DOE OSTI · 2588867
Elastocaloric evidence for a multicomponent superconductor stabilized within the nematic state in Ba(Fe 1− x Co x ) 2 As 2
Abstract
The iron-based high-$T_c$ superconductors (SCs) exhibit rich phase diagrams with intertwined phases, including magnetism, nematicity, and superconductivity. The superconducting $T_c$ in many of these materials is maximized in the regime of strong nematic fluctuations, making the role of nematicity in influencing the superconductivity a topic of intense research. Here, in this study, we use the AC elastocaloric effect (ECE) to map out the phase diagram of Ba(Fe 1− x Co x ) 2 As 2 near optimal doping. The ECE signature at $T_c$ on the overdoped side, where superconductivity condenses without any nematic order, is quantitatively consistent with other thermodynamic probes that indicate a single-component superconducting state. In contrast, on the slightly underdoped side, where superconductivity condenses within the nematic phase, ECE reveals a second thermodynamic transition proximate to and below $T_c$. We rule out magnetism and reentrant tetragonality as the origin of this transition and find that our observations strongly suggest a phase transition into a multicomponent superconducting state. This implies the existence of a subdominant pairing instability that competes strongly with the dominant $s^±$ instability. Our results highlight the significant role of nematic order in determining the pairing symmetry close to optimal doping in this extensively studied iron-based SC, while also demonstrating the power of ECE in uncovering strain-tuned phase diagrams of quantum materials.
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Ghosh, Sayak [Stanford University, CA (United States)] (ORCID:0000000341687198), Ikeda, Matthias S. [Stanford University, CA (United States)] (ORCID:0000000243672937), Chakraborty, Anzumaan R. [University of Minnesota, Minneapolis, MN (United States); University of Illinois at Urbana-Champaign, IL (United States)] (ORCID:0009000642646805), Worasaran, Thanapat [Stanford University, CA (United States)], Theuss, Florian [Cornell University, Ithaca, NY (United States)] (ORCID:0000000279968510), Peralta, Luciano B. [Cornell University, Ithaca, NY (United States); Universidad de Los Andes, Bogotá (Colombia)], Lozano, P. M. [Argonne National Laboratory (ANL), Argonne, IL (United States). Advanced Photon Source (APS)] (ORCID:0000000166024118), Kim, Jong-Woo [Argonne National Laboratory (ANL), Argonne, IL (United States). Advanced Photon Source (APS)] (ORCID:0000000196412947), Thompson, P. J. [University of Liverpool (United Kingdom)] (ORCID:0000000296976141), Ryan, Philip J. [Argonne National Laboratory (ANL), Argonne, IL (United States). Advanced Photon Source (APS)] (ORCID:000000015247349X), Ye, Linda [Stanford University, CA (United States)] (ORCID:0000000179491356), Kapitulnik, Aharon [Stanford University, CA (United States)] (ORCID:0000000225693582), Kivelson, Steven A. [Stanford University, CA (United States)] (ORCID:0000000191499502), Ramshaw, B. J. [Cornell University, Ithaca, NY (United States); Canadian Institute for Advanced Research (CIFAR), Toronto, ON (Canada)] (ORCID:0000000232225007), Fernandes, Rafael M. [University of Minnesota, Minneapolis, MN (United States); University of Illinois at Urbana-Champaign, IL (United States)], Fisher, Ian R. [Stanford University, CA (United States)] (ORCID:0000000212787862). 2025-09-08. Elastocaloric evidence for a multicomponent superconductor stabilized within the nematic state in Ba(Fe 1− x Co x ) 2 As 2. https://doi.org/10.1073/pnas.2424833122
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