Bipolaronic superconductivity is an exotic pairing mechanism proposed for materials like Ba1-xKxBiO3 (BKBO); however, conclusive experimental evidence for a (bi)polaron metallic state in this material remains elusive. Here, we combine resonant inelastic x-ray and neutron total scattering techniques with advanced modeling to study the local lattice distortions, electronic structure, and electron-phonon (e-ph) coupling in BKBO as a function of doping. Data for the parent compound (x = 0) indicate that the electronic gap opens in predominantly oxygen-derived states strongly coupled to a long-range ordered breathing distortion of the oxygen sublattice. Upon doping, short-range breathing distortions and sizable e-ph coupling persist into the superconducting regime (x = 0.4). Comparisons with exact diagonalization and determinant quantum Monte Carlo calculations further support this conclusion. Our results provide compelling evidence that BKBO's metallic phase hosts a liquid of small (bi)polarons derived from local breathing distortions of the lattice, with implications for understanding the low-temperature superconducting instability.

The room temperature conversion of gaseous methanol to carbon monoxide and hydrogen on a polycrystalline Au film at ambient pressure has been triggered and characterized by oxygen K-edge excitation and vibrationally resolved resonant inelastic X-ray scattering. The rate-limiting first methanol dehydrogenation step is driven by ultrafast O-H dissociation and deprotonation of O K-edge excited CH3OH. The Au surface further dehydrogenates the CH3O+ photoradical created by X-rays via electron transfer from the Au surface. With vibrationally resolved resonant inelastic X-ray scattering, we trace the CO molecular potential energy surface along the C-O coordinate. The CO bond softens, and the C-O stretch frequency changes from 2250 to 2065 cm-1 at a CO chemisorption energy of 38-58 kJ/mol. This constitutes weak chemisorption as compared to the transition metals but also stronger bonding than the physisorbed CO species on single-crystal Au surfaces. In liquid methanol, the recombination of the CH3O+ photoradical created by X-rays with protons quenches this conversion.

Bipolaronic superconductivity is an exotic pairing mechanism proposed for materials like Ba_{1−x}K_{x}BiO_{3} (BKBO); however, conclusive experimental evidence for a (bi)polaron metallic state in this material remains elusive. Here, we combine resonant inelastic x-ray and neutron total scattering techniques with advanced modeling to study the local lattice distortions, electronic structure, and electron-phonon (e-ph) coupling in BKBO as a function of doping. Data for the parent compound (x=0) indicate that the electronic gap opens in predominantly oxygen-derived states strongly coupled to a long-range ordered breathing distortion of the oxygen sublattice. Upon doping, short-range breathing distortions and sizable e-ph coupling persist into the superconducting regime (x=0.4). Comparisons with exact diagonalization and determinant quantum Monte Carlo calculations further support this conclusion. Our results provide compelling evidence that BKBO’s metallic phase hosts a liquid of small (bi)polarons derived from local breathing distortions of the lattice, with implications for understanding the low-temperature superconducting instability.

Bipolaronic superconductivity is an exotic pairing mechanism proposed for materials like Ba_1-xK_xBiO_3 (BKBO); however, conclusive experimental evidence for a (bi)polaron metallic state in this material remains elusive. Here, we combine resonant inelastic x-ray and neutron total scattering techniques with advanced modelling to study the local lattice distortions, electronic structure, and electron-phonon coupling (e-ph) in BKBO as a function of doping. Data for the parent compound (x = 0) indicates that the electronic gap opens in predominantly oxygen-derived states strongly coupled to a long-range ordered breathing distortion of the oxygen sublattice. Upon doping, short-range breathing distortions and sizable (e-ph) coupling persist into the superconducting regime (x = 0.4). Comparisons with exact diagonalization and determinant quantum Monte Carlo calculations further support this conclusion. Our results provide compelling evidence that BKBO's metallic phase hosts a liquid of small (bi)polarons derived from local breathing distortions of the lattice, with implications for understanding the low-temperature superconducting instability

We have investigated the 3d orbital excitations in CaCuO_{2} (CCO), Nd_{2}CuO_{4} (NCO), and La_{2}CuO_{4} (LCO) using high-resolution resonant inelastic x-ray scattering. In LCO they behave as well-localized excitations, similarly to several other cuprates. On the contrary, in CCO and NCO the d_{xy} orbital clearly disperses, pointing to a collective character of this excitation (orbiton) in compounds without apical oxygen. We ascribe the origin of the dispersion as stemming from a substantial next-nearest-neighbor (NNN) orbital superexchange. Such an exchange leads to the liberation of the orbiton from its coupling to magnons, which is associated with the orbiton hopping between nearest neighbor copper sites. Finally, we show that the exceptionally large NNN orbital superexchange can be traced back to the absence of apical oxygens suppressing the charge transfer energy.

The fluctuating hydrogen bridge bonded network of liquid water at ambient conditions entails a varied ensemble of the underlying constituting H 2 O molecular moieties. This is mirrored in a manifold of the H 2 O molecular potentials. Subnatural line width resonant inelastic X-ray scattering allowed us to quantify the manifold of molecular potential energy surfaces along the H 2 O symmetric normal mode and the local asymmetric O–H bond coordinate up to 1 and 1.5 Å, respectively. The comparison of the single H 2 O molecular potentials and spectroscopic signatures with the ambient conditions liquid phase H 2 O molecular potentials is done on various levels. In the gas phase, first principles, Morse potentials, and stepwise harmonic potential reconstruction have been employed and benchmarked. In the liquid phase the determination of the potential energy manifold along the local asymmetric O–H bond coordinate from resonant inelastic X-ray scattering via the bound state oxygen 1 s to 4 a 1 resonance is treated within these frameworks. The potential energy surface manifold along the symmetric stretch from resonant inelastic X-ray scattering via the oxygen 1 s to 2 b 2 resonance is based on stepwise harmonic reconstruction. We find in liquid water at ambient conditions H 2 O molecular potentials ranging from the weak interaction limit to strongly distorted potentials which are put into perspective to established parameters, i.e., intermolecular O–H, H–H, and O–O correlation lengths from neutron scattering.

It has been well established experimentally that the interplay of electronic correlations and spin-orbit interactions in Ir4+ and Ir5+ oxides results in insulating J(eff) = 1/2 and J(eff) = 0 ground states, respectively. However, in compounds where the structural dimerization of iridium ions is favorable, the direct Ir d-d hybridization can be significant and takes a key role. Here, we investigate the effects of direct Ir d-d hybridization in comparison with electronic correlations and spin-orbit coupling in Ba5AlIr2O11, a compound with Ir dimers. Using a combination of ab initio many-body wave-function quantum chemistry calculations and resonant inelastic x-ray scattering experiments, we elucidate the electronic structure of Ba5AlIr2O11. We find excellent agreement between the calculated and the measured spin-orbit excitations. Contrary to expectations, the analysis of the many-body wave function shows that the two Ir (Ir4+ and Ir5+) ions in the Ir2O9 dimer unit in this compound preserve their local J(eff) character close to 1/2 and 0, respectively. The local point group symmetry at each of the Ir ions plays an important role, significantly limiting the direct d-d hybridization. Our results emphasize that minute details in the local crystal field environment can lead to dramatic differences in the electronic states in iridates and 5d oxides in general.

The parent compounds of iron-based superconductors are magnetically ordered bad metals, with superconductivity appearing near a putative magnetic quantum critical point. The presence of both Hubbard repulsion and Hund's coupling leads to rich physics in these multiorbital systems, and motivated descriptions of magnetism in terms of itinerant electrons or localized spins. The NaFe1-xCuxAs series consists of magnetically ordered bad metal (x = 0), superconducting (x approximate to 0.02) and magnetically ordered semiconducing/insulating (x approximate to 0.5) phases, providing a platform to investigate the connection between superconductivity, magnetism and electronic correlations. Here we use x-ray absorption spectroscopy and resonant inelastic x-ray scattering to study the valence state of Fe and spin dynamics in two NaFe1-xCuxAs compounds (x = 0 and 0.47). We find that magnetism in both compounds arises from Fe2+ atoms, and exhibits underdamped dispersive spin waves in their respective ordered states. The dispersion of spin excitations in NaFe0.53Cu0.47As is consistent with being quasi-one-dimensional. Compared to NaFeAs, the band top of spin waves in NaFe0.53Cu0.47As is slightly softened with significantly more spectral weight of the spin excitations. Our results indicate the spin dynamics in NaFe0.53Cu0.47As arise from localized magnetic moments and suggest the iron-based superconductors are proximate to a correlated insulating state with localized iron moments.

The interplay of nearly degenerate orders in quantum materials can lead to a myriad of emergent phases. A prominent case is that of the high- T c cuprates for which the relationship between superconductivity and a short-ranged, incommensurate charge density wave in the CuO 2 planes involving the $$d_{x^2 - y^2}$$ d x 2 − y 2 orbitals (Cu-CDW) is a subject of great current interest. Strong modifications of the strength and coherence of this Cu-CDW have been achieved by applying large magnetic fields, uniaxial pressure, or via the interfacial coupling in cuprate/manganite multilayers. However, such modifications do not alter the dominant orbital character. Here we investigate cuprate/manganite multilayers with resonant inelastic X-ray scattering (RIXS) and show that a new kind of Cu-based density wave order can be induced that has not been previously observed in the cuprates. This order has an unusually small in-plane wave vector in the range of Q ||  < 0.1 reciprocal lattice units (r.l.u.), a large correlation length of about 40 nm, and a predominant $$d_{z^2}$$ d z 2 orbital character, instead of the typical $$d_{x^2 - y^2}$$ d x 2 − y 2 one. Its appearance is determined by the hole doping of the manganite which is a key parameter controlling the interfacial charge transfer and orbital reconstruction. We anticipate that the observation of a previously unknown type of density wave order at the YBCO interface will allow for fresh perspectives on the enigmatic relation between superconductivity and charge order (CO) in the cuprates.

A Correction to this paper has been published: https://doi.org/10.1038/s41535-021-00323-8

Quenching of vibrational excitations in resonant inelastic X-ray scattering (RIXS) spectra of liquid acetic acid is observed. At the oxygen core resonance associated with localized excitations at the O–H bond, the spectra lack the typical progression of vibrational excitations observed in RIXS spectra of comparable systems. We interpret this phenomenon as due to strong rehybridization of the unoccupied molecular orbitals as a result of hydrogen bonding, which however cannot be observed in x-ray absorption but only by means of RIXS. This allows us to address the molecular structure of the liquid, and to determine a lower limit for the average molecular chain length.

A Correction to this paper has been published: https://doi.org/10.1038/s41535-021-00323-8

The interplay of nearly degenerate orders in quantum materials can lead to a myriad of emergent phases. A prominent case is that of the high-Tc cuprates for which the relationship between superconductivity and a short-ranged, incommensurate charge density wave in the CuO2 planes involving the $$d_{x^2 - y^2}$$ orbitals (Cu-CDW) is a subject of great current interest. Strong modifications of the strength and coherence of this Cu-CDW have been achieved by applying large magnetic fields, uniaxial pressure, or via the interfacial coupling in cuprate/manganite multilayers. However, such modifications do not alter the dominant orbital character. Here we investigate cuprate/manganite multilayers with resonant inelastic X-ray scattering (RIXS) and show that a new kind of Cu-based density wave order can be induced that has not been previously observed in the cuprates. This order has an unusually small in-plane wave vector in the range of Q|| < 0.1 reciprocal lattice units (r.l.u.), a large correlation length of about 40 nm, and a predominant $$d_{z^2}$$ orbital character, instead of the typical $$d_{x^2 - y^2}$$ one. Its appearance is determined by the hole doping of the manganite which is a key parameter controlling the interfacial charge transfer and orbital reconstruction. We anticipate that the observation of a previously unknown type of density wave order at the YBCO interface will allow for fresh perspectives on the enigmatic relation between superconductivity and charge order (CO) in the cuprates.

A theoretical and experimental study of the gas phase and liquid acetic acid based on resonant inelastic x-ray scattering (RIXS) spectroscopy is presented. We combine and compare different levels of theory for an isolated molecule for a comprehensive analysis, including electronic and vibrational degrees of freedom. The excitation energy scan over the oxygen K-edge absorption reveals nuclear dynamic effects in the core-excited and final electronic states. The theoretical simulations for the monomer and two different forms of the dimer are compared against high-resolution experimental data for pure liquid acetic acid. We show that the theoretical model based on a dimer describes the hydrogen bond formation in the liquid phase well and that this bond formation sufficiently alters the RIXS spectra, allowing us to trace these effects directly from the experiment. Multimode vibrational dynamics is accounted for in our simulations by using a hybrid time-dependent stationary approach for the quantum nuclear wave packet simulations, showing the important role it plays in RIXS.

A Correction to this paper has been published: https://doi.org/10.1038/s41535-021-00323-8

Phase transitions driven by ultrashort laser pulses have attracted interest both for understanding the fundamental physics of phase transitions and for potential new data storage or device applications. In many cases these transitions involve transient states that are different from those seen in equilibrium. To understand the microscopic properties of these states, it is useful to develop elementally selective probing techniques that operate in the time domain. Here we show fs-time-resolved measurements of V Ledge Resonant Inelastic X-Ray Scattering (RIXS) from the insulating phase of the Mott- Hubbard material V2O3 after ultrafast laser excitation. The probed orbital excitations within the d-shell of the V ion show a sub-ps time response, which evolve at later times to a state that appears electronically indistinguishable from the high-temperature metallic state. Our results demonstrate the potential for RIXS spectroscopy to study the ultrafast orbital dynamics in strongly correlated materials.

Advanced Functional MaterialsVolume 30, Issue 51 2070339 FrontispieceFree Access Binary Oxide Superlattices: Versatile Tunability of the Metal Insulator Transition in (TiO2)m/(VO2)m Superlattices (Adv. Funct. Mater. 51/2020) Gyula Eres, Gyula Eres Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN, 37831 USASearch for more papers by this authorShinbuhm Lee, Shinbuhm Lee Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN, 37831 USA Department of Emerging Materials Science, Daegu-Gyeongbuk Institute of Science and Technology, Daegu, 42988 Republic of KoreaSearch for more papers by this authorJohn Nichols, John Nichols Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN, 37831 USASearch for more papers by this authorChanghee Sohn, Changhee Sohn Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN, 37831 USASearch for more papers by this authorJong Mok Ok, Jong Mok Ok Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN, 37831 USASearch for more papers by this authorAlessandro R. Mazza, Alessandro R. Mazza Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN, 37831 USASearch for more papers by this authorChenze Liu, Chenze Liu Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN, 37996 USASearch for more papers by this authorGerd Duscher, Gerd Duscher Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN, 37996 USASearch for more papers by this authorHo Nyung Lee, Ho Nyung Lee Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN, 37831 USASearch for more papers by this authorDaniel E. McNally, Daniel E. McNally Paul Scherrer Institut, Photon Science Division, Villigen PSI, CH-5232 SwitzerlandSearch for more papers by this authorXingye Lu, Xingye Lu Paul Scherrer Institut, Photon Science Division, Villigen PSI, CH-5232 SwitzerlandSearch for more papers by this authorMilan Radovic, Milan Radovic Paul Scherrer Institut, Photon Science Division, Villigen PSI, CH-5232 SwitzerlandSearch for more papers by this authorThorsten Schmitt, Thorsten Schmitt Paul Scherrer Institut, Photon Science Division, Villigen PSI, CH-5232 SwitzerlandSearch for more papers by this author Gyula Eres, Gyula Eres Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN, 37831 USASearch for more papers by this authorShinbuhm Lee, Shinbuhm Lee Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN, 37831 USA Department of Emerging Materials Science, Daegu-Gyeongbuk Institute of Science and Technology, Daegu, 42988 Republic of KoreaSearch for more papers by this authorJohn Nichols, John Nichols Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN, 37831 USASearch for more papers by this authorChanghee Sohn, Changhee Sohn Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN, 37831 USASearch for more papers by this authorJong Mok Ok, Jong Mok Ok Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN, 37831 USASearch for more papers by this authorAlessandro R. Mazza, Alessandro R. Mazza Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN, 37831 USASearch for more papers by this authorChenze Liu, Chenze Liu Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN, 37996 USASearch for more papers by this authorGerd Duscher, Gerd Duscher Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN, 37996 USASearch for more papers by this authorHo Nyung Lee, Ho Nyung Lee Oak Ridge National Laboratory, Materials Science and Technology Division, Oak Ridge, TN, 37831 USASearch for more papers by this authorDaniel E. McNally, Daniel E. McNally Paul Scherrer Institut, Photon Science Division, Villigen PSI, CH-5232 SwitzerlandSearch for more papers by this authorXingye Lu, Xingye Lu Paul Scherrer Institut, Photon Science Division, Villigen PSI, CH-5232 SwitzerlandSearch for more papers by this authorMilan Radovic, Milan Radovic Paul Scherrer Institut, Photon Science Division, Villigen PSI, CH-5232 SwitzerlandSearch for more papers by this authorThorsten Schmitt, Thorsten Schmitt Paul Scherrer Institut, Photon Science Division, Villigen PSI, CH-5232 SwitzerlandSearch for more papers by this author First published: 15 December 2020 https://doi.org/10.1002/adfm.202070339AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract The fabrication of binary oxide superlattices is undertaken as a general approach for exploring novel concepts and phenomena in reduced dimensionality systems of strongly correlated oxides. In article number 2004914, Gyula Eres, Milan Radovic, Thorsten Schmitt, and co-workers achieve a wide range of tunability of the metal insulator transition in VO2 while reducing oxygen vacancy formation that is detrimental to electrical properties. The design was prepared by Yun-Yi Pai and Gyula Eres both of ORNL. Volume30, Issue51December 15, 20202070339 RelatedInformation