Seminar: Observation of electron quadrupling condensates and fractionalization of the flux quantum

Abstract: Electron pairing gives rise to a new state of matter: superconductivity. Within the reigning paradigm of the mean-field Bardeen-Cooper-Schrieffer theory, electron quadrupling cannot occur, as pairing tendencies always dominate. In 2002 and 2004, we proposed a fluctuations-based theoretical mechanism for how electron quadrupling can occur [1]. Quadrupling gives a much richer spectrum of states of matter than pairing (for a review, including the review of early works, see [2]). The most interesting quadrupling states are associated with dissipationless counterflows, while electric DC currents are dissipative. This results in a class of states principally different from superconductors and superfluid as they are described by an effective model different from Ginzburg-Landau and Gross-Pitaevskii functionals [3], resulting in a plethora of new effects. One such state where the dissipationless counterflows occur because electron quadrupling leads to spontaneous breaking of time-reversal symmetry was predicted to occur in Ba1-xKxFe2As2 [4]. Experimental evidence for that state was reported in 2021 in Ba1-xKxFe2As2  [5,6]. The first evidence came from transport, calorimetric, magnetic, thermal transport, and ultrasound measurements. The theoretical predictions of fermion quadrupling in materials like Ba1-xKxFe2As2 required the existence of novel quantum vortices carrying an arbitrary fraction of magnetic flux quantum [7] in superconducting states of the compounds that support fermion quadrupling condensates. In more than half a century of studies of quantum vortex physics, such effects were not seen. The recent experiment on Ba1-xKxFe2As2 [8] observed vortices violating that quantization, i.e. carrying flux, which is not a function of fundamental constants.

References:

[1] E. Babaev, A. Sudbo, N.W. Ashcrosft Nature 431 (7009), 666-668 (2004). E. Babaev cond-mat/0201547

[2]  BV Svistunov, ES Babaev, NV Prokof'ev Superfluid states of matter. CRC press, available on ResearchGate (2015) Chapter 6.10

[3] J Garaud, E Babaev Physical Review Letters 129 (8), 087602 (2022)

[4] TA Bojesen, E Babaev, A Sudbø Physical Review B 88 (22), 220511 (2013) TA Bojesen, E Babaev, A Sudbø Physical Review B 89 (10), 104509 (2014)

[5] Vadim Grinenko, Daniel Weston, Federico Caglieris, Christoph Wuttke, Christian Hess, Tino Gottschall, Ilaria Maccari, Denis Gorbunov, Sergei Zherlitsyn, Jochen Wosnitza, Andreas Rydh, Kunihiro Kihou, Chul-Ho Lee, Rajib Sarkar, Shanu Dengre, Julien Garaud, Aliaksei Charnukha, Ruben Hühne, Kornelius Nielsch, Bernd Büchner, Hans-Henning Klauss, Egor Babaev Nature Physics 17 (11), 1254-1259 (2021)

[6] Ilya Shipulin, Nadia Stegani, Ilaria Maccari, Kunihiro Kihou, Chul-Ho Lee, Yongwei Li, Ruben Hühne, Hans-Henning Klauss, Marina Putti, Federico Caglieris, Egor Babaev, Vadim Grinenko arXiv:2212.13515

[7] E. Babaev Phys. Rev. Lett. 89, 67001 (2002); E. Babaev, N.W. Ashcrosft Nature Physics 3 (8), 530-533 (2007);

[8] Yusuke Iguchi, Ruby A Shi, Kunihiro Kihou, Chul-Ho Lee, Mats Barkman, Andrea L Benfenati, Vadim Grinenko, Egor Babaev, Kathryn A Moler Science Magazine 380, 6651 1244-1247 (2023)