Takahiro Iijima

Optical pumping NMR in the compensated semiconductor InP:Fe

The optical pumping nuclear-magnetic-resonance effect in the compensated semiconductor InP:Fe has been investigated in terms of the dependences of photon energy E p , helicity σ ′ , and exposure time τ L of infrared lights.The 3 1 P and 1 5 In signal enhancements show large σ ′ asymmetries and anomalous oscillations as a function of E p . We find that (i) the oscillation period as a function of E p is similar for 3 1 P and 1 1 5 In and almost field independent in spite of significant reduction of the enhancement in higher fields. (ii) A characteristic time for buildup of the 3 1 P polarization under the light exposure shows strong E p dependence, but is almost independent of σ ′ . (iii) The buildup times for 3 1 P and 1 1 5 In are of the same order (10 3 s), although the spin-lattice relaxation times T 1 are different by more than three orders of magnitude between them. The results are discussed in terms of (1) discrete energy spectra due to donor-acceptor pairs in compensated semiconductors and (2) interplay between 3 1 P and dipolar ordered indium nuclei, which are optically induced.

High-Field NMR up to 30 T with a Hybrid Magnet

NMR measurements up to 30 T were performed with the hybrid magnet installed in the National Institute for Materials Science (NIMS). The field profile and stability of superconducting, resistive and their hybrid magnets were measured using 63Cu NMR of a Cu metal. The field homogeneity of the hybrid magnet at 30 T is 186±4 ppm in the region ±5 mm from the field center along the z-axis. The magnetic field fluctuates with a total amplitude of about 30 G at 30 T (100 ppm). The present status of the hybrid magnet for high-resolution solid-state NMR measurements is discussed.

75As, 113,115In, and 123Sb NMR Studies of Indirect Nuclear Spin–Spin Coupling in InX (X=As, Sb)

Indirect nuclear spin–spin coupling in InX (X=As, Sb) semiconductors has been investigated by magic-angle spinning (MAS) NMR for 75As, 113,115In and 123Sb spins. The coupling constants, J, of the isotropic indirect interaction for neighboring 113,115In–X (X=75As, 123Sb) spins were obtained as 478±10 and 730±40 Hz, respectively, from triple-resonance NMR experiments employing cross polarization from rare 113In to X and 115In decoupling during acquisition. The J values above and those for the 113,115In–121Sb spins were also estimated by a second moment analysis for the MAS NMR spectra of 75As, 113,115In and 123Sb spins, but the moment analysis provided values that deviated from the correct ones by a factor of up to 1.7. The reduced indirect coupling constant K was calculated from the above J values for InX (X=As, Sb) and the previously obtained value of J=224±5 Hz for InP [T. Iijima et al.: J. Phys. Soc. Jpn. 73 (2004) 1045]. The increase in K with decreasing band gap energy of the semiconductors of InX (X=P, As, Sb) was explained by considering that indirect coupling is dominated by the second-order perturbation energy of the nuclear–electron interaction with occupied and unoccupied states being in the valence and conduction bands, respectively.

Indirect Spin–Spin Coupling in InP Investigated by Triple-Resonance NMR under Magic-Angle Spinning

The 31 P NMR spectra were measured using 31 P, 113 In and 115 In triple-resonance and magic-angle spinning (MAS) to investigate an indirect spin–spin interaction in undoped and Fe-doped InP semiconductors. The linewidth of the static NMR spectrum obtained by cross polarization (CP) of 115 In → 31 P with 115 In decoupling was explained by the lattice sum of 31 P– 31 P dipolar couplings. On the other hand, the linewidth of the MAS NMR spectrum obtained by CP of 115 In → 31 P without 115 In decoupling was interpreted by the isotropic indirect interaction of neighboring 115 In– 31 P spins. We obtained 10 equally spaced lines corresponding to 113 In spin state from the MAS NMR spectrum measured with CP from rare 113 In to 31 P under 115 In decoupling. From these experiments, the isotropic contributions of the electron-mediated nuclear coupling between the nearest neighboring 113, 115 In– 31 P spins was obtained as | 1 J iso ( I , S )| = 224 ±5 Hz for undoped and Fe-doped InP samples. The 1 J iso ( I , S ) valu...

Development of an optical pumping nuclear spin polarizer

We have been developing an optical pumping nuclear spin polarizer (OPNSP) to enhance nuclear magnetic resonance (NMR) signals from small amount of nano and biological materials. In this scheme, a semiconductor and a target (nano or biological) material are in close contact with each other, and high nuclear spin polarization in the semiconductor created by the optical pumping method is transferred to the nuclei in the target material by a cross-polarization at the interfaces and a spin diffusion inside the target material. As a result, high nuclear spin polarization is realized in the target material. In this paper, we present profiles of the nuclear spin polarization in the target material of the OPNSP calculated for the multilayered and multi-columnar configurations, from which suitable designs of the OPNSP to achieve high nuclear polarization in the target materials are discussed.

Homonuclear and Heteronuclear Indirect Spin-Spin Couplings in InP Studied Using 31P Cross Polarization NMR Spectra under Magic-Angle Spinning

The homonuclear and heteronuclear indirect spin-spin interactions in InP semiconductors have been studied using 31P cross polarization (CP) NMR spectra under magic-angle spinning (MAS). The isotropic contributions of the electron-coupled nuclear coupling between next-nearest neighboring 31P-31P spins and the nearest neighboring 113, 115In-31P spins were obtained as |2J(31P, 31P)| = 13±5 Hz and |1J(113, 115In, 31P)| = 224±5 Hz, respectively, for undoped and Fe-doped InP. We find no dependence of these 1, 2J values on the carrier density in the range of 107-1015 cm- 3.

Development of a nuclear spin polarizer with the optical pumping method

Abstract We have developed an optical pumping NMR system for semiconductors as an effective nuclear spin polarizer for a solid-state NMR quantum computer. The system was successfully applied to enhancexa031P nuclear spin polarization in InP doped with Fe, and the enhancement of thexa031P NMR signal by more than two orders of magnitude was achieved. We also observed the strong dependences of the enhancement on the helicity and the photon energy of incident light. The most effective enhancement was achieved with the helicity σ+ and photon energy smaller than the bandgap by 10∼15xa0meV.

Electron localization of polyoxomolybdates with ε-Keggin structure studied by solid-state 95Mo NMR and DFT calculation.

We report electron localization of polyoxomolybdates with ε-Keggin structure investigated by solid-state (95)Mo NMR and DFT calculation. The polyoxomolybdates studied are the basic ε-Keggin crystals of [Me3NH]6[H2Mo12O28(OH)12{MoO3}4] · 2H2O (1), the crystals suggested to have a disordered {ε-Mo12} core of [PMo12O36(OH)4{La(H2O)2.75Cl1.25}4]·27H2O (2), and the paramagnetic Keggin crystals of [H2Mo12O30(OH)10{Ni(H2O)3}4] · 14H2O (3). The spectra of (95)Mo static NMR of these samples were measured under moderate (9.4 and 11.7 T) and ultrahigh magnetic fields (21.8 T). From spectral simulation and quantum chemical calculation, the NMR parameters of the chemical shift and quadrupole interactions for (95)Mo were estimated. By the analysis based on the result for 1, it was found for 2 that although the {ε-Mo12} core is disordered, the eight d(1) electrons in it are not completely localized on four Mo-Mo bonds. Furthermore, it was shown for 3 that the d(1) electrons are localized to make the Mo-Mo bonds, while the unpaired electrons are also almost localized on the paramagnetic Ni(II) ions.

Shallow Donor Impurity States of InP Studied by 31P NMR Spectra under Magic-Angle Spinning

The impurity dependence of 115In-decoupled 31P NMR spectra under magic-angle spinning (MAS) have been studied in InP semiconductors. In the spectra of S-doped n-type and Zn-doped p-type InP samples with the carrier density of the order of 1018 cm-3, the line shift caused by the distribution of the strength of the electron-nuclear interaction were observed. The spectrum for the S-doped sample could be simulated by considering the Fermi-contact interaction of 31P spins with the delocalized electron of shallow donor existing in InP with the carrier density of the order of 1017 cm-3.

High field NMR study of Yb0.9Y0.1InCu4 up to 30 T

NMR measurements of 115 In in Yb 0.9 Y 0.1 InCu 4 were performed at 4.2 K up to 30 T with a hybrid magnet installed in National Institute for Materials Science (NIMS). Fourier-transformed (FT) spectrum at a constant magnetic field was obtained by summing a set of FT spectra with shifting the transient frequency. A first-order valence transition was observed at 22 T. Coexistence of the valence-fluctuating phase and the field-induced phase was confirmed. It was found that the proportion of the field-induced phase continues to increase even at 30 T. The effect of 4 f -electron spin correlations in the field-induced phase on the spin–spin relaxation time T 2 was discussed.