Connection between in-plane upper critical field $H_{c2}$ and gap   symmetry in layered $d$-wave superconductors revisited
  Angle-resolved upper critical field $H_{c2}$ provides an efficient tool to probe the gap symmetry of unconventional superconductors. We revisit the behavior of in-plane $H_{c2}$ in $d$-wave superconductors by considering both the orbital effect and Pauli paramagnetic effect. After carrying out systematic analysis, we show that the maxima of $H_{c2}$ could be along either nodal or antinodal directions of a $d$-wave superconducting gap, depending on the specific values of a number of tuning parameters. This behavior is in contrast to the common belief that the maxima of in-plane $H_{c2}$ are along the direction where the superconducting gap takes its maximal value. Therefore, identifying the precise $d$-wave gap symmetry through fitting experiments results of angle-resolved $H_{c2}$ with model calculations at a fixed temperature, as widely used in previous studies, is difficult and practically unreliable. However, our extensive analysis of angle-resolved $H_{c2}$ show that there is a critical temperature $T^{*}$: in-plane $H_{c2}$ exhibits its maxima along nodal directions at $T < T^{*}$ and along antinodal directions at $T^{*} < T < T_c$. The concrete value of $T^{*}$ may change as other parameters vary, but the existence of $\pi/4$ shift of $H_{c2}$ at $T^{\ast}$ appears to be a general feature. Thus a better method to identify the precise $d$-wave gap symmetry is to measure $H_{c2}$ at a number of different temperatures, and examine whether there is a $\pi/4$ shift in its angular dependence at certain $T^{*}$. We further show that Landau level mixing does not change this general feature. However, in the presence of Fulde-Ferrell-Larkin-Ovchinnikov state, the angular dependence of $H_{c2}$ becomes quite complicated, which makes it more difficult to determine the gap symmetry by measuring $H_{c2}$. 
Jing-Rong Wang, Guo-Zhu Liu, and Chang-Jin Zhang
cond-mat.supr-con