Chirality Induced Tilted-Hill Giant Nernst Signal

We reveal a novel source of a giant Nernst response exhibiting strong nonlinear temperature and magnetic field dependence, including the mysterious tilted-hill temperature profile observed in a pleiad of materials. The phenomenon results directly from the formation of a chiral ground state, e.g., a chiral $d$-density wave, which is compatible with the eventual observation of diamagnetism and is distinctly different from the usual quasiparticle and vortex Nernst mechanisms. Our picture provides a unified understanding of the anomalous thermoelectricity observed in materials as diverse as the hole-doped cuprates and heavy-fermion compounds like ${\mathrm{URu}}_2{\mathrm{Si}}_2$.

Figure 1

(a) Symmetric current loops generated from a $d_{x^2-y^2}$ density wave. (b) Domains of current loops in a masked chiral $d_{xy}+i{d}_{x^2-y^2}$ density wave. (c) In the previous cases time reversal is violated only locally, while in the presence of a field, macroscopic chirality is generated accompanied by a strong paramagnetic or even diamagnetic response.

Figure 2

(a) For low temperatures the Nernst signal and thermopower increase exponentially while the Hall angle is large. At the crossing point (I) where ${\sigma }_{xx}={\sigma }_{xy}$, the Nernst signal equals the thermopower (II) while their sign may be the same or opposite. At this crossover regime, the usually large magnitude of the thermopower ensures that the Nernst signal will be enhanced since they necessarily become equal. Just after the crossing temperature, ${\sigma }_{xx}\gg {\sigma }_{xy}$ forces the Nernst signal to decrease rapidly, giving rise to the tilted-hill Nernst profile. (b) Nernst signal dependence on doping. Tuning of the chemical potential inverts the Nernst signal’s sign. The Nernst signal is symmetric around an offset $\mu \simeq -0.8\mathrm{meV}$ needed to compensate the term $-m_z\mathcal{B}$ ($\mathcal{B}=5\mathrm{T}$).

Figure 3

(a) Magnetic field evolution of the tilted-hill Nernst signal temperature profile. The chirality driven Nernst response may reach the enormous values of $50\mu \mathrm{V}/\mathrm{K}$ in the high-field limit. (b) The magnetic field enhances the Nernst signal while it shifts the Nernst peak to high temperatures. (c) The Nernst coefficient decreases upon raising the field.

Figure 4

Magnetic field dependence of the Nernst response for different temperatures. The Nernst signal demonstrates a strong nonlinearity for $T<56\mathrm{K}$ that is qualitatively very similar to the data of underdoped ${\mathrm{La}}_{1.93}{\mathrm{Sr}}_{0.07}{\mathrm{CuO}}_4$ for $T<12\mathrm{K}$. The slope change after a critical temperature ($T=56\mathrm{K}$ here) is consistent with the experimental findings.