Neutron stars as type-I superconductors

In a recent paper by Link, it was pointed out that the standard picture of the neutron star core composed of a mixture of a neutron superfluid and a proton type-II superconductor is inconsistent with observations of a long period precession in isolated pulsars. In the following we will show that an appropriate treatment of the interacting two-component superfluid (made of neutron and proton Cooper pairs), when the structure of proton vortices is strongly modified, may dramatically change the standard picture, resulting in a type-I superconductor. In this case the magnetic field is expelled from the superconducting regions of the neutron star, leading to the formation of the intermediate state when alternating domains of superconducting matter and normal matter coexist.     

Resistivity of a superconductor in the intermediate state

The resistivity of a superconductor in the intermediate state is known for some limiting cases such as low temperatures, small mean free path, etc. In this paper, the resistivity is derived in the general case; it is a function of the temperature, the mean free path, the thickness of the normal and superconducting layers. The existence of a discontinuity of the electric potential at the interface between a normal and a superconducting layer is shown.     

Zn取代Cu抑制YBa2Cu3O7-y超导电性的机理研究

通过精心制备样品,系统地研究分析YBa₂Cu₃O_7-y体系的超导转变、电阻率、热电势、晶体结构,以及氧含量等随Zn浓度变化的规律。并指出,Zn取代Cu导致CuO面上Cu的数目减少,载流子浓度下降;正是以Zn杂质为中心的正常芯及随之产生的渗流性破坏抑制了YBa₂Cu_3-xO_7-y的超导电性;以Zn杂质散射为基础的渗流超导电性模型与实验结果一致。     

Stability of normal state bubbles in the intermediate state of type I superconductors

The intermediate state in a type I superconducting Indium slab is observed with the high resolution magneto-optical imaging technique. The patterns consist of coexisting cylindrical-shape (bubbles) and elongated-shape (lamellae) normal state domains surrounded by the superconducting phase. We show that the shape cylindrical or elongated of the domains is correlated to their surface area. Those first experimental results indicate that there is a maximum surface area above which bubbles are no more observed. This maximum surface area is found to coincide with the stability limit of the bubble shape.     

Theory of optical conductivity and photoemission intensity for Cu---O plane superconductors

The quasiparticle self-energy and the dynamic spin and charge susceptibilities are calculated self-consistently in RPA for the two-dimensional Hubbard model with additional electron-phonon interaction. Vertex corrections lead to an enhancement of charge fluctuations and a suppression of spin fluctuations, thus increasing T_c. The resulting optical reflectivity in the normal and superconducting state is found to be in qualitative agreement with the experiment data on YBa₂Cu₃O₇ for intermediate values of λ_ph and U/t. We calculate also the photoemission intensity in the normal and superconducting state.     

Magnetic-field effects on the size of vortices below the surface of NbSe2 detected using low energy beta-NMR

A low energy radioactive beam of polarized 8Li has been used to observe the vortex lattice near the surface of superconducting NbSe2. The inhomogeneous magnetic-field distribution associated with the vortex lattice was measured using depth-resolved beta-detected NMR. Below Tc, one observes the characteristic line shape for a triangular vortex lattice which depends on the magnetic penetration depth and vortex core radius. The size of the vortex core varies strongly with the magnetic field. In particular, in a low field of 10.8 mT, the core radius is much larger than the coherence length. The possible origin of these giant vortices is discussed.     

Diamagnetic contact in superheated superconducting suspension of microspheres

This paper shows the existence of an intermediate state in the transition of superheated superconducting microspheres in strong diamagnetic interactions. Such an interaction can lead to “iamagnetic contact” which is an almost complete suppression of the superheated state even in the absence of geometrical contact. Experimental evidences are presented for collections of tin microspheres with large filling factor in superconducting metal.     

Mesoscopic superconducting disks

Using the nonlinear Ginzburg–Landau (GL) equations, type I superconducting disks of finite radius (R) and thickness (d) are studied in a perpendicular magnetic field. Depending on R and d, first- or second-order phase transitions are found for the normal to superconducting state. For sufficiently large R, several transitions in the superconducting phase are found corresponding to different angular momentum giant vortex states. In an increasing magnetic field the superconductor is in its ground state, while in a field down sweep it is possible to drive the system into metastable states. We also present a quantitative analysis of the relation between the detector output and the sample magnetization. The latter, and the incorporation of the finite thicknesses of the disks, are essential in order to obtain quantitative agreement with experiment.     

磁屏蔽感应型超导故障限流器的仿真研究

介绍了磁屏蔽感应型超导故障限流器 (SCFCL)的结构 ,分析了它的工作原理 ,并对一样机进行了仿真实验 ,获得了与该样机的实验近乎一致的结果。并通过仿真 ,研究了相关参数对限流效果的影响 ,从而为设计该 SCFCL提供了新的方法。     

Localized superconductivity in the quantum-critical region of the disorder-driven superconductor-insulator transition in TiN thin films

We investigate low-temperature transport properties of thin TiN superconducting films in the vicinity of the disorder-driven superconductor-insulator transition. In a zero magnetic field, we find an extremely sharp separation between superconducting and insulating phases, evidencing a direct superconductor-insulator transition without an intermediate metallic phase. At moderate temperatures, in the insulating films we reveal thermally activated conductivity with the magnetic field-dependent activation energy. At very low temperatures, we observe a zero-conductivity state, which is destroyed at some depinning threshold voltage V{T}. These findings indicate the formation of a distinct collective state of the localized Cooper pairs in the critical region at both sides of the transition.     

Very low temperature specific heat of crystalline zirconium in normal and superconducting states

Specific heat measurements of zirconium between 0.03 and 1.2 K in both normal and superconducting states are reported. In the normal state a purely linear electronic contribution is observed down to 0.1 K; at lower temperatures there appears the onset of a nuclear hyperfine contribution which is unobservable in the superconducting state within our experimental time scale.     

Superconducting properties of the attractive Hubbard model

A self-consistent set of equations for the one-electron self-energy in the ladder approximation is derived for the attractive Hubbard model in the superconducting state. The equations provide an extension of a T-matrix formalism recently used to study the effect of electron correlations on normal-state properties. An approximation to the set of equations is solved numerically in the intermediate coupling regime, and the one-particle spectral functions are found to have four peaks. This feature is traced back to a peak in the self-energy, which is related to the formation of real-space bound states. For comparison we extend the moment approach to the superconducting state and discuss the crossover from the weak (BCS) to the intermediate coupling regime from the perspective of single-particle spectral densities.     

The thermoelectric power of type I superconductors in the intermediate state

Measurements of the thermoelectric power of superconducting indium in the intermediate state are reported. A large maximum in the thermoelectric power of crystalline indium is observed with the intermediate state value exceeding the normal state value in the temperature range 0.3 < T/T_c < 1. No maximum is observed in polycrystalline indium.     

Even-odd effect in Andreev transport through a carbon nanotube quantum dot

We have measured the current (I)-voltage (V) characteristics of a single-wall carbon nanotube quantum dot coupled to superconducting source and drain contacts in the intermediate coupling regime. Whereas the enhanced differential conductance dI/dV due to the Kondo resonance is observed in the normal state, this feature around zero-bias voltage is absent in the superconducting state. Nonetheless, a pronounced even-odd effect appears at finite bias in the dI/dV subgap structure caused by Andreev reflection. The first-order Andreev peak appearing around V=Delta/e is markedly enhanced in gate-voltage regions, in which the charge state of the quantum dot is odd. This enhancement is explained by a "hidden" Kondo resonance, pinned to one contact only. A comparison with a single-impurity Anderson model, which is solved numerically in a slave-boson mean-field approach, yields good agreement with the experiment.     

Hole core in superconductors and the origin of the Spin Meissner effect

It is proposed that superconductors possess a hidden ‘hole core’ buried deep in the Fermi sea. The proposed hole core is a small region of the Brillouin zone (usually at the center of the zone), where the lowest energy states in the normal state reside. We propose that in the superconducting state these energy states become singly occupied with electrons of a definite spin helicity. In other words, that holes of a definite spin helicity condense from the top to the bottom of the band in the transition to superconductivity, and electrons of that spin helicity ‘float’ on top of the hole core, thus becoming highly mobile. The hole core has radius q₀ = 1/2λ_L, with λ_L the London penetration depth, and the electrons expelled from the hole core give an excess negative charge density within a London penetration depth of the real space surface of the superconductor. The hole core explains the development of a spin current in the transition to superconductivity (Spin Meissner effect) and the associated negative charge expulsion from the interior of metals in the transition to superconductivity, effects we have proposed in earlier work to exist in all superconductors and to be at the root of the Meissner effect.     

Calorimetric evidence for a Fulde-Ferrell-Larkin-Ovchinnikov superconducting state in the layered organic superconductor kappa-(BEDT-TTF)2Cu(NCS)2

The specific heat of the layered organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2), where BEDT-TTF is bisethylenedithio-tetrathiafulvalene, has been studied in magnetic fields up to 28 T applied perpendicular and parallel to the superconducting layers. In parallel fields above 21 T, the superconducting transition becomes first order, which signals that the Pauli-limiting field is reached. Instead of saturating at this field value, the upper-critical-field increases sharply and a second first-order transition line appears within the superconducting phase. Our results give strong evidence that the phase, which separates the homogeneous superconducting state from the normal state is a realization of a Fulde-Ferrell-Larkin-Ovchinnikov state.     

Quantum metallicity on the high-field side of the superconductor-insulator transition

We investigate ultrathin superconducting TiN films, which are very close to the localization threshold. Perpendicular magnetic field drives the films from the superconducting to an insulating state, with very high resistance. Further increase of the magnetic field leads to an exponential decay of the resistance towards a finite value. In the limit of low temperatures, the saturation value can be very accurately extrapolated to the universal quantum resistance h/e2. Our analysis suggests that at high magnetic fields a new ground state, distinct from the normal metallic state occurring above the superconducting transition temperature, is formed. A comparison with other studies on different materials indicates that the quantum metallic phase following the magnetic-field-induced insulating phase is a generic property of systems close to the disorder-driven superconductor-insulator transition.     

Vibrations in Magnet/Superconductor Levitation Systems

The problem of a small magnet levitating above a very thin superconducting disc in the Meissner state is analysed. The dipole-dipole interaction model is employed to derive analytical expressions for the interaction energy, levitation force, magnetic stiffness and frequency of small vibrations about the equilibrium position in two different configurations, i.e. with the magnetic moment parallel and perpendicular to the superconductor. The results show that the frequency of small vibrations decreases with the increasing levitation height for a particular radius of the superconducting disc, which is in good agreement with the experimental results. However, the frequency increases monotonically up to saturation by increasing the radius of the disc for a particular height of the magnet. In addition, the frequency of vibrations is higher when the system is in the vertical configuration than that when the system is in the horizontal configuration.     

Quasiparticle density of states of 2H-NbSe2 single crystals revealed by low-temperature specific heat measurements according to a two-component model

Low-temperature specific heat in a dichalcogenide superconductor 2H-NbSe2 is measured in various magnetic fields. It is found that the specific heat can be described very well by a simple model concerning two components corresponding to vortex normal core and ambient superconducting region, separately. For calculating the specific heat outside the vortex core region, we use the Bardeen-Cooper Schrieffer （BCS） formalism under the assumption of a narrow distribution of the superconducting gaps. The field-dependent vortex core size in the mixed state of 2H-NbSe2, determined by using this model, can explain the nonlinear field dependence of specific heat coefficient γ（H）, which is in good agreement with the previous experimental results and more formal calculations. With the high-temperature specific heat data, we can find that, in the multi-band superconductor 2H-NbSe2, the recovered density of states （or Fermi surface） below Tc under a magnetic field seems not to be gapped again by the charge density wave （CDW） gap, which suggests that the superconducting gap and the CDW gap may open on different Fermi surface sheets.     

Type of phase transitions in a mesoscopic superconducting disc

The Ginzburg–Landau equations coupled with the three-dimensional Maxwell equations are solved for the disc geometry in order to explain recent magnetization experiments. In order to explain the experimental results on a 0.5 m radius Al disc, we have to assume that the superconducting state stays in the lowest angular momentum giant-vortex state even in regions where it is not the lowest energy state.