744 research outputs found
Invariants of the Haldane-Shastry Chain
Using a formalism developed by Polychronakos, we explicitly construct a set
of invariants of the motion for the Haldane-Shastry chain.Comment: 11 pages, UVA-92-0
Electronic Structure of LuRh2Si2: "Small" Fermi Surface Reference to YbRh2Si2
We present band structure calculations and quantum oscillation measurements
on LuRh2Si2, which is an ideal reference to the intensively studied quantum
critical heavy-fermion system YbRh2Si2. Our band structure calculations show a
strong sensitivity of the Fermi surface on the position of the silicon atoms
zSi within the unit cell. Single crystal structure refinement and comparison of
predicted and observed quantum oscillation frequencies and masses yield zSi =
0.379c in good agreement with numerical lattice relaxation. This value of zSi
is suggested for future band structure calculations on LuRh2Si2 and YbRh2Si2.
LuRh2Si2 with a full f electron shell represents the "small" Fermi surface
configuration of YbRh2Si2. Our experimentally and ab initio derived quantum
oscillation frequencies of LuRh2Si2 show strong differences with earlier
measurements on YbRh2Si2. Consequently, our results confirm the contribution of
the f electrons to the Fermi surface of YbRh2Si2 at high magnetic fields. Yet
the limited agreement with refined fully itinerant local density approximation
calculations highlights the need for more elaborated models to describe the
Fermi surface of YbRh2Si2.Comment: 12 pages 10 figure
Starburst Galaxies: Why the Calzetti Dust Extinction Law?
The empirical reddening function for starburst galaxies generated by Calzetti
and her co-workers has proven very successful, and is now used widely in the
observational literature. Despite its success, however, the physical basis for
this extinction law, or more correctly, attenuation law remains weak. Here we
provide a physical explanation for the Calzetti Law based on a turbulent
interstellar medium. In essence, this provides a log-normal distribution of
column densities, giving a wide range of column densities in the dusty
foreground screen. Therefore, extended sources such as starburst regions or HII
regions seen through it suffer a point-to-point stochastic extinction and
reddening. Regions of high column densities are "black" in the UV, but
translucent in the IR, which leads to a flatter extinction law, and a larger
value of the total to selective extinction, R_V. We fit the Calzetti Law, and
infer that the variance sigma of the log-normal distribution lies in the range
0.6<sigma<2.2. The absolute to selective extinction R_V is found to be in the
range 4.3 to 5.2 consistent with R_V=4.05+/-0.80 of the Calzetti Law.Comment: accepted for publication in ApJ
Taxonomy of particles in Ising spin chains
The statistical mechanics of particles with shapes on a one-dimensional
lattice is investigated in the context of the Ising chain with uniform
nearest-neighbor coupling, quadratic single-site potential, and magnetic field,
which supports four distinct ground states:
, ,
, . The complete
spectrum is generated from each ground state by particles from a different set
of six or seven species. Particles and elements of pseudo-vacuum are
characterized by motifs (patterns of several consecutive site variables).
Particles are floating objects that can be placed into open slots on the
lattice. Open slots are recognized as permissible links between motifs. The
energy of a particle varies between species but is independent of where it is
placed. Placement of one particle changes the open-slot configuration for
particles of all species. This statistical interaction is encoded in a
generalized Pauli principle, from which the multiplicity of states for a given
particle combination is determined and used for the exact statistical
mechanical analysis. Particles from all species belong to one of four
categories: compacts, hosts, tags, or hybrids. Compacts and hosts find open
slots in segments of pseudo-vacuum. Tags find open slots inside hosts. Hybrids
are tags with hosting capability. In the taxonomy of particles proposed here,
`species' is indicative of structure and `category' indicative of function. The
hosting function splits the Pauli principle into exclusion and accommodation
parts. Near phase boundaries, the state of the Ising chain at low temperature
is akin to that of miscible or immiscible liquids with particles from one
species acting as surfactant molecules.Comment: 12 pages, 6 tables, 6 figure
Statistically interacting quasiparticles in Ising chains
The exclusion statistics of two complementary sets of quasiparticles,
generated from opposite ends of the spectrum, are identified for Ising chains
with spin s=1/2,1. In the s=1/2 case the two sets are antiferromagnetic domain
walls (solitons) and ferromagnetic domains (strings). In the s=1 case they are
soliton pairs and nested strings, respectively. The Ising model is equivalent
to a system of two species of solitons for s=1/2 and to a system of six species
of soliton pairs for s=1. Solitons exist on single bonds but soliton pairs may
be spread across many bonds. The thermodynamics of a system of domains spanning
up to lattice sites is amenable to exact analysis and shown to become
equivalent, in the limit M -> infinity, to the thermodynamics of the s=1/2
Ising chain. A relation is presented between the solitons in the Ising limit
and the spinons in the XX limit of the s=1/2 XXZ chain.Comment: 18 pages and 4 figure
Are the Narrow Line Regions in Active Galaxies Dusty and Radiation Pressure Dominated?
The remarkable similarity between emission spectra of narrow line regions
(NLR) in Seyfert Galaxies has long presented a mystery. In photoionization
models, this similarity implies that the ionization parameter is nearly always
the same, about U ~ 0.01. Here we present dusty, radiation-pressure dominated
photoionization models that can provide natural physical insight into this
problem. In these models, dust and the radiation pressure acting on it provide
the controlling factor in moderating the density, excitation and surface
brightness of photoionized NLR structures. Additionally, photoelectric heating
by the dust is important in determining the temperature structure of the
models. These models can also explain the coexistence of the low-,
intermediate- and coronal ionization zones within a single self-consistent
physical structure. The radiation pressure acting on dust may also be capable
of driving the fast (~3000 km/s) outflows such as are seen in the HST
observations of NGC 1068.Comment: 23 pages, 8 figures, Accepted by Ap
Thermodynamics of statistically interacting quantum gas in D dimensions
We present the exact thermodynamics (isochores, isotherms, isobars, response
functions) of a statistically interacting quantum gas in D dimensions. The
results in D=1 are those of the thermodynamic Bethe ansatz for the nonlinear
Schroedinger model, a gas with repulsive two-body contact potential. In all
dimensions the ideal boson and fermion gases are recovered in the weak-coupling
and strong-coupling limits, respectively. For all nonzero couplings ideal
fermion gas behavior emerges for D>>1 and, in the limit D->infinity, a phase
transition occurs at T>0. Significant deviations from ideal quantum gas
behavior are found for intermediate coupling and finite D.Comment: 12 pages and 19 figure
Strong in-plane anisotropy in the electronic structure of fixed-valence -LuAlB
The origin of intrinsic quantum criticality in the heavy-fermion
superconductor -YbAlB has been attributed to strong Yb valence
fluctuations and its peculiar crystal structure. Here, we assess these
contributions individually by studying the isostructural but fixed-valence
compound -LuAlB. Quantum oscillation measurements and DFT
calculations reveal a Fermi surface markedly different from that of
-YbAlB, consistent with a `large' Fermi surface there. We also find
an unexpected in-plane anisotropy of the electronic structure, in contrast to
the isotropic Kondo hybridization in -YbAlB.Comment: 6 pages, 4 figure
Ground State and Excitations of Spin Chain with Orbital Degeneracy
The one dimensional Heisenberg model in the presence of orbital degeneracy is
studied at the SU(4) symmetric viewpoint by means of Bethe ansatz. Following
Sutherland's previous work on an equivalent model, we discuss the ground state
and the low-lying excitations more extensively in connection to the spin
systems with orbital degeneracy. We show explicitly that the ground state is a
SU(4) singlet. We study the degeneracies of the elementary excitations and the
spectra of the generalized magnons consisting of these excitations. We also
discuss the complex 2-strings in the context of the Bethe ansatz solutions.Comment: Revtex, 9 pages, 3 figures; typos correcte
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