# The Linac Coherent Light Source: Recent Developments and Future Plans
## 3 LCLS-II Science Opportunities
### Quantum Materials  
\"Quantum materials\" are materials where charged particles behave collectively in ways we are unable to predict from the conventional single-electron band models that effectively describe simple metals and semiconductors. For quantum materials, reductionist approaches that consider only individual atoms, electrons and their orbitals are inadequate. Rather, hallmarks of quantum materials are competing or entwined order, phase separation, and heterogeneity (e.g., fluctuating nanoscale texture of charge, spin and orbitals) that result from strong coupling between constituent particles (charge, spin, orbitals, and phonons). These quantum interactions give rise to important \"emergent\" macroscopic properties such as high-temperature superconductivity, colossal magnetoresistivity, and topologically protected phases. One fundamental model for understanding the charged collective modes of an interacting electron system is the two-particle dynamic structure factor, \(S_{e}(q,\omega)\)-\(\chi(q,\omega)\) that describes inter-particle correlations as a function of momentum-transfer (\(q\)) and energy (\(\omega\)). Although the study of quasiparticles in quantum materials is now well advanced, we still lack effective experimental methods to directly probe \(S_{e}(q,\omega)\) in relevant materials. Because of the subtle balance among competing interactions in quantum materials, the important ground states are determined by collective modes in the 1 to 100 meV energy range as illustrated in Figure 10. In this region, modern X-ray sources and X-ray emission spectrometers struggle to achieve the combination of photon flux and energy resolution required for incisive measurements. This capability gap for measuring the essential observable of an interacting electron system substantially limits our understanding of quantum materials. High repetition rate XFELs will bridge this capability gap and offer transformative capabilities; for both characterizing ground-state collective modes (energy and momentum dependence throughout the Brillouin zone), and for following their response to tailored excitations. Momentum-resolved resonant inelastic X-ray scattering (qRIXS) records the energy and momentum-transfer of scattered X-rays and is a powerful tool to map the energy-momentum dispersion of collective excitations [41]. This approach has been applied to characterize the energy-momentum dispersion of a range of collective excitations (such as magnons [42], paramagnons [43], triplons [44], two-spinons [45], phonons [46], orbitons [45] etc.) at a routinely available energy resolution of -100 meV (resolving power R \(\sim\)10,000). State-of-the art RIXS instruments (e.g., ESRF ID32 [47], NSLS-II 2-ID SIX [48]) provide R \(>\) 30,000 to investigate collective excitations at energy scales comparable to the superconducting gap and pseudogap in unconventional high-Tc superconductors (<50 meV). In spite of these advances, the inherent lack of longitudinal coherence (and therefore limited spectral flux, ph/s/meV) of synchrotron X-ray sources limits the ultimate scientific impact of qRIXS. High repetition rate seeded XFELs are anticipated to drive a qualitative advance in applications of qRIXS to quantum materials by providing nearly a 1000-fold increase in available X-ray spectral flux for studies at unprecedented resolution and sensitivity. Beyond conventional qRIXS methods, _time-domain_ approaches have tremendous potential to provide an important new perspective on collective excitations by selectively perturbing (or suppressing) one of the interacting degrees of freedom; e.g., by transient charge, spin, or vibrational excitation. Of particular interest is the stimulation of materials directly on the low-energy scales characteristic of the collective excitations (phonons, plasmons, magnons etc.). Ultrafast pulses spanning the visible-to-THz regimes are effective stimuli of coherent collective excitations and can transiently disrupt intertwined degrees of freedom.