A. L. Chernyshev

Neél order in square and triangular lattice Heisenberg models.

We show that the density matrix renormalization group can be used to study magnetic ordering in two-dimensional spin models. Local quantities should be extrapolated with the truncation error, not with its square root. We introduce sequences of clusters, using cylindrical boundary conditions with pinning fields, which provide for rapidly converging finite-size scaling. We determine the magnetization for both the square and triangular Heisenberg lattices with errors comparable to the best alternative approaches.

Generalized t - t ′- J model: Parameters and single-particle spectrum for electrons and holes in copper oxides

A microscopically based Hamiltonian of the generalized {ital t}-{ital t}{sup {prime}}-{ital J} model is presented. Two types of the additional {ital t}{sup {prime}} terms are discussed. The numerical range of the amplitudes corresponding to the additional {ital t}{sup {prime}} terms for the real CuO{sub 2} planes is derived from the three-band model calculations. Using the variational spin-polaron approach the single-carrier dispersions in the generalized {ital t}-{ital t}{sup {prime}}-{ital J} model are calculated both for the hole- and electron-doped systems. The hole and electron band minima are found to be at points ({plus_minus}{pi}/2,{plus_minus}{pi}/2), (0,{plus_minus}{pi}), and ({plus_minus}{pi},0), respectively. The band minima shifts {parallel}{Delta}{sub (0,{pi}){minus}({pi}/2,{pi}/2)}{parallel} are not small ({approximately}{ital J}). The bandwidths for both cases of doping are found to be 1.5{endash}4.0 times larger than those in the {ital t}-{ital J} model. {copyright} {ital 1996 The American Physical Society.}

Breakdown of magnons in a strongly spin-orbital coupled magnet

The description of quantized collective excitations stands as a landmark in the quantum theory of condensed matter. A prominent example occurs in conventional magnets, which support bosonic magnons—quantized harmonic fluctuations of the ordered spins. In striking contrast is the recent discovery that strongly spin-orbital-coupled magnets, such as α-RuCl3, may display a broad excitation continuum inconsistent with conventional magnons. Due to incomplete knowledge of the underlying interactions unraveling the nature of this continuum remains challenging. The most discussed explanation refers to a coherent continuum of fractional excitations analogous to the celebrated Kitaev spin liquid. Here, we present a more general scenario. We propose that the observed continuum represents incoherent excitations originating from strong magnetic anharmonicity that naturally occurs in such materials. This scenario fully explains the observed inelastic magnetic response of α-RuCl3 and reveals the presence of nontrivial excitations in such materials extending well beyond the Kitaev state.The observation of magnetic excitation continua in putative Kitaev spin liquid compounds is consistent with predictions of exotic quantum behavior, attracting significant interest. Here, the authors show the observed continua could instead be a consequence of realistic non-Kitaev interactions.

Hole-hole superconducting pairing in the t-J model induced by spin-wave exchange.

We study numerically the hole pairing induced by spin-wave exchange. The contact hole-hole interaction is taken into account as well. Consideration is given to the assumption of preservation of antiferromagnetic order at all relevant pairing scales. The strongest pairing is obtained for the [ital d]-wave symmetry of the gap. Dependence of the value of the gap on hole concentration and temperature is presented. For the critical temperature we obtain [ital T][sub [ital c]][similar to]100 K at the hole concentration [delta][similar to]0.2--0.3.

Metallic Stripe in Two Dimensions: Stability and Spin-Charge Separation

The problems of charge stripe formation, spin-charge separation, and stability of the antiphase domain wall (ADW) associated with a stripe are addressed using an analytical approach to the t- J(z) model. We show that a metallic stripe together with its ADW is the ground state of the problem in the low doping regime. The stripe is described as a system of spinons and magnetically confined holons strongly coupled to the two dimensional spin environment with holon-spin-polaron elementary excitations filling a one-dimensional band.

Disorder-Induced Mimicry of a Spin Liquid in YbMgGaO4

We suggest that a randomization of the pseudodipolar interaction in the spin-orbit-generated low-energy Hamiltonian of YbMgGaO_{4} due to an inhomogeneous charge environment from a natural mixing of Mg^{2+} and Ga^{3+} can give rise to orientational spin disorder and mimic a spin-liquid-like state. In the absence of such quenched disorder, 1/S and density matrix renormalization group calculations both show robust ordered states for the physically relevant phases of the model. Our scenario is consistent with the available experimental data, and further experiments are proposed to support it.

SINGLE-HOLE DISPERSION RELATION FOR THE REAL CUO2 PLANE

Dispersion relation for the CuO{sub 2} hole is calculated based on the {ital generalized} {ital t}-{ital t{prime}}-{ital J} model, recently derived from the three-band one. Numerical ranges for all model parameters, {ital t}/{ital J}=2.4{endash}2.7, {ital t{prime}}/{ital t}=0.0 to {minus}0.25, {ital t{double_prime}}/{ital t}=0.1{endash}0.15, and three-site terms 2{ital t}{sub {ital N}}{approximately}{ital t}{sub {ital S}}{approximately}{ital J}/4 have been strongly justified previously. Physical reasons for their values are also discussed. A self-consistent Born approximation is used for the calculation of the hole dispersion. Good agreement between calculated {ital E}{sub {ital k}} and one obtained from the angle-resolved photoemission experiments is found. A possible explanation of the broad peaks in the experimental energy distribution curves at the top of the hole band is presented. {copyright} {ital 1996 The American Physical Society.}

Two-hole problem in the t-J model: A canonical transformation approach

The t-J model in the spinless-fermion representation is studied. An effective Hamiltonian for the quasiparticles is derived using a canonical transformation approach. It is shown that the rather simple form of the transformation generator allows one to take into account the effect of hole interactions with the short-range spin waves and to describe the single-hole ground state. Obtained results are very close to ones of the self-consistent Born approximation. Further accounting of the long-range spin-wave interaction is possible on a perturbative basis. Spin-wave exchange and an effective interaction due to minimization of the number of broken antiferromagnetic bonds are included in the effective quasiparticle Hamiltonian. The two-hole bound state problem is solved using a Bethe-Salpeter equation. The only bound state found to exist in the region of 1{lt}(t/J){lt}5 is the d wave. Both types of the hole-hole interaction are important for its formation. A discussion of the possible relation of the obtained results to the problem of superconductivity in real systems is presented. {copyright} {ital 1997} {ital The American Physical Society}

Stripe as an effective one-dimensional band of composite excitations

The microscopic structure of a charge stripe in an antiferromagnetic insulator is studied within the

Diluted quantum antiferromagnets: Spin excitations and long-range order

{t\ensuremath{-}J}_{z}