A first-principles study of the influence of helium atoms on the optical response of small silver clusters

Optical excitation spectra of Ag(n) and Ag(n)@He(60) (n = 2, 8) clusters are investigated in the framework of the time-dependent density functional theory (TDDFT) within the linear response regime. We have performed the ab initio calculations for two different exact exchange functionals (GGA-exact and LDA-exact). The computed spectra of Ag(n)@He(60) clusters with the GGA-exact functional accounting for exchange-correlation effects are found to be generally in a relatively good agreement with the experiment. A strategy is proposed to obtain the ground-state structures of the Ag(n)@He(60) clusters and in the initial process of the geometry optimization, the He environment is simulated with buckyballs. A redshift of the silver clusters spectra is observed in the He environment with respect to the ones of bare silver clusters. This observation is discussed and explained in terms of a contraction of the Ag-He bonding length and a consequent confinement of the s valence electrons in silver clusters. Likewise, the Mie-Gans predictions combined with our TDDFT calculations also show that the dielectric effect produced by the He matrix is considerably less important in explaining the redshifting observed in the optical spectra of Ag(n)@He(60) clusters.     

Interplay between the magnetic field and the dipolar interaction on a magnetic nanoparticle system: A Monte Carlo study

We have studied the influence of the applied magnetic field on the blocking temperature (T_B) of a fine magnetic particle system. By means of a Monte Carlo technique we have simulated zero field cooling (ZFC) curves under different applied fields, obtaining the respective T_B as a function of H. We have focused our study on the limit H → H_K (where H_K is the anisotropy field), since the results found in the literature usually lack a detailed study of this range. The simulations were done at different sample concentration of the nanoparticles, with the purpose of observing how the magnetic dipolar interaction affects the field dependence of T_B. The classical expression predicts T_B to disappear for H ? H_K, independently of the dipolar interaction strength. Our simulations show that at strong interacting conditions T_B exists even for fields H > H_K.     

Temperature behavior of bound magnetic polarons in antiferromagnetic chain

The behavior of a one-dimensional antiferromagnetic (AF) chain doped by non-magnetic donor impurities is analyzed, in the limit of low impurity density n  . The doping leads to the formation of ferromagnetically correlated regions localized near impurities (bound magnetic polarons or ferrons). The temperature evolution of the chain is calculated using an approximate variational method, and a Monte Carlo simulation. Both these methods give the similar results. The analysis of correlation functions for neighboring local spins demonstrates that the ferromagnetic correlations inside a ferron are significant even at high temperatures. The AF correlations in the rest part of the chain decay much faster with temperature. So, the ferron is a stable object that does not disappear even above the Néel temperature _TN$T_{\mathrm{N}}$. At rather small values of the electron–impurity coupling energy V$V$ (for V$V$ lower then the electron hopping integral t  ), the bound ferron depins from impurity retaining its magnetic structure. Such a depinning occurs at T∼V$T\sim V$.     

Electronic structure of dimerized spinel ZnV2O4

Electronic structure calculations were performed for ZnV₂O₄, a material close to a metal-insulator transition. Structural optimization leads to the formation of V-V dimers along the off-plane chains. A strong spin-lattice coupling is expected close to the transition to itinerancy. No orbital ordering is observed in such a structure, and the experimentally found magnetic structure is naturally explained.     

Synthesis and characterization of CoFe2O4–PVP nanocomposites

Cobalt ferrite–poly(N-vinyl-2-pyrrolidone) nanocomposites were prepared by drying a dispersion of cobalt ferrite (CoFe₂O₄) nanoparticles and poly(N-vinyl-2-pyrrolidone). Magnetic measurements indicate a superparamagnetic behavior. Zero-field-cooling magnetization experiments at 100 Oe show different trends depending on the CoFe₂O₄ nanoparticles size. For the smaller ones (3.9 nm), the blocking temperatures shift to lower temperatures with increasing concentration; however, this shift is not observed for the larger ones (6.6 nm). These differences can be related to the anisotropy constant of the CoFe₂O₄ nanoparticles and the interparticle dipolar interactions.     

High-pressure magnetic and structural properties of TiOX (X=Cl,Br)

We report high-pressure diffraction and magnetization measurements to demonstrate that the partial collapse of electronic gap at high-pressure insulator to metal transition reported in TiOCl (C. Kuntscher et al. Phys. Rev. B 74 184402 (2006).) corresponds to a Ti³⁺–Ti³⁺ dimerization at room temperature within the space group P2₁/m. The shortest Ti–Ti distance is comparable to that of the Ti metal, but a Peierls-like distortion prevents a metallic behaviour.     

Orbital and magnetic structure of quasi-1D cobaltites: Ab initio calculations and experiment

The magnetic properties of the orbitally degenerate quasi-one-dimensional cobaltites Sr_xBa_1−xCoO₃ are explained on the basis of a phase separation phenomenon. Noninteracting magnetic particles embedded in a nonferromagnetic matrix develop in the system. Details are given about the electronic and magnetic structure for x=0,0.2$x=0,0.2$ and 0.5. At x=0.5$x=0.5$, the geometry of the CoO₆ trigonally distorted octahedra changes by about 1–2%, but magnetic particles get 3 times bigger, compared to the parent compound, with the corresponding changes in the magnetic properties. The electronic structure of the Co⁴⁺ ion, however, stays roughly unchanged.     

Study of the pressure effects in TiOCl by ab initio calculations

Electronic structure calculations on the low-dimensional spin?1/2 compound TiOCl were performed at several pressures in the orthorhombic phase, finding that the structure is quasi-one-dimensional. The Ti³⁺ (d¹) ions have one t_2g orbital occupied (d_yz) with a large hopping integral along the b-direction of the crystal. The most important magnetic coupling is Ti–Ti along the b-axis. The transition temperature (T_c) has a linear evolution with pressure, and at about to 10 GPa this T_c is close to room temperature, leading to a room temperature spin-Peierls insulator–insulator transition, with an important reduction of the charge gap in agreement with the experiment. On the high-pressure monoclinic phase, TiOCl presents two possible dimerized structures with a long or short dimerization. Long dimerized state occurs above 15 GPa, and below this pressure the short dimerized structure is the more stable phase.     

On some interfacial fundamental characteristics for a general model of giant magnetoresistance

We present an analytical model for introducing interface roughness into giant magnetoresistance. The roughness acts mainly on the spin currents via the mixing relaxation time, in such a way, that the theoretical results suggest a direct experimental procedure.     

Stabilization of magnetic polarons in antiferromagnetic semiconductors by extended spin distortions

We study the problem of a magnetic polaron in an one-dimensional antiferromagnetic semiconductor (ferron). We obtain an analytical solution for the distortion produced in the antiferromagnetic structure due to the presence of a charge carrier bound to an impurity. The region in which the charge carrier is trapped is of the order of the lattice constant (small ferron) but the distortion of the magnetic structure extends over a much larger distance. It is shown that the presence of this distortion makes the ferron more stable, and introduces a new length scale in the problem.Received: 12 February 2004, Published online: 23 July 2004PACS: 75.10.Pq Spin chain models - 75.50.Pp Magnetic semiconductors - 75.30.Hx Magnetic impurity