1,586 research outputs found
Two-channel Kondo effect and renormalization flow with macroscopic quantum charge states
Many-body correlations and macroscopic quantum behaviors are fascinating
condensed matter problems. A powerful test-bed for the many-body concepts and
methods is the Kondo model which entails the coupling of a quantum impurity to
a continuum of states. It is central in highly correlated systems and can be
explored with tunable nanostructures. Although Kondo physics is usually
associated with the hybridization of itinerant electrons with microscopic
magnetic moments, theory predicts that it can arise whenever degenerate quantum
states are coupled to a continuum. Here we demonstrate the previously elusive
`charge' Kondo effect in a hybrid metal-semiconductor implementation of a
single-electron transistor, with a quantum pseudospin-1/2 constituted by two
degenerate macroscopic charge states of a metallic island. In contrast to other
Kondo nanostructures, each conduction channel connecting the island to an
electrode constitutes a distinct and fully tunable Kondo channel, thereby
providing an unprecedented access to the two-channel Kondo effect and a clear
path to multi-channel Kondo physics. Using a weakly coupled probe, we reveal
the renormalization flow, as temperature is reduced, of two Kondo channels
competing to screen the charge pseudospin. This provides a direct view of how
the predicted quantum phase transition develops across the symmetric quantum
critical point. Detuning the pseudospin away from degeneracy, we demonstrate,
on a fully characterized device, quantitative agreement with the predictions
for the finite-temperature crossover from quantum criticality.Comment: Letter (5 pages, 4 figures) and Methods (10 pages, 6 figures
Optical network physical layer parameter optimization for digital backpropagation using Gaussian processes
We present a novel methodology for optimizing fiber optic network performance by determining the ideal values for attenuation, nonlinearity, and dispersion parameters in terms of achieved signal-to-noise ratio (SNR) gain from digital backpropagation (DBP). Our approach uses Gaussian process regression, a probabilistic machine learning technique, to create a computationally efficient model for mapping these parameters to the resulting SNR after applying DBP. We then use simplicial homology global optimization to find the parameter values that yield maximum SNR for the Gaussian process model within a set of a priori bounds. This approach optimizes the parameters in terms of the DBP gain at the receiver. We demonstrate the effectiveness of our method through simulation and experimental testing, achieving optimal estimates of the dispersion, nonlinearity, and attenuation parameters. Our approach also highlights the limitations of traditional one-at-a-time grid search methods and emphasizes the interpretability of the technique. This methodology has broad applications in engineering and can be used to optimize performance in various systems beyond optical networks
Bose Hubbard Models with Synthetic Spin-Orbit Coupling: Mott Insulators, Spin Textures and Superfluidity
Motivated by the experimental realization of synthetic spin-orbit coupling
for ultracold atoms, we investigate the phase diagram of the Bose Hubbard model
in a non-abelian gauge field in two dimensions. Using a strong coupling
expansion in the combined presence of spin-orbit coupling and tunable
interactions, we find a variety of interesting magnetic Hamiltonians in the
Mott insulator (MI), which support magnetic textures such as spin spirals and
vortex and Skyrmion crystals. An inhomogeneous mean field treatment shows that
the superfluid (SF) phases inherit these exotic magnetic orders from the MI and
display, in addition, unusual modulated current patterns. We present a slave
boson theory which gives insight into such intertwined spin-charge orders in
the SF, and discuss signatures of these orders in Bragg scattering, in situ
microscopy, and dynamic quench experiments.Comment: 4 pages + references + supplementary inf
Particle Dynamics in a Mass-Conserving Coalescence Process
We consider a fully asymmetric one-dimensional model with mass-conserving
coalescence. Particles of unit mass enter at one edge of the chain and
coalescence while performing a biased random walk towards the other edge where
they exit. The conserved particle mass acts as a passive scalar in the reaction
process , and allows an exact mapping to a restricted ballistic
surface deposition model for which exact results exist. In particular, the
mass- mass correlation function is exactly known. These results complement
earlier exact results for the process without mass. We introduce a
comprehensive scaling theory for this process. The exact anaytical and
numerical results confirm its validity.Comment: 5 pages, 6 figure
Phase-Dependent Spontaneous Spin Polarization and Bifurcation Delay in Coupled Two-Component Bose-Einstein Condensates
The spontaneous spin polarization and bifurcation delay in two-component
Bose-Einstein condensates coupled with laser or/and radio-frequency pulses are
investigated. We find that the bifurcation and the spontaneous spin
polarization are determined by both physical parameters and relative phase
between two condensates. Through bifurcations, the system enters into the
spontaneous spin polarization regime from the Rabi regime. We also find that
bifurcation delay appears when the parameter is swept through a static
bifurcation point. This bifurcation delay is responsible for metastability
leading to hysteresis.Comment: Improved version for cond-mat/021157
Perfect Fluids and Bad Metals: Transport Analogies Between Ultracold Fermi Gases and High Superconductors
In this paper, we examine in a unified fashion dissipative transport in
strongly correlated systems. We thereby demonstrate the connection between "bad
metals" (such as the high temperature superconductors) and "perfect fluids"
(such as the ultracold Fermi gases, near unitarity). One motivation of this
work is to communicate to the high energy physics community some of the central
unsolved problems in high superconductors. Because of interest in the
nearly perfect fluidity of the cold gases and because of new tools such as the
AdS/CFT correspondence, this better communication may lead to important
progress in a variety of different fields. A second motivation is to draw
attention to the great power of transport measurements which more directly
reflect the excitation spectrum than, say, thermodynamics and thus strongly
constrain microscopic theories of correlated fermionic superfluids. Our
calculations show that bad metal and perfect fluid behavior is associated with
the presence of a normal state excitation gap which suppresses the effective
number of carriers leading to anomalously low conductivity and viscosity above
the transition temperature . Below we demonstrate that the
condensate collective modes ("phonons") do not couple to transverse probes such
as the shear viscosity. As a result, our calculated shear viscosity at low
becomes arbitrarily small as observed in experiments. In both homogeneous and
trap calculations we do not find the upturn in or (where is
the entropy density) found in most theories. In the process of these studies we
demonstrate compatibility with the transverse sum rule and find reasonable
agreement with both viscosity and cuprate conductivity experiments.Comment: 21 pages, 11 figure
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