Exploration of iron-chalcogenide superconductors
Iron-chalcogenide compounds with FeSe(Te,S) layers did not attract much attention until the discovery of high- superconductivity (SC) in the iron-pnictide compounds at the begining of 2008. Compared with FeAs-based superconductors, iron-chalcogenide superconductors have aroused enormous enthusiasm to study the relationship between SC and magnetisms with several distinct features, such as different antiferromagnetic ground states with relatively large moments in the parents, indicating possibly different superconducting mechanisms, the existence of the excess Fe atoms or Fe vacancies in the crystal lattice. Another reason is that the large single crystals are easily grown for the iron-chalcogenide compounds. This review will focus on our exploration for the iron-chalcogenide superconductors and discussion on several issues, including the crystal structure, magnetic properties, superconductivity, and phase separation. Some of them reach a consensus but some important questions still remain to be answered.
Keywords: Fe-based superconductors; Fe(Te,Se,S) compounds; (Tl,K,Rb)FeSe compounds
PACS: 74.10. +v; 74.25. -q; 74.25. DW; 74.72. Cj
The emergence of superconductivity (SC) with superconducting transition temperature (T=26K) in the LaFeAs[OF] compounds LaOFFeAs, following the same group’s earlier discovery of superconductivity with T 5K in LaFePOF, is quite astonishing because the iron element is always considered to be ferromagnetic which is always detrimental to SC with spin singlet pairing. Soon after this discovery, Chinese scientists devoted great effort to pursuing new iron-based superconductors with new structures and higher T. Significant progress was made in several months later. For example, various stackings of antifluorite FeAs building blocks interleaved with alkali, alkaline earth, or rare earth oxides layers form varieties of Fe-As based superconductors, including ’1111’ chenxh2008superconductivity; ChenGF2008Ce; CaoW56K, ’122’BaK; Co122; LJLiNi122, ’111’LiFeAs; TappLiFeAs and ’32522’WenHH32522, ’42622’Wen42622 system. The parent compound of FeAs-based superconductor undergoes a structural transition from tetragonal (T) to orthorhombic (O) lattice accompanied with an antiferromagnetic (AFM) transition occuring simultaneously or at a lower temperature () than the structural transition temperature () LaOFFeAsNeutron; CePhasediagram; Ba122Neutron. The in-plane wave vector of AFM order in the FeAs-based parent compound is universally characterized by Q=(, )=(1, 0) which is the same as the vector connecting the hole Fermi pocket at the zone center and the electron pocket at the corner of the Brillouin zoneHongDBa122ARPES; Co122ARPES. It was suggested that the FeAs-based material is an s-wave superconductor with an unconventional spin fluctuation mechanism, that is, superconducting order parameter has opposite signs at the hole pocket and the electron pocket MazinPRL; KurokiTheorySwave; FaWTheory; BaKNeutron.
The iron-chalcogenide compounds are much simpler in structure due to the neutrality of the FeSe(Te,S) layer than the Fe-As based compounds. The first discovery of SC with T8K in FeSe compound was reported by Hsu et al.maokun on 15th July, 2008, and quickly followed by the reports of FeTeSe (14K) by Fang et al.FangFeTeSe on 30th July, 2008 and FeTeS (T10K) fangFeTeS; FeTeS. KFeSe with T 30K was firstly reported by Guo et al.xiaolong on the 14th Dec., 2010 and considered to be of isostructure to the well-known ThCrSi structure with I4/mmm space group. On 23rd Dec., 2010, our groupFang122 independently reported on the bulk SC with T 31K and a trace of SC at 40K in (Tl,K)FeSe system, and pointed out that the SC in this system is close to an AFM insulator, which is associated with the Fe-vacancy ordering in the Fe square lattice. Two kinds of Fe-vacancy orderings were suggested. Shortly after, SC with T30K in AFeSe (A=KGFChenFeSe122, RbXHRb122; WenHHFeSe122,CsCs, and Tl/RbTlRb) compounds were reported. Electronic band structures and magnetic ground states of the AFeSe compounds with Fe-vacancy orderings were systematically investigatedCaoCTheory; TaoXiangTheory; YZhouTheory, e.g., the first-principles calculations suggested that the ground state of (Tl,K)FeSe is of the checkerboard antiferromagnetically coupled blocks of the minimal Fe square, which was confirmed by the neutron diffraction experiments later.
This review will focus on our exploration for the iron-chalcogenide superconductors and discussion on the several issues, including their crystal structures, magnetic properties, superconductivities, and phase separations. Some of them reach a consensus but some still remain to be answered.
Ii Fe(Te,Se,S) System
ii.1 Discovery of the Superconductivity in Fe(Te,Se,S) system
Iron selenium binary compounds have several phases with different crystal structures. SC occurs only in FeSe with the lowest excess FeFeSeSCvsExcess, so-called the phase, which crystallizes into the anti-PbO tetragonal structure at ambient pressure (P4/nmm ) maokun and is considered to be the compound with the simplest structure in the Fe-based superconductors. The key ingredient of SC is a quasi-two-dimensional(2D) layer consisting of a square lattice of iron atoms with tetrahedrally coordinated bonds to the selenium anions which are staggered above and below the iron lattice [Fig. 1(a)]. These slabs, which are simply stacked and combined together with Van der Waals force, are believed to be responsible for the SC in this compound.
FeSe was first found to be a superconductor with T=8K by Hsu et al.maokun, their results are summarized in Fig. 1(c). Then Medvedev et al.FeSepressure found that the of FeSe increases from 8.5 K to 36.7 K under an applied pressure of 8.9 Gpa. Although the structural transition from a tetragonal to hexagonal NiAs-type was observed above 7 Gpa, Mössbauer measurements did not reveal any static magnetic order for the whole p-T phase diagram. However, short range spin fluctuations, which were strongly enhanced near T, were observed by neutron magnetic resonance (NMR) measurements and thought to play an important role for the emergence of SC in this compoundFeSeNMR.
Soon after the report of SC in FeSe by Hsu et al.maokun, we first foundFangFeTeSe that the partial isovalent substitution of Te for Se in Fe(Te,Se) compounds results in the increase of the T to 14 K, and first reported that the end compound FeTe is an AFM semiconductor or metal, which depends on the quantity of the excess Fe atoms in the lattice, instead of a superconductor, although the band calculation TheoryFeTe indicated that it should be a superconductor with higher than that of FeSe. Neutron diffraction experiments BaoweiFeTe soon revealed that the AFM transition in FeTe is also accompanied with a structural transition and that the AFM ground state at lower temperatures has two types, one is a commensurate AFM order in the sample with less excess Fe atoms, and the other is an incommensurate AFM order in the sample with more excess Fe atoms. It is important that the FeTe be in a bicollinear AFM order, characterized by an in-plane propagation wave vector Q=(, 0), which is different from the Fe-pnictide (1111 and 122 types) system. But for the superconducing FeTeSe sample, neutron scattering measurement revealed a prominent short-range quasi-elastic magnetic scattering at the incommensurate wave vector (0.438, 0). These short-range correlations are enhanced significantly as the temperature decreases below 40 K (Fig. 3(c)), thereby leading to an anomalous temperature dependence in the Hall coefficient. These results indicate strongly that the SC in Fe(Te,Se) compounds may be mediated by a magnetic fluctuation.
ii.2 Effect of excess Fe atoms on the structure and magnetism in Fe(Te,Se,S) system
Although the Fe(Te,Se,S) system is considered to be the simplest structure in the iron-based superconductors, there are excess iron atoms partially occupying the interstitial sites between adjacent FeX (X = Te,Se,S) layers BaoweiFeTe as denoted by Fe(2) in Fig. 3(a), which is similar to the location of Li in the LiFeAs superconductor that shares the same space group P4/nmm with FeTe TappLiFeAs.
Zhang et al.excessFetheory studied the electronic and magnetic properties of the excess Fe atoms in FeTe by means of density functional calculations. They found that the excess Fe atom has a monovalence in the FeTe compound, i.e. Fe, and thus provides electron doping of approximately one e/Fe. The excess Fe ion is suggested to be magnetic with 2.4 moment, bigger than that of the Fe (1.6-1.8) cation at the Fe(1) position. The interaction between the local moment of the excess Fe ion and the moment of Fe on the layers is expected to persist even after the AFM order has been suppressed.
Generally, the excess Fe ions existing in both FeTe and FeSe, as well as Fe(Te,Se,S) lattices have an effect on their crystal structures and magnetic properties at low temperatures. McQueen et al.FeSeStruvsExcessFe found that FeSe with less excess Fe atoms undergoes a structural transition at 90 K from a tetragonal structure to an orthorhombic structure, while there is no structural transition for FeSe with more excess Fe atoms . The structure of FeSe compound below 90 K can be identified to be orthorhombic (space group Cmma). The unit cell is enlarged into a supercell similar to that of the iron-pnictide parent compound. There is no peak splitting or other significant change in the Mössbauer spectrum corresponding to any magnetic transition between 50 and 100 K. However, the NMR measurement FeSeNMR for the FeSe compound demonstrated that the electronic properties of FeSe are very similar to those of an optimally electron-doped FeAs-based superconductor and the AFM spin fluctuations are very strongly enhanced near T.
As reported by Bao et al.BaoweiFeTe, the parent compound FeTe exhibits quite a different scenario. At the room temperature, FeTe has a tetragonal structure (P4/nmm), which is the same as that of FeSe, regardless of the content of excess Fe atoms. But at lower temperatures, FeTe compounds with different quantities of the excess Fe atoms undergo different structural transitions. For example, for FeTe, a structural transition to an orthorhombic structure (Pmmn) occurs at T63 K, with the a axis expanding and the b axis contracting. While FeTe with less excess atoms undergoes a first order transition to a monoclinic structure (space group P2/m) at T75 K. For the low temperature phase, except for the difference between the a and b axis, the angle between a and c axes is less than 90. Unlike the structural transition in FeAs-based parent compounds, neither the orthorhombic nor monoclinic distortion in FeTe compound doubles or rotates the the tetragonal cell.
As discussed above, there is a discernable AFM transition coupled to the structural transition in FeTe. The wave vector of AFM ordering observed in FeTe is different from the ubiquitous in-plane wave vector of (, ) observed in the FeAs-based parent. Although the moments between the adjacent FeTe layers are antiferromagnetically aligned, the in-plane magnetic structure depends on the quantity of the excess Fe atoms. For example, FeTe develops a bicollinear AFM order with an commensurate in-plane wave vector Q = (, 0) shiliangFeTeN; BaoweiFeTe. The Fe ions in the plane have a total moment of 2.25(8) with a major component (2.0(7) ) along the b-axis as shown in Fig. 4(a), which is rotated 45 from a-axis in the Fe-As materials. There are also projections of the moment along the a and c axes with -0.7(2) and 0.7(1) respectively. Li et al.shiliangFeTeN attribute the finite moments along the c-axis to the finite moments of the excess Fe ion. As the content of the excess iron increases, the AFM order becomes incommensurate with an in-plane wave vector Q = (, 0). The magnetic moments along the b-axis are still ferromagnetically aligned. The row of the moments of Fe ions in the plane is modulated with the propagating vector 2/a. Bao et al.BaoweiFeTe also found that incommensurability can be tuned by varying the excess Fe in the orthorhombic phase as shown in the left inset of Fig. 3(c). It reaches a commensurate value of 0.5 for the composition FeTe.
For the superconducting FeSeTe compound, neutron scattering measurements FeTeSeInelastic revealed low energy spin fluctuations with a characteristic wave vector (, ) which corresponds to Fermi surface nesting wave vector, but differs from the bi-collinear AFM order vector Q=(, 0) in the parent compound FeTe. In addition, a pair of nearly compensated Fermi pockets FeTeARPES, one hole pocket at the point of the Brillouin zone, the other electron pockets at the M point were observed by the angle resolved photo emission spectroscopy (ARPES) experiments, which is similar to those expected from density functional theory (DFT) calculationTheoryFeSe. However,no signature of Fermi surface nesting instability associated with Q=(/2, /2) was observed, Fe(Te,Se) system may harbor an unusual mechanism for superconductivity.
ii.3 Effect of the excess Fe atoms on superconductivity in Fe(Te,Se,S) system
The excess Fe atoms existing in Fe(Te,Se,S) lattice not only affect the crystal and magnetic structure, but also suppress their SC. Liu et al. LTJExcess studied the SCs in the optimally doped FeTeSe samples with different quantities of the excess Fe atoms. They found that FeTeSe (denoted as SC2) exhibits a lower superconducting volume fraction, a lower T and broader transition width as shown in Figs. 5(a) and (b), while FeTeSe (denoted as SC1) with less excess Fe atoms exhibits better SC. In addition, it was found that the resistivity in the normal state of SC1 shows a metallic behavior, whereas the resistivity of SC2 exhibits a logarithmic behavior, which is a characteristic of the