Electron spin relaxation in pseudo-Jahn-Teller low-symmetry Cu(II) complexes in diaqua(L-aspartate)Zn(II).H(2)O crystals.

Low-temperature (4-55 K) pulsed EPR measurements were performed with the magnetic field directed along the z-axis of the g-factor of the low-symmetry octahedral complex [(63)Cu(L-aspartate)(2)(H2O)2] undergoing dynamic Jahn-Teller effect in diaqua(L-aspartate)Zn(II) hydrate single crystals. Spin-lattice relaxation time T(1) and phase memory time T(M) were determined by the electron spin echo (ESE) method. The relaxation rate 1/T(1) increases strongly over 5 decades in the temperature range 4-55 K. Various processes and mechanisms of T(1)-relaxation are discussed, and it is shown that the relaxation is governed mainly by Raman relaxation processes with the Debye temperature Theta(D)=204 K, with a detectable contribution from disorder in the doped Cu(2+) ions system below 12 K. An analytical approximation of the transport integral I(8) is given in temperature range T=0.025-10Theta(D) and applied for computer fitting procedures. Since the Jahn-Teller distorted configurations differ strongly in energy (delta(12)=240 cm(-1)), there is no influence of the classical vibronic dynamics mechanism on T(1). Dephasing of the ESE (phase relaxation) is governed by instantaneous diffusion and spectral diffusion below 20 K with resulting rigid lattice value 1/T(0)(M)=1.88 MHz. Above this temperature the relaxation rate 1/T(M) increases upon heating due to two mechanisms. The first is the phonon-controlled excitation to the first excited vibronic level of energy Delta=243 cm(-1), with subsequent tunneling to the neighbor potential well. This vibronic-type dynamics also produces a temperature-dependent broadening of lines in the ESEEM spectra. The second mechanism is produced by the spin-lattice relaxation. The increase in T(M) is described in terms of the spin packets forming inhomogeneously broadened EPR lines.     

Coherent manipulation of electron spins up to ambient temperatures in Cr5+ (S = 1/2) doped K3NbO8

We report coherent spin manipulation on Cr(5+) (S = 1/2, I = 0) doped K(3)NbO(8), which constitutes a dilute two-level model relevant for use as a spin qubit. Rabi oscillations are observed for the first time in a spin system based on transition metal oxides up to room temperature. At liquid helium temperature the phase coherence relaxation time T2 reaches approximately 10 micros and, with a Rabi frequency of 20 MHz, yields a single-qubit figure of merit Q(M) of about 500. This shows that a diluted ensemble of Cr(5+) (S = 1/2) doped K(3)NbO(8) is a potential candidate for solid-state quantum information processing.     

Spin-lattice relaxation and dynamic polarization of protons in ethyleneglycol-methanol samples

We report measurements of nuclear spin-lattice relaxation and dynamic polarization of protons in ethyleneglycol-alcohol mixtures at 9 kG and 4·2 K and 1·3 K. Comparison of experimental results with a proposed model of spin-lattice relaxation indicates that the role of dipolar and exchange reservoirs of paramagnetic ions in nuclear relaxation is not necessarily negligible, particularly in the presence of high concentration of paramagnetic impurities. The influence of methanol is to enhance the dynamic polarization of protons compared to pure ethyleneglycol, in the most favourable cases by almost 50% in our experimental conditions.     

Submillimeter-wave ESR measurements of Ca1−xCuO2 (x = 0.164) with edge-sharing CuO2 chains

Quasi-one-dimensional (1-D) cupric oxide Ca_1?xCuO₂ (x = 0.164) is the system with 25–40% hole-doped edge-sharing CuO₂ chains. However, the holes are almost localized in Ca_1?xCuO₂ and its magnetic susceptibility with a peak at 30 K was explained by the model considering both 1-D antiferromagnetic chains and spin dimers (Z. Hiroi, M. Okumura, Y. Nabeshima, T. Yamada, M. Takano: J. Phys. Soc. Jpn.69, 1824, 2000). To clarify the magnetic nature of Ca_1?xCuO₂, we performed submillimeter-wave electron spin resonance (ESR) measurements on a powder sample of Ca_0.83?6CuO₂. The resonance above 12 K showed typical powder ESR of Cu²⁺ and theg-values were determined to be g∥= 2.33 and g_⊥ = 2.06 from the analysis. The resonance below 12 K changed completely from ESR. The frequency-field relation of ESR at 1.8 K clearly showed the easy-axis type antiferromagnetic resonance.     

Magnetic anisotropy and low-frequency dielectric response of weak ferromagnetic phase in κ-(BEDT-TTF)2Cu[N(CN)2]Cl, where BEDT-TTF is Bis(ethylenedithio)tetrathiafulvalene

We report a detailed characterization of the magnetism and AC transport in single crystals of the organic conductor -(BEDT-TTF)₂Cu[N(CN)₂]Cl by means of magnetic anisotropy measurements and low-frequency dielectric spectroscopy. Magnetic anisotropy obeys Curie-Weiss law with negative Curie-Weiss temperature in the temperature range 300 K-70 K. An antiferromagnetic transition with concomitant canted antiferromagnetic state is established at 22 K. A large hysteresis in the spin-flop transition and magnetic field reversal of the weak ferromagnetic magnetization are documented for the first time. A broad dielectric relaxation mode of moderate strength () emerges at 32 K, and weakens with temperature. The mean relaxation time, much larger than that expected for single-particle excitations, is thermally activated in a manner similar to the DC conductivity and saturates below 22 K. These features suggest the origin of the broad relaxation as an intrinsic property of the weak ferromagnetic ground state. We propose a charged domain wall in a random ferromagnetic domain structure as the relaxation entity. We argue that the observed features might be well described if Dzyaloshinsky-Moriya interaction is taken into account. A Debye relaxation with similar temperature dependence was also observed and seems to be related to an additional ferromagnetic-like, most probably, field-induced phase. We tentatively associate this phase, whose tiny contribution was sample dependent, with a Cu²⁺ magnetic subsystem. Received 15 June 1998 and Received in final form 1 February 1999     

Spin-lattice relaxation for rare-earth ions

Relaxation arising from one-phonon processes is considered for crystals in which the rare-earth ion (minor component) is enclosed in a cube consisting of eight diamagnetic particles. The relaxation time is estimated for Yb³⁺ in CaF₂; the discrepancy between theory and experiment is discussed.I am indebted to A. M. Prokhorov and K. K. Svidzinskii for discussions.     

Magnetic field dependence of the superconducting fluctuation contribution to NMR-NQR relaxation

The dependence of the Maki-Thompson and of the density of states (DOS) depletion contributions from superconducting fluctuations to nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) relaxation is derived in the framework of the diagrammatic theory, applied to layered three-dimensional (3-D) high-T_c superconductors. The regularization procedure devised for the conductivity (A. I. Buzdin, A. A. Varlamov: Phys. Rev. B58, 14195, 1998) is used in order to avoid the divergence of the DOS term. The theoretical results are discussed in the light of NMR-NQR measurements in YBCO and compared with the recent theory (M. Eschrig et al.: Phys. Rev. B59, 12095, 1999), on the basis of the assumption of a purely 2-D spectrum of fluctuations.     

Broad band dielectric behaviour of plasticized PEO-based solid polymer electrolytes

The conductivity and dielectric response of poly(ethylene oxide) (PEO) based plasticized polymer electrolyte systems were studied in the broad frequency range from 5 Hz to 1.8 GHz and in the temperature range from 248 K to 353 K. Propylene carbonate (PC) and ethylene carbonate (EC) were used as conventional plasticizers while poly(perfluorinated ethylene methylene oxide) (M03) was used as a new type of plasticizer. PEO-LiN(CF₃SO₂)₂ plasticized with M03 shows high enough conductivity values to be used as electrolyte in rechargeable lithium polymer batteries. At high frequency a dielectric relaxation is observed for pure PEO as well as for the salt containing systems in the GHz region that is assumed to be due to segmental motion of the polymer chains. In the salt containing systems, this relaxation is shifted to lower frequencies relative to that of pure PEO, this is attributed to transient cross-linking. However, at lower frequencies another dielectric response peak was detected in all samples containing salts. The effect of the plasticizer on this relaxation is complex. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Dephasing relaxation of the electron spin echo of the vibronic Cu(H(2)O)(6) complexes in Tutton salt crystals at low temperatures.

Two-pulse electron spin echo (ESE) measurements of the phase relaxation (phase memory time T(M)) were performed in a series of Tutton salt crystals M(I)(2)M(II)(SO(4))(2).6X(2)O (M(I)=NH(4), K, Cs; M(II)=Zn, Mg; X=H, D) weakly doped with Cu(2+) ions (c approximately equal to 10(18) ions/cm(3)) in temperature range 4-60 K where ESE signals were detectable. The ESE decay was strongly modulated with proton (or deuteron) frequencies and described by the decay function V(2tau)=V(0)exp(-btau-mtau(2)) with the mtau(2) term being temperature independent and negligible above 20 K. Various mechanisms leading to the tau- or tau(2)-type ESE decay are reviewed. The m and b coefficients for nuclear spectral diffusion (NSD), electron spectral diffusion (SD), and instantaneous diffusion (ID) were calculated in terms of existing theories and the resulting rigid lattice T(0)(M) times were found to be close one to another within the crystal family with average values: 17.5 micros (NSD protons), 200 micros (NSD deuterons), 8 micros (SD), and 5 micros (ID). The ID dominates but the calculated effective T(M)(0) is longer than the experimental T(M)(0)=2 micros. This is due to a nonuniform distribution of the Cu(2+) ions with a various degree of the disorder in the studied crystals. The acceleration of the dephasing rate 1/T(M) with temperature is due to the mechanisms producing exp(-btau) decay. They are reviewed and two of them were found to be operative in Tutton salt crystals: (a) Excitations to the vibronic levels of energy Delta leading to the temperature dependence 1/T(M)=B exp(-Delta/kT), with the vibronic levels produced by strong Jahn-Teller effect, and (b) spin-lattice relaxation processes being effective above 50 K. Based on the Delta values being on the order of 100 cm(-1), the scheme of vibronic levels in the Tutton salts is presented, and the independence of the Delta on temperature proves that the adiabatic potential surface shape of Jahn-Teller active Cu(H(2)O)(6) complexes is not affected by temperature below 65 K.     

Will spin-relaxation times in molecular magnets permit quantum information processing?

Using X-band pulsed electron-spin resonance, we report the intrinsic spin-lattice (T1) and phase-coherence (T2) relaxation times in molecular nanomagnets for the first time. In Cr7M heterometallic wheels, with M=Ni and Mn, phase-coherence relaxation is dominated by the coupling of the electron spin to protons within the molecule. In deuterated samples T2 reaches 3 micros at low temperatures, which is several orders of magnitude longer than the duration of spin manipulations, satisfying a prerequisite for the deployment of molecular nanomagnets in quantum information applications.     

A nuclear magnetic resonance study of the phase transitions and electric quadrupole Raman processes of M5H3(SO4)4·H2O (M=Na, K, Rb, and Cs) single crystals

The spin–lattice relaxation times and spin–spin relaxation times for ¹H and M in M₅H₃(SO₄)₄·H₂O (M=Na, K, Rb, and Cs) single crystals grown using the slow-evaporation method were measured as functions of temperature. Two kinds of protons were identified in the M₅H₃(SO₄)₄·H₂O structure: acid protons and water protons. Our experimental results show that the acid and water protons in Cs₅H₃(SO₄)₄·H₂O are involved in phase transitions of this crystal, whereas neither type of proton is involved in the phase transitions of the other three crystal type (M₅H₃(SO₄)₄·H₂O; M=Na, K, and Rb). Moreover, the relaxation times for the M (=Na, K, and Rb) nuclei in these crystals were found to decrease with increasing temperature and can be described with (k=2). The T₁ results for M (=Na, K, and Rb) in M₅H₃(SO₄)₄·H₂O crystals can be explained in terms of a relaxation mechanism in which the lattice vibrations are coupled to the nuclear electric quadrupole moments.     

Magnetic properties of NpM4Al8−x (M=Cr,Fe, Cu) intermetallics: Mössbauer effect and neutron diffraction studies

Mössbauer effect and neutron diffraction studies on the tetragonal NpM₄Al_8?x (M=Cr, Fe, Cu) of the 14/mmm space group are reported. In NpFe₄Al_8?x, both the Np and Fe sites order magnetically at close temperatures: the Np order ferromagnetically at 115(15) K (μ(Np)) ～ 0.6 μ_B and the Fe moments order antiferromagnetically at 130(10) K (μ(Fe) = 1.05(15) μ_B) with a ++?? sequence. In NpCr₄Al_8?x, the Np order magnetically around 50K. From the susceptibility data we conclude possible antiferromagnetic order of both Np and Cr sites. NpCu₄Al_8?x does not order magnetically down to at least 2 K. The magnetic hyperfine splitting observed below 45 K is explained by slow paramagnetic relaxation. The Np isomer shifts and also its magnetic behavior point to a trivalent ion (⁵I₄ Hund's rule ground state). The observed relaxation and magnetic phenomena are discussed in terms of crystalline electric field effects. In contrast to the isostructural rare-earth RM₄Al₈ compounds with practically independent order of R and M magnetic sublattices, we show that Np and M sublattices in NpM₄Al₈ are strongly coupled. This is caused by hybridization of both Np and M atoms with Al.     

NMR observation of rattling phonons in the pyrochlore superconductor KOs2O6

We report nuclear magnetic resonance studies on the beta-pyrochlore oxide superconductor KOs2O6. The nuclear relaxation at the K sites is entirely caused by fluctuations of the electric field gradient, which we ascribe to highly anharmonic low frequency oscillation (rattling) of K ions. A phenomenological analysis shows a crossover from overdamped to underdamped behavior of the rattling phonons with decreasing temperature and its sudden sharpening below the superconducting transition temperature T(c). Suppression of the Hebel-Slichter peak in the relaxation rate at the O sites below T(c) also indicates strong electron-phonon coupling.     

Measurement of the lifetime difference in D0 meson decays

We report a measurement of the D0-D macro(0) mixing parameter y(CP) using 23.4 fb(-1) of data collected near the Upsilon(4S) resonance with the Belle detector at KEKB. y(CP) is measured from the lifetime difference of D0 mesons decaying into the K(-)pi(+) state and the CP-even eigenstate K(-)K(+). We find y(CP) = (-0.5+/-1.0(+0.7)(-0.8))x10(-2), where the first error is statistical and the second is systematic, corresponding to a 95% confidence interval -0.030相似文献   

Temperature Dependence of Electron Spin Relaxation of 2,2-Diphenyl-1-Picrylhydrazyl in Polystyrene

The electron spin relaxation rates for the stable radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) doped into polystyrene were studied by inversion recovery and electron spin echo at X-band and Q-band between 20 and 295 K. At low concentration (340 μM, 0.01 %), spin–lattice relaxation was dominated by the Raman process and a local mode. At high concentration (140 mM, 5 %), relaxation is orders of magnitude faster than at the lower concentration, and 1/T ₁ is approximately linearly dependent on the temperature. Spin lattice relaxation rates are similar at X-band and Q-band. The temperature dependence of spin echo dephasing was faster at about 140 K than at higher or lower temperatures, which is attributed to a wagging motion of the phenyl groups.     

Long-range ferromagnetic dipolar ordering of high-spin molecular clusters

We report the first example of a transition to long-range magnetic order in a purely dipolarly interacting molecular magnet. For the magnetic cluster compound Mn6O4Br4(Et2dbm)6, the anisotropy experienced by the total spin S = 12 of each cluster is so small that spin-lattice relaxation remains fast down to the lowest temperatures, thus enabling dipolar order to occur within experimental times at T(c) = 0.16 K. In high magnetic fields, the relaxation rate becomes drastically reduced and the interplay between nuclear- and electron-spin lattice relaxation is revealed.     

Rapid annealing of Fe-Si-B amorphous tapes by joule heating: Effects on magnetic and mechanical properties

We report a systematic study of relaxation effects in Fe-Si-B amorphous tapes rapidly annealed for short time at T≈800 K by Joule heating. It is found that this type of annealing in air can result in nearly the same degree of relaxation as obtained by two hours of conventional relaxation annealing in a furnace under inert gas or vacuum. The extent of relaxation as a function of current density and duration has been assessed by high precision calorimetric measurements and found to be highly reproducible. The effect of such rapid annealing on magnetic and mechanical properties are compared to those of conventional relaxation annealing for two hours at 655 K and found to be advantageous.     

Chain dynamics in the low-temperature phases of lipid membranes by electron spin-echo spectroscopy

Spin-echo decays of spin-labelled phospholipids have been recorded to study the chain dynamics in the low-temperature phases of dipalmitoyl phosphatidylcholine membranes with and without 50 mol% cholesterol. The phase-memory relaxation time, T(2M), depends on the position of spin-labelling in the sn-2 chain, and on the presence of cholesterol. A biphasic temperature dependence of T(2M) is obtained over the range 150-270 K. Echo-detected field-swept absorption EPR spectra were recorded as a function of the echo delay time, tau. The echo-detected EPR lineshapes show a strong dependence on tau, revealing anisotropic phase relaxation arising from torsional chain motions. Cholesterol has a large effect on torsional oscillations about the chain long axis. Small-amplitude chain motions in the low-temperature phases may be important for cryopreservation of membranes.     

Spin dynamics in the quasi-1 dimensional spin-glass FeMgBo4

The results of an inelastic neutron scattering investigation performed on the insulating title compound are presented. The spin dynamic is well described by a relaxation model leading to a quasi-elastic Lorentzian neutron-scattering line. The relaxation rate $\text{1}\text{τ}{}_{\mathrm{R}}$ is a linear temperature function for T>20 K, which is ascribed to the isotropic and 1-D character of the spin correlations. At T = 20 ? 5 K a drastic drop in $\text{1}\text{τ}{}_{\mathrm{R}}$ is observed reflecting a “crossover” to a highly frustrated 3D-spin glass state.     

Proton NMR and susceptibility measurements on the magnetic core of ferritin

We report an investigation of the magnetic core of the biomolecule ferritin by means of proton nuclear magnetic resonance (NMR) and relaxation, magnetic susceptibility and scanning electron microscope (SEM) measurements. SEM images show that the outer protein shell is taken out completely by an appropriate chemical treatment and indicate particle sizes ranging from 10² to 10⁴ nm. Susceptibility measurements show a maximum in the zero-field-cooled data which is strongly field-dependent and can be ascribed to superparamagnetic behavior, whereas the hysteresis curve is different from normal ferritin. Proton NMR and spin-lattice relaxation data as a function of temperature at 4.7 T suggest the presence of an antiferromagnetic transition around 100 K.