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Register a new userCoexistence of magnetic order and valence fluctuations in the Kondo lattice system Ce$_2$Rh$_3$Sn$_5$
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We report on the electronic band structure, structural, magnetic and thermal properties of Ce$_2$Rh$_3$Sn$_5$. Ce $L_{mathrm{III}}$-edge XAS spectra give direct evidence for an intermediate valence behaviour. Thermodynamic measurements reveal magnetic transitions at $T_{mathrm{N1}}approx$ 2.9 K and $T_{mathrm{N2}}approx$ 2.4 K. Electrical resistivity shows behaviour typical for Kondo lattices. The coexistence of magnetic order and valence fluctuations in a Kondo lattice system we attribute to a peculiar crystal structure in which Ce ions occupy two distinct lattice sites. Analysis of the structural features of Ce$_2$Rh$_3$Sn$_5$, together with results of electronic band structure calculations and thermodynamic data indicate that Ce2 ions are in an intermediate valence state with the ground state electronic configuration close to 4$f^0$, whereas Ce1 ions are trivalent (4$f^1$) and contribute to the low temperature magnetic ordering. Thus, our joined experimental and theoretical investigations classify Ce$_2$Rh$_3$Sn$_5$ as a multivalent charge-ordered system.
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We report the results of high pressure x-ray diffraction, x-ray absorption, and electrical transport measurements of Kondo insulator Ce$_3$Bi$_4$Pt$_3$ up to 42 GPa, the highest pressure reached in the study of any Ce-based KI. We observe a smooth decrease in volume and movement toward intermediate Ce valence with pressure, both of which point to increased electron correlations. Despite this, temperature-dependent resistance data show the suppression of the interaction-driven ambient pressure insulating ground state. We also discuss potential ramifications of these results for the predicted topological KI state.
$^{115}$In Nuclear magnetic resonance data are presented for a series of Ce$_{1-x}$La$_x$CoIn$_5$ crystals with different La dilutions, $x$. Multiple In(1) sites associated with different numbers of nearest-neighbor cerium atoms exhibit different Knight shifts and spin lattice relaxation rates. Analysis of the temperature dependence of these sites reveals both an evolution of the heavy electron coherence as a function of dilution, as well as spatial inhomogeneity associated with a complete suppression of antiferromagnetic fluctuations in the vicinity of the La sites. Quantum critical fluctuations persist within disconnected Ce clusters with dilution levels up to 75%, despite the fact that specific heat shows Fermi liquid behavior in dilute samples.
Our theoretical understanding of heavy-fermion compounds mainly derives from iconic models, such as those due to Kondo or Anderson. While providing invaluable qualitative insight, detailed comparisons to experiments are encumbered by the materials complexity, including the spin-orbit coupling, crystal fields, and ligand hybridizations. Here, we study the paradigmatic Kondo insulator Ce$_3$Bi$_4$Pt$_3$ with a first principles dynamical mean-field method that includes these complications. We find that salient signatures of many-body effects in this material---large effective masses, the insulator-to-metal crossover, the concomitant emergence of Curie-Weiss behaviour and notable transfers of optical spectral weight---are captured quantitatively. With this validation, we elucidate the fabric of the many-body state. In particular, we extent the phenomenology of the Kondo crossover to time-scales of fluctuations: We evidence that spin and charge degrees of freedom each realize two regimes in which fluctuations adhere to vastly different decay laws. We find these regimes to be separated by a {it common} temperature $T^{max}_chi$, linked to the onset of Kondo screening. Interestingly, below (above) $T^{max}_chi$, valence fluctuations become faster (slower) than the dynamical screening of the local moments. Overall, however, spin and charge fluctuations occur on comparable time-scales of $mathcal{O}(0.5-12hbox{ fs})$, placing them on the brink of detection for modern time-resolved probes.
The Ce(Co,Rh,Ir)In$_5$ family of ``Ce-115 materials hosts an abundance of correlated electron behavior, including heavy-fermion physics, magnetism, superconductivity and nematicity. The complicated behavior of these entangled phenomena leads to a variety of exotic physical properties, which, despite the seemingly simple crystal structure of these compounds, remain poorly understood. It is generally accepted that the interplay between the itinerant and local character of Ce-$4f$ electrons is the key to their exotic behavior. Here, we report theoretical evidence that the Ce-115 materials are also topological semi-metals, with Dirac fermions around well-separated nodes. Dirac nodes in each compound are present on the $Gamma-Z$ plane close to the Fermi level. As the Dirac bands are derived from In-orbitals, they occur in all family members irrespective of the transition metal (Co,Rh,Ir). We present the expected Fermi-arc surface state patterns and show the close proximity of a topological Lifshitz transition, which possibly explains the high field physics of Ce-115 materials. Experimentally, we highlight the surprising similarity of Ce(Co,Rh,Ir)In$_5$ in high magnetic fields, despite the distinctly different states of the Ce-$4f$ electrons. These results raise questions about the role Dirac fermions play in exotic transport behavior, and we propose this class of materials as a prime candidate for unconventional topological superconductivity.
Transition metal oxides of the $4d$ and $5d$ block have recently become the targets of materials discovery, largely due to their strong spin-orbit coupling that can generate exotic magnetic and electronic states. Here we report the high pressure synthesis of Lu$_2$Rh$_2$O$_7$, a new cubic pyrochlore oxide based on $4d^5$ Rh$^{4+}$ and characterizations via thermodynamic, electrical transport, and muon spin relaxation measurements. Magnetic susceptibility measurements reveal a large temperature-independent Pauli paramagnetic contribution, while heat capacity shows an enhanced Sommerfeld coefficient, $gamma$ = 21.8(1) mJ/mol-Rh K$^2$. Muon spin relaxation measurements confirm that Lu$_2$Rh$_2$O$_7$ remains paramagnetic down to 2 K. Taken in combination, these three measurements suggest that Lu$_2$Rh$_2$O$_7$ is a correlated paramagnetic metal with a Wilson ratio of $R_W = 2.5$. However, electric transport measurements present a striking contradiction as the resistivity of Lu$_2$Rh$_2$O$_7$ is observed to monotonically increase with decreasing temperature, indicative of a nonmetallic state. Furthermore, although the magnitude of the resistivity is that of a semiconductor, the temperature dependence does not obey any conventional form. Thus, we propose that Lu$_2$Rh$_2$O$_7$ may belong to the same novel class of non-Fermi liquids as the nonmetallic metal FeCrAs.
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