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Exact analytical diagonalization is used to study the bipartite entanglement of the antiferromagnetic mixed spin-(1/2,$S$) Heisenberg dimer (MSHD) with the help of negativity. Under the assumption of uniaxial single-ion anisotropy affecting higher spin-$S$ ($S\!>\!1/2$) entities only, the ground-state degeneracy $2S$ is partially lifted and the ground state is two-fold degenerate with the total magnetization per dimer $\pm(S\!-\!1/2)$. It is shown that the largest quantum entanglement is reached for the antiferromagnetic ground state of MSHD with arbitrary half-odd-integer spins $S$, regardless of the exchange and single-ion anisotropies. Contrary to this, the degree of a quantum entanglement in MSHD with an integer spin $S$ for the easy-plane single-ion anisotropy, exhibits an increasing tendency with an obvious spin-$S$ driven crossing point. It is shown that the increasing spin magnitude is a crucial driving mechanism for an enhancement of a threshold temperature above which the thermal entanglement vanishes. The easy-plane single-ion anisotropy together with an enlargement of the spin-$S$ magnitude is other significant driving mechanism for an enhancement of the thermal entanglement in MSHD., Comment: 4 pages, 4 figures

Liquid crystals (LCs) composed of mesogens play important roles in various scientific and engineering problems. How a system with many mesogens can enter a LC state is an interesting and important problem. Using stiff and free-joint Lennard-Jones chain molecules as mesogens, we study the conditions under which the mesogens can enter various LC phases. The guideline is to eliminate the unwanted translational orders under a controlled fine-tuning procedure across a sequence of systems. Instead of monitoring the growth of order out of the disorder, we prepare a configuration of high orientation ordering and find out where it relaxes to. Such a procedure begins with a reference system, consisting of short chains of homogeneous soft spheres, in a liquid-vapor coexistence situation, at which the thermodynamic instability triggers a fast spontaneous growing process. By applying a short pulse of auxiliary field to align the dispersedly oriented clusters, followed by reducing the volume and, finally, changing the homogeneous molecules into heterogeneous chains, we are able to obtain a range of systems, including nematic and smectic LCs, at their stable ordered states. The model can be extended to study the influence of nanoparticles or external field on the LC structure., Comment: 20 pages, 9 figures

An exhaustive ground-state analysis of extended two-dimensional (2D) correlated spin-electron model consisting of the Ising spins localized on nodal lattice sites and mobile electrons delocalized over pairs of decorating sites is performed within the framework of rigorous analytical calculations. The investigated model, defined on an arbitrary 2D doubly decorated lattice, takes into account the kinetic energy of mobile electrons, the nearest-neighbor Ising coupling between the localized spins and mobile electrons, the further-neighbor Ising coupling between the localized spins and the Zeeman energy. The ground-state phase diagrams are examined for a wide range of model parameters for both ferromagnetic as well as antiferromagnetic interaction between the nodal Ising spins and non-zero value of external magnetic field. It is found that non-zero values of further-neighbor interaction leads to a formation of new quantum states as a consequence of competition between all considered interaction terms. Moreover, the new quantum states are accompanied with different magnetic features and thus, several kinds of discontinuous field-driven phase transitions are observed., Comment: 9 pages, 5 figures

Magnetic properties of a ferronematic, i.e., nematic liquid crystal doped with magnetic nanoparticles in low volume concentration are studied, with the focus on the ac magnetic susceptibility. A weak dc bias magnetic field (units of Oe) applied to the ferronematic in its isotropic phase increases the ac magnetic susceptibility considerably. Passage of the isotropic-to-nematic phase transition resets this enhancement irreversibly (unless the dc bias field is applied again in the isotropic phase). These experimental findings pave a way to application possibilities, such as low magnetic field sensors, or basic logical elements for information storage., Comment: 7 pages, 6 figures, revised version appeared in Soft Matter

The response in capacitance to low external magnetic fields (up to 0.1 T) of suspensions of spherical magnetic nanoparticles, single-wall carbon nanotubes (SWCNT), SWCNT functionalized with carboxyl group (SWCNT-COOH) and SWCNT functionalized with Fe$_3$O$_4$ nanoparticles in a nematic liquid crystal has been studied experimentally. The volume concentration of nanoparticles was $\phi_1$ = 10$^{-4}$ and $\phi_2$ = 10$^{-3}$. Independent of the type and the volume concentration of the nanoparticles, a linear response to low magnetic fields (far below the magnetic Fr\'eederiksz transition threshold) has been observed, which is not present in the undoped nematic.

The radiation stability of biocompatibile magnetic fluid used in nanomedicine after electron irradiation was studied. Two types of the water-based magnetic fluids were prepared. The first one was based on the magnetite nanoparticles stabilized by one surfactant natrium oleate. The second one was biocompatibile magnetic fluid stabilized with two surfactants, natrium oleate as a first surfactant and Poly(ethylene glycol) (PEG) as a second surfactant. The magnetization measurements showed that electron irradiation up to 1000Gy caused 50% reduction of saturation magnetization in the case of the first sample with only one surfactant while in the case of the second biocompatibile magnetic fluid, only 25% reduction of saturation magnetization was observed. In the first magnetic fluid the radiation caused the higher sedimentation of the magnetic particles than in the second case, when magnetic particles are covered also with PEG. The obtained results show that PEG behave as a protective element., Comment: 15 pages, 8 figures

The systematic study of ground-state properties of the three-dimensional Falicov-Kimball model is performed by a well-controlled numerical method. The results obtained are used to categorize the ground-state configurations according to common features for weak, intermediate and strong interactions. It is shown that only a few configuration types form the basic structure of the phase diagram. In particular, the largest regions of stability correspond to phase segregated configurations, striped configurations and configurations in which electrons are distributed in diagonal planes with incomplete chessboard structure. Near half-filling, mixtures of two phases with complete and incomplete chessboard structure are determined. The relevance of these results for a description of real material is discussed., Comment: 9 pages, 4 figures