Spatially Inhomogeneous Superconducting State near H_c2 in UPd₂Al₃

Abstract

We have performed 27 Al-NMR measurements on single-crystalline UPd2Al3 with the field parallel to the c axis to investigate the superconducting (SC) properties near the upper critical field of superconductivity Hc2. The broadening of the NMR linewidth below 14 K indicates the appearance of the internal field at the Al site, which originates from the antiferromagnetically ordered moments of U 5f electrons. In the SC state well below µ0Hc2 = 3.4 T, the broadening of the NMR linewidth due to the SC diamagnetism and a decrease in the Knight shift are observed, which are well-understood by the framework of spin-singlet superconductivity. In contrast, the Knight shift does not change below Tc(H), and the NMR spectrum is broadened symmetrically in the SC state in the field range of 3 T < µ0H < µ0Hc2. The unusual NMR spectrum near Hc2 suggests that a spatially inhomogeneous SC state such as the Fulde-Ferrell-LarkinOvchinnikov (FFLO) state would be realized.Superconductivity in the presence of magnetic fields close to the upper critical field (H c2 ) is the critical state where the intriguing physical phenomena are anticipated. In general, there are two well-known pair-breaking mechanisms in a type-II superconductor under magnetic fields. One is the orbital pair-breaking effect related to the emergence of Abrikosov vortices, and the superconductivity is destroyed at the vortex cores. The other is the Pauli pair-breaking effect, which originates from the Zeeman splitting of spin-singlet Cooper pairs. When the Zeeman-splitting energy is as large as the condensation energy of superconductivity, the superconductivity becomes unstable and transitions to the normal state. The strong contribution of the Pauli pair-breaking effect leads to many interesting issues, such as the realization of the spatially modulated superconducting (SC) state, which is called the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. 1, 2) For the realization of the FFLO state, a very clean system is necessary; thus, not many compounds have been pointed out as candidates for the FFLO state.UPd 2 Al 3 is one such candidate for the FFLO state. UPd 2 Al 3 has a hexagonal PrNi 2 Al 3 -type structure with the space group P 6/mmm and exhibits antiferromagnetic (AFM) order at the Néel temperature T N = 14.5 K with a commensurate wave vector Q AF = (0, 0, 0.5) and well-localized magnetic moment of ∼ 0.85 µ B /U. 3, 4) After the AFM transition occurs, superconductivity is observed below the SC transition temperature T c = 2 K. 5) NMR and µSR measurements provide evidence for a spin-singlet Cooper pairing from the clear decrease in the Knight shift below T c . 6, 7) Large specific-heat jump at T c 5) indicates that heavy electrons form Cooper pairs; thus, the orbital pair-breaking field becomes large. As a result, the suppression of superconductivity by the Pauli pair-breaking effect is expected. Moreover, a pronounced hysteresis behavior has been observed in a narrow field range below H c2 , H * < H < H c2 , with several different experiments, such as magnetizatio...

Unconventional superconductivity in UTe2

“…In UTe 2 , by adopting Δγ el ∼ 60 mJ mol −1 K −2 [6] and A hf determined from a K -χ plot [44], ΔK γ along the b and c axes are estimated and are shown in table 3, where we used (μ eff ) 2 = g 2 j( j + 1)μ 2 B = 3μ 2 B for the free-electron value (g = 2, j = 1/2) and R = 1. In the table 3, the experimental reduction of K in the SC state ΔK spin and these values in UPd 2 Al 3 [125,126] and URu 2 Si 2 [127] are also shown. It is noted that ΔK γ determined by this method is in good agreement with ΔK spin in UPd 2 Al 3 [124] and URu 2 Si 2 [127], which are identified as a spin-singlet superconductors.…”

Review of U-based Ferromagnetic Superconductors: Comparison between UGe2, URhGe, and UCoGe

“…Actually, a clear decrease in the Knight shift was reported in the U-based superconductor UPd 2 Al 3 , which is a spin-singlet superconductor coexisting with the antiferromagnetism. 127,128) For a spin-triplet superconductor, the spin component of the Cooper pair is expressed by the SC d vector, which is defined to be perpendicular to the spin component of the Cooper pair, and the Knight shift decreases below T SC when H is applied parallel to the d vector fixed along a certain crystal axis. However, the situation is not so simple in the present FM superconductors.…”

Evidence for the Presence of the Fulde-Ferrell-Larkin-Ovchinnikov State in CeCu2Si2 Revealed Using Kitagawa1, Nakamine2, Ishida3 et al. 2018Phys. Rev. Lett.Self Cite453190View full textAdd to dashboardCite

“…It is noted that such an enhancement of 1/T 1 T was not observed in UPd 2 Al 3 which is also a candidate for observing the FFLO state. We recently performed 27 Al-NMR measurements on single-crystalline UPd 2 Al 3 with the field parallel to the c axis, and we discovered an unexpected symmetrical broadening in the NMR spectra in the field range 3 T < µ 0 H < µ 0 H c2 = 3.6 T, suggesting the presence of a spatially inhomogeneous SC state such as the FFLO state [37]. However in the case of UPd 2 Al 3 , 1/T 1 T did not increase, remaining at a constant value just below µ 0 H c2 (T ) before beginning to decrease below 3 T, in contrast to the results obtained using CeCu 2 Si 2 and κ-(BEDT-TTF) 2 Cu(NCS) 2 .…”