A new record-high room temperature electron Hall mobility (µ RT = 194 cm 2 /V•s at n ~ 8×10 15 cm -3 ) for β-Ga 2 O 3 is demonstrated in the unintentionally doped thin film grown on (010) semi-insulating substrate via metalorganic chemical vapor deposition (MOCVD). A peak electron mobility of ~9500 cm 2 /V•s is achieved at 45 K. Further investigation on the transport properties indicate the existence of sheet charges near the epi-layer/substrate interface. Si is identified as the primary contributor to the background carrier in both the epi-layer and the interface, originated from both surface contamination as well as growth environment. Pre-growth hydrofluoric acid cleaning of the substrate lead to an obvious decrease of Si impurity both at interface and in epi-layer. In addition, the effect of MOCVD growth condition, particularly the chamber pressure, on the Si impurity incorporation is studied. A positive correlation between the background charge concentration and the MOCVD growth pressure is confirmed. It is noteworthy that in a β-Ga 2 O 3 film with very low bulk charge concentration, even a reduced sheet charge density can play an important role in the charge transport properties.
We present neutron-scattering measurements of the excitations of liquid 4 He confined in Vycor glass focussing on wavevectors in the range 1.55 Å Ϫ1 рQр1.80 Å Ϫ1 and temperatures around the superfluid-normal fluid critical temperature T c. 4 He adsorption isotherms have been measured on the Vycor sample in order to control filling. The superfluid density of 4 He in the sample was measured using a sound velocity technique yielding a superfluid to normal fluid critical temperature of T c ϭ2.05 K at full filling. The liquid 4 He in Vycor supports well-defined phonon-roton excitations above the critical temperature in Vycor suggesting the existence of localized Bose-Einstein condensation ͑BEC͒ in the normal phase above T c. The phonon-roton excitations ͑localized BEC͒ appear to exist up to temperatures of T ϭ2.17 K. At full filling the phonon-roton excitations have energies that are the same as those in bulk superfluid 4 He within precision. The liquid also supports layer modes for wave vectors in the roton region (1.90 Å Ϫ1 рQр2.10 Å Ϫ1) as observed previously in Vycor, aerogel, MCM, and Geltech silica.
Neutron scattering measurements of the fundamental excitations of liquid 4He confined in 44 A pore diameter gelsil glass at pressures up to 40 bars in the wave vector range 0.41.6 A(-1), especially the rotons, are observed up to complete solidification of all the liquid at a pressure of approximately 40 bars where the roton vanishes. At and above a pressure of 35.1 bars, Bragg peaks are observed, indicating coexistence of liquid and solid in the pores at pressures 35 less than or approximately equal P less than or approximately equal 40 bars.
We describe a digital signal processing method for high precision frequency evaluation of approximately sinusoidal signals based on a computationally efficient method. We demonstrate frequency measurement enabling sensitive measurement of the oscillatory force exerted on a micromechanical cantilever. We apply this technique to detection of the force signal arising in a micromechanically detected magnetic resonance force microscopy electron spin resonance signal. Our frequency detection measurements agree well with the theoretical noise analysis presented here, and we find that due to the excellent sensitivity of optical displacement detection, our sensitivity is limited only by the thermal displacement noise of the cantilever.
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