Off-axis MgB2 films using an in situ annealing pulsed laser deposition method

Highly smooth and c-axis oriented superconducting MgB2 thin films were prepared by pulsed laser deposition (PLD) with off-axis geometry. The films were deposited on Al2O3-C substrates perpendicularly aligned to a stoichiometric MgB2 target in a 120 mTorr high purity Ar background gas. An in situ annealing was carried out at 650C for 1 min in a 760 Torr Ar atmosphere. Despite the short annealing time, an x-ray theta-2 theta scan shows fairly good crystallization, according to the clear c-axis oriented peaks for the films. Both atomic force microscopy and the x-ray diffraction results indicate that the crystallite size is less than 50nm. The root mean square roughness of our off-axis film is ~4 nm in a 5x5 square micron area. The Tc onset value of the best off-axis film reaches 33.1 K with a narrow transition width of 0.9 K. The films showed no anisotropy in Hc2-T curves when parallel and perpendicular fields were applied. The slope of Hc2-T curves in low field regime is 1 T/K, which is among the highest reported values.Comment: 7 pages 7 figure

Superconducting Properties of Carbonaceous Chemical Doped MgB2

The discovery of superconductivity in magnesium diboride (MgB2: 39 K, in January 2001) (Nagamatsu et al., 2001) has generated enormous interest and excitement in the superconductivity community and the world in general, but especially among researchers into superconductivity in non-oxide and boron related compounds

Multiple Fermi pockets revealed by Shubnikov-de Haas oscillations in WTe2

We use magneto-transport measurements to investigate the electronic structure of WTe2 single crystals. A non-saturating and parabolic magnetoresistance is observed in the temperature range between 2.5 to 200 K and magnetic fields up to 8 T. Shubnikov - de Haas oscillations with beating patterns are observed. The fast Fourier transform of the SdH oscillations reveals three oscillation frequencies, corresponding to three pairs of Fermi pockets with comparable effective masses , m* ~ 0.31 me. By fitting the Hall resistivity, we infer the presence of one pair of electron pockets and two pairs of hole pockets, together with nearly perfect compensation of the electron-hole carrier concentration. These magnetotransport measurements reveal the complex electronic structure in WTe2, explaining the nonsaturating magnetoresistance.Comment: Submitted to journal on 1 April, 2015, 4 Figure

Observation of topological transition of Fermi surface from a spindle-torus to a torus in large bulk Rashba spin-split BiTeCl

The recently observed large Rashba-type spin splitting in the BiTeX (X = I, Br, Cl) bulk states due to the absence of inversion asymmetry and large charge polarity enables observation of the transition in Fermi surface topology from spindle-torus to torus with varying the carrier density. These BiTeX systems with high spin-orbit energy scales offer an ideal platform for achieving practical spintronic applications and realizing non-trivial phenomena such as topological superconductivity and Majorana fermions. Here we use Shubnikov-de Haas oscillations to investigate the electronic structure of the bulk conduction band of BiTeCl single crystals with different carrier densities. We observe the topological transition of the Fermi surface (FS) from a spindle-torus to a torus. The Landau level fan diagram reveals the expected non-trivial {\pi} Berry phase for both the inner and outer FSs. Angle-dependent oscillation measurements reveal three-dimensional FS topology when the Fermi level lies in the vicinity of the Dirac point. All the observations are consistent with large Rashba spin-orbit splitting in the bulk conduction band.Comment: 28 pages, supplementary informatio

Critical currents and vortex pinning in 235U doped Ag/Bi2223 tapes

Systematic analysis of the transport critical current densities J_c at 77 K for ^235U-oxide (UO_2·2H_2O) doped Ag-sheathed (Bi,Pb)_2Sr_2Ca_2Cu_3O_x tapes (Bi2223) irradiated with thermal neutrons is presented. Within the explored range of U-oxide concentrations (c £ 2 wt%) and neutron fluences (ϕ £ 6·10_19 n/m^2), the pinning of vortices in irradiated tapes increases approximately linearly with fission track density (proportional to cϕ), as evidenced by progressively weaker magnetic field dependence of Jc on increasing cf. Accordingly, the crossover field B* at which the pinning force density F_p reaches its maximum value F_pmax = J_cB* and the irreversibility field B_irr(F_p → 0) increase approximately linearly with cϕ. Therefore, J_c-B and F_p-B variations for all tapes with vortex pinning dominated by fission tracks show universal scaling behaviour with the scaling parameter B* (proportional to cϕ). Taking into account the adverse effects of c and f on Jc of our tapes, we estimate a maximum B* and B_irr that can be reached in ^235U-oxide doped Bi2223 tapes at 77 K

Kritične struje i zapinjanje magnetskih vrtloga u Ag/Bi2223 vrpcama dopiranim s 235U i ozračenim termičkim neutronima

Systematic analysis of the transport critical current densities Jc at 77 K for 235Uoxide (UO2 · 2H2O) doped Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox tapes (Bi2223) irradiated with thermal neutrons is presented. Within the explored range of U-oxide concentrations (c ≤ 2 wt%) and neutron fluences (φ ≤ 6 · 1019 n/m2), the pinning of vortices in irradiated tapes increases approximately linearly with fission track density (proportional to cφ), as evidenced by progressively weaker magnetic field dependence of Jc on increasing cφ. Accordingly, the crossover field B∗ at which the pinning force density Fp reaches its maximum value Fp max = JcB∗ and the irreversibility field Birr(Fp → 0) increase approximately linearly with cφ. Therefore, Jc − B and Fp − B variations for all tapes with vortex pinning dominated by fission tracks show universal scaling behaviour with the scaling parameter B∗ (proportional to cφ). Taking into account the adverse effects of c and φ on Jc of our tapes, we estimate a maximum B∗ and Birr that can be reached in 235U-oxide doped Bi2223 tapes at 77 K.Predstavljamo sustavnu analizu transportnih gustoća kritične struje Jc na 77 K sa 235U-oksidom (UO2 · 2H2O) dopiranih, srebrom obloženih (Bi,Pb)2Sr2Ca2Cu3Ox vrpci ozračenih termičkim neutronima. U istraživanom području koncentracija U-oksida (c ≤ 2 tež.%) i toka neutrona (φ ≤ 6 · 1019 n/m2), zapinjanje magnetskih vrtloga u vrpcama raste približno linearno s gustoćom fisijskih tragova (razmjernoj sa cφ), što se ogleda u sve slabijoj ovisnosti Jc o magnetskom polju pri porastu cφ. U skladu s time, prijelazno polje B∗ na kojem gustoća sile zapinjanja Fp postiže najveću vrijednost Fp max = JcB∗ i ireverzibilno polje Birr(Fp → 0) rastu približno linearno s cφ. Zbog toga, Jc − B i Fp − B ovisnosti za sve vrpce u kojima je zapinjanje vrtloga pretežito zbog fisijskih tragova pokazuju univerzalno skaliranje s parametrom skaliranja B∗ (razmjerno cφ). Uzimajući u obzir negativni utjecaj c i φ na Jc naših vrpci, procijenili smo najveća polja B∗ i Birr koja se mogu postići u srebrom obloženim (Bi,Pb)2Sr2Ca2Cu3Ox vrpcama dopiranim s 235U-oksidom na 77 K