arXiv subjects

Yoshiteru Maeno

Publications and source records attributed to Yoshiteru Maeno.

At least 19 recordsLinked to original sources

arXiv2026

Substitution effects in RuO$_2$ single crystals

RuO$_2$ has emerged as one of the leading candidates for investigating altermagnetism. Recent quantum oscillation and angle-resolved photoemission spectroscopy measurements found the absence of altermagnetism or antiferromagnetism in pure RuO$_2$ crystals. The continuing debate over intrinsic magnetic order in RuO$_2$ may reflect its proximity to an altermagnetic instability. In this work, we grow single crystals of Ru$_{0.9}$V$_{0.1}$O$_2$ and investigate their structural, transport, and magnetic properties. X-ray photoelectron spectroscopy reveals an average V oxidation state near $+4$. The paramagnetic susceptibility remains nearly unchanged up to room temperature, with no evidence of magnetic ordering. Thus, the 10\% V-substitution in RuO$_2$ does not induce altermagnetism. Electronic structure calculations for the V-substituted systems using two methods suggest that a higher level of V-substitution leads to a significant change in the density of states. These findings underscore the potential of nonmagnetic substitution in RuO$_2$ as an attractive candidate for probing altermagnetic transitions and their experimental signatures.

cond-mat.mtrl-sci↗

arXiv2026

Fermi-liquid behavior and characteristic temperature-dependent susceptibility in clean RuO$_2$ crystal

The magnetic nature of the altermagnet candidate RuO$_2$ remains under debate. It has been recently shown from quantum oscillations and angle-resolved photoemission spectroscopy (ARPES) that the high-quality RuO$_2$ bulk single crystal is a paramagnetic metal. Here we report the specific heat and magnetic susceptibility in ultra-clean RuO$_2$ single crystals with residual resistivity ratio up to 1200. The magnetic susceptibility increases with temperature and is phenomenologically fitted with an inclusion of $T\textrm{ln}(T/T_0)$ over a wide temperature range up to 400 K. In contrast, the energy dependence of the density of states and thermal activation of quasiparticles lead to a decrease with temperature. Such characteristic temperature dependence, similar to that observed in other $d$-electron metals, is attributable to an enhanced orbital contribution arising from lattice-expansion-induced changes in the band structure. The electronic specific heat, the magnetic susceptibility, and the $T^2$ coefficient in resistivity point to a weakly-correlated 3D Fermi-liquid state with a modest electron correlation, as supported by the Wilson and Kadowaki-Woods ratios.

cond-mat.mtrl-sci↗

arXiv2025

Origins of spontaneous magnetic fields in Sr$_2$RuO$_4$

The nature of the broken time reversal symmetry (BTRS) state in Sr$_2$RuO$_4$ remains elusive, and its relation to superconductivity remains controversial. There are various universal predictions for the BTRS state when it is associated with a multicomponent superconducting order parameter. In particular, in the BTRS superconducting state, spontaneous fields appear around crystalline defects, impurities, superconducting domain walls and sample surfaces. However, this phenomenon has not yet been experimentally demonstrated for any BTRS superconductor. Here, we aimed to verify these predictions for Sr$_2$RuO$_4$ by performing muon spin relaxation ($μ$SR) measurements on Sr$_{2-y}$La$_{y}$RuO$_4$ single crystals at ambient pressure and stoichiometric Sr$_2$RuO$_4$ under hydrostatic pressure. The study allowed us to conclude that spontaneous fields in the BTRS superconducting state of Sr$_2$RuO$_4$ appear around non-magnetic inhomogeneities and, at the same time, decrease with the suppression of $T_{\rm c}$. The observed behaviour is consistent with the prediction for multicomponent BTRS superconductivity in Sr$_2$RuO$_4$. The results of the work are relevant to understanding BTRS superconductivity in general, as they demonstrate, for the first time, the relationship among the superconducting order parameter, the BTRS transition, and crystal-structure inhomogeneities.

cond-mat.supr-con↗

arXiv2025

Muon Knight shift as a precise probe of the superconducting symmetry of Sr$_2$RuO$_4$

Muon spin rotation ($μ$SR) measurements of internal magnetic field shifts, known as the muon Knight shift, is used for determining pairing symmetries in superconductors. While this technique has been especially effective for $f$-electron-based heavy-fermion superconductors, it remains challenging in $d$-electron-based superconductors such as Sr$_2$RuO$_4$, where the Knight shift is intrinsically small. Here, we report high-precision muon Knight shift measurements of superconducting Sr$_2$RuO$_4$. We observe that using multiple pieces of crystals, a common practice in $μ$SR measurements, induces a substantial paramagnetic shift below the superconducting transition temperature, $T_c$, when a weak magnetic field is applied. We attribute such an unresolved paramagnetic shift to stray fields generated by neighboring diamagnetic crystals. To avoid this, one piece of crystal was used in this study. We experimentally determine the muon Knight shift of Sr$_2$RuO$_4$ in the normal state to be -116$\pm$7 ppm. By combining the observed muon Knight shift with independently determined bulk magnetization data from the same crystal used in $μ$SR and carefully separating various contributions to the shift, we confirm a significant reduction in the spin Knight shift below $T_c$, consistent with spin-singlet-like pairing. This result constitutes the precise muon Knight shift measurement in a $d$-electron-based superconductor. Our results highlight the potential of $μ$SR as a powerful complementary technique to the established method of nuclear magnetic resonance for probing the spin susceptibility in superconductors.

cond-mat.supr-con↗

arXiv2025

Electronic state of superconductivity in line nodal material CaSb2 under pressure up to 4.2 GPa

We report the results of resistance measurements under pressure up to $4.2\,\mathrm{GPa}$ on single-crystalline $\mathrm{CaSb}_2$ , which shows the maximum of superconducting transition temperature $T_\mathrm{c}$ at $3.1\,\mathrm{GPa}$. At room temperature, $R(P)$ shows a subtle anomaly at $3.1\,\mathrm{GPa}$. However, Bloch-Grüneisen analysis of $R(T)$ indicates that the electronic state does not change significantly across $3.1\,\mathrm{GPa}$.

cond-mat.supr-con↗

arXiv2025

Direct evidence for the absence of coupling between shear strain and superconductivity in Sr2RuO4

The superconducting symmetry of Sr2RuO4 has been intensely debated for many years. A crucial controversy recently emerged between shear-mode ultrasound experiments, which suggest a two-component order parameter, and some uniaxial pressure experiments that suggest a one-component order parameter. To resolve this controversy, we use a new approach to directly apply three different kinds of shear strain to single crystals of Sr2RuO4 and investigate the coupling to superconductivity. After characterising the strain by optical imaging, we observe variations of the transition temperature Tc smaller than 10mK/% as measured by low-frequency magnetic susceptibility, indicating that shear strain has little to no coupling to superconductivity. Our results are consistent with a one-component order parameter model, but such a model cannot consistently explain other experimental evidence such as time-reversal symmetry breaking, superconducting domains, and horizontal line nodes, thus calling for alternative interpretations.

cond-mat.supr-con↗

arXiv2025

Growth of ultra-clean single crystals of RuO2

We report the details of the growth of ultra-clean single crystals for RuO2, a candidate material for altermagnetism. By using a crystal-growth tube with a necking structure and precisely controlling the conditions of the sublimation transport method, it is possible to control the morphology of the crystals. We obtained crystals in mainly three kinds of morphology: thick plate-like crystals typically 5 x 3 x 2mm3 and up to 10 x 5 x 2mm3 with a large (101) facet, rhombohedral columnar crystals elongating along the [001] direction, and fiber and needle crystals of length up to 8 mm and the width of 0.1-0.4 mm. These crystals show residual resistivity of about 30 nOhmcm and a residual resistivity ratio (RRR) up to 1200. The crystals do not exhibit any signs of magnetic ordering down to low temperatures.

cond-mat.str-el↗

arXiv2025

The Fermi surface of RuO2 measured by quantum oscillations

The metallic oxide RuO$_2$ has emerged as a promising altermagnet candidate, owing to reports of this material hosting antiferromagnetic ordering accompanied by a spin-split electronic band structure characteristic of time-reversal symmetry-breaking. However, recent studies have robustly questioned this scenario. Here we map the Fermi surface of pristine single-crystalline RuO$_2$. By measuring magnetic quantum oscillations of a bulk thermodynamic property, our study resolves the electronic structure present in the bulk of RuO$_2$. Several Fermi sheets are discerned, with a range of effective quasiparticle masses up to five times that of the bare electron mass. We compare our measurements with the predictions for altermagnetic and nonmagnetic Fermi surfaces deduced from density functional theory calculations. The quantum oscillatory frequency spectra correspond very poorly to the profile expected for the case of altermagnetism; by contrast, they correspond well to the nonmagnetic scenario. Our findings place significant constraints on the bulk magnetic properties of RuO$_2$, and strongly suggest that this material is a paramagnet.

cond-mat.mtrl-sci↗

arXiv2025

Subtle Structural Anomaly under Compression in Line-Nodal CaSb$_2$

We report X-ray diffraction patterns and calculated electronic band structures of the Dirac line-nodal material CaSb$_2$ under pressure. Its superconducting transition temperature ($T_{\mathrm{c}}=1.7$ K) increases under pressure and reaches a maximum at 3.4 K at around 3 GPa. We observed subtle anomalies in lattice parameters accompanied by a jump in bulk modulus without any change in crystal symmetry at around 3 GPa. First-principles calculations revealed that the distorted lattice of Sb(1) site deforms in the pressure range of 0-3 GPa. Those results suggest the existence of a first-order structural transition and arouse expectations for unusual phononic properties affecting the superconducting state. The calculated pressure dependence of the electronic density of states (DOS) confirms that it is not the change in the DOS that governs the variations in $T_{\mathrm{c}}$.

cond-mat.mtrl-sci↗

arXiv2024

Hydride superconductivity: here to stay

The field of hydride superconductivity has recently been mired in a controversy that might divert attention from the question of central importance: do hydrides support genuine superconductivity or not? We examine some key papers from the field, and conclude that hydride superconductivity is real.

cond-mat.supr-con↗

arXiv2024

Anomalous Fermi pockets on Hund's metal surface of Sr2RuO4 induced by the correlation-enhanced spin-orbit coupling

The electronic structure of the topmost layer in Sr2RuO4 in the close vicinity of the Fermi level is investigated by angle-resolved photoemission spectroscopy (ARPES) with a 7-eV laser. We find that the spin-orbit coupling (SOC) predicted as 100 meV by the density functional theory (DFT) calculations is enormously enhanced in a real material up to 250 meV, even more than that of bulk state (200 meV), by the electron-correlation effect increased by the octahedral rotation in the crystal structure. This causes the formation of highly orbital-mixing small Fermi pockets and reasonably explains why the orbital-selective Mott transition (OSMT) is not realized in perovskite oxides with crystal distortion. Interestingly, Hund's metal feature allows the quasiparticle generation only near EF, restricting the spectral gap opening derived by band hybridization within an extremely small binding energy (< 10 meV). Furthermore, it causes coherent-incoherent crossover, making the Fermi pockets disappear at elevated temperatures. The anomalous Fermi pockets are characterized by the dichotomy of the orbital-isolating Hund's coupling and the orbital-mixing SOC, which is key to understanding the nature of Sr2RuO4.

cond-mat.str-el↗

arXiv2024

Negatively enhanced thermopower near a Van Hove singularity in electron-doped Sr$_2$RuO$_4$

The layered perovskite Sr$_2$RuO$_4$ serves as a model material of the two-dimensional (2D) Fermi liquid but also exhibits various emergent phenomena including the non-Fermi-liquid (NFL) behavior under external perturbations such as uniaxial pressure and chemical substitutions. Here we present the thermoelectric transport of electron-doped system Sr$_{2-y}$La$_{y}$RuO$_4$, in which a filling-induced Lifshitz transition occurs at the Van Hove singularity (VHS) point of $y\approx 0.2$. We find that the sign of the low-temperature thermopower becomes negative only near the VHS point, where the NFL behavior has been observed in the earlier work. This observation is incompatible with either a numerical calculation within a constant relaxation-time approximation or a toy-model calculation for the 2D Lifshitz transition adopting an elastic carrier scattering. As a promising origin of the observed negatively enhanced thermopower, we propose a skewed NFL state, in which an inelastic scattering with a considerable odd-frequency term plays a crucial role to negatively enhance the thermopower.

cond-mat.str-el↗

arXiv2024

Challenges in extracting nonlinear current-induced phenomena in Ca2RuO4

An appealing direction to change the properties of strongly correlated materials is to induce nonequilibrium steady states by the application of a direct current. While access to these novel states is of high scientific interest, Joule heating due to current flow often constitutes a hurdle to identify nonthermal effects. The biggest challenge usually resides in measuring accurately the temperature of a sample subjected to direct current, and to use probes that give direct information of the material. In this work, we exploit the simultaneous measurement of electrical transport and magnetisation to probe non-equilibrium steady states in Ca2RuO4. In order to reveal non-thermal current-induced effects, we employ a simple model of Joule self-heating to remove the effects of heating and discuss the importance of temperature inhomogeneity within the sample. Our approach provides a solid basis for investigating current-induced phenomena in highly resistive materials.

cond-mat.str-el↗

arXiv2024

Unveiling unconventional magnetism at the surface of Sr$_2$RuO$_4$

Materials with strongly correlated electrons exhibit physical properties that are often difficult to predict as they result from the interactions of large numbers of electrons combined with several quantum degrees of freedom. The layered oxide perovskite Sr$_2$RuO$_4$ is a strongly correlated electron material that has been intensively investigated since its discovery due to its unusual physical properties. Whilst recent experiments have reopened the debate on the exact symmetry of the superconducting state in Sr$_2$RuO$_4$, a deeper understanding of the Sr$_2$RuO$_4$ normal state appears crucial as this is the background in which electron pairing occurs. Here, by using low-energy muon spin spectroscopy we discover the existence of magnetism at the surface of Sr$_2$RuO$_4$ in its normal state. We detect static weak dipolar fields yet manifesting below a relatively high onset temperature larger than 50 K, which reveals the unconventional nature of the observed magnetism. We relate the origin of this phase breaking time reversal symmetry to electronic ordering in the form of orbital loop currents that originate at the reconstructed Sr$_2$RuO$_4$ surface. Our observations set a reference for the discovery of the same magnetic phase in other materials and unveil an electronic ordering mechanism that can influence unconventional electron pairing with broken time reversal symmetry in those materials where the observed magnetic phase coexists with superconductivity.

cond-mat.str-el↗

arXiv2024

Still mystery after all these years -- Unconventional superconductivity of Sr2RuO4 --

This review describes recent significant research developments made on the layered perovskite Sr2RuO4 and discusses current issues from both experimental and theoretical perspectives. Since the discovery of superconductivity in Sr2RuO4 in 1994, studies using high-quality single crystals quickly revealed it to be an archetypal unconventional superconductor among strongly correlated electron systems. In particular, it was thought that the spin-triplet chiral p-wave superconducting state, which breaks time-reversal symmetry, was a prominent possibility. In 2019, however, a new development overturned the past experimental results, and spin-singlet-like behavior became conclusive. Furthermore, innovation in uniaxial strain devices has stimulated researchers to explore changes in the superconducting state by controlling the symmetry and dimensionality of the Fermi surfaces and enhancing the superconducting transition temperature Tc from 1.5 K to 3.5 K. A spin-singlet chiral d-wave superconducting state is consistent with most of these recent experimental results. Nevertheless, there are still unnatural aspects that remain to be explained. The focus of this review is on unraveling this mystery. Unlike many other unconventional superconductors, the normal state of Sr2RuO4 exhibits typical Fermi liquid behavior. Nevertheless, to elucidate its superconducting state, it may be essential to go beyond the traditional framework of unconventional superconductivity and recast the theory explicitly considering the multi-orbital aspects of its electronic states. In this review, we describe the frontiers of superconductivity research in Sr2RuO4 and discuss how the remaining issues may be resolved.

cond-mat.supr-con↗

arXiv2023

Superconducting Penetration Depth Through a Van Hove Singularity: Sr$_2$RuO$_4$ Under Uniaxial Stress

In the unconventional superconductor Sr$_2$RuO$_4$, uniaxial stress along the $[100]$ direction tunes the Fermi level through a Van Hove singularity (VHS) in the density of states, causing a strong enhancement of the superconducting critical temperature $T_\textrm{c}$. Here, we report measurements of the London penetration depth $λ$ as this tuning is performed. We find that the zero-temperature superfluid density, here defined as $λ(0)^{-2}$, increases by $\sim$15%, with a peak that coincides with the peak in $T_\textrm{c}$. We also find that the low temperature form of $λ(T)$ is quadratic over the entire strain range. Using scanning tunneling microscopy, we find that the gap increases from $Δ_0 \approx 350~μ$eV in unstressed Sr$_2$RuO$_4$ to $Δ_0 \approx 600~μ$eV in a sample strained to near the peak in $T_c$. With a nodal order parameter, an increase in the superconducting gap could bring about an increase in the superfluid density through reduced sensitivity to defects and through reduced non-local effects in the Meissner screening. Our data indicate that tuning to the VHS increases the gap throughout the Brillouin zone, and that non-local effects are likely more important than reduced scattering.

cond-mat.supr-con↗

arXiv2023

Anisotropic Seebeck coefficient of $\mathrm{Sr}_2\mathrm{Ru}\mathrm{O}_4$ in the incoherent regime

Intuitive entropic interpretations of the thermoelectric effect in metals predict an isotropic Seebeck coefficient at high temperatures in the incoherent regime even in anisotropic metals since entropy is not directional. $\mathrm{Sr}_2\mathrm{Ru}\mathrm{O}_4$ is an enigmatic material known for a well characterised anisotropic normal state and unconventional superconductivity. Recent ab-initio transport calculations of $\mathrm{Sr}_2\mathrm{Ru}\mathrm{O}_4$ that include the effect of strong electronic correlations predicted an enhanced high-temperature anisotropy of the Seebeck coefficient at temperatures above 300 K, but experimental evidence is missing. From measurements on clean $\mathrm{Sr}_2\mathrm{Ru}\mathrm{O}_4$ single crystals along both crystallographic directions, we find that the Seebeck coefficient becomes increasingly isotropic upon heating towards room temperature as generally expected. Above 300 K, however, $S$ acquires a new anisotropy which rises up to the highest temperatures measured (750 K), in qualitative agreement with calculations. This is a challenge to entropic interpretations and highlights the lack of an intuitive framework to understand the anisotropy of thermopower at high temperatures.

cond-mat.str-el↗

arXiv2023

Constraints on a split superconducting transition under uniaxial strain in Sr$_2$RuO$_4$ from scanning SQUID microscopy

More than two decades after the discovery of superconductivity in Sr$_2$RuO$_4$, it is still unclear whether the order parameter has a single component or two degenerate components. For any two-component scenario, application of uniaxial strain is expected to lift the degeneracy, generating two distinct phase transitions. The presence of a second (lower-temperature) transition may be observable by probes that are sensitive to changes in the London penetration depth, $λ$, as a function of temperature, $T$. Here, we use scanning SQUID microscopy combined with a uniaxial strain device to test for a second transition under strain. We only observe a single transition. Within the temperature range where a second transition has been suggested by $μ$SR measurements, we further place a tight upper bound of less than 1% on the change in the zero temperature superfluid density $n_s\proptoλ^{-2}(0)$ due to a second transition, suggesting that such a transition does not occur. These results constrain theories of the order parameter in Sr$_2$RuO$_4$.

cond-mat.supr-con↗