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Register a new userThermoelectric response near a quantum critical point: the case of CeCoIn5
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We present a study of thermoelectric coefficients in CeCoIn_5 down to 0.1 K and up to 16 T in order to probe the thermoelectric signatures of quantum criticality. In the vicinity of the field-induced quantum critical point, the Nernst coefficient nu exhibits a dramatic enhancement without saturation down to lowest measured temperature. The dimensionless ratio of Seebeck coefficient to electronic specific heat shows a minimum at a temperature close to threshold of the quasiparticle formation. Close to T_c(H), in the vortex-liquid state, the Nernst coefficient behaves anomalously in puzzling contrast with other superconductors and standard vortex dynamics.
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This paper demonstrates the anisotropic response of quantum critical fluctuations with respect to the direction of the magnetic field $B$ in Ni-doped CeCoIn$_5$ by measuring the magnetization $M$ and specific heat $C$. The results show that $M/B$ at $B=0.1 {rm T}$ for both the tetragonal $c$ and $a$ directions exhibits $T^{-eta}$ dependencies, and that $C/T$ at $B=0$ follows a $-ln T$ function, which are the characteristics of non-Fermi-liquid (NFL) behaviors. For $B,||,c$, both the $M/Bpropto T^{-eta}$ and $C/T propto -ln T$ dependencies change into nearly temperature-constant behaviors by increasing $B$, indicating a crossover from the NFL state to the Fermi-liquid state. For $B,||,a$, however, the NFL behavior in $C/T$ persists up to $B=7 {rm T}$, whereas $M/B$ exhibits temperature-independent behavior for $Bge 1 {rm T}$. These contrasting characteristics in $M/B$ and $C/T$ reflect the anisotropic nature of quantum critical fluctuations; the $c$-axis spin component significantly contributes to the quantum critical fluctuations. We compare this anisotropic behavior of the spin fluctuations to superconducting properties in pure CeCoIn$_5$, especially to the anisotropy in the upper critical field and the Ising-like characteristics in the spin resonance excitation, and suggest a close relationship between them.
The thermoelectric coefficients have been measured on the Yb-based heavy fermion compounds beta-YbAlB4 and YbRh2Si2 down to a very low temperature. We observe a striking difference in the behavior of the Seebeck coefficient, S in the vicinity of the Quantum Critical Point (QCP) in the two systems. As the critical field is approached, S/T enhances in beta-YbAlB4 but is drastically reduced in YbRh2Si2. While in the former system, the ratio of thermopower-to-specific heat remains constant, it drastically drops near the QCP in YbRh2Si2. In both systems, on the other hand, the Nernst coefficient shows a diverging behavior near the QCP. The results provide a new window to the way various energy scales of the system behave and eventually vanish near a QCP.
Measurements of specific heat and electrical resistivity in magnetic fields up to 9 T along [001] and temperatures down to 50 mK of Sn-substituted CeCoIn5 are reported. The maximal -ln(T) divergence of the specific heat at the upper critical field H_{c2} down to the lowest temperature characteristic of non-Fermi liquid systems at the quantum critical point (QCP), the universal scaling of the Sommerfeld coefficient, and agreement of the data with spin-fluctuation theory, provide strong evidence for quantum criticality at H_{c2} for all x < 0.12 in CeCoIn5-xSnx. These results indicate the accidental coincidence of the QCP located near H_{c2} in pure CeCoIn5, in actuality, constitute a novel quantum critical point associated with unconventional superconductivity.
The thermal conductivity kappa of the heavy-fermion metal CeCoIn5 was measured in the normal and superconducting states as a function of temperature T and magnetic field H, for a current and field parallel to the [100] direction. Inside the superconducting state, when the field is lower than the upper critical field Hc2, kappa/T is found to increase as T approaches absolute zero, just as in a metal and in contrast to the behavior of all known superconductors. This is due to unpaired electrons on part of the Fermi surface, which dominate the transport above a certain field. The evolution of kappa/T with field reveals that the electron-electron scattering (or transport mass m^*) of those unpaired electrons diverges as H approaches Hc2 from below, in the same way that it does in the normal state as H approaches Hc2 from above. This shows that the unpaired electrons sense the proximity of the field-tuned quantum critical point of CeCoIn5 at H^* = Hc2 even from inside the superconducting state. The fact that the quantum critical scattering of the unpaired electrons is much weaker than the average scattering of all electrons in the normal state reveals a k-space correlation between the strength of pairing and the strength of scattering, pointing to a common mechanism, presumably antiferromagnetic fluctuations.
The elastic neutron scattering experiments were carried out on the solid solutions CeRh_{1-x}Co_xIn_5 to clarify the nature of the antiferromagnetic (AF) state in the vicinity of the quantum critical point (QCP): x_c ~0.8. The incommensurate AF order with the wave vector of q_h=(1/2,1/2,~0.3) observed in pure CeRhIn_5 is weakly suppressed upon doping with Co, and a commensurate q_c=(1/2,1/2,1/2) and an incommensurate q_1=(1/2,1/2,~0.42) AF structures evolve at intermediate Co concentrations. These AF orders are enhanced at x=0.7, and furthermore the q_h AF order vanishes. These results suggest that the AF correlations with the q_c and q_1 modulations are significantly enhanced in the intermediate x range, and may be connected with the evolution of the superconductivity observed above x~0.3.
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