124 research outputs found
Mid-Infrared Conductivity from Mid-Gap States Associated with Charge Stripes
The optical conductivity of La(2-x)Sr(x)NiO(4) has been interpreted in
various ways, but so far the proposed interpretations have neglected the fact
that the holes doped into the NiO(2) planes order in diagonal stripes, as
established by neutron and X-ray scattering. Here we present a study of optical
conductivity in La(2)NiO(4+d) with d=2/15, a material in which the charge
stripes order three-dimensionally. We show that the conductivity can be
decomposed into two components, a mid-infrared peak that we attribute to
transitions from the filled valence band into empty mid-gap states associated
with the stripes, and a Drude peak that appears at higher temperatures as
carriers are thermally excited into the mid-gap states. The shift of the mid-IR
peak to lower energy with increasing temperature is explained in terms of the
Franck-Condon effect. The relevance of these results to understanding the
optical conductivity in the cuprates is discussed.Comment: final version of paper (minor changes from previous version
Optical absorption and single-particle excitations in the 2D Holstein t-J model
To discuss the interplay of electronic and lattice degrees of freedom in
systems with strong Coulomb correlations we have performed an extensive
numerical study of the two-dimensional Holstein t-J model. The model describes
the interaction of holes, doped in a quantum antiferromagnet, with a
dispersionsless optical phonon mode. We apply finite-lattice Lanczos
diagonalization, combined with a well-controlled phonon Hilbert space
truncation, to the Hamiltonian. The focus is on the dynamical properties. In
particular we have evaluated the single-particle spectral function and the
optical conductivity for characteristic hole-phonon couplings, spin exchange
interactions and phonon frequencies. The results are used to analyze the
formation of hole polarons in great detail. Links with experiments on layered
perovskites are made. Supplementary we compare the Chebyshev recursion and
maximum entropy algorithms, used for calculating spectral functions, with
standard Lanczos methods.Comment: 32 pages, 12 figures, submitted to Phys. Rev.
Narrow-band high-lying excitons with negative-mass electrons in monolayer WSe<sub>2</sub>.
Monolayer transition-metal dichalcogenides (TMDCs) show a wealth of exciton physics. Here, we report the existence of a new excitonic species, the high-lying exciton (HX), in single-layer WSe2 with an energy of ~3.4âeV, almost twice the band-edge A-exciton energy, with a linewidth as narrow as 5.8âmeV. The HX is populated through momentum-selective optical excitation in the K-valleys and is identified in upconverted photoluminescence (UPL) in the UV spectral region. Strong electron-phonon coupling results in a cascaded phonon progression with equidistant peaks in the luminescence spectrum, resolvable to ninth order. Ab initio GW-BSE calculations with full electron-hole correlations explain HX formation and unmask the admixture of upper conduction-band states to this complex many-body excitation. These calculations suggest that the HX is comprised of electrons of negative mass. The coincidence of such high-lying excitonic species at around twice the energy of band-edge excitons rationalizes the excitonic quantum-interference phenomenon recently discovered in optical second-harmonic generation (SHG) and explains the efficient Auger-like annihilation of band-edge excitons
Berry phases and pairing symmetry in Holstein-Hubbard polaron systems
We study the tunneling dynamics of dopant-induced hole polarons which are
self-localized by electron-phonon coupling in a two-dimensional antiferro-
magnet. Our treatment is based on a path integral formulation of the adia-
batic approximation, combined with many-body tight-binding, instanton, con-
strained lattice dynamics, and many-body exact diagonalization techniques. Our
results are mainly based on the Holstein- and, for comparison, on the
Holstein-Hubbard model. We also study effects of 2nd neighbor hopping and
long-range electron-electron Coulomb repulsion. The polaron tunneling dynamics
is mapped onto an effective low-energy Hamiltonian which takes the form of a
fermion tight-binding model with occupancy dependent, predominant- ly 2nd and
3rd neighbor tunneling matrix elements, excluded double occupan- cy, and an
effective intersite charge interactions. Antiferromagnetic spin correlations in
the original many-electron Hamiltonian are reflected by an attractive
contribution to the 1st neighbor charge interaction and by Berry phase factors
which determine the signs of effective polaron tunneling ma- trix elements. In
the two-polaron case, these phase factors lead to polaron pair wave functions
of either -wave symmetry or p-wave symme- try with zero and
nonzero total pair momentum, respectively. Implications for the doping
dependent isotope effect, pseudo-gap and Tc of a superconduc- ting polaron pair
condensate are discussed/compared to observed in cuprates.Comment: 23 pages, revtex, 13 ps figure
Competition of Zener and polaron phases in doped CMR manganites
Inspired by the strong experimental evidence for the coexistence of localized
and itinerant charge carriers close to the metal-insulator transition in the
ferromagnetic phase of colossal magnetoresistive manganese perovskites, for a
theoretical description of the CMR transition we propose a two-phase scenario
with percolative characteristics between equal-density polaron and Zener
band-electron states. We find that the subtle balance between these two states
with distinctly different electronic properties can be readily influenced by
varying physical parameters, producing various ``colossal'' effects, such as
the large magnetization and conductivity changes in the vicinity of the
transition temperature.Comment: 8 pages, 5 figure
A search for point sources of EeV photons
Measurements of air showers made using the hybrid technique developed with
the fluorescence and surface detectors of the Pierre Auger Observatory allow a
sensitive search for point sources of EeV photons anywhere in the exposed sky.
A multivariate analysis reduces the background of hadronic cosmic rays. The
search is sensitive to a declination band from -85{\deg} to +20{\deg}, in an
energy range from 10^17.3 eV to 10^18.5 eV. No photon point source has been
detected. An upper limit on the photon flux has been derived for every
direction. The mean value of the energy flux limit that results from this,
assuming a photon spectral index of -2, is 0.06 eV cm^-2 s^-1, and no celestial
direction exceeds 0.25 eV cm^-2 s^-1. These upper limits constrain scenarios in
which EeV cosmic ray protons are emitted by non-transient sources in the
Galaxy.Comment: 28 pages, 10 figures, accepted for publication in The Astrophysical
Journa
Reconstruction of inclined air showers detected with the Pierre Auger Observatory
We describe the method devised to reconstruct inclined cosmic-ray air showers
with zenith angles greater than detected with the surface array of
the Pierre Auger Observatory. The measured signals at the ground level are
fitted to muon density distributions predicted with atmospheric cascade models
to obtain the relative shower size as an overall normalization parameter. The
method is evaluated using simulated showers to test its performance. The energy
of the cosmic rays is calibrated using a sub-sample of events reconstructed
with both the fluorescence and surface array techniques. The reconstruction
method described here provides the basis of complementary analyses including an
independent measurement of the energy spectrum of ultra-high energy cosmic rays
using very inclined events collected by the Pierre Auger Observatory.Comment: 27 pages, 19 figures, accepted for publication in Journal of
Cosmology and Astroparticle Physics (JCAP
On the self-trapping problem of electrons or excitons in one dimension
We present a detailed numerical study of the one-dimensional Holstein model
with a view to understanding the self-trapping process of electrons or excitons
in crystals with short-range particle-lattice interactions. Applying a very
efficient variational Lanczos method, we are able to analyze the ground-state
properties of the system in the weak-- and strong-coupling, adiabatic and
non-adiabatic regimes on lattices large enough to eliminate finite-size
effects. In particular, we obtain the complete phase diagram and comment on the
existence of a critical length for self-trapping in spatially restricted
one-dimensional systems. In order to characterize large and small polaron
states we calculate self-consistently the lattice distortions and the
particle-phonon correlation functions. In the strong-coupling case, two
distinct types of small polaron states are shown to be possible according to
the relative importance of static displacement field and dynamic polaron
effects. Special emphasis is on the intermediate coupling regime, which we also
study by means of direct diagonalization preserving the full dynamics and
quantum nature of phonons. The crossover from large to small polarons shows up
in a strong decrease of the kinetic energy accompanied by a substantial change
in the optical absorption spectra. We show that our numerical results in all
important limiting cases reveal an excellent agreement with both analytical
perturbation theory predictions and very recent density matrix renormalization
group data.Comment: submitted to Phys. Rev.
The Pierre Auger Observatory III: Other Astrophysical Observations
Astrophysical observations of ultra-high-energy cosmic rays with the Pierre
Auger ObservatoryComment: Contributions to the 32nd International Cosmic Ray Conference,
Beijing, China, August 201
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