Effective Mass of the Four Flux Composite Fermion at $\nu =1\mathrm{/}4$

We have measured the effective mass ($m^{\ast }$) of the four flux composite fermion at Landau level filling factor $\nu =1\mathrm{/}4$ (${}^4$CF), using the activation energy gaps at the fractional quantum Hall effect (FQHE) states $\nu$ = 2/7, 3/11, and 4/15 and the temperature dependence of the Shubnikov-de Haas (SdH) oscillations around $\nu =1\mathrm{/}4$. We find that the energy gaps show a linear dependence on the effective magnetic field $B_{eff}$ ($\equiv B-B_{\nu=1/4}$),andfromthislineardependenceweobtain$m^* = 1.0 m_e$ and a disorder broadening $\mathrm{\Gamma }\sim$ 1 K for a sample of density $n = 0.87 \times 10^{11}$\mathrm{/}cm$^2$\mathrm{.}The$m^$ deduced from the temperature dependence of the SdH effect shows large differences for $\nu >1\mathrm{/}4$ and $\nu <1\mathrm{/}4$. For $\nu >1\mathrm{/}4$, $m^{\ast }\sim 1.0m_e$. It scales as $\sqrt{B_{\nu }}$ with the mass derived from the data around $\nu =1\mathrm{/}2$ and shows an increase in $m^{\ast }$ as $\nu \to 1/4$,resemblingthefindingsaround$\nu =1/2$\mathrm{.}For$\nu < 1/4$,$m^$increasesrapidlywithincreasing$B_{eff}$andcanbedescribedby$m^*/m_e = -3.3 + 5.7 \times B_{eff}$\mathrm{.}Thisanomalousdependenceon$B_{eff}$ is precursory to the formation of the insulating phase at still lower filling.