VOLUME 78, NUMBER 7 PHYSICAL REVIEW LETTERS 17 FEBRUARY 1997 Mott Insulating Ground State on a Triangular Surface Lattice H. H. Weitering, 1,2 X. Shi, 1 P. D. Johnson, 3 J. Chen, 4 N. J. DiNardo, 5,6 and K. Kempa 7 1 Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996 2 Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 3 Department of Physics, Brookhaven National Laboratory, Upton, New York 11973 4 Department of Physics, University of Pennsylvania, Philadelphia, Pennsylvania 19104 5 Department of Physics and Atmospheric Science, Drexel University, Philadelphia, Pennsylvania 19104 6 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 7 Department of Physics, Boston College, Chestnut Hill, Massachusetts 02167 (Received 20 September 1996) Momentum-resolved direct and inverse photoemission spectra of the KySis111d-s p 3 3 p 3 dR30 ± -B interface reveals the presence of strongly localized surface states. The K overlayer remains nonmetallic up to the saturation coverage. This system most likely presents the first experimental realization of a frustrated spin 1y2 Heisenberg antiferromagnet on a two-dimensional triangular lattice. [S0031-9007(97)02423-X] PACS numbers: 73.20.At, 71.30.+h, 75.30.Pd, 79.60.Dp The band theory of solids has been remarkably success- ful in describing the ground state properties of many crys- talline materials. Nonetheless, limitations to this theory were already pointed out in 1937 by de Boer and Ver- wey who argued that NiO, which is an optically transpar- ent insulator, would be metallic according to band theory [1]. Fascinating examples of the breakdown of band the- ory include the high T c superconductors [2] and doped fullerenes [3]. In simple terms, band theory breaks down when the Coulomb repulsion U between electrons on a lattice site is comparable to or larger than the single par- ticle bandwidth, W [1]. If the ratio UyW * 1, the charge carriers no longer delocalize but condense at the ion cores, leading to an insulating antiferromagnetic ground state at half filling (i.e., a Mott-Hubbard insulator). Surfaces and interfaces represent a special class of nar- row band systems. Many semiconductor surfaces possess dangling-bond-derived surface states with bandwidths &1 eV. Estimates indicate that the effective Coulomb in- teractions, U eff , within the dangling bonds of an ideally truncated Si(111) surface are on the order of l1 eV [4,5] which brings the Si(111) surface in the Mott-Hubbard regime. However, Si(111) does not become an anti- ferromagnetic insulator because it largely eliminates its dangling bonds by forming a s7 3 7d superstructure [6]. Likewise, it was believed that the Sis111d-s2 3 1d surface reconstruction is a buckled antiferromagnetic insulator [7] until Pandey introduced the p -bonded chain model and showed that the single-particle band theory does provide an accurate description of the electronic properties [8]. In fact, only a very few claims on Mott insulating surfaces have withstood the test of time: some room temperature (RT) saturated alkali-metalyGaAs(110) interfaces are be- lieved to be Mott insulators [9–11]. Most recently, it was argued that the charge-density-wave phase of PbyGe(111) is a Mott insulator [12]. Model calculations for the two-dimensional rectangular lattice of the alkaliyGaAs(110) interfaces produce an “up- per Hubbard band” (UHB) and a “lower Hubbard band” (LHB), separated by a distinct energy gap when UyW . 2 [11]. The metal-insulator transition is predicted to occur near UyW l 1.6. From an experimental point of view, the situation is not so clear. DiNardo and co-workers have in- terpreted their electron energy loss spectra (EELS) of a saturated Cs overlayer on GaAs(110) in terms of charge transfer excitations involving the Hubbard U [9]. Nonethe- less, problems exist. Inverse photoemission spectroscopy (IPES) [13] shows features that could perhaps be identified as the UHB but the LHB has not been seen in photoemis- sion spectroscopy (PES) [13]. In this Letter, we report on momentum-resolved PES and IPES data of the KySis111d-s p 3 3 p 3 dR30 ± -B (henceforth KySi) interface. The interface consists of a monatomic alkali layer on top of a Si(111) surface with a boron underlayer (Fig. 1) [14]. At zero alkali coverage, the dangling bond surface states are completely empty [14]. When the alkali coverage saturates at 1y3 mono- layer [15–17], the surface consists of a s p 3 3 p 3 dR30 ± arrangement of half-filled dangling bonds and, conse- quently, the interface should be metallic according to band theory [16]. Instead, the single-particle excitation spectra (PESyIPES) show two prominent features near the Fermi energy, E F , which are identified as the Hubbard bands of a 2D Hubbard system. We analyze the spectra following Harrison’s scheme of incorporating the Hubbard U into tight-binding theory [4] and discuss important implications for the electrical transport and magnetic properties. Experiments were carried out in two different ultrahigh vacuum systems. Momentum-resolved PES data were acquired at the beam line U12B of the National Syn- chrotron Light Source [16]. The overall resolution of the PES spectra is l0.1 eV. Momentum-resolved IPES 0031-9007y 97y 78(7) y1331(4)$10.00 © 1997 The American Physical Society 1331