Effect of Impurities on Supersolid Condensate: A Ginzburg–Landau Approach

Basing our arguments on a wavefunction that contains both positional and superfluid order, we propose a Ginzburg–Landau functional for a supersolid with the two order parameters necessary to describe such a phase: density n _B (r) and supersolid order parameter ψ _V . We argue that adding lighter. ³He atoms to a ⁴He supersolid produces attractive regions for vacancies, leading to patches of higher T _c. On the other hand, the supersolid stiffness decreases in this granular state with increased ³He disorder. Both effects are linear in ³He concentration.     

Ramp‐Reversal Memory and Phase‐Boundary Scarring in Transition Metal Oxides

Transition metal oxides are complex electronic systems that exhibit a multitude of collective phenomena. Two archetypal examples are VO₂ and NdNiO₃, which undergo a metal–insulator phase transition (MIT), the origin of which is still under debate. Here this study reports the discovery of a memory effect in both systems, manifested through an increase of resistance at a specific temperature, which is set by reversing the temperature ramp from heating to cooling during the MIT. The characteristics of this ramp‐reversal memory effect do not coincide with any previously reported history or memory effects in manganites, electron‐glass or magnetic systems. From a broad range of experimental features, supported by theoretical modelling, it is found that the main ingredients for the effect to arise are the spatial phase separation of metallic and insulating regions during the MIT and the coupling of lattice strain to the local transition temperature of the phase transition. We conclude that the emergent memory effect originates from phase boundaries at the reversal temperature leaving “scars” in the underlying lattice structure, giving rise to a local increase in the transition temperature. The universality and robustness of the effect shed new light on the MIT in complex oxides.     

The one-dimensionalization of three-dimensional organic chain conductors by impurities

The effect of impurities on the phase transition caused by weak electron-electron interactions in a 3D array of linear conducting chains is considered. Inasmuch as the impurities destroy the correlations between electrons on different chains, they suppress the 3D phase transition and enhance the superconductive fluctuations on single chains, leading to a sort of precursor temperature behavior above the 3D phase transition temperature. The possibility of the observation of this effect in real materials is discussed, and arguments are given showing that this phenomenon could be responsible for the unusual properties of the nonstoichiometric compound (TTT)₂I_+;.This work was performed at the Aspen Center for Physics while the authors were participants in the joint US-USSR research group in the theory of condensed matter, which was supported by a National Science Foundation grant.Supported in part by NSF grant DMR 76-23330 AO1.     

Of Mutual Interest

In which the author of the report to the BBC reflects on the issues raised by contributors to this colloquy.     

Odd frequency pairing in superconductors

The properties of a superconductor having an energy gap function _k( _n ) which is an odd function of frequency _n, or time, are investigated. One finds (a) the allowed pair orbital angular momenta for spin zero and spin one are interchanged, permittingS= 0, l=1 and S=1, l=0 condensates; (b) there exists an equal-time order parameter corresponding to condensation of a spin wave coupled to two fermions, in contrast to the conventional timereversed fermion pair condensate; an explicit energy gap equation is derived for this composite operator condensate; (c) the coherence factors are shown to be reversed from those of conventional pairing, for>2, removing the peak in thea+C conductivity() and generating a peak in the>2 acoustic attenuation as a function of temperature; (d) the Josephson current vanishes between an even and odd frequency superconductor, and vanishes to leading order in the tunneling rates t ₁₂ ² .The authors wish to acknowledge the support of the National Science Foundation, grant No. DMR92-13295 (J.R.S.) and No. DMR 92-205027 (D.J.S.). They would like to thank Leo Lilly and William Putikka for helpful discussions.     

Looking for Trouble

A liason between broadcasters and academics proposes areas where social research can help improve media products and policy.