arXiv subjects

M. M. Ferreira-Carvalho

Publications and source records attributed to M. M. Ferreira-Carvalho.

3 recordsLinked to original sources

arXiv2026

Single-Crystal Growth and Physical Properties of the Antiferromagnetic Double Perovskite Ba_2CoMoO_6

This work presents a comprehensive investigation of the structural, magnetic, and electronic properties of the double perovskite Ba$_2$CoMoO$_6$ (BCMO). Single crystals were grown via floating-zone and Czochralski growth techniques and characterized using a set of complementary methods. X-ray diffraction analysis confirmed that BCMO crystallizes in a face-centered cubic (FCC) structure with space group $Fm\overline{3}m$. Magnetic susceptibility measurements exhibit antiferromagnetic ordering below $T_{\mathrm{N}} = 20.1(1)$~K. A spin-flop transition is observed at 26.5~kOe. Heat capacity measurements and entropy analysis are consistent with a $J_{\mathrm{eff}} = 1/2$ ground state for Co$^{2+}$ ions. X-ray absorption spectroscopy provided insight into the local electronic structure, revealing the spin-orbit and crystal field effects. The cluster-model analysis yields a $g$-factor of 4.36, consistent with a spin-orbit-entangled ground state. Surface photovoltage spectroscopy demonstrates a strong near surface optical response with an onset near 1.5~eV and a pronounced feature at 2.65~eV, consistent with visible range Co--O--Mo transitions in line with related Ba-based double perovskites, highlighting its potential for spintronic and energy conversion applications.

cond-mat.str-el↗

arXiv2026

Direct Evidence of a Near-Ideal Jeff = 1/2 Ground State in Triangular-Lattice Na2BaCo(PO4)2

We investigated the local Co 3d electronic structure of Na2BaCo(PO4)2 using polarization-dependent X-ray absorption spectroscopy (XAS) in combination with full multiplet cluster calculations. We employed the line-fitting inverse partial fluorescence yield (IPFY) technique to obtain accurate XAS spectra from strong insulating materials. Our combined experimental and theoretical analysis reveals a very small effective trigonal distortion of only 11 meV in the CoO6 octahedra, indicating a close to ideal condition to render a ground state with the Jeff = 1/2 character. With our cluster model we were also able to simulate magnetic susceptibility measurements along different directions in the crystal. These findings highlight Na2BaCo(PO4)2 as a promising platform for exploring exotic magnetic phenomena associated with Jeff = 1/2 ground states on triangular lattices.

cond-mat.str-el↗

arXiv2025

Trigonal distortion in the Kitaev candidate honeycomb magnet BaCo2(AsO4)2

We conducted x-ray absorption (XAS) and magnetic circular dichroism (XMCD) measurements at the Co $L_{2,3}$ edges on single crystals of the Kitaev candidate honeycomb lattice compound BaCo$_2$(AsO$_4$)$_2$. The measurements employed the inverse partial fluorescence yield technique, which is ideal for acquiring reliable x-ray absorption spectra from highly insulating samples, enabling precise quantitative analysis. Our experimental results revealed a significant linear dichroic signal, indicating strong trigonal distortion in the CoO$_{6}$ octahedra in BaCo$_2$(AsO$_4$)$_2$. We performed a detailed analysis of the experimental XAS and XMCD spectra using a full-multiplet configuration-interaction cluster model. This analysis unveiled that the $t_{2g}$ hole density is predominantly localized in the $a_{1g}$ orbital. Through XMCD sum rules and theoretical calculations, we quantified both the spin and orbital magnetic moments. Our study demonstrates that the local electronic structure of the CoO$_{6}$ octahedra displays an effective trigonal distortion of approximately $-0.114$ eV. This distortion is larger than the Co $3d$ spin-orbit coupling constant, emphasizing the crucial impact of local structural distortions on the electronic and magnetic properties of BaCo$_2$(AsO$_4$)$_2$.

cond-mat.str-el↗