Laboratoire National des Champs Magnétiques Pulsés CNRS – INSA

Metals and Superconductors
Contributors: A. Demuer, I. Sheikin, F. Levy (post-doc)
Collaborators: A. Huxley (University of Edinburgh, U.K.), R. Lortz (Hong Kong University, Hong Kong), K.
Behnia (ESPCI, Paris), D. Aoki (CEA-Grenoble), P. Haen, R. Rodiere, P. Lejay (CNRS-Institut Néel,
Grenoble), R. Settai (University of Osaka, Japan)
The most remarkable and significant results were obtained on the ferromagnetic superconductor
URhGe. This compound undergoes a ferromagnetic transition at TCurie of 9.5 K, below which the magnetic
moments align along the c-axis of the orthorhombic crystal structure. Superconductivity then emerges in
high quality samples below Tc of 250 mK, a temperature much lower than that of the ferromagnetic
transition. We found that in URhGe there is a first order magnetic transition where the direction of the spin
axis changes when a magnetic field of 12 Tesla is applied parallel to the crystal b-axis. We then discovered
that a second pocket of superconductivity occurs at low temperature for a range of fields enveloping this
magnetic transition, well above the field of 2 Tesla at which superconductivity is first destroyed.
Remarkably, the re-entrant superconductivity is of an origin different from the Jaccarino-Peter effect. When
the magnetic field is inclined towards the c-axis, both the spin rotation transition and the re-entrant
superconductivity move to higher fields. The transition becomes broader and the spins are no longer aligned
with the field above the transition. The magnetic transition changes to second order and then vanishes when
the magnetic field is at a few degrees from b- to the c-axis giving rise to a quantum critical point. The fieldinduced
superconductivity then disappears at a small angle of about 7 degrees. The established phase
diagram implies that it is the quantum critical point that is at the origin of the field-induced superconductivity
in URhGe. As the next step of our study, we applied the magnetic field in the crystal (ab) plane; a-axis is the
hard magnetic axis. We found that in this case the spin-rotation transition does not depend on the acomponent
of the applied field and always occurs when the b-axis projection of the field reaches 12 Tesla.
As regards the re-entrant superconductivity, it follows the spin rotation transition over a wide angular range
and was observed up to 32 Tesla, the highest field available in the laboratory at the time of the experiment.
Our findings strongly suggest that excitations in which the spins rotate stimulate superconductivity in the
neighborhood of a quantum phase transition under high magnetic field. The analysis of the coherence length
suggests that even the low field superconductivity is due to the tail of the excitations developing at the
quantum critical point. This implies that both low and high field superconducting phases are of the same
origin. This finding sheds light on the origin of superconductivity in other ferromagnets, notably recently
discovered UCoGe.
The specific heat and magnetic torque of the layered organic superconductor κ-(BEDT-
TTF)2Cu(NCS)2 have been studied in magnetic fields up to 28 T applied perpendicular and parallel to the
superconducting layers. In parallel fields above 21 T, the superconducting transition becomes first order,
which signals that the Pauli-limiting field is reached. Instead of saturating at this field value, the upper
critical field increases sharply and a second first-order transition line appears within the superconducting
phase. Our results give strong evidence that the phase, which separates the homogeneous superconducting
state from the normal state, is a realization of a Fulde-Ferrel-Larkin-Ovchinnikov state.
In CeCoIn5, an intriguing case of a heavy fermion superconductor, we performed high field
measurements of the Nernst and de Haas-van Alphen effects at low temperature and up to 28 Tesla. The
field-dependence of the Nernst coefficient in the 12 - 28 Tesla field range reveals new features emerging at
high magnetic field. The anomaly detected at HK ~ 23 T is similar to what is observed in CeRu2Si2 at Hm ~
7.8 T where a metamagnetic transition occurs. Fourier spectra of the de Haas-van Alphen oscillations
indicate that new frequencies also appear above 23 Tesla. Furthermore, the Dingle temperature was found to
decrease by about 30% at 23 Tesla. Thus, two independent sets of evidence point to the existence of a new
field scale in CeCoIn5 at 23 T. Based on the Nernst coefficient anomalies and de Haas-van Alphen results,
the previously determined magnetic phase diagram of CeCoIn5 is revised. Later, we observed a similar
behavior of the high field de Haas-van Alphen effect in another compound of the same family, CeIrIn5. This
suggests that it might be a common feature for the whole family of Ce 115 compounds.
In CeRh2Si2 we performed low temperature transport, specific heat, magnetostriction measurements
in fields up to 28 Tesla as well de Haas-van Alphen effect measurements up to 32 Tesla. When a magnetic
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