detailed in Chapter 2, �𝑑𝐼 𝑑𝑉 � � �𝐼 𝑉 � � � vs. 𝑉 , from different region with differing lattice arrangements. The spectra for the more honeycombed (relaxed) “ α ” region of the graphene sample resembled the spectra found in mechanically exfoliated graphene and are Dirac-like (V-shaped) except a substantially larger zero bias conductance and an addition of several weak conductance peaks. In contrast, the typical spectra associated with the square lattice (strongly strained) “ β ” region appeared to be asymmetric relative to those in the “ α ” region, with sharp conductance peaks more closely spaced. For “ α ” and “ β ” regions and the representative spectra found in each region, the energy interval between consecutive conductance peaks appears to decrease with increasing energy. For disordered, amorphous “ γ ” regions without atomic resolution, almost completely parabolic spectra that deviates fundamentally from the linear energy dependence of the Dirac fermions was found. While the “ γ ” region spectra does also demonstrate conductance peaks, the conductance peaks seem randomly spaced. Figure 6.10 shows topography images of each region type and the representative spectra for each region. Figure 6.11 shows similar results achieved and verified on a second graphene sample prepared exactly as the first. The spatial variations in the spectra (tunneling conductance) depends very sensitively on the local strain of the lattice.
This thesis would not have been completed without the help of many individuals. First of all, I would like to thank my advisor Professor Nai-Chang Yeh, for her guidance of my adventure into the vast high-temperature superconductivity field, for the enlightening discussions, her constant encour- agement, and incomparable optimism despite frequent setbacks with instrumentation and fundings. Nai-Chang is unique in her comprehension and implementation of the theory that complements her experimentalist’s intuition. Therefore, she has supported both the theoretical and experimental aspects of the thesis project, which makes this Ph.D. experience exceptional.
this thesis, as well as previous results from our group , find no evidence of a zero-bias conduction peak for any orientation of the grains in La-112 while the gap values determined from one half of the peak-to-peak energy separation also exhibit small variations. Although the zero-bias conductance peak (ZBCP) is a signature for superconductors with nodes, its presence is under the premise of the clean limit. Namely, the mean free path is much longer than the superconducting coherence length. In Y-123 the clean limit condition is indeed satisfied and the ZBCP has been clearly detected. On the other hand, in the event that the mean free path is comparable to or shorter than the superconducting coherence length, as in the case of La-112, it becomes difficult to assert the pairing symmetry solely based on the absence of the ZBCP. Moreover, based on other measurements on electron-type cuprate superconductors, the possibility of a d x 2 −y 2 -wave superconductor coexisting
provided a spectroscopic statement that HOPG has a high-mobility quasi-2DEG, which is eligible to show the quantum Hall effect. The first observation of Dirac fermion’s Landau level was realized by Andrei’s group of Rutgers University . They found evidence of the coexistence of both massless and massive Dirac fermions in graphite, which is proposed to arise from different stacking structures of the graphite. After single-layer graphene was eventually isolated in 2004, it has become an ideal model system for theoretical and experimental study of Dirac fermions. On the surface of graphene grown on silicon carbide, Stroscio’s group of NIST directly observed the non-equally-spaced energy spectrum of Landau levels, including the hallmark zero- energy state of graphene which is independent of the applied magnetic field . Notably, a complete linear collapse of the Landau level energies was obtained by plotting E n versus (|n|B) 1/2 , confirming massless Dirac
In service components such as steam pipes, pipe branches, gas and steam turbine blades, etc. which operate in engineering applications such as power plant, aero-engines, chemical plant etc., can operate at temperatures which are high enough for creep to occur. Often, only nominal operating conditions (i.e. pressure, temperatures, system load, etc.) are known and hence precise life predictions for these components, which may be complex in terms of geometry or weld characteristics, are not possible. Within complex components it can also be the case that the proportion of the material creep life consumed may vary from position to position within the component. It is therefore important that non-destructive techniques are available for assisting in the making of decisions on whether to repair, continue operating or replace certain components. Small specimen creep testing is a technique which can allow such analyses to be performed. Small samples of material are removed from the component to make small creep test specimens. These specimens can then be tested to give information on the remaining creep life of the component. This paper presents the results of small ring specimens tested under creep conditions and shows the comparison to standard (full size) creep testing for materials used under hightemperature in industry.
Abstract Piezoelectric sensors and actuators are a mature technology, commonplace amongst a plethora of industrial fields including automotive, maritime and non-destructive testing. However the environments that these devices are required to serve in are becoming more demanding, with temperatures being driven higher to increase efficiencies and reduce shut-downs. Materials to survive these tem- peratures have been the focus of many research groups over the last decade, but there still remains no standard for the measurement of piezoelectric materials at high tem- perature. This is required to effectively determine compa- rable Figures of Merit into which devices can be successfully designed. As part of a recent European effort to establish metrological techniques for hightemperature evaluation of electro-mechanical properties, we present here a review of the most promising hightemperature polycrystalline materials. Where their properties allow operation above that of the ubiquitous commercial material lead zirconate titanate, as well as work done to modify a promising hightemperature system, for use as a material standard.
Fig.4:This image is a side-by-side comparison of the PtIr tip produced in this project (left) and a standard tungsten tip (right). Both images were taken using a scanning electron microscope and include the magnification used in the bottom corner. It should be noted that the images are on different scales. As can be seen, the PtIr tip has a profile roughly 3 times as large as the tungsten tip. Although this may seem significant, it is actually within an acceptable range and will still work as a STM tip.
ScanningTunneling Microscopy (STM) is a technique where the surface topography of a conducting sample is probed by a scanning metallic tip. The tip-to-surface distance is controlled by monitoring the electronic tunneling current between the two metals. The aim of this work is to extend the temporal range of this instrument by characterising the time fluctuations of this current on different surfaces. The current noise power spectral density is dominated by a characteristic 1/f component, the physical origin of which is not yet clearly identified, despite a number of investigations. A new I-V preamplifier was developed in order to characterise these fluctuations of the tunnelling current and to obtain images of their spatial repartition. It is observed that their intensity is correlated with some topographical features. This information can be used to get insights on the physical phenomena involved that are not accessible by the usual STM set-up, which is limited to low frequencies.
To measure the magnetization of a sample, it must be mounted in a non-magnetic holder. The sample holder is attached to a sample transport rod, which allows the entire construction to be inserted into the sample space. The top end of the sample rod is attached to a stepper motor that drives the sample rod vertically through the detection coils in discrete steps. Discrete steps are possible because the components of the detection circuit are all made of superconducting materials - the induced current in the coils due to the motion of the magnetized sample does not decay as it would in a normal conductor. During a single measurement the sample is moved symmetrically about the centre of the pick-up coils, typically over a scan length of 4 cm divided into 32 points. The induced current in the detection coils couples inductively to the SQUID sensor, which is configured to work as a highly linear current to voltage converter. Thus the output voltage of the SQUID electronics is directly proportional to the magnetization of the sample. Calibration using a sample with known mass and magnetic susceptibility (typically Palladium) thus provides a highly accurate way of determining the magnetization of unknown samples.
The phenomenological Ginzburg-Landau theory (GL), which can be derived as an expansion of BCS theory around the critical temperature, has also had a multiband generalisation which was first derived by Tilley. Peeter’s and co-workers recently used the Gor’kov technique to find the next order corrections to the theory, extending the validity to slightly lower temperatures. We restrict ourselves to the field-free case and find extensions to very high order. We show that in the one-band theory and most multiband cases the superconducting gap in this extended GL theory converges to the BCS theory over almost the entire temperature range. However there are some cases in the multiband theory where the GL expansion diverges. This divergence is related to the appearance of a second critical temperature in the uncoupled limit.
Artefacts present in individual STM images can include affine-distortion (shear/stretch), non-linear scanning distortion, and abrupt image contrast changes resulting from structural tip changes. These artefacts arise due to thermal drift between the sample and the tip, piezo-scan- ner hysteresis, or from the laboratory environment, and are generally exacerbated as field-of-view or scanning time increase. Thus, the resolution of a single STM image is determined by the combination of imaging/instru- mental artefacts and the intrinsic limit to resolution due to quantum mechanical interactions. In reality, it is the artefacts that limit the resolving power of most STMs. Imaging artefacts are often hidden by using filters such as median (real-space), Wiener and low-pass (Fourier- space) filters . Nevertheless, median filters can distort the lattice or blur structural features. Wiener filters and low-pass filters involving Fourier transforms can intro- duce artificial periodicities or modify existing periodic features. A further point is that filters contain subjective elements where the researcher selects a specific filter to give the impression of resolution enhancement. However, we have shown in this paper that the MFA approach does not require any filtering steps. The scan-corrected images are reliable and highly reproducible due to the simple averaging process.
The expression for the electron density of states (EDOS) of hightemperaturesuperconductors (HTS) has been derived taking the disorder and anharmonicity effects as a central problem. This has been dealt with the help of double time thermodynamic Green’s function theory for electrons via a generalized Hamiltonian which consists of the contribution due to 1) unperturbed electrons; 2) unperturbed phonons; 3) isotopic impurities; 4) anharmonicities (no BCS type Ham- iltonian has been taken up in the formulation); and 5) electron-phonon interactions. The renormalization effects and emergence of pairons appears as a unique feature of the theory and dependence of EDOS on impurity concentration and temperature has been discussed in details with special reference to the HTS.
The sample is carried by a convenient sample holder with heating and temperature measurement integrated. A transport system makes it easy to introduce new samples into the system and to bring samples to separate vacuum chambers (see Figure 7) for sputter cleaning, various forms of deposition, Low- Energy Electron Diffraction, Auger Electron Spectroscopy, X-Ray Photoelectron Spectroscopy (not shown in Figure 7) and ScanningTunneling Microscopy. When the sample is in the STM-position, a Kalrez seal is located between the surface of the sample and the reactor body (see Figure 6). Between the reactor and the piezo element of the STM is a Viton seal. Together, the two seals make it possible to fill up the tiny reactor volume with gas mixtures up to a total pressure of 6 bar, while the pressure in the surrounding ultrahigh vacuum chamber remains unaffected. Presently, the setup is being modified to accommodate pressures up to 20 bar. The only STM components that are exposed to the gases are the tip and its gold-coated holder, which can slide in sub-micrometer steps in order to provide coarse z-positioning, bringing the surface into the z-control range of the piezo element. The piezo element itself is kept in vacuum. The reactor is usually operated in flow mode and the gases are led into and out of the small reactor volume via capillaries. These capillary gas lines are thin enough that they are easily integrated with a traditional spring suspension and eddy current damping system that decouples the system mechanically from external vibrations. Live observations of the structure of an oxidation catalyst in action
(Received 24 January 2018; revised manuscript received 14 August 2018; published 17 September 2018) We present a unified theory of charge carrier transport in 2D Dirac systems with broken mirror inversion and time-reversal symmetries (e.g., as realized in ferromagnetic graphene). We find that the entanglement between spin and pseudospin SU(2) degrees of freedom stemming from spin-orbit effects leads to a distinctive gate voltage dependence (change of sign) of the anomalous Hall conductivity approaching the topological gap, which remains robust against impurity scattering and thus is a smoking gun for magnetized 2D Dirac fermions. Furthermore, we unveil a robust skew scattering mechanism, modulated by the spin texture of the energy bands, which causes a net spin accumulation at the sample boundaries even for spin- transparent disorder. The newly unveiled extrinsic spin Hall effect is readily tunable by a gate voltage and opens novel opportunities for the control of spin currents in 2D ferromagnetic materials.
involved at the preparation of aqueous solution of the required cation, the chelation of cations in solution by addition of tartaric acid then, raising the temperature of the solution until formation the precursor. The precursor was calcined at low temperature compared by other methods to form the powders. The tartaric acid was not only used to form stable complexes with starting metallic ions, but also it was used as organic rich fuel. The process is very simple and available for most of piezoelectric materials. Pure chemical grade of Aluminum nitrate, Strontium carbonate, in the presence of stoichiometric amount of tartaric acid were used as starting materials. The mixture of Sr-Al, solution was firstly prepared and then stirred for 15 minute on hot plate magnetic stirrer, followed with addition of an aqueous solution of tartaric acid to the mixture with stirring. Then, the solution was evaporated to 80 o C with constant stirring until dryness and then dried in a dryer at 100 o C overnight. The dried powders obtained as aluminates precursors. Thermal analysis of the un-annealed precursors was carried.
following such a deposition is shown in Fig. 3共c兲. The sur- face is characterized by the formation of ⬃1.1 Å high Fe islands, covering roughly half of the surface area. A room temperature dI/ dV spectra for this surface is presented in Fig. 3共d兲. It is apparent that even this ultrathin Fe film results in the complete closing of the semiconducting band gap of the magnetite 共001兲 surface. Therefore, in order to obtain the true electronic signature of magnetite one must ensure that measurements are obtained only from well-ordered surfaces, and specifically the presence of Fe rich areas of the surface, which can induce metallic states in the gap region, must be avoided. We note that large area averaging techniques, such as PES, would be affected by surface disorder such as step bunching, or localized stoichiometry deviations, possibly re- sulting in the presence of states in the gap region. This offers an explanation as to why some PES experiments of magne- tite surfaces have observed metal states, 18 while others have
compounds promises to yield significant advances toward the development of hightemperature superconductivity as well as the realization of technological applications of these materials on a broad scale such as the applicable ability of HTSrs to conduct the electricity with zero resistance can be used in many applications. Scientists are working to take HTSrs at room temperature which can make many resolutions in the electricity generation, utilization and many other fields. Possibly in near future HTSrs will come to room temperature or ambient temperature and will serve in many applications. Growing demand of energy for the human needs have forced to develop energy efficient technologies to reduce environmental pollutions and to energy conservation. So, technological applications if the HTSrs help to meet in energy conservation and environmental pollution reduction targets. Despite their advantages, superconductors also suffer from a number of limitations. These include their restricted range of operating temperatures, brittleness, and sensitivity to changing magnetic fields. As previously noted, the world record for the highest Tc currently stands at 138K. Thus, there is still a long way to go before superconductors are available for applications such as consumer electronics. It is impractical for handheld consumer devices to have liquid nitrogen running through them.
temporal (up to 10 µs) resolution [16–18]. The signal-to-noise is significantly enhanced by differencing signals from two lines-of-sight on poloidal planes that are toroidally separated by ∼ 4 cm, to measure simultaneously both on-beam and background (off-beam) emission. Measurements are made at one radial location, for one impurity species, per discharge. However, due to high reproducibility, compilation of results from various similar discharges in MST is possible. The custom-built duo spectrometer  used for the measurements has been calibrated for radiant sensitivity, using a tungsten– halogen lamp which is pre-calibrated in the range λ = 300 to 1100 nm, for absolute impurity density calculations from charge-exchange emission brightness. The charge-exchange and the electron-impact excitation/de-excitation emissions are modeled accurately using fine structure in corresponding atomic transitions, obtained from ADAS, to extract Doppler- broadened emission parameters. The model is necessary for the accurate estimation of the ion temperature when the wavelength span of the fine structure manifold is comparable to or more than the Doppler broadening, which is the case presented in this paper. Details of the model are given elsewhere .
rock crystal BPY-4 with the aim of having artificial hold over the physical character of these materials seem valid with the experimental observance and theoretical investigation as per ambition. The various physical parameters such as Hall coefficient, electrical carrier density, electron concentration, plasma frequency are deeply influenced by the exposure of 241 Am 95 radiation source