# On the nonlinear NMR and magnon BEC in antiferromagnetic materials with coupled electron-nuclear spin precession

## TL;DR

This paper investigates nonlinear NMR and magnon Bose-Einstein Condensation in antiferromagnetic MnCO3, revealing that coupled electron-nuclear spin precession exhibits behaviors inconsistent with traditional models, and proposing a new theoretical framework.

## Contribution

It introduces a novel theoretical description of coupled electron-nuclear spin precession that accounts for indirect nuclear relaxation, challenging conventional Bloch equation-based models.

## Key findings

- NMR signals contradict traditional noninteracting spin models.
- Nuclear magnetization magnitude remains conserved at high excitation powers.
- Experimental evidence supports BEC of nuclear spin waves with k=0.

## Abstract

We present a new study of nonlinear NMR and Bose-Einstein Condensation (BEC) of nuclear spin waves in antiferromagnetic MnCO3 with coupled electron and nuclear spins. In particular, we show that the observed behaviour of NMR signals strongly contradicts the conventional description of paramagnetic ensembles of noninteracting spins based on the phenomenological Bloch equations. We present a new theoretical description of the coupled electron-nuclear spin precession, which takes into account an indirect relaxation of nuclear spins via the electron subsystem. We show that the magnitude of the nuclear magnetization is conserved for arbitrary large excitation powers, which is drastically different from the conventional heating scenario derived from the Bloch equations. This provides strong evidence that the coherent precession of macroscopic nuclear magnetization observed experimentally can be identified with BEC of nuclear spin waves with k=0.