Effect of Surface Morphology and Magnetic Impurities on the Electronic Structure in Cobalt-Doped BaFe₂As₂ Superconductors

Abstract

Combined scanning tunneling microscopy, spectroscopy and local barrier height (LBH) studies show that low-temperature-cleaved optimally-doped Ba(Fe 1-x Co x ) 2 As 2 crystals with x=0.06, with T c = 22 K, have complicated morphologies. Although the cleavage surface and hence the morphologies are variable, the superconducting gap maps show the same gap widths and nanometer size inhomogeneities irrelevant to the morphology. Based on the spectroscopy and LBH maps, the bright patches and dark stripes in the morphologies are identified as Ba and As dominated surface terminations, respectively. Magnetic impurities, possibly due to cobalt or Fe atoms, are believed to create local in-gap state and in addition suppress the superconducting coherence peaks. This study will clarify the confusion on the cleavage surface terminations of the Fe-based superconductors, and its relation with the electronic structures.One major difference between the iron-based superconductors (FeSC) and conventional superconductors is the necessary involvement of magnetic elements. For a long time magnetic elements had been thought to suppress the superconductivity because of the competitive nature of superconductivity and magnetism, but with the cuprates and now FeSC, there are two cases of high-temperature superconductivity that apparently are magnetic in origin [1][2][3][4] .FeSC share a common Fe 2 X 2 layer structure where X is a pnictogen (P, As) or a chalcogen (S, Se, Te). With the incorporation of intercalation layers between the Fe 2 X 2 layers, FeSC are classified into five families: '11', '111', '122', '1111' and complex structure materials 3-4 . To understand the causes of this superconductivity, the relationship between the global properties and local behavior must be understood. These local signatures include chemical defects, electronic variations, dopant distributions, and strain. The '11' and '111' families have been studied extensively using scanning tunneling microscopy and spectroscopy (STM/STS) and the results are clear due to the cleanness of the cleavage as there is unique and symmetric cleavage plane, 1 and a non-polar surface with minimal surface effect [5][6] . On the contrary, the STM/STS studies on the '122' family are still controversial although great efforts have been put in the studies [5][6][7][8][9][10][11][12] , with main debate focused on where and how the crystal cleaves. For example, within the BaFe 2 As 2 (Ba122) unit cell, the Fe-As layers have a mirror symmetry around the Ba layer as shown in Fig. 1 (a). As FeAs layer is a highly polar plane, the physical cleavage through this plane might force Ba ions to be divided to both sides of the cleavage surface to alleviate the polar nature, associated with surface reconstructing. As a result, rich morphologies or "terminations" exist on the Ba122 cleavage surface. Some groups report that the crystal cleaves at Ba plane with ½ monolayer of Ba atoms to achieve the charge neutral surface 7-10 , while others suggest As atoms terminate the surface by breaking the we...

Competitive and cooperative electronic states in Ba(Fe1−xTx)2As2 with T = Co, Ni, Cr

“…At the temperature of experiments in this work (4 K), Cr-122 has a G-AFM ground state (TN~ 100 K); optimally doped Co-122 is in SC state (Tc ~ 22 K) 24 ; and the composition of the Ni-122 was picked to locate it in the coexistence range of SC and AFM orders (Tc ~ 19 K, TN~ 45 K). After low temperature cleavage, the three sets of samples show similar topography, which is the coexistence of As terminated 2×1 and Ba terminated √2 × √2 reconstructions, as reported for Co-122 before 25 . Fig.…”

“…The above coexistence of S-shape and in-gap states with superconducting states are not affected by the surface termination; they are also observed on 2 × 1 surfaces. A STM study of BaFe2As2 has found both √2 × √2 and 2 × 1 surface reconstruction coexistence on the cleavage surface 25,27 . As reported recently, although different terminations with different LDOS coexist on the surface of the transition metal substituted BaFe2As2 (√2 × √2 and 2 × 1), the superconductivity gap width is not affected by the surface reconstructions owing to the global nature of the superconductivity 25 .…”

Nanoscale Superconducting States in the Fe-Based Filamentary Superconductor of Pr-Doped CaFe2As2

“…To visualize the local superconductivity, a superconducting gap map serves better than an STS dI/dV map. As an example, Figure 3 shows a STM topographic image and corresponding superconducting gap map, which is calculated from the current imaging tunneling spectroscopy (CITS) image at 4.2 K [11,12]. The majority area of the sample does not show superconductivity, which is consistent with a low superconducting volume fraction from the bulk measurement (about 3%).…”

“…However, these Pr dopants, in parallel, create lattice defects with some net magnetic moment [18]. The local magnetic moment can also destroy superconductivity [11]. Therefore, the Pr-Ca122 sample is not able to become a bulk superconductor by simply increasing the Pr dopant concentration.…”

Lattice disorder effect on magnetic ordering of iron arsenides

“…3. Using a similar method as STM report on Co-doped BaFe2As2 [37], the 2 × 1 and √2 × √2 reconstructed surfaces can be assigned to arsenic termination and europium termination, respectively, as shown in Fig. 3a.…”