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Atomlike defect levels in silicon carbide (SiC) polytypes have been proposed and proven to be an excellent platform for various quantum technology applications. Single-photon emitters, coherent control at room temperature, and temperature and magnetic field sensing at the nanoscale have already been demonstrated for the case of vacancy and divacancy defects in SiC and more recently proposed for negatively charged NV centers. NV centers, which allow a better control of their generation, offer in addition a further shift of the spectral range in the near infrared, i.e., in the $O$- and $S$-band telecom range. We demonstrate here that the association of high resolution optical spectroscopy and electron paramagnetic resonance spectroscopy combined with first-principles calculations allow the identification of the microscopic structure of the six distinct NV centers in $6H\ensuremath{-}\mathrm{SiC}$ and the assignment of their associated zero-phonon photoluminescence lines. Time resolved photo-EPR measurements at $T=4\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ show that NV centers in $6H\ensuremath{-}\mathrm{SiC}$ present spin lattice relaxation times of several seconds. These excellent qubit properties should enable their application and implementation in quantum information devices.

We have investigated the optical properties of the ${(\mathrm{NV})}^{\ensuremath{-}}$ center in 3C-SiC to determine the photoluminscence zero phonon line (ZPL) associated with the $^{3}\mathrm{E}\ensuremath{\rightarrow}^{3}\mathrm{A}_{2}$ intracenter transition. Combining electron paramagnetic resonance and photoluminescence spectroscopy, we show that the ${\mathrm{NV}}^{\ensuremath{-}}\phantom{\rule{0.28em}{0ex}}\mathrm{center}$ in 3C-SiC has a ZPL line at 1.468 $\ensuremath{\mu}\mathrm{m}$ in excellent agreement with theoretical predictions. The ZPL line can be observed up to $T=100$ K. The negatively charged NV center in 3C-SiC is the structural isomorphe of the NV center in diamond and has equally a spin $S=1$ ground state and a spin $S=1$ excited state, long spin lattice relaxation times and presents optically induced groudstate spin polarization. These properties make it already a strong competitor to the NV center in diamond, but as its optical domain is shifted in the near infrared at $1.5\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$, the NV center in 3C-SiC is compatible with quantum photonic networks and silicon based microelectronics.

It has become widely accepted that particles with long-range hopping do not undergo Anderson localization. However, several recent studies demonstrated localization of particles with long-range hopping. In particular, it was recently shown that the effect of long-range hopping in 1D lattices can be mitigated by cooperative shielding, which makes the system behave effectively as one with short-range hopping. Here, we show that cooperative shielding, demonstrated previously for 1D lattices, extends to 3D lattices with isotropic long-range $r^{-\alpha}$ hopping, but not to 3D lattices with dipolar-like anisotropic long-range hopping. We demonstrate the presence of localization in 3D lattices with uniform ($\alpha=0$) isotropic long-range hopping and the absence of localization with uniform anisotropic long-range hopping by using the scaling behaviour of eigenstate participation ratios. We use the scaling behaviour of participation ratios and energy level statistics to show that the existence of delocalized, non-ergodic extended, or localized states in the presence of disorder depends on both the exponents $\alpha$ and the isotropy of the long-range hopping amplitudes., Comment: 12 pages, 9 figures. Extensive rewrite and new perspective

Fluorescent paramagnetic defects in solids have become attractive systems for quantum information processing in recent years. One of the leading contenders is the negatively charged nitrogen-vacancy (NV) defect in diamond with visible emission, but an alternative solution in a technologically mature host is an immediate quest for many applications in this field. It has been recently found that various polytypes of silicon carbide (SiC), that are standard semiconductors with wafer scale technology, can host a NV defect that could be an alternative qubit candidate with emission in the near infrared region. However, there is much less known about this defect than its counterpart in diamond. The inequivalent sites within a polytype and the polytype variations offer a family of NV defects. However, there is an insufficient knowledge on the magneto-optical properties of these configurations. Here we carry out density functional theory calculations, in order to characterize the numerous forms of NV defects in the most common polytypes of SiC including $3C, 4H$, and $6H$, and we also provide new experimental data in $4H$ SiC. Our calculations mediate the identification of individual NV qubits in SiC polytypes. In addition, we discuss the formation of NV defects in SiC, providing detailed ionization energies of NV defects in SiC, which reveals the critical optical excitation energies for ionizing these qubits in SiC. Our calculations unravel the challenges to produce NV defects in SiC with a desirable spin bath.

The outstanding magneto-optical properties of the nitrogen-vacancy (NV) center in diamond have stimulated the search for similar systems. We show here that NV triplet centers can also be generated in all the main SiC polytypes. We have identified by electron paramagnetic resonance spectroscopy and first-principles calculations the axial ${\mathrm{NV}}^{\ensuremath{-}}$ pairs in $3C,4H$, and $6H$ SiC, showing polytype and lattice site-specific magnetic and optical properties. We demonstrate very efficient room-temperature spin polarization of the ground state upon near infrared optical excitation for the NV center in $3C$ SiC and axial NV centers in the hexagonal $(4H,6H)$ polytypes; the signals of basal pairs are much lower in intensity. Axial NV centers in hexagonal SiC polytypes and thus constitute unidirectional ensembles which may be useful in nanosensing applications.

The microscopic structure and introduction rate of point defects in n-type 6H- and 4H-SiC generated by room-temperature proton implantation have been studied by the electron paramagnetic resonance technique. In order to selectively study the effects of defect introduction in the trace region, 12-MeV implantation in 300-\ensuremath{\mu}m-thick samples was employed, for which the protons completely cross the sample. In both polytypes we observe three dominant paramagnetic defects attributed to the Si monovacancy in the negative charge state and the neutral Si monovacancy in the hexagonal and quasicubic lattice sites, respectively. The concentration of all three defects increases linearly with proton dose. Their total introduction rate is \ensuremath{\sim}19 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which amounts only to 4% of the concentration expected from SRIM simulations. No carbon-vacancy-related defect is observed. Thermal annealing at 1100 \ifmmode^\circ\else\textdegree\fi{}C is sufficient to anneal out the ${V}_{\mathrm{Si}}$ defects and to restore n-type conductivity. The observation of the neutral Si vacancy at hexagonal and quasicubic sites under thermal equilibrium conditions at 4 K does not support their previous assignment to an excited state.

The intrinsic defects in p-type $6H\ensuremath{-}\mathrm{S}\mathrm{i}\mathrm{C}:\mathrm{A}\mathrm{l}$ generated by electron irradiation at 300 keV, which is close to the threshold of the silicon atom displacement, have been studied by electron paramagnetic resonance spectroscopy. We observed two dominant irradiation-induced paramagnetic defects: (i) a silicon-vacancy-related spin $S=3/2$ defect with a zero-field splitting of $D=68.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1},$ which is tentatively attributed to a Si Frenkel pair aligned parallel to the c axis and (ii) a carbon-vacancy-related spin $S=1/2$ defect already previously attributed to ${V}_{\mathrm{C}}^{+}.$ A slight increase of the electron energy to 350 keV creates in addition a lower-symmetry $S=3/2$ spectrum equally attributed to Si Frenkel pairs with the interstitial being located at six equivalent off-axis sites. High-energy irradiation (2 MeV) creates only isolated Si vacancies with no zero-field splitting.

The $(100){\mathrm{S}\mathrm{i}/\mathrm{S}\mathrm{i}}_{3}{\mathrm{N}\mathrm{}}_{4}$ interface, formed by the deposition of ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$ layers on bulk (100) Si by distributed electron-cyclotron-resonance plasma-enhanced chemical vapor deposition, has been studied by electron-paramagnetic-resonance spectroscopy. Only one interface defect, the ${P}_{b0}$ center, characterized by trigonal point symmetry and principal values of the g tensor, ${g}_{\ensuremath{\parallel}}=2.0018\ifmmode\pm\else\textpm\fi{}0.0002$ and ${g}_{\ensuremath{\perp}}=2.0089\ifmmode\pm\else\textpm\fi{}0.0002$, has been detected. Its area concentration is of the order of $(2--5)\ifmmode\times\else\texttimes\fi{}{10}^{11} {\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}.$ The characteristic defect of the $(100){\mathrm{S}\mathrm{i}/\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ interface, the ${P}_{b1}$ center, is not observed. The ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$ layers contain one main native bulk defect, the K center, at concentrations of $5\ifmmode\times\else\texttimes\fi{}{10}^{17} {\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}.$

The adsorption of ethylene on $\mathrm{Si}(001)\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}1$ and $\mathrm{Si}(111)\ensuremath{-}7\ifmmode\times\else\texttimes\fi{}7$ has been studied at 300 K using core-level (Si $2p,$ C $1s)$ and valence-band x-ray photoemission with synchrotron radiation. Dangling bonds play a very different role in the chemisorption process on these two surfaces. While for $\mathrm{Si}(001)\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}1$ the quenching of the surface states ``monitors'' the molecule uptake, this is not the case for $\mathrm{Si}(111)\ensuremath{-}7\ifmmode\times\else\texttimes\fi{}7:$ the adsorption process continues well after all surface states have disappeared, and carbon coverages greater than that deduced from the rest-atom/adatom bridge model are obtained. An alternative model is proposed, in which molecules are bonded directly to the rest plane. The thermal stability of both surfaces covered by ethylene is also studied. We relate the formation of carbon-rich layers at 820\char21{}860 K to the strong changes in the electronic structure of the silicon surface.

The magnetism in \ensuremath{\sim}1 at$%$ Mn-doped nanometric Bi${}_{2}$Se${}_{3}$ epitaxial layers has been studied by ferromagnetic resonance (FMR), magnetometry, and polarized neutron reflectivity (PNR) measurements. The FMR results reveal the formation of an intrinsic homogeneous FM bulk phase, as confirmed by the PNR measurements. Its critical temperature is low: ${T}_{c}$ \ensuremath{\approx} 6 K; no surface magnetic phase is detected above this temperature. The FMR measurements at $T$ $=$ 4.2 K show a single uniform mode spectrum with a strong uniaxial in-plane/out-of-plane symmetry. The uniaxial anisotropy field at $T$ $=$ 4 K is ${H}_{a}$ \ensuremath{\sim} \ensuremath{-}2500 Oe, corresponding to a uniaxial anisotropy constant of \ensuremath{-}9 \ifmmode\times\else\texttimes\fi{} 10${}^{3}$ erg/cm${}^{3}$. The easy axis of magnetization is oriented in the film plane for temperatures between 4 K and ${T}_{c}$. Conduction electron-mediated FM in this (very) diluted magnetic semiconductor is considered as the most probable origin of the FM phase formation.

Ga1-xMnxAs films with exceptionally high saturation magnetizations of approximate to 100 emu/cm(3) corresponding to effective Mn concentrations of x(eff)approximate to 0.10 still have a Curie temperature T-C smaller than 195 K contradicting mean-field predictions. The analysis of the critical exponent beta of the remnant magnetization-beta = 0.407(5)-in the framework of the models for disordered/amorphous ferromagnets suggests that this limit on T-C is intrinsic and due to the short range of the ferromagnetic interactions resulting from the small mean-free path of the holes. This result questions the perspective of room-temperature ferromagnetism in highly doped GaMnAs.

The nearest-neighbor antisite pair defects in 4H-SiC, 6H-SiC, and 3C-SiC single crystals have been identified using electron paramagnetic resonance spectroscopy in combination with a nonperturbative ab initio scheme for the electronic g tensor. Based on the theoretical predictions, the positively charged defect has been found experimentally also in the cubic 3C-SiC polytype where it is characterized by spin 1/2 and highly anisotropic g values of g(xx)=2.0030, g(yy)=2.0241, and g(zz)=2.0390 within C-1h symmetry. The exceptional large g values are explained by details of the spin-orbit coupling causing a strongly anisotropic quenching of the orbital angular momentum of the p-like unpaired electron.

The magnetic properties of annealed, epitaxial Ga0.93Mn0.07As layers under tensile and compressive stress have been investigated by X-band (9 GHz) and Q-band (35 GHz) ferromagnetic resonance (FMR) spectroscopy. From the analysis of the linewidths of the uniform mode spectra the FMR Gilbert damping factor alpha has been determined. At T=4 K we obtain a minimum damping factor of alpha=0.003 for the compressively stressed layer. Its value is not isotropic. It has a minimum value for the easy axes orientations of the magnetic field and increases with the measuring temperature. Its average value is for both type of films on the order of 0.01 in spite of strong differences in the inhomogeneous linewidth which vary between 20 and 600 Oe for the layers grown on GaAs and GaInAs substrates, respectively.

The magnetic properties of ferromagnetic ${\mathrm{Ga}}_{0.93}{\mathrm{Mn}}_{0.07}\mathrm{As}$ thin films have been investigated by $X$-band ferromagnetic resonance (FMR) spectroscopy as a function of hole concentration. The hole concentration $[p]$ was varied from ${10}^{21}\phantom{\rule{0.3em}{0ex}}\text{to}l{10}^{18}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ by hydrogen passivation. We determined the limit for the onset of ferromagnetism as $[p]\ensuremath{\approx}1\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, the critical temperatures as a function of hole concentration, and the easy and hard axes for magnetization as a function of temperature and hole concentration. The temperature and hole concentration dependences of the magnetocrystalline anisotropy constants were determined. The dominant constant ${K}_{2\ensuremath{\perp}}$ changes for $T=4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ from $\ensuremath{-}8\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}+1\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{0.3em}{0ex}}\mathrm{erg}∕{\mathrm{cm}}^{3}$ with decreasing the hole concentration. The anisotropy coefficient ${\ensuremath{\kappa}}_{2}$ varies with the reduced magnetization with an expected quadratic power law. The effective $g$ factor is different from the paramagnetic value of ${\mathrm{Mn}}^{2+}$ of $g=2.00$, due to the hole contribution; it changes from 1.90 to 1.97 with decreasing the hole concentration. In addition to the resonance positions, the FMR linewidth and intensity were analyzed. The FMR linewidth varies with the inverse of the hole concentration. Free energy surfaces are calculated for the simulation of magnetization switching.

Contradictory results concerning the formation of a ferromagnetic state in Mn doped ZnO layers have been reported in the last years. We present the results of an electron paramagnetic resonance study on homogeneous single phase $n$-type conductive ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{\mathrm{x}}\mathrm{O}$ $(xl0.07)$ thin films. Based on the observation of the existence of a narrow Mn composition range, $xl0.05$, where a ferromagnetic interaction is dominant, whereas for larger $x$ nearest neighbor antiferromagnetic interactions dominate, we propose a magnetic polaron model for the ferromagnetic Mn-Mn interaction in $n$-type films.

The magnetic properties of ${\mathrm{Co}}^{2+}$ ions in epitaxial (Zn,Co)O layers with 10% Co concentration have been studied by electron paramagnetic resonance spectroscopy. The Co-related EPR spectrum is characterized by a 200-G broad anisotropic single line with g factors close to those of the isolated ${\mathrm{Co}}^{2+}$ ion. The temperature dependence of the EPR signal follows a Curie-Weiss law with a critical temperature of +12 K. We find no evidence for a high-temperature ferromagnetic state. Magnetocrystalline anisotropy is observed and a canted spin arrangement is suggested.

The phosphorus related defects in n-type 6H-SiC monocrystals doped by neutron transmutation have been studied by electron paramagnetic resonance (EPR) spectroscopy. After thermal annealing at 1900°C two sets of three phosphorous related EPR spectra are observed, which are attributed to the isolated shallow P donor on the two quasicubic (c1,c2) and the hexagonal (h) sites as well as to a deep P center on the c1, c2, h sites. The deep P center is tentatively attributed to the P S i -carbon vacancy complex. Our results show in agreement with theoretical predictions that the previous suggestions that the two types of centers are the ground and excited states of the isolated P donor can be ruled out. The donor-monovacancy complexes are characterized by an exceptional thermal stability.