Non-local correlation effects and metal-insulator transition in the s-d exchange model

The metal-insulator transition within the s-d exchange model is studied by the Dynamical Mean Field Theory and Dual Fermion approaches. The latter takes into account nonlocal correlation effects which are shown to be essential. In particular, the critical values of the s-d exchange coupling constant for these two methods turn out to be different by a factor of more than 2 (for the case of square lattice and spin 1/2 localized electrons). The calculations were performed using a Continuous-Time Quantum Monte Carlo method. The difference of the quantum spin-1/2 case and the classical-spin case is discussed. For the quantum case the sign of s-d exchange coupling constant is relevant which demonstrates the importance of the Kondo effect.     

Reduced s-d model with antiferromagnetically coupled impurity spins

An antiferromagnetic equivalent-neighbour Heisenberg interaction H_i between impurity spins is added to the reduced s-d Hamiltonian H_r previously introduced by simplifying the Kondo s-d exchange Hamiltonian H_K. Asymptotic mean-field theory is developed for H_r + H_i, in the presence and absence of external magnetic field, and applied to (La_1−xCe_x)Al₂ alloys. Specific heat c_i(T) and zero-field susceptibility χ_i(0,T) curves for (La_1−xCe_x)Al₂ are depicted. The coupling constants of H_r + H_i and conduction bandwidth are adjusted so that T_c temperatures for x = 0.2, 0.1 are equal to the experimental values. c_i(T) exhibits a jump at T_c and is decreasing for T < T_c. χ_i(0,T) has a first order pole at T_c which corresponds to the maximum of experimental susceptibility and χ_i(0,0) > 0. These results improve those obtained earlier on the grounds of H_r theory.     

Electronic spectra in the s-d model of ferromagnetic semiconductors using the method of moments

An approximate scheme determining basic parameters of the electronic spectral density in the s-d model of ferromagnetic semiconductors is suggested which well interpolates between the low and high temperature régimes. The method is based on fitting the lowest momenta of the spectral density and yields no exceptional behaviour of the electronic spectrum at the Curie temperature.     

Garvey-Kelson mass formula in the s-d shell

A literal interpretation of the Garvey-Kelson mass formula is used to relate deviations from the simple predictions for certain sextuplets of nuclei. The large discrepancies between prediction and experiment for ²⁵Ne, ²⁹Mg and ³³Si are, in this way, removed.     

Correlation coefficients of effective interactions in the s-d shell

Ten effective interactions in the s-d shell have been compared via their correlation coefficients in scalar (m) and scalar-T (m, T) spaces. Higher-order spectroscopic averages have been used to evaluate the average spread (dispersion) of an eigenstate of one Hamiltonian with respect to another Hamiltonian.     

Specific features of indirect interaction in an impurity graphene bilayer in the framework of the s-d model

The specific features of the Ruderman-Kittel-Kasuya-Yosida interaction in an impurity graphene bilayer have been calculated in the framework of the s-d model. Hydrogen atoms are considered as impurities. The calculations have demonstrated that the antiferromagnetic impurity-spin ordering is preferable at short distances, while as the distance increases, the ordering becomes ferromagnetic. The dependences of the exchange interaction constant on the problem parameters are shown.     

Antiferromagnetic s-d exchange coupling in GaMnAs

Measurements of coherent electron spin dynamics in Ga1-xMnxAs/Al0.4Ga0.6As quantum wells with 0.0006%相似文献   

Impurity contribution to the specific heat in the Kondo model

Summary Using the Lanczos scheme (tridiagonalization) we investigate the impurity contribution to the single-impurity Kondo model. This is accomplished by obtaining the ground-state energy of aN-particle system and a (N+1)-particle system, and obtaining the functional dependence ofg(J) (whereJ is the strength of the interaction). Hereg(J) is the coefficient of a correction term which is of order 1/N. The density of states at the Fermi level is effectively changed by the factorg(J). The excess heat capacity associated with the impurity is then proportional tog(J). To speed up publication, the authors of this papers have agreed to not receive the proofs for correction.     

Structure of neutron-rich s-d shell nuclei

States in neutron-rich s-d shell nuclei were populated in the reaction of a ¹⁴C beam at E _lab=22 MeV on a ¹⁴C target. Coincidences between γ rays and either other γ rays or light charged particles were measured. γ rays in coincidence with protons established levels at 66.8, 1730, 1823, and 2219 keV in ²⁷Na. The states are compared with calculations based on the s-d shell model and the cranked Nilsson-Strutinsky model. A number of levels in ²⁴Ne were observed in both α-γ and α-γ-γ coincidences and are compared with shell-model calculations.     

(p, γ) Resonance strengths in the s-d shell

The strengths of selected resonances in the range E_p = 0.5–2.0 MeV in the (p, γ) reactions on Mg, ³⁰Si, ³⁴S, ³⁷C1, ³⁹K and ⁴⁰Ca have been found relative to the E_p = 632 and 992 keV resonances in ²⁷Al(p, γ)²⁸Si by relative yield measurements. Targets were made from mixtures or chemical compounds such that each contained at least two of the isotopes of interest and their chemical composition was determined by Rutherford back-scattering of α-particles. Absolute measurements were conducted on the selected resonances in ²⁷A1(p, γ)²⁸Si and ³⁰Si(p, γ)³¹P by semi-thick target and thin target techniques with the target thickness, needed for the latter technique, found by Rutherford back-scattering of protons. Absolute strengths for all of the resonances treated, together with one from each of ²³Na, ³¹P and ³⁵Cl, reported in a previous paper, were deduced by normalizing to the absolute measurements on the Al(p, γ)²⁸Si resonances.     

Impurity transport in the model of a dual-porosity regularly heterogeneous medium with colloids

The problem of impurity advection in a channel surrounded by a weakly permeable medium with drifting colloidal particles present in the channel is solved. A regime in which the front of the impurity cloud moves with a constant velocity and the concentration behind the front decreases as a power law is established due to the impurity adsorption on colloids and its diffusive exchange with the matrix on moderate time scales. On long time scales, the transport in the channel is quasi-diffusive as a result of the impurity desorption and its eventual escape into the matrix.     

Multi-band spectra of odd and even nuclei of the s-d shell

The energy levels of^{21, 23}Na,^{22, 23}Ne,^{24, 28}Mg and^{25, 29}Al are obtained by mixing various bands using the projected deformed Hartree-Fock (DHF) method. Solutions having minimum energies are found to be prolate for all the nuclei considered here. Higher bands are obtained either by considering particle-hole excitations or oblate solutions. These various bands are mixed using the projection method and care has been taken to orthogonalize the bands. The interactions used in this study are those given by Kuo, Preedom-Wildenthal (PW) and WHMK interactions. The last one seems to give good results for most of the nuclei considered here. Not only are the lowest bands well-reproduced but the second lowest bands agree reasonably well in most nuclei. The third lowest ones obtained in some nuclei are not yet observed as complete bands. However, K obtained for the third band seems to be correct. A comparison with shell model calculations—which are numerically exhausting—shows similar results for the lowest band. However, the agreement of the second band varies from nucleus to nucleus. A comparison between matrix elements of the interactions is made to analyze the results.     

Two-body density matrix for closed s-d shell nuclei

The two-body density matrix for ⁴He,¹⁶O and ⁴⁰Ca within the Low-order approximation of the Jastrow correlation method is considered. Closed analytical expressions for the two-body density matrix, the center of mass and relative local densities and momentum distributions are presented. The effects of the short-range correlations on the two-body nuclear characteristics are investigated. Received: 11 September 1999 / Revised version: 20 December 1999     

同位旋效应对F同位素s-d能级间隙的影响

在单粒子势模型的框架下, 计算了O同位素和F同位素的基态和某些低激发态的能级值、均方根半径值及密度分布。通过在伍兹 萨克逊势中引入同位旋依赖项, 研究了同位旋效应对F同位素s-d能级间隙的影响。理论计算得到的均方根半径值和自旋宇称值与实验结果一致。尤其是, 理论计算能够实现16F, 15F和14F中2s_1/2与1d_5/2质子能级间的反转, 且F同位素中s-d质子能级间隙及其整体变化趋势与已知实验结果符合较好。此外, 理论结果表明17F的(1/2)+第一激发态为单质子晕态。 The level structures of O and F isotopes are investigated in the framework of the single particle potential model. The isospin effect on the s d level spacing in F isotopes is studied by introducing an isospin dependent term in the depth of the Woods Saxon potential. The theoretical results of RMS radii and spin parity values are in agreement with the experimental data. In particular, the level inversion between proton levels of 2s_1/2 and 1d_5/2 in 16F, 15F and 14F is reproduced. The global tendency of the s-d level spacing agrees with the observed fact. In addition, the study confirms the one proton halo structure in 17F(2s_1/2).     

Estimates of the s-d interaction in CdCr from superconducting transition temperature and magnetic susceptibility

Cr has a well developed magnetic moment in a Cd host and the superconducting transition temperature, T_c, is strongly depressed. We estimate a spin, S = 1.5 leading to a depression rate - $\text{dT}{}_{\mathrm{c}}\text{dc}\text{= 140 K/at.\%}$. The s-d interaction strength is estimated to 0.5–0.7 eV leading to a Kondo temperature in the milliKelvin range.     

Effect of s-d hybridization in residual resistance of binary ordered alloys

Calculations are made of the residual resistance of the ordered binary alloys stemming from the difference between the d levels of the components. The residual resistance is found to be dependent on the degree of the long-range order. It is established that in alloys in which the Fermi energy E_F is far from the d resonances decreases monotonically as increases. When E_F approaches one of the resonance levels the resistance increases severalfold and varies monotonically with . The results are used in interpreting the anomalies of the temperature dependence of the resistance of iron-palladium alloys.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 39–44, April, 1979.The author is indebted to A. A. Kuranov and P. N. Syutkin for providing us with detailed experimental data, and to S. I. Masharov for his fruitful discussion.     

Off-diagonal s-d coupling and red shift of absorption spectra in magnetic semiconductors

Using the method of moments, it is argued that the strong coupling s-d model gives an appreciably enhanced red shift of the quasiparticle density of states with temperature due to even a weak off-diagonal s-d coupling contribution as compared to the case with only a diagonal coupling. The result is used to interpret the red shift in chromium chalcogenide spinels.