2,386 research outputs found
Microscopic observation of superconducting fluctuations in -(BEDT-TTF)Cu[N(CN)]Br by C NMR spectroscopy
We performed C-NMR experiment and measured spin-lattice relaxation
rate divided by temperature near the superconducting (SC) transition
temperature in -(BEDT-TTF)Cu[N(CN)]Br (-Br
salt), and -(BEDT-TTF)Cu(NCS) (-NCS salt). We
observed the reduction of starting at the temperature higher than
in -Br salt. Microscopic observation of quasi-particle density of
states in the fluctuating SC state revealed the effects of short-range Cooper
pairs induced in the normal state to the quasi-particle density of states. We
also performed systematic measurements in the fields both parallel and
perpendicular to the conduction plane in -Br and -NCS salts,
and confirmed that the reduction of above is observed only
in -Br salt regardless of the external field orientation.Comment: Accepted for publication in PR
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
Anisotropic Behavior of Knight Shift in Superconducting State of Na_xCoO_2yH_2O
The Co Knight shift was measured in an aligned powder sample of
Na_xCoO_2yH_2O, which shows superconductivity at T_c \sim 4.6 K. The
Knight-shift components parallel (K_c) and perpendicular to the c-axis (along
the ab plane K_{ab}) were measured in both the normal and superconducting (SC)
states. The temperature dependences of K_{ab} and K_c are scaled with the bulk
susceptibility, which shows that the microscopic susceptibility deduced from
the Knight shift is related to Co-3d spins. In the SC state, the Knight shift
shows an anisotropic temperature dependence: K_{ab} decreases below 5 K,
whereas K_c does not decrease within experimental accuracy. This result raises
the possibility that spin-triplet superconductivity with the spin component of
the pairs directed along the c-axis is realized in Na_xCoO_2yH_2O.Comment: 5 pages, 5 figures, to be published in Journal of Physical Society of
Japan vol. 75, No.
Explicit towers of Drinfeld modular curves
We give explicit equations for the simplest towers of Drinfeld modular curves
over any finite field, and observe that they coincide with the asymptotically
optimal towers of curves constructed by Garcia and Stichtenoth.Comment: 10 pages. For mini-symposium on "curves over finite fields and codes"
at the 3rd European Congress in Barcelona 7/2000 Revised to correct minor
typographical and grammatical error
Irregular behaviour of class numbers and Euler-Kronecker constants of cyclotomic fields: the log log log devil at play
Kummer (1851) and, many years later, Ihara (2005) both posed conjectures on
invariants related to the cyclotomic field with a
prime. Kummer's conjecture concerns the asymptotic behaviour of the first
factor of the class number of and Ihara's the positivity
of the Euler-Kronecker constant of (the ratio of the
constant and the residue of the Laurent series of the Dedekind zeta function
at ). If certain standard conjectures in
analytic number theory hold true, then one can show that both conjectures are
true for a set of primes of natural density 1, but false in general.
Responsible for this are irregularities in the distribution of the primes. With
this survey we hope to convince the reader that the apparently dissimilar
mathematical objects studied by Kummer and Ihara actually display a very
similar behaviour.Comment: 20 pages, 1 figure, survey, to appear in `Irregularities in the
Distribution of Prime Numbers - Research Inspired by Maier's Matrix Method',
Eds. J. Pintz and M. Th. Rassia
Phonon and Elastic Instabilities in MoC and MoN
We present several results related to the instability of MoC and MoN in the
B1 (sodium chloride) structure. These compounds were proposed as potential
superconductors with moderately high transition temperatures. We show that the
elastic instability in B1-structure MoN, demonstrated several years ago,
persists at elevated pressures, thus offering little hope of stabilizing this
material without chemical doping. For MoC, another material for which
stoichiometric fabrication in the B1-structure has not proven possible, we find
that all of the cubic elastic constants are positive, indicating elastic
stability. Instead, we find X-point phonon instabilities in MoC (and in MoN as
well), further illustrating the rich behavior of carbo-nitride materials. We
also present additional electronic structure results for several transition
metal (Zr, Nb and Mo) carbo-nitride systems and discuss systematic trends in
the properties of these materials. Deviations from strict electron counting
dependencies are apparent.Comment: 5 pages and 4 trailing figures. Submitted to PR
Specific Heat and Superfluid Density for Possible Two Different Superconducting States in NaxCoO2.yH2O
Several thermodynamic measurements for the cobaltate superconductor,
NaxCoO2.yH2O, have so far provided results inconsistent with each other. In
order to solve the discrepancies, we microscopically calculate the temperature
dependences of specific heat and superfluid density for this superconductor. We
show that two distinct specific-heat data from Oeschler et al. and Jin et al.
are reproduced, respectively, for the extended s-wave state and the p-wave
state. Two different superfluid-density data are also reproduced for each case.
These support our recent proposal of possible two different pairing states in
this material. In addition, we discuss the experimentally proposed large
residual Sommerfeld coefficient and extremely huge effective carrier mass.Comment: 5 pages, 4 figures, Submitted to J. Phys. Soc. Jp
London's urban heat island: Impact on current and future energy consumption in office buildings
This article is available open access and shared under a Creative Commons license: (http://creativecommons.org/licenses/by/3.0/). Copyright @ 2011 Elsevier B.V.This paper presents the results of a computational study on the energy consumption and related CO2 emissions for heating and cooling of an office building within the Urban Heat Island of London, currently and in the future. The study developed twenty weather files in an East-West axis through London; the weather files were constructed according to future climate change scenario for 2050 suitable for the UK which have been modified to represent specific locations within the London UHI based on measurements and predictions from a program developed for this purpose (LSSAT). The study simulated an office with typical construction, heat gains and operational patterns with an advanced thermal simulation program (IESVE). The predictions confirm that heating load decreases, cooling load and overheating hours increase as the office location moves from rural to urban sites and from present to future years. It is shown that internal heat gains are an important factor affecting energy performance and that night cooling using natural ventilation will have a beneficial effect at rural and city locations. As overheating will increase in the future, more buildings will use cooling; it is shown that this might lead to a five-fold increase of CO2 emission for city centre offices in London in 2050. The paper presents detailed results of the typical office placed on the East-West axis of the city, arguing the necessity to consider using weather files based on climate projections and urbanheat island for the design of currentbuildings to safeguard their efficiency in the future.EPSR
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