Superconductivity in Mesoscopic Metal Particles

Recently, it has been possible to construct single-electron transistors to study electronic properties, including superconductivity, in metallic grains of nanometer size. Among several theoretical results are suppression of superconductivity with decreasing grain size and parity effect, that is, dependence on the parity of the number of electrons on the grain. We study how these results are affected by degeneracy of energy levels. In addition to the time-reversal symmetry, for certain energy spectra and more generally for lattice symmetries, energy levels are strongly degenerate near the Fermi energy. For a parabolic dispersion, degeneracy d is of the order of k_FL, whereas the typical distance between the levels is of the order of _F/(k_FL)² where k_F and _F are the Fermi wave vector and energy, respectively, and L is the particle size. First, using an exact solution method for BCS Hamiltonian with finite number of energy levels, we find a new feature for the well studied nondegenerate case. In that case, parity effect exhibits a minimum instead of a monotonic behavior. For d-fold degenerate states we find that the ratio of two successive parity-effect parameters _p is nearly 1 + 1/d. Our numerical solutions for the exact ground state energy of negative-U Hubbard model on a cubic cluster also give very similar results. Hence we conclude that parity effect is a general property of small Fermi systems with attractive interaction, and it is closely related to degeneracy of energy levels.     

纳米金属氧化物气敏薄膜的最佳晶粒尺寸和介观效应

纳米金属氧化物气敏薄膜存在灵敏度的最佳膜厚l*和最佳晶粒尺寸r0*,其值与材料和工艺有关.最佳晶粒尺寸的存在是一种新的介观效应,它来自晶粒尺寸缩小到一定程度时反映吸附氧负离子的Fermi统计失效;根据当晶粒尺寸缩小到特征长度rm时晶粒将保持电中性的Kubo理论对Fermi统计公式进行修正,从而给出了最佳晶粒尺寸r0*=2rm的结果,并用Kubo理论电子逸出功公式估算了rm与r0*的数值.     

Triplet Superconductivity in a Nutshell

The triplet superconductors have been around us since 1980, when Jerome et al. discovered the Bechgaard salts (TMTSF)₂PF₆ and (TMTSF)₂ClO₄. Now there are more than 20 or so triplet superconductors discovered. Recently we found that most of them with the known gap symmetries can be mapped to the superfluid phase of ³He-A and ³He-A₁. Further, in most of them, (the chiral vector, i.e. the quantization axis of the pair angular momentum) is fixed parallel to one of the crystal axes and all the topological defects are considered in terms of -textures, where is the spin vector. Then Sr₂RuO₄, UPt₃, PrOs₄Sb₁₂ and (TMTSF)₂ClO₄ belong to type-A, which is an analog of superfluid ³He-A. In these superconductors, a vortex splits into a pair of half quantum vortices (HQVs) at low temperatures. On the other hand, CePt₃Si, CeIrSi₃, CeRhSi₃, UIr and Li₂Pt₃B (those in non-centrosymmetric crystals) belong to type-A₁, which is an analog of superfluid ³He-A₁. In all of these triplet superconductors, vortices harbor the zero mode or the Majorana fermions, the implications of which deserves further exploration.     

Superconductivity in Doped Systems Beyond Migdal''s Theorem

The basic system of equations for a theory of superconductivity in systems with a chaotically distributed paramagnetic impurity of substitution, in which Migdal theorem is violated (it cannot be supposed ₀ E _F), is presented. We take into account electron–phonon and impurity diagrams as well as supplement ones corresponding to intersection of electron–phonon and electron–impurity lines. Decrease of the quantity T _C with increase of impurity concentration is shown to weaken in comparison with the case of usual superconductors. Coefficient of isotope effect , energy gap, and order parameter at T = 0 are also calculated. Behavior of these quantities as a function of impurity concentration depends on Migdal's parameter m and transferred momentum q _c.     

High-Temperature Superconductivity Without Cuprate Planes

Sr₂YRuO₆ and Ba₂YRuO₆, both doped with Cu on Ru sites, superconduct at onset temperatures of ∼49 and ∼93 K, despite having no cuprate planes. Sr₂YRuO₆ has two sister cuprate compounds, GdSr₂Cu₂RuO₈ and Gd_2-z Ce _z Sr₂Cu₂RuO₁₀, that begin superconducting at nearly the Sr₂YRuO₆ temperature, which suggests that all of these materials superconduct not in their cuprate planes (which do not exist in Sr₂YRuO₆ or Ba₂YRuO₆), but in their SrO layers (BaO layers for Ba₂YRuO₆). Generalization of this result to all high-T _c superconductors implies that the cuprate planes do not superconduct, and that the SrO layers (or equivalent layers) do.Sr₂YRuO₆ superconducts with less than 1% Cu, too few Cu atoms to form a significant number of CuO₂ planes [1].Cu-doped Sr₂YRuO₆ becomes fully superconducting at 23 K. See Ref. [2].     

Interfacial Superconductivity on the Topological Semimetal Tungsten Carbide Induced by Metal Deposition

Interfaces between materials with different electronic ground states have become powerful platforms for creating and controlling novel quantum states of matter, in which inversion symmetry breaking and other effects at the interface may introduce additional electronic states. Among the emergent phenomena, superconductivity is of particular interest. Here, by depositing metal films on a newly identified topological semimetal tungsten carbide (WC) single crystal, interfacial superconductivity is obtained, evidenced from soft point-contact spectroscopy. This very robust phenomenon is demonstrated for a wide range of metal/WC interfaces, involving both nonmagnetic and ferromagnetic films, and the superconducting transition temperatures are surprisingly insensitive to the magnetism of thin films. This method offers an opportunity to explore the long-sought topological superconductivity and has potential applications in topological-state-based spin devices.     

Superconductivity in Lithium Under Pressure

We use an ab initio calculation, within the framework of the density functional theory, to calculate total energy, electronic energy bands, and the phonon dispersion curves of an fcc structure of lithium as a function of the unit cell volume. We find that the fcc structure is stable starting from a reduced volume of 0.88 V₀, where V₀ is the known unit cell volume at ambient pressure and it remains so until a volume of 0.40 V₀. The electronic structure and the vibrational modes are calculated, as well as the variation of T_c with the volume. At a reduced volume of 0.40 V₀, T_c is calculated to be 20 K, which is in agreement with the experimental value. Our calculations indicate that superconductivity in Li under high-pressure occurs through intervalley scattering of electrons, between pockets centered at the L-points of the Brillouin zone (BZ), mediated by phonons in the vicinity of the X-points.     

Superconductivity in compensated and uncompensated semiconductors

Abstract

We investigate the localization and superconductivity in heavily doped semiconductors. The crossover from the superconductivity in the host band to that in the impurity band is described on the basis of the disordered three-dimensional attractive Hubbard model for binary alloys. The microscopic inhomogeneity and the thermal superconducting fluctuation are taken into account using the self-consistent 1-loop order theory. The superconductor-insulator transition accompanies the crossover from the host band to the impurity band. We point out an enhancement of the critical temperature T_c around the crossover. Further localization of electron wave functions leads to the localization of Cooper pairs and induces the pseudogap. We find that both the doping compensation by additional donors and the carrier increase by additional acceptors suppress the superconductivity. A theoretical interpretation is proposed for the superconductivity in the boron-doped diamond, SiC, and Si.     

Possible Superconductivity in TiSe2 Intercalated by Transition Metals

Electric conductivity and magnetoresistence data for TiSe₂ intercalated by Fe²⁺ and Cr³⁺ are interpreted as superconducting state with T _c ∼ 5 and 4 K, respectively.     

六氟化钨中金属粒子的分析

结合六氟化钨的性质,对六氟化钨中的金属粒子进行分析。由于六氟化钨具有腐蚀性而且遇水迅速反应生成三氧化钨和氟化氢,同时金属粒子密度大,只有测定液态六氟化钨中的金属粒子含量,分析结果才是稳定准确的,所以设计了一个密闭的半定量取样管路,可以准确地完成六氟化钨中金属粒子的取样。完成取样后,根据各种金属粒子的溶解性质,将所取金属粒子配成可以用于原子发射光谱分析的待测溶液,然后利用原子发射光谱分析即可以得到六氟化钨样品中各种金属粒子的含量。     

基于细观仿真建模的CFRP细观破坏

碳纤维增强树脂基复合材料(CFRP)在细观上呈现纤维、树脂及界面组成的混合态,其切削加工过程的实质为刀具作用下材料细观层面的破坏至切屑宏观形成的演化过程。为了揭示CFRP切削加工过程中材料的细观破坏,建立了CFRP切削的细观有限元模型。该模型在几何上包含了纤维、基体及界面等组成相,而不是使用传统的等效均质建模方法。各组成相不仅考虑了各自不同的材料本构,而且为了能够模拟材料破坏,还将各组成相材料的失效及演化准则考虑其中。该模型可从细观层面更真实地模拟不同纤维角度CFRP单向板切削过程中纤维/基体断裂、界面开裂及演化的过程。仿真结果表明:不同纤维角度下CFRP细观破坏不同,切削0℃FRP时以界面开裂和纤维弯断为主;切削45°/90℃FRP时主要是刀具侵入工件,纤维基体被压断;切削135℃FRP时则以纤维弯曲断裂为主,断裂面往往在加工面以下。通过实验显微在线观测手段验证了模拟结果的正确性。      

Vortex Interaction with Mesoscopic Surface Cavities in Superconductors

The conformal mapping method is used to study the problem of flux line interaction with surface cavities having cylindrical profile and characteristic size , i.e., within mesoscopic scale, where λ is the penetration length. It is shown that the metastable states are achieved when the dimensions of the surface irregularities do not exceed the coherence length ξ. Our study shows that the surface barrier may vanish at some weak point at which the surface irregularities have mesoscopic scales. On the other hand, a remarkable decrease in the surface barrier occurs when the surface defects size . Our results are compared with the available experimental data and theoretical results.     

The Mechanism of High-Tc Superconductivity: “Nonlinear” Superconductivity

The main purpose of the paper is to present an overview of the current situation in the development of the understanding of the mechanism of high-T _c superconductivity which arises due to moderately strong, nonlinear electron–phonon interactions and due to spin fluctuations. The former are responsible for electron pairing, and the latter mediate the phase coherence. In addition, a key experiment for superconductivity in cuprates is proposed.     

Transistor Doping-Predictions for Superconductivity in the Charge Order Model

Transistor doping of potential superconductors promises to illuminate inherent levels and limits for the materials in question. Here some of these limits are quantitatively predicted on a charge order or radical model for a variety of experiments. This model is based on a universal T _c scaling with anionic radical concentration. It is applicable to systems with covalently bonded, high electronegativity components. For the cuprates its general features are a linear increase of T _c with holes, which are normalized per total hole carrying species (e.g., O^– radical density). This dependence proceeds up to optima, corresponding to a uniform alternate hole charge order limit and T _c decreases thereafter with various slopes. The optima are quantitatively calibrated through the deleterious influence of the Blocking layer as measured by its O content. Amongst the predictions is a general limit of T _c ^max 167 K for oxide superconductors, obtainable in infinite layer type compounds (CaCuO₂). Other predictions are optimal T _c 83 K for La₂CuO₄ and maximum values potentially near 500 K for borides or carbides.     

Two-Dimensional d-Waves Superconductivity

We consider a two-dimensional (2D) Fermi liquid with attraction in the d channel. The critical temperature, the order parameter at T = 0 and the temperature dependence of the order parameter have been calculated analytically for a constant, an ^r-energy dependent, and a van Hove density of states.     

Sketching the Theory of Copper Oxide High Temperature Superconductivity

The role of the antibonding state in the electron correlations in copper oxide HTSC is analyzed. Then the t-J Hamiltonian is used to establish the formation of the charge stripes in underdoped oxides. It is proposed that these stripes make up the boundaries between the two degenerate antiferromagnetic (AFM) states, and that they are a key factor in switching between these states. We also provide a theoretical expression to the charge driven AFM magnons that have been observed by Neutron scattering experiments. Finally, the double correlation theory is applied to the stripe phase of holes to result in the superconductive gap and in the “pseudogap.”     

Superconductivity and the Chemical Bond

Experimental evidence suggests the operation of electron rules based on bond ordering of doped radical CuO bonds (BO) and lattice pressure (LP) in cuprate superconductivity. Ideas are developed for understanding the crystal chemical principles involved in these BO. They are then extended to materials on the basis of other electronegative elements. For cuprates, a quantitative T _c formalism is based on radical bond or hole concentration in the CuO₂ plane (h_p) per O in the extended planar system including the apical O (radical formalism or RF). The deleterious effect of the apical O is ascribed to bond resonance with the CuO₂ plane. A range in the critical hole concentration exists for optimal T _c depending on LP. Accordingly the small Sr analogs tend to be limited at smaller hole concentrations h _p = 1/6, while the Ba analogs can optimize at 1/3. Selected experimental evidence is used to construct plausible BO patterns. Superconductivity is related to strings of local pairs of trijugate half bonds. Electronic freezing can occur in domain walls of alternating single holes, e.g., in La₂CuO₄ type materials at h _p = 1/8. Predictions for this BO model have been corroborated by details in the transistor doping of CaCuO₂ and C ₆₀ as a universal sequence of T _c development with hole doping becomes apparent for both cases. Hallmarks include linear portions up to an optimal BO (RF slopes), which extrapolate to the origin, and a sharp peak with another linear region thereafter. They further include T _c onset at a critical BO, where an adverse BO is cooperatively transformed to a beneficial one with a slope related to RF by rational numbers. Tendencies to electronic freezing [e.g., h _p = 1/8 in CaCuO₂ and an analogous one in C ₆₀] occur near the optimal BO because of increasing LP. Some of these features occur at identical numbers of atoms per hole for both systems and others are related numerically. T _c–hole slopes can supply hints for the mechanisms of BO transformations. The universality of this behavior suggests a common origin for all high T _c materials in pairs of doped covalent bonds of radical character and a musical BO phenomenology with respect to levels and limits.     

纳米颗粒的超导转变温度

首次给出一组适合声子特点的共轭复量子数算符,由此建立声子的量子算符代数理论,根据该理论得到声子的量子化能量、纳米颗粒的晶格点阵热容和纳米颗粒的超导转变温度的具体表示。结果表明,声子的量子化能量、纳米颗粒的晶格点阵热容和纳米颗粒的超导转变温度均与声子的量子尺寸、状态量子数及时间量子数有关。