Superconductivity and Quantum Criticality in CeCoIn_5

Here we present our experimental and theoretical study of the effects of pressure on the transport properties of the heavy-fermion alloy Ce(1-x)Yb(x)CoIn5 with x~0.07. We specifically choose this value of ytterbium concentration because the magnetic-field-induced quantum critical point, which separates the antiferromagnetic and paramagnetic states at zero temperature, approaches zero, as has been established in previous studies. Our measurements show that pressure further sup...

We study single-crystal CeCoIn5 with uniaxial pressure up to 3.97 kbar applied along the c-axis. We find a non-linear dependence of the superconducting transition temperature Tc on pressure, with a maximum close to 2 kbar. The transition also broadens significantly as pressure increases. We discuss the temperature dependence in terms of the general trend that Tc decreases in anisotropic heavy-fermion compounds as they move towards three-dimensional behavior.

We report the discovery of exotic superconductivity (SC) and novel magnetism in heavy-fermion (HF) compounds, CeCu$_2$Si$_2$, CeRhIn$_5$ and CeIn$_3$ through nuclear-quadrupole-resonance (NQR) measurements under pressure ($P$). The exotic SC in a homogeneous CeCu$_2$Si$_2$ revealed antiferromagnetic critical fluctuations at the border to antiferromagnetism (AFM) or marginal AFM. The uniform mixed phase of SC and AFM in CeCu$_2$(Si$_{1-x}$Ge$_x$)$_2$ emerges on a microscopic l...

The nature of charge degrees-of-freedom distinguishes scenarios for interpreting the character of a second order magnetic transition at zero temperature, that is, a magnetic quantum critical point (QCP). Heavy-fermion systems are prototypes of this paradigm, and in those, the relevant question is where, relative to a magnetic QCP, does the Kondo effect delocalize their $f$-electron degrees-of-freedom. Herein, we use pressure-dependent Hall measurements to identify a finite-te...

Heat and charge transport were used to probe the magnetic field-tuned quantum critical point in the heavy-fermion metal CeCoIn$_5$. A comparison of electrical and thermal resistivities reveals three characteristic energy scales. A Fermi-liquid regime is observed below $T_{FL}$, with both transport coefficients diverging in parallel and $T_{FL}\to 0$ as $H\to H_c$, the critical field. The characteristic temperature of antiferromagnetic spin fluctuations, $T_{SF}$, is tuned to ...

The normal-state Hall effect and magnetoresisitance (MR) have been measured in the quasi-2D heavy fermion superconductor CeCoIn_5. In the non-Fermi liquid region where the reistivity rho_xx exhibits an almost perfect T-linear dependence, the Hall angle varies as cot theta_H propto T^2 and the MR displays a strong violation of Kohler's rule. We demonstrate a novel relation between the MR and the Hall conductivity, Delta rho_xx/rho_xx propto (sigma_xy rho_xx)^2. These results b...

We report magnetic neutron-diffraction and electrical resistivity studies on single crystals of the heavy-fermion antiferromagnet CeRhIn$_{5}$ at pressures up to 2.3 GPa. These experiments show that the staggered moment of Ce and the incommensurate magnetic structure change weakly with applied pressure up to 1.63 GPa, where resistivity, specific heat and NQR measurements confirm the presence of bulk superconductivity. This work places new constraints on an interpretation of t...

We measure the ac susceptibility of single-crystal CeCoIn5 in dc field parallel to the c axis and find further evidence for a high-field phase transition within the superconducting phase in this orientation. We apply up to 2.3 kbar uniaxial pressure along the c axis and discuss the pressure dependence of the high-field phase. We also report the behavior of Hc2 under uniaxial pressure for field along c.

It has been proposed that there are two types of superconductivity in CeCu2Si2, mediated by spin fluctuations at ambient pressure, and by critical valence fluctuations around a charge instability at a pressure P_v \simeq 4.5 GPa. We present in detail some of the unusual features of this novel type of superconducting state, including the coexistence of superconductivity and huge residual resistivity of the order of the Ioffe-Regel limit, large and pressure dependent resistive ...

The main properties and the type of the field-tuned quantum critical point in the heavy-fermion metal CeCoIn$_5$ arisen upon applying magnetic fields $B$ are considered within the scenario based on the fermion condensation quantum phase transition. We analyze the behavior of the effective mass, resistivity, specific heat, charge and heat transport as functions of applied magnetic fields $B$ and show that in the Landau Fermi liquid regime these quantities demonstrate the criti...