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Register a new userReal space imaging of metastable Bragg glass states in a weakly pinned Type II superconductor
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Added by
Pratap Raychaudhuri
Publication date
2014
fields
Physics
and research's language is
English
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The breakdown of crystalline order in a disordered background connects to some of the most challenging problems in condensed matter physics. For a superconducting vortex lattice, the equilibrium state in the presence of impurities is predicted to be a Bragg glass (BG), where the local crystalline order is maintained everywhere and yet the global positional order decays algebraically. Here, using scanning tunnelling spectroscopy (STS) we image the vortex lattice in a weakly pinned NbSe2 single crystal. We present direct evidence that the ordered state of the VL is a BG, consisting of a large number of degenerate metastable states, which is a hallmark of a glassy state. These results are a significant step towards understanding the disordering of a lattice under the influence of quenched random disorder with a direct impact on various fields, including charge density waves, colloidal crystals and self-organised periodic structures on a substrate.
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A review is given on the theory of vortex-glass phases in impure type-II superconductors in an external field. We begin with a brief discussion of the effects of thermal fluctuations on the spontaneously broken U(1) and translation symmetries, on the global phase diagram and on the critical behaviour. Introducing disorder we restrict ourselves to the experimentally most relevant case of weak uncorrelated randomness which is known to destroy the long-ranged translational order of the Abrikosov lattice in three dimensions. Elucidating possible residual glassy ordered phases, we distinguish betwee positional and phase-coherent vortex glasses. The discussion of elastic vortex glasses, in two and three dimensions occupy the main part of our review. In particular, in three dimensions there exists an elastic vortex-glass phase which still shows quasi-long-range translational order: the `Bragg glass. It is shown that this phase is stable with respect to the formation of dislocations for intermediate fields. Preliminary results suggest that the Bragg-glass phase may not show phase-coherent vortex-glass order. The latter is expected to occur in systems with weak disorder only in higher dimensions. We further demonstrate that the linear resistivity vanishes in the vortex-glass phase. The vortex-glass transition is studied in detail for a superconducting film in a parallel field. Finally, we review some recent developments concerning driven vortex-line lattices moving in a random environment.
In a recent letter Klein et al. [Nature 413, 404 (2001); cond-mat/0110018] provide experimental evidence for the existence of the Bragg glass phase in impure type II superconductors. Here we show that a more complete consideration of recent theoretical findings allows an even better interpretation of the experimental data.
We have used small-angle-neutron-scattering (SANS) and ac magnetic susceptibility to investigate the global magnetic field H vs temperature T phase diagram of a single crystal Nb in which a first-order transition of Bragg-glass melting (disordering), a peak effect, and surface superconductivity are all observable. It was found that the disappearance of the peak effect is directly related to a multicritical behavior in the Bragg-glass transition. Four characteristic phase boundary lines have been identified on the H-T plane: a first-order line at high fields, a mean-field-like continuous transition line at low fields, and two continuous transition line associated with the onset of surface and bulk superconductivity. All four lines are found to meet at a multicritical point.
The vortex lattice in a Type II superconductor provides a versatile model system to investigate the order-disorder transition in a periodic medium in the presence of random pinning. Here, using scanning tunnelling spectroscopy in a weakly pinned Co0.0075NbSe2 single crystal, we show that at low temperatures, the vortex lattice in a 3-dimensional superconductor disorders in two steps across the peak effect. At the onset of the peak effect, the equilibrium Bragg glass transforms into an orientational glass through the proliferation of dislocations. At a higher field, the dislocations dissociate into isolated disclination giving rise to an amorphous vortex glass. We also show the existence of a variety of additional non-equilibrium metastable states, which can be accessed through different thermomagnetic cycling.
We report the impact of Ni doping on superconductivity of PdTe superconductor. The superconducting parameters like critical temperature (Tc), upper critical field (Hc2) and normalized specific-heat jump are reported for Ni doped Pd1-xNixTe. The samples of series Pd1-xNixTe with nominal compositions x=0, .01, 0.05, 0.07, 0.1, 0.15, 0.2, 0.3 and 1.0 are synthesized via solid state reaction route. All the studied samples of series Pd1-xNixTe (x = 0.0 to 1.0) are crystallized in hexagonal crystal structure within the space group P63/mmc. Unit cell volume shrinks almost linearly upon Ni doping in Pd1-xNixTe. The normal state residual resistivity increases with Ni substitution on Pd site. Both the electrical resistivity and magnetic measurements revealed that Tc decreases with increase of Ni concentration in Pd1-xNixTe and is not observed down to 2K for x=0.30 i.e., 30% of Ni doping at Pd site. Interestingly, this is unusual for magnetic Ni doping in a known type-II BCS type superconductor. Magnetic Ni must suppress the superconductivity much faster. Interestingly, the isothermal magnetization measurements for NiTe revealed that Ni is non-magnetic in Pd1-xNixTe structure and hence the Tc depression is mainly due to disorder. The magneto-transport measurements revealed that flux is better pinned for 20% Ni doped PdTe as compared to other compositions of Pd1-xNixTe. The magnetic field dependence of specific heat of Pd1-xNixTe for x=0.01 was studied and the estimated value of the normalized specific-heat jump,is found to be 1.42, which is under BCS weak-coupling limit. Summarily, we report the impact of Ni doping in Pd1-xNixTe superconductor and conclude that Ni substitutes at Pd site, suppress superconductivity moderately and is of non magnetic nature in this system. To best of our knowledge this is the first study on Ni substitution in PdTe superconductor.
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