3,578 research outputs found
Zn-impurity effect and interplay of s± and s++ pairings in iron-based superconductors
published_or_final_versio
Observation of the Nernst signal generated by fluctuating Cooper pairs
Long-range order is destroyed in a superconductor warmed above its critical
temperature (Tc). However, amplitude fluctuations of the superconducting order
parameter survive and lead to a number of well established phenomena such as
paraconductivity : an excess of charge conductivity due to the presence of
short-lived Cooper pairs in the normal state. According to an untested theory,
these pairs generate a transverse thermoelectric (Nernst) signal. In amorphous
superconducting films, the lifetime of Cooper pairs exceeds the elastic
lifetime of quasi-particles in a wide temperature range above Tc; consequently,
the Cooper pairs Nernst signal dominate the response of the normal electrons
well above Tc. In two dimensions, the magnitude of the expected signal depends
only on universal constants and the superconducting coherence length, so the
theory can be unambiguously tested. Here, we report on the observation of a
Nernst signal in such a superconductor traced deep into the normal state. Since
the amplitude of this signal is in excellent agreement with the theoretical
prediction, the result provides the first unambiguous case for a Nernst effect
produced by short-lived Cooper pairs
Electronic Origin of High Temperature Superconductivity in Single-Layer FeSe Superconductor
The latest discovery of high temperature superconductivity signature in
single-layer FeSe is significant because it is possible to break the
superconducting critical temperature ceiling (maximum Tc~55 K) that has been
stagnant since the discovery of Fe-based superconductivity in 2008. It also
blows the superconductivity community by surprise because such a high Tc is
unexpected in FeSe system with the bulk FeSe exhibiting a Tc at only 8 K at
ambient pressure which can be enhanced to 38 K under high pressure. The Tc is
still unusually high even considering the newly-discovered intercalated FeSe
system A_xFe_{2-y}Se_2 (A=K, Cs, Rb and Tl) with a Tc at 32 K at ambient
pressure and possible Tc near 48 K under high pressure. Particularly
interesting is that such a high temperature superconductivity occurs in a
single-layer FeSe system that is considered as a key building block of the
Fe-based superconductors. Understanding the origin of high temperature
superconductivity in such a strictly two-dimensional FeSe system is crucial to
understanding the superconductivity mechanism in Fe-based superconductors in
particular, and providing key insights on how to achieve high temperature
superconductivity in general. Here we report distinct electronic structure
associated with the single-layer FeSe superconductor. Its Fermi surface
topology is different from other Fe-based superconductors; it consists only of
electron pockets near the zone corner without indication of any Fermi surface
around the zone center. Our observation of large and nearly isotropic
superconducting gap in this strictly two-dimensional system rules out existence
of node in the superconducting gap. These results have provided an unambiguous
case that such a unique electronic structure is favorable for realizing high
temperature superconductivity
High glucose up-regulates ENaC and SGK1 expression in HCD-cells
Background/Aim: Diabetic nephropathy is associated with progressive renal damage, leading to impaired function and end-stage renal failure. Secondary hypertension stems from a deranged ability of cells within the kidney to resolve and appropriately regulate sodium resorption in response to hyperglycaemia. However, the mechanisms by which glucose alters sodium re-uptake have not been fully characterised.
Methods: Here we present RT-PCR, western blot and immunocytochemistry data confirming mRNA and protein expression of the serum and glucocorticoid inducible kinase (SGK1) and the a conducting subunit of the epithelial sodium channel (ENaC) in a model in vitro system of the human cortical collecting duct (HCD). We examined changes in expression of these elements in response to glucose challenge, designed to mimic hyperglycaemia associated with type 2 diabetes mellitus. Changes in Na+ concentration were assessed using single-cell microfluorimetry.
Results: Incubation with glucose, the Ca2+-ionophore ionomycin and the cytokine TGF-beta 1 were all found to evoke significant and time-dependent increases in both SGK1 and alpha ENaC protein expression. These molecular changes were correlated to an increase in Na+-uptake at the single-cell level.
Conclusion: Together these data offer a potential explanation for glucose-evoked Na+-resorption and a potential contributory role of SGK1 and ENaCs in development of secondary hypertension, commonly linked to diabetic nephropathy
Artificial Topological Superconductor by the Proximity Effect
published_or_final_versio
Enhancement of the Nernst effect by stripe order in a high-Tc superconductor
The Nernst effect in metals is highly sensitive to two kinds of phase
transition: superconductivity and density-wave order. The large positive Nernst
signal observed in hole-doped high-Tc superconductors above their transition
temperature Tc has so far been attributed to fluctuating superconductivity.
Here we show that in some of these materials the large Nernst signal is in fact
caused by stripe order, a form of spin / charge modulation which causes a
reconstruction of the Fermi surface. In LSCO doped with Nd or Eu, the onset of
stripe order causes the Nernst signal to go from small and negative to large
and positive, as revealed either by lowering the hole concentration across the
quantum critical point in Nd-LSCO, or lowering the temperature across the
ordering temperature in Eu-LSCO. In the latter case, two separate peaks are
resolved, respectively associated with the onset of stripe order at high
temperature and superconductivity near Tc. This sensitivity to Fermi-surface
reconstruction makes the Nernst effect a promising probe of broken symmetry in
high-Tc superconductors
A transient homotypic interaction model for the influenza A virus NS1 protein effector domain
Influenza A virus NS1 protein is a multifunctional virulence factor consisting of an RNA binding domain (RBD), a short linker, an effector domain (ED), and a C-terminal 'tail'. Although poorly understood, NS1 multimerization may autoregulate its actions. While RBD dimerization seems functionally conserved, two possible apo ED dimers have been proposed (helix-helix and strand-strand). Here, we analyze all available RBD, ED, and full-length NS1 structures, including four novel crystal structures obtained using EDs from divergent human and avian viruses, as well as two forms of a monomeric ED mutant. The data reveal the helix-helix interface as the only strictly conserved ED homodimeric contact. Furthermore, a mutant NS1 unable to form the helix-helix dimer is compromised in its ability to bind dsRNA efficiently, implying that ED multimerization influences RBD activity. Our bioinformatical work also suggests that the helix-helix interface is variable and transient, thereby allowing two ED monomers to twist relative to one another and possibly separate. In this regard, we found a mAb that recognizes NS1 via a residue completely buried within the ED helix-helix interface, and which may help highlight potential different conformational populations of NS1 (putatively termed 'helix-closed' and 'helix-open') in virus-infected cells. 'Helix-closed' conformations appear to enhance dsRNA binding, and 'helix-open' conformations allow otherwise inaccessible interactions with host factors. Our data support a new model of NS1 regulation in which the RBD remains dimeric throughout infection, while the ED switches between several quaternary states in order to expand its functional space. Such a concept may be applicable to other small multifunctional proteins
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Encapsulation of graphene transistors and vertical device integration by interface engineering with atomic layer deposited oxide
We demonstrate a simple, scalable approach to achieve encapsulated graphene transistors with negligible gate hysteresis, low doping levels and enhanced mobility compared to as-fabricated devices. We engineer the interface between graphene and atomic layer deposited (ALD) AlO by tailoring the growth parameters to achieve effective device encapsulation whilst enabling the passivation of charge traps in the underlying gate dielectric. We relate the passivation of charge trap states in the vicinity of the graphene to conformal growth of ALD oxide governed by gaseous HO pretreatments. We demonstrate the long term stability of such encapsulation techniques and the resulting insensitivity towards additional lithography steps to enable vertical device integration of graphene for multi-stacked electronics fabrication.This work was supported by the EPSRC (Grant Nos. EP/K016636/1, GRAPHTED and EP/L020963/1) and the ERC (Grant No. 279342, InsituNANO). JAA-W acknowledges a Research Fellowship from Churchill College, Cambridge. JS acknowledges support from NUDT. ZAVV acknowledges funding from ESPRC grant EP/L016087/1. ACV acknowledges the Conacyt Cambridge Scholarship and the Roberto Rocca Fellowship. RW acknowledges EPSRC Doctoral Training Award (EP/M506485/1)
First trimester placenta accreta: a rare clinical entity and diagnostic dilemma
First trimester placenta accreta is a rare occurrence but potential life threatening and catastrophic. Most of these cases ended up with emergency hysterectomy. We report a case of incidental finding of placental accreta during evacuation of retained product of conception (ERPOC) for missed miscarriage. A 33-year-old, Gravida 4 Para 2 + 1 at 15 weeks’ gestation admitted for missed miscarriage, failed medical evacuation requiring ERPOC. There was excessive bleeding during the procedure and required hysterectomy and bilateral internal iliac artery ligation. Histopathological examination confirmed products of conception with evidence of placenta accreta. This case highlighted the diagnostic dilemma and importance of early accurate diagnosis of placental accreta prior to any surgical intervention for miscarriage
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
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