Research / Annual reports / Main research results of the Institute for Condensed Matter Physics of NAS of Ukraine in 2019

# Main research results of the Institute for Condensed Matter Physics of NAS of Ukraine in 2019

We propose a method that allows one to determine how far an electron system is from the thermal equilibrium after it is excited to nonequilibrium in the pump-probe experiment. The idea is to compare time evolution of fermionic and collective bosonic temperatures, which must agree in equilibrium. We find effective fermionic temperature from the pump-probe photoemission spectroscopy (single-particle excitations) by extracting it from the Fermi-Dirac distribution. Effective bosonic temperature is defined from the Stokes-anti-Stokes ratio in the pump-probe electronic Raman spectroscopy (two-particle excitations). As an example, we study the Falicov-Kimball model which describes both metallic and Mott-insulator phases, and has an exact solution within the nonequilibrium dynamical mean-field theory. We also found, that in contradiction to metallic and insulating phases, an inverse Raman effect occurs for the case of near to critical insulator phase (and effective fermionic temperature remains negative even after the pump is gone). (O.P. Matveev, A.M. Shvaika in collaboration with J. Freericks (Georgetown University, U.S.A.) and T. Devereaux (Stanford University, U.S.A.))

The mixed spin-1 and spin-1/2 Heisenberg octahedral chain with regularly alternating monomeric spin-1 sites and square-plaquette spin-1/2 sites in a magnetic field shows an extraordinarily rich ground-state phase diagram, which includes the uniform and cluster-based Haldane phases, two ferrimagnetic phases of Lieb-Mattis type, two quantum spin liquids and two bound magnon crystals in addition to the fully polarized ferromagnetic phase. The lowest-energy eigenstates in a highly-frustrated parameter region belong to flat bands and hence, low-temperature thermodynamics above the bound magnon-crystal ground states can be satisfactorily described within the localized-magnon approach. The most spectacular quantum ground states relate to three cluster-based Haldane phases, which exhibit a higher-period of a magnetic unit cell due to a spontaneous breaking of the translational symmetry. The cluster-based Haldane phases are constituted from a finite spin cluster in a triplet state (a few connected octahedra), which can be effectively described by the open spin-1 Heisenberg chains with odd number of spins separated from each other by plaquette-singlet (nonmagnetic) state. The cluster-based Haldane phases are subject of current intense interest from the viewpoint of quantum processing of information and quantum computing, because appropriate modification of them could possibly lead to a creation of topologically protected edge states. The results are published in K. Karl'ová, J. Strečka, T. Verkholyak, Phys. Rev. B 100, 094405 (2019).

A schematic of the mixed spin-1 and spin-1/2 Heisenberg octahedral chain and the equivalent two-component lattice-gas model of hard-core monomers valid in a highly frustrated region J2/J13. Black (green) and shaded (violet) parallelograms denote one-magnon and two-magnon states of a square plaquette. Unoccupied white parallelogram denotes fully polarized (zero-magnon) state of a square plaquette.

Molecular dynamics simulations of pristine and carboxylated (6,6) carbon nanotubes (CNTs) immersed in water-chloroform, water-formaldehyde and water-nitromethane mixtures were performed at ambient conditions. The selected co-solvents differed in their degree of polarity (chloroform < formaldehyde < nitromethane) and proticity (formaldehyde is a polar aprotic solvent, while nitromethane is a polar protic solvent). The hydrophobic character of the CNTs was altered via functionalizing one of the nanotube endings with three −COO− groups, or by decorating both endings with three −COO− groups on each side. The immiscibility of water with chloroform and nitromethane leads to a sandwich-like liquid-liquid structure of the solvent mixture. In both cases the pristine CNT preferred to be fully burred inside the organic phase, while partial or complete carboxylation of the CNT openings oriented the nanotube in such a way as to form a channel-like pathway through the organic layer (i.e. −COO− groups touching aqueous phase and the hydrophobic part immersed in the organic phase). Since the nonpolar chloroform molecules were too large to enter the interior of the (6,6) CNT, the nanotube presented a pathway for water transport between the layers. In the case of water-formaldehyde mixture (no phase separation) the hydrophobic character of the CNT was the only decisive factor for the preferential solvation of the tube. Here, −COO− groups were predominantly hydrated, while formaldehyde molecules preferentially solvated the hydrophobic parts of the CNT's exterior and interior (M.Druchok, M.Lukšič, J. Mol. Liq. 291 (2019) 111287).

By applying mapped averaging framework in the case of hard spheres, the alternative ensemble averages for the pressure, the singlet and pair densities are found. The obtained expressions differ from those conventionally used for hard spheres providing an exact correction to simple approximations for the property of interest. In particular, the pressure formula is constructed such that it gives an ensemble average that exactly corrects the second-order virial equation of state. It is shown how this finding is important to improve the accuracy of computer simulation data (Trokhymchuk A.D.).

Magnetic systems, which can be described by the random anisotropy model (RAM), include a wide class of amorphous materials, in particular, the properties of rare earth – transition metal alloys are described using RAM. The influence of structural disorder on the critical behavior of the magnetic systems with random anisotropy is investigated. In particular, it is studied how the critical properties of RAM depend on the distribution of the local anisotropy axis. The most general case of random local anisotropy is considered, where the local anisotropy axis obeys the combined distribution, including extreme cases of isotropic distribution and distribution along the axes of the hypercube. Previous studies for such a system suggest that ferromagnetic ordering exists at space dimension D=3. We reconsider the problem using two different field theoretical renormalization group approaches. Both methods exclude the possibility of a continuous phase transition in this model, that is, confirm an absence of ferromagnetic ordering in the system under study.

Fig. Sketch plot of the two-dimensional RAM. Red discs depict sites of the lattice with spins (black arrows) on them. Random local anisotropy axis direction on each site is shown by light blue lines.

One of the most intriguing subjects in the field of quantum magnetism is the study of effects caused by geometrical frustration. Moreover, a consideration of three-dimensional geometrically frustrated lattices, where those effects lead to the absence of a phase transition between magnetically ordered and paramagnetic phases at finite temperature, appears to be very challending. Besides, because of geometrical frustration, there is no magnetic order at the ground state too. One of such geometrically frustrated lattices is the three-dimensional network of corner sharing tetrahedra, which is also known as pyrochlore lattice. The nature of the ground state of the Heisenberg antiferromagnet on this lattice is still under debate, and results for any thermodynamic quantities are rather scarce. We used the so-called entropy method (modified Pade analysis of high-temperature series) to calculate the heat capacity and the uniform magnetic susceptibility of the spin-half antiferromagnetic Heisenberg model . One of the main results is surprisingly low temperature of the main maximum of the heat capacity which is . Moreover, we have found that the low-temperature behaviour of the heat capacity looks like , the ground-state energy (per site) is about -0.52J, and that a finite value of the uniform magnetic susceptibility at zero temperature agrees with the recent diagrammatic Monte Carlo simulations available at temperatures (Derzhko O.V., Hutak T.I., Krokhmalskyj T.J.).

Star polymers are simplest representatives of the class of branched polymer architectures, consisting of a number f (functionality) of linear branches radiating from a central core. Such structures undergo conformational transformations from polymeric (low number f) to colloidal (large f) type of behavior. This change affects solubility, aggregation and rheological behavior and is of much practical interest. We study the range of size and shape properties of the star molecule and of its individual arms upon this transformation. The quantities like the relative gyration radius pg(f) and asphericity a(f) of single branch are evaluated both theoretically within the frames of continuous chain model and numerically. Computer simulations are done by the dissipative particle dynamics and in part by Monte Carlo method. Theory and simulations provide qualitatively similar trends upon increase of the number of arms. We quantitatively describe the increase of asphericity, elongation and relative size of individual branch of star polymers in approaching the colloidal regime (with increasing the star functionality f)(O. Kalyuzhnyi, K. Haidukivska, V. Blavatska, Ja. Ilnytskyi). O. Kalyuzhnyi, K. Haidukivska, V. Blavatska, Ja. Ilnytskyi, Macromol. Theory Simul. 28, 1900012 (2019) .

A short overview of the problem of research evaluation in Ukraine and the applicability of scientometrics to this end is prepared. Several examples of ambiguity of practical usage of metrics are discussed. In particular, case studies of h-index-based evaluations and rating of researchers and entire institutions are analysed. It is hard to compare even individuals basing exclusively only on this particular metric: the results are not absolute and context-dependent. But even more problematic is to interpret the rating results of entire Institutions relying only on their group h-index. The size (the number of staff contributing to publications) is one of the factors influencing the score (Mryglod O.I.).

ЗDependence of group h-index of 40 Institutions of the Ministery of Education and Science of Ukraine on the number of affiliated authors according to their Scopus profiles (data are collected in April, 2019).