3,850 research outputs found
Inverse photoemission in strongly correlated electron systems
Based on exact results for small clusters of 2D t-J model we demonstrate the
existence of several distinct `channels' in its inverse photoemission (IPES)
spectrum. Hole-like quasiparticles can either be annihilated completely, or
leave behind a variable number of spin excitations, which formed the `dressing
cloud' of the annihilated hole. In the physical parameter regime the latter
processes carry the bulk of IPES weight and although the Fermi surface takes
the form of hole pockets, the distribution of spectal weight including these
`magnon-bands' in the IPES spectrum is reminiscent of free electrons. The
emerging scenario for Fermiology and spectral weight distribution is shown to
be consistent with photoemission, inverse photemission and de Haas--van Alphen
experiments on cuprate superconductors.Comment: Revtex file, 4 PRB pages + three figures appended as uu-encoded
postscript. Hardcopies of figures (or the entire manuscript) can also be
obtained by e-mail request to: [email protected]
Landau mapping and Fermi liquid parameters of the 2D t-J model
We study the momentum distribution function n(k) in the 2D t-J model on small
clusters by exact diagonalization. We show that n(k) can be decomposed
systematically into two components with Bosonic and Fermionic doping
dependence. The Bosonic component originates from the incoherent motion of
holes and has no significance for the low energy physics. For the Fermionic
component we exlicitely perform the one-to-one Landau mapping between the low
lying eigenstates of the t-J model clusters and those of an equivalent system
of spin-1/2 quasiparticles. This mapping allows to extract the quasiparticle
dispersion, statistics, and Landau parameters. The results show conclusively
that the 2D t-J model for small doping is a Fermi liquid with a `small' Fermi
surface and a moderately strong attractive interaction between the
quasiparticles.Comment: Revtex file, 5 pages with 5 embedded eps-files, hardcopies of figures
(or the entire manuscript) can be obtained by e-mail request to:
[email protected]
Spectral density for a hole in an antiferromagnetic stripe phase
Using variational trial wave function based on the string picture we study
the motion of a single mobile hole in the stripe phase of the doped
antiferromagnet. The holes within the stripes are taken to be static, the
undoped antiferromagnetic domains in between the hole stripes are assumed to
have alternating staggered magnetization, as is suggested by neutron scattering
experiments. The system is described by the t-t'-t''-J model with realistic
parameters and we compute the single particle spectral density.Comment: RevTex-file, 9 PRB pages with 15 .eps and .gif files. To appear in
PRB. Hardcopies of figures (or the entire manuscript) can be obtained by
e-mail request to: [email protected]
Temperature dependent band structure of the Kondo insulator
We present a Qantum Monte Carlo (QMC) study of the temperature dependent
dynamics of the Kondo insulator. Working at the so-called symmetrical point
allows to perform minus-sign free QMC simulations and thus reach temperatures
of less than 1% of the conduction electron bandwidth. Study of the temperature
dependence of the single particle Green's function and dynamical spin
correlation function shows a surprisingly intricate low temperature band
structure and gives evidence for two characteristic temperatures, which we
identify with the Kondo and coherence temperature, respectively. In particular,
the data show a temperature induced metal-insulator transition at the coherence
temperature.Comment: RevTex-file, 4 PRB pages with 4 eps figures. Hardcopies of figures
(or the entire manuscript) can be obtained by e-mail request to:
[email protected]
A Tableaux Calculus for Reducing Proof Size
A tableau calculus is proposed, based on a compressed representation of
clauses, where literals sharing a similar shape may be merged. The inferences
applied on these literals are fused when possible, which reduces the size of
the proof. It is shown that the obtained proof procedure is sound,
refutationally complete and allows to reduce the size of the tableau by an
exponential factor. The approach is compatible with all usual refinements of
tableaux.Comment: Technical Repor
Excitation spectrum of the homogeneous spin liquid
We discuss the excitation spectrum of a disordered, isotropic and
translationally invariant spin state in the 2D Heisenberg antiferromagnet. The
starting point is the nearest-neighbor RVB state which plays the role of the
vacuum of the theory, in a similar sense as the Neel state is the vacuum for
antiferromagnetic spin wave theory. We discuss the elementary excitations of
this state and show that these are not Fermionic spin-1/2 `spinons' but spin-1
excited dimers which must be modeled by bond Bosons. We derive an effective
Hamiltonian describing the excited dimers which is formally analogous to spin
wave theory. Condensation of the bond-Bosons at zero temperature into the state
with momentum (pi,pi) is shown to be equivalent to antiferromagnetic ordering.
The latter is a key ingredient for a microscopic interpretation of Zhang's
SO(5) theory of cuprate superconductivityComment: RevTex-file, 16 PRB pages with 13 embedded eps figures. Hardcopies of
figures (or the entire manuscript) can be obtained by e-mail request to:
[email protected]
Gas-Rich Dwarf Galaxies from the PSS-II --- II. Optical Properties
We describe the optical properties of a sample of 101 gas-rich field dwarf
galaxies found on PSS-II (Second Palomar Sky Survey) plates, most newly
discovered as part of a survey to investigate the clustering properties of
dwarf galaxies relative to giants. These galaxies have low surface brightnesses
and are relatively distant, with recession velocities ranging up to 10,000
km/s. They have bluer V-I colors (median value of 0.75) than either actively
star-forming giant galaxies or low metallicity globular clusters, implying that
these dwarfs have both low metallicities and little past star formation. These
galaxies are also extremely gas rich, with a median HI mass to V luminosity
ratio of approximately 2 in solar units. We divide the sample into two groups:
true dwarfs with diameters (at 25 I mag arcsec^-2) less than 7.5 kpc and
Magellanic dwarfs with diameters greater than that value. The true dwarfs have
greater HI mass to V luminosity ratios and slightly bluer V-I colors than the
Magellanic dwarfs. Overall, the optical properties of our sample of dwarf
galaxies point towards their being quiescent objects that have undergone little
star formation over the age of the universe. They are not faded objects, but
instead may be going through one of their first periods of weak star formation.Comment: 27 pages, to appear in 20 May 1997 ApJ, paper also available at
http://www.astro.nwu.edu/astro/pildis/dwarfphot.html and
http://zebu.uoregon.edu/~js/dwarf.htm
Dynamics of spin ladders
We derive an approximate theory for Heisenberg spin ladders with two legs by
mapping the spin dynamics onto the problem of hard-core `bond-Bosons'. The
parameters of the Bosonic Hamiltonian are obtained by matching anomalous
Green's functions to Lanczos results and we find evidence for a strong
renormalization due to quantum fluctuations. Various dynamical spin correlation
functions are calculated and found to be in good agreement with Lanczos
results. We then enlarge the effective Hamiltonian to describe the coupling of
the bond-Bosons to a single hole injected into the system and treat the
hole-dynamics within the `rainbow-diagram' approximation by Schmidt-Rink et.
al. Theoretical predictions for the single hole spectral function are obtained
and found to be in good agreement with Lanczos results.Comment: RevTex-file, 10 PRB pages with 7 eps files. Hardcopies of figures (or
the entire manuscript) can be obtained by e-mail request to:
[email protected]
Advanced materials design based on waste wood and bark
Trees belong to the largest living organisms on Earth and plants in general are one of our main renewable resources. Wood as a material has been used since the beginning of humankind. Today, forestry still provides raw materials for a variety of applications, for example in the building industry, in paper manufacturing and for various wood products. However, many parts of the tree, such as reaction wood, branches and bark are often discarded as forestry residues and waste wood, used as additives in composite materials or burned for energy production. More advanced uses of bark include the extraction of chemical substances for glues, food additives or healthcare, as well as the transformation to advanced carbon materials. Here, we argue that a proper understanding of the internal fibrous structure and the resulting mechanical behaviour of these forest residues allows for the design of materials with greatly varying properties and applications. We show that simple and cheap treatments can give tree bark a leather-like appearance that can be used for the construction of shelters and even the fabrication of woven textiles. This article is part of the theme issue ‘Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 1)’
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