NMR and Raman Scattering Studies of Temperature- and Pressure-Driven Phase Transitions in CH3NH2NH2PbCl3 Perovskite

Three-dimensional methylhydrazinium (CH3NH2NH2+, MHy+) lead halides, related to the famous methylammonium (CH3NH3+, MA+) and formamidinium (CH(NH2)2+, FA) perovskites, are attractive optoelectronic materials crystallizing in polar structures. In this work, temperature-dependent 1H and 207Pb magic-angle spinning (MAS) NMR, Raman as well as high-pressure Raman studies of CH3NH2NH2PbCl3 (MHyPbCl3) are reported. Raman spectroscopy reveals many similarities between phonon properties of MHy lead halides and the MA and FA analogues. In particular, these families of compounds show an increase in wavenumber of cage modes when large I- ions are replaced by smaller Br- and then Cl- ones. They also show strong sensitivity of the CH3 torsional mode on size of the cavity occupied by MHy+ cation that decreases with decreasing size of the halide anion. The cage modes of MHyPbCl3 are, however, observed at significantly lower wavenumbers than similar modes of MAPbCl3 and FAPbCl3, indicating higher softness of MHyPbCl3. Temperature-dependent Raman and NMR studies demonstrate that the MHy+ cations in MHyPbCl3 are significantly less affected by the temperature-induced phase transition than the Pb-Cl framework. This suggests a displacive type of the phase transition dominated by tilting and deformation of the PbCl6 octahedra. Analysis of the 207Pb MAS NMR spectra reveals the presence of two differently distorted PbCl6 octahedra and diminishing (increasing) distortion of the less (more) distorted octahedra in the high-temperature phase. Pressure-dependent Raman studies reveal the presence of a single first-order pressure-induced phase transition between 0.72 and 1.27 GPa. Analysis of the spectra indicates that the driving forces for the pressure-induced phase transition in MHyPbCl3 are tilting and distortion of the PbCl6 octahedra accompanied by reorientation of MHy+ cations. Raman spectra do not show evidence of any additional phase transition or amorphization up to 6.95 GPa

Electron paramagnetic resonance study of ferroelectric phase transition and dynamic effects in a Mn²⁺ doped [NH₄][Zn(HCOO)₃] hybrid formate framework

We present an X- and Q-band continuous wave (CW) and pulse electron paramagnetic resonance (EPR) study of a manganese doped [NH4][Zn(HCOO)3] hybrid framework, which exhibits a ferroelectric structural phase transition at 190 K. The CW EPR spectra obtained at different temperatures exhibit clear changes at the phase transition temperature. This suggests a successful substitution of the Zn2+ ions by the paramagnetic Mn2+ centers, which is further confirmed by the pulse EPR and 1H ENDOR experiments. Spectral simulations of the CW EPR spectra are used to obtain the temperature dependence of the Mn2+ zero-field splitting, which indicates a gradual deformation of the MnO6 octahedra indicating a continuous character of the transition. The determined data allow us to extract the critical exponent of the order parameter (β = 0.12), which suggests a quasi two-dimensional ordering in [NH4][Zn(HCOO)3]. The experimental EPR results are supported by the density functional theory calculations of the zero-field splitting parameters. Relaxation time measurements of the Mn2+ centers indicate that the longitudinal relaxation is mainly driven by the optical phonons, which correspond to the vibrations of the metal–oxygen octahedra. The temperature behavior of the transverse relaxation indicates a dynamic process in the ordered ferroelectric phase

Spin Textures of Polariton Condensates in a Tunable Microcavity with Strong Spin-Orbit Interaction

We report an extended family of spin textures in coexisting modes of zero-dimensional polariton condensates spatially confined in tunable open microcavity structures. The coupling between photon spin and angular momentum, which is enhanced in the open cavity structures, leads to new eigenstates of the polariton condensates carrying quantised spin vortices. Depending on the strength and anisotropy of the cavity confinement potential and the strength of the spin-orbit coupling, which can be tuned via the excitonic/photonic fractions, the condensate emissions exhibit either spin-vortex-like patterns or linear polarization, in good agreement with theoretical modelling

Tunable polaritonic molecules in an open microcavity system

We experimentally demonstrate tunable coupled cavities based upon open access zero-dimensional hemispherical microcavities. The modes of the photonic molecules are strongly coupled with quantum well excitons forming a system of tunable polaritonic molecules. The cavity-cavity coupling strength, which is determined by the degree of modal overlap, is controlled through the fabricated centre-to-centre distance and tuned in-situ through manipulation of both the exciton-photon and cavity-cavity detunings by using nanopositioners to vary the mirror separation and angle between them. We demonstrate micron sized confinement combined with high photonic Q-factors of 31 000 and lower polariton linewidths of 150 μeV at resonance along with cavity-cavity coupling strengths between 2.5 meV and 60 μeV for the ground cavity state

Highly nonlinear trion-polaritons in a monolayer semiconductor

Highly nonlinear optical materials with strong effective photon-photon interactions are required for ultrafast and quantum optical signal processing circuitry. Here we report strong Kerr-like nonlinearities by employing efficient optical transitions of charged excitons (trions) observed in semiconducting transition metal dichalcogenides (TMDCs). By hybridising trions in monolayer MoSe2 at low electron densities with a microcavity mode, we realise trion-polaritons exhibiting significant energy shifts at small photon fluxes due to phase space filling. We find the ratio of trion- to neutral exciton–polariton interaction strength is in the range from 10 to 100 in TMDC materials and that trion-polariton nonlinearity is comparable to that in other polariton systems. The results are in good agreement with a theory accounting for the composite nature of excitons and trions and deviation of their statistics from that of ideal bosons and fermions. Our findings open a way to scalable quantum optics applications with TMDCs

Condensation of 2D exciton-polaritons in an open-access microcavity

International audienceWe establish a tunable open-access microcavity consisting of two planar distributed Bragg reflectors (DBRs) individually controlled by nanopositioners. By varying the cavity length, such configuration enables variation of the light–matter coupling strength by a factor of 2, while keeping in microresonators the same active region and cavity mirrors. Polariton condensation was demonstrated over a large range of Rabi splittings and the corresponding threshold diagram was derived as a function of cavity-exciton detuning, which fits well with theoretical simulations. The results show that for various light-matter coupling strengths, optimal detunings featured by the lowest condensation threshold always occur at a fixed depth of energy trap between the exciton reservoir and the polariton ground state, which enables the most efficient exciton–exciton scattering into the condensate state in the driven-dissipative polaritonic system

Spin Textures of Exciton-Polaritons in a Tunable Microcavity with Large TE-TM Splitting.

We report an extended family of spin textures of zero-dimensional exciton-polaritons spatially confined in tunable open microcavity structures. The transverse-electric-transverse-magnetic (TE-TM) splitting, which is enhanced in the open cavity structures, leads to polariton eigenstates carrying quantized spin vortices. Depending on the strength and anisotropy of the cavity confining potential and of the TE-TM induced splitting, which can be tuned via the excitonic or photonic fractions, the exciton-polariton emissions exhibit either spin-vortex-like patterns or linear polarization, in good agreement with theoretical modeling

Strong exciton-photon coupling in open semiconductor microcavities

We present a method to implement 3-dimensional polariton confinement with in-situ spectral tuning of the cavity mode. Our tunable microcavity is a hybrid system consisting of a bottom semiconductor distributed Bragg reflector (DBR) with a cavity containing quantum wells (QWs) grown on top and a dielectric concave DBR separated by a micrometer sized gap. Nanopositioners allow independent positioning of the two mirrors and the cavity mode energy can be tuned by controlling the distance between them. When close to resonance, we observe a characteristic anticrossing between the cavity modes and the QW exciton demonstrating strong coupling. For the smallest radii of curvature concave mirrors of 5.6 μm and 7.5 μm, real-space polariton imaging reveals submicron polariton confinement due to the hemispherical cavity geometry