Surface energy of the one-dimensional supersymmetric <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="d1e2023" altimg="si7.svg"><mml:mrow><mml:mi>t</mml:mi><mml:mo linebreak="goodbreak" linebreakstyle="after">−</mml:mo><mml:mi>J</mml:mi></mml:mrow></mml:math> model with

In this paper, we study the surface energy of the one-dimensional supersymmetric $t-J$ model with unparallel boundary magnetic fields, which is a typical $U(1)$-symmetry broken quantum integrable strongly correlated electron system. It is shown that at the ground state, the contribution of inhomogeneous term in the Bethe ansatz solution of eigenvalues of transfer matrix satisfies the finite size scaling law $L^{\beta}$ where $\beta<0$. Based on it, the physical quantities of the system in the thermodynamic limit are calculated. We obtain the patterns of Bethe roots and the analytical expressions of density of states, ground state energy and surface energy. We also find that there exist the stable boundary bound states if the boundary fields satisfy some constraints.