Anna F. Kusmartseva

Electronic tuning of two metals and colossal magnetoresistances in EuWO1+ xN2- x perovskites

A remarkable electronic flexibility and colossal magnetoresistance effects have been discovered in the perovskite oxynitrides EuWO(1+x)N(2-x). Ammonolysis of Eu(2)W(2)O(9) yields scheelite-type intermediates EuWO(4-y)N(y) with a very small degree of nitride substitution (y = 0.04) and then EuWO(1+x)N(2-x) perovskites that show a wide range of compositions -0.16 <or= x <or= 0.46. The cubic lattice parameter varies linearly with x, but electron microscopy reveals a tetragonal superstructure. The previously unobserved x < 0 regime corresponds to oxidation of Eu (hole doping of the Eu:4f band), whereas x > 0 materials have chemical reduction of W (electron doping of the W:5d band). Hence, both the Eu and W oxidation states and the hole/electron doping are tuned by varying the O/N ratio. EuWO(1+x)N(2-x) phases order ferromagnetically at 12 K, and colossal magnetoresistances (CMR) are observed in the least doped (x = -0.04) sample. Distinct mechanisms for the hole and electron magnetotransport regimes are identified.

Piezochromism in Nickel Salicylaldoximato Complexes: Tuning Crystal-Field Splitting with High Pressure

The crystal structures of bis(3-fluoro-salicylaldoximato)nickel(II) and bis(3-methoxy-salicylaldoximato)nickel(II) have been determined at room temperature between ambient pressure and approximately 6 GPa. The principal effect of pressure is to reduce intermolecular contact distances. In the fluoro system molecules are stacked, and the Ni⋅⋅⋅Ni distance decreases from 3.19 Å at ambient pressure to 2.82 Å at 5.4 GPa. These data are similar to those observed in bis(dimethylglyoximato)nickel(II) over a similar pressure range, though contrary to that system, and in spite of their structural similarity, the salicyloximato does not become conducting at high pressure. Ni-ligand distances also shorten, on average by 0.017 and 0.011 Å for the fluoro and methoxy complexes, respectively. Bond compression is small if the bond in question is directed towards an interstitial void. A band at 620 nm, which occurs in the visible spectrum of each derivative, can be assigned to a transition to an antibonding molecular orbital based on the metal 3d(x(2)-y(2)) orbital. Time-dependent density functional theory calculations show that the energy of this orbital is sensitive to pressure, increasing in energy as the Ni-ligand distances are compressed, and consequently increasing the energy of the transition. The resulting blueshift of the UV-visible band leads to piezochromism, and crystals of both complexes, which are green at ambient pressure, become red at 5 GPa.

Large magnetoresistances and non-Ohmic conductivity in EuWO1+xN2−x

The magnetic field and voltage dependent electronic transport properties of EuWO1+xN2−x ceramics are reported. Large negative magnetoresistances are observed at low temperatures, up to 70% in the least doped (x=0.09) material. Non-Ohmic conduction emerges below the 12 K Curie transition. This is attributed to a microstructure of ferromagnetic conducting and antiferromagnetic insulating regions resulting from small spatial fluctuations in the chemical doping.

Current-voltage characteristics of Weyl semimetal semiconducting devices, Veselago lenses, and hyperbolic Dirac phase

The current-voltage characteristics of a new range of devices built around Weyl semimetals has been predicted using the Landauer formalism. The potential step and barrier have been reconsidered for a three-dimensional Weyl semimetals, with analogies to the two-dimensional material graphene and to optics. With the use of our results we also show how Veselago lens can be made from Weyl semimetals, e.g. from NbAs and NbP. Such lens may have many practical applications and can be used as a probing tip in a scanning tunneling microscope (STM). Because of the ballistic character of Weyl fermion transport inside the semimetal tip and ideal focusing of the Weyl fermions by Veselago lens on the surface of the tip one may create a very narrow electron beam from the tip to the surface of the studied material. With such Weyl semimetal probing tip the resolution of the present STMs can be improved significantly, and one may image not only individual atoms but also individual electron orbitals and chemical bonding and therewith to resolve the long-term issue of the chemical and hydrogen bonds formation. We show that applying a pressure to the Weyl semimental having no centre of spacial inversion one may model a matter at extreme conditions such as that arising in vicinity of a Black Hole. As the materials Cd3As2 and Na3Bi show an asymmetry in their Dirac cones, a scaling factor was used to model this asymmetry. The scaling factor created additional regions of no propagation and condensed the appearance of resonances. We argue that under external pressure there in Weyl semimetals may arise a topological phase transition, where the electron transport changes character and becomes anisotropic. There a hyperbolic Dirac phases occurs where there is a strong light absorption and photo-current generation.

Effects of high pressure on the magnetism of ErCo2

This paper presents the results of a high pressure study on the ferrimagnetic compound ErCo2, which exhibits at ambient pressure magnetic ordering below TC = 33 K, where the localized Er magnetic moments order ferromagnetically, and the itinerant moments in the Co sublattice, align antiparallel to the Er moments. In the paramagnetic range, clustering of the Co moments is observed below Tf ∼ 100 K for which the term “parimagnetism” has been proposed. In contrast to the pronounced peak in the AC susceptibility data at TC only a tiny “bump” shows up at Tf. The decrease of both TC and Tf with applied hydrostatic pressure has been observed, the first one being in agreement with previously published data. At higher pressures, an asymmetry in the TC-related AC susceptibility peak has been observed owing to a decoupling of the magnetic ordering phenomena in the Er and Co sublattices. Compared to earlier published results, our magnetization measurements at higher pressures above PC reveal a small but clearly obser...

Novel metallic states at low temperatures

We present an overview of unconventional metallic states arising close to magnetic quantum critical points with a focus on d-electron systems. The applicability and potential breakdowns of traditional self-consistent field theories of such materials are discussed as well as related phenomena in other systems.

Pressure suppression of charge order without metallisation in Cs2Au2I6

Resistivity and powder X-ray diffraction measurements on Cs(2)Au(2)I(6) up to 320 kbar pressure show that the suppression of Au(+)/Au(3+) charge order above 55 kbar results in a non-metallic phase containing localised Au(2+) states that is irreversibly amorphised above 120 kbar, and a gradual metallisation observed above 175 kbar may result from decomposition within the amorphous material.

Spin-orbital polarons in electron doped copper oxides

Abstract Present work demonstrates the formation of spin-orbital polarons in electron doped copper oxides, that arise due to doping-induced polarisation of the oxygen orbitals in the CuO 2 planes. The concept of such polarons is fundamentally different from previous interpretations. The novel aspect of spin–orbit polarons is best described by electrons becoming self-trapped in one-dimensional channels created by polarisation of the oxygen orbitals. The one-dimensional channels form elongated filaments with two possible orientations, along the diagonals of the elementary CuO 2 square plaquette. As the density of doped electrons increases multiple filaments are formed. These may condense into a single percolating filamentary phase. Alternatively, the filaments may cross perpendicularly to create an interconnected conducting quasi-one-dimensional web. At low electron doping the antiferromagnetic (AFM) state and the polaron web coexist. As the doping is increased the web of filaments modifies and transforms the AFM correlations leading to a series of quantum phase transitions - which affect the normal and superconducting state properties.