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Superconductivity and magnetism in 11-structure iron chalcogenides in relation to the iron
pnictides
View the table of contents for this issue, or go to the journal homepage for more
2012 Sci. Technol. Adv. Mater. 13 054304
(http://iopscience.iop.org/1468-6996/13/5/054304)
Home Search Collections Journals About Contact us My IOPscience
Received 29 October 2012Accepted for publication 29 October 2012Published 28 December 2012Online at stacks.iop.org/STAM/13/054304
AbstractThis is a review of the magnetism and superconductivity in ‘11’-type Fe chalcogenides, ascompared to the Fe-pnictide materials. The chalcogenides show many differences from thepnictides, as might be anticipated from their very varied chemistries. These differencesinclude stronger renormalizations that might imply stronger correlation effects as well asdifferent magnetic ordering patterns. Nevertheless the superconducting state and mechanismfor superconductivity are apparently similar for the two classes of materials. Unansweredquestions and challenges to theory are emphasized.
Keywords: superconductivity, magnetism, 11-structure iron chalcogenides, iron pnictides
1. Introduction
The discovery of high-temperature superconductivity(Tc ∼ 26 K) in electron-doped LaFeAsO1−x Fx [1] hasresulted in widespread interest and exploration of relatedmaterials, some of which have Tc exceeding 55 K. Thisrepresented the first finding of superconductivity above 50 Kin a non-cuprate material. Furthermore, while importantdifferences from cuprates emerged over time, theseiron-based superconductors were found to have similaritieswith the cuprates as well, particularly the presence ofsuperconductivity ‘near’ magnetism [2]. This is in contrastto other high-temperature superconductors discovered afterthe cuprates, such as (Ba,K)BiO3 [3, 4], MgB2 [5] andprobably Ax C60 [6, 7] (in the last case, antiferromagnetismdoes exist [8] but may not be very intimately connected withsuperconductivity; most likely it is a competing phase toelectron–phonon superconductivity).
This makes the Fe-based materials perhaps the mostinteresting new superconductors since the discovery of thecuprates. Key questions include of course the mechanismand nature of the superconductivity, the relationship withthe cuprates, and the chemistry of the superconductivity,
i.e., what range of compounds display this phenomenon andwhat is the relationship between the superconductivity andthe chemical composition and structure. At present there isstrong a interest in finding new high-temperature Fe-basedsuperconductors and especially in finding materials withhigher critical temperature.
As mentioned above, high-temperature Fe-basedsuperconductivity was first discovered in electron–dopedLaFeAsO1−x Fx . Subsequently, high-temperature super-conductivity was also found in various iron-basedoxyarsenides by replacing La with other rare-earth metals[9–13], as well as oxygen-free arsenides such as dopedBaFe2As2 [14, 15] SrFe2As2 [16], CaFe2As2 [17, 18] andLiFeAs [19–21]. The common structural feature of this familyof materials is the appearance of Fe–As layers. These consistof an Fe square planar sheet tetrahedrally coordinated by Asatoms above and below the Fe planes.
In addition, superconductivity of an apparently similarnature occurs in doped LaFePO [22–24]. This shows thatAs is not essential to the superconductivity, although itshould be noted that the Tc is much lower in the phosphide.There are no known Fe-based superconductors based onSb, although not surprisingly there has been speculation
Figure 1. Crystal structure of the ‘11’ Fe-chalcogenides showingthe site for the additional Fe as small spheres.
about this possibility and compounds that might realize it[25, 26]. Low-temperature superconductivity has also beenfound in isostructural Ni compounds, including LaNiPO[27], LaNiAsO [28], LaNiBiO [29], BaNi2As2 [30] andSrNi2As2 [31, 32]. However, while like Fe, Ni is a 3delemental ferromagnet, these Ni-based materials are readilyexplained as conventional electron–phonon superconductors[33–35], while, as was shown by direct calculations of theelectron–phonon coupling, the Fe-based materials are not[36, 37].
The cuprates are a large family of high-Tc
superconductors. They are layered compounds containingCuO2 square planes, with Cu near, but doped away from, thenominal 2+ oxidation state. Many different superconductingcompounds that contain stacks of these CuO2 layers withother blocks have been discovered. However, substitutionof Cu or the in-plane O is not compatible with high-Tc
superconductivity. Similarly, in the case of the Fe-basedmaterials it was known very soon that they also are a largefamily of materials, and are based on Fe square planescoordinated by As, or with much weaker superconductivity,by P. In contrast with the cuprates, superconductivity can beinduced by partial substitution of Fe with dopants such as Coor Ni [38–41] or with isovalent impurities like Ru [42], whichsuppresses magnetism without doping [43, 44]. This retentionof superconductivity with very substantial Fe substitution ismost remarkable for an unconventional superconductor and isstill not fully understood.
The chemistry of chalcogens is very different from thatof the pnictides, and in particular selenides are generallydifferent in properties from arsenides. For example, Se israther more electronegative than As or P (Pauling scale,2.55 versus 2.18 or 2.19). Thus it came as a surprise whenPbO, so-called ‘11’ structure Fe1+δSe [45–47], was discoveredto be a superconductor. Direct calculations showed thatsimilar to the pnictides [36], this is not an electron–phononsuperconductor [48].
Similar to the pnictides, Fe1+δSe features an Fe squarelattice and a tetrahedral coordination of the Fe, though notwith As, as shown in figure 1. Furthermore, the criticaltemperature of Fe1+δSe (initially written as FeSe1−x ) increasesstrongly with either Te substitution [49] or pressure, reaching∼37 K under pressure [46, 50–53]. The high values ofTc under pressure imply a relationship with the Fe-As
Figure 2. Generic high-Tc cuprate phase diagram as a function ofdoping. Note the prominent antiferromagnetic Mott insulating phaseat zero doping.
superconductors. Here we overview some aspects of thesuperconductivity and related properties of these 11-structurecompounds, mainly from a theoretical perspective.
2. Features of Fe-pnictide superconductors
It is useful to very briefly mention some general aspects of theFe-pnictide superconductors to set the stage for the discussionof the chalcogenides. A particularly detailed review may befound in [54], while a summary of the electronic structures,which are a focus here, can be found in [55]. Sefat andsingh [56] present the chemistry of these materials. Asmentioned above, the Fe-based superconductors have thehighest known Tc outside of the cuprates, and a genericcuprate phase diagram is shown in figure 2. It contains aprominent antiferromagnetic Mott insulating state at zerodoping that is suppressed by doping and gives way to ahigh-temperature superconducting state. This Mott insulatingphase is universal, and doping is essential to destroy it in favorof superconductivity.
It was recognized early on that the Fe-based materialsalso have antiferromagnetism that is in apparent competitionwith a high-Tc superconducting state [2, 57]. However,the antiferromagnetic state of the Fe-based materials isvery different from that of the cuprates. First of all, itis unambiguously metallic as seen from spectroscopy [57],transport [58] and quantum oscillations [59]. Furthermore,as shown in figure 3, the order is different fromthe nearest-neighbor antiferromagnetism of the cuprates,implying more subtle interactions. The particular orderobserved in the arsenides is a zone-corner ordering, which inthe unit cell containing two Fe atoms means a magnetic cellinvolving four Fe atoms. This order consists of alternatingspin-up and spin-down chains of Fe atoms, and lowersthe symmetry from tetragonal to orthorhombic. Thesesystems have substantial magnetoelastic coupling and sothis symmetry lowering is readily observed in the latticeparameters below the ordering temperature. Remarkably,while in some cases the structural transition and the magnetic
Figure 3. Magnetic order of different phases: nearest-neighbor antiferromagnetism, typical of cuprates (left), the stripe order with lines ofaligned Fe moments, as in LaFeAsO (middle), and the so-called double-stripe order of Fe1+δTe (right).
Figure 4. Fermi surfaces of doped LaFeAsO (left) and calculated real part of the non-interacting Lindhard susceptibility χ0(q), from [37].
ordering transition are coincident, in many cases, includingthe first material studied, LaFeAsO, the structural distortionprecedes the magnetic ordering as the temperature is lowered[2, 60, 61]. While various explanations have been offered forthis symmetry breaking before magnetic order, i.e., nematicbehavior, the details remain unclear [62–65].
In the cuprates, the Mott insulating antiferromagnet isdestroyed by doping, giving way to a conducting state. Atleast at high doping levels this state is a Fermi liquid,while at low doping levels there is nanoscale inhomogeneity.In the Fe-based materials, the antiferromagnetic andsuperconducting states appear to be more intimatelyconnected. In particular, superconductivity and magneticorder coexist in the phase diagrams [66] and there is arather continuous evolution of the spin fluctuations seen innuclear magnetic resonance (NMR) spectra of the magneticto the superconducting phases [67]. Also, unlike cuprates, theantiferromagnetic order in the Fe-based superconductors canbe suppressed in favor of superconductivity by various means,such as pressure, isovalent alloying, etc—not only doping asin cuprates.
Turning to the superconducting state, the electronicstructure of Fe pnictides is also very different from that ofthe cuprates. Initially it as assumed that Coulomb correlationsmay be dominant and play a role similar to the cuprates,producing Hubbard bands and a nearby antiferromagneticMott insulating state [68, 69]. However, early experimentsquickly showed differences from cuprates. They revealed ametallic state exhibiting quantum oscillations [59, 70, 71] and
metallic-like band dispersions around EF [72, 73], and lackingthe Hubbard bands [74].
More recent theoretical results obtained by incorporatingcorrelations are in good accord with these experimental data.The essential aspects are that the effective Hubbard interactionU is weaker than in the cuprates, the multi-orbital nature of theelectronic structure works against Mott states, and the on-siteHund’s interaction J plays a stronger role [75–78].
This is not to say that the materials are uncorrelated.Photoemission does show significant renormalizationsrelative to standard density functional theory (DFT)calculations [72] and this is also seen in a reduction ofthe optical Drude weight [79]. Depending on doping levela renormalization of ∼1.4–2.7 was needed to bring localdensity approximation (LDA) calculations into agreementwith angle-resolved photoemission (ARPES) results for(Ba,K)Fe2As2 and Ba(Fe, Co)2As2 [80]. The difference fromthe cuprates is that this represents a redistribution of spectralweight in the Fe d bands, rather than a transfer to Hubbardbands (consistent with an important role for the Hund’scoupling) and furthermore, the renormalization is momentumdependent. Note that in a paper on FeSe based on dynamicalmean field calculations, Aichhorn et al [81] referred to thisredistribution of spectral weight inside the Fe d bands dueto Hund’s coupling as producing a lower Hubbard band,although plainly the feature is not a Hubbard band in thesense that is normally reserved for that characterization.
The band structure and Fermi surfaces have beencalculated by many authors. In these compounds Fe occurs
nominally as Fe2+, i.e. with six d electrons. The Fermi surfacesof all the materials consist of compensating electron andhole sheets derived from Fe d bands. This was first foundby Lebegue [82] who studied the superconductor LaFePOfinding hole-like Fermi surfaces around the tetragonal zonecenter and compensating electron-like sheets around the zonecorner. Studies following the discovery of high-temperaturesuperconductivity in arsenides showed similar structuresin the various compounds [55, 83–88]. Therefore, thepnictides have a semimetallic electronic structure consistingof disconnected electron and hole sheets of Fermi surface.
Importantly, the Fermi surface structure is not dominatedby a single orbital as in the cuprates, but rather containscontributions from several orbitals. The electron sheets havethe same dxz/dyz/dxy mixed character in all compounds,while the hole sheets differ more. However, in all cases theycontain sections of dxz/dyz character mixed with or in additionto sections of other d orbital character [88]. Therefore, themultiorbital nature of the electronic structure is an essentialfeature of these materials.
Returning to cuprates, the Mott insulating state is notdescribed by standard band structure calculations, and froma band structure point of view these materials are incorrectlydescribed as not being close to magnetism. The pnictideson the other hand have relatively low Fermi velocities [83]that lead to high densities of states ∼1 eV−1 on a per Feper spin basis. This does put them near itinerant magnetism.In fact, they are overly magnetic within standard DFTcalculations [89]. The strong overestimate of the magneticmoments in such calculations is an unusual feature of thesematerials, and may be a consequence of strong competitionbetween different magnetic states leading to renormalizationby quantum fluctuations [90]. Another possibility that hasbeen discussed is a more complex magnetic state involvingintra-atomic non-collinearity [91]. In any case, from anelectronic point of view, these are metals with low carrierconcentrations and high densities of states, which approachband magnetism.
As mentioned above, the Fermi surfaces consist ofdisconnected electron and hole parts, with the hole partsaround the zone center and the electron parts around the zonecorner. These have similar orbital character and are separatedby a 2D wavevector, ( 1
2 , 12 )2π/a, where a is the in-plane
lattice parameter. This leads to a nesting that is reflected in thesusceptibility (figure 4). The dominant magnetic instabilityin iron pnictides is a zone-corner instability correspondingto this connection vector, and is known as the spin densitywave (SDW) ordering [2], although this does not mean thatthe instability comes only from states very close to the Fermienergy [92], as in classical descriptions of weak spin densitywaves. This ordering pattern, as mentioned above, consists oflines of Fe-atoms with parallel spins, as shown in the middlepanel of figure 3, and is the ground state predicted by densityfunctional calculations [37, 93–95]. Various experiments [54]showed an intimate connection between the magnetism andthe superconductivity of these materials.
Within a picture of spin-fluctuation mediated super-conductivity a susceptibility peaked at the connection vector
Figure 5. Phase diagrams based on literature for ‘11’ structure ironchalcogenides. The left side shows the behavior of Fe1+δSe underpressure following [47, 50], while the right side shows the behaviorof Fe1+δTex Se1−x as a function of x following [47, 103, 104]Superconductivity can also be produced by alloying with FeS[105, 106].
between such disconnected Fermi surfaces can lead tosinglet superconductivity provided that the sign of the orderparameter is opposite on the electron and hole sheets.Considering the small sizes of the Fermi surfaces this favorsa sign-changing s-wave state, denoted s+−, while other statesthat do not have this sign change are disfavored [37, 85]. Thissign-changing s-wave superconductivity is strongly supportedby a variety of experiments as discussed in [96]. Perhapsmost important is the observation of the predicted [97, 98]neutron resonance around (π/a, π/a) [99], which is aninelastic neutron scattering peak seen in the superconductingstate at energies above the superconducting gap and atmomenta connecting parts of the Fermi surface where theorder parameter has opposite sign.
3. Iron chalcogenides: similarities and differencesfrom the pnictides
Phase diagrams as a function of pressure and compositionof the ‘11’ Fe-chalcogenides are shown in figure 5. Twonotable features are (i) a strong increase to ∼37 K in thecritical temperature of nominal Fe1+δSe under pressure, sothat this is very much a high-temperature superconductor [46,50–53], and (ii) the only ordered magnetic phase is that nearFe1+δTe, and it has a different magnetic order [100–102] thanthe SDW-type order seen in pnictides such as LaFeAsO.
One complication is that the ‘11’ Fe-chalcogenidesgenerally form with excess iron, particularly when Te rich,and so the chemical formula should be written as, e.g. Fe1+δTe[107, 108]. The superconductivity, especially for the selenide,which has a low Tc, is very sensitive to stoichiometry [109].The excess Fe occurs roughly in the chalcogen plane, in sitesthat can be regarded as the complementary positions to thechalcogen atoms, i.e. partially occupying the corners of thetetrahedron that would be obtained by an inversion aboutthe center of the chalcogen tetrahedron (see figure 1).
Density functional calculations for Fe1+δTe with excessFe [110] showed that the excess Fe has a valence of
approximately 1+ and therefore provides electron doping,and furthermore that the excess Fe is strongly magnetic.It provides local moments that interact with the Fe in theplanes. Various experiments also show effects of excess Fe onthe magnetic and other properties. As mentioned above, thesuperconductivity of Fe1+δSe is very sensitive to stoichiometry[109]; this is also the case in other parts of the phase diagramincluding both the Te-rich side [111], and the region near themaximum Tc [112, 113].
Han and Savrasov [114] performed band structurecalculations for FeTe and noted that a nesting feature wouldarise at the wavevector of the observed double-stripe magneticstate if the electron count were 0.5 per formula unit higher,which they ascribed to the excess Fe. This explanation isalmost certainly incorrect, first of all because other studiesshowed the double stripe to be the ground state without excessFe [92, 100], and because the 8+ valence state of the excess Fe(i.e. Fe8+) assumed by Han and Savrasov is clearly unphysicaland is not supported by calculations [110, 115].
Neutron scattering studies on Fe1+δTe1−x Sex , x = 0.38showed that the interstitial Fe induces a magnetic Friedel-likeoscillation, extending over ∼50 Fe sites [116]. This indicatesthat the excess Fe introduces moments that interactsubstantially with the Fe planes. Inelastic measurements onsamples with different Fe concentrations at x = 0.3 showedchanges in the magnetic scattering correlated with a decreasein the superconducting volume fraction [117], and changesare also seen in the magnetic behavior at x = 0 [118]. Liuet al [119] reported transport, susceptibility and specific heatmeasurements for samples with different amounts of excessFe at both x = 0.4 and 0. They found both a suppression ofsuperconductivity due to excess Fe and evidence for carrierlocalization, which they interpreted as reflecting a magneticinteraction between the excess Fe and the Fe planes andleading to local magnetic order. This conclusion is supportedby annealing experiments [120]. Interestingly, first principlescalculations show that anion vacancies in FeSe, i.e. inFeSe1−x , also induce magnetism on at least eight neighboringFe atoms [121].
Besides magnetic effects, x-ray experiments for x = 0showed strong diffuse scattering indicative of strong localdistortions caused by the excess Fe [122]. We note thatthese materials show strong coupling between magnetism andstructure, as seen for example in the strong lattice distortionsupon or preceding magnetic order [2], and so these structuraldistortions may be important for understanding the interplaybetween the excess Fe and the magnetism of the Fe planes.
Another complication is the structural distortion ofsuperconducting Fe1+δSe at ∼90 K depending on composition[123, 124]. Experiments on thin films with differentorientations, which affect the structure distortion, showsensitivity of the superconductivity to the structure transition[125], although the sensitivity of superconductivity to strain(as seen, e.g., in the pressure dependence) hinders evaluationof the effect of tetragonal to orthorhombic transition. Inany case, this structure distortion lowers the symmetry fromtetragonal to orthorhombic, as does the nematic transitionin e.g. LaFeAsO, but in Fe1+δSe it is not followed by
magnetic ordering at lower temperatures, nor is the transitionitself caused by magnetic order. It is, however, accompaniedby a resistivity anomaly, although not nearly as strong asthat in, for example, LaFeAsO [123]. This tetragonal toorthorhombic transition then seems to be different in naturefrom the transition in the pnictides and at this time is poorlyunderstood.
This tetragonal to orthorhombic distortion on the Se-richside of the alloy system is also different from the distortionseen for Fe1+δTe, which is directly connected with andcoincident with the double-stripe magnetic order [102, 104].The Fe1+δSe structural transition is suppressed both byTe alloying and by pressure, both of which increase thesuperconducting Tc.
Photoemission experiments in comparison with LDAcalculations show stronger renormalization in Fe1+δSe than inthe arsenides, and in particular a larger mass enhancement,Z−1
' 3.6 [126]. The renormalization in Fe1+δSe is nearly afactor of two larger than that in doped LaFeAsO [127]. Asin the arsenides, there is a large Fe 3d spectral weight in thevicinity of the Fermi level with a clear Fermi edge. In contrastto the pnictides, substitutions on the Fe site are detrimental tothe superconductivity of the ‘11’ phases [106, 128], althoughin the case of Cu-substituted Fe1+δSe, which is electron doped[129], superconductivity can be restored under pressure [130].Density functional calculations show that Cu substitution ishighly disruptive to the electronic structure and induces localmoments around the Cu site [129].
Therefore, from the points of view of renormalizationseen in the electronic structure, sensitivity to substitutionson the Fe site and observed ordered magnetism, theFe-chalcogenide superconductors appear different from thepnictides. However, both these classes of materials havesquare lattices of Fe and similarly high Tc values.
Density functional calculations performed shortly afterthe discovery of superconductivity in Fe1+δSe showed somesimilar aspects to the electronic structure of the chalcogenidesand pnictides [48, 131], as illustrated in figures 6 and 7.In particular, the density of states shows predominant Fecharacter involving multiple orbitals near EF, with a highdensity of states. It also shows that the Se occurs nominallyas Se2−, so that the Fe is in a d6 configuration. Also, there isa dip in the density of states at this electron count, in contrastto what would be expected in a tetrahedral crystal field, wherethe d bands would split into a lower-lying manifold holdingfour electrons per Fe and an upper manifold of six. Thisresult was supported by photoemission experiments [126,131], showing that the calculated assignment of partial densityof states (Fe and Se) is in accord with measurements, althoughstronger renormalization near the Fermi energy was foundin comparison to the pnictides [126]. Also, first principlescalculations show a tendency towards the same type ofmagnetic order in FeSe as in the pnictides [48, 132, 133].Again there is substantial coupling to the lattice reflected instatic distortion and sizable shifts of phonon frequencies atleast at the level of static magnetism [132, 134].
Dynamical mean-field theory calculations, usingrealistic parameters obtained from first principles via a
Figure 6. Calculated electronic density of states and projections forLaFeAsO (top) and FeSe (bottom) following references [48, 83].
constrained random-phase approximation method, founda spectral function in qualitative accord with this strongerrenormalization in relation to the pnictides [77, 81]. Thecalculations also found a feature at higher binding energy,though still within the Fe d bands that was identified as alower Hubbard band, which is nomenclature usually appliedto correlated materials with Mott physics, as in the cuprates.The parameters obtained by the constrained random-phaseapproximation method do suggest stronger correlation than inthe pnictides −U = 4.01 eV and J = 0.91 eV as compared toU = 2.7 eV and J = 0.8 eV for LaFeAsO [77, 135].
This result is consistent with the larger renormalizationsin the chalcogenide. However, it is to be noted that theorigin of the lower band is still with the Hund’s couplingrather than the Coulomb U , and so this may be viewed asa redistribution of spectral weight inside the d-bands dueto Hund’s (magnetic) coupling rather than a conventionalHubbard band. Unlike a Mott system, features, which arequalitatively similar to this spectral weight at higher bindingenergies in the d-bands, do occur in standard DFT calculationswhen magnetism is included [92]. However, this comes at theexpense of sizable stable ordered moments that do not exist in
Figure 7. Calculated Fermi surfaces of Ba(Fe, Co)2As2 and FeSefollowing [39, 48].
experiment. Therefore the issue may relate to understandingwhy and how the large moments in DFT calculations, whichcould be regarded for this purpose as a static mean field likemethod, are suppressed. As mentioned above, one possibilityis that the suppression of the magnetic moments is dueto quantum fluctuations as is the case in materials near amagnetic quantum critical point [89, 90].
Band structure calculations show disconnected Fermisurface sections, with hole sections around the zone centerand electron sections at the zone corner, very similar to thepnictides. It should be noted that recent ARPES experimentssee the electron sheets but not the predicted zone centerhole sheets [136]. However, this may be an artifact ofthe measurements; such an electronic structure, if correct,would strongly violate Luttinger’s theorem implying that thereare unseen compensating hole sections. The disconnectedsheets observed in the pnictides are essential for the s+−
superconducting state.In any case, the calculated electronic structure suggests
similar magnetic behavior to the pnictides as was noted earlyon [48], but experiment did not see the SDW order andinstead revealed a different order in Fe1+δTe. This is theso-called ‘double-stripe’ order, as shown in figure 3. Whilethis different order is also given by standard density functionalcalculations [92, 100], it does raise the question of whether thesuperconductivity could really be similar between these twodifferent families within a spin-fluctuation framework giventhat the observed magnetic order is not the same.
Many experiments showed a similar type ofsuperconductivity in the pnictides and the chalcogenides.Early muon spin rotation (µSR) measurements indicatedgapless s-wave superconductivity, with two gaps [137]. Also,similar to the pnictides, NMR experiments on Fe1+δSe donot detect a Hebel–Slichter peak, indicating unconventionalsuperconductivity, and furthermore they show a connectionbetween antiferromagnetic fluctuations and superconductivityas the spin fluctuations are enhanced approaching Tc, and alsounder pressure, where Tc increases [138]. More directly, thesuperconducting resonance characteristic of a sign change inthe order parameter at (π/a, π/a) was observed in severalinelastic neutron scattering experiments on various samplesat different alloy compositions [139–143].
Neutron scattering studies of the evolution of the spinexcitations as a function of composition in Fe1+δTe1−x Sex
showed that even though Fe1+δTe has a different magneticorder than the pnictides, when this order is suppressedby Se alloying, magnetic fluctuations characteristic ofproximity to the SDW-type order emerge showing qualitativesimilarity to the pnictides [117, 139, 144, 145]. Bulksuperconductivity was associated with the appearance ofthe SDW-type magnetic scattering in the experiments, whilethe competing double-stripe-type magnetism was found tobe antagonistic to superconductivity [145]. Additionally,scattering was observed at wavevectors that correspondto nearest-neighbor magnetism as in the cuprates [144],suggesting that besides the double stripe, nearest-neighborantiferromagnetism is a competing phase. This was alsosuggested by DFT calculations for TlFe2Se2 [146], which isa strongly antiferromagnetic compound [147].
Thus it seems that the superconductivity of the ‘11’-phasechalcogenides is similar both in nature (i.e. order parameter)and in relation to magnetism to that of the pnictides in spiteof the fact that SDW magnetic order is not seen in thechalcogenides.
4. Some open questions
The superconductivity of the ‘11’ Fe-chalcogenides is clearlyclosely related to that of the pnictides, but as discussed, showssome key differences. These differences may be particularlyuseful for understanding Fe-based superconductivity, andperhaps more generally high-temperature superconductivity.In this regard, it is important that large high-qualitysingle crystals can be made, especially of the Se–Te alloy[120, 148, 149].
One set of important questions about the Fe-basedsuperconductors revolve around the nature of the magnetismin these materials. As discussed above, both the chalcogenidesand pnictides show strongly reduced magnetism in orderedform relative to density functional calculations, and both showevidence of strong spin fluctuations. However, the drivingforce for the fluctuations as opposed to ordered magnetismhas not yet been properly clarified. Furthermore, while themagnetism and spin fluctuations clearly couple to electronsnear the Fermi surface, as seen for example in resistivity, theextent of the coupling and the role of the Fermi surface inthe magnetism remain controversial. One aspect that couldbe important is competition of different magnetic states.In contrast to the pnictides, where SDW order dominatesthe phase diagrams, phases such as the double stripe areseen in the chalcogenides, as is nearness to nearest-neighborantiferromagnetism; understanding these nearby phases, thefluctuations associated with them and their role in competingwith the SDW type magnetism may be very helpful.
Another set of questions revolves around the strength,nature and role of electron correlations. No Mott insulatingstate that is plausibly close to the Fe-based superconductorshas been identified, and as discussed above it is also not clearwhether electronic features characteristic of Mott–Hubbardphysics (e.g. Hubbard bands) exist in these materials.However, electron correlations are important as seen forexample in the renormalizations of the bands. These
correlations or at least their manifestations are different fromthose in cuprates. The multiorbital nature of the electronicstructure may be a critically important aspect of the physicsfor understanding these differences. The multiple orbitalsavailable at low energy can open channels for inter-orbitalcharge fluctuations that work against the Mott state. Sucheffects were approximately quantified by Gunnarsson et al[150], who arrived at an approximate factor 1/
√N , to be
multiplied by U/W when assessing the proximity of amaterial to a Mott transition (U is the effective Coulombrepulsion, W is the band width and N is the number oforbitals). In any case, as discussed above, the chalcogenidesappear to be more strongly correlated than the pnictidesand may therefore offer a more convenient window into thecorrelated electron physics of the Fe-based superconductors.
5. Summary and outlook
This article provided a brief overview of some aspectsof the ‘11’-type Fe-based superconductors, which willhopefully stimulate research addressing the many outstandingquestions about the physics of these materials. As notedabove chalcogens are very different from pnictogens from achemical point of view. The fact that the maximum Tc of thechalcogenides is not much lower than that of the pnictidesprovides opportunities for better understanding Fe-basedsuperconductivity as well as hope that additional materialsmay be found. Thus we may anticipate discoveries of newfamilies of high-temperature superconductors related to theknown Fe-based superconductors.
Acknowledgments
I am grateful for helpful discussions with I I Mazin aswell as many present and former colleagues at Oak RidgeNational Laboratory. This work was supported by theDepartment of Energy, Office of Science, Materials Sciencesand Engineering Division.
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