379 research outputs found
Dynamical electron transport through a nanoelectromechanical wire in a magnetic field
We investigate dynamical transport properties of interacting electrons moving
in a vibrating nanoelectromechanical wire in a magnetic field. We have built an
exactly solvable model in which electric current and mechanical oscillation are
treated fully quantum mechanically on an equal footing. Quantum mechanically
fluctuating Aharonov-Bohm phases obtained by the electrons cause nontrivial
contribution to mechanical vibration and electrical conduction of the wire. We
demonstrate our theory by calculating the admittance of the wire which are
influenced by the multiple interplay between the mechanical and the electrical
energy scales, magnetic field strength, and the electron-electron interaction
Ground state properties of ferromagnetic metal/conjugated polymer interfaces
We theoretically investigate the ground state properties of ferromagnetic
metal/conjugated polymer interfaces. The work is partially motivated by recent
experiments in which injection of spin polarized electrons from ferromagnetic
contacts into thin films of conjugated polymers was reported. We use a
one-dimensional nondegenerate Su-Schrieffer-Heeger (SSH) Hamiltonian to
describe the conjugated polymer and one-dimensional tight-binding models to
describe the ferromagnetic metal. We consider both a model for a conventional
ferromagnetic metal, in which there are no explicit structural degrees of
freedom, and a model for a half-metallic ferromagnetic colossal
magnetoresistance (CMR) oxide which has explicit structural degrees of freedom.
The Fermi energy of the magnetic metallic contact is adjusted to control the
degree of electron transfer into the polymer. We investigate electron charge
and spin transfer from the ferromagnetic metal to the organic polymer, and
structural relaxation near the interface. Bipolarons are the lowest energy
charge state in the bulk polymer for the nondegenerate SSH model Hamiltonian.
As a result electrons (or holes) transferred into the bulk of the polymer form
spinless bipolarons. However, there can be spin density in the polymer
localized near the interface.Comment: 7 figure
Doping dependence of the exchange energies in bilayer manganites: Role of orbital degrees of freedom
Recently, an intriguing doping dependence of the exchange energies in the
bilayer manganites has been observed in the neutron
scattering experiments. The intra-layer exchange only weakly changed with
doping while the inter-layer one drastically decreased. Here we propose a
theory which accounts for these experimental findings. We argue, that the
observed striking doping dependence of the exchange energies can be attributed
to the evaluation of the orbital level splitting with doping. The latter is
handled by the interplay between Jahn-Teller effect (supporting an axial
orbital) and the orbital anisotropy of the electronic band in the bilayer
structure (promoting an in-plane orbital), which is monitored by the Coulomb
repulsion. The presented theory, while being a mean-field type, describes well
the experimental data and also gives the estimates of the several interesting
energy scales involved in the problem.Comment: Added references, corrected typos. To appear in Phys. Rev.
Optical Investigations of Charge Gap in Orbital Ordered La1/2Sr3/2MnO4
Temperature and polarization dependent electronic structure of La1/2Sr3/2MnO4
were investigated by optical conductivity analyses. With decreasing
temperature, for E//ab, a broad mid-infrared (MIR) peak of La1/2Sr3/2MnO4
becomes narrower and moves to the higher frequency, while that of
Nd1/2Sr3/2MnO4 nearly temperature independent. We showed that the MIR peak in
La1/2Sr3/2MnO4 originates from orbital ordering associated with CE-type
magnetic ordering and that the Jahn-Teller distortion has a significant
influence on the width and the position of the MIR peak.Comment: 10 pages, 4 figure
Essential Role of the Cooperative Lattice Distortion in the Charge, Orbital and Spin Ordering in doped Manganites
The role of lattice distortion in the charge, orbital and spin ordering in
half doped manganites has been investigated. For fixed magnetic ordering, we
show that the cooperative lattice distortion stabilize the experimentally
observed ordering even when the strong on-site electronic correlation is taken
into account. Furthermore, without invoking the magnetic interactions, the
cooperative lattice distortion alone may lead to the correct charge and orbital
ordering including the charge stacking effect, and the magnetic ordering can be
the consequence of such a charge and orbital ordering. We propose that the
cooperative nature of the lattice distortion is essential to understand the
complicated charge, orbital and spin ordering observed in doped manganites.Comment: 5 pages,4 figure
Polaronic Signatures in Mid-Infrared Spectra: Prediction for LaMnO3 and CaMnO3
Hole-doped LaMnO3 and electron-doped CaMnO3 form self-trapped electronic
states. The spectra of these states have been calculated using a two orbital
(Mn eg Jahn-Teller) model, from which the non-adiabatic optical conductivity
spectra are obtained. In both cases the optical spectrum contains weight in the
gap region, whose observation will indicate the self-trapped nature of the
carrier states. The predicted spectra are proportional to the concentration of
the doped carriers in the dilute regime, with coefficients calculated with no
further model parameters.Comment: 6 pages with 3 figures imbedde
Anisotropic Optic Conductivities due to Spin and Orbital Orderings in LaVO3 and YVO3: First-Principles Studies
The anisotropy of low energy (05eV) optical excitations in strongly
correlated transition-metal oxides is closely related to the spin and orbital
orderings. The recent successes of LDA+ method in describing the magnetic
and electronic structures enable us to calculate the optical conductivity from
first-principles. The LaVO and YVO, both of which have
configuration and have various spin and orbital ordered phases at low
temperature, show distinct anisotropy in the optical spectra. The effects of
spin and orbital ordering on the anisotropy are studied in detail based on our
first-principles calculations. The experimental spectra of both compounds at
low temperature phases can be qualitatively explained with our calculations,
while the studies for the intermediate temperature phase of YVO suggest the
substantial persistence of the low temperature phase at elevated temperature.Comment: 6 pages, 3 figures, accepted by PR
Temperature Dependence of Low-Lying Electronic Excitations of LaMnO_3
We report on the optical properties of undoped single crystal LaMnO_3, the
parent compound of the colossal magneto-resistive manganites. Near-Normal
incidence reflectance measurements are reported in the frequency range of
20-50,000 cm-1 and in the temperature range 10-300 K. The optical conductivity,
s_1(w), is derived by performing a Kramers-Kronig analysis of the reflectance
data. The far-infrared spectrum of s_1(w) displays the infrared active optical
phonons. We observe a shift of several of the phonon to high frequencies as the
temperature is lowered through the Neel temperature of the sample (T_N = 137
K). The high-frequency s_1(w) is characterized by the onset of absorption near
1.5 eV. This energy has been identified as the threshold for optical
transitions across the Jahn-Teller split e_g levels. The spectral weight of
this feature increases in the low-temperature state. This implies a transfer of
spectral weight from the UV to the visible associated with the paramagnetic to
antiferromagnetic state. We discuss the results in terms of the double exchange
processes that affect the optical processes in this magnetic material.Comment: 7 pages, 5 figure
Manipulating the Tomonaga-Luttinger exponent by electric field modulation
We establish a theoretical framework for artificial control of the power-law
singularities in Tomonaga-Luttinger liquid states. The exponent governing the
power-law behaviors is found to increase significantly with an increase in the
amplitude of the periodic electric field modulation applied externally to the
system. This field-induced shift in the exponent indicates the tunability of
the transport properties of quasi-one-dimensional electron systems.Comment: 7 pages, 3 figure
Manganites at Quarter Filling: Role of Jahn-Teller Interactions
We have analyzed different correlation functions in a realistic spin-orbital
model for half-doped manganites. Using a finite-temperature diagonalization
technique the CE phase was found in the charge-ordered phase in the case of
small antiferromagnetic interactions between electrons. It is shown
that a key ingredient responsible for stabilization of the CE-type spin and
orbital-ordered state is the cooperative Jahn-Teller (JT) interaction between
next-nearest Mn neighbors mediated by the breathing mode distortion of
Mn octahedra and displacements of Mn ions. The topological phase
factor in the Mn-Mn hopping leading to gap formation in one-dimensional models
for the CE phase as well as the nearest neighbor JT coupling are not able to
produce the zigzag chains typical for the CE phase in our model.Comment: 16 pages with 16 figures, contains a more detailed parameter estimate
based on the structural data by Radaelli et al. (accepted for publication in
Phys. Rev. B
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