Laboratoire de Physique Théorique
Toulouse - UMR 5152

In the two consecutive works published in the Physical Review Letters, experimentalists and theorists have demonstrated that, contrary to previous expectations, disorder can help ordering of quantum matter. To show this, they have studied the spin-chain based material $\mathrm{Ni}(\mathrm{Cl}_{1-x}\mathrm{Br}_x)_2-4\mathrm{SC}(\mathrm{NH}_2)_2$, also called “DTN”, which at low temperature presents a magnetic-field-induced ordered phase, described as a Bose-Einstein condensate (BEC). So far it was believed that, close to this BEC phase, chemical disorder created by doping Br impurities to substitute Cl ions would lead to localization, namely the so-called Bose-Glass state. However, building on Nuclear Magnetic Resonance experiments at high magnetic field, combined with state-of-the-art quantum Monte Carlo simulations, it has been shown that the impurity-induced localized bosonic degrees of freedom are indeed at work, but their mutual interaction plays a new unsuspected major role.

Their pairwise effective interaction leads to a global quantum coherence over the full sample, which results in a new type of BEC ordering of these impurity states, in sharp contrast with a localized Bose-Glass. This discovery is rewarding a very successful collaboration between experimental (LNCMI Grenoble) and theoretical (LPT Toulouse) teams.