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61.
A two-site single electron double exchange model incorporating orbital degeneracy, superexchange and electron-phonon (e-ph) interaction is studied using exact diagonalization method. The core spins are treated quantum mechanically. We study the ground state phase diagram as well as the magnetic susceptibility and the kinetic energy of the system as a function of temperature. Effect of difference in site energies, which mimics the role of site-diagonal disorder, is investigated. The susceptibility shows a peak at a characteristic temperature which we have referred to as T0. The variation of T0 with e-ph coupling and that of the isotope-shift exponent (α) with T0 are obtained. We also investigate the field-induced change in the kinetic energy, which is related to the colossal magnetoresistance (CMR) of the system, and find that the disorder enhances the CMR even for the two-site system. 相似文献
62.
Monte‐Carlo simulations predict that a local correlated disorder is responsible for many of the novel transport and magnetic properties of colossal magnetoresistance (CMR) materials such as manganites. One important prediction of these models is that the resistivity at the metal–insulator transition (MIT) in manganites depends strongly on the correlated quenched disorder. However, experimental confirmation has been challenging since it is difficult to control the amount of disorder in these compounds. We carried out experiments on Sm0.55Sr0.45MnO3, a prototypical CMR manganite with a sharp MIT, whereby the oxygen‐related disorder is systematically enhanced by low temperature thermal activation. We observe dramatic changes in the temperature dependence of resistivity at the MIT as the amount of quenched disorder is increased, occurring in a manner that is in agreement with theoretical predictions.
63.
Inter-Layer Energy Transfer through Wetting-Layer States in Bi-layer InGaAs/GaAs Quantum-Dot Structures with Thick Barriers 下载免费PDF全文
The inter-layer energy transfer in a bi-layer InGaAs/GaAs quantum dot structure with a thick GaAs barrier is studied using temperature-dependent photoluminescence. The abnormal enhancement of the photoluminescence of the QDs in the layer with a larger amount of coverage considering the resonant Forster energy transfer between the at 110K is observed, which can be explained by wetting layer states at elevated temperatures. 相似文献
64.
Z.F. Zi Y.P. Sun X.B. Zhu J.M. Dai 《Journal of magnetism and magnetic materials》2009,321(15):2378-2381
La0.7Sr0.3MnO3 nanoparticles were prepared by a simple chemical coprecipitation route. Structural, magnetoresistance (MR), and magnetic properties were investigated. Rietveld refinement of X-ray powder diffraction result shows that the sample is single-phase with the space group of R3¯C. The result of field-emission scanning electronic microscopy shows that most of the grain sizes are distributed from 50 to 200 nm. The composition determined by energy-dispersive spectroscopy is the stoichiometry of La0.7Sr0.3MnO3. The ferromagnetic to paramagnetic transition is sharp with Curie temperature TC=367 K, which further confirms that the sample is single-phase. The steep change in MR at low fields is attributed to the alignment of the magnetization, while the high-field MR is due to the grain boundary effect. 相似文献
65.
MgO-based magnetic tunnel junctions (MTJs) with a layer sequence Ir22Mn78 or Fe50Mn50 (10 nm)/CoFe (2 nm)/Ru (0.85 nm)/CoFeB (0.5?t<2 nm)/MgO (2.5 nm)/CoFeB (3 nm) have been fabricated. The bias voltage dependence of tunneling magnetoresistance (TMR) is given as a function of the annealing temperature for these MTJs, which shows the TMR ratio changes its sign from inverted to normal at a critical bias voltage (VC) when an unbalanced synthetic antiferromagnetic stack CoFe/Ru/CoFeB is used. VCs change with the thickness of the pinned CoFeB and annealing temperature, which implies one can achieve different VCs by artificial control. The asymmetric VC values suggest that a strong density-of-states modification occurs at bottom oxide/ferromagnet interface. 相似文献
66.
The role of the background in the extraction of resonance contributions from meson–baryon scattering
The separation of background and resonance contributions in pion–nucleon scattering is an often discussed issue. We investigate to what extent the background can be separated from the pole contribution. For illustration we use results from an analytic model for the meson–baryon interaction derived from meson exchange. We focus on the two distinct cases of an elastic and a highly inelastic resonance, namely the Δ(1232) and the Δ∗(1700). Our results are also relevant for studies of dynamically generated resonances and attempts to extract bare quantities from hadronic models to be compared to quark model results. 相似文献
67.
68.
Large negative magnetoresistance (MR)(|ΔR/R|=|[R(H)-Rmax]/Rmax|>50%) has been observed in the broken cold-pressed CrO2 powder samples near the room temperature, which is ascribed to the magnetic field-induced mechanical contact variation. This large, room-temperature negative MR might easily lead to a wrong conclusion that the spin-polarization is still very high at high temperatures. This observation may also point to the possibility to develop field sensors based on the mechanical MR. 相似文献
69.
A. Sboychakov A. Rakhmanov K. Kugel I. González J. Castro D. Baldomir 《Journal of magnetism and magnetic materials》2006
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. At rather small values of the electron–impurity coupling energy V (for 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. 相似文献
70.
S. Teuber T. Döppner T. Fennel J. Tiggesbäumker K.H. Meiwes-Broer 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,16(1):59-63
The photoionization of metal clusters in intense femtosecond laser fields has been studied. In contrast to an experiment on
atoms, the interaction in this case leads to a very efficient and high charging of the particle where tens of electrons per
atom are ejected from the cluster. The recoil energy distribution of the atomic fragment ions was measured which in the case
of lead clusters exceeds 180 keV. Enhanced charging efficiency which we observed earlier for specific pulse conditions is
not reflected in the recoil energy spectra. Both the average and the maximum energies decrease with increasing laser pulse
width. This is in good agreement with molecular dynamics calculations.
Received 20 December 2000 相似文献