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DTSTAMP:20260908T141351Z
LAST-MODIFIED:20170906T184607Z
DTSTART:20170808T190000Z
DTEND:20170808T200000Z
UID:event1780@bu.edu
URL:http://physics.bu.edu/internal/events/show/1780
SUMMARY:Pseudoparticle approach to strongly correlated systems
DESCRIPTION:Featuring Garry Goldstein\, University of Cambridge \nHosted by
	: Claudio Rebbi\n\nPart of the Condensed Matter Theory Seminar Series.\n\nI
	n the first part we present a slave-particle meanfield study of the mixed
	 boson+fermion quantum dimer model introduced by Punk\, Allais\, and Sachde
	v [PNAS 112\, 9552 (2015)] to describe the physics of the pseudogap phase i
	n cuprate superconductors. Our analysis naturally leads to four charge e fe
	rmion pockets whose total area is equal to the hole doping p\, for a range 
	of parameters consistent with the t−J model for high temperature supercon
	ductivity. Here we find that the dimers are unstable to d-wave superconduc
	tivity at low temperatures. The region of the phase diagram with d-wave rat
	her than s-wave superconductivity matches well with the appearance of the f
	our fermion pockets. In the superconducting regime\, the dispersion contain
	s eight Dirac cones along the diagonals of the Brillouin zone. \n\nIn the s
	econd part we present an approach for studying systems with hard constraint
	s such that certain positive semidefinite operators must vanish. The diffi
	culty with mean-field treatments of such cases is that imposing that the c
	onstraint is zero only in average is problematic for a quantity that is alw
	ays non-negative. We reformulate the hard constraints by adding an auxiliar
	y system such that some of the states to be projected out from the total sy
	stem are at finite negative energy and the rest at finite positive energy
	\, we dubbed this method the soft constraint. This auxiliary system comes w
	ith an extra coupling that we are free to vary and that parametrizes a whol
	e family of mean-field theories. We argue that this variational-type param
	eter for the family of mean-field theories should be fixed by matching a 
	given experimental observation\, with the quality of the resulting mean-fi
	eld approximation measured by how it fits other data. We test these ideas 
	in the well-understood single-impurity Kondo problem\, where we fix the pa
	rameter via the TK (Kondo temperature) obtained from the magnetic susceptib
	ility value\, and score the quality of the approximation by its predicted W
	ilson ratio. We also study a continuum version of the soft constraint metho
	d. To show its usefulness we study the Laughlin fractional quantum Hall sta
	tes at filling fraction ν = 1 2m+1. We produce the Landau Ginzburg Cherns
	 Simons theory for hard core bosons(arising in this formulation of the Laug
	hlin functions0 for these filling fractions. We handle the hard core const
	raint explicitly and show that to leading order it is equivalent to a theor
	y of soft core bosons with an effective density density interaction betwee
	n the particles of the form of a Coulomb interaction coming from the electr
	ons in the original theory and an effective interaction coming from the ha
	rd core constraint that can be tuned by hand which we denote by λ(q\,ω). 
	We show that we can use this effective interaction to reproduce the known 
	spectra of both inter Landau level and intra Landau level neutral excitatio
	ns for the Laughlin states as well as the energy gap to charged excitations
	 and the electromagnetic response tensor.
LOCATION:SCI 328\, 590 Commonwealth Avenue\, 02215
STATUS:CONFIRMED
CLASS:PUBLIC
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