Recently it was demonstrated that Sr intercalation provides a new route to induce superconductivity in the topological insulator Bi2Se3. Topological superconductors are predicted to be unconventional with an odd-parity pairing symmetry. An adequate probe to test for unconventional superconductivity is the upper critical field, Bc2. For a standard BCS layered superconductor Bc2 shows an anisotropy when the magnetic field is applied parallel and perpendicular to the layers, but is isotropic when the field is rotated in the plane of the layers. Here we report measurements of the upper critical field of superconducting SrxBi2Se3 crystals (Tc = 3.0 K). Surprisingly, field-angle dependent magnetotransport measurements reveal a large anisotropy of Bc2 when the magnet field is rotated in the basal plane. The large two-fold anisotropy, while six-fold is anticipated, cannot be explained with the Ginzburg-Landau anisotropic effective mass model or flux flow induced by the Lorentz force. The rotational symmetry breaking of Bc2 indicates unconventional superconductivity with odd-parity spin-triplet Cooper pairs (Δ4-pairing) recently proposed for rhombohedral topological superconductors, or might have a structural nature, such as self-organized stripe ordering of Sr atoms.

We report a high-pressure single crystal study of the topological superconductor Cu{x}Bi{2}Se{3}. Resistivity measurements under pressure show superconductivity is depressed smoothly. At the same time the metallic behavior is gradually lost. The upper-critical field data B{c2}(T) under pressure collapse onto a universal curve. The absence of Pauli limiting and the comparison of B{c2}(T) to a polar-state function point to spin-triplet superconductivity, but an anisotropic spin-singlet state cannot be discarded completely.

The Kibble-Zurek (KZ) mechanism has been applied to a variety of systems ranging from low temperature Bose-Einstein condensations to grand unification scales in particle physics and cosmology and from classical phase transitions to quantum phase transitions. Here we show that finite-time scaling (FTS) provides a detailed improved understanding of the mechanism. In particular, the finite time scale, which is introduced by the external driving (or quenching) and results in FTS, is the origin of the division of the adiabatic regimes from the impulse regime in the KZ mechanism. The origin of the KZ scaling for the defect density, generated during the driving through a critical point, is not that the correlation length ceases growing in the nonadiabatic impulse regime, but rather, is that it is taken over by the effective finite length scale corresponding to the finite time scale. We also show that FTS accounts well for and improves the scaling ansatz proposed recently by Liu, Polkovnikov, and Sandvik [Phys. Rev. B 89, 054307 (2014)]. Further, we show that their universal power-law scaling form applies only to some observables in cooling but not to heating. Even in cooling, it is invalid either when an appropriate external field is present. However, this finite-time-finite-size scaling calls for caution in application of FTS. Detailed scaling behaviors of the FTS and finite-size scaling, along with their crossover, are explicitly demonstrated, with the dynamic critical exponent z being estimated for two-and three-dimensional Ising models under the usual Metropolis dynamics. These values of z are found to give rise to better data collapses than the extant values do in most cases but take on different values in heating and cooling in both twoand three-dimensional spaces.

Abstract:We demonstrate mode-division multiplexed WDM transmission over 50-km of few-mode fiber using the fiber's LP 01 and two degenerate LP 11 modes. A few-mode EDFA is used to boost the power of the output signal before a few-mode coherent receiver. A 6×6 time-domain MIMO equalizer is used to recover the transmitted data. We also experimentally characterize the 50-km few-mode fiber and the few-mode EDFA.

Recently, there was a letter [1] posted on arxiv.org/abs/1904.06463 by M. Kayyalha et al. reporting a transport study on a quantum anomalous Hall insulator (QAHI)superconductor (SC) structure. In their arXiv letter, the authors used the device structure similar to that used in our previous report [2], with different substrate and details. Yet in their devices, the two-terminal conductance (σ12) was found to always near but below ~0.5e 2 /h (half-plateaus), and could not go above this value at the quantum anomalous Hall temperature over a wide range of external magnetic field, i.e. from 0 to ~ 3 T as shown in Fig. 3 (a) and 4 of [1]. The authors attributed their observation to the shunting effect of two quantum anomalous Hall edges by the SC layer. Here, we would like to point out that the half-plateaus observed in their arXiv letter are, in fact, different from those observed from our previous results [2], in which the half-plateaus occur near the magnetization reversal regions ( Fig. 1 of this note, reproduced from Ref. [2]).In clarifying and addressing this issue, we present additional magneto-transport data for our similar QAHI-SC samples fabricated after Ref.[2] was published. The results taken from these new samples are shown in Fig. 2 of this note. It can be clearly seen that the halfplateaus are well developed before the magnetization reversal regions, again validating our previous results. Furthermore, by varying the conditions of samples as well as the materials properties, the widths of the half-plateaus can be adjusted. Our studies and our results on the new structures not only demonstrate that the appearance of the half-plateaus is reproducible over different devices, but also further point out that the QAHI-SC coupling, the details of processing, and material properties play important roles in obtaining the halfplateaus. More investigative experiments continue. Acknowledgment