We propose an approach to realize a hybrid quantum system composed of a diamond nitrogen-vacancy (NV) center spin coupled to a magnon mode of the low-damping, low-moment organic ferrimagnet vanadium tetracyanoethylene. We derive an analytical expression for the spin-magnon cooperativity as a function of NV position under a micron-scale perpendicularly magnetized disk, and show that, surprisingly, the cooperativity will be higher using this magnetic material than in more conventional materials with larger magnetic moments, due to in part to the reduced demagnetization field. For reasonable experimental parameters, we predict that the spin-magnon-mode coupling strength is g ∼ 2π×10 kHz. For isotopically pure 12C diamond we predict strong coupling of an NV spin to the unoccupied magnon mode, with cooperativity C λ = 15 for a wide range of NV spin locations within the diamond, well within the spatial precision of NV center implantation. Thus our proposal describes a practical pathway for single-spin-state-to-single-magnon-occupancy transduction and for entangling NV centers over micron length scales.

No abstract

We present a paradoxical finding that, in the vicinity of a topological phase transition in a quantum anomalous Hall system (Chern insulator), topology nearly always (except when the system obeys charge-conjugation symmetry) results in a significant extension of the edge-state structure beyond the minimal one required to satisfy the Chern numbers. The effect arises from the universal gapless linear-in-momentum Hamiltonian of the nodal semimetal describing the system right at the phase transition, whose form is enforced by the change of the Chern number. Its emergent approximate chiral symmetry results in an edge-state band in the vicinity of the node, in the region of momenta where such form is dominant. Upon opening the gap, this edge-state band is modified in the gap region, becoming "protected" (connected to the valence bulk band with one end and conduction band with the other) in the topologically nontrivial phase and "nonprotected" (connected to either the valence or conduction band with both ends) in the trivial phase. The edge-state band persists in the latter as long as the gap is small enough. arXiv:1807.05111v2 [cond-mat.mes-hall]

Effective spin-orbit (SO) Hamiltonians for conduction electrons in wurtzite heterostructures are lacking in the literature, in contrast to zincblende structures. Here we address this issue by deriving such an effective Hamiltonian valid for quantum wells, wires, and dots with arbitrary confining potentials and external magnetic fields. We start from an 8×8 Kane model accounting for the s-p z orbital mixing important to wurtzite structures, but absent in zincblende, and apply both quasi-degenerate perturbation theory (Löwdin partitioning) and the folding down approach to derive an effective 2×2 electron Hamiltonian. Focusing on wurtzite quantum wells, we later on also perform a self-consistent Poisson-Schrödinger calculation in the Hartree approximation to determine the relevant SO couplings. We obtain the usual k-linear Rashba term arising from the structural inversion asymmetry of the wells and, differently from zincblende structures, a bulk Rashba-type term induced by the inversion asymmetry of the wurtzite lattice. Our results show this latter term to be the main contribution to the Rashba coupling in wurtzite wells. We also find linear-and cubic-in-momentum Dresselhaus contributions. Both the bulk Rashba-type term and the Dresselhaus terms originate exclusively from the admixture of s-and p z -like states in wurtzites structures. Interestingly, in these systems the linear Rashba and the Dresselhaus terms have the same symmetry and can in principle cancel each other out completely, thus making the spin a conserved quantity. We determine the intrasubband (intersubband) Rashba α ν (η) and linear Dresselhaus β ν (Γ) SO strengths of GaN/AlGaN single and double wells with one and two occupied subbands (ν = 1, 2). For the GaN/Al 0.3 Ga 0.7 N single well with one occupied subband, we obtain the total spin splitting coefficient α eff 1 = α 1 + β 1 ∼ 7.16 meV·Å, in agreement with weak antilocalization measurements. In the case of two occupied subbands, we observe that the intersubband Rashba η is much weaker than the intrasubband coupling α ν . For double wells even in the presence of strong built-in electric fields (spontaneous and piezoelectric, crucial in GaN/AlGaN wells), we find a seemingly symmetric potential configuration at which both the Rashba η and Dresselhaus Γ intersubband couplings exhibit their highest strengths. On the other hand, we observe that the intrasubband Dresselhaus coefficients β 1 and β 2 interchange their values as the gate voltage V g varies across zero; a similar behavior, though less pronounced, is seen for the Rashba couplings α 1 and α 2 . We believe our general effective Hamiltonian for electrons in wurtzite heterostructures put forward here, should stimulate additional theoretical works on wurtzite quantum wells, wires, and dots with variously defined geometries and external magnetic fields.