Spatial dynamics of hybrid electromagnetic spin waves in a lateral multiferroic microwaveguide

Regimes of the formation of spatial structures at the propagation of hybrid electromagnetic spin waves in a system of laterally coupled multiferroics, which consist of parallel ferromagnetic microwaveguides with a ferroelectric layer, are studied experimentally and theoretically. Brillouin spectroscopy measurements at frequencies near the ferromagnetic resonance by the method of selection of mode patterns reveal a sharp increase in the spatial scales of transfer of power between microwaveguides. The calculations of the characteristics of propagation of electromagnetic spin wave in a lateral multiferroic structure with a finite width show that energy exchange between films is due to the features of intermodal coupling between waves. Higher transverse modes and electric-field-induced transformation of spectra of the electromagnetic spin waves in the adjacent multiferroics are studied experimentally and numerically.     

Charge order and low frequency spin dynamics in lanthanum cuprates revealed by Nuclear Magnetic Resonance

We report detailed ¹⁷O, ¹³⁹La, and ^63,65Cu Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) measurements in a stripe ordered La_1.875Ba_0.125CuO₄ single crystal and in oriented powder samples of La_1.8−x Eu_0.2Sr _x CuO₄. We observe a partial wipeout of the ¹⁷O NMR intensity and a simultaneous drop of the ¹⁷O electric field gradient (EFG) at low temperatures where the spin stripe order sets in. In contrast, the ^63,65Cu intensity is completely wiped out at the same temperature. The drop of the ¹⁷O quadrupole frequency is compatible with a charge stripe order. The ¹⁷O spin lattice relaxation rate shows a peak similar to that of the ¹³⁹La, which is of magnetic origin. This peak is doping dependent and is maximal at x ≈ 1/8.     

Pseudogap and symmetry of superconducting order parameter in cuprates

The phase diagram, nature of the normal state pseudogap, type of the Fermi surface, and behavior of the superconducting gap in various cuprates are discussed in terms of a correlated state with valence bonds. The variational correlated state, which is a band analogue of the Anderson (RVB) states, is constructed using local unitary transformations. Formation of valence bonds causes attraction between holes in the d-channel and corresponding superconductivity compatible with antiferromagnetic spin order. Our calculations indicate that there is a fairly wide range of doping with antiferromagnetic order in isolated CuO₂ planes. The shape of the Fermi surface and phase transition curve are sensitive to the value and sign of the hopping interaction t′ between diagonal neighboring sites. In underdoped samples, the dielectrization of various sections of the Fermi boundary, depending on the sign of t′, gives rise to a pseudogap detected in photoemission spectra for various quasimomentum directions. In particular, in bismuth-and yttrium-based ceramics (t′>0), the transition from the normal state of overdoped samples to the pseudogap state of underdoped samples corresponds to the onset of dielectrization on the Brillouin zone boundary near k=(0,π) and transition from “large” to “small” Fermi surfaces. The hypothesis about s-wave superconductivity of La-and Nd-based ceramics has been revised: a situation is predicted when, notwithstanding the d-wave symmetry of the superconducting order parameter, the excitation energy on the Fermi surface does not vanish at all points of the phase space owing to the dielectrization of the Fermi boundary at k _x=± k _y. The model with orthorhombic distortions and two peaks on the curve of T _c versus doping is discussed in connection with experimental data for the yttrium-based ceramic. Zh. éksp. Teor. Fiz. 115, 649–674 (February 1999)     

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 gaps and spin dynamics in superconducting cuprates

Spin gap effects, consisting of a declining uniform susceptibility and spin paramagnetic NMR shift at low temperaturesin the normal state and associatedT ₁ behavior, are discussed and documented in several cuprate superconductors. Dynamic spin magnetism in these systems is further reviewed in the light of mean-field models, where we note that detailed results from the model by Millis, Monien, and Pines are not borne out in recent neutron data on YBa₂Cu₃O_6.92.T ₁ data on¹⁷O in La_1.85Sr_0.15CuO₄ are presented, showing consistency with neutron dynamic susceptibility data forT≧80 K, but exhibiting a strong spin gap character below 80 K which is not present in the neutron data. Data for Zn-doped YBCO withT _c≈60 K are also presented, showing strong RKKY broadening from localized moments in the planes, but no spin gap effect such as that found in theT _c=60 K oxygen-deficient phase.     

Effect of electron-phonon interaction on optical response in one-dimensional cuprates

We examine the single-particle excitation and linear optical absorption spectra in the one-dimensional (1D) extended Hubbard-Holstein model. We perform dynamical density matrix renormalization group calculations with use of pseudo-site representation of phonons. We focus on the interplay among phonons and elementary excitations in 1D Mott insulators. The excitations in the Mott insulators are easily modified by the phonons. We discuss implications of the present results in light of spectroscopic measurements in 1D cuprates.     

Fractionalization in the cuprates: detecting the topological order

The precise theoretical characterization of a fractionalized phase in spatial dimensions higher than one is through the concept of "topological order." We describe a physical effect that is a robust and a direct consequence of this hidden order that should enable a precise experimental characterization of fractionalized phases. In particular, we propose specific "smoking-gun" experiments to unambiguously settle the issue of electron fractionalization in the underdoped cuprates.     

Chiral domains in cycloidal multiferroic thin films: switching and memory effects

Cycloidal magnetic order occurring in some AMnO(3) perovskites is known to induce ferroelectricity. The polarization is perpendicular to the propagation vector direction of the cycloid and its chirality, and therefore it is directly related to the chiral domain structure. We show that the switching process of chiral domains is sensitively dependent on the magnetoelectric history of the sample. Moreover, by appropriate field cycling, magnetic order can display partial chiral memory. We argue that memory results from electric field coupling of cycloidal domain and nucleation and pinning of chiral domain walls, much like the domain structure in other ferroic systems.     

Chiral order and fluctuations in multi-flavour QCD

Multi-flavour chiral perturbation theory (PT) may exhibit instabilities due to vacuum fluctuations of sea pairs. Keeping the fluctuations small would require a very precise fine tuning of the low-energy constants and to , . A small deviation from these critical values - like the one suggested by the phenomenology of OZI-rule violation in the scalar channel - is amplified by huge numerical factors inducing large effects of vacuum fluctuations. This would lead in particular to a strong N_f dependence of chiral symmetry breaking (SB) and a suppression of the multi-flavour chiral order parameters. A simple resummation is shown to cure the instability of PT, but it modifies the standard expressions of some O(p ² ) and O(p ⁴ ) low-energy parameters in terms of observables. On the other hand, for r= m _s /m > 15, the two-flavour condensate is not suppressed, due to the contribution induced by massive vacuum pairs. Thanks to the latter, the standard two-flavour PT is protected from multi-flavour instabilities and could provide a well-defined expansion scheme in powers of non-strange quark masses. Received: 31 July 2002 / Revised version: 6 November 2002 / Published online: 15 January 2003 RID="a" ID="a" e-mail: sdg@hep.phys.soton.ac.uk RID="b" ID="b" e-mail: luca.girlanda@pd.infn.it ^* Present address: ECT^*, Villa Tambosi, Strada delle Tabarelle 286, 38050 Trento, Italy RID="c" ID="c" e-mail: stern@ipno.in2p3.fr     

Competing phases in the cuprates: Charge vs spin order

We have investigated magnetic and charge instabilities of the cuprates within the Gutzwiller approximation RPA (GA+RPA). Interestingly, in GA the dressed Hubbard U is not a single parameter, but has different forms in the spin and charge responses, with distinct doping dependencies. While there are a number of competing magnetic instabilities for hole-doped cuprates, we fail to find any purely electronic charge density waves. The dominant magnetic instabilities are associated with ‘double nesting’, and the phase diagrams are material dependent, with LSCO differing from other cuprates.     

Chiral electromagnetic meson exchange currents

Properties of electromagnetic isovector meson exchange currents are discussed starting from the concept of chiral symmetry and vector meson dominance. The 4-currents are constructed in the framework of theS-matrix method and satisfy the current conservation also at the relativistic level. The comparison with the standard approach is made in detail. The case when only current conservation and Lorentz invariance are taken into account is also considered. Cross sections are compared with the new data.Lectures held at the Indian Summer School on Electromagnetic and Weak Interactions of Particles with Nuclei, Sázava, Czechoslovakia, 6–11 September 1992I am indebted to the organizers for inviting me to the school.     

Spin order in one-dimensional Kondo and Hund lattices

We study numerically the one-dimensional Kondo and Hund lattices consisting of localized spins interacting antiferromagnetically or ferromagnetically with the itinerant electrons, respectively. Using the density-matrix renormalization group we find, for both models and in the small coupling regime, the existence of new magnetic phases where the local spins order forming ferromagnetic islands coupled antiferromagnetically. Furthermore, by increasing the interaction parameter |J| we find that this order evolves toward the ferromagnetic regime through a spiral-like phase with longer characteristic wavelengths. These results shed new light on the zero temperature magnetic phase diagram for these models.     

Envelope solitons of electromagnetic spin waves in an artificial layered multiferroic

The nonlinear wave properties of an artificial multiferroic medium consisting of a ferroelectric layer and a ferromagnetic layer have been studied. The simultaneous effect of wave nonlinearities of both layers on the formation and propagation of envelope solitons has been analyzed. It has been shown that bright envelope solitons of electromagnetic spin waves can appear in such a medium owing to the wave nonlinearity of ferroelectric layer. In order to confirm the theoretical results, the coefficients of the nonlinear Schrödinger equation have been calculated and this equation has been solved numerically.     

Surface enhanced strong visible photoluminescence from one-dimensional multiferroic BiFeO3 nanostructures

The surface defect dominated visible emission from one-dimensional multiferroic BiFeO₃ (BFO) nanowires (NWs) is characterized by photoluminescence (PL) spectroscopy. The PL spectra of BFO NWs exhibit a weak near band emission (NBE) along with a strong defect-level emission (DLE). It is suggested that excess surface defects exist in BFO NWs which are responsible for strong visible green emission. Passivation of BFO NW surface with H₂ significantly improves the NBE emission while suppressing surface recombination. Such a surface enhanced emission promises many potential applications of BFO NWs not only in photonic devices such as LEDs but also in fluorescence-based chemical sensing.     

Chiral and antichiral order in bent-core liquid crystals

Recent experiments have found a bent-core liquid crystal in which the layer chirality alternates from layer to layer, giving a racemic or "antichiral" material, even though the molecules are uniformly chiral. To explain this effect, we map the liquid crystal onto an Ising model, analogous to a model for chiral order in polymers. We calculate the phase diagram for this model and show that it has a second-order phase transition between antichiral order and homogeneous chiral order. We discuss how this transition can be studied by further chemical synthesis or by doping experiments.     

Chiral criticality and glue dynamics

The chiral order-parameter σ field and its higher-order cumulants of fluctuations are calculated within the functional renormalization group approach by adopting the local potential approximation in this study. The influence of glue dynamics on fluctuations of the σ field is investigated, and we find that they are weakly affected. This is in sharp contrast to the baryon number fluctuations, which are sensitive to the glue dynamics and involve information on the color confinement. The implications of our calculated results are discussed from the viewpoint of the theoretical and experimental efforts in the search for the QCD critical end point.     

The electromagnetic Brillouin precursor in one-dimensional photonic crystals

We have calculated the electromagnetic Brillouin precursor that arises in a one-dimensional photonic crystal that consists of two homogeneous slabs which each have a single electron resonance. This forerunner is compared with the Brillouin precursor that arises in a homogeneous double-electron resonance medium. In both types of medium, the precursor consists of the components of the applied pulse that have their frequencies below the lowest of the two electron resonances. In the inhomogeneous medium however, the slab contrast starts affecting the precursor field after a certain rise time of the precursor: its spectrum starts to peak at the geometric scattering resonances of the medium whereas minima appear at the Bragg-scattering frequencies.     

Intensity distribution of electromagnetic radiation in one-dimensional superstructures

Distribution function of density of particles in one-dimensional sample with weak disorder has been found using Berezinsky diagram technique. For open systems, when a particle having reached the end of the sample can leave it, logarithmically normal distribution for density of particles has been obtained. It has been shown that the function obtaineddescribes radiation intensity distribution in weakly disordered superlattice.     

Charge dynamics in electron-underdoped high-Tc cuprates

We investigate charge dynamics in the antiferromagnetic (AF) phase of electron-doped cuprates by using numerically exact diagonalization technique for an electron-doped t-t′-J model. When AF correlation develops with decreasing temperature, a gap-like behavior emerges in the optical conductivity accompanied by the enhancement of the coherent motion of carriers due to the same sublattice hoppings. This is a remarkable contrast to the behavior of a hole-doped t-t′-J model.