Spin dynamics in the stripe phase of the cuprate superconductors

Within a model that supports stripe spin and charge order coexisting with a d(x2-y2)-wave superconducting phase, we study the self-consistently obtained electronic structure and the associated transverse dynamical spin susceptibility. In the coexisting phase of superconducting and static stripe order, the resulting particle-hole continuum can strongly damp parts of the low-energy spin-wave branches. This provides insight into recent inelastic neutron scattering data revealing the dispersion of the low-energy collective magnetic modes of lanthanum based cuprate superconductors.     

Spin correlations in 2 + ε dimensions - ordered and disordered phase

Closed form expressions for the correlation functions of fixed length spins in 2 and 2 + ? dimensions below and above the critical temperature are obtained by a skeleton graph approach.     

c-axis optical response in the static stripe ordered phase of the cuprates

The c-axis far-infrared reflectivity spectra in the superconducting state were investigated for La1.85-yNd(y)Sr0.15CuO4. The Josephson plasma edge rapidly shifts towards lower frequency with increasing y, and almost disappears when y exceeds a critical value ( y(c) approximately 0.12) above which the low-temperature-tetragonal (LTT) deformation switches on, stabilizing the spin/charge ordered stripe phase. In the vicinity of y(c), a "normal state" reentrant behavior was observed with lowering temperature below Tc. This clearly demonstrates that the static stripe order pinned by the LTT distortion suppresses the interlayer phase coherence.     

Spin correlations in Ni-Mn-Ga

Results of measuring small-angle neutron scattering and neutron depolarization in a Ni_49.1Mn_29.4Ga_21.5 single crystal in the temperature range 15<T<400 K and in the range of magnetic fields 0<H<4.5 kOe are presented. The characteristic temperatures of the alloy were found to be as follows: T _C=373.7 K and the martensite transition temperature T _m=301–310 K. The magnetic critical scattering at T _C and the scattering at T<T _C were adequately described by the relationship I _m=A(q ²+κ²)^?2 (q is the transferred wave vector and R _c=1/κ is the correlation radius), and the temperature dependences of the A and R _c scattering parameters were determined. Left-right asymmetry was observed at 150<T< T _m in the scattering of neutrons polarized along or opposite to the applied field. This asymmetry was due to the inelastic magnetic interaction of neutrons in the sample. The magnetization of the alloy at T _m, critical scattering at T?T _C, anomalies in scattering, and the softening of magnetic excitations at 150 <T<T _m are discussed.     

Spin dynamics in the stripe phase of high-T c cuprates with the modulation of superconducting order

Possible superconducting order modulation and its effect on the spin susceptibility in the coexisting phase of the stripe and superconducting orders are investigated. It is shown that the superconducting order modulation is mainly caused by the spin-domain-derived scattering, while the charge-domain-derived scattering tends to suppress it in a wide parameter region. The modulation leads to a two-peak structure in the spin excitation spectrum, which is qualitatively consistent with the recent experimental observations in La_{2- x} Sr _x CuO₄. This result suggests the importance of the superconducting order modulation for the understanding of the multiform spin excitations in these cuprate superconductors.     

Spin correlations in the paramagnetic phase and ring exchange in La2CuO4

Spin correlations in the paramagnetic phase of La(2)CuO(4) have been studied using polarized neutron scattering, with two important results. First, the temperature dependence of the characteristic energy scale of the fluctuations and the amplitude of the neutron structure factor are shown to be in quantitative agreement with the predictions of the quantum nonlinear sigma model. Second, a comparison of a high-temperature series expansion of the equal-time spin correlations with the diffuse neutron intensity provides definitive experimental evidence for ring exchange.     

Spin correlations in Au-Fe alloys

Measurements of the (111) Bragg peak of ferromagnetic Au-Fe alloys reveal a remarkable three-step increase to saturation with decreasing temperature. The final “RKKY step” around ≈ 50 K is accompanied by broadening of the Bragg peak resulting in the observed q-dependent maxima in the low-q intensity. Viewed with increasing temperature the results suggest a continuous process of de-freezing of magnetic clusters of spins accompanied by “dynamic narrowing” of the Bragg peak.     

Evidence for charge orbital and spin stripe order in an overdoped manganite

Overdoped La0.42Sr1.58MnO4 exhibits a complex ordering of charges, orbitals, and spins. Neutron diffraction experiments reveal three incommensurate and one commensurate order parameters to be tightly coupled. The position and the shape of the distinct superstructure scattering as well as higher-order signals are inconsistent with a harmonic charge and spin-density-wave picture but point to a stripe arrangement in which ferromagnetic zigzag chains are disrupted by excess Mn(4+).     

Spin correlations and magnetonuclear cross-correlation in Sm(Sr)-Mn-O perovskites in the low-temperature phase

The results of measurements of spin correlations and magnetonuclear cross-correlations (MNCCs) in the system ¹⁵⁴Sm_1−x Sr_xMnO₃ with the perovskite structure (x=0.25,0.4) in the low-temperature phase in magnetic fields 0<H<1 kOe are presented. These are the first measurements performed by the small-angle polarized-neutron scattering method (SAPNS). It is shown that ferromagnetic correlations with a scale of 180–250 Å and MNCCs characterizing the intercoupling of the magnetic and lattice subsystems on this scale exist in the system. It is found that in the low-temperature phase the system exhibits spin-glass properties. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 4, 323–328 (25 February 1999)     

Spin transresistivity—a probe of the opposite-spin correlations in an electron system

We investigate the spin drag effect in spin polarized transport beyond the random phase approximation by considering an effective scattering potential that incorporates self-consistently the effects of the short range Coulomb interaction through momentum dependent local field corrections. We find that the first order many-body correction to the spin transresistivity is determined by the local field factor for opposite-spin correlations.     

Spin polarization of charge carriers and Andreev reflection in (LaCa)MnO/superconductor point contacts

Spin polarization of charge carriers in La_0.65Ca_0.35MnO₃ (LCMO) is studied using point-contact Andreev spectroscopy. Pb and MgB₂ are used to make superconducting electrodes. In all cases, the transport spin polarization obtained from the conductivity of LCMO/superconductor point contacts does not exceed 80–85%. Different models of the current flow through the superconductor-ferromagnetic metal contact and possible reasons for noncomplete spin polarization of a current in manganites are explored. The level of spin polarization observed in Sharvin contacts (contact area ～10⁴ Ų) is most naturally explained in terms of a model that suggests separation of the crystal into nanosized magnetic phases, only one of which is a ferromagnetic metal with full spin polarization of charge carriers.     

Direct evidence of the charge ordered phase transition of indium nanowires on Si111

A controversial issue of the driving force for the phase transition of the one-dimensional (1D) metallic In wires on Si(111) is studied by low-temperature scanning tunneling microscopy and spectroscopy. The energy gap opening and the longitudinal charge ordering through charge transfer at the Fermi level are unambiguously observed. The vacancy defects induce a local charge ordering decoupled from a lattice distortion above T(c), and pin the phase of charge order below T(c). All these results below and above T(c) including the detailed features such as local fluctuations strongly support the 1D charge-density-wave mechanism for the phase transition.     

Electrostatic stabilization of an ordered phase in titanium monoxide

The influence of the ordering of titanium and oxygen atoms on the Madelung energy has been studied using the computer simulation. The dependence of the Madelung constant on the crystal size for the titanium monoxide with ordered and disordered distributions of atoms and vacancies has been obtained for the first time. It has been demonstrated that the Madelung energy in the ordered titanium monoxide is considerably (by 6%) lower than that in the disordered monoxide. The electrostatic interaction between titanium and oxygen ions stabilizes the ordered phase in titanium monoxide up to high temperatures and should substantially affect the disordering processes that occur in this compound at high temperatures.     

Vertex corrections near the stripe phase

We calculate the vertex corrections within a model for fermion quasiparticles coupled with charge and spin fluctuations, which provide the relevant scattering mechanism near the stripe instability in high- T(c) cuprates. The logarithmic divergence of the vertex, which characterizes the spin-fermion model near the antiferromagnetic instability, is ruled out, due to the incommensuration of the charge and spin modulation within the stripe phase, as revealed by neutron scattering. This simplifies the skeleton structure of the problem. The vertex is negative in the relevant kinematical regime, effectively reducing the interaction strength. Our results apply to generic incommensurate instabilities of electronic origin.