arXiv subjects

P. A. Montano

Publications and source records attributed to P. A. Montano.

5 recordsLinked to original sources

arXiv2007

Spin moment over 10-300 K and delocalization of magnetic electrons above the Verwey transition in magnetite

In order to probe the magnetic ground state, we have carried out temperature dependent magnetic Compton scattering experiments on an oriented single crystal of magnetite (Fe$_3$O$_4$), together with the corresponding first-principles band theory computations to gain insight into the measurements. An accurate value of the magnetic moment $μ_S$ associated with unpaired spins is obtained directly over the temperature range of 10-300K. $μ_S$ is found to be non-integral and to display an anomalous behavior with the direction of the external magnetic field near the Verwey transition. These results reveal how the magnetic properties enter the Verwey energy scale via spin-orbit coupling and the geometrical frustration of the spinel structure, even though the Curie temperature of magnetite is in excess of 800 K. The anisotropy of the magnetic Compton profiles increases through the Verwey temperature $T_v$ and indicates that magnetic electrons in the ground state of magnetite become delocalized on Fe B-sites above $T_v$.

cond-mat.str-el↗

arXiv2007

Magnetic momentum density, Fermi surface and directional magnetic Compton profiles in LaSr$_{2}$Mn$_{2}$O$_{7}$ and La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$

We have carried out first principles, all-electron computations of the magnetic momentum density $ρ_{mag}({\mathbf p})$ and magnetic Compton profiles (MCPs) for momentum transfer along the [100], [001], and [110] directions in LaSr$_2$Mn$_2$O$_7$ and La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ within the local spin density approximation (LSDA) based band theory framework. Parallel measurements of these three MCPs from a single crystal of La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ at 5 K in a magnetic field of 7 T are also reported. $ρ_{mag}({\mathbf p})$ is shown to contain distinct peaks arising from the occupied majority-spin $t_{2g}$ electrons and to display images of the Fermi surface (FS) in the first and higher Brillouin zones (BZs). The overall shape of the MCPs, $J_{mag}(p_z)$, obtained by integrating $ρ_{mag}({\mathbf p})$ over $p_x$ and $p_y$, is found to be dominated by the majority-spin $t_{2g}$ states. The FS-related fine structure in the MCPs is however substantial only in the [100] MCP, which contains features arising from the large majority-spin hole sheets. The overall shapes and widths of the experimental MCPs along all three directions investigated are in reasonably good accord with theoretical predictions, although some discrepancies indicating inadequacy of the LSDA in treating the magnetic states can be identified. We discuss details of the FS-related signatures in the first and higher BZs in the [100] MCP and show that high resolution magnetic Compton scattering experiments with a momentum resolution of 0.1 a.u. FWHM (full-width-at-half-maximum) or better will be necessary to observe this fine structure. We comment also on the feasibility of using positron annihilation spectroscopy in this connection.

cond-mat.str-el↗

arXiv2005

Extracting d-orbital occupancy from magnetic Compton scattering in bilayer manganites

We consider the shape of the magnetic Compton profile (MCP), $J_{mag}(p_z)$, in La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ for momentum transfer $p_z$ along the [110] direction and the associated reciprocal form factor $B(r)$ defined by taking the one-dimensional Fourier transform of $J_{mag}(p_z)$. $B(r)$ is shown to contain a prominent dip at $r\approx$ 1 $Å$, where the minimum value $B_{min}$ of $B(r)$ can be related to the occupancies of the $e_g$ orbitals of $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ symmetry in the system. We illustrate our procedure in detail by analyzing the measured MCP at 5K and the MCP computed within the framework of the local spin density approximation (LSDA) and comment on the differences between the measured and computed $e_g$ occupancies as a reflection of the limitations of the LSDA in treating electron correlation effects.

cond-mat.str-el↗

arXiv2004

Temperature dependent orbital degree of freedom in a bilayer manganite by magnetic Compton scattering

We have measured temperature-dependent magnetic Compton profiles (MCPs) from a single crystal of La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$. The MCPs, which involved the scattering of circularly polarized x-rays, are in general related to the momentum density of all the unpaired spins in the system. Nevertheless, we show that when the x-ray scattering vector lies along the [110] direction, the number of magnetic electrons of a specific symmetry, i.e. $d$-electrons of $x^2-y^2$ symmetry, yield a distinct signature in the MCP, allowing us to monitor substantial changes in the occupancy of the $d_{x^2-y^2}$ states over the investigated temperature range of 5-200K. This study indicates that magnetic Compton scattering can provide a powerful window on the properties of specific magnetic electrons in complex materials.

cond-mat.str-el↗

arXiv2001

Momentum-Resolved Charge Excitations in a Prototype One Dimensional Mott Insulator

We report momentum resolved charge excitations in a one dimensional (1-D) Mott insulator studied using high resolution (~ 325 meV) inelastic x-ray scattering over the entire Brillouin zone for the first time. Excitations at the insulating gap edge are found to be highly dispersive (momentum dependent)compared to excitations observed in two dimensional Mott insulators. The observed dispersion in 1-D is consistent with charge excitations involving holons which is unique to spin-1/2 quantum chain systems. These results point to the potential utility of inelastic x-ray scattering in providing valuable information about electronic structure of strongly correlated insulators.

cond-mat.str-el↗