Antiferromagnetic Mott Insulator Research Articles

Absence of charge order in the dimerizedκ-phase BEDT-TTF salts

Utilizing infrared vibrational spectroscopy we have investigated dimerized two-dimensional organic salts in order to search for possible charge redistribution that might cause electronic dipoles and ferroelectricity: the quantum spin liquid \kappa-(BEDT-TTF)2Cu2(CN)3, the antiferromagnetic Mott insulator \kappa-(BEDT-TTF)2$Cu[N(CN)2]Cl, and the superconductor \kappa-(BEDT-TTF)2$Cu[N(CN)2]Br. None of them exhibit any indication of charge disproportionation upon cooling down to low temperatures. No modification in the charge distribution is observed around T = 6 K where a low-temperature anomaly has been reported for the spin-liquid \kappa-(BEDT-TTF)2$Cu2(CN)3. In this compound the in-plane optical response and vibrational coupling are rather anisotropic, indicating that the tilt of the BEDT-TTF molecules in c-direction and their coupling to the anion layers has to be considered in the explanation of the electromagnetic properties.

Power-law dependence of the optical conductivity observed in the quantum spin-liquid compoundκ-(BEDT-TTF)2Cu2(CN)3

The Mott-insulator $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu${}_{2}$(CN)${}_{3}$ is the prime candidate of a quantum spin-liquid with puzzling magnetic properties. Our terahertz and infrared investigations reveal that also the charge dynamics does not follow the expectations for a Mott insulator. We observe a large in-gap absorption where the excess conductivity exhibits a power-law behavior ${\ensuremath{\sigma}}_{1}^{\mathrm{exc}}(\ensuremath{\omega})\ensuremath{\propto}{\ensuremath{\omega}}^{n}$ that grows stronger as the temperature decreases and extends all the way through the far infrared. With $n\ensuremath{\approx}0.8$ to 1.5, the exponent is significantly smaller than predicted by Ng and Lee [T.-K. Ng and P. A. Lee, Phys. Rev. Lett. 99, 156402 (2007)] for spinon contributions to the optical conductivity. We suggest fluctuations become important in the spin-liquid state and couple to the electrodynamic properties differently compared to the antiferromagnetic Mott insulator $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu[N(CN)${}_{2}$]Cl. We discuss the various possibilities of how charge fluctuations are influenced by the presence or absence of magnetic order.

Quantum phase transitions in the Kane-Mele-Hubbard model

We study the two-dimensional Kane-Mele-Hubbard model at half filling by means of quantum Monte Carlo simulations. We present a refined phase boundary for the quantum spin liquid. The topological insulator at finite Hubbard interaction strength is adiabatically connected to the groundstate of the Kane-Mele model. In the presence of spin-orbit coupling, magnetic order at large Hubbard U is restricted to the transverse direction. The transition from the topological band insulator to the antiferromagnetic Mott insulator is in the universality class of the three-dimensional XY model. The numerical data suggest that the spin liquid to topological insulator and spin liquid to Mott insulator transitions are both continuous.

Effect of disorder on the charge‐carrier dynamics in κ ‐(D8‐ET)2Cu[N(CN)2]Br

AbstractWe have applied resistance fluctuation spectroscopy to study the intrinsic low‐frequency dynamics of the charge carriers in the system κ ‐(D8‐ET)2Cu[N(CN)2]Br for two different cooling rates. A strong increase of both the resistance and the noise level is observed below the thermodynamic glass‐transition temperature Tg. Surprisingly, the data of the normalized spectral weight 〈(δR)2〉/R2 fall on top of each other for both cooling rates except for temperatures near the Mott critical point (T0 ≃ 35 K) and below 28 K. The suppression of the peak in 〈(δR)2〉/R2 associated with the Mott transition suggests that disorder is an additional parameter in the generalized phase diagram. The enhanced noise level below 28 K might be explained by a disorder‐induced staggered magnetic moment in the metallic state close to the antiferromagnetic Mott insulator. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Interplay between crystal-field splitting and superexchange interaction in thet2gorbital Mott insulator

With a model of the ${d}^{1}({t}_{2g})$-electron antiferromagnetic Mott insulator a competition of two typical interactions for orbital states of the ${t}_{2g}$-triplet levels has been investigated: The on-site electronic coupling with a local crystal field (LCF) tends orbitals to order, while the intersite superexchange (SE) induces their fluctuations. Both interactions coexist in perovskites: The LCF is induced by GdFeO${}_{3}$-type structural deformations. In turn, the SE originates from virtual electronic hoppings, allowed by the Pauli principle, between nonorthogonal orbital states of adjacent sites. The dependence of the state of the orbitals (namely, a dispersion of their excitations and the quadrupole polarization) on a relation between the energy $\ensuremath{\Delta}$ of the triplet level splitting, caused by the ${D}_{3d}$-symmetry LCF, and the exchange energy ${J}_{\text{SE}}$ is analyzed. Two qualitatively different regimes of the collective orbital behavior have been established: the induced order and fast fluctuations. It has been found that a crossover between them occurs when $\ensuremath{\Delta}$ comes close to ${J}_{\text{SE}}$, and in the parameter region about $\ensuremath{\Delta}\ensuremath{\approx}{J}_{\text{SE}}$ the order and fluctuations affect the wave function in a comparable extent. Previous approaches based on either of the interactions fail in this window. The present study identifies validity ranges of the former theories, embracing the physics of the orbitals by the unified description.

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

Competition between 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 interaction, the system firstly transits from a paramagnetic (PM) metal to a Néel antiferromagnetic (AFM) Mott insulator, or to a nonmagnetic orbital insulator, depending on the competition of crystal field splitting and the Hund’s rule coupling. The AFM Mott insulating, PM metallic and orbital insulating phases are not, partially and fully orbital polarized, respectively. For a small J H and a finite crystal field, the orbital insulator is robust. 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.

First-principles study of the electronic structure and magnetism of CaIrO3

I study the electronic structure and magnetism of postperovskite CaIrO$_3$ using first-principles calculations. The density functional calculations within the local density approximation without the combined effect of spin-orbit coupling and on-site Coulomb repulsion show the system to be metallic, which is in disagreement with the recent experimental evidences that show CaIrO$_3$ to be an antiferromagnetic Mott insulator in the $J_\textrm{eff}$ = 1/2 state. However, when spin-orbit coupling is taken into account, the Ir $t_{2g}$ bands split into fully filled $J_\textrm{eff}$ = 3/2 bands and half-filled $J_\textrm{eff}$ = 1/2 bands. I find that spin-orbit coupling along with a modest on-site Coulomb repulsion opens a gap leading to a Mott insulating state. The ordering is antiferromagnetic along the c axis with total moments aligned antiparallel along the c axis and canted along the b axis.

High-TcSuperconductivity and Antiferromagnetism in Multilayered Copper Oxides –A New Paradigm of Superconducting Mechanism–

High-temperature superconductivity (HTSC) in copper oxides emerges on a layered CuO 2 plane when an antiferromagnetic Mott insulator is doped with mobile hole carriers. We review extensive studies of multilayered copper oxides by site-selective nuclear magnetic resonance (NMR), which have uncovered the intrinsic phase diagram of antiferromagnetism (AFM) and HTSC for a disorder-free CuO 2 plane with hole carriers. We present our experimental findings such as the existence of the AFM metallic state in doped Mott insulators, the uniformly mixed phase of AFM and HTSC, and the emergence of d -wave SC with a maximum T c just outside a critical carrier density, at which the AFM moment on a CuO 2 plane disappears. These results can be accounted for by the Mott physics based on the t – J model. The superexchange interaction J in among spins plays a vital role as a glue for Cooper pairs or mobile spin-singlet pairs, in contrast to the phonon-mediated attractive interaction among electrons established in the Bardeen–Cooper–Schrieffer (BCS) theory. We remark that the attractive interaction for raising the T c of HTSC up to temperatures as high as 160 K is the large J in (∼0.12 eV), which binds electrons of opposite spins to be on neighboring sites, and that there are no bosonic glues. It is the Coulomb repulsive interaction U (> 6 eV) among Cu-3 d electrons that plays a central role in the physics behind high- T c phenomena. A new paradigm of the SC mechanism opens to strongly correlated electron matter.

Chiral symmetry breaking in graphene

The question of whether the Coulomb interaction is strong enough to break the sublattice symmetry of undoped graphene is discussed. We formulate a strong coupling expansion where the ground state of the Coulomb Hamiltonian is found exactly and the kinetic hopping Hamiltonian is treated as a perturbation. We argue that many of the properties of the resulting system would be shared by graphene with a Hubbard model interaction. In particular, the best candidate sublattice symmetry-breaking ground state is an antiferromagnetic Mott insulator. We discuss the results of some numerical simulations which indicate that the Coulomb interaction is indeed subcritical. We also point out the curious fact that if the electron did not have spin degeneracy, the tendency to break chiral symmetry would be much greater and even relatively weak Coulomb interactions would likely gap the spectrum.

Particle-hole symmetry and interaction effects in the Kane-Mele-Hubbard model

We prove that the Kane-Mele-Hubbard model with purely imaginary next-nearest-neighbor hoppings has a particle-hole symmetry at half-filling. Such a symmetry has interesting consequences including the absence of charge and spin currents along open edges, and the absence of the sign problem in the determinant quantum Monte-Carlo simulations. Consequentially, the interplay between band topology and strong correlations can be studied at high numeric precisions. The process that the topological band insulator evolves into the antiferromagnetic Mott insulator as increasing interaction strength is studied by calculating both the bulk and edge electronic properties. In agreement with previous theory analyses, the numeric simulations show that the Kane-Mele-Hubbard model exhibits three phases as increasing correlation effects: the topological band insulating phase with stable helical edges, the bulk paramagnetic phase with unstable edges, and the bulk antiferromagnetic phase.

Displacement-Type Ferroelectricity with Off-Center Magnetic Ions in PerovskiteSr1−xBaxMnO3

We report a ferroelectric transition driven by the off-centering of magnetic Mn(4+) ions in antiferromagnetic Mott insulators Sr(1-x)Ba(x)MnO(3) with a perovskite structure. As x increases, the perovskite lattice shows the typical soft-mode dynamics, as revealed by the momentum-resolved inelastic x-ray scattering and far-infrared spectroscopy, and the ferroelectricity shows up for x ≥ 0.45. The observed polarization is comparable to that for a prototypical ferroelectric BaTiO(3). We further demonstrate that the magnetic order suppresses the ferroelectric lattice dilation by ∼70% and increases the soft-phonon energy by ∼50%, indicating the largest magnetoelectric effects yet attained.

Spin liquids in graphene

We reveal that local interactions in graphene allow novel spin liquids between the semi-metal and antiferromagnetic Mott insulating phases, identified with algebraic spin liquid and Z$_{2}$ spin liquid, respectively. We argue that the algebraic spin liquid can be regarded as the two dimensional realization of one dimensional spin dynamics, where antiferromagnetic correlations show exactly the same power-law dependence as valence bond correlations. Nature of the Z$_{2}$ spin liquid turns out to be $d + i d'$ singlet pairing, but time reversal symmetry is preserved, taking $d + i d'$ in one valley and $d - i d'$ in the other valley. We propose the quantized thermal valley Hall effect as an essential feature of this gapped spin liquid state. Quantum phase transitions among the semi-metal, algebraic spin liquid, and Z$_{2}$ spin liquid are shown to be continuous while the transition from the Z$_{2}$ spin liquid to the antiferromagnetic Mott insulator turns out to be the first order. We emphasize that both algebraic spin liquid and $d \pm id'$ Z$_{2}$ spin liquid can be verified by the quantum Monte Carlo simulation, showing the enhanced symmetry in the algebraic spin liquid and the quantized thermal valley Hall effect in the Z$_{2}$ spin liquid.

Magnetic Properties of Ab initio Model of Iron-Based Superconductors LaFeAsO

By using variational Monte Carlo method, we examine an effective low-energy model for LaFeAsO derived from an ab initio downfolding scheme. We show that quantum and many-body fluctuations near a quantum critical point largely reduce the antiferromagnetic (AF) ordered moment and the model not only quantitatively reproduces the small ordered moment in LaFeAsO, but also explains the diverse dependence on LaFePO, BaFe2As2 and FeTe. We also find that LaFeAsO is under large orbital fluctuations, sandwiched by the AF Mott insulator and weakly correlated metals. The orbital fluctuations and Dirac-cone dispersion hold keys for the diverse magnetic properties.

Variational cluster approximation study of Mott transition with strong spin-orbit coupling

Motivated by recent experiments on Sr2IrO4, the ground state magnetic and electronic structures are studied theoretically for a two-dimensional three-band Hubbard model with strong spin-orbit coupling. To treat spin-orbit coupling, local Coulomb interactions, and band structure effects on the same footing, the variational cluster approximation based on the self-energy functional theory is employed. It is found that for a relatively large coupling region, the ground state is an anisotropic antiferromagnetic Mott insulator of an effective local angular momentum Jeff = 1/2 with xy plane as an easy plane. This anisotropy is caused by the strong spin-orbit coupling along with the inter-orbital Hund's coupling. The momentum resolved one-particle excitations are also studied for the Mott insulating phase. It is found that the low-energy one-particle excitations consist mostly of the Jeff = 1/2 state, a direct evidence of a novel Jeff = 1/2 Mott insulator.

Anisotropic hyperfine coupling in vanadium oxides

We have investigated orbital occupancies in metallic states of the vanadium oxides by 51V NMR measurements on the single crystals of V2O3, K2V8016, and LiV204, having ground states of an antiferromagnetic Mott insulator, a spin singlet insulator, and a heavy-mass Fermi liquid, respectively. The Knight shift K vs the magnetic susceptibility plots for the principal axes give the axially anisotropic and isotropic parts of the hyperfine coupling constant, Aax and Aiso In V2O3, Aax diminishes owing to the equivalent contribution of three-hold t2g orbitals to the spin susceptibility. The orbital mixing is also significant in K2V8O16 having a small Aax/Aiso ~ 0.1, which is expected to have 0.5 – 0.6 for 3dxy1. In LiV204, Aax/Aiso has a large negative value, as expected in the a1g orbital. The result suggests the presence of the localized spin hidden in the itinerancy, giving the high-entropy state of LiV2O4 at low temperatures.

Pressure-induced superconductivity and Mott transition in spin-liquidκ−(ET)2Cu2(CN)3probed byC13NMR

Pressure-induced superconductivity and Mott transition in the spin-liquid Mott insulator $\ensuremath{\kappa}\text{\ensuremath{-}}{(\text{ET})}_{2}{\text{Cu}}_{2}{(\text{CN})}_{3}$ have been investigated by $^{13}\text{C}$ NMR measurements. The Mott transition from the spin liquid to the Fermi liquid is observed as a sudden decrease in $1/{T}_{1}T$, where ${T}_{1}$ is the nuclear-spin-lattice relaxation time. Pseudogap behavior is absent in the Fermi-liquid state, unlike in the metallic phase adjacent to the antiferromagnetic Mott insulator. In the superconducting state, $1/{T}_{1}$ have a cubic temperature dependence with no Hebel-Slichter peak, which is consistent with non-$s$-wave superconducting pairing.

Detecting the tunneling rates for strongly interacting fermions on optical lattices

Strongly interacting fermionic atoms on optical lattices are studied through a Hubbard-like model Hamiltonian, in which tunneling rates of atoms and molecules between neighboring sites are assumed to be different. In the limit of large onsite repulsion U, the model is shown to reproduce the t-J Hamiltonian, in which the J coefficient of the Heisenberg term depends on the particle-assisted tunneling rate g: explicitly, $J=4 g^2/U$. At half-filling, g drives a crossover from a Brinkman-Rice paramagnetic insulator of fully localized atoms (g=0) to the antiferromagnetic Mott insulator of the standard Hubbard case (g=t). This is observed already at the intermediate coupling regime in the number of doubly occupied sites, thus providing a criterion to extract from measurements the effective value of g.

Observation of a d-wave nodal liquid in highly underdoped Bi2Sr2CaCu2O8+δ

We use angle resolved photoemission spectroscopy to probe the electronic excitations of the non-superconducting state that exists between the antiferromagnetic Mott insulator at zero doping and the superconducting state at larger dopings in Bi_2Sr_2CaCu_2O_{8+\delta}. We find that this state is a nodal liquid whose excitation gap becomes zero only at points in momentum space. Despite exhibiting a resistivity characteristic of an insulator and the absence of coherent quasiparticle peaks, this material has the same gap structure as the d-wave superconductor. We observe a smooth evolution of the spectrum across the insulator-to-superconductor transition, which suggests that high temperature superconductivity emerges when quantum phase coherence is established in a non-superconducting nodal liquid.

Metallization of quasi-two-dimensional Mott insulator CaIrO 3 withS=12spins

Abstract We report on three attempts to metallize a S = 1 2 antiferromagnetic Mott insulator CaIrO3 with the quasi-two-dimensional post-perovskite structure. By controlling the dimensionality and the electron filling, we can successfully drive a metal–insulator transition, while an insulating state is robust against the applied pressure. These results are compared with related experiments for the post-perovskite Mg0.9Fe0.1SiO4, the layered perovskite Ca2RuO4, and high- T c cuprates. The interplay between spin, charge, and orbital degrees of freedom is discussed.

Effects of Coulomb interaction on the electronic structure and lattice dynamics of the Mott insulatorFe2SiO4spinel

Using the ab initio density-functional generalized gradient approximation with varying on-site Coulomb interaction $U$, the electronic and crystal structures of ${\text{Fe}}_{2}{\text{SiO}}_{4}$ spinel were optimized at high pressure. It was found that the ground state changes from the metallic ferromagnetic state at $U=0$ into the antiferromagnetic state, stable in the regime of Mott insulating state for $Ug2\text{ }\text{eV}$. We found that ${\text{Fe}}_{2}{\text{SiO}}_{4}$ spinel is an antiferromagnetic Mott insulator at the realistic value of $U\ensuremath{\sim}4.5\text{ }\text{eV}$, with a distinct gap which opens in the minority-$\text{Fe}(3d)$ states. We point out that the low value of the N\'eel temperature ${T}_{N}\ensuremath{\sim}12\text{ }\text{K}$ follows from the geometrical frustration of magnetic interactions, while the exchange constant $J\ensuremath{\sim}1.7\text{ }\text{meV}$ estimated from the energies of ordered magnetic phases gives a rather high value of the Curie-Weiss temperature ${\ensuremath{\theta}}_{\text{CW}}\ensuremath{\simeq}340\text{ }\text{K}$. The phonon-dispersion curves and phonon density of states were derived for both the metallic and insulating antiferromagnetic phase of ${\text{Fe}}_{2}{\text{SiO}}_{4}$ spinel, and significant changes of energy and dispersion of certain phonon modes, particularly with large Fe character, caused by local electron correlations were detected and analyzed. We point out some similarities with the electronic and dynamical properties of magnetite.