prev

next

out of 6

Published on

04-Jul-2016View

222Download

5

Embed Size (px)

Transcript

Physics Letters A 303 (2002) 223228www.elsevier.com/locate/pla

Modeling of the thermopower of electron-doped manganites

Raymond Frsard a,, Sylvie Hbert a, Antoine Maignan a, Li Pi a, Jiri Hejtmanek ba Laboratoire Crismat, UMR 6508 du Centre National de la Recherche Scientifique et de lInstitut des Sciences de la Matire et

du Rayonnement, 6, Bld. du Marchal Juin, 14050 Caen Cedex, Franceb Institute of Physics, Cukrovarnicka 10, 16200 Prague 6, Czech Republic

Received 19 August 2002; accepted 4 September 2002

Communicated by J. Flouquet

Abstract

The temperature dependence of the thermopower of electron-doped manganites is analyzed in terms of a microscopicHamiltonian. We show that the experimental data can be understood for a series of materials over a broad temperature rangeusing two parameters, namely the effective bandwidth for the quasiparticles, and the high temperature limit of the appropriatecorrelation function. These parameters are material-specific, and vary modestly over the series of considered materials. 2002 Elsevier Science B.V. All rights reserved.

PACS: 72.20.Pa; 71.27.+a; 71.30.+h; 74.62.Dh

1. Introduction

Materials with thermoelectrical applications keep on receiving a lot of attention. New developments in this areaare numerous, primarily following from the discovery of new skutterudites and oxides. In particular spectaculardevelopments have been achieved thanks to the discovery of large thermopower values in layered cobaltites [1] andlow electron-doped titanates [2] and manganites [3].

When it comes to interpret experimental data of thermopower out of a microscopic Hamiltonian, severe difficultytakes place, in particular for the above systems. Indeed the rather involved band structure, together with the strongcorrelation effects following from the large Coulomb and electronphonon interactions look pretty discouragingwhen envisaging evaluating the correlation functions entering the expression for the thermopower (S). Neverthelessour understanding of the temperature dependence of S improved in recent years, in particular in the context of thecuprates [4,5]. In the latter work it was shown for a large domain of density and temperature, that the ratio ofthe energy currentparticle current correlation function to the particle current autocorrelation function, is mostlytemperature independent, implying that the T -dependence of S results from the one of the chemical potential.More recently, Oudovenko and Kotliar evaluated this ratio in the high temperature regime for a two-band Hubbard

* Corresponding author.E-mail address: raymond.fresard@ismra.fr (R. Frsard).

0375-9601/02/$ see front matter 2002 Elsevier Science B.V. All rights reserved.PII: S0375-9601(02)0 12 25 -2

224 R. Frsard et al. / Physics Letters A 303 (2002) 223228

model [6]. This gives us confidence that the T -dependence of S for lightly electron-doped manganites can beanalyzed in terms of a simple model. Such an attempt is done here.

2. Model for the temperature dependence of the thermopower

We now attempt at interpreting the temperature dependence of the Seebeck coefficient data for lightly electron-doped manganites in simple terms. We resort to a microscopic effective Hamiltonian for oxides which takes intoaccount the bands which are crossing the Fermi level. For the manganites they result from the hybridization of thetwo eg levels on the manganese ions with the oxygen 2p levels. The bands resulting from the manganese t2g levelsare half-filled and are therefore not expected to play an important role, since the Hunds rule coupling is strong inthese materials. They are left out in the following. At this stage the kinetic energy part of the Hamiltonian T isgiven by:

(1)T =k,

k,

d 0 2itzx sinkxa

2 2itzy sin

kya

2 2itzz sin

kza2

0 d 2itxx sinkxa

2 2itxy sin

kya

2 0

2itzx sin kxa2 2itxx sin kxa2 p 0 02itzy sin kya2 2itxy sin kya2 0 p 02itzz sin kza2 0 0 0 p

k, ,

with k, = (dz,k, , dx,k, ,px,k, ,py,k, ,pz,k, ). It is obtained by Fourier transformation of dz,i, (dx,i, ) whichis the creation operator for an electron in the 3z2 r2 (x2 y2) orbital on a manganese atom at site i , andp

,i, which is the creation operator for an electron in the ( = x, y, z) 2p orbital on a neighboring oxygen

atom pointing towards the corresponding Mn atom. In a cubic environment, the overlap integrals t are given interm of one single Slater and Koster parameter [7] pd = tzz as tzx = tzy = 12 tzz , and txx =txy =

3

2 tzz . Once these

relationships are satisfied, cubic symmetry is obeyed. Typical values for pd for transition metal oxide-basedperovskites can be found in Ref. [8]. According to their calculation they are in the range of 1 eV, while the energydifference between the manganese eg- and oxygen 2p-levels is about 2 eV [8]. Once it is recognized that oneof the five eigenvalues of the kinetic energy operator is given by p d , one can easily get the other foureigenvalues. They yield two sets of two bands, one dispersing above d , and the other one below p . None of thebands disperse over the whole Brillouin zone in a way similar to what would be obtained from an effective simplecubic model. When considering the upper two bands, one disperses as k2 at the bottom of the band, while the otherone does not disperse in the X directions. The same behavior is found at the top of the band by symmetry, withboth bands interchanged. For each of the resulting bands, the bandwidth is given by:

(2)W = 12

(2 + 24tz2z

),

even though the bands are non-degenerate. Please note that the two parameters of the model, tzz and , enter theexpression for the bandwidth. In turn the density of states also depends on both parameters. It is seen in Fig. 1that its energy dependence is much stronger for large ratio of tzz / than for smaller ones. Regardless of this ratiothe density of states remains finite at both band edges. This results from the particular dispersion relations foreg electrons. As a result, the chemical potential strongly depends on temperature, as shown in the inset of Fig. 1for several values of tzz . Since the density of states at the bottom of the band decreases with increasing tzz , thetemperature dependence of is enhanced by increasing tzz .

On top of the above discussed kinetic energy term, one expects strong interaction terms in the Hamiltonianof the system. They primarily consist of Hubbard and Hunds rule interactions between the eg and t2g electrons

R. Frsard et al. / Physics Letters A 303 (2002) 223228 225

Fig. 1. Density of states for the eg electrons, for = 2 eV, tzz = 0.2 eV (full line) and tzz = 1 eV (dashed line). Inset: temperature dependenceof the chemical potential for 6% of electrons, = 2 eV, tzz = 0.5 eV (full line), tzz = 0.33 eV (dotted line), tzz = 0.25 eV (dashed line), andtzz = 0.2 eV (dashed-dotted-dotted line).

on the manganese ions. The additional interaction terms responsible for the JahnTeller effect, which are playingan important role in the Mn3+-rich side, are not expected to play a decisive role in the Mn4+-rich side. Theirinfluence is discussed below. The Hubbard and Hunds rule interactions could be taken care of in the framework ofGutzwiller approximation [9] or in slave-boson mean-field approximation [10,11]. Nevertheless this would implydealing with a five band model, obviously a very involved task. Still, the expected output of such a calculation is,at the first place, that the one-particle bands split off into quasiparticle bands and lower and upper Hubbard bands.The quasiparticle bands are dispersing in a way that is similar to the original bands, but with a renormalized tzz .Instead of determining it from a detailed calculation, which would require introducing additional parameters suchas U and JH , we rather determine the renormalization of tzz out of the experimental data.

In order to evaluate the Seebeck coefficient, we use the standard expression [12]

(3)S = 1eT

L12

L11

where L11 is the particle current autocorrelation function, and L12 the heat currentparticle current correlationfunction. Writing the heat current operator as jQ = jE j , Eq. (3) can be cast into two terms as

(4)S = kB|e|(E0)where E0 represents the contribution following from the energy current operator jE . Our analysis assumes that thetemperature dependence of E0 can be neglected. Thus an expression for it can be derived from a particular modelusing, e.g., a high temperature expansion. This has been done in particular for a degenerate two-band Hubbardmodel [6]. Here we keep E0 as a parameter accounting for the combined result of disorder and correlation effects.In this way one may gain some insight in the origin of particular structures in the T -dependence of S. ObviouslyE0should go to (T = 0) in the low T limit. This cannot happen within the above assumption, since we determine E0from its high T limit. This implicitly sets a low temperature bound to the validity of our approach.

3. Numerical application

We now turn to the analysis of experimental data. They consists of both the A site-, and B site-, substitutedmanganite CaMnO3, in the low doping regime. In this regime, the JahnTeller effect, which is primarily connected

226 R. Frsard et al. / Physics Letters A 303 (2002) 223228

Fig. 2. Thermopower of CaMn0.94Ru0.06O3 (circles), CaMn0.94Re0.06O3 (triangles), and CaMn0.96Mo0.04O3 (diamonds). The original dataare in V/K. Here the thermopower for the Ru-substituted sample is divided by three, while 25 V/K are subtracted from the ones for theRe-substituted ones, for clarity. Theoretical calculation (rescaled as above) for (tzz = 0.42 eV and E0 = 16 meV) (full line), (tzz = 0.58 eVand E0 =27 meV) (dashed line), and (tzz = 0.60 eV and E0 =23 meV) (dashed-dotted line).

to Mn3+ ions, is not expected to play any dominant role. Let us start with the B site-substituted manganites. Hereone brings in charge carriers in this band insulator by substituting Mn4+ ions by either 6+ ions, such as Mo [13], or5+ ions such as Ru, and Re [14]. We apply our analysis to CaMn0.94Ru0.06O3, CaMn0.94Re0.06O3 (correspondingto a density of 6% of electrons in the eg bands), and CaMn0.96Mo0.04O3 (corresponding to a density of 8% ofelectrons in the eg bands). The result of this analysis is shown in Fig. 2. Manifestly the calculated S reproducesthe experimental data over a wide T -range. This range extends from room temperature down to 100 K, or evenbelow. Rather remarkably there is a fairly small scattering of the value of tzz needed to describe this set of data.It ranges from 0.42 eV for Mo, to 0.60 eV for Re, thus a factor two smaller than the predictions of the bandstructure calculation [8]. The theoretical curves are not equally sensitive to all parameters. In this T -range, hasvery little influence on S, once large enough, namely as long as the lower bands are not thermally excited. On thecontrary, the slope of S at room temperature is very sensitive to the value of tzz , and a 2% change of the latter quiteseverely affects the quality of the agreement. Therefore one can make use of the S data to accurately determinethe bandwidth of a particular compound. This also represents a clear signature of eg electrons. Indeed, carryingout the same analysis using free electron dispersion (or equivalently, for this purpose, t2g electrons) would yield avery different result since in this case the chemical potential has a far weaker temperature dependence. Generically,the thermopower has a fairly linear temperature dependence around room temperature. Here we see that even thedownturn is reproduced by our calculation as well. It is therefore not related to any particular physical process, butrather to the temperature at which E0 becomes the dominant contribution to S. Here we see that the T -dependenceof E0 becomes relevant for T < Tlow, with Tlow = 80 K for the Ru-substituted sample, 90 K (Re), and 110 K (Mo).Thus Tlow increases with decreasing values of |E0|. Remarkably, the magnetic transition taking place at 110 K inthe Ru-substituted sample only has a small influence on S, since only a small departure from the calculated curvecan be observed below 110 K. On the contrary, Tlow corresponds to the magnetic transition temperatures in Re andMo.

We now turn to A site-substituted manganites, taking as examples SmxCa1xMnO3, with x = 0.05 andx = 0.10. The results are presented in Fig. 3. For x = 0.05, the numerical result and the experimental dataagree above the Curie temperature of 110 K, where an abrupt change in the T -dependence of S takes place.For x = 0.10, the curves start to depart smoothly at 120 K, reflecting the contribution of the large ferromagneticfluctuations to E0. We note that the experimental data for x = 0.05 correspond to tzz = 0.45 eV, and the ones forx = 0.10 to tzz = 0.51 eV. Therefore increasing the charge carrier density corresponds to an increase of the effectivebandwidth. This appears to be in contradiction to what one expects when modeling the series SmxCa1xMnO3 with

R. Frsard et al. / Physics Letters A 303 (2002) 223228 227

Fig. 3. Thermopower of Sm0.05Ca0.95MnO3 (diamonds), and Sm0.10Ca0.90MnO3 (circles) compared to Eq. (4) with (tzz = 0.496 eV,E0 =22 meV and = 0.1) (full line), tzz = 0.45 eV, E0 =10 meV, and = 0.05) (dashed line).

an extended Hubbard model with fixed parameters t , U and JH , where an increase in the charge carrier densityleads to a decrease of the quasiparticle bandwidth. However, one should keep in mind that increasing the Sm contentalso induces an increase of the MnOMn angle, and therefore of the relevant overlap integrals. This effects seemsto overcompensate the reduction of the quasiparticle bandwidth due to the electronelectron interaction.

The values of E0 in the above analysis are ranging from 10 to 25 meV. They appear to be too small to beexplained with the result of Oudovenko and Kotliar [6]. Indeed, using U 3 eV, and (in their notation) n 4%,one obtains E0 120 meV. The difference, together with the scattering of the results, should be attributed todisorder effects, which may hardly be ignored in these ceramic materials.

4. Summary

In summary we have presented a model calculation of the temperature dependence of the thermopower for bothA site-, and B site-, substituted CaMnO3, in the low electron-doped regime. Based on a microscopic Hamiltonian,this calculation proved to accurately reproduce a series of experimental data over a broad temperature range. Theparameters entering the calculation are seen to be modestly varying over this series of materials. Calculations forother systems involving t2g electrons are in progress.

Acknowledgements

We are grateful to Ch. Simon, Ch. Martin, and P. Prelovek for helpful discussions.

References

[1] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12 685.[2] T. Okuda, K. Nakanishi, S. Miyasaka, Y. Tokura, Phys. Rev. B 63 (2001) 113104.[3] A. Maignan, C. Martin, F. Damay, B. Raveau, J. Hejtmanek, Phys. Rev. B 58 (1998) 2758.[4] G. Hildebrand, T.J. Hagenaars, W. Hanke, S. Grabowski, J. Sch...