Exchange and Quadrupole Broadening of Nuclear Acoustic Resonance Line Shapes in the III-V Semiconductors

Abstract

Nuclear acoustic resonance (NAR) and nuclear magnetic resonance (NMR) are used to study the linewidths, second moments, and line shapes of nuclear-spin systems of InAs, InSb, GaAs, GaSb, and AlSb. These cw measurements are made at frequencies of 8-10 MHz and at 300 °K in single crystals with intrinsic or near intrinsic concentrations of impurities. Different NAR and NMR linewidths for the same nuclear-spin system are explained by the different interaction Hamiltonians for spin-phonon and spin-photon couplings to the nuclear-spin system. When the magnetic field is along (001) directions, the resonance line shapes are broadened by dipole-dipole and isotropic nuclear-exchange interactions. When the magnetic field is rotated from (001) directions, increased broadening of the resonance line shapes is explained by small anisotropic dipole-dipole and much larger anisotropic quadrupole interactions. The measured like-spin and unlike-spin exchange constants agree with an exchange-constant dependence on the inverse fourth power of the internuclear distance. Like-spin and unlike-spin exchange constants are determined at the nuclear positions in each compound and used with a theory for indirect nuclear-spin exchange to predict the ^-character electronic wave-function density. The strongly anisotropic quadrupole broadening is explained by electric field gradients produced by the electric fields associated with ionized substitutional impurities. From the measured field gradients, antishielding constants at the In, Sb, Ga, and As nuclear positions are determined relative to each other. 9 R. M. Sternheimer, Phys. Rev. 146, 140 (1966).

Semiclassical model for the dephasing of a two-electron spin qubit coupled to a coherently evolving nuclear spin bath

“…The spread of the Larmor frequencies δω 2 α that was found from fitting to the experimental results 18 was equivalent to 3 Gauss effective spread of magnetic field, somewhat bigger than the values of NMR line-widths, typically about 1 Gauss equivalent spread, reported for bulk GaAs in the literature 39,40 . In addition to a random contribution to the local magnetic field coming from the dipolar interaction with neighboring nuclei, the nuclear spins in a GaAs quantum dot experience a quadrupolar splitting due to electric field gradients originating from the confined electrons, which we estimate to be at the order of a few Gauss (see Ref.…”

“…All of these parameters affect the grouping of spins into "giant spins," and the matrix elements T lk , calculated according to Eq. (40).…”

Dephasing time of GaAs electron-spin qubits coupled to a nuclear bath exceeding 200 μs

“…The measured NMR linewidth in pure GaAs is about 0.1 mT (ref. 25). A possible origin for the larger field inhomogeneity found here is the quadrupole splitting arising from the presence of the two localized electrons 27 .…”

“…This field dependence can be accounted for by including a spread of the Larmor precession frequencies for each nuclear species. Such a variation is also manifest in the width of NMR lines and naturally arises from dipolar and other interactions between nuclei 25 . We model it as a shift of the magnetic field acting on each individual nuclear spin by an amount B loc , where B loc is a Gaussian random variable with standard deviation δB loc .…”

Electron spin-flip correlations due to nuclear dynamics in driven GaAs double dots

“…Previous investigations suggest that dipole-dipole interactions and superexchange interactions give comparable contributions to the frequency broadening of the NMR lines. [45,48] If one assumes that the frequency shift of any given nucleus is the sum of contributions from a large number of neighboring nuclei, then one recovers immediately a Gaussian distribution for the individual frequencies and consequently a Gaussian time-dependence of the correlation function g λ defined in Eq. (20).…”