Resolved Hyperfine Interactions Research Articles

Electron nuclear interactions in spin-3/2 color centers in silicon carbide: A high-field pulse EPR and ENDOR study

High-frequency pulsed electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) were used to determine electron nuclear interactions on remote ligand shells of silicon and carbon in spin-3/2 color centers with an optically induced high-temperature spin alignment in hexagonal $4H$-, $6H$-, and rhombic $15R$-silicon carbide (SiC) polytypes. The EPR and ENDOR experimental data relate unambiguously to spin-3/2 centers in which the optically induced alignment of the spin-level populations occurs. The identification is based on resolved ligand hyperfine interactions with carbon and silicon nearest, next-nearest, and the more distant neighbors and on the determination of the unpaired electron spin densities. The hyperfine interactions with $^{29}\mathrm{Si}$ and $^{13}\mathrm{C}$ nuclei are unambiguously separated due to the selective population of the fine-structure levels with certain values of ${\mathrm{M}}_{\mathrm{S}}$. The signs of these interactions and, as a result, the signs of oscillating spin density at $^{29}\mathrm{Si}$ and $^{13}\mathrm{C}$ nuclei, are determined. On the basis of the EPR and optically induced ENDOR measurements, signs of the fine-structure splitting for all the centers were demonstrated, which made it possible to establish the character of optically induced spin alignment, including the inverse populations of the spin levels for these centers. The values of hyperfine interaction with $^{29}\mathrm{Si}$ and $^{13}\mathrm{C}$ nuclei, including those remote from the localization of the spin-3/2 center, are tabulated, which can be used by a number of algorithms in quantum information processing as long-term memory.

Zn acceptors in β-Ga2O3 crystals

Electron paramagnetic resonance (EPR) is used to identify and characterize neutral zinc acceptors in Zn-doped β-Ga2O3 crystals. Two EPR spectra are observed at low temperatures, one from Zn ions at tetrahedral Ga(1) sites (the ZnGa10 acceptor) and one from Zn ions at octahedral Ga(2) sites (the ZnGa20 acceptor). These Zn acceptors are small polarons, with the unpaired spin localized in each case on a threefold coordinated oxygen O(I) ion adjacent to the Zn ion. Resolved hyperfine interactions with neighboring 69Ga and 71Ga nuclei allow the EPR spectra from the two acceptors to be easily distinguished: ZnGa10 acceptors interact equally with two Ga(2) ions and ZnGa20 acceptors interact unequally with a Ga(1) ion and a Ga(2) ion. The as-grown crystals are compensated, with the Zn ions initially present as singly ionized acceptors (ZnGa1− and ZnGa2−). Exposing a crystal to 325 nm laser light, while being held at 140 K, primarily produces neutral ZnGa20 acceptors when photoinduced holes are trapped at ZnGa2− acceptors. This suggests that there may be significantly more Zn ions at Ga(2) sites than at Ga(1) sites. Warming the crystal briefly to room temperature, after removing the light, destroys the EPR spectrum from the shallower ZnGa20 acceptors and produces the EPR spectrum from the more stable ZnGa10 acceptors. The ZnGa20 acceptors decay in the 240–260 K region with a thermal activation energy near 0.65 eV, similar to MgGa20 acceptors, whereas the slightly deeper ZnGa10 acceptors decay close to room temperature with an approximate thermal activation energy of 0.78 eV.

Identification of a Nitrogen-related acceptor in ZnO nanowires.

Nanostructured ZnO, such as ZnO nanowires (NWs), is a promising material system for a wide range of electronic applications ranging from light emission to water splitting. Utilization of ZnO requires development of effective and controllable p-type doping. Nitrogen is considered among key p-type dopants though the exact origin of N-induced acceptors is not fully understood, especially in the case of nanostructured ZnO. In this work we employ electron paramagnetic resonance (EPR) spectroscopy to characterize N-related acceptors in ZnO NWs. N doping was achieved using ion implantation commonly employed for these purposes. We show that the Fermi level position is lowered in the N implanted NWs, indicating the formation of compensating acceptors. The formed acceptor is unambiguously proven to involve an N atom based on a resolved hyperfine interaction with a 14N nucleus with a nuclear spin I = 1. The revealed center is shown to act as a deep acceptor with an energy level located at about 1.1 eV above the top of the valence band. This work represents the first unambiguous identification of acceptors deliberately introduced in ZnO nanostructures. It also shows that the configuration and electronic structure of the N-related acceptors in nanostructures differ from those in ZnO bulk and thin-films. The present findings are of importance for understanding the electronic properties of nanostructured ZnO required for its future electronic applications.

Dual role of Sb ions as electron traps and hole traps in photorefractive Sn_2P_2S_6 crystals

Doping photorefractive single crystals of Sn2P2S6 with antimony introduces both electron and hole traps. In as-grown crystals, Sb3+ (5s2) ions replace Sn2+ ions. These Sb3+ ions are either isolated (with no nearby perturbing defects) or they have a charge-compensating Sn2+ vacancy at a nearest-neighbor Sn site. When illuminated with 633 nm laser light, isolated Sb3+ ions trap electrons and become Sb2+ (5s25p1) ions. In contrast, Sb3+ ions with an adjacent Sn vacancy trap holes during illumination. The hole is primarily localized on the (P2S6)4− anionic unit next to the Sb3+ ion and Sn2+ vacancy. These trapped electrons and holes are thermally stable below ∼200 K, and they are observed with electron paramagnetic resonance (EPR) at temperatures below 150 K. Resolved hyperfine interactions with 31P, 121Sb, and 123Sb nuclei are used to establish the defect models.

Sn vacancies in photorefractive Sn2P2S6 crystals: An electron paramagnetic resonance study of an optically active hole trap

Electron paramagnetic resonance (EPR) is used to identify the singly ionized charge state of the Sn vacancy (VSn−) in single crystals of Sn2P2S6 (often referred to as SPS). These vacancies, acting as a hole trap, are expected to be important participants in the photorefractive effect observed in undoped SPS crystals. In as-grown crystals, the Sn vacancies are doubly ionized (VSn2−) with no unpaired spins. They are then converted to a stable EPR-active state when an electron is removed (i.e., a hole is trapped) during an illumination below 100 K with 633 nm laser light. The resulting EPR spectrum has g-matrix principal values of 2.0079, 2.0231, and 1.9717. There are resolved hyperfine interactions with two P neighbors and one Sn neighbor. The isotropic portions of these hyperfine matrices are 167 and 79 MHz for the two 31P neighbors and 8504 MHz for the one Sn neighbor (this latter value is the average for 117Sn and 119Sn). These VSn− vacancies are shallow acceptors with the hole occupying a diffuse wave function that overlaps the neighboring Sn2+ ion and (P2S6)4− anionic unit. Using a general-order kinetics approach, an analysis of isothermal decay curves of the VSn− EPR spectrum in the 107–115 K region gives an activation energy of 283 meV.

Identification of native defects (vacancies and antisites) in CdSiP2 crystals

Electron paramagnetic resonance (EPR) is used to identify four native defects in single crystals of CdSiP2. This nonlinear optical material is used in optical parametric oscillators to generate tunable output in the mid-infrared. The performance of these frequency-conversion devices is limited when infrared absorption bands associated with native defects overlap a pump wavelength. Cadmium, silicon, and phosphorus vacancies and also silicon-on-cadmium antisites are present in the as-grown undoped CdSiP2 crystals. Using near-band-edge 632.8 nm light from a He-Ne laser, a paramagnetic charge state, and thus an EPR spectrum, is formed at liquid-helium temperatures for three of the four defects. The EPR spectrum from the singly ionized silicon vacancy (VSi−) is present without light and has five hyperfine lines due to equal interactions with the four neighboring 31P nuclei. In contrast, the photoinduced EPR spectrum from the singly ionized cadmium vacancy (VCd−) has a three-line hyperfine pattern due to equal interactions with only two of its four neighboring 31P nuclei. The light-induced spectrum from the singly ionized silicon-on-cadmium antisite (Si Cd+) also has a three-line hyperfine pattern, thus indicating that the unpaired spin interacts primarily with only two 31P neighbors. For the neutral phosphorus vacancy (VP 0), the unpaired spin is primarily localized on the nearest-neighbor silicon ions and the photoinduced EPR spectrum has no resolved 31P hyperfine interactions. The silicon and cadmium vacancies are acceptors, and the silicon-on-cadmium antisite and the phosphorus vacancy are donors.

Spectrally resolved hyperfine interactions between polaron and nuclear spins in organic light emitting diodes: Magneto-electroluminescence studies

We use spectrally resolved magneto-electroluminescence (EL) measurements to study the energy dependence of hyperfine interactions between polaron and nuclear spins in organic light-emitting diodes. Using layered devices that generate bright exciplex emission, we show that the increase in EL emission intensity I due to small applied magnetic fields of order 100 mT is markedly larger at the high-energy blue end of the EL spectrum (ΔI/I ∼ 11%) than at the low-energy red end (∼4%). Concurrently, the widths of the magneto-EL curves increase monotonically from blue to red, revealing an increasing hyperfine coupling between polarons and nuclei and directly providing insight into the energy-dependent spatial extent and localization of polarons.

Evidence for a phosphorus-related interfacial defect complex at a GaP/GaNP heterojunction

Optically detected magnetic resonance (ODMR) studies of molecular beam epitaxial GaNP/GaP structures reveal presence of a P-related complex defect, evident from its resolved hyperfine interaction between an unpaired electronic spin $(S=1/2)$ and a nuclear spin $(I=\frac{1}{2})$ of a $^{31}\text{P}$ atom. The principal axis of the defect is concluded to be along a $⟨111⟩$ crystallographic direction from angular dependence of the ODMR spectrum, restricting the P atom (either a ${\text{P}}_{\text{Ga}}$ antisite or a ${\text{P}}_{i}$ interstitial) and its partner in the complex defect to be oriented along this direction. The principal values of the electronic $g$ tensor and hyperfine interaction tensor are determined as: ${g}_{\ensuremath{\perp}}=2.013$, ${g}_{\ensuremath{\parallel}}=2.002$, and ${A}_{\ensuremath{\perp}}=130\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\text{ }{\text{cm}}^{\ensuremath{-}1}$, ${A}_{\ensuremath{\parallel}}=330\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\text{ }{\text{cm}}^{\ensuremath{-}1}$, respectively. The interface nature of the defect is clearly manifested by the absence of the ODMR lines originating from two out of four equivalent $⟨111⟩$ orientations. Defect formation is shown to be facilitated by nitrogen ion bombardment under nonequilibrium growth conditions and the defect is thermally stable upon post-growth thermal annealing.

Depth resolved hyperfine interactions with standing waves in [W/Si]10/Si/Ag/57Fe/Ag/Si multilayer

Delayed nuclear resonant reflectivity (NRR) of [W/Si]10 /Si(z nm)/Ag/57Fe/Ag/Si multilayer as well as the time spectra at several grazing angles have been measured at the nuclear resonance beamline ID18/ID22N of the ESRF. NRR of 57Fe layer has been enhanced by standing waves created by Bragg reflection from [W/Si]10 backing and by multiple reflections from Ag adjacent layers. Fitting of the data was done with the program package REFTIM. The reconstructed HFI depth distributions show an essential asymmetry of the 57Fe interfaces. The total 57Fe density depth-profiles turn out to be in a good agreement with the measured Fe Kα fluorescence curve.

Universal control of nuclear spins via anisotropic hyperfine interactions

We propose a method for completely controlling a nuclear spin subsystem using only microwave irradiation of resolved anisotropic hyperfine interactions with a single electron spin. This paradigm of control has important applications for spin based solid-state quantum information processing. In particular we argue that indirect addressing of the nuclear spins via an electron spin acting as a spin actuator allows for nuclear spin quantum logic gates whose operational times are significantly faster than either gates based on rf fields resonant with nuclear spin flips or nuclear-nuclear dipolar interactions. We demonstrate experimental aspects of this method with one electron and one nuclear spin of a single crystal of irradiated malonic acid.

Appearance of copper d9 defect centres in wide‐gap CdSe nanoparticles: A high‐frequency EPR study

AbstractWhen preparing CdSe nanoparticles in polybutadiene‐based polymer matrices, quantum confinement in the smaller particles leads to the usual increase in the band‐gap energy, with associated, well defined, band‐edge luminescence emission. However, for samples whose size is smaller than 3 nm, new broad luminescence features appear, with the simultaneous appearance of a new paramagnetic defect centre that has not previously been reported in the literature. This work concentrates on analyzing the electron paramagnetic resonance of the new defect, which is found to be located at a trigonally‐distorted tetrahedral site, with a spin S = 1/2 and g‐values g|| = 2.321 ± 0.006 and g⊥ = 2.053 ± 0.006. With the attribution of very poorly resolved hyperfine interaction to a nucleus of spin I = 3/2, the identity of the defect is ascribed to copper in its d9 state, which would be expected to act as a simple acceptor centre in CdSe. Conclusions are drawn to the effect that the defect normally resides within the valence band for bulk and large particles, only becoming apparent as a defect within the band‐gap as the energy increases above 2.6 eV in the smaller particles. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Silicon vacancy annealing andDIluminescence in6H−SiC

Combining electron paramagnetic resonance measurements with ab initio calculations, we identify the ${V}_{\mathrm{C}}{\mathrm{C}}_{\mathrm{Si}}({\mathrm{Si}}_{\mathrm{C}}{\mathrm{C}}_{\mathrm{Si}})$ complex as a second annealing product of the silicon vacancy via an analysis of resolved carbon hyperfine interactions and of the zero-field splitting parameter of its excited triplet state. At high temperatures the carbon vacancy can dissociate from this complex leaving behind a diamagnetic ${\mathrm{Si}}_{\mathrm{C}}{({\mathrm{C}}_{\mathrm{Si}})}_{2}$ complex, which is proposed to be an excellent candidate to cause the ${D}_{\mathrm{I}}$ photoluminescence spectrum: the calculated local vibration modes fit very well with the characteristic phonon assisted structure of the photoluminescence-spectra. Furthermore, our model provides a detailed explanation for the excitonic electron-hole recombination responsible for the ${D}_{\mathrm{I}}$ luminescence.

Identification of the carbon dangling bond center at the 4H-SiC/SiO(2) interface by an EPR study in oxidized porous SiC.

We report the observation of a paramagnetic interface defect in thermally oxidized porous n-type doped 4H-SiC/SiO(2). Based on its axial symmetry and resolved hyperfine interactions it is attributed to an sp(3) carbon dangling bond center situated at the SiC side of the interface. This center is electrically active and pins the Fermi level in the oxidized samples. No silicon related paramagnetic dangling bond centers are observed. The formation of dangling bond centers seems to be related to interstitial oxygen diffusion at the interface during the oxidation process.

Optical detection of electron paramagnetic resonance in low-dislocation-content GaN grown by hydride vapor-phase epitaxy

Three broad overlapping photoluminescence bands, centered at $\ensuremath{\sim}1.75 \mathrm{eV}$ (red), $\ensuremath{\sim}2.2 \mathrm{eV}$ (yellow), and $\ensuremath{\sim}2.33 \mathrm{eV}$ (green), are observed in low-dislocation-content GaN grown by the hydride vapor-phase epitaxy method. Optical detection of electron paramagnetic resonance (ODEPR) studies reveal that each is fed by a spin-dependent electron transfer from a shallow donor to a deeper defect, which is different for each of the bands, and different from defects previously found in GaN grown by the more conventional epitaxy methods (metal-organic vapor-phase epitaxy and molecular beam epitaxy). The g-value progression for the deeper defects suggests a two-stage luminescence process in which the luminescence arises from hole capture at the specific defect in each case, after the spin-dependent electron capture process observed by ODEPR. This model also fits for the deep level usually found in the yellow band for more heavily dislocated materials, as originally suggested by Glaser. An additional weak anisotropic ODEPR signal is also observed in the red and yellow bands. None of the signals show resolved hyperfine interactions, and their chemical and/or lattice structures remain unknown.

Interpretation of the EPR Spectra of Nitrogen-Containing Compounds Adsorbed on Copper-Exchanged Zeolites

Electron paramagnetic resonance (EPR) spectroscopy was used to investigate adsorption of a series of amines (ammonia, methylamine, triethylamine) on Cu−ZSM5 and Cu−Beta. Several changes were observed in the EPR spectra after adsorption of the amine: an increase in spectral broadening, a shift in the copper hyperfine features, and the appearance of nitrogen hyperfine splittings in the perpendicular region of the EPR spectrum for some of the samples. For samples that exhibited resolved nitrogen hyperfine couplings, the EPR spectra were easily interpreted in terms of the number of coordinated amine molecules. For samples in which resolved nitrogen hyperfine interactions were not observed, the interpretation of the EPR spectrum was more complex. A second approach for the interpretation of the EPR spectrum involved using an empirical model correlating the EPR parameters for a series of model compounds to the number of coordinated nitrogen atoms and the charge of the copper complex. This approach was very useful for determining the number of coordinated nitrogen atoms if the charge of the complex was known or vice versa. It was determined that four molecules of ammonia or methylamine bind to the Cu2+ centers in Cu−ZSM5 and Cu−Beta, but that only one molecule of triethylamine is able to bind to the Cu2+ centers in these zeolites. For cases in which the charge of the complex and the number of ligands are both unknown, there is more uncertainty in using this empirical approach to interpret the EPR spectra. This empirical approach was further assessed by application to the EPR spectra of NO adsorbed on Cu−ZSM5 and Cu−Beta.

Negatively charged Si vacancy in4HSiC: A comparison between theory and experiment

We use electron paramagnetic resonance and electron nuclear double resonance to identify the negatively charged Si vacancy in neutron-irradiated $4H$ SiC. The identification is based on resolved ligand hyperfine interactions with carbon and silicon nearest and next nearest neighbors and on the determination of the spin state, which is $S=3/2.$ The magnetic resonance parameters of ${V}_{\mathrm{Si}}^{\ensuremath{-}}$ are almost identical for the polytypes $3C,$ $4H,$ and $6H.$ The experimental findings are supported by theoretical ligand hyperfine interaction data based on a total-energy calculation using the standard local-density approximation of the density-functional theory.

Optical detection of magnetic resonance in electron-irradiated GaN

Four new optically detected magnetic resonances are observed in photoluminescence bands at \ensuremath{\sim} 0.85 and \ensuremath{\sim}0.93 eV, which are produced in epitaxial wurtzite films of GaN by 2.5-MeV electron irradiation. One of these reveals a resolved hyperfine interaction with a single Ga nucleus, indicating a displaced Ga atom of some kind. It is tentatively identified as a complex involving interstitial ${\mathrm{Ga}}_{\mathrm{i}}^{2+}$.

Electron Paramagnetic Resonance of d-Xylose Isomerase: Evidence for Metal Ion Movement Induced by Binding of Cyclic Substrates and Inhibitors

The interactions of substrates and inhibitors with the Mn2+ ions in the binuclear active center of D-xylose isomerase (XylI) were investigated by EPR spectroscopy at X- and Q-band frequencies. The metal binding site 1 (A site) was specifically occupied with Mn2+ ions by blocking the high-affinity metal binding site 2 (B-site) either with Co2+ ions, resulting in a catalytically active enzyme, or with Cd2+ or Pb2+ ions yielding an inactive enzyme species. Incubation of both the Co2+/Mn2+- and the Cd2+/Mn2+-XylI with the acyclic inhibitor xylitol revealed EPR spectra with well-resolved hyperfine patterns, but with increased zero field splitting (zfs) parameter D compared to the spectra without inhibitor. D was estimated by spectral simulation of the central --1/2<-->1/2 fine structure transition. D values of 33 and 50 mT were obtained for the Co2+/Mn2+-XylI and the Cd2+/Mn2+-XylI samples, respectively. These results indicate direct interaction of the xylitol with the Mn2+ in the A-site. More drastic changes are observed with the substrates D-xylose and D-glucose and with the cyclic inhibitors 5-thio-alpha-D-glucose and 2-desoxy-D-glucose. For Cd2+/Mn2+-XylI, the EPR spectra with substrates and cyclic inhibitors are similar to each other but different from the spectra with the acylic inhibitor xylitol. They exhibit well-resolved line patterns with a relative large zero field splitting, which was estimated to be in the range of D = 65-85 mT in the various complexes. Binding of substrates or of cyclic inhibitors to the Co2+/ Mn2+-XylI yields EPR spectra without resolved hyperfine interactions, indicative of dipolar interaction between the two paramagnetic metal ions. This can be explained with a decrease in the metal-metal distance. Furthermore, the EPR data strongly suggest that the corresponding metal ion movement is induced by binding of the cyclic conformation of either substrates or cyclic inhibitors and not by binding of the extended form of the sugars.

Electron-Nuclear Double Resonance Study of the Zinc Vacancy in Zinc GERMANIUM PHOSPHIDE (ZnGeP2)

AbstractAs-grown crystals of ZnGeP2 are highly compensated and contain significant concentrations of donors and acceptors. The dominant acceptor in ZnGeP2 is believed to be the zinc vacancy. This center is paramagnetic in its normal singly ionized state, and gives rise to an electron paramagnetic resonance (EPR) signal characterized by a resolved primary hyperfine interaction with two equivalent phosphorus nuclei adjacent to the vacancy. The present investigation has focused on electron-nuclear double resonance (ENDOR) measurements of additional hyperfine interactions which are not resolved in the regular EPR spectra. Principal values and principal axes directions for four additional phosphorus nuclei are determined from the ENDOR angular dependence. These parameters support the zinc-vacancy assignment for the acceptor and they provide an experimental check of wave functions generated in future computational modeling efforts.

Calculated paramagnetic properties of matrix isolated Au 3 cluster

Non-relativistic (NR-Xα) and Dirac (DSW-Xα) spin-restricted calculations on the matrix isolated Au 3 cluster are reported. It is shown that the resolved hyperfine interactions arising from the apical gold atom and the anisotropic effects on the spin distribution and on the paramagnetic properties could be adequately interpreted by using Dirac operators and Dirac cluster wave functions. The DSW-Xα calculations are in reasonable agreement with empirical and resolved paramagnetic resonance data of the Au 3 cluster isolated on the C 6D 6 matrix.