One evening,a husband and wife are working in their garden. While watering the grass, the wife holds the hose horizontally and the water flows from the end of the hose. She twists the nozzle of the hose to partially close it and suddenly the water shoots farther from the end of the hose. This seems odd to her husband, who wonders, "Why does the stream of water travel farther, since it appears that less water is flowing from the end of the hose?" My challenge is to give a scientific explanation for this unnatural phenomenom.
Professor Budakian earned his bachelor's, master's, and Ph.D degrees in physics from the University of California, Los Angeles. From 2002 to 2005, he was a visiting scientist at the IBM Almaden Research Center in San Jose, California, where his research focused on detecting the spin of a single defect in glass using magnetic resonance force microscopy. The long-term goal of this work is to achieve three-dimensional sub-surface imaging of atomic structure, a capability that would transform our understanding in areas ranging from the determination of protein structure to the characterization of buried interfaces in semiconductor devices.
Professor Budakian's research focuses on developing ultra sensitive spin detection techniques for single spin imaging and quantum readout. Current areas of research include (i) design and fabrication of micro-machined silicon cantilevers for sub-attonewton force detection, (ii) development of spin detection/manipulation protocols that enable force detection at the thermal limit, (iii) imaging single dopants and defects in semiconductors, (iv) spin control via active feedback, and (v) combining MRFM with electron nuclear double resonance (ENDOR) for high sensitivity nuclear spin detection.
Magnetic Resonance Force Microscopy (MRFM):
MRFM is a powerful new technique that combines the high sensitivity and spatial resolution of scanning probe microscopy with the three-dimensional imaging capabilities of magnetic resonance imaging. In MRFM detection, a micron-sized permanent magnet is attached to the end of a sensitive silicon cantilever and brought near a sample containing electron or nuclear spins. In the presence of the inhomogeneous field from the tip, the Larmor frequency of the spins is a strong function of position with respect to the tip. A microwave field applied to the sample resonantly excites only those spins that lie within a thin slice near the probe tip. The spins located within the “resonant slice” experience a force caused by the strong field gradient. This force can be detected by measuring either the cantilever amplitude or frequency.
Current state of the art in MRFM detection is capable of detecting a single electron spin, which can be as deep as 100 nm below the sample surface, with 25-nm lateral resolution. The force generated by a single spin in these measurements is approximately 2 atto-newtons (10–18 N), which is of order 106 times smaller than the electrostatic and van der Waals tip-surface interactions. Through the use of magnetic resonance, we are able to unambiguously distinguish the minute spin signal from all other interactions.
Controlling spins with real-time feedback: Magnetic resonance force microscopy can not only be used to detect spins but also to monitor, manipulate, and control their orientation. Recently, we have demonstrated a spin manipulation protocol that creates spin order by controlling the naturally occurring statistical fluctuations in small ensembles of electron spins. In a manner reminiscent of a Maxwell's demon, we can apply real-time feedback to the spin ensemble and rectify the random fluctuations to create a hyperpolarized state. Furthermore, we can capture and store this ordered state and later read it out. Thus far, we have demonstrated this capability on small spin ensembles. As part of my research program, I intend to apply this spin manipulation protocol for the initialization and readout of single spins.
Electron nuclear double resonance (ENDOR): While direct detection of a single nuclear spin is beyond the capabilities of the current state of the art in ultrasensitive force detection, it may be possible to use an electron spin to measure the state of a nearby nuclear spin. Electron nuclear double resonance (ENDOR) is a powerful technique used in NMR spectroscopy to gain detailed spectroscopic information on nuclear spins. In ENDOR, the longitudinal state of the nuclear spin determines the electron resonance frequency through the hyperfine coupling. Provided the hyperfine coupling is comparable to the thickness of the resonant slice, a transition in the nuclear spin state could be monitored by measuring the MRFM signal from the electron spin. The expected increase in nuclear spin detection sensitivity is of order the ratio of the gyromagnetic ratios of the electron and nuclear spins (~103).
106 Seitz Materials Research Lab
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