Demagnetization of Quantum Dot Nuclear Spins: Breakdown of the Nuclear Spin Temperature Approach

The physics of interacting nuclear spins arranged in a crystalline lattice is typically described using a thermodynamic framework: a variety of experimental studies in bulk solid-state systems have proven the concept of a spin temperature to be not only correct but also vital for the understanding of experimental observations. Using demagnetization experiments we demonstrate that the mesoscopic nuclear spin ensemble of a quantum dot (QD) can in general not be described by a spin temperature. We associate the observed deviations from a thermal spin state with the presence of strong quadrupolar interactions within the QD that cause significant anharmonicity in the spectrum of the nuclear spins. Strain-induced, inhomogeneous quadrupolar shifts also lead to a complete suppression of angular momentum exchange between the nuclear spin ensemble and its environment, resulting in nuclear spin relaxation times exceeding an hour. Remarkably, the position dependent axes of quadrupolar interactions render magnetic field sweeps inherently non-adiabatic, thereby causing an irreversible loss of nuclear spin polarization.Comment: 15 pages, 3 figure

Spatial imaging of the spin Hall effect and current-induced polarization in two-dimensional electron gases

Spin-orbit coupling in semiconductors relates the spin of an electron to its momentum and provides a pathway for electrically initializing and manipulating electron spins for applications in spintronics and spin-based quantum information processing. This coupling can be regulated with quantum confinement in semiconductor heterostructures through band structure engineering. Here we investigate the spin Hall effect and current-induced spin polarization in a two-dimensional electron gas confined in (110) AlGaAs quantum wells using Kerr rotation microscopy. In contrast to previous measurements, the spin Hall profile exhibits complex structure, and the current-induced spin polarization is out-of-plane. The experiments map the strong dependence of the current-induced spin polarization to the crystal axis along which the electric field is applied, reflecting the anisotropy of the spin-orbit interaction. These results reveal opportunities for tuning a spin source using quantum confinement and device engineering in non-magnetic materials.Comment: Accepted for publication (2005

Full coherent control of nuclear spins in an optically pumped single quantum dot

Highly polarized nuclear spins within a semiconductor quantum dot (QD) induce effective magnetic (Overhauser) fields of up to several Tesla acting on the electron spin or up to a few hundred mT for the hole spin. Recently this has been recognized as a resource for intrinsic control of QD-based spin quantum bits. However, only static long-lived Overhauser fields could be used. Here we demonstrate fast redirection on the microsecond time-scale of Overhauser fields of the order of 0.5 T experienced by a single electron spin in an optically pumped GaAs quantum dot. This has been achieved using full coherent control of an ensemble of 10^3-10^4 optically polarized nuclear spins by sequences of short radio-frequency (rf) pulses. These results open the way to a new class of experiments using rf techniques to achieve highly-correlated nuclear spins in quantum dots, such as adiabatic demagnetization in the rotating frame leading to sub-micro K nuclear spin temperatures, rapid adiabatic passage, and spin squeezing

Evidence for reversible control of magnetization in a ferromagnetic material via spin-orbit magnetic field

Conventional computer electronics creates a dichotomy between how information is processed and how it is stored. Silicon chips process information by controlling the flow of charge through a network of logic gates. This information is then stored, most commonly, by encoding it in the orientation of magnetic domains of a computer hard disk. The key obstacle to a more intimate integration of magnetic materials into devices and circuit processing information is a lack of efficient means to control their magnetization. This is usually achieved with an external magnetic field or by the injection of spin-polarized currents. The latter can be significantly enhanced in materials whose ferromagnetic properties are mediated by charge carriers. Among these materials, conductors lacking spatial inversion symmetry couple charge currents to spin by intrinsic spin-orbit (SO) interactions, inducing nonequilibrium spin polarization tunable by local electric fields. Here we show that magnetization of a ferromagnet can be reversibly manipulated by the SO-induced polarization of carrier spins generated by unpolarized currents. Specifically, we demonstrate domain rotation and hysteretic switching of magnetization between two orthogonal easy axes in a model ferromagnetic semiconductor.Comment: 10 pages including supplemental materia

Photo-Induced Spin Dynamics in Semiconductor Quantum Wells

We experimentally investigate the dynamics of spins in GaAs quantum wells under applied electric bias by photoluminescence (PL) measurements excited with circularly polarized light. The bias-dependent circular polarization of PL (PPL) with and without magnetic field is studied. ThePPLwithout magnetic field is found to be decayed with an enhancement of increasing the strength of the negative bias. However,PPLin a transverse magnetic field shows oscillations under an electric bias, indicating that the precession of electron spin occurs in quantum wells. The results are discussed based on the electron–hole exchange interaction in the electric field