Effective mass and combination frequencies of de Haas-van Alphen oscillations in κ-(BEDT-TTF)2Cu(NCS)2

De Haas-van Alphen (dHvA) measurements in the two dimensional organic superconductor κ-(BEDT-TTF) 2Cu(NCS)2 have been performed down to .44K in a wide magnetic field range up to 30T. In the range between 20T and 30T, we find dHvA frequencies corresponding to the closed (α) and magnetic breakdown (β) orbits, and many combination frequencies of α and β as well. The observation of the combination frequencies β-α and 2β-2α is not understood by conventional theories because both the torque and magnetic interaction effects are negligibly small. The effective masses and the field dependence of these oscillations are reported.