Quartz Fork Research Articles

P-Type conductivity of hexagonal boron nitride as a dielectrically tunable monolayer: modulation doping with magnesium.

Hexagonal boron nitride (h-BN) is the widest band gap 2D material (>6 eV), which has attracted extensive attention. For exploring potential applications in optoelectronic devices, electrical conductivity modulation (n or p type) is of extreme importance. Here, we report the achievement of a large-scale and high quality h-BN monolayer with p-type conductivity by modulation doping of Mg using a low pressure chemical vapor deposition method. A large-scale monolayer h-BN (>10 inches) was grown by using a wound Cu foil roll on a multi-prong quartz fork. Magnesium nitride is used as a dopant precursor in a separate line due to its appropriate melting point and decomposition temperature. Density functional theory calculations revealed that the acceptor level introduced by Mg is almost pinned into the valence band and the activated holes are highly delocalized into the surrounding h-BN lattices. The h-BN:Mg monolayer showed a p-type conductivity with a considerable surface current of over 12 μA and a hole density of 1.7 × 1014 cm-2. The dielectrically tunable h-BN monolayer makes the fabrication of advanced 2D optoelectronic devices in short wavelength possible.

Heat Transfer in $$^{3}\hbox {He}$$ 3 He – $$^{4}\hbox {He}$$ 4 He Mixtures in Cylindrical Geometry

The paper presents the results of theoretical studies of the transport processes that take place in the newly proposed experiments on study of a vibrating quartz fork in superfluid \(^{3}\hbox {He}\)–\(^{4}\hbox {He}\) mixtures. In addition to known mechanisms of energy loss from a vibrating quartz fork such as first sound radiation or interaction with thermal excitations, two more mechanisms specific for \(^{3}\hbox {He}\)–\(^{4}\hbox {He}\) mixtures are proposed and studied in the paper. The relative contribution of these mechanisms: second sound and effective diffusion, is considered, and experimental conditions under which these mechanisms become effective are discussed.

Large-roll growth of 25-inch hexagonal BN monolayer film for self-release buffer layer of free-standing GaN wafer.

Hexagonal boron nitride (h-BN) is known as promising 2D material with a wide band-gap (~6 eV). However, the growth size of h-BN film is strongly limited by the size of reaction chamber. Here, we demonstrate the large-roll synthesis of monolayer and controllable sub-monolayer h-BN film on wound Cu foil by low pressure chemical vapor deposition (LPCVD) method. By winding the Cu foil substrate into mainspring shape supported by a multi-prong quartz fork, the reactor size limit could be overcome by extending the substrate area to a continuous 2D curl of plane inward. An extremely large-size monolayer h-BN film has been achieved over 25 inches in a 1.2” tube. The optical band gap of h-BN monolayer was determined to be 6.0 eV. The h-BN film was uniformly transferred onto 2” GaN or 4” Si wafer surfaces as a release buffer layer. By HVPE method, overgrowth of thick GaN wafer over 200 μm has been achieved free of residual strain, which could provide high quality homo-epitaxial substrate.

A Multi-Fork Z-Axis Quartz Micromachined Gyroscope

A novel multi-fork z-axis gyroscope is presented in this paper. Different from traditional quartz gyroscopes, the lateral electrodes of the sense beam can be arranged in simple patterns; as a result, the fabrication is simplified. High sensitivity is achieved by the multi-fork design. The working principles are introduced, while the finite element method (FEM) is used to simulate the modal and sensitivity. A quartz fork is fabricated, and a prototype is assembled. Impedance testing shows that the drive frequency and sense frequency are similar to the simulations, and the quality factor is approximately 10,000 in air. The scale factor is measured to be 18.134 mV/(°/s) and the nonlinearity is 0.40% in a full-scale input range of ±250 °/s.

UNCERTAINTY QUANTIFICATION IN STOCHASTIC SYSTEMS USING POLYNOMIAL CHAOS EXPANSION

In recent years, extensive research has been reported about a method which is called the generalized polynomial chaos expansion. In contrast to the sampling methods, e.g., Monte Carlo simulations, polynomial chaos expansion is a nonsampling method which represents the uncertain quantities as an expansion including the decomposition of deterministic coefficients and random orthogonal bases. The generalized polynomial chaos expansion uses more orthogonal polynomials as the expansion bases in various random spaces which are not necessarily Gaussian. A general review of uncertainty quantification methods, the theory, the construction method, and various convergence criteria of the polynomial chaos expansion are presented. We apply it to identify the uncertain parameters with predefined probability density functions. The new concepts of optimal and nonoptimal expansions are defined and it demonstrated how we can develop these expansions for random variables belonging to the various random spaces. The calculation of the polynomial coefficients for uncertain parameters by using various procedures, e.g., Galerkin projection, collocation method, and moment method is presented. A comprehensive error and accuracy analysis of the polynomial chaos method is discussed for various random variables and random processes and results are compared with the exact solution or/and Monte Carlo simulations. The method is employed for the basic stochastic differential equation and, as practical application, to solve the stochastic modal analysis of the microsensor quartz fork. We emphasize the accuracy in results and time efficiency of this nonsampling procedure for uncertainty quantification of stochastic systems in comparison with sampling techniques, e.g., Monte Carlo simulation.

Generation of turbulence by vibrating forks and other structures in superfluidH4e

A study of the drag on the prongs of a number of quartz forks vibrating in the superfluid phase of $^{4}\text{H}\text{e}$ is reported, and particular attention is paid to the transitions from laminar to turbulent flow over a wide range of temperature. Behavior in the normal phase is consistent with that for a classical fluid, as has already been reported [Phys. Rev. E 75, 025302 (2007)]. Behavior in the superfluid phase is compared to that of other structures vibrating in superfluid $^{4}\text{H}\text{e}$, and similarities and differences are noted. We focus on the observed behavior of the drag coefficient as a function of velocity, and the problems posed by this behavior are explored. There is evidence for a sharp critical velocity at which significant turbulence starts to be generated in the superfluid component. At high velocities the drag coefficient tends to that observed in a classical fluid, suggesting that the two fluids, strongly coupled by mutual friction, are then behaving like a single classical viscous fluid. Behavior in the intermediate region seems to vary from one case to another. Evidence is presented that in the case of some structures the transition to single-fluid behavior takes place rather abruptly at a velocity that is only slightly greater than the sharp superfluid critical velocity, but that in other structures the transition is more gradual. Observed values of both the superfluid critical velocity and the effective viscosity of the fully coupled fluids are presented and discussed. It is suggested that the critical superfluid velocity is always closely similar to that at which the coupled fluids would be expected to undergo a classical transition between a flow that is strictly laminar and one that displays the first instability, and a possible reason is discussed.

The use of vibrating quartz forks in cryogenic helium research – On their ability to detect an externally applied flow in superfluid 4He

Quartz tuning forks mass-produced as frequency standards for watches proved to serve as very useful tools for generating and probing flows of gaseous and liquid helium. Their cryogenic use as thermometers, pressure- and viscometers as well as generators and detectors of cavitation and turbulence has been recently widely discussed in the literature [JLTP 136, 1 (2004); 146, 537 (2007); 150, 525 (2008)]. Here we report our preliminary experiments where the vibrating fork is used to detect the externally applied flow in superfluid 4He.

On cavitation in liquid helium in a flow due to a vibrating quartz fork

Cavitation in normal and superfluid liquid He4 at saturated vapor pressure and slightly elevated pressures has been experimentally studied in a flow due to quartz forks vibrating at high amplitudes. Above the temperature- and pressure-dependent critical velocity, heterogeneous cavitation is observed both visually and electrically, as a breakdown of the resonance response of the fork. We compare our results with available experimental and discuss them using existing theoretical models. In particular, we show that thermal effects leading to local overheating of the vicinity of the fork have to be taken into account, especially in normal liquid He4.

Cavitation in Liquid Helium Observed in a Flow Due to a Vibrating Quartz Fork

A quartz fork vibrating at high amplitude is used to study cavitation in He I and He II along the saturated vapor pressure (SVP) curve and at slightly elevated pressures. Cavitation is observed as a breakdown of the resonance response at critical velocity when slowly sweeping the frequency of the drive across the resonance and confirmed by visual observation of a bubble occurring in He II in the space between the prongs of the fork. On decreasing the temperature from 4.2 K along the SVP curve the critical velocity slowly increases from about 0.4 m/s to 1 m/s, until a steep increase up to about 2 m/s occurs within about 20 mK just below the superfluid transition. We discuss our results, including the measured dependence of the critical velocity versus overpressure at fixed bath temperature.

Quantum Turbulence Generated and Detected by a Vibrating Quartz Fork

Flow due to a commercially available vibrating quartz fork is studied in gaseous helium, He I, He II and 3He–B, over a wide range of temperatures and pressures. On increasing the driving force, the flow changes in character from laminar (characterized by a linear velocity versus drive dependence) to turbulent (characterized by a square root velocity versus drive dependence). In classical fluids, we characterize this transition by a critical Reynolds number, Re c =U cr δ/ν, where U cr is the critical velocity, ν stands for the kinematic viscosity, $\delta=\sqrt{2\nu/\omega}$ is the viscous penetration depth and ω is the angular frequency of oscillations. U cr of order 10 cm/s observed in He II and 1 mm/s in 3He–B agree with those found with other vibrating objects such as spheres, wires and grids, as well as with available numerical simulations of vortex motion in an applied ac flow.

Transition from laminar to turbulent drag in flow due to a vibrating quartz fork

Flow due to a commercially available vibrating quartz fork is studied in gaseous helium, He I and He II, over a wide range of temperatures and pressures. On increasing the driving force applied to the fork, the drag changes in character from laminar (characterized by a linear drive vs velocity dependence) to turbulent (characterized by a quadratic drive vs velocity dependence). We characterize this transition by a critical Reynolds number Recrdelta=Ucrdelta/nu, where Ucr is the critical velocity, nu stands for the kinematic viscosity, delta=sqrt[2nu/omega] is the viscous penetration depth, and omega is the angular frequency of oscillations. We have experimentally verified that the corresponding scaling Ucr proportional, sqrt[nuomega] holds in a classical viscous fluid over two decades of nu.

Demonstration of low-temperature atomic force microscope with atomic resolution using piezoresistive cantilevers

Compared to current optical-lever methods adopted in atomic force microscopes, nonoptical methods such as piezoresistive methods and quartz fork methods can be more advantageous due to their smaller installation size and the lack of electromagnetic effects from laser light during electrical conductivity experiments. As a technological demonstration of nonoptical methods, a low-temperature atomic force microscope using piezoresistive cantilevers was developed and operated at liquid-He4 temperatures (5K). The cantilever and sample can be transferred from atmosphere to the microscope head operating at low temperatures. Both contact mode and noncontact mode were used for testing the system while carrying out atomic resolution studies on clean Si(111) and clean Si(100) surfaces.

THE INFLUENCE OF HUMIDITY ON THE SHEAR FORCE BETWEEN TIP AND SAMPLE IN NSOM USING PIEZOELECTRIC FORK

The distance between tip and sample can be regulated using piezoelectric quartz fork glued with micro optic fiber probe. A biquadrate vibration equation for the fork–probe–sample system is established to theoretically analyze the relations of the electric current flow through fork versus tip–sample (T–S) distance (I–d). The I–d curve and the action distance for shear force are influenced by environmental humidity. The results reinforce the opinions in the earlier works by other researches that the physical origin of the shear force is due to a material filling the tip–sample gap. Furthermore, the intrinsic reasons for shear force damping between tip and sample have been confirmed and developed, i.e. the water and hydrocarbon coupled between optic fiber probe and sample due to the capillary cohesion force.

A novel ultrasonic resonance sample-tip distance regulation for near field optical microscopy and shear force microscopy

We have developed a novel technique of sample-tip distance regulation by using ultrasonic resonance method to sense the shear force in the nano-Newton range. A piezoelectric quartz fork is driven by its resonance frequency without an additional dither vibrator. By combining the vibration amplitude and phase shift of the fiber tip attached to the quartz fork, the sample-tip distance can be reliably controled at 0–50 nm. The shear force images show that the lateral resolution is better than 50 nm depending of the aperture size of the tip. Being a non-optical detection mechanism, this method is very promising in the near field scanning optical microscopy, especially in the cases of reflective mode and spectroscopy, as well as being an independent shear force microscope for insulation samples.