arXiv subjects

Ziqiang Wang

Publications and source records attributed to Ziqiang Wang.

At least 199 records · Page 11Linked to original sources

arXiv2014

Carrier localization and electronic phase separation in a doped spin-orbit driven Mott phase in Sr3(Ir1-xRux)2O7

Interest in many strongly spin-orbit coupled 5d-transition metal oxide insulators stems from mapping their electronic structures to a J=1/2 Mott phase. One of the hopes is to establish their Mott parent states and explore these systems' potential of realizing novel electronic states upon carrier doping. However, once doped, little is understood regarding the role of their reduced Coulomb interaction U relative to their strongly correlated 3d-electron cousins. Here we show that, upon hole-doping a candidate J=1/2 Mott insulator, carriers remain localized within a nanoscale phase separated ground state. A percolative metal-insulator transition occurs with interplay between localized and itinerant regions, stabilizing an antiferromagnetic metallic phase beyond the critical region. Our results demonstrate a surprising parallel between doped 5d- and 3d-electron Mott systems and suggest either through the near degeneracy of nearby electronic phases or direct carrier localization that U is essential to the carrier response of this doped spin-orbit Mott insulator.

cond-mat.str-el↗

arXiv2013

Dynamical Mass Generation of Composite Dirac Fermions and Fractional Quantum Hall Effects near Charge Neutrality in Graphene

We develop a composite Dirac fermion theory for the fractional quantum Hall effects (QHE) near charge neutrality in graphene. We show that the interactions between the composite Dirac fermions lead to dynamical mass generation through exciton condensation. The four-fold spin-valley degeneracy is fully lifted due to the mass generation and the exchange effects such that the odd-denominator fractional QHE observed in the vicinity of charge neutrality can be understood in terms of the integer QHE of the composite Dirac fermions. At the filling factor $ν=1/2$, we show that the massive composite Dirac fermion liquid is unstable against chiral p-wave pairing for weak Coulomb interactions and the ground state is a paired nonabelian state described by the Moore-Read Pfaffian in the long wavelength limit.

cond-mat.str-el↗

arXiv2013

Imaging the evolution of metallic states in a spin-orbit interaction driven correlated iridate

The Ruddlesden-Popper (RP) series of iridates (Srn+1IrnO3n+1) have been the subject of much recent attention due to the anticipation of emergent physics arising from the cooperative action of spin-orbit (SO) driven band splitting and Coulomb interactions[1-3]. However an ongoing debate over the role of correlations in the formation of the charge gap and a lack of understanding of the effects of doping on the low energy electronic structure have hindered experimental progress in realizing many of the predicted states[4-8] including possible high-Tc superconductivity[7,9]. Using scanning tunneling spectroscopy we map out the spatially resolved density of states in the n=2 RP member, Sr3Ir2O7 (Ir327). We show that the Ir327 parent compound, argued to exist only as a weakly correlated band insulator in fact possesses a substantial ~130meV charge excitation gap driven by an interplay between structure, SO coupling and correlations. A critical component in distinguishing the intrinsic electronic character within the inhomogeneous textured electronic structure is our identification of the signature of missing apical oxygen defects, which play a critical role in many of the layered oxides. Our measurements combined with insights from calculations reveal how apical oxygen vacancies transfer spectral weight from higher energies to the gap energies thereby revealing a path toward obtaining metallic electronic states from the parent-insulating states in the iridates.

cond-mat.mtrl-sci↗

arXiv2013

Majorana fermions in spin-singlet nodal superconductors with coexisting non-collinear magnetic order

Realizations of Majorana fermions in solid state materials have attracted great interests recently in connection to topological order and quantum information processing. We propose a novel way to create Majorana fermions in superconductors. We show that an incipient non-collinear magnetic order turns a spin-singlet superconductor with nodes into a topological superconductor with a stable Majorana bound state in the vortex core; at a topologically-stable magnetic point defect; and on the edge. We argue that such an exotic non-Abelian phase can be realized in extended t-J models on the triangular and square lattices. It is promising to search for Majorana fermions in correlated electron materials where nodal superconductivity and magnetism are two common caricatures.

cond-mat.str-el↗

arXiv2012

Spin ordering and electronic texture in the bilayer iridate Sr$_3$Ir$_2$O$_7$

Through a neutron scattering, charge transport, and magnetization study, the correlated ground state in the bilayer iridium oxide Sr$_3$Ir$_2$O$_7$ is explored. Our combined results resolve scattering consistent with a high temperature magnetic phase that persists above 600 K, reorients at the previously defined $T_{AF}=280$ K, and coexists with an electronic ground state whose phase behavior suggests the formation of a fluctuating charge or orbital phase that freezes below $T^{*}\approx70$ K. Our study provides a window into the emergence of multiple electronic order parameters near the boundary of the metal to insulator phase transition of the 5d $J_{eff}=1/2$ Mott phase.

cond-mat.str-el↗

arXiv2012

Effect of uniaxial strain on the structural and magnetic phase transitions in BaFe$_2$As$_2$

We report neutron scattering experiments probing the influence of uniaxial strain on both the magnetic and structural order parameters in the parent iron pnictide compound, BaFe$_2$As$_2$. Our data show that modest strain fields along the in-plane orthorhombic b-axis can affect significant changes in phase behavior simultaneous to the removal of structural twinning effects. As a result, we demonstrate in BaFe$_2$As$_2$ samples detwinned via uniaxial strain that the in-plane C$_4$ symmetry is broken by \textit{both} the structural lattice distortion \textit{and} long-range spin ordering at temperatures far above the nominal (strain-free), phase transition temperatures. Surprising changes in the magnetic order parameter of this system under relatively small strain fields also suggest the inherent presence of magnetic domains fluctuating above the strain-free ordering temperature in this material.

cond-mat.supr-con↗

arXiv2011

Possible interaction driven topological phases in (111) bilayers of LaNiO3

We use the variational mean-field approach to systematically study the phase diagram of a bilayer heterostructure of the correlated transition metal oxide LaNiO3, grown along the (111) direction. The Ni 3+ ions with d7 (or eg1) configuration form a buckled honeycomb lattice. We show that as a function of the strength of the on-site interactions, various topological phases emerge. In the presence of a reasonable size of the Hund's coupling, as the correlation is tuned from intermediate to strong, the following sequence of phases is found: (1) a Dirac half-semimetal phase, (2) a quantum anomalous hall insulator (QAHI) phase with Chern number one, and (3) a ferromagnetic nematic phase breaking the lattice point group symmetry. The spin-orbit couplings and magnetism are both dynamically generated in the QAHI phase.

cond-mat.str-el↗

arXiv2011

Extended Hubbard model of superconductivity driven by charge fluctuations in iron-pnictides

We present a scenario for iron-pnictide superconductivity mediated by charge fluctuations that are strongly enhanced by Fe-As intersite electronic interactions. Deriving an eight-band extended Hubbard model including Fe 3$d$ and As 4$p$ orbitals for the LaOFeAs family, we show that charge fluctuations induced by $p$-$d$ charge transfer and As orbital polarization interactions in the Fe-pnictogen structure peak at wavevectors $(0, 0)$, and ($π$, 0) and ($π$, $π$) respectively. Intraorbital spin-singlet pairing attraction develops at these wavevectors and the solution of the linearized gap equation shows robust s-wave superconductivity with both $s_\pm$ and $s_{++}$ gap functions.

cond-mat.supr-con↗

arXiv2011

Inter-layer Superconducting Pairing Induced c-axis Nodal Lines in Iron-based Superconductors

A layered superconductor with a full pairing energy gap can be driven into a nodal superconducting (SC) state by inter-layer pairing when the SC state becomes more quasi-3D. We propose that this mechanism is responsible for the observed nodal behavior in a class of iron-based SCs. We show that the intra- and inter-layer pairings generally compete and the gap nodes develop on one of the {\em hole} Fermi surface pockets as they become larger in the iron-pnictides. Our results provide a natural explanation of the c-axis gap modulations and gap nodes observed by angle resolved photoemission spectroscopy. Moreover, we predict that an anti-correlated c-axis gap modulations on the hole and electron pockets should be observable in the $S^{\pm}$-wave pairing state.

cond-mat.supr-con↗

arXiv2011

Electron-Spin Excitation Coupling in an Electron Doped Copper Oxide Superconductor

High-temperature (high-Tc) superconductivity in the copper oxides arises from electron or hole doping of their antiferromagnetic (AF) insulating parent compounds. The evolution of the AF phase with doping and its spatial coexistence with superconductivity are governed by the nature of charge and spin correlations and provide clues to the mechanism of high-Tc superconductivity. Here we use a combined neutron scattering and scanning tunneling spectroscopy (STS) to study the Tc evolution of electron-doped superconducting Pr0.88LaCe0.12CuO4-delta obtained through the oxygen annealing process. We find that spin excitations detected by neutron scattering have two distinct modes that evolve with Tc in a remarkably similar fashion to the electron tunneling modes in STS. These results demonstrate that antiferromagnetism and superconductivity compete locally and coexist spatially on nanometer length scales, and the dominant electron-boson coupling at low energies originates from the electron-spin excitations.

cond-mat.supr-con↗

arXiv2011

Abelian and non-abelian anyons in integer quantum anomalous Hall effect and topological phase transitions via superconducting proximity effect

We study the quantum anomalous Hall effect described by a class of two-component Haldane models on square lattices. We show that the latter can be transformed into a pseudospin triplet p+ip-wave paired superfluid. In the long wave length limit, the ground state wave function is described by Halperin's (1,1,-1) state of neutral fermions analogous to the double layer quantum Hall effect. The vortex excitations are charge e/2 abelian anyons which carry a neutral Dirac fermion zero mode. The superconducting proximity effect induces `tunneling' between `layers' which leads to topological phase transitions whereby the Dirac fermion zero mode fractionalizes and Majorana fermions emerge in the edge states. The charge e/2 vortex excitation carrying a Majorana zero mode is a non-abelian anyon. The proximity effect can also drive a conventional insulator into a quantum anomalous Hall effect state with a Majorana edge mode and the non-abelian vortex excitations.

cond-mat.str-el↗

arXiv2010

Correlation-hole induced paired quantum Hall states in lowest Landau level

A theory is developed for the paired even-denominator fractional quantum Hall states in the lowest Landau level. We show that electrons bind to quantized vortices to form composite fermions, interacting through an exact instantaneous interaction that favors chiral p-wave pairing. Two canonically dual pairing gap functions are related by the bosonic Laughlin wavefunction (Jastraw factor) due to the correlation holes. We find that the ground state is the Moore-Read pfaffian in the long wavelength limit for weak Coulomb interactions, a new pfaffian of an oscillatory pairing function for intermediate interactions, and a Read-Rezayi composite Fermi liquid beyond a critical interaction strength. Our findings are consistent with recent experimental observations of the 1/2 and 1/4 fractional quantum Hall effects in asymmetric wide quantum wells.

cond-mat.str-el↗

arXiv2010

Electron correlation and spin density wave order in iron pnictides

We study the correlation effects on the electronic structure and spin density wave order in Fe-pnictides. Using the multiorbital Hubbard model and Gutzwiller projection, we show that nonperturbative correlation effects are essential to stabilize the metallic spin density wave phase for the intermediate correlation strengths appropriate for pnictides. We find that the ordered moments depend sensitively on the Hund's rule coupling $J$ but weakly on the intraorbital Coulomb repulsion $U$, varying from $0.3μ_B$ to $1.5μ_B$ in the range $J=0.3\sim0.8$ eV for $U=3\sim4$ eV. We obtain the phase diagram and discuss the effects of orbital order and electron doping, the evolution of the Fermi surface topology with the ordered moment, and compare to recent experiments.

cond-mat.str-el↗

arXiv2009

Non-Abelian Quantum Hall States and their Quasiparticles: from the Pattern of Zeros to Vertex Algebra

In the pattern-of-zeros approach to quantum Hall states, a set of data {n;m;S_a|a=1,...,n; n,m,S_a in N} (called the pattern of zeros) is introduced to characterize a quantum Hall wave function. In this paper we find sufficient conditions on the pattern of zeros so that the data correspond to a valid wave function. Some times, a set of data {n;m;S_a} corresponds to a unique quantum Hall state, while other times, a set of data corresponds to several different quantum Hall states. So in the latter cases, the patterns of zeros alone does not completely characterize the quantum Hall states. In this paper, We find that the following expanded set of data {n;m;S_a;c|a=1,...,n; n,m,S_a in N; c in R} provides a more complete characterization of quantum Hall states. Each expanded set of data completely characterize a unique quantum Hall state, at least for the examples discussed in this paper. The result is obtained by combining the pattern of zeros and Z_n simple-current vertex algebra which describes a large class of Abelian and non-Abelian quantum Hall states Φ_{Z_n}^sc. The more complete characterization in terms of {n;m;S_a;c} allows us to obtain more topological properties of those states, which include the central charge c of edge states, the scaling dimensions and the statistics of quasiparticle excitations.

cond-mat.str-el↗

arXiv2009

Mott and Wigner-Mott transitions in doped correlated electron systems: effects of superlattice potential and inter-site correlation

We introduce the notion of superstructure Mottness to describe the Mott and Wigner-Mott transition in doped strongly correlated electron systems at commensurate filling fractions away from one electron per site. We show that superstructure Mottness emerges in an inhomogeneous electron system when the superstructure contains an odd number of electrons per supercell. We argue that superstructure Mottness exists even in the absence of translation symmetry breaking by a superlattice, provided that the extended or intersite Coulomb interaction is strong. In the latter case, superstructure Mottness offers a unifying framework for the Mott and Wigner physics and a nonperturbative, strong coupling description of the Wigner-Mott transition. We support our proposal by studying a minimal single-band ionic Hubbard $t$-$U$-$V$-$Δ$ model with nearest neighbor Coulomb repulsion $V$ and a two-sublattice ionic potential $Δ$. The model is mapped onto a Hubbard model with two effective ``orbitals'' representing the two sites within the supercell, the intra and interorbital Coulomb repulsion $U$ and $U^\prime \sim V$, and a crystal field splitting $Δ$. Charge order on the original lattice corresponds to orbital order. Developing a cluster Gutzwiller approximation, we study the effects and the interplay between $V$ and $Δ$ on the Mott and Wigner-Mott transitions at quarter-filling. We provide the mechanism by which the superlattice potential enhances the correlation effects and the tendency towards local moment formation, construct and elucidate the phase diagram in the unifying framework of superstructure Mottness.

cond-mat.str-el↗

arXiv2009

Unveiling the Atomic and Electronic Structure at the Surface of the Parent Pnictide SrFe2As2

The parent compounds of the recently discovered iron-arsenic (pnictide) high temperature superconductors transition into an intriguing spin density wave (SDW) phase at low temperatures. Progress in understanding this SDW state has been complicated by a complex band structure and by the fact that the spin, electronic, and structural degrees of freedom are closely intertwined in these compounds. Scanning tunneling microscopy (STM) measurements have added to this complexity by revealing different topographies with no consensus on the surface structure. In this paper, we use a combination of high-resolution STM imaging and spectroscopy, and low energy electron diffraction (LEED) to determine the atomic and electronic structure of the parent pnictide SrFe2As2. Our data present a compelling picture of the existence of two coexisting homotopic structures on the surface. Based on this, we construct a simple model for the surface, which offers an explanation of the two classes of topographies seen by STM. STM spectroscopy shows that while the high energy density of states (DOS) profile is consistent with the Fe 3d and As 4p-electrons predicted by LDA it is in better agreement with calculations that include electron correlations beyond LDA. Importantly, we find a gap of ~15 meV in the low energy density of states on both structures which may be linked with the SDW or the observed surface reconstruction.

cond-mat.supr-con↗

arXiv2009

Valence bond glass -- A unified theory of electronic disorder and pseudogap phenomena in high temperature superconductors

We show that the low-energy fluctuations of the valence bond in underdoped high-T_c cuprates, originating from quantum fluctuations of the superexchange interaction, are pinned by the electronic disorder due to off-stoichiometric dopants, leading to a valence bond glass (VBG) pseudogap phase. The antinodal Fermi surface sections are gapped out, giving rise to a normal state Fermi arc whose length shrinks with underdoping. Below T_c, the superexchange interaction induces a d-wave superconducting gap that coexists with the VBG pseudogap. The evolution of the local and momentum-space spectroscopy with doping and temperature captures the salient properties of the pseudogap phenomena and the electronic disorder. The unified theory elucidates the important interplay between strong correlation and the intrinsic electronic disorder in doped transition metal oxides.

cond-mat.str-el↗

arXiv2009

An exactly soluble model with tunable p-wave paired fermion ground states

Motivated by the work of Kitaev, we construct an exactly soluble spin-$\frac{1}2$ model on honeycomb lattice whose ground states are identical to $Δ_{1x}p_x+Δ_{1y}p_y+i(Δ_{2x}p_x+Δ_{2y}p_y)$-wave paired fermions on square lattice, with tunable paring order parameters. We derive a universal phase diagram for this general p-wave theory which contains a gapped A phase and a topologically non-trivial B phase. We show that the gapless condition in the B phase is governed by a generalized inversion (G-inversion) symmetry under $p_x\leftrightarrow {Δ_{1y}\over Δ_{1x}} p_y$. The G-inversion symmetric gapless B phase near the phase boundaries is described by 1+1-dimensional gapless Majorana fermions in the asymptotic long wave length limit, i.e. the $c=1/2$ conformal field theory. The gapped B phase has G-inversion symmetry breaking and is the weak pairing phase described by the Moore-Read Pfaffian. We show that in the gapped B phase, vortex pair excitations are separated from the ground state by a finite energy gap.

cond-mat.str-el↗