Quantum fluctuations in the effective pseudospin-1/2 model for magnetic pyrochlore oxides

The effective quantum pseudospin-1/2 model for interacting rare-earth magnetic moments, which are locally described with atomic doublets, is studied theoretically for magnetic pyrochlore oxides. It is derived microscopically for localized Pr^{3+} 4f moments in Pr_2TM_2O_7 (TM = Zr, Sn, Hf, and Ir) by starting from the atomic non-Kramers magnetic doublets and performing the strong-coupling perturbation expansion of the virtual electron transfer between the Pr 4f and O 2p electrons. The most generic form of the nearest-neighbor anisotropic superexchange pseudospin-1/2 Hamiltonian is also constructed from the symmetry properties, which is applicable to Kramers ions Nd^{3+}, Sm^{3+}, and Yb^{3+} potentially showing large quantum effects. The effective model is then studied by means of a classical mean-field theory and the exact diagonalization on a single tetrahedron and on a 16-site cluster. These calculations reveal appreciable quantum fluctuations leading to quantum phase transitions to a quadrupolar state as a melting of spin ice for the Pr^{3+} case. The model also shows a formation of cooperative quadrupole moment and pseudospin chirality on tetrahedrons. A sign of a singlet quantum spin ice is also found in a finite region in the space of coupling constants. The relevance to the experiments is discussed.Comment: 18 pages including 14 figures; Comparison with the magnetization curve on Pr2Ir2O7 included; to appear in Phys. Rev.

Phenomenological theory of spin excitations in La- and Y-based cuprates

Motivated by recent inelastic neutron scattering (INS) experiments on La-based cuprates and based on the fermiology theories, we study the spin susceptibility for La-based (e.g., La$_{2-x}$Sr$_x$CuO$_4$) and Y-based (e.g., YBa$_2$Cu$_3$O$_y$) cuprates, respectively. The spin excitation in YBa$_2$Cu$_3$O$_y$ is dominated by a sharp resonance peak at the frequency 40 meV in the superconducting state. Below and above the resonance frequency, the incommensurate (IC) peaks develop and the intensity of the peaks decreases dramatically. In the normal state, the resonant excitation does not occur and the IC peaks are merged into commensurate ones. The spin excitation of La$_{2-x}$Sr$_x$CuO$_4$ is significantly different from that of Y-based ones, namely, the resonance peak does not exist due to the decreasing of the superconducting gap and the presence of the possible spin-stripe order. The spectra are only enhanced at the expected resonance frequency (about 18 meV) while it is still incommensurate. On the other hand, another frequency scale at the frequency 55 meV is also revealed, namely the spectra are commensurate and local maximum at this frequency. We elaborate all the results based on the Fermi surface topology and the d-wave superconductivity, and suggest that the spin-stripe order be also important in determining the spin excitation of La-based cuprates. A coherent picture for the spin excitations is presented for Y-based and La-based cuprates.Comment: 8 pages, 8 figure

Spin susceptibility in bilayered cuprates: resonant magnetic excitations

We study the momentum and frequency dependence of the dynamical spin susceptibility in the superconducting state of bilayer cuprate superconductors. We show that there exists a resonance mode in the odd as well as the even channel of the spin susceptibility, with the even mode being located at higher energies than the odd mode. We demonstrate that this energy splitting between the two modes arises not only from a difference in the interaction, but also from a difference in the free-fermion susceptibilities of the even and odd channels. Moreover, we show that the even resonance mode disperses downwards at deviations from ${\bf Q}=(\pi,\pi)$. In addition, we demonstrate that there exists a second branch of the even resonance, similar to the recently observed second branch (the $Q^*$-mode) of the odd resonance. Finally, we identify the origin of the qualitatively different doping dependence of the even and odd resonance. Our results suggest further experimental test that may finally resolve the long-standing question regarding the origin of the resonance peak.Comment: 8 pages, 5 figure

Drude weight and total optical weight in a t-t'-J model

We study the Drude weight D and the total optical weight K for a t-t'-J model on a square lattice that exhibits a metallic phase-modulated antiferromagnetic ground state close to half-filling. Within a suitable 1/N expansion that includes leading quantum-fluctuation effects, D and K are found to increase linearly with small hole doping away from the Mott metal-insulator transition point at half-filling. The slow zero-sound velocity near the latter transition identifies with the velocity of the lower-energy branch of the twofold excitation spectrum. At higher doping values, D and K eventually saturate and then start to decrease. These features are in qualitative agreement with optical conductivity measurements in doped antiferromagnets.Comment: 7 pages, REVTEX file (3 Postscript figures). To appear in J. Phys.: Condens. Mattte

Strongly Correlated Cerium Systems: Non-Kondo Mechanism for Moment Collapse

We present an ab initio based method which gives clear insight into the interplay between the hybridization, the coulomb exchange, and the crystal-field interactions, as the degree of 4f localization is varied across a series of strongly correlated cerium systems. The results for the ordered magnetic moments, magnetic structure, and ordering temperatures are in excellent agreement with experiment, including the occurence of a moment collapse of non-Kondo origin. In contrast, standard ab initio density functional calculations fail to predict, even qualitatively, the trend of the unusual magentic properties.Comment: A shorter version of this has been submitted to PR

Multi-k magnetic structures in USb_{0.9}Te_{0.1} and UAs_{0.8}Se_{0.2} observed via resonant x-ray scattering at the U M4 edge

Experiments with resonant photons at the U M4 edge have been performed on a sample of USb_{0.9}Te_{0.1}, which has an incommensurate magnetic structure with k = 0.596(2) reciprocal lattice units. The reflections of the form , as observed previously in a commensurate k = 1/2 system [N. Bernhoeft et al., Phys. Rev. B 69 174415 (2004)] are observed, removing any doubt that these occur because of multiple scattering or high-order contamination of the incident photon beam. They are clearly connected with the presence of a 3k configuration. Measurements of the reflections from the sample UAs_{0.8}Se_{0.2} in a magnetic field show that the transition at T* ~ 50 K is between a low-temperature 2k and high-temperature 3k state and that this transition is sensitive to an applied magnetic field. These experiments stress the need for quantitative theory to explain the intensities of these reflections.Comment: submitted to Phys. Rev.

Identifying Collective Modes via Impurities in the Cuprate Superconductors

We show that the pinning of collective charge and spin modes by impurities in the cuprate superconductors leads to qualitatively different fingerprints in the local density of states (LDOS). In particular, in a pinned (static) spin droplet, the creation of a resonant impurity state is suppressed, the spin-resolved LDOS exhibits a characteristic spatial pattern, and the LDOS undergoes significant changes with increasing magnetic field. Since all of these fingerprints are absent in a charge droplet, impurities are a new probe for identifying the nature and relative strength of collective modes.Comment: 4 pages, 4 figure

Doping dependence of spin excitations in the stripe phase of high-Tc superconductors

Based on the time-dependent Gutzwiller approximation for the extended Hubbard model we calculate the energy and momentum dependence of spin excitations for striped ground states. Our starting point correctly reproduces the observed doping dependence of the incommensurability in La-based cuprates and the dispersion of magnetic modes in the insulating parent compound. This allows us to make quantitative predictions for the doping evolution of the dispersion of magnetic modes in the stripe phase including the energy and intensity of the resonance peak as well as the velocity of the spin-wave like Goldstone mode. In the underdoped regime $n_h<1/8$ we find a weak linear dependence of $\omega_{res}$ on doping whereas the resonance energy significantly shifts to higher values when the charge concentration in the stripes starts to deviate from half-filling for $n_h>1/8$. The velocity $c$ is non-monotonous with a minimum at 1/8 in coincidence with a well known anomaly in $T_c$. Our calculations are in good agreement with available experimental data. We also compare our results with analogous computations based on linear spin-wave theory.Comment: 18 pages, 14 figures, revised and extended versio

Linear dependence of peak width in \chi(\bq, \omega) vs T_c for YBCO superconductors

It is shown that the momentum space width of the peak in the spin susceptibility, Im$\chi(q,\omega)$, is linearly proportional to the superconducting $T_c$: $T_c = \hbar v^*\Delta q$ with $\hbar v^* \simeq 35 meV$\AA. This relation is similar to the linear relation between incommensurate peak splitting and $T_c$ in LaSrCuO superconductors, as first proposed by Yamada et al. (Phys. Rev. B 57, 6165, (1998)). The velocity $\hbar v^*$ is smaller than Fermi velocity or the spin-wave velocity of the parent compound and remains the same for a wide doping range. This result points towards strong similarities in magnetic state of YBCO and LaSrCuO.Comment: 5 pages, 3 figures, latex fil

Spin Waves in Striped Phases

In many antiferromagnetic, quasi-two-dimensional materials, doping with holes leads to "stripe" phases, in which the holes congregate along antiphase domain walls in the otherwise antiferromagnetic texture. Using a suitably parametrized two-dimensional Heisenberg model on a square lattice, we study the spin wave spectra of well-ordered spin stripes, comparing bond-centered antiphase domain walls to site-centered antiphase domain walls for a range of spacings between the stripes and for stripes both aligned with the lattice ("vertical") and oriented along the diagonals of the lattice ("diagonal"). Our results establish that there are qualitative differences between the expected neutron scattering responses for the bond-centered and site-centered cases. In particular, bond-centered stripes of odd spacing generically exhibit more elastic peaks than their site-centered counterparts. For inelastic scattering, we find that bond-centered stripes produce more spin wave bands than site-centered stripes of the same spacing and that bond-centered stripes produce rather isotropic low energy spin wave cones for a large range of parameters, despite local microscopic anisotropy. We find that extra scattering intensity due to the crossing of spin wave modes (which may be linked to the "resonance peak" in the cuprates) is more likely for diagonal stripes, whether site- or bond-centered, whereas spin wave bands generically repel, rather than cross, when stripes are vertical.Comment: 12 pages, 12 figures, for some high-res.pics, see http://physics.bu.edu/~yaodx/spinwave/spinw.htm