Giuseppe Santoro

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Current research topics

Recent publications

An ongoing research project concerns the role of the Jahn-Teller effect in alkali doped C60 compounds. We are currently interested in the interplay between strong electron-electron correlations and Jahn-Teller effect in AC60, in particular the quenched low-temperature phase of CsC60, and in K3C60, which turns insulating and antiferromagnetic upon increasing the lattice constant with insertion of NH3.
 
With the aim of better understanding electronic conduction in systems where the lattice sites have orbital degeneracy and Jahn-Teller effect, we have studied simple hopping models where the orbital degeneracy is mimicked by an extra spin variable, plus a constraint. Interestingly, one find that these ingredients tend to favor superconductivity even for purely repulsive interactions, especially at low electron density. See Refs.[ 2 , 3 ].
 
When a system with local orbital degeneracy is in a Mott insulating state due to large on-site repulsion, the low-energy excitation (spin and orbital excitations) are governed by an effective Hamiltonian involving the ordinary spin variables (like in the non degenerate case) and extra pseudo-spin variables, related to the orbital degrees of freedom. We have considered a particular instance in which a twofold degenerate orbital with a strong Jahn-Teller coupling overwinning the Hund's coupling, leads to an exchange Hamiltonian of the type
HST = -J Sum(ij) ( Si.Sj  -1/2 ) ( Ti.Tj  -1/2 )

We have shown that in one dimension this Hamiltonian has a spin-Peierls-like dimerized ground state, Ref. [ 7 ]. In two dimensions, we find that the ground state is a spin liquid.

We have recently studied the low temperature instabilities of the alpha-phase of tetravalent adatoms on (111) semiconductor surfaces. The cases of interest include the surface charge density wave (CDW) systems Pb/Ge(111) and Sn/Ge(111), as well as the Mott insulators Si/SiC(0001) and K/Si(111):B. We have approached the problem in two ways: first, by employing a one-band model Hamiltonian of the Hubbard-Holstein type, in order to understand general features of the phase diagram as a function of the strength of electron-electron and electron-phonon interactions; second, by performing realistic ab-initio calculations within the local spin density approximation (LSDA) for the case of Sn/Si(111), and of a hypothetical alpha-Si/Si(111) mimicking K/Si(111):B. The collinear LSDA calculation for both Sn/Ge(111) and Si/Si(111) predicts a spin density wave state with a uniform magnetization mz=1/3 and a small secondary CDW. We discuss and stress the likely important role played by electron-electron interactions in explaining the phenomenology of all these systems, as opposed to the secondary role played by the electron-phonon coupling, which would at most drive the lattice, for Pb-Sn/Ge(111), after the electrons have caused the transition. See Refs.[ 5 , 10 ].

Semiconductor quantum dots (QD) are an area of great technological interest in mesoscopic physics. Effects played by Coulomb interaction are expected to be crucial for electrons confined in a QD. For instance, in a transport measurement (depending on the relative transparencies of the barriers which tune the tunnelling rates), non-linearities in the I-V characteristic, such as Coulomb blockade and Coulomb staircase, give evidence that charging effects in adding a single electron to the system strongly influence the behaviour of the system itself. Charging effects, together with the discreteness of the energy spectrum, are precisely the ingredients which make QD's of potentially large relevance in future electronic devices (single-electron memories, single-electron transistors, etc.). We plan to focus on the interplay between quantum coherence in transport across the structure and the electron-electron interactions among the particles in the Dot. Exact diagonalization of a few electron Dot realistic Hamiltonian (N up to 6) will be performed comparing the results with perturbative methods in the Green's function formalism. These methods will be used, in conjunction with explict calculations of the tunneling rates obtained within a Tunneling Hamiltonian scheme, to evaluate the current in linear and non-linear regimes. Until now, an exact diagonalization code written by us has been used to attribute to the relevant many body states features revealed by magneto-tunneling measurements performed on vertical QDs. (See Ref.[11].)

This project started in 1994, and aimed at describing the physics of disordered flat surfaces, and the ensuing pre-roughening transition. Classical statistical mechanics models for surface phase transitions can be often mapped onto quantum problems in one dimension. The first line of approach, consisted in studying particular restricted solid-on-solid models of the BCSOS-type with competing interactions, by mapping them onto 1D Heisenberg chain problems. (See Refs. [1, 4].) These studies lead to the conclusion that the disordered flat phase of these BCSOS models is nothing but the dimer phase of a Heisenberg chain. A second line of approach regards the problem of a disordered surface as an assembly of interacting up and down steps. Such steps can be viewed as imaginary-time world-lines of hard-core bosons in 1D. We have then studied the role of step-step interactions in giving rise to a disordered flat phase (DOF) by using one-dimensional techniques. (See Refs. [ 6, 8, 9].)


 
  1. G. Santoro and M. Fabrizio

    2. Disordered flat phase in a solid on solid model of fcc(110) surfaces and dimes states in quantum spin-1/2 chains,
    Physical Review B 49 , 13886 (1994) .( cond-mat/9402038 )
  2. G.Santoro, M.Airoldi, N.Manini, E.Tosatti and A.Parola

    3. The Phase Diagram of Correlated Electrons in a Lattice of Berry Molecules ,
    Physical Review Letters 74 , 4039 (1995). ( cond-mat/9410051 )
  3. G. Santoro, N. Manini, A Parola, and E. Tosatti

    4. Phase Diagram of a Model of Correlated Hopping of Electrons in a Lattice of Berry Molecules,
    Physical Review B 53 , 828 (1996). ( cond-mat/9511093 )
  4. G. Santoro, M. Vendruscolo, S. Prestipino, and E. Tosatti,

    5. Disordered flat phase and phase diagram for restricted solid on solid models of fcc(110) surfaces,
    Physical Review B 53, 13169 (1996). ( cond-mat/9512138 )
  5. G. Santoro, S. Sorella, F. Becca, S. Scandolo and E. Tosatti

    6. Metallic charge density waves and surface Mott insulators for adlayer structures
    on semiconductors:  extended Hubbard modeling
    Surface Science 402-404, 802 (1998). ( cond-mat/9802014 )
  6. G. Santoro, A. Laio, M. Fabrizio and E. Tosatti,

    7. Interacting hard-core bosons and surface physics,
    Surf. Sci. 377-379, 514 (1997).
  7. G. Santoro, L. Guidoni, A. Parola, and E. Tosatti,

    8. Valence-bond states in dynamical Jahn-Teller molecular systems,
    Physical Review B 55, 16168 (1997).( cond-mat/9809017 )
  8. G. Santoro, A. Laio and E. Tosatti,

    9. Step-step interactions and correlations from 1D hard-core boson mapping,
    Surface Science 402-404, 880 (1998).
  9. A. Laio, G. Santoro, and E. Tosatti,

    10. Interacting hard-core bosons and surface preroughening,
    Phys. Rev. B 58, 13151 (1998).( cond-mat/9809091 )
  10. G. Santoro, S. Scandolo, and E. Tosatti,

    11. Charge density waves and surface Mott insulators for adlayer structures on semiconductors: extended Hubbard modeling,
    Phys. Rev. B (Jan. 1999).( cond-mat/9809016 )
  11. B. Jouault, G. Faini, A. Angelucci, M. Di Stasio, G. Santoro, A. Tagliacozzo, F. Laruelle, R. Werner,

    12. Shell filling in non-linear magneto-tunneling spectroscopy of vertical quantum dots,
    submitted to Phys. Rev. Lett. (nov. 1998).( cond-mat/9810094 )
  12. L. Guidoni, G. Santoro, S. Sorella, A. Parola, and E. Tosatti,

    13. Spin gap in low-dimensional Mott insulators with orbital degeneracy,
    to appear on J. Appl. Phys. (1999).( cond-mat/9809013 )

Revised on jan. 8, 1999

 santoro@sissa.it