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This article is part of the themed collection: Recent Open Access Articles, A metal–insulator transition is observed in spin–orbit-coupled IrO2 thin films upon reduction of the film thickness. In the epitaxially grown samples, the critical thickness (t ∼ 1.5–2.2 nm) is found to depend on growth orientation (001), (100) or (110). Interestingly from the applied point of view, the insulating behavior is found even in polycrystalline ultrathin films. By analyzing the experimental electrical response with various theoretical models, we find good fits to the Efros–Shklovskii-VRH and the Arrhenius-type behaviors, which suggests an important role of electron correlations in determining the electrical properties of IrO2. Our magnetic measurements also point to a significant role of magnetic order. Altogether, our results would point to a mixed Slater- and Mott-type of insulator., This work was partially supported by the Spanish MINECO projects MAT2014-54425-R (MINECO/FEDER, UE), MAT2017-82970-C2-R (AEI/FEDER, UE), MAT2017-83468-R (AEI/FEDER, UE), MAT2017-87134-C02-01-R (AEI/FEDER, UE) and MAT2017-87134-C02-02-R (AEI/FEDER, UE), the Spanish MICINN project PID2020-115159GB-I00 / AEI / 10.13039/501100011033 and by the Aragon Regional Government (Projects No. E12-20R and E28-20R). E. A-E acknowledges the Spanish MINECO and the European Social Fund for an FPI (Formación de Personal Investigador, 2015) grant. R. B. acknowledges funding support from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement no. 665919. This research used resources of the APS, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We acknowledge Diamond Light Source for time on I20-Scanning under Proposal SP-17266. The research leading to this result has been supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation Horizon 2020. The authors acknowledge funding from the project Quantox of QuantERA ERA-NET Cofund of Quantum Technologies (Grant Agreement No. 731473) implemented within the European Union's Horizon 2020 Programme.

X-ray magnetic circular dichroism (XMCD) is a technique commonly used to probe magnetic properties of materials with element and orbital selectivity, which requires the use of circularly polarized (CP) X-rays. It is possible to accomplish XMCD experiments with fixed CP and alternating the magnetic field orientation, but most reliable data are obtained when alternating the magnetization orientation and the polarization between right and left helicities. A versatile strategy has been developed to perform XMCD experiments using a hard X-ray quarter-wave plate, at both polychromatic dispersive and conventional monochromatic optics, in combination with synchronous data acquisition. The switching frequency waveform is fed into a lock-in amplifier to detect and amplify the XMCD signal. The results on a reference sample demonstrate an improvement in data quality and acquisition time. The instrumentation successfully generated 98% of CP X-rays switching the beam helicity at 13 Hz, with the possibility of faster helicity switching once it is installed at the new Brazilian fourth-generation source, SIRIUS.

We use x-ray spectroscopy at the Ir ${L}_{3}/{L}_{2}$ absorption edge to study powder samples of the intercalated honeycomb magnet ${\mathrm{Ag}}_{3}\mathrm{Li}{\mathrm{Ir}}_{2}{\mathrm{O}}_{6}$. Based on x-ray absorption and resonant inelastic x-ray scattering measurements, and exact diagonalization calculations including nearest-neighbor Ir-Ir electron hopping integrals, we argue that the intercalation of Ag atoms results in a delocalized electronic structure with enhanced Ir-O hybridization, departing from the local relativistic ${j}_{\mathrm{eff}}=1/2$ state. We find that the relative orbital contribution to the magnetic moment is increased and the magnetization density is spatially extended and asymmetric in this hybridized state. Our results confirm the importance of metal-ligand hybridization in the magnetism of transition metal oxides and provide empirical guidance for understanding the collective magnetism in intercalated honeycomb iridates.

As a hallmark of electronic correlation, spin-charge interplay underlies many emergent phenomena in doped Mott insulators, such as high-temperature superconductivity, whereas the half-filled parent state is usually electronically frozen with an antiferromagnetic order that resists external control. We report on the observation of a new positive magnetoresistance that probes the staggered susceptibility of a pseudospin-half square-lattice Mott insulator built as an artificial SrIrO3/SrTiO3 superlattice. Its size is particularly large in the high-temperature insulating paramagnetic phase near the N\'eel transition. This novel magnetoresistance originates from a collective charge response to the large longitudinal spin fluctuations under a linear coupling between the external magnetic field and the staggered magnetization enabled by strong spin-orbit interaction. Our results demonstrate a magnetic control of the binding energy of the fluctuating particle-hole pairs in the Slater-Mott crossover regime analogous to the BCS-to-Bose-Einstein condensation crossover of ultracold-superfluids., Comment: Accepted by Nature communications

In this study, we employ bulk electronic properties characterization and x-ray scattering/spectroscopy techniques to map the structural, magnetic and electronic properties of (Eu$_{1-x}$Ca$_{x}$)$_{2}$Ir$_{2}$O$_{7}$ as a function of Ca-doping. As expected, the metal-insulator transition temperature, $T_{MIT}$, decreases with Ca-doping until a metallic state is realized down to 2 K. In contrast, $T_{AFM}$ becomes decoupled from the MIT and (likely short-range) AFM order persists into the metallic regime. This decoupling is understood as a result of the onset of an electronically phase separated state, the occurrence of which seemingly depends on both synthesis method and rare earth site magnetism. PDF analysis suggests that electronic phase separation occurs without accompanying chemical phase segregation or changes in the short-range crystallographic symmetry while synchrotron x-ray diffraction confirms that there is no change in the long-range crystallographic symmetry. X-ray absorption measurements confirm the $J_{eff}$ = 1/2 character of (Eu$_{1-x}$Ca$_{x}$)$_{2}$Ir$_{2}$O$_{7}$. Surprisingly these measurements also indicate a net electron doping, rather than the expected hole doping, indicating a compensatory mechanism. Lastly, XMCD measurements show a weak Ir magnetic polarization that is largely unaffected by Ca-doping., 18 pages, 10 figures

The anomalous Hall effect (AHE) is an intriguing transport phenomenon occurring typically in ferromagnets as a consequence of broken time reversal symmetry and spin-orbit interaction. It can be caused by two microscopically distinct mechanisms, namely, by skew or side-jump scattering due to chiral features of the disorder scattering, or by an intrinsic contribution directly linked to the topological properties of the Bloch states. Here we show that the AHE can be artificially engineered in materials in which it is originally absent by combining the effects of symmetry breaking, spin orbit interaction and proximity-induced magnetism. In particular, we find a strikingly large AHE that emerges at the interface between a ferromagnetic manganite (La_(0.7)Sr_(0.3)MnO_3) and a semimetallic iridate (SrIrO_3). It is intrinsic and originates in the proximity-induced magnetism present in the narrow bands of strong spin-orbit coupling material SrIrO_3, which yields values of anomalous Hall conductivity and Hall angle as high as those observed in bulk transition-metal ferromagnets. These results demonstrate the interplay between correlated electron physics and topological phenomena at interfaces between 3d ferromagnets and strong spin-orbit coupling 5d oxides and trace an exciting path towards future topological spintronics at oxide interfaces. The anomalous Hall effect (AHE) occurs in ferromagnets caused by intrinsic and extrinsic mechanisms. Here, Yoo et al. report large anomalous Hall conductivity and Hall angle at the interface between a ferromagnet La_(0.7)Sr_(0.3M)nO_3 and a semimetallic SrIrO_3, due to the interplay between correlated physics and topological phenomena.

$\mathrm{Cu_2IrO_3}$ is among the newest layered honeycomb iridates and a promising candidate to harbor a Kitaev quantum spin liquid state. Here, we investigate the pressure and temperature dependence of its structure through a combination of powder x-ray diffraction and x-ray absorption fine structure measurements, as well as $ab$-$initio$ evolutionary structure search. At ambient pressure, we revise the previously proposed $C2/c$ solution with a related but notably more stable $P2_1/c$ structure. Pressures below 8 GPa drive the formation of Ir-Ir dimers at both ambient and low temperatures, similar to the case of $\mathrm{Li_2IrO_3}$. At higher pressures, the structural evolution dramatically depends on temperature. A large discontinuous reduction of the Ir honeycomb interplanar distance is observed around 15 GPa at room temperature, likely driven by a collapse of the O-Cu-O dumbbells. At 15 K, pressures beyond 20 GPa first lead to an intermediate phase featuring a continuous reduction of the interplanar distance, which then collapses at 30 GPa across yet another phase transition. However, the resulting structure around 40 GPa is not the same at room and low temperatures. Remarkably, the reduction in interplanar distance leads to an apparent healing of the stacking faults at room temperature, but not at 15 K. Possible implications on the evolution of electronic structure of $\mathrm{Cu_2IrO_3}$ with pressure are discussed., Comment: 11 pages, 7 figures, see ancillary file for Supplemental Material

We report the observation of a two-dimensional (2D) checkerboard charge density wave (CDW) in the low-dimensional superconductor Ta 4 Pd 3 Te 16 . By determining its CDW properties across the temperature-pressure ( T − P ) phase diagram and comparing with prototypical CDW materials, we conclude that Ta 4 Pd 3 Te 16 features (a) an incommensurate CDW with a mixed character of dimensions [quasi-1D (Q1D) considering its needlelike shape along the b axis, Q2D as the CDW has checkerboard wave vectors, and 3D because of CDW projections along all three axes], and (b) one of the weakest CDWs compared to its superconductivity (SC), i.e., enhanced SC with respect to CDW, suggesting an interesting interplay of the two orders.

Novel Ir1–xCrxO2 thin films have been prepared by reactive co–sputtering deposition. Composition, structure and electric and magnetic behavior have been analyzed by different techniques including EDX, XRR, XRD, SQUID magnetometry, electrical resistivity and XANES and XMCD spectroscopies. Despite the difficulty in growing CrO2 by physical deposition techniques, an Ir1–xCrxO2 solid solution phase could be achieved for 0 ≤ x ≤ 0.8, where the oxidation state of Cr is found to remain as 4+. Both the electrical and the magnetic behavior are shown to starkly depart from those of the parent IrO2 (paramagnetic metal) and CrO2 (half–metal ferromagnet) compounds. In particular, they show a semiconducting behavior, dρ/dT < 0 and giant magnetic coercivity at low temperatures. XMCD reveals a significant contribution of Ir to the magnetic response of the Ir1–xCrxO2 films. In addition, the nature of the magnetic moment of the Ir4+ ion (〈ml〉/〈ms〉 = 0.09) is completely different from the large orbital moment that is a hallmark of insulating Ir4+ oxides. This suggests a Cr–induced magnetic moment, which is a remarkably surprising result for an oxide., This work was partially supported by the Spanish Ministry of Science and Innovation under contract projects MAT2014–54425–R and MAT2017–83468–R. The authors would like to acknowledge the use of Servicio General de Apoyo a la Investigación–SAI, Universidad de Zaragoza. This research used resources of the APS, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE–AC02–06CH11357. E.A.–E. acknowledges the Spanish MINECO and the European Social Fund for a FPI (Formación de Personal Investigador, 2015) grant.

Chiral interactions in magnetic systems can give rise to rich physics manifested, for example, as nontrivial spin textures. The foremost interaction responsible for chiral magnetism is the Dzyaloshinskii-Moriya interaction (DMI), resulting from inversion symmetry breaking in the presence of strong spin-orbit coupling. However, the atomistic origin of DMIs and their relationship to emergent electrodynamic phenomena, such as topological Hall effect (THE), remain unclear. Here, we investigate the role of interfacial DMIs in 3d-5d transition metal-oxide-based LaMnO3/SrIrO3 superlattices on THE from a chiral spin texture. By additively engineering the interfacial inversion symmetry with atomic-scale precision, we directly link the competition between interfacial collinear ferromagnetic interactions and DMIs to an enhanced THE. The ability to control the DMI and resulting THE points to a pathway for harnessing interfacial structures to maximize the density of chiral spin textures useful for developing high-density information storage and quantum magnets for quantum information science.

Oxygen vacancies in complex oxides are indispensable for information and energy technologies. There are several means to create oxygen vacancies in bulk materials. However, the use of ionic interfaces to create oxygen vacancies has not been fully explored. Herein, we report an oxide nanobrush architecture designed to create high-density interfacial oxygen vacancies. An atomically well-defined (111) heterointerface between the fluorite CeO2 and the bixbyite Y2O3 is found to induce a charge modulation between Y3+ and Ce4+ ions enabled by the chemical valence mismatch between the two elements. Local structure and chemical analyses, along with theoretical calculations, suggest that more than 10% of oxygen atoms are spontaneously removed without deteriorating the lattice structure. Our fluorite–bixbyite nanobrush provides an excellent platform for the rational design of interfacial oxide architectures to precisely create, control, and transport oxygen vacancies critical for developing ionotronic and memristive devices for advanced energy and neuromorphic computing technologies., Oxygen vacancies can impart interesting properties in complex oxides, but specific architectures designed to create high-density oxygen vacancies are largely unknown. Here the authors report a fluorite-bixbyite nanobrush platform to tune interfacial oxygen and show that an atomically well-defined heterointerface can induce charge modulation.

In this work, we analyze the role of a thin Cr spacer between Fe and Gd layers on the structure and magnetic properties of a [Fe(35 Å)/Cr(tCr)/Gd(50 Å)/Cr(tCr)]12 superlattice. Samples without the Cr spacer (tCr = 0) and with a thin spacer (tCr = 4 Å) are investigated using X-ray diffraction, polarized neutron and resonance X-ray magnetic reflectometry, static magnetometry, magneto-optical Kerr effect, and ferromagnetic resonance techniques. Magnetic properties are studied experimentally in a wide temperature range 4–300 K and analyzed theoretically using numerical simulation on the basis of the mean-field model. We show that a reasonable agreement with the experimental data can be obtained considering temperature dependence of the effective field parameter in gadolinium layers. The analysis of the experimental data shows that besides a strong reduction of the antiferromagnetic coupling between Fe and Gd, the introduction of Cr spacers into Fe/Gd superlattice leads to modification of both structural and magnetic characteristics of the ferromagnetic layers. © 2018, Pleiades Publishing, Inc.

A fundamental difference between antiferromagnets and ferromagnets is the lack of linear coupling to a uniform magnetic field due to the staggered order parameter. Such coupling is possible via the Dzyaloshinskii-Moriya (DM) interaction but at the expense of reduced antiferromagnetic (AFM) susceptibility due to the canting-induced spin anisotropy. We solve this long-standing problem with a top-down approach that utilizes spin-orbit coupling in the presence of a hidden SU(2) symmetry. We demonstrate giant AFM responses to sub-Tesla external fields by exploiting the extremely strong two-dimensional critical fluctuations preserved under a symmetry-invariant exchange anisotropy, which is built into a square-lattice artificially synthesized as a superlattice of SrIrO3 and SrTiO3. The observed field-induced logarithmic increase of the ordering temperature enables highly efficient control of the AFM order. As antiferromagnets promise to afford switching speed and storage security far beyond ferromagnets, our symmetry-invariant approach unleashes the great potential of functional antiferromagnets., Comment: 3 figures

The development of revolutionary new alloys and composites is crucial to meeting materials requirements for next generation nuclear reactors. The newly developed amorphous silicon oxycarbide (SiOC) and crystalline Fe composite system has shown radiation tolerance over a wide range of temperatures. To advance understanding of this new composite, we investigate the structure and thermal stability of the interface between amorphous SiOC and crystalline Fe by combining various experimental techniques and simulation methods. We show that the SiOC/Fe interface is thermally stable up to at least 400 °C. When the annealing temperature reaches 600 °C, an intermixed region forms at this interface. This region appears to be a crystalline phase that forms an incoherent interface with the Fe layer. Density functional theory (DFT) Molecular dynamics (MD) is performed on the homogeneous SiFeOC phase to study the early stages of formation of the intermixed layer. Both experimental and simulation results suggest this phase has the fayalite crystal structure. The physical processes involved in the formation of the intermixed region are discussed.

Single crystals of a honeycomb lattice antiferromagnet, ${\mathrm{Tb}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Ga}}_{9}$, were synthesized, and the physical properties have been studied. From magnetometry, a long-range antiferromagnetic ordering at $\ensuremath{\approx}12.5$ K with highly anisotropic magnetic behavior was found. Neutron powder diffraction confirms that the Tb spins lie along the $\mathbf{a}$-axis, parallel to the shortest Tb-Tb contact. Two field-induced spin-flip transitions are observed when the field is applied parallel to this axis, separated by a plateau corresponding roughly to $M\ensuremath{\approx}{M}_{\mathrm{s}}/2$. Transport measurements show the resistivity to be metallic with a discontinuity at the onset of N\'eel order. Heat capacity shows a $\ensuremath{\lambda}$-like transition confirming the bulk nature of the magnetism. We propose a phenomenological spin Hamiltonian that describes the magnetization plateau as a result of strong Ising character arising from a quasidoublet ground state of the ${\mathrm{Tb}}^{3+}$ ion in a site of ${C}_{s}$ symmetry and expressing a significant bond-dependent anisotropy.

Significance Heterojunctions between dissimilar materials provide an opportunity to generate emergent properties not present in their individual components. These properties are often linked to interfacial charge transfer, an important mechanism allowing one to access and control novel quantum states. Here, we have grown unit-cell scale superlattices composed of 5 d S r I r O 3 and 3 d L a N i O 3 . We have discovered a massive interfacial charge transfer from Ir to Ni, triggering dramatic electronic and magnetic reconstructions at the interface that suppress strong spin–orbit coupling effects normally present in the iridates. These findings call for careful evaluation and reinterpretation of experiments on spin–orbit-driven physics in thin films and heterostructures based on prototypical 5 d transition metal oxides.

Anyonic excitations emerging from a Kitaev spin liquid can form a basis for quantum computers. Searching for such excitations motivated intense research on the honeycomb iridate materials. However, access to a spin liquid ground state has been hindered by magnetic ordering. Cu2IrO3 is a new honeycomb iridate without thermodynamic signatures of a long-range order. Here, we use muon spin relaxation to uncover the magnetic ground state of Cu2IrO3. We find a two-component depolarization with slow and fast relaxation rates corresponding to distinct regions with dynamic and static magnetism, respectively. X-ray absorption spectroscopy and first principles calculations identify a mixed copper valence as the origin of this behavior. Our results suggest that a minority of Cu2+ ions nucleate regions of static magnetism whereas the majority of Cu+/Ir4+ on the honeycomb lattice give rise to a Kitaev spin liquid.

A pressure-induced collapse of magnetic ordering in $\ensuremath{\beta}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ at ${P}_{m}\ensuremath{\sim}1.5--2$ GPa has previously been interpreted as evidence for possible emergence of spin liquid states in this hyperhoneycomb iridate, raising prospects for experimental realizations of the Kitaev model. Based on structural data obtained at room temperature, this magnetic transition is believed to originate in small lattice perturbations that preserve crystal symmetry, and related changes in bond-directional anisotropic exchange interactions. Here we report on the evolution of the crystal structure of $\ensuremath{\beta}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ under pressure at low temperatures ($T\ensuremath{\le}50$ K) and show that the suppression of magnetism coincides with a change in lattice symmetry involving Ir-Ir dimerization. The critical pressure for dimerization shifts from 4.4(2) GPa at room temperature to $\ensuremath{\sim}1.5--2$ GPa below 50 K. While a direct $Fddd\ensuremath{\rightarrow}C2/c$ transition is observed at room temperature, the low temperature transitions involve new as well as coexisting dimerized phases. Further investigation of the Ir (${L}_{3}$/${L}_{2}$) isotropic branching ratio in x-ray absorption spectra indicates that the previously reported departure of the electronic ground state from a ${J}_{\mathrm{eff}}=1/2$ state is closely related to the onset of dimerized phases. In essence, our results suggest that the predominant mechanism driving the collapse of magnetism in $\ensuremath{\beta}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ is the pressure-induced formation of $\mathrm{Ir}{}_{2}$ dimers in the hyperhoneycomb network. The results further confirm the instability of the ${J}_{\mathrm{eff}}=1/2$ moments and related noncollinear spiral magnetic ordering against formation of dimers in the low-temperature phase of compressed $\ensuremath{\beta}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$.

This study explores a filling-controlled metal-insulator transition in the pyrochlore-based iridium oxide Nd${}_{2}$Ir${}_{2}$O${}_{7}$. Through measurements of the local site symmetry, average structure, and electronic properties, the authors comment on the interplay of the lattice, charge gap, and magnetism in this material. The work holds important implications for tuning this compound toward its anticipated quantum critical regime, which has been predicted to host a Weyl semimetal state.

We describe how first principle calculations can play a key role in the interpretation of X-ray absorption near-edge structure (XANES) and X-ray magnetic circular dichroism (XMCD) spectra for a better understanding of emergent phenomena in condensed matter physics at high applied pressure. Eu compounds are used as case study to illustrate the advantages of this methodology, ranging from studies of electronic charge transfer probed by quadrupolar and dipolar contributions, to accurately determining electronic valence, and to inform about the influence of pressure on RKKY interactions and magnetism. This description should help advance studies where the pressure dependence of XANES and XMCD data must be tackled with the support of theoretical calculations for a proper understanding of the electronic properties of materials.

In order to rationalize and predict the behavior of compounds containing 5d transition metal ions, an understanding of the local moments and superexchange interactions from which their magnetic properties are derived is necessary. The magnetic and electrical properties of the ferrimagnetic double perovskites Ca2CoOsO6 and Ca2NiOsO6 studied here provide critical insight toward that goal. First-principles density functional theory (DFT) calculations indicate, and experimental measurements confirm, that the Os(VI) moments are directed antiparallel to the Co/Ni moments. X-ray magnetic circular dichroism (XMCD) measurements reveal that the orbital moment on osmium has a magnitude that is approximately 30% of the spin moment, and the two contributions oppose each other. Both the size and direction of the orbital moment are confirmed by the DFT calculations. The size of the Os(VI) total moment is predicted to be 0.6–0.7 μB by DFT calculations. The ferrimagnetic ground state is stabilized by strong antiferromagne...

Materials with strong magnetoresistive responses are the backbone of spintronic technology, magnetic sensors, and hard drives. Among them, manganese oxides with a mixed valence and a cubic perovskite structure stand out due to their colossal magnetoresistance (CMR). A double exchange interaction underlies the CMR in manganates, whereby charge transport is enhanced when the spins on neighboring Mn3+ and Mn4+ ions are parallel. Prior efforts to find different materials or mechanisms for CMR resulted in a much smaller effect. Here we show an enormous CMR at low temperatures in EuCd2P2 without manganese, oxygen, mixed valence, or cubic perovskite structure. EuCd2P2 has a layered trigonal lattice and exhibits antiferromagnetic ordering at 11 K. The magnitude of CMR (104 percent) in as-grown crystals of EuCd2P2 rivals the magnitude in optimized thin films of manganates. Our magnetization, transport, and synchrotron X-ray data suggest that strong magnetic fluctuations are responsible for this phenomenon. The realization of CMR at low temperatures without heterovalency leads to a new regime for materials and technologies related to antiferromagnetic spintronics.

GdN and SmN are two of the lanthanide nitrides, most of which are intrinsic ferromagnetic semiconductors. Superlattices comprising the pair offer a unique opportunity to investigate heterojunctions that feature simultaneous conductivity and magnetic interface influences. Here we report an investigation of these influences, using magnetisation and X-ray magnetic circular dichroism for magnetic effects, and magnetoresistance and Hall effect studies of their electrical conductance. Magnetic data show clear signatures of a conflicting Zeeman vs. interfacial exchange and the magnetic disruption that results, while resistivity and Hall measurements show conduction in both GdN and SmN.

The recent discovery of two-dimensional (2D) magnets, with a number of interesting magnetic properties, has drawn much interest due to their potential for future 2D spintronic device applications. CrI3, a van der Waals magnet, exhibits two-dimensional ferromagnetism even in monolayer form, stabilized by strong magnetic anisotropy. Its interlayer magnetic ordering is coupled to structural layer stacking, implying that the charge density distribution mediating van der Waals interactions plays a key role in the magnetic interaction between the layers. Using polarization-dependent x-ray spectroscopy, we investigated the response of the electronic environment around Cr and I sites to structural changes of layer stacking order. The highly anisotropic nature of the Cr local environment is significantly enhanced and is accompanied by changes in the valence band, in the rhombohedral phase. Magnetic spectroscopy measurements reveal a sizable iodine orbital moment, indicating the iodine contribution to magnetic anisotropy. Our results uncover an important role for the extended nature of anisotropic Cr orbital states in dictating interlayer magnetic interactions and the iodine contribution to magnetic anisotropy.

We report the observation of multiple phonon satellite features in ultra thin superlattices of form $n$SrIrO$_3$/$m$SrTiO$_3$ using resonant inelastic x-ray scattering. As the values of $n$ and $m$ vary the energy loss spectra show a systematic evolution in the relative intensity of the phonon satellites. Using a closed-form solution for the cross section, we extract the variation in the electron-phonon coupling strength as a function of $n$ and $m$. Combined with the negligible carrier doping into the SrTiO$_3$ layers, these results indicate that tuning of the electron-phonon coupling can be effectively decoupled from doping. This work showcases both a feasible method to extract the electron-phonon coupling in superlattices and unveils a potential route for tuning this coupling which is often associated with superconductivity in SrTiO$_3$-based systems., 4 pages, 5 figures

The hyper-kagome lattice iridate ${\mathrm{Na}}_{3}{\mathrm{Ir}}_{3}{\mathrm{O}}_{8}$, closely related to spin liquid candidate ${\mathrm{Na}}_{4}{\mathrm{Ir}}_{3}{\mathrm{O}}_{8}$, is unusual in that spin-orbit interactions acting on the 1/3-filled ${J}_{\mathrm{eff}}=1/2$ state lead to a semimetallic ground state, in contrast to the conventional insulating Mott state stabilized by S-O interactions in the 1/2-filled ${J}_{\mathrm{eff}}=1/2$ state of other iridates including ${\mathrm{Na}}_{4}{\mathrm{Ir}}_{3}{\mathrm{O}}_{8}$. We have studied the evolution of crystal structure, electronic structure, and transport in ${\mathrm{Na}}_{3}{\mathrm{Ir}}_{3}{\mathrm{O}}_{8}$ under high pressure using x-ray diffraction, x-ray absorption near edge structure, and electrical resistance measurements in a diamond anvil cell. The study was augmented by the use of ab initio calculations, which provided insight into pressure-induced changes in crystal and electronic structure. We found that ${\mathrm{Na}}_{3}{\mathrm{Ir}}_{3}{\mathrm{O}}_{8}$ transforms from a semimetal to an insulator under pressure, with an estimated energy gap that increases to about 130 meV at $P\ensuremath{\sim}$ 9 GPa. At approximately 10 GPa, a cubic-to-monoclinic structural transition takes place between two insulating phases. This structural transition features a sizable volume collapse and a high-pressure phase characterized by apparent dimerization of Ir-Ir distances, wide dispersion of Ir-O-Ir bond angles, and an increase in the occupation of $5d$ states. Although the energy gap is reduced in the high-pressure phase, insulating behavior remains to the highest pressures $\ensuremath{\sim}1$ Mbar. The strongly pressure-dependent insulating gap shows a positive correlation with the expectation value of the angular part of the S-O interaction, $\ensuremath{\langle}\mathbf{L}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{S}\ensuremath{\rangle}$, which may indicate an active role of S-O interactions in stabilizing the insulating state that emerges in the compressed ${\mathrm{Na}}_{3}{\mathrm{Ir}}_{3}{\mathrm{O}}_{8}$ structure.

We studied the effect of physical pressure on the electronic and magnetic properties of ferrimagnetic double perovskites ${A}_{2}{\mathrm{FeReO}}_{6}$ ($A=\mathrm{Ca}$, Ba) using Re ${L}_{2,3}$ edge x-ray absorption spectroscopy and powder diffraction measurements. Volume compression is shown to dramatically increase the magnetic coercivity (${H}_{c}$) in polycrystalline samples of both compounds with $\mathrm{\ensuremath{\Delta}}{H}_{c}/\mathrm{\ensuremath{\Delta}}V\ensuremath{\sim}150$--200 $\mathrm{Oe}/{\AA{}}^{3}$. A nearly eight-fold increase in ${H}_{c}$, from 0.2 to 1.55 T, is obtained in ${\mathrm{Ba}}_{2}{\mathrm{FeReO}}_{6}$ at $P=29$ GPa. While no signs of structural phase transitions are seen in either sample to $\ensuremath{\sim}30$ GPa, the structural data points to a pressure-driven increase in tetragonal distortion of ${\mathrm{ReO}}_{6}$ octahedra. A sizable but pressure-independent Re orbital-to-spin magnetic moment ratio is observed, pointing to the critical role of spin-orbit interactions at Re sites. We present a ${j}_{\mathrm{eff}}$ description of the electronic structure that combines effects of crystal field and spin-orbit coupling on the Re $5{d}^{2}$ orbitals and use this description to provide insight into the pressure-induced enhancement of magnetic anisotropy.

Compounds with honeycomb structures occupied by strong spin orbit coupled (SOC) moments are considered to be candidate Kitaev quantum spin liquids. Here we present the first example of Os on a honeycomb structure, Li2.15(3)Os0.85(3)O3 (C2/c, a = 5.09 Å, b = 8.81 Å, c = 9.83 Å, β = 99.3°). Neutron diffraction shows large site disorder in the honeycomb layer and X-ray absorption spectroscopy indicates a valence state of Os (4.7 ± 0.2), consistent with the nominal concentration. We observe a transport band gap of Δ = 243 ± 23 meV, a large van Vleck susceptibility, and an effective moment of 0.85 μB, much lower than expected from 70% Os(+5). No evidence of long range order is found above 0.10 K but a spin glass-like peak in ac-susceptibility is observed at 0.5 K. The specific heat displays an impurity spin contribution in addition to a power law ∝T(0.63±0.06). Applied density functional theory (DFT) leads to a reduced moment, suggesting incipient itineracy of the valence electrons, and finding evidence that Li over stoichiometry leads to Os(4+)−Os(5+) mixed valence. This local picture is discussed in light of the site disorder and a possible underlying quantum spin liquid state.

We demonstrate that the action of physical pressure, chemical compression, or aliovalent substitution in ACo2As2 (A = Eu and Ca) has a general consequence of causing these antiferromagnetic materials to become ferromagnets. In all cases, the mixed valence triggered at the electropositive A site results in the increase of the Co 3d density of states at the Fermi level. Remarkably, the dramatic alteration of magnetic behavior results from the very minor (0.15 electron) change in the population of the 3d orbitals. The mixed valence state of Eu observed in the high-pressure (HP) form of EuCo2As2 exhibits a remarkable stability, achieving the average oxidation state of +2.25 at 12.6 GPa. In the case of CaCo2As2, substituting even 10% of Eu or La into the Ca site causes ferromagnetic ordering of Co moments. Similar to HP-EuCo2As2, the itinerant 3d ferromagnetism emerges from electronic doping into the Co layer because of chemical compression of Eu sites in Ca0.9Eu0.1Co1.91As2 or direct electron doping in Ca0.85La0.15Co1.89As2. The results reported herein demonstrate the general possibility of amplifying minor localized electronic effects to achieve major changes in material's properties via involvement of strongly correlated electrons.

The application of pressure reveals a rich phase diagram for the quantum $S=1/2$ spin chain material TiOCl. We performed x-ray diffraction on single-crystal samples in a diamond-anvil cell down to $T=4$ K and pressures up to 14.5 GPa. Remarkably, the magnetic interaction scale increases dramatically with increasing pressure, as indicated by the high onset temperature of the spin-Peierls phase. The spin-Peierls phase was probed at $\ensuremath{\sim}6$ GPa up to 215 K but possibly extends in temperature to above $T=300$ K, indicating the possibility of a quantum singlet state at room temperature. Near the critical pressure for the transition to the more metallic phase, coexisting phases are exemplified by incommensurate order in two directions. Further comparisons are made with the phase diagrams of related spin-Peierls systems that display metallicity and superconductivity under pressure.

The multifaceted character of 5f electrons in actinide materials, from localized to itinerant and in between, together with their complex interactions with 6d and other conduction electron states, has thwarted efforts for fully understanding this class of compounds. While theoretical efforts abound, direct experimental probes of relevant electronic states and their hybridization are limited. Here we exploit the presence of sizable quadrupolar and dipolar contributions in the uranium L3-edge X-ray absorption cross section to provide unique information on the extent of spin-polarized hybridization between 5f and 6d electronic states by means of X-ray magnetic circular dichroism. As a result, we show how this 5f-6d hybridization regulates the magnetism of each sublattice in UCu2Si2 and UMn2Si2 compounds, demonstrating the potentiality of this methodology to investigate a plethora of magnetic actinide compounds., Study and identification of the actinide electronic structure is complicated and crucial. Here the authors probe the hybridization between 5f to 6d orbitals in uranium compounds using X-ray magnetic circular dichroism near U-L3 edge through the dipolar and quadrupolar spectral contributions.

We explore the response of Ir $5d$ orbitals to pressure in $\ensuremath{\beta}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$, a hyperhoneycomb iridate in proximity to a Kitaev quantum spin-liquid (QSL) ground state. X-ray absorption spectroscopy reveals a reconstruction of the electronic ground state below 2 GPa, the same pressure range where x-ray magnetic circular dichroism shows an apparent collapse of magnetic order. The electronic reconstruction, which manifests a reduction in the effective spin-orbit interaction in $5d$ orbitals, pushes $\ensuremath{\beta}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ further away from the pure ${J}_{\mathrm{eff}}=1/2$ limit. Although lattice symmetry is preserved across the electronic transition, x-ray diffraction shows a highly anisotropic compression of the hyperhoneycomb lattice which affects the balance of bond-directional Ir-Ir exchange interactions driven by spin-orbit coupling at Ir sites. An enhancement of symmetric anisotropic exchange over Kitaev and Heisenberg exchange interactions seen in theoretical calculations that use precisely this anisotropic Ir-Ir bond compression provides one possible route to the realization of a QSL state in this hyperhoneycomb iridate at high pressures.

We report an experimental investigation of the two-dimensional ${J}_{\text{eff}}=1/2$ antiferromagnetic Mott insulator by varying the interlayer exchange coupling in [${({\mathrm{SrIrO}}_{3})}_{1}$, ${({\mathrm{SrTiO}}_{3})}_{m}$] ($m=1$, 2 and 3) superlattices. Although all samples exhibited an insulating ground state with long-range magnetic order, temperature-dependent resistivity measurements showed a stronger insulating behavior in the $m=2$ and $m=3$ samples than the $m=1$ sample which displayed a clear kink at the magnetic transition. This difference indicates that the blocking effect of the excessive ${\mathrm{SrTiO}}_{3}$ layer enhances the effective electron-electron correlation and strengthens the Mott phase. The significant reduction of the N\'eel temperature from 150 K for $m=1$ to 40 K for $m=2$ demonstrates that the long-range order stability in the former is boosted by a substantial interlayer exchange coupling. Resonant x-ray magnetic scattering revealed that the interlayer exchange coupling has a switchable sign, depending on the ${\mathrm{SrTiO}}_{3}$ layer number $m$, for maintaining canting-induced weak ferromagnetism. The nearly unaltered transition temperature between the $m=2$ and the $m=3$ demonstrated that we have realized a two-dimensional antiferromagnet at finite temperatures with diminishing interlayer exchange coupling.

Honeycomb iridates such as $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ are argued to realize Kitaev spin-anisotropic magnetic exchange, along with Heisenberg and possibly other couplings. While systems with pure Kitaev interactions are candidates to realize a quantum spin-liquid ground state, in $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ it has been shown that the presence of competing magnetic interactions leads to an incommensurate spiral spin order at ambient pressure below 38 K. We study the pressure sensitivity of this magnetically ordered state in single crystals of $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ using resonant x-ray scattering (RXS) under applied hydrostatic pressures of up to 3 GPa. RXS is a direct probe of electronic order, and we observe the abrupt disappearance of the ${\mathbf{q}}_{\mathrm{sp}}\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}(0.57,\phantom{\rule{0.28em}{0ex}}0,\phantom{\rule{0.28em}{0ex}}0)$ spiral order at a critical pressure ${P}_{c}=1.4$ GPa with no accompanying change in the symmetry of the lattice.

Four-point electrical resistivity measurements were carried out on Nd metal and dilute magnetic alloys containing up to 1 at.% Nd in superconducting Y for temperatures 1.5--295 K under pressures to 210 GPa. The magnetic ordering temperature ${T}_{\text{o}}$ of Nd appears to rise steeply under pressure, increasing ninefold to 180 K at 70 GPa before falling rapidly. Y(Nd) alloys display both a resistivity minimum and superconducting pair breaking $\mathrm{\ensuremath{\Delta}}{T}_{\text{c}}$ as large as 38 K/at.% Nd. The present results give evidence that for pressures above 30--40 GPa, the exchange coupling $J$ between Nd ions and conduction electrons becomes negative, thus activating Kondo physics in this highly correlated electron system. The rise and fall of ${T}_{\text{o}}$ and $\mathrm{\ensuremath{\Delta}}{T}_{\text{c}}$ with pressure can be accounted for in terms of an increase in the Kondo temperature.

We investigate doping- and pressure-induced changes in the electronic state of Mn $3d$ and As $4p$ orbitals in II-II-V-based diluted magnetic semiconductor $(\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}){(\mathrm{Z}{\mathrm{n}}_{1\ensuremath{-}y}\mathrm{M}{\mathrm{n}}_{y})}_{2}\mathrm{A}{\mathrm{s}}_{2}$ to shed light into the mechanism of indirect exchange interactions leading to high ferromagnetic ordering temperature ($T\mathrm{c}=230\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in optimally doped samples). A suite of x-ray spectroscopy experiments (emission, absorption, and dichroism) show that the emergence and further enhancement of ferromagnetic interactions with increased hole doping into the As $4p$ band is accompanied by a decrease in local $3d$ spin density at Mn sites. This is a result of increasing Mn $3d--\mathrm{As} 4p$ hybridization with hole doping, which enhances indirect exchange interactions between Mn dopants and gives rise to induced magnetic polarization in As $4p$ states. On the contrary, application of pressure suppresses exchange interactions. While Mn K $\ensuremath{\beta}$ emission spectra show a weak response of $3d$ states to pressure, clear As $4p$ band broadening (hole delocalization) is observed under pressure, ultimately leading to loss of ferromagnetism concomitant with a semiconductor to metal transition. The pressure response of As $4p$ and Mn $3d$ states is intimately connected with the evolution of the As-As interlayer distance and the geometry of the $\mathrm{MnA}{\mathrm{s}}_{4}$ tetrahedral units, which we probed with x-ray diffraction. Our results indicate that hole doping increases the degree of covalency between the anion (As) $p$ states and cation (Mn) $d$ states in the $\mathrm{MnA}{\mathrm{s}}_{4}$ tetrahedron, a crucial ingredient to promote indirect exchange interactions between Mn dopants and high $T\mathrm{c}$ ferromagnetism. The instability of ferromagnetism and semiconducting states against pressure is mainly dictated by delocalization of anion $p$ states.

The perovskite SrIrO3 is an exotic narrow-band metal owing to a confluence of the strengths of the spin-orbit coupling (SOC) and the electron-electron correlations. It has been proposed that topological and magnetic insulating phases can be achieved by tuning the SOC, Hubbard interactions, and/or lattice symmetry. Here, we report that the substitution of nonmagnetic, isovalent Sn4+ for Ir4+ in the SrIr1-xSnxO3 perovskites synthesized under high pressure leads to a metal-insulator transition to an antiferromagnetic (AF) phase at TN > 225 K. The continuous change of the cell volume as detected by x-ray diffraction and the lamda-shape transition of the specific heat on cooling through TN demonstrate that the metal-insulator transition is of second-order. Neutron powder diffraction results indicate that the Sn substitution enlarges an octahedral-site distortion that reduces the SOC relative to the spin-spin exchange interaction and results in the type-G AF spin ordering below TN. Measurement of high-temperature magnetic susceptibility shows the evolution of magnetic coupling in the paramagnetic phase typical of weak itinerant-electron magnetism in the Sn-substituted samples. A reduced structural symmetry in the magnetically ordered phase leads to an electron gap opening at the Brillouin zone boundary below TN in the same way as proposed by Slater., 15 pages, 4 figures

The performance of a diamond X-ray phase retarder to enable the production of circularly polarized X-rays has been quantitatively measured and magnetic dichroism contrast in transmission and diffraction geometries has been demonstrated. Feasibility tests for dichroic Bragg coherent diffractive imaging experiments were performed and showed that the diamond X-ray phase retarder does not produce significant distortions to the X-ray wavefront and that Bragg coherent diffractive imaging reconstructions are achievable., Recent progress in the development of dichroic Bragg coherent diffractive imaging, a new technique for simultaneous three-dimensional imaging of strain and magnetization at the nanoscale, is reported. This progress includes the installation of a diamond X-ray phase retarder at beamline 34-ID-C of the Advanced Photon Source. The performance of the phase retarder for tuning X-ray polarization is demonstrated with temperature-dependent X-ray magnetic circular dichroism measurements on a gadolinium foil in transmission and on a Gd5Si2Ge2 crystal in diffraction geometry with a partially coherent, focused X-ray beam. Feasibility tests for dichroic Bragg coherent diffractive imaging are presented. These tests include (1) using conventional Bragg coherent diffractive imaging to determine whether the phase retarder introduces aberrations using a nonmagnetic gold nanocrystal as a control sample, and (2) collecting coherent diffraction patterns of a magnetic Gd5Si2Ge2 nanocrystal with left- and right-circularly polarized X-rays. Future applications of dichroic Bragg coherent diffractive imaging for the correlation of strain and lattice defects with magnetic ordering and inhomogeneities are considered.

We used x-ray magnetic circular dichroism (XMCD) to probe the ferromagnetic properties of As $p$-symmetric ($4p$) states in the recently synthesized diluted magnetic semiconductor $(\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}){(\mathrm{Z}{\mathrm{n}}_{1\ensuremath{-}y}\mathrm{M}{\mathrm{n}}_{y})}_{2}\mathrm{A}{\mathrm{s}}_{2}$ system under ambient- and high-pressure conditions. The As $K$-edge XMCD signal scales with the sample magnetization (dominated by Mn) and scales with the ferromagnetic ordering temperature $T\mathrm{c}$, and hence it is representative of the bulk magnetization. The XMCD intensity gradually decreases upon compression and vanishes at around 25 GPa, indicating quenching of ferromagnetism at this pressure. Transport measurements show a concomitant increase in conductivity with pressure, leading to a nearly metallic state at about the same pressure where magnetic order collapses. High-pressure x-ray diffraction shows an absence of structural transitions to 40 GPa. The results indicate that the mobility of doped holes, probed by both transport and x-ray absorption spectroscopy ($4p$ band broadening), is intimately connected with the mechanism of magnetic ordering in this class of compounds and that its control using external pressure provides an alternative route for tuning the magnetic properties in diluted magnetic semiconductor materials.

Using synchrotron-based Mossbauer and x-ray emission spectroscopies, we explore the evolution of magnetism in elemental (divalent) europium as it gives way to superconductivity at extreme pressures. Magnetic order in Eu is observed to collapse just above 80 GPa as superconductivity emerges, even though Eu cations retain their strong local 4f(7) magnetic moments up to 119 GPa with no evidence for an increase in valence. We speculate that superconductivity in Eu may be unconventional and have its origin in magnetic fluctuations, as has been suggested for high-T-c cuprates, heavy fermions, and iron-pnictides.

Ferromagnetic insulators (FMIs) are one of the most important components in developing dissipationless electronic and spintronic devices. However, FMIs are innately rare to find in nature as ferromagnetism generally accompanies metallicity. Here, novel room-temperature FMI films that are epitaxially synthesized by deliberate control of the ratio between two B-site cations in the double perovskite Sr2 Fe1+ x Re1- x O6 (-0.2 ≤ x ≤ 0.2) are reported. In contrast to the known FM metallic phase in stoichiometric Sr2 FeReO6 , an FMI state with a high Curie temperature (Tc ≈ 400 K) and a large saturation magnetization (MS ≈ 1.8 µB f.u.-1 ) is found in highly cation-ordered Fe-rich phases. The stabilization of the FMI state is attributed to the formation of extra Fe3+ Fe3+ and Fe3+ Re6+ bonding states, which originate from the relatively excess Fe ions owing to the deficiency in Re ions. The emerging FMI state created by controlling cations in the oxide double perovskites opens the door to developing novel oxide quantum materials and spintronic devices.

We investigate the magnetic and electronic properties of the $\mathrm{Gd}{T}_{2}{\mathrm{Zn}}_{20}$ ($T=\mathrm{Fe}$ and Co) compounds using x-ray resonant magnetic scattering (XRMS), x-ray absorption near-edge structure (XANES), and x-ray magnetic circular dichroism (XMCD). The XRMS measurements reveal that ${\mathrm{GdCo}}_{2}{\mathrm{Zn}}_{20}$ has a commensurate antiferromagnetic spin structure with a magnetic propagation vector $\stackrel{P\vec}{\ensuremath{\tau}}=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ below the N\'eel temperature (${T}_{N}\ensuremath{\sim}\phantom{\rule{4.pt}{0ex}}5.7$ K). Only the Gd ions carry a magnetic moment forming an antiferromagnetic structure with magnetic representation ${\mathrm{\ensuremath{\Gamma}}}_{6}$. For the ferromagnetic ${\mathrm{GdFe}}_{2}{\mathrm{Zn}}_{20}$ compound, an extensive investigation was performed at low temperature and under magnetic field using XANES and XMCD. A strong XMCD signal of about $12.5%$ and $9.7%$ is observed below the Curie temperature (${T}_{C}\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}85\mathrm{K}$) at the Gd ${L}_{2}$ and ${L}_{3}$ edges, respectively. In addition, a small magnetic signal of about $0.06%$ of the jump is recorded at the Zn $K$ edge, suggesting that the Zn $4p$ states are spin polarized by the Gd $5d$ extended orbitals.

Some of the most exotic material properties derive from electronic states with short correlation length (~10-500 {\AA}), suggesting that the local structural symmetry may play a relevant role in their behavior. Here we discuss the combined use of polarized x-ray absorption fine structure and x-ray diffraction at high pressure as a powerful method to tune and probe structural and electronic orders at multiple length scales. Besides addressing some of the technical challenges associated with such experiments, we illustrate this approach with results obtained in the cuprate La$_{1.875}$Ba$_{0.125}$CuO$_4$, in which the response of electronic order to pressure can only be understood by probing the structure at the relevant length scales., Comment: 11 pages, 4 figures, accepted for publication in High Pressure Research

The simultaneous presence of sizable spin-orbit interactions and electron correlations in iridium oxides has led to predictions of novel ground states including Dirac semimetals, Kitaev spin liquids, and superconductivity. Electron and hole doping studies of spin-orbit assisted Mott insulator Sr2IrO4 are being intensively pursued due to extensive parallels with the La2CuO4 parent compound of Cuprate superconductors. In particular, the mechanism of charge doping associated with replacement of Ir with Rh ions remains controversial with profound consequences for the interpretation of electronic structure and transport data. Using x-ray absorption near edge structure (XANES) measurements at the Rh L, K- and Ir L- edges we observe anomalous evolution of charge partitioning between Rh and Ir with Rh doping. The partitioning of charge between Rh and Ir sites progresses in a way that holes are initially doped into the Jeff=1/2 band at low x only to be removed from it at higher x values. This anomalous hole doping naturally explains the re-entrant insulating phase in the phase diagram of Rh doped Sr2IrO4 and ought to be considered when searching for superconductivity and other emergent phenomena in iridates doped with 4d elements, Comment: Accepted to be published in PRB Rapid Communications, https://journals.aps.org/prb/accepted/75073OcbL8cE901092c69c34262f259d8151b3cff

Strong Coulomb repulsion and spin–orbit coupling are known to give rise to exotic physical phenomena in transition metal oxides. Initial attempts to investigate systems, where both of these fundamental interactions are comparably strong, such as 3d and 5d complex oxide superlattices, have revealed properties that only slightly differ from the bulk ones of the constituent materials. Here we observe that the interfacial coupling between the 3d antiferromagnetic insulator SrMnO3 and the 5d paramagnetic metal SrIrO3 is enormously strong, yielding an anomalous Hall response as the result of charge transfer driven interfacial ferromagnetism. These findings show that low dimensional spin–orbit entangled 3d–5d interfaces provide an avenue to uncover technologically relevant physical phenomena unattainable in bulk materials., Whilst superlattices containing thin films of 5d transition metal oxides are expected to yield strong interfacial coupling, only weak effects have been observed. Here, the authors report strong coupling between 3d SrMnO3 and 5d SrIrO3 due to the interplay of strong Coulomb and spin orbit interactions.

The sudden decrease in molar volume exhibited by most lanthanides under high pressure is often attributed to changes in the degree of localization of their 4f-electrons. We give evidence, based on electrical resistivity measurements of dilute Y(Gd) and Y(Tb) alloys to 120 GPa, that the volume collapse transitions in Gd and Tb metals have different origins, despite their being neighbors in the periodic table. Remarkably, the change under pressure in the magnetic state of isolated Pr or Tb impurity ions in the nonmagnetic Y host appears to closely mirror corresponding changes in pure Pr or Tb metals. The collapse in Tb appears to be driven by an enhanced negative exchange interaction between 4f and conduction electrons under pressure (Kondo resonance) which, in the case of Y(Tb), dramatically alters the superconducting properties of the Y host, much like previously found for Y(Pr). In Gd our resistivity measurements suggest that a Kondo resonance is not the main driver for its volume collapse. X-ray absorption and emission spectroscopies clearly show that 4f local moments remain largely intact across both volume collapse transitions ruling out 4f band formation (delocalization) and valence transition models as possible drivers. The results highlight the richness of behavior behind the volume collapse transition in lanthanides and demonstrate the stability of the 4f level against band formation to extreme pressure., 8 pages, 5 figures, accepted for publication in the Physical Review B

In order to probe the changes in the valence state and magnetic properties of Eu metal under extreme pressure, x-ray absorption near-edge spectroscopy, x-ray magnetic circular dichroism and synchrotron Mossbauer spectroscopy experiments have been carried out. The Mossbauer isomer shift exhibits an anomalous pressure dependence, passing through a maximum near 20 GPa. Density functional theory has been applied to give insight into the pressure-induced changes in both Eu’s electronic structure and Mossbauer isomer shift. Contrary to previous reports, Eu is found to remain nearly divalent to the highest pressures reached (87 GPa) with magnetic order persisting to at least 50 GPa. These results should lead to a better understanding of the nature of the superconducting state found above 75 GPa and of the sequence of structural phase transitions observed to 92 GPa.