The effective activation energy Ueff(T,B,J) in Hg-1223 single phase superconductors

We review the methods of calculating the effective activation energy U_eff(T,B,J) for both transport measurements and magnetic decay, together with some theoretical models. Then, we apply these methods to our Hg-1223 single-phase superconductor to obtain the activation energy. Transport results give that the magnetic field and temperature dependence of the U_eff can be well described as U₀B^−α(1−T/T_c)^m. Magnetic relaxation shows that the current density dependence of U(J) can be scaled onto a single curve, which can be considered as the activation energy at some temperature T₀. The pinning mechanism in the measured temperature range does not change, and the activation energy depends separately on the three variables: T, B, and J, are responsible for the magnetic decay data scaling onto a single curve at various temperatures. As temperatures close to zero and near T_c, thermally assisted flux motion model is no longer valid since other processes dominate.     

Superconductivity in a correlated two-dimensional Hubbard model: A theoretical study

In this work we study the superconductivity within an attractive two-dimensional one-band Hubbard model. We consider a d-wave superconducting gap and a Hubbard-I approximation to describe the strongly correlated superconducting regime. We use Green's function method to obtain the order parameter Δ and the superconducting critical temperature T_c. The results show that for fixed values of the superconducting attractive potential U (U<0), the gap increases for low temperatures, whereas diminishes abruptly as the temperature increases. The effect of pressure can be discussed, varying the next-nearest-neighbors hopping t₂, yielding a change in T_c, and also in Δ₀.     

Double fluctuations on the attractive Hubbard model: ladder approximation

We explore, for the first time, the effect of double fluctuations on both the diagonal and off-diagonal selfenergy. We use the T-Matrix equations below T _c , developed recently by the Zürich group (M.H. Pedersen et al) for the local pair attraction Hamiltonian. Here, we include as well the effect of fluctuations on the order parameter (beyond the BCS solution) up to second order in U/t. This is equivalent to approximating the effective interaction by U in the off-diagonal self-energy. For U/t = ?6.0, T/t = 0.05, μ/t = ?5.5 and Δ/t = 1.5, we find four peaks both for the diagonal, A(n(π/16, π/16), ω), the far left peak has a vanishing small weight; (b) in B(n(π/16, π/16), ω) the far left and far right peaks have very small weights. The physical picture is, then, that the pair physics in the normal phase (T > T _c is still valid below T _c . However, the condensation of the e-h pairs produces an additional gap around the chemical potential as in BCS, in other words, superconductivity opens a gap in the lower branch of a Hubbard-type-I solution.     

The non-linear paramagnetic effect in CdCr2Se4 near the Curie temperature

Results are presented of measurements made of alternating current susceptibility changes Δχ = χ_B - χ₀ induced by external application of a constant magnetic field using the beat method (f ～ 1MHz). For 1 K < (T ? T_c) < 50 K and taking the appropriate values of magnetic fields the relation Δχ ～ B² was satisfied. Making use of the Debye theory adapted for the non-linear dielectric effect, it was shown that the mean number n of elementary cells in a cluster varies with temperature according to the relation n ～ (T ? T_c)^-1.31.     

Vortex pinning in YBa2Cu3O7?x films

Evidence that pinning on linear or planar defects dominates the vortex dynamics in YBa₂Cu₃O_7−x (YBCO) films is provided by complex impedance measurements at temperature 8 K<T<T _c and magnetic field 0<B<6 T. Below the vortex lattice melting transition B_g(T) but above a threshold field B_p≈8(1-T/T _c ) T, the inductance of vortices increases as B², much less rapidly than predicted for collective pinning of vortices by point defects. Above the vortex melting line, critical scaling persists over the region B_g(T<B<B^*(T) where the vortex correlation length ξ exceeds a characteristic length scale ξ^*≡ξ(B=B^*)≈450?. The value of ξ^* is not sensitive to Al-doping in the Cu sites in the lattice and is close to the size of twin domains in the film. The nature of the observed crossovers is discussed in terms of available theoretical models for a glass-liquid transition at B_g.     

Anomalies in the surface impedance penetration depth in ferromagnetic superconductors

We have investigated the surface impedance penetration depth, Λ, of ErRh₄B₄ and Er_0.5Ho_0.5Rh₄B₄ both experimentally and theoretically. For ErRh₄B₄, owing to the critical spin fluctuations just above T_s ( > T_c2), the critical temperature at which surface ferromagnetism appears, Λ^?1 decreases smoothly as T decreases toward T_s. For Er_0.5Ho_0.5Rh₄B₄, the decrease in Λ^?1 owing to spin fluctuations for T ? T_c2 is very small, and Λ^?1 decreases abruptly at T_c2. Theoretical values of Λ^?1 are in good agreement with the data.     

Magnetic relaxation near a fishtail peak of isothermal magnetization in B1 NbCy encapsulated in multiwall carbon nanocages

Isothermal magnetization near a fishtail peak in nanocrystalline B1 NbC_y encapsulated in multiwall carbon nanocages is studied within the time window of 100 < t < 4000 s. The current density J exhibits a linear logarithmic time decay. The effective activation energy U_eff increases linearly with temperature T and is independent of applied magnetic field H. The results of J(t) and U_eff (T, H) are consistent with the Anderson–Kim flux–creep model for thermally activated motion of uncorrelated vortices or vortex bundles over a net potential barrier U_eff. U_eff at a fishtail peak field H_fp evolves quickly above a fishtail peak temperature T_fp, but slowly below that temperature. The result suggests that a decrease of flux viscosity coefficient above T_fp at H_fp is the origin of the fishtail peak in nanocrystalline B1 NbC_y encapsulated in multiwall carbon nanocages.     

Time dependence of isothermal magnetization in single-phase B1 MoCy encapsulated in multiwall carbon nanocages

Isothermal magnetization M(t) in nanocrystalline single-phase B1 MoC_y encapsulated in multiwall carbon nanocages is studied within the time window of 100 < t < 5000 s. The current density J exhibits a linear logarithmic time decay. The effective activation energy U_eff increases linearly with increasing temperature T, and decreases linearly with increasing J. The behaviors of J(t), U_eff(T), and U_eff(J) can be described by the Anderson–Kim flux-creep model for thermally activated motion of uncorrelated vortices or vortex bundles over a net potential barrier U_eff. The slower relaxation of current density above the broad peak field in the isothermal magnetization curves suggests that the peak is a result of vortex dynamics.     

Die einwirkung der struktur auf die kristallisationskinetik des glasigen selens

By means of a calorimetric method the rate of crystallization of vitreous selenium samples remelted at various temperatures t_U (225–3502C) was studied at a temperature of 90.0°C. To describe the kinetics the Avrami equation was used. The samples remelted at t_U < 260°C crystallized in one stage, A. For the samples remelted at t_U ? 260°C two stages, A and B, were observed. The rate constants k_Kr = z(L · c_Kr)$\text{1}\text{2}$ and the coefficients z were calculated from the Avrami equation. For the samples remelted at t_U = 225, 250 and 330°C, the activation energy e_kr was determined.The values of z, k_Kr and z · E_Kr were found to decrease as the temperature t_U increased. The kinetics of crystallization probably depend upon the structure of vitreous selenium (i.e. on the number of crystalline regions and on content of the small rings), which in turn is dependent on the temperature _U. This supposition was confirmed mathematically.     

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.     

Ising ferromagnet with biquadratic exchange

A spin one Ising system with biquadratic exchange, is investigated, using Green's function technique in random phase approximation (RPA). Transition temperature T_c and <(S^z)²> at T_c, are found to increase with biquadratic exchange parameter α for sc, bcc and fcc lattices. The variation of <(S^z)²> at T_c with α is found to be the same for the above lattices.     

Dynamic melting of driven vortex matter below 1 K

We report on measurements of a mode-locking resonance in amorphous Mo_xGe_1?x films at different temperatures T down to 0.8 K, which is well below the superconducting transition (6 K). We observe dynamic ordering of driven vortex matter for all the temperatures studied. As the field exceeds a certain critical field B_c,dyn at fixed T, moving vortices do not exhibit dynamic ordering. At high T, this field B_c,dyn(T), so-called dynamic melting, nearly coincides with a characteristic field B_c(T) where the linear resistivity vanishes. At low T, however, B_c,dyn(T) is significantly suppressed compared to B_c(T), suggesting intrinsic quantum melting in the absence of pinning.     

Elastic scattering of antiprotons on protons at 5 GeV/c evidence for a backward peak

Antiproton-proton elastic scattering has been measured at 5 GeV/c. A total of 30 000 events were observed in the angular range 17° < θ_cm < 136°, corresponding to 0.3 < −t < 7.7 (GeV/c²). In addition to the known dip at −t = 0.5 (GeV/c)², we observe a structure at about −t = 2 (GeV/c)² and a backward peak with a slope4.1 ± 0.6 (GeV/c)². The extrapolated differential cross-section at u = 0 is 1.3 ± 0.8 μb/(GeV/c)².     

Relaxation Kinetics of the Microhardness of KDP Crystals after Their Exposure to a Magnetic Field

It has been shown that potassium dihydrogen phosphate crystals change their microhardness reversibly after their exposure to a magnetic field of B = 0.8 or 1.2 T for t_m = 7–90 min. It has been found that the magnetic effect can be conveniently characterized by the quantity B²t_m, because the variation of the parameters conserving B²t_m=const does not change the result. At B²t_m < 10 T² min, the effect is almost absent. Above this threshold, the amplitude of changes in the microhardness increases and approaches a constant value of ~10% at B²t_m ≈ 19 T² min. The responses of samples of the same crystal from the faces of the prismatic and pyramidal growth sectors to exposure are different. In the former case, they soften; in the latter case, the hardening stage follows the softening stage. However, in both cases, the microhardness returns to the initial value. At B²t_m values from 19 to 37 T² min, the amplitudes and durations of the effect do not change, but in the narrow range of 37–43 T² min, the lifetime of the modified state increases sharply with transition to a new level: “sharp” peaks with a half-width of ~2 days are transformed to trapezoids with the width of the horizontal side of ~1–2 weeks. A physical scheme of the observed effects has been proposed.     

表面与界面的绝缘铁磁理论

采用Green函数技术,我们讨论了温度从T=0直到T=T_c范围内铁磁体表面和界面的磁化强度随温度的变化情况,所得到表面磁化强度的结果与其他作者的结果一致;而在界面计算中,我们发现界面的T_c与体内的不同,并且找到了一个关系式:T_cc^A,T_c^B)。 关键词：     

Static properties of type-II superconductors at arbitrary temperature and induction

A concise expression for the free energy of clean type-II superconductors is given from which magnetization curves, gap and field are easily calculated for 0 < T < T_c and 0 < B < H_c2.     

Ultrasonic studies of the high Tc superconductor (La1−xSrx)2CuO4−δ

We synthesized (La_1−xSr_x)₂CuO_4−δ crystals with 0 ≦ x ≦ 0.4 by the sintering method, and obtained several samples with good homogeneity, high onset superconducting transition temperature T_c ≲ 38 K and narrow transition width ΔT_c. For the sample with the maximum T_c among them, ultrasonic measurements were performed. In the curve of the sound velocity change ΔV_s(T) versus temperature T, there was observed an anomalously large decrease of sound velocity as T decreases in the range 150 K < T < 240 K. Furthermore, in the plots of the ultrasonic attenuation coefficient α(T) as a function of T, there were observed a broad and large peak located at T ∼ 100 K and a small peak located at T ∼ 200 K. These results show the existence of an optical mode of the energy ℏω_op/k_B ∼ 100 K and, probably, also of that of ∼ 200 K. On the basis of these experimental results of elastic properties, we suggest the origin of high T_c of this material.     

Flow equations for the ionic Hubbard model

Taking the site-diagonal terms of the ionic Hubbard model (IHM) in one and two spatial dimensions, as H₀, we employ Continuous Unitary Transformations (CUT) to obtain a “classical” effective Hamiltonian in which hopping term has been renormalized to zero. For this Hamiltonian spin gap and charge gap are calculated at half-filling and subject to periodic boundary conditions. Our calculations indicate two transition points. In fixed Δ, as U increases from zero, there is a region in which both spin gap and charge gap are positive and identical; characteristic of band insulators. Upon further increasing U, first transition occurs at U=Uc₁, where spin and charge gaps both vanish and remain zero up to U=Uc₂. A gap-less state in charge and spin sectors characterizes a metal. For U>Uc₂ spin gap remains zero and charge gap becomes positive. This third region corresponds to a Mott insulator in which charge excitations are gaped, while spin excitations remain gap-less.     

Temporal correlations and persistence in the kinetic Ising model: the role of temperature

We study the statistical properties of the sum S _t = dt'σ _t', that is the difference of time spent positive or negative by the spin σ _t, located at a given site of a D-dimensional Ising model evolving under Glauber dynamics from a random initial configuration. We investigate the distribution of S_t and the first-passage statistics (persistence) of this quantity. We discuss successively the three regimes of high temperature ( T > T _c), criticality ( T = T _c), and low temperature ( T < T _c). We discuss in particular the question of the temperature dependence of the persistence exponent , as well as that of the spectrum of exponents (x), in the low temperature phase. The probability that the temporal mean S _t/t was always larger than the equilibrium magnetization is found to decay as t ^{- - ?}. This yields a numerical determination of the persistence exponent in the whole low temperature phase, in two dimensions, and above the roughening transition, in the low-temperature phase of the three-dimensional Ising model. Received 4 December 2000     

Electron temperature measurements in low-density plasmas by helium spectroscopy

Methods of measuring the electron temperature in low-density plasmas by He spectroscopy are examined. These utilize either the relative intensities of singlet and triplet lines or the absolute intensities of single lines. Calculations from measured and theoretical data show that both methods are seriously influenced by secondary processes, the most important of which are excitation from the metastable levels 2¹S and 2³S, and excitation transfer in electron-atom collisions combined with imprisonment of resonance radiation. The calculations give parameter limits for the validity of different methods and combinations of lines. Due to the secondary processes, the determination of T_e from relative line intensities is restricted to low-density, short-duration plasmas (typically n_e < 2 × 10¹⁶ m^-3, t_ex < 5 × 10^-6 s) or to even lower densities that depend on the apparatus dimensions (typically n_e < 3 × 10¹⁵ m^-3, L ≈ 0.1 m). For the determination of T_e from absolute line intensities, the situation is more favourable and, with a suitable choice of lines, typical restrictions on n_e and t_ex are n_e < 5 × 10¹⁷ m^-3, t_ex < 10^-5 s or n_e < 10¹⁷ m^-3, L ≈ 0.1 m for electron temperatures above 10 eV. For temperatures below 10 eV and degrees of imprisonment below 7% measurements are possible for electron densities up to 10¹⁹–10²⁰ m^-3, without any limits on t_ex or L.