The quasiparticle density of states in correlated and quantum-critical metals directly probes the effect of electronic correlations on the Fermi surface. Measurements of the nuclear spin-lattice relaxation rate provide one such experimental probe of quasiparticle mass through the electronic density of states. By far the most common way of accessing the spin-lattice relaxation rate is via nuclear magnetic resonance and nuclear quadrupole resonance experiments, which require resonant excitation of nuclear spin transitions. Here we report nonresonant access to spin-lattice relaxation dynamics in AC-calorimetric measurements. The nuclear spin-lattice relaxation rate is inferred in our measurements from its effect on the frequency dispersion of the thermal response of the calorimeter-sample assembly. We use fast, lithographically defined nanocalorimeters to access the nuclear spin-lattice relaxation times in metallic indium from 0.3 to 7 K and in magnetic fields up to 35 T.

The electronic density of states, and, hence, the quasiparticle mass on the Fermi surface, is strongly enhanced through electronic correlations in quantum-critical metals. The nature of electronic correlations in such systems can be constrained by comparing different probes of the electronic density of states. Comparative studies in high-Tc superconductors present a significant challenge because of the masking effect of the superconducting phase. In contrast, the normal state can be readily accessed in the unconventional superconductor CeCoIn₅ because the energy scale associated with superconductivity is small. Here, we use thermal impedance spectroscopy to simultaneously access the electronic density of states in CeCoIn₅ in two independent ways; via the nuclear spin-lattice relaxation rate and via the electronic specific heat. We establish that the temperature and magnetic field dependence of the nuclear spin-lattice relaxation rate is determined entirely by the electronic density of states on the Fermi surface, where mass enhancement is cut off at high magnetic fields. Surprisingly, the specific heat reveals excess entropy in addition to that associated with the density of states on the Fermi surface. The electronic nature of this excess entropy is evidenced by its suppression in the superconducting state. We postulate that a second 'flavor' of boson generates the observed quantum critical physics beyond the mass renormalization on the Fermi surface in CeCoIn₅ and suggest such a multi-flavor character for a broader range of quantum critical metals.

Employing the elemental sensitivity of x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), we study the valence and magnetic order in the heavy fermion superconductor CeCoIn₅. We probe spin population of the f-electrons in Ce and d-electrons in Co as a function of temperature (down to 0.1 K) and magnetic field (up to 6 T). From the XAS we find a pronounced contribution of Ce⁴⁺ component at low temperature and a clear temperature dependence of the Ce valence below 5 K, suggesting enhanced valence fluctuations, an indication for the presence of a nearby quantum critical point (QCP). We observe no significant corresponding change with magnetic field. The XMCD displays a weak signal for Ce becoming clear only at 6 T. This splitting of the Kramers doublet ground state of Ce³⁺ is significantly smaller than expected for independent but screened ions, indicating strong antiferromagnetic pair interactions. The unconventional character of superconductivity in CeCoIn₅ is evident in the extremely large specific heat step at the superconducting transition.

Rare-earth element containing aperiodic quasicrystals and their related periodic approximant crystals can exhibit non-trivial physical properties at low temperatures. Here, we investigate the 1/1 and 2/1 approximant crystal phases of the Ce-Au-Al system by studying the ac-susceptibility and specific heat at low temperatures and in magnetic fields up to 12 T. We find that these systems display signs of quantum criticality similar to the observations in other claimed quantum critical systems, including the related Yb-Au-Al quasicrystal. In particular, the ac-susceptibility at low temperatures shows a diverging behavior χ ∝1/T as the temperature decreases as well as cutoff behavior in magnetic field. Notably, the field dependence of χ closely resembles that of quantum critical systems. However, the ac-susceptibility both in zero and nonzero magnetic fields can be understood from the splitting of a ground state Kramers doublet of Ce³⁺. The high-temperature Curie-Weiss fit yields an effective magnetic moment of approximately 2.54µB per Ce for both approximant systems, which is reduced to∼2.0µB at temperatures below 10 K. The low-temperature specific heat is dominated by the Schottky anomaly originating from the splitting of the Ce³⁺ Kramers doublet, resulting in an entropy of Rln2 at around 10 K.

In strongly correlated systems, interactions give rise to critical fluctuations surrounding the quantum critical point (QCP) of a quantum phase transition. Quasicrystals allow the study of quantum critical phenomena in aperiodic systems with frustrated magnetic interactions. Here, we study the magnetic field and temperature scaling of the low-temperature specific heat for the quantum critical Yb-Au-Al quasicrystal. We devise a scaling function that encapsulates the limiting behaviors as well as the area where the system goes from a temperature-limited to a field-limited quantum critical region, where magnetic field acts as a cutoff for critical fluctuations. The zero-field electronic specific heat is described by a power-law divergence, Cel/T ∝T^−0.54, aligning with previously observed ac-susceptibility and specific heat measurements. The field dependence of the electronic specific heat at high magnetic fields shows a similar power-law Cel/T ∝B^−0.50. In the zero-field and low-field region, we observe two small but distinct anomalies in the specific heat, located at 0.7 K and 2.1 K.

Spin ice compounds enable the exploration of the dynamics of magnetic monopoles in condensed matter systems. In this study, we use modulation calorimetry to probe the dynamical response of the heat capacity of the classical spin-ice compounds Dy₂Ti₂O₇ at low temperatures (0.5− 5 K). Using modulation frequencies of 0.01-500 Hz we find a strong frequency dependence in the measured heat capacity and are able to study thermal relaxation effects on the corresponding timescales. From the frequency dependence of the heat capacity, the relaxation time τ is extracted from the frequency below which the probed heat capacity is independent of the frequency. The relaxation time shows a divergent behavior below 1 K reaching∼6 s at 0.65 K, similar to the spin relaxation time seen in previous studies. Corresponding specific heat shows a maximum around this temperature. Performing dynamic Monte Carlo simulations we verify that the specific heat frequency response has its origin in the slow magnetic monopole dynamics indigenous to spin ice. We find a timescale of 20 ms per Monte Carlo step at 4 K in contrast to 2.5 ms mentioned in previous studies by other techniques.