Transport properties, thermodynamic properties, and electronic structure of SrRuO3

Lattice effects on the physical properties of half-doped perovskite ruthenates

We investigate the unusual phase transitions in SrRuO₃and Sr_0.5Ca_0.5Ru1-xCr_xO₃(x = 0, 0.05 and 0.1) employing x-ray diffraction, resistivity, magnetic studies and x-ray photoemission spectroscopy. Our results show the compounds undergo a crossover from itinerant ferromagnetism to localized ferromagnetism. The combined studies suggest Ru and Cr be in the 4+ valence state. A Griffith phase and an enhancement in Curie temperature (T_c) from 38 K to 107 K are observed with Cr doping. A shift in the chemical potential towards the valence band is observed with Cr doping. In the metallic samples, interestingly, a direct link between the resistivity and orthorhombic strain is observed. We also observe a connection between orthorhombic strain andT_cin all the samples. Detailed studies in this direction will be helpful to choose suitable substrate materials for thin-film/device fabrication and hence manoeuvre its properties. In the non-metallic samples, the resistivity is mainly governed due to disorder, electron-electron correlation effects and a reduction in the number of electrons at the Fermi level. The value of the resistivity for the 5% Cr doped sample suggests semi-metallic behaviour. Understanding its nature in detail using electron spectroscopic techniques could unravel the possibility of its utility in high-mobility transistors at room temperature and its combined property with ferromagnetism will be helpful in making spintronic devices.

Light-Induced Mott-Insulator-to-Metal Phase Transition in Ultrathin Intermediate-Spin Ferromagnetic Perovskite Ruthenates

Light control of emergent quantum phenomena is a widely used external stimulus for quantum materials. Generally, perovskite strontium ruthenate SrRuO₃ has an itinerant ferromagnetism with a low-spin state. However, the phase of intermediate-spin (IS) ferromagnetic metallic state has never been seen. Here, by means of UV-light irradiation, a photocarrier-doping-induced Mott-insulator-to-metal phase transition is shown in a few atomic layers of perovskite IS ferromagnetic SrRuO_{3- δ} . This new metastable IS metallic phase can be reversibly regulated due to the convenient photocharge transfer from SrTiO₃ substrates to SrRuO_{3- δ} ultrathin films. These dynamical mean-field theory calculations further verify such photoinduced electronic phase transformation, owing to oxygen vacancies and orbital reconstruction. The optical manipulation of charge-transfer finesse is an alternative pathway toward discovering novel metastable phases in strongly correlated systems and facilitates potential light-controlled device applications in optoelectronics and spintronics.

Evidence of electron correlation and weak bulk plasmon in SrMoO3

We investigate the electronic structure of highly conducting perovskite SrMoO₃using valence band photoemission spectroscopy and electronic structure calculations. Large intensity corresponding to coherent feature close to Fermi level is captured by density functional theory (DFT) calculation. An additional satellite at ∼3 eV binding energy remains absent in DFT, hybrid functional (DFT-hybrid) and dynamical mean field theory (DFT + DMFT) calculations. Mo 4dspectra obtained with different surface sensitive photoemission spectroscopy suggest different surface and bulk electronic structures. DFT + DMFT spectral function is in excellent agreement with the coherent feature in the bulk Mo 4dspectra, revealing moderate electron correlation strength. A large plasmon satellite and signature of strong electron correlation are observed in the surface spectra, while the bulk spectra exhibits aweakplasmon satellite.

Mooij Law Violation from Nanoscale Disorder

Nanoscale inhomogeneity can profoundly impact properties of two-dimensional van der Waals materials. Here, we reveal how sulfur substitution on the selenium atomic sites in Fe_1-ySe_1-xS_x (0 ≤ x ≤ 1, y ≤ 0.1) causes Fe-Ch (Ch = Se, S) bond length differences and strong disorder for 0.4 ≤ x ≤ 0.8. There, the superconducting transition temperature T_c is suppressed and disorder-related scattering is enhanced. The high-temperature metallic resistivity in the presence of strong disorder exceeds the Mott limit and provides an example of the violation of Matthiessen's rule and the Mooij law, a dominant effect when adding moderate disorder past the Drude/Matthiessen's regime in all materials. The scattering mechanism responsible for the resistivity above the Mott limit is unrelated to phonons and arises for strong Se/S atom disorder in the tetrahedral surrounding of Fe. Our findings shed light on the intricate connection between the nanostructural details and the unconventional scattering mechanism, which is possibly related to charge-nematic or magnetic spin fluctuations.

Stranger than metals

In traditional metals, the temperature ( T ) dependence of electrical resistivity vanishes at low or high T , albeit for different reasons. Here, we review a class of materials, known as “strange” metals, that can violate both of these principles. In strange metals, the change in slope of the resistivity as the mean free path drops below the lattice constant, or as T → 0, can be imperceptible, suggesting continuity between the charge carriers at low and high T . We focus on transport and spectroscopic data on candidate strange metals in an effort to isolate and identify a unifying physical principle. Special attention is paid to quantum criticality, Planckian dissipation, Mottness, and whether a new gauge principle is needed to account for the nonlocal transport seen in these materials.

Structural Phase Transition and In-Situ Energy Storage Pathway in Nonpolar Materials: A Review

Benefitting from exceptional energy storage performance, dielectric-based capacitors are playing increasingly important roles in advanced electronics and high-power electrical systems. Nevertheless, a series of unresolved structural puzzles represent obstacles to further improving the energy storage performance. Compared with ferroelectrics and linear dielectrics, antiferroelectric materials have unique advantages in unlocking these puzzles due to the inherent coupling of structural transitions with the energy storage process. In this review, we summarize the most recent studies about in-situ structural phase transitions in PbZrO₃-based and NaNbO₃-based systems. In the context of the ultrahigh energy storage density of SrTiO₃-based capacitors, we highlight the necessity of extending the concept of antiferroelectric-to-ferroelectric (AFE-to-FE) transition to broader antiferrodistortive-to-ferrodistortive (AFD-to-FD) transition for materials that are simultaneously ferroelastic. Combining discussion of the factors driving ferroelectricity, electric-field-driven metal-to-insulator transition in a (La_1−xSr_x)MnO₃ electrode is emphasized to determine the role of ionic migration in improving the storage performance. We believe that this review, aiming at depicting a clearer structure–property relationship, will be of benefit for researchers who wish to carry out cutting-edge structure and energy storage exploration.

Infinite-layer/perovskite oxide heterostructure-induced high-spin states in SrCuO2/SrRuO3 bilayer films

Heterostructures composed of dissimilar oxides with different properties offer opportunities to develop emergent devices with desired functionalities. A key feature of oxide heterostructures is interface electronics and orbital reconstructions. Here, we combined infinite-layered SrCuO₂ and perovskite SrRuO₃ into heterostructures. A rare high spin state as large as 3.0 μ_B f.u^-1 and an increase in Curie temperature by 12 K are achieved in an ultrathin SrRuO₃ film capped by a SrCuO₂ layer. Atomic-scale lattice imaging shows the uniform CuO₂-plane-to-RuO₅-pyramid connection at the interface, where the regularly arranged RuO₅ pyramids were elongated along the out-of-plane direction. As revealed by theoretical calculations and spectral analysis, these features finally result in an abnormally high spin state of the interfacial Ru ions with highly polarized e_g orbitals. The present work demonstrates that oxygen coordination engineering at the infinite-layer/perovskite oxide interface is a promising approach towards advanced oxide electronics.

Orbital Isotropy of Magnetic Fluctuations in Correlated Electron Materials Induced by Hund's Exchange Coupling

Characterizing nonlocal magnetic fluctuations in materials with strong electronic Coulomb interactions remains one of the major outstanding challenges of modern condensed matter theory. In this Letter, we address the spatial symmetry and orbital structure of magnetic fluctuations in perovskite materials. To this aim, we develop a consistent multiorbital diagrammatic extension of dynamical mean-field theory, which we apply to an anisotropic three-orbital model of cubic t_{2g} symmetry. We find that the form of spatial spin fluctuations is governed by the local Hund's coupling. For small values of the coupling, magnetic fluctuations are anisotropic in orbital space, which reflects the symmetry of the considered t_{2g} model. Large Hund's coupling enhances collective spin excitations, which mixes orbital and spatial degrees of freedom, and magnetic fluctuations become orbitally isotropic. Remarkably, this effect can be seen only in two-particle quantities; single-particle observables remain anisotropic for any value of the Hund's coupling. Importantly, we find that the orbital isotropy can be induced both at half filling and for the case of four electrons per lattice site, where the magnetic instability is associated with different, antiferromagnetic and ferromagnetic, modes, respectively.

Local Kondo scattering in 4d-electron RuOx nanoclusters on atomically-resolved ultrathin SrRuO3 films

Perovskite SrRuO₃ is a unique 4d transition metal oxide with coexisting spin-orbit coupling (SOC) and electron-electron correlation. However, the intrinsic, non-reconstructed surface structure of SrRuO₃ has not been reported so far. Here we report an atomic imaging of the non-reconstructed, SrO-terminated SrRuO₃ surface by scanning tunneling microscopy/spectroscopy. Moreover, a Kondo resonant behavior is revealed in RuO_x clusters located on top of the nonmagnetic SrO surface layer. The density functional theory calculations confirm that RuO_x clusters possess localized 4d-electron-involved spin moments and hybridize with the conduction electrons in the metal host, resulting in the appearance of the Kondo resonance features around the Fermi level. Our work demonstrates that artificially-engineered transition metal oxides provide new opportunities to explore the Kondo physics in 4d multi-orbital systems.

Thermoelectric materials taking advantage of spin entropy: lessons from chalcogenides and oxides

The interplay between charges and spins may influence the dynamics of the carriers and determine their thermoelectric properties. In that respect, magneto-thermoelectric power MTEP, i.e. the measurements of the Seebeck coefficient S under the application of an external magnetic field, is a powerful technique to reveal the role of magnetic moments on S. This is illustrated by different transition metal chalcogenides: CuCrTiS4 and CuMnTiS4 magnetic thiospinels, which are compared with magnetic oxides, Curie-Weiss (CW) paramagnetic misfit cobaltites, ruthenates, either ferromagnetic perovskite or Pauli paramagnet quadruple perovskites, and CuGa1-x Mn x Te2 chalcopyrite telluride and Bi1.99Cr0.01Te3 in which diluted magnetism is induced by 3%-Mn and 1%-Cr substitution, respectively. In the case of a ferromagnet (below TC) and CW paramagnetic materials, the increase of magnetization at low T when a magnetic field is applied is accompanied by a decrease of the entropy of the carriers and hence S decreases. This is consistent with the lack of MTEP in the Pauli paramagnetic quadruple perovskites. Also, no significant MTEP is observed in CuGa1-x Mn x Te2 and Bi1.99Cr0.01Te3, for which Kondo-type interaction between magnetic moments and carriers prevails. In contrast, spin glass CuCrTiS4 exhibits negative MTEP like in ferromagnetic ruthenates and paramagnetic misfit cobaltites. This investigation of some chalcogenides and oxides provides key ingredients to select magnetic materials for which S benefits from spin entropy.

Epitaxial lift-off of freestanding (011) and (111) SrRuO3 thin films using a water sacrificial layer

Two-dimensional freestanding thin films of single crystalline oxide perovskites are expected to have great potential in integration of new features to the current Si-based technology. Here, we showed the ability to create freestanding single crystalline (011)- and (111)-oriented SrRuO₃ thin films using Sr₃Al₂O₆ water-sacrificial layer. The epitaxial Sr₃Al₂O₆(011) and Sr₃Al₂O₆(111) layers were realized on SrTiO₃(011) and SrTiO₃(111), respectively. Subsequently, SrRuO₃ films were epitaxially grown on these sacrificial layers. The freestanding single crystalline SrRuO₃(011)_pc and SrRuO₃(111)_pc films were successfully transferred on Si substrates, demonstrating possibilities to transfer desirable oriented oxide perovskite films on Si and arbitrary substrates.

Manipulating Berry curvature of SrRuO3 thin films via epitaxial strain

Berry curvature plays a crucial role in exotic electronic states of quantum materials, such as the intrinsic anomalous Hall effect. As Berry curvature is highly sensitive to subtle changes of electronic band structures, it can be finely tuned via external stimulus. Here, we demonstrate in SrRuO₃ thin films that both the magnitude and sign of anomalous Hall resistivity can be effectively controlled with epitaxial strain. Our first-principles calculations reveal that epitaxial strain induces an additional crystal field splitting and changes the order of Ru d orbital energies, which alters the Berry curvature and leads to the sign and magnitude change of anomalous Hall conductivity. Furthermore, we show that the rotation of the Ru magnetic moment in real space of a tensile-strained sample can result in an exotic nonmonotonic change of anomalous Hall resistivity with the sweeping of magnetic field, resembling the topological Hall effect observed in noncoplanar spin systems. These findings not only deepen our understanding of anomalous Hall effect in SrRuO₃ systems but also provide an effective tuning knob to manipulate Berry curvature and related physical properties in a wide range of quantum materials.

Analysis of the temperature- and fluence-dependent magnetic stress in laser-excited SrRuO3

We use ultrafast x-ray diffraction to investigate the effect of expansive phononic and contractive magnetic stress driving the picosecond strain response of a metallic perovskite SrRuO3 thin film upon femtosecond laser excitation. We exemplify how the anisotropic bulk equilibrium thermal expansion can be used to predict the response of the thin film to ultrafast deposition of energy. It is key to consider that the laterally homogeneous laser excitation changes the strain response compared to the near-equilibrium thermal expansion because the balanced in-plane stresses suppress the Poisson stress on the picosecond timescale. We find a very large negative Grüneisen constant describing the large contractive stress imposed by a small amount of energy in the spin system. The temperature and fluence dependence of the strain response for a double-pulse excitation scheme demonstrates the saturation of the magnetic stress in the high-fluence regime.

Subterahertz Momentum Drag and Violation of Matthiessen's Rule in an Ultraclean Ferromagnetic SrRuO_{3} Metallic Thin Film

SrRuO_{3}, a ferromagnet with an approximately 160 K Curie temperature, exhibits a T^{2}-dependent dc resistivity below ≈30  K. Nevertheless, previous optical studies in the infrared and terahertz range show non-Drude dynamics at low temperatures, which seem to contradict Fermi-liquid predictions. In this work, we measure the low-frequency THz range response of thin films with residual resistivity ratios, ρ_{300K}/ρ_{4K}≈74. At temperatures below 30 K, we find both a sharp zero frequency mode which has a width narrower than k_{B}T/ℏ as well as a broader zero frequency Lorentzian that has at least an order of magnitude larger scattering. Both features have temperature dependences consistent with a Fermi liquid with the wider feature explicitly showing a T^{2} scaling. Above 30 K, there is a crossover to a regime described by a single Drude peak that we believe arises from strong interband electron-electron scattering. Such two channel Drude transport sheds light on reports of the violation of Matthiessen's rule and extreme sensitivity to disorder in metallic ruthenates.

Towards electrical-current control of quantum states in spin-orbit-coupled matter

Novel materials, which often exhibit surprising or even revolutionary physical properties, are necessary for critical advances in technologies. Simultaneous control of structural and physical properties via a small electrical current is of great significance both fundamentally and technologically. Recent studies demonstrate that a combination of strong spin-orbit interactions and a distorted crystal structure in magnetic Mott insulators is sufficient to attain this long-desired goal. In thistopical review, we highlight underlying properties of this class of materials and present two representative antiferromagnetic Mott insulators, namely, 4d-electron based Ca2RuO4and 5d-electron based Sr2IrO4, as model systems. In essence, a small, applied electrical current engages with the lattice, critically reducing structural distortions, which in turn readily suppresses the antiferromagnetic and insulating state and subsequently results in emergent new states. While details may vary in different materials, at the heart of these phenomena are current-reduced lattice distortions, which, via spin-orbit interactions, dictate physical properties. Electrical current, which joins magnetic field, electric field, pressure, light, etc as a new external stimulus, provides a new, key dimension for materials research, and also pose a series of intriguing questions that may provide the impetus for advancing our understanding of spin-orbit-coupled matter. ThisTopical Reviewprovides a brief introduction, a few hopefully informative examples and some general remarks. It is by no means an exhaustive report of the current state of studies on this topic.

Electronic properties in itinerant ferromagnet SrRu1-x Ti x O3

Here, we study the electrical transport and specific heat in 4d based ferromagnetic material SrRuO₃ and its Ti substituted SrRu_1-x Ti _x O₃ series (x ⩽ 0.7). The SrRuO₃ is a metal and shows itinerant ferromagnetism with transition temperature T _c ∼ 160 K. The nonmagnetic Ti⁴⁺ (3d ⁰) substitution would not only weaken the active Ru-O-Ru channel but is also expected to tune the electronic density and electron correlation effect. A metal to insulator transition has been observed around x ∼ 0.4. The nature of charge transport in paramagnetic-metallic state (x ⩽ 0.4) and in insulating state (x > 0.4) follows modified Mott's variable range hopping model. In ferromagnetic-metallic state, resistivity shows a T ² dependence below T _c which though modifies to T ^3/2 dependence at low temperature. In Ti substituted samples, temperature range for T ^3/2 dependence extends to higher temperature. Interestingly, this T ^3/2 dependence dominates in whole ferromagnetic regime in presence of magnetic field. This evolution of electronic transport behavior can be explained within the framework of Fermi liquid theory and electron-magnon scattering mechanism. The negative magnetoresistance exhibits a hysteresis and a crossover between negative and positive value with magnetic field which is connected with magnetic behavior in series. The decreasing electronic coefficient of specific heat with x supports the increasing insulating behavior in present series. We calculate a high Kadowaki-Woods ratio (x ⩽ 0.3) for SrRuO₃ which increases with substitution concentration. This signifies an increasing electronic correlation effect with substitution concentration.

Atomic origin of spin-valve magnetoresistance at the SrRuO3 grain boundary

Defects exist ubiquitously in crystal materials, and usually exhibit a very different nature from the bulk matrix. Hence, their presence can have significant impacts on the properties of devices. Although it is well accepted that the properties of defects are determined by their unique atomic environments, the precise knowledge of such relationships is far from clear for most oxides because of the complexity of defects and difficulties in characterization. Here, we fabricate a 36.8° SrRuO3 grain boundary of which the transport measurements show a spin-valve magnetoresistance. We identify its atomic arrangement, including oxygen, using scanning transmission electron microscopy and spectroscopy. Based on the as-obtained atomic structure, the density functional theory calculations suggest that the spin-valve magnetoresistance occurs because of dramatically reduced magnetic moments at the boundary. The ability to manipulate magnetic properties at the nanometer scale via defect control allows new strategies to design magnetic/electronic devices with low-dimensional magnetic order.

Interfacial Control of Ferromagnetism in Ultrathin SrRuO3 Films Sandwiched between Ferroelectric BaTiO3 Layers

Interfaces between materials provide an intellectually rich arena for fundamental scientific discovery and device design. However, the frustration of magnetization and conductivity of perovskite oxide films under reduced dimensionality is detrimental to their device performance, preventing their active low-dimensional application. Herein, by inserting the ultrathin 4d ferromagnetic SrRuO₃ layer between ferroelectric BaTiO₃ layers to form a sandwich heterostructure, we observe enhanced physical properties in ultrathin SrRuO₃ films, including longitudinal conductivity, Curie temperature, and saturated magnetic moment. Especially, the saturated magnetization can be enhanced to ∼3.12 μ_B/Ru in ultrathin BaTiO₃/SrRuO₃/BaTiO₃ trilayers, which is beyond the theoretical limit of bulk value (2 μ_B/Ru). This observation is attributed to the synergistic ferroelectric proximity effect (SFPE) at upper and lower BaTiO₃/SrRuO₃ heterointerfaces, as revealed by the high-resolution lattice structure analysis. This SFPE in dual-ferroelectric interface cooperatively induces ferroelectric-like lattice distortions in RuO₆ oxygen octahedra and subsequent spin-state crossover in SrRuO₃, which in turn accounts for the observed enhanced magnetization. Besides the fundamental significance of interface-induced spin-lattice coupling, our findings also provide a viable route to the electrical control of magnetic ordering, taking a step toward low-power applications in all-oxide spintronics.

Atomic-Scale Metal-Insulator Transition in SrRuO3 Ultrathin Films Triggered by Surface Termination Conversion

The metal-insulator transition (MIT) in transition-metal-oxide is fertile ground for exploring intriguing physics and potential device applications. Here, an atomic-scale MIT triggered by surface termination conversion in SrRuO₃ ultrathin films is reported. Uniform and effective termination engineering at the SrRuO₃ (001) surface can be realized via a self-limiting water-leaching process. As the surface termination converts from SrO to RuO₂ , a highly insulating and nonferromagnetic phase emerges within the topmost SrRuO₃ monolayer. Such a spatially confined MIT is corroborated by systematic characterizations on electrical transport, magnetism, and scanning tunneling spectroscopy. Density functional theory calculations and X-ray linear dichroism further suggest that the surface termination conversion breaks the local octahedral symmetry of the crystal field. The resultant modulation in 4d orbital occupancy stabilizes a nonferromagnetic insulating surface state. This work introduces a new paradigm to stimulate and tune exotic functionalities of oxide heterostructures with atomic precision.

Correlation between structural phase transition and surface chemical properties of thin film SrRuO3/SrTiO3 (001)

The correlation between the structural phase transition (SPT) and oxygen vacancy in SrRuO₃ (SRO) thin films was investigated by in situ X-ray diffraction (XRD) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). In situ XRD shows that the SPT occurs from a monoclinic SRO phase to a tetragonal SRO phase near ∼200 °C, regardless of the pressure environment. On the other hand, significant core level shifts in both the Ru and Sr photoemission spectra are found under ultrahigh vacuum, but not under the oxygen pressure environment. The directions and behavior of the core level shift of Ru and Sr are attributed to the formation of oxygen vacancy across the SPT temperature of SRO. The analysis of in situ XRD and AP-XPS results provides an evidence for the formation of metastable surface oxide possibly due to the migration of internal oxygen atoms across the SPT temperature, indicating the close relationship between oxygen vacancy and SPT in SRO thin films.

Site dilution and charge disorder effect on physical properties in SrRu1-x Ga x O3

Here, we report an evolution of structural, magnetic and transport behavior in doped SrRu_1-x Ga _x O₃ (x [Formula: see text] 0.2). The nonmagnetic dopant Ga³⁺ (3d ¹⁰) not only acts for magnetic site dilution in SrRuO₃ but also modifies the Ru charge state and electronic density. Our studies show that Ga³⁺ substitution does not affect the original orthorhombic- Pbnm structure of SrRuO₃ which is due to its matching ionic radii with Ru⁴⁺ . However, Ga³⁺ has a substantial effect on the magnetic behavior of SrRuO₃ where it decreases both magnetic moment as well as magnetic transition temperature [Formula: see text]. Further, this dilution induces Griffiths phase behavior across [Formula: see text] and cluster-glass behavior at low temperature with higher concentration of doping. The magnetic critical exponent [Formula: see text] increases with x due to this site dilution effect. The Ga³⁺ induces an insulating state in SrRuO₃ with x  >  0.05. The charge transport in paramagnetic as well as in insulating state of samples can be well described with Mott's modified variable-range-hopping model. The metallic charge transport just below [Formula: see text] in SrRuO₃ obeys Fermi liquid behavior which breaks down at low temperature. We further find a correlation between field dependent magnetoresistance and magnetization through power-law behavior over the series.

Magnon profile on SrRuO3 films studied by inelastic neutron scattering

In this study, the inelastic neutron scattering probe of SIKA in ANSTO is employed to investigate the magnon dispersion curve in ferromagnetic SrRuO3 single crystal epitaxial films and to better understand the underlying mechanisms. This report presents the successful measurement of a magnon peak from the SrRuO3 films which contained an amount of material of only 0.9 mg. We reveal one significant magnon dispersion curve along [002] following the quadraticE ∝ Q 2 ) relation, which shows a magnon gap of 0.32 meV. We have discussed several possible mechanisms, such as the higher symmetry structure and the impurity levels, which may contribute to this smaller gap.

Heteroepitaxial vertical perovskite hot-electron transistors down to the monolayer limit

Two-dimensional heterostructures combined with vertical geometries are candidates to probe and utilize the physical properties of atomically-thin materials. The vertical configuration enables a unique form of hot-carrier spectroscopy as well as atomic-scale devices. Here, we present the room-temperature evolution of heteroepitaxial perovskite hot-electron transistors using a SrRuO₃ base down to the monolayer limit (∼4 Å). As a fundamental electronic probe, we observe an abrupt transition in the hot-electron mean free path as a function of base thickness, coinciding with the thickness-dependent resistive transition. As a path towards devices, we demonstrate the integrated synthesis of perovskite one-dimensional electrical edge contacts using water-soluble and growth-compatible Sr₃Al₂O₆ hard masks. Edge-contacted monolayer-base transistors exhibit on/off ratios reaching ∼10⁸, complete electrostatic screening by the base manifesting pure hot-electron injection, and excellent scaling of the output current density with device dimensions. These results open new avenues for incorporating emergent phenomena at oxide interfaces and in heterostructures.

Two-dimensional heterostructures combined with vertical geometries trigger superior functionalities in fundamental studies and applications. Here, the authors report vertical perovskite hot-electron transistors integrated with perovskite one-dimensional edge contacts down to the monolayer limit.

Interplay of Electronic and Spin Degrees in Ferromagnetic SrRuO_{3}: Anomalous Softening of the Magnon Gap and Stiffness

The magnon dispersion of ferromagnetic SrRuO_{3} was studied by inelastic neutron scattering experiments on single crystals as a function of temperature. Even at low temperature the magnon modes exhibit substantial broadening pointing to strong interaction with charge carriers. We find an anomalous temperature dependence of both the magnon gap and the magnon stiffness, which soften upon cooling in the ferromagnetic phase. Both effects trace the temperature dependence of the anomalous Hall effect and can be attributed to the impact of Weyl points, which results in the same relative renormalization in the spin stiffness and magnon gap.

Role of electron-magnon interaction in non-Fermi liquid behavior of SrRuO3

SrRuO₃ is a popular material extensively used as a bottom electrode in various applications, however, a few problems which will certainly change the interface band structure and greatly alter the device's property are still not fully understood, such as the change of carrier types at a certain temperature and the quasiparticle scattering for non-Fermi liquid behavior below ferromagnetic transition temperature. In this study, magnetic, transport (electrical and thermal) properties and x-ray photoemission spectra have been used to understand the role of quasiparticle interactions in the SrRuO₃ bulk system. At the Fermi level, the hybridization of Ru4dt _2g   ↓  and O2p  bands form a typical two band system. In order to explain the problems as mentioned, our present work reveals that there must be an impurity band that couples with the bands around Fermi level and serves as a charge reservoir. In the present case, the impurity is attributed to the Ru vacancies. As a result, the conduction electrons scatter strongly with the Ru vacancies and couple with the Ru magnons to give rise to a dominant electron-magnon coupling that overwhelms the electron-phonon coupling in the temperature range of 90-150 K.

Lattice Energetics and Correlation-Driven Metal-Insulator Transitions: The Case of Ca_{2}RuO_{4}

This Letter uses density functional, dynamical mean field, and Landau-theory methods to elucidate the interplay of electronic and structural energetics in the Mott metal-insulator transition. A Landau-theory free energy is presented that incorporates the electronic energetics, the coupling of the electronic state to local distortions and the coupling of local distortions to long-wavelength strains. The theory is applied to Ca_{2}RuO_{4}. The change in lattice energy across the metal-insulator transition is comparable to the change in electronic energy. Important consequences are a strongly first order transition, a sensitive dependence of the phase boundary on pressure and that the geometrical constraints on in-plane lattice parameter associated with epitaxial growth on a substrate typically change the lattice energetics enough to eliminate the metal-insulator transition entirely.

Temperature- and field-driven spin reorientations in triple-layer ruthenate Sr4Ru3O10

Sr₄Ru₃O₁₀, the n = 3 member of the Ruddlesden-Popper type ruthenate Sr_n+1Ru_nO_3n+1, is known to exhibit a peculiar metamagnetic transition in an in-plane magnetic field. However, the nature of both the temperature- and field-dependent phase transitions remains as a topic of debate. Here, we have investigated the magnetic transitions of Sr₄Ru₃O₁₀ via single-crystal neutron diffraction measurements. At zero field, we find that the system undergoes a ferromagnetic transition with both in-plane and out-of-plane magnetic components at T_c ≈ 100 K. Below T ^* = 50 K, the magnetic moments incline continuously toward the out-of-plane direction. At T = 1.5 K, where the spins are nearly aligned along the c axis, a spin reorientation occurs above a critical field B_c, giving rise to a spin component perpendicular to the plane defined by the field direction and the c axis. We suggest that both the temperature- and field-driven spin reorientations are associated with a change in the magnetocrystalline anisotropy, which is strongly coupled to the lattice degrees of freedom. This study elucidates the long-standing puzzles on the zero-field magnetic orders of Sr₄Ru₃O₁₀ and provides new insights into the nature of the field-induced metamagnetic transition.

Enhanced magnetic and thermoelectric properties in epitaxial polycrystalline SrRuO3 thin films

Transition metal oxide thin films show versatile electric, magnetic, and thermal properties which can be tailored by deliberately introducing macroscopic grain boundaries via polycrystalline solids. In this study, we focus on the modification of magnetic and thermal transport properties by fabricating single- and polycrystalline epitaxial SrRuO₃ thin films using pulsed laser epitaxy. Using the epitaxial stabilization technique with an atomically flat polycrystalline SrTiO₃ substrate, an epitaxial polycrystalline SrRuO₃ thin film with the crystalline quality of each grain comparable to that of its single-crystalline counterpart is realized. In particular, alleviated compressive strain near the grain boundaries due to coalescence is evidenced structurally, which induced the enhancement of ferromagnetic ordering of the polycrystalline epitaxial thin film. The structural variations associated with the grain boundaries further reduce the thermal conductivity without deteriorating the electronic transport, and lead to an enhanced thermoelectric efficiency in the epitaxial polycrystalline thin films, compared with their single-crystalline counterpart.

Spin-Orbit Coupling and Interband Transitions in the Optical Conductivity of Sr_{2}RhO_{4}

The prototypical correlated metal Sr_{2}RhO_{4} was studied using optical and photoemission spectroscopy. At low energies and temperatures, the optical data reveal a complex, multicomponent response that on the surface points to an unconventional metallic state in this material. Via a comparison with photoemission, the anomalous optical response may be attributed to an unexpectedly strong interband transition near 180 meV between spin-orbit coupled bands that are nearly parallel along ΓX. This spin-orbit coupling effect is shown to occur in a number of related metallic ruthenates and explains the previously puzzling optical properties reported for these materials.

Effect of Gd and Cr substitution on the structural, electronic and magnetic phases of SrRuO3: a case study of doping and chemical phase separation

We explore the crystal structure, electrical resistivity and magnetic behavior of the compositional series (SrRuO₃)[Formula: see text] (GdCrO₃) _x (where [Formula: see text]), which resides between orthorhombic ferromagnetic (FM) metal SrRuO₃ ([Formula: see text] K) and orthorhombic antiferromagnetic (AFM) insulator GdCrO₃ ([Formula: see text] K). Crystal structure analysis reveals that complete solid solution exists only up to [Formula: see text], above which chemical phase separation of two/three phases occurs, and persists up to [Formula: see text]. X-ray photoelectron spectroscopy measurement also corroborates the existence of [Formula: see text] for the intermediate composition [Formula: see text], which reinforces the astonishing scheelite-type GdCrO₄ formation (at ambient pressure) for [Formula: see text] compositions. Electrical resistivity measurements affirm the temperature driven metal to insulator (M-I) transition for [Formula: see text] and [Formula: see text] samples. Low temperature insulating state in these samples is interpreted by electron-electron interaction of weak disordered systems. Precise analysis of temperature dependent resistivity for [Formula: see text] samples (which have insulating ground state) dictate that the transport phenomenon is mainly associated with Arrhenius-type charge conduction, Mott's variable range hopping, short-range and long-range Coulomb interaction mediated hopping processes, due to the high degree of randomness. Interruption of magnetic Ru-O-Ru interaction by Ru-O-Cr and Cr-O-Cr interactions lowers the FM transition temperature (T _C), and thereby introduces Griffiths phase in phase separated samples. Furthermore, we believe that a sharp rise in magnetization at low temperature for [Formula: see text] samples is due to the formation of AFM GdCrO₄ phase. Prominent thermal hysteresis in temperature dependent magnetization curves for [Formula: see text], and appearance of spin-reorientation transition for [Formula: see text] are the distinct indications for transformation into canted AFM GdCrO₃ oxide at higher x. The effective magnetic moment ([Formula: see text]) continuously increases with the incorporation of higher moment elements (Gd and Cr); while coercive field (H _C) exhibits an abrupt variation as a function of x at the onset of phase separation.

Itinerant magnetic metals

In this review, an overview of itinerant magnets without magnetic elements is presented, beginning with a comparison of the local and itinerant moment pictures, the two extremes of magnetism. Then, the theoretical developments leading up to the self-consistent renormalization theory of spin fluctuations will be discussed, followed by an introduction to quantum criticality and the experimental signatures associated with systems near a quantum critical point. Three itinerant magnets without magnetic elements, ZrZn₂, Sc_3.1In, and TiAu are the focus of this review, as their empty d shells set them apart in their purely itinerant character, while several enhanced Pauli paramagnets and intermediate moment magnets are also discussed to put the overall comparison into perspective.

Designing functionality in perovskite thin films using ion implantation techniques: Assessment and insights from first-principles calculations

Recent experimental findings have demonstrated that low doses of low energy helium ions can be used to tailor the structural and electronic properties of single crystal films. These initial studies have shown that changes to lattice expansion were proposed to be the direct result of chemical pressure originating predominantly from the implanted He applying chemical pressure at interstitial sites. However, the influence of possible secondary knock-on damage arising from the He atoms transferring energy to the lattice through nuclear-nuclear collision with the crystal lattice remains largely unaddressed. Here, we study SrRuO₃ to provide a comprehensive examination of the impact of common defects on structural and electronic properties. We found that, while interstitial He can modify the properties, a dose significantly larger than those reported in experimental studies would be required. Our study suggests that true origin of the observed changes is from combination of secondary defects created during He implantation. Of particular importance, we observe that different defect types can generate greatly varied local electronic structures and that the formation energies and migration energy barriers vary by defect type. Thus, we may have identified a new method of selectively inducing controlled defect complexes into single crystal materials.

Site dilution in SrRuO3: effects on structural and magnetic properties

We have investigated the effect of site dilution with substitution of nonmagnetic element in SrRu_1-x Ti _x O₃ (x  ⩽  0.7). The nature of ferromagnetic state in SrRuO₃ is believed to be of itinerant type with transition temperature [Formula: see text] K. Crystallographically, SrRuO₃ has a distorted orthorhombic structure. Substitution of [Formula: see text] (3d ⁰) for Ru⁺⁴ (4d ⁴), however, does not introduce significant structural modification due to their matching ionic radii. This substitution, on the other hand, is expected to tune the electronic correlation effect and the d electron density in the system. With Ti substitution, we find that magnetic moment and Curie temperature decreases but T _c remains unchanged which has been attributed to opposite tuning of electron correlation effect and density of states within the framework of itinerant ferromagnetism. The estimated critical exponent (β) related to magnetization implies a mean-field type of magnetic nature in SrRuO₃. The value of β further increases with x which is understood from the dilution effect of magnetic lattice. The system evolves to exhibit Griffiths phase like behavior above T _c which is usually realized in diluted ferromagnet following local moment model of magnetism. Our detail analysis of magnetization data indicates that magnetic state in SrRuO₃ has contribution from both itinerant and local moment model of magnetism.

Transport Properties of Metallic Ruthenates: A DFT+DMFT Investigation

We present a systematical theoretical study on the transport properties of an archetypal family of Hund's metals, Sr_{2}RuO_{4}, Sr_{3}Ru_{2}O_{7}, SrRuO_{3}, and CaRuO_{3}, within the combination of first principles density functional theory and dynamical mean field theory. The agreement between theory and experiments for optical conductivity and resistivity is good, which indicates that electron-electron scattering dominates the transport of ruthenates. We demonstrate that in the single-site dynamical mean field approach the transport properties of Hund's metals fall into the scenario of "resilient quasiparticles." We explain why the single layered compound Sr_{2}RuO_{4} has a relative weak correlation with respect to its siblings, which corroborates its good metallicity.

Tuning magnetic anisotropy by interfacially engineering the oxygen coordination environment in a transition metal oxide

Strong correlations between electrons, spins and lattices--stemming from strong hybridization between transition metal d and oxygen p orbitals--are responsible for the functional properties of transition metal oxides. Artificial oxide heterostructures with chemically abrupt interfaces provide a platform for engineering bonding geometries that lead to emergent phenomena. Here we demonstrate the control of the oxygen coordination environment of the perovskite, SrRuO3, by heterostructuring it with Ca0.5Sr0.5TiO3 (0-4 monolayers thick) grown on a GdScO3 substrate. We found that a Ru-O-Ti bond angle of the SrRuO3 /Ca0.5Sr0.5TiO3 interface can be engineered by layer-by-layer control of the Ca0.5Sr0.5TiO3 layer thickness, and that the engineered Ru-O-Ti bond angle not only stabilizes a Ru-O-Ru bond angle never seen in bulk SrRuO3, but also tunes the magnetic anisotropy in the entire SrRuO3 layer. The results demonstrate that interface engineering of the oxygen coordination environment allows one to control additional degrees of freedom in functional oxide heterostructures.

Correlation among disorder, electronic and magnetic phases of SrRuO3

Electric and magnetic properties of Sr1-xBaxRu1-xTixO3 (0 ⩽ x ⩽ 0.8) have been investigated to find the interrelationship between metallicity and ferromagnetism in SrRuO3 (SRO). The simultaneous doping of Sr and Ru with Ba and Ti results in single phase SRO at x = 0.1 and mixed phase of SRO and hexagonal BaTiO3 (h-BTO) at x ⩾ 0.2. Co-doping at Sr and Ru sites gives rise to oxygen vacancy and mixed valency of Ru (Ru(3+) and Ru(4+)). Room temperature resistivity increases due to modification of p(O)-d(Ru) hybridization and phase segregation. Temperature dependent resistivity reveals metal-insulator transition around 232 K at x = 0.1 and insulator down to 2 K at x ⩾ 0.2. The insulating state (x = 0.1) at low temperature is well described by weak localization and electron-electron interaction. Temperature dependence of resistivity (x ⩾ 0.2) follows Mott's three dimensional variable range hopping model. Localization length and average hopping distance decrease with the increase of x, indicating the presence of more disorder. Ferromagnetic transition temperature decreases to 149 K at x = 0.1 and remains constant up to x = 0.5. The Curie-Wiess (CW) temperature (ΘCW) decreases monotonically and becomes negative at x = 0.5. The effective magnetic moment estimated from CW law is smaller than that of pure SRO due to the formation of Ru(3+) ions. The saturation magnetization diminishes, suggesting the demagnetization factor owing to diamagnetic h-BTO. The coercivity increases from 6700 Oe (x = 0) to 12 500 Oe (x = 0.4) and then decreases to 3700 Oe (x = 0.5). Ferromagnetic cluster comprising of doped SRO gives rise to the formation of a Griffith-like phase. The co-occurrence of high jump in resistivity ratio and disappearance of ferromagnetism suggests an interplay between transport process and magnetism at low temperature.

Tuning the metal-insulator crossover and magnetism in SrRuO₃ by ionic gating

Reversible control of charge transport and magnetic properties without degradation is a key for device applications of transition metal oxides. Chemical doping during the growth of transition metal oxides can result in large changes in physical properties, but in most of the cases irreversibility is an inevitable constraint. Here we report a reversible control of charge transport, metal-insulator crossover and magnetism in field-effect devices based on ionically gated archetypal oxide system - SrRuO₃. In these thin-film devices, the metal-insulator crossover temperature and the onset of magnetoresistance can be continuously and reversibly tuned in the range 90–250 K and 70–100 K, respectively, by application of a small gate voltage. We infer that a reversible diffusion of oxygen ions in the oxide lattice dominates the response of these materials to the gate electric field. These findings provide critical insights into both the understanding of ionically gated oxides and the development of novel applications.

First-principles study of the lattice dynamical properties of strontium ruthenate

By means of first-principles calculations, various properties of SrRuO3 are investigated, focusing on its lattice dynamical properties. Despite having a Goldschmidt tolerance factor very close to 1, the phonon dispersion curves of the high-temperature cubic phase of SrRuO3 show strong antiferrodistortive instabilities. The energetics of metastable phases with different tilt patterns are discussed, concluding that the coupling of oxygen rotation modes with anti-polar Sr motion plays a key role in stabilizing the Pnma phase with respect to alternative rotation patterns. Our systematic analysis confirms previous expectations and contributes to rationalizing better why many ABO3 perovskites, including metallic compounds, exhibit an orthorhombic ground state. The zone-center phonon modes of the Pnma phase have been computed, from which we propose partial reassignment of available experimental data. The full dispersion curves have also been obtained, constituting benchmark results for the interpretation of future measurements and providing access to thermodynamical properties.

Structural, magnetic and electrical properties of SrRuO3 films and SrRuO3/SrTiO3 superlattices

SrRuO3 films and SrRuO3/SrTiO3 superlattices grown on SrTiO3(001) were studied by structural, magnetic, magnetoresistance and Hall effect measurements. The superlattices showed heteroepitaxial growth with coherent interfaces and a Ru/Ti diffusion region of 1-1.5 unit cells. The resistivity had metallic character above a critical thickness of 3-4 unit cells, becoming insulating below. There was no hint of conduction processes along the interfaces. Both magnetization and magnetoresistance measurements showed an increase of the magnetic anisotropy, consistent with magnetostriction effects. The magnetostriction coefficient was estimated as λ100 ∼ 1.4 × 10(-4). Three unit cell thick SrRuO3 layers in SrRuO3/SrTiO3 superlattices were found to have tetragonal crystal symmetry, as deduced from the sign change of the anomalous Hall constant.

Large in-plane deformation of RuO6 octahedron and ferromagnetism of bulk SrRuO3

SrRuO3 is a ferromagnetic metal with several unusual physical properties such as zero thermal expansion below Tc, so-called Invar behavior. Another anomalous feature is that the a-axis lattice constant is larger than the b-axis lattice constant, a clear deviation from the predictions of the Glazer structural description with rigid RuO6 octahedron motion. Using high resolution neutron diffraction techniques, we show how these two structural anomalies arise from the irregular in-plane deformation, i.e. plastic behavior of the RuO6 octahedron, a weak band Jahn-Teller distortion. We further demonstrate that the ferromagnetic instability of SrRuO3 is related to the temperature-induced localization of Ru 4d bands.

Orbital-ordering-induced ferroelectricity in SrCrO3

Using density functional theory calculations, ultrathin films of SrVO3(d1) and SrCrO3(d2) on SrTiO3 substrates have been studied as possible multiferroics. Although both are metallic in the bulk limit, they are found to be insulating as a result of orbital ordering driven by lattice distortions at the ultrathin limit. While the distortions in SrVO3 have a first-order Jahn-Teller origin, those in SrCrO3 are ferroelectric in nature. This route to ferroelectricity results in polarizations comparable with conventional ferroelectrics.

Epitaxial strain induced magnetic transitions and phonon instabilities in tetragonal SrRuO3

Using density-functional theory calculations, we investigate the magnetic as well as the dynamical properties of tetragonal SrRuO3 (SRO) under the influence of epitaxial strain. It is found that both tensile and compressive strain in the xy-plane can induce an abrupt change in the magnetic moment of the Ru atom. In particular, under an in-plane compressive strain of ~4%, a ferromagnetic to nonmagnetic transition is induced, whereas for a tensile strain larger than 3%, the magnetic moment of Ru drops gradually with increase of the strain, exhibiting a weak ferromagnetic state. We find that these magnetic transitions can be qualitatively explained by the Stoner model. In addition, frozen-phonon calculations at the Γ point and phonon dispersion calculations reveal that structural instabilities can occur under both compressive and tensile strain. These instabilities are very similar to those of the ferroelectric perovskite oxides, even though SRO remains metallic in the range we studied. This might have an influence on the physical properties of oxide supercells having SRO as a constituent.

Quasiparticle mass enhancement and temperature dependence of the electronic structure of ferromagnetic SrRuO3 thin films

We report high-resolution angle-resolved photoemission studies of epitaxial thin films of the correlated 4d transition metal oxide ferromagnet SrRuO(3). The Fermi surface in the ferromagnetic state consists of well-defined Landau quasiparticles exhibiting strong coupling to low-energy bosonic modes which contributes to the large effective masses observed by transport and thermodynamic measurements. Upon warming the material through its Curie temperature, we observe a substantial decrease in quasiparticle coherence but negligible changes in the ferromagnetic exchange splitting, suggesting that local moments play an important role in the ferromagnetism in SrRuO(3).

Correlation between structural properties and resistivity critical behavior in SrRuO3 thin films

SrRuO(3) is a strong itinerant ferromagnet showing many features of 'bad metals' together with Fermi liquid behavior at very low temperature. The intriguing magnetic and transport properties of SrRuO(3) are tightly related to structure, as commonly observed in transition metal oxides. Here we report on the correlation of structural parameters (lattice constant and surface roughness) with the critical behavior of resistivity at the Curie point in SrRuO(3) thin films deposited on (001) SrTiO(3) by PLD. By varying the deposition conditions we were able to obtain a wide variety of structural properties in our samples, thus allowing us to perform a systematic study. Our analysis demonstrates the direct correlation between the critical temperature T(P) and the lattice out-of-plane parameter. Furthermore, the value of the critical exponent λ is proved to be a good physical parameter to quantify the microscopic order of SrRuO(3) samples.

Ferroelectric PbTiO3/SrRuO3 superlattices with broken inversion symmetry

We have fabricated PbTiO3/SrRuO3 superlattices with ultrathin SrRuO3 layers. Because of the superlattice geometry, the samples show a large anisotropy in their electrical resistivity, which can be controlled by changing the thickness of the PbTiO3 layers. Therefore, along the ferroelectric direction, SrRuO3 layers can act as dielectric, rather than metallic, elements. We show that, by reducing the concentration of PbTiO3, an increasingly important effect of polarization asymmetry due to compositional inversion symmetry breaking occurs. The results are significant as they represent a new class of ferroelectric superlattices, with a rich and complex phase diagram. By expanding our set of materials we are able to introduce new behaviors that can only occur when one of the materials is not a perovskite titanate. Here, compositional inversion symmetry breaking in bicolor superlattices, due to the combined variation of A and B site ions within the superlattice, is demonstrated using a combination of experimental measurements and first principles density functional theory.

Anomalous Hall effect and electron transport in ferromagnetic MnBi films

The electron transport properties of highly c-axis oriented MnBi thin films of various thicknesses have been investigated. Samples are metallic but the low temperature resistivity shows an unusual T(3) dependence. Transverse Hall effect measurements show that both the ordinary and anomalous Hall coefficients decrease with decreasing temperature below 300 K, but the ordinary Hall coefficient (R(0)) undergoes a sign reversal around 105 K, where the magnetic anisotropy also changes sign. Analysis of the Hall data for various samples shows that the anomalous Hall coefficient (R(s)) exhibits a strong ρ(2) dependence, where ρ is the longitudinal resistivity.

Structural phase transitions and magnetic order in SrTcO3

The structure of the perovskite SrTcO(3) has been investigated using both synchrotron X-ray and neutron powder diffraction. At room temperature SrTcO(3) is orthorhombic as a consequence of cooperative tilting of the corner sharing TcO(6) octahedra. The tilts are sequentially removed as the sample is heated with the oxide displaying the sequence of structres Pnma→Imma→I4/mcm→Pm ̅3m. Neutron powder diffraction data collected in the temperature range 4-1023 K indicate that SrTcO(3) has G-type antiferromagnetic structure, in which each Tc moment is antiparallel to its six nearest neighbours, below ∼1000 K. The magnetic structure is collinear antiferromagnetic with the technetium moments parallel to c-axis and can be described by the propagation vector k = [0,0,0] and the basis vector (0,0,A(z)). The same magnetic structure is observed in each of the four crystal structures.

A size-dependent nanoscale metal-insulator transition in random materials

Insulators and conductors with periodic structures can be readily distinguished, because they have different band structures, but the differences between insulators and conductors with random structures are more subtle. In 1958, Anderson provided a straightforward criterion for distinguishing between random insulators and conductors, based on the 'diffusion' distance ζ for electrons at 0 K (ref. 3). Insulators have a finite ζ, but conductors have an infinite ζ. Aided by a scaling argument, this concept can explain many phenomena in disordered electronic systems, such as the fact that the electrical resistivity of 'dirty' metals always increases as the temperature approaches 0 K (refs 4-6). Further verification for this model has come from experiments that measure how the properties of macroscopic samples vary with changes in temperature, pressure, impurity concentration and applied magnetic field, but, surprisingly, there have been no attempts to engineer a metal-insulator transition by making the sample size less than or more than ζ. Here, we report such an engineered transition using six different thin-film systems: two are glasses that contain dispersed platinum atoms, and four are single crystals of perovskite that contain minor conducting components. With a sample size comparable to ζ, transitions can be triggered by using an electric field or ultraviolet radiation to tune ζ through the injection and extraction of electrons. It would seem possible to take advantage of this nanometallicity in applications.