arXiv subjects

Liang-Jian Zou

Publications and source records attributed to Liang-Jian Zou.

At least 55 records · Page 3Linked to original sources

arXiv2011

A Three-Dimensional Tight-Binding Model and Magnetic Instability of KFe2e2

For a newly discovered iron-based high T_c superconducting parent material KFe2Se2, we present an effective three-dimensional five-orbital tight-binding model by fitting the band structures. The three t2g-symmetry orbitals of the five Fe 3d orbitals mainly contribute to the electron-like Fermi surface, in agreement with recent angle-resolved photoemission spectroscopy experiments. To understand the groundstate magnetic structure, the two- and three-dimensional dynamical spin susceptibilities within the random phase approximation are investigated. It obviously shows a sharp peak at wave vector $\mathbf{Q}$ $\thicksim$ ($π$, $π$), indicating the magnetic instability of {\it N$\acute{e}$el}-type antiferromagnetic rather than ($π$/2, $π$/2)-type antiferromagnetic ordering. While along $\emph{c}$ axis, it exhibits a ferromagnetic coupling between the nearest neighboring FeSe layers. The difference between the present results and the experimental observation in KxFe2-ySe2 is attributed to the presence of Fe vacancy in the latter.

cond-mat.supr-con↗

arXiv2011

Influence of electronic correlations on orbital polarizations in the parent and doped iron pnictides

Orbital polarization and electronic correlation are two essential aspects in understanding the normal state and superconducting properties of multiorbital FeAs-based superconductors. In this Letter, we present a systematical study on the orbital polarization of iron pnictides from weak to strong Coulomb correlations within the KRSB approach. The magnetic phase diagram of the two-orbital model for LaFeAsO clearly shows that the striped antiferromagnetic metallic phase with considerable orbital polarization exists over a wide doping range. The reversal of the orbital polarization occurs in the intermediate correlation regime.A small crystal field splitting enhances the orbital density wave order.

cond-mat.supr-con↗

arXiv2011

Magnetic properties in the doped spin-1/2 honeycomb-lattice compound In$_3$Cu$_2$VO$_9$

We report the magnetic properties in the Co- and Zn-doped spin-1/2 honeycomb-lattice compound In$_3$Cu$_2$VO$_9$. The magnetic susceptibility and specific heat experiments show no long-range ordering down to 2 K in In$_3$Cu$_2$VO$_9$. In the low temperature range, approximately T$^{2}$-dependent magnetic specific heat and linearly T-dependent spin susceptibility were observed, suggesting a spin liquid candidate with a S = 1/2 honeycomb lattice. When Cu$^{2+}$ ions are partially substituted by Co$^{2+}$ ions, both impurity potential scattering and magnetic impurity scattering induced by magnetic Co$^{2+}$ ions break the homogenous spin-singlet spin liquid state, and lead to an antiferromagnetic(AFM) long-range correlation. While replacing Cu$^{2+}$ with nonmagnetic Zn$^{2+}$ ions, the frustration and antiferromagnetic correlation between Cu$^{2+}$ ions is weakened, leading to breakage of a spin liquid state and suppression of the low-dimensional AFM.

cond-mat.str-el↗

arXiv2011

Effect of Spin Correlations on Multi-orbital Metal-Insulator Transitions and Suppression of Orbital Selective Mott Transitions

We present the influence of spin correlation on the metal-insulator transitions (MIT) in two-orbital Hubbard models by the Kotliar-Ruckenstein slave-boson approach. In the asymmetric half-filling situation, the two orbits simultaneously transit from conducting to insulating states with the increase of Coulomb correlation, accompanied by a paramagnetic (PM)-antiferromagnetic (AFM) transition. The orbital selective Mott transition found in the PM condition is completely suppressed over a wide correlation range, though it may exist in the systems away from half-filling. In the insulating state, the system crosses over from a partially-polarized spin-gapped phase in the intermediate correlation regime to an almost fully-polarized Mott insulating phase in the strong correlation regime. These results demonstrate that the spin modulation to the quasiparticle spectra brings much rich and more interesting MIT scenario in multi-orbital correlated systems.

cond-mat.str-el↗

arXiv2011

Competition of crystal field splitting and Hund's rule coupling in two-orbital magnetic metal-insulator transitions

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 Néel} 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.

cond-mat.str-el↗

arXiv2011

Orbital density wave induced by electron-lattice coupling in orthorhombic iron pnictides

In this paper we explore the magnetic and orbital properties closely related to a tetragonal-orthorhombic structural phase transition in iron pnictides based on both two- and five-orbital Hubbard models. The electron-lattice coupling, which interplays with electronic interaction, is self-consistently treated. Our results reveal that the orbital polarization stabilizes the spin density wave (SDW) order in both tetragonal and orthorhombic phases. However, the ferro-orbital density wave (F-ODW) only occurs in the orthorhombic phase rather than in the tetragonal one. Magnetic moments of Fe are small in the intermediate Coulomb interaction region for the striped antiferromangnetic phase in the realistic five orbital model. The anisotropic Fermi surface in the SDW/ODW orthorhombic phase is well in agreement with the recent angle-resolved photoemission spectroscopy experiments. These results suggest a scenario that the magnetic phase transition is driven by the ODW order mainly arising from the electron-lattice coupling.

cond-mat.supr-con↗

arXiv2009

Two-orbital Systems with Crystal Field Splitting and Interorbital Hopping

The nondegenerate two-orbital Hubbard model is studied within the dynamic mean-field theory to reveal the influence of two important factors, i.e. crystal field splitting and interorbital hopping, on orbital selective Mott transition (OSMT) and realistic compound Ca$_{2-x}$Sr$_{x}$RuO$_{4}$. A distinctive feature of the optical conductivity of the two nondegenerate bands is found in OSMT phase, where the metallic character of the wide band is indicated by a nonzero Drude peak, while the insulating narrow band has its Drude peak drop to zero in the mean time. We also find that the OSMT regime expands profoundly with the increase of interorbital hopping integrals. On the contrary, it is shown that large and negative level splitting of the two orbitals diminishes the OSMT regime completely. Applying the present findings to compound Ca$_{2-x}$Sr$_{x}$RuO$_{4}$, we demonstrate that in the doping region from $x=0.2$ to 2.0, the negative level splitting is unfavorable to the OSMT phase.

cond-mat.str-el↗

arXiv2008

Superconducting Properties of Two-Orbital t-t'-J-J' Models

Motivated by the recent contradiction of the superconducting pairing symmetry in the angle-resolved photoemission spectra (ARPES) and the nuclear magnetic resonance (NMR) data in the FeAs superconductors, we present the theoretical results on the phase diagram, the temperature dependent Fermi surfaces in normal state, the ARPES character of quasiparticles and the spin-lattice relaxation 1/T$_{1}$ of the two-orbital t-t$^{'}$-J-J$^{'}$ models. Our results show that most of the properties observed in iron-based superconductors could be comprehensively understood in the present scenario qualitatively, indicating that the pairing symmetry of the ironpnictides is anisotropic nodeless s-wave, mainly originating from the band structures and the Fermi surface topology.

cond-mat.supr-con↗

arXiv2008

Orbital Selective Superconductivity in Two-Orbital Asymmetric t-J Models

We present the zero-temperature superconducting (SC) ground states of the two-orbital asymmetric $t-J$ model on a square lattice by means of the auxiliary-boson approach. Besides the two-gap SC phase, we find an orbital selective SC (OSSC) phase, which is simultaneously SC in one orbit and normal in another orbit. The novel OSSC phase is stable only for sufficient asymmetric degree in orbital space and doping concentration. The pairing symmetry of the SC phase is s-wave-like in most doping regime, against the d-wave symmetry of the single-orbital $t-J$ model in a square lattice. The implication of the present scenario on multi-orbital heavy fermion and iron-based superconductors is also discussed.

cond-mat.supr-con↗

arXiv2008

Metal-insulator transition in half-filling two-orbital Hubbard model on triangular lattice

We have investigated the half-filling two-orbital Hubbard model on a triangular lattice by means of the dynamical mean-field theory (DMFT). The densities of states and optical conductivity clearly show the occurence of metal-insulating transition (MIT) at U$_{c}$, U$_{c}$=18.2, 16.8, 6.12 and 5.85 for J=0, 0.01U, U/4 and U/3, respectively. The distinct continuities of double occupation of electrons, local square moments and local susceptibility of the charge, the spin and the orbital at J > 0 suggest that the MIT is the first-order; however at J=0, the MIT is the second-order in the half-filling two-orbital Hubbard model on triangular lattices. We attribute the first-order nature of the MIT to the low symmetry of the systems with finite Hund's coupling J.

cond-mat.str-el↗

arXiv2008

Anharmonic effect on lattice distortion, orbital ordering and magnetic properties in Cs2AgF4

We develop the cluster self-consistent field method incorporating both electronic and lattice degrees of freedom to study the origin of ferromagnetism in Cs$_{2}$AgF$_{4}$. After self-consistently determining the harmonic and anharmonic Jahn-Teller distortions, we show that the anharmonic distortion stabilizes the staggered x$^{2}$-z$^{2}$/y$^{2}$-z$^{2}$ orbital and ferromagnetic ground state, rather than the antiferromagnetic one. The amplitudes of lattice distortions, Q$_{2}$ and Q$_{3}$, the magnetic coupling strengthes, J$_{x,y}$, and the magnetic moment, are in good agreement with the experimental observation.

cond-mat.str-el↗

arXiv2007

Phase transition and critical properties of spin-orbital interacting systems

Phase transition and critical properties of Ising-like spin-orbital interacting systems in 2-dimensional triangular lattice are investigated. We first show that the ground state of the system is a composite spin-orbital ferro-ordered phase. Though Landau effective field theory predicts the second-order phase transition of the composite spin-orbital order, however, the critical exponents obtained by the renormalization group approach demonstrate that the spin-orbital order-disorder transition is far from the second-order, rather, it is more close to the first-order, implying that the widely observed first-order transition in many transition-metal oxides may be intrinsic. The unusual critical behavior near the transition point is attributed to the fractionalization of the composite order parameter.

cond-mat.stat-mech↗

arXiv2007

Orbital Insulators and Orbital Order-disorder Induced Metal-Insulator Transition in Transition-Metal Oxides

The role of orbital ordering on metal-insulator transition of transition-metal oxides is investigated by the cluster self-consistent field approach in the strong correlation regime. A clear dependence of the insulating gap on the orbital order parameter is found in the single-particle excitation spectra. The thermal fluctuation drives the orbital order-disorder transition, diminishes the gap and leads to the metal-insulator transition. The interplay between spins and orbits results in unusual temperature dependence of the orbital polarization in the orbital insulator, which can be seen in the resonant x-ray scattering intensity.

cond-mat.str-el↗

arXiv2006

Entanglements in Systems with Multiple Degrees of Freedom

We present the entanglement properties of the spin-orbital coupling systems with multiple degrees of freedom. After constructing the maximally entangled spin-orbital basis of bipartite, we find that the quantum entanglement length in the noninteracting itinerant Fermion system with spin and orbit is considerably larger than that in the system with only spin. In the SU(2)$\otimes$SU(2) spin-orbital interacting system, the entanglement, expressed in terms of the spin-orbital correlation functions, clearly manifests the close relationship with the quantum phases in strongly correlated systems; and the entanglement phase diagram of the finite-size systems is in agreement with the magnetic and orbital phase diagram of the infinite systems. The application of the present theory on nucleon systems is suggested.

cond-mat.str-el↗

arXiv2006

Orbital Order Instability and Orbital Excitations in Degenerate Itinerant Electron Systems

We present the theory of orbital ordering in orbital-degenerate itinerant electron systems. After proposing the criterion of instability for orbital ordering or orbital density wave ordering, we find that the orbital and the spin-orbital collective excitation spectra in ferro-orbital ordered phase exhibit finite gaps. The anomalous electronic energy spectra manifested in the angle-resolved photoemission spectroscopy (ARPES) and the orbital occupation in the resonant X-ray scattering (RXS) intensities are also presented for the orbital-ordered phase.

cond-mat.mtrl-sci↗

arXiv2005

Orthorhombic crystalline field splitting on orbital and magnetic orders in KCuF$_3$

Magnetic and orbital structures in KCuF$_{3}$ are revisited by the cluster self-consistent field approach developed recently. We clearly showed that due to the inherent frustration, the ground state of the system with the superexchange and Jahn-Teller phonon-mediated orbital couplings is highly degenerate without broken symmetry; the orthorhombic crystalline field splitting arising from static Jahn-Teller distortion stabilizes the orbital ordering, about 42% in the $x^{2}-y^{2}$ orbit and 58% in the $3z^{2}-r^{2}$ orbit in sublattices. The magnetic moment of Cu is considerably reduced to 0.49$μ_{B}$, and the magnetic coupling strengths are highly anisotropic, J$_{c}$/J$_{ab}$ $\approx$ 26. These results are in agreement with the experiments, implying that as an orbital selector, the crystalline field plays an essential role in stabilizing the ground state of KCuF$_{3}$. The 1s-3d resonant X-ray scattering amplitudes in KCuF$_{3}$ with the {\it type-a} and {\it type-d} structures are also presented.

cond-mat.str-el↗

arXiv2005

Effects of interorbital hopping on orbital fluctuations and metal-insulator transitions: Extended linearized dynamical mean-field theory

We study the effects of interorbital hopping on orbital fluctuations and Mott-Hubbard metal-insulator transition (MIT) in the two-orbital Hubbard model within the extended linearized dynamical mean-field theory. By mapping the model onto an effective model with different bandwidths through the canonical transformation, we find that at half-filling, the increases of the interorbital Coulomb interaction $U^{\prime}$ and the Hund's coupling $J$ drive the MIT, and the critical $J_{c}$ for MIT increases with the lift of the inter-orbital hopping integral $t_{ab}$. Meanwhile at quarter filling and in the strong correlation regime, the system without $t_{ab}$ exhibits MIT with the decreasing of $J$, and favors the orbital liquid ground state. However, the system transits from metal to insulator with the increasing of t$_{ab}$, accompanied with the rising of the orbital order parameter. These results show the important role of the interorbital hopping in the orbital fluctuation and orbital ordering.

cond-mat.str-el↗

arXiv2004

Electronic States and Magnetism of Mn Impurities and Dimers in Narrow-Gap and Wide-Gap III-V Semiconductors

Electronic states and magnetic properties of single $Mn$ impurity and dimer doped in narrow-gap and wide-gap $III$-$V$ semiconductors have been studied systematically. It has been found that in the ground state for single $Mn$ impurity, $Mn$-$As(N)$ complex is antiferromagnetic (AFM) coupling when $p$-$d$ hybridization $V_{pd}$ is large and both the hole level $E_{v}$ and the impurity level $E_{d}$ are close to the midgap; or very weak ferromagnetic (FM) when $V_{pd}$ is small and both $E_{v}$ and $E_d$ are deep in the valence band. In $Mn$ dimer situation, the $Mn$ spins are AFM coupling for half-filled or full-filled $p$ orbits; on the contrast, the Mn spins are double-exchange-like FM coupling for any $p$-orbits away from half-filling. We propose the strong {\it p-d} hybridized double exchange mechanism is responsible for the FM order in diluted $III$-$V$ semiconductors.

cond-mat.str-el↗