488 research outputs found
A form factor approach to finite temperature correlation functions in CFT
The excitation spectrum of specific conformal field theories (CFT) with
central charge can be described in terms of quasi-particles with charges
and fractional statistics properties. Using the language of Jack
polynomials, we compute form factors of the charge density operator in these
CFTs. We study a form factor expansion for the finite temperature
density-density correlation function, and find that it shows a quick
convergence to the exact result. The low-temperature behavior is recovered from
a form factor with particles, while the high-temperature limit is
recovered from states containing no more than 3 particles.Comment: 15 pp, 6 fi
Spin glass behavior upon diluting frustrated magnets and spin liquids: a Bethe-Peierls treatment
A Bethe-Peierls treatment to dilution in frustrated magnets and spin liquids
is given. A spin glass phase is present at low temperatures and close to the
percolation point as soon as frustration takes a finite value in the dilute
magnet model; the spin glass phase is reentrant inside the ferromagnetic phase.
An extension of the model is given, in which the spin glass / ferromagnet phase
boundary is shown not to reenter inside the ferromagnetic phase asymptotically
close to the tricritical point whereas it has a turning point at lower
temperatures. We conjecture similar phase diagrams to exist in finite
dimensional models not constraint by a Nishimori's line. We increase
frustration to study the effect of dilution in a spin liquid state. This
provides a ``minimal'' ordering by disorder from an Ising paramagnet to an
Ising spin glass.Comment: 16 pages, 5 figures (size reduced and other small corrections),
accepted in Eur. Phys. J.
Kinetic modelling of runaway electron avalanches in tokamak plasmas
Runaway electrons (REs) can be generated in tokamak plasmas if the
accelerating force from the toroidal electric field exceeds the collisional
drag force due to Coulomb collisions with the background plasma. In ITER,
disruptions are expected to generate REs mainly through knock-on collisions,
where enough momentum can be transferred from existing runaways to slow
electrons to transport the latter beyond a critical momentum, setting off an
avalanche of REs. Since knock-on runaways are usually scattered off with a
significant perpendicular component of the momentum with respect to the local
magnetic field direction, these particles are highly magnetized. Consequently,
the momentum dynamics require a full 3-D kinetic description, since these
electrons are highly sensitive to the magnetic non-uniformity of a toroidal
configuration. A bounce-averaged knock-on source term is derived. The
generation of REs from the combined effect of Dreicer mechanism and knock-on
collision process is studied with the code LUKE, a solver of the 3-D linearized
bounce-averaged relativistic electron Fokker-Planck equation, through the
calculation of the response of the electron distribution function to a constant
parallel electric field. This work shows that the avalanche effect can be
important even in non-disruptive scenarios. RE formation through knock-on
collisions is found to be strongly reduced when taking place off the magnetic
axis, since trapped electrons cannot contribute to the RE population. The
relative importance of the avalanche mechanism is investigated as a function of
the key parameters for RE formation; the plasma temperature and the electric
field strength. In agreement with theoretical predictions, the simulations show
that in low temperature and E-field knock-on collisions are the dominant source
of REs and can play a significant role for RE generation, including in
non-disruptive scenarios.Comment: 23 pages, 12 figure
Exact spin dynamics of the 1/r^2 supersymmetric t-J model in a magnetic field
The dynamical spin structure factor S^{zz}(Q,omega) in the small momentum
region is derived analytically for the one-dimensional supersymmetric t-J model
with 1/r^2 interaction. Strong spin-charge separation is found in the spin
dynamics. The structure factor S^{zz}(Q,omega) with a given spin polarization
does not depend on the electron density in the small momentum region. In the
thermodynamic limit, only two spinons and one antispinon (magnon) contribute to
S^{zz}(Q,omega). These results are derived via solution of the SU(2,1)
Sutherland model in the strong coupling limit.Comment: 20 pages, 8 figures. Accepted for publication in J.Phys.
Toy models of crossed Andreev reflection
We propose toy models of crossed Andreev reflection in multiterminal hybrid
structures containing out-of-equilibrium conductors. We apply the description
to two possible experiments: (i) to a device containing a large quantum dot
inserted in a crossed Andreev reflection circuit. (ii) To a device containing
an Aharonov-Bohm loop inserted in a crossed Andreev reflection circuit.Comment: 5 pages, 9 figures, minor modification
3rd harmonic ECRH absorption enhancement by 2nd harmonic heating at the same frequency in a tokamak
The fundamental mechanisms responsible for the interplay and synergy between the absorption dynamics of extraordinary-mode electron cyclotron waves at two different harmonic resonances (the 2nd and 3rd) are investigated in the TCV tokamak. An enhanced 3rd harmonic absorption in the presence of suprathermal electrons generated by 2nd harmonic heating is predicted by Fokker-Planck simulations, subject to complex alignment requirements in both physical space and momentum space. The experimental signature for the 2nd/3rd harmonic synergy is sought through the suprathermal bremsstrahlung emission in the hard x-ray range of photon energy. Using a synthetic diagnostic, the emission variation due to synergy is calculated as a function of the injected power and of the radial transport of suprathermal electrons. It is concluded that in the present experimental setup a synergy signature has not been unambiguously detected. The detectability of the synergy is then discussed with respect to variations and uncertainties in the plasma density and effective charge in view of future optimized experiments
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