Dynamic transitions and hysteresis

When an interacting many-body system, such as a magnet, is driven in time by an external perturbation, such as a magnetic field,the system cannot respond instantaneously due to relaxational delay. The response of such a system under a time-dependent field leads to many novel physical phenomena with intriguing physics and important technological applications. For oscillating fields, one obtains hysteresis that would not occur under quasistatic conditions in the presence of thermal fluctuations. Under some extreme conditions of the driving field, one can also obtain a non-zero average value of the variable undergoing such dynamic hysteresis. This non-zero value indicates a breaking of symmetry of the hysteresis loop, around the origin. Such a transition to the spontaneously broken symmetric phase occurs dynamically when the driving frequency of the field increases beyond its threshold value which depends on the field amplitude and the temperature. Similar dynamic transitions also occur for pulsed and stochastically varying fields. We present an overview of the ongoing researches in this not-so-old field of dynamic hysteresis and transitions.Comment: 30 Pages Revtex, 10 Postscript figures. To appear in Reviews of Modern Physics, April, 199

Analysis and Design of a Valve-Controlled Hydraulic Device

When the load of an actuator consists of a mass and friction for a valvecontrolled hydraulic device, the exact responses are obtained analytically for rectilinear inputs. As any type of input can be approximated by a connection of straight lines, the responses to various types of inputs can be calculated. An important application of the analysis is to obtain the frequency characteristics. As the sinusoidal input can be approximated by a proper trapezoid, the rectilinear inputs are applied to each straight range of the trapezoid. By this approach, the frequency characteristics for the case of considerably high frequency or heavy load for common hydraulic servos are made clear. The analytical results are shown in normalized forms and are approximated by the responses which are calculated by the equivalent transfer functions. The analytical results were verified by experiments. Another important application of the analysis is to make clear the effects of the dither applied to the spool. The wave shape of the dither is also approximated by a proper trapezoid, and the responses to the rectilinear inputs are applied to each straight range of the trapezoid. The results of calculations are represented by an equivalent transfer function, and it was made clear how the dither affects the gain and the time constant. The equivalent transfer functions are very useful to design the hydraulic servomechanism. In this paper, the design criterion which is based on the performance of the hydraulic device, particularly that of the frequency characteristics, is given

Stability of a Servomechanism Operated by PWM Mode

In a servomechanism operated by the PWM mode, the output contains the dither corresponding to the carrier pulse. When the loop gain becomes high, the dither component is fed back and modulates the pulse width. Therefore, we must consider not only the ordinary stability problem of the servo loop but also the stability of the dither. First, the stability of the fundamental circuit was analyzed. In this case, it was shown that the various waveshapes took place depending on the ratio of the pulse transmission lag to the pulse period and the loop gain. The ranges of the loop gain were also obtained for various waves. Next, the stability was studied with regard to the servomechanism in which two types of compensating circuits were used. One was a first-order system and the other was a phase lead network. It was shown that these compensating circuits were effective for stabilizing the servomechanism, if they were selected properly. The optimum time constants were obtained for these compensating circuits. Finally, the theoretical results were verified by experiments which were carried out by an electrohydraulic servomechanism

Bang-Bang Control of Electrohydraulic Servomechanisms

The optimum control is given for an electro-hydraulic servomechanism. It is approximated by two quasi-optimum controls which are reasonable in point of the control sensitivity and the practical construction of control system. The optimum control is obtained by the bang-bang control except in a very special case. The optimum switching points are determined as intersections of the phase trajectories and the switching surface in the phase space. We determine the switching points by the projections on the phase plane. The switching based on the fixed mathematical model is not favorable for considering the sensitivities to parameter variations. Two quasi-optimum controls are considered. One is obtained by approximating the optimum switching curve by two lines on the phase plane, the other by approximating the switching surface by a switching plane in the phase space. The errors due to the approximations are investigated. The satisfactory results are obtained in experiments and the advantages of this method are verified

Strain distribution analysis of sputter-formed strained Si by tip-enhanced Raman spectroscopy

Simultaneous nanometer-scale measurements of the strain and surface undulation distributions of strained Si (s-Si) layers on strain-relief quadruple-Si1-xGex-layer buffers, using a combined atomic force microscopy (AFM) and tip-enhanced Raman spectroscopy (TERS) system, clarify that an s-Si sample formed by our previously proposed sputter epitaxy method has a smoother and more uniformly strained surface than an s-Si sample formed by gas-source molecular beam epitaxy. The TERS analyses suggest that the compositional fluctuation of the underlying Si1-xGex buffer layer is largely related to the weak s-Si strain fluctuation of the sputtered sampl

Si/Ge hole-tunneling double-barrier resonant tunneling diodes formed on sputtered flat Ge layers

We have demonstrated Si/Ge hole-tunneling double-barrier resonant tunneling diodes (RTDs) formed on flat Ge layers with a relaxation rate of 89% by our proposed method; in this method, the flat Ge layers can be directly formed on highly B-doped Si(001) substrates using our proposed sputter epitaxy method. The RTDs exhibit clear negative differential resistance effects in the static current–voltage (I–V) curves at room temperature. The quantized energy level estimation suggests that resonance peaks that appeared in the I–V curves are attributed to hole tunneling through the first heavy- and light-hole energy levels