810 research outputs found
Viking X-band telemetry experiment
In order to uncover operational and design problems in the use of X-band by the 1977 Mariner Jupiter-Saturn mission and future spacecraft using the Deep Space Network, the Viking X-band telemetry experiment was conducted. The experiment was conducted during the months of December 1975 and January 1976. During each of the five successful passes, a periodic sequence (in lieu of ranging) was transmitted to the spacecraft and returned by the spacecraft transponder on both S- and X-bands. These telemetry-like signals were received, demodulated, and detected. From a variety of measurements at the station, four independent measurements were made of the received signal-to-noise ratio (SNR). These four SNRs were later compared with each other and the predicted SNR. The principal result of the experiment is that X-band telemetry works as expected. That is, the measured SNRs were consistent relative to each other and to the predicted values within the accuracy of the experiment
Ferromagnetic Luttinger Liquids
We study weak itinerant ferromagnetism in one-dimensional Fermi systems using
perturbation theory and bosonization. We find that longitudinal spin
fluctuations propagate ballistically with velocity v_m << v_F, where v_F is the
Fermi velocity. This leads to a large anomalous dimension in the spin-channel
and strong algebraic singularities in the single-particle spectral function and
in the transverse structure factor for momentum transfers q ~ 2 Delta/v_F,
where 2 Delta is the exchange splitting.Comment: 4 pages, 3 figure
Hopping on the Bethe lattice: Exact results for densities of states and dynamical mean-field theory
We derive an operator identity which relates tight-binding Hamiltonians with
arbitrary hopping on the Bethe lattice to the Hamiltonian with nearest-neighbor
hopping. This provides an exact expression for the density of states (DOS) of a
non-interacting quantum-mechanical particle for any hopping. We present
analytic results for the DOS corresponding to hopping between nearest and
next-nearest neighbors, and also for exponentially decreasing hopping
amplitudes. Conversely it is possible to construct a hopping Hamiltonian on the
Bethe lattice for any given DOS. These methods are based only on the so-called
distance regularity of the infinite Bethe lattice, and not on the absence of
loops. Results are also obtained for the triangular Husimi cactus, a recursive
lattice with loops. Furthermore we derive the exact self-consistency equations
arising in the context of dynamical mean-field theory, which serve as a
starting point for studies of Hubbard-type models with frustration.Comment: 14 pages, 9 figures; introduction expanded, references added;
published versio
Ferromagnetism in the two dimensional t-t' Hubbard model at the Van Hove density
Using an improved version of the projection quantum Monte Carlo technique, we
study the square-lattice Hubbard model with nearest-neighbor hopping t and
next-nearest-neighbor hopping t', by simulation of lattices with up to 20 X 20
sites. For a given R=2t'/t, we consider that filling which leads to a singular
density of states of the noninteracting problem. For repulsive interactions, we
find an itinerant ferromagnet (antiferromagnet) for R=0.94 (R=0.2). This is
consistent with the prediction of the T-matrix approximation, which sums the
most singular set of diagrams.Comment: 10 pages, RevTeX 3.0 + a single postscript file with all figure
Exact analytic results for the Gutzwiller wave function with finite magnetization
We present analytic results for ground-state properties of Hubbard-type
models in terms of the Gutzwiller variational wave function with non-zero
values of the magnetization m. In dimension D=1 approximation-free evaluations
are made possible by appropriate canonical transformations and an analysis of
Umklapp processes. We calculate the double occupation and the momentum
distribution, as well as its discontinuity at the Fermi surface, for arbitrary
values of the interaction parameter g, density n, and magnetization m. These
quantities determine the expectation value of the one-dimensional Hubbard
Hamiltonian for any symmetric, monotonically increasing dispersion epsilon_k.
In particular for nearest-neighbor hopping and densities away from half filling
the Gutzwiller wave function is found to predict ferromagnetic behavior for
sufficiently large interaction U.Comment: REVTeX 4, 32 pages, 8 figure
Ferromagnetism in Correlated Electron Systems: Generalization of Nagaoka's Theorem
Nagaoka's theorem on ferromagnetism in the Hubbard model with one electron
less than half filling is generalized to the case where all possible
nearest-neighbor Coulomb interactions (the density-density interaction ,
bond-charge interaction , exchange interaction , and hopping of double
occupancies ) are included. It is shown that for ferromagnetic exchange
coupling () ground states with maximum spin are stable already at finite
Hubbard interaction . For non-bipartite lattices this requires a hopping
amplitude . For vanishing one obtains as in
Nagaoka's theorem. This shows that the exchange interaction is important
for stabilizing ferromagnetism at finite . Only in the special case
the ferromagnetic state is stable even for , provided the lattice allows
the hole to move around loops.Comment: 13 pages, uuencoded postscript, includes 1 table and 2 figure
Improved stability regions for ground states of the extended Hubbard model
The ground state phase diagram of the extended Hubbard model containing
nearest and next-to-nearest neighbor interactions is investigated in the
thermodynamic limit using an exact method. It is found that taking into account
local correlations and adding next-to-nearest neighbor interactions both have
significant effects on the position of the phase boundaries. Improved stability
domains for the -pairing state and for the fully saturated ferromagnetic
state at half filling have been constructed. The results show that these states
are the ground states for model Hamiltonians with realistic values of the
interaction parameters.Comment: 21 pages (10 figures are included) Revtex, revised version. To be
published in Phys. Rev. B. E-mail: [email protected]
Borrowing against the future: the response to the public consultation on the NHS bursary
This paper discusses the UK Government's public consultation into the NHS bursary and the response from the Nursing and Midwifery Council. A public consultation stipulated that the current arrangements for funding, by the State, were not to be considered for discussion. Instead, the consultation only appraised views that would lead to the successful introduction of student finance loans for NHS professional education. Testimonies from nurses, midwives and nursing students expressed concern that the new funding arrangements were unaffordable, dis-incentivising and biased towards the marketisation of student loans in the UK, yet the changes went ahead. The changes to NHS bursary funding resulted from the UK Government's desire for growth in student numbers (and ostensibly not growth in financial figures), and the fact that nursing (and other healthcare) students will become more fiscally indebted, despite society's moral obligation to them
Relaxation of a one-dimensional Mott insulator after an interaction quench
We obtain the exact time evolution for the one-dimensional integrable
fermionic 1/r Hubbard model after a sudden change of its interaction parameter,
starting from either a metallic or a Mott-insulating eigenstate. In all cases
the system relaxes to a new steady state, showing that the presence of the Mott
gap does not inhibit relaxation. The properties of the final state are
described by a generalized Gibbs ensemble. We discuss under which conditions
such ensembles provide the correct statistical description of isolated
integrable systems in general. We find that generalized Gibbs ensembles do
predict the properties of the steady state correctly, provided that the
observables or initial states are sufficiently uncorrelated in terms of the
constants of motion.Comment: 9 pages, 1 figure; published versio
Thermodynamically consistent equilibrium properties of normal-liquid Helium-3
The high-precision data for the specific heat C_{V}(T,V) of normal-liquid
Helium-3 obtained by Greywall, taken together with the molar volume V(T_0,P) at
one temperature T_0, are shown to contain the complete thermodynamic
information about this phase in zero magnetic field. This enables us to
calculate the T and P dependence of all equilibrium properties of normal-liquid
Helium-3 in a thermodynamically consistent way for a wide range of parameters.
The results for the entropy S(T,P), specific heat at constant pressure
C_P(T,P), molar volume V(T,P), compressibility kappa(T,P), and thermal
expansion coefficient alpha(T,P) are collected in the form of figures and
tables. This provides the first complete set of thermodynamically consistent
values of the equilibrium quantities of normal-liquid Helium-3. We find, for
example, that alpha(T,P) has a surprisingly intricate pressure dependence at
low temperatures, and that the curves alpha(T,P) vs T do not cross at one
single temperature for all pressures, in contrast to the curves presented in
the comprehensive survey of helium by Wilks.
Corrected in cond-mat/9906222v3: The sign of the coefficient d_0 was
misprinted in Table I of cond-mat/9906222v1 and v2. It now correctly reads
d_0=-7.1613436. All results in the paper were obtained with the correct value
of d_0. (We would like to thank for E. Collin, H. Godfrin, and Y. Bunkov for
finding this misprint.)Comment: 19 pages, 19 figures, 9 tables; published version; note added in
proof; v3: misprint correcte
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