For distribution - thanks

[CM ICTP - Trieste]

JOINT ICTP/SISSA STATISTICAL PHYSICS SEMINAR




Friday, 7 March    -    11:00 a.m.



Lecture Room 'D' - SISSA Main Building




H. NISHIMORI   ( Tokyo Institute of Technology )


" Griffiths singularities in finite-dimensional spin glasses "



Abstract

The distribution of zeros of the partition function is studied for the 
two- and three-dimensional Ising spin glasses on the complex field 
plane.  We estimate the density of zeros on the imaginary axis by an 
importance-sampling Monte Carlo algorithm, which enables us to sample 
very rare events.  Our result suggests that the density has an 
essential singularity at the origin, giving the first evidence for a 
Griffiths singularity in spin glass systems in equilibrium.


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CONDENSED MATTER AND STATISTICAL PHYSICS SECTION
	
Joint ICTP/Democritos

INFORMAL SEMINAR on Chemical Physics




Tuesday, 11 March   -    11:00 a.m.




Seminar Room - ICTP Main Building  (first floor)



Yansun YAO   ( University of Saskatchewan, Saskatoon )



"Structures and superconductivity of selected high pressure solids"


Abstract

Recent achievements in synchrotron instrumentations have greatly 
improved the quality of powder diffraction data obtained from a diamond 
anvil cell.  With structural refinement techniques, many new structures 
of solid materials at high pressures have been discovered.  In a number 
of solids, these structural changes also induce metallization and lead 
to the appearance of superconductivity upon compressing.  On the 
theoretical side, typical approaches for predicting/determining unknown 
high-pressure crystal structures usually involve dynamical processes.  
We have tried another approach based on evolutionary algorithms 
proposed recently.  This is an attempt to predict the most stable 
crystal structure and energetically favourable meta-stable structures 
without any approximation on initial structures.  During evolutionary 
procedures, randomly generated structures evolve gradually into lower 
energy phases after a few generations.  We have investigated the 
high-pressure structures of calcium and predicted two new stable 
structures that might explain the diffraction pattern of Ca-IV and 
Ca-V.  Another example will be presented is the prediction of a 
metastable single-bonded phase of nitrogen.  The search of a synthetic 
strategy for novel materials with high-Tc has been a continuing 
challenge.  Recently there have been several new proposals relating 
superconductivity with some tailgate signatures for prospective 
candidate materials.  Among these proposals are (i) the occurrence of 
low vibrational frequencies (ii) simultaneous occurrence of dynamical 
and electronic instabilities.  Here we demonstrated successfully that 
these conditions can be satisfied in a high pressure phase of SnH4.  We 
predicted this material would be superconducting with a Tc close to 80 
K at 120 GPa.  Some other high-pressure superconductors will be briefly 
reviewed.
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* Work done in collaboration with John S. Tse