Thermopower across the stripe critical point of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>La</mml:mtext></mml:mrow><mml:mrow><mml:mn>1.6</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mtext>Nd</mml:mtext></mml:mrow><mml:mrow><mml:mn>0.4</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mtext>Sr</mml:mtext></mml:mrow><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mrow><mml:mtext>CuO</mml:mtext></mml:mrow><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>: Evidence for a quantum critical point in a hole-doped high-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi>T</mml:mi><mml:mi>c</mml:mi></mml:msub></mml:mrow></mml:math>superconductor

The thermopower $S$ of the high-${T}_{c}$ superconductor ${\text{La}}_{1.6\ensuremath{-}x}{\text{Nd}}_{0.4}{\text{Sr}}_{x}{\text{CuO}}_{4}$ was measured as a function of temperature $T$ near its quantum critical point, the critical hole doping ${p}^{\ensuremath{\star}}$ where all characteristic temperatures go to zero. Just above ${p}^{\ensuremath{\star}}$, $S/T$ varies as $\text{ln}(1/T)$ over a decade of temperature. Below ${p}^{\ensuremath{\star}}$, $S/T$ undergoes a large increase at low temperature. As with the temperature dependence of the resistivity, which is linear just above ${p}^{\ensuremath{\star}}$ and undergoes a large upturn at low temperature, these are typical signatures of a quantum phase transition. This suggests that ${p}^{\ensuremath{\star}}$ is a quantum critical point below which some order sets in, causing a reconstruction of the Fermi surface, whose fluctuations are presumably responsible for the linear-$T$ resistivity and logarithmic thermopower. All the evidence points to ``stripe'' order, a form of spin/charge modulation known to exist in this material.

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