735 research outputs found
Quantum Hall resistance standards from graphene grown by chemical vapor deposition on silicon carbide
Replacing GaAs by graphene to realize more practical quantum Hall resistance
standards (QHRS), accurate to within in relative value, but operating
at lower magnetic fields than 10 T, is an ongoing goal in metrology. To date,
the required accuracy has been reported, only few times, in graphene grown on
SiC by sublimation of Si, under higher magnetic fields. Here, we report on a
device made of graphene grown by chemical vapour deposition on SiC which
demonstrates such accuracies of the Hall resistance from 10 T up to 19 T at 1.4
K. This is explained by a quantum Hall effect with low dissipation, resulting
from strongly localized bulk states at the magnetic length scale, over a wide
magnetic field range. Our results show that graphene-based QHRS can replace
their GaAs counterparts by operating in as-convenient cryomagnetic conditions,
but over an extended magnetic field range. They rely on a promising hybrid and
scalable growth method and a fabrication process achieving low-electron density
devices.Comment: 12 pages, 8 figure
Bmi1+ Progenitor Cell Dynamics in Murine Cornea During Homeostasis and Wound Healing
The outermost layer of the eye, the cornea, is renewed continuously throughout life. Stem cells of the corneal epithelium reside in the limbus at the corneal periphery and ensure homeostasis of the central epithelium. However, in young mice, homeostasis relies on cells located in the basal layer of the central corneal epithelium. Here, we first studied corneal growth during the transition from newborn to adult and assessed Keratin 19 (Krt19) expression as a hallmark of corneal maturation. Next, we set out to identify a novel marker of murine corneal epithelial progenitor cells before, during and after maturation, and we found that Bmi1 is expressed in the basal epithelium of the central cornea and limbus. Furthermore, we demonstrated that Bmi1+ cells participated in tissue replenishment in the central cornea. These Bmi1+ cells did not maintain homeostasis of the cornea for more than 3 months, reflecting their status as progenitor rather than stem cells. Finally, after injury, Bmi1+ cells fueled homeostatic maintenance, whereas wound closure occurred via epithelial reorganization. Stem Cells 2018
Wiedemann-Franz law and abrupt change in conductivity across the pseudogap critical point of a cuprate superconductor
The thermal conductivity of the cuprate superconductor
LaNdSrCuO was measured down to 50 mK in seven
crystals with doping from to , both in the superconducting
state and in the magnetic field-induced normal state. We obtain the electronic
residual linear term as across the pseudogap critical
point . In the normal state, we observe an abrupt drop in
upon crossing below , consistent with a drop in carrier
density from to , the signature of the pseudogap phase inferred
from the Hall coefficient. A similar drop in is observed at ,
showing that the pseudogap critical point and its signatures are unaffected by
the magnetic field. In the normal state, the Wiedemann-Franz law,
, is obeyed at all dopings, including at the critical
point where the electrical resistivity is -linear down to . We conclude that the non-superconducting ground state of the pseudogap
phase at is a metal whose fermionic excitations carry heat and charge as
conventional electrons do.Comment: 10 pages, including Supplementary Materia
Investigation of ferrofluid cooling in modular permanent magnet machines
Compared with conventional non-modular machines, the flux gaps in alternate stator teeth of the modular machines can increase the winding factor and provide flux focusing effect, and hence can increase the torque/power density. In addition, the flux gaps can also be used as cooling channels to improve the thermal performance. This paper investigates an advanced cooling approach using ferrofluid as coolant to fill in the cavity around end-windings and flux gaps. The multiphysics modelling in this paper has shown that the influence of the flux gaps on machine thermal performances comes from two aspects: (1) the gravitational effect, the flux gaps allow more self-circulating coolant to transfer heat to the housing. This helps to reduce machine temperature by around 5°C for a flux gap width of 2mm; (2) the magnetic body force, i.e., the thermomagnetic effect. This is very effective in non-modular machine cooling. But its efficiency slightly deteriorates in the modular machines. This is because the self-circulation of ferrofluid in the flux gaps due to the gravitational effect has been negatively affected by the thermomagnetic effect
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