Application of Finite Strain Landau Theory To High Pressure Phase Transitions

In this paper we explain how to set up what is in fact the only possible consistent construction scheme for a Landau theory of high pressure phase transitions that systematically allows to take into account elastic nonlinearities. We also show how to incorporate available information on the pressure dependence of elastic constants taken from experiment or simulation. We apply our new theory to the example of the high pressure cubic-tetragonal phase transition in Strontium Titanate, a model perovskite that has played a central role in the development of the theory of structural phase transitions. Armed with pressure dependent elastic constants calculated by density functional theory, we give a both qualitatively as well as quantitatively satisfying description of recent high precision experimental data. Our nonlinear theory also allows to predict a number of additional elastic transition anomalies that are accessible to experiment.Comment: submitted to Phys. Rev. Let

Contact resistance and overlapping capacitance in flexible sub-micron long oxide thin-film transistors for above 100 MHz operation

In recent years new forms of electronic devices such as electronic papers, flexible displays, epidermal sensors, and smart textiles have become reality. Thin-film transistors (TFTs) are the basic blocks of the circuits used in such devices and need to operate above 100 MHz to efficiently treat signals in RF systems and address pixels in high resolution displays. Beyond the choice of the semiconductor, i.e., silicon, graphene, organics, or amorphous oxides, the junctionless nature of TFTs and its geometry imply some limitations which become evident and important in devices with scaled channel length. Furthermore, the mechanical instability of flexible substrates limits the feature size of flexible TFTs. Contact resistance and overlapping capacitance are two parasitic effects which limit the transit frequency of transistors. They are often considered independent, while a deeper analysis of TFTs geometry imposes to handle them together; in fact, they both depend on the overlapping length (LOV) between source/drain and the gate contacts. Here, we conduct a quantitative analysis based on a large number of flexible ultra-scaled IGZO TFTs. Devices with three different values of overlap length and channel length down to 0.5 μm are fabricated to experimentally investigate the scaling behavior of the transit frequency. Contact resistance and overlapping capacitance depend in opposite ways on LOV. These findings establish routes for the optimization of the dimension of source/drain contact pads and suggest design guidelines to achieve megahertz operation in flexible IGZO TFTs and circuits

HMcode-2020::Improved modelling of non-linear cosmological power spectra with baryonic feedback

We present an updated version of the HMcode augmented halo model that can be used to make accurate predictions of the non-linear matter power spectrum over a wide range of cosmologies. Major improvements include modelling of BAO damping in the power spectrum and an updated treatment of massive neutrinos. We fit our model to simulated power spectra and show that we can match the results with an RMS error of 2.5 per cent across a range of cosmologies, scales $k < 10\,h\mathrm{Mpc}^{-1}$, and redshifts $z<2$. The error rarely exceeds 5 per cent and never exceeds 16 per cent. The worst-case errors occur at $z\simeq2$, or for cosmologies with unusual dark-energy equations of state. This represents a significant improvement over previous versions of HMcode, and over other popular fitting functions, particularly for massive-neutrino cosmologies with high neutrino mass. We also present a simple halo model that can be used to model the impact of baryonic feedback on the power spectrum. This six-parameter physical model includes gas expulsion by AGN feedback and encapsulates star formation. By comparing this model to data from hydrodynamical simulations we demonstrate that the power spectrum response to feedback is matched at the $<1$ per cent level for $z<1$ and $k<20\,h\mathrm{Mpc}^{-1}$. We also present a single-parameter variant of this model, parametrized in terms of feedback strength, which is only slightly less accurate. We make code available for our non-linear and baryon models at https://github.com/alexander-mead/HMcode and it is also available within CAMB and soon within CLASS.Comment: 17 pages, 5 figures, 4 appendices; v2 - matches accepted version, new appendix with comparisons between HMcode and 6 different emulator

Microcanonical Determination of the Interface Tension of Flat and Curved Interfaces from Monte Carlo Simulations

The investigation of phase coexistence in systems with multi-component order parameters in finite systems is discussed, and as a generic example, Monte Carlo simulations of the two-dimensional q-state Potts model (q=30) on LxL square lattices (40<=L<=100) are presented. It is shown that the microcanonical ensemble is well-suited both to find the precise location of the first order phase transition and to obtain an accurate estimate for the interfacial free energy between coexisting ordered and disordered phases. For this purpose, a microcanonical version of the heatbath algorithm is implemented. The finite size behaviour of the loop in the curve describing the inverse temperature versus energy density is discussed, emphasizing that the extrema do not have the meaning of van der Waals-like "spinodal points" separating metastable from unstable states, but rather describe the onset of heterophase states: droplet/bubble evaporation/condensation transitions. Thus all parts of these loops, including the parts that correspond to a negative specific heat, describe phase coexistence in full thermal equilibrium. However, the estimates for the curvature-dependent interface tension of the droplets and bubbles suffer from unexpected and unexplained large finite size effects which need further study.Comment: submitted to special issue "Liquid Matter" of Journal of Physics C: Condensed Matter on occasion of the 8th Liquid Matter Conference held Sept. 6-10, 2011 in Vienna, Austri

Improvement of contact resistance in flexible a-IGZO thin-film transistors by CF4/O2 plasma treatment

In this work, we analyze the effect of CF4/O2 plasma treatment on the contact interface between the amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) semiconductor and Titanium-Gold electrodes. First, the influence of CF4/O2 plasma treatment is evaluated using transmission line structures and compared to pure O2 and CF4 plasma, resulting in a reduction of the contact resistance RC by a factor of 24.2 compared to untreated interfaces. Subsequently, the CF4/O2 plasma treatment is integrated in the a-IGZO thin-film transistor (TFT) fabrication process flow. We achieve a reduction of the gate bias dependent RC by a factor up to 13.4, which results in an increased current drive capability. Combined with an associated channel length reduction, the effective linear field-effect mobility is increased by up to 74.6% for the CF4/O2 plasma treated TFTs compared to untreated reference devices

Influence of Heat-Treatment of Selective Laser Melting Products - e.g. Ti6Al4V

Usually additive manufactured metal parts are showing a different mechanical behavior compared to conventionally produced parts used the same material. Apart from process-related macroscopic part imperfections (pores, surface roughness, etc.) the microstructure has a decisive influence on the mechanical properties of the materials. Thus, in order to optimize mechanical properties of metal parts a heat treatment for changing microstructures is routinely applied in most production lines to meet the product requirements. By means of the Titanium alloy Ti6Al4V the optimization of the static- and the fracture mechanical behavior by changing the microstructure with a heat treatment after the SLM process is discussed on the present work.Mechanical Engineerin

Finite strain Landau theory of high pressure phase transformations

The properties of materials near structural phase transitions are often successfully described in the framework of Landau theory. While the focus is usually on phase transitions, which are induced by temperature changes approaching a critical temperature T-c, here we will discuss structural phase transformations driven by high hydrostatic pressure, as they are of major importance for understanding processes in the interior of the earth. Since at very high pressures the deformations of a material are generally very large, one needs to apply a fully nonlinear description taking physical as well as geometrical nonlinearities (finite strains) into account. In particular it is necessary to retune conventional Landau theory to describe such phase transitions. In Troster et al (2002 Phys. Rev. Lett. 88 55503) we constructed a Landau-type free energy based on an order parameter part, an order parameter-(finite) strain coupling and a nonlinear elastic term. This model provides an excellent and efficient framework for the systematic study of phase transformations for a wide range of materials up to ultrahigh pressures

Geometry-based tunability enhancement of flexible thin-film varactors

In this letter, flexible voltage-controlled capacitors (varactors) based on an amorphous Indium–Gallium–Zinc–Oxide (a-IGZO) semiconductor are presented. Two different varactor designs and their influence on the capacitance tuning characteristics are investigated. The first design consists of a top electrode finger structure which yields a maximum capacitance tunability of 6.9 at 10 kHz. Second, a novel interdigitated varactor structure results in a maximum tunability of 93.7 at 100 kHz. The design- and frequency-dependencies of the devices are evaluated through C–V measurements. Furthermore, we show bending stability of the devices down to a tensile radius of 6 mm without altering the performance. Finally, a varactor is combined with a thin-film resistor to demonstrate a tunable RC-circuit for impedance matching and low-pass filtering applications. The device fabrication flow and material stack are compatible with standard flexible thin-film transistor fabrication which enables parallel implementation of analog or logic circuitry and varactor devices