Institute of Molecular Physics

Analytic expressions for the equation of state in terms of the compressibility factor, Z, and excess entropy, sex, of inverse power (IP) potential fluids are derived and parameterized using molecular dynamics simulation data. The IP pair potential is ϕ(r) ∼ r-n, where r is the pair separation and n is an exponent that governs the steepness of the potential. A number of parameterizations of the dependence of Z on number density and n are proposed and compared. These include multiparameter global series expansion fits and more simple formulas based on the very soft (small n) and hard sphere particle representation limits. The excess entropy can be represented over the whole fluid and n > 3+ range well by a single analytic expression with n-dependent parameters. It is shown that sex is a concave function of density for n greater than about 6 and convex for smaller values of the exponent. The excess entropy varies with n for a constant value of the density normalized by its freezing point value.

We present the structural, magnetic, and magnetocaloric properties of thin films with stoichiometry Tb$_{31}$Co$_{69}$ and Dy$_{31}$Co$_{69}$ deposited on naturally oxidized silicon Si (100) substrates. Samples with a thickness $d=50$ nm covered with a protective Au overlayer with a thickness $d_{\rm Au} =5 $ nm were produced using the pulsed laser deposition technique. X-ray diffraction analysis indicated the presence of crystallized Laves phases and amorphous phases in the prepared materials. Magnetization measurements as a function of temperature revealed ferrimagnetic behavior in both samples. We estimated the compensation temperature $T_{\rm comp}$ of the amorphous phase for Tb$_{31}$Co$_{69}$ at 81.5 K and for Dy$_{31}$Co$_{69}$ at 88.5 K, while we found the Curie temperature $T_{\rm C,\ Laves}$ of the crystallized Laves phases at 204.5 K and at 117 K, respectively. We investigated the magnetocaloric effect in a wide temperature range, covering $T_{\rm comp}$ of amorphous phases and $T_{\rm C,\ Laves}$ of crystallized Laves phases. The analysis for the magnetic field change of $\Delta \mu_0H=5$ T showed values of the magnetic entropy change of $-\Delta S_{\rm M}=4.9$ mJ cm$^{-3}$ K$^{-1}$ at $T_{\rm comp}$ and $-\Delta S_{\rm M}=6.6$ mJ cm$^{-3}$ K$^{-1}$ at $T_{\rm C,\ Laves}$ for Tb$_{31}$Co$_{69}$, while for Dy$_{31}$Co$_{69}$, we determined the values of $-\Delta S_{\rm M}=35$ mJ cm$^{-3}$ K$^{-1}$ at $T_{\rm comp}$ and $-\Delta S_{\rm M}=28$ mJ cm$^{-3}$ K$^{-1}$ at $T_{\rm C,\ Laves}$.

Landau theory relates phase transitions to the minimization of the Landau functional (e.g., free energy functional), which is expressed as a power series of the order parameter. It has been shown that the critical behavior of certain physical systems can be described using Landau functionals that include nonanalytic terms, corresponding to odd or even noninteger powers of the absolute value of the order parameter. In particular, these nonanalytic terms can determine the order of the phase transition and the values of the critical exponents. Here, we show that such terms can also shape the finite-size scaling behavior of fluctuations of observables (e.g., of energy or magnetization) or the response functions (e.g., heat capacity or magnetic susceptibility) at the continuous phase transition point. We demonstrate this on two examples, the equilibrium molecular zipper and the nonequilibrium version of the Curie-Weiss model.

Amidequats have attracted considerable attention in research; however, their surface activity remains largely unknown, and their potential as emulsifiers is still unexplored. In this work, series of novel surface-active amidequats based on ecofriendly caprylic acid were synthesized and their micellization behavior in water was explored systematically. The surface tension, wettability, foamability and stability, and measurements of melting and crystallization temperatures were employed to characterize the compounds. The functionalization of amidequats containing an alkoxymethyl substituent significantly enhances micellization properties compared to the structurally analogous anionic surfactant, sodium caprylate. Moreover, for optimized molecule geometries, electrical dipole moments were determined and correlated with surface activity. Experimental and theoretical studies indicate that amidequats with 12 carbon atoms in the alkyl chain exhibit the highest surface and foam-forming activity, making them promising emulsifiers. Due to this fact, oil-in-water (o/w) or water-in-oil (w/o) emulsions using the mentioned amidequat were prepared, and their rheological analysis, ζ potential, topography analysis, and particle size were studied. A detailed investigation of the surface properties of amidequats was undertaken to assess their potential application in the cleaning agent and cosmetic industries.

High-pressure induced structural transition from zircon-type phase into scheelite-type phase in YVO$_4$ is studied using ab initio calculations. Several structures with compressed volumes are evaluated, where for every considered volume the c/a ratio and atomic positions are optimised. The transition pressure and transition volume change are calculated. The reports on YVO$_4$ electronic structure and electronic band gap behaviour are followed by results of X-Ray photoemission spectra XPS calculations. Most of our theoretical predictions are compared with experimental results taken from literature.

Topologically nontrivial adiabatic loops of the orientation of a homogeneous external magnetic field drive the walking of paramagnetic colloidal bipeds above a deformed quasi-periodic magnetic square pattern. Depending on the topological properties of the loop we can simultaneously control the walking directions of colloidal bipeds as a function of their size and as a function of the size of a deformed unit cell of the pattern. The bipeds walk performing steps with their two feet alternatingly grounding one foot and lifting the other. The step width of the bipeds is given by a set of winding numbers (wx, wy) ∈ 2 - a set of topological invariants - that can only change by integers as we continuously increase the length of the bipeds. We experimentally use this discrete size dependence for the robust sorting of bipeds according to their length.

This study investigates lead-free organic-inorganic hybrids (C3NH8)2[BiCl5] (ABC) and (C3NH8)2[BiBr5] (ABB), focusing on their structural, dielectric, ferroelectric, and optical properties. Both compounds exhibit paraelectric (I) to ferroelectric (II) phase transitions (PTs) at 230/233 K and 228/229 K, respectively, transitioning from orthorhombic (Pnma) to monoclinic (P21) phases, with distorted [BiX6]3- octahedra forming 1D chains. Quasielastic neutron scattering and solid-state 1H NMR studies reveal the localized motion of azetidinium cations. Dielectric measurements of ABC and ABB show step-like permittivity changes by 11-12 units post-transition I → II, demonstrating reversible switching behavior. Absorption studies reveal band gaps of 3.24 eV for ABC and 2.76 eV for ABB, classifying them as insulators. Luminescence spectra at 77K display 578 nm (ABC) and 708 nm (ABB) emissions, attributed to 3P1,0 → 1S0 transitions. Both compounds demonstrate stable second harmonic generation (SHG) switching of the on-off type. The switching performance is evaluated over multiple thermal cycles using the treq metric, which decreases with increasing temperature change rate and indicates that ABB's SHG switching is approximately 30% faster than that of ABC.

Magnesium-based materials are an interesting solution in terms of medical applications. Alloys that are hard to obtain via standard means may be manufactured via mechanical alloying (MA), which allows the production of materials with complex a chemical composition and non-equilibrium structures. This work aimed to investigate materials obtained by the MA process for 5, 8, 13, and 20 h in terms of their phase composition and changes during heating. The results of thermal XRD analysis were in the temperature range between 25 and 360 °C, which revealed MgZn2, PrZn11, Ca2Mg5Zn13, and Ca phases as well as α-Mg and α-Zn solid solution. The structural analysis features the powder morphology of the analyzed samples, showing cold-welding and fracturing processes leading to their homogenization, which is supported by the EDS results. The base Mg-Zn-Ca alloy was modified by different additions, but a thorough analysis of the influence of praseodymium on its thermal properties has not yet been performed. We chose to focus on Pr addition because it belongs to low-toxicity rare earth metals, which is an essential feature of biomaterials. Also, the Ca2Mg5Zn13 phase is not fully known, as there are no crystallographic data (hkl). Therefore, the investigation is important and scientifically justified.