Interplay betweenR4fand Fe3dstates in charge-ordered

Abstract

The electronic structures of R 4f and Fe 3d states of RFe 2 O 4 (R = Er, Tm, Lu) have been investigated by employing soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism at the Fe 2p and R 3d absorption edges. It is found that the valence states of Fe and R ions are nearly Fe 2.5+ and R 3+ , and that the net magnetic moments of Fe 2+ and Fe 3+ ions are antiparallel to each other. Both R 3d and O 1s XAS spectra show that the localized R 4f states do not contribute to the multiferroicity of RFe 2 O 4 . On the other hand, the magnetization data for RFe 2 O 4 at low temperature (T ) reveal the cluster glass behavior for R = Tm and Lu, but not for R = Er. This work suggests that the T -dependent structural differences among RFe 2 O 4 , caused by different R ions, are closely related to the magnetoelectricity at low temperature.

Hubbard Gap Modulation in Vanadium Dioxide Nanoscale Tunnel Junctions

“…With the addition of Hubbard U term, the d -orbitals split into a lower Hubbard band (LHB) and an upper Hubbard band (UHB) and eventually produce a Mott–Hubbard gap. The electronic state as projected in our PDOS computations agrees well with the orbital distributions as shown by Quackenbush et al From our first-principles study, we observe that U = 4 eV produces a Mott gap of 0.56 eV, which is in excellent agreement with the experimental as well as previous theoretical results of 0.6 V. ,,, Therefore, we considered DFT+U instead of conventional DFT along with NEGF formalism in our transport calculations of the M1 phase.…”

Optical and Electro-Optical Properties of YbFe2O4 Thin Films

“…[9,11,13,14] The ferrimagnetic transition due to the ordering of the Fe 2+ and Fe 3+ moments occurs around 240 K in this family. Similarly, the Fe 2+ -Fe 3+ charge-ordered state has been reported in LuFe 2 O 4 [15,16,17] and in YbFe 2 O 4 [18,19,20]. Such charge ordering is believed to break the space-inversion symmetry, leading to macroscopic ferroelectric polarization.…”

Metallic Diluted Dimerization in VO2 Tweeds

“…Although no general consensus exists on the reliability of band‐theory methods to predict energy gaps and magnetic orderings of VO 2 phases, [ 46–48 ] including an electronic Hubbard repulsion term on vanadium 3d orbitals has been shown to give accurate results for structural and vibrational properties. [ 9,49 ] Here, we use a U = 4 eV on top of the Perdew–Burke–Ernzerhof (PBE) exchange‐correlation functional. In agreement with previous results, in the harmonic approximation we observed the presence of soft phonons in the rutile phase that has been used to explain the transition to the M1 phase.…”

“…In agreement with previous results, in the harmonic approximation we observed the presence of soft phonons in the rutile phase that has been used to explain the transition to the M1 phase. [ 49 ] A finite‐difference phonon calculation using a supercell compatible with the ×3 periodicity gives a soft phonon mode that favors dimerization with an intermediate “isolated” V site. Even though these soft phonons could be stabilized by anharmonic contributions, [ 9 ] guided by the above experimental findings, we performed full relaxation of the structure following the vibration eigenmodes, resulting in a metallic phase with two out of three V ions forming dimers along c R ‐axis.…”