3,747 research outputs found
From GM Law to A Powerful Mean Field Scheme
A new and powerful mean field scheme is presented. It maps to a
one-dimensional finite closed chain in an external field. The chain size
accounts for lattice topologies. Moreover lattice connectivity is rescaled
according to the GM law recently obtained in percolation theory. The associated
self-consistent mean-field equation of state yields critical temperatures which
are within a few percent of exact estimates. Results are obtained for a large
variety of lattices and dimensions. The Ising lower critical dimension for the
onset of phase transitions is . For the Ising hypercube it
becomes the Golden number . The scheme recovers the
exact result of no long range order for non-zero temperature Ising triangular
antiferromagnets.Comment: 3M Conference Proceedings, San Jose, California (November, 1999
Observation of Low Temperature Magneto-Mechanic Effects in Crystalline Resonant Phonon Cavities
We observe magnetic effects in ultra-high quality factor crystalline quartz
Bulk Acoustic Wave resonators at milli-Kelvin temperature. The study reveals
existence of hysteresis loops, jumps and memory effects of acoustical resonance
frequencies. These loops arise as a response to the external magnetic field and
span over few Hertz range for modes with linewidths of about mHz, which
constitute a frequency shift of order 60 linewidths. The effects are broadband
but get stronger towards higher frequencies where both nonlinear effects and
losses are limited by two level systems. This suggests that the observed
effects are due to ferromagnet-like phase of a spin ensemble coupled to
mechanical modes. The observed coupling between mechanical and spin degrees of
freedom in the ultra low loss regime brings new possibilities for the emerging
class of quantum hybrid systems
Inducing Strong Non-Linearities in a Phonon Trapping Quartz Bulk Acoustic Wave Resonator Coupled to a Superconducting Quantum Interference Device
A quartz Bulk Acoustic Wave resonator is designed to coherently trap phonons
in a way that they are well confined and immune to suspension losses so they
exhibit extremely high acoustic -factors at low temperature, with products of order Hz. In this work we couple such a resonator to a
SQUID amplifier and investigate effects in the strong signal regime. Both
parallel and series connection topologies of the system are investigated. The
study reveals significant non-Duffing response that is associated with the
nonlinear characteristics of Josephson junctions. The nonlinearity provides
quasi-periodic structure of the spectrum in both incident power and frequency.
The result gives an insight into the open loop behaviour of a future Cryogenic
Quartz Oscillator in the strong signal regime
Critical points and non-Fermi liquids in the underscreened pseudogap Kondo model
Numerical Renormalization Group simulations have shown that the underscreened
spin-1 Kondo impurity model with power-law bath density of states (DOS)
\rho(\w) \propto |\w|^r possesses various intermediate-coupling fixed points,
including a stable non-Fermi liquid phase. In this paper we discuss the
corresponding universal low-energy theories, obtain thermodynamic quantities
and critical exponents by renormalization group analysis together with suitable
-expansions, and compare our results with numerical data. Whereas the
particle-hole symmetric critical point can be controlled at weak coupling using
a simple generalization of the spin-1/2 model, we show that the stable
non-Fermi liquid fixed point must be accessed near strong coupling via a
mapping onto an effective ferromagnetic S_\mr{eff}=1/2 model with singular
bath DOS with exponent r_\mr{eff}=-r<0. In addition, we consider the
particle-hole asymmetric critical fixed point, for which we propose a universal
field theory involving the crossing between doublet and triplet levels.Comment: 10 pages, 8 figures. Minor modifications in updated versio
Triggered star formation in a molecular shell created by a SNR?
We present a study of a new molecular shell, G126.1-0.8-14, using available
multiwavelegth Galactic plane surveys and optical Gemini observations. A well
defined shell-like structure is observed in the CO(1--0) line emission at (l,b)
= (126.1, -0.8), in the velocity range --10.5 to --15.5 km/s. The HI, emission
shows a region of low emissivity inside G126.1-0.8-14, while radio continuum
observations reveal faint non-thermal emission possibly related to this shell.
Optical spectra obtained with Gemini South show the existence of B-type stars
likely to be associated with G126.1-0.8-14. An estimate of the stellar wind
energy injected by these stars show that they alone can not be able to create
such a structure. On the other hand, one supernova explosion would provide
enough energy to generate the shell. Using the MSX, IRAS, and WISE Point Source
Catalogues we have found about 30 young stellar objects candidates, whose birth
could have been triggered by the expansion of G126.1-0.8-14. In this context,
Sh2-187 could be a consequence of the action on its surroundings of the most
massive (and thus most evolve) of the stars formed by the expanding molecular
shell.Comment: 13 pages, 10 figures, accepted in MNRA
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