On the "spin-freezing" mechanism in underdoped superconducting cuprates

The letter deals with the spin-freezing process observed by means of NMR-NQR relaxation or by muon spin rotation in underdoped cuprate superconductors. This phenomenon, sometimes referred as coexistence of antiferromagnetic and superconducting order parameters, is generally thought to result from randomly distributed magnetic moments related to charge inhomogeneities (possibly stripes) which exhibit slowing down of their fluctuations on cooling below T$_c$ . Instead, we describe the experimental findings as due to fluctuating, vortex-antivortex, orbital currents state coexisting with d-wave superconducting state. A direct explanation of the experimental results, in underdoped Y$_{1-x}$Ca$_x$Ba$_2$Cu$_3$O$_{6.1}$ and La$_{2-x}$Sr$$CuO$_4$, is thus given in terms of freezing of orbital current fluctuations

Magnetic degeneracy and hidden metallicity of the spin density wave state in ferropnictides

We analyze spin density wave (SDW) order in iron-based superconductors and electronic structure in the SDW phase. We consider an itinerant model for Fe-pnictides with two hole bands centered at $(0,0)$ and two electron bands centered at $(0,\pi)$ and $(\pi,0)$ in the unfolded BZ. A SDW order in such a model is generally a combination of two components with momenta $(0,\pi)$ and $(\pi,0)$, both yield $(\pi,\pi)$ order in the folded zone. Neutron experiments, however, indicate that only one component is present. We show that $(0,\pi)$ or $(\pi,0)$ order is selected if we assume that only one hole band is involved in the SDW mixing with electron bands. A SDW order in such 3-band model is highly degenerate for a perfect nesting and hole-electron interaction only, but we show that ellipticity of electron pockets and interactions between electron bands break the degeneracy and favor the desired $(0,\pi)$ or $(\pi,0)$ order. We further show that stripe-ordered system remains a metal for arbitrary coupling. We analyze electronic structure for parameters relevant to the pnictides and argue that the resulting electronic structure is in good agreement with ARPES experiments. We discuss the differences between our model and $J_1-J_2$ model of localized spins.Comment: reference list updated, typos are correcte

Polaron Effects on Superexchange Interaction: Isotope Shifts of TNT_N, TCT_C, and T∗T^* in Layered Copper Oxides

A compact expression has been obtained for the superexchange coupling of magnetic ions via intermediate anions with regard to polaron effects at both magnetic ions and intermediate anions. This expression is used to analyze the main features of the behavior of isotope shifts for temperatures of three types in layered cuprates: the Neel temperatures ($T_N$), critical temperatures of transitions to a superconducting state ($T_C$), and characteristic temperatures of the pseudogap in the normal state ($T^*$).Comment: 4 pages, 1 figur

Dynamical charge susceptibility in layered cuprates: the influence of screened inter-site Coulomb repulsion

The analytical expression for dynamical charge susceptibility in layered cuprates has been derived in the frame of singlet-correlated band model beyond random-phase-approximation (RPA) scheme. Our calculations performed near optimal doping regime show that there is a peak in real part of the charge susceptibility $\chi({\bf q},\omega)$ at {\bf Q} = ($\pi$, $\pi$) at strong enough inter-site Coulomb repulsion. Together with the strong maximum in the Im $\chi({\bf Q},\omega)$ at 15 meV it confirms the formation of low-energetic plasmons or charge fluctuations. This provides a jsutification that these excitations are important and together with a spin flcutuations can contribute to the Cooper pairing in layered cuprates. Analysing the charge susceptibilitiy with respect to an instability we obtain a new plasmon branch, $\omega_{\bf q}$, along the Brillouin Zone. In particular, we have found that it goes to zero near {\bf Q}$_{CDW} \approx (2\pi/3, 2\pi/3)$

Determination of glass transition temperature for polymers by methods of thermoactivation spectroscopy

For rapid determination of glass transition temperature for polymers, we propose a method of thermally stimulated luminescence. The experiments were carried for epoxy polymers dyed and undyed with organic dyes. It is shown that glass transition temperature depends on curing temperature and concentration of the dye. The comparison with the thermogravimetric analysis showed coincidence of the results obtained

RESULTS OF EXPEDITIONS CARRIED OUT BY KRYMSK EXPERIMENT BREEDING STATION OF VIR TO COLLECT WILD AND RUN-WILD FRUIT AND BERRY PLANTS IN SOUTHERN REGIONS OF RUSSIA

Since the beginning of the 2000s, the ecological and climatic situation in southern regions of Russia has deteriorated sig­nificantly due to the increased anthropogenic impact. In particular, the Republic of Crimea, especially its steppe zone, has been experiencing a real ecological disaster since 2016 because of water deficiency due to the drying up of the North-Crimean Canal after it has been blocked by Ukraine. All this makes it urgent to search for and collect wild and run-wild plant species in their natural habitats, and con­serve them in controlled conditions at VIR’s branches.  Krymsk Experiment Breeding Station has developed a long-term plan for conducting expeditions to collect stone fruit and berry genetic resources for augmenting VIR’s collec­tions. This plan is being implemented on a regular basis. The goal of this work is to explore natural habitats, collect fruit and berry plant diversity, further maintain it in field collections at Krymsk Experiment Breeding Station, and comprehensively study it with the aim of identifying sources of economically important traits for creating new highly adaptive and technologically advanced varieties and root­stocks.  Expeditions were conducted in 2016–2018 in southern areas of the steppe zone and northern steppe regions of Krasno­dar Territory, Volgograd Province, in southeastern regions of Rostov Province, and in the Republic of Crimea. It was es­tablished by field surveys that wild stone fruit plants are found everywhere much less frequently than it was ob­served in previous years, when similar expeditions were launched to these areas. All in all, this work resulted in col­lecting 222 samples of typical representatives of the studied fruit and berry species from the areas of their natural oc­currence. All of them were placed in the nursery of Krymsk Experiment Breeding Station, got established well, and thus augmented the VIR collection of fruit and berry crops.  In view of the observed tendency of the natural distribution areas being reduced for a number of wild stone fruit species due to global climatic changes and recent increase of the an­thropogenic impact, it is necessary to continue conducting expeditions to collect genotypes, especially of such species as ground cherry and Russian almond. In the near future, it is planned to explore the mountainous and Caspian regions in the Republic of Dagestan