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To investigate the static and dynamic magnetic properties on the uranium-based superconductor UTe$_{2}$, we measured the NMR Knight shift $K$ and the nuclear spin-lattice relaxation rate $1/T_{1}$ in $H \parallel a$ by $^{125}$Te-NMR on a $^{125}$Te-enriched single-crystal sample. $1/T_1T$ in $H \parallel a$ is much smaller than $1/T_1T$ in $H \parallel b$ and $c$, and magnetic fluctuations along each axis are derived from the $1/T_1T$ measured in $H$ parallel to all three crystalline axes. The magnetic fluctuations are almost identical at two Te sites and isotropic at high temperatures, but become anisotropic below 40 K, where heavy-fermion state is formed. The character of magnetic fluctuations in UTe$_2$ is discussed with the comparison to its static susceptibility and the results on other U-based superconductors. It is considered that the magnetic fluctuations probed with the NMR measurements are determined by the magnetic properties inside the two-leg ladder formed by U atoms, which are dominated by the $q_a$ = 0 ferromagnetic fluctuations., 10 pages, 6 figures

Spin susceptibility in the superconducting (SC) state was measured in the higher-quality sample of uranium-based superconductor UTe$_2$ by using Knight-shift measurements for a magnetic field $H$ along all three crystalline axes. In the higher-quality sample, the SC transition temperature $T_{\rm c}$ is about 2.1 K, and the residual electronic term in the specific heat is almost zero. The NMR linewidth becomes narrower and is almost half of that in the previous sample with $T_{\rm c} \sim 1.6$ K when $H \parallel a$ and $c$. Although the Knight-shift behavior was not so different from the previous results for $H \parallel b$, and $c$, a large reduction in Knight shift along the $a$ axis was observed, in contrast with the previous $a$-axis Knight shift result. We discuss the origin of the difference between the previous and present results, and the possible SC state derived from the present results., Comment: 7 pages, 6 figures, including supplemental materials

We report the first observation of the de Haas-van Alphen (dHvA) effect in the novel spin-triplet superconductor UTe2 using high quality single crystals with the high residual resistivity ratio (RRR) over 200. The dHvA frequencies, named alpha and beta, are detected for the field directions between c and a-axes. The frequency of branch beta increases rapidly with the field angle tilted from c to a-axis, while branch alpha splits, owing to the maximal and minimal cross-sectional areas from the same Fermi surface. Both dHvA branches, alpha and beta reveal two kinds of cylindrical Fermi surfaces with a strong corrugation at least for branch alpha. The angular dependence of the dHvA frequencies is in very good agreement with that calculated by the generalized gradient approximation (GGA) method taking into account the on-site Coulomb repulsion of U = 2 eV, indicating the main Fermi surfaces are experimentally detected. The detected cyclotron effective masses are large in the range from 32 to 57 m0 . They are approximately 10-20 times lager than the corresponding band masses, consistent with the mass enhancement obtained from the Sommerfeld coefficient and the calculated density of states at the Fermi level. The local density approximation (LDA) calculations of ThTe2 assuming U4+ with the 5f^2 localized model are in less agreement with our experimental results, in spite of the prediction for two cylindrical Fermi surfaces, suggesting a mixed valence states of U4+ and U3+ in UTe2., 7 pages, 8 figures, submitted to J. Phys. Soc. Jpn

This study investigates the spin susceptibility in U-based superconductor UTe$_2$ in the superconducting (SC) state by using Knight shift measurements for a magnetic field $H$ along the $a$ axis, which is the magnetic easy axis of UTe$_2$. Although a tiny anomaly ascribed to the SC diamagnetic effect was observed just below the SC transition temperature $T_{\rm c}$, the $a$-axis Knight shift in the SC state shows no significant decrease, following the extrapolation from the normal-state temperature dependence. This indicates that the spin susceptibility is nearly unchanged below $T_{\rm c}$. Considering the previous Knight shift results for $H \parallel b$ and $H \parallel c$, the dominant SC state is determined to be $B_{\rm 3u}$ in the spin-triplet pairing, which is consistent with the spin anisotropy in the normal state. The present result shows that UTe$_2$ is a spin-triplet superconductor with spin degrees of freedom., 6 pages, 8 figures

$^{125}$Te NMR experiments in field ($H$) applied along the easy magnetization axis (the $a$-axis) revealed slow electronic dynamics developing in the paramagnetic state of UTe$_2$. The observed slow fluctuations are concerned with a successive growth of long-range electronic correlations below 30$-$40 K, where the spin susceptibility along the hard magnetization axis (the $b$-axis) shows a broad maximum. The experiments also imply that tiny amounts of disorder or defects locally disturb the long-range electronic correlations and develop an inhomogeneous electronic state at low temperatures, leading to a low temperature upturn observed in the bulk-susceptibility in $H\|a$. We suggest that UTe$_2$ would be located on the paramagnetic side near an electronic phase boundary, where either magnetic or Fermi-surface instability would be the origin of the characteristic fluctuations., Comment: 5 pages, 5 figures, accepted for publication in J. Phys. Soc. Jpn

Magnetic-field-angle-resolved specific heat and magnetization measurements were conducted on a ferromagnetic superconductor UCoGe with remarkable anisotropic upper critical field $H_{\rm c2}$. Although $H_{\rm c2}$ reaches a high magnetic field ($\sim 20$~T) along the $b$ axis, it is small ($\sim~0.6$~T) when a magnetic field is applied along the magnetic easy $c$-axis. This study indicates that the specific heat is abruptly suppressed when the magnetic field is applied toward the $c$ axis from the $a$ and $b$ axes in the ferromagnetic state. The field response of density of states (DOS) is anisotropic, relative to the $c$ axis, and its angle dependence is slightly singular. The Ising-type magnetic anisotropy of the ferromagnetic state is dominant even in the anisotropic reinforced superconducting state. These facts indicate that the suppression of DOS may closely relate to the superconducting state. We theoretically analyze these findings together with URhGe and UTe$_2$ by highlighting the common and distinctive features among three compounds., Comment: This paper is accepted in Phys. Rev. B

High-sensitivity capacitive Faraday magnetometers were developed for static DC magnetization measurements in a sub-Kelvin region that can be used with

Metamagnetic transitions in the novel spin-triplet superconductor UTe$_2$ were investigated through the newly developed simultaneous measurements of magnetization and sample temperature for the field along the orthorhombic $b$-axis and close to the [011] direction, where reentrant superconductivity (RSC) is detected below and above the first-order metamagnetic transition field $H_{\rm m}$. Combining the Maxwell's relation and the Clausius-Clapeyron equation, we obtained the field dependence of Sommerfeld coefficient $\gamma$ through the first-order metamagnetic transition. A significant enhancement of the effective mass toward $H_{\rm m}$ were detected for both field directions. On the other hand, above $H_{\rm m}$, the effective mass discontinuously decreases for $H~||~b$, while it discontinuously increases for $H~||~\sim [011]$, which seems to make a crucial role for the RSC., Comment: 8 pages, 11 figures, including supplemental material. To be published in Journal of Physical Society of Japan

We report the magnetoresistance in the novel spin-triplet superconductor UTe2 under pressure close to the critical pressure Pc, where the superconducting phase terminates, for field along the three a, b and c-axes in the orthorhombic structure. The superconducting phase for H // a-axis just below Pc shows a field-reentrant behavior due to the competition with the emergence of magnetic order at low fields. The upper critical field Hc2 for H // c-axis shows a quasi-vertical increase in the H-T phase diagram just below Pc, indicating that superconductivity is reinforced by the strong fluctuations which persist even at high fields above 20T. Increasing pressure leads to the disappearance of superconductivity at zero field with the emergence of magnetic order. Surprisingly, field-induced superconductivity is observed at high fields, where a spin-polarized state is realized due to the suppression of the magnetic ordered phases; the spin-polarized state is favorable for superconductivity, whereas the magnetic ordered phase at low field seems to be unfavorable. The huge Hc2 in the spin-polarized state seems to imply a spin-triplet state. Contrary to the a- and c-axes, no field-reinforcement of superconductivity occurs for magnetic field along the b-axis. We compare the results with the field-reentrant superconductivity above the metamagnetic field, Hm for the field direction tilted by about 30 deg. from b to c-axis at ambient pressure as well as the field-reentrant (-reinforced) superconductivity in ferromagnetic superconductors, URhGe and UCoGe., 7 pages, 5 figures, submitted to J. Phys. Soc. Jpn

We report the magnetic susceptibility and the magnetization under pressures up to 1.7 GPa above the critical pressure, Pc ~ 1.5 GPa, for H || a, b, and c-axes in the novel spin triplet superconductor UTe2. The anisotropic magnetic susceptibility at low pressure with the easy magnetization a-axis changes to the quasi-isotropic behavior at high pressure, revealing a rapid suppression of the susceptibility for a-axis, and a gradual increase of the susceptibility for the b-axis. At 1.7 GPa above Pc, magnetic anomalies are detected at TMO ~ 3K and TWMO ~ 10 K. The former anomaly corresponds to long-range magnetic order, most likely antiferromagnetism, while the latter shows a broad anomaly, which is probably due to the development of short range order. The unusual decrease and increase of the susceptibility below TWMO for H || a and b-axes, respectively, indicate the complex magnetic properties at low temperatures above Pc. This is related to the interplay between multiple fluctuations dominated by antiferromagnetism, ferromagnetism, valence and Fermi surface instabilities., Journal of the Physical Society of Japan, 90 (7), ISSN:0031-9015, ISSN:1347-4073

To investigate spin susceptibility in a superconducting (SC) state, we measured the $^{125}\mathrm{Te}$-NMR Knight shifts at magnetic fields ($H$) up to 6.5 T along the $b$ and $c$ axes of single-crystal ${\mathrm{UTe}}_{2}$, a promising candidate for a spin-triplet superconductor. In the SC state, the Knight shifts along the $b$ and $c$ axes (${K}_{b}$ and ${K}_{c}$, respectively) decreased slightly, and the decrease in ${K}_{b}$ was almost constant up to 6.5 T. The reduction in ${K}_{c}$ decreased with increasing $H$, and ${K}_{c}$ was unchanged through the SC transition temperature at 5.5 T, excluding the possibility of spin-singlet pairing. Our results indicate that spin susceptibilities along the $b$ and $c$ axes slightly decrease in the SC state in low $H$, and the $H$ response of SC spin susceptibility is anisotropic on the $bc$ plane. We discuss the possible $\mathbit{d}$-vector state within the spin-triplet scenario and suggest that the dominant $\mathbit{d}$-vector component for the case of $H\ensuremath{\parallel}b$ changes above 13 T, where ${T}_{\mathrm{c}}$ increases with increasing $H$.

Chirality, which is a fundamental property of symmetry, can produce unique electronic and magnetic properties. The problem is that many of the chiral inorganic compounds form a racemic mixture consisting of right- and left-handed enantiomers. Here, the authors succeeded in growing the enantiopure single crystal of Nd-based monoaxial chiral magnet, demonstrating the nontrivial magnetic phase diagram in this rare-earth chiral magnet with the DM interaction. This ternary rare-earth platinum boride system provides an attractive platform for examining chiral magnetism and the role of DM interaction.

We report an unconventional class of pressure-induced quantum phase transitions, possessing two bicritical points at 6.25 GPa in URhSn. This unique transformation accompanies a Fermi surface reconstruction, demarcating competing ordered phases suitably described with a localized and itinerant description of the magnetic $5f$ electrons. Ferromagnetic fluctuations over a wide range of temperatures and pressures in the pressure-induced low temperature phase are evidenced by a robust ${T}^{5/3}$ temperature dependence of resistivity up to 11 GPa, which is a characteristic of an elusive marginal Fermi-liquid state.

${\mathrm{CeIr}}_{3}$ is a Ce-based superconductor with a superconducting transition temperature ${T}_{\mathrm{c}}=3.4$ K in an intermediate-valence state. We grew high-quality single crystals of ${\mathrm{CeIr}}_{3}$ using the Czochralski method, and measured the electrical resistivity, magnetic torque, and specific heat. The anisotropy of the superconducting upper critical field ${H}_{\mathrm{c}2}$ was determined. The temperature dependence of ${H}_{\mathrm{c}2}$, obtained from the resistivity measurements, suggests the multiband character of the superconductivity in ${\mathrm{CeIr}}_{3}$. Different field-angle dependencies of ${H}_{\mathrm{c}2}$ in electrical transport and thermodynamic measurements indicate the robust surface effect in the bulk superconductivity of ${\mathrm{CeIr}}_{3}$. We analyzed the surface superconductivity based on simple models to reveal the bulk superconducting properties. Our results support the isotropic bulk superconducting state and robust surface superconductivity in ${\mathrm{CeIr}}_{3}$ single crystals.

The physical properties of URhSn with quasi-kagome structure are studied using single-crystalline samples via electrical resistivity, magnetic susceptibility, heat capacity, thermal expansion, and high-field magnetization measurements. Remarkable magnetic anisotropy is found in the ferromagnetic (FM) state below ${T}_{\mathrm{C}}=16\phantom{\rule{4pt}{0ex}}\mathrm{K}$ as well as in the ordered state between ${T}_{\mathrm{C}}$ and ${T}_{\mathrm{O}}=54\phantom{\rule{4pt}{0ex}}\mathrm{K}$, where the easy and hard magnetization directions are the hexagonal [0001] and $[10\overline{1}0]$ axes. In the paramagnetic state, the magnetic susceptibility shows a Curie-Weiss behavior; the Weiss temperatures are positive and negative for [0001] and $[10\overline{1}0]$, respectively, indicating the presence of both FM and antiferromagnetic (AFM) correlations. The entropy release for $5f$ electrons is approximately $R\phantom{\rule{0.16em}{0ex}}\mathrm{ln}3$ at ${T}_{\mathrm{O}}$. The thermal expansion coefficient is strongly anisotropic around ${T}_{\mathrm{O}}$ between the hexagonal basal plane and the [0001] axis, indicating its remarkable anisotropic magnetoelastic response and uniaxial stress dependences. Interestingly, the magnetic field response of the higher-temperature ordered state is unusual: ${T}_{\mathrm{O}}(H)$ increases and the heat-capacity jump is enhanced with the magnetic field for $H\phantom{\rule{0.16em}{0ex}}||\phantom{\rule{4pt}{0ex}}[0001]$. Based on the established thermodynamic evidence for the second-order transition at ${T}_{\mathrm{O}}(H)$, a plausible scenario is the occurrence of a canted AFM ordering or a conical state under magnetic fields, which is stabilized when coupled with field-induced magnetic moments along the [0001] axis. Another possibility is the occurrence of quadrupole ordering at ${T}_{\mathrm{O}}(H)$.

We report the single-crystal growth of the noncentrosymmetric cubic heavy-fermion paramagnet ${\mathrm{UPt}}_{5}$ and comparative Fermi surface studies of de Haas-van Alphen (dHvA) experiments and theoretical band calculations. We also present the results of single-crystal x-ray diffraction, electrical resistivity, heat capacity, and magnetization measurements using single crystals of ${\mathrm{UPt}}_{5}$. The dHvA experiments reveal nearly spherical split hole Fermi surfaces and largely split electron Fermi surfaces, which are in agreement with the results of a band-structure calculation. The largely split electron Fermi surfaces may be understood through the antisymmetric spin-orbit interaction derived from U-$5f$ and Pt-$5d$ electrons. We explain the observed unexpected dHvA frequencies quasiquantitatively based on orbital crossing using the coupled network analysis in the split hole Fermi surface pair. Furthermore, the temperature dependence of the dHvA amplitude reveals enhanced cyclotron effective masses of up to $29{m}_{0}$ and a peculiar anisotropic effective mass in the paramagnetic cubic heavy-fermion ${\mathrm{UPt}}_{5}$.

The gap structure of a novel uranium-based superconductor UTe$_2$, situated in the vicinity of ferromagnetic quantum criticality, has been investigated via specific-heat $C(T,H,\Omega)$ measurements in various field orientations. Its angular $\Omega(\phi,\theta)$ variation shows a characteristic shoulder anomaly with a local minimum in $H \parallel a$ at moderate fields rotated within the $ab$ and $ac$ planes. Based on the theoretical calculations, these features can be attributed to the presence of point nodes in the superconducting gap along the $a$ direction. Under the field orientation along the easy-magnetization $a$ axis, an unusual temperature dependence of the upper critical field at low fields together with a convex downward curvature in $C(H)$ were observed. These anomalous behaviors can be explained on the basis of a nonunitary triplet state model with equal-spin pairing whose $T_{\rm c}$ is tuned by the magnetization along the $a$ axis. From these results, the gap symmetry of UTe$_2$ is most likely described by a vector order parameter of $d(k)=(b + ic)(k_b + ik_c)$., Comment: 6 pages, 4 figures (main text) + 7 pages, 5 figures (Supplementary Material), accepted for publication in Phys. Rev. Research

We studied the anisotropy of the superconducting upper critical field $H_{\rm c2}$ in the heavy-fermion superconductor UTe$_2$ under hydrostatic pressure by magnetoresistivity measurements. In agreement with previous experiments we confirm that superconductivity disappears near a critical pressure $p_{\rm c} \approx 1.5$~GPa, and a magnetically ordered state appears. The unusual $H_{\rm c2}(T)$ at low temperatures for $H \parallel a$ suggests that the multiple superconducting phases which appear under pressure have quite different $H_{\rm c2}$. For a field applied along the hard magnetization $b$ axis $H_{\rm c2} (0)$ is glued to the metamagnetic transition $H_{\rm m}$ which is suppressed near $p_{\rm c}$. The suppression of $H_{\rm m}$ with pressure follows the decrease of temperature $T_{\chi}^{\rm max}$, at the maximum in the susceptibility along $b$. The strong reinforcement of $H_{\rm c2}$ at ambient pressure for $H \parallel b$ above 16~T is rapidly suppressed under pressure due to the increase of $T_{\rm sc}$ and the decrease of $H_{\rm m}$. The change in the hierarchy of the anisotropy of $H_{\rm c2}(0)$ on approaching $p_{\rm c}$ points out that the $c$ axis becomes the hard magnetization axis., Comment: 5 pages and 4 figures, supplemental material: 9 figures

We performed AC calorimetry and magnetoresistance measurements under pressure for H || a-axis (easy-magnetization axis) in the novel heavy-fermion superconductor UTe2. Thanks to the thermodynamic information, multiple superconducting phases have been revealed under pressure and magnetic field. The (H,T) phase diagram of superconductivity under pressure displays an abrupt increase of the upper critical field (Hc2) at low temperature and in the high field region, and a strong convex curvature of Hc2 at high temperature. This behavior of Hc2 and the multiple superconducting phases require a state for the superconducting order parameter more complex than a spin-triplet equal spin pairing. Above the superconducting critical pressure, Pc, we find strong indications that the possible magnetic order is closer to antiferromagnetism than to ferromagnetism., 6 pages, 5 figures, accepted for publication in J. Phys. Soc. Jpn

Low-temperature thermodynamic investigation for UBe13 and UPd2Al3 were performed in order to gain insight into their unusual ground states of 5 f electrons. Our heat-capacity data for the cubic UBe13 strongly suggest that nodal quasiparticles are absent and its superconducting (SC) gap is fully open over the Fermi surface. Moreover, two unusual thermodynamic anomalies are also observed in UBe13 at ∼ 3 T and ∼ 9 T; the lower-field anomaly is seen only in the SC mixed state by dc magnetization M ( H ) as well as heat-capacity C ( H ) , while the higher-field anomaly appears for C ( H ) in the normal phase above the upper critical field. On the other hand, field-orientation dependence of the heat capacity in the hexagonal UPd2Al3 shows a significantly anisotropic behavior of C ( H ) ∝ H 1 / 2 , reflecting the nodal gap structure of this system. Our result strongly suggests the presence of a horizontal line node on the Fermi surface with heavy effective mass in UPd2Al3.

UTe$_2$ is a recently discovered promising candidate for a spin-triplet superconductor. In contrast to conventional spin-singlet superconductivity, spin-triplet superconductivity possesses spin and angular momentum degrees of freedom. To detect these degrees of freedom and obtain the solid evidence of spin-triplet superconductivity in UTe$_2$, we performed $^{125}$Te-NMR measurement. We previously reported that the shoulder signal appears in NMR spectra below the superconducting (SC) transition temperature $T_{\rm c}$ in $H \parallel b$, and that a slight decrease in the Knight shift along the $b$ and $c$ axes ($K_b$ and $K_c$, respectively) below $T_{\rm c}$ at a low magnetic field $H$. To clarify the origin of the shoulder signal and the trace of the decrease in $K_b$, we compared the $^{125}$Te-NMR spectra obtained when $H~\parallel~b$ and $H~\parallel~c$ and measured the $^{125}$Te-NMR spectra for $H~\parallel~b$ up to 14.5~T. The intensity of the shoulder signal observed for $H~\parallel~b$ has a maximum at $\sim 6$~T and vanishes above 10~T, although the superconductivity is confirmed by the $\chi_{\rm AC}$ measurements, which can survive up to 14.5~T (maximum $H$ in the present measurement). Moreover, the decrease in $K_b$ in the SC state starts to be small around 7~T and almost zero at 12.5~T. This indicates that the SC spin state gradually changes with the application of $H$. Meanwhile, in $H~\parallel~c$, unexpected broadening without the shoulder signals was observed below $T_{\rm c}$ at 1~T, and this broadening was quickly suppressed with increasing $H$. We construct the $H$--$T$ phase diagram for $H~\parallel~b$ and $H~\parallel~c$ based on the NMR measurements and discuss possible SC states with the theoretical consideration. We suggest that the inhomogeneous SC state characterized by the broadening of the NMR spectrum originates from the spin degrees of freedom., Comment: 8 pages, 7 figures

We report an observation of metamagnetism in the $5f$-electron heavy-fermion antiferromagnets ${\mathrm{UNi}}_{2}{\mathrm{Al}}_{3}$ at 78 T and $\mathrm{U}{({\mathrm{Pd}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2}{\mathrm{Al}}_{3}$ at 30 T (42 T) for $x=0.5$ ($x=0.75$) using single-crystalline samples. The magnetization curves for $\mathrm{U}{({\mathrm{Pd}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2}{\mathrm{Al}}_{3}$ ($x=0.5$ and 0.75) show a sharp increase at the metamagnetic transition (MMT) field ${B}_{\mathrm{m}}$, strongly indicating their first-order nature. The obtained $B\ensuremath{-}T$ phase diagram for $x=0.5$ suggests the possible presence of the tricritical point, while the MMT still occurs at high temperatures above ${T}_{\mathrm{N}}$ from paramagnetic to polarized-paramagnetic states as a crossover. The results indicate that the antiferromagnetic order is closely connected to the MMT in $\mathrm{U}{({\mathrm{Pd}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2}{\mathrm{Al}}_{3}$, but the metamagnetic behavior cannot be explained by the conventional spin flip of antiferromagnetic ordered moments. Our data for $\mathrm{U}{({\mathrm{Pd}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2}{\mathrm{Al}}_{3}$ have revealed an empirical linear relation of ${B}_{\mathrm{m}}(x)\ensuremath{\propto}\phantom{\rule{4pt}{0ex}}{T}_{{\ensuremath{\chi}}_{\mathrm{max}}}(x)$ and a scaling behavior in magnetic susceptibility of $\ensuremath{\chi}(T/{T}_{\ensuremath{\chi}\mathrm{max}})/{\ensuremath{\chi}}_{\mathrm{max}}$, where ${T}_{\ensuremath{\chi}\mathrm{max}}$ is the temperature of the susceptibility maximum [${\ensuremath{\chi}}_{\mathrm{max}}\ensuremath{\equiv}\ensuremath{\chi}({T}_{\ensuremath{\chi}\mathrm{max}})$]. Importantly, the critical value of the magnetic polarization at the MMT ${M}_{\mathrm{cr}}$ is roughly unchanged, although the value of susceptibility is notably suppressed with increasing $x$.

The spin-triplet state is most likely realized in uranium ferromagnetic superconductors, UGe2, URhGe, UCoGe. The microscopic coexistence of ferromagnetism and superconductivity means that the Cooper pair should be realized under the strong internal field due the ferromagnetism, leading to the spin-triplet state with equal spin pairing. The field-reinforced superconductivity, which is observed in all three materials when the ferromagnetic fluctuations are enhanced, is one of the strong evidences for the spin-triplet superconductivity. We present here the results of a newly discovered spin-triplet superconductor, UTe2, and compare those with the results of ferromagnetic superconductors. Although no magnetic order is found in UTe2, there are similarities between UTe2 and ferromagnetic superconductors. For example, the huge upper critical field exceeding the Pauli limit and the field-reentrant superconductivity for H || b-axis are observed in UTe2, URhGe and UCoGe. We also show the specific heat results on UTe2 in different quality samples, focusing on the residual density of states in the superconducting phase., 6 pages, 3 figures, accepted for publication in JPSJ Conference Proceedings

We report 125Te-NMR studies on a newly discovered heavy fermion superconductor UTe2. Using a single crystal, we have measured the 125Te-NMR Knight shift K and spin-lattice relaxation rate 1/T1 for fields along the three orthorhombic crystal axes. The data confirm a moderate Ising anisotropy for both the static (K) and dynamical susceptibilities (1/T1) in the paramagnetic state above about 20 K. Around 20 K, however, we have observed a sudden loss of NMR spin-echo signal due to sudden enhancement of the NMR spin-spin relaxation rate 1/T2, when the field is applied along the easy axis of magnetization (=a axis). This behavior suggests the development of longitudinal magnetic fluctuations along the a axis at very low frequencies below 20 K., 5 pages, 7 figures, accepted for publication in J. Phys. Soc. Jpn

We have studied the magnetization of the recently discovered heavy fermion superconductor UTe$_2$ up to 56 T in pulsed-magnetic fields. A first-order metamagnetic transition has been clearly observed at $H_{\rm m}$ =34.9 T when the magnetic field $H$ is applied along the orthorhombic hard-magnetization $b$-axis. The transition has a critical end point at $\sim$11 K and 34.8 T, where the first order transition terminates and changes into a crossover regime. Using the thermodynamic Maxwell relation, we have evaluated the field dependence of the Sommerfeld coefficient of the specific heat directly related to the superconducting pairing. From the analysis, we found a significant enhancement of the effective mass centered at $H_{\rm m}$, which is reminiscent of the field-reentrant superconductivity of the ferromagnet URhGe in transverse fields. We discuss the origin of their field-robust superconductivity., 5 pages, 5 figures, accepted for publication in J. Phys. Soc. Jpn

To unravel the nature of the superconducting symmetry of the enigmatic $5f$ heavy-fermion ${\mathrm{UBe}}_{13}$, the pressure dependence of the upper critical field and of the normal state are studied up to 10 GPa. Remarkably, the pressure evolution of the anomalous ${H}_{c2}(T,P)$ over the entire pressure range up to 5.9 GPa can be successfully explained by the gradual admixture of a field-pressure-induced ${E}_{u}$ component in an ${A}_{1u}$ spin-triplet ground state. This result provides strong evidence for parallel-spin pairing in ${\mathrm{UBe}}_{13}$, which is also supported by the recently observed fully gapped excitation spectrum at ambient pressure. Moreover, we have also found a novel non-Fermi-liquid behavior of the resistivity, $\ensuremath{\rho}(T)\ensuremath{\sim}{T}^{n}$ ($n\ensuremath{\lesssim}1$), which disappears with the collapse of the negative magnetoresistance behavior and the existence of a superconducting ground state around $P=6\text{ }\text{ }\mathrm{GPa}$, suggesting a close interplay between Kondo scattering and superconductivity.

The physical properties of the single-crystalline ${\mathrm{U}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Si}}_{5}$, a new ternary uranium compound with ${\mathrm{U}}_{2}{\mathrm{Co}}_{3}{\mathrm{Si}}_{5}$-type orthorhombic structure, are investigated by means of magnetic susceptibility $\ensuremath{\chi}(T)$, specific heat $C(T)$, electrical resistivity $\ensuremath{\rho}(T)$, and high-field magnetization $M(H,T)$ measurements. ${\mathrm{U}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Si}}_{5}$ undergoes an antiferromagnetic transition at ${T}_{N}=36$ K followed by a first-order phase transition at ${T}_{0}=25.5$ K. The sharp peak in $C(T)$ at ${T}_{0}$ and the obvious hysteresis behavior in $\ensuremath{\chi}(T),\ensuremath{\rho}(T)$, and $M(H,T)$ around ${T}_{0}$ provide strong evidence for the first-order phase transition. The $\ensuremath{\rho}(T)$ measurements along the $a$ and $b$ axes reveal the negative temperature coefficients of resistance over a wide temperature range, which can be understood based on the semiconductorlike narrow band gap model or the Kondo effect. The $M(H)$ curve measured at 4.2 K along the $b$ axis shows three-step metamagnetic transitions within a narrow field region around 200 kOe and a large hysteresis near the first transition field, while the ${M}_{H\ensuremath{\parallel}a}(H)$ and ${M}_{H\ensuremath{\parallel}c}(H)$ curves show no transitions up to 560 kOe suggesting the strong magnetic anisotropy. A possible mechanism of the first-order phase transition at ${T}_{0}$ is the occurrence of a magnetic quadrupolar order, resulting from the quasi-one-dimensional uranium zigzag chain.

We have performed the $^{125}$Te-nuclear magnetic resonance (NMR) measurement in the field along the $b$ axis on the newly discovered superconductor UTe$_2$, which is a candidate of a spin-triplet superconductor. The nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ abruptly decreases below a superconducting (SC) transition temperature $T_c$ without showing a coherence peak, indicative of UTe$_2$ being an unconventional superconductor. It was found that the temperature dependence of $1/T_1T$ in the SC state cannot be understood by a single SC gap behavior but can be explained by a two SC gap model. The Knight shift, proportional to the spin susceptibility, decreases below $T_c$, but the magnitude of the decrease is much smaller than the decrease expected in the spin-singlet pairing. Rather, the small Knight-shift decrease as well as the absence of the Pauli-depairing effect can be interpreted by the spin triplet scenario., Comment: 10 pages, 8 figures, including supplemental materials, chosen as Editors' choice, open access

The effect of differently oriented magnetic field on chiral incommensurate helimagnet UPtGe is studied both experimentally and theoretically. The magnetization measurements up to the field above the saturation have revealed an isotropic magnetic response below 20 T and a remarkable nonmonotonic anisotropy in high fields. Moreover, the two principally different phase transitions from the noncollinear incommensurate to the field-induced ferromagnetic state have been observed. These properties are successfully explained by density-functional theory calculations taking into account the noncollinearity of the magnetic structures, arbitrary directed magnetic field, and relativistic effects. We also estimate the strength of different competing magnetic interactions and discuss possible scenarios of the field-induced phase transformations., 7 pages, 6 figures, 1 table

In order to gain insight into the superconducting (SC) gap of UBe 13 , we studied its quasiparticle excitations by means of heat-capacity measurements. Quite unexpectedly, we found the isotropic C ( H ) ∝ H behavior in low fields at low temperatures, implying the absence of nodal quasiparticle excitations. This result indicates that the SC gap in UBe 13 is fully open over the Fermi surfaces. Furthermore, we observed a characteristic oscillation of heat capacity both in the SC and non-Fermi-liquid normal states above ∼2 T, and the angular variation of heat capacity possibly originates from anisotropic magnetic response of the heavy-electron state. Our result regarding the low-energy quasiparticle excitations in the SC and normal states will be a clue to understand the unusual nature of UBe 13 .

A new ternary uranium germanide U2Rh3Ge5 has been successfully synthesized and investigated by means of magnetic susceptibility χ(T, H), isothermal magnetization M(T, H), electrical resistivity ρ(T), and specific heat C(T, H) measurements. This compound is found to crystallize in the U2Co3Si5-type orthorhombic structure. The low-field χ(T) shows a clear peak at T N = 41.5 K corresponding to an antiferromagnetic transition. The M(H) curve measured up to 70 kOe exhibits an H-linear behavior at 2 K with very small induced magnetic moments, while it shows upward curvature with increasing temperature, implying the possible presence of a metamagnetic transition in high-field region above 70 kOe. As the temperature decreases, ρ(T) increases slowly at T > T N and decreases rapidly at T < T N, which can be understood based on a semiconductor-like narrow band gap model (or the c-f hybridization effect) and an antiferromagnetic spin-wave model, respectively. No evidence of heavy-fermion behavior or superconductivity transition is observed at temperatures as low as 0.4 K. The obtained experimental results are discussed by comparing with those reported for the isomorphic compound U2Ir3Si5 and the quasi-isomorphic compound U2Rh3Si5.