741 research outputs found
A result on q-series and its application to quadratic forms
AbstractThe purpose of this paper is to prove a conjectured q-identity. The result is then applied to estimating the local density of solutions of certain systems of quadratic form identities
Density of States and NMR Relaxation Rate in Anisotropic Superconductivity with Intersecting Line Nodes
We show that the density of states in an anisotropic superconductor with
intersecting line nodes in the gap function is proportional to for , where is the maximum value of
the gap function and is constant, while it is proportional to if
the line nodes do not intersect. As a result, a logarithmic correction appears
in the temperature dependence of the NMR relaxation rate and the specific heat,
which can be observed experimentally. By comparing with those for the heavy
fermion superconductors, we can obtain information about the symmetry of the
gap function.Comment: 7 pages, 4 PostScript Figures, LaTeX, to appear in J. Phys. Soc. Jp
Evidence for ferromagnetic spin-pairing superconductivity in UGe: A Ge-NQR study under pressure
We report that a novel type of superconducting order parameter has been
realized in the ferromagnetic states in UGe via Ge
nuclear-quadrupole-resonance (NQR) experiments performed under pressure ().
Measurements of the nuclear spin-lattice relaxation rate have
revealed an unconventional nature of superconductivity such that the up-spin
band is gapped with line nodes, but the down-spin band remains gapless at the
Fermi level. This result is consistent with that of a ferromagnetic
spin-pairing model in which Cooper pairs are formed among ferromagnetically
polarized electrons. The present experiment has shed new light on a possible
origin of ferromagnetic superconductivity, which is mediated by ferromagnetic
spin-density fluctuations relevant to the first-order transition inside the
ferromagnetic states.Comment: 5 pages, 5 figure
Spin Susceptibility of Noncentrosymmetric Heavy-fermion Superconductor CeIrSi3 under Pressure: 29Si-Knight Shift Study on Single Crystal
We report 29Si-NMR study on a single crystal of the heavy-fermion
superconductor CeIrSi3 without an inversion symmetry along the c-axis. The
29Si-Knight shift measurements under pressure have revealed that the spin
susceptibility for the ab-plane decreases slightly below Tc, whereas along the
c-axis it does not change at all. The result can be accounted for by the spin
susceptibility in the superconducting state being dominated by the strong
antisymmetric (Rashba-type) spin-orbit interaction that originates from the
absence of an inversion center along the c-axis and it being much larger than
superconducting condensation energy. This is the first observation which
exhibits an anisotropy of the spin susceptibility below Tc in the
noncentrosymmetric superconductor dominated by strong Rashba-type spin-orbit
interaction.Comment: 4 pages, 4 figures, Accepted for publication in Phys. Rev. Let
Optimal branching asymmetry of hydrodynamic pulsatile trees
Most of the studies on optimal transport are done for steady state regime
conditions. Yet, there exists numerous examples in living systems where supply
tree networks have to deliver products in a limited time due to the pulsatile
character of the flow. This is the case for mammals respiration for which air
has to reach the gas exchange units before the start of expiration. We report
here that introducing a systematic branching asymmetry allows to reduce the
average delivery time of the products. It simultaneously increases its
robustness against the unevitable variability of sizes related to
morphogenesis. We then apply this approach to the human tracheobronchial tree.
We show that in this case all extremities are supplied with fresh air, provided
that the asymmetry is smaller than a critical threshold which happens to fit
with the asymmetry measured in the human lung. This could indicate that the
structure is adjusted at the maximum asymmetry level that allows to feed all
terminal units with fresh air.Comment: 4 pages, 4 figure
Enhancement of Superconducting Transition Temperature due to the strong Antiferromagnetic Spin Fluctuations in Non-centrosymmetric Heavy-fermion Superconductor CeIrSi3 :A 29Si-NMR Study under Pressure
We report a 29Si-NMR study on the pressure-induced superconductivity (SC) in
an antiferromagnetic (AFM) heavy-fermion compound CeIrSi3 without inversion
symmetry. In the SC state at P=2.7-2.8 GPa, the temperature dependence of the
nuclear-spin lattice relaxation rate 1/T_1 below Tc exhibits a T^3 behavior
without any coherence peak just below Tc, revealing the presence of line nodes
in the SC gap. In the normal state, 1/T_1 follows a \sqrt{T}-like behavior,
suggesting that the SC emerges under the non-Fermi liquid state dominated by
AFM spin fluctuations enhanced around quantum critical point (QCP). The reason
why the maximum Tc in CeIrSi3 is relatively high among the Ce-based
heavy-fermion superconductors may be the existence of the strong AFM spin
fluctuations. We discuss the comparison with the other Ce-based heavy-fermion
superconductors.Comment: 4 pages, 5 figures, To be published in Phys. Rev. Let
Realization of odd-frequency p-wave spin-singlet superconductivity coexisting with antiferromagnetic order near quantum critical point
A possibility of the realization of the p-wave spin-singlet superconductivity
(SS), whose gap function is odd both in momentum and in frequency, is
investigated by solving the gap equation with the phenomenological interaction
mediated by the antiferromagnetic spin fluctuation. The SS is realized
prevailing over the d-wave singlet superconductivity (SS) in the vicinity of
antiferromagnetic quantum critical pint (QCP) both on the paramagnetic and on
the antiferromagnetic sides. Off the QCP in the paramagnetic phase, however,
the SS with line-nodes is realized as \textit{conventional} anisotropic
superconductivity. For the present SS state, there is no gap in the
quasiparticle spectrum everywhere on the Fermi surface due to its odd
frequency. These features can give a qualitative understanding of the anomalous
behaviors of NQR relaxation rate on CeCuSi or CeRhIn where the
antiferromagnetism and superconductivity coexist on a microscopic level.Comment: 20 pages with 12 figures. To appear in J. Phys. Soc. Jpn. Vol. 72,
No. 1
Instability of Magnons in Two-dimensional Antiferromagnet at High Magnetic Fields
Spin dynamics of the square lattice Heisenberg antiferromagnet, \BaMnGeO, is
studied by a combination of bulk measurements, neutron diffraction, and
inelastic neutron scattering techniques. Easy plane type antiferromagnetic
order is identified at K. The exchange interactions are estimated
as = 27.8(3)eV and = 1.0(1) eV, and the saturation
field is 9.75 T. Magnetic excitation measurements with high
experimental resolution setup by triple axis neutron spectrometer reveals the
instability of one magnon excitation in the field range of .Comment: 5 pgase, 5 figuers, to be published in PRB R
Planar CuO_2 hole density estimation in multilayered high-T_c cuprates
We report that planar CuO_2 hole densities in high-T_c cuprates are
consistently determined by the Cu-NMR Knight shift. In single- and bi-layered
cuprates, it is demonstrated that the spin part of the Knight shift K_s(300 K)
at room temperature monotonically increases with the hole density from
underdoped to overdoped regions, suggesting that the relationship of K_s(300 K)
vs. p is a reliable measure to determine p. The validity of this K_s(300 K)-p
relationship is confirmed by the investigation of the p-dependencies of
hyperfine magnetic fields and of spin susceptibility for single- and bi-layered
cuprates with tetragonal symmetry. Moreover, the analyses are compared with the
NMR data on three-layered Ba_2Ca_2Cu_3O_6(F,O)_2, HgBa_2Ca_2Cu_3O_{8+delta},
and five-layered HgBa_2Ca_4Cu_5O_{12+delta}, which suggests the general
applicability of the K_s(300 K)-p relationship to multilayered compounds with
more than three CuO_2 planes. We remark that the measurement of K_s(300 K)
enables us to separately estimate p for each CuO_2 plane in multilayered
compounds, where doped hole carriers are inequivalent between outer CuO_2
planes and inner CuO_2 planes.Comment: 7 pages, 5 figures, 2 Tables, to be published in Physical Review
Orbital ordering in frustrated Jahn-Teller systems
We consider the superexchange in `frustrated' Jahn-Teller systems, such as
the transition metal oxides NaNiO_2, LiNiO_2, and ZnMn_2O_4, in which
transition metal ions with doubly degenerate orbitals form a triangular or
pyrochlore lattice and are connected by the 90-degree metal-oxygen-metal bonds.
We show that this interaction is much different from a more familiar exchange
in systems with the 180-degree bonds, e.g. perovskites. In contrast to the
strong interplay between the orbital and spin degrees of freedom in
perovskites, in the 90-degree exchange systems spins and orbitals are
decoupled: the spin exchange is much weaker than the orbital one and it is
ferromagnetic for all orbital states. Due to frustration, the mean-field
orbital ground state is strongly degenerate. Quantum orbital fluctuations
select particular ferro-orbital states, such as the one observed in NaNiO_2. We
also discuss why LiNiO_2 may still behave as an orbital liquid.Comment: 5 pages, 3 figure
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