Electron correlation, metallization, and Fermi-level pinning at ultrathin K/Si(111) interfaces

Ultra-shallow p-type doping of silicon by performing atomic layer deposition of Al2O3 thin films onto SiO2/Si

We report an ultra-shallow p-type doping of silicon resulting from the rapid thermal annealing of thin Al2O3 films deposited on intrinsic silicon with a native SiO2 layer, using a common atomic layer deposition process. Characterization revealed a two-stage decrease in sheet resistance, providing insights into the doping mechanism.

Identification of single-layer metallic structure of indium on Si(1 1 1)

We have studied the In/Si(1 1 1) ([Formula: see text])-hex and striped phases by low-energy electron diffraction (LEED) and angle-resolved photoelectron spectroscopy (ARPES). The two phases are formed by different processes, and hence the different names have been used conventionally. We, however, found that LEED I-[Formula: see text] curves of the two phases agree with each other, indicating that they have an identical atomic structure. Our ARPES measurement revealed that the ([Formula: see text])-hex phase has metallic surface states. The observed Fermi surface was found to be very similar to the theoretical one calculated for an indium single-layer model.

Uncertainty principle for experimental measurements: Fast versus slow probes

The result of a physical measurement depends on the time scale of the experimental probe. In solid-state systems, this simple quantum mechanical principle has far-reaching consequences: the interplay of several degrees of freedom close to charge, spin or orbital instabilities combined with the disparity of the time scales associated to their fluctuations can lead to seemingly contradictory experimental findings. A particularly striking example is provided by systems of adatoms adsorbed on semiconductor surfaces where different experiments--angle-resolved photoemission, scanning tunneling microscopy and core-level spectroscopy--suggest different ordering phenomena. Using most recent first principles many-body techniques, we resolve this puzzle by invoking the time scales of fluctuations when approaching the different instabilities. These findings suggest a re-interpretation of ordering phenomena and their fluctuations in a wide class of solid-state systems ranging from organic materials to high-temperature superconducting cuprates.

Dynamical screening effects in correlated electron materials-a progress report on combined many-body perturbation and dynamical mean field theory: 'GW + DMFT'

We give a summary of recent progress in the field of electronic structure calculations for materials with strong electronic Coulomb correlations. The discussion focuses on developments beyond the by now well established combination of density functional and dynamical mean field theory dubbed 'LDA + DMFT'. It is organized around the description of dynamical screening effects in the solid. Indeed, screening in the solid gives rise to dynamical local Coulomb interactions U(ω) (Aryasetiawan et al 2004 Phys. Rev. B 70 195104), and this frequency dependence leads to effects that cannot be neglected in a truly first principles description. We review the recently introduced extension of LDA + DMFT to dynamical local Coulomb interactions 'LDA + U(ω) + DMFT' (Casula et al 2012 Phys. Rev. B 85 035115, Werner et al 2012 Nature Phys. 1745-2481). A reliable description of dynamical screening effects is also a central ingredient of the 'GW + DMFT' scheme (Biermann et al 2003 Phys. Rev. Lett. 90 086402), a combination of many-body perturbation theory in Hedin's GW approximation and dynamical mean field theory. Recently, the first GW + DMFT calculations including dynamical screening effects for real materials have been achieved, with applications to SrV O3 (Tomczak et al 2012 Europhys. Lett. 100 67001, Tomczak et al Phys. Rev. B submitted (available electronically as arXiv:1312.7546)) and adatom systems on surfaces (Hansmann et al 2013 Phys. Rev. Lett. 110 166401). We review these and comment on further perspectives in the field. This review is an attempt to put elements of the original works into the broad perspective of the development of truly first principles techniques for correlated electron materials.

Temperature dependence of the electronic structure of two-dimensional Na gas on the Si(111)-7 × 7 surface

The temperature dependence of the irreversible phase transition from a two-dimensional gas to an ordered zero-dimensional solid on the Si(111)-7 × 7 surface was studied using photoemission spectroscopy. With increasing Na coverage, the two-dimensional Na gas, which is a state of highly mobile Na atoms, undergoes a phase transition into ordered zero-dimensional magic nanoclusters at room temperature. The critical Na coverage of the phase transition was found to increase with reduced temperature. This was used to develop a gas-solid phase diagram of Na atoms on the Si(111)-7 × 7 surface as a function of Na coverage and sample temperature based on the electronic structure. The temperature dependence of the phase transition can be ascribed to the suppression of the thermal energy that is required to overcome the energetic barrier between the two-dimensional gas and the zero-dimensional solid at low temperature, where three different hopping mechanisms are related to the phase transition.

Long-range Coulomb interactions in surface systems: a first-principles description within self-consistently combined GW and dynamical mean-field theory

Systems of adatoms on semiconductor surfaces display competing ground states and exotic spectral properties typical of two-dimensional correlated electron materials which are dominated by a complex interplay of spin and charge degrees of freedom. We report a fully ab initio derivation of low-energy Hamiltonians for the adatom systems Si(111):X, with X=Sn, Si, C, Pb, that we solve within self-consistently combined GW and dynamical mean-field theory. Calculated photoemission spectra are in agreement with available experimental data. We rationalize experimentally observed trends from Mott physics toward charge ordering along the series as resulting from substantial long-range interactions.

Understanding the insulating nature of alkali-metal/Si(111):B interfaces

We have recently revisited the phase diagram of alkali-metal/Si(111):B semiconducting interfaces previously suggested as the possible realization of a Mott-Hubbard insulator on a triangular lattice. The insulating character of the 2√[3] × 2√[3]R30 surface reconstruction observed at the saturation coverage, i.e. 0.5 ML, has been shown to find its origin in a giant alkali-metal-induced vertical distortion. Low energy electron diffraction, photoemission spectroscopy and scanning tunneling microscopy and spectroscopy experiments coupled with linear augmented plane-wave density functional theory calculations allow a full understanding of the k-resolved band structure, explaining both the inhomogeneous charge transfers into an Si-B hybridized surface state and the opening of a band gap larger than 1 eV. Moreover, √[3] × √[3]R30, 3 × 3 and 2√[3] × 2√[3]R30 surface reconstructions observed as a function of coverage may reveal a filling-controlled transition from a half-filled correlated magnetic material to a strongly distorted band insulator at saturation.

What about U on surfaces? Extended Hubbard models for adatom systems from first principles

Electronic correlations together with dimensional constraints lead to some of the most fascinating properties known in condensed matter physics. As possible candidates where these conditions are realized, semiconductor (111) surfaces and adatom systems on surfaces have been under investigation for quite some time. However, state-of-the-art theoretical studies on these materials that include many-body effects beyond the band picture are rare. First principles estimates of inter-electronic Coulomb interactions for the correlated states are missing entirely, and usually these interactions are treated as adjustable parameters. In this work, we report on calculations of the interaction parameters for the group IV surface-adatom systems in the α-phase series of Si(111):C, Si, Sn, Pb. For all systems investigated, the inter-electronic Coulomb interactions are indeed large compared to the kinetic energies of the states in question. Moreover, our study reveals that intersite interactions cannot be disregarded. We explicitly construct an extended Hubbard model for the series of group IV surface-adatom systems on silicon, which can be used for further many-body calculations.

Surface-state bipolaron formation on a triangular lattice in the sp-type alkali-metal/Si(111) Mott insulator

We report on new low-energy electron diffraction, scanning tunneling microscopy, and angle-resolved photoemission spectroscopy studies of alkali-metal/Si(111) previously established as having a Mott-insulating ground state at surface. The observation of a strong temperature dependent Franck-Condon broadening of the surface band together with the novel sqrt[3] x sqrt[3] --> 2(sqrt[3] x sqrt[3]) charge and lattice ordering below 270 K evidence a surface charge density wave in the strong electron-phonon coupling limit (g approximately 8). Both the adiabatic ratio variant Planck's over 2piomega_{0}/t approximately 0.8 and the effective pairing energy V_{eff} = U - 2gvariant Planck's over 2piomega_{0} approximately -800 meV are consistent with the possible formation of a bipolaronic insulating phase consisting of alternating doubly occupied and unoccupied dangling bonds as expected in the Holstein-Hubbard model.

Novel electronically driven surface phase predicted in C/Si(111)

We predict a novel electronically driven phase for the recently created C/Si(111) surface at 1/3 monolayer coverage. Whereas the isoelectronic surface Sn/Ge(111) is a 3 x 3 distorted metal and Si/SiC(0001) is an undistorted magnetic Mott insulator, the new phase combines both features. Two of three adatoms in C/Si(111) should form a distorted (3 x 3) honeycomb sublattice, the third an undistorted insulating and magnetic triangular sublattice. The generally conflicting elements, namely, band energy, favoring distortion, and strong electron correlations favoring a Mott state, actually conspire in this case. This kind of state represents the surface analog of the Fazekas-Tosatti state in the charge density wave compound 1T-TaS2.

Na adsorption on the Si111-(7 x 7) surface: from two-dimensional gas to nanocluster array

We have systematically investigated Na adsorption on the Si(111)-(7 x 7) surface at room temperature using scanning tunneling microscopy (STM). Below the critical coverage of 0.08 monolayer, we find intriguing contrast modulation instead of localized Na adsorbates, coupled with streaky noise in the STM images, which is accompanied by monotonic work function drop. Above the critical coverage, Na clusters emerge and form a self-assembled array. Combined with first-principles theoretical simulations, we conclude that the Na atoms on the (7 x 7) surface are, while strongly bound ( approximately 2.2 eV) to the surface, highly mobile in "basins" around the Si rest atoms, forming a two-dimensional gas phase at the initial coverage, and that the cluster at the higher coverage consists of six Na atoms together with three Si adatoms.

Band splitting for Si(557)-Au: is it spin-charge separation?

It has been proposed that the Si(557)-Au surface exhibits spin-charge separation in a one-dimensional electron liquid. Two narrowly spaced bands are found which exhibit a well-defined splitting at the Fermi level. That is incompatible with the assignment to a spinon-holon pair in a Luttinger liquid. Instead, we propose that the two bands are associated with two nearly degenerate atomic chains, or a chain of step atoms with two broken bonds. Such an assignment explains why the surface is metallic despite an even number of electrons per unit cell.

Observation of disorder-induced 2D mott-hubbard states of the alkali-earth metal (Mg,Ba)-adsorbed Si(111) surface

We report evidence of a disorder-driven Mott-Hubbard-type localization on the alkali-earth metal (AEM) (Mg,Ba)-adsorbed Si(111)-(7x7) surface. The clean metallic Si(111) surface is found to undergo a two-dimensional (2D) metal-insulator transition as randomly distributed AEM adsorbates cause disorder on the surface. A well-defined electron-energy-loss peak unique to the insulating phase is attributed to an interband excitation between the split Hubbard bands originated from a metallic surface band at Fermi energy. A quantitative analysis of the loss peak reveals that the AEM-induced insulating surfaces are of a Mott-Hubbard type driven essentially by disorder.