Electronic states of PrCoO3_3: X-ray photoemission spectroscopy and LDA+U density of states studies

Electronic states of PrCoO$_3$ are studied using x-ray photoemission spectroscopy. Pr 3d$_{5/2}$ core level and valence band (VB) were recorded using Mg K$_\beta$ source. The core level spectrum shows that the 3d$_{5/2}$ level is split into two components of multiplicity 4 and 2, respectively due to coupling of the spin states of the hole in 3d$_{5/2}$ with Pr 4f holes spin state. The observed splitting is 4.5 eV. The VB spectrum is interpreted using density of states (DOS) calculations under LDA and LDA+U. It is noted that LDA is not sufficient to explain the observed VB spectrum. Inclusion of on-site Coulomb correlation for Co 3d electrons in LDA+U calculations gives DOS which is useful in qualitative explanation of the ground state. However, it is necessary to include interactions between Pr 4f electrons to get better agreement with experimental VB spectrum. It is seen that the VB consists of Pr 4f, Co 3d and O 2p states. Pr 4f, Co 3d and O 2p bands are highly mixed indicating strong hybridization of these three states. The band near the Fermi level has about equal contributions from Pr 4f and O 2p states with somewhat smaller contribution from Co 3d states. Thus in the Zaanen, Sawatzky, and Allen scheme PrCoO$_3$ can be considered as charge transfer insulator. The charge transfer energy $\Delta$ can be obtained using LDA DOS calculations and the Coulomb-exchange energy U' from LDA+U. The explicit values for PrCoO$_3$ are $\Delta$ = 3.9 eV and U' = 5.5 eV; the crystal field splitting and 3d bandwidth of Co ions are also found to be 2.8 and 1.8 eV, respectively.Comment: 12 pages, 7 figures; to appear J. Phys.: Condens. Matte

Local distortion in LaCoO3 and PrCoO3: EXAFS, XRD and XANES studies

Room temperature Co K-edge extended x-ray absorption fine structure (EXAFS), x-ray absorption near edge structure (XANES) including pre-edge and x-ray diffraction (XRD) studies are carried out on LaCoO3 and PrCoO3. The Co-O, Co-La/Pr and Co-Co bond lengths are obtained from EXAFS analysis and compared with those obtained from XRD. The EXAFS analysis of data indicates that CoO6 octahedron is distorted in both LaCoO3 and PrCoO3. Such distortion is expected in orthorhombic PrCoO3 but not in rhombohedral LaCoO3. This distortion in CoO6 octahedron is attributed to Jahn-Teller active Co3+ ion in intermediate spin state in these compounds. Higher shell studies reveal that Debye-Waller (DW) factors of Co-Pr and Co-Co bonds in PrCoO3 are more in comparison to Co-La and Co-Co bonds in LaCoO3 indicating that these bonds are structurally more disordered in PrCoO3. The comparison of Co-Co bond lengths and corresponding DW factors indicates that the structural disorder plays an important role in deciding the insulating properties of these compounds. XANES studies have shown changes in the intensities and positions of different near edge features.Comment: 22 pages, 8 figures, 2 tables. To appear in J. Phys.: Condens. Matte

A study of transition metal K-edge x-ray absorption spectra of LaBO3 (B=Mn, Fe, Co, Ni), La2CuO4 and SrMnO3 using partial density of states

The transition metal K-edge x-ray absorption near edge structure (XANES) studies have been carried on LaBO3 (B=Mn, Fe, Co, Ni), La2CuO4 and SrMnO3 compounds. The theoretical spectra have been calculated using transition metal (TM) 4p density of states (DOS) obtained from full-potential LMTO density functional theory. The exchange-correlation functional used in this calculation is taken under local density approximation (LDA). The comparison of experimental spectra with the calculated ones indicates that single-particle transitions under LDA are sufficient to generate all the observed XANES including those which have earlier been attributed to many-body shake-up transitions and core-hole potentials. The present study reveals that all the experimentally observed features are mainly due to distribution in TM 4p DOS influenced by hybridization with other orbitals. Specifically, for LaMnO3, the feature earlier attributed to shake-up process is seen to arise from hybridization of Mn 4p with La 6p and O 2p orbitals; in La2CuO4 the features attributed to core hole potential correspond to hybridization of Cu 4p with La 6p, La 5d and O 2p orbitals. To see the effect of inhomogeneous electronic charge distribution and on-site Coulomb and exchange interaction (U) on the XANES of these compounds generalized-gradient approximation and U corrections are incorporated in the calculations. These corrections do not generate any new features in the spectra but affect the detailed intensity and positions of some of the features.Comment: 23 pages, 8 figures. To appear in J. Phys.: Condens. Matte

Neutron powder diffractometer beam line with a toroidally bent asymmetric crystal monochromator: Monte Carlo simulation studies

Monte Carlo simulation studies are carried out for beam lines for neutron powder diffraction. Programs are developed for studying the propagation of neutrons starting from reactor end of beam tube and progressing through various optical elements-collimators, slits, monochromator crystal, sample and finally detector. The simulation results are seen to be in fair agreement for an existing beam line using a plane crystal monochromator on Dhruva reactor, BARC, Mumbai. The programs are then used to design a beam line using a toroidally bent monochromator crystal and open collimator geometry. The simulation results show that a perfect bent crystal monochromator based diffractometer provides better resolution and higher intensity as compared to that based on a plane crystal monochromator. A complete optical design of the beam line is worked out taking into consideration vertical and horizontal radii of curvature for the asymmetrically cut monochromator crystal, sample size and detector resolution function

<span style="font-size:22.0pt;mso-bidi-font-size:15.0pt; line-height:115%;font-family:"Times New Roman","serif";mso-fareast-font-family: "Times New Roman";mso-fareast-theme-font:minor-fareast;mso-ansi-language:EN-US; mso-fareast-language:EN-US;mso-bidi-language:AR-SA;mso-bidi-font-weight:bold">EXAFS study of Sr doped Y_0.4Pr_{<span style="font-size:17.0pt; mso-bidi-font-size:10.0pt;line-height:115%;font-family:"Times New Roman","serif"; mso-fareast-font-family:"Times New Roman";mso-fareast-theme-font:minor-fareast; mso-ansi-language:EN-US;mso-fareast-language:EN-US;mso-bidi-language:AR-SA; mso-bidi-font-weight:bold">0.6</span>}<span style="font-size:22.0pt; mso-bidi-font-size:15.0pt;line-height:115%;font-family:"Times New Roman","serif"; mso-fareast-font-family:"Times New Roman";mso-fareast-theme-font:minor-fareast; mso-ansi-language:EN-US;mso-fareast-language:EN-US;mso-bidi-language:AR-SA; mso-bidi-font-weight:bold">Ba_{<span style="font-size:17.0pt; mso-bidi-font-size:10.0pt;line-height:115%;font-family:"Times New Roman","serif"; mso-fareast-font-family:"Times New Roman";mso-fareast-theme-font:minor-fareast; mso-ansi-language:EN-US;mso-fareast-language:EN-US;mso-bidi-language:AR-SA; mso-bidi-font-weight:bold">2</span>}<span style="font-size:22.0pt; mso-bidi-font-size:15.0pt;line-height:115%;font-family:"Times New Roman","serif"; mso-fareast-font-family:"Times New Roman";mso-fareast-theme-font:minor-fareast; mso-ansi-language:EN-US;mso-fareast-language:EN-US;mso-bidi-language:AR-SA; mso-bidi-font-weight:bold">Cu_{<span style="font-size:17.0pt; mso-bidi-font-size:10.0pt;line-height:115%;font-family:"Times New Roman","serif"; mso-fareast-font-family:"Times New Roman";mso-fareast-theme-font:minor-fareast; mso-ansi-language:EN-US;mso-fareast-language:EN-US;mso-bidi-language:AR-SA; mso-bidi-font-weight:bold">3</span>}<span style="font-size:22.0pt; mso-bidi-font-size:15.0pt;line-height:115%;font-family:"Times New Roman","serif"; mso-fareast-font-family:"Times New Roman";mso-fareast-theme-font:minor-fareast; mso-ansi-language:EN-US;mso-fareast-language:EN-US;mso-bidi-language:AR-SA; mso-bidi-font-weight:bold">O_{<span style="font-size:17.0pt; mso-bidi-font-size:10.0pt;line-height:115%;font-family:"Times New Roman","serif"; mso-fareast-font-family:"Times New Roman";mso-fareast-theme-font:minor-fareast; mso-ansi-language:EN-US;mso-fareast-language:EN-US;mso-bidi-language:AR-SA; mso-bidi-font-weight:bold">7-δ</span>}<span style="font-size:26.0pt; mso-bidi-font-size:19.0pt;line-height:115%;font-family:"Arial","sans-serif"; mso-fareast-font-family:"Times New Roman";mso-fareast-theme-font:minor-fareast; mso-ansi-language:EN-US;mso-fareast-language:EN-US;mso-bidi-language:AR-SA; mso-bidi-font-weight:bold">*</span></span></span></span></span>

272-276<span style="font-size: 16.0pt;mso-bidi-font-size:9.0pt;font-family:" times="" new="" roman","serif""="">The Cu K-edge <span style="font-size:16.0pt;mso-bidi-font-size:9.0pt;font-family: " times="" new="" roman","serif""="">extended X-ray absorption line structure (EXAFS) studies in Y<span style="font-size:22.0pt;mso-bidi-font-size:15.0pt;font-family: " times="" new="" roman","serif";mso-bidi-font-weight:bold"="">Ba2<span style="font-size:22.0pt; mso-bidi-font-size:15.0pt;font-family:" times="" new="" roman","serif";mso-bidi-font-weight:="" bold"="">Cu<span style="font-size:17.0pt;mso-bidi-font-size:10.0pt; font-family:" times="" new="" roman","serif";mso-bidi-font-weight:bold"="">3O<span style="font-size:17.0pt; mso-bidi-font-size:10.0pt;font-family:" times="" new="" roman","serif";mso-bidi-font-weight:="" bold"="">7-δ<span style="font-size:13.0pt;mso-bidi-font-size:6.0pt; font-family:" times="" new="" roman","serif""=""> <span style="font-size:16.0pt; mso-bidi-font-size:9.0pt;font-family:" times="" new="" roman","serif""="">and Y0.4Pr0.6Ba2-x SrxCu3O7-δ<span style="font-size:13.0pt; mso-bidi-font-size:6.0pt;font-family:" times="" new="" roman","serif""=""> <span style="font-size: 13.5pt;mso-bidi-font-size:6.5pt;font-family:" times="" new="" roman","serif""=""> <span style="font-size:16.0pt; mso-bidi-font-size:9.0pt;font-family:" times="" new="" roman","serif""="">with 0≤ x≤1.25 have been carried out to examine the changes in near neighbour environment of Cu as a function of Sr doping. There is a sudden shift of apical oxygen O(4) towards the CuO planes in compounds wherein superconductivity is restored. This contraction in the Cu(2)-O(4) bond length seems to weaken the Pr-O hybridization and thereby restoring superconductivity. Infrared absorption studies on these compounds also support this picture. </span