Condensed Matter Physics Seminar
2 p.m., Thursday, April 22, 2004
Room 1201, Physics Building
Theory of Insulating Diluted Magnetic Semiconductors
Victor Galitski
(Condensed Matter Theory Center,
University of Maryland)
Abstract: An important recent experimental discovery is that a
large number of semiconductors become ferromagnetic when doped carefully with
1-10% of magnetic impurities. Some examples of such "diluted magnetic
semiconductors" (DMS) are GaMnAs, GeMn, InMnAs, GaMnN, GaMnP. Ferromagnetic
semiconductors, where magnetic and transport properties can be controlled and
tuned at will, are projected to form the basic ingredients in the emerging field
of spintronics. I will discuss in the talk, the physical mechanisms underlying
DMS ferromagnetism within a minimal theoretical model where magnetic coupling
between the impurity local moments is mediated by the semiconductor carriers
(mostly valence band or impurity band holes). Ferromagnetism occurs irrespective
of whether the system is metallic or insulating, leading to a novel percolation
theory of DMS ferromagnetism. I will emphasize the effects of disorder in DMS
materials which may lead to the appearance of a very unusual Griffiths phase
above the ferromagnetic transition point. Such a phase appears due to rare
disorder fluctuations, which yield non-perturbative intermittent contributions
to various thermodynamic and dynamic response quantities. Within the percolation
model, the complicated magnetic phase diagram of DMS systems will be discussed.
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Host: Das Sarma
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