- Email: [email protected]
Switching between coherent and incoherent electronic states
Journal of Physics and Chemistry of Solids 67 (2006) 254–258 www.elsevier.com/locate/jpcs
Switching between coherent and incoherent electronic states Alexander Menzel a,*, Zhenrong Zhang a, Mariana Minca a, Erminald Bertel a, Josef Redinger b, Rinaldo Zucca b a
b
Institute f Physical Chemistry, University of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria Center for Computational Materials Science, Vienna University of Technology, Getreidemarkt 9/134, A-6010 Vienna, Austria
Abstract A quasi-one-dimensional surface resonance on the Pt(110)-(1!2) missing-row reconstructed surface is investigated by angle-resolved photoemission for different hydrogen coverages. On the H covered surface the one-dimensional (1D) electronic states located on the close-packed atom rows are shown to be phase-coherent resulting in a pronounced quasi-particle (QP) peak at EF. Desorption of hydrogen destroys the coherence, thus quenching the QP peak. The intensity is—partially inelastically—redistributed into a broad range of k space. q 2006 Elsevier Ltd. All rights reserved.
Dimensional reduction introduces strong correlation even in materials which are uncorrelated in the extended bulk. Hence correlation is an important issue in all nanostructured materials. One of the most powerful experimental methods to probe the electronic structure is angle-resolved ultraviolet photoemission spectroscopy (ARUPS). However, the extraction of the relevant information from ARUPS of correlated systems is non-trivial. In correlated systems any excitation is associated with a strong many-body response. Accordingly, energy and momentum conservation loses its significance for correlated D systems toEthe extent to which the quasi-particle ð N K1jckN ð weight k; approaches zero. We show that on the highly anisotropic Pt(110) surface the ARUPS spectra are modified by correlation effects, present an analysis of the energy dependent ARUPS intensity distribution in k space and demonstrate how the correlation strength is changed by hydrogen. On metal surfaces a reduction of electronic states from 2D to 1D can be achieved by reducing the lateral coupling of surface states in one direction (which we call the y direction). The lateral coupling ty can be engineered in a variety of ways: by creating anisotropic disorder [1], uniaxial stress domains [2–4], or step arrays [5] on a vicinal surface. On the other hand, when ty is already small, a dimensional transition can also take place as a function of temperature, if kT approaches ty. In the present study we investigate the effect of a H induced dimensional transition on a Pt(110)-(1!2) missing-row * Corresponding author. Tel.: C512 507 5099; fax: C512 507 2925. E-mail address: [email protected] (A. Menzel).
0022-3697/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.10.140
reconstructed surface. This surface is strongly anisotropic as the top layer is organized into close-packed atom rows with a ˚ , whereas the row-to-row nearest-neighbour distance of 2.77 A ˚ distance amounts to 7.84 A. Of course, as Pt is a metal, one expects a strong electronic through-substrate coupling of the topmost atom rows. On the other hand, if d derived Tamm surface states exist, their through-substrate coupling will be small provided the energy separation from the parental bulk d bands is sufficiently large. The experiments have been carried out in a UHV chamber with a base pressure of 8!10K11 mbar. The main residual gas component is hydrogen. Cleaning of the Pt(110) crystal was performed by standard procedures, but in addition cycles of low-temperature oxygen adsorption followed by thermal desorption were carried out as described in Ref. [6]. The photon source consisted of a high-intensity plasma source at the entrance of a toroidal monochromator which yields a high degree of polarization (w90%). The spectra were recorded with HeI photons (21.22 eV) unless otherwise stated. The analyser was mounted on a two-axis goniometer. For the spectra shown here, its resolution was set to 60 meV, the angular resolution was G0.68. The chamber was also equipped with a low-energy electron diffraction (LEED) unit, which was used for structure determination by dynamical (I,V-) LEED measurements. Coverages are given with respect to the number of surface atoms in the Pt(110)-(1!1) surface. The first step is to locate a surface state or resonance in k space. Fig. 1a shows the photoemission intensity distribution along the G Xline, i.e. parallel to the close-packed atom rows, as shown in the sketch of the surface Brillouin zone (SBZ) in Fig. 1c. At X we find a band reaching a maximum just below EF. In Fig. 2b we follow the dispersion of this band along X S.
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along the SBZ boundary is a surface state band (or, more precisely, a surface resonance), which strongly samples the additional Fourier component of the surface potential stemming from the (1!2) missing row reconstruction. Additional evidence for the surface resonance character is the constant binding energy of the X quasi-particle peak if measured with photon energies between 16 and 70 eV. The orbital character of the topmost band at X can be deduced from experimental and theoretical considerations. The quasi-particle feature at X is intense if excited with photons polarized parallel to the close-packed rows, but strongly suppressed if the polarisation is perpendicular to the rows. Hence it belongs to the totally symmetric A1 representation of the C2v point group. Inspection of the bulk band structure shows that at X the band is close to the X5 and the X2 bulk bands, which both yield A1 representations, if projected onto the surface. As this analysis neglects the effect of spin– orbit interaction, we have also carried out a fully relativistic LDA calculation (including spin–orbit interaction) for a free-standing chain of Pt atoms, employing
12000
_
(a)
T= 170 K T= 310 K
X 6000
0 –1.5
The band disperses upwards through the Fermi level and In order to determine the comes down again at S. dispersion more quantitatively, we also measured the ˚ K1, where the band has sufficiently dispersion at kx Z1.3 A dropped below EF, so that the entire ky dispersion can be mapped (inset in Fig. 1b). From these measurements we find a bandwidth of Ww180 meV perpendicular to the close-packed rows, whereas the kx dispersion is more of the order of eV (it is difficult to specify a total bandwidth in that direction due to intervening bands at higher binding energies). In any case, the band has a saddle point just below EF at X and a strongly anisotropic dispersion. Furthermore, it is symmetric around a point halfway between X and Swith almost equivalent intensity at both principal points of the SBZ. This proves that the band
–1.0
–0.5
0.0
0.5
Binding Energy (eV) 1200
(b)
1100 1000
Intensity (cps)
Fig. 1. (a) Grey plot of the ARUPS intensity distribution (TZ120 K) along G X. The red diamond marks the position of the X point, the other colour symbols refer to the calculated bandstructure of a free-standing Pt atom chain. Note the (b) The ARUPS intensity distribution (TZ120 K) along intense emission at X. Red diamond: X; dashed line: ky dispersion of the band green rectangle: S; X S. Inset: dispersion of the same band giving rise to the quasi-particle peak at X. (c) The surface Brillouin zone with the principal measured slightly beyond X. points and the k space position of the dispersion measured in the inset of (b).
900 800 700 600 500 400 200
250
300
350
Temperature (K) Fig. 2. (a) The increase of the quasi-particle peak intensity at X as a function of H coverage. The black line is the spectrum measured at 310 K for a hydrogenfree surface. (b) Blue symbols: Decrease of the quasi-particle peak intensity from a surface with QHZ0.25 ml (b2 state saturated) as the temperature is ramped (0.2 K sK1) to 330 K. The abrupt quenching of the peak intensity occurs at the onset of the H desorption from the b2 state. Red line: Temperature programmed desorption of H from the b2 state (3 K sK1).
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the all-electron full-potential linearized augmented plane wave code FLAIR [7]. In FLAIR, core states are treated fully relativistically and in all iterations spin–orbit interaction is included for the valence states in a second-variation step. No shape approximation is assumed for the potential or wavefunc˚ corresponds tions. The distance between the Pt atoms 2.765 A ˚ , close to the to a bulk LDA minimum lattice constant of 3.91 A experimental value. In Fig. 1a the calculated bandstructure is imposed onto the photoemission intensity distribution pattern. Surprisingly, the calculated and observed bands agree qualitatively quite well (with exception of the band at w1 eV binding energy), although the calculation describes only a free-standing chain. In agreement with the symmetry arguments given above, the topmost band at X is correlated with a totally symmetric d derived state. The intensity of the quasi-particle state at X shows a strong dependence on H coverage. This is illustrated in Fig. 2: Fig. 2a shows the variation of the peak as the H coverage is gradually increased at 170 K. The coverage at the highest intensity shown was determined by temperature programmed desorption (TPD) as w0.2 ML. For comparison, a spectrum recorded at 310 K is shown (lowest peak intensity). At this temperature, H is not stable on the surface. The reverse experiment is illustrated in Fig. 2b. It shows the intensity of the quasi-particle peak after depositing about 0.5 ML H at 170 K and then slowly ramping the temperature up to 330 K (0.2 K sK1). At 265 K the intensity suddenly drops to a lower level. The TPD spectrum shown on the same scale indicates clearly that the drop in intensity is associated with hydrogen desorption from the so-called b2 state [8], which will be described further below. We have found previously a similar unusual intensity change of a quasi-particle peak at EF on the c(2!2)-Br/Pt(110) surface. Here, the peak faded away within a narrow temperature range as the c(2!2) structure was driven into one-dimensional disorder [9]. We interpreted this change as indicating a dimensional cross-over from a two-dimensional wavefunction into a one-dimensional state. There is, however, a much broader pattern in a variety of systems, showing a quasi-particle peak at the Fermi level, which is quenched, as the system goes through a phase transition. Already in [9] we pointed out the similarity to high-Tc superconductors, but analogous observations have been made by ARUPS in Kondosystems and heavy-fermion materials [10], in doped Mott– Hubbard insulators [11], in 2D metals [12] and in 1D organic conductors [13], to name just a few examples. The common feature in all these cases is the cross-over from coherent to incoherent emission. In Kondo lattices, for instance, the conduction-electron screening cloud of the localised moments overlaps at low temperature, giving rise to a phase-coherent delocalised state. Above the Kondo temperature the screened moments are decoupled and photoemission probes an ensemble of incoherent localised states. This corresponds to a 3D/0D dimensional cross-over. In 1D organic conductors the interchain hopping matrix element is small, but sufficient to allow 3D Bloch states at low temperature. With increasing temperature, the chains loose their phase coherence. This
corresponds to a 3D/1D transition. In layered metals, a 3D/ 2D cross-over is observed at a critical temperature. Irrespective of dimensionality, the fingerprint of the coherent–incoherent transition is strikingly similar in all these cases. The photoemission pattern observed here closely parallels the behaviour of Fermi-level quasi-particle peaks in all these systems as they go through the coherent–incoherent transition. This indicates that on Pt(110), too, a coherent-incoherent transition takes place. It involves quasi-one-dimensional d derived surface resonances, which are localised on the closepacked atom chains. These states are weakly coupled through the conduction electron states of the substrate, giving rise to the observed ky dispersion of w180 meV. The dispersion of Ww180 meV remains constant upon H adsorption within our experimental accuracy. As the H desorbs, the quasi-particle residue is drastically reduced and the ARUPS spectrum develops the hallmarks of an incoherent superposition of single-chain emission. In the following discussion, we first concentrate on possible alternative explanations and show, why we can rule them out. Then we investigate the implications for the coherent–incoherent transition. In search for conventional explanations of the H induced growth of the quasi-particle peak one could think about a photoemission feature, which resides just above EF on clean Pt(110) and is lowered to just below EF by H adsorption. Indeed it is well known that H generally induces a down-shift of surface states [14,15]. In the present case this explanation can be ruled out, because Fig. 2a shows if anything then a slight shift from higher binding energy towards the Fermi level as the peak grows. This is supported by measurements at kx values slightly off the X point, where the band position is clearly below EF (see the inset in Fig. 1b), and where no H induced shift was observed. In the b2 state, the H atoms are known to reside in the short bridge sites on top of the close-packed atom rows [8]. The surface resonance at X derives from totally symmetric orbitals, but it exhibits nodal planes midway between the atoms in the close-packed rows. Therefore direct interaction of the H 1s orbital with this surface state is symmetry forbidden and no H induced shift is observed. Another explanation could be a redistribution of the photoemission intensity within the k space due to a H overstructure, i.e. a H induced final-state umklapp, which scatters intensity from the S into the X point. This can be excluded, because H is an extremely weak scatterer as compared to Pt. Furthermore, H forms a (1!1) structure at the relevant coverages [8]. The Pt(110) reconstruction remains unchanged. Hence, a final-state umklapp can be ruled out as well. However, quite dramatic intensity changes in photoemission have been observed as a function of temperature due to diffraction effects [16]. Thus, diffraction could play a role in our case due to the following mechanism. In the present system, H adsorption gives rise to an outward relaxation of the ˚ [8]. The intensity of the topmost layer of the order of 0.1 A quasi-particle peak could vary as a function of layer-to-layer distance, depending on the occurrence of constructive or destructive interference between the direct and the second-
A. Menzel et al. / Journal of Physics and Chemistry of Solids 67 (2006) 254–258
layer-backscattered wave. In order to assess the effect of the relaxation on the quasi-particle peak, we recorded the quasiparticle peak on the clean and the H covered surface with photon energies ranging from hn Z16.85 up to 72 eV. At all photon energies within this range the peak intensity increases upon population of the b2 phase, although the strength of the effect varies for different photon energies. Hence, a diffraction effect, and indeed any final state effect cannot be the origin of the H induced increase of the quasi-particle peak. Further insight is obtained from the redistribution of the intensity in k-space shown in Fig. 3. The low-temperature photoemission intensity distribution (Fig. 3a; TZ120 K) is
Fig. 3. (a) 3D plot of the ARUPS intensity distribution along X S measured at 120 K with H populating the b2 state. (b) 3D plot of the ARUPS intensity distribution along X S measured at 300 K for a hydrogen-free surface.
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recorded on a sample, where the b2 state is populated, whereas the distribution obtained at TZ300 K (Fig. 3b) represents the clean sample. In Fig. 3a the quasi-particle peak at X is well developed. In Fig. 3b the peak is almost completely suppressed and the intensity is transferred into an extended range in k-space around the S point. However, much of the intensity is not elastically transferred, but goes into an inelastic background ranging down to w1 eV below EF. This intensity transfer reveals two important properties of the electronic system: in the upper panel we observe a sharp quasi-particle peak well localised in k-space, hence the photo-hole is delocalised in real space as is expected for an ordinary twodimensional Bloch state. In the lower panel, the intensity is transferred into a rather broad range in k-space (note that the broad binding energy range corresponds to a broad momentum distribution). Accordingly, the photo-hole wavefunction is much more localised in real space. Furthermore, there is considerable intensity at larger binding energies reflecting a whole spectrum of excited final states (analogous to shake-ups in core hole spectroscopy). This is typical of an increased many-body response occurring in a more localised final state. The substantial inelastic intensity redistribution also rules out a diffraction effect as the origin of the quasi-particle peak suppression. Thus we conclude from the intensity-transfer pattern that in Fig. 3a the quasi-one-dimensional surface states are coherently emitting due to a non-negligible inter-chain coupling, while in Fig. 3b the individual chain states begin to lose their phase coherence. The angular pattern of the emission intensity is then no longer related to the SBZ but to the angular emission characteristics of the individual chain states. However, the transition from coherent to incoherent emission reflected in Fig. 3 is not complete: In the upper panel there is while in the still a residual diffuse inelastic intensity close toS, lower panel the X quasi-particle weight at EF is strongly, but not completely, suppressed. According to the experimental data, both, the H-induced energy shifts and the change in dispersion appear to be very small, the main effect being the transfer of quasi-particle weight. As mentioned above, there are several systems showing a similar coherent–incoherent transition, ranging from Mott insulators to Kondo systems and high-Tc superconductors. In all these systems, the phase transitions represent the delicate balance between electrons delocalised or localised on particular sites. For Pt(110), the localised ‘sites’ are the onedimensional chains and the surface resonance is on the very edge between localization and delocalization. The chain states with energy 30 have no direct overlap but are weakly coupled by a hybridisation matrix element V0k to a delocalised conduction band 3k provided by the metallic substrate, similar to the Kondo-lattice problem. The small effective bandwidth then allows a temperature-induced decoherence. This is consistent with the unusually strong intensity decrease of the coherent QP peak before H desorption between 160 and 260 K shown in Fig. 2. Desorption of H further reduces the phase coherence. A temperature-induced decoherence can take place without any change of the coupling due to an Anderson type localization of the wavefunction as a result of increasing
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disorder. The effect of H, which apparently tilts this balance towards the coherent state is most easily interpreted in terms of an increased coupling between the chains. The present data then indicate that small changes in the coupling, which hardly change the overall dispersion are sufficient for a substantial reduction of phase coherence length and hence the quasiparticle weight. The similarity to the Kondo-lattice problem may raise the question whether the chains can develop a magnetic moment. Although scalar relativistic calculations yielded no magnetic ordering on Pt surfaces [17], a recent theoretical study of Pt nanowires does predict ferromagnetic ˚ [18]. ordering along the chains even for a bond length of 2.48 A On the basis of this result and our ARUPS observations we speculate that ferromagnetic ordering occurs along the closepacked rows and that ferromagnetic or antiferromagnetic rowto-row ordering can be obtained at appropriate H coverages and sufficiently low temperature. Hydrogen could tip the balance in the model in different ways, so the system merits a closer theoretical and experimental analysis along these lines. In summary, we report the observation of a d derived quasi1D surface state or resonance on Pt(110). On the clean surface the state is localised on the close-packed atom chains. The photoemission spectra show the fingerprint of incoherent emission from the individual chain states, in particular the near absence of a quasi-particle peak at EF and a rather diffuse and partially inelastic photoemission intensity distribution off the G X symmetry plane. Upon H adsorption into the b2 state, coherence between the chain states is established, and a prominent quasi-particle peak appears at EF on the X point of the SBZ. The observed changes are qualitatively similar to those we discussed recently for the Shockley surface state on the striped O/Cu(110) phase [4,19], although they are much more pronounced in the present case.
Acknowledgements Financial support by the Austrian Science Fund is gratefully acknowledged. References [1] U. Bischler, E. Bertel, Phys. Rev. Lett. 71 (1993) 2296. [2] K. Kern, H. Niehus, A. Schatz, P. Zeppenfeld, J. Goerge, G. Comsa, Phys. Rev. Lett. 67 (1991) 855. [3] E. Bertel, J. Lehmann, Phys. Rev. Lett. 80 (1998) 1497. [4] K. Berge, A. Gerlach, G. Meister, A. Goldmann, E. Bertel, Phys. Rev. B, submitted. [5] A. Mugarza, A. Mascaraque, V. Repain, S. Rousset, K.N. Altmann, F.J. Himpsel, Yu. M. Koroteev, E.V. Chulkov, F.J. Garcia de Abajo, J.E. Ortega, Phys. Rev B 66 (2002) 245419. [6] P. Hanesch, E. Bertel, Phys. Rev. Lett. 79 (1997) 1523. [7] (a) E. Wimmer, H. Krakauer, M. Weinert, A.J. Freeman, Phys. Rev. B 24 (1981) 864; (b) M. Weinert, E. Wimmer, A.J. Freeman, Phys. Rev. B 26 (1982) 4571. [8] Zh. Zhang, M. Minca, C. Deisl, T. Loerting, A. Menzel, E. Bertel, R. Zucca, J. Redinger, Phys. Rev. B 70 (2004) 121401(R). [9] A. Menzel, R. Beer, E. Bertel, Phys. Rev. Lett. 89 (2002) 076803. [10] D. Malterre, M. Grioni, Y. Baer, Adv. Phys. 45 (2001) 299. [11] A.Y. Matsuura, H. Watanabe, C. Kim, S. Doniach, X. Shen, Phys. Rev. B 58 (1998) 3690. [12] T. Valla, P.D. Johnson, Z. Yusof, B. Wells, Q. Li, S.M. Loureiro, R.J. Cava, M. Mikami, Y. Mori, M. Yoshimura, T. Sasaki, Nature 417 (2002) 627. [13] F. Zwick, D. Je´rome, G. Margaritondo, M. Onellion, J. Voit, M. Grioni, Phys. Rev. Lett. 81 (1998) 2974. [14] U. Bischler, E. Bertel, Phys. Rev. Lett. 71 (1993) 2296. [15] F. Greuter, W. Plummer, Solid State Commun. 48 (1983) 37. [16] R. Matzdorf, R. Paniago, G. Meister, A. Goldmann, Ch. Zubra¨gel, J. Braun, G. Borstel, Surf. Sci. 352–354 (1996) 670. [17] S. Blu¨gel, Phys. Rev.Lett. 68 (1992) 852. [18] A. Delin, E. Tosatti, Phys. Rev. B 68 (2003) 144431–144432. [19] In the striped O/Cu(110) phase the coherent-incoherent transition occurs as a function of temperature.
Switching between coherent and incoherent electronic states Alexander Menzel a,*, Zhenrong Zhang a, Mariana Minca a, Erminald Bertel a, Josef Redinger b, Rinaldo Zucca b a
b
Institute f Physical Chemistry, University of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria Center for Computational Materials Science, Vienna University of Technology, Getreidemarkt 9/134, A-6010 Vienna, Austria
Abstract A quasi-one-dimensional surface resonance on the Pt(110)-(1!2) missing-row reconstructed surface is investigated by angle-resolved photoemission for different hydrogen coverages. On the H covered surface the one-dimensional (1D) electronic states located on the close-packed atom rows are shown to be phase-coherent resulting in a pronounced quasi-particle (QP) peak at EF. Desorption of hydrogen destroys the coherence, thus quenching the QP peak. The intensity is—partially inelastically—redistributed into a broad range of k space. q 2006 Elsevier Ltd. All rights reserved.
Dimensional reduction introduces strong correlation even in materials which are uncorrelated in the extended bulk. Hence correlation is an important issue in all nanostructured materials. One of the most powerful experimental methods to probe the electronic structure is angle-resolved ultraviolet photoemission spectroscopy (ARUPS). However, the extraction of the relevant information from ARUPS of correlated systems is non-trivial. In correlated systems any excitation is associated with a strong many-body response. Accordingly, energy and momentum conservation loses its significance for correlated D systems toEthe extent to which the quasi-particle ð N K1jckN ð weight k; approaches zero. We show that on the highly anisotropic Pt(110) surface the ARUPS spectra are modified by correlation effects, present an analysis of the energy dependent ARUPS intensity distribution in k space and demonstrate how the correlation strength is changed by hydrogen. On metal surfaces a reduction of electronic states from 2D to 1D can be achieved by reducing the lateral coupling of surface states in one direction (which we call the y direction). The lateral coupling ty can be engineered in a variety of ways: by creating anisotropic disorder [1], uniaxial stress domains [2–4], or step arrays [5] on a vicinal surface. On the other hand, when ty is already small, a dimensional transition can also take place as a function of temperature, if kT approaches ty. In the present study we investigate the effect of a H induced dimensional transition on a Pt(110)-(1!2) missing-row * Corresponding author. Tel.: C512 507 5099; fax: C512 507 2925. E-mail address: [email protected] (A. Menzel).
0022-3697/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.10.140
reconstructed surface. This surface is strongly anisotropic as the top layer is organized into close-packed atom rows with a ˚ , whereas the row-to-row nearest-neighbour distance of 2.77 A ˚ distance amounts to 7.84 A. Of course, as Pt is a metal, one expects a strong electronic through-substrate coupling of the topmost atom rows. On the other hand, if d derived Tamm surface states exist, their through-substrate coupling will be small provided the energy separation from the parental bulk d bands is sufficiently large. The experiments have been carried out in a UHV chamber with a base pressure of 8!10K11 mbar. The main residual gas component is hydrogen. Cleaning of the Pt(110) crystal was performed by standard procedures, but in addition cycles of low-temperature oxygen adsorption followed by thermal desorption were carried out as described in Ref. [6]. The photon source consisted of a high-intensity plasma source at the entrance of a toroidal monochromator which yields a high degree of polarization (w90%). The spectra were recorded with HeI photons (21.22 eV) unless otherwise stated. The analyser was mounted on a two-axis goniometer. For the spectra shown here, its resolution was set to 60 meV, the angular resolution was G0.68. The chamber was also equipped with a low-energy electron diffraction (LEED) unit, which was used for structure determination by dynamical (I,V-) LEED measurements. Coverages are given with respect to the number of surface atoms in the Pt(110)-(1!1) surface. The first step is to locate a surface state or resonance in k space. Fig. 1a shows the photoemission intensity distribution along the G Xline, i.e. parallel to the close-packed atom rows, as shown in the sketch of the surface Brillouin zone (SBZ) in Fig. 1c. At X we find a band reaching a maximum just below EF. In Fig. 2b we follow the dispersion of this band along X S.
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along the SBZ boundary is a surface state band (or, more precisely, a surface resonance), which strongly samples the additional Fourier component of the surface potential stemming from the (1!2) missing row reconstruction. Additional evidence for the surface resonance character is the constant binding energy of the X quasi-particle peak if measured with photon energies between 16 and 70 eV. The orbital character of the topmost band at X can be deduced from experimental and theoretical considerations. The quasi-particle feature at X is intense if excited with photons polarized parallel to the close-packed rows, but strongly suppressed if the polarisation is perpendicular to the rows. Hence it belongs to the totally symmetric A1 representation of the C2v point group. Inspection of the bulk band structure shows that at X the band is close to the X5 and the X2 bulk bands, which both yield A1 representations, if projected onto the surface. As this analysis neglects the effect of spin– orbit interaction, we have also carried out a fully relativistic LDA calculation (including spin–orbit interaction) for a free-standing chain of Pt atoms, employing
12000
_
(a)
T= 170 K T= 310 K
X 6000
0 –1.5
The band disperses upwards through the Fermi level and In order to determine the comes down again at S. dispersion more quantitatively, we also measured the ˚ K1, where the band has sufficiently dispersion at kx Z1.3 A dropped below EF, so that the entire ky dispersion can be mapped (inset in Fig. 1b). From these measurements we find a bandwidth of Ww180 meV perpendicular to the close-packed rows, whereas the kx dispersion is more of the order of eV (it is difficult to specify a total bandwidth in that direction due to intervening bands at higher binding energies). In any case, the band has a saddle point just below EF at X and a strongly anisotropic dispersion. Furthermore, it is symmetric around a point halfway between X and Swith almost equivalent intensity at both principal points of the SBZ. This proves that the band
–1.0
–0.5
0.0
0.5
Binding Energy (eV) 1200
(b)
1100 1000
Intensity (cps)
Fig. 1. (a) Grey plot of the ARUPS intensity distribution (TZ120 K) along G X. The red diamond marks the position of the X point, the other colour symbols refer to the calculated bandstructure of a free-standing Pt atom chain. Note the (b) The ARUPS intensity distribution (TZ120 K) along intense emission at X. Red diamond: X; dashed line: ky dispersion of the band green rectangle: S; X S. Inset: dispersion of the same band giving rise to the quasi-particle peak at X. (c) The surface Brillouin zone with the principal measured slightly beyond X. points and the k space position of the dispersion measured in the inset of (b).
900 800 700 600 500 400 200
250
300
350
Temperature (K) Fig. 2. (a) The increase of the quasi-particle peak intensity at X as a function of H coverage. The black line is the spectrum measured at 310 K for a hydrogenfree surface. (b) Blue symbols: Decrease of the quasi-particle peak intensity from a surface with QHZ0.25 ml (b2 state saturated) as the temperature is ramped (0.2 K sK1) to 330 K. The abrupt quenching of the peak intensity occurs at the onset of the H desorption from the b2 state. Red line: Temperature programmed desorption of H from the b2 state (3 K sK1).
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the all-electron full-potential linearized augmented plane wave code FLAIR [7]. In FLAIR, core states are treated fully relativistically and in all iterations spin–orbit interaction is included for the valence states in a second-variation step. No shape approximation is assumed for the potential or wavefunc˚ corresponds tions. The distance between the Pt atoms 2.765 A ˚ , close to the to a bulk LDA minimum lattice constant of 3.91 A experimental value. In Fig. 1a the calculated bandstructure is imposed onto the photoemission intensity distribution pattern. Surprisingly, the calculated and observed bands agree qualitatively quite well (with exception of the band at w1 eV binding energy), although the calculation describes only a free-standing chain. In agreement with the symmetry arguments given above, the topmost band at X is correlated with a totally symmetric d derived state. The intensity of the quasi-particle state at X shows a strong dependence on H coverage. This is illustrated in Fig. 2: Fig. 2a shows the variation of the peak as the H coverage is gradually increased at 170 K. The coverage at the highest intensity shown was determined by temperature programmed desorption (TPD) as w0.2 ML. For comparison, a spectrum recorded at 310 K is shown (lowest peak intensity). At this temperature, H is not stable on the surface. The reverse experiment is illustrated in Fig. 2b. It shows the intensity of the quasi-particle peak after depositing about 0.5 ML H at 170 K and then slowly ramping the temperature up to 330 K (0.2 K sK1). At 265 K the intensity suddenly drops to a lower level. The TPD spectrum shown on the same scale indicates clearly that the drop in intensity is associated with hydrogen desorption from the so-called b2 state [8], which will be described further below. We have found previously a similar unusual intensity change of a quasi-particle peak at EF on the c(2!2)-Br/Pt(110) surface. Here, the peak faded away within a narrow temperature range as the c(2!2) structure was driven into one-dimensional disorder [9]. We interpreted this change as indicating a dimensional cross-over from a two-dimensional wavefunction into a one-dimensional state. There is, however, a much broader pattern in a variety of systems, showing a quasi-particle peak at the Fermi level, which is quenched, as the system goes through a phase transition. Already in [9] we pointed out the similarity to high-Tc superconductors, but analogous observations have been made by ARUPS in Kondosystems and heavy-fermion materials [10], in doped Mott– Hubbard insulators [11], in 2D metals [12] and in 1D organic conductors [13], to name just a few examples. The common feature in all these cases is the cross-over from coherent to incoherent emission. In Kondo lattices, for instance, the conduction-electron screening cloud of the localised moments overlaps at low temperature, giving rise to a phase-coherent delocalised state. Above the Kondo temperature the screened moments are decoupled and photoemission probes an ensemble of incoherent localised states. This corresponds to a 3D/0D dimensional cross-over. In 1D organic conductors the interchain hopping matrix element is small, but sufficient to allow 3D Bloch states at low temperature. With increasing temperature, the chains loose their phase coherence. This
corresponds to a 3D/1D transition. In layered metals, a 3D/ 2D cross-over is observed at a critical temperature. Irrespective of dimensionality, the fingerprint of the coherent–incoherent transition is strikingly similar in all these cases. The photoemission pattern observed here closely parallels the behaviour of Fermi-level quasi-particle peaks in all these systems as they go through the coherent–incoherent transition. This indicates that on Pt(110), too, a coherent-incoherent transition takes place. It involves quasi-one-dimensional d derived surface resonances, which are localised on the closepacked atom chains. These states are weakly coupled through the conduction electron states of the substrate, giving rise to the observed ky dispersion of w180 meV. The dispersion of Ww180 meV remains constant upon H adsorption within our experimental accuracy. As the H desorbs, the quasi-particle residue is drastically reduced and the ARUPS spectrum develops the hallmarks of an incoherent superposition of single-chain emission. In the following discussion, we first concentrate on possible alternative explanations and show, why we can rule them out. Then we investigate the implications for the coherent–incoherent transition. In search for conventional explanations of the H induced growth of the quasi-particle peak one could think about a photoemission feature, which resides just above EF on clean Pt(110) and is lowered to just below EF by H adsorption. Indeed it is well known that H generally induces a down-shift of surface states [14,15]. In the present case this explanation can be ruled out, because Fig. 2a shows if anything then a slight shift from higher binding energy towards the Fermi level as the peak grows. This is supported by measurements at kx values slightly off the X point, where the band position is clearly below EF (see the inset in Fig. 1b), and where no H induced shift was observed. In the b2 state, the H atoms are known to reside in the short bridge sites on top of the close-packed atom rows [8]. The surface resonance at X derives from totally symmetric orbitals, but it exhibits nodal planes midway between the atoms in the close-packed rows. Therefore direct interaction of the H 1s orbital with this surface state is symmetry forbidden and no H induced shift is observed. Another explanation could be a redistribution of the photoemission intensity within the k space due to a H overstructure, i.e. a H induced final-state umklapp, which scatters intensity from the S into the X point. This can be excluded, because H is an extremely weak scatterer as compared to Pt. Furthermore, H forms a (1!1) structure at the relevant coverages [8]. The Pt(110) reconstruction remains unchanged. Hence, a final-state umklapp can be ruled out as well. However, quite dramatic intensity changes in photoemission have been observed as a function of temperature due to diffraction effects [16]. Thus, diffraction could play a role in our case due to the following mechanism. In the present system, H adsorption gives rise to an outward relaxation of the ˚ [8]. The intensity of the topmost layer of the order of 0.1 A quasi-particle peak could vary as a function of layer-to-layer distance, depending on the occurrence of constructive or destructive interference between the direct and the second-
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layer-backscattered wave. In order to assess the effect of the relaxation on the quasi-particle peak, we recorded the quasiparticle peak on the clean and the H covered surface with photon energies ranging from hn Z16.85 up to 72 eV. At all photon energies within this range the peak intensity increases upon population of the b2 phase, although the strength of the effect varies for different photon energies. Hence, a diffraction effect, and indeed any final state effect cannot be the origin of the H induced increase of the quasi-particle peak. Further insight is obtained from the redistribution of the intensity in k-space shown in Fig. 3. The low-temperature photoemission intensity distribution (Fig. 3a; TZ120 K) is
Fig. 3. (a) 3D plot of the ARUPS intensity distribution along X S measured at 120 K with H populating the b2 state. (b) 3D plot of the ARUPS intensity distribution along X S measured at 300 K for a hydrogen-free surface.
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recorded on a sample, where the b2 state is populated, whereas the distribution obtained at TZ300 K (Fig. 3b) represents the clean sample. In Fig. 3a the quasi-particle peak at X is well developed. In Fig. 3b the peak is almost completely suppressed and the intensity is transferred into an extended range in k-space around the S point. However, much of the intensity is not elastically transferred, but goes into an inelastic background ranging down to w1 eV below EF. This intensity transfer reveals two important properties of the electronic system: in the upper panel we observe a sharp quasi-particle peak well localised in k-space, hence the photo-hole is delocalised in real space as is expected for an ordinary twodimensional Bloch state. In the lower panel, the intensity is transferred into a rather broad range in k-space (note that the broad binding energy range corresponds to a broad momentum distribution). Accordingly, the photo-hole wavefunction is much more localised in real space. Furthermore, there is considerable intensity at larger binding energies reflecting a whole spectrum of excited final states (analogous to shake-ups in core hole spectroscopy). This is typical of an increased many-body response occurring in a more localised final state. The substantial inelastic intensity redistribution also rules out a diffraction effect as the origin of the quasi-particle peak suppression. Thus we conclude from the intensity-transfer pattern that in Fig. 3a the quasi-one-dimensional surface states are coherently emitting due to a non-negligible inter-chain coupling, while in Fig. 3b the individual chain states begin to lose their phase coherence. The angular pattern of the emission intensity is then no longer related to the SBZ but to the angular emission characteristics of the individual chain states. However, the transition from coherent to incoherent emission reflected in Fig. 3 is not complete: In the upper panel there is while in the still a residual diffuse inelastic intensity close toS, lower panel the X quasi-particle weight at EF is strongly, but not completely, suppressed. According to the experimental data, both, the H-induced energy shifts and the change in dispersion appear to be very small, the main effect being the transfer of quasi-particle weight. As mentioned above, there are several systems showing a similar coherent–incoherent transition, ranging from Mott insulators to Kondo systems and high-Tc superconductors. In all these systems, the phase transitions represent the delicate balance between electrons delocalised or localised on particular sites. For Pt(110), the localised ‘sites’ are the onedimensional chains and the surface resonance is on the very edge between localization and delocalization. The chain states with energy 30 have no direct overlap but are weakly coupled by a hybridisation matrix element V0k to a delocalised conduction band 3k provided by the metallic substrate, similar to the Kondo-lattice problem. The small effective bandwidth then allows a temperature-induced decoherence. This is consistent with the unusually strong intensity decrease of the coherent QP peak before H desorption between 160 and 260 K shown in Fig. 2. Desorption of H further reduces the phase coherence. A temperature-induced decoherence can take place without any change of the coupling due to an Anderson type localization of the wavefunction as a result of increasing
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disorder. The effect of H, which apparently tilts this balance towards the coherent state is most easily interpreted in terms of an increased coupling between the chains. The present data then indicate that small changes in the coupling, which hardly change the overall dispersion are sufficient for a substantial reduction of phase coherence length and hence the quasiparticle weight. The similarity to the Kondo-lattice problem may raise the question whether the chains can develop a magnetic moment. Although scalar relativistic calculations yielded no magnetic ordering on Pt surfaces [17], a recent theoretical study of Pt nanowires does predict ferromagnetic ˚ [18]. ordering along the chains even for a bond length of 2.48 A On the basis of this result and our ARUPS observations we speculate that ferromagnetic ordering occurs along the closepacked rows and that ferromagnetic or antiferromagnetic rowto-row ordering can be obtained at appropriate H coverages and sufficiently low temperature. Hydrogen could tip the balance in the model in different ways, so the system merits a closer theoretical and experimental analysis along these lines. In summary, we report the observation of a d derived quasi1D surface state or resonance on Pt(110). On the clean surface the state is localised on the close-packed atom chains. The photoemission spectra show the fingerprint of incoherent emission from the individual chain states, in particular the near absence of a quasi-particle peak at EF and a rather diffuse and partially inelastic photoemission intensity distribution off the G X symmetry plane. Upon H adsorption into the b2 state, coherence between the chain states is established, and a prominent quasi-particle peak appears at EF on the X point of the SBZ. The observed changes are qualitatively similar to those we discussed recently for the Shockley surface state on the striped O/Cu(110) phase [4,19], although they are much more pronounced in the present case.
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