Observation of metal–insulator transition in Al–Pd–Re quasicrystals
by x-ray absorption and photoemission spectroscopy
Y. Y. Lay, J. C. Jan, J. W. Chiou, H. M. Tsai, and W. F. Ponga)
Department of Physics, Tamkang University, Tamsui, Taiwan 251, Republic of China M.-H. Tsai
Department of Physics, National Sun Yat-Sen University, Kaohsiung, Taiwan 804, Republic of China T. W. Pi, J. F. Lee, C. I. Ma, and K. L. Tseng
Synchrotron Radiation Research Center, Hsinchu, Taiwan 300, Republic of China C. R. Wangb)and S. T. Lin
Department of Physics, National Cheng Kung University, Tainan, Taiwan 701, Republic of China 共Received 21 October 2002; accepted 5 February 2003兲
Using x-ray absorption and valence-band photoelectron spectroscopy 共PES兲 we investigated the electronic structures of icosahedral (i)-Al70Pd22.5Re7.5 quasicrystals 共QCs兲 with a 4.2 K/300 K
resistivity ratio, r, ranging from 8.3 to 107 obtained under various annealing conditions. Our Al K-and Pd L3-edge x-ray absorption results show that the density of states, N(E), near the Fermi level, EF, jumps to a larger value when r decreases down to below about 20.6. The valence-band PES results show that N(E) near EF is greatly reduced in i-Al70Pd22.5Re7.5QCs relative to that of the
pure metal, which confirms the existence of the pseudogap. The PES spectrum has a sharp cutoff at EF for r⫽8.3 QC, while it decreases smoothly down to zero at EF for larger r’s. The combined results suggest the occurrence of metal–insulator transition at an r between 13 and 20.6. © 2003 American Institute of Physics. 关DOI: 10.1063/1.1565183兴
Quasicrystalline alloys have received increasing interest owing to their unusual physical properties.1,2 For example, icosahedral (i) phases of Al-based quasicrystals共QCs兲 have high electrical resistivity共兲 and a negative temperature co-efficient of resistivity.3,4The stability or metastability of QCs was attributed to the Hume–Rothery effect, which causes the formation of a pseudogap in the electronic density of states, N(E), at the Fermi level (EF).3,5,6Low-temperature photo-emission studies of Stadnik et al. demonstrated that the pseudogap plays an important role in determining the stabil-ity of QCs, though it was argued not to be the major cause of the observed highin QCs.7Recent studies of i-Al–Pd– Re QCs also revealed that these QCs have the highestat 4.2 K and the evidence of a metal–insulator transition 共MIT兲 at a critical resistivity ratio r⫽共4.2 K兲/共300 K兲 obtained under some appropriate annealing condition.4,8,9Studies using soft x-ray emission and absorption spectroscopy provided further insight into the electronic structures of quasicrystalline Al–Pd–Mn10and i-Al–Pd– Re7,11,12alloys. In this study we have performed a systematic investigation of the electronic structures of both occupied and unoccupied valence states as functions of r for i-Al–Pd– Re QCs using Al K-, Pd and Re L3-edges x-ray absorption near edge structure 共XANES兲
and valence-band photoelectron spectroscopy 共PES兲 mea-surements.
XANES and PES spectra were obtained at the Synchro-tron Radiation Research Center, Hsinchu, Taiwan. For all samples, the Al K-edge XANES spectra were obtained from high-energy spherical grating monochromator beamline us-ing the fluorescence yield method, while the Pd and
Re L3-edges XANES spectra were obtained using the double crystal monochromator and wiggler-C beamlines by the sample drain current and fluorescence methods, respectively. Valence-band PES measurements were performed using the low-energy spherical grating monochromator beamline. The fabrication and characterization of the samples are described elsewhere.13
Figure 1 shows the Al K-edge XANES spectra of the various i-Al70Pd22.5Re7.5QC samples and pure Al. The
spec-tra were normalized using the incident beam intensity, I0,
keeping fixed the area under the spectra in an energy range between 1605 and 1627 eV共not fully shown in the figure兲. The leading near-edge feature was argued to be due to strong hybridization between Al p and Pd/Re d orbitals.14,15 For r⫽107, 75.6, 52, and 20.6 samples the leading feature as indicated by the position of the maximum intensity, which is marked by an arrow, in these spectra consistently shifts to lower energies when r decreases from 107 to 20.6 as shown in Fig. 1. For pure Al, r⬍1 because the electric resistivity increases with the temperature for a metal. Thus, the leading feature in the spectra of r⫽13 and 8.3 and pure Al samples also consistently show a shift to lower energy when r is decreased. There is a discontinuity in the shift of the leading feature between r⫽20.6 and r⫽13, which indicates a sudden change 共or transition兲 of the electronic property. All spectra extend up to about 30 eV above the edge in agreement with the finding of Tamura et al.16 The enlarged near edge fea-tures shown in the inset of Fig. 1 are obtained by subtracting an arctangent type background 共represented by the dotted curve兲 from the measured spectra. The inset of Fig. 1 shows that the near edge spectra, which reflect the density of unoc-cupied Al 3 p-derived states, of the QCs with r⫽13 and 20.6 have the largest and smallest intensities, respectively.
Figures 2 and 3 show the normalized Pd and Re L3-edges a兲Author to whom correspondence should be addressed; electronic mail:
b兲Present address: Institute of Physics, Academic Sinica, Taipei, Taiwan.
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x-ray absorption spectra of i-Al70Pd22.5Re7.5QCs and refer-ence Pd and Re metals. In the insets of Figs. 2 and 3 the spectra have been subtracted with an arctangent type back-ground as shown by the dotted curves to better illustrate the contributions from unoccupied Pd 4d and Re 5d derived states in QCs and pure Pd and Re metals. In contrast to the Al K-edge XANES spectra, the near edge features in the Pd L3-edge spectra of QCs and reference Pd are more
sym-metric and narrower, which suggest that these features are dominantly contributed by unoccupied Pd 4d orbitals be-cause a free Pd atom has a 4d10 configuration without 6s electrons.17 The Re L3-edge spectrum of the Re metal as
shown in Fig. 3 has two features located about 10538 and 10552 eV. The occurrence of two prominent features shows that this spectrum is an overlapping of two unoccupied Re 5d subbands split by the crystal field. Re L3-edge spectra extend
to about 30 eV similar to Al K-edge spectra, while the Pd L3-edge spectra extend only up to about 10 eV. The Pd
and Re L3-edge XANES spectra of QCs are also shifted by ⬃3.5 and ⬃0.8 eV, respectively, toward the higher energy side and the near edge features are broader than those of corresponding Pd and Re metals. The intensities of the near edge features for QCs are reduced relative to that for the pure metal for Pd, while they are enhanced for Re. The observed large Pd L3-edge shift was attributed to the large work func-tion difference between QCs and pure Pd.18The reduction of the numbers of unoccupied Pd 4d-derived states in QCs can be understood by a much larger electronegativity for Pd 共2.2兲17 than for Al共1.61兲,17 which causes a transfer of
elec-trons from Al to Pd and reduces the number of unoccupied Pd 4d states. Since Re also has a larger electronegativity 共1.9兲,17the enhancement of unoccupied Re 5d-derived states
may be due to the increase of electrostatic potential given rise by electron transfer from Al to the Re 6s-derived states. The near edge features in the Al K- and Pd and Re L3-edges spectra were integrated between 1560.3 and
1602.3 eV, 3170.7 and 3181.9 eV, and 10510 and 10566 eV, respectively, as shown in Fig. 4. For a semiconductor/ insulator with an energy gap, the increase of the
concentra-tions of electron and hole carriers is proportional to an ex-ponential function of the temperature, which greatly outweighs the increase of resistive electron-phonon interac-tions. Thus, 共300 K兲 is much less than 共4.2 K兲 and rⰇ1 if the energy gap is not too large 共if the energy gap is too large, is still very large at 300 K兲. For metals without an energy gap, the concentration of electron carriers does not increase with temperature as rapidly as that of the semiconductor/insulator, so that r is greatly reduced. Since QCs are composed of metal atoms, N(E) is not expected to have a real energy gap, so that the large r values of QCs was interpreted as the existence of a pseudogap.7,11,12 The pseudogap manifests itself as a diminishing N(E) or deple-tion of states near EF and was argued as one of the factors responsible for the anomalously low electrical conductivity of i-Al–Pd– Re QCs.3,5,6Thus, an increase of r means that a greater number of states near EF are depleted, which will be
FIG. 1. Normalized Al K-edge x-ray absorption spectra of i-Al70Pd22.5Re7.5 QCs and pure Al. The lower inset shows the enlarged near edge features after background subtraction.
FIG. 2. Normalized Pd L3-edge x-ray absorption spectra of i-Al70Pd22.5Re7.5 QCs and pure Pd. The lower inset shows the enlarged near edge features after background subtraction.
FIG. 3. Normalized Re L3-edge x-ray absorption spectra of i-Al70Pd22.5Re7.5 QCs and pure Re. The lower inset shows the enlarged near edge features after background subtraction.
2036 Appl. Phys. Lett., Vol. 82, No. 13, 31 March 2003 Layet al.
accompanied by an enhancement of unoccupied states and the peak area of the near edge feature in the XANES spec-trum will increase monotonically with the increase of r. Though Fig. 4 shows that the peak area of the Re 5d feature is nearly a constant, both the peak areas of Al 3 p and Pd 4d features increase monotonically with r for an r greater than about 20.6 just like we have discussed. In the region of r between 8.3 and 20.6, the peak areas of the Al 3 p and Pd 4d near edge features jump to significantly higher values, which suggests a significant change in the electronic structure near EF. The sudden increase of the near edge XANES feature means an increase of N(E) near and above EF. For a gap-less electronic structure near EF, this property implies an increase of N(E) near and below EF. Thus, our XANES results suggest that in the region r⬍20.6 QCs are metallic and QC has a MIT around r⫽20.6. This finding supports the low-temperature conductivity and magnetoconductivity mea-surements for i-Al70Pd22.5Re7.5QCs.9,19
Figure 5 presents the valence-band PES spectra of QCs for r⫽8.3, 20.6, and 107 with an incident photon energy of 80 eV. The maximum intensity was normalized to unity for comparison. An interesting property of these spectra is that they rise slowly from EF 共0 eV兲 down to about ⫺3 eV and then rise sharply to form the prominent feature at ⬃⫺6 eV. The PES spectra of QCs can be divided into共⫺3 eV, EF) and 共⫺14, ⫺3 eV兲 two regions, which were attributed primarily to the Re 5d- and Pd 4d-derived states, respectively.7,11,12,20 The spectra were also pointed out to contain contributions from Al 3 p and 3s – d states distributed between ⫺14 and ⫺3 eV.10 One can also expect significant contribution from
Re 6s states to the PES spectrum because an isolated Re atom has a 6s25d5 electron configuration.17The feature be-tween⫺3 eV and EF is in sharp contrast to that of the pure Re metal shown in the inset共a兲 of Fig. 5. The section of the PES spectra of QCs between⫺3.5 eV and EF has been en-larged in the inset 共b兲 of Fig. 5, which shows a markedly lower spectral intensity of the r⫽20.6 sample than those of the r⫽8.3 and 107 samples. This result is consistent with the absorption spectra shown in Fig. 4, in which Al 3 p and Pd 4d near edge features have the lowest intensities around r⫽20.6. The r⫽8.3 sample exhibits a distinctive sharp Fermi
edge cutoff, which shows that this sample is metallic. In contrast, the PES spectra of the r⫽20.6 and 107 samples decrease smoothly down to zero at EF, which is a charac-teristics of an insulator. Thus, we have observed MIT in i-Al70Pd22.5Re7.5QCs.
The author 共W.F.P.兲 wishes to acknowledge support by the National Science Council of the Republic of China under Contract No. NSC-91-2112-M-032-015.
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FIG. 4. Integrated intensities of the near edge features in the Al K- and Pd and Re L3-edge XANES spectra as functions of r for i-Al70Pd22.5Re7.5QCs.
FIG. 5. Representative valence-band PES spectra of r⫽8.3, 20.6, and 107 QC samples for an incident photon energy of 80 eV. The insets共a兲 and 共b兲 show the enlarged spectra near EF for the reference Re metal and i-Al70Pd22.5Re7.5QCs, respectively.
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