Motivated by mathematical structures appearing in gauge and string theories with N=2 supersymmetry, I’ll consider the behavior of certain generalized theta series under Kontsevich-Soibelman transformations. In Calabi-Yau string vacua, such theta series encode instanton corrections from NS5-branes, and their transformation properties ensure the mutual consistency of NS5-instantons, D-instantons and wallcrossing. It turns out that the transformations are captured by Faddeev’s quantum dilogarithm, and lead to a new type of quantum dilogarithm identities with the quantization parameter inversely proportional to the NS5-brane charge.

The Dzyaloshinskii-Moriya Interaction (DMI) has recently attracted considerable interest owing to its fundamental role in the stabilization of chiral spin textures in ultrathin ferromagnets, which are interesting candidates for future spintronic technologies. Here we employ a scanning nano-magnetometer based on a single nitrogen-vacancy (NV) defect in diamond to locally probe the strength of the interfacial DMI in CoFeB$|$MgO ultrathin films grown on different heavy metal underlayers X=Ta,TaN, and W. By measuring the stray field emanating from DWs in micron-long wires of such materials, we observe deviations from the Bloch profile for TaN and W underlayers that are consistent with a positive DMI value favoring right-handed chiral spin structures. Moreover, our measurements suggest that the DMI constant might vary locally within a single sample, illustrating the importance of local probes for the study of magnetic order at the nanoscale.

Among a variety of layered materials used as building blocks in van der Waals heterostructures, hexagonal boron nitride (hBN) appears as an ideal platform for hosting optically-active defects owing to its large bandgap ($\sim 6$~eV). Here we study the optical response of a high-purity hBN crystal under green laser illumination. By means of photon correlation measurements, we identify individual defects emitting a highly photostable fluorescence under ambient conditions. A detailed analysis of the photophysical properties reveals a high quantum efficiency of the radiative transition, leading to a single photon source with very high brightness ($\sim 4\times 10^6$ counts.s$^{-1}$). These results illustrate how the wide range of applications offered by hBN could be further extended to photonic-based quantum information science and metrology.

We study the structure of inactive states in a prototypical model glass, the Kob-Andersen binary Lennard-Jones mixture. These inactive states are obtained by transition path sampling and are at dynamical phase coexistence with an active equilibrium state. Configurations in the inactive states are kinetically stable and are located in deeper basins of the energy landscape than their active counterparts. By analyzing trajectory-to-trajectory fluctuations within the inactive state, we assess correlations between kinetic stability, energy and other structural properties. We show that measures of local order associated to stable local packings and bond-orientational order are weakly correlated with energy and kinetic stability. We discuss what kinds of structural measurement might capture the relevant dynamical features of the inactive state.

Self-assembled AlyGa1%yN quantum dots (QDs), with y = 0 and 0.1, have been grown by molecular beam epitaxy on Al0.5Ga0.5N(0001) oriented layers using sapphire substrates. The QD formation has been followed in situ by reflection high energy electron diffraction (RHEED). A two- to three-dimensional (2D–3D) transition of the layer morphology is observed, characterized by a change of the RHEED pattern from streaky lines to Bragg spots. High QD densities, from 1010 up to near 1012 cm%2, have been obtained. By decreasing the GaN QD size and incorporating Al inside the QDs, a strong variation in the photoluminescence (PL) emission has been observed, enabling to cover a large spectral range from near UV (3 eV) to UV-B (3.95 eV). By combining temperature-dependent and time-resolved PL measurements, the internal quantum efficiency of the QDs has been determined at both low and high temperatures as a function of the PL energy.

The Auger effect and its impact on the internal quantum efficiency (IQE) of yellow light emitters based on silicon-doped InGaN–AlGaN–GaN quantum wells are investigated by power dependence measurement and using an ABC model. Photoluminescence intensity recorded as a function of excitation power density follows a linear dependence up to a threshold PT that depends on the design of the sample. Above this threshold, the variation of the intensity becomes sublinear, which is characteristic of the onset of Auger recombination processes. After extracting the evolution of IQE with pump power from the experimental data, we use a modified ABC modeling that includes the residual n-type doping to estimate the contribution of different recombination channels. We find that the Auger effect dominates in the high-excitation regime. In addition, we find that intercalating an AlGaN-strain-compensating layer reduces not only the coefficient of nonradiative recombination rates but also reduces the onset of Auger recombination.