Calculation of Penetration Depth and $T_c$ in $κ $-$($ET$)_2$Cu(NCS)$_2$ under Pressure

Details Calculation of Penetration Depth and $T_c$ in $κ $-$($ET$)_2$Cu(NCS)$_2$ under Pressure Calculation of Penetration Depth and $T_c$ in $κ $-$($ET$)_2$Cu(NCS)$_2$ under Pressure

2004-09-24

arxiv.org/abs/cond-mat/0409633v1

Physics

Condensed Matter

Superconductivity

23pages, 16figures

Scientific paper

10.1143/JPSJ.73.3394

The pressure dependence of the inverse square of the magnetic penetration depth $\lambda ^{- 2}$ in $\kappa$-$($ET$)_2$Cu(NCS)$_2$ was measured by Larkin, et al According to the paper, $\lambda ^{- 2}$ behaves differently under low pressure and under high pressure. Under low pressure, the development of $\lambda ^{- 2}$ just below $T=T_c$ is rapid compared to the case under high pressure. Moreover, $T_c$ in $\kappa$-$($ET$)_2$Cu(NCS)$_2$ increases under c-axis pressure up to 1kbar 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 $\lambda ^{- 2}$ for $\kappa$-$($ET$)_2$Cu(NCS)$_2$ under pressure. In the calculation we mainly use an effective dimer Hubbard model. In conclusion, the behavior of $\lambda ^{- 2}$ 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 $\lambda ^{- 2}$ in cuprates which we observe when the doping varies. Moreover, we explain the increase in $T_c$ under the c-axis pressure up to 1kbar and the decrease in $T_c$ over 1kbar 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.

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