1,308 research outputs found
Two-channel Kondo physics in odd impurity chains
We study odd-membered chains of spin-(1/2) impurities, with each end
connected to its own metallic lead. For antiferromagnetic exchange coupling,
universal two-channel Kondo (2CK) physics is shown to arise at low energies.
Two overscreening mechanisms are found to occur depending on coupling strength,
with distinct signatures in physical properties. For strong inter-impurity
coupling, a residual chain spin-(1/2) moment experiences a renormalized
effective coupling to the leads; while in the weak-coupling regime, Kondo
coupling is mediated via incipient single-channel Kondo singlet formation. We
also investigate models where the leads are tunnel-coupled to the impurity
chain, permitting variable dot filling under applied gate voltages. Effective
low-energy models for each regime of filling are derived, and for even-fillings
where the chain ground state is a spin singlet, an orbital 2CK effect is found
to be operative. Provided mirror symmetry is preserved, 2CK physics is shown to
be wholly robust to variable dot filling; in particular the single-particle
spectrum at the Fermi level, and hence the low-temperature zero-bias
conductance, is always pinned to half-unitarity. We derive a Friedel-Luttinger
sum rule and from it show that, in contrast to a Fermi liquid, the Luttinger
integral is non-zero and determined solely by the `excess' dot charge as
controlled by gate voltage. The relevance of the work to real quantum dot
devices, where inter-lead charge-transfer processes fatal to 2CK physics are
present, is also discussed. Physical arguments and numerical renormalization
group techniques are used to obtain a detailed understanding of these problems.Comment: 21 pages, 19 figure
Recommended from our members
Regional analysis of Residual Oil Zone potential in the Permian Basin
textThis study provides independent analysis of Residual Oil Zones (ROZs) in the Permian Basin from a regional perspective, focusing on the formation mechanism and present ROZ locations. Results demonstrate widespread potential for ROZs, defined here as thick volumes of reservoir rock containing near-residual saturations of predominantly immobile oil formed by natural imbibition and displacement of oil by dynamic buoyant or hydrodynamic forces. Previous work suggests hydrodynamic forces generated by regional tectonic uplift drove widespread oil remobilization and ROZ creation. To test the hypothesis, uplift and tilting are quantified and the resulting peak regional potentiometric gradient used as a physical constraint to compute and compare predicted ROZ thicknesses from hydrodynamics for several ROZ-bearing San Andres fields with known ROZ thicknesses. Late-Albian Edwards Group geologic contacts, which are interpreted to have been deposited near sea level prior to uplift, are used as a regional datum. Approximate elevations determined for the present datum show ~1800 m of differential uplift since Edwards deposition, with an average regional slope of ~0.128˚. This post-Edwards tilting increased the pre-existing regional structural gradient of the San Andres Formation to ~0.289˚. Using the calculated post-Edwards gradient results in to prediction of ROZ thicknesses from hydrodynamics that is consistent with measured ROZ thicknesses at several fields. When compared with countervailing buoyancy forces, hydrodynamics is calculated to be the more dominant driving force of oil movement for reservoirs with structural dips less than 1.5˚, which is the common dip for San Andres Formation platform deposits where ROZs have been identified. To predict the location of ROZs, ROZ-related oil field properties were identified and analyzed for over 2,800 Permian Basin reservoirs. A strong basin-wide correlation between API and crude sulfur content is consistent with the expected outcome of oil degradation driven by oil-water interaction, and supports the use of API and sulfur content as proxies for ROZ potential in the Permian Basin. Spatial analysis of sulfur data shows that the highest probability for ROZ existence exists in Leonardian through Guadalupian-age reservoirs, distributed primarily in shelf and platform areas of Permian structures. Combined, these results support the widespread potential for ROZs across the Permian Basin generated primarily by regional scale tilting and resultant hydrodynamic forces.Geological Science
Recommending Learning Algorithms and Their Associated Hyperparameters
The success of machine learning on a given task dependson, among other
things, which learning algorithm is selected and its associated
hyperparameters. Selecting an appropriate learning algorithm and setting its
hyperparameters for a given data set can be a challenging task, especially for
users who are not experts in machine learning. Previous work has examined using
meta-features to predict which learning algorithm and hyperparameters should be
used. However, choosing a set of meta-features that are predictive of algorithm
performance is difficult. Here, we propose to apply collaborative filtering
techniques to learning algorithm and hyperparameter selection, and find that
doing so avoids determining which meta-features to use and outperforms
traditional meta-learning approaches in many cases.Comment: Short paper--2 pages, 2 table
Two-channel Kondo physics in two-impurity Kondo models
We consider the non-Fermi liquid quantum critical state of the spin-S
two-impurity Kondo model, and its potential realization in a quantum dot
device. Using conformal field theory (CFT) and the numerical renormalization
group (NRG), we show the critical point to be identical to that of the
two-channel Kondo model with additional potential scattering, for any spin-S.
Distinct conductance signatures are shown to arise as a function of device
asymmetry; with the `smoking gun' square-root behavior, commonly believed to
arise at low-energies, dominant only in certain regimes.Comment: 4.5 pages (with 3 figures) + 9 pages (with 4 figures) supplementary
materia
The History of Air Quality in Utah: A Narrative Review
Utah has a rich history related to air pollution; however, it is not widely known or documented. This is despite air quality being a top issue of public concern for the state’s urban residents and acute episodes that feature some of the world’s worst short-term particulate matter exposure. As we discuss in this narrative review, the relationship between air pollution and the state’s residents has changed over time, as fuel sources shifted from wood to coal to petroleum and natural gas. Air pollution rose in prominence as a public issue in the 1880s as Utah’s urban areas grew. Since then, scientific advances have increased the understanding of air quality impacts on human health, groups of concerned citizens worked to raise public awareness, policy makers enacted legislation to improve air quality, and courts upheld rights to clean air. Utah’s air quality future holds challenges and opportunities and can serve as useful case for other urbanizing regions struggling with air quality concerns. Population growth and changing climate will exacerbate current air quality trends, but economically viable clean energy technologies can be deployed to reduce air pollution, bringing substantial public health and economic benefits to the state’s residents and other settings with similar public health concerns
Generalized Wilson Chain for solving multichannel quantum impurity problems
The Numerical Renormalization Group is used to solve quantum impurity
problems, which describe magnetic impurities in metals, nanodevices, and
correlated materials within DMFT. Here we present a simple generalization of
the Wilson Chain, which improves the scaling of computational cost with the
number of channels/bands, bringing new problems within reach. The method is
applied to calculate the t-matrix of the three-channel Kondo model at T=0,
which shows universal crossovers near non-Fermi liquid critical points. A
non-integrable three-impurity problem with three bands is also studied,
revealing a rich phase diagram and novel screening/overscreening mechanisms.Comment: 5 pages + 5 pages supplementary materia
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