Search

Using DFT-based molecular dynamics simulation of Cu$^+$ cations flipping and In$^{3+}$ cations displacive dynamics, we clarify the dipole ordering of CuInP$_2$S$_6$ ferrielectrics through the second order Jahn-Teller effect which determines the double-well potential for copper cations as well as three-well potential for indium cations inside the structural layers. The temperature dependence of the spatial distribution of Cu$^+$ and In$^{3+}$ cations is also described within the quantum anharmonic oscillators model. In$^{3+}$ cations play a decisive role in the character of the polar ordering: at ambient or for positive pressures, the first order transition occurs; with the rise of negative compression, the ferrielectric transition evolves to the second order. The peculiarities of spontaneous polarization switching are related to the contributions of copper and indium cationic sublattices.

Superconducting parametric amplifiers are commonly fabricated using planar transmission lines with a non-linear inductance provided by either Josephson junctions or the intrinsic kinetic inductance of the thin film. However, Banys et al. [1] reported non-linear behaviour in a niobium pillbox cavity, hypothesising that below Tc, the pair iris-bulk resonator would act as a superconducting contact surface exploiting a Josephson-like non-linearity. This work investigates this effect further by applying Keysight Technologies' Advanced Design System (ADS) to simulate the cavity using an equivalent circuit model that includes a user defined Josephson inductance component. The simulations show that for a resonance centred at nu0 = 30.649 GHz, when two tones (pump and signal) are injected into the cavity, mixing and parametric gain occur. The maximum achievable gain is explored when the resonator is taken to its bifurcation energy. These results are compared to cryogenic measurements where the pump and signal are provided by a Vector Network Analyzer.

Kinetic inductance travelling wave parametric amplifiers (KI-TWPAs) have been simulated using Keysight's Advanced Design System (ADS). A lumped element representation of a superconducting transmission line has been developed using nonlinear inductor elements to represent the current dependency of the intrinsic kinetic inductance. This provides a fast, simple and easily modifiable method for analysing the parametric processes that occur in nonlinear kinetic inductance devices such as parametric oscillators or amplifiers, while also allowing the accompanying RF network to be optimised. This methodology is used to model the behaviour of a select number of KI-TWPA designs operating in the 3-wave mixing (3WM) and 4-wave mixing (4WM) regimes. S-parameters and dispersion properties are extracted from large signal S-parameter simulations and the gain curve of each amplifier design is generated from harmonic balance simulations. Results of these are compared to the gain expression from the analytic solutions of the coupled mode equations, while making reference to the gain curves of the equivalent KI-TWPAs from literature., Comment: 9 pages, 8 figures

It is well known that disordered relaxor ferroelectrics exhibit local polar correlations. The origin of localized fields that disrupt long range polar order for different substitution types, however, is unclear. Currently, it is known that substituents of the same valence as Ti4+ at the B-site of barium titanate lattice produce random disruption of Ti-O-Ti chains that induces relaxor behavior. On the other hand, investigating lattice disruption and relaxor behavior resulting from substituents of different valence at the B-site is more complex due to the simultaneous occurrence of charge imbalances and displacements of the substituent cation. The existence of an effective charge mediated mechanism for relaxor behavior appearing at low (< 10%) substituent contents in heterovalent modified barium titanate ceramics is presented in this work. These results will add credits to the current understanding of relaxor behavior in chemically modified ferroelectric materials and also acknowledge the critical role of defects (such as cation vacancies) in lattice disruption, paving the way for chemistry-based materials design in the field of dielectric and energy storage applications.

We present the design and simulation methodology of a superconducting ridge-gap waveguide (RGWG) as a potential basis for mm-wave kinetic inductance travelling wave parametric amplifiers (KI-TWPAs). A superconducting RGWG was designed using Ansys HFSS to support a quasi-TEM mode of transmission over a bandwidth of 20 to 120 GHz with its internal dimensions optimised for integration with W-band rectangular waveguide. A design of an impedance loaded travelling wave structure incorporating periodic perturbations of the ridge was described. A method to simulate the nonlinear kinetic inductance via user-defined components in Keysight's ADS was outlined, which yielded the power dependent S-parameters and parametric signal gain. A RGWG with a 30 nm NbTiN coating and 5 um conductor spacing, corresponding to a kinetic inductance fraction $\alpha \sim 60\%$ was used for the description of a KI-TWPA with 900 perturbations equivalent to a physical length 25 cm that achieved more than 10 dB of signal gain over a 75--110 GHz bandwidth via 4-wave mixing (4WM)., Comment: 9 pages, 3 figures, submitted to the 19th International Workshop on Low Temperature Detectors (LTD19) Proceedings

The dipole ordering in Sn(Pb)$_2$P$_2$S(Se)$_6$ materials may be tuned by chemical substitution realizing a ferroelectric quantum phase transition and quantum glassy or relaxor type phenomena on different parts of the phase diagram. The introduction of Ge impurity increases the temperature of the phase transitions and initiates a more pronounced Ising type critical anomaly in Sn$_2$P$_2$S$_6$ crystal, does not shift the coordinate of the Lifshitz point $x_{\textrm {LP}}$ in Sn$_2$P$_2$(Se$_x$S$_{1-x}$)$_6$ mixed crystals, induces the appearance of a ferroelectric phase transition in quantum paraelectrics Pb$_2$P$_2$S$_6$ and inhomogeneous polar ordering in (Pb$_{0.7}$Sn$_{0.3}$)$_2$P$_2$S(Se)$_6$ crystals. For Pb$_2$P$_2$S$_6$ crystal, the real part of the dielectric susceptibility in the quantum critical regime varies as $1/T^2$ instead of the expected $1/T^3$ behavior for uniaxial materials. This can be partially explained by a screening phenomenon in the semiconductor materials of the Sn(Pb)$_2$P$_2$S(Se)$_6$ system, which weakens the long range electric dipole interactions, and also provides, at high temperatures, a critical behavior near the Lifshitz point (studied by thermal diffusivity) similar to the one predicted in the case of systems with short range interactions. At low temperatures, a quantum critical behavior in Pb$_2$P$_2$S$_6$ crystal can be established by the nonlinear coupling between polar and antipolar fluctuations. An increase in thermal conductivity is induced by Ge impurity in Pb$_2$P$_2$S$_6$ crystal, which is explained through the weakening of the acoustic phonons resonance scattering by soft optic phonons because of the appearance of ferroelectric phase polar clusters.

For the first time in a bulk proper uniaxial ferroelectrics, double antiferroelectric-like hysteresis loops have been observed in the case of Sn$_2$P$_2$S$_6$ crystal. The quantum anharmonic oscillator model was proposed for description of such polarization switching process. This phenomenon is related to three-well local potential of spontaneous polarization fluctuations at peculiar negative ratio of coupling constants which correspond to inter-site interaction in given sublattice and interaction between two sublattices of Sn$_2$P$_2$S$_6$ modeled crystal structure. Obtained data can be used for development of triple-level cell type memory technology.

The valence fluctuations which are related to the charge disproportionation of phosphorous ions $P^{4+} + P^{4+}\rightarrow P^{3+} + P^{5+}$ are the origin of ferroelectric and quantum paraelectric states in Sn(Pb)$_2$P$_2$S$_6$ semiconductors. They involve recharging of SnPS$_3$ or PbPS$_3$ structural groups which could be represented as half-filled sites in the crystal lattice. Temperature-pressure phase diagram for Sn$_2$P$_2$S$_6$ compound and temperature-composition phase diagram for (Pb$_y$Sn$_{1-y}$)$_2$P$_2$S$_6$ mixed crystals, which include tricritical points and where a temperature of phase transitions decrease to 0 K, together with the data about some softening of low energy optic phonons and rise of dielectric susceptibility at cooling in quantum paraelectric state of Pb$_2$P$_2$S$_6$, are analyzed by GGA electron and phonon calculations and compared with electronic correlations models. The anharmonic quantum oscillators model is developed for description of phase diagrams and temperature dependence of dielectric susceptibility.