Material characterization of superconducting T′-Nd2CuO4 films synthesized by metal organic decomposition

Recently we have achieved superconductivity in T′-RE₂CuO₄ (RE = Pr, Nd, Sm, Eu, and Gd), films by metal organic decomposition (MOD). In this article, we first report the cation off-stoichiometry effect, which aims at screening out the possibility of hole doping by cation deficiencies. We also investigated the structure and microstructure of superconducting T′-Nd₂CuO₄ films synthesized by MOD. This investigation aims at elucidating why MOD-grown Nd₂CuO₄ films become superconducting whereas our previous Nd₂CuO₄ films grown by molecular beam epitaxy were not superconducting.     

Thermodynamic properties of tunneling quasiparticles in graphene-based structures

Thermodynamic properties of quasiparticles in a graphene-based structures are investigated. Two graphene superconducting layers (one superconducting component is placed on the top layered-graphene structure and the other component in the bottom) separated by oxide dielectric layers and one normal graphene layer in the middle. The quasiparticle flow emerged due to external gate voltage, we considered it as a gas of electron–hole pairs whose components belong to different layers. This is a striking result in view of the complexity of these systems: we have established that specific heat exhibits universal (?T³) behavior at low T, independent from the gate voltage and the superconducting gap. The experimental observation of this theoretical prediction would be an important step towards our understanding of critical massless matter.     

Superconducting and normal state properties of tantalumcarbonitrides under hydrostatic pressure

We have investigated the formation and the normal- and superconducting properties of TaC_xN_y. By analogy to NbCN the superconducting transition temperature of the nitrogen-rich samples increases with pressure and decreases for carbon-rich samples. This is correlated with a different resistivity vs temperature behaviour.     

Terahertz conductivity measurement of FeSe0.5Te0.5 and Co-doped BaFe2As2 thin films

The terahertz (THz) conductivity of FeSe_0.5Te_0.5 (‘11’-type) and Co-doped BaFe₂As₂ (‘122’-type) thin films are investigated. For ‘11’-type, the frequency dependence of the complex conductivity can be understood as that of BCS-type superconductor near the superconducting gap energy, and we estimated the superconducting gap energy to be 0.6 meV. For ‘122’-type, we estimated the superconducting gap energy to be 2.8 meV, which is considered to be the superconducting gap opened at the electron-type Fermi surface near the M point.     

Thickness dependence of the perpendicular critical magnetic fields of Pb/Cu double layers

We have investigated the proximity effect of a normal metal (Cu) on the perpendicular critical magnetic fields H_c⊥ of superconducting Pb-films. Special attention was devoted to the dependence of H_c⊥ on the thickness of the superconducting layers. The experimental results are compared with calculations based on the Ginzburg-Landau theory.     

Theory of superconducting contacts

Metallic contacts between superconducting and normal films are known to have a pair-breaking effect on the electron pairs. By limiting our considerations to the gapless regime and to contacts between superconducting and paramagnetic metals the features of this pair-breaking mechanism are investigated. We compute the normalization of the order parameter and with it such quantities as the tunneling conductance, electrical and thermal conductivity and the critical current parallel to the contact. Furthermore we investigate the influence of a magnetic field. By calculating the? ₂(t)-parameter we demonstrate the interplay between the different pair-breaking mechanisms in perpendicular and parallel fields. In the latter case we discuss the influence of the contact on the superconducting sheath proposed bySaint-James andde Gennes. Experiments to test the present theory are suggested.     

Coherent transmission of nodal Dirac fermions through a graphene-based superconducting double barrier junction

Transport characteristics of relativistic electrons through graphene-based d-wave superconducting double barrier junction and ferromagnet/d-wave superconductor/normal metal double junction have been investigated based on the Dirac–Bogoliubov–de Gennes equation. We have first presented the results of superconducting double barrier junction. In the subgap regime, both the crossed Andreev and nonlocal tunneling conductance all oscillate with the bias voltage due to the formation of Andreev bound states in the normal metal region. Moreover, the critical voltage beyond which the crossed Andreev conductance becomes to zero decreases with increasing value of superconducting pair potential α. In the presence of the ferromagnetism, the MR through graphene-based ferromagnet/ d-wave superconductor/normal metal double junction has been investigated. It is shown that the MR increases from exchange splitting h ₀=0 to h ₀=E _F (Fermi energy), and then it goes down. At h ₀=E _F, MR reaches its maximum 100. In contrast to the case of a single superconducting barrier, Andreev bound states also manifest itself in the zero bias MR, which result in a series of peaks except the maximum one at h ₀=E _F. Besides, the resonance peak of the MR can appear at certain bias voltage and structure parameter. Those phenomena mean that the coherent transmission can be tuned by superconducting pair potential, structure parameter, and external bias voltage, which benefits the spin-polarized electron device based on the graphene materials.     

Tunneling conductance in graphene-based normal metal-insulator-superconductor junctions

Based on the transfer-matrix method, we have investigated the coherent quantum transport in a graphene-based normal metal-insulator-superconductor (NG/IG/SG) junctions, in the limit of a thin barrier. It is found that the conductance spectra of such system, in contrast to a π periodic oscillatory behavior is shown for aligned Fermi surfaces of the normal and superconducting regions, exhibit a new π/2 periodic oscillatory behavior as a function of barrier strength (χ) for a large Fermi surface mismatch.     

Comprehensive study of high-Tc interface superconductivity

Using ALL-MBE technique, we have synthesized different heterostructures consisting of an insulator La₂CuO₄ (I) and a metal La_1.56Sr_0.44CuO₄ (M) layer neither of which is superconducting by itself. The M-I bilayers were superconducting with a critical temperature T_c≈30-36 K. This highly robust phenomenon is confined within 1-2 nm from the interface and is primarily caused by the redistribution of doped holes across the interface. In this paper, we present a comprehensive study of the interface superconductivity by a range of experimental techniques including transport measurements of superconducting properties.     

Low temperature magneto-resistance,logarithmic resistivity rise and anisotropic superconductivity in TaS2(pyridine)12

The superconducting intercalation complex TaS₂(pyridine)${}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ has received considerable attention as a prototype of two dimensional superconductivity. In this material we have detected a substantial, anisotropic magneto-resistance 11 K° above the superconducting transition and an anisotropic logarithmic resistivity rise below 20°K. The resistively measured superconducting transition temperature is also anisotropic. T_c is the lowest when the current is perpendicular to the layers.     

Superconductivity and calorimetric studies of Pr1.85Ce0.15CuO4+δ under different annealing conditions

Polycrystalline samples of electron-doped Pr_1.85Ce_0.15CuO_4+δ have been prepared under different annealing conditions and investigated by means of X-ray-diffraction, oxygen content analysis, electrical resistivity, magnetic susceptibility and low temperature specific heat measurements. X-ray-diffraction patterns show that samples contain a single T′ phase. The superconducting transition temperatures T_cm taken with the onset of diamagnetism in magnetic-susceptibility measurements are 20 and 19.5 K for sample annealed in flowing Ar gas and in vacuum (∼10⁻³ torr), respectively. The data of the samples, which are annealed in flowing Ar gas, show clear evidence for an αT² term at zero magnetic field in superconducting electronic specific heat, and are consistent with d-wave superconductivity. However, this behavior is not observed in the other sample, which is annealed in vacuum. These results indicate that different heat treatments affect the oxygen content, homogeneity, superconducting transition temperature T_c, superconducting volume fraction, and the superconducting pairing symmetry of Pr_1.85Ce_0.15CuO_4+δ.     

Coexistence of magnetism and superconductivity in the hole doped FeAs-based superconducting compound

The magnetic and superconducting properties of the Sm-doped FeAs-based superconducting compound were investigated under wide ranges of temperature and magnetic field. After the systematical magnetic ion substitution, the superconducting transition temperature decreases with increasing magnetic moment. The hysteresis loop of the La_0.87?xSm_xSr_0.13FeAsO sample shows a superconducting hysteresis and a paramagnetic background signal. The paramagnetic signal is mainly attributed to the Sm moments. The experiment demonstrates that the coexistence of magnetism and superconductivity in the hole doped FeAs-based superconducting compounds is possible. Unlike the electron doped FeAs-based superconducting compounds SmFeAsOF, the hole doped superconductivity is degraded by the substitution of La by Sm. The hole-doped and electron-doped sides are not symmetric.     

Probing the ground state properties of iron-based superconducting pnictides and related systems by muon-spin spectroscopy

In this short review, we attempt to give a comprehensive discussion of studies performed to date by muon-spin spectroscopy (more precisely the relaxation and rotation technique, also know as μSR) on the recently discovered layered iron-based superconductors. On one side, μSR has been used to characterize the magnetic state of different families of layered iron-based systems. Similarly the subtle interplay of the magnetic state and the structural transition present in some families has been investigated. We will also discuss the information provided by this technique on the interaction between the magnetic state and the superconducting phase. Finally the μSR technique has been used to investigate the magnetic penetration depth of the superconducting ground state. The study of its absolute value, temperature and magnetic field dependence provides crucial tests for investigating possible unconventional superconducting states in such systems.     

Spezifische Wärme und magnetische Suszeptibilität supraleitender,binärer komplexer Phasen von Übergangsmetallen

Measurements of the electronic specific heat in the normal and superconducting state of 15 superconducting binary complex phases of theσ- andχ-structure are presented. The alloys have been prepared under high vacuum in an electron-beam melting apparatus described in detail. In the investigated range between 6 and 7 valence-electrons, the obvious correlation betweenT _c, the superconducting critical temperature, andγ, the coefficient of the electronic specific heat, leads to agreement with the empirical rules, found byMatthias. Recently,Morel andAnderson andGarland have calculated the values of the deviation of the normal isotope-effect. With these values it is possible to relate the observedT _c-data for most of the transition metal alloys investigated so far to the density of states at the Fermi level and to a systematically varying electron-phonon interaction parameter. In the superconducting state, an exponential dependence of the electronic specific heat on 1/T is found in the range betweenT _c/2 andT _c/6. However the parameters are somewhat different from those predicted by theory. The values ofγ observed also account for the lack of any correlation between the total magnetic susceptibility and the superconducting critical temperature for these phases.     

Intermittent superconductivity in three-dimensional mesoscopic rings

Three-dimensional mesoscopic rings are investigated by the phenomenological Ginzburg–Landau theory in the presence of an externally applied magnetic-field. By solving an eigenvalue problem and minimizing systematic free energy, we find that the superconducting and normal states cyclically appear, and that between the state L and the state L + 1 the field extent of existing normal state increases with increase of vorticity L. The dimensional phase diagram with the maximum vorticity and existing normal state between two superconducting states is given. The normal state can be controlled by the medium with surface enhancement or suppression of superconductivity.     

Microscopic description of superconductor-normal-metal superlattices

We study a model for superconductor-normal-metal superlattices in which adjacent layers are coupled via single-particle hopping. Examples include the high-T _c superconductor Bi₂Sr₂CaCu₂O_8+δ, where the BiO sheets seem to have normal metallic character. Using a BCS treatment, we investigate the influence of the interlayer hopping between the superconducting and the normal-metal slabs on the superconducting density of states, the tunneling characteristics for tunneling into both superconducting and normal-metal slabs as well as the temperature dependence of the London penetration depth.     

Identification and preparation of single phase 90 K oxide superconductor and structural determination by lattice imaging

We have investigated various compositions of Ba-Y-Cu-O, resulting in the preparation of single phase 90 K superconducting material of composition (Ba_0.67Y_0.33)CuO_3−δ and the identification of the structure of this superconducting phase by lattice imaging as a layered perovskite. The single phase material had a resistive transition with a midpoint at 92 K and a 1.3 K width (10%–90%) as well as a sharp low-field (H = 0.01 Oe) susceptibility transition.     

Superconductivity of vapour quenched beryllium and beryllium-based alloys

The properties of Be films, quench-condensed upon a³He cooled substrate, have been investigated by resistance and tunneling measurements. The superconducting transition temperature,T _c , of Be films increased with thickness and a thick film limit of 9.95 K could be estimated. Alloying with Al or Pb decreasedT _c. The ratios between energy gaps andT _c 's indicated that Be is a weak coupling superconductor, and no phonon induced structure could be traced in tunneling curves neither in pure Be nor in the Be based alloys. Resistance change during annealing as well as superconducting data indicated that the vapour quenched Be films were amorphous as deposited.     

Spin dynamics in the stripe phase of high-T c cuprates with the modulation of superconducting order

Possible superconducting order modulation and its effect on the spin susceptibility in the coexisting phase of the stripe and superconducting orders are investigated. It is shown that the superconducting order modulation is mainly caused by the spin-domain-derived scattering, while the charge-domain-derived scattering tends to suppress it in a wide parameter region. The modulation leads to a two-peak structure in the spin excitation spectrum, which is qualitatively consistent with the recent experimental observations in La_{2- x} Sr _x CuO₄. This result suggests the importance of the superconducting order modulation for the understanding of the multiform spin excitations in these cuprate superconductors.     

Superconducting properties of tin embedded in nanometer-sized pores of glass

The electrical resistance of tin embedded from a melt in porous glasses with an average pore diameter of ??7 nm has been investigated at low temperatures in magnetic fields up to 2 T. The temperatures of the transition to the superconducting state for nanocrystalline tin have been determined in magnetic fields of 0, 0.3, 0.5, 1.0, 1.5, and 2.0 T. It has been found that the temperature and magnetic-field dependences of the electrical resistance of the nanocomposite under investigation exhibit two transitions to the superconducting state. The nature of the double superconducting transitions has been discussed. The H _c -T _c phase diagram has been constructed using the entire set of data on the magnetic-field and temperature dependences of the electrical resistance of nanostructured tin. This phase diagram indicates that the upper critical magnetic field H _c2(0) for nanostructured tin is almost two orders of magnitude higher than the corresponding field for bulk tin.