Orava J.,7 Charles Babbage Road |
Orava J.,Tohoku University |
Hewak D.W.,University of Southampton |
Greer A.L.,7 Charles Babbage Road |
Greer A.L.,Tohoku University
Advanced Functional Materials | Year: 2015
Phase-change random-access memory relies on the reversible crystalline-glassy phase change in chalcogenide thin films. In this application, the speed of crystallization is critical for device performance: there is a need to combine ultrafast crystallization for switching at high temperature with high resistance to crystallization for non-volatile data retention near to room temperature. In phase-change media such as nucleation-dominated Ge2Sb2Te5, these conflicting requirements are met through the highly "fragile" nature of the temperature dependence of the viscosity of the supercooled liquid. The present study explores, using ultrafast-heating calorimetry, the equivalent temperature dependence for the growth-dominated medium Ag-In-Sb-Te. The crystallization shows (unexpectedly) Arrhenius temperature dependence over a wide intermediate temperature range. Here it is shown that this is evidence for a fragile-to-strong crossover on cooling the liquid. Such a crossover has many consequences for the interpretation and control of phase-change kinetics in chalcogenide media, helping to understand the distinction between nucleation- and growth-dominated crystallization, and offering a route to designing improved device performance. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Loudon J.C.,7 Charles Babbage Road |
Yazdi S.,Technical University of Denmark |
Kasama T.,Technical University of Denmark |
Zhigadlo N.D.,ETH Zurich |
And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2015
We demonstrate that images of flux vortices in a superconductor taken with a transmission electron microscope can be used to measure the penetration depth and coherence length in all directions at the same temperature and magnetic field. This is particularly useful for MgB2, where these quantities vary with the applied magnetic field and values are difficult to obtain at low field or in the c direction. We obtained images of flux vortices from a MgB2 single crystal cut in the ac plane by focused ion beam milling and tilted to 45? with respect to the electron beam about the crystallographic a axis. A new method was developed to simulate these images that accounted for vortices with a nonzero core in a thin, anisotropic superconductor and a simplex algorithm was used to make a quantitative comparison between the images and simulations to measure the penetration depths and coherence lengths. This gave penetration depths Λab=100±35 nm and Λc=120±15 nm at 10.8 K in a field of 4.8 mT. The large error in Λab is a consequence of tilting the sample about a and had it been tilted about c, the errors on Λab and Λc would be reversed. Thus obtaining the most precise values requires taking images of the flux lattice with the sample tilted in more than one direction. In a previous paper [J. C. Loudon, Phys. Rev. B 87, 144515 (2013)PRBMDO1098-012110.1103/PhysRevB.87.144515], we obtained a more precise value for Λab using a sample cut in the ab plane. Using this value gives Λab=107±8 nm, Λc=120±15 nm, ξab=39±11 nm, and ξc=35±10 nm, which agree well with measurements made using other techniques. The experiment required two days to conduct and does not require large-scale facilities. It was performed on a very small sample, 30×15 μm and 200-nm thick, so this method could prove useful for superconductors where only small single crystals are available, as is the case for some iron-based superconductors. © 2015 American Physical Society.
Li W.,University of Nottingham |
Coulson J.,University of Nottingham |
Marrow P.,University of Nottingham |
Smith R.J.,University of Nottingham |
And 3 more authors.
Journal of Physics: Conference Series | Year: 2016
Spatially resolved acoustic spectroscopy (SRAS) is a laser ultrasonic technique that shows qualitative contrast between grains of different orientation, illustrating the sensitivity of acoustic waves to the material structure. The technique has been improved significantly on determining the full orientation of multigrain cubic metals, by comparing the measured surface acoustic wave (SAW) velocity to a pre-calculated model. In this paper we demonstrate the ability of this technique to determine the orientation of hexagonal structure crystals, such as magnesium and titanium based alloys. Because of the isotropy of the SAW velocity on the basal plane (0001) of hexagonal crystals, the slowness surface is shown as a circle. As the plane moves from (0001) towards (1120) or towards (1010), the slowness surface gradually turns into an oval. These acoustic properties increase the difficulty in orientation determination. The orientation results of a grade 1 commercially pure titanium by SRAS is presented, with comparison with electron backscattered diffraction (EBSD) results. Due to the nature of SAWs on hexagonal structure crystals, only the results of Euler angles 1 and 2 are discussed. The error between SRAS and EBSD is also investigated.
Saxton W.O.,7 Charles Babbage Road
Ultramicroscopy | Year: 2015
This paper lists simple closed-form expressions estimating aberration coefficients (defocus, astigmatism, three-fold astigmatism, coma / misalignment, spherical aberration) on the basis of image shift or diffractogram shape measurements as a function of injected beam tilt. Simple estimators are given for a large number of injected tilt configurations, optimal in the sense of least-squares fitting of all the measurements, and so better than most reported previously. Standard errors are given for most, allowing different approaches to be compared. Special attention is given to the measurement of the spherical aberration, for which several simple procedures are given, and the effect of foreknowledge of this on other aberration estimates is noted. Details and optimal expressions are also given for a new and simple method of analysis, requiring measurements of the diffractogram mirror axis direction only, which are simpler to make than the focus and astigmatism measurements otherwise required. © 2015.
Campbell J.J.,University of Cambridge |
Botos L.-A.,University of Cambridge |
Sargeant T.J.,University of Cambridge |
Davidenko N.,7 Charles Babbage Road |
And 2 more authors.
Integrative Biology (United Kingdom) | Year: 2014
Involution is a process whereby the mammary gland undergoes extensive tissue remodelling involving exquisitely coordinated cell death, extracellular matrix degradation and adipose tissue regeneration following the weaning of offspring. These processes are mediated in part through Jak/Stat signalling pathways, which can be deregulated in breast cancer. Synthetic in vitro analogues of the breast could become important tools for studying tumorigenic processes, or as personalized drug discovery platforms and predictors of therapeutic response. Ideally, such models should support 3D neo-tissue formation, so as to recapitulate physiological organ function, and be compatible with high-throughput screening methodologies. We have combined cell lines of epithelial, stromal and immunological origin within engineered porous collagen/hyaluronic acid matrices, demonstrating 3D-specific molecular signatures. Furthermore seeded cells form mammary-like branched tissues, with lobuloalveolar structures that undergo inducible involution phenotypes reminiscent of the native gland under hormonal/cytokine regulation. We confirm that autophagy is mediated within differentiated mammary epithelial cells in a Stat-dependent manner at early time points following the removal of a prolactin stimulus (H/WD). In addition, epithelial cells express markers of an M2 macrophage lineage under H/WD, a process that is attenuated with the introduction of the monocyte/macrophage cell line RAW 264.7. Thus, such 3D models are suitable platforms for studying cell-cell interactions and cell death mechanisms in relation to cancer. © 2014 the Partner Organisations 2014.
Reid B.P.L.,University of Oxford |
Kocher C.,University of Oxford |
Kocher C.,University of Konstanz |
Zhu T.,7 Charles Babbage Road |
And 5 more authors.
Applied Physics Letters | Year: 2014
Experimental observation of Rabi rotations between an exciton excited state and the crystal ground state in a single non-polar InGaN quantum dot is presented. The exciton excited state energy is determined by photoluminescence excitation spectroscopy using two-photon excitation from a pulsed laser. The population of the exciton excited state is seen to undergo power dependent damped Rabi oscillations. © 2014 AIP Publishing LLC.
Hasan H.S.,University of Technology, Iraq |
Peet M.J.,7 Charles Babbage Road |
Avettand-Fenoel M.-N.,CNRS Materials and Transformations Unit of UMET |
Bhadeshia H.K.D.H.,7 Charles Babbage Road
Materials Science and Engineering A | Year: 2014
The tensile properties of a nanostructured carbide-free bainitic steel formed at 200-250. °C are compared against those after tempering sufficiently to remove the retained austenite. Although significant ductility is observed following tempering, a comparison of tempered and untempered samples shows that it is in fact reduced when a comparison is made at identical strength. The shape of the stress-strain curves shows clear evidence that the capacity for work hardening is reduced with the loss of austenite. The nanostructure of the steel transformed at 250. °C is examined by transmission electron microscopy, to compare the as-transformed to the tempered structure. In this case after tempering at 500. °C the energy absorbed during the tensile test is lower, due to the lower strength. Reduction of strength is caused by the slight coarsening of the bainite plates, and lower dislocation density after tempering. Considering the formation of carbide particles in high strength steel, impressive ductility is exhibited even in the tempered condition. © 2014 Elsevier B.V.