Low-temperature Incommensurate Phase Research Articles

Temperature dependence of the propagation vector in Ni3−xCoxV2O8 with x=0.1 and 0.5

We present the susceptibility and neutron diffraction data on multiferroic Ni3−xCoxV2O8 with x=0.1 and 0.5. The temperature dependence of the susceptibility indicates that the magnetic order–disorder transition occurs at THTI=8.5K in both samples. Below THTI, the high-temperature incommensurate magnetic structure is realized, which undergoes a transition to the low-temperature incommensurate phase when the sample is cooled. The temperature evolution of the propagation vector k for x=0.1 is very weak, which confirms previous studies showing that substitution of 3.5% Ni ions by Co ions suppresses the explicit temperature dependence of k that is observed in the parent compound. On the other hand, we found that the vector k for x=0.5 exhibits a definite temperature dependence, which differs from the case of the undoped sample (x=0).

Role of magnetic chirality in polarization flip upon a commensurate-incommensurate magnetic phase transition in YMn2O5

We have performed simultaneous measurements of magnetic chirality by using polarized neutrons and electric polarization along the b-axis of single crystals of YMn$^{4+}$(Mn$_{1-x}$Ga$_{x}$)$^{3+}$O$_{5}$ with $x=0.047$ and 0.12, in which nonmagnetic Ga-ions dilute Mn$^{3+}$ spins. The $x=0.047$ sample exhibits high-temperature incommensurate (HT-ICM), commensurate (CM), and low-temperature incommensurate (LT-ICM) magnetic phases in order of decreasing temperature, whereas the $x=0.12$ sample exhibits only HT-ICM and LT-ICM phases. Here, the CM and LT-ICM phases are ferroelectric and weak-ferroelectric, respectively. Measurements conducted under zero field heating after various field-cooling conditions evidence that the microscopic mechanisms of the spin-driven ferroelectricity in the CM and LT-ICM phases are different: the magnetic chirality of Mn$^{4+}$ cycloidal spins plays a dominant role in the LT-ICM phase, whereas the magnetic exchange striction by the Mn$^{4+}$-Mn$^{3+}$ chain plays a dominant role in the CM phase. The polarization of YMn$_{2}$O$_{5}$ flips upon CM to LT-ICM phase transition because the ferroelectricity driven by the magnetic chirality and the exchange striction provides opposite directions of polarization.

Strong magnetoelastic effect on the magnetoelectric phenomena of TbMn2O5

Comparative studies of magnetoelectric susceptibility ($\alpha$), magnetization ($M$), and magnetostriction ($u$) in TbMn$_{2}$O$_{5}$ reveal that the increment of $M$ owing to the field-induced Tb$^{3+}$ spin alignment coins a field-asymmetric line shape in the $\alpha(H)$ curve, being conspicuous in a low temperature incommensurate phase but persistently subsisting in the entire ferroelectric phase. Correlations among electric polarization, $u$, and $M^{2}$ variation represent linear relationships, unambiguously showing the significant role of Tb magnetoelastic effects on the low field magnetoelectric phenomena of TbMn$_{2}$O$_{5}$. An effective free energy capturing the observed experimental features is also suggested.

Spin chirality and electric polarization in multiferroic compounds RMn2O5 (R=Ho, Er)

Polarized neutron diffraction experiments have been performed on multiferroic materials $R$Mn$_{2}$O$_{5}$ ($R=$Ho, Er) under electric fields in the ferroelectric commensurate (CM) and the low-temperature incommensurate (LT-ICM) phases, where the former has the highest electric polarization and the latter has reduced polarization. It is found that, after cooling in electric fields down to the CM phase, the magnetic chirality is proportional to the electric polarization. Also we confirmed that the magnetic chirality can be switched by the polarity of the electric polarization in both the CM and LT-ICM phases. These facts suggest an intimate coupling between the magnetic chirality and the electric polarization. However, upon the transition from the CM to LT-ICM phase, the reduction of the electric polarization is not accompanied by any reduction of the magnetic chirality, implying that the CM and LT-ICM phases contain different mechanisms of the magnetoelectric coupling.

Incommensurate magnetic structure ofYMn2O5: A stringent test of the multiferroic mechanism

We have determined the magnetic structure of the low-temperature incommensurate phase of multiferroic ${\text{YMn}}_{2}{\text{O}}_{5}$ using single-crystal neutron diffraction. By employing corepresentation analysis, we have ensured full compliance with both symmetry and physical constraints, so that the electrical polarization must lie along the $b$ axis, as observed. The evolution of the spin components and propagation through the commensurate-incommensurate phase boundary identifies the exchange-striction mechanism as the primary driving force for ferroelectricity.

Non-Resonant and Resonant X-Ray Scattering Studies on MultiferroicTbMn2O5

Comprehensive x-ray scattering studies, including resonant scattering at Mn L, Tb L, and M edges, were performed on single crystals of TbMn2O5 for crystallographic data to elucidate the nature of its commensurate and incommensurate phases. The scattering results provide direct evidence of symmetry lowering to the ferroelectric phase driven by magnetically induced lattice modulations and show the presence of multiple magnetic orders. The competing orders under spin-frustrated geometry are believed to cause discommensuration and result in the commensurate-to-incommensurate phase transition around 24 K. It is proposed that the low temperature incommensurate phase consists of commensurate domains separated by antiphase domain walls which change both signs of spontaneous polarizations and x-ray scattering amplitudes for forbidden reflections.

Magnetic phase diagrams of the Kagomé staircase compoundsCo3V2O8 and Ni3V2O8

An extensive low temperature magnetization study of high quality single crystals of the Kagomé staircasecompounds Ni3V2O8 and Co3V2O8 has beenperformed, and the H–T phase diagrams have been determined from these measurements. The magnetization and susceptibilitycurves for Co3V2O8 are analysed in terms of their compatibility with the different ferromagneticand antiferromagnetic structures proposed for this compound. ForNi3V2O8, thephase diagram is extended to magnetic fields higher than previously reported; for a field applied alongthe a axis, the low temperature incommensurate phase is found to close at around 90 kOe.

Molecular probing of low-temperature incommensurate phases

Two-dimensional (2D) excitation–emission spectra of biphenyl doped with free-base chlorin are measured at 5 K under various pressures up to 350 MPa. Besides the features related to zero-phonon lines and their phonon sidebands, a broad spectral band amounting to 80% of the total intensity at 5 K is revealed in the 2D spectra. The inhomogeneous distribution function obtained shows drastic changes with increasing pressure—the triplet structure observable at normal pressure in the incommensurate phase ICIII of biphenyl converges to a singlet in the high-pressure commensurate phase CI. These observations are assumed to reflect a relaxation specific to incommensurate phases after optical excitation of the probe molecules and their interaction with the incommensurate modulation wave.

Magnetic phase diagram ofCsCuCl3for in-plane magnetic fields up to 14 T

High-resolution specific-heat and magnetocaloric-effect measurements have been utilized to complement the magnetic phase diagram of the hexagonal antiferromagnet ${\mathrm{CsCuCl}}_{3}$ in fields up to 14 T aligned perpendicular to the c axis. The recently found additional incommensurate phase induced by thermal fluctuations could be resolved completely. It appears at about 4 T in a narrow temperature range, with a maximum width of only $\ensuremath{\sim}70 \mathrm{mK}$ and narrows towards higher fields extending up to $\ensuremath{\sim}11 \mathrm{T}.$ Within the low-temperature incommensurate phase, the magnetocaloric-effect data confirm another phase transition that may be explained by quantum fluctuations.

Specific Heat Anomalies near 147 K for K2ZnCl4

Single crystals of K 2 ZnCl 4 with high purity were grown by the Czochralski method in order to avoid the strong influence of water remaining in the crystal. The heat capacity for a single crystal was measured for cooling as well as heating runs, using a home-built adiabatic calorimeter with a temperature precision of 1 mK. The existence of a low-temperature incommensurate phase was revealed in these measurements for the first time. On heating, two sharp peaks were found at 146.983 ± 0.002 K (peak I) and at 146.956 ± 0.002 K (peak II), and the temperature interval between the two peaks amounted to approximately 0.027 K. These peaks showed a thermal hysteresis of about 0.1 K. The shapes of peak II obtained on both heating and cooling measurement are characteristic of a commensurate - incommensurate phase transition.

Competing structural phase sequences in (C3H7NH3)2CuCl4: reconstructive phase transitions and the low-temperature incommensurate phase

The structural phase sequence of (C3H7NH3)2CuCl4 (abbreviated to C3CuCl) has been determined between 20 and 300 K by optical and x-ray methods. The Jahn-Teller compound undergoes two types of reconstructive phase transition between antiferrodistortive (AF) and ferrodistortive (F) structures: the first may be characterized by a spontaneous 90 degrees switching of the long Jahn-Teller axes of the octahedra within every second layer ('octahedron-axes switching' (OAS)), and the second by a spontaneous reorientation of the hydrogen-bonding pattern ('hydrogen-bond switching' (HBS)) in the whole crystal. With these mechanisms, the highly unusual sequence of the phases delta (Pbca, Z=4), epsilon (P21/n11, Z=2), zeta (IC(P21/n11)), and eta (P21/c11, Z=4) is delta -180 K(OAS) to zeta -132 K(HBS) to eta , delta from 192 K(OAS)- epsilon from 180 K- zeta from 141 K(HBS)- eta . On cooling, the OAS transition at 180 K leads directly to the incommensurate (IC) zeta phase with a modulation wavevector qzeta =0.58t2*+0.08t3*. On heating, the modulation disappears at the same temperature and, as a new phase, the symmetrical epsilon phase occurs; at 192 K, the compound switches back to the room-temperature delta phase. The structure of the low-temperature eta phase, generated by the HBS mechanism, may be considered a distorted version of the twin structure of the delta phase. No symmetry relation exists between the IC zeta phase and the delta and eta phases. We find that the complex transition sequence of C3CuCl is in fact the result of two interfering phase sequences, namely those of the AF and F states, respectively. X-ray data on the high-temperature IC gamma phase are also presented.

Interatomic potentials and the phase diagram of Xe/Pt(111)

We present a microscopic model for the Xe/Pt(111) system that is consistent with the experimental desorption energy, the experimental vibrational frequency of the Xe atom in the direction normal to the Pt(111) surface, and salient features of the experimental phase diagram of Xe/Pt(111). The interatomic potentials in this model were obtained using a simple technique that we have developed for generalizing the typical pairwise-additive atom–atom central potentials used in modeling physisorption systems to make them noncentral and more flexible in their functional form. We applied this technique to the Lennard-Jones pair potential and fit the parameters to reproduce the experimental binding energy, the frequency for vibration of the adsorbate normal to the surface, and a reasonable choice of the binding distance. We adjusted the corrugation of the potential ΔV, defined as the energy barrier for motion of an adsorbate atom from one binding site to another, in order to fit as much of the phase diagram as possible. Our model for the Xe–Pt interaction was constructed on the basis of the assumption that the binding site is located in the threefold site of Pt(111). When the Xe–Xe interaction was represented by the form appropriate for atoms in the gas phase, we were unable to find a stable commensurate phase for any choice of the corrugation that predicted a low temperature incommensurate phase. When a substrate mediated contribution to the Xe–Xe interaction was included in the model, we found that the commensurate phase was stable in an intermediate temperature range with an incommensurate phase stable at low temperature for a range of values of the corrugation. For a choice of ΔV=171 K, the striped incommensurate phase is stable at low temperature, there is a phase transition to a √3 ×√3R30° phase at T=65±5 K, and the commensurate phase melts to become a liquid at T≊120 K. These results are in reasonably good agreement with the experiments.

Structural instabilities and electronic structures of the D(M(dmit)2)2 molecular conductors and superconductors

Structural studies of some molecular conductors and superconductors D(M(dmit)2)2 (D=TTF,Cs,NMe4; M=Ni,Pd) are presented and related to the calculated band electronic structure of these compounds. The charge density wave (CDW) instabilities observed in TTF(M(dmit)2)2 (M=Ni,Pd) as well as the low temperature incommensurate phase stabilized in Cs(Pd(dmit)2)2 are well accounted for by the properties of their computed Fermi surfaces. We emphasize that the originality of these compounds mainly stems from the weak energy difference between the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) of the organometallic M(dmit)2 molecule.

Superspace symmetry modes and incommensurate-to-incommensurate phase transition in NbTe4.

The symmetry modes transforming according to the irreducible representations of the little superspace group of the wave vector characterizing the incommensurate-to-incommensurate phase transition in ${\mathrm{NbTe}}_{4}$ are derived. Some of these modes are considered as possible order parameters, and possible Landau theories of the phase transition are considered. The superspace symmetries of the possible low-temperature incommensurate phases are found, and the systematic absences predicted for each case are compared with experiments and used to identify the symmetry of the appropriate order parameter. In addition, some fundamental questions concerning the invariance of the energy of the physical crystal with respect to superspace-group transformations, and concerning the appropriateness of characterizing the symmetry modes by a wave vector in physical space, are examined.

The incommensurate-commensurate phase transition in akermanite, Ca2MgSi2O7, observed by in-situ 29Si MAS NMR spectroscopy

29Si MAS NMR spectra of synthetic akermanite, Ca2MgSi2O7, exhibit a splitting into three or more lines in the stability feld of the low-temperature incommensurate phase. With increasing temperature, these lines continuously merge into a single line which is then characteristic of the high-temperature, melilite-type structure. The data point to the non-equivalence of Si sites in the low-temperature phase and the gradual structural changes as the phase transition temperature is approached.

Analysis of a theoretical model for the incommensurate-to-incommensurate phase transition in NbTe4.

We use a theoretical model for ${\mathrm{NbTe}}_{4}$ introduced previously, to examine the stability of the \ensuremath{\surd}2 a\ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}2 a incommensurate state with respect to domain-wall distortions that correspond to all possible wave vectors q in the basal plane, and to homogeneous phason strains, at low temperatures in the incommensurate phase. We find that for certain values of the model parameters either the previously discussed single-q, q=(\ensuremath{\pi}/a)x^, state or a homogeneous phason strain state is stable in the low-temperature incommensurate phase relative to the \ensuremath{\surd}2 a\ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}2 a state of the high-temperature incommensurate phase. We determine (for a certain range of the model parameters) the phase boundary between the \ensuremath{\surd}2 a\ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}2 a state and the single-q, q=(\ensuremath{\pi}/a)x^, state; we show that the phase transition corresponding to this boundary is second order.

Electron diffraction evidence for domain structures in the low-temperature incommensurate phase in CuV 2S 4

A low-temperature electron diffraction study of the sulfo spinel CuV 2S 4 is presented. From the diffraction geometry analysis a deformation modulated structure of the room temperature spinel phase below 90 K is proposed. Taking the low-temperature reflection intensities into account qualitatively, evidence is presented for a domain structure consisting of six orientation variants, corresponding with a symmetry lowering from cubic (order 48) to orthorhombic (order 8). The observation of pairs of low-temperature reflections located at incommensurate positions leads to a structure model consisting of quasi-periodically stacked antiphase boundaries characterized by a displacement vector R =12[101]. The deformation modulation is interpreted in terms of dimerized 〈110〉 types of copper-atom zig-zag chains, possibly associated with a charge density wave-type distortion.

An experimental study of the effect of random fields in BaMnF4: a material with continuous symmetry

X-ray diffraction measurements on two samples of BaMnF4 with different defect concentrations show that the defects destroy the long-range order of the low-temperature incommensurate phase. There is however, a well defined structural phase transition, and the scattering has lorentzian-squared component in this d=3 system, with a continuous symmetry.

Molecular dynamics simulation of an overfilled Kr monolayer on graphite

A microscopic model for Kr physisorbed on graphite is studied by the molecular dynamics technique. The model contains a Lennard-Jones potential describing the interaction among the Kr atoms and an external potential with sixfold symmetry describing the interaction between the Kr atoms and the graphite surface. The number of Kr atoms and the area of the surface are chosen to correspond to a five per cent overfilling of the perfect Kr monolayer. The Kr atoms are confined to move in a plane implying that second-layer promotion is inhibited in this study. Most simulations are performed with rectangular lattices of dimensions up to 170.4 AA*147.6 AA with periodic boundary conditions. A few simulations have also been performed with a circular lattice with reflecting walls in order to study the effects of the boundary conditions on the domain structures. The structure factors indicate two phases for the model: a low-temperature incommensurate phase and a high-temperature fluid phase. The transition between the two phases proceeds via a first-order transition with coexistence of the two phases in a temperature interval. No commensurate phase is seen in the simulation, in contrast to the experiments.

Neutron observations of phasons and amplitudons in incommensurate phases

Abstract We present the first observations of phase modes in low-temperature incommensurate phases. The experiments were performed using thermal and cold neutron triple-axis-spectrometers at I.L.L on biphenyl and thorium bromide. Biphenyl is a molecular crystal exhibiting two distinct low-temperature incommensurate phases but no lock-in transition. Despite its structural complexity two low-lying phonon branches corresponding to torsional motions of the biphenyl molecule can be well studied. In the high-temperature phase one of these modes becomes soft and condenses at an incommensurate lattice position near the zone boundary. Within the incommensurate phases this mode splits into a phason with a linear dispersion and a soft-mode-like amplitudon. The other mode also splits into two distinct branches in the incommensurate phase. Thorium bromide is a structurally much simpler system which displays an incommensurate phase below 95 K. Here too a high-temperature soft mode is clearly observed, below T c it splits into a phason and an amplitudon branch.