A detailed high pressure X-ray diffraction and Raman spectroscopy study is carried out on monolayer WS2 and nanocrystalline WS2. The monolayer sample is obtained by liquid exfoliation. Photoluminescence and Raman measurements show it to consist of a monolayer. Careful analysis of ambient and high pressure data indicates the emergence of a triclinic phase at about 5.8 GPa in patches embedded in the parent hexagonal phase. This raises a question mark over the structural purity of the exfoliated monolayer materials beyond certain stress conditions. Raman mode values and their full width at half maximum of the monolayer sample show anomalous changes at about 27 GPa, the pressure where the sample completely gets converted to the triclinic structure indicating the importance of strain in structural as well as electronic properties of two dimensional materials.

We have carried out detailed experimental investigations on polycrystalline CuO using dielectric constant, dc resistance, Raman spectroscopy and X-ray diffraction measurements at high pressures. Observation of anomalous changes both in dielectric constant and dielectric loss in the pressure range 3.7–4.4 GPa and reversal of piezoelectric current with reversal of poling field direction indicate to a change in ferroelectric order in CuO at high pressures. A sudden jump in Raman integrated intensity of Ag mode at 3.4 GPa and observation of Curie-Weiss type behaviour in dielectric constant below 3.7 GPa lends credibility to above ferroelectric transition. A slope change in the linear behaviour of the Ag mode and a minimum in the FWHM of the same indicate indirectly to a change in magnetic ordering. Since all the previous studies show a strong spin-lattice interaction in CuO, observed change in ferroic behaviour at high pressures can be related to a reentrant multiferroic ordering in the range 3.4 to 4.4 GPa, much earlier than predicted by theoretical studies. We argue that enhancement of spin frustration due to anisotropic compression that leads to change in internal lattice strain brings the multiferroic ordering to room temperature at high pressures.

We report high pressure x-ray diffraction and systematic Raman measurements on a ReS 2 sample, which is mechanically exfoliated from a single crystal. A few new Bragg peaks are observed to emerge above 6 GPa indicating a structural transition from distorted 1T to distorted 1T 0 in a triclinic structure. The same is corroborated by the appearance of new Raman modes in the same pressure range. Softening of the Raman modes corresponding to Re atom vibrations is observed in the distorted 1T 0 phase in the pressure range of 15-25 GPa. In the same pressure range, the anomalous change in the volume is found to be induced by the lattice expansion. The volume expansion is related to the sliding of layers leading to octahedral distortion and an increase in octahedral volume. The sample is found to be very incompressible above 25 GPa with respect to below 15 GPa data. The same is also reflected in the Raman mode shifts with pressure.

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We report a new classical spin liquid in which the collective flux degrees of freedom break the translation symmetry of the honeycomb lattice. This exotic phase exists in frustrated spin-orbit magnets where a dominant off-diagonal exchange, the so-called Γ term, results in a macroscopic ground-state degeneracy at the classical level. We demonstrate that the system undergoes a phase transition driven by thermal order-by-disorder at a critical temperature Tc ≈ 0.04|Γ|. At first sight, this transition reduces an emergent spherical spin-symmetry to a cubic one: spins point predominantly toward the cubic axes at T < Tc. However, this seems to simply restore the cubic symmetry of the Γ model, and the non-coplanar spins remain disordered below Tc. We show that the phase transition actually corresponds to plaquette ordering of hexagonal fluxes and the cubic symmetry is indeed broken, a scenario that is further confirmed by our extensive Monte Carlo simulations.

Detailed high pressure Resonance Raman (RR) Spectroscopy and x-ray diffraction (XRD) studies are carried out on 3-4 layered MoSe 2 obtained by liquid exfoliation. Analysis of ambient XRD pattern and RR spectra indicate the presence of a triclinic phase along with its parent hexagonal phase. Slope change in the linear behavior of reduced pressure (H) with respect to Eulerian strain ( f E ) is observed at about 13 GPa in hexagonal phase and at about 17 GPa for the triclinic phase. High pressure Raman measurements using two different pressure transmitting media (PTM) show three linear pressure regions, separated by pressure values around which anomalies in the structure are observed. A broad minimum in the FWHM values of E g 2 1 mode at about 10-12 GPa indicate to an electron-phonon coupling. Above 33 GPa the sample completely gets converted to the triclinic structure, which indicates the importance of strain in structural as well as electronic properties of two dimensional materials.

We report a detailed high pressure study involving X-ray diffraction, Raman spectroscopy, and photoluminescence measurements on a model Pb-free solar cell material Cs3Bi2Br9 halide perovskite. The sample starts showing photoluminescence in a broad range of 550–900 nm above 1.4 GPa, due to an isostructural transition to a distorted unit cell. Further enhancement in intensity with pressure is found to be driven by an increase in distortion of BiBr6 octahedra. Electronic band structure calculations show the sample in the high pressure phase to be an indirect band gap semiconductor. The photoluminescence peak shows a kink at higher energy and a broad asymmetric peak at lower energies due to the recombination of free excitons and self-trapped excitons, respectively. The blue shift of the PL peaks until about 4.4 GPa can be related to the extensive structural distortion before the transition to lowest symmetry triclinic phase.

Supernova remnants (SNRs) have a variety of overall morphology as well as rich structures over a wide range of scales. Quantitative study of these structures can potentially reveal fluctuations of density and magnetic field originating from the interaction with ambient medium and turbulence in the expanding ejecta. We have used 1.5GHz (L band) and 5GHz (C band) VLA data to estimate the angular power spectrum C ℓ of the synchrotron emission fluctuations of the Kepler SNR. This is done using the novel, visibility based, Tapered Gridded Estimator of C ℓ . We have found that, for ℓ = (1.9 − 6.9) × 10 4 , the power spectrum is a broken power law with a break at ℓ = 3.3 × 10 4 , and power law index of −2.84 ± 0.07 and −4.39 ± 0.04 before and after the break respectively. The slope −2.84 is consistent with 2D Kolmogorov turbulence and earlier measurements for the Tycho SNR. We interpret the break to be related to the shell thickness of the SNR (0.35 pc) which approximately matches ℓ = 3.3 × 10 4 (i.e., 0.48 pc). However, for ℓ > 6.9 × 10 4 , the estimated C ℓ of L band is likely to have dominant contribution from the foregrounds while for C band the power law slope −3.07 ± 0.02 is roughly consistent with 3D Kolmogorov turbulence like that observed at large ℓ for Cas A and Crab SNRs.