Improved critical eigenfunction restriction estimates on Riemannian surfaces with nonpositive curvature

We show that one can obtain improved $L^4$ geodesic restriction estimates for eigenfunctions on compact Riemannian surfaces with nonpositive curvature. We achieve this by adapting Sogge's strategy in proving improved critical $L^p$ estimates. We first combine the improved $L^2$ restriction estimate of Blair and Sogge and the classical improved $L^\infty$ estimate of B\'erard to obtain an improved weak-type $L^4$ restriction estimate. We then upgrade this weak estimate to a strong one by using the improved Lorentz space estimate of Bak and Seeger. This estimate improves the $L^4$ restriction estimate of Burq, G\'erard and Tzvetkov and Hu by a power of $(\log\log\lambda)^{-1}$. Moreover, in the case of compact hyperbolic surfaces, we obtain further improvements in terms of $(\log\lambda)^{-1}$ by applying the ideas from recent works of Chen, Sogge and Blair, Sogge. We are able to compute various constants that appeared in the work of Chen and Sogge explicitly, by proving detailed oscillatory integral estimates and lifting calculations to the universal cover $\mathbb H^2$.