45 research outputs found
Emergent nanoscale superparamagnetism at oxide interfaces
Atomically sharp oxide heterostructures exhibit a range of novel physical
phenomena that do not occur in the parent bulk compounds. The most prominent
example is the appearance of highly conducting and superconducting states at
the interface between the band insulators LaAlO3 and SrTiO3. Here we report a
new emergent phenomenon at the LaMnO3/SrTiO3 interface in which an
antiferromagnetic insulator abruptly transforms into a magnetic state that
exhibits unexpected nanoscale superparamagnetic dynamics. Upon increasing the
thickness of LaMnO3 above five unit cells, our scanning nanoSQUID-on-tip
microscopy shows spontaneous formation of isolated magnetic islands of 10 to 50
nm diameter, which display random moment reversals by thermal activation or in
response to an in-plane magnetic field. Our charge reconstruction model of the
polar LaMnO3/SrTiO3 heterostructure describes the sharp emergence of
thermodynamic phase separation leading to nucleation of metallic ferromagnetic
islands in an insulating antiferromagnetic matrix. The model further suggests
that the nearby superparamagnetic-ferromagnetic transition can be gate tuned,
holding potential for applications in magnetic storage and spintronics
Hundredfold Enhancement of Light Emission via Defect Control in Monolayer Transition-Metal Dichalcogenides
Two dimensional (2D) transition-metal dichalcogenide (TMD) based
semiconductors have generated intense recent interest due to their novel
optical and electronic properties, and potential for applications. In this
work, we characterize the atomic and electronic nature of intrinsic point
defects found in single crystals of these materials synthesized by two
different methods - chemical vapor transport and self-flux growth. Using a
combination of scanning tunneling microscopy (STM) and scanning transmission
electron microscopy (STEM), we show that the two major intrinsic defects in
these materials are metal vacancies and chalcogen antisites. We show that by
control of the synthetic conditions, we can reduce the defect concentration
from above to below . Because these point
defects act as centers for non-radiative recombination of excitons, this
improvement in material quality leads to a hundred-fold increase in the
radiative recombination efficiency
Probing dynamics and pinning of single vortices in superconductors at nanometer scales
The dynamics of quantized magnetic vortices and their pinning by materials
defects determine electromagnetic properties of superconductors, particularly
their ability to carry non-dissipative currents. Despite recent advances in the
understanding of the complex physics of vortex matter, the behavior of vortices
driven by current through a multi-scale potential of the actual materials
defects is still not well understood, mostly due to the scarcity of appropriate
experimental tools capable of tracing vortex trajectories on nanometer scales.
Using a novel scanning superconducting quantum interference microscope we
report here an investigation of controlled dynamics of vortices in lead films
with sub-Angstrom spatial resolution and unprecedented sensitivity. We
measured, for the first time, the fundamental dependence of the elementary
pinning force of multiple defects on the vortex displacement, revealing a far
more complex behavior than has previously been recognized, including striking
spring softening and broken-spring depinning, as well as spontaneous hysteretic
switching between cellular vortex trajectories. Our results indicate the
importance of thermal fluctuations even at 4.2 K and of the vital role of
ripples in the pinning potential, giving new insights into the mechanisms of
magnetic relaxation and electromagnetic response of superconductors.Comment: 15 pages and 5 figures (main text) + 15 pages and 11 figures
(supplementary material
Quantitative nanoscale vortex-imaging using a cryogenic quantum magnetometer
Microscopic studies of superconductors and their vortices play a pivotal role
in our understanding of the mechanisms underlying superconductivity. Local
measurements of penetration depths or magnetic stray-fields enable access to
fundamental aspects of superconductors such as nanoscale variations of
superfluid densities or the symmetry of their order parameter. However,
experimental tools, which offer quantitative, nanoscale magnetometry and
operate over the large range of temperature and magnetic fields relevant to
address many outstanding questions in superconductivity, are still missing.
Here, we demonstrate quantitative, nanoscale magnetic imaging of Pearl vortices
in the cuprate superconductor YBCO, using a scanning quantum sensor in form of
a single Nitrogen-Vacancy (NV) electronic spin in diamond. The sensor-to-sample
distance of ~10nm we achieve allows us to observe striking deviations from the
prevalent monopole approximation in our vortex stray-field images, while we
find excellent quantitative agreement with Pearl's analytic model. Our
experiments yield a non-invasive and unambiguous determination of the system's
local London penetration depth, and are readily extended to higher temperatures
and magnetic fields. These results demonstrate the potential of quantitative
quantum sensors in benchmarking microscopic models of complex electronic
systems and open the door for further exploration of strongly correlated
electron physics using scanning NV magnetometry.Comment: Main text (5 pages, 4 figures) plus supplementary material (5 pages,
6 figures). Comments welcome. Further information under
http://www.quantum-sensing.c
The mathematical analysis for peristaltic flow of nano fluid in a curved channel with compliant walls
Quantized magnetic vortices driven by electric current determine key electromagnetic
properties of superconductors. While the dynamic behavior of slow vortices has been
thoroughly investigated, the physics of ultrafast vortices under strong currents remains
largely unexplored. Here, we use a nanoscale scanning superconducting quantum
interference device to image vortices penetrating into a superconducting Pb film at rates of
tens of GHz and moving with velocities of up to tens of km/s, which are not only much larger
than the speed of sound but also exceed the pair-breaking speed limit of superconducting
condensate. These experiments reveal formation of mesoscopic vortex channels which
undergo cascades of bifurcations as the current and magnetic field increase. Our
numerical simulations predict metamorphosis of fast Abrikosov vortices into mixed
Abrikosov-Josephson vortices at even higher velocities. This work offers an insight into the
fundamental physics of dynamic vortex states of superconductors at high current densities,
crucial for many applications
Advancing global health through development and clinical trials partnerships: a randomized, placebo-controlled, double-blind assessment of safety, tolerability, and Immunogenicity of Plasmodium falciparum sporozoites vaccine for malaria in healthy Equatoguinean men
Equatorial Guinea (EG) has implemented a successful malaria control program on Bioko Island. A highly effective vaccine would be an ideal complement to this effort and could lead to halting transmission and eliminating malaria. Sanaria® PfSPZ Vaccine (Plasmodium falciparum sporozoite Vaccine) is being developed for this purpose. To begin the process of establishing the efficacy of and implementing a PfSPZ Vaccine mass vaccination program in EG, we decided to conduct a series of clinical trials of PfSPZ Vaccine on Bioko Island. Because no clinical trial had ever been conducted in EG, we first successfully established the ethical, regulatory, quality, and clinical foundation for conducting trials. We now report the safety, tolerability, and immunogenicity results of the first clinical trial in the history of the country. Thirty adult males were randomized in the ratio 2:1 to receive three doses of 2.7 × 105 PfSPZ of PfSPZ Vaccine (N = 20) or normal saline placebo (N = 10) by direct venous inoculation at 8-week intervals. The vaccine was safe and well tolerated. Seventy percent, 65%, and 45% of vaccinees developed antibodies to Plasmodium falciparum (Pf) circumsporozoite protein (PfCSP) by enzyme-linked immunosorbent assay, PfSPZ by automated immunofluorescence assay, and PfSPZ by inhibition of sporozoite invasion assay, respectively. Antibody responses were significantly lower than responses in U.S. adults who received the same dosage regimen, but not significantly different than responses in young adult Malians. Based on these results, a clinical trial enrolling 135 subjects aged 6 months to 65 years has been initiated in EG; it includes PfSPZ Vaccine and first assessment in Africa of PfSPZ-CVac. ClinicalTrials.gov identifier: NCT02418962