541 research outputs found
Spin susceptibility and fluctuation corrections in the BCS-BEC crossover regime of an ultracold Fermi gas
We investigate magnetic properties and effects of pairing fluctuations in the
BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover
regime of an ultracold Fermi gas. Recently, Liu and Hu, and Parish, pointed out
that the strong-coupling theory developed by Nozi\`eres and Schmitt-Rink (NSR),
which has been extensively used to successfully clarify various physical
properties of cold Fermi gases, unphysically gives negative spin susceptibility
in the BCS-BEC crossover region. The same problem is found to also exist in the
ordinary non-self-consistent T-matrix approximation. In this paper, we clarify
that this serious problem comes from incomplete treatment in term of pseudogap
phenomena originating from strong pairing fluctuations, as well as effects of
spin fluctuations on the spin susceptibility. Including these two key issues,
we construct an extended T-matrix theory which can overcome this problem. The
resulting positive spin susceptibility agrees well with the recent experiment
on a 6Li Fermi gas done by Sanner and co-workers. We also apply our theory to a
polarized Fermi gas to examine the superfluid phase transition temperature Tc,
as a function of the polarization rate. Since the spin susceptibility is an
important physical quantity, especially in singlet Fermi superfluids, our
results would be useful in considering how singlet pairs appear above and below
Tc in the BCS-BEC crossover regime of cold Fermi gases.Comment: 21 pages, 9 figure
Transport, magnetic and superconducting properties of RuSr2RCu2O8 (R= Eu, Gd) doped with Sn
Ru{1-x}Sn{x}Sr2EuCu2O8 and Ru{1-x}Sn{x}Sr2GdCu2O8 have been comprehensively
studied by microwave and dc resistivity and magnetoresistivity and by the dc
Hall measurements. The magnetic ordering temperature T_m is considerably
reduced with increasing Sn content. However, doping with Sn leads to only
slight reduction of the superconducting critical temperature T_c accompanied
with the increase of the upper critical field B_c2, indicating an increased
disorder in the system and a reduced scattering length of the conducting holes
in CuO2 layers. In spite of the increased scattering rate, the normal state
resistivity and the Hall resistivity are reduced with respect to the pure
compound, due to the increased number of itinerant holes in CuO2 layers, which
represent the main conductivity channel. Most of the electrons in RuO2 layers
are presumably localized, but the observed negative magnetoresistance and the
extraordinary Hall effect lead to the conclusion that there exists a small
number of itinerant electrons in RuO layers that exhibit colossal
magnetoresistance.Comment: 10 pages, 9 figure
Nitrogen Fertilization and Harvest Management Improve Forage and Crude Protein Content in Crabgrass
Crabgrass (Digitaria spp.) is an annual summer grass that can provide high-quality forage, but optimal management strategies are unclear. Our objective was to compare the yield and quality of crabgrass (Mojo and Quick-N-Big) under different nitrogen rates and harvest management. The experimental design was a randomized complete block with five treatments and three replications for each crabgrass variety, totaling fifteen experimental units for both Mojo and Quick-N-Big, in adjacent sites. Treatments were nitrogen rates (0, 100, and 200 lb N/acre) and harvest management (cut once or twice per year) for two growing seasons (2020 and 2021). Total forage accumulation (TFA) increased with nitrogen fertilization for both cultivars. Mojo had the highest TFA in the first year (7000 lb DM/a/yr) while Quick-N-Big TFA was the highest in the second year (7635 lb DM/a/yr). The highest crude protein (CP) content was obtained with the highest N dose, ranging from 10.5 to 13% for both cultivars. Based on these results, N fertilization and harvest management can contribute to improving forage yield and crude protein of crabgrass varieties during the growing season in forage systems
Opening of a pseudogap in a quasi-two dimensional superconductor due to critical thermal fluctuations
We examine the role of the anisotropy of superconducting critical thermal
fluctuations in the opening of a pseudogap in a quasi-two dimensional
superconductor such as a cuprate-oxide high-temperature superconductor. When
the anisotropy between planes and their perpendicular axis is large enough and
its superconducting critical temperature T_c is high enough, the fluctuations
are much developed in its critical region so that lifetime widths of
quasiparticles are large and the energy dependence of the selfenergy deviates
from that of Landau's normal Fermi liquids. A pseudogap opens in such a
critical region because quasiparticle spectra around the chemical potential are
swept away due to the large lifetime widths. The pseudogap never smoothly
evolves into a superconducting gap; it starts to open at a temperature higher
than T_c while the superconducting gap starts to open just at T_c. When T_c is
rather low but the ratio of varepsilon_G(0)/k_BT_c, with varepsilon_G(0) the
superconducting gap at T=0K and k_B the Boltzmann constant, is much larger than
a value about 4 according to the mean-field theory, the pseudogap must be
closing as temperature T approaches to the low T_c because thermal fluctuations
become less developed as T decreases. Critical thermal fluctuations cannot
cause the opening of a prominent pseudogap in an almost isotropic three
dimensional superconductor, even if its T_c is high.Comment: 25 pages, 5 figures (14 subfigures
A Universal Intrinsic Scale of Hole Concentration for High-Tc Cuprates
We have measured thermoelectric power (TEP) as a function of hole
concentration per CuO2 layer, Ppl, in Y1-xCaxBa2Cu3O6 (Ppl = x/2) with no
oxygen in the Cu-O chain layer. The room-temperature TEP as a function of Ppl,
S290(Ppl), of Y1-xCaxBa2Cu3O6 behaves identically to that of La2-zSrzCuO4 (Ppl
= z). We argue that S290(Ppl) represents a measure of the intrinsic equilibrium
electronic states of doped holes and, therefore, can be used as a common scale
for the carrier concentrations of layered cuprates. We shows that the Ppl
determined by this new universal scale is consistent with both hole
concentration microscopically determined by NQR and the hole concentration
macroscopically determined by the Cu valency. We find two characteristic
scaling temperatures, TS* and TS2*, in the TEP vs. temperature curves that
change systematically with doping. Based on the universal scale, we uncover a
universal phase diagram in which almost all the experimentally determined
pseudogap temperatures as a function of Ppl fall on two common curves; upper
pseudogap temperature defined by the TS* versus Ppl curve and lower pseudogap
temperature defined by the TS2* versus Ppl curve. We find that while pseudogaps
are intrinsic properties of doped holes of a single CuO2 layer for all high-Tc
cuprates, Tc depends on the number of layers, therefore the inter-layer
coupling, in each individual system.Comment: 11 pages, 9 figures, accepted for publication in Physical Review
Doping Dependent Density of States and Pseudogap Behavior in La2-xSrxCuO4
We have made a high-resolution photoemission study of La2-xSrxCuO4 in a wide
hole concentration (x) range from a heavily overdoped metal to an undoped
insulator. As x decreases, the spectral density of states at the chemical
potential (\mu) is suppressed with an x-dependence similar to the suppression
of the electronic specific heat coefficient. In the underdoped region, the
spectra show a pseudogap structure on the energy scale of 0.1 eV. The width of
the pseudogap increases with decreasing x following the x-dependence of the
characteristic temperatures of the magnetic susceptibility and the Hall
coefficient.Comment: 4 pages, 3 EPS figures, uses RevTex; few references have been update
Evidence on anti-malarial and diagnostic markets in Cambodia to guide malaria elimination strategies and policies
BACKGROUND: Understanding Cambodia's anti-malarial and diagnostic landscape in 2015 is critical for informing and monitoring strategies and policies as Cambodia moves forward with national efforts to eliminate malaria. The aim of this paper is to present timely and key findings on the public and private sector anti-malarial and diagnostic landscape in Cambodia. This evidence can serve as a baseline benchmark for guiding implementation of national strategies as well as other regional initiatives to address malaria elimination activities. METHODS: From August 17th to October 1st, 2015, a cross sectional, nationally-representative malaria outlet survey was conducted in Cambodia. A census of all public and private outlets with potential to distribute malaria testing and/or treatment was conducted among 180 communes. An audit was completed for all anti-malarials, malaria rapid diagnostic tests (RDT) and microscopy. RESULTS: A total of 26,664 outlets were screened, and 1303 outlets were eligible and interviewed. Among all screened outlets in the public sector, 75.9% of public health facilities and 67.7% of community health workers stocked both malaria diagnostic testing and a first-line artemisinin-based combination therapy (ACT). Among anti-malarial-stocking private sector outlets, 64.7% had malaria blood testing available, and 70.9% were stocking a first-line ACT. Market share data illustrate that most of the anti-malarials were sold or distributed through the private sector (58.4%), including itinerant drug vendors (23.4%). First-line ACT accounted for the majority of the market share across the public and private sectors (90.3%). Among private sector outlets stocking any anti-malarial, the proportion of outlets with a first-line ACT or RDT was higher among outlets that had reportedly received one or more forms of 'support' (e.g. reportedly received training in the previous year on malaria diagnosis [RDT and/or microscopy] and/or the national treatment guidelines for malaria) compared to outlets that did not report receiving any support (ACT: 82.1 and 60.6%, respectively; RDT: 78.2 and 64.0%, respectively). CONCLUSION: The results point to high availability and distribution of first-line ACT and widespread availability of malaria diagnosis, especially in the public sector. This suggests that there is a strong foundation for achieving elimination goals in Cambodia. However, key gaps in terms of availability of malaria commodities for case management must be addressed, particularly in the private sector where most people seek treatment. Continued engagement with the private sector will be important to ensure accelerated progress towards malaria elimination
Charged Vortices in High Temperature Superconductors Probed by NMR
We report a first experimental evidence that a vortex in the high temperature
superconductors (HTSC) traps a finite electric charge from the high resolution
measurements of the nuclear quadrupole frequencies. In slightly overdoped
YBa_2Cu_3O_7 the vortex is negatively charged by trapping electrons, while in
underdoped YBa_2Cu_4O_8 it is positively charged by expelling electrons. The
sign of the trapped charge is opposite to the sign predicted by the
conventional BCS theory. Moreover, in both materials, the deviation of the
magnitude of the charge from the theory is also significant. These unexpected
features can be attributed to the novel electronic structure of the vortex in
HTSC.Comment: 6 pages, 7 figures, to be published in Phys Rev.
Spin dynamics and antiferromagnetic order in PrBa2Cu4O8 studied by Cu nuclear respnance
Results of the nuclear resonance experiments for the planar Cu sites in
PrBa2Cu4O8 are presented. The NMR spectrum at 1.5 K in zero magnetic field
revealed an internal field of 6.1 T, providing evidence for an
antiferromagnetic order of the planar Cu spins. This confirms that the CuO2
planes are insulating, therefore, the metallic conduction in this material is
entirely due to the one-dimensional zigzag Cu2O2 chains. The results of the
spin-lattice relaxation rates measured by zero field NQR above 245 K in the
paramagnetic state are explained by the theory for a Heisenberg model on a
square lattice.Comment: 4 pages, 2 figure
Doping Dependence of the Pseudogap in La(2-x)Sr(x)CuO(4)
We report the results of Raman scattering experiments on single crystals of
La(2-x)Sr(x)CuO(4) that span the range from underdoped (x = 0.10) to overdoped
(x =0.22). The spectra are consistent with the existence of a strong
anisotropic quasiparticle interaction that results in a normal state depletion
of spectral weight from regions of the Fermi surface located near the zone
axes. The strength of the interaction decreases rapidly with increasing hole
concentration and the spectral evidence for the pseudogap vanishes when the
optimum doping level is reached. The results suggest that the pseudogap and
superconducting gap arise from different mechanisms.Comment: 7 pages, 6 eps figures, added new sections, figures, reference
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