We have measured the effective mass (m∗) of the four flux composite
fermion at Landau level filling factor ν=1/4 (4CF), using the
activation energy gaps at the fractional quantum Hall effect (FQHE) states
ν = 2/7, 3/11, and 4/15 and the temperature dependence of the Shubnikov-de
Haas (SdH) oscillations around ν=1/4. We find that the energy gaps show a
linear dependence on the effective magnetic field Beff (≡B−Bν=1/4), and from this linear dependence we obtain m∗=1.0me and
a disorder broadening Γ∼ 1 K for a sample of density n=0.87×1011 /cm2. The m∗ deduced from the temperature dependence of
the SdH effect shows large differences for ν>1/4 and ν<1/4. For
ν>1/4, m∗∼1.0me. It scales as Bν with the mass
derived from the data around ν=1/2 and shows an increase in m∗ as ν→1/4, resembling the findings around ν=1/2. For ν<1/4, m∗
increases rapidly with increasing Beff and can be described by m∗/me=−3.3+5.7×Beff. This anomalous dependence on Beff is
precursory to the formation of the insulating phase at still lower filling.Comment: 5 pages, 3 figure