Applied Physics Seminar Series

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Abstract:

Materials exhibiting multiple superconducting phases are exceptionally rare in nature. The few known examples of multiphase superconductors display complex phase diagrams, where distinct phases can be independently induced by means of external stimuli such as pressure or magnetic fields. Here we report the coexistence of two superconducting condensates with different spatial localization in the van der Waals 4Hb-TaSSe polytype. Its layered structure consisting of alternating layers of the T-type and H-type polymorphs enables the development of two effectively decoupled superconducting phases with marked distinct microscopic properties. Using high-resolution quasiparticle tunneling and Andreev reflection spectroscopy in the two polymorph layers, we identify two different superconducting gaps in size in each layer, with signatures compatible with weakly coupled condensates, potentially of different pairing symmetry. The coexistence of these condensates is further corroborated by our measured critical temperatures and upper critical magnetic fields, which significantly differ in each polymorph layer. We explore the possible superconducting ground states using a minimal model based on ab initio calculations that captures many of the experimental features (1). Lastly, I will compare this phenomenology with that occurring in related layered compounds showing conventional superconductivity (2). Our results challenge the current understanding of superconductivity in low-dimensional superconductors and open new pathways for customizable superconducting devices that could independently operate several superconducting states.

References

(1) H. Guo, S. Sajan, et al. Arxiv:2507.15647 (2025).

(2) S. Sajan, H. Guo, et al. Nano Letters 25, 6654 (2025).

More about the Speaker:

Miguel M. Ugeda is an Ikerbasque Research Professor at the Donostia International Physics Center (DIPC), Spain, where he leads a research group studying emergent electronic phenomena in low-dimensional quantum materials. His research focuses on scanning tunneling microscopy and spectroscopy of two-dimensional systems, with particular emphasis on superconductivity, charge order, magnetism, and correlated states in transition metal dichalcogenides and related materials. He received his PhD in Physics from the Universidad Autónoma de Madrid in 2011 and carried out postdoctoral research at the University of California, Berkeley, in the Crommie group (2012-2016). He later held research positions at the Materials Physics Center (MPC) and DIPC in San Sebastián, where he established his own research group through an ERC Starting Grant and was subsequently awarded an ERC Consolidator Grant to investigate superconductivity in moiré quantum matter.

His work has led to important advances in the physics of two-dimensional quantum materials, including early contributions to magnetism in graphene, the study of superconductivity in TMDs in the monolayer limit, and the observation of novel collective states in transition metal dichalcogenides, ranging from superconducting collective modes to coherent heavy-fermion behavior.