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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 ( T 1 ) 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 T 1 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. Understanding the connection between qubit coherence and microscopic materials properties is vital for improving device performance. Here, the relaxation times of superconducting transmon qubits are found to be directly correlated with Nb film properties such as grain size and surface oxide composition.

The metal–insulator transition and the intriguing physical properties of rare-earth perovskite nickelates have attracted considerable attention in recent years. Nonetheless, a complete understanding of these materials remains elusive. Here we combine X-ray absorption and resonant inelastic X-ray scattering (RIXS) spectroscopies to resolve important aspects of the complex electronic structure of rare-earth nickelates, taking NdNiO 3 thin film as representative example. The unusual coexistence of bound and continuum excitations observed in the RIXS spectra provides strong evidence for abundant oxygen holes in the ground state of these materials. Using cluster calculations and Anderson impurity model interpretation, we show that distinct spectral signatures arise from a Ni 3 d 8 configuration along with holes in the oxygen 2 p valence band, confirming suggestions that these materials do not obey a conventional positive charge-transfer picture, but instead exhibit a negative charge-transfer energy in line with recent models interpreting the metal–insulator transition in terms of bond disproportionation. Rare-earth perovskite nickelates show intriguing metal–insulator transitions, whose mechanism remains elusive. Here, Bisogni et al . evidenced a 3 d 8 Ni configuration together with abundance of oxygen 2 p holes in the ground state of a NdNiO 3 thin film, suggesting a negative charge-transfer scenario.

Single-photon emitters serve as building blocks for many emerging concepts in quantum photonics. The recent identification of bright, tunable and stable emitters in hexagonal boron nitride (hBN) has opened the door to quantum platforms operating across the infrared to ultraviolet spectrum. Although it is widely acknowledged that defects are responsible for single-photon emitters in hBN, crucial details regarding their origin, electronic levels and orbital involvement remain unknown. Here we employ a combination of resonant inelastic X-ray scattering and photoluminescence spectroscopy in defective hBN, unveiling an elementary excitation at 285 meV that gives rise to a plethora of harmonics correlated with single-photon emitters. We discuss the importance of N π* anti-bonding orbitals in shaping the electronic states of the emitters. The discovery of elementary excitations in hBN provides fundamental insights into quantum emission in low-dimensional materials, paving the way for future investigations in other platforms. Combining resonant inelastic X-ray scattering and photoluminescence spectroscopy, an elementary excitation in hexagonal-boron-nitride-based single-photon emitters has been demonstrated, giving rise to multiple regular harmonics that can explain the wide frequency range of these emitters.

Spin excitations of magnetic thin films are the founding element for magnetic devices in general. While spin dynamics have been extensively studied in bulk materials, the behaviour in mesoscopic films is less known due to experimental limitations. Here, we employ resonant inelastic X-ray scattering to investigate the spectrum of spin excitations in mesoscopic Fe films, from bulk-like films down to three unit cells. In bulk samples, we find isotropic, dispersive ferromagnons consistent with previous neutron scattering results for bulk single crystals. As the thickness is reduced, these ferromagnetic spin excitations renormalize to lower energies along the out-of-plane direction while retaining their dispersion in the in-plane direction. This thickness dependence is captured by simple Heisenberg model calculations accounting for the confinement in the out-of-plane direction through the loss of Fe bonds. Our findings highlight the effects of mesoscopic scaling on spin dynamics and identify thickness as a knob for fine tuning and controlling magnetic properties. The thickness dependence of the dispersion of spin waves in mesoscopic iron films is measured and found to be renormalized in the out-of-plane direction as the thickness is reduced. The results are captured by a Heisenberg model that accounts for the confinement in the out-of-plane direction through the loss of Fe bonds.

Strongly correlated materials respond sensitively to external perturbations such as strain, pressure, and doping. In the recently discovered superconducting infinite-layer nickelates, the superconducting transition temperature can be enhanced via only ~ 1% compressive strain-tuning with the root of such enhancement still being elusive. Using resonant inelastic x-ray scattering (RIXS), we investigate the magnetic excitations in infinite-layer PrNiO 2 thin films grown on two different substrates, namely SrTiO 3 (STO) and (LaAlO 3 ) 0.3 (Sr 2 TaAlO 6 ) 0.7 (LSAT) enforcing different strain on the nickelates films. The magnon bandwidth of PrNiO 2 shows only marginal response to strain-tuning, in sharp contrast to the enhancement of the superconducting transition temperature T c in the doped superconducting samples. These results suggest the bandwidth of spin excitations of the parent compounds is similar under strain while T c in the doped ones is not, and thus provide important empirics for the understanding of superconductivity in infinite-layer nickelates. Nickelates have been shown to host unconventional superconductivity, and recently it has been found that the choice of substrate can significantly change the superconducting critical temperature. This suggests, that like some Cuprates, strain could be important. Here Gao, Fan, Wang, and coauthors find that magnetic excitations in a parent Nickelate are insensitive to substrate choice, and therefore strain, which differs markedly from the case of Cuprates.

A ferroelectric material often exhibits a soft transverse optical (TO) phonon mode which governs its phase transition. Charge coupling to this ferroelectric soft mode may further mediate emergent physical properties, including superconductivity and defect tolerance in semiconductors. However, direct experimental evidence for such coupling is scarce. Here we show that a photogenerated coherent phonon couples strongly to the electronic transition above the bandgap in the van der Waals (vdW) two-dimensional (2D) ferroelectric semiconductor NbOI 2 . Using terahertz time-domain spectroscopy and first-principles calculations, we identify this mode as the TO phonon responsible for ferroelectric order. This exclusive coupling occurs only with the above-gap electronic transition and is absent in the valence band as revealed by resonant inelastic X-ray scattering. Our findings suggest a new role of the soft TO phonon mode in electronic and optical properties of ferroelectric semiconductors. Experimental evidence for charge coupling to ferroelectric soft mode is scarce. Here, the authors find a photogenerated coherent phonon coupling to the electronic transition above the bandgap in the van der Waals ferroelectric semiconductor NbOI2.

Geometrically frustrated lattices can display a range of correlated phenomena, ranging from spin frustration and charge order to dispersionless flat bands due to quantum interference. One particularly compelling family of such materials is the half-valence spinel Li B 2 O 4 materials. On the B -site frustrated pyrochlore sublattice, the interplay of correlated metallic behavior and charge frustration leads to a superconducting state in LiTi 2 O 4 and heavy fermion behavior in LiV 2 O 4 . To date, however, LiTi 2 O 4 has primarily been understood as a conventional BCS superconductor despite a lattice structure that could host more exotic ground states. Here, we present a multimodal investigation of LiTi 2 O 4 , combining ARPES, RIXS, proximate magnetic probes, and ab-initio many-body theoretical calculations. Our data reveals a novel mobile polaronic ground state with spectroscopic signatures that underlie co-dominant electron-phonon coupling and electron-electron correlations also found in the lightly doped cuprates. The cooperation between the two interaction scales distinguishes LiTi 2 O 4 from other superconducting titanates, suggesting an unconventional origin to superconductivity in LiTi 2 O 4 . Our work deepens our understanding of the rare interplay of electron-electron correlations and electron-phonon coupling in unconventional superconducting systems. In particular, our work identifies the geometrically frustrated, mixed-valence spinel family as an under-explored platform for discovering unconventional, correlated ground states. The authors study epitaxial thin films of the pyrochlore-sublattice compound LiTi2O4 by RIXS and ARPES. They observe cooperation between strong electron correlations and strong electron-phonon coupling, giving rise to a mobile polaronic ground state in which charge motion and lattice distortions are coupled.

Understanding the spin dynamics in antiferromagnetic (AFM) thin films is fundamental for designing novel devices based on AFM magnon transport. Here, we study the magnon dynamics in thin films of AFMS=52α−Fe2O3by combining resonant inelastic x-ray scattering, Anderson impurity model plus dynamical mean-field theory, and Heisenberg spin model. Below 100 meV, we observe the thickness-independent (down to 15 nm) acoustic single-magnon mode. At higher energies (100–500 meV), an unexpected sequence of equally spaced, optical modes is resolved and ascribed toΔSz=1, 2, 3, 4, and 5 magnetic excitations corresponding to multiple, noninteracting magnons. Our study unveils the energy, character, and momentum dependence of single and multimagnons inα−Fe2O3thin films, with impact on AFM magnon transport and its related phenomena. From a broader perspective, we generalize the use ofL-edge resonant inelastic x-ray scattering as a multispin-excitation probe up toΔSz=2S. Our analysis identifies the spin-orbital mixing in the valence shell as the key element for accessing excitations beyondΔSz=1, and up to, e.g.,ΔSz=5. At the same time, we elucidate the novel origin of the spin excitations beyond theΔSz=2, emphasizing the key role played by the crystal lattice as a reservoir of angular momentum that complements the quanta carried by the absorbed and emitted photons.

Strongly correlated insulators are broadly divided into two classes: Mott–Hubbard insulators, where the insulating gap is driven by the Coulomb repulsion U on the transition-metal cation, and charge-transfer insulators, where the gap is driven by the charge-transfer energy Δ between the cation and the ligand anions. The relative magnitudes of U and Δ determine which class a material belongs to, and subsequently the nature of its low-energy excitations. These energy scales are typically understood through the local chemistry of the active ions. Here we show that the situation is more complex in the low-dimensional charge-transfer insulator Li 2 CuO 2 , where Δ has a large non-electronic component. Combining resonant inelastic X-ray scattering with detailed modelling, we determine how the elementary lattice, charge, spin and orbital excitations are entangled in this material. This results in a large lattice-driven renormalization of Δ, which significantly reshapes the fundamental electronic properties of Li 2 CuO 2 . In transition metal oxides, an insulating band gap is found when the energy scales related to ionic charge excitations dominate over electronic itinerancy. Here, the authors demonstrate strong electron-phonon interactions in Li 2 CuO 2 and their effect on the insulating band gap.

The sustained interest in investigating magnetism in the 2D limit of insulating antiferromagnets is driven by the possibilities of discovering, or engineering, novel magnetic phases through layer stacking. However, due to the difficulty of directly measuring magnetic interactions in 2D antiferromagnets, it is not yet understood how intralayer magnetic interactions in insulating, strongly correlated, materials can be modified through layer proximity. Herein, the impact of reduced dimensionality in the model van der Waals antiferromagnet NiPS3 is explored by measuring electronic excitations in exfoliated samples using Resonant Inelastic X‐ray Scattering (RIXS). The resulting spectra shows systematic broadening of NiS6 multiplet excitations with decreasing layer count from bulk down to three atomic layers (3L). It is shown that these trends originate from a decrease in transition metal‐ligand and ligand–ligand hopping integrals, and by charge‐transfer energy evolving from Δ = 0.83 eV in the bulk to 0.37 eV in 3L NiPS3. Relevant intralayer magnetic exchange integrals computed from the electronic parameters exhibit a decrease in the average interaction strength with thickness. This study underscores the influence of interlayer electronic interactions on intralayer ones in insulating magnets, indicating that magnetic Hamiltonians in few‐layer insulating magnets can greatly deviate from their bulk counterparts. The impact of reduced dimensionality in the model van der Waals antiferromagnet NiPS3 is explored by measuring electronic excitations in exfoliated samples using Resonant Inealstic X‐ray Scattering (RIXS). The systematic broadening of RIXS spectra with decreasing thickness revealed an electronic structure intricately linked to dimensionality. This work showcases RIXS as an essential tool to investigate exfoliated 2D magnets.