自旋极化对Kondo系统基态的影响

Anderson模型中的自旋极化效应是一个普遍存在的问题.本文从Anderson杂质模型出发,利用变分及对角化方法分析了自旋极化所引起的系统基态性质的改变,分别研究了自旋极化对Kondo单态以及高温超导两分量模型中Zhang-Rice单态稳定性的影响问题.     

Collisions of alkali K and Rb atoms in ground state: Modification of interaction potentials

The interaction of alkali K and Rb atoms that reside in the ground state is considered in the range of collision energies E = 10⁻⁴ to 10⁻² au. The singlet (X ¹Σ⁺) and triplet (a ³Σ⁺) interaction potentials available in the literature are analyzed and modified. For the KRb dimer in the range of interatomic distances 15–21a ₀, we chose analytical representations of the singlet and triplet potentials that more accurately describe the interaction of alkali Rb and K atoms in the ground state. Complex cross sections of the spin exchange are calculated for the first time that permit one to calculate the processes of polarization transfer and relaxation times, as well as shifts in the magnetic resonance frequencies caused by K-Rb spin exchange collisions.     

Stretched Exponential Fixation in Stochastic Ising Models at Zero Temperature

We study a class of continuous time Markov processes, which describes ± 1 spin flip dynamics on the hypercubic latticeℤ ^d , d≥ 2, with initial spin configurations chosen according to the Bernoulli product measure with density p of spins + 1. During the evolution the spin at each site flips at rate c= 0, or 0 < α≤ 1, or 1, depending on whether, respectively, a majority of spins of nearest neighbors to this site exists and agrees with the value of the spin at the given site, or does not exist (there is a tie), or exists and disagrees with the value of the spin at the given site. These dynamics correspond to various stochastic Ising models at 0 temperature, for the Hamiltonian with uniform ferromagnetic interaction between nearest neighbors. In case α= 1, the dynamics is also a threshold voter model. We show that if p is sufficiently close to 1, then the system fixates in the sense that for almost every realization of the initial configuration and dynamical evolution, each site flips only finitely many times, reaching eventually the state + 1. Moreover, we show that in this case the probability q(t) that a given spin is in state − 1 at time t satisfies the bound: for arbitrary ɛ > 0, q(t) ≤ exp(−t ^{(1/ d ) −ɛ}), for large t. In d= 2 we obtain the complementary bound: for arbitrary ɛ > 0, q(t) ≥ exp(−t ^{(1/2) +ɛ}), for large t. Received: 12 July 2001 / Accepted: 1 February 2002     

Spin-Selective Electron Transfer and Charge Recombination in Self-Assembled Porphyrin Naphthalenediimide Dyads

The spin selectivity of electron transfer in a series of metalloporphyrin pyridyl-linked naphthalenediimides (MTPP-Pyr(CH₂) _n NDI, where M = Zn, n = 2, 4, 7, and M = Al(OCOPh), n = 7) is studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy in the nematic liquid crystal 4-cyano-4′-pentylbiphenyl (5CB). Following pulsed laser excitation, all of the complexes show a narrow antiphase doublet that is assigned to the triplet state of the radical pair MTPP^•+NDI^•−. Initially, the antiphase doublet has an emission/absorption (E/A) polarization pattern characteristic of singlet electron transfer. At later times the polarization inverts to an A/E pattern. The intensity of the late signal depends very strongly on the nature of the metal in the porphyrin. A qualitative model that rationalizes this result is presented. It is proposed that both singlet and triplet electron transfer occur in the dyads and that the differences in the intensity of the polarization are the result of differences in the spin selectivity of intersystem crossing for the different metals. The consequences of this model for magnetic field effects in such systems are briefly discussed.     

On the longitudinal magnetic susceptibility of fractal ferrodielectrics

The longitudinal component of the magnetic susceptibility tensor χ _zz of a fractal ferrodielectric with dimension d _F = 3 − ɛ, where ɛ > 0, is calculated. The magnon dispersion in this case is shown to be a strongly anisotropic function of the parameter ɛ which, in turn, leads to nontrivial frequency dependences of χ _zz .     

Proposal on the measurements of d–p elastic scattering analyzing powers at 0.3–2.0 GeV at the Internal Target Station of the Nuclotron

A new high-energy beam polarimeter is proposed for Nuclotron, which uses the Internal Target Station (ITS). This polarimeter based on a measurement of asymmetry for the d–p elastic scattering will allow one to measure simultaneously both vector and tensor components of deuteron beam polarization. For this purpose an analyzing powers measurement for the d–p elastic scattering at energies T _d = 0.88–2 GeV is proposed. The precise measurements of the deuteron analyzing powers over the energy range T _d = 300–2000 MeV can give an irreplaceable clue to the study of the spin dependence of three nucleon forces. The text was submitted by the authors in English.     

Magnetic properties of the ordered and disordered double perovskite Sr<Subscript>2</Subscript>Fe<Subscript>1+x</Subscript>Mo<Subscript>1−x</Subscript>O<Subscript>6</Subscript> (−1 ≤ x ≤ 1/3)

We have studied the effect of cationic disorder on the spin polarization of the double perovskite system Sr₂Fe_1+x Mo_1−x O₆ with  −1 ≤ x ≤ 1/3. The composition x = 0 corresponds to the well-known double-perovskite Sr₂FeMoO₆, which is expected to have complete spin polarization, however all samples present some degree of Fe/Mo disorder which reduces the tunneling magnetoresistance in granular samples. We consider an electronic model within the renormalized perturbation expansion Green’s functions, consisting in a correlated electron picture with localized Fe-ions and itinerant electrons interacting with the local spins via a double-exchange type mechanism. Our results show the influence of disorder on the density of states and the ground-state properties, particularly on the spin polarization over the whole range of x.     

Colossal magnetodielectric effect in SmFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> multiferroic

The colossal (more than threefold) decrease in the dielectric constant ɛ in the easy-plane SmFe₃(BO₃)₄ ferroborate in a magnetic field of ∼5 kOe applied in the basal ab plane of the crystal has been found. A close relation of this effect to anomalies in the field dependence of the electric polarization has been established. It has been shown that this magnetodielectric effect is due to the contribution to ɛ from the electric susceptibility, which is related to the rotation of spins in the ab plane, arises in the region of the antiferromagnetic ordering T < T _N = 33 K, and is suppressed by the magnetic field. A theoretical model describing the main features of the behavior of ɛ and electric polarization in the magnetic field has been proposed, taking into account the additional anisotropy in the basal plane induced by the magnetoelastic stresses.     

Singlet-triplet Hamiltonian for spin excitations in a Kondo insulator

In the nonsymmetric version of the periodic Anderson model for a Kondo insulator, an effective singlet-triplet Hamiltonian Ĥ _s−t with indirect antiferromagnetic f-f exchange is constructed, which makes it possible to analyze the dynamic magnetic susceptibility χ _f (k, ω) of f electrons. Hamiltonian Ĥ _s−t is used to describe the experimentally observed dispersion of the three-level spin excitation spectrum in YbB₁₂. A distinguishing feature of this analysis is the introduction of small-radius singlet and triplet collective f-d excitations that form low- and high-lying spin bands during motion over the lattice.     

Spin-wave resonance in multilayer films (one-dimensional magnon crystals). Identification rules

The characteristic modification of the spectrum of the exchange spin waves has been revealed in ferromagnet/ferromagnet multilayer films with a thickness of N(d ₁ + d ₂) by the spin-wave resonance method. This modification is due to the first bandgap at the wavenumber k _b = π/(d ₁ + d ₂) of a magnon crystal, which is formed by one-dimensional modulation of the magnetization. It has been shown that the transformation of the multilayer film with thermal annealing to the film of a single-phase alloy is accompanied by the disappearance of this modification of the spectrum.     

Critical Behavior of Massless Free Field at Depinning Transition

We consider the d-dimensional massless free field localized by a δ-pinning of strength ɛ. We study the asymptotics of the variance of the field (when d= 2), and of the decay-rate of its 2-point function (when d≥ 2), as ɛ goes to zero, for general Gaussian interactions. Physically speaking, we thus rigorously obtain the critical behavior of the transverse and longitudinal correlation lengths of the corresponding d+ 1-dimensional effective interface model in a non-mean-field regime. We also describe the set of pinned sites at small ɛ, for a broad class of d-dimensional massless models. Received: 1 November 2000 / Accepted: 15 June 2001     

Ferromagnetic effect of a Mn delta layer in the GaAs barrier on the spin polarization of carriers in an InGaAs/GaAs quantum well

The polarization properties of the luminescence of an undoped InGaAs quantum well in InGaAs/GaAs het-erojunctions with a Mn delta layer in the GaAs barrier have been studied in a wide range of temperatures and magnetic fields. It has been found that the s, p-d exchange interaction of carriers in the quantum well with Mn ions in the δ layer leads to the ferromagnetic behavior of both the Zeeman splitting and spin polarization of the carriers with a Curie temperature typical of the Mn delta layer in the GaAs barrier. The saturation of the spin polarization of holes associated with their Fermi degeneracy has been observed at low temperatures (T < 20 K).     

Transmission of electrons through ferromagnetic material and applications to detection of electron spin polarization

The salient features of the total low energy inelastic electron scattering cross section in transition metals are described by a constant term σ₀ plus a term σ_d that is proportional to the number of unoccupied d-orbitals. This simple model predicts that low energy electrons transmitted through a ferromagnetic ultrathin film acquire a transport spin polarization a(χ). Using the ratio σ₀/σ_d as the only adjustable parameter, the model predicts the enhancement of the spin polarization of the low energy cascade electrons as well as a(χ) in reasonable agreement with the existing observations on Fe, Co and Ni. A detector for electron spin polarization P based on the spin dependent transmission of electrons through ferromagnetic material is proposed which should be superior to existing P-detectors by 1–2 orders of magnitude.     

<Emphasis Type="Italic">Λ</Emphasis>N scattering length from the reaction <Emphasis Type="Italic">γ</Emphasis>d→K<Superscript>+</Superscript><Emphasis Type="Italic">Λ</Emphasis>n

The prospects of utilizing the strangeness production reaction γd→K ⁺ Λn for the determination of the Λn low-energy scattering parameters are investigated. The spin observables that need to be measured in order to isolate the Λn singlet (¹ S ₀) and triplet (³ S ₁) states are identified. Possible kinematical regions where the extraction of the Λn scattering lengths might be feasible are discussed.     

Antiferromagnetism and superconductivity in a model with extended pairing interactions

The competition between antiferromagnetism and the d + id superconducting state is studied in a model with near and next near neighbour interactions in the absence of any on-site repulsion. A mean field study shows that it is possible to have simultaneous occurrence of an antiferromagnetic and a singlet d + id superconducting state in this model. In addition, such a coexistence generates a triplet d + id superconducting order parameter with centre of mass momentum Q = (π,π) dynamically having the same orbital symmetry as the singlet superconductor. Inclusion of next nearest neighbour hopping in the band stabilises the d_xy superconducting state away from half filling, the topology of the phase diagram, though, remains similar to the near neighbour model. In view of the very recent observation of a broad region of coexistence of antiferromagnetic and unconventional superconducting states in organic superconductors, the possibility of observation of the triplet state has been outlined. Received 30 November 2000 and Received in final form 27 March 2001     

Spin-wave resonance in a multilayer Co1 ? xPx/Co1 ? yPy structure as a method of detection of the Bragg gaps in a spin wave spectrum

The method of spin-wave resonance has been used to detect in multilayer (Co₉₈P₂/Co₉₅P₅) _N structures a modification of the exchange spin wave spectrum due to the formation of the first, second, and third Brillouin zones in a one-dimensional magnon crystal formed by a periodic modulation of the exchange. The band gaps have been measured for wave vectors k _b = π/(d ₁ + d ₂) and 2k _b .     

Photon polarization in the two-photon decay of heavy hydrogen-like ions

We have applied the density matrix and second-order perturbation theory in order to re-analyze the two-photon decay of hydrogen-like ions for the polarization of emitted light. Special attention is paid to the linear polarization of one of the photons, while the spin state of the second photon is supposed to be unobserved. For such an “angle-polarization” correlation of the two photons we investigate the contributions that arise from non electric-dipole terms in the expansion of the electron-photon interaction. Detailed calculations are performed for the 2s_1/2 →1s_1/2 and 3d_5/2 →1s_1/2 transitions in neutral hydrogen as well as in hydrogen-like uranium.     

The transient EPR spectra and spin dynamics of coupled three-spin systems in photosynthetic reaction centres

The concept of introducing an additional, stable paramagnetic species into photosynthetic reaction centres to increase the information content of their spin polarized transient EPR spectra is investigated theoretically. The light-induced electron transfer in such systems generates a series of coupled three-spin states consisting of sequential photoinduced radical pairs coupled to the stable spin which acts as an “observer”. The spin polarized transient EPR spectra are investigated using the coupled three-spin system P⁺I⁻Q⁻ _A in pre-reduced bacterial reaction centres as a specific example which has been studied experimentally. The evolution of the spin system and the spin polarized EPR spectra of P⁺I⁻Q⁻ _A and Q⁻ _A following recombination of the radical pair (P = primary donor, I = primary acceptor, Q_A = quinone acceptor) are calculated numerically by solving the equations of motion for the density matrix. The net polarization of the observer spin is also calculated analytically by perturbation theory for the case of a single, short-lived, charge-separated state. The result bears a close resemblance to the chemically induced nuclear polarization (CIDNP) generated in photolysis reactions in which a nuclear spin plays the role of the observer interacting with the radical pair intermediates. However, because the Zeeman frequencies of the three electron spins involved are usually quite similar, the polarization of the electron observer spin in strong magnetic fields can reflect features of the CIDNP effect in both, high and low magnetic fields. The dependence of the quinone spin polarization on the exchange couplings in the three-spin system is investigated by numerical simulations, and it is shown that the observed emissive polarization pattern is compatible with either sign, positive or negative, for a range of exchange couplings, J_PI, in the primary pair. The microwave frequency and orientation dependence of the spectra are discussed as two of several possible criteria for determining the sign of J_PI.     

The μCF Experiments at PSI – A Conclusive Review

During 25 years pioneering μCF experiments were performed at PSI. After initial study of the Wolfenstein–Gershtein effect in H/D, an intense research program on dμd fusion led to the early discovery of resonant dμd formation at low temperature and to the first direct observation of μd spin flip. With the Gatchina ionisation chamber absolute precisions of ∼1% on the determination of dμd formation and spin flip rates were recently obtained in good agreement with the theory. In a very large effort the highly resonant dμt fusion cycle was investigated. Record cycle rates up to 2×10⁸ s⁻¹ and yields up to 124 fusions per muon were measured. By slope analysis and by direct observation, effective sticking ω _s = (0.505 ± 0.029)% is the final PSI result. Clear experimental evidence of large epithermal resonances in D/T and H/D/T mixtures was found. This revised version was published online in August 2006 with corrections to the Cover Date.     

Muon charge exchange and muonium spin exchange in gases

The spin dynamics of the positive muon undergoing cyclic charge exchange (μ⁺⇌Mu) while it is slowing down is compared with that in the case of repeated muonium spin exchange after thermalization. The expectation value of the spin polarization at timet aftern spin exchange collisions (att ₁,t ₂, ...,t _n ) are calculated explicitly from time dependent wave functions, and the quantity is averaged over the statistical distribution of the times of collisions and over all possible numbers of collisions betweent=0 and the observation timet. This result is complementary and equivalent to the conventional density matrix formalism, but offers an insight into the roles of spin flip and spin non-flip processes. The neutral fraction during slowing down is also discussed.