Sr2RuO4 Research Articles

Toward a Reliable Synthesis of Strontium Ruthenate: Parameter Control and Property Investigation of Submicrometer-Sized Structures

SrRuO3 (SRO) has been studied extensively at the bulk and thin film scales for a variety of applications, such as multilayer devices, field-effect devices, multiferroics, and even ferroelectric random access memory (FeRAM). Conversely, the exploration of SRO nanoscale and submicrometer scale structures in terms of both synthesis and applications has been significantly limited. Herein, we are the first to report on the synthesis of SRO submicrometer-sized particles via a molten-salt method. We have accomplished this by systematically probing experimental parameters such as precursors, type of salt, annealing time, annealing temperature, surfactant, cooling rate, and reaction atmosphere in an effort to predictably control the resulting size, shape, and morphology of the SRO product particles. In particular, by quenching the reaction at a cooling rate of 100 °C/min, we can produce rounded SRO particles averaging 149 ± 100 nm in size. Moreover, with the addition of a mixture of mineral oil in Triton X-100, the final SRO particles are highly faceted, single-crystalline octahedra, averaging 126 ± 45 nm in size. Apart from the choice of molten salt which primarily controls chemical composition of SRO, we have determined the most important experimental parameter for shape and aggregation control is the surfactant, because of a combination of its hydrophobic and hydrophilic characteristics. We have found that the decomposition of the surfactant promotes the formation of SrCO3, which is consistent with the generation of SRO through the reaction of SrCO3 with RuO2. Our successful syntheses have allowed us to explore physical properties, that is, magnetic and electronic, of these submicrometer-sized SRO particles, which are in good agreement with bulk SRO. Furthermore, we have also explored the potential of our as-synthesized particles as effective methanol oxidation reaction (MOR) catalysts for direct methanol fuel cells (DMFCs). Not only are our as-synthesized SRO particles MOR active but also our well-defined faceted octahedra exhibit a 4-fold increase in mass-activity and a 4-fold increase in surface area by comparison with the rounded particles, thereby emphasizing a clear advantage of faceted submicrometer SRO for electrochemical applications.

The expanding search for Majorana particles

The reports in this issue about half-quantum vortices in strontium ruthenate (page 17) and about spin-orbit coupling in a nanowire (page 19) both involve a system in which theorists have proposed that one might find an elusive Majorana particle. A system of Majorana particles might be a good candidate for a topological quantum computer.

Change in oxygen concentration in Sr2RuO4 single crystals after laser processing

In order to check the possibility of realization of high-temperature superconductivity in strontium ruthenate, we studied experimentally the influence of laser radiation on single-crystal samples of Sr2RuO4. Results obtained by the technique of electron-probe X-ray microanalysis showed an enhancement of the oxygen concentration in the near-surface layer of samples, which may most probably be caused by the change in the ruthenium valence as a result of laser treatment.

Effects of post‐growth annealing on physical properties of SrRuO3 thin film grown by MOCVD

AbstractWe report on the annealing effects on strontium ruthenate thin films deposited on SrTiO3 using liquid‐delivery metalorganic chemical vapor deposition (MOCVD). The results of high resolution X‐ray diffraction (HR‐XRD), Raman spectroscopy, atomic force microscopy (AFM), and electrical resistivity, before and after a post‐growth annealing process were analyzed and compared. XRD and Raman spectroscopy for the as‐deposited film revealed that the film had c‐axis orientation and contained RuO2 as secondary phase. Single phase SrRuO3 could be obtained by annealing the film at 700 °C. Annealing at higher temperatures up to 850 °C leads to ruthenium deficiency, which is revealed by an increase in the out‐of‐plane lattice parameter, while at even higher annealing temperatures a decrease in the out‐of‐plane lattice parameter was detected might be due to diffusion of Ti from the substrate. The electrical resistivity increased with increasing annealing temperature up to 950 °C and showed typical metallic temperature dependence, while for the film annealed at 1000 °C a semiconductor‐like behavior was observed.

Why I like condensed-matter physics

<xhtml:span xmlns:xhtml="http://www.w3.org/1999/xhtml" xml:lang="en"> In 2001, I edited my first feature article for <em>Physics Today</em> about condensed-matter physics, "<a href="http://dx.doi.org/10.1063/1.1349611">The Intriguing Superconductivity of Strontium Ruthenate</a>," by Yoshiteru Maeno, Maurice Rice, and Manfred Sigrist. My previous engagement with condensed matter took place in 1982. Then, as an undergraduate at Imperial College in London, I took my first and only course on what the lecturer John Pendry and others referred to as solid-state physics.</xhtml:span>

Mapping Out an Entropic Landscape

![Figure][1] CREDIT: ROST ET AL. Quantum critical points are continuous phase transitions occurring near absolute zero displaying interesting properties that may have valuable applications. Experimental thermodynamic information on quantum critical systems is sparse, partly because the better known systems are tuned using hydrostatic pressure, and thermodynamic measurements are difficult to perform under such conditions. Rost et al. (p. [1360][2], published online 6 August 2009; see the Perspective by [Fisk][3] ) built and calibrated bespoke apparatus for a thermodynamic study of strontium ruthenate using a magnetic field as the tuning parameter. The specific heat and magnetocaloric measuremements were combined to map an “entropy landscape” of the quantum criticality and phase formation. This technique should offer a way to probe other materials to reveal unusual properties, such as novel metallic states and superconductivity. [1]: pending:yes [2]: /lookup/doi/10.1126/science.1176627 [3]: /lookup/doi/10.1126/science.1179046

Toward intrinsic functionalities of bilayered ruthenateSr3Ru2O7

Polymorphic materials are known for being prone to intergrowth. Remarkable examples are strontium ruthenates whose properties are dramatically tuned by impurities and disorder. In particular, ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$ shows a strong variation in transport and magnetic properties depending on synthesis process. It is therefore crucial to correlate atomic structure and properties to identify the functionalities of individual nanostructural constituents. We report a comparative study between ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$ crystals grown as single phase and in ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}{\text{-Sr}}_{2}{\text{RuO}}_{4}$ eutectics. Our analysis by transmission electron microscopy reveals that ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$ domains of the eutectic have a significantly lower level of impurities compared to ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$ single-phase crystals, where intergrowths of ${\text{Sr}}_{4}{\text{Ru}}_{3}{\text{O}}_{10}$ and ${\text{SrRuO}}_{3}$ phase are seen. This is confirmed by magnetic measurements. These results identify the eutectic solidification as a fruitful way to grow highly pure crystals of polymorphic materials which, in combination with recent technological developments allowing the extraction of embedded features of crystals, opens a pathway for understanding of their physical properties and applications.

Entropy Landscape of Phase Formation Associated with Quantum Criticality in Sr 3 Ru 2 O 7

Low-temperature phase transitions and the associated quantum critical points are a major field of research, but one in which experimental information about thermodynamics is sparse. Thermodynamic information is vital for the understanding of quantum many-body problems. We show that combining measurements of the magnetocaloric effect and specific heat allows a comprehensive study of the entropy of a system. We present a quantitative measurement of the entropic landscape of Sr3Ru2O7, a quantum critical system in which magnetic field is used as a tuning parameter. This allows us to track the development of the entropy as the quantum critical point is approached and to study the thermodynamic consequences of the formation of a novel electronic liquid crystalline phase in its vicinity.

Suppression of spontaneous supercurrents in a chiralp-wave superconductor

The superconducting state of strontium ruthenate (SRO) is widely believed to have chiral $p$-wave order that breaks time-reversal symmetry. Such a state is expected to have a spontaneous magnetization, both at sample edges and at domain walls between regions of different chirality. Indeed, muon spin-resonance experiments are interpreted as evidence of spontaneous magnetization due to domain walls or defects in the bulk. However, recent magnetic-microscopy experiments place upper limits on the magnetic fields at the sample edge and surface which are as much as 2 orders of magnitude smaller than the fields predicted theoretically for a somewhat idealized chiral $p$-wave superconductor. We investigate the effects on the spontaneous supercurrents and magnetization of rough and pair-breaking surfaces for a range of parameters within a Ginzburg-Landau formalism. The effects of competing orders nucleated at the surface are also considered. We find the conditions under which the edge currents are significantly reduced while leaving the bulk domain-wall currents intact, are quite limited. The implications for interpreting the existing body of experimental results on superconducting SRO within a chiral $p$-wave model are discussed.

Is Sr2RuO4 a chiral p-wave superconductor?

Much excitement surrounds the possibility that strontium ruthenate exhibits chiral p-wavesuperconducting order. Such order would be a solid state analogue of the A phase of He-3,with the potential for leading to exotic physics relevant to quantum computing. We take acritical look at the evidence for such time reversal symmetry breaking order. The possiblesuperconducting order parameter symmetries and the evidence for and againstchiral p-wave order are reviewed, with an emphasis on the most recent theoreticalpredictions and experimental observations. In particular, attempts to reconcileexperimental observations and theoretical predictions for the spontaneous supercurrentsexpected at sample edges and domain walls of a chiral p-wave superconductor andfor the polar Kerr effect, a key signature of broken time reversal symmetry, arediscussed.

Effect of Sr–Ruthenate Seed Layer on Dielectric Properties of SrTiO3 Thin Films Prepared by Plasma-Enhanced Atomic Layer Deposition

The effect of a strontium ruthenate seed layer on the dielectric properties of thin films was investigated. The thin films were deposited by plasma-enhanced atomic layer deposition using titanium tetra-isopropoxide and bis(dipivaloylmethanato) strontium as precursors and oxygen as an oxidant. A strontium ruthenate seed layer was formed through the deposition of ultrathin SrO and a postannealing process. The thin films deposited on a seed layer, prepared by deposition of SrO and postannealing prior to deposition, showed considerably enhanced dielectric properties in comparison to films deposited on Ru directly; that was attributed to enhancement of the films’ crystallinites and a reduction of low- interfacial layers. For optimization of seed-layer formation, the dependency of the inserted SrO layer thickness on the dielectric properties of the films was investigated for a SrO thickness range of . The thick thin film on the seed layer formed by deposition of a thick SrO layer and postannealing exhibited a dielectric constant of about 75 [equivalent oxide thickness ], which meets the requirement of dynamic random access memory technology.

Collective modes and electromagnetic response of a chiral superconductor

Motivated by the recent controversy surrounding the Kerr effect measurements in strontium ruthenate [J. Xia et al., Phys. Rev. Lett. 97, 167002 (2006)], we examine the electromagnetic response of a clean chiral $p$-wave superconductor. When the contributions of the collective modes are accounted for, the Hall response in a clean chiral superconductor is smaller by several orders of magnitude than previous theoretical predictions and is too small to explain the experiment. We also uncover some unusual features of the collective modes of a chiral superconductor, namely, that they are not purely longitudinal and couple to external transverse fields.

Superconducting phases in strontium ruthenate

AbstractWe review our recent work concerning the search for the superconducting state realised in Sr2RuO4. The orbital symmetry of the order parameter in this spin triplet superconductor is widely believed to be chiral one with d ‐vector along the crystal c‐axis. The three band, three dimensional model with two different phenomenological interaction parameters is able to explain most of the existing data in quantitative way. This is also true for the recently observed “rotation” of the d ‐vector in magnetic field directed along c‐axis. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Ferroelectric Bi 3.25La 0.75Ti 3O 12 thin films on a conductive Sr 4Ru 2O 9 electrode obtained by pulsed laser deposition

Strontium ruthenate and Bi 3.25La 0.75Ti 3O 12 (BLT) layers were grown on Si(100) substrate using pulsed laser deposition technique. Starting from a Sr 2RuO 4 target, we obtained single phase films composed of Sr 4Ru 2O 9; on these strontium ruthenate electrodes, textured and non-textured BLT were grown at 700 °C. Structural characterizations of these double layers were done by X-ray diffraction, scanning electron microscopy, normal and high-resolution transmission electron microscopy. The Van der Pauw's resistivity measurements indicate that Sr 4Ru 2O 9 can be used as a back electrode. The temperature dependence of the resistivity at low temperatures is ρ ( T ) = ρ 0 exp ( T 0 T ) 1 / 2 , which corresponds to a variable-range hopping mechanism.

Structural and electrical properties of room temperature pulsed laser deposited and post-annealed thin SrRuO 3 films

Good quality strontium ruthenate (SrRuO 3) thin continuous films (15 to 125 nm thick) have been synthesized on silicon (100) substrates by room temperature pulsed laser deposition under vacuum followed by a post-deposition annealing, a route unexplored and yet not reported for SrRuO 3 film growth. The presence of an interfacial Sr 2SiO 4 layer has been identified for films annealed at high temperature, and the properties of this interface layer as well as the properties of the SrRuO 3 film have been analyzed and characterized as a function of the annealing temperature. The room temperature resistivity of the SrRuO 3 films deposited by laser ablation at room temperature and post-annealed is 2000 μΩ·cm. A critical thickness of 120 nm has been determined above which the influence of the interface layer on the resistivity becomes negligible.

Formation of a Nematic Fluid at High Fields in Sr 3 Ru 2 O 7

In principle, a complex assembly of strongly interacting electrons can self-organize into a wide variety of collective states, but relatively few such states have been identified in practice. We report that, in the close vicinity of a metamagnetic quantum critical point, high-purity strontium ruthenate Sr3Ru2O7 possesses a large magnetoresistive anisotropy, consistent with the existence of an electronic nematic fluid. We discuss a striking phenomenological similarity between our observations and those made in high-purity two-dimensional electron fluids in gallium arsenide devices.

Electronic structure of [formula omitted] studied by angle-resolved photoemission spectroscopy

Electronic structure of the monolayer strontium ruthenate Sr 2 RuO 4 was investigated by high-resolution angle-resolved photoemission spectroscopy. We present photon-energy ( h ν ) dependence of the electronic structure near the Fermi level along the Γ M line. The h ν dependence has shown a strong spectral weight modulation of the Ru 4 d xy and 4 d zx bands.

Growth of textured MgO through e-beam evaporation and inclined substratedeposition

Long length textured MgO template on Hastelloy C276™ (HC) has been successfullydeposited in a reel-to-reel (R2R) electron beam (e-beam) evaporationsystem by inclined substrate deposition (ISD). High deposition rate up to10 nm s−1 with exposure length of 7 cm has been realized. The MgO template showed good in-plane texture of9.5°–11.5° measured from the (002) phi scans. Experimental results reveal that MgO in-planetexture is formed by the preferred growth direction of normal and one of the MgO {200} planes rotates to the in-flux direction. A newexpression, termed the ‘two-thirds relationship’, between the inclination angleα and the tilted angleof the (00l) plane fromthe substrate normal, β, has been summarized. YBa2Cu3O7−δ (YBCO) film deposited by pulsed laser deposition (PLD) on strontium ruthenate (SRO) buffered ISD MgOshowed Tc of 91 K with transition width of 1 K. Critical current measurement indicated anIc of110 A cm−1 at 77 K in self-fieldfor 0.68 µm YBCO film,corresponding to a Jc of 1.6 MA cm−2.

Proposal to stabilize and detect half-quantum vortices in strontium ruthenate thin films: Non-Abelian braiding statistics of vortices in apx+ipysuperconductor

We propose a simple way to stabilize half-quantum vortices in superconducting strontium ruthenate, assuming the order parameter is of chiral ${p}_{x}+i{p}_{y}$ symmetry, as is suggested by recent experiments. The method, first given by Salomaa and Volovik in the context of Helium-3, is very naturally suited for strontium ruthenate, which has a layered, quasi-two-dimensional, perovskite crystal structure. We propose possible experiments to detect their non-Abelian braiding statistics. These experiments are of potential importance for topological quantum computation.

Fermi surfaces and kink in the energy dispersion of Sr2RuO4

We have performed a high-resolution angle-resolved photoemission spectroscopy on the monolayer strontium ruthenate Sr2RuO4, to elucidate the origin of the kink. We present the Fermi surface mapping and the detailed energy dispersions near the Fermi level. The kink was clearly observed for γ Fermi surface sheet derived from the in-plane Ru 4dxy orbital. The kink of γ Fermi surface sheet was still observed at points far from (0, π), indicating a rather weak momentum dependence of the kink of Sr2RuO4.