57 research outputs found
Quantum critical point shifts under superconductivity: the pnictides and the cuprates
We compare the position of an ordering transition in a metal to that in a
superconductor. For the spin density wave (SDW) transition, we find that the
quantum critical point shifts by order |Delta|, where Delta is pairing
amplitude, so that the region of SDW order is smaller in the superconductor
than in the metal. This shift is larger than the ~ |Delta|^2 shift predicted by
theories of competing orders which ignore Fermi surface effects. For
Ising-nematic order, the shift from Fermi surface effects remains of order
|Delta|^2. We discuss implications of these results for the phase diagrams of
the cuprates and the pnictides. We conclude that recent observations imply that
the Ising-nematic order is tied to the square of the SDW order in the
pnictides, but not in the cuprates.Comment: 11 pages, 8 figure
Exciton condensations in thin film topological insulator
We study the many-body physics in thin film topological band insulator, where
the inter-edge Coulomb interaction can lead to an exciton condensation
transition. We investigate the universality class of the exciton condensation
quantum critical point. With different chemical potentials and interactions,
the exciton condensation can belong to z = 2 mean field, or 3d XY, or
Yukawa-Higgs universality classes. The interplay between exciton condensate and
the time-reversal symmetry breaking is also discussed. Predictions of our work
can be tested experimentally by tuning the chemical potentials on both surfaces
of the thin film through gate voltage. We also show that all the analysis of
the exciton condensate can be directly applied to a spin-triplet superconductor
phase with attractive inter-edge interaction.Comment: 5 pages, 3 figures, published versio
Novel Quantum Criticality in Two Dimensional Topological Phase transitions
Topological quantum phase transitions intrinsically intertwine
self-similarity and topology of many-electron wave-functions, and divining them
is one of the most significant ways to advance understanding in condensed
matter physics. Our focus is to investigate an unconventional class of the
transitions between insulators and Dirac semimetals whose description is beyond
conventional pseudo relativistic Dirac Hamiltonian. At the transition without
the long-range Coulomb interaction, the electronic energy dispersion along one
direction behaves like a relativistic particle, linear in momentum, but along
the other direction it behaves like a non-relativistic particle, quadratic in
momentum. Various physical systems ranging from TiO-VO
heterostructure to organic material -(BEDT-TTF) I under
pressure have been proposed to have such anisotropic dispersion relation. Here,
we discover a novel quantum criticality at the phase transition by
incorporating the 1/r long-range Coulomb interaction. Unique interplay between
the Coulomb interaction and electronic critical modes enforces not only the
anisotropic renormalization of the Coulomb interaction but also marginally
modified electronic excitation. In connection with experiments, we investigate
several striking effects in physical observables of our novel criticality.Comment: 12 pages + 31 pages, 5 figures, 2 tables, published versio
A New Type of Quantum Criticality in the Pyrochlore Iridates
Magnetic fluctuations and electrons couple in intriguing ways in the vicinity
of zero temperature phase transitions - quantum critical points - in conducting
materials. Quantum criticality is implicated in non-Fermi liquid behavior of
diverse materials, and in the formation of unconventional superconductors. Here
we uncover an entirely new type of quantum critical point describing the onset
of antiferromagnetism in a nodal semimetal engendered by the combination of
strong spin-orbit coupling and electron correlations, and which is predicted to
occur in the iridium oxide pyrochlores. We formulate and solve a field theory
for this quantum critical point by renormalization group techniques, show that
electrons and antiferromagnetic fluctuations are strongly coupled, and that
both these excitations are modified in an essential way. This quantum critical
point has many novel features, including strong emergent spatial anisotropy, a
vital role for Coulomb interactions, and highly unconventional critical
exponents. Our theory motivates and informs experiments on pyrochlore iridates,
and constitutes a singular realistic example of a non-trivial quantum critical
point with gapless fermions in three dimensions.Comment: 5 pages + 8 pages of Supplementary Material, 3 figures + 1
supplementary figur
Superfluid-insulator transitions of the Fermi gas with near-unitary interactions in a periodic potential
We consider spin-1/2 fermions of mass m with interactions near the unitary
limit. In an applied periodic potential of amplitude V and period a, and with a
density of an even integer number of fermions per unit cell, there is a
second-order quantum phase transition between superfluid and insulating ground
states at a critical V=Vc. We compute the universal ratio Vc m a^2 / h^2 at
N=infinity in a model with Sp(2N) spin symmetry. The insulator interpolates
between a band insulator of fermions and a Mott insulator of fermion pairs. We
discuss implications for recent experiments.Comment: 5 pages, 3 figure
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