Fermi surface topology of Ca1.5Sr0.5RuO4 determined by angle-resolved photoelectron spectroscopy

We report angle-resolved photoelectron spectroscopy results of the Fermi surface of Ca1.5Sr0.5RuO4, which is at the boundary of magnetic/orbital instability in the phase diagram of the Ca-substituted Sr ruthenates. Three t(2g) energy bands and the corresponding Fermi surface sheets are observed, which are also present in the Ca-free Sr2RuO4. We find that while the Fermi surface topology of the alpha,beta (d(yz,zx)) sheets remains almost the same in these two materials, the gamma (d(xy)) sheet exhibits a holelike Fermi surface in Ca1.5Sr0.5RuO4 in contrast to being electronlike in Sr2RuO4. Our observation of all three volume conserving Fermi surface sheets clearly demonstrates the absence of orbital-selective Mott transition, which was proposed theoretically to explain the unusual transport and magnetic properties in Ca1.5Sr0.5RuO4.     

Fermi surface nesting and structural transition on a metal surface: In /Cu(001)

Peierls-type instability and structural phase transition are shown to occur on the surface of a normal metal. An In overlayer on Cu(001) undergoes a reversible transition at approximately 350 K. Scanning tunneling microscopy of the low-temperature, reduced-symmetry phase indicates a strong periodic lattice distortion (PLD). Angle-resolved photoemission of the high-temperature phase reveals that the In-derived surface resonance constitutes a square-shaped, quasi-two-dimensional Fermi surface within the projected bulk Cu bands. The Fermi surface exhibits one-dimensional nesting upon the transition, which is in agreement with the PLD periodicity.     

Quasi-two-dimensional mott transition system Ca2-xSrxRuO4

We have revealed the phase diagram of Ca2-xSrxRuO4: the quasi-two-dimensional Mott transition system that connects the Mott insulator Ca2RuO4 with the spin-triplet superconductor Sr2RuO4. Adjacent to the metal/nonmetal transition at x approximately 0.2, we found an antiferromagnetically correlated metallic region where non-Fermi-liquid behavior in resistivity is observed. Besides this, the critical enhancement of susceptibility toward the region boundary at x(c) approximately 0.5 suggests the crossover of magnetic correlation to a nearly ferromagnetic state, which evolves into the spin-triplet superconductor Sr2RuO4.     

Negative volume thermal expansion via orbital and magnetic orders in Ca₂Ru₁-(x)Cr(x)O₄(0 < x < 0.13)

Ca?RuO? undergoes a metal-insulator transition at T(MI)=357 K, followed by a well-separated transition to antiferromagnetic order at T(N)=110 K. Dilute Cr doping for Ru reduces the temperature of the orthorhombic distortion at T(MI) and induces ferromagnetic behavior at T(C). The lattice volume V of Ca?Ru?-(x)Cr(x)O? (0 < x < 0.13) abruptly expands with cooling at both T(MI) and T(C), giving rise to a total volume expansion ΔV/V ≈ 1%, which sharply contrasts the smooth temperature dependence of the few known examples of negative volume thermal expansion driven by anharmonic phonon modes. In addition, the near absence of volume thermal expansion between T(C) and T(MI) represents an Invar effect. The two phase transitions, which surprisingly mimic the classic freezing transition of water, suggest an exotic ground state driven by an extraordinary coupling between spin, orbit, and lattice degrees of freedom.     

Soft phonon, dynamical fluctuations, and a reversible phase transition: indium chains on silicon

The In/Si(111)-(4 x 1) surface is a paradigmatic example of a quasi-one-dimensional system showing a reversible structural and electronic (metal-insulator) phase transition when the temperature is lowered. In this work, we use first-principles simulation techniques to uncover the atomic and electronic origin of this controversial transition. Our calculations show that the ground state consists of insulating (4 x 2) indium chains with a weak interchain coupling that induces opposite shear distortions in alternate chains. First-principles molecular dynamics simulations show that the (4 x 1) <--> (8 x 2) phase transition is due to the "dynamical fluctuations" the system undergoes when, at high temperature, it fluctuates chaotically between degenerate ground states. The metallicity of the In/Si(111)-(4 x 1) surface is related to the low energy cost for the shear distortion.     

Strong orbital-dependent d-band hybridization and Fermi-surface reconstruction in metallic Ca2-xSrxRuO4

We study the effects of RuO6 rotation on Ru 4d band structures in metallic Ca2-xSrxRuO4 (0.5 < or = x < or = 2) by first-principles electronic structure calculations. We show that the RuO6 rotation leads to the strong hybridization between dxy and dx2-y2 bands, resulting in orbital-dependent changes in the band structure. The dxy band near the Fermi level is significantly modified and thereby a severely reconstructed Fermi surface with nested sections appears at x=0.5. In contrast, the dyz and dzx bands are found to be insensitive to the rotational distortions induced by the Ca substitution. Our results imply that the progressive changes in the magnetic, optical, and thermal properties of Ca2-xSrxRuO4 are associated with the dxy band.     

Structural properties of Pb2MnW(1-x)Re(x)O6 double perovskites

Pb2MnW(1-x)Re(x)O6 samples have been synthesized and their structure determined by powder x-ray diffraction. These samples undergo a first order structural phase transition between 413 and 445 K depending on the composition. Above this temperature, the samples are cubic. Below the transition temperature, solid solutions are found for x ≤ 0.2 and x ≥ 0.5. The W-rich samples adopt an orthorhombic cell whereas the Re-rich compounds are monoclinic. In the intermediate region, 0.2 < x < 0.5, both phases coexist. X-ray absorption spectra did not reveal significant changes in the local structure for Pb, Mn or Re atoms across the structural phase transition. All the atoms exhibit distorted environments in the whole series. In the case of Pb and W(Re) atoms, the local distortion remains in the high temperature phase. Samples with x ≤ 0.2 also show a sharp discontinuity in the dielectric permittivity at the phase transition temperature indicating the presence of a concomitant electrical ordering in the bulk grains. Such an anomaly in the dielectric constant is not observed for the x ≥ 0.5 samples, compatible with the lack of dipole ordering for this composition range. The different electrical behaviours also explain the differences in the entropy content for the two types of transition.     

Interplay among superconductivity,charge density waves,and magnetism in EuMo6S8

In a one-dimensional metal, the energy of the electrons can always be lowered by opening an energy gap around the Fermi energy (the Peierls instability): all occupied states are then in the lower-energy band, while the higher-energy band is empty. The opening of such a gap requires a structural distortion, resulting in the formation of a charge density wave. In a three-dimensional system, the gapping takes place in the region where the Fermi surface is nested (i.e., large parallel areas of the Fermi surface are spanned by a certain wave vector), giving rise to partial gapping of the Fermi surface, accompanied by a structural distortion. In this case, a charge density wave can coexist with superconductivity. Both charge-density-wave and superconducting transitions involve the formation of an energy gap at the Fermi energy. A charge-density-wave gap is formed at a region of the Fermi surface where there is a high density of electronic states. In such a material, there is also a strong electronphonon interaction. A region with high density of states and a high electron-phonon interaction is just the portion of the Fermi surface that will enhance the superconducting transition temperature, according to the BCS (Bardeen-Cooper-Schrieffer) theory. When a charge-density-wave gap opens up at the Fermi surface these electronic states are no longer available to form Cooper pairs and to enhance the superconducting transition temperature. The opposite is also true; if a superconducting gap opens, the states involved in forming this gap are no longer available to take part in a charge-density-wave transition. It appears that charge density waves and superconductivity compete for the same portion of the Fermi surface and thus inhibit each other. In this paper, we will review a unique situation with respect to the competition between these two ground states and will also discuss how this competition affects the anomalous behavior of critical field in EuMo6S, at high pressure.     

Surface phase transitions induced by electron mediated adatom-adatom interaction

We propose that the indirect adatom-adatom interaction mediated by the conduction electrons of a metallic surface is responsible for the sqrt[3]xsqrt[3]<==>3x3 structural phase transitions observed in Sn/Ge (111) and Pb/Ge (111). When the indirect interaction overwhelms the local stress field imposed by the substrate registry, the system suffers a phonon instability, resulting in a structural phase transition in the adlayer. Our theory is capable of explaining all the salient features of the sqrt[3]xsqrt[3]<==>3x3 transitions observed in Sn/Ge (111) and Pb/Ge (111), and is in principle applicable to a wide class of systems whose surfaces are metallic before the transition.     

Interband proximity effect and nodes of superconducting gap in Sr2RuO4

The power-law temperature dependences of the specific heat, the nuclear relaxation rate, and the thermal conductivity suggest the presence of line nodes in the superconducting gap of Sr2RuO4. These recent experimental observations contradict the scenario of a nodeless (k(x)+ik(y))-type superconducting order parameter. We propose that interaction of superconducting order parameters on different sheets of the Fermi surface is a key to understanding the above discrepancy. A full gap exists in the active band, which drives the superconducting instability, while line nodes develop in passive bands by the interband proximity effect.     

Strongly enhanced magnetic fluctuations in a large-mass layered ruthenate

We have studied the magnetic excitations in Ca2-xSrxRuO4, x=0.52 and 0.62, which exhibit an anomalously high susceptibility and heavy mass Fermi liquid behavior. Our inelastic neutron scattering experiments reveal strongly enhanced magnetic fluctuations around an incommensurate wave-vector (0.22,0,0) pointing to a magnetic instability. The magnetic fluctuations show no correlation in the c direction and also along the RuO2 planes the signal is extremely broad, Deltaq=0.45 A(-1). These fluctuations can quantitatively account for the high specific heat coefficient and relate to the high macroscopic susceptibility. The magnetic scattering is attributed to the d(xy) band, the active band for spin triplet superconductivity in Sr2RuO4.     

RuO2/TiO2复合光催化剂的制备及性能研究

采用溶胶-凝胶-浸渍法制备了RuO2/TiO2复合光催化剂, 以紫外灯为光源, 直接耐晒黑G溶液的光催化降解为模型反应, 研究了RuO2/TiO2的光催化性能. 结果表明 掺杂量ω(RuO2)为0.16%、煅烧温度500 ℃、催化剂投加量为5.00 g·L-1时, RuO2/TiO2复合光催化剂催化活性最高. 直接耐晒黑G降解反应遵从Langmuir-Hinshelwood动力学模型, 测得反应速率常数为4.94×10-3 mmol(L·min)-1和吸附常数14.2 L·mmol-1.     

Dilute 57Fe and 151Eu as Probes for Magnetism in Ruthenium Magneto-Superconductors

Hyperfine Interactions - Mössbauer studies of 151Eu and/or dilute 57Fe in ruthenium oxides; R2?x Ce x Sr2RuCu2O10 (R=Eu, Gd), SrRuO3, CaRuO3, EuSr2RuCu2O8, EuSr2RuO6, Eu2RuO5 and...     

Structural aspects of metamagnetism in Ca2-xSrxRuO4: evidence for field tuning of orbital occupation

The crystal structure of Ca(2-x)Sr(x)RuO(4) with 0.2 < or = x < or = 1.0 has been studied by diffraction techniques and by high resolution capacitance dilatometry as a function of temperature and magnetic field. Upon cooling in zero magnetic field, the crystal structure and the octahedra shrink along the c direction and elongate in the a and b planes, whereas the opposite occurs upon cooling at high field (x = 0.2 and 0.5). These findings yield evidence for an orbital rearrangement driven by temperature and magnetic field, which accompanies the metamagnetic transition at low temperature. The temperature and magnetic-field dependencies are found to be governed by the same energy scale.     

Observation of a Mott insulating ground state for Sn/Ge(111) at low temperature

We report an investigation on the properties of 0.33 ML of Sn on Ge(111) at temperatures down to 5 K. Low-energy electron diffraction and scanning tunneling microscopy show that the (3x3) phase formed at approximately 200 K, reverts to a new ((square root 3)x(square root 3))R30 degrees phase below 30 K. The vertical distortion characteristic of the (3x3) phase is lost across the phase transition, which is fully reversible. Angle-resolved photoemission experiments show that, concomitantly with the structural phase transition, a metal-insulator phase transition takes place. The ((square root 3)x(square root 3))R30 degrees ground state is interpreted as the formation of a Mott insulator for a narrow half-filled band in a two-dimensional triangular lattice.     

Severe Fermi surface reconstruction at a metamagnetic transition in Ca2-xSrxRuO4 (for 0.2 <or=x<or=0.5)

We report an electrical transport study in Ca2-xSrxRuO4 single crystals at high magnetic fields (B). For x=0.2, the Hall constant Rxy decreases sharply at an anisotropic metamagnetic transition, reaching its value for Sr2RuO4 at high fields. A sharp decrease in the coefficient of the resistivity T2 term and a change in the structure of the angular magnetoresistance oscillations for B rotating in the planes confirms the reconstruction of the Fermi surface. Our observations and local-density-approximation calculations indicate a strong dependence of the Fermi surface on Ca concentration and suggest the coexistence of itinerant and localized electronic states in single layered ruthenates.     

Missing xy-band Fermi surface in 4d transition-metal oxide Sr2RhO4: effect of the octahedra rotation on the electronic structure

Electronic structures of the 4d transition-metal oxide compound Sr2RhO4 are investigated by angle-resolved photoemission spectroscopy and density-functional electronic structure calculations. In the measured Fermi surfaces (FS) of Sr2RhO4, the xy-band FS sheet expected from the well-established results of the FS of Sr2RuO4 is missing, the volume of which should be different only by one additional electron for Sr2RhO4. The apparent contradiction is resolved by a careful analysis of the band structure where the rotation of octahedra results in the hybridization of e(g) and t(2g) states and thus plays a key role in the determination of the electronic structure near EF. The modification of the FS structure due to the distorted lattice is related to the charge transfer among the orbital states and suggested to be relevant to the metal-insulator transition in Ca(2-x)Sr(x)RuO4.     

High-pressure X-Ray diffraction and optical absorption studies of CsI

Static-compression data and absorption spectra for CsI have been collected to 61 GPa (610 kbar) at room temperature. The band gap closes with increasing pressure and CsI is expected to metallize at 105 (± 15) GPa. A second order phase transition to the CuAu I structure is observed at 39 (± 1) GPa. The elastic constants measured at low pressures do not predict that an elastic instability, and hence a structural distortion, would occur at elevated pressures. Similarly, an ionic pair-potential model which reproduces the properties of CsI at low pressures does not show the distortion to be stabilized at high pressures.     

Peierls mechanism of the metal-insulator transition in ferromagnetic hollandite K2Cr8O16

Synchrotron x-ray diffraction experiment shows that the metal-insulator transition occurring in a ferromagnetic state of a hollandite K(2)Cr(8)O(16) is accompanied by a structural distortion from the tetragonal I4/m to monoclinic P112(1)/a phase with a √2×√2×1 supercell. Detailed electronic structure calculations demonstrate that the metal-insulator transition is caused by a Peierls instability in the quasi-one-dimensional column structure made of four coupled Cr-O chains running in the c direction, leading to the formation of tetramers of Cr ions below the transition temperature. This provides a rare example of the Peierls transition of fully spin-polarized electron systems.     

Dopant-induced nanoscale electronic inhomogeneities in Ca(2-x)Sr(x)RuO4

Ca(2-x)Sr(x)RuO4 single crystals with 0.1 < or = x < or = 2.0 have been studied systematically using scanning tunneling microscopy (STM) and spectroscopy, low-energy electron diffraction, and angle resolved photoelectron spectroscopy (ARPES). In contrast with the well-ordered lattice structure, the local density of states at the surface clearly shows a strong doping dependent nanoscale electronic inhomogeneity, regardless of the fact of isovalent substitution. Remarkably, the surface electronic roughness measured by STM and the inverse spectral weight of quasiparticle states determined by ARPES are found to vary with x in the same manner as the bulk in-plane residual resistivity, following the Nordheim rule. For the first time, the surface measurements--especially those with STM--are shown to be in good agreement with the bulk transport results, all clearly indicating a doping-induced electronic disorder in the system.