arXiv subjects

Sen Zhou

Publications and source records attributed to Sen Zhou.

36 records · Page 2Linked to original sources

arXiv2020

Role of anion in the pairing interaction of iron-based superconductivity

High-temperature iron-based superconductivity develops in a structure with unusual lattice-orbital geometry, based on a planar layer of Fe atoms with 3d orbitals and tetrahedrally coordinated by anions. Here we elucidate the electronic role of anions in the iron-based superconductors utilizing state-of-the-art scanning tunneling microscopy. By measuring the local electronic structure, we find that As anion in Ba0.4K0.6Fe2As2 has a striking impact on the electron pairing. The superconducting electronic feature can be switched off/on by removing/restoring As atoms on Fe layer at the atomic scale. Our analysis shows that this remarkable atomic switch effect is related to the geometrical cooperation between anion mediated hopping and unconventional pairing interaction. Our results uncover that the local Fe-anion coupling is fundamental for the pairing interaction of iron-based superconductivity, and promise the potential of bottom-up engineering of electron pairing.

cond-mat.supr-con↗

arXiv2020

t-J model on the effective brick-wall lattice for the recently discovered high-temperature superconductor Ba$_2$CuO$_{3+δ}$

Layered copper oxides have highest superconducting transition temperatures at ambient pressure. Its mechanism remains a grand challenge in condensed matter physics. The essential physics lying in 2-dimensional copper-oxygen layers is well described by a single band Hubbard model or its strong coupling limit t-J model in 2-dimensional square lattice. Recently discovered high temperature superconductor Ba$_2$CuO$_{3+δ}$ with $δ\sim 0.2$ has different crystal structure with large portion of in-plane oxygen vacancies. We observe that an oxygen vacancy breaks the bond of its two neighboring copper atoms, and propose ordered vacancies in Ba$_2$CuO$_{3+δ}$ lead to extended t-J model on an effective brick-wall lattice. For the nearest neighbor hopping, the brick-wall model can be mapped onto t-J model on honeycomb lattice. Our theory explains the superconductivity of Ba$_2$CuO$_{3+δ}$ at high charge carrier density, and predict a time reversal symmetry broken pairing state.

cond-mat.supr-con↗

arXiv2019

Dynamical slave-boson mean-field study of the Mott transition in the Hubbard model in the large-$z$ limit

The Mott metal-insulator transition in the Hubbard model is studied by constructing a dynamical slave-boson mean-field theory in the limit of large lattice coordination number $z$ that incorporates the binding between doubly occupied (doublon) and empty (holon) sites. On the Mott insulating side where all doublons and holons bond in real space into excitonic pairs leading to the charge gap, the theory simplifies considerably to leading order in $1/\sqrt{z}$, and becomes exact on the infinite-$z$ Bethe lattice. An asymptotic solution is obtained for a continuous Mott transition associated with the closing of the charge gap at a critical value of the Hubbard $U_c$ and the corresponding doublon density $n_d^c$, hopping $χ_d^c$ and doublon-holon pairing $Δ_d^c$ amplitudes. We find $U_c=U_{\rm BR} [1 -2n_d^c -\sqrt{z} (χ_d^c +Δ_d^c))] \simeq0.8U_{\rm BR}$, where $U_{\rm BR}$ is the critical value for the Brinkman-Rice transition in the Gutzwiller approximation captured in the static mean-field solution of the slave-boson formulation of Kotliar and Ruckenstein. Thus, the Mott transition can be viewed as the quantum correction to the Brinkman-Rice transition due to doublon-holon binding. Quantitative comparisons are made to the results of the dynamical mean-field theory, showing good agreement. In the absence of magnetic order, the Mott insulator is a $U(1)$ quantum spin liquid with nonzero intersite spinon hopping that survives the large-$z$ limit and lifts the $2^N$-fold degeneracy of the local moments. We show that the spinons are coupled to the doublons/holons by a dissipative compact $U(1)$ gauge field in the deconfined phase, realizing the spin-charge separated gapless spin liquid Mott insulator.

cond-mat.str-el↗

arXiv2018

Antiferromagnetic Chern insulators in non-centrosymmetric systems

We investigate a new class of topological antiferromagnetic (AF) Chern insulators driven by electronic interactions in two-dimensional systems without inversion symmetry. Despite the absence of a net magnetization, AF Chern insulators (AFCI) possess a nonzero Chern number $C$ and exhibit the quantum anomalous Hall effect (QAHE). Their existence is guaranteed by the bifurcation of the boundary line of Weyl points between a quantum spin Hall insulator and a topologically trivial phase with the emergence of AF long-range order. As a concrete example, we study the phase structure of the honeycomb lattice Kane-Mele model as a function of the inversion-breaking ionic potential and the Hubbard interaction. We find an easy $z$-axis $C=1$ AFCI phase and a spin-flop transition to a topologically trivial $xy$-plane collinear antiferromagnet. We propose experimental realizations of the AFCI and QAHE in correlated electron materials and cold atom systems.

cond-mat.str-el↗

arXiv2017

Correlation effects and hidden spin-orbit entangled electronic order in parent and electron-doped iridates Sr$_2$IrO$_4$

Analogs of the high-T$_c$ cuprates have been long sought after in transition metal oxides. Due to the strong spin-orbit coupling (SOC), the $5d$ perovskite iridates Sr$_2$IrO$_4$ exhibit a low-energy electronic structure remarkably similar to the cuprates. Whether a superconducting state exists as in the cuprates requires understanding the correlated spin-orbit entangled electronic states. Recent experiments discovered hidden order in the parent and electron doped iridates, some with striking analogies to the cuprates, including Fermi surface pockets, Fermi arcs, and pseudogap. Here, we study the correlation and disorder effects in a five-orbital model derived from the band theory. We find that the experimental observations are consistent with a $d$-wave spin-orbit density wave order that breaks the symmetry of a joint two-fold spin-orbital rotation followed by a lattice translation. There is a Berry phase and a plaquette spin flux due to spin procession as electrons hop between Ir atoms, akin to the intersite SOC in quantum spin Hall insulators. The associated staggered circulating $J_\text{eff}=1/2$ spin current can be probed by advanced techniques of spin-current detection in spintronics. This electronic order can emerge spontaneously from the intersite Coulomb interactions between the spatially extended iridium $5d$ orbitals, turning the metallic state into an electron doped quasi-2D Dirac semimetal with important implications on the possible superconducting state suggested by recent experiments.

cond-mat.str-el↗

arXiv2016

Doublon-holon binding as origin of Mott transition and fractionalized spin liquid -- Asymptotic solution of the Hubbard model in the limit of large coordination

An analytical solution of the Mott transition is obtained for the Hubbard model on the Bethe lattice in the large coordination number ($z$) limit. The excitonic binding of doublons (doubly occupied sites) and holons (empty sites) is shown to be the origin of a continuous Mott transition between a metal and an emergent quantum spin liquid insulator. The doublon-holon binding theory enables a different large-$z$ limit and a different phase structure than the dynamical meanfield theory by allowing intersite spinon correlations to lift the $2^N$-fold degeneracy of the local moments in the insulating phase. We show that the spinons are coupled to doublons/holons by a dissipative compact U(1) gauge field that is in the deconfined phase, stabilizing the spin-charge separated gapless spin liquid Mott insulator.

cond-mat.str-el↗

arXiv2015

Chiral spin density wave order on frustrated honeycomb and bilayer triangle lattice Hubbard model at half-filling

We study the Hubbard model on the frustrated honeycomb lattice with nearest-neighbor $t_1$ and second nearest-neighbor hopping $t_2$, which is isomorphic to the bilayer triangle lattice, using the SU(2)-invariant slave boson theory. We show that the Coulomb interaction $U$ induces antiferromagnetic (AF) chiral spin-density wave ($χ$-SDW) order in a wide range of $κ=t_2/t_1$ where both the two-sublattice AF order at small $κ$ and the decoupled three-sublattice 120$^\circ$ order at large $κ$ are strongly frustrated, leading to three distinct phases with different anomalous Hall responses. We find a continuous transition from a $χ$-SDW semimetal with anomalous Hall effect to a topological chiral Chern insulator exhibiting quantum anomalous Hall effect, followed by a discontinuous transition to a $χ$-SDW insulator with zero total Chern number but anomalous ac Hall effect.The $χ$-SDW is likely a generic phase of strongly correlated and highly frustrated hexagonal lattice electrons.

cond-mat.str-el↗

arXiv2014

Textured electronic states of the triangular lattice Hubbard model and Na$_x$CoO$_2$

We show that geometric frustration and strong correlation in the triangular lattice Hubbard model lead a rich and novel phase structure of $\sqrt{3}\times\sqrt{3}$ spin-charge textured electronic states over a wide region of electron doping $0\le x \le 0.40$. In addition to the 120$^\circ$ Néel ordered insulator at half-filling, we found a novel spin-charge ordered insulator at $x=1/3$ with collinear antiferromagnetic (AF) order on the underlying unfrustrated honeycomb lattice. Separating the two insulating phases is a Lifshitz transition between a noncollinear AF ordered metal and one with coexisting charge order. We obtain the phase diagram and the evolution of the Fermi surface (FS). Remarkably, the correlated ground states near $x=1/3$ emerges as doping the "1/3 AF insulator" by excess carriers, leading to electron and hole FS pockets with important implications for the cobaltate superconducting state.

cond-mat.str-el↗

arXiv2014

Doublon-holon binding, Mott transition, and fractionalized antiferromagnet in the Hubbard model

We argue that the binding between doubly occupied (doublon) and empty (holon) sites governs the incoherent excitations and plays a key role in the Mott transition in strongly correlated Mott-Hubbard systems. We construct a new saddle point solution with doublon-holon binding in the Kotliar-Ruckenstein slave-boson functional integral formulation of the Hubbard model. On the half-filled honeycomb lattice and square lattice, the ground state is found to exhibit a continuous transition from the paramagnetic semimetal/metal to an antiferromagnetic ordered Slater insulator with coherent quasiparticles at $U_{c1}$, followed by a Mott transition into an electron-fractionalized AF$^*$ phase without coherent excitations at $U_{c2}$. Such a phase structure appears generic of bipartite lattices without frustration. We show that doublon-holon binding unites the three important ideas of strong correlation: the coherent quasiparticles, the incoherent Hubbard bands, and the deconfined Mott insulator.

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↗

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↗

arXiv2008

Nodal $d+id$ pairing and topological phases on the triangular lattice: unconventional superconducting state of Na$_x$CoO$_2\cdot y$H$_2$O

We show that finite angular momentum pairing chiral superconductors on the triangular lattice have point zeroes in the complex gap function. A topological quantum phase transition takes place through a nodal superconducting state at a specific carrier density $x_c$ where the normal state Fermi surface crosses the isolated zeros. For spin singlet pairing, we show that the second nearest neighbor $d+id$-wave pairing can be the dominant pairing channel. The gapless critical state at $x_c\simeq0.25$ has six Dirac points and is topologically nontrivial with a $T^3$ spin relaxation rate below $T_c$. This picture provides a possible explanation for the unconventional superconducting state of Na$_x$CoO$_2\cdot y$H$_2$O. Analyzing a pairing model with strong correlation using the Gutzwiller projection and symmetry arguments, we study these topological phases and phase transitions as a function of Na doping.

cond-mat.str-el↗

arXiv2007

Itinerant and localized magnetism on the triangular lattice: sodium rich phases of Na$_x$CoO$_2$

We study the interplay between correlation, itinerant ferromagnetism and local moment formation on the electron doped triangular lattice of sodium cobaltates Na$_x$CoO$_2$. We find that strong correlation renormalizes the Stoner criterion and stabilizes the paramagnetic state for $x x_c$, ferromagnetic (FM) order emerges. The enhanced Na dopant potential fluctuations play a crucial role in the sodium rich phases and lead to an inhomogeneous FM state, exhibiting nonmagnetic Co$^{3+}$ patches, antiferromagnetic (AF) correlated regions, and FM clusters with AF domains. Hole doping the band insulator at x=1 leads to the formation of local moments near the Na vacancies and AF correlated magnetic clusters. We explain recent observations by neutron, $μ$SR, and NMR experiments on the evolution of the magnetic properties in the sodium rich phases.

cond-mat.str-el↗

arXiv2007

Charge and spin order on the triangular lattice -- Na$_x$CoO$_2$ at $x=0.5$

The nature of electronic states due to strong correlation and geometric frustration on the triangular lattice is investigated in connection to the unconventional insulating state of Na$_x$CoO$_2$ at $x=0.5$. We study an extended Hubbard model using a spatially unrestricted Gutzwiller approximation. We find a new class of charge and spin ordered states at $x=1/3$ and $x=0.5$ where antiferromagnetic (AF) frustration is alleviated via weak charge inhomogeneity. At $x=0.5$, we show that the $\sqrt{3}a\times2a$ off-plane Na dopant order induces weak $\sqrt{3}a\times1a$ charge order in the Co layer. The symmetry breaking enables successive $\sqrt{3}a\times1a$ AF and $2a\times2a$ charge/spin ordering transitions at low temperatures. The Fermi surface is truncated by the $2a\times2a$ hexagonal zone boundary into small electron and hole pockets. We study the phase structure and compare to recent experiments.

cond-mat.str-el↗

arXiv2007

Correlating off-stoichiometric doping with nanoscale electronic disorder and quasiparticle interference pattern in high-$T_c$ superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

A microscopic theory is presented for the observed electronic disorder in superconducting Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. The essential phenomenology is shown to be consistent with the existence of two types of interstitial oxygen dopants: those serving primarily as charge reserviors and those close to the apical plane contributing both carriers and electrostatic potential to the CuO$_2$ plane. The nonlinear screening of the latter produces nanoscale variations in the doped hole concentration, leading to electronic inhomogeneity. Based on an unrestricted Gutzwiller approximation of the extended t-$J$ model, we provide a consistent explanation of the correlation between the observed dopant location and the pairing gap and its spatial evolutions. We show that the oxygen dopants are the primary cause of both the pairing gap disorder and the quasiparticle interference pattern.

cond-mat.str-el↗

arXiv2006

Inhomogeneous states with checkerboard order in the t-J Model

We study inhomogeneous states in the t-J model using an unrestricted Gutzwiller approximation. We find that $pa\times pa$ checkerboard order, where $p$ is a doping dependent number, emerges from Fermi surface instabilities of both the staggered flux phase and the Fermi liquid state with realistic band parameters. In both cases, the checkerboard order develops at wave vectors $(\pm 2π/pa,0)$, $(0,\pm2π/pa)$ that are tied to the peaks of the wave-vector dependent susceptibility, and is of the Lomer-Rice-Scott type. The properties of such periodic, inhomogeneous states are discussed in connection to the checkerboard patterns observed by STM in underdoped cuprates.

cond-mat.str-el↗

arXiv2005

Electron correlation and Fermi surface topology of Na$_x$CoO$_2$

The electronic structure of Na$_x$CoO$_2$ revealed by recent photoemission experiments shows important deviations from band theory predictions. The six small Fermi surface pockets predicted by LDA calculations have not been observed as the associated $e_g^\prime$ band fails to cross the Fermi level for a wide range of sodium doping concentration $x$. In addition, significant bandwidth renormalizations of the $t_{2g}$ complex have been observed. We show that these discrepancies are due to strong electronic correlations by studying the multi-orbital Hubbard model in the Hartree-Fock and strong-coupling Gutzwiller approximation. The quasiparticle dispersion and the Fermi surface topology obtained in the presence of strong local Coulomb repulsion are in good agreement with experiments.

cond-mat.str-el↗

arXiv2004

Pseudogap, competing order and coexistence of staggered flux and d-wave pairing in high-temperature superconductors

We study the t-J-V model of a doped Mott insulator in connection to high-T_c superconductors. The nearest neighbor Coulomb interaction (V) is treated quantum mechanically on equal footing as the antiferromagnetic exchange interaction (J). Motivated by the SU(2) symmetry at half-filling, we construct a large-N theory which allows a systematic study of the interplay between staggered flux order and superconductivity upon doping. We solve the model in the large-N limit and obtain the ground state properties and the phase diagram as a function of doping. We discuss the competition and the coexistence of the staggered flux and the d-wave superconductivity in the underdoped regime and the disappearance of superconductivity in the overdoped regime

cond-mat.str-el↗