Superconductivities of Quasi-One-Dimensional and Quasi-Two-Dimensional Tight-Binding Electrons in Magnetic Field

The superconductivity of the tight binding electrons in a magnetic field is studied. We can treat both cases of quasi-one-dimension and quasi-two-dimension in the same manner. We study a various kind of anisotropic superconductivity with line nodes of the energy gap by taking attractive interaction between electrons in nearest sites along each axis. The magnetic field dependence of the transition temperature is calculated for a various pairing symmetry of superconductivity. When a magnetic field is applied in the conducting plane, the transition temperature is shown to increase as the magnetic field increases. In the strong magnetic field the eigenstates approach to those in the absence of hopping between planes, resulting in no orbital frustration.     

Experimental Evidence for Spin-Triplet Superconductivity in Sr2RuO4

We review the experimental evidence for spin-triplet superconductivity in Sr₂RuO₄. The Knight shift experiments decisively demonstrated that the Cooper-pair symmetry is spin triplet. We discuss the most probable wave function of the Cooper pairs on the basis of the results of a number of key experiments. We point out that a simple picture of the unitary state with the isotropic gap is not compatible with the observed behavior of the specific heat, and that a profound modification of the pairing wave function is needed. We also present the in-plane angular dependence of the upper critical field, with the field applied exactly parallel to the quasi-two-dimensional plane. We discuss the possible implication of the observed fourfold anisotropy in connection with the proposed second superconducting state with a line-node gap.     

Theoretical Consideration on Triplet Superconductivity in UPt3 and Sr2RuO4

Two representative superconductors, UPt₃ and Sr₂RuO₄, are comparatively studied on the basis of a general symmetry argument. The existing experimental data set for both systems strongly and commonly points to a triplet pairing. For the former, heavy fermion material, UPt₃, a non-unitary bipolar state is the most possible candidate, whereas for the latter system, Sr₂RuO₄, determination of a precise pairing symmetry class, either unitary or non-unitary, is debated.     

SrRuO: Normal-State Properties and the Effect of Disorder

A brief review is given of the normal-state properties of Sr₂RuO₄, combined with a discussion of some aspects of the superconductivity. Particular attention is paid to the significance of high sample purity to many of the discoveries that have been made. Attention is also drawn to the simplicity of the electronic structure of the material, which is likely to play a key role in future investigation of both the normal and the superconducting states.     

p-Wave Superconductors in Sr2RuO4 and Bechgaard Salts

We review our recent works on the vortex state in p-wave superconductors. First, in a magnetic field parallel to the c-axis, the square vortex lattice is most stable, except in the immediate vicinity of T = T _c0. Second, the effect of impurities on H _c2 is studied, which exhibits characteristics of unconventional superconductors. Finally, the a–b anisotropy in the upper critical field in a magnetic field is considered. This anisotropy provides important information about the fourfold term arising from the Fermi surface effect.     

Superconducting Properties of the 3-K Phase of Sr2RuO4 near Hc2

The superconducting transition temperature, T _c , of the impurity-free, intrinsic Sr₂RuO₄ is as high as 1.50 K. However, we recently showed that T _c is remarkably increased up to 3 K in the Sr₂RuO₄–Ru eutectic system, in which plate-like microdomains of Ru metal are embedded in the primary-phase Sr₂RuO₄. The phase diagram of the anisotropic upper critical field of the 3-K phase indicates that H _c2 for the field parallel to the RuO₂ plane is strongly suppressed at low temperatures. We argue that the reorientation of the Cooper-pair spin direction near the Sr₂RuO₄–Ru interface may be responsible for this suppression. In addition, we observed unusual hysteresis in the out-of-plane resistivity, _c , at low temperatures and near H _c2, only when the field was applied parallel to the RuO₂ plane.     

The Specific Heat and the Fermi Surface Anisotropy in Sr2RuO4

The influence of the Fermi surface anisotropy on the specific heat jump is studied for the p-wave superconductor with a gap function d = z^(k _x ± ik _y ) in connection with Sr₂RuO₄. It is shown that the normalized specific heat jump, C/T _c , is reduced from its universal value, 1.43, in accordance with experimental findings. We show that this behavior fits well into our present understanding of the quasiparticle spectrum and the impurity effects.     

Superconductivity in an Organic Conductor Stabilized by a High Magnetic Field

The application of a sufficiently strong magnetic field to a superconductor will in general destroy the superconducting state. However, for the quasi‐two‐dimensional organic conductor λ‐(BETS)₂FeCl₄, where BETS is bis(ethylenedithio)tetraselenafulvalene, we find that superconductivity is induced under magnetic fields in the range between 18 T and 42 T when the magnetic fields are applied exactly parallel to the conducting layers of the crystals, although it has an insulating ground state below 10 T. This is the first material whose superconducting phase is stabilized only under a magnetic field. The resulting phase diagram indicates that the internal magnetic field due to the negative exchange interaction between the localized Fe moments and the conduction electron spins is crucial.     

Phonon-Induced Superconductivity and the Staggered Magnetic Field on a Square Lattice

We have considered phonon-induced superconductivity on a square lattice. The antiferromagnetic correlations that are present in high-temperature superconductors (HTSC) have been simulated by the staggered magnetic field. The Eliashberg equations have been generalized to account for the momentum dependent order parameter, and the superconducting transition temperature T _c has been evaluated within the strong-coupling theory. The two-dimensional band structure has explicitly been taken into account. It occurs that the results obtained for T _c when considering d-wave symmetry of the superconducting state much better reflect the experimentally observed behavior than those derived for s-wave symmetry. The isotope shift exponent has also been evaluated. One can observe that the introduction of the staggered magnetic field leads to the decrease of this quantity. Therefore, the presence of the staggered magnetic field may contribute to experimentally observed values of smaller than the BCS value of 1/2     

Superconductivity in Na-doped Bi2Sr2CaCu2O y system

Samples with the nominal composition Bi₂Sr₂Ca_1−x Na _x Cu₂O _y (x=0, 0·1, 0·2 and 0·3) were prepared by solid-state reaction of the individual compounds. X-ray diffraction patterns indicate that the samples have a majority 2212 phase with 2223 also being present. From the DC four-probe resistance data, we have observed that the furnace-cooled samples show metallic behaviour while the quenched samples show superconductivity up to 97 K.     

Superconductivity and Magnetism in RuSr2GdCu2O8

The ruthenocuprate RuSr₂GdCu₂O₈ orders magnetically at 135 K and then becomes superconducting at 45 K. These transitions have been observed by several groups, but the intrinsically complex nature of the compound, as well as the unexpected coexistence of magnetism with high-temperature superconductivity, makes it uncertain what the magnetic, electronic, and structural character of its ground state is. We find that density functional-based evaluations give a magnetic and structural character that is consistent with the latest data: the RuO₆ octahedra rotate (by 7° around the c^ axis) causing a doubling of the cell, and an antiferromagnetic structure that has this same cell doubling is favored over a ferromagnetic alignment of Ru spins. The minority Ru d _xy states are partially occupied, leaving a metallic RuO₂ layer that dopes the CuO₂ bilayer and may lead to canting of the Ru spins.     

Tuning Heavy Fermion Systems into Quantum Criticality by Magnetic Field

We discuss a series of thermodynamic, magnetic, and electrical transport experiments on the two heavy fermion compounds CeNi₂Ge₂ and YbRh₂Si₂ in which magnetic fields, B, are used to tune the systems from a non-Fermi liquid (NFL) into a field-induced FL state. Upon approaching the quantum-critical points from the FL side by reducing B we analyze the heavy quasiparticle (QP) mass and QP-QP scattering cross sections. For CeNi₂Ge₂ the observed behavior agrees well with the predictions of the spin-density wave (SDW) scenario for three-dimensional (3D) critical spin-fluctuations. By contrast, the observed singularity in YbRh₂Si₂ cannot be explained by the itinerant SDW theory for neither 3D nor 2D critical spinfluctuations. Furthermore, we investigate the magnetization M(B) at high magnetic fields. For CeNi₂Ge₂ a metamagnetic transition is observed at 43 T, whereas for YbRh₂Si₂ a kink-like anomaly occurs at 10 T in M vs B (applied along the easy basal plane) above which the heavy fermion state is completely suppressed.     

Superconductivity at 14 K in SmFe0.9Co0.1AsO

We report superconductivity in the SmFe_0.9Co_0.1AsO system being prepared by most easy and versatile single-step solid-state reaction route. The parent compound SmFeAsO is non-superconducting but shows the spin density wave (SDW) like antiferromagnetic ordering at around 140 K. To destroy the antiferromagnetic ordering and to induce the superconductivity in the parent system, the Fe²⁺ is partially substituted by Co³⁺. Superconductivity appears in SmFe_0.9Co_0.1AsO system at around 14 K. The Co doping suppresses the SDW anomaly in the parent compound and induces the superconductivity. Magnetization measurements show clearly the onset of superconductivity with T _c^dia at 14 K. The isothermal magnetization measurements exhibit the lower critical fields (H _c1) to be around 200 Oe at 2 K. The bulk superconductivity of the studied SmFe_0.9Co_0.1AsO sample is further established by open diamagnetic M(H) loops at 2 and 5 K. Normal-state (above T _c) linear isothermal magnetization M(H) plots excluded the presence of any ordered magnetic impurity in the studied compound.      

Superconductivity characterization by a magnetic pendulum

A magnetic pendulum device was designed, developed, and used to characterize high-T_c materials. A correlation between the volume fraction of superconducting phase in a composite and the force of expulsion was found. The device is sensitive enough to detect a volume of 0.5% superconducting phase in a composite. The application of the device for characterizing composites, incipient superconductivity, and films has been discussed.     

Superconductivity at differentTc in CdBa2Ca2Cu3Oy

A Cd analogue of the Tl and Hgn =3 series with nominal composition CdBa₂Ca₂Cu₃O_y has been synthesized. The samples were superconducting according to magnetic susceptibility measurements. The critical temperature was 103 or 107 K depending on the preparation conditions. The EDX analysis revealed the presence of Cd-1111, Cd-1121, and Cd-2333 as minor phases. The observed diamagnetic effects were attributed to the differentT _c of these phases.     

Low Temperature Properties and Superconductivity of LuB12

We have investigated the low temperature properties of LuB₁₂ by measuring its magnetic susceptibility, heat capacity, and electrical resistivity, as well as by point-contact spectroscopy using both the spear-anvil type technique and mechanically controllable break junctions. Our specific heat measurements and point - contact spectroscopy results indicate that LuB₁₂ is a simple weak-coupling BCS-type superconductor with T_C ≈ 0.4 K, a superconducting energy gap of 2 Δ ≈ 0.12 meV, and a very small critical field B_C ≈ 1 mT. From the dU/dI(U) characteristics in the superconducting state, the energy gap 2Δ, the critical current I_C and the Andreev-reflection excess current I_ex as a function of normal-state point contact resistance R_N have been determined. At low R_N all three parameters are strongly suppressed, possibly due to the self-magnetic field. At contacts with large R_N the model of resistively and capacitively shunted Josephson junctions (RCSJ) allowed us to estimate the superconducting current plasma frequency and the Josephson coupling energy. Moreover, from the d²U/d I²(U) spectra in the normal state the (point-contact) electron-phonon interaction function and the characteristic phonon energies of LuB₁₂ have been determined.     

磁性纳米四氧化三铁在生物医学中的应用

近年来,由于磁性纳米粒子在实际应用中发挥越来越重要的作用,有关磁性纳米粒子的应用受到科学界广泛关注,特别是生物医学领域。由于磁性纳米Fe_3O_4粒子制作简单且晶体对细胞无毒,在生物医药领域大量应用,磁性纳米Fe_3O_4粒子主要通过表面包覆成为免疫磁性微球进行使用。简述了磁性纳米Fe_3O_4粒子的制备方法,重点综述了近些年磁性纳米Fe_3O_4粒子在生物医学上的应用,包括磁共振成像技术、磁分离技术、靶向药物载体技术、肿瘤热疗技术、造影剂技术,并且阐述了磁性纳米Fe_3O_4粒子的发展前景。     

Band structure of Bi2Sr2CuO6: Strong effects due to structural modulation

The electronic structure of idealized tetragonal Bi₂Sr₂CuO₆ is compared with that of an orthorhombic 2×2 model that includes atomic displacements due to the structural modulation. The electronic structure, particularly that related to Bi and O(3) atoms, is strongly modified near the Fermi energy. In particular the shape and topology of the Bi-O(3) related Fermi surfaces near the ¯M point are changed. With the modulation, bands occur near the Fermi energy over a significant part of the zone near this point. A method is proposed to distinguish between lattice distortions and antiferromagnetic correlations as the cause of recently observed shadow Fermi surfaces.     

Pairing, Magnetic Spin Fluctuations, and Superconductivity Near a Quantum Critical Point

The properties of a wide variety of intermetallic compounds exhibiting magnetic localized spin and superconducting fluctuations near a quantum critical point (QCP) are reviewed. They show highly anomalous critical indices (anomalously small). Laws of corresponding are observed in these materials and a theory is presented which gives a fully quantitative explanation of these laws. The theory employs a gauge transformation which rotates the electron spin quantization axis into the direction of the instantaneous staggered localized spin direction , where is the localized spin array wave vector. Many properties of these materials are worked out on the basis of this theory. The technological promise of these substances is truly immense, including energy generation, storage and transmission, MRI magnets, industrial and scientific magnets, maglev, cellular communications, -wave electronics, etc.