Nanoribbons are model systems for studying nanoscale effects in graphene. For ribbons with zigzag edges, tunable bandgaps have been predicted due to coupling of spin-polarized edge states, which have yet to be systematically demonstrated experimentally. Here we synthesize zigzag nanoribbons using Fe nanoparticle-assisted hydrogen etching of epitaxial graphene/ SiC(0001) in ultrahigh vacuum. We observe two gaps in their local density of states by scanning tunnelling spectroscopy. For ribbons wider than 3 nm, gaps up to 0.39 eV are found independent of width, consistent with standard density functional theory calculations. Ribbons narrower than 3 nm, however, exhibit much larger gaps that scale inversely with width, supporting quasiparticle corrections to the calculated gap. These results provide direct experimental confirmation of electron-electron interactions in gap opening in zigzag nanoribbons, and reveal a critical width of 3 nm for its onset. Our findings demonstrate that practical tunable bandgaps can be realized experimentally in zigzag nanoribbons.

The band structure of the recently synthesized (3 × 3) silicene monolayer on (4 × 4) Ag(111) is investigated using density functional theory. A k-projection technique that includes the k⊥-dependence of the surface bands is used to separate the contributions arising from the silicene and the substrate, allowing a consistent comparison between the calculations and the angle-resolved photoemission experiments. Our calculations not only reproduce the observed gap and linear dispersion across the K point of (1 × 1) silicene but also demonstrate that these originate from the k⊥-dependence of Ag(111) substrate states (modified by interactions with the silicene) and not from a Dirac state.

We propose a guideline for exploring substrates that stabilize the monolayer honeycomb structure of silicene and germanene while simultaneously preserve the Dirac states: in addition to have a strong binding energy to the monolayer, a suitable substrate should be a large-gap semiconductor with a proper workfunction such that the Dirac point lies in the gap and far from the substrate states when their bands align. We illustrate our idea by performing first-principles calculations for silicene and germanene on the Al-terminated (0001) surface of Al2O3. The overlaid monolayers on Al-terminated Al2O3(0001) retain the main structural profile of the low-buckled honeycomb structure via a binding energy comparable to the one between silicene and Ag(111). Unfolded band structure derived from the k-projection method reveals that gapped Dirac cone is formed at the K point due to the structural distortion and the interaction with the substrate. The gaps of 0.4 eV and 0.3 eV respectively for the supported silicene and germanene suggest that they may have potential applications in nanoelectronics.

We present an investigation of the magnetic structure for iron-based superconductors (FeSCs) when inversion symmetry is broken, such as in substrate-supported monolayers or in the presence of a c-axis electric field. We perform group-, mean-field-, and density-functional-theoretic analyses on a model system of monolayer iron selenide (FeSe) on a strontium titanate (SrTiO3(001)) substrate. Our group-and mean-field-theoretic calculations are more generally applicable to thin films of the rest of the 11 (e.g., FeSe) family of iron-based superconductors, as well as to thin films of the 111 (e.g., LiFeAs) and 1111 (e.g., LaOFeAs) families, as these all belong to the same space group. We find that in systems with a collinear antiferromagnetic phase in bulk, when inversion symmetry is broken the transition is instead into a "spin vortex crystal" phase, and that a further phase transition can occur at a lower temperature in some circumstances. The spin vortex crystal is a C4 symmetric magnetic phase which is related to this parent C2 symmetric collinear antiferromagnetic (stripe) phase which is ubiquitous among the iron-based superconductors.

Reliable evaluation of the lattice thermal conductivity is of importance for optimizing the figureof-merit of thermoelectric materials. Traditionally, when deriving the phonon mediated thermal conductivity κ ph = κ − κ el from the measured total thermal conductivity κ the constant Lorenz number L0 of the Wiedemann-Franz law κ el = T L0σ is chosen. The present study demonstrates that this procedure is not reliable when the Seebeck coefficient |S| becomes large which is exactly the case for a thermoelectric material of interest. Another approximation using L0 − S 2 , which seem to work better for medium values of S 2 also fails when S 2 becomes large, as is the case when the system becomes semiconducting/insulating. For a reliable estimation of κ el it is proposed, that a full firstprinciples calculations by combining density functional theory with Boltzmann's transport theory has to be made. For the present study such an approach was chosen for investigating the clathrate type-I compound Ba8Au6−xGe40+x for a series of dopings or compositions x. For a doping of 0.8 electrons corresponding to x = 0.27 the calculated temperature dependent Seebeck coefficient agrees well with recent experiments corroborating the validity of the density functional theory approach.PACS numbers: 72.15. Jf, 72.15.Eb, Thermal conductivity plays an important role for the thermoelectric performance of a material as expressed by the figure-of-merit ZT = T S 2 σ/(κ el + κ ph ) which includes the Seebeck coefficient S, the electrical conductivity σ, and the thermal conductivity κ = κ el + κ ph summing up the contributions of electronic states and phonon mediated processes. Consequently, a low thermal conductivity in combination with large values of S and σ are desirable in order to achieve large values of ZT . Considerable efforts for lowering κ by reducing κ ph were made by utilizing structural properties, such as building up superlattices 1-4 and incorporating suitable filler atoms into structural cages 5-12 . These concepts rely on the strong scattering of heat-transporting phonon modes. However, neither κ el nor κ ph are directly measured. Rather, κ ph is derived by subtracting κ el from the measured total thermal conductivity, i.e., κ ph ≈ κ meas. − κ el in which the electronic thermal conductivity is estimated via the Wiedemann-Franz (WF) relation for simple metals, κ el ≈ T L 0 σ 6-11,13-21 . In this expression, L 0 is a universal constant and does not depend on temperature and materials properties. In the present work it is shown by a density functional theory (DFT) study for a typical thermoelectric material that the application of the WF law leads to unreliable estimates of κ el in particular when the Seebeck coefficient of the material is large, which is exactly the case of interest.The present theoretical study is based on the same DFT concept as applied for first-principles calculations of Seebeck coefficients (for example, see Ref. 22). In the present work the WF law is generalized by introducing a material and temperature depend...