Field-induced paramagnons at the metamagnetic transition of Ca1.8Sr0.2RuO4

The magnetic excitations in Ca1.8Sr0.2RuO4 were studied across the metamagnetic transition and as a function of temperature using inelastic neutron scattering. At low temperature and low magnetic field the magnetic response is dominated by a complex superposition of incommensurate antiferromagnetic fluctuations. Upon increasing the magnetic field across the metamagnetic transition, paramagnon and finally well-defined magnon scattering is induced, partially suppressing the incommensurate signals. The high-field phase in Ca1.8Sr0.2RuO4, therefore, has to be considered as an intrinsically ferromagnetic state stabilized by the magnetic field.     

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.     

Incommensurate magnetic ordering in Sr2Ru(1-x)TixO4

In Sr2RuO4 the spin excitation spectrum is dominated by incommensurate fluctuations at q = (0.3 0.3q(z)), which arise from Fermi-surface nesting. We show that upon Ti substitution, known to suppress superconductivity, a short range magnetic order develops with a propagation vector (0.307 0.307 1). In Sr2Ru0.91Ti0.09O4, the ordered moment points along the c direction. This finding shows that superconducting Sr2RuO4 is extremely close to an incommensurate spin density wave instability.     

Anisotropy of the incommensurate fluctuations in Sr2RuO4: a study with polarized neutrons

The anisotropy of the magnetic incommensurate fluctuations in Sr2RuO4 has been studied by inelastic neutron scattering with polarized neutrons. We find a sizable enhancement of the out-of-plane component by a factor of 2 for intermediate energy transfer, which appears to decrease for higher energies. Our results qualitatively confirm calculations of the spin-orbit coupling, but the experimental anisotropy and its energy dependence are weaker than predicted.     

Gap structure of the spin-triplet superconductor Sr2RuO4 determined from the field-orientation dependence of the specific heat

We report the field-orientation dependent specific heat of the spin-triplet superconductor Sr2RuO4 under the magnetic field aligned parallel to the RuO2 planes with high accuracy. Below about 0.3 K, striking fourfold oscillations of the density of states reflecting the superconducting gap structure have been resolved for the first time. We also obtained strong evidence of multiband superconductivity and concluded that the superconducting gap in the active band, responsible for the superconducting instability, is modulated with a minimum along the [100] direction.     

High-energy spin excitations in the electron-doped superconductor Pr(0.88)LaCe(0.12)CuO(4-delta) with T(c) = 21 K

We use high-resolution inelastic neutron scattering to study the low-temperature magnetic excitations of the electron-doping superconductor Pr(0.88)LaCe(0.12)CuO(4-delta) (T(c) = 21 +/- 1 K) over a wide energy range (4 meV < or = homega < or = 330 meV). The effect of electron doping is to cause a wave vector (Q) broadening in the low-energy (homega < or = 80 meV) commensurate spin fluctuations at (0.5, 0.5) and to suppress the intensity of spin-wave-like excitations at high energies (homega > or = 100 meV). This leads to a substantial redistribution in the spectrum of the local dynamical spin susceptibility chi'(omega), and reveals a new energy scale similar to that of the lightly hole-doped YB2Cu3O(6.353) (T(c) = 18 K).     

Measurement of the 101Ru-Knight shift of superconducting Sr2RuO4 in a parallel magnetic field

101Ru-Knight shift (101K) in the spin-triplet superconductor Sr2RuO4 was measured under magnetic fields parallel to the c axis (perpendicular to the RuO2 plane), which is the promising superconducting (SC) d-vector direction in a zero field. We succeeded in measuring K(c) in the field range from 200 to 1200 Oe and at temperatures down to 80 mK, using nuclear-quadrupole-resonance spectra. We found that (101)K(c) is invariant with respect to the field and temperature on passing through H(c2) and T(c) above 200 Oe. This indicates that the spin susceptibility along the c axis does not change in the SC state, at least, in the field greater than 200 Oe. The results imply that the SC d vector is in the RuO2 plane when the magnetic field is applied to the c axis.     

Non-fermi-liquid behavior in Sr2RuO4 with nonmagnetic impurities

We report that the quasi-two-dimensional Fermi-liquid behavior of the spin-triplet superconductor Sr2RuO4 breaks down in the vicinity of the critical impurity concentration for the onset of magnetic order induced by nonmagnetic Ti4+ impurities. The non-Fermi-liquid behavior is interpreted in terms of the two-dimensional antiferromagnetic fluctuations, which arise mainly from the nesting within one of the Fermi-surface sheets. We argue against the main role of such magnetic fluctuations in the pairing mechanism.     

Interference of magnetic and anisotropic tensor susceptibility reflections in resonant X-ray scattering of GdB4

Resonant x-ray scattering experiments at the Gd L3 edge show interference between magnetic and anisotropic tensor susceptibility (ATS) reflections in GdB4. Energy profiles obtained from the magnetic and ATS resonances exhibited approximately 10 eV separation between the maximum resonance energies. The findings show that the Gd 5d band experienced hybridization giving rise to a significant split into isotropic lower energy band and distorted upper band states that account for the magnetic and ATS scattering, respectively.     

The concentration metamagnetic transition in Tm1−x TbxCo2 compounds

The Tm_1?x Tb_xCo₂ (0 ≤ x ≤ 1) system was studied by measuring the magnetic susceptibility, electrical resistance, and neutron diffraction. In the compounds with 0 < x ≤ 0.15, an inhomogeneous magnetic state characterized by the existence of large regions (up to 100 Å in size) with short-range ferrimagnetic order was found to occur. The maximum of the residual electrical resistance observed in the compound with x = 0.1 at the magnetic ordering temperature was established to be due to the scattering of conduction electrons by localized spin fluctuations associated with f-d exchange fluctuations caused by the substitution of terbium for thulium. The increase in the terbium concentration to x ≥ 0.15 leads to a sharp increase in the Co sublattice magnetization and the establishment of a long-range ferromagnetic order, which indicates a concentration metamagnetic transition in the band subsystem.     

Off-site repulsion-induced triplet superconductivity: a possibility for chiral p(x+y)-wave pairing in Sr2RuO4

In order to probe the effect of charge fluctuations on triplet pairing, we study the pairing symmetry in the one-band Hubbard model having the off-site Coulomb repulsion (V) on top of the on-site repulsion as a model for the gamma band of Sr2RuO4, a strong candidate for a triplet pairing superconductor. The result, obtained with the dynamical cluster approximation combined with the quantum Monte Carlo method, and confirmed from the fluctuation exchange approximation, shows that while d(x(2)-y(2)) pairing dominates over p in the absence of V, introduction of V makes p(x+y) and d(xy) dominant. The gap function for the chiral p(x+y)+ip(x-y) has nodes that are consistent with the recent measurement of specific heat in rotated magnetic fields in the ruthenate. This suggests that the off-site repulsion may play an essential role in triplet superconductivity in this material.     

Heavy-mass fermi liquid near a ferromagnetic instability in layered ruthenates

Low temperature magnetic, thermal, and transport measurements in Ca2-xSrxRuO4 clarify the appearance of a cluster glass phase, after the evolution of a nearly ferromagnetic heavy-mass Fermi liquid from the spin-triplet superconductor Sr2RuO4. As the Mott transition is approached across a 2nd-order structural transition, both the magnetization and specific heat decrease considerably while the transport scattering rate diverges. A metamagnetic transition to a highly spin polarized state, with a local moment S=1/2, is observed. We argue that an orbital rearrangement with Ca substitution changes itinerant ferromagnetism to antiferromagnetism of localized moments.     

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.     

Spin-orbit coupling in the Mott insulator Ca(2)RuO(4)

O 1s x-ray absorption study of the Mott insulator Ca(2)RuO(4) shows that the orbital population of the 4d t(2g) band dramatically changes with temperature. In addition, spin-resolved circularly polarized photoemission study of Ca(2)RuO(4) shows that a substantial orbital angular momentum is induced in the Ru 4d t(2g) band. Based on the experimental results and model Hartree-Fock calculations, we argue that the cooperation between the strong spin-orbit coupling in the Ru 4d t(2g) band and the small distortion of the RuO(6) octahedra causes the interesting changeover of the spin and orbital anisotropy as a function of temperature.     

Quasiparticle line shape of Sr2RuO4 and its relation to anisotropic transport

The bulk-representative low-energy spectrum of Sr2RuO4 can be directly measured by angle-resolved photoemission. We find that the quasiparticle spectral line shape of Sr2RuO4 is sensitive to both temperature and momentum. Along the (0,0)-(pi,0) direction, both gamma and beta bands develop a sharp quasiparticle peak near k(F) at low temperatures, but as the temperature increases the spectra quickly lose coherent weight and become broad backgrounds above approximately 130 K, which is the metal-nonmetal crossover temperature, T(M), in the c-axis resistivity. However, spectra along the (0,0)-(pi,pi) direction evolve smoothly across T(M). A simple transport model can describe both in-plane and c-axis resistivity in terms of the quasiparticle line shape. Comparisons are also made to the cuprates, with implications for two dimensionality, magnetic fluctuations, and superconductivity.     

Magnetic fluctuations of filled skutterudites emerging in the transition region between singlet and triplet states

In order to clarify magnetic properties of filled skutterudites, we analyze the Anderson model including seven f orbitals hybridized with an au conduction band using a numerical technique. For n=2 corresponding to Pr-based filled skutterudites, where n is the local f-electron number, even if the ground state is a singlet, there remain significant magnetic fluctuations from a triplet state with a small excitation energy. This result can be understood by the fact that f-electron states are clearly distinguished as itinerant and localized ones in the filled skutterudite structure. This picture also explains the complex results for f-electron magnetic susceptibility and entropy for n=1-13.     

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.     

Band-dependent normal-state coherence in Sr2RuO4: evidence from Nernst effect and thermopower measurements

We present the first measurement on the Nernst effect in the normal state of the odd-parity, spin-triplet superconductor Sr2RuO4. Below 100 K, the Nernst signal was found to be negative, large, and, as a function of magnetic field, nonlinear. Its magnitude increases with the decreasing temperature until reaching a maximum around T* approximately equal to 20-25 K, below which it starts to decrease linearly as a function of temperature. The large value of the Nernst signal appears to be related to the multiband nature of the normal state and the nonlinearity to band-dependent magnetic fluctuation in Sr2RuO4. We argue that the sharp decrease in the Nernst signal below T* is due to the suppression of quasiparticle scattering and the emergence of band-dependent coherence in the normal state.     

A quantitative determination of magnetic fluctuations in CuGeO3

A polarised neutron scattering investigation has been carried out on a powder sample of CuGeO₃ within the temperature range of 1.5 K to 600 K. The magnetic scattering has been separated from all other contributions by using polarised neutrons and polarisation analysis and placed onto an absolute scale. At low temperatures the long wavelength components of the paramagnetic response are suppressed consistent with the formation of Cu dimers in which the magnetic moments are correlated antiferromagnetically. This form of the scattering persists to temperatures well above the dimerisation temperature T _sp ∼ 14 K. However as the temperature is raised the intensity of the long wavelength spin fluctuations increases and above ∼150 K they are the dominant feature in the wave vector dependence of the response. At all temperatures the observed scattering extrapolates smoothly to the Q = 0 value given by the uniform susceptibility. Consequently the thermal variation of the uniform susceptibility arises from the evolution of the long wavelength magnetic fluctuations. At large wave vectors the energy dependence of the scattering revealed that the response occurs below 16 meV in agreement with the reported maximum magnetic excitation energy at the zone boundary in the ground state. However the total magnetic scattering is significantly less than that expected for a local moment system suggesting that the spectrum of thermal and quantum fluctuations overlap. Received 30 May 2000 and Received in final form 22 March 2001     

Spin fluctuations, susceptibility, and the dipole oscillation of a nearly ferromagnetic fermi gas

We discuss the spin fluctuations and the role played by the magnetic susceptibility in an atomic Fermi gas interacting with a positive scattering length. Both thermal and zero-temperature quantum fluctuations are considered. Using a sum rule approach and recent ab initio Monte Carlo results for the magnetic susceptibility of uniform matter, we provide explicit predictions for the frequency of the spin dipole oscillation of a gas trapped by a harmonic potential and discuss the deviations from the ideal gas behavior when the system approaches the ferromagnetic transition. The role of the Landau's parameters in the characterization of the magnetic properties is also discussed.