Phonons, which are collective excitations in a lattice of atoms or molecules, play a major role in determining various physical properties of condensed matter, such as thermal and electrical conductivities. In particular, phonons in graphene interact strongly with electrons; however, unlike in usual metals, these interactions between phonons and massless Dirac fermions appear to mirror the rather complicated physics of those between light and relativistic electrons. Therefore, a fundamental understanding of the underlying physics through systematic studies of phonon interactions and excitations in graphene is crucial for realising graphene-based devices. In this study, we demonstrate that the local phonon properties of graphene can be controlled at the nanoscale by tuning the interaction strength between graphene and an underlying Pt substrate. Using scanning probe methods, we determine that the reduced interaction due to embedded Ar atoms facilitates electron–phonon excitations, further influencing phonon-assisted inelastic electron tunnelling.

We report spectroscopic measurements of discrete two-level systems (TLSs) coupled to a dc SQUID phase qubit with a 16 µm 2 area Al/AlO x /Al junction. Applying microwaves in the 10 GHz to 11 GHz range, we found eight avoided level crossings with splitting sizes from 10 MHz to 200 MHz and spectroscopic lifetimes from 4 ns to 160 ns. Assuming the transitions are from the ground state of the composite system to an excited state of the qubit or an excited state of one of the TLS states, we fit the location and spectral width to get the energy levels, splitting sizes and spectroscopic coherence times of the phase qubit and TLSs. The distribution of splittings is consistent with non-interacting individual charged ions tunneling between random locations in the tunnel barrier and the distribution of lifetimes is consistent with the AlO x in the junction barrier having a frequency-independent loss tangent. To check that the charge of each TLS couples independently to the voltage across the junction, we also measured the spectrum in the 20-22 GHz range and found tilted avoided level crossings due to the second excited state of the junction and states in which both the junction and a TLS were excited.

Bi2−xSbxTe3−ySey has been argued to exhibit both topological surface states and insulating bulk states, but has not yet been studied with local probes on the atomic scale. Here we report on the atomic and electronic structures of Bi1.5Sb0.5Te1.7Se1.3 studied using scanning tunnelling microscopy (STM) and spectroscopy (STS). Although there is significant surface disorder due to alloying of constituent atoms, cleaved surfaces of the crystals present a well-ordered hexagonal lattice with 10 Å high quintuple layer steps. STS results reflect the band structure and indicate that the surface state and Fermi energy are both located inside the energy gap. In particular, quasi-particle interference patterns from electron scattering demonstrate that the surface states possess linear dispersion and chirality from spin texture, thus verifying its topological nature. This finding demonstrates that alloying is a promising route to achieve full suppression of bulk conduction in topological insulators whilst keeping the topological surface state intact.

Direct contacts of a metal with atomically thin two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors have been found to suppress device performance by producing a high contact resistance. NbS is a 2D TMDC and a conductor. It is expected to form ohmic contacts with 2D semiconductors because of its high work function and the van der Waals interface it forms with the semiconductor, with such an interface resulting in weak Fermi level pinning. Despite the usefulness of NbS as an electrode, previous synthesis methods could not control the thickness, uniformity, and shape of the NbS film and hence could not find practical applications in electronics. Here, we report a patternable method for carrying out the synthesis of NbS films in which the number of NbS layers formed over a large area was successfully controlled, which is necessary for the production of customized electrodes. The synthesized NbS films were shown to be highly transparent and uniform in thickness and conductivity over the large area. Furthermore, the synthesized NbS showed half the contact resistance than did the molybdenum metal in MoS field effect transistors (FETs) on a large transparent quartz substrate. The MoS device with NbS showed an electron mobility as high as 12.7 cm V s, which was three times higher than that found for the corresponding molybdenum-contacted MoS device. This result showed the high potential of the NbS thin film as a transparent electrode for 2D transition metal dichalcogenide (TMDC) semiconductors with low contact resistance.