Title: Stellar atmospheric parameters of $\sim$ 11,000 RR Lyrae stars from LAMOST Spectra

Abstract: Accurate determination of the stellar atmospheric parameters of RR Lyrae stars (RRLs) requires short individual exposures of the spectra to mitigate pulsation effects. We present improved template matching methods to determine the stellar atmospheric parameters of RRLs from single-epoch spectra of LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope, also known as the Guoshoujing telescope). We determine the radial velocities and stellar atmospheric parameters (effective temperature: $T_\mathrm{eff}$, surface gravity: $\log{g}$, and metallicity: [M/H]) of 10,486 and 1,027 RRLs from 42,729 low-resolution spectra (LRS) and 7,064 medium-resolution spectra (MRS) of LAMOST, respectively. Our results are in good agreement with the parameters of other databases, where the external uncertainties of $T_\mathrm{eff}$, $\log{g}$, and [M/H] for LRS/MRS are estimated to be 314/274 K, 0.42/0.29 dex, and 0.39/0.31 dex, respectively. We conclude with the variation characteristics of the radial velocities ($RV$) and stellar atmospheric parameters for RRLs during the pulsation phase. There is a significant difference of $28\pm21$ km/s between the peak-to-peak amplitude ($A_\mathrm{ptp}$) of $RV$ from H$\alpha$ line ($RV_\mathrm{H\alpha}$) and from metal lines ($RV_\mathrm{metal}$) for RRab, whereas it is only $4\pm17$ km/s for RRc. The $A_\mathrm{ptp}$ of $T_\mathrm{eff}$ is $930\pm456$ and $409\pm375$ K for RRab and RRc, respectively. The $\log{g}$ of RRab show mild variation of approximately $0.23\pm0.42$ dex near the phase of $\varphi = 0.9$, while that of RRc almost remains constant. The [M/H] of RRab and RRc show a minor variation of about $0.25\pm0.50$ and $0.28\pm0.55$ dex, respectively, near the phase of $\varphi = 0.9$.