Nematic spin correlations in the tetragonal state of uniaxial strained BaFe_2-xNi_xAs₂

Understanding the microscopic origins of electronic phases in high-transition temperature (high-Tc) superconductors is important when it comes to elucidating the mechanism of superconductivity. In the paramagnetic tetragonal phase of BaFe2−xTxAs2 (where T is Co or Ni) iron pnictides, an in-plane resistivity anisotropy has been observed. Here, we use inelastic neutron scattering to show that low-energy spin excitations in these materials change from fourfold symmetric to twofold symmetric at temperatures corresponding to the onset of the in-plane resistivity anisotropy. Because resistivity and spin excitation anisotropies both vanish near optimal superconductivity, we conclude that they are likely to be intimately connected.

Nematic spin correlations - Fig. 1 Figure 1: (a) Schematic of nematic phase in liquid crystal. (b) Resistivity anisotropy in uniaxial-stress detwinned Ba(Fe1-xNix)2As2. (x=0.017) [6]. (c) The collinear C-type AF structure in the Fe plane. The green dashed box marks the tetragonal unit cell in the paramagnetic state and the magenta dashed box indicates the orthorhombic magnetic unit cell. (d) Magnetic Bragg peak intensity at the (1,0,1) and (0,1,1) positions for the BaFe2As2 at zero pressure (green) and P ~ 15 MPa uniaxial pressure along the bo axis.

Figure 1: (a) Schematic of nematic phase in liquid crystal. (b) Resistivity anisotropy in uniaxial-stress detwinned Ba(Fe1-xNix)2As2. (x=0.017) [6]. (c) The collinear C-type AF structure in the Fe plane. The green dashed box marks the tetragonal unit cell in the paramagnetic state and the magenta dashed box indicates the orthorhombic magnetic unit cell. (d) Magnetic Bragg peak intensity at the (1,0,1) and (0,1,1) positions for the BaFe2As2 at zero pressure (green) and P ~ 15 MPa uniaxial pressure along the bo axis.

Correlated electron materials such as high temperature superconductors harbour various exotic properties driven by electron correlations. One of them is the electronic nematic phase in iron pnictides [1]. A nematic phase was proposed to describe a state of cigar-shaped molecules in liquid crystal in which the molecules have no positional order, but their long axis is preferentially aligned in one direction, as shown in Figure 1(a). Molecules in nematic phase can flow like a liquid. Meanwhile, they form a symmetry broken (anisotropic) pattern like that of crystal. Thus, the nematic phase can be viewed as an intermediate state between an isotropic liquid and a highly ordered crystal in view of the broken symmetry. The nematicity in iron pnictides was first discovered as in-plane resistivity anisotropy in Ba(Fe1-xTx)2As2 (T=Co, Ni). An example is shown in Figure 1 (b). This anisotropy indicates that electrons transport much more easily in one direction (a axis) than the other (b axis), even in the case of the tetragonal state of uniaxial-stress detwinned sample (Figure 1 (b)). This nematicity was also discovered as a splitting of dxz and dyz orbitals in the uniaxial-strain detwinned samples at a temperature above Ts [2]. It has been generally believed that the nematic phase is driven by a certain electronic degree of freedom such as charge/orbital fluctuations and spin fluctuations [3]. However, it has also been explained as arising from the anisotropic scattering of Co dopants rather than intrinsic properties of the system [4].

Nematic spin correlations - Fig. 2 Figure 2: (a) Temperature dependence of the spin excitations at E = 6 meV for (1, 0, 1) and (0, 1, 1).The anisotropy in spin excitations vanishes around T = 160±10 K. Inset: Detwinning device mounted on the supporting sample holder. Uniaxial pressure can be applied by a steel spring driven by a screw. (b) The electronic phase diagram of BaFe2−xNixAs2 from resistivity anisotropy ratio ρbo=ρao obtained under uniaxial pressure [6]. The spin excitation anisotropy temperatures are marked as T*.

Figure 2: (a) Temperature dependence of the spin excitations at E = 6 meV for (1, 0, 1) and (0, 1, 1).The anisotropy in spin excitations vanishes around T = 160±10 K. Inset: Detwinning device mounted on the supporting sample holder. Uniaxial pressure can be applied by a steel spring driven by a screw. (b) The electronic phase diagram of BaFe2−xNixAs2 from resistivity anisotropy ratio ρbo=ρao obtained under uniaxial pressure [6]. The spin excitation anisotropy temperatures are marked as T*.

In the antiferromagnetically (AF) ordered state of a detwinned sample, magnetic Bragg peaks and low energy spin waves should occur at Q = (±1,0,L) (L = odd). By contrast, in the paramagnetic tetragonal phase (T > Ts ≥ TN), one would expect the spin excitations at the (±1, 0) and (0,±1) positions to have equal intensities. This dictates a symmetry change from C4 to C2 of the spin excitations in a detwinned sample across Ts and TN, similar to resistivity. Thus, temperature dependence of the difference between spin excitations at (1, 0, 1) and (0, 1, 1) across TN and Ts can conclusively determine whether there are nematic spin correlations in the tetragonal state. Searching for nematic spin correlations above TN will be significant for understanding the origin of the nematic phase [3].
By carrying out a series of inelastic neutron scattering experiments on uniaxial-strain detwinned annealed BaFe2-xNixAs2 (x=0, TN = 138 K, underdoped x=0.085, Tc = 16.5 K, TN = 44 K and overdoped x=0.12, Tc = 18.6 K) samples using the thermal triple-axis spectrometer PUMA, we have discovered nematic spin correlations in the tetragonal state.
Figure 2(a) shows the temperature dependence of the spin excitations (signal above background scattering) across TN and Ts. In the AF ordered state, we see only spin waves from the wave vector (1,0,1). On warming to the paramagnetic tetragonal state above TN and Ts, we see clear differences (nematic spin correlations) between (1,0,1) and (0,1,1) that vanish above 165 K, well above Ts, the same temperature below which anisotropy is observed in the in-plane resistivity. We conclude that the four-fold to two-fold symmetry change in spin excitations in BaFe2As2 occurs alongside the resistivity anisotropy. To compare nematic spin correlations with resistivity data, we summarize our INS experiments on uniaxial strain detwinned BaFe2−xNixAs2 in Figure 2(b). The square red symbols indicate the temperature below which spin excitations at an energy transfer of E = 6 meV exhibit a difference in intensity between the (±1,0) and (0,±1) positions for undoped and electron underdoped BaFe2−xNixAs2. For electron overdoped BaFe1.88Ni0.12As2, the same uniaxial pressure has no effect on spin excitations at wave vectors (±1,0) and (0,±1). As shown in Figure 2(b), the resistivity anisotropy occurs near the spin excitation anisotropy temperature T∗ determined from INS, indicating that the nematicity revealed by resistivity anisotropy is an intrinsic property and may have the same origin as nematic spin correlations. Our results revealed nematicity in the spin-excitation channel and shed light on our understanding of the origin of nematic phase in iron pnictides. For detailed discussions, see ref. [5].

Experiments at Puma

The success of the experiments presented depends on the high flux and excellent performance of the PUMA thermal-triple axis spectrometer. The configuration of the sample is shown in the inset of Figure 2(a). In all experiments, horizontally and vertically curved pyrolytic graphite (PG) crystals were used as a monochromator and analyzer. To eliminate contamination from epithermal or higher-order neutrons, a sapphire filter was added in front of the monochromator and two PG filters were installed in front of the analyzer. All measurements were carried out with a fixed final wave vector, kf = 2.662 Å-1.