Electronic Liquid Crystal Physics of Underdoped Cuprates

Recent observations of broken symmetries have partly demystified the pseudogap phase. Here we review evidence for long-range intra-unit-cell(IUC) nematic order and its unexpectedly strong coupling to the phase of the fluctuating stripes in the pseudogap states of underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. In particular, we focus on the analysis techniques that reveal this evidence in scanning tunneling spectroscopy data, the definition of the extracted IUC nematic order parameter, and a phenomenological theory of the coupling between the IUC nematic order and the previously reported coexisting fluctuating stripes. We also present a microscopic mechanism of IUC nematic order driven by on-site and near-neighbor repulsions. Finally we discuss open questions in the context of these results.Comment: 26 pages, 10 figures. To be published in Physica

Thermal Wave Characterization of Semiconductors and Superconductors

Thermal wave technology has proven to be a very effective means for investigating the near surface region of several different materials. Although there are many methods for generating and detecting thermal waves the most desirable for quantitative NDE are the noncontact and nondamaging laser methods. When a material is excited with an intensity-modulated laser pump beam a thermal wave is generated within the near surface of the sample. Since the complex refractive index of most materials depends on temperature, the laser pump induced modulations in the local temperature of the sample will induce a corresponding modulation in the local refractive index. This variation in refractive index can in turn be detected through the modulation in the reflectance of a laser probe beam from the surface of the material [1,2]. This method is not only a highly effective method for generating and detecting thermal waves, but also permits thermal wave measurements to be performed with micron scale spatial resolution by utilizing highly focused pump and probe laser beams

Studies of Microscopic Scale Strains in Nickel Alloys Resulting from Mechanical and Chemical Forces

The nickel-based Alloy 600, also known as Inconel 600 has been found to be susceptible to stress corrosion cracking (SCC) in high temperature aqueous environments. Despite extensive research, the mechanisms by which this process occurs remain in question. It is known that SCC results from the simultaneous effects of a chemically corrosive environment and a tensile stress. Many studies have been conducted on the microscopic chemical changes associated with SCC. There have been fewer studies of the microscopic stress/strain process. The main objective of this thesis is to use the new x-ray based diffraction technique – polychromatic x-ray microdiffraction (PXM) to study changes of the microstructure and elastic/plastic deformation introduced by SCC in Alloy 600, thus leading us to an understanding of the mechanisms of SCC. More traditional techniques such as neutron diffraction and electron backscatter diffraction (EBSD) were also used in this thesis. Neutron diffraction measures information down to a depth resolution of a few mm, while EBSD is considered to be a surface measurement (1-2 μm). By comparison, PXM examines strain to a depth of several grains in alloy 600 (~ 60 μm), which is particularly appropriate for detecting information from intergranular interactions under typical stress corrosion conditions. The capability of measuring strain directions (compressive or tensile) is another advantage of PXM in SCC study. Besides the microstructure and elastic strain information, the local plastic deformation can also be examined by assessing the streaking/splitting of the Laue spots and modeling of the Laue diffraction images. In this thesis, PXM was first validated by comparing the data to those from neutron diffraction and EBSD for a simple case – uniaxially stressed tensile specimens. Then studies were carried out on C-ring samples before and after cracked by SCC. Torsional stressing of an Alloy 600 C-ring specimen results in significant tensile strain in the stress axis direction. Intergranular SCC (IGSCC) was observed in Alloy 600 C-ring specimens electrochemically corroded in a high temperature caustic solution. Tensile strains with respect to the stress axis and plastic strain could be identified in regions ahead of the crack tip. These regions iv correspond to triple junctions where crack arrest occurs and re-initiation requires a buildup of the strains. Keywords Alloy 600, Polychromatic X-ray Microdiffraction (PXM), neutron diffraction, Electron Backscatter Diffraction (EBSD), Stress Corrosion Cracking (SCC), deviatoric strain, plastic deformation, dislocatio

Tensile deformation of silver micro-wires of small thickness-to-grain-size ratios

A recent letter from us has indicated that the tensile proof strength of polycrystalline Ag micro-wires, with thicknesses in the range from 20 to 50. μm, depends strongly on the specimen size (t) and weakly on the grain size (d) when t/. d becomes smaller than about 3. In this work, we report further coupled effects of specimen size and grain size in the regime when their ratio is small. At a given grain size from 3 to 40. μm, the tensile elongation was found to decrease as the wire thickness decreases, and as the t/. d ratio gets smaller than about 3, the tensile elongation loses its dependence on the grain size. The work-hardening rate was found to scale approximately with the proof strength, and so they should be controlled by the same metallurgical factors. Transmission electron microscopy examination of the dislocation microstructures showed that in the regime where the grain size dominates strength and ductility, the dislocation density rises rapidly on deformation, but in the regime where the specimen thickness dominates strength and ductility, the dislocation density remains on the same order of magnitude as the undeformed state. The easy escape of dislocations from the specimen is thought to be the reason for the observed size effect of strength. © 2012 Elsevier B.V.postprin

Application of an electron microscope conductive mode of operation for the study of optoelectronic devices

Imperial Users onl

Novel effects of strains in graphene and other two dimensional materials

The analysis of the electronic properties of strained or lattice deformed graphene combines ideas from classical condensed matter physics, soft matter, and geometrical aspects of quantum field theory (QFT) in curved spaces. Recent theoretical and experimental work shows the influence of strains in many properties of graphene not considered before, such as electronic transport, spin-orbit coupling, the formation of Moir\'e patterns, optics, ... There is also significant evidence of anharmonic effects, which can modify the structural properties of graphene. These phenomena are not restricted to graphene, and they are being intensively studied in other two dimensional materials, such as the metallic dichalcogenides. We review here recent developments related to the role of strains in the structural and electronic properties of graphene and other two dimensional compounds.Comment: 75 pages, 15 figures, review articl

Regularities of formation of structure, texture and properties under the combined plastic deformation of the low-carbon and ultralow-carbon steels for cold press forming

The current paper reports the new solutions intended to enhance the complex of mechanical properties and tendency to the press forming of the hot-rolled low-carbon and ultralow-carbon steels with subsequent cold deformation via upsetting and torsion under hydrostatic pressure (THP).У роботі одержано нові розв’язки науково-практичної задачі, яка полягає у підвищенні комплексу механічних властивостей і схильності до штампування гарячевальцьованих низько- та ультранизьковуглецевих сталей з подальшою холодною деформацією методами осаду та кручення під гідростатичним тиском (КГТ).В работе получены новые решения научно-практической задачи, которая заключается в повышении комплекса механических свойств и склонности к штамповке горячекатаных низко- и ультранизкоуглеродистых сталей с последующей холодной деформацией методами осадки и кручения под гидростатическим давлением (КГД)