Y.Q. Wang

Magnetic torque evidence for the Fulde-Ferrell-Larkin-Ovchinnikov state in the layered organic superconductor κ − ( BEDT − TTF ) 2 Cu ( NCS ) 2

We present magnetic-torque measurements of the organic superconductor k-(BEDT-TTF)2Cu(NCS)2 for in-plane magnetic fields up to 32 T. In this layered two-dimensional compound the superconductivity can persist even in fields above the Pauli limit of about 21 T. There, a pronounced upturn of the upper-critical-field line occurs and the superconducting phase-transition line splits and forms an additional high magnetic field phase. k-(BEDT-TTF)2Cu(NCS)2 is a spin-singlet superconductor, therefore, such a superconducting high-field phase beyond the Pauli limit can originate only from Cooper pairing with finite center-of-mass momentum. The measurements are discussed in connection with a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, in accordance with earlier specific-heat observations. The torque experiments allow us to investigate the high magnetic-field phase diagram and the FFLO state of k-(BEDT-TTF)2Cu(NCS)2 in great detail.

Superconducting transitions of intrinsic arrays of weakly coupled one-dimensional superconducting chains: the case of the extreme quasi-1D superconductor Tl2Mo6Se6

Tl2Mo6Se6 represents a model system for quasi-one-dimensional (quasi-1D) superconductors. We investigate its superconducting transition in detail by means of electrical transport experiments on high-quality single crystalline samples with onset Tcxa0=xa06.8xa0K. Our measurements indicate a highly complex superconducting transition that occurs in different stages, with a characteristic bump in the resistivity and distinct plateau structures in the supercurrent gap imaged by V–I curves. We interpret these features as fingerprints of the gradual establishment of global phase coherence in an array of weakly coupled parallel 1D superconducting bundles. In this way, we demonstrate that superconducting Tl2Mo6Se6 behaves like an intrinsic array of proximity or Josephson junctions, undergoing a complex superconducting phase-ordering transition at 4.5xa0K that shows many similarities to the Berezinskii–Kosterlitz–Thouless transition.

The Fulde–Ferrell–Larkin–Ovchinnikov state in the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 as observed in magnetic-torque experiments

Abstract We present magnetic-torque experiments on the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 for magnetic fields applied parallel to the 2D superconducting layers. The experiments show a crossover from a second-order to a first-order transition when the upper critical field reaches 21xa0T. Beyond this field, which we interpret as the Pauli limit for superconductivity, the upper critical field line shows a pronounced upturn and a phase transition line separates the superconducting state into a low- and a high-field phase. We interpret the data in the framework of a Fulde–Ferrell–Larkin–Ovchinnikov state.

Optical anisotropy of (11N) GaAs/GaAlAs superlattices

Abstract The in-plane anisotropy of a series of (113), (115), (001) vicinal, and singular (001) oriented GaAs/Ga 0.7 Al 0.3 As superlattices (SLs) has been studied by reflectance difference spectroscopy (RDS) in the photon energy range covering both the E 0 and the E 1 energies of GaAs, and from 80 to 300 K. The polarity and the energy position of the observed RDS resonances near the E 0 energy confirm that these resonances are indeed originated from the heavy hole (HH), the light hole (LH), and the Γ 7 LH subbands in the GaAs wells. The transition strength anisotropy is in agreement with the multiband k * p model calculations. Sizable RD resonances have been observed in the (100) singular SLs at low temperatures, which are believed to be due to the HH and the LH exciton bound to anisotropic interface structural defects. The optical anisotropy of the SLs near the E 1 critical energy of GaAs shows complicated resonance patterns. Some of the resonances occur below the E 1 energy, which the simple effective-mass theory for the L-point states cannot explain.

Thermodynamic evidence for pressure-induced bulk superconductivity in the Fe–As pnictide superconductor CaFe2As2

We report on specific-heat and resistivity experiments performed in parallel in a Bridgman-type pressure cell in order to investigate the nature of pressure-induced superconductivity in the iron pnictide compound CaFe2As2. The presence of a pronounced specific-heat anomaly at Tc reveals the bulk nature of the superconducting state. The thermodynamic transition temperature differs dramatically from the onset of the resistive transition. Our data indicate that superconductivity occurs in the vicinity of a crystallographic phase transition. We discuss the discrepancy between the two methods as caused by strain-induced superconducting precursors formed above the bulk thermodynamic transition due to the vicinity of the structural instability.

Factors affecting the shape of MBE-grown laterally aligned Fe nanowires.

Various microstructural and chemical analysis techniques were applied to study two types (type-A and B) of self-assembled laterally aligned Fe nanowires (NWs) fabricated by molecular beam epitaxy on a ZnS buffer layer. The formation of the three-dimensional shapes of these NWs was found to be driven by the principle of surface energy minimization. We have provided phenomenological models to address the factors affecting the observed topological shape of these NWs, including the role of the lattice relationship between the Fe NWs and the underlying buffer layer, growth temperature, Fe nominal coverage and substrate orientation. Magnetic hysteresis measurements were performed at different temperature, demonstrating the Fe NWs possess a coercivity about 30 times larger than that of a Fe thin film. The observed gradual magnetization reversal indicates the magnetization process is accomplished by the rotation of magnetic moments within a single domain.

The current-voltage characteristics of weakly coupled /spl delta/-doped GaAs/Al/sub x/Ga/sub 1-x/As superlattices

The /spl delta/-doped GaAs/Al/sub x/Ga/sub 1-x/As superlattice samples are grown by molecular beam epitaxy. The current- voltage, l(V). characteristics of the samples are measured at 1.5 K. The I(V) curves show the current-plateaus corresponding to the formation of the low- and high-electric field domains due to the ground state resonance and the first excited state to ground state resonance, respectively, in the quantum well. In addition to the typical sawtooth-like current branches, a new small current peak is observed accompanying each sawtooth-like current branch in our samples. Possible origins of the additional peaks are discussed in terms of cross-domain boundary tunneling and phonon emissions.

The study of optical properties of Zn/sub 1-x/MN/sub x/Se thin films grown by MOCVD on GaAs substrates

A series of Zn/sub 1-x/Mn/sub x/Se thin film samples with different Mn compositions were grown by metal-organic chemical vapor deposition. X-ray diffraction measurements indicated the good crystallinity of samples. Two emission peaks were observed in photoluminescence (PL) spectra close to the band edge for the samples. The temperature dependence of PL shows the change of the relative intensity of the two emission peaks, which is attributed to the competition between band-to-band excitonic transitions and transitions of localized excitons bound to Mn-induced impurity bound states. Time-resolved PL measurements and PL under different excitation power also support this analysis. Further investigation implies that the impurity bound states are associated with Mn incorporation.

Novel heterojunction bipolar transistors (HBTs) with significantly reduced emitter current crowding effect

HBTs with integrated vertical ballasting resistors for improving the thermal stability and current handling capability have been fabricated. Both computer simulation and experimental results show that the performance of these transistors are very promising in microwave power and mobile communication applications.