Novel Experimental Method Measures the Higgs Mode in High-Temperature Superconductors

An international research team including Tomke Glier (University of Hamburg), Stefan Kaiser (TU Dresden), Dirk Manske (Max Planck Institute for Solid State Research, Stuttgart), and Michael Rübhausen (University of Hamburg) has developed a groundbreaking experimental method to directly measure the Higgs mode in superconductors [1].

In superconductors, electrons can pair up in a remarkable way: they move collectively in a shared quantum state in which electric current flows without any resistance. Researchers can selectively excite this state using ultrashort laser pulses, setting it into oscillation. This process gives rise to so-called Higgs modes — collective oscillations of the superconducting state itself.

The term "Higgs modes" is no coincidence: it evokes the famous Higgs boson from particle physics, as both phenomena involve the oscillation of a field, governed by the same physical principle of symmetry breaking. In superconductors, the Higgs mode offers valuable insight into the hidden symmetries and internal structure of this extraordinary state. One can think of the vibrational spectrum of the Higgs modes as the fingerprint of a superconductor — a kind of characteristic echo that reveals its properties.

The newly developed spectroscopic method now enables researchers to directly and selectively observe Higgs modes. This represents a major step forward, as many fundamental questions about superconductivity, particularly high-temperature superconductivity, which persists even at relatively high temperatures, remain unresolved.

"Terahertz lasers have enabled significant advances in experimental Higgs spectroscopy over the past decade, [2]" explains Stefan Kaiser, Professor at TU Dresden. "What has been lacking, however, is a precise tool for probing the symmetry properties of these excitations — this is precisely where Raman spectroscopy comes into play."

© Tomke Glier

To that end, the team developed Non-Equilibrium Anti-Stokes Raman Scattering (NEARS). The method involves initiating a controlled “soft quench” of the so-called Mexican hat potential, which leads to the targeted excitation of metastable Higgs states. "This excitation induces a characteristic population inversion, which manifests in the spectrum as an additional anti-Stokes Raman signal," explains Tomke Glier, first author of the study. "This polarization-dependent Raman spectroscopy enables, for the first time, the experimental determination of the symmetry of Higgs modes - crucial step," adds Dirk Manske, who, together with theorists at the MPI Stuttgart and the Max Planck–UBC–UTokyo Center for Quantum Materials, developed a classification scheme for possible Higgs modes in superconductors [3].

Using the NEARS technique, the team was able to detect, for the first time, the symmetry-dependent Higgs modes in A_1g and B_1g symmetry at around 25 meV in Bi₂Sr₂CaCu₂O₈₊ₓ (Bi-2212), a high-temperature superconductor. The measurements were carried out as a function of quench strength. The results show that the energy of these modes corresponds to Cooper pair coherence lengths below 5 nm in Bi-2212. In addition, a BCS-based model of electronic Raman scattering was developed by the team around Dirk Manske, and a comparative Ginzburg-Landau theory was applied to support the experimental findings.

© Michael Rübhausen

NEARS measurements are conducted using a unique, newly developed Raman spectrometer located in Hamburg at the Center for Free-Electron Laser Science (CFEL, Rübhausen Group) [4]. The spectrometer developed in Hamburg enables time-resolved Raman spectroscopy in resonance with the relevant energy scales of condensed matter. "This broad spectral range, combined with very high energy resolution at low excitation energies, is achieved through a setup that has been continuously improved over many years," says Michael Rübhausen.

NEARS opens up a systematic approach for analyzing amplitude modes across a wide range of quantum condensates — from superconductors with competing orders to transiently induced superconducting states or interface superconductors. In the future, the presence of the Higgs mode may even serve as a criterion for identifying superconductivity and be used to characterize coherence lengths and order parameter symmetries in new superconductors.

Funding and Acknowledgments:

This work was supported by the Max Planck Society, the German Federal Ministry of Education and Research (Bundesministerium für Bildung und Forschung 05K19GU5, 05K22GU2), the German Research Foundation (DFG) within SFB 1143 (project ID 247310070), and the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter (ct.qmat, EXC 2147). Additional funding was provided by the European Union (ERC, T-Higgs, GA 101044657). Further support came from the University of British Columbia (Andrea Damascelli) and the University of Minnesota (Martin Greven).

References:

[1] Tomke E. Glier, Sida Tian, Mika Rerrer, Lea Westphal, Garret Lüllau, Liwen Feng, Jakob Dolgner, Rafael Haenel, Marta Zonno, Hiroshi Eisaki, Martin Greven, Andrea Damascelli,
Stefan Kaiser, Dirk Manske, Michael Rübhausen; Non-equilibrium anti-Stokes Raman spectroscopy for investigating Higgs modes in superconductors; Nature Commun. 16, 7027 (2025).

[2] Kim, M.-J. et al. Tracing the dynamics of superconducting order via transient terahertz third-harmonic generation. Sci. Adv.10, eadi7598 (2024). DOI:10.1126/sciadv.adi7598

[3] Schwarz, L. et al. Classification and characterization of nonequilibrium Higgs modes in unconventional superconductors. Nat Commun 11, 287 (2020). https://doi.org/10.1038/s41467-019-13763-5

[4] Schulz, B. et al., Fully reflective deep ultraviolet to near infrared spectrometer and entrance optics for resonance Raman spectroscopy. Rev. Sci. Instrum. 76, 073107 (2005). https://doi.org/10.1063/1.1946985