197 research outputs found
Exact solution of A-D Temperley-Lieb Models
We solve for the spectrum of quantum spin chains based on representations of
the Temperley-Lieb algebra associated with the quantum groups {\cal U}_q(X_n }
for X_n = A_1,B_n,C_nD_n$. We employ a generalization of the coordinate
Bethe-Ansatz developed previously for the deformed biquadratic spin one chain.
As expected, all these models have equivalent spectra, i.e. they differ only in
the degeneracy of their eigenvalues. This is true for finite length and open
boundary conditions. For periodic boundary conditions the spectra of the lower
dimensional representations are containded entirely in the higher dimensional
ones. The Bethe states are highest weight states of the quantum group, except
for some states with energy zero
Recording from two neurons: second order stimulus reconstruction from spike trains and population coding
We study the reconstruction of visual stimuli from spike trains, recording
simultaneously from the two H1 neurons located in the lobula plate of the fly
Chrysomya megacephala. The fly views two types of stimuli, corresponding to
rotational and translational displacements. If the reconstructed stimulus is to
be represented by a Volterra series and correlations between spikes are to be
taken into account, first order expansions are insufficient and we have to go
to second order, at least. In this case higher order correlation functions have
to be manipulated, whose size may become prohibitively large. We therefore
develop a Gaussian-like representation for fourth order correlation functions,
which works exceedingly well in the case of the fly. The reconstructions using
this Gaussian-like representation are very similar to the reconstructions using
the experimental correlation functions. The overall contribution to rotational
stimulus reconstruction of the second order kernels - measured by a chi-squared
averaged over the whole experiment - is only about 8% of the first order
contribution. Yet if we introduce an instant-dependent chi-square to measure
the contribution of second order kernels at special events, we observe an up to
100% improvement. As may be expected, for translational stimuli the
reconstructions are rather poor. The Gaussian-like representation could be a
valuable aid in population coding with large number of neurons
Quantum spin chains of Temperley-Lieb type: periodic boundary conditions, spectral multiplicities and finite temperature
We determine the spectra of a class of quantum spin chains of Temperley-Lieb
type by utilizing the concept of Temperley-Lieb equivalence with the S=1/2 XXZ
chain as a reference system. We consider open boundary conditions and in
particular periodic boundary conditions. For both types of boundaries the
identification with XXZ spectra is performed within isomorphic representations
of the underlying Temperley-Lieb algebra. For open boundaries the spectra of
these models differ from the spectrum of the associated XXZ chain only in the
multiplicities of the eigenvalues. The periodic case is rather different. Here
we show how the spectrum is obtained sector-wise from the spectra of globally
twisted XXZ chains. As a spin-off, we obtain a compact formula for the
degeneracy of the momentum operator eigenvalues. Our representation theoretical
results allow for the study of the thermodynamics by establishing a
TL-equivalence at finite temperature and finite field.Comment: 29 pages, LaTeX, two references added, redundant figures remove
Scaling limit of the one-dimensional attractive Hubbard model: The half-filled band case
The scaling limit of the higher level Bethe Ansatz (HLBA) equations for a
macroscopically half-filled Hubbard chain is considered. These equations
practically decouple into three disjoint sets which are again of the BA type,
and correspond to the secular equations of three different kinds of dressed
particles (one massive and two massless). The finite size corrections and the
fine structure of the spectrum show that the massless sector corresponds to a
conformal field with central charge c=1 and Gaussian anomalous dimensions. The
zero temperature free energy is also calculated and is found to be in perfect
agreement with the results of a perturbative calculation in the SU(2) chiral
Gross-Neveu (CGN) model.Comment: LATEX, uses Revtex, 39 page
Generalized Particle Statistics in Two-Dimensions: Examples from the Theory of Free Massive Dirac Field
In the framework of algebraic quantum field theory we analyze the anomalous
statistics exhibited by a class of automorphisms of the observable algebra of
the two-dimensional free massive Dirac field, constructed by fermionic gauge
group methods. The violation of Haag duality, the topological peculiarity of a
two-dimensional space-time and the fact that unitary implementers do not lie in
the global field algebra account for strange behaviour of statistics, which is
no longer an intrinsic property of sectors. Since automorphisms are not inner,
we exploit asymptotic abelianness of intertwiners in order to construct a
braiding for a suitable -tensor subcategory of End(). We
define two inequivalent classes of path connected bi-asymptopias, selecting
only those sets of nets which yield a true generalized statistics operator.Comment: 24 page
Clinical benefit of fulvestrant in postmenopausal women with advanced breast cancer and primary or acquired resistance to aromatase inhibitors: final results of phase II Swiss Group for Clinical Cancer Research Trial (SAKK 21/00)
Background: The aim of this study was to evaluate the efficacy and tolerability of fulvestrant, an estrogen receptor antagonist, in postmenopausal women with hormone-responsive tumors progressing after aromatase inhibitor (AI) treatment. Patients and methods: This is a phase II, open, multicenter, noncomparative study. Two patient groups were prospectively considered: group A (n = 70) with AI-responsive disease and group B (n = 20) with AI-resistant disease. Fulvestrant 250 mg was administered as intramuscular injection every 28 (±3) days. Results: All patients were pretreated with AI and 84% also with tamoxifen or toremifene; 67% had bone metastases and 45% liver metastases. Fulvestrant administration was well tolerated and yielded a clinical benefit (CB; defined as objective response or stable disease [SD] for ≥24 weeks) in 28% (90% confidence interval [CI] 19% to 39%) of patients in group A and 37% (90% CI 19% to 58%) of patients in group B. Median time to progression (TTP) was 3.6 (95% CI 3.0 to 4.8) months in group A and 3.4 (95% CI 2.5 to 6.7) months in group B. Conclusions: Overall, 30% of patients who had progressed following prior AI treatment gained CB with fulvestrant, thereby delaying indication to start chemotherapy. Prior response to an AI did not appear to be predictive for benefit with fulvestran
Multi-particle structure in the Z_n-chiral Potts models
We calculate the lowest translationally invariant levels of the Z_3- and
Z_4-symmetrical chiral Potts quantum chains, using numerical diagonalization of
the hamiltonian for N <= 12 and N <= 10 sites, respectively, and extrapolating
N to infinity. In the high-temperature massive phase we find that the pattern
of the low-lying zero momentum levels can be explained assuming the existence
of n-1 particles carrying Z_n-charges Q = 1, ... , n-1 (mass m_Q), and their
scattering states. In the superintegrable case the masses of the n-1 particles
become proportional to their respective charges: m_Q = Q m_1. Exponential
convergence in N is observed for the single particle gaps, while power
convergence is seen for the scattering levels. We also verify that
qualitatively the same pattern appears for the self-dual and integrable cases.
For general Z_n we show that the energy-momentum relations of the particles
show a parity non-conservation asymmetry which for very high temperatures is
exclusive due to the presence of a macroscopic momentum P_m=(1-2Q/n)/\phi,
where \phi is the chiral angle and Q is the Z_n-charge of the respective
particle.Comment: 22 pages (LaTeX) plus 5 figures (included as PostScript),
BONN-HE-92-3
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