46 research outputs found
Charge Friedel oscillations in a Mott insulator
When a metal undergoes a transition to an insulator it will lose its
electronic Fermi surface. Interestingly in some situations a `ghost' Fermi
surface of electrically neutral spin carrying fermions may survive into the
insulator. Such a novel ghost Fermi surface has been proposed to underlie the
properties of a few different materials but its direct detection has proven
elusive. In this paper we show that the ghost Fermi surface leads to slowly
decaying spatial oscillations of the electron density near impurities or other
defects. These and related oscillations stem from the sharpness of the ghost
Fermi surface and are direct analogs of the familiar Friedel oscillations in
metals. The oscillation period contains geometric information about the shape
of the ghost Fermi surface which can be potentially exploited to detect its
existence.Comment: 6 pages, 3 figure
Stripe melting and quantum criticality in correlated metals
We study theoretically quantum melting transitions of stripe order in a
metallic environment, and the associated reconstruction of the electronic Fermi
surface. We show that such quantum phase transitions can be continuous in
situations where the stripe melting occurs by proliferating pairs of
dislocations in the stripe order parameter without proliferating single
dislocations. We develop an intuitive picture of such phases as "Stripe Loop
Metals" where the fluctuating stripes form closed loops of arbitrary size at
long distances. We obtain a controlled critical theory of a few different
continuous quantum melting transitions of stripes in metals . At such a
(deconfined) critical point the fluctuations of the stripe order parameter are
strongly coupled, yet tractable. They also decouple dynamically from the
Fermi-surface. We calculate many universal properties of these quantum critical
points. In particular we find that the full Fermi-surface and the associated
Landau quasiparticles remain sharply defined at the critical point. We discuss
the phenomenon of Fermi surface reconstruction across this transition and the
effect of quantum critical stripe fluctuations on the superconducting
instability. We study possible relevance of our results to several phenomena in
the cuprates.Comment: 22 pages, 22 figure
The two-impurity Kondo model with spin-orbit interactions
We study the two-impurity Kondo model (TIKM) in two dimensions with
spin-orbit coupled conduction electrons. In the first part of the paper we
analyze how spin-orbit interactions of Rashba as well as Dresselhaus type
influence the Kondo and RKKY interactions in the TIKM, generalizing results
obtained by H. Imamura {\em et al.} (2004) and J. Malecki (2007). Using our
findings we then explore the effect from spin-orbit interactions on the
non-Fermi liquid quantum critical transition between the RKKY-singlet and
Kondo-screened RKKY-triplet states. We argue that spin-orbit interactions under
certain conditions produce a line of critical points exhibiting the same
leading scaling behavior as that of the ordinary TIKM. In the second part of
the paper we shift focus and turn to the question of how spin-orbit
interactions affect the entanglement between two localized RKKY-coupled spins
in the parameter regime where the competition from the direct Kondo interaction
can be neglected. Using data for a device with two spinful quantum dots
patterned in a gated InAs heterostructure we show that a gate-controlled
spin-orbit interaction may drive a maximally entangled state to one with
vanishing entanglement, or vice versa (as measured by the concurrence). This
has important implications for proposals using RKKY interactions for nonlocal
control of qubit entanglement in semiconductor heterostructures.Comment: Revised version; new title and introduction in response to referee
suggestion, expanded discussion of results, added references. 14 pages, 5
figure
Spin and pair density wave glasses
Spontaneous breaking of translational symmetry---known as `density wave'
order---is common in nature. However such states are strongly sensitive to
impurities or other forms of frozen disorder leading to fascinating glassy
phenomena. We analyze impurity effects on a particularly ubiquitous form of
broken translation symmetry in solids: a Spin Density Wave (SDW) with spatially
modulated magnetic order. Related phenomena occur in Pair Density Wave (PDW)
superconductors where the superconducting order is spatially modulated. For
weak disorder, we find that the SDW / PDW order can generically give way to a
SDW / PDW glass---new phases of matter with a number of striking properties,
which we introduce and characterize here. In particular, they exhibit an
interesting combination of conventional (symmetry-breaking) and spin glass
(Edwards-Anderson) order. This is reflected in the dynamic response of such a
system, which---as expected for a glass---is extremely slow in certain
variables, but---surprisingly---is fast in others. Our results apply to all
uniaxial metallic SDW systems where the ordering vector is incommensurate with
the crystalline lattice. In addition, the possibility of a PDW glass has
important consequences for some recent theoretical and experimental work on
.Comment: 10 pages, 5 figure
Bosonic Analogue of Dirac Composite Fermi Liquid
We introduce a particle-hole-symmetric metallic state of bosons in a magnetic
field at odd-integer filling. This state hosts composite fermions whose energy
dispersion features a quadratic band touching and corresponding Berry
flux protected by particle-hole and discrete rotation symmetries. We also
construct an alternative particle-hole symmetric state---distinct in the
presence of inversion symmetry---without Berry flux. As in the Dirac composite
Fermi liquid introduced by Son, breaking particle-hole symmetry recovers the
familiar Chern-Simons theory. We discuss realizations of this phase both in 2D
and on bosonic topological insulator surfaces, as well as signatures in
experiments and simulations.Comment: 8 pages, 5 figure
Symmetry and duality in bosonization of two-dimensional Dirac fermions
Recent work on a family of boson-fermion mappings has emphasized the
interplay of symmetry and duality: Phases related by a particle-vortex duality
of bosons (fermions) are related by time-reversal symmetry in their fermionic
(bosonic) formulation. We present exact mappings for a number of concrete
models that make this property explicit on the operator level. We illustrate
the approach with one- and two-dimensional quantum Ising models, and then
similarly explore the duality web of complex bosons and Dirac fermions in (2+1)
dimensions.Comment: 31 pages, 9 figure
Anomalous Quasiparticle Symmetries and Non-Abelian Defects on Symmetrically Gapped Surfaces of Weak Topological Insulators
We show that boundaries of 3D weak topological insulators can become gapped
by strong interactions while preserving all symmetries, leading to Abelian
surface topological order. The anomalous nature of the weak topological
insulators manifests itself in a non-trivial action of symmetries on the
quasiparticles; most strikingly, translations change the anyon types in a
manner impossible in strictly 2D systems with the same symmetry. As a further
consequence, screw dislocations form non-Abelian defects that trap
parafermion zero modes.Comment: 6 pages, 4 figure
Spin-charge separation in two dimensions: spinon-chargon gauge theories from duality
Strong interactions between electrons in two dimensions can realize phases
where their spins and charges separate. We capture this phenomenon within a
dual formulation. Focusing on square lattices, we analyze the long-wavelength
structure of vortices when the microscopic particles -- electrons or spinful
bosons -- are near half-filling. These conditions lead to a compact gauge
theory of spinons and chargons, which arise as the fundamental topological
defects of the low-energy vortices. The gauge theory formulation is
particularly suitable for studying numerous exotic phases and transitions. We
support the general analysis by an exact implementation of the duality of a
coupled-wire array. Finally, we demonstrate how the latter can be exploited to
construct parent Hamiltonians for fractional phases and their transitions
Vacancies in generic Kitaev spin liquids
The Kitaev honeycomb model supports gapless and gapped quantum spin liquid
phases. Its exact solvability relies on extensively many locally conserved
quantities. Any real-world manifestation of these phases would include
imperfections in the form of disorder and interactions that break
integrability. We show that the latter qualitatively alters the properties of
vacancies in the gapless Kitaev spin liquid: (i) Isolated vacancies carry a
magnetic moment, which is absent in the exactly solvable case. (ii) Pairs of
vacancies on even/opposite sublattices gap each other with distinct power laws
that reveal the presence of emergent gauge flux.Comment: 8 pages, 9 figure