90 research outputs found
The enigma of the pseudogap phase of the cuprate superconductors
The last few years have seen significant experimental progress in
characterizing the copper-based hole-doped high temperature superconductors in
the regime of low hole density, p. Quantum oscillations, NMR, X-ray, and STM
experiments have shed much light on the nature of the ordering at low
temperatures. We review evidence that the order parameter in the
non-Lanthanum-based cuprates is a d-form factor density-wave. This novel order
acts as an unexpected window into the electronic structure of the pseudogap
phase at higher temperatures in zero field: we argue in favor of a
`fractionalized Fermi liquid' (FL*) with 4 pockets of spin S=1/2, charge +e
fermions enclosing an area specified by p.Comment: 37 pages, 19 figures, 100+ references. Proceedings of the 50th
Karpacz Winter School of Theoretical Physics, 2-9 March 2014, Karpacz, Polan
Higgs criticality in a two-dimensional metal
We analyze a candidate theory for the strange metal near optimal hole-doping
in the cuprate superconductors. The theory contains a quantum phase transition
between metals with large and small Fermi surfaces of spinless fermions
carrying the electromagnetic charge of the electron, but the transition does
not directly involve any broken global symmetries. The two metals have emergent
SU(2) and U(1) gauge fields respectively, and the transition is driven by the
condensation of a real Higgs field, carrying a finite lattice momentum and an
adjoint SU(2) gauge charge. This Higgs field measures the local
antiferromagnetic correlations in a "rotating reference frame". We propose a
global phase diagram around this Higgs transition, and describe its
relationship to a variety of recent experiments on the cuprate superconductors.Comment: 30 pages, 7 figures; (v2) added new figur
Onset of many-body chaos in the model
The growth of commutators of initially commuting local operators diagnoses
the onset of chaos in quantum many-body systems. We compute such commutators of
local field operators with components in the -dimensional
nonlinear sigma model to leading order in . The system is taken to be in
thermal equilibrium at a temperature above the zero temperature quantum
critical point separating the symmetry broken and unbroken phases. The
commutator grows exponentially in time with a rate denoted . At
large the growth of chaos as measured by is slow because the
model is weakly interacting, and we find . The
scaling with temperature is dictated by conformal invariance of the underlying
quantum critical point. We also show that operators grow ballistically in space
with a "butterfly velocity" given by where is the
Lorentz-invariant speed of particle excitations in the system. We briefly
comment on the behavior of and in the neighboring symmetry
broken and unbroken phases.Comment: (1+55) pages, 13 figures; (v2) Final published versio
Synchronization of oscillators with long range power law interactions
We present analytical calculations and numerical simulations for the
synchronization of oscillators interacting via a long range power law
interaction on a one dimensional lattice. We have identified the critical value
of the power law exponent across which a transition from a
synchronized to an unsynchronized state takes place for a sufficiently strong
but finite coupling strength in the large system limit. We find .
Frequency entrainment and phase ordering are discussed as a function of . The calculations are performed using an expansion about the aligned
phase state (spin-wave approximation) and a coarse graining approach. We also
generalize the spin-wave results to the {\it d}-dimensional problem.Comment: Final published versio
Quantum oscillations in insulators with neutral Fermi surfaces
We develop a theory of quantum oscillations in insulators with an emergent
fermi sea of neutral fermions minimally coupled to an emergent gauge
field. As pointed out by Motrunich (Phys. Rev. B 73, 155115 (2006)), in the
presence of a physical magnetic field the emergent magnetic field develops a
non-zero value leading to Landau quantization for the neutral fermions. We
focus on the magnetic field and temperature dependence of the analogue of the
de Haas-van Alphen effect in two- and three-dimensions. At temperatures above
the effective cyclotron energy, the magnetization oscillations behave similarly
to those of an ordinary metal, albeit in a field of a strength that differs
from the physical magnetic field. At low temperatures the oscillations evolve
into a series of phase transitions. We provide analytical expressions for the
amplitude and period of the oscillations in both of these regimes and simple
extrapolations that capture well their crossover. We also describe oscillations
in the electrical resistivity of these systems that are expected to be
superimposed with the activated temperature behavior characteristic of their
insulating nature and discuss suitable experimental conditions for the
observation of these effects in mixed-valence insulators and triangular lattice
organic materials.Comment: 20 pages, 9 figures, 1 tabl
Mixed-valence insulators with neutral Fermi surfaces
Samarium hexaboride is a classic three-dimensional mixed valence system with
a high-temperature metallic phase that evolves into a paramagnetic charge
insulator below 40 kelvin. A number of recent experiments have suggested the
possibility that the low-temperature insulating bulk hosts electrically neutral
gapless fermionic excitations. Here we show that a possible ground state of
strongly correlated mixed valence insulators - composite exciton Fermi liquid -
hosts a three dimensional Fermi surface of a neutral fermion, that we name the
"composite exciton". We describe the mechanism responsible for the formation of
such excitons, discuss the phenomenology of the composite exciton Fermi liquids
and make comparison to experiments in SmB.Comment: Final published versio
Effect of magnetization on the tunneling anomaly in compressible quantum Hall states
Tunneling of electrons into a two-dimensional electron system is known to
exhibit an anomaly at low bias, in which the tunneling conductance vanishes due
to a many-body interaction effect. Recent experiments have measured this
anomaly between two copies of the half-filled Landau level as a function of
in-plane magnetic field, and they suggest that increasing spin polarization
drives a deeper suppression of tunneling. Here we present a theory of the
tunneling anomaly between two copies of the partially spin-polarized
Halperin-Lee-Read state, and we show that the conventional description of the
tunneling anomaly, based on the Coulomb self-energy of the injected charge
packet, is inconsistent with the experimental observation. We propose that the
experiment is operating in a different regime, not previously considered, in
which the charge-spreading action is determined by the compressibility of the
composite fermions.Comment: (5+1) pages, 1 figure; (v2) minor changes and added reference to our
accompanying paper arXiv:1712.02357; (v3) Final published versio
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