Helimagnetism in gallium substituted LuMn<Subscript>6</Subscript>Ge<Subscript>6</Subscript> studied by nuclear magnetic resonance

Zero-field nuclear magnetic resonance (NMR) of all NMR isotopes (¹⁷⁵Lu, ⁵⁵Mn, ⁷³Ge, ^69,71Ga) in LuMn₆Ge_6-xGa_x, 0 ≤ x ≤ 1, is used to monitor the variation of the hyperfine interaction with the sequence of antiferromagnetic - helimagnetic - ferromagnetic arrangement of the manganese moments of subsequent Kagomé net planes achieved by variation of the gallium content x. According to the ⁵⁵Mn-NMR results, the local Mn moment varies by less than ±5% in this series. ¹⁷⁵Lu-NMR proves canting of the antiferromagnetic sublattices in LuMn₆Ge₆. The anisotropy of the Ge magnetic hyperfine interaction decreases with increasing separation from the hexagonal Lu plane, whereas the absolute value of its isotropic part is only qualitatively correlated with the average separation of the six nearest Mn neighbours. Due to the anisotropic magnetic and electric hyperfine interaction at Ge and Ga sites, the non-collinear magnetic structures are clearly reflected by the NMR spectra, which are described quantitatively in this contribution. The preferred Mn moment direction rotates away from the c direction with x. The conduction or bonding electron spin polarization at the Ga nuclei is increased by 35–80% compared to the Ge nuclei. We argue that this is related with the variation of the magnetic order with the Ga content.     

ENDOR investigation of Ni3+ in GaP

With ENDOR measurements the ligand hyperfine and quadrupole interactions of Ni³⁺ in GaP with 3 shells of P neighbours and 3 shells of Ga neighbours could be resolved. It was established that the effective spin is $\text{3}\text{2}$, thus the defect is in the 3d⁷ state, which confirms the earlier assignment of the centre as an Ni³⁺ from ESR measurements. From the quadrupole interaction of the nearest Ga neighbours it can be concluded that Ni³⁺ is substitutional for Ga. From ENDOR-induced ESR experiments an upper limit for the u-parameter of the B·S³ term in the Spin Hamiltonian was determined. The unpaired spin density distribution shows a pronounced tetrahedral symmetry along the [111]-directions.     

Unusual freezing and melting of gallium encapsulated in carbon nanotubes

The freezing and melting behavior of gallium (Ga) encapsulated in carbon nanotubes was investigated through in situ observation in a transmission electron microscope. It is shown that Ga remains liquid up to -80 degrees C when encapsulated in carbon nanotubes. Results of detailed electron diffraction analysis show that the encapsulated Ga can crystallize in either beta phase or gamma phase rather than the common alpha phase upon freezing. Both beta-Ga and gamma-Ga melt at around -20 degrees C. While this is very close to the melting point of bulk beta-Ga (-16 degrees C), it is considerably higher than that of bulk gamma-Ga (-35.6 degrees C). It was observed that upon solidification, Ga has its unique crystallographic orientation relative to the host carbon nanotube.     

ESR study of exchange coupled pairs in copper diethyldithiocarbamate—explanation of the low temperature hyperfine anomaly

A study of the hyperfine interaction in the ESR of Cu-Cu pairs in single crystals of copper diethyldithiocarbamate as a function of temperature has shown distinct differences in the hyperfine structure in the two fine structure transitions at 20 K, the spectrum not having the same hyperfine intensity pattern in the low field fine structure transition in contrast to that of the high field transition. The details of the structure of both the fine structure transitions in the 20 K spectrum have now been explained by recognizing the fact that the mixing of the nuclear spin states caused by the anisotropic hyperfine interaction affects the electron spin states | + 1 > and | −> differently. This has incidentally led to a determination of the sign ofD confirming the earlier model. The anomalous hyperfine structure is found to become symmetric at 77 K and 300 K. It is proposed that the reason for this lies in the dynamics of spin-lattice interaction which limits the lifetime of the spin states in each of the electronic levels | − 1 >, | 0 > and | + 1 > The estimate of spin-lattice relaxation time agrees with those indicated from other studies. The model proposed here for the hyperfine interaction of pairs in the electronic triplet state is of general validity.     

Magnetization-recovery experiments for static and MAS-NMR of I=3/2 nuclei

Multifrequency pulsed NMR experiments on quadrupole-perturbed I=3/2 spins in single crystals are shown to be useful for measuring spin-lattice relaxation parameters even for a mixture of quadrupolar plus magnetic relaxation mechanisms. Such measurements can then be related to other MAS-NMR experiments on powders. This strategy is demonstrated by studies of (71)Ga and (69)Ga (both I=3/2) spin-lattice relaxation behavior in a single-crystal (film) sample of gallium nitride, GaN, at various orientations of the axially symmetric nuclear quadrupole coupling tensor. Observation of apparent single-exponential relaxation behavior in I=3/2 saturation-recovery experiments can be misleading when individual contributing rate processes are neglected in the interpretation. The quadrupolar mechanism (dominant in this study) has both a single-quantum process (T(1Q1)) and a double-quantum process (T(1Q2)), whose time constants are not necessarily equal. Magnetic relaxation (in this study most likely arising from hyperfine couplings to unpaired delocalized electron spins in the conduction band) also contributes to a single-quantum process (T(1M)). A strategy of multifrequency irradiation with observation of satellite and/or central transitions, incorporating different initial conditions for the level populations, provides a means of obtaining these three relaxation time constants from single-crystal (71)Ga data alone. The (69)Ga results provide a further check of internal consistency, since magnetic and quadrupolar contributions to its relaxation scale in opposite directions compared to (71)Ga. For both perpendicular and parallel quadrupole coupling tensor symmetry axis orientations small but significant differences between T(1Q1) and T(1Q2) were measured, whereas for a tensor symmetry axis oriented at the magic-angle (54.74 degrees ) the values were essentially equal. Magic-angle spinning introduces a number of complications into the measurement and interpretation of the spin-lattice relaxation. Comparison of (71)Ga and (69)Ga MAS-NMR saturation-recovery curves with both central and satellite transitions completely saturated by a train of 90 degrees pulses incommensurate with the rotor period provides the simplest means of assessing the contribution from magnetic relaxation, and yields results for the quadrupolar mechanism contribution that are consistent with those obtained from the film sample.     

准一维半导体量子点中电偶极自旋共振的物理机理

半导体量子点中的电子自旋具有较长相干时间以及可扩展性的特点, 在近十几年来引起了人们的广泛兴趣. 人们常常利用电子自旋共振技术来对单个自旋进行操纵. 这样不但需要一个静磁场来使电子产生赛曼劈裂, 同时还需要一个与之垂直的局域振荡磁场. 但是, 在实验上产生足够强且具有固定频率的局域磁场是比较困难的. 后来人们发现, 局域的振荡电场也可以操纵单个电子自旋, 也就是所谓的电偶极自旋共振. 众所周知, 自旋只有自旋磁矩, 不会与电场有任何直接的相互作用. 所以, 电偶极自旋共振的发生必须依赖于某些媒质. 这些媒质包括：量子点材料中的自旋轨道耦合作用, 量子点中的局域磁场梯度, 以及量子点中电子自旋与核自旋的超精细相互作用. 这些媒质能诱导出自旋与电场之间间接的相互作用, 从而外电场操纵单个电子自旋得以实现. 本文总结归纳了目前半导体量子点系统中发生电偶极自旋共振的三种主要物理机理.     

Relaxation of the shallow acceptor center in germanium

Polarized negative muons were used to study relaxation mechanisms of shallow acceptors in germanium. In p-type germanium at low temperatures relaxation of the muon spin was observed, indicating that the muonic atom (gallium-like acceptor center) formed via capture of the negative muon by a host atom is in the paramagnetic state and its magnetic moment is relaxing. The relaxation rate of the muon spin was found to depend on temperature and on concentration of gallium impurity. We conclude that to the relaxation of the magnetic moment of the Ga acceptor in Ge there contribute both scattering of phonons and quadrupole interaction between the acceptors. We estimate, for the first time, the hyperfine interaction constant for the gallium acceptor in germanium as 0.11 MHz.     

Temperature and field dependence of the order parameter in the antiferroquadrupolar phase of CeB6 from mu+ Knight shift measurements

The Fermi contact hyperfine contribution to the Knight shift of positive muons, implanted at the interstitial 3d sites in CeB6, is found to exhibit the same temperature dependence below T(Q) in phase II as the quadrupolar order parameter determined from resonant and nonresonant x-ray scattering. Furthermore, the contact coupling parameter is shown to be anisotropic and field dependent. These unanticipated features are interpreted to arise from the RKKY induced conduction electron spin polarization, which depends on the orientation and expectation value of the ordered 4f quadrupole moments.     

Stabilization of the electron-nuclear spin orientation in quantum dots by the nuclear quadrupole interaction

The nuclear quadrupole interaction eliminates the restrictions imposed by hyperfine interaction on the spin coherence of an electron and nuclei in a quantum dot. The strain-induced nuclear quadrupole interaction suppresses the nuclear spin flip and makes possible the zero-field dynamic nuclear polarization in self-organized InP/InGaP quantum dots. The direction of the effective nuclear magnetic field is fixed in space, thus quenching the magnetic depolarization of the electron spin in the quantum dot. The quadrupole interaction suppresses the zero-field electron spin decoherence also for the case of nonpolarized nuclei. These results provide a new vision of the role of the nuclear quadrupole interaction in nanostructures: it elongates the spin memory of the electron-nuclear system.     

Intrinsic vacancy-induced nanoscale wire structure in heteroepitaxial Ga2Se3/Si(001)

A highly anisotropic growth morphology is found for heteroepitaxial gallium sesquiselenide (Ga2Se3) on the lattice matched substrate, arsenic-terminated Si(001). Scanning tunneling microscopy of Ga2Se3 films reveals nanoscale, wirelike structures covering the surface in parallel lines, less than 1 nm wide and up to 30 nm long. Core-level photoemission spectroscopy and diffraction reveals the local structure of buried Ga and Se atoms to reflect the bulk, defected zinc-blende structure of beta-Ga2Se3, which contains ordered 110 arrays of Ga vacancies. These ordered vacancy lines are proposed to be responsible for the observed growth anisotropy in heteroepitaxial Ga2Se3.     

Ti：Al2O3晶体的电子自旋共振谱研究

Ｔｉ：Ａｌ２Ｏ３晶体中的顺磁中心Ｔｉ＾３＋由于强烈的晶格自旋耦合而使其电子自旋共振吸收只有在液拟温度附近才能看到，本实验在液氮温度附近记录到由许多强的的吸收峰迭加于Ｔｉ＾３＋宽吸收线所组成的电子自旋共振谱，这些吸吸收峰被认为是Ａｌ２Ｏ３基质中的Ｆｅ＾３＋，Ｍｎ＾２＋，Ｃｒ＾３＋，Ｍｏ＾３＋，Ｆｅ＾２＋，Ｃｏ＾２＋等杂质的共振吸收及Ｔｉ＾２＋３Ａ２ｇ基态的双量子跃迁造成的。     

Electron spin-echo envelope modulation at S-band. 3: Analysis of nuclear quadrupole interaction effects

In the present paper the nuclear modulation of electron spin echo signals at S-band is investigated in the case of interacting nuclei with a quadrupole moment high enough to cause nuclear quadrupole couplings not negligible with respect to the nuclear Zeeman and dipolar hyperfine couplings. Both the two-pulse and three-pulse electron spin echo envelope modulation (ESEEM) due to²⁷Al and¹⁴N are simulated at different values of the nuclear quadrupole coupling by numerical diagonalization of the nuclear Hamiltonians. The behavior of their amplitude and periods is discussed on the basis of the ratios between the strengths of the nuclear quadrupole interaction and the nuclear Zeeman and the dipolar hyperfine interactions. The interpretation of their trends in terms of the eigenfunctions and eigenvectors of the nuclear Hamiltonians is carried out by using analytical equations obtained by perturbation approaches. First order perturbation treatments for integer and half-integer nuclear spin quantum numbers are developed when the nuclear quadrupole coupling is the main interaction. A discussion on the limits of the interpretation based on the perturbation approach is also given by comparing the magnitude Fourier transform of the patterns calculated by exact diagonalization and analytical equations.     

Subsecond spin relaxation times in quantum dots at zero applied magnetic field due to a strong electron-nuclear interaction

A key to ultralong electron spin memory in quantum dots (QDs) at zero magnetic field is the polarization of the nuclei, such that the electron spin is stabilized along the average nuclear magnetic field. We demonstrate that spin-polarized electrons in n-doped (In,Ga)As/GaAs QDs align the nuclear field via the hyperfine interaction. A feedback onto the electrons occurs, leading to stabilization of their polarization due to formation of a nuclear spin polaron [I. A. Merkulov, Phys. Solid State 40, 930 (1998)]. Spin depolarization of both systems is consequently greatly reduced, and spin memory of the coupled electron-nuclear spin system is retained over 0.3 sec at temperature of 2 K.     

Decoherence in solid-state qubits

The interaction of solid-state qubits with environmental degrees of freedom strongly affects the qubit dynamics, and leads to decoherence. In quantum information processing with solid-state qubits, decoherence significantly limits the performances of such devices. Therefore, it is necessary to fully understand the mechanisms that lead to decoherence. In this review, we discuss how decoherence affects two of the most successful realizations of solid-state qubits, namely, spin qubits and superconducting qubits. In the former, the qubit is encoded in the spin 1/2 of the electron, and it is implemented by confining the electron spin in a semiconductor quantum dot. Superconducting devices show quantum behaviour at low temperatures, and the qubit is encoded in the two lowest energy levels of a superconducting circuit. The electron spin in a quantum dot has two main decoherence channels, a (Markovian) phonon-assisted relaxation channel, due to the presence of a spin–orbit interaction, and a (non-Markovian) spin bath constituted by the spins of the nuclei in the quantum dot that interact with the electron spin via the hyperfine interaction. In a superconducting qubit, decoherence takes place as a result of fluctuations in the control parameters, such as bias currents, applied flux and bias voltages, and via losses in the dissipative circuit elements.     

Optical polarization of nuclei and ODNMR in GaAs/AlGaAs quantum wells

Optical orientation of electrons was used to polarize the crystal lattice nuclei in quantum-size heterostructures and to study the effect of the conduction band spin splitting on the spin states of quasi-two-dimensional (2D) electrons drifting in an external electric field. High (～1%) nuclear polarization was registered using polarized luminescence and ODNMR in single GaAs/AlGaAs quantum wells. Measurement was made of the hyperfine interaction fields created by polarized nuclei on electrons and by electrons on nuclei. The spin-lattice relaxation of nuclei on the non-degenerate 2D electron gas was calculated. A comparison of the theoretical and experimental longitudinal relaxation times permitted the conclusion that the localized charge carriers are responsible for nuclear polarization in quantum wells in the temperature range of 2–77 K. A new effect has been studied, i.e. induction of an effective magnetic field acting on 2D electron spins when electrons drift in an external electric field in the quantum well plane. This effective field B_eff is due to the spin splitting of the conduction band of 2D electrons. The paper discusses possible registration of an ODNMR signal when the field B_eff is modulated by an electric current during optical orientation.     

Determination of the 14N quadrupole coupling constant of nitroxide spin probes by W-band ELDOR-detected NMR

Nitroxide spin probe electron paramagnetic resonance (EPR) has proven to be a very successful method to probe local polarity and solvent hydrogen bonding properties at the molecular level. The g(xx) and the (14)N hyperfine A(zz) principal values are the EPR parameters of the nitroxide spin probe that are sensitive to these properties and are therefore monitored experimentally. Recently, the (14)N quadrupole interaction of nitroxides has been shown to be also highly sensitive to polarity and H-bonding (A. Savitsky et al., J. Phys. Chem. B 112 (2008) 9079). High-field electron spin echo envelope modulation (ESEEM) was used successfully to determine the P(xx) and P(yy) principal components of the (14)N quadrupole tensor. The P(zz) value was calculated from the traceless character of the quadrupole tensor. We introduce here high-field (W-band, 95 GHz, 3.5 T) electron-electron double resonance (ELDOR)-detected NMR as a method to obtain the (14)N P(zz) value directly, together with A(zz). This is complemented by W-band hyperfine sublevel correlation (HYSCORE) measurements carried out along the g(xx) direction to determine the principal P(xx) and P(yy) components. Through measurements of TEMPOL dissolved in solvents of different polarities, we show that A(zz) increases, while |P(zz)| decreases with polarity, as predicted by Savitsky et al.     

Nuclear and ion spins in semiconductor nanostructures

Spin interactions are studied between conduction band electrons in GaAs heterostructures and local moments, specifically the spins of constituent lattice nuclei and of partially filled electronic shells of impurity atoms. Nuclear spin polarizations are addressed through the contact hyperfine interaction resulting in the development of a method for high-field optically detected nuclear magnetic resonance sensitive to 10⁸ nuclei. This interaction is then used to generate nuclear spin polarization profiles within a single parabolic quantum well; the position of these nanometer-scale sheets of polarized nuclei can be shifted along the growth direction using an externally applied electric field. In order to directly investigate ion spin dynamics, doped GaMnAs quantum wells are fabricated in the regime of very low Mn concentrations. Measurements of coherent electron spin dynamics show an antiferromagnetic exchange between s-like conduction band electrons and electrons localized in the d-shell of the Mn impurities, which varies as a function of well width.     

Investigation of the spin generation using the perturbation theory in the long wavelength limit

An investigation of the electronic spin-generation in gallium arsenide is performed using the perturbation theory of the spin transport model in the long wavelength limit, where electron–electron and electron–phonon interactions are ignored. The spin polarizations of the conduction-band-electrons in dependences of the excited one- and two-photon energies are estimated. For both cases, the spin-polarization is found to depolarize for excitation energies equal to or larger than the energy gap of the split-off band to the conduction band. The results, however, show that an enhancement of the spin-polarization is achieved by multiphoton excitation of the bulk semiconductors. The calculated results are compared with those obtained in recent experiments. A good agreement between theory and experiment is obtained.     

The electric field gradient in heavy rare earth metals

Estimates of the electric field gradient in heavy rare earth metals have been evaluated from experimental hyperfine interaction data. In addition, the magnetic hyperfine fields are analyzed. In the metals the effective radial integrals 〈r ^?3〉_4f of the magnetic and quadrupole hyperfine interaction are reduced at most by 10% compared with the free ion values. The electric field gradients due to the crystalline field have been found to be 200 times larger than those predicted from point charge calculations. This antishielding effect can be explained by an enhanced conduction electron density at the interstitial sites and an increase of the Sternheimer factor γ_∞ in the metallic environment.     

Investigation of Hyperfine Interactions in GdNiIn Compound

Perturbed gamma–gamma angular correlation technique was used to measure the hyperfine interactions in the compound GdNiIn using the ¹¹¹InCd → and ¹⁴⁰La¹⁴⁰Ce probe nuclei at the In and Gd sites, respectively. A unique quadrupole frequency with asymmetry parameter η = 0.78 was observed for ¹¹¹Cd probe at In sites for the measurements above Curie temperature. Below T _C , the spectra for ¹¹¹Cd show combined magnetic dipole and electric quadrupole interaction. Below 85 K, a unique magnetic interaction is observed at ¹⁴⁰Ce. A linear relationship between the saturated magnetic hyperfine field and the magnetic transition temperature was observed for both probes, indicating that the main contribution to the mhf comes from the conduction electron polarization.