This is part of a virtual seminar series in theoretical condensed matter and atomic physics, with the aim to connect researchers working at different Nordic universities.
Egor Babaev (KTH Stockholm)
abstract: Electron pairing gives rise to a distinct state of matter: superconductivity, described by an order parameter bilinear in fermions <cc>. In a series of works, we proposed a fluctuations-based theoretical mechanism for how electron quadrupling can occur (see, e.g. [1]). The prerequisites for this mechanism is strong fluctuations and multiple broken symmetries (for a review, including the review of early works, see [2]). Quadrupling gives a much richer spectrum of states of matter than pairing. Especially intriguing ones are the quadrupling states are associated with dissipationless counterflows, with order parameter <c1c1c2^\dagger c2^\dagger> 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, but related to a Skyrme-like model [3], resulting in a plethora of new effects, ranging from unconventional magnetic properties to new type of hydrodynamics [9]. 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 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 quantum vortices carrying an arbitrary fraction of magnetic flux quantum [7] in superconducting states of the compounds that support fermion quadrupling condensates. The recent experiment on Ba1-xKxFe2As2 [8] observed vortices violating that quantization, i.e. carrying flux, which is not a function of fundamental constants. If time permits, I will also briefly discuss a topological counterpart of the electron quadrupling state [10]
[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 Nature Communications 14 (1), 6734
[7] E. Babaev Phys. Rev. Lett. 89, 67001 (2002)
[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)
[9] E Babaev, B Svistunov arXiv preprint arXiv:2311.04340
[10] E Babaev arXiv preprint arXiv:2401.02551