We have identified and characterized a charge-density wave transition ͑T c ϳ 260 ± C͒ in the lowcoverage a phase of the Sn͞Ge(111) interface both experimentally and theoretically. Charge ordering is accompanied by a structural distortion from ͑ p 3 3p 3 ͒R30 ± to ͑3 3 3͒ symmetry. Density-functional theory calculations are unable to correctly reproduce the observed ground state and, more importantly, indicate that Fermi surface nesting does not play a role in this transition. Both signal the importance of many-body effects in this system. Experiment and theory indicate that the Sn͞Ge(111) overlayer is fundamentally different from the Pb͞Ge (111) overlayer previously reported. [S0031-9007(97)04249-X] PACS numbers: 73.20.At, 61.16.Ch, 68.35.Md, 71.45.LrA charge-ordered state, or charge-density wave (CDW), incorporates a symmetry-lowering periodic redistribution of valence charge driven by a reduction in the system's total electronic energy, resulting in a small periodic lattice distortion [1]. This phenomenon is most likely to occur in reduced dimensions [2,3], for example, in the layered perovskites [4]. The loss of coordination at a crystal surface might also be expected to invite CDW formation, but few genuine instances of surface charge-density waves have been reported to date. One clear example was recently discovered at the Pb͞Ge(111)-a vacuum interface [5]. This low-density overlayer transforms from the room-temperature (RT) ͑ p 3 3p 3 ͒R30 ± structure to the low-temperature (LT) ͑3 3 3͒ ground state. Charge ordering (ϳ0.5 Å corrugation) and accompanying lattice distortion occur gradually and reversibly with T c ϳ 220 ± C. Density function theory calculations indicate the chargeordered ͑3 3 3͒ structure is the ground state of this system [5]. The opening of a E g ϳ 65 meV band gap below T c (later confirmed by photoemission measurements [6]) must be the consequence of correlation effects [5]. We therefore conjectured that although enhanced electron-phonon coupling (enabled by Fermi surface nesting) drives this transition, many-body interactions stabilize the ground state.Our motivation to study the a phase of Sn͞Ge(111) was threefold. First, seeking verification that Pb͞Ge(111) charge ordering is not an isolated quirk of nature resulting from Pb's exotic properties, we considered the other isostructural adsorbate/substrate combinations that exist. Al, Ga, In, Sn, and Pb all form the same low-density overlayer atop both Si(111) and Ge(111) [7,8]. However, overlayers composed of the trivalent species (Al, Ga, In) will probably not undergo a CDW distortion, as they are semiconducting [7] with an even number of electrons per unit cell at RT.Second, it is imperative to test several theoretically derived concepts, including Fermi surface nesting, and the idea that the CDW ground state is a Mott-Hubbard insulator [5]. Intuitively, the Sn overlayer bandwidth w should be less than that of the Pb overlayer, because the metallic r Sn is ϳ10% smaller than r Pb . For the same reason one expects the on-site Coulomb repuls...

A new method of refrigeration is proposed. Cooling is obtained by thermionic emission of electrons over periodic barriers in a multilayer geometry. These could be either Schottky barriers between metals and semiconductors or else barriers in a semiconductor superlattice. The same device is an efficient power generator. A complete theory is provided.

The full equations are derived for the resistances to the transport of heat and electricity through boundaries of thermoelectrics. We show that the boundary resistances of heat and electricity are proportional. This relationship is a boundary form of the Wiedemann–Franz law. We also show there is a boundary Seebeck coefficient.

Symmetry, dimensionality, and disorder play a pivotal role in critical phenomena. The atomic imaging capabilities of the scanning tunneling microscope were used to directly visualize the interaction between charge density oscillations and lattice defects in a two-dimensional charge density wave (CDW) system. Point defects act as nucleation centers of the CDW, which, as the temperature is lowered, results in the formation of pinned CDW domains that are separated by atomically abrupt charge boundaries. Incomplete freezing of substitutional disorder at low temperature indicates a novel CDW-mediated hopping of pinning centers.

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