44 research outputs found
Observation of first-order quantum phase transitions and ferromagnetism in twisted double bilayer graphene
Twisted graphene multilayers are highly tunable flatband systems for
developing new phases of matter. Thus far, while orbital ferromagnetism has
been observed in valley polarized phases, the long-range orders of other
correlated phases as well as the quantum phase transitions between different
orders mostly remain unknown. Here, we report an observation of Coulomb
interaction driven first-order quantum phase transitions and ferromagnetism in
twisted double bilayer graphene (TDBG). At zero magnetic field, the transitions
are revealed in a series of step-like abrupt resistance jumps with prominent
hysteresis loop when either the displacement field (D) or the carrier density
(n) is tuned across symmetry-breaking boundary near half filling, indicating a
formation of ordered domains. It is worth noting that the good turnability and
switching of these states gives a rise to a memory performance with a large
on/off ratio. Moreover, when both spin and valley play the roles at finite
magnetic field, we observe abundant first-order quantum phase transitions among
normal metallic states from charge neutral point, orbital ferromagnetic states
from quarter filling, and spin-polarized states from half filling. We interpret
these first-order phase transitions in the picture of phase separations and
spin domain percolations driven by multi-field tunable Coulomb interactions, in
agreement with Lifshitz transition from Hartree-Fock calculations. The observed
multi-filed tunable domain structure and its hysteresis resembles the
characteristics of multiferroics, revealing intriguing magnetoelectric
properties. Our result enriches the correlated phase diagram in TDBG for
discovering novel exotic phases and quantum phase transitions, and it would
benefit other twisted moir\'e systems as well
Layer-by-Layer Epitaxy of Multilayer MoS2 Wafers
Two-dimensional (2D) semiconductor of MoS2 has great potential for advanced
electronics technologies beyond silicon1-9. So far, high-quality monolayer MoS2
wafers10-12 are already available and various demonstrations from individual
transistors to integrated circuits have also been shown13-15. In addition to
the monolayer, multilayers have narrower band gaps but improved carrier
mobilities and current capacities over the monolayer5,16-18. However, achieving
high-quality multilayer MoS2 wafers remains a challenge. Here we report the
growth of high quality multilayer MoS2 4-inch wafers via the layer-by-layer
epitaxy process. The epitaxy leads to well-defined stacking orders between
adjacent epitaxial layers and offers a delicate control of layer numbers up to
6. Systematic evaluations on the atomic structures and electronic properties
were carried out for achieved wafers with different layer numbers. Significant
improvements on device performances were found in thicker-layer field effect
transistors (FETs), as expected. For example, the average field-effect mobility
({\mu}FE) at room temperature (RT) can increase from ~80 cm2V-1s-1 for
monolayer to ~110/145 cm2V-1s-1 for bilayer/trilayer devices. The highest RT
{\mu}FE=234.7 cm2V-1s-1 and a record-high on-current densities of 1.704
mA{\mu}m-1 at Vds=2 V were also achieved in trilayer MoS2 FETs with a high
on/off ratio exceeding 107. Our work hence moves a step closer to practical
applications of 2D MoS2 in electronics.Comment: 13 pages,4 Figure
Room-temperature correlated states in twisted bilayer MoS
Moir\'e superlattices have emerged as an exciting condensed-matter quantum
simulator for exploring the exotic physics of strong electronic correlations.
Notable progress has been witnessed, but such correlated states are achievable
usually at low temperatures. Here, we report the transport evidences of
room-temperature correlated electronic states and layer-hybridized SU(4)
Hubbard model simulator in AB-stacked MoS homo-bilayer moir\'e
superlattices. Correlated insulating states at moir\'e band filling factors v =
1, 2, 3 are unambiguously established in twisted bilayer MoS. Remarkably,
the correlated electronic states can persist up to a record-high critical
temperature of over 285 K. The realization of room-temperature correlated
states in twisted bilayer MoS can be understood as the cooperation effects
of the stacking-specific atomic reconstruction and the resonantly enhanced
interlayer hybridization, which largely amplify the moir\'e superlattice
effects on electronic correlations. Furthermore, extreme large non-linear Hall
responses up to room-temperature are uncovered near correlated insulating
states, demonstrating the quantum geometry of moir\'e flat conduction band.Comment: 13 pages, 3 figure
Comment on âDisentangling Orbital and Valley Hall Effects in Bilayers of Transition Metal Dichalcogenidesâ ()
Funding Information: We thank Yanchong Zhao and Zhipei Sun for valuable discussions. We gratefully acknowledge the financial support from Academy of Finland (Grant No. 333099). Publisher Copyright: © 2021 American Physical SocietyA Comment on the Letter by 1T.-P. Cysne, Phys. Rev. Lett.126, 056601 (2021).PRLTAO0031-900710.1103/PhysRevLett.126.056601 The authors of the Letter offer a Reply.Non peer reviewe
The Effect of Twin Grain Boundary Tuned by Temperature on the Electrical Transport Properties of Monolayer MoS2
Theoretical calculation and experimental measurement have shown that twin grain boundary (GB) of molybdenum disulphide (MoS2) exhibits extraordinary effects on transport properties. Precise transport measurements need to verify the transport mechanism of twin GB in MoS2. Here, monolayer molybdenum disulphide with a twin grain boundary was grown in our developed low-pressure chemical vapor deposition (CVD) system, and we investigated how the twin GB affects the electrical transport properties of MoS2 by temperature-dependent transport studies. At low temperature, the twin GB can increase the in-plane electrical conductivity of MoS2 and the transport exhibits variable-range hopping (VRH), while at high temperature, the twin GB impedes the electrical transport of MoS2 and the transport exhibits nearest-neighbor hopping (NNH). Our results elucidate carrier transport mechanism of twin GB and give an important indication of twin GB in tailoring the electronic properties of MoS2 for its applications in next-generation electronics and optoelectronic devices
Twisting for Tunable Nonlinear Optics
Recently, Yang et al. introduces the twisting degree of freedom into nonlinear optics and demonstrates tunable second-order nonlinear optical response in twisted bilayer graphene. The results open a new route to future nonlinear photonic and optoelectronic applications.Peer reviewe
Dual-gated mono-bilayer graphene junctions
| openaire: EC/H2020/820423/EU//S2QUIP | openaire: EC/H2020/834742/EU//ATOPA lateral junction with an atomically sharp interface is extensively studied in fundamental research and plays a key role in the development of electronics, photonics and optoelectronics. Here, we demonstrate an electrically tunable lateral junction at atomically sharp interfaces between dual-gated mono- and bilayer graphene. The transport properties of the monoâbilayer graphene interface are systematically investigated with IdsâVds curves and transfer curves, which are measured with bias voltage Vds applied in opposite directions across the asymmetric monoâbilayer interface. Nearly 30% difference between the output IdsâVds curves of graphene channels measured at opposite Vds directions is observed. Furthermore, the measured transfer curves confirm that the conductance difference of graphene channels greatly depends on the doping level, which is determined by dual-gating. The Vds direction dependent conductance difference indicates the existence of a gate tunable junction in the monoâbilayer graphene channel,due to different band structures of monolayer graphene with zero bandgap and bilayer graphene with a bandgap opened by dual-gating. Simulation of the IdsâVds curves based on a new numerical model validates the gate tunable junction at the monoâbilayer graphene interface from another point of view. The dual-gated monoâbilayer graphene junction and new protocol for IdsâVds curve simulation pave a possible way for functional applications of graphene in next-generation electronics.Peer reviewe
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Moiré photonics and optoelectronics
Moiré superlattices, the artificial quantum materials, have provided a wide range of possibilities for the exploration of completely new physics and device architectures. In this Review, we focus on the recent progress on emerging moiré photonics and optoelectronics, including but not limited to moiré excitons, trions, and polaritons; resonantly hybridized excitons; reconstructed collective excitations; strong mid- and far-infrared photoresponses; terahertz single-photon detection; and symmetry-breaking optoelectronics. We also discuss the future opportunities and research directions in this field, such as developing advanced techniques to probe the emergent photonics and optoelectronics in an individual moiré supercell; exploring new ferroelectric, magnetic, and multiferroic moiré systems; and using external degrees of freedom to engineer moiré properties for exciting physics and potential technological innovations
Switchable Photoresponse Mechanisms Implemented in Single van der Waals Semiconductor/Metal Heterostructure
| openaire: EC/H2020/820423/EU//S2QUIP | openaire: EC/H2020/834742/EU//ATOP | openaire: EC/H2020/965124/EU//FEMTOCHIPvan der Waals (vdW) heterostructures based on two-dimensional (2D) semiconducting materials have been extensively studied for functional applications, and most of the reported devices work with sole mechanism. The emerging metallic 2D materials provide us new options for building functional vdW heterostructures via rational band engineering design. Here, we investigate the vdW semiconductor/metal heterostructure built with 2D semiconducting InSe and metallic 1T-phase NbTe2, whose electron affinity ÏInSe and work function ΊNbTe2 almost exactly align. Electrical characterization verifies exceptional diode-like rectification ratio of >103 for the InSe/NbTe2 heterostructure device. Further photocurrent mappings reveal the switchable photoresponse mechanisms of this heterostructure or, in other words, the alternative roles that metallic NbTe2 plays. Specifically, this heterostructure device works in a photovoltaic manner under reverse bias, whereas it turns to phototransistor with InSe channel and NbTe2 electrode under high forward bias. The switchable photoresponse mechanisms originate from the band alignment at the interface, where the band bending could be readily adjusted by the bias voltage. In addition, a conceptual optoelectronic logic gate is proposed based on the exclusive working mechanisms. Finally, the photodetection performance of this heterostructure is represented by an ultrahigh responsivity of âŒ84 A/W to 532 nm laser. Our results demonstrate the valuable application of 2D metals in functional devices, as well as the potential of implementing photovoltaic device and phototransistor with single vdW heterostructure.Peer reviewe