87 research outputs found

    Fermi Surface Instabilities in Ferromagnetic Superconductor URhGe

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    The field-reentrant (field-reinforced) superconductivity on ferromagnetic superconductors is one of the most interesting topics in unconventional superconductivity. The enhancement of effective mass and the induced ferromagnetic fluctuations play key roles for reentrant superconductivity. However, the associated change of the Fermi surface, which is often observed at (pseudo-) metamagnetic transition, can also be a key ingredient. In order to study the Fermi surface instability, we performed Hall effect measurements in the ferromagnetic superconductor URhGe. The Hall effect of URhGe is well explained by two contributions, namely by the normal Hall effect and by the large anomalous Hall effect due to skew scattering. The large change in the Hall coefficient is observed at low fields between the paramagnetic and ferromagnetic states for H // c-axis (easy-magnetization axis) in the orthorhombic structure, indicating that the Fermi surface is reconstructed in the ferromagnetic state below the Curie temperature (T_Curie=9.5K). At low temperatures (T << T_Curie), when the field is applied along the b-axis, the reentrant superconductivity was observed in both the Hall resistivity and the magnetoresistance below 0.4K. Above 0.4K, a large jump with the first-order nature was detected in the Hall resistivity at a spin-reorientation field H_R ~ 12.5T, demonstrating that the marked change of the Fermi surface occurs between the ferromagnetic state and the polarized state above H_R. The results can be understood by the Lifshitz-type transition, induced by the magnetic field or by the change of the effective magnetic field.Comment: 7 pages, 6 figures, accepted for publication in J. Phys. Soc. Jp

    Collapse of ferromagnetism and Fermi surface instability near reentrant superconductivity of URhGe

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    We present thermoelectric power and resistivity measurements in the ferromagnetic superconductor URhGe for magnetic field applied along the hard magnetization b axis of the orthorhombic crystal. Reentrant superconductivity is observed near the the spin reorientation transition at HRH_{R}=12.75 T, where a first order transition from the ferromagnetic to the polarized paramagnetic state occurs. Special focus is given to the longitudinal configuration, where both electric and heat current are parallel to the applied field. The validity of the Fermi-liquid T2T^2 dependence of the resistivity through HRH_R demonstrates clearly that no quantum critical point occurs at HRH_R. Thus the ferromagnetic transition line at HRH_R becomes first order implying the existence of a tricritical point at finite temperature. The enhancement of magnetic fluctuations in the vicinity of the tricritical point stimulates the reentrance of superconductivity. The abrupt sign change observed in the thermoelectric power with the thermal gradient applied along the b axis together with the strong anomalies in the other directions is a definitive macroscopic evidence that in addition a significant change of the Fermi surface appears through HRH_R.Comment: 6 pages, 5 figure

    Unconventional Strong Spin-Fluctuation Effects around the Critical Pressure of the Itinerant Ising-Type Ferromagnet URhAl

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    Resistivity measurements were performed for the itinerant Ising-type ferromagnet URhAl at temperatures down to 40 mK under high pressure up to 7.5 GPa, using single crystals. We found that the critical pressure of the Curie temperature exists at around PcP_c ~ 5.2 GPa. Near PcP_c, the AA-coefficient of the AT2AT^{2} Fermi-liquid resistivity term below TT^* is largely enhanced with a maximum around 5.2-5.5 GPa. Above PcP_c, the exponent of the resistivity ρ(T)\rho(T) deviates from 2. At PcP_c, it is close to n=5/3n = 5/3, which is expected by the theory of three-dimensional ferromagnetic spin fluctuations for a 2nd-order quantum-critical point (QCP). However, TC(P)T_C(P) disappears as a 1st-order phase transition, and the critical behavior of resistivity in URhAl cannot be explained by the theory of a 2nd-order QCP. The 1st-order nature of the phase transition is weak, and the critical behavior is still dominated by the spin fluctuation at low temperature. With increasing pressure, the non-Fermi-liquid behavior is observed in higher fields. Magnetic field studies point out a ferromagnetic wing structure with a tri-critical point (TCP) at ~ 4.8-4.9 GPa in URhAl. One open possibility is that the switch from the ferromagnetic to the paramagnetic states does not occur simply but an intermediate state arises below the TCP as suggested theoretically recently. Quite generally, if a drastic Fermi-surface change occurs through PcP_c, the nature of the interaction itself may change and lead to the observed unconventional behavior.Comment: 9 pages, 9 figure

    Magnetic-Field Control of Quantum Critical Points of Valence Transition

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    We study the mechanism how critical end points of first-order valence transitions are controlled by a magnetic field. We show that the critical temperature is suppressed to be a quantum critical point (QCP) by a magnetic field and unexpectedly the QCP exhibits nonmonotonic field dependence in the ground-state phase diagram, giving rise to emergence of metamagnetism even in the intermediate valence-crossover regime. The driving force of the field-induced QCP is clarified to be cooperative phenomena of Zeeman effect and Kondo effect, which create a distinct energy scale from the Kondo temperature. This mechanism explains peculiar magnetic response in CeIrIn5 and metamagnetic transition in YbXCu4 for X=In as well as sharp contrast between X=Ag and Cd.Comment: 4 pages, 4 figure

    Electronic nematicity in URu2Si2 revisited

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    The nature of the hidden-order (HO) state in URu2Si2 remains one of the major unsolved issues in heavy-fermion physics. Recently, torque magnetometry, x-ray diffraction and elastoresistivity data have suggested that the HO phase transition at THO = 17.5 K is driven by electronic nematic effects. Here, we search for thermodynamic signatures of this purported structural instability using anisotropic thermal-expansion, Young\'s modulus, elastoresistivity and specific-heat measurements. In contrast to the published results, we find no evidence of a rotational symmetry-breaking in any of our data. Interestingly, our elastoresistivity measurements, which are in full agreement with published results, exhibit a Curie-Weiss divergence, which we however attribute to a volume and not to a symmetry-breaking effect. Finally, clear evidence for thermal fluctuations is observed in our heat-capacity data, from which we estimate the HO correlation length.Comment: 4 Figures, 5 page

    Thermodynamic Investigation of Metamagnetism in Pulsed High Magnetic Fields on Heavy Fermion Superconductor UTe2_2

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    We investigated the thermodynamic property of the heavy fermion superconductor UTe2_2 in pulsed high magnetic fields. The superconducting transition in zero field was observed at TcT_{\rm c}=1.65 K as a sharp heat capacity jump. Magnetocaloric effect measurements in pulsed-magnetic fields obviously detected a thermodynamic anomaly accompanied by a first-order metamagnetic transition at μ\mu0_{0}HmH_{\rm m}=36.0 T when the fields are applied nearly along the hard-magnetization bb-axis. From the results of heat capacity measurements in magnetic fields, we found a drastic diverging electronic heat capacity coefficient of the normal state γ\gammaN_{\rm N} with approaching HmH_{\rm m}. Comparing with the previous works via the magnetic Clausius-Clapeyron relation, we unveil the thermodynamic details of the metamagnetic transition. The enhancement of the effective mass observed as the development of γN\gamma_{\rm N} indicates that quantum fluctuation strongly evolves around HmH_{\rm m}; it assists the superconductivity emerging even in extremely high fields.Comment: 6 pages, 6 figures, accepted for publication in JPS
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