Fractal Magneto-conductance Fluctuations in Mesoscopic Semiconductor Billiards

Negatively biased surface-gates allow electrostatic depletion of selected regions of a 2DEG, forming confined regions of specific geometry called billiards, in which ballistic transport occurs. At millikelvin temperatures, the electron phase coherence length is sufficient that quantum interference effects produce reproducible magneto-conductance fluctuations (MCF) that act as a 'magneto-fingerprint' of the scattering dynamics in the billiard. It has been predicted that billiard MCF are fractal. Fractal MCF in mesoscopic semiconductor billiards are investigated experimentally. The MCF of a Sinai billiard displayed exact self-similarity (ESS). A correlation function analysis is used to quantify the presence of ESS. A model for the Sinai billiard MCF based on a Weierstrass function is presented. Using a bridging interconnect, a continuous transition between the Sinai and an empty square geometry is achieved. The removal of the circle induces a transition from ESS to statistical self-similarity (SSS), suggesting that ESS is due to the presence of an obstacle at the center of the billiard. The physical dependencies of SSS are investigated and show variation in the fractal dimension, rather than the fractal scaling range. SSS obeys a unified picture where the fractal dimension depends only on the ratio between the average spacing and broadening of billiard energy levels, irrespective of other billiard parameters. The semiclassical origin of SSS is demonstrated and the suppression of SSS is observed in both the quantum and classical limits. The influence of soft-wall potential profile on fractal MCF is investigated using double-2DEG billiards. Detailed reviews of semiconductor billiard fabrication, low-temperature electrical measurements and fractal analysis are also presented.Comment: Ph.D. Thesis - Completed May 2000. 241 pages as 3.43MB PDF file. Higher quality version available at http://www.phys.unsw.edu.au/~mico/downloads/downloads.html . Questions, comments, etc to [email protected]

Realizing lateral wrap-gated nanowire FETs: Controlling gate length with chemistry rather than lithography

An important consideration in miniaturizing transistors is maximizing the coupling between the gate and the semiconductor channel. A nanowire with a coaxial metal gate provides optimal gate-channel coupling, but has only been realized for vertically oriented nanowire transistors. We report a method for producing laterally oriented wrap-gated nanowire field-effect transistors that provides exquisite control over the gate length via a single wet etch step, eliminating the need for additional lithography beyond that required to define the source/drain contacts and gate lead. It allows the contacts and nanowire segments extending beyond the wrap-gate to be controlled independently by biasing the doped substrate, significantly improving the sub-threshold electrical characteristics. Our devices provide stronger, more symmetric gating of the nanowire, operate at temperatures between 300 to 4 Kelvin, and offer new opportunities in applications ranging from studies of one-dimensional quantum transport through to chemical and biological sensing.Comment: 16 pages, 5 figures. Submitted version, published version available at http://http://pubs.acs.org/journal/nalef

Three personal barriers to teaching transformation

The benefits of active learning over traditional teaching methods have been well known for over a decade. Yet, uptake in courses taught by lecturers not familiar with the literature is very slow. Existing research has highlighted several institutional and structural barriers to change, but few go into the personal experience of the lecturer undergoing this transformation. In 2021, a second-year course on quantum mechanics was transformed to a flipped classroom style course employing constructive alignment. The lecturer (Micolich) had taught the course with good student feedback for several years and was not previously experienced with modern teaching methods before collaborating with a Physics Education Research academic (Lindstrøm). All meetings were recorded, and all written communication and notes were collated, resulting in a large data set capturing the process. In this talk, we will focus on three clear barriers to transformation faced by the lecturer: 1) being convinced of the research evidence for active teaching methods; 2) being convinced that the research literature was relevant to the lecturer’s specific context; and 3) as a late career researcher accustomed to being the expert, fundamentally changing his teaching approach required moving outside his comfort zone and having the courage to be a novice again

Digital video as a resource for teaching physics – A preliminary evaluation of effectiveness and some tips on how to do it better

Recent developments in digital video technologies allow video footage to be captured, edited and presented far more easily than was possible with older analogue techniques (e.g. 35mm film, VCR, etc.), making the widespread use of video in lectures a more viable possibility. Here I will discuss my recent experiences with using digital video to improve the effectiveness of examples and anecdotes in my lectures and to enhance, supplement or replace live physics demonstrations. I will include some tips on how to better use digital video as a teaching tool along with a preliminary evaluation of the success of digital video in lectures based on student feedback. Of particular note, the feedback shows that students almost always prefer live demonstrations to videos, even if the demonstration is unsuccessful or difficult to see, suggesting that digital video is not an effective ‘low-cost’ substitute for demonstrations in the teaching of physics

Using polymer electrolyte gates to set-and-freeze threshold voltage and local potential in nanowire-based devices and thermoelectrics

We use the strongly temperature-dependent ionic mobility in polymer electrolytes to 'freeze in' specific ionic charge environments around a nanowire using a local wrap-gate geometry. This enables us to set both the threshold voltage for a conventional doped substrate gate and the local disorder potential at temperatures below 200 Kelvin, which we characterize in detail by combining conductance and thermovoltage measurements with modeling. Our results demonstrate that local polymer electrolyte gates are compatible with nanowire thermoelectrics, where they offer the advantage of a very low thermal conductivity, and hold great potential towards setting the optimal operating point for solid-state cooling applications.Comment: Published in Advanced Functional Materials. Includes colour versions of figures and supplementary informatio

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Towards bioelectronic logic(Conference Presentation)

One of the critical tasks in realising a bioelectronic interface is the transduction of ion and electron signals at high fidelity, and with appropriate speed, bandwidth and signal-to-noise ratio [1]. This is a challenging task considering ions and electrons (or holes) have drastically different physics. For example, even the lightest ions (protons) have mobilities much smaller than electrons in the best semiconductors, effective masses are quite different, and at the most basic level, ions are ‘classical’ entities and electrons ‘quantum mechanical’. These considerations dictate materials and device strategies for bioelectronic interfaces alongside practical aspects such as integration and biocompatibility [2]. In my talk I will detail these ‘differences in physics’ that are pertinent to the ion-electron transduction challenge. From this analysis, I will summarise the basic categories of device architecture that are possibilities for transducing elements and give recent examples of their realisation. Ultimately, transducing elements need to be combined to create ‘bioelectronic logic’ capable of signal processing at the interface level. In this regard, I will extend the discussion past the single element concept, and discuss our recent progress in delivering all-solids-state logic circuits based upon transducing interfaces. [1] “Ion bipolar junction transistors”, K. Tybrandt, K.C. Larsson, A. Richter-Dahlfors and M. Berggren, Proc. Natl Acad. Sci., 107, 9929 (2010). [2] “Electronic and optoelectronic materials and devices inspired by nature”, P Meredith, C.J. Bettinger, M. Irimia-Vladu, A.B. Mostert and P.E. Schwenn, Reports on Progress in Physics, 76, 034501 (2013)

Single-material OECT-based flexible complementary circuits featuring polyaniline in both conducting channels

The organic electrochemical transistor (OECT) with a conjugated polymer as the active material is the elementary unit of organic bioelectronic devices. Improved functionalities, such as low power consumption, can be achieved by building complementary circuits featuring two or more OECTs. Complementary circuits commonly combine both p- and n-type transistors to reduce power draw. While p-type OECTs are readily available, n-type OECTs are less common mainly due to poor stability of the n-type active channel material in aqueous electrolyte. Here, a complementary circuit is made using a pair of OECTs having polyaniline (PANI) as the channel material in both transistors. PANI, with a finite electrochemical window accessible at voltages lower than 1 V, exhibits a peak in current versus gate voltage when used as an active channel in an OECT. The current peak has two slopes, one n-like and one p-like, which correspond to different electrochemical regimes of the same underlying conjugated polymer. The electrochemistry enables the design of a complementary circuit using only PANI as the channel material. The PANI-based circuit is shown to have excellent performance with gain of ≈7 and is transferred on a flexible biocompatible chitosan substrate with demonstrated operation in aqueous electrolyte