432 research outputs found
Electronic Raman scattering in correlated materials: exact treatment of nonresonant, mixed, and resonant scattering with dynamical mean field theory
We solve for the electronic Raman scattering response functions on an
infinite-dimensional hypercubic lattice employing dynamical mean field theory.
This contribution extends previous work on the nonresonant response to include
the mixed and resonant contributions. We focus our attention on the spinless
Falicov-Kimball model, where the problem can be solved exactly, and the system
can be tuned to go through a Mott-Hubbard-like metal-insulator transition.
Resonant effects vary in different scattering geometries, corresponding to the
symmetries of the charge excitations scattered by the light. We do find that
the Raman response is large near the double resonance, where the transfered
frequency is close to the incident photon frequency. We also find a joint
resonance of both the charge-transfer peak and the low-energy peak when the
incident photon frequency is on the order of the interaction strength. In
general, the resonance effects can create order of magnitude (or more)
enhancements of features in the nonresonant response, especially when the
incident photon frequency is somewhat larger than the frequency of the
nonresonant feature. Finally, we find that the resonant effects also exhibit
isosbestic behavior, even in the A1g and B2g sectors, and it is most prominent
when the incident photon frequency is on the order of the interaction energy.Comment: (20 pages, 13 figures
Strong diamagnetic response and specific heat anomaly above T_c in underdoped La_(2-x)Sr_xCuO_4
By measuring AC susceptibility using a very low amplitude of the AC field (<1
mG) it is shown that underdoped samples of La_(2-x)Sr_xCuO_4 (LASCO), are
diamagnetic in a temperature region above T_c up to a temperature T^*. This
behavior is only observed with AC fields along the c-axis whereas for fields in
the ab-plane no diamagnetism above Tc was detected. The diamagnetism is almost
frequency independent in the frequency range 0.1-10 kHz. At T* a broad step
anomaly in the specific heat is inferred through measurements of the elastic
constant c33. We suggest that the observed diamagnetism and the anomaly in the
elastic constant are associated with the existence of phase incoherent Cooper
pairs between Tc and T*.Comment: 5 pages 7 figures, to appear in Phys. rev
Chapter 6 Apprenticeship Learning in Preparation for Meeting the Unforeseen
"The chapter analyzes teaching where the goal is to enable students at the
Armed Forces Staff College to master unforeseen events. The students are all participants
in a program for joint operations in which different military branches are placed
together to solve complex tasks. How can the supervisor contribute to increased
samhandling when facing the unforeseen? The goal is for students to be able to cope
with the roles that exist in a normal NATO headquarters and to learn how to use
NATO’s operational planning strategy. The group supervisor becomes a form of
master who greatly influences the approach of students to the training community.
From a sample of one hundred students, five groups consisting of four to five students
of both sexes, with varied defense-force affiliations, backgrounds and experience
were selected to be interviewed. A total of 23 informants participated in the
interviews. In addition, observations were carried out. Apprenticeship Learning as a
method is appropriate to prepare the students better for samhandling in anticipation
of the unforeseen. The way the supervisor manages his or her role has a great deal of
impact on samhandling and learning outcomes. The supervisor’s insight and expertise
in what is needed to make groups work together is decisive. Strengthening and
developing samhandling in exercises is a suitable educational method for military
forces in meeting unforeseen events, provided that it is done properly.
Raman scattering through a metal-insulator transition
The exact solution for nonresonant A1g and B1g Raman scattering is presented
for the simplest model that has a correlated metal-insulator transition--the
Falicov-Kimball model, by employing dynamical mean field theory. In the general
case, the A1g response includes nonresonant, resonant, and mixed contributions,
the B1g response includes nonresonant and resonant contributions (we prove the
Shastry-Shraiman relation for the nonresonant B1g response) while the B2g
response is purely resonant. Three main features are seen in the nonresonant
B1g channel: (i) the rapid appearance of low-energy spectral weight at the
expense of higher-energy weight; (b) the frequency range for this low-energy
spectral weight is much larger than the onset temperature, where the response
first appears; and (iii) the occurrence of an isosbestic point, which is a
characteristic frequency where the Raman response is independent of temperature
for low temperatures. Vertex corrections renormalize away all of these
anomalous features in the nonresonant A1g channel. The calculated results
compare favorably to the Raman response of a number of correlated systems on
the insulating side of the quantum-critical point (ranging from Kondo
insulators, to mixed-valence materials, to underdoped high-temperature
superconductors). We also show why the nonresonant B1g Raman response is
``universal'' on the insulating side of the metal-insulator transition.Comment: 12 pages, 11 figures, ReVTe
Overestimation of Vitamin a Supplementation Coverage from District Tally Sheets Demonstrates Importance of Population-Based Surveys for Program Improvement: Lessons from Tanzania.
Tanzania has conducted a national twice-yearly Vitamin A supplementation (VAS) campaign since 2001. Administrative coverage rates based on tally sheets consistently report >90% coverage; however the accuracy of these rates are uncertain due to potential errors in tally sheets and their aggregation, incomplete or inaccurate reporting from distribution sites, and underestimating the target population. The post event coverage survey in Mainland Tanzania sought to validate tally-sheet based national coverage estimates of VAS and deworming for the June 2010 mass distribution round, and to characterize children missed by the national campaign. WHO/EPI randomized cross-sectional cluster sampling methodology was adapted for this study, using 30 clusters by 40 individuals (n = 1200), in addition to key informant interviews. Households with children 6-59 months of age were included in the study (12-59 months for deworming analysis). Chi-squared tests and logistic regression analysis were used to test differences between children reached and not reached by VAS. Data was collected within six weeks of the June 2010 round. A total of 1203 children, 58 health workers, 30 village leaders and 45 community health workers were sampled. Preschool VAS coverage was 65% (95% CI: 62.7-68.1), approximately 30% lower than tally-sheet coverage estimates. Factors associated with not receiving VAS were urban residence [OR = 3.31; p = 0.01], caretakers who did not hear about the campaign [OR = 48.7; p<0.001], and Muslim households [OR<3.25; p<0.01]. There were no significant differences in VAS coverage by child sex or age, or maternal age or education. Coverage estimation for vitamin A supplementation programs is one of most powerful indicators of program success. National VAS coverage based on a tally-sheet system overestimated VAS coverage by ∼30%. There is a need for representative population-based coverage surveys to complement and validate tally-sheet estimates
Pseudogap in the microwave response of YBa_2Cu_3O_{7-x}
The in-plane and out-of-plane surface impedance and microwave conductivity
components of one and the same YBa_2Cu_3O_{7-x} (0.07\le x\le 0.47) single
crystal are determined in the wide ranges of temperature T and carrier
concentration p in CuO_2 planes. The following features of the superfluid
density n_s(T,p)\propto\lambda_{ab}^{-2}(T,p) are observed at T<Tc/2 and
0.078\le p\le 0.16: (i) n_s(0,p) depends linearly on p, (ii) the derivative
|dn_s(T,p)/dT|_{T\to 0} depends on p slightly in the optimally and moderately
doped regions (0.10<p\le 0.16); however, it rapidly increases with p further
lowering and (iii) the latter finding is accompanied by the linear
low-temperature dependence \Delta n_s(T)\propto(-T) changing to \Delta
n_s(T)\propto(-\sqrt{T}). For optimum oxygen content the temperature dependence
of the normalized imaginary part of the c-axis conductivity
\lambda_c^2(0)/\lambda_c^2(T) is found to be strikingly similar to that of
\lambda_{ab}^2(0)/\lambda_{ab}^2(T) and becomes more convex with p lowering.
\lambda_c^{-2}(0,p) values are roughly proportional to the normal state
conductivities \sigma_c(T_c,p) along the c-axis. All these properties can be
treated in the framework of d-density wave order of pseudogap.Comment: 7 pages, 9 figures, presented at EUCAS 2003 (September 14-18),
submitted to SUS
Evolution of magnetic polarons and spin-carrier interactions through the metal-insulator transition in EuGdO
Raman scattering studies as functions of temperature, magnetic field, and
Gd-substitution are used to investigate the evolution of magnetic polarons and
spin-carrier interactions through the metal-insulator transition in
EuGdO. These studies reveal a greater richness of phase behavior
than have been previously observed using transport measurements: a
spin-fluctuation-dominated paramagnetic (PM) phase regime for T T
T, a two-phase regime for T T in which magnetic polarons
develop and coexist with a remnant of the PM phase, and an inhomogeneous
ferromagnetic phase regime for T T
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