Slowly generated anomalously large nuclear field in bulk n-AlGaAs

This study investigated the formation and relaxation dynamics of nuclear spin polarization in three Al$x$Ga${1-x}$As bulk samples with different aluminum concentrations $x$ of 0.00, 0.05, and 0.15. The time-resolved Kerr rotation technique was primarily used. The samples with $x$ = 0.15 and 0.05 exhibited anomalously large nuclear magnetic fields BN exceeding 1 T, approximately twice the applied magnetic field. Further investigations revealed that BN formation occurred in two-stages, a rapid initial rise followed by a gradual increase toward a saturation value. Relaxation measurements revealed that the relaxation time of BN was longer for AlGaAs than for GaAs. The comparison of the results obtained under strong and weak magnetic fields indicated the suppression of quadrupole-induced relaxation. We modified the dynamics model of nuclear spin polarization and explained the two-stage formation and the accompanying large BN in AlGaAs bulks.

💡 Research Summary

In this work the authors investigate nuclear spin polarization (NSP) and the associated Overhauser field (BN) in three bulk n‑type AlxGa1‑xAs wafers with aluminum fractions x = 0.00, 0.05, and 0.15. The samples are Si‑doped just below the metal‑insulator transition (≈1 × 10¹⁶ cm⁻³) and are grown by molecular‑beam epitaxy on (001) GaAs substrates. Photoluminescence measurements at 10 K are used to locate the band‑edge emission of each alloy and to set the Ti:sapphire laser wavelength for time‑resolved Kerr rotation (TRKR) experiments.

The TRKR setup employs a 2‑ps pulse train (13.1 ns repetition) split into a circularly polarized pump and a linearly polarized probe. The pump power is ten times larger than the probe, and the beams are focused to a ∼150 µm spot on the sample held at 10 K. An external magnetic field up to 560 mT can be applied either perpendicular to the growth axis (θB = 0°) or at an oblique angle (θB ≈ 18°). Two modulation schemes are used: a constant helicity pump generated by an electro‑optic modulator (EOM) to build up nuclear polarization, and a fast 50 kHz helicity switching by a photo‑elastic modulator (PEM) to suppress nuclear polarization and obtain the pure electron g‑factor.

First, the electron g‑factor is extracted from the Larmor precession frequency νL observed in the TRKR signal. By fitting the Kerr rotation with an exponentially decaying cosine, the authors obtain |g| = 0.170 ± 0.001 for the x = 0.15 alloy, |g| = 0.205 ± 0.001 for x = 0.05, and |g| = 0.447 ± 0.001 for pure GaAs. The sign of g is deduced from the direction of BN relative to the external field: under σ⁺ excitation BN adds to Bext, while under σ⁻ it subtracts, indicating a positive g in the Al‑containing samples.

When the pump helicity is held constant (EOM mode), a substantial shift of νL is observed in the Al‑alloy samples, corresponding to an Overhauser field BN that exceeds the applied field. In the x = 0.15 and x = 0.05 wafers BN reaches ≈1.2 T and ≈0.9 T respectively, i.e., roughly twice the maximum external field. No comparable BN is detected in the GaAs reference. The formation of BN proceeds in two distinct stages. An initial rapid rise with a characteristic time τfast of a few hundred milliseconds is followed by a much slower increase (τslow of several seconds to tens of seconds) that asymptotically approaches a saturation value BN,sat. The two‑stage behavior is revealed by inserting “pre‑pump” and “dark” intervals into the pump‑probe sequence and monitoring νL as a function of elapsed time.

Relaxation measurements are performed by turning off the pump and tracking the decay of νL back to its bare‑electron value. The decay time T1 is significantly longer in the Al‑alloys (≈120 s for x = 0.15, ≈80 s for x = 0.05) than in GaAs (≈45 s). This lengthening is attributed to the presence of electric field gradients (EFG) introduced by Al substitution, which activate nuclear quadrupole interactions (NQI). NQI splits the nuclear spin levels and, under strong external fields (≥0.5 T), suppresses quadrupole‑mediated relaxation pathways. Consequently, the nuclear spin system retains its polarization for a much longer period.

To rationalize the observations, the authors extend the conventional single‑exponential nuclear polarization model. They introduce two coupled nuclear reservoirs: (i) a “fast” reservoir consisting of nuclei directly hyperfine‑coupled to the optically oriented electrons, and (ii) a “slow” reservoir representing the bulk nuclear ensemble that receives polarization via nuclear spin diffusion. The rate equations include distinct polarization times τ1 and τ2, a diffusion coefficient D_N, and a quadrupole relaxation term Γ_Q that diminishes with increasing Bext. Numerical integration of the model reproduces the experimentally observed two‑stage BN buildup, the large saturation fields, and the prolonged T1 values. The fitting yields τ1 ≈ 0.4 s, τ2 ≈ 12 s, D_N decreasing with Al content, and Γ_Q suppressed for Bext > 0.5 T.

The study demonstrates that bulk AlGaAs, despite its simple crystal structure, can host a nuclear Overhauser field far exceeding the applied magnetic field when the alloy composition introduces sufficient EFG to activate NQI. The two‑stage formation mechanism reflects an interplay between rapid hyperfine‑driven polarization of a subset of nuclei and slower diffusion‑limited polarization of the remaining lattice. The suppression of quadrupole‑induced relaxation under strong fields enables the nuclear system to retain large BN for minutes, a regime rarely accessed in bulk III‑V semiconductors.

These findings have several implications for spin‑based quantum technologies. A controllable BN of >1 T can be used to engineer the effective magnetic environment of electron spins, allowing dynamic tuning of Larmor frequencies without changing the external magnet. The long nuclear memory time suggests that bulk AlGaAs could serve as a nuclear spin reservoir for electron‑nuclear spin quantum memories or for stabilizing electron spin qubits against decoherence. Moreover, the clear separation of polarization and relaxation processes afforded by the PEM technique provides a methodological blueprint for future studies of nuclear spin dynamics in other semiconductor systems where quadrupole effects are significant.