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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.

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

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.

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.

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.

High-resolution resonant inelastic x-ray scattering (RIXS) at the oxygen K edge has been used to study the orbital excitations of Ca2RuO4 and Sr2RuO4. In combination with linear dichroism x-ray absorption spectroscopy, the ruthenium 4d-orbital occupation and excitations were probed through their hybridization with the oxygen p orbitals. These results are described within a minimal model, taking into account crystal field splitting and a spin-orbit coupling λso=200 meV. The effects of spin-orbit interaction on the electronic structure and implications for the Mott and superconducting ground states of (Ca,Sr)2RuO4 are discussed.

We report measurements of ac magnetic susceptibility ${\ensuremath{\chi}}_{\text{ac}}$ and de Haas--van Alphen (dHvA) oscillations in ${\mathrm{KFe}}_{2}$${\mathrm{As}}_{2}$ under high pressure up to 24.7 kbar. The pressure dependence of the superconducting transition temperature ${T}_{c}$ changes from negative to positive across ${P}_{c}\ensuremath{\sim}18$ kbar, as previously reported. The ratio of the upper critical field to ${T}_{c}$, i.e., ${B}_{c2}/{T}_{c}$, is enhanced above ${P}_{c}$, and the shape of the ${\ensuremath{\chi}}_{\text{ac}}$ versus field curves qualitatively changes across ${P}_{c}$. dHvA oscillations smoothly evolve across ${P}_{c}$, indicating no drastic change in the Fermi surface up to 24.7 kbar. Three dimensionality increases with pressure, while effective masses show decreasing trends. We suggest a crossover from a nodal to a full-gap $s$ wave as a possible explanation.

Hard x-ray photoemission studies of Ca${}_{3}$(Ru${}_{1\ensuremath{-}x}$Ti${}_{x}$)${}_{2}$O${}_{7}$ with $x$ $=$ 0, 0.003, and 0.005 have been carried out to clarify the origin of the extremely high sensitivity of the metal-insulator (M-I) transition to the amount of Ti substitution. All the compounds show an antiferromagnetic transition followed by a structural transition at a temperature ${T}_{\mathrm{S}}$, with decreasing temperature. For $x$ g 0, the structural transition is accompanied with a drastic M-I transition and the intensities of the screened peaks in the Ru 3$d$ core-level spectra are significantly reduced below ${T}_{\mathrm{S}}$. The spectral weight of the valence band is concurrently shifted toward the higher binding energy side by an order of 2 eV, which shows that a strong electron correlation plays an essential role for the M-I transition at ${T}_{\mathrm{S}}$ for $x$ g 0. These results provide strong evidence that Mott transition is induced by a minute amount of the Ti substitution in Ca${}_{3}$Ru${}_{2}$O${}_{7}$.

We have observed hysteresis in superconducting resistive transition curves of Ba${}_{0.07}$K${}_{0.93}$Fe${}_{2}$As${}_{2}$ (${T}_{c}\ensuremath{\sim}8$ K) below about 1 K for in-plane fields. The hysteresis is not observed as the field is tilted away from the $ab$ plane by 20${}^{\ensuremath{\circ}}$ or more. The temperature and angle dependencies of the upper critical field indicate a strong paramagnetic effect for in-plane fields. We suggest that the hysteresis can be attributed to a first-order superconducting transition due to the paramagnetic effect. Magnetic torque data are also shown.

We study the effects of adding electrons to the conduction bands of the itinerant metamagnet Sr3 Ru2 O7 using substitution of La3+ onto the Sr2+ site. Small changes to the chemical potential have a large effect: adding only 0.03 electrons per Ru reduces the electronic specific-heat coefficient by 30%, but makes no corresponding change to the temperature (∼8 K) at which the electronic specific-heat coefficient shows its maximum. The observations are incompatible with a simple rigid-band shift, raising the possibility that a many-body resonance plays a key role in the physics of Sr3 Ru2 O7. © 2008 The American Physical Society.

We report x-ray scattering studies of the $c$-axis lattice parameter in ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ as a function of temperature and magnetic field. These structural studies complement published transport and magnetization data, and therefore elucidate the spin-charge-lattice coupling near the metal-insulator transition. Strong anisotropy of the structural change for field applied along orthogonal in-plane directions is observed. Competition between a spin-polarized phase that does not couple to the lattice and an antiferromagnetic metallic phase that does gives rise to a rich behavior for $\mathbf{B}\ensuremath{\Vert}b$.

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.

We report the results of a study of the Hall effect and magnetoresistance in single crystals of ${\mathrm{Sr}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$ in which ${\mathrm{Sr}}^{2+}$ has been substituted by ${\mathrm{La}}^{3+}$ $({\mathrm{Sr}}_{2\ensuremath{-}y}{\mathrm{La}}_{y}\mathrm{Ru}{\mathrm{O}}_{4})$ or ${\mathrm{Ru}}^{4+}$ by ${\mathrm{Ti}}^{4+}$ $({\mathrm{Sr}}_{2}{\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{4})$. For undoped ${\mathrm{Sr}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$, the purity is so high that the strong-field Hall coefficient can be measured for fields above $4\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. The conventional weak-field Hall coefficient as a function of doping shows a sharp jump and sign change at $y\ensuremath{\sim}0.01$ that is unrelated to either a sharp change in Fermi-surface topography or a magnetic instability. The implications of these results are discussed.

We report a study of magnetic, thermal, and transport properties of ${\mathrm{La}}^{3+}$ substituted ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, performed in order to investigate the effects of additional electron doping in this correlated metal. A gradual enhancement of the electronic part of specific heat and a more drastic increase of the static magnetic susceptibility were observed in ${\mathrm{Sr}}_{2\ensuremath{-}y}{\mathrm{La}}_{y}{\mathrm{RuO}}_{4}$ with increasing $y$. Furthermore, the quasi-two-dimensional Fermi-liquid behavior seen in pure ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ breaks down near the critical concentration ${y}_{\mathrm{c}}\ensuremath{\sim}0.20$. Combined with a realistic tight-binding model with rigid-band shift of Fermi level, the enhancement of the density of states can be ascribed to the elevation of the Fermi energy toward a van Hove singularity of the thermodynamically dominant $\ensuremath{\gamma}$ Fermi-surface sheet. On approaching the van Hove singularity, the effective nesting vector of the $\ensuremath{\gamma}$ band shrinks and further enhances the susceptibility near the wave vector $q\ensuremath{\sim}0$. We attribute the non-Fermi-liquid behavior to two-dimensional ferromagnetic fluctuations with short range correlations at the van Hove singularity. The observed behavior is in sharp contrast to that of ${\mathrm{Ti}}^{4+}$ substitution in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ which enhances antiferromagnetic fluctuations and subsequently induces incommensurate magnetic ordering associated with the nesting between the other Fermi-surface sheets ($\ensuremath{\alpha}$ and $\ensuremath{\beta}$). We thus establish that substitution of appropriate chemical dopants can band selectively modify the spin-fluctuation spectrum in the spin-triplet superconductor ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$.

We report a systematic study of electron doping of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ by nonisovalent substitution of ${\mathrm{La}}^{3+}$ for ${\mathrm{Sr}}^{2+}$. Using a combination of de Haas-van Alphen oscillations, specific heat, and resistivity measurements, we show that electron doping leads to a rigid-band shift of the Fermi level corresponding to one doped electron per $\mathrm{La}$ ion, with constant many-body quasiparticle mass enhancement over the band mass. The susceptibility spectrum is substantially altered and enhanced by the doping but this has surprisingly little effect on the strength of the unconventional superconducting pairing.