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The three-dimensional charge density wave (CDW) compound ${\mathrm{Lu}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Si}}_{5}$ undergoes a first-order CDW phase transition at around 200 K. An atypical CDW state is found, that is characterized by an incommensurate CDW with $\mathbf{q}=[0.2499(3),0.4843(4),0.2386(2)]$ at 60 K, and a large orthorhombic-to-triclinic lattice distortion with $\ensuremath{\beta}=91.945{(2)}^{\ensuremath{\circ}}$. We present the modulated crystal structure of the incommensurate CDW state. Structural analysis shows that the CDW resides on the zigzag chains of iridium atoms along $\mathbf{c}$. The structural distortions are completely similar between nonmagnetic ${\mathrm{Lu}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Si}}_{5}$ and previously studied isostructural magnetic ${\mathrm{Er}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Si}}_{5}$ with the small differences explained by the different values of the atomic radii of Lu and Er. Such a similarity is unique to ${R}_{2}{\mathrm{Ir}}_{3}{\mathrm{Si}}_{5}$ $(R=\mathrm{rare}\phantom{\rule{0.28em}{0ex}}\mathrm{earth})$. It differs from, for example, the rare-earth CDW compounds ${R}_{5}{\mathrm{Ir}}_{4}{\mathrm{Si}}_{10}$ for which ${\mathrm{Lu}}_{5}{\mathrm{Ir}}_{4}{\mathrm{Si}}_{10}$ and ${\mathrm{Er}}_{5}{\mathrm{Ir}}_{4}{\mathrm{Si}}_{10}$ possess entirely different CDW states. We argue that the mechanism of CDW formation, thus, is different for ${R}_{2}{\mathrm{Ir}}_{3}{\mathrm{Si}}_{5}$ and ${R}_{5}{\mathrm{Ir}}_{4}{\mathrm{Si}}_{10}$.

YILDIZ, Yasin Gokturk/0000-0002-1391-1888 WOS: 000471732300017 In this paper, we investigated the lattice distortion displacement effects (LDDE) on magnetizations versus temperature in a monolayer nanographene (MLNG) by using effective field theory developed by Kaneyoshi. To obtain the LDDE, we used the twinning angle (theta) of the zigzag edge of the MLNG and obtain the M-T curves for theta = 10 degrees-180 degrees with 10 steps. We find that MLNG has a minimum Curie temperature (T-C(min) = 2.738) for a certain critical twinning angle (theta(CTA) = 130) of the zigzag edge of the MLNG. The T-C increases (T-C > T-C(min)) as the theta increases or decreases from the theta(CTA) = 130 degrees (both theta < theta(CTA) and at theta > theta(CTA) cases). Since the hardness is directly proportional with the T-C, the hardness of the MLNG increases for both theta < theta(CTA) and at theta > theta(CTA) cases. Thus, the MLNG has a resistance against to the any changing in the theta(CTA). Therefore, we suggest that the origin of the hardness in nanographene is theta(CTA). (C) 2019 Elsevier B.V. All rights reserved. Scientific Research Projects Unit of Kirikkale UniversityKirikkale University [2018/060] This study was supported by Scientific Research Projects Unit of Kirikkale University under grant No. 2018/060.

The interplay between interlayer van der Waals interaction and intralayer lattice distortion can lead to structural reconstruction in slightly twisted bilayer graphene (TBG) with the twist angle being smaller than a characteristic angle θ_{c}. Experimentally, the θ_{c} is demonstrated to be very close to the magic angle (θ≈1.08°). Here we address the transition between reconstructed and unreconstructed structures of the TBG across the magic angle by using scanning tunneling microscopy (STM). Our experiment demonstrates that both structures are stable in the TBG around the magic angle. By using a STM tip, we show that the two structures can be changed to each other and a triangular network of chiral one-dimensional states hosted by domain boundaries can be switched on and off. Consequently, the bandwidth of the flat band, which plays a vital role in the emergent strongly correlated states in the magic angle TBG, is tuned. This provides an extra control knob to manipulate the exotic electronic states of the TBG near the magic angle.

Various technical developments extended the potential of angle-resolved photoemission (ARPES) tremendously during the last twenty years. In particular improved momentum, energy and spin resolution as well as the use of photon energies from few eV up to several keV makes ARPES a rather unique tool to investigate the electronic properties of solids and surfaces. With our work we present a generalization of the state of the art description of the photoemission process, the so called one-step model that describes excitation, transport to the surface and escape into the vacuum in a coherent way. In particular, we present a theoretical description of temperature-dependent ARPES with a special emphasis on spin fluctuations. Finite temperature effects are included within the so called alloy analogy model which is based on the coherent potential approximation and this way allows to describe uncorrelated lattice vibrations in combination with spin fluctuations quantitatively on the same level of accuracy. To demonstrate the applicability of our approach a corresponding numerical analysis has been applied to spin- and angle-resolved photoemission of Fe(100) at finite temperatures., Submitted to PRB

We have studied the compression behavior of ${\mathrm{H}}_{2}\text{\ensuremath{-}}\mathrm{He}$ mixtures in comparison with pure ${\mathrm{H}}_{2}$ and He using powder synchrotron x-ray diffraction, and we present the pressure-volume (PV) compression data of ${\mathrm{H}}_{2}\text{\ensuremath{-}}\mathrm{He}$ mixtures to 160 GPa. The results indicate that both ${\mathrm{H}}_{2}$ and He in ${\mathrm{H}}_{2}\text{\ensuremath{-}}\mathrm{He}$ mixtures remain in hcp to the maximum pressure studied, yet they develop a substantial level of lattice distortion in the (100) plane, most profound in He-rich solids and below 66 GPa. The measured PV data also indicate the softening of an He (or ${\mathrm{H}}_{2}$) -rich lattice upon increasing the level of the guest ${\mathrm{H}}_{2}$ (or He) concentration. We suggest that the observed softening and lattice distortion are due to a substitutional incorporation of ${\mathrm{H}}_{2}$ (guest) molecules into the basal plane of the hcp-He (host) lattice, and thereby reflect the miscibility between ${\mathrm{H}}_{2}$ and He in ${\mathrm{H}}_{2}\text{\ensuremath{-}}\mathrm{He}$ mixtures. Interestingly, solid He exhibits a lesser degree of preferred orientation in ${\mathrm{H}}_{2}\text{\ensuremath{-}}\mathrm{He}$ mixtures than in pure He, likely due to the presence of solid ${\mathrm{H}}_{2}$ disturbing the crystalline ordering of He-rich solids. Finally, the present PV compression data of ${\mathrm{H}}_{2}$-rich and He-rich solids to 160 GPa deviate from those of pure ${\mathrm{H}}_{2}$ and pure He above \ensuremath{\sim}70 and 45 GPa respectively, providing new constraints for the development of the equation of state for ${\mathrm{H}}_{2}\text{\ensuremath{-}}\mathrm{He}$ mixtures for planetary models.

In crystals with time-reversal symmetry but without inversion symmetry, the phonon angular momentum can be generated by the temperature gradient, and it is called phonon thermal Edelstein effect. On the other hand, when both symmetries are broken and their product is conserved, the phonon angular momentum for any phonon modes at any wave vectors vanishes, and the phonon thermal Edelstein effect does not occur. In this paper, we propose another mechanism of generation of the phonon angular momentum. We show that in such crystals the electric field generates the phonon angular momentum, via the lattice distortion due to the electric field. This effect is in the same symmetry class with the magnetoelectric effect, and we call this effect phonon rotoelectric effect. We discuss the temperature dependence of the phonon angular momentum generated by the temperature gradient and by the electric field in the high- and the low-temperature limits., Comment: 8 pages, 3 figures

The optimal superconductivity (${T}_{c}\ensuremath{\approx}30$ K) in ${\mathrm{BaFe}}_{2}{({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x})}_{2}$ can be reached when the coupled antiferromagnetic (AF) order $({T}_{N})$ and orthorhombic lattice distortion $({T}_{s})$ are suppressed to zero temperature with increasing of P concentration or hydrostatic pressure. Here we use transport and neutron scattering to study the $c$-axis pressure effects on electronic phases in underdoped ${\mathrm{BaFe}}_{2}{({\mathrm{As}}_{0.72}{\mathrm{P}}_{0.28})}_{2}$, which has ${T}_{N}={T}_{s}\ensuremath{\approx}40$ K and ${T}_{c}\ensuremath{\approx}28$ K at zero pressure. With increasing $c$-axis pressure, ${T}_{N}$ and ${T}_{s}$ are slightly enhanced around ${P}_{c}\ensuremath{\sim}20$ MPa. Upon further increasing pressure, AF order is gradually suppressed to zero, while ${T}_{c}$ is enhanced to 30 K. Our results reveal the importance of magnetoelastic couplings in ${\mathrm{BaFe}}_{2}{({\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x})}_{2}$, suggesting that the $c$-axis pressure can be used as a tuning parameter to manipulate the electronic phases in iron pnictides.

The authors identify the modalities enabling an electronically induced distortion, which is identical across the board of 3d elements showing electronic degenerate states in the high symmetry cubic cell. This constitutes the fingerprint of a Jahn-Teller effect. Materials without electronic instabilities such as LaMnO3 display an alternate lattice distortion simply resulting from lattice mode couplings with the sterically induced distortions.

Nematic states are characterized by rotational symmetry breaking without translational ordering. Recently, nematic superconductivity, in which the superconducting gap spontaneously lifts the rotational symmetry of the lattice, has been discovered. In nematic superconductivity, multiple superconducting domains with different nematic orientations can exist, and these domains can be controlled by a conjugate external stimulus. Domain engineering is quite common in magnets but has not been achieved in superconductors. Here, we report control of the nematic superconductivity and their domains of SrxBi2Se3, through externally-applied uniaxial stress. The suppression of subdomains indicates that it is the Δ4y state that is most favoured under compression along the basal Bi-Bi bonds. This fact allows us to determine the coupling parameter between the nematicity and lattice distortion. These results provide an inevitable step towards microscopic understanding and future utilization of the unique topological nematic superconductivity., Nematic superconductors, exhibiting rotational-symmetry breaking, can form domains; a counterpart of common magnetic domains. Here, the authors report control of nematic superconductivity and their domains of SrxBi2Se3 by uniaxial deformation, a step toward superconductive domain engineering.

The $S=1/2$ twisted three-leg spin tube with the lattice distortion from the regular triangle to the isosceles one is investigated using the numerical diagonalization of finite-size clusters and the phenomenological renormalization group analysis. It is found that the quantum phase transition occurs from the spin gap phase to another one with respect to this lattice distortion., to be published in JPS Conference Proceedings

We investigate the states of triplet pairing in a candidate nematic superconductor versus typical material parameters, using the mean field theory for two- and three-dimensional tight-binding models with local triplet pairing in the $E_u$ representation. In the two-dimensional model, the system favors the fully gapped chiral state for weaker warping or lower filling level, while a nodal and nematic $\Delta_{4x}$ state is favorable for stronger warping or higher filling, with the $d$-vector aligned along the principle axis. In the presence of lattice distortion, relative elongation along one of the principle axes, ${\bf a}$, tends to rotate the nematic $d$-vector orthogonal to ${\bf a}$, resulting in the nematic $\Delta_{4y}$ state at sufficient elongation. Three-dimensionality is seen to suppress the chiral state in favor of the nematic ones. Our results may explain the variety in the probed direction of the $d$-vector in existing experiments., Comment: 5 pages, 4 color figures

Phase transformation between bcc and fcc in Fe–Ni nanoparticle has been studied by using molecular dynamics simulation with an embedded atom method. The transformation has been explored by designing the nanoparticles with different initial structures, sizes and elemental distributions at various Ni concentrations. The results show that the structural transformation is strongly related to the Ni content and elemental distribution. Initial fcc structure transforms to bcc for a lower Ni content and bcc transforms to fcc for a higher Ni content. The transformation is accompanied with a sharp reduction in energy even for the nanoparticle with a large size. Furthermore, lattice distortion first occurs before the transformation. The transformation from fcc to bcc is occurred by elongating fcc (100) to bcc (110) and that from bcc to fcc by compressing bcc (110) to fcc (100). The reason is that the nanoparticle has a low energy state for bcc structure with a lower Ni content and also for fcc structure with a higher Ni content. The coexistence of bcc and fcc phases appears with the change of elemental distribution.

We investigate the dynamics of an active particle in two-dimensional spherical crystals, which provide an ideal environment to illustrate the interplay of active particle and crystallographic defects. A moving active particle is observed to be surrounded by localized topological defects, becoming a dressed active particle. Such a physical picture characterizes both the lattice distortion around the moving particle and the healing of the distorted lattice in its trajectory. We find that the dynamical behaviors of an active particle in both random and ballistic motions uniformly conform to this featured scenario, whether the particle is initially a defect or not. We further observe that the defect pattern around a dressed ballistic active particle randomly oscillates between two well-defined wing-like defect motifs regardless of its speed. The established physical picture of dressed active particles in this work partially deciphers the complexity of the intriguing nonequilibrium behaviors in active crystals, and opens the promising possibility of introducing the activity to engineer defects, which has strong connections with the design of materials., Comment: 8 pages, 5 figures

The spin-$\frac{1}{2}$ kagome antiferromagnet is an archetypal frustrated system predicted to host a variety of exotic magnetic states. We show using neutron scattering measurements that deuterated vesignieite ${\mathrm{BaCu}}_{3}{\mathrm{V}}_{2}{\mathrm{O}}_{8}(\mathrm{OD}{)}_{2}$, a fully stoichiometric $S=1/2$ kagome magnet with $l1%$ lattice distortion, orders magnetically at ${T}_{N}=9\text{ }\text{ }\mathrm{K}$ into a multi-$\mathbf{k}$ coplanar variant of the predicted triple-$\mathbf{k}$ octahedral structure. We find that this structure is stabilized by a dominant antiferromagnetic third-neighbor exchange ${J}_{3}$ with minor first- or second-neighbor exchanges. The spin-wave spectrum is well described by a ${J}_{3}$-only model including a tiny symmetric exchange anisotropy.

Topological crystalline insulators (TCI) are insulating electronic phases of matter with nontrivial topology originating from crystalline symmetries. Recent theoretical advances have provided powerful guidelines to search for TCIs in real materials. Using density functional theory, we identify a class of new TCI states in the tetragonal lattice of the Ca$_2$As material family. On both top and side surfaces, we observe topological surface states protected independently by rotational and mirror symmetries. We show that a particular lattice distortion can single out the newly proposed topological protection by the rotational symmetry. As a result, the Dirac points of the topological surface states are moved to generic locations in momentum space away from any high symmetry lines. Such topological surface states have not been seen before. Moreover, the other family members, including Ca$_2$Sb, Ca$_2$Bi and Sr$_2$Sb, feature different topological surface states due to their distinct topological invariants. We thus further propose topological phase transitions in the pseudo-binary systems such as (Ca$_{1-x}$Sr$_x$)$_2$As and Ca$_2$As$_x$Sb$_{1-x}$. Our work reveals rich and exotic TCI physics across the Ca$_2$As family of materials, and suggests the feasibility of materials database search methods to discover new TCIs., 5 pages, 4 figures

The effects of using a traction-free (plane-stress) assumption to obtain the full distortion tensor from high-resolution EBSD measurements are analyzed. Equations are derived which bound the traction-free error arising from angular misorientation of the sample surface; the error in recovered distortion is shown to be quadratic with respect to that misorientation, and the maximum 'safe' angular misorientation is shown to be 2.7 degrees. The effects of localized stress fields on the traction-free assumption are then examined by a numerical case study, which uses the Boussinesq formalism to model stress fields near a free surface. Except in cases where localized stress field sources occur very close to sample points, the traction-free assumption appears to be admirably robust.

Based on the density functional theory within the generalized-gradient approximation, the electronic structure and magnetic properties of CsAX2 (A = Mg, Ca, Sr, and Ba; X = N, and O) full- Heusler compounds have been studied. The spin-polarized calculations show that CsAN2/CsAO2 are newly designed d 0 half-metals with an integer magnetic moment of 3 μ B /1 μ B in their ferrimagnetic ground states. Importantly, the strong spin-polarization of the p states of N and O atoms is found to be the origin of the magnetic moment and the half-metallicity. Also, the half-metallic gaps of CsAX2 are quite large, thus, the half-metallicity is robust against lattice distortion.

Synchrotron x-ray studies of single-layer $\mathrm{TiS}{\mathrm{e}}_{2}$ reveal displacements of the Ti and Se atoms as a function of temperature. The measurements, with a high sensitivity of 0.001 \AA{}, show a (2 \ifmmode\times\else\texttimes\fi{} 2) charge-density-wave (CDW) structure at temperatures below a critical temperature of ${T}_{\mathrm{C}1}=233\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The temperature dependence follows a BCS-like second-order mean-field behavior. A five-layer $\mathrm{TiS}{\mathrm{e}}_{2}$ film also exhibits a CDW transition of the same character but at a lower transition temperature of ${T}_{\mathrm{C}5}=204\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is the same as that for bulk $\mathrm{TiS}{\mathrm{e}}_{2}$. The results demonstrate that lattice distortion is an integral part of the CDW transition that must also involve renormalization of the electronic structure.

We propose a tight-binding one-band model for the iron-based superconductors. The Hamiltonian incorporates Jahn-tellar (JT) distortion in the degenerate dXZ and dYZ orbitals and superconductivity in both the orbitals with s± pairing symmetry. The electron Green’s functions are calculated by using Zubarev’s Green’s function technique from which the temperature dependent gap equations for superconductivity and JT distortion are calculated from the corresponding electron Green’s functions. The gap equations are computed numerically and self-consistently taking 100×100 grid points for the electron momentum. Interplay of orbital ordering and superconductivity is investigated by varying different model parameters of the system.

Iron telluride (FeTe) is known to display bicollinear magnetic order at low temperatures together with a monoclinic lattice distortion. Because the bicollinear order can involve two different wave vectors $(\ensuremath{\pi}/2,\ensuremath{\pi}/2)$ and $(\ensuremath{\pi}/2,\ensuremath{-}\ensuremath{\pi}/2)$, symmetry considerations allow for the possible stabilization of a nematic state with short-range bicollinear order coupled to monoclinic lattice distortions at a ${T}_{S}$ higher than the temperature ${T}_{N}$ where long-range bicollinear order fully develops. As a concrete example, the three-orbital spin-fermion model for iron telluride is studied with an additional coupling ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\lambda}}}_{12}$ between the monoclinic lattice strain and an orbital-nematic order parameter with ${B}_{2g}$ symmetry. Monte Carlo simulations show that with increasing ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\lambda}}}_{12}$ the first-order transition characteristic of FeTe splits and bicollinear nematicity is stabilized in a (narrow) temperature range. In this new regime, the lattice is monoclinically distorted and short-range spin and orbital order breaks rotational invariance. A discussion of possible realizations of this exotic state is provided.

Antiferromagnetic (AFM) materials, with robust rigidity to magnetic field perturbations and ultrafast spin dynamics, show great advantages in information storage and have developed into a fast-emerging field of AFM spintronics. However, a direct characterization of spin alignments in AFM films has been challenging, and their manipulation by lattice distortion and magnetic proximity is inevitably accompanied by ``ferromagnetic'' features within the AFM matrix. Here we resolve the $G$-type AFM structure of $\mathrm{SrCo}{\mathrm{O}}_{2.5}$ and find that the interfacial AFM structure could be modulated intrinsically from in plane to out of plane with a canted angle of ${60}^{\ensuremath{\circ}}$ by the charge transfer and orbital reconstruction in $\mathrm{SrCo}{\mathrm{O}}_{2.5}/\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ heterostructures both experimentally and theoretically. Such an interfacial AFM reconfiguration caused by electronic reconstruction does not cause the ferromagnetic feature and changes the magnetization switching process of $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ from in plane to perpendicular to the plane, in turn. Our study not only reveals the coupling between charge, orbital, and AFM structure, but also provides a unique approach to manipulating AFM structure.

High-resolution (or “cross-correlation”) electron backscatter diffraction analysis (HR-EBSD) utilizes cross-correlation techniques to determine relative orientation and distortion of an experimental electron backscatter diffraction pattern with respect to a reference pattern. The integrity of absolute strain and tetragonality measurements of a standard Si/SiGe material have previously been analyzed using reference patterns produced by kinematical simulation. Although the results were promising, the noise levels were significantly higher for kinematically produced patterns, compared with real patterns taken from the Si region of the sample. This paper applies HR-EBSD techniques to analyze lattice distortion in an Si/SiGe sample, using recently developed dynamically simulated patterns. The results are compared with those from experimental and kinematically simulated patterns. Dynamical patterns provide significantly more precision than kinematical patterns. Dynamical patterns also provide better estimates of tetragonality at low levels of distortion relative to the reference pattern; kinematical patterns can perform better at large values of relative tetragonality due to the ability to rapidly generate patterns relating to a distorted lattice. A library of dynamically generated patterns with different lattice parameters might be used to achieve a similar advantage. The convergence of the cross-correlation approach is also assessed for the different reference pattern types.

What characterises a solid is its way to respond to external stresses. Ordered solids, such crystals, display an elastic regime followed by a plastic one, both well understood microscopically in terms of lattice distortion and dislocations. For amorphous solids the situation is instead less clear, and the microscopic understanding of the response to deformation and stress is a very active research topic. Several studies have revealed that even in the elastic regime the response is very jerky at low temperature, resembling very much the one of disordered magnetic materials. Here we show that in a very large class of amorphous solids this behaviour emerges by decreasing the temperature as a phase transition where standard elastic behaviour breaks down. At the transition all non-linear elastic modulii diverge and standard elasticity theory does not hold anymore. Below the transition the response to deformation becomes history and time-dependent., Comment: 3 figures

Perovskite-type manganites exhibit various interesting phenomena arising from complex interplay among spin, charge, orbital, and lattice degrees of freedom. One such example is the keen competition between phases with different spin/charge/orbital orders. Keen competition between antiferromagnetic metal and orbital-ordered insulator is found in the slightly electron-doped regime near Mn4+ state which is stabilized by the high oxygen-pressure condition. Another one is the emergence of ferroelectricity either induced by the magnetic ordering or independently of the magnetic ordering. As the respective examples, perovskite-type YMnO3 and Sr1−xBaxMnO3 are discussed. In the YMnO3, the ferroelectric lattice distortion associated with the E-type spin order is observed for the first time. Displacement-type ferroelectricity with off-center magnetic ions is discovered for Sr0.5Ba0.5MnO3, which shows both large polarization value and strong coupling between ferroelectricity and magnetism.

Ab initio calculations show that the Dzyaloshinskii–Moriya interaction(DMI)and net magnetization per unit cell in BiFeO 3 are reduced when U is increasing from 0 to 2.9 eV, and independent of J . Interestingly, the DMI is even destroyed as U exceeds a critical value of 2.9 eV. We propose a simple model to explain this phenomenon and present the nature of the rotation of the magnetization corresponding to altered antiferrodistortive distortions under DMI in BiFeO 3 .

Researchers have theoretically investigated spin–current generation from lattice distortion dynamics in metals in the presence of spin–orbit coupling and discovered that a charge-to-spin current co...

The magnetoelastic instability in nanostructured ring-shaped Ising-like spin 1/2 model has been investigated by using the exact diagonalization method. It is found that there exists two critical anisotropy parameters Δ c ( 1 ) and Δ c ( 2 ) ( Δ c ( 1 ) Δ c ( 2 ) ) in the phase diagram. As the anisotropy parameter Δ > Δ c ( 2 ) , the magnetoelastic transition from dimerized phase to uniform phase is a first order transition with an increase of the lattice spring constant. While for Δ Δ c ( 2 ) , the transition is continuous. Another critical value Δ c ( 1 ) divides the different lattice distortion behavior as the anisotropy is strengthened.

The effect of electric field, applied on systems in the nanoscale regime has attracted a lot of research in recent times. We highlight some of the recent results in the field of single molecule electronics and then move on to focus on some of our own results in this area. We have first shown how important it is to obtain the spatial profile of the external bias potential across the system, and how this would change in the presence of electron-electron interactions. We have also studied different kinds of insulators in the presence of the spatially varying external bias, and have explicitly shown that a two sublattice structure, caused either by a lattice distortion, or by the presence of substituents with strong dipolar nature, can result in negative differential resistance (NDR) in the transport characteristics. We also find this to be true in case of correlated insulators. Additionally, we have shown clear NDR behavior in a correlated double quantum dot by tuning the electron-electron interaction strength in the system., Comment: 53 pages, 15 figures, accepted for publication in Journal of Physical Chemistry C as a feature article

Recent observation by Xiao et al. [L. Xiao, B. Tollberg, X. Hu, L. Wang, J. Chem. Phys. 124 (2006) 114309.] suggested substantial lattice distortion in linear gold nanowires of finite chain length up to 2 nm and remarkably they have predicted the disappearance of Peierls distortion in the infinite limit. In the present study we adopt the method of constrained relaxation search to find out the existence of true dimerization in these systems. The role of scalar relativistic effect on lattice dimerization in these nanowires has been investigated using first-principle density functional calculations at zero-order regular approximation level. It is well known that real lattice dimerization in gold nanowires are perceptible just immediately before the breakage of the wires. While exploring the role of scalar relativity, it has been observed that scalar relativity decreases the dimerization amplitude of each of the linear metastable gold nanowires compared to its non-relativistic counterpart. Another interesting observation is the greater stabilization of Au 5 and Au 7 in constrained relaxation search compared to that of the unconstrained relaxation scheme in relativistic method. The dramatic even–odd oscillations are observed in the values of the dimerization amplitudes, HOMO–LUMO energy gap, and second differences of wire energies. The results are very stable and consistent up to Au 9 .

Katsuaki Kodama1, Naoki Igawa1, Shin-ichi Shamoto1, Kazutaka Ikeda2, Hidetoshi Ohshita2, Naokatsu Kaneko2, Toshiya Otomo2, Kentaro Suzuya3, Akinori Hoshikawa4, and Toru Ishigaki4 1Quantum Beam Science Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan 2Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan 3J-PARC Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan 4 Frontier Research Center for Applied Atomic Sciences, Ibaraki University, Tokai, Ibaraki 319-1106, Japan

Insulator-to-metal (MI) phase transition in vanadium dioxide (VO2) thin films with controlled lattice distortion was investigated by thermopower measurements. VO2 epitaxial films with different crystallographic orientations, grown on (0001) alpha-Al2O3, (11-20) alpha-Al2O3, and (001) TiO2 substrates, showed significant decrease of absolute value of Seebeck coefficient (S) from ~200 to 23 microV K-1, along with a sharp drop in electrical resistivity (rho), due to the transition from an insulator to a metal. The MI transition temperatures observed both in rho (Trho) and S (TS) for the VO2 films systematically decrease with lattice shrinkage in the pseudo-rutile structure along c-axis, accompanying a broadening of the MI transition temperature width. Moreover, the onset TS, where the insulating phase starts to become metallic, is much lower than onset Trho. This difference is attributed to the sensitivity of S for the detection of hidden metallic domains in the majority insulating phase, which cannot be detected in rho-measurements. Consequently, S-measurements provide a straightforward and excellent approach for a deeper understanding of the MI transition process in VO2., To be published in Physical Review B

The A-phase of La 1 - x Ca x MnO 3 , stable for small x, is studied using a mean-field slave–boson formulation for e g -electrons in two orbitals with excluded multiple occupancy. The classical t 2 g -spins are ordered ferromagnetically (F) in the xy planes, which are antiferromagnetically (AF) coupled along the z -direction. All 3d-electrons are coupled F via Hund's rule. The F double-exchange, competing with the AF superexchange, can lead to a lattice distortion via Jahn–Teller coupling of the e g -electrons to the lattice degrees of freedom. Conditions for staggered orbital long-range order are discussed.

Depending on the lattice distortion, the dielectric properties of PZT and PT ceramics are studied from a structural phase transition point of view. The study involves a more profound physical process than the study of the relation between the dielectric properties and composition x. Two equations connecting the dielectric properties and the lattice distortions are established in the tetragonal and rhombohedral phase regions. In particular, the relation between the dielectric properties and the lattice distortion is investigated in the phase coexistence region of PZT ceramics using a phase statistical distribution model, and we determine the fitting value of the volume fraction of the tetragonal phase VTto composition x in the equation. All the fitting results indicate that our results are very consistent with the related experimental data for PZT and PT ceramics.PACS Nos.: 64.70.–p, 77.22.–d, 77.84.–s

A mechanism of the magnetization plateau observed in the high-field magnetizatiom measurement of the organic S = 1 spin ladder system BIP-TENO is investigated. Assuming possible lattice distortion, we propose a possible scenario of the plateau formation on the basis of the bond-alternating ladder. Some unsolved problems are also discussed.

In the present paper, we continue our investigation on the antiferromagnetic origin of the charge order observed in the half-doped manganese. By introducing a Su–Schrieffer–Heeger (SSH) type of perturbation interaction to the double-exchange Hamiltonian, we calculate again its ground-state phase diagram at filling x = 0.5 by the unrestricted real-space Hartree–Fock approximation method. We find that, as the SSH electron-phonon interaction increases, the charge order parameter decreases to zero rapidly but the CE-type antiferromagnetic order becomes more stable. In other words, the charge order is much more fragile than the CE-type or the Neel-type antiferromagnetic orders under the electron-phonon perturbation. These results support the proposed theory in the recent publications that the charge order in these systems is induced by the antiferromagnetic correlations.

Les modifications structurales de l'eau confinee dans le gel de silice en poudre en fonction du taux d'hydratation sont etudiees par calorimetrie differentielle a balayage et par diffusion des rayons X. L'analyse thermique par DSC a permis de montrer que pour les faibles taux d'hydratation (≤50%) l'eau est completement confinee dans les pores alors que pour les forts taux d'hydratation (>50%) l'eau se trouve aussi bien dans les pores qu'autour des grains de la silice. L'apparition de la glace cubique lors du refroidissement de l'eau confinee dans le gel de silice prouve que l'interaction eau-silice ne peut etre negligee lors de la determination du signal de l'eau. L'extraction du signal, diffuse uniquement par l'eau confinee, a partir des intensites diffusees par le gel de silice a deux taux d'hydratation distincts, revele dans les fonctions de correlation de paire un pic a 3.7A qui ne peut etre attribue qu'a une correlation oxygene-oxygene inexistante dans l'eau massique et resultant de la deformation du reseau tetraedrique des molecules d'eau sous l'effet du confinement et de la proximite d'une surface.

We investigated the origin of the energy splitting observed in the resonant x-ray scattering (RXS) in manganites. Using thin film samples with controlled lattice parameters and orbital states at a fixed orbital filling, we estimated that the contribution of the interatomic Coulomb interaction relative to the Jahn-Teller mechanism is insignificant and at most 0.27. This indicates that RXS probes mainly Jahn-Teller distortion in manganites.

The phonon dispersion relations in the two-dimensional Peierls phase (Peierls insulator) are studied numerically, focusing on a two-dimensional SSH (Su–Shrieffer–Heeger) model having a half-filled ...

In order to get a realistic view of the origin of the antiferroorbital (AFO) order in LaMnO 3 , MnF 3 and KCuF 3 , both the cooperative Jahn–Teller effect (CJTE) and the orbital-dependent exchange interaction (OEI) are taken into account for the d γ electron–lattice system in compounds with the perovskite-type structure. The phonon-mediated orbital interaction in the CJTE and the d γ electron-mediated one in the OEI are treated by use of the mean-field approximation with preserving the symmetry of the crystal, and are joined to give an effective Hamiltonian. It is stressed that the CJTE favors any of the FO and AFO orders, while the OEI does only the AFO order. The present theory is consistent with the observed fact that the AFO order is characterized by an arrangement of alternate mixed orbitals of d x 2 - y 2 and d 3 z 2 - r 2 , the AFO phase survives even at sufficiently high temperatures in the paramagnetic phase, and the AFO order is always accompanied by the ferrodistortion and the antiferrodistortion.

L'etude de la liaison chimique et son evolution sous conditions extremes de temperature et de pression est un challenge important de la physique de la matiere condensee. Nous etudions les systemes (elements de groupes V, VI et VII)qui presentent une brisure spontanee de symetrie, la distorsion de Peierls, dans l'etat cristallin et nous etudions si cette distorsion persiste dans l'etat liquide. Nous montrons que le terme repulsif joue un role important. Enfin, dans les composees IV-VI, GeSe et GeTe, nous montrons l'existence d'une transition de phase reentrante.

The current-driven dynamics of disordered Josephson junction networks (JJN) in a magnetic field is studied numerically. It is found that the depinning of vortices is caused by the elastic deformation of vortex lattices or by the excitation of vortex-antivortex pairs, depending on the strength of the disorder. The vortex depinning mechanisms and the driven vortex dynamics after depinning are discussed in detail.

The ground state phase transition in the distorted S = 1 kagom´e Heisenberg antiferromagnet (KHAF) is studied by means of the perturbational calculation and numerical exact diagonalization method. For strong √3×√3 lattice distortion, the hexgonal singlet solid (HSS) ground state of the uniform KHAF is destroyed and a new singlet state, the large HSS (LHSS) state, which is globally different from the HSS state is realized. The quantum phase transition between these two singlet states is analogous to the Haldane-dimer transition in the S = 1 antiferromagnetic Heisenberg chain. The presence of this transition is fully consistent with the HSS picture of the ground state of the uniform S = 1 KHAF and supports the validity of this picture., Copyright(c)2002 The Physical Society of Japan