394 research outputs found
Superconductivity in Ca-doped graphene
Graphene, a zero-gap semimetal, can be transformed into a metallic,
semiconducting or insulating state by either physical or chemical modification.
Superconductivity is conspicuously missing among these states despite
considerable experimental efforts as well as many theoretical proposals. Here,
we report superconductivity in calcium-decorated graphene achieved by
intercalation of graphene laminates that consist of well separated and
electronically decoupled graphene crystals. In contrast to intercalated
graphite, we find that Ca is the only dopant that induces superconductivity in
graphene laminates above 1.8 K among intercalants used in our experiments such
as potassium, caesium and lithium. Ca-decorated graphene becomes
superconducting at ~ 6 K and the transition temperature is found to be strongly
dependent on the confinement of the Ca layer and the induced charge carrier
concentration. In addition to the first evidence for superconducting graphene,
our work shows a possibility of inducing and studying superconductivity in
other 2D materials using their laminates
Magnetoresistance in Co-hBN-NiFe tunnel junctions enhanced by resonant tunneling through single defects in ultrathin hBN barriers
Hexagonal boron nitride (hBN) is a prototypical high-quality two-dimensional
insulator and an ideal material to study tunneling phenomena, as it can be
easily integrated in vertical van der Waals devices. For spintronic devices,
its potential has been demonstrated both for efficient spin injection in
lateral spin valves and as a barrier in magnetic tunnel junctions (MTJs). Here
we reveal the effect of point defects inevitably present in mechanically
exfoliated hBN on the tunnel magnetoresistance of Co-hBN-NiFe MTJs. We observe
a clear enhancement of both the conductance and magnetoresistance of the
junction at well-defined bias voltages, indicating resonant tunneling through
magnetic (spin-polarized) defect states. The spin polarization of the defect
states is attributed to exchange coupling of a paramagnetic impurity in the
few-atomic-layer thick hBN to the ferromagnetic electrodes. This is confirmed
by excellent agreement with theoretical modelling. Our findings should be taken
into account in analyzing tunneling processes in hBN-based magnetic devices.
More generally, our study shows the potential of using atomically thin hBN
barriers with defects to engineer the magnetoresistance of MTJs and to achieve
spin filtering, opening the door towards exploiting the spin degree of freedom
in current studies of point defects as quantum emitters
Revealing common artifacts due to ferromagnetic inclusions in highly-oriented pyrolytic graphite
We report on an extensive investigation to figure out the origin of
room-temperature ferromagnetism that is commonly observed by SQUID magnetometry
in highly-oriented pyrolytic graphite (HOPG). Electron backscattering and X-ray
microanalysis revealed the presence of micron-size magnetic clusters
(predominantly Fe) that are rare and would be difficult to detect without
careful search in a scanning electron microscope in the backscattering mode.
The clusters pin to crystal boundaries and their quantities match the amplitude
of typical ferromagnetic signals. No ferromagnetic response is detected in
samples where we could not find such magnetic inclusions. Our experiments show
that the frequently reported ferromagnetism in pristine HOPG is most likely to
originate from contamination with Fe-rich inclusions introduced presumably
during crystal growth.Comment: 8 pages, 7 figure
Micromagnetometry of two-dimensional ferromagnets
The study of atomically thin ferromagnetic crystals has led to the discovery
of unusual magnetic behaviour and provided insight into the magnetic properties
of bulk materials. However, the experimental techniques that have been used to
explore ferromagnetism in such materials cannot probe the magnetic field
directly. Here, we show that ballistic Hall micromagnetometry can be used to
measure the magnetization of individual two-dimensional ferromagnets. Our
devices are made by van der Waals assembly in such a way that the investigated
ferromagnetic crystal is placed on top of a multi-terminal Hall bar made from
encapsulated graphene. We use the micromagnetometry technique to study
atomically thin chromium tribromide (CrBr3). We find that the material remains
ferromagnetic down to monolayer thickness and exhibits strong out-of-plane
anisotropy. We also find that the magnetic response of CrBr3 varies little with
the number of layers and its temperature dependence cannot be described by the
simple Ising model of two-dimensional ferromagnetism.Comment: 19 pages, 12 figure
Growth of a vortex polycrystal in type II superconductors
We discuss the formation of a vortex polycrystal in type II superconductors
from the competition between pinning and elastic forces. We compute the elastic
energy of a deformed grain boundary, that is strongly non-local, and obtain the
depinning stress for weak and strong pinning. Our estimates for the grain size
dependence on the magnetic field strength are in good agreement with previous
experiments on NbMo. Finally, we discuss the effect of thermal noise on grain
growth.Comment: 4 pages, 2 figure
Giant oscillations in a triangular network of one-dimensional states in marginally twisted graphene
The electronic properties of graphene superlattices have attracted intense
interest that was further stimulated by the recent observation of novel
many-body states at "magic" angles in twisted bilayer graphene (BLG). For very
small ("marginal") twist angles of 0.1 deg, BLG has been shown to exhibit a
strain-accompanied reconstruction that results in submicron-size triangular
domains with the Bernal stacking. If the interlayer bias is applied to open an
energy gap inside the domain regions making them insulating, marginally-twisted
BLG is predicted to remain conductive due to a triangular network of chiral
one-dimensional (1D) states hosted by domain boundaries. Here we study electron
transport through this network and report giant Aharonov-Bohm oscillations
persisting to temperatures above 100 K. At liquid helium temperatures, the
network resistivity exhibits another kind of oscillations that appear as a
function of carrier density and are accompanied by a sign-changing Hall effect.
The latter are attributed to consecutive population of the flat minibands
formed by the 2D network of 1D states inside the gap. Our work shows that
marginally twisted BLG is markedly distinct from other 2D electronic systems,
including BLG at larger twist angles, and offers a fascinating venue for
further research.Comment: 11 pages, 8 figure
Long-range nonlocal flow of vortices in narrow superconducting channels
We report a new nonlocal effect in vortex matter, where an electric current
confined to a small region of a long and sufficiently narrow superconducting
wire causes vortex flow at distances hundreds of inter-vortex separations away.
The observed remote traffic of vortices is attributed to a very efficient
transfer of a local strain through the one-dimensional vortex lattice, even in
the presence of disorder. We also observe mesoscopic fluctuations in the
nonlocal vortex flow, which arise due to "traffic jams" when vortex
arrangements do not match a local geometry of a superconducting channel.Comment: a slightly longer version of a tentatively accepted PR
Small Scale Anisotropy Predictions for the Auger Observatory
We study the small scale anisotropy signal expected at the Pierre Auger
Observatory in the next 1, 5, 10, and 15 years of operation, from sources of
ultra-high energy (UHE) protons. We numerically propagate UHE protons over
cosmological distances using an injection spectrum and normalization that fits
current data up to \sim 10^{20}\eV. We characterize possible sources of
ultra-high energy cosmic rays (UHECRs) by their mean density in the local
Universe, Mpc, with between 3 and 6.
These densities span a wide range of extragalactic sites for UHECR sources,
from common to rare galaxies or even clusters of galaxies. We simulate 100
realizations for each model and calculate the two point correlation function
for events with energies above 4 \times 10^{19}\eV and above 10^{20}\eV, as
specialized to the case of the Auger telescope. We find that for r\ga 4,
Auger should be able to detect small scale anisotropies in the near future.
Distinguishing between different source densities based on cosmic ray data
alone will be more challenging than detecting a departure from isotropy and is
likely to require larger statistics of events. Combining the angular
distribution studies with the spectral shape around the GZK feature will also
help distinguish between different source scenarios.Comment: 15 pages, 6 figures, 6 tables, submitted to JCA
Myopia and its complications
According to the data of World Health Organization, weak sight at uncorrected anomalies of refringence is one of the main directions of liquidation of preventable blindness to the 2020. The article presents the review of present-day literature devoted to the etiology, pathogenesis of myopia and its complications. Data of Russian and foreign researchers are presented. The estimation of the methods of research in myopia diagnostics and associated complications is given. The attention is given to the unsolved problems of early diagnostics of myopia complications
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