Large Crystal Field Splitting Research Articles

Ce[formula omitted] ion-induced distortion in a perovskite-structured KMgF3 single crystal

We investigate the local structural distortion induced by ▪ ion in a (cubic) perovskite-structured KMgF3 crystal for potential vacuum ultraviolet (VUV) applications. The crystal with 1.0 mol% nominal doping concentration exhibits five absorption bands centered at 196, 203, 209, 227, and 233 nm and two emission bands centered at 266 and 282 nm. These absorption and emission bands are attributed to the 4f−5d transitions of the ▪ ion. Our Ce K- and ▪-edge XAS analyses confirm the presence of ▪ oxidation state in the KMgF3 crystal. Our analyses reveal a significantly distorted coordination environment which has a coordination number (CN) of 5.03±0.55 and an average Ce–F bond length of 2.44±0.01 Å. Strong mixing of the 4f and 5d states and large ligand-field effects are also implied in the Ce ▪-edge XANES spectrum, as evidenced by an intense pre-edge peak. Optically, these mixing and ligand field effects conform to the large crystal field splitting of 1.03 eV (8285 ▪ ). Although this was the case, ▪ -doped KMgF3 remains a candidate VUV optical material due to its small Stokes shift of 0.64eV (5141 ▪ ).

Spin–orbit exciton–induced phonon chirality in a quantum magnet

The interplay of charge, spin, lattice, and orbital degrees of freedom in correlated materials often leads to rich and exotic properties. Recent studies have brought new perspectives to bosonic collective excitations in correlated materials. For example, inelastic neutron scattering experiments revealed non-trivial band topology for magnons and spin-orbit excitons (SOEs) in a quantum magnet CoTiO3 (CTO). Here, we report phonon properties resulting from a combination of strong spin-orbit coupling, large crystal field splitting, and trigonal distortion in CTO. Specifically, the interaction between SOEs and phonons endows chirality to two [Formula: see text] phonon modes and leads to large phonon magnetic moments observed in magneto-Raman spectra. The remarkably strong magneto-phononic effect originates from the hybridization of SOEs and phonons due to their close energy proximity. While chiral phonons have been associated with electronic topology in some materials, our work suggests opportunities may arise by exploring chiral phonons coupled to topological bosons.

Geometry and electronic structure of Yb(III)[CH(SiMe3)2]3 from EPR and solid-state NMR augmented by computations.

Characterization of paramagnetic compounds, in particular regarding the detailed conformation and electronic structure, remains a challenge, and - still today it often relies solely on the use of X-ray crystallography, thus limiting the access to electronic structure information. This is particularly true for lanthanide elements that are often associated with peculiar structural and electronic features in relation to their partially filled f-shell. Here, we develop a methodology based on the combined use of state-of-the-art magnetic resonance spectroscopies (EPR and solid-state NMR) and computational approaches as well as magnetic susceptibility measurements to determine the electronic structure and geometry of a paramagnetic Yb(III) alkyl complex, Yb(III)[CH(SiMe3)2]3, a prototypical example, which contains notable structural features according to X-ray crystallography. Each of these techniques revealed specific information about the geometry and electronic structure of the complex. Taken together, both EPR and NMR, augmented by quantum chemical calculations, provide a detailed and complementary understanding of such paramagnetic compounds. In particular, the EPR and NMR signatures point to the presence of three-centre-two-electron Yb-γ-Me-β-Si secondary metal-ligand interactions in this otherwise tri-coordinate metal complex, similarly to its diamagnetic Lu analogues. The electronic structure of Yb(III) can be described as a single 4f13 configuration, while an unusually large crystal-field splitting results in a thermally isolated ground Kramers doublet. Furthermore, the computational data indicate that the Yb-carbon bond contains some π-character, reminiscent of the so-called α-H agostic interaction.

Realization of flat bands by lattice intercalation in kagome metals

Recently there has been intense interest in kagome metals, which are expected to host flat bands (FBs). However, the observed ``FBs'' are not flat over the whole two-dimensional Brillouin zone and overlap strongly with other bands. In fact, the FB does not truly exist in a default $d$-orbital kagome lattice, and the conditions for its existence in kagome metals are unknown. Here, based on tight-binding model analyses of the interplay between orbital and lattice symmetry, we establish such conditions. We show that for a single $d$-orbital kagome lattice assuming large crystal field splitting (CFS), only the ${d}_{{z}^{2}}$ orbital gives rise to a FB, while ${d}_{xy}, {d}_{{x}^{2}\ensuremath{-}{y}^{2}}, {d}_{xz}$, and ${d}_{yz}$ orbitals can only produce a FB with a rotated $d$-orbital basis so that they conform with the underlying kagome lattice symmetry. Most importantly, we demonstrate that both conditions of $d$-orbital rotation and large CFS can be ideally satisfied by intercalating the kagome lattice with a hexagonal sublattice without disrupting the destructive interference of FB wave function. Furthermore, we propose layered metalorganic frameworks as promising candidate kagome metals to realize FBs.

Understanding the Magnetic Relaxation Mechanism in Mixed-Valence Dilanthanide Complexes with Metal-Metal Bonding: A Theoretical Investigation.

Theoretical investigations on mixed-valence dilanthanide complexes (CpiPr5)2Ln2I3 (Ln = Tb, Dy, and Ho) indicate that the total spin of the 4f shell couples preferentially to the σ electron spin and then to the orbital angular momentum, improving the strength of spin-orbit coupling (SOC) for each magnetic center. On the other hand, the concentration of negative charges containing the delocalized σ electron in the axial direction leads to a large crystal-field (CF) splitting. Both strong SOC and large CF splitting lead to the largest energy barrier Ueff of such complexes up to now. In addition, our calculations show that the introduction of σ electron can better suppress the quantum tunneling of magnetization in the ground spin-orbit state, and the Ueff of (CpiPr5)2Ln2I3 is expected to originate from the contribution of both Ln ions under such strong Ln-σ exchange coupling.

Effects of the surface termination and the oxygen vacancy position on LaNiO3 ultra-thin films: First-principles study

While ultra-thin layers of the ${\mathrm{LaNiO}}_{3}$ film exhibit a remarkable metal-insulator transition as the film thickness becomes smaller than a few unit cell (u.c.), the formation of possible oxygen vacancies and their effects on the correlated electronic structure have been rarely studied using first principles. Here, we investigate the effects of the surface termination and the oxygen vacancy position on the electronic properties and vacancy energetics of ${\mathrm{LaNiO}}_{3}$ ultra-thin films under the compressive strain using density functional theory plus U $(\mathrm{DFT}+\mathrm{U})$. We find that oxygen vacancies can be easily formed in the Ni layers with the ${\mathrm{NiO}}_{2}$ terminated surface (0.5 u.c. and 1.5 u.c. thickness) compared to the structures with the LaO terminated surface and the in-plane vacancy is energetically favored than the out-of-plane vacancy. When two vacancy sites are allowed, the Ni square plane geometry is energetically more stable in most cases as two oxygen vacancies tend to stay near a Ni ion. Strong anisotropy between the in-plane and out-of-plane vacancy formation as well as the layer and orbital dependent electronic structure occur due to strain, surface termination, charge reconstruction, and quantum confinement effects. The in-plane vacancy of the ${\mathrm{NiO}}_{2}$ terminated structure is favored since the released charge due to the oxygen vacancy can be easily accommodated in the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbital, which is less occupied than the ${d}_{{z}^{2}}$ orbital. Remarkably, the oxygen vacancy structure containing the Ni square-plane geometry becomes an insulating state in $\mathrm{DFT}+\mathrm{U}$ with a sizable band gap of 1.2 eV because the large crystal field splitting between ${d}_{{z}^{2}}$ and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbitals in the square-plane favors an insulating state and the Mott insulating state is induced in other Ni sites due to strong electronic correlations. In the thin-film structure without oxygen vacancies, the strong correlation effect in $\mathrm{DFT}+\mathrm{U}$ drives a pseudogap ground state at the Fermi energy, similarly as the experimental photo-emission spectra; however, the variation of the $\mathrm{DFT}+\mathrm{U}$ electronic structure depending on the surface termination becomes weaker compared to those obtained in DFT.

Off‐Stoichiometry Quaternary Heusler‐Like Semiconductors with Magnetism and Disorder

AbstractThe theoretical finding of a large group of intrinsic off‐stoichiometry quaternary Heusler‐like semiconductors with mixing and tunable occupation of 4c‐4d atomic sites, such as FexCoyTiSb with non‐integer x + y > 1 are reported. Those semiconductors are never reported before, and they radically break the well‐recognized 18‐ (or 24‐) valence electron counting (VEC) rule and the exact 4c‐ (or both 4c‐ and 4d‐) atomic sites stoichiometry for the traditional quaternary half‐Heusler (or full‐Heusler) semiconductors. Physically, the novel semiconductors can be designed by following a d‐orbital‐determined compensation rule. The extra atoms fill in the tetrahedral vacancies of the half‐Heusler structure, introduce symmetry‐constrained d orbitals near the Fermi level, large crystal field splitting, and then form a bandgap at appropriate fractional compositions, which leads to an exact compensated electronic structure. The newly established compensation rule reveals an abundant phase space of the quaternary Heusler‐like (2x + 3y = 3 for VECY+Z = 9, 2x + 3y = 4 for VECY+Z = 8, and 2x + 3y = 5 for VECY+Z = 7) compounds. High‐throughput screenings reveal many new FexCoyYZ semiconductors, all of which possess strong disorder, weak magnetism, spin compensation, and wide composition‐range tunability. The intertwined orders may either find functionalities surpassing the existing materials or give rise to new potential applications.

Theoretical analysis of single-ion anisotropy in d3 Mott insulators

An effective spin model for Mott insulators is determined by the symmetries involved among magnetic sites, electron fillings, and their interactions. Such a spin Hamiltonian offers insight to mechanisms of magnetic orders and magnetic anisotropy beyond the Heisenberg model. For a spin moment S bigger than 1/2, single-ion anisotropy is in principle allowed. However, for $d^3$ Mott insulators with large cubic crystal field splitting, the single-ion anisotropy is absent within the LS coupling, despite S = 3/2 local moment. On the other hand, preferred magnetic moment directions in $d^3$ materials have been reported, which calls for a further theoretical investigation. Here we derive the single-ion anisotropy interaction using the strong-coupling perturbation theory. The cubic crystal field splitting including $e_g$ orbitals, trigonal distortions, Hund's coupling, and spin-orbit coupling beyond the LS scheme are taken into account. For compressed distortion, the spin-orbit coupling at magnetic sites can favor either the easy-axis or the easy-plane while that of anions leads to easy-axis anisotropy. We apply the theory on $\rm{CrX}_3$ with X = Cl and I, and show the dependence of the single-ion anisotropy on the strength of the spin-orbit couplings of both magnetic and anion sites. Significance of the single-ion anisotropy in ideal two-dimensional magnets is also discussed.

Strongly anisotropic electronic and magnetic structures in oxide dichlorides RuOCl2 and OsOCl2

Here, using density functional theory and density matrix renormalization group methods, we investigate the electronic and magnetic properties of RuOCl$_2$ and OsOCl$_2$ with $d^4$ electronic configurations. Different from a previous study using VOI$_2$ with $d^1$ configuration, these systems with $4d^4$ or $5d^4$ do not exhibit a ferroelectric instability along the $a$-axis. Due to the fully-occupied $d_{xy}$ orbital in RuOCl$_2$ and OsOCl$_2$, the Peierls instability distortion disappears along the $b$-axis, leading to an undistorted I${\rm mmm}$ phase (No. 71). Furthermore, we observe strongly anisotropic electronic and magnetic structures along the $a$-axis. The large crystal-field splitting energy (between $d_{xz/yz}$ and $d_{xy}$ orbitals) and large hopping between nearest-neighbor Ru and Os atoms suppresses the spin-orbital effect in $M$OCl$_2$ ($M$ = Ru or Os) with electronic density $n = 4$, resulting in a spin-1 system instead of a $J = 0$ singlet ground state. Moreover, we find staggered antiferromagnetic order with $\pi$ wavevector along the $M$-O chain direction ($a$-axis) while the magnetic coupling along the $b$-axis is weak. Based on Wannier functions from first-principles calculations, we calculated the relevant hopping amplitudes and crystal-field splitting energies of the $t_{2g}$ orbitals for the Os atoms to construct a multi-orbital Hubbard model for the $M$-O chains. Staggered AFM with $\uparrow$-$\downarrow$-$\uparrow$-$\downarrow$ spin structure dominates in our DMRG calculations, in agreement with DFT calculations.

Magnetic anisotropy of the van der Waals ferromagnet Cr2Ge2Te6 studied by angular-dependent x-ray magnetic circular dichroism

The van der Waals ferromagnet ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ (CGT) has a two-dimensional crystal structure where each layer is stacked through van der Waals force. We have investigated the nature of the ferromagnetism and the weak perpendicular magnetic anisotropy (PMA) of CGT by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) studies of CGT single crystals. The XMCD spectra at the Cr ${L}_{2,3}$ edge for different magnetic field directions were analyzed on the basis of the cluster-model multiplet calculation. The Cr valence is confirmed to be $3+$ and the orbital magnetic moment is found to be nearly quenched, as expected for the high-spin ${t}_{2g}^{3}$ configuration of the ${\mathrm{Cr}}^{3+}$ ion. A large ($\ensuremath{\sim}0.2$ eV) trigonal crystal-field splitting of the ${t}_{2g}$ level caused by the distortion of the $\mathrm{Cr}{\mathrm{Te}}_{6}$ octahedron has been revealed, while the single-ion anisotropy (SIA) of the Cr atom is found to have a sign opposite to the observed PMA and too weak compared to the reported anisotropy energy. The present result suggests that anisotropic exchange coupling between the Cr atoms through the ligand Te $5p$ orbitals having strong spin-orbit coupling has to be invoked to explain the weak PMA of CGT, as in the case of the strong PMA of ${\mathrm{CrI}}_{3}$.

Ferromagnetic and ferroelectric 5d1 insulator Ba5(OsO5)3Cl

Poly- and single-crystalline samples of ${\mathrm{Ba}}_{5}{(\mathrm{Os}{\mathrm{O}}_{5})}_{3}\mathrm{Cl}$ have been successfully synthesized. ${\mathrm{Ba}}_{5}{(\mathrm{Os}{\mathrm{O}}_{5})}_{3}\mathrm{Cl}$ crystallizes in the noncentrosymmetric space group $P{6}_{3}cm$. ${\mathrm{Ba}}_{5}{(\mathrm{Os}{\mathrm{O}}_{5})}_{3}\mathrm{Cl}$ shows a ferromagnetic order below 5 K and a possible ferroelectric order below 150 K. A multiferroic state with both ferromagnetic and ferroelectric ordering is established, although the ferromagnetic and ferroelectric orders are not coupled. The experimental effective moment and the saturated magnetization for ${\mathrm{Ba}}_{5}{(\mathrm{Os}{\mathrm{O}}_{5})}_{3}\mathrm{Cl}$ are close to the spin-only value, indicating that the spin-orbit coupling does not play an important role, which is due to the large crystal field and spin splitting in ${\mathrm{Ba}}_{5}{(\mathrm{Os}{\mathrm{O}}_{5})}_{3}\mathrm{Cl}$.

Rapid Synthesis of Red‐Emitting Sr2Sc0.5Ga1.5O5:Eu2+ Phosphors and the Tunable Photoluminescence Via Sr/Ba Substitution

AbstractDiscovering new Eu2+‐doped red‐emitting phosphors in oxide‐based materials is a challenge for white light‐emitting diode (WLED) applications. Herein, a highly efficient high‐frequency induction heating method is employed to rapidly prepare the red‐emitting Sr2Sc0.5Ga1.5O5:Eu2+ phosphors peaking at 614 nm and exhibiting a high photoluminescence quantum yield of 78.4% under the excitation of 440 nm. The structural and spectral analyses suggest that Eu2+ ions tend to enter the [Sc1/Ga1O6] and [Ga2O6] polyhedrons with small coordination numbers, leading to the broadband red emission originated from large crystal field splitting of Eu2+ 5d level. The chemical substitution of Ba in the Sr site enhances the thermal stability and helps to the photoluminescence tuning from 614 to 728 nm in SrBaSc0.5Ga1.5O5:Eu2+. The WLED device fabricated by blending the red Sr1.7Ba0.3Sc0.5Ga1.5O5:Eu2+ and yellow Y3(Al, Ga)5O12:Ce3+ phosphors shows a high color‐rendering index (Ra = 91.1), and low color‐correlated temperature (CCT = 4750 K). This study aims to provide a new synthesis method and design principle for guiding the development of Eu2+‐doped oxide‐based red phosphors with low preparation cost; moreover, the photoluminescence tuning strategy via cation substitutions is essential to achieve tunable emission, even the near‐infrared luminescence.

Strong enhancement of magnetic order from bulk to stretched monolayer FeSe as Hund\u2019s metals

Despite of the importance of magnetism in possible relation to other key properties in iron-based superconductors, its understanding is still far from complete especially for FeSe systems. On one hand, the origin of the absence of magnetic orders in bulk FeSe is yet to be clarified. On the other hand, it is still not clear how close monolayer FeSe on SrTiO3, with the highest transition temperature among iron-based superconductors, is to a magnetic instability. Here we investigate magnetic properties of bulk and monolayer FeSe using dynamical mean-field theory combined with density-functional theory. We find that suppressed magnetic order in bulk FeSe is associated with the reduction of interorbital charge fluctuations, an effect of Hund’s coupling, enhanced by a larger crystal-field splitting. Meanwhile, spatial isolation of Fe atoms in expanded monolayer FeSe leads into a strong magnetic order, which is completely destroyed by a small electron doping. Our work provides a comprehensive understanding of the magnetic order in iron-based superconductors and other general multi-orbital correlated systems as Hund’s metals.

Elastic strain control of electronic structure, and magnetic properties of [Pr1−xCaxMnO3/SrTiO3]15 superlattices

We report the growth, electronic structure, and in-plane magnetic properties of pulsed laser deposition grown 2D superlattice structures [Pr0.7Ca0.3MnO3/SrTiO3]15 and [Pr0.5Ca0.5MnO3/SrTiO3]15 on (001) oriented SrTiO3 and LaAlO3 single crystal substrates. The x-ray reflectivity measurements reveal well-defined interfaces between the manganite and titanate layers along with the existence of Kiessig fringes, providing the evidence for the smooth periodic superlattice structure. The reciprocal space mapping provides signature of tetragonal distortion in all the superlattices. The electronic structure determined from the x-ray photoelectron spectroscopy reveals divalent Sr and Ca, tetravalent Ti, and mixed valent Mn with a pronounce shift of binding energy peaks toward the higher energy side in the superlattices grown on (001) oriented LaAlO3 as compared to those grown on SrTiO3. These superlattices exhibit highly anisotropic ferromagnetic character. We used the law of approach to saturation to determine the anisotropy field (HK) and cubic anisotropy constant (K1) for all the investigated superlattices. This analysis yields the highest HK∼9 kOe and K1∼8×105 erg/cc for the [Pr0.7Ca0.3MnO3/SrTiO3]15 superlattice system. Furthermore, significant enhancement of the overall magnetic moment and a decrease in TC (<100 K) was observed in the case of LaAlO3 grown superlattice, which indicates a substantial role of residual elastic strain on the magnetic ordering. Our results indicate that the strain induced elongation of MnO6 octahedra leads to finite possibility of non-orthogonal overlapping of orbitals in the presence of large crystal field splitting of eg levels, which, in turn, causes suppression of the ferromagnetic double exchange interaction.

Massive red shift of Ce3+ in Y3Al5O12 incorporating super-high content of Ce.

In light emitting diodes, Y3Al5O12:Ce (YAG:Ce) is used as a yellow phosphor in combination with blue LEDs but lacks a red component in emission. Therefore, considerable efforts have been directed toward shifting the emission of YAG:Ce to longer wavelengths. In this study, a Y3Al5O12 (YAG) crystal incorporating a high content of Ce, (Y1−xCex)3Al5O12 (0.006 ≦ x ≦ 0.21), was successfully prepared by a polymerized complex method in which low-temperature annealing (650–750 °C) was employed prior to sintering at 1080 °C. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis indicated that the obtained sample was a single phase YAG crystal with x ≤ 0.21. Interestingly, orange-red emission was observed with x ≥ 0.07 with UV-blue light irradiation. With excitation at 450 nm, the emission peak increases from 538 nm (x = 0.006) to 606 nm (x = 0.21). This massive red shift in the high-x region was not observed without the 1st step of low-temperature annealing, which implied that low-temperature annealing was essential for incorporating a high concentration of Ce. The precursor formed by low-temperature annealing was amorphous at x = 0.04, whereas CeO2 nanocrystals were formed in the amorphous material with x ≥ 0.11, based on the XRD and TEM results. CeLIII X-ray absorption edge structure revealed that Ce existed as Ce4+ in the precursor and Ce3+ in the obtained crystal. It was speculated that CeO2 was formed at low temperature, releasing oxygen, with sintering at 1080 °C, leading to the incorporation of Y3+ in the Ce–O framework. The lattice constant increased significantly from 12.024 Å to 12.105 Å with increasing x, but the crystal field splitting did not increase and was constant from x = 0.06 to x = 0.21. Hence, the massive red shift in emission was not explained by the large crystal field splitting, but instead by the Stokes shift.

Odd-Parity Multipoles by Staggered Magnetic Dipole and Electric Quadrupole Orderings in CeCoSi

We investigate a possibility of odd-parity multipole orderings in a locally noncentrosymmetric tetragonal compound CeCoSi. By performing symmetrical and microscopic mean-field analyses on a two-orbital tight-binding model, we propose potential odd-parity multipoles hidden in staggered antiferromagnetic and antiferroquadrupole orderings in CeCoSi. We show that $3z^2-r^2$ type of the magnetic quadrupole is induced by the staggered magnetic dipole ordering for a large crystal-field splitting between two orbitals, while $xy$ type of the electric toroidal quadrupole is emergent by the staggered electric quadrupole ordering for a small crystal-field splitting. Furthermore, we discuss a magneto-electric effect and elastic-electric effect due to the odd-parity multipoles, which will be useful to identify order parameters in CeCoSi.

Performance evaluation of Ce3+ doped flexible PVDF fibers for efficient optical pressure sensors

This work proposes a highly novel centrifugally spun lanthanide doped nonwoven fibrous mat as a non-contact optical pressure sensor with a wide linear dynamic range and good pressure sensitivity. The sensor properties are based on the spectral shift, broadening and intensity enhancement of Ce3+ ion in Ce doped PVDF fiber upto significantly high pressure. Two different systems: Ce(NO3)3·6H2O and (NH4)4Ce(SO4)4·2H2O doped PVDF flexible fibers (CeN-PF and CeS-PF) were produced using the Forcespinning® technique. Both CeN-PF and CeS-PF fibers displayed violet-blue emission under UV irradiation due to a 5d-4f transition of Ce3+ ions. Our emission results show that both CeN-PF and CeS-PF spectral characteristics are influenced by high pressures, inducing significant spectral ref shift in 5d-4f. The pressure-induced monotonous changes in bandwidth and emission intensity enhancement along with red shift suggesting the potential application of these fibers for pressure sensing applications. The CeN-PF fiber exhibits a high-pressure sensitivity (dλ/dP ≈ 0.28 nm/GPa) under a comprehensive linear dynamic range (0–64 GPa) with no pressure-induced luminescence quenching. The changes in CeS-PF is less pronounced with a lower pressure sensitivity of 0.10 nm/GPa compared to CeN-PF due to large crystal field splitting energy of nitrate ion compared to sulphate ion. This work presents a highly efficient, cost effective, scalable lanthanide doped flexible fibrous based system with negligible high pressure quenching and a wider linear dynamic range for optical pressure sensing applications.

High electron mobility with significant spin-orbit coupling in rock-salt YbO epitaxial thin film

We studied the optical and electrical properties of ytterbium monoxide (YbO) epitaxial thin films with unusual Yb2+ (4f145d0) valence grown by the pulsed laser deposition method. The narrow bandgap of 0.25 eV and the large crystal field splitting of 5d orbitals in YbO determined from absorption spectra were consistent with the chemical trends of ytterbium monochalcogenides (YbChs). Electrical resistivity was tunable upon electron doping via introduction of oxygen vacancies. Electron mobility at room temperature increased up to ∼13 cm2 V−1 s−1 with increasing electron carrier density. The heavily electron-doped YbO showed weak antilocalization at low temperature, suggesting significant spin-orbit coupling owing to the heavy Yb nucleus.

Anomalous orbital structure in two-dimensional titanium dichalcogenides

Generally, lattice distortions play a key role in determining the electronic ground states of materials. Although it is well known that trigonal distortions are generic to most two dimensional transition metal dichalcogenides, the impact of this structural distortion on the electronic structure and topological properties has not been understood conclusively. Here, by using a combination of polarization dependent X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) and atomic multiplet cluster calculations, we have investigated the electronic structure of titanium dichalcogenides TiX2 (X = S, Se, Te), where the magnitude of the trigonal distortion increase monotonically from S to Se and Te. Our results reveal the presence of an anomalously large crystal field splitting. This unusual kind of crystal field splitting is likely responsible for the unconventional electronic structure of TiX2 compounds and ultimately controls the degree of the electronic phase protection. Our findings also indicate the drawback of the distorted crystal field picture in explaining the observed electronic ground state and emphasize the key importance of trigonal symmetry, metal-ligand hybridization and electron-electron correlations in defining the electronic structures at the Fermi energy.

A Terminal Fluoride Ligand Generates Axial Magnetic Anisotropy in Dysprosium Complexes.

The first dysprosium complexes with a terminal fluoride ligand are obtained as air-stable compounds. The strong, highly electrostatic dysprosium-fluoride bond generates a large axial crystal-field splitting of the J=15/2 ground state, as evidenced by high-resolution luminescence spectroscopy and correlated with the single-molecule magnet behavior through experimental magnetic susceptibility data and ab initio calculations.