804 research outputs found
The lifetime of electrons, holes and excitons before self-trapping
In this paper we discuss the self-trapping of a carrier or exciton in an insulator.
The qualitative differences between small self-trapped molecular polarons and dielectric
polarons are stressed. We point out that, for the formation of a molecular polaron or selftrapped
exciton, a potential barrier must be penetrated or surmounted by the configuration
coordinate, leading to a delay in the self-trapping process. This does not exist for dielectric
polarons. The observable consequence of the delay time before self-trapping is discussed,
and applications are made to alkali halides and to SOz
Fermi-Hubbard physics with atoms in an optical lattice
The Fermi-Hubbard model is a key concept in condensed matter physics and
provides crucial insights into electronic and magnetic properties of materials.
Yet, the intricate nature of Fermi systems poses a barrier to answer important
questions concerning d-wave superconductivity and quantum magnetism. Recently,
it has become possible to experimentally realize the Fermi-Hubbard model using
a fermionic quantum gas loaded into an optical lattice. In this atomic approach
to the Fermi-Hubbard model the Hamiltonian is a direct result of the optical
lattice potential created by interfering laser fields and short-ranged
ultracold collisions. It provides a route to simulate the physics of the
Hamiltonian and to address open questions and novel challenges of the
underlying many-body system. This review gives an overview of the current
efforts in understanding and realizing experiments with fermionic atoms in
optical lattices and discusses key experiments in the metallic,
band-insulating, superfluid and Mott-insulating regimes.Comment: Posted with permission from the Annual Review of of Condensed Matter
Physics Volume 1 \c{opyright} 2010 by Annual Reviews,
http://www.annualreviews.or
Solid-state physics : a historic experiment redesigned
PostprintPeer reviewe
Finite temperature phase transition for disordered weakly interacting bosons in one dimension
It is commonly accepted that there are no phase transitions in
one-dimensional (1D) systems at a finite temperature, because long-range
correlations are destroyed by thermal fluctuations. Here we demonstrate that
the 1D gas of short-range interacting bosons in the presence of disorder can
undergo a finite temperature phase transition between two distinct states:
fluid and insulator. None of these states has long-range spatial correlations,
but this is a true albeit non-conventional phase transition because transport
properties are singular at the transition point. In the fluid phase the mass
transport is possible, whereas in the insulator phase it is completely blocked
even at finite temperatures. We thus reveal how the interaction between
disordered bosons influences their Anderson localization. This key question,
first raised for electrons in solids, is now crucial for the studies of atomic
bosons where recent experiments have demonstrated Anderson localization in
expanding very dilute quasi-1D clouds.Comment: 8 pages, 5 figure
Metal-insulator transition in vanadium dioxide nanobeams: probing sub-domain properties of strongly correlated materials
Many strongly correlated electronic materials, including high-temperature
superconductors, colossal magnetoresistance and metal-insulator-transition
(MIT) materials, are inhomogeneous on a microscopic scale as a result of domain
structure or compositional variations. An important potential advantage of
nanoscale samples is that they exhibit the homogeneous properties, which can
differ greatly from those of the bulk. We demonstrate this principle using
vanadium dioxide, which has domain structure associated with its dramatic MIT
at 68 degrees C. Our studies of single-domain vanadium dioxide nanobeams reveal
new aspects of this famous MIT, including supercooling of the metallic phase by
50 degrees C; an activation energy in the insulating phase consistent with the
optical gap; and a connection between the transition and the equilibrium
carrier density in the insulating phase. Our devices also provide a
nanomechanical method of determining the transition temperature, enable
measurements on individual metal-insulator interphase walls, and allow general
investigations of a phase transition in quasi-one-dimensional geometry.Comment: 9 pages, 3 figures, original submitted in June 200
Thermally driven spin injection from a ferromagnet into a non-magnetic metal
Creating, manipulating and detecting spin polarized carriers are the key
elements of spin based electronics. Most practical devices use a perpendicular
geometry in which the spin currents, describing the transport of spin angular
momentum, are accompanied by charge currents. In recent years, new sources of
pure spin currents, i.e., without charge currents, have been demonstrated and
applied. In this paper, we demonstrate a conceptually new source of pure spin
current driven by the flow of heat across a ferromagnetic/non-magnetic metal
(FM/NM) interface. This spin current is generated because the Seebeck
coefficient, which describes the generation of a voltage as a result of a
temperature gradient, is spin dependent in a ferromagnet. For a detailed study
of this new source of spins, it is measured in a non-local lateral geometry. We
developed a 3D model that describes the heat, charge and spin transport in this
geometry which allows us to quantify this process. We obtain a spin Seebeck
coefficient for Permalloy of -3.8 microvolt/Kelvin demonstrating that thermally
driven spin injection is a feasible alternative for electrical spin injection
in, for example, spin transfer torque experiments
Unbalanced Holographic Superconductors and Spintronics
We present a minimal holographic model for s-wave superconductivity with
unbalanced Fermi mixtures, in 2+1 dimensions at strong coupling. The breaking
of a U(1)_A "charge" symmetry is driven by a non-trivial profile for a charged
scalar field in a charged asymptotically AdS_4 black hole. The chemical
potential imbalance is implemented by turning on the temporal component of a
U(1)_B "spin" field under which the scalar field is uncharged. We study the
phase diagram of the model and comment on the eventual (non) occurrence of
LOFF-like inhomogeneous superconducting phases. Moreover, we study "charge" and
"spin" transport, implementing a holographic realization (and a generalization
thereof to superconducting setups) of Mott's two-current model which provides
the theoretical basis of modern spintronics. Finally we comment on possible
string or M-theory embeddings of our model and its higher dimensional
generalizations, within consistent Kaluza-Klein truncations and brane-anti
brane setups.Comment: 45 pages, 15 figures; v2: two paragraphs below eq. (3.1) slightly
modified, figure 5 (left) replaced, references added; v3: typos corrected,
comments added, figure 12 replace
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