Field dependence of the electron spin relaxation in quantum dots

The interaction of the electron spin with local elastic twists due to transverse phonons is studied. The universal dependence of the spin-relaxation rate on the strength and direction of the magnetic field is obtained in terms of the electron gyromagnetic tensor and macroscopic elastic constants of the solid. The theory contains no unknown parameters and it can be easily tested in experiment. At high magnetic field it provides a parameter-free lower bound on the electron spin relaxation in quantum dots.     

NMR determination of 2D electron spin polarization at nu = 1/2

Using a "standard" NMR spin-echo technique we determined the spin polarization P of two-dimensional electrons, confined to GaAs quantum wells, from the hyperfine shift of Ga nuclei located in the wells. Concentrating on the temperature ( 0.05 less, similarT less, similar10 K) and magnetic field ( 7 less, similarB less, similar17 T) dependencies of P at Landau level filling factor nu = 1/2, we find that the results are described well by a simple model of noninteracting composite fermions, although some inconsistencies remain when the two-dimensional electron system is tilted in the magnetic field.     

Low-field theory of nuclear spin relaxation in paramagnetic low-symmetry complexes for electron spin systems of S = 1, 3/2, 2, 5/2, 3 and 7/2

(Molecular Physics, 2000, 98, 1617)     

Paramagnetic proton nuclear spin relaxation theory of low-symmetry complexes for electron spin quantum number S = 52

A generalization of the modified Solomon-Bloembergen-Morgan (MSBM) equations has been derived in order to describe paramagnetic relaxation enhancement (PRE) of paramagnetic complexes characterized by both a transient (DeltaZFSt) and a static (DeltaZFSs) zero-field splitting (ZFS) interaction. The new theory includes the effects of static ZFS, hyperfine coupling, and angular dependence and is presented for the case of electron spin quantum number S = 52, for example, Mn(II) and Fe(III) complexes. The model gives the difference from MSBM theory in terms of a correction term delta which is given in closed analytical form. The theory may be important in analyzing the PRE of proton spin-lattice relaxation dispersion measurements (NMRD profiles) of low-symmetry aqua-metal complexes which are likely to be formed upon transition metal ions associated with charged molecular surfaces of biomacromolecules. The theory has been implemented with a computer program which calculates solvent water proton T1 NMRD profiles using both MSBM and the new theory.     

Low-temperature spin diffusion in a highly ideal S=1/2 Heisenberg antiferromagnetic chain studied by muon spin relaxation

The organic radical-ion salt DEOCC-TCNQF4 contains linear chains of stacked molecules with significant Heisenberg antiferromagnet interactions along the chain and extremely weak interactions between the chains. Zero-field muSR has confirmed the absence of long-range magnetic order down to 20 mK and field-dependent muSR is found to be consistent with diffusive motion of the spin excitations. The anisotropic spin dynamics and the upper boundary for magnetic ordering temperature both indicate interchain magnetic coupling /J'/<7 mK. As the intrachain coupling J is 110 K, /J'/J/ is significantly less than 10(-4). This system therefore provides one of the most ideal examples of the one-dimensional S=1/2 Heisenberg antiferromagnet yet discovered.     

Decay of tripositronium with spin 3/2 and orbital angular momentum L=1 into an electron and a photon

It is shown that the probability for decay into an electron and a photon of a negative ion of positronium with the quantum numbers S=3/2, L=1, S_z=3/2, and L_z=0 is equal to 0.7·10^–7sec-^–1.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 101–105, October, 1978.The author thanks V. S. Vanyashin for suggesting this problem and for useful discussions.     

Frequency dependence of the spin-lattice relaxation time in solid H2 with very low concentration of ortho-molecules

By extending the previous theory of Hama et al. for the spectral density in the system of randomly distributed spins with R^–n-interactions, the frequency dependence of the spin-lattice relaxation time of o-H₂ in solid H₂ at very low concentration is studied, with results in excellent agreement with measurements. In the present extension the effects of the anisotropy and rotational Zeeman energies have been taken into account by the use of the measured constants relevant to both energies.     

Spin degree of freedom in the nu=1 bilayer electron system investigated by nuclear spin relaxation

The nuclear-spin-relaxation rate 1/T(1) has been measured in a bilayer electron system at and around total Landau level filling factor nu=1. The measured 1/T(1), which probes electron spin fluctuations, is found to increase gradually from the quantum Hall (QH) state at low fields through a phase transition to the compressible state at high fields. Furthermore, 1/T(1) in the QH state shows a noticeable increase away from nu=1. These results demonstrate that, as opposed to common assumption, the electron spin degree of freedom is not completely frozen either in the QH or the compressible states.     

Frequency (250 MHz to 9.2 GHz) and viscosity dependence of electron spin relaxation of triarylmethyl radicals at room temperature

Electron spin relaxation times for four triarylmethyl (trityl) radicals at room temperature were measured by long-pulse saturation recovery, inversion recovery, and electron spin echo at 250 MHz, 1.5, 3.1, and 9.2 GHz in mixtures of water and glycerol. At 250 MHz T(1) is shorter than at X-band and more strongly dependent on viscosity. The enhanced relaxation at 250 MHz is attributed to modulation of electron-proton dipolar coupling by tumbling of the trityl radicals at rates that are comparable to the reciprocal of the resonance frequency. Deuteration of the solvent was used to distinguish relaxation due to solvent protons from the relaxation due to intra-molecular electron-proton interactions at 250 MHz. For trityl-CD(3), which contains no protons, modulation of dipolar interaction with solvent protons dominates T(1). For proton-containing radicals the relative importance of modulation of intra- and inter-molecular proton interactions varies with solution viscosity. The viscosity and frequency dependence of T(1) was modeled based on dipolar interaction with a defined number of protons at specified distances from the unpaired electron. At each of the frequencies examined T(2) decreases with increasing viscosity consistent with contributions from T(1) and from incomplete motional averaging of anisotropic hyperfine interaction.     

Nuclear spin relaxation in itinerant electron antiferromagnetic Cr1−xVx system

Nuclear spin relaxation rate T^?1₁ for ⁵¹V in an incommensurate antiferromagnetic Cr_1?xV_x system has been measured in a temperature range between 1.3 and 4.2 K and in a range of magnetic field from 0 to 13.3 kOe by using a field-cycling nuclear magnetic resonance technique. In the (T₁T)^?1 vs x curve a pronounced maximum was observed near the critical concentration (x_c～0.040). Furthermore for alloys with x = 0.038 and 0.040 a deviation from the Korringa relation, T₁T = constant, was observed. The experimental results of (T₁T)^?1 are interpreted in terms of the spin-fluctuation and d-orbital contributions.     

Intensity of cross-peaks in hyscore spectra of S = 1/2, I = 1/2 spin systems

The cross-peak intensity for a S = 1/2, I = 1/2 spin system in two-dimensional HYSCORE spectra of single-crystals and powders is analyzed. There is a fundamental difference between these two cases. For single crystals, the cross-peak intensity is distributed between the two (+, +) and (+, -) quadrants of the hyperfine sublevel correlation (HYSCORE) spectrum by the ratio c(2):s(2) (C. Gemperle, G. Aebli, A. Schweiger, and R. R. Ernst, J. Magn. Reson. 88, 241 (1990)). However, for powder spectra another factor becomes dominant and governs cross-peak intensities in the two quadrants. This factor is the phase interference between modulation from different orientations of the paramagnetic species. This can lead to essentially complete disappearance of the cross-peak in one of the two (+, +) or (+, -) quadrants. In the (+, +) quadrant, cross-peaks oriented parallel to the main (positive) diagonal of the HYSCORE spectrum are suppressed, while the opposite is true in the (+, -) quadrant where cross-peaks nearly perpendicular to the main (negative) diagonal of HYSCORE spectra are suppressed. Analytical expressions are derived for the cross-peak intensity profiles in powder HYSCORE spectra for both axial and nonaxial hyperfine interactions (HFI). The intensity is a product of two terms, one depending only on experimental parameter (tau) and the other only on the spin Hamiltonian. This separation provides a rapid way to choose tau for maximum cross-peak intensity in a region of interest in the spectrum. For axial HFI, the Hamiltonian-dependent term has only one maximum and decreases to zero at the canonical orientations. For nonaxial HFI, this term produces three separate ridges which outline the whole powder lineshape. These three ridges have the majority of the intensity in the HYSCORE spectrum. The intensity profile of each ridge resembles that observed for axial HFI. Each ridge defines two principal values of the HFI similar to the ridges from an axial HFI.     

Spin-1 chain doped with mobile S = 1/2 fermions

We investigate the doping of a two-orbital chain with mobile S = 1/2 fermions as a valid model for Y2-xCaxBaNiO5. The S = 1 spins are stabilized by strong, ferromagnetic Hund's rule couplings. We calculate correlation functions and thermodynamic quantities by density matrix renormalization group methods and find a new hierarchy of energy scales in the spin sector upon doping. Gapless spin excitations are generated at a lower energy scale by interactions among itinerant polarons created by each hole and coexist with the larger scale of the gapful spin-liquid background of the S = 1 chain accompanied by a finite string order parameter.     

Generalized Elliott-Yafet theory of electron spin relaxation in metals: origin of the anomalous electron spin lifetime in MgB2

The temperature dependence of the electron-spin relaxation time in MgB2 is anomalous as it does not follow the resistivity above 150 K; it has a maximum around 400 K and decreases for higher temperatures. This violates the well established Elliot-Yafet theory of spin relaxation in metals. The anomaly occurs when the quasiparticle scattering rate (in energy units) is comparable to the energy difference between the conduction and a neighboring bands. The anomalous behavior is related to the unique band structure of MgB2 and the large electron-phonon coupling. The saturating spin relaxation is the spin transport analogue of the Ioffe-Regel criterion of electron transport.     

Entanglement between an electron and a nuclear spin 1/2

We report on the preparation and detection of entangled states between an electron spin 1/2 and a nuclear spin 1/2 in a molecular single crystal. These were created by applying pulses at ESR (9.5 GHz) and NMR (21 MHz, 46 MHz) frequencies. Entanglement was detected by using a special entanglement detector sequence based on a unitary back transformation including phase rotation.     

A method for investigations of solid surfaces by nuclear spin relaxation

A method is presented which allows to investigate hot metal surfaces (1,000 K to 1,600 K) by use of nuclear spin polarized alkali atom beams (⁶Li,⁷Li,²³Na). At these temperatures the atoms are adsorbed for about 10^–4 s to 1 s. During the residence time on the surface the nuclear moments interact with the electromagnetic fields of their environment. The nuclear spin polarization of the ions desorbing from the surface is detected by beam foil spectroscopy. The depolarization gives information on electric field gradients and magnetic fields originating in the surface and on the electronic state of the absorbed alkali atoms.Supported partly by the Deutsche Forschungsgemeinschaft and the Bundesministerium für Forschung und Technologie, both Bonn, FRG     

Internuclear distance determination of S = 1, I = 1/2 spin pairs using REAPDOR NMR

A universal function is proposed to describe REAPDOR dephasing curves of an observed spin-1/2 nucleus dipole-recoupled to a spin-1 quadrupolar nucleus ((2)H or (14)N). Previous work had shown that, in contrast to REDOR, the shape of the dephasing curve depends on a large number of parameters including the quadrupolar coupling constant and asymmetry parameter, the sample rotation speed, the RF amplitude, and the relative orientations of the quadrupole tensor and the internuclear vector. Here we demonstrate by numerical simulations that the actual dispersion of REAPDOR dephasing curves is quite small, provided the rotation speed and the RF amplitude applied to the quadrupolar nucleus satisfy an adiabaticity condition. The condition is easily met for (2)H and is also practically achievable for virtually any (14)N-containing compound. This allows the REAPDOR curves to be approximated by a simple universal gaussian-type function, comparison of which with experimental data yields internuclear distances with less than 4% error. The spin dynamics of the recoupling mechanism is discussed. The critical importance of a stable spinning speed for optimizing the signal-to-noise ratio of the (13)C echoes is demonstrated and practical suggestions for achieving high stability are presented. Examples of applications of the universal curve are given for (2)H/(13)C and (14)N/(13)C REAPDOR in alanine.