Artur Barasiński

Interplay of complete wetting, critical adsorption, and capillary condensation.

The excess adsorption Gamma in two-dimensional Ising strips (infinityxL), subject to identical boundary fields at both one-dimensional surfaces decaying in the orthogonal direction j as -h1j(-p), is studied for various values of p and along various thermodynamic paths below the bulk critical point by means of the density-matrix renormalization-group method. The crossover behavior between the complete-wetting and critical-adsorption regimes, occurring in semi-infinite systems, is strongly influenced by confinement effects. Along isotherms T=const the asymptotic power-law dependences on the external bulk field, which characterize these two regimes, are pre-empted by capillary condensation. Along the pseudo-first-order phase-coexistence line of the strips, which varies with temperature, we find a broad crossover regime in which both the thickness of the wetting film and Gamma increase as functions of the reduced temperature tau but do not follow any power law. Above the wetting temperature the order-parameter profiles are not slablike but exhibit wide interfacial variations and pronounced tails.

Quantum correlations and entanglement in a model comprised of a short chain of nonlinear oscillators

We discuss a model comprised of a chain of three Kerr-like nonlinear oscillators pumped by two modes of external coherent field. We show that the system can be treated as nonlinear quantum scissors and behave as a three-qubit model. For such situation, different types of tripartite entangled states can be generated, even when damping effects are present in the system. Some amount of such entanglement can survive even in a long-time limit. The flow of bipartite entanglement between subsystems of the model and relations among first-order correlations, second-order correlations, and the entanglement are discussed.

Ground-state entanglement of spin-1 bosons undergoing superexchange interactions in optical superlattices

We will discuss a model with ultracold atoms confined in optical superlattices. In particular, we will study the ground-state properties of two spin-1 bosons trapped in a double-well potential. Depending on the external magnetic field and biquadratic interactions, different phases of magnetic order are realized. Applying von Neumann entropy and the number of relevant orbitals, we will quantify the bipartite entanglement between particles. Changing the values of the parameters determining the superlattices, we can switch the system between differently entangled states.

Magnetization-based assessment of correlation energy in canted single-chain magnets

We demonstrate numerically that for the strongly anisotropic homometallic S=2 canted single-chain magnet described by the quantum antiferromagnetic Heisenberg model the correlation energy and exchange coupling constant can be directly estimated from the in-field-magnetization profile found along the properly selected crystallographic direction. In the parameter space defined by the spherical angles (\phi, \theta) determining the axes orientation, four regions are identified with different sequences of the characteristic field-dependent magnetization profiles representing the antiferromagnetic, metamagnetic and weak ferromagnetic type behavior. These sequences provide a criterion for the applicability of the anisotropic quantum Heisenberg model to a given experimental system. Our analysis shows that the correlation energy decreases linearly with field and vanishes for a given value H_{cr} which defines a special coordinates in the metamagnetic profile relevant for the zero-field correlation energy and magnetic coupling. For the single-chain magnet formed by the strongly anisotropic manganese(III) acetate meso-tetraphenylporphyrin complexes coupled to the phenylphosphinate ligands, the experimental metamagnetic-type magnetization curve in the c direction yields an accurate estimate of the values of correlation energy \Delta_{\xi}/k_B = 7.93 K and exchange coupling J/k_B=1.20 K.

Quantum effects and Haldane gap in magnetic chains with alternating anisotropy axes

Abstract The isotropic quantum Heisenberg model with alternating uniaxial anisotropy axes is analyzed numerically by the density-matrix renormalization-group (DMRG) method. In the classical version, the model is applied to describe the magnetic properties of the S = 2 zigzag chain containing Mn(III) acetate meso-tetraphenylporphyrin complexes coupled by the phenylphosphinate ligands which transmit antiferromagnetic interactions. Although the tensors representing the uniaxial magnetic anisotropy D and g factors are non-diagonal in the global coordination system, the DMRG approach has been successfully applied to this complex model in the entire temperature region studied. The predictions of our quantum approach are compared to those previously obtained from the classical one and the importance of quantum effects for analysis of the single-crystal susceptibility and magnetization is demonstrated. At low temperatures the magnetization in the field applied along the c direction increases much more slowly than the classical counterpart. The magnetization behavior is very sensitive to temperature. Moreover, the presence of a magnetization jump in the limit T → 0 at the field H = 3.8 Tesla can be an indication of the Haldane gap of the order of 10.2 K. The considerable differences are demonstrated for the temperature dependent single-crystal susceptibilities, but surprisingly they disappear after averaging over the three crystallographic directions which has not been reported before.

Solvation forces in Ising films with long-range boundary fields: density-matrix renormalization-group study

Using the quasi-exact density-matrix renormalization-group method we calculate the solvation forces in two-dimensional Ising films of thickness L subject to identical algebraically decaying boundary fields with various decay exponents p. At the bulk critical point the solvation force acquires a universal contribution which is long-ranged in L due to the critical fluctuations, a phenomenon known as the critical Casimir effect. For p = 2, 3 and 50, we study the scaling behaviour of the solvation force along the pseudo-phase coexistence and along the critical and sub-critical isotherms.

CsLiSO4/NH4LiSO4 system investigated by EPR

The single crystals of CsLiSO4 and the solid solutions Cs x (NH4)(1− x )LiSO4 have been investigated using the EPR technique. Two kinds of vanadyl doped samples were considered – as-grown from the aqueous solutions of appropriate compounds and after a thermal treatment. It has been established that in as-grown crystals the EPR spectra of vanadyl ions were very complex due to the presence of more than one paramagnetic site in the lattice. At least two such sites were identified and spin Hamiltonian parameters were calculated for all the sites. After annealing the crystals at about 400 K, a significant decrease in the intensities of the EPR lines of VO2+ complexes was observed. In well-annealed samples the complexity of the EPR spectra has been found to disappear. Additionally, the third kind of vanadyl complexes were created during this treatment. It has also been shown that the EPR spectra of Cr3+ ions in CsLiSO4 could be observed without any additional sample treatment reported in literature like an irradiation or thermal decomposition. It was established that the VO2+ and Cr3+, being sensitive to structural phase transition in the CsLiSO4, can be used as a probe for the structural changes in the CsLiSO4/NH4LiSO4 system.

Localizable entanglement as a necessary resource of controlled quantum teleportation

We analyze the controlled teleportation protocol through three-qubit mixed states. In particular, we investigate the relation between the faithfulness of the controlled teleportation scheme and entanglement. While our knowledge concerning controlled teleportation and entanglement in pure states is well established, for mixed states it is considerably much harder task and very little has been done in this field. Here, we present counterintuitive results that provide a new light on controlled teleportation protocol. It is shown that even mixed biseparable states are useful for this protocol along with genuine entangled three-qubit states.

Restriction on the local realism violation in three-qubit states and its relation with tripartite entanglement

Quantum entanglement and non-locality are two special aspects of quantum correlations. The relationship between multipartite entanglement and non-locality is at the root of the foundations of quantum mechanics but there is still no general quantitative theory. In order to address this issue we analyze the relationship between tripartite non-locality and tripartite entanglement measure, called the three-tangle. We describe the states which give the extremal quantum values of a Bell-type inequality for a given value of the tripartite entanglement. Moreover, we show that such extremal states can be reached if one introduced an appropriate order induced by the three-π entanglement measure. Finally, we derive an analytical expression relating tripartite entanglement to the maximal violations of the Bell-type inequalities.

Symmetry restoring and ancilla-driven entanglement for ultra-cold spin-1 atoms in a three-site ring

The spin-change dynamics of a model with ultra-cold hyperfine-spin-1 atoms confined in an optical superlattice is discussed. First, the disturbance of the two-site dynamics by coupling the dimer to a spin-1 ancilla is analyzed. When the dimer is coupled to the ancilla, even by a weak coupling, the significant changes in the system’s time-evolution processes are observed. Next, we show that for the two-particle case the total hyperfine-spin-singlet state is generated by exploiting a quadratic Zeeman shift with realistic values of the strength of external magnetic field and evolution period of time. Moreover, even in a weak coupling regime, the proper choice of the additional ancilla–dimer interaction results in generating the wave function which is characteristic of the homogeneous three-site ring. In consequence, such wave function exhibits translational invariance symmetry despite the strong asymmetry of the lattice. Furthermore, we present our proposal for extracting various kinds of maximally entangled states (MES) for three-site spin-1 systems, starting from initial product states. In particular, we show that the type of generated MES can be unambiguously recognized by the measurement performed on the ancilla.