824 research outputs found
Violating the string winding number maximally in Anti-de Sitter space
We study n-string scattering amplitudes in three-dimensional Anti-de Sitter
space (AdS3). We focus our attention on the processes in which the winding
number conservation is violated maximally; that is, those processes in which it
is violated in n-2 units. A worldsheet conformal field theory calculation leads
us to confirm a previous conjecture about the functional form of these
observables.Comment: 15 pages. V2 references added and minor typo corrected. V3, V4
addendum on an alternative derivation of the main formula for short string
Systematic Study of Short Range Antiferromagnetic Order and The Spin-Glass State in Lightly Doped La2-xSrxCuO4
Systematic measurements of the magnetic susceptibility were performed on
single crystals of lightly doped La2-xSrxCuO4 (x=0.03, 0.04 and 0.05). For all
samples the temperature dependence of the in-plane magnetic susceptibility
shows typical spin-glass features with spin-glass transition temperatures Tg of
6.3K, 5.5K and 5.0K for x=0.03, 0.04 and 0.05, respectively. The canonical
spin-glass order parameter extracted from the in-plane susceptibility of all
the samples follows a universal scaling curve. On the other hand, the
out-of-plane magnetic susceptibility deviates from Curie law below a
temperature Tdv, higher than Tg. Comparing with previous neutron scattering
results with an instrumental energy resolution of 0.25 meV from Wakimoto et
al., the x-dependence of Tdv is qualitatively the same as that of Tel, the
temperature below which the elastic magnetic scattering develops around (pi,
pi). Thus, a revised magnetic phase diagram in the lightly doped region of
La2-xSrxCuO4 is proposed. The Curie constants calculated from the in-plane
susceptibility are independent of the Sr concentration. On the basis of the
cluster spin-glass model, this fact might reflect an inhomogeneous distribution
of doped holes in the CuO2 plane, such as in a stripe structure.Comment: 7 pages, 6 figure
Free Field Realization of Cylindrically Symmetric Einstein Gravity
Cylindrically reduced Einstein gravity can be regarded as an
sigma model coupled to 2D dilaton gravity. By using the corresponding 2D
diffeomorphism algebra of constraints and the asymptotic behaviour of the Ernst
equation we show that the theory can be mapped by a canonical transformation
into a set of free fields with a Minkowskian target space. We briefly discuss
the quantization in terms of these free-field variables, which is considerably
simpler than in the other approaches.Comment: 8 pages, no figures, discussions on the dual metric and on the
free-field expansion are adde
Inelastic neutron scattering study on the resonance mode in an optimally doped superconductor LaFeAsOF
An optimally doped iron-based superconductor LaFeAsOF with
K has been studied by inelastic powder neutron scattering. The
magnetic excitation at \AA is enhanced below , leading to
a peak at meV as the resonance mode, in addition to the
formation of a gap at low energy below the crossover energy . The peak energy at \AA corresponds to in
good agreement with the other values of resonance mode observed in the various
iron-based superconductors, even in the high- cuprates. Although the
phonon density of states has a peak at the same energy as the resonance mode in
the present superconductor, the -dependence is consistent with the resonance
being of predominately magnetic origin.Comment: 4 pages, 5 Postscript figure
Competing ferromagnetism in high temperature copper oxide superconductors
The extreme variability of observables across the phase diagram of the
cuprate high temperature superconductors has remained a profound mystery, with
no convincing explanation of the superconducting dome. While much attention has
been paid to the underdoped regime of the hole-doped cuprates because of its
proximity to a complex Mott insulating phase, little attention has been paid to
the overdoped regime. Experiments are beginning to reveal that the
phenomenology of the overdoped regime is just as puzzling. For example, the
electrons appear to form a Landau Fermi liquid, but this interpretation is
problematic; any trace of Mott phenomena, as signified by incommensurate
antiferromagnetic fluctuations, is absent, and the uniform spin susceptibility
shows a ferromagnetic upturn. Here we show and justify that many of these
puzzles can be resolved if we assume that competing ferromagnetic fluctuations
are simultaneously present with superconductivity, and the termination of the
superconducting dome in the overdoped regime marks a quantum critical point
beyond which there should be a genuine ferromagnetic phase at zero temperature.
We propose new experiments, and make new predictions, to test our theory and
suggest that effort must be mounted to elucidate the nature of the overdoped
regime, if the problem of high temperature superconductivity is to be solved.
Our approach places competing order as the root of the complexity of the
cuprate phase diagram.Comment: The expanded published version with very minor difference
Unusual magnetic susceptibility anisotropy in untwinned La_{2-x}Sr_xCuO_4 single crystals in the lightly-doped region
We present a study of the magnetic susceptibility in carefully detwinned
La_{2-x}Sr_{x}CuO_4 single crystals in the lightly-doped region (x=0-0.03),
which demonstrates a remarkable in-plane anisotropy of the spin system. This
anisotropy is found to persist after the long-range antiferromagnetic (AF)
order is destroyed by hole doping, suggesting that doped holes break the AF
order into domains in which the spin alignment is kept essentially intact. It
turns out that the freezing of the spins taking place at low temperatures is
also notably anisotropic, implying that the "spin-glass" feature is governed by
the domain structure as well.Comment: 4 pages, 3 figures, accepted for publication in Phys.Rev.Let
On classification of Poisson vertex algebras
We describe a conjectural classification of Poisson vertex algebras of CFT
type and of Poisson vertex algebras in one differential variable (= scalar
Hamiltonian operators)
Diagonal static spin correlation in the low temperature orthorhombic Pccn phase of La1.55Nd0.4Sr0.05CuO4
Elastic neutron scattering measurements have been performed on Nd, Sr
co-doped La1.55Nd0.4Sr0.05CuO4, which exhibits a structural phase transition at
Ts ~ 60K from the low temperature orthorhombic Bmab phase (labelled LTO1) to
the low temperature orthorhombic Pccn phase (labelled LTO2). At low
temperatures, well below Ts, elastic magnetic peaks are observed at the
``diagonal'' incommensurate (IC) positions (0, 1+/-0.055, 0), with modulation
direction only along the orthorhombic b-axis just as in Nd-free
La1.95Sr0.05CuO4. In the present study, the one-dimensionality of the IC
modulation, which is naturally explained by a stripe model, is clearly
demonstrated with our "single-domain" crystal. The temperature dependence of
the IC peak intensity suggests a substantial contribution from the Nd3+ spins
below ~3K. Consistent with this, the L dependence of the magnetic scattering is
accurately accounted for by a model in which the contribution of the Nd3+ spins
is explicitly included.Comment: 8 pages, 8 figure
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