arXiv subjects

A. L. Chernyshev

Publications and source records attributed to A. L. Chernyshev.

At least 73 records · Page 4Linked to original sources

arXiv2005

Edge States in Doped Antiferromagnetic Nano-Structures

We study competition between different phases in a strongly correlated nano-structure with an edge. Making use of the self-consistent Green's function and density matrix renormalization group methods, we study a system described by the t-Jz and t-J models on a strip of a square lattice with a linear hole density n_||. At intermediate interaction strength J/t we find edge stripe-like states, reminiscent of the bulk stripes that occur at smaller J/t. We find that stripes attach to edges more readily than hole pairs, and that the edge stripes can exhibit a peculiar phase separation.

cond-mat.str-el↗

arXiv2004

Higher order effective low-energy theories

Three well-known perturbative approaches to deriving low-energy effective theories, the degenerate Brillouin-Wigner perturbation theory (projection method), the canonical transformation, and the resolvent methods are compared. We use the Hubbard model as an example to show how, to fourth order in hopping t, all methods lead to the same effective theory, namely the t-J model with ring exchange and various correlated hoppings. We emphasize subtle technical difficulties that make such a derivation less trivial to carry out for orders higher than second. We also show that in higher orders, different approaches can lead to seemingly different forms for the low-energy Hamiltonian. All of these forms are equivalent since they are connected by an additional unitary transformation whose generator is given explicitly. The importance of transforming the operators is emphasized and the equivalence of their transformed structure within the different approaches is also demonstrated.

cond-mat.str-el↗

arXiv2003

Spin Diffusion in Double-Exchange Manganites

The theoretical study of spin diffusion in double-exchange magnets by means of dynamical mean-field theory is presented. We demonstrate that the spin-diffusion coefficient becomes independent of the Hund's coupling JH in the range of parameters JH*S >> W >> T, W being the bandwidth, relevant to colossal magnetoresistive manganites in the metallic part of their phase diagram. Our study reveals a close correspondence as well as some counterintuitive differences between the results on Bethe and hypercubic lattices. Our results are in accord with neutron scattering data and with previous theoretical work for high temperatures.

cond-mat.str-el↗

arXiv2002

Spin polarons and high-Tc superconductivity

In this short review article, we consider some aspects of the spin-polaron concept as they relate to high-Tc superconductivity. The article is intended to be an overview in which a flavor of the research on spin polarons is given.

cond-mat.str-el↗

arXiv2002

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 t-Jz model using analytical and numerical approaches. We demonstrate that a stripe in an antiferromagnet should be viewed as a system of composite holon-spin-polaron excitations condensed at the self-induced antiphase domain wall (ADW) of the antiferromagnetic spins. The properties of such excitations are studied in detail with numerical and analytical results for various quantities being in very close agreement. A picture of the stripe as an effective one-dimensional (1D) band of such excitations is also in very good agreement with numerical data. These results emphasize the primary role of kinetic energy in favoring the stripe as a ground state. A comparative analysis suggests the effect of pairing and collective meandering on the energetics of the stripe formation to be secondary. The implications of this microscopic picture of fermions bound to the 1D antiferromagnetic ADW for the effective theories of the stripe phase in the cuprates are discussed.

cond-mat.str-el↗

arXiv2002

Acoustic Phonon Anomaly in MgB2

Recent first principles calculations of the phonon dispersion curves in MgB_2 have suggested the presence of anomalies in some of the curves, particularly in the longitudinal acoustical (LA) branch in the Gamma to A direction. Similar behavior has been observed in numerous other superconductors with T_c's higher than those of standard electron-phonon BCS superconductors. Phenomenological calculations of the Gamma->A LA dispersion based on both an acoustical plasmon and a ``resonant polarization'' mechanism are given here to emphasize the importance of these similarities.

cond-mat.supr-con↗

arXiv2001

Charge Stripe in an Antiferromagnet: 1d Band of Composite Excitations

With the help of analytical and numerical studies of the $t$-$J_z$ model we argue that the charge stripe in an antiferromagnetic insulator should be understood as a system of holon-spin-polaron excitations condensed at the self-induced antiphase domain wall. The structure of such a charge excitation is studied in detail with numerical and analytical results for various quantities being in a very close agreement. An analytical picture of these excitations occupying an effective 1D stripe band is also in a very good accord with numerical data. The emerging concept advocates the primary role of the kinetic energy in favoring the stripe as a ground state. A comparative analysis suggests the effect of pairing and collective meandering on the energetics of the stripe formation to be secondary.

cond-mat.str-el↗

arXiv2001

Effect of impurities on quasi-2D quantum antiferromagnet

We have studied the static and dynamic properties of quasi-two-dimensional quantum antiferromagnets (AF) diluted with spinless impurities using spin-wave theory and $T$-matrix approximation. We show that the spectrum of a 2D AF at long wavelengths is overdamped at arbitrary concentration of spinless impurities. The scattering leads to a new length scale $\ell/a \sim e^{π/4x}$, $x$ being impurity concentration and $a$ the lattice spacing, beyond which the influence of impurities on the spectrum is dominant. Although the dynamical properties are significantly modified we show that 2D is not the lower critical dimension for this problem. Thus, in low-dimensional systems with disorder the connection between static and dynamic quantities is not straightforward. Our results are in quantitative agreement with the recent Monte Carlo simulations and experimental data for $S=1/2$, S=1, and $S=5/2$. We have also proposed experiments which can further test the results of our theory.

cond-mat.str-el↗

arXiv2001

Diluted quantum antiferromagnets: spin excitations and long-range order

We have studied the static and dynamic magnetic properties of two-dimensional (2D) and quasi-two-dimensional, spin-S, quantum Heisenberg antiferromagnets (QHAF) diluted with spinless vacancies. Using spin-wave theory and T-matrix approximation we have calculated the staggered magnetization, M(x,T), the neutron scattering dynamical structure factor, S(k,omega), the 2D magnetic correlation length, ξ(x,T), and the Neel temperature, T_N(x), for the quasi-2D case. We find that in 2D the hydrodynamic description of excitations in terms of spin-waves breaks down at the wavelength larger than \ell/a ~ exp(π/4x), x being impurity concentration and "a" the lattice spacing. The density of states acquires a constant term and exhibits an anomalous peak at omega omega_0 associated with the low-energy localized excitations. These anomalies lead to a substantial enhancement of the magnetic specific heat, C_M, at low temperatures. Although the dynamical properties are significantly modified we show that 2D is not the lower critical dimension for this problem. Our results are in quantitative agreement with the recent Monte Carlo simulations and experimental data for S=1/2, S=1, and S=5/2.

cond-mat.str-el↗

arXiv2001

Long-range order and low-energy spectrum of diluted 2D quantum AF

The problem of diluted two-dimensional (2D) quantum antiferromagnet (AF) on a square lattice is studied using spin-wave theory. The influence of impurities on static and dynamic properties is investigated and a good agreement with experiments and Monte Carlo (MC) data is found. The hydrodynamic description of spin-waves breaks down at characteristic wavelengths $Λ\agt\exp(\frac{const}{x})$, $x$ being an impurity concentration, while the order parameter is free from anomalies. We argue that this dichotomy originates from strong scattering of the low-energy excitations in 2D.

cond-mat.str-el↗

arXiv2000

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 (2D) spin environment with holon-spin-polaron elementary excitations filling a one-dimensional band.

cond-mat.str-el↗

arXiv2000

Charge ordering and long-range interactions in layered transition metal oxides: a quasiclassical continuum study

The competition between long-range and short-range interactions among holes moving in an antiferromagnet (AF), is studied within a model derived from the spin density wave picture of layered transition metal oxides. A novel numerical approach is developed which allows one to solve the problem at finite hole densities in very large systems (of order hundreds of lattice spacings), albeit in a quasiclassical limit, and to correctly incorporate the long-range part of the Coulomb interaction. The focus is on the problem of charge ordering and the charge phase diagram: at low temperatures four different phases are found, depending on the strength of the magnetic (dipolar) interaction generated by the spin-wave exchange, and the density of holes. The four phases are the Wigner crystal, diagonal stripes, a grid phase (horizontal-vertical stripe loops) and a glassy-clumped phase. In the presence of both in-plane and out-of-plane charged impurities the stripe ordering is suppressed, although finite stripe segments persist. At finite temperatures multiscale (intermittency) dynamics is found, reminiscent of that in glasses. The dynamics of stripe melting and its implications for experiments is discussed.

cond-mat↗

arXiv1999

Instability of antiferromagnetic magnons in strong fields

We predict that spin-waves in an ordered quantum antiferromagnet (AFM) in a strong magnetic field become unstable with respect to spontaneous two-magnon decays. At zero temperature, the instability occurs between the threshold field $H^*$ and the saturation field $H_c$. As an example, we investigate the high-field dynamics of a Heisenberg antiferromagnet on a square lattice and show that the single-magnon branch of the spectrum disappears in the most part of the Brillouin zone.

cond-mat.str-el↗

arXiv1999

Holes in the t-J_z model: a thorough study

The t-J_z model is the strongly anisotropic limit of the t-J model which captures some general properties of the doped antiferromagnets (AF). The absence of spin fluctuations simplifies the analytical treatment of hole motion in an AF background and allows us to calculate the single- and two-hole spectra with high accuracy using regular diagram technique combined with real-space approach. At the same time, numerical studies of this model via exact diagonalization (ED) on small clusters show negligible finite size effects for a number of quantities, thus allowing a direct comparison between analytical and numerical results. Both approaches demonstrate that the holes have tendency to pair in the p- and d-wave channels at realistic values of t/J. The interactions leading to pairing and effects selecting p and d waves are thoroughly investigated. The role of transverse spin fluctuations is considered using perturbation theory. Based on the results of the present study, we discuss the pairing problem in the realistic t-J-like model. Possible implications for preformed pairs formation and phase separation are drawn.

cond-mat.str-el↗

arXiv1998

A comprehensive numerical and analytical study of two holes doped into the 2D t-J model

We report on a detailed examination of numerical results and analytical calculations devoted to a study of two holes doped into a two-dimensional, square lattice described by the t-J model. Our exact diagonalization numerical results represent the first solution of the exact ground state of 2 holes in a 32-site lattice. Using this wave function, we have calculated several important correlation functions, notably the electron momentum distribution function and the hole-hole spatial correlation function. Further, by studying similar quantities on smaller lattices, we have managed to perform a finite-size scaling analysis. We have augmented this work by endeavouring to compare these results to the predictions of analytical work for two holes moving in an infinite lattice. This analysis relies on the canonical transformation approach formulated recently for the t-J model. From this comparison we find excellent correspondence between our numerical data and our analytical calculations. We believe that this agreement is an important step helping to justify the quasiparticle Hamiltonian, and in particular, the quasiparticle interactions, that result from the canonical transformation approach. Also, the analytical work allows us to critique the finite-size scaling ansatzes used in our analysis of the numerical data. One important feature that we can infer from this successful comparison involves the role of higher harmonics in the two-particle, d-wave symmetry bound state -- the conventional (\cos(k_x) - \cos(k_y)) term is only one of many important contributions to the d-wave symmetry pair wave function.

cond-mat.str-el↗

arXiv1997

A numerical and analytical study of two holes doped into the 2D t--J model

Exact diagonalization numerical results are presented for a 32-site square cluster, with two holes propagating in an antiferromagnetic background described by the t-J model. We characterize the wave function of the lowest energy bound state found in this calculation, which has d_{x^2-y^2} symmetry. Analytical work is presented, based on a Lang-Firsov-type canonical transformation derived quasiparticle Hamiltonian, that accurately agrees with numerically determined values for the electron momentum distribution function and the pair correlation function. We interpret this agreement as strong support for the validity of this description of the hole quasiparticles.

cond-mat.str-el↗

arXiv1996

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 canonical transformation approach. It is shown that the rather simple form of the transformation generator allows to take into account effect of hole interaction with the short-range spin waves and to describe the single-hole groundstate. Obtained results are very close to ones of the self-consistent Born approximation. Further accounting for the long-range spin-wave interaction is possible on the perturbative basis. Both spin-wave exchange and an effective interaction due to minimization of the number of broken antiferromagnetic bonds are included in the effective quasiparticle interaction. Two-hole bound state problem is solved using Bethe-Salpeter equation. The only d-wave bound state is found to exist in the region of 1< (t/J) <5. Combined effect of the pairing interactions of both types is important to its formation. Discussion of the possible relation of the obtained results to the problem of superconductivity in real systems is presented.

cond-mat↗

arXiv1996

Single hole dispersion relation for the real CuO$_2$ plane

Dispersion relation for the CuO$_2$ hole is calculated basing on the {\it generalized} $t$-$t^{\prime}$-$J$ model, recently derived from the three-band one. Numerical ranges for all model parameters, $t/J=2.4..2.7$, $t^{\prime}/t = 0..-0.25$, $t^{\prime\prime}/t=0.1..0.15$, and three-site terms $2t_N\sim t_S\sim J/4$, have been strongly justified previously. Physical reasons for their values are also discussed. Self-consistent Born approximation is used for the calculation of the hole dispersion. An excellent agreement between calculated $E_{\bf k}$ and one obtained from the angle-resolved photoemission experiments is found.

cond-mat↗