Incommensurate Positions Research Articles

Spin Dynamics in the t–t′–J Model: Dynamical Density-Matrix Renormalization Group Study

The ground state of a hole-doped t-t'-J ladder with four legs favors a striped charge distribution. Spin excitation from the striped ground state is known to exhibit incommensurate spin excitation near q=(pi,pi) along the leg direction (qx direction). However, an outward dispersion from the incommensurate position toward q=(0,pi) is strong in intensity, inconsistent with inelastic neutron scattering (INS) experiment in hole-doped cuprates. Motivated by this inconsistency, we use the t-t'-J model with m x n=96 lattice sites by changing lattice geometry from four-leg (24x4) to rectangle (12x8) shape and investigate the dynamical spin structure factor by using the dynamical density matrix renormalization group. We find that the outward dispersion has weak spectral weights in the 12x8 lattice, accompanied with the decrease of excitation energy close to q=(pi,pi), being consistent with the INS data. In the 12x8 lattice, weakening of incommensurate spin correlation is realized even in the presence of the striped charge distribution. For further investigation of geometry related spin dynamics, we focus on direction dependent spin excitation reported by recent resonant inelastic x-ray scattering (RIXS) for cuprate superconductors and obtain a consistent result with RIXS by examining an 8x8 t-t'-J square lattice.

Charge transfer enhanced magnetic correlations in type-II multiferroic Co3TeO6.

Magnetic structure of the Co ions in monoclinic Co3TeO6 in the antiferroelectric state at 16 K has been determined by neutron powder together with single-crystal diffractions. The indices of the magnetic reflections that appear at the incommensurate positions were determined by diffractions from a single crystal, which allow to uniquely identify the magnetic modulation vector. There are two crystallographically distinct Co layers. Magnetic incommensurability appears in the Co spins in the layers comprising zig-zag chains, with a magnetic modulation vector of (0.357, 0.103, 0.121) at 3 K but changes to (0.4439, 0, 0.137) at 16 K, while the Co ions in the honeycomb webs form a collinear antiferromagnetic structure. Thermal reduction rate of the Co moments in the honeycomb webs was found to be much smaller than those in the zigzag chains. Shifting of large amounts of electronic charge into the Co─O bonds in the honeycomb webs on warming is used to understand the behavior.

Relaxing Kondo-screened Kramers doublets in CeRhSi3

CeRhSi$_{3}$ is a superconductor under pressure coexisting with a weakly antiferromagnetic phase characterized by a Bragg peak at $\vec{q}_{0}$=($\sim$ 0.2, 0, 0.5) (N. Aso et al. J. Magn. Magn. Mater. 310, 602 (2007)). The compound is also a heavy fermion material with a large specific heat coefficient $\gamma$=110 mJ $\cdot$ mol$^{-1}$ $\cdot$ K$^{-2}$ and a high Kondo temperature of $T_{K}$=50 K indicative that CeRhSi$_{3}$ is in a strongly Kondo screened state. We apply high resolution neutron spectroscopy to investigate the magnetic fluctuations in the normal phase, at ambient pressures, and at low temperatures. We measure a commensurate dynamic response centered around the $\vec{Q}$=(0, 0, 2) position that gradually evolves to H$\sim$ 0.2 with decreasing temperature and/or energy transfers. The response is broadened both in momentum and energy and not reminiscent of sharp spin wave excitations found in insulating magnets where the electrons are localized. We parameterize the excitation spectrum and temperature dependence using a heuristic model utilizing the random phase approximation to couple relaxing Ce$^{3+}$ ground state Kramers doublets with a Kondo-like dynamic response. With a Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction within the $ab$ plane and an increasing single site susceptibility, we can qualitatively reproduce the neutron spectroscopic results in CeRhSi$_{3}$ and namely the trade-off between scattering at commensurate and incommensurate positions. We suggest that the antiferromagnetic phase in CeRhSi$_{3}$ is driven by weakly correlated relaxing localized Kramers doublets and that CeRhSi$_{3}$ at ambient pressures is on the border between a Rudderman-Kittel-Yosida antiferromagnetic state and a Kondo screened phase where static magnetism is predominately absent.

Al-impurity-induced magnetic excitations in heavily over-doped La1.7Sr0.3Cu0.95Al0.05O4

By means of inelastic neutron scattering, we measured magnetic excitations in a sizable single crystal of La1.7Sr0.3Cu0.95Al0.05O4, which is an Al-substituted system of the heavily hole-doped cuprate system La2-xSrxCuO4 (LSCO) with an effective concentration of holes of xeff = 0.25. At incommensurate positions Qort = (1+δ, ±δ)/(1-δ, ±δ) around the magnetic Γ point QM = (1, 0), magnetic correlations were induced, and the incommensurability δ was 0.139 (r.l.u), which is in contrast with the δ of 0.125 (r.l.u) for LSCO x = 0.25, suggesting that the Al substitution caused an increase in δ. We also observed the same effect in the Fe-substituted system La1.7Sr0.3Cu0.95Fe0.05O4 (δ = 0.144). In addition to the obvious effect on the IC excitations, we observed continuum magnetic excitation with a ring-like shape in a two-dimmensional momentum space around QM = (1, 0). This was also observed in the Fe-substituted system, meaning that the induction/enhancement of the magnetic correlations originates from the impurity substitution, regardless of whether the impurities have magnetism. Considering our recent experimental results, revealing that the ring-like excitation also evolves in impurity-free LSCO x = 0.25 with increasing temperature, these facts suggest an intimate relationship between the temperature change and impurity substitution on the magnetic correlations and may present clues for investigating the magnetic correlations through an effective approach using impurities.

Fe-impurity-induced magnetic excitations in heavily over-doped La1.7Sr0.3Cu0.95Fe0.05O4

Inelastic neutron scattering experiments were performed to investigate the effect of Fe substitution on the magnetic excitation spectra in a heavily hole-doped cuprate system La1.7Sr0.3Cu0.95Fe0.05O4, with an effective concentration of holes of xeff = 0.25. At T = 7 K, well-defined low-energy magnetic excitations, associated with a static magnetic order, were observed at the incommensurate positions Qtet = (0.5±δ, 0.5)/(0.5, 0.5±δ). Although the incommensurate structure resembles that observed in La2-xSrxCuO4 (LSCO) systems, the incommensurability (δ) of 0.144 (r.l.u) was larger than that of ∼0.125 (r.l.u) in LSCO with x = 0.25, suggesting an increase in δ by Fe substitution. In addition to the obvious effect on IC excitations, we show the emergence of ring-shaped magnetic excitations centered at Qtet = (0.5, 0.5) in a constant energy spectrum by Fe-substitution. These ring-shaped excitations were observed for energies below ∼8 meV, yielding a cylindrical continuum excitation in the energy and momentum spaces. Similar continuum excitations were recently observed in pristine LSCO with x = 0.25 at high temperatures, in our neutron experiments [1]. These results, which show the enhancement of δ (higher than δ = 1/8) and the stabilization of continuum excitations by Fe substitution, are incompatible with a simple pinning scenario of the stripe order, which is characterized by local spin correlations. The metallic aspect of the low-energy part of the magnetic excitation, which underlies high-temperature superconductivity in over-doped cuprate systems, would be enhanced/induced by Fe substitution.

Incommensurate spin correlations in highly oxidized cobaltates La2-xSrxCoO4.

We observe quasi-static incommensurate magnetic peaks in neutron scattering experiments on layered cobalt oxides La2−xSrxCoO4 with high Co oxidation states that have been reported to be paramagnetic. This enables us to measure the magnetic excitations in this highly hole-doped incommensurate regime and compare our results with those found in the low-doped incommensurate regime that exhibit hourglass magnetic spectra. The hourglass shape of magnetic excitations completely disappears given a high Sr doping. Moreover, broad low-energy excitations are found, which are not centered at the incommensurate magnetic peak positions but around the quarter-integer values that are typically exhibited by excitations in the checkerboard charge ordered phase. Our findings suggest that the strong inter-site exchange interactions in the undoped islands are critical for the emergence of hourglass spectra in the incommensurate magnetic phases of La2−xSrxCoO4.

Ising Incommensurate Spin Resonance of CeCoIn5: A Dynamical Precursor of the Q Phase.

It is shown through detailed inelastic neutron scattering experiments that the gapped collective magnetic excitation of the unconventional superconductor CeCoIn5, the spin resonance mode, is incommensurate and that the corresponding fluctuations are of an Ising nature. The incommensurate peak position of these fluctuations corresponds to the propagation vector of the adjacent field induced static magnetic ordered phase, the so-called Q phase. Furthermore, the direction of the magnetic moment fluctuations is also the direction of the ordered magnetic moments of the Q phase. Hence, the resonance mode and the Q phase share the same symmetry and this strongly supports a scenario where the static order is realized by a condensation of the magnetic excitation.

Field-induced interplanar magnetic correlations in the high-temperature superconductorLa1.88Sr0.12CuO4

We present neutron scattering studies of the inter-planar correlations in the high-temperature superconductor La1.88Sr0.12CuO4 (T_c=27 K). The correlations are studied both in a magnetic field applied perpendicular to the CuO2 planes, and in zero field under different cooling conditions. We find that the effect of the magnetic field is to increase the magnetic scattering signal at all values of the out-of-plane wave vector L, indicating an overall increase of the magnetic moments. In addition, weak correlations between the copper oxide planes develop in the presence of a magnetic field. This effect is not taken into account in previous reports on the field effect of magnetic scattering, since usually only L~0 is probed. Interestingly, the results of quench-cooling the sample are similar to those obtained by applying a magnetic field. Finally, a small variation of the incommensurate peak position as a function of L provides evidence that the incommensurate signal is twinned with the dominating and sub-dominant twin displaying peaks at even or odd L, respectively.

Field Dependence of the Magnetic Propagation Vector of the Heavy Fermion Compound CeCu2Ge2 Studied by Neutron Diffraction

CeCu2Ge2, the counterpart of the heavy-fermion superconductor CeCu2Si2, exhibits an in-commensurate antiferromagnetically long-range ordered ground state with τ = (0.28 0.28 0.54) below TN = 4.15K. The magnetism is strongly affected by a Kondo screening of the Ce 4f-moments by conduction electrons. The similar energy scale of both, Kondo and exchange interactions, results in a complex magnetic phase diagram and gives rise to potential quantum critical phenomena at very low temperatures. We present elastic neutron diffraction data obtained on a CeCu2Ge2 single crystal employing the cold triple axis spectrometer PANDA at MLZ and the diffractometer D23 at ILL.The field dependence of the magnetic propagation vector was measured at T ≤ 400 mK in the [110]/[001] plane with vertical magnetic fields applied along [1̄10]. We observe a low-field incommensurate magnetic phase AF1, a first order phase transition around 7.8 T with the coexistence of two phases AF1 and AF2 with slightly different propagation vectors, the disappearance of AF1 at 8 T and the existence of AF2 up to 12 T with a possible modification at 10 T. At 12.6 T, yet still well below the value of 26 T of the saturation for magnetic fields in [110] direction, the AF2-type magnetic order is lost and magnetic intensities are not to be found at incommensurate positions in the [110]/[001] plane any more. These new results contradict a previously suggested scenario with a QCP located at 8 T and contribute new information to the B − T phase diagram of CeCu2Ge2 in [110] direction.

Transformation between spin-Peierls and incommensurate fluctuating phases of Sc-doped TiOCl

Single crystals of ${\mathrm{Sc}}_{x}$${\mathrm{Ti}}_{1\ensuremath{-}x}\text{OCl}(x=0.005)$ have been grown by the vapor phase transport technique. Specific heat measurements prove the absence of phase transitions for 4--200 K. Instead, an excess entropy is observed over a range of temperatures that encompasses the incommensurate phase transition at 90 K and the spin-Peierls transition at 67 K of pure TiOCl. Temperature-dependent x-ray diffraction on ${\mathrm{Sc}}_{x}$${\mathrm{Ti}}_{1\ensuremath{-}x}$OCl gives broadened diffraction maxima at incommensurate positions between ${T}_{c1}=61.5\phantom{\rule{0.16em}{0ex}}(3)$ and $\ensuremath{\sim}90$ K, and at commensurate positions below 61.5 K. These results are interpreted as due to the presence of an incommensurate phase without long-range order at intermediate temperatures, and of a highly disturbed commensurate phase without long-range order at low temperatures. The commensurate phase is attributed to a fluctuating spin-Peierls state on an orthorhombic lattice. The monoclinic symmetry and local structure of the fluctuations are equal to the symmetry and structure of the ordered spin-Peierls state of TiOCl. A novel feature of ${\mathrm{Sc}}_{x}$${\mathrm{Ti}}_{1\ensuremath{-}x}\text{OCl}(x=0.005)$ is a transformation from one fluctuating phase (the incommensurate phase at intermediate temperatures) to another fluctuating phase (the spin-Peierls-like phase). This transformation is not a phase transition occurring at a critical temperature, but it proceeds gradually over a temperature range of $\ensuremath{\sim}10$ K wide. The destruction of long-range order requires much lower levels of doping in TiOCl than in other low-dimensional electronic crystals, like the canonical spin-Peierls compound ${\mathrm{CuGeO}}_{3}$. An explanation for the higher sensitivity to doping has not been found, but it is noticed that it may be the result of an increased two-dimensional character of the doped magnetic system. The observed fluctuating states with long correlation lengths are reminiscent of Kosterlitz--Thouless-type phases in two-dimensional systems.

Direction of atom displacement in incommensurate state of Ti–32Pd–18Fe shape memory alloy

Displacement of atoms causing diffuse satellites in a Ti–32Pd–18Fe alloy (in at%) has been investigated by transmission electron microscopy (TEM). In this alloy, diffuse satellites appear at incommensurate positions given by gB2+q when the zone axis is [111] and [001], but they are missing when [110]. This results imply that the satellites appear because of a transverse wave with a propagation vector of q∼1/5〈ζζ¯0〉⁎ and displacement in 〈110〉 direction. Possibly, this transverse wave occurs as a result of phonon softening of the TA2 phonon branch, which is a common feature for B2-type Ti-based shape memory alloys.

Theorising about ageing, family and immigration

ABSTRACTThis article critically reviews the ways in which theories in social gerontology have been used to explain the influence of ethnicity and immigration on older adults. The purpose of this cursory examination is to explore how these theories can be used for further theory advancement without serious modifications to existing theories. Current theories do not have the capacity to capture the effects of immigration at the structural level and its link to the social, psychological and family levels, let alone the physical ageing of individuals or societies. The overriding complexity of ageing and immigration requires at minimum, a long-term view and an integrating framework with multiple levels that can accommodate a variety of theoretical interests. The conclusions are that a lifecourse perspective, in conjunction with existing theories, can be employed in two different ways to further this theoretical agenda. A lifecourse perspective provides scaffolding for other theories where seemingly incommensurate epistemological positions can be easily accommodated and the principles of the lifecourse can be integrated into existing theories for a more fine-grained analysis of ethnicity and immigration. These approaches leave the theoretical door open to everyone including the positivists, the constructionists and critical and postmodern scholars with the possibility for interdisciplinary theory building to advance the understanding of the lives of immigrant families.

Relation between incommensurate satellites and phonon softening in Ti–Ni-based shape memory alloys

In Ti–44.0 Ni–6.0 Fe alloy, diffuse satellites appear at incommensurate positions given by gB2 + q when the zone axis is [1 1 1] or [0 0 1], but not [1 1 0]. This result implies that the satellites appear because of a transverse wave consistent with the displacement of atoms in the 〈1 1 0〉 direction, which is perpendicular to the propagation vector q = 〈ζ ζ ¯ 0〉∗, for ζ ∼ 1/3. This transverse wave occurs as a result of the phonon softening of the TA2 branch.

The credit crunch: Ideological, psychological and epistemological perspectives

Two economic interpretations of the credit crunch are outlined and the question posed whether these are incommensurate ideological positions. Psychological perspectives are then explored including insights from cognitive and social psychology. The argument is made that policy options depend on what constitutes the ‘good society’ and whether the culture of financial institutions can be changed by government intervention, social pressure and human agency. It is concluded that those interested in socio-economics have a duty to engage with alternative discourses.

Fe-doping effects on magnetism in hole-type superconductors of (Bi,Pb)2Sr2CuO6

We studied Fe-dopant effects on spin correlations in Bi1.75Pb0.35Sr1.90CuO6+δ′. Magnetic neutron elastic scattering, which is absent in a pristine sample, has been observed at incommensurate positions with an incommensurability of δ~0.2. Surprisingly, this anomalously large S follows a linear relation δ~p even in the overdoped region, unlike for La2−xSrxCuO4. We discuss this specific feature observed in the overdoped phase from a dynamical stripe viewpoint.

Theoretical study of the magnetism in the incommensurate phase of TiOCl

Abstract Going beyond a recently proposed microscopic model [D. Mastrogiuseppe, A. Dobry, arXiv:0810.3018v1] for the incommensurate transition in the spin-Peierls TiOX (X=Cl, Br) compounds, in the present work we start by studying the thermodynamics of the model with XY spins and adiabatic phonons. We find that the system enters an incommensurate phase by a first order transition at a low temperature T c 1 . At a higher temperature T c 2 a continuous transition to a uniform phase is found. Furthermore, we study the magnetism in the incommensurate phase by density matrix renormalization group (DMRG) calculations on a one-dimensional Heisenberg model where the exchange is modulated by the incommensurate atomic position pattern. When the wave vector q of the modulation is near π , we find local magnetized zones (LMZ) in which spins abandon their singlets as a result of the domain walls induced by the modulated distortion. When q moves far away enough from π , the LMZ disappear and the system develops incommensurate magnetic correlations induced by the structure. We discuss the relevance of this result regarding previous and future experiments in TiOCl.

Neutron diffraction study on stress-induced X-phase in Ni 2MnGa

Neutron diffraction profiles of the X-phase in Ni 2 MnGa, which is induced by applying a compressive stress, have revealed that two satellite reflections appear at incommensurate positions of [ h 2 - h 0 ] P ∗ , with h = 0.360 and 1.638 at 240 K under a compressive stress of about 30 MPa. The satellite positions of the X-phase are clearly different from those of the martensite and intermediate phases.

Origin of diffuse scattering appearing in iron-doped Ti-Ni shape memory alloys

We have investigated diffuse scattering appearing in Ti-Ni based shape memory alloys to understand the origin of instability of the B2-type structure in the system. Transmission electron microscope observation and a first-principle electronic structure calculation have been made for Ti-(50-x)Ni-x Fe (x = 6, 7, 8, 10). They exhibit diffuse scattering in the electron diffraction pattern along 〈ξξ0 〉* direction on cooling with the intensity peak located at incommensurate positions of qdiffuse = 〈ζζ0 〉*. The value ζ increases with decreasing temperature for all alloys. The temperature at which the incommensurate scattering start to appear is close to the one at which the electrical resistivity exhibits a local minimum, Tmin, for all the alloys. The value of qdiffuse at Tmin is nearly the same as the nesting vector of Fermi surface at Tmin, which is obtained by calculating the generalized susceptibility of the B2-type structure along the 〈ξξ0 〉* direction. These results strongly suggest that the nesting effect of Fermi surface is responsible for the instability of the B2-type structure in Ti-Ni based shape memory alloys.

Crystal structure of martensite and intermediate phases in Ni2MnGa studied by neutron diffraction

We have investigated crystal structures of martensite and intermediate phases in stoichiometric Ni2MnGa. The neutron diffraction profile of the martensite phase measured at T = 4.2 K exhibits four satellites between [2 0 0]P* and [0 2 0]P* reflections (P stands for the parent phase) at incommensurate positions of [h 2-h 0]P* with h = 0.428, 0.863, 1.136 and 1.572. The profile of the intermediate phase measured at T = 210K exhibits two satellites between [2 0 0]P* and [0 2 0]P* reflections at incommensurate positions of [h 2-h 0]P* with h = 0.343 and 1.657. Although each satellite of the martensite phase moves toward its nearest fundamental reflection as temperature increases, that of the intermediate phase does not move significantly. On the contrary, the intensity of each satellite decreases significantly in the intermediate phase as temperature increases while not in the martensite phase. A synchrotron X-ray diffraction and a Rietveld analysis of the result reveal that, for both the phases, the displacement of atoms from the parent phase are represented by a sine wave whose propagation vector is parallel to [1 1 0]P.

Iron content and temperature dependences of diffuse scattering in Ti–(50–x)Ni–xFe (6 ≤ x ≤ 10) alloys

Diffuse scatterings appearing in electron diffraction patterns of Ti–(50 − x)Ni–xFe (x = 6, 7, 8, 10, in at.%) alloys were investigated. In the alloys, martensitic (R-phase) transformation is suppressed down to 4.2 K, but the electrical resistivity exhibits a local minimum at T min (210 K, 195 K, 180 K and 140 K for x = 6, 7, 8, 10, respectively). The following results were obtained for all the alloys. Diffuse scattering appears below T min and its intensity maximum is located at an incommensurate position of g + ⟨ζζ0⟩*, where g is a reciprocal lattice vector of the B2-phase. The value ζ at T min is significantly smaller than 1/3 and increases with decreasing temperature; it decreases with increasing Fe content. The value of ζ at T min agrees with the length of nesting vector obtained by a band calculation, suggesting that the diffuse scattering is caused by the nesting effect of the Fermi surface in the B2-type structure.