Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userBulk Band Gaps in Divalent Hexaborides: A Soft X-Ray Emission Study
56
0
0.0
(
0
)
Added by
Jonathan D. Denlinger
Publication date
2001
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
Boron K-edge soft x-ray emission and absorption are used to address the fundamental question of whether divalent hexaborides are intrinsic semimetals or defect-doped bandgap insulators. These bulk sensitive measurements, complementary and consistent with surface-sensitive angle-resolved photoemission experiments, confirm the existence of a bulk band gap and the location of the chemical potential at the bottom of the conduction band.
rate research
Read More
Complementary angle-resolved photoemission and bulk-sensitive k-resolved resonant inelastic x-ray scattering of divalent hexaborides reveal a >1 eV X-point gap between the valence and conduction bands, in contradiction to the band overlap assumed in several models of their novel ferromagnetism. This semiconducting gap implies that carriers detected in transport measurements arise from defects, and the measured location of the bulk Fermi level at the bottom of the conduction band implicates boron vacancies as the origin of the excess electrons. The measured band structure and X-point gap in CaB_6 additionally provide a stringent test case for proper inclusion of many-body effects in quasi-particle band calculations.
We present a study of resonant inelastic X-ray scattering (RIXS) spectra collected at the rare-earth $L$ edges of divalent hexaborides YbB$_6$ and EuB$_6$. In both systems, RIXS-active features are observed at two distinct resonances separated by $sim10$ eV in incident energy, with angle-dependence suggestive of distinct photon scattering processes. RIXS spectra collected at the divalent absorption peak strongly resemble the unoccupied 5$d$ density of states calculated using density functional theory, an occurrence we ascribe to transitions between weakly-dispersing 4$f$ and strongly dispersing 5$d$ states. In addition, anomalous resonant scattering is observed at higher incident energy, where no corresponding absorption feature is present. Our results suggest the far-reaching utility of $L$-edge RIXS in determining the itinerant-state properties of $f$-electron materials.
The evolution of electron correlation in Sr$_{x}$Ca$_{1-x}$VO$_3$ has been studied using a combination of bulk-sensitive resonant soft x-ray emission spectroscopy (RXES), surface-sensitive photoemission spectroscopy (PES), and ab initio band structure calculations. We show that the effect of electron correlation is enhanced at the surface. Strong incoherent Hubbard subbands are found to lie ~ 20% closer in energy to the coherent quasiparticle features in surface-sensitive PES measurements compared with those from bulk-sensitive RXES, and a ~ 10% narrowing of the overall bandwidth at the surface is also observed.
Using resonant soft X-ray techniques we follow the magnetic behavior of a strained epitaxial film of CoCr2O4, a type-II multiferroic. The film is [110]-oriented, such that both the ferroelectric and ferromagnetic moments can coexist in plane. X-ray magnetic circular dichroism (XMCD) is used in scattering and in transmission modes to probe the magnetization of Co and Cr separately. The transmission measurements utilized X-ray excited optical luminescence from the substrate. Resonant soft X-ray diffraction (RSXD) was used to study the magnetic order of the low temperature phase. The XMCD signals of Co and Cr appear at the same ordering temperature Tc~90K, and are always opposite in sign. The coercive field of the Co and of Cr moments is the same, and is approximately two orders of magnitude higher than in bulk. Through sum rules analysis an enlarged Co2+ orbital moment (m_L) is found, which can explain this hardening. The RSXD signal of the (q q 0) reflection appears below Ts, the same ordering temperature as the conical magnetic structure in bulk, indicating that this phase remains multiferroic under strain. To describe the azimuthal dependence of this reflection, a slight modification is required to the spin model proposed by the conventional Lyons-Kaplan-Dwight-Menyuk theory for magnetic spinels. Lastly, a slight increase in reflected intensity is observed below Ts=27K when measuring at the Cr edge (but not at the Co edge).
This paper has been withdrawn by the authors due to new theoretical evidence and experimental proof that the semiconducting bandgap reported in this paper and ascribed to a surface region is in fact a bulk property of divalent hexaborides. As reported in J. D. Denlinger et al., cond-mat/0107429, which supercedes this paper, bulk-sensitive boron K-edge soft x-ray emission provides a complementary confirmation of the X-point band gap identified by angle-resolved photoemission.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
