Magnetic fluctuation and suppression in high-temperature superconductor Ba(Fe1-xCox)2As2

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Abstract

Using the first principle calculations, electronic structure and magnetic properties of Ba(Fe1-xCox)2As2 (x = 0, 0.125) are studied. The collinear antiferromagnetic order is the ground states of the parent compound BaFe2As2. Only hole pockets appear near points Γ and Z for BaFe2As2, while there are both hole and electron pockets nesting near the Γ and Z for Ba(Fe0.875Co0.125)2As2 due to the up shifting of the Fermi level. After Co doping, both the interactions of the nearest neighbour J1 and next nearest neighbour J2 of magnetic ions decrease simultaneously with the decreasing of magnetic moment of Fe, and considering the reduction of J1/2J2, indicating antiferromagnetic order and magnetic fluctuations tend to be suppressed.

Introduction

The discovery of iron-based superconductors was an important progress in the field of superconductivity, which provided an alternative system to understand mechanism of high temperature superconductors [1], [2], [3]. LaO1−xFxFeAs with a critical temperature up to 26 K was reported [4] and the transition temperature of SmO1−xFxFeAs was found to be as high as 55 K [5]. According to their stoichiometry, iron-based superconductors can be divided into several categories, such as the series of 1111 [4], [5], [6], 122 [7], [8], [9], 111 [10] and 11 [11], etc.

Generally, superconductivity can be derived by doping in iron-pnictide compounds. Therefore, it is very important to study the ground state of the parent compound for understanding the mechanism of superconductivity. Earlier, there was a debate on whether LaOFeAs was a nonmagnetic metal [12,13] or an antiferromagnetic (AFM) semimetal [14,15]. Then, optical measurement and theoretical calculation showed that LaOFeAs had spin density wave (SDW) instability below 150 K, and the superconductivity in the material was due to the suppression of SDW order [14]. The competition between AFM order and superconductivity is the basic feature of iron-based superconductors [16], [17], [18], and the spin fluctuations were suggested to be the key to induce superconductivity [1,7,19].

BaFe2As2 is a promising system, which shows superconductivity in K doping [20], and presents collinear AFM order [21] when the temperature is below the Neel temperature TN. Superconductivity appear when x is from 0.025 to 0.18 in Ba(Fe1-xCox)2As2, and the maximum transition temperature is about x = 0.06 [22]. Fe-As layer plays an important role for the superconductivity of iron-based superconductors [15,23,24]. The competition of AFM order and superconducting state in the Fe-As layer is strikingly similar to that in the superconductor of cuprates (the Fe-As layer is compared with the Cu-O layer). The magnetic interaction comes from inside the iron layer, and the main exchange path between the spins of Fe is along a and b axes and along the diagonal via As atoms [25]. This shows the commonness of the magnetic interaction in the stacking Fe-As layers. Therefore, the interaction along the c axis has no great contribution to the whole magnetic properties [26]. However, the variation of AFM order and fluctuation is seldom obtained by analyzing the interaction between the nearest neighbour (NN) and the next nearest neighbour (NNN) of the magnetic ions. The works using Heisenberg model to describe the suppression of AFM order and fluctuation in the doping of BaFe2As2 were very few.

In this work, electronic structures of collinear AFM order of Ba(Fe1-xCox)2As2 are studied by using first principle simulation. The influence of Co doping on the AFM order and fluctuation is studied. The changes of electronic structure and Fermi surface are analyzed in detail. Heisenberg model is used to reflect the interaction between the NN (J1) and the NNN (J2) of the magnetic ions. The results will be helpful to understand the relationship between superconductivity and magnetic fluctuation and the depression of AFM order, and provide support for further understanding of superconducting mechanism.

Section snippets

Model and calculation method

Our calculations used the CASTEP package which based on density functional theory. The lattice constants (Å) of BaFe2As2 were a = 5.6157(2), b = 5.5718(2) and c = 12.9424(4). The space group is Fmmm [27]. 1 × 1 × 1 unit cell (as shown in Fig. 1) with 20 atoms was used to perform calculations. Four different magnetic order, checkerboard AFM order (AF1, in Fig. 2(a)), collinear AFM order (AF2, in Fig. 2(b)), ferromagnetic order (FM, in Fig. 2(c)) and nonmagnetic state (NM) were under

Results and discussions

The geometry of Ba(Fe1-xCox)2As2 (at x = 0 and 0.125 respectively) were optimized and the coordinates were shown in Table 1. Among the energies of the four different magnetic states (shown in Fig. 2), we found that the energy in collinear AFM order (Fig. 2(b)) was the lowest (see Table 2). Thus the collinear AFM order was the ground state, which is in consistent with the experimental results [26]. In collinear antiferromagnetic order, lattice constants were compared in Table 3, and the lattice

Conclusions

Electronic structure of Ba(Fe1-xCox)2As2 (x = 0 and 0.125) were studied by the first principle calculations. Band structure and the topology of the Fermi surface changed with the doping of Co. The nesting of the hole pockets and the electron Fermi pockets at the Γ and Z point was clearly observed. The strong AFM superexchange between the nearest neighbour (J1) and the next nearest neighbour (J2) of Fe was found in BaFe2As2. The magnitude of magnetic moment of Fe and the values of J1 and J2

Acknowledgments

This work was sponsored by National Natural Science Foundation of China under Grant nos. 51872034 and 51722205, Liaoning Revitalization Talents Programunder Grant no. XLYC1807173, the Key Laboratory of New Energy and Rare Earth Resource Utilization of State Ethnic Affairs Commissionunder Grant no. NERE201905, and Dalian Technology Innovation Fund(Applied Basic Research).

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