Effect of Oxygen Concentration on Superconducting Properties of Rubidium Tungsten Bronzes Rb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>WO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

Superconducting transition temperature (T _c) as a function of oxygen concentration for hexagonal rubidium tungsten bronzes Rb _x WO _y with 2.80 ≤ y ≤ 3.07 and x = 0.19, 0.23, and 0.27 has been systematically investigated. Three regions corresponding to T _c < 2 K (denoted as superconductivity suppressed region), T _c∼ 3 K (superconductivity uniform region) and T _c > 3 K (superconductivity enhanced region) were identified in T _c–y phase diagram for Rb_0.19WO _y and Rb_0.23WO _y . No superconductivity enhanced region was observed for Rb_0.27WO _y . The superconductivity suppressed region shifts toward higher oxygen content as rubidium concentration increases. The local ordering of the intercalated rubidium atoms might be responsible for the intriguing T _c–y phase diagram of Rb _x WO _y .     

Double Correlations in HTSC

The Nambu–Gorkov generalized Hartree–Fock theory of superconductivity is further generalized to treat simultaneously two kinds of correlations: the correlations that lead to superconductivity, as well as the correlations that produce the pseudogap in the normal state. The treatment of these simultaneous double correlations is done by enlarging the dimension of the propagator and self-energy matrices, from two to four. The off-diagonal self-energies and the DOS are calculated. The results are compatible with tunneling and PES experiments.     

High-T c superconductivity in thed-p electron system

The relaxation time with spin flipτ _s and the parametersξ, δ, χ of superconducting phase have been calculated on the basis of the kinematical mechanism of superconductivity in strongly correlated oxide models. An inter-relation between the superconducting gap Δ₀ and the specific heat jump Δ _c allowing the experimental verification was obtained and the Ginsburg-Landau equation derived.     

A Possible 2D H x WO3 Superconductor with a T c of 120 K

Magnetic susceptibility measurements of WO₃ crystals hydrogen doped on the surface suggest 2D local non-percolated superconductivity with an onset temperature of 120 K. Observed zero field cooled vs. field cooled magnetization response is characteristic of type II superconductivity. The diamagnetic response at the critical temperature is field dependent, and is suppressed by a magnetic field of ∼1700 Oe.     

Influence of Antiferromagnetic Fluctuations on the Fulde–Ferrell–Larkin–Ovchinnikov State in CeCoIn5

The Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state is a spatially inhomogeneous superconducting (SC) phase with a periodically modulated order parameter predicted to appear in sufficiently clean type-II superconductors, close to the upper critical field, if the orbital pair breaking is small relative to the Pauli-limiting effect. The heavy-fermion superconductor CeCoIn₅ is the first material, where different physical probes show strong experimental evidence pointing to the realization of the FFLO state, even though strong antiferromagnetic (AFM) spin-fluctuations (SF) are present at atmospheric pressure. To study the influence of the AFM-SF on the FFLO state we performed heat-capacity experiments under pressure. We utilized a newly developed miniature piston-cylinder type pressure cell specially suited for measuring small samples at high-magnetic fields and low temperatures (0 GPa ≤ P ≤ 1.5 GPa, 0 kOe ≤ H ≤ 140 kOe, and 100 mK ≤ T ≤ 4 K). We found the second anomaly inside the SC state in CeCoIn5 can still be observed with pressure, which suppress the strong AFM-SF. The FFLO phase extends to higher fields and temperatures on applying pressure while the Pauli-limiting effect is becoming weaker and the SF are suppressed. This reveals the detrimental effect of the AFM-SF on the FFLO phase stability.      

Local Electronic Structure, O–T Phase Transition and Superconductivity in the Ba-site Nd-substituted YBCO System

A series of YBa_2-xNd_xCu₃O_y (x = 0–0.4) samples have been systematically studied by means of X-ray diffraction, transport property measurements and positron annihilation technology. The positron lifetime parameters show strong Nd substitution dependence. There is an obvious change of positron lifetime parameters around the O–T phase transition. The local electron density n_e and vacancy concentration C_v as a function of x were calculated from the positron lifetime results. The correlations between local electronic structure, O–T phase transition and superconductivity are discussed. The results confirmed that n_e mainly has an effect on high-T_c superconductivity by affecting the charge transfer between CuO₂ planes and Cu–O chains region or Ba–O layer. The vacancy properties in the orthorhombic phase and tetragonal phase are two intrinsic different types. Positron lifetime is very sensitive to the O–T phase transition in the YBCO systems that can be used as a useful technique to determine the O–T phase transition in these systems.     

Specific Heat of Mg11B2, a Two-Gap Superconductor

We report measurements of the specific heat of Mg¹¹B₂, from 1 to 50 K and in magnetic fields to 9 T, and give the values of parameters relevant to the superconductivity. The superconducting-state electron contribution is dramatically different from that of other superconductors, but the general features are consistent with predictions for a two-gap superconductor, and it can be quantitatively represented by a two-gap model based on BCS thermodynamics. Parameters characterizing the gaps are in good agreement with some spectroscopic determinations, and also with theoretical calculations. An unusually strong magnetic field dependence of the temperature-proportional term in the electron contribution to the vortex-state specific heat is evidently another manifestation of the two gaps.     

Nuclear magnetic properties of aluminium

We have demagnetized a 2.92 mole Al sample of 6 N purity and measured its nuclear specific heat at temperatures 21 μK≤T≤20 mK and in magnetic fields of 11.4 mT≤B≤69.0 mT. In addition, we have performed nuclear magnetic resonance experiments on it atT≤175 μK as well as on an Al powder and an AuAl₂ sample at 200 μK≤T≤22 mK. No anomalies could be detected in these temperature and field regions. This shows that Al is a good candidate for a nuclear demagnetization stage and that its nuclear specific heat can be used as a primary thermometer in the μK- and mK-temperature regimes. The NMR data indicate that the nuclear dipole-dipole interaction is dominant in Al. The absence of any anomalies in the spin lattice relaxation time τ₁ toT=34 μK indicates that a measurement of τ₁ also can be used for primary thermometry.     

Mean Field Superconductivity Approach in Two Dimensions

Within the BCS theory of superconductivity we calculate the superconducting gap at a zero-temperature for metallic hydrogen–graphene system in order to estimate the superconducting critical temperature of quasi-two-dimensional highly oriented pyrolytic graphite. The obtained results are given as a function of the hydrogen-induced density of carriers n and their effective mass m ^⋆ . The obtained gap shows a Maxwell-like distribution with a maximum of ∼60 K at n∼3×10¹⁴ cm⁻² and m ^⋆ /m=1. The theoretical results are discussed taking into account recent experimental evidence for granular superconductivity in graphite.      

π-d Interaction in the Field Induced Superconductor λ-(BETS)2FeCl4: Studied by 77Se NMR

⁷⁷Se NMR measurements have been performed in the normal state of the organic, magnetic field induced superconductor λ(BETS)₂FeCl₄, to examine the interaction between Fe local spins and π conduction electrons. Large, negative Knight shift values were observed at fields lower than the one corresponding to onset of field induced superconductivity. This evidences that the conduction electrons feel a negative internal field coming from the Fe spins through the exchange interaction { } · { }. This suggests that the superconductivity induced by high magnetic fields has its origin in the Jaccarino-Peter compensation mechanism. The estimated value of the exchange interaction J, is 30% smaller than that predicted by the resistivity measurements.     

Normalized Free Energy and Normalized Entropy Applied to Some Lambda (λ) Transitions

The λ-anomaly occurs for a system that can undergo a boson – fermion thermodynamic equilibrium. It is shown that a λ-transition figure can be interpreted in terms of the normalized Gibbs–Helmholtz equation, the Maxwell–Boltzmann energy distribution function, and properties of the statistics of the relevant species. There are three variations of a “λ-transition” curve. These are: (A) the classical λ curve, (B) a saw-tooth line shape that is characteristic of the Bardeen–Cooper–Schrieffer theory of superconductivity, and (C) a single line δ type figure. The low temperature He-4 transition, and Type II superconductor transitions are typical of the line shape A. Type I superconductors typically have type B line shapes. The line shapes for variations A and C result from classical thermodynamic equilibria. The type B line shape occurs in systems that do not have a classical thermodynamic equilibrium at the superconducting transition. Analysis of type B line shapes provides interesting concepts and data for some low- and high-temperature superconductors. Several applications and physical property consequences of these line shapes are discussed.     

Phase transitions and hard magnetic properties for rapidly solidified MnAl alloys doped with C,B, and rare earth elements

MnAl alloys are attractive candidates to potentially replace rare earth hard magnets because of their superior mechanical strength, reasonable magnetic properties, and low cost. In this study, the phase transitions and magnetic properties of melt spun Mn₅₅Al₄₅ based alloys doped with C, B, and rare earth (RE) elements were investigated. As-spun Mn–Al, Mn–Al–C, and Mn–Al–C–RE ribbons possessed a hexagonal ε crystal structure. Phase transformations between the ε and the L1₀ (τ) phase are of interest. The ε → τ transformation occurred at ~500 °C and the reversed τ → ε transformation was observed at ~800 °C. Moderate carbon addition promoted the formation of the desired hard magnetic L1₀ τ-phase and improved the hard magnetic properties. The Curie temperature T _C of the τ phase is very sensitive to the C concentration. Dy or Pr doping in MnAlC alloy had no significant effect on T _C. Pr addition can slightly improve the magnetic properties of MnAlC alloy, especially J_S. Doping B could not enhance the magnetic properties of MnAl alloy since B is not able to stabilize either the ε phase or the L1₀ hard magnetic τ phase.     

Superconductivity,Magnetism, and Magnetoresistancein RuSr<Subscript>2</Subscript>R<Subscript>1.4</Subscript>Ce<Subscript>0.6</Subscript>Cu<Subscript>2</Subscript>O<Subscript>10–δ</Subscript> (R = Eu,Sm)

Samples with nominal compositions RuSr₂R_1.4Ce_0.6Cu₂O_10–δ (R = Eu, Sm) were synthesized and their superconducting (SC) and magnetic properties were compared. A coexistence of AFM and FM ordering below the FM transition temperature was established in the two samples. It was shown that their SC properties are affected by a spontaneous vortex phase (SVP). The first critical fields of the two samples were calculate — H_cl≈ 70 Oe for R=Eu and 60 Oe for R=Sm. It was shown that the improved superconducting properties of the Sm-containing sample do not affect its FM behavior. A sizable magnetoresistance was observed in the two samples at T<T _c.     

Theoretical Study of Interplay of Superconductivity and Ferromagnetism in Hybrid Rutheno–Cuprate Superconductors RuSr2RCu2O8

We consider the extended two-band s–f model with additional terms, describing intersite Cooper pairs’ interaction between 4f (5f) and conduction electrons. Following Green’s function technique and equation of motion method, self-consistent equations for superconducting order parameter (Δ) and magnetic order parameter (m _f ) are derived. The expressions for specific heat, density of states, and free energy are also derived. The theory has been applied to explain the coexistence of superconductivity and ferromagnetism in hybrid rutheno-cuprate superconductors RuSr₂RECu₂O₈ (RE = Gd, Eu). The theory shows that it is possible to become superconducting if the system is already ferromagnetic. A study of specific heat, density of states and free energy is also presented. The agreement between theory and experimental observations is quite satisfactory.      

Dielectric Properties of Na<Subscript>0.7</Subscript>CoO<Subscript>2</Subscript> and of the Superconducting Na<Subscript>0.3</Subscript>CoO<Subscript>2</Subscript>·1.3H<Subscript>2</Subscript>O

We present dielectric properties of ceramic anhydrous Na_0.7CoO₂ and the superconducting Na_0.3CoO₂·1.3H₂O materials. The presence of water which induces superconductivity also may increase the dielectric constant (ε) of the hydrated material. This is consistent with the predicted relationship between the highε and the enhancement ofT _c in highT _c superconductors. The anhydrous sample is porous and the transport is due to some percolation via the pores. The porosity is much higher for the hydrated material and the transport is ionic inside bulk water.     

Theoretical Study of Magnetic Behavior in Ru-based Ferromagnetic Superconductors

A microscopic theory of interplay of superconductivity and ferromagnetism in hybrid rutheno-cuprate superconductors RuSr₂RECu₂O₈ (Ru-1212) (RE=Gd, Eu) is developed from first principles considering s–f model. Self-consistent equations for superconducting order parameter Δ and ferromagnetic order parameter 〈S ^Z 〉 are obtained using Green’s function technique and equation of motion method. The theory is applied to explain the experimental results for RuSr₂RECu₂O₈ system. This model clearly predicts the coexistence of superconductivity and spatially uniform ferromagnetism in RuSr₂RECu₂O₈. The theory shows that it is possible to become superconducting if the system is already ferromagnetic. Study of specific heat, density of states, free energy, critical field, and ac spin susceptibility is also presented. Agreement between experimental results and theory is quite convincing.     

Possibility of Superconductivity of the 2212 Phase in the Bi(Pb)-Sr-Ce-Cu-O System

Synthesis of the Bi-2212 compound in the Bi-Sr-Ce-Cu-O system has been already known. But, there has been no report on superconductivity of the compound yet. We have prepared many Bi-2212 samples partially substituted by Pb for Bi in the Bi-Sr-Ce-Cu-O system. The nominal composition is (Bi_2−y Pb _y )Sr₂(Sr_1−x Ce _x )Cu₂O _z . Then, we have investigated possibility of superconductivity for the samples. As a result, we find that a sample with nominal composition ofx=0.23 andy=0.1, which is of almost the single 2212 phase, shows an anomaly at about 70 K in addition to temperature dependence of the resistivity like a semiconductor. Furthermore, the sample also shows a decrease of magnetic susceptibility starting at about 70 K with decreasing temperature. These experimental results can be considered to result from superconductivity of the 2212 phase in the Bi(Pb)-Sr-Ce-Cu-O system.     

Charge Expulsion, Spin Meissner Effect, and Charge Inhomogeneity in Superconductors

Superconductivity occurs in systems that have a lot of negative charge: the highly negatively charged (CuO₂)⁼ planes in the cuprates, negatively charged (FeAs)⁻ planes in the iron arsenides, and negatively charged B ⁻ planes in magnesium diboride. And, in the nearly filled (with negative electrons) bands of almost all superconductors, as evidenced by their positive Hall coefficient in the normal state. No explanation for this charge asymmetry is provided by the conventional theory of superconductivity, within which the sign of electric charge plays no role. Instead, the sign of the charge carriers plays a key role in the theory of hole superconductivity, according to which metals become superconducting because they are driven to expel negative charge (electrons) from their interior. This is why NIS tunneling spectra are asymmetric, with larger current for negatively biased samples. The theory also offers a compelling explanation of the Meissner effect: as electrons are expelled towards the surface in the presence of a magnetic field, the Lorentz force imparts them with azimuthal velocity, thus generating the surface Meissner current that screens the interior magnetic field. In type II superconductors, the Lorentz force acting on expelled electrons that do not reach the surface gives rise to the azimuthal velocity of the vortex currents. In the absence of applied magnetic field, expelled electrons still acquire azimuthal velocity, due to the spin–orbit interaction, in opposite direction for spin-up and spin-down electrons: the “Spin Meissner effect.” This results in a macroscopic spin current flowing near the surface of superconductors in the absence of applied fields, of magnitude equal to the critical charge current (in appropriate units). Charge expulsion also gives rise to an interior outward-pointing electric field and to excess negative charge near the surface. In strongly type II superconductors this physics should give rise to charge inhomogeneity and spin currents throughout the interior of the superconductor, to large sensitivity to (non-magnetic) disorder and to a strong tendency to phase separation.     

Influence of grain properties on the Abrikosov vortices interaction with Josephson junction

The problem of the displacement of an Abrikosov vortex is investigated when the magnetic field decreases to H _cl ^G in a grain of a type II superconductor. It is shown that near grain boundaries the vortex line generates an intergranular current which depends on the normalized grain size τ and the anisotropy ratio ν. These parameters strongly influence the conditions of the Josephson vortex generation as a result of the interaction of the Abrikosov vortex with the Josephson junction. We support our theory with calculations of the intergrain critical current taking into account two types of the vortex configuration, triangular and square, for different grain characteristics. The results are of interest for the charge transport in granular superconductors as well as the relaxation process in devices that contain Josephson junctions in micro- and nanoelectronics in magnetic field.     

Anisotropy of Superconducting Single Crystal SmFeAsO0.8F0.2 Studied by Torque Magnetometry

Single crystals of the oxypnictide superconductor SmFeAsO_0.8F_0.2 with T _c≃45(1) K were investigated by torque magnetometry. The crystals of mass ≤0.1 μg were grown by a high-pressure cubic anvil technique. The use of a high-sensitive piezoresistive torque sensor made it possible to study the anisotropic magnetic properties of these tiny crystals. The anisotropy parameter γ was found to be field independent, but varies strongly with temperature ranging from γ≃8 at T≲T _c to γ≃23 at T≃0.4T _c. This unusual behavior of γ signals unconventional superconductivity.