IC Phase Research Articles

Far-infrared spectroscopy in the spin-Peierls compoundCuGeO3under high magnetic fields

Polarized far-infrared (FIR) spectroscopic measurements and FIR magneto-optical studies were performed on the inorganic spin-Peierls compound ${\mathrm{CuGeO}}_{3}.$ An absorption line, which was found at $98 {\mathrm{cm}}^{\ensuremath{-}1}$ in the dimerized phase $(D$ phase), was assigned to a folded phonon mode of ${B}_{3u}$ symmetry. The splitting of the folded mode into two components in the incommensurate phase (IC phase) has been observed. A broad absorption centered at $63 {\mathrm{cm}}^{\ensuremath{-}1}$ was observed only in the $\mathbf{E}\ensuremath{\Vert}b$ axis polarization, which was assigned to a magnetic excitation from singlet ground state to a continuum state.

Peculiar Ferroelectric Domain Patterns Observed on Quenched Sodium Nitrite

Ferroelectric domain patterns which appear under three different quenching conditions have been studied by means of an etching technique. When a b -plate sample is quenched from above T C , a three-dimensional pattern which varies dramatically along the thickness direction is often observed. When single-domain samples are quenched from temperatures between about T C -30°C and T C , the polarization reversal occurs locally and two-dimensional domain patterns are formed. The polarization reversal is caused by the depolarization field which appears during quenching even when the electrodes are short-circuited. When a single-domain sample is heated and quenched in the middle of the C-to-IC transition, the nucleated IC phase regions are converted to ferroelectric domains. These domains and those produced by the polarization reversal in the commesurate phase regions are superposed to one another and make up complicated domain patterns.

Nucleation and growth of incommensurate phase in the C-to-IC transition of Sodium nitrite

The process of nucleation and growth of the incommensurate phase in the C-to-IC transition has been studied. In the course of the transition, a sample is quenched to room temperature. During quenching, the nucleated IC regions are converted to ferroelectric domains. Among them the domains having the polarity opposite to that of the bulk can be delineated by etching. The IC phase is nucleated in the form of thin layers perpendicular to the α-axis and grows rapidly along the bc-plane. The layer is thinner than 1.3 μm. The C-to-IC transition is completed when the sample is covered with such IC layers.

Mechanism of transition between 1q and 3q incommensurate phases in two-dimensional crystal model

The one-dimensional elastically hinged molecule (EHM) microscopic model of a crystal was generalised to two dimensions on the base of the hexagonal lattices. The two-dimensional model contains two anharmonic terms which allow, depending on parameters of the model, the stability of 1q, 2q or 3q modulated phase. The phase transition between 1q and 3q IC phases characteristic to the hexagonal system was studied numerically by the molecular dynamics technique. The model shows the sequence of phases as it is in quartz on cooling and explains the increase of the 1q IC phase temperature range in quartz under uniaxial stress.

New azo-dye containing side group copolymethacrylates

A new type of side group copolymethacrylates was synthesized, characterized and tested with rcspect to its behavior on irradiation with an Ar + -laser beam. The side groups consist of methylene spacer units with different length and of three-ring mesogens with an ester group between the first and the second aromatic ring and on the other hand of two-ring mesogens with an azo-structure between the aromatic rings, respectively. The azochromophoric structure contains different substituents in the terminal aromatic ring. The homopolymcrs of the azo-dye containing monomers do not display liquid crystalline (Ic) mesophases, but the three-ring mesogens induce lc-phases in opposition to the equimolaric copolymers with two-ring ester group mesogens. The Ic phases are important for the stability of the recorded change of birefringence (photorecording process) which is influenced by irradiation of thc polymers.

Soliton dynamics in a type-II incommensurate-commensurate system: Betaine calcium chloride dihydrate (BCCD)

The results of dielectric experiments on pure and Br-doped betaine calcium chloride dihydrate (BCCD) at variable temperatures $T$, hydrostatic pressures $p$, and quasistatic electric fields $E$ are presented, which show the occurrence of field-induced commensurate (c) phases proving the existence of a square-wave modulated polarization at low $T$. With increasing $T$ the shape of the modulation wave turns rather smoothly into an almost harmonic wave in the incommensurate (ic) region. Near the transition temperature c$(\ensuremath{\delta}=$$\frac{1}{4}$)$\ensuremath{\rightarrow}$ic at ${T}_{1/4}$ indications for the onset of a gliding polarization wave (phason) in the ic phase, but also in the narrow c phases above ${T}_{1/4}$ with an almost harmonic modulation were detected in BCCD. This range extends up to ${T}_{i}.$

Influence of the rate of temperature change on the dielectric tail of single crystal Rb 2CoCl 4

The influence of very small rates of temperature change on the dielectric permittivity of incommensurate crystal Rb 2CoCl 4 was studied. It is shown that in the commensurate phase the permittivity excess depends on the cooling rate and the tail is present even for rates as low as 0.002 K min −1. Relaxation processes of the permittivity were studied in both the temperature range where C and IC phases coexist and at temperatures where the discommensurations are not stable. Results are compared with data of Rb 2ZnCl 4 and K 2ZnCl 4. Data suggest that the dielectric tail is more stable in Rb 2CoCl 4 than in K 2ZnCl 4 and is a consequence of revival discommensurations.

Synchrotron x-ray-scattering study of magnetic-field-induced transitions in Cu1-x(Zn, Ni)xGeO3.

We have studied the high-field phase behavior of the spin-chain compound ${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$(Zn, Ni${)}_{\mathit{x}}$${\mathrm{GeO}}_{3}$ by synchrotron x-ray scattering in magnetic fields up to 13 T. The rich magnetic phase diagram of this system includes uniform, low-field commensurate spin-Peierls (C), high-field incommensurate spin-Peierls (IC), and N\'eel phases. For the pure system (x=0), we have determined the shape of the phase boundaries as well as the incommensurability and harmonic content of the incommensurate lattice distortion in the IC phase as a function of field and temperature. The results are in good qualitative agreement with predictions based on the soliton lattice model of the IC phase, but significant quantitative discrepancies are found. Dilution of the copper oxide spin-1/2 chains with very small amounts (x\ensuremath{\sim}0.01) of spin-0 (Zn) or spin-1 (Ni) impurities results in a short-range-ordered IC phase with an anisotropic correlation length comparable to the average impurity separation. This presumably reflects strong pinning of the magnetic solitons to the impurities. For larger x the long-range order is disrupted also in the C phase, and the system undergoes a N\'eel transition at low temperatures. Aided by supplementary neutron-diffraction measurements, we construct magnetic phase diagrams for the diluted systems. For these systems we also present detailed measurements of the incommensurabilities and correlation lengths as a function of both field and temperature. The data are interpreted in terms of a simple phenomenological model based on lifetime broadening of the spin excitations in finite-sized chain segments. \textcopyright{} 1996 The American Physical Society.

Soft and folded modes in incommensurate Sn2P2Se6 studied by raman spectroscopy

Abstract Sn2Se6 is one of the few semiconductive proper ferroelectrics with an incommensurate phase. The IC phase (193 K < T < 221 K) is intermediate between a high temperature paraelectric (PE) and ferroelectric (FE) phase. Raman spectra studies have been performed to investigate the relation in lattice dynamics of the phase sequence PE-IC-FE. The soft mode in the FE phase could be distinguished. This mode together with a central peak contribute to the strong low frequency scattering. In the IC phase two new modes appear at 8 and 19 cm−1. The first could be identified as a folded acoustic mode, whereas the other might be a folded optical mode.

Defect density waves and special realizations of memory effects in crystals with incommensurate phases

Abstract New realizations of memory effects linked with defect density waves have been established by X-ray investigations of Ag3AsS3, SC(NH2)2, and TMAZnCl4 crystals. They involve the presence of additional wave modulation with temperature independent wave vector, interference of the normal and additional waves, the presence of the new intermediate state in the temperature region of two-phase state, the reverse hysteresis at the N<->IC phase transition.

Structural modulation and low-frequency excitations in the incommensurate phases of biphenyl

The incommensurate phases IC II and IC III of biphenyl are investigated by means of NMR. It is shown that the structural modulation appears along one direction, and can be described by a periodic twisting of the phenyl rings about the molecular axis. The temperature dependence of the maximum twist angle below the N - IC phase transition follows a power law with a critical exponent . A two-dimensional NMR exchange spectrum in the IC II phase of biphenyl is recorded. It clearly excludes a `floating' of the modulation wave. The temperature and Larmor frequency dependences of the NMR spin - lattice relaxation rates in the IC phases of biphenyl are measured. A distinct dependence of on the Larmor frequency is found and analysed on the basis of a relaxatory dynamics of the order parameter (OP). A reliable estimate of about 5 MHz for the characteristic frequencies of the OP dynamics is derived. In the IC III phase, the spectra as well as the spin - lattice relaxation rates indicate that the structure of biphenyl is not a truly incommensurate one.

Commensurate-incommensurate phase transition in one-dimensional quantum sine-Gordon model based on a lattice massive thirring model

The commensurate-incommensurate (C-IC) phase transition in the one dimensional quantum sine-Gordon model at zero temperature is exactly solved with the use of the Bethe ansatz technique for the lattice massive Thirring model. The energy difference between C and IC phases is derived based on the same ground state which is valid in the whole parameter region. It is due to the fact that there is no change in the ground state of the lattice massive Thirring model even in the strongly repulsive region in contrast to the continuum massive Thirring model even in the strongly repulsive region in contrast to the continuum massive Thirring model. It is proved in the whole parameter region that the IC phase can be realized with the soliton density proportional to\(\sqrt { - E_s } \) (Es: formation energy of soliton), whenEs becomes negative.

An amphiphile system forming a lyotropic blue phase

Abstract A partial phase diagram of the potassium N-tetradecanoyl-L-alaninate/decanol/water (+10 per cent CsCI) system has been charted using polarizing microscopy, laser diffraction, and NMR spectroscopy. At low amphiphile concentrations, a chiral nematic ChD phase with disc-like micelles occurs and at higher concentrations there occur a chiral nematic, ChC phase with cylindrical micelles and a chiral version of a dimensionally-ordered (probably rectangular or hexagonal) phase. Between the Chc phase and the genuine isotropic liquid there is a chiral isotropic phase which appears to be a micellar analogue of a thermotropic blue phase. This isotropic chiral Ic phase showed flow birefringence.

Electronic bonding and optical properties of the H2-H2O phase at high pressure.

The electronic structure and the optical properties of the cubic hydrate ${\mathrm{H}}_{2}$-${\mathrm{H}}_{2}$O at high pressure are elucidated by means of first-principles local-density calculations. The results are compared with the Ic phase of ice. It is shown that the encapsulated ${\mathrm{H}}_{2}$ molecules interact weakly with the ${\mathrm{H}}_{2}$O network. The valence-band structure is only slightly modified while the conduction band becomes very different. A number of important physical properties including the bulk modulus, optical-absorption spectrum, refractive index, pressure dependence of the gap, etc., are predicted for ${\mathrm{H}}_{2}$-${\mathrm{H}}_{2}$O.

The incommensurate phase of 4,4'-dichlorobiphenyl sulphone reinvestigated by 35Cl NQR

In order to acquire new information about the normal-incommensurate (IC) phase transition and to check the existence of a lock-in transition in 4,4'-dichlorobiphenyl sulphone (4,4'-DCBS), a comprehensive 35Cl nuclear quadrupole resonance (NQR) study is reported. Detailed NQR measurements of frequency, line width and spin relaxation time (Tl) were obtained in the temperature range 80-270 K. The results show that the IC phase is present from 150 K down to 80 K. The behaviour of the NQR spectrum in the transition region suggests that the IC modulation is dominated by the first-order term in the order parameter. The tl values at the peaks of the IC spectrum are quite different, suggesting strong anharmonic effects. No lock-in transition was detected at 115 K. Small changes in the temperature behaviour of the frequency and the tl of the low-frequency IC peak were observed from temperatures lower than 110 K. Neither impurities nor thermal hysteresis effects were detected.

Raman-and far-infrared-investigations of betaine-calciumchloride-dihydrate under hydrostatic pressure

Abstract Raman and infrared spectra of BCCD were investigated under hydrostatic pressure at various temperatures. In the regions of structure branching processes the frequencies of the lowest modes becoming active in the modulated phases exhibit a quasi-continuous change in frequency interrupted by small plateaus indicating the lock-in phases 2/9 and 2/11. This is the first direct experimental evidence for the existence of new c and ic phases in the branching region. These new modes can be understood as folded acoustic and optic modes.

Interaction of defects with the incommensurate structure in (N(CH3)4)2MeCl4 crystals using dielectric and birefringence measurements

Abstract The crystals with the IC phase assuming the character of “devil's staircase” were investigated using dielectric and optical methods.

EPR study of incommensurate and ferroelastic phases of [Mg(H2O)6]GeF6

The temperature and angular dependence of the EPR spectra of Mn2+ doped single crystal of [Mg(H2O6)]GeF6are studied. Two succesive phase transitions were found to occur, and intermediate phase was found to be incommensurately modulated. The modulation is sinusoidal throughout the IC phase, and lock-in transition is first order.

Molecular Rotation of NO2Radicals and Dipole Interaction in NaNO2

The permanent dipole of the NO 2 radical and the relative displacement between the ions are considered about the ferroelectricity in NaNO 2 . In this case the possibility of the rotation of NO 2 can be explained as the mixed double rotations around the a -axis and the c -axis. The IC phase between the ferroelectric and paraelectric phases is considered to be constructed by the interaction between the 2D electron system on the c -domain surface of the crystal and the NO 2 dipole, whose long periodicity can be estimated by the magnitude of spontaneous polarization.

Accumulation of structural branching processes in BCCD

Abstract The dielectric constants of BCCD have been measured under hydrostatic pressure (190 MPa ≤ p ≤ 320 Mpa) and at varying temperatures (140 K ≤ T ≤ 210 K) in the region of structure branching processes between the ⟨22⟩– and ⟨23⟩– commensurate (c) phases. Along the various phase boundaries to incommensurate (ic) phases the dielectric permittivity exhibits maxima, which have been attributed to an accumulation of structure combination branching processes, as predicted by statistical spin models (e.g. the ANNNI- model).- We discuss on a heuristic basis the relations between these accumulation points and the observed dielectric susceptibility.