Orbital Band Research Articles

Synthesis, Structure, Chemical Bonding, and Magnetism of the Series RELiGe2 (RE = La–Nd, Sm, Eu)

This article focuses on the synthesis and the crystal chemistry of six members of a series of rare-earth metal based germanides with general formula RELiGe(2) (RE = La-Nd, Sm, and Eu). The structures of these compounds have been established by single-crystal X-ray diffraction (CaLiSi(2) structure type, space group Pnma, Z = 4, Pearson symbol oP16). The chemical bonding within this atomic arrangement can be rationalized in terms of anionic germanium zigzag chains, conjoined via chains of edge-shared LiGe(4) tetrahedra and separated by rare-earth metal cations. The structure can also be viewed as an intergrowth of AlB(2)-like and TiNiSi-like fragments, or as the result of the replacement of 50% of the rare-earth metal atoms by lithium in the parent structure of the REGe monogermanides. Except for LaLiGe(2) and SmLiGe(2), the remaining four RELiGe(2) phases exhibit Curie-Weiss paramagnetism above about 50 K. In the low temperature regime, the localized 4f electrons in CeLiGe(2), PrLiGe(2), and SmLiGe(2) order ferromagnetically, while antiferromagnetic ordering is observed for NdLiGe(2) and EuLiGe(2). The calculated effective magnetic moments confirm RE(3+) ground states in all cases excluding EuLiGe(2), in which the magnetic response is consistent with Eu(2+) configuration (J = S = 7/2). The experimental results have been complemented by tight-binding linear muffin-tin orbital (TB-LMTO) band structure calculations.

Topological semimetal in a fermionic optical lattice

Optical lattices have an important role in advancing our understandingof correlated quantum matter. The recent implementation of orbital degrees of freedom in chequerboard1,2 and hexagonal3 optical lattices opens up a new avenue towards discovering novel quantum states of matter that have no prior analogues in solid-state electronic materials. Here, we predict that an exotic topological semimetal emerges as a parity-protected gapless state in the orbital bands of a two-dimensional fermionic optical lattice. This new quantum state is characterized by a parabolic band-degeneracy point with Berry flux 2π, in sharp contrast to the π flux of Dirac points as in graphene. We show that the appearance of this topological liquid is universal for all lattices with D4 point-group symmetry, as long as orbitals with opposite parities hybridize strongly with each other and the band degeneracy is protected by odd parity. Turning on inter-particle repulsive interactions, the system undergoes a phase transition to a topological insulator whose experimental signature includes chiral gapless domain-wall modes, reminiscent of quantum Hall edge states.

Conjugated polymers based on quinoxaline for polymer solar cells

Quinoxaline-based π-conjugated donor polymers with different substituent groups (phenyl, p-fluorophenyl, p-methoxyphenyl, methyl, or hydrogen) on the quinoxaline unit were synthesized and characterized, focusing on the effects of these substituent groups on the performance of bulk heterojunction (BHJ)-based polymer solar cells fabricated with phenyl C71 butyric acid methyl ester ([70]PCBM) as an acceptor. Polymers with phenyl groups or p-methoxyphenyl groups showed nanometer-scale phase separation or uniform morphology in BHJ film, together with high field effect transistor (FET) hole mobility. On the other hand, polymers with methyl or hydrogen and a polymer with fluorine exhibited micrometer-scale domains in BHJ and low hole mobility. It is suggested that the mostly steric hindrance of the substituents regulate the polymer's aggregation properties and compatibility with [70]PCBM, which probably affects the hole transport properties and BHJ morphologies. We also investigated the effects of substituent groups on highest occupied molecular orbital energy level and optical band gap of the polymers.

Direct measurement of density of states in pentacene thin film transistors

We report on direct high lateral resolution measurements of density of states in pentacene thin film transistors using Kelvin probe force microscopy. The measurements were conducted on passivated (hexamethyldisilazane) and unpassivated field effect transistors with 10- and 30-nm-thick pentacene polycrystalline layers. The analysis takes into account both the band bending in the organic film and the trapped charge at the SiO${}_{2}$--pentacene interface. We found that the density of states for the highest occupied molecular orbital band of pentacene film on the treated substrate is Gaussian with a width (variance) of $\ensuremath{\sigma}=0.07\ifmmode\pm\else\textpm\fi{}0.01\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ and an exponential tail. The concentration of the density of states in the gap for pentacene on bare SiO${}_{2}$ substrate was larger by one order of magnitude, had a different energy distribution, and induced Fermi level pinning. The results are discussed in view of their effect on pentacene thin film transistors' performance.

Electronic Structure of Aromatic Monomolecular Films: The Effect of Molecular Spacers and Interfacial Dipoles

The electronic structure of several arene-based monomolecular films on Au(111), with and without an alkyl linker between the aromatic unit and thiol headgroup, has been investigated by photoemission and inverse photoemission. While the HOMO–LUMO gap in these films was found to depend on the aromatic backbone, the molecular band offset of the electronic states was strongly affected by the interfacial dipole. The smallest HOMO–LUMO gap was found for the strongly conjugated anthracene moiety, intermediate for terphenyl, and widest for the perfluorinated terphenyl. The perfluorinated terphenyl-based films appear to be more n-type as a semiconductor than the terphenyl or anthracene-based monolayers, as indicated by the placement of the Fermi level (chemical potential) relative to the conduction or lowest molecular orbital band edge. Accordingly, the occupied electronic states related to the aromatic rings sink to greater binding energies, well below those for the alkyl linker, and thus for the perfluorinated t...

Electronic and magnetic properties of (Zn1−xVx)O diluted magnetic semiconductors elucidated from x-ray magnetic circular dichroism at VL2,3edges and first-principles calculations

The electronic structure of (Zn,V)O dilute magnetic semiconductors (DMSs) was investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method and the Korringa-Kohn-Rostoker Green's function approach within the local spin-density approximation. X-ray magnetic circular dichroism (XMCD) spectra at the V ${L}_{2,3}$ edges were calculated. Comparing the experimental results and the theoretical simulations we discuss possible crystal and magnetic structures of (Zn,V)O and reach the best agreement with the experimental XMCD spectra for the antiferromagnetically ordered DMSs in the presence of an oxygen vacancy. The corresponding structure was used to determine the electronic and magnetic properties of (Zn,V)O.

Self-Assembly, Optical, and Electrical Properties of a Novel Water-Soluble Perylene Bisimide

Highly fluorescent and water-soluble N,N′-bis(2-((5-((dimethylamino)methyl) furan-2-yl)methylthio)ethyl)perylene-3,4,9,10-tetracarboxylic diimide was synthesized, and characterized using various spectral techniques. The compound was highly fluorescent and exhibited strong absorption and emission in the visible spectral region. Computational studies predicted the highest occupied molecular orbital to lowest unoccupied molecular orbital band gap as 1.96 eV. The molecule self-assembled into a novel ball-like structure unlike the expected one-dimensional nanowire or nanobelt of other perylene bisimides. Molecular geometry optimization and computational studies at density functional theory as well as semiempirical levels strongly supported the observed self-assembly. New highly water-soluble N,N′-bis(2-((5-((dimethylamino)methyl)furan-2-yl) methylthio)ethyl)perylene-3,4,9,10-tetracarboxylic diimide molecule self-assembled as globules through π-stacking and rolling of its polar tail.

Interference-induced electron- and hole-conduction asymmetry

Principles established by Shephard and Paddon-Row for optimizing and controlling intramolecular electron transport through the modulation of interfering pathways are employed to design new molecules for steady-state conduction experiments aimed at manifesting electron–hole conduction asymmetry in a unique way. First, a review of the basic principles is presented through application to a pertinent model system in which a molecule containing donor and acceptor terminal linking groups with an internal multiple-pathway bridge is used to span two metal electrodes. Different interference patterns are produced depending on whether the through-molecule coupling pathways are symmetric or antisymmetric with respect to a topological bisecting plane, giving rise to asymmetric electron and hole conductances at the tight-binding (Huckel) level; this process is also described from a complementary molecular-orbital viewpoint. Subsequently, a new molecular system based on organic polyradicals is designed to allow such asymmetry to be realized in single-molecule conduction experiments. These polyradicals are analyzed using analogous simple models, density-functional theory (DFT) calculations of steady-state transmission, and intermediate neglect of differential overlap (INDO) calculations of intramolecular connectivity, verifying that polyradicals at low temperatures should show experimentally measureable electron–hole conduction asymmetry. A key feature of this system is that the polyradicals form a narrow partially occupied band of orbitals that lie within and well separated from the HOMO and LUMO orbitals of the surrounding molecular scaffold, allowing for holes and electrons to be transported through the same molecular band.

Complex and real unconventional Bose-Einstein condensations in high orbital bands

We perform a theoretical study on the recently observed unconventional Bose-Einstein condensations (UBEC) in the high bands of optical lattices. These exotic states are characterized by complex-valued condensate wave functions with nodal points or real-valued wave functions with nodal lines; thus, they are beyond the ``no-node'' theorem of conventional BECs. A quantum phase transition is driven by the competition between the single-particle band and interaction energies. The complex UBECs spontaneously break time-reversal symmetry, exhibiting a vortex-antivortex lattice structure.

High resolution X-ray absorption on metallicity selected C60 peapods, single-, and double walled carbon nanotubes

We have investigated the C1s core level X-ray absorption of bulk samples of C60-peapods and double wall carbon nanotubes (DWNTs) derived from either metallic or semiconducting single-walled carbon nanotubes (SWNTs). The clear cut separation of the different species unveils their distinct electronic van Hove singularities (VHS). The core level spectra of peapods are a quantitative superpositon of the conduction band of nanotubes and unoccupied molecular orbitals of C60. The outer shell metallicity selected DWNTs reveal changes in the π* resonance due to the inner shell VHS.

Eight-Coordinated Arsenic in the Zintl Phases RbCd4As3 and RbZn4As3: Synthesis and Structural Characterization

Reported are two new series of Zintl phases, ACd(4)Pn(3) and AZn(4)Pn(3) (A = Na, K, Rb, Cs; Pn = As, P), whose structures feature complex atomic arrangements based on four- and eight-coordinated arsenic and phosphorus. A total of 12 compounds have been synthesized from the corresponding elements via high temperature reactions, and their structures have been established by X-ray diffraction. RbCd(4)As(3), KCd(4)As(3), NaCd(4)As(3), NaZn(4)As(3), KCd(4)P(3), and KZn(4)P(3) crystallize with a new rhombohedral structure (space group R3m, Z = 3, Pearson symbol hR24), while the isoelectronic RbZn(4)As(3), CsCd(4)As(3), CsZn(4)As(3), KZn(4)As(3), CsZn(4)P(3), and RbZn(4)P(3) adopt the tetragonal KCu(4)S(3)-type structure (space group P4/mmm, Z = 1, Pearson symbol tP8). Both structures are very closely related to the ubiquitous CaAl(2)Si(2) and ThCr(2)Si(2) structure types, and the corresponding relationships are discussed. The experimental results have been complemented by linear muffin-tin orbital (LMTO) tight-binding band structure calculations. Preliminary transport properties measurements on polycrystalline samples suggest that the compounds of these families could be promising thermoelectric materials.

Stable Fulde-Ferrell-Larkin-Ovchinnikov pairing states in two-dimensional and three-dimensional optical lattices

We present the study of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing states in the $p$-orbital bands in both two and three-dimensional optical lattices. Due to the quasi one-dimensional band structure which arises from the unidirectional hopping of the orthogonal $p$-orbitals, the pairing phase space is not affected by spin imbalance. Furthermore, interactions build up high dimensional phase coherence which stabilizes the FFLO states in 2D and 3D optical lattices in a large parameter regime in phase diagram. These FFLO phases are stable with imposing the inhomogeneous trapping potential. Their entropies are comparable to those of the normal states at finite temperatures.

Frustrated Cooper pairing andf-wave supersolidity in cold-atom optical lattices

Geometric frustration in quantum magnetism refers to that magnetic interactions on different bonds cannot be simultaneously minimized. The usual Cooper pairing systems favor the uniform distribution of the pairing phase among lattice sites without frustration. In contrast, we propose "frustrated Cooper pairing" in non-bipartite lattices which leads to frustrated supersolid states with non-uniform distributions of the Cooper pair phase and density. This exotic pairing state naturally occurs in the $p$-orbital band in optical lattices with ultra-cold spinless fermions. In the triangular lattice, it exhibits an unconventional supersolid state with the $f$-wave symmetry.

Quantum anomalous Hall states in thep-orbital honeycomb optical lattices

We study the quantum anomalous Hall states in the $p$-orbital bands of the honeycomb optical lattices loaded with the single component fermions. Such an effect has not been realized in both condensed matter and cold atom systems yet. By applying the available experimental technique developed by Gemelke \textit{et al.} to rotate each lattice site around its own center, the band structures become topologically non-trivial. At a certain rotation angular velocity $\Omega$, a flat band structure appears with localized eigenstates carrying chiral current moments. With imposing the soft confining potential, the density profile exhibits a wedding-cake shaped distribution with insulating plateaus at commensurate fillings. Moreover, the inhomogeneous confining potential induces dissipationless circulation currents whose magnitudes and chiralities vary with the distance from the trap center. In the insulating regions the Hall conductances are quantized, and in the metallic regions the directions and magnitudes of chiral currents cannot be described by the usual local-density-approximation. The quantum anomalous Hall effects are robust at temperature scales small compared to band gaps, which increases the feasibility of experimental realizations.

Physical properties of GdFe2(AlxZn1−x)20

The high ferromagnetic ordering temperature of the dilute, rare-earth-bearing, intermetallic compound ${\mathrm{GdFe}}_{2}{\mathrm{Zn}}_{20}$ has been understood as being the consequence of the Gd${}^{3+}$ moment being embedded in a nearly ferromagnetic Fermi liquid. To test this understanding in detail, single crystals of the pseudoternary series ${\mathrm{GdFe}}_{2}({\mathrm{Al}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}{)}_{20}$ ($x\ensuremath{\leqslant}0.122$) and ${\mathrm{YFe}}_{2}({\mathrm{Al}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}{)}_{20}$ ($x\ensuremath{\leqslant}0.121$) were grown out of Zn-rich solution. Magnetization, heat capacity, and resistivity measurements show that, with Al doping, the ferromagnetic phase transition temperatures of the ${\mathrm{GdFe}}_{2}({\mathrm{Al}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}{)}_{20}$ compounds decrease from 86 K ($x=$ 0) to 10 K ($x=$ 0.122); for the nonmagnetic analog, the ${\mathrm{YFe}}_{2}({\mathrm{Al}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}{)}_{20}$ series, the Stoner enhancement factor $Z$ decreases from 0.88 ($x=$ 0) to 0.35 ($x=$ 0.121) in a similar manner. Tight-binding linear-muffin-tin orbital atomic-sphere approximation band structure calculations are used to rationalize this trend. These results, together with the earlier studies of the $R({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{)}_{2}{\mathrm{Zn}}_{20}$ ($R=$ Gd and Y) series, clearly highlight the importance of band filling and the applicability of even a simple, rigid-band model to these compounds.

Photoemission study on the valence band of a β-FeSi2 thin film using synchrotron radiation

Resonant and constant-initial state photoemission spectroscopies using synchrotron radiation were applied to investigate the valence-band electronic structure of a semi-conducting β-type iron-disilicide (β-FeSi(2)) thin film. The results clearly indicated that the component elements, iron (Fe) and silicon (Si), contribute differently to the valence band features; the Fe 3d orbitals mainly concentrate in the top region of the valence band while the Si 3s and 3p orbitals spread over the wide region of the valence band. The β-FeSi(2) thin film showed a typical p-type semi-conducting nature with a work function of 4.78 eV. The β-FeSi(2) film showed the Fe M(1)VV Auger lines around the kinetic energy of 88 eV. It would be expected from these observations that there exist strong interactions between iron and silicon atoms in the β-FeSi(2) film resulting in orbital mixing and band formation.

Generic redefinition, key and two new species of Acorystus Townes (Hymenoptera: Ichneumonidae: Cryptinae)

The taxonomic limits of Acorystus are reviewed. The genus is characterized by a body surface shiny and mostly impunctate, supra-antennal area with a conical horn, epomia and posterior transverse carina of propodeum entirely absent, ovipositor tip very slender, without nodus and with very weak teeth. Two new taxa, A. albicornis Scherrer et Santos, sp. nov., and A.circumflexus Scherrer et Santos, sp. nov., are described and illustrated. Acorystus albicornis is diagnosed mainly by the white band of the female flagellum covering only about five flagellomeres, supra-antennal area with a white central horn, with a very weak and slightly rugose longitudinal swelling, orbital band reaching supra-clypeal and supra-antennal areas, propodeum with anterior transverse carina almost straight, and pleural carina very weak, with short transverse wrinkles. It is recorded from two close localities at Espírito Santo State (19º57’55”S 40º32’25”W and 20º16’21”S 40º28’40”W), in the Atlantic Forest of Southeastern Brazil. Acorystus circumflexus is diagnosed mainly by the white band of the female flagellum covering about fifteen flagellomeres, supra-antennal area with a black central horn, with a strong and rugose longitudinal swelling, orbital band absent, propodeum with anterior transverse carina centrally arched forwards, and pleural carina entirely absent. It is recorded only from Caxiuanã (2º4’54”S 51º51’5”), Eastern Amazon Basin. A key to all three known species of the genus is provided.

Blue fluorescent polyamides containing naphthalene and oxadiazole rings

AbstractNew polyamides containing 1,3,4‐oxadiazole and naphthalene rings were prepared by low‐temperature solution polycondensation reaction of a new diamine containing preformed oxadiazole ring with various aromatic diacid chlorides. Elemental analysis, mass, infrared, and nuclear magnetic resonance spectroscopy were used to confirm the structure of the monomers and corresponding polymers. The thermal stability and glass transition temperatures of these poly(oxadiazole‐amide)s were measured and compared with those of related polymers. Their good solubility allows them to be processed in very thin films with smooth surfaces, without pinholes or cracks, when studied by atomic force microscopy. Upon irradiation with UV light the polymers showed photoluminescence maxima in the blue spectral range, both in solution and in solid state. Cyclic voltammetry (CV) was performed in order to obtain information about the electrochemical stability and reversibility of the redox processes of these polyamides. The highest occupied molecular orbital and the lowest unoccupied molecular orbital energy levels, and electrochemical and optical band gap values were calculated by using the results of CV and UV/vis, respectively, showing very good electron and hole injection and transport characteristics. These properties make the present polymers suitable for application in electroluminescent devices. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Proposed realization of itinerant ferromagnetism in optical lattices

We propose a scheme to realize the itinerant ferromagnetism of two-component cold fermionic atoms in the $p$-orbital bands in optical lattices. The band flatness in the two-dimensional honeycomb lattice dramatically amplifies the interaction effect, driving the ferromagnetic transition even with a relatively weak repulsive interaction. This scheme has the advantage that the stability of the system can be maintained without causing decay to the molecular state as one approaches the Feshbach resonance from the side with the positive scattering length. Experimental signatures and detections are also discussed.

F-wave pairing of cold atoms in optical lattices

The tremendous development of cold atom physics has opened up a whole new opportunity to study novel states of matter which are not easily accessible in solid-state systems. Here we propose to realize the $f$-wave pairing superfluidity of spinless fermions in ${p}_{x,y}$-orbital bands of two-dimensional honeycomb optical lattices. The nontrivial orbital band structure, rather than strong correlation effects, gives rise to the unconventional pairing with the nodal lines of the $f$-wave symmetry. With a confining harmonic trap, zero-energy Andreev bound states appear around the circular boundary with a sixfold symmetry. The experimental realization and detection of this novel pairing state are feasible.