1,976 research outputs found
Transport through two-level quantum dots weakly coupled to ferromagnetic leads
Spin-dependent transport through a two-level quantum dot in the sequential
tunneling regime is analyzed theoretically by means of a real-time diagrammatic
technique. It is shown that the current, tunnel magnetoresistance, and shot
noise (Fano factor) strongly depend on the transport regime, providing a
detailed information on the electronic structure of quantum dots and their
coupling to external leads. When the dot is asymmetrically coupled to the
leads, a negative differential conductance may occur in certain bias regions,
which is associated with a super-Poissonian shot noise. In the case of a
quantum dot coupled to one half-metallic and one nonmagnetic lead, one finds
characteristic Pauli spin blockade effects. Transport may be also suppressed
when the dot levels are coupled to the leads with different coupling strengths.
The influence of an external magnetic field on transport properties is also
discussed.Comment: 12 pages, 8 figure
Ferromagnetic resonance with a magnetic Josephson junction
We show experimentally and theoretically that there is a coupling via the
Aharonov-Bohm phase between the order parameter of a ferromagnet and a singlet,
s-wave, Josephson supercurrent. We have investigated the possibility of
measuring the dispersion of such spin waves by varying the magnetic field
applied in the plane of the junction and demonstrated the electromagnetic
nature of the coupling by the observation of magnetic resonance side-bands to
microwave induced Shapiro steps.Comment: 6 pages, 5 figure
Dilution Effects in Two-dimensional Quantum Orbital System
We study dilution effects in a Mott insulating state with quantum orbital
degree of freedom, termed the two-dimensional orbital compass model. This is a
quantum and two-dimensional version of the orbital model where the interactions
along different bond directions cause frustration between different orbital
configurations. A long-range correlation of a kind of orbital at each row or
column, termed the directional order, is studied by means of the quantum
Monte-Carlo method. It is shown that decrease of the ordering temperature due
to dilution is much stronger than that in spin models. Quantum effect enhances
the effective dimensionality in the system and makes the directional order
robust against dilution. We discuss an essential mechanism of the dilute
orbital systems.Comment: 5pages, 4 figure
Sliding Singlet Mechanism Revisited
We show that the unification of the doublet Higgs in the standard model (SM)
and the Higgs to break the grand unified theory (GUT) group stabilizes the
sliding singlet mechanism which can solve the doublet-triplet (DT) splitting
problem. And we generalize this attractive mechanism to apply it to many
unified scenarios. In this paper, we try to build various concrete E_6 unified
models by using the generalized sliding singlet mechanism.Comment: 13 page
Low energy electronic states and triplet pairing in layered cobaltates
The structure of the low-energy electronic states in layered cobaltates is
considered starting from the Mott insulating limit. We argue that the coherent
part of the wave-functions and the Fermi-surface topology at low doping are
strongly influenced by spin-orbit coupling of the correlated electrons on the
level. An effective t-J model based on mixed spin-orbital states is
radically different from that for the cuprates, and supports unconventional,
pseudospin-triplet pairing.Comment: 4 pages, 3 figure
Orbital Compass Model as an Itinerant Electron System
Two-dimensional orbital compass model is studied as an interacting itinerant
electron model. A Hubbard-type tight-binding model, from which the orbital
compass model is derived in the strong coupling limit, is identified. This
model is analyzed by the random-phase approximation (RPA) and the
self-consistent RPA methods from the weak coupling. Anisotropy for the orbital
fluctuation in the momentum space is qualitatively changed by the on-site
Coulomb interaction. This result is explained by the fact that the dominant
fluctuation is changed from the intra-band nesting to the inter-band one by
increasing the interaction.Comment: 7 pages, 8 figure
Effects of antiferromagnetic planes on the superconducting properties of multilayered high-Tc cuprates
We propose a mechanism for high critical temperature (T_c) in the coexistent
phase of superconducting- (SC) and antiferromagnetic (AF) CuO_2 planes in
multilayered cuprates. The Josephson coupling between the SC planes separated
by an AF insulator (Mott insulator) is calculated perturbatively up to the
fourth order in terms of the hopping integral between adjacent CuO_2 planes. It
is shown that the AF exchange splitting in the AF plane suppresses the
so-called pi-Josephson coupling, and the long-ranged 0-Josephson coupling leads
to coexistence with a rather high value of T_c.Comment: 4 pages including 4 figure
Randomly Diluted e_g Orbital-Ordered Systems
Dilution effects on the long-range ordered state of the doubly degenerate
orbital are investigated. Quenched impurities without the orbital degree
of freedom are introduced in the orbital model where the long-range order is
realized by the order-from-disorder mechanism. It is shown by the Monte-Carlo
simulation and the cluster-expansion method that a decrease in the orbital
ordering temperature by dilution is remarkable in comparison with that in the
randomly diluted spin models. Tiltings of orbitals around impurity cause this
unique dilution effects on the orbital systems. The present theory provides a
new view point for the recent experiments in KCuZnF.Comment: 4 pages, 4 figure
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