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Recently synthesized Kagome compounds AV$_3$Sb$_5$ attract great attention due to the unusual coexistence of the topology, charge density wave and superconductivity. In this {\it Letter}, based on the band structures for CsV$_3$Sb$_5$ in pristine phase, we fit an effective 6-band model for the low-energy processes; utilizing the random phase approximation (RPA) on the effective minimal model, we obtain the momentum-resolved static spin susceptibility attributing the spin-fluctuation pairing mechanism, we find that the superconducting pairing strengths increase with the lift of the Coulomb correlation, and the superconductive pairing symmetry is singlet, the gap functions are antisymmetric with respect to the x-axis and the y-axis in the intermediate to strong Coulomb correlated regime,indicating the unconventional superconductivity in Kagome compounds AV$_3$Sb$_5$., 6 pages, 6 figures

In this paper we present the microscopic origin of the ferromagnetism of Fe0.25TaS2 and its finite-temperature magnetic properties. We first obtain the band structures of Fe0.25TaS2 by the first-principles calculations and find that both conventional and Dirac carriers coexist in metallic Fe0.25TaS2. Accordingly, considering the spin-orbit coupling of Fe 3d ion, we derive an effective RKKY-type Hamiltonian between Fe spins in the presence of both the conventional parabolic-dispersion and the Dirac linear-dispersion carriers, which contains a Heisenberg-like, an Ising-like and an XY-like term. In addition, we obtain the ferromagnetic Curie temperature Tc by using the cluster self-consistent field method. Our results could address not only the high ferromagnetic Curie temperature, but also the large magnetic anisotropy in FexTaS2., Comment: 13 pages, 4 figures

The electronic states near the Fermi level of recently discovered superconductor ${\mathrm{Ba}}_{2}{\mathrm{CuO}}_{4\ensuremath{-}\ensuremath{\delta}}$ consist primarily of the Cu ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ and ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ orbitals. We investigate the electronic correlation effect and the orbital polarization of an effective two-orbital Hubbard model mimicking the low-energy physics of ${\mathrm{Ba}}_{2}{\mathrm{CuO}}_{4\ensuremath{-}\ensuremath{\delta}}$ in the hole-rich regime by utilizing the dynamical mean-field theory with the Lanczos method as the impurity solver. We find that the hole-overdoped ${\mathrm{Ba}}_{2}{\mathrm{CuO}}_{4\ensuremath{-}\ensuremath{\delta}}$ with $3{d}^{8}$ (${\mathrm{Cu}}^{3+}$) is in the orbital-selective Mott phase (OSMP) at half-filling, and the typical two-orbital feature remains in ${\mathrm{Ba}}_{2}{\mathrm{CuO}}_{4\ensuremath{-}\ensuremath{\delta}}$ when the electron filling approaches ${n}_{e}\ensuremath{\sim}2.5$, which closely approximates to the experimental hole doping for the emergence of the high-${T}_{c}$ superconductivity. We also obtain that the orbital polarization is very stable in the OSMP, and the multiorbital correlation can drive orbital polarization transitions. These results indicate that in hole-overdoped ${\mathrm{Ba}}_{2}{\mathrm{CuO}}_{4\ensuremath{-}\ensuremath{\delta}}$ the OSMP physics and orbital polarization, local magnetic moment, and spin or orbital fluctuations still exist. We propose that our present results are also applicable to ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{4\ensuremath{-}\ensuremath{\delta}}$ and other two-orbital cuprates, demanding an unconventional multiorbital superconducting scenario in hole-overdoped high-${T}_{c}$ cuprates.

Searching for spin liquids on the honeycomb J1-J2 Heisenberg model has been attracting great attention in the past decade. In this Paper we investigate the topological properties of the J1-J2 Heisenberg model by introducing nearest-neighbour and next-nearest-neighbour bond parameters. We find that there exist two topologically different phases in the spin disordered regime 0.20.32, it is a pi-flux chiral spin liquid, which is topological nontrivial and gapped. These results suggest that there exist two topologically different spin disorder phases in honeycomb J1-J2 Heisenberg model., Comment: 11 pages, 4 figures

The Su--Schrieffer--Heeger (SSH) model is one of the simplest topological lattice models and is a promising candidate for quantum simulations of topological systems. In this study we investigate an interacting SSH model with nearest-neighbor interactions using mean field theory and an exact diagonalization method after the Jordan-Wigner transformation. We obtain a rich phase diagram with six different phases. Majorana bound states are found and confirmed by analyzing wave functions and Majorana number. In addition, we propose to use superconducting devices to construct a one-dimensional quantum circuit, where Cooper pair boxes and Josephson junctions are alternately connected in series, to theoretically simulate the interacting SSH model. This approach also provides us a toolkit to simulate correlated topological systems.

We present the modulation of magnetic order on the orbital selective Mott phases (OSMP) and the metal-insulator transitions (MIT) of multi-orbital Hubbard models by employing the rotationally invariant slave-boson methods. We show that at half filling, the well-known paramagnetic (PM) OSMP is completely covered by an antiferromagnetic (AFM) Slater insulator, and the PM Mott phase by an AFM Mott insulator when electron correlation strength varies from intermediate to strong both in two- and three-orbitals Hubbard systems. Away from half-filling, we find that a partial-polarized AFM orbital-selective Slater phase appears in the intermediate correlation regime, and an almost full-polarized AFM OSMP fully covers the paramagnetic OSMP. In addition, the ferromagnetic phase in the three-orbital case is more robust than that in the two-orbital case. These results demonstrate that the modulation of magnetic correlation to the quasiparticle spectra leads to much rich and more interesting MIT scenario in multiorbital correlated systems.

Effects of longitudinal ( J Z ) and transverse (including spin-flip J X and pair-hopping J P terms) Hund’s couplings on Mott transitions of two-orbital Hubbard models are studied by the rotationally invariant slave boson approach. We show that in the half-filled asymmetric systems, the orbital selective Mott phase (OSMP) expands with increasing J X , P / J Z when J X , P / J Z 1 , and has the largest region in the isotropic case ( J X , P / J Z = 1 ); and further increasing spin-flip Hund’s coupling to J X , P / J Z > 1 may quickly suppress the OSMP state. In other near-half-filled systems, the transverse Hund’s coupling favors or unfavors the OSMP state, depending on the electronic correlation strength of the systems. In the quarter-filled and around systems, a small J X , P / J Z 1 has less effect on Mott transition, while a large J X , P / J Z > 1 enhances the electron itineracy and considerably increases the critical correlation strength of the Mott transition both in symmetric and asymmetric systems. These results could be addressed by different spin–orbital states favored by J X , J P and J Z components, respectively; and the competing longitudinal and transverse Hund’s coupling terms lead to most strong quantum fluctuations in the isotropic system.

We theoretically study the coexistence of spin density wave (SDW) and superconductivity (SC) in ironpnictide superconductors based on a three-orbital model, focusing on the momentum-space and real-space distributions of SDW and SC order parameters in the coexistence region. We show that a SDW–SC coexisting state lies in the T–n phase diagram, in qualitative agreement with those of NaFe1 − xCoxAs and Ba(Fe1 − xCox)2As2. In the SC state the pairing wavefunction has s ± symmetry with s x 2 y 2 and s x 2 + y 2 components. In the coexisting state, the SDW and SC order parameters display strong orbital-selective competitions in momentum space, which also result in real-space modulation and spin singlet–triplet mixing in the Cooper pairing amplitude. We expect that the obtained features may be observed in future experiments.

We employ the self-consistent Bogoliubov-de Gennes (BdG) formulation to investigate the effect of single interstitial nonmagnetic/magnetic impurity in iron-based superconductors with s ± -wave pairing symmetry. We find that both the nonmagnetic and magnetic impurities can induce bound states within the superconducting (SC) gap and a π phase shift of SC order parameter at the impurity site. However, different from the interstitial-nonmagnetic-impurity case characterized by two symmetric peaks with respect to zero energy, the interstitial magnetic one only induces single bound-state peak. In the strong scattering regime this peak can appear at the Fermi level, which has been observed in the recent scanning tunneling microscope (STM) experiment of Fe(Te,Se) superconductor with interstitial Fe impurities (Yin et al. 2015 [44]). This novel single in-gap peak feature also distinguishes the interstitial case from the substitutional one with two peaks. These results provide important information for comparing the different impurity effects in the iron-based superconductors.

In this paper, we present the electronic structures and orbital-resolved electronic properties of structurally distorted $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ bulk and monolayer within density functional theory approaches. The relaxed commensurate-charge-density-wave (CCDW) structure shows that 13 Ta atoms condense into a star-of-David cluster, accompanied by a buckling of neighboring $S$ planes. Through detailed analyses to the orbital characters near the Fermi level, we show that there exists an orbital-density-wave (ODW) order, which is predominantly contributed by Ta-$5{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ orbital in the central Ta of the star-of-David cluster. We further demonstrate that the structural distortion, together with the Coulomb interaction, stabilizes the CCDW insulating ground state with an ODW order. The results obtained from dynamical mean-field theory confirm the role of the electronic correlation. Moreover, such an ODW ground state favors an intralayer ferromagnetic order in bulk and monolayer $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, and an interlayer antiferromagnetic order in bulk. We propose that $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ monolayer may pave new ways to study exciton physics, flat-band physics, and potential applications in luminescence.

We develop a generalized numerical optimization algorithm for the rotationally invariant multi-orbital slave boson approach, which is applicable for arbitrary boundary constraints of high-dimensional objective function by combining several classical optimization techniques. After constructing the calculation architecture of rotationally invariant multi-orbital slave boson model, we apply this optimization algorithm to find the stable ground state and magnetic configuration of two-orbital Hubbard models. The numerical results are consistent with available solutions, confirming the correctness and accuracy of our present algorithm. Furthermore, we utilize it to explore the effects of the transverse Hund’s coupling terms on metal–insulator transition, orbital selective Mott phase and magnetism. These results show the quick convergency and robust stable character of our algorithm in searching the optimized solution of strongly correlated electron systems.

For a newly discovered iron-based high T c superconducting parent material KFe 2 Se 2 , we present an effective three-dimensional five-orbital tight-binding model by fitting the band structures. The three t 2 g -symmetry orbitals of the five Fe 3d orbitals mainly contribute to the electron-like Fermi surface, which has a three-dimensional character, in agreement with recent angle-resolved photoemission spectroscopy experiments. To understand the groundstate magnetic properties, the two- and three-dimensional spin and charge susceptibilities within the random phase approximation are investigated. It obviously shows a sharp peak at wave vector q ∼ ( π , π ) , indicating the magnetic instability of Neel-type antiferromagnetic rather than ( π / 2 , π / 2 )-type or stripe ( π , 0)-type antiferromagnetic ordering, while it exhibits a ferromagnetic instability between the nearest neighboring FeSe layers along c axis. The three-dimensional spin fluctuation may play an important role in pairing symmetry in superconducting materials A y Fe 2 − x Se 2 ( A = Rb , Cs , K , etc.).

We study the magnetic phase diagram within an extended half-filled Hubbard model, focusing on the roles of the next-nearest-neighbor (NNN) and the next-next-nearest-neighbor (3rd NN) hoppings in the magnetic configurations. We find that due to the spin frustration from the long range hopping and the competition between long-range hopping and Coulomb correlation, the striped antiferromagnetic (AFM) phase is stable when the NNN hopping is dominant, while the bicollinear AFM phase is robust when the 3rd NN hopping is considerably large. The triple points are found in various magnetic phase diagrams. Possible applications of the present theory on intermediately correlated LaFeAsO and strongly correlated FeTe are discussed.

A simple analytic expression of the two-dimensional Child–Langmuir law is derived for non-zero injection velocities and Lau's result is obtained in our model by setting the injection velocity equal to zero. The calculation results show that the modify term in our model is larger than Lau's with a non-zero electronic initial energy, and it is twice as large as Lau's when the electronic initial energy is much greater than the potential energy of the gap.

Taking account of the metallicity dependence of the s-process nucleosynthesis in the AGB stars, we adopted the wind accretion model with the condition of total angular momentum conservation and used the Monte-Carlo method to study the variations and the distributions of the orbital elements of the mild and strong Ba stars. The calculated results show that the level of heavy-element overabundance in a Ba star depends on the orbital period. Since there is a strong dependence of s-process yields on the initial stellar metallicity of the AGB star and a strong increase of the s-process yields in AGB stars with decreasing metallicity, the calculated results strongly suggest that the initial metallicity of the Ba star systems is another important parameter for the level of heavy-element overabundance in a Ba star. The strong Ba stars generally have lower metallicities than mild Ba stars. The masses of AGB progenitor and Ba star are other two parameters which also have some impact on the heavy-element overabundance in the Ba star.

We studied the Kane-Mele-Hubbard model at half filling with Zeeman magnetic field using rotationally invariant slave boson method. The competition between external magnetic field and spin orbital coupling at large Hubbard U is explored. It is found that different canted antiferromagnetic phases exist in the presence of longitudinal and transverse magnetic fields. With longitudinal external magnetic field and strong electron correlation, the z component of the local moment is preferred by the finite spin-orbital coupling.

In this paper we investigate the nature of ferromagnetic (FM) phase in three-orbital Hubbard model by using Kotliar-Ruckenstein slave-boson approach. We find that the FM metallic phase widely exists and stabilizes in magnetic phase diagram for large Hund’s rule coupling, and FM insulator is not seen. An FM orbital-selective Mott phase (OSMP), mainly due to large crystal field splitting and strong electronic correlation, is observed and identified as double-exchange FM. An antiferromagnetic (AFM) OSMP is also found. We conclude that large Coulomb correlation and Hund’s rule coupling is crucial for stable FM phase, and further crystal field splitting also enhance the stability of AFM and FM OSMP.

The electronic and magnetic properties of BaTi$_{2}$As$_{2}$O have been investigated using both the first-principles and analytical methods. The full-potential linearized augmented plane-wave calculations show that the most stable state is a site-selective antiferromagnetic (AFM) metal with a $\text{2}\times \text{1}\times \text{1}$ magnetic unit cell containing two nonmagnetic Ti atoms and two other Ti atoms with antiparallel moments. Further analysis to Fermi surface and spin susceptibility shows that the site-selective AFM ground state is driven by the Fermi surface nesting and the Coulomb correlation. Meanwhile, the charge density distribution remains uniform, suggesting that the phase transition at $200$ K in experiment is a spin-density-wave (SDW) transition., Comment: 3 page, 3 figures; conference paper submitted to MMM58, and accepted to J. Appl. Phys 2014

In this paper we investigate the electronic and magnetic properties of K$_{x}$Fe$_{2-y}$Se$_{2}$ materials at different band fillings utilizing the multi-orbital Kotliar-Ruckenstein's slave-boson mean field approach. We find that at three-quarter filling, corresponding to KFe$_{2}$Se$_{2}$, the ground state is a paramagnetic bad metal. Through band renormalization analysis and comparison with the angle-resolved photoemission spectra data, we identify that KFe$_{2}$Se$_{2}$ is also an intermediate correlated system, similar to iron-pnictide systems. At two-third filling, corresponding to the Fe$^{2+}$-based systems, the ground state is a striped antiferromagnetic (SAFM) metal with spin density wave gap partially opened near the Fermi level. In comparison, at half filling case, corresponding to the Fe$^{3+}$-based compounds, besides SAFM, a $N\acute{e}el$ antiferromagnetic metallic ground state without orbital ordering is observed in the intermediate correlation range, and an orbital selective Mott phase (OSMP) accompanied with an intermediate-spin to high-spin transition is also found. These results demonstrate that the band filling and correlation control the electronic state, Fermi surface topology and magnetism in K$_{x}$Fe$_{2-y}$Se$_{2}$., Comment: 21 pages, 12 figures

Orbital polarization and electronic correlation are two essential aspects in understanding the normal-state and superconducting properties of multi-orbital FeAs-based superconductors. In this paper, we present a systematic study on the orbital polarization of iron pnictides from weak to strong Coulomb correlations within the Kotliar-Ruckenstein slave boson approach. The magnetic phase diagram of the two-orbital model for LaFeAsO clearly shows that a striped antiferromagnetic metallic phase with orbital polarization exists over a wide doping range, in addition to the Slater-type insulator, Mott insulator and paramagnetic phases. A reversal of the orbital polarization occurs in the intermediate correlation regime in the absence of the crystal field splitting; however, a small crystal field splitting considerably enhances the orbital polarization, and stabilizes the xz-type orbital order. We argue that the ferro-orbital polarization is characteristic of a density wave, and leads to a pseudogap-like behavior in the density of states.

In this paper we explore the magnetic and orbital properties in iron pnictides based on the two-orbital as well as the five-orbital Hubbard models. These properties are closely related to a tetragonal-orthorhombic structural phase transition. The electron-lattice coupling, interplaying with electron-electron interaction, is self-consistently treated. Our results reveal that the orbital polarization favors the spin-density wave (SDW) in the orthorhombic phase. The ferro-orbital (FO) order only occurs in the orthorhombic phase rather than in the tetragonal one. For the five-orbital model, magnetic moments of Fe are small in the intermediate Coulomb interaction region in the striped antiferromagnetic phase. We also calculate the Fermi surface, which is anisotropic in the SDW/FO orthorhombic phase and agrees well with the recent angle-resolved photoemission spectroscopy experiments. These results suggest that the magnetic phase transition is driven by the FO order arising from the electron-lattice coupling.

Competition of crystal field splitting and Hund's rule coupling in magnetic metal-insulator transitions of half-filled two-orbital Hubbard model is investigated by multi-orbital slave-boson mean field theory. We show that with the increase of Coulomb correlation, the system firstly transits from a paramagnetic (PM) metal to a {\it N\'{e}el} antiferromagnetic (AFM) Mott insulator, or a nonmagnetic orbital insulator, depending on the competition of crystal field splitting and the Hund's rule coupling. The different AFM Mott insulator, PM metal and orbital insulating phase are none, partially and fully orbital polarized, respectively. For a small $J_{H}$ and a finite crystal field, the orbital insulator is robust. Although the system is nonmagnetic, the phase boundary of the orbital insulator transition obviously shifts to the small $U$ regime after the magnetic correlations is taken into account. These results demonstrate that large crystal field splitting favors the formation of the orbital insulating phase, while large Hund's rule coupling tends to destroy it, driving the low-spin to high-spin transition., Comment: 4 pages, 4 figures

We present a study on the Jahn—Teller (JT) distortions of the TiO6, NiO6 and MnO6 complexes in NaTiO2, NaNiO2 and NaMnO2 triangular compounds with a C2/m structure. The JT vibronic normal modes are found to be Q3, Q'4 and Q6 by the group symmetry on the C2/m structure. The magnitude of the normal coordinates (Q3, Q'4, Q6) and the structural parameters of distorted octahedra MO6 (M=Ti, Ni, Mn) are obtained and in good agreement with experimental data. The energy level splitting of 3d orbitals and the highest occupied molecular orbital (HOMO) character in the MO6 complex are also calculated in accordance with the JT distortions. These results provide a first insight into the groundstate and magnetic properties of distorted triangular compounds AMO2.