Comparative Study of Electronic structure of New Superconductors (Sr,Ca)Pd2As2 and related compound BaPd2As2
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Comparative Study of Electronic structure of New Superconductors(Sr,Ca)Pd As and related compound BaPd As . a I. A. Nekrasov , a,b M. V. Sadovskii a Institute for Electrophysics, Russian Academy of Sciences, Ural Branch, Amundsen str. 106, Ekaterinburg, 620016, Russia b Institute for Metal Physics, Russian Academy of Sciences, Ural Branch, S.Kovalevskoi str. 18, Ekaterinburg, 620990, Russia
This paper presents the comparative study of LDA calculated electronic structure of new isostructural toiron based systems superconductors (Sr,Ca)Pd As with T c about 1K and similar but structurally differentsystem BaPd As . Despite chemical formula looks similar to iron superconductors and even main structuralmotif is the same - layers of Fe square lattices, electronic structure of (Sr,Ca)Pd As and BaPd As differs fromFe(As,Se)-HTSC completely. All these systems have essentially three dimensional Fermi surfaces in contrastto Fe(As,Se) materials. The Fermi level is crossed by low intensive tails of Pd-4d and As-4p states. However(Sr,Ca)Pd As and BaPd As materials have rather well developed peaks of Pd-4d( x − y ) band. Thus bydoping of about 2 holes per unit cell one can increase density of states at the Fermi level by a factor about2.5. Since experimentally these compounds were found to be simple BCS superconductors the hole dopingmay considerably increase T c . LDA calculated total densities of states at the Fermi level for stoichiometricsystems perfectly agree with experimental estimates signifying rather small role of electronic correlations.PACS: 71.20.-b, 71.18.+y 74.70.-b Novel noncuprate HTSC FeAs based systems aftertheir discovery in 2008 [1] stimulated an avalanche of ex-perimental and theoretical investigations [2, 3, 4]. More-over long series of related perspective compounds wassynthesized.Electronic stucture of many FeAs systems was inves-tigated by us [5, 6, 7, 8, 9] and other groups [10, 11].Also we made some effort to investigate FeSe based su-perconductors (chalcogenides) [12, 13, 14, 15] and tocompare them with pnictides [16]. Electronic struc-tures of some related systems were studied during recentyears: APt P [17], BaFe Se [18], SrPt As [19].This work was motivated by recent detailed workof Anand et al. [20] on superconducting and normalstate properties of (Sr,Ca)Pd As and BaPd As singlecrystals. SrPd As and CaPd As systems were foundto be superconductors with T c As showed only tracesof superconductivity. These Pd systems can be consid-ered as a physically interesting end-point compounds ofA(Fe − x M x ) As with M=Cr,Mn,Co,Ni,Cu,Ru,Rh andA=Ca,Sr,Ba series with respect to maximal number ofcharge carriers in such systems. Changes of magneticand superconducting properties of A(Fe − x M x ) As se-ries are quite non trivial. Detailed overview of electronicproperties of these compounds is presented in the paper E-mail: [email protected] E-mail: [email protected] by Anand et al. [20]. However theoretical band struc-tures of SrPd As , CaPd As and BaPd As materialswere not investigated yet to our knowledge.Here we present the detailed comparison of bandstructures of SrPd As , CaPd As and BaPd As ma-terials with respect to each other and also to isovalent(Sr,Ba)Ni As compounds [21, 22, 23] and iron pnictidesand chalcogenides in general.For our band structure calculations within the localdensity approximation (LDA) we used refined crystalstructure data of Ref. [20]. SrPd As and CaPd As have tetragonal body centred crystal structure withthe space group I/ mmm the same as typical pnic-tide representative BaFe As [24] where two mirroredFeAs tetrahedra layers are contained in the elemen-tary cell [6] (see also Fig. 1 on the left side). Al-though in most cases 122-pnictide systems belong tothe space group P /nmm [9]. The lattice parame-ters are a =4.2824˚A and 4.3759˚A and c =10.088˚A and10.1671˚A for Ca and Sr systems respectively. As com-pared to Ba122 pnictide the Ca and Sr systems havelarger a and smaller c parameters. Wyckoff positionsof ions are identical to 122 systems: Ca,Sr – 2a(0,0,0);Pd – 4d(0,0.5,0.25); As – 4e (0,0,0.3763) for Ca and(0,0,0.3768) for Sr systems.Being chemically similar to Ba122 pnictide theBaPd As system has very different crystal structure.According to Ref. [20] the space group of BaPd As is P/ mmm . Ions positions are: Ba – 1a(0,0,0); Pd– 2e(0,0.5,0.5); As – 2h(0.5,0.5,0.2505). This crystal1 I. A. Nekrasov, M. V. Sadovskii
Fig. 1. Crystal structure of (Sr,Ca)Pd As (left) andBaPd As (right). Blue balls are Sr,Ba ions, green – Asand red – Pd. structure is drawn in Fig. 1 on the right side. The crys-tal structure also consists from layers. But in contrastto Ba122 and other Fe-pnictides and chalcogenides Feions here (which form square lattice) are surrounded byAs rectangles (see the right panel of Fig. 1) and nottetrahedrons (e.g. see Fig. 1, left panel).Electronic structure calculations were performedwithin the linearized muffin-tin orbitals method(LMTO) [25] with default settings. LDA calculatedband dispersions plotted along high-symmetry Brillouinzone directions are shown in Fig. 2 for SrPd As (toppanel) and BaPd As (bottom panel) on the right sides.Since band structure of CaPd As obtained by us isonly slightly different from that of SrPd As below wepresent only theoretical data for SrPd As system.For the Sr system our band dispersions are quite sim-ilar to those obtained in Ref. [23] for isovalent SrNi As compound except a little bit shifted up in energy partof As-4p states within X-P direction. This leads to aslightly simpler than in Ni case Fermi surface, whichis plotted in Fig. 4 (upper row) and described below.Around Γ we observe small hole pocket while aroundX-point we see large electronic pocket. Close to N-pointthere is another electronic pocket of the Fermi surface.Fermi level is crossed by bands containing many contri-butions of different states without any dominant orbital.It is interesting that band dispersions and Fermisurfaces of isovalent BaNi As compound reported inRefs. [21, 22] are quite different from those of SrNi As and SrPd As . Which agrees with our calculations.This is rather puzzling since BaNi As crystal struc-ture is very much similar to SrNi As and SrPd As compounds. At the same time densities of states ofBaNi As are rather close to those plotted on the rightside of upper panel of Fig. 2 for SrPd As .From SrPd As density of states one can see thatmost of the spectral weight is formed by Pd-4d and As-4p states. Pd-4d states are located between -4 and -0.5 eV (see also the upper panel of Fig. 3), As-4p statesbelong to the interval (-6;-4) eV. Significant hybridiza-tion between Pd-4d and As-4p states is also evident.Comparing this Sr system to e.g. Ba122 Fe-pnictidewe see that Pd-4d states are obviously more extendedin energy than Fe-3d. Overall shape of the SrPd As total DOS as expected is similar to that of SrNi As and to Ba122 Fe-pnictide. However the value of totalDOS at the Fermi level N ( E F )=1.93 states/eV/f.u. ismore than twice lower than for Ba122 Fe-pnictide be-cause additional electron doping moves the Fermi levelup into lower DOS region. Indirect estimates of N ( E F )from experiments gives 1.89 states/eV/f.u. [20] whichagrees well with our calculated value, signifying thatcorrelation effects are more or less not important in theSrPd As . This can be due to more extended in energyPd-4d states in contrast to Fe-3d states e.g. in Ba122Fe-pnictide.To understand the structure of Pd-4d DOS of theSrPd As we present (on the upper panel of Fig. 3)the orbitally resolved DOSes. Here we see that about0.8eV below the Fermi level there is peak formed by Pd-4d( x − y ). Thus by doping of about 2 holes per unitcell one can increase N ( E F ) more than twice. Con-sequently one can expect increase of the T c since thissystem was experimentally found to be a simple BCSlike superconductor [20].Lower panel of Fig. 2 demonstrates LDA electronicband dispersions and densities of states of BaPd As .Since its crystal structure is totally different from allother Fe pnictides and chalcogenides no wonder that itselectronic structure is different too. Around Γ and Rpoints we found large and small hole-like pockets andaround M point there are two electron-like pockets (seealso lower line of the Fig. 4). Similar to the Sr materialin case of Ba compound Fermi level is also crossed bybands containing many contributions of different stateswithout any dominant orbital contribution and finallygiving quite low DOS.Despite different band dispersions, the shape of to-tal and partial DOSes for Ba compound is somewhatsimilar to Sr ones. Pd-4d states are located between -3eV and -0.5 eV, while As-4p states are situated rightbelow Pd-4d states and continue down to -6 eV. Con-siderable admixture (hybridisation) between Pd-4d andAs-4p states is observed. The LDA calculated valueof total density of states at the Fermi level N ( E F ) forBaPd As is 2.29 states/eV/f.u. (taking into accountthe fact that there is only one f.u. in the unit cell for Bamaterial). This value agrees well with experimental es-timates of Ref. [20] giving 2.03 states/eV/f.u.. Howeverone can guess that electron-phonon coupling constant lectronic structure: (Sr,Ca)Pd As vs. BaPd As As about 0.7eVbelow the Fermi level there is peak in the Pd-4d DOSdue to the band of x − y symmetry which is clearlyseen at this energy within dispersions. Thus we also canexpect here that hole doping can lead to appearance ofsuperconductivity (its traces were actually observed ex-perimentally in Ref. [20]).In Fig. 4 we present Fermi surfaces of SrPd As (up-per row) and BaPd As (lower row) obtained withinLDA calculations. For both compounds panels (a) givegeneral view of the Fermi surfaces in corresponding firstBrillouin zone. Fermi surface of the Sr system is es-sentially three-dimensional in contrast to e.g. 122 Fe-pnictides (see Ref. [6]). The structure of the Fermi sur-face is quite complicated and consists of three sheets.On the panel (b) (upper row of Fig. 4) there is hole-likesheet in according to performed above band structureanalysis but this sheet does not cross k z = 0 plane. Onpanels (c) and (d) hole and electronic sheets are pre-sented. The latter one cross the k z = 0 plane formingthe Fermi surface presented on the panel (e).LDA calculated Fermi surface for the Ba system isquite simple with respect to the shape of different sheets.That can be seen on the panel (a) of lower row of theFig. 4. Again the Fermi surface is essentially three-dimensional and also has three sheets. In the centrethere is large hole-like Fermi surface sheet (see panel(b) in the lower line of Fig. 4) while in the corners thereare two electron-like sheets.In conclusion we performed LDA calculations of elec-tronic band structure for recently reported in Ref. [20]materials (Sr,Ca)Pd As and BaPd As related to the122 Fe-pnictide systems. In general band structure ofthe Sr system is very similar to that of isovalent ma-terial SrNi As reported elsewhere [23]. However theband structure of another isovalent system BaNi As issurprisingly quite different to the SrPd As compoundthough the crystal structure is very similar. The bandstructure of BaPd As system is very dissimilar to anyother 122 systems because of its different crystal struc-ture. It is interesting that considered in this work Sr andBa systems has one similarity within the band structure.Namely, in the DOS of both materials there is ratherintensive peak formed by Pd-4d( x − y ) states (about0.7 eV below the Fermi level). Thus doping of about 2holes per unit cell may lead to a considerable increase(by a factor 2.5) of the total DOS value at the Fermilevel N ( E F ) giving rise to T c for the Sr compound andperhaps appearing of superconductivity in the Ba com- E n e r gy , e V Γ X P N-7-6-5-4-3-2-101
SrPd As Total Pd-4d As-4p E n e r gy , e V Γ X M Γ Z R A M-7-6-5-4-3-2-101
BaPd As Total Pd-4d As-4p
Fig. 2. LDA calculated band dispersions and densitiesof states of (SrPd As (top) and BaPd As (bottom).The Fermi level is zero. pound (traces of superconductivity were observed hereexperimentally in Ref. [20]). Obtained in LDA calcu-lations values of N ( E F ) for stoichoimetric systems arein perfect agreement with experimental estimates [20]thus signifying minor role of correlation effects in thesesystems.This work is partly supported by RFBR grant 11-02-00147 and was performed within the framework of pro-grams of fundamental research of the Russian Academyof Sciences (RAS) “Quantum mesoscopic and disorderedstructures” (12-Π-2-1002). IAN acknowledges SB-UBRAS grant.
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