Wangchun Chen

Production of Highly Polarized 3He Using Spectrally Narrowed Diode Laser Array Bars

We have produced 70%–75% 3He polarization by spin-exchange optical pumping in cells ≈100 cm3 in volume. The polarization achieved is consistent with known spin-exchange and spin-relaxation rates, but only when the recently discovered temperature dependence of 3He relaxation is included. Absolute 3He polarization measurements were performed using two different methods in two different laboratories. The results were obtained with either a spectrally narrowed laser or one type of broadband laser. Based on tests of several larger cells at pressures near 1 bar, we find that the power required to reach the same polarization is typically three times lower for the spectrally narrowed laser. This last result indicates that spectrally narrowed lasers will be important for obtaining the highest polarization in large volume neutron spin filters. Polarization in excess of 55% as obtained in cells up to 640 cm3 in volume and 70% polarization is anticipated with available increases in spectrally narrowed laser power.

Coupled Magnetic and Ferroelectric Domains in Multiferroic Ni3V2O8

Electric control of multiferroic domains is demonstrated through polarized magnetic neutron diffraction. Cooling to the cycloidal multiferroic phase of Ni3V2O8 in an electric field E causes the incommensurate Bragg reflections to become neutron spin polarizing, the sense of neutron polarization reversing with E. Quantitative analysis indicates the E-treated sample has a handedness that can be reversed by E. We further show a close association between cycloidal and ferroelectric domains through E-driven spin and electric polarization hysteresis. We suggest that a definite cycloidal handedness is achieved through magnetoelastically induced Dzyaloshinskii-Moriya interactions.

Polarized He3 gas compression system using metastability-exchange optical pumping

Dense samples (10–100barcm) of nuclear spin polarized He3 are utilized in high energy physics, neutron scattering, atomic physics, and magnetic resonance imaging. Metastability exchange optical pumping can rapidly produce high He3 polarizations (≈80%) at low pressures (few mbar). We describe a polarized He3 gas compressor system which accepts 0.26barlh−1 of He3 gas polarized to 70% by a 4W neodymium doped lanthanum magnesium hexaluminate (Nd:LMA) laser and compresses it into a 5barcm target with final polarization of 55%. The spin relaxation rates of the system’s components have been measured using nuclear magnetic resonance and a model of the He3 polarization loss based on the measured relaxation rates and the gas flow is in agreement with a He3 polarization measurement using neutron transmission.

Polarized neutron reflectometry of a patterned magnetic film with a 3He analyzer and a position-sensitive detector

We have employed a polarized 3He spin filter in conjunction with a position-sensitive detector (PSD) to perform efficient polarization analysis of neutron diffuse reflectivity. This work was carried out on the NG-1 polarized neutron reflectometer at the National Institute of Standards and Technology Center for Neutron Research. We measured the specular and diffuse reflectivity of a patterned magnetic array that has periodic square holes in a Co film. Analysis of the data yielded spin-analyzed two-dimensional Qx−Qz reciprocal space maps for the sample in magnetized and demagnetized states. We compared the measurements obtained with a 3He analyzer and a PSD with those obtained using a conventional supermirror analyzer and a 3He proportional counter. The results are in good agreement. For this experiment, 3He gas was polarized by the spin-exchange optical pumping method and stored in a uniform magnetic field provided by a shielded solenoid. Improved optical pumping using a spectrally narrowed diode laser arr...

Neutron Beam Effects on Spin-Exchange-Polarized 3He

We have observed depolarization effects when high intensity cold neutron beams are incident on alkali-metal spin-exchange-polarized 3He cells used as neutron spin filters. This was first observed as a reduction of the maximum attainable 3He polarization and was attributed to a decrease of alkali-metal polarization, which led us to directly measure alkali-metal polarization and spin relaxation over a range of neutron fluxes at Los Alamos Neutron Science Center and Institute Laue-Langevin. The data reveal a new alkali-metal spin-relaxation mechanism that approximately scales as sqrt[phi_{n}], where phi_{n} is the neutron capture-flux density incident on the cell. This is consistent with an effect proportional to the concentration of electron-ion pairs but is much larger than expected from earlier work.

Polarized 3He analyzers for neutron reflectometry

Abstract At the National Institute of Standards and Technology (NIST) and Indiana University, we are developing polarized 3 He analyzers for neutron reflectometry. We have employed 3 He analyzers at two polarized neutron reflectometers, NG1 at the NIST Center for Neutron Research (NCNR) and POSY I at the Intense Pulsed Neutron Source (IPNS), Argonne National Laboratory. The long-term goal for both efforts is to perform efficient studies of magnetic diffuse scattering. At the NCNR, we tested a 3 He analyzer by comparing measurements of specular scattering obtained with a supermirror analyzer to those obtained with a 3 He analyzer. For this test, we measured the spin-flip and nonspin-flip scattering from a Mn0.52Pd0.48/Fe thin film. The results with the 3 He analyzer show very good agreement with those obtained with the supermirror analyzer. We have also carried out tests of the 3 He analyzer for application to magnetic diffuse scattering experiments. We discuss the development of a 3 He analyzer for IPNS that will be employed for studies of patterned magnetic arrays.

Electrical heating for SEOP-based polarized 3He system

Development of neutron spin filters based on polarized 3He is underway at Spallation Neutron Source (SNS). We report the progress of electrical heating tests in polarized 3He based on Spin-Exchange Optical Pumping (SEOP) method. We first test the system performance based on electrical heating via non-inductance heating pads. We observe a contribution of 955 hours to the relaxation time T1 from the heating pads. We then test the electrical heating SEOP pumping system at the SNS beamline Magnetic Reflectometer. We currently obtain 73% 3He polarization in a cell with 820 cm3 in volume.

Direct observation of the Higgs amplitude mode in a two-dimensional quantum antiferromagnet near the quantum critical point

Spontaneous symmetry-breaking quantum phase transitions play an essential role in condensed matter physics. The collective excitations in the broken-symmetry phase near the quantum critical point can be characterized by fluctuations of phase and amplitude of the order parameter. The phase oscillations correspond to the massless Nambu

Demonstration of Focusing Wolter Mirrors for Neutron Phase and Magnetic Imaging

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Spin-Analyzed SANS for Soft Matter Applications

Goldstone modes whereas the massive amplitude mode, analogous to the Higgs boson in particle physics, is prone to decay into a pair of low-energy Nambu