Speaker
Description
The interlayer magnetothermopower of the organic superconductor $\kappa-\rm (ET)_2Cu(NCS)_2$ is studied at temperatures down to 0.5 K and fields up to 32 T. Analysis of the background magnetothermopower show that at low temperatures it is negative and exhibits an upturn at higher field, producing a dip at a field close to the upper critical field $B_{c2}$. There are clear magnetothermopower quantum oscillations visible above 5 T. The obtained oscillation frequencies are in a good agreement with those previously reported on the magnetoresistance and magnetization quantum oscillations. According to our results, the magnetothermopower in $\kappa-\rm (ET)_2Cu(NCS)_2$ presents features which have already been detected in YBCO and other high-$T_c$ cuprates indicating that some kind of a charge density wave order is also present in the normal state of $\kappa-\rm (ET)_2Cu(NCS)_2$. Most strikingly, our measurements show that there is another dip, again followed by an upturn, in the magnetothermopower of $\kappa-\rm (ET)_2Cu(NCS)_2$ occurring at much higher fields than $B_{c2}$, around the magnetic breakdown field of $\sim 21$ T that is not present in $\rm YBCO$. We propose that the two induced successive phase transitions, consisting of two similarly ordered states each restricted to a finite magnetic field window are in fact charge density wave ordered states arising as a result of the layer-stacking mechanism in the interlayer direction. Our results support and advance some of the previous findings that the superconductivity in the organic superconductor $\kappa-\rm (ET)_2Cu(NCS)_2$ is mediated by a charge density wave order rather than antiferromagnetic fluctuation.
