5,224 research outputs found
Quantum Topological Excitations: from the Sawtooth Lattice to the Heisenberg Chain
The recently elucidated structure of the delafossite YCuO reveals a
Cu-O network with nearly independent chains having different
interactions between the spins. Motivated by this result, we study the
chain for various ratios of the base-base and
base-vertex interactions. By exact diagonalization and extrapolation, we show
that the elementary excitation spectrum, which (within numerical error) is the
same for total spins and 1, has a gap only in the interval
. The gap is dispersionless
for , but has increasing -dependence as moves away from unity, related to the instability of dimers in
the ground state.Comment: 4 pages, 6 figures (revtex twocolumn
Finite Temperature Behavior of Small Silicon and Tin Clusters: An Ab Initio Molecular Dynamics Study
The finite temperature behavior of small Silicon (Si, Si, and
Si) and Tin (Sn and Sn) clusters is studied using
isokinetic Born-Oppenheimer molecular dynamics. The lowest equilibrium
structures of all the clusters are built upon a highly stable tricapped
trigonal prism unit which is seen to play a crucial role in the finite
temperature behavior of these clusters. Thermodynamics of small tin clusters
(Sn and Sn) is revisited in light of the recent experiments on
tin clusters of sizes 18-21 [G. A. Breaux et. al. Phys. Rev. B {\bf 71} 073410
(2005)]. We have calculated heat capacities using multiple histogram technique
for Si, Sn and Si clusters. Our calculated specific heat
curves have a main peak around 2300 K and 2200 K for Si and Sn
clusters respectively. However, various other melting indicators such as root
mean square bond length fluctuations, mean square displacements show that
diffusive motion of atoms within the cluster begins around 650 K. The finite
temperature behavior of Si and Sn is dominated by isomerization
and it is rather difficult to discern the temperature range for transition
region. On the other hand, Si does show a liquid like behavior over a
short temperature range followed by the fragmentation observed around 1800 K.
Finite temperature behavior of Si and Sn show that these clusters
do not melt but fragment around 1200 K and 650 K respectively.Comment: 9 figure
Field-theory calculation of the electric dipole moment of the neutron and paramagnetic atoms
Electric dipole moments (edms) of bound states that arise from the
constituents having edms are studied with field-theoretic techniques. The
systems treated are the neutron and a set of paramagnetic atoms. In the latter
case it is well known that the atomic edm differs greatly from the electron edm
when the internal electric fields of the atom are taken into account. In the
nonrelativistic limit these fields lead to a complete suppression, but for
heavy atoms large enhancement factors are present. A general bound-state field
theory approach applicable to both the neutron and paramagnetic atoms is set
up. It is applied first to the neutron, treating the quarks as moving freely in
a confining spherical well. It is shown that the effect of internal electric
fields is small in this case. The atomic problem is then revisited using
field-theory techniques in place of the usual Hamiltonian methods, and the
atomic enhancement factor is shown to be consistent with previous calculations.
Possible application of bound-state techniques to other sources of the neutron
edm is discussed.Comment: 21 pages, 5 figure
Measurement of the 6S-7S transition polarizablility in atomic cesium and an improved test of the standard model
The ratio of the off-diagonal hyperfine amplitude to the tensor transition
polarizability (Mhf/beta) for the 6S-7S transition in cesium has been measured.
The value of beta=27.024(43)(expt)(67)(theory)a_0^3 is then obtained using an
accurate semi-empirical value of Mhf. This is combined with a previous
measurement of parity nonconservation in atomic cesium and previous atomic
structure calculations to determine the value of the weak charge. The
uncertainties in the atomic structure calculations are updated (and reduced) in
light of new experimental tests. The result Q_W=-72.06(28)(expt) (34)(theory)
differs from the prediction of the standard model of elementary particle
physics by 2.5 sigma.Comment: 12 pages, 1 figur
The Compton-thick quasar at the heart of the high-redshift giant radio galaxy 6C 0905+39
Our XMM-Newton spectrum of the giant, high-redshift (z=1.88) radio galaxy 6C
0905+39 shows that it contains one of the most powerful, high-redshift,
Compton-thick quasars known. Its spectrum is very hard above 2 keV. The steep
XMM spectrum below that energy is shown to be due to extended emission from the
radio bridge using Chandra data. The nucleus of 6C 0905+39 has a column density
of 3.5 (+1.4,-0.4) X 10^24 cm^-2 and absorption-corrected X-ray luminosity of
1.7 (+0.9,-0.1) X 10^45 erg/s in the 2-10 keV band. A lower redshift active
galaxy in the same field, SDSS J090808.36+394313.6, may also be Compton-thick.Comment: 5 pages, 5 figures, accepted to MNRA
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