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Register a new userEvidence for freezing of charge degrees of freedom across a critical point in CeCoIn$_5$
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The presence of a quantum critical point separating two distinct zero-temperature phases is thought to underlie the `strange metal state of many high-temperature superconductors. The nature of this quantum critical point, as well as a description of the resulting strange metal, are central open problems in condensed matter physics. In large part, the controversy stems from the lack of a clear broken symmetry to characterize the critical phase transition, and this challenge is no clearer than in the example of the unconventional superconductor CeCoIn$_5$. Through Hall effect and Fermi surface measurements of CeCoIn$_5$, in comparison to ab initio calculations, we find evidence for a critical point that connects two Fermi surfaces with different volumes without apparent symmetry-breaking, indicating the presence of a transition that involves an abrupt localization of one sector of the charge degrees of freedom. We present a model for the anomalous electrical Hall resistivity of this material based on the conductivity of valence charge fluctuations.
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Quantum criticality in the normal and superconducting state of the heavy-fermion metal CeCoIn$_5$ is studied by measurements of the magnetic Gr{u}neisen ratio, $Gamma_H$, and specific heat in different field orientations and temperatures down to 50 mK. Universal temperature over magnetic field scaling of $Gamma_H$ in the normal state indicates a hidden quantum critical point at zero field. Within the superconducting state the quasiparticle entropy at constant temperature increases upon reducing the field towards zero, providing additional evidence for zero-field quantum criticality.
The thermal Hall conductivity $kappa_{xy}$ and Hall conductivity $sigma_{xy}$ in CeCoIn$_5$ are used to determine the Lorenz number ${cal L}_H$ at low temperature $T$. This enables the separation of the observed thermal conductivity into its electronic and non-electronic parts. We uncover evidence for a charge-neutral, field-dependent thermal conductivity, which we identify with spin excitations. At low $T$, these excitations dominate the scattering of charge carriers. We show that suppression of the spin excitations in high fields leads to a steep enhancement of the electron mean-free-path, which leads to an interesting scaling relation between the magnetoresistance, thermal conductivity and $sigma_{xy}$.
The Ce compounds CeCoIn$_5$ and CeRhIn$_5$ are ideal model systems to study the competition of antiferromagnetism (AF) and superconductivity (SC). Here we discuss the pressure--temperature and magnetic field phase diagrams of both compounds. In CeRhIn$_5$ the interesting observation is that in zero magnetic field a coexistence AF+SC phase exist inside the AF phase below the critical pressure $p_{rm c}^star approx 2$ GPa. Above $p_{rm c}^star$ AF is suppressed in zero field but can be re-induced by applying a magnetic field. The collapse of AF under pressure coincides with the abrupt change of the Fermi surface. In CeCoIn$_5$ a new phase appears at low temperatures and high magnetic field (LTHF) which vanishes at the upper critical field $H_{rm c2}$. In both compounds the paramagnetic pair breaking effect dominates at low temperature. We discuss the evolution of the upper critical field under high pressure of both compounds and propose a simple picture of the glue of reentrant magnetism to the upper critical field in order to explain the interplay of antiferromagnetic order and superconductivity.
We present nuclear magnetic resonance (NMR) measurements on the three distinct In sites of CeCoIn$_5$ with magnetic field applied in the [100] direction. We identify the microscopic nature of the long range magnetic order (LRO) stabilized at low temperatures in fields above 10.2 T while still in the superconducting (SC) state. We infer that the ordered moment is oriented along the $hat c$-axis and map its field evolution. The study of the field dependence of the NMR shift for the different In sites indicates that the LRO likely coexists with a modulated SC phase, possibly that predicted by Fulde, Ferrell, Larkin, and Ovchinnikov. Furthermore, we discern a field region dominated by strong spin fluctuations where static LRO is absent and propose a revised phase diagram.
The heavy-fermion superconductor CeCoIn$_5$ displays an additional transition within its superconducting (SC) state, whose nature is characterized by high-precision studies of the isothermal field dependence of the entropy, derived from combined specific heat and magnetocaloric effect measurements at temperatures $Tgeq 100$ mK and fields $Hleq 12$ T aligned parallel, perpendicular and $18^circ$ off the tetragonal [100] direction. For any of these conditions, we do not observe an additional entropy contribution upon tuning at constant temperature by magnetic field from the homogeneous SC into the presumed Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) SC state. By contrast, for $Hparallel [100]$ a negative isothermal entropy contribution, compatible with spin-density-wave (SDW) ordering, is found. Our data exclude the formation of a FFLO state in CeCoIn$_5$ for out-of-plane field directions, where no SDW order exists.
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