Divergent Nematic Susceptibility in an Iron Arsenide Superconductor

Abstract

Within the Landau paradigm of continuous phase transitions, ordered states of matter are characterized by a broken symmetry. Although the broken symmetry is usually evident, determining the driving force behind the phase transition is often a more subtle matter due to coupling between otherwise distinct order parameters. In this paper we show how measurement of the divergent nematic susceptibility of an iron pnictide superconductor unambiguously distinguishes an electronic nematic phase transition from a simple ferroelastic distortion. These measurements also reveal an electronic nematic quantum phase transition at the composition with optimal superconducting transition temperature. [6][7][8][9] and iron pnictides [10][11][12] have been proposed as candidate platforms that might harbour an electronic nematic phase, which opens up exciting new possibilities related to the interplay of nematic order with high temperature superconductivity. However, one of the key doubts accompanied by the experimental discoveries is that the crystal lattice of these two systems does not retain a fourfold symmetry. In particular, in iron pnictides there is an orthorhombic structural distortion accompanying the rapid increase of resistivity anisotropy, which puts the legitimacy of the term "electronic nematic" into question. Here we report measurements of the resistivity anisotropy of Ba(Fe 1−x Co x ) 2 As 2 induced by a tunable uni-axial strain, which exhibits a divergent behaviour as the system approaches the phase transition from the high temperature side. Our result explicitly shows that the structural phase transition in Ba(Fe 1−x Co x ) 2 As 2 is purely driven by the instability in the electronic part of the free energy, and furthermore reveals an electronic nematic quantum phase transition at the composition with optimal superconducting transition temperature.We apply a tuneable in-plane uniaxial strain to single crystal samples of Ba(Fe 1−x Co x ) 2 As 2 to probe the nematic response. As shown in Fig. 1(A), by gluing the sample on the side wall of a piezostack, strains can be applied by the deformation of the piezo, which is controlled by an applied voltage(V P ) [13]. The strain (i.e. the fractional change of length along the current direction, ǫ P = ∆L/L) was monitored via a strain gauge glued on the back side of the piezo stack. Both ǫ P and the fractional change of resistivity (η = ∆ρ/ρ 0 , where ρ 0 is the resistivity of the free standing sample before gluing on the piezo stack) were measured at constant temperature while the applied voltage was swept, as shown in Fig.

In-plane uniaxial stress effects on the structural and electronic properties of BaFe2As2and CaFe2AsTomić1, Jeschke2, Fernandes3 et al. 2013Phys. Rev. B285231View full textAdd to dashboardCite

“…In the tetragonal phase, rather small uniaxial stress P < 10 MPa is enough to completely detwin the sample, giving rise to a single domain. 6,8,13 This can be understood as fluctuations above the structural transition temperature giving rise to long-range order in the presence of a symmetry-breaking field. 11 The situation is however very different deep in the orthorhombic phase, where twin domains are already formed.…”

“…[9][10][11] Theoretically, although it is clear that in the tetragonal phase the applied uniaxial pressure acts as a conjugate field to the orthorhombic order parameter, condensing a single domain, 12 the nature of the detwinning process deep inside the orthorhombic phase remains an open question, since different mechanisms might be at play -such as twin boundary motion or reversal of the order parameter inside the domains. 8,13 Besides promoting detwinning, uniaxial strain has also been shown to affect the thermodynamic properties of the iron pnictides. Recent neutron scattering experiments on BaFe 2 As 2 under compressive stress along the in-plane b direction reported a progressive shift to higher temperatures of the magnetic transition 6 -a behavior also seen in BaFe 2 (As 1−x P x ) 2 by thermodynamic measurements.…”

Phase stability and large in-plane resistivity anisotropy in the 112-type iron-based superconductor Ca1−xLaxFeAs2

“…they belong to different space groups) the orthorhombic phases realized below the structural transitions in, for example, LaFeAsO, where the transition is from the tetragonal P 4/nmm to the orthorhombic Cmma (# 67) structure [48]. Similarly, in the 122 compounds that have a tetragonal structure with the ThCr 2 Si 2 type (space group F mmm) at high temperature, an electronic nematic phase breaks the four-fold rotational symmetry and hence induces the structural transition to an orthorhombic space group [49][50][51]. In the case of Ca 1−x La x FeAs 2 , the monoclinic structure is stable up to at least 450 K [14], and no transition to the tetragonal phase is reported.…”

Observation of the magnetic C4 phase in Ca1−xNaxFe2As2

“…[2][3][4][5] Indeed, measurements of different properties have found support of either model with magnetic torque, resistivity anisotropy and orbital ordering supporting orbital mechanics while measurements of the dynamic spin excitations, strain magnetism coupling and magnetoelastic scaling strongly suggesting a spin-driven scenario. [6][7][8][9][10][11] Yet determining the correct model, and consequently the underlying primary order parameter, is vital to understanding the mechanism of superconductivity in these systems. The recent discovery of a new magnetic phase with fourfold C 4 symmetry in the hole-doped '122' FBS evidenced the vital role of itinerant electronic behavior in determining the behavior of these materials.…”