5,133 research outputs found
A photonic bandgap resonator to facilitate GHz frequency conductivity experiments in pulsed magnetic fields
We describe instrumentation designed to perform millimeter-wave conductivity
measurements in pulsed high magnetic fields at low temperatures. The main
component of this system is an entirely non-metallic microwave resonator. The
resonator utilizes periodic dielectric arrays (photonic bandgap structures) to
confine the radiation, such that the resonant modes have a high Q-factor, and
the system possesses sufficient sensitivity to measure small samples within the
duration of a magnet pulse. As well as measuring the sample conductivity to
probe orbital physics in metallic systems, this technique can detect the sample
permittivity and permeability allowing measurement of spin physics in
insulating systems. We demonstrate the system performance in pulsed magnetic
fields with both electron paramagnetic resonance experiments and conductivity
measurements of correlated electron systems.Comment: Submitted to the Review of Scientific instrument
Force Dynamics in Weakly Vibrated Granular Packings
The oscillatory force F_b^ac on the bottom of a rigid, vertically vibrated,
grain filled column, reveals rich granular dynamics, even when the peak
acceleration of the vibrations is signicantly less than the gravitational
acceleration at the earth's surface. For loose packings or high frequencies,
F_b^ac 's dynamics are dominated by grain motion. For moderate driving
conditions in more compact samples, grain motion is virtually absent, but
F_b^ac nevertheless exhibits strongly nonlinear and hysteretic behavior,
evidencing a granular regime dominated by nontrivial force-network dynamics.Comment: 4 pages, 5 figure
Charges, Monopoles and Duality Relations
A charge-monopole theory is derived from simple and self-evident postulates.
Charges and monopoles take an analogous theoretical structure. It is proved
that charges interact with free waves emitted from monopoles but not with the
corresponding velocity fields. Analogous relations hold for monopole equations
of motion. The system's equations of motion can be derived from a regular
Lagrangian function.Comment: 17 pages + 3 figures
An Exact String Theory Model of Closed Time-Like Curves and Cosmological Singularities
We study an exact model of string theory propagating in a space-time
containing regions with closed time-like curves (CTCs) separated from a finite
cosmological region bounded by a Big Bang and a Big Crunch. The model is an
non-trivial embedding of the Taub-NUT geometry into heterotic string theory
with a full conformal field theory (CFT) definition, discovered over a decade
ago as a heterotic coset model. Having a CFT definition makes this an excellent
laboratory for the study of the stringy fate of CTCs, the Taub cosmology, and
the Milne/Misner-type chronology horizon which separates them. In an effort to
uncover the role of stringy corrections to such geometries, we calculate the
complete set of alpha' corrections to the geometry. We observe that the key
features of Taub-NUT persist in the exact theory, together with the emergence
of a region of space with Euclidean signature bounded by time-like curvature
singularities. Although such remarks are premature, their persistence in the
exact geometry is suggestive that string theory theory is able to make physical
sense of the Milne/Misner singularities and the CTCs, despite their
pathological character in General Relativity. This may also support the
possibility that CTCs may be viable in some physical situations, and may be a
natural ingredient in pre-Big-Bang cosmological scenarios.Comment: 37 pages, 4 figures. V2: discussion of computation of metric refined,
references adde
Magnetic quantum oscillations in YBaCuO and YBaCuO in fields of up to 85 T; patching the hole in the roof of the superconducting dome
We measure magnetic quantum oscillations in the underdoped cuprates
YBaCuO with , 0.69, using fields of up to 85 T. The
quantum-oscillation frequencies and effective masses obtained suggest that the
Fermi energy in the cuprates has a maximum at . On either
side, the effective mass may diverge, possibly due to phase transitions
associated with the T=0 limit of the metal-insulator crossover (low- side),
and the postulated topological transition from small to large Fermi surface
close to optimal doping (high side)
Chiral Plaquette Polaron Theory of Cuprate Superconductivity
Ab-initio density functional calculations on explicitly doped
La(2-x)Sr(x)CuO4 find doping creates localized holes in out-of-plane orbitals.
A model for superconductivity is developed based on the assumption that doping
leads to the formation of holes on a four-site Cu plaquette composed of the
out-of-plane A1 orbitals apical O pz, planar Cu dz2, and planar O psigma. This
is in contrast to the assumption of hole doping into planar Cu dx2-y2 and O
psigma orbitals as in the t-J model. Interaction of holes with the d9 spin
background leads to chiral polarons with either a clockwise or anti-clockwise
charge current. When the polaron plaquettes percolate through the crystal at
x~0.05 for LaSrCuO, a Cu dx2-y2 and planar O psigma band is formed. Spin
exchange Coulomb repulsion with chiral polarons leads to D-wave
superconductivity. The equivalent of the Debye energy in phonon
superconductivity is the maximum energy separation between a chiral polaron and
its time-reversed partner. An additive skew-scattering contribution to the Hall
effect is induced by chiral polarons and leads to a temperature dependent Hall
effect that fits the measured values for LaSrCuO. The integrated imaginary
susceptibility satisfies omega/T scaling due to chirality and spin-flip
scattering of polarons along with a uniform distribution of polaron energy
splittings. The derived functional form is compatible with experiments. The
static spin structure factor is computed and is incommensurate with a
separation distance from (pi,pi) given by ~(2pi)x. Coulomb scattering of the
x2-y2 band with polarons leads to linear resistivity. Coupling of the x2-y2
band to the undoped Cu d9 spins leads to the ARPES pseudogap and its doping and
temperature dependence.Comment: 32 pages, 17 figure
Instability-induced ordering, universal unfolding and the role of gravity in granular Couette flow
Linear stability theory and bifurcation analysis are used to investigate the role of gravity in shear-band formation in granular Couette flow, considering a kinetic-theory rheological model. We show that the only possible state, at low shear rates, corresponds to a 'plug' near the bottom wall, in which the particles are densely packed and the shear rate is close to zero, and a uniformly sheared dilute region above it. The origin of such plugged states is shown to be tied to the spontaneous symmetry-breaking instabilities of the gravity-free uniform shear flow, leading to the formation of ordered bands of alternating dilute and dense regions in the transverse direction, via an infinite hierarchy of pitchfork bifurcations. Gravity plays the role of an 'imperfection', thus destroying the 'perfect' bifurcation structure of uniform shear. The present bifurcation problem admits universal unfolding of pitchfork bifurcations which subsequently leads to the formation of a sequence of a countably infinite number of 'isolas', with the solution structures being a modulated version of their gravity-free counterpart. While the solution with a plug near the bottom wall looks remarkably similar to the shear-banding phenomenon in dense slow granular Couette flows, a 'floating' plug near the top wall is also a solution of these equations at high shear rates. A two-dimensional linear stability analysis suggests that these floating plugged states are unstable to long-wave travelling disturbances.The unique solution having a bottom plug can also be unstable to long waves, but remains stable at sufficiently low shear rates. The implications and realizability of the present results are discussed in the light of shear-cell experiments under 'microgravity' conditions
On a Bosonic-Parafermionic Realization of
We realize the current algebra at arbitrary level in
terms of one deformed free bosonic field and a pair of deformed parafermionic
fields. It is shown that the operator product expansions of these parafermionic
fields involve an infinite number of simple poles and simple zeros, which then
condensate to form a branch cut in the classical limit . Our
realization coincides with those of Frenkel-Jing and Bernard when the level
takes the values 1 and 2 respectively.Comment: 8 pages, CRM-220
Specific and general autobiographical knowledge in adults with autism spectrum disorders: The role of personal goals
Autobiographical knowledge is stored hierarchically, at both specific and general levels of representation. It has also been proposed that the self is the structure around which autobiographical memories are organised. The current series of studies assessed whether the autobiographical memory difficulties observed in adults with autism spectrum disorders (ASD) could be due to problems in using the self as an effective memory cue. A series of cueing paradigms were used to assess the accessibility of both specific and general autobiographical knowledge relating to (i) currently pursued goals (either high or low in self-concordance) and (ii) goals that participants were not currently pursuing. Results demonstrated that while event-specific knowledge was impaired in the ASD group, general event knowledge appeared relatively intact. Moreover, while both event-specific and general event knowledge were organised around goals of the self in control participants, a corresponding relationship was only observed for general event knowledge in the ASD group
The target space geometry of N=(2,1) string theory
We describe the constraints on the target space
geometry of the heterotic superstring due to the left-moving
supersymmetry and currents. In the fermionic description of the internal
sector supersymmetry is realized quantum mechanically, so that both tree-level
and one-loop effects contribute to the order
constraints. We also discuss the physical interpretation of the resulting
target space geometry in terms of configurations of a -dimensional object
propagating in a -dimensional spacetime with a null isometry, which has
recently been suggested as a unified description of string and M theory.Comment: 41 pages, 5 figures, standard LaTeX, uses epsf.tex. Some typos
corrected, discussion in footnote 1 correcte
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