Research on glass cells for He neutron spin filters

Glass cells play an important role in polarized ³He neutron spin filters. To evaluate the scattering and absorption contribution from glass cells during neutron scattering experiments, we measured small-angle scattering and neutron transmission in GE180 and other glasses. The small-angle neutron scattering measurements revealed that the glasses used for ³He spin filters have acceptably lower scattering: dΣ(Q)/dΩ=4-7×10⁻⁴ cm⁻¹ at Q=0.03-0.12 Å⁻¹. The transmission measurement was performed at J-PARC. Neutron transmission of about 92% through empty GE180 cells was observed over a wide wavelength range 0.014-7.0 Å. To pursue the possibility of being a structural influence on ³He spin relaxation in GE180 glass cells, we performed precise X-ray diffraction measurement using synchrotron radiation at SPring-8. From these measurements, a structural difference was observed among GE180 glasses with different thermal treatments.     

Tuning the bottleneck effect in Ag metallic host doped with magnetic Gd and non-magnetic Sb ions

The Ag metallic host doped with Gd and Sb is an excellent model system to study the bottleneck effect associated to the conduction-electron (c-e) spin-flip scattering mechanism. Electron spin resonance of Gd³⁺ in both, Ag-(Gd doped)- and Ag-(Gd and Sb doped)-systems, reveal the presence of bottleneck which can be tuned by the amount of Gd and Sb impurities. The increase of the Gd concentration leads to a c-e spin-flip relaxation rate to the magnetic Gd³⁺ ions larger than that to the lattice, favoring the bottleneck regime. Whereas the effect of the non-magnetic impurities (Sb ions) is to increase, via spin–orbit scattering, the spin-flip relaxation rate of the c-e to the lattice, weakening the bottleneck regime.     

Theoretical model of spin transfer torque in multilayers

We present a model of spin transport in a Co/Cu(1 1 1)/Co pseudo-spin-valve (PSV) structure where current is flowing in the current perpendicular-to-plane (CPP) geometry. The model considers ballistic spin-dependent transmission at the two Co–Cu interfaces, as well as diffusive spin relaxation within the Cu spacer and free Co layer. In the latter, the spin relaxation process is composed of the usual longitudinal spin relaxation due to spin flip scattering, as well as transverse spin relaxation due to spin precession. The resulting spin transfer torque exerted on the moments within the free Co layer is composed of two contributions, the main contribution coming from “absorbed” spins in the interfacial regions. The second contribution arises from the relaxation of spin accumulation within the free Co layer. The calculated critical current density for switching is estimated to be approximately between 3.3×10⁷ and 1.1×10⁸ A/cm², which is in agreement with available experimental results.     

Hyperfine switching triggered by resonant tunneling for the detection of a single nuclear spin qubit

A novel detection mechanism and a robust control of a single nuclear spin-flip by hyperfine interactions between the nuclear spin and tunneling electron spin are proposed on the basis of ab initio non-equilibrium Green's function calculations. The calculated relaxation times of the nuclear spin of proton in a nano-contact system, Pd(electrode)-H₂-Pd(electrode), show that ON/OFF switching of hyperfine interactions is effectively triggered by resonant tunneling mediated through the d-orbitals of Pd. The relaxation times at ON-resonance are ∼¹⁰³ times faster than those at OFF-resonance, indicating that ON-resonance is suitable for the detection (read-out) of nuclear spin states. In addition, the effectiveness of bias voltage applications at OFF-resonance for selective operations on the proton qubit is demonstrated in the calculations of the resonant frequencies of proton using the gauge-invariant atomic orbital method.     

Observation of spin-lattice relaxations of dilute Fe3+ in MgO by Mössbauer spectroscopy

We present a method to describe the temperature dependence of emission Mössbauer spectra showing slow spin-lattice relaxations of Fe^3?+? in MgO single crystals, obtained after implantation of ⁵⁷Mn at ISOLDE/CERN. The analysis is based on the Blume-Tjon model for the line-shape of relaxing paramagnetic sextets with the spin relaxation rate, τ ^???1 as a parameter. The temperature dependent spin relaxation rate of Fe^3?+? in MgO is found to increase to ~10⁸ s^???1 at 647 K by assuming a relaxation rate of τ ^???1?< 10⁶ s^???1 at 77 K. The results are in accordance with those obtained by electron paramagnetic resonance spectroscopy demonstrating the possibility of retrieving spin-lattice relaxation rates of dilute Fe^3?+? from emission Mössbauer spectroscopy of Mn/Fe-implanted oxides.     

Molecular dynamics and phase transitions in paramagnetic [Mn(H2O)6][SiF6] investigated by H NMR

The temperature dependences of ²H NMR spectra and spin-lattice relaxation time T₁ have been measured for paramagnetic [Mn(H₂O)₆][SiF₆]. The obtained ²H NMR spectra were simulated by considering the quadrupole interaction and paramagnetic shift. The variation of the spectra measured in phase III was explained by the 180° flip of water molecules. The activation energy E_a and the jumping rate at infinite temperature k₀ for the 180° flip of H₂O were obtained as 35 kJ mol⁻¹ and 4×10¹⁴ s⁻¹, respectively. The spectral change in phases I and II was ascribed to the reorientation of [Mn(H₂O)₆]²⁺ around the C₃ axis where the E_a and k₀ values were estimated as 45 kJ mol⁻¹ and 1×10¹³ s⁻¹, respectively. From the almost temperature independent and short T₁ value, the correlation time for electron-spin flip-flops, τ_e, and the exchange coupling constant J were obtained as 3.0×10⁻¹⁰ s and 2.9×10⁻³ cm⁻¹, respectively. The II-III phase transition can be caused by the onset of the jumping motion of [Mn(H₂O)₆]²⁺ around the C₃ axis.     

On the environmental decoherence and spin interference in mesoscopic loop structures

Mechanisms of ‘environmental decoherence’ such as surface scattering, Elliot–Yafet process and precession mechanisms, as well as their influence on the spin phase relaxation are considered and compared. It is shown that the ‘spin ballistic’ regime is possible, when the phase relaxation length for the spin part of the wave function (L^(s)) is much greater than the phase relaxation length for the ‘orbital part’ (L^(e)). In the presence of an additional magnetic field, the spin part of the electron's wave function (WF) acquires a phase shift due to additional spin precession about that field. If the structure length L is chosen to be L^(s)>L>L^(e), it is possible to ‘wash out’ the quantum interference related to the phase coherence of the ‘orbital part’ of the WF, retaining at the same time that related to the phase coherence of the spin part and, hence, to reveal corresponding conductance oscillations.     

Resonant photoemission study of Eu1−xGdxTe layers

Resonant photoemission study of electronic structure of molecular beam epitaxy grown Eu_1−xGd_xTe layers without and with cover protected layer of Te were performed using synchrotron radiation. The analysis of the valence band and shallow core levels spectra of the clean surface of Eu_1−xGd_xTe obtained in situ under UHV conditions showed the existence of Eu²⁺ and Eu³⁺ ions in the layers. The trivalent europium ions mostly are located at the surface and its amount strongly depends on sample surface preparation conditions. The prolonged annealing of Eu_1−xGd_xTe layers covered with protected layer of Te leads to formation of clean surface of the sample not changing the stoichiometry of it and without the accumulation of Eu³⁺ ions at the surface region.     

Spin relaxation of conduction electrons in highly-doped n-type germanium at low temperature

We present a theory of the spin relaxation time of the conduction electrons in highly-doped n-type germanium at liquid helium temperature. The theory is compared with some of our measurements and the experimental data available in the literature on As-doped germanium. The observed linewidth at T = 10 K is accounted for in the whole metallic concentration range (N_D > 3 × 10¹⁷cm^?3).In the lower concentration range (3 × 10¹⁷ < N_D < 10¹⁸cm^?3), the relevant mechanism is the random jumping of the g factor upon intervalley scattering. The agreement with experiment is good without any adjustable parameter.In the higher concentration range (N_D > 10¹⁸cm^?3), the dominant process for the linewidth is the spin-flip scattering by ionized donors (Elliott process); the usual theory is shown to be insufficient and the greater effectiveness of scattering by the localized part of the donor potentials is pointed out. The calculated linewidth is related to the intervalley scattering time T_iv. The agreement with experiment is good and predictions are given for the linewidth in the case of other shallow donors in the same concentration range.     

Luminescence properties of Tb implanted ZnO

ZnO [0 0 0 1] crystals were irradiated at room temperature with Tb⁺ ions of 400 keV with fluences from 1×10¹⁶ to 2×10¹⁷ cm⁻². The implanted layer was examined by several methods, including radioluminescence (RL), Rutherford backscattering spectrometry (RBS) and optical spectroscopy. The optical extinction spectra were simulated using Mie scattering theory. Absorption spectra predicted by Mie theory for particles of decreasing diameter were compared with those obtained experimentally. Some qualitative agreement between theoretical and experimental data was achieved. It was also shown that the intensities of the characteristic green emission bands associated with Tb produced by ⁵D₄→⁷F_j=5,4 transitions have increased about 8 times after annealing. Optical spectroscopy and radioluminescence data have revealed that the ion implantation is a promising tool for synthesizing Tb nanoparticles in the ZnO surface. The Tb nanoparticles exhibit a rather weak plasma resonance.     

Electron hopping mechanism in hematite (α-Fe2O3)

The frequency dependence of the real (?′) and imaginary (?″) parts of the dielectric constant of polycrystalline hematite (α-Fe₂O₃) has been investigated in the frequency range 0-100 kHz and the temperature range 190-350 K, in order to reveal experimentally the electron hopping mechanism that takes place during the Morin transition of spin-flip process. The dielectric behaviour is described well by the Debye-type relaxation (α-dispersion) in the temperature regions T<233 K and T>338 K. In the intermediate temperature range 233 K<T<338 K a charge carrier mechanism takes place (electron jump from the O²⁻ ion into one of the magnetic ions Fe³⁺) which gives rise to the low frequency conductivity and to the Ω-dispersion. The temperature dependence of relaxation time (τ) in the −ln τ vs 10³/T plot shows two linear regions. In the first, T<238 K, τ increases with increasing T implying a negative activation energy −0.01 eV, and in the second region T>318 K τ decreases as the temperature increases implying a positive activation energy 0.12 eV. The total reorganization energy (0.12-0.01) 0.11 eV is in agreement with the adiabatic activation energy 0.11 eV given by an ab initio model in the literature. The temperature dependence of the phase shift in the frequencies 1, 5, 10 kHz applied shows clearly an average Morin temperature T_Mo=284±1 K that is higher than the value of 263 K corresponding to a single crystal due to the size and shape of material grains.     

Light scattering observations of spin reversal excitations in the fractional quantum Hall regime

Resonant inelastic light scattering experiments access the low lying excitations of electron liquids in the fractional quantum Hall regime in the range 2/5≥ν≥1/3. Modes associated with changes in the charge and spin degrees of freedom are measured. Spectra of spin reversed excitations at filling factor ν?1/3 and at ν?2/5 identify a structure of lowest spin-split Landau levels of composite fermions (CFs) that is similar to that of electrons. Observations of spin wave excitations enable determinations of energies required to reverse spin. The spin reversal energies obtained from the spectra illustrate the significant residual interactions of composite fermions. At ν=1/3 energies of spin reversal modes are larger but relatively close to spin conserving excitations that are linked to activated transport. Predictions of composite fermion theory are in good quantitative agreement with experimental results.     

Femtosecond laser-induced damage of gold films

Single- and multi-shot ablation thresholds of gold films in the thickness range of 31-1400 nm were determined employing a Ti:sapphire laser delivering pulses of 28 fs duration, 793 nm center wavelength at 1 kHz repetition rate. The gold layers were deposited on BK7 glass by an electron beam evaporation process and characterized by atomic force microscopy and ellipsometry. A linear dependence of the ablation threshold fluence F_th on the layer thickness d was found for d ≤ 180 nm. If a film thickness of about 180 nm was reached, the damage threshold remained constant at its bulk value. For different numbers of pulses per spot (N-on-1), bulk damage thresholds of ∼0.7 J cm⁻² (1-on-1), 0.5 J cm⁻² (10-on-1), 0.4 J cm⁻² (100-on-1), 0.25 J cm⁻² (1000-on-1), and 0.2 J cm⁻² (10000-on-1) were obtained experimentally indicating an incubation behavior. A characteristic layer thickness of L_c ≈ 180 nm can be defined which is a measure for the heat penetration depth within the electron gas before electron-phonon relaxation occurs. L_c is by more than an order of magnitude larger than the optical absorption length of α⁻¹ ≈ 12 nm at 793 nm wavelength.     

Ferromagnetism in amorphous Ge1−xMnx grown by low temperature vapor deposition

Magnetic properties of amorphous Ge_1−xMn_x thin films were investigated. The thin films were grown at 373 K on (100) Si wafers by using a thermal evaporator. Growth rate was ∼35 nm/min and average film thickness was around 500 nm. The electrical resistivities of Ge_1−xMn_x thin films are 5.0×10⁻⁴∼100 Ω cm at room temperature and decrease with increasing Mn concentration. Low temperature magnetization characteristics and magnetic hysteresis loops measured at various temperatures show that the amorphous Ge_1−xMn_x thin films are ferromagnetic but the ferromagnetic magnetizations are changing gradually into paramagnetic as increasing temperature. Curie temperature and saturation magnetization vary with Mn concentration. Curie temperature of the deposited films is 80-160 K, and saturation magnetization is 35-100 emu/cc at 5 K. Hall effect measurement at room temperature shows the amorphous Ge_1−xMn_x thin films have p-type carrier and hole densities are in the range from 7×10¹⁷ to 2×10²² cm⁻³.     

Porous silicon as efficient surface enhanced Raman scattering (SERS) substrate

Silver nanocrystallites are obtained through immersion of porous silicon samples in AgNO₃ solutions and a successive thermal annealing. The efficiency of nanostructures as surface enhanced Raman scattering (SERS) substrates is checked on cyanine-based dyes and horseradish peroxidase, evidencing detectable concentrations as low as 10⁻⁷ to 10⁻⁸ M. The substrate efficiency is strictly related to the Ag particle morphology, which could yield to either local surface plasmons (LSP) coupled to individual particles or to inter-particle short-range interaction.     

Effects of lattice and local dynamics in EPR spectra and electron spin relaxation of vibronic Cu(imidazole)6 complexes in Zn(imidazole)6Cl2·4H2O crystals

Cu(im)₆ complexes in Zn(im)₆Cl₂·4H₂O exhibit a strong Jahn-Teller effect which is static below 100 K and the complex in localized in the two low-energy potential wells. We have reinvestigated electron paramagnetic resonance (EPR) spectra in the temperature range 4.2-300 K and determined the deformation directions produced by the Jahn-Teller effect, energy difference 11 cm⁻¹ between the wells and energy 300 cm⁻¹ of the third potential well. The electron spin relaxation was measured by electron spin echo (ESE) method in the temperature range of 4.2-45 K for single crystal and powder samples. The spin-lattice relaxation is dominated by a local mode of vibration with energy 11 cm⁻¹ at low temperatures. We suppose that this mode is due to reorientations (jumps) of the Cu(im)₆ complex between the two lowest energy potential wells. At intermediate temperatures (15-35 K), the T₁ relaxation is determined by the two-phonon Raman processes in acoustic phonon spectrum with Debye temperature Θ_D=167 K, whereas at higher temperatures the relaxation is governed by the optical phonon of energy 266 cm⁻¹. The ESE dephasing is produced by an instantaneous diffusion below 15 K with the temperature-independent phase memory time , then it grows exponentially with temperature with an activation energy of 97 cm⁻¹. This is the energy of the first excited vibronic level. The thermal population of this level leads to a transition from anisotropic to isotropic EPR spectrum observed around 90 K. FT-ESE gives ESEEM spectrum dominated by quadrupole peaks from non-coordinating ¹⁴N atom of the imidazole rings and the peak from double quantum transition ν_dq. We show that the amplitude of the ν_dq transition can be used to determine the number of non-coordinating nitrogen atoms.     

Single crystal EPR investigation on Mn(II) doped biomineral: cobalt potassium phosphate hexahydrate

Single crystal EPR study of Mn(II) doped in cobalt potassium phosphate hexahydrate has been carried out at room temperature. The impurity shows a 30 line pattern EPR spectra along a particular crystallographic axis suggesting the existence of only one type of impurity in place of Co(II) ion in the host lattice. The spin Hamiltonian parameters have been estimated as: g₁₁=2.011, g₂₂=1.998, g₃₃=1.991, and A₁₁=−8.9, A₂₂=−8.8, A₃₃=−8.4 mT and D₁₁=−15.2, D₂₂=−9.4, D₃₃=24.6 mT, respectively. The sign of A is designated as negative and D as positive. The covalency of metal-oxygen bond has been estimated. The relaxation times, calculated as a function of temperature, indicate spin-lattice relaxation narrowing at room temperature.     

H and K spin-lattice relaxation, ionic motion and Raman processes in the proton conducting KHSeO4 single crystal

The spin-lattice relaxation rates of ¹H and ³⁹K nuclei in KHSeO₄ crystals were studied in the temperature range 160-400 K. The spin-lattice relaxation recovery of ¹H nucleus in this crystal can be represented with a single exponential function, and the relaxation T₁⁻¹ curve of ¹H can be represented with the Bloembergen-Purcell-Pound (BPP) function. The relaxation process of ³⁹K with dominant quadrupole relaxation can be described by a linear combination of two exponential functions. T₁⁻¹ for the ³⁹K nucleus was found to have a very strong temperature dependence, T₁⁻¹=βT⁷. Rapid variations in relaxation rates are associated with critical fluctuations in the electronic spin system. The T⁷ temperature dependence of the Raman relaxation rate is shown here to be due to phonon-magnon coupling.     

Raman scattering from spin fluctuations and phonons in the heavy-fermion superconductor UPt3

Quasielastic scattering from spin fluctuations has been observed in UPt₃ by Raman spectroscopy. The experiments for wave vectors q≈0 show a nearly temperature independent linewidth for 5 K ⩽ T ⩽ 300 K Complementary to neutron scattering results this establishes the q independence of the spin relaxation rate, indicating the localized nature of the spin fluctuations. A Raman-active phonon near 79 cm^-1 (10 meV) shows a drastic increase in line-width with decreasing temperature, demonstrating strong electron-phonon coupling.     

Theoretical investigations of the local structure distortion and the spin Hamiltonian parameters of Cr ions at tetragonal charge-compensation defect CrF5O site in Cr: KMgF3 crystals

The local structure distortion and the spin Hamiltonian (SH) parameters, including the zero-field splitting (ZFS) parameter D and the Zeeman g-factors g_‖ and g_⊥, are theoretically investigated by means of complete diagonalization method (CDM) and the microscopic spin Hamiltonian theory for tetragonal charge compensation CrF₅O defect center in Cr³⁺:KMgF₃ crystals. The superposition model (SPM) calculations are carried out to provide the crystal field (CF) parameters. This investigation reveals that the replacement of O²⁻ for F⁻ and its induced lattice relaxation Δ₁(O²⁻) combined with an inward relaxation of the nearest five fluorine Δ₂(F⁻) give rise to a strong tetragonal crystal field, which in turn results in the large ZFS and large anisotropic g-factor Δg. The experimental SH parameters D and Δg can be reproduced well by assuming that O²⁻ moves towards the central ion Cr³⁺ by Δ₁(O²⁻)=0.172R₀ and the five F⁻ ions towards the central ion Cr³⁺ by Δ₂(F⁻)=0.022R₀. Our approach takes into account the spin-orbit (SO) interaction as well as the spin-spin (SS), spin-other-orbit (SOO), and orbit-orbit (OO) interactions omitted in previous studies. This shows that although the SO interaction is the most important one, the contributions to the SH parameters from other three magnetic interactions are appreciable and should not be omitted, especially for the ZFS parameter D.