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Unambiguous identification of the superconducting order parameter symmetry of Sr$_2$RuO$_4$ has remained elusive for more than a quarter century. While a chiral $p$-wave ground state analogue to superfluid $^3$He-$A$ was ruled out only very recently, other proposed $p$-wave scenarios are still viable. Here, field-dependent $^{17}$O Knight shift measurements are compared to corresponding specific heat measurements, previously reported. We conclude that the shift results can be accounted for by the expected field-induced quasiparticle response only. An upper bound for the condensate magnetic response of $, Main Article: 4 pages 3 figures Supplement: 6 pages 4 figures

There is considerable evidence that the superconducting state of Sr2RuO4 breaks time reversal symmetry. In the experiments showing time reversal symmetry breaking, its onset temperature, TTRSB, is generally found to match the critical temperature, Tc, within resolution. In combination with evidence for even parity, this result has led to consideration of a dxz ± idyz order parameter. The degeneracy of the two components of this order parameter is protected by symmetry, yielding TTRSB = Tc, but it has a hard-to-explain horizontal line node at kz = 0. Therefore, s ± id and d ± ig order parameters are also under consideration. These avoid the horizontal line node, but require tuning to obtain TTRSB ≈ Tc. To obtain evidence distinguishing these two possible scenarios (of symmetry-protected versus accidental degeneracy), we employ zero-field muon spin rotation/relaxation to study pure Sr2RuO4 under hydrostatic pressure, and Sr1.98La0.02RuO4 at zero pressure. Both hydrostatic pressure and La substitution alter Tc without lifting the tetragonal lattice symmetry, so if the degeneracy is symmetry-protected, TTRSB should track changes in Tc, while if it is accidental, these transition temperatures should generally separate. We observe TTRSB to track Tc, supporting the hypothesis of dxz ± idyz order., Nature Communications, 12 (1), ISSN:2041-1723

Funding: This work is supported by a KAKENHI Grants-in-Aids for Scientific Research (Grant Nos.17H06136, 18K04715, and 21H01033) from the Japan Society for the Promotion of Science (JSPS) and by a JST-Mirai Program (Grant No. JPMJMI18A3). We report the single-crystal growth of the unconventional superconductor Sr2RuO4, on which research has reached a turning point recently. In order to optimize the quality of crystals grown by the floating-zone method using an infrared image furnace, we focus on an improvement of the structure of the filament in the halogen lamps. By reducing the thickness of the total filament, the form of the molten zone was narrowed. More importantly, the molten zone was observed to be more stable during the growth process. Finally, we obtained the crystals with a length of 12 cm. Additionally, the grown crystal has high quality, displaying the 1.5 K transition temperature expected only for the purest crystals. We also discuss the availability of the newly developed halogen lamps. Publisher PDF

This work has been financially supported by the Deutsche Forschungsgemeinschaft (GR 4667/1, GRK 1621 and SFB 1143 projects C02 and C09) and the Max Planck Society. Y.M., T.M. and J.S.B. acknowledge the financial support of JSPS Kakenhi (JP15H5852, JP15K21717 and JP17H06136) and the JSPS Core-to-Core Program. N.K. acknowledges the financial support from JSPS Kakenhi (no. JP18K04715) and JST-Mirai Program (no. JPMJMI18A3). A.N. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 701647. Strontium ruthenate (Sr2RuO4) continues to present an important test of our understanding of unconventional superconductivity, because while its normal-state electronic structure is known with precision, its superconductivity remains unexplained. There is evidence that its order parameter is chiral, but reconciling this with recent observations of the spin part of the pairing requires an order parameter that is either finely tuned or implies a new form of pairing. Therefore, a definitive resolution of whether the superconductivity of Sr2RuO4 is chiral is important for the study of superconductivity. Here we report the measurement of zero-field muon spin relaxation—a probe sensitive to weak magnetism—on samples under uniaxial stresses. We observe stress-induced splitting between the onset temperatures of superconductivity and time-reversal symmetry breaking—consistent with the qualitative expectations for a chiral order parameter—and argue that this observation cannot be explained by conventional magnetism. In addition, we report the appearance of bulk magnetic order under higher uniaxial stress, above the critical pressure at which a Lifshitz transition occurs in Sr2RuO4. Postprint

We report resistance and elastoresistance measurements on $({\mathrm{Ba}}_{0.5}{\mathrm{K}}_{0.5}){\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$, ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$, and ${\mathrm{KCa}}_{2}{\mathrm{Fe}}_{4}{\mathrm{As}}_{4}{\mathrm{F}}_{2}$. The Fe-site symmetry is ${D}_{2d}$ in the first compound but ${C}_{2v}$ in the latter two, which lifts the degeneracy of the Fe ${d}_{xz}$ and ${d}_{yz}$ orbitals. The temperature dependence of the resistance and elastoresistance is similar between the three compounds. Especially, the [110] elastoresistance is enhanced with decreasing temperature irrespective of the Fe-site symmetry. This appears to be in conflict with recent Raman scattering studies on ${\mathrm{CaKFe}}_{4}{\mathrm{As}}_{4}$, which suggest the absence of nematic fluctuations. We consider possible ways of reconciliation and suggest that the present result is important in elucidating the origin of in-plane resistivity anisotropy in iron-based superconductors.

The cubic Kondo lattice ${\mathrm{YbCo}}_{2}{\mathrm{Zn}}_{20}$ is one of the heaviest known Fermi liquids. We have measured the low-temperature electrical resistivity $\ensuremath{\rho}(T)$, magnetic susceptibility $\ensuremath{\chi}(T)$, and heat capacity $C(T)$ in single crystals of $\mathrm{Yb}{({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2}{\mathrm{Zn}}_{20}$ ($x\ensuremath{\le}0.126$) and $\mathrm{Yb}{({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{y})}_{2}{\mathrm{Zn}}_{20}$ ($y\ensuremath{\le}0.07$). While pure ${\mathrm{YbCo}}_{2}{\mathrm{Zn}}_{20}$ displays maxima in $\ensuremath{\rho}(T)$ and $\ensuremath{\chi}(T)$, ascribed to an enhanced crystal electric field (CEF) degeneracy, the maxima are suppressed by small amounts ($\ensuremath{\approx}6$ at.%) of substitutions of the Co atoms. This goes along with a suppression of superheavy Fermi liquid behavior as manifested in the divergence of $C/T$. We ascribe the observations to local distortions at the Yb site due to the chemical disorder in the environment, rather than to a chemical pressure effect. This fragileness to the local distortion indicates that superheavy Fermion behavior in ${\mathrm{YbCo}}_{2}{\mathrm{Zn}}_{20}$ is closely linked to an enhanced CEF degeneracy.

Polar distortions in solids give rise to the well-known functionality of switchable macroscopic polarisation in ferroelectrics and, when combined with strong spin-orbit coupling, can mediate giant spin splittings of electronic states. While typically found in insulators, ferroelectric-like distortions can remain robust against increasing itineracy, giving rise to so-called "polar metals". Here, we investigate the temperature-dependent electronic structure of Ca$_3$Ru$_2$O$_7$, a correlated oxide metal in which octahedral tilts and rotations combine to mediate pronounced polar distortions. Our angle-resolved photoemission measurements reveal the destruction of a large hole-like Fermi surface upon cooling through a coupled structural and spin-reorientation transition at 48 K, accompanied by a sudden onset of quasiparticle coherence. We demonstrate how these result from band hybridisation mediated by a hidden Rashba-type spin-orbit coupling. This is enabled by the bulk structural distortions and unlocked when the spin reorients perpendicular to the local symmetry-breaking potential at the Ru sites. We argue that the electronic energy gain associated with the band hybridisation is actually the key driver for the phase transition, reflecting a delicate interplay between spin-orbit coupling and strong electronic correlations, and revealing a new route to control magnetic ordering in solids., Comment: Contains 6+5 pages, including supplementary information

Sr2RuO4 has stood as the leading candidate for a spin-triplet superconductor for 26 years1. However, recent NMR experiments have cast doubt on this candidacy2,3 and it is difficult to find a theory of superconductivity that is consistent with all experiments. The order parameter symmetry for this material therefore remains an open question. Symmetry-based experiments are needed that can rule out broad classes of possible superconducting order parameters. Here, we use resonant ultrasound spectroscopy to measure the entire symmetry-resolved elastic tensor of Sr2RuO4 through the superconducting transition. We observe a thermodynamic discontinuity in the shear elastic modulus c66, which implies that the superconducting order parameter has two components. A two-component p-wave order parameter, such as px + ipy, naturally satisfies this requirement. As this order parameter appears to have been precluded by recent NMR experiments, we suggest that two other two-component order parameters, namely $$\{{d}_{xz},{d}_{yz}\}$$ and $$\{{d}_{{x}^{2}-{y}^{2}},{g}_{xy({x}^{2}-{y}^{2})}\}$$ , are now the prime candidates for the order parameter of Sr2RuO4. Ultrasound measurements show that the superconducting order parameter in strontium ruthenate must have two components.

The authors would like to acknowledge the financial support from the Max-Planck Society. E.H. and M.N. acknowledge support from Deutsche Forschungsgemeinschaft (DFG) through the Project No. 107745057 (TRR80: From Electronic Correlations to Functionality). This work is also supported by JSPS KAKENHI (No. 18K04715). A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation Cooperative Agreements No. DMR-1157490 and No. DMR-1644779 and the State of Florida. The delafossite series of layered oxides includes some of the highest conductivity metals ever discovered. Of these, PtCoO2, with a room-temperature resistivity of 1.8 μΩcm for in-plane transport, is the most conducting of all. The high conduction takes place in triangular lattice Pt layers, separated by layers of Co-O octahedra, and the electronic structure is determined by the interplay of the two types of layers. We present a detailed study of quantum oscillations in PtCoO2, at temperatures down to 35 mK and magnetic fields up to 30 T. As for PdCoO2 and PdRhO2, the Fermi surface consists of a single cylinder with mainly Pt character and an effective mass close to the free-electron value. Due to Fermi-surface warping, two close-lying high frequencies are observed. Additionally, a pronounced difference frequency appears. By analyzing the detailed angular dependence of the quantum-oscillation frequencies, we establish the warping parameters of the Fermi surface. We compare these results to the predictions of first-principles electronic-structure calculations including spin-orbit coupling on Pt and Co and on-site correlation U on Co, and hence demonstrate that electronic correlations in the Co-O layers play an important role in determining characteristic features of the electronic structure of PtCoO2. Publisher PDF

Phases of matter are usually identified through spontaneous symmetry breaking, especially regarding unconventional superconductivity and the interactions from which it originates. In that context, the superconducting state of the quasi-two-dimensional and strongly correlated perovskite Sr2RuO4 is considered to be the only solid-state analogue to the superfluid 3He-A phase1,2, with an odd-parity order parameter that is unidirectional in spin space for all electron momenta and breaks time-reversal symmetry. This characterization was recently called into question by a search for an expected ‘split’ transition in a Sr2RuO4 crystal under in-plane uniaxial pressure, which failed to find any such evidence; instead, a dramatic rise and a peak in a single-transition temperature were observed3,4. Here we use nuclear magnetic resonance (NMR) spectroscopy of oxygen-17, which is directly sensitive to the order parameter via hyperfine coupling to the electronic spin degrees of freedom, to probe the nature of superconductivity in Sr2RuO4 and its evolution under strain. A reduction of the Knight shift is observed for all strain values and at temperatures below the critical temperature, consistent with a drop in spin polarization in the superconducting state. In unstrained samples, our results contradict a body of previous NMR work reporting no change in the Knight shift5 and the most prevalent theoretical interpretation of the order parameter as a chiral p-wave state. Sr2RuO4 is an extremely clean layered perovskite and its superconductivity emerges from a strongly correlated Fermi liquid, and our work imposes tight constraints on the order parameter symmetry of this archetypal system. Postprint

This work was supported in part by the Laboratory Directed Research and Development (LDRD) program of Los Alamos National Laboratory under Project No. 20170204ER. Y. L. acknowledges partial support through the LDRD and 1000 Youth Talents Plan of China. N. K. acknowledges the support from JSPS KAKNHI (Grant No. 18K04715). I. I.M. is supported by ONR through the NRL basic research program. This work is supported in part by the National Science Foundation (Grants No. DMR-1410343 and No. DMR-1709304).

Nonequilibrium steady state conditions induced by a dc current can alter the physical properties of strongly correlated electron systems. In this regard, it was recently shown that dc current can trigger novel electronic states, such as current-induced diamagnetism, which cannot be realized in equilibrium conditions. However, reversible control of diamagnetism has not been achieved yet. Here, we demonstrate reversible in situ control between a Mott insulating state and a diamagnetic semimetal-like state by a dc current in the Ti-substituted bilayer ruthenate ${\mathrm{Ca}}_{3}({\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x}{)}_{2}{\mathrm{O}}_{7}$ ($x=0.5%$). By performing simultaneous magnetic and resistive measurements, we map out the temperature vs current-density phase diagram in the nonequilibrium steady state of this material. The present results open up the possibility of creating novel electronic states in a variety of strongly correlated electron systems under dc current.

We perform de Haas-van Alphen measurements and quasiparticle self-consistent \textit{GW} (QS\textit{GW}) calculations on FeS. The calculated Fermi surface (FS) consists of two hole and two electron cylinders. We observe all the eight predicted FS cross sections experimentally. With momentum-independent band-energy adjustments of less than 0.1 eV, the maximum deviation between the calculated and observed cross sections is less than 0.2\% of the Brillouin zone area for $B \parallel c$. The carrier density is $\sim$0.5 carriers/Fe. The mass enhancements are nearly uniform across the FS cylinders and moderate, $\sim$2. The absence of a third hole cylinder with $d_{xy}$ character is favorable for the formation of a nodal superconducting gap., 7 pages, 3 figures

Pressure represents a clean tuning parameter for traversing the complex phase diagrams of interacting electron systems, and as such has proved of key importance in the study of quantum materials. Application of controlled uniaxial pressure has recently been shown to more than double the transition temperature of the unconventional superconductor Sr2RuO4, leading to a pronounced peak in Tc versus strain whose origin is still under active debate. Here we develop a simple and compact method to passively apply large uniaxial pressures in restricted sample environments, and utilise this to study the evolution of the electronic structure of Sr2RuO4 using angle-resolved photoemission. We directly visualise how uniaxial stress drives a Lifshitz transition of the γ-band Fermi surface, pointing to the key role of strain-tuning its associated van Hove singularity to the Fermi level in mediating the peak in Tc. Our measurements provide stringent constraints for theoretical models of the strain-tuned electronic structure evolution of Sr2RuO4. More generally, our experimental approach opens the door to future studies of strain-tuned phase transitions not only using photoemission but also other experimental techniques where large pressure cells or piezoelectric-based devices may be difficult to implement.

We report magnetic torque measurements on iron-pnictide superconductors ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($x\ensuremath{\approx}$ 0.69 and 0.76) to an applied field of ${B}_{a}=45$ T. The peak effect is observed in torque-vs-field curves below the irreversibility field. It is enhanced and becomes asymmetric as the field is tilted from the $c$ axis. For field directions close to the $ab$ plane, increasing- and decreasing-field curves peak at markedly different fields and exhibit a sharp jump, suggestive of a first-order phase transition, on the high- and the low-field sides of the peak, respectively. Complicated history dependence of the torque is observed in the peak-effect region. We construct and discuss the temperature-- ($T$) applied-magnetic-field (${B}_{a}$) phase diagram. Since the upper critical field for the $ab$-plane direction is comparable to the Pauli limit, we also consider possible influence of the spin paramagnetic effect on the anomalous peak effect.

High-quality single crystals are essentially needed for the investigation of the novel bulk properties of unconventional superconductors. The availability of such crystals grown by the floating-zone method has helped to unveil the unconventional superconductivity of the layered perovskite Sr$_2$RuO$_4$, which is considered as a strong candidate of a topological spin-triplet superconductor. Yet, recent progress of investigations urges further efforts to obtain ultimately high-quality crystalline samples. In this paper, we focus on the method of preparation of feed rods for the floating-zone melting and report on the improvements of the crystal growth. We present details of the improved methods used to obtain crystals with superconducting transition temperatures $T_\mathrm{c}$ that are consistently as high as 1.4 K, as well as the properties of these crystals., 9 pages, 6 figures

The Ce atoms in CeIrSn form a quasikagome lattice in the hexagonal $c$ plane. Our single-crystal x-ray diffraction analysis has confirmed the absence of superstructure in the ZrNiAl-type structure. Motivated by the recent observation of quantum critical behaviors in the isostructural and isoelectronic compound CeRhSn, we have studied low-temperature properties of single-crystalline samples of CeIrSn by measuring the electrical resistivity, magnetic susceptibility $\ensuremath{\chi}$, magnetization $M$, and specific heat $C$ down to 0.05 K. Our study does not show evidence of a long-range magnetic ordering down to the lowest temperature. The $\ensuremath{\chi}(T)$ data show an Ising character, ${\ensuremath{\chi}}_{c}\ensuremath{\gg}{\ensuremath{\chi}}_{a}$, and power-law behaviors at low temperatures: ${\ensuremath{\chi}}_{c}\ensuremath{\propto}{T}^{\ensuremath{-}0.8}$ and ${\ensuremath{\chi}}_{a}\ensuremath{\propto}{T}^{\ensuremath{-}0.6}$. When external field $B$ is applied along the $a$ axis, both $M(B)$ and $C(B)/T$ exhibit metamagnetic anomalies at 5.5 T, which resemble those observed in CeRhSn at 3.5 T. We attribute the metamagnetic crossover under in-plane magnetic fields to the alleviation of magnetic frustration inherent in the quasikagome Ising Kondo lattice.

Resistance and magnetic torque measurements are reported in a layered organic superconductor, $\ensuremath{\beta}''\ensuremath{-}{(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF})}_{4}[({\mathrm{H}}_{3}\mathrm{O})\mathrm{Ga}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{3}]{\mathrm{C}}_{6}{\mathrm{H}}_{5}{\mathrm{NO}}_{2}$ with ${T}_{c}=4.8$ K, where BEDT-TTF stands for bis(ethylenedithio)tetrathiafulvalene. Because of the large anion between the BEDT-TTF conducting layers, the superconductivity of this salt is highly anisotropic. In magnetic fields parallel to the conducting layers for $T=0.4$ K, the magnetic torque shows a large diamagnetic signal associated with hysteresis up to $\ensuremath{\sim}21$ T, suggesting the upper critical field ${H}_{c2}\ensuremath{\gtrsim}21$ T at 0.4 K. The large reduction of the diamagnetic signal is observed above 16 T, which shows a Fulde and Ferrell and Larkin and Ovchinnikov (FFLO) phase transition. For $T=0.5$ K, the interlayer resistance has nonzero value in a wide field region up to ${H}_{c2}$, arising from the Josephson vortex dynamics. Successive dips in the second derivative curves of the resistance are observed between 16 T and ${H}_{c2}$, which are ascribed to the commensurability effect between the Josephson vortex lattice and the order parameter oscillation of the FFLO phase. The commensurability effect is observed only in nearly parallel fields, showing that the FFLO phase is stable in a very limited field angle region. The temperature-field phase diagram is determined.

The strongly correlated insulator Ca2RuO4 is considered as a paradigmatic realization of both spin-orbital physics and a band-Mott insulating phase, characterized by orbitally selective coexistence of a band and a Mott gap. We present a high resolution oxygen K-edge resonant inelastic x-ray scattering study of the antiferromagnetic Mott insulating state of Ca2RuO4. A set of low-energy (about 80 and 400 meV) and high-energy (about 1.3 and 2.2 eV) excitations are reported, which show strong incident light polarization dependence. Our results strongly support a spin-orbit coupled band-Mott scenario and explore in detail the nature of its exotic excitations. Guided by theoretical modeling, we interpret the low-energy excitations as a result of composite spin-orbital excitations. Their nature unveils the intricate interplay of crystal-field splitting and spin-orbit coupling in the band-Mott scenario. The high-energy excitations correspond to intra-atomic singlet-triplet transitions at an energy scale set by Hund’s coupling. Our findings give a unifying picture of the spin and orbital excitations in the band-Mott insulator Ca2RuO4.

CeRhSn with a quasikagome lattice of Ce atoms in the hexagonal $c$ plane has been expected to be in close vicinity to a zero-field quantum criticality derived from magnetic frustration. We have studied how the ground state changes with substitution of Pd for Rh in $\mathrm{CeR}{\mathrm{h}}_{1\ensuremath{-}x}\mathrm{P}{\mathrm{d}}_{x}\mathrm{Sn}$ $(x\ensuremath{\le}0.75)$ by measuring the specific heat $C$, magnetic susceptibilities ${\ensuremath{\chi}}_{\mathrm{dc}}$ and ${\ensuremath{\chi}}_{\mathrm{ac}}$, magnetization $M$, electrical resistivity \ensuremath{\rho}, and magnetoresistance. For $x=0$, the field dependence of ${\ensuremath{\chi}}_{\mathrm{ac}}$ at $T=0.03\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ shows a peak at $B\ensuremath{\parallel}a=3.5\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, confirming the spin-flop crossover in the field applied along the hard axis. The temperature dependence of ${\ensuremath{\chi}}_{\mathrm{ac}}$ shows a broad maximum at 0.1 K whereas $C/T$ continues to increase down to 0.08 K. For $x\ensuremath{\geqq}0.1,\ensuremath{\rho}(T)$ is dominated by incoherent Kondo scattering and both $C/T$ and ${\ensuremath{\chi}}_{\mathrm{ac}}(T)$ exhibit peaks, indicating the development of an antiferromagnetic order. The ordering temperature rises to 2.5 K as $x$ is increased to 0.75. Our results indicate that the ground state in the quasikagome Kondo lattice $\mathrm{CeR}{\mathrm{h}}_{1\ensuremath{-}x}\mathrm{P}{\mathrm{d}}_{x}\mathrm{Sn}$ leaves the quantum critical point at $x=0$ with increasing $x$ as a consequence of suppression of both the magnetic frustration and Kondo effect.

Magnetic torque measurements using a microcantilever have been performed to investigate the superconducting phase of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ down to 40 mK. For high-quality single crystals with the transition temperature $({T}_{\mathrm{c}})$ of 1.48--1.49 K, an abrupt jump of the torque signal is found near 1.5 T in field parallel to the conducting ${\mathrm{RuO}}_{2}$ planes below $\ensuremath{\sim}0.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The jump corresponds to the first order transition recently revealed by magnetocaloric and magnetization measurements [Yonezawa et al., Phys. Rev. Lett. 110, 077003 (2013); Kittaka et al., Phys. Rev. B 90, 220502(R) (2014)]. Furthermore, weak diamagnetic and irreversible signals are found to persist above the first order transition up to 1.85 T. The result indicates the presence of a subphase boundary separating low- and high-field phases in the superconducting phase. The high-field subphase disappears when the field is tilted from the conducting planes only by a few degrees. Quantum oscillation measurements are also reported to clarify the strong sample-quality dependence of the high-field subphase.

The resistivity $\rho$ and Hall resistivity $\rho_H$ are measured on FeSe at pressures up to $P$ = 28.3 kbar in magnetic fields up to $B$ = 14.5 T. The $\rho(B)$ and $\rho_H(B)$ curves are analyzed with multicarrier models to estimate the carrier density and mobility as a function of $P$ and temperature ($ T\leqslant$ 110 K). It is shown that the pressure-induced antiferromagnetic transition is accompanied by an abrupt reduction of the carrier density and scattering. This indicates that the electronic structure is reconstructed significantly by the antiferromagnetic order., Comment: 6 pages, 5 figures + SM ( 2 pages, 2 figures)

The gap structure of Sr$_2$RuO$_4$, which is a longstanding candidate for a chiral p-wave superconductor, has been investigated from the perspective of the dependence of its specific heat on magnetic field angles at temperatures as low as 0.06 K ($\sim 0.04T_{\rm c}$). Except near $H_{\rm c2}$, its fourfold specific-heat oscillation under an in-plane rotating magnetic field is unlikely to change its sign down to the lowest temperature of 0.06 K. This feature is qualitatively different from nodal quasiparticle excitations of a quasi-two-dimensional superconductor possessing vertical lines of gap minima. The overall specific-heat behavior of Sr$_2$RuO$_4$ can be explained by Doppler-shifted quasiparticles around horizontal line nodes on the Fermi surface, whose in-plane Fermi velocity is highly anisotropic, along with the occurrence of the Pauli-paramagnetic effect. These findings, in particular, the presence of horizontal line nodes in the gap, call for a reconsideration of the order parameter of Sr$_2$RuO$_4$., Comment: 5 pages, 3 figures (main text) + 3 pages, 3 figures (supplemental material), accepted for publication in J. Phys. Soc. Jpn

The magnetic field-induced changes in the conductivity of metals are the subject of intense interest, both for revealing new phenomena and as a valuable tool for determining their Fermi surface. Here, we report a hitherto unobserved magnetoresistive effect in ultra-clean layered metals, namely a negative longitudinal magnetoresistance that is capable of overcoming their very pronounced orbital one. This effect is correlated with the inter-layer coupling disappearing for fields applied along the so-called Yamaji angles where the inter-layer coupling vanishes. Therefore, it is intrinsically associated with the Fermi points in the field-induced quasi-one-dimensional electronic dispersion, implying that it results from the axial anomaly among these Fermi points. In its original formulation, the anomaly is predicted to violate separate number conservation laws for left- and right-handed chiral- (e.g. Weyl) fermions. Its observation in PdCoO$_2$, PtCoO$_2$ and Sr$_2$RuO$_4$ suggests that the anomaly affects the transport of clean conductors, particularly near the quantum limit., Comment: Nature Communications (in press)

We report Shubnikov-de Haas (SdH) oscillation measurements on FeSe under high pressure up to $P$ = 16.1 kbar. We find a sudden change in SdH oscillations at the onset of the pressure-induced antiferromagnetism at $P$ $\sim$ 8 kbar. We argue that this change can be attributed to a reconstruction of the Fermi surface by the antiferromagnetic order. The negative d$T_c$/d$P$ observed in a range between $P$ $\sim$ 8 and 12 kbar may be explained by the reduction in the density of states due to the reconstruction. The ratio of the transition temperature to the effective Fermi energy remains high under high pressure: $k_BT_c/E_F$ $\sim$ 0.1 even at $P$ = 16.1 kbar., 12 pages , 10 figures

Hard X-ray photoemission studies are carried out to evaluate the electronic structure of iron-based superconductors BaFe2(As1−xPx)2 in a wide range of the P content 0 ≤ x ≤ 0.61. As x increases, th...

We have observed Shubnikov-de Haas oscillations in FeSe. The Fermi surface deviates significantly from predictions of band-structure calculations and most likely consists of one electron and one hole thin cylinder. The carrier density is in the order of 0.01 carriers/ Fe, an order-of-magnitude smaller than predicted. Effective Fermi energies as small as 3.6 meV are estimated. These findings call for elaborate theoretical investigations incorporating both electronic correlations and orbital ordering., Comment: 6 pages, 6 figures, 1 table, published in Phys. Rev. B

We show that the metamagnetic transition in ${\mathrm{Sr}}_{4}{\mathrm{Ru}}_{3}{\mathrm{O}}_{10}$ bifurcates into two transitions as the field is rotated away from the conducting planes. This two-step process comprises partial or total alignment of moments in ferromagnetic bands followed by an itinerant metamagnetic transition whose critical field increases with rotation. Evidence for itinerant metamagnetism is provided by the Shubnikov--de Haas effect, which shows a nontrivial evolution of the geometry of the Fermi surface and an enhancement of the quasiparticles effective mass across the transition. We conclude that the metamagnetic response of ${\mathrm{Sr}}_{4}{\mathrm{Ru}}_{3}{\mathrm{O}}_{10}$ is orbital dependent and involves both ferromagnetic and metamagnetic bands.