High Frequency Conductivity in the Quantum Hall Regime

We have measured the complex conductivity $\sigma_{xx}$ of a two-dimensional electron system in the quantum Hall regime up to frequencies of 6 GHz at electron temperatures below 100 mK. Using both its imaginary and real part we show that $\sigma_{xx}$ can be scaled to a single function for different frequencies and for all investigated transitions between plateaus in the quantum Hall effect. Additionally, the conductivity in the variable-range hopping regime is used for a direct evaluation of the localization length $\xi$. Even for large filing factor distances $\delta \nu$ from the critical point we find $\xi \propto \delta \nu^{-\gamma}$ with a scaling exponent $\gamma=2.3$

Hopping conductivity in the quantum Hall effect -- revival of universal scaling

We have measured the temperature dependence of the conductivity $\sigma_{xx}$ of a two-dimensional electron system deep into the localized regime of the quantum Hall plateau transition. Using variable-range hopping theory we are able to extract directly the localization length $\xi$ from this experiment. We use our results to study the scaling behavior of $\xi$ as a function of the filling factor distance $|\delta \nu|$ to the critical point of the transition. We find for all samples a power-law behavior $\xi\propto|\delta\nu|^{-\gamma}$ with a universal scaling exponent $\gamma = 2.3$ as proposed theoretically

Conductance fluctuations at the quantum Hall plateau transition

We analyze the conductance fluctuations observed in the quantum Hall regime for a bulk two-dimensional electron system in a Corbino geometry. We find that characteristics like the power spectral density and the temperature dependence agree well with simple expectations for universal conductance fluctuations in metals, while the observed amplitude is reduced. In addition, the dephasing length $L_\Phi \propto T^{-1/2}$, which governs the temperature dependence of the fluctuations, is surprisingly different from the scaling length $L_{sc}\propto T^{-1}$ governing the width of the quantum Hall plateau transition

Geographies of Indigenous-based Team Name and Mascot Use in American Secondary Schools

This dissertation identifies the geographic locations and characteristics of high schools using Native American nicknames and, through the examination of Indigenous-based iconography, analyzes the portrayal of Indigenous peoples in these learning environments. Primary and secondary data collected from archives, newspapers, yearbooks, directories, school websites, and fieldwork are utilized in various ways, including cartographic and quantitative analyses of 1,368 school locations and their attributes, numerous case studies highlighting concocted histories and stereotypical depictions of Native Americans, and a content analysis of Indigenous-based imagery photographed in 125 schools with predominately white student bodies. The first chapter defines the dissertation topic and provides a literature review of relevant geographic and anti-mascot literature, and Chapter Two stresses the relevance of studying team names. Chapter Three examines the Indian’s role in the development of American identity and argues that the tradition of portraying of Native Americans as “Others” in literature, art, Wild West shows, film, and television continues to this day through the use of team names and mascots. Chapter Four applies Renato Rosaldo’s concept of imperialist nostalgia, Mary Louise Pratt’s idea of anti-conquest, and other theories to explain the selection and continued use of Native American mascots. National, regional, and local geographic patterns of mascot use are mapped and analyzed in Chapter Five, and Chapter Six critiques the portrayal of diverse Native American groups as stereotypical Plains Indians, noble and ignoble savages, masculine warriors, Indian princesses, and other depictions in secondary schools. The study complements existing geographic and anti-mascot literature in three ways: by approaching the Native American mascot issue from a spatial perspective and mapping the locations and characteristics of these schools for analysis, by strengthening the anti-mascot argument by examining the practice in secondary schools, the next logical step in a movement that has focused largely on professional and collegiate sports, and by advancing knowledge in geography and in the interdisciplinary realm of antimascot literature by using textual evidence from numerous historical periods, and at several geographic scales, to emphasize how race-based ideologies become manifest on the landscape and in life through the use of iconography and ritual

Specific Heat of a Fractional Quantum Hall System

Using a time-resolved phonon absorption technique, we have measured the specific heat of a two-dimensional electron system in the fractional quantum Hall effect regime. For filling factors $\nu = 5/3, 4/3, 2/3, 3/5, 4/7, 2/5$ and 1/3 the specific heat displays a strong exponential temperature dependence in agreement with excitations across a quasi-particle gap. At filling factor $\nu = 1/2$ we were able to measure the specific heat of a composite fermion system for the first time. The observed linear temperature dependence on temperature down to $T = 0.14$ K agrees well with early predictions for a Fermi liquid of composite fermions.Comment: 4 pages, 4 figures (version is 1. resubmission: Added a paragraph to include the problems which arise by the weak temperature dependence at \nu = 1/2, updated affiliation

Terahertz response of organic amorphous systems: Experimental concerns and perspectives

We summarize practical considerations for performing highly accurate terahertz spectroscopy measurements of amorphous systems. We also outline possible perspectives for a further development of terahertz spectroscopy and its applications. This includes a deeper understanding of terahertz dynamics of amorphous systems and the development of a theory for a comparison of terahertz spectra to the results commonly obtained from well-established scattering experiments.The authors gratefully acknowledge S. Capaccioli, K. Ngai and W. Schirmacher for stimulating discussions, as well as G. Cinque for his assistance with the measurements at the Diamond Lights Source. This work was carried out with the support of the Diamond Light Source. We would also like to acknowledge the U.K. Engineering and Physical Sciences Research Council (EP/J007803/1) as well as MedImmune for funding. We thank to Bristol-Myers Squibb for the generous donation of Kollidon VA 64

Direct measurement of molecular mobility and crystallisation of amorphous pharmaceuticals using terahertz spectroscopy.

Despite much effort in the area, no comprehensive understanding of the formation and behaviour of amorphous solids has yet been achieved. This severely limits the industrial application of such materials, including drug delivery where, in principle, amorphous solids have demonstrated their great usefulness in increasing the bioavailability of poorly aqueous soluble active pharmaceutical ingredients. Terahertz time-domain spectroscopy is a relatively novel analytical technique that can be used to measure the fast molecular dynamics of molecules with high accuracy in a non-contact and non-destructive fashion. Over the past decade a number of applications for the characterisation of amorphous drug molecules and formulations have been developed and it has been demonstrated how this technique can be used to determine the onset and strength in molecular mobility that underpins the crystallisation of amorphous drugs. In this review we provide an overview of the history, fundamentals and future perspective of pharmaceutical applications related to the terahertz dynamics of amorphous systems.We would like to acknowledge the U.K. Engineering and Physical Sciences Research Council (EP/J007803/1) as well as MedImmune for funding.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.addr.2015.12.02

Transport Gap in Suspended Bilayer Graphene at Zero Magnetic Field

We report a change of three orders of magnitudes in the resistance of a suspended bilayer graphene flake which varies from a few k$\Omega$s in the high carrier density regime to several M$\Omega$s around the charge neutrality point (CNP). The corresponding transport gap is 8 meV at 0.3 K. The sequence of appearing quantum Hall plateaus at filling factor $\nu=2$ followed by $\nu=1$ suggests that the observed gap is caused by the symmetry breaking of the lowest Landau level. Investigation of the gap in a tilted magnetic field indicates that the resistance at the CNP shows a weak linear decrease for increasing total magnetic field. Those observations are in agreement with a spontaneous valley splitting at zero magnetic field followed by splitting of the spins originating from different valleys with increasing magnetic field. Both, the transport gap and $B$ field response point toward spin polarized layer antiferromagnetic state as a ground state in the bilayer graphene sample. The observed non-trivial dependence of the gap value on the normal component of $B$ suggests possible exchange mechanisms in the system.Comment: 8 pages, 5 figure

Long-run Behaviour of Multi-fidelity Bayesian Optimisation

Multi-fidelity Bayesian Optimisation (MFBO) has been shown to generally converge faster than single-fidelity Bayesian Optimisation (SFBO) (Poloczek et al. (2017)). Inspired by recent benchmark papers, we are investigating the long-run behaviour of MFBO, based on observations in the literature that it might under-perform in certain scenarios (Mikkola et al. (2023), Eggensperger et al. (2021)). An under-performance of MBFO in the long-run could significantly undermine its application to many research tasks, especially when we are not able to identify when the under-performance begins. We create a simple benchmark study, showcase empirical results and discuss scenarios and possible reasons of under-performance.Comment: NeurIPS 2023 Workshop on Adaptive Experimental Design and Active Learning in the Real Worl