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BEGIN:VEVENT
DTSTAMP:20261020T094542Z
LAST-MODIFIED:20151006T135231Z
DTSTART:20151014T160000Z
DTEND:20151014T170000Z
UID:event1502@bu.edu
URL:http://physics.bu.edu/internal/events/show/1502
SUMMARY:Building and understanding magnetic nano-structures\, one atom at a
	 time
DESCRIPTION:Featuring A. Feiguin\, Northeastern University\n\nPart of the C
	ondensed Matter Theory Seminar Series.\n\nUnderstanding magnetism is a comp
	lex undertaking: it relies on our knowledge of the exact position of magnet
	ic ions in a crystal and their interactions. More important\, at its core\,
	 this is fundamentally a quantum problem.\nIn general\, knowledge of the ma
	gnetic properties of a single atom will not tell much about the magnetic pr
	operties of a material\, and requires understanding the cooperative effects
	 of many degrees of freedom\, particularly the spin. Starting from the chem
	istry\, "cooking" a single crystal with enough purity and without defects i
	s already an enormous challenge. In addition\, being able to "design" a mat
	erial with the desired geometry and interactions is only possible in a few 
	cases\, usually using organic molecules as a fundamental building block. In
	 the past decade we have witnessed enormous progress in experiments that co
	nsist of placing magnetic atoms at predetermined positions on substrates\, 
	and building magnetic nanostructures\, one atom at a time. The electrons in
	 the substrate mediate the interactions between the spins\, and scanning tu
	nneling microscopy allows one to study their properties. In order to unders
	tand these interactions\, we rely on a theory developed decades ago by Rude
	rman\, Kittel\, Kasuya\, and Yosida\, dubbed "RKKY theory"\, which applies 
	when the spins are classical. The quantum nature of the electronic spin int
	roduces another degree of complexity\, and competition with other quantum p
	henomena: the Kondo effect. This competition is quite subtle and non-trivia
	l\, and can only be studied by numerical means. We have found that there is
	 a critical distance at which the Kondo effect dominates\, translating into
	 a finite range for the RKKY interaction. We study this mechanism on differ
	ent lattice geometries in 2 and 3 dimensions by introducing an exact mappin
	g onto an effective one-dimensional problem that we can solve with the dens
	ity matrix renormalization group method (DMRG). We show a clear and departu
	re from the conventional RKKY theory\, and important differences that can b
	e attributed to the dimensionality and geometry. In particular\, for dimens
	ion d>1\, Kondo physics dominates even at short distances\, while the ferro
	magnetic RKKY state is energetically unfavorable\, which can have important
	 implications in our understanding of heavy fermion materials and magnetic 
	semiconductors.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
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