We measure the donor-bound electron longitudinal spin-relaxation time (T1) as a function of magnetic field (B) in three high-purity direct-bandgap semiconductors: GaAs, InP, and CdTe, observing a maximum T1 of 1.4 ms, 0.4 ms and 1.2 ms, respectively. In GaAs and InP at low magnetic field, up to ∼2 T, the spin-relaxation mechanism is strongly density and temperature dependent and is attributed to the random precession of the electron spin in hyperfine fields caused by the lattice nuclear spins. In all three semiconductors at high magnetic field, we observe a power-law dependence T1 ∝ B −ν with 3 ν 4. Our theory predicts that the direct spin-phonon interaction is important in all three materials in this regime in contrast to quantum dot structures. In addition, the "admixture" mechanism caused by Dresselhaus spin-orbit coupling combined with single-phonon processes has a comparable contribution in GaAs. We find excellent agreement between high-field theory and experiment for GaAs and CdTe with no free parameters, however a significant discrepancy exists for InP.

Monolayer transition metal dichalcogenides, a new class of atomically thin semiconductors, possess optically coupled 2D valley excitons. The nature of exciton relaxation in these systems is currently poorly understood. Here, we investigate exciton relaxation in monolayer MoSe_{2} using polarization-resolved coherent nonlinear optical spectroscopy with high spectral resolution. We report strikingly narrow population pulsation resonances with two different characteristic linewidths of 1 and <0.2  μeV at low temperature. These linewidths are more than 3 orders of magnitude narrower than the photoluminescence and absorption linewidth, and indicate that a component of the exciton relaxation dynamics occurs on time scales longer than 1 ns. The ultranarrow resonance (<0.2  μeV) emerges with increasing excitation intensity, and implies the existence of a long-lived state whose lifetime exceeds 6 ns.

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The nitrogen vacancy (NV) center in diamond is a sensitive probe of magnetic field and a promising qubit candidate for quantum information processing. The performance of many NV-based devices improves by aligning the NV(s) parallel to a single crystallographic direction. Using ab initio theoretical techniques, we show that NV orientation can be controlled by high-temperature annealing in the presence of strain under currently accessible experimental conditions. We find that (89 ± 7)% of NVs align along the [111] crystallographic direction under 2% compressive biaxial strain (perpendicular to [111]) and an annealing temperature of 970• C.The prospect of nanoscale sensing at ambient conditions has spurred renewed interest in the mature field of point-defect physics. Studying point-defect physics using ab initio computational techniques expands our understanding of important crystal defects and widens the scope of applicability for defect based devices. In this work, we show that the orientation of the nitrogenvacancy (NV) center in diamond can be controlled by annealing in the presence of strain.The diamond NV center is a technologically-relevant and well-studied defect consisting of a substitutional nitrogen and nearest-neighbor vacancy (Fig. 1).1-3 The center's electron spin-triplet ground state has an unusually long spin coherence time, exceeding 1 ms at room temperature.4 This long coherence time, coupled with the ability to perform optically detected magnetic resonance, enables the NV center to be used as a sensitive probe of temperature, 5 electric field, 6 magnetic field, 7-10 strain, 11and pressure. 12 The NV center is also a promising qubit candidate for quantum information applications. 13-15Many NV-based devices would benefit from simultaneous control over the NV position and orientation in the host diamond. Due to the tetrahedral coordination of the diamond lattice, the NV has four possible orientations, two of which are shown in Fig. 1. Control over orientation increases NV homogeneity and improves the performance of sensors based on ensembles. In magnetic sensing for example, alignment inhomogeneity increases noise because only the magnetic field projection on the NV symmetry axis is measured.9,16 Similarly, quantum information applications typically require qubits with identical properties, including alignment, to facilitate qubit coupling and entanglement generation.17-19 Finally, both sensing and quantum information may benefit from coupling the NV to an optical resonator, where NV dipole alignment to the resonant mode polarization is critical. 20-22Current techniques to create NVs provide control over either NV position or orientation, but not both simultaneously. In chemical vapor deposition grown diamond, NV defects may align to a preferential direction during growth, over spatial location. Conversely, implantation and annealing positions NVs to nanoscale accuracy, 25 but leaves them randomly oriented. Our method aligns pre-existing NV centers through strain engineering, thus enabling simulta...

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