Thermopower Quantum Oscillations in the Charge Density Wave State of the Organic Conductor $$\upalpha $$ α -(BEDT-TTF) $$_{2}\text {KHg}(\text {SCN})_{4}$$ 2 KHg ( SCN ) 4

Abstract

We report on experimental studies of magnetic quantum oscillations of the interlayer thermopower in the charge density wave state of the multiband organic conductor $$\upalpha $$ -(BEDT-TTF) $$_{2}\text {KHg}(\text {SCN})_4$$ . The magnetic field is along the direction perpendicular to the conducting layers. The interlayer thermopower or Seebeck effect has been measured in magnetic fields of up to 32 T and temperatures down to 0.5 K. The thermopower magnetic field dependence was measured at two temperatures 0.5 K and 4 K. Quantum oscillations observed in thermopower, at fields above 8 T and temperature 4 K, originate only from the $$\alpha $$ orbit, whereas at 0.5 K the energy spectrum consists of Landau levels of not only the $$\alpha $$ orbit but also the second harmonic as obtained from the fast Fourier transform analysis. In addition to the $$\alpha $$ and 2 $$\alpha $$ frequencies, the oscillation spectrum reveals existence of a low-frequency peak at $$F_{\lambda } = 180$$ T which was previously observed in both magnetoresistance and magnetization. The behavior of thermopower magnetic quantum oscillations amplitude is analyzed using the Lifshitz–Kosevich formula, and the influence of different damping factors such as the temperature, spin splitting, Dingle and magnetic breakdown factors is studied. In addition, we analyze how the second harmonic of fundamental $$\alpha $$ frequency observed at low temperatures in the $$\text {CDW}_0$$ state at field perpendicular to the conducting layers affects the thermopower quantum oscillations amplitude. At low temperature, we find that on entering the low-field state, the scattering rate is observed to increase dramatically. We also observe an apparent increase in the effective mass of first harmonic from $$m_{\alpha }^*=1.7 m_{\text{e}}$$ , in the low-field state to $$m_{\alpha }^*=3.3 m_{\text{e}}$$ , in the high-field state. For the second harmonic, a constant effective mass of $$m_{2\alpha }^*=3.2 m_{\text{e}}$$ is estimated above and below the kink field.