A combination of angle-resolved photoemission and scanning tunneling microscopy is used to explore the possibilities for tailoring the electronic structure of gold atom chains on silicon surfaces. It is shown that the interchain coupling and the band filling can be adjusted systematically by varying the step spacing via the tilt angle from Si͑111͒. Planes with odd Miller indices are stabilized by chains of gold atoms. Metallic bands and Fermi surfaces are observed. These findings suggest that atomic chains at stepped semiconductor substrates make a highly flexible class of solids approaching the one-dimensional limit.

The crystal and magnetic structures of single-crystalline hexagonal LuFeO(3) films have been studied using x-ray, electron, and neutron diffraction methods. The polar structure of these films are found to persist up to 1050 K; and the switchability of the polar behavior is observed at room temperature, indicating ferroelectricity. An antiferromagnetic order was shown to occur below 440 K, followed by a spin reorientation resulting in a weak ferromagnetic order below 130 K. This observation of coexisting multiple ferroic orders demonstrates that hexagonal LuFeO(3) films are room-temperature multiferroics.

We present a variable temperature scanning tunneling microscopy and spectroscopy study of the Si(553)-Au atomic chain reconstruction. This quasi-one-dimensional system undergoes at least two charge density wave (CDW) transitions, which can be attributed to electronic instabilities in the fractionally filled 1D bands of the high-symmetry phase. Upon cooling, Si(553)-Au first undergoes a single-band Peierls distortion, resulting in period doubling along the chains. This Peierls state is ultimately overcome by a competing 3 CDW, which is accompanied by a 2 periodicity in between the chains. These locked-in periodicities indicate small charge transfer between the nearly 1=2-filled and 1=4-filled bands. The presence and the mobility of atomic-scale dislocations in the 3 CDW state indicates the possibility of manipulating phase solitons carrying a (spin, charge) of 1=2; e=3 or 0; 2e=3 . DOI: 10.1103/PhysRevLett.96.076801 PACS numbers: 73.20.At, 68.37.Ef, 71.10.Pm, 73.20.Mf According to the Mermin-Wagner theorem [1], thermodynamic fluctuations preclude the formation of a longrange ordered broken symmetry state in one dimension, except at T 0 K [2]. For all practical purposes, however, thermodynamic phase transitions may still be possible in finite size 1D systems. Furthermore, fluctuations are inevitably suppressed if the 1D chains are weakly coupled, or if the chains are coupled to a substrate [2,3]. Prototypical 1D metallic systems such as the transition metal trichalcogenides, organic charge transfer salts, blue bronzes, and probably all atomic Au-chain reconstructions on vicinal Si substrates exhibit symmetry breaking phase transitions at finite temperatures [4,5]. For a band filling of 1=n, the phase transition opens up a gap in the single particle excitation spectrum at wave vector k F =na, and the corresponding broken symmetry state adopts the new periodicity of =k F na, where a is the lattice parameter of the high-symmetry phase [4].Fractional band fillings other than half filling provide an interesting subset of 1D systems which often exhibit exotic physical phenomena. Depending on the relative magnitude of bandwidth and electron-electron interaction, charge density wave (CDW) states often compete with spin density waves, Mott insulating states, or a Luttinger liquid state. Atomic-scale STM observations of surface phase transitions provide important insights into the complexity of symmetry breaking phenomena in reduced dimensionality [6]. For instance, the recently reported 4 1-to-8 2 phase transition in quasi-1D indium chains on Si(111) [6] involves a gap opening in a complex triple band Peierls system, resulting in a doubling of the periodicity along the atom chains. Another recently discovered system with three fractionally filled bands is the Si(553)-Au surface. Angle-resolved photoemission spectroscopy (ARPES) [7] revealed three metallic bands, but despite theoretical efforts to understand the electronic structure [7,8], the atomic structure and real space location of the surface state orbitals remain...

Many quasi-one-dimensional ͑1D͒ materials are experimental approximations to the textbook models of Peierls instabilities and collective excitations in 1D electronic systems. The recently observed self-assembly of atom wires on solid surfaces has provided fascinating new insights into the nature of their structural and electronic instabilities, from both real-space and momentum-space perspectives. In this Colloquium, three of the most studied atom wire arrays are highlighted, all featuring multiple surface-state bands. One of these is made of indium atoms on a flat silicon ͑111͒ surface, while the two others consist of gold atoms on surfaces that are vicinal to Si͑111͒. The experimental and theoretical results are discussed with a focus on the detailed mechanisms of the observed phase transitions and on the role of microscopic defects.

We report on the use of helium ion implantation to independently control the out-of-plane lattice constant in epitaxial La 0.7 Sr 0.3 MnO 3 thin films without changing the in-plane lattice constants. The process is reversible by a vacuum anneal. Resistance and magnetization measurements show that even a small increase in the out-of-plane lattice constant of less than 1% can shift the metal-insulator transition and Curie temperatures by more than 100°C. Unlike conventional epitaxy-based strain tuning methods which are constrained not only by the Poisson effect but by the limited set of available substrates, the present study shows that strain can be independently and continuously controlled along a single axis. This permits novel control over orbital populations through Jahn-Teller effects, as shown by Monte Carlo simulations on a double-exchange model. The ability to reversibly control a single lattice parameter substantially broadens the phase space for experimental exploration of predictive models and leads to new possibilities for control over materials' functional properties. The crystal lattice is one of the most accessible degrees of freedom in materials. In complex oxides, effective control over lattice parameters not only facilitates the understanding of multiple interactions in strongly correlated systems, but also creates new phases and emergent functionalities [1][2][3][4]. Lattice engineering has played an extremely important role in attempts to design strongly correlated systems and has led to many important discoveries [1,[5][6][7][8]. Control over lattice strain in films using different substrates [9] has revealed enhanced ferroelectricity [10] and superconductivity [11], as well as induced superconductivity in otherwise nonsuperconducting compounds [12]. However, the basic nature of the broken translational symmetry in the crystal lattice also entails a rigidity against arbitrary control [13]. There is so far no experimental technique that allows one to alter the lattice parameter solely along a single crystal axis, i.e., with an effective Poisson's ratio of zero. For strain engineering in systems with a nonzero Poisson ratio, the lattice constant, and hence the electronic structure, necessarily change in all three directions, clouding the cause-effect relations between single degrees of freedom and order parameters.We demonstrate an approach using helium implantation to effectively "strain dope" the lattice along a single axis of a La 0.7 Sr 0.3 MnO 3 (LSMO) film that is epitaxially latticelocked to a substrate. The out-of-plane (c-axis) lattice constant can be modified independently of the in-plane lattice constants. The c-axis strain can be continuously manipulated, and is thus not restricted by the limited collection of substrates that dictate conventional epitaxial strain engineering. The change in materials' properties, while reversible via a high temperature anneal, is persistent even well above room temperature. No continuous external actuation is required as with transient pressure-induc...

The localized acid−base properties of different, aluminum oxide thin layer surfaces have been evaluated with X-ray photoelectron spectroscopy (XPS). Five types of oxide layers were studied, which were produced by oxidizing aluminum in a vacuum, with an alkaline and acidic pretreatment, and in boiling water. The photoelectron core level binding energies, as measured with XPS, are evaluated for this purpose, while taking into consideration the initial and final state effects. For the structurally comparable oxides, the shifts in the O 1s binding energies are determined by their initial state chemistry. The values of the O 1s binding energy can be directly related to the surface-averaged charge on the O anions. For the Al cations, a correlation between the photoelectron core level binding energy shift and changes in the initial state chemistry was observed, but the Al 2p binding energy shifts were found to be partially due to changes in extra-atomic relaxation. The measured Al 2p binding energies and the binding energies of the resolved OH and O components in the O 1s peak showed that the studied aluminum oxides have OH sites with the same Brönsted/Lewis acid−base properties, O sites with the same Lewis base properties, and Al sites with very similar Lewis acid properties. The pseudoboehmite oxide, obtained by boiling aluminum in water, exhibits more basic O, OH, and Al sites. This oxide deviates structurally from the other oxides studied, resulting in a lower extra-atomic relaxation and Madelung potential contribution to the binding energies.

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Bulk rutile RuO2 has long been considered a Pauli paramagnet. Here we report that RuO2 exhibits a hitherto undetected lattice distortion below approximately 900 K. The distortion is accompanied by antiferromagnetic order up to at least 300 K with a small room temperature magnetic moment of approximately 0.05 µB as evidenced by polarized neutron diffraction. Density functional theory plus U (DFT+U ) calculations indicate that antiferromagnetism is favored even for small values of the Hubbard U of the order of 1 eV. The antiferromagnetism may be traced to a Fermi surface instability, lifting the band degeneracy imposed by the rutile crystal field. The combination of high Néel temperature and small itinerant moments make RuO2 unique among ruthenate compounds and among oxide materials in general.PACS numbers: 74.70. Pq,75.50.Ee,75.30.Fv Theories of magnetism in 3d transition metal oxides (TMOs) are usually framed in the context of strong Coulomb repulsions and Hund's rule coupling in the 3d orbitals of the transition metal cation, and their covalent bonding with the oxygen 2p orbitals. Strong on-site electron interactions tend to inhibit double occupancy of the 3d orbital and the overall Coulomb energy of the crystal is lowered by localizing the valence charge of the cation. Covalent bonding delocalizes the d-electron charge and thus lowers the kinetic energy. The former mechanism favors the formation of local magnetic moments while the latter decreases the moment but increases the exchange coupling between the moments through virtual hopping processes. In particular, the anion-mediated KramersAnderson "superexchange" between half-filled 3d orbitals often gives rise to strong antiferromagnetism. Many 3d transition metal oxides can be classified as antiferromagnetic Mott insulators where the on-site Coulomb repulsion U exceeds the electronic band width W . has been reported to host hightemperature antiferromagnetism with a Néel temperature T N = 563 K [5]. Ruthenium dioxide (RuO 2 ), on the other hand, has been thought to fall in line with other binary 4d transition metal oxides [6]; it is a good metal [7] and believed to be Pauli paramagnetic [8]. From the point of view of correlated electron physics and magnetism, RuO 2 seems to be one of the least interesting 4d TMOs. From a technology perspective, however, RuO 2 is by far one of the most important oxides. It has numerous applications in electro-and heterogeneous catalysis, as electrode material in electrolytic cells, supercapacitors, batteries and fuel cells, and as diffusion barriers in microelectronic devices [9]. It owes its usefulness in part to its relatively high electrical conductivity combined with its excellent thermal and chemical stability [10]. For the technological applications little attention has been paid to the potential role of magnetism (with the exception of Ref.[11]), presumably because magnetism is generally believed to be non-existent in bulk RuO 2 .In this letter we report on the finding that RuO 2 is distorted from the rutile symmetry...