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Dynamical behaviour of Josephson and pancake vortices (JVs and PVs) in intrinsic Josephson junctions of Bi 2 Sr 2 CaCu 2 O 8+ y (Bi-2212) single crystal with a nano-size hole-array has been studied to measure the flow-resistance of the vortices. In the magnetic field perpendicular to the superconducting layers, flow resistance of PVs measured with the in-plane current shows a matching behaviour as usually observed at the matching fields. After the measurements, the sample was fabricated into the in-line shaped structure for the c -axis current measurements to obtain the JV flow-resistance. Instead of the usually-observed lock-in phenomenon of JVs in Bi-2212, several peaks are observed with changing the angle from the in-plane magnetic field to show the enhancement of the flow-resistance at some typical angles. When PVs are introduced into the sample with changing the angle, are trapped into holes, and are interacted with JVs, it is clearly seen that the well-aligned PVs cause the enhancement of the JV flow-resistance.

The conventional Ginzburg–Landau (GL) theory is extended to a nonlocal one. In this nonlocal GL theory the superconducting current becomes nonlocal. The magnetic field distribution calculated in the single vortex state indicates that the interaction between vortices has an attractive part in low- κ type II superconductors with κ κ c ∼ 1. The attractive interaction causes the first-order transition at H c 1 .

In this paper we study some superconducting properties of a flat disk by taking into account the roughness of the surface. Also, the sample is surrounded by a thin layer of a metallic material and submitted to and external magnetic field applied perpendicular to its plane. The boundary condition is taking into account via the de Gennes extrapolation length b. We evaluate the Gibbs free energy, magnetization, vorticity, the first and second critical thermodynamical fields as a function of the external magnetic field and b parameter. We also evaluate the first critical field as a function of the surface rugosity.

We study the vortex structures and quasi-particle structures in nano-structured superconductors. We used the Bogoliubov–de Gennes equation and the finite element method and obtained stable magnetic flux structures and the quasi-particle states. We found the vortex configurations are affected by the interference of the quasi-particle bound states around the vortices. In order to clarify the interference between the quasi-particle wave-functions around two vortices we have developed a numerical method using the elliptic coordinates and the Mathieu functions. We apply this method to two singly quantized vortex state in a conventional s -wave superconductor and a pair of half-quantum vortices in a chiral p -wave superconductor.

We analyze the magnetic properties through two-orbital Hubbard model with the spin–orbit coupling (SOC) interaction in the iron-based superconductors. With the help of the Ising approximation for the Hund’s coupling between the itinerant electrons and the localized spins, we give a self-consistent account of the various magnetic orders observed in pnictides and the pairing symmetry. We also calculate the local density of states (LDOS) of the vortex state when a magnetic field is applied. The LDOS without SOC shows no resonant peak at the vortex core center in the superconducting state, while it shows an obvious resonant peak when SOC is applied.

We study local density of states (LDOS) of vortex state in mesoscopic square superconductors with Bogoliubov-de Gennes (BdG) equation. We develop one effective numerical method based on the finite element method to self-consistently solve the BdG equation. The LDOS for various vortex states is obtained. Our results about the single vortex show that the LDOS has the particle-hole asymmetry and the results for one- and two-vortex state agree very well with the experimental observation. Besides, we predict the LDOS of multi-vortex states, which is crucial for the further STM/STS experimental study of vortex state in mesoscopic superconducting system.

Using scanning Hall probe microscopy a direct visualization of the flux distribution in a Pb film covering a fivefold Penrose array of Co dots is obtained. We demonstrate that stable vortex configurations can be found for fields H ∼ 0.8 H 1 , H 1 and 1.6 H 1 , where H 1 corresponds to one flux quantum per pinning site. The vortex pattern at 0.8 H 1 corresponds to one vacancy in one of the vertices of the thin tiles whereas at 1.6 H 1 the vortex structure can be associated with one interstitial vortex inside each thick tile. Strikingly, for H = 1.6 H 1 interstitial and pinned vortices arrange themselves in ring-like structures (“vortex corrals”) which favor the formation of a giant vortex state at their center.

The dynamic nature of the vortex state of MgB 2 single crystals near the peak effect (PE) region, which is very different either from that of conventional low-temperature superconductors or from that of high-temperature cuprate superconductors, is introduced in this article. Relaxation from a disordered, metastable field-cooled (FC) state to an ordered, stable zero-field-cooled (ZFC) state of the MgB 2 single crystals under an applied magnetic field and current is investigated. From an analysis of the noise properties in the ZFC state, a dynamic vortex phase diagram of the MgB 2 is obtained near the PE region. Between the onset and the peak region in the critical current vs. magnetic field diagram, crossovers from a high-noise state to a noise-free state are observed with increasing current. Above the peak, however, an opposite phenomenon, crossovers from a noise-free to a high-noise state, is observed which has not been observed in any other superconductors. The hysteresis in the I – V curves and the two-level random telegraph noise in the time evolution of the voltage response under an constant applied current at the ZFC state are also studied in detail.

We have investigated the behavior of vortex lattice in a weakly pinned YBa2Cu2O7−δ crystal using rotational magnetization measurements. At low applied magnetic fields parallel to the ab plane of the crystal, we have found that the vortex state undergoes a crossover from a relatively strongly pinned vortex regime to a weakly pinned vortex regime in the Bragg glass phase. In the weakly pinned vortex regime of the Bragg glass phase, vortices were described to be completely depinned and aligned parallel to the applied field.

We have measured the ultrasound velocity of quasi-two-dimensional superconductor CeCoIn 5 with extremely large Pauli paramagnetic susceptibility. The results indicate that the new high field superconducting phase, which is revealed by the recent heat capacity measurements, is characterized by the unusual softening of flux line lattice. The softening is most likely due to the collapse of the flux line lattice tilt modulus and transition to quasi-two-dimensional vortex state. These results provide a strong evidence that the high field phase is the Fulde–Ferrell–Larkin–Ovchinnikov phase, in which the order parameter is spatially modulated and has planar nodes aligned perpendicularly to the vortices.

We investigate the change of the vortex structure in the presence of the induced stripe or checkerboard pattern modulation of the incommensurate spin density wave (SDW) and charge density wave (CDW) around the vortex core. The order parameter and the quasiparticle structure in the vortex lattice state are calculated by Bogoliubov-de Gennes theory on a two-dimensional Hubbard model with nearest-neighbor site pairing interaction. By the induced SDW and CDW, low-energy excitations around the vortex core are suppressed, producing “empty core” structure, as observed by the scanning tunneling microscopy. In the relation to these quasiparticle structures, we discuss the site-selective nuclear spin relaxation time T 1 and thermal conductivity in the vortex state.

We have studied a magnetic phase diagram of Josephson vortices in slightly ( T c = 86.0 K) and moderately ( T c = 82.5 K) over-doped Bi 2 Sr 2 CaCu 2 O 8+ δ single crystals by measuring a Josephson-vortex flow resistance. Doping of this range shows a significant change in the lower boundary of the 3D-ordered vortex phase, which was confirmed by the periodic oscillations, and a weak one in the upper boundary, which exists at about 50 kOe at higher temperatures and above 70 kOe at lower temperatures than 30 K in both samples. Above this boundary, the flow resistance shows a different behavior, which may be related to the strength of the interlayer coupling along the c -axis, and to the excitation/annihilation of pancake vortex pairs thermally and magnetically.

The upper critical magnetic field for the vortex state, H c2 , for the bulk crystals YC 2 and the critical field for the state of surface superconductivity, H c3 , for the single-crystals YC 2 encapsulated in the multiwall carbon nanocages as a function of t ( T / T c , T c = 3.838 K) are reported. The ratio H c3 / H c2 increases with increasing t from 1.6 ± 0.2 at t = 0.6 to 2.0 ± 0.3 at t = 0.99. The present data accord with the reported experimental results for the type-II superconductors surrounded by the insulators. The role of the interface between the single-crystal YC 2 and the multiwall carbon nanocage in surface superconductivity is discussed in terms of the boundary effects.

A systematic investigation of the electrical parameters of Bi2Sr2CaCu2Ox thick tapes in the vortex and superconducting states was carried out. The tapes were grown on either single crystals of MgO or silver foil under different heat treatment conditions. The temperature dependence of the electrical resistivity in the vortex state was found to follow an Arrhenius-like relation of the form ρ(T, H) = ρ0exp(−U0(H)/kBT). The activation energy, U0, showed a strong dependence on the applied field and the growth conditions of the sample. Its average values were between 10 and 300 meV. The current–voltage characteristics (I–V) for various samples of different growth conditions and applied fields were also investigated. The study revealed a current dependence on the applied voltage was found to fit a power law of the form: I ∼ Vα, where α is an exponent whose values are strongly dependent on growth conditions. At constant voltages, the current was found to decrease exponentially with the applied magnetic field according to I(B) ∼ exp(−cB/B∗), where c is a constant depending on growth conditions and B∗ is a characteristic field of the sample. This exponential decay of the current is attributed to strong flux pinning centers. The dependence of the current on field and voltage could be written as I ∼ Vαexp(−cB/B∗). Furthermore, the heat treatment conditions and annealing atmosphere had strong influence on the I–V characteristic of the sample.

We have studied low-frequency noise in the normal and mixed states of high-Tc thin films. In the normal state, resistance fluctuations with a 1/f power spectrum are observed from 100 K up to 300 K. The normal state noise is largely attributable to hopping or reordering of oxygen vacancies which in turn lead to fluctuations in local carrier density in the copper oxygen planes. Analysis shows a substantial density of oxygen defect states with activation energies below 0.5 eV, followed by a rapid increase at higher energies. In the mixed state, we have measured the current-induced, vortex-motion voltage noise. When the current–voltage characteristics are nonlinear there exist two distinct regimes. The first is a low magnetic field regime where measurements of the variations of voltage noise with magnetic field reveal reproducible, microstructure-dependent noise magneto-fingerprints. Rapid variations of the noise as a function of magnetic field and ac current drive experiments indicated that the noise is associated with metastability of pinned vortex configurations. Above magnetic fields which roughly match vortex separation with the grain size in the films, the magnitude and fine structure of the noise magneto-fingerprints are suppressed, suggesting a crossover to a more weakly pinned vortex state. The second regime is a low bias voltage regime where the observed noise scales linearly with bias voltage and has a 1/f-like spectrum. The linear voltage scaling is reminiscent of vortex shot noise although no characteristic transit time could be determined. At higher voltage biases, the noise power drops off rapidly. We attribute the sharp drop off to a loss of pinning at higher temperatures and fields which results in the absence of metastability in the vortex pinned states. The temperature and magnetic field dependence of the noise are discussed in the context of various models including vortex shot noise, channel, critical current fluctuation, and vortex-glass models.

Time-dependent Ginzburg–Landau (TDGL) approach is used to investigate vortex states at the vicinity of T c in three-component superconductors with time-reversal symmetry breaking (TRSB). When the nucleation field H n is slightly larger than the thermodynamic field H tc , vortices form a cluster indicating clearly that this is an exotic superconducting state which cannot be categorized to type-I and type-II.

Using the finite element method we have solved the Ginzburg–Landau equation for several nano-scaled composite structures with s- and d-wave superconductors, which is called d-dot, and we have obtained the superconducting state and the magnetic field distribution under zero magnetic field. For the right triangle and the square shaped d-dot, the magnetic field is induced broadly along the hypotenuses, which is different from the case of the corner junction and the square d-dot. And we show possibility of the manipulation of the magnetic flux structure by the external field or the electric current.

We found the periodic oscillations in Josephson-vortex-flow resistance with the current perpendicular to the Cu–O layers of Bi 2 Sr 2 CaCu 2 O 8+ δ . As the flow resistance is closely related to the configurations of the Josephson vortices, we have observed clear boundaries between the triangular-lattice phase and others. It is strongly suggested that the flow resistance measurements open the window to investigate magnetic phase diagram of Josephson vortices in strongly anisotropic high T c superconductors.

The vortex state in highly anisotropic overdoped Bi 2 Sr 2 CaCu 2 O 8+δ superconductor with T c = 84.5 K has been studied by the local ac-permeability (μ = μ' - iμ) measurements in magnetic fields tilted away from the c-axis to the ab-plane. In the present measurements, we have observed the vortex lattice melting transition, the peak effect, and several anomalies of the local ac-response in the vortex solid phase that suggest the novel phase transitions in the vortex crossing lattice state.

We have studied vortex states in a superconductor with a geometrical constraint such as squares and triangles with sizes of 30 and 50 μm using a scanning SQUID microscope. The quantized magnetic flux enters the superconductors discretely as a function of magnetic field accompanied by the sequential step-wise jump of magnetization of the superconductor. Quantitative as well as qualitative analyses of the spatial vortex arrangement as a function of the number of vortices are shown in comparison with the recent theoretical predictions. In the 30-μm-sized triangle, the magnetic flux penetrates the center of the triangle and its integrated intensity shows Φ 0 , 2 Φ 0 , and 3 Φ 0 as the applied magnetic field increases, suggesting formation of the concentrated multi-vortex or giant vortex states.

Recent studies on cobalt and permalloy rings have shown that a totally flux-closed magnetic vortex structure is stable at remanence. The two chiralities of the vortex, clockwise and anti-clockwise, have been proposed as the carriers for the stored information that could be read in a magneto-resistance-based device. Here, we present the results of systematic characterization of arrays of micron size narrow permalloy ring elements of different symmetry with magnetic force microscopy (MFM) to determine the magnetic patterns inside the rings and to visualize the reversal process during a magnetization cycle. Those experimental data are in a good agreement with the results of micromagnetic calculations.

Spectroscopy based on the Andreev reflection (AR) process at the interface between the normal metal tip and the superconductor has become one of the very successful methods for studies in novel exotic superconductors. The method is capable to address the size, symmetry as well as multiplicity of the superconducting order parameter. The method provided one of the first evidences of the two-gap superconductivity in MgB 2 with a detailed temperature dependence of the both gaps. A theory treating the Andreev reflection spectroscopy in the mixed state is missing. We analyse the AR spectra of MgB 2 in the mixed state via modelling the magnetic pair-breaking by the increasing spectral broadening parameter Γ . As a result a non-trivial pair breaking effect in the π-band is found.

We have prepared fully textured (Hg0.9Re0.1)Ba2CaCu2O6+δ (HgRe-1212) thin films by pulsed laser deposition (PLD) and post-annealing. The films exhibit sharp superconducting transitions at Tc=124 K with transition width ΔTc≃2 K. Conditions for reproducible film preparation have been found. The resistive transitions have been investigated in magnetic fields up to 8 T (parallel to the c-axis) and 10 T (perpendicular to the c-axis). We have determined the activation energy of thermally activated flux-motion for both magnetic field orientations. The Hall resistivity (ρxy) of HgRe-1212 superconductor thin films has been measured in the ab-plane at 8 T. In the mixed state a power-law behavior is observed, where ρxy scales to a power-law function of ρxx: ρxy=Aρβxx, with β=1.4±0.1. Above 130 K the T2 dependence of the cotangent of the Hall angle is observed. At different temperatures magnetization loops were measured with a magnetic field applied normal to the film in a SQUID magnetometer and the corresponding critical current densities Jc were calculated. Under variation of the angle θ between the field direction and the c-axis of the film the anisotropic properties of the vortex state and the depinning field Bdp(θ) have been studied. The measured angular dependence Bdp(θ) shows a cusp for θ=90°. The films exhibit a rather low critical field anisotropy of 7.7.

Vortex state metastability near the order–disorder transition in the mixed state of weakly pinned superconductors is described using the standard phenomenology of first order transitions. It is shown that vortex motion establishes a dynamical equilibrium between the ordered and disordered vortex phases and results in the broadening of the order–disorder transition. The discussion in this paper is consistent with the recent phenomenological model proposed [Phys. Rev. B 63 (2001) 22452] to describe various experiments manifesting the metastability in vortex state near order–disorder transition.

Low-temperature scanning tunneling spectroscopy (STS) has been performed on YBa 2 Cu 3 O y (YBCO) in the magnetic field μ 0 H =1 T. Clear vortex images are obtained on the surface chemically etched with 1% Br-ethanol. We measured on the twinned crystals and directly observed the effect of twin boundary on the vortex arrangement. Vortices close to the twin boundary align themselves parallel to it, suggesting that vortices are effectively trapped by the twin boundary. The vortices very close to the twin boundary form almost square lattice. Such tendency decreases with increasing distance and the vortices form a hexagonal lattice in the region far from the twin boundary. However, the orientation of hexagonal lattice is not consistent with the previous reports. Possible origins of such disagreement are described in this paper. We also show spectra observed in the scan region and discuss their features.

We highlight interesting thermomagnetic history effects across the transition line between the quasi-ordered and disordered vortex states in single crystal YBa 2 Cu 3 O 7− δ (YBCO), and argue that these features are indicative of the first-order nature of the transition line. We suggest that the destruction of the ordered vortex state in YBCO leading to vortex liquid (at high temperatures and low fields) and amorphous vortex solid (at low temperatures and high fields), takes place along a unified first-order transition line. The non-monotonic behavior of this first-order transition line gives rise to the possibility of more than one Kauzmann point where the entropies of the ordered and disordered vortex states are equal. In the high temperature region, one may order the vortex lattice by warming it, giving rise to an inverse melting effect.

The peak effect in superconductors like NbSe 2 , V 3 Si, is characterized by the history dependence of the critical current density J c . A phenomenological model was proposed and a numerical solution obtained to account for the history dependence in J c near peak effect. We present an exact solution of the model. Recently, history dependence in J c is reported also in the low field region in V 3 Si system. We present an experimental study, through the measurement of minor magnetization curves, of the history effects across the entire field range encompassing low field and high field order–disorder transitions. We compare the experimental observations with the results of our model calculations.

A microscopic calculation for the charge distribution in the single vortex state in type II superconductors is presented, in which the charge screening effect is microscopically incorporated within linear response of the scalar potential. The Poisson equation is numerically solved with the Bogoliubov–de Gennes equation in the single vortex state, and the charge screening inside the core is clarified. The Coulomb interaction between core electrons induces the oscillations in the charge distribution inside the vortex core.

Josephson vortices in Bi 2 Sr 2 CaCu 2 O 8+ δ single crystals have been studied to measure the vortex-flow resistance with the current perpendicular to the superconducting layers, sweeping the parallel magnetic fields. In the flow resistance, we have found novel periodic oscillations. The period of the oscillations corresponds to adding “one” flux quantum per “two” intrinsic Josephson junctions. It is considered that the oscillations take place with a coherent motion of the vortices in the dynamical flow. These results suggest that the Josephson vortices form a triangular lattice in the ground state, related to the matching effect between the sample width and the lattice spacing along the layers. It is also suggested that they will be applicable to a new type of magnetic sensor.

We studied the DC current-driven vortex state of a Bi 2 Sr 2 CaCu 2 O 8+ δ model in the presence of pointlike disorder via a Monte Carlo simulation. We found that the vortex drift velocity v as a function of temperature T drastically changes twice at a depinning ( T dp ) and a melting ( T m ) temperature, as is consistent with our two-stage melting scenario. At the low field region, whereas v remains almost zero for T T dp , it rapidly grows for T dp T T m and the Bragg peak intensity survives for T T m . We thus give numerical evidence that a moving Bragg glass phase exists for the intermediate temperature regime of T dp T T m .

We have measured the Josephson vortex flow resistance with the current perpendicular to the Cu–O layers of Bi 2 Sr 2 CaCu 2 O 8+ δ . It has been found that novel periodic oscillations appear in the flow resistance with sweeping the magnetic field parallel to the layers. The oscillations have been observed in a wide range of parallel fields larger than 6 kOe and only in a smaller current density than about 100 A/cm 2 . The period of the oscillations corresponds to adding “one” flux quantum per “two” intrinsic Josephson junctions. These results suggest that the Josephson vortices form a triangular lattice as a ground state.

The oxygen doped and Pb-substituted single crystalline Bi 2 Sr 2 CaCu 2 O 8+ δ samples having T c,on and anisotropy ( γ 2 ) ranging respectively from 65 to 93 K and from 773 to 49,715 have been studied in the basis of magneto-resistance and magnetization data. It is found that the lowering of anisotropy due to higher oxygen content or Pb substitution is responsible for the reduction of magnetic field-induced transitional broadening and a shift of H irr ( T ) and H g ( T ) to higher temperatures and magnetic field. Meanwhile, the increasing disorder manifested by decreasing T c also leads to the extended range of appearance for the second magnetization peak (SMP) and enlarged critical glassy region. It is shown that decreasing anisotropy does not necessarily lead to the lowering of T c in contrast to the monotonic dependence of T c on the disorder measured in the temperature range of SMP effect. Additionally, a remarkable monotonic correlation between T c and the glassy exponent s has been established.

In the vortex-flow resistance measured with the current perpendicular to the superconducting layers of Bi 2 Sr 2 CaCu 2 O 8+ δ , we have found novel periodic oscillations with sweeping the magnetic field parallel to the layers. The oscillations have been observed in a wide range of parallel fields larger than 6 kOe and only in a smaller current region than 200 μA. The period of the oscillations corresponds to adding “one” flux quantum per “two” intrinsic Josephson junctions. These results suggest that the Josephson vortices form a triangular lattice in moving vortices, related to the matching between the sample width and the lattice spacing along the layers, and supply important evidence for new applications.

The low temperature heat capacity C p and microwave surface impedance Z s in the vortex state of YNi 2 B 2 C have been studied. In contrast to conventional s-wave superconductors, C p shows a nearly H 1/2 -dependence. This H 1/2 -dependence is little affected by the introduction of the columnar defects. On the other hand, flux flow resistivity obtained from Z s increases linearly with H . These results indicate that in the vortex state of YNi 2 B 2 C the delocalized quasiparticle states around the vortex core play a much more important role, similar to d-wave superconductors.

The Josephson plasma resonance (JPR), which is a phase collective excitation mode representing the Cooper-pair oscillation mode, is a most powerful means to study the vortex state in Josephson coupled high-Tc superconductors (HTSC). In this paper, we report the detailed and quantitative study of the interlayer quantum phase coherence in the vortex liquid, Bragg glass and vortex glass phases of Bi2Sr2CaCu2O8+δ by the JPR. We also provide a quantitative discussion on the nature of the phase transitions among these vortex phases. On the basis of these results, we discuss the vortex phase diagram of HTSC.

We have performed current–voltage ( I – V ) measurements on intrinsic Josephson junctions of Bi 2 Sr 2 CaCu 2 O 8+ δ single crystals to study the excitation of a Josephson plasma by a Josephson vortex flow. Junctions have been fabricated by using a focused ion beam in the in-line symmetric structure. In the I–V characteristics under magnetic field parallel to the Cu–O layers, an enhancement of the Josephson current at constant voltage was observed on the vortex flow branches in the narrow range of magnetic field between 5 to 9 kOe. Although no signals in the detection limit of the detectors was observed, it is suggested that the enhancement of the current might be a sign of the emission of Josephson plasma. It is also pointed out that problems are in emitting the electromagnetic wave to free space from the junction.

(Tl0.5Pb0.5)(Ba0.2Sr0.8)2Ca2Cu3Oy (Tl,Pb-1223) has been prepared, characterized and oriented for NMR studies. For comparison, we also fabricated (Tl0.5Pb0.5)(Ba0.1Sr0.9)2CaCu2Oy (Tl,Pb-1212). In the case of (Tl0.5Pb0.5)(Ba0.2Sr0.8)2Ca2Cu3Oy the 205Tl NMR spectra and the spin–lattice relaxation time were measured in the temperature range from 68 to 300 K. For (Tl0.5Pb0.5)(Ba0.1Sr0.9)2CaCu2Oy the 205Tl spectra were measured at room temperature only. The 207Pb NMR spectra for both compounds were also obtained at room temperature. From the chemical shift measurements it was concluded that the valence states of Tl and Pb were 3+ and 4+, respectively. In addition, a linear relationship was found between the shift and the critical temperatures. At low temperatures an anisotropic melting of the vortex state was observed.

We present magnetization data on a weakly pinned single crystal of CeRu 2 , showing the existence of a stable state of the vortex matter in the peak effect (PE) region. The stable state is achieved by cycling the magnetic field by a small amplitude. It is characterized by a unique value of the critical current density, independent of the magnetic history of the sample. The results support a recent phenomenological model proposed by Ravikumar et al. [Phys. Rev. B 61 (2000) R6479] which postulates a specific recipe to incorporate the history dependence of the current density. Our data also shows the occurrence of a first order phase transition from an ordered vortex phase to a disordered phase across the PE.

We present results of transport properties measurement on a single crystal of V$_3$Si showing distinct signature of peak-effect in its vortex state. The field variation of the electrical resistance in the flux-line lattice prepared by different experimental path, namely zero field cooling (ZFC) and field cooling (FC), shows a distinct path dependence in the vicinity of the peak-effect regime. In the field cooled state, small cycling of magnetic field modifies the resistance drastically around the peak-effect regime, highlighting the metastable nature of that state in the concerned regime., 17 pages, 5 figures; to appear in Physica C

The superconducting properties of a two-dimensional superconducting wire network with a new geometry have been measured as a function of the external magnetic field. The extreme localization effect recently predicted for this periodic lattice is revealed as a suppression of the critical current when the applied magnetic field corresponds to half a flux quantum per unit cell. For this particular magnetic field, the observed vortex state configuration is highly disordered.

The vortex state at the lower critical field, H_{c1}, in clean anisotropic superconductors placed in an external field tilted with respect to the axis of anisotropy (c-axis) is considered assuming two possible arrangements: dilute vortex-lines or dilute vortex-line chains. By minimizing the Gibbs free energies in the London limit for each possibility we obtain the corresponding lower critical fields as a function of the tilt angle. The equilibrium configuration at H_{c1} for a given tilt angle is identified with that for which H_{c1} is the smallest. We report results for parameter values typical of strong and moderate anisotropy. We find that for strong anisotropy vortex-line chains are favored for small tilt angles (< 7.9^o) and that at 7.9^o there is coexistence between this configuration and a vortex-line one. For moderate anisotropy we find that there is little difference between the vortex-line and the vortex-chain lower critical fields., 5 pages, 4 figures, accepted to appear on Physica C

We study phase shifts in a Josephson junction induced by vortices in superconducting mesoscopic electrodes. The position of the vortices are controlled by suitable geometry of a nano-scale Nb–Pt 1− x Ni x –Nb junction of the overlap type made by Focused Ion Beam (FIB) sculpturing. The vortex is kept outside the junction, parallel to the junction plane. From the measured Fraunhofer characteristics the entrance and exit of vortices are detected. By changing the bias current through the junction at constant magnetic field the vortices can be manipulated and the system can be switched between two consecutive vortex states which are characterized by different critical currents of the junction. A mesoscopic superconductor thus acts as a non-volatile memory cell in which the junction is used both for reading and writing information (vortex). Furthermore, we observe that the critical current density of Nb–Pt 1− x Ni x –Nb junctions decreases non-monotonously with increasing Ni concentration. It exhibits a minimum at ∼40 at.% Ni, which is an indication of switching into the π state.

Based on a Ginzburg-Landau theory of a superconductor with singlet pairing in the d x 2 − y 2− and d xy - channels and a Zeeman Coupling, we study the vortex structure of the field-induced d x 2 − y 2 + id xy state. The induced time-reversal-symmetry-broken state may lead to a full gap on Fermi surface of the superconductor. The magnetic field and temperature dependencies of the d xy -component are analyzed quantitatively. The results may be related to the thermal conductivities observed in some high temperature superconductors.

Magnetization and resistivity measurements on untwinned YBa 2 Cu 3 O y single crystals are performed in high magnetic fields up to 30T. The upper critical point H ucp at which three different phases (the vortex liquid, the vortex glass, and the Bragg glass) meet strongly depends on the oxygen content y . For fully oxidized YBa 2 Cu 3 O y ( y ⋍ 7, T c = 87.5 K), the first-order vortex lattice melting line H m ( T ) is observed up to 30 T. In the low field region below 5 T, on the other hand, H m ( T ) changes to the second peak and the entropy jump disappears; the results imply the existence of the lower critical point. For optimally doped YBa 2 Cu 3 O y ( y ⋍ 6.92, T c ⋍ 93 K), the vortex glass transition line H g ( T ) approaches to the first-order transition line well below H acp , indicating the new vortex state such as a vortex slush (a pinned liquid) regime.

Current–voltage (I–V) measurements on mesa-structured Bi2Sr2CaCu2O8+δ single crystals have been performed to study the interlayer coupling between Cu–O superconducting layers and vortex states in presence of the Josephson current and magnetic fields along the c-axis. At lower temperatures than 25 K, a pronounced effect of magnetic fields to the I–V characteristics appears in multiple-branching behaviour, when magnetic fields exceed the dimensional-crossover field B2D. This is considered as a consequence of missing a long-range order of correlation of the vortex lines along the c-axis, reflecting the transition to vortex glass state from vortex lattice state. At higher temperatures, the effect is observed in an abrupt change of the Josephson critical current at magnetic fields lower than the B2D and the melting field Bm. This might be a possible evidence of the reduction in the magnetic fields of the peak effect and the melting transition in presence of the Josephson current.

STM is a very useful tool to study the vortex state, and the local density of states (LDOS), in superconducting samples. It is interesting to study the vortex LDOS in different surface conditions: in flat, clean and free-of-defects regions, or in the vicinity of a terrace or defects. In order to observe these situations, it is necessary to be able to probe large and different regions of the sample. We have built a low-temperature STM with an X – Y positioning system, which allows us to cover an entire area of 2×2 mm 2 in steps as small as a few tens of nanometers, without impairing its mechanical stability. This capability is also of great interest for the study of the local distribution of the order parameter and other electronic properties in microscopic and mesoscopic scale structures. In these preliminary studies, we investigate the evolution of the conductance (d I /d V vs. V ) curves, taken along the transition from tunneling to point-contact regime, on a NbSe 2 single crystal. In the presence of an external magnetic field, the evolution observed in the conductance curves suggests the creation of a pinning center when the tip contacts the sample.

The peak effect of the current density j vs. magnetic field B and its relation to the vortex phase diagram is investigated in a twin-free YBa 2 Cu 3 O 7− δ single crystal. The fully oxidized crystal without any j ( B ) peak and with a first-order melting transition of the vortex lattice was subsequently oxygen-reduced. The solid/liquid transition, the upper critical point, the evolution of the peak, and the transition from the lattice below the peak into the amorphous state above are investigated as a function of random disorder from oxygen reduction. The influence from disorder and thermal fluctuation on the lattice/glass transition in the solid vortex state is compared with the model of field-driven introduced dislocation into the vortex solid.

Using a high-angular resolution vector magnetometer, we have studied the components of the magnetic moment parallel and transverse to the CuO planes induced by vortex lock-in phenomena in YBa2Cu3Oy single crystals. The full shielding of the transverse field component originating from the infinite tilt modulus of the vortex locked state, is observed for the lowest fields only. The transverse susceptibility decreases linearly with the magnetic field. We show that this is connected with entropy-induced fragmentation of the locked vortex state by thermally activated kinks. We introduce a simple model of this phenomenon that accounts for the reduced shielding, as well as describes the angular behavior of the critical current.

Using the nonlinear Ginzburg–Landau (GL) theory, we compute the magnetization and the dipole magnetic moment of cylinders and disks in the field-cooled (FC) regime. When cooling down, first a diamagnetic response is obtained, which can be followed by the paramagnetic Meissner effect (PME) at high external magnetic fluxes. The second-order transition from a giant vortex state to a multivortex state with the same vorticity leads to the suppression of PME.

For analyzing the checker-board like modulation of the local density of states (LDOS) around a vortex observed in the slightly overdoped Bi2Sr2CaCu2Ox, we examined the effect of pseudogap state of high-Tc superconductors to the LDOS around the vortex. We first derived the Bogoliubov-de Gennes equation for d-wave superconductivity (d-SC) in the presence of d-spin density wave (d-SDW). Using the Fourier–Bessel expansion, we solved this equation for a single vortex state, numerically. We found that the peak of the bound states around E = 0 becomes small and modulation of the LDOS is observed for larger d-SDW order parameter.