Effective Mass of the Four Flux Composite Fermion at v=1/4

We have measured the effective mass (m sup *) of the four flux composite fermion at Landau level filling factor v=1/4 (sup 4 CF), using the activation energy gaps at the fractional quantum Hall effect (FQHE) states v=2/7, 3/11, and 4/15 and the temperature dependence of the Shubnikov- de Has (SdH) oscillations around v=1/4. We find that the energy gaps show a linear dependence on the effective magnetic field B sub (eff) (is identical with B-B sub (v=1/4)), and from this linear dependence we obtain m sup * = 1.0 m sub e and a disorder broadening GAMMA ~ 1K for a sample of density n=0.87x10 sup (11)/cm sup 2. The m sup * deduced from the temperature dependence of the SdH effect shows large differences for v > 1/4 and v 1/4. For v > 1/4, m sup * ~ 1.0 m sub e. It scales as sqrtB sub v with the mass derived from the data around v=1/2 and shows an increase in m sup * as v -> 1/4, resembling the findings around v=1/2. For v 1/4, m sup * increases rapidly with increasing B sub (eff) and can be described by m sup */m sub e = 3.3+5.7xB sub (eff). This anomalous dependence on B sub (eff) is precursory to the formation of the insulating phase at still lower filling.