Electron-doped Sr Research Articles

Magnetic and electronic properties of spin-orbit coupled Dirac electrons on a (001) thin film of double-perovskite Sr2FeMoO6

We present an interacting model for the electronic and magnetic behavior of a strained $(001)$ atomic layer of Sr$_2$FeMoO$_6,$ which shows room-temperature ferrimagnetism and magnetoresistance with potential spintronics application in the bulk. We find that the strong spin-orbit coupling in the molybdenum 4$d$ shell gives rise to a robust ferrimagnetic state with an emergent spin-polarized electronic structure consisting of flat bands and four massive or massless Dirac dispersions. Based on the spin-wave theory, we demonstrate that the magnetic order remains intact for a wide range of doping, leading to the possibility of exploring flat band physics, such as Wigner crystallization in electron-doped Sr$_{2-x}$La$_{x}$FeMoO$_6.$

Spectroscopic Evidence for Electron-Boson Coupling in Electron-Doped Sr_{2}IrO_{4}.

The pseudogap, d-wave superconductivity and electron-boson coupling are three intertwined key ingredients in the phase diagram of the cuprates. Sr_{2}IrO_{4} is a 5d-electron counterpart of the cuprates in which both the pseudogap and a d-wave instability have been observed. Here, we report spectroscopic evidence for the presence of the third key player in electron-doped Sr_{2}IrO_{4}: electron-boson coupling. Akink in nodal dispersion is observed with an energy scale of ∼50 meV. The strength of the kink changes with doping, but the energy scale remains the same. These results provide the first noncuprate platform for exploring the relationship between the pseudogap, d-wave instability, and electron-boson coupling in doped Mott insulators.

Non-Fermi-liquid types of behavior associated with a magnetic quantum critical point in Sr(Co1−xNix)2As2 single crystals

Electron-doped Sr(Co{1-x}Ni{x})2As2 single crystals with compositions x = 0 to 0.9 were grown out of self-flux and SrNi2As2 single crystals out of Bi flux. The crystals were characterized using single-crystal x-ray diffraction (XRD), magnetic susceptibility chi(H,T), isothermal magnetization M(H,T), heat capacity Cp(H,T), and electrical resistivity ho(H,T) measurements versus applied magnetic field H and temperature T. The chi(T) data show that the crystals exhibit an antiferromagnetic (AFM) ground state almost immediately upon Ni doping on the Co site. Ab-initio electronic-structure calculations for x = 0 and x = 0.15 indicate that a flat band with a peak in the density of states just above the Fermi energy is responsible for this initial magnetic-ordering behavior on Ni doping. The Curie-Weiss-like T dependence of \chi in the paramagnetic (PM) state indicates dominant ferromagnetic (FM) interactions. The small ordered moments ~0.1 muB per transition metal atom and the values of the Rhodes-Wohlfarth ratio indicate that the magnetism is itinerant. The Cp(T) at low T exhibits Fermi-liquid behavior for 0 < x < 0.15 whereas an evolution to a logarithmic non-Fermi-liquid (NFL) behavior is found for x = 0.2 to 0.3. The logarithmic dependence is suppressed in an applied magnetic field. The low- T rho(H = 0,T) data show a T^2 dependence for 0 < x < 0.20 and a power-law dependence with n < 2 for x = 0.20 and 0.30. These low-T NFL behaviors observed in the Cp and rho measurements are most evident near the quantum-critical concentration x ~ 0.3 at which a T = 0 composition-induced transition from the AFM phase to the PM phase occurs.

Momentum-resolved lattice dynamics of parent and electron-doped Sr2IrO4

The mixing of orbital and spin character in the wave functions of the $5d$ iridates has led to predictions of strong couplings among their lattice, electronic and magnetic degrees of freedom. As well as realizing a novel spin-orbit assisted Mott-insulating ground state, the perovskite iridate Sr$_{2}$IrO$_{4}$ has strong similarities with the cuprate La$_{2}$CuO$_{4}$, which on doping hosts a charge-density wave that appears intimately connected to high-temperature superconductivity. These phenomena can be sensitively probed through momentum-resolved measurements of the lattice dynamics, made possible by meV-resolution inelastic x-ray scattering. Here we report the first such measurements for both parent and electron-doped Sr$_{2}$IrO$_{4}$. We find that the low-energy phonon dispersions and intensities in both compounds are well described by the same nonmagnetic density functional theory calculation. In the parent compound, no changes of the phonons on magnetic ordering are discernible within the experimental resolution, and in the doped compound no anomalies are apparent due to charge-density waves. These measurements extend our knowledge of the lattice properties of (Sr$_{1-x}$La$_{x}$)$_{2}$IrO$_{4}$ and constrain the couplings of the phonons to magnetic and charge order.

Odd-parity superconductivity driven by octahedra rotations in iridium oxides

Iridium oxides have provided a playground to study novel phases originating from spin-orbit coupling and electron-electron interactions. Among them, the d-wave singlet superconductor was proposed for electron-doped Sr$_2$IrO$_4$, containing two Ir atoms in a unit cell due to the staggered rotation of oxygen octahedra about the c-axis. It was also noted that such oxygen octahedra rotation affects electronic transports. Here we study the role of octahedra tilting away from the c-axis, in determining superconducting pairing symmetry. We show that the octahedra tilting changes the large Fermi surface to a Dirac point, which strongly suppresses the conventional d-wave pairing. Furthermore, it also promotes effective spin-triplet interactions in the strong Hubbard interaction limit, leading to a transition from the even-parity to odd-parity superconducting phase. Thus, tuning octahedra distortions can be used as a tool to engineer a spin triplet superconductor in strongly correlated systems with strong spin-orbit coupling.

Formation of an Antiferromagnetic Metal Phase in the Electron-Doped Sr0.98La0.02MnO3 Oxide According to 17O NMR Data

The spin density distribution of itinerant electrons, neg, and their effect on pairwise correlations of localized spins S(t2g) of Mn4+ ions in cubic Sr1 − xLaxMnO3 antiferromagnet (x = 0.02, TN = 230 K, G type magnetic structure) is studied experimentally by nuclear magnetic resonance on 17O nuclei. The regions with neg > x, in which the local spin susceptibility of pairs of Mn atoms follows the dependence χ ∼ (T − Θ)−1 with Θ = 20(5) K, indicating the growth of ferromagnetic spin correlations of neighboring magnetic ions in these domains, are found in the paramagnetic phase. The fraction of Mn-O-Mn bonds (neg > x) increases with the decrease in the temperature. The interpenetrating meshes of Mn-O-Mn bonds with different densities of itinerant eg electrons form an antiferromagnetic metal phase below TN. The role of weak localization effects is discussed as the main origin of the magnetic inhomogeneity of the antiferromagnetic metal phase in Sr0.98La0.02MnO3.

Effect of Cobalt on the Crystal Structure and Magnetism of Electron-Doped Sr0.8Ce0.2MnO3 Oxide

The Sr0.8Ce0.2Mn1 – yCoyO3 – δ (y = 0.2, 0.3, 0.4, 0.5, and 0.6) complex oxides with the perovskite structure obtained from simple oxides by the solid–state reaction technique have been examined using structural analysis and magnetic measurements. Substitution of Co for Mn in the dilute Sr0.8Ce0.2MnO3 antiferromagnet with a Neel temperature of TN = 210 K leads to a decrease in the degree of tetragonal distortion of the crystal structure and transition to the cubic cell. The degeneracy of the antiferromagnetic interaction (TN = 138 K at y = 0.2) observed at the first stage of the substitution of Co for Mn changes for its gradual enhancement with an increase in the magnetic transformation temperature up to 239 K at y = 0.6. An increase in the Co content weakens the competition between the ferromagnetic and antiferromagnetic couplings and reduces the temperature of the transition to the spin-glass-like state. The magnetic inhomogeneity and formation of Co2+–Mn4+ ferromagnetic clusters in Sr0.8Ce0.2Mn0.4Co0.6O2.69 at 140 K have been observed.

Magnetotransport studies of optimally doped Sr(Fe1−xCox)2As2

We report magnetotransport measurements and their scaling analysis for the optimally electron-doped Sr(Fe0.88CoAs2 system. We observe that both the Kohler’s and modified Kohler’s scalings are violated. Interestingly, the Hall angle displays a quadratic temperature dependence (cot T2) similar to many cuprates and heavy fermion systems. The fact that this T2 dependence is seen in spite of the violation of modified Kohler’s scaling suggests that the Hall angle and magnetoresistance are not governed by the same scattering mechanism. We also observe a linear magnetoresistance in this system, which does not harbor a spin density wave ground state. Implications of our observations are discussed in the context of existing models for the magnetotransport of these strongly correlated electron systems.

Persistent Paramagnons Deep in the Metallic Phase of Sr_{2-x}La_{x}IrO_{4}.

We have studied the magnetic excitations of electron-doped Sr_{2-x}La_{x}IrO_{4} (0≤x≤0.10) using resonant inelastic x-ray scattering at the Ir L_{3} edge. The long-range magnetic order is rapidly lost with increasing x, but two-dimensional short-range order (SRO) and dispersive magnon excitations with nearly undiminished spectral weight persist well into the metallic part of the phase diagram. The magnons in the SRO phase are heavily damped and exhibit anisotropic softening. Their dispersions are well described by a pseudospin-1/2 Heisenberg model with exchange interactions whose spatial range increases with doping. We also find a doping-independent high-energy magnetic continuum, which is not described by this model. The spin-orbit excitons arising from the pseudospin-3/2 manifold of the Ir ions broaden substantially in the SRO phase, but remain largely separated from the low-energy magnons. Pseudospin-1/2 models are therefore a good starting point for the theoretical description of the low-energy magnetic dynamics of doped iridates.

Anisotropic softening of magnetic excitations in lightly electron-dopedSr2IrO4

The magnetic excitations in electron doped (Sr$_{1-x}$La$_x$)$_2$IrO$_4$ with $x = 0.03$ were measured using resonant inelastic X-ray scattering at the Ir $L_3$-edge. Although much broadened, well defined dispersive magnetic excitations were observed. Comparing with the magnetic dispersion from the parent compound, the evolution of the magnetic excitations upon doping is highly anisotropic. Along the anti-nodal direction, the dispersion is almost intact. On the other hand, the magnetic excitations along the nodal direction show significant softening. These results establish the presence of strong magnetic correlations in electron doped Sr$_{1-x}$La$_x$)$_2$IrO$_4$ with close analogies to the hole doped cuprates, further motivating the search for high temperature superconductivity in this system.

Inhomogeneous magnetic state in the electron-doped Sr0.98La0.02MnO3 Manganite According to 55Mn NMR data

55Mn NMR spectra in the magnetically ordered state in Sr0.98La0.02MnO3 manganite have been obtained and the magnetic susceptibility has been measured. It has been shown that the microscopic phase separation into the antiferromagnetic matrix and ferromagnetic clusters, which can be presented as magnetic polarons, is observed in the long-range magnetic order region.

Observation of a d-wave gap in electron-doped Sr2IrO4

High temperature superconductivity in cuprates emerges out of a highly enigmatic `pseudogap' metal phase. The mechanism of high temperature superconductivity is likely encrypted in the elusive relationship between the two phases, which spectroscopically is manifested as Fermi arcs---disconnected segments of zero-energy states---collapsing into $d$-wave point nodes upon entering the superconducting phase. Here, we reproduce this distinct cuprate phenomenology in the 5$d$ transition-metal oxide Sr$_2$IrO$_4$. Using angle-resolved photoemission, we show that clean, low-temperature phase of 6-8$\%$ electron-doped Sr$_2$IrO$_4$ has gapless excitations only at four isolated points in the Brillouin zone with a predominant $d$-wave symmetry of the gap. Our work thus establishes a connection between the low-temperature $d$-wave instability and the previously reported high-temperature Fermi arcs in electron-doped Sr$_2$IrO$_4$. Although the physical origin of the $d$-wave gap remains to be understood, Sr$_2$IrO$_4$ is a first non-cuprate material to spectroscopically reproduce the complete phenomenology of the cuprates, thus offering a new material platform to investigate the relationship between the pseudogap and the $d$-wave gap.

Superconducting Properties and Phase Diagram of Indirectly Electron-Doped $(\hbox{Sr}_{1 - x}\hbox{La}_{x})\hbox{Fe}_{2}\hbox{As}_{2}$ Epitaxial Films Grown by Pulsed Laser Deposition

A non-equilibrium phase (Sr1-xLax)Fe2As2 was formed by epitaxial film-growth. The resulting films emerged superconductivity along with suppression of the resistivity anomaly that is associated with magnetic and structural phase transitions. The maximum critical temperature was 20.8 K, which is almost the same as that of directly electron-doped Sr(Fe1-xCox)2As2. Its electronic phase diagram is much similar to that of Sr(Fe1-xCox)2As2, indicating that the difference in the electron doping sites does not influence the superconducting properties of 122-type SrFe2As2.

Thickness dependence of structural and electrical properties of electron-doped Sr1−xLaxCuO2 infinite-layer thin films grown by pulsed laser deposition

As the building blocks for all of the high-Tc cuprate superconductors, infinite layer (IL) compounds have the simplest structures and the highest Tc of electron-doped superconductors. However, IL structure is one of the high-pressure forms, which makes it difficult to synthesize a single crystal. Therefore, it is highly desirable to obtain high-quality epitaxial thin films making use of epitaxial effect. Although there are many reported attempts to grow IL thin films on different substrates, no one has systematically studied the thickness dependence of structural and electrical properties of IL thin films. In this report, electron-doped Sr1−xLaxCuO2 thin films of various thicknesses were deposited on (001) KTaO3 substrate by pulsed laser deposition. It is shown that IL peak shifts to a lower angle with increase of film thickness, indicating the reduction of the tensile strain. With further increase of thickness, there emerges an impurity phase. Transport measurements showed strong influence of the sample thickness on resistivity and Tc. For a certain thickness range we have demonstrated that superconductivity will occur. The resistive superconducting transition with Tc onset = 14.2 K and Tc (ρ =0) = 10 K has been observed. A moderate thickness is required to obtain IL thin films with superconductivity.

Signature of an antiferromagnetic metallic ground state in heavily electron-doped Sr2FeMoO6

Sr${}_{2}$FeMoO${}_{6}$ is a well-known double perovskite with exciting high-temperature magnetic properties. Through various magnetic and spectroscopic measurements, we collect compelling evidence here that this compound can be driven into a rare three-dimensional antiferromagnetic metallic state by heavy electron doping (70$%$ Sr${}^{2+}$ substitution by La${}^{3+}$). Moreover, local structural study of these Sr${}_{2\ensuremath{-}x}$La${}_{x}$FeMoO${}_{6}$ (1.0 $\ensuremath{\le}x\ensuremath{\le}$ 1.5) compounds reveals unusual atomic scale phase distribution in terms of La,Fe- and Sr,Mo-rich regions driven by strong La-O covalency, a phenomenon hitherto undisclosed in double perovskites. The general trend of our findings is in agreement with theoretical calculations carried out on realistic structures having local chemical fluctuations, which reconfirms the relevance of the kinetic-energy-driven magnetic model.

ELECTRON DOPING EFFECTS ON ELECTRONIC AND MAGNETIC PROPERTIES IN Sr2FeReO6

We have investigated the electronic and magnetic properties of electron-doped Sr 2-x La x FeReO 6 (x = 0.0, 0.25, 0.5) using first-principles density functional theory within the generalized gradient approximation (GGA) and GGA + U schemes. Our results reveal that the symmetry of the La -doped compounds is decreased from tetragonal I4/m for perfect sample to monoclinic P21/n. With increasing La content the absolute magnetic moment of the Re site increases distinctly and the doped electrons are considered to occupy mainly the down-spin Re 5d band from the band calculation. Electronic doping is found to enhance the Curie temperature (T c ) and stabilize the ferromagnetic half-metallic ground states of Sr 2 FeReO 6. And it is found that the increase of T c is mainly caused by the increase of ferromagnetic interaction between the Fe – O – Fe .

Influence of oxygen reduction on the structural and electronic properties of electron-doped Sr 1−xLa xCuO 2 thin films

Single-phase, c-axis oriented, e-doped, Sr 1− x La x CuO 2 thin films were epitaxially grown on KTaO 3 and DyScO 3 substrates by reactive rf sputtering. As-grown films being insulating due to oxygen excess, oxygen reduction is necessary to observe superconductivity. Two different procedures were employed to reach superconductivity. On one hand an in situ reduction process was conducted on a series of films deposited on both type of substrates. On the other hand, an ex situ reduction procedure was performed sequentially on a single film deposited on DyScO 3. The study of the influence of oxygen reduction on the structural and electronic properties of the thin films is presented and discussed.

Theoretical study of the density of states and magnetic properties of electron-doped Sr 1−xCe xCoO 3 compounds

Within the unrestricted Hartree–Fock approximation and the real space recursion method, we calculated the electronic structure of the Sr 1− x Ce x CoO 3 in the whole doping range 0.0⩽ x⩽1.0. Various spin states in an enlarged double cell are investigated and five of them are the ground states at different doping concentration. The magnetic ground state of the doped system takes an intermediate-spin state for x<0.02, a low-spin–intermediate-spin ferromagnetically ordered state for 0.02⩽ x<0.38, and a low-spin–high-spin ferromagnetically ordered state for 0.38⩽ x<0.79. For 0.79⩽ x<0.9, the ground state of the system is the intermediate-spin chain-type antiferromagnetically ordered state, which is followed by the intermediate-spin nearest neighbor antiferromagnetically ordered state up to x=1.0. The densities of states of all magnetic ground states are computed and their electronic and magnetic properties are discussed.

First-principles study of electron doping and disorder effects on electronic and magnetic properties in Sr 2− xNd xFeMoO 6 double perovskites

We have investigated the electronic and magnetic properties of electron-doped Sr 2− x Nd x FeMoO 6 ( x = 0.0 , 0.25 , 0.5 ) using first-principles density functional theory within the generalized gradient approximation (GGA) and GGA + U schemes. Our results reveal that the structural-phase transition occurs for the composition x = 0.5 and the unit cell volume remains nearly constant from x = 0.0 to x = 0.25 . The magnetic moment of the Mo site increases distinctly on electron doping and the doped electrons are considered to occupy mainly the down-spin Mo 4d band from the band calculation. We also have performed the electronic structure calculations for pure and doped Sr 2FeMoO 6 with positional disorder at the Fe/Mo sites. Disordering is found to destroy the half-metallic nature of the ordered compound; moreover, disorder leads to a significant reduction in the net magnetic moment. It is found that the rapid reduction in the magnetic moment is mainly caused by the antiferromagnetic interaction between Fe–O–Fe and the electron doping effects.

Structural and electronic properties of electron-doped Sr1−xLaxCuO2 epitaxial thin films grown by sputtering

We report on the synthesis of electron-doped Sr1−xLaxCuO2 thin films by a sputtering technique. Single phase c-axis oriented thin films were epitaxially grown on various oxide substrates, with better results on DyScO3 and KTaO3. Our preliminary results on the structural and electronic properties of the thin films are presented.