70 research outputs found
Beyond Gross-Pitaevskii Mean Field Theory
A large number of effects related to the phenomenon of Bose-Einstein
Condensation (BEC) can be understood in terms of lowest order mean field
theory, whereby the entire system is assumed to be condensed, with thermal and
quantum fluctuations completely ignored. Such a treatment leads to the
Gross-Pitaevskii Equation (GPE) used extensively throughout this book. Although
this theory works remarkably well for a broad range of experimental parameters,
a more complete treatment is required for understanding various experiments,
including experiments with solitons and vortices. Such treatments should
include the dynamical coupling of the condensate to the thermal cloud, the
effect of dimensionality, the role of quantum fluctuations, and should also
describe the critical regime, including the process of condensate formation.
The aim of this Chapter is to give a brief but insightful overview of various
recent theories, which extend beyond the GPE. To keep the discussion brief,
only the main notions and conclusions will be presented. This Chapter
generalizes the presentation of Chapter 1, by explicitly maintaining
fluctuations around the condensate order parameter. While the theoretical
arguments outlined here are generic, the emphasis is on approaches suitable for
describing single weakly-interacting atomic Bose gases in harmonic traps.
Interesting effects arising when condensates are trapped in double-well
potentials and optical lattices, as well as the cases of spinor condensates,
and atomic-molecular coupling, along with the modified or alternative theories
needed to describe them, will not be covered here.Comment: Review Article (19 Pages) - To appear in 'Emergent Nonlinear
Phenomena in Bose-Einstein Condensates: Theory and Experiment', Edited by
P.G. Kevrekidis, D.J. Frantzeskakis and R. Carretero-Gonzalez (Springer
Verlag
Artificial graphene as a tunable Dirac material
Artificial honeycomb lattices offer a tunable platform to study massless
Dirac quasiparticles and their topological and correlated phases. Here we
review recent progress in the design and fabrication of such synthetic
structures focusing on nanopatterning of two-dimensional electron gases in
semiconductors, molecule-by-molecule assembly by scanning probe methods, and
optical trapping of ultracold atoms in crystals of light. We also discuss
photonic crystals with Dirac cone dispersion and topologically protected edge
states. We emphasize how the interplay between single-particle band structure
engineering and cooperative effects leads to spectacular manifestations in
tunneling and optical spectroscopies.Comment: Review article, 14 pages, 5 figures, 112 Reference
Universal Spin Transport in a Strongly Interacting Fermi Gas
Transport of fermions is central in many elds of physics. Electron transport runs modern technology,
de ning states of matter such as superconductors and insulators, and electron spin, rather
than charge, is being explored as a new carrier of information [1]. Neutrino transport energizes
supernova explosions following the collapse of a dying star [2], and hydrodynamic transport of the
quark-gluon plasma governed the expansion of the early Universe [3]. However, our understanding
of non-equilibrium dynamics in such strongly interacting fermionic matter is still limited. Ultracold
gases of fermionic atoms realize a pristine model for such systems and can be studied in real time
with the precision of atomic physics [4, 5]. It has been established that even above the super
uid
transition such gases
ow as an almost perfect
uid with very low viscosity [3, 6] when interactions
are tuned to a scattering resonance. However, here we show that spin currents, as opposed to
mass currents, are maximally damped, and that interactions can be strong enough to reverse spin
currents, with opposite spin components reflecting off each other. We determine the spin drag coefficient, the spin di usivity, and the spin susceptibility, as a function of temperature on resonance and
show that they obey universal laws at high temperatures. At low temperatures, the spin di usivity
approaches a minimum value set by ħ/m, the quantum limit of di usion, where ħ is the reduced
Planck's constant and m the atomic mass. For repulsive interactions, our measurements appear to
exclude a metastable ferromagnetic state [7{9].National Science Foundation (U.S.)United States. Office of Naval ResearchUnited States. Army Research Office (DARPA OLE programme)Alfred P. Sloan FoundationUnited States. Air Force Office of Scientific Research. Multidisciplinary University Research InitiativeUnited States. Army Research Office. Multidisciplinary University Research InitiativeUnited States. Defense Advanced Research Projects Agency. Young Faculty AwardDavid & Lucile Packard Foundatio
Metastability and Coherence of Repulsive Polarons in a Strongly Interacting Fermi Mixture
Ultracold Fermi gases with tuneable interactions represent a unique test bed
to explore the many-body physics of strongly interacting quantum systems. In
the past decade, experiments have investigated a wealth of intriguing
phenomena, and precise measurements of ground-state properties have provided
exquisite benchmarks for the development of elaborate theoretical descriptions.
Metastable states in Fermi gases with strong repulsive interactions represent
an exciting new frontier in the field. The realization of such systems
constitutes a major challenge since a strong repulsive interaction in an atomic
quantum gas implies the existence of a weakly bound molecular state, which
makes the system intrinsically unstable against decay. Here, we exploit
radio-frequency spectroscopy to measure the complete excitation spectrum of
fermionic 40K impurities resonantly interacting with a Fermi sea of 6Li atoms.
In particular, we show that a well-defined quasiparticle exists for strongly
repulsive interactions. For this "repulsive polaron" we measure its energy and
its lifetime against decay. We also probe its coherence properties by measuring
the quasiparticle residue. The results are well described by a theoretical
approach that takes into account the finite effective range of the interaction
in our system. We find that a non-zero range of the order of the interparticle
spacing results in a substantial lifetime increase. This major benefit for the
stability of the repulsive branch opens up new perspectives for investigating
novel phenomena in metastable, repulsively interacting fermion systems.Comment: 11 pages, 9 figure
Physics of Neutron Star Crusts
The physics of neutron star crusts is vast, involving many different research
fields, from nuclear and condensed matter physics to general relativity. This
review summarizes the progress, which has been achieved over the last few
years, in modeling neutron star crusts, both at the microscopic and macroscopic
levels. The confrontation of these theoretical models with observations is also
briefly discussed.Comment: 182 pages, published version available at
<http://www.livingreviews.org/lrr-2008-10
Spin transport and spin torque in antiferromagnetic devices
Ferromagnets are key materials for sensing and memory applications. In contrast, antiferromagnets which represent the more common form of magnetically ordered materials, have found less practical application beyond their use for establishing reference magnetic orientations via exchange bias. This might change in the future due to the recent progress in materials research and discoveries of antiferromagnetic spintronic phenomena suitable for device applications. Experimental demonstration of the electrical switching and detection of the Néel order open a route towards memory devices based on antiferromagnets. Apart from the radiation and magnetic-field hardness, memory cells fabricated from antiferromagnets can be inherently multilevel, which could be used for neuromorphic computing. Switching speeds attainable in antiferromagnets far exceed those of ferromagnetic and semiconductor memory technologies. Here we review the recent progress in electronic spin-transport and spin-torque phenomena in antiferromagnets that are dominantly of the relativistic quantum mechanical origin. We discuss their utility in pure antiferromagnetic or hybrid ferromagnetic/antiferromagnetic memory devices
Fermi polaron-polaritons in charge-tunable atomically thin semiconductors
The dynamics of a mobile quantum impurity in a degenerate Fermi system is a fundamental problem in many-body physics. The interest in this field has been renewed due to recent ground-breaking experiments with ultracold Fermi gases. Optical creation of an exciton or a polariton in a two-dimensional electron system embedded in a microcavity constitutes a new frontier for this field due to an interplay between cavity coupling favouring ultralow-mass polariton formation6 and exciton–electron interactions leading to polaron or trion formation. Here, we present cavity spectroscopy of gate-tunable monolayer MoSe2 exhibiting strongly bound trion and polaron resonances, as well as non-perturbative coupling to a single microcavity mode. As the electron density is increased, the oscillator strength determined from the polariton splitting is gradually transferred from the higher-energy repulsive exciton-polaron resonance to the lower-energy attractive exciton-polaron state. Simultaneous observation of polariton formation in both attractive and repulsive branches indicates a new regime of polaron physics where the polariton impurity mass can be much smaller than that of the electrons. Our findings shed new light on optical response of semiconductors in the presence of free carriers by identifying the Fermi polaron nature of excitonic resonances and constitute a first step in investigation of a new class of degenerate Bose–Fermi mixtures.Physic
Superfluidity and spin superfluidity in spinor Bose gases
\u3cp\u3eWe show that spinor Bose gases subject to a quadratic Zeeman effect exhibit coexisting superfluidity and spin superfluidity, and study the interplay between these two distinct types of superfluidity. To illustrate that the basic principles governing these two types of superfluidity are the same, we describe the magnetization and particle-density dynamics in a single hydrodynamic framework. In this description spin and mass supercurrents are driven by their respective chemical potential gradients. As an application, we propose an experimentally accessible stationary state, where the two types of supercurrents counterflow and cancel each other, thus resulting in no mass transport. Furthermore, we propose a straightforward setup to probe spin superfluidity by measuring the in-plane magnetization angle of the whole cloud of atoms. We verify the robustness of these findings by evaluating the four-magnon collision time, and find that the time scale for coherent (superfluid) dynamics is separated from that of the slower incoherent dynamics by one order of magnitude. Comparing the atom and magnon kinetics reveals that while the former can be hydrodynamic, the latter is typically collisionless under most experimental conditions. This implies that, while our zero-temperature hydrodynamic equations are a valid description of spin transport in Bose gases, a hydrodynamic description that treats both mass and spin transport at finite temperatures may not be readily feasible.\u3c/p\u3
Magnon spin Hall magnetoresistance of a gapped quantum paramagnet
\u3cp\u3eMotivated by recent experimental work, we consider spin transport between a normal metal and a gapped quantum paramagnet. We model the latter as the magnonic Mott-insulating phase of an easy-plane ferromagnetic insulator. We evaluate the spin current mediated by the interface exchange coupling between the ferromagnet and the adjacent normal metal. For the strongly interacting magnons that we consider, this spin current gives rise to a spin Hall magnetoresistance that strongly depends on the magnitude of the magnetic field, rather than its direction. This Letter may motivate electrical detection of the phases of quantum magnets and the incorporation of such materials into spintronic devices.\u3c/p\u3
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