2,097 research outputs found
Berry Curvature on the Fermi Surface: Anomalous Hall Effect as a Topological Fermi-Liquid Property
The intrinsic anomalous Hall effect in metallic ferromagnets is shown to be
controlled by Berry phases accumulated by adiabatic motion of quasiparticles on
the Fermi surface, and is purely a Fermi-liquid property, not a ``bulk'' Fermi
sea property like Landau diamagnetism, as has been previously supposed. Berry
phases are a new topological ingredient that must be added to Landau
Fermi-liquid theory in the presence of broken inversion or time-reversal
symmetry.Comment: 4 pages, 0 figures; to appear in Physical Review Letters; cleaner
form of main formula+note added confirming continued validity of result in
interacting Fermi liquids: + improved summary paragraph stating result; final
published version (minor changes
Dynamic Fracture in Single Crystal Silicon
We have measured the velocity of a running crack in brittle single crystal
silicon as a function of energy flow to the crack tip. The experiments are
designed to permit direct comparison with molecular dynamics simulations;
therefore the experiments provide an indirect but sensitive test of interatomic
potentials. Performing molecular dynamics simulations of brittle crack motion
at the atomic scale we find that experiments and simulations disagree showing
that interatomic potentials are not yet well understood.Comment: 4 pages, 4 figures, 19 reference
Density Functional Theory of Multicomponent Quantum Dots
Quantum dots with conduction electrons or holes originating from several
bands are considered. We assume the particles are confined in a harmonic
potential and assume the electrons (or holes) belonging to different bands to
be different types of fermions with isotropic effective masses. The density
functional method with the local density approximation is used. The increased
number of internal (Kohn-Sham) states leads to a generalisation of Hund's first
rule at high densities. At low densitites the formation of Wigner molecules is
favored by the increased internal freedom.Comment: 11 pages, 5 figure
Necessary and sufficient condition for longitudinal magnetoresistance
Since the Lorentz force is perpendicular to the magnetic field, it should not
affect the motion of a charge along the field. This argument seems to imply
absence of longitudinal magnetoresistance (LMR) which is, however, observed in
many materials and reproduced by standard semiclassical transport theory
applied to particular metals. We derive a necessary and sufficient condition on
the shape of the Fermi surface for non-zero LMR. Although an anisotropic
spectrum is a pre-requisite for LMR, not all types of anisotropy can give rise
to the effect: a spectrum should not be separable in any sense. More precisely,
the combination , where is the radial
component of the momentum in a cylindrical system with the z-axis along the
magnetic field and ) is the radial (tangential) component
of the velocity, should depend on the momentum along the field. For some
lattice types, this condition is satisfied already at the level of
nearest-neighbor hopping; for others, the required non-separabality occurs only
if next-to-nearest-neighbor hopping is taken into account.Comment: 7 pages, 2 figure
Berry phase effect in anomalous thermoelectric transport
We develop a theory of Berry phase effect in anomalous transport in
ferromagnets driven by statistical forces such as the gradient of temperature
or chemical potential. Here a charge Hall current arises from the Berry phase
correction to the orbital magnetization rather than from the anomalous velocity
which does not exist in the absence of a mechanical force. A finite-temperature
formula for the orbital magnetization is derived, which enables us to provide
an explicit expression for the off-diagonal thermoelectric conductivity, to
establish the Mott relation between the anomalous Nernst and Hall effects, and
to reaffirm the Onsager relations between reciprocal thermoelectric
conductivities. A first-principles evaluation of our expression is carried out
for the material CuCrSeBr, obtaining quantitative agreement
with a recent experiment.Comment: Published version in PR
Dynamical stability of the crack front line
Dynamical stability of the crack front line that propagates between two
plates is studied numerically using the simple two-dimensional mass-spring
model. It is demonstrated that the straight front line is unstable for low
speed while it becomes stable for high speed. For the uniform model, the
roughness exponent in the slower speed region is fairly constant around 0.4 and
there seems to be a rough-smooth transition at a certain speed. For the
inhomogeneous case with quenched randomness, the transition is gradual.Comment: 14 pages, 7 figure
Weak localization in mesoscopic hole transport: Berry phases and classical correlations
We consider phase-coherent transport through ballistic and diffusive
two-dimensional hole systems based on the Kohn-Luttinger Hamiltonian. We show
that intrinsic heavy-hole light-hole coupling gives rise to clear-cut
signatures of an associated Berry phase in the weak localization which renders
the magneto-conductance profile distinctly different from electron transport.
Non-universal classical correlations determine the strength of these Berry
phase effects and the effective symmetry class, leading even to
antilocalization-type features for circular quantum dots and Aharonov-Bohm
rings in the absence of additional spin-orbit interaction. Our semiclassical
predictions are quantitatively confirmed by numerical transport calculations
Large thermoelectric figure of merit for 3D topological Anderson insulators via line dislocation engineering
We study the thermoelectric properties of three-dimensional topological
Anderson insulators with line dislocations. We show that at high densities of
dislocations the thermoelectric figure of merit ZT can be dominated by
one-dimensional topologically-protected conducting states channeled through the
lattice screw dislocations in the topological insulator materials with a
non-zero time-reversal-invariant momentum such as Bi_{1-x}Sb_x. When the
chemical potential does not exceed much the mobility edge the ZT at room
temperatures can reach large values, much higher than unity for reasonable
parameters, hence making this system a strong candidate for applications in
heat management of nano-devices.Comment: 4 pages, 3 figure
Nonlinear optical properties of push–pull polyenes for electro-optics
Improved nonlinear organic chromophores of varying conjugation length with either thiobarbituric acid or 3-dicyanomethylene-2,3-dihydrobenzothiophene-1,1-dioxide (FORON® Blue) acceptors have been synthesized and investigated for their nonlinear optical properties. Very large quadratic hyperpolarizabilities β(−2ω; ω, ω) have been found, up to 25,700×10^(−48) esu at λ=1.91 μm. In a guest–host polymer very high electro-optic (EO) coefficients, of up to 55 pm/V, have been determined at λ=1.31 μm with 20-wt % chromophore loading. We find good agreement between molecular parameters evaluated by electric-field-induced second-harmonic generation (EFISH) and the measurements of guest–host solid–solid solutions. The latter method is well suited to the determination of the product of dipole moment μ and hyperpolarizability β quickly and reliably at the wavelength of interest for EO applications without the complications associated with EFISH measurements
- …