873 research outputs found
Mesoscopic interplay of superconductivity and ferromagnetism in ultra-small metallic grains
We review the effects of electron-electron interactions on the ground-state
spin and the transport properties of ultra-small chaotic metallic grains. Our
studies are based on an effective Hamiltonian that combines a superconducting
BCS-like term and a ferromagnetic Stoner-like term. Such terms originate in
pairing and spin exchange correlations, respectively. This description is valid
in the limit of a large dimensionless Thouless conductance. We present the
ground-state phase diagram in the fluctuation-dominated regime where the
single-particle mean level spacing is comparable to the bulk BCS pairing gap.
This phase diagram contains a regime in which pairing and spin exchange
correlations coexist in the ground-state wave function. We discuss the
calculation of the tunneling conductance for an almost-isolated grain in the
Coulomb-blockade regime, and present measurable signatures of the competition
between superconductivity and ferromagnetism in the mesoscopic fluctuations of
the conductance.Comment: 6 pages, 3 figures, To be published in the proceedings of the NATO
Advance Research Workshop "Recent Advances in Nonlinear Dynamics and Complex
System Physics.
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
Role of the Ce valence in the coexistence of superconductivity and ferromagnetism of CeOFBiS revealed by Ce -edge x-ray absorption spectroscopy
We have performed Ce -edge x-ray absorption spectroscopy (XAS)
measurements on CeOFBiS, in which the superconductivity of the
BiS layer and the ferromagnetism of the CeOF layer are induced
by the F-doping, in order to investigate the impact of the F-doping on the
local electronic and lattice structures. The Ce -edge XAS spectrum of
CeOBiS exhibits coexistence of (Ce) and (Ce)
state transitions revealing Ce mixed valency in this system. The spectral
weight of the state decreases with the F-doping and completely
disappears for where the system shows the superconductivity and the
ferromagnetism. The results suggest that suppression of Ce-S-Bi coupling
channel by the F-doping appears to drive the system from the valence
fluctuation regime to the Kondo-like regime, leading to the coexistence of the
superconducting BiS layer and the ferromagnetic CeOF layer.Comment: 5 pages, 5 figure
- …