RETRACTED ARTICLE: Cooperative Time-Reversal Symmetry Breaking Induced by a Magnetic Field in a Quasi-2D d-Wave Superconductor

A model of quasi-two-dimensional d-wave superconductor, with strong nesting properties of the Fermi surface is considered. The orbital effect of a moderate magnetic field applied perpendicularly to the conducting planes is studied in the mean field approximation. It is shown that the field can induce a time-reversal symmetry-breaking spin density wave order coexisting with the superconducting order and can open a gap over the whole Fermi surface. The anomalies recently observed in the heat conductivity, penetration depth, and zero-bias conduction in cuprates might be ascribed to this effect.     

Cooperative Time-Reversal Symmetry Breaking Induced by a Magnetic Field in a Quasi-2D d-Wave Superconductor

A model of quasi-two-dimensional d-wave superconductor, with strong nesting properties of the Fermi surface is considered. The orbital effect of a moderate magnetic field applied perpendicularly to the conducting planes is studied in the mean field approximation. It is shown that the field can induce a time-reversal symmetry-breaking spin density wave order coexisting with the superconducting order and can open a gap over the whole Fermi surface. The anomalies recently observed in the heat conductivity, penetration depth, and zero-bias conduction in cuprates might be ascribed to this effect.     

Electronic Structure in Underdoped Cuprates Due to the Emergence of a Pseudogap

The phenomenological Green’s function developed in the works of Yang, Rice, and Zhang has been very successful in understanding many of the anomalous superconducting properties of the deeply underdoped cuprates. It is based on considerations of the resonating valence bond spin liquid approximation and is designed to describe the underdoped regime of the cuprates. Here, we emphasize the region of doping, x, just below the quantum critical point at which the pseudogap develops. In addition to Luttinger hole pockets centered around the nodal direction, there are electron pockets near the antinodes which are connected to the hole pockets by gapped bridging contours. We determine the contours of nearest approach as would be measured in angular resolved photoemission experiments and emphasize signatures of the Fermi surface reconstruction from the large Fermi contour of Fermi liquid theory (which contains 1+x hole states) to the Luttinger pocket (which contains x hole states). We find that the quasiparticle effective mass renormalization increases strongly toward the edge of the Luttinger pockets beyond which it diverges.     

Full Relativistic Electronic Structure and Fermi Surface Sheets of the First Honeycomb-Lattice Pnictide Superconductor SrPtAs

We report full-potential density functional theory (DFT)-based ab initio band structure calculations to investigate electronic structure properties of the first pnictide superconductor with a honeycomb-lattice structure: SrPtAs. As a result, electronic bands, density of states, Fermi velocities and the topology of the Fermi surface for SrPtAs are obtained. These quantities are discussed in comparison to the first available experimental data. Predictions for future measurements are provided.     

Superconductivity and the Van Hove Scenario

We give a review of the role of the Van Hove singularities in superconductivity. Van Hove singularities (VHs) are a general feature of low-dimensional systems. They appear as divergences of the electronic density of states (DOS). Jacques Friedel and Jacques Labbé were the first to propose this scenario for the A15 compounds. In NbTi, for example, Nb chains give a quasi-1D electronic structure for the d-band, leading to a VHs. They developed this model and explained the high T _C and the many structural transformations occurring in these compounds. This model was later applied by Jacques Labbé and Julien Bok to the cuprates and developed by Jacqueline Bouvier and Julien Bok. The high T _C superconductors cuprates are quasi-bidimensional (2D) and thus lead to the existence of Van Hove singularities in the band structure. The presence of VHs near the Fermi level in the cuprates is now well established. In this context we show that many physical properties of these materials can be explained, in particular the high critical temperature T _C, the anomalous isotope effect, the superconducting gap and its anisotropy, and the marginal Fermi liquid properties, they studied these properties in the optimum and overdoped regime. These compounds present a topological transition for a critical hole doping p≈0.21 hole per CuO₂ plane.     

Quantum criticality and incipient phase separation in the thermodynamic properties of the Hubbard model

Transport measurements on the cuprates suggest the presence of a quantum critical point (QCP) hiding underneath the superconducting dome near optimal hole doping. We provide numerical evidence in support of this scenario via a dynamical cluster quantum Monte Carlo study of the extended two-dimensional Hubbard model. Single-particle quantities, such as the spectral function, the quasi-particle weight and the entropy, display a crossover between two distinct ground states: a Fermi liquid at low filling and a non-Fermi liquid with a pseudo-gap at high filling. Both states are found to cross over to a marginal Fermi-liquid state at higher temperatures. For finite next-nearest-neighbour hopping t', we find a classical critical point at temperature T(c). This classical critical point is found to be associated with a phase-separation transition between a compressible Mott gas and an incompressible Mott liquid corresponding to the Fermi liquid and the pseudo-gap state, respectively. Since the critical temperature T(c) extrapolates to zero as t' vanishes, we conclude that a QCP connects the Fermi liquid to the pseudo-gap region, and that the marginal Fermi-liquid behaviour in its vicinity is the analogue of the supercritical region in the liquid-gas transition.     

Water-gated charge doping of graphene induced by mica substrates

We report on the existence of water-gated charge doping of graphene deposited on atomically flat mica substrates. Molecular films of water in units of ~0.4 nm thick bilayers were found to be present in regions of the interface of graphene/mica heterostacks prepared by micromechanical exfoliation of kish graphite. The spectral variation of the G and 2D bands, as visualized by Raman mapping, shows that mica substrates induce strong p-type doping in graphene with hole densities of (9 ± 2) × 10(12) cm(-2). The ultrathin water films, however, effectively block interfacial charge transfer, rendering graphene significantly less hole-doped. Scanning Kelvin probe microscopy independently confirmed a water-gated modulation of the Fermi level by 0.35 eV, which is in agreement with the optically determined hole density. The manipulation of the electronic properties of graphene demonstrated in this study should serve as a useful tool in realizing future graphene applications.     

Spectral Properties of a Doped Antiferromagnet with Pairing Correlations

In this paper, we study the spectral properties of a phenomenological model for a weakly doped antiferromagnet. In this model, it is assumed that each carrier moves in one of the two sublattices where it was introduced. Such a situation corresponds to a case of underdoped high-temperature superconductors with the free carrier spectra maximum at k=(±π/2,±π/2) and with a four-pocket Fermi surface. We study the spectral properties of the model by taking into account both the fluctuations of the phases of the superconducting order parameter and spins of the antiferromagnetic background. It is shown that the hole spectral function and the density of states are strongly affected by these fluctuations. In particular, we argue that these fluctuations can be responsible for the temperature evolution of the Fermi pockets in cuprate superconductors.     

Electronic Structure of BaFe<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As<Subscript>2</Subscript> Revealed by Angle-Resolved Photoemission Spectroscopy

We report an angle-resolved photoemission spectroscopy study of BaFe_2−x Co _x As₂. For x=0, above the structural and magnetic transition temperature (T _s ), the spectral weight near the Fermi level is considerably suppressed around the Γ point where the Fe 3d yz/zx orbital degeneracy is expected. This observation suggests that the Jahn–Teller type instability is playing an important role in the tetragonal phase above T _s . Below T _s , the spectral weight of 0–100 meV is reconstructed to form flat bands at 70–100 meV and Fermi surfaces, consistent with the orbital-dependent excitonic coupling. In the optimally doped and overdoped regimes, the hole pocket around the Γ point and the electron pocket around the M point are apparently nested, indicating that the doping dependence of the superconducting transition temperature cannot be explained by the nesting scenario and that the unusual electron–lattice fluctuation due to the orbital degeneracy is important.     

Influence of substitutional doping on the electronic properties of carbon nanotubes with Stone Wales defects: density functional calculations

Abstract

In this work, we implemented density function theory to investigate the structural and the electronic properties of nitrogen doped single walled carbon nanotube under different orientations of Stone Wales defect. We have found that, the doped defected structures are more stable than the non-doped defected structures. Furthermore, doping defected carbon nanotubes with a nitrogen atom has significantly narrowed the band gap and slightly shifted the Fermi level toward the conduction band. Moreover, nitrogen substitution creates new band levels just above the Fermi level which exemplifies an n-type doping. However, the induced band gap is indirect band gap compared to direct band gap as in pristine carbon nanotubes. Furthermore, the electronic and structural properties of nitrogen doped carbon nanotube with Stone Wales defects is crucially affected by the dopant site as well as the orientations of Stone Wales defects.     

High-pressure Study of a Doped-type Organic Superconductor, κ-(BEDT-TTF)4Hg2.89Br8

Physical properties of the organic charge-transfer complex that may be regarded as a doping system are presented. We measured resistivity of an ambient-pressure superconductor, κ-(BEDT-TTF)₄Hg_2.89Br₈, as functions of temperature, magnetic field, and pressure. Metallic and superconducting states of this salt are possibly attributable to the doping effect, which originates from incommensurability of Hg chain because band structure calculation predicts strong electron-electron correlation enough to localize the itinerant electrons. We uncovered anomalous pressure and magnetic field dependences of superconductivity as well as non-Fermi liquid behavior in the normal-state resistivity at low pressures. In addition, we observed a pressure-induced crossover to the Fermi liquid behavior, which is seen in non-doped κ-type salts at any pressures.     

The Non-Metal-to-Metal and Vice-Versa Transitions in a GaMnAs Layer

A self-consistent calculation of the density of states and the spectral density function is performed in a two-dimensional spin-polarized hole system based on a multiple-scattering approximation. Using parameters corresponding to GaMnAs thin layers, a wide range of Mn concentrations and hole densities have been explored to understand the nature, localized or extended, of the spin-polarized holes at the Fermi level for several values of the average magnetization of the Mn system. If the impurity concentration is below a certain threshold, the increase of the magnetization does not lead to a significant change in the localized character of the spin-polarized states. Above this threshold, we reach conditions allowing the occurrence of spin-polarized extended states at the Fermi level—more properly extended states in a very dirty metal—which are very sensitive to the average magnetization. The occurrence of a metal-to-non-metal transition has been surveyed around an optimum combination of the Mn concentration and the free carriers density.      

Effect of an Elliptical Inclusion on Critical Current Density of a Long Cylindrical High-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> Superconductor

An analytical investigation is presented to display the distribution of critical current flow and trapped magnetic field around an elliptical nonsuperconducting inclusion within a long cylindrical superconductor. The current streamlines, the critical current density, and the trapped field around the inclusion in the superconductor without deformation are obtained based on the Bean model and the method of conformal mapping. The results show that the critical current density of a superconductor will be decreased dramatically due to a macroscopic nonsuperconducting inclusion. Besides, the maximum trapped magnetic field is limited by the inclusion.     

First-Principles Study on the Stability and Electronic Properties of Bi-Doped Sr3Ti2O7

The electronic and crystal structural properties of Bi-doped Sr_3Ti_2O_7are studied using the first principles density functional theory(DFT)based on pseudopotentials basis and plane-wave method.Our results show that the formation energy of Bi doping in Site-1 and Site-2 of Sr_3Ti_2O_7increases with increasing doping concentration.And at the same doping concentration,the formation energy of Bi doping in Site-2 is lower than that in Site-1.The undoped Sr_3Ti_2O_7is found to be an insulator and its Fermi level stays at the top of the valence band.While the Fermi level of the Bi-doped Sr_3Ti_2O_7moves into the bottom of conduction band,the system undergoes an insulator-to-metal phase transition.Furthermore,our calculation results demonstrated that the Fermi level of the Bi-doped Sr_3Ti_2O_7goes deeper into the bottom of conduction band with increasing doping concentration.     

Structural, electronic and magnetic properties of manganese doping in the upper layer of bilayer graphene

First-principles spin-polarized calculations have been conducted to investigate the structural, electronic and magnetic properties of 3d transition metal Mn doping into two typical sites in the upper layer of bilayer graphene with the AB Bernal structure. One of the doping sites is above the center of a carbon hexagon of the lower graphene layer (called the H site) and the other is directly on top of a carbon atom of the lower graphene layer (called the T site). We found that Mn doping enlarges the interlayer distance in bilayer graphene. Charge density distribution indicates that the region between the upper and lower graphene layer has apparent covalent-bonding characters due to the Mn doping. In the spin-polarized band structure of H?site doping, the π and π(*) bands separate from each other at the Dirac point both in majority spin and minority spin. In the band structure of T site doping, the Fermi level is located above the Dirac point and moves to the conduction bands in majority spin and minority spin, making the bilayer graphene n doped. A high spin polarization of 95% is achieved due to the H site doping. The local moment of Mn for H and T site doping is reduced to 1.76?μ(B) and 1.88?μ(B), respectively, which are smaller than the value (5?μ(B)) in the free state.     

Interpolative approximation for the Hubbard and the emery model

Calculations in Green's function technique apply an interpolation ansatz for the self-energy between the weak and strong coupling regime to the doped case of correlated models. The DOS of the one-band Hubbard model with the Kondo-like peak at the chemical potential is in good agreement with recent numerical results. In the doped three-band model a singlet correlation band from copper and oxygen states appears at the Fermi surface with dispersion similar to the free bands but with strongly renormalized bandwidth. The Fermi surface changes from closed hole type to closed electron type upon doping and is perfectly nested for some doping valuen1.25.     

Josephson Current in a Gapped Graphene Superconductor/Barrier/Superconductor Junction: Case of Massive Electrons

The Josephson effect in a gapped graphene-based superconductor/barrier/superconductor junction is studied. The superconductivity in gapped graphene may be achieved by depositing conventional superconductor on the top of the gapped graphene such as graphene grown on SiC substrate. In gapped graphene system, the carriers exhibit massive Dirac fermions. We focus on the effect of pseudo-Dirac-like mass on the supercurrent. In contrast to that in the gapless graphene superconductor/barrier/superconductor junction, we find that the supercurrent exhibits dependency of the Fermi energy. Also, the massive supercurrent anomalously oscillates as a function of the gate potential. This novel behavior is due to the effect of electrons acquiring mass in gapped graphene.     

Extraction of the short-range defect potential parameters from available experimental data on the graphene resistance

Abstract

We consider a problem of obtaining information about the scattering potentials of the monolayer graphene sample using available experimental data on its resistance. For this purpose, we study theoretically the dependence of graphene resistance on Fermi energy having in mind to compare it with experimental data where super-high mobility electrons in suspended graphene samples without chemical doping were investigated. As far as practical absence of the doping impurities in this case makes the Coulomb scattering negligible, we consider models of the short-range scattering potentials. The model of short-range potential is assumed to be supported by the close vicinity of the ring or the circumference of a circle. The diameter of circles is supposed to be of the order of the crystal lattice spacing. The empty core of the model potential guarantees the suppression of nonphysical shortwave modes. Two models are investigated: the delta function on the circumference of a circle (delta shell) and the annual well. An advantage of the former is simplicity, while a virtue of the latter is regularity. We consider scattering of electrons by these potentials and obtain exact explicit formulae for the scattering data. We here discuss application of these formulae for calculation of observables. Namely, we analyze the contribution of this scattering into the graphene resistance and plot the resistivity as a function of the Fermi energy according to our theoretical formulae. The obtained results are consistent with experiment, where the resistance was measured as a function of the Fermi momentum on the suspended annealed graphene. This fact gives a possibility to find parameters of the modeled potential on the basis of the available experimental data on resistance of the suspended graphene sample with the gate voltage controlled Fermi level position. It is clear to be very important for applications.     

Spin Structures of the Hubbard Clusters and the Pseudogap

The spin and charge structures formed in the Hubbard model for a finite two-dimensional cluster have been studied in the mean field approximation. The self-consistent iterative procedure reduces an uncorrelated initial spin distribution into stable structures with spectral properties typical of stripes. It is shown that the density of states of the system for any doping has a sharp minimum at the Fermi level, the pseudogap. The pinning of the gap at the Fermi level is characteristic not only of a superconducting state, but also typical of a normal state of spin glasses. Our results support conception of the PG state as a state with frozen locally nematic spin structures of antiferromagnetic domains.