Nano-wires with surface disorder: Giant localization lengths and quantum-to-classical crossover

We investigate electronic quantum transport through nano-wires with one-sided surface roughness. A magnetic field perpendicular to the scattering region is shown to lead to exponentially diverging localization lengths in the quantum-to-classical crossover regime. This effect can be quantitatively accounted for by tunneling between the regular and the chaotic components of the underlying mixed classical phase space.Comment: 4 pages, 3 figures; final version (including added references

Picosecond timing of Microwave Cherenkov Impulses from High-Energy Particle Showers Using Dielectric-loaded Waveguides

We report on the first measurements of coherent microwave impulses from high-energy particle-induced electromagnetic showers generated via the Askaryan effect in a dielectric-loaded waveguide. Bunches of 12.16 GeV electrons with total bunch energy of $\sim 10^3-10^4$ GeV were pre-showered in tungsten, and then measured with WR-51 rectangular (12.6 mm by 6.3 mm) waveguide elements loaded with solid alumina ($Al_2 O_3$) bars. In the 5-8 GHz $TE_{10}$ single-mode band determined by the presence of the dielectric in the waveguide, we observed band-limited microwave impulses with amplitude proportional to bunch energy. Signals in different waveguide elements measuring the same shower were used to estimate relative time differences with 2.3 picosecond precision. These measurements establish a basis for using arrays of alumina-loaded waveguide elements, with exceptional radiation hardness, as very high precision timing planes for high-energy physics detectors.Comment: 16 pages, 15 figure

The Unusual Superconducting State at 49 K in Electron-Doped CaFe2As2 at Ambient

We report the detection of unusual superconductivity up to 49 K in single crystalline CaFe2As2 via electron-doping by partial replacement of Ca by rare-earth. The superconducting transition observed suggests the possible existence of two phases: one starting at ~ 49 K, which has a low critical field ~ 4 Oe, and the other at ~ 21 K, with a much higher critical field > 5 T. Our observations are in strong contrast to previous reports of doping or pressurizing layered compounds AeFe2As2 (or Ae122), where Ae = Ca, Sr or Ba. In Ae122, hole-doping has been previously observed to generate superconductivity with a transition temperature (Tc) only up to 38 K and pressurization has been reported to produce superconductivity with a Tc up to 30 K. The unusual 49 K phase detected will be discussed.Comment: 11 pages, 8 figure

Shot-noise limited monitoring and phase locking of the motion of a single trapped ion

We perform high-resolution real-time read-out of the motion of a single trapped and laser-cooled Ba ion. By using an interferometric setup we demonstrate shot-noise limited measurement of thermal oscillations with resolution of 4 times the standard quantum limit. We apply the real-time monitoring for phase control of the ion motion through a feedback loop, suppressing the photon recoil-induced phase diffusion. Due to the spectral narrowing in phase-locked mode, the coherent ion oscillation is measured with resolution of about 0.3 times the standard quantum limit

Iron pnictides: Single crystal growth and effect of doping on structural, transport and magnetic properties

We demonstrate the preparation of large, free standing iron pnictide single crystals with a size up to 20 x 10 x 1 mm3 using solvents in zirconia crucibles under argon atmosphere. Transport and magnetic properties are investigated to study the effect of potassium doping on the structural and superconducting property of the compounds. The spin density wave (SDW) anomaly at Ts ~138 K in BaFe2As2 single crystals from self-flux shifts to Ts ~85 K due to Sn solvent growth. We show direct evidence for an incorporation of Sn on the Fe site. The electrical resistivity data show a sharp superconducting transition temperature Tc~38.5 K for the single crystal of Ba0.68K0.32Fe2As2. A nearly 100% shielding fraction and bulk nature of the superconductivity for the single crystal were confirmed by magnetic susceptibility data. A sharp transition Tc~25 K occurred for the single crystal of Sr0.85K0.15Fe2As2. There is direct evidence for a coexistence of the SDW and superconductivity in the low doping regime of Sr1-xKxFe2As2 single crystals. Structural implications of the doping effects as well as the coexistence of the two order parameters are discussed.Comment: 22 pages, 9 figure

Nonlinear acousto-electric transport in a two-dimensional electron system

We study both theoretically and experimentally the nonlinear interaction between an intense surface acoustic wave and a two-dimensional electron plasma in semiconductor-piezocrystal hybrid structures. The experiments on hybrid systems exhibit strongly nonlinear acousto-electric effects. The plasma turns into moving electron stripes, the acousto-electric current reaches its maximum, and the sound absorption strongly decreases. To describe the nonlinear phenomena, we develop a coupled-amplitude method for a two-dimensional system in the strongly nonlinear regime of interaction. At low electron densities the absorption coefficient decreases with increasing sound intensity, whereas at high electron density the absorption coefficient is not a monotonous function of the sound intensity. High-harmonic generation coefficients as a function of the sound intensity have a nontrivial behavior. Theory and experiment are found to be in a good agreement.Comment: 27 pages, 6 figure

Electron doped Ca10(Pt3As8)(Fe2As2)5 and Ca10(Pt4As8)(Fe2As2)5 - High Tc superconductors with skutterudite intermediary layers

It has been argued that the very high transition temperatures of the highest Tc cuprate superconductors are facilitated by enhanced CuO2 plane coupling through the (Bi,Tl,Hg)-O intermediary layers. Whether enhanced coupling through intermediary layers can also influence Tc in the iron arsenide superconductors has never been tested due the lack of appropriate systems for study. Here we report the crystal structures and properties of two iron arsenide superconductors, Ca10(Pt3As8)(Fe2As2)5 (the 10-3-8 phase) and Ca10(Pt4As8)(Fe2As2)5 (the 10-4-8 phase). Based on -Ca-(PtnAs8)-Ca-Fe2As2- layer stacking, the most important difference in the structures lies in the structural and electronic characters of the intermediary platinum arsenide layers. Electron doping through partial substitution of Pt for Fe in the Fe2As2 layers leads to Tc of 11 K in the 10-3-8 phase and 25 K in the 10-4-8 phase. Using the chemical concepts of Zintl ion electron counting and the stability of Pt in the 5d8 configuration we argue that the dramatic difference in Tc arises because the intermediary layer is semiconducting in the 10-3-8 phase but metallic in the 10-4-8 phase, leading to enhanced interlayer coupling in the 10-4-8 phase. The results suggest that metallic intermediary layers may offer a new road to higher Tc in iron arsenide superconductors

Bound states in the continuum in open Aharonov-Bohm rings

Using formalism of effective Hamiltonian we consider bound states in continuum (BIC). They are those eigen states of non-hermitian effective Hamiltonian which have real eigen values. It is shown that BICs are orthogonal to open channels of the leads, i.e. disconnected from the continuum. As a result BICs can be superposed to transport solution with arbitrary coefficient and exist in propagation band. The one-dimensional Aharonov-Bohm rings that are opened by attaching single-channel leads to them allow exact consideration of BICs. BICs occur at discrete values of energy and magnetic flux however it's realization strongly depend on a way to the BIC's point.Comment: 5 pgaes, 4 figure