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Hund’s metals: from strong correlations to emergent orders

I am happy to share that our review article with Massimo Capone, “Hund’s metals: the interplay of correlations and emergent orders,” has now been published in Contemporary Physics.

Hund’s metals are a fascinating class of strongly correlated materials in which Hund’s coupling, the intra-atomic exchange interaction that favours high-spin configurations, plays a central role in shaping the electronic properties. They combine strong correlations, orbital selectivity, suppressed coherence scales, and partially incoherent electronic excitations, while remaining metallic.

Several excellent reviews have already established the key normal-state phenomenology of Hund’s metals. The distinctive perspective of this review is to move beyond that normal-state characterization and ask how Hund-driven correlations shape the ordered phases and low-temperature instabilities that emerge from this peculiar metallic state.

In particular, we discuss how Hund’s coupling can affect magnetic, nematic, and superconducting tendencies not simply as a direct ordering mechanism, but by reshaping the low-energy electronic spectrum, the coherence scales, orbital differentiation, and the effective fluctuation vertices entering the instability channels.

A central message is that, in Hund’s metals, identifying the microscopic driver of an ordered phase is not always sufficient to interpret its experimental fingerprints. The observable signatures of an order are filtered through a strongly correlated, dynamical electronic background. As a result, Hund-driven correlations can qualitatively affect the ordered state that is ultimately realized, reshaping both the instability itself and its manifestation in the electronic spectrum. One example discussed in the review is that even a simple orbital perturbation can be transformed into a much richer spectral response, with strongly energy-dependent anisotropies and non-trivial structures.

The review is intended to be pedagogical and accessible, while still giving a physically detailed account of the mechanisms that make Hund’s metals relevant for broader questions in correlated quantum materials.

Article link:
https://doi.org/10.1080/00107514.2026.2647572

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Spectral Nematicity – Linking weak and strong correlations at UIUC

I will spend three weeks at the University of Illinois Urbana–Champaign to work with Rafael Fernandes on a project investigating nematicity from a spectral perspective. The aim is to move beyond static characterizations and examine how nematic fluctuations are distributed in momentum and energy, and how they interact with superconductivity.

The approach brings together complementary viewpoints. Low-energy models grounded in spin-fluctuation physics capture the momentum structure of the instability, while strong-correlation methods such as DMFT encode local interaction effects and orbital selectivity. By combining these, we seek a coherent description of the nematic susceptibility as a spectral object and a clearer identification of the energy scales that govern the onset of nematic order and its coupling to superconducting degrees of freedom.

The expected outcome is a framework that connects theory with spectroscopy, in and out of equilibrium, and clarifies the respective roles of local and non-local interactions in shaping nematic responses in correlated superconductors. I will follow up here as we proceed!

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Superconductivity from Repulsion: Resolving the Saddle Point Paradox

In conventional BCS theory, superconductivity arises when electrons attract each other: no attraction-no pairing. But multiband systems change the rules of the game.

In these systems, the band degree of freedom acts as a powerful ally. Even a purely repulsive interaction, if it acts between bands (interband), can lead to superconductivity. This idea sparked a lot of interest in the early days of iron-based superconductors, especially regarding how to define fluctuations and eigenmodes in such unconventional settings. (See our early contributions from 2009 and 2013)

More recently, this setting has revealed a subtle and somewhat paradoxical feature: while superconductivity clearly exists in these systems, certain mean-field treatments make the superconducting solution appearing at a saddle point rather than a minimum of the mean-field free energy. This raised a conceptual puzzle: how can a physically stable state appear unstable in theory?

This apparent paradox sparked recent proposals to restore stability by projecting out repulsive modes, effectively restricting the problem to the attractive sector. However, this kind of projection is physically unjustified, as there is no underlying symmetry or dominant interaction that would naturally restrict the space of superconducting order parameters.

In our work, Superconductivity with repulsion: a variational approach, we address this problem at its root. By adopting Bogoliubov’s variational principle, we construct a consistent and physically meaningful free energy functional. Within this framework, the superconducting state is correctly identified as a true minimum, resolving the paradox and reaffirming the stability of superconductivity even in the presence of purely repulsive interband interactions.

We further discuss the relevance of considering the full BCS kernel, not just the interaction matrix, in correctly identifying the collective modes, and explicitly show how simplified treatments can lead to incorrect conclusions in model cases.

This work provides a robust and general framework for analyzing stability and fluctuations in multichannel superconductors. We’re proud that it was selected as an Editors’ Choice on Physical Review B, a recognition that highlights its clarity and pedagogical intent. We hope it will serve as a useful reference for those working on multiband superconductivity and related topics.

Free energy landscape for a two-band model with purely interband repulsion. The mean-field free-energy is unbounded along the repulsive direction and the BCS lay at the saddle-point of the energy landscape, the variational free energy instead remains bounded and exhibits a true minimum at the BCS solution.

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Anticipating SCES 2025 in Montréal

I am delighted and honored to have been invited to speak at the International Conference on Strongly Correlated Electron Systems 2025 – SCES 2025 – which will be held in Montréal, Canada, from July 6 – 11, 2025. This renowned meeting has long been a cornerstone for our community, bringing together researchers working on heavy-fermion physics, oxides, topological and spin-orbit phenomena, and the broader landscape of quantum materials.

SCES 2025 promises an outstanding scientific program and an ideal forum to exchange ideas on the latest developments in strongly correlated electron systems. Montréal, vibrant, multicultural, and home to world-class universities, offers a fitting backdrop for inspiring discussions and new collaborations.

I look forward to sharing recent results from my work on electronic correlations and their interplay with emergent orders, and I am eager to hear about the advances you and others have made across the field. If you plan to attend, please feel free to reach out or catch me during the sessions and coffee breaks; it will be a pleasure to connect in person.

See you in Montréal!

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The Science of Complex Systems – 20 Years of Collaboration

On February 6-7, I had the pleasure of participating in the workshop The Science of Complex Systems, hosted at the CNR headquarters in Rome. The event celebrated the 20th anniversary of the Institute for Complex Systems (ISC-CNR), marking two decades of interdisciplinary research and collaboration.

My contribution focused on the synergy between electronic correlations and superconductivity, an area where complex interactions continue to challenge and expand our understanding of quantum materials.

The workshop brought together researchers from across disciplines, offering a stimulating environment for exchanging ideas and reflecting on the evolution of complexity in science.

It was also a valuable opportunity to finally meet in person colleagues from the various ISC units: Rome, Florence (Sesto Fiorentino), and Turin (Politecnico). These exchanges enriched the discussions and highlighted the strength of our collaborative network.

The event was a meaningful occasion to celebrate past achievements and to look ahead to the scientific questions that will shape the future of the field.

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Hund’s Metals: DMFT-QE Symposium

I have been recently invited to give one talk within the online DMFT-QE Symposium organized by the Flatiron Institute. This is a monthly event bringing together the community of researchers interested in Dynamical Mean Field Theory/Quantum Embedding methods, covering a broad range of recent topics from methodological developments to applications in physics, chemistry, and materials science. 

The session focused on Hund’s metals. Together with Luca de’Medici (ESPCI) we embarked in a extensive overview covering both the characterization of the Hund’s physics and its connection with Mott physics and the interplay of Hund driven correlation and low-energy modes (superconductivity and nematicity).

The whole session is available online:

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Quantum Materials With and Without Quasiparticles

It has been a pleasure coming back to the Kavli Institute for Theoretical Physics of Santa Barbara to participate this amazing program and the conference associated with.

The event brought together theorists and experimentalists working on unconventional materials, triggering a discussion of new theoretical and experimental results that continue to change our view of quantum matter. We had exciting talks and very pedagogical round tables. We discussed about electron transport beyond the quasiparticle paradigm, the nature of Cooper pairing in correlated systems and the behavior of collective modes in systems without well-defined quasiparticles.

If you want to find out more, the conference’s talks are available online! Enjoy !

Electron Correlations beyond the Quasiparticle Paradigm: Theory and Experiment

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Modeling pair density wave

In weakly coupled BCS superconductors, only electrons within a tiny energy window around the Fermi energy, form Cooper pairs. This may not be the case in strong coupling superconductors where the pairing scale, becomes comparable or even larger than the Fermi one.

During my time at UF I collaborated with Chandan Setty and Peter Hirshfeld on this topic and our work got finally published on Nature Communications!

We study an analytically solvable model to examine pairing in the strongly coupled regime and in the presence of anisotropic interactions. Already for moderate coupling we find an unusual finite temperature phase, below an instability temperature, where local pair correlations have non-zero center-of-mass momentum but lack long-range order. At low temperature, this fluctuating pair density wave can condense either to a uniform d-wave superconductor or the widely postulated pair-density wave phase depending on the interaction strength. Our minimal model offers a unified framework to understand the emergence of both fluctuating and long range pair density waves in realistic systems.

Full article available here

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New role as EPL Co-Editor!

I am glad to announce that I’ve recently entered the Editorial Board of EPL.

I am honored to be part of such a diverse, truly international and scientifically sound board. Have a look to the board members here!

As a Co-Editor I will be responsible for overseeing the review process, selecting referees and making publication decision for every manuscript.

Those are quite hard times for scientific publications…Predatory journals, questionable review procedures and impact factor obsession are just a few of several problems that we need to address.

Especially in this context, I think that the role of the Editorial Board is fundamental in order to maintain a high quality of the scientific dissemination. I hope I will be able to act as a good editor fro EPL and provide a good service to the scientific community.

Wish me luck!

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A new chapter of my scientific adventure

I’m excited to announce that I just started a new position as research scientist at the Institute of Complex Systems of the Italian National Research Council (ISC-CNR).

After my experience as MSCA global fellow, at the University of Florida and the International School for Advanced Studies of Trieste, I am ready to join the permanent staff of ISC, a CNR institute characterized by a lively intellectual environment and an outstanding quality of research. 

I can’t wait to see where this new chapter of my career will take me.