First-principles study of the spin-gap system Sr2Cu(BO3)2

Abstract

We have employed first-principles electronic structure calculations based on the $N$th-order muffin-tin orbital downfolding method to derive a low energy spin model for the spin-gap compound ${\mathrm{Sr}}_{2}\mathrm{Cu}{({\mathrm{BO}}_{3})}_{2}$. Our calculations reveal that this compound is a coupled dimer system with the strongest Cu-Cu interaction mediated by a pair of ${\mathrm{BO}}_{3}$ triangular units. The appreciable interdimer interactions are mediated via super-super exchange due to short O-O distances in the exchange pathway. The validity of the model is checked by calculating the magnetic susceptibility as a function of temperature and magnetization both as a function of temperature as well as field using quantum Monte Carlo technique and comparing them with the available experimental data. This comparison establishes the suitability of the coupled dimer model for the description of the low energy physics of ${\mathrm{Sr}}_{2}\mathrm{Cu}{({\mathrm{BO}}_{3})}_{2}$.