Antiferromagnetic fluctuations in the normal state of LiFeAs

We present a detailed study of 75As NMR Knight shift and spin-lattice relaxation rate in the normal state of stoichiometric polycrystalline LiFeAs. Our analysis of the Korringa relation suggests that LiFeAs exhibits strong antiferromagnetic fluctuations, if transferred hyperfine coupling is a dominant interaction between 75As nuclei and Fe electronic spins, whereas for an on-site hyperfine coupling scenario, these are weaker, but still present to account for our experimental observations. Density-functional calculations of electric field gradient correctly reproduce the experimental values for both 75As and 7Li sites.Comment: 5 pages, 3 figures, thoroughly revised version with refined experimental data, accepted for publication as a Rapid Communication in Physical Review B

Materials with Colossal Dielectric Constant: Do They Exist?

Experimental evidence is provided that colossal dielectric constants, epsilon >= 1000, sometimes reported to exist in a broad temperature range, can often be explained by Maxwell-Wagner type contributions of depletion layers at the interface between sample and contacts, or at grain boundaries. We demonstrate this on a variety of different materials. We speculate that the largest intrinsic dielectric constant observed so far in non-ferroelectric materials is of order 100.Comment: 3 figure

Tuning a sign of magnetoelectric coupling in paramagnetic NH2(CH3)2Al1-xCrx(SO4)*6H2O crystals by metal ion substitution

Hybrid organometallic systems offer a wide range of functionalities, including magnetoelectric interactions. However, the ability to design on-demand ME coupling remains challenging despite a variety of host-guest configurations and ME phases coexistence possibilities. Here, we report the effect of metal-ion substitution on the magnetic and electric properties in the paramagnetic ferroelectric DMAAS crystals. Doing so we are able to induce and even tune a sign of the ME interactions in the paramagnetic ferroelectric state. Both studied samples with 6.5% and 20% of Cr become paramagnetic, contrary to the initial diamagnetic compound. Due to the isomorphous substitution with Cr the ferroelectric phase transition temperature increases nonlinearly, with the shift being larger for the sample with Cr content of 6.5%. A magnetic field applied along the polar c axis increases ferroelectricity for this sample and shifts Tc to higher values, while inverse effects are observed for sample containing 20% of Cr. The ME coupling coefficient of 1.7ns/m found for a crystal with 20% of Cr is among the highest reported up to now. The observed sign change of ME coupling coefficient with a small change in Cr content paves the way for ME coupling engineering.Comment: 7 pages, 6 figures. New Organometallic Magnetoelectric and Ferroelectric compoun

Tricyanidoferrates(−IV) and ruthenates(−IV) with non‐innocent cyanido ligands

Exceptionally electron-rich, nearly trigonal-planar tricyanidometalate anions [Fe(CN)(3)](7-) and [Ru(CN)(3)](7-) were stabilized in LiSr3[Fe(CN)(3)] and AE(3.5)[M(CN)(3)] (AE=Sr, Ba; M=Fe, Ru). They are the first examples of group 8 elements with the oxidation state of -IV. Microcrystalline powders were obtained by a solid-state route, single crystals from alkali metal flux. While LiSr3[Fe(CN)(3)] crystallizes in P6(3)/m, the polar space group P6(3) with three-fold cell volume for AE(3.5)[M(CN)(3)] is confirmed by second harmonic generation. X-ray diffraction, IR and Raman spectroscopy reveal longer C-N distances (124-128 pm) and much lower stretching frequencies (1484-1634 cm(-1)) than in classical cyanidometalates. Weak C-N bonds in combination with strong M-C pi-bonding is a scheme also known for carbonylmetalates. Instead of the formal notation [Fe-IV(CN-)(3)](7-), quantum chemical calculations reveal non-innocent intermediate-valent CN1.67- ligands and a closed-shell d(10) configuration for Fe, that is, Fe2-

Sr2V3O9 and Ba2V3O9: quasi one-dimensional spin-systems with an anomalous low temperature susceptibility

The magnetic behaviour of the low-dimensional Vanadium-oxides Sr2V3O9 and Ba2V3O9 was investigated by means of magnetic susceptibility and specific heat measurements. In both compounds, the results can be very well described by an S=1/2 Heisenberg antiferromagnetic chain with an intrachain exchange of J = 82 K and J = 94 K in Sr2V3O9 and Ba2V3O9, respectively. In Sr2V3O9, antiferromagnetic ordering at T_N = 5.3 K indicate a weak interchain exchange of the order of J_perp ~ 2 K. In contrast, no evidence for magnetic order was found in Ba2V3O9 down to 0.5 K, pointing to an even smaller interchain coupling. In both compounds, we observe a pronounced Curie-like increase of the susceptibility below 30 K, which we tentatively attribute to a staggered field effect induced by the applied magnetic field. Results of LDA calculations support the quasi one-dimensional character and indicate that in Sr2V3O9, the magnetic chain is perpendicular to the structural one with the magnetic exchange being transferred through VO4 tetrahedra.Comment: Submitted to Phy. Rev.

Principle of work of the vortex heat generator installation

Представленная статья содержит краткий анализ вихревых теплогенерирующих установок (ВТУ), обзор актуальности темы исследования, патентноинформационное обоснование, а также проанализированы современные научнообоснованные факты и результаты, как теоретических, так и экспериментальных работ. Исследованы основные основополагающие (физико-химические, технологические процессы, технико-экономические и законодательные обоснования), а также и другие актуальные вопросы касательно ВТУ. The presented article contains a brief analysis of vortex heat-generating plants, an overview of the relevance of the research topic, patent information justification, and also analyzes modern scientifically-based facts and results of both theoretical and experimental work. The basic fundamental (physicochemical, technological processes, feasibility and legislative justifications), as well as other relevant issues regarding vortex heat-generating installations are investigated

Anisotropic superconductivity and quantum oscillations in the layered dichalcogenide TaSnS2

TaSnS2 single crystal and polycrystalline samples are investigated in detail by magnetization, electrical resistivity, and specific heat as well as Raman spectroscopy and nuclear magnetic resonance (NMR). Studies are focused on the temperature and magnetic field dependence of the superconducting state. We determine the critical fields for both directions B∥c and B⊥c. Additionally, we investigate the dependence of the resistivity, the critical temperature, and the structure through Raman spectroscopy under high pressure up to 10 GPa. At a pressure of ≈3GPa the superconductivity is suppressed below our minimum temperature. The Sn NMR powder spectrum shows a single line which is expected for the TaSnS2 phase and confirms the high sample quality. Pronounced de Haas-van Alphen oscillations in the ac susceptibility of polycrystalline sample reveal two pairs of frequencies indicating coexisting small and large Fermi surfaces. The effective mass of the smaller Fermi surface is ≈0.5me. We compare these results with the band structures from DFT calculations. Our findings on TaSnS2 are discussed in terms of a quasi-two-dimensional BCS superconductivity

Colossal dielectric constants in transition-metal oxides

Many transition-metal oxides show very large ("colossal") magnitudes of the dielectric constant and thus have immense potential for applications in modern microelectronics and for the development of new capacitance-based energy-storage devices. In the present work, we thoroughly discuss the mechanisms that can lead to colossal values of the dielectric constant, especially emphasising effects generated by external and internal interfaces, including electronic phase separation. In addition, we provide a detailed overview and discussion of the dielectric properties of CaCu3Ti4O12 and related systems, which is today's most investigated material with colossal dielectric constant. Also a variety of further transition-metal oxides with large dielectric constants are treated in detail, among them the system La2-xSrxNiO4 where electronic phase separation may play a role in the generation of a colossal dielectric constant.Comment: 31 pages, 18 figures, submitted to Eur. Phys. J. for publication in the Special Topics volume "Cooperative Phenomena in Solids: Metal-Insulator Transitions and Ordering of Microscopic Degrees of Freedom

Dielectric signature of charge order in lanthanum nickelates

Three charge-ordering lanthanum nickelates La2-xAxNiO4, substituted with specific amounts of A = Sr, Ca, and Ba to achieve commensurate charge order, are investigated using broadband dielectric spectroscopy up to GHz frequencies. The transition temperatures of the samples are characterized by additional specific heat and magnetic susceptibility measurements. We find colossal magnitudes of the dielectric constant for all three compounds and strong relaxation features, which partly are of Maxwell-Wagner type arising from electrode polarization. Quite unexpectedly, the temperature-dependent colossal dielectric constants of these materials exhibit distinct anomalies at the charge-order transitions.Comment: 7 pages, 6 figure