Ying Xing

Direct Observation of High-Temperature Superconductivity in One-Unit-Cell FeSe Films

We prepared one-unit-cell (1-UC) thick FeSe films on insulating SrTiO3 substrates with non-superconducting FeTe protection layers by molecular beam epitaxy for ex situ studies. By direct transport and magnetic measurements, we provide definitive evidence for high temperature superconductivity in the 1-UC FeSe films with an onset TC above 40 K and an extremely large critical current density JC~1.7×106 A/cm2 at 2 K, which are much higher than TC~8 K and JC~104 A/cm2 for bulk FeSe, respectively. Our work may pave the way to enhancing and tailoring superconductivity by interface engineering.

Quantum Griffiths singularity of superconductor-metal transition in Ga thin films

Cooling to see the effects of disorder In sufficiently strong external magnetic fields, thin superconducting films typically become insulating. The presence of disorder can affect this phase transition. Theorists have proposed that disorder can cause the so-called Griffiths singularity, where the behavior of the system is determined by a small number of superconducting islands that form above the critical magnetic field. Xing et al. observed a signature of such a singularity in thin films of gallium by analyzing transport data taken at very low temperatures (see the Perspective by Markovic). In this regime, thermal fluctuations were not strong enough to homogenize the system, which allowed the rare islands to form. Science, this issue p. 542; see also p. 509 Systematic transport measurements at low temperatures and in magnetic fields indicate the divergence of the dynamical critical exponent. [Also see Perspective by Markovic] The Griffiths singularity in a phase transition, caused by disorder effects, was predicted more than 40 years ago. Its signature, the divergence of the dynamical critical exponent, is challenging to observe experimentally. We report the experimental observation of the quantum Griffiths singularity in a two-dimensional superconducting system. We measured the transport properties of atomically thin gallium films and found that the films undergo superconductor-metal transitions with increasing magnetic field. Approaching the zero-temperature quantum critical point, we observed divergence of the dynamical critical exponent, which is consistent with the Griffiths singularity behavior. We interpret the observed superconductor-metal quantum phase transition as the infinite-randomness critical point, where the properties of the system are controlled by rare large superconducting regions.

High temperature superconducting FeSe films on SrTiO3 substrates

Interface enhanced superconductivity at two dimensional limit has become one of most intriguing research directions in condensed matter physics. Here, we report the superconducting properties of ultra-thin FeSe films with the thickness of one unit cell (1-UC) grown on conductive and insulating SrTiO3 (STO) substrates. For the 1-UC FeSe on conductive STO substrate (Nb-STO), the magnetization versus temperature (M-T) measurement shows a drop crossover around 85 K. For the FeSe films on insulating STO substrate, systematic transport measurements were carried out and the sheet resistance of FeSe films exhibits Arrhenius TAFF behavior with a crossover from a single-vortex pinning region to a collective creep region. More intriguing, sign reversal of Hall resistance with temperature is observed, demonstrating a crossover from hole conduction to electron conduction above TC in 1-UC FeSe films.

Demonstration of surface transport in a hybrid Bi2Se3/Bi2Te3 heterostructure.

In spite of much work on topological insulators (TIs), systematic experiments for TI/TI heterostructures remain absent. We grow a high quality heterostructure containing single quintuple layer (QL) of Bi2Se3 on 19 QLs of Bi2Te3 and compare its transport properties with 20 QLs Bi2Se3 and 20 QLs Bi2Te3. All three films are grown on insulating sapphire (0001) substrates by molecular beam epitaxy (MBE). In situ angle-resolved photoemission spectroscopy (ARPES) provides direct evidence that the surface state of 1 QL Bi2Se3/19 QLs Bi2Te3 heterostructure is similar to the surface state of the 20 QLs Bi2Se3 and different with that of the 20 QLs Bi2Te3. In ex situ transport measurements, the observed linear magnetoresistance (MR) and weak antilocalization (WAL) of the hybrid heterostructure are similar to that of the pure Bi2Se3 film and not the Bi2Te3 film. This suggests that the single Bi2Se3 layer on top of 19 QLs Bi2Te3 dominates its transport properties.

Anisotropic Fermi surface and quantum limit transport in high mobility three-dimensional Dirac semimetal Cd3As2

Analyzing changes in resistivity is one component of condensed-matter physics research that has applications in the electronics industry. Now, researchers experimentally show that the resistivity of a Cd

Detection of a superconducting phase in a two-atom layer of hexagonal Ga film grown on semiconducting GaN(0001).

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On the origin of critical temperature enhancement in atomically thin superconductors

As

Ising Superconductivity and Quantum Phase Transition in Macro-Size Monolayer NbSe2

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Superconductivity in topologically nontrivial material Au 2 Pb

crystal can be modulated by the geometry of the materials Fermi surface.

Observation of quantum Griffiths singularity and ferromagnetism at the superconducting LaAl O 3 / SrTi O 3 ( 110 ) interface

The recent observation of the superconducting state at atomic scale has motivated the pursuit of exotic condensed phases in two-dimensional (2D) systems. Here we report on a superconducting phase in two-monolayer crystalline Ga films epitaxially grown on wide-band-gap semiconductor GaN(0001). This phase exhibits a hexagonal structure and only 0.552 nm in thickness, nevertheless, brings about a superconducting transition temperature Tc as high as 5.4 K, confirmed by in situ scanning tunneling spectroscopy and ex situ electrical magnetotransport and magnetization measurements. The anisotropy of critical magnetic field and Berezinski-Kosterlitz-Thouless-like transition are observed, typical for the 2D superconductivity. Our results demonstrate a novel platform for exploring atomic-scale 2D superconductors, with great potential for understanding the interface superconductivity.