Optical anisotropy in the electronic nematic phase of FeSe

At ambient pressure, FeSe undergoes a structural, tetragonal-to-orthorhombic, phase transition at Ts≃90 K without any magnetic ordering on further cooling. FeSe thus provides an arena for examining the nematic phase without the complications following the reconstruction of the Fermi surface due to the antiferromagnetic order within the orthorhombic state. We perform an optical-reflectivity investigation across the structural transition, as a function of uniaxial stress in order to detwin the specimen. These measurements reveal a hysteretic behavior of the anisotropic optical response to uniaxial stress for T≤Ts, which extends to energy scales of about 0.5 eV. The sign changes of the optical anisotropy between distinct energy intervals suggest a complex evolution of the polarized electronic structure in the nematic phase. The temperature dependence of the optical anisotropy for the fully detwinned specimen is furthermore acting as a proxy for the order parameter of nematicity. Disciplines Condensed Matter Physics Comments This article is published as Chinotti, Manuel, Anirban Pal, Leonardo Degiorgi, Anna E. Böhmer, and Paul C. Canfield. "Optical anisotropy in the electronic nematic phase of FeSe." Physical Review B 96, no. 12 (2017): 121112. DOI: 10.1103/PhysRevB.96.121112. Posted with permission. This article is available at Iowa State University Digital Repository: https://lib.dr.iastate.edu/physastro_pubs/538 RAPID COMMUNICATIONS PHYSICAL REVIEW B 96, 121112(R) (2017) Optical anisotropy in the electronic nematic phase of FeSe M. Chinotti, A. Pal, and L. Degiorgi* Laboratorium für Festkörperphysik, ETH Zürich, 8093 Zürich, Switzerland A. E. Böhmer and P. C. Canfield Ames Laboratory, Ames, Iowa 50010, USA (Received 13 June 2017; revised manuscript received 30 August 2017; published 18 September 2017) At ambient pressure, FeSe undergoes a structural, tetragonal-to-orthorhombic, phase transition at Ts 90 K without any magnetic ordering on further cooling. FeSe thus provides an arena for examining the nematic phase without the complications following the reconstruction of the Fermi surface due to the antiferromagnetic order within the orthorhombic state. We perform an optical-reflectivity investigation across the structural transition, as a function of uniaxial stress in order to detwin the specimen. These measurements reveal a hysteretic behavior of the anisotropic optical response to uniaxial stress for T Ts , which extends to energy scales of about 0.5 eV. The sign changes of the optical anisotropy between distinct energy intervals suggest a complex evolution of the polarized electronic structure in the nematic phase. The temperature dependence of the optical anisotropy for the fully detwinned specimen is furthermore acting as a proxy for the order parameter of nematicity. DOI: 10.1103/PhysRevB.96.121112 Superconductivity at high temperatures often develops in proximity to symmetry-breaking states. The nematic state, in which the electron system breaks a discrete rotational symmetry of the crystal lattice without altering the existing translational symmetry, has emerged as a key concept in iron-based superconductors and may be intimately related to the pairing mechanism of superconductivity itself [1]. Nematicity was originally invoked in order to account for the anisotropy in the dc transport properties of the archetypal Co-underdoped 122 materials Ba(Fe1−xCox)2As2 below the structural tetragonal-to-orthorhombic phase transition at Ts [2,3], which is larger than expected from the tiny orthorhombic lattice distortion alone. Recently, we studied the optical reflectivity [4,5] as a function of temperature (T ) across the structural phase transition for underdoped compositions of Ba(Fe1−xCox)2As2, with uniaxial and in situ tunable applied stress which acts as a conjugate field to the orthorhombic distortion and circumvents sample twinning below Ts [6]. Above Ts , this stress induces a finite value of the orthorhombic distortion [2]. At temperatures T < Ts , the optical anisotropy exhibits a remarkable hysteretic response to the applied stress even at energies deep into the electronic structure. The anisotropy turns into a reversible linear stress dependence at T Ts . Our results indicate an important polarization of the electronic structure in the nematic phase below Ts and a significant stress-induced one above Ts , related to the substantial nematic susceptibility [7]. In Ba(Fe1−xCox)2As2, as in most Fe arsenide and chalcogenide superconductors, the structural phase transition either accompanies or precedes the onset of long-range magnetic order, and therefore the nematic and magnetic states are likely intertwined with each other. Consequently, the effects of the Fermi-surface folding and thus of its reconstruction because of the antiferromagnetic order make it difficult to unambiguously address the intrinsic properties *Author to whom correspondence and requests for materials should be addressed: degiorgi@solid.phys.ethz.ch of the nematic state over a large temperature range. In this context, FeSe is very well suited in order to shed light on nematicity, since it harbors a tetragonal-to-orthorhombic structural phase transition at Ts 90 K, where the lattice breaks the C4 rotational symmetry, in the absence of any subsequent, ambient pressure long-range magnetic order. FeSe is then superconducting below Tc = 8 K [8,9]. Here, we describe results of reflectivity [R(ω)] measurements on FeSe that probe the optical response to variable uniaxial stress at temperatures below and above Ts , in the energy interval from the infrared up to the visible spectral range. Our measurements at T < Ts reveal a strong polarization dependence of the reflectivity spectra with respect to the orthorhombic axes, and clearly demonstrate that the electronic anisotropy associated with the orthorhombicity extends far from the Fermi energy and also persists in the superconducting state. While the observed hysteretic behavior of the stress-dependent optical anisotropy in FeSe at T Ts shares some qualitative similarities with our earlier findings in the electron-doped 122 materials, it yet displays an even more complex behavior than in Ba(Fe1−xCox)2As2 [4,5], with two sign changes on increasing frequency [i.e., the polarization dependence of R(ω) is opposite in distinct energy intervals]. Moreover, we discover that the stress-induced orthorhombic distortion above Ts may couple differently to the complex orbital order occurring in FeSe than in Ba(Fe1−xCox)2As2. This reveals important peculiarities of the two classes of materials in the response of their electronic structure to nematicity. Our results favor models for the nematic phase, which have to go beyond a simple ferro-orbital order scenario, as recently envisaged by angle-resolved photoemission spectroscopy (ARPES) data [10–13]. Large single crystals of FeSe were grown using chemical vapor transport, similarly to the description in Ref. [14]. The samples were mounted into our homemade device for applying stress and placed inside a Janis cryostat coupled to a Bruker Vertex 80v, Fourier-transform infrared interferometer. The device, described in detail in Refs. [4,5], consists of a spring bellows, which can be extended/retracted by flushing 2469-9950/2017/96(12)/121112(6) 121112-1 ©2017 American Physical Society RAPID COMMUNICATIONS CHINOTTI, PAL, DEGIORGI, BÖHMER, AND CANFIELD PHYSICAL REVIEW B 96, 121112(R) (2017) FIG. 1. Representative data of the optical reflectivity [R(ω)] of FeSe for ZPC experiments: (a) R(ω) measured at 10 K with applied uniaxial stress of pbellows = 1.2 bars (i.e., psample = 18.5 MPa felt by the sample) along the orthorhombic a [Ra(ω)] and b [Rb(ω)] axes, displaying the optical anisotropy in the infrared and midinfrared spectral range. The inset shows Ra(ω) and Rb(ω) from the far-infrared up to the ultraviolet range with a logarithmic frequency scale. (b)–(g) p dependence of Rratio(ω) (see text) at 10, 60, and 100 K for (b), (d), (f) increasing and (c), (e), (g) decreasing p. Released p is noted by (r). The lower left panel shows a typical FeSe sample and emphasizes its orientation with respect to the directions of the applied stress p and polarization (Pol) of light [16]. He gas into its volume or evacuating it, thus exerting and releasing uniaxial stress on the lateral side of the specimen. The specimens of 0.3 mm thickness and approximate in-plane dimensions of 2 × 1.2 mm2 were cut and oriented such that the uniaxial stress (p), detwinning the samples, is applied parallel to the short orthorhombic axis, which at T < Ts is preferentially aligned along the direction of a compressive stress (lower left panel of Fig. 1). As in Refs. [4,5], we refer here to the He-gas pressure inside the volume of the bellows (pbellows): The effective stress felt by the sample (psample) depends on its size and thickness, so that pbellows = 0.1 bar corresponds to an effective uniaxial stress of about psample ∼ 1.5 MPa on our FeSe crystals. It has been widely established that an effective stress of at least 10 MPa is enough to fully detwin the specimen and thus reveal the underlying symmetry breaking [2]. R(ω) as a function of T and in situ tunable p was measured at nearly normal incidence [15] with the electromagnetic radiation polarized along the orthorhombic elongated a [Ra(ω)] and short b [Rb(ω)] axes in a broad spectral range from the far infrared up to the ultraviolet. Our results were obtained from zero-pressure-cooled (ZPC) “pressure-loop” (at fixed T ) and pressure-cooled (PC) “fixedpressure” (as a function of T ) experiments [4,5]. Further details about this experiment can be found in the Supplemental Material (SM) [16]. Representative R(ω) data of FeSe in the infrared and midinfrared (MIR) spectral range (i.e., for 500 ω < 6000 cm−1) are shown in the main panel of Fig. 1(a) at 10 K and with pbellows = 1.2 bars following an initial ZPC protocol. Even though we mainly focus ou