Search

The search for one-dimensional electron systems with a giant Rashba-type spin splitting is of importance for the application of spin transport. Here we report, based on a first-principles density-functional-theory calculation, that Bi zigzag chains formed on a heterogeneous GaAs(110) surface have a giant spin splitting of surface states. This giant spin splitting is revealed to originate from spin-orbit coupling (SOC) and electric dipole interaction that are significantly enhanced by (i) the asymmetric surface charge distribution due to the strong SOC-induced hybridization of the Bi px, py, and pz orbitals and (ii) the large out-of-plane and in-plane potential gradients generated by two geometrically and electronically inequivalent Bi atoms bonding to Ga and As atoms. The results demonstrate an important implication of the in-plane and out-of-plane asymmetry of the Bi/GaAs(110) interface system in producing the giant spin splitting with the in-plane and out-of-plane spin components. [ABSTRACT FROM AUTHOR]

Recent experimental evidence that fluorinated graphene creates local magnetic moments around F adatoms has not been supported by semilocal density-functional theory (DFT) calculations where the adsorption of an F adatom induces no magnetic moment in graphene. Here, we show that such an incorrect prediction of the nonmagnetic ground state is due to the self-interaction error inherent in semilocal exchange-correlation functionals. The present hybrid DFT calculation for an F adatom on graphene predicts not only a spin-polarized ground state with a spin moment of ∼1 μB, but also a long-range spin polarization caused by the bipartite nature of the graphene lattice as well as the induced spin polarization of the graphene states. The results provide support for the experimental observations of local magnetic moments in fluorinated graphene. [ABSTRACT FROM AUTHOR]

We present theoretical investigations of the self-assembled growth of one-dimensional (1D) molecular lines directed across the dimer rows on the H-terminated Si(001)-(2 × 1) surface. Based on density-functional theory calculations, a growth approach of the 1D acetylacetone line is proposed, which involves the radical chain reaction initiated at two dangling-bond sites on one side of two adjacent Si dimers. It is also enabled that, if an H-free Si dimer were employed as the initial reaction site, a 1D acetylacetone line can grow along the dimer row. Our findings represent a novel insight into the growth of 1D molecular lines not only across but also along the dimer rows on the H-terminated Si(001)-(2 × 1) surface. [ABSTRACT FROM AUTHOR]

Using first-principles density-functional calculations, we investigate the structure and growth mechanism of allyl mercaptan lines on the H-terminated Si(100)-2 × 1 surface. The earlier structural model (termed the linear structure), where the terminal C atom of the C=C bond initially reacts with a single Si dangling bond, has been competing with a new structural model (termed the branched structure) where the medial C atom of the C=C bond initially reacts with a single Si dangling bond. We find that formation of the branched structure is kinetically unfavored over that of the linear structure. Moreover, the simulated scanning tunneling microscopy (STM) image of the branched structure cannot reproduce the features observed in the STM experiment, such as the position and origin of the bright protrusion. Thus, the present results do not support the branched structure in many respects. [ABSTRACT FROM AUTHOR]

A recent density-functional calculation for fcc C60Hn (n = odd) [K. W. Lee and C. E. Lee, Phys. Rev. Lett. 106, 166402 (2011)] proposed the existence of Stoner ferromagnetism based on an itinerant band model. However, our density-functional calculation shows that the antiferromagnetic (AFM) configuration is slightly more stable than the ferromagnetic (FM) one. This preference for antiferromagnetism over ferromagnetism is analogous to the case of a dimer (C60H)2, where each C60H is spin polarized by an intramolecular exchange and the two magnetic moments are antiferromagnetically coupled with each other. The results demonstrate that the underlying mechanism of the magnetic order in fcc C60Hn is associated with the AFM superexchange between the magnetic moments created by H dopants. [ABSTRACT FROM AUTHOR]

van der Waals energy-corrected density functional theory (DFT + vdW) as well as the exact exchange with electron correlation in the random-phase approximation are used to study the adsorption of benzene on the Si(001) surface with respect to two controversial adsorption structures (termed "butterfly" and "tight bridge"). Our finding that the tight-bridge structure is energetically favored over the butterfly structure agrees with standard DFT but conflicts with previous vdW-inclusive calculations. However, the inclusion of zero-point energy and thermal vibrations reverses the stability of the two structures with increasing temperature. Our results provide an explanation for the recent experimental observation that both structures coexist at room temperature. [ABSTRACT FROM AUTHOR]

First-principles calculations combined with kinetic Monte Carlo simulations are carried out to unambiguously demonstrate the vital role of van der Waals (vdW) interactions in the self-assembly of styrene nanowires on H-terminated Si(100) surfaces. We find that, only with the inclusion of London dispersion forces, accounting for the attractive parts of vdW interactions, are the effective intermolecular interactions reversed from repulsive to attractive. Such attractive interactions, in turn, ensure the preferred growth of long wires under physically realistic conditions as observed experimentally. We further propose a cooperative scheme, invoking the application of an electric field and the selective creation of Si dangling bonds, to drastically improve the ordered arrangement of the molecular nanowires. The present paper represents a significant step forward in the fundamental understanding and precise control of molecular self-assembly guided by London dispersion forces. [ABSTRACT FROM AUTHOR]

Using first-principle density-functional calculations, we investigate the competition between Peierls instability and spin ordering in a zigzag C chain generated in an H-passivated graphene. We find that such a quasi-one-dimensional (1D) C chain of infinite length stabilizes a Peierls instability showing bond alternation with a band-gap opening. As the chain length becomes finite, the Peierls distortion is enhanced at even-numbered chains, while a ferrimagnetic spin ordering with the atomic structure of a topological soliton is stabilized at odd-numbered chains. These results indicate the existence of complex interplay between electron-lattice and electron-electron interactions with respect to the parity of the quasi-1D C chains. [ABSTRACT FROM AUTHOR]

It was recently proposed that the stress state of a material can also be altered via electron or hole doping, a concept termed electronic stress (ES), which is different from the traditional mechanical stress (MS) due to lattice contraction or expansion. Here we demonstrate the equivalence of ES and MS in structural stabilization, using In wires on Si(l 11) as a prototypical example. Our systematic density-functional theory calculations reveal that, first, for the same degrees of carrier doping into the In wires, the ES of the high-temperature metallic 4 × 1 structure is only slightly compressive, while that of the low-temperature insulating 8 × 2 structure is much larger and highly anisotropic. As a consequence, the intrinsic energy difference between the two phases is significantly reduced towards electronically phase-separated ground states. Our calculations further demonstrate quantitatively that such intriguing phase tunabilities can be achieved equivalently via lattice-contraction induced MS in the absence of charge doping. We also validate the equivalence through our detailed scanning tunneling microscopy experiments. The present findings have important implications for understanding the underlying driving forces involved in various phase transitions of simple and complex systems alike. [ABSTRACT FROM AUTHOR]

The Sn overlayer on the Si (111) surface has been considered as a prototypical system for exploring two- dimensional (2D) correlated physics on the triangular lattice. Most of the previous theoretical studies were based on the presumption that the surface state dominantly originates from Sn dangling-bond (DB) electrons, leading to a strongly correlated 2D electronic system. By contrast, our density-functional theory calculations show that the Sn DB state significantly hybridizes with Si substrate states to form a resonant state. The strong resonance between the Sn 5pz and Si 3pz orbitals facilitates the recently observed antiferromagnetic order through superexchange interactions, giving rise to a band-gap opening. It is thus demonstrated that the insulating ground state of Sn/Si(111) can be characterized as a Slater-type insulator via band magnetism. [ABSTRACT FROM AUTHOR]

Based on first-principles density-functional theory calculations, we present a comparative study of the electronic structures of ultranarrow zigzag graphene nanoribbons (ZGNRs) embedded in a hexagonal boron nitride (BN) sheet and fully hydrogenated graphene (graphane) as a function of their width N (the number of zigzag C chains composing the ZGNRs). We find that ZGNRs/BN have the nonmagnetic ground state except at N=5 and 6 that weakly stabilize as a half-semimetallic state, whereas ZGNRs/graphane with N =2 exhibit a strong antiferromagnetic coupling between ferromagnetically ordered edge states on each edge. It is revealed that the disparate magnetic properties of the two classes of ZGNRs are attributed to the contrasting interedge superexchange interactions arising from different interface structures: that is, the asymmetric interface structure of ZGNRs/BN gives a relatively short-range and weak superexchange interaction between the two inequivalent edge states, while the symmetric interface structure of ZGNRs/graphane gives a long-range, strong interedge superexchange interaction. [ABSTRACT FROM AUTHOR]