Electronic structure of the quasi-two-dimensional spin-gap system SrCu$_2$(BO$_3$)$_2$

During the last decade, a lot of theoretical and experimental work has been devoted to the study of the magnetic properties of SrCu$_2$(BO$_3$)$_2$. This compound crystallizes in a tetragonal structure where layers of CuBO$_3$ alternate with planes of Sr atoms along the(001) direction. Due to this unusual structure where Cu$\sp{2+}$ atoms are arranged in the layers to form a network of orthogonal dimers, SrCu$_2$(BO$_3$)$_2$ appears as the first realization of a 2D Heisenberg model known as the Shastry-Sutherland model and exhibits a number of unique features such as a spin gap behavior, unusual magnetic excitations or a magnetization {\it plateaux}. In this work, the electronic structure of this system has been investigated using first-principles band structure calculations within the local-density approximation (LDA)+U method as implemented in the {\tt wien2k} code. The comparison of our calculations with available experimental data (exchange integrals estimated from magnetic susceptibility measurements, optical gap from reflectance measurements, and O-K edge recorded in EELS) shows that the (LDA)+U method with a single value of the parameter $U$ can provide an accurate description of both low-energy-scale (magnetic) and high-energy-scale (electronic) properties of this magnetic insulator.