Path 7: Non-Equilibrium Enhancement (Photo-induced & Dynamic Superconductivity)

Idea: Use external stimuli (laser pulses, terahertz fields, rapid pressure quenches) to induce or sustain superconductivity at higher temperatures than normally possible.

Rationale: Superconductivity is ordinarily an equilibrium ground-state property, but recent experiments show we can “boost” it transiently. In 2014, an infrared laser pulse aimed at YBa₂Cu₃O₇ (T_c ~90 K) was reported to induce a transient superconducting-like state at room temperature for a few picoseconds. The IR pulse excited certain lattice vibrations, which presumably enhanced the pairing interaction temporarily. Similarly, in 2016, researchers observed signatures of superconductivity in K₃C₆₀ (T_c ~20 K) persisting up to 100 K under mid-IR laser excitation. These findings suggest “out-of-equilibrium” pathways: one can use light or ultrafast perturbations to change the electronic structure or drive oscillations that favor pairing (a concept called Floquet engineering in physics). Another approach is pressure quenching – rapidly compressing and decompressing a material to trap a high-pressure phase metastably at ambient conditions (this was attempted, for example, to trap metallic hydrogen or to stabilize high-T_c hydrides without constant pressure). Non-equilibrium methods won’t directly yield a static room-T superconductor, but they could demonstrate that high-T_c pairing interactions exist and guide how to make them permanent.

Prerequisites: Ultrafast optics and spectroscopy, knowledge of how superconductors respond to high-frequency fields (time-dependent Ginzburg–Landau theory), and lattice dynamics. Also, familiarity with experimental setups like pump–probe measurements that detect fleeting states is needed to interpret results.

Dependencies: Insights from Path 2 and 4 might inform what modes to drive (e.g. if a certain lattice mode competes with a charge order, driving it could tip the balance to superconductivity). Path 7 might also leverage materials from Path 1–6 as testbeds (e.g. using lasers on a high-pressure hydride to see if one can lower the needed pressure).

Signs of Progress: The current milestones are transient and in exotic setups. Progress would be extending the duration and scale of photo-induced superconductivity – for example, achieving a state lasting nanoseconds or more, or inducing global zero resistance in a thin film (so far, evidence comes from optical properties or c-axis conductivity changes in cuprates). If researchers manage to stabilize a high-T_c phase – even in a thin slice of material or under continuous pumping – that would be a major leap. Another sign would be repeatable non-equilibrium switching of a material from normal to superconducting at high T by an external field, effectively creating a superconducting switch. While practical implementation (like a continually laser-driven superconducting cable) is far-fetched, demonstrating that superconductivity can be turned on and off at, say, 200 K with a stimulus would prove the principle. Over 30 years, these dynamic experiments could also reveal hidden superconducting states (so-called “hidden phases”) that equilibrium never accesses, guiding the creation of new materials that mimic those driven conditions in static form.

Base Camp 7.1: Time-Dependent SC and Floquet Theory

• A. Cavalleri. “Photo-Induced Superconductivity.” Contemporary Physics, vol. 59, 2018, pp. 31–46. – Review by the leader in this field, summarizing experiments on light-induced effects in cuprates, fullerides, etc. and theoretical interpretations. A must-read to understand what has been achieved and the theoretical frameworks (like Floquet Hamiltonians) used to analyze them.
• M. A. Sentef, et al. “Theory of Floquet Band Formation and Local Pseudospin Textures in Pump-Probed Graphene.” Nature Communications, vol. 6, 2015, 7047. – While about graphene, this is a primer on Floquet theory and band engineering via light. Provides technical tools that can be applied to superconductors (e.g. how a periodic drive renormalizes interactions). Good preparation for understanding how one might boost pairing by driving certain modes (the language of Floquet modes is used in analyzing pumped superconductors too).
• T. Oka and S. Kitamura. “Floquet Engineering of Quantum Materials.” Annual Review of Condensed Matter Physics, vol. 10, 2019, pp. 387–408. – Overview of Floquet engineering (periodic driving) in various contexts, including inducing topological states and possibly superconducting-like states. It helps generalize the idea of Path 7: we are essentially trying to engineer a Hamiltonian with enhanced pairing via time-periodic fields. This shows the breadth of that approach beyond just superconductivity experiments.

Base Camp 7.2: Ultrafast Experimental Techniques

• D. Fausti, et al. “Light-Induced Superconductivity in a Stripe-Ordered Cuprate.” Science, vol. 331, 2011, pp. 189–191. – The first report of transient superconductivity in 1/8-doped LESCO cuprate. Used mid-IR pulses to melt charge stripes and enhance conductivity, interpreted as possible SC up to 50 K (above equilibrium T_c ~ 8 K). Crucial for seeing how such experiments are done and how results are interpreted. It’s a cornerstone of Path 7’s experimental side.
• R. Mankowsky, et al. “Nonlinear Lattice Dynamics as a Basis for Enhanced Superconductivity in YBa₂Cu₃O_6.5.” Nature, vol. 516, 2014, pp. 71–73. – The Nature paper that reported ~3 ps-long signatures of superconductivity at 300 K in YBCO after IR pumping. Key to study: what was measured (c-axis conductivity via THz probe), what was inferred (Josephson plasma resonance appeared, indicating coherence between CuO₂ planes). Understanding this helps us design future experiments and also highlights controversies (proper subtraction of non-SC signals).
• M. Mitrano, et al. “Possible Light-Induced Superconductivity in K₃C₆₀ at High Temperature.” Nature, vol. 530, 2016, pp. 461–464. – Report of optical signatures of a superconducting state in K₃C₆₀ up to 100 K when driven by ultrafast pulses. It’s valuable as a non-cuprate example, suggesting the phenomenon is broader. It also describes detecting a Meissner-like response (via THz probe showing inductive behavior). Studying this paper teaches how to interpret complex optical data for superconductivity.

Base Camp 7.3: Metastable Phase Creation & Quenching

• K. Shimizu, et al. “Superconductivity in Compressed Lithium at 20 K.” Nature, vol. 419, 2002, pp. 597–599. – Superconducting lithium under high pressure (20 K at ~50 GPa). Not a non-equilibrium example per se, but relevant as lithium can be quenched to metastable phases at ambient once cooled under pressure (demonstrating one approach to metastability). It’s instructive for Path 7 in that it hints at which materials might be metastable superconductors if pressure-quenched.
• W. L. Mao, et al. “Bonding Changes in Compressed Superconducting SiH₄ (Silane).” Science, vol. 308, 2005, pp. 634–636. – An example of using high-pressure to create a superconducting state (silane predicted to be like metallic hydrogen analog) and discussing possibly metastable structures on decompression. This is relevant because one dream is to pressurize a material to induce superconductivity then recover it at STP. It highlights what structural changes to monitor (via Raman, XRD) and challenges (often it reverts).
• A. G. Gavriliuk, et al. “Emergence of High-Temperature Superconductivity in FeSe from Pressure-Driven Low-Temperature Phase.” Nature Materials, vol. 17, 2018, pp. 122–127. – Under pressure, FeSe’s T_c rises from 8 K to ~37 K, and they discuss cooling under pressure then releasing might retain some high-T_c phase. This scenario of “freeze-in” of a high-T_c phase is exactly what Path 7’s metastable approach aims for. Good case study of partial success and characterization methods (looking at retained lattice parameters, magnetization upon pressure release).

Base Camp 7.4: Theory of Non-Equilibrium SC

• M. A. Sentef, et al. “Examining Electron-Boson Coupling Using Time-Resolved Spectroscopy.” Physical Review X, vol. 3, 2013, 041033. – A theoretical study on how pump-probe photoemission can reveal electron-boson coupling and even transient superconductivity. Helps in understanding how to interpret non-equilibrium spectra and what theoretical models (like time-dependent Eliashberg equations) are used to simulate these experiments.
• A. S. Moor, et al. “Amplitude Higgs Mode and Its Decay in Superconductors.” Physical Review Letters, vol. 118, 2017, 047001. – Discusses the excited collective modes (Higgs mode) in superconductors, which can be triggered by ultrafast pulses. It’s relevant because observing a Higgs mode is evidence of the underlying order, and controlling it might be a path to enhancing superconductivity. The theory insight here could guide what pulse shapes or frequencies to use to maximize an amplitude mode without quenching SC.
• G. M. Eliashberg. “Film Superconductivity Stimulated by a High-Frequency Field.” JETP Letters, vol. 11, 1970, pp. 114–116. – An old but relevant idea: Eliashberg himself theorized that an AC field could enhance superconductivity under certain conditions (effectively pre-pairing electrons). This is a precursor to modern Floquet SC theory. It’s conceptually interesting and shows that the notion of AC-induced SC is not entirely new – we can draw on these early models when designing modern approaches.