15 research outputs found
Gate-controlled reversible rectifying behaviour in tunnel contacted atomically-thin MoS transistor
Atomically-thin 2D semiconducting materials integrated into van der Waals
heterostructures have enabled architectures that hold great promise for next
generation nanoelectronics. However, challenges still remain to enable their
full acceptance as compliant materials for integration in logic devices. Two
key-components to master are the barriers at metal/semiconductor interfaces and
the mobility of the semiconducting channel, which endow the building-blocks of
diode and field effect transistor. Here, we have devised a reverted
stacking technique to intercalate a wrinkle-free h-BN tunnel layer between
MoS channel and contacting electrodes. Vertical tunnelling of electrons
therefore makes it possible to suppress the Schottky barriers and Fermi level
pinning, leading to homogeneous gate-control of the channel chemical potential
across the bandgap edges. The observed unprecedented features of ambipolar
to diode, which can be reversibly gate tuned, paves the way for
future logic applications and high performance switches based on atomically
thin semiconducting channel.Comment: 23 pages, 5 main figures + 9 SI figure
A gate-programmable van der Waals metal-ferroelectric-semiconductor memory
Ferroelecticity, one of the keys to realize nonvolatile memories owing to the
remanent electric polarization, has been an emerging phenomenon in the
two-dimensional (2D) limit. Yet the demonstrations of van der Waals (vdW)
memories using 2D ferroelectric materials as an ingredient are very limited.
Especially, gate-tunable ferroelectric vdW memristive device, which holds
promises in future neuromorphic applications, remains challenging. Here, we
show a prototype gate-programmable memory by vertically assembling graphite,
CuInP2S6, and MoS2 layers into a metal-ferroelectric-semiconductor
architecture. The resulted devices exhibit two-terminal switchable
electro-resistance with on-off ratios exceeding 105 and long-term retention,
akin to a conventional memristor but strongly coupled to the ferroelectric
characteristics of the CuInP2S6 layer. By controlling the top gate, Fermi level
of MoS2 can be set inside (outside) of its band gap to quench (enable) the
memristive behaviour, yielding a three-terminal gate programmable nonvolatile
vdW memory. Our findings pave the way for the engineering of
ferroelectric-mediated memories in future implementations of nanoelectronics
Probing the fractional quantum Hall phases in valley-layer locked bilayer MoS
Semiconducting transition-metal dichalcogenides (TMDs) exhibit high mobility,
strong spin-orbit coupling, and large effective masses, which simultaneously
leads to a rich wealth of Landau quantizations and inherently strong electronic
interactions. However, in spite of their extensively explored Landau levels
(LL) structure, probing electron correlations in the fractionally filled LL
regime has not been possible due to the difficulty of reaching the quantum
limit. Here, we report evidence for fractional quantum Hall (FQH) states at
filling fractions 4/5 and 2/5 in the lowest LL of bilayer MoS, manifested
in fractionally quantized transverse conductance plateaus accompanied by
longitudinal resistance minima. We further show that the observed FQH states
sensitively depend on the dielectric and gate screening of the Coulomb
interactions. Our findings establish a new FQH experimental platform which are
a scarce resource: an intrinsic semiconducting high mobility electron gas,
whose electronic interactions in the FQH regime are in principle tunable by
Coulomb-screening engineering, and as such, could be the missing link between
atomically thin graphene and semiconducting quantum wells.Comment: 10 pages, 4 figure
In-plane magnetic domains and N\'eel-like domain walls in thin flakes of the room temperature CrTe van der Waals ferromagnet
The recent discovery of magnetic van der Waals materials has triggered a
wealth of investigations in materials science, and now offers genuinely new
prospects for both fundamental and applied research. Although the catalogue of
van der Waals ferromagnets is rapidly expanding, most of them have a Curie
temperature below 300 K, a notable disadvantage for potential applications.
Combining element-selective x-ray magnetic imaging and magnetic force
microscopy, we resolve at room temperature the magnetic domains and domains
walls in micron-sized flakes of the CrTe van der Waals ferromagnet.
Flux-closure magnetic patterns suggesting in-plane six-fold symmetry are
observed. Upon annealing the material above its Curie point (315 K), the
magnetic domains disappear. By cooling back down the sample, a different
magnetic domain distribution is obtained, indicating material stability and
lack of magnetic memory upon thermal cycling. The domain walls presumably have
N\'eel texture, are preferentially oriented along directions separated by 120
degrees, and have a width of several tens of nanometers. Besides microscopic
mapping of magnetic domains and domain walls, the coercivity of the material is
found to be of a few mT only, showing that the CrTe compound is
magnetically soft. The coercivity is found to increase as the volume of the
material decreases
Synergistic interplay between Dirac fermions and long-wavelength order parameters in graphene-insulator heterostructures
In this work, we theoretically study the electronic structure, topological
properties, and interaction effects of the low-energy Dirac electrons in
band-aligned heterostructures consisted of graphene and some correlated
insulating substrates. By virtue of the band alignment, charge carriers can be
transferred from graphene to the insulating substrate under the control of gate
voltages. This may yield a long-wavelength charge order at the surface of the
substrate through the Wigner-crystallization mechanism. The long-wavelength
charge order in turn exerts a superlattice Coulomb potential to the Dirac
electrons in graphene, which reduces the non-interacting Fermi velocity, such
that - Coulomb interactions would play an important role. Consequently,
the Dirac points are spontaneously gapped out by interactions leading to a
sublattice polarized insulator state. Meanwhile, the Fermi velocities around
the Dirac points are drastically enhanced to more than twice of the bare value
by interaction effects, which can give rise to large Landau-level spacing with
robust quantization plateaus of Hall resistivity under weak magnetic fields and
at high temperatures. We have further performed high-throughput first
principles calculations, and suggested a number of promising insulating
materials as candidate substrates for graphene, which could realize the gapped
Dirac state concomitant with low-field, high-temperature quantum Hall effects.Comment: Main text: 7 pages with 3 figures and 2 tables; Supplement info: 16
pages with 8 figures and 4 table
Large scale integration of CVD-graphene based NEMS with narrow distribution of resonance parameters
International audienc
On-demand assembly of optically-levitated nanoparticle arrays in vacuum
Realizing a large-scale fully controllable quantum system is a challenging
task in current physical research and has broad applications. Ultracold atom
and molecule arrays in optical tweezers in vacuum have been used for quantum
simulation, quantum metrology and quantum computing. Recently, quantum ground
state cooling of the center-of-mass motion of a single optically levitated
nanoparticle in vacuum was demonstrated, providing unprecedented opportunities
for studying macroscopic quantum mechanics and precision measurements. In this
work, we create a reconfigurable optically-levitated nanoparticle array in
vacuum. Our optically-levitated nanoparticle array allows full control of
individual nanoparticles to form an arbitrary pattern and detect their motion.
As a concrete example, we choose two nanoparticles without rotation signals
from an array to synthesize a nanodumbbell in-situ by merging them into one
trap. The nanodumbbell synthesized in-situ can rotate beyond 1 GHz. Our work
provides a new platform for studying macroscopic many-body physics.Comment: 6 pages, 4 figure
An Optical Spectroscopic Study of Air-Degradation of van der Waals Magnetic Semiconductor Cr<sub>2</sub>Ge<sub>2</sub>Te<sub>6</sub>
Two-dimensional (2D) magnetic semiconductors exhibit unique combination of electronic and magnetic properties, holding great promise in potential applications such as spintronics and magneto-optics. However, many of them are air-sensitive, and their properties can be significantly altered upon exposure to air. Here, we showed an optical spectroscopic investigation of the effects of air-degradation on few-layered van der Waals (vdW) magnetic semiconductor Cr2Ge2Te6. It was found that although the partially degraded few-layered Cr2Ge2Te6 showed a significant Raman redshift and a split of Eg peak at room temperature, the magneto-optic Kerr hysteresis loop can remain largely unchanged below the Curie temperature. Temperature-dependent Raman measurements further revealed characteristic blueshifts of phonon energy, which were associated with the ferromagnetic phase transition in partially degraded Cr2Ge2Te6, in agreement with Kerr measurements. Our results provide an optical spectroscopic insight into the air-instability of 2D magnetic semiconductors, and contribute to a better understanding of the relationship between phonon modes and long-range spin order at the presence of defects in ultra-thin vdW magnetic semiconductors