137 research outputs found
Optimal Hypercontractivity for Fermi Fields and Related Non-Commutative Integration
Optimal hypercontractivity bounds for the fermion oscillator semigroup are
obtained. These are the fermion analogs of the optimal hypercontractivity
bounds for the boson oscillator semigroup obtained by Nelson. In the process,
several results of independent interest in the theory of non-commutative
integration are established. {}.Comment: 18 p., princeton/ecel/7-12-9
Surface Magnetization of Aperiodic Ising Quantum Chains
We study the surface magnetization of aperiodic Ising quantum chains. Using
fermion techniques, exact results are obtained in the critical region for
quasiperiodic sequences generated through an irrational number as well as for
the automatic binary Thue-Morse sequence and its generalizations modulo p. The
surface magnetization exponent keeps its Ising value, beta_s=1/2, for all the
sequences studied. The critical amplitude of the surface magnetization depends
on the strength of the modulation and also on the starting point of the chain
along the aperiodic sequence.Comment: 11 pages, 6 eps-figures, Plain TeX, eps
Super-Hubbard models and applications
We construct XX- and Hubbard- like models based on unitary superalgebras
gl(N|M) generalising Shastry's and Maassarani's approach of the algebraic case.
We introduce the R-matrix of the gl(N|M) XX model and that of the Hubbard model
defined by coupling two independent XX models. In both cases, we show that the
R-matrices satisfy the Yang--Baxter equation, we derive the corresponding local
Hamiltonian in the transfer matrix formalism and we determine the symmetry of
the Hamiltonian. Explicit examples are worked out. In the cases of the gl(1|2)
and gl(2|2) Hubbard models, a perturbative calculation at two loops a la Klein
and Seitz is performed.Comment: 26 page
Making Almost Commuting Matrices Commute
Suppose two Hermitian matrices almost commute (). Are they close to a commuting pair of Hermitian matrices, ,
with ? A theorem of H. Lin
shows that this is uniformly true, in that for every there exists
a , independent of the size of the matrices, for which almost
commuting implies being close to a commuting pair. However, this theorem does
not specify how depends on . We give uniform bounds relating
and . We provide tighter bounds in the case of block
tridiagonal and tridiagonal matrices and a fully constructive method in that
case. Within the context of quantum measurement, this implies an algorithm to
construct a basis in which we can make a {\it projective} measurement that
approximately measures two approximately commuting operators simultaneously.
Finally, we comment briefly on the case of approximately measuring three or
more approximately commuting operators using POVMs (positive operator-valued
measures) instead of projective measurements.Comment: 22 pages; tighter bounds; Note: fixed mistake in proof pointed out by
Filonov and Kachkovski
Spin-Charge Separation in the Model: Magnetic and Transport Anomalies
A real spin-charge separation scheme is found based on a saddle-point state
of the model. In the one-dimensional (1D) case, such a saddle-point
reproduces the correct asymptotic correlations at the strong-coupling
fixed-point of the model. In the two-dimensional (2D) case, the transverse
gauge field confining spinon and holon is shown to be gapped at {\em finite
doping} so that a spin-charge deconfinement is obtained for its first time in
2D. The gap in the gauge fluctuation disappears at half-filling limit, where a
long-range antiferromagnetic order is recovered at zero temperature and spinons
become confined. The most interesting features of spin dynamics and transport
are exhibited at finite doping where exotic {\em residual} couplings between
spin and charge degrees of freedom lead to systematic anomalies with regard to
a Fermi-liquid system. In spin dynamics, a commensurate antiferromagnetic
fluctuation with a small, doping-dependent energy scale is found, which is
characterized in momentum space by a Gaussian peak at (, ) with
a doping-dependent width (, is the doping
concentration). This commensurate magnetic fluctuation contributes a
non-Korringa behavior for the NMR spin-lattice relaxation rate. There also
exits a characteristic temperature scale below which a pseudogap behavior
appears in the spin dynamics. Furthermore, an incommensurate magnetic
fluctuation is also obtained at a {\em finite} energy regime. In transport, a
strong short-range phase interference leads to an effective holon Lagrangian
which can give rise to a series of interesting phenomena including linear-
resistivity and Hall-angle. We discuss the striking similarities of these
theoretical features with those found in the high- cuprates and give aComment: 70 pages, RevTex, hard copies of 7 figures available upon request;
minor revisions in the text and references have been made; To be published in
July 1 issue of Phys. Rev. B52, (1995
Many body physics from a quantum information perspective
The quantum information approach to many body physics has been very
successful in giving new insight and novel numerical methods. In these lecture
notes we take a vertical view of the subject, starting from general concepts
and at each step delving into applications or consequences of a particular
topic. We first review some general quantum information concepts like
entanglement and entanglement measures, which leads us to entanglement area
laws. We then continue with one of the most famous examples of area-law abiding
states: matrix product states, and tensor product states in general. Of these,
we choose one example (classical superposition states) to introduce recent
developments on a novel quantum many body approach: quantum kinetic Ising
models. We conclude with a brief outlook of the field.Comment: Lectures from the Les Houches School on "Modern theories of
correlated electron systems". Improved version new references adde
Thermal transport in one-dimensional spin gap systems
We study thermal transport in one dimensional spin systems both in the
presence and absence of impurities. In the absence of disorder, all these spin
systems display a temperature dependent Drude peak in the thermal conductivity.
In gapless systems, the low temperature Drude weight is proportional to
temperature and to the central charge which characterizes the conformal field
theory that describes the system at low energies. On the other hand, the low
temperature Drude weight of spin gap systems shows an activated behavior
modulated by a power law. For temperatures higher than the spin gap, one
recovers the linear T behavior akin to gapless systems. For temperatures larger
than the exchange coupling, the Drude weight decays as 1/T^2. We argue that
this behavior is a generic feature of quasi one dimensional spin gap systems
with a relativistic-like low energy dispersion. We also consider the effect of
a magnetic field on the Drude weight with emphasis on the
commensurate-incommensurate transition induced by it. We then study the effect
of nonmagnetic impurities on the thermal conductivity of the dimerized XY chain
and the spin-1/2 two leg ladder. Impurities destroy the Drude peak and the
thermal conductivity exhibits a purely activated behavior at low temperature,
with an activation gap renormalized by disorder. The relevance of these results
for experiments is briefly discussed.Comment: 13 pages, 6 eps figures, RevTeX
TRY plant trait database â enhanced coverage and open access
Plant traitsâthe morphological, anatomical, physiological, biochemical and phenological characteristics of plantsâdetermine how plants respond to environmental factors, affect other trophic levels, and influence ecosystem properties and their benefits and detriments to people. Plant trait data thus represent the basis for a vast area of research spanning from evolutionary biology, community and functional ecology, to biodiversity conservation, ecosystem and landscape management, restoration, biogeography and earth system modelling. Since its foundation in 2007, the TRY database of plant traits has grown continuously. It now provides unprecedented data coverage under an open access data policy and is the main plant trait database used by the research community worldwide. Increasingly, the TRY database also supports new frontiers of traitâbased plant research, including the identification of data gaps and the subsequent mobilization or measurement of new data. To support this development, in this article we evaluate the extent of the trait data compiled in TRY and analyse emerging patterns of data coverage and representativeness. Best species coverage is achieved for categorical traitsâalmost complete coverage for âplant growth formâ. However, most traits relevant for ecology and vegetation modelling are characterized by continuous intraspecific variation and traitâenvironmental relationships. These traits have to be measured on individual plants in their respective environment. Despite unprecedented data coverage, we observe a humbling lack of completeness and representativeness of these continuous traits in many aspects. We, therefore, conclude that reducing data gaps and biases in the TRY database remains a key challenge and requires a coordinated approach to data mobilization and trait measurements. This can only be achieved in collaboration with other initiatives
Autoantibodies against type I IFNs in patients with life-threatening COVID-19
Interindividual clinical variability in the course of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is vast. We report that at least 101 of 987 patients with life-threatening coronavirus disease 2019 (COVID-19) pneumonia had neutralizing immunoglobulin G (IgG) autoantibodies (auto-Abs) against interferon-w (IFN-w) (13 patients), against the 13 types of IFN-a (36), or against both (52) at the onset of critical disease; a few also had auto-Abs against the other three type I IFNs. The auto-Abs neutralize the ability of the corresponding type I IFNs to block SARS-CoV-2 infection in vitro. These auto-Abs were not found in 663 individuals with asymptomatic or mild SARS-CoV-2 infection and were present in only 4 of 1227 healthy individuals. Patients with auto-Abs were aged 25 to 87 years and 95 of the 101 were men. A B cell autoimmune phenocopy of inborn errors of type I IFN immunity accounts for life-threatening COVID-19 pneumonia in at least 2.6% of women and 12.5% of men
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