Origin of the Resistivity Anisotropy in the Nematic Phase of FeSe

Abstract

The in-plane resistivity anisotropy is studied in strain-detwinned single crystals of FeSe. In contrast to other iron-based superconductors, FeSe does not develop long-range magnetic order below the nematic/structural transition at Ts ≈90 K. This allows for the disentanglement of the contributions to the resistivity anisotropy due to nematic and magnetic orders. Comparing direct transport and elastoresistivity measurements, we extract the intrinsic resistivity anisotropy of strainfree samples. The anisotropy peaks slightly below Ts and decreases to nearly zero on cooling down to the superconducting transition. This behavior is consistent with a scenario in which the in-plane resistivity anisotropy in FeSe is dominated by inelastic scattering by anisotropic spin fluctuations.PACS numbers: 74.70. Xa, 74.25.Ld Electronic nematicity has emerged as a key concept in iron-based superconductors since the observation of inplane resistivity anisotropy in stress-detwinned crystals of Co-doped BaFe 2 As 2 [1, 2]. The fact that the resistivity anisotropy is much larger than what is expected from the small lattice distortion led to the proposal that the tetragonal-to-orthorhombic transition in the iron pnictides is driven not by phonons, but by an electronic nematic phase. Subsequent experiments revealed an intricate dependence of the resistivity anisotropy on doping (a sign change between electron-and hole-doped materials [2-6]), and disorder [7,8], sparking hot debates about its microscopic origins (see Refs. [9 and 10] for reviews).Electronic contributions involved in the in-plane resistivity anisotropy [10] can be separated into the Drude weight and/or of the scattering rate anisotropies. Fermisurface anisotropies arising, for instance, from the ferroorbital order triggered at the nematic transition, affect mostly the Drude weight [11][12][13]. Anisotropic scattering, can be due to elastic processes, such as the development of local magnetic order around an impurity [14,15], or inelastic processes, such as the scattering of electrons by anisotropic magnetic fluctuations [16,17] known to exist below T s [18]. Recent stress-dependent optical reflectivity studies in Co-doped BaFe 2 As 2 point to a dominant effect of the Drude weight [19,20]. However, stripe magnetic order appearing at the magnetic transition severely complicates the analysis. This is because the magnetic state breaks tetragonal symmetry leading to an anisotropic reconstruction of the Fermi surface [7,21] and to the appearance of "Dirac cones" [22], which may dramatically alter the resistivity anisotropy [23]. Disentangling these contributions is fundamental to reveal the origin of the resistivity anisotropy and, consequently, of the nematic state.In this context, the stoichiometric FeSe [24] is an ideal system. It is rather clean (residual resistivity ratios as high as 50 [25]) and its orthorhombic/nematic phase transition at T s ≈ 90 K is not accompanied by a longrange magnetic order [26] eliminating effects of Fermi surface folding.In this Letter we rep...

Polarized Light Microscopy Study on the Reentrant Phase Transition in a (Ba1 – xKx)Fe2As2 Single Crystal with x = 0.24

“…The temperature-dependent resistivity derivative-dρ/dT vs. T (inset in Figure 1b In previous studies, the tetragonal-to-orthorhombic reentrant transition was revealed by neutron and X-ray diffraction measurements [9,11] and high-resolution thermal-expansion and specific-heat measurements [8,10]. It should be pointed out that the orthorhombic distortion leads to the formation of structural domains in the LTO state, revealed and quantified by using polarized light microscopy [16][17][18][19][20]. Here, we use polarized light microscopy to study the temperature evolution of orthorhombic domain patterns in a single crystal of (Ba1 − xKx)Fe2As2 (x = 0.24).…”

“…The contrast of the domain images is higher for larger difference in the rotation of the polarization plane between neighboring domains, proportional to the degree of orthorhombic distortion, δ = (a O − b O ) / (a O + b O ). Quantitative analysis of the images enables in determination of the orthorhombic order parameter [19,20]. This was not possible due to low contrast of the images.…”

“…It should be pointed out that the orthorhombic distortion leads to the formation of structural domains in the LTO state, revealed and quantified by using polarized light microscopy [16][17][18][19][20]. Here, we use polarized light microscopy to study the temperature evolution of orthorhombic domain patterns in a single crystal of (Ba 1 − x K x )Fe 2 As 2 (x = 0.24).…”

In-plane anisotropy of transport coefficients in electronic nematic states: Universal origin of nematicity in Fe-based superconductors

“…The relation ρ < 0 is observed in the nonmagnetic nematic state for T S > T > T N , and even for T T S under the weak uniaxial stress. Remarkably, the opposite anisotropy ρ > 0 is realized in FeSe [18,19]. According to these observations, one may expect that the origin of nematicity in FeSe is special.…”

“…To answer this question, the strong in-plane anisotropy of transport coefficients has been studied intensively as a key electronic property in the nematic state [2,[15][16][17][18][19][20][21][22][23][24]. In Ba(Fe 1−x Co x ) 2 As 2 , Ba(As 1−x P x ) 2 and EuFe 2 (As 1−x P x ) 2 , large C 2 anisotropy in the resistivity ρ ≡ ρ x − ρ y < 0 appears in detwinned samples below T S , where ρ μ is the resistivity along the μ axis [15][16][17].…”