The non-Fermi liquid behaviour of the Heusler-type l'b2. xYi XA1
alloys was investigated with the use of ab initio and many-body methods.
Calculations have shown that the narrow d band originating from the
impurity Fe atoms is responsible for the unusual temperature dependence
of different physical properties of these materials

Borgieł, Władysław

Deniszczyk, Józef

Acta Physica Polonica B

English

Repozytorium Uniwersytetu Śląskiego RE-BUŚ

             Title: Application of the polaronic heavy fermion to the properties of the Fe2+xV1-xAl alloys  Author: Józef Deniszczyk, Władysław Borgieł  Citation style: Deniszczyk Józef, Borgieł Władysław. (2003). Application of the polaronic heavy fermion to the properties of the Fe2+xV1-xAl alloys. "Acta Physica Polonica. B" (2003, no. 2, s. 1257-1260). A PPL IC A T IO N  OF T H E  PO LA RO N IC HEAVY FER M IO N  A PPR O A C H  T O  T H E  PR O P E R T IE S  OF T H E  Fe2+XV !_XA1 ALLOYS*J .  D e n i s z c z y kInstitute of Physics and Chemistry of Metals, University of Silesia Bankowa 12, 40-007 Katowice, PolandAND W . BORGIELChelkowski Institute of Physics, Silesian University of Silesia Uniwersytecka 4, 40-007 Katowice, Poland(Received July 10, 2002)The non-Fermi liquid behaviour of the Heusler-type l'b2. xYi XA1 alloys was investigated with the use of ab initio and many-body meth­ods. Calculations have shown that the narrow d band originating from the impurity Fe atoms is responsible for the unusual temperature dependence of different physical properties of these materials.PACS numbers: 71.20.Be, 72.80.Ga, 71.10.Hf1. In tro d u ctio nExperim ental investigations [1] have shown th a t several properties ob­served in the Fe2 VAl Heusler compound and Fe2 +xV i_ xAl (x > 0) alloys resemble those of the non-magnetic narrow-gap semiconductor FeSi known as the d-electron “Kondo” insulator [2]. The most intriguing, supporting the classification of Fe2 +xV i_ xAl m aterials to the group of the 3d heavy fermion m aterials, are: the semiconductor-like behaviour of the resistivity in the param agnetic state; the large (compared with the normal metal) value of the low tem perature electronic specific heat coefficient 7  showing an up­tu rn  at tem peratures below few Kelvins and no traces of the energv-gap on the valence-band XPS spectrum . Our recent ab initio electronic structure calculations [3] have shown th a t the anti-site (AS) defects of Fe atoms at nominalv V positions of the Fe2 VAl compound (hereafter denoted as FeQ* Presented a t th e  International Conference on Strongly Correlated Electron Systems, (SCES02), Cracow, Poland, July 10-13, 2002.give rise to the narrow resonance peak in the DOS located at Fermi level ep. In the paper we present the results for the tem perature dependence of the electronic specific heat, resistivity and therm oelectric power calculated for the off-stoichiometric Fe2 VAl within the m anv-bodv approach derived by Liu [4] for heavv-electron systems.2. C a lcu la tio n s  a n d  re su ltsThe electronic structure calculations [5] have shown th a t the Fe2 VAl compound is semi-metallic and non-magnetic having the band-structure with the ~  0.5 eV wide pseudogap located symmetrically around the £p.  Our recent electronic structure calculations [3] performed with the use of the approxim ate TB-LM TO m ethod of Andersen et al., [6] and verified within the more accurate FP-LA PW  m ethod (WIEN97 code) [7] have proved th a t in the concentration range 0 <  x  < 0.5 the Fe^ defects in the non-magnetic Fe2+xV i-xA l give rise to the narrow, strongly correlated d-like band located ju st a t the Fermi level (£p). The spin-polarized calculations have shown th a t the Fe^ atoms together with the surrounding Fe atoms form the magnetic clusters (with effective moment of 3 .5 -4 //b) embedded in the non-magnetic host. Fig. l a  shows the to ta l DOS for the Fe2 .0fiV0.94 with the partial d-DOSFig. 1. (a) Total DOS for the nonmagnetic Fe2 .0eV0.94 (solid line) and partialFe^ DOS (dash line), (b) The quasiparticle DOS in the vicinity of £f at different temperatures (ij = 0.0087 eV). Inset shows the model Bloch-DOS of the c-like (very flat parabola) and d-like (sharp peak) electrons.of the Fe^ atoms (dash line) calculated by means of the TB-LM TO m ethod. The narrow band located ju st at £f is composed mainly of Fe^-d states with eg symmetry. The Fe^-f2g states form the wide band in the energy range of —4 to — leV  and the sharp structure above £p. In the whole energy range the DOS is dom inated by the contributions from the d-states of transition m etal atoms. Except the energy region near the £f the shape of the DOS is almost the same as calculated for stoichiometric Fe2 VAl [5]. In the vicinity of £f the calculated DOS structure of the off-stoichiometric Fe2 VAl resemblesth a t of the heavv-fermion (mixed valent) /-electron  systems with the Fermi level pinned at an energy where the narrow /-b a n d  forms. Based on th a t sim ilarity we guess th a t the peculiar properties of the Fe2 VAl are due to the Fe^ defects and may have the same physical origin as th a t proposed for heavv-fermion /-e lectron  systems.One of the approach used in the description of the non-Fermi liquid be­haviour of the heavv-fermion systems, based on the polaronic effects, was derived by Liu [4]. In application to our system of Fe2 VAl with Fe-AS de­fects the assum ptions of the approach can be formulated as follows. The d-eg electrons localized at Fe^ atoms (hereafter denoted as d electrons) can prop­agate and participate in the transport only via the hybridization with the states of the broad conduction band (c-electrons). W hen the d-eg electron leaves the Fe^ site the remaining hole, a ttracting  the conduction electrons, forms the polaron (the accompanying dynamical processes were discussed in details in [4,8]). The corresponding Hamiltonian reads [4]H  =  e k 'd f A 'k 'T + e^  y] d),jd"i.T — I '<•/ y] y] d h7(i):7<-'k 'T,<-k / TtC'‘ lk  k ^ ,{k a  i , a  i , a a ' k , k '+  ¿ E ( cL d- e - M + h .c .)  , (1 )i k awhere the Ucd term  stands for the d-hole-c-electron Coulomb attraction, and the V  term  is the c d hybridization. F irst two term s represent the c-electron kinetic energy and the d-electron atomic energy, respectively. The c-band quasiparticle spectrum  is determined from the poles of the c-electron Green function: oj — ek — V 2Gg{oj) =  0 with the d-electron Green functiondefined as Gflu)) =  f  Sd^ J c° ■ The ground-state form of the Sd function was assumed in the shape proposed in [4] and is shown in the inset of Fig. 1. At finite tem peratures the tem perature dependent Sd function yields the Dyson equation for the c-band quasiparticle spectrum  in the form [4]^  JO  wW £fe “ 4r ( a )ow here /3= (fcsT )"1, x =  (u /- e d)/iy  The energy r] (=  [27rF2A csc(7ra)/W ]«)determines the energy scale of the model and depends on the interaction strengths V,  Ucd (a  oc Ucd) and c-band width W. In the presented inves­tigations the c-electron Bloch-DOS was used in the form of flat parabola simulating th a t of the Fe2 VAl near the e+. The position of the atomic d-level ed =  0 was assumed and the c d hybridization V  =  0.1 eV was used. The value of the a  was taken close to  th a t used by Liu [4]. For the calcula­tions of the electronic specific heat (Cei), therm oelectric power (TEP) and resistivity we used the formulae given in [4].a—I o,/3xcosh(/3(r) +  cos(7ra)sm7TTTa —ld r .Tem peratu re [K] Tem peratu re [K]Tem peratu re [K]Fig. 2. Temperature dependence of: (a) — chemical potential; (b) specific heat coefficient 7 ; (c) — TEP; (d) — resistivity.100 200 300 400 500 600Tem peratu re [K]electronic3. C on clu sion sFig. 2 collects the tem perature variation of different calculated quanti­ties. The calculated tem perature dependences of 7 , T E P  and resistivity re­produce qualitatively those observed in the param agnetic off-stoichiometric Fc2VA1. It can be concluded th a t the narrow d-eg band due to  Fe^ AS atoms and the polaronic effects can be the origin of the unusual properties of the Fe2+xV i-xA l m aterials.REFERENCES[1] Y. Nishino, M. Kato, S. Asano, K. Soda, M. Hayasaki, U. Mizutani, Phys. Rev. Lett. 79, 1909 (1997); M. Kato, Y. Nishino, U. Mizutani, S. Asano, J.Phys.: Condens. Matter 12 , 1769 (2000). S. Nishigori, Y. Yamada, T. Ito, H. Abe, A. Matsushita, Physica P  281, 686 (2000).[2] Z. Schlesinger, Z. Fisk, H.T. Zhang, M.B. Maple, J.F. DiTusa, G. Aeppli, Phys. Rev. Lett. 71, 1748 (1993).[3] J. Deniszczyk, W. Borgiel, Acta Phys. Pol. A  98, 551 (2000).[4] S.H. Liu, Phys. Rev. B37, 3542 (1988); S.H. Liu, Physica P  240, 49 (1997).[5] D.J. Singh, I.I. Mazin, Phys. Rev. B57, 14352 (1998).[6] O.K. Andersen, O. Jepsen, Phys. Rev. Lett. 53, 2571 (1984).[7] P. Blaha, K. Schwarz, P. Sorantin, S.B. Trickey, Comput. Phys. Commun. 59, 399 (1990).[8] G.D. Mahan, Phys. Rev. 163, 612 (1967); P.W. Anderson, Phys. Rev. Lett. 18, 1049 (1967); P. Nozieres, C.T. De Dominicis, Phys. Rev. 178, 1097 (1969).

Application of the polaronic heavy fermion to the properties of the Fe2+xV1-xAl alloys

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Application of the polaronic heavy fermion to the properties of the Fe2+xV1-xAl alloys

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