46,180 research outputs found
Fourier-transform spectroscopy of Sr2 and revised ground state potential
Precise potentials for the ground state X1Sigma+g and the minimum region of
the excited state 2_1Sigma+u of Sr2 are derived by high resolution
Fourier-transform spectroscopy of fluorescence progressions from single
frequency laser excitation of Sr2 produced in a heat pipe at 950 Celsius. A
change of the rotational assignment by four units compared to an earlier work
(G. Gerber, R. M\"oller, and H. Schneider, J. Chem. Phys. 81, 1538 (1984)) is
needed for a consistent description leading to a significant shift of the
potentials towards longer inter atomic distances. The huge amount of ground
state data derived for the three different isotopomers 88Sr2, 86Sr88Sr and
87Sr88Sr (almost 60% of all excisting bound rovibrational ground state levels
for the isotopomer 88Sr2) fixes this assignment undoubtedly. The presented
ground state potential is derived from the observed transitions for the radial
region from 4 to 11 A (9 cm-1 below the asymptote) and is extended to the longe
range region by the use of theoretical dispersion coefficients together with
already available photoassociation data. New estimations of the scattering
lengths for the complete set of isotopic combinations are derived by mass
scaling with the derived potential. The data set for the excited state
2_1Sigma+u was sufficient to derive a potential energy curve around the
minimum.Comment: 10 pages, 7 figures, some small corrections done especially to the
potential description of the excited state (already included in the published
journal version
Investigation to develop a process for production of oxide fibers by melt draw technique Final report
Process for production of oxide fibers by melt draw techniqu
Reducing the numerical effort of finite-temperature density matrix renormalization group transport calculations
Finite-temperature transport properties of one-dimensional systems can be
studied using the time dependent density matrix renormalization group via the
introduction of auxiliary degrees of freedom which purify the thermal
statistical operator. We demonstrate how the numerical effort of such
calculations is reduced when the physical time evolution is augmented by an
additional time evolution within the auxiliary Hilbert space. Specifically, we
explore a variety of integrable and non-integrable, gapless and gapped models
at temperatures ranging from T=infty down to T/bandwidth=0.05 and study both
(i) linear response where (heat and charge) transport coefficients are
determined by the current-current correlation function and (ii) non-equilibrium
driven by arbitrary large temperature gradients. The modified DMRG algorithm
removes an 'artificial' build-up of entanglement between the auxiliary and
physical degrees of freedom. Thus, longer time scales can be reached
Elliptic Genera of Symmetric Products and Second Quantized Strings
In this note we prove an identity that equates the elliptic genus partition
function of a supersymmetric sigma model on the N-fold symmetric product
of a manifold M to the partition function of a second quantized
string theory on the space . The generating function of these
elliptic genera is shown to be (almost) an automorphic form for O(3,2,Z). In
the context of D-brane dynamics, this result gives a precise computation of the
free energy of a gas of D-strings inside a higher-dimensional brane.Comment: 17 pages, latex, 1 figure, to appear in Commun. Math. Phy
Finite temperature dynamical DMRG and the Drude weight of spin-1/2 chains
We propose an easily implemented approach to study time-dependent correlation
functions of one dimensional systems at finite temperature T using the density
matrix renormalization group. The entanglement growth inherent to any
time-dependent calculation is significantly reduced if the auxiliary degrees of
freedom which purify the statistical operator are time evolved with the
physical Hamiltonian but reversed time. We exploit this to investigate the long
time behavior of current correlation functions of the XXZ spin-1/2 Heisenberg
chain. This allows a direct extraction of the Drude weight D at intermediate to
large T. We find that D is nonzero -- and thus transport is dissipationless --
everywhere in the gapless phase. At low temperatures we establish an upper
bound to D by comparing with bosonization
Unbounded growth of entanglement in models of many-body localization
An important and incompletely answered question is whether a closed quantum
system of many interacting particles can be localized by disorder. The time
evolution of simple (unentangled) initial states is studied numerically for a
system of interacting spinless fermions in one dimension described by the
random-field XXZ Hamiltonian. Interactions induce a dramatic change in the
propagation of entanglement and a smaller change in the propagation of
particles. For even weak interactions, when the system is thought to be in a
many-body localized phase, entanglement shows neither localized nor diffusive
behavior but grows without limit in an infinite system: interactions act as a
singular perturbation on the localized state with no interactions. The
significance for proposed atomic experiments is that local measurements will
show a large but nonthermal entropy in the many-body localized state. This
entropy develops slowly (approximately logarithmically) over a diverging time
scale as in glassy systems.Comment: 4 pages, 2 figures, v2. added more dat
Logarithmic terms in entanglement entropies of 2D quantum critical points and Shannon entropies of spin chains
Universal logarithmic terms in the entanglement entropy appear at quantum
critical points (QCPs) in one dimension (1D) and have been predicted in 2D at
QCPs described by 2D conformal field theories. The entanglement entropy in a
strip geometry at such QCPs can be obtained via the "Shannon entropy" of a 1D
spin chain with open boundary conditions. The Shannon entropy of the XXZ chain
is found to have a logarithmic term that implies, for the QCP of the
square-lattice quantum dimer model, a logarithm with universal coefficient . However, the logarithm in the Shannon entropy of the transverse-field
Ising model, which corresponds to entanglement in the 2D Ising conformal QCP,
is found to have a singular dependence on replica or R\'enyi index resulting
from flows to different boundary conditions at the entanglement cut.Comment: 4 pages and 4 page appendix, 4 figure
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