Anisotropy of the Upper Critical Field in the Heavy-Fermion Superconductor UTe 2 under Pressure
Résumé
We studied the anisotropy of the superconducting upper critical field H 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 c ≈ 1.5 GPa, and a magnetically ordered state appears. The unusual H c2 (T) at low temperatures for H k a suggests that the multiple superconducting phases which appear under pressure have quite different H c2. For a field applied along the hard magnetization b axis H c2 (0) is glued to the metamagnetic transition H m , which is suppressed near p c. The suppression of H m with pressure follows the decrease of temperature T max , at the maximum in the susceptibility along b. The strong reinforcement of H c2 at ambient pressure for H k b above 16 T is rapidly suppressed under pressure due to the increase of T sc and the decrease of H m. The change in the hierarchy of the anisotropy of H c2 (0) on approaching p c points out that the c axis becomes the hard magnetization axis. In many strongly correlated electron systems unconven-tional superconductivity (SC) appears close to a quantum phase transition where a long range ordered phase is suppressed by tuning a control parameter of the system such as pressure, doping, or charge carrier number. 1,2) It is believed that the enhancement of the magnetic and electronic fluctuations are the glue for the superconducting pairing. This has been most impressively shown for ferromagnetic super-conductors such as URhGe and UCoGe, where the pairing strength itself can be tuned by the magnetic field. 3-5) A magnetic field applied along the easy magnetization axis of these orthorhombic systems lowers the superconducting pairing, while a magnetic field applied along the hard magnetization axis enhances the superconducting pairing strength as the field drives the system to a collapse of the ferromagnetism. This enhancement of the pairing strength for H k b results in the reentrance of SC in the field range of 9-12 T in URhGe 6) and an enhancement of SC in UCoGe. 7) The strong internal exchange field in these ferromagnetic superconductors and the extremely high ratio of the upper critical field H c2 compared to the superconducting transition temperature T sc imposes a non-unitary spin triplet state with equal spin pairing (ESP), which is a superconducting state very rarely realized in bulk materials. Recently the superconducting state below T sc ¼ 1:6 K of the paramagnetic heavy fermion compound UTe 2 has also been proposed to be a spin triplet. 8,9) Evidence for this is obtained from the very large and strongly anisotropic H c2 , which is 6 T for the field applied along the easy magnetiza-tion axis a, 11 T for the c-axis but is extremely field-enhanced up to 35 T for the field along the hard magnetization b-axis (exceeding by far the Pauli limitation for a singlet super-conductor). Above 35 T, where a metamagnetic transition with a huge jump of the magnetization M occurs, 10,11) SC is abruptly suppressed. An additional singularity of UTe 2 is that under pressure multiple superconducting phases occur. 12) T sc is initially linearly suppressed with pressure, but for p > 0:3 GPa two specific heat anomalies are observed and the upper superconducting anomaly increases up to 3 K while the lower continues decreasing in temperature. In that experiment SC is suppressed near 1.7 GPa and a new magnetically ordered phase appears. The increase of T sc by a factor of 2 compared to the ambient pressure value and the collapse of the superconducting regime near 1.7 GPa has been confirmed by resistivity experiments. 12-14) In the present work we first present the pressure dependence of the susceptibility. Next we concentrate on the anisotropy of H c2 under pressure. We show that (i) H c2 for H k a is unusually enhanced at low temperature, (ii) for H k b SC is suppressed at a metamagnetic transition, which decreases in field by increasing pressure, and (iii) for H k c, H c2 crosses that for H k b for p > 1:2 GPa. This could be related to a change in the magneto-crystalline anisotropy and we speculate that the c axis becomes the hardest magnetiza-tion axis at high pressure. Experimental details are given in the Supplemental Material. 15) In Fig. 1(a) we show the magnetic susceptibility as M=H for a field of 1 T applied along the b axis as a function of temperature for different pressures up to 0.9 GPa. At zero pressure, the susceptibility has a maximum at T max % 31:5 K, slightly lower than outside the pressure cell. At zero pressure T max is linked to the metamagnetic transition at H m % 35 T. 10,16) It shows that the same energy scale is responsible for the formation of a correlated electronic regime in zero magnetic field and pushes the system under magnetic field to a metamagnetic transition. 17) In UTe 2 a huge jump of the magnetization ÁM ¼ 0:6 B is observed at H m at p ¼ 0, while the susceptibility @M=@H is almost unchanged below and above H m. The maximum of the susceptibility T max decreases under pressure, and at 0.9 GPa we find T max % 20 K. The absolute value of M=H at low temperature is inversely proportional to T max and increases under pressure
Domaines
Matière Condensée [cond-mat]
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