Basal Ab Plane Research Articles

Peculiar crystal structure and long-range order of spin-1/2 ladders in MoOBr3

Tetragonal crystal structure of molybdenum oxytribromide, MoOBr3, consists of non-planar Mo2Br6 units joined by non-equidistant Mo-O-Mo bonds into infinite ladders, which are connected to each other by the van der Waals forces. Within unit cell, the apical Mo atoms in any given ladder are shifted along the c axis either above (+z) or below (−z) the basal ab plane of the Br atoms. Magnetic susceptibility taken at various frequencies χac(T) and magnetic fields χdc(T) as well as specific heat Cp(T) data evidence the formation of a long-range antiferromagnetic order at TN = 34 ± 1 K with largely reduced effective magnetic moments µeff = 1.3 ± 0.1 µB. First principles GGA+U calculations point to the predominance of intra-ladder ferromagnetic exchange interaction on both rungs Jrung = − 10 meV and legs Jleg = − 1 meV of the ladder, while much weaker inter-ladder antiferromagnetic exchange interaction Jinter = 0.2 meV couples the ladders. The anisotropic part of the exchange K = 0.6 meV in combination with Jinter allows estimating the spin-flop transition field µ0Hspin-flop ∼ 6 T in good correspondence with the magnetization M(H) data.

Study of the Antiferromagnetic State Nematics in EuFe2As2 by Using Spin-Resonance and Magnetic Measurements

Using electron spin resonance spectroscopy and SQUID-magnetometry we obtained direct evidence of the occurrence of magnetic domains in the antiferromagnetically ordered state of a {text{EuF}}{{{text{e}}}_{{text{2}}}}{text{A}}{{{text{s}}}_{{text{2}}}} single crystal. The resonance spectra of europium ions were measured in the temperature range from 4 to 200 K. Using an equation for the resonance field in an antiferromagnet that takes into account the exchange and anisotropy fields, we have performed an analysis of the angular dependence of the spectrum at a temperature of 4.8 K, measured upon the crystal rotation around the c axis. Data analysis showed that {text{EuF}}{{{text{e}}}_{{text{2}}}}{text{A}}{{{text{s}}}_{{text{2}}}} is the antiferromagnet with easy anisotropy plane. Besides, we found in the ab-plane the second order axes of easy magnetization for each of the two types of magnetic domains, related to the structural transition and the formation of twins. Magnetic anisotropy caused by the exchange interaction of europium ions with iron ions indicates the occurrence of nematic magnetic ordering in the basal ab plane. An estimate of the magnitude of the exchange field and the anisotropy field is obtained from the angular dependence of the resonance fields.

Effects of annealing on the fluctuation conductivity and pseudogap in slightly doped HoBa2Cu3O7–δ single crystals

The effect of annealing at room temperature on the fluctuation conductivity (FLC) σ′(T) and pseudogap (PG) Δ*(7) in the basal ab plane of ReBa2Cu3O7–δ (Re = Ho) single crystals with a lack of oxygen has been studied. It is shown that at all stages of annealing, the FLC near Tc can be described by the Aslamazov-Larkin and Maki-Thompson fluctuation theories, demonstrating a 3D-2D crossover with increasing temperature. The crossover temperature T0 was used to determine the coherence length along the c axis, ξc(0) = (2.82 ± 0.2) Å. At the intermediate stage of annealing, an anomalous increase in 2D FLC was revealed, which is associated with the influence of uncompensated magnetic moments in HoBa2Cu3O7–5 (HoBCO): μeff, Ho = 9.7μΒ. For the quenched sample S1, the temperature dependence of the PG has a shape typical of single crystals with a large number of defects. However, Δ*(T) has two small additional maxima at high temperature, which is a feature of HoBCO single crystals with pronounced twins and indicates the two-phase nature of the sample. Upon annealing, the shape of Δ*(T) noticeably changes, very likely due to an increase in the magnetic interaction (sample S2). More important is the change in the slope of the data at high temperatures, which has become about 3.5 times steeper. The ordering of the oxygen distribution due to the diffusion process during annealing somewhat compensates for the influence of magnetic interaction. But the slope does not change (sample S3). Interestingly, the slope turns out to be the same as for FeAs-based superconductors, suggesting the possibility of the existence of spin density waves in HoBCO in the PG state. The comparison of the pseudogap parameter Δ∗(T)/Δmax∗ near Tc with the Peters-Bauer theory revealed a slight increase in the density of local pairs < n↑n↓>, which should explain the observed increase in Tc by 9 K during annealing.

Impact of temperature and pressure on phase separation in the basal plane of Y0.77Pr0.23Ba2Cu3O7–δ single crystals

Here, we study the impact of high hydrostatic pressure (up to 11 kbar) on the conductivity in the basal ab plane of Y0.77Pr0.23Ba2Cu3O7–δ single crystals. Conversely to YBa2Cu3O7–δ single crystals, it was found that high pressure leads to phase separation in the basal plane of Y0.77Pr0.23Ba2Cu3O7–δ single crystals. The mechanisms of the influence of praseodymium and high hydrostatic pressure on the two-step resistive transition to the superconducting state are discussed.

Antiferromagnetic structure and magnetic properties of Dy2O2Te : An isostructural analog of the rare-earth superconductors R2O2Bi

The rare-earth compounds R2O2Bi (R=Tb, Dy, Er, Lu, Y) are newly discovered superconductors in the vicinity of a rare-earth magnetic long-range order. In this work, we determine the magnetic order of the parent compound Dy2O2Te by neutron scattering as the A-type antiferromagnetic structure below the N\'eel temperature TN=9.7K. The large staggered magnetic moment 9.4(1) {\mu}B per Dy at T=3.5K lies in the basal ab plane. In a magnetic field, anomalous magnetic properties including the bifurcation between zero-field- and field-cooling magnetization, a butterfly-shaped magnetic hysteresis, and slow magnetic relaxation emerge, which are related to the field-induced metamagnetic transitions in Dy2O2Te. Our experimental findings could stimulate further research on the relation between antiferromagnetism and superconductivity in these rare-earth compounds.

Effect of high pressure on temperature dependences of the resistivity in the ab-plane of Y0.77Pr0.23Ba2Cu3O7-δ single crystals

In the present work, we investigated the influence of high hydrostatic pressure up to 11 kbar on the conductivity in the basal ab-plane of medium-doped with praseodymium (x≈0.23) single-crystal Y1-xPrxBa2Cu3O7-δ samples. It was determined that, in contrast to the pure YBa2Cu3O7-δsamples with the optimal oxygen content, the application of high pressure leads to the formation of phase separation in the basal plane of Y0.77Pr0.23Ba2Cu3O7-δ single crystals. Possible mechanisms of the effect of Pr doping and high pressure on the two-step resistive transition to the superconducting state are discussed. It was determined that in the normal state, the conductivity is metallic and is limited by phonons scattering (Bloch-Grüneisen regime) and defects. The fluctuation conductivity is considered within the Lorentz-Doniach model. Hydrostatic pressure, accompanied by a decrease in anisotropy, leads to a decrease in the residual and phonon resistances. Debye temperature and coherence length are independent of pressure. The applicability of the McMillan formula in the presence of significant anisotropy is discussed.

Magnetic anisotropy in the van der Waals ferromagnet VI3

A comprehensive study of magnetocrystalline anisotropy of a layered van-der-Waals ferromagnet VI3 was performed. We measured angular dependences of the torque and magnetization with respect to the direction of the applied magnetic field within the "ac" plane perpendicular to and within the basal ab plane, respectively. A two-fold butterfly-like signal was detected by magnetization in the perpendicular "ac" plane. This signal symmetry remains conserved throughout all magnetic regimes as well as through the known structural transition down to the lowest temperatures. The maximum of the magnetization signal and the resulting magnetization easy axis is significantly tilted from the principal c axis by ~40{\deg}. The close relation of the magnetocrystalline anisotropy to the crystal structure was documented. In contrast, a two-fold-like angular signal was detected in the paramagnetic region within the ab plane in the monoclinic phase, which transforms into a six-fold-like signal below the Curie temperature TC. With further cooling, another six-fold-like signal with an angular shift of ~30{\deg} grows approaching TFM. Below TFM, in the triclinic phase, the original six-fold-like signal vanishes, being replaced by a secondary six-fold-like signal with an angular shift of ~30{\deg}.

Short notes: Effect of hydrostatic pressure up to 12 kbar on the electrical resistance of Y0.77Pr0.23Ba2Cu3O7−δ single crystals

The effect of high hydrostatic pressure up to 12 kbar on the electrical resistivity in the basal ab plane of an optimally doped Y0.77Pr0.23Ba2Cu3O7−δ single crystal was studied. An inhomogeneity of the sample is found that does not depend on the applied pressure, P. The Tc(P) dependence increases nonlinearly with increasing P, and the value of the baric derivative, dTc/dP, decreases. Possible mechanisms of the effect of high pressure on Tc are discussed, taking into account the presence of features in the electronic spectrum of charge carriers.

Influence of high pressure on the temperature dependence of electrical resistivity of Y1-xPrxBa2Cu3O7-δ single crystals

The effect of hydrostatic pressure up to 12 kbar on electrical resistance in the basal ab-plane of optimally oxygen-doped Y1–xPrxBa2Cu3O7–δ single crystals doped with praseodymium (х≈0.23) has been investigated. It was determined that the introduction of praseodymium leads to a fivefold increase in the baric derivative dTc/dP at zero pressure; however, as the pressure rises, dTc/dP decreases to a level characteristic of undoped Y1–xPrxBa2Cu3O7–δ samples with an optimal oxygen content.

Upper Critical Field of a Two-Band SrFe2 − xNixAs2 Superconductor

The upper critical field of an iron-based SrFe2 − xNixAs2 superconductor single crystal is measured for the first time in longitudinal and transverse magnetic fields up to 16 T in the basal ab plane and to it along the c axis. The Hc2(0) values are 18 and 25 T for H ∥ c and H ∥ ab, respectively. The anisotropy of the upper critical field γ(T) = Hc2 / Hc2 decreases monotonically to 1.4 with decreasing temperature. The temperature dependence of Hc2 is not described fully by the Werthamer–Helfand–Hohenberg model taking into account the influence of orbital and spin paramagnetic effects. However, an effective two-band model reproduces well the measured temperature dependence. This indicates the two-band nature of this superconductor, where one of the bands is almost isotropic. According to the two-band model approximation, the contribution from the anisotropic band dominates near Tc, whereas the main contribution at low temperatures comes from the isotropic band with a lower diffusion coefficient.

Pressure effect on the electrical resistance of Y0.77Pr0.23Ba2Cu3O7-δ single crystals

The effect of hydrostatic pressure up to 12 kbar on the electrical resistance in the basal ab-plane of optimally oxygen-doped (δ<0.1) single crystals Y1–xPrxBa2Cu3O7–δ moderately doped with praseodymium (x≈0.23) with a critical temperature Tc≈67 K. Compared to undoped single-crystal YBa2Cu3O7–δ, doping with praseodymium led to a decrease in the critical temperature by ≈24 K with a simultaneous increase in ρab (300 K) by ≈130 μΩcm. In the region of the transition to the superconducting state, several clearly pronounced peaks are observed on the dρ/dT – T curves, which indicates the presence of several phases with different critical temperatures in the sample. The application of high hydrostatic pressure leads to an increase in Tc by about 3 K. This increase slows down with increasing pressure, and the baric derivatives, dTc/dP, decrease from 0.44 K/kbar at atmospheric pressure to 0.14 K/kbar at 11 kbar. The comparatively weak change in the critical temperature under the action of hydrostatic pressure is due to the relatively small value of the orthorhombic distortion, (a–b)/a. The change in the baric derivative dTc/dP upon all-round compression of the sample is due to the fact that, along with an increase in the Debye temperature, the matrix element of the electron-phonon interaction also increases. Possible mechanisms of the effect of high pressure on Tc are discussed taking into account the presence of features in the electronic spectrum of carriers.

Temperature induced structural phase transition in Sr3-xCaxFe2TeO9 (0 ≤ x ≤ 1) probed by Raman and Mossbauer techniques

A series of perovskites Sr3–xCaxFe2TeO9 (0 ≤ x ≤ 1) have been prepared in polycrystalline form by solid–state reaction method in air. These materials have been studied by X–ray powder diffraction method (XRPD) and Raman spectroscopy. An analysis of the XRD patterns at room temperature has shown that these compounds crystallize in a tetragonal system, space group I4/m. The structure contains alternating (Fe/Te)2aO6 and (Fe/Te)2bO6 octahedra, tilted in anti–phase in the basal ab–plane. The study of Raman spectroscopy at various temperatures shows a transition from tetragonal to cubic phase: I4/m → Fm3¯m. This phase transition occurs at high–temperature. Analysis of Raman spectra recorded at several temperatures shows that this phase transition appears near ∼375 °C for (x = 0), ∼435 °C for (x = 0.5) and ∼451 °C for (x = 1). A proportional and gradual increase of temperature phase transition is observed as function of the calcium amount.

Up-up-down-down magnetic chain structure of the spin- 12 tetragonally distorted spinel GeCu2O4

GeCu2O4 exhibits a tetragonal spinel structure due to the strong Jahn-Teller distortion associated with Cu2+ ions. We show that its magnetic structure can be described as slabs composed of a pair of layers with orthogonally oriented spin 1/2 Cu chains in the basal ab plane. The spins between the two layers within a slab are collinearly aligned while the spin directions of neighboring slabs are perpendicular to each other. Interestingly, we find that spins along each chain form an unusual up-up-down-down (UUDD) pattern, suggesting a non-negligible nearest-neighbor biquadratic exchange interaction in the effective classical spin Hamiltonian. We hypothesize that spin-orbit coupling and orbital mixing of Cu2+ ions in this system is non-negligible, which calls for future calculations using perturbation theory with extended Hilbert (spin and orbital) space and calculations based on density functional theory including spin-orbit coupling and looking at the global stability of the UUDD state.

Effect of Long Aging on the Resistivity Properties of Optimally Doped <inline-formula> <tex-math notation="TeX">$\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7-\delta}$</tex-math></inline-formula> Single Crystals

We investigate conducting properties in the basal abplane before and after a long time exposure in air atmosphere of optimally oxygen doped YBa 2 Cu 3 O 7-δ single crystals. Prolonged exposure leads to an increase of the effective scattering centers of the normal carriers. The excess conductivity in a wide temperature range has exponential temperature dependence and near the critical temperature is well described within the AslamazovLarkin theoretical model. The prolonged exposure increases to a great extent the temperature range of the implementation of the pseudogap state, narrowing the linear section of the temperature dependence of the resistivity in the ab-plane, ρ ab (T).

First-principles study of structural, electronic and magnetic properties of double perovskite oxides Ba2CoMO6 (M=Mo and W)

Ba2CoMO6 (M=Mo and W) ordered double perovskite oxides have been investigated by using the first-principles of full potential linearized muffin-tin orbital (FP-LMTO) computational method. The structural, electronic and magnetic properties of Ba2CoMO6 were calculated using both the local spin density approximation (LSDA) and generalized gradient approximation (GGA) methods. The room temperature crystal structures of Ba2CoMO6 are face-centered cubic (space group Fm3¯m and tilt system a0a0a0) with lattice constants of (a=8.011Å) and (a=8.031Å) for (M=Mo) and (M=W), respectively. The crystals of Ba2CoMO6 contain alternating CoO6 and MO6 octahedra, almost fully ordered in the basal ab planes. It was shown that the obtained lattice constants agree well with the experimental data. The influence of M-cation on structural, electronic and magnetic properties of Ba2CoMO6 compounds is analyzed. The total and partial densities of states (DOSs) and partial and total spin magnetic moments are calculated, and the valence states of Co and M ions are examined. The magnetic and electronic properties and bond valence sums are consistent with the electronic configuration Co2+ (3d7)–M6+ (4d/5d)0 with Co2+ in high spin states.

Electron transport and stability of the oxygen subsystem of YBa2Cu3O7−δ single crystals upon prolonged exposure to air

Conductivity in the basal ab-plane of optimally oxygen-doped YBa2Cu3O7−δ single crystals was investigated before and after a prolonged exposure to air. The prolonged exposure was shown to increase the number of effective scattering centers for normal carriers. The excess conductivity for the samples studied was found to be an exponential function of temperature in a wide temperature range, while near the critical temperature Tc it is satisfactorily described by the Aslamazov–Larkin model. It was found that a prolonged exposure to air results in a considerable extension of the temperature range in which the pseudogap state is realized and a narrowing of the linear part of the ρab(T) dependence.

Effect of pressure on the critical temperature of single-crystal Y0.95Pr0.05Ba2Cu3O7–δwith a specified planar defect geometry

The effect of high hydrostatic pressures up to 17 kbar on the conductivity in the basal ab-plane of single crystal samples of Y1–xPrxBa2Cu3O7–δ with a system of unidirectional twin boundaries and lightly doped with praseodymium (x = 0.05) is studied. It is found that, in contrast with undoped YBa2Cu3O7-δ samples with an optimum oxygen content, high pressure leads to a double increase in the pressure derivative dTc/dP. Possible mechanisms for the effect of high pressure on Tc are discussed taking the existence of singularities in the carrier electronic spectrum into account.

Phase segregation and the effect of high pressure on the electro-transport in Y0.95Pr0.05Ba2Cu3O7-δ single crystals

The effect of high hydrostatic pressures of up to 17 kbar on the basal ab-plane conductivity of lightly praseodymium-doped (x ≈ 0.05) Y 1-x Pr x Ba 2 Cu 3 O 7-δ single crystals with unidirectional twin boundaries (TBs) is investigated. It is observed that the application of a high pressure leads to a doubling of the pressure derivative value dTc/dP for Pr -doped samples compared to non-doped YBa 2 Cu 3 O 7-δ samples with optimal oxygen content. Possible mechanisms of the high pressure effect on the critical temperature Tc and the phase segregation in the sample volume are discussed.

Effect of size reduction on the structural and magnetic order in LaMnO3+δ (δ ≈ 0.03) nanocrystals: a neutron diffraction study

We report a structural transition from the orthorhombic to the rhombohedral phase upon size reduction in nanocrystalline LaMnO3+δ (δ ≈ 0.03) as revealed through neutron diffraction studies. The transition occurs when the average particle (crystallite) size is taken below ∼50 nm without change of δ, which is fixed at around 0.03 as measured by a number of characterization tools. The change in the crystallographic structure is accompanied by a change in the magnetic order, where the canted antiferromagnetic order with moments in the basal (ab) plane for the bulk changes to collinear ferromagnetic order with spins along the c-axis for the nanocrystals. The spontaneous ferromagnetic moment ∼3 μB and the transition temperature of 260 K in LaMnO3+δ nanocrystals are similar to those found in La0.67Ca0.33MnO3 which has a much higher Mn4+ content. The likely origin is traced to change in magnetic exchange interactions due to change in Mn–O bond lengths which become almost identical in the MnO6 octahedron in the rhombohedral structure in the absence of Jahn–Teller distortion. The study provides an example of structural and magnetic phase transition driven purely by size reduction and with no change in the chemical constituents.

Effect of Long Aging on the Resistivity Properties of Aluminum Doped YBa2Cu3−y Al y O7−δ Single Crystals with a Given Twin Boundary Topology

We investigate the conducting properties in the basal ab-plane before and after a long time exposure in air atmosphere of aluminum doped YBa2Cu3−y Al y O7−δ single crystals. Prolonged exposure leads to an increase of the effective scattering centers of the normal carriers. The excess conductivity in a wide temperature range has exponential temperature dependence and near the critical temperature (T c ) can be adequately described within the Aslamazov-Larkin theoretical model. Nevertheless, the description of the excess conductivity through the relation $\Delta\sigma\sim (1-T/T ^{*}) \exp (\Delta^{*}_{ab} /T)$ , can be interpreted in terms of the mean-field theory, where T ∗ is the mean field temperature of the superconducting transition and the temperature dependence of the pseudo-gap is satisfactorily described by the BEC-BCS crossover theory. It is determined that the prolonged exposure increases significantly the temperature range of the implementation of the pseudogap state, thus narrowing the linear section of the resistivity dependence ρ ab (T).