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Our measurements of $^{125}$Te NMR relaxations reveal an enhancement of electronic spin fluctuations above $\mu_0H^*\sim15$ T, leading to their divergence in the vicinity of the metamagnetic transition at $\mu_0H_m\approx35$ T, below which field-reinforced superconductivity appears when a magnetic field ($H$) is applied along the crystallographic $b$ axis. The NMR data evidence that these fluctuations are dominantly longitudinal, providing a key to understanding the peculiar superconducting phase diagram in $H\|b$, where such fluctuations enhance the pairing interactions., Comment: 7 pages, 4 figures

We performed de Haas-van Alphen (dHvA) experiments in the spin-triplet superconductor UTe2 for magnetic field along the c-axis above 15T. Three fundamental dHvA frequencies, named alpha1, alpha2 and beta corresponding to the cross sections of cylindrical Fermi surfaces (FSs) with large cyclotron effective masses (33-43 m0) were detected. No other fundamental dHvA frequencies were detected at high frequency range, suggesting a cylindrical-shaped electron FS without connecting at the Z point of the Brillouin zone. However, the existence of small pocket FSs associated with extremely heavy masses cannot be fully excluded., Comment: accepted for publication in J. Phys. Soc. Jpn. as "short notes", 3 pages, 2 figures

The potential spin-triplet heavy-fermion superconductor UTe$_2$ exhibits signatures of multiple distinct superconducting phases. For field aligned along the $b$ axis, a metamagnetic transition occurs at $\mu_0 H_\mathrm{m}\approx35\,$T. It is associated with magnetic fluctuations that may be beneficial for the field-reinforced superconductivity surviving up to $H_\mathrm{m}$. Once the field is tilted away from the $b$ towards the $c$ axis, a reentrant superconducting phase emerges just above $H_\mathrm{m}$. In order to better understand this remarkably field-resistant superconducting phase, we conducted magnetic-torque and magnetotransport measurements in pulsed magnetic fields. We determine the record-breaking upper critical field of $\mu_0 H_\mathrm{c2}\approx 73\,$T and its evolution with angle. Furthermore, the normal-state Hall effect experiences a drastic suppression indicative of a reduced band polarization above $H_\mathrm{m}$ in the angular range around $30^\circ$ caused by a partial compensation between the applied field and an exchange field. This promotes the Jaccarino-Peter effect as a likely mechanism for the reentrant superconductivity above $H_\mathrm{m}$., Comment: Main text: 29 pages, 4 figure; Supplement: 12 pages, 6 figures

The recently discovered superconductor UTe$_2$, with a T$_c$ between 1.5~K and 2~K, is attracting much attention due to strong suspicion of spin-triplet and topological superconductivity. Its properties under magnetic field are also remarkable, with field-reinforced and field-induced superconducting phases. Here, we report the first complete thermodynamic determination of the phase diagram for fields applied along the three crystallographic directions. Measurements were performed up to 36~T along the hard magnetisation $b$~axis in order to follow the superconducting transition up to the metamagnetic transition at $H_{m} = 34.75$~T. They reveal the existence of a phase transition line within the superconducting phase, and drastic differences occurring between these two phases. Detailed analysis supports a different spin state between the two phases, implying a low-field spin-triplet to high-field spin-singlet transition, a unique case among superconductors, giving insight on the mechanisms leading to spin-triplet superconductivity., Comment: Main paper : 17 pages and 15 figures, supplementary material : 17 pages and 17 figures

The normal-state out of which unconventional superconductivity in UTe$_2$ emerges is studied in detail using a variety of thermodynamic and transport probes. Clear evidence for a broad Schottky-like anomaly with roughly R ln 2 entropy around $T^{*} \approx 12$K is observed in all measured quantities. Comparison with high magnetic field transport data allows the construction of an $H\text{-}T$ phase diagram resembling that of the ferromagnetic superconductor URhGe. The low field electronic Gr\"uneisen parameter of $T^{*}$ and that of the metamagnetic transition at $H_m \approx 35$T are comparable pointing to a common origin of both phenomena. Enhanced Wilson and Korringa ratios suggests that the existence of short range ferromagnetic fluctuations cannot be ruled out.

Thermoelectric power ($S$) and Hall effect ($R_\mathrm{H}$) measurements on the paramagnetic superconductor UTe$_2$ with magnetic field applied along the hard magnetization $b$-axis are reported. The first order nature of the metamagnetic transition at $H_\mathrm{m}=H^b_\mathrm{c2}=35$~T leads to drastic consequences on $S$ and $R_\mathrm{H}$. In contrast to the field dependence of the specific heat in the normal state through $H_\mathrm{m}$, $S(H)$ is not symmetric with respect to $H_\mathrm{m}$. This implies a strong interplay between ferromagnetic (FM) fluctuations and a Fermi-surface reconstruction at $H_\mathrm{m}$. $R_\mathrm{H}$ is very well described by incoherent skew scattering above the coherence temperature $T_\mathrm{m}$ corresponding roughly to the temperature of the maximum in the susceptibility $T_{\chi_\mathrm{max}}$ and coherent skew scattering at lower temperatures. The discontinuous field dependence of both, $S(H)$ and the ordinary Hall coefficient $R_0$, at $H_\mathrm{m}$ and at low temperature, provides evidence of a change in the band structure at the Fermi level., Comment: 5 figures in the main text; 9 figures in the supplemental material

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

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., Comment: 6 pages, 3 figures, accepted for publication in JPSJ Conference Proceedings

We present different transport measurements up to fields of 29~T in the recently discovered heavy-fermion superconductor UTe$_{2}$ with magnetic field $H$ applied along the easy magnetization a-axis of the body-centered orthorhombic structure. The thermoelectric power varies linearly with temperature above the superconducting transition, $T_{SC}= 1.5$ K, indicating that superconductivity develops in a Fermi liquid regime. As a function of field the thermolelectric power shows successive anomalies which are attributed to field-induced Fermi surface instabilities. These Fermi-surface instabilities appear at critical values of the magnetic polarization. Remarkably, the lowest magnetic field instability for $H\parallel a$ occurs for the same critical value of the magnetization (0.4 $\mu_B$) than the first order metamagnetic transition at 35~T for field applied along the $b$-axis. The estimated number of charge carriers at low temperature reveals a metallic ground state distinct from LDA calculations indicating that strong electronic correlations are a major issue in this compound., Comment: 5 pages, 5 figures

We present a study of the upper critical field of the newly discovered heavy fermion superconductor UTe$_2$ by magnetoresistivity measurements in pulsed magnetic fields up to 60~T and static magnetic fields up to 35~T. We show that superconductivity survives up to the metamagnetic transition at $H_{\rm m} \approx 35$~T at low temperature. Above $H_{\rm m}$ superconductivity is suppressed. At higher temperature superconductivity is enhanced under magnetic field leading to reentrance of superconductivity or an almost temperature independent increase of $H_{\rm c2}$. By studying the angular dependence of the upper critical field close to the $b$ axis (hard magnetization axis) we show that the maximum of the reentrant superconductivity temperature is depinned from the metamagnetic field. A key ingredient for the field-reinforcement of superconductivity on approaching $H_{\rm m}$ appears to be an immediate interplay with magnetic fluctuations and a possible Fermi-surface reconstruction., Comment: 5 pages + Supplemental Material, accepted for publication in J. Phys. Soc. Jpn

We present magnetoresistivity measurements on the heavy-fermion superconductor UTe$_{2}$ in pulsed magnetic fields $\mu_0H$ up to 68~T and temperatures $T$ from 1.4 to 80~K. Magnetic fields applied along the three crystallographic directions $\mathbf{a}$ (easy magnetic axis), $\mathbf{b}$, and $\mathbf{c}$ (hard magnetic axes), are found to induce different phenomena - depending on the field direction - beyond the low-field suppression of the superconducting state. For $\mathbf{H}\parallel\mathbf{a}$, a broad anomaly in the resistivity is observed at $\mu_0H^*\simeq10$~T and $T = 1.4$~K. For $\mathbf{H}\parallel\mathbf{c}$, no magnetic transition nor crossover are observed. For $\mathbf{H}\parallel\mathbf{b}$, a sharp first-order-like step in the resistivity indicates a metamagnetic transition at the field $\mu_0H_m \simeq 35$~T. When the temperature is raised signature of first-order metamagnetism is observed up to a critical endpoint at $T_{CEP}\simeq7$~K. At higher temperatures a crossover persists up to 28~K, i.e., below the temperature $T_\chi^{max} = 35$~K where the magnetic susceptibility is maximal. A sharp maximum in the Fermi-liquid quadratic coefficient $A$ of the low-temperature resistivity is found at $H_m$. It indicates an enhanced effective mass associated with critical magnetic fluctuations, possibly coupled with a Fermi surface instability. Similarly to the URhGe case, we show that UTe$_{2}$ is a candidate for field-induced reentrant superconductivity in the proximity of $H_m$., Comment: 5 pages, 4 Figures, to be published in the Journal of the Physical Society of Japan includes Supplementary Materials (15 pages, 11 Figures)

The $g$-factor anisotropy of the heavy quasiparticles in the hidden order state of URu$_2$Si$_2$ has been determined from the superconducting upper critical field and microscopically from Shubnikov-de Haas (SdH) oscillations. We present a detailed analysis of the $g$-factor for the $\alpha$, $\beta$ and $\gamma$ Fermi-surface pockets. Our results suggest a strong $g$-factor anisotropy between the $c$ axis and the basal plane for all observed Fermi surface pockets. The observed anisotropy of the $g$-factor from the quantum oscillations is in good agreement with the anisotropy of the superconducting upper critical field at low temperatures, which is strongly limited by the paramagnetic pair breaking along the easy magnetization axis $c$. However, the anisotropy of the initial slope of the upper critical field near $T_c$ cannot be explained by the anisotropy of the effective masses and Fermi velocities derived from quantum oscillations., Comment: 17 pages, 11 figures, accepted for publication in Phys. Rev. B

We grew single crystals of the recently discovered heavy fermion superconductor UTe2, and measured the resistivity, specific heat and magnetoresistance. Superconductivity (SC) was clearly detected at Tsc=1.65K as sharp drop of the resistivity in a high quality sample of RRR=35. The specific heat shows a large jump at Tsc indicating strong coupling. The large Sommerfeld coefficient, 117mJ K-2mol-1 extrapolated in the normal state and the temperature dependence of C/T below Tsc are the signature of unconventional SC. The discrepancy in the entropy balance at Tsc between SC and normal states points out that hidden features must occur. Surprisingly, a large residual value of the Sommerfeld coefficient seems quite robust (gamma_0/gamma ~ 0.5). The large upper critical field Hc2 along the three principal axes favors spin-triplet SC. For H // b-axis, our experiments do not reproduce the huge upturn of Hc2 reported previously. This discrepancy may reflect that Hc2 is very sensitive to the sample quality. A new perspective in UTe2 is the proximity of a Kondo semiconducting phase predicted by the LDA band structure calculations., Comment: 5 pages, 8 figures, accepted for publication in J. Phys. Soc. Jpn

Despite almost 40 years of research, the origin of heavy-fermion superconductivity is still strongly debated. Especially, the pressure-induced enhancement of superconductivity in CeCu$_2$Si$_2$ away from the magnetic breakdown is not sufficiently taken into consideration. As recently reported in CeCu$_2$Si$_2$ and several related compounds, optimal superconductivity occurs at the pressure of a valence crossover, which arises from a virtual critical end point at negative temperature $T_{\rm cr}$. In this context, we did a meticulous analysis of a vast set of top-quality high-pressure electrical resistivity data of several Ce-based heavy fermion compounds. The key novelty is the salient correlation between the superconducting transition temperature $T_{\rm c}$ and the valence instability parameter $T_{\rm cr}$, which is in line with theory of enhanced valence fluctuations. Moreover, it is found that, in the pressure region of superconductivity, electrical resistivity is governed by the valence crossover, which most often manifests in scaling behavior. We develop the new idea that the optimum superconducting $T_{\rm c}$ of a given sample is mainly controlled by the compound's $T_{\rm cr}$ and limited by non-magnetic disorder. In this regard, the present study provides compelling evidence for the crucial role of critical valence fluctuations in the formation of Cooper pairs in Ce-based heavy fermion superconductors besides the contribution of spin fluctuations near magnetic quantum critical points, and corroborates a plausible superconducting mechanism in strongly correlated electron systems in general., Comment: Supplementary Material follows after the bibliography

We present the pressure-temperature phase diagram of the antiferromagnet CeAg$_2$Si$_2$ established via resistivity and calorimetry measurements under quasi-hydrostatic conditions up to 22.5~GPa. With increasing pressure, the N\'eel temperature [$T_{\mathrm{N}}(p=0)=8.6$~K] slowly increases up to $T_{\mathrm{N}}=13.4$~K at 9.4~GPa and then vanishes abruptly at the magnetic critical pressure $p_{\mathrm{c}}\sim13$~GPa. For the first time, heavy fermion superconductivity is observed in CeAg$_2$Si$_2$ . Superconductivity emerges at $\sim 11$~GPa and persists over roughly 10~GPa. Partial- and bulk-transition temperatures are highest at $p=16$~GPa, with a maximal $T_{\mathrm{c}^{\rm bulk}}=1.25$~K. In the pressure region of superconductivity, Kondo and crystal-field splitting energies become comparable and resistivity exhibits clear signatures of a Ce-ion valence crossover. The crossover line is located at a rapid collapse in resistivity as function of pressure and extrapolates to a valence transition critical endpoint at critical pressure and temperature of $p_{\rm cr}\sim 17$~GPa and $T_{\rm cr}\sim-13~K$, respectively. Both critical spin and valence fluctuations may build up superconductivity in CeAg$_2$Si$_2$.

We have performed Raman scattering measurements on URu$_2$Si$_2$ single crystals on a large energy range up to $\sim$ 1300 cm$^{-1}$ and in all the Raman active symmetries as a function of temperature down to 15 K. A large excitation, active only in the E$_{g}$ symmetry, is reported. It has been assigned to a crystal electric field excitation on the Uranium site. We discuss how this constrains the crystal electric field scheme of the Uranium ions. Furthermore, three excitations in the A$_{1g}$ symmetry are observed. They have been associated to double Raman phonon processes consistently with ab initio calculations of the phonons dispersion.

The pressure-temperature-phase diagram of the new heavy-fermion superconductor CeAu$_2$Si$_2$ is markedly different from those studied previously. Indeed, superconductivity emerges, not on the verge, but deep inside the magnetic phase. In this context, we have carried out ac-calorimetry, resistivity and thermoelectric power measurements on a CeAu$_2$Si$_2$-single crystal under high-pressure. The principal novelties of this experiment are the observation of a new transition line, presumably a magnetic structure rearrangement at $T_{\mathrm{M}}^{\mathrm{mod}}$ inside the antiferromagnetic phase, and the occurrence of quantum critical behavior in resistivity linked to superconductivity. Strong non-Fermi-liquid behavior is observed around the maximum of superconductivity and enhanced scattering rates are observed close to both the emergence and the maximum of superconductivity. Intriguingly, $T_{\mathrm{M}}^{\mathrm{mod}}$ almost coincides with the onset of the superconducting transition over a broad pressure range, where mysteriously Tc increases with the strengthening of magnetism. A comparison of the features in CeAu$_2$Si$_2$ and its parent compounds CeCu$_2$Si$_2$ and CeCu$_2$Ge$_2$ plotted as function of the unit-cell volume leads us to promote that critical fluctuations of a valence crossover play a crucial role in the superconducting paring mechanism. Our study reveals new intriguing features of magnetism and illustrates a complex interplay between magnetism, quantum criticality and superconductivity., Comment: 10 pages, 10 figures

In the generic phase diagram of heavy fermion systems, tuning an external parameter such as hydrostatic or chemical pressure modifies the superconducting transition temperature. The superconducting phase forms a dome in the temperature-tuning parameter phase diagram, which is associated with a maximum of the superconducting pairing interaction. Proximity to antiferromagnetism suggests a relation between the disappearance of antiferromagnetic order and superconductivity. We combine muon spin rotation, neutron scattering, and x-ray absorption spectroscopy techniques to gain access to the magnetic and electronic structure of CeCo(In$_{1-x}$Cd$_x$)$_5$ at different time scales. Different magnetic structures are obtained that indicate a magnetic order of itinerant character, coexisting with bulk superconductivity. The suppression of the antiferromagnetic order appears to be driven by a modification of the bandwidth/carrier concentration, implying that the electronic structure and consequently the interplay of superconductivity and magnetism is strongly affected by hydrostatic and chemical pressure., Comment: Article + Supplementary information 33 pages, 13 figures

We discuss recent results on the heavy fermion superconductor CeRhIn$_5$ which presents ideal conditions to study the strong coupling between the suppression of antiferromagnetic order and the appearance of unconventional superconductivity. The appearance of superconductivity as function of pressure is strongly connected to the suppression of the magnetic order. Under magnetic field, the re-entrance of magnetic order inside the superconducting state shows that antiferromagnetism nucleates in the vortex cores. The suppression of antiferromagnetism in CeRhIn$_5$ by Sn doping is compared to that under hydrostatic pressure., Comment: 6 pages, 8 figures, to be published in Proc. Int. Conf. Heavy Electrons (ICHE2010) J. Phys. Soc. Jpn. 80 (2011)

We present a detailed analysis of the upper critical field for CeCoIn5 under high pressure. We show that, consistently with other measurements, this system shows a decoupling between maximum of the superconducting transition temperature Tc and maximum pairing strength. This puts forward CeCoIn5 as an important paradigm for this class of unconventional, strongly correlated superconductors., Comment: 15 pages, 5 figures, author version, published in New J. Phys

We report detailed very low temperature resistivity measurements on the heavy fermion compounds Ce_{1-x}La_{x}CoIn5 (x=0 and x=0.01), with current applied in two crystallographic directions [100] (basal plane) and [001] (perpendicular to the basal plane) under magnetic field applied in the [001] or [011] direction. We found a Fermi liquid (\rho \propto T^{2}) ground state, in all cases, for fields above the superconducting upper critical field. We discuss the possible location of a field induced quantum critical point with respect to Hc2(0), and compare our measurements with the previous reports in order to give a clear picture of the experimental status on this long debated issue., Comment: 17 pages, 7 figures accepted for publication in JPSJ

We have determined HC2(T) for FeSe1-xTex (x=0.52) single crystals using resistivity measurements at high static and pulsed magnetic field, as well as specific heat measurements up to 9T. We find that the significant anisotropy of the initial slope of HC2(T) determined from resistivity measurements, is not present when HC2 is determined from the specific heat results. This suggests that the thermodynamic upper critical field is almost isotropic, and that anisotropic vortex dynamics play a role. Further evidence of anisotropic vortex dynamics is found in the behaviour in pulsed field. We also find that Pauli limiting must be included in order to fit the temperature dependence of HC2, indicating probably higher effective mass in FeSe1-xTex than in other Fe superconductors.

The Ce compounds CeCoIn$_5$ and CeRhIn$_5$ are ideal model systems to study the competition of antiferromagnetism (AF) and superconductivity (SC). Here we discuss the pressure--temperature and magnetic field phase diagrams of both compounds. In CeRhIn$_5$ the interesting observation is that in zero magnetic field a coexistence AF+SC phase exist inside the AF phase below the critical pressure $p_{\rm c}^\star \approx 2$ GPa. Above $p_{\rm c}^\star$ AF is suppressed in zero field but can be re-induced by applying a magnetic field. The collapse of AF under pressure coincides with the abrupt change of the Fermi surface. In CeCoIn$_5$ a new phase appears at low temperatures and high magnetic field (LTHF) which vanishes at the upper critical field $H_{\rm c2}$. In both compounds the paramagnetic pair breaking effect dominates at low temperature. We discuss the evolution of the upper critical field under high pressure of both compounds and propose a simple picture of the glue of reentrant magnetism to the upper critical field in order to explain the interplay of antiferromagnetic order and superconductivity., Comment: 6 pages, 7 figures, Manuscript for Proceedings of the International Conference on Quantum Criticality and Novel Phases (QCNP09, Dresden); to appear in pss(b)

The itinerant quasi-ferromagnetic metal MnSi has been studied by detailed thermal expansion measurements under pressures and magnetic fields. A sudden decrease of the volume at the critical pressure Pc ~1.6 GPa has been observed and is in good agreement with the pressure variation of the volume fraction of the spiral magnetic ordering. This confirms that the magnetic order disappears by a first order phase transition. The energy change estimated by the volume discontinuity on crossing Pc is of similar order as the Zeeman energy of the transition from the spiral ground state to a polarized paramagnetic one under magnetic field. In contrast to the strong pressure dependence of the transition temperature, the characteristic fields are weakly pressure dependent, indicating that the strength of the ferromagnetic and the Dzyaloshinskii-Moriya interactions do not change drastically around Pc. The evaluated results of the thermal expansion coefficient and the magnetostriction are analyzed thermodynamically. The Sommerfeld coefficient of the linear temperature term of the specific heat is enhanced just below Pc. The magnetic field-temperature phase diagrams in the ordered and paramagnetic phases are also compared. Comparison is made with other heavy fermion compounds with first order phase transition at 0 K., Comment: 9 pages, 13 figures, accepted to be published in JPSJ

The pressure- and anion-dependent electronic structure of EuX (X=Te, Se, S, O) monochalcogenides is probed with element- and orbital-specific X-ray absorption spectroscopy in a diamond anvil cell. An isotropic lattice contraction enhances the ferromagnetic ordering temperature by inducing mixing of Eu 4{\it f} and 5{\it d} electronic orbitals. Anion substitution (Te $\to$ O) enhances competing exchange pathways through spin-polarized anion {\it p} states, counteracting the effect of the concomitant lattice contraction. The results have strong implications for efforts aimed at enhancing FM exchange interactions in thin films through interfacial strain or chemical substitutions., Comment: To appear in Phys. Rev. Lett

The superconducting state of the heavy fermion PrOs4Sb12 is studied by heat transport measure- ments on a highly homogeneous single crystal exhibiting only one transition peak in the specific heat. The field and temperature dependence of the thermal conductivity confirm multiband super- conductivity and point to fully open gaps on the whole Fermi surface.