Understanding high-Tc cuprates based on the phase string theory of doped antiferromagnet

We present a self-consistent RVB theory which unifies the metallic (superconducting) phase with the half-filling antiferromagnetic (AF) phase. Two crucial factors in this theory include the RVB condensation which controls short-range AF spin correlations and the phase string effect introduced by hole hopping as a key doping effect. We discuss both the uniform and non-uniform mean-field solutions and show the unique features of the characteristic spin energy scale, superconducting transition temperature, and the phase diagram, which are all consistent with the experimental measurements of high-$T_c$ cuprates.Comment: 4 pages, 4 embeded eps figures, minor typos are corrected, to appear in the proceedings of M2S-HTSC-VI conferenc

Non-collinear magnetic structure and multipolar order in Eu2_2Ir2_2O7_7

The magnetic properties of the pyrochlore iridate material Eu$_2$Ir$_2$O$_7$ (5$d^5$) have been studied based on the first principle calculations, where the crystal field splitting $\Delta$, spin-orbit coupling (SOC) $\lambda$ and Coulomb interaction $U$ within Ir 5$d$ orbitals are all playing significant roles. The ground state phase diagram has been obtained with respect to the strength of SOC and Coulomb interaction $U$, where a stable anti-ferromagnetic ground state with all-in/all-out (AIAO) spin structure has been found. Besides, another anti-ferromagnetic states with close energy to AIAO have also been found to be stable. The calculated nonlinear magnetization of the two stable states both have the d-wave pattern but with a $\pi/4$ phase difference, which can perfectly explain the experimentally observed nonlinear magnetization pattern. Compared with the results of the non-distorted structure, it turns out that the trigonal lattice distortion is crucial for stabilizing the AIAO state in Eu$_2$Ir$_2$O$_7$. Furthermore, besides large dipolar moments, we also find considerable octupolar moments in the magnetic states.Comment: 6 pages, 4 figures, supplemental material is included in the source file, accepted for publication in PR

Lower Pseudogap Phase: A Spin/Vortex Liquid State

The pseudogap phase is considered as a new state of matter in the phase string model of the doped Mott insulator, which is composed of two distinct regimes known as upper and lower pseudogap phases, respectively. The former corresponds to the formation of spin singlet pairing and the latter is characterized by the formation of the Cooper pair amplitude and described by a generalized Gingzburg-Landau theory. Elementary excitation in this phase is a charge-neutral object carrying spin-1/2 and locking with a supercurrent vortex, known as spinon-vortex composite. Here thermally excited spinon-vortices destroy the phase coherence and are responsible for nontrivial Nernst effect and diamagnetism. The transport entropy and core energy associated with a spinon-vortex are determined by the spin degrees of freedom. Such a spontaneous vortex liquid phase can be also considered as a spin liquid with a finite correlation length and gapped S=1/2 excitations, where a resonancelike non-propagating spin mode emerges at the antiferromagnetic wavevector with a doping-dependent characteristic energy. A quantitative phase diagram in the parameter space of doping, temperature, and magnetic field is determined. Comparisons with experiments are also made.Comment: 22 pages, 12 figure

Mutual Chern-Simons Theory of Spontaneous Vortex Phase

We apply the mutual Chern-Simons effective theory (Phys. Rev. B 71, 235102) of the doped Mott insulator to the study of the so-called spontaneous vortex phase in the low-temperature pseudogap region, which is characterized by strong unconventional superconducting fluctuations. An effective description for the spontaneous vortex phase is derived from the general mutual Chern-Simons Lagrangian, based on which the physical properties including the diamagnetism, spin paramagnetism, magneto-resistance, and the Nernst coefficient, have been quantitatively calculated. The phase boundaries of the spontaneous vortex phase which sits between the onset temperature $T_{v}$ and the superconducting transition temperature $T_{c}$, are also determined within the same framework. The results are consistent with the experimental measurements of the cuprates.Comment: 12 pages, 8 figure

Confinement-deconfinement interplay in quantum phases of doped Mott insulators

It is generally accepted that doped Mott insulators can be well characterized by the t-J model. In the t-J model, the electron fractionalization is dictated by the phase string effect. We found that in the underdoped regime, the antiferromagnetic and superconducting phases are dual: in the former, holons are confined while spinons are deconfined, and {\it vice versa} in the latter. These two phases are separated by a novel phase, the so-called Bose-insulating phase, where both holons and spinons are deconfined. A pair of Wilson loops was found to constitute a complete set of order parameters determining this zero-temperature phase diagram. The quantum phase transitions between these phases are suggested to be of non-Landau-Ginzburg-Wilson type.Comment: 4 pages, 1 figure, accepted by Phys. Rev. Let

The electronic structure of NaIrO3_3, Mott insulator or band insulator?

Motivated by the unveiled complexity of nonmagnetic insulating behavior in pentavalent post-perovskite NaIrO$_3$, we have studied its electronic structure and phase diagram in the plane of Coulomb repulsive interaction and spin-orbit coupling (SOC) by using the newly developed local density approximation plus Gutzwiller method. Our theoretical study proposes the metal-insulator transition can be generated by two different physical pictures: renormalized band insulator or Mott insulator regime. For the realistic material parameters in NaIrO$_3$, Coulomb interaction $U=2.0 (J=U/4)$ eV and SOC strength $\eta=0.33$ eV, it tends to favor the renormalized band insulator picture as revealed by our study.Comment: 5 pages, 4 figure

Theoretical Prediction of Two-Dimensional Functionalized MXene Nitrides as Topological Insulators

Recently, two-dimensional (2D) transition metal carbides and nitrides, namely, MXenes have attracted lots of attention for electronic and energy storage applications. Due to a large spin-orbit coupling (SOC) and the existence of a Dirac-like band at the Fermi energy, it has been theoretically proposed that some of the MXenes will be topological insulators (TIs). Up to now, all of the predicted TI MXenes belong to transition metal carbides, whose transition metal atom is W, Mo or Cr. Here, on the basis of first-principles and Z2 index calculations, we demonstrate that some of the MXene nitrides can also be TIs. We find that Ti3N2F2 is a 2D TI, whereas Zr3N2F2 is a semimetal with nontrivial band topology and can be turned into a 2D TI when the lattice is stretched. We also find that the tensile strain can convert Hf3N2F2 semiconductor into a 2D TI. Since Ti is one of the mostly used transition metal element in the synthesized MXenes, we expect that our prediction can advance the future application of MXenes as TI devices