Abstract
The mechanism behind the nematicity of FeSe is not known. Through elastoresitivity measurements it has been shown to be an electronic instability. However, so far measurements have extended only to small strains, where the response is linear. Here, we apply large elastic strains to FeSe, and perform two types of measurements. (1) Using applied strain to control twinning, the nematic resistive anisotropy at temperatures below the nematic transition temperature Ts is determined. (2) Resistive anisotropy is measured as nematicity is induced through applied strain at fixed temperature above Ts. In both cases, as nematicity strengthens the resistive anisotropy peaks about about 7%, then decreases. Below ~40 K, the nematic resistive anisotropy changes sign. We discuss possible implications of this behaviour for theories of nematicity. We report in addition: (1) Under experimentally accessible conditions with bulk crystals, stress, rather than strain, is the conjugate field to the nematicity of FeSe. (2) At low temperatures the twin boundary resistance is ~10% of the sample resistance, and must be properly subtracted to extract intrinsic resistivities. (3) Biaxial inplane compression increases both in-plane resistivity and the superconducting critical temperature Tc, consistent with a strong role of the yz orbital in the electronic correlations.
Highlights
At an electronic-nematic transition, electronic interactions drive a spontaneous reduction in rotational symmetry without introducing translational or time-reversal symmetry breaking
The T-dependent nematic resistive anisotropy has been reported previously for a few iron-based compounds [19,24,25,26,27], these previous measurements have relied upon assumptions that twin boundary resistance is negligible and/or that a sustained stress applied to detwin samples is weak enough not to substantially alter the electronic structure, even though the iron-based superconductors are extremely sensitive to uniaxial stress [25,28]
Within our strain range only a linear component of the strain dependence is resolved, with slope dTs=dε110 1⁄4 750 K. This slope is due to the A1g component of the applied strain: under the tetragonal symmetry of FeSe at T > Ts, reversal of the sign of εB2g gives a symmetrically equivalent strain, so coupling to εB2g can give only strain-even components in the strain dependence of Ts
Summary
At an electronic-nematic transition, electronic interactions drive a spontaneous reduction in rotational symmetry without introducing translational or time-reversal symmetry breaking. Measurements of the strain dependence of resistivity, i.e., the elastoresistivity, have shown that the nematicity of FeSe, like that of other iron-based superconductors, is an electronic instability. (2) Strain tuning is employed to control the twinning as samples are cooled, allowing measurement of the intrinsic resistive anisotropy at temperatures below Ts. the T-dependent nematic resistive anisotropy has been reported previously for a few iron-based compounds [19,24,25,26,27], these previous measurements have relied upon assumptions that twin boundary resistance is negligible and/or that a sustained stress applied to detwin samples is weak enough not to substantially alter the electronic structure, even though the iron-based superconductors are extremely sensitive to uniaxial stress [25,28]. Spectroscopic probes [17,21,30,31,32,33], and the hole pocket becomes elongated along the ky direction
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