Two-leg Ladder System Research Articles

The energy spectrum and magnetization profile of the decorated spin ladder systems

The peculiarities of the energy spectrum of the decorated mixed spin two-leg ladder system having three spin unit cells have been studied by the perturbation theory in the limit of dominant spin coupling in ladder rungs. It is shown, that despite the singlet ground state, this spin ladder may have an intermediate plateau in a low-temperature magnetization profile. The existence of a similar plateau was shown also in the case of some intermediate values of coupling parameters by means of exact diagonalization study of small ladder clusters and by the density-matrix renormalization group calculations. We also found that the Ising type of interactions in ladder rungs may lead to the disappearance of the above intermediate magnetization plateau for the corresponding isotropic spin ladder.

Josephson effects between the Kitaev ladder superconductors

The two-leg ladder system consisting of the Kitaev chains is known to exhibit a richer phase diagram than that of the single chain. We theoretically investigate the variety of the Josephson effects between the ladder systems. We consider the Josephson phase difference \thetaθ between these two ladder systems as well as the phase difference \phiϕ between the parallel chains in each ladder system. The total energy of the junction at T = 0T=0 is calculated by a numerical diagonalization method as functions of \thetaθ, \phiϕ, and also a transverse hopping t_{\perp}t⊥ in the ladders. We find that, by controlling t_{\perp}t⊥ and \phiϕ, the junction exhibits not only the fractional Josephson effect for the phase difference \thetaθ, but also the usual 0-junction and even \piπ-junction properties.

Quench dynamics of two-leg ladders with magnetic flux

The realization and detection of two-leg ladders with the interplay of the artificial gauge field provide an ideal platform for investigations of the quantum phase transition and quantum dynamics. Here, we realize the spatio-temporal evolution dynamics of two-leg ladder systems with magnetic flux analytically and numerically. The quench dynamics induced by giving sudden quenches of initial density distributions of left and right leg components, rung-to-leg coupling ratios and magnetic flux from the ground state are discussed in detail. Affected by the chirality of ladder systems together with the tunneling between the two legs, rich and interesting quench dynamics observed, which including the Josephson Oscillations (JOs), Chiral Density Wave (CDW), Scissors Mode (SM), Periodic Torsional Vortex (PTV) and Periodic Torsional-Scissors Vortex (PTSV). Remarkably, we find that the oscillation frequency of the quench dynamics associated with coupling ratios and magnetic flux can be used to identify the phase transition between the Meissner phase and Vortex phase. Therefore, our work may provide an effective way to distinguish phase transitions by using quench dynamics.

Pressure-induced orbital-selective metal from the Mott insulator BaFe2Se3

A general understanding of the mechanism underlying the pressure-induced Mott insulator-metal transition in strongly correlated materials is still lacking. Here we explore the pressure-induced electronic reconstruction in BaFe 2 Se 3 , a potential two-leg ladder system for unconventional (non-BCS) superconductivity. We stress the importance of multiorbital Coulomb interactions in concert with first-principles band-structure calculations for a consistent understanding of its intrinsic Mott-Hubbard insulating state both at ambient and under pressure. We elucidate the nature of pressure-induced insulator-metal transition seen in experiment, showing that it is driven by bandwidth broadening under pressure. We reveal an orbital-selective electronic state where Mott localized and itinerant electrons coexist in compressed BaFe 2 Se 3 , which incorporates orbital-resolved scattering rates and renormalization factors hidden in the normal state at high pressures.

Iron telluride ladder compounds: Predicting the structural and magnetic properties of BaFe2Te3

Since the discovery of pressure-induced superconductivity in the two-leg ladder system BaFe$_2X_3$ ($X$=S, Se), with the 3$d$ iron electronic density $n = 6$, the quasi-one-dimensional iron-based ladders have attracted considerable attention. Here, we use Density Functional Theory (DFT) to predict that the novel $n = 6$ iron ladder BaFe$_2$Te$_3$ could be stable with a similar crystal structure as BaFe$_2$Se$_3$. Our results also indicate that BaFe$_2$Te$_3$ will display the complex 2$\times$2 Block-type magnetic order. Due to the magnetic striction effects of this Block order, BaFe$_2$Te$_3$ should be a magnetic noncollinear ferrielectric system with a net polarization $0.31$ $\mu$C/cm$^2$. Compared with the S- or Se-based iron ladders, the electrons of the Te-based ladders are more localized, implying that the degree of electronic correlation is enhanced for the Te case which may induce additional interesting properties. The physical and structural similarity with BaFe$_2$Se$_3$ also suggests that BaFe$_2$Te$_3$ could become superconducting under high pressure.

Emergence of orbital-selective marginal Fermi liquidness in compressed RbFe2Te3

Driving a Mott insulator towards an orbital-selective metal with coexisting Mott localized and marginal Fermi liquid electronic states remains central to understanding various correlated emergent phenomena. Here, using density functional dynamical mean-field theory we explore correlation and hole-doping-induced electronic reconstruction in ${\mathrm{RbFe}}_{2}{\mathrm{Te}}_{3}$, a potential two-leg ladder system for hole carrier superconductivity under pressure. We stress the importance of multiorbital Coulomb interactions in concert with band structure calculations for a consistent understanding of intrinsic Mott localization at ambient pressure. We elucidate the nature of pressure-induced Mottness upon tuning the correlation to bandwidth ratio away from the Mott boundary. As a by-product of our analysis, a quantum critical phenomenon with coexisting Mott localized and marginal Fermi liquid electrons is predicted to exist in compressed ${\mathrm{RbFe}}_{2}{\mathrm{Te}}_{3}$ metal.

Solitons in two-leg bosonic ladder subject to an artificial magnetic field

SynopsisThe nonlinear dynamics in two-leg ladder system subject to an artificial magnetic field are discussed in detail. The critical conditions for the formation of soliton and for the transition from diffusion to self-trapping are presented, and confirmed by the numerical simulations of the full discrete nonlinear Schrödinger equation.

Quantum phases of biased bosonic ladders subject to a magnetic field

SynopsisThe quantum phase transition of the Plane Wave I phase, the Plane Wave II phase, and interestingly, a new phase named Moving Biased Vortex phase are revealed in the biased bosonic two-leg ladder system.

Triplet excitations in the frustrated spin ladder Li2Cu2O(SO4)2

Magnetic excitations of the recently discovered frustrated spin-1/2 two-leg ladder system Li$_2$Cu$_2$O(SO$_4$)$_2$ are investigated using inelastic neutron scattering, magnetic susceptibility and infrared absorption measurements. Despite the presence of a magnetic dimerization concomitant with the tetragonal-to-triclinic structural distortion occurring below 125 K, neutron scattering experiments reveal the presence of dispersive triplet excitations above a spin gap of $\Delta = 10.6$ meV at 1.5 K, a value consistent with the estimates extracted from magnetic susceptibility. The likely detection of these spin excitations in infrared spectroscopy is explained by invoking a dynamic Dzyaloshinskii-Moriya mechanism in which light is coupled to the dimer singlet-to-triplet transition through an optical phonon. These results are qualitatively explained by exact diagonalization and higher-order perturbation calculations carried out on the basis of the dimerized spin Hamiltonian derived from first-principles.

Chiral Mott insulators in frustrated Bose-Hubbard models on ladders and two-dimensional lattices: A combined perturbative and density matrix renormalization group study

We study the fully gapped chiral Mott insulator (CMI) of frustrated Bose-Hubbard models on ladders and two-dimensional lattices by perturbative strong-coupling analysis and density matrix renormalization group (DMRG). First we show the existence of a low-lying exciton state on all geometries carrying the correct quantum numbers responsible for the condensation of excitons and formation of the CMI in the intermediate interaction regime. Then we perform systematic DMRG simulations on several two-leg ladder systems with $\pi$-flux and carefully characterize the two quantum phase transitions. We discuss the possibility to extend the generally very small CMI window by including repulsive nearest-neighbour interactions or changing density and coupling ratios.

Thermal Conductivity due to Spins in the Frustrated Two-Leg Spin Ladder System BiCu2PO6

We have measured the thermal conductivity in the frustrated two-leg spin ladder system of Bi(Cu1−xZnx)2PO6 single crystals with x = 0 and 0.02 with the spin quantum number S = 1/2 to investigate th...

Two-leg ladder systems with dipole–dipole Fermion interactions

The ground-state phase diagram of a two-leg fermionic dipolar ladder with inter-site interactions is studied using density matrix renormalization group (DMRG) techniques. We use a state-of-the-art implementation of the DMRG algorithm and finite size scaling to simulate large system sizes with high accuracy. We also consider two different model systems and explore stable phases in half and quarter filling factors. We find that in the half filling, the charge and spin gaps emerge in a finite value of the dipole–dipole and on-site interactions. In the quarter filling case, s-wave superconducting state, charge density wave, homogenous insulating and phase separation phases occur depend on the interaction values. Moreover, in the dipole–dipole interaction, the D-Mott phase emerges when the hopping terms along the chain and rung are the same, whereas, this phase has been only proposed for the anisotropic Hubbard model. In the half filling case, on the other hand, there is either charge-density wave or charged Mott order phase depends on the orientation of the dipole moments of the particles with respect to the ladder geometry.

Majorana spin liquids, topology, and superconductivity in ladders

We theoretically address spin chain analogs of the Kitaev quantum spin model on the honeycomb lattice. The emergent quantum spin liquid phases or Anderson resonating valence bond (RVB) states can be understood, as an effective model, in terms of p-wave superconductivity and Majorana fermions. We derive a generalized phase diagram for the two-leg ladder system with tunable interaction strengths between chains allowing us to vary the shape of the lattice (from square to honeycomb ribbon or brickwall ladder). We evaluate the winding number associated with possible emergent (topological) gapless modes at the edges. In the Az phase, as a result of the emergent Z2 gauge fields and pi-flux ground state, one may build spin-1/2 (loop) qubit operators by analogy to the toric code. In addition, we show how the intermediate gapless B phase evolves in the generalized ladder model. For the brickwall ladder, the $B$ phase is reduced to one line, which is analyzed through perturbation theory in a rung tensor product states representation and bosonization. Finally, we show that doping with a few holes can result in the formation of hole pairs and leads to a mapping with the Su-Schrieffer-Heeger model in polyacetylene; a superconducting-insulating quantum phase transition for these hole pairs is accessible, as well as related topological properties.

Extended Bose-Hubbard model for two-leg ladder systems in artificial magnetic fields

We investigate the ground state properties of ultracold atoms with long range interactions trapped in a two leg ladder configuration in the presence of an artificial magnetic field. Using a Gross-Pitaevskii approach and a mean field Gutzwiller variational method, we explore both the weakly interacting and strongly interacting regime, respectively. We calculate the boundaries between the density-wave/supersolid and the Mott-insulator/superfluid phases as a function of magnetic flux and uncover regions of supersolidity. The mean-field results are confirmed by numerical simulations using a cluster mean field approach.

Floquet engineering of Haldane Chern insulators and chiral bosonic phase transitions

The realization of synthetic gauge fields has attracted a lot of attention recently in relation with periodically driven systems and the Floquet theory. In ultra-cold atom systems in optical lattices and photonic networks, this allows to simulate exotic phases of matter such as quantum Hall phases, anomalous quantum Hall phases and analogs of topological insulators. In this paper, we apply the Floquet theory to engineer anisotropic Haldane models on the honeycomb lattice and two-leg ladder systems. We show that these anisotropic Haldane models still possess a topologically non-trivial band structure associated with chiral edge modes (without the presence of a net unit flux in a unit cell), then referring to the quantum anomalous Hall effect. Focusing on (interacting) boson systems in s-wave bands of the lattice, we show how to engineer through the Floquet theory, a quantum phase transition between a uniform superfluid and a BEC (Bose-Einstein Condensate) analog of FFLO (Fulde-Ferrell-Larkin-Ovchinnikov) states, where bosons condense at non-zero wave-vectors. We perform a Ginzburg-Landau analysis of the quantum phase transition on the graphene lattice, and compute observables such as chiral currents and the momentum distribution. The results are supported by exact diagonalization calculations and compared with those of the isotropic situation. The validity of high-frequency expansion in the Floquet theory is also tested using time-dependent simulations for various parameters of the model. Last, we show that the anisotropic choice for the effective vector potential allows a bosonization approach in equivalent ladder (strip) geometries.

Tellurium-bridged two-leg spin ladder inBa2CuTeO6

We present single-crystal growth and magnetic property studies of tellurium-bridged copper spin-$\frac{1}{2}$ system ${\mathrm{Ba}}_{2}{\mathrm{CuTeO}}_{6}$. The spin-exchange interaction among copper spins via $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Te}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Cu}$ super-superexchange route leads to a novel two-leg spin ladder system. Spin susceptibility $\ensuremath{\chi}(T)$ data indicate that the triclinic ${\mathrm{Ba}}_{2}{\mathrm{CuTeO}}_{6}$ undergoes a stepwise crossover for exchange couplings revealed by a broad maximum of $\ensuremath{\chi}(T)$ near ${T}_{max}\ensuremath{\sim}75$ K and an anisotropic cusp in $\frac{d\ensuremath{\chi}}{dT}(T)$ at ${T}_{N}\ensuremath{\sim}15$ K to signify a three-dimensional (3D) antiferromagnetic long-range ordering (LRO). The 3D LRO has been suggested from the anisotropic behavior of $\ensuremath{\chi}(T)$ with strong $c$-axis spin anisotropy and the signature of spin-flop transition from the isothermal magnetization below ${\mathit{T}}_{N}$. Analysis of magnetic heat capacity (${C}_{m}$) at ${T}_{N}\ensuremath{\sim}15$ K indicates that most of the spin entropy $(\ensuremath{\sim}92%)$ has already been released above ${T}_{N}$, which supports the picture of consecutive spin entropy reduction upon cooling with Te-bridged two-leg spin ladder system with strong intraladder and interladder couplings. Theoretical $\mathrm{DFT}+U$ calculations have been performed to search for the ground-state magnetic configuration and also to evaluate exchange-coupling constants that support the magnetic model deduced from the combined spin susceptibility and crystal structure symmetry analysis.

Ground-state and spectral properties of an asymmetric Hubbard ladder

We investigate a ladder system with two inequivalent legs, namely a Hubbard chain and a one-dimensional electron gas. Analytical approximations, the density matrix renormalization group method, and continuous-time quantum Monte Carlo simulations are used to determine ground-state properties, gaps, and spectral functions of this system at half-filling. Evidence for the existence of four different phases as a function of the Hubbard interaction and the rung hopping is presented. First, a Luttinger liquid exists at very weak interchain hopping. Second, a Kondo-Mott insulator with spin and charge gaps induced by an effective rung exchange coupling is found at moderate interchain hopping or strong Hubbard interaction. Third, a spin-gapped paramagnetic Mott insulator with incommensurate excitations and pairing of doped charges is observed at intermediate values of the rung hopping and the interaction. Fourth, the usual correlated band insulator is recovered for large rung hopping. We show that the wavenumbers of the lowest single-particle excitations are different in each insulating phase. In particular, the three gapped phases exhibit markedly different spectral functions. We discuss the relevance of asymmetric two-leg ladder systems as models for atomic wires deposited on a substrate.

Superconductivity in a single-layer alkali-doped FeSe: A weakly coupled two-leg ladder system

We prepare single-layer potassium-doped iron selenide (110) film by molecular-beam expitaxy. Such a single-layer film can be viewed as a two-dimensional system composed of weakly coupled two-leg iron ladders. Scanning tunneling spectroscopy reveals that superconductivity is developed in this two-leg ladder system. The superconducting gap is similar to that of the multilayer films. However, the Fermi-surface topology in this quasi-one-dimensional system is remarkably different from that of the bulk materials. Our results suggest that superconducting pairing is very short ranged or takes place rather locally in iron chalcogenides. The superconductivity is most likely driven by electron-electron correlation effect and is insensitive to the change of Fermi surfaces.

The bis(ethylenedithio)tetrathiafulvalene-based ionic charge-transfer complex with 2,3-dichloro-5,6-dicyano-p-benzoquinone

In the ionic charge-transfer (CT) complex composed of bis(ethylenedithio)tetrathiafulvalene (ET) and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), C10H8S8·C8Cl2N2O2, the donor and acceptor molecules both form centrosymmetric dimers associated by strong face-to-face π-π interactions. The disordered DDQ molecules form a one-dimensional π-stacked column, while the ET molecules form a two-leg ladder through additional short S···S contacts between adjacent π-π-bonded dimers. The crystal structure of ET-DDQ revealed in this study will provide a valuable example of the two-leg spin ladder system, which has rarely been reported for ET-based CT complexes.

Inverse magnetocaloric effects in two-leg spin ladder system

Magnetocaloric properties of the two-leg frustrated spin ladder system are investigated by the use of transfer-matrix method. Exact results for the magnetization, entropy, entropy change and magnetocaloric cooling rate versus temperature and field are presented and discussed in detail. Large inverse magnetocaloric effects induced by the field arise at low temperature, and exhibit strong dependency relationship on the exchange couplings. In the vicinity of the critical field, the enhanced inverse magnetocaloric effects and normal magnetocaloric effects are both observed. Besides, the influences of the single-ion anisotropy on the entropy change are discussed as well.