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Seminar - An order parameter for impurity systems at quantum criticality


on 18-09-2014

... Abolfazl Bayat, University College London

18/09/2014, 11:30
Room P9, Mathematics Building

* * * Please note exceptional day and time. * * *


A quantum phase transition may occur in the ground state of a system at zero temperature when a controlling field or interaction is varied. The resulting quantum fluctuations which trigger the transition produce scaling behavior of various observables, governed by universal critical exponents. A particularly interesting class of such transitions appear in systems with quantum impurities where a non-extensive term in the free energy becomes singular at the critical point. Curiously, the notion of a conventional order parameter which exhibits scaling at the critical point is generically missing in these systems. We here explore the possibility to use the Schmidt gap, which is an observable obtained from the entanglement spectrum, as an order parameter. A case study of the two-impurity Kondo model confirms that the Schmidt gap faithfully captures the scaling behavior by correctly predicting the critical exponent of the dynamically generated length scale at the critical point.


Quantum Computation and Information Seminar
http://math.ist.utl.pt/seminars/qci/?action=next

Support: Phys-Info (IT), SQIG (IT), CFIF and CAMGSD, with support from FCT, FEDER and EU FP7, namely via projects PEst-OE/EEI/LA0008/2013, CQVibes, Landauer (GA 318287) and PAPETS (323901). More Information..

Seminar - Discrete Breathers and Negative Temperature States


on 11-09-2014

... Stefano Iubini, CBM-CNRS Orléans

11/09/2014, 14:00
Room P8, Mathematics Building, IST

* * * Please note exceptional day and place. * * *


Since the pioneering work of Onsager and Ramsey in the 1940s and '50s, physical states at negative (absolute) temperatures have attracted the curiosity of researchers and shown how science can challenge common sense. In negative-temperature regimes, the temperature is above infinity and high-energy states are more populated than low-energy ones.

After many years elapsed since the first experimental evidences of negative temperatures in quantum nuclear-spin systems, recent experiments have realized a negative temperature state in a system of ultracold bosons trapped in optical lattice, modeled by a Bose-Hubbard Hamiltonian.

I will discuss the statistical behavior of a semi-classical limit of the Bose-Hubbard model, namely the Discrete Nonlinear Schroedinger Equation. By monitoring the microcanonical temperature, it is possible to show that there exists a parameter region where the system evolves towards a state characterized by a finite density of spatially localized nonlinear excitations (discrete breathers) and a negative temperature. Such a state persists over very long (astronomical) times since the convergence to equilibrium becomes increasingly slower as a consequence of a coarsening process.

I will also discuss possible mechanisms for the generation of negative-temperature states in experimental setups.

Quantum Computation and Information Seminar
http://math.ist.utl.pt/seminars/qci/?action=next

Support: Phys-Info (IT), SQIG (IT), CFIF and CAMGSD, with support from FCT, FEDER and EU FP7, namely via projects PEst-OE/EEI/LA0008/2013, CQVibes, Landauer (GA 318287) and PAPETS (323901). More Information..