Magnon-Mediated Indirect Exciton Condensation through Antiferromagnetic Insulators

Abstract

Electrons and holes residing on the opposing sides of an insulating barrier and experiencing an attractive Coulomb interaction can spontaneously form a coherent state known as an indirect exciton condensate. We study a trilayer system where the barrier is an antiferromagnetic insulator. The electrons and holes here additionally interact via interfacial coupling to the antiferromagnetic magnons. We show that by employing magnetically uncompensated interfaces, we can design the magnon-mediated interaction to be attractive or repulsive by varying the thickness of the antiferromagnetic insulator by a single atomic layer. We derive an analytical expression for the critical temperature T c of the indirect exciton condensation. Within our model, anisotropy is found to be crucial for achieving a finite T c , which increases with the strength of the exchange interaction in the antiferromagnetic bulk. For realistic material parameters, we estimate T c to be around 7 K, the same order of magnitude as the current experimentally achievable exciton condensation where the attraction is solely due to the Coulomb interaction. The magnon-mediated interaction is expected to cooperate with the Coulomb interaction for condensation of indirect excitons, thereby providing a means to significantly increase the exciton condensation temperature range.Introduction.-Interactions between fermions result in exotic states of matter. Superconductivity is a prime example, where the negatively charged electrons can have an overall attractive coupling mediated by individual couplings to the vibrations, known as phonons, of the positively charged lattice. In addition to charge, the electron also has a spin degree of freedom. The electron spin can interact with localized magnetic moments through an exchange interaction exciting the magnetic moment by transfer of angular momentum. These excitations are quasiparticles known as magnons. Theoretical predictions of electron-magnon interactions have shown that these can also induce effects such as superconductivity [1][2][3][4][5][6][7][8][9][10].Research interest in antiferromagnetic materials is surging [11,12]. This enthusiasm is due to the promising properties of antiferromagnets such as high resonance frequencies in the THz regime and a vanishing net magnetic moment. Much of this research focuses on interactions involving magnons or spin waves at magnetic interfaces in hybrid structures. Examples of this are spin pumping [13-19], spin transfer [15, 20-22], and spin Hall magnetoresistance [23-28] at normal metal interfaces, and magnon-mediated superconductivity [9,10]. Recently, an experiment has also demonstrated spin transport in an antiferromagnetic insulator over distances up to 80 µm [29]. Moreover, antiferromagnetic materials are also of interest since it is believed that high-temperature superconductivity in cuprates is intricately linked to magnetic fluctuations near an antiferromagnetic Mott insulating phase [30,31]. Thus it is crucial to achieve a good understanding of antiferromagnetic magn...

Schwinger boson study of superconductivity mediated by antiferromagnetic spin fluctuations

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Magnon-mediated superconductivity on the surface of a topological insulator

“…This enhancement can be understood from the picture of antiferromagnetic magnons as squeezed states, revealing that an antiferromagnetic magnon is associated with a large spin located at each sublattice [22]. Coupling to only one of the two sublattices of an AFMI thereby involves coupling to a large spin, leading to a strong enhancement of the coupling interaction [16,23].…”

Coherent spin-wave transport in an antiferromagnet

“…is also interesting from a fundamental point of view, as it is predicted to be a prerequisite for the occurrence of exotic phenomena such as magnetic solitons 12 , Bose-Einstein condensates 13,14 and spin-superfluidity [15][16][17] . These prospects call for efficient methods for the excitation, manipulation, and detection of short-wavelength coherent antiferromagnetic magnons.…”