Rotational Phase Transition Research Articles

Rotational phase transitions in antifluorite-type osmate and iridate compounds

We present temperature-dependent single-crystal diffraction results on seven antifluorite-type A2MeX6 compounds with Me = Os or Ir: K2OsCl6, A 2OsBr6 with A = K, Rb, Cs and NH4, and K2IrX 6 with X = Cl and Br. The structural transitions in this family arise from MeX 6 octahedron rotations that generate a rich variety of symmetries depending on the rotation axis and stacking schemes. In order to search for local distortions in the high-symmetry phase we perform refinements of anharmonic atomic displacement parameters with comprehensive data sets. Even at temperatures close to the onset of structural distortions, these refinements only yield a small improvement indicating only small anharmonic effects. The phase transitions in these antifluorites are essentially of displacive character. However, some harmonic displacement parameters are very large reflecting soft phonon modes with the softening covering large parts of the Brillouin zone. The occurrence of the rotational transitions in the antifluorite-type family can be remarkably well analyzed in terms of a tolerance factor of ionic radii.

Temperature‐Reliant Dynamic Properties and Elasto‐Plastic to Plastic Crystal (Rotator) Phase Transition in a Metal Oxyacid Salt

AbstractAlthough, dynamic crystals are attractive for use in many technologies, molecular level mechanisms of various solid‐state dynamic processes and their interdependence, remain poorly understood. Here, we report a rare example of a dynamic crystal (1), involving a heavy transition metal, rhenium, with an initial two‐face elasticity (within ≈1 % strain), followed by elasto‐plastic deformation, at room temperature. Further, these crystals transform to a rotator (plastic) crystal phase at ≈105 °C, displaying exceptional malleability. Qualitative and quantitative mechanical tests, X‐ray diffraction, μ‐Raman and polarized light microscopy experiments reveal that the elasto‐plastic deformation involves both partial molecular rotations and slip, while malleability in the rotator phase is facilitated by reorientational motions and increased symmetry (slip planes). Our work, connecting the plastically bendable (1D or 2D) crystals with the rotator phases (3D), is important for designing multi‐functional dynamic crystals.

Temperature-Reliant Dynamic Properties and Elasto-Plastic to Plastic Crystal (Rotator) Phase Transition in a Metal Oxyacid Salt.

Although, dynamic crystals are attractive for use in many technologies, molecular level mechanisms of various solid-state dynamic processes and their interdependence, remain poorly understood. Here, we report a rare example of a dynamic crystal (1), involving a heavy transition metal, rhenium, with an initial two-face elasticity (within ≈1 % strain), followed by elasto-plastic deformation, at room temperature. Further, these crystals transform to a rotator (plastic) crystal phase at ≈105 °C, displaying exceptional malleability. Qualitative and quantitative mechanical tests, X-ray diffraction, μ-Raman and polarized light microscopy experiments reveal that the elasto-plastic deformation involves both partial molecular rotations and slip, while malleability in the rotator phase is facilitated by reorientational motions and increased symmetry (slip planes). Our work, connecting the plastically bendable (1D or 2D) crystals with the rotator phases (3D), is important for designing multi-functional dynamic crystals.

Investigation of the Order–Disorder Rotator Phase Transition in KSiH3 and RbSiH3

The β–α (order–disorder) transition in the silanides ASiH3 (A = K, Rb) was investigated by multiple techniques, including neutron powder diffraction (NPD, on the corresponding deuterides), Raman spectroscopy, heat capacity (Cp), solid-state 2H NMR spectroscopy, and quasi-elastic neutron scattering (QENS). The crystal structure of α-ASiH3 corresponds to a NaCl-type arrangement of alkali metal ions and randomly oriented, pyramidal, SiH3– moieties. At temperatures below 200 K ASiH3 exist as hydrogen-ordered (β) forms. Upon heating the transition occurs at 279(3) and 300(3) K for RbSiH3 and KSiH3, respectively. The transition is accompanied by a large molar volume increase of about 14%. The Cp(T) behavior is characteristic of a rotator phase transition by increasing anomalously above 120 K and displaying a discontinuous drop at the transition temperature. Pronounced anharmonicity above 200 K, mirroring the breakdown of constraints on SiH3– rotation, is also seen in the evolution of atomic displacement paramet...

Phase transitions for rotational states within an algebraic cluster model

The ground state and excited, rotational phase transitions are investigated within the Semimicroscopic Algebraic Cluster Model (SACM). The catastrophe theory is used to describe these phase transitions. Short introductions to the SACM and the catastrophe theory are given. We apply the formalism to the case of 16O+α→20Ne.

Bias-driven spontaneous spin-valley polarization in monolayer transition-metal dichalcogenides

A physical mechanism that may enable electrical control of carrier spin-valley polarization is theoretically examined in a monolayer transition-metal dichalcogenide (TMD) structure. The idea is based on the interplay between the strongly spin-orbit coupled nature of the TMD band structure and the exchange interaction with a proximate magnet that can spontaneously lift the valley degeneracy when the carrier density exceeds a certain threshold. The analysis based on the free energy of the system clearly illustrates the desired spin-valley polarization in the TMD layer as well as the accompanying rotational phase transition in the magnetization. Numerical estimates utilizing the ${\mathrm{WS}}_{2}$ parameters as an example indicate a sharp transition in the spin-valley polarization over tens of percent at room temperature with only a modest change in the electrochemical potential of a few meV via electrostatic bias. Detection of the predicted phenomenon is expected to be straightforward through the corresponding modification in the TMD channel conductance.

Rotation dynamics of C60 molecules in a monolayer fullerene film on the WO2/W(110) surface near the rotational phase transition

The rotation dynamics of C60 molecules in monolayer fullerene films grown on the WO2/W(110) surface is studied by scanning tunneling microscopy. The formation of molecule clusters, which have a high libron vibration amplitude, is detected near the rotational phase transition temperature. The energy parameters that determine a change in the molecule orientation, namely, the energy difference between the nearest minima of the C60 molecule energy (30 meV) as a function of the molecule orientation and the potential barrier between them (610 meV), are determined. The results are discussed in terms of the mean-field approximation.

Vortex Lattice Studies in Type-II Superconductors by SANS

When a type-II superconductor is placed in a magnetic field, it is threaded by swirling whirlpools of electric current known as vortices. The vortices behave like massive entities and provide a unique probe into the nature of the superconducting state in the host material. Furthermore, the collective vortex behavior is of crucial importance for practical applications since vortex motion will lead to dissipation. I will discuss two recent vortex lattice (VL) studies using small-angle neutron scattering (SANS) that exemplify the continuous evolution of this technique. In the first example we used SANS to determine the superconducting anisotropy of Sr2RuO4 (SRO) that is among the few superconductors believed to exhibit p-wave pairing [1]. While there is significant experimental support for unconventional pairing, there is a discrepancy between the anisotropies of the upper critical field and the Fermi surface. Taking advantage of the significant transverse VL field component that arises due to the large anisotropy of SRO we measured the superconducting anisotropy ≍ 60, roughly three times greater than the upper critical field anisotropy. This result imposes significant constraints on possible models of triplet pairing in SRO. In the second example we used a stop-motion technique to study VL transition dynamics in MgB2 [2,3]. Here the VL exhibit extensive metastability in connection with a second order rotational phase transition that cannot be understood based on the single VL domain free energy or vortex pinning. Instead, we have proposed that a jamming of VL domains acting as granular entities is responsible for the metastability. We have performed extensive SANS experiments, as the VL is driven from the metastable to the ground state by small-amplitude AC magnetic field oscillations. This shows a dual power-law behavior indicating a two-step process for the transition to the ground state. Supported by the US Department of Energy (Grant No. DE-FG02-10ER46783).

Does rotational melting make molecular crystal surfaces more slippery?

The surface of a crystal made of roughly spherical molecules exposes, above its bulk rotational phase transition at T = Tr, a carpet of freely rotating molecules, possibly functioning as "nanobearings" in sliding friction. We explored by extensive molecular dynamics simulations the frictional and adhesion changes experienced by a sliding C60 flake on the surface of the prototype system C60 fullerite. At fixed flake orientation both quantities exhibit only a modest frictional drop of order 20% across the transition. However, adhesion and friction drop by a factor of ∼2 as the flake breaks its perfect angular alignment with the C60 surface lattice suggesting an entropy-driven aligned-misaligned switch during pull-off at Tr. The results can be of relevance for sliding Kr islands, where very little frictional differences were observed at Tr, but also to the sliding of C60-coated tip, where a remarkable factor ∼2 drop has been reported.

Persistence of metastable vortex lattice domains in MgB2 in the presence of vortex motion.

Recently, extensive vortex lattice metastability was reported in MgB2 in connection with a second-order rotational phase transition. However, the mechanism responsible for these well-ordered metastable vortex lattice phases is not well understood. Using small-angle neutron scattering, we studied the vortex lattice in MgB2 as it was driven from a metastable to the ground state through a series of small changes in the applied magnetic field. Our results show that metastable vortex lattice domains persist in the presence of substantial vortex motion and directly demonstrate that the metastability is not due to vortex pinning. Instead, we propose that it is due to the jamming of counterrotated vortex lattice domains which prevents a rotation to the ground state orientation.

Distinct Molecular Motions in a Switchable Chromophore Dielectric 4‐N,N‐Dimethylamino‐4′‐N′‐methylstilbazolium Trifluoromethanesulfonate

AbstractMolecular motion associated with rotational and orientational phase transitions is one of the prominent structural strategies for assembling the functional materials such as artificial motors and tunable molecular dielectrics. Here, a new organic chromophore molecule, 4‐N,N‐dimethylamino‐4′‐N′‐methylstilbazolium trifluoromethanesulfonate (complex 1), which undergoes an exceptional order‐disorder phase transition at 322 K, is successfully synthesized. The single crystal X‐ray diffraction analysis, thermal analysis, and dielectric measurements are used to characterize its dielectric dynamic behaviors. The results reveal that 1 behaves as a molecular rotator with the obviously distorted bipyramidal geometry of trifluoromethanesulfonate anions. In addition to its disorderings, two very distinct motions of the anionic moieties are confirmed, namely the “earth rotation” of partial units and the “earth revolution” of the whole molecule. Such unique molecular motions are found to be mainly responsible for the order–disorder phase transition together with the abrupt dielectric anomaly and anisotropy. The charge‐transfer cationic parts enhance the molecular motions behaving as the stator and give rise to its excellent third‐order nonlinearities. The concept may allow for the remote control of molecular events to explore functional materials in organic chromophores.

Fluctuation-driven first-order crystal to rotator phase transition

This article develops renormalization-group techniques to analyze the phase transition between the crystal and the rotator-V (Rv), phase in n-alkanes. An explicit free energy is constructed and it is verified that the unstable fixed point corresponds to a fluctuation-driven first-order crystal to Rv phase transition. Available experimental data are consistent with our model.

Theory of the reentrant quantum rotational phase transition in high-pressure HD

Theory of the reentrant quantum rotational phase transition in high-pressure HD

Tricritical behavior of the R I–R V rotator phase transition in a mixture of alkanes with nanoparticles

A phenomenological theory is presented, which describes the tricritical behavior of the R(I)-R(V) rotator phase transition in the mixture of alkanes with nanoparticles. The influence of the nanoparticles on the R(I)-R(V) transition in alkanes is discussed by varying the coupling between the order parameters of the rotator phases and nanoparticle. When nanoparticle solutes are added to pure alkanes, the R(I)-R(V) transition temperature is increased. It was observed from the theoretical calculations that for a particular value of the concentration of the nanoparticles, the first order R(I)-R(V) transition becomes second order at a tricritical point. Calculations based on this model agree qualitatively with experiment.

Elastic properties of the RIV– RIII rotator phases of alkanes

A model free energy has been constructed to describe the R IV – R III rotator phase transition in alkanes in terms of the elastic strains and order parameter. The conditions for the R IV – R III phase transition are discussed. From the free energy, the order parameter and the elastic strains are determined. The model free energy describes the first or second order character of the R IV – R III transition depending on the strength of the coupling. The elastic properties in the vicinity of the R IV – R III transition are discussed on the basis of a free energy expansion. The temperature dependence of the elastic constants is calculated on both sides of the transition. The coupling between the order parameter and elastic stains is shown to have a crucial influence on the phase behavior and the order of the transition.

Renormalization-group analysis of the RI − RV rotator phase transition

A model for coupled tilt angle and lattice distortion parameter is proposed to describe the R(I)-R(V) transition in n-alkane. The model is treated in the framework of a Landau mean-field theory and renormalization-group theory. The influence of gauche conformations and molecular flexibility on the R(I)-R(V) transition is discussed within the mean-field theory. The fluctuations on the R(I)-R(V) transition are discussed by the renormalization-group theory. Renormalization-group theory predicts that the R(I)-R(V) transition can be driven first order by fluctuations and becomes second order at a tricritical point. Available experimental data are consistent with our model.

Understanding nuclear shape phase transitions at the nucleon level with a boson mapping approach

Implementing a shell model Hamiltonian with monopole-pair, quadrupole-pair and quadrupole–quadrupole interactions between nucleons, we study the shape phases of nuclei and their transitions, with the Dyson boson mapping approach. The investigation concentrates on the influence of each interaction on the nuclear shape phases. A correspondence between the strength of each of the interactions and the nuclear shape phases is obtained. The investigation also indicates that increasing the quadrupole-pair interaction strength can induce the vibrational to the axially prolate rotational shape phase transition and enhancing the quadrupole–quadrupole interaction can drive the phase transition from the axially oblate rotational to the axially prolate rotational, with the γ-soft rotational being the critical point.

Studies of Domain and Twin Patterns in NaNbO3-Gd1/3NbO3 Solid Solution Crystals

Studies of (1-x)NaNbO3-xGd1/3NbO3 crystals in polarized light revealed the independence of the temperature of rotational phase transition into the cubic phase on x. This correlates with the similarity of Na+ and Gd3+ ionic radii. Increase of Gd content leads to the diffusion of permittivity maxima and a dramatic decrease of a mean size of twins (domains). This makes both crystallooptic studies and rotating polarizer method ineffective. Multifractal analysis of false-colour image patterns obtained by rotating polarizer method, using the Metripol microscope system enables to detect the changes in domain (twin) pattern in the temperature range of diffuse phase transition.

ROTATIONAL DRIVEN NUCLEAR SHAPE PHASE TRANSITION OF THE YRAST STATES OF INDIVIDUAL NUCLEUS IN INTERACTING BOSON MODEL

With the intrinsic coherent state formalism and the angular momentum projection, we study the shape phase structure of the yrast states in the dynamical symmetries of the IBM. We found that the states in the U (5) symmetry can undergo a rotation driven vibrational to axially rotational shape phase transition if the interaction parameters take negative values smaller than the critical ones. It shows that the U (5) symmetry of the IBM1 is an appropriate approach to describe the rotation driven shape phase transition along the yrast line of individual nucleus as the interaction parameters are taken in a special region. The O (6) symmetric yrast states may involve a phase transition from γ-soft rotation to triaxial rotation as the angular momentum increases, if the interaction parameters are specially chosen. And the yrast states in SU (3) symmetry always appear in the axially prolate shape phase.

Vibration-rotation transitional patterns in theSD-pair shell model

Patterns of the vibrational to rotational phase transition in the SD-pair shell model are studied. The results show that the U(5)-SU(3) transitional patterns of the interacting boson model can be reproduced well in the SD-pair shell model.