Anomalous field-symmetric Nernst signal in striped cuprate La$_{2-x}$Ba$_x$CuO$_4$

Starting at the structural transition temperature $T_{d2}$ = 54 K, the striped cuprate La$_{2-x}$Ba$_x$CuO$_4$ ($x = \frac18$) displays a remarkable cascade of transitions$^1$ at the characteristic temperatures $T_{d2} > T_1^{**} > T_{BKT} >T_c$, before settling down to 3D superconductivity with long-range coherence at $T_c$= 5 K. The Nernst signal $e_N$ and thermopower $S$ have been investigated in detail in these multiple states. As in pure LaSrCuO, the Nernst coefficient $N = lim_{B\to 0} e_N/B$ (initially negative) acquires a positive vortex contribution at 120 K that grows rapidly. However, here, $N$ saturates in the interval $T_{d2}$ (54 K) $\to T_1^{**}$ (34 K). As the vortex liquid becomes increasingly stabilized below $T_1^{**}$, $N$ resumes increasing at an even steeper rate. Surprisingly, below 34 K, $e_N$ acquires a $B$-symmetric component that is very large and oscillatory in $B$. We have excluded $S$ and quasiparticles as the source of the anomalous term. We will discuss various origins including the possibility of vortex formation mechanisms that break time-reversal invariance.\\ 1) J. M. Tranquada \emph{et al.}, Phys. Rev. B 78, 174529 (2008).