190 research outputs found

    Antiferromagnetism in two-dimensional t-J model: pseudospin representation

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    We discuss a pseudospin representation of the two-dimensional t-J model. We introduce pseudospins associated with empty sites, deriving a new representation of the t-J model that consists of local spins and spinless fermions. We show, within a mean-field approximation, that our representation of t-J model corresponds to the {\it isotropic} antiferromagnetic Heisenberg model in an effective magnetic field. The strength and the direction of the effective field are determined by the hole doping δ{\delta} and the orientation of pseudospins associated with empty sites, respectively. We find that the staggered magnetization in the standard representation corresponds to the component of magnetization perpendicular to the effective field in our pseudospin representation. Using a many-body Green's function method, we show that the staggered magnetization decreases with increasing hole doping δ{\delta} and disappears at δ0.060.15{\delta \approx 0.06-0.15} for t/J=25t/J=2-5. Our results are in good agreement with experiments and numerical calculations in contradistinction to usual mean-field methods.Comment: 6 pages, 3 figure

    Nonzero macroscopic magnetization in half-metallic antiferromagnets at finite temperatures

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    Combining density-functional theory calculations with many-body Green's-function technique, we reveal that the macroscopic magnetization in half-metallic antiferromagnets does not vanish at finite temperature as for the T=0 limit. This anomalous behavior stems from the inequivalent magnetic sublattices which lead to different intrasublattice exchange interactions. As a consequence, the spin fluctuations suppress the magnetic order of the sublattices in a different way leading to a ferrimagnetic state at finite temperatures. Computational results are presented for the half-metallic antiferromagnetic CrMnZ (Z=P,As,Sb) semi-Heusler compounds.Comment: 4 pages, 2 figure

    Magnetization Relaxation and Collective Spin Excitations in Correlated Double--Exchange Ferromagnets

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    We study spin relaxation and dynamics of collective spin excitations in correlated double--exchange ferromagnets. For this, we introduce an expansion of the Green's functions equations of motion that treats non--perturbativerly all correlations between a given number of spin and charge excitations and becomes exact within a sub--space of states. Our method treats relaxation beyond Fermi's Golden Rule while recovering previous variational results for the spin--wave dispersion. We find that the momentum dependence of the spin--wave dephasing rate changes qualitatively due to the on--site Coulomb interaction, in a way that resembles experiment, and depends on its interplay with the magnetic exchange interaction and itinerant spin lifetime. We show that the collective spin relaxation and its dependence on the carrier concentration depends sensitively on three--body correlations between a spin excitation and a Fermi sea electron and hole. The above spin dynamics can be controlled via the itinerant carrier population.Comment: 13 pages, 10 figures, published in Phys. Rev.

    Green's function theory of quasi-two-dimensional spin-half Heisenberg ferromagnets: stacked square versus stacked kagom\'e lattice

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    We consider the thermodynamic properties of the quasi-two-dimensional spin-half Heisenberg ferromagnet on the stacked square and the stacked kagom\'e lattices by using the spin-rotation-invariant Green's function method. We calculate the critical temperature TCT_C, the uniform static susceptibility χ\chi, the correlation lengths ξν\xi_\nu and the magnetization MM and investigate the short-range order above TCT_C. We find that TCT_C and MM at T>0T>0 are smaller for the stacked kagom\'e lattice which we attribute to frustration effects becoming relevant at finite temperatures.Comment: shortened version as published in PR

    Thermodynamic properties of Holstein polarons and the effects of disorder

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    The ground state and finite temperature properties of polarons are studied considering a two-site and a four-site Holstein model by exact diagonalization of the Hamiltonian. The kinetic energy, Drude weight, correlation functions involving charge and lattice deformations, and the specific heat have been evaluated as a function of electron-phonon (e-ph) coupling strength and temperature. The effects of site diagonal disorder on the above properties have been investigated. The disorder is found to suppress the kinetic energy and the Drude weight, reduces the spatial extension of the polaron, and makes the large-to-small polaron crossover smoother. Increasing temperature also plays similar role. For strong coupling the kinetic energy arises mainly from the incoherent hopping processes owing to the motion of electrons within the polaron and is almost independent of the disorder strength. From the coherent and incoherent contributions to the kinetic energy, the temperature above which the incoherent part dominates is determined as a function of e-ph coupling strength.Comment: 17 pages. 17 figure

    The t-J model on a semi-infinite lattice

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    The hole spectral function of the t-J model on a two-dimensional semi-infinite lattice is calculated using the spin-wave and noncrossing approximations. In the case of small hole concentration and strong correlations, tJt\gg J, several near-boundary site rows appear to be depleted of holes. The reason for this depletion is a deformation of the magnon cloud, which surrounds the hole, near the boundary. The hole depletion in the boundary region leads to a more complicated spectral function in the boundary row in comparison with its bulk shape.Comment: 8 pages, 5 figure

    Superlight small bipolarons

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    Recent angle-resolved photoemission spectroscopy (ARPES) has identified that a finite-range Fr\"ohlich electron-phonon interaction (EPI) with c-axis polarized optical phonons is important in cuprate superconductors, in agreement with an earlier proposal by Alexandrov and Kornilovitch. The estimated unscreened EPI is so strong that it could easily transform doped holes into mobile lattice bipolarons in narrow-band Mott insulators such as cuprates. Applying a continuous-time quantum Monte-Carlo algorithm (CTQMC) we compute the total energy, effective mass, pair radius, number of phonons and isotope exponent of lattice bipolarons in the region of parameters where any approximation might fail taking into account the Coulomb repulsion and the finite-range EPI. The effects of modifying the interaction range and different lattice geometries are discussed with regards to analytical strong-coupling/non-adiabatic results. We demonstrate that bipolarons can be simultaneously small and light, provided suitable conditions on the electron-phonon and electron-electron interaction are satisfied. Such light small bipolarons are a necessary precursor to high-temperature Bose-Einstein condensation in solids. The light bipolaron mass is shown to be universal in systems made of triangular plaquettes, due to a novel crab-like motion. Another surprising result is that the triplet-singlet exchange energy is of the first order in the hopping integral and triplet bipolarons are heavier than singlets in certain lattice structures at variance with intuitive expectations. Finally, we identify a range of lattices where superlight small bipolarons may be formed, and give estimates for their masses in the anti-adiabatic approximation.Comment: 31 pages. To appear in J. Phys.: Condens. Matter, Special Issue 'Mott's Physics

    Fingerprints of Spin-Orbital Physics in Crystalline O2_2

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    The alkali hyperoxide KO2_2 is a molecular analog of strongly-correlated systems, comprising of orbitally degenerate magnetic O2_2^- ions. Using first-principles electronic structure calculations, we set up an effective spin-orbital model for the low-energy \textit{molecular} orbitals and argue that many anomalous properties of KO2_2 replicate the status of its orbital system in various temperature regimes.Comment: 4 pages, 2 figures, 1 tabl

    Absence of long-range order in a spin-half Heisenberg antiferromagnet on the stacked kagome lattice

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    We study the ground state of a spin-half Heisenberg antiferromagnet on the stacked kagome lattice by using a spin-rotation-invariant Green's-function method. Since the pure two-dimensional kagome antiferromagnet is most likely a magnetically disordered quantum spin liquid, we investigate the question whether the coupling of kagome layers in a stacked three-dimensional system may lead to a magnetically ordered ground state. We present spin-spin correlation functions and correlation lengths. For comparison we apply also linear spin wave theory. Our results provide strong evidence that the system remains short-range ordered independent of the sign and the strength of the interlayer coupling

    Thermodynamics of layered Heisenberg magnets with arbitrary spin

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    We present a spin-rotation-invariant Green-function theory of long- and short-range order in the ferro- and antiferromagnetic Heisenberg model with arbitrary spin quantum number S on a stacked square lattice. The thermodynamic quantities (Curie temperature T_C, N\'eel temperature T_N, specific heat C_V, intralayer and interlayer correlation lengths) are calculated, where the effects of the interlayer coupling and the S dependence are explored. In addition, exact diagonalizations on finite two-dimensional (2D) lattices with S>=1 are performed, and a very good agreement between the results of both approaches is found. For the quasi-2D and isotropic 3D magnets, our theory agrees well with available quantum Monte Carlo and high-temperature series-expansion data. Comparing the quasi-2D S=1/2 magnets, we obtain the inequalities T_N>T_C and, for small enough interlayer couplings, T_N<T_C. The results for C_V and the intralayer correlation length are compared to experiments on the quasi-2D antiferromagnets Zn_2VO(PO_4)_2 with S=1/2 and La_2NiO_4 with S=1, respectively.Comment: 10 pages, 8 figures, 3 tables, submitted to Physical Review
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