31 research outputs found
Branch-cut Singularities in Thermodynamics of Fermi Liquid Systems
The recently measured spin susceptibility of the two dimensional electron gas
exhibits a strong dependence on temperature, which is incompatible with the
standard Fermi liquid phenomenology. Here we show that the observed temperature
behavior is inherent to ballistic two dimensional electrons. Besides the
single-particle and collective excitations, the thermodynamics of Fermi liquid
systems includes effects of the branch-cut singularities originating from the
edges of the continuum of pairs of quasiparticles. As a result of the
rescattering induced by interactions, the branch-cut singularities generate
non-analyticities in the thermodynamic potential which reveal themselves in
anomalous temperature dependences. Calculation of the spin susceptibility in
such a situation requires a non-perturbative treatment of the interactions. As
in high-energy physics, a mixture of the collective excitations and pairs of
quasiparticles can be effectively described by a pole in the complex momentum
plane. This analysis provides a natural explanation for the observed
temperature dependence of the spin susceptibility, both in sign and magnitude.Comment: 8 pages, 3 figure
Theory of Superconductivity in the Cuprates
The quantum critical fluctuations of the time-reversal breaking order
parameter which is observed in the pseudogap regime of the Cuprates are shown
to couple to the lattice equivalent of the local angular momentum of the
fermions. Such a coupling favors scattering of fermions through angles close to
which is unambiguously shown to promote d-wave pairing. The right
order of magnitude of both and the normalized zero temperature gap
are calculated using the same fluctuations which give the
temperature, frequency and momentum dependence of the the anomalous normal
state properties for dopings near the quantum-critical value and with two
parameters extracted from fit to such experiments.Comment: Accepted for publication in PRB with the title "Theory of the
coupling of quantum-critical fluctuations to fermions and d-wave
superconductivity in the cuprates
Thermodynamic constraints on the amplitude of quantum oscillations
Magneto-quantum oscillation experiments in high temperature superconductors
show a strong thermally-induced suppression of the oscillation amplitude
approaching critical dopings---in support of a quantum critical origin of their
phase diagrams. We suggest that, in addition to a thermodynamic mass
enhancement, these experiments may directly indicate the increasing role of
quantum fluctuations that suppress the oscillation amplitude through inelastic
scattering. We show that the traditional theoretical approaches beyond
Lifshitz-Kosevich to calculate the oscillation amplitude in correlated metals
result in a contradiction with the third law of thermodynamics and suggest a
way to rectify this problem.Comment: PRB Rapid commun. (2017
Screening of point charge impurities in highly anisotropic metals: application to spin relaxation in underdoped cuprates
We calculate the screening charge density distribution due to a point charge,
such as that of a positive muon (), placed between the planes of a
highly anisotropic layered metal. In underdoped hole cuprates the screening
charge converts the charge density in the metallic-plane unit cells in the
vicinity of the to nearly its value in the insulating state. The
current-loop ordered state observed by polarized neutron diffraction then
vanishes in such cells, and also in nearby cells over a distance of order the
intrinsic correlation length of the loop-ordered state. This in turn strongly
suppresses the loop-current field at the site. We estimate this
suppressed field in underdoped YBaCuO and
LaSrCuO, and find consistency with the observed 0.2--0.3 G
field in the former case and the observed upper bound of 0.2 G in the
latter case. This resolves the controversy between the neutron diffraction and
SR experiments. The screening calculation also has relevance for the
effect of other charge impurities in the cuprates, such as the dopants
themselves
Weak Ferromagnetism Accompanying Loop current order in Underdoped Cuprates
We discuss the necessary symmetry conditions and the different ways in which
they can be physically realized for the occurrence of ferromagnetism
accompanying the loop current orbital magnetic order observed by polarized
neutron-diffraction experiments or indeed any other conceivable principal order
in the under-doped phase of cuprates. We contrast the Kerr effect experiments
in single crystals observing ferromagnetism with the direct magnetization
measurements in large powder samples, which do not observe it. We also suggest
experiments to resolve the differences among the experiments, all of which we
believe to be correct.Comment: 4 page
Universality of the single-particle spectra of cuprate superconductors
All the available data for the dispersion and linewidth of the
single-particle spectra above the superconducting gap and the pseudogap in
metallic cuprates for any doping has universal features. The linewidth is
linear in energy below a scale and constant above. The cusp in the
linewidth at mandates, due to causality, a "waterfall", i.e., a
vertical feature in the dispersion. These features are predicted by a recent
microscopic theory. We find that all data can be quantitatively fitted by the
theory with a coupling constant and an upper cutoff at
which vary by less than 50% among the different cuprates and for varying
dopings. The microscopic theory also gives these values to within factors of
O(2).Comment: 4 pages, 4 figures; accepted by Phys. Rev. Let
Extent of Fermi-surface reconstruction in the high-temperature superconductor HgBaCuO
High magnetic fields have revealed a surprisingly small Fermi-surface in
underdoped cuprates, possibly resulting from Fermi-surface reconstruction due
to an order parameter that breaks translational symmetry of the crystal
lattice. A crucial issue concerns the doping extent of this state and its
relationship to the principal pseudogap and superconducting phases. We employ
pulsed magnetic field measurements on the cuprate HgBaCuO to
identify signatures of Fermi surface reconstruction from a sign change of the
Hall effect and a peak in the temperature-dependent planar resistivity. We
trace the termination of Fermi-surface reconstruction to two hole
concentrations where the superconducting upper critical fields are found to be
enhanced. One of these points is associated with the pseudogap end-point near
optimal doping. These results connect the Fermi-surface reconstruction to both
superconductivity and the pseudogap phenomena.Comment: 5 pages. 3 Figures. PNAS (2020
One-Component Order Parameter in URuSi Uncovered by Resonant Ultrasound Spectroscopy and Machine Learning
The unusual correlated state that emerges in URuSi below T =
17.5 K is known as "hidden order" because even basic characteristics of the
order parameter, such as its dimensionality (whether it has one component or
two), are "hidden". We use resonant ultrasound spectroscopy to measure the
symmetry-resolved elastic anomalies across T. We observe no anomalies in
the shear elastic moduli, providing strong thermodynamic evidence for a
one-component order parameter. We develop a machine learning framework that
reaches this conclusion directly from the raw data, even in a crystal that is
too small for traditional resonant ultrasound. Our result rules out a broad
class of theories of hidden order based on two-component order parameters, and
constrains the nature of the fluctuations from which unconventional
superconductivity emerges at lower temperature. Our machine learning framework
is a powerful new tool for classifying the ubiquitous competing orders in
correlated electron systems