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BEGIN:VEVENT
DTSTAMP:20261020T043607Z
LAST-MODIFIED:20171013T200332Z
DTSTART:20171113T170000Z
DTEND:20171113T180000Z
UID:event1846@bu.edu
URL:http://physics.bu.edu/internal/events/show/1846
SUMMARY:Magnetic Field Effects in Low-Dimensional Quantum Magnets
DESCRIPTION:Featuring Adam Iaizzi\n\nPart of the PhD Final Oral Exams.\n\nE
	xamining Committee: Anders Sandvik\, David Campbell\, Robert Carey\, Shyam 
	Erramilli\, Claudio Chamon\n\nAbstract:\n\nWe present a comprehensive study
	 of a low-dimensional spin-half quantum antiferromagnet\, the J-Q model\, i
	n the presence of an external  (Zeeman)  magnetic field using numerical \nm
	ethods\, chiefly stochastic series expansion  quantum Monte Carlo with dire
	cted loop updates and \nquantum  replica exchange. The  J-Q model is a many
	-body Hamiltonian acting on a lattice of \nlocalized spin-half degrees of f
	reedom; it augments the Heisenberg exchange with a \nfour-spin interaction 
	of strength Q.  This model has been extensively studied at zero field\, whe
	re \nthe Q term drives a quantum phase transition  from a Neel-like  state 
	 to  a valence-bond  solid (a nonmagnetic state consisting of a long-range-
	ordered arrangement of local singlet bonds \nbetween sites).  This transiti
	on is believed to be an example of deconfined quantum criticality\, \nwhere
	 the excitations are spinons—exotic spin-half bosons.  We study the J-Q m
	odel in the presence \nof a magnetic field in both one and two dimensions. 
	In one dimension\, there is metamagnetism  above \na critical coupling rati
	o (Q/J )min.  Metamagnetism is a first-order quantum phase transition \ncha
	racterized by discontinuities in the magnetization as a function of field (
	magnetization jumps). \n We derive an exact expression for (Q/J)min  = 2/9\
	, and show that the metamagnetism is caused by \nthe onset of attractive in
	teractions between magnons (flipped spins on a polarized\nbackground). We  
	predict  that the same mechanisms  will produce metamagnetism in the unfrus
	trated \nantiferromagnetic J1-J2  model with anisotropy. Below (Q/J)min\,\n
	the saturation transition is continuous and we show that it is governed by 
	the expected zero-scale-factor universality.  In two dimensions\, we also f
	ind metamagnetism above a \ncritical coupling ratio (Q/J)min=0.417\, caused
	 by the same mechanism  as in the one-dimensional \ncase.  In two dimension
	s we also show evidence of an anomalous temperature dependence of specific 
	\nheat arising from field-induced Bose-Einstein condensation of spinons at 
	the deconfined quantum \ncritical point.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
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