Time-delay control of a magnetic levitated linear positioning system

In this paper, a high accuracy linear positioning system with a linear force actuator and magnetic levitation is proposed. By locating a permanently magnetized rod inside a current-carrying solenoid, the axial force is achieved by the boundary effect of magnet poles and utilized to power the linear motion, while the force for levitation is governed by Ampere's Law supplied with the same solenoid. With the levitation in a radial direction, there is hardly any friction between the rod and the solenoid. The high speed motion can hence be achieved. Besides, the axial force acting on the rod is a smooth function of rod position, so the system can provide nanometer resolution linear positioning to the molecule size. Since the force-position relation is highly nonlinear, and the mathematical model is derived according to some assumptions, such as the equivalent solenoid of the permanently magnetized rod, so there exists unknown dynamics in practical application. Thus 'robustness' is an important issue in controller design. Meanwhile the load effect reacts directly on the servo system without transmission elements, so the capability of 'disturbance rejection; is also required. With the above consideration, a time-delay control scheme is chosen and applied. By comparing the input-output relation and the mathematical model, the time-delay controller calculates an estimation of unmodeled dynamics and disturbances and then composes the desired compensation into the system. Effectiveness of the linear positioning system and control scheme are illustrated with simulation results

Quasiparticle dynamics and phonon softening in FeSe superconductors

Quasiparticle dynamics of FeSe single crystals revealed by dual-color transient reflectivity measurements ({\Delta}R/R) provides unprecedented information on Fe-based superconductors. The amplitude of fast component in {\Delta}R/R clearly tells a competing scenario between spin fluctuations and superconductivity. Together with the transport measurements, the relaxation time analysis further exhibits anomalous changes at 90 K and 230 K. The former manifests a structure phase transition as well as the associated phonon softening. The latter suggests a previously overlooked phase transition or crossover in FeSe. The electron-phonon coupling constant {\lambda} is found to be 0.16, identical to the value of theoretical calculations. Such a small {\lambda} demonstrates an unconventional origin of superconductivity in FeSe.Comment: Final published version; 5 pages; 4 figure

Spatially-resolved relaxation dynamics of photoinduced quasiparticles in underdoped YBasub2sub 2Cusub3sub 3Osub7−deltasub {7-delta}

The spatially-resolved relaxation characteristics of photoinduced quasiparticles (QPs) in CuO$sub 2$ planes of underdoped YBCO are disclosed by polarized fs time-resolved spectroscopy. The relaxation time (tau) along b axis diverges at Tc, and appears to be governed by a temperature-dependent gap Delta(T) at T Tc, a monotonic increase of tau with decreasing T along the b axis and ab diagonal was observed and can be attributed to a temperature-independent gap Delta$sub p$. The results lend support to recombination dominant scenario of QP dynamics. However, the QP thermalization may take part along the nodal direction in the highly underdoped samples.Comment: 16 pages, 4 figures. To be published in Physical Review B, Brief Repor

Investigation of reactive‐ion‐etch‐induced damage of InP/InGaAs multiple quantum wells by photoluminescence

The effects of CH4/H2 reactive ion etching (RIE) on the optical properties of an InP/InGaAs multiple‐quantum‐well structure have been investigated by low‐temperature photoluminescence (PL). The structure consisted of eight InGaAs quantum wells, lattice matched to InP, with nominal thicknesses of 0.5, 1, 2, 3, 5, 10, 20, and 70 monolayers, respectively, on top of a 200‐nm‐thick layer of InGaAs for calibration. The design of this structure allowed etch‐induced damage depth to be obtained from the PL spectra due to the different confinement energies of the quantum wells. The samples showed no significant decrease of luminescence intensity after RIE. However, the observed shift and broadening of the PL peaks from the quantum wells indicate that intermixing of well and barrier material increased with etch time. © 1995 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70403/2/JAPIAU-78-3-1528-1.pd

Behaviors of beryllium compensation doping in InGaAsP grown by gas source molecular beam epitaxy

We report structural properties as well as electrical and optical behaviors of beryllium (Be)-doped InGaAsP lattice-matched to InP grown by gas source molecular beam epitaxy. P type layers present a high degree of compensation on the order of 1018 cm−3, and for Be densities below 9.5×1017 cm−3, they are found to be n type. Enhanced incorporation of oxygen during Be doping is observed by secondary ion mass spectroscopy. Be in forms of interstitial donors or donor-like Be-O complexes for cell temperatures below 800°C is proposed to account for such anomalous compensation behaviors. A constant photoluminescence energy of 0.98 eV without any Moss-Burstein shift for Be doping levels up to 1018 cm−3 along with increased emission intensity due to passivation effect of Be is also observed. An increasing number of minority carriers tend to relax via Be defect state-related Shockley-Read-Hall recombination with the increase of Be doping density