G. Knebel

Coexistence of antiferromagnetism and superconductivity in CeRhIn5 under high pressure and magnetic field

We report on detailed ac calorimetry measurements under high pressure and magnetic field of CeRhIn5. Under hydrostatic pressure the antiferromagnetic order vanishes near p_c*=2 GPa due to a first order transition. Superconductivity is found for pressures above 1.5 GPa inside the magnetic ordered phase. However, the superconductivity differ from the pure homogeneous superconducting ground state above 2 GPa. The application of an external magnetic field H || ab induces a transition inside the superconducting state above pc* which is strongly related to the re-entrance of the antiferromagnetism with field. This field-induced supplementary state vanishes above the quantum critical point in this system. The analogy to CeCoIn5 is discussed.

Similarity of the Fermi surface in the hidden order state and in the antiferromagnetic state of URu₂Si₂.

Shubnikov-de Haas measurements of high quality URu2Si2 single crystals reveal two previously unobserved Fermi surface branches in the so-called hidden order phase. Therefore, about 55% of the enhanced mass is now detected. Under pressure in the antiferromagnetic state, the Shubnikov-de Haas frequencies for magnetic fields applied along the crystalline c axis show little change compared with the zero pressure data. This implies a similar Fermi surface in both the hidden order and antiferromagnetic states, which strongly suggests that the lattice doubling in the antiferromagnetic phase due to the ordering vector Q(AF)=(001) already occurs in the hidden order. These measurements provide a good test for existing or future theories of the hidden order parameter.

Tricritical point and wing structure in the itinerant ferromagnet UGe

Precise resistivity measurements on the ferromagnetic superconductor UGe2 under pressure p and magnetic field H reveal a previously unobserved change of the anomaly at the Curie temperature. Therefore, the tricritical point (TCP) where the paramagnetic-to-ferromagnetic transition changes from a second order to a first order transition is located in the p-T phase diagram. Moreover, the evolution of the TCP can be followed under the magnetic field in the same way. It is the first report of the boundary of the first order plane which appears in the p-T-H phase diagram of weak itinerant ferromagnets. This line of critical points starts from the TCP and will terminate at a quantum critical point. These measurements provide the first estimation of the location of the quantum critical point in the p-H plane and will inspire similar studies of the other weak itinerant ferromagnets.

Localization of 4f state in YbRh2Si2 under magnetic field and high pressure : Comparison with CeRh2Si2

We present detailed measurements of the electrical resistivity and microcalorimetry under magnetic field and high pressure which confirm the proximity of YbRh 2 Si 2 to a quantum critical point at ...

Itinerant metamagnetism of CeRu2Si2: bringing out the dead. Comparison with the new Sr3Ru2O7 case

Focus is given on the macroscopic and microscopic experimental works realized during a decade on the clear case of itinerant metamagnetism in the heavy fermion paramagnetic compound CeRu2Si2. Emphasis is made on the feedback between the band structure, the exchange coupling and the lattice instability. Sweeps in magnetic field, pressure and temperature feel the pseudogap of this strongly correlated electronic system as well as its equivalent CeRu2Ge2 at a fictitious negative pressure. Some mysteries persist as the complete observation of the Fermi surface above the metamagnetic field HM and the detection of the dynamical ferromagnetic fluctuation near HM. The novelty of the bilayer ruthenate Sr3Ru2O7 is discussed by comparison. Despite differences in spin and electronic dimensionality many common trends emerge.

Observation of Spin Susceptibility Enhancement in the Possible Fulde-Ferrell-Larkin-Ovchinnikov State of CeCoIn 5

We report (115)In nuclear magnetic resonance measurements of the heavy-fermion superconductor CeCoIn(5) in the vicinity of the superconducting critical field H(c2) for a magnetic field applied perpendicular to the ĉ axis. A possible inhomogeneous superconducting state, the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, is stabilized in this part of the phase diagram. In an 11 T applied magnetic field, we observe clear signatures of the two phase transitions: the higher temperature one to the homogeneous superconducting state and the lower temperature phase transition to a FFLO state. We find that the spin susceptibility in the putative FFLO state is significantly enhanced as compared to the value in a homogeneous superconducting state. The implications of this finding for the nature of the low temperature phase are discussed.

Spin-fluctuation-dominated electrical transport of Ni 3 Al at high pressure

We present the first study of a magnetic quantum phase transition in the itinerant-electron ferromagnet Ni3Al at high pressures. Electrical resistivity measurements in a diamond anvil cell at hydrostatic pressures up to 100 kbar and temperatures as low as 50 mK indicate that the Curie temperature collapses towards absolute zero at a critical pressure pc=82(2) kbar. Over wide ranges in pressure and temperature, both in the ferromagnetic and paramagnetic states, the temperature variation of the resistivity is found to deviate from the conventional Fermi-liquid form. We consider the extent to which this deviation can be understood in terms of a mean-field model of enhanced spin fluctuations on the border of ferromagnetism in three dimensions.

Superconductivity Reinforced by Magnetic Field and the Magnetic Instability in Uranium Ferromagnets

We review our recent results on ferromagnetic superconductors, URhGe and UCoGe. High quality single crystals of both compounds were successfully grown. The specific heat shows a clear jump related to the superconducting transition in UCoGe. The finite values of C / T at 0 K are discussed in terms of the self-induced vortex state and the value of the ordered moment. With increasing fields for H ∥ b -axis in URhGe, the jump of thermal expansion increases and shifts to lower temperature. The re-entrant and S-shaped superconducting phases for URhGe and UCoGe respectively are explained by the unusual field dependence of the effective mass, which is induced by the ferromagnetic instability when the field is applied along the hard magnetization b -axis. The magnetic fluctuations are very sensitive to the field orientation. This is reflected in the H c2 and the anisotropy of the effective mass.

Magnetically ordered Kondo lattice in YbNi2Ge2 at high pressure

We have studied the electrical resistivity of YbNi2Ge2 under hydrostatic pressure up to 100 kbar. With increasing pressure the system is continuously tuned from an intermediate valence state at low pressure to a magnetically ordered Kondo lattice at high pressure. The critical pressure for the appearance of a magnetic ground state is near 50 kbar. Below Pc the resistivity shows a T2 dependence, characteristic of a Fermi liquid description at low temperatures. The coefficient of the T2 term increases drastically on approaching Pc. Above Pc, the magnetic transition temperature Tm increases linearly with pressure. The magnetoresistance at high pressure suggests the importance of ferromagnetic coupling.

Field-induced non-Fermi-liquid resistivity of stoichiometric YbAgGe single crystals

We have investigated hexagonal