Incommensurate Correlations Research Articles

Frustration-induced eta inversion in the S=1/2 bond-alternating spin chain.

We study the frustration-induced enhancement of the incommensurate correlation for a bond-alternating quantum spin chain in a magnetic field, which is associated with a quasi-one-dimensional organic compound F5PNN. We investigate the temperature dependence of the staggered susceptibilities by using the density matrix renormalization group, and then find that the incommensurate correlation becomes dominant in a certain range of the magnetic field. We also discuss the mechanism of this enhancement on the basis of the mapping to the effective S=1/2 XXZ chain and a possibility of the field-induced incommensurate long-range order.

Shift of incommensurate antiferromagnetic peaks inLa2−xSrxCuO4

Neutron scattering experiments on ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ (LSCO) have revealed that the incommensurate antiferromagnetic peaks do not lie exactly on the symmetry axes ${(q}_{x}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}$ or ${q}_{y}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}),$ but are slightly shifted from them. In this paper, a scenario for such a ``shift'' is presented in terms of the anisotropy of ${t}^{\ensuremath{'}}$ (diagonal hopping integral on the square lattice) in the slave-boson scheme of the two-dimensional $t\ensuremath{-}J$ model. The predictions of the present theory based on fermiology are found to be different from those based on the ``spin-charge stripes'' hypothesis. This difference will serve to clarify a factor responsible for incommensurate antiferromagnetic correlations in LSCO systems.

Magnetic field effects and magnetic anisotropy in lightly dopedLa2−xSrxCuO4

The effects of the application of a magnetic field on the diagonal stripe spin-glass phase is studied in lightly doped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ $(x=0.014$ and 0.024). With increasing magnetic field, the magnetic elastic intensity at the diagonal incommensurate (DIC) positions $(1,\ifmmode\pm\else\textpm\fi{}\ensuremath{\epsilon},0)$ decreases as opposed to the increase seen in superconducting samples. This diminution in intensity with increasing magnetic field originates from a spin reorientation transition, which is driven by the antisymmetric exchange term in the spin Hamiltonian. On the other hand, the transition temperature, the incommensurability, and the peak width of the diagonal incommensurate correlations are not changed with magnetic field. This result suggests that the magnetic correlations are determined primarily by the charge disproportionation and that the geometry of the diagonal incommensurate magnetism is also determined by effects---that is, stripe formation---which are not purely magnetic in origin. The Dzyaloshinskii-Moriya antisymmetric exchange is nevertheless important in determining the local spin structure in the DIC stripe phase.

Neutron and x-ray evidence of charge melting in ferromagnetic layered colossal magnetoresistance manganites (invited)

Recent x-ray and neutron scattering studies have revealed static diffuse scattering due to polarons in the paramagnetic phase of the colossal magnetoresistive manganites La2−2xSr1+2xMn2O7, with x=0.40 and 0.44. We show that the polarons exhibit short-range incommensurate correlations that grow with decreasing temperature, but disappear abruptly at the combined ferromagnetic and metal–insulator transition in the x=0.40 system because of the sudden charge delocalization, while persisting at low temperature in the antiferromagnetic x=0.44 system. The “melting” of the polaron ordering as we cool through TC occurs with the collapse of the polaron scattering itself in the x=0.40 system. This short-range polaron order is characterized by an ordering wave vector q=(0.3,0,1) that is almost independent of x for x⩾0.38, and is consistent with a model of disordered stripes.

POLARON ORDERING IN FERROMAGNETIC COLOSSAL MAGNETORESISTIVE OXIDES

We review our recent x-ray and neutron scattering studies that reveal static diffuse scattering due to polarons in the paramagnetic phase of the colossal magnetoresistive manganites La 2-2x Sr 1+2x Mn 2 O 7, with x=0.40 and 0.44. We show that the polarons exhibit short-range incommensurate correlations that grow with decreasing temperature, but disappear abruptly at the combined ferromagnetic and metal-insulator transition in the x=0.40 system because of the sudden charge delocalization, while persisting at low temperature in the antiferromagnetic x=0.44 system. The "melting" of the polaron ordering as we cool through TC occurs with the collapse of the polaron scattering itself in the x=0.40 system. The polaron order is characterized by an ordering wave vector q=(0.3,0,1) that is almost independent of x for x≥0.40, and is consistent with a model of disordered stripes.

Commensurate and incommensurate correlations in Haldane-gap antiferromagnets

We analyze the onset of incommensurabilities around the VBS point of the $S=1$ bilinear-biquadratic model. We propose a simple effective field theory which is capable of reproducing all known properties of the commensurate-incommensurate transition at the disorder point ${\ensuremath{\theta}}_{\mathrm{VBS}}.$ Moreover, the theory predicts another special point ${\ensuremath{\theta}}_{\mathrm{disp}},$ distinct from the VBS point, where the Haldane gap behaves singularly. The ground state energy density is an analytic function of the model parameters everywhere, thus we do not have phase transitions in the conventional sense.

Numerical method for detecting incommensurate correlations in the Heisenberg zigzag ladder

We study two Heisenberg spin-1/2 chains coupled by a frustrating ``zigzag'' interaction. We are particularly interested in the regime of weak interchain coupling, which is difficult to analyse by either numerical or analytical methods. Previous density matrix renormalisation group (DMRG) studies of the isotropic model with open boundary conditions and sizeable interchain coupling have established the presence of incommensurate correlations and of a spectral gap. By using twisted boundary conditions with arbitrary twist angle, we are able to determine the incommensurabilities both in the isotropic case and in the presence of an exchange anisotropy by means of exact diagonalisation of relatively short finite chains of up to 24 sites. Using twisted boundary conditions results in a very smooth dependence of the incommensurabilities on system size, which makes the extrapolation to infinite systems significantly easier than for open or periodic chains.

Magnetization Process in Heavy Electron Systems

The magnetization process in the one-dimensional Kondo lattice model away from half filling ( n c <1) is studied on the basis of the density matrix renormalization group method. A rapid increase of the magnetization is caused by the collapse of the intersite incommensurate correlation of f spins. On the contrary, the intrasite f-c singlet correlation survives in the larger magnetic field. The crossover from large to small Fermi surfaces for majority and minority spins is observed, whereas the Fermi surfaces are always contributed by f spins. A magnetization plateau appears with the magnitude of 1- n c . The lower end and the upper end of the plateau are related to the coherence temperature and the Kondo temperature, respectively, which play the essential role in heavy electron systems as the characteristic energies.

Phase coexistence in the deuteron glass Rb0.9(ND4)0.1D2AsO4 proven by neutron diffraction

Neutron diffraction technique has been used to study the transitions from the paraelectric (PE) phase to the ferroelectric (FE) one in the deuteron glass crystal Rb0.9(ND4)0.1D2AsO4 (DRADA-10). The crystal shows PE hk0 diffraction spots splitting into spot patterns from domains, 4 spots from 4 FE domain types. A more complex phase behavior that the one corresponding to a typical PE-FE phase transition has been found. Coexistence of FE and PE features together with incommensurate correlations in the low temperature phase has been discovered. The intensity of the spots associated with the FE domains indicates the fraction of the crystal in the low-temperature phase. For DRADA-10 there is a fairly wide temperature range (7-10 K) over which these spots coexisted with spots remaining from the paraelectric phase

Transition from Haldane Phase to Spin Liquid and Incommensurate Correlation in Spin-1/2 Heisenberg Chains

The bulk properties and impurity effect in the one-dimensional S=1/2 Heisenberg model with dimerization and next-nearest-neighbor exchange are studied by the density matrix renormalization group (DMRG) and exact diagonalization methods. This model widely covers the S=1 Heisenberg model, two-leg ladders, bond-alternating chains and the S=1/2 Heisenberg model. Almost the whole parameter space belongs to the Haldane phase. In the non-dimerizing limit a transition to a kind of spin liquid takes place, characterized by the delocalized spin polarization. This transition causes the distinct impurity effects between a spin-Peierls compound CuGeO_{3} and the Haldane systems. Experimental aspects of a bond-alternating compound CuNb_{2}O_{6} are also discussed. Furthermore, the incommensurate correlation arising in this model is extensively studied, and a phase diagram is constructed.

Incommensurate Spin Correlations in Spin-1/2Frustrated Two-Leg Heisenberg Ladders

We develop a low-energy effective theory for spin-1/2 frustrated two-leg Heisenberg spin ladders. We obtain a new type of interchain coupling that breaks parity symmetry. In the presence of an XXZ-type anisotropy, this interaction gives rise to a novel ground state, characterized by incommensurate correlations. In the case of a single ladder, this state corresponds to a spin nematic phase. For a frustrated quasi-one-dimensional system of infinitely many weakly coupled chains, this state develops true three dimensional spiral order. We apply our theory to recent neutron scattering experiments on $Cs_2CuCl_4$.

Variational and density-matrix renormalization-group studies of the frustrated antiferromagnetic Heisenberg S=1 quantum spin chain

We study a frustrated antiferromagnetic isotropic Heisenberg $S=1$ chain using a variational ansatz and the DMRG. At $\alpha_D=0.284(1)$, there is a disorder point of the second kind, marking the onset of incommensurate correlations in the chain. At $\alpha_L=0.3725(25)$ there is a Lifshitz point, at which the excitation spectrum develops a doubly degenerate structure. These points are the quantum remnants of the transition from antiferromagnetic to spiral order in the classical frustrated chain. At $\alpha_T=0.7444(6)$ there is a first order phase transition from an AKLT phase to a next-nearest neighbor generalization of the AKLT model. At the transition, the string order parameter shows a discontinuous jump of 0.085 to 0; the correlation length and the gap are both finite at the transition. The problem of edge states in open frustrated chains is discussed at length.

Sr impurity effects on the magnetic correlations ofLa2−xSrxCuO4

We examine the low-temperature magnetic properties of moderately doped LaSrCuO paying particular attention to the spin-glass (SG) phase and the C-IC transition as they are affected by Sr impurity disorder. New measurements of the low-temperature susceptibility in the SG phase show an increase of an anomalously small Curie constant with doping. This behaviour is explained in terms of our theoretical work that finds small clusters of AFM correlated regions separated by disordered domain walls. The domain walls lead to a percolating sequence of paths connecting the impurities. We predict that for this spin morphology the Curie constant should scale as $1/(2 \xi(x,T=0)^2)$, a result that is quantitatively in agreement with experiment. Also, we find that the magnetic correlations in the ground states in the SG phase are commensurate, and that this behaviour should persist at higher temperatures where the holes should move along the domain walls. However, our results show that incommensurate correlations develop continuously around 5 % doping, consistent with recent measurements by Yamada.

ChemInform Abstract: Neutron Scattering Studies of YBa2Cu3O6.6 and CuGeO3

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Roughening of anisotropically reconstructed surfaces and the Hubbard model.

We consider a model of reconstructed crystal surfaces originally considered by Villain and Vilfan [Europhys. Lett. 12, 253 (1990); 13, 285 (1990); Surf. Sci. 257, 368 (1991)] for the gold (110) surface, in which roughening occurs via the formation of anisotropic steps traversing the entire length of the crystal. The model is studied by a mapping to a spin-1/2 fermion system in 1+1 dimensions which, in the absence of islands, is precisely the Hubbard model. We consider a general p\ifmmode\times\else\texttimes\fi{}1 reconstruction, in the presence of interstep interactions and closed islands. Our analysis predicts the existence of a new type of rough phase, with incommensurate correlations in the reconstruction order parameter and unusual momentum space singularities at a characteristic ``Fermi momentum'' and its harmonics. The new phase is analogous to the Luttinger liquid of one-dimensional fermions. The general phase structure is as follows: For p&gt;2, there is a flat ordered, a rough incommensurate, and a flat incommensurate (FI) phase. For p=2, the FI phase is replaced by a flat disordered phase, and there is a rough disordered-incommensurate phase. The universality classes of all the connecting phase transitions are determined.

Incommensurate correlations in quantum spin models.

Frustrated quantum antiferromagnets in two dimensions are studied by analyzing, using a 1/N expansion, the phase properties of associated (2+1)-dimensional nonlinear continuum models. The spin system on square lattice with isotropic couplings that include up to the third-nearest neighbors is considered. Periodic, commensurate, or incommensurate correlations are found to be stable. The relevance in some phase regions of effective (1+1)-dimensional nonlinear models is discussed.

Incommensurate correlations in the t-J and frustrated spin-1/2 Heisenberg models.

We analyze the possible existence of incommensurate correlations in the t-J model as a function of doping and in the ${\mathit{J}}_{1}$-${\mathit{J}}_{2}$-${\mathit{J}}_{3}$ spin-1/2 Heisenberg model as a function of frustration using exact diagonalization techniques on small lattices. For the t-J model we found that when holes are introduced the spin structure factor S(q) shifts the position of its maximum away from (\ensuremath{\pi},\ensuremath{\pi}). However, we do not observe indications of the existence of long-range incommensurate order. The frustrated Heisenberg model is analyzed using spin-wave techniques. We present the phase diagram including 1/S corrections. It has special regions where there is no magnetic order that are natural candidates for new nonclassical ground states. Using a Lanczos method we study the line ${\mathit{J}}_{3}$=${\mathit{J}}_{2}$/2, which is completely disordered in the spin-wave analysis for ${\mathit{J}}_{2}$/${\mathit{J}}_{1}$&gt;1/4. S(q) behaves as in the t-J model replacing doping by frustration. We also study the square of order parameters related with N\'eel, twisted, dimer, and chiral order. After the N\'eel order is destabilized by frustration, the twisted and dimer states are enhanced. On the other hand, there are no indications that the uniform or staggered chiral state can be the ground state of the frustrated Heisenberg model.

Static and dynamic central peaks in the freezing of a structural glass.

Combining dielectric, Brillouin, and neutron measurements, two spectral components related to freezing are identified in single crystals of ${\mathrm{Rb}}_{0.38}$${(\mathrm{N}{\mathrm{D}}_{4})}_{0.62}$${\mathrm{D}}_{2}$P${\mathrm{O}}_{4}$. The first corresponds to the incommensurate correlations that develop upon cooling and saturate near 80 K. These seem to be mostly static. A second, long-wave dynamic component continues to evolve markedly down to \AA{} 30 K, developing a broad distribution of slow relaxations. The former may provide the hierarchy of constraints leading to the latter.

Diffuse scattering of RADP glasses

Abstract X-ray and neutron scattering have been used to study the temperature, wavevector, and frequency dependences of short-range correlations developing in various crystals of Rb1−x(NH4)xH2PO4, and in Rb0.35(ND4)0.65D2pO4 a deuterated homolog, as they freeze into a glass-like state. Diffuse incommensurate peaks grow on cooling for wavectors along the [100] and [010] directions for values of × in the glass-forming range. This diffuse scattering, including the x-dependence of its wavevector, is qualitatively well explained by a model Hamiltonian that takes into account the crystal structure and the competing ferroelectric (FE) and antiferroelectric (AFE) interactions, without adjustable parameter. Observations close to the FE boundary indicate that the incommensurate correlations are very much quenched by FE ordering. Close to the AFE boundary, diffuse peaks also appear at the AFE wavevectors. Neutron scattering reveals a dynamical origin of the incommensurate correlations, with a temperature-dependence of their characteristic frequency that so far seems in agreement with the dynamics observed in macroscopic (q = 0) measurements.

The structure of ?nearly? ferroelectric RADP

X-ray scattering techniques have been used to study the structures of two crystals of Rb1−x(NH4)xH2PO4 withx≃0.2, near to the boundary between ferroelectric and “structural-glass” behavior at low temperature. Below about 83K, both crystals develop shortrange incommensurate correlations with a wavevectorq≃0.13 a* which are presumably related to the glass properties. On cooling below 70 K, the crystal with the slightly larger NH4+ concentrationx, develops the ferroelectric structure in a small fraction of the crystal, while the bulk of the crystal retains the tetragonal structure. The other crystal transforms almost wholly to the ferroelectric phase. The transition to the ferroelectric structure shows considerable hysteresis on heating and cooling, and is spread over about 20 K. The transition is certainly of first order, and the spread in temperature may arise from concentration fluctuations. These results and the structure of the incommensurate modulations are compared with the predictions of a theoretical model for this system.