Pseudoentanglement of spin states in the multilevel 15N@C60 system

We have prepared combined electron and nuclear spin pseudoentangled states Psi+/-27 and Phi+/-18 out of the total number of eight quantum states in the multilevel quantum system of a nitrogen atom with electron spin 3/2 and nuclear spin 1/2 encaged in the endohedral fullerene (15)N@C(60). Density matrix tomography was applied to verify the degree of entanglement.     

Electron paramagnetic resonance investigation of endohedral fullerenes N@C(60) and N@C(70) in a liquid crystal

Endohedral fullerenes N@C(60) and N@C(70) were dissolved in the liquid crystal 4-methoxybenzylidene-4'-n-butylaniline (MBBA) and investigated by electron paramagnetic resonance. In both cases well resolved EPR spectra give proof for molecular orientation in the nematic mesophase. Spectral features are dominated by a nonvanishing zero-field interaction, indicating a deviation from spherical spin density distribution at the encased nitrogen atom. In N@C(70), a maximum order parameter O(33) = 0.18(3), correlated with the long axis of the cage, and a zero-field-splitting parameter D = -2.6(4) MHz were determined. A persistent zero-field splitting is also observed in C(60) via the quartet spin of the encapsulated nitrogen, although no assignment of the director with respect to the molecular frame is possible. The observed line splitting is indicative of pseudo orientation of the rapidly rotating cage in this case.     

Coherent state transfer between an electron and nuclear spin in (15)N@C(60)

Electron spin qubits in molecular systems offer high reproducibility and the ability to self-assemble into larger architectures. However, interactions between neighboring qubits are "always on," and although the electron spin coherence times can be several hundred microseconds, these are still much shorter than typical times for nuclear spins. Here we implement an electron-nuclear hybrid scheme which uses coherent transfer between electron and nuclear spin degrees of freedom in order to both effectively turn on or off interqubit coupling mediated by dipolar interactions and benefit from the long nuclear spin decoherence times (T(2n)). We transfer qubit states between the electron and (15)N nuclear spin in (15)N@C(60) with a two-way process fidelity of 88%, using a series of tuned microwave and radio frequency pulses and measure a nuclear spin coherence lifetime of over 100 ms.     

Spin polarization of the neutral exciton in a single quantum dot

While efficient nuclear polarization has earlier been reported for the charged exciton in InAs/GaAs quantum dots at zero external magnetic field, we report here on a surprisingly high degree of circular polarization, up to ≈60%$\approx 60\%$, for the neutral exciton emission in individual InAs/GaAs dots. This high degree of polarization is explained in terms of the appearance of an effective nuclear magnetic field which stabilizes the electron spin. The nuclear polarization is manifested in experiments as a detectable Overhauser shift. In turn, the nuclei located inside the dot are exposed to an effective electron magnetic field, the Knight field. This nuclear polarization is understood as being due to the dynamical nuclear polarization by an electron localized in the QD. The high degree of polarization for the neutral exciton is also suggested to be due to separate in-time capture of electrons and holes into the QD.     

Room temperature electrical detection of spin coherence in C60

An experimental demonstration of electrical detection of coherent spin motion of weakly coupled, localized electron spins in thin fullerene C60 films at room temperature is presented. Pulsed electrically detected magnetic resonance experiments on vertical photocurrents through Al/C(60)/ZnO samples showed that an electron spin Rabi oscillation is reflected by transient current changes. The nature of possible microscopic mechanisms responsible for this spin to charge conversion as well as its implications for the readout of endohedral fullerene (N@C(60)) spin qubits are discussed.     

Polarized beam of unstable nuclei via ion beam surface interaction at grazing incidence

We have measured the polarization of unstable nuclei produced by the technique of ion beam surface interaction at grazing incidence (IBSIGI). A 60 keV ¹²⁴Cs beam from the on-line mass separator was used. The NMR technique was employed for the observation of the nuclear polarization. A small polarization of 0.22(13)% was observed. The small value was interpreted in terms of the velocity dependence of nuclear polarization, comparing with the results observed with stable nuclei. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ih和D5h对称的三金属氮化物富勒烯Sc3N@C80的几何结构、电子结构和磁学特性

采用密度泛函理论（density functional theory, DFT）方法对具有Ih和D5h对称的三金属氮化物富勒烯Sc3 N＠C80的几何结构、电子结构及其磁学特性进行了计算研究。几何结构优化显示掺杂Sc3 N之后,C80的结构只是发生了细微的变化,仍然保持了Ih 和D5h对称性。能级图和局部态密度图表明Sc原子对能级的变化贡献最大,掺杂之后能隙增加,简并度下降,增强了两种异构体的稳定性。磁学特性分析指出掺杂之后,Sc3 N的磁性完全淬灭,两种异构体均没有磁矩,都不能作为磁性材料。     

Residual polarization of negative muons implanted in C60 and K3C60

Negative muons captured by atoms with zero nuclear spin usually retain about 1/6 of their initial spin polarization P_0 after cascading down to their lowest Bohr orbital. This is particularly observed for μ^- implanted into graphite and diamond. However, μ^- implanted into a powder of the fullerene C₆₀ are found to show essentially zero residual polarization. A residual polarization of \sim P_0/6 is again seen in metallic K₃C₆₀. We speculate that these results are to be explained on the basis of hyperfine effects after completion of the muonic cascade. This revised version was published online in August 2006 with corrections to the Cover Date.     

High-frequency dynamic nuclear polarization in the nuclear rotating frame

A proton dynamic nuclear polarization (DNP) NMR signal enhancement (epsilon) close to thermal equilibrium, epsilon = 0.89, has been obtained at high field (B(0) = 5 T, nu(epr) = 139.5 GHz) using 15 mM trityl radical in a 40:60 water/glycerol frozen solution at 11 K. The electron-nuclear polarization transfer is performed in the nuclear rotating frame with microwave irradiation during a nuclear spin-lock pulse. The growth of the signal enhancement is governed by the rotating frame nuclear spin-lattice relaxation time (T(1rho)), which is four orders of magnitude shorter than the nuclear spin-lattice relaxation time (T(1n)). Due to the rapid polarization transfer in the nuclear rotating frame the experiment can be recycled at a rate of 1/T(1rho) and is not limited by the much slower lab frame nuclear spin-lattice relaxation rate (1/T(1n)). The increased repetition rate allowed in the nuclear rotating frame provides an effective enhancement per unit time(1/2) of epsilon(t) = 197. The nuclear rotating frame-DNP experiment does not require high microwave power; significant signal enhancements were obtained with a low-power (20 mW) Gunn diode microwave source and no microwave resonant structure. The symmetric trityl radical used as the polarization source is water-soluble and has a narrow EPR linewidth of 10 G at 139.5 GHz making it an ideal polarization source for high-field DNP/NMR studies of biological systems.     

Ih和D5h对称的三金属氮化物富勒烯Sc3N@C80的几何结构、电子结构和磁学特性

采用密度泛函理论（density functional theory, DFT）方法对具有Ih和D5h对称的三金属氮化物富勒烯Sc3N@C80的几何结构、电子结构及其磁学特性进行了计算研究.几何结构优化显示掺杂Sc3N之后,C80的结构只是发生了细微的变化,仍然保持了Ih和D5h对称性.能级图和局部态密度图表明Sc原子对能级的变化贡献最大,掺杂之后能隙增加,简并度下降,增强了两种异构体的稳定性.磁学特性分析指出掺杂之后,Sc3N的磁性完全淬灭,两种异构体均没有磁矩,都不能作为磁性材料.     

The Energy Spectrum and Optical Absorption Spectrum of Endohedral Fullerenes Lu3N@C80 and Y3N@С80 within the Hubbard Model

Physics of the Solid State - The anticommutator Green’s functions and energy spectra of fullerene С80 and endohedral fullerenes Lu3N@C80 and Y3N@С80 with symmetry group Ih are...     

Mechanism of Polarization Production by Means of the Tilted-Foil Technique Studied Using Beta-Radioactive Nuclei

In order to disclose the polarization mechanism of the tilted-foil technique, systematic experimental studies were performed through direct observations of the nuclear polarization of short-lived beta-emitters. The present studies concentrated on the mechanisms of the following processes: (1) production of atomic polarization in the beam–tilted-foil interaction, (2) polarization transfer between an atom and a nucleus, and (3) enhancement of nuclear polarization by the beam multi-tilted-foil interaction. The present results have confirmed the plausibility of a previous model in which the atomic-polarization process in the tilted-foil technique is attributed to the right–left asymmetry of the electron-capture process at the final surface of a tilted foil. The enhancement effect of nuclear polarization by the multi-tilted-foil technique could be consistently explained by a model proposed by Goldring and Niv. Further information on atomic processes is required to make a definite statement on hyperfine interactions and atomic configurations, which contribute to polarization transfer.     

Surface SelectiveH/Si CP NMR by NOE Enhancement from Laser Polarized Xenon

The surface proton spin polarization created by the spin-polarization-induced nuclear Overhauser effect from optically polarized xenon can be transferred in a subsequent step by solid-state cross polarization to another nuclear spin species such as²⁹Si. The technique exploits the dipolar interactions of xenon nuclear spins with high γ nuclei such as¹H, and is experimentally simpler than direct polarization transfer from¹²⁹Xe to heteronuclei such as¹³C and²⁹Si.     

From quantum Hall ferromagnetism to huge longitudinal resistance at the 2/3 fractional quantum Hall state

We observe the transition from a spin-unpolarized to a polarized nu=2/3 fractional quantum Hall state at low currents (<5 nA), recently described in terms of quantum Hall ferromagnetism, versus density and parallel magnetic field. At larger currents the time and current dependent huge longitudinal resistance (HLR) is always initiated at the transition. Transport in the HLR regime is linear and the amount of current-induced nuclear polarization in the HLR is comparable to the thermal nuclear polarization at approximately 20 mK and 10 T. A current-induced disorder in the nuclear polarization is speculated to cause the enhanced resistance in the HLR regime.     

High proton polarization by microwave-induced optical nuclear polarization at 77 K

Protons in naphthalene and p-terphenyl doped with pentacene have been polarized up to 32% and 18%, respectively, at liquid nitrogen temperature in a magnetic field of 0.3 T by means of microwave-induced optical nuclear polarization. The polarization was measured by nuclear magnetic resonance as well as by the neutron transmission method. The relaxation time of the proton polarization at 0.0007 T and 77 K was found to be almost 3 h and the polarization enhancement reached a record value of 8x10(4). The usefulness of the method in many areas of physics and chemistry is discussed.     

Nuclear relaxation in dynamic nuclear polarization

The behavior of nuclear polarization in a substance, i.e., a solution of the complex HMBA(Cr^V)Na⁺ in 1,2-propylene glycol used in polarized nuclear targets is experimentally investigated by magnetic spectroscopic methods under conditions of dynamic nuclear polarization at hv_S/kT=≈1.5−3.2. Nuclear polarization is measured and analyzed as a function of time at different values of the saturating microwave signal and temperature. It is shown that the process of decreasing the nuclear polarization involving free nuclear relaxation is described by a nonmonoexponential law with two damping decrements, which determine the time of reaching equilibrium between the Zeeman nuclear subsystem, the dipole-dipole pool, and the lattice. Specific features of dynamic processes proceeding in the electronic-nuclear system of the substance investigated are discussed. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 363–366, May–June, 1997.     

Role of spin noise in the detection of nanoscale ensembles of nuclear spins

When probing nuclear spins in materials on the nanometer scale, random fluctuations of the spin polarization will exceed the mean Boltzmann polarization for sample volumes below about (100 nm){3}. In this Letter, we use magnetic resonance force microscopy to observe nuclear spin fluctuations in real time. We show how reproducible measurements of the polarization variance can be obtained by controlling the spin correlation time and rapidly sampling a large number of independent spin configurations. This allows significant improvement in the signal-to-noise ratio for nanometer-scale magnetic resonance imaging.     

A spectrometer for EPR,DNP, and multinuclear high-resolution NMR

We describe a magnetic resonance spectrometer capable of EPR, dynamic nuclear polarization, and multinuclear high-resolution NMR. The operating field is 1.4 T, corresponding to Larmor frequencies of 40 GHz and 60 MHz for electrons and protons, respectively. The microwave side of the probe is based on a Fabry-Perot resonator (FPR ), an open structure that enhances power-to-field conversion for efficient saturation of the EPR for dynamic polarization, and further permits in situ detection for EPR. This allows the external field to be set at, rather than scanned for, the optimal DNP position. Moreover, we have found that adjustments necessary for maximizing DNP may be done via optimization of the EPR signal, a feature of particular significance for samples which exhibit NMR signals on the borderline of detectability, i.e., samples for which DNP is of special importance. 'H and '3C polarization enhancements achieved using the FPR are compared with devices used by others, in particular the horn /reflector system used by Wind and co-workers. Direct '3C enhancements large enough to detect 2.5 x 10'6 spins in (fluoranthenyl)2 PF6 after a single one-second polarization period have been obtained, and the first high-field 'Li DNP results are also presented.     

Zur Vakuumpolarisation in Myonenatomen

The various approximations of vacuum polarization potential and the higher order corrections up to α³ are reviewed and quantitatively dicussed. The quadrupol part of the vacuum polarization is established. It leads rather straight forward to a small contribution of vacuum polarization to nuclear polarization. These effects are quantitatively investigated.     

Polarization of nuclear spins from the conductance of quantum wire

We devise an approach to measure the polarization of nuclear spins via conductance measurements. Specifically, we study the combined effect of external magnetic field, nuclear spin polarization, and Rashba spin-orbit interaction on the conductance of a quantum wire. Nonequilibrium nuclear spin polarization affects the electron energy spectrum making it time dependent. Changes in the extremal points of the spectrum result in time dependence of the conductance. The conductance oscillation pattern can be used to obtain information about the amplitude of the nuclear spin polarization and extract the characteristic time scales of the nuclear spin subsystem.