Superconducting critical temperature, Tc, and phase equilibrium diagram of A3B β-W) type compounds

A correlation between the superconducting critical temperature, T_c, and solid solution range (SSR) appearing in the phase equilibrium diagram of A₃B (β-W) type compounds has been observed. Among the compounds of incongruent melting (peritectic or peritectoid) type, the T_c is directly proportional to the solid solution range. Further, in the low temperature range, the solid solution range invariably is found on the A element-rich side of the stoichiometric composition (75 at. % A: 25 at. % B), suggesting a fundamental importance in the “A-chain integrity,” viz., an A element can substitute for a B element which is not a part of the chain, but a B element cannot replace an A element which forms a part of the chain) which is believed to be a key to superconductivity.     

Possible preferential order-disorder in A15(β-W type) compounds

A possible existence of preferential disorder in the B sites in A₃B(β-W type) compounds is demonstrated. This appears to support the “A-chain integrity” recognized in these compounds. Therefore, the correlation between the order-disorder parameters and the superconducting critical temperatures, investigated in the past, should be reexamined.     

Stoichiometry of the chevrel phase SnMo6S8

The phase field of the Chevrel phase Sn_xMo_yS₈ was investigated by metallographic methods. A shift of the phase field away from the “ideal” compound SnMo₆S₈ towards the molybdenum-rich side of the phase diagram was observed. The degree of off-stoichiometry could not yet be determined, however, the c/a ratio of the hexagonal lattice parameters can be used to characterize the samples. Measurements of the superconducting transition temperature indicate two superconducting “phases”, which differ in the c/a ratio of the lattice parameters. While phase A showed a linear variation of T^a_c with c/a in the region from 11.0 to 13.0 K phase B was independent of the c/a ratio with a T^b_c = 10.8 ± 0.2 K.     

Effect of nuclear irradiation on the superconducting transition temperatures of A-15 materials

A mechanism for the degradation of superconducting transition temperatures T_c by high energy nuclear irradiation in A-15 materials is proposed. The radiation is supposed to produce small “disordered” regions having a very low T_c, in a matrix having the original T_c of the material. The composite system has a well defined but lower T_c due to the proximity effect of the “disordered” regions.     

A study of Fe-doped Bi-Sr-Ca-Cu superconductors

We have substituted 1.5% of Fe for Cu in several “2212” and “2223” Bi-Sr-Ca-Cu superconductors. All of the samples show a reduction ofT _c about 13 K due to the Fe impurities. Mössbauer measurements at room temperature reveal the structural characteristics such as stacking faults and intergrowth of different phases in these Bi-based compounds on the microscopic scale. The susceptibility ofT _c to Fe-doping in the Bi-“2212” or “2223” system is comparable to that of the “123” system but much smaller than that of the “214” system. The interplanar correlation existing in the “123” and the Bi-“2212” and “2223” systems seems to play an important role in sustaining the high temperature superconductivity and weakening the detrimental effect of impurity elements on superconductivity in these two systems.     

Round-robin study of Tc measurement of superconducting “Nb3Ge” thin films

To determine the reproductibility in the measurement of T_c a round-robin experiment was conducted. The samples consisted of five high T_c “Nb₃Ge” thin films. The results of this experiment show that only the midpoint of the T_c curve is reproducible to within ≈±0.2 K.     

Resistance maximum across the spin glass to Kondo transition

In a recent theory of the noise model of alloys like AuFe a singular point at zero temperature was found to separate a spin glass phase at high concentrations and a Kondo phase at low concentrations. Despite this there is a resistance maximum in both “phases”, although of different characters. In the present letter a relation is given between the temperature of the maximum, T_m, the noise temperature, Δ_c, and the Kondo temperature, T_K. This extends a previously given expression, that is only valid in the spin glass limit Δ_c >> T_K, across the transition at Δ_c = T_K into the Kondo phase and values of Δ_c less than T_K.     

The temperature dependence of the ALO-modes in Pb5Ge3O11

The temperature dependence of the LO-TO splitting of the A-modes of Pb₅Ge₃O₁₁ between room temperature and T_c=450K was determined from the Raman spectrum. The finite splitting of two A-modes above T_c leads to the conclusion that the space group of Pb₅Ge₃O₁₁ in the paraelastic phase deviates from P6 which was determined by X-ray and other physical methods.     

Ll2超导体的Tc表达式

通过对L1₂超导体的研究,我们得出了这类超导体临界温度的表达式T_c=(15.9T_BV(B)G_A)/(M^1/2V(L1₂)G_B)。其中T_B是B组元的超导转变温度,V(B)是B组元的原子体积,V(L1₂)为L1₂化合物的平均原子体积,M是L1₂化合物的平均原子量,G_A,G_B分别为A组元、B组元的Gordy电负性值。 关键词：     

B NMR relaxation in superconductors: YRuB2 and LuRuB2

To investigate the electronic states in YRuB₂ and LuRuB₂, we have carried out ¹¹B NMR measurements. In the normal state, the spin-lattice relaxation rates 1/T₁'s in these compounds are proportional to the temperature T. 1/T₁'s show a small coherence peak just below the superconducting transition temperature T_c and decrease exponentially well below T_c. YRuB₂ and LuRuB₂ are found to be BCS superconductors with the energy gap 2Δ(0)=3.52 k_BT_c.     

Supraconductivite et chaleur specifique dans les alliages A15 a base de titane

Experimental data on the superconducting transition temperature, and low temperature specific heat, together with X-ray investigations, are reported for binary and pseudo-binary compounds of Ti with the A 15-type structure. A “true” relative maximum of the coefficient of the electronic specific heat, γ, as well as the superconducting transition temperature, T_c, occurs in the Ti₃Ir_?xPt_x system near x = 0.2. Ti₃Ir_0.8Pt_0.2 shows the lowest Debye temperature, θ₀ , of all A 15-type compounds known to date. The anomalous temperature dependence of the lattice specific heat may be reproduced by a model phonon spectrum similar to that of Nb₃Sn.     

An asymptotically free Reggeon field theory

Renormalization group techniques are used to examine Reggeon field theories with derivative triple Pomeranchuk interactions in accord with the decoupling theorems. The simplest asymptotically free “weak coupling” solution has total cross sections which behave as σ_tot^AB(s) ∽ g_Ag_B?c/log s +…, and the triple Pomeranchuk vertex has a nonsense wrong signature zero asymptotically ～(t?t₁?t₂). Another “strong coupling” solution reproduces the results of Migdal, Polyakov, Ter-Martirosyan, Abarbanel and Bronzan to lowest order in the ?-expansion.     

The upper critical magnetic field Bc2(T) of amorphous molybdenum films

Molybdenum films are obtained by quenched condensation of the metal vapour onto a He-cooled substrate. During annealing the electrical resistance decreases over a small temperature range; this is the typical behaviour of amorphous metals. The films, about 300 Å thick and with a high resistivity, have a positive Hall effect. The transition temperature T_c = 8.0 K is strongly enhanced compared to crystalline Mo. The upper critical field strongly enhanced compared to crystalline Mo. The upper critical field B_c2(T) has been measured. In the examined field range up to 80 kG, B_c2 is a linear function of temperature; the slope has the large value dB_c2/dT = -45 kG/K.     

Microwave spectrum of tertiary-butylphosphine

The microwave spectra of tertiarybutylphosphine (CH₃)₃CPH₂, (CH₃)₃CPHD, and (CH₃)₃CPD₂ have been recorded in the region 26.5–40.3 GHz. Both a- and c-type transitions were observed and assigned for the “light” and “heavy” molecules and a-type transitions were observed and assigned for the d₁ species. The rigid rotor rotational constants were determined to be A = 4397.63 ± 0.04, B = 2878.88 ± 0.02, and C = 2870.86 ± 0.02 MHz for (CH₃)₃CPH₂ and A= 4261.98 ± 0.04, B = 2769.82 ± 0.02, and C = 2752.71 ± 0.02 MHz for (CH₃)₃CPD₂ and A = 4330 ± 2, B = 2831.45 ± 0.02, and C = 2801.50 ± 0.02 MHz for (CH₃)₃CPHD. Dipole moment components of |μ_a| = 1.06 ± 0.02, |μ_c| = 0.49 ± 0.02 and |μ_t| = 1.17 ± 0.02D were determined from the Stark effect. By assuming reasonable structural parameters for the tertiarybutyl and phosphine groups, a least-squares fit of the rotational constants gave $\text{λ}{}_{\mathrm{<mtext>P-C</mtext>}}\text{= 1.896}\text{A}\text{?}$ and $\text{?CPH = 95.7°}$. No splitting was observed of the first excited state of the phosphine torsional mode.     

51V nuclear magnetic resonance in polycrystalline ferroelastic BiVO4

Nuclear magnetic resonance studies on polycrystalline ferroelastic BiVO₄ indicate that the ⁵¹V electric field gradient asymmetry parameter is an order parameter in the ferroelastic transition. Using ∩ = A(T?T_c)^B, B is found to be 0.48(5), in good agreement with earlier studies of this material. Near the phase transition above and below, the vanadium nuclear quadrupole coupling is constant with a value of 4.8(1) MHz.     

The classification of superconductors using muon spin rotation

In this paper we explore the role of muon spin rotation (μSR) techniques in the characterization and classification of superconducting materials. In particular we focus upon the Uemura classification scheme which considers the correlation between the superconducting transition temperature,T _c, and the effective Fermi temperature,T _F, determined from μSR measurements of the penetration depth. Within this scheme strongly correlated “exotic” superconductors, i.e, hightT _C cuprates, heavy fermions, Chevrel phases and the organic superconductors, form a common but distinct group, characterized by a universal scaling ofT _C withT _F such that 1/100 <T _C/T _F<1/10. For conventional BCS superconductorsT _C/T _F<1/1000. The results of new μSR measurements of the penetration depth in superconducting Y(Ni_1?x Co _x )₂B₂C and YB₆ are also presented. In Y(Ni_1?x Co _x )₂B₂C the decrease ofT _C with increasing Co concentration is linked to a marked decrease in the carrier density from 2.9·10²⁸ m^?3 atx=0 to 0.6·10²⁸ m^?3, atx=0.1, while the carrier mass enhancement remains almost constant at approximately 10. For YB₆ we find evidence of a modest enhancement of the carrier mass (m ^*/m=3), and a relatively low carrier density of 0.24·10²⁸ m^?3. These results are discussed within the Uemura classification scheme. It is found that neither Y(Ni_1?x Co _x )₂B₂C withT _c/T _F>>1/250 nor YB₆ withT _C/T _F>>1/340 can be definitively classified as either “exttic” or “conventional”, but instead the compounds display behavior which interpolates between the two regimes.     

Neutron diffraction study of Ho(Rh0.3Ir0.7)4B4

Using powder neutron diffraction techniques, we have examined the magnetic order of the pseudoternary compound Ho(Rh_0.3Ir_0.7)₄B₄ below the Néel temperature T_N=2.7K. The magnetic structure consists of stacked antiferromagnetic basal plane sheets forming a body centered tetragonal unit cell, with a sublattice magnetization corresponding to 9.6±0.6μ_B per Ho³⁺ion at 1.5 K. Magnetic intensity versus temperature measurements indicate that the transition is second order and reveal no anomalous effects when the compound becomes superconducting at T_c=1.34K.     

Superconductivity in the ternary borides CeOs3B2 and CeRu3B2: Magnetic susceptibility and specific heat measurements

The compounds CeOs₃B₂ and CeRu₃B₂ show superconductivity with T_c's of 3.5 and 1.1 K respectively. The magnetic susceptibility of both these compounds may be described as the sum of a Curie-Weiss term dominating at low temperature and a large temperature independent term. The heat capacity measurements yield the value of electronic specific heat coefficient γ of 40.1 and 15.6 mJ/mol K² for Os and Ru compounds, respectively. These compounds appear to be conventional bulk superconductors in which Ce is in a strongly mixed valent state.     

Correlations in inhomogeneous Ising models

Using general methods developed in a previous treatment we study correlations in inhomogeneous Ising models on a square lattice. The nearest neighbour couplings can vary both in strength and sign such that the coupling distribution is translationally invariant in diagonal direction. We calculate correlations parallel to the layering in the diagonally layered model with periodv=2, the so-called “general square lattice” model (GS). If the model has a finite critical temperature,T _c>0, we have a spontaneous magnetization belowT _c vanishing atT _c with the Ising exponent β=1/8. AtT _c correlations decay algebraically with critical exponnet η=1/4 and exponentially forT>T _c. In the frustrated case we have oscillatory behaviour superposed on the exponential decay where the wavevector of the oscillations changes at some “disorder temperature”T _D(>T _c) from commensurate to temperature-dependent in commensurate periods. If the critical temperature vanishes,T _c=0 we always have exponential decay at finite temperatures, while atT=T _c=0 we encounter either long-range order or algebraic decay with critical index η=1/2, i.e.T=0 is thus a critical point.     

Neutron diffraction studies of the magnetic ordering in the rare earth compounds TbNi2Si2, HoCo2Si2 and TbCo2Si2

Neutron diffraction studies and magnetic measurements on the compounds TbNi₂Si₂ (1), HoCo₂Si₂ (2) and TbCo₂Si₂ (3) revealed a collinear antiferromagnetic order below T_N = 10 ± 1 K (1), T_N = 13 ± 1 K (2) and T_N = 30 ± 2 K (3) with the rare earths moments oriented along the c-axis [m₀ = 8.8 ± 0.2 μ_B (1), m₀ = 8.1 ± 0.2 μ_B (2), m₀ = 8.8 ± 0.2 μ_B (3)] and the corresponding wavevector are $\text{k}\text{= [}\text{1}\text{2}\text{1}\text{2}\text{0] (1)}\text{and}\text{k}\text{= [ 0 0 1] (2) (3)}$. The magnetic structure of the compounds HoCo₂Si₂ and TbCo₂Si₂ consists of ferromagnetic layers perpendicular to the c-axis coupled antiferromagnetically (+?+?) while for TbNi₂Si₂ the ordering within (0 0 1) plane is antiferromagnetic and the planes (0 0 1) are indeed decoupled.