Dimerized Phase Research Articles

Dipolar origin of the gas-liquid coexistence of the hard-core 1:1 electrolyte model.

We present a systematic study of the effect of the ion pairing on the gas-liquid phase transition of hard-core 1:1 electrolyte models. We study a class of dipolar dimer models that depend on a parameter R(c), the maximum separation between the ions that compose the dimer. This parameter can vary from sigma(+/-) that corresponds to the tightly tethered dipolar dimer model to R(c)--> infinity that corresponds to the Stillinger-Lovett description of the free ion system. The coexistence curve and critical point parameters are obtained as a function of R(c) by grand-canonical Monte Carlo techniques. Our results show that this dependence is smooth but nonmonotonic and converges asymptotically towards the free ion case for relatively small values of R(c). This fact allows us to describe the gas-liquid transition in the free ion model as a transition between two dimerized fluid phases. The role of the unpaired ions can be considered as a perturbation of this picture.

MAGNETIZATION CURVES OF QUASI-ONE-DIMENSIONAL HALDANE SYSTEMS

We study the magnetization process of a quasi-one-dimensional S=1 antiferromagnet with bond alternation by using the density matrix renormalization group method combined with interchain mean-field approximation. Particularly, we discuss how the interchain couplings affect the magnetization curve around a plateau structure in both of the Haldane phase and the dimer phase. It is shown that the antiferromagnetic correlation induced by the interchain couplings reduces the region of the plateau significantly, which will play an important role when the theoretical results are compared with actual measurements of the magnetization.

HIGH FIELD NMR STUDIES OF NaV2O5 to 44.7 T

23Na NMR studies have been carried out to map the uniform to dimerized phase boundary up to 44.7 T in the NHMFL hybrid. The transition is observed by the splitting of the NMR line ( H //a axis) by the lattice distortion. The field dependence of Tc up to 44.7 T, in agreement with previous magnetization data to 30 T, is very small (α = 0.092) compared to 0.4 for conventional SP systems and shows no deviation within experimental error from the h2 dependence, while optical data show a decrease of the slope in the H-T phase diagram beginning around 40 T. The region of an anomaly near T 33 T and 32 K observed in optical data taken in the long pulse magnet at LANL1 was also explored. The only evidence in the NMR of the observed optical anomaly near 33 T and 32 K was line broadening over a 2 T interval.

Dimer liquid state in the quantum antiferromagnet compoundLiCu2O2

Results of systematic structural, magnetic, electron-spin resonance, and specific-heat investigations of the $S=1/2$ quantum antiferromagnet compound ${\mathrm{LiCu}}_{2}{\mathrm{O}}_{2}$ are presented. A spin-singlet ground state is revealed at temperatures above $T\ensuremath{\sim}23\mathrm{K}.$ In this temperature region ${\mathrm{LiCu}}_{2}{\mathrm{O}}_{2}$ may be regarded as a quantum $S=1/2$ chain with competing nearest- and next-nearest-neighbor interactions in the intermediate range of the ${J}_{2}{/J}_{1}$ ratio $({\ensuremath{\alpha}}_{c1}<{J}_{2}{/J}_{1}<{\ensuremath{\alpha}}_{c2}).$ The size of the energy gap between the spin-singlet ground state and the first excited triplet, \ensuremath{\Delta}, is found to be \ensuremath{\sim}72 K. The study confirms a magnetic phase transition in ${\mathrm{LiCu}}_{2}{\mathrm{O}}_{2},$ which results in the collapse of the spin-singlet ground state at a temperature below 23 K. In addition, a second low-temperature transition was found at $T\ensuremath{\sim}9\mathrm{K}.$ Peculiarities of the magnetic excitation spectrum in the dimer phase of ${\mathrm{LiCu}}_{2}{\mathrm{O}}_{2}$ and its magnetic structure are discussed.

Ground state properties of a generalized spin-1/2distorted diamond chain

With the help of the bond operator representation for three S = 1 2 spins, westudy the effective Hamiltonian and the phase diagram of a generalized spin-12distorted diamond chain. In the weak-intertriangle-coupling limit, the magnetismof the effective Hamiltonian is studied with second-order perturbation theory andmean-field decoupling. Various phases such as the spin-fluid phase, thedimerized phase and the ferrimagnetic phase are shown to compete. Forlarger intertriangle interactions, the spin-fluid phase and the dimerizedphase can be also described by the effective spin–orbital model and theregion of the dimerized phase enlarges around the symmetric point ofJ1 = J2 = J3(J1,J2 andJ3are the intratriangle interactions). The magnetization plateaus at 13Ms and23Msin the magnetic field are also studied.

Quantum criticalities in a two-leg antiferromagneticS=12ladder induced by a staggered magnetic field

We study a two-leg antiferromagnetic spin-1/2 ladder in the presence of a staggered magnetic field. We consider two parameter regimes: strong (weak) coupling along the legs and weak (strong) coupling along the rungs. In both cases, the staggered field drives the Haldane spin-liquid phase of the ladder towards a Gaussian quantum criticality. In a generalized spin ladder with a non-Haldane, spontaneously dimerized phase, the staggered magnetic field induces an Ising quantum critical regime. In the vicinity of the critical lines, we derive low-energy effective field theories and use these descriptions to determine the dynamical response functions, the staggered spin susceptibility, and the string order parameter.

Ground state properties of S = 1 antiferromagnetic chain systems studied by ESR

Electron spin resonance (ESR) was used to study the ground state properties of two kinds of spin (S) one Heisenberg antiferromagnetic chain systems, namely a uniform chain system (HAUC), which is well known as the Haldane system, and a bond alternating chain system (HABA). To investigate and compare the features of the ground state, two nickel chain compounds doped with non-magnetic Zn2+ impurities were studied. The HAUC was modelled with Ni(1,3-pn)2(μ-NO2)(ClO4)(1,3-pn = 1,3-propanediamine), abbreviated as NINO, while the HABA was modelled with Ni(333-tet)(μ-NO2)(ClO4) (333-tet = bis-(3-aminopropyl)-1,3-propanediamine), abbreviated as NTENP. Both systems have a singlet ground state with an excitation gap. The ground state of NINO approximates well to the valence bond solid state, thus producing S = 1/2 spins at the sites neighbouring the impurities. The angular dependence of the ESR signals of NINO:Zn is explained by the anisotropy of the g tensor for spin 1/2. On the other hand, the ground state of NTENP is expected to be in the singlet dimer phase based on the ratio of alternating bond strengths. In this case, it is expected that the S = 1 spins will appear at the sites neighbouring the impurities without forming the singlet dimer. From ESR studies of NTENP:Zn was observed the triplet state (S = 1), induced by the impurity doping, which is consistent with the above picture.

Spin-phonon chains with bond coupling

We investigate the antiadiabatic limit of an antiferromagnetic S=1/2 Heisenberg chain coupled to Einstein phonons via a bond coupling. The flow equation method is used to decouple the spin and the phonon part of the Hamiltonian. In the effective spin model longer range spin-spin interactions are generated. The effective spin chain is frustrated. The resulting temperature dependent couplings are used to determine the magnetic susceptibility and to determine the phase transition from a gapless state to a dimerized gapped phase. The susceptibilities and the phase diagram obtained via the effective couplings are compared with independently calculated quantum Monte Carlo results.

Dimerized spin fluid in a one-dimensional electron system

The ground state of a one-dimensional Hubbard model with a bond-charge attraction W term at half-filling is investigated by the density matrix renormalization group method. It is confirmed that the spin gap will be closed at U>8W. But the long-range bond order wave survives even when the spin gap is closed. It indicates that the ground state is a novel dimerized spin fluid at U>8W. By a charge-spin transformation, it is shown that there should be a dimerized metallic phase at U<-8W. Furthermore, it is found that the Hubbard interaction U enhances initially the dimerization for a weak bond charge attraction W whereas it reduces monotonously the dimerization for a stronger bond charge attraction W.

DMRG study of the S=1/2 Heisenberg chains with bond alternation and randomness on the strong bonds

The S=1/2 Heisenberg chains with bond alternation and randomness on the strong bonds are studied by the density matrix renormalization group method. It is assumed that the eventh bond is antiferromagnetic with strength J and oddth bond can take the values ± J′( J′> J>0) randomly. The ground state of this model interpolates between the Haldane phase and the dimer phase via a randomness dominated intermediate phase. From energy gap distribution and mean field treatment of the interchain coupling, we discuss the possibility of magnetic order induced by randomness in the intermediate regime. The relationship with the recent experiment for (CH 3) 2CHNH 3Cu(Cl x Br 1− x ) 3 is also discussed.

Finite-temperature phase diagram of mixed-stack charge-transfer complexes

The mixed-stack organic charge-transfer complex, (BEDO–TTF)(Cl 2TCNQ), has shown a phase transition accompanied with a sharp drop in the magnetic susceptibility and with an increase in the intermolecular overlap, as the temperature is lowered. Here, we theoretically consider the possibility for a phase transition from the ionic phase to a neutral phase, which is driven by charge-transfer fluctuations. Using finite-temperature density-matrix renormalization-group calculations for the one-dimensional extended Hubbard model with alternating potentials, we show that, with increasing transfer integral t, a transition from the neutral phase to the ionic phase is induced by spin fluctuations for small t, while another transition from the ionic phase to the neutral phase is induced by charge-transfer fluctuations for large t. Near the phase boundary, the free energy is easily lowered by dimerization of transfer integrals or by staggered magnetic field. Thus, a further transition is also possible to a dimerized nonmagnetic phase or to an antiferromagnetic phase at a lower temperature. In two dimensions, we expect that the effect of charge-transfer fluctuations is larger and that the transition to the dimerized nonmagnetic phase is suppressed.

Excitations and Dynamics of Spin-Orbital Chains

The low-energy excitations and dynamics of a spin-orbital chain are calculated within a variational framework based on dimer states, exact diagonalization and the dynamical Density Matrix Renormalization Group (DMRG). In its dimerized phase, the spin-orbital chain exhibits a rich interplay of solitonic and magnonic excitations. While a solitonic excitation is the lowest excitation in a narrow region around the exactly solvable dimer point, magnonic excitations govern the phase transitions to neighbouring phases. Numerical calculations confirm that the curvature of the magnonic excitations is exactly flat on the so-called dimer line, changing sign across the line. This statement is not true for the solitonic excitations, which determine the lower edge of the excitation spectrum in the vicinity of the dimer line. DMRG calculations suggest that low-lying solitonic excitations may be experimentally hard to observe, with most spectral weight in higher-lying magnons.

First-Order Phase Transitions in Frustrated Spin Systems

We give a short review of our recent works on the first-order quantum phase transitions in frustrated spin chains with orthogonal-dimer structure. When the ratio of the competing antiferromagnetic exchange couplings is varied, a first-order transition occurs between the dimer phase and the plaquette phase, which is accompanied by the discontinuity in the spin excitation gap. We further show that strong frustration triggers the phase transitions in a magnetic field, which exhibit plateaus and jumps in the magnetization curve. The hole-doping effect is also addressed for the orthogonal-dimer chain with linked-tetrahedra structure. It is found that the competing antiferromagnetic interactions result in a first-order metal-insulator transition upon hole doping.

Magnetic Properties ofS=1/2 Heisenberg Chains with Bond Alternation and Randomness on the Strong Bonds

The $S=1/2$ Heisenberg chains with bond alternation and randomness on the strong bonds are studied by the density matrix renormalization group method. It is assumed that the odd-th bond is antiferromagnetic with strength $J$ and even-th bond can take the values $\JA$ and $\JF$ $ (\JA > J > 0 > \JF)$ randomly. The ground state of this model interpolates between the Haldane and dimer phases via a randomness dominated intermediate phase. Based on the scaling of the low energy spectrum and mean field treatment of the interchain coupling, it is found that the magnetic long range order is induced by randomness in the intermediate regime. In the magnetization curves, there appears a plateau at the fractional value of the saturated magnetization. The fine structures of the magnetization curves and low energy spectrum are understood based on the cluster picture. The relation with the recent experiment for (CH$_3)_2$CHNH$_3$Cu(Cl$_x$Br$_{1-x})_3$ is discussed.

AnS=1/2 Impurity Spin in the AntiferromagneticS=1 Bond-Alternating Chain

We explore low-lying excited states as well as the ground state of the antiferromagnetic S=1 bond-alternating chain with an S = ½ impurity spin. For the case where the ground-state phase of the host system is the Haldane phase, we review a numerical analysis of the electron-spin-resonance experimental results on the NENP:Cu 2 + system. For the case where the ground-state phase of the host system is the dimer phase, on the other hand, we calculate, using the exact-diagonalization method, the dependences of the energy differences between the ground and low-lying excited states upon both the impurity-host exchange constant and the single-ion-type anisotropy constant, and also calculate, using the density-matrix renormalization-group method, the external-magnetic-field dependence of the impurity-spin magnetizaion in the ground state. In these calculations, we keep the NTENP:Cu 2 + system in mind to choose the value of the bond-alternation parameter. We find that a few low-lying excited states which are expected from the valence-bond-solid picture appear as the impurity states in the energy gap between the singlet ground and triplet first-excited states (the dimer gap). Furthermore, for certain values of the above constants, we find that the impurity-spin magnetizaion shows a clear jump at a magnetic field which is in the dimer-gap region or in the magnetization-plateau region of the host system, and also that the impurity-spin magnetizaion has a magnetic-field region where it decreases as a function of the magnetic field.

Laser Oscillation in Aggregates of Ultrasmall Si Nanoparticles

AbstractWe dispersed electrochemically etched Si into ultrabright ultrasmall nanoparticles, with brightness higher than fluorescein or rhodamine. The emission from single particles is readily detectable. Aggregates or films of the particles exhibit emission with highly nonlinear characteristics. We observe directed blue beams at ∼ 410 nm between faces of aggregates excited by femtosecond radiation at 780 nm; and at ∼ 610 nm from aggregates of red luminescent Si nanoparticles excited by radiation at 550-570 nm from a mercury lamp. Intense directed Gaussian beams, a pumping threshold, spectral line narrowing, and speckle patterns manifest the emission. The results are analyzed in terms of population inversion and stimulated emission in quantum confinement-induced Si-Si dimer phase, found only on ultrasmall Si nanoparticles. This microlasing constitutes an important step towards the realization of a laser on a chip.

Exact Bounds on the Critical Frustration in the Shastry–Sutherland Model

We discuss the phase diagram of the Shastry–Sutherland model for arbitrary spin S and derive rigorous lower and upper bounds on the phase boundaries of the dimer phase by using various versions of a variational ansatz in combination with the exact diagonalisation method.

Field theory for generalized Shastry-Sutherland models

We consider the bosonic dimer representation for Generalized Shastry-Sutherland models that have the same symmetries as the original Shastry-Sutherland model and preserve the exact dimer eigenstate. Various phases with differing types of magnetic order are found within mean-field theory for the corresponding low-energy effective dimer field theory. Transitions are allowed between any of these mean-field phases, which are dimer bose condensates, and with the dimer phase, which is the dimer bose vacuum. The Neel state, absent from this mean-field study, is described as a bosonic Mott insulator induced by the coupling to the underlying lattice. Moreover, dimer bose condensates with local Neel order are found to be unstable to spiral states. Instead of a direct phase transition between the dimer and the Neel phases, we propose an intermediate weakly incommensurate spin-density wave (WISDW) phase. The stability of the mean-field transitions is studied by renormalization techniques in d=2, the upper critical dimension. While the transition from the Neel phase is found to be stable, the transition point from the dimer phase is not perturbatively accessible. We argue that the latter renormalization results point to the possibility of an intermediate phase of a different kind.

Magnetization Process of One-Dimensional Kondo Necklace Model with Next-Nearest-Neighbor Interaction

The magnetization process of one-dimensional Kondo necklace model with next-nearest-neighbor interaction has been studied by the use of exact-diagonalization method. The magnetization plateaus appear at zero ( m =0) and at half ( m =1/2) of the saturation magnetization. The ground state in the absence of a magnetic field is regarded as an aggregate of intra-atomic Kondo singlets. On the other hand, the spin configuration at the 1/2-plateau represents the intra-chain dimer (σ-dimer) phase with broken translational symmetry. The picture for spin configurations in the uniform magnetic fields other than m =0 and 1/2 are also given. We also determined the phase diagram for the ground state of the system in the magnetic field.

η-pairing superconductivity in the Hubbard chain with pair hopping

The ground-state phase diagram of the one-dimensional Hubbard chain with pair-hopping interaction is studied. The analysis of the model is performed using the continuum-limit field theory approach and exact diagonalization studies. At half-filling the phase diagram is shown to consist of two superconducting states with Cooper-pair center-of-mass momentum $Q=0 (\mathrm{BCS}\ensuremath{-}{\ensuremath{\eta}}_{0}$ phase) and $Q=\ensuremath{\pi} ({\ensuremath{\eta}}_{\ensuremath{\pi}}$ phase) and four insulating phases corresponding to the Mott antiferromagnet, the Peierls dimerized phase, the charge-density-wave (CDW) insulator, and an unconventional insulating phase characterized by the coexistence of a CDW and a bond-located staggered magnetization. Away from half-filling the phase diagram consists of the superconducting $\mathrm{BCS}\ensuremath{-}{\ensuremath{\eta}}_{0}$ and ${\ensuremath{\eta}}_{\ensuremath{\pi}}$ phases and the metallic Luttinger-liquid phase. The $\mathrm{BCS}\ensuremath{-}{\ensuremath{\eta}}_{0}$ phase exhibits a smooth crossover from a weak-coupling BCS type to a strong-coupling local-pair regime. The ${\ensuremath{\eta}}_{\ensuremath{\pi}}$ phase shows the properties of the doublon (zero-size Cooper-pair) superconductor with Cooper-pair center-of-mass momentum $Q=\ensuremath{\pi}.$ The transition into the ${\ensuremath{\eta}}_{\ensuremath{\pi}}$-paired state corresponds to an abrupt change in the ground-state structure. After the transition the conduction band is completely destroyed and a new ${\ensuremath{\eta}}_{\ensuremath{\pi}}$-pair band, corresponding to the strongly correlated doublon motion, is created.