829 research outputs found
Pair-Hopping Mechanism for Layered Superconductors
We propose a possible charge fluctuation effect expected in layered
superconducting materials. In the multireference density functional theory,
relevant fluctuation channels for the Josephson coupling between
superconducting layers include the interlayer pair hopping derived from the
Coulomb repulsion. When interlayer single-electron tunneling processes are
irrelevant in the Kohn-Sham electronic band structure calculation, the two-body
effective interactions stabilize a superconducting phase. This state is also
regarded as a valence-bond solid in a bulk electronic state. The hidden order
parameters coexist with the superconducting order parameter when the charging
effect of a layer is comparable to the pair hopping. Relevant materials
structures favorable for the pair-hopping mechanism are discussed.Comment: 24 pages, 2 figures, to be published in J. Phys. Soc. Jpn. (2009
Self-consistent description of Andreev bound states in Josephson quantum dot devices
We develop a general perturbative framework based on a superconducting atomic
limit for the description of Andreev bound states (ABS) in interacting quantum
dots connected to superconducting leads. A local effective Hamiltonian for
dressed ABS, including both the atomic (or molecular) levels and the induced
proximity effect on the dot is argued to be a natural starting point. A
self-consistent expansion in single-particle tunneling events is shown to
provide accurate results even in regimes where the superconducting gap is
smaller than the atomic energies, as demonstrated by a comparison to recent
Numerical Renormalization Group calculations. This simple formulation may have
bearings for interpreting Andreev spectroscopic experiments in superconducting
devices, such as STM measurements on carbon nanotubes, or radiative emission in
optical quantum dots.Comment: 12 pages, 11 figures. Last version: we added several extra
references, modified two figures, and discussed recent proposals for Andreev
spectroscop
Composite-Fermion Picture for the Spin-Wave Excitation in the fractional quantum Hall system
Spin-wave excitation mode from the spin-polarized ground state in the
fractional quantum Hall liquid with odd fractions () numerically
obtained by the exact diagonalization of finite systems is shown to be
accurately described, for wavelengths exceeding the magnetic length, in terms
of the composite-fermion mean-field approximation for the spin-wave (magnon)
theory formulated in the spherical geometry. This indicates that the composite
picture extends to excited states, and also provides the spin stiffness in
terms of peculiar exchange interactions.Comment: 10 pages, typeset in LATEX, NA-94-05, 2 figures available upon
request at [email protected]
Quantum phase transition in a minimal model for the Kondo effect in a Josephson junction
We propose a minimal model for the Josephson current through a quantum dot in
a Kondo regime. We start with the model that consists of an Anderson impurity
connected to two superconducting (SC) leads with the gaps
, where for the lead at left and right. We show that, when one of the SC gaps is
much larger than the others , the starting model can
be mapped exactly onto the single-channel model, which consists of the right
lead of and the Anderson impurity with an extra onsite SC gap of
. Here and are
defined with respect to the starting model, and is the level width
due to the coupling with the left lead. Based on this simplified model, we
study the ground-state properties for the asymmetric gap, , using the numerical renormalization group (NRG) method. The
results show that the phase difference of the SC gaps , which induces the Josephson current, disturbs the screening of the
local moment to destabilize the singlet ground state typical of the Kondo
system. It can also drive the quantum phase transition to a magnetic doublet
ground state, and at the critical point the Josephson current shows a
discontinuous change. The asymmetry of the two SC gaps causes a re-entrant
magnetic phase, in which the in-gap bound state lies close to the Fermi level.Comment: 23 pages, 13 figures, typos are correcte
Cross-validated methods for promoter/transcription start site mapping in SL trans-spliced genes, established using the Ciona intestinalis troponin I gene
In conventionally-expressed eukaryotic genes, transcription start sites (TSSs) can be identified by mapping the mature mRNA 5′-terminal sequence onto the genome. However, this approach is not applicable to genes that undergo pre-mRNA 5′-leader trans-splicing (SL trans-splicing) because the original 5′-segment of the primary transcript is replaced by the spliced leader sequence during the trans-splicing reaction and is discarded. Thus TSS mapping for trans-spliced genes requires different approaches. We describe two such approaches and show that they generate precisely agreeing results for an SL trans-spliced gene encoding the muscle protein troponin I in the ascidian tunicate chordate Ciona intestinalis. One method is based on experimental deletion of trans-splice acceptor sites and the other is based on high-throughput mRNA 5′-RACE sequence analysis of natural RNA populations in order to detect minor transcripts containing the pre-mRNA’s original 5′-end. Both methods identified a single major troponin I TSS located ∼460 nt upstream of the trans-splice acceptor site. Further experimental analysis identified a functionally important TATA element 31 nt upstream of the start site. The two methods employed have complementary strengths and are broadly applicable to mapping promoters/TSSs for trans-spliced genes in tunicates and in trans-splicing organisms from other phyla
Contrast of LiFeAs with isostructural, isoelectronic, and non-superconducting MgFeGe
Stoichiometric LiFeAs at ambient pressure is an 18 K superconductor while
isoelectronic MgFeGe is not, despite their extremely similar electronic
structures. To investigate possible sources of this distinctively different
superconducting behavior, we quantify the differences using first principles
density functional theory, establishing first that the Fe total 3d occupations
are identical in the two compounds. Individual 3d orbital occupations also
differ very little (). The differences in Fermi surfaces (FSs) do
not seem significant; however a redistribution of bands just above the Fermi
level does represent a possibly significant distinction. Because the bands and
FSs of LiFeAs are less in agreement with experiment than for other
iron-pnictides, we study the effects of additional exchange-correlations
effects beyond GGA (the generalized gradient approximation) by applying the
modified Becke-Johnson potential (mBJ) exchange potential, which gives much
improved bandgaps in insulators compared to GGA and might be useful for
semimetals such as the Fe-based superconductors. Overall, we conclude that the
mBJ corrections do not improve the description of LiFeAs as compared to
experiment
Ferromagnetism in the one-dimensional Hubbard model with orbital degeneracy: From low to high electron density
We studied ferromagnetism in the one-dimensional Hubbard model with doubly
degenerate atomic orbitals by means of the density-matrix renormalization-group
method and obtained the ground-state phase diagrams. It was found that
ferromagnetism is stable from low to high (0< n < 1.75) electron density when
the interactions are sufficiently strong. Quasi-long-range order of triplet
superconductivity coexists with the ferromagnetic order for a strong Hund
coupling region, where the inter-orbital interaction U'-J is attractive. At
quarter-filling (n=1), the insulating ferromagnetic state appears accompanying
orbital quasi-long-range order. For low densities (n<1), ferromagnetism occurs
owing to the ferromagnetic exchange interaction caused by virtual hoppings of
electrons, the same as in the quarter-filled system. This comes from separation
of the charge and spin-orbital degrees of freedom in the strong coupling limit.
This ferromagnetism is fragile against variation of band structure. For high
densities (n>1), the phase diagram of the ferromagnetic phase is similar to
that obtained in infinite dimensions. In this case, the double exchange
mechanism is operative to stabilize the ferromagnetic order and this long-range
order is robust against variation of the band-dispersion. A partially polarized
state appears in the density region 1.68<n<1.75 and phase separation occurs for
n just below the half-filling (n=2).Comment: 16 pages, 16 figures, final version, references adde
Exchange Interaction and in Alkaline-earth-metal-oxide-based DMS without Magnetic Impurities: First Principle Pseudo-SIC and Monte Carlo Calculation
The prospects of half-metallic ferromagnetism being induced by the
incorporation of C atoms into alkaline-earth-metal-oxides are investigated by
the first principle calculation. The origin of the ferromagnetism is discussed
through the calculation of the electronic structure and exchange coupling
constant by using the pseudo-potential-like self-interaction-corrected local
spin density method. The Curie temperature () is also predicted by
employing the Monte Carlo simulation. It is shown that by taking the electron
self-interaction into account, the half-metallic ferromagnetism induced by C in
the host materials is more stabilized in comparison with the standard LDA case,
and the C's electron states in the bandgap become more localized resulting
in the predominance of the short-ranged exchange interaction. While the
ferromagnetism in MgOC is stabilized due to the exchange
interaction of the -nearest neighbor pairs and might be suppressed by the
anti-ferromagnetic super-exchange interaction at higher , the ferromagnetism
in CaOC, SrOC, and BaOC is stabilized by
both the - and -nearest neighbor pairs, and monotonously
increases with the C concentration.Comment: 5 pages, 5 figure
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