V. B. Geshkenbein

Pinning and creep in layered superconductors

Abstract The thermal- and disorder-induced effects in the mixed state of a layered high- T c superconductor (in a field H ‖ c ) are studied. The flux line lattice is of a quasi-2D type at sufficiently high field H ⪆ H 0 where H 0 is proportional to the mass anisotropy m / M and estimated to be around 3 T for the Bi- and Tl-based superconductors. At H 》 H 0 the FLL melts at a temperature T m close to the temperature of 2D dislocation-mediated melting. At T > T m the system is in the normal state. At low temperatures T T g ⋍ T m / In ( H / H 0 ) the system is in the vortex glass state with zero linear resistivity: ∂ V /∂ j →0 as j →0. In the intermediate temperature range T g T T m the energy barriers for the plastic motion of vortices are finite and thermally activated flux flow (TAFF) occurs. In the vortex glass state, different regimes of flux pinning and creep are identified and the behaviour of the critical current j c as a function of temperature and magnetic field is estimated. Power-law behaviour of the effective pinning energy U ( j ) at j ⪌ j c is obtained for the case of 2D collective creep.

Environmentally decoupled sds -wave Josephson junctions for quantum computing

Quantum computers have the potential to outperform their classical counterparts in a qualitative manner, as demonstrated by algorithms which exploit the parallelism inherent in the time evolution of a quantum state. In quantum computers, the information is stored in arrays of quantum two-level systems (qubits), proposals for which include utilizing trapped atoms and photons, magnetic moments in molecules and various solid-state implementations. But the physical realization of qubits is challenging because useful quantum computers must overcome two conflicting difficulties: the computer must be scalable and controllable, yet remain almost completely detached from the environment during operation, in order to maximize the phase coherence time. Here we report a concept for a solid-state ‘quiet’ qubit that can be efficiently decoupled from the environment. It is based on macroscopic quantum coherent states in a superconducting quantum interference loop. Our two-level system is naturally bistable, requiring no external bias: the two basis states are characterized by different macroscopic phase drops across a Josephson junction, which may be switched with minimal external contact.A combination of elements for use in surgery, particularly suturing. The combination includes a conventional needle holder with a second element, namely a needle-suture assembly combining a suture storage chamber and a hollow needle through which the suture is drawn. The needle-suture assembly is associated with the needle holder in order that suturing is accomplished by manipulation of the handles of the needle holder. The needle-suture assembly is attached to the needle holder by grasping the needle with the needle holder and also by means associated with the suture storage chamber, which attaches the unit to the needle holder on the opposite side of the pivot point from the needle. The latter attaching means serves also to effectively lock the needle holder against inadvertent opening during the suturing operation. The suture storage chamber includes a storage spool which serves to seal the storage chamber, store the thread, and to restrain the suture against rapid, undesirable dispensing.

Superconductivity in a system with preformed pairs

We discuss the phenomenology of superconductivity resulting from the Bose condensation of preformed pairs coexisting with unpaired fermions. We show that this transition is more mean-field-like than the usual Bose condensation, i.e., it is characterized by a relatively small value of the Ginzburg parameter. We consider the Hall effect in the vortex-flow regime and in the fluctuational regime above

Imaging the vortex-lattice melting process in the presence of disorder

{\mathrm{T}}_{\mathrm{c}}

Plastic Vortex Creep in YBa2Cu3O7-x Crystals.

, and show that in this situation it is much less than in the transition driven entirely by Bose condensation but much larger than in usual superconductivity. We analyze the available Hall data and conclude that this phenomenology describes reasonably well the data in the underdoped materials of Y-Ba-Cu-O family but is not an appropriate description of optimally doped materials or underdoped La-Sr-Cu-O.

Plastic Vortex Creep in YBa{sub {bold 2}}Cu{sub {bold 3}}O{sub {bold 7{minus}}}{ital x} Crystals

General arguments suggest that first-order phase transitions become less sharp in the presence of weak disorder, while extensive disorder can transform them into second-order transitions; but the atomic level details of this process are not clear. The vortex lattice in superconductors provides a unique system in which to study the first-order transition on an inter-particle scale, as well as over a wide range of particle densities. Here we use a differential magneto-optical technique to obtain direct experimental visualization of the melting process in a disordered superconductor. The images reveal complex behaviour in nucleation, pattern formation, and solid–liquid interface coarsening and pinning. Although the local melting is found to be first-order, a global rounding of the transition is observed; this results from a disorder-induced broad distribution of local melting temperatures, at scales down to the mesoscopic level. We also resolve local hysteretic supercooling of microscopic liquid domains, a non-equilibrium process that occurs only at selected sites where the disorder-modified melting temperature has a local maximum. By revealing the nucleation process, we are able to experimentally evaluate the solid–liquid surface tension, which we find to be extremely small.

Flux pinning and creep in high-Tc superconductors

Local magnetic relaxation measurements in YBa2Cu3O72x crystals show evidence for plastic vortex creep associated with the motion of dislocations in the vortex lattice. This creep mechanism governs the vortex dynamics in a wide range of temperatures and fields below the melting line and above the field corresponding to the peak in the “fishtail” magnetization. In this range the activation energy Upl, which decreases with field, drops below the elastic (collective) creep activation energy, Uel, which increases with field. A crossover in flux dynamics from elastic to plastic creep is shown to be the origin of the fishtail in YBa 2Cu3O72x. [S0031-9007(96)00878-2]

Nature of the Irreversibility Line in Bi2Sr2CaCu2O8

Local magnetic relaxation measurements in YBa2Cu3O72x crystals show evidence for plastic vortex creep associated with the motion of dislocations in the vortex lattice. This creep mechanism governs the vortex dynamics in a wide range of temperatures and fields below the melting line and above the field corresponding to the peak in the “fishtail” magnetization. In this range the activation energy Upl, which decreases with field, drops below the elastic (collective) creep activation energy, Uel, which increases with field. A crossover in flux dynamics from elastic to plastic creep is shown to be the origin of the fishtail in YBa 2Cu3O72x. [S0031-9007(96)00878-2]

Weak to Strong Pinning Crossover

Abstract The theory of collective pinning in the presence of thermal fluctuations of vortex lines is developed. Critical current jc is shown to decrease rapidly with temperature. Flux creep under the action of weak (j

Low-field phase diagram of layered superconductors: The role of electromagnetic coupling.

Local vortex dynamics in Bi2Sr2CaCu2O8 single crystals was studied using novel microscopic GaAs/AlGaAs Hall-sensor arrays. The irreversibility line (IL) is found to exist in the absence of bulk pinning. At high temperatures the IL is due to geometrical barriers whereas at intermediate temperatures the irreversible behavior is determined by surface barriers. Bulk pinning governs the IL only at T < 22 K.