Dependence of Josephson Current on Orientational Angle in Diffusive p-Wave Superconductor Junctions

We calculate Josephson current in p-wave superconductor (P)/diffusive normal metal (DN)/insulator/diffusive normal metal/p-wave superconductor junctions, using quasi-classical Green’s function theory. We study the dependence of the critical current on the orientational angle at various temperatures. We find that the critical current in these junctions are proportional to cos?αcos?β. Here, α and β are angles between the direction perpendicular to the interfaces, located between the DN and P, and the crystal axis of left and right P respectively.     

Josephson Current in a Gapped Graphene Superconductor/Barrier/Superconductor Junction: Case of Massive Electrons

The Josephson effect in a gapped graphene-based superconductor/barrier/superconductor junction is studied. The superconductivity in gapped graphene may be achieved by depositing conventional superconductor on the top of the gapped graphene such as graphene grown on SiC substrate. In gapped graphene system, the carriers exhibit massive Dirac fermions. We focus on the effect of pseudo-Dirac-like mass on the supercurrent. In contrast to that in the gapless graphene superconductor/barrier/superconductor junction, we find that the supercurrent exhibits dependency of the Fermi energy. Also, the massive supercurrent anomalously oscillates as a function of the gate potential. This novel behavior is due to the effect of electrons acquiring mass in gapped graphene.     

Spin-Orbit Coupling Effects in the dc Josephson Current in the s Wave Superconductor/Insulator/d Wave Superconductor Junction

The Bogoliubov-de Gennes equation is applied to the study of the spin-orbit coupling (SOC) effects in s-wave superconductor/insulator/d-wave superconductor junctions. It is found that the dc Josephson currents show a nonmonotonic dependence on SOC scattering strength. The phase-current relationship for the spin-up and spin-down channels deviates substantially from sin2φ because of the spin-orbit scattering, so the periods of oscillation curves are 2π for the total Josephson current at α=π/4. An alternative method to confirm the effects of the spin-orbit scattering is the measurement of the phase-current relationship.     

Free Energy of a d-Wave Superconductor in the Nearly Antiferromagnetic Fermi Liquid Model

A very general Hamiltonian which describes the coupling of charge carriers via a spin susceptibility is used to derive a formula for the free energy difference between the superconducting and the normal state of a d-wave superconductor. The formula is then specialized to the nearly antiferromagnetic Fermi liquid (NAFL) model and evaluated numerically. The comparison with specific heat data measured for optimally doped YBa₂Cu₃O_6.95 reveals the necessity to include even contributions far away from the Fermi surface to the free energy. The usual restriction to a narrow shell around the Fermi surface in deriving the free energy formula is proved to be inadequate.     

Detecting Current Noise with a Josephson Junction in the Macroscopic Quantum Tunneling Regime

We discuss the use of a hysteretic Josephson junction to detect current fluctuations with frequencies below the plasma frequency of the junction. These adiabatic fluctuations are probed by switching measurements observing the noise-affected average rate of macroscopic quantum tunneling of the detector junction out of its zero-voltage state. In a proposed experimental scheme, frequencies of the noise are limited by an on-chip filtering circuit. The third cumulant of current fluctuations at the detector is related to an asymmetry of the switching rates.      

Field Dependence of the Vortex Structure in d-Wave and s-Wave Superconductors

The vortex structure and its field dependence are studied in the clean limit on the basis of the quasi-classical Eilenberger theory to find their difference between - and s-wave pairings. We show the -wave nature and the vortex lattice effect on the local density of states around the vortex, and on the field dependence of the spatially averaged density of states. The -wave pairing introduces a fourfold symmetric structure around each vortex core in the pair potential and the internal field. With increasing field, their contribution becomes significant to the whole structure of the vortex lattice state.     

Anisotropy in Quasiparticle Density of States in the Vortex State of an Orthorhombic d-Wave Superconductor

The quasiparticle density of states is calculated in the vortex state of an orthorhombic d-wave superconductor as a function of the orientation of the external magnetic field (H ) in the CuO ₂ -plane. Because of the existence of chains oriented along the y-axis in YBCO, its in-plane properties are highly anisotropic and in the superconducting state a predominantly order parameter will also contain a subdominant s-wave admixture allowed by the orthorhombic crystal space group. In a simple anisotropic effective mass model for the band structure and a d + s gap we compute the quasiparticle density of states as a function of the orientation () of H with respect to the a-axis in the CuO ₂ plane. As a function of we find a two fold pattern of behavior with minima at the angles where the Fermi velocity at the nodal momentum is in the direction of the external field. For an untwinned single crystal the resulting anisotropy of the specific heat at low temperatures should be measurable. It is greatly reduced in twinned samples     

Electrical-Thermal-Structural Coupled Finite Element Model of High Temperature Superconductor for Resistive Type Fault Current Limiters

A multi-physics finite element model of high-temperature superconductors (HTS) will be presented in this article. The electrical-thermal model is mainly based on Maxwell’s equation and basic heat transfer equation to calculate the temperature propagation along the length of the superconducting tapes. According to the calculated temperature profile during quench, the surface thermal stress of the tape is obtained by a structural model. This model can be used to evaluate the degradation of tape performance due to the surface thermal stress, and the results can be used to minimize these effects in the SFCL system. The proposed model is used to investigate the uneven tape, the thickness of the copper layer and YBCO layer which is non-uniform. Methods of smoothing the degradation effect by adding auxiliary fixed points to the tape and changing the basic thickness of copper layer are discussed.     

Duality in the Quantum Dissipative Villain Model and Application to Mesoscopic Josephson Junction Circuits

We study exact self-duality in the model of a Brownian particle in a washboard (WB) potential which describes a Josephson junction (JJ) coupled to an environment, for arbitrary temperature and arbitrary form of the spectral density of the environment. To this end we introduce the quantum dissipative Villain model (QDVM), which models tunneling of a degree of freedom coupled to a linear quantum environment through an infinite set of states. We derive general exact mappings on various dual discrete representations (one-dimensional Coulomb gases or surface-roughening models) which are exactly self-dual. Then we show how the QDVM maps exactly onto the WB model and use duality relations to calculate the leading terms of the total impedance of a JJ circuit, for general frequency dependence of the spectral density of the environment and arbitrary temperature.     

Various Charge-Ordered States in the Extended Hubbard Model with On-Site Attraction in the Zero-Bandwidth Limit

The extended Hubbard model in the zero-bandwidth limit with intersite density–density interactions (nearest- and next-nearest-neighbor) is analyzed in the site-dependent mean-field approximation. In this paper, we investigate a case of on-site attraction U<0 for arbitrary values of intersite interactions as well as chemical potential (or electron density). We present ground state and finite temperature phase diagrams obtained in the four-sublattice assumption (e.g., 1D chain, 2D square, or 3D body-centered cubic lattices). Our results for U<0 show that in the system various phases emerge: three different types of charge-ordered phases (checkerboard, laminar/stripe, and four-subllatice-type) and non-ordered phases.     

Thermodynamics of the Frustrated 2D Hubbard Model

The thermodynamic properties of the quarter filled 2D Hubbard model extended by next-nearest-neighbor (NNN) hopping have been studied in an exact diagonalization method on a tilted square cluster. The NNN hopping interaction (i.e. frustration) is prominent at the low-temperature region. A positive (negative) diagonal hopping t′ favors (suppresses) double occupancy and hence decreases (increases) local moment. At higher temperatures thermal excitations favor the formation of doublans against the strong Coulomb repulsion U and the weak diagonal hopping, hence decreasing local moment. A two-peak structure is observed in the specific heat curves with the inclusion of negative NNN hopping. The low-T peak in specific heat is highly suppressed in comparison to the high-T peak. It has also been argued that t′ influences the ‘pseudogap’ energy scale. In the present case of quarter filling, a weak and short range antiferromagnetic order is present at very low temperatures, which is slightly suppressed (enhanced) by positive (negative) t′. The effect of t′ is more pronounced in the non-interacting limit.     

提高MgB2超导体临界电流密度的研究进展

临界电流是超导体载流能力大小的重要表征,论述了制备工艺、元素掺杂和物理场对MgB2超导体临界电流密度的影响.使用非晶态高纯硼粉制备致密的MgB2试样有利于样品临界电流密度的提高,添加SiC或者Ti也能起到提高临界电流密度的作用,质子照射使MgB2试样在高磁场条件下的临界电流密度增加,强磁场下烧结则有利于提高MgB2样品临界的传输电流密度.     

Influence of Long-Range Coulomb Interaction and On-Site Hubbard Repulsion on the Formation of d-Wave Copper-Pairing in High-T c Cuprates

We develop a diagram technique for the self-consistent treatment of the long-range Coulomb interaction and on-site Hubbard repulsion in the normal and superconducting state of high-T _c cuprates. The resultant analytical expression for the “screened” matrix elements taking into account long-range and on-site repulsion has been derived. In particular, it accounts for processes with and without spin-flip due to an exchange of spin and charge density fluctuations. Furthermore, we derive the expressions for the normal and anomalous self-energy parts near the superconducting transition temperature T _c that takes into account the vertex corrections including crossing diagrams. The contribution of the crossing parts is taken within the ladder approximation (similar to Fluctuation-Exchange approximation) where the role of Hubbard on-site interaction is replaced by the Coulomb matrix element with a spin-flip averaged over the momentum. Finally, the developed scheme allows to analyze the formation of d-wave superconductivity and its stability in presence of the long-range Coulomb repulsion within a self-consistent anisotropic Eliashberg-like approach.     

Influence of Grain Properties on the Abrikosov Vortices Interaction with Josephson Junction

No Heading The problem of the displacement of an Abrikosov vortex is investigated when the magnetic field decreases to H^G_c1 in a grain of a type II superconductor. It is shown that near grain boundaries the vortex line generates an intergranular current which depends on the normalized grain size and the anisotropy ratio . These parameters strongly influence the conditions of the Josephson vortex generation as a result of the interaction of the Abrikosov vortex with the Josephson junction. We support our theory with calculations of the intergrain critical current taking into account two types of the vortex configuration, triangular and square, for different grain characteristics. The results are of interest for the charge transport in granular superconductors as well as the relaxation process in devices that contain Josephson junctions in micro- and nanoelectronics in magnetic field.PACS numbers: 74.25.Qt, 74.25Sv, 74.50.+r.     

Effect of Virtual Andreev Reflection on Tunneling in Normal/Superconductor Graphene Junction

The Andreev reflection (AR) process and the tunneling conductance in graphene-based normal metal/superconductor junction are studied, where both massless and massive fermions are considered. As a result, the evanescent type of AR or the virtual Andreev reflection (VAR), is found to take place at the interface above the critical angle or in case where AR decays due to massive energy gap. Significantly, current of the junction is found to exist in the evanescent AR regime. In this work, we modify tunneling conductance formalism by including the current induced by the evanescent type of AR process (or VAR), which has not been taken into account in previous studies. In contrast to the previous work, our formula leads to a new result due to the influence of the VAR when the energy of the quasiparticles greater than the superconducting gap.     

超导体MgB2的高温电阻率及热重-差热分析

利用Mg扩散法制备出单相多晶的MgB2 超导体 ,其超导转变的零电阻温度为 3 8 4K左右 ,转变宽度小于 1K。正常态电阻率温度关系为金属型 ,在 2 70K -4 5 0K的高温范围内电阻率与温度成很好的线性关系。差热和热重分析表明在空气气氛中MgB2 在 90 0℃左右开始急剧氧化。     

Ground-State Properties of the Frustrated 2D Quarter-Filled Hubbard Model

The Hubbard model extended by next-nearest-neighbor (NNN) hopping t′ at quarter filling has been investigated numerically using exact diagonalization technique on a small cluster. We report the role of frustration (NNN hopping) on the ground-state properties of the system. The effects of both positive and negative t′ are considered. Results show that a short range antiferromagnetic order is present in the system, which is favored (suppressed) by negative (positive) t′. It appears that electron-electron correlation decreases with NNN hopping interaction. Inclusion of t′ facilitates electron hopping. Though pairing in the extended s-wave channel is dominant for quarter-filled unfrustrated system, d-wave pairing becomes dominant with the inclusion of negative t′.