urgent distribution seminars

[CM ICTP - Trieste]

JOINT ICTP/SISSA CONDENSED MATTER SEMINARS
Academic Year 2006/07


Seminar Room - Main Building  (first floor)


Wednesday, 25 July -     4:00 p.m.


G. BASKARAN   ( The Institute of Mathematical Sciences, Chennai )


" Theory of supersolid phenomena "

Abstract

Quantum fluctuations in Solid He 4 leads to a variety of interesting 
phenomena.  One such possibility is a supersolid phase, where a fragile 
superfluidity survives in the crystalline state. Recent experimental 
observations of non classical moment of inertia by Kim and Chan claim 
to confirm this exotic phase.  After giving a background I will review 
my recent theory [1] of the supersolid phenomena.  In my theory, 
quantized vortices play a fundamental role. Quantized vortices in He4 
have a very small core energy and core size (about 1 Au). Quantized 
vortex loops are also present in the superfluid, as quantum 
fluctuations. Pressure beyond 25 atmosphere generates a spontaneous 
vortex-antivortex lattice, also self consistently develops an atom 
density wave with hcp symmetry.  Supersolid He4 is a vortex solid.  
Many of the existing phenomenology gets explained and some interesting 
predictions are made.

[1] And quiet flows the supersolid, G. Baskaran, cond-mat/0505160


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CONDENSED MATTER SECTION

	
INFORMAL SEMINAR



Thursday, 26 July   -    11:00 a.m.



Lecture Room 'C' -  Main Bldg. - terrace level



T.I. BATURINA   ( Institute of Semiconductor Physics, Novosibirsk )

"Quantum critical region of the disorder-driven 
superconductor-insulator transition"

Abstract

I present results of the experimental study of the low-temperature 
transport properties of disordered thin TiN superconducting films 
across the localization threshold.  The films exhibit an extremely 
sharp separation between the well-defined superconducting- and 
insulating phases, demonstrating thus unambiguously the direct 
disorder-driven superconductor-insulator transition.  Although the 
films choose unequivocally between the either superconducting or 
insulating ground states, the temperature- and magnetic-field behaviors 
of the resistance reveal an insulating trend in the superconducting 
films and a superconducting trend in the insulating films.  At zero and 
low magnetic fields the conductivity of insulating films is thermally 
activated, with the activation energy being nonmonotonic and 
magnetic-field dependent.  In all samples, including insulating films, 
magnetoresistance varies non-monotonously with the magnetic field, 
starting with the positive magnetoresistance at low fields, then 
reaching the maximum, followed first by the rapid drop and the eventual 
saturation at higher magnetic fields.  At these high fields the 
difference between the insulating and superconducting samples vanishes 
and all curves converge to the resistance close to the quantum value.  
At very low temperatures a sharp depinning voltage threshold in 
conductivity is observed.  Our results and the comparison with other 
studies on InOx clearly indicate that, in the vicinity of the 
disorder-driven superconductor-insulator transition, the response to 
the applied magnetic and/or electric fields, is the same irrespectively 
to whether the underlying ground state is superconducting or 
insulating.   The observed similarity allows for conjecture that in the 
critical region of the transition a peculiar highly inhomogeneous 
"superconducting insulator" forms.  This inhomogeneity may be 
considered as self-induced granularity, coming from the strong 
mesoscopic fluctuations in disordered thin superconducting films and/or 
the fractal character of the electronic wave functions near the 
localization threshold.