1,146 research outputs found
Asymptotic tunneling conductance in Luttinger liquids
Conductance through weak constrictions in Luttinger liquids is shown to
vanish with frequency as , where
is a dimensionless parameter characterizing the Luttinger liquid phase, and
and are nonuniversal constants. The first term arises from the ^^
Coulomb blockade' effect and dominates for , whereas the second
results from eliminating high-energy modes and dominates for .Comment: Latex file + one appended postcript figur
A search for ferromagnetism in transition-metal-doped piezoelectric ZnO
We present the results of a computational study of ZnO in the presence of Co
and Mn substitutional impurities. The goal of our work is to identify potential
ferromagnetic ground states within the (Zn,Co)O or (Zn,Mn)O material systems
that are also good candidates for piezoelectricity. We find that, in contrast
to previous results, robust ferromagnetism is not obtained by substitution of
Co or Mn on the Zn site, unless additional carriers (holes) are also
incorporated. We propose a practical scheme for achieving such -type doping
in ZnO
Late-Glacial Paleoecology of the Middle Susitna Valley, Alaska: Environmental Context for Human Dispersal
We present here the results of multi-proxy analyses (sediment geochemistry, diatoms, and pollen) from sediment cores collected at four lakes in the middle Susitna Valley, Alaska. These lakes form a transect from the tundra to the boreal forest. The retrieved cores span from ∼12,000 cal yr BP to the present, with age control provided by radiometric dates and tephra deposits, some of which are newly identified. Results indicate that deglaciation occurred before 12,000 cal yr BP and that by that time, the lakes were deep, productive, and surrounded by shrub tundra. The lake with the highest sampling resolution indicates a brief climatic reversal ∼11,500 cal yr BP with decreased diatom-inferred lake level and lowered lake productivity, and reduced shrub presence. During the early to middle Holocene, all of the sedimentary records provide evidence of climatic amelioration with tree expansion and productive lakes. A middle to late Holocene climatic deterioration with reduced trees and a shallower, less productive lake is also indicated. In addition, the prominent Watana tephra at ∼4,000 cal yr BP likely reduced lake productivity and affected the vegetation. Even though the region was relatively productive soon after deglaciation, people did not occupy the region until ∼11,000 cal yr BP, about 1000 years later, and then only sparsely. By the middle and late Holocene, the region was more densely populated and this shift in human occupancy presumably reflects changes in resource abundance, especially caribou. Whether the Watana ashfall influenced caribou abundance and thus people, is still under investigation, but given the tephra’s effect on vegetation and lake productivity, it seems likely
Hydrogen molecule in a magnetic field: The lowest states of the Pi manifold and the global ground state of the parallel configuration
The electronic structure of the hydrogen molecule in a magnetic field is
investigated for parallel internuclear and magnetic field axes. The lowest
states of the manifold are studied for spin singlet and triplet as well as gerade and ungerade parity for a broad range of field
strengths For both states with gerade parity we
observe a monotonous decrease in the dissociation energy with increasing field
strength up to and metastable states with respect to the
dissociation into two H atoms occur for a certain range of field strengths. For
both states with ungerade parity we observe a strong increase in the
dissociation energy with increasing field strength above some critical field
strength . As a major result we determine the transition field strengths
for the crossings among the lowest , and
states. The global ground state for is the strongly
bound state. The crossings of the with the
and state occur at and , respectively. The transition between the and
state occurs at Therefore, the global ground state of the
hydrogen molecule for the parallel configuration is the unbound
state for The ground state for is the strongly bound state. This result is of great
relevance to the chemistry in the atmospheres of magnetic white dwarfs and
neutron stars.Comment: submitted to Physical Review
Surface Core Level Shifts of Clean and Oxygen Covered Ru(0001)
We have performed high resolution XPS experiments of the Ru(0001) surface,
both clean and covered with well-defined amounts of oxygen up to 1 ML coverage.
For the clean surface we detected two distinct components in the Ru 3d_{5/2}
core level spectra, for which a definite assignment was made using the high
resolution Angle-Scan Photoelectron Diffraction approach. For the p(2x2),
p(2x1), (2x2)-3O and (1x1)-O oxygen structures we found Ru 3d_{5/2} core level
peaks which are shifted up to 1 eV to higher binding energies. Very good
agreement with density functional theory calculations of these Surface Core
Level Shifts (SCLS) is reported. The overriding parameter for the resulting Ru
SCLSs turns out to be the number of directly coordinated O atoms. Since the
calculations permit the separation of initial and final state effects, our
results give valuable information for the understanding of bonding and
screening at the surface, otherwise not accessible in the measurement of the
core level energies alone.Comment: 16 pages including 10 figures. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
The electronic structure of amorphous silica: A numerical study
We present a computational study of the electronic properties of amorphous
SiO2. The ionic configurations used are the ones generated by an earlier
molecular dynamics simulations in which the system was cooled with different
cooling rates from the liquid state to a glass, thus giving access to
glass-like configurations with different degrees of disorder [Phys. Rev. B 54,
15808 (1996)]. The electronic structure is described by a tight-binding
Hamiltonian. We study the influence of the degree of disorder on the density of
states, the localization properties, the optical absorption, the nature of
defects within the mobility gap, and on the fluctuations of the Madelung
potential, where the disorder manifests itself most prominently. The
experimentally observed mismatch between a photoconductivity threshold of 9 eV
and the onset of the optical absorption around 7 eV is interpreted by the
picture of eigenstates localized by potential energy fluctuations in a mobility
gap of approximately 9 eV and a density of states that exhibits valence and
conduction band tails which are, even in the absence of defects, deeply located
within the former band gap.Comment: 21 pages of Latex, 5 eps figure
The physics of dynamical atomic charges: the case of ABO3 compounds
Based on recent first-principles computations in perovskite compounds,
especially BaTiO3, we examine the significance of the Born effective charge
concept and contrast it with other atomic charge definitions, either static
(Mulliken, Bader...) or dynamical (Callen, Szigeti...). It is shown that static
and dynamical charges are not driven by the same underlying parameters. A
unified treatment of dynamical charges in periodic solids and large clusters is
proposed. The origin of the difference between static and dynamical charges is
discussed in terms of local polarizability and delocalized transfers of charge:
local models succeed in reproducing anomalous effective charges thanks to large
atomic polarizabilities but, in ABO3 compounds, ab initio calculations favor
the physical picture based upon transfer of charges. Various results concerning
barium and strontium titanates are presented. The origin of anomalous Born
effective charges is discussed thanks to a band-by-band decomposition which
allows to identify the displacement of the Wannier center of separated bands
induced by an atomic displacement. The sensitivity of the Born effective
charges to microscopic and macroscopic strains is examined. Finally, we
estimate the spontaneous polarization in the four phases of barium titanate.Comment: 25 pages, 6 Figures, 10 Tables, LaTe
Ab-Initio Calculation of Molecular Aggregation Effects: a Coumarin-343 Case Study
We present time-dependent density functional theory (TDDFT) calculations for
single and dimerized Coumarin-343 molecules in order to investigate the quantum
mechanical effects of chromophore aggregation in extended systems designed to
function as a new generation of sensors and light-harvesting devices. Using the
single-chromophore results, we describe the construction of effective
Hamiltonians to predict the excitonic properties of aggregate systems. We
compare the electronic coupling properties predicted by such effective
Hamiltonians to those obtained from TDDFT calculations of dimers, and to the
coupling predicted by the transition density cube (TDC) method. We determine
the accuracy of the dipole-dipole approximation and TDC with respect to the
separation distance and orientation of the dimers. In particular, we
investigate the effects of including Coulomb coupling terms ignored in the
typical tight-binding effective Hamiltonian. We also examine effects of orbital
relaxation which cannot be captured by either of these models
Charge Transfer from Regularized Symmetry-Adapted Perturbation Theory
16 pages, 16 figure
Bonding, Moment Formation, and Magnetic Interactions in Ca14MnBi11 and Ba14MnBi11
The ``14-1-11'' phase compounds based on magnetic Mn ions and typified by
Ca14MnBi11 and Ba14MnBi11 show unusual magnetic behavior, but the large number
(104) of atoms in the primitive cell has precluded any previous full electronic
structure study. Using an efficient, local orbital based method within the
local spin density approximation to study the electronic structure, we find a
gap between a bonding valence band complex and an antibonding conduction band
continuum. The bonding bands lack one electron per formula unit of being
filled, making them low carrier density p-type metals. The hole resides in the
MnBi4 tetrahedral unit and partially compensates the high spin d^5 Mn moment,
leaving a net spin near 4 \mu_B that is consistent with experiment. These
manganites are composed of two disjoint but interpenetrating `jungle gym'
networks of spin 4/2 MnBi4^{9-} units with ferromagnetic interactions within
the same network, and weaker couplings between the networks whose sign and
magnitude is sensitive to materials parameters. Ca14MnBi11 is calculated to be
ferromagnetic as observed, while for Ba14MnBi11 (which is antiferromagnetic)
the ferro- and antiferromagnetic states are calculated to be essentially
degenerate. The band structure of the ferromagnetic states is very close to
half metallic.Comment: 17 pages, containing 10 postscript figures and 5 tables. Two
additional figures (Fig.8 and 11 of the paper) are provided in JPG format in
separate files. Submitted to Phys. Rev. B on September 20th 200
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