One of the suggested ways of controlling the electronic properties of graphene is to establish a periodic potential modulation on it, which could be achieved by self-assembly of ordered molecular lattices. We have studied the self-assembly of cobalt phthalocyanines (CoPc) on chemical vapor deposition (CVD) grown graphene transferred onto silicon dioxide (SiO2) and hexagonal boron nitride (h-BN) substrates. Our scanning tunneling microscopy (STM) experiments show that, on both substrates, CoPc forms a square lattice. However, on SiO2, the domain size is limited by the corrugation of graphene, whereas on h-BN, single domain extends over entire terraces of the underlying h-BN. Additionally, scanning tunneling spectroscopy (STS) measurements suggest that CoPc molecules are doped by the substrate and that the level of doping varies from molecule to molecule. This variation is larger on graphene on SiO2 than on h-BN. These results suggest that graphene on h-BN is an ideal substrate for the study of molecular self-assembly toward controlling the electronic properties of graphene by engineered potential landscapes.

The detection of mechanical vibrations near the quantum limit is a formidable challenge since the displacement becomes vanishingly small when the number of phonon quanta tends towards zero. An interesting setup for on-chip nanomechanical resonators is that of coupling them to electrical microwave cavities for detection and manipulation. Here we show how to achieve a large cavity coupling energy of up to (2π) 1 MHz/nm for metallic beam resonators at tens of MHz. We used focused ion beam (FIB) cutting to produce uniform slits down to 10 nm, separating patterned resonators from their gate electrodes, in suspended aluminum films. We measured the thermomechanical vibrations down to a temperature of 25 mK, and we obtained a low number of about twenty phonons at the equilibrium bath temperature. The mechanical properties of Al were excellent after FIB cutting and we recorded a quality factor of Q ∼ 3 × 10 5 for a 67 MHz resonator at a temperature of 25 mK. Between 0.2K and 2K we find that the dissipation is linearly proportional to the temperature. Keywords nanomechanics, NEMS, quantum limit, detection, dissipationThe measurement of small-amplitude vibrations in mechanical systems is becoming an increasingly interesting problem [1,2]. From the point of view of basic science, the study of mechanical systems close to the quantum limit has attracted a lot of interest recently [3,4]. The endeavor towards the ground state of the harmonic phonon oscillations has been going on in various physical systems such as in optomechanics [5][6][7], or in electrically coupled beam resonators which have been measured using single-electron transistors [4,8], or lately, with on-chip microwave cavities [9][10][11][12].The quantum challenge is posed by several issues, including the relatively low frequency (f 0 ∼ 10 MHz), of the lowest modes in suspended beams. The quantum limit implies stringent requirements on temperature, since hf 0 needs to be small in comparison to k B T . On the other hand, at higher frequencies, the coupling to measuring systems diminishes rapidly. Third, the zero-point vibration amplitudes x ZP = /2mω 0 , where m is the effective mass and ω 0 = 2πf 0 is the angular frequency, are vanishingly small even at the atomic scale. Very recently, O'Connell et al.[13] demonstrated a piezoelectric mechanical mode at the quantum ground state by using a coupling to a superconducting qubit. However, bringing a purely mechanical mode to the quantum limit remains an ongoing quest, with the goal becoming a reality probably in the near future.Micromechanical resonators are also used in applications as detectors. The best devices take advantage of the trend to smaller size and higher frequencies, and will soon approach sensing at the atomic mass unit level [14,15]. They could also be operated as sensors of position, force, or high-frequency electromagnetic fields.For conductive resonators, capacitive coupling to an electrical measuring apparatus is useful for readout. In contrast to magnetomotive or optical detection, one can obtain v...

We study inelastic energy relaxation in graphene for low energies to find out how electrons scatter with acoustic phonons and other electrons. By coupling the graphene to superconductors, we create a strong dependence of the measured signal, i.e., critical Josephson current, on the electron population on different energy states. Since the relative population of high-and low-energy states is determined by the inelastic scattering processes, the critical current becomes an effective probe for their strength. We argue that the electron-electron interaction is the dominant relaxation method and we estimate a scattering time τ e−e = 0.1 . . . 1 ps at T = 500 mK, 1-2 orders of magnitude smaller than predicted for normal two-dimensional diffusive systems.

Single electron transistors (SETs) fabricated from single-walled carbon nanotubes (SWNTs) can be operated as highly sensitive charge detectors reaching sensitivity levels comparable to metallic radio frequency SETs (rf-SETs). Here, we demonstrate how the charge sensitivity of the device can be improved by using the mechanical oscillations of a single-walled carbon nanotube quantum dot. To optimize the charge sensitivity δQ, we drive the mechanical resonator far into the nonlinear regime and bias it to an operating point where the mechanical third order nonlinearity is canceled out. This way we enhance δQ, from 6 μe/(Hz)(1/2) for the static case to 0.97 μe/(Hz)(1/2) at a probe frequency of ∼1.3 kHz.

We have investigated the microwave response of nanotube Josephson junctions at 600-900 MHz at microwave powers corresponding to currents from 0 to $2\times I_{\mathrm C}$ in the junction. Compared with theoretical modeling, the response of the junctions correspond well to the lumped element model of resistively and capacitively shunted junction. We demonstrate the operation of these superconducting FETs as charge detectors at high frequencies without any matching circuits. Gate-voltage-induced charge $Q_{\rm G}$ modifies the critical current $I_{\rm C}$, which changes the effective impedance of the junction under microwave irradiation. This change, dependent on the transfer characteristics $dI_{\mathrm C}/dQ_{\rm G}$, modifies the reflected signal and it can be used for wide band electrometry. We measure a sensitivity of $3.1\times10^{-5}$ $e/\sqrt{\mathrm{Hz}}$ from a sample which has a maximum switching current of 2.6 nA.Comment: 4 pages, 4 figure