Christopher J. Mellor

High Broad‐Band Photoresponsivity of Mechanically Formed InSe–Graphene van der Waals Heterostructures

High broad‐band photoresponsivity of mechanically formed InSe–graphene van der Waals heterostructures is achieved by exploiting the broad‐band transparency of graphene, the direct bandgap of InSe, and the favorable band line up of InSe with graphene. The photoresponsivity exceeds that for other van der Waals heterostructures and the spectral response extends from the near‐infrared to the visible spectrum.

Nonlinear modal coupling in a high-stress doubly-clamped nanomechanical resonator

We present results from a study of the nonlinear inter-modal coupling between different flexural vibrational modes of a single high-stress, doubly-clamped silicon nitride nanomechanical beam. Using the magnetomotive technique and working at 100 mK we explored the nonlinear behaviour and modal couplings of the first, third and fifth modes of a 25.5 μm long beam. We find very good agreement between our results and a simple analytical model which assumes that the different modes of the resonator are coupled to each other by displacement induced tension in the beam. The small size of our resonator leads to relatively strong nonlinear couplings, for example we find a shift of about 7 Hz in the third mode for a 1 nm displacement in the first mode and frequency shifts ~ 20 times larger than the linewidth (130 Hz) are readily observed.

Dissipation due to tunneling two-level systems in gold nanomechanical resonators

We present measurements of the dissipation and frequency shift in gold nanomechanical resonators at temperatures down to 10 mK. The resonators were fabricated as doubly clamped beams above a GaAs substrate and actuated magnetomotively. Measurements on beams with frequencies 7.95 and 3.87 MHz revealed that from 30 to 500 mK the dissipation increases with temperature as T 0.5 , with saturation occurring at higher temperatures. The relative frequency shift of the resonators increases logarithmically with temperature up to at least 400 mK. Similarities with the behavior of bulk amorphous solids suggest that the dissipation in our resonators is dominated by two-level systems.

Graphical computing in the undergraduate laboratory: Teaching and interfacing with LabVIEW

We describe the development and implementation of an undergraduate physics laboratory course based on National Instruments’ LabVIEW application. LabVIEW, a graphical programming language, provides an intuitive interface with which to teach fundamental computer-based data acquisition techniques. To convey the importance of these techniques in modern experimental physics, during our course the students complete a variety of tasks and experiments based around LabVIEW virtual instruments that they have constructed. Furthermore, LabVIEW is a powerful signal processing and waveform analysis tool, it may be used to reinforce core physics concepts taught in an analytical fashion in other courses. Foremost among these is Fourier analysis. We discuss the efficacy of LabVIEW as a pedagogical tool in a number of Fourier-related areas. Other important pure and applied physics topics covered in our LabVIEW course and briefly described here include resonance, filtering and lock-in techniques, thermal diffusivity, chaos,...

Current-voltage characteristics of zinc-blende (cubic) Al0.3Ga0.7N/GaN double barrier resonant tunneling diodes

Measurements of the current-voltage characteristics of zinc-blende (cubic) Al0.3Ga0.7N/GaN, double barrier resonant tunneling diodes are presented. Clear and reproducible negative differential resistance effects are observed, with room temperature peak-to-valley ratios up to 4 and peak currents up to about 1000 A cm−2.

Resistance noise scaling in a dilute two-dimensional hole system in GaAs

The 1/f resistance noise of a two-dimensional (2D) hole system in a high mobility GaAs quantum well has been measured on both sides of the 2D metal-insulator transition (MIT) at zero magnetic field (B=0), and deep in the insulating regime. The two measurement methods used are described: I or V fixed, and measurement of resp. V or I fluctuations. The normalized noise magnitude SR/R^2 increases strongly when the hole density is decreased, and its temperature (T) dependence goes from a slight increase with T at the largest densities, to a strong decrease at low density. We find that the noise magnitude scales with the resistance, SR /R^2 ~ R^2.4. Such a scaling is expected for a second order phase transition or a percolation transition. The possible presence of such a transition is investigated by studying the dependence of the conductivity as a function of the density. This dependence is consistent with a critical behavior close to a critical density p* lower than the usual MIT critical density pc.The 1/f resistance noise of a two-dimensional (2D) hole system in a high mobility GaAs quantum well has been measured on both sides of the 2D metal-insulator transition (MIT) at zero magnetic field (B = 0), and deep in the insulating regime. The two measurement methods used are described: I or V fixed, and measurement of resp. Vor I fluctuations. The normalized noise magnitude SR/R2 increases strongly when the hole density is decreased, and its temperature (T) dependence goes from a slight increase with T at the largest densities, to a strong decrease at low density. We find that the noise magnitude scales with the resistance, SR/R2 ~ R2.4. Such a scaling is expected for a second order phase transition or a percolation transition. The possible presence of such a transition is investigated by studying the dependence of the conductivity as a function of the density. This dependence is consistent with a critical behavior close to a critical density p* lower than the usual MIT critical density pc.

A Comparison of Self-Potential Tomography with Electrical Resistivity Tomography for the Detection of Abandoned Mineshafts

We present a comparison of the abilities of Electrical Resistivity Tomography (ERT) and Self-Potential Tomography (SPT) to detect and characterize buried mineshafts at the site of a former colliery. Surface electrical resistivity and self-potential (SP) surveys were carried out at two test sites, each containing a hidden shaft. The ERT survey results indicate that both sites had a highly heterogeneous subsurface resistivity distribution, which we attribute to colliery spoil and former infrastructure. ERT managed to distinguish an air-filled, highly resistive mineshaft from this background, but failed to detect the second shaft, which was backfilled and therefore had a much lower resistivity contrast with the surrounding formation. However, SPT located both shafts, gave an indication of their size, shape and depth of burial, and was able to distinguish the open- from the backfilled mineshaft due to the strength of the associated SP anomalies. We argue that these SP anomalies are likely to be due to changes in the streaming potential caused by preferential drainage into the shafts.

Strain-Engineered Graphene Grown on Hexagonal Boron Nitride by Molecular Beam Epitaxy

Graphene grown by high temperature molecular beam epitaxy on hexagonal boron nitride (hBN) forms continuous domains with dimensions of order 20 μm, and exhibits moiré patterns with large periodicities, up to ~30 nm, indicating that the layers are highly strained. Topological defects in the moiré patterns are observed and attributed to the relaxation of graphene islands which nucleate at different sites and subsequently coalesce. In addition, cracks are formed leading to strain relaxation, highly anisotropic strain fields, and abrupt boundaries between regions with different moiré periods. These cracks can also be formed by modification of the layers with a local probe resulting in the contraction and physical displacement of graphene layers. The Raman spectra of regions with a large moiré period reveal split and shifted G and 2D peaks confirming the presence of strain. Our work demonstrates a new approach to the growth of epitaxial graphene and a means of generating and modifying strain in graphene.

Spin polarization of (Ga,Mn)As measured by Andreev Spectroscopy : The role of spin-active scattering

We investigate the spin polarization of the ferromagnetic semiconductor (Ga,Mn)As by point-contact Andreev reflection spectroscopy. The conductance spectra are analyzed using a recent theoretical model that accounts for momentum- and spin-dependent scattering at the interface. This allows us to fit the data without resorting, as in the case of the standard spin-dependent Blonder-Tinkham-Klapwijk (BTK) model, to an effective temperature or a statistical distribution of superconducting gaps. We find a transport polarization PC ≈ 57%, in considerably better agreement with the � ·� p kinetic-exchange model of (Ga,Mn)As, than the significantly larger estimates inferred from the BTK model. The temperature dependence of the conductance spectra is fully analyzed.

Flux-coherent series SQUID array magnetometers operating above 77 K with superior white flux noise than single-SQUIDs at 4.2 K

A very promising direction to improve the sensitivity of magnetometers based on superconducting quantum interference devices (SQUIDs) is to build a series-array of N non-interacting SQUIDs operating flux-coherently, because in this case their voltage modulation depth, ΔV, linearly scales with N whereas the white flux noise SΦ1/2 decreases as 1/N1/2. Here, we report the realization of both these improvements in an advanced layout of very large SQUID arrays made of YBa2Cu3O7. Specially designed with large area narrow flux focusers for increased field sensitivity and improved flux-coherency, our arrays have extremely low values for SΦ1/2 between (0.25 and 0.44) μΦ0/Hz1/2 for temperatures in the range (77–83) K. In this respect, they outperform niobium/aluminium trilayer technology-based single-SQUIDs operating at 4.2 K. Moreover, with values for ΔV and transimpedance in the range of (10–17) mV and (0.3–2.5) kΩ, respectively, a direct connection to a low-noise room temperature amplifier is allowed, while matc...