Giant Rydberg excitons in the copper oxide Cu2O

• Published in: Nature (London) Vol. 514; no. 7522; pp. 343 - 347
• Main Authors: Kazimierczuk, T., Fröhlich, D., Scheel, S., Stolz, H., Bayer, M.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 16.10.2014
• Subjects: 140/125; 639/766/119/1000; 639/766/36; Humanities and Social Sciences; letter; multidisciplinary; Science
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature13832

Rydberg excitons (condensed-matter analogues of hydrogen atoms) are shown to exist in single-crystal copper oxide with principal quantum numbers as large as n = 25 and giant wavefunctions with extensions of around two micrometres; this has implications for research in condensed-matter optics. Giant excitons in copper oxide Excitons, electron–hole pairs that play an essential role in the optical properties of semiconductors, can be viewed as condensed-matter analogues of hydrogen atoms, with a similar excitation spectrum. Dietmar Fröhlich and colleagues extend the series of excitations from the previous record of principal quantum number n = 12, to n = 25 for excitons in single crystal cuprous oxide. At such high quantum numbers, the wave function of the excitons becomes giant, around 2 micrometres, and it is expected that these giant excitons (also called Rydberg excitons) strongly interact with each other. The authors observe evidence for a blockade effect where the presence of an exciton prevents excitation of another exciton in its vicinity. This work opens new research directions for optics in condensed matter. A highly excited atom having an electron that has moved into a level with large principal quantum number is a hydrogen-like object, termed a Rydberg atom. The giant size of Rydberg atoms 1 leads to huge interaction effects. Monitoring these interactions has provided insights into atomic and molecular physics on the single-quantum level. Excitons—the fundamental optical excitations in semiconductors 2 , consisting of an electron and a positively charged hole—are the condensed-matter analogues of hydrogen. Highly excited excitons with extensions similar to those of Rydberg atoms are of interest because they can be placed and moved in a crystal with high precision using microscopic energy potential landscapes. The interaction of such Rydberg excitons may allow the formation of ordered exciton phases or the sensing of elementary excitations in their surroundings on a quantum level. Here we demonstrate the existence of Rydberg excitons in the copper oxide Cu 2 O, with principal quantum numbers as large as n = 25. These states have giant wavefunction extensions (that is, the average distance between the electron and the hole) of more than two micrometres, compared to about a nanometre for the ground state. The strong dipole–dipole interaction between such excitons is indicated by a blockade effect in which the presence of one exciton prevents the excitation of another in its vicinity.