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14-A-20

BL-9A

Temperature Dependence of Fermi Surface in
Mixed-Valent EuPtP
H. Anzaia, K. Ichikia, M. Aritab, H. Namatameb, M. Taniguchib,c,M. Mitsudad, M. Umedad, H. Wadad, H. Ikedae, and K. Mimuraa,bab

Graduate School of Engineering, Osaka Prefecture University, Sakai 599-8531, Japan
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046, Japanc
Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, JapandDepartment of Physics, Graduate School of Science, Kyushu University, Fukuoka 812-8581, Japane
Department of Physics, Ritsumeikan University, Kusatsu Shiga 525-8577, Japan

Keywords: f-electron, Electronic structure, Angle-resolved photoemission spectroscopy.

The ternary rare-earth compound EuPtP shows two first-order valence transitions at T1 = 235 K and T2 = 190
K [1,2,3], and the mean Eu valence changes from a magnetic divalent 4f 7 state to a non-magnetic trivalent 4f 6state with decreasing temperature. These two valence transitions cause an anomalous enhancement of the c-axis
resistivity between two valence transitions (T2 < T < T1), and the resistivity approaches the limit of metallicconduction. In general, the conductivity of a free electron meal at low temperature is proportional to the densityof conduction electrons near Fermi energy (EF) [4]. Therefore, a comprehensive understanding of topology ofFermi surface (FS) and its volume would provide important insight into the origin of the mechanism for thevalence transition in EuPtP.
Angle-resolved photoemission spectroscopy (ARPES) has played a leading role in the investigating theelectric state near EF. However, the study of the valence transition has been hindered, because only a few Eubased compounds have good cleavage plane, which is required for the ARPES measurement. While a recent
ARPES study for mixed-valent EuNi2P2 has revealed that the Ni 3d bands indeed cross the EF, the FS topology
has still been unclear [5].
Here, we report the high-resolution ARPES study of temperature dependence of the FS in EuPtP for the firsttime. The ARPES measurements were performed at BL-9A of Hiroshima Synchrotron Radiation Center, where
the two different linear polarizations (s- and p-polarized light) were achieved from APPLE II type undulator.
Total instrumental energy and momentum resolutions were set at 10 meV and 0.008 Å−1, respectively. The
samples were cleaved in situ and kept under ultrahigh vacuum better than 5 x 10-11 Torr.
Figures 1(a) and 1(b) show the ARPES spectra measured at T = 260 K (above T1) along the -M direction (cut#1) and off the direction (cut #2), respectively. The Eu 4f6 multiplets are observed as a featureless continuum inthe region from -0.5 to -1.0 eV due to finite resolution effects. Near EF, three dispersive hole-like bands, , ,and , can be identified. They mainly originate from Pt 5d orbitals and overlap the multiplet terms. As shownby energy distribution curves (EDCs) in Fig. 1(d) near K point, the  and  bands clearly cross EF. Therefore,
there are two hole-type FSs around K point, and one hole-type FS around  point, as shown in Fig. 1(c). The 
FS is nearly circular with enclosed area of ~ 11% of the Brillouin zone area. The  and  FSs are triangle-likeshape with an area of ~ 4% and ~ 23%, respectively.
By comparing the electric structure above and below T1, downward band shifts could be identified for the and the  bands. As shown in Fig. 1(g), the  and the  bands moves away from EF, and the top of the  band isat about -0.2 eV below EF. There are now one hole-like  FS around  point and one triangle-like  FS. The
estimated area of the  and the  FSs with respect to the Brillouin zone is ~11% and ~18%, respectively. Theshrinking FS implies that the Pt 5d electron is disappearing from the Fermi volume, and as a result, electricalresistivity may increase between the two-valence transitions.

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Figure 1 Electric structure of EuPtP taken with s polarization and photon energy of h = 34 eV. (a)(b) ARPES intensity plots
as indicated the black arrows in panel (c). Green dashed lines in (a) and (b) are guide to the eye. (c) Fermi surface mapping in
the Brillouin zone integrated over 0 < || < 300 meV. (d) Energy distribution curves around K point for the data in panel (b).
Data were taken at T = 260 K (aboveT1). (f), (g), (h), and (i) are the same as in panel (a), (b), (c), (d) and (e), respectively, buttaken at T = 220 K (T2 < T < T1). (e)(j) The calculated Fermi surface above T1 and within T2 < T < T1, respectively.

In Figs. 1(e) and 1(j), we show FSs predicted by the first-principle calculation. At first glance, the bands do
not show much kz dispersion, because the EuPtP is quasi-two dimensional system. Around the K point, theshrinking FS with the valence transition is roughly reproduced by the calculations. However, one finds that thenumber and topology of the calculated FSs around  point is different from that of the measured one. We notethat in order to obtain accurate FS topology, it is crucial to compare the data taken with p- and s-polarized light
following the dipole selection rule.
In summary, we have revealed the temperature dependence of the FS in EuPtP from the high-resolution
ARPES for the first time. The experimentally determined FS above T1 consists of three types of hole pockets,
one centered at the  point, two around the K point. The area of FS shrinks due to the downward energy shift ofthe bands, resulting in the reduction of the Fermi volume which is likely responsible for the anomalousenhancement of c-axis resistivity.
References[1] N. Lossau et al., Z. Phys. B 74, 227 (1989).
[2] N. Lossau et al., Z. Phys. B 77, 393 (1989).
[3] G. Michels et al., Z. Phys. B 98, 75 (1995).
[4] C. Kittel, Solid State Physics (John Wiley & Sons, New York, 1996).
[5] S. Danzenbächer et al., Phys. Rev. Lett. 102, 026403 (2009).

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