Session M08: X-ray spectroscopy study of correlated electronic systems

Magnetically propagating Hund’s exciton in van der Waals antiferromagnet NiPS3

Magnetic van der Waals materials have opened new frontiers for realizing novel many-body phenomena. Recently NiPS₃ has received intense interest since it hosts an excitonic quasiparticle whose properties appear to be intimately linked to the magnetic state of the lattice. Despite extensive studies, the electronic character, mobility, and magnetic interactions of the exciton remain unresolved. Here we address these issues by measuring NiPS₃ with ultra-high energy resolution resonant inelastic x-ray scattering. We find that Hund's exchange interactions are primarily responsible for the energy of formation of the exciton. Measuring the dispersion of the Hund's exciton reveals that it propagates in a way that is analogous to a double magnon. We trace this unique behavior to fundamental similarities between the NiPS₃ exciton hopping and spin exchange processes, underlining the unique magnetic characteristics of this novel quasiparticle.   

Investigation of the surface chemical state of SrFeO2.5 films during topotactic transition by ambient pressure X-ray photoelectron spectroscopy

Topotactic transition between a metallic perovskite SrFeO₃ (SFO) and an insulating brownmillerite SrFeO_2.5 has been extensively studied due to its potential applications, such as synaptic components for neuromorphic computing and memory devices. Only ordered oxygen vacancies (OVCs) are formed and removed during the transition while maintaining the crystallographic framework. The formation of OVCs is known to be strongly influenced by the strain and surface energy that can affect the capability to dissociate gas molecules of a material. However, there are still few studies on the effect of the strain on the surface chemistry and the topotactic transition.

In this study, we investigated the correlation between the surface chemistry and the topotactic transition of SFO films with different strain states. SFO films were grown on SrTiO₃(001) and (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7(001), LSAT substrates using the pulsed laser deposition. Structural analysis based on X-ray diffraction confirmed that both films have high crystallinity, and the SFO film grown on STO(001) showed weaker in-plane compressive strain than the film grown on LSAT(001) due to the small lattice mismatch between the film and STO substrate. We investigate the change in the surface chemical state and the corresponding electrical resistance of the films during the topotactic transition using Ambient-pressure X-ray photoelectron spectroscopy. We observed a larger surface SrO state in the film grown on the STO(001) substrate, where the phase transition occurs more slowly than the film grown on the LSAT(001) substrate. We will discuss the correlation between strain state, surface SrO, and resistance on the topotactic transition.   

Resolving local lattice distortions in strongly correlated systems by total x-ray scattering

Competing interactions in strongly correlated systems are known to lead to the emergence of collectively ordered electronic states such as unconventional superconductivity, charge density waves, complex magnetic order, nematic order, and Kondo lattice, to name a few. The orders appear intertwined in intricate phase diagrams and are often coupled to lattice degrees of freedom, including local lattice distortions. Local lattice distortions are, however, difficult to assess using traditional crystallographic techniques, rendering it difficult to quantify how they interact with the emerging orders. We will show that the problem can be resolved by employing non-standard experimental techniques such as total x-ray scattering coupled to 3D structure modeling. Examples will include results from our recent studies on UPt₂Si₂ [1] exhibiting Kondo lattice order and the charge density wave system 1T-TaS₂ [2].   

Resonant Inelastic X-ray Scattering (RIXS) Spectra for Ladder Cuprates

The ladder compound Sr14Cu24O41 is of interest both as a quasi-one-dimensional analog of the superconducting cuprates and as a superconductor in its own right when Sr is substituted by Ca. In order to model resonant inelastic x-ray scattering (RIXS) spectra for this compound, we investigate the simpler SrCu2O3 system in which the crystal structure contains very similar ladder planes. We approximate the LDA dispersion of SrCu2O3 by a Cu only two-band tight-binding model. Strong correlation effcts are incorporated by assuming an anti-ferromagnetic ground state. The available angle-resolved photoemission (ARPES) and RIXS data on the ladder compound are found to be in reasonable accord with our theoretical predictions   

Incommensurate spin and charge order in cuprate systems

In recent years, charge order (CO) has been found to complement spin order (SO) as an integral part of the low energy physics of cuprate superconductors. Oftentimes the respective ordering vectors become incommensurate, like, e.g., the CO wavevector in the electron-doped cuprate Nd_2-xCe_xCuO₄ (NCCO) or the SO wavevector in La-based cuprates. Based on the extended one-band Hubbard model, we apply slave-boson mean field theory and evaluate fluctuations around magnetic ground states to study the interplay of spin and charge order in these systems. Thereby, we achieve close agreement with experimental evidence from resonant inelastic x-ray scattering (RIXS) and shed light on the driving mechanisms of incommensurate order.   

Oxygen Isotope Study of Y1-xPrxBa2Cu3O7 Thin Films

Oxygen isotope effect (OIE) studies have been used to probe the extent of electron-phonon interactions in the pairing mechanism [1] and in the pseudogap formation [2] in cuprate superconductors. In this talk, we present a new OIE study of Pr-doped YBa₂Cu₃O_7-δ (YPrBCO), which is the only member of the RBCO (R = rare earth) family where its superconductivity can be systematically suppressed (by Pr-doping) without changing its oxygen content. Recently, YPrBCO has also been shown to exhibit incommensurate charge-density-waves (CDWs) all the way to the insulating Mott limit [3]. This study uses epitaxial YPrBCO thin films grown by pulsed laser deposition to maximize doping and isotope homogeneity. The samples were characterized using X-ray diffraction, X-ray reflectometry, scanning transmission electron microscopy (STEM), and scanning tunning microscopy. The substitution of ¹⁶O by ¹⁸O was done by gas-phase diffusion and confirmed using Raman spectroscopy and STEM. Electrical transport measurements and resonant X-ray scattering were used to determine the superconducting critical temperature (T_c), the pseudogap onset temperature (T^*), and the CDW onset temperature (T_CDW) respectively. Isotope shifts of T_CDW are used to probe the extent of electron-phonon interactions in the charge order, and correlations between the isotope shifts of T_c, T^* and T_CDW are used to determine the degree of interaction between the respective electronic orders. Our results are discussed in the context of the pseudogap and its relationship to the superconductivity and charge order in oxygen-underdoped YBCO and YPrBCO.

[1] G. Zhao et al., PRB 51 16487(R) (1995)

[2] D. Temperano et al., PRL 84 1990 (1999)

[3] M. Kang et al., PNAS 120 e2302099120 (2023)   

Unraveling the L3 edge RIXS spectrum of lightly manganese doped Sr3Ru2O7

The local nature of resonant inelastic x-ray scattering (RIXS) spectroscopy permits the use of single-site or cluster-based theories to simulate experimental spectra. These theories focus on the central atom (mostly the transition metal) and treat the correlations with surrounding ligands or adjacent transition metal complex through some hybridization parameters determined by fitting with the experimental spectra. Simulated spectra are mostly compared with experimental ones where the probed elements are the dominant species. It is still interesting to examine how far one can extend this kind of local treatment to the case of a dilute dopant system where the dopants themselves are the primary focus. We consider the experimental RIXS spectrum of a 10% Mn-doped Sr₃Ru₂O₇ compound at the Mn L₃ edge resonance. Using a single-ion based RIXS theory for Mn³⁺ (the dopant oxidation state) in conjunction with the experimental data, we unravel the microscopic interaction parameters (crystal field energies, superexchange couplings) of the Mn³⁺ ion in the host material. Based on our theoretical modeling and fit, we estimate the energy scale of the non-spin-flip and spin-flip dd excitation along with identifying the charge transfer excitation energy boundary.    

Soft Polar-Acoustic Excitations Reveal an Incipient Modulated Phase in SrTiO3

Enhanced fluctuations are one of the hallmarks of continuous phase transitions, however in some cases these fluctuations inhibit long-range order down to zero temperature. Even when not fully ordered, the associated elementary excitations of the system inherit properties of the underlying ``hidden order''. Here we study quantum paraelectric SrTiO₃, a quintessential example of such frustrated orders. We use ultrafast x-ray scattering at a free electron laser, together with a novel approach to probe inversion symmetry breaking using intense terahertz pulses, to resolve the polar excitations of SrTiO₃. We find that the relevant low-energy excitations at low temperature are polar-acoustic and particularly soft at the tens of nanometer lengthscale. These soft polar-acoustic modes indicate precursor fluctuations of an incipient polar state in which the polarization and strain are spatially modulated, and is distinct from the known homogeneous ferroelectric state. Our results provide a glimpse into potential modulated phases of SrTiO₃ and highlight the importance of probing fluctuations with atomic resolution both in equilibrium and at ultrafast timescales.