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Viewing upcoming talks containing the keyword: 4
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Cond Mat Seminars
Andreev spin qubits in multichannel Rashba nanowires
Speaker: Sunghun Park (Universidad Autonoma de Madrid)
A Josephson junction is a weak link between two superconductors. Andreev bound states (ABSs) in Josephson junctions are nowadays extensively studied both theoretically and experimentally because of the nontrivial combined effect of Rashba spin-orbit coupling and a Zeeman field on ABSs, which gives rise to Majorana states, self-conjugate zero-energy ABSs [1].In this talk, we theoretically analyze the spin structure of ABSs in superconductor-nanowire-superconductor Josephson junctions where the nanowire possesses multichannel structure, Rashba spin-orbit coupling, and Zeeman fields [2]. We show that these ingredients lead to spin-split ABSs, and enable transition between them by microwave irradiation. These features may provide a way for the Andreev spin qubit manipulation.
[1] Y. Oreg, G. Refael, and F. von Oppen, PRL 105, 177002 (2010)[2] S. Park and A. Levy Yeyati, PRB 96, 125416 (2017)
On: October 4, 2017 From: 12h00 To: 13h00
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EaStCHEM Colloquia
Modifications of ZIFs – role of guest molecules, pressure and temperature
Speaker: Simon Redfern (Cambridge)
Zeolitic imidazolate frameworks (ZIFs) are a class of metal organic frameworks that have structural similarities to aluminosilicate zeolites but show much greater framework flexibility. By tuning these structures selective guest incorporation can be achieved. In the case of ZIF -7, we have used neutron and synchrotron X -ray methods to understand the relationship between structural transformations of the host framework and CO2 incorporation. The potential for recycle CO2 capture and release from this material is remarkable, and further applications of these materials, from ferroelectric properties to their potential for drug delivery, are gaining wider appreciation. Here, I discuss the structural response of these materials to external and interna l pressure, to temperature, and to guest molecule incorporation. Download PDFOn: October 11, 2017 From: 15h30 To: 16h30
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Cond Mat Seminars
The Effect of Losses on the Dicke Superradiance Transition
Speaker: Peter Kirton (University of St Andrews)
When the effective coupling between matter and light is large enough, a phase transition to a "superradiant'' state can occur. The archetypal model for this is the Dicke model, describing N emitters coupled to a single photon mode. Such a transition has been realised in a driven system, with ultracold atoms in an optical cavity [1]. These experiments prompt questions about the effects of loss and dissipation on the phase transition. While the effect of photon losses is well understood, there are conflicting statements in the literature about the effects of losses on the emitters [2,3]. We show that the presence of pure dephasing can kill the superradiance transition, but it can be restored by loss terms [4]. The fact that adding atom loss can restore a phase transition to an ordered state is a surprising result, which provides a crucial example where the precise form of dissipation dramatically changes the collective behaviour. To solve this problem, we present a method to give an exact solution for the N emitter problem which scales polynomially with N. By comparing finite size scaling of our exact solution to a cumulant expansion, we confirm the destruction and restoration of the superradiance transition holds in the thermodynamic limit. We also show how, by adding incoherent pumping processes we are able to distinguish between regions of phase diagram where superradiance occurs and those where coherence is generated by regular lasing. This leads to regions of the phase diagram where the mean field equations show chaotic behaviour.[1] K. Baumann, C. Guerlin, F. Brennecke, and T. Esslinger, Nature 464, 1301 (2010)[2] M. H. Szymanska, P. B. Littlewood, and B. D. Simons, Phys. Rev. A 68, 013818 (2003)[3] E. G. Dalla Torre, Y. Shchadilova, E. Y. Wilner, M. D. Lukin, and E. Demler, Phys. Rev. A 94, 061802 (2016).[4] P. Kirton and J. Keeling, Phys. Rev. Lett. 118, 123602 (2017)
On: October 11, 2017 From: 12h00 To: 13h00
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EaStCHEM Colloquia
Shaping the density of states in organic hole conductors with switchable traps (Joint EaStCHEM/Physics Colloquium)
Speaker: Klaus Meerholz (Cologne)
[1] R. Shallcross, P. Körner, E. Maibach, A. Köhnen, K. Meerholz, Adv. Mater. 2013, 25, 4807 -4813.
Shaping the density of states in organic hole conductors with switchable traps
Abstract:
Despite the success of organic materials in applications such as OLED, a fundamental understanding of charge transport is far from being complete. Partly this is due to the difficulty of probing the density of states (DOS) in organic semiconductors directly. Thus, it is a challenge to differentiate between intrinsic limitations of the material under investigation and “extrinsic” effects like structural or electronic defects, traps stemming from impurities during synthesis or device operation. To better understand the influence of traps on the charge transport properties of organic semiconductors our strategy is to design the DOS of the transport layer on purpose. We synthesized a number of crosslinkable hole transport molecules with identical core structure and varying side groups to tune the ionization potential (IP, “HOMO energy”) between 5.1 eV and 5.8 eV. Devices were prepared by mixing variable amounts of a molecule with low IP (trap) into a host molecule with high IP. This approach allows to deliberately change th e depth of the trap and shape of the combined DOS more or less freely.
The analysis of current -voltage characteristics and impedance spectroscopy allows to identify the crossover from space -charge limited to trap limited transport and to test charge trans port models. Although, intentional mixing of traps serves it purpose, the device in its trap -free state cannot be recovered. To broaden the scope of charge transport investigations we show that the use of switchable traps has some interesting benefits. We present a unique hole trap, dithienlyethene (DTE), which can be photochemically switched [1]. DTE features two stable isomers that differ in HOMO energy. In a hole -conducting matrix with a certain HOMO energy, one DTE isomer acts as a hole trap, while the other isomer has a negligible influence on charge transport. The energetic trap depth is varied by blending DTE with various TPD -type hole conductors featuring different HOMO levels. In this way the amount of traps can be controlled and the initial, native , state of the device can be restored. Download PDFOn: October 13, 2017 From: 10h00 To: 11h00
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History of Mathematics
Mathematics in Biology – Has D’Arcy Thompson been vindicated?
Speaker: Evelyn Fox Keller (Massachusetts Institute of Technology)
The Fauvel Lecture, supported by the British Society for the History of Mathematics
Free, but booking essentialÂ
Evelyn Fox Keller is one of the most internationally respected historians of science. A physicist, author and feminist, she is currently Professor Emerita of the History & Philosophy of Science at MIT. Beginning her career in theoretical physics, she moved on to work in molecular biology before becoming renowned for her work as a feminist critic of science. Her books include Keywords in Evolutionary Biology (1998), The Century of the Gene (2000) and Making Sense of Life: Explaining Biological Development with Models, Metaphors, and Machines (2002). The latter has a particular focus on mathematical biology and in her lecture she will discuss the legacy of D’Arcy Thompson’s work.
This free public lecture is associated with the 3-day conference celebrating the centenary of D'Arcy Thompson's On Growth and Form. Full programme and registration
On: October 14, 2017 From: 18h30 To: 19h30
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EaStCHEM Colloquia
Dissecting Interactions In Solution
Speaker: Scott Cockroft (Edinburgh)
DISSECTING INTERACTI ONS IN SOLUTION Scott L. COCKROFT EaStCHEM School of Chemistry, University of Edinburgh, Edinburgh, UK. scott.cockroft@ed.ac.uk , http://homepages.ed.ac.uk/scockrof/ research .php
The study of non -covalent interactions is often complicated by their fundamentally weak nature and the role of solvent effects. This talk will illustrate how synthetic molecular torsion balances [1]-[3][5]-[8] and supramolecular complexes [4] can be used to quantify difficult -to-study interactions includi ng van der Waals dispersion forces, [2][3] aromatic stacking, [4] hydrogen -bonding cooperativity, [5] solvent ,[6]-[8] and solvophobic effects. [8] I will show how the information gleaned from such studies can be used to develop our theoretical understanding of molecular recognition phenomenon.
References : [1] I. K. Mati, S. L. Cockroft , Chem. Soc. Rev., 2010 , 39, 4195 -205 . [2] L. Yang, C. Adam, G. S. Nichol s, S. L. Cockroft , Nature Chem ., 2013 , 5(12),1006 -10. [3] C. Adam, L. Yang , S. L. Cockroft , Angew. Chem. Int. Ed., 2015 , 54(4), 1164 -7. [4] L. Yang, J. Brazier, T. Hubbard, D. M. Rogers & S. L. Cockroft , Angew. Chem. Int. Ed ., 2016 , 55(3), 912 -6. [5] N. Dominelli -Whiteley, J. J. Brown, K. B. Muchowska, I. K. Mati, C. Adam, T. A. Hubbard, A. Elmi, A. J. Brown, I. A. W. Bell, S. L. Co ckroft , Angew. Chem. Int. Ed., 2017 , DOI: 10.1002/anie.201703757 . [6] K. B. Muchowska, C. Adam, I. K. Mati , S. L. Cockroft , J. Am. Chem. Soc ., 2013 , 135, 9976 -79 . [7] I. K. Mati, C. Adam , S. L. Cockroft , Chem. Sci. , 2013 , 4, 3965 -72 . [8] L. Yang, C. Adam , S. L. Cockroft , J. Am. Chem. Soc. , 2015 , 137(32), 10084 -7. Download PDFOn: October 18, 2017 From: 15h30 To: 16h30
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