Overhauser shift of the electron spin-resonance line of Si:P at the metal-insulator transition: II. 29 Si contribution

The electron-spin resonance (ESR) line of delocalised electrons shifts upon saturation due to the hyperfine interaction with the dynamically polarized nuclear spins. The ²⁹ Si part of the Overhauser shift of the ESR line of phosphorus doped silicon (Si:P) is separated in the concentration range 2.7 ... 7.3×10 ¹⁸ / cm ³ covering the metal-insulator transition. The Overhauser shift profiles, recorded versus ²⁹ Si nuclear magnetic resonance (NMR) frequency, are asymmetric. Their dependence on temperature and ESR saturation compares reasonably with simulations. Time and NMR frequency dependence of the dynamic nuclear polarization is studied in detail. No pronounced variation of the ²⁹ Si Overhauser shift profiles with P concentration is observed, but the maximum value of the shift profile decreases with increasing P concentration. In contrast to standard ²⁹ Si NMR results, these measurements reveal the behaviour of the ²⁹ Si nuclei close to the P doping sites. Received 8 November 2001     

Specific heat of neutron-compensated heavily doped Si:P near the metal-insulator transition

The specific heat of neutron-irradiated heavily doped Si:P (3.3·10¹⁸ cm^–3Nd7.3·10¹⁸ cm^–3) was measured between 0.07 and 3K and in magnetic fields between 0 and 6T. The compensation ratio was varied systematically by isochronal annealing. Characterization was done by temperature dependent measurements of Hall coefficient and electrical resistivity. The specific heat displays a minimum of the linear coefficient at the carrier concentration where the P impurity band is starting to be occupied. The concentration dependence of localized moments inferred from Schottky anomalies can be interpreted in terms of localized magnetic moments arising from the defect structure introduced by the irradiation and from P-derived electrons. As in uncompensated Si:P, local moments survive on the metallic side of the transition.     

Observation of non-Gaussian conductance fluctuations at low temperatures in Si:P(b) at the metal-insulator transition

We report investigations of conductance fluctuations (with 1/f(alpha) power spectra) in doped silicon at low temperatures (T<20 K) as it is tuned through the metal-insulator transition (MIT) by changing the carrier concentration n. The scaled magnitude of noise, gamma(H), increases with decreasing T following an approximate power law gamma(H) approximately T-beta. At low T, gamma(H) diverges as n decreases through the critical concentration n(c), accompanied by a growth of low-frequency spectral weight. The second spectrum and probability density of the fluctuations show strong non-Gaussian behavior below 20 K as n/n(c) decreases through 1. This is interpreted as the onset of a glassy freezing of the electronic system across the MIT.