2,650 research outputs found
Surface magnetic ordering in topological insulators with bulk magnetic dopants
We show that a three dimensional topological insulator doped with magnetic
impurities in the bulk can have a regime where the surface is magnetically
ordered but the bulk is not. This is in contrast to conventional materials
where bulk ordered phases are typically more robust than surface ordered
phases. The difference originates from the topologically protected gapless
surface states characteristic of topological insulators. We study the problem
using a mean field approach in two concrete models that give the same
qualitative result, with some interesting differences. Our findings could help
explain recent experimental results showing the emergence of a spectral gap in
the surface state of Bi2Se3 doped with Mn or Fe atoms, but with no measurable
bulk magnetism.Comment: 8 pages, 6 figure
Observability of a projected new state of matter: a metallic superfluid
Dissipationless quantum states, such as superconductivity and superfluidity,
have attracted interest for almost a century. A variety of systems exhibit
these macroscopic quantum phenomena, ranging from superconducting electrons in
metals to superfluid liquids, atomic vapours, and even large nuclei. It was
recently suggested that liquid metallic hydrogen could form two new unusual
dissipationless quantum states, namely the metallic superfluid and the
superconducting superfluid. Liquid metallic hydrogen is projected to occur only
at an extremely high pressure of about 400 GPa, while pressures on hydrogen of
320 GPa having already been reported. The issue to be adressed is if this state
could be experimentally observable in principle. We propose four experimental
probes for detecting it.Comment: in print in Phys. Rev. Let
Nonlinear instability of density-independent orbital-free kinetic energy functionals
We study in this article the mathematical properties of a class of
orbital-free kinetic energy functionals. We prove that these models are
linearly stable but nonlinearly unstable, in the sense that the corresponding
kinetic energy functionals are not bounded from below. As a matter of
illustration, we provide an example of an electronic density of simple shape
the kinetic energy of which is negative.Comment: 14 pages, 1 figur
Electron-ion and ion-ion potentials for modeling warm-dense-matter: applications to laser-heated or shock-compressed Al and Si
The pair-interactions U_{ij}(r) determine the thermodynamics and linear
transport properties of matter via the pair-distribution functions (PDFs),
i.e., g_{ij}(r). Great simplicity is achieved if U_{ij}(r) could be directly
used to predict material properties via classical simulations, avoiding
many-body wavefunctions. Warm dense matter (WDM) is encountered in
quasi-equilibria where the electron temperature differs from the ion
temperature T_i, as in laser-heated or in shock-compressed matter. The electron
PDFs g_{ee}(r) as perturbed by the ions are used to evaluate fully non-local
exchange-correlation corrections to the free energy, using Hydrogen as an
example. Electron-ion potentials for ions with a bound core are discussed with
Al and Si as examples, for WDM with T_e \ne T_i, and valid for times shorter
than the electron-ion relaxation time. In some cases the potentials develop
attractive regions, and then become repulsive and `Yukawa-like' for higher
. These results clarify the origin of initial phonon-hardening and rapid
release. Pair-potentials for shock-heated WDM show that phonon hardening would
not occur in most such systems. Defining meaningful quasi-equilibrium static
transport coefficients consistent with the dynamic values is addressed. There
seems to be no meaningful `static conductivity' obtainable by extrapolating
experimental or theoretical \sigma(\omega, T_i, T_e) to \omega \to 0, unless
T_i \to T_e as well. Illustrative calculations of quasi-static resistivities
R(T_i,T_e) of laser-heated as well as shock-heated Aluminum and Silicon are
presented using our pseudopotentials, pair-potentials and classical integral
equations. The quasi-static resistivities display clear differences in their
temperature evolutions, but are not the strict \omega \to 0 limits of the
dynamic values.Comment: 12 pages, 6 figues, Latex file
Confinement-induced Berry phase and helicity-dependent photocurrents
The photocurrent in an optically active metal is known to contain a component
that switches sign with the helicity of the incident radiation. At low
frequencies, this current depends on the orbital Berry phase of the Bloch
electrons via the "anomalous velocity" of Karplus and Luttinger. We consider
quantum wells in which the parent material, such as GaAs, is not optically
active and the relevant Berry phase only arises as a result of quantum
confinement. Using an envelope approximation that is supported by numerical
tight-binding results, it is shown that the Berry phase contribution is
determined for realistic wells by a cubic Berry phase intrinsic to the bulk
material, the well width, and the well direction. These results for the
magnitude of the Berry-phase effect suggest that it may already have been
observed in quantum well experiments.Comment: 4 pages, 2 figure
Break-junction experiments on the zero-bias anomaly of non-magnetic and ferromagnetically ordered metals
We have investigated break junctions of normal non-magnetic metals as well as
ferromagnets at low temperatures. The point contacts with radii 0.15 - 15 nm
showed zero-bias anomalies which can be attributed to Kondo scattering at a
single Kondo impurity at the contact or to the switching of a single conducting
channel. The Kondo temperatures derived from the width of the anomalies varied
between 10 and 1000 K. These results agree well with literature data on
atomic-size contacts of the ferromagnets as well as with spear-anvil type
contacts on a wide variety of metals.Comment: 7 pages, 4 figures, submitted to Proceedings of the 26th Conference
on Low Temperature Physic
Fast-field cycling NMR is sensitive to the method of cross-linking in BSA gels
This work was supported by ARUK (grant number 19689).Non peer reviewedPublisher PD
Achievable Qubit Rates for Quantum Information Wires
Suppose Alice and Bob have access to two separated regions, respectively, of
a system of electrons moving in the presence of a regular one-dimensional
lattice of binding atoms. We consider the problem of communicating as much
quantum information, as measured by the qubit rate, through this quantum
information wire as possible. We describe a protocol whereby Alice and Bob can
achieve a qubit rate for these systems which is proportional to N^(-1/3) qubits
per unit time, where N is the number of lattice sites. Our protocol also
functions equally in the presence of interactions modelled via the t-J and
Hubbard models
The role of quasi-momentum in the resonant dynamics of the atom-optics kicked rotor
We examine the effect of the initial atomic momentum distribution on the
dynamics of the atom-optical realisation of the quantum kicked rotor. The atoms
are kicked by a pulsed optical lattice, the periodicity of which implies that
quasi-momentum is conserved in the transport problem. We study and compare
experimentally and theoretically two resonant limits of the kicked rotor: in
the vicinity of the quantum resonances and in the semiclassical limit of
vanishing kicking period. It is found that for the same experimental
distribution of quasi-momenta, significant deviations from the kicked rotor
model are induced close to quantum resonance, while close to the classical
resonance (i.e. for small kicking period) the effect of the quasi-momentum
vanishes.Comment: 10 pages, 4 figures, to be published in J. Phys. A, Special Issue on
'Trends in Quantum Chaotic Scattering
A comprehensive study of electric, thermoelectric and thermal conductivities of Graphene with short range unitary and charged impurities
Motivated by the experimental measurement of electrical and hall
conductivity, thermopower and Nernst effect, we calculate the longitudinal and
transverse electrical and heat transport in graphene in the presence of unitary
scatterers as well as charged impurities. The temperature and carrier density
dependence in this system display a number of anomalous features that arise due
to the relativistic nature of the low energy fermionic degrees of freedom. We
derive the properties in detail including the effect of unitary and charged
impurities self-consistently, and present tables giving the analytic
expressions for all the transport properties in the limit of small and large
temperature compared to the chemical potential and the scattering rates. We
compare our results with the available experimental data. While the qualitative
variations with temperature and density of carriers or chemical potential of
all transport properties can be reproduced, we find that a given set of
parameters of the impurities fits the Hall conductivity, Thermopower and the
Nernst effect quantitatively but cannot fit the conductivity quantitatively. On
the other hand a single set of parameters for scattering from Coulomb
impurities fits conductivity, hall resistance and thermopower but not Nernst
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