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The quasiparticle properties resulting from charge and spin are clearly identified in nitrogen-substituted silicenes, for which a theoretical framework is successfully developed from first-principles calculations. Such systems create extremely non-uniform chemical and physical environments through the distribution of the guest atoms. They present unusual geometric, electronic, and magnetic properties, which can be identified from the optimal honeycomb lattices, the atom- and spin-dominated energy spectra, the spatial charge density distributions, and the atom-, orbital- and spin-projected van Hove singularities [the net magnetic moments]. The complicated relations between the highly hybridized sp2-N–Si bonds and the ferromagnetic/non-magnetic configurations are responsible for the p-type or semiconducting behavior, the significant modifications to the Dirac cone structures, the difficulty in identifying the π and σ bands, and the vanishing or finite magnetic moments. The theoretical predictions could be verified by high-resolution experimental measurements.

Coiled-coil proteins consisting of multiple copies of helices take part in transmembrane transportation and oligomerization, and are used for drug delivery. Cross-alpha amyloid-like coiled-coil structures, in which tens of short helices align perpendicular to the fibril axis, often resist molecular replacement due to the uncertainty to position each helix. Eight coiled-coil structures already solved and posted in the protein data bank are reconstructed ab initio to demonstrate the direct phasing results. Non-crystallographic symmetry and intermediate-resolution diffraction data are considered for direct phasing. The retrieved phases have a mean phase error around 30∼40°. The calculated density map is ready for model building, and the reconstructed model agrees with the deposited structure. The results indicate that direct phasing is an efficient approach to construct the protein envelope from scratch, build each helix without model bias which is also used to confirm the prediction of AlphaFold and RosettaFold, and solve the whole structure of coiled-coil proteins.

The magneto-transport properties of phosphorene are investigated by employing the generalized tight-binding model to calculate the energy bands. For bilayer phosphorene, a composite magnetic and electric field is shown to induce a feature-rich Landau level (LL) spectrum which includes two subgroups of low-lying LLs. The two subgroups possess distinct features in level spacings, quantum numbers, as well as field dependencies. These together lead to anomalous quantum Hall (QH) conductivities which include a well-shape, staircase and composite quantum structures with steps having varying heights and widths. The Fermi energy-magnetic field-Hall conductivity (EF−Bz−σxy) and Fermi energy-electric field-Hall conductivity (EF−Ez−σxy) phase diagrams clearly exhibit oscillatory behaviors and cross-over from integer to half-integer QH effect. The predicted results should be verifiable by magneto-transport measurements in a dual-gated system.

A thyroid receptor alpha structure (PDB ID: 4LNW) was studied for ab initio phasing. With the diffraction intensity data, protein sequence, and ligand structure as the only input, a high-resolution structure was successfully reconstructed by using an iterative projective method based on the hybrid input&ndash, output (HIO) algorithm.

An iterative projection algorithm proposed previously for direct phasing of high-solvent-content protein crystals is extended to include non-crystallographic symmetry (NCS) averaging. For proper NCS, when the NCS axis is positioned, the molecular envelope can be automatically rebuilt. For improper NCS, when the NCS axis and the translation vector are known, the molecular envelope can also be automatically reconstructed. Some structures with a solvent content of around 50% could be directly solved using this ab initio phasing method. Trial calculations are described to illustrate the methodology. Real diffraction data are used and the calculated phases are good for automatic model building. The refinement of approximate NCS parameters is discussed.

An iterative transform algorithm is proposed to improve the conventional molecular-replacement method for solving the phase problem in X-ray crystallography. Several examples of successful trial calculations carried out with real diffraction data are presented., An iterative transform method proposed previously for direct phasing of high-solvent-content protein crystals is employed for enhancing the molecular-replacement (MR) algorithm in protein crystallography. Target structures that are resistant to conventional MR due to insufficient similarity between the template and target structures might be tractable with this modified phasing method. Trial calculations involving three different structures are described to test and illustrate the methodology. The relationship of the approach to PHENIX Phaser-MR and MR-Rosetta is discussed.

For direct phasing of protein crystals, a method based on the hybrid-input-output (HIO) algorithm has been proposed and tested on a variety of structures. So far, however, the diffraction data have been limited to high-resolution ones, i.e., higher than 2 Å. In principle, the methodology can be applied to data of lower resolutions, which might be particularly useful for phasing membrane protein crystals. For resolutions higher than 3.5 Å, it seems the atomic structure is solvable. For data of lower resolutions, information of the secondary structures and the protein boundary can still be obtained. Examples are given to support the conclusions. Real experimental data are used. Two aspects of the observed data have been discussed: removal of the measured low-resolution reflections and involvement of the unmeasured high-resolution reflections. The ab initio phasing employs histogram matching for density modification. A question arises whether the reference histogram used should match the resolution of the diffraction data or not. It seems that there is an optimal histogram which is good to use for data at various resolutions.

The magneto-optical properties of bilayer phosphorene is investigated by the generalized tight-binding model and the gradient approximation. The vertical inter-Landau-level transitions, being sensitive to the polarization directions, are mainly determined by the spatial symmetries of sub-envelope functions on the distinct sublattices. The anisotropic excitations strongly depend on the electric and magnetic fields. A uniform perpendicular electric field could greatly diversify the selection rule, frequency, intensity, number and form of symmetric absorption peaks. Specifically, the unusual magneto-optical properties appear beyond the critical field as a result of two subgroups of Landau levels with the main and side modes. The rich and unique magnetoabsorption spectra arise from the very close relations among the geometric structures, multiple intralayer and interlayer hopping integrals and composite external fields.

To study the Hofstadter spectrum of monolayer molybdenum disulfide (MoS2), we systematically examine the magnetic energy spectra due to various hoppings between d-orbital electrons to illustrate the link between the Bloch bands and the corresponding magnetoelectronic spectra. The magnetoelectronic spectrum shows a mirror (inversion) symmetry as a result of the particle–hole symmetry (asymmetry) in the Bloch bands. At small field, specific Landau fan diagrams can be ascribed to certain Bloch-band singularities. In the spectrum of real MoS2, we further find a breaking of spectral symmetry, the spin and valley polarization, and a flux-dependent energy gap. Our numerical results can facilitate a qualitative understanding of the topological nature of d-bands and provide a basis for exploring the Landau levels in transition-metal dichalcogenides.

In an iterative projection algorithm proposed forab initiophasing, the error metrics typically exhibit little improvement until a sharp decrease takes place as the iteration converges to the correct high-resolution structure. Related to that is the small convergence probability for certain structures. As a remedy, a variable weighting scheme on the diffraction data is proposed. It focuses on phasing low- and medium-resolution data first. The weighting shifts to incorporate more high-resolution reflections when the iteration proceeds. It is found that the precipitous drop in error metrics is replaced by a less dramatic drop at an earlier stage of the iteration. It seems that once a good configuration is formed at medium resolution, convergence towards the correct high-resolution structure is almost guaranteed. The original problem of phasing all diffraction data at once is reduced to a much more manageable one due to the dramatically smaller number of reflections involved. As a result, the success rate is significantly enhanced and the speed of convergence is raised. This is illustrated by applying the new algorithm to several structures, some of which are very difficult to solve without data weighting.

The magneto-electronic properties of bilayer zigzag graphene nanoribbons are investigated by the Peierls tight-binding method. In the presence of magnetic fields, Landau quantization leads to the formation of Landau subbands. For the bilayer nanoribbons, these subbands are partially dispersionless in k-space and are called quasi-Landau levels (QLLs). Perpendicular electric fields, serving as the top gate, push the QLLs to higher state energy and split the flat subbands. From the evidence of band structure and density of states, the QLLs remain dispersionless and the corresponding peaks are still the main structure of density of states, which means that the material properties related to the QLLs are unchanged. However, the wave functions present a totally different evidence that the Landau wave functions are severely mixed, and the corresponding material properties would be strongly affected or destroyed. The wave functions provide an effective way to comprehend the characteristics of the flat subbands and Landau subbands. The energy spectra, density of states, and wave functions are discussed in detail.

A square hole array is fabricated over a micro-bridge of NbN thin film by electron beam lithography and reactive ion milling. Magneto-resistance is measured across the micro-bridge filled with a hole array near the superconducting transition temperature. It is found that magneto-resistance minima occur when the number of vortices is an integer multiple or a fractional multiple of the number of holes. The temperature and the current dependences of the matching effect are studied.

To understand several peculiar features of d-wave superconductors such as the cuprates, we have studied a two-dimensional tight-binding (extended Hubbard) model in which the electrons interact with a potential which is attractive at nearest neighbor distance and repulsive elsewhere. Such a model is designed to produce d-wave superconductivity. It also yields antiferromagnetism and phase separation. In the two-phase region, due to vanishing compressibility the system can be easily rendered inhomogeneous by weak perturbations. Thus inhomogeneous superconductivity is a likely consequence of d-wave pairing. In addition to inhomogeneity the model also reproduces a phase diagram strikingly similar to the one observed in the cuprates. As such it throws some light on the nature of the pseudogap state.

As a non-phonon mechanism of superconductivity (SC), Little has proposed models in which the pairing force is induced by electronic polarization of molecules embeded in the conductor. Hirsch and Scalapino have studied a tight-binding model of this type in one dimension (1D) and have found that superconductivity is dominated by charge–density-wave (CDW). We extend their calculations to two dimensions (2D). Our quantum Monte Carlo calculations suggest that the charge–density-wave formation is limited to a region close to half-filling. Outside of that superconductivity flourishes even in the presence of a finite onsite repulsion, demonstrating the Little model as a viable model for electronic mechanism of superconductivity. The results might be relevant to high-temperature superconductors.

We have studied a correlated fermion model with nearest neighbor attractive interactions on a square lattice by means of quantum Monte Carlo simulation. At half-filling this model exhibits rather strong correlation effects. Charge fluctuations are suppressed and the ground state is an insulator with long-range anitferromagnetic correlations. When doped with holes this model exihibits phase separation which seems to be triggered by the antiferromagnetic long-range correlations.

An iterative transform method is proposed for solving the phase problem in protein crystallography. In each iteration, a weighted average electron-density map is constructed to define an estimated protein mask. Solvent flattening is then imposed through the hybrid input–output algorithm [Fienup (1982).Appl. Opt.21, 2758–2769]. Starting from random initial phases, after thousands of iterations the mask evolves into the correct shape and the phases converge to the correct values with an average error of 30–40° for high-resolution data for several protein crystals with high solvent content. With the use of non-crystallographic symmetry, the method could potentially be extended to phase protein crystals with less than 50% solvent fraction. The new phasing algorithm can supplement and enhance the traditional refinement tools.

Cooperative absorption transitions between two Eu 3+ ions are reported from measurements of the 80 K absorption spectrum of Eu 2 O 2 S. The transitions are 4 to 5 times weaker, relative to single ion f–f transitions, than previously reported from measurements of the excitation spectrum by Imanaga et al., J. Lumin. 16 (1978) 77, attributed to saturation effects.

The structure of complex [PyH][Ni(mnt)2], as determined by X-ray single crystal analysis, consists of pyridinium cations and [Ni(mnt)2]− anions. Crystallographic data for the complex: monoclinic P21/c, a = 16.5430(18) A, b = 6.3847(9) A, c = 17.430(2) A, β = 118.040(7)°, V = 1625.4(3) A3, Z = 4. The [Ni(mnt)2]− anion is not a planar and the CN groups bend away from the S4 plane. The interactions between anions and cations via the formation of bifurcated N–H···N≡C hydrogen bonds give rise to an 1D zigzag chain of alternating cations and anions.

Distinct species U(III), U(IV), U(V) and U(VI) have been identified when U(VI) or U(IV) species are doped into the cubic elpasolite lattice. The band intensities, and derived vibrational frequencies, in the luminescence and absorption spectra of the uranyl ion situated in this lattice are compared with those of Cs2UO2Cl4. In particular, the intensity enhancement of the B2g→A1g(D4h) hypersensitive transition shows that the uranyl ion occupies a majority site of C2v or lower symmetry.

Using an auxiliary-field quantum Monte Carlo (AFQMC) method, we have studied a two-dimensional tight-binding model in which the conduction electrons can polarize an adjacent layer of molecules through electron–electron repulsion. Calculated average conduction electron density as a function of chemical potential exhibits a clear break characteristic of phase separation. Compared to the noninteracting system, the d -wave pair-field correlation function shows significant enhancement. The simultaneous presence of phase separation and d -wave superconductivity suggests that an effective extended pairing force is induced by the electron–exciton coupling.

Using a symmetrized density-matrix renormalization-group formulation, we have investigated the electronic binding energy of the lowest optically allowed exciton (1B{sub u}) within an extended Hubbard-Peierls model (with parameters U, V, and {delta}) for conjugated chains with N=80 sites. Three symmetries, the C{sub 2}, spin parity, and electron-hole symmetries, have been applied to construct the projector operator. Analysis of our results on the ratios between exciton binding energies and exciton energies, sheds light on the current experimental and theoretical controversy on exciton binding for polyacetylene and polyparaphenylene vinylene. We show that in the absence of dimerization, the exciton binding energy is vanishingly small for U/t up to 5 and V/t up to 2.3; for a finite dimerization, it is only when V is large enough that the exciton gets to be significantly bound. This result questions the applicability to conjugated polymers of the strong correlation picture, in which the exciton binding energy is equal to V. {copyright} {ital 1997} {ital The American Physical Society}

In this paper we present a microscopic hierarchy theory of the fractional quantum Hall effect. The wave functions of the ground state and the collective excitation states are obtained in terms of the electron coordinates. Working in the subspace spanned by the quasiparticles of the 1/m L Laughlin ground state, with m L an odd integer, it is shown that there exists a simple mapping between electron states in the quasiparticle subspace and states of an auxiliary boson system which is defined such that the number of the bosons is the same as that of the quasiparticles and the total magnetic flux quanta seen by the bosons equals the number of electrons. For the auxiliary boson system, one can write down the Laughlin state as well as the density wave states, analogous to the electron system at filling factor 1/m L . By mapping these states onto the quasiparticle subspace of the electrons, we find that the resulting wave functions provide a quite good description for the ground state and the collective excitations respectively of the original electron system at filling factor ν=1/(m L (± 1/2p)) with p a positive integer. This construction of the ground state and the collective excitation states can be repeated for higher filling factors. The theory presented in this paper can be viewed as a microscopic realization of Haldane's original hierarchy picture.

We adopt the Su-Schrieffer-Heeger (SSH) Hamiltonian supplemented by a Hubbard term, which we call the SSHH model, for systems with a degenerate ground state, and the Brazovskii-Kirova (BK) Hamiltonian supplemented by a Hubbard term (BKH model) for systems with a nondegenerate ground state, to study the effect of electron correlation on doping in conjugated polymers. The density matrix renormalization group (DMRG) technique developed by White has proved to be a very powerful tool for treating low-dimensional systems with strong correlation. This method is applied to the above models. We first study the metal-insulator transition within the SSHH model. With the BKH model, we investigate the formation of bipolarons versus separated polarons in a nondegenerate system. Optical signatures of polarons are also calculated.

We discuss how simulated annealing can be employed to solve the X-ray phase problem. Examples ranging from small molecules to proteins are given to demonstrate the capability of the method.

Low-temperature scanning electron microscopy (LTSEM) is a promising measuring technique for probing the spatial distribution of the superconducting properties of the high-temperature superconducting (HTS) thin films. A theoretical analysis of the electron-beam irradiation on the HTS thin films in the LTSEM has been carried out. An inhomogeneously distributed grain array model has been applied in the analysis, and some numerical simulations have been carried out on the electron-beam induced voltage (EIV) signals in the LTSEM experiments. The comparisons of our numerical results with the LTSEM experimental data indicate that it is quite reasonable to use a two-dimensional Josephson junction array for stimulating the inhomogeneous HTS thin film sample. Our numerical results also show that the EIV signals are influenced by the electron-beam power used in the LTSEM, and a reduction of the electron-beam power is suggested in order to eliminate the errors in estimating the local values of critical temperature Tc and critical current Ic by the sample temperature and the bias current at which the first EIV signal occurs.

The effect of a parallel magnetic field on the quantum Hall state of a double-layer system at total filling factor \ensuremath{\nu}=(4n+1), with n being the Landau-level index, is studied. While our result is consistent with previous ones, which indicate that the system at \ensuremath{\nu}=1 undergoes a parallel-magnetic-field-induced phase transition, we find that the phase transition also takes place at higher filling factors. For a certain parameter range, we observe an instability of the quantum Hall state towards probably a charge-density-wave state at a smaller parallel magnetic field before the phase transition takes place.

We have carried out a theoretical study of the lattice relaxation of some lox-lying even-parity singlet exited states in polyacetyelene. Both degenerate perturbation and strong coupling calcualtions give very similar results which indicate the relaxed configuration of the lowest Ag state is a 4-soliton bound state, with a binding energy of about 0.05 eV against dissociation into four free neutral solitons. Experimental implications of the theoretical findings are discussed.

Within a theoretical approach including both electron-phonon and electron-electron interaction terms, we investigate the relaxation processes of photoexcited states in oligomers and long chains of poly(paraphenylene vinylene), PPV. Our direct calculations indicate that in PPV the difference in relaxation energy between two polarons and one singlet [triplet] polaron-exciton decreases [increases] with the Hubbard U term; the long-range interaction V terms stabilize both the singlet and triplet polaron-excitons. The theoretical simulation of the photoinduced absorption spectra both for singlet and triplet states are in very a good agreement with experiment.

A YBa2Cu3O7-δ thin film is modified by a probe electric field of an atomic force microscope to form a ridge with the width of only a grain cell. The modification varies with the operation parameters of the bias voltage, the moving velocity of the probe and the ambient humidity. Energy dispersive spectroscopy analysis shows only oxygen deficiency in the modified YBCO thin film. As a result, the suppressed superconductivity was found in the junction crossing the ridge.

A tight-binding model is diagonalized to elucidate the magnetoelectronic properties of bulk graphite. This method reveals far more significant results than previous treatments based on simplified effective-mass approximation. The wave functions are explicitly presented and their spatial distributions appropriately characterize the Landau subbands. The calculated density of states and optical absorption spectra agree well with experiments. Our accurate results enable one to assess to what degree the bulk graphite resembles monolayer and bilayer graphenes. The inclusion of all interlayer coupling leads to rich spectral features awaiting future experimental verifications.

Nonlinear optical spectra of conjugated polymers are theoretically studied in the Su-Schrieffer-Heeger model supplemented by long-range Coulomb interactions. Excitonic correlation for electron-hole excitations in explicitly taken into account by a standard method. Nonlinear susceptibilities χ (3) are calculated numerically with a standard sum-over-states method for finite chains (up to 1000 sites). Using moderate interaction strength, our calculations can reproduce many distinct features observed in the linear and nonlinear spectra (two-photon absorption, third-harmonic generation, and electroabsorption) of polydiacetylenes and some other polymers. Especially, a hump in the spectrum of third-harmonic generation on the high energy side of its main peak is shown to be due to the three-photon resonance to weakly bound or unbound electron-hole states around the exciton ionization threshold.

A recently developed hierarchy theory of the fractional quantum Hall effect (FQHE) is reviewed in this article. We use a many quasiparticle (QP) wave function to obtain the effective partition function of a FQHE system in the path-integral representation, which has been shown to be formally equivalent to an anyon system confined to the lowest Landau level. However the two systems are different in the dimension of the Hilbert space, which implies that the standard hierarchy theory is invalid beyond second hierarchical states. The only way to overcome the difficulty is to place anyons in higher Landau levels. By making a mean-field approximation to the statistical gauge field, the QPs are converted to bosons and a constraint boson system is derived. We find a new type of condensate of the bosonic QPs, which results in a new type of incompressible states of the electrons. These states which involve only two hierarchies should have higher stability over others. That explains why some sequences of filling fractions occur prominently in experiment. Finally, we show that there is a one-to-one correspondence between the new hierarchy theory and the composite fermion theory proposed by Jain.

The doping dependence of spin excitations in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ is studied based on a two-orbital model under RPA approximation. The interplay between the spin-density-wave (SDW) and superconductivity (SC) is considered in our calculation. Our results for the spin susceptibility are in good agreement with neutron scattering (NS) experiments in various doping ranges at temperatures (T) above and below the superconducting transition temperature T$_{c}$. For the overdoped sample where one of the two hole pockets around $\Gamma$ point disappears according to ARPES, we show that the imaginary part of the spin susceptibility in both SC and normal phases exhibits a gap-like behavior. This feature is consistent with the "pseudogap" as observed by recent NMR and NS experiments., Comment: 5 pages, 2 figures

We study interorbital pairings of iron pnictide superconductors within a minimal two-orbital tight-binding model. We find that in real space, a set of self-consistently determined pairing order parameters forms two sublattices with a relative phase of $\ensuremath{\pi}$ and the pairing symmetry is ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}\ensuremath{\sim}\text{cos}\text{ }{k}_{x}\ensuremath{-}\text{cos}\text{ }{k}_{y}$. In momentum space, it corresponds to the $\ensuremath{\eta}$ pairing proposed by Yang [Phys. Rev. Lett. 63, 2144 (1989)], with nonzero momenta of Cooper pairs. One physical consequence of this type of pairing is the existence of a significant amount of zero energy (gapless) states around the Fermi surface even in the absence of disorder, which contradicts current experiments thus excluding such a pairing in iron pnictide superconductors.

A quantum Hall system is investigated by exact numerical diagonalization of the Hamiltonian. The energy, off-diagonal long-range order, crystal long-range order, and the mean occupation number p(j)= of the jth Landau orbital for v=1/2 ground state are calculated. Our results indicate no sign of either electron pairing or phase separation, the two possible features previously suggested for the v=1/2 ground state. A possible quasiparticle pairing mechanism is proposed in trying to account for the numerical results and experimental observations

Short polyenes exhibit optical properties similar to those of nondegenerate polymers. The broad subgap absorption tail, the vibronic features in the absorption as well as in the luminescence spectra together with the large Stokes' shift are evidence of nonlinear quantum lattice fluctuations. We have carried out a detailed and careful study of those effects in the context of the SSH model. Implications of this on the optical properties of conjugated polymers are discussed. Another interesting theoretical result is the dispersion of the resonant Raman-line profile due to those intrinsic lattice fluctuations. This contrasts with the interpretation of the same effect in terms of sample inhomogeneity.

Nonlinear optical susceptibilities of conjugated polymers are studied in the Su-Schrieffer-Heeger model supplemented by long-range Coulomb interactions, taking account of the formation of excitons and the influence of disorder. In the calculated third-harmonic generation spectrum, on the high energy side of the main peak (due to the lowest singlet exciton) a second peak (due to the three-photon resonance to the electron-hole continuum) is found that is strong enough to remain significant even in the presence of disorder. The results are in good agreement with experiments in polydiacetylene and polythiophene.

Third-order nonlinear optical susceptibilities of σ-conjugated one-dimensional polymers have been calculated in a tight-binding model by taking account of the formation of excitons due to long-range Coulomb interactions. The spectrum of third-harmonic generation (THG) exhibits peaks due to excitons as well as unbound electron—hole states, in contrast to the linear absorption spectrum which is dominated by the lowest exciton state. The results are in excellent agreement with recent experiments in polysilanes not only for THG but also for linear absorption, two-photon absorption, and electroabsorption in a mutually consistent way.

We proposed a modified Schwinger boson theory for quantum Heisenberg model which naturally satisfies the spin identity [Formula: see text], and in which the redundant degrees of freedom due to an average treatment of the constraints are discarded by introducing new boson operators. The results, either for free energy or for spin-spin correlation are exactly the same as those of Takahashi’s modified spin wave theory.7 This theory provides a unified approach to study the ferromagnet and antiferromagnet on equal footing. As such it would be a good starting point for discussing antiferromagnetism in the presence of holes, where ferromagnetic component plays an important role.

A study of the Meissner effect and the plasmon spectrum for an interacting anyon system is presented. It is shown that the Meissner effect is not sensitive to, and still persists in, the presence of repulsive Coulomb interaction among anyons. Within the random phase approximation we show that the collective excitation of the system in the long-wavelength limit corresponds to the plasmon mode of a two dimensional interacting electron gas and it does not depend on the statistics associated with the anyons. There is a roton minimum of the collective mode at a finite value of the wavevector. A similar analysis on an array of layered anyon systems is also reported.

The ν=1/q quantum Hall states are characterized by a perfect cancellation of the magnetic (Aharonov-Bohm) and the Fermi statistical phases. The lowering of the ground state energy due to this phase cancellation is illustrated by using a partial analogy with the Helium II problem and via a path integral formulation. For general filling fractions ν=p/q, a cluster of p electrons is considered instead of a single electron. There is again a perfect cancellation of the effective Aharonov-Bohm phase and the statistical phase of the clusters provided that q is odd.

The one– and two–photon absorption spectra of one–dimensional Peierls semiconductors have been calculated by taking account of the effect of exciton. In the one–photon spectrum, the lowest exciton state gains anomalously large oscillator strength as a result of one–dimensionality. In the two–photon absorption spectrum, interband transitions are as important as exciton transitions.

The do I–V characteristics of orthorhombic TaS3 have been measured up to the field as high as 7000 V/cm inn the temperature range of 77 K – 300 K. Some new features have been revealed. The results support the single-particle excitation picture of transport in quasi-lD conductors.

Sun Guozhu,1,* Wang Yiwen,2 Cao Junyu,1 Chen Jian,1 Ji Zhengming,1 Kang Lin,1 Xu Weiwei,1 Yu Yang,2 Han Siyuan,1,3 and Wu Peiheng1 1Research Institute of Superconductor Electronics, Department of Electronic Science and Engineering, Nanjing University, Nanjing 210093, People’s Republic of China 2National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China 3Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045, USA Received 2 November 2007; revised manuscript received 7 January 2008; published 28 March 2008

We study the formation of charge density waves (CDW) in ${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{K}}_{\mathrm{x}}$${\mathrm{BiO}}_{3}$ within a tight-binding model proposed by Prelovsek, Rice, and Zhang. Light doping leads to the formation of immobile bipolarons. The charge density wave amplitude diminishes gradually as doping concentration increases and disappears at about 35% doping. We have calculated the density of states and optical properties for various doping concentrations. We also present corresponding results in two dimensions that are applicable to a CDW system with the ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ geometry.

To understand the interplay of d-wave superconductivity and antiferromagnetism, we consider a two-dimensional extended Hubbard model with nearest neighbor attractive interaction. The Hamiltonian is solved in the mean-field approximation on a finite lattice. In the impurity-free case, the minimum energy solutions show phase separation as predicted previously based on free energy argument. The phase separation tendency implies that the system can be easily rendered inhomogeneous by a small external perturbation. Explicit solutions of a model including weak impurity potentials are indeed inhomogeneous in the spin-density-wave and d-wave pairing order parameters. Relevance of the results to the inhomogeneous cuprate superconductors is discussed., Comment: 9 pages, 8 figures

Following our previous work [Ho et al., Phys. Rev. B 89, 155316 (2014)], we systematically calculate the magneto-optical properties of various transition metal dichalcogenide monolayers. The intrinsic spin-orbit coupling gives rise to the spin-split electronic states, while a perpendicular magnetic field further induces a valley splitting. In magneto-optical spectra with linearly polarized light, spectral features are spin and valley-polarized. Compounds are different from one another in terms of transition energies and appearance of twin peaks. Our numerical results can serve as a guide for future experimental identification.