Nuclear Hyperfine Interactions Research Articles

Rotational and Vibrational Spectra of the Pyridyl Radicals: A Coupled-Cluster Study.

Pyridyl is a prototypical nitrogen-containing aromatic radical that may be a key intermediate in the formation of nitrogen-containing aromatic molecules under astrophysical conditions. On meteorites, a variety of complex molecules with nitrogen-containing rings have been detected with nonterrestrial isotopic abundances, and larger nitrogen-containing polycyclic aromatic hydrocarbons (PANHs) have been proposed to be responsible for certain unidentified infrared emission bands in the interstellar medium. In this work, the three isomers of pyridyl (2-, 3-, and 4-pyridyl) have been investigated with coupled cluster methods. For each species, structures were optimized at the CCSD(T)/cc-pwCVTZ level of theory and force fields were calculated at the CCSD(T)/ANO0 level of theory. Second-order vibrational perturbation theory (VPT2) was used to derive anharmonic vibrational frequencies and vibrationally corrected rotational constants, and resonances among vibrational states below 3500 cm-1 were treated variationally with the VPT2+K method. The results yield a complete set of spectroscopic parameters needed to simulate the pure rotational spectrum of each isomer, including electron-spin, spin-spin, and nuclear hyperfine interactions, and the calculated hyperfine parameters agree well with the limited available data from electron paramagnetic resonance spectroscopy. For the handful of experimentally measured vibrational frequencies determined from photoelectron spectroscopy and matrix isolation spectroscopy, the typical agreement is comparable to experimental uncertainty. The predicted parameters for rotational spectroscopy reported here can guide new experimental investigations into the yet-unobserved rotational spectra of these radicals.

Tuning the adiabaticity of spin dynamics in diamond nitrogen vacancy centers

We study the spin dynamics of diamond nitrogen vacancy (NV) centers in an oscillating magnetic field along the symmetry axis of the NV in the presence of transverse magnetic fields. It is well-known that the coupling between the otherwise degenerate Zeeman levels |M S = ±1⟩ due to strain and electric fields is responsible for a Landau–Zener process near the pseudo-crossing of the adiabatic energy levels when the axial component of the oscillating magnetic field changes sign. We derive an effective two-level Hamiltonian for the NV system that includes coupling between the two levels via virtual transitions into the third far-detuned level |M S = 0⟩ induced by transverse magnetic fields. This coupling adds to the coupling due to strain and electric fields, with a phase that depends on the direction of the transverse field in the plane perpendicular to the NV axis. Hence, the total coupling of the Zeeman levels can be tuned to control the adiabaticity of spin dynamics by fully or partially compensating the effect of the strain and electric fields, or by enhancing it. Moreover, by varying the strength and direction of the transverse magnetic fields, one can determine the strength and direction of the local strain and electric fields at the position of the NV center, and even the external stress and electric field. The nuclear spin hyperfine interaction is shown to introduce a nuclear spin dependent offset of the axial magnetic field for which the pseudo-crossing occurs, while the adiabaticity remains unaffected by the nuclear spin. If the NV center is coupled to the environment, modeled by a bath with a Gaussian white noise spectrum, as appropriate for NVs near the diamond surface, then the spin dynamics is accompanied by relaxation of the Zeeman level populations and decoherence with a non-monotonic decrease of the purity of the system. The results presented here have important impact for metrology with NV centers, quantum control of spin systems in solids and coupled dynamics of spin and rotations in levitated nano-objects in the presence of magnetic fields.

Using Molecular Design to Enhance the Coherence Time of Quintet Multiexcitons Generated by Singlet Fission in Single Crystals.

Multiexciton quintet states, 5(TT), photogenerated in organic semiconductors using singlet fission (SF), consist of four quantum entangled spins, promising to enable new applications in quantum information science. However, the factors that determine the spin coherence of these states remain underexplored. Here, we engineer the packing of tetracene molecules within single crystals of 5,12-bis(tricyclohexylsilylethynyl)tetracene (TCHS-tetracene) to demonstrate a 5(TT) state that exhibits promising spin qubit properties, including a coherence time, T2, = 3 μs at 10 K, a population lifetime, Tpop, = 130 μs at 5 K, and stability even at room temperature. The single-crystal platform also enables global alignment of the spins and, consequently, individual addressability of the spin-sublevel transitions. Decoherence mechanisms, including exciton diffusion, electronic dipolar coupling, and nuclear hyperfine interactions, are elucidated, providing design principles for increasing T2 and the operational temperature of 5(TT). By dynamically decoupling 5(TT) from the surrounding spin bath, T2 = 10 μs is achieved. These results demonstrate the viability of harnessing singlet fission to initiate multiple electron spins in a well-defined quantum state for next-generation molecular-based quantum technologies.

Extraction of dipolar coupling constants from low-frequency electrically detected magnetic resonance and near-zero field magnetoresistance spectra via least squares fitting to models developed from the stochastic quantum Liouville equation

We report low-frequency electrically detected magnetic resonance (EDMR) and near-zero field magnetoresistance (NZFMR) measurements observed through spin-dependent trap-assisted tunneling on unpassivated 28Si/28SiO2 metal–insulator–semiconductor (MIS) capacitors. This study both experimentally and theoretically explores the effects of the low-frequency EDMR response and the mechanisms responsible for spin-mixing, which leads to the NZFMR response in the absence of electron–nuclear hyperfine interactions. Previous reports that utilized high-frequency EDMR and NZFMR on these devices indicated that the observed trap-assisted tunneling spectra are dominated by silicon dangling bonds back bonded to silicon at the Si/SiO2 interface, Pb0 and Pb1 centers. These previous results also suggest that the rate limiting step in trap-assisted tunneling is the interface to an oxide trapping event. In this work, we extend the theory to show the explicit connection of the defects observed between the NZFMR response and the EDMR, which has not yet been demonstrated. We also extend a theoretical approach to the analysis of both the EDMR and NZFMR spectra and match the theory to experimental observations made in 28Si/28SiO2 MIS capacitors. The method utilizes a least squares fitting algorithm of models developed from the stochastic quantum Liouville equation. We find that we can extract a dipolar coupling constant by fitting both the NZFMR and EDMR spectra. Our experimental results and resulting fitted spectra from our quantitative model suggest the mechanism responsible for spin-mixing, which leads to the NZFMR response in the absence of electron–nuclear hyperfine interactions, is predominately magnetic dipolar interactions between Pb centers at the interface.

On the Biot–Savart law of electromagnetism applied to the atomic circulation current *

The law of Biot and Savart, derived by observation of the magnetic field produced by electric current flowing in a macroscopic conductor, is shown to yield the correct expression for the nuclear hyperfine interaction, when applied to the electron circulation current derived for hydrogenic atoms from the theories of Pauli and Dirac, as further developed independently by Darwin, Gordon and Hartree. The law thus applies also on the microscopic scale to the magnetic field generated by the electron probability flux of the various atomic eigenstates, and may clearly be generalised formally for application to many-electron atoms, as outlined by Hartree. The present, fully relativistic, treatment is advantageous in avoiding the usual artificial splitting of the hyperfine interaction into a magnetic dipole–dipole contribution and a separate contribution arising from electron orbital motion. The formula obtained forms the basis for the inclusion of higher order (quantum electrodynamic and nuclear recoil) corrections. Interest in this subject is enhanced by observations of ‘strongly forbidden’ (magnetic dipole) atomic transitions arising from the nuclear magnetic interaction, notably the 21 cm line of hydrogen, which has played an important role in astrophysics, in both measurements of galactic rotation and studies of the early Universe.

Electronic Structure of the Primary Electron Donor P700+• in Photosystem I Studied by Multifrequency HYSCORE Spectroscopy at X- and Q-Band.

The primary electron donor P700 of the photosystem I (PSI) is a heterodimer consisting of two chlorophyll molecules. A series of electron-transfer events immediately following the initial light excitation leads to a stabilization of the positive charge by its cation radical form, P700+•. The electronic structure of P700+• and, in particular, its asymmetry with respect to the two chlorophyll monomers is of fundamental interest and is not fully understood up to this date. Here, we apply multifrequency X- (9 GHz) and Q-band (35 GHz) hyperfine sublevel correlation (HYSCORE) spectroscopy to investigate the electron spin density distribution in the cation radical P700+• of PSI from a thermophilic cyanobacterium Thermosynechococcus elongatus. Six 14N and two 1H distinct nuclei have been resolved in the HYSCORE spectra and parameters of the corresponding nuclear hyperfine and quadrupolar hyperfine interactions were obtained by combining the analysis of HYSCORE spectral features with direct numerical simulations. Based on a close similarity of the nuclear quadrupole tensor parameters, all of the resolved 14N nuclei were assigned to six out of total eight available pyrrole ring nitrogen atoms (i.e., four in each of the chlorophylls), providing direct evidence of spin density delocalization over the both monomers in the heterodimer. Using the obtained experimental values of the 14N electron-nuclear hyperfine interaction parameters, the upper limit of the electron spin density asymmetry parameter is estimated as RA/Bupper = 7.7 ± 0.5, while a tentative assignment of 14N observed in the HYSCORE spectra yields RB/A = 3.1 ± 0.5.

Revealing the hidden hyperfine interactions in ε -iron

Herein, evidence for the long-sought finite hyperfine interaction in the high-pressure hexagonal close-packed $\ensuremath{\epsilon}$-iron is gained through synchrotron radiation perturbed angular correlation spectroscopy. This method yields an energy splitting of $3.5(5)\phantom{\rule{4.pt}{0ex}}\text{neV}$ between the ${m}_{{I}_{\text{e}}}=\ifmmode\pm\else\textpm\fi{}1/2$ and ${m}_{{I}_{\text{e}}}=\ifmmode\pm\else\textpm\fi{}3/2$ nuclear sublevels of the iron-57 $14.412\text{-keV}$ nuclear excited state at $30(1)\phantom{\rule{4.pt}{0ex}}\text{GPa}$ and room temperature. This energy splitting is related to a nuclear quadrupole hyperfine interaction with an electric field gradient of $eq=1.2(2)\ifmmode\times\else\texttimes\fi{}{10}^{16}{\phantom{\rule{0.16em}{0ex}}\mathrm{V}/\mathrm{cm}}^{2}$. However, there is still a possibility that the splitting of the iron-57 nuclear levels is related to a modest magnetic hyperfine interaction of ca. $0.40(5)\phantom{\rule{4.pt}{0ex}}\text{T}$.

First experimental evidence of non-collinear spin structure in CaCu2.3Ti3.3Fe1.4O12 quadruple perovskite through low-temperature, in-field 57Fe Mossbauer spectroscopy

A polycrystalline sample of quadruple perovskite oxide, CaCu2.3Ti3.3Fe1.4O12, was synthesized and characterized by zero-field and in-field (H̄app = 50 kOe) 57Fe Mossbauer spectroscopy at 5 K. The presence of ΔmI = 0 spectral lines in Mossbauer spectrum recorded with H̄app parallel to the direction of γ-ray transmission provides unambiguous affirmation for the presence of canting angles among individual moments and the direction of net-sublattice magnetization. The observed non-collinear spin structure is well supported by the M(H) loop characteristic. The canting angle, θc = 67.8°, determined from Mossbauer spectral analysis is in accordance with the canting angle, θM = 71.2°, calculated from the site magnetic moments. This led to cluster-spin-glass like magnetic ordering well reflected in the M(T) curve. The effects of low-temperature and applied magnetic field on the line shape of the Mossbauer spectra and nuclear hyperfine interaction parameters have also been discussed in detail.

Effect of Electron-Nuclear Hyperfine Interactions on Multiple-Quantum Coherences in Photogenerated Covalent Radical (Qubit) Pairs.

Ultrafast photodriven electron transfer reactions starting from an excited singlet state in an organic donor-acceptor molecule can generate a spin-correlated radical pair (RP) with an initially entangled spin state that may prove useful as a two-qubit pair in quantum information protocols. Here we investigate the effects of modulating electron-nuclear hyperfine coupling by rapidly transferring an electron between two equivalent sites comprising the reduced acceptor of the RP. A covalent electron donor-acceptor molecule including a tetrathiafulvalene (TTF) donor, a 4-aminonaphthalene-1,8-imide (ANI) chromophoric primary acceptor, and an m-xylene bridged cyclophane having two equivalent pyromellitimides (PI2), TTF-ANI-PI2, as a secondary acceptor was synthesized along with an analogous molecule having one pyromellitimide (PI) acceptor, TTF-ANI-PI. Photoexcitation of ANI within each molecule results in sub-nanosecond formation of TTF+•-ANI-PI-• and TTF+•-ANI-PI2-•. The effect of reducing electron-nuclear hyperfine interactions in TTF+•-ANI-PI2-• relative to TTF+•-ANI-PI-• on decoherence of multiple-quantum coherences has been measured by pulse-EPR spectroscopy. This contribution is especially relevant in the absence of modulation of exchange or dipolar interactions, as with the RP at a fixed distance in the molecules in this work. The theoretical prediction of the contribution from an ensemble of hyperfine interactions to decoherence in these RPs is shown to be less than the full width at half-maximum of the quantum beat frequencies measured experimentally. Pulse bandwidth and off-resonant excitation by square microwave pulses are proposed as larger contributors to decoherence in these molecules than the hyperfine interactions, and specific pulse shapes relevant to arbitrary waveform generation are introduced.

Hyperfine interaction study of pressure induced phase transformations in Hafnia

Time Differential Perturbed Angular correlation (TDPAC) study of pressure dependent phase transition in HfO2 has been performed for the first time to reveal the change in local structure around Hf atom. The results show that the nuclear hyperfine interaction parameters namely, quadrupole interaction frequency and asymmetry parameter at atmospheric pressure are in agreement with literature while that at 45 GPa are significantly different from that at atmospheric pressure.

FeCr2O4spinel to near megabar pressures: Orbital moment collapse and site-inversion facilitated spin crossover

The interplay between lattice, orbital, and spin degrees of freedom in iron chromite (spinel $[\mathrm{Fe}]{\mathrm{C}{\mathrm{r}}_{2}}{\mathrm{O}}_{4})$ has been investigated to near megabar pressures. The cubic-to-tetragonal transition, from static Jahn-Teller distortions at the Fe locality, rises from ${T}_{\mathrm{J}\text{\ensuremath{-}}\mathrm{T}}\ensuremath{\sim}135\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at ambient pressure to 300 K by $\ensuremath{\sim}12\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. The tetragonal distortion progressively increases and orbital moment quenching is triggered beyond $\ensuremath{\sim}24\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, as monitored by the magnetic hyperfine field ${H}_{hf}$ at Fe sites. In the range 30--60 GPa, original $\mathrm{F}{\mathrm{e}}^{2+}$ tetrahedral sites with an unquenched orbital moment $({H}_{hf}\ensuremath{\sim}20\phantom{\rule{0.16em}{0ex}}\mathrm{T})$ coexist with newly evolved $\mathrm{F}{\mathrm{e}}^{2+}$ at tetrahedral sites having the orbital moment quenched and a resultant large ${H}_{hf}\ensuremath{\sim}35\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. Additionally, new $\mathrm{F}{\mathrm{e}}^{2+}$ sites having distinguishable orbital moment quenching signatures $({H}_{hf}\ensuremath{\sim}42\phantom{\rule{0.16em}{0ex}}\mathrm{T})$ are discerned. Those sites also have other Fe nuclear hyperfine interaction parameter values typical of $\mathrm{F}{\mathrm{e}}^{2+}$ in octahedral coordination. There is a concurrent change to a steeper decrease of unit-cell volume as pressure rises above $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. These electronic and lattice responses are interpreted as signatures of progressive partial spinel inversion from high-spin $\mathrm{Fe}\ensuremath{\leftrightarrow}\mathrm{Cr}$ tetrahedral/octahedral site exchange, triggered near $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Beyond 60 GPa a new diamagnetic low-spin M\ossbauer spectral component emerges. This is preceded by an inflection and discontinuity in the pressure dependence of the resistance and tetragonal unit-cell volume, respectively. By $\ensuremath{\sim}93\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, half of the iron is low spin in octahedral sites from evolved tetragonal-spinel inversion processes. The remainder reside in tetrahedral high-spin sites with ${H}_{hf}\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. The charge gap from electron correlations does not close, despite an anticipated appreciable band broadening from a $\ensuremath{\sim}30%$ unit-cell volume reduction upon pressurization to $\ensuremath{\sim}93\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. This is attributable to an increase in effective on-site repulsion (Hubbard ${U}_{\mathrm{eff}}$) with increasing pressure or at spin crossover, specific to the $\mathrm{C}{\mathrm{r}}^{3+}({d}^{3})$ and $\mathrm{F}{\mathrm{e}}^{2+}({d}^{6})$ electronic configurations, respectively. Thus a $\ensuremath{\sim}200\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ ``Mott'' gap prevails in the mixed spin-state of the partially inverted tetragonal phase $[\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{r}}_{x}]{\mathrm{F}{\mathrm{e}}_{x}\mathrm{C}{\mathrm{r}}_{2\ensuremath{-}x}}{\mathrm{O}}_{4}\phantom{\rule{0.28em}{0ex}}(x\ensuremath{\sim}0.5)$ up to near a megabar (100 GPa) densification.

Optical response from terahertz to visible light of electronuclear transitions inLiYF4:Ho3+

Because of its role as a model system with tunable quantum fluctuations and quenched disorder, and the desire for optical control and readout of its states, we have used high-resolution optical absorption spectroscopy to measure the crystal-field excitations for Ho^(3+) ions in LiHo_xY_(1−x)F_4 from the terahertz to visible regimes. We show that many of the excitations yield very narrow lines visibly split even by the nuclear hyperfine interaction, making Ho^(3+) in LiHo_xY_(1−x)F_4 a candidate host for optically addressable electronuclear qubits with quality factors as high as Q = 4.7 × 10^5, where the higher-lying levels are electronic singlets. Optical transitions in the easily accessible near- and mid-infrared are narrow enough to allow readout of the ground-state electronuclear qubits responsible for the interesting magnetism of LiHo_xY_(1−x)F_4. While many of the higher-lying states have been observed previously, we also report here detailed spectra of terahertz excitations. The strengths of the electric and magnetic dipole crystal-field transition lines of five of the lowest excited spin-orbit manifolds of dilute LiYF_4:Ho^(3+) were calculated and compared with measurement. The magnitude of the nuclear hyperfine coupling was used to assign the correct upper and lower states to transition lines.

Electron Paramagnetic Resonance (EPR) studies on the photo-thermo ionization process of photo-thermo-refractive glasses

Photo-thermo-refractive (PTR) glass is an optically transparent photosensitive sodium alumino silicate glass, containing NaF and KBr additives, along with cerium, silver, tin and antimony oxide dopants. UV-exposed regions of this glass produce NaF nanocrystals upon heating, giving rise to a permanent localized refractive index change. In this article we examine the initial stages of this crystallization process by continuous-wave and pulsed X-band electron paramagnetic resonance (EPR) spectroscopy. UV exposure of PTR glass produces unpaired electrons whose EPR spectrum is characterized by pronounced peak splitting arising from nuclear magnetic hyperfine interactions with spin-5/2 and spin-7/2 nuclei suggesting close proximity of the unpaired electrons with 121Sb and 123Sb nuclei. These results indicate that the Sb2O3 dopant plays a key role in the initial stages of the crystallization mechanism. Upon thermal annealing, leading to the crystallization of NaF, these species disappear, indicating their transient nature. A number of other unpaired electron species identified in the dopant free matrix appear to be unrelated to the crystallization process. These results clearly challenge the classical mechanism proposed decades ago to explain the complex crystallization process of PTR glass. Together with more recent results from optical spectroscopy they support the Nikonorov model involving (1) photoionization of Ce3+, (2) transfer of this electron to Sb5+ species to create a Sb4+ species, (3) upon annealing electron transfer from Sb4+ to Ag+ ions, producing silver atoms, (4) coalescence of these species into Ag clusters, which (5) serve as nucleation catalysts for NaF nanocrystals.

Solid-state EPR strategies for the structural characterization of paramagnetic NO adducts of frustrated Lewis pairs (FLPs).

Anisotropic interactions present in three new nitroxide radicals prepared by N,N addition of NO to various borane-phosphane frustrated Lewis pairs (FLPs) have been characterized by continuous-wave (cw) and pulsed X-band EPR spectroscopies in solid FLP-hydroxylamine matrices at 100 K. Anisotropic g-tensor values and (11)B, (14)N, and (31)P hyperfine coupling tensor components have been extracted from continuous-wave lineshape analyses, electron spin echo envelope modulation (ESEEM), and hyperfine sublevel correlation spectroscopy (HYSCORE) experiments with the help of computer simulation techniques. Suitable fitting constraints are developed on the basis of density functional theory (DFT) calculations. These calculations reveal that different from the situation in standard nitroxide radicals (TEMPO), the g-tensors are non-coincident with any of the nuclear hyperfine interaction tensors. The determination of these interaction parameters turns out to be successful, as the cw- and pulse EPR experiments are highly complementary in informational content. While the continuous-wave lineshape is largely influenced by the anisotropic hyperfine coupling to (14)N and (31)P, the ESEEM and HYSCORE spectra contain important information about the (11)B hyperfine coupling and nuclear electric quadrupolar interaction. The set of cw- and pulsed EPR experiments, with fitting constraints developed by DFT calculations, defines an efficient strategy for the structural analysis of paramagnetic FLP adducts.

Pulsed electron spin nutation spectroscopy for weakly exchange-coupled multi-spin molecular systems with nuclear hyperfine couplings: a general approach to bi- and triradicals and determination of their spin dipolar and exchange interactions

Weakly exchange-coupled biradicals have attracted much attention in terms of their dynamic nuclear polarisation application in NMR spectroscopy for biological systems or the use of synthetic electron-spin qubits in quantum information processing/quantum-computing technology. Analogues multi-partite molecular systems are important in entering a new phase of the relevant fields. Many stable organic biradicals known so far have nitrogen nuclei at their electron spin sites, where singly occupied molecular orbitals are dominating and large hyperfine couplings occur. A salient feature of such weakly exchange-coupled molecular systems in terms of electronic spin structures is underlain by small zero-field splitting (ZFS) parameters comparable with nuclear hyperfine and/or exchange interactions. Pulse-based electron spin nutation (ESN) spectroscopy of weakly exchange-coupled biradicals, applicable to oriented or non-oriented media, has proven to be a useful and facile approach to the determination of ZFS parameters, which reflect relatively short distances between unpaired electron spins. In the present study, we first treat two-dimensional single-crystal ESN spectroscopy (Q-band) of a 15N-labelled weakly exchange-coupled biradical, showing the nuclear hyperfine effects on the ESN phenomena from both the experimental and theoretical side. ESN spectroscopy is transition moment spectroscopy, in which the nutation frequency as a function of the microwave irradiation strength ω1 (angular frequency) for any cases of weakly exchange-coupled systems can be treated. The results provide a testing ground for the simplified but general approach to the ESN analysis. In this study, we have invoked single-crystal electron-electron double resonance measurements on a typical biradical well incorporated in a diamagnetic host lattice and checked the accuracy of our ESN analysis for the spin dipolar tensor and exchange interaction. Next, we extend the general approach to analogues multi-partite molecular systems such as stable organic triradical, in which the exchange interaction can be governed by a significant amount of the delocalisation of three unpaired spins over the molecular frame of a triangular structure. The triangular structure maintains π-conjugation in which each spin-bearing nitroxide at the vertex participates and the exchange interaction is greatly controlled by the dihedral angle between the π-conjugation plane and nitroxide moiety at the vertex. In this context, the ZFS parameters do not correspond to spin distances (1.0 nm) in a straightforward manner, but reflect a salient electronic structure associated with both the π-electron network and the symmetry property of the triradical under study. Thus, both the D-value and exchange interaction J have been controlled in this study. In order to interpret the experimental ZFS parameters and exchange interaction, which is three-order of magnitude reduced in the present poly(methyl methacrylate) polymer matrix compared with that in the crystal, sophisticated quantum chemical calculations of the ZFS tensor and exchange interaction were carried out and reproduced the experimental values, concluding that the present triradical of the triangular structure undergoes significant twisting at the nitroxide sites in the polymer matrix. In this study, we observed electron spin resonance forbidden transitions between the Ms-manifolds belonging to the spin-doublet ground state and spin-quartet excited state. The observation enables us to derive the magnitude of the exchange coupling.

Lifetimes of the hyperfine levels of 3d94s 3D3 in high-Z Ni-like ions

Based on the multi-configuration Dirac–Hartree–Fock method and using the GRASPVU package, a theoretical investigation was performed to study the lifetimes of hyperfine levels of the first excited level 3d94s 3D3 in Ni-like ions (Z = 72–79) for all stable isotopes with nuclear spin. Comparisons between hyperfine-induced electric quadrupole transition rates and the pure magnetic octupole transition rates show that the extra electric quadrupole transition channel caused by the nuclear magnetic dipole and electric quadrupole hyperfine interaction is important for most hyperfine levels in each individual ion. Lifetimes of most hyperfine levels are sensitive to this extra decay channel. Extreme cases are found in 181Ta, 185Re and 187Re, where lifetimes of some hyperfine levels are shortened by more than an order of magnitude.

Sensitive Cu2+–Cu2+Distance Measurements in a Protein–DNA Complex by Double-Quantum Coherence ESR

Double quantum coherence (DQC) ESR spectroscopy is applied to measure the Cu(2+)-Cu(2+) distance in the EcoRI-DNA complex. A simple method is proposed to reduce the contribution of nuclear hyperfine and quadrupole interactions to such data. The effects of such interactions between the electron spin of Cu(2+) and neighboring nuclei on the DQC data make it difficult to measure the nanometer range interspin distance. The DQC data is in good agreement with results obtained by double electron electron resonance (DEER) spectroscopy. At the same time, the signal-to-noise ratio per shot in DQC is high. Taken together, these results provide impetus for further development of paramagnetic metal ion-based DQC techniques.

Zero-field detection of spin dependent recombination with direct observation of electron nuclear hyperfine interactions in the absence of an oscillating electromagnetic field

Electrically detected magnetic resonance (EDMR) involves the electron paramagnetic resonance (EPR) study of spin dependent transport mechanisms such as spin dependent tunneling and spin dependent recombination (SDR) in solid state electronics. Conventional EPR measurements generally require strong static magnetic fields, typically 3 kG or greater, and high frequency oscillating electromagnetic fields, typically 9 GHz or higher. In this study, we directly demonstrate that, in the absence of the oscillating electromagnetic field, a very large SDR response (≈1%) can be detected at zero magnetic field with associated hyperfine interactions at extremely low magnetic fields in a silicon carbide (SiC) diode at room temperature. The zero-field SDR (ZFSDR) response that we detect is unexpected in the conventional detection scheme of SDR via EDMR. We believe that our observations provide fundamental physical understanding of other recently reported zero-field phenomena such as singlet triplet mixing in double quantum dots and low-field giant magnetoresistance in organic semiconductors. Our work provides an unambiguous demonstration that the zero-field phenomenon we observe involves SDR. Measurements reported herein indicate that extremely useful low-field SDR and ZFSDR results can be acquired simply and inexpensively in systems of technological importance. This work also suggests the potential use of this new physics in applications including absolute magnetometry with self-calibration, spin based memories, quantum computation, and inexpensive low-field EDMR spectrometers for wafer/probing stations.

Temperature-dependent hyperfine interactions at 111Cd-C complex in germanium

The temperature dependent nuclear hyperfine interaction of 111Cd-carbon complex in germanium has been studied using the perturbed γ–γ angular correlation (PAC) method. The parameters of the hyperfine interaction representing substitutional carbon–cadmium complex in germanium (νQ1=207(1) MHz (η=0.16)) shows dependence on temperature. The formation and thermal stability of the complex has been reported by the same author earlier. It was found in this study that the quadrupole coupling constant of the interaction increases at sample temperature below 293 K. The results are encouraging toward better understanding of the complex in the host matrix.

Temperature-scanned magnetic resonance and the evidence of two-way transfer of a nitrogen nuclear spin hyperfine interaction in coupled NV-N pairs in diamond

New method for the detection of magnetic resonance signals versus temperature is developed on the basis of the temperature dependence of the spin Hamiltonian parameters of the paramagnetic system under investigation. The implementation of this technique is demonstrated on the nitrogen-vacancy (NV) centers in diamonds. Single NV defects and their ensembles are suggested to be almost inertialess temperature sensors. The hyperfine structure of the 14N nitrogen nuclei of the nitrogen-vacancy center appears to be resolved in the hyperfine structure characteristic of the hyperfine interaction between NV and an Ns center (substitutional nitrogen impurity) in the optically detected magnetic resonance spectra of the molecular NV-Ns complex. Thus, we show that a direct evidence of the two-way transfer of a nitrogen nuclear spin hyperfine interaction in coupled NV-Ns pairs was observed. It is shown that more than 3-fold enhancement of the NV optically detected magnetic resonance signal can be achieved by using water as a collection optics medium.