Breakdown of the nuclear-spin-temperature approach in quantum-dot demagnetization experiments

Abstract

The physics of interacting nuclear spins arranged on a crystalline lattice is generally described using a thermodynamic framework 1 and the concept of spin temperature. In the past, experimental studies in bulk solid-state systems have proven this concept to be not only correct 2,3 but also vital for the understanding of experimental observations 4 . Here we show, using demagnetization experiments, that the concept of spin temperature in general fails to describe the mesoscopic nuclear-spin ensemble of a quantum dot. We associate the observed deviations from a thermal spin state with the presence of strong quadrupolar interactions within the quantum dot, which cause significant anharmonicity in the spectrum of the nuclear spins. Strain-induced, inhomogeneous quadrupolar shifts also lead to a complete suppression of angularmomentum exchange between the nuclear-spin ensemble and its environment, resulting in nuclear-spin relaxation times exceeding an hour. Remarkably, the position-dependent axes of the quadrupolar interactions render magnetic-field sweeps inherently non-adiabatic, thereby causing an irreversible loss of nuclear-spin polarization.The study of nuclear-spin physics by optical orientation experiments in bulk semiconductor materials has been an active field of research over recent decades [5][6][7] . These research efforts have shown that, using the electron as a mediator, it is possible to transfer angular momentum from light onto nuclei, thereby establishing a nuclear-spin polarization that is orders of magnitude higher than the equilibrium nuclear polarization at cryogenic temperatures. As a result, the effective nuclear-spin temperature in such an optically pumped system can be pushed to the microkelvin regime. Combining these optical pumping schemes with nuclear adiabatic demagnetization techniques borrowed from bulk nuclear magnetic resonance experiments 3 would be a natural extension to these experiments that could lead to a significant further reduction of the nuclear-spin temperature. This approach, previously demonstrated in bulk semiconductors 5,8 , suffers from the fact that, in most systems where optical orientation of nuclear spins is possible, nuclear-spin relaxation is too fast to allow for a significant reduction of magnetic fields in an adiabatic way. Here, we use the exceedingly long nuclear-spin relaxation time in self-assembled quantum dots (QDs) (ref. 9) to implement an 'adiabatic' demagnetization experiment on the system of ∼10 5 nuclear spins.The mesoscopic ensemble of nuclear spins in a QD can be conveniently polarized and measured by optical means 5,9-12 . To this end, we use the photoluminescence of the negatively charged exciton (X −1 ) under resonant excitation of an excited QD state. It has been shown previously 13 that, under appropriate excitation conditions, 20-50% of the QD nuclear spins can be efficientlyInstitute of Quantum Electronics, ETH-Zurich, CH-8093 Zurich, Switzerland. *e-mail: patrickm@phys.ethz.ch; imamoglu@phys.ethz.ch. Lin.-3 0 3 B ext = 2 T for an unchar...

Voltage control of electron-nuclear spin correlation time in a single quantum dot

“…Our results strongly suggest that the presence of a spin polarized electron in the dot, given by τ c , has a direct influence on the nuclear spin relaxation time T d . This agrees with the observation of long nuclear spin memory times only for unpopulated QDs [28]. Nuclear spin depolarisation mechanisms linked to the presence of the electron in the dot could be direct through Knight field fluctuations [29], or indirect through local fluctuations of the electric field gradients, i.e.…”

Vanishing electrongfactor and long-lived nuclear spin polarization in weakly strained nanohole-filled GaAs/AlGaAs quantum dots

“…From the measurements of the nuclear spin depolarization in the dark, we observe decay times of up to 1000 s in both QDs type A and B. This is significantly longer than in natural fluctuation GaAs/AlGaAs dots where decay times of ∼ 40 s were found, 37 and is comparable to the decay times in self-assembled InP 47 and InGaAs 48 QDs. The enhanced stability of the nuclear spin polarization in the NFDE dots can be understood from the NMR spectra in Figs.…”

Nuclear spin effects in semiconductor quantum dots

“…Nuclear spin depolarization can occur via flip-flops between interacting (neighboring) nuclei, giving rise to nuclear spin diffusion. However, in most QDs the nuclear Zeeman splitting matching required for such nuclear-nuclear flip-flops is not fulfilled owing to quadrupole interactions occurring mainly as a result of strain 13,19,60,[64][65][66] . Thus nuclear spin diffusion is typically suppressed in QDs 36,60,61,63 .…”

“…This in principle can be achieved by cooling nuclear spins to ultra-low sub-μK temperatures using adiabatic demagnetization (AD), although first attempts in self-assembled dots experienced difficulties owing to strong quadrupole effects 65 . In future, similar experiments could be attempted in unstrained GaAs dots.…”