Both the long-duration gamma-ray bursts (LGRBs) and the Type I superluminous supernovae (SLSNe~I) have been proposed to be primarily powered by central magnetars. A correlation, proposed between the initial spin period ($P_0$) and the surface magneti
c field ($B$) of the magnetars powering the X-ray plateaus in LGRB afterglows, indicates a possibility that the magnetars have reached an equilibrium spin period due to the fallback accretion. The corresponding accretion rates are inferred as $dot{M}approx10^{-4}-10^{-1}$ M$_odot$ s$^{-1}$, and this result holds for the cases of both isotropic and collimated magnetar wind. For the SLSNe~I and a fraction of engine-powered normal type Ic supernovae (SNe~Ic) and broad-lined subclass (SNe~Ic-BL), the magnetars could also reach an accretion-induced spin equilibrium, but the corresponding $B-P_0$ distribution suggests a different accretion rate range, i.e., $dot{M}approx 10^{-7}-10^{-3}$ M$_odot$ s$^{-1}$. Considering the effect of fallback accretion, magnetars with relatively weak fields are responsible for the SLSNe~I, while those with stronger magnetic fields could lead to SNe~Ic/Ic-BL. Some SLSNe~I in our sample could arise from compact progenitor stars, while others that require longer-term accretion may originate from the progenitor stars with more extended envelopes or circumstellar medium.
SN 2018hti is a Type I superluminous supernova (SLSN~I) with an absolute $g$-band magnitude of $-22.2$ at maximum brightness, discovered in a metal-poor galaxy at a redshift of 0.0612. We present extensive photometric and spectroscopic observations o
f this supernova, covering the phases from $sim -35$ days to more than +340 days from the $r$-band maximum. Combining our $BVgri$-band photometry with {it Swift} UVOT optical/ultraviolet photometry, we calculated the peak luminosity as $sim 3.5times10^{44}$ erg s$^{-1}$. Modeling the observed light curve reveals that the luminosity evolution of SN 2018hti can be produced by an ejecta mass of 5.8 $M_odot$ and a magnetar with a magnetic field of $B=1.8times10^{13}$~G having an initial spin period of $P_0=1.8$ ms. Based on such a magnetar-powered scenario and a larger sample, a correlation between the spin of the magnetar and the kinetic energy of the ejecta can be inferred for most SLSNe~I, suggesting a self-consistent scenario. Like for other SLSNe~I, the host galaxy of SN 2018hti is found to be relatively faint ($M_{g} = -17.75$ mag) and of low metallicity ($Z=0.3~Z_odot$), with a star-formation rate of 0.3 $M_odot$ yr$^{-1}$. According to simulation results of single-star evolution, SN 2018hti could originate from a massive, metal-poor star with a zero-age main sequence (ZAMS) mass of 25--40 $M_odot$, or from a less massive rotating star with $M_mathrm{ZAMS} approx 16$--25 $M_odot$. For the case of a binary system, its progenitor could also be a star with $M_mathrm{ZAMS} gtrsim 25$ $M_odot$.
