We used resonant inelastic x-ray scattering (RIXS) with and without analysis of the scattered photon polarization, to study dispersive spin excitations in the high temperature superconductor YBa2Cu3O6+x over a wide range of doping levels (0.1 ≤ x ≤ 1). The excitation profiles were carefully monitored as the incident photon energy was detuned from the resonant condition, and the spin excitation energy was found to be independent of detuning for all x. These findings demonstrate that the largest fraction of the spin-flip RIXS profiles in doped cuprates arises from magnetic collective modes, rather than from incoherent particle-hole excitations as recently suggested theoretically [Benjamin et al. Phys. Rev. Lett. 112, 247002(2014)]. Implications for the theoretical description of the electron system in the cuprates are discussed. PACS numbers:Electronic spin fluctuations are of central importance for current models of unconventional superconductivity in d-and f -electron compounds [1]. Inelastic neutron scattering (INS) provides comprehensive maps of the spin fluctuation intensity at energies and momenta that are well matched to the intrinsic collective response of correlated-electron systems, and has thus played a pivotal role in motivating and guiding theoretical work on unconventional superconductors [2]. Because of the limited intensity of primary neutron beams, however, INS can only be applied to materials of which large single crystals can be grown, and it is unsuitable as a probe of spin excitations in atomically thin heterostructures of complex materials, which provide perspectives for control -and ultimately design -of unconventional superconductivity [3].Resonant inelastic x-ray scattering (RIXS) at transition-metal L 2,3 -edges has recently emerged as a powerful momentum-resolved spectroscopic probe of collective spin excitations in crystals of sub-millimeter dimensions, and in thin films and multilayers [4,5]. Recent examples of RIXS studies of spin excitations include cuprates [6][7][8][9][10][11][12][13][14][15][16][17], iron-based superconductors [18] or iridates [19], where the intrinsic energy scale of the spin dynamics exceeds 100 meV. Initial RIXS data on the dispersion of magnons in the antiferromagnetic "parent compounds" of the cuprate high-temperature superconductors are fully consistent with prior INS data [6,12]. Remarkably, further RIXS studies revealed that magnon-like collective spin excitations persist in almost undiminished form even in optimally doped and overdoped cuprates, [12][13][14][15][16] where INS data are very limited. This indicates that strong electronic correlations persist even in a regime where Fermi-liquid properties have been well documented [20,21]. Motivated by these results, soft x-ray RIXS spectrometers with greatly enhanced resolution are currently under construction at many synchrotron facilities worldwide.To realize the potential of RIXS as a probe of unconventional superconductors and other correlated-electron systems, it is imperative to develop a quantitative...

The strongly correlated insulator Ca2RuO4 is considered as a paradigmatic realization of both spin-orbital physics and a band-Mott insulating phase, characterized by orbitally selective coexistence of a band and a Mott gap. We present a high-resolution oxygen K-edge resonant inelastic X-ray scattering study of the antiferromagnetic Mott insulating state of Ca2RuO4. A set of lowenergy (∼80 and 400 meV) and high-energy (∼ 1.3 and 2.2 eV) excitations are reported that show strong incident light polarization dependence. Our results strongly support a spin-orbit coupled band-Mott scenario and explore in detail the nature of its exotic excitations. Guided by theoretical modelling, we interpret the low-energy excitations as a result of composite spin-orbital excitations. Their nature unveil the intricate interplay of crystal-field splitting and spin-orbit coupling in the band-Mott scenario. The high-energy excitations correspond to intra-atomic singlet-triplet transitions at an energy scale set by the Hund's coupling. Our findings give a unifying picture of the spin and orbital excitations in the band-Mott insulator Ca2RuO4.Introduction. Spin-orbit coupling (SOC) is a central thread in the search for novel quantum material physics [1]. A particularly promising avenue is the combination of SOC and strong electron correlations in multiorbital systems. This scenario is realized in heavy transition metal oxides composed of 4d and 5d elements. Iridium-oxides (iridates) such as Sr 2 IrO 4 are prime examples of systems where SOC plays a defining role in shaping the Mott insulating ground state [2]. In fact, spin-orbit entanglement essentially outplays the effectiveness of the usually influential crystal field δ. Of equal interest is the complex regime where SOC and crystal field energy scales are comparable. Here Ca 2 RuO 4 is a topical material that displays a wealth of physical properties. A record high non-superconducting diamagnetic response has, for example, been reported recently [3]. Superconductivity emerges in strained films [4] or upon application of hydrostatic pressure to bulk crystals [5]. Neutron and Raman scattering experiments have demonstrated both phase and amplitude spin-excitation modes consistent with the existence of a spin-orbit exciton [6][7][8]. Moreover, measurements of the paramagnetic insulating band structure [9] were interpreted in favor of an orbitally differentiated band-Mott insulating ground state [10,11]. This rich phenomenology of Ca 2 RuO 4 is a manifestation of the interplay between multiple energy scales, specifically, the Coulomb interaction U , the Hund's coupling J H , the crystal field splitting δ and SOC λ. In particular, a tendency towards an orbital selective Mott state is expected to be driven by the Hund's coupling [12]. Furthermore, the band-Mott scenario is triggered by a

We have used Resonant Inelastic X-ray Scattering (RIXS) and dynamical susceptibility calculations to study the magnetic excitations in NaFe 1−x Co x As (x = 0, 0.03 and 0.08). Despite a relatively low ordered magnetic moment, collective magnetic modes are observed in parent compounds (x = 0) and persist in optimally (x = 0.03) and overdoped (x = 0.08) samples. Their magnetic bandwidths are unaffected by doping within the range investigated. High energy magnetic excitations in iron pnictides are robust against doping, and present irrespectively of the ordered magnetic moment. Nevertheless, Co doping slightly reduces the overall magnetic spectral weight, differently from previous studies on hole-doped BaFe 2 As 2 , where it was observed constant.Finally, we demonstrate that the doping evolution of magnetic modes is different for the dopants being inside or outside the Fe-As layer.

We investigate electronic excitations in La2−x(Br,Sr)xCuO4 using resonant inelastic x-ray scattering (RIXS) at the oxygen K edge. RIXS spectra of the hole-doped cuprates show clear momentum dependence below 1 eV. The spectral weight exhibits positive dispersion and shifts to higher energy with increasing hole concentration. Theoretical calculation of the dynamical charge structure factor on oxygen orbitals in a three-band Hubbard model is consistent with the experimental observation of the momentum and doping dependence, and therefore the dispersive mode is ascribed to intraband charge excitations which have been observed in electron-doped cuprates.PACS numbers: 71.27.+a,74.25.Jb,74.72.Gh,78.70.Ck Strongly correlated transition-metal oxides display various interesting physical properties including metalinsulator transition, high-temperature superconductivity and colossal magnetoresistance, and some of the oxides are classified into doped Mott insulators where electron correlation significantly modifies their band structure which diverges from that of the noninteracting simple metal [1]. Among the doped Mott insulators, superconducting cuprates are most intensively studied [2]. This is mainly due to the superconductivity at high transition temperature and related phenomena such as pseudogap and a competing phase with charge order [3]. In addition to the interest of superconductivity, doped cuprates are important and suitable for the study of electronic structure of the doped Mott insulator because relatively simple theoretical models with a few orbitals are applicable to describe the electronic structure near the Fermi energy. They represent the benchmark of the doped Mott insulators and the clarification of the fundamental electronic structure is essential for understanding the mechanism of the physical phenomena in the doped cuprates.In the undoped cuprates, only the spin degree of freedom contributes to the low-energy electron dynamics. When carriers are doped, the charge degree of freedom becomes active and the electron dynamics is characterized by the motion of spin and charge. Therefore, we consider that both spin and charge excitations must be investigated on equal footing in order to understand the electron dynamics characterizing the physics of the cuprates. Inelastic neutron scattering (INS) has been widely used for studying the spin dynamics in the reciprocal lattice space, and high-resolution resonant inelastic x-ray scattering (RIXS) at the Cu L 3 -edge has recently become an alternative to measure momentum-resolved spin excitations up to several hundreds meV [4,5]. Charge excitations in the doped cuprates extend to higher energy than the spin excitations and the electron correlation affects the charge excitations of the order of a few eV. Optical studies [6,7] demonstrated that spectral weight of the intraband charge excitations emerges below the charge-transfer gap in the doped cuprates and the weight characterizes the charge excitations in the doped Mott insulators. The high-energy part of mome...

We studied the magnetic ordering of thin films and bulk crystals of rutile RuO2 using resonant X-ray scattering across the Ru L2 absorption edge. Combining polarization analysis and azimuthalangle dependence of the magnetic Bragg signal, we have established the presence and characteristic of collinear antiferromagnetism in RuO2 with TN > 300 K. In addition to revealing a spin-ordered ground state in the simplest ruthenium oxide compound, the persistence of magnetic order even in nanometer-thick films lays the ground for potential applications of RuO2 in antiferromagnetic spintronics.

Despite mounting evidence that materials imperfections are a major obstacle to practical applications of superconducting qubits, connections between microscopic material properties and qubit coherence are poorly understood. Here, we combine measurements of transmon qubit relaxation times (T1) with spectroscopy and microscopy of the polycrystalline niobium films used in qubit fabrication. By comparing films deposited using three different techniques, we reveal correlations between T1 and intrinsic film properties such as grain size, enhanced oxygen diffusion along grain boundaries, and the concentration of suboxides near the surface. Qubit and resonator measurements show signatures of two-level system defects, which we propose to be hosted in the grain boundaries and surface oxides. We also show that the residual resistance ratio of the polycrystalline niobium films can be used as a figure of merit for qubit lifetime. This comprehensive approach to understanding qubit decoherence charts a pathway for materials-driven improvements of superconducting qubit performance.

The discovery of superconductivity in a d 9−δ nickelate has inspired disparate theoretical perspectives regarding the essential physics of this class of materials. A key issue is the magnitude of the magnetic superexchange, which relates to whether cuprate-like high-temperature nickelate superconductivity could be realized. We address this question using Ni L-edge and O K-edge spectroscopy of the reduced d 9−1=3 trilayer nickelates R 4 Ni 3 O 8 (where R ¼ La, Pr) and associated theoretical modeling. A magnon energy scale of ∼80 meV resulting from a nearest-neighbor magnetic exchange of J ¼ 69ð4Þ meV is observed, proving that d 9−δ nickelates can host a large superexchange. This value, along with that of the Ni-O hybridization estimated from our O K-edge data, implies that trilayer nickelates represent an intermediate case between the infinite-layer nickelates and the cuprates. Layered nickelates thus provide a route to testing the relevance of superexchange to nickelate superconductivity.

High-resolution resonant inelastic x-ray scattering (RIXS) at the oxygen K edge has been used to study the orbital excitations of Ca 2 RuO 4 and Sr 2 RuO 4 . In combination with linear dichroism x-ray absorption spectroscopy, the ruthenium 4d-orbital occupation and excitations were probed through their hybridization with the oxygen p orbitals. These results are described within a minimal model, taking into account crystal field splitting and a spin-orbit coupling λ so = 200 meV. The effects of spin-orbit interaction on the electronic structure and implications for the Mott and superconducting ground states of (Ca,Sr) 2 RuO 4 are discussed.