Institute of Theoretical Physics Workshop

Quantum Magnetism in Novel Materials and Geometries



  Books on Magnetism



Research into the Quantum Mechanical Behavior of the Electron's Spin

Workshop Organizers

Prof. Rajiv R. P. Singh

Prof. Warren E. Pickett

Prof. Daniel Khomskii

Prof. George Sawatzky

The Workshop Will Focus on Five Topics

Spin Gap & Spin Peierls Materials

Recently a very large number of new materials have been discovered in which the magnetic atoms are locked cooperatively into a non-magnetic spin-singlet ground state. The signature of this state is that the spin-susceptibilities develop an activated temperature dependence at low temperatures. Such a spin-gap phase can arise from the high temperature paramagnetic phase through a sharp phase transition or through a gradual crossover as the temperature is lowered. Materials of interest include Calcium and Sodium Vanadates, Copper Germanates, Strontium Cuprates, Strontium Boron Cuprates and a number of organic materials. Many of these systems exhibit a complex interplay of spin, phonon and orbital degrees of freedom. Understanding the magnetic behavior of these systems requires input from quantum chemistry, electronic structure, many-body model Hamiltonians and quantum field theory. The physics of insulating spin-gap materials may also have implications for high temperature superconductivity. These systems will be one of the central topics of the ITP program.

Magnetic Nanoparticles and Quantum Tunneling

Recent advances in fabrication of nanomaterials raises several important questions. How does one understand the magnetism at decreasing length scales and at what length scale will the magnetism become unstable? What role do quantum fluctuations play in the dynamics of magnetic nanoparticles? How does one characterize the behavior of individual nanoparticles and what is its relation to the behavior of an ensemble of nanoparticles? How does one determine their size distributions? What novel applications may result from manipulations of magnetism at mesoscopic and smaller length scales in semiconductors, molecular magnets and other related systems?

Half-Metallic Ferromagnets

Whether a material is a conductor or an insulator is one of its most fundamental characteristics. Ferromagnets are usually metallic, but ferromagnetic insulators also exist. In either case both up and down spin electrons have the same electrical behavior: both spins are metallic, or both are insulating. Half-metallic ferromagnets, however, are schizophrenic: electrons with one spin direction (say, up) are metallic, while spin-down electrons are insulating. This peculiar electronic state has become the focus of recent research, both for its intrinsic scientific interest and because of the promise of optimum `spin-electronics' devices that could provide an extra magnetic degree of freedom in manipulating electrical signals.

Half-metallicity, and no doubt many other consequences, can arise in materials in very high magnetic fields. The quantum Hall effect is one consequence that has been thoroughly studied. How to describe a metal, and what phenomena to expect, in high fields needs theoretical attention.

Colossal Magnetoresistive Materials

Very unusual magnetotransport behavior discovered in Mn-based perovskites has stimulated intensive study of the many phonomena that occur, not only in this system but in a variety of transition metal oxides. The seed of the phenomenon seems to lie in the double exchange mechanism of Zener, but the remarkable range of other phenomena observed in these systems (such as magnetic field induced structural transitions) indicates that these materials will provide great scientific interest for some time. The behavior of the spin dynamics is central to their behavior.

Spin Electronics and Coherent Spin Systems

Manipulation of the spin degree of freedom of conduction electrons leads to a new form of electronics, now dubbed Spintronics. This form of current and voltage control uses low resistance (hence low voltage) magnetic metals rather than high resistance (high voltage) semiconductors such as Si. Thhe spin-polarized current also offers entirely new possibilities, such as manipulations of electronic signals by magnetic fields (or vice versa), or novel effects in ferromagnet/superconductor/ferromagnet sandwiches or multilayers. Quantum information storage and quantum computation are related phenomena that require further study.

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