Evolution of electronic structure in Eu1−xLaxFe2As2

Journal of Physics and Chemistry of Solids 72 (2011) 474–478

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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs

Evolution of electronic structure in Eu1  xLaxFe2As2 Bo Zhou a, L.X. Yang a, Fei Chen a, Min Xu a, Tao Wu b, Gang Wu b, X.H. Chen b, D.L. Feng a, a

State Key Laboratory of Surface Physics, Department of Physics, and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People’s Republic of China Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China b

a r t i c l e i n f o

a b s t r a c t

Available online 21 October 2010

We report an angle-resolved photoemission spectroscopy study of electronic structures of Eu1  xLaxFe2As2 single crystals, in which the spin density wave transition is suppressed with La doping. In the paramagnetic state, the Fermi surface maps are similar for all dopings, with chemical potential shifts corresponding to the extra electrons introduced by the La doping. In the spin density wave state, we identify electronic structure signatures that relate to the spin density wave transition. Bands around M show that the energy of the system is saved by the band shifts towards high energies, and the shifts decrease with increasing doping, in agreement with the weakened magnetic order. & 2010 Elsevier Ltd. All rights reserved.

1. Introduction Iron-based superconductors have attracted great interests as a new family of high-temperature superconductors [1,2]. The parent compounds of both cuprates and iron-based superconductors show antiferromagnetic ordered ground state, which hints the intimate relation between superconductivity and magnetism. Therefore, in order to understand the superconducting mechanism, it is imperative to investigate the magnetic properties in the iron pnictides. The ubiquitous spin density wave (SDW) in the parent compounds of iron pnictides has been studied extensively [3–11]. Numerous studies by angle-resolved photoemission spectroscopy (ARPES) have reported that the observed unconventional band splittings might be responsible for the SDW [3–5,9–11]. Therefore, it is rather intriguing to reveal the common characters with the SDW in various materials. EuFe2As2 is a parent compound of the so-called ‘‘122’’ series of iron pnictide, which shows not only an SDW transition (TS ¼188 K) due to the antiferromagnetic ordering of Fe2 + ions, but also an antiferromagnetic transition (TN ¼ 20 K) of Eu2 + ordering [12–16]. With La doping, the SDW transition temperature decreases as shown in Fig. 1, whereas the antiferromagnetic transition of Eu2 + around TN ¼20 K is preserved for all samples. In our early photoemission work of EuFe2As2 [5], the electronic structure does not show any noticeable changes across TN, implying a weak coupling between the Eu and FeAs layers, in consistency with other studies [15–17]. Therefore, here we will focus on the SDW transition,

 Corresponding author.

E-mail address: [email protected] (D.L. Feng). 0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.10.075

especially the evolution of electronic structure with La doping in Eu1  xLaxFe2As2. We report ARPES measurements in Eu1  xLaxFe2As2 single crystals. We show that La concentration alters the electronic structure in many aspects. For example, it introduces more electrons, and weakens the band folding related to the SDW, which is probably due to the introduced disorder. In the SDW state, the energy is saved by band shifts to higher binding energies, and the saved energy decreases with doping, which naturally explains the weakening of the SDW.

TS=188 K 0.8

x=0

x=0.15 ρ ab (mΩcm)

Keywords: D. Electronic structure D. Spin density wave C. Photoelectron spectroscopy

0.6

TS=143 K

TN=20 K

x=0.18 0.4

TS=110 K 0.2

0

50

100

150 T (K)

200

250

300

Fig. 1. In-plane resistivities of Eu1  xLaxFe2As2 for x¼ 0 (squares), x¼ 0.15 (circles), and x¼ 0.18 (triangles) (Ref. [14]).

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2. Experimental

3. Results and discussion

High quality Eu1  xLaxFe2As2 (x ¼0, 0.15, and 0.18) single crystals were synthesized by self-flux method, and details are described in Ref. [14]. Their stoichiometry was confirmed by energy dispersive X-ray (EDX) analysis. ARPES measurements were performed with 21.2 eV photons from a helium discharge lamp. Scienta R4000 electron analyzer is equipped in the setup. The overall energy resolution is 9 meV, and angular resolution is 0.31. The samples were cleaved in situ, and measured in ultrahigh vacuum below 4  10  11 mbar. Measurements were carefully monitored to minimize the aging effect. The high quality sample surfaces were confirmed by clear patterns of low-energy electron diffraction (LEED).

Fig. 2 shows the Fermi surface maps for different La dopings (x ¼0, 0.15, and 0.18) in the paramagnetic (PM) and SDW states. As demonstrated in Ref. [5], the Fermi contours are depicted in Fig. 2(a) for the undoped sample. The observed PM state Fermi surface consists of two hole pockets around G, an elongated electron pocket around M and some Fermi crossings which may be originated from another electron pocket around M; while the Fermi surface in the SDW state becomes quite complex due to band splitting, folding and hybridization. With La doping, the Fermi surface topology in the PM state [Figs. 2(b) and (c)] does not show obvious changes from the undoped case [Fig. 2(a)], partly because the spectra are broader most likely due to the disorder introduced by doping. In the SDW state, the most intense features in the Fermi surface are the two intense spots and two lobes of intensity around M [Fig. 2(d)], which have been observed in several ARPES studies before [5,9,10,18]. These features become blurred out and more like the PM state Fermi surface topology as La doping increases, which may be resulted from partially the disorder effect and partially the

SDW state

M

Γ

M

x=0

X

Γ

M

X

M

E-EF (meV)

PM state

high

0

-100 195K

x=0.15

X

Γ

X

α

E-EF (meV)

Γ

low A(k,EF)

Γ

Γ

X

γ α1 α2 β δ’

0

x=0 σγ δ

-100

0

-100 150K

x = 0.15

22K

x = 0.15

115K

x = 0.18

22K

x = 0.18

x=0.15 x=0.18 0.8

1.6

k// (Å-1) Fig. 2. Four-fold symmetrized Fermi surface maps of Eu1  xLaxFe2As2 integrated over an energy window of 10 meV around EF for (a) x ¼ 0, (b) x¼ 0.15, and (c) x¼ 0.18 in the PM state at 195, 150, and 115 K, respectively. (d), (e), and (f) are the corresponding SDW state data taken at 22 K. (g) MDCs along G-M are extracted from Fermi surface maps in panels d–f. Labels are explained in the text.

E-EF (meV)

M x=0

0

E-EF (meV)

x=0.18

X

Intensity (arb. units)

high

δ

22K

M

M

Γ

β

low A(k, ω)

x=0

0

-100

Γ

M

Γ

M

Fig. 3. Photoemission intensity plots of Eu1  xLaxFe2As2 along G2M divided by the resolution-convoluted Fermi–Dirac function for (a) and (b) x¼ 0, (e) and (f) x¼ 0.15, (g) and (h) x¼ 0.18. The left panels are in the PM state and the right panels are in the SDW state. (c) and (d) Band structures corresponding to panels a and b, respectively, which are taken from Ref. [5]. Labels are explained in the text.

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B. Zhou et al. / Journal of Physics and Chemistry of Solids 72 (2011) 474–478

E-EF (meV)

0

-100

Intensity (arb. units)

-200

195K

x=0

150K

EF

115K

-80meV

M

-0.5

0.5

x = 0.18

EF

EF

-80meV

-0.5

x = 0.15

-80meV

M

0.5

-0.5

M

0.5

k// (Å-1)

Intensity (arb. units)

Intensity (arb. units)

x = 0.18 x = 0.18 x = 0.15 x=0

x = 0.15

x=0

-200

-100 E-EF (meV)

0

-0.5

M

0.5 -1

k// (Å )

E-EF (meV)

0

-100

-200

22K

x=0

-0.5

M

22K

-0.5

0.5

x = 0.15

M

0.5

22K

x = 0.18

-0.5

M

0.5

k// (Å-1)

Intensity (arb. units)

high x = 0.18

M

x = 0.15 low x=0

-200

#1

Γ

X

A(k, ω)

-100

0

E-EF (meV) Fig. 4. Photoemission data near M. (a) The PM state photoemission intensity plots of Eu1  xLaxFe2As2 (x ¼0, 0.15, and 0.18) along cut 1 as marked in panel f, which have been divided by the resolution-convoluted Fermi–Dirac function, and the corresponding MDCs are shown in the second row. (b) EDCs taken at the M point in the PM state. (c) Compares the MDCs at EF in the PM state near M. The fitting results with two Lorentzian peaks are shown. (d) Photoemission intensity plots along cut 1 in the SDW state. (e) EDCs at M in the SDW state. (f) Two dimensional Brillouin zone. Labels are explained in the text.

B. Zhou et al. / Journal of Physics and Chemistry of Solids 72 (2011) 474–478

Eu1-xLaxFe2As2 (x = 0)

CaFe2As2, Ref. [11]

Eu1-xLaxFe2As2 (x = 0.15)

BaFe2As2, Ref. [10]

Eu1-xLaxFe2As2 (x= 0.18)

BaFe2As2, Ref. [18]

50

band shift (meV)

40

30

20

10

0 0

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TSDW (K) 0.7 0.6 0.5

Δk (Å-1)

suppressed SDW. The locations of the intense spots are marked by circles in Figs. 2(d)-(f), which get closer to the M point with increasing doping. This is illustrated more clearly by the momentum distribution curves (MDCs) along G-M in Fig. 2(g), where the peak positions indicated by the arrows show the locations of the spots. The separation between them is 0.62 A˚  1 (x¼0), 0.45 A˚  1 (x¼0.15), or 0.36 A˚  1 (x ¼0.18). As illustrated in Ref. [18], the temperature dependent Fermi surface maps of BaFe2As2 show the similar behavior in the SDW state for the intense spots, where the spots are closer to M at higher temperatures. Since the spots are resulted from the band folding and certain band hybridization, it is straightforward that they are strongly related to the strength of magnetic ordering in the SDW state. The photoemission data along the high symmetry direction G-M are shown in Fig. 3. Generally, they are quite similar for all dopings. The band structures of the undoped sample are illustrated in Figs. 3(c) and (d), where band splitting, folding, and hybridization all appear in the SDW state [5]. We note that du is the folding of d and the right half of b is folded from the left half, which are clearly observed in the undoped sample as indicated by white arrows in Fig. 3(b). However, these two features are much less intense when doped [Figs. 3(f) and (h)]. The weak folding is consistent with the suppressed SDW order in the doped cases. Fig. 4 presents the electronic structure near M along cut 1 indicated in Fig. 4(f). The PM state spectra are divided by the resolution-convoluted Fermi-Dirac function to reveal the band dispersion in the vicinity of the Fermi energy (EF), each of which exhibits a simple parabolic electronlike band [Fig. 4(a)]. The dispersions are marked in the corresponding MDCs as well. The energy distribution curves (EDCs) at the M point in the PM state for different dopings are stacked in Fig. 4(b), where the arrows indicate the peak positions (or the electronlike band bottom). The band bottom moves towards higher energies as the doping increases, which are located at  5075 meV (x ¼0), 55 75 meV (x ¼0.15), and  6575 meV (x¼0.18), respectively. Fig. 4(c) shows the MDCs at EF, and each MDC is fitted by two Lorentzians whose peak positions mark the Fermi crossings. The separation of the Fermi crossings increases with the doping, which is 0.16 A˚  1, 0.18 A˚  1, or 0.22 A˚  1 for x¼0, 0.15, or 0.18, respectively. We note that the separation is not linearly dependent on the doping. Nevertheless, as the doping increases, the electronlike band around M moves towards higher binding energy, since La3 + provides one more electron than Eu2 + . As shown by the detailed temperature dependence data for EuFe2As2 in Ref. [5], the electronlike band in the PM state splits in the SDW state, and shifts towards high energies. In the SDW state of the doped cases, two M-like dispersions are observed below EF as well [Fig. 4(d)]. Although not crossing EF, the residual spectral weight is observed as the two lobes of intensities around M in Fig. 2. The EDCs at M in the SDW state are stacked in Fig. 4(e). The splitting of the parabolic band gives two peaks in each EDC. However, the binding energies of the two peaks are fixed at  70 and 95 meV, respectively. Compared with the position of the single peak in the PM state [Fig. 4(b)], the amplitudes of the band shifts (in other words, the electronic energy saved in the SDW state) decrease with increasing doping. This clearly show from the electronic structure perspective that the strength of the SDW ordering is weakened with doping. The band shifts taken from Fig. 4 and Refs. [10,11,18] are summarized in Fig. 5(a), which shows an intriguing monotonic relation with the SDW temperature. Similar behavior is observed in Fig. 5(b) for the separation between the two spots around M, although samples with different cations have slightly different band structure. The common behavior in different ‘‘122’’ series suggests that the band shift at M and the separation between the two intense spots intimately relate to the magnetic ordering.

477

0.4 0.3 0.2 0.1 0 0

50

100 TSDW (K)

Fig. 5. (a) Summary of the band shift at M vs. the SDW temperature, where the solid (empty) markers indicate the shift between the band in the PM state and the split upper (lower) band in the SDW state. (b) Summary of the separation between the two intense spots around M vs. the SDW temperature. Data are taken from Figs. 2 and 4 and Refs. [10,11,18].

4. Conclusion To summarize, we have shown electronic structures of Eu1  xLaxFe2As2 measured by ARPES. In the PM state, the electronic structures are generally similar for all dopings, and the chemical potential shift is consistent with the electron carrier doping. In the SDW state, the electronic structure is reconstructed drastically. As La doping increases, the two intense spots in the Fermi surface shrink towards M and the band shifts at M decrease. We show that

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the behaviors of the separation between the two spots and the shifts at M are common in various ‘‘122’’ iron pnictides, and intimately relate to the magnetic ordering. Acknowledgments This work was supported by the NSFC, MOE, MOST (National Basic Research Program nos. 2006CB921300 and 2006CB601002), STCSM of China. References [1] Y. Kamihara, et al., J. Am. Chem. Soc. 130 (2008) 3296.

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