Neutron powder diffraction study of the phase transitions in deuterated methylammonium lead iodide

We report the results of a neutron powder diffraction study of the phase transitions in deuterated methylammonium lead iodide, with a focus on the system of orientational distortions of the framework of PbI6 octahedra. The results are analysed in terms of symmetry-adapted lattice strains and normal mode distortions. The higher-temperature cubic–tetragonal phase transition at 327 K is weakly discontinuous and nearly tricritical. The variations of rotation angles and spontaneous strains with temperature are consistent with a standard Landau theory treatment. The lower-temperature transition to the orthorhombic phase at 165 K is discontinuous, with two systems of octahedral rotations and internal distortions that together can be described by five order parameters of different symmetry. In this paper we quantify the various symmetry breaking distortions and their variation with temperature, together with their relationship to the spontaneous strains, within the formalism of Landau theory. A number of curious results in the low-temperature phase are identified, particularly regarding distortion amplitudes that decrease rather than increase with lowering temperature.

Multi-mode excitation drives disorder during the ultrafast melting of a C4-symmetry-broken phase

Spontaneous C4-symmetry breaking phases are ubiquitous in layered quantum materials, and often compete with other phases such as superconductivity. Preferential suppression of the symmetry broken phases by light has been used to explain non-equilibrium light induced superconductivity, metallicity, and the creation of metastable states. Key to understanding how these phases emerge is understanding how C4 symmetry is restored. A leading approach is based on time-dependent Ginzburg-Landau theory, which explains the coherence response seen in many systems. However, we show that, for the case of the single layered manganite La0.5Sr1.5MnO4, the theory fails. Instead, we find an ultrafast inhomogeneous disordering transition in which the mean-field order parameter no longer reflects the atomic-scale state of the system. Our results suggest that disorder may be common to light-induced phase transitions, and methods beyond the mean-field are necessary for understanding and manipulating photoinduced phases.

Freedericksz transition in Ferronematic Liquid Crystal under weak anchoring conditions

Freedericksz effect is investigated theoretically for a ferronematic liquid crystal, which is a colloidal suspension of ferromagnetic nanoparticles in a nematic fluid. Considering a splay type Freedericksz geometry, weak anchoring conditions are assumed at the cell boundaries. The specific nature of this anchoring reveals a rich variety of stable ferronematic phases, which include uniform, distorted and saturated states. Apart from weak anchoring conditions at the cell boundaries, soft planar anchoring is assumed for the mesogenic molecules at the surface of a nanoparticle. The interplay between these two anchoring phenomena along with Frank type elastic theory determine the values of Freedericksz threshold between various ferronematic states. It is found that compared to relatively strong anchoring for the mesogens both at the cell boundaries and at the surface of the nanoparticles, weak anchoring significantly reduces the Freedericksz threshold field. Landau theory is then utilized to understand the nature of transition between different ferronematic states. Based on the phenomenon of segregation effect, these transitions are found to be either first order or second order in nature. The present theory is also extended to non-ferromagnetic nanoparticles and significant reduction in Freedericksz threshold is obtained. Finally, these results are corroborated with experimental findings.

Reliability of the Ginzburg–Landau Theory in the BCS-BEC Crossover by Including Gaussian Fluctuations for 3D Attractive Fermions

We calculate the parameters of the Ginzburg–Landau (GL) equation of a three-dimensional attractive Fermi gas around the superfluid critical temperature. We compare different levels of approximation throughout the Bardeen–Cooper–Schrieffer (BCS) to the Bose–Einstein Condensate (BEC) regime. We show that the inclusion of Gaussian fluctuations strongly modifies the values of the Ginzburg–Landau parameters approaching the BEC regime of the crossover. We investigate the reliability of the Ginzburg–Landau theory, with fluctuations, studying the behavior of the coherence length and of the critical rotational frequencies throughout the BCS-BEC crossover. The effect of the Gaussian fluctuations gives qualitative correct trends of the considered physical quantities from the BCS regime up to the unitary limit of the BCS-BEC crossover. Approaching the BEC regime, the Ginzburg–Landau equation with the inclusion of Gaussian fluctuations turns out to be unreliable.

Reliability of the Ginzburg–Landau Theory in the BCS-BEC Crossover by Including Gaussian Fluctuations for 3D Attractive Fermions

We calculate the parameters of the Ginzburg–Landau (GL) equation of a three-dimensional attractive Fermi gas around the superfluid critical temperature. We compare different levels of approximation throughout the Bardeen–Cooper–Schrieffer (BCS) to the Bose–Einstein Condensate (BEC) regime. We show that the inclusion of Gaussian fluctuations strongly modifies the values of the Ginzburg–Landau parameters approaching the BEC regime of the crossover. We investigate the reliability of the Ginzburg–Landau theory, with fluctuations, studying the behavior of the coherence length and of the critical rotational frequencies throughout the BCS-BEC crossover. The effect of the Gaussian fluctuations gives qualitative correct trends of the considered physical quantities from the BCS regime up to the unitary limit of the BCS-BEC crossover. Approaching the BEC regime, the Ginzburg–Landau equation with the inclusion of Gaussian fluctuations turns out to be unreliable.

High-throughput analysis of magnetic phase transition by combining table-top sputtering, photoemission electron microscopy, and Landau theory

Recent progress in materials informatics has triggered growing interest in combinatorial experimental systems for materials development. We demonstrate a novel high-throughput experiment combining compact materials synthesis, synchrotron radiation measurements, and statistical data analysis. This technique focuses on not only drawing phase diagrams but also analysing phase transitions for exploring the functions of magnetic materials. This study involved the rapid preparation of a composition-gradient Fe–Co–Cr ternary thin film using a table-top sputtering system and 3D printer, followed by measurement of the chemical components and magnetic contrast by photoemission electron microscopy, through the acquisition of one million spectral datasets within 10 min. The ternary magnetic phase diagram of Fe–Co–Cr obtained by statistical analysis of the magnetic circular dichroism (MCD) contrast images was in perfect agreement with previous studies. The MCD histogram was fitted based on Landau theory, and the estimated critical exponent β (0.36 ± 0.028) showed excellent agreement with previous theoretical and experimental studies. This study successfully demonstrates universal physical parameter analysis that characterizes magnetic properties by a high-throughput approach combined with a simple experimental apparatus.

Effect of Co and Mg doping at Cu site on structural, magnetic and dielectric properties of α-Cu2V2O7

We have studied the effect of doping of both magnetic (Co) and nonmagnetic (Mg) ions at the Cu site on phase transition in polycrystalline α-Cu2V2O7 through structural, magnetic, and electrical measurements. x-ray diffraction reveals that Mg doping triggers an onset of α- to β-phase structural transition in Cu2−xMgxV2O7 above a critical Mg concentration xc=0.15, and both the phases coexist up to x=0.25. Cu2V2O7 possesses a non-centrosymmetric(NCSM) crystal structure and antiferromagnetic (AFM) ordering along with a non-collinear spin structure in the α phase, originated from the microscopic Dzyaloshinskii-Moriya(DM) interaction between the neighboring Cu spins. Accordingly, a weak ferromagnetic behavior has been observed up to x=0.25. However, beyond this concentration, Cu2−xMgxV2O7 exhibits complex magnetic properties. A clear dielectric anomaly is observed in α-Cu2−xMgxV2O7 around the magnetic transition temperature, which loses its prominence with the increase in Mg doping. The analysis of experimental data shows that the magnetoelectric coupling is nonlinear, which is in agreement with the Landau theory of continuous phase transitions. Co doping, on the other hand, initiates a sharp α to β phase transition around the same critical concentration xc=0.15 in Cu2−xCoxV2O7 but the ferromagnetic behavior is very weak and can be detected only up to x=0.10. We have drawn the magnetic phase diagram which indicates that the rate of suppression in transition temperature is the same for both types of doping, magnetic (Co) and nonmagnetic (Zn/Mg).

Ginzburg–Landau Theory of Condensates

Ginzburg–Landau theory is an important tool in condensed matter physics research, describing the ordered phases of condensed matter, including the dynamics, elasticity, and thermodynamics of the condensed configurations. In this systematic introduction to Ginzberg–Landau theory, both common and topological excitations are considered on the same footing (including their thermodynamics and dynamical phenomena). The role of the topological versus energetic considerations is made clear. Required mathematics (symmetry, including lattice translation, topology, and perturbative techniques) are introduced as needed. The results are illustrated using arguably the most fascinating class of such systems, high Tc superconductors subject to magnetic field. This book is an important reference for both researchers and graduate students working in condensed matter physics or can act as a textbook for those taking advanced courses on these topics.