Institute for Complex Systems - Sapienza - CNR

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ISC Sapienza Strong Correlation

Strong Correlation

Research on strongly correlated electronic systems and cold quantum matter.


Volume collapse transition in Ce

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Crystalline Ce has a remarkable phase diagram in that is the solid state analogue of a Van der Walls system with a line of discontinuous transitions ending at a critical point.

The two phases alpha and gamma have the same cubic symmetry and differ only on density just like water and vapor but in the solid state!.

Fig 1: Phase diagram of Ce metal after Gschneidner Jr KA, Eyring L, editors. Handbook on the physics and chemistry of rare earths, vol. I. Amsterdam: North-Holland; 1979

However unlike water and vapor the critical exponents close to the critical point are not three dimensional Ising critical exponents, as one would naively guess from symmetries, but  classical (Gaussian) exponents. Also Ce has several other thermodynamics anomalies which we have explained with a non-linear elastic model.

Last Updated on Wednesday, 25 March 2015 11:58

Polarons in strongly correlated systems

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In system with strong electron-phonon interaction, the carriers loose mobility, ultimately acquiring polaronic character. A polaron is a state in which the phonon and electron degrees of freedom are strongly entangled, and the presence of an electron is associated to a finite lattice distortion, which in turn binds the electron leading to the so-called self-trapping effect. Polarons also tend to create bound pairs, (called bipolarons). Of course the presence of Coulomb repulsion destabilizes bipolarons in favor of a pure polaronic state at finite densities. Coulomb interaction (even screened in the form of Hubbard on-site repulsion) has been found to give interesting interplay with the attractive el-ph interaction in the regime in which the repulsion is so strong to produce strong antiferromagnetic correlations.In this case the increase of the exchange coupling J gives rise to polaron formation reflected in. fig. 1 in the shrinking of the magnetic polaron size as well as in a sudden change in the local phonon distribution associated to the carrier site.

Last Updated on Thursday, 03 March 2011 11:07

Frustrated Phase Separation

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A large variety of systems with competing short and long range interactions self-organizes in domain patterns as reviewed by Seul and Andelman. Examples range from magnetic systems (left figure A) to organic systems (left figure B).

Frustrated phase separation in magnetic and organic films


Inhomogeneous states display a simple set of predominant morphologies like circular droplets and stripes in two-dimensional (2D) systems, and layers, cylindrical rods and spherical droplets in three-dimensional (3D) systems. This tendency for a common behavior across different systems calls for simple models which neglect the specific details of each system and capture the general properties.

Last Updated on Wednesday, 06 September 2017 16:57