Ru 4d Electrons Research Articles

Revelation of Mott insulating state in layered honeycomb lattice Li2RuO3

We investigate the role of electron correlation in the electronic structure of honeycomb lattice Li2RuO3 using photoemission spectroscopy and band structure calculations. Monoclinic Li2RuO3 having Ru network as honeycomb lattice undergoes magneto-structural transition at T c ∼ 540 K from high temperature phase C2/m to low temperature dimerized phase P21/m. Room temperature valence band photoemission spectra reveal an insulating ground state with no intensity at Fermi level (E F). Ru 4d band extracted from high and low photon energy valence band photoemission spectra reveal that the surface and bulk electronic structures are very similar in this system. Band structure calculations using generalized gradient approximation leads to metallic ground state while screened hybrid (YS-PBE0) functional reveals opening up of a gap in almost degenerate d zx /d yz orbitals, whereas d xy orbital is already gapped. Ru 3d core level spectra with prominent unscreened feature provides direct evidence of strong electron correlation among Ru 4d electrons which is also manifested by |E − E F|2 dependence of spectral density of states in the vicinity of E F in the high-resolution spectra, establishing Li2RuO3 as Mott insulator.

A Noncentrosymmetric Polymorph of LuRuGe.

We report a new polymorph of LuRuGe, obtained in indium flux. This phase exhibits the noncentrosymmetric ZrNiAl-type structure with the space group P6̅2m as determined by single-crystal X-ray diffraction. This polymorph can convert into another centrosymmetric polymorph (TiNiSi-type structure, space group Pnma) at high temperatures. We performed electrical transport, magnetization, and specific heat measurements on this new phase. It shows metallic behavior with a Hall sign change from negative at 2 K to positive at 125 K. LuRuGe exhibits Pauli paramagnetism as the ground state with no local magnetic moments from either the Ru or Lu site. The Debye temperature Θ = 348 K and electronic coefficient γe = 3.6 mJ K-2 mol-1 are extracted from the low-temperature specific heat data in LuRuGe. We also carried out first-principles density functional theory calculations to map out the electronic band structure and density of states. There are several electronic bands crossing the Fermi level, supporting a multiband scenario consistent with the Hall sign change. The density of states around the Fermi level is mainly from Ru 4d and Ge 4p electrons, indicating a strong hybridization between those atomic orbitals.

First principle calculations of structural and electronic properties of pyrochlore Y2Ru2O7 and Y1-xMxRu2O7−δ (M=Mg, Ca, Sr, Ba, Zn, Cd and Hg)

The electronic structural and electronic properties of pyrochlore Y2Ru2O7 (YRO) and Y1-xMxRu2O7−δ (M = Mg, Ca, Sr, Ba, Zn, Cd and Hg) were investigated using first-principle calculation based on GGA + U and HSE06 hybrid functional. The schematic energy level diagram of YRO was drawn, and its band gap was found to locate in π* antibonding orbital, dominated by Ru-4d electrons. RuO6 octahedrons in YRO undergo a compressing trigonal distortion, with a local symmetry group of D3d, which is quite different from RuO2 and perovskite ruthenium oxide. When Y is substituted by divalent cation M (MY), O vacancies can achieve low formation energy and introduce the MY-VO and 2MY-VO complex. Further calculation showed that MY-VO complex in YRO can induce an impurity energy level into forbidden band, and narrow the band gap. As a result, high electrical conductivity of Y1-xMxRu2O7−δ is expected, and MY-VO complex may be the origin of electrocatalytic activity in Y1-xMxRu2O7.

Metal Substitution Steering Electron Correlations in Pyrochlore Ruthenates for Efficient Acidic Water Oxidation.

Exploring the advanced oxygen evolution reaction (OER) electrocatalysts is highly desirable toward sustainable energy conversion and storage, yet improved efficiency in acidic media is largely hindered by its sluggish reaction kinetics. Herein, we rationally manipulate the electronic states of the strongly electron correlated pyrochlore ruthenate Y2Ru2O7 alternative through partial A-site substitution of Sr2+ for Y3+, efficiently improving its intrinsic OER activity. The optimized Y1.7Sr0.3Ru2O7 candidate observes a highly intrinsic mass activity of 1018 A gRu-1 at an overpotential of 300 mV with excellent durability in 0.5 M H2SO4 electrolyte. Combining synchrotron-radiation X-ray spectroscopic investigations with theoretical simulations, we reveal that the electron correlations in the Ru 4d band are weakened through coordinatively geometric regulation and charge redistribution by the exotic Sr2+ cation, enabling the delocalization of Ru 4d electrons via an insulator-to-metal transition. The induced Ru-O covalency promotion and band alignment rearrangement decreases the charge transfer energy to accelerate interfacial charge transfer kinetics. Meanwhile, the chemical affinity of oxygen intermediates is also rationalized to weaken the metal-oxygen binding strength, thus lowering the energy barrier of the overall reaction. This work offers fresh insights into designing advanced solid-state electrocatalysts and underlines the versatility of electronic structure manipulation in tuning catalytic activity.

The structural, electronic and thermodynamic properties of LaRuX (X = Si, Ge) compounds

The structural, electronic and thermodynamic properties of LaRuX (X = Si, Ge) compounds are investigated using density functional theory by the Wien2k code. Using the first-principles procedure, the Hubbard parameter of La 5d electrons and Ru 4d electrons of LaRuX (X = Si, Ge) compounds is calculated. In these calculations, the exchange–correlation potential is calculated using the generalized gradient approximation (GGA) and generalized gradient approximation plus Hubbard parameter (GGA + U). The calculated results indicate that LaRuX (X = Si, Ge) compounds are stable in the nonmagnetic phase. The electron density of states and band structure of LaRuX (X = Si, Ge) compounds within GGA and GGA + U approaches in the presence of spin orbit coupling are calculated. The obtained results of the electronic band structure show that LaRuX (X = Si, Ge) compounds have metallic behavior. Furthermore, thermodynamic properties of LaRuX (X = Si, Ge) compounds using the quasi-harmonic Debye model within GGA and GGA + U approaches are investigated.

Avoided quantum criticality and cluster-glass formation in itinerant ferromagnet Sr1−x(La0.5K0.5)xRuO3

We demonstrate that the cluster-glass state emerges as ferromagnetic quantum criticality is avoided in itinerant ferromagnet Sr1-x(La0.5K0.5)xRuO3. In this compound, the ferromagnetic order is suppressed by increasing x, and then disappears at the critical concentration: x=0.5. In this x range, the present study reveals that no prominent feature is ascribed to the quantum critical fluctuations in specific heat. Instead, ac magnetic susceptibility exhibits a broad peak due to spontaneous spin freezing, and the peak temperature depends significantly on the frequency of the applied ac magnetic field. Furthermore, specific heat is enhanced within a wide temperature range, whereas specific heat shows no salient anomaly associated with spin freezing. These features are characteristics of the formation of cluster-glass; in particular, the observed frequency variations in ac magnetic susceptibility are well described by the Vogel-Fulcher law. We compare the features concerning the suppression of the ferromagnetic order in this doped compound with those in isostructural Ca- and La-doped SrRuO3, and suggest that a local correlated disorder effect and the very small coherence of itinerant Ru 4d electrons are responsible for the cluster-glass formation instead of the quantum phase transition in Sr1-x(La0.5K0.5)xRuO3.

Superconductivity at 4.8 K and Violation of Pauli Limit in La2IRu2 Comprising Ru Honeycomb Layer.

We discovered superconductivity at 4.8 K in the hexagonal layered compound La2IRu2 comprising a triangular lattice of the La and a honeycomb lattice of the Ru atoms. First-principles calculations reveal a two-dimensional band structure made up of La 5d and Ru 4d electrons and formal oxidation states +1.5 for the La and the uncommon oxidation state -1 for the Ru atoms. The temperature dependence of the specific heat indicates fully gapped superconductivity. Nevertheless, the upper critical field of this compound violates the Pauli limit. We argue that the high upper critical field is ascribed to an antisymmetric spin-orbit coupling in the unique multilayer structure.

Ultrathin Scale Tailoring of Anisotropic Magnetic Coupling and Anomalous Magnetoresistance in SrRuO3-PrMnO3 Superlattices.

A strong perpendicular magnetocrystalline anisotropy (PMA) in antiferromagnetically coupled SrRuO3(17 uc (unit cell))/PrMnO3( n uc) superlattices effectively reconstructs the interfacial spin ordering. The occurrence of significant anisotropic interfacial antiferromagnetic coupling between the Ru and Mn ions is systematically tuned by varying the PrMnO3 layer thickness in ultrathin scale from 3 to 12 uc, which is associated with a rise in PMA energy from 0.28 × 106 to 1.60 × 106 erg/cm3. The analysis using the Stoner-Wohlfarth model and density functional theory confirm that the exchange anisotropy is the major contribution to the PMA. The superlattices with PrMnO3 layer thickness ≥7 uc exhibit the tunneling-like transport of Ru 4d electrons, which is rather expected in the stronger antiferromagnetically coupled superlattices with thinner PrMnO3 layer. Tunneling-like transport at thicker spacer layer in the SrRuO3-PrMnO3 superlattice system is an unique feature of two ferromagnet-based superlattices. Our investigations show that the technologically important interfacial magnetic coupling, PMA, and tunneling magnetoresistance could be achieved in a periodically stacked bilayer and can be precisely manipulated by the size effect in ultrathin scale.

Investigation of optoelectronic and thermoelectric properties of half-metallic BaRuO3 using DFT+U

We have studied the structural, electronic, magnetic, thermoelectric and optical properties of the half-metal BaRuO3 using the accurate full-potential linearized augmented plane wave (FP-LAPW) method based on the density functional theory (DFT). The generalized gradient approximation (GGA) was used to treat the exchange and correlation potential. The GGA[Formula: see text] approximation was also used to enhance the description of the electronic structure after calculating theoretically the Coulomb repulsion ([Formula: see text][Formula: see text]eV). The ferromagnetic (FM) phase of BaRuO3 is more stable. This result is in accordance with experimental and theoretical calculations. The calculated magnetic moments in BaRuO3 were found to arise especially from the Ru-4d state electrons. We have obtained the semiconductor gap (0.9[Formula: see text]eV) in spin-up while in spin down, the metal character was dominant, and therefore BaRuO3 has a half-metallic behavior. The thermoelectric efficiency was 0.12 at room temperature. Here we have considered only the electronic thermal conductivity, we have not included the lattice thermal conductivity. The relaxation time was assumed constant. The [Formula: see text] approximation was also used to analyze the optical properties by determining the complex dielectric function from which are derived the other parameters.

The structural, magnetic and electrical properties of cobalt-doped SrRuO3

The influence of Co doing at Ru site on the structural, magnetic and electrical properties of polycrystalline SrRu1-xCoxO3 (0≤x≤0.3) have been investigated. All the compounds have single phase with Pnma space group. The chemical homogeneity of synthesized samples are confirmed by scanning electron microscopy and energy-dispersive spectroscopy. The X-ray photoelectron spectra of Co and Ru reveal that both two transition metal elements exist in the mixed ionic pair, that is Co3+/ Co2+ and Ru5+/ Ru4+. With Co doping, the ferromagnetic transition temperatures Tc decreases moderately, but the magnetic moment drops substantially. The coercive field Hc is enhanced by the substitution of Co, which may originates from the high degree randomness of Co and Ru ions, as well as from the improved competition between ferromagnetic Ru4+-O-Ru4+ and antiferromagnetic Co2+-O-Ru5+ and/or Co3+-O-Co3+ iterations. Co substitution in SrRu1-xCoxO3 system weakens the itinerant properties of Ru 4d electrons and results in localization of mobile electrons, and thus gives rise to the metal-insulator transition at around x = 0.15.

Effect of Pb doping on metallic state of cubic pyrochlore Cd2Ru2O7

Many exotic phenomena in strongly correlated electron systems, such as unconventional superconductivity, metal-insulator transition, and quantum criticality, take place in the intermediate regime between localized and itinerant electronic state. To understand the electronic behaviors near the localized-to-itinerant crossover remains a challenging problem in condensed matter physics. The Ru5+ cubic pyrochlores A2Ru2O7 (A=Cd, Cd, Hg) constitute such a system that the Ru-4d electrons acquire characters of both itinerancy and localization. In addition, the magnetic Ru5+ ions that are situated on the vertices of corner-shared tetrahedral lattice are expected to experience strong geometrical frustration given an antiferromagnetic (AF) arrangement. In this work, we investigate the cubic pyrochlore Cd2Ru2O7, which develops a peculiar metallic state below the AF transition. We synthesize a series of Pb-doped Cd2-xPbxRu2O7 (0 x 2) polycrystalline samples under high-pressure condition, and study the effects of Pb doping on their crystal structure and physical properties. Although the Pb2Ru2O7 pyrochlore is a Pauli paramagnetic metal, we find that the substitution of 10% Pb2+ for Cd2+ in Cd1.8Pb0.2Ru2O7 converts the metallic state of Cd2Ru2O7 into an insulating ground state, in a manner similar to the consequence of exerting hydrostatic pressure or substituting 10% Ca2+ for Cd2+ ions as we found recently. We propose that the electronic state of Cd2Ru2O7 be located at the itinerancy to localization crossover, and the introduction of chemical disorder via Pb2+ substitution may enhance the localized character and induce the metal-to-insulator transition. Our results further demonstrate that the cubic Ru5+ pyrochlore oxides offer an important paradigm for studying the exotic physics of correlated electrons on the border of (de)localization in the presence of strong geometrical frustration.

Negligible spin-orbit coupling effect in the Mott-insulating antiferromagnet KRuO4

A comprehensive investigation of the electronic and magnetic properties of KRuO4 has been performed using the first-principles calculations in order to clarify the importance of Coulomb interaction and spin-orbit coupling effect. The results indicate that its ground state is of a G-type Mott-insulating antiferromagnet with nearest-neighbor antiferromagnetic coupling, and the computed magnetic moment of Ru7+ ion is 0.50 μB, in nice agreement with the observed value of 0.57(7) μB. In addition, the electronic structure near the Fermi level is dominated by strong hybridized Ru 4d and O 2p states. In sharply contrast with KOsO4, the significantly weaker spin orbit coupling of Ru 4d electrons has negligible impact on the electronic and magnetic properties of KRuO4, and the orbital contribution to the total moment is minor. On the other hand, the on-site Coulomb repulsion affects the band structure significantly, and is indispensable for appraising the electronic properties, opening the band gap, establishing the orbital ordering of the quarter-filled eg manifold.

Correlation effect in Sr1−xLaxRuO3 studied by soft x-ray photoemission spectroscopy

To clarify how the electronic state of Sr1-xLaxRuO3 evolves with La doping, we conducted photoemission (PES) experiments using soft x-rays. The spectral shape of the Ru 4d derived peak near the Fermi level changes significantly with increasing x. This variation indicates that a spectral weight transfer from the coherent to incoherent component occurs due to an enhancement of the electron correlation effect. Resonant PES experiments at the La 3d_{5/2} edge have confirmed that there is no significant contribution of the La 5d state in the energy range where the spectral weight transfer is observed. Using the dependence of the photoelectron mean free path on the photon energy, we subtracted the surface components from the PES spectra and confirmed that the enhancement of the electron correlation effect with La doping is an intrinsic bulk phenomenon. On the other hand, a large portion of the coherent component remains at the Fermi level up to x = 0.5, reflecting that the Ru 4d state still has itinerant characteristics. Moreover, we found that the PES spectra hardly depend on the temperature and do not exhibit a discernible change with magnetic ordering, suggesting that the temperature variation of the exchange splitting does not follow the prediction of the Stoner theory. The presently obtained experimental results indicate that the electron correlation effect plays an important role in Sr1-xLaxRuO3 and that the Ru 4d electrons possess both local and itinerant characteristics.

Enhanced metallic properties of SrRuO3 thin films via kinetically controlled pulsed laser epitaxy

Metal electrodes are a universal element of all electronic devices. Conducting SrRuO3 (SRO) epitaxial thin films have been extensively used as electrodes in complex-oxide heterostructures due to good lattice mismatches with perovskite substrates. However, when compared to SRO single crystals, SRO thin films have shown reduced conductivity and Curie temperatures (TC), which can lead to higher Joule heating and energy loss in the devices. Here, we report that high-quality SRO thin films can be synthesized by controlling the plume dynamics and growth rate of pulsed laser epitaxy (PLE) with real-time optical spectroscopic monitoring. The SRO thin films grown under the kinetically controlled conditions, down to ca. 16 nm in thickness, exhibit both enhanced conductivity and TC as compared to bulk values, due to their improved stoichiometry and a strain-mediated increase of the bandwidth of Ru 4d electrons. This result provides a direction for enhancing the physical properties of PLE-grown thin films and paves a way to improved device applications.

Effect of Si Substitution on the Antiferromagnetic Ordering in the Kondo Semiconductor CeRu2Al10

We have studied the effect of 3p electron doping on the unusual antiferromagnetic (AFM) order in the Kondo semiconductor CeRu2Al10 with TN = 27 K by measuring the magnetic susceptibility χ, specific heat C, and electrical resistivity ρ for polycrystalline samples of CeRu2Al10−ySiy. The large decrease in the absolute value of paramagnetic Curie temperature |θP| with increasing y indicates the suppression of c–f hybridization. The thermal activation behavior in ρ(T) above TN disappears for y ≥ 0.3 and TN decreases to 12 K for y = 0.38. These systematic changes in |θP|, ρ(T), and TN coincide with those reported in the 4d-electron doped system Ce(Ru1−xRhx)2Al10 with respect to the number of doped electrons per formula unit. This coincidence indicates that the Al 3p- and Ru 4d-electrons in CeRu2Al10 play the equivalent role in both the formation of hybridization gap and the unusual AFM ordering.

Experimental measure of metal–alkynyl electronic structure interactions by photoelectron spectroscopy: (η5-C5H5)Ru(CO)2C[tbnd]CMe and [(η5-C5H5)Ru(CO)2]2(μ-C[tbnd]C)

The gas-phase He I and He II photoelectron spectra of the propynylruthenium molecule CpRu(CO)2CCMe (Cp=η5-C5H5) and the ethynediyldiruthenium molecule [CpRu(CO)2]2(μ-CC) are compared with the spectrum of CpRu(CO)2Cl to experimentally determine electronic structure interactions of the alkynyl ligands with the metal. The spectra indicate that the interaction between the filled metal-dπ and filled alkynyl-π orbitals dominates the metal-alkynyl π electronic structure, mirroring previously characterized CpFe(CO)2 alkynyls. All valence ionizations of the Ru molecules are stabilized with respect to similar Fe compounds, contrary to the common expectation of lower ionization energies with atomic substitution down a column of the periodic table. Ab initio electronic structure calculations suggest that this stabilization traces to the greater inherent electronic relaxation energy associated with removal of Fe 3d electrons compared to removal of Ru 4d electrons. Destabilization of the first two ionization bands of the diruthenium molecule are a result of filled–filled interactions between alkynyl π-bonds with the symmetric combination of metal–metal-dπ orbitals, showing electronic communication between the metals through the alkynyl bridge. From the photoelectron spectrum, this communication was calculated to have a minimum electron-transfer integral of 0.56eV. The stabilization of the antisymmetric combination of the metal–metal-dπ orbitals gives a direct and unique experimental measure of the interaction with the alkynyl π∗ orbitals. The stabilization caused by the alkynyl π∗ orbitals was found to be approximately one-third of the destabilization caused by the filled–filled interaction with the alkynyl π-bonds and about one-fourth to one-third the stabilization provided by back-bonding to a carbonyl ligand.

Ferromagnetic Cluster-Glass State in Itinerant Electron System Sr1−xLaxRuO3

The magnetic and electronic properties of Sr1-xLaxRuO3 were studied by means of dc-magnetization, ac-susceptibility, specific heat, and electrical resistivity measurements. The dc-magnetization and ac-susceptibility measurements have revealed that the transition temperature and the ordered moment of the ferromagnetic order are strongly suppressed as La is substituted for Sr. The ac-susceptibility exhibits a peak at T* due to the occurrence of spontaneous spin polarization. Furthermore, we observed that T* shows clear frequency variations for x>= 0.3. The magnitude of the frequency shifts of T* is comparable to that of cluster-glass systems, and the frequency dependence is well described in terms of the Vogel-Fulcher law. On the other hand, it is found that the linear specific heat coefficient gamma enhances with the suppression of the ferromagnetic order. The relatively large gamma values reflect the presence of the Ru 4d state at Fermi level, and hence, the magnetism of this system is considered to be tightly coupled with the itinerant characteristics of the Ru 4d electrons. The present experimental results and analyses suggest that the intrinsic coexistence of the spatially inhomogeneous magnetic state and the itinerant nature of the Ru 4d electrons is realized in this system, and such a feature may be commonly involved in La- and Ca-doped SrRuO3.

Insight of volume-compression-induced changes in SrRuO3 from first-principles calculations

The changes in magnetic stabilization energy with volume for SrRuO3 are examined by means of the density functional theory calculations with on-site Coulomb interaction corrections to Ru 4d electrons. A robust magnetic ground state for SrRuO3 is found even when its volume is compressed to a value close to the experimental volume of the nonmagnetic (NM) CaRuO3. Nevertheless, the important ferromagnetic (FM) and half-metallic (HM) characters of SrRuO3 are found to destabilize during the process of volume compression. Corresponding to the change from FM to antiferromagnetic (AFM) character, the electrical property of the compound changes from conducting into insulating. The electronic structure calculations for SrRuO3 under various volume-compression cases show that the loss of HM is due to Ru 4dt2g states in minority band of SrRuO3 being pushed away from Fermi level. Further examination to the atomic positions before and after volume compression suggests the dramatic change of density of states at Fermi level may be due to the giant internal atomic displacements of Sr/O in SrRuO3.

Redox chemistry and magnetism of LaSrM0.5Ru0.5O4±δ (M = Co, Ni and Zn) Ruddlesden–Popper phases

The n = 1 Ruddlesden-Popper (RP) phases LaSrM(0.5)Ru(0.5)O(4±δ) (M = Co, Ni and Zn) have been prepared by solid state reactions and structurally characterized by powder X-ray and electron diffraction. All the samples adopt the tetragonal I4/mmm space group with random M and Ru cation occupation on the B-sites. The potential causes of no cation ordering are discussed. A combined analysis of the tolerance factors, the distortion of the octahedral coordination of M and Ru cations and the magnetic interactions between M and Ru cations provide a better understanding for forming a phase with 3D cation ordering on the B-sites in the n = 1 RP phases. The investigation of XPS spectra suggests that the transition element species exist as mixed ion pairs, Ru((4-δ)+)-Ru(4+)↔ Co(2+)-Co(3+) in LaSrCo(0.5)Ru(0.5)O(4), and Ru(4+)-Ru((4+δ)+)↔ Ni(+)-Ni(2+) in LaSrNi(0.5)Ru(0.5)O(4), which is consistent with cation disorder over the B sites. LaSrCo(0.5)Ru(0.5)O(4) shows a weakly ferromagnetic behaviour below 50 K; LaSrNi(0.5)Ru(0.5)O(4) is evidenced by the presence of long-range magnetic ordering at a Néel temperature of 125 K, and LaSrZn(0.5)Ru(0.5)O(4) exhibits a paramagnetic behaviour down to 5 K. Due to atomic disorder, Ru4d, O2p covalent coupling is weakened, strengthening the intraatomic spin-spin coupling among the π* electrons. Charge transfer between Ru and Co or Ru and Ni, as well as the increasing overlap of both nearest-neighbour and next-nearest-neighbour Ru 4d electrons due to atomic disorder, favour the formation of ferromagnetic interactions. Although antiferromagnetism is dominant, particularly in LaSrNi(0.5)Ru(0.5)O(4), ferromagnetic interactions are stronger in the title compounds than in the related La(2)MRuO(6) (M = Co, Ni) double perovskites where the B-site cations are ordered.

Strong electron correlation effect of Ru 4d electrons in La 2NiRuO 6 and La 2ZnRuO 6

Double perovskites La 2NiRuO 6 and La 2ZnRuO 6 were studied by use of the density functional method. The unquenched orbital moments were found for both Ni and Ru after spin–orbit coupling effect is included. Electron correlation U is found to be important in reproducing the experimental results. Both compounds are found to be half metallic when U of Ru is less than 1.36 eV, but they are semiconductors for larger U, in agreement with the experimental observation. This reveals that the semiconducting behavior is primarily driven by the electron correlation U.