Showing 1–50 of 140 results for author: Tjeng, L H

Synthesis and structural validation of close-to-stoichiometric NiTe$_2$ single crystals

Authors: T. Tirasutt, L. H. Tjeng, A. C. Komarek

Abstract: We report the synthesis of stoichiometric NiTe$_{2}$ single crystals via chemical vapor transport together with precise structural characterization. A combined analysis using powder X-ray diffraction (XRD) and single crystal XRD was performed to determine lattice parameters and structural details. High-resolution single crystal XRD reveals no evidence of interstitial nickel, confirming a close-to-… ▽ More We report the synthesis of stoichiometric NiTe$_{2}$ single crystals via chemical vapor transport together with precise structural characterization. A combined analysis using powder X-ray diffraction (XRD) and single crystal XRD was performed to determine lattice parameters and structural details. High-resolution single crystal XRD reveals no evidence of interstitial nickel, confirming a close-to-stoichiometric composition. This work establishes a reliable benchmark for the crystal structure of stoichiometric NiTe$_{2}$, offering a reference for future studies on its physical properties. △ Less

Submitted 3 August, 2026; originally announced August 2026.

Probing La-based nickelates with Ni 1$s$ core-level photoelectron spectroscopy

Authors: Daisuke Takegami, Naoki Ito, Koto Fujinuma, Masato Yoshimura, Grace A. Pan, Dan Ferenc Segedin, Qi Song, Hanjong Paik, Charles M. Brooks, Hanjie Guo, Alexander C. Komarek, Takanori Taniguchi, Masaki Fujita, Julia A. Mundy, Takashi Mizokawa, Liu Hao Tjeng, Berit H. Goodge, Atsushi Hariki

Abstract: We present a comparative Ni core level photoemission study of La$_3$Ni$_2$O$_7$, Nd$_3$Ni$_2$O$_7$, and LaNiO$_3$ using both the Ni $2p$ and the Ni $1s$. We address the challenges in analyzing the widely investigated Ni $2p$ spectra arising from the substantial overlap in energy of the Ni $2p$ with the La $3d$. We show that on the other hand the deep Ni $1s$ core level does provide a clean view on… ▽ More We present a comparative Ni core level photoemission study of La$_3$Ni$_2$O$_7$, Nd$_3$Ni$_2$O$_7$, and LaNiO$_3$ using both the Ni $2p$ and the Ni $1s$. We address the challenges in analyzing the widely investigated Ni $2p$ spectra arising from the substantial overlap in energy of the Ni $2p$ with the La $3d$. We show that on the other hand the deep Ni $1s$ core level does provide a clean view on the intrinsic electronic excitations and we highlight its potential to resolve detailed differences in the electronic structure within the strongly correlated Ruddlesden-Popper series La$_{n+1}$Ni$_n$O$_{3n+1}$. △ Less

Submitted 16 June, 2026; originally announced June 2026.

The Kondo effect in ferromagnetic quantum critical CeRh$_6$Ge$_4$

Authors: Martin Sundermann, Joe D. Thompson, Eric D. Bauer, Chun Fu Chang, Sheng-Huai Chen, Chang-Yang Kuo, Liu Hao Tjeng, Getrud Zwicknagl, Andrea Severing

Abstract: The mechanism of a pressure-induced quantum critical point in the heavy fermion ferromagnet CeRh$_6$Ge$_4$ has attracted interest, as ferromagnetic quantum criticality in a clean itinerant Ce compound is typically avoided. The localized versus itinerant character of the 4\textit{f} electrons is a key aspect for understanding this behavior. We investigated the electronic structure of the 4\textit{f… ▽ More The mechanism of a pressure-induced quantum critical point in the heavy fermion ferromagnet CeRh$_6$Ge$_4$ has attracted interest, as ferromagnetic quantum criticality in a clean itinerant Ce compound is typically avoided. The localized versus itinerant character of the 4\textit{f} electrons is a key aspect for understanding this behavior. We investigated the electronic structure of the 4\textit{f} shell in CeRh$_6$Ge$_4$ using core-level photoelectron and x-ray absorption spectroscopy, demonstrating the hybridization of Ce 4\textit{f} with the conduction electrons. Linearly polarized x-ray absorption reveals a temperature-dependent linear dichroism consistent with the crystal-electric-field (CEF) sequence as inferred from the static susceptibility. This dichroism cannot be described by an ionic full-multiplet model alone, but is reproduced by including the Kondo effect within a single-impurity Anderson model in the non crossing approximation (SIAM/NCA). The Kondo effect mixes higher lying crystal-field states into a resulting multiorbital ground state with 4\textit{f} occupancy \textit{n}$_f$\,$\sim$\,0.9. Deviations at low temperatures between the measured linear dichroism and calculated dichroism suggest an orbital-dependent Kondo effect. A scenario in which there is a multiorbital ground state and orbital-dependent Kondo hybridization should be a starting point for a model of pressure-induced criticality in CeRh$_6$Ge$_4$. △ Less

Submitted 10 June, 2026; originally announced June 2026.

Comments: 9 pages, 7 Figures, accepted in Phys. Rev. B

Journal ref: Phys. Rev. B 113, 245149 (2026)

Unveiling orbital landscapes in strongly correlated bulk nickelates with $s$-NIXS

Authors: Edgar Abarca Morales, Martin Sundermann, Brett Leedahl, Vignesh Sundaramurthy, Georg Poelchen, Ulrich Burkhardt, Raul Cardoso, Pascal Puphal, Alexander Komarek, Bernhard Keimer, Matthias Hepting, Liu Hao Tjeng, Berit H. Goodge

Abstract: We leverage $s$-orbital non-resonant inelastic X-ray scattering ($s$-NIXS) to perform orbital imaging on three bulk rare-earth nickelates spanning a range of formal nickel valence (3$d$ electron filling) from Ni$^{3+}$ (3$d^7$) to Ni$^{1+}$ (3$d^9$). Our results directly reveal the ground states of these compounds all with minimal theoretical input. In particular, we demonstrate the low-spin orbit… ▽ More We leverage $s$-orbital non-resonant inelastic X-ray scattering ($s$-NIXS) to perform orbital imaging on three bulk rare-earth nickelates spanning a range of formal nickel valence (3$d$ electron filling) from Ni$^{3+}$ (3$d^7$) to Ni$^{1+}$ (3$d^9$). Our results directly reveal the ground states of these compounds all with minimal theoretical input. In particular, we demonstrate the low-spin orbital configuration of trivalent LaNiO$_3$, the $d_{x^2-y^2}$ configuration of monovalent LaNiO$_2$, and resolve the effective $e_g$ crystal field splitting in the distorted octahedral environment of divalent La$_2$NiO$_4$. This work illustrates the potential of $s$-NIXS to study the ground state and excited states of strongly correlated materials without needing complex theoretical analysis of spectroscopic data. △ Less

Submitted 4 June, 2026; originally announced June 2026.

Comments: 3 figures, 10 supplemental figures, 1 table

Disentangling the contributions of individual cations to magnetic order in a spinel high entropy oxide

Authors: Mario Ulises González-Rivas, Chun-Fu Chang, Martin Bluschke, Jessica Freese, Peter Bencok, Ronny Sutarto, Teak D. Boyko, Robert J. Green, George A. Sawatzky, Liu Hao Tjeng, Alannah M. Hallas

Abstract: High entropy oxides (HEOs) can possess long-range ordered magnetic states despite their extreme chemical disorder. Very little is known about how the different chemical constituents in HEOs contribute to the emergence of these magnetic states. In this work, we leverage element-specific magnetometry attained via x-ray magnetic circular dichroism (XMCD) to understand how magnetic order is driven in… ▽ More High entropy oxides (HEOs) can possess long-range ordered magnetic states despite their extreme chemical disorder. Very little is known about how the different chemical constituents in HEOs contribute to the emergence of these magnetic states. In this work, we leverage element-specific magnetometry attained via x-ray magnetic circular dichroism (XMCD) to understand how magnetic order is driven in two ferrimagnetic spinel-structured HEOs with compositions (Cr,Mn,Fe,Co,Ni)$_3$O$_4$ and (Cr,Mn,Fe,Co,Ni)$_{2.4}$Ga$_{0.6}$O$_4$. We find that while the magnetic transition is simultaneous for all chemical species, the rate at which their magnetic moments grow is strongly cation dependent. This behavior is explained by the varying $\textit{3d}$ crystal field level fillings of the magnetic cations, which in turn determine their ability to participate in the different magnetic exchange pathways available in the spinel structure. Dominant $A$-$B$ sublattice exchange enables some species to harden rapidly ($\textit{e.g.}$ tetrahedral Fe$^{3+}$ and octahedral Ni$^{2+}$) while others exhibit a sluggish transition due to frustration from competing interactions ($\textit{e.g.}$ octahedral Fe$^{3+}$ and Cr$^{3+}$). Non-magnetic substitution suppresses these differences, introducing broken magnetic linkages that relieve frustration. Tailoring the magnetism of HEO spinels therefore requires detailed knowledge of both their site selectivities and their exchange pathways. △ Less

Submitted 25 May, 2026; originally announced May 2026.

UCd$_{11}$: A strongly localized 5$f^3$ material

Authors: Martin Sundermann, Naoki Ito, Daisuke Takegami, Chun-Fu Chang, Sheng-Huai Chen, Chang-Yang Kuo, Simone G. Altendorf, Andrei Gloskovskii, Hlynur Gretarsson, Eric D. Bauer, Jan Kuneš, Liu Hao Tjeng, Andrea Severing, Atsushi Hariki

Abstract: UCd$_{11}$ is an antiferromagnetic uranium intermetallic compound ($T_{\rm N}$ = 5.3K) with enhanced electron mass and uranium-uranium spacings nearly twice the Hill limit, suggesting a weakly hybridized 5$f$ electronic character. Various x-ray spectroscopy techniques indicate that uranium in UCd$_{11}$ adopts the formal U$^{3+}$ 5$f^3$ configuration, while core-level photoemission spectroscopy (P… ▽ More UCd$_{11}$ is an antiferromagnetic uranium intermetallic compound ($T_{\rm N}$ = 5.3K) with enhanced electron mass and uranium-uranium spacings nearly twice the Hill limit, suggesting a weakly hybridized 5$f$ electronic character. Various x-ray spectroscopy techniques indicate that uranium in UCd$_{11}$ adopts the formal U$^{3+}$ 5$f^3$ configuration, while core-level photoemission spectroscopy (PES) data of UCd$_{11}$ reveal only a weak satellite feature, typically interpreted as a signature of itinerancy. In this work, we present density functional theory (DFT) combined with dynamical mean-field theory (DMFT) calculations of UCd$_{11}$, using material-specific parameters tuned to reproduce valence-band PES spectra at different photon energies, thereby exploiting the energy dependence of photoionization cross sections. Our results demonstrate that UCd$_{11}$ is a highly localized uranium 5$f^3$ system. Furthermore, core-level spectra obtained from a DFT+DMFT Anderson impurity model reveal that, contrary to common assumptions, the presence or absence of satellite structures is not a reliable indicator of strong correlations or itinerant 5$f$ behavior. △ Less

Submitted 18 April, 2026; originally announced April 2026.

Journal ref: Phys. Rev. Research 8, 023008 (2026)

Single-Crystal Growth and Magnetic, Electronic Properties of the FCC Antiferromagnet Ba_2CoMoO_6

Authors: A. R. N. Hanna, M. M. Ferreira-Carvalho, S. H. Chen, C. F. Chang, C. Y. Kuo, A. T. M. N. Islam, R. Feyerherm, L. H. Tjeng, B. Lake

Abstract: 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 techniques and characterized using a set of complementary methods. X-ray diffraction analysis confirmed that BCMO crystallizes in a face-centered cubic structure with space group… ▽ More 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 techniques and characterized using a set of complementary methods. X-ray diffraction analysis confirmed that BCMO crystallizes in a face-centered cubic structure with space group $Fm\bar{3}m$. Magnetic susceptibility measurements reveal antiferromagnetic ordering below $T_\mathrm{N} = 20.1(1)$~K, with a spin-flop transition at $μ_0 H = 2.65$~T. Heat-capacity measurements and entropy analysis, $ΔS \approx 0.95\,R\ln 2$, are consistent with a $J_\mathrm{eff} = \tfrac{1}{2}$ effective ground state for Co$^{2+}$ ions. X-ray absorption spectroscopy at the Co~$L_{2,3}$ edges provides insight into the local electronic structure, revealing spin-orbit and crystal-field splitting effects; cluster-model calculations constrained by the XAS spectra yield a Landé $g$ factor $g = 4.52$, consistent with a spin-orbit-entangled $J_\mathrm{eff} = \tfrac{1}{2}$ ground state. Surface photovoltage spectroscopy reveals a strong optical response with a prominent feature at $2.65$~eV. These findings advance the understanding of face-centered cubic lattice antiferromagnets with strong spin-orbit coupling and suggest the technological promise of BCMO for spintronic and energy-conversion applications. △ Less

Submitted 16 March, 2026; originally announced March 2026.

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

Authors: M. M. Ferreira-Carvalho, S. H. Chen, Y. C. Ku, Anagha Jose, Ryan Morrow, C. Y. Kuo, C. F. Chang, Z. Hu, M. W. Haverkort, L. H. Tjeng

Abstract: 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 v… ▽ More 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. △ Less

Submitted 9 February, 2026; originally announced February 2026.

High-resolution valence band RIXS at the actinide M$_{4,5}$-edges

Authors: Martin Sundermann, Henrik Hahn, Denise S. Christovam, Maurits W. Haverkort, Roberto Caciuffo, Bernhard Keimer, Liu Hao Tjeng, Andrea Severing, Hlynur Gretarsson

Abstract: Understanding the electronic structure of actinide materials is crucial for both fundamental research and nuclear applications. The partially filled 5f shells exhibit complex behavior due to strong correlations and ligand hybridization, requiring advanced spectroscopic techniques. Here, we report on the development and application of high-resolution valence-band resonant inelastic x-ray spectrosco… ▽ More Understanding the electronic structure of actinide materials is crucial for both fundamental research and nuclear applications. The partially filled 5f shells exhibit complex behavior due to strong correlations and ligand hybridization, requiring advanced spectroscopic techniques. Here, we report on the development and application of high-resolution valence-band resonant inelastic x-ray spectroscopy (VB-RIXS) experiments at the uranium M$_{4,5}$ edges (3551 and 3725\,eV). We present data of UO$_2$, a well-established model actinide compound. VB-RIXS is particularly well suited for probing the 5f-shell electronic structure, as it probes, in contrast to core-to-core RIXS, excitations without leaving a high-energy core hole in the final state. In VB-RIXS, we achieve energy resolutions of 50\,meV (M$_5$) and 90\,meV (M$_4$), enabling the resolution of multiplet excitations and crystal-field effects, as well as charge-transfer and fluorescence-like features with unprecedented clarity. As such, high resolution VB-RIXS offers direct insights into both low-energy, near ground-state properties and high-energy hybridization and covalency effects. Our results demonstrate the power of VB-RIXS as a versatile and powerful tool for probing the strongly correlated electronic structure of actinide materials, providing essential input for quantitative modeling and the validation of theoretical concepts. △ Less

Submitted 27 October, 2025; v1 submitted 22 September, 2025; originally announced September 2025.

Comments: 10 pages, 6 figures

Journal ref: Physical Review Research 7, 043081 (2025)

UTe$_2$: a narrow band superconductor

Authors: Martin Sundermann, Takaki Okauchi, Naoki Ito, Denise S. Christovam, Andrea Marino, Daiskue Takegami, Andrei Golskovskii, Priscila F. S. Rosa, Jan Kuneš, Shin ichi Fujimori, Liu Hao Tjeng, Andrea Severing, Atsushi Hariki

Abstract: We investigate the nature of the 5$f$ electrons in the unconventional odd-parity superconductor UTe$_2$, focusing on the degree of covalency, localization versus itinerancy, and dominant electronic configuration. This is achieved using density functional theory (DFT) in combination with dynamical mean-field theory (DMFT) calculations. A key aspect of our approach is the material-specific tuning of… ▽ More We investigate the nature of the 5$f$ electrons in the unconventional odd-parity superconductor UTe$_2$, focusing on the degree of covalency, localization versus itinerancy, and dominant electronic configuration. This is achieved using density functional theory (DFT) in combination with dynamical mean-field theory (DMFT) calculations. A key aspect of our approach is the material-specific tuning of the double-counting correction parameter, $μ_{\rm dc}$, within the DFT+DMFT part. This tuning is guided by the energy dependence of photo-ionization cross-sections in valence band photoelectron spectroscopy. The reliability of the parameters is confirmed by the accurate reproduction of the angle-resolved valence-band photoemission spectra and the U 4$f$ core-level data. The DFT+DMFT model reveals that in UTe$_2$ U 5$f^n$ configurations with n=1 to 4 contribute to the ground state, with the 5$f^2$ configuration being most prevalent and an average 5$f$ shell fillings close to 2.5. The model further suggests that the 5$f$ electrons form narrow bands and that charge fluctuations due to degeneracy play a role in addition to coherent valence dynamics arising from hybridization with the conduction bath. Additionally, the significance of the U 6$d$ states in UTe$_2$ is discussed. △ Less

Submitted 12 December, 2025; v1 submitted 9 September, 2025; originally announced September 2025.

Comments: 12 pages, 11 figures, submitted as regular article

Journal ref: Phys. Rev. Research 7, 043195 (2025)

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

Authors: M. M. Ferreira-Carvalho, S. Rößler, C. F. Chang, Z. Hu, S. M. Valvidares, P. Gargiani, M. W. Haverkort, Prashanta K. Mukharjee, P. Gegenwart, A. A. Tsirlin, L. H. Tjeng

Abstract: 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 quant… ▽ More 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$_4$)$_2$. △ Less

Submitted 8 September, 2025; originally announced September 2025.

Journal ref: Phys. Rev. B 112, 125135 (2025)

Valency, charge-transfer, and orbital-dependent correlation in bilayer nickelates Nd3Ni2O7

Authors: Daisuke Takegami, Takaki Okauchi, Edgar Abarca Morales, Kouto Fujinuma, Mizuki Furo, Masato Yoshimura, Ku-Ding Tsuei, Grace A. Pan, Dan Ferenc Segedin, Qi Song, Hanjong Paik, Charles M. Brooks, Julia A. Mundy, Takashi Mizokawa, Liu Hao Tjeng, Berit H. Goodge, Atsushi Hariki

Abstract: We examine the bulk electronic structure of Nd3Ni2O7 using Ni 2p core-level hard x-ray photoemission spectroscopy combined with density functional theory + dynamical mean-field theory. Our results reveal a large deviation of the Ni 3d occupation from the formal Ni2.5+ valency, highlighting the importance of the charge-transfer from oxygen ligands. We find that the dominant d8 configuration is acco… ▽ More We examine the bulk electronic structure of Nd3Ni2O7 using Ni 2p core-level hard x-ray photoemission spectroscopy combined with density functional theory + dynamical mean-field theory. Our results reveal a large deviation of the Ni 3d occupation from the formal Ni2.5+ valency, highlighting the importance of the charge-transfer from oxygen ligands. We find that the dominant d8 configuration is accompanied by nearly equal contributions from d7 and d9 states, exhibiting an unusual valence state among Ni-based oxides. Finally, we discuss the Ni dx2-y2 and dz2 orbital-dependent hybridization, correlation and local spin dynamics. △ Less

Submitted 16 February, 2025; originally announced February 2025.

Journal ref: Phys. Rev. B 111, 165101 (2025)

Magnetic Ni-N-Ni Centers in N-substituted NiO

Authors: Simon Godin, Ilya S. Elfimov, Fengmiao Li, Bruce A. Davidson, Ronny Sutarto, Jonathan D. Denlinger, Liu Hao Tjeng, George A. Sawatzky, Ke Zou

Abstract: To explore how anion substitution modifies the existing magnetism in strongly correlated oxides, we investigate local electronic states and magnetic ordering in nickel oxide (NiO) induced by substituting oxygen (O) with nitrogen (N). Each N introduces an additional N 2p hole and modifies the magnetic moment of a neighboring nickel (Ni) cation site, as the exchange interaction between this hole and… ▽ More To explore how anion substitution modifies the existing magnetism in strongly correlated oxides, we investigate local electronic states and magnetic ordering in nickel oxide (NiO) induced by substituting oxygen (O) with nitrogen (N). Each N introduces an additional N 2p hole and modifies the magnetic moment of a neighboring nickel (Ni) cation site, as the exchange interaction between this hole and the Ni eg electrons exceeds the Ni-O-Ni superexchange interaction. This leads to the formation of Ni-N-Ni centers consisting of five spins, without perturbing the antiferromagnetic NiO lattice. These centers are studied using density functional theory and confirmed through high-resolution spectroscopy on N-substituted NiO thin films grown by molecular beam epitaxy. This type of magnetic design may enable future advances in quantum technologies based on strongly correlated materials, such as quantum sensors and spin-based qubits. △ Less

Submitted 25 July, 2025; v1 submitted 26 December, 2024; originally announced December 2024.

Comments: 13 pages, 14 figures. We are updating the arXiv version due to the paper being now published

Journal ref: Phys. Rev. Lett. 135, 046706 (2025)

Quantifying the U $5f$ covalence and degree of localization in U intermetallics

Authors: Andrea Marino, Denise S. Christovam, Daisuke Takegami, Johannes Falke, Miguel M. F. Carvalho, Takaki Okauchi, Chun-Fu Chang, Simone G. Altendorf, Andrea Amorese, Martin Sundermann, Andrei Gloskovskii, Hlynur Gretarsson, Bernhard Keimer, Alexandr V. Andreev, Ladislav Havela, Andreas Leithe-Jasper, Andrea Severing, Jan Kunes, Liu Hao Tjeng, Atsushi Hariki

Abstract: A procedure for quantifying the U $5f$ electrons' covalence and degree of localization in U intermetallic compounds is presented. To this end, bulk sensitive hard and soft x-ray photoelectron spectroscopy were utilized in combination with density-functional theory (DFT) plus dynamical mean-field theory (DMFT) calculations. The energy dependence of the photoionization cross-sections allows the dise… ▽ More A procedure for quantifying the U $5f$ electrons' covalence and degree of localization in U intermetallic compounds is presented. To this end, bulk sensitive hard and soft x-ray photoelectron spectroscopy were utilized in combination with density-functional theory (DFT) plus dynamical mean-field theory (DMFT) calculations. The energy dependence of the photoionization cross-sections allows the disentanglement of the U\,$5f$ contribution to the valence band from the various other atomic subshells so that the computational parameters in the DFT\,+\,DMFT can be reliably determined. Applying this method to UGa$_2$ and UB$_2$ as model compounds from opposite ends of the (de)localization range, we have achieved excellent simulations of the valence band and core-level spectra. The width in the distribution of atomic U\,$5f$ configurations contributing to the ground state, as obtained from the calculations, quantifies the correlated nature and degree of localization of the U\,5$f$. The findings permit answering the longstanding question why different spectroscopic techniques give seemingly different numbers for the U 5$f$ valence in intermetallic U compounds. △ Less

Submitted 30 July, 2024; v1 submitted 9 April, 2024; originally announced April 2024.

Comments: 14 pages, 9 figures

Journal ref: Phys. Rev. Research 6, 033068 (2024), Editors' Suggestions

Stabilization of U 5$f^2$ configuration in UTe$_2$ through U 6d dimers in the presence of Te2 chains

Authors: Denise S. Christovam, Martin Sundermann, Andrea Marino, Daisuke Takegami, Johannes Falke, Paulius Dolmantas, Manuel Harder, Hlynur Gretarsson amd Bernhard Keimer, Andrei Gloskovskii, Maurits W. Haverkort, Ilya Elfimov, Gertrud Zwicknagl, Alexander V. Andreev, Ladislav Havela, Mitchell M. Bordelon, Eric D. Bauer, Priscila F. S. Rosa, Andrea Severing, Liu Hao Tjeng

Abstract: We investigate the topological superconductor candidate UTe$_2$ using high-resolution valence-band resonant inelastic x-ray scattering at the U $M_{4,5}$-edges. We observe atomic-like low-energy excitations that support the correlated nature of this unconventional superconductor. These excitations originate from the U $5f^2$ configuration, which is unexpected since the short Te2-Te2 distances excl… ▽ More We investigate the topological superconductor candidate UTe$_2$ using high-resolution valence-band resonant inelastic x-ray scattering at the U $M_{4,5}$-edges. We observe atomic-like low-energy excitations that support the correlated nature of this unconventional superconductor. These excitations originate from the U $5f^2$ configuration, which is unexpected since the short Te2-Te2 distances exclude Te2 being 2-. By utilizing the photoionization cross-section dependence of the photoemission spectra in combination with band structure calculations, we infer that the stabilization of the U $5f^2$ configuration is due to the U $6d$ bonding states in the U-dimers acting as a charge reservoir. Our results emphasize that the description of the physical properties should commence with a $5f^2$ $ansatz$. △ Less

Submitted 6 February, 2024; originally announced February 2024.

Comments: 9 pages, 5 figures, article

Linear dichroic x-ray absorption response of Ti-Ti dimers along the $c$ axis in Ti$_2$O$_3$ upon Mg substitution

Authors: M. Okawa, D. Takegami, D. S. Christovam, M. Ferreira-Carvalho, C. -Y. Kuo, C. T. Chen, T. Miyoshino, K. Takasu, T. Okuda, C. F. Chang, L. H. Tjeng, T. Mizokawa

Abstract: Corundum oxide Ti$_2$O$_3$ shows the metal-insulator transition around 400-600 K accompanying the nearest Ti$^{3+}$-Ti$^{3+}$ bond ($a_{1g}a_{1g}$ singlet state) formation along the $c$ axis. In order to clarify the hole-doping effect for the $a_{1g}a_{1g}$ singlet bond in Ti$_2$O$_3$, we investigated Ti $3d$ orbital anisotropy between corundum-type Ti$_2$O$_3$ and ilmenite-type MgTiO$_3$ using li… ▽ More Corundum oxide Ti$_2$O$_3$ shows the metal-insulator transition around 400-600 K accompanying the nearest Ti$^{3+}$-Ti$^{3+}$ bond ($a_{1g}a_{1g}$ singlet state) formation along the $c$ axis. In order to clarify the hole-doping effect for the $a_{1g}a_{1g}$ singlet bond in Ti$_2$O$_3$, we investigated Ti $3d$ orbital anisotropy between corundum-type Ti$_2$O$_3$ and ilmenite-type MgTiO$_3$ using linear dichroism of soft x-ray absorption spectroscopy of the Ti $L_{2,3}$ edge. From the linear dichroic spectral weight in Mg$_y$Ti$_{2-y}$O$_3$, we confirmed that the $a_{1g}a_{1g}$ state is dominant not only in $y=0.01$ (almost Ti$_2$O$_3$), but also in $y = 0.29$, indicating that the Ti-Ti bond survives against a certain level of hole doping. In $y=0.63$ corresponding to 46% hole doping per Ti, the $3d$ orbital symmetry changes from $a_{1g}$ to $e_g^π$. △ Less

Submitted 8 November, 2023; originally announced November 2023.

Comments: 5 pages, 5 figures

Journal ref: Phys. Rev. B 108, 195108 (2023)

Fe substitution in URu$_2$Si$_2$: singlet magnetism in an extended Doniach phase diagram

Authors: Andrea Marino, Denise S. Christovam, Chun-Fu Chang, Johannes Falke, Chang-Yang Kuo, Chi-Nan Wu, Martin Sundermann, Andrea Amorese, Hlynur Gretarsson, Eric Lee Wong, Camilla M. Moir, Yuang Deng, M. Brian Maple, Peter Thalmeier, Liu Hao Tjeng, Andrea Severing

Abstract: The application of pressure as well as the successive substitution of Ru with Fe in the hidden order (HO) compound URu$_2$Si$_2$ leads to the formation of the large moment antiferromagnetic phase (LMAFM). Here we have investigated the substitution series URu$_{2-x}$Fe$_x$Si$_2$ from $x$\,=\,0.0 to 2.0 by U\,4$f$ core-level photoelectron spectroscopy and have observed non-monotonic changes in the s… ▽ More The application of pressure as well as the successive substitution of Ru with Fe in the hidden order (HO) compound URu$_2$Si$_2$ leads to the formation of the large moment antiferromagnetic phase (LMAFM). Here we have investigated the substitution series URu$_{2-x}$Fe$_x$Si$_2$ from $x$\,=\,0.0 to 2.0 by U\,4$f$ core-level photoelectron spectroscopy and have observed non-monotonic changes in the spectra. The initial increase and subsequent decrease of the spectral weight of the 4$f$ core level satellite with increasing $x$ stands for a non-monotonic 5$f$ filling across the substitution series. The competition of chemical pressure and increase of the density of states at the Fermi energy, both due to substitution of Ru with Fe, can explain such a behavior. An extended Doniach phase diagram including the $x$ dependence of the density of states is proposed. Also in URu$_{2-x}$Fe$_x$Si$_2$ the ground state is a singlet or quasi-doublet state consisting of two singlets. Hence, the formation of magnetic order in the URu$_{2-x}$Fe$_x$Si$_2$ substitution series must be explained within a singlet magnetism model. △ Less

Submitted 23 August, 2023; originally announced August 2023.

Comments: 10 pages, 7 figures

Journal ref: Phys. Rev. B 108, 085128 (2023)

Spectroscopic evidence of Kondo-induced quasi-quartet in CeRh$_2$As$_2$

Authors: Denise S. Christovam, Miguel Ferreira-Carvalho, Andrea Marino, Martin Sundermann, Daisuke Takegami, Anna Melendez-Sans, Ku Ding Tsuei, Zhiwei Hu, Sahana Roessler, Manuel Valvidares, Maurits W. Haverkort, Yu Liu, Eric D. Bauer, Liu Hao Tjeng, Gertrud Zwicknagl, Andrea Severing

Abstract: CeRh$_2$As$_2$ is a new multiphase superconductor with strong suggestions for an additional itinerant multipolar ordered phase. The modeling of the low temperature properties of this heavy fermion compound requires a quartet Ce$^{3+}$ crystal-field ground state. Here we provide the evidence for the formation of such a quartet state using x-ray spectroscopy. Core-level photoelectron and x-ray absor… ▽ More CeRh$_2$As$_2$ is a new multiphase superconductor with strong suggestions for an additional itinerant multipolar ordered phase. The modeling of the low temperature properties of this heavy fermion compound requires a quartet Ce$^{3+}$ crystal-field ground state. Here we provide the evidence for the formation of such a quartet state using x-ray spectroscopy. Core-level photoelectron and x-ray absorption spectroscopy confirm the presence of Kondo hybridization in CeRh$_2$As$_2$. The temperature dependence of the linear dichroism unambiguously reveils the impact of Kondo physics for coupling the Kramer's doublets into an effective quasi-quartet. Non-resonant inelastic x-ray scattering data find that the $|Γ_7^- \rangle$ state with its lobes along the 110 direction of the tetragonal structure ($xy$ orientation) contributes most to the multi-orbital ground state of CeRh$_2$As$_2$. △ Less

Submitted 23 January, 2024; v1 submitted 21 August, 2023; originally announced August 2023.

Comments: 8 pages, 7 figures

Journal ref: Physical Review Letters 132, 046401 (2024)

Singlet magnetism in intermetallic UGa$_2$ unveiled by inelastic x-ray scattering

Authors: Andrea Marino, Martin Sundermann, Denise S. Christovam, Andrea Amorese, Chun-Fu Chang, Paulius Dolmantas, Ayman H. Said, Hlynur Grrtarsson, Bernhard Keimer, Maurits W. Haverkort, Alexander V. Andreev, Ladilav Havela, Peter Thalmeier, Liu Hao Tjeng, Andrea Severing

Abstract: Using high resolution tender-x-ray resonant inelastic scattering and hard-x-ray non-resonant inelastic scattering beyond the dipole limit we were able to detect electronic excitations in intermetallic UGa$_2$ that are highly atomic in nature. Analysis of the spectral lineshape reveals that the local $5f^2$ configuration characterizes the correlated nature of this ferromagnet. The orientation and d… ▽ More Using high resolution tender-x-ray resonant inelastic scattering and hard-x-ray non-resonant inelastic scattering beyond the dipole limit we were able to detect electronic excitations in intermetallic UGa$_2$ that are highly atomic in nature. Analysis of the spectral lineshape reveals that the local $5f^2$ configuration characterizes the correlated nature of this ferromagnet. The orientation and directional dependence of the spectra indicate that the ground state is made of the $Γ_1$ singlet and/or $Γ_6$ doublet symmetry. With the ordered moment in the $ab$ plane, we infer that the magnetism originates from the higher lying $Γ_6$ doublet being mixed with the $Γ_1$ singlet due to inter-site exchange, qualifying UGa$_2$ to be a true quantum magnet. The ability to observe atomic excitations is crucial to resolve the on-going debate about the degree of localization versus itineracy in U intermetallics. △ Less

Submitted 27 July, 2023; originally announced July 2023.

Comments: 9 pages, 7 figures

Journal ref: Phys. Rev. B 108, 045142 (2023)

Orbital selective coupling in CeRh$_3$B$_2$: co-existence of high Curie and high Kondo temperature

Authors: Andrea Amorese, Philipp Hansmann, Andrea Marino, Peter Korner, Thomas Willers, Andrew Walters, Kejin Zhou, Kurt Kummer, Nicholas B. Brooks, Hong-Ji Lin, Cien-Te Chen, Pascal Lejay, Maurits W. Haverkort, Liu Hao Tjeng, Andrea Severing

Abstract: We investigated the electronic structure of the enigmatic CeRh$_3$B$_2$ using resonant inelastic scattering and x-ray absorption spectroscopy in combination with $ab$ $initio$ density functional calculations. We find that the Rh 4$d$ states are irrelevant for the high-temperature ferromagnetism and the Kondo effect. We also find that the Ce 4$f$ crystal-field strength is too small to explain the s… ▽ More We investigated the electronic structure of the enigmatic CeRh$_3$B$_2$ using resonant inelastic scattering and x-ray absorption spectroscopy in combination with $ab$ $initio$ density functional calculations. We find that the Rh 4$d$ states are irrelevant for the high-temperature ferromagnetism and the Kondo effect. We also find that the Ce 4$f$ crystal-field strength is too small to explain the strong reduction of the Ce magnetic moment. The data reveal instead the presence of two different active Ce 4$f$ orbitals, with each coupling selectively to different bands in CeRh$_3$B$_2$. The inter-site hybridization of the |J=5/2,Jz=+/-1/2> crystal-field state and Ce 5$d$ band combined with the intra-site Ce 4$f$-5$d$ exchange creates the strong ferromagnetism, while hybridization between the |J=5/2,Jz=+/-5/2> and the B $sp$ in the $ab$-plane contributes to the Kondo interaction which causes the moment reduction. This orbital selective coupling explains the unique and seemingly contradictory properties of CeRh$_3$B$_2$. △ Less

Submitted 22 March, 2023; v1 submitted 28 November, 2022; originally announced November 2022.

Comments: 15 pages, 14 figures

Journal ref: Phys. Ref. B 107 115164 (2023)

Single-crystal epitaxial europium iron garnet films with strain-induced perpendicular magnetic anisotropy: structural, strain, magnetic, and spin transport properties

Authors: M. X. Guo, C. K. Cheng, Y. C. Liu, C. N. Wu, W. N. Chen, T. Y Chen, C. T. Wu, C. H. Hsu, S. Q. Zhou, C. F. Chang, L. H. Tjeng, S. F. Lee, C. F. Pai, M. Hong, J. Kwo

Abstract: Single-crystal europium iron garnet (EuIG) thin films epitaxially strain-grown on gadolinium gallium garnet (GGG)(100) substrates using off-axis sputtering have strain-induced perpendicular magnetic anisotropy (PMA). By varying the sputtering conditions, we have tuned the europium/iron (Eu/Fe) composition ratios in the films to tailor the film strains. The films exhibited an extremely smooth, part… ▽ More Single-crystal europium iron garnet (EuIG) thin films epitaxially strain-grown on gadolinium gallium garnet (GGG)(100) substrates using off-axis sputtering have strain-induced perpendicular magnetic anisotropy (PMA). By varying the sputtering conditions, we have tuned the europium/iron (Eu/Fe) composition ratios in the films to tailor the film strains. The films exhibited an extremely smooth, particle-free surface with roughness as low as 0.1 nm as observed using atomic force microscopy. High-resolution x-ray diffraction analysis and reciprocal space maps showed in-plane epitaxial film growth, very smooth film/substrate interface, excellent film crystallinity with a small full width at half maximum of 0.012$^{\circ}$ in the rocking curve scans, and an in-plane compressive strain without relaxation. In addition, spherical aberration-corrected scanning transmission electron microscopy showed an atomically abrupt interface between the EuIG film and GGG. The measured squarish out-of-plane magnetization-field hysteresis loops by vibrating sample magnetometry in conjunction with the measurements from angle-dependent x-ray magnetic dichroism demonstrated the PMA in the films. We have tailored the magnetic properties of the EuIG thin films, including saturation magnetization ranging from 71.91 to 124.51 emu/c.c. (increase with the (Eu/Fe) ratios), coercive field from 27 to 157.64 Oe, and the strength of PMA field ($H_\bot$) increasing from 4.21 to 18.87 kOe with the in-plane compressive strain from -0.774 to -1.044%. We have also investigated spin transport in Pt/EuIG bi-layer structure and evaluated the real part of spin mixing conductance to be $3.48\times10^{14} Ω^{-1}m^{-2}$. We demonstrated the current-induced magnetization switching with a low critical switching current density of $3.5\times10^6 A/cm^2$, showing excellent potential for low-dissipation spintronic devices. △ Less

Submitted 11 January, 2022; originally announced January 2022.

Comments: 29 pages, 9 figures, 1 table

CaCu$_3$Ru$_4$O$_{12}$: a high Kondo-temperature transition metal oxide

Authors: D. Takegami, C. Y. Kuo, K. Kasebayashi, J. -G. Kim, C. F. Chang, C. E. Liu, C. N. Wu, D. Kasinathan, S. G. Altendorf, K. Hoefer, F. Meneghin, A. Marino, Y. F. Liao, K. D. Tsuei, C. T. Chen, K. -T. Ko, A. Günther, S. G. Ebbinghaus, J. W. Seo, D. H. Lee, G. Ryu, A. C. Komarek, S. Sugano, Y. Shimakawa, A. Tanaka , et al. (4 additional authors not shown)

Abstract: We present a comprehensive study of CaCu$_3$Ru$_4$O$_{12}$ using bulk sensitive hard and soft x-ray spectroscopy combined with local-density approximation (LDA) + dynamical mean-field theory (DMFT) calculations. Correlation effects on both the Cu and Ru ions can be observed. From the Cu $2p$ core level spectra we deduce the presence of magnetic Cu$^{2+}$ ions hybridized with a reservoir of itinera… ▽ More We present a comprehensive study of CaCu$_3$Ru$_4$O$_{12}$ using bulk sensitive hard and soft x-ray spectroscopy combined with local-density approximation (LDA) + dynamical mean-field theory (DMFT) calculations. Correlation effects on both the Cu and Ru ions can be observed. From the Cu $2p$ core level spectra we deduce the presence of magnetic Cu$^{2+}$ ions hybridized with a reservoir of itinerant electrons. The strong photon energy dependence of the valence band allows us to disentangle the Ru, Cu, and O contributions and thus to optimize the DMFT calculations. The calculated spin and charge susceptibilities show that the transition metal oxide CaCu$_3$Ru$_4$O$_{12}$ must be classified as a Kondo system and that the Kondo temperature is in the range of 500-1000 K. △ Less

Submitted 6 December, 2021; originally announced December 2021.

Comments: Accepted for publication in Phys. Rev. X

Journal ref: Phys. Rev. X 12, 011017 (2022)

Magnetic Frustration in a Zeolite

Authors: Danrui Ni, Zhiwei Hu, Guangming Cheng, Xin Gui, Wenzhu Yu, Chunjiang Jia, Xiao Wang, Javier Herrero-Martín, Nan Yao, Liu Hao Tjeng, Robert J. Cava

Abstract: Zeolites are so well known in real world applications and after decades of scientific study that they hardly need any intro-duction: their importance in chemistry cannot be overemphasized. Here we add to the remarkable properties that they dis-play by reporting our discovery that the simplest zeolite, sodalite, when doped with Cr3+ in the \b{eta}-cage, is a frustrated magnet. Soft X-ray absorption… ▽ More Zeolites are so well known in real world applications and after decades of scientific study that they hardly need any intro-duction: their importance in chemistry cannot be overemphasized. Here we add to the remarkable properties that they dis-play by reporting our discovery that the simplest zeolite, sodalite, when doped with Cr3+ in the \b{eta}-cage, is a frustrated magnet. Soft X-ray absorption spectroscopy and magnetic measurements reveal that the Cr present is Cr(III). Cr(III), with its isotropic 3d3 valence electron configuration, is well-known as the basis for many geometrically frustrated magnets, but it is especially surprising that a material like the Ca8Al12Cr2O29 zeolite is a frustrated magnet. This finding illustrates the value of exploring the properties of even well-known materials families. △ Less

Submitted 7 October, 2021; originally announced October 2021.

Emergent 1/3 magnetization plateaus in pyroxene CoGeO$_3$

Authors: H. Guo, L. Zhao, M. Baenitz, X. Fabrèges, A. Gukasov, A. Melendez Sans, D. I. Khomskii, L. H. Tjeng, A. C. Komarek

Abstract: Despite the absence of an apparent triangular pattern in the crystal structure, we observe unusually well pronounced 1/3 magnetization plateaus in the quasi one-dimensional Ising spin chain compound CoGeO$_3$ which belongs to the class of pyroxene minerals. We succeeded in uncovering the detailed microscopic spin structure of the 1/3 magnetization plateau phase by means of neutron diffraction. We… ▽ More Despite the absence of an apparent triangular pattern in the crystal structure, we observe unusually well pronounced 1/3 magnetization plateaus in the quasi one-dimensional Ising spin chain compound CoGeO$_3$ which belongs to the class of pyroxene minerals. We succeeded in uncovering the detailed microscopic spin structure of the 1/3 magnetization plateau phase by means of neutron diffraction. We observed changes of the initial antiferromagnetic zero-field spin structure that are resembling a regular formation of antiferromagnetic "domain wall boundaries", resulting in a kind of modulated magnetic structure with 1/3-integer propagation vector. The net ferromagnetic moment emerges at these "domain walls" whereas two third of all antiferromagnetic chain alignments can be still preserved. We propose a microscopic model on the basis of an anisotropic frustrated square lattice to explain the observations. △ Less

Submitted 15 September, 2021; originally announced September 2021.

Journal ref: Physical Review RESEARCH 3, L032037 (2021)

Single crystal growth and physical properties of pyroxene CoGeO$_3$

Authors: L. Zhao, Z. Hu, H. Guo, C. Geibel, H. J. Lin, C. T. Chen, D. I. Khomskii, L. H. Tjeng, A. C. Komarek

Abstract: We report on the synthesis and physical properties of cm-sized CoGeO$_3$ single crystals grown in a high pressure mirror furnace at pressures of 80~bar. Direction dependent magnetic susceptibility measurements on our single crystals reveal highly anisotropic magnetic properties that we attribute to the impact of strong single ion anisotropy appearing in this system with T$_N$~$\sim$~33.5~K. Furthe… ▽ More We report on the synthesis and physical properties of cm-sized CoGeO$_3$ single crystals grown in a high pressure mirror furnace at pressures of 80~bar. Direction dependent magnetic susceptibility measurements on our single crystals reveal highly anisotropic magnetic properties that we attribute to the impact of strong single ion anisotropy appearing in this system with T$_N$~$\sim$~33.5~K. Furthermore, we observe effective magnetic moments that are exceeding the spin only values of the Co ions which reveals the presence of sizable orbital moments in CoGeO$_3$. △ Less

Submitted 26 March, 2021; originally announced March 2021.

Journal ref: Crystals 11(4), 378 (2021)

Crystal structure and properties of iron-based spin-chain compound Ba9Fe3Se15

Authors: Jun Zhang, Alexander C. Komarek, Meiling Jin, Xiancheng Wang, Yating Jia, Jianfa Zhao, Wenmin Li, Zhiwei Hu, Wei Peng, L. H. Tjeng, Zheng Deng, Runze Yu, Shaomin Feng, Sijia Zhang, Min Liu, Yi-feng Yang, Hong-ji Lin, Chien-Te Chen, Xiaodong Li, Jinlong Zhu, Changqing Jin

Abstract: We report the synthesis of a new quasi one-dimensional (1D) iron selenide. Ba9Fe3Se15 was synthesized at high temperature and high pressure of 5.5 GPa and systematically studied via structural, magnetic and transport measurements at ambient and at high-pressures. Ba9Fe3Se15 crystallizes in a monoclinic structure and consists of face-sharing FeSe6 octahedral chains along the c axis. At ambient pres… ▽ More We report the synthesis of a new quasi one-dimensional (1D) iron selenide. Ba9Fe3Se15 was synthesized at high temperature and high pressure of 5.5 GPa and systematically studied via structural, magnetic and transport measurements at ambient and at high-pressures. Ba9Fe3Se15 crystallizes in a monoclinic structure and consists of face-sharing FeSe6 octahedral chains along the c axis. At ambient pressure it exhibits an insulating behavior with a band gap ~460 meV and undergoes a ferrimagnet-like phase transition at 14 K. Under high pressure, a complete metallization occurs at ~29 GPa, which is accompanied by a spin state crossover from high spin (HS) state to low spin (LS) state. The LS appears for pressures P >36 GPa. △ Less

Submitted 6 February, 2021; originally announced February 2021.

Journal ref: Phys. Rev. Materials 5, 054606 (2021)

Selective orbital imaging of excited states with x-ray spectroscopy: the example of $α$-MnS

Authors: A. Amorese, B. Leedahl, M. Sundermann, H. Gretarsson, Z. Hu, H. -J. Lin, C. T. Chen, M. Schmidt, H. Borrmann, Yu. Grin, A. Severing, M. W. Haverkort, L. H. Tjeng

Abstract: Herein we show that non-resonant inelastic x-ray scattering involving an $s$ core level is a powerful spectroscopic method to characterize the excited states of transition metal compounds. The spherical charge distribution of the $s$ core hole allows the orientational dependence of the intensities of the various spectral features to produce a spatial charge image of the associated multiplet states… ▽ More Herein we show that non-resonant inelastic x-ray scattering involving an $s$ core level is a powerful spectroscopic method to characterize the excited states of transition metal compounds. The spherical charge distribution of the $s$ core hole allows the orientational dependence of the intensities of the various spectral features to produce a spatial charge image of the associated multiplet states in a straightforward manner, thereby facilitating the identification of their orbital character. In addition, the $s$ core hole does not add an extra orbital angular momentum component to the multiplet structure so that the well-established Sugano-Tanabe-Kamimura diagrams can be used for the analysis of the spectra. For $α$-MnS we observe the spherical charge density corresponding to its high spin $3d^5$ ($^6A_1$) ground state configuration and we were able to selectively image its excited states and identify them as $t_{2g}$ ($^5T_2$) and $e_g$ ($^5E$) with an energy splitting $10Dq$ of 0.78\,eV. △ Less

Submitted 6 January, 2021; originally announced January 2021.

Journal ref: Physical Review X 11, 011002 (2021)

Possible multi-orbital ground state in CeCu$_2$Si$_2$

Authors: Andrea Amorese, Andrea Marino, Martin Sundermann, Kai Chen, Zhiwei Hu, Thomas Willers, Fadi Choukani, Philippe Ohresser, Javier Herrero-Martin, Stefano Agrestini, Chien-Te Chen, Hong-Ji Lin, Maurits W. Haverkort, Silvia Seiro, Christoph Geibel, Frank Steglich, Liu Hao Tjeng, Gertrud Zwicknagl, Andrea Severing

Abstract: The crystal-field ground state wave function of CeCu$_2$Si$_2$ has been investigated with linear polarized $M$-edge x-ray absorption spectroscopy from 250mK to 250K, thus covering the superconducting ($T_{\text{c}}$=0.6K), the Kondo ($T_{\text{K}}$$\approx$20K) as well as the Curie-Weiss regime. The comparison with full-multiplet calculations shows that the temperature dependence of the experiment… ▽ More The crystal-field ground state wave function of CeCu$_2$Si$_2$ has been investigated with linear polarized $M$-edge x-ray absorption spectroscopy from 250mK to 250K, thus covering the superconducting ($T_{\text{c}}$=0.6K), the Kondo ($T_{\text{K}}$$\approx$20K) as well as the Curie-Weiss regime. The comparison with full-multiplet calculations shows that the temperature dependence of the experimental linear dichroism is well explained with a $Γ_7^{(1)}$ crystal-field ground-state and the thermal population of excited states at around 30meV. The crystal-field scheme does not change throughout the entire temperature range thus making the scenario of orbital switching unlikely. Spectroscopic evidence for the presence of the Ce 4$f^0$ configuration in the ground state is consistent with the possibility for a multi-orbital character of the ground state. We estimate from the Kondo temperature and crystal-field splitting energies that several percents of the higher lying $Γ_6$ state and $Γ_7^{(2)}$ crystal-field states are mixed into the primarily $Γ_7^{(1)}$ ground state. This estimate is also supported by re-normalized band-structure calculations that uses the experimentally determined crystal-field scheme. △ Less

Submitted 5 January, 2021; v1 submitted 5 October, 2020; originally announced October 2020.

Comments: 8 pages, 5 figures

Journal ref: Phys. Rev. B 102, 245146 (2020)

New highly-anisotropic Rh-based Heusler compound for magnetic recording

Authors: Yangkun He, Gerhard H. Fecher, Chenguang Fu, Yu Pan, Kaustuv Manna, Johannes Kroder, Ajay Jha, Xiao Wang, Zhiwei Hu, Stefano Agrestini, Javier Herrero-Martin, Manuel Valvidares, Yurii Skourski, Walter Schnelle, Plamen Stamenov, Horst Borrmann, Liu Hao Tjeng, Rudolf Schaefer, S. S. P. Parkin, J. M. D. Coey, Claudia Felser

Abstract: The development of high-density magnetic recording media is limited by the superparamagnetism in very small ferromagnetic crystals. Hard magnetic materials with strong perpendicular anisotropy offer stability and high recording density. To overcome the difficulty of writing media with a large coercivity, heat assisted magnetic recording (HAMR) has been developed, rapidly heating the media to the C… ▽ More The development of high-density magnetic recording media is limited by the superparamagnetism in very small ferromagnetic crystals. Hard magnetic materials with strong perpendicular anisotropy offer stability and high recording density. To overcome the difficulty of writing media with a large coercivity, heat assisted magnetic recording (HAMR) has been developed, rapidly heating the media to the Curie temperature Tc before writing, followed by rapid cooling. Requirements are a suitable Tc, coupled with anisotropic thermal conductivity and hard magnetic properties. Here we introduce Rh2CoSb as a new hard magnet with potential for thin film magnetic recording. A magnetocrystalline anisotropy of 3.6 MJm-3 is combined with a saturation magnetization of μ0Ms = 0.52 T at 2 K (2.2 MJm-3 and 0.44 T at room-temperature). The magnetic hardness parameter of 3.7 at room temperature is the highest observed for any rare-earth free hard magnet. The anisotropy is related to an unquenched orbital moment of 0.42 μB on Co, which is hybridized with neighbouring Rh atoms with a large spin-orbit interaction. Moreover, the pronounced temperature-dependence of the anisotropy that follows from its Tc of 450 K, together with a high thermal conductivity of 20 Wm-1K-1, makes Rh2CoSb a candidate for development for heat assisted writing with a recording density in excess of 10 Tb/in2. △ Less

Submitted 2 September, 2020; originally announced September 2020.

Journal ref: Advanced Materials 2020

Charge disproportionation and nano phase separation in $R$SrNiO$_4$

Authors: H. Guo, Z. W. Li, C. F. Chang, Z. Hu, C. -Y. Kuo, T. G. Perring, W. Schmidt, A. Piovano, K. Schmalzl, H. C. Walker, H. J. Lin, C. T. Chen, S. Blanco-Canosa, J. Schlappa, C. Schüßler-Langeheine, P. Hansmann, D. I. Khomskii, L. H. Tjeng, A. C. Komarek

Abstract: We have successfully grown centimeter-sized layered $R$SrNiO$_4$ single crystals under high oxygen pressures of 120 bar by the floating zone technique. This enabled us to perform neutron scattering experiments where we observe close to quarter-integer magnetic peaks below $\sim$77 K that are accompanied by steep upwards dispersing spin excitations. Within the high-frequency Ni-O bond stretching ph… ▽ More We have successfully grown centimeter-sized layered $R$SrNiO$_4$ single crystals under high oxygen pressures of 120 bar by the floating zone technique. This enabled us to perform neutron scattering experiments where we observe close to quarter-integer magnetic peaks below $\sim$77 K that are accompanied by steep upwards dispersing spin excitations. Within the high-frequency Ni-O bond stretching phonon dispersion, a softening at the propagation vector for a checkerboard modulation can be observed. Together with our spin wave simulations these observations reveal that this Ni$^{3+}$ system exhibits charge disproportionation with charges segregating into a checkerboard pattern within a nano phase separation scenario rather than showing a Jahn-Teller effect. △ Less

Submitted 7 July, 2020; originally announced July 2020.

Journal ref: Sci. Rep. 10, 18012 (2020)

Interfacing topological insulators and ferrimagnets: Bi$_2$Te$_3$ and Fe$_3$O$_4$ heterostructures grown by molecular beam epitaxy

Authors: V. M. Pereira, C. N. Wu, C. -A. Knight, A. Choa, L. H. Tjeng, S. G. Altendorf

Abstract: Relying on the magnetism induced by the proximity effect in heterostructures of topological insulators and magnetic insulators is one of the promising routes to achieve the quantum anomalous Hall effect. Here we investigate heterostructures of Bi$_2$Te$_3$ and Fe$_3$O$_4$. By growing two different types of heterostructures by molecular beam epitaxy, Fe$_3$O$_4$ on Bi$_2$Te$_3$ and Bi$_2$Te$_3$ on… ▽ More Relying on the magnetism induced by the proximity effect in heterostructures of topological insulators and magnetic insulators is one of the promising routes to achieve the quantum anomalous Hall effect. Here we investigate heterostructures of Bi$_2$Te$_3$ and Fe$_3$O$_4$. By growing two different types of heterostructures by molecular beam epitaxy, Fe$_3$O$_4$ on Bi$_2$Te$_3$ and Bi$_2$Te$_3$ on Fe$_3$O$_4$, we explore differences in chemical stability, crystalline quality, electronic structure, and transport properties. We find the heterostructure Bi$_2$Te$_3$ on Fe$_3$O$_4$ to be a more viable approach, with transport signatures in agreement with a gap opening in the topological surface states. △ Less

Submitted 26 June, 2020; originally announced June 2020.

Comments: accepted for publication in APL Materials

Journal ref: APL Materials 8, 071114 (2020)

Charge transfer energy in iridates: a hard x-ray photoelectron spectroscopy study

Authors: D. Takegami, D. Kasinathan, K. K. Wolff, S. G. Altendorf, C. F. Chang, K. Hoefer, A. Melendez-Sans, Y. Utsumi, F. Meneghin, T. D. Ha, C. H. Yen, K. Chen, C. Y. Kuo, Y. F. Liao, K. D. Tsuei, R. Morrow, S. Wurmehl, B. Büchner, B. E. Prasad, M. Jansen, A. C. Komarek, P. Hansmann, L. H. Tjeng

Abstract: We have investigated the electronic structure of iridates in the double perovskite crystal structure containing either Ir$^{4+}$ or Ir$^{5+}$ using hard x-ray photoelectron spectroscopy. The experimental valence band spectra can be well reproduced using tight binding calculations including only the Ir $5d$, O $2p$ and O $2s$ orbitals with parameters based on the downfolding of the density-function… ▽ More We have investigated the electronic structure of iridates in the double perovskite crystal structure containing either Ir$^{4+}$ or Ir$^{5+}$ using hard x-ray photoelectron spectroscopy. The experimental valence band spectra can be well reproduced using tight binding calculations including only the Ir $5d$, O $2p$ and O $2s$ orbitals with parameters based on the downfolding of the density-functional band structure results. We found that regardless of the A and B cations, the A$_2$BIrO$_6$ iridates have essentially zero O $2p$ to Ir $5d$ charge transfer energies. Hence, double perovskite iridates turn out to be extremely covalent systems with the consequence being that the magnetic exchange interactions become very long-ranged, thereby hampering the materialization of the long-sought Kitaev physics. Nevertheless, it still would be possible to realize a spin-liquid system using the iridates with a proper tuning of the various competing exchange interactions. △ Less

Submitted 25 May, 2020; originally announced May 2020.

Comments: 9 pages, 7 figures, submitted to Physical Review B

Journal ref: Phys. Rev. B 102, 045119 (2020)

Impact of Fe substitution on the electronic structure of URu$_2$Si$_2$

Authors: Martin Sundermann, Andrea Amorese, Daisuke Takegami, Hlynur Gretarsson, Hasan Yavaş, Andrei Gloskovskii, Christoph Schlueter, Sheng Ran, M. Brian Maple, Peter Thalmeier, Liu Hao Tjeng, Andrea Severing

Abstract: The application of pressure as well as the successive substitution of Ru with Fe in the hidden order (HO) compound URu$_2$Si$_2$ leads to the formation of the large moment antiferromagnetic phase (LMAFM). Here we have investigated the substitution series URu$_{2-x}$Fe$_x$Si$_2$ with $x$ = 0.2 and 0.3 with non-resonant inelastic x-ray scattering (NIXS) and 4$f$ core-level photoelectron spectroscopy… ▽ More The application of pressure as well as the successive substitution of Ru with Fe in the hidden order (HO) compound URu$_2$Si$_2$ leads to the formation of the large moment antiferromagnetic phase (LMAFM). Here we have investigated the substitution series URu$_{2-x}$Fe$_x$Si$_2$ with $x$ = 0.2 and 0.3 with non-resonant inelastic x-ray scattering (NIXS) and 4$f$ core-level photoelectron spectroscopy with hard x-rays (HAXPES). NIXS shows that the substitution of Fe has no impact on the symmetry of the ground-state wave function. In HAXPES we find no shift of spectral weight that would be indicative for a change of the 5$f$-electron count. Consequently, changes in the exchange interaction $\cal{J}$ due to substitution must be minor so that the conjecture of chemical pressure seems unlikely. An alternative scenario is discussed, namely the formation of long range magnetic order due the substitution induced local enhancement of the magnetization in the vicinity of the $f$-electron ions while the overall electronic structure remains unchanged. △ Less

Submitted 8 May, 2020; originally announced May 2020.

Comments: 6 pages, 3 figures

Dual nature of 5$f$ electrons in the isostructural UM$_2$Si$_2$ family: from antiferro- to Pauli paramagnetism via hidden order

Authors: Andrea Amorese, Martin Sundermann, Brett Leedahl, Andrea Marino, Daisuke Takegami, Hlynur Gretarsson, Andrei Hloskovsky, Christoph Schlüter, Maurits W. Haverkort, Yingkai Huang, Maria Szlawska, Dariusz Kaczorowski, Sheng Ran, M. Brian Maple, Eric D. Bauer, Andreas Leithe-Jasper, Peter Thalmeier, Liu Hao Tjeng, Andrea Severing

Abstract: Using inelastic x-ray scattering beyond the dipole limit and hard x-ray photoelectron spectroscopy we establish the dual nature of the U $5f$ electrons in UM$_2$Si$_2$ (M = Pd, Ni, Ru, Fe), regardless of their degree of delocalization. We have observed that the compounds have in common a local atomic-like state that is well described by the U $5f^2$ configuration with the $Γ_1^{(1)}$ and $Γ_2$ qua… ▽ More Using inelastic x-ray scattering beyond the dipole limit and hard x-ray photoelectron spectroscopy we establish the dual nature of the U $5f$ electrons in UM$_2$Si$_2$ (M = Pd, Ni, Ru, Fe), regardless of their degree of delocalization. We have observed that the compounds have in common a local atomic-like state that is well described by the U $5f^2$ configuration with the $Γ_1^{(1)}$ and $Γ_2$ quasi-doublet symmetry. The amount of the U 5$f^3$ configuration, however, varies considerably across the UM$_2$Si$_2$ series, indicating an increase of U5$f$ itineracy in going from M=Pd to Ni to Ru, and to the Fe compound. The identified electronic states explain the formation of the very large ordered magnetic moments in UPd$_2$Si$_2$ and UNi$_2$Si$_2$, the availability of orbital degrees of freedom needed for the hidden order in URu$_2$Si$_2$ to occur, as well as the appearance of Pauli paramagnetism in UFe$_2$Si$_2$. A unified and systematic picture of the U$M_2$Si$_2$ compounds may now be drawn, thereby providing suggestions for new experiments to induce hidden order and/or superconductivity in U compounds with the tetragonal body-centered ThCr$_2$Si$_2$ structure. △ Less

Submitted 15 December, 2020; v1 submitted 28 April, 2020; originally announced April 2020.

Journal ref: Proceedings of the National Academy of Sciences Dec 2020, 117 (48) 30220-30227

Room-temperature ferrimagnetism of anti-site-disordered Ca2MnOsO6

Authors: Hai L. Feng, Madhav Prasad Ghimire, Zhiwei Hu, Sheng-Chieh Liao, Stefano Agrestini, Jie Chen, Yahua Yuan, Yoshitaka Matsushita, Yoshihiro Tsujimoto, Yoshio Katsuya, Masahiko Tanaka, Hong-Ji Lin, Chien-Te Chen, Shih-Chang Weng, Manuel Valvidares, Kai Chen, Francois Baudelet, Arata Tanaka, Martha Greenblatt, Liu Hao Tjeng, Kazunari Yamaura

Abstract: Room-temperature ferrimagnetism was discovered for the anti-site-disordered perovskite Ca2MnOsO6 with Tc = 305 K. Ca2MnOsO6 crystallizes into an orthorhombic structure with a space group of Pnma, in which Mn and Os share the oxygen-coordinated-octahedral site at an equal ratio without a noticeable ordered arrangement. The material is electrically semiconducting with variable-range-hopping behavior… ▽ More Room-temperature ferrimagnetism was discovered for the anti-site-disordered perovskite Ca2MnOsO6 with Tc = 305 K. Ca2MnOsO6 crystallizes into an orthorhombic structure with a space group of Pnma, in which Mn and Os share the oxygen-coordinated-octahedral site at an equal ratio without a noticeable ordered arrangement. The material is electrically semiconducting with variable-range-hopping behavior. X-ray absorption spectroscopy confirmed the trivalent state of the Mn and the pentavalent state of the Os. X-ray magnetic circular dichroism spectroscopy reveals that the Mn and Os magnetic moments are aligned antiferromagnetically, thereby classifying the material as a ferrimagnet which is in accordance with band structure calculations. It is intriguing that the magnetic signal of the Os is very weak, and that the observed total magnetic moment is primarily due to the Mn. The Tc = 305 K is the second highest in the material category of so-called disordered ferromagnets such as CaRu1-xMnxO3, SrRu1-xCrxO3, and CaIr1-xMnxO3, and hence, may support the development of spintronic oxides with relaxed requirements concerning the anti-site disorder of the magnetic ions. △ Less

Submitted 10 March, 2020; originally announced March 2020.

Molecular beam epitaxy preparation and in situ characterization of FeTe thin films

Authors: V. M. Pereira, C. N. Wu, C. E. Liu, S. -S. Liao, C. F. Chang, C. -Y. Kuo, C. Koz, U. Schwarz, H. -J. Lin, C. T. Chen. L. H. Tjeng, S. G. Altendorf

Abstract: We have synthesized Fe$_{1+y}$Te thin films by means of molecular beam epitaxy (MBE) under Te-limited growth conditions. We found that epitaxial layer-by-layer growth is possible for a wide range of excess Fe values, wider than expected from what is known on the bulk material. Using x-ray magnetic circular dichroism spectroscopy at the Fe L$_{2,3}$ and Te M$_{4,5}$ edges, we observed that films wi… ▽ More We have synthesized Fe$_{1+y}$Te thin films by means of molecular beam epitaxy (MBE) under Te-limited growth conditions. We found that epitaxial layer-by-layer growth is possible for a wide range of excess Fe values, wider than expected from what is known on the bulk material. Using x-ray magnetic circular dichroism spectroscopy at the Fe L$_{2,3}$ and Te M$_{4,5}$ edges, we observed that films with high excess Fe contain ferromagnetic clusters while films with lower excess Fe remain nonmagnetic. Moreover, x-ray absorption spectroscopy showed that it is possible to obtain films with very similar electronic structure as that of a high quality bulk single crystal Fe$_{1.14}$Te. Our results suggest that MBE with Te-limited growth may provide an opportunity to synthesize FeTe films with smaller amounts of excess Fe as to come closer to a possible superconducting phase. △ Less

Submitted 21 February, 2020; originally announced February 2020.

Comments: 6 pages, 6 figures accepted for publication in Physical Review Materials

Journal ref: Phys. Rev. Materials 4, 023405, 2020

Origin of Ising magnetism in Ca3Co2O6 unveiled by orbital imaging

Authors: Brett Leedahl, Martin Sundermann, Andrea Amorese, Andrea Severing, Hlynur Gretarsson, Lunyong Zhang, Alexander C. Komarek, Maurits W. Haverkort, Antoine Maignan, Liu Hao Tjeng

Abstract: The one-dimensional cobaltate Ca3Co2O6 is an intriguing material having an unconventional magnetic structure, displaying quantum tunneling phenomena in its magnetization. Using a newly developed experimental method, s-core-level non-resonant inelastic x-ray scattering (s-NIXS), we were able to image the atomic Co 3d orbital that is responsible for the Ising magnetism in this system. We show that w… ▽ More The one-dimensional cobaltate Ca3Co2O6 is an intriguing material having an unconventional magnetic structure, displaying quantum tunneling phenomena in its magnetization. Using a newly developed experimental method, s-core-level non-resonant inelastic x-ray scattering (s-NIXS), we were able to image the atomic Co 3d orbital that is responsible for the Ising magnetism in this system. We show that we can directly observe that it is the complex d2 orbital occupied by the sixth electron at the high-spin Co-trig{3+} (d6) sites that generates this behavior. This is extremely rare in the research field of transition metal compounds, and is only made possible by the delicately balanced prismatic trigonal coordination. The ability to directly relate the orbital occupation with the local crystal structure is essential to model the magnetic properties of this system. △ Less

Submitted 30 October, 2019; originally announced October 2019.

Journal ref: Nature Communications. 10, 5447 (2019)

Nature of the magnetism of iridium in the double perovskite Sr2CoIrO6

Authors: S. Agrestini, K. Chen, C. -Y. Kuo, L. Zhao, H. -J. Lin, C. -T. Chen, A. Rogalev, P. Ohresser, T. -S. Chan, S. -C. Weng, A. C. Komarek, K. Yamaura, M. W. Haverkort, Z. Hu, L. H. Tjeng

Abstract: We report on our investigation on the magnetism of the iridate double perovskite Sr$_2$CoIrO$_6$, a nominally Ir$^{5+}$ Van Vleck $J_{eff}=0$ system. Using x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy at the Ir-$L_{2,3}$ edges, we found a nearly zero orbital contribution to the magnetic moment and thus an apparent breakdown of the $J_{eff}=0$ ground state. By ca… ▽ More We report on our investigation on the magnetism of the iridate double perovskite Sr$_2$CoIrO$_6$, a nominally Ir$^{5+}$ Van Vleck $J_{eff}=0$ system. Using x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy at the Ir-$L_{2,3}$ edges, we found a nearly zero orbital contribution to the magnetic moment and thus an apparent breakdown of the $J_{eff}=0$ ground state. By carrying out also XAS and XMCD experiments at the Co-$L_{2,3}$ edges and by performing detailed full atomic multiplet calculations to simulate all spectra, we discovered that the compound consists of about 90% Ir$^{5+}$ ($J_{eff}=0$) and Co$^{3+}$ ($S=2$) and 10% Ir$^{6+}$ ($S=3/2$) and Co$^{2+}$ ($S=3/2$). The magnetic signal of this minority Ir$^{6+}$ component is almost equally strong as that of the main Ir$^{5+}$ component. We infer that there is a competition between the Ir$^{5+}$-Co$^{3+}$ and the Ir$^{6+}$-Co$^{2+}$ configurations in this stoichiometric compound. △ Less

Submitted 7 June, 2019; originally announced June 2019.

Journal ref: Phys. Rev. B 100, 014443 (2019)

Spin-orbit coupling and crystal-field distortions for a low-spin $3d^5$ state in BaCoO$_{3}$

Authors: Y. Y. Chin, Z. Hu, H. -J. Lin, S. Agrestini, J. Weinen, C. Martin, S. Hébert, A. Maignan, A. Tanaka, J. C. Cezar, N. B. Brookes, Y. -F. Liao, K. -D. Tsuei, C. T. Chen, D. I. Khomskii, L. H. Tjeng

Abstract: We have studied the electronic structure of BaCoO$_3$ using soft x-ray absorption spectroscopy at the Co-$L_{2,3}$ and O-$K$ edges, magnetic circular dichroism at the Co-$L_{2,3}$ edges, as well as valence band hard x-ray photoelectron spectroscopy. The quantitative analysis of the spectra established that the Co ions are in the formal low-spin tetravalent 3$d^5$ state and that the system is a neg… ▽ More We have studied the electronic structure of BaCoO$_3$ using soft x-ray absorption spectroscopy at the Co-$L_{2,3}$ and O-$K$ edges, magnetic circular dichroism at the Co-$L_{2,3}$ edges, as well as valence band hard x-ray photoelectron spectroscopy. The quantitative analysis of the spectra established that the Co ions are in the formal low-spin tetravalent 3$d^5$ state and that the system is a negative charge transfer Mott insulator. The spin-orbit coupling plays also an important role for the magnetism of the system. At the same time, a trigonal crystal field is present with sufficient strength to bring the 3$d^5$ ion away from the $J_{eff} = 1/2$ state. The sign of this crystal field is such that the $a_{1g}$ orbital is doubly occupied, explaining the absence of a Peierl's transition in this system which consists of chains of face-sharing CoO$_6$ octahedra. Moreover, with one hole residing in the $e_g^π$, the presence of an orbital moment and strong magneto-crystalline anisotropy can be understood. Yet, we also infer that crystal fields with lower symmetry must be present to reproduce the measured orbital moment quantitatively, thereby suggesting the possibility for orbital ordering to occur in BaCoO$_3$. △ Less

Submitted 23 May, 2019; originally announced May 2019.

Journal ref: Phys. Rev. B 100, 205139 (2019)

Direct imaging of orbitals in quantum materials

Authors: Hasan Yavaş, Martin Sundermann, Kai Chen, Andrea Amorese, Andrea Severing, Hlynur Gretarsson, Maurits W. Haverkort, Liu Hao Tjeng

Abstract: The spectacular physical properties of quantum materials based on transition metal, rare earth, and actinide elements continue to challenge our comprehension of solid state physics and chemistry. The electronic states of these materials are dominated by the $d$ and $f$ wave functions intertwined with the strong band formation of the solid. In order to estimate which wave functions contribute to th… ▽ More The spectacular physical properties of quantum materials based on transition metal, rare earth, and actinide elements continue to challenge our comprehension of solid state physics and chemistry. The electronic states of these materials are dominated by the $d$ and $f$ wave functions intertwined with the strong band formation of the solid. In order to estimate which wave functions contribute to the ground state formation, we have had to rely, until now, on theoretical calculations combined with spectroscopy. Here we show that $s$-core-level non-resonant inelastic x-ray scattering ($s$-NIXS) can directly image the active orbital in real space, without the necessity of any modeling. The power and accuracy of this new technique is shown using the text-book example, x$^2$-y$^2$/3$z^2$-r$^2$ orbital of the Ni$^{2+}$ ion in NiO single crystal. △ Less

Submitted 19 February, 2019; originally announced February 2019.

Comments: 6 pages, 4 figures

Journal ref: Nature Physics 15, 559 (2019)

Orientation of ground-state orbital in CeCoIn$_5$ and CeRhIn$_5$

Authors: M. Sundermann, A. Amorese, F. Strigari, B. Leedahl, M. W. Haverkort, H. Gretarsson, L. H. Tjeng, M. Moretti Sala, H. Yavş, E. D. Bauer, P. F. S. Rosa, J. D. Thompson, A. Severing

Abstract: We present core level non-resonant inelastic x-ray scattering (NIXS) data of the heavy fermion compounds CeCoIn$_5$ and CeRhIn$_5$ measured at the Ce $N_{4,5}$-edges. The higher than dipole transitions in NIXS allow determining the orientation of the $Γ_7$ crystal-field ground-state orbital within the unit cell. The crystal-field parameters of the Ce$M$In$_5$ compounds and related substitution pha… ▽ More We present core level non-resonant inelastic x-ray scattering (NIXS) data of the heavy fermion compounds CeCoIn$_5$ and CeRhIn$_5$ measured at the Ce $N_{4,5}$-edges. The higher than dipole transitions in NIXS allow determining the orientation of the $Γ_7$ crystal-field ground-state orbital within the unit cell. The crystal-field parameters of the Ce$M$In$_5$ compounds and related substitution phase diagrams have been investigated in great detail in the past; however, whether the ground-state wavefunction is the $Γ_7^+$ ($x^2\,-\,y^2$) or $Γ_7^-$ ($xy$ orientation) remained undetermined. We show that the $Γ_7^-$ doublet with lobes along the (110) direction forms the ground state in CeCoIn$_5$ and CeRhIn$_5$. For CeCoIn$_5$, however, we find also some contribution of the first excited state crystal-field state in the ground state due to the stronger hybridization of 4$f$ and conduction electrons, suggesting a smaller $α^2$ value than originally anticipated from x-ray absorption. A comparison is made to the results of existing density functional theory plus dynamical mean-field theory calculations. △ Less

Submitted 25 June, 2019; v1 submitted 18 February, 2019; originally announced February 2019.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 99, 235143 (2019)

A RIXS investigation of the crystal-field splitting of Sm$^{3+}$ in SmB$_6$

Authors: Andrea Amorese, Oliver Stockert, Kurt Kummer, Nickolas B. Brookes, Dae-Jeong Kim, Zachary Fisk, Maurits W. Haverkort, Peter Thalmeier, Liu Hao Tjeng, Andrea Severing

Abstract: The crystal-field (CF) splitting of the $^6H_{5/2}$ Hund's rule ground state of Sm$^{3+}$ in the strongly correlated topological insulator SmB$_6$ has been determined with high resolution resonant inelastic x-ray scattering (RIXS) at the Sm M$_5$ edge. The valence selectivity of RIXS allows isolating the crystal-field-split excited multiplets of the Sm$^{3+}$ (4$f^5$) configuration from those of S… ▽ More The crystal-field (CF) splitting of the $^6H_{5/2}$ Hund's rule ground state of Sm$^{3+}$ in the strongly correlated topological insulator SmB$_6$ has been determined with high resolution resonant inelastic x-ray scattering (RIXS) at the Sm M$_5$ edge. The valence selectivity of RIXS allows isolating the crystal-field-split excited multiplets of the Sm$^{3+}$ (4$f^5$) configuration from those of Sm$^{2+}$ (4$f^6$) in intermediate valent SmB$_6$. The very large energy range of RIXS allows the crystal-field analysis of a high lying multiplet at about 2.4\,eV that has the same total angular momentum $J$ as the ground state so that ambiguities due to the elastic tail can be avoided. We find that the $Γ_7$ doublet and $Γ_8$ quartet of the $^6H_{5/2}$ Hund's rule ground state are split by $Δ^{CF}_{^6H_{5/2}}$\,=\,20$\pm$10\,meV which sets an upper limit for the 4$f$ band width. This indicates an extremely large mass renormalization from the band structure value, pointing out the need to consider the coefficients of fractional parentage for the hopping of the 4$f$ electrons. △ Less

Submitted 25 June, 2019; v1 submitted 30 January, 2019; originally announced January 2019.

Comments: 8 pages, 6 figures

Journal ref: Phys. Rev. B 100, 241107 (2019)

Determining the local low-energy excitations in the Kondo semimetal CeRu$_4$Sn$_6$ using resonant inelastic x-ray scattering

Authors: Andrea Amorese, Kurt Kummer, Nickolas B. Brookes, Oliver Stockert, Devashibhai T. Adroja, Andre M. Stryodm, Andrey Sidorenko, Hannes Winkler, Diego A. Zocco, Andrey Prokofiev, Silke Paschen, Maurits W. Haverkort, Liu Hao Tjeng, Andrea Severing

Abstract: We have investigated the local low-energy excitations in CeRu$_4$Sn$_6$, a material discussed recently in the framework of strongly correlated Weyl semimetals, by means of Ce $M_5$ resonant inelastic x-ray scattering (RIXS). The availability of both $^2$F$_\frac{5}{2}$ and $^2$F$_\frac{7}{2}$ excitations of the Ce $4f^1$ configuration in the spectra allows for the determination of the crystal-elec… ▽ More We have investigated the local low-energy excitations in CeRu$_4$Sn$_6$, a material discussed recently in the framework of strongly correlated Weyl semimetals, by means of Ce $M_5$ resonant inelastic x-ray scattering (RIXS). The availability of both $^2$F$_\frac{5}{2}$ and $^2$F$_\frac{7}{2}$ excitations of the Ce $4f^1$ configuration in the spectra allows for the determination of the crystal-electric field parameters that explain quantitatively the temperature dependence and anisotropy of the magnetic susceptibility. The absence of an azimuthal dependence in the spectra indicates that all crystal-electric field states are close to being rotational symmetric. We show further that the non-negligible impact of the $\check A_6^0$ parameter on the ground state of CeRu$_4$Sn$_6$ leads to a reduction of the magnetic moment due to multiplet intermixing. The RIXS results are consistent with inelastic neutron scattering (INS) data and are compared to the predictions from \textsl{ab-initio} based electronic structure calculations. △ Less

Submitted 13 June, 2018; originally announced June 2018.

Comments: 9 pages and 7 figures, submitted

Journal ref: Phys. Rev. B 98, 081116 (2018)

Probing the $J_{eff}=0$ ground state and the Van Vleck paramagnetism of the Ir$^{5+}$ ions in the layered Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$

Authors: S. Agrestini, C. -Y. Kuo, K. Chen, Y. Utsumi, D. Mikhailova, A. Rogalev, F. Wilhelm, T. Förster, A. Matsumoto, T. Takayama, H. Takagi, M. W. Haverkort, Z. Hu, L. H. Tjeng

Abstract: We report a combined experimental and theoretical x-ray magnetic circular dichroism (XMCD) spectroscopy study at the Ir-$L_{2,3}$ edges on the Ir$^{5+}$ ions of the layered hybrid solid state oxide Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$ with the K$_2$NiF$_4$ structure. From theoretical simulation of the experimental Ir-$L_{2,3}$ XMCD spectrum, we found a deviation from a pure $J_{eff}=0$ ground state wit… ▽ More We report a combined experimental and theoretical x-ray magnetic circular dichroism (XMCD) spectroscopy study at the Ir-$L_{2,3}$ edges on the Ir$^{5+}$ ions of the layered hybrid solid state oxide Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$ with the K$_2$NiF$_4$ structure. From theoretical simulation of the experimental Ir-$L_{2,3}$ XMCD spectrum, we found a deviation from a pure $J_{eff}=0$ ground state with an anisotropic orbital-to-spin moment ratio ($L_x/2S_x$ = 0.43 and $L_z/2S_z$ = 0.78). This deviation is mainly due to multiplet interactions being not small compared to the cubic crystal field and due to the presence of a large tetragonal crystal field associated with the crystal structure. Nevertheless, our calculations show that the energy gap between the singlet ground state and the triplet excited state is still large and that the magnetic properties of the Ir$^{5+}$ ions can be well described in terms of singlet Van Vleck paramagnetism. △ Less

Submitted 23 February, 2018; originally announced February 2018.

Strain induced changes of electronic properties of B-site ordered double perovskite Sr$_2$CoIrO$_6$ thin films

Authors: S. Esser, C. F. Chang, C. -Y. Kuo, S. Merten, V. Roddatis, T. D. Ha, A. Jesche, V. Moshnyaga, H. -J. Lin, A. Tanaka, C. T. Chen, L. H. Tjeng, P. Gegenwart

Abstract: B-site ordered thin films of double perovskite Sr$_2$CoIrO$_6$ were epitaxially grown by a metal-organic aerosol deposition technique on various substrates, actuating different strain states. X-ray diffraction, transmission electron microscopy and polarized far-field Raman spectroscopy confirm the strained epitaxial growth on all used substrates. Polarization dependent Co $L_{2,3}$ X-ray absorptio… ▽ More B-site ordered thin films of double perovskite Sr$_2$CoIrO$_6$ were epitaxially grown by a metal-organic aerosol deposition technique on various substrates, actuating different strain states. X-ray diffraction, transmission electron microscopy and polarized far-field Raman spectroscopy confirm the strained epitaxial growth on all used substrates. Polarization dependent Co $L_{2,3}$ X-ray absorption spectroscopy reveals a change of the magnetic easy axis of the antiferromagnetically ordered (high-spin) Co$^{3+}$ sublattice within the strain series. By reversing the applied strain direction from tensile to compressive, the easy axis changes abruptly from in-plane to out-of-plane orientation. The low-temperature magnetoresistance changes its sign respectively and is described by a combination of weak anti-localization and anisotropic magnetoresistance effects. △ Less

Submitted 16 April, 2018; v1 submitted 11 January, 2018; originally announced January 2018.

Comments: 19 pages, 29 figures (including supplemental material)

Journal ref: Phys. Rev. B 97, 205121 (2018)

The role of nonmagnetic d0 vs. d10 B-type cations on the magnetic exchange interactions in osmium double perovskites

Authors: Hai L. Feng, Kazunari Yamaura, Liu Hao Tjeng, Martin Jansen

Abstract: Polycrystalline samples of double perovskites Ba2BOsO6 (B = Sc, Y, In) were synthesized by solid state reactions. They adopt the cubic double perovskite structures (space group, Fm-3m) with ordered B and Os arrangements. Ba2BOsO6 (B = Sc, Y, In) show antiferromagnetic transitions at 93 K, 69 K, and 28 K, respectively. The Weiss-temperatures are -590 K for Ba2ScOsO6, -571 K for Ba2YOsO6, and -155 K… ▽ More Polycrystalline samples of double perovskites Ba2BOsO6 (B = Sc, Y, In) were synthesized by solid state reactions. They adopt the cubic double perovskite structures (space group, Fm-3m) with ordered B and Os arrangements. Ba2BOsO6 (B = Sc, Y, In) show antiferromagnetic transitions at 93 K, 69 K, and 28 K, respectively. The Weiss-temperatures are -590 K for Ba2ScOsO6, -571 K for Ba2YOsO6, and -155 K for Ba2InOsO6. Sc3+ and Y3+ have the open-shell d0 electronic configuration, while In3+ has the closed-shell d10. This indicates that a d0 B-type cation induces stronger overall magnetic exchange interactions in comparison to a d10. Comparison of Ba2BOsO6 (B = Sc, Y, In) to their Sr and Ca analogues shows that the structural distortions weaken the overall magnetic exchange interactions. △ Less

Submitted 6 December, 2017; originally announced December 2017.

Journal ref: J. Solid State Chem. 243 (2016) 119-123

The $c$-axis dimer and its electronic break-up: the insulator-to-metal transition in Ti$_2$O$_3$

Authors: C. F. Chang, T. C. Koethe, Z. Hu, J. Weinen, S. Agrestini, J. Gegner, H. Ott, G. Panaccione, Hua Wu, M. W. Haverkort, H. Roth, A. C. Komarek, F. Offi, G. Monaco, Y. -F. Liao, K. -D. Tsuei, H. -J. Lin, C. T. Chen, A. Tanaka, L. H. Tjeng

Abstract: We report on our investigation of the electronic structure of Ti$_2$O$_3$ using (hard) x-ray photoelectron and soft x-ray absorption spectroscopy. From the distinct satellite structures in the spectra we have been able to establish unambiguously that the Ti-Ti $c$-axis dimer in the corundum crystal structure is electronically present and forms an $a_{1g}a_{1g}$ molecular singlet in the low tempera… ▽ More We report on our investigation of the electronic structure of Ti$_2$O$_3$ using (hard) x-ray photoelectron and soft x-ray absorption spectroscopy. From the distinct satellite structures in the spectra we have been able to establish unambiguously that the Ti-Ti $c$-axis dimer in the corundum crystal structure is electronically present and forms an $a_{1g}a_{1g}$ molecular singlet in the low temperature insulating phase. Upon heating we observed a considerable spectral weight transfer to lower energies with orbital reconstruction. The insulator-metal transition may be viewed as a transition from a solid of isolated Ti-Ti molecules into a solid of electronically partially broken dimers where the Ti ions acquire additional hopping in the $a$-$b$ plane via the $e_g^π$ channel, the opening of which requires the consideration of the multiplet structure of the on-site Coulomb interaction. △ Less

Submitted 24 October, 2017; originally announced October 2017.

Comments: 7 pages, 4 figures

Journal ref: Phys. Rev. X 8, 021004 (2018)

Electronic signature of the vacancy ordering in NbO (Nb3O3)

Authors: A. K. Efimenko, N. Hollmann, K. Hoefer, J. Weinen, D. Takegami, K. K. Wolff, S. G. Altendorf, Z. Hu, A. D. Rata, A. C. Komarek, A. A. Nugroho, Y. F. Liao, K. -D. Tsuei, H. H. Hsieh, H. -J. Lin, C. T. Chen, L. H. Tjeng, D. Kasinathan

Abstract: We investigated the electronic structure of the vacancy-ordered 4d-transition metal monoxide NbO (Nb3O3) using angle-integrated soft- and hard-x-ray photoelectron spectroscopy as well as ultra-violet angle-resolved photoelectron spectroscopy. We found that density-functional-based band structure calculations can describe the spectral features accurately provided that self-interaction effects are t… ▽ More We investigated the electronic structure of the vacancy-ordered 4d-transition metal monoxide NbO (Nb3O3) using angle-integrated soft- and hard-x-ray photoelectron spectroscopy as well as ultra-violet angle-resolved photoelectron spectroscopy. We found that density-functional-based band structure calculations can describe the spectral features accurately provided that self-interaction effects are taken into account. In the angle-resolved spectra we were able to identify the so-called vacancy band that characterizes the ordering of the vacancies. This together with the band structure results indicates the important role of the very large inter-Nb-4d hybridization for the formation of the ordered vacancies and the high thermal stability of the ordered structure of niobium monoxide. △ Less

Submitted 20 September, 2017; originally announced September 2017.

Journal ref: Phys. Rev. B 96, 195112 (2017)

4$f$ crystal field ground state of the strongly correlated topological insulator SmB$_6$

Authors: Martin Sundermann, Hasan Yavaş, Kai Chen, Dae Jeong Kim, Zachary Fisk, Deepa Kasinathan, Maurits Wim Haverkort, peter Thalmeier, Andrea Severing, Liu Hao Tjeng

Abstract: We investigated the crystal-electric field ground state of the 4$f$ manifold in the strongly correlated topological insulator SmB$_6$ using core level non-resonant inelastic x-ray scattering (NIXS). The directional dependence of the scattering function that arises from higher multipole transitions establishes unambiguously that the $Γ_8$ quartet state of the Sm $f^5$ $J$=$5/2$ configuration govern… ▽ More We investigated the crystal-electric field ground state of the 4$f$ manifold in the strongly correlated topological insulator SmB$_6$ using core level non-resonant inelastic x-ray scattering (NIXS). The directional dependence of the scattering function that arises from higher multipole transitions establishes unambiguously that the $Γ_8$ quartet state of the Sm $f^5$ $J$=$5/2$ configuration governs the ground-state symmetry and hence the topological properties of SmB$_6$. Our findings contradict the results of density functional calculations reported so far. △ Less

Submitted 8 January, 2018; v1 submitted 25 June, 2017; originally announced June 2017.

Comments: 9 pages, 9 figures

Journal ref: Phys. Rev. Lett. 120, 016402 (2018)

Intricacies of the Co$^{3+}$ spin state in Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$: an x-ray absorption and magnetic circular dichroism study

Authors: S. Agrestini, C. -Y. Kuo, D. Mikhailova, K. Chen, P. Ohresser, T. W. Pi, H. Guo, A. C. Komarek, A. Tanaka, Z. Hu, L. H. Tjeng

Abstract: We report on a combined soft x-ray absorption and magnetic circular dichroism (XMCD) study at the Co-$L_{3,2}$ on the hybrid 3$d$/5$d$ solid state oxide Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$ with the K$_2$NiF$_4$ structure. Our data indicate unambiguously a pure high spin state $(S=2)$ for the Co$^{3+}$ (3$d^6$) ions with a significant unquenched orbital moment $L_z/2S_z=0.25$ despite the sizeable elong… ▽ More We report on a combined soft x-ray absorption and magnetic circular dichroism (XMCD) study at the Co-$L_{3,2}$ on the hybrid 3$d$/5$d$ solid state oxide Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$ with the K$_2$NiF$_4$ structure. Our data indicate unambiguously a pure high spin state $(S=2)$ for the Co$^{3+}$ (3$d^6$) ions with a significant unquenched orbital moment $L_z/2S_z=0.25$ despite the sizeable elongation of the CoO$_6$ octahedra. Using quantitative model calculations based on parameters consistent with our spectra, we have investigated the stability of this high spin state with respect to the competing low spin and intermediate spin states. △ Less

Submitted 12 May, 2017; originally announced May 2017.

Comments: 7 pages, 4 figures

Journal ref: Phys. Rev. B 95, 245131 (2017)