51,442 research outputs found
Mesoscopic competition of superconductivity and ferromagnetism: conductance peak statistics in metallic grains
We investigate the competition between superconductivity and ferromagnetism
in chaotic ultra-small metallic grains in a regime where both phases can
coexist. We use an effective Hamiltonian that combines a BCS-like pairing term
and a ferromagnetic Stoner-like spin exchange term. We study the transport
properties of the grain in the Coulomb blockade regime and identify signatures
of the coexistence between pairing and exchange correlations in the mesoscopic
fluctuations of the conductance peak spacings and peak heights.Comment: 4 pages, 3 figure
The coexistence of superconductivity and ferromagnetism in nano-scale metallic grains
A nano-scale metallic grain in which the single-particle dynamics are chaotic
is described by the so-called universal Hamiltonian. This Hamiltonian includes
a superconducting pairing term and a ferromagnetic exchange term that compete
with each other: pairing correlations favor minimal ground-state spin, while
the exchange interaction favors maximal spin polarization. Of particular
interest is the fluctuation-dominated regime where the bulk pairing gap is
comparable to or smaller than the single-particle mean level spacing and the
Bardeen-Cooper-Schrieffer theory of superconductivity breaks down.
Superconductivity and ferromagnetism can coexist in this regime. We identify
signatures of the competition between superconductivity and ferromagnetism in a
number of quantities: ground-state spin, conductance fluctuations when the
grain is weakly coupled to external leads and the thermodynamic properties of
the grain, such as heat capacity and spin susceptibility.Comment: 13 pages, 13 figures, Proceedings of the Conference on the Frontiers
of Quantum and Mesoscopic Thermodynamics (FQMT11
Coherent center domains from local Polyakov loops
We analyze properties of local Polyakov loops using quenched as well as
dynamical SU(3) gauge configurations for a wide range of temperatures. It is
demonstrated that for both, the confined and the deconfined regime, the local
Polyakov loop prefers phase values near the center elements 1, exp(i 2 pi/3),
exp(-i 2 pi/3). We divide the lattice sites into three sectors according to
these phases and show that the sectors give rise to the formation of clusters.
For a suitable definition of these clusters we find that in the quenched case
deconfinement manifests itself as the onset of percolation of the clusters. A
possible continuum limit of the center clusters is discussed
Time delay in the Kuramoto model with bimodal frequency distribution
We investigate the effects of a time-delayed all-to-all coupling scheme in a
large population of oscillators with natural frequencies following a bimodal
distribution. The regions of parameter space corresponding to synchronized and
incoherent solutions are obtained both numerically and analytically for
particular frequency distributions. In particular we find that bimodality
introduces a new time scale that results in a quasiperiodic disposition of the
regions of incoherence.Comment: 5 pages, 4 figure
A two-dimensional representation of four-dimensional gravitational waves
The Einstein equation in D dimensions, if restricted to the class of
space-times possessing n = D - 2 commuting hypersurface-orthogonal Killing
vectors, can be equivalently written as metric-dilaton gravity in 2 dimensions
with n scalar fields. For n = 2, this results reduces to the known reduction of
certain 4-dimensional metrics which include gravitational waves. Here, we give
such a representation which leads to a new proof of the Birkhoff theorem for
plane-symmetric space--times, and which leads to an explanation, in which sense
two (spin zero-) scalar fields in 2 dimensions may incorporate the (spin two-)
gravitational waves in 4 dimensions. (This result should not be mixed up with
well--known analogous statements where, however, the 4-dimensional space-time
is supposed to be spherically symmetric, and then, of course, the equivalent
2-dimensional picture cannot mimic any gravitational waves.) Finally, remarks
on hidden symmetries in 2 dimensions are made.Comment: 12 pages, LaTeX, no figures, Int. J. Mod. Phys. D in prin
Magnetic structure of the Eu2+ moments in superconducting EuFe2(As1-xPx)2 with x = 0.19
The magnetic structure of the Eu2+ moments in the superconducting
EuFe2(As1-xPx)2 sample with x = 0.19 has been determined using neutron
scattering. We conclude that the Eu2+ moments are aligned along the c direction
below T_C = 19.0(1) K with an ordered moment of 6.6(2) mu_B in the
superconducting state. An impurity phase similar to the underdoped phase exists
within the bulk sample which orders antiferromagnetically below T_N = 17.0(2)
K. We found no indication of iron magnetic order, nor any incommensurate
magnetic order of the Eu2+ moments in the sample.Comment: Accepted for publication in Phys. Rev. B (regular article
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