Harbor Development Study Progress Report for August-October, 1950

This report summarizes the results of the first phase of the Laboratory's current comprehensive study of harbor design. This phase comprises the study of the transmission of wave energy through, and the distribution of wave energy behind, breakwater openings. The results obtained to date relate the effect of three major variables; width of breakwater opening, direction of wave approach, and breakwater alignment, on the two quantities mentioned above. The results of this study are subject to certain limitations, notably the small number of cases studied, the idealization of harbor and breakwater configuration, and experimental error. However, the results are considered to be far more quantitative than qualitative and to be of definite usefulness and value in harbor layout and design

Truncation method for Green's functions in time-dependent fields

We investigate the influence of a time dependent, homogeneous electric field on scattering properties of non-interacting electrons in an arbitrary static potential. We develop a method to calculate the (Keldysh) Green's function in two complementary approaches. Starting from a plane wave basis, a formally exact solution is given in terms of the inverse of a matrix containing infinitely many 'photoblocks' which can be evaluated approximately by truncation. In the exact eigenstate basis of the scattering potential, we obtain a version of the Floquet state theory in the Green's functions language. The formalism is checked for cases such as a simple model of a double barrier in a strong electric field. Furthermore, an exact relation between the inelastic scattering rate due to the microwave and the AC conductivity of the system is derived which in particular holds near or at a metal-insulator transition in disordered systems.Comment: to appear in Phys. Rev. B., 21 pages, 3 figures (ps-files

Harbor Development Study Interim Report December, 1951

A general objective of the Harbor Development study is the investigation of the wave energy distribution in harbor areas. Treated in a general way, the energy distribution in a harbor can be considered in two parts. The first concerns the amount and distribution of energy entering the harbor through the breakwater opening. Second is the consideration of the redistribution of energy by reflection and absorption at the harbor boundaries. The first part, that of diffraction through breakwater openings, has been presented by this Laboratory in previous progress reports(l). The second part, that of reflection and absorption at harbor boundaries, is the subject of this report. These factors are important in harbor design because the resultant wave pattern in a harbor is determined by both the incident and reflected waves. In any harbor with reflecting boundaries the resultant wave pattern is usually complex and an exact solution by graphical or mathematical treatment would prove very difficult. However, an approximate graphical solution, developed recently by this Laboratory, appears promising. This report presents the results of extensive measurements of wave disturbances in two idealized harbors and compares these results with those of the graphical analysis

Interim Report

Recent experimental work has concentrated on the study of resultant force and pressure distribution on the stepped barrier designated as Case 5 (W. M. Simpson, Serial 1057, 6 Aug 1953 / J. H . Carr). Case 5 barrier has a plane vertical front face extending the two feet from bottom to still-water surface. Above still-water level are five steps, each of 2. 4-in. rise and tread. These dimensions, to a 20:1 scale, represent the prescribed prototype depth of 40 feet and the rise and tread of four feet each. The separate model barriers for resultant force and for pressure distribution measurements are each cast of aluminum. Vertical ribs at the rear provide rigidity and serve as convenient mounting surfaces

Spin Susceptibility of a 2D Electron System in GaAs towards the Weak Interaction Region

We determine the spin susceptibility $\chi$ in the weak interaction regime of a tunable, high quality, two-dimensional electron system in a GaAs/AlGaAs heterostructure. The band structure effects, modifying mass and g-factor, are carefully taken into accounts since they become appreciable for the large electron densities of the weak interaction regime. When properly normalized, $\chi$ decreases monotonically from 3 to 1.1 with increasing density over our experimental range from 0.1 to $4\times10^{11} cm^{-2}$. In the high density limit, $\chi$ tends correctly towards $\chi\to 1$ and compare well with recent theory.Comment: Submitted to Physical Review

Interim Report November 1953

Recent experimental work has concentrated on the study of resultant force and pressure distribution on the stepped barrier designated as Case 5 (W. M. Simpson, Serial 1057, 6 Aug 1953 / J. H . Carr). Case 5 barrier has a plane vertical front face extending the two feet from bottom to still-water surface. Above still-water level are five steps, each of 2.4-in. rise and tread. These dimensions, to a 20:1 scale, represent the prescribed prototype depth of 40 feet and the rise and tread of four feet each. The separate model barriers for resultant force and for pressure distribution measurements are each cast of aluminum. Vertical ribs at the rear provide rigidity and serve as convenient mounting surfaces

Interim Report December 1952-March 1953

Change Order "O" to Contract NOy-12561, under which the Laboratory is now operating, provides as a general objective the investigation of the forces and pressures exerted by waves on fixed plane barriers inclined at any angle to the sea surface, and on stepped or off-set plane barriers. In keeping with this objective, the Laboratory is endeavoring to develop a program which will combine a basic investigation of the mechanics of wave-induced forces with the production of immediately useful data. It is expected, for instance, that the results of experimental farce and pressure measurements on plane barriers, aside from their scientific value, can be used with profit in calculating the stability of gravity-type or caisson breakwaters against sliding and overturning. Since this program represents a new field of work for this Laboratory, the initial portion of the contract period has been devoted to the alteration of existing laboratory equipment, and the design and construction of required new equipment. In all of the~ modification and development of force and pressure measuring equipment, an effort has been made to design for maximum flexibility. Thus, the resulting basic units can be easily adapted to the various specific phases of the current or future investigations, such as breaking and non-breaking wave conditions, and many configurations of structures for which force or pressure data will be of interest. At this time, all equipment for the pressure measuring and orbit investigation programs has been completed, and work on these programs has begun. The force measuring apparatus is under construction, with completion estimated as June 1, 1953. In view of the considerable investment in time and funds made to equip the Laboratory for this new type of investigation, this report has been prepared to describe in some detail the alteration of existing equipment and the development of new equipment

Circular polarization dependent study of the microwave photoconductivity in a two-dimensional electron system

The polarization dependence of the low field microwave photoconductivity and absorption of a two-dimensional electron system has been investigated in a quasi-optical setup in which linear and any circular polarization can be produced in-situ. The microwave induced resistance oscillations and the zero resistance regions are notedly immune to the sense of circular polarization. This observation is discrepant with a number of proposed theories. Deviations only occur near the cyclotron resonance absorption where an unprecedented large resistance response is observed.Comment: 5 pages, 3 figure

Harbor Development Study Interim Report January - July, 1952

This report summarizes experimental and analytical work conducted in the Laboratory during the first half of 1952. During this period the first task of the current change order, the preparation of a manual of harbor design, was also accomplished. Sections II and III of this report describe work done in connection with the second task assignment - the obtaining of further information concerning wave disturbances in harbors. The experimental part of this study was terminated for lack of time before positive results could be obtained. The study was nonetheless interesting and valuable since it pointed out the importance of further investigation of the stability of standing wave patterns in closed basins. Sections IV and V describe initial phases of two-and three-dimensional studies of scour, a program undertaken in accordance with the general provisions of the third task assignment of the change order. These studies are to be continued until the termination of the current contract period, and have been proposed for further study in the pending contract extension proposal

Gate-Controlled Electron Spin Resonance in a GaAs/AlGaAs Heterostructure

The electron spin resonance (ESR) of two-dimensional electrons is investigated in a gated GaAs/AlGaAs heterostructure. We found that the ESR resonance frequency can be turned by means of a gate voltage. The front and back gates of the heterostructure produce opposite g-factor shift, suggesting that electron g-factor is being electrostatically controlled by shifting the equilibrium position of the electron wave function from one epitaxial layer to another with different g-factors