Toward the ultimate superconductivity solver

A complete understanding of nature rarely precedes its technological exploitation. More often the reverse is true. Promising applications stimulate development long before theory reaches consensus. Superconductivity is perhaps the most striking example. Decades after the discovery of high-temperature superconductors by Bednorz and Müller in 1986 triggered an unprecedented wave of research (Bussmann-Holder and Keller, 2020) the quest for materials capable of carrying electrical current without resistance continues to outpace efforts to construct a unified explanation of why they work.

The field is vibrant: New superconducting systems continue to emerge, from twisted graphene (Cao et al., 2018) to Kagome compounds (Wilson and Ortiz, 2024) and transition-metal dichalcogenides (Simon et al., 2024; Steinberg et al., 2025). The promise of superconductors operating at practical temperatures is difficult to overstate. Yet, nearly 4 decades after the discovery of high-temperature superconductivity, its underlying physics remains stubbornly elusive.

Part of the difficulty stems from the fact that superconductivity rarely exists in isolation (Fradkin et al., 2015; Dai, 2015; Seamus Davis and Lee, 2013). It competes and coexists with charge order, magnetism, lattice distortions, disorder and strain. Modern thinking increasingly views superconductors not as simple electronic systems but as complex ecosystems in which several phenomena interact simultaneously. Understanding such systems requires more than a single experimental technique. It requires a coordinated view.

The history of angle-resolved photoemission spectroscopy (ARPES) in the study of high-temperature superconductors illustrates this point well. Early in the field’s development, P. W. Anderson famously identified high-quality photoemission measurements as a potential “smoking gun” capable of revealing the microscopic origins of superconductivity (Anderson and Schrieffer, 1991). The remark proved remarkably visionary. Advances in photoemission instrumentation soon transformed ARPES into one of the most powerful probes of electronic structure, providing unprecedented insight into the many-body interactions underlying superconductivity. Dispersion kinks, quasiparticle dynamics and signatures of many-body interactions became visible for the first time (Valla et al., 1999a; Valla et al., 1999b; Lanzara et al., 2001; Johnson et al., 2001).

Yet the hoped-for resolution of the superconductivity puzzle never arrived. Improved measurements generated new questions alongside new answers. Were observed interactions driven by phonons, magnetic excitations or something else entirely?

Meanwhile, scanning tunnelling microscopy added an unexpected complication to the interpretation of photoemission data. Many unconventional superconductors proved electronically inhomogeneous on nanometre length scales (Hanaguri et al., 2004; McElroy et al., 2005). Rather than exhibiting a single electronic structure, they often consist of a patchwork of regions with subtly different local properties. Conventional ARPES averages over such variations, raising a fundamental question: to what extent do measured spectra represent intrinsic behaviour rather than a spatial average over distinct electronic environments?

Nano-ARPES emerged as a natural response (Avila and Asensio, 2014). By focusing the photon beam to sub-micron dimensions, the technique promised direct access to the electronic structure of individual regions within an inhomogeneous sample. The concept was compelling. In practice, however, the required focusing optics imposed a significant penalty in photon flux, and with it a reduction in achievable energy resolution. As a result, nano-ARPES succeeded in revealing spatial variations of electronic structure but often lacked the spectral precision needed to resolve the subtle low-energy phenomena most relevant to superconductivity.

Progress continues. Recent developments have demonstrated that the traditional trade-off between spatial and energy resolution is not unchangeable (Miyai et al., 2024). Nevertheless, achieving simultaneously nanometre-scale spatial sensitivity and meV or sub-meV energy resolution remains one of the major instrumental challenges facing photoemission science.

The lesson is straightforward. Resolution alone is not enough. Future progress requires correlation.

This challenge points toward a broader conclusion. The objective need not be to make ARPES alone resolve every aspect of an inhomogeneous superconductor. A more practical strategy may be to combine ultra-high-resolution photoemission with complementary probes capable of mapping structural and electronic heterogeneity. In this framework, diffraction determines where the patches are, while photoemission reveals the electronic states associated with them.

Recent advances in instrumentation suggest a possible path forward. Imagine a facility in which high-resolution ARPES, micro- or nano-scale X-ray diffraction and resonant inelastic X-ray scattering (RIXS) probe precisely the same sample and, ideally, the same microscopic region. Diffraction would map structural variations (Campi et al., 2015). ARPES would reveal the local electronic landscape. RIXS would provide access to phonons, magnons and other collective excitations (Ament et al., 2011). Together they would deliver a multidimensional picture of superconductivity unavailable to any individual technique.

The concept is technically ambitious but no longer implausible. Modern synchrotron sources provide exceptional brightness across broad energy ranges. Laser-based ARPES systems routinely achieve sub-meV energy resolution (Iwasawa et al., 2017), in some cases approaching hundreds of micro-electron-volts. Diffraction and RIXS instrumentation continue to improve in both spatial and energy resolution. The necessary ingredients already exist. What remains is their integration.

Such a facility would not by itself solve the mystery of superconductivity. No instrument can guarantee discovery. But it would provide something the field has long lacked: a means of simultaneously connecting structure, electronic states and collective excitations within the same material. In a discipline increasingly defined by complexity and coexistence, such correlations may prove more valuable than yet another incremental improvement in resolution.

A useful precedent can be found in structural biology. Determining complex biomolecular structures increasingly relies on correlative approaches combining cryo-electron microscopy, X-ray diffraction and computational analysis. Each technique contributes a distinct perspective, while data integration provides insights unavailable from any single probe. A similar philosophy may prove valuable for superconductivity research, where structural, electronic and collective degrees of freedom are deeply intertwined.

The history of superconductivity is rich with examples in which advances in instrumentation preceded advances in understanding. To open the next chapter, the community may benefit from building an experimental ecosystem capable of seeing the whole problem at once, rather than searching for a single decisive experiment. An integrated platform combining diffraction, RIXS, and ultra-high-resolution photoemission could become not merely another set of instruments, but the ultimate superconductivity solver.

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Footnotes

Edited and reviewed by: Ctirad Uher, University of Michigan, Ann Arbor, United States