Hyperfine Interaction Of Electron Research Articles

Decoherence of nuclear spins in the frozen core of an electron spin

Hybrid qubit systems combining electronic spins with nearby (``proximate'') nuclear spin registers offer a promising avenue towards quantum information processing, with even multispin error-correction protocols recently demonstrated in diamond. However, for the important platform offered by spins of donor atoms in cryogenically cooled silicon, decoherence mechanisms of $^{29}\mathrm{Si}$ proximate nuclear spins are not yet well understood. The reason is partly because proximate spins lie within a so-called ``frozen core'' region where the donor electronic hyperfine interaction strongly suppresses nuclear dynamics. We investigate the decoherence of a central proximate nuclear qubit arising from quantum spin baths outside, as well as inside, the frozen core around the donor electron. We consider the effect of a very large nuclear spin bath comprising many $(\ensuremath{\gtrsim}{10}^{8})$ weakly contributing pairs outside the frozen core (the ``far bath''). We also propose that there may be an important contribution from a few (of order 100) symmetrically sited nuclear spin pairs (``equivalent pairs''), which were not previously considered because their effect is negligible outside the frozen core. If equivalent pairs represent a measurable source of decoherence, nuclear coherence decays could provide sensitive probes of the symmetries of electronic wave functions. For the phosphorus donor system, we obtain ${T}_{2n}$ values of order 1 second for both the far-bath and equivalent-pair models, confirming the suitability of proximate nuclei in silicon as very-long-lived spin qubits.

Spin noise of localized electrons: Interplay of hopping and hyperfine interaction

The theory of spin fluctuations is developed for an ensemble of localized electrons taking into account both hyperfine interaction of electron and nuclear spins and electron hopping between the sites. The analytical expression for the spin noise spectrum is derived for arbitrary relation between the electron spin precession frequency in the field of the nuclear fluctuation and the hopping rate. An increase in the hopping rate results in the drastic change of the spin noise spectrum. The effect of an external magnetic field is briefly addressed.

Density Functional Theory Studies of Electronic Structures and Hyperfine Interactions of Muonium in Imidazole

We carried out ab initio electronic structure calculations in the frameworks of the Density Functional Theory (DFT) to study the electronic structures and hyperfine interaction of muonium (Mu) in imidazole (C3H4N2) and 1–methylimidazole (CH3C3H3N2). The local energy minima and hyperfine interactions of the Mu trapped at the three studies sites were determined by performing geometry optimization procedure. The results show the total energies for all three studied sites are close to one another. The Mu hyperfine interactions were also determined, with the corresponding values vary from 343.00 MHz to 471.28 MHz for the imidazole–Mu cluster, and from 380.21 MHz – 465.57 MHz to 475.93 MHz for the cluster of 1–methylimidazole–Mu, respectively.

Calibration of Recoil-In-Vacuum attenuations from first principles: comparison with recent experimental data on Fe isotopes

Precession of aligned nuclear spin systems in ions recoiling from the target into vacuum (RIV) with consequent attenuation of angular distributions of emitted radiation is, in principle, a versatile method for measurement of g-factors of nuclear excited states of lifetimes in the pico-second range (Stone et al., Phys. Rev. Lett., 94, 192501, 2005 and Stuchbery and Stone, Phys. Rev. C, 76, 034307, 2007). Calibration of the observed attenuations has been achieved in favourable cases through comparison with measurements on states having previously known g-factors and lifetimes. The general lack of suitable states with known g-factors has limited application of the RIV method. This paper concerns the present status of efforts to describe the states of excited ions recoiling into vacuum in detail so that the average interaction can be estimated with useful precision from a-priori theory. The calculations use the GRASP2K package (Froese-Fischer et al. 1997 and Jonsson, Comp. Phys. Comm., 177, 597, 2007 & 184, 2197, 2013) to obtain, for each recoiling ion change state, the individual possible electronic states, their configurations, lifetimes and hyperfine interactions. It is assumed that all possible ionic states are produced, up to a chosen excitation energy. This energy is selected to approximate the energy at which all states have lifetimes far shorter than the nuclear state of interest. It is further assumed that the ionic state total electron angular momenta are randomly oriented in space. The first estimates of the average attenuation of emission distributions, as a function of the product g τ of the nuclear state g-factor and mean lifetime, used an averaged precession frequency obtained neglecting transitions between electronic states. Improved calculations, which include such transitions, are described.

Homogeneous linewidth narrowing of the charged exciton via nuclear spin screening in an InAs/GaAs quantum dot ensemble

In semiconductor quantum dots, the electron hyperfine interaction with the nuclear spin bath is the leading source of spin decoherence at cryogenic temperature. Using high-resolution two-color differential transmission spectroscopy, we demonstrate that such electron-nuclear coupling also imposes a lower limit for the positively charged exciton dephasing rate, \gamma, in an ensemble of InAs/GaAs quantum dots. We find that the dephasing rate is sensitive to the strength of the hyperfine interaction, which can be controlled through the application of an external magnetic field in the Faraday configuration. At zero applied field, strong electron-nuclear coupling induces additional dephasing beyond the radiative limit and \gamma = 230 MHz (0.95 \mu eV). Screening of the hyperfine interaction is achieved for an external field of ~1 T, resulting in \gamma = 172 MHz (0.71 \mu eV) limited only by spontaneous recombination. On the other hand, application of a Voigt magnetic field mixes the spin eigenstates, which increases the dephasing rate by up to 75%. These results are reproduced with a simple and intuitive model that captures the essential features of the electron hyperfine interaction and its influence on \gamma.

Paramagnetic copper centers in ferroelectric lead germanate with halogens

The electron paramagnetic resonance spectrum of triclinic bivalent copper centers in copper-doped Pb5Ge3O11 single crystals has been investigated. The spectrum intensity increases after thermal treatment in a chlorine- or fluorine-containing atmosphere. The parameters of the electron Zeeman, hyperfine, and nuclear quadrupole interactions have been determined. The model of the observed centers has been proposed.

Nuclear spin dynamics in double quantum dots: Multistability, dynamical polarization, criticality, and entanglement

We theoretically study the nuclear spin dynamics driven by electron transport and hyperfine interaction in an electrically-defined double quantum dot (DQD) in the Pauli-blockade regime. We derive a master-equation-based framework and show that the coupled electron-nuclear system displays an instability towards the buildup of large nuclear spin polarization gradients in the two quantum dots. In the presence of such inhomogeneous magnetic fields, a quantum interference effect in the collective hyperfine coupling results in sizable nuclear spin entanglement between the two quantum dots in the steady state of the evolution. We investigate this effect using analytical and numerical techniques, and demonstrate its robustness under various types of imperfections.

Electron electric dipole moment and hyperfine interaction constants for ThO

A recently implemented relativistic four-component configuration interaction approach to study P- and T-odd interaction constants in atoms and molecules is employed to determine the electron electric dipole moment effective electric field in the Ω=1 first excited state of the ThO molecule. We obtain a value of Eeff=75.2GVcm with an estimated error bar of 3% and 10% smaller than a previously reported result (Skripnikov et al., 2013). Using the same wavefunction model we obtain an excitation energy of TvΩ=1=5410 (cm-1), in accord with the experimental value within 2%. In addition, we report the implementation of the magnetic hyperfine interaction constant A|| as an expectation value, resulting in A||=-1339 (MHz) for the Ω=1 state in ThO. The smaller effective electric field increases the previously determined upper bound (Baron et al., 2014) on the electron electric dipole moment to |de|<9.7×10-29ecm and thus mildly mitigates constraints to possible extensions of the Standard Model of particle physics.

Computational Studies of Electronic Structures and Hyperfine Interactions of Muonium in Tetraphenylgermane

The equilibrium structure of muoniatedtetraphenylgermane (GePh4Mu) was studied using the first principle Density Functional Theory (DFT) method. Three muonium (Mu) trapping sites were considered, namely ortho, meta, and para positions on one of the phenyl rings. Geometry optimization procedure was utilized to determine the local energy minimum for all the systems. The total energies corresponding to Mu at the three positions are very similar to each other. For the meta case, the corresponding energy is higher than the other two sites by only about 0.03 eV. The hyperfine parameters of Mu were also calculated. The Mu isotropic hyperfine coupling constants were found to be 441.85 MHz, 449.80 MHz, and 439.01 MHz for the ortho, meta, and para cases, respectively. The anisotropic value was calculated to be very small.

Long-lived electron spin coherence in CdSe/Zn(S,Se) self-assembled quantum dots

The electron spin coherence in $n$-doped and undoped, self-assembled CdSe/Zn(S,Se) quantum dots has been studied by time-resolved pump-probe Kerr rotation. Long-lived spin coherence persisting up to 13 ns after spin orientation has been found in the $n$-doped quantum dots, outlasting significantly the lifetimes of charge neutral and negatively charged excitons of 350--530 ps. The electron spin dephasing time as long as 5.6 ns has been measured in a magnetic field of 0.25 T. Hyperfine interaction of resident electrons with nuclear spin fluctuations is suggested as the main limiting factor for the dephasing time. The efficiency of this mechanism in II-VI and III-V quantum dots is analyzed.

Dominant channels of exciton spin relaxation in photoexcited self-assembled (In,Ga)As quantum dots

We present a comprehensive theoretical investigation of spin relaxation processes of excitons in photoexcited self-assembled quantum dots. The exciton spin relaxations are considered between dark- and bright-exciton states via the channels created by various spin-admixture mechanisms, including electron Rashba and Dresselhaus spin-orbital couplings (SOCs), hole linear and hole cubic SOCs, and electron hyperfine interactions, incorporated with single- and double-phonon processes. The hole-Dresselhaus SOC is identified as the dominant spin-admixture mechanism, leading to relaxation rates as fast as $~$${10}^{\ensuremath{-}2}$ ns${}^{\ensuremath{-}1}$, consistent with recent observations. Moreover, due to significant electron-hole exchange interactions, single-phonon processes are unusually dominant over two-phonon ones in a photoexcited dot even at temperatures as high as $15 \mathrm{K}$.

Oxygen vacancies adjacent to Cu2+ ions in TiO2 (rutile) crystals

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are used to characterize Cu2+ ions substituting for Ti4+ ions in nominally undoped TiO2 crystals having the rutile structure. Illumination at 25 K with 442 nm laser light reduces the concentration of Cu2+ ions by more than a factor of 2. The laser light also reduces the EPR signals from Fe3+ and Cr3+ ions and introduces signals from Ti3+ ions. Warming in the dark to room temperature restores the crystal to its preilluminated state. Monitoring the recovery of the photoinduced changes in the Cu2+ ions and the other paramagnetic electron and hole traps as the temperature is raised from 25 K to room temperature provides evidence that the Cu2+ ions have an adjacent doubly ionized oxygen vacancy. These oxygen vacancies serve as charge compensators for the substitutional Cu2+ ions and lead to the formation of electrically neutral Cu2+-VO complexes during growth of the crystals. The Cu2+-VO complexes act as electron traps and convert to nonparamagnetic Cu+-VO complexes when the crystals are illuminated at low temperature. Complete sets of spin-Hamiltonian parameters describing the electron Zeeman, hyperfine, and nuclear electric quadrupole interactions for both the 63Cu and 65Cu nuclei are obtained from the EPR and ENDOR data. This study suggests that other divalent cation impurities in TiO2 such as Co2+ and Ni2+ may also have an adjacent oxygen vacancy for charge compensation.

Hyperfine-mediated static polarizabilities of monovalent atoms and ions

We apply relativistic many-body methods to compute static differential polarizabilities for transitions inside the ground-state hyperfine manifolds of monovalent atoms and ions. Knowledge of this transition polarizability is required in a number of high-precision experiments, such as microwave atomic clocks and searches for $\mathit{CP}$-violating permanent electric dipole moments. While the traditional polarizability arises in the second order of interaction with the externally applied electric field, the differential polarizability involves an additional contribution from the hyperfine interaction of atomic electrons with nuclear moments. We derive formulas for the scalar and tensor polarizabilities including contributions from magnetic dipole and electric quadrupole hyperfine interactions. Numerical results are presented for Al, Rb, Cs, ${\mathrm{Yb}}^{+}$, ${\mathrm{Hg}}^{+}$, and Fr.

Measurement of a Heavy-Hole Hyperfine Interaction in InGaAs Quantum Dots Using Resonance Fluorescence

We measure the strength and the sign of hyperfine interaction of a heavy hole with nuclear spins in single self-assembled quantum dots. Our experiments utilize the locking of a quantum dot resonance to an incident laser frequency to generate nuclear spin polarization. By monitoring the resulting Overhauser shift of optical transitions that are split either by electron or exciton Zeeman energy with respect to the locked transition using resonance fluorescence, we find that the ratio of the heavy-hole and electron hyperfine interactions is -0.09 ± 0.02 in three quantum dots. Since hyperfine interactions constitute the principal decoherence source for spin qubits, we expect our results to be important for efforts aimed at using heavy-hole spins in quantum information processing.

First-principles cluster study of electronic structures, locations and hyperfine interactions of isolated atoms and ions in silicon

The electronic structures of the dilute transition metal (TM) impurities, V 2+ ,C r + ,M n 2+ ,a nd Mn 0 in silicon have been studied using the Hartree-Fock (HF) procedure combined with many-body perturbation theory (MBPT). The systems studied involved the TM impurities at the hexagonal interstitial (Hi), tetrahedral interstitial (Ti) and substitutional (S) locations. Investigations of the binding energy and local potential energy surface of the TM impurity-Si systems indicate that the TM impurities are binding at the Ti location. Hyperfine interaction constants (A) of the TM impurities at the Ti and S sites are presented and compared with available experimental results (Woodbury and Ludwig, J Phys Rev 117:102, 1960a, Phys Rev Lett 5:98, b) from Electron Paramagnetic Resonance (EPR) measurements.

Spins coupled to a spin bath: From integrability to chaos

Motivated by the hyperfine interaction of electron spins with surrounding nuclei, we investigate systems of central spins coupled to a bath of noninteracting spins in the framework of random matrix theory. With increasing number of central spins a transition from Poissonian statistics to the Gaussian orthogonal ensemble occurs which can be described by a generalized Brody distribution. These observations are unaltered upon applying an external magnetic field. In the transition region, the classical counterparts of the models studied have mixed phase space.

The role of magnetic fields on organic light-emitting devices based on aluminum tris(8-hydroxyquinoline) (Alq3) at room temperature

We report the magnetic-field effect (MFE) in Alq 3 based organic light-emitting device (OLED) with the structure of ITO/NPB/LiF/Alq 3 /LiF/Al.When an external magnetic field of 50 mT and insertion of LiF thickness of 0.8 nm is applied, the current efficiency and the brightness of the device increase to 12.4% and 17%, respectively.The origin of the improvements could be attributed to the Zeeman effect,which reduces the hyperfine interactions of electrons and holes, inhibiting energy transfer from the singlet polaron pairs to triplet polaron pairs and increasing the population of singlets.Similar MFE was observed in phosphorescent organic light-emitting diodes(PhOLEDs).The current efficiency of ITO/NPB/LiF(0.8 nm)/CBP:6 wt% Ir(ppy) 3 /BCP/Alq 3 /LiF/Al increases to 12.1% at 50 mT.Magnetic-field-induced increase of singlet excitons in the CBP matrix reflects increase of triplet excitons in the dispersed Ir(ppy) 3 molecules in the CBP+Ir(ppy) 3 blend.

Electron-nuclear double resonance study of [formula omitted] ions in [formula omitted] crystals

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are used to identify and characterize V 4+ ions in a bulk single crystal of vanadium dioxide (V O 2). These S = 1 / 2 defects are observed in the as-grown crystal because an adjacent nonmagnetic M 4+ impurity, e.g., a Si 4+ ion, has destroyed the normal antiferromagnetic coupling associated with the close pairs of V 4+ ions that occur in the low-temperature monoclinic phase of V O 2. EPR spectra taken near 5 K with the magnetic field along the [110] and [001] axes show resolved hyperfine patterns due to one 51V nucleus. ENDOR spectra taken at 5 K with the magnetic field parallel to the [001] axis have large nuclear electric quadrupole splittings as a result of a significant electric field gradient at the 51V nucleus. Spin-Hamiltonian parameters describing the electron Zeeman, hyperfine, and nuclear electric quadrupole interactions are reported.

Spin dynamics in double quantum dots in the spin blockade regime

We analyze theoretically the effect of dynamical nuclear spin polarization on the electronic transport through a double quantum dot in the spin blockade (SB) regime in the presence of hyperfine interaction. The electron and nuclei hyperfine interaction produces electron spin flip which partially removes SB. Induced nuclei spin polarization produces a finite magnetic field which acts on the electrons generating an additional Zeeman splitting of the electronic spin up and down levels in each quantum dot. This additional Zeeman splitting changes dynamically with the electronic level occupation. Then, strong feedback between the induced nuclear polarization within each quantum dot and the electronic charge distribution occurs and it produces strong non-linearities in the current as a function of both, external magnetic field and dc voltage. We analyze as well electron spin resonance in a double quantum dot driven by crossed dc and ac magnetic fields. The time dependent magnetic field produces coherent electron spin rotations within each quantum dot. In this configuration, spatial Rabi oscillations due to inter-dot tunnel compete with Rabi oscillations between spin up and down states induced by the time dependent magnetic field, giving rise to a complicated time dependent behavior of the tunneling current.

Electron paramagnetic resonance and electron‐nuclear double resonance of nonequivalent Yb3+ centers in stoichiometric lithium niobate

AbstractLithium niobate crystals doped with ytterbium were studied using Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR). The tremendous narrowing of EPR lines in nearly stoichiometric samples, when compared to those in congruent samples, allowed us to distinguish nine non‐equivalent centers, as well as line splitting caused by the hyperfine interaction of ytterbium electrons with the nuclear spins of two magnetic isotopes, 171Yb and 173Yb. Eight of the nine centers are described for the first time. It was found that three of the centers have axial C3 symmetry, and all others have the lowest C1 symmetry due to the presence of intrinsic defects and/or charge compensation defects in the near neighborhood of Yb3+. Characteristics of the g ‐tensor for all of the centers and hyperfine tensors for axial centers were determined. The ENDOR observations of Nb nuclei in the nearest neighborhood of Yb13+ gave direct evidence that the dominated axial Yb1 center has no charge compensator in its nearest surroundings (distant charge compensation mechanism). Both the EPR and ENDOR data for the main axial ytterbium center are explained by a supposition that Yb3+ ions substitute for Li+. Possible models for low‐symmetry centers are discussed. The obtained numerous spectroscopic parameters can be used as cornerstones for model calculations of Yb3+ centers in lithium niobate. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)