No abstract

With the ability to couple them to electromagnetic and plasmonic modes, they hold the promise to be the key building blocks in future quantum information technology. Graphene based resonators are of interest for technological applications due to their high resonant frequencies, multiple mechanical modes, and low mass.1-7 The tension mediated non-linear coupling between various modes of the resonator can be excited in a controllable manner. 8-11Here, we engineer a graphene resonator to have large frequency tunability at low temperatures resulting in large intermodal coupling strength. We observe the emergence of new eigenmodes and amplification of the coupled modes using red and blue parametric excitation respectively. We demonstrate that the dynamical intermodal coupling is tunable. A cooperativity of 60 between two resonant modes of ∼100 MHz is achieved in the strong coupling regime.The ability to dynamically control the coupling between high frequency eigenmodes of a mechanical system opens up possibility for quantum mechanical experiments at low temperatures. 12,13Experiments in cavity optomechanics, 14,15 where a low frequency mechanical oscillator parametrically modulates the resonant frequency of an electromagnetic mode of a cavity, have demonstrated the ability to prepare a mechanical system in its ground state, [16][17][18][19] and entanglement between propagating photons and phonons. 20 Since mechanical resonators support a large number of vibrational modes, a natural extension of the optomechanical scheme is to use coupled modes of a mechanical resonator where a high frequency mode plays the role of the cavity. 21,22 There has been a considerable interest in exploring coupling among eigenmodes of a mechanical system, demonstrating coherent coupling between low frequency modes. [21][22][23][24] The eventual goal of such experiments is to demonstrate coupling in the quantum regime. 12,13 To achieve a quantum coherent coupling, graphene mechanical resonators offer many advantages. Firstly, the high frequencies of the graphene drum resonators yields lesser phonons once cooled down to low temperatures. Secondly, the large frequency dispersion with gate voltage results in large inter-modal coupling offering advantage while using higher frequency modes as phononic cavity. Furthermore, the large quantum zero-point motion of graphene resonators offers advantage of efficient coupling with electromagnetic cavities. 4-7Here we fabricate graphene drum tension yielding large frequency tunability at low temperatures (see Methods for details on fabrication). The actuation and detection of the mechanical modes of graphene is implemented using an all electrical scheme. The drums we study consist of a graphene flake contacted by Cr/Au electrodes with a central region of diameter 3.5 µm suspended 300 nm above a local gate electrode of Ti/Pt (Figure 1a,b). The local gate is on a sapphire substrate and the graphene is transferred onto electrodes on SiO 2 . The region of SiO 2 over the gate electrode is etched out prior to p...

Highly doped graphene holds promise for next-generation electronic and photonic devices.However, chemical doping cannot be precisely controlled, and introduces external disorder that significantly diminishes the carrier mobility and therefore the graphene conductivity. Here, we show that monolayer tungsten oxyselenide (TOS) created by oxidation of WSe2 acts as an efficient and low-disorder hole-dopant for graphene. When the TOS is directly in contact with graphene, the induced hole density is 3 × 10 13 cm -2 , and the room-temperature mobility is 2,000 cm 2 /V•s, far exceeding that of chemically-doped graphene. Inserting WSe2 layers between the TOS and graphene tunes the induced hole density as well as reduces charge disorder such that the mobility exceeds 20,000 cm 2 /V•s and reaches the limit set by acoustic phonon scattering, resulting in sheet resistance below 50 /□. An electrostatic model based on work-function mismatch accurately describes the tuning of the carrier density with WSe2 interlayer thickness. These films show unparalleled performance as transparent conductors at telecommunication wavelengths, as shown by measurements of transmittance in thin films and insertion loss in photonic ring resonators. This work opens up new avenues in optoelectronics incorporating two-dimensional heterostructures including infrared transparent conductors, electro-phase modulators, and various junction devices.

Quantum Hall effect provides a simple way to study the competition between single particle physics and electronic interaction. However, electronic interaction becomes important only in very clean graphene samples and so far the trilayer graphene experiments are understood within non-interacting electron picture. Here, we report evidence of strong electronic interactions and quantum Hall ferromagnetism seen in Bernal-stacked trilayer graphene. Due to high mobility ∼500,000 cm2 V−1 s−1 in our device compared to previous studies, we find all symmetry broken states and that Landau-level gaps are enhanced by interactions; an aspect explained by our self-consistent Hartree–Fock calculations. Moreover, we observe hysteresis as a function of filling factor and spikes in the longitudinal resistance which, together, signal the formation of quantum Hall ferromagnetic states at low magnetic field.

The high contact resistance to 2D semiconductors must be reduced to attain their full potential for next-generation applications. In this work, we report the lowest total p-type contact resistance (642 Ω•μm) to sub-5 nm thin WSe 2 using a monolayer dopant, namely tungsten oxyselenide (TOS), that induces degenerate doping densities as high as ∼4 × 10 13 cm −2 . Moreover, this doping remains active even at temperatures as low as 77 K, enabling a pathway toward high-quality contacts for low-temperature applications. Electrical measurements taken four months apart on TOS−WSe 2 devices kept in ambient conditions show less than an order of magnitude reduction in the hole density and demonstrate promising stability of the doping. In addition, we show a drastic improvement in device characteristics through areaselective doping of the contact, which opens an avenue toward achieving high-performance p-type transistors with ultrathin 2D WSe 2 .