75,608 research outputs found
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
Unifying Magnons and Triplons in Stripe-Ordered Cuprate Superconductors
Based on a two-dimensional model of coupled two-leg spin ladders, we derive a
unified picture of recent neutron scattering data of stripe-ordered
La_(15/8)Ba_(1/8)CuO_4, namely of the low-energy magnons around the
superstructure satellites and of the triplon excitations at higher energies.
The resonance peak at the antiferromagnetic wave vector Q_AF in the
stripe-ordered phase corresponds to a saddle point in the dispersion of the
magnetic excitations. Quantitative agreement with the neutron data is obtained
for J= 130-160 meV and J_cyc/J = 0.2-0.25.Comment: 4 pages, 4 figures included updated version taking new data into
account; factor in spectral weight corrected; Figs. 2 and 4 change
Creation and Manipulation of Anyons in the Kitaev Model
We analyze the effect of local spin operators in the Kitaev model on the
honeycomb lattice. We show, in perturbation around the isolated-dimer limit,
that they create Abelian anyons together with fermionic excitations which are
likely to play a role in experiments. We derive the explicit form of the
operators creating and moving Abelian anyons without creating fermions and show
that it involves multi-spin operations. Finally, the important experimental
constraints stemming from our results are discussed.Comment: 4 pages, 3 figures, published versio
Optimal cooperative control synthesis of active displays
A technique is developed that is intended to provide a systematic approach to synthesizing display augmentation for optimal manual control in complex, closed-loop tasks. A cooperative control synthesis technique, previously developed to design pilot-optimal control augmentation for the plant, is extended to incorporate the simultaneous design of performance enhancing displays. The technique utilizes an optimal control model of the man in the loop. It is applied to the design of a quickening control law for a display and a simple K/s(2) plant, and then to an F-15 type aircraft in a multi-channel task. Utilizing the closed loop modeling and analysis procedures, the results from the display design algorithm are evaluated and an analytical validation is performed. Experimental validation is recommended for future efforts
Auxiliary Field Diffusion Monte Carlo calculation of nuclei with A<40 with tensor interactions
We calculate the ground-state energy of 4He, 8He, 16O, and 40Ca using the
auxiliary field diffusion Monte Carlo method in the fixed phase approximation
and the Argonne v6' interaction which includes a tensor force. Comparison of
our light nuclei results to those of Green's function Monte Carlo calculations
shows the accuracy of our method for both open and closed shell nuclei. We also
apply it to 16O and 40Ca to show that quantum Monte Carlo methods are now
applicable to larger nuclei.Comment: 4 pages, no figure
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