Meng-Yue Li

Linewidth dependence of coherent terahertz emission from Bi2Sr2CaCu2O8 intrinsic Josephson junction stacks in the hot-spot regime

We report on measurements of the linewidthf of terahertz radiation emitted from intrinsic Josephson junction stacks, using a Nb/AlN/NbN integrated receiver for detection. Previous resolution-limited measurements indicated thatf may be below 1 GHz—much smaller than expected from a purely cavity-induced synchronization. While at low bias we foundf to be not smaller than ∼500 MHz, at high bias, where a hot spot coexists with regions which are still superconducting, �f turned out to be as narrow as 23 MHz. We attribute this to the hot spot acting as a synchronizing element. �f decreases with increasing bath temperature, a behavior reminiscent of motional narrowing in NMR or electron spin resonance (ESR), but hard to explain in standard electrodynamic models of Josephson junctions.

Growth of Single-Crystal Ca10(Pt4As8)(Fe1.8Pt0.2As2)5 Nanowhiskers with Superconductivity up to 33 K

Single-crystal Ca(10)(Pt(4)As(8))(Fe(1.8)Pt(0.2)As(2))(5) superconducting (SC) nanowhiskers with widths down to hundreds of nanometers were successfully grown in a Ta capsule in an evacuated quartz tube by a flux method. Magnetic and electrical properties measurements demonstrate that the whiskers have excellent crystallinity with critical temperature of up to 33 K, upper critical field of 52.8 T, and critical current density of J(c) of 6.0 × 10(5) A/cm(2) (at 26 K). Since cuprate high-T(c) SC whiskers are fragile ceramics, the present intermetallic SC whiskers with high T(c) have better opportunities for device applications. Moreover, although the growth mechanism is not understood well, the technique can be potentially useful for growth of other whiskers containing toxic elements.

Direct observation of the depairing current density in single-crystalline Ba0.5K0.5Fe2As2 microbridge with nanoscale thickness

We investigated the critical current density (Jc) of Ba0.5K0.5Fe2As2 single-crystalline microbridges with thicknesses ranging from 276 to 18 nm. The Jc of the microbridge with thickness down to 91 nm is 10.8 MA/cm2 at 35 K, and reaches 944.4 MA/cm2 by extrapolating Jc(T) to T = 0 K using a two-gap s-wave Ginzburg-Landau model, well in accordance with the depairing current limit. The temperature, magnetic field, and angular-dependence of Jc(T,H,θ) indicated weaker field dependence and weakly anisotropic factor of 1.15 (1 T) and 1.26 (5 T), which also yielded the validity of the anisotropic Ginzburg-Landau scaling.

Nematic superconducting state in iron pnictide superconductors

Nematic order often breaks the tetragonal symmetry of iron-based superconductors. It arises from regular structural transition or electronic instability in the normal phase. Here, we report the observation of a nematic superconducting state, by measuring the angular dependence of the in-plane and out-of-plane magnetoresistivity of Ba0.5K0.5Fe2As2 single crystals. We find large twofold oscillations in the vicinity of the superconducting transition, when the direction of applied magnetic field is rotated within the basal plane. To avoid the influences from sample geometry or current flow direction, the sample was designed as Corbino-shape for in-plane and mesa-shape for out-of-plane measurements. Theoretical analysis shows that the nematic superconductivity arises from the weak mixture of the quasi-degenerate s-wave and d-wave components of the superconducting condensate, most probably induced by a weak anisotropy of stresses inherent to single crystals.Nematic electronic order is rare and its onset often indicates a phase transition. Here, Li et al. report a nematic superconducting state in Ba0.5K0.5Fe2As2 by measuring the angular dependence of the in-plane and out-of-plane magnetoresistivity.

Evolution of internal energy distribution and terahertz radiation of a Josephson junctions stack

Being frequency tunable sources, Josephson junctions are attractive for the generation of high frequency electromagnetic radiation. Recently, coherent off-chip THz radiation with an extrapolated output power of some μW was observed. Different mechanisms, like geometry resonance and heating effect, have been raised to explain how the junctions in the stack are synchronized, while there is no consensus which has been reached. To get further understanding on this issue, it is expected to demonstrate the process that the electromagnetic field and thermal distribution evolve as the detected terahertz(THz) emission signal varies in experiment. To realize this goal, we developed a home-made system, by combining a low-temperature scanning laser microscope (LTSLM) and a THz interferometer. With this setup, we are able to simultaneously detect THz emission and observe LTSLM images. In this talk, we will report the unambiguous observation of the correlation between the standing wave patterns, the hotspot formation and the THz radiation characteristics of the Bi2Sr2CaCu2O8 (BSCCO) junctions stack with a mesa structure. We stress that this joint measurement setup will play an important role in studying solid state microwave or terahertz devices.

Nonlinear and nonlocal electrodynamics of a d-wave superconductor in the Meissner state

We study the competition among thermal, nonlinear and nonlocal effects on the magnetic penetration depth in the Meissner state of a d-wave superconductor. We show that both the nonlocal and nonlinear effects modify the linear T behavior of the penetration depth to a higher power at low T, and the nonlocal effects cut off the nonlinear Meissner effect at low T and low magnetic field.

Results of relaxation function theory for the NMR measurements in La2CuO4

Abstract The spin-lattice relaxation rates and the spin-echo decay rates for the two-dimensional Heisenberg model are interpreted in a framework of a self-consistent relaxation function theory. The results are compared with the experiments on La2CuO4 and the numerical simulations of quantum Monte Carlo method.

Magnetization Curve for One-Dimensional Hubbard Model in Critical Region

A power series expression of the exact solution of the one-dimensional (1D) Hubbard model in an external magnetic field is derived to study the antiferromagnet-ferromagnet transition in the critical region. Aside from the magnetization curve, the critical field and susceptibility are analytically evaluated, and the results of these quantities in some limiting cases are discussed.