Calculation of Penetration Depth andTcin κ-(ET)2Cu(NCS)2under Pressure

Abstract

The pressure dependence of the inverse square of the magnetic penetration depth λ -2 in κ-(ET) 2 Cu(NCS) 2 was measured by Larkin et al. [Phys. Rev. B 64 (2001) 144514]. According to the paper, λ -2 behaves differently under low pressure and under high pressure. Under low pressure, the development of λ -2 just below T = T c is rapid compared to the case under high pressure. Moreover, T c in κ-(ET) 2 Cu(NCS) 2 increases under c -axis pressure up to 1 kbar and decreases under higher pressure, while T c decreases monotonically under the hydrostatic pressure, or under the uniaxial pressure parallel to other axes. In order to explain these behaviors, we calculate T c and λ -2 for κ-(ET) 2 Cu(NCS) 2 under pressure. In the calculation we mainly use an effective dimer Hubbard model. In conclusion, the behavior of λ -2 results from three effects: the variation of the bandwidth of quasiparticles, the change of the Fermi surfaces, and the effect of vertex correction. This is a different mechanism from that of λ -2 in cuprates which we observe when the doping varies. Moreover, we explain the increase in T c under the c -axis pressure up to 1 kbar and the decrease in T c over 1 kbar from our calculation. With the increase in the c -axis pressure, two competitive effects with respect to T c appear. One is the approach of the Fermi surface to the antiferromagnetic Brillouin zone boundary, and the other is the suppression of the electron correlation. Under the low c -axis pressure, T c increases since the former effect is dominant. On the other hand, T c decreases since the latter effect is dominant under the high c -axis pressure.