3,766 research outputs found
Quantum Hall conductance of two-terminal graphene devices
Measurement and theory of the two-terminal conductance of monolayer and
bilayer graphene in the quantum Hall regime are compared. We examine features
of conductance as a function of gate voltage that allow monolayer, bilayer, and
gapped samples to be distinguished, including N-shaped distortions of quantum
Hall plateaus and conductance peaks and dips at the charge neutrality point.
Generally good agreement is found between measurement and theory. Possible
origins of discrepancies are discussed
Counting statistics and super-Poissonian noise in a quantum dot
We present time-resolved measurements of electron transport through a quantum
dot. The measurements were performed using a nearby quantum point contact as a
charge detector. The rates for tunneling through the two barriers connecting
the dot to source and drain contacts could be determined individually. In the
high bias regime, the method was used to probe excited states of the dot.
Furthermore, we have detected bunching of electrons, leading to
super-Poissonian noise. We have used the framework of the full counting
statistics (FCS) to model the experimental data. The existence of
super-Poissonian noise suggests a long relaxation time for the involved excited
state, which could be related to the spin relaxation time
Angular dependent vortex pinning mechanisms in YBCO coated conductors and thin films
We present a comparative study of the angular dependent critical current
density in YBa2Cu3O7 films deposited on IBAD MgO and on single crystal MgO and
SrTiO3 substrates. We identify three angular regimes where pinning is dominated
by different types of correlated and uncorrelated defects. We show that those
regimes are present in all cases, indicating that the pinning mechanisms are
the same, but their extension and characteristics are sample dependent,
reflecting the quantitative differences in texture and defect density. In
particular, the more defective nature of the films on IBAD turns into an
advantage as it results in stronger vortex pinning, demonstrating that the
critical current density of the films on single crystals is not an upper limit
for the performance of the IBAD coated conductors.Comment: 14 pages, 3 figures. Submitted to AP
Extensional Flow of a Free Film of Nematic Liquid Crystal with Moderate Elasticity
Motivated by problems arising in tear film dynamics, we present a model for
the extensional flow of thin sheets of nematic liquid crystal. The rod-like
molecules of these substances impart an elastic contribution to its response.
We rescale a weakly elastic model due to Cummings et al. [European Journal of
Applied Mathematics 25 (2014): 397-423] to describe a case of moderate
elasticity. The resulting system of two nonlinear partial differential
equations for sheet thickness and axial velocity is nonlinear and fourth order
in space, but still represents a significant reduction of the full system. We
analyze solutions arising from several different boundary conditions, motivated
by the underlying application, with particular focus on dynamics and underlying
mechanisms under stretching. We solve the system numerically, via collocation
with either finite difference or Chebyshev spectral discretization in space,
together with implicit time stepping. At early times, depending on the initial
film shape, pressure either aids or opposes extensional flow, which changes the
shape of the sheet and may result in the loss of a minimum or maximum at the
moving end. We contrast this finding with the cases of weak elasticity and
Newtonian flow, where the sheet retains all extrema from the initial condition
throughout time.Comment: Copyright (2023) M.J. Taranchuk, L.J. Cummings, T.A. Driscoll, and
R.J. Braun. This article is distributed under a Creative Commons Attribution
(CC BY) License. The following article has been submitted to Physics of
Fluids. After it is published, it will be found at
https://aip.scitation.org/journal/ph
Critical Fields and Critical Currents in MgB2
We review recent measurements of upper (Hc2) and lower (Hc1) critical fields
in clean single crystals of MgB2, and their anisotropies between the two
principal crystallographic directions. Such crystals are far into the "clean
limit" of Type II superconductivity, and indeed for fields applied in the
c-direction, the Ginzburg-Landau parameter k is only about 3, just large enough
for Type II behaviour. Because m0Hc2 is so low, about 3 T for fields in the
c-direction, MgB2 has to be modified for it to become useful for high-current
applications. It should be possible to increase Hc2 by the introduction of
strong electron scattering (but because of the electronic structure and the
double gap that results, the scatterers will have to be chosen carefully). In
addition, pinning defects on a scale of a few nm will have to be engineered in
order to enhance the critical current density at high fields.Comment: BOROMAG Conference Invited paper. To appear in Supercond. Sci. Tec
Synthesis of a new hollandite-type manganese oxide with framework and interstitial Cr(III)
Hollandite with Cr(III) in both tunnel and framework sites has been prepared hydrothermally from layered manganese oxide precursors
Progress and perspective on different strategies to achieve wake-up-free ferroelectric hafnia and zirconia-based thin films
In the last decade orthorhombic hafnia and zirconia films have attracted tremendous attention arising from the discovery of ferroelectricity at the nanoscale. However, an initial wake-up pre-cycling is usually needed to achieve a ferroelectric behaviour in these films. Recently, different strategies, such as microstructure tailoring, defect, bulk and interface engineering, doping, NH3 plasma treatment and epitaxial growth, have been employed to obtain wake-up free orthorhombic ferroelectric hafnia and zirconia films. In this work we review recent developments in obtaining polar hafnia and zirconia-based thin films without the need of any wake-up cycling. In particular, we discuss the rhombohedral phase of hafnia/ zirconia, which under a constrained environment exhibits wake-up-free ferroelectric behaviour. This phase could have a strong impact on the current investigations of ferroelectric binary oxide materials and pave the way toward exploiting ferroelectric behaviour for next-generation memory and logic gate applications.This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding Contract UIDB/04650/2020 and by DST-SERB, Govt. of India through Grant Nr. ECR/2017/00006. R. F. Negrea and L. Pintilie acknowledge funding through project CEPROFER/ PN-III-P4-ID-PCCF-2016-0047 (contract 16/2018, funded by UEFISCDI). J.L.M-D. thanks the Royal Academy of Engineering Chair in Emerging Technologies Grant, CIET1819_24, the EPSRC grant EP/T012218/1- ECCS – EPSRC, and the grant EU-H2020-ERC-ADG # 882929, EROS
Flow Separation Associated with 3-D Shock-Boundary Layer Interaction (SBLI)
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140423/1/6.2014-1138.pd
Fluid Acquisition and Resupply Experiments on Space Shuttle Flights STS-53 and STS-57
The Fluid Acquisition and Resupply Experiment (FARE) program, managed by the Marshall Space Flight Center Space Propulsion Branch with Martin Marietta Civil Space and Communications as the contractor, consisted of two flights designated FARE I and FARE II. FARE I flew in December 1992 on STS-53 with a screen channel liquid acquisition device (LAD) and FARE II flew in June 1993 on STS-57 with a vane-type LAD. Thus, the FARE I and II flights represent the two basic LAD categories usually considered for in-space fluid management. Although both LAD types have been used extensively, the usefulness of the on-orbit data has been constrained by the lack of experimentation beyond predicted performance limits, including both propellant fill and expulsion. Therefore, the FARE tests were designed to obtain data that would satisfy two primary objectives: (1) Demonstrate the performance of the two types of LADs, screen channel and vane, and (2) support the anchoring of analytical models. Both flights were considered highly successful in meeting these two primary objectives
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