First principle study of the electronic and structural properties of NaFeAs superconductor (2023)

After finding of the superconducting properties in iron (Fe) containing materials (LaFeAsO doped with Fluorine) [1], a rich number of superconductors based on these iron called pnictides and chalcogenides had been observed to superconduct at different transition temperature [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. This discovery stimulates intense research to explore new superconducting materials of this kind and to reveal the physics behind these superconducting properties. The highest critical temperature T_C in this family is already reached to 55K in Sm[O_1-xF_x]FeAs [12] and 56K in Gd_1−xTh_xFeAsO [13]. These pnictides are mainly classified into four subgroups [14]. The first one is the 1111-type such as RFePnO where R is the rare earth element and Pn denotes the pnictide elements such as phosphorus (P) or Arsenic (As), second one is the AeFe₂Pn₂ abbreviated as 122-type and here Ae denotes the alkaline earth metal, AFePn is the third one denoted by 111-type, where A is the alkaline metal and the last one is the FePn or the 11-type ironbased superconductors. There are also observed some iron based materials having a thick blocking layer known as 32522 or the 42622-type iron based superconductors (FeSCs) reported in [15]. All of these iron based superconductors are found to have a identical layered type configuration created from a Fe²⁺ square planar layer that are tetrahedrally coordinated with anions of pnictogen i.e., P or As [16], [17], [18]. Similar to the pnictogen, the chalcogenids are attached to chalcogen (Ch); Ch=S, Se or Te [19], [20], [21]. It is found that this Fe²⁺ layers are mainly responsible for the superconducting properties in FeSCs, similar to that of the CuO₂ layer in cuprates [22]. In normal state, these iron based families show some metallic behaviour and most of the undoped materials or the parent compounds of FeSC undergo a spin density wave (SDW) magnetic transition which disappear upon electrons or holes doping in the ionic layer (FeAs) or by applying pressure [23], [24]. At low temperature, a phase transition is also observed from tetragonal to orthorhombic crystal structure. While among all of the four series most of the 111 family found to be an exception; not showing any phase transition and the absence of SDW order [25], [26]. This family also show relatively low transition temperature T_C in comparison with other FeSCs. It was also found that to describe the physical mechanism and the observed high transition temperature of this FeSCs, the coupling provided by the electron–phonon is too weak [27], [28]. So to achieve a theoretical understanding of superconductors, many theories have been proposed [29], [30], [31], [32], [33], [34], [35], [36], [37], but the one which bring a good explanation of all the properties is still far from the scientific community. It was also observed that the superconductivity appears close to antiferromagnetic phase [38], [39], [40] both in cuprates and iron based superconductors, so we believed that the spin fluctuations i.e. based on magnetism are most promising, which was also supported by many theoretical and experimental works [41], [42], [43], [44]. Because of the metallic parent state, this iron based family can be well described by the first principle density functional theory and before studying the superconducting properties, it is very much important to study the parent state of the compound, whose description is lacking in numbers. So here we study the general features of parent compound of NaFeAs system. For NaFeAs, LiFeAs, KFeAs and LiFeP i.e. the 111-type iron based superconductors, some work have already reported [45], [46], [47], [48], [49], which includes their electrical and superconducting properties.

In this paper, we investigated and reported the structural and electronic behaviour of NaFeAs compound. The NaFeAs compound exhibit superconducting properties at ambient pressure and without doping at a critical temperature of T_C=12–25K [50], [51]. The Na_1-xFeAs compound found to exhibit the SDW magnetic state [52] and due to the evaporation loss of the sodium (Na), the synthesis of NaFeAs is quite difficult [53]. So here we use the first principle calculation to study the structural and electronic properties of NaFeAs compound. Several theoretical examinations of the properties of NaFeAs compound using the first principle density functional calculation have been reported [54], [55], [56], [57], [58]. For better understanding of the superconducting behaviour of a compound, it is highly essential to study their fundamental properties of the undoped or parent material of that compound. So here with the help of density functional studies, we look into the different properties of NaFeAs material. In particular here the main objective of this work is to study the structural and electronic properties including their electronic band structure diagram, the total density of states (DOS) and projected/partial density of states (PDOS) of NaFeAs system. Here the convergences of the plane wave cutoff and Brilliouin zone sampling for NaFeAs have been done and by using the optimized cell parameters, the self-consistent field (SCF), band structure and density of state (DOS) calculations have been performed. The total energy, Fermi energy of the material is observed and the band structure which provides the energy range of an electron and the band gap is also calculated along high symmetric direction in the Brillion Zone. Similarly the DOS which give the information regarding the number of different states that can be occupied by electrons in a particular energy range and projected density of state (PDOS) that provide the information about the contributions of different orbitals are also mentioned. In Section 2, we briefly described the methodology or the computational approach of our studies. Section 3 represents the results and discussions part including the crystal structure in 3.1 subsection, band structure in 3.2 and density of state (DOS) in 3.3 respectively. Finally the conclusion is presented in Section 4.