Superconductivity of compressed platinum powder at very low temperatures

Superconductivity of compressed, high-purity platinum powder (average grain size 2–3 μm) was found by measurements of resistivity, AC susceptibility and magnetization. The transition temperature into the superconducting state T_c and the critical magnetic field B_c strongly depend on the packing fraction f of the samples: we found 0.62T_c(0)1.38 mK and 6.6B_c(0)67 μT for 0.8f0.5, respectively. The temperature dependence of the critical magnetic fields can be described by B_c(T)=B_c(0)(1−(T/T_c)²). The discussion of these results includes possible explanations for the origin of superconductivity in this new superconducting material.     

Fabrication, microstructure and critical current density of screen-printed Ag-(Bi, Pb)2Sr2Ca2Cu3Ox superconducting tapes

The influence of the sintering conditions on the microstructure and critical current density J_c has been studied on screen-printed Ag-(Bi, Pb)₂Sr₂Ca₂Cu₃O_x tapes with a ceramics mono-layer core. Three kinds of fabrication processes, which consist of a combination of cold working (rolling and/or pressing) and sintering, are applied. Four times repetition of pressing and sintering after the pre-sintering produces the highest c-axis alignment and achieves J_c= 1.5 × 10⁴ A/cm² (77 K, 0 T). The J_c versus θ data with an angle θ between B and the c-axis elucidate the relation between the anisotropy ratio γ=J_c(Bc)/J_c(B|c and the half-height angular width Δθ of a peak for Bc. This is related to both grain alignment and the J_c value. An increase in J_c, which comes from an improvement for grain alignment, enhances γ and narrows Δθ. The J_c versus θ data are fitted to the expression J_c(B, θ)=J _c(B, 90°)/[(γ−1)|cos θ|ⁿ+1] by regarding both γ and n as adjustable parameters. Fabrication of screen-printed tapes with multilayers (1≤N≤5) is presented, where the critical current increases from 8.0 A to 30.2 A at 77 K and 0 T as N increases.     

The contribution of Bc mesons to the search for B → τ ντ decays at LEP

We study the contribution of B_c mesons to the search for B → τν_τ decays. We find that at LEP the contributions from B_u and B_c mesons can be comparable. This observation can have a relevant impact on the extraction of constraints on new physics (such as charged-Higgs contributions) from current LEP limits on B → τν final states. Inclusion of the B_c contribution can reduce the current L3 limit on tan β/M_H from 0.38 GeV⁻¹ (90% CL) down to 0.27 GeV⁻¹ (90% CL).     

Reduced s-d model with antiferromagnetically coupled impurity spins

An antiferromagnetic equivalent-neighbour Heisenberg interaction H_i between impurity spins is added to the reduced s-d Hamiltonian H_r previously introduced by simplifying the Kondo s-d exchange Hamiltonian H_K. Asymptotic mean-field theory is developed for H_r + H_i, in the presence and absence of external magnetic field, and applied to (La_1−xCe_x)Al₂ alloys. Specific heat c_i(T) and zero-field susceptibility χ_i(0,T) curves for (La_1−xCe_x)Al₂ are depicted. The coupling constants of H_r + H_i and conduction bandwidth are adjusted so that T_c temperatures for x = 0.2, 0.1 are equal to the experimental values. c_i(T) exhibits a jump at T_c and is decreasing for T < T_c. χ_i(0,T) has a first order pole at T_c which corresponds to the maximum of experimental susceptibility and χ_i(0,0) > 0. These results improve those obtained earlier on the grounds of H_r theory.     

Disruption of antiferromagnetic order in Zn-doped La2CuO4

The magnetic phase diagram of La₂(Cu_1−xZn_x)O₄ has been determined from zero-field muon-spin-rotation (ZF-μSR) data taken at LAMPF for 0 ≤ x ≤ 0.10. Antiferromagnetic onset temperatures follow T_N(x) from susceptibility measurements on the same samples. However, the order becomes long range, as evidenced by a well-defined internal magnetic field, only at temperatures well below T_N. Extrapolation of our results yields T_N → 0 K at x = 0.11 for a single (Cu_1−xZn_x)O₂ plane, and comparison with YBa₂(Cu_1−xZn_x)₃O₆ implies identical disruption of magnetism by Zn doping in the single- and double-plane systems.     

Density profile and flow of miscible fluid with dissimilar constituent masses

A computer simulation model is used to study the density profile and flow of a miscible gaseous fluid mixture consisting of differing constituent masses (m_A=m_B/3) through an open matrix. The density profile is found to decay with the height ∝exp (−m_A(B)h), consistent with the barometric height law. The flux density shows a power-law increase ∝(p_c−p)^μ with μ2.3 at the porosity 1−p above the pore percolation threshold 1−p_c.     

Flux creep and flux flow for thallium ceramics in a pulsed magnetic field

The resistive properties of Tl-ceramics with T_c|=0=114 K were investigated in a pulsed magnetic field (B) up to 30 T and in the temperature range of 4.2 K<T<140 K. It was shown that the character of the field dependence of the resistance differs qualitatively in high-and low-temperature regions. At low (T80K) temperatures the dynamic magnetoresistance arising in the sample is analogous to that observed earlier in LaSrCuO [1] and YBaCuO [2] ceramics. This magnetoresistance is defined by the magnetic field variation rate and leads to the appearance of a minimum at the maximum of the magnetic field pulse, i.e. at . In the region of high temperatures (80 K T<T_c) or magnetic fields (at T60K) the sample resistance rises monotonically with B increase, and dynamic resistance is not observed. In this temperature range the existence of a scaling relation is shown (here B^* and T^* meet the condition k=(B^*, T^*)/ _n(T^*)=const) for the ceramics resistance (B,T), which can be represented as . An estimate for the upper critical field B_c2(0)B_o=1030±40 T is obtained.     

Influence of magnetization on lattice constants of 3d transition metal alloys

A simple relation is found in 3d transitional metal alloys between the lattice constant and the magnetization, which can be described with an equation: a(x) = a₀^A· (1 − x) + a₀^B. x + C μ(x). It is proposed that studies of lattice constants at high temperatures may serve as an experimental method to detect the existence of localized moments above T_c. The anomalous thermal expansion of the Invar alloy is explained as a result of the collapse of localized moments above T_c.     

Surface barrier and magnetic hysteresis of ac permeability in YBaCuO single crystal

The nature of hysteretic behavior of the flux line lattice (FLL) contribution to ac magnetic permeability (μ_v) is analyzed for the case of YBa₂Cu₃O_x single crystal (at applied magnetic field Hc axis). It is shown that hysteresis loops μ_v(H) corresponding to different temperatures (T=70–84 K) are scaled to a universal curve in normalized coordinates. Such a behavior is interpreted in terms of the FLL interaction with the crystal surface. The explicit relationship between μ_v and magnetic induction B is found for the near-surface region of the superconductor. It is shown that the μ_v(H) loops are closely related to the hysteresis of B at cycling of applied magnetic field. The latter hysteresis stems from the Bean–Livingston surface barrier. The estimates demonstrate strong suppression of the surface barrier in YBa₂Cu₃O_x crystal in comparison to that expected for the ideal surface. As a result, the lower branch of the hysteresis loop corresponding to the increasing field is very close to the equilibrium μ_v(H) curve and the surface barrier appreciably affects only the upper branch when magnetic flux leaves the sample. The comparison of theoretical predictions and experimental data provides an opportunity to refine the actual range of stability H_max(B)–H_min(B) for the FLL at fixed B for YBa₂Cu₃O_x crystal in the case of Hc.     

Ising spin glass on a D = 3 hierarchical lattice: Effects of tailed coupling distributions

Within a real-space renormalization-group framework, we approach the cubic lattice through a D = 3 diamond-like hierarchical lattice. The model is a standard, nearest-neighbor, Ising spin glass with coupling constants {J_ij} distributed according to the family of continuous probability distributions P_q(J_ij) ∝ 1/[1 + (q − 1)J_ij²/2J²]^{1/(q − 1)} (if 1 + (q − 1) J_ij²/2J² > 0, and zero otherwise; q ). Such distributions, which arise naturally in the treatment, within the recently proposed nonextensive thermostatistics, of anomalous diffusion, reproduce the usual, Gaussian case, for q → 1. Moreover, they present a second moment J_ij² proportional to (5 − 3q)⁻¹ for q < 5/3, diverging for q ≥ 5/3, but keeping a finite width at midheight. In the limit q → 3, P_q(J_ij) collapses with the abscissa, and so the width at midheight diverges. We compute the q-dependence of the spin-glass critical temperature T_c. We show numerically that T_c does not scale with J_ij^21/2 (contrary to the usual belief), but rather with the width at midheight of P_q(J_ij). Our results suggest that T_c vanishes as −1/q when q → −∞; furthermore, we verified that T_c diverges exponentially when q approaches 3 from below.     

Itinerant magnetism in TiMxBe2-x (M = Cu, Fe)

Magnetization and Mössbauer studies of TiCu_xFe_yBe_2-x-y (x = 0, 0.03, 0.4, y = 0, 0.02) show that TiBe₂ is an enhanced paramagnet, 0.02 Fe or 0.03 Cu reduce the susceptibility. On the other hand, TiCu_0.4Be_1.6 is ferromagnetic (T_c = 20 K) and 0.02 Fe reduces the magnetization and Curie point (T_c = 16 K). The magnetic properties of all samples are extremely sensitive to sample preparation and heat treatments.

The Mössbauer studies show that the itinerant magnetism resides on the Ti site, all Ti sites have the same local spin density irrespective of local environment.     

Influence of high magnetic fields on the low temperature heat capacity of the itinerant electron ferromagnet Sc3In

The low temperature (1.3–20 K) heat capacity of the weak itinerant electron ferromagnet Sc₃In was measured in magnetic fields up to 10 T. The measurements were made on three different samples containing 24.1, 24.3 and 24.4 at.% In with Curie temperatures at H = 0 of 5.5, 6.0 and 6.3 K, respectively. The heat capacity peak at T_c becomes smaller with increasing fields and at 9.98 T the magnetic entropies are only 11 to 19% of the zero field value. Above T_c the spin fluctuation contribution to heat capacity, which is enhanced by the magnetic field at low fields (<≈ 5 T), is quenched at high fields ( ≈ 5 T). Our results show that the spin fluctuations in Sc₃In are almost completely quenched by the magnetic fields of ≈ 10 T, and that the characteristics spin fluctuation temperature, T_s, of this itinerant electron ferromagnet is about the same as T_c.     

Magnetic order in M2Mo3O8 single crystals (M = Mn, Fe, Co, Ni)

Magnetic measurements on hexagonal single crystals of M₂Mo₃O₈ (M = Mn, Fe, Co and Ni) are reported. The Mn compound orders ferrimagnetically at T_c = 41.5 K; the Fe and Co compounds antiferrimagnetically at T_N = 59.5 K and 40.8 K, respectively. No magnetic ordering was found in the Ni compound down to 2 K. All the compounds showed strong magnetic anisotropy when ordered, the results indicating that the magnetic spins' preferred orientation is along the hexagonal axis. Mn₂Mo₃O₈ shows a temperature dependence of the spontaneous magnetization in the ferrimagnetic regime which is rarely observed: as T → 0, M_s → 0.     

Temperature dependence of the critical current in YBa2Cu3O7−δ and Bi2Sr2Ca1Cu2O8 Josephson junctions

We have studied the stationary Josephson effect on YBa₂Cu₃O_7−δ (T_c=90 K) and Bi₂Sr₂Ca₁Cu₂ O₈ (T_c=80 K and 87 K for two samples of different origin) ceramic based junctions. The temperature dependence of the critical current near T_c has been found as I_c≈(T_c-T) for the Y-Ba-Cu-O samples indicating that they should be classified as S-N-I-N-S type junctions. The I-V curves of the Bi-Sr-Ca-Cu samples show the typical behaviour of S-I-S structures. Using Ambegaokar-Baratoff's theory for Bi₂Sr₂Ca₁Cu₂O₈, the temperature dependence of the superconducting state gap Δ(T) was calculated and it was evaluated that 1.452Δ(0)/k_BT_c3.5.     

The pion velocity at chiral restoration and the vector manifestation

We study the effects of Lorentz non-invariance on the physical pion velocity at the critical temperature T_c in an effective theory of hidden local symmetry (HLS) with the “vector manifestation” fixed point. We match at a “matching scale” Λ_M the axial-vector current correlator in the HLS with the one in the operator product expansion for QCD, and present the matching condition to determine the bare pion velocity. We find that the physical pion velocity, which is found to be one at T=T_c when starting from the Lorentz invariant bare HLS, remains close to one with the Lorentz non-invariance, v_π(T_c)=0.83–0.99. This result is quite similar to the pion velocity in dense matter.     

Anomalous magnetotransport phenomena in θ-(BEDT-TTF)2I3

Anomalous magnetotransport phenomena have been observed in θ-(BEDT-TTF)₂I₃ crystals at temperatures below 15 K. The magnetoresistance M : (1) is a linear function of the magnetic field H, (2) is not affected by the angle between the electric current and the magnetic field, (3) but depends on the magnetic field orientation with respect to the crystal axis. Magnetoresistance is expressed as M = (aH²_a + bH²_b + cH²_c)₀^-3/2/H in terms of H = (H_a, H_b, H_c), the zero field resistivity ₀, and parameters a, b, and c which are independent of temperature and magnetic field. We have found that b a > c. Magnetoresistance up to 40 is observed for H = 7T along the b-axis at T = 1.5K.     

Tracking the formation of 2223 BiSrCaCu oxygen compounds by resistivity and susceptibility measurements reaching Tc(0)=115 K

Unsubstituted and lead-antimony substituted 2223 BiSrCaCu Oxygen compounds have been synthesized and compared by resistivity R(T) and susceptibility χ(T) measurements and chemical analysis. The R(T) and χ(T) measurements show that interface effects occur which are reflected in the R(T) characteristics. The chemical analysis of samples with a resistance zero T_c(0)=114 K and a diamagnetic onset at 115 K yields a severe loss of lead from nominally 40 at% down to about 3 at%, whereas the T_c(0)=110 K samples contain up to about 30 at% of lead. It is assumed that for the formation of the 115 K phase lead oxide acts as a catalyst not being incorporated to a considerable amount into the lattice.     

Effect of Pr and Ca substitution on superconductivity in Y(1−x−yPrxCay)Ba2Cu3O7−δ

Pr substituted at constant Ca concentration for Y in (Y_1−x−yPr_xCa_y)Ba₂Cu₃O_7−δ superconductors have been prepared under identical conditions and the temperature dependence of the electrical resistivity of these samples are measured. The resistively determined values of T_c decrease linearly with increasing x (0 ≤ x ≤ 0.2) for constant y = 0.10 and 0.15 which provides convincing evidence that the suppression of superconductivity by Pr is mainly due to magnetic pair breaking. The suppression of superconductivity can also be correlated to the observed changes in oxygen content determined by iodometric analysis and to the average Cu-valences. However, it is found that the observed suppression of T_c cannot be compensated by appropriate hole doping with Ca.     

Study of the magnetic properties of single-crystal Ba0.6K0.4BiO3

The magnetic properties of single-crystal Ba_0.6K_0.4BiO₃ were studied. The results show that this isotropic superconductor (cubic structure with T_c ≈ 32 K) exhibits irreversibility and relaxation properties similar to those observed in the layered, high temperature superconductors. For fields above 0.1 T, an irreversibility line B_irr = B_irr (0) (1 − t)ⁿ with n = 3/2 and B_irr (0) = 20 T is observed. The comparison among several superconducting systems with different anisotropies suggests that the irreversibility line is unlikely to be the melting line for the YBa₂Cu₃O_7−δ.     

On the Curie temperature of a spin one XY ferromagnet with uniaxial anisotropy

A spin one XY ferromagnet with uniaxial anisotropy has been investigated, using Green's function technique in random phase approximation (RPA). The Green functions associated with the anisotropy energy are treated without decoupling. A set of coupled equations have been obtained to find the critical temperature T_c and (S^Z)² at T_c as function of the uniaxial anisotropy parameter D. T_c and (S^Z)² at T_c are found to increase with D. The results are compared with the earlier results obtained in the Narath type of RPA.