6,414 research outputs found
Effective theory of Fermi pockets in fluctuating antiferromagnets
We describe fluctuating two-dimensional metallic antiferromagnets by
transforming to a rotating reference frame in which the electron spin
polarization is measured by its projections along the local antiferromagnetic
order. This leads to a gauge-theoretic description of an `algebraic charge
liquid' involving spinless fermions and a spin S=1/2 complex scalar. We propose
a phenomenological effective lattice Hamiltonian which describes the binding of
these particles into gauge-neutral, electron-like excitations, and describe its
implications for the electron spectral function across the entire Brillouin
zone. We discuss connections of our results to photoemission experiments in the
pseudogap regime of the cuprate superconductors.Comment: 28 pages, 8 figure
Fermi surfaces and Luttinger's theorem in paired fermion systems
We discuss ground state properties of a mixture of two fermion species which
can bind to form a molecular boson. When the densities of the fermions are
unbalanced, one or more Fermi surfaces can appear: we describe the constraints
placed by Luttinger's theorem on the volumes enclosed by these surfaces in such
Bose-Fermi mixtures. We also discuss the nature of the quantum phase
transitions involving changes in the number of Fermi surfaces.Comment: 7 pages with one figure embedded. V2: Minor modifications. Final
version as appeared in prin
Metallic spin glasses
Recent work on the zero temperature phases and phase transitions of strongly
random electronic system is reviewed. The transition between the spin glass and
quantum paramagnet is examined, for both metallic and insulating systems.
Insight gained from the solution of infinite range models leads to a quantum
field theory for the transition between a metallic quantum paramagnetic and a
metallic spin glass. The finite temperature phase diagram is described and
crossover functions are computed in mean field theory. A study of fluctuations
about mean field leads to the formulation of scaling hypotheses.Comment: Contribution to the Proceedings of the ITP Santa Barbara conference
on Non-Fermi liquids, 25 pages, requires IOP style file
Fluctuating spin density waves in metals
Recent work has used a U(1) gauge theory to describe the physics of Fermi
pockets in the presence of fluctuating spin density wave order. We generalize
this theory to an arbitrary band structure and ordering wavevector. The
transition to the large Fermi surface state, without pockets induced by local
spin density wave order, is described by embedding the U(1) gauge theory in a
SU(2) gauge theory. The phase diagram of the SU(2) gauge theory shows that the
onset of spin density wave order in the Fermi liquid occurs either directly, in
the framework discussed by Hertz, or via intermediate non-Fermi liquid phases
with Fermi surfaces of fractionalized excitations. We discuss application of
our results to the phase diagram of the cuprates.Comment: 15 pages, 2 figures; (v2) Improved figure
Frustrated quantum Ising spins simulated by spinless bosons in a tilted lattice: from a quantum liquid to antiferromagnetic order
We study spinless bosons in a decorated square lattice with a near-diagonal
tilt. The resonant subspace of the tilted Mott insulator is described by an
effective Hamiltonian of frustrated quantum Ising spins on a non-bipartite
lattice. This generalizes an earlier proposal for the unfrustrated quantum
Ising model in one dimension which was realized in a recent experiment on
ultracold Rb atoms in an optical lattice. Very close to diagonal tilt,
we find a quantum liquid state which is continuously connected to the
paramagnet. Frustration can be reduced by increasing the tilt angle away from
the diagonal, and the system undergoes a transition to an antiferromagnetically
ordered state. Using quantum Monte Carlo simulations and exact diagonalization,
we find that for realistic system sizes the antiferromagnetic order appears to
be quasi-one-dimensional; however, in the thermodynamic limit the order is
two-dimensional.Comment: 27 pages, 14 figure
Instabilities near the onset of spin density wave order in metals
We discuss the low energy theory of two-dimensional metals near the onset of
spin density wave order. It is well known that such a metal has a
superconducting instability induced by the formation of spin-singlet pairs of
electrons, with the pairing amplitude changing sign between regions of the
Fermi surface connected by the spin density wave ordering wavevector. Here we
review recent arguments that there is an additional instability which is nearly
as strong: towards the onset of a modulated bond order which is locally an
Ising-nematic order. This new instability is a consequence of an emergent
"pseudospin" symmetry of the low energy theory---the symmetry maps the
sign-changing pairing amplitude to the bond order parameter.Comment: 14 pages, 9 figures; contribution to the special issue of the New
Journal of Physics on "Fermiology of Cuprates", edited by Mike Norman and
Cyril Prous
Wilson Loops in Non-Compact U(1) Gauge Theories at Criticality
We study the properties of Wilson loops in three dimensional non-compact U(1)
gauge theories with global abelian symmetries. We use duality in the continuum
and on the lattice, to argue that close to the critical point between the Higgs
and Coulomb phases, all correlators of the Wilson loops are periodic functions
of the Wilson loop charge, Q. The period depends on the global symmetry of the
theory, which determines the magnetic flux carried by the dual particles. For
single flavour scalar electrodynamics, the emergent period is Q = 1. In the
general case of N complex scalars with a U(1)^{N-1} global symmetry, the period
is Q = N. We also give some arguments why this phenomenon does not generalize
to theories with a full non-abelian SU(N) symmetry, where no periodicity in Q
is expected. Implications for lattice simulations, as well as for physical
systems, such as easy plane antiferromagnets and disordered superfluids, are
noted.Comment: 25 pages, 1 figur
Valence bond solid order near impurities in two-dimensional quantum antiferromagnets
Recent scanning tunnelling microscopy (STM) experiments on underdoped
cuprates have displayed modulations in the local electronic density of states
which are centered on a Cu-O-Cu bond (Kohsaka et. al., cond-mat/0703309). As a
paradigm of the pinning of such bond-centered ordering in strongly correlated
systems, we present the theory of valence bond solid (VBS) correlations near a
single impurity in a square lattice antiferromagnet. The antiferromagnet is
assumed to be in the vicinity of a quantum transition from a magnetically
ordered Neel state to a spin-gap state with long-range VBS order. We identify
two distinct classes of impurities: i) local modulation in the exchange
constants, and ii) a missing or additional spin, for which the impurity
perturbation is represented by an uncompensated Berry phase. The `boundary'
critical theory for these classes is developed: in the second class we find a
`VBS pinwheel' around the impurity, accompanied by a suppression in the VBS
susceptibility. Implications for numerical studies of quantum antiferromagnets
and for STM experiments on the cuprates are noted.Comment: 41 pages, 6 figures; (v2) Minor changes in terminology, added
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