567 research outputs found
Electron-Phonon mechanism for Superconductivity in NaCoO: Valence-Band Suhl-Kondo effect Driven by Shear Phonons
To study the possible mechanism of superconductivity in NaCoO,
we examine the interaction between all the relevant optical phonons (breathing
and shear phonons) and -electrons of Co-ions, and study
the transition temperature for a s-wave superconductivity. The obtained is very low when the -valence-bands are far below the Fermi level.
However, is strongly enhanced when the top of the
-valence-bands is close to the Fermi level (say -50meV), thanks to
interband hopping of Cooper pairs caused by shear phonons. This ``valence-band
Suhl-Kondo mechanism'' due to shear phonons is significant to understand the
superconductivity in NaCoO. By the same mechanism, the kink
structure of the band-dispersion observed by ARPES, which indicates the strong
mass-enhancement () due to optical phonons, is also explained.Comment: 5 pages, 4 figures; v2:Added references, published in J. Phys. Soc.
Jp
The Josephson current in Fe-based superconducting junctions: theory and experiment
We present theory of dc Josephson effect in contacts between Fe-based and
spin-singlet -wave superconductors. The method is based on the calculation
of temperature Green's function in the junction within the tight-binding model.
We calculate the phase dependencies of the Josephson current for different
orientations of the junction relative to the crystallographic axes of Fe-based
superconductor. Further, we consider the dependence of the Josephson current on
the thickness of an insulating layer and on temperature. Experimental data for
PbIn/BaK(FeAs) point-contact Josephson junctions are
consistent with theoretical predictions for symmetry of an order
parameter in this material. The proposed method can be further applied to
calculations of the dc Josephson current in contacts with other new
unconventional multiorbital superconductors, such as and
superconducting topological insulator .Comment: 16 pages, 14 figure
The prosegment catalyzes native folding of Plasmodium falciparum plasmepsin II
Plasmepsin II is a malarial pepsin-like aspartic protease produced as a zymogen containing an N-terminal prosegment domain that is removed during activation. Despite structural similarities between active plasmepsin II and pepsin, their prosegments adopt different conformations in the respective zymogens. In contrast to pepsinogen, the proplasmepsin II prosegment is 80 residues longer, contains a transmembrane region and is non-essential for recombinant expression in an active form, thus calling into question the prosegment's precise function. The present study examines the role of the prosegment in the folding mechanism of plasmepsin II. Both a shorter (residues 77–124) and a longer (residues 65–124) prosegment catalyze plasmepsin II folding at rates more than four orders of magnitude faster compared to folding without prosegment. Native plasmepsin II is kinetically trapped and requires the prosegment both to catalyze folding and to shift the folding equilibrium towards the native conformation. Thus, despite low sequence identity and distinct zymogen conformations, the folding landscapes of plasmepsin II and pepsin, both with and without prosegment, are qualitatively identical. These results imply a conserved and unusual feature of the pepsin-like protease topology that necessitates prosegment-assisted folding
Orbital-Controlled Superconductivity in f-Electron Systems
We propose a concept of superconductivity controlled by orbital degree of
freedom taking CeMIn5 (M= Co, Rh, and Ir) as typical examples. A microscopic
multiorbital model for CeMIn5 is analyzed by fluctuation exchange
approximation. Even though the Fermi-surface structure is unchanged, the ground
state is found to change significantly among paramagnetic, antiferromagnetic,
and d-wave superconducting phases, depending on the dominant orbital component
in the band near the Fermi energy. We show that our picture naturally explains
the different low-temperature properties of CeMIn5 by carefully analyzing the
crystalline electric field states.Comment: 5 pages, 4 figure
Deformation of Electronic Structures Due to CoO6 Distortion and Phase Diagrams of NaxCoO2.yH2O
Motivated by recently reported experimental phase diagrams, we study the
effects of CoO6 distortion on the electronic structure in NaxCoO2.yH2O. We
construct the multiband tight-binding model by employing the LDA result.
Analyzing this model, we show the deformation of band dispersions and
Fermi-surface topology as functions of CoO2-layer thickness. Considering these
results together with previous theoretical ones, we propose a possible
schematic phase diagram with three successive phases: the extended s-wave
superconductivity (SC), the magnetic order, and the spin-triplet SC phases when
the Co valence number s is +3.4. A phase diagram with only one phase of
spin-triplet SC is also proposed for the s=+3.5 case.Comment: 4 pages, 5 figure
The characteristics of Hayabusa returned samples and their distributions for NASA and international announcement of opportunity (AO).
第3回極域科学シンポジウム/第35回南極隕石シンポジウム 11月30日(金) 国立国語研究所 2階講
Random Spin-orbit Coupling in Spin Triplet Superconductors: Stacking Faults in Sr_2RuO_4 and CePt_3Si
The random spin-orbit coupling in multicomponent superconductors is
investigated focusing on the non-centrosymmetric superconductor CePt_3Si and
the spin triplet superconductor Sr_2RuO_4. We find novel manifestations of the
random spin-orbit coupling in the multicomponent superconductors with
directional disorders, such as stacking faults. The presence of stacking faults
is indicated for the disordered phase of CePt_3Si and Sr_2RuO_4. It is shown
that the d-vector of spin triplet superconductivity is locked to be d = k_y x -
k_x y with the anisotropy \Delta T_c/T_c0 \sim \bar{\alpha}^2/T_c0 W_z, where
\bar{\alpha}, T_c0, and W_z are the mean square root of random spin-orbit
coupling, the transition temperature in the clean limit, and the kinetic energy
along the c-axis, respectively. This anisotropy is much larger (smaller) than
that in the clean bulk Sr_2RuO_4 (CePt_3Si). These results indicate that the
helical pairing state d = k_y x - k_x y in the eutectic crystal
Sr_2RuO_4-Sr_3Ru_2O_7 is stabilized in contrast to the chiral state d = (k_x
\pm i k_y) z in the bulk Sr_2RuO_4. The unusual variation of T_c in CePt_3Si is
resolved by taking into account the weak pair-breaking effect arising from the
uniform and random spin-orbit couplings. These superconductors provide a basis
for discussing recent topics on Majorana fermions and non-Abelian statistics.Comment: J. Phys. Soc. Jpn. 79 (2010) 08470
d-Wave Spin Density Wave phase in the Attractive Hubbard Model with Spin Polarization
We investigate the possibility of unconventional spin density wave (SDW) in
the attractive Hubbard model with finite spin polarization. We show that
pairing and density fluctuations induce the transverse d-wave SDW near the
half-filling. This novel SDW is related to the d-wave superfluidity induced by
antiferromagnetic spin fluctuations, in the sense that they are connected with
each other through Shiba's attraction-repulsion transformation. Our results
predict the d-wave SDW in real systems, such as cold Fermi atom gases with
population imbalance and compounds involving valence skipper elements
Asteroid Itokawa sample curation and distribution in the Planetary Material Sample Curation Facility of JAXA.
第2回極域科学シンポジウム/第34回南極隕石シンポジウム 11月17日(木) 国立国語研究所 2階講
Symmetry and Topology in Superconductors - Odd-frequency pairing and edge states -
Superconductivity is a phenomenon where the macroscopic quantum coherence
appears due to the pairing of electrons. This offers a fascinating arena to
study the physics of broken gauge symmetry. However, the important symmetries
in superconductors are not only the gauge invariance. Especially, the symmetry
properties of the pairing, i.e., the parity and spin-singlet/spin-triplet,
determine the physical properties of the superconducting state. Recently it has
been recognized that there is the important third symmetry of the pair
amplitude, i.e., even or odd parity with respect to the frequency. The
conventional uniform superconducting states correspond to the even-frequency
pairing, but the recent finding is that the odd-frequency pair amplitude arises
in the spatially non-uniform situation quite ubiquitously. Especially, this is
the case in the Andreev bound state (ABS) appearing at the surface/interface of
the sample. The other important recent development is on the nontrivial
topological aspects of superconductors. As the band insulators are classified
by topological indices into (i) conventional insulator, (ii) quantum Hall
insulator, and (iii) topological insulator, also are the gapped
superconductors. The influence of the nontrivial topology of the bulk states
appears as the edge or surface of the sample. In the superconductors, this
leads to the formation of zero energy ABS (ZEABS). Therefore, the ABSs of the
superconductors are the place where the symmetry and topology meet each other
which offer the stage of rich physics. In this review, we discuss the physics
of ABS from the viewpoint of the odd-frequency pairing, the topological
bulk-edge correspondence, and the interplay of these two issues. It is
described how the symmetry of the pairing and topological indices determines
the absence/presence of the ZEABS, its energy dispersion, and properties as the
Majorana fermions.Comment: 91 pages, 38 figures, Review article, references adde
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