Nernst and Seebeck Coefficients of the Cuprate Superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathbf{O}}_{6.67}$: A Study of Fermi Surface Reconstruction

The Seebeck and Nernst coefficients $S$ and $\nu$ of the cuprate superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ (YBCO) were measured in a single crystal with doping $p=0.12$ in magnetic fields up to $H=28\mathrm{T}$. Down to $T=9\mathrm{K}$, $\nu$ becomes independent of field by $H\simeq 30\mathrm{T}$, showing that superconducting fluctuations have become negligible. In this field-induced normal state, $S/T$ and $\nu /T$ are both large and negative in the $T\to 0$ limit, with the magnitude and sign of $S/T$ consistent with the small electronlike Fermi surface pocket detected previously by quantum oscillations and the Hall effect. The change of sign in $S(T)$ at $T\simeq 50\mathrm{K}$ is remarkably similar to that observed in ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$, ${\mathrm{La}}_{2-x-y}{\mathrm{Nd}}_y{\mathrm{Sr}}_x{\mathrm{CuO}}_4$, and ${\mathrm{La}}_{2-x-y}{\mathrm{Eu}}_y{\mathrm{Sr}}_x{\mathrm{CuO}}_4$, where it is clearly associated with the onset of stripe order. We propose that a similar density-wave mechanism causes the Fermi surface reconstruction in YBCO.

Figure 1

Thermoelectric coefficients of YBCO at $p=0.12$ as a function of magnetic field $H$. Upper panel: Seebeck coefficient $S$ plotted as $S/T$ vs $H$, for temperatures as indicated. Lower panel: Nernst coefficient $\nu$ plotted as $\nu /T$ vs $H$, for temperatures as indicated. Inset: Derivative of the 9-K isotherm (in arbitrary units), showing that $d\nu /dH\to 0$ as $H\to 30\mathrm{T}$.

Figure 2

Upper panel: Hall angle ${\theta }_H$ plotted as $\tan {\theta }_H={\rho }_{xy}/{\rho }_{xx}$ vs $T$. The arrow marks the onset of the drop to large negative values. Lower panel: Nernst coefficient $\nu$ plotted as $\nu /T$ vs $T$ for magnetic fields as indicated. The vertical line marks the zero-field superconducting transition ($T_c$). Inset: Zoom on data at $H=1\mathrm{T}$. The arrow marks the onset of the positive signal due to superconducting fluctuations.

Figure 3

Thermopower of four hole-doped cuprates at $p\simeq 1/8$, plotted as $S/T$ vs $T$: ${\mathrm{YBa}}_2{\mathrm{CuO}}_{6.67}$ [YBCO, $H=28\mathrm{T}$ (circles) and $H=0$ (squares); this work], ${\mathrm{La}}_{1.675}{\mathrm{Eu}}_{0.2}{\mathrm{Sr}}_{0.125}{\mathrm{CuO}}_4$ (Eu-LSCO, $H=0$, triangles; this work), ${\mathrm{La}}_{1.48}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.12}{\mathrm{CuO}}_4$ (Nd-LSCO, $H=0$, dotted line; Ref. 32), and ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{CuO}}_4$ (LBCO, $H=9\mathrm{T}$, solid line; Ref. 31). The arrow marks the onset of stripe order in Eu-LSCO at $p=0.125$ measured by x-ray diffraction [14, 33].