  The thermal conductivity kappa of the quasi-two-dimensional (Q2D) organic
superconductor lambda-(BETS)_2 GaCl_4 was studied in the magnetic field H
applied parallel to the Q2D plane. The phase diagram determined from this bulk
measurement shows notable dependence on the sample quality. In dirty samples
the upper critical field H_{c2} is consistent with the Pauli paramagnetic
limiting, and a sharp change is observed in kappa(H) at H_{c2 parallel}. In
contrast in clean samples H_{c2}(T) shows no saturation towards low
temperatures and the feature in kappa(H) is replaced by two slope changes
reminiscent of second-order transitions. The peculiarity was observed below ~
0.33T_c and disappeared on field inclination to the plane when the orbital
suppression of superconductivity became dominant. This behavior is consistent
with the formation of a superconducting state with spatially modulated order
parameter in clean samples.
