3,319 research outputs found
Manifestation of spin-charge separation in the dynamic dielectric response of one--dimensional Sr2CuO3
We have determined the dynamical dielectric response of a one-dimensional,
correlated insulator by carrying out electron energy-loss spectroscopy on
Sr2CuO3 single crystals. The observed momentum and energy dependence of the
low-energy features, which correspond to collective transitions across the gap,
are well described by an extended one-band Hubbard model with moderate nearest
neighbor Coulomb interaction strength. An exciton-like peak appears with
increasing momentum transfer. These observations provide experimental evidence
for spin-charge separation in the relevant excitations of this compound, as
theoretically expected for the one-dimensional Hubbard model.Comment: RevTex, 4 pages+2 figures, to appear in PRL (July 13
The Littlest Higgs
We present an economical theory of natural electroweak symmetry breaking,
generalizing an approach based on deconstruction. This theory is the smallest
extension of the Standard Model to date that stabilizes the electroweak scale
with a naturally light Higgs and weakly coupled new physics at TeV energies.
The Higgs is one of a set of pseudo Goldstone bosons in an
nonlinear sigma model. The symmetry breaking scale is around a TeV, with
the cutoff \Lambda \lsim 4\pi f \sim 10 TeV. A single electroweak doublet,
the ``little Higgs'', is automatically much lighter than the other pseudo
Goldstone bosons. The quartic self-coupling for the little Higgs is generated
by the gauge and Yukawa interactions with a natural size ,
while the top Yukawa coupling generates a negative mass squared triggering
electroweak symmetry breaking. Beneath the TeV scale the effective theory is
simply the minimal Standard Model. The new particle content at TeV energies
consists of one set of spin one bosons with the same quantum numbers as the
electroweak gauge bosons, an electroweak singlet quark with charge 2/3, and an
electroweak triplet scalar. One loop quadratically divergent corrections to the
Higgs mass are cancelled by interactions with these additional particles.Comment: 15 pages. References added. Corrected typos in the discussion of the
top Yukawa couplin
Effectiveness of one-to-one volunteer support for patients with psychosis: protocol of a randomised controlled trial
Programme Grant for Applied Research from the NIHR; grant no.
RP-PG-00611-20002
Corrections to the universal behavior of the Coulomb-blockade peak splitting for quantum dots separated by a finite barrier
Building upon earlier work on the relation between the dimensionless interdot
channel conductance g and the fractional Coulomb-blockade peak splitting f for
two electrostatically equivalent dots, we calculate the leading correction that
results from an interdot tunneling barrier that is not a delta-function but,
rather, has a finite height V and a nonzero width xi and can be approximated as
parabolic near its peak. We develop a new treatment of the problem for g much
less than 1 that starts from the single-particle eigenstates for the full
coupled-dot system. The finiteness of the barrier leads to a small upward shift
of the f-versus-g curve at small values of g. The shift is a consequence of the
fact that the tunneling matrix elements vary exponentially with the energies of
the states connected. Therefore, when g is small, it can pay to tunnel to
intermediate states with single-particle energies above the barrier height V.
The correction to the zero-width behavior does not affect agreement with recent
experimental results but may be important in future experiments.Comment: Title changed from ``Non-universal...'' to ``Corrections to the
universal...'' No other changes. 10 pages, 1 RevTeX file with 2 postscript
figures included using eps
Little Higgses from an Antisymmetric Condensate
We construct an SU(6)/Sp(6) non-linear sigma model in which the Higgses arise
as pseudo-Goldstone bosons. There are two Higgs doublets whose masses have no
one-loop quadratic sensitivity to the cutoff of the effective theory, which can
be at around 10 TeV. The Higgs potential is generated by gauge and Yukawa
interactions, and is distinctly different from that of the minimal
supersymmetric standard model. At the TeV scale, the new bosonic degrees of
freedom are a single neutral complex scalar and a second copy of SU(2)xU(1)
gauge bosons. Additional vector-like pairs of colored fermions are also
present.Comment: 13 page
Decay of Superconducting and Magnetic Correlations in One- and Two-Dimensional Hubbard Models
In a general class of one and two dimensional Hubbard models, we prove upper
bounds for the two-point correlation functions at finite temperatures for
electrons, for electron pairs, and for spins. The upper bounds decay
exponentially in one dimension, and with power laws in two dimensions. The
bounds rule out the possibility of the corresponding condensation of
superconducting electron pairs, and of the corresponding magnetic ordering. Our
method is general enough to cover other models such as the t-J model.Comment: LaTeX, 8 pages, no figures. A reference appeared after the
publication is adde
A general moment NRIXS approach to the determination of equilibrium Fe isotopic fractionation factors: application to goethite and jarosite
We measured the reduced partition function ratios for iron isotopes in
goethite FeO(OH), potassium-jarosite KFe3(SO4)2(OH)6, and hydronium-jarosite
(H3O)Fe3(SO4)2(OH)6, by Nuclear Resonant Inelastic X-Ray Scattering (NRIXS,
also known as Nuclear Resonance Vibrational Spectroscopy -NRVS- or Nuclear
Inelastic Scattering -NIS) at the Advanced Photon Source. These measurements
were made on synthetic minerals enriched in 57Fe. A new method (i.e., the
general moment approach) is presented to calculate {\beta}-factors from the
moments of the NRIXS spectrum S(E). The first term in the moment expansion
controls iron isotopic fractionation at high temperature and corresponds to the
mean force constant of the iron bonds, a quantity that is readily measured and
often reported in NRIXS studies.Comment: 38 pages, 2 tables, 8 figures. In press at Geochimica et Cosmochimica
Acta. Appendix C contains new derivations relating the moments of the iron
PDOS to the moments of the excitation probability function measured in
Nuclear Resonant Inelastic X-ray Scatterin
Correlational Origin of the Roton Minimum
We present compelling evidence supporting the conjecture that the origin of
the roton in Bose-condensed systems arises from strong correlations between the
constituent particles. By studying the two dimensional bosonic dipole systems a
paradigm, we find that classical molecular dynamics (MD) simulations provide a
faithful representation of the dispersion relation for a low- temperature
quantum system. The MD simulations allow one to examine the effect of coupling
strength on the formation of the roton minimum and to demonstrate that it is
always generated at a sufficiently high enough coupling. Moreover, the
classical images of the roton-roton, roton-maxon, etc. states also appear in
the MD simulation spectra as a consequence of the strong coupling.Comment: 7 pages, 4 figure
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