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Anomalous Hall effect (AHE) plays important role in the rapidly developing field of antiferromagnetic spintronics. It has been recently discussed that it can be a feature of not only uncompensated magnetic systems but also in altermagnetic materials. Hexagonal MnTe belongs to this appealing group of compounds exhibiting AHE and is commonly perceived as magnetically compensated. Here, we demonstrate that bulk form of MnTe exhibits small but detectable magnetic moment correlating with hysteretic behaviour of the AHE. We formulate a phenomenological model which explains how this feature allows to create a disbalance between states with opposite N\'eel vector and prevent the AHE signal from averaging out to zero. Moreover, we show how the dependence of AHE on the N\'eel vector arises on microscopical level and highlight the differences in Berry curvature between magnetically compensated and uncompensated systems.

This paper introduces a new approach to neuromorphic photonics in which microcavities exhibiting strong exciton-photon interaction may serve as building blocks of optical spiking neurons. The experimental results demonstrate the intrinsic property of exciton-polaritons to resemble the Leaky Integrate-and-Fire spiking mechanism. It is shown that exciton-polariton microcavities when non-resonantly pumped with a pulsed laser exhibit leaky-integration due to relaxation of the excitonic reservoir, threshold-and-fire mechanism due to transition to Bose-Einstein condensate, and resetting due to stimulated emission of photons. These effects, evidenced in photoluminescence characteristics, arise within sub-ns timescales. The presented approach provides means for energy-efficient ultrafast processing of spike-like laser pulses at the level below 1 pJ/spike.

The photonics involving II-VI epitaxial layers was limited so far to structures based on a single planar microcavity. Here, we present double, vertically coupled, ZnTe optical microcavities in planar and 3-D photonic molecule geometry. We design the structures with the help of transfer matrix method calculations and we establish their fabrication technology by molecular beam epitaxy. We characterize the samples by reflectivity spatial mapping and study them by measurements of angle-integrated and angle-resolved photoluminescence and reflectivity. We efficiently tailor the interaction strength of the cavities optical modes by an adjustment of the spatial separation between the microcavities, their thickness ratio and by the size of micropillars etched out of the planar structure. Coupling constants extracted from our measurements agree well with those determined in calculations in the frame of a tight-binding approach applied to one-dimensional photonic structures.

Dopants of transition metal ions in II-VI semiconductors exhibit native 2+ valency. Despite this, 3+ or mixed 3+/2+ valency of iron ions in ZnO was reported previously. Several contradictory mechanisms have been put forward for explanation of this fact so far. Here, we analyze Fe valency in ZnO by complementary theoretical and experimental studies. Our calculations within the generalized gradient approximation (GGA+U) indicate that the Fe ion is a relatively shallow donor. Its stable charge state is Fe2+ in ideal ZnO, however, the high energy of the (+/0) transition level enhances the compensation of Fe2+ to Fe3+ by non-intentional acceptors in real samples. Using several experimental methods like electron paramagnetic resonance, magnetometry, conductivity, excitonic magnetic circular dichroism and magneto-photoluminescence we confirm the 3+ valency of the iron ions in polycrystalline (Zn,Fe)O films with the Fe content attaining 0.2%.We find a predicted increase of n-type conductivity upon the Fe doping with the Fe donor ionization energy of 0.25 +/- 0.02 eV consistent with the results of theoretical considerations. Moreover, our magnetooptical measurements confirm the calculated non-vanishing s,p-d exchange interaction between band carriers and localized magnetic moments of the Fe3+ ions in the ZnO, being so far an unsettled issue., Comment: 17 pages, 10 figures

Design, epitaxial growth, and resonant spectroscopy of CdSe Quantum Dots (QDs) embedded in an innovative (Zn,Cd)Se barrier are presented. The (Zn,Cd)Se barrier enables shifting of QDs energy emission down to 1.87 eV, that is below the energy of Mn$^{2+}$ ions internal transition (2.1 eV). This opens a perspective for implementation of epitaxial CdSe QDs doped with several Mn ions as, e. g., the light sources in high quantum yield magnetooptical devices. Polarization resolved Photoluminescence Excitation measurements of individual QDs reveal sharp ($\Gamma <$ 150 $\mu$eV) maxima and transfer of optical polarization to QD confining charged exciton state with efficiency attaining 26 %. The QD doping with single Mn$^{2+}$ ions is achieved., Comment: 6 pages, 5 figures

Solotronics, optoelectronics based on solitary dopants, is an emerging field of research and technology reaching the ultimate limit of miniaturization. It aims at exploiting quantum properties of individual ions or defects embedded in a semiconductor matrix. As already shown, optical control of a spin of a magnetic ion is feasible employing photo-generated carriers confined in a quantum dot. A non-radiative recombination, regarded as a severe problem, limited development of quantum dots with magnetic ions. Our photoluminescence studies on, so far unexplored, individual CdTe dots with single cobalt ions and individual CdSe dots with single manganese ions show, however, that even if energetically allowed, the single ion related non-radiative recombination is negligible in such zero-dimensional structures. This opens solotronics for a wide range of even not yet considered systems. Basing on the results of our single spin relaxation experiments and on the material trends, we identify optimal magnetic ion-quantum dot systems for implementation of a single-ion based spin memory., Comment: Main text and supplementary information: 15 pages, 6 figures, 3 tables

Magnetooptical properties of (Ga,Mn)N layers containing various concentrations of Fe-rich nanocrystals embedded in paramagnetic (Ga,Fe)N layers are reported. Previous studies of such samples demonstrated that magnetization consists of a paramagnetic contribution due to substitutional diluted Fe ions as well as of ferromagnetic and antiferromagnetic components originating from Fe-rich nanocrystals, whose relative abundance can be controlled by the grow conditions. The nanocrystals are found to broaden and to reduce the magnitude of the excitonic features. However, the ferromagnetic contribution, clearly seen in SQUID magnetometry, is not revealed by magnetic circular dichroism (MCD). Possible reasons for differences in magnetic response determined by MCD and SQUID measurements are discussed., Comment: 4 pages, 3 figures

The question of the correlation between magnetization, band splittings, and magnetic circular dichroism (MCD) in the fundamental gap region of dilute magnetic semiconductors is examined experimentally and theoretically taking the case of wurtzite Ga(1-x)FexN as an example. Magnetization and polarization-resolved reflectivity measurements have been performed down to 2K and up to 7T for x = 0.2%. Optical transitions originating from all three free excitons A, B and C, specific to the wurtzite structure, have been observed and their evolution with the magnetic field determined. It is demonstrated that the magnitude of the exciton splittings evaluated from reflectivity-MCD data can be overestimated by more than a factor of 2, as compared to the values obtained by describing the polarization-resolved reflectivity spectra with appropriate dielectric functions. A series of model calculations shows that the quantitative inaccuracy of MCD originates from a substantial influence of the magnetization-dependent exchange interactions not only on the spin splittings of excitons but also upon their linewidth and oscillator strength. At the same time, a method is proposed that allows to evaluate the field and temperature dependencies of the magnetization from MCD spectra. The accurate values of the excitonic splittings and of the magnetization reported here substantiate the magnitudes of the apparent $sp-d$ exchange integrals in (Ga,Fe)N previously determined., Comment: 9 pages, 8 figures

We propose a theoretical design of a structure combining a diluted magnetic semiconductor (DMS) layer with one-dimensional photonic crystal and show that it allows for a sizable (a few-fold) enhancement of the magneto-optical Kerr effect (MOKE) magnitude. We base our calculations on the transfer matrix method assuming realistic, and optimal with respect to a sample growth, parameters. We investigate representative cases of (Ga,Mn)N or (Cd,Mn)Te DMS layers deposited on the top of a respectively GaN- or CdTe-based distributed Bragg reflector, and highlight the applicability of the proposed design for both: III-V wurtzite and II-VI zinc-blende DMS., Comment: 10 pages, 5 fugures

This work presents results of near-band gap magnetooptical studies on (Zn,Mn)O epitaxial layers. We observe excitonic transitions in reflectivity and photoluminescence, that shift towards higher energies when the Mn concentration increases and split nonlinearly under the magnetic field. Excitonic shifts are determined by the s,p-d exchange coupling to magnetic ions, by the electron-hole s-p exchange, and the spin-orbit interactions. A quantitative description of the magnetoreflectivity findings indicates that the free excitons A and B are associated with the Gamma_7 and Gamma_9 valence bands, respectively, the order reversed as compared to wurtzite GaN. Furthermore, our results show that the magnitude of the giant exciton splittings, specific to dilute magnetic semiconductors, is unusual: the magnetoreflectivity data is described by an effective exchange energy N_0(beta-alpha)=+0.2+/-0.1 eV, what points to small and positive N_0 beta. It is shown that both the increase of the gap with x and the small positive value of the exchange energy N_0 beta corroborate recent theory describing the exchange splitting of the valence band in a non-perturbative way, suitable for the case of a strong p-d hybridization., Comment: 8 pages, 8 figures

Near band-gap photoluminescence and reflectivity in magnetic field are employed to determine the exchange-induced splitting of free exciton states in paramagnetic wurtzite Ga1-xMnxN, x < 1%, grown on sapphire substrates by metal-organic vapor phase epitaxy. The band gap is found to increase with x. The giant Zeeman splitting of all three excitons A, B and C is resolved, enabling the determination of the apparent exchange integrals N0alpha(app) = 0.0 +/- 0.1 eV and N0beta(app) = +0.8 +/- 0.2 eV. These non-standard values and signs of the s - d and p - d exchange energies are explained in terms of recent theories that suggest a contribution of the electron-hole exchange to the spin splitting of the conduction band and a renormalization of the free hole spin-splitting by a large p - d hybridization. According to these models, in the limit of a strong p - d coupling, the band gap of (Ga,Mn)N increases with x and the order of hole spin subbands is reversed, as observed., Comment: 8 pages, 5 figures

The sign, magnitude, and range of the exchange couplings between pairs of Mn ions is determined for (Ga,Mn)N and (Ga,Mn)N:Si with x < 3%. The samples have been grown by metalorganic vapor phase epitaxy and characterized by secondary-ion mass spectroscopy; high-resolution transmission electron microscopy with capabilities allowing for chemical analysis, including the annular dark-field mode and electron energy loss spectroscopy; high-resolution and synchrotron x-ray diffraction; synchrotron extended x-ray absorption fine-structure; synchrotron x-ray absorption near-edge structure; infra-red optics and electron spin resonance. The results of high resolution magnetic measurements and their quantitative interpretation have allowed to verify a series of ab initio predictions on the possibility of ferromagnetism in dilute magnetic insulators and to demonstrate that the interaction changes from ferromagnetic to antiferromagnetic when the charge state of the Mn ions is reduced from 3+ to 2+., Comment: 12 pages, 14 figures; This version contains the detailed characterization of the crystal structure as well as of the Mn distribution and charge state

The origin of the emission within the optical mode of a coupled quantum dot-micropillar system is investigated. Time-resolved photoluminescence is performed on a large number of deterministically coupled devices in a wide range of temperature and detuning. The emission within the cavity mode is found to exhibit the same dynamics as the spectrally closest quantum dot state. Our observations indicate that fast dephasing of the quantum dot state is responsible for the emission within the cavity mode. An explanation for recent photon correlation measurements reported on similar systems is proposed., Comment: 4 pages, 4 figures

Asymmetrical horizontal interdot coupling was demonstrated to exist in a system of a single plane of self-assembled CdTe/ZnTe quantum dots. Photoluminescence excitation (PLE), second order photon correlation and optical orientation were used as main experimental tools. Each individual absorbing dot was identified by a sharp PLE resonance, assigned to neutral exciton transition, while the corresponding emission contained several excitonic transitions of different charge states in another single quantum dot different from the absorbing one. Energy and spin transfer dynamics were studied. A high efficiency of spin transfer was found from optical orientation in a vertical magnetic field (70%) as well as without the magnetic field (40%), in spite of a significant anisotropic exchange splitting of the absorbing dot. Coherent mechanism of linear-to-circular polarization conversion was identified, with an efficiency (43%) close to the theoretical limit of 50%.

Using far field optical lithography, a single quantum dot is positioned within a pillar microcavity with a 50 nm accuracy. The lithography is performed in-situ at 10 K while measuring the quantum dot emission. Deterministic spectral and spatial matching of the cavity-dot system is achieved in a single step process and evidenced by the observation of strong Purcell effect. Deterministic coupling of two quantum dots to the same optical mode is achieved, a milestone for quantum computing., Comment: Modified version: new title, additional experimental data in figure 3

Spin dynamics of a single electron and an exciton confined in CdTe/ZnTe quantum dot is investigated by polarization-resolved correlation spectroscopy. Spin memory effects extending over at least a few tens of nanoseconds have been directly observed in magnetic field and described quantitatively in terms of a simple rate equation model. We demonstrate an effective (68%) all-optical read-out of the single carrier spin state through probing the degree of circular polarization of exciton emission after capture of an oppositely charged carrier. The perturbation introduced by the pulsed optical excitation serving to study the spin dynamics has been found to be the main source of the polarization loss in the read-out process. In the limit of low laser power the read-out efficiency extrapolates to a value close to 100%. The measurements allowed us as well to determine neutral exciton spin relaxation time ranging from 3.4 +/- 0.1 ns at B = 0 T to 16 +/- 3 ns at B = 5 T., Comment: to appear in Phys. Rev. B

Polarization resolved photoluminescence (PL) spectroscopy of individual CdTe/ZnTe quantum dots is investigated in the presence of external in-plane magnetic field. We find that the excitonic fine structure strongly depends on the magnitude and direction of applied field. The splitting between "bright" and "dark" states increases with the magnetic field, whereas the anisotropic exchange splitting of the bright excitons can be reduced or enhanced, depending on the field direction. Increase (decrease) is observed when the field is applied parallel to the PL polarization direction of the lower (upper) energy exciton. For intermediate fields, we observe a rotation of the PL polarization orientation. The results are discussed in terms of an effective spin Hamiltonian derived for the exciton ground state., Comment: 6 pages, 6 figures

Systematic measurements of auto- and cross-correlations of photons emitted from individual CdTe/ZnTe quantum dots under pulsed excitation were used to elucidate non-resonant excitation mechanisms in this self-assembled system. Memory effects extending over a few excitation pulses have been detected in agreement with previous reports and quantitatively described by a rate equation model, fitting a complete set of correlation and PL intensity results. The important role of single carrier trapping in the quantum dot was established. An explanation was suggested for the unusually wide antibunching dip observed previously in X-X autocorrelation experiments on quantum dots under cw excitation., Comment: 8 pages, 8 figures