87 research outputs found
Topological change of the Fermi surface in ternary iron-pnictides with reduced c/a ratio: A dHvA study of CaFe2P2
We report a de Haas-van Alphen effect study of the Fermi surface of CaFe2P2
using low temperature torque magnetometry up to 45 T. This system is a close
structural analogue of the collapsed tetragonal non-magnetic phase of CaFe2As2.
We find the Fermi surface of CaFe2P2 to differ from other related ternary
phosphides in that its topology is highly dispersive in the c-axis, being
three-dimensional in character and with identical mass enhancement on both
electron and hole pockets (~1.5). The dramatic change in topology of the Fermi
surface suggests that in a state with reduced (c/a) ratio, when bonding between
pnictogen layers becomes important, the Fermi surface sheets are unlikely to be
nested
Quantum oscillations in the parent pnictide BaFeAs : itinerant electrons in the reconstructed state
We report quantum oscillation measurements that enable the direct observation
of the Fermi surface of the low temperature ground state of \ba122. From these
measurements we characterize the low energy excitations, revealing that the
Fermi surface is reconstructed in the antiferromagnetic state, but leaving
itinerant electrons in its wake. The present measurements are consistent with a
conventional band folding picture of the antiferromagnetic ground state,
placing important limits on the topology and size of the Fermi surface.Comment: 5 pages, 3 figure
Evolution of the Fermi surface of BaFe_2(As_{1-x}P_x)_2 on entering the superconducting dome
Using the de Haas-van Alphen effect we have measured the evolution of the
Fermi surface of BaFe_2(As_{1-x}P_x)_2 as function of isoelectric substitution
(As/P) for 0.41<x<1 (T_c up to 25 K). We find that the volume of electron and
hole Fermi surfaces shrink linearly with decreasing x. This shrinking is
accompanied by a strong increase in the quasiparticle effective mass as x is
tuned toward the maximum T_c. It is likely that these trends originate from the
many-body interaction which give rise to superconductivity, rather than the
underlying one-electron bandstructure.Comment: 4 page
13C NMR study of superconductivity near charge instability realized in beta"-(BEDT-TTF)4[(H3O)Ga(C2O4)3]C6H5NO2
To investigate the superconducting (SC) state near a charge instability, we
performed ^{13}C NMR experiments on the molecular superconductor
beta"-(BEDT-TTF)_{4}[(H_{3}O)Ga(C_{2}O_{4})_{3}]C_{6}H_{5}NO_{2}, which
exhibits a charge anomaly at 100 K. The Knight shift which we measured in the
SC state down to 1.5 K demonstrates that Cooper pairs are in spin-singlet
state. Measurements of the nuclear spin-lattice relaxation time reveal strong
electron-electron correlations in the normal state. The resistivity increase
observed below 10 K indicates that the enhanced fluctuation has an electric
origin. We discuss the possibility of charge-fluctuation-induced
superconductivity.Comment: 5 pages, 4 figure
Fermi surface of superconducting LaFePO determined by quantum oscillations
We report extensive measurements of quantum oscillations in the normal state
of the Fe-based superconductor LaFePO, (Tc ~ 6 K) using low temperature torque
magnetometry and transport in high static magnetic fields (45 T). We find that
the Fermi surface is in broad agreement with the band-structure calculations
with the quasiparticle mass enhanced by a factor ~2. The quasi-two dimensional
Fermi surface consist of nearly-nested electron and hole pockets, suggesting
proximity to a spin/charge density wave instability.Comment: to appear in Physical Review Letter
Interplay between localized and itinerant d electrons in a frustrated metallic antiferromagnet, 2H-AgNiO2
We report the electronic and magnetic behaviour of the frustrated triangular
metallic antiferromagnet 2H-AgNiO2 in high magnetic fields (54 T) using
thermodynamic and transport measurements. Here localized d electrons are
arranged on an antiferromagnetic triangular lattice nested inside a honeycomb
lattice with itinerant d electrons. When the magnetic field is along the easy
axis we observe a cascade of field-induced transitions, attributed to the
competition between easy-axis anisotropy, geometrical frustration and coupling
of the localized and itinerant system. The quantum oscillations data suggest
that the Fermi surface is reconstructed by the magnetic order but in high
fields magnetic breakdown orbits are possible. The itinerant electrons are
extremely sensitive to scattering by spin fluctuations and a significant mass
enhancement (~ 3) is found.Comment: 4 page
Phenomenology of the normal state in-plane transport properties of high- cuprates
In this article, I review progress towards an understanding of the normal
state (in-plane) transport properties of high- cuprates in the light of
recent developments in both spectroscopic and transport measurement techniques.
Against a backdrop of mounting evidence for anisotropic single-particle
lifetimes in cuprate superconductors, new results have emerged that advocate
similar momentum dependence in the transport decay rate ({\bf k}). In
addition, enhancement of the energy scale (up to the bare bandwidth) over which
spectroscopic information on the quasiparticle response can be obtained has led
to the discovery of new, unforeseen features that surprisingly, may have a
significant bearing on the transport properties at the dc limit. With these two
key developments in mind, I consider here whether all the ingredients necessary
for a complete phenomenological description of the anomalous normal state
transport properties of high- cuprates are now in place.Comment: 31 pages, 10 figure
Quantum oscillations from Fermi arcs
When a metal is subjected to strong magnetic field B nearly all measurable
quantities exhibit oscillations periodic in 1/B. Such quantum oscillations
represent a canonical probe of the defining aspect of a metal, its Fermi
surface (FS). In this study we establish a new mechanism for quantum
oscillations which requires only finite segments of a FS to exist. Oscillations
periodic in 1/B occur if the FS segments are terminated by a pairing gap. Our
results reconcile the recent breakthrough experiments showing quantum
oscillations in a cuprate superconductor YBCO, with a well-established result
of many angle resolved photoemission (ARPES) studies which consistently
indicate "Fermi arcs" -- truncated segments of a Fermi surface -- in the normal
state of the cuprates.Comment: 8 pages, 5 figure
The power of peers: An effectiveness evaluation of a cluster-controlled trial of group antenatal care in rural Nepal
Background: Reducing the maternal mortality ratio to less than 70 per 100,000 live births globally is one of the Sustainable Development Goals. Approximately 830 women die from pregnancy-or childbirth-related complications every day. Almost 99% of these deaths occur in developing countries. Increasing antenatal care quality and completion, and institutional delivery are key strategies to reduce maternal mortality, however there are many implementation challenges in rural and resource-limited settings. In Nepal, 43% of deliveries do not take place in an institution and 31% of women have insufficient antenatal care. Context-specific and evidence-based strategies are needed to improve antenatal care completion and institutional birth. We present an assessment of effectiveness outcomes for an adaptation of a group antenatal care model delivered by community health workers and midwives in close collaboration with government staff in rural Nepal. Methods: The study was conducted in Achham, Nepal, via a public private partnership between the Nepali non-profit, Nyaya Health Nepal, and the Ministry of Health and Population, with financial and technical assistance from the American non-profit, Possible. We implemented group antenatal care as a prospective non-randomized, cluster-controlled, type I hybrid effectiveness-implementation study in six village clusters. The implementation approach allowed for iterative improvement in design by making changes to improve the quality of the intervention. We evaluated effectiveness through a difference in difference analysis of institutional birth rates between groups prior to implementation of the intervention and 1 year after implementation. Additionally, we assessed the change in knowledge of key danger signs and the acceptability of the group model compared with individual visits in a nested cohort of women receiving home visit care and home visit care plus group antenatal care. Using a directed content and thematic approach, we analyzed qualitative interviews to identify major themes related to implementation. Results: At baseline, there were 457 recently-delivered women in the six village clusters receiving home visit care and 214 in the seven village clusters receiving home visit care plus group antenatal care. At endline, there were 336 and 201, respectively. The difference in difference analysis did not show a significant change in institutional birth rates nor antenatal care visit completion rates between the groups. There was, however, a significant increase in both institutional birth and antenatal care completion in each group from baseline to endline. We enrolled a nested cohort of 52 participants receiving home visit care and 62 participants receiving home visit care plus group antenatal care. There was high acceptability of the group antenatal care intervention and home visit care, with no significant differences between groups. A significantly higher percentage of women who participated in group antenatal care found their visits to be 'very enjoyable' (83.9% vs 59.6%, p = 0.0056). In the nested cohort, knowledge of key danger signs during pregnancy significantly improved from baseline to endline in the intervention clusters only (2 to 31%, p < 0.001), while knowledge of key danger signs related to labor and childbirth, the postpartum period, and the newborn did not in either intervention or control groups. Qualitative analysis revealed that women found that the groups provided an opportunity for learning and discussion, and the groups were a source of social support and empowerment. They also reported an improvement in services available at their village clinic. Providers noted the importance of the community health workers in identifying pregnant women in the community and linking them to the village clinics. Challenges in birth planning were brought up by both participants and providers. Conclusion: While there was no significant change in institutional birth and antenatal care completion at the population level between groups, there was an increase of these outcomes in both groups. This may be secondary to the primary importance of community health worker involvement in both of these groups. Knowledge of key pregnancy danger signs was significantly improved in the home visit plus group antenatal care cohort compared with the home visit care only group. This initial study of Nyaya Health Nepal's adapted group care model demonstrates the potential for impacting women's antenatal care experience and should be studied over a longer period as an intervention embedded within a community health worker program. Trial registration: ClinicalTrials.gov Identifier: NCT02330887, registered 01/05/2015, retroactively registered
Evidence for a Fermi liquid in the pseudogap phase of high-Tc cuprates
Cuprate high-T_c superconductors on the Mott-insulating side of "optimal
doping" (with respect to the highest T_c's) exhibit enigmatic behavior in the
non-superconducting state. Near optimal doping the transport and spectroscopic
properties are unlike those of a Landau-Fermi liquid. For carrier
concentrations below optimal doping a pseudogap removes quasi-particle spectral
weight from parts of the Fermi surface, and causes a break-up of the Fermi
surface into disconnected nodal and anti-nodal sectors. Here we show that the
near-nodal excitations of underdoped cuprates obey Fermi liquid behavior. Our
optical measurements reveal that the dynamical relaxation rate 1/tau(omega,T)
collapses on a universal function proportional to (hbar omega)^2+(1.5 pi k_B
T)^2. Hints at possible Fermi liquid behavior came from the recent discovery of
quantum oscillations at low temperature and high magnetic field in underdoped
YBa2Cu3O6+d and YBa2Cu4O8, from the observed T^2-dependence of the DC
({\omega}=0) resistivity for both overdoped and underdoped cuprates, and from
the two-fluid analysis of nuclear magnetic resonance data. However, the direct
spectroscopic determination of the energy dependence of the life-time of the
excitations -provided by our measurements- has been elusive up to now. This
observation defies the standard lore of non-Fermi liquid physics in high T_c
cuprates on the underdoped side of the phase diagram.Comment: 15 pages, 11 figure
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