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A. Sherman

Publications and source records attributed to A. Sherman.

52 records · Page 3Linked to original sources
arXiv2004

Quantum interference of electrons in Nb_{5-δ}Te_4 single crystals

The compound $Nb_{5-δ}Te_4$ ($δ=0.23$) with quasi-one-dimensional crystal structure undergoes a transition to superconductivity at $T_c$=0.6--0.9 K. Its electronic transport properties in the normal state are studied in the temperature range 1.3--270 K and in magnetic fields up to 11 T. The temperature variation of the resistivity is weak ($<2%$) in the investigated temperature range. Nonmonotonic behavior of the resistivity is observed which is characterized by two local maxima at $T\sim$2 K and $\sim$30 K. The temperature dependence of the resistivity is interpreted as an interplay of weak localization, weak antilocalization, and electron-electron interaction effects in the diffusion and the Cooper channel. The temperature dependence of the dephasing time $τ_ϕ$ extracted from the magnetoresistance data is determined by the electron-phonon interaction. The saturation of $τ_ϕ$ in the low-temperature limit correlates with $T_c$ of the individual crystal and is ascribed to the scattering on magnetic impurities.

cond-mat.mes-hall↗
arXiv2004

Magnetic properties of the two-dimensional Heisenberg model on a triangular lattice

The spin Green's function of the antiferromagnetic Heisenberg model on a triangular lattice is calculated using Mori's projection operator technique. At T=0 the spin excitation spectrum is shown to have gaps at the wave vectors of the classical Neel ordering. This points to the absence of the antiferromagnetic long-range order in the ground state. The calculated spin correlation on the neighboring sites of the same sublattice is in good agreement with the value derived from exact diagonalization. The temperature dependencies of the spin correlations and the gaps are calculated.

cond-mat.str-el↗
arXiv2004

Magnetic Properties of Cuprate Perovskites

The magnetic susceptibility of underdoped yttrium and lanthanum cuprates is interpreted based on the self-consistent solution of the t-J model of a Cu-O plane. The calculations reproduce correctly the frequency dependencies of the susceptibility in YBa2Cu3O_{7-y} and La_{2-x}Sr_xCuO4 attributing their dissimilarity to the difference in the damping of spin excitations. In YBa2Cu3O_{7-y} these excitations are well defined at the antiferromagnetic wave vector Q=(π,π) even in the normal state which manifests itself in a pronounced maximum -- the resonance peak -- in the susceptibility. In La_{2-x}Sr_xCuO4 the spin excitations are overdamped which leads to a broad low-frequency feature in the susceptibility. The low-frequency incommensurability in the magnetic response is attributed to a dip in the magnon damping at Q. The calculated concentration and temperature dependencies of the incommensurability parameter conform with experimental observations. Generally the incommensurate magnetic response is not accompanied with an inhomogeneity of the carrier density.

cond-mat.str-el↗
arXiv2004

The t-J model: From light to heavy doping

The regions of hole concentrations $0 \leq x \alt 0.3$ and temperatures $0.005|t| \leq T \leq 0.02|t|$ are studied in the t-J model of Cu-O planes of perovskite high-$T_c$ superconductors. For this purpose self-energy equations for hole and spin Green's functions are derived using Mori's projection operator technique and these equations are self-consistently solved. The calculated hole band transforms radically at $x\approx 0.08$. A narrow low-concentration band with minima near $(\pm\fracπ{2},\pm\fracπ{2})$ is converted to a band resembling the case of weak electron correlations, with the minimum at $(π,π)$ or $(0,0)$. The hole Fermi surface is respectively changed from small ellipses at $(\pm\fracπ{2},\pm\fracπ{2})$ to a large rhombus centered at $(π,π)$ or $(0,0)$. The decrease of the magnetic susceptibility at the antiferromagnetic wave vector and spin correlations with doping is determined by the growth of the frequency of spin excitations at this momentum. The shape of the frequency dependence of the susceptibility depends heavily on the hole damping and varies from a broad feature similar to that observed in La$_{2-x}$Sr$_x$CuO$_4$ to a pronounced maximum which resembles the normal-state resonance peak in YBa$_2$Cu$_3$O$_{7-y}$.

cond-mat.str-el↗
arXiv2004

Low-frequency incommensurate magnetic response in strongly correlated systems

It is shown that in the t-J model of Cu-O planes at low frequencies the dynamic spin structure factor is peaked at incommensurate wave vectors (1/2+-delta,1/2)$, (1/2,1/2+-delta). The incommensurability is connected with the momentum dependencies of the magnon frequency and damping near the antiferromagnetic wave vector. The behavior of the incommensurate peaks is similar to that observed in La_{2-x}(Ba,Sr)_xCuO_{4+y} and YBa_2Cu_3O_{7-y}: for hole concentrations 0.02 0.12 it tends to saturation. The incommensurability disappears with increasing temperature. Generally the incommensurate magnetic response is not accompanied by an inhomogeneity of the carrier density.

cond-mat.str-el↗
arXiv2003

Resonance peak in underdoped cuprates

The magnetic susceptibility measured in neutron scattering experiments in underdoped YBa$_2$Cu$_3$O$_{7-y}$ is interpreted based on the self-consistent solution of the t-J model of a Cu-O plane. The calculations reproduce correctly the frequency and momentum dependencies of the susceptibility and its variation with doping and temperature in the normal and superconducting states. This allows us to interpret the maximum in the frequency dependence -- the resonance peak -- as a manifestation of the excitation branch of localized Cu spins and to relate the frequency of the maximum to the size of the spin gap. The low-frequency shoulder well resolved in the susceptibility of superconducting crystals is connected with a pronounced maximum in the damping of the spin excitations. This maximum is caused by intense quasiparticle peaks in the hole spectral function for momenta near the Fermi surface and by the nesting.

cond-mat.str-el↗
arXiv2003

Two-dimensional t-J model at moderate doping

Using the method which retains the rotation symmetry of spin components in the paramagnetic state and has no preset magnetic ordering, spectral and magnetic properties of the two-dimensional t-J model in the normal state are investigated for the ranges of hole concentrations 0 <= x <= 0.16 and temperatures 0.01t <= T <= 0.2t. The used hopping t and exchange J parameters of the model correspond to hole-doped cuprates. The obtained solutions are homogeneous which indicates that stripes and other types of phase separation are not connected with the strong electron correlations described by the model. A series of nearly equidistant maxima in the hole spectral function calculated for low T and x is connected with hole vibrations in the region of the perturbed short-range antiferromagnetic order. The hole spectrum has a pseudogap in the vicinity of (0,π) and (π,0). For x \approx 0.05 the shape of the hole Fermi surface is transformed from four small ellipses around (\pmπ/2,\pmπ/2) to two large rhombuses centered at (0,0) and (π,π). The calculated temperature and concentration dependencies of the spin correlation length and the magnetic susceptibility are close to those observed in cuprate perovskites. These results offer explanations for the observed scaling of the static uniform susceptibility and for the changes in the spin-lattice relaxation and spin-echo decay rates in terms of the temperature and doping variations in the spin excitation spectrum of the model.

cond-mat.str-el↗
arXiv2003

Magnetic properties of cuprate perovskites in the normal state

Normal-state magnetic properties of cuprate high-T_c superconductors are interpreted based on the self-consistent solution of the t-J model of Cu-O planes. The solution method retains the rotation symmetry of spin components in the paramagnetic state and has no preset magnetic ordering. The obtained solution is homogeneous. The calculated temperature and concentration dependencies of the magnetic susceptibility are close to those observed in experiment. These results offer explanations for the observed scaling of the static uniform susceptibility and for the changes in the spin correlation length, spin-lattice and spin-echo decay rates in terms of the temperature and doping variations in the spin excitation spectrum.

cond-mat.str-el↗
arXiv2002

Magnetic properties of the doped two-dimensional antiferromagnet

The variety of the normal-state magnetic properties of cuprate high-Tc superconductors is interpreted based on the self-consistent solution of the self-energy equations for the two-dimensional t-J model. The observed variations of the spin correlation length with the hole concentration x, of the spin susceptibility with x and temperature T and the scaling of the static uniform susceptibility are well reproduced by the calculated results. The nonmonotonic temperature dependence of the Cu spin-lattice relaxation rate is connected with two competing tendencies in the low-frequency susceptibility: its temperature decrease due to the increasing spin gap and the growth of the susceptibility in this frequency region with the temperature broadening of the maximum in the susceptibility.

cond-mat.str-el↗
arXiv2002

The rotationally invariant approximation for the two-dimensional t-J model

Using the description in terms of the Hubbard operators hole and spin Green's functions of the two-dimensional t-J model are calculated in an approximation which retains the rotation symmetry of the spin susceptibility in the paramagnetic state and has no predefined magnetic ordering. In this approximation, Green's functions are represented by continued fractions which are interrupted with the help of the decoupling corrected by the constraint of zero site magnetization in the paramagnetic state. Results obtained in this approach for an undoped 32x32 lattice (the Heisenberg model) and for one hole in a 4x4 lattice are in good agreement with Monte Carlo and exact diagonalization data, respectively. In the limit of heavy doping the hole spectrum described by the obtained formulas acquires features of the spectrum of weakly correlated excitations.

cond-mat.str-el↗
arXiv2001

Peculiarities of the spectrum of strongly correlated electrons

Hole and spin Green's functions of the two-dimensional t-J model of the Cu-O planes are calculated in an approximation which retains the rotation symmetry in the paramagnetic state and has no presumed magnetic ordering. In this approximation Green's functions are represented by continued fractions which are interrupted by decouplings with a vertex correction determined from the constraint of zero site magnetization in the paramagnetic state. Obtained results are shown to be in good agreement with Monte Carlo and exact diagonalization data. The calculated spectra demonstrate a pseudogap and a band splitting which are similar to those observed in Bi-based cuprate perovskites.

cond-mat.str-el↗
arXiv2000

One-loop approximation for the Heisenberg antiferromagnet

We use the diagram technique for spin operators to calculate Green's functions and observables of the spin-1/2 quantum Heisenberg antiferromagnet on a square lattice. The first corrections to the self-energy and interaction are taken into account in the chain diagrams. The approximation reproduces main results of Takahashi's modified spin-wave theory [Phys. Rev. B 40, 2494 (1989)] and is applicable in a wider temperature range. The energy per spin calculated in this approximation is in good agreement with the Monte Carlo and small-cluster exact-diagonalization calculations in the range 0 <= T < 1.2J where J is the exchange constant. For the static uniform susceptibility the agreement is good for T < 0.6J and becomes somewhat worse for higher temperatures. Nevertheless the approximation is able to reproduce the maximum in the temperature dependence of the susceptibility near T = 0.9J.

cond-mat.str-el↗
arXiv1999

Temperature behavior of the magnon modes of the square lattice antiferromagnet

A spin-wave theory of short-range order in the square lattice Heisenberg antiferromagnet is formulated. With growing temperature from T=0 a gapless mode is shown to arise simultaneously with opening a gap in the conventional spin-wave mode. The spectral intensity is redistributed from the latter mode to the former. For low temperatures the theory reproduces results of the modified spin-wave theory by M.Takahashi, J.E.Hirsch et al. and without fitting parameters gives values of observables in good agreement with Monte Carlo results in the temperature range 0 <= T < 0.8J where J is the exchange constant.

cond-mat.str-el↗
arXiv1998

Ferron-like states in YBa_2Cu_3O_(6+x)

With the use of the Hubbard model bound hole states in YBa_2Cu_3O_(6+x) are studied. For the parameters of this crystal the exchange interaction between the spin-carrying chain ion O^- and Cu-O plane sites is shown to ensure the formation of a large ferromagnetically ordered clusters around holes in the plane.

cond-mat.str-el↗
arXiv1998

Pseudogaps and magnetic properties of the two-dimensional t-J model

We apply the modified spin-wave theory with the constraint of zero staggered magnetization to investigate normal-state spectral and magnetic properties of the 2D t-J model in the paramagnetic state. A set of self-energy equations for hole and magnon Green's functions is solved numerically in the self-consistent Born approximation. The constraint can be fulfilled in the ranges of hole concentrations 0.02 < x < 0.17 and temperatures T < 150 K. In this region the hole spectrum differs from a conventional metallic spectrum which is manifested in the variation with x of the quasiparticle weights of states and in the violation of Luttinger's theorem. With decreasing x from x = 0.17 hidden parts appear in the hole Fermi surface which can be interpreted as the opening of a pseudogap near (pi,0). Obtained size, symmetry and concentration dependence of the pseudogap are in agreement with photoemission data in Bi2212. Calculated temperature dependencies of the spin correlation length, spin-lattice relaxation times at the Cu and O sites, and static susceptibility are typical for the quantum disordered regime with a pseudogap in the spectrum of magnetic excitations. These quantities are in qualitative and in some cases in quantitative agreement with experiment in underdoped YBa2Cu3O(6+y). At x > 0.12 the considered phase borders the phase of conventional metal.

cond-mat.str-el↗