https://hal.archives-ouvertes.fr/hal-00120432/document

Raman Spectroscopy of Nanomaterials: How Spectra Relate to Disorder, Particle Size and Mechanical Properties

Valence fluctuation phenomena occur in rare-earth compounds in which the proximity of the 4f level to the Fermi energy leads to instabilities of the charge configuration (valence) and/or of the magnetic moment. The authors review the experimental results observed in the subset of such systems for which the 4f ions form a lattice with identical valence on each site. The discussion includes key thermodynamic experiments, such as susceptibility and lattice constant, and spectroscopic experiments such as XPS and neutron scattering. This is followed by a review of existing theoretical work concerning both the ground states and the isomorphic phase transitions which occur in such compounds; the emphasis is on those aspects which make valence fluctuation phenomena such a challenging many-body problem.

https://iopscience.iop.org/article/10.1088/0034-4885/44/1/001/pdf

Valence fluctuation phenomena

Vibrational thermodynamics of materials

We describe a new calorimeter for measuring heat capacity in the range 1–35 K, using a silicon chip bolometer as sample holder, temperature sensor, and sample heater. The apparatus is capable of measuring very small samples (1–500 mg), and may be used with a number of different experimental methods.

Heat Capacity Measurements on Small Samples at Low Temperatures

Critical examination of heat capacity measurements made on a Quantum Design physical property measurement system

We propose a model for the electronic structure of Sm${\mathrm{B}}_{6}$ suggested by new transport measurements in combination with analyses of the magnetic susceptibility and lattice energetics. Our model successfully describes the resistivity, Hall effect, magnetic susceptibility, and M\"ossbauer isomer shift in this material. In addition, it predicts an anomaly in the low-temperature specific heat. Analysis of the lattice energetics shows that the rigidity of the boron lattice is responsible for the fact that Sm ions on equivalent sites can occur with the $4f$ shell in either the $4{f}^{5}$ or the $4{f}^{6}$ configuration. A relationship among the compressibilities of the rare-earth hexaborides is predicted on the basis of this analysis.

Physical properties of SmB 6

We present measurements of the specific heat and resistivity $\ensuremath{\rho}$ of Pr${\mathrm{B}}_{6}$ at low temperatures. The extreme sharpness of the specific-heat anomaly at the antiferromagnetic ordering temperature indicates that Pr${\mathrm{B}}_{6}$ may undergo a first-order transition of the type predicted by Wang and Cooper. A comparison of $\frac{d\ensuremath{\rho}}{\mathrm{dT}}$ with the heat capacity in the critical region gives qualitative agreement with the theory of Fisher and Langer.

Magnetic Ordering in Pr B 6

We report measurements of the low-temperature specific heat of Ce${\mathrm{B}}_{6}$ and discuss their significance in the light of earlier studies of this compound. The large specific heat above the antiferromagnetic ordering at 2.31 K is interpreted with the aid of a decorated Ising model. This model permits us to combine exchange and crystal field interactions in a simplified, but exact, calculation of the thermodynamic properties of a magnetic system including the effects of fluctuations. The analysis gives independent support to the proposal by Nickerson and White that the exchange in the crystal field excited state is substantially stronger than the ground-state exchange in Ce${\mathrm{B}}_{6}$. We also comment on the possibility of extending such an analysis to other magnetic rare-earth hexaborides.

Exchange Interactions and Fluctuations in Ce B 6

Magnetic and transport measurements in ferromagnetic EuB6 show (a) evidence for a Fisher‐Langer‐type resistance anomaly near Tc, (b) that EuB6 may not be a simple ferromagnet, and (c) that the magnetic coupling might be influenced by a temperature‐dependent conduction electron concentration.

K. N. Lee

Papers

Heat Capacity Measurements on Small Samples at Low Temperatures

Physical properties of SmB 6

Magnetic Ordering in Pr B 6

Exchange Interactions and Fluctuations in Ce B 6

Spin Scattering and Magnetic Ordering in EuB6