71 research outputs found
Negative to Positive Crossover of Magnetoresistance in Layered WS2 with Ohmic Contact
The discovery of graphene has ignited intensive investigation on two
dimensional (2D) materials. Among them, transition metal dichalcogenide (TMDC),
a typical representative, attracts much attention due to the excellent
performance in field effect transistor (FET) related measurements and
applications. Particularly, when TMDC eventually reaches few-layer dimension, a
wide range of electronic and optical properties, in striking contrast to bulk
samples, are detected. In this Letter, we synthesized single crystalline WS2
nanoflakes by physical vapor deposition (PVD) method and carried out a series
of transport measurements of contact resistance and magnetoresistance. Focused
ion beam (FIB) technology was applied to deposit Pt electrodes on WS2 flakes.
Different from the electron beam lithography (EBL) fabricated electrodes,
FIB-deposited leads exhibited ohmic contact, resolving the dilemma of Schottky
barrier. Furthermore, a temperature-modulated negative-to-positive transition
of magnetoresistance (MR) associated with a crossover of carrier type at
similar temperature was demonstrated. Our work offers a pathway to optimize the
contact for TMDC and reveals the magnetoresistance characteristics of WS2
flakes, which may stimulate further studies on TMDC and corresponding potential
electronic and optoelectronic applications
A New Luminous blue variable in the outskirt of the Andromeda Galaxy
The hot massive luminous blue variables (LBVs) represent an important
evolutionary phase of massive stars. Here, we report the discovery of a new LBV
-- LAMOST J0037+4016 in the distant outskirt of the Andromeda galaxy. It is
located in the south-western corner (a possible faint spiral arm) of M31 with
an unexpectedly large projection distance of 22 kpc from the center. The
optical light curve shows a 1.2 mag variation in band and its outburst and
quiescence phases both last over several years. The observed spectra indicate
an A-type supergiant at epoch close to the outburst phase and a hot B-type
supergiant with weak [Fe II] emission lines at epoch of much dimmer brightness.
The near-infrared color-color diagram further shows it follows the distribution
of Galactic and M31 LBVs rather than B[e] supergiants. All the existing data
strongly show that LAMOST J0037+4016 is an LBV. By spectral energy distribution
fitting, we find it has a luminosity ()
and an initial mass , indicating its nature of less
luminosity class of LBV.Comment: 7 pages, 4 figures, 3 tables, accepted by ApJ
Observation of Quantum Griffiths Singularity and Ferromagnetism at Superconducting LaAlO3/SrTiO3(110) Interface
Diverse phenomena emerge at the interface between band insulators LaAlO3 and
SrTiO3, such as superconductivity and ferromagnetism, showing an opportunity
for potential applications as well as bringing fundamental research interests.
Particularly, the two-dimensional electron gas formed at LaAlO3/SrTiO3
interface offers an appealing platform for quantum phase transition from a
superconductor to a weakly localized metal. Here we report the
superconductor-metal transition in superconducting two-dimensional electron gas
formed at LaAlO3/SrTiO3(110) interface driven by a perpendicular magnetic
field. Interestingly, when approaching the quantum critical point, the dynamic
critical exponent is not a constant but a diverging value, which is a direct
evidence of quantum Griffiths singularity raised from quenched disorder at
ultralow temperatures. Furthermore, the hysteretic property of
magnetoresistance was firstly observed at LaAlO3/SrTiO3(110) interfaces, which
suggests potential coexistence of superconductivity and ferromagnetism
Ising Superconductivity and Quantum Phase Transition in Macro-Size Monolayer NbSe2
Two-dimensional (2D) transition metal dichalcogenides (TMDs) have a range of
unique physics properties and could be used in the development of electronics,
photonics, spintronics and quantum computing devices. The mechanical
exfoliation technique of micro-size TMD flakes has attracted particular
interest due to its simplicity and cost effectiveness. However, for most
applications, large area and high quality films are preferred. Furthermore,
when the thickness of crystalline films is down to the 2D limit (monolayer),
exotic properties can be expected due to the quantum confinement and symmetry
breaking. In this paper, we have successfully prepared macro-size atomically
flat monolayer NbSe2 films on bilayer graphene terminated surface of
6H-SiC(0001) substrates by molecular beam epitaxy (MBE) method. The films
exhibit an onset superconducting critical transition temperature above 6 K, 2
times higher than that of mechanical exfoliated NbSe2 flakes. Simultaneously,
the transport measurements at high magnetic fields reveal that the parallel
characteristic field Bc// is at least 4.5 times higher than the paramagnetic
limiting field, consistent with Zeeman-protected Ising superconductivity
mechanism. Besides, by ultralow temperature electrical transport measurements,
the monolayer NbSe2 film shows the signature of quantum Griffiths singularity
when approaching the zero-temperature quantum critical point
1-Deoxynojirimycin promotes cardiac function and rescues mitochondrial cristae in mitochondrial hypertrophic cardiomyopathy
Based on the Foot–Ground Contact Mechanics Model and Velocity Planning Buffer Control
In order to reduce the impact of the leg joint motors and body electric devices of a falling robot, active flexible control based on force and velocity is proposed. A velocity planning buffer method based on a virtual model is proposed. We established a mechanical model of leg and ground contact. Then, we controlled the knee joint angular velocity change after the robot contacted the ground to reduce the collision impact force and to protect the robot’s joint motors and body’s internal parts. First, the relationship between contact force and velocity was analyzed through the contact mechanical model between leg and ground, and the target was determined. Then, by planning the velocity of the robot’s thigh and hip joint, the velocity mutation during contact was reduced so that the impact on the robot was reduced. This method can avoid complex accurate physics model building and complex torque signal interference filtering processing, the control process is simple and its effectiveness is verified by ADAMS simulation and experimental verification. The velocity planning buffer strategy was tested in experimental studies which showed that the contact force of the buffer strategy was 0.671 times that of no buffer. Additionally, the contact impact acceleration of velocity planning was 1.5505 g, which was less than the force 1.7 g of virtual model control. The velocity planning buffer strategy was better to protect the robot
Based on the Foot–Ground Contact Mechanics Model and Velocity Planning Buffer Control
In order to reduce the impact of the leg joint motors and body electric devices of a falling robot, active flexible control based on force and velocity is proposed. A velocity planning buffer method based on a virtual model is proposed. We established a mechanical model of leg and ground contact. Then, we controlled the knee joint angular velocity change after the robot contacted the ground to reduce the collision impact force and to protect the robot’s joint motors and body’s internal parts. First, the relationship between contact force and velocity was analyzed through the contact mechanical model between leg and ground, and the target was determined. Then, by planning the velocity of the robot’s thigh and hip joint, the velocity mutation during contact was reduced so that the impact on the robot was reduced. This method can avoid complex accurate physics model building and complex torque signal interference filtering processing, the control process is simple and its effectiveness is verified by ADAMS simulation and experimental verification. The velocity planning buffer strategy was tested in experimental studies which showed that the contact force of the buffer strategy was 0.671 times that of no buffer. Additionally, the contact impact acceleration of velocity planning was 1.5505 g, which was less than the force 1.7 g of virtual model control. The velocity planning buffer strategy was better to protect the robot
Entropic Interactions in Semiflexible Polymer Nanocomposite Melts
By
employing molecular dynamics simulations, we explored the effective
depletion zone for nanoparticles (NP) immersed in semiflexible polymer
melts and calculated the entropic depletion interactions between a
pair of NPs in semiflexible polymer nanocomposite melts. The average
depletion zone volumes rely mainly on polymer chain stiffness and
increase with chain stiffness increasing. In the semiflexible polymer
nanocomposite melts, the entropic depletion interactions are attractive
and anisotropic, and increase with chain stiffness increasing. Meanwhile,
the attractive interactions between NPs and polymers can also affect
strongly the entropic depletion interactions. For the semiflexible
polymer nanocomposite melts in the athermal system, the entropic depletion
interactions change from anisotropic to isotropic when the NP/polymer
interactions increase. For NPs in the rodlike polymer melts, a mixture
structure of contact/“bridging” aggregations for NPs
is formed at a strong attractive NP/polymer interaction. Our calculations
can provide an effective framework to predict the morphology of NPs
immersed in semiflexible polymer melts
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