News and events
Landau Seminar Series 2017-18
Wednesday 06 December 2017 - 14.00, N1.12 Haslegrave Building
According to the current cosmological model a recombination of hydrogen and helium occurred approximately 300000 years after the Big Bang. The primordial H and He recombination lines that had appeared at this stage should be visible now redshifted to the Rayleigh-Jeans part of the Cosmic Microwave Background (CMB) blackbody spectrum. Assuming that T_cmb is 2.726K, primordial recombination lines are expected to have a magnitude about 10-30 nK and will manifest themselves as the tiny low-contrast "ripples" on top of the smooth CMB Planck spectrum.
When trying to observe primordial recombination lines one has to take into account not only the foreground emission of different origin (our own Galaxy, the distant clusters of galaxies) but also RFI and the numerous systematic effects.
Wednesday 29 November 2017 - 11.00, SCH.1.05 Schofield Building
The emergence of single quantum emitters in layered transition metal dichalcogenide semiconductors offers new opportunities to construct a scalable quantum architecture with a coherent light-matter inter- face. Here I will present results taking steps in this direction. First, using nanoscale strain engineering, we deterministically achieve a two-dimensional lattice of quantum emitters in an atomically thin semi- conductor. We create point-like strain perturbations in mono-and bi- layer WSe2 which locally modify the band-gap, leading to efficient funnelling of excitons towards isolated strain-tuned quantum emitters. These emitters exhibit high-purity single photon emission that is stable and bright, yielding detected count rates up to 3 MHz. Next, we perform time-resolved photoluminescence, resonance fluorescence, and high-resolution photoluminescence excitation spectroscopy of these isolated, localized 2D excitons to characterize their dephasing mechanisms and unravel their origin. Finally, I will provide an outlook on investigations of the spin and valley coherence and prospects for integrated photonic chips incorporating quantum emitters in atomically-thin TMD semiconductors.
Wednesday 22 November 2017 - 14.00, SMB104 Stewart Mason Building
ISIS Neutron and Muon Facility, Harwell Science and Innovation Campus, Science and Technologies Facility Council, Didcot, Oxon., United Kingdom
Quantifying the interfacial behaviour of nanoscale magnetic systems is key to understanding a wide range of solid state physics phenomena. Moreover, such behaviour often results in useful functionality that can be exploited in devices. Spin diffusion lengths in ferromagnetic metals are typically much less than 100nm. This lengthscale gives an indication of the resolution required to spatially resolve interfacial physics such as: proximity magnetism, the spin-Seebeck effect, doped topological insulators and superconducting spintronics. Providing a quantitative understanding of such phenomena presents a significant experimental challenge. The interfaces of interest are often buried and not readily accessible to more conventional macroscopic techniques. Neutron techniques provide unparalleled quantitative access to such systems through the exploitation of polarized neutrons in reflection and small angle scattering geometries.
In this talk we shall briefly introduce the capabilities of the current suite of polarized instrumentation at ISIS for nanoscience research and highlight some of the science it has enabled. Examples will be taken from recent studies of topological matter [1-3], materials for magnonics , synthetic antiferromagnets  and thin film rare-earths  for spin triplet generation.
 C. Pappas et al. Phys. Rev. Lett. 119, 47203 (2017).
 N.A. Porter et al. Phys. Rev. B 92, 144402 (2015).
 L.J. Collins-McIntyre et al. Europhys. Lett. 107, 57009 (2014).
 A. Mitra et al. Sci. Rep. 7, 11774 (2017). J.F.K. Cooper Phys. Rev. B 96, 104404 (2017).
 A. Fernández-Pacheco, A. et al. Adv. Mater. Interfaces 3, 1600097 (2016).
 J.D.S. Witt et al. Sci. Rep. 6, 39021 (2016).
Wednesday 08 November 2017 - 14.00, SMB104 Stewart Mason Building
Deceptively Complex: Understanding, Controlling and Exploiting Stochastic Behaviour in Pseudo-1D Magnetic Systems, by Dr Tom Hayward, University of Sheffield
The behaviour of domain walls (DWs) in soft ferromagnetic nanowires has been a topic of intense research since proposals for using DWs to represent data in non-volatile and solid state logic  and memory devices  were made around the turn of the century. These applications were made attractive by the apparent pseudo-1D nature of the nanowires, with the magnetisation of magnetic domains strongly favouring alignment along their length, thus offering a binary degree of freedom for encoding data, and the high mobility of DWs allowing data to be propagated through the nanowires to perform read and write operations. However, despite their apparent technological potential practical nanowire devices have yet to be realised because, while appearing simple, they exhibit highly complex, stochastic switching behaviours, that make it almost impossible to predict how they will behave from one operation to the next .
In this talk I first will present the of results experimental measurements and numerical simulations that explain why these apparently simple bistable magnetic systems exhibit such phenomenally complex magnetic behaviours. I will then go on to suggest methods by which stochastic effects may be suppressed, thus allowing the realisation of new devices. Finally, I will discuss the possibility of harnessing stochastic behaviour as a functional, rather than inhibitive phenomena, so as to create computer architectures that mimic the operation of the human brain.
 S. S. P. Parkin, M. Hayashi, and L. Thomas, Science,320, 190-194, (2008).
 D. A. Allwood, G. Xiong, C. C. Faulkner, D. Atkinson, D. Petit, and R. P. Cowburn, Science 309, 1688 (2005).
 T.J. Hayward and K.A. Omari, J. Phys. D. Appl. Phys. 50, 084006 (2017).
Wednesday 03 November 2017 - 14.00, WAV057 Wavy Top Building
Many topological phenomena first proposed and observed in the context of electrons in solids have recently found counterparts in optical and acoustic systems. In this talk I will discuss non-Abelian Berry phases that can accumulate when coherent states of light are injected into "topological guided modes" in specially-fabricated photonic waveguide arrays. These modes are photonic analogues of topological zero modes
in electronic systems. Light traveling inside spatially well-separated topological guided modes can be braided, leading to the accumulation of non-Abelian phases, which depend on the order that the guided beams are wound around each other. Notably, these effects survive the macroscopic photon occupation limit, and can be understood as wave phenomena and thus predicted directly from Maxwell's equations without resorting to quantization of light. We propose an optical interference experiment to probe this non-Abelian braiding of light directly.
Wednesday 25 October 2017 - 14.00, N1.12 Haslegrave Building
Spatial filtering of radiation from terahertz wire lasers, by Dr Ekaterina Orlova, Institute of Microwaves and Photonics, School of Electrical and Electronics Engineering, University of Leeds and Institute for Physics of Microstructures RAS, Russia University of Leeds
Abstract: Terahertz lasers are based on quantum cascade semiconductor hetero-structures (THz QCL-s) are attractive for numerous applications due to their compact size, high output power, ability to control the radiation spectrum by methods of band-gap engineering, and the possibility of continuous generation regime. However, broad application of such lasers is hindered due to peculiarities of their beam profile. Typical waveguides of THz QCL-s have a wire geometry with sub-wavelength transverse dimensions and the length much larger than the radiation wavelength (wire lasers). Sub-wavelength transverse size of laser waveguides leads to high radiation divergence. Additionally, strong intensity modulations accompanied by rapid phase shifts have been observed in the far field of such lasers . It was shown that the far-field pattern of wire lasers is formed by the interference of radiation from the longitudinal distribution of sources along the laser waveguide . Directive emission from THz QCL wire lasers has been achieved experimentally using gratings with appropriate periodicity, acting as a discrete array of phased sources . However, this approach is not robust. Indeed, the modes producing directive beam exhibit higher radiation losses leading to an increase of the radiation threshold and to suppression of directive emission by competing modes. Here we review the methods of improvement of the beam profile of wire lasers based on spatial filtering of wire laser radiation by external optical elements. These methods include application of specially designed phase plates , formation of a limited divergence beam as an image of a wire laser placed along the axis of a spherical lens , and separation of one of the image field maxima with a diaphragm .
 A. J. L. Adam, et al., Appl. Phys. Lett., 88, 151105, (2006).
 E. E. Orlova, et al., Phys. Rev. Lett., 96, 173904 (2006).
 M. I. Amanti, et al., Nature Photonics, 3, 586, (2010)
 E. E. Orlova, Phys. Scr. T162, 014006 (2014).
 E. E. Orlova, J. N. Hovenier, P.J. de Visser, J. R. Gao, Phys. Rev. A, 91, 51802(R) (2015).
 E. E. Orlova, et al. Laser Physics Letters 14, 045001 (2017).
Wednesday 18 October 2017 - 14.00, N.1.12 Haselgrave Building
Nonequilibrium dynamics and thermalisation of closed quantum systems of cold bosons, by Dr Anna Posazhennikova, Royal Holloway
Abstract: If and how an isolated quantum system thermalizes despite its unitary time evolution is a long-standing, open problem of many-body physics. We consider a Bose-Einstein condensate (BEC) trapped in a double-well potential with an initial population imbalance. We find that the system thermalizes under certain conditions. With respect to the non equilibrium dynamics we identify three regimes. After an initial regime of undamped Josephson oscillations, the subsystem of incoherent excitations or quasiparticles (QP) becomes strongly coupled to the BEC subsystem by means of a dynamically generated, parametric resonance. When the energy stored in the QP system reaches its maximum, the number of QPs becomes effectively constant, and the system enters a quasi-hydrodynamicregime where the two subsystems are weakly coupled. In this final regime the BEC acts as a grand-canonical heat reservoir for the QP system (and vice versa), resulting in thermalization. We term this mechanism dynamical bath generation (DBG). We discuss applications of our mechanism to optical lattices and realistic double-well potentials.
Thursday 12 October 2017 - 11.00, WAV011 (Wavy Top Building)
Ferroic superglasses, by Prof. Wolfgang Kleemann, Physics Department, University Duisburg-Essen, Germany
Abstract: 'Ferroic superglasses' have entered mesoscopic solid state physics in different versions - as matrix isolated nanoparticular ‘superspin glasses’  and as defect-activated intrinsic ‘ferroic glasses’ , respectively. Both undergo glassy dynamic criticality as T approaches Tg and non-ergodicity at T < Tg. While superspin glasses largely resemble atomic spin glasses , ferroic nanoregional glass states are proposed to emerge from random field-controlled nanoclustering and subsequent polar glass formation. Well-known examples are ferroelectric relaxors like PbMg1/3Nb2/3/O3  and ferroelastic martensites like Ti48.5Ni51.5 . Current approaches to modelling will be discussed.
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 S. Bedanta and W. Kleemann, J. Phys. D: Appl. Phys. 42 (2009) 013001.
 W. Kleemann and J. Dec, Phys. Rev. B 94 (2016) 174203.
 S. Sarkar, X. Ren, and K. Otsuka, Phys. Rev. Lett. 95 (2005) 205702.
Wednesday 4 October 2017 - 11.00, SCH.0.13 (Schofield Building)
Wigner function reconstruction of experimental three-qubit GHZ and W states, by Professor Todd Tilma, Tokyo Institute of Technology
Abstract: Assessing the quality of a multi-qubit state is a multi-faceted problem, It involves considering disparate aspects, and, while some of these are collective indicators such as purity and degree of entanglement, others look more in detail at the form of the correlations between subsystems. Obtaining a visualisation of these details would give at least a qualitative indication, but the density matrix, with its large number of parameters, hardly offers insights. Here we show how relying on the Wigner function of a multipartite system provides rich information on the state, both at an illustrative level, by choosing proper slicing on the phase space, and at a quantitative one, by introducing new figures of merit for the state quality. We test this concept on a GHZ and W state produced in a photonic architecture making use of the multiple degrees of freedom of two photons.