu et al. observed a two-dimensional superconducting state in the material MoS2 that was surprisingly immune to a magnetic field applied in the plane of the sample (see the Perspective by Suderow). The band structure of MoS2 and its spin-orbit coupling conspired to create an effective magnetic field that reinforced the electron pairing, with spins aligned perpendicular to the sample. Science, this issue p. 1353; see also p. 1316 Transport measurements are used to reveal a superconducting state that is only weakly affected by an in-plane magnetic field. [Also see Perspective by Suderow] The Zeeman effect, which is usually detrimental to superconductivity, can be strongly protective when an effective Zeeman field from intrinsic spin-orbit coupling locks the spins of Cooper pairs in a direction orthogonal to an external magnetic field. We performed magnetotransport experiments with ionic-gated molybdenum disulfide transistors, in which gating prepared individual superconducting states with different carrier dopings, and measured an in-plane critical field Bc2 far beyond the Pauli paramagnetic limit, consistent with Zeeman-protected superconductivity. The gating-enhanced Bc2 is more than an order of magnitude larger than it is in the bulk superconducting phases, where the effective Zeeman field is weakened by interlayer coupling. Our study provides experimental evidence of an Ising superconductor, in which spins of the pairing electrons are strongly pinned by an effective Zeeman field.

Possible topological superconducting phases of MoS2.

Molybdenum disulphide (MoS2) has attracted much interest in recent years due to its potential applications in a new generation of electronic devices. Recently, it was shown that thin films of MoS2 can become superconducting with a highest Tc of 10 K when the material is heavily gated to the conducting regime. In this work, using the group theoretical approach, we determine the possible pairing symmetries of heavily gated MoS2. Depending on the electron-electron interactions and Rashba spin-orbit coupling, the material can support an exotic spin-singlet p+ip-wavelike, an exotic spin-triplet s-wavelike, and a conventional spin-triplet p-wave pairing phase. Importantly, the exotic spin-singlet p+ip-wave phase is a topological superconducting phase that breaks time-reversal symmetry spontaneously and possesses nonzero Chern numbers where the Chern number determines the number of branches of chiral Majorana edge states.

Realization of 2D Spin-Orbit Interaction and Exotic Topological Orders in Cold Atoms

Majorana zero bound mode exists in the vortex core of a chiral

Selective equal-spin Andreev reflections induced by Majorana fermions.

p+ip

Two-dimensional superconductivity at the interface of a Bi 2 Te 3 /FeTe heterostructure

superconductor or superfluid, which can be driven from an s-wave pairing state by two-dimensional (2D) spin-orbit (SO) coupling. We propose here a novel scheme based on realistic cold atom platforms to generate 2D SO interactions in a blue-detuned square optical lattice, and predict both the quantum anomalous Hall effect and chiral topological superfluid phase in the experimentally accessible parameter regimes. This work opens a new direction with experimental feasibility to observe non-Abelian topological orders in cold atom systems.

Majorana Kramers doublets in d x 2 −y 2 -wave superconductors with Rashba spin-orbit coupling

In this work, we find that Majorana fermions induce selective equal spin Andreev reflections (SESARs), in which incoming electrons with certain spin polarization in the lead are reflected as counterpropagating holes with the same spin. The spin polarization direction of the electrons of this Andreev reflected channel is selected by the Majorana fermions. Moreover, electrons with opposite spin polarization are always reflected as electrons with unchanged spin. As a result, the charge current in the lead is spin polarized. Therefore, a topological superconductor which supports Majorana fermions can be used as a novel device to create fully spin-polarized currents in paramagnetic leads. We point out that SESARs can also be used to detect Majorana fermions in topological superconductors.

Ising Superconductivity and Majorana Fermions in Transition Metal Dichalcogenides

The realization of superconductivity at the interface between a topological insulator and an iron-chalcogenide compound is highly attractive for exploring several recent theoretical predictions involving these two new classes of materials. Here we report transport measurements on a Bi2Te3/FeTe heterostructure fabricated via van der Waals epitaxy, which demonstrate superconductivity at the interface, which is induced by the Bi2Te3 epilayer with thickness even down to one quintuple layer, though there is no clear-cut evidence that the observed superconductivity is induced by the topological surface states. The two-dimensional nature of the observed superconductivity with the highest transition temperature around 12u2009K was verified by the existence of a Berezinsky-Kosterlitz-Thouless transition and the diverging ratio of in-plane to out-plane upper critical field on approaching the superconducting transition temperature. With the combination of interface superconductivity and Dirac surface states of Bi2Te3, the heterostructure studied in this work provides a novel platform for realizing Majorana fermions.