616,275 research outputs found
Role of Orbitals in the Physics of Correlated Electron Systems
Rich properties of systems with strongly correlated electrons, such as
transition metal oxides, is largely connected with an interplay of different
degrees of freedom in them: charge, spin, orbital ones, as well as crystal
lattice. Specific and often very important role is played by orbital degrees of
freedom. In this comment I will shortly summarize the main concepts and discuss
some of the well-known manifestations of orbital degrees of freedom, but will
mostly concentrate on a recent development in this field.Comment: To be published in "Comments on Solid State Physics", part of
"Physica Scripta
Coupling the valley degree of freedom to antiferromagnetic order
Conventional electronics are based invariably on the intrinsic degrees of
freedom of an electron, namely, its charge and spin. The exploration of novel
electronic degrees of freedom has important implications in both basic quantum
physics and advanced information technology. Valley as a new electronic degree
of freedom has received considerable attention in recent years. In this paper,
we develop the theory of spin and valley physics of an antiferromagnetic
honeycomb lattice. We show that by coupling the valley degree of freedom to
antiferromagnetic order, there is an emergent electronic degree of freedom
characterized by the product of spin and valley indices, which leads to
spin-valley dependent optical selection rule and Berry curvature-induced
topological quantum transport. These properties will enable optical
polarization in the spin-valley space, and electrical detection/manipulation
through the induced spin, valley and charge fluxes. The domain walls of an
antiferromagnetic honeycomb lattice harbors valley-protected edge states that
support spin-dependent transport. Finally, we employ first principles
calculations to show that the proposed optoelectronic properties can be
realized in antiferromagnetic manganese chalcogenophosphates (MnPX_3, X = S,
Se) in monolayer form.Comment: 6 pages, 5 figure
Classification of atomic-scale multipoles under crystallographic point groups and application to linear response tensors
Four types of atomic-scale multipoles, electric, magnetic, magnetic toroidal,
and electric toroidal multipoles, give a complete set to describe arbitrary
degrees of freedom for coupled charge, spin, and orbital of electrons. We here
present a systematic classification of these multipole degrees of freedom
towards the application in condensed matter physics. Starting from the
multipole description under the rotation group in real space, we generalize the
concept of multipoles in momentum space with the spin degree of freedom. We
show how multipoles affect the electronic band structures and linear responses,
such as the magneto-electric effect, magneto-current (magneto-gyrotropic)
effect, spin conductivity, Piezo-electric effect, and so on. Moreover, we
exhibit a complete table to represent the active multipoles under 32
crystallographic point groups. Our comprehensive and systematic analyses will
give a foundation to identify enigmatic electronic order parameters and a guide
to evaluate peculiar cross-correlated phenomena in condensed matter physics
from microscopic point of view.Comment: 37 pages, 4 figures, 32 table
Confinement, Vacuum Structure: from QCD to Quantum Gravity
A minimal Lorentz gauge gravity model with R^2-type Lagrangian is proposed.
In the absence of torsion the model admits a topological phase with unfixed
metric. The model possesses a minimal set of dynamical degrees of freedom for
the torsion. Remarkably, the torsion has the same number of dynamical of-shell
degrees of freedom as the metric tensor. We trace an analogy between the
structure of the quantum chromodynamics and the structure of possible theory of
quantum gravity.Comment: 7 pages; reduced version of talk given at IV International Symposium
on Symmetries in Subatomic Physics (SSP 2009), plenary session in Honor of
Yongmin Cho's 65th Birthday, Taipei, Taiwan, 2-5 June 2009; to appear in
"Symmetries in Subatomic Physics", ed. P. Hwang
Consistent effective description of nucleonic resonances in an unitary relativistic field-theoretic way
High energy strong interaction physics is successfully described by the local
renormalizable gauge theory called Quantum-Chromo-Dynamics (QCD) with quarks
and gluons as ``elementary'' degrees of freedom, while intermediate energy
strong interaction physics shows up to be determined by a non-local,
non--renormalizable effective field theory (EFT) of ``effective'' degrees of
freedom like mesons, ground state baryons and resonances. Within the picture of
an effective field theory of strong interaction at intermediate energies I
present a ``toy-model'' in which fermionic and bosonic resonances are
considered to be ``particles'', i.e. they consistently are described by
(anti-)commuting effective field-operators (containing dynamics of infinitely
many quark-gluon or meson-nucleon diagrams) which are comfortably treated by
Wick's Theorem in a covariant framework and obey unitarity. Non-trivial
implications to couplings of non-local interactions are shown.Comment: 8 pages, 1 figure; invited talk given at XIV. Int. Sem. on High
Energy Physics Probl., 17.-22.8.1998, Dubna (to be published in the
proceedings
Quantum transport in carbon nanotubes
Carbon nanotubes are a versatile material in which many aspects of condensed
matter physics come together. Recent discoveries, enabled by sophisticated
fabrication, have uncovered new phenomena that completely change our
understanding of transport in these devices, especially the role of the spin
and valley degrees of freedom. This review describes the modern understanding
of transport through nanotube devices.
Unlike conventional semiconductors, electrons in nanotubes have two angular
momentum quantum numbers, arising from spin and from valley freedom. We focus
on the interplay between the two. In single quantum dots defined in short
lengths of nanotube, the energy levels associated with each degree of freedom,
and the spin-orbit coupling between them, are revealed by Coulomb blockade
spectroscopy. In double quantum dots, the combination of quantum numbers
modifies the selection rules of Pauli blockade. This can be exploited to read
out spin and valley qubits, and to measure the decay of these states through
coupling to nuclear spins and phonons. A second unique property of carbon
nanotubes is that the combination of valley freedom and electron-electron
interactions in one dimension strongly modifies their transport behaviour.
Interaction between electrons inside and outside a quantum dot is manifested in
SU(4) Kondo behavior and level renormalization. Interaction within a dot leads
to Wigner molecules and more complex correlated states.
This review takes an experimental perspective informed by recent advances in
theory. As well as the well-understood overall picture, we also state clearly
open questions for the field. These advances position nanotubes as a leading
system for the study of spin and valley physics in one dimension where
electronic disorder and hyperfine interaction can both be reduced to a very low
level.Comment: In press at Reviews of Modern Physics. 68 pages, 55 figure
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