Browsing by Subject "Electron Spin Resonance"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
Item Study of Impurity-Helium Condensates Formed by Multishell Nanoclusters(2014-12-17) Mao, ShunImpurity-helium condensates (IHCs) are porous gel-like materials created by injecting a mixed beam of helium gas and an impurity gas into super fluid 4He. Van der Waals forces lead to the formation of clusters of impurities each surrounded by a thin layer of solid helium. Inside super fluid helium the clusters tend to aggregate into a gel-like structure with wide distribution of pore sizes. Matrix isolation of free radical impurities in IHCs leads to unusually high concentrations of these impurities. Impurity-helium condensates (IHCs) containing nitrogen and krypton atoms immersed in super fluid 4He have been studied via a CW electron spin resonance (ESR) technique. It was found that the addition of krypton atoms to the nitrogen-helium gas mixture used for preparation of IHCs increases efficiency of stabilization of nitrogen atoms. We have achieved high average (5x10^19 cm^-3) and local (2x10^21 cm^-3) concentrations of nitrogen atoms in krypton-nitrogen-helium condensates. High concentrations of nitrogen atoms achieved in IHCs provide an important step in the search for magnetic ordering effects at low temperatures. Impurity-helium condensates created by injection of hydrogen (deuterium) atoms and molecules as well as rare gas (RG) atoms (Ne and Kr) into super fluid 4He also have been studied via electron spin resonance (ESR) techniques. Measurements of the ground-state spectroscopic parameters of hydrogen and deuterium atoms show that the nanoclusters have a shell structure. H and D atoms reside in solid molecular layers of H2 and D2, respectively. By monitoring the recombination of H atoms in the collection of hydrogen-neon nanoclusters, we show that nanoclusters form a gel-like porous structure which enables the H atoms to be transported through the structure via percolation. Observation of percolation in the collection of nanoclusters containing stabilized hydrogen atoms opens new possibilities for a search for macroscopic collective quantum phenomena at ultralow temperatures accessible by a dilution refrigerator.Item Ultrasensitive Magnetometry and Imaging with NV Diamond(2011-08-08) Kim, ChangdongNV centers in a diamond are proving themselves to be good building blocks for quantum information, electron spin resonance (ESR) imaging, and sensor applications. The key feature of the NV is that it has an electron spin that can be polarized and read out at room temperature. The readout is optical, thus the magnetic field imaging can also be done easily. Magnetic field variation with feature sizes below 0.3 microns cannot be directly resolved, and so in this region magnetic resonance imaging must be employed. To realize the full sensitivity of NV diamond, the spin transition linewidth must be as narrow as possible. Additionally, in the case of NV ensembles for micron-sized magnetometers, there must be a high concentration of NV. To this end three techniques are explored: (1) Electron paramagnetic resonance (EPR) imaging with microwave field gradients, (2) Magic angle rotation of magnetic field, and (3) TEM irradiation to optimize the yield of NV in a diamond. For the EPR imaging demonstration a resonant microwave field gradient is used in place of the usual DC magnetic gradient to obtain enough spatial resolution to resolve two very close "double NV" centers in a type Ib bulk diamond. Microfabrication technology enabled the micron-size wire structure to sit directly on the surface of millimeter-scale diamond plate. In contrast to conventional magnetic resonance imaging pulsed ESR was used to measure the Rabi oscillations. From the beating of Rabi oscillations from a "double NV," the pair was resolved using the one-dimension EPR imaging (EPRI) and the spatial distance was obtained. To achieve high sensitivity in nitrogen-doped diamond, the dipole-dipole coupling between the electron spin of the NV center and the substitutional nitrogen (14N) electron must be suppressed because it causes linewidth broadening. Magic angle spinning is an accepted technique to push T2 and T2 * down toward the T1 limit. An experiment was performed using the HPHT diamond with a high concentration of nitrogen, and a rotating field was applied with a microfabricated wire structure to reduce line broadening. In this experiment, ~50% suppression of the linewidth was observed and the effective time constant T2* improved from 114 ns to 227 ns. To achieve the highest possible sensitivity for micro-scale magnetic sensors the concentration of NV should be large. Since the unconverted N are magnetic impurities they shorten T2 and T2*, giving a tradeoff between NV (and therefore N) concentration and sensitivity. To construct a damage monitor, a type Ib HPHT sample was irradiated with electrons from a transmission electron microscope (TEM) and the effects on the ESR transition were seen well before physical damage appeared on the diamond and thus this proved to be a sensitive metric for irradiation damage.