Nuclear Spin Fluctuations Research Articles

Pseudo-spin-gap and slow spin fluctuation inLa2−xSrxCuO4(x=0.13and 0.18) via63Cuand139Lanuclear quadrupole resonance

We analyzed nonexponential ${}^{63}\mathrm{Cu}$ nuclear spin-lattice relaxation curves for ${}^{63}\mathrm{Cu}$-enriched high-${T}_{c}$ superconductors: ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ with $x=0.13$ (slightly underdoped) and 0.18 (slightly overdoped), and studied the applicability of an impurity-induced nuclear spin-lattice relaxation theory. We found a remnant of pseudo-spin-gap effect on the host ${}^{63}\mathrm{Cu}$ nuclear spin-lattice relaxation time and slow inhomogeneous spin fluctuation via the impurity-induced relaxation time. The effect of slow spin dynamics was also observed in ${}^{139}\mathrm{La}$ nuclear spin-lattice relaxation. The inhomogeneous electron-spin fluctuation, which is associated with randomly distributed staggered moments on the ${\mathrm{CuO}}_{2}$ plane, smears the pseudo-spin-gap. The fact that the optimal ${T}_{c}\ensuremath{\sim}38\mathrm{K}$ is smaller than ${T}_{c}\ensuremath{\sim}96\mathrm{K}$ of ${\mathrm{HgBa}}_{2}{\mathrm{CuO}}_{4+\ensuremath{\delta}}$ can be attributed to the depairing effect due to the slow spin fluctuation.

Theory of nuclear-induced spectral diffusion: Spin decoherence of phosphorus donors in Si and GaAs quantum dots

We propose a model for spectral diffusion of localized spins in semiconductors due to the dipolar fluctuations of lattice nuclear spins. Each nuclear spin flip-flop is assumed to be independent, the rate for this process being calculated by a method of moments. Our calculated spin decoherence time $T_{M}=0.64$ ms for donor electron spins in Si:P is a factor of two longer than spin echo decay measurements. For $^{31}$P nuclear spins we show that spectral diffusion is well into the motional narrowing regime. The calculation for GaAs quantum dots gives $T_{M}=10-50$ $\mu$s depending on the quantum dot size. Our theory indicates that nuclear induced spectral diffusion should not be a serious problem in developing spin-based semiconductor quantum computer architectures.

Temperatur Dependence of Spin-Density Fluctuations in Liquid 3He

The dynamic structure factor S(Q,E) in normal liquid3He, which contains information on both density and spin-density fluctuations, has been measured by inelastic neutron scattering both well below and close to the Fermi temperature. The data extend to smaller wave vectors and energies than covered in earlier experiments, and give a much better determination of the line shape of the spin-density response. Together with the temperature dependence, this allows to discriminate between different theoretical models for the nuclear spin fluctuations, and provides information on the quasiparticle dispersion and effective mass.

A study of excited state dynamics of rare earth ions in solids by coherent transients and spectral holeburning

Barriers to spectral resolution imposed by instrumental limits and inhomogeneous broadening can often be removed by linear and nonlinear laser spectroscopy. This is illustrated by the case of LaF 3:Pr 3+. Six orders of magnitude increase in resolution has been demonstrated, and this uncovered new mechanisms for optical dephasing such as coupling to nuclear spin fluctuations. The importance of the time-scale of the experimental probe is stressed, and the information obtained from spectral holeburning and coherent transients can be very different. This was found in CaF 2:Pr 3+ where the fluorine nuclear spin-flips exhibit a wide range of time-scales.

Electron-spin resonance in insulating doped semiconductors.

Spin dynamics of shallow impurity electrons in the insulating phase of $n$-doped semiconductors is examined within the framework of the scaling theory of a disordered spin-\textonehalf{} Heisenberg antiferromagnet. Quantum fluctuations of the donor nuclear spins are included and found to be important in determining the ESR line shape and position. With use of the known hyperfine constant, a quantitative fit to the experimental results is obtained, including the dramatic temperature and frequency dependence of the linewidth and resonance field seen in phosphorus-doped silicon at millikelvin temperatures.

The NSW interaction with nuclear local modes in antiferromagnets

The nuclear spin wave (NSW) relaxation rates due to the interaction with nuclear impurity local modes (NILM) in antiferromagnets are considered in the framework of the Keldysh formalism based on the spin operator diagram technique. It is shown that the NSW relaxation frequency due to the scattering on the fluctuations of impurity nuclear spins is of resonance character. The NSW damping due to the resonance absorption by NILM depends strongly on the NSW amplitude N, which accounts for the “hard” excitation of NSW's in parallel pumping experiments /6/. The NSW relaxation rates due to the processes involving two NSW's and one impurity nuclear excitation are also calculated.

A comparison of phase sensitive and phase insensitive apertures for scattering measurements

At 8.5 T, the polarization of an ensemble of electron spins is essentially 100% at 2 K, and decreases to 30% at 20 K. The strong temperature dependence of the electron spin polarization between 2 and 20 K leads to the phenomenon of spin bath quenching: temporal fluctuations of the dipolar magnetic fields associated with the energy-conserving spin “flip-flop” process are quenched as the temperature of the spin bath is lowered to the point of nearly complete spin polarization. This work uses pulsed electron paramagnetic resonance (EPR) at 240 GHz to investigate the effects of spin bath quenching on the phase memory times (TM) of randomly-distributed ensembles of nitroxide molecules below 20 K at 8.5 T. For a given electron spin concentration, a characteristic, dipolar flip-flop rate (W) is extracted by fitting the temperature dependence of TM to a simple model of decoherence driven by the spin flip-flop process. In frozen solutions of 4-Amino-TEMPO, a stable nitroxide radical in a deuterated water–glass, a calibration is used to quantify average spin–spin distances as large as r¯=6.6nm from the dipolar flip-flop rate. For longer distances, nuclear spin fluctuations, which are not frozen out, begin to dominate over the electron spin flip-flop processes, placing an effective ceiling on this method for nitroxide molecules. For a bulk solution with a three-dimensional distribution of nitroxide molecules at concentration n, we find W∝n∝1/r¯3, which is consistent with magnetic dipolar spin interactions. Alternatively, we observe W∝n32 for nitroxides tethered to a quasi two-dimensional surface of large (Ø ∼ 200 nm), unilamellar, lipid vesicles, demonstrating that the quantification of spin bath quenching can also be used to discern the geometry of molecular assembly or organization.

Nuclear Magnetic Relaxation and Spin Fluctuations in α-Mn by N.M.R.

The temperature dependence of the nuclear spin-lattice ( T 1 ) and spin-spin ( T 2 ) relaxation times in α-Mn metal has been studied for the four different Mn sites between 95 K ( T N ) and room temperature by means of cw and pulsed NMR techniques. The 1/ T 1 for sites I and II estimated from cw NMR linewidth was found to be almost temperature independent as ∼1.9 ×10 5 (s -1 ). This high relaxation rate is interpreted in terms of the existence of localized spin fluctuations at sites I and II. The temperature dependence of 1/ T 1 for sites III and IV was found to be 1/ T 1 =0.50 ×10 3 +4.4 T (s -1 ) from 120 K to room temperature. The first term arises from the localized spin fluctuations at sites I and II via RKKY type interaction. The second T-linear term is Korringa type relaxation effect due to the band electron in α-Mn.

Nuclear spin dynamics near ferromagnetic phase transitions. I. Isotropic systems

The mode-coupling theory of critical dynamics has been applied to both electron and nuclear spin fluctuations of isotropic ferromagnets with electron-nuclear spin interactions. By treating nuclear and electron spin fluctuations in a similar manner, the influence of hyperfine interactions on nuclear as well as electron spin motion could be accounted for. It has thus been possible to derive the asymptotic q-k- omega dependence of the self-energies of electron and nuclear spin fluctuations in the entire critical region above Tc.

Nuclear spin dynamics near ferromagnetic phase transitions. II. Anisotropic systems

The mode-coupling theory of critical dynamics has been applied to both electron and nuclear spin fluctuations of isotropic ferromagnets with electron-nuclear spin interactions. By treating nuclear and electron spin fluctuations in a similar manner, the influence of hyperfine interactions on nuclear as well as electron spin motion could be accounted for. It has thus been possible to derive the asymptotic q-k- omega dependence of the self-energies of electron and nuclear spin fluctuations in the entire critical region above Tc.

Theory of Spin-Wave Relaxation in Antiferromagnets

The relaxation frequencies associated with spin waves in flopped antiferromagnets have been investigated for various mechanisms. It is found that the three-magnon dipolar interaction, which is important in ferromagnets, does not contribute to the relaxation of antiferromagnetic spin waves, because of sublattice cancellations. The two processes that appear to make the dominant contributions to the relaxation process are the scattering of spin waves from nuclear spin fluctuations through the hyperfine interaction and three-magnon non-momentum-conserving scattering induced by spatial inhomogeneities. The latter process gives good agreement with experimental data on RbMn${\mathrm{F}}_{3}$.