Incommensurate Magnetic Response Research Articles

Magnetic Structure of Electronic Inhomogeneities in Cuprates: Competition between Stripes and Spirals

It is shown that the magnetic structure of high-Tc superconductors is strongly in uenced by the next-nearest neighbor hopping parameter t′ which distinguishes di erent families of cuprates. Our investigations indicate that uniform spirals get favored by a large t′/t ratio but are unstable at small doping towards stripes with spin canting. For large |t′/t| spirals can be stabilized under certain conditions in the overdoped regime which may explain the elastic incommensurate magnetic response recently observed in iron-co-doped Bi2201 materials.

Low-Frequency Magnetic Response in the Pseudogap Phase of Cuprate Perovskites

We calculate the magnetic susceptibility in the pseudogap phase of cuprates using the two-dimensional t-J model and the Mori projection operator technique. In this phase, the Fermi arcs lead to a quasi-elastic incommensurate magnetic response for low temperatures. The theory accounts for the reorientation of the susceptibility maxima from the axial to the diagonal direction occurring at small hole concentrations in La2−xSrxCuO4. A small cusp of the hole dispersion near the Fermi level, which is connected with the spin-polaron band, affords the growth of the maxima with decreasing frequency. As in the superconducting phase, the susceptibility maxima have an hourglass dispersion. The assumption of the phase separation into regions of superconducting and pseudogap phases in underdoped cuprates explains the lack of the superconducting gap in their susceptibility. The observed rapid change of widths of the susceptibility maxima with frequency is related to the contribution of the pseudogap phase. This contribution may also explain the strengthening of the quasi-elastic response by modest magnetic fields and by small impurity concentrations.

MAGNETIC INCOMMENSURABILITY IN p-TYPE CUPRATE PEROVSKITES

For the superconducting phase with a d-wave order parameter and zero temperature, the magnetic susceptibility of the t–J model is calculated using the Mori projection operator technique. Conditions for the appearance of an incommensurate magnetic response below the resonance frequency are identified. A fast decay of the tails of the hole coherent peaks and a weak intensity of the hole incoherent continuum near the Fermi level are enough to produce an incommensurate response using different hole dispersions established for p-type cuprates, in which such response was observed. In this case, the nesting of the itinerant-electron theory or the charge modulation of the stripe theory is unnecessary for the incommensurability. The theory reproduces the hourglass dispersion of the susceptibility maxima with their location in the momentum space similar to that observed experimentally. The upper branch of the dispersion stems from the excitations of localized spins, while the lower one is due to the incommensurate maxima of their damping. The narrow and intensive resonance peak arises if the frequency of these excitations at the antiferromagnetic momentum lies below the edge of the two-fermion continuum; otherwise the maximum is broad and less intensive.

Spin canting as a result of the competition between stripes and spirals in cuprates

Based on the extended Hubbard model we calculate the energy of stripe and spiral ground states. We find that uniform spirals get favored by a large $t'/t$ ratio but are unstable at small doping towards stripes and checkerboard textures with spin canting. The structure of these inhomogeneities also depends on t'/t and the associated spin currents may induce a small lattice distortion associated with local dipole moments. We discuss a new kind of stripe which appears as a domain wall of the antiferromagnetic (AF) order parameter with a fractional change of the phase of the AF order. For large |t'/t| spirals can be stabilized under certain conditions in the overdoped regime which may explain the elastic incommensurate magnetic response recently observed in iron-codoped Bi2201 materials.

Resonance peak and incommensurability in cuprate perovskites

The magnetic susceptibility of yttrium and lanthanum cuprates is interpreted based on the self-consistent solution of the t-J model of Cu-O planes. To take a proper account of strong electron correlations inherent in cuprates, the calculations were carried out in the formalism of the Hubbard operators and Mori’s projection operator technique. The calculations correctly reproduce the frequency and momentum dependences of the resonance peak in YBa2Cu3O7 − y and its variation with doping and temperature in the normal and superconducting states. In lanthanum cuprates, spin excitations near the antiferromagnetic wave vector Q = (π, π) are overdamped and, therefore, a broad maximum is observed in the susceptibility instead of a sharp peak. For low frequencies, temperatures, and hole concentrations x ≥ 0.02, the susceptibility is peaked at incommensurate wave vectors (π ± 2πδ, π), (π, π ± 2πδ). The incommensurability is connected with the minimum in the magnon damping at Q in the crystal with a short-range antiferromagnetic order. Generally, the incommensurate magnetic response is not accompanied by an inhomogeneity of the charge-carrier density.

Incommensurate spin dynamics in underdoped cuprate perovskites

The incommensurate magnetic response observed in normal-state cuprate perovskites is interpreted based on Mori's projection operator formalism and the t– J model of Cu–O planes. In agreement with experiment, the calculated dispersion of maxima in the susceptibility has the shape of two parabolas with upward- and downward-directed branches which converge at the antiferromagnetic wave vector. The upper parabola reflects the dispersion of magnetic excitations of the localized Cu spins, while the lower parabola arises due to the weakness of the interaction between the spin excitations and holes near the hot spots. Also in conformity with experiment, the low-frequency incommensurability parameter δ grows with the hole concentration x for x ≲ 0.12 and then saturates.

Incommensurate Magnetic Response in Cuprate Perovskites

AbstractFor Abstract see ChemInform Abstract in Full Text.

INCOMMENSURATE SPIN DYNAMICS IN UNDERDOPED CUPRATE PEROVSKITES

The incommensurate magnetic response observed in normal-state cuprate perovskites is interpreted based on the projection operator formalism and the t–J model of Cu-O planes. In agreement with experiment the calculated dispersion of maxima in the susceptibility has the shape of two parabolas with upward and downward branches which converge at the antiferromagnetic wave vector. The maxima are located at the momenta (½, ½ ± δ), (½ ± δ, ½) and at (½ ± δ, ½ ± δ), (½ ± δ, ½ ∓ δ) in the lower and upper parabolas, respectively. The upper parabola reflects the dispersion of magnetic excitations of the localized Cu spins, while the lower parabola arises due to a dip in the spin-excitation damping at the antiferromagnetic wave vector. For moderate doping this dip stems from the weakness of the interaction between the spin excitations and holes near the hot spots. The frequency dependence of the susceptibility is shown to depend strongly on the hole bandwidth and damping and varies from the shape observed in YBa 2 Cu 3 O 7-y to that inherent in La 2-x Sr x CuO 4.

Incommensurate magnetic response in cuprate perovskites

The incommensurate magnetic response of the normal-state cuprate perovskites is interpreted based on Mori's memory function approach and the t– J model of Cu–O planes. In agreement with experiment the calculated dispersion of the susceptibility maxima has the shape of two parabolas with upward and downward branches which converge at the antiferromagnetic wave vector. The maxima are located at ( 1 2 , 1 2 ± δ ) , ( 1 2 ± δ , 1 2 ) and at ( 1 2 ± δ , 1 2 ± δ ) , ( 1 2 ± δ , 1 2 ∓ δ ) in the lower and upper parabolas, respectively. The upper parabola reflects the dispersion of magnetic excitations of the localized Cu spins, while the lower parabola arises due to a dip in the spin-excitation damping at the antiferromagnetic wave vector. For moderate doping this dip stems from the weakness of the interaction between the spin excitations and holes near the hot spots.

Low-frequency incommensurate magnetic response in strongly correlated systems

It is shown that in the $t\ensuremath{-}J$ model of Cu-O planes at low frequencies the dynamic spin structure factor is peaked at incommensurate wave vectors $(\frac{1}{2}\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta},\frac{1}{2}),$ $(\frac{1}{2},\frac{1}{2}\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}).$ 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 ${\mathrm{La}}_{2\ensuremath{-}x}(\mathrm{B}\mathrm{a},\mathrm{S}\mathrm{r}{)}_{x}{\mathrm{CuO}}_{4+y}$ and ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}y}:$ for hole concentrations $0.02<x\ensuremath{\lesssim}0.12$ we find that $\ensuremath{\delta}$ is nearly proportional to x, while for $x>0.12$ 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.

Magnetic coherence as a universal feature of cuprate superconductors

Recent inelastic neutron scattering (INS) experiments on La$_{2-x}$Sr$_x$CuO$_4$ have established the existence of a {\it magnetic coherence effect}, i.e., strong frequency and momentum dependent changes of the spin susceptibility, $\chi''$, in the superconducting phase. We show, using the spin-fermion model for incommensurate antiferromagnetic spin fluctuations, that the magnetic coherence effect establishes the ability of INS experiments to probe the electronic spectrum of the cuprates, in that the effect arises from the interplay of an incommensurate magnetic response, the form of the underlying Fermi surface, and the opening of the d-wave gap in the fermionic spectrum. In particular, we find that the magnetic coherence effect observed in INS experiments on La$_{2-x}$Sr$_x$CuO$_4$ requires that the Fermi surface be closed around $(\pi,\pi)$ up to optimal doping. We present several predictions for the form of the magnetic coherence effect in YBa$_2$Cu$_3$O$_{6+x}$ in which an incommensurate magnetic response has been observed in the superconducting state.

Magnetic coherence in cuprate superconductors

Recent inelastic neutron scattering (INS) experiments on La$_{2-x}$Sr$_x$CuO$_4$ observed a {\it magnetic coherence effect}, i.e., strong frequency and momentum dependent changes of the spin susceptibility, $\chi''$, in the superconducting phase. We show that this effect is a direct consequence of changes in the damping of incommensurate antiferromagnetic spin fluctuations due to the appearance of a d-wave gap in the fermionic spectrum. Our theoretical results provide a quantitative explanation for the weak momentum dependence of the observed spin-gap. Moreover, we predict {\bf (a)} a Fermi surface in La$_{2-x}$Sr$_x$CuO$_4$ which is closed around $(\pi,\pi)$ up to optimal doping, and {\bf (b)} similar changes in $\chi''$ for all cuprates with an incommensurate magnetic response.

Dynamic nesting and the incommensurate magnetic response in superconducting YBa 2Cu 3O 6+ y

The dynamic magnetic susceptibility χ″( q , ω) of the t– t′– J-model for YBCO compounds is studied in slave-boson mean-field theory. Within a renormalized random-phase approximation χ″ is compared for different fixed energies ω in the superconducting state. At the energy ω= ω 0, where χ″(( π, π), ω) shows a sharp peak (the `41 meV resonance'), the response is commensurate in wave vector space. At lower energies around ω i =0.7 ω 0, however, we find four peaks at q =( π± δ, π) and ( π, π± δ). The results are in agreement with inelastic neutron scattering experiments, in particular with the incommensurate response recently observed in YBa 2Cu 3O 6.6 by Mook et al. We argue that dynamic nesting in the dispersion of quasi particles causes this effect.

New slow and short range magnetic correlations in superconducting

Inelastic neutron scattering measurements have been performed on high quality single crystals of La2-xSrxCuO4ce> in order to study the spin dynamics of this compound. In addition to the well-established incommensurate magnetic response, we show the existence of a new set of low energy excitations present in the whole superconducting region of the phase diagram. This new feature of the dynamical cross section is characterized, below about 10 K, by very short range (~ 3 lattice spacing) antiferromagnetic correlations and by a low energy scale of ~ 1 meV. At higher temperatures these fluctuations become nearly Q-independent. Different possible origins of these new spin correlations are discussed.