157 research outputs found
Generalized conductance sum rule in atomic break junctions
When an atomic-size break junction is mechanically stretched, the total
conductance of the contact remains approximately constant over a wide range of
elongations, although at the same time the transmissions of the individual
channels (valence orbitals of the junction atom) undergo strong variations. We
propose a microscopic explanation of this phenomenon, based on Coulomb
correlation effects between electrons in valence orbitals of the junction atom.
The resulting approximate conductance quantization is closely related to the
Friedel sum rule.Comment: 4 pages, 1 figure, appears in Proceedings of the NATO Advanced
Research Workshop ``Size dependent magnetic scattering'', Pecs, Hungary, May
28 - June 1, 200
Order by disorder and spiral spin liquid in frustrated diamond lattice antiferromagnets
Frustration refers to competition between different interactions that cannot
be simultaneously satisfied, a familiar feature in many magnetic solids. Strong
frustration results in highly degenerate ground states, and a large suppression
of ordering by fluctuations. Key challenges in frustrated magnetism are
characterizing the fluctuating spin-liquid regime and determining the mechanism
of eventual order at lower temperature. Here, we study a model of a diamond
lattice antiferromagnet appropriate for numerous spinel materials. With
sufficiently strong frustration a massive ground state degeneracy develops
amongst spirals whose propagation wavevectors reside on a continuous
two-dimensional ``spiral surface'' in momentum space. We argue that an
important ordering mechanism is entropic splitting of the degenerate ground
states, an elusive phenomena called order-by-disorder. A broad ``spiral
spin-liquid'' regime emerges at higher temperatures, where the underlying
spiral surface can be directly revealed via spin correlations. We discuss the
agreement between these predictions and the well characterized spinel MnSc2S4
The Magic Angle "Mystery" in Electron Energy Loss Spectroscopy: Relativistic and Dielectric Corrections
Recently it has been demonstrated that a careful treatment of both
longitudinal and transverse matrix elements in electron energy loss spectra can
explain the mystery of relativistic effects on the {\it magic angle}. Here we
show that there is an additional correction of order where is
the atomic number and the fine structure constant, which is not
necessarily small for heavy elements. Moreover, we suggest that macroscopic
electrodynamic effects can give further corrections which can break the
sample-independence of the magic angle.Comment: 10 pages (double column), 6 figure
Sine-Gordon Model - Renormalization Group Solutions and Applications
The sine-Gordon model is discussed and analyzed within the framework of the
renormalization group theory. A perturbative renormalization group procedure is
carried out through a decomposition of the sine-Gordon field in slow and fast
modes. An effective slow modes's theory is derived and re-scaled to obtain the
model's flow equations. The resulting Kosterlitz-Thouless phase diagram is
obtained and discussed in detail. The theory's gap is estimated in terms of the
sine-Gordon model paramaters. The mapping between the sine-Gordon model and
models for interacting electrons in one dimension, such as the g-ology model
and Hubbard model, is discussed and the previous renormalization group results,
obtained for the sine-Gordon model, are thus borrowed to describe different
aspects of Luttinger liquid systems, such as the nature of its excitations and
phase transitions. The calculations are carried out in a thorough and
pedagogical manner, aiming the reader with no previous experience with the
sine-Gordon model or the renormalization group approach.Comment: 44 pages, 7 figure
Entropy flow in near-critical quantum circuits
Near-critical quantum circuits are ideal physical systems for asymptotically
large-scale quantum computers, because their low energy collective excitations
evolve reversibly, effectively isolated from the environment. The design of
reversible computers is constrained by the laws governing entropy flow within
the computer. In near-critical quantum circuits, entropy flows as a locally
conserved quantum current, obeying circuit laws analogous to the electric
circuit laws. The quantum entropy current is just the energy current divided by
the temperature. A quantum circuit made from a near-critical system (of
conventional type) is described by a relativistic 1+1 dimensional relativistic
quantum field theory on the circuit. The universal properties of the
energy-momentum tensor constrain the entropy flow characteristics of the
circuit components: the entropic conductivity of the quantum wires and the
entropic admittance of the quantum circuit junctions. For example,
near-critical quantum wires are always resistanceless inductors for entropy. A
universal formula is derived for the entropic conductivity:
\sigma_S(\omega)=iv^{2}S/\omega T, where \omega is the frequency, T the
temperature, S the equilibrium entropy density and v the velocity of `light'.
The thermal conductivity is Real(T\sigma_S(\omega))=\pi v^{2}S\delta(\omega).
The thermal Drude weight is, universally, v^{2}S. This gives a way to measure
the entropy density directly.Comment: 2005 paper published 2017 in Kadanoff memorial issue of J Stat Phys
with revisions for clarity following referee's suggestions, arguments and
results unchanged, cross-posting now to quant-ph, 27 page
Flavour-coherent propagators and Feynman rules: Covariant cQPA formulation
We present a simplified and generalized derivation of the flavour-coherent
propagators and Feynman rules for the fermionic kinetic theory based on
coherent quasiparticle approximation (cQPA). The new formulation immediately
reveals the composite nature of the cQPA Wightman function as a product of two
spectral functions and an effective two-point interaction vertex, which
contains all quantum statistical and coherence information. We extend our
previous work to the case of nonzero dispersive self-energy, which leads to a
broader range of applications. By this scheme, we derive flavoured kinetic
equations for local 2-point functions , which are
reminiscent of the equations of motion for the density matrix. We emphasize
that in our approach all the interaction terms are derived from first
principles of nonequilibrium quantum field theory.Comment: 20 pages, 3 figures. Minor modifications, version published in JHE
Large Anomalous Hall effect in a silicon-based magnetic semiconductor
Magnetic semiconductors are attracting high interest because of their
potential use for spintronics, a new technology which merges electronics and
manipulation of conduction electron spins. (GaMn)As and (GaMn)N have recently
emerged as the most popular materials for this new technology. While Curie
temperatures are rising towards room temperature, these materials can only be
fabricated in thin film form, are heavily defective, and are not obviously
compatible with Si. We show here that it is productive to consider transition
metal monosilicides as potential alternatives. In particular, we report the
discovery that the bulk metallic magnets derived from doping the narrow gap
insulator FeSi with Co share the very high anomalous Hall conductance of
(GaMn)As, while displaying Curie temperatures as high as 53 K. Our work opens
up a new arena for spintronics, involving a bulk material based only on
transition metals and Si, and which we have proven to display a variety of
large magnetic field effects on easily measured electrical properties.Comment: 19 pages with 5 figure
Isoperimetric inequalities for some integral operators arising in potential theory
In this paper we review our previous isoperimetric results for the
logarithmic potential and Newton potential operators. The main reason why the
results are useful, beyond the intrinsic interest of geometric extremum
problems, is that they produce a priori bounds for spectral invariants of
operators on arbitrary domains. We demonstrate these in explicit examples.Comment: This conference paper gives a review of our previous results in the
subjec
Non-Fermi-liquid d-wave metal phase of strongly interacting electrons
Developing a theoretical framework for conducting electronic fluids
qualitatively distinct from those described by Landau's Fermi-liquid theory is
of central importance to many outstanding problems in condensed matter physics.
One such problem is that, above the transition temperature and near optimal
doping, high-transition-temperature copper-oxide superconductors exhibit
`strange metal' behaviour that is inconsistent with being a traditional Landau
Fermi liquid. Indeed, a microscopic theory of a strange-metal quantum phase
could shed new light on the interesting low-temperature behaviour in the
pseudogap regime and on the d-wave superconductor itself. Here we present a
theory for a specific example of a strange metal---the 'd-wave metal'. Using
variational wavefunctions, gauge theoretic arguments, and ultimately
large-scale density matrix renormalization group calculations, we show that
this remarkable quantum phase is the ground state of a reasonable microscopic
Hamiltonian---the usual t-J model with electron kinetic energy and two-spin
exchange supplemented with a frustrated electron `ring-exchange' term,
which we here examine extensively on the square lattice two-leg ladder. These
findings constitute an explicit theoretical example of a genuine
non-Fermi-liquid metal existing as the ground state of a realistic model.Comment: 22 pages, 12 figures: 6 pages, 7 figures of main text + 16 pages, 5
figures of Supplementary Information; this is approximately the version
published in Nature, minus various subedits in the main tex
On the (anisotropic) uniform metallic ground states of fermions interacting through arbitrary two-body potentials in d dimensions
We demonstrate that the skeleton of the Fermi surface S_{F;s} pertaining to a
uniform metallic ground state (corresponding to fermions with spin index s) is
determined by the Hartree-Fock contribution to the dynamic self-energy. The
Fermi surface S_{F;s} consists of all points which in addition to satisfying
the quasi-particle equation in terms of the Hartree-Fock self-energy, fulfill
the equation S_{s}(k) = 0, where S_{s}(k) is defined in the main text; the set
of k points which satisfy the Hartree-Fock quasi-particle equation but fail to
satisfy S_{s}(k) = 0, constitute the pseudo-gap region of the putative Fermi
surface of the interacting system. We consider the behaviour of the
ground-state momentum-distribution function n_{s}(k) for k in the vicinity of
S_{F;s} and show that whereas for the uniform metallic ground states of the
conventional Hubbard Hamiltonian n_{s}(k) is greater/less than 0.5 for k
approaching S_{F;s} from inside/outside the Fermi sea, for interactions of
non-zero range these inequalities can be violated (without thereby contravening
the condition of the non-negativity of the possible jump in n_{s}(k) on k
crossing S_{F;s} from directly inside to directly outside the Fermi sea). We
discuss, in the light of the findings of the present work, the growing
experimental evidence with regard to the `frustration' of the kinetic energy of
the charge carriers in the normal states of the copper-oxide-based
high-temperature superconducting compounds. [Short abstract]Comment: 30 pages, 3 postscript figures. Brought into conformity with the
published versio
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