We report a photoelectron spectroscopic study of the valence bands of double hexagonal-close-packed (dhcp) α-La(0001) films epitaxially grown on W(110) at room temperature. The La 5 d photoemission cross section in the photon energy region from 20 eV to 130 eV was obtained and the valence-band structure of α-La was determined. Except for 4 f-related structures, the valence-band structures of dhcp α-La and dhcp β-Ce were found to resemble each other. From the band structure, the crystal structure of the La film was confirmed. No evidence for the existence of a 5 d-like surface state near the Fermi energy at the point of the surface Brillouin zone was obtained and a 6s band bottom was identified.
The unusual physical properties of the various phases of metallic La have been the subject of many experimental and theoretical investigations [
The ground-state electronic configuration is [Xe]4f 05d16s2 for La atom and can be denoted as [Xe]4f 0(5d6s)3 for La metal considering possible hybridization. Compared with the isoelectronic elements Y and Sc, which are not superconducting under atmospheric pressure, La has a high superconducting transition temperature TC of 4.88 K (dhcp α) and 6.05 K (fcc β). It is worth noting that the physical properties of La are quite unusual in the β phase which is metastable below ~609 K at atmospheric pressure, rather than in the stable α phase. Under pressure, TC for the β phase rises sharply from ~6 K at ambient pressure and saturates at a value of ~13 K around 2 × 107 Pa. This rapid rise of TC with pressure for fcc β-La is the most dramatic among all the elements. In addition, the temperature dependence of the thermal-expansion coefficient is quite anomalous for fcc β-La: it becomes negative at ~40 K, and reaches its largest negative value at ~18 K.
These remarkable properties have led to speculations and suggestions about their electronic origin that a mechanism involving 4f electrons is responsible and the electronic wave functions at the Fermi level contain a significant admixture of 4f character. However, bremsstrahlung-isochromat [
Understanding the properties of La in relation to the electronic structure is, of course, important. Furthermore, in addition to its intrinsic interest, La is of importance as a reference system for the chemically similar 4f metals. As the first member of the rare-earth series of elements, the properties of La are frequently compared with those of other rare earths to help interpret properties be associated with the occupied 4f levels. The valence-band photoemission of Ce exhibits two peaks attributed to 4f emission. To understand better the electronic properties of Ce, it is useful to study the neighboring element La with no 4f electrons. Several photoemission experiments have been done to determine the valence-band electronic structure of La [
In this paper, we report the results of angle-resolved (ARPES) and resonant valence-band photoemission of epitaxial thin films of La grown on W(110) using synchrotron radiation. Photon-energy (hν) dependence of the intensities of the photoemission peaks is measured to characterize the energy bands of La and the 5d6s valence- band structures of La(0001) film are determined.
The experiments were performed at the beamline BL-3B of the Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK). ARPES spectra were measured at room temperature using a hemispherical electron analyzer with an acceptance of ±1˚. Total instrumental energy resolution was 50 - 100 meV, depending on the photon energy (hν) in the range of 20 - 130 eV. Each series of spectra was normalized to the relative flux of incident photons. The light incidence angle from the surface normal was fixed to be 45˚.
The clean W(110) surface, having sharp (1 × 1) low-energy electron diffraction (LEED) patterns with low backgrounds, was prepared by repeated heating it to ~2300˚C in ultrahigh vacuum. The amounts of impurities were below the detection limit of Auger electron spectroscopy (AES).
La (purity 99.99%) was deposited in situ on the W(110) surface by electron-beam evaporation, with the substrate held at room temperature, after a long outgassing of the La source. The base pressure in the experimental chamber was 1.3 × 10−8 Pa, rising to 4.2 × 10−8 Pa during deposition. No traces of surface impurities including oxygen and carbon contaminations were found by AES. The films (up to 15 monolayers) grown under these conditions showed sharp hexagonal LEED patterns corresponding to the formation of well-ordered dhcp αLa(0001) or fcc β-La(111) surfaces. The LEED patterns could not be further improved by sample annealing at 370 - 570 K. The interplanar spacing of dhcp La(0001) (3.04 Å) is almost equal to that of fcc La(111) (3.06 Å), and therefore the distinguishing between dhcp La and fcc La phases is difficult in the LEED experiment. As stated above, the possibility of coexisting of fcc β-La in a metastable form cannot be denied, but our ARPES band dispersion data are consistent with the dhcp structure rather than the fcc structure. Therefore, throughout this paper, only the stable dhcp α-phase below 609 K is referred. The amounts of deposits were determined from the Auger-peak-intensity ratio ILa(78eV)/IW(169eV), as described previously [
To obtain information about the character of peaks A and B, we examine the hν dependence of ARPES spectrum of La.
As seen in
bottoms at EB ~ 0.85 eV. Thereafter, band A2 disperses upward and bands A1 and A2 are merged into one at k// ~ 1.5 Å−1. Band B disperses upward as k// increases up to ~0.35 Å−1 and then is split into two bands: upper band B1 and lower band B2. Thereafter, band B1 still disperses upward until ~ 0.5 Å−1 and then downward: bands B1 and A2 unite to one labeled A2 + B1 between ~0.5 and ~1.35 Å−1. Band B2 disperses downward to reach the rather flat bottom at EB ~ 1.27 eV between ~0.6 and ~1.3 Å−1. Thus, the bands B1 and B2 show very complicated dis-
persions, up and down. Peak C is weak in intensity (see
Here, we note that the band structure is symmetrical with respect to the point of k// = 0.94 Å−1. If this turning k//-point is M ( _ ) in the fcc(111) SBZ, the corresponding lattice constant is estimated to be 5.46 Å since k Γ ( _ ) M ( _ ) = 2(2/3)1/2π/a. This a value is larger than that for fcc β-La (afcc = 5.3 Å) by 3.0%. If this turning k//-point is M ( _ ) in the dhcp(0001) SBZ, the corresponding lattice constant is estimated to be 3.86 Å since k Γ ( _ ) M ( _ ) = 2(1/3)1/2π/a. This a value is larger than that for dhcp α-La (adhcp = 3.77 Å) by 2.4%. This comparison may indicate that the dhcp phase is slightly favorable, but the difference between 2.4 and 3.0 is too small to distinguish definitely between fcc La and dhcp La phases. Looking on fcc(111) as dhcp(0001), afcc/adhcp = . Note that 5.3 Å/3.77 Å = 1.41. Therefore, the lattice-constant consideration alone cannot determine the crystal structure of the present La film.
It is the electronic energy dispersion relation E(k) which determines the phase of La film. A great number of band-structure calculations of La have been reported so far, but most of them were for fcc β-phase, and for dhcp α-phase one and only one has been reported by Jarlborg et al. [
In the present case, as stated above, k^ is not a good quantum number, and it would be expected that k//-re- solved structures in the one-dimensional density of bulk states (k//-resolved DOS) are observed by tuning θe. Singularities in the density of states (i.e., high occupied density of states) are prominent in the k//-resolved DOS spectrum and therefore the k//-resolved DOS spectrum may reflect, not rigorously but approximately, the E(k//) dispersion [
Γ-X, and L-U can explain experimental band A2 + B1, but are inconsistent with other bands. More important, in sharp contrast to observations, the bulk bands of fcc β-La are not symmetrical about M ( _ ) .
In the case of dhcp α phase, the ( 1 ( _ ) 2 1 ( _ ) 0) mirror plane, defined by the (0001) surface normal and the [10 1 ( _ ) 0] ( Γ ( _ ) - M ( _ ) ) direction, contains the Γ-M and A-L axes, which are parallel to [10 1 ( _ ) 0] and furthermore have the same one-dimensional periodicity as the Γ ( _ ) - M ( _ ) axis. That is, the bulk bands along Γ-M and A-L are symmetrical about M ( _ ) in agreement with the observations. Comparing the experimental band structure (
The band structure calculation for dhcp La predicts a 6s-like band along Γ-M (from EB ~ 2 eV at M to ~ 3.4 eV at Γ) [
Rare earth elements are characterized by their partially filled 4f-shell. In the case of Ce with the ground-state electronic configuration of [Xe]4f1(5d6s)3, its valence-band photoelectron spectrum reveals a characteristic double-peaked structure: one near EF and one at EB ~ 2 eV [
Finally, we want to examine band A and the possibility of its relation to a surface state. Recent development in valence-band photoemission studies of rare-earth metals revealed that a d-like surface state exists just below EF around the center ( Γ ( _ ) ) of the SBZ for the close-packed surfaces of almost all rare-earth metals including La [
The electronic structure of well-ordered and atomically clean dhcp α-La(0001) epitaxially grown on a W(110) surface has been studied by means of photoelectron spectroscopy. The photoemission cross sections of the La 5d states were clarified in the hν region from 20 eV to 130 eV, confirming character of the photoemission peaks. The energy-band structure of the dhcp α-La(0001) film was determined along the [10 1 ( _ ) 0] ( Γ ( _ ) - M ( _ ) ) azimuth direction and was found to be consistent with the theoretical calculation [
We are pleased to thank the staff of Photon Factory for their excellent support. This work has been performed under the approval of the Photon Factory Program Advisory Committee (Grant Nos. 2000G168, 2002G180, 2004G187, 2006G220, and 2008G102), and partly supported by JSPS KAKENHI Grant Number 15K04618 and Research Project from Graduate School of Science and Technology, Hirosaki University.
YoshiharuEnta,OsamuMorimoto,HirooKato,YasuoSakisaka, (2016) Angle-Resolved and Resonant Photoemission Study of the Valence Bands of α-La(0001) on W(110). World Journal of Condensed Matter Physics,06,17-26. doi: 10.4236/wjcmp.2016.61003