Incommensurate Research Articles

X-Ray Photoelectron Spectroscopy of Ferroelectrics

The paper presents the X-ray photoelectron spectra (XPS) of the valence band (VB) and of the principal core levels (CL) for SbSI-type and incommensurate TlInS 2 single crystals. Form and electronic structure of the VB and CL splitting in the quasi-one-dimensional SbSI, SbSeI and BiSI, and in the incommensurate (IC) phase of TlInS 2 crystals are discussed. Experimentally obtained energies were compared with the results of theoretical ab initio calculations. Large shifts (3 ÷ 5 eV) in the CL binding energies in SbSI show a strong dependence on the surface crystallographic plane. The splitting of the CL was also obtained in the IC phase of TlInS 2 crystals.

Spatial inhomogeneity of magnetic moments in the cobalt oxide spinelCo3O4

The antiferromagnetic (AF) nature of the normal spinel ${\mathrm{Co}}_{3}{\mathrm{O}}_{4}$ with N\'eel temperature $({T}_{\mathrm{N}})=30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ was investigated by means of positive muon spin rotation and relaxation $({\ensuremath{\mu}}^{+}\mathrm{SR})$ techniques using a polycrystalline sample. Clear muon spin precession signals due to a quasistatic long-range AF order were found in the zero-field ${\ensuremath{\mu}}^{+}\mathrm{SR}$ spectra below ${T}_{\mathrm{N}}$. The spectra consist of two oscillating signals with frequencies at $T\ensuremath{\rightarrow}0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ of 80 and $60\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, respectively, indicating an incommensurate (IC) AF order in ${\mathrm{Co}}_{3}{\mathrm{O}}_{4}$. A possible reason for the appearance of the IC-AF order in ${\mathrm{Co}}_{3}{\mathrm{O}}_{4}$ would be local structural transitions due to a charge and/or a spin state change of Co ions.

Electrosorbed carbon monoxide monolayers on Pt(1 1 1)

We review structures of high-density CO monolayers on Pt(1 1 1) surfaces in CO-saturated electrolytes or in gaseous CO at near atmospheric pressure, using surface X-ray scattering (SXS) and scanning tunneling microscopy (STM). In electrolytes, we confirmed the well-known (2 × 2)-3CO and (√19 × √19)-13CO structures and were able to study the transition between them. For gas-phase studies, we were able to stabilize extremely well-ordered CO monolayers by emersion transfer from an electrochemical cell. We found that the hexagonal close-packed (2 × 2)-3CO structure is the equilibrium phase at room temperature in ∼1 atm CO gas pressure. This commensurate (C) phase transforms continuously to an incommensurate (IC) phase at elevated temperature (a second-order phase transition). We also confirm that the (√19 × √19)-13CO structure is stable at lower CO partial pressure. This C phase transforms discontinuously to an IC phase (a first-order phase transition). A tentative phase diagram and a brief review of structure details of the (2 × 2)-3CO and (√19 × √19)-13CO phases will be presented.

メリライト族鉱物のインコメンシュレート構造における大規模規則性とその温度変化

The structures of the melilite (three compounds) have been determined in the range from room temperature up to 773 K. The structures of incommensurate (IC) phase are characterized by bundles of four arrays of the six-coordinated Ca polyhedra with T 1 tetrahedra along the c-axis. The IC phases undergo the phase transition into the normal phases at elevated temperatures. The change of the modulated structure has been clarified by construction of the structures using the desired q-values and the modulation amplitudes determined at 293 K. The change of IC structures is characterized by variable distribution of bundle and octagonal arrangements of them. Large-scale regularities with sizes much larger than the modulation wavelength are also formed in the structures. The structure of commensurate phase of Ca2CoSi2O7 has been determined based on the results of simulation.

On the Sociological Significance of the American Jewish Experience: Boundary Blurring, Assimilation, and Pluralism

In thinking about the results reported here from the 2000/01 NJPS, it is crit ical to have a clear-eyed view of how the social landscape in which American Jews find themselves has changed in the post-World War II period. Some of the papers (Phillips and Fishman; Phillips and Keiner) argue that a soft version of rational choice can help to shed light on such key indicators of the contempo rary Jewish situation as intermarriage, and I agree that it can. But a choice-based framework is useful only insofar as one has a firm conceptual grip on the range of options faced by minority individuals, along with their perceived benefits, risks, and costs. If that configuration has changed substantially over time, past research may not give useful guidance, indeed may be fundamentally incom mensurate with the findings from new data. The applicability of past models is, quite appropriately, called into question by some of the papers, most notably, Phillips and Keiner. The concept of a social boundary provides a useful way of specifying some of the major changes that have taken place in the last six decades or so, as well as of identifying the value of the American Jewish case for comparative studies. By a boundary, I mean a social distinction that individuals make in their everyday lives and that shapes their actions and mental orientations towards others; it is typically embedded in a variety of social and cultural differences between groups that give a boundary concrete significance (so that those on one side think of those on the other, are not like us because ...). When we discuss ethnici ty, the kinds of boundaries we invoke have the character that Weber attributed to the ethnic group: namely, that they are rooted in a belief in com mon descent?i.e., in a shared history based on a common point of origin in the past, which may be real or putative. Admittedly, this subjective belief in a shared history may be felt more by one side of a boundary than the other and thus, to a greater or lesser extent, the boundary may be imposed, as is typically the case with race-like distinctions.

Dielectric constant, loss factor and ac conductivity of Ni2+-doped K2ZnCl4 crystals in the ferroelectric-commensurate, incommensurate and normal phases

The dielectric constant (ϵ), dielectric loss (tanδ) and ac conductivity (σac) of virgin and thermally cycled undoped and Ni2+-doped K2ZnCl4 (KZC) crystals in the ferroelectric-commensurate (FC), incommensurate (IC) and normal phases (N) have been studied. Anomalous behaviour at the two-phase transition points was observed while measuring ϵ along the polar a-axis for the undoped sample. With increasing Ni2+concentration a systematic shift of the phase transition temperature towards lower values and a continuous inhibition of the peak height were detected. ϵ changed linearly with lnf up to f =105 Hz. tanδ along the a-axis declared the phase transitions by peak changes. After Ni2+-doping this behaviour was preserved at the FC-IC phase transition point while the IC-N phase transition was manifested by a change in the slope of the straight line representing the tanδ-T dependence. The ac conductivity changed lineally with frequency according to a relation of the form σac = σof β where 0>β>1.9. σac increased monotonically with increasing temperature and doping concentration in the low-temperature phases tending to merge in one straight-line with high activation energy that might be due to superionic dc conduction in the high temperature N-phase. Doping with Ni2+ pinned stripples, decreased the soliton-soliton interaction, weakened discommensuration effects, shortened the IC-phase and strongly affected the ‘chaotic' behaviour at the TC-IC. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron Localization or Delocalization in Incommensurate Helical Magnets

The electronic states in incommensurate helical magnets are studied theoretically from the viewpoint of the localization or delocalization. It is found that in the multiband system with a relativistic spin-orbit interaction, the electronic wave functions show both an extended and localized nature along the helical axis depending on the orbital, helical wave number, and the direction of the plane on which spins rotate. The possible realization of this localization is discussed.

Magnetic excitations and their anisotropy inYBa2Cu3O6+x: Slave-boson mean-field analysis of the bilayert-Jmodel

We perform a comprehensive analysis of the dynamical magnetic susceptibility $\ensuremath{\chi}(\mathbf{q},\ensuremath{\omega})$ in the slave-boson mean-field scheme of the bilayer $t$-$J$ model. We use model parameters appropriate for ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ (YBCO), a typical bilayer high-${T}_{c}$ cuprate compound well studied by neutron scattering experiments. In the $d$-wave pairing state, the strongest magnetic spectral weight appears at $\mathbf{q}=\mathbf{Q}\ensuremath{\equiv}(\ensuremath{\pi},\ensuremath{\pi})$ and $\ensuremath{\omega}={\ensuremath{\omega}}_{\mathbf{Q}}^{\mathrm{res}}$, and spreads into a diamond-shaped shell around $\mathbf{Q}$ in $\mathbf{q}$ space for $\ensuremath{\omega}<{\ensuremath{\omega}}_{\mathbf{Q}}^{\mathrm{res}}$. This weight is due to a collective mode, namely a particle-hole bound state, which has a downward $\ensuremath{\omega}$ versus $\mathbf{q}$ dispersion around $\mathbf{Q}$. Within the high intensity shell, the incommensurate (IC) signals at $\mathbf{q}=(\ensuremath{\pi},\ensuremath{\pi}\ifmmode\pm\else\textpm\fi{}2\ensuremath{\pi}\ensuremath{\eta})$ and $(\ensuremath{\pi}\ifmmode\pm\else\textpm\fi{}2\ensuremath{\pi}\ensuremath{\eta},\ensuremath{\pi})$ tend to be stronger than the diagonal incommensurate (DIC) signals at $\mathbf{q}=(\ensuremath{\pi}\ifmmode\pm\else\textpm\fi{}2\ensuremath{\pi}{\ensuremath{\eta}}^{\ensuremath{'}},\ensuremath{\pi}\ifmmode\pm\else\textpm\fi{}2\ensuremath{\pi}{\ensuremath{\eta}}^{\ensuremath{'}})$, especially for a large hole density $\ensuremath{\delta}$. For $\ensuremath{\omega}\ensuremath{\ll}{\ensuremath{\omega}}_{\mathbf{Q}}^{\mathrm{res}}$ the IC signals completely disappear and the weight remains only around the DIC positions. For $\ensuremath{\omega}>{\ensuremath{\omega}}_{\mathbf{Q}}^{\mathrm{res}}$ strong signals of $\mathrm{Im}\phantom{\rule{0.2em}{0ex}}\ensuremath{\chi}(\mathbf{q},\ensuremath{\omega})$ tracing an upward dispersion are found and interpreted as an overdamped collective excitation near ${\ensuremath{\omega}}_{\mathbf{Q}}^{\mathrm{res}}$. In the normal state, $\mathrm{Im}\ensuremath{\chi}(\mathbf{q},\ensuremath{\omega})$ has a broad peak at $\mathbf{q}=\mathbf{Q}$. That is, the IC and DIC signals appear only in the $d$-wave pairing state. We also study effects of a small orthorhombic anisotropy, which is intrinsic in untwinned YBCO crystals. Because of electron-electron correlations favoring $d$-wave shaped Fermi surface deformations, we expect an enhanced anisotropy of magnetic excitation spectra. This effect is particularly pronounced for low $\ensuremath{\delta}$ and at relatively high temperature. The present theory provides a rather detailed microscopic explanation of the most salient properties of magnetic excitations observed in YBCO.

De Haas–van Alphen effect of [formula omitted] in commensurate and incommensurate phases

We have studied the de Haas–van Alphen effect in the commensurate (C) and incommensurate (IC) phases of PrB 6 . It is found that the amplitudes and effective masses of the two oscillations α 1 and α 2 from the main Fermi surface are approximately the same in the C phase while they are considerably different in the IC phase.

Possible evidence for electromagnons in multiferroic manganites

Magnetodielectric materials are characterized by a strong coupling of magnetic and dielectric properties and in rare cases simultaneously exhibit both, magnetic and polar order. Among other multiferroics, TbMnO3 and GdMnO3 reveal a strong magneto-dielectric (ME) coupling and as a consequence fundamentally new spin excitations exist: Electro-active magnons, or electromagnons, i. e. spin waves which can be excited by ac electric fields. Here we show that these excitations appear in the phase with an incommensurate (IC) magnetic structure of the manganese spins. In external magnetic fields this IC structure can be suppressed and the electromagnons are wiped out, thereby inducing considerable changes in the index of refraction from dc up to THz frequencies. Hence, besides adding a new creature to the zoo of fundamental excitations, the refraction index can be tuned by moderate magnetic fields, which allows the design of a new generation of optical switches and optoelectronic devices.

New orthorhombic multiferroics R1–xYx MnO3 (R = Eu, Gd)

AbstractA tendency to form multiferroic states with a spatially modulated antiferromagnetic structure and electric polarization in pure orthorhombic manganites RMnO3 upon decreasing of the rare‐earth ionic radii has been realized in single crystals of substituted compounds Eu1–x Yx MnO3 (0.2 ≤ x ≤ 0.5) and Gd1–y Yy MnO3 (0 ≤ y ≤ 0.2). While the parent pure and slightly substituted (x ≤ 0.1) compounds exhibited a spontaneous transition from the incommensurate (IC) to the canted antiferromagnetic (CAF) state at T CA < T N = 45–50 K, the transitions from IC to commensurate ferroelectric (C/FE) phase were observed at T lock < T N for x ≥ 0.3 and y ≥ 0.05 accompanied by dielectric anomalies. Various phase transitions were observed in magnetic fields up to 250 kOe along the a ‐, b ‐, c ‐axes by magnetization, magnetostriction and electric polarization measurements. The latter indicate the existence of a spontaneous electric polarization below T lock. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic, dielectric and magnetoelectric properties of new family of orthorhombic multiferroic Eu 1−xY xMNO 3 manganites

Multiferroic ground states with a spatially modulated antiferromagnetic structure and electric polarization have been revealed in Eu 1− x Y x MnO 3 ( 0.2 ⩽ x ⩽ 0.5 ) single crystals. While the slightly substituted ( x ⩽ 0.1 ) compounds exhibited a transition from the incommensurate (IC) to the canted antiferromagnetic (CAF) state at T CA < T N , the transitions from IC to commensurate ferroelectric (C/FE) phase were observed at T lock < T N for x > 0.2 . Various phase transitions were observed in the magnetic fields up to 250 kOe along a, b, c axes by magnetization, magnetostriction and electric polarization measurements which show an existence of a spontaneous electric polarization below T lock.

Incommensurate helix magnetic order in quasi-1D chain cuprates LiCu 2O 2 and NaCu 2O 2 as seen by NMR

NMR investigation of magnetic structure and phase transitions in two isostructural quasi-one-dimensional cuprates LiCu 2O 2 and NaCu 2O 2 has been performed. LiCu 2O 2 exhibits a unique sequence of phase transitions at T c = 2 4 , 22.5 and 9 K. NMR evidence of low temperature incommensurate (IC) in-chain spin structure in LiCu 2O 2 has been obtained. It is shown that below the magnetic ordering temperature T c = 2 4 K the 7Li NMR lineshape is characteristic for an IC static modulation of the local magnetic field caused by the helical spin structure of Cu magnetic moments twisted along the chain axis. The proposed helix structure model involving modulation of both helix angles θ and φ shows good agreement with experimental 7Li line profile. The absence of the central peak at 4.2 K provides unambiguous evidence that the plane wave IC regime is well established for helix magnetic structure of LiCu 2O 2 at T < 9 K . Therefore the transition at 9 K can be assigned to freezing out the phason-like fluctuations. First 23Na NMR measurements confirmed the existence of IC magnetic structure in NaCu 2O 2 at 3 K.

Anisotropy of incommensurate spin and charge fluctuations in detwinnedYBa2Cu3O6+δ

Motivated by a recent neutron scattering experiment on detwinned YBa$_2$Cu$_3$O$_{6+\delta}$ superconductor, we examine the frequency and doping dependence of the anisotropy in the spin and charge fluctuation arising from the coupling between the plane and the chain. Starting from the two-dimensional $t$-$t^{'}$-$J$ model and using the random-phase approximation (RPA), we find a pronounced anisotropy of the incommensurate (IC) peaks in the spin channel, namely the peak intensity at the $(\pi\pm\delta,\pi)$ direction is stronger than that at the $(\pi,\pi\pm\delta)$ direction in a wide frequency range from $\omega=0.2J$ to the resonance frequency $\omega_r=0.35J$. Above the resonance frequency, the IC peaks reemerge. Their intensities shift to the diagonal direction and no anisotropy exists along the two diagonal directions. We find that this anisotropy is robust with respect to the possible variation of the RPA correction factor and to the dopings. The charge fluctuation is also found to be incommensurate for all energies considered and peak at $(0,\delta)$ and $(\delta,0)$. An anisotropy in its IC peak intensity along the $k_x$ and the $k_y$ direction exists, but in sharp contrast to the spin channel, the maximum intensity of the IC peak is along the $k_y$ direction. Moreover, the IC peak in the charge channel exhibits an upward dispersion, in contrast to the downward dispersion below the spin resonance frequency for the spin IC peak. We explain these features based on the effect of the plane-chain coupling on the topology of the Fermi surface.

Rapid Suppression of the Commensurate Magnetic Ordered Phase of PrB6 by La Doping

We have studied the magnetization, electrical resistivity and magnetic phase diagram of Pr x La 1- x B 6 . The x dependence of the incommensurate (IC) magnetic ordering temperature, T IC1 , is simi...

High temperature phase transitions in barium sodium niobate: the wall roughening1q–2q incommensurate transition and mean field tricritical behaviour in a disordered exclusionmodel

Barium sodium niobate (Ba2NaNb5O15) is a tungsten bronze structure that exhibits a complicated sequence ofsix structural phase transitions, including three incommensurate (IC)phases. The phases are unusual in that all but the highest temperatureP4/mbm structure are ferroelectric. Unlike the situation for most incommensurate insulators, inwhich ferroelectricity develops at low temperatures along the modulationdirection, the polarization direction in barium sodium niobate is orthogonal tothe modulation(s), permitting some unusual phenomena. In the present studywe analyse the thermal and dielectric behaviour at the Curie temperatureTC near 830 K as wellas that at the Ccm 21–IC(1q) transitionnear 543 K, the IC(1q)–IC(2q) transition near565 K and the IC(2q)–P4bm transition at 582 K. The entropy change at 565 K is related to the wallroughening model of Rice et al (1981 Phys. Rev. B 24 2751). Data nearTC = 830 K indicate close proximity to a tricritical point, and discussions of criticalexponents are presented, all of which are found to be mean field. Because ofNa vacancies, transition temperature variation is found among specimensBa2Na1−xNb5O15 (830 K<TC(x)<865 K), and the system appears to be describable by the disordered exclusion model as a slightlyfirst-order intrinsic system whose dynamics are suppressed by weak disorder. NearTC the specificheat C(T) is compared with the random bond prediction of Harris (1974 J. Phys. C: Solid State Phys. 7 1671):C(T) = C0(T)/[1+bx2C0(T)],where C0(T) is the intrinsic specific heat of the vacancy-free crystal varying as(TC−T)−1/2 and x is the sodium vacancy concentration. In agreement with Harris’s model, the shifts inTC(x) are to lowerT with increasingx and scale asx; the broadeningscales as x2; and the effective critical exponent remains unchanged atα = 1/2.

Modulated phases in NaNbO3: Raman scattering, synchrotron x-ray diffraction, and dielectric investigations

Raman spectra of sodium niobate(NaNbO3) were obtained in all phases and revealed a significant disorder in the high-temperature U,T2 and T1 phases and a complicated folding of the Brillouin zone at the transitions intomodulated S, R, P and N phases associated with the competitive zone-boundary softmodes (in-phase and out-of phase octahedral tilts) along the M–T–R line. Anextensive Raman study combined with x-ray diffraction (XRD) and dielectricmeasurements confirmed the presence of the incommensurate (INC) phase insodium niobate. XRD experiments revealed the invar effect in the temperatureinterval 410–460 K corresponding to the INC phase associated with rotations of theNbO6 octahedramodulated along the b-direction. Our experiments suggest that the phase P consists of three phases: monoclinic(Pm) between 250 and 410 K, INC between 410 and 460 K, and orthorhombic(Po) between 460 and 633 K. At the low-temperature transition to the ferroelectric rhombohedralN phase all folded modes originating from the M- and T-points of the Brillouin zoneabruptly disappear, Raman spectra in the N phase become much simpler and all peakswere assigned.

Incommensurate magnetism in the coupled spin tetrahedra system Cu2Te2O5Cl2

Neutron scattering studies on powder and single crystals have provided new evidence for unconventional magnetism in Cu2Te2O5Cl2. The compound is built from tetrahedral clusters of S=1/2 Cu2+ spins located on a tetragonal lattice. Magnetic ordering, emerging at TN=18.2 K, leads to a very complex multi-domain, most likely degenerate, ground state, which is characterized by an incommensurate (ICM) wave vector k∼[0.15,0.42,1/2]. The Cu2+ ions carry a magnetic moment of 0.67(1)μB/Cu2+ at 1.5 K and form a four-helix spin arrangement with two canted pairs within the tetrahedra. A domain redistribution is observed when a magnetic field is applied in the tetragonal plane (Hc≈0.5 T), but not for H∥c up to 4 T. The excitation spectrum is characterized by two well-defined modes, one completely dispersionless at 6 meV, the other strongly dispersing to a gap of 2 meV. The reason for such complex ground state and spin excitations may be geometrical frustration of the Cu2+ spins within the tetrahedra, intra- and inter-tetrahedral couplings having similar strengths and strong Dzyaloshinski–Moriya anisotropy. Candidates for the dominant intra- and inter-tetrahedral interactions are proposed.

Pseudospin domain of ν=1 double-layer quantum Hall state near commensurate–incommensurate transition

We investigate the quantum Hall (QH) state at the total Landau level filling factor ν=1 in double layer systems around the commensurate (C)–incommensurate (IC) transition point. We find a broad maximum in the magnetoresistance ( R xx ) between the C and IC phases. The temperature dependence of R xx shows that the maximum remains finite when the data are extrapolated down to 0 K. This fact suggests that there is a dissipative phase between the C and IC phases such as the theoretically predicted soliton lattice phase.

Neutron scattering study on stripe correlations in La 2− xBa xCuO 4

We have performed neutron scattering experiment on the single crystals of La 2− x Ba x CuO 4 ( x = 0.10 and 0.125) in order to study the doping dependence of stripe correlations. At low temperatures, clear incommensurate (IC) peaks from spin-density-wave orders (SDW) were observed in both samples. The incommensurability ( δ) of the SDW in the x = 0.10 sample was found to be 0.110, which slightly deviates from the simple relation of δ = x seen in La 2− x Sr x CuO 4, possibly due to the large stripe pinning effect in the low-temperature tetragonal (LTT) structure. The inelastic magnetic signal at the low-energy region below 6 meV drastically changes around the structural transition temperature between LTT and low-temperature orthorhombic (LTO) phases T d2. Temperature dependences of the peak-width and the incommensurability also show an anomalous behavior at T d2 rather than the spin ordering temperature, T sp. These results suggest that spin correlations are well stabilized in the LTT phase as expected from the stripe model. With further increasing temperature in LTO phase, the incommensurability decreases and the peak-width grows due to the degradation of stripe correlations. However, the stiffness of stripe correlation seen in the low-energy IC spin fluctuations depends on the doping. In the x = 0.10 sample no well-defined magnetic excitation peak was observed below 6 meV at 200 K, while a flat top shaped single peak remains at 6 meV in the x = 0.125 sample. Deviation from the 1/8 doping more easily lose the spatial coherence of spin correlations above T d2.