Sensitivity to disorder of the metallic state in the ruthenates

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.

High-sensitivity of initial SrO growth on the residual resistivity in epitaxial thin films of SrRuO[Formula: see text] on SrTiO[Formula: see text] (001)

The growth of SrRuO[Formula: see text] (SRO) thin film with high-crystallinity and low residual resistivity (RR) is essential to explore its intrinsic properties. Here, utilizing the adsorption-controlled growth technique, the growth condition of initial SrO layer on TiO[Formula: see text]-terminated SrTiO[Formula: see text] (STO) (001) substrate was found to be crucial for achieving a low RR in the resulting SRO film grown afterward. The optimized initial SrO layer shows a c(2 [Formula: see text] 2) superstructure that was characterized by electron diffraction, and a series of SRO films with different thicknesses (ts) were then grown. The resulting SRO films exhibit excellent crystallinity with orthorhombic-phase down to [Formula: see text] 4.3 nm, which was confirmed by high resolution X-ray measurements. From X-ray azimuthal scan across SRO orthorhombic (02 ± 1) reflections, we uncover four structural domains with a dominant domain of orthorhombic SRO [001] along cubic STO [010] direction. The dominant domain population depends on t, STO miscut angle ([Formula: see text]), and miscut direction ([Formula: see text]), giving a volume fraction of about 92 [Formula: see text] for [Formula: see text] 26.6 nm and [Formula: see text] (0.14[Formula: see text], 5[Formula: see text]). On the other hand, metallic and ferromagnetic properties were well preserved down to t [Formula: see text] 1.2 nm. Residual resistivity ratio (RRR = [Formula: see text]/[Formula: see text]) reduces from 77.1 for t [Formula: see text] 28.5 nm to 2.5 for t [Formula: see text] 1.2 nm, while [Formula: see text] increases from 2.5 [Formula: see text]cm for t [Formula: see text] 28.5 nm to 131.0 [Formula: see text]cm for t [Formula: see text] 1.2 nm. The ferromagnetic onset temperature ([Formula: see text]) of around 151 K remains nearly unchanged down to t [Formula: see text] 9.0 nm and decreases to 90 K for t [Formula: see text] 1.2 nm. Our finding thus provides a practical guideline to achieve high crystallinity and low RR in ultra-thin SRO films by simply adjusting the growth of initial SrO layer.

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.

Analysis of Metal-Insulator Crossover in Strained SrRuO3 Thin Films by X-ray Photoelectron Spectroscopy

The electronic properties of strontium ruthenate SrRuO3 perovskite oxide thin films are modified by epitaxial strain, as determined by growing on different substrates by pulsed laser deposition. Temperature dependence of the transport properties indicates that tensile strain deformation of the SrRuO3 unit cell reduces the metallicity of the material as well as its metal-insulator-transition (MIT) temperature. On the contrary, the shrinkage of the Ru–O–Ru buckling angle due to compressive strain is counterweighted by the increased overlap of the conduction Ru-4d orbitals with the O-2p ones due to the smaller interatomic distances resulting into an increased MIT temperature, i.e., a more conducting material. In particular, in the more metallic samples, the core level X-ray photoemission spectroscopy lineshapes show the occurrence of an extra-peak at the lower binding energies of the main Ru-3d peak that is attributed to screening, as observed in volume sensitive photoemission of the unstrained material.

Theory of the strange metal Sr3Ru2O7

The bilayer perovskite Sr3Ru2O7 has been widely studied as a canonical strange metal. It exhibits T-linear resistivity and a T log(1/T) electronic specific heat in a field-tuned quantum critical fan. Criticality is known to occur in "hot" Fermi pockets with a high density of states close to the Fermi energy. We show that while these hot pockets occupy a small fraction of the Brillouin zone, they are responsible for the anomalous transport and thermodynamics of the material. Specifically, a scattering process in which two electrons from the large, "cold" Fermi surfaces scatter into one hot and one cold electron renders the ostensibly noncritical cold fermions a marginal Fermi liquid. From this fact the transport and thermodynamic phase diagram is reproduced in detail. Finally, we show that the same scattering mechanism into hot electrons that are instead localized near a 2D van Hove singularity explains the anomalous transport observed in strained Sr2RuO4.

Coulomb correlations in 4d and 5d oxides from first principles-or how spin-orbit materials choose their effective orbital degeneracies

The interplay of spin-orbit coupling and Coulomb correlations has become a hot topic in condensed matter theory and is especially important in 4d and 5d transition metal oxides, like iridates or rhodates. Here, we review recent advances in dynamical mean-field theory (DMFT)-based electronic structure calculations for treating such compounds, introducing all necessary implementation details. We also discuss the evaluation of Hubbard interactions in spin-orbit materials. As an example, we perform DMFT calculations on insulating strontium iridate (Sr₂IrO₄) and its 4d metallic counterpart, strontium rhodate (Sr₂RhO₄). While a Mott-insulating state is obtained for Sr₂IrO₄ in its paramagnetic phase, the spectral properties and Fermi surfaces obtained for Sr₂RhO₄ show excellent agreement with available experimental data. Finally, we discuss the electronic structure of these two compounds by introducing the notion of effective spin-orbital degeneracy as the key quantity that determines the correlation strength. We stress that effective spin-orbital degeneracy introduces an additional axis into the conventional picture of a phase diagram based on filling and on the ratio of interactions to bandwidth, analogous to the degeneracy-controlled Mott transition in d¹ perovskites.

Unusual ferromagnetism enhancement in ferromagnetically optimal manganite La0.7-yCa0.3+yMn1-yRuyO3 (0≤y<0.3): the role of Mn-Ru t2g super-exchange

The e_g-orbital double-exchange mechanism as the core of physics of colossal magnetoresistance (CMR) manganites is well known, which usually covers up the role of super-exchange at the t_2g-orbitals. The role of the double-exchange mechanism is maximized in La_0.7Ca_0.3MnO₃, leading to the concurrent metal-insulator transition and ferromagnetic transition as well as CMR effect. In this work, by a set of synchronous Ru-substitution and Ca-substitution experiments on La_0.7–yCa_0.3+yMn_1–yRu_yO₃, we demonstrate that the optimal ferromagnetism in La_0.7Ca_0.3MnO₃ can be further enhanced. It is also found that the metal-insulator transition and magnetic transition can be separately modulated. By well-designed experimental schemes with which the Mn³⁺-Mn⁴⁺ double-exchange is damaged as weakly as possible, it is revealed that this ferromagnetism enhancement is attributed to the Mn-Ru t_2g ferromagnetic super-exchange. The present work allows a platform on which the electro-transport and magnetism of rare-earth manganites can be controlled by means of the t_2g-orbital physics of strongly correlated transition metal oxides.

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.

Similarity of scattering rates in metals showing T-linear resistivity

Many exotic compounds, such as cuprate superconductors and heavy fermion materials, exhibit a linear in temperature (T) resistivity, the origin of which is not well understood. We found that the resistivity of the quantum critical metal Sr(3)Ru(2)O(7) is also T-linear at the critical magnetic field of 7.9 T. Using the precise existing data for the Fermi surface topography and quasiparticle velocities of Sr(3)Ru(2)O(7), we show that in the region of the T-linear resistivity, the scattering rate per kelvin is well approximated by the ratio of the Boltzmann constant to the Planck constant divided by 2π. Extending the analysis to a number of other materials reveals similar results in the T-linear region, in spite of large differences in the microscopic origins of the scattering.

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.

Electron localization and magnetism in SrRuO3 with non-magnetic cation substitution

The destruction of the ferromagnetism of alloyed SrRuO(3) can be caused by electron localization at the substitution sites. Among all the non-magnetic cations that enter the B site, Zr(4+) is the least disruptive to conductivity and ferromagnetism. This is because Zr(4+) does not cause any charge disorder, and its empty d electron states which are poorly matched in energy with the Ru t(2g)(4) states cause the least resonance scattering of Ru's d electrons. Conducting Sr(Ru, Zr)O(3) may be used as an electrode for perovskite-based thin film devices, while its insulating counterpart provides unprecedented magnetoresistance, seldom seen in other non-manganite and non-cobaltite perovskites.

Metallic ground state and glassy transport in single crystalline URh2Ge2: enhancement of disorder effects in a strongly correlated electron system

We present a detailed study of the electronic transport properties on a single crystalline specimen of the moderately disordered heavy-fermion system URh2Ge2. For this material, we find glassy electronic transport in a single crystalline compound. We derive the temperature dependence of the electrical conductivity and establish metallicity by means of optical conductivity and Hall effect measurements. The overall behavior of the electronic transport properties closely resembles that of metallic glasses, with at low temperatures an additional minor spin disorder contribution. We argue that this glassy electronic behavior in a crystalline compound reflects the enhancement of disorder effects as a consequence of strong electronic correlations.

Multiple first-order metamagnetic transitions and quantum oscillations in ultrapure Sr3Ru2O7

We present measurements on ultraclean single crystals of the bilayered ruthenate metal Sr3Ru2O7, which has a magnetic-field-tuned quantum critical point. Quantum oscillations of differing frequencies can be seen in the resistivity both below and above its metamagnetic transition. This frequency shift corresponds to a small change in the Fermi surface volume that is qualitatively consistent with the small moment change in the magnetization across the metamagnetic transition. Very near the metamagnetic field, unusual behavior is seen. There is a strong enhancement of the resistivity in a narrow field window, with a minimum in the resistivity as a function of temperature below 1 K that becomes more pronounced as the disorder level decreases. The region of anomalous behavior is bounded at low temperatures by two first-order phase transitions. The implications of the results are discussed.

Quantum phase transition in quasi-one-dimensional BaRu6O12

We report the first systematic study of the electrical transport and magnetic properties of BaRu6O12, which has a quasi-one-dimensional (quasi-1D) hollandite structure. We show that BaRu6O12 is quasi-1D electronically as well. Its physical properties were found to be extremely sensitive to disorder. Furthermore, a transition from being metallic with a resistance drop around 2 K to being weakly insulating as the applied magnetic field was increased was also found. We propose that these two features are related to the possible presence of a quantum phase transition in this material system.