SVIBOR - Project code: 1-03-172

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Project code: 1-03-172


STRONGLY CORRELATED ELECTRONIC SYSTEMS


Main researcher: HAMZIĆ, AMIR (15234)


Assistants
Type of research: basic
Duration from: 01/01/91. to 12/31/94.
Papers on project (total): 14
Papers on project quoted in Current Contents: 10
Institution name: Prirodoslovno-matematički fakultet, Prirodoslovni odjeli, Zagreb (119)
Department/Institute: Department of Physics, Faculty of Science
Address: Bijenička 32
City: 10000 - Zagreb, Croatia
Communication
Phone: 385 (1) 4555 730
Phone: 385 (1) 432 482
Fax: 385 (1) 432 525
E-mail: hamzic@dominis.phy.hr
Summary: The strongly correlated fermion systems have been intensively studied in the solid state physics for several years now. The problem of electron interactions is present, among others, in He3, quantum Hall effect, the physics of heavy fermion (HF) systems, high temperature superconductivity (HTSC), magnetic multilayers and organic conductors. The heavy fermion systems (HF) represent a new class of intermetallic compounds, in which one of the constituents has an unfilled f-shell. At high temperatures the HF systems behave as a collection of independent localized 4f moments (Kondo systems), whereas at low temperatures these moments delocalize and form a new, heavy fermion, "coherent" state. In this regime, HF can be either nonmagnetic or antiferromagnets, and in some cases, at even lower temperatures, superconducting. The nature of this superconducting state is very unusual and controversial; there have been indications that the pairing mechanism is due to the electron-electron interaction. The high temperature superconductivity (HTSC) is the one of the physical phenomena which origin is still not fully understood, in spite of the discovery of numerous new materials and extensive experimental and theoretical research. These oxide materials are characterised with a crystal structure with Cu-O planes and a reduced dimensionality of the relevant electronic states. Small changes of their chemical composition can lead to a long-range magnetic correlations, indicating the importance of the electron-electron interactions. The anisotropic organic conductors have so far demonstrated a variety of phenomena (superconductivity, spin density waves - SDW, anion ordering, etc) when different physical parameters are varied.Their crystal structure consists of chains of donor molecules separated by chains of acceptor anions and important value differences between their nearest neighbour tight-binding transfer integral along crystallographic axis.give rise to their anisotropic properties. Particulary interesting are the studies of their SDW and insulating phases under high magnetic fields. Magnetic multilayer superlattices are the artificial epitaxial or sputtered structures which consist of several thin monocrystalline layers (e.g. Fe/Cr, Co/Cu, Co/Ru, Fe/V, Cu/Ni, etc.). Due to the reduced thickness they represent quasi-twodimensional magnetic systems, characterised with a strong magnetic anisotropy (which can favour certain orientation of the magnetization), a giant magnetoresistance effect (exceeding, in some cases, 100 %) and oscillatory magnetic correlations between neighbouring ferromagnetic layers. Equally, in the domain of nanomagnetism and the studies of different mesoscopic systems (electronically lithographed multilayers or aggregates) one of the most important aspect is the investigation of the dynamics of the orientation of magnetisation of small monodomain particles, due to its important impact on the development of new magnetic memories.

Keywords: strongly correlated electronic systems, heavy fermion systems, high temperature superconductors, magnetic multilayers (magnetic superlattices), organic conductors, electrical resistivity, magnetoresistivity, Hall effect, low temperatures, high magnetic fields

Research goals: The aim of the project has been the systematic experimental investigations of the similarities and differences of the electronic properties of systems with strong correlations. The Hall effect and magnetoresistance studies of the HF systems in the high temperature incoherent state have established the existence of an universal temperature and field dependence, which can be well described by the skew scattering of conduction electrons on independent magnetic moments. On the other hand, the transition to the low temperature coherent regime and some of its additional features (magnetic correlations, effects of chemical and structural doping) have not yet been fully understood. Thus, one of the aims of the project would be to investigate those HF systems in which such effects appear in the coherent state, when the external parameters (magnetic field, doping) are varied. Particular interest will be given to studies of different types of asymmetric scattering (anomalous velocity term, expected to dominate in the disordered compounds), which could be clearly separated in the low temperature Hall effect data. In order to clarify the magnetic phases in some HF systems, the galvanomagnetic studies should also be continued, since they have proven to be very sensitive to small changes of the electronic or magnetic structures. The properties of normal and superconducting states of the available high quality HTSC materials will be investigated, and a special interest will be focused on the studies of the vortex structure (measurements of the slow magnetic relaxation at ultralow temperatures). The magnetotransport results will be correlated with other (e.g. magnetic) types of measurements. The possible applications of HTSC materials will be considered. The high field galvanomagnetic investigations (magnetoresistivity and its anisotropy, Hall effect) of different anisotropic organic conductors (TMTCF)2X (C = Se, S; X = NO3, Br) will be done. Particular attention will be given to the studies of these systems in the spin density wave (SDW) phase (the possible influence of the applied magnetic field on SDW, semimetallic nature of the SDW ground state, temperature variation of the carrier concentration), as well as in their low temperature insulating phase. The interest for the studies of magnetic multilayered superlattices (such as Fe/Cr, Co/Cu) or granular thin films is twofold. They represent a new, artificially produced, class of magnetic systems characterised with a strong magnetic anisotropy, giant magnetoresistance and oscillatory type of magnetic correlations. At the same time, these properties make them extremely interesting for possible applications ("perpendicular" storage of data, magnetic memories, magnetic field detectors). The influence of different parameters (layer thickness, surface roughness, temperature, doping, applied magnetic field), on giant magnetoresistance will be investigated in details, as well as a search for new systems with even better characteristics. Using the new preparation methods by electronic lithography or preparation of aggregates enables the production of the samples on the nanometric scale. One of the fundamental issue in nanomagnetism is the physical origin for the dynamics of the orientation of the magnetisation of small monodomain particles at very low temperatures: whether it is due to the thermally activated superparamagnetism or to the tunnel effect. This dynamics will be studied by investigating the temperature dependence of the giant magnetoresistance.

COOPERATION - PROJECTS


  1. Name of project: DOE JF 817-31 Electronic Properties of Heavy Fermion Systems (do 31.08.1992.)
    Name of institution: Los Alamos National Laboratory
    City: Los Alamos, SAD

  2. Name of project: CI1*-0576M(A) Magnetic and Transport Properties of Heavy Fermion Intermetallic Compounds (EEC projekt; do 30.06.1993.)
    Name of institution: Laboratoire de Physique des Solides
    City: 91405 - Orsay, Francuska

  3. Name of project: CI1*-0576M(A) Magnetic and Transport Properties of Heavy Fermion Intermetallic Compounds (EEC projekt; do 30.06.1993.)
    Name of institution: Imperial College of Science
    City: London, Velika Britanija

  4. Name of project: CI1*-0576M(A) Magnetic and Transport Properties of Heavy Fermion Intermetallic Compounds (EEC projekt; do 30.06.1993.)
    Name of institution: Institut za fiziku
    City: 10000 - Zagreb, Croatia

  5. Name of project: CI1*-0568C(EDB) Single Crystal Studies of High Temperature Superconductors (EEC projekt)
    Name of institution: Institut za fiziku
    City: 10000 - Zagreb, Croatia

  6. Name of project: CI1*-0568C(EDB) Single Crystal Studies of High Temperature Superconductors (EEC projekt)
    Name of institution: Institut Jožef Štefan
    City: Ljubljana, Slovenija

  7. Name of project: CI1*-0568C(EDB) Single Crystal Studies of High Temperature Superconductors (EEC projekt)
    Name of institution: Imperial College of Science
    City: London, Velika Britanija

  8. Name of project: CI1*-0568C(EDB) Single Crystal Studies of High Temperature Superconductors (EEC projekt)
    Name of institution: CNRS
    City: 75000 - Paris, Francuska

  9. Name of project: Magnetisme des composes intermetalliques et des couches minces
    Name of institution: Ministere francais de la Recherche et de la Technologie
    City: 75000 - Paris, Francuska

COOPERATION - INSTITUTIONS


  1. Name of institution: Laboratoire de Physique des Solides (associe au CNRS)
    Type of institution: University/Faculty
    Type of cooperation: Joint project
    City: 91405 - Orsay, Francuska

  2. Name of institution: Institut za fiziku Sveučilišta
    Type of institution: University/Faculty
    Type of cooperation: Joint publishing of scientific papers
    City: 10000 - Zagreb, Croatia

  3. Name of institution: Los Alamos National Laboratory
    Type of institution: State institute
    Type of cooperation: Joint project
    City: Los Alamos, SAD

  4. Name of institution: Imperial College of Science
    Type of institution: University/Faculty
    Type of cooperation: Joint project
    City: London, Velika Britanija

  5. Name of institution: Institut Jožef Štefan
    Type of institution: State institute
    Type of cooperation: Joint project
    City: Ljubljana, Slovenija
Other information about the project.
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