Y Maeno - Academia.edu (original) (raw)
Papers by Y Maeno
Physical Review Letters, 2004
We have studied the magnetic excitations in Ca 2ÿx Sr x RuO 4 , x 0:52 and 0.62, which exhibit an... more We have studied the magnetic excitations in Ca 2ÿx Sr x RuO 4 , 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 RuO 2 planes the signal is extremely broad, q 0:45 A ÿ1. 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 Sr 2 RuO 4 .
Physical Review Letters, 2006
We report low-temperature thermal expansion measurements on the bilayer ruthenate Sr3Ru2O7 as a f... more We report low-temperature thermal expansion measurements on the bilayer ruthenate Sr3Ru2O7 as a function of magnetic field applied perpendicular to the Ruthenium-oxide planes. The fielddependence of the c-axis expansion coefficient indicates the accumulation of entropy close to 8 Tesla, related to an underlying quantum critical point. The latter is masked by two first-order metamagnetic transitions which bound a regime of enhanced entropy. Outside this region the singular thermal expansion behavior is compatible with the predictions of the itinerant theory for a two-dimensional metamagnetic quantum critical end point.
Physical Review Letters, 2006
We investigated optical spectra of quasi-two-dimensional multi-band Ca2−xSrxRuO4 systems. The ext... more We investigated optical spectra of quasi-two-dimensional multi-band Ca2−xSrxRuO4 systems. The extended Drude model analysis on the ab-plane optical conductivity spectra indicates that the effective mass should be enhanced near x = 0.5. Based on the sum rule argument, we showed that the orbital-selective Mott-gap opening for the d yz/zx bands, the widely investigated picture, could not be the origin of the mass enhancement. We exploited the multi-band effects in the extended Drude model analysis, and demonstrated that the intriguing heavy mass state near x = 0.5 should come from the renormalization of the dxy band.
Physical Review Letters, 2002
We have measured the temperature and field dependence of the resistivity of the unconventional su... more We have measured the temperature and field dependence of the resistivity of the unconventional superconductor Sr 2 RuO 4 at pressures up to 3.3 GPa. Using the Shubnikov-de Haas effect, we find that the Fermi surface sheet believed to be primarily responsible for superconductivity becomes more twodimensional with increasing pressure, a surprising result that is, however, consistent with a recent model of orbital-dependent superconductivity in this system. Many-body enhancements and the superconducting transition temperature all fall gradually with increasing pressure, contrary to previous suggestions of a ferromagnetic quantum critical point at 3 GPa.
Physical Review Letters, 2006
Strongly frustrated magnetism of the metallic pyrochlore oxide Pr 2 Ir 2 O 7 has been revealed by... more Strongly frustrated magnetism of the metallic pyrochlore oxide Pr 2 Ir 2 O 7 has been revealed by single crystal study. While Pr 4f moments have an antiferromagnetic RKKY interaction energy scale of jT j 20 K mediated by Ir 5d-conduction electrons, no magnetic long-range order is found except for partial spin freezing at 120 mK. Instead, the Kondo effect, including a lnT dependence in the resistivity, emerges and leads to a partial screening of the moments below jT j. Our results indicate that the underscreened moments show spin-liquid behavior below a renormalized correlation scale of 1.7 K.
Physical Review B, 2014
We report two-dimensionality and effective thickness of the superconductivity of a SrTiO 3 single... more We report two-dimensionality and effective thickness of the superconductivity of a SrTiO 3 single-crystal surface induced by electric double-layer gating. The carrier density was tuned from 3 × 10 13 to 1.1 × 10 14 cm −2 by gating, where superconductivity appears with T c of around 0.4 K. Typical two-dimensional behavior perfectly described by the Ginzburg-Landau equation was observed in the angular and temperature dependence of the upper critical magnetic field. The effective thickness of the superconducting layer remains nearly invariant, ranging from 11 to 13 nm, with increasing charge carrier density. This invariance contradicts the expected reduction in the thickness of the accumulation layer in a triangular quantum well model. This unexpected invariance of the superconducting layer thickness is probably a unique nature for a two-dimensional electron system in the incipient ferroelectric SrTiO 3 .
Physical Review B, 2012
The organic materials (TMTSF) 2 X are unique unconventional superconductors with archetypal quasi... more The organic materials (TMTSF) 2 X are unique unconventional superconductors with archetypal quasi-onedimensional (Q1D) electronic structures. Here, based on our comprehensive field-angle-resolved calorimetry of (TMTSF) 2 ClO 4 , we succeeded in mapping the nodal gap structure by discriminating between the Fermi wave vectors and Fermi velocities. In addition, the thermodynamic phase diagrams of (TMTSF) 2 ClO 4 for all principal field directions are obtained. These findings, providing strong evidence of nodal spin-singlet superconductivity, serves as solid bases for the further elucidation of anomalous superconducting phenomena in (TMTSF) 2 X.
Physical Review B, 2012
The low-temperature magnetic behavior of the double-layered ruthenate Sr 3 Ru 2 O 7 , as grown fr... more The low-temperature magnetic behavior of the double-layered ruthenate Sr 3 Ru 2 O 7 , as grown from a eutectic Sr 2 RuO 4-Sr 3 Ru 2 O 7 system, was investigated via zero-and transverse-field muon-spin rotation. The gradual increase of the muon relaxation rate observed below 2.5 K, even in the absence of applied magnetic fields, indicates the occurrence of a spontaneous breaking of time-reversal symmetry. The onset of the latter at a temperature above 1.5 K, the T c of the single phase Sr 2 RuO 4 , provides evidence about an unconventional superconducting state in the eutectic phase, which most likely takes place at the interface between the Sr 2 RuO 4 and Sr 3 Ru 2 O 7 domains, or even inside the Sr 3 Ru 2 O 7 phase. We show that the superconducting state manifests a two-component behavior in the transverse-field response with changeover at about T = 2.5 K and T = 1.5 K. The comparison with zero-field μSR data in the Ru-Sr 2 RuO 4 eutectic system rules out the possibility of spurious effects due to embedded Ru islands.
Physical Review B, 2005
ARPES spectra hold a wealth of information about the many-body interactions in a correlated mater... more ARPES spectra hold a wealth of information about the many-body interactions in a correlated material. However, the quantitative analysis of ARPES spectra to extract the various coupling parameters in a consistent manner is extremely challenging, even for a model Fermi liquid system. We propose a fitting procedure which allows quantitative access to the intrinsic lineshape, deconvolved of energy and momentum resolution effects, of the correlated 2-dimensional material Sr2RuO4. For the first time in correlated 2-dimensional materials, we find an ARPES linewidth that is narrower than its binding energy, a key property of quasiparticles within Fermi liquid theory. We also find that when the electron-electron scattering component is separated from the electron-phonon and impurity scattering terms it decreases with a functional form compatible with Fermi liquid theory as the Fermi energy is approached. In combination with the previously determined Fermi surface, these results give the first complete picture of a Fermi liquid system via ARPES. Furthermore, we show that the magnitude of the extracted imaginary part of the self-energy is in remarkable agreement with DC transport measurements.
physica status solidi (b), 2013
We report high-resolution measurements of the in-plane thermal expansion anisotropy in the vicini... more We report high-resolution measurements of the in-plane thermal expansion anisotropy in the vicinity of the electronic nematic phase in Sr 3 Ru 2 O 7 down to very low temperatures and in varying magnetic field orientation. For fields applied along the c-direction, a clear second-order phase transition is found at the nematic phase, with critical behavior compatible with the two-dimensional Ising universality class (although this is not fully conclusive). Measurements in a slightly tilted magnetic field reveal a broken four-fold in-plane rotational symmetry, not only within the nematic phase, but extending towards slightly larger fields. We also analyze the universal scaling behavior expected for a metamagnetic quantum critical point, which is realized outside the nematic region. The contours of the magnetostriction suggest a relation between quantum criticality and the nematic phase.
Physica C: Superconductivity, 2013
Superconductors with noncentrosymmetric crystal structures such as Li 2 T 3 B(T:Pd,Pt) have been ... more Superconductors with noncentrosymmetric crystal structures such as Li 2 T 3 B(T:Pd,Pt) have been the focus of in-depth research with their parity mixing nature. In this study, we focused our research on non-magnetic impurity effect in Li 2 T 3 B (T: Pd, Pt). The nature of the pair breaking by non-magnetic impurity in the parity mixing superconducting state is still unclear. We prepared different quality samples of Li 2 Pd 3 B and Li 2 Pt 3 B by changing conditions in synthesizing, and sample qualities were estimated by residual resistivity. Spin singlet dominant superconductor Li 2 Pd 3 B exhibits the weak T c suppression attributed by nonmagnetic impurity and defects, while H c2 (0) value increased. This behavior is similar in ordinary s-wave superconductor. On the other hand, for the spin triplet dominant superconductor Li 2 Pt 3 B, it was suggested that the Cooper pair was broken and superconducting gap was decreased by non-magnetic impurities and defects. Li 2 Pt 3 B is similar to unconventional superconducting state.
Physica C: Superconductivity, 2000
We report on the temperature dependence of the penetration depth of single crystals of Sr2RuO4 (T... more We report on the temperature dependence of the penetration depth of single crystals of Sr2RuO4 (To ~ 1.44 K). We observe a quadratic temperature dependence in the low temperature region down to 40 mK, with no sign of a second phase transition. The result is not consistent with some proposed models of multi-band superconductivity. We discuss the implications of this result with respect to the symmetry of the order parameter in this material.
Physica C: Superconductivity, 2001
We have carried out detailed angle dependent studies of the normal state microwave (40±112 GHz) m... more We have carried out detailed angle dependent studies of the normal state microwave (40±112 GHz) magneto-conductivity of several single crystal samples of the perovskite superconductor Sr 2 RuO 4. As previously reported [Phys. Rev. Lett. 84 (2000) 3374], we observe a series of resonant absorptions which we attribute to cyclotron resonance of quasiparticles belonging to the three well-known Fermi surfaces for this material. From the angle dependence, we con®rm the two-dimensional character of these resonances, i.e. the cyclotron frequencies scale as the inverse cosine of the angle between the magnetic ®eld and the normal to the conducting layers. Furthermore, by performing measurements on several samples, and in dierent electromagnetic ®eld con®gurations, we are able to couple to dierent cyclotron modes (harmonics) which derive from deformations (warpings) of the Fermi surfaces from perfect cylinders. These mode couplings will be discussed in the light of recent angle dependent de Haas±van Alphen measurements.
Physica C: Superconductivity, 1991
Physica C: Superconductivity, 2011
Atomic-resolution scanning tunneling spectroscopy is used to study the spectrum of quasiparticle ... more Atomic-resolution scanning tunneling spectroscopy is used to study the spectrum of quasiparticle states in superconducting Sr2RuO4. The measured temperature dependence of the quasiparticle spectrum is consistent with a multi-band superconducting order parameter with one band exhibiting a line of nodes, if a constant density of states offset can be ascribed to a surface effect. In magnetic fields H < Hc2, a vortex lattice, square and oriented parallel to the 110 direction of the RuO2 lattice, is detected by spectroscopic imaging. Each vortex exhibits one flux quantum and a strong zero-bias conductance peak.
Physica B: Condensed Matter, 1999
We have found a sharp transition in the vortex creep rates at a temperature T * = 0.05T c in a si... more We have found a sharp transition in the vortex creep rates at a temperature T * = 0.05T c in a single crystal of Sr 2 RuO 4 (T c = 1.03 K) by means of magnetic relaxation measurements. For T < T * , the initial creep rates drop to undetectable low levels. One explanation for this transition into a phase with such extremely low vortex creep is that the low-temperature phase of Sr 2 RuO 4 breaks time reversal symmetry. In that case, degenerate domain walls separating discreetly degenerate states of a superconductor can act as very strong pinning centers.[1]
Journal of the Physical Society of Japan, 2003
Co NQR studies were performed in recently discovered superconductor NaxCoO2•yH2O to investigate p... more Co NQR studies were performed in recently discovered superconductor NaxCoO2•yH2O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same NaxCoO2 were examined, SC bilayer-hydrate sample with Tc ∼ 4.7 K and non-SC monolayer-hydrate sample. From the measurement of nuclear-spin lattice relaxation rate 1/T1 in the SC sample, it was found that the coherence peak is absent just below Tc and that 1/T1 is proportional to temperature far below Tc. These results, which are in qualitatively agreement with the previous result by Fujimoto et al., suggest strongly that unconventional superconductivity is realized in this compound. In the normal state, 1/T1T of the SC sample shows gradual increase below 100K down to Tc, whereas 1/T1T of the non-SC sample shows the Korringa behavior in this temperature range. From the comparison between 1/T1T and χ bulk in the SC sample, the increase of 1/T1T is attributed to nearly ferromagnetic fluctuations. These remarkable findings suggest that the SC sample possesses nearly ferromagnetic fluctuations, which are possibly related with the unconventional superconductivity in this compound. The implication of this finding is discussed.
Journal of Low Temperature Physics, 1996
Wehavesynthesized anew bilayered ruthenate solid solution Sr3-x Ca x Ru 2 0 7 (0$t~2.0), which is... more Wehavesynthesized anew bilayered ruthenate solid solution Sr3-x Ca x Ru 2 0 7 (0$t~2.0), which is homologous to copper-free layered perovskite superconductor Sr 2 Ru0 4 (Tc =1.5 K), with the aim of searching for new superconductors. We have also succeeded in growing single crystals of Sr 3 Ru 2 0 7 by a floating zone (FZ) method. A systematic measurement has been carried out for the x dependence of the lattice parameters, magnetic susceptibility X(1), magnetization M(H), and electrical resistivity AT). No indication of superconductivity has been observed down to 100 mK for x=O (single crystals), 300 mK for x=l, and 2 K for other compositions. The measured values of lattice parameters and of magnetic quantities for the present single crystal samples are in good agreement with those for polycrystalline samples. Polycrystalline samples for 0.6<x~2.0 show magnetic ordering below 50 K. It is intriguing that the magnetic correlations vary with increasing Ca content, from nearly ferromagnetic, to itinerant weakly ferromagnetic, and finally to localized antiferromagnetic. For x(Ca)=O, we have concluded that the ground state is the strongly-correlated Fermi liquid accompanied by enhanced ferromagnetic correlations. The lowest magnetic ordering temperature in this series is observed at 3.2 K in the sample with x (Ca) =1.0. This transition is described well as that due to a weakly ferromagnetic ordering by the self-consistent renormalization (SCR) theory of spin fluctuations. To the best of our knowledge, this is the first example to demonstrate an applicability of the SCR theory to a weak ferromagnet with a quasi-two dimensional crystal structure.
EPL (Europhysics Letters), 2011
Comprehensive experimental studies by magnetic, thermal and neutron measurements have clarified t... more Comprehensive experimental studies by magnetic, thermal and neutron measurements have clarified that Rb4Mn(MoO4)3 is a model system of a quasi-2D triangular Heisenberg antiferromagnet with an easy-axis anisotropy, exhibiting successive transitions across an intermediate collinear phase. As a rare case for geometrically frustrated magnetism, quantitative agreement between experiment and theory is found for complete, anisotropic phase diagrams as well as magnetic properties.
Physical Review Letters, 2004
We have studied the magnetic excitations in Ca 2ÿx Sr x RuO 4 , x 0:52 and 0.62, which exhibit an... more We have studied the magnetic excitations in Ca 2ÿx Sr x RuO 4 , 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 RuO 2 planes the signal is extremely broad, q 0:45 A ÿ1. 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 Sr 2 RuO 4 .
Physical Review Letters, 2006
We report low-temperature thermal expansion measurements on the bilayer ruthenate Sr3Ru2O7 as a f... more We report low-temperature thermal expansion measurements on the bilayer ruthenate Sr3Ru2O7 as a function of magnetic field applied perpendicular to the Ruthenium-oxide planes. The fielddependence of the c-axis expansion coefficient indicates the accumulation of entropy close to 8 Tesla, related to an underlying quantum critical point. The latter is masked by two first-order metamagnetic transitions which bound a regime of enhanced entropy. Outside this region the singular thermal expansion behavior is compatible with the predictions of the itinerant theory for a two-dimensional metamagnetic quantum critical end point.
Physical Review Letters, 2006
We investigated optical spectra of quasi-two-dimensional multi-band Ca2−xSrxRuO4 systems. The ext... more We investigated optical spectra of quasi-two-dimensional multi-band Ca2−xSrxRuO4 systems. The extended Drude model analysis on the ab-plane optical conductivity spectra indicates that the effective mass should be enhanced near x = 0.5. Based on the sum rule argument, we showed that the orbital-selective Mott-gap opening for the d yz/zx bands, the widely investigated picture, could not be the origin of the mass enhancement. We exploited the multi-band effects in the extended Drude model analysis, and demonstrated that the intriguing heavy mass state near x = 0.5 should come from the renormalization of the dxy band.
Physical Review Letters, 2002
We have measured the temperature and field dependence of the resistivity of the unconventional su... more We have measured the temperature and field dependence of the resistivity of the unconventional superconductor Sr 2 RuO 4 at pressures up to 3.3 GPa. Using the Shubnikov-de Haas effect, we find that the Fermi surface sheet believed to be primarily responsible for superconductivity becomes more twodimensional with increasing pressure, a surprising result that is, however, consistent with a recent model of orbital-dependent superconductivity in this system. Many-body enhancements and the superconducting transition temperature all fall gradually with increasing pressure, contrary to previous suggestions of a ferromagnetic quantum critical point at 3 GPa.
Physical Review Letters, 2006
Strongly frustrated magnetism of the metallic pyrochlore oxide Pr 2 Ir 2 O 7 has been revealed by... more Strongly frustrated magnetism of the metallic pyrochlore oxide Pr 2 Ir 2 O 7 has been revealed by single crystal study. While Pr 4f moments have an antiferromagnetic RKKY interaction energy scale of jT j 20 K mediated by Ir 5d-conduction electrons, no magnetic long-range order is found except for partial spin freezing at 120 mK. Instead, the Kondo effect, including a lnT dependence in the resistivity, emerges and leads to a partial screening of the moments below jT j. Our results indicate that the underscreened moments show spin-liquid behavior below a renormalized correlation scale of 1.7 K.
Physical Review B, 2014
We report two-dimensionality and effective thickness of the superconductivity of a SrTiO 3 single... more We report two-dimensionality and effective thickness of the superconductivity of a SrTiO 3 single-crystal surface induced by electric double-layer gating. The carrier density was tuned from 3 × 10 13 to 1.1 × 10 14 cm −2 by gating, where superconductivity appears with T c of around 0.4 K. Typical two-dimensional behavior perfectly described by the Ginzburg-Landau equation was observed in the angular and temperature dependence of the upper critical magnetic field. The effective thickness of the superconducting layer remains nearly invariant, ranging from 11 to 13 nm, with increasing charge carrier density. This invariance contradicts the expected reduction in the thickness of the accumulation layer in a triangular quantum well model. This unexpected invariance of the superconducting layer thickness is probably a unique nature for a two-dimensional electron system in the incipient ferroelectric SrTiO 3 .
Physical Review B, 2012
The organic materials (TMTSF) 2 X are unique unconventional superconductors with archetypal quasi... more The organic materials (TMTSF) 2 X are unique unconventional superconductors with archetypal quasi-onedimensional (Q1D) electronic structures. Here, based on our comprehensive field-angle-resolved calorimetry of (TMTSF) 2 ClO 4 , we succeeded in mapping the nodal gap structure by discriminating between the Fermi wave vectors and Fermi velocities. In addition, the thermodynamic phase diagrams of (TMTSF) 2 ClO 4 for all principal field directions are obtained. These findings, providing strong evidence of nodal spin-singlet superconductivity, serves as solid bases for the further elucidation of anomalous superconducting phenomena in (TMTSF) 2 X.
Physical Review B, 2012
The low-temperature magnetic behavior of the double-layered ruthenate Sr 3 Ru 2 O 7 , as grown fr... more The low-temperature magnetic behavior of the double-layered ruthenate Sr 3 Ru 2 O 7 , as grown from a eutectic Sr 2 RuO 4-Sr 3 Ru 2 O 7 system, was investigated via zero-and transverse-field muon-spin rotation. The gradual increase of the muon relaxation rate observed below 2.5 K, even in the absence of applied magnetic fields, indicates the occurrence of a spontaneous breaking of time-reversal symmetry. The onset of the latter at a temperature above 1.5 K, the T c of the single phase Sr 2 RuO 4 , provides evidence about an unconventional superconducting state in the eutectic phase, which most likely takes place at the interface between the Sr 2 RuO 4 and Sr 3 Ru 2 O 7 domains, or even inside the Sr 3 Ru 2 O 7 phase. We show that the superconducting state manifests a two-component behavior in the transverse-field response with changeover at about T = 2.5 K and T = 1.5 K. The comparison with zero-field μSR data in the Ru-Sr 2 RuO 4 eutectic system rules out the possibility of spurious effects due to embedded Ru islands.
Physical Review B, 2005
ARPES spectra hold a wealth of information about the many-body interactions in a correlated mater... more ARPES spectra hold a wealth of information about the many-body interactions in a correlated material. However, the quantitative analysis of ARPES spectra to extract the various coupling parameters in a consistent manner is extremely challenging, even for a model Fermi liquid system. We propose a fitting procedure which allows quantitative access to the intrinsic lineshape, deconvolved of energy and momentum resolution effects, of the correlated 2-dimensional material Sr2RuO4. For the first time in correlated 2-dimensional materials, we find an ARPES linewidth that is narrower than its binding energy, a key property of quasiparticles within Fermi liquid theory. We also find that when the electron-electron scattering component is separated from the electron-phonon and impurity scattering terms it decreases with a functional form compatible with Fermi liquid theory as the Fermi energy is approached. In combination with the previously determined Fermi surface, these results give the first complete picture of a Fermi liquid system via ARPES. Furthermore, we show that the magnitude of the extracted imaginary part of the self-energy is in remarkable agreement with DC transport measurements.
physica status solidi (b), 2013
We report high-resolution measurements of the in-plane thermal expansion anisotropy in the vicini... more We report high-resolution measurements of the in-plane thermal expansion anisotropy in the vicinity of the electronic nematic phase in Sr 3 Ru 2 O 7 down to very low temperatures and in varying magnetic field orientation. For fields applied along the c-direction, a clear second-order phase transition is found at the nematic phase, with critical behavior compatible with the two-dimensional Ising universality class (although this is not fully conclusive). Measurements in a slightly tilted magnetic field reveal a broken four-fold in-plane rotational symmetry, not only within the nematic phase, but extending towards slightly larger fields. We also analyze the universal scaling behavior expected for a metamagnetic quantum critical point, which is realized outside the nematic region. The contours of the magnetostriction suggest a relation between quantum criticality and the nematic phase.
Physica C: Superconductivity, 2013
Superconductors with noncentrosymmetric crystal structures such as Li 2 T 3 B(T:Pd,Pt) have been ... more Superconductors with noncentrosymmetric crystal structures such as Li 2 T 3 B(T:Pd,Pt) have been the focus of in-depth research with their parity mixing nature. In this study, we focused our research on non-magnetic impurity effect in Li 2 T 3 B (T: Pd, Pt). The nature of the pair breaking by non-magnetic impurity in the parity mixing superconducting state is still unclear. We prepared different quality samples of Li 2 Pd 3 B and Li 2 Pt 3 B by changing conditions in synthesizing, and sample qualities were estimated by residual resistivity. Spin singlet dominant superconductor Li 2 Pd 3 B exhibits the weak T c suppression attributed by nonmagnetic impurity and defects, while H c2 (0) value increased. This behavior is similar in ordinary s-wave superconductor. On the other hand, for the spin triplet dominant superconductor Li 2 Pt 3 B, it was suggested that the Cooper pair was broken and superconducting gap was decreased by non-magnetic impurities and defects. Li 2 Pt 3 B is similar to unconventional superconducting state.
Physica C: Superconductivity, 2000
We report on the temperature dependence of the penetration depth of single crystals of Sr2RuO4 (T... more We report on the temperature dependence of the penetration depth of single crystals of Sr2RuO4 (To ~ 1.44 K). We observe a quadratic temperature dependence in the low temperature region down to 40 mK, with no sign of a second phase transition. The result is not consistent with some proposed models of multi-band superconductivity. We discuss the implications of this result with respect to the symmetry of the order parameter in this material.
Physica C: Superconductivity, 2001
We have carried out detailed angle dependent studies of the normal state microwave (40±112 GHz) m... more We have carried out detailed angle dependent studies of the normal state microwave (40±112 GHz) magneto-conductivity of several single crystal samples of the perovskite superconductor Sr 2 RuO 4. As previously reported [Phys. Rev. Lett. 84 (2000) 3374], we observe a series of resonant absorptions which we attribute to cyclotron resonance of quasiparticles belonging to the three well-known Fermi surfaces for this material. From the angle dependence, we con®rm the two-dimensional character of these resonances, i.e. the cyclotron frequencies scale as the inverse cosine of the angle between the magnetic ®eld and the normal to the conducting layers. Furthermore, by performing measurements on several samples, and in dierent electromagnetic ®eld con®gurations, we are able to couple to dierent cyclotron modes (harmonics) which derive from deformations (warpings) of the Fermi surfaces from perfect cylinders. These mode couplings will be discussed in the light of recent angle dependent de Haas±van Alphen measurements.
Physica C: Superconductivity, 1991
Physica C: Superconductivity, 2011
Atomic-resolution scanning tunneling spectroscopy is used to study the spectrum of quasiparticle ... more Atomic-resolution scanning tunneling spectroscopy is used to study the spectrum of quasiparticle states in superconducting Sr2RuO4. The measured temperature dependence of the quasiparticle spectrum is consistent with a multi-band superconducting order parameter with one band exhibiting a line of nodes, if a constant density of states offset can be ascribed to a surface effect. In magnetic fields H < Hc2, a vortex lattice, square and oriented parallel to the 110 direction of the RuO2 lattice, is detected by spectroscopic imaging. Each vortex exhibits one flux quantum and a strong zero-bias conductance peak.
Physica B: Condensed Matter, 1999
We have found a sharp transition in the vortex creep rates at a temperature T * = 0.05T c in a si... more We have found a sharp transition in the vortex creep rates at a temperature T * = 0.05T c in a single crystal of Sr 2 RuO 4 (T c = 1.03 K) by means of magnetic relaxation measurements. For T < T * , the initial creep rates drop to undetectable low levels. One explanation for this transition into a phase with such extremely low vortex creep is that the low-temperature phase of Sr 2 RuO 4 breaks time reversal symmetry. In that case, degenerate domain walls separating discreetly degenerate states of a superconductor can act as very strong pinning centers.[1]
Journal of the Physical Society of Japan, 2003
Co NQR studies were performed in recently discovered superconductor NaxCoO2•yH2O to investigate p... more Co NQR studies were performed in recently discovered superconductor NaxCoO2•yH2O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same NaxCoO2 were examined, SC bilayer-hydrate sample with Tc ∼ 4.7 K and non-SC monolayer-hydrate sample. From the measurement of nuclear-spin lattice relaxation rate 1/T1 in the SC sample, it was found that the coherence peak is absent just below Tc and that 1/T1 is proportional to temperature far below Tc. These results, which are in qualitatively agreement with the previous result by Fujimoto et al., suggest strongly that unconventional superconductivity is realized in this compound. In the normal state, 1/T1T of the SC sample shows gradual increase below 100K down to Tc, whereas 1/T1T of the non-SC sample shows the Korringa behavior in this temperature range. From the comparison between 1/T1T and χ bulk in the SC sample, the increase of 1/T1T is attributed to nearly ferromagnetic fluctuations. These remarkable findings suggest that the SC sample possesses nearly ferromagnetic fluctuations, which are possibly related with the unconventional superconductivity in this compound. The implication of this finding is discussed.
Journal of Low Temperature Physics, 1996
Wehavesynthesized anew bilayered ruthenate solid solution Sr3-x Ca x Ru 2 0 7 (0$t~2.0), which is... more Wehavesynthesized anew bilayered ruthenate solid solution Sr3-x Ca x Ru 2 0 7 (0$t~2.0), which is homologous to copper-free layered perovskite superconductor Sr 2 Ru0 4 (Tc =1.5 K), with the aim of searching for new superconductors. We have also succeeded in growing single crystals of Sr 3 Ru 2 0 7 by a floating zone (FZ) method. A systematic measurement has been carried out for the x dependence of the lattice parameters, magnetic susceptibility X(1), magnetization M(H), and electrical resistivity AT). No indication of superconductivity has been observed down to 100 mK for x=O (single crystals), 300 mK for x=l, and 2 K for other compositions. The measured values of lattice parameters and of magnetic quantities for the present single crystal samples are in good agreement with those for polycrystalline samples. Polycrystalline samples for 0.6<x~2.0 show magnetic ordering below 50 K. It is intriguing that the magnetic correlations vary with increasing Ca content, from nearly ferromagnetic, to itinerant weakly ferromagnetic, and finally to localized antiferromagnetic. For x(Ca)=O, we have concluded that the ground state is the strongly-correlated Fermi liquid accompanied by enhanced ferromagnetic correlations. The lowest magnetic ordering temperature in this series is observed at 3.2 K in the sample with x (Ca) =1.0. This transition is described well as that due to a weakly ferromagnetic ordering by the self-consistent renormalization (SCR) theory of spin fluctuations. To the best of our knowledge, this is the first example to demonstrate an applicability of the SCR theory to a weak ferromagnet with a quasi-two dimensional crystal structure.
EPL (Europhysics Letters), 2011
Comprehensive experimental studies by magnetic, thermal and neutron measurements have clarified t... more Comprehensive experimental studies by magnetic, thermal and neutron measurements have clarified that Rb4Mn(MoO4)3 is a model system of a quasi-2D triangular Heisenberg antiferromagnet with an easy-axis anisotropy, exhibiting successive transitions across an intermediate collinear phase. As a rare case for geometrically frustrated magnetism, quantitative agreement between experiment and theory is found for complete, anisotropic phase diagrams as well as magnetic properties.