arXiv subjects

E. Pomjakushina

Publications and source records attributed to E. Pomjakushina.

At least 145 records · Page 8Linked to original sources

arXiv2010

Magnetic excitations of Fe_{1+y}Se_xTe_{1-x} in magnetic and superconductive phases

We have used inelastic neutron scattering and muon-spin rotation to compare the low energy magnetic excitations in single crystals of superconducting Fe1.01Se0.50Te0.50 and non-superconducting Fe1.10Se0.25Te0.75. We confirm the existence of a spin resonance in the superconducting phase of Fe1.01Se0.50Te0.50, at an energy of 7 meV and a wavevector of (1/2,1/2,0). The non-superconducting sample exhibits two incommensurate magnetic excitations at (1/2,1/2,0)\pm(0.18,-0.18,0) which rise steeply in energy, but no resonance is observed at low energies. A strongly dispersive low-energy magnetic excitation is also observed in Fe1.10Se0.25Te0.75 close to the commensurate antiferromagnetic ordering wavevector (1/2-δ,0,1/2) where δ\approx 0.03. The magnetic correlations in both samples are found to be quasi-two dimensional in character and persist well above the magnetic (Fe1.10Se0.25Te0.75) and superconducting (Fe1.01Se0.50Te0.50) transition temperatures.

cond-mat.supr-con↗

arXiv2009

Pressure Induced Static Magnetic Order in Superconducting FeSe_1-x

We report on a detailed investigation of the electronic phase diagram of FeSe_1-x under pressures up to 1.4GPa by means of AC magnetization and muon-spin rotation. At a pressure \simeq0.8GPa the non-magnetic and superconducting FeSe_1-x enters a region where long range static magnetic order is realized above T_c and bulk superconductivity coexists and competes on short length scales with the magnetic order below T_c. For even higher pressures an enhancement of both the magnetic and the superconducting transition temperatures as well as of the corresponding order parameters is observed. These exceptional properties make FeSe1-x to be one of the most interesting superconducting systems investigated extensively at present.

cond-mat.supr-con↗

arXiv2009

Evolution of two-gap behavior of the superconductor FeSe_1-x

The superfluid density, ρ_s, of the iron chalcogenide superconductor, FeSe_1-x, was studied as a function of pressure by means of muon-spin rotation. The zero-temperature value of ρ_s increases with increasing transition temperature T_c (increasing pressure) following the tendency observed for various Fe-based and cuprate superconductors. The analysis of ρ_s(T) within the two-gap scheme reveals that the effect on both, T_c and ρ_s(0), is entirely determined by the band(s) where the large superconducting gap develops, while the band(s) with the small gap become practically unaffected.

cond-mat.supr-con↗

arXiv2009

Coexistence of incommensurate magnetism and superconductivity in Fe_{1+y}Se_xTe_{1-x}

We report an investigation into the superconducting and magnetic properties of Fe_{1+y}Se_{x}Te_{1-x} single crystals by magnetic susceptibility, muon spin rotation, and neutron diffraction. We find three regimes of behavior in the phase diagram for 0\leq x\leq 0.5: (i) commensurate magnetic order for x< 0.1, (ii) bulk superconductivity for $x\lesssim 0.1$, and (iii) a range \sim 0.25\leq x\leq 0.45 in which superconductivity coexists with static incommensurate magnetic order. The results are qualitatively consistent with a two-band mean-field model in which itinerant magnetism and extended s-wave superconductivity are competing order parameters.

cond-mat.supr-con↗

arXiv2009

Direct observation of charge order and orbital glass state in multiferroic LuFe2O4

Geometrical frustration of the Fe ions in LuFe2O4 leads to intricate charge and magnetic order and a strong magnetoelectric coupling. Using resonant x-ray diffraction at the Fe K edge, the anomalous scattering factors of both Fe sites are deduced from the (h/3 k/3 l/2) reflections. The chemical shift between the two types of Fe ions equals 4.0(1) eV corresponding to full charge separation into Fe2+ and Fe3+. Polarization and azimuthal angle dependence of the superlattice reflections demonstrates the absence of anisotropic scattering revealing random orientations of the Fe2+ orbitals characteristic of an orbital glass state.

cond-mat.str-el↗

arXiv2009

Probing the Yb$^{3+}$ spin relaxation in Y$_{0.98}$Yb$_{0.02}$Ba$_{2}$Cu$_{3}$O$_{x}$ by Electron Paramagnetic Resonance

The relaxation of Yb$^{3+}$ in YBa$_{2}$Cu$_{3}$O$_{x}$ ($6<x<7$) was studied using Electron Paramagnetic Resonance (EPR). It was found that both electronic and phononic processes contribute to the Yb$^{3+}$ relaxation. The phononic part of the relaxation has an exponential temperature dependence, which can be explained by a Raman process via the coupling to high-energy ($\sim$500 K) optical phonons or an Orbach-like process via the excited vibronic levels of the Cu$^{2+}$ ions (localized Slonczewski-modes). In a sample with a maximum oxygen doping $x$=6.98, the electronic part of the relaxation follows a Korringa law in the normal state and strongly decreases below $T_{c}$. Comparison of the samples with and without Zn doping proved that the superconducting gap opening is responsible for the sharp decrease of Yb$^{3+}$ relaxation in YBa$_{2}$Cu$_{3}$O$_{6.98}$. It was shown that the electronic part of the Yb$^{3+}$ relaxation in the superconducting state follows the same temperature dependence as $^{63}$Cu and $^{17}$O nuclear relaxations despite the huge difference between the corresponding electronic and nuclear relaxation rates.

cond-mat.supr-con↗

arXiv2008

LaBaNiO4: A Fermi glass

Polycrystalline samples of LaSr1-xBaxNiO4 show a crossover from a state with metallic transport properties for x = 0 to an insulating state as x -> 1. The end member LaBaNiO4 with a nominal nickel Ni 3d7 configuration might therefore be regarded as a candidate for an antiferromagnetic insulator. However, we do not observe any magnetic ordering in LaBaNiO4 down to 1.5 K, and despite its insulating transport properties several other physical properties of LaBaNiO4 resemble those of metallic LaSrNiO4. Based on an analysis of electrical and thermal-conductivity data as well as magnetic-susceptibility and low-temperature specific-heat measurements we suggest that LaBaNiO4 is a Fermi glass with a finite electron density of states at the Fermi level but these states are localized.

cond-mat.str-el↗

arXiv2008

Muon-spin rotation study of the in-plane magnetic penetration depth of FeSe_0.85: evidence for nodeless superconductivity

The in-plane magnetic penetration depth λ_{ab} of the iron selenide superconductor with the nominal composition FeSe_0.85 was studied by means of muon-spin rotation. The measurements of λ_{ab}^{-2}(T) are inconsistent with the presence of nodes in the gap as well as with a simple isotropic s-wave type of the order parameter, but can be equally well described within a two-gap (s+s) and an anisotropic s-wave gap picture. This implies that the superconducting energy gap in FeSe_0.85 contains no nodes.

cond-mat.supr-con↗

arXiv2008

Microscopic Evidence of Spin State Order and Spin State Phase Separation in Layered Cobaltites RBaCo2O5.5 with R=Y, Tb, Dy, and Ho

We report muon spin relaxation measurements on the magnetic structures of RBaCo_2O_5.5 with R=Y, Tb, Dy, and Ho. Three different phases, one ferrimagnetic and two antiferromagnetic, are identified below 300 K. They consist of different ordered spin state arrangements of high-, intermediate-, and low-spin Co^3+ of CoO_6 octahedra. Phase separation into well separated regions with different spin state order is observed in the antiferromagnetic phases. The unusual strongly anisotropic magnetoresistance and its onset at the FM-AFM phase boundary is explained.

cond-mat.str-el↗

arXiv2008

Spin-state polaron in lightly hole-doped LaCoO_3

Inelastic neutron scattering (INS), electron spin (ESR) and nuclear magnetic resonance (NMR) measurements were employed to establish the origin of the strong magnetic signal in lightly hole-doped La_{1-x}Sr_xCoO_3, x=0.002. Both, INS and ESR low temperature spectra show intense excitations with large effective g-factors ~10-18. NMR data indicate the creation of extended magnetic clusters. From the Q-dependence of the INS magnetic intensity we conclude that the observed anomalies are caused by the formation of octahedrally shaped spin-state polarons comprising seven Co ions.

cond-mat.str-el↗

arXiv2008

A Resonant soft x-ray powder diffraction study to determine the orbital ordering in A-site ordered SmBaMn2O6

Soft X-ray resonant powder diffraction has been performed at the Mn L2,3 edges of A-site ordered SmBaMn2O6. The energy and polarization dependence of the (1/2 1/2 0) reflection provide direct evidence for a (x2-z2)/(y2-z2) type orbital ordering in contrast to the single layer manganite. The temperature dependence of the reflection indicates an orbital reorientation transition at 210 K, below which the charge and orbital ordered MnO2 sheets show AAAA type of stacking. The concurring reduction of the ferromagnetic super exchange correlations leads to further charge localization.

cond-mat.str-el↗

arXiv2007

Oxygen isotope effect on superconductivity and magnetism in Y_1-xPr_xBa_2Cu_3O_7-δ

Oxygen isotope (^{16}O/^{18}O) effects (OIE's) on the superconducting transition (T_c), the spin-glass ordering (T_g), and the antiferromagnetic ordering (T_N) temperatures were studied for Y_1-xPr_xBa_2Cu_3O_7-δas a function of Pr content (0.0\leq x \leq 1.0). The OIE on T_c increases with increasing x up to x\approx0.55, where superconductivity disappears. For decreasing x the OIE's on T_N and T_g increase down to x\approx 0.7 where antiferromagnetic order and down to x\approx0.3 where spin-glass behavior vanish, respectively. The OIE's on T_g and T_N are found to have {\it opposite signs} as compared to the OIE on T_c. All OIE's are suggested to arise from the isotope dependent mobility (kinetic energy) of the charge carriers.

cond-mat.supr-con↗

arXiv2007

Oxygen-isotope effect on the superconducting gap in the cuprate superconductor Y_{1-x}Pr_xBa_2Cu_3O_{7-δ}

The oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the zero-temperature superconducting energy gap Δ_0 was studied for a series of Y_{1-x}Pr_xBa_2Cu_3O_{7-δ} samples (0.0\leq x\leq0.45). The OIE on Δ_0 was found to scale with the one on the superconducting transition temperature. These experimental results are in quantitative agreement with predictions from a polaronic model for cuprate high-temperature superconductors and rule out approaches based on purely electronic mechanisms.

cond-mat.supr-con↗

arXiv2007

Magnetic excitations in the spin-trimer compounds Ca3Cu3-xNix(PO4)4 (x=0,1,2)

Inelastic neutron scattering experiments were performed for the spin-trimer compounds Ca3Cu3-xNix(PO4)4 (x=0,1,2) in order to study the dynamic magnetic properties. The observed excitations can be associated with transitions between the low-lying electronic states of linear Cu-Cu-Cu, Cu-Cu-Ni, and Ni-Cu-Ni trimers which are the basic constituents of the title compounds. The exchange interactions within the trimers are well described by the Heisenberg model with dominant antiferromagnetic nearest-neighbor interactions J. For x=0 we find JCu-Cu=-4.74(2) meV which is enhanced for x=1 to JCu-Cu=-4.92(6) meV. For x=1 and x=2 we find JCu-Ni=-0.85(10) meV and an axial single-ion anisotropy parameter DNi=-0.7(1) meV. While the x=0 and x=1 compounds do not exhibit long-range magnetic ordering down to 1 K, the x=2 compound shows antiferromagnetic ordering below TN=20 K, which is compatible with the molecular-field parameter 0.63(12) meV derived by neutron spectroscopy.

cond-mat.other↗

arXiv2007

Magnetism in Geometrically Frustrated YMnO3 Under Hydrostatic Pressure Studied with Implanted Muons

The rare-earth ferroelectromagnet YMnO3 consists of weakly coupled triangular layers of S=2 spins. Below T_N ~ 70 K muon-spin relaxation data show two oscillatory relaxing signals due to magnetic order, with no relaxing signal resolvable (which would be expected for different coexisting spin distributions). The transition temperature T_N increases with applied hydrostatic pressure, even though the ordered moment decreases. These results suggest that pressure increases both the exchange coupling between the layers and the frustration within the layers.

cond-mat.str-el↗

arXiv2006

Magnetic field dependence of the oxygen isotope effect on the magnetic penetration depth in hole-doped cuprate superconductors

The magnetic field dependence of the oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic field penetration depth λ_{ab} was studied in the hole-doped high-temperature cuprate superconductors YBa_2Cu_4O_8, Y_0.8Pr_0.2Ba_2Cu_3O_7-δ, and Y_0.7Pr_0.3Ba_2Cu_3O_7-δ. It was found that λ_ab for the ^{16}O substituted samples increases stronger with increasing magnetic field than for the ^{18}O ones. The OIE on λ_ab decreases by more than a factor of two with increasing magnetic field from μ_0H=0.2 T to μ_0H=0.6 T. This effect can be explained by the isotope dependence of the in-plane charge carrier mass m^\ast_{ab}.

cond-mat.supr-con↗

arXiv2006

Effect of light Sr doping on the spin-state transition in LaCoO_3

We present an inelastic neutron scattering study of the low energy crystal-field excitations in the lightly doped cobalt perovskite La_0.998Sr_0.002CoO_3. In contrast to the parent compound LaCoO_3 an inelastic peak at energy transfer ~0.75 meV was found at temperatures below 30 K. This excitation apparently corresponds to a transition between a ground state orbital singlet and a higher excited orbital doublet, originating from a high-spin triplet split by a small trigonal crystal field. Another inelastic peak at an energy transfer ~0.6 meV was found at intermediate temperatures starting from T > 30 K. This confirms the presence of a thermally induced spin-state transition from the low-spin Co^3+ to a magnetic high-spin state in the non-disturbed LaCoO_3 matrix. We suggest that hole doping of LaCoO_3 leads to the creation of a magnetic polaron and hence to the low-to-high spin state transition on the relevant Co sites.

cond-mat.str-el↗

arXiv2005

Reduction of the ordered magnetic moment in YMnO3 with hydrostatic pressure

YMnO3 exhibits a ferroelectric transition at high temperature (~ 900 K) and magnetic ordering at T_N ~ 70 K where the dielectric constant shows an anomaly indicative of the magneto-dielectric effect. Here we report powder neutron diffraction experiments in this compound that show that the magnetic moment at saturation is reduced by application of hydrostatic pressure. Our results yield further insight about the nature of the spin-lattice interaction in ferroic materials.

cond-mat.str-el↗