12,635 research outputs found
Superconducting Fluctuations in a Multi-Band 1D Hubbard Model
A renormalization-group and bosonization approach for a multi-band Hubbard
Hamiltonian in one dimension is described. Based on the limit of many bands, it
is argued that this Hamiltonian with bare repulsive electron-electron
interactions is scaled under specific conditions to a model in which
superconducting fluctuations dominate.Comment: 12 pages + 1 fig, Revtex, Preprint - Los Alamo
Effective s- and p-Wave Contact Interactions in Trapped Degenerate Fermi Gases
The structure and stability of dilute degenerate Fermi gases trapped in an
external potential is discussed with special emphasis on the influence of s-
and p-wave interactions. In a first step an Effective Contact Interaction for
all partial waves is derived, which reproduces the energy spectrum of the full
potential within a mean-field model space. Using the s- and p-wave part the
energy density of the multi-component Fermi gas is calculated in Thomas-Fermi
approximation. On this basis the stability of the one- and two-component Fermi
gas against mean-field induced collapse is investigated. Explicit stability
conditions in terms of density and total particle number are given. For the
single-component system attractive p-wave interactions limit the density of the
gas. In the two-component case a subtle competition of s- and p-wave
interactions occurs and gives rise to a rich variety of phenomena. A repulsive
p-wave part, for example, can stabilize a two-component system that would
otherwise collapse due to an attractive s-wave interaction. It is concluded
that the p-wave interaction may have important influence on the structure of
degenerate Fermi gases and should not be discarded from the outset.Comment: 18 pages, 11 figures (using RevTEX4
The cleaning of burned and contaminated archaeological maize prior to 87Sr/86Sr analysis
Accurate trace-metal and strontium-isotope analyses of archaeological corn cobs require that metal contaminants be removed prior to chemical analysis. Archaeological cobs are often coated with construction debris, dust, or soilwhich contains mineral particles. In addition, most archaeological cobs are partially or completely burned and the burned parts incorporate mineral debris in their hardened residual structures. Unburned cobs are weak ion exchangers and most metals within a cob are not firmly bound to cob organic matter; therefore, immersing cobs in acids and rinsing them in deionized water to remove mineral contaminants may result in the undesirable loss of metals, including strontium, from the cob.
In this paper we show that some cob metal-pair ratios are not substantially changed when the cob is ‘‘cleaned’’ with deionized water, if the water-cob contact time does not exceed five minutes. Additionally, we introduce a method for eliminating mineral contaminants in both burned and unburned cobs, thus rendering them acceptable for strontium-isotope analysis. However, the decontamination procedure results in the rapid non-stoichiometric leaching of trace metals from the unburned cobs and it is possible that most metals will be extracted from the cobs during the lengthy decontamination process. Trace metals, in particular Al and Ca, should be analyzed in order to determine the presence and level of mineral contamination after cleaning
Learn your opponent's strategy (in polynomial time)!
Agents that interact in a distributed environment might increase their utility by behaving optimally given the strategies of the other agents. To do so, agents need to learn about those with whom they share the same world. This paper examines interactions among agents from a game theoretic perspective. In this context, learning has been assumed as a means to reach equilibrium. We analyze the complexity of this learning process. We start with a restricted two-agent model, in which agents are represented by finite automata, and one of the agents plays a fixed strategy. We show that even with this restrictions, the learning process may be exponential in time. We then suggest a criterion of simplicity, that induces a class of automata that are learnable in polynomial time
Noise-mitigated randomized measurements and self-calibrating shadow estimation
Randomized measurements are increasingly appreciated as powerful tools to
estimate properties of quantum systems, e.g., in the characterization of hybrid
classical-quantum computation. On many platforms they constitute natively
accessible measurements, serving as the building block of prominent schemes
like shadow estimation. In the real world, however, the implementation of the
random gates at the core of these schemes is susceptible to various sources of
noise and imperfections, strongly limiting the applicability of protocols. To
attenuate the impact of this shortcoming, in this work we introduce an
error-mitigated method of randomized measurements, giving rise to a robust
shadow estimation procedure. On the practical side, we show that error
mitigation and shadow estimation can be carried out using the same session of
quantum experiments, hence ensuring that we can address and mitigate the noise
affecting the randomization measurements. Mathematically, we develop a picture
derived from Fourier-transforms to connect randomized benchmarking and shadow
estimation. We prove rigorous performance guarantees and show the functioning
using comprehensive numerics. More conceptually, we demonstrate that, if
properly used, easily accessible data from randomized benchmarking schemes
already provide such valuable diagnostic information to inform about the noise
dynamics and to assist in quantum learning procedures.Comment: 6+20 pages, 6 figure
Non-Fermi-liquid scattering rates and anomalous band dispersion in ferropnictides
Angle-resolved photoemission spectroscopy (ARPES) is used to study the band
dispersion and the quasiparticle scattering rates in two ferropnictides
systems. Our ARPES results show linear-in-energy dependent scattering rates
which are constant in a wide range of control parameter and which depend on the
orbital character of the bands. We demonstrate that the linear energy
dependence gives rise to weakly dispersing band with a strong mass enhancement
when the band maximum crosses the chemical potential. In the superconducting
phase the related small effective Fermi energy favors a
Bardeen-Cooper-Schrieffer (BCS)\,\cite{Bardeen1957}-Bose-Einstein
(BE)\,\cite{Bose1924} crossover state.Comment: 5 pages, 4 figures Supplement 4 pages, 6 figure
Estimating gate-set properties from random sequences
With quantum computing devices increasing in scale and complexity, there is a
growing need for tools that obtain precise diagnostic information about quantum
operations. However, current quantum devices are only capable of short
unstructured gate sequences followed by native measurements. We accept this
limitation and turn it into a new paradigm for characterizing quantum
gate-sets. A single experiment - random sequence estimation - solves a wealth
of estimation problems, with all complexity moved to classical post-processing.
We derive robust channel variants of shadow estimation with close-to-optimal
performance guarantees and use these as a primitive for partial, compressive
and full process tomography as well as the learning of Pauli noise. We discuss
applications to the quantum gate engineering cycle, and propose novel methods
for the optimization of quantum gates and diagnosing cross-talk.Comment: 10+18 pages, two figures, substantially rewritten (made more
intuitive, connected better to common experimental prescriptions, equipped
with stronger numerical analysis
A Quantum Monte Carlo Method and Its Applications to Multi-Orbital Hubbard Models
We present a framework of an auxiliary field quantum Monte Carlo (QMC) method
for multi-orbital Hubbard models. Our formulation can be applied to a
Hamiltonian which includes terms for on-site Coulomb interaction for both
intra- and inter-orbitals, intra-site exchange interaction and energy
differences between orbitals. Based on our framework, we point out possible
ways to investigate various phase transitions such as metal-insulator, magnetic
and orbital order-disorder transitions without the minus sign problem. As an
application, a two-band model is investigated by the projection QMC method and
the ground state properties of this model are presented.Comment: 10 pages LaTeX including 2 PS figures, to appear in J.Phys.Soc.Jp
Ab initio determination of the lifetime of the state f or by relativistic many-body theory
Relativistic coupled-cluster(RCC) theory has been employed to calculate the
life time of the state of single ionized lead() to an
accurac y of 3% and compared with the corresponding value obtained using second
order r elativistic many-body perturbation theory(RMBPT). This is one of the
very few ap plications of this theory to excited state properties of heavy
atomic systems. C ontributions from the different electron correlation effects
are given explicitl y
- …