142 research outputs found
Chiral spin-order in some purported Kitaev spin-liquid compounds
We examine recent magnetic torque measurements in two compounds,
-LiIrO and RuCl, which have been discussed as possible
realizations of the Kitaev model. The analysis of the reported discontinuity in
torque, as an external magnetic field is rotated across the axis in both
crystals, suggests that they have a translationally-invariant chiral spin-order
of the from in the ground
state and persisting over a very wide range of magnetic field and temperature.
An extra-ordinary dependence of the torque for small fields, beside
the usual part, is predicted due to the chiral spin-order, and found to
be consistent with experiments upon further analysis of the data. Other
experiments such as inelastic scattering and thermal Hall effect and several
questions raised by the discovery of chiral spin-order, including its
topological consequences are discussed.Comment: Clearer figures of the experimental data provided. Also clearer
exposition and comment on related recent wor
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
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
Multi-flavor quantum criticality
In a quantum critical metal, the electronic density of states, or
quasiparticle mass on the Fermi surface, is strongly enhanced through
electronic correlations. The density of states in the quantum critical
unconventional superconductor CeCoIn, can be readily accessed in the normal
state because all energy scales are small. However, the experimental challenges
associated with large nuclear specific heat and long nuclear spin-lattice
relaxation times have impeded unveiling a more detailed physical picture. Here
we report an extensive thermal impedance spectroscopy study of CeCoIn that
assesses the density of states in two independent ways, via the nuclear
spin-lattice relaxation rate and via the specific heat. We establish that the
temperature- and magnetic field dependence of the nuclear spin-lattice
relaxation rate is determined entirely by the energy-scale competition near the
quantum critical point. In particular, mass enhancement is cut off at finite
magnetic fields. However, the specific heat measurements reveal excess entropy
in addition to that associated with the density of states on the Fermi surface.
This excess entropy is direct thermodynamic evidence for a "second flavor" of
fluctuating boson in CeCoIn. The electronic nature of this excess entropy
is evidenced by its suppression in the superconducting state. We suggest such a
multi-flavour character for a broader class of quantum critical metals.Comment: 39 page
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
Charge transport through weakly open one dimensional quantum wires
We consider resonant transmission through a finite-length quantum wire
connected to leads via finite transparency junctions. The coherent electron
transport is strongly modified by the Coulomb interaction. The low-temperature
current-voltage () curves show step-like dependence on the bias voltage
determined by the distance between the quantum levels inside the conductor, the
pattern being dependent on the ratio between the charging energy and level
spacing. If the system is tuned close to the resonance condition by the gate
voltage, the low-voltage curve is Ohmic. At large Coulomb energy and low
temperatures, the conductance is temperature-independent for any relationship
between temperature, level spacing, and coupling between the wire and the
leads
Coulomb Blockade Peak Spacings: Interplay of Spin and Dot-Lead Coupling
For Coulomb blockade peaks in the linear conductance of a quantum dot, we
study the correction to the spacing between the peaks due to dot-lead coupling.
This coupling can affect measurements in which Coulomb blockade phenomena are
used as a tool to probe the energy level structure of quantum dots. The
electron-electron interactions in the quantum dot are described by the constant
exchange and interaction (CEI) model while the single-particle properties are
described by random matrix theory. We find analytic expressions for both the
average and rms mesoscopic fluctuation of the correction. For a realistic value
of the exchange interaction constant J_s, the ensemble average correction to
the peak spacing is two to three times smaller than that at J_s = 0. As a
function of J_s, the average correction to the peak spacing for an even valley
decreases monotonically, nonetheless staying positive. The rms fluctuation is
of the same order as the average and weakly depends on J_s. For a small
fraction of quantum dots in the ensemble, therefore, the correction to the peak
spacing for the even valley is negative. The correction to the spacing in the
odd valleys is opposite in sign to that in the even valleys and equal in
magnitude. These results are robust with respect to the choice of the random
matrix ensemble or change in parameters such as charging energy, mean level
spacing, or temperature.Comment: RevTex, 11 pages, 9 figures. v2: Conclusions section expanded.
Accepted for publication in PR
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