Shih-Hui Chao

Force-detected magnetic resonance in a field gradient of 250 000 Tesla per meter

We report the detection of slice-selective electron spin resonance with an external magnetic field gradient comparable to local interatomic gradients, using the techniques of magnetic resonance force microscopy. An applied microwave field modulated the spin-gradient force between a paramagnetic DPPH sample and a micrometer-scale ferromagnetic tip on a force microscope cantilever. A sensitivity equivalent to 184 polarized electron moments in a one-Hertz detection bandwidth was attained. We mapped the tip magnetic field with a resonant slice thickness of order one nanometer, thereby demonstrating magnetic resonance on length scales comparable to molecular dimensions.

Nanometer-scale magnetic resonance imaging

Magnetic resonance force microscopy (MRFM) images the three-dimensional spatial distribution of resonant spins by mechanical force detection. Image reconstruction in MRFM is challenging because the resonance occurs in a strongly curved shell that extends beyond the scan range. In contrast with conventional magnetic resonance imaging, where Fourier techniques work well, the curved-shell resonant geometry inherent to MRFM requires novel reconstruction methods. Here, we show the application of iterative reconstruction in an electron spin resonance imaging experiment with 80 nm voxels. The reconstructed image has a total scan volume of 0.5 cubic micrometers, and was generated by a magnetic resonant shell with a curvature radius of 2.3 μm. The imaged object was a paramagnetically doped solid with an obliquely tilted surface. The reconstructed image correctly identified the location and orientation of the surface, and mapped the spin distribution within the solid. Applications of MRFM include three-dimensional nanometer-scale mapping of dopant distributions in semiconductors, studies of magnetism of thin films, and spin diffusion physics. An ultimate goal of MRFM is the direct observation of molecular structure at the atomic scale.

Thermal tuning of a fiber-optic interferometer for maximum sensitivity

We describe a fiber-optic interferometer that employs wavelength changes to achieve maximum sensitivity. Wavelength changes are induced by adjusting the operating temperature of the laser, eliminating the need for an actuator to vary the spacing between the sensing fiber and the object to be monitored. The instrument and techniques described are suitable for cryogenic, high vacuum applications such as magnetic resonance force microscopy, where space is limited and micromanipulation can be challenging. The noise floor of 1.6×10−3u2002nm/Hz is adequate for monitoring subangstrom displacement of force microscope cantilevers.

The classical and quantum theory of thermal magnetic noise, with applications in spintronics and quantum microscopy

Thermal fluctuations generate magnetic noise in the vicinity of any conductive and/or magnetically permeable solid. This magnetic noise plays a fundamental role in the design of spintronic devices: namely, it sets the time scale during which electron spins retain their coherence. This paper presents a rigorous classical and quantum analysis of thermal magnetic noise, together with practical engineering examples. Starting with the fluctuation-dissipation theorem and Maxwells equations, a closed-form expression for the spectral density of thermal magnetic noise is derived. Quantum decoherence, as induced by thermal magnetic noise, is analyzed via the independent oscillator heat bath model of Ford et al. The resulting quantum Langevin equations yield closed-form expressions for the spin relaxation times. For realistic experiments in spintronics, magnetic resonance force microscopy, Bose-Einstein condensates, atomic physics, and solid-state quantum computing, the predicted relaxation rates are rapid enough that substantial experimental care must be taken to minimize them. At zero temperature, the quantum entanglement between a spin state and a thermal reservoir is computed. The same Hamiltonian matrix elements that govern fluctuation and dissipation are shown to also govern entanglement and renormalization, and a specific example of a fluctuation-dissipation-entanglement theorem is constructed. We postulate that this theorem is independent of the detailed structure of thermal reservoirs, and therefore expresses a general thermodynamic principle.

The design and control of a three-dimensional piezoceramic tube scanner with an inertial slider

Inertial sliders are often used to produce coarse relative positioning for scanning probe microscopes. This article describes the design, dynamic analysis, and control of a compact four-segment piezoceramic tube scanner employing an inertial slider. Velocity feedback control, implemented using two-segment piezoelectric sensing, was used to suppress undesired vibrations in the tube, and to improve scanner step uniformity. The control analysis was based on an empirical open-loop identification of the as-built tube behavior, which was also measured using the two-segment sensing technique. A reset integrator friction simulation predicted the overall system performance, and showed good agreement with experimental results.