3 CMSP seminars coming up soon

Fri Feb 1 14:52:06 CET 2013

Joint ICTP/SISSA INFORMAL SEMINAR on     Statistical Physics

Monday, 4 February -  3:00 p.m.

SISSA, Santorio Building,   Room 5 (ground floor)

Subodh R. SHENOY    ( Indian Institute of Science Education and Research, Trivandrum )

"Entropy barriers and glass-like behaviour in  martensitic models without extrinsic disorder"

Abstract

A 2D model of three-valued discrete-strain pseudospins S (r)=0, +1,-1 is studied in Monte Carlo simulations, under systematic temperature quenches, without extrinsic disorder [1].  The reduced model is from Landau minima of a continuum-strain free energy of the square-rectangle martensitic transition. The pseudospins interact via a power-law anisotropic potential obtained from the St Venant Compatibility constraint.  The same procedure yields pseudospin models for other 2D and 3D transitions [2].  After a temperature quench, dilute martensitic seeds in an austenite background, induce sequential  domain-wall phases of a 'vapour' of a martensite droplet in austenite; a  'liquid' of disordered martensite domain walls; and a ‘crystal’ of oriented-wall twins.  The evolution is tracked through Temperature-Time –Transformation (TTT) curves, with time scales for conversion to martensite t_m (T);  and Compatibility-driven domain-wall orientation, t_C (T).  Depending on elastic constants, t_m (T)  can either be thermally activated and slow, as in 'isothermal' martensites; or non-activated and explosive, as in 'athermal' martensites below a martensite start temperature ~ T_1.  For quenching to above such a temperature, we find (generically)  a delay tail, that rises towards a temperature T=T_4  > T_1.  This is identified with the puzzling delay tail in t_m (T) found by Kakeshita [3].  The mean time shows glass-like Vogel-Fulcher behaviour, t_m ~ exp [ 1/( T_4 - T)], and distributions are log-normal [1].  The times are insensitive to the Hamiltonian energy scale E_0, and are attributed to entropy barriers that vanish/ diverge at T_1 and T_4.  The conversion delays are from searches in Fourier space on constant-energy surfaces, for rare energy-lowering pathways.  The 'vapour' phase structure-factor peak has to find pathways to distort and roll into a small anisotropic 'golf hole'; to be then guided by a 'funnel' into a 'liquid' phase;  and finally symmetry-broken, to a 'crystal' phase.  A time- dependent  effective temperature re-equilibrates to the bath temperature.  The entropic golf-hole, and energetic funnel, are concepts from protein folding [4].

 [1] N. Shankaraiah, K.P.N. Murthy, T. Lookman and S.R .Shenoy, Europhys. Lett. 92, 36002 (2010); Phys. Rev. B 84, 064119 (2011) ; and unpublished.
[2]  S.R. Shenoy, T. Lookman and A. Saxena, Phys. Rev. B 82, (2010).
[3] T. Kakeshita, T. Fukuda and T. Saburi, Scr. Mater. 34, 1 (1996); L. Mueller, U. Klemradt and T.R. Finlayson, Mater. Sci. Eng. A, 438, 122 (2006).
[4] P.G. Wolynes, et al. Science 267, 1619 (1995); D.J. Bicout and A. Szabo, Protein Science 9, 452 (2000); N. Nakagawa, Phys. Rev. Lett.98, 128104 (2007).

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JOINT ICTP/SISSA STATISTICAL PHYSICS SEMINAR

Tuesday, 5 February -   11:30 hrs.

Luigi Stasi Seminar Room -    ICTP Leonardo Building - 1st floor

Andreas LAUCHLI     ( Institute for Theoretical Physics - Innsbruck University )

"Entanglement spectra: A novel spectroscopic tool to investigate quantum many body wave functions"

Abstract

The entanglement spectrum, ie the logarithm of the eigenvalues of reduced density matrices of quantum many body wave functions, has been the focus of a rapidly expanding research endeavor recently.  Initially introduced by Li & Haldane in the context of the fractional quantum Hall effect, its usefulness has been shown to extend to many more fields, such as topological insulators, fractional Chern insulators,
spin liquids, continuous symmetry breaking states, etc.

After a general introduction to the field we review some of our own contributions to the field, in particular the perturbative structure of the entanglement spectrum in gapped phases, the entanglement spectrum across the Mott-insulator transition in the Bose-Hubbard model, and the relation of the entanglement spectrum of (1+1) dimensional quantum critical systems to the operator content of their underlying CFT.

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SEMINAR on   Disorder and strong electron correlations

Thursday, 7 February -  11:30 a.m.

Luigi Stasi Seminar Room,  Leonardo Building - first floor

Yun LI    (INO-CNR Center and University of Trento)

"Bose-Einstein condensed gas with equal Rashba and Dresselhaus spin-orbit coupling"

Abstract

We consider a spin-orbit (SO) coupled configuration of spin-1/2 interacting bosons with equal Rashba and Dresselhaus couplings. The phase diagram of the system at T=0 is discussed with special emphasis on the role of the interaction, treated in the mean-field approximation. For a critical value of the density and of the Raman coupling we predict the occurrence of a characteristic tricritical point separating the stripe, the spin polarized and the zero momentum states of the Bose gas. The role of the breaking of spin symmetry in the interaction Hamiltonian, as well as the compressibility and the spin polarizability of the system are discussed.

By using the hydrodynamic formalism we identify the excitation spectrum at zero temperature. The propagation of phonons is strongly affected by the SO coupling and the velocity of sound vanishes when one approaches the second order phase transition between the spin polarized and the zero momentum quantum phases. We also point out the emergence of a roton minimum in the excitation spectrum for small values of the Raman coupling, when one approaches the transition to the stripe phase.