789 research outputs found
Quantum Phase Transitions in the Itinerant Ferromagnet ZrZn
We report a study of the ferromagnetism of ZrZn, the most promising
material to exhibit ferromagnetic quantum criticality, at low temperatures
as function of pressure . We find that the ordered ferromagnetic moment
disappears discontinuously at =16.5 kbar. Thus a tricritical point
separates a line of first order ferromagnetic transitions from second order
(continuous) transitions at higher temperature. We also identify two lines of
transitions of the magnetisation isotherms up to 12 T in the plane where
the derivative of the magnetization changes rapidly. These quantum phase
transitions (QPT) establish a high sensitivity to local minima in the free
energy in ZrZn, thus strongly suggesting that QPT in itinerant
ferromagnets are always first order
Cosmology as Geodesic Motion
For gravity coupled to N scalar fields with arbitrary potential V, it is
shown that all flat (homogeneous and isotropic) cosmologies correspond to
geodesics in an (N+1)-dimensional `augmented' target space of Lorentzian
signature (1,N), timelike if V>0, null if V=0 and spacelike if V<0.
Accelerating cosmologies correspond to timelike geodesics that lie within an
`acceleration subcone' of the `lightcone'. Non-flat (k=-1,+1) cosmologies are
shown to evolve as projections of geodesic motion in a space of dimension
(N+2), of signature (1,N+1) for k=-1 and signature (2,N) for k=+1. This
formalism is illustrated by cosmological solutions of models with an
exponential potential, which are comprehensively analysed; the late-time
behviour for other potentials of current interest is deduced by comparison.Comment: 26 pages, 2 figures, journal version with additional reference
Gravity a la Born-Infeld
A simple technique for the construction of gravity theories in Born-Infeld
style is presented, and the properties of some of these novel theories are
investigated. They regularize the positive energy Schwarzschild singularity,
and a large class of models allows for the cancellation of ghosts. The possible
correspondence to low energy string theory is discussed. By including curvature
corrections to all orders in alpha', the new theories nicely illustrate a
mechanism that string theory might use to regularize gravitational
singularities.Comment: 21 pages, 2 figures, new appendix B with corrigendum: Class. Quantum
Grav. 21 (2004) 529
Thermodynamic Studies on Non Centrosymmetric Superconductors by AC Calorimetry under High Pressures
We investigated the non centrosymmetric superconductors CePtSi and UIr by
the ac heat capacity measurement under pressures. We determined the pressure
phase diagrams of these compounds. In CePtSi, the N\'{e}el temperature
= 2.2 K decreases with increasing pressure and becomes zero at the
critical pressure 0.6 GPa. On the other hand, the
superconducting phase exists in a wider pressure region from ambient pressure
to 1.5 GPa. The phase diagram of CePtSi is very
unique and has never been reported before for other heavy fermion
superconductors. In UIr, the heat capacity shows an anomaly at the Curie
temperature = 46 K at ambient pressure, and the heat capacity
anomaly shifts to lower temperatures with increasing pressure. The present
pressure dependence of was consistent with the previous studies by
the resistivity and magnetization measurements. Previous ac magnetic
susceptibility and resistivity measurements suggested the existence of three
ferromagnetic phases, FM1-3. shows a bending structure at 1.98,
2.21, and 2.40 GPa .The temperatures where these anomalies are observed are
close to the phase boundary of the FM3 phase.Comment: This paper was presented at the international workshop ``Novel
Pressure-induced Phenomena in Condensed Matter Systems(NP2CMS)" August 26-29
2006, Fukuoka Japa
A novel coupler design and analysis with shielding material tests for a CPT system of electric vehicles based on electromagnetic resonant coupling
In this paper, a contactless power transfer (CPT) system using a novel geometrically enhanced energy transfer coupler with three different shielding materials has been built and analysed, along with the evaluations from aspects of electromagnetics and RMS power transmitting based on electromagnetic resonant coupling. A CPT system design improvement with the proposed H-shape ferromagnetic cores and the combined semi-enclosed passive electromagnetic shielding methods have been investigated in terms of generated electromagnetic field characteristics, system power transfer ratings, system efficiency optimization and performances of shielding materials. The results have shown that, across the range of operating frequency of the CPT system, aluminium shielding as a metallic material method could deliver better overall CPT system performance than other two ferromagnetic materials, steel 1010 and ferrite. In addition, the coupler prototype design limitations, misalignment tolerance and the passive shielding design considerations including distance between windings and inner surfaces of shielding shells have been discussed
Cosmology from Moduli Dynamics
We investigate moduli field dynamics in supergravity/M-theory like set ups
where we turn on fluxes along some or all of the extra dimensions. As has been
argued in the context of string theory, we observe that the fluxes tend to
stabilize the squashing (or shape) modes. Generically we find that at late
times the shape is frozen while the radion evolves as a quintessence field. At
earlier times we have a phase of radiation domination where both the volume and
the shape moduli are slowly evolving. However, depending on the initial
conditions and the parameters of the theory, like the value of the fluxes,
curvature of the internal manifold and so on, the dynamics of the internal
manifold can be richer with interesting cosmological consequences, including
inflation.Comment: 38 pages, 6 figures; references adde
A note on accelerating cosmologies from compactifications and S-branes
We give a simple interpretation of the recent solutions for cosmologies with
a transient accelerating phase obtained from compactification in hyperbolic
manifolds, or from S-brane solutions of string/M-theory. In the
four-dimensional picture, these solutions correspond to bouncing the radion
field off its exponential potential. Acceleration occurs at the turning point,
when the radion stops and the potential energy momentarily dominates. The
virtues and limitations of these approaches become quite transparent in this
interpretation.Comment: 9 pages, 1 figure. References adde
Ferromagnetic Properties of ZrZn
The low Curie temperature (T_C approx 28K) and small ordered moment (M_0
approx 0.17 mu_B f.u.^-1) of ZrZn2 make it one of the few examples of a weak
itinerant ferromagnet. We report results of susceptibility, magnetization,
resistivity and specific heat measurements made on high-quality single crystals
of ZrZn2. From magnetization scaling in the vicinity of T_C
(0.001<|T-T_C|/T_C<0.08), we obtain the critical exponents beta=0.52+/-0.05 and
delta=3.20+/-0.08, and T_C=27.50+/-0.05K. Low-temperature magnetization
measurements show that the easy axis is [111]. Resistivity measurements reveal
an anomaly at T_C and a non-Fermi liquid temperature dependence
rho(T)=rho_0+AT^n, where n=1.67+/-0.02, for 1<T<14K. The specific heat
measurements show a mean-field-like anomaly at T_C. We compare our results to
various theoretical models.Comment: submitted to PR
Accelerating Cosmologies from Exponential Potentials
An exponential potential of the form arising from
the hyperbolic or flux compactification of higher-dimensional theories is of
interest for getting short periods of accelerated cosmological expansions.
Using a similar potential but derived for the combined case of hyperbolic-flux
compactification, we study the four-dimensional flat (and open) FLRW
cosmologies and give analytic (and numerical) solutions with exponential
behavior of scale factors. We show that, for the M-theory motivated potentials,
the cosmic acceleration of the universe can be eternal if the spatial curvature
of the 4d spacetime is negative, while the acceleration is only transient for a
spatially flat universe. We also comment on the size of the internal space and
its associated geometric bounds on massive Kaluza-Klein excitations.Comment: 17 pages, 6 figures; minor typos fixe
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