High-T_c Superconductivity

The mechanism of unconventional high-temperature superconductivity in copper oxides remains one of the great challenges in condensed matter physics. Spectroscopic and scattering probes in our department provide detailed information about atomic-scale dynamical correlation functions that are required to assess theoretical models of unconventional superconductivity. We are pursuing this question via experiments on single crystals and epitaxial thin films with systematically optimized crystallographic perfection, increasingly taking advantage of uniaxial strain as a powerful method to modulate the electron system in-situ. We also perform measurements on the electronic properties of different nickelate compounds, which are rapidly developing into a new model system for high-T_c superconductivity with some commonalities, but also key differences to the cuprates. High-quality cuprate and nickelate crystals and thin-film structures synthesized in our department have enabled collaborations with multiple groups worldwide.
 

Superconductivity in infinite-layer and Ruddlesden–Popper nickelates
P. Puphal, T. Schäfer, B. Keimer, and M. Hepting
Nature Reviews Physics 8, 70–85 (2026)
Charge ordering in superconducting copper oxides
Alex Frano, Santiago Blanco-Canosa, Bernhard Keimer and Robert J Birgeneau
J. Phys.: Condens. Matter 32, 374005 (2020)
From quantum matter to high-temperature superconductivity in copper oxides
B. Keimer, S. A. Kivelson, M. R. Norman, S. Uchida, J. Zaanen
Nature 518, 179, 12 February 2015
 

Over the past few years, experiments in our group as well as others have demonstrated that charge order is the leading competitor of high-temperature superconductivity in moderately doped cuprate superconductors. We also found that uniaxial strain is a highly effective control parameter for this interplay, and are currently following up on our recent discovery of a long-range ordered charge density wave state in highly strained YBa₂Cu₃O_6+x.

Using strain to uncover the interplay between two- and three-dimensional charge density waves in high-temperature superconducting YBa₂Cu₃O_y
I. Vinograd, S.M. Souliou, A.-A. Haghighirad, T. Lacmann, Y. Caplan, M. Frachet, M. Merz, G. Garbarino, Y. Liu, S. Nakata, K. Ishida, H.M.L. Noad, M. Minola, B. Keimer, D. Orgad, C.W. Hicks, and M. Le Tacon
Nature Communications 15, 3277 (2024)

Resonant x-ray scattering study of charge density wave correlations in YBa₂Cu₃O_6+𝑥 under uniaxial stress
S. Nakata, D. Betto, E. Schierle, S. Hameed, Y. Liu, H.-H. Kim, S.M. Souliou, T. Lacmann, K. Fürsich, T. Loew, E. Weschke, A.P. Mackenzie, C.W. Hicks, M. Le Tacon, B. Keimer, and M. Minola
Physical Review B 111, 115152 (2025)

 

Chemical disorder is a major influence on charge density wave formation and its interplay with superconductivity and other quantum states, but most theories are regarding hypothetical clean systems.  To facilitate quantitative understanding of superconductivity in the cuprates, we have recently focused on systems such as YBa₂Cu₄O₈ and La_2-xCa_xCuO₄ where uncontrolled disorder is minimal.

Coincident onset of charge order and pseudogap in a homogeneous high-temperature superconductor
D. Betto, S. Nakata, F. Pisani, Y. Liu, S. Hameed, M. Knauft, C.T. Lin, R. Sant, K. Kummer, F. Yakhou, N.B. Brookes, M. Le Tacon, B. Keimer, and M. Minola
Nature Communications 16, 3579 (2025)

Interplay between electronic and atomic short-range order in La_2−𝑥⁢Ca_𝑥⁢CuO_4−𝛿 : Temperature and doping dependence>
S. Hameed, Y. Liu, K. S. Rabinovich, G. Kim, M. Neumann, P. Wochner, G. Christiani, G. Logvenov, K. Higuchi, N. B. Brookes , E. Weschke, F. Yakhou-Harris, A. V. Boris, B. Keimer, and M. Minola
Physical Review B 112, L161114 (2025)

 

Nickelate perovskites have recently emerged as a new platform for unconventional superconductivity and correlated-electron phenomena. We are exploring this platform in a multi-pronged approach comprising synthesis of single crystals, thin films, and heterostructures, as well as spectroscopic experiments, and model calculations.

Unconventional Crystal Structure of the High-Pressure Superconductor La₃Ni₂O₇
P. Puphal, P. Reiss, N. Enderlein, Y.-M. Wu, G. Khaliullin, V. Sundaramurthy, T. Priessnitz, M. Knauft, A. Suthar, L. Richter, M. Isobe, P.A. van Aken, H. Takagi, B. Keimer, Y.E. Suyolcu, B. Wehinger, P. Hansmann, and M. Hepting
Phys. Rev. Lett. 133, 146002 (2024)

Oxygen sublattice disorder and valence state modulation in infinite-layer nickelate superlattices
R. A. Ortiz, N. Enderlein, K. Fürsich, R. Pons, P. Radhakrishnan, E. Schierle, P. Wochner, G. Logvenov, G. Cristiani, P. Hansmann , B. Keimer, and E. Benckiser
Physical Review Materials 9, 054801 (2025)