30 research outputs found
Spin Frustration and Magnetic Exchange in Cobalt Aluminum Oxide Spinels
We report on x-ray diffraction, magnetic susceptibility, electron- spin
resonance and heat- capacity studies of Co[Al_1-xCo_x]_2O_4 for Co
concentrations 0<x<1. In this spinel system only the A-site Co^2+ cation is
magnetic, while the non-magnetic B-site Al^3+ is substituted by the low-spin
non-magnetic Co^3+, and it is possible to investigate the complete phase
diagram from Co^2+Al^3+_2O_4 to Co^2+Co^3+_2O_4. All samples reveal large
negative Curie-Weiss temperatures Theta_CW of the order of -110 K independent
of concentration, which is attributed to a high multiplicity of the
superexchange interactions between the A-site Co^2+ cations. A pure
antiferromagnetic state is found for x = 1.0 and 0.9 with Neel temperatures T_N
= 29.5 K and 21.2 K, respectively, as evidenced by lambda-like anomalies in the
specific heat. Compositions with 0.3<x<0.75 show smeared out strongly reduced
magnetic ordering temperatures. At low temperatures, a T^2.5 dependence of the
specific heat is indicative of a spin-liquid state. For x < 0.2 a T^2
dependence of the specific heat and a spin-glass like behavior of the
susceptibility below T_f = 4.7 K are observed. The high value of the
frustration parameter f = |Theta_CW|/T_f > 10 indicates the presence of strong
spin frustration at least for x < 0.6. The frustration mechanism is attributed
to competing nearest neighbor and next-nearest neighbor superexchange
interactions between the A-site Co^2+ ions.Comment: 19 pages, 9 figures, 46 reference
Hole-doping dependence of percolative phase separation in Pr_(0.5-delta)Ca_(0.2+delta)Sr_(0.3)MnO_(3) around half doping
We address the problem of the percolative phase separation in polycrystalline
samples of PrCaSrMnO for (hole doping between 0.46 and 0.54). We perform
measurements of X-ray diffraction, dc magnetization, ESR, and electrical
resistivity. These samples show at a paramagnetic (PM) to ferromagnetic
(FM) transition, however, we found that for there is a coexistence of
both of these phases below . On lowering below the charge-ordering
(CO) temperature all the samples exhibit a coexistence between the FM
metallic and CO (antiferromagnetic) phases. In the whole range the FM phase
fraction () decreases with increasing . Furthermore, we show that only
for the metallic fraction is above the critical percolation
threshold . As a consequence, these samples show very
different magnetoresistance properties. In addition, for we
observe a percolative metal-insulator transition at , and for
the insulating-like behavior generated by the enlargement of
with increasing is well described by the percolation law , where is a critical exponent. On the basis of
the values obtained for this exponent we discuss different possible percolation
mechanisms, and suggest that a more deep understanding of geometric and
dimensionality effects is needed in phase separated manganites. We present a
complete vs phase diagram showing the magnetic and electric properties
of the studied compound around half doping.Comment: 9 text pages + 12 figures, submitted to Phys. Rev.
An efficient pair natural orbital based configuration interaction scheme for the calculation of open-shell ionization potentials
A spin adapted configuration interaction scheme is proposed for the evaluation of ionization potentials in α high spin open shell reference functions. There are three different ways to remove an electron from such a reference, including the removal of an alpha or a beta electron from doubly occupied or an alpha electron from singly occupied molecular orbitals. Ionization operators are constructed for each of these cases, and the resulting second quantized expressions are implemented using an automated code generator environment. To achieve greater computational efficiency, the virtual space is reduced using an averaged pair natural orbital machinery developed earlier and applied with great success in the calculation of X-ray absorption spectra [D. Manganas et al., J. Chem. Phys. A 122, 1215 (2018)]. Various approximate integral evaluation schemes including the resolution of identity and seminumerical techniques are also invoked to further enhance the computational efficiency. Although the resulting method is not particularly accurate in terms of predicting absolute energy values, with a simple shift in the ionization potentials, it is still possible to use it for the qualitative characterization of the basic features of X-ray photoionization spectra. While satellite intensities cannot be computed with the current method, the inclusion of vibrational effects using a path integral technique allows for the computation of vibrational transitions corresponding to main peaks
Near-infrared spectroscopy patterns of cortical activity during gait in Parkinson’s disease patients treated with DBS STN
Disorders of gait seriously affect the functional state and quality of life of patients with Parkinson’s disease (PD). Impaired brain function underlies disorders of movement control in PD, however functional brain imaging with magnetic resonance (fMRI) is not feasible during gait. Near-Infrared Spectroscopy (NIRS) is a portable imaging method for measuring brain activity. It uses low-energy optical radiation to detect local changes of (de)oxyhemoglobin concentration in the cerebral cortex, like a fMRI. We included 8 patients with advanced PD chronically treated with DBS STN. Brain activity was recorded with the NIRSport. Gait was examined in 10 cycles, during which the active and resting phases alternated. Changes in oxyhemoglobin concentration were calculated from the native NIRS signal using a modified transformation of the Lambert-Beer Law. The signals were filtered in the 0.015–0.3 Hz band and the least-squares algorithm was fitted with the HRF function for each cycle separately, from which the median was finally calculated. The activity of the motor cortex was significantly higher during gait in the OFF compared to ON state (p = 0.02). In contrast, in other regions no differences were found. A higher motor cortex activity shown in the DBS OFF compared to ON state may reflect the impairment of gait control in PD. In general terms, the present study demonstrates the potential utility of the NIRS method in detecting functional changes of the brain during gait in patients with PD