Thermal transport in the oxygenated magnetic superconductor, Eu1.5Ce0.5RuSr2Cu2O10+δ

The thermal conductivity and thermopower are reported for a hole doped Eu_1.5Ce_0.5RuSr₂Cu₂O_10+δ sample that has been annealed at 1100 K under an oxygen pressure of 54 atm. At T_c=45 K superconductivity and weak ferromagnetism coexist (T_m=180 K). Weak features in the thermopower, S(T), and thermal conductivity, κ(T), are observed both at T_m and at T^*=140 K. The thermopower begins to decrease sharply toward zero at T_c, and there is an extremely sharp increase of about 30% in the thermal conductivity at T_c. This “first order” transition may be related to the sudden appearance of a spontaneous vortex phase at T_c. A small shoulder is observed in κ(T) in the temperature range T=5–13 K.     

Geometrical approach to the thermodynamical theory of phase transitions of the second kind

A geometrical approach to the phenomenological theory of phase transitions of the second kind at constant pressure P and variable temperature T is proposed. Equilibrium states of a system at zero external field and fixed P and T are described by points in three-dimensional space with coordinates η, the order parameter, T, the temperature and /gf, the thermodynamic potential. These points form the so-called zero field curve in the (η, T, /gf) space. Its branch point coincides with the critical point of the system. The small parameter of the theory (the distance from the critical point along the zero-field curve) is shown to be more convenient than the small parameter of the Landau theory. It is emphasized that no explicit functional dependency of /gf on η and T is imposed.

It is shown that using (η, T, /gf) space one cannot overcome well-known difficulties of the Landau theory of phase transitions and describe non-analytical behavior of real systems in the vicinity of the critical point. This becomes possible only if one increases the dimensionality of the space, taking into account the dependency of the thermodynamic potential not only on η and T, but also on near (local) order parameters λ_i. In this case under certain conditions it is possible to describe anomalous increase of the specific heat when the temperature of the system approaches the critical point from above as well as from below the critical temperature T_c.     

The resistivity maximum in the ternary spin glass system Au-Cu-Mn

The temperature of the resistivity maximum, T_m, in the ternary spin glass system Au-Cu-Mn has been analysed in terms of Larsen's theory in order to highlight the contribution from the Kondo effect and the RKKY interaction energy to the resistivity maximum in spin glasses. The competition between these two contributions has been effectively illustrated and a good agreement with theory is obtained for samples with varying magnetic and nonmagnetic atom concentration. A comparison of the dependence of T_m and the RKKY interaction energy on the lattice pressure generated due to addition of Au with reported pressure studies on Mn alloys shows that there is a close relation between the lattice pressure and the externally applied pressure.     

Linear and nonlinear critical magnetic properties and transport in Nd0.75Ba0.25MnO3 single crystal: evidence for its anomalous critical behavior near TC

The data on the resistance and magnetoresistance (MR) as well as measurements of the linear and nonlinear susceptibilities are presented for a Nd_0.75Ba_0.25MnO₃ single crystal with the Curie temperature T_C≈129 K. Although this compound remains insulating in the ferromagnetic state, its resistance has an anomaly near T_C and it reveals the colossal magnetoresistance. The data on the magnetic response are well described by the dynamic scaling theory for 3D isotropic ferromagnets in the paramagnetic critical region at τ>τ^*≈0.11, τ=(T−T_C)/T_C. Below τ^* an anomalous critical behavior is found that suggests the coexistence of two magnetic phases. This behavior is discussed in terms of a phase separation which can occur in the moderately doped manganites exhibiting an orbital ordering.     

非晶纳米Ni500团簇等温晶化过程中的结构与动力学研究

采用分子动力学方法和镶嵌原子势, 模拟了500个Ni原子(简称Ni₅₀₀)组成的纳米团簇的等温晶化过程. 通过对纳米Ni团簇的动力学行为和微观结构演变的研究, 发现Ni₅₀₀在高温时是一步晶化的, 在低温时则呈现出多步晶化的特征. 在多步晶化的过程中, 团簇结构会陷入多个亚稳态结构, 经过原子重排, 进入能量更低的亚稳态, 最后完成晶化. 在多步晶化过程中, 原子的位置重排是通过协同跳跃运动实现的, 其协同运动方式不但有常见的线型协同运动, 也有多个原子的集体平移运动等其他形式. 关键词： 分子动力学模拟 纳米Ni团簇 协同运动     

Effects of milling-induced disorder on the lattice parameters and magnetic properties of hematite

A ball-milling treatment in air for 30 min is enough to reduce anhydrous bulk hematite (-Fe₂O₃) grains to nanometric sizes. For milling times, t_m, of 15, 30 min, 1 and 2 h, the crystals suffer an anisotropic lattice dilation, which is more pronounced for the smaller average grain sizes attained. Mössbauer and susceptibility results show that the process alters the effective Morin temperatures, T_M. The transition occurs less sharply than for the non-milled material and spreads over a maximum extent of 50 K for t_m=30 min. The susceptibility data indicate that the T_M for the fraction of material undergoes the transition shifts toward values not lower than 235 K. For t_m=10 h the transition is suppressed down to 12 K. Our results do not allow distinguishing bulk from surface regions of the grains. In addition, the disorder brought about by the milling reduces the magnetic response in the weak ferromagnetic state of -Fe₂O₃. The modification of T_M seems to depend mainly on the anisotropic dilation of the unit cell that affects the whole grain and it is related to the nanometric grain sizes achieved.     

Upper critical fields of ferromagnet/superconductor layered structures

The temperature dependence of the upper critical fields, both perpendicular H_c2 and parallel H_c2 to layer planes of ferromagnet/superconductor bi- and multilayers, is theoretically investigated. The secular equation of the superconducting order parameter for determining the phase diagram (H, T) is obtained by solving exactly the linearized Usadel equations in the multimode method taking into account the material parameter values. For the bilayers system, the influence of the boundary resistivity on the critical fields, and the dimensional crossover behavior of H_c2(T) are studied in details. For the multilayered structure, the effects of the π-phase state on both the superconducting transition temperature T_c and the upper critical fields (H_c2, and H_c2) are also considered. The nonmonotonic T_c behaviors are predicted. The interplay between 0- and π-phases leading to the strong oscillations of T_c as well as the temperature dependence of the zero temperature critical fields on the ferromagnetic layer thickness are investigated theoretically.     

Magnetic study of the UPt3 superconducting phases

In order to distinguish the UPt₃ superconducting (s.c.) phases we have studied their magnetic properties at low fields in a SQUID magnetometer and up to fields >H_c2(0) with a capacitive torque-meter. With the SQUID we measure the magnetic penetration depth and find the second s.c. transition at T_c⁻ when the field is applied along the c-axis, but not with . This result, combined with power-law behavior at low temperature T, is most consistent with the two-dimensional E_2u s.c. order parameter. Below 20 mK we find an additional diamagnetic signal that we ascribe to the normal state magnetism. In high fields our torque measurements show a kink of the perpendicular magnetization component at the B–C phase line, pointing to an enhanced Ginzburg–Landau parameter in the C phase.     

Properties of a local pairing model of high-Tc superconductivity

A simulation technique is used to study the properties of the “hole modulated hopping” model introduced by Hirsch. The superconducting order parameter, energy gap and pair size have been determined for a range of particle densities and temperatures in the neighbourhood of the superconducting phase transition. Results are consistent with the interpretation of the superconducting transition to be Bose-like at low hole densities and BCS-like at high hole densities, with a crossover near the T_c maximum in the T_c versus hole density curve. This behaviour is related to the existence of small non-overlapping pairs at low hole densities and large strongly interpenetrating pairs at hole densities above the T_c maximum.     

A possible mechanism of superconductivity in high-Tc cuprates

This paper derives a generic T_c formula by using the long-range phase coherence condition in quantum phase fluctuation of the order parameter. Taking the two-local-spin-mediated interaction (TLSMI) proposed by Liu and Chen [Phys. Rev. B 58 (1998) 8812] as a Cooper pair potential, and the T_c formula, this paper explains five basic experimental facts in high-T_c cuprates. The aim of this paper is to show that TLSMI is a possible pairing mechanism of superconductivity in high-T_c cuprates.     

Structural changes in RNiO3 perovskites (R=rare earth) across the metal–insulator transition

Rare earth nickel oxide perovskites (RNiO₃, R=rare earth) have, except for LaNiO₃, a metal–insulator (MI) phase transition as temperature decreases. The transition temperature (T_MI) increases as the R-ion becomes smaller. They also present, at low temperatures, a complex antiferromagnetic order. For lighter R-ions (e.g. Pr and Nd), the antiferromagnetic transition temperature (T_N) is close to T_MI, while for heavier R-ions (e.g. Eu, Sm), T_MI and T_N are very far apart, suggesting that the magnetic and electronic behaviors are not directly coupled. Even though Ni³⁺ is a Jahn–Teller ion, no distortion in the NiO₆ octahedra was found for RNiO₃ perovskites with R=Pr, Nd, Sm and Eu. In this work we have measured EXAFS at Ni K edge for samples of PrNiO₃, NdNiO₃ and EuNiO₃. The Fourier transform spectra for the three samples show a clear splitting in the first peak at the insulating phase. This splitting corresponds to two or more different Ni–O distances. This indicates that there is either a distortion in the NiO₆ octahedra or there are two different Ni sites at the insulating phase.     

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.     

钙钛矿型锰氧化物(La$lt;sub$gt;0.8$lt;/sub$gt;Eu$lt;sub$gt;0.2$lt;/sub$gt;)$lt;sub$gt;4/3$lt;/sub$gt;Sr$lt;sub$gt;5/3$lt;/sub$gt;Mn$lt;sub$gt;2$lt;/sub$gt;O$lt;sub$gt;7$lt;/sub$gt;的磁性和电性研究

采用传统固相反应法制备钙钛矿型锰氧化物 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇多晶样品, X-射线衍射分析表明, 样品(La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇结构呈现良好的单相. 通过磁化强度随温度的变化曲线(M-T)、不同温度下磁化强度随磁场的变化曲线(M-H)和电子自旋共振谱发现: 在300 K以下, 随着温度的降低, 样品先后经历了二维短程铁磁有序转变 (T_C^2D ≈ 282 K)、三维长程铁磁有序转变(T_C^3D ≈ 259 K)、奈尔转变(T_N ≈ 208K)和电荷有序转变(T_CO ≈ 35 K); 样品 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇在T_N以下, 主要处于反铁磁态; 在T_C^3D达到370 K时, 样品处于铁磁-顺磁共存态, 在370 K以上时样品进入顺磁态. 此外, 分析电阻率随温度的变化曲线(ρ-T)得到: 样品在金属-绝缘转变温度(T_P ≈ 80 K)附近出现最大磁电阻值, 其位置远离T_C^3D, 表现出非本征磁电阻现象, 其磁电阻值约为61%. 在T_CO以下, 电阻率出现明显增长, 这是由于温度下降使原本在高温部分巡游的e_g电子开始自发局域化增强所致. 通过对 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇的ρ-T 曲线拟合, 发现样品在高温部分的导电方式基本遵循小极化子的导电方式. 关键词： 磁性 电性 金属-绝缘转变温度 电子自旋共振     

非自治物质畸形波的传播操控

研究了(1+1)维的变系数Gross-Pitaevskii方程, 获得了该方程的精确畸形波解. 基于该精确畸形波解, 深入研究了非自治物质畸形波在随时间指数变化的相互作用下的传播动力学行为, 发现非自治畸形波除具有“来无影、去无踪”的不可预测特性外, 也可实现完全激发、抑制激发以及维持激发等操控. 研究表明, 畸形波操控的关键是对累积时间的最大值T_max 与峰值位置T₀ (或T_I,T_II)值大小关系的调节. 当T_max > T₀ (或T_I,T_II)时畸形波被快速地完全激发, 热原子团中的原子增加到凝聚体中. 当T_max = T₀ (或T_I,T_II) 时畸形波激发到最大振幅, 可以维持相当长的时间而不消失, 热原子团中的原子增加到凝聚体中. 当T_max < T₀ (或T_I,T_II)时畸形波没有充足的时间来激发而被抑制甚至消失, 凝聚体中的原子减少. 这些结果在理论和实际应用上具有启迪意义.     

Influence of the ferroelastic twin domain structure on the {100} surface morphology of LaAlO3 HTSC substrates

LaAlO₃ crystals have been investigated with differential scanning calorimetry (DSC), high-precision X-ray powder diffraction (XRD) and scanning force microscopy (SFM). The DSC measurements show the second-order phase transition of LaAlO₃ at 544°C, where LaAlO₃ changes its symmetry from the cubic Pm3m high-temperature phase to the pseudocubic rhombohedral low-temperature phase. This paraelastic to improper ferroelastic phase transition causes twinning in the {100} and {110} planes of the pseudocubic lattice. The twin angles between the surface {100}_pseudocubic planes of twin domains were measured by SFM on the surface of a macroscopic (100)_cubic cut crystal plate. The misorientation angle ω₁₀₀ between {100} twins is 0.195(8)°, while {110} twinning gives an angle of ω₁₁₀=0.276(7)°. The two twin kink angles correspond to a rhombohedral angle of the pseudocubic cell of the phase as ₁=90.0973(40)° and ₂=90.0975(30)°, respectively. The XRD result for this rhombohedral angle is =90.096(1)°. The orientation of the misfit steps formed during annealing after mechanical surface polishing depends on the domain orientation and pattern during polishing. Any heating close to or above T_c changes the domain pattern. Footprints of previous domain patterns can thus be found on the surface in the form of surface corrugation and changes in the shape and orientation of misfit steps.     

Crystal structure of Tl0.5Pb0.5Sr2Ca2Cu3O9 at 300 K and around Tc (118 K)

The crystal structure including the cation distribution, of a polycrystalline sample of nominal composition Tl_0.5Pb_0.5Sr₂Ca₂Cu₃O₉ with T_c = 118.2 K has been determined using resonant synchrotron X-ray diffraction data collected at the Cu K, Tl L_III and Sr K edges and time-of-flight powder neutron diffraction data. No oxygen deficiency was observed, but cation disorder at all the non copper sites according to the formula (Tl_0.60Pb_0.40)(Sr_1.60Ca_0.40)(Ca_1.93Tl_0.07) Cu₃O₉ gives a mean hole concentration of 0.18(1) per Cu atom for the three CuO₂ planes, consistent with the high T_c for this material. Analysis of five time-of-flight powder neutron diffraction data between 80 and 150 K have revealed a possible discontinuity in the variation of the c lattice parameter at T_c, due to an anomaly in the position of the apical oxygen atoms.     

Temperature range for critical scaling behavior in YBCO thin films

The magnetic and transport properties of thin films and single crystals of YBa₂Cu₃O_7−δ are compared. For measurements on thin films, the apparent critical scaling behavior is observed to exist over a temperature range from 87 K down to the vortex-glass transition T_g = 84.2 K at 2.5 kOE and from 83 K to T_g = 70.4 K at 50 kOe. The inflection point (T_inf) in temperature dependent resistivity measurements R(T) coincides with the highest temperature at which current-voltage (I–V) characteristics are found to scale. The region between T_g and T_inf shows a behavior characteristics of thermally activated flux motion, while above T_inf I–V curves show ohmic behavior. No similar scaling region is observed in some single crystal results, supporting recent claims that the phase transition in some single crystals may not be critical in nature (of order greater than one).     

Electron paramagnetic resonance measurements for (La,Y)2/3(Ca,Ba)1/3MnO3

We have measured the paramagnetic (PM) resonance behavior as a function of temperature for various manganese perovskites (La,Y)_2/3(Ca,Ba)_1/3MnO₃ with an average A-site size r_A basically covering the whole region of ferromagnetic (FM) metallic ground state. We show that at least three regions with different magnetic behaviors can be distinguished by two phase lines: T_onset−r_A and T_c−r_A. For each given r_A, the complete PM and long-range FM behaviors appear above T_onset and below T_c, respectively; while some anomalous PM behaviors appear for the intermediate temperature range of T_onset<T<T_c . A possible magnetic transition process is discussed in order of decreasing temperature from high-T complete paramagnetism to low-T long-range ferromagnetism.     

Calorimetric investigation on the paramagnetic–antiferromagnetic phase transition in CoO

The paramagnetic–antiferromagnetic phase transition of a single crystal of CoO, whose first- or second-order character is controversial, has been studied using a high sensitive calorimetric technique. Although both specific heat and differential thermal analysis (DTA) trace obtained at very low temperature rate (0.1 K h⁻¹) show strong anomalies at the Néel temperature T_N, the DTA trace and that calculated from the specific heat anomaly coincides indicating a continuous phase transition. In agreement with the theoretical predictions, the specific heat follows the 3D Ising model in a temperature range of 3 K below T_N. Fisher relation for antiferromagnets is also obeyed in the same temperature range.