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Viewing upcoming talks containing the keyword: 3
Cond Mat Seminars
A Scanning Single-Electron Transistor Array Microscope Probes the Hall Potential Profile in the Fractional Quantum Hall Effect
Speaker: Andreas Gauß (Max Planck Institute for Solid State Research Stuttgart)
The Hall potential distribution and thus the current distribution in integer quantum Hall samples have been measured since 1999  by an electrostatic potential probing scanning force microscope, limited to temperatures above 1.4 K. The results contradict the widely used edge-state picture and have given a new microscopic picture of the QHE . Similar measurements on the fractional quantum Hall effect require much lower temperature and a strongly enhanced sensitivity. Thus we have built a scanning SET microscope using a 1D array of eight single-electron transistors (SET), acting as local electrometers . The instrument is working below 40 mK electron temperature and up to applied magnetic fields of 18 T.We present Hall potential distribution measurements in the integer quantum Hall regime. The data confirms our previous results  obtained at 1.4 K. Further we present novel results for the Hall potential profiles and therefore the current distribution in the ν=2/3 fractional quantum Hall state. P. Weitz, E. Ahlswede, J. Weis, K. v. Klitzing, and K. Eberl, Physica E 6, 1 (2000). J. Weis, K. von Klitzing, Phil. Trans. of the Royal Society A 369, 3954 (2011) J. Weber, J. Weis, M. Hauser, and K. v. Klitzing, Nanotechnology 19, 37 (2008).
On: April 17, 2019 From: 13h00 To: 14h00View talk
History of Mathematics
“The Etherealisation of Common Sense”? Late Victorian Opposition to Maxwell’s Mathematics of Measurement
Speaker: Daniel Jon Mitchell (Institute for Theoretical Physics and Cosmology, RWTH Aachen)
The late nineteenth century gradually witnessed a liberalization of the kinds of mathematical object and forms of mathematical reasoning permissible in physical argumentation. The spread of an “algebraic” mode of mathematical intelligibility into elementary arithmetical pedagogy, experimental physics, and fields of physical practice like telegraphic engineering was nonetheless slow and difficult. Developed by leading mathematicians from the 1830s onwards, the algebraic mode permitted mathematical operations between non-numerical entities, which, according to James Clerk Maxwell, were indispensable in articulating the relationship between derived and fundamental units in a so-called “absolute” system of measurement.
A watershed event was a clash that took place during 1878 between J. D. Everett, an acolyte of Maxwell’s, and James Thomson, Lord Kelvin’s brother, over the meaning and algebraic manipulation of the “dimensional” formulae invented by Maxwell for this purpose. Buoyed by dramatic changes to science education in Britain during the 1870s, and the rising economic importance of electrical science and technology during the 1880s, this clash precipitated the emergence of rival “Maxwellian” and “Thomsonian” approaches towards interpreting and applying “dimensional” equations.
What at first looks like a dispute over a seemingly esoteric mathematical tool for unit conversion turns out to concern Everett’s break with arithmetical algebra in the representation and manipulation of physical quantities. This move prompted a vigorous rebuttal from Thomsonian defenders of an orthodox “arithmetical empiricism,” who, for epistemological, semantic, and pedagogical reasons, insisted upon retaining physical correlates for mathematical entities and operations. Their resolute stance illustrates a deep gulf in terms of conceptions of mathematical intelligibility in Victorian Britain between leading mathematicians and those who employed mathematics to further experimental or practical goals.
On: April 23, 2019 From: 16h00 To: 17h30View talk
Speaker: Graham Smith (University of St Andrews)
In this talk I will describe our recent work on mm-wave antennas to create specialised optical output beams from single mode waveguide. I will show how combinations of HE1n modes, Gaussian beams (LG0n modes), and Airy beams can all be accurately synthesized using mode transformation techniques over wide bandwidths and how they can be used in a wide variety of metrology, instrumentation, radar and radiometric applications. In particular I will show how it is possible to continuously refocus a Gaussian beam without using mirrors or lenses, as an alternative to conventional quasi-optical techniques.
On: April 23, 2019 From: 13h00 To: 14h00View talk
RSC Award Lectures
Speaker: Artem Bakulin (Imperial College London)
Ultrafast photocurrent spectroscopy of molecular electronic devices Artem A. Bakulin Department of Chemistry, Imperial College London London SW7 2AZ, UK firstname.lastname@example.org
I will present an old but well -forgotten advanced time -resolved spectroscopic technique based on the ultrafast optical control of molecular excited states followed by photocurrent detection. The method is very useful to study electronic dynamics in molecular semiconducting materials and organic optoelectronic devices , like plastic solar cells, under working conditions. 1 Using th is method , we were able to identify and track in time the precursor and charge trapping states in a variety of material systems, including polymer -fullerene blends, polyme r-oxide hybrids, colloidal quantum dots and perovskites. We show that a careful management of charge and energy dynamics in the excited st ate can substantially suppress the loss pathways and improve the device performance.
1 Bakulin A.A., Silva C., Ve lla E., Ultrafast Spectroscopy with Photocurrent Detection: Watching Excitonic Optoelectronic Systems at Work, J. Phys. Chem. Lett. 7, 250 (2016) Download PDF
On: April 24, 2019 From: 14h00 To: 15h00View talk
RSC Award Lectures
Speaker: Daniele Leonori (Manchester)
Photoinduced Assembly of C -N Bonds Dr Daniele Leonori School of Chemistry, University of Manchester, Manchester M13 9PL, UK email@example.com Nitrogen -containing compounds are a privileged class of molecules, which have applications in medicines, agrochemicals, dyes and materials. This relevance makes the construction of C –N bonds an extremely active area of research. Nitrogen -radicals are versa tile synthetic intermediates that can engage in a broad range of chemical reactions. 1 However, the difficulties associated with their generation have somewhat thwarted their use in synthetic chemistry. Development of Photoinduced Radical -Transposition Reactions We have developed a class of easy -to -make oximes and hyroxy -amides that upon photoredox oxidation enable access to iminyl and amidyl radicals. 2,3 These species have been used in radical transposition reactions for the site -selective functionaliza tion of unactivated sp 3-carbons. These strategies have been applied to the deconstruction –functionalization of complex steroids (radical ring -opening) 2 and to the preparation of unnatural aminoacids (1,5 -HAT) .3 Development of Photoinduced Aromatic C –H Ami nation Reactions Aminated aromatics are a widespread motif in high -value products. In general, these structures are assemble d by Pd or Cu -catalysed cross -couplings between aryl halides/organoboron and amines. We have developed an umpolung approach where electrophilic amidyl and aminyl radicals are generated by photoredox reduction of electron poor N-aryloxy -amides and -amines. 4,5 These radical specie s undergo highly selective addition to a broad range of electron rich aromatics thus enabling direct C – H amination. References 1. Review: S. Z. Zard Chem. Soc. Rev. 2008 , 37 , 1603. 2. Angew. Chem. Int. Ed. 2018 , 57 , 744. 3. Angew. Chem. Int. Ed. 2018 , 57 , 12495. 4. J. Am. Chem. Soc. 2016 , 138 , 8092. 5. Angew. Chem. Int. Ed. 2017 , 56 , 14948. Download PDF
On: May 1, 2019 From: 14h00 To: 15h00View talk
Cond Mat Seminars
Speaker: Henry Legg (University of Cologne)
In contrast to usual metals, where quasiparticles obey a Schrödinger-like Hamiltonian, a large number of materials such as graphene, topological insulators, and even helium-3 have low-energy excitations well described by a massless Dirac Hamiltonian. These "Dirac materials" often have unique responses to impurities and symmetry breaking perturbations - such as a magnetic field -, for example, electrons on the surface of a topological insulator are protected from backscattering due to spin-momentum locking. In this talk I will discuss three phenomena in 2D and 3D Dirac materials both theoretically and in comparison to experiments:(1) I will show how spin-momentum locking can be probed using quasiparticle interference (QPI). A comparison of theory and experiment enables a detailed characterisation of a topological insulator surface.(2) Protection from backscattering can be lifted by a magnetic field parallel to the topological insulator surface. I will demonstrate that such a field retains the Dirac physics of the surface but enables backscattering predominantly in the direction parallel to the magnetic field. The resulting anisotropy of magnetoresistance is a sensitive probe of this loss of topological protection. We compare our theory with transport measurements on bulk-insulating Bi2−xSbxTe3, where we find very good agreement, in particular as a function of gate voltage.(3) Crystal symmetries often mean that 3D Dirac materials contain multiple Dirac nodes within their Brillouin zone. In a large magnetic field the Dirac quasiparticles form effective 1D channels parallel to the magnetic field, meaning only the dispersion parallel to the field is retained. I will argue that a positive magnetoresistance occurs when the magnetic field is orientated in a direction perpendicular to a vector connecting a pair of Dirac nodes i.e. when the two nodes are located at the same location in momentum space projected parallel to the field. This transport mechanism allows one to "spectroscopically" map the bulk electronic structure of a 3D Dirac or Weyl semi-metal with multiple nodes. Our theory will be compared to experiments on the Dirac semi-metal Pb1-xSnxSe.Further details on (2) can be found in Nat. Comms. vol. 8, 1340 (2017). Manuscripts for (1) & (3) are in preparation.
On: May 1, 2019 From: 13h00 To: 14h00View talk