Incommensurate crystals

An attempt has first been made at systemitizing and analyzing the currently available data on almost all the known incommensurate-dielectrics and their physical properties with the aim of elucidating the features of the crystals behaviour in incommensurate phase and some of their structure and dynamic characteristics. The analysis performed allowed one to reveal some regularitic characteristic of incommensurate crystals only.     

Incommensurate SDW in cuprates

The incommensurate antiferromagnetism (AF) in metallic underdoped cuprates is interpreted with several complementary approaches, unified by a consistent disambiguation of the copper and oxygen degrees of freedom. Collinear (with respect to the copper lattice) peaks in the neutron response at low frequency are due to the oxygen-dominated arcs, while the diagonal peaks at high frequency or low doping come from the copper-dominated vH points. The latter switch from collinear to diagonal as the frequency increases. The direct O–O overlap induces an AF which is weaker and thermodynamically preferred to the AF of strongly localized copper sites, and which coexists with Fermi arcs. A theoretical understanding of the strongly k-dependent$\mathbf{k}\mathrm{-}\mathrm{dependent}$ AF gap observed in ARPES and STM measurements is proposed.     

Incommensurate diffusion in confined systems

Molecular simulations corroborate the existence of the disputed window effect, i.e., an increase in diffusion rate by orders of magnitude when the alkane chain length increases so that the shape of the alkane is no longer commensurate with that of a zeolite cage. This window effect is shown to be characteristic for molecular sieves with pore openings that approach the diameter of the adsorbate. Furthermore, the physical compatibility between the adsorbate and the adsorbent has a direct effect on the heat of adsorption, the Henry coefficients, the activation energy, and the frequency factors.     

Optical properties of KCP at room temperature

A fit of the reflectivity data of KCP is given using the expressions of the small polaron theory. It is further shown that small polaron theory gives the same optical properties as either a two site model with a deformation coordinate and Franck Condon transitions between the sites or a Peierls subband model for nearly free electrons.     

Incommensurate magnetic order in TbTe3

We report a neutron diffraction study of the magnetic phase transitions in the charge-density wave (CDW) TbTe(3) compound. We discover that in the paramagnetic phase there are strong 2D-like magnetic correlations, consistent with the pronounced anisotropy of the chemical structure. A long-range incommensurate magnetic order emerges in TbTe(3) at T(mag1) = 5.78 K as a result of continuous phase transitions. We observe that near the temperature T(mag1) the magnetic Bragg peaks appear around the position (0, 0, 0.24) (or its rational multiples), that is fairly close to the propagation vector (0,0,0.29) associated with the CDW phase transition in TbTe(3). This suggests that correlations leading to the long-range magnetic order in TbTe(3) are linked to the modulations that occur in the CDW state.     

Existence of double-pinning force maxima for a single pinning mechanism

Critical current measurements on a cold rolled Pb₆₀In₄₀ foil show two pinning maxima, one close to H_c2 in the neighborhood of h = 0.95, and the other at low fields around h = 0.15. The high field pinning force maximum shifts to lower fields, and the low field maximum shifts to higher fields with increasing pinning strength. The data is explained in terms of Kramer's pinning force model incorporating Brandt's expression for C>₆₆.     

Incommensurate valence modulation in high-Tc cuprates

An incommensurate modulation has been observed in a Cu-rich La2CuO4.003 crystal. It is shown that the modulation results from a periodically distributed holes lying on the CuO2 planes, and that the hole modulation may be regarded to be a kind of valence modulation. It is shown that appreciable valence modulation contrast may be generated by the mechanisms of hole scattering alone when the period of the modulation is of the order of 2 nm.     

Incommensurate magnetic structures in rhombohedral Heisenberg antiferromagnet

The phase diagram of the ground state has been calculated for a rhombohedral antiferromagnet of the R3m symmetry with frustrated exchange in the base plane and competition of exchanges between the nearest and next-nearest planes. The diagram contains phases of collinear antiferromagnetic ordering of various types separated by five incommensurate magnetic states of the helicoidal type, differing in the ordering type and in the direction of the modulation vector. The commensurate and incommensurate phases converge at multi-critical points lying on a line corresponding to an antiferromagnet with an undistorted simple cubic lattice.     

Incommensurate Antiferromagnetism in the Extended t-J Model

The effect of the extra second neighbor hopping t‘ on the incommensurate spin correlation in the t-J modelin the underdoped regime is studied within the fermion-spin theory. It is shown that although the extra second neighborhopping t‘ is systematically accompanied with the increasing of the weight of the incommensurate peaks in the dynamicalspin structure factor, for the physical reasonable small value of t‘ the qualitative behavior of the incommensurate spincorrelation in the t-t‘-J model is the same as in the case of t-J model.     

Incommensurate spin density waves in iron aluminides

Neutron diffraction in Fe(Al) reveals incommensurate spin density waves (SDWs) in alloys known to be spin glasses. The wave vectors for crystals of Fe(34Al), Fe(40Al), and Fe(43Al) show n varying from 11 to 6 for q-->=2pi(h+/-1/n,k+/-1/n,l+/-1/n)/a(0), where (h,k,l) and a(0) characterize the parent bcc lattice of the CsCl structure. The magnetic reflections are present far above the spin-glass freezing temperatures. These SDWs keep the spins on nearest-neighbor Fe atoms close to parallel, in contrast with SDWs in Cr, which keep nearest-neighbor spins close to antiparallel.     

Anisotropic pinning in Nb thin films with triangular pinning arrays

A superconducting thin film with regular triangular arrays has been fabricated to explore the anisotropic vortex pinning behavior. We found that the critical currents for these films depend strongly on the current directions. Some interesting temperature-dependent phenomena are observed and discussed. We also made molecular dynamic simulations to study the dynamics of the vortex motion. The simulation results confirm the anisotropic features we found in the experiments.     

Incommensurate modulated magnetic structure in orthorhombic EuPdSb

Orthorhombic EuPdSb is known to undergo two magnetic transitions, at 12 K and at T _N≃ 18 K, and in phase III (T < 12 K), single crystal magnetisation data have shown that the spin structure is collinear antiferromagnetic, with magnetic moments along the crystal a axis. From a ¹⁵¹Eu M?ssbauer absorption study, we show that, at any temperature within phase III, all the moments have equal sizes, and that in phase II (12 K< T <18 K) the magnetic structure is modulated and incommensurate with the lattice spacings. The modulation is close to a pure sine-wave just below T _N = 18 K, and it squares up as temperature is lowered. We measured the thermal variations of the first and third harmonics of the moment modulation, and we could determine the first and third harmonics of the exchange coupling. We furthermore show that the antiferromagnetic-incommensurate transition at 12 K is strongly first order, with a hysteresis of 0.05 K, and that the incommensurate-paramagnetic transition at 18 K is weakly first order. Finally, we present an explanation of the spin-flop transition observed in the single crystal magnetisation data in phase III when || in terms of an anisotropic molecular field tensor. Received 17 January 2001 and Received in final form 20 March 2001     

Incommensurate structure in the lattice-gas ANNNI model

The phase diagram of the two-dimensional lattice-gas ANNNI model is investigated using the cluster transfer-matrix method. The numerical calculations have shown commensurate, disordered, and floating incommensurate phases. The properties of the incommensurate phase are studied in detail and the position of the Lifshitz point is discussed.     

Incommensurate structures in a two-subsystem partially frustrated ferrimagnet

A new incommensurate magnetic structure with a locally triangular orientation of spins has been proposed for a two-subsystem magnet with frustrated intersystem exchange and competition between exchanges in one of the subsystems. When the temperature is lowered, this structure appears from the antiferromagnetic state after a first-order phase transition. It transfers to the Yafet-Kittel triangular structure when the threshold conditions for the exchange interactions are fulfilled. An increase in the length of the frustrated exchange bonds leads to the appearance of an incommensurate phase with the local antiferromagnetic orientation of the sublattices in each subsystem between the commensurate antiferromagnetic and Yafet-Kittel phases.     

Local impurity phase pinning and pinning force in charge density waves

Starting from the static Fukuyama-Lee-Rice equation for a three-dimensional incommensurate charge density wave (CDW) in quasi one-dimensional conductors a solvable model for local phase pinning by impurities is defined and studied. We find that average CDW energy and average pinning force show critical behaviour with respect to the pinning parameter h. Specifically the pinning force exhibits a threshold at h=1 with exponent . Our model exemplifies a general concept of local impurity pinning in which the force exerted by the impurity on the periodic CDW structure becomes multivalued and metastable states appear beyond a threshold. It is found that local impurity pinning becomes less effective at low temperatures and may eventually cease completely. These results are independent of spatial dimensionality as expected for local impurity pinning. Comparison with Larkin's model is also made. Received 8 July 1998