Transport Measurements of the Heavy Fermion Superconductor CeCu2Si2 Under Pressure

Superconducting and normal state properties of a single crystal of CeCu₂Si₂ have been investigated by resistivity measurements under hydrostatic pressure and magnetic field. The low temperature resistivity behaviour ( = ₀ + AT ², which is observed up to a temperature T _A) is considered within the self-consistent renormalized (SCR) spin fluctuation model. Contrary to the model predictions, the product AT ² _A increases on approaching the quantum critical point (QCP). The temperature dependence of the upper critical field is analyzed assuming strong coupling and an intermediate regime between the clean and dirty limits. Both the low temperature resistivity behaviour at high pressure and the parameters deduced from the fits of H _c2 point to a pressure induced decrease of the quasi-particle effective mass m*. The anisotropy of the initial slope of H _c2(T) and therefore that of the effective mass was found to change under pressure.     

Influence of Structural Defects on Properties of Reactor Graphite

Data are presented on defect formation in structural graphite materials in response to high-temperature heat treatment, oxidation, and neutron irradiation. The influence of defects on the structure and properties of graphite is analyzed.     

Influence of Structural Defects on the Thermophysical and Magnetic Properties of Polyaniline

The temperature dependences of the thermophysical and paramagnetic properties of polyaniline that contains variously structured impurity centers have been investigated. The results obtained have been explained on the basis of the theory of phase transitions of second order.     

Superconducting Properties of Ag and Sb Substitution on Low-Density YBa2Cu3O δ Superconductor

The effects of silver (Ag) and antimony (Sb) substitution on low-density YBa₂Cu₃O _δ (YBCO) superconductor were investigated. Two series of sample with a nominal composition of YBa_2?x Ag _x Cu₃O _δ and YBa_2?x Sb _x Cu₃O _δ where x=0.05, 0.10, 0.15, 0.20, 0.30, 0.40 and 0.50 were synthesized and characterized. All Ag-doped samples showed metallic behavior at the normal state and T _C?onset was found at 90 K. T _C?zero decreased as the Ag concentration increased. Optimum Ag concentration was achieved at x=0.20 where T _C?zero has the highest value of 87 K and J _C at 70 K is 16.50 A/cm². For Sb-doped case with x≤0.30, the samples showed metallic behavior above T _C?onset while semiconducting behavior was shown for x≥0.40. The optimum Sb concentration was achieved at x=0.15 where T _C?zero is 85 K and J _C value measured at 70 K is 2.75 A/cm². T _C?onset and T _C?zero were found to decrease toward higher Sb concentration. The crystallographic structure transformed to tetragonal in Sb-doped samples of x≥0.30 while other samples remain orthorhombic.     

The Influence of Postannealing on Structural and Superconducting Properties of Bi-2212 Ceramics

The effect of postannealing on the structural and superconducting properties of Bi-2212 sintered ceramic samples prepared by the solid state reaction method has been investigated. Postannealing times were varied from 0 h to 192 h at 700 ^°C. Electrical resistivity studies showed that postannealed samples at 96 h have the lowest room temperature values while critical transition temperature does not change significantly. XRD data have shown that the Bi-2212 phase content does not change noticeably, independently of the postannealing length. In addition, J _c values, calculated from the hysteresis loops using the Bean’s model, increased with increasing the postannealing time until 48 h and decrease for longer ones.     

Theory of Unusual Superconducting Phase Transitions in Heavy Fermion Metals at High Magnetic Fields

We present a non-perturbative theory of paramagnetically-driven superconducting (SC) phase transitions in heavy-fermion metals, which reliably determines their stable SC phases, treats properly the corresponding finite jumps of the order parameter, and can account for unusual features reported recently for this type of materials. It is found that for quasi-2D heavy-fermion metals, such as CeCoIn₅, at high magnetic fields oriented perpendicular to the highly conducting planes, the effect of the Fulde-Ferrel (FF) modulation is too weak to prevent a direct first-order phase transition from the normal to the uniform SC state. For 3D heavy-fermion metals, such as URu₂Si₂, the FF modulation stabilizes, under a decreasing magnetic field, a non-uniform SC state via a second-order phase transition from the normal state. However, at a slightly lower field the modulated phase becomes unstable, transforming to a uniform SC state via a first-order transition.     

Superconducting phase diagram of UPt3 studied by thermal expansion and specific heat

The specific heat and thermal expansion of UPt₃ were measured in the vicinity of the superconducting transition for magnetic fields up to 10 kOe, applied along the hexagonalc axis. Both thermodynamic techniques show clear evidence of two transitions in zero field, which merge at a critical field of approximately 8 kOe. The upper critical field curve determined by the resistivity for the upper transition exhibits a small but abrupt change of slope at a comparable value of the field. The phase diagram that emerges forHc is therefore qualitatively similar to that previously found forHc, this time with a critical point (or region) aroundH*=8 kOe,T*=0.36 K. The implications of a nearly isotropic phase diagram are discussed in connection with recent theories.     

Role of Aniline Addition in Structural and Superconducting Properties of MgB<Subscript>2</Subscript> Bulk Superconductor

In this study, the structural and superconducting properties of aniline-added MgB₂ superconductors were investigated by X-ray diffraction (XRD), thermal analysis techniques, and ac susceptibility measurements. The amount of aniline was changed from 0 to 1 mol%. Phase analysis and lattice parameters were determined from XRD measurements. X-ray diffraction analysis indicates that the main phase is MgB₂ and that there is a small amount of Mg as the secondary phase in aniline-added samples. According to the determination of lattice parameters, it is seen that the addition of aniline does not give a proper distribution with the contribution amount of a and c lattice parameters. From DSC curves, two exothermic peaks and one endothermic peak were observed in all samples. Pure and aniline-added samples were found to be dependent on the magnetic field in the ac susceptibility measurements, and the superconducting transition temperature (T _c ) was found to decrease to lower temperatures due to an increase in the amount of aniline. It has been determined that changes in the in-phase (χ ^′) and out-of-phase (χ ^″) components of the ac susceptibility by increasing the aniline amount have weakened the MgB₂ phase structure and thus cause changes in the pinning mechanism. In addition, ac losses of all the samples were calculated under external fields ranging from 160 to 1280 A/m and at 25 K.     

Y2BaCuO5粒子在准单畴Y-Ba-Cu-O超导块材中的分布及其对超导性能的影响

采用顶部籽晶熔融织构法制备了Y-Ba-Cu-O(YBCO)准单畴超导块材,研究了Y2BaCuO5(Y211)粒子在块材中的分布及其对临界电流密度(Jc)和磁悬浮力性能的影响.扫描电子显微镜(SEM)观察发现,Y211颗粒在准单畴超导块材中的分布是不均匀的.实验结果表明,Y211平均粒径的大小与样品的磁悬浮力和临界电流密度成反比,Y211相粒子在母体中的分布越均匀,粒子平均粒径越小,越有利于提高临界电流密度和磁悬浮力.对于φ20mm的YBCO单畴块材,磁悬浮力可以达到33N(77K,0.55T),临界电流密度达到6.6×104 A/cm2.     

Thermal Conductivity of the Accidental Degeneracy and Enlarged Symmetry Group Models for Superconducting UPt3

We present theoretical calculations of the thermal conductivity for the accidental degeneracy and enlarged symmetry group models that have been proposed to explain the phase diagram of UPt ₃ . The order parameters for these models possess point nodes or cross nodes, reflecting the broken symmetries of the ground state. These broken symmetries lead to robust predictions for the ratio of the low-temperature thermal conductivity for heat flow along the axis and in the basal plane. The anisotropy of the heat current response at low temperatures is determined by the phase space for scattering by impurities. The measured anisotropy ratio, _c/ _b, provides a strong constraint on theoretical models for the ground state order parameter. The accidental degeneracy and enlarged symmetry group models based on no spin-orbit coupling do not account for the thermal conductivity of UPt ₃. The models for the order parameter that fit the experimental data for the and directions of the heat current are the 2D E_1g and E _2u models, for which the order parameters possess line nodes in the ab-plane and point nodes along the axis, and the A_1gE _1g model of Zhitomirsky and Ueda. This model spontaneously breaks rotational symmetry in the ab-plane below T_c2 and predicts a large anisotropy for the ab-plane heat current.     

Influence of Ag-doping in La0.5Sr0.5CoO3 on Its Structural and Magnetic Properties

The structural and magnetic properties of the mixed valent cobaltites La_0.5Sr_0.5?x Ag _x CoO₃ (0≤x≤0.15) with perovskite structure have been investigated. X-ray diffraction (XRD) analysis using Rietveld refinement shows that all our samples crystallize in the orthorhombic structure with $R\overline{3}c$ space group. Although the unit cell volume remains almost constant, the Co–O bond length and the Co–O–Co bond angle are sensitive to the Ag addition. All our studied samples undergo a paramagnetic-ferromagnetic transition with decreasing temperature. The zero field cooled (ZFC) and field cooled (FC) magnetization curves at 50 mT of La_0.5Sr_0.5CoO₃ (x=0) sample exhibits thermomagnetic irreversibility indicating frustration and competition of both antiferromagnetic and ferromagnetic interactions. It is suggested that Co³⁺ ions are in the intermediate spin state but Co⁴⁺ ions stay in a mixture of intermediate and high spin states. Ag addition leads to dramatic changes in magnetic properties. An interesting result has been obtained for La_0.5Sr_0.4Ag_0.1CoO₃, where the ZFC and FC curves coincide. For La_0.5Sr_0.35Ag_0.15CoO₃ (x=0.15) sample, the observed thermomagnetic irreversibility is much higher than that observed in x=0 sample. Hysteresis loops were recorded for x=0, 0.05, 0.1, and 0.15. The coercitive field is found to increase with Ag content reaching 0.26 T for x=0.15 whereas the remanent magnetization decreases.     

Effect of Eu2O3 Nanoparticles Addition on Structural and Superconducting Properties of BSCCO

To study the effects of Eu₂O₃ nanoparticles addition to BSCCO superconducting system, four bulk polycrystalline samples with general formula of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O _y +xEu₂O₃ (where x=0.0, 0.3, 0.5, 1.0 wt%) were prepared by chemical sol–gel method. X-ray diffraction, SEM, and TEM were used for structural characterization of the samples. DC electrical resistivity, critical current, and AC magnetic susceptibility were measured. XRD analysis showed that both (Bi,Pb)-2223 and Bi-2212 phases coexist in the samples having orthorhombic crystal structure. DC electrical resistivity, J _c , and AC magnetic susceptibility measurements reveal that adding Eu nanoparticles to BSCCO improves superconducting properties of this system and enhances its critical current density. The enhancement of the J _c may be caused by improvement of the grain connectivity with Eu nanoparticle additions.     

Investigation of Structural and Superconducting Properties of Cr Added Bi-2212 Superconducting Ceramics

This study reports the effect of Cr addition on the structural and superconducting properties of Bi_1.8Sr_2.0CrxCa_1.1Cu_2.1O _y superconductor with x=0, 0.1, 0.3, 0.5 0.7, and 1 by means of X-ray analysis (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and resistivity measurements. The samples studied in this work were prepared using the standard solid-state reaction method. Zero resistivity transition temperatures (T _c ) are qualitatively estimated from the dc resistivity measurements. The zero resistivity transition temperatures are obtained to decrease from 81 K to 53 K with the increase in Cr addition. Moreover, the phase and lattice parameters were determined from XRD measurements. Based on the refinement of cell parameters done by considering the structural modulation, the Cr addition was confirmed by an increase of the lattice parameter a and decrease of the cell parameter a of the samples in comparison with that of the undoped sample (Cr0). As for the microstructure and element composition analyses on the surface of the samples produced, SEM and EDS measurements were investigated. According to the measurements, not only were the grain sizes of the samples noted to decrease, but also the surface morphology and grain connectivity were obtained to degrade with increase in the Cr addition. The possible reasons for the observed degradation in microstructural and superconducting properties due to Cr addition were also discussed.     

Effect of Ti Doping on Structural and Superconducting Property of YBa2Cu3O7−y High T c Superconductor

Dimensional fluctuations of superconducting order parameters in YBa₂(Cu_1?x Ti _x )₃O_7?y (x=0.01, 0.02, 0.04, 0.05) have been analyzed. SEM micrographs reveal the reduced grain size and the formation of TiO₂ nanowires covering over the grains of YBCO matrix. XRD graphs show the unchanged orthorhombic structure. With the increase of TiO₂%, it is found that the superconducting transition temperatures determined from standard four-probe method decrease gradually. Excess conductivity fluctuation analysis using the Aslamazov–Larkin model fitting reveals transition of two dominant regions (2D and 3D) above T _c . 2D to 3D crossover temperature, i.e., the Lawerence–Doniach temperature that demarcates dimensional nature of fluctuation inside the grains is influenced by Ti incorporation in YBCO matrix. The decrease in the Lawerence–Doniach temperature in the mean field region has been observed as a consequent dominance of 3D region with increase in Ti%.     

The High-Temperature Superconductor EuBa2Cu3O6 + x: Role of Thermal History on Microstructure and Superconducting Properties

We present here combined resistivity and micro-Raman spectroscopy measurements for the superconductor EuBa₂Cu₃O_{6 + x} (Eu-123, EuBCO). We found an important dependence of microstructure and critical temperature on preparation temperature. We also show a strong orientation effect and a T _c of 93 K in samples prepared at high temperature relative to those prepared at lower temperature, which exhibit lower T _c, broad transitions, and no grain orientation. In addition, we show the influence of annealing conditions on superconducting properties by preparing samples with the same oxygen content (x = 0.38) but with various T and P(O₂). The peculiar behavior of Eu-123 with temperature by considering a different hole injection sequence for Eu-123 with respect to Y-123 is explained. Micro-Raman measurements seem to support this hypothesis.     

不同热处理制度对NbTiTa超导线材性能的影响

通过调整超导线材的最终附加应变、时效热处理时间和时效热处理温度3个参数来提高NbTiTa超导线材的J_c性能.在法国国家高场实验室通过标准四引线法,在4.2K、0～11T的磁场中测量了线材的临界电流.结果表明,适度升高时效热处理温度、延长时效热处理时间、增加最终附加应变,有益于提高NbTiTa超导线材的临界电流密度.经4次时效热处理385~C×(40～70)h,最终附加应变从2.5增加到3.5,Φ1.25mm超导线材的J_c在4.2K、8T下达到777A/nm~2,与国外文献报道的J_c值相当.     

Frustration and the Kondo Effect in Heavy Fermion Materials

The observation of a separation between the antiferromagnetic phase boundary and the small-large Fermi surface transition in recent experiments has led to the proposal that frustration is an important additional tuning parameter in the Kondo lattice model of heavy fermion materials. The introduction of a Kondo (K) and a frustration (Q) axis into the phase diagram permits us to discuss the physics of heavy fermion materials in a broader perspective. The current experimental situation is analyzed in the context of this combined “QK” phase diagram. We discuss various theoretical models for the frustrated Kondo lattice, using general arguments to characterize the nature of the f-electron localization transition that occurs between the spin liquid and heavy Fermi liquid ground-states. We concentrate in particular on the Shastry–Sutherland Kondo lattice model, for which we establish the qualitative phase diagram using strong coupling arguments and the large-N expansion. The paper closes with some brief remarks on promising future theoretical directions.     

A Study of the Influence Exerted by Structural Defects on Photoluminescence Spectra in n-3C-SiC

Technical Physics Letters - Photoluminescence (PL) spectra have been studied in 3C-SiC/4H-SiC heterostructures and 3C‑SiC single crystals. It was shown that epitaxial 3C-SiC layers grown on...     

衬底温度对掺锆酸钡高温超导YBCO薄膜结构和超导电性能的影响

高温超导氧化物YBCO外延薄膜在掺入少量锆酸钡(BaZrO3,BZO)后,其超导性能会得到较大幅度的提高.研究了不同衬底温度对掺BZO的YBCO薄膜的外延特性及超导性能的影响.以固相反应法制备了YBCO及BZO原材料,利用固相烧结工艺制备了BZO含量为2%(质量分数)的YBCO-BZO复合靶材,采用脉冲激光沉积技术(PLD)在LaAlO3(100)基片上外延生长YBCO薄膜.以不同的衬底温度制备了YBCO薄膜,用X射线衍射和高分辨透射电子显微技术对所制备的YBCO薄膜的外延特性进行了分析.最后比较了在不同衬底温度下制备的YBCO薄膜的超导性能,得到了最佳衬底温度.