Density Of Low-energy Excitations Research Articles

Entropy Bottlenecks at T $$\,\rightarrow \,$$ → 0 in Ce-Lattice and Related Compounds

A number of specific heat \(C_\mathrm{m}\) anomalies are reported in Ce- and Yb-lattice compounds around 1 K which cannot be associated to usual phase transitions despite of their robust magnetic moments. Instead of a \(C_\mathrm{m}(T)\) jump, these anomalies show coincident morphology: (i) a significant tail in \(C_\mathrm{m}/T\), with similar power law decay above their maxima (\(T>T_\mathrm{m}\)), (ii) whereas a \(C_\mathrm{m}(T^2)\) dependence is observed below \(T_\mathrm{m}\). (iii) The comparison of their respective entropy gain \(S_\mathrm{m}(T)\) indicates that \(\approx 0.7R\)ln2 is condensed within the \(T>T_\mathrm{m}\) tail, in coincidence with an exemplary spin-ice compound. Such amount of entropy arises from a significant increase of the density of low energy excitations, reflected in a divergent \(C_\mathrm{m}(T>T_\mathrm{m})/T\) dependence. (iv) Many of their lattice structures present conditions for magnetic frustration. The origin of these anomalies can be attributed to an interplay between the divergent density of magnetic excitations at \(T\rightarrow 0\) and the limited amount of degrees of freedom: \(S_\mathrm{m}\) = \(R\)ln2 for a doublet-ground state. Due to this “entropy bottleneck,” the paramagnetic minimum of energy blurs out and the system slides into an alternative minimum through a continuous transition. A relevant observation in these very heavy fermion systems is the possible existence of an upper limit for \(C_\mathrm{m}/T_{\mathrm{Lim} T\rightarrow 0} ~\approx 7~\)J/mol K\(^2\) observed in four Yb- and Pr-based compounds.

Thermodynamic Behavior of Ce Compounds Close to a $T \to 0$ Critical Point

There is a reduced group of Ce very heavy Fermions (VHF) which do not order magnetically down to at least T ≈ 500 mK because they are very close to a Tord = 0 critical point. These compounds are at the top of the limT→ 0 Cm/T specific heat values because they collect very high density of low energy excitations. From the analysis of Cm(T)/T and entropy Sm(T) dependencies performed on selected CePd3Mx ternaries (where M = B and Be) a quantitative evaluation of an upper limit for the density of excitations can be proposed. These observations exclude any evidence of Cm(T)/T divergency as T→ 0 in agreement with thermodynamic laws. A comparison with selected Yb-base VHF supports these features.

Thermodynamic properties of ferromagnetic Mott-insulator GaV 4S 8

We present the results of the magnetic and specific heat measurements on V 4 tetrahedral-cluster compound GaV 4S 8 between 2 and 300 K. We find two transitions related to a structural change at 42 K followed by ferromagnetic order at 12 K on cooling. Remarkably similar properties were previously reported for the cluster compounds of Mo 4. These compounds show an extremely high density of low energy excitations in their electronic properties. We explain this behavior in a cluster compound as due to the reduction of coulomb repulsion among electrons that occupy highly degenerate orbits of different clusters.

Excitation Spectrum of Andreev Billiards with a Mixed Phase Space

We present a semiclassical theory for the excitation spectrum of a ballistic quantum dot weakly coupled to a superconductor, for the generic situation that the classical motion gives rise to a phase space containing islands of regularity in a chaotic sea. The density of low-energy excitations is determined by quantum energy scales that are related in a simple way to the morphology of the mixed phase space. An exact quantum mechanical computation for the annular billiard shows good agreement with the semiclassical predictions, in particular for the reduction of the excitation gap when the coupling to the regular regions is maximal.

Dipole gap effects in low energy excitation spectrum of amorphous solids. Theory for dielectric relaxation

The influence of the long-range interaction of two-level systems in dielectric glasses on the density of low-energy excitations and the dielectric properties is studied. The AC dielectric susceptibiliy is analyzed under rapid switching on the external DC electric field. The instantaneous increase in the dielectric susceptibility and its subsequent logarithmic relaxation, recently observed in Ref. 1, are qualitatively explained in a framework of a weakly interacting two-level systems model. The temperature and frequency dependencies of the logarithmic relaxation rate are discussed.

Low-energy excitations in the mixed crystal Ba1-xLaxF2+x.

We have studied the evolution of a glasslike distribution and density of low-energy excitations in the mixed crystal ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{F}}_{2+x}$. Increase in the concentration $x$ from 0 to 0.46 gradually changes the behavior of the low-temperature thermal conductivity, specific heat, and internal friction from those of a pure crystal to the characteristic properties of an amorphous solid.

Low-temperature dielectric constant of near-stoichiometric Naβ-alumina

Measurements of the dielectric constant of Na $\ensuremath{\beta}$-alumina at temperatures below 2 K reveal that the density of low-energy excitations is not a function of the Na stoichiometric excess for the range 7-35% excess Na.

Effect of neutron irradiation on the density of low-energy excitations in vitreous silica

Neutron irradiation of vitreous silica produces changes of the same relative magnitude in the low-temperature excess specific heat ${C}_{\mathrm{ex}}$, the anomalous thermal conductivity $\ensuremath{\kappa}$, and the anomalous temperature dependence of ultrasonic velocity, $\frac{\ensuremath{\Delta}v}{v}$. It is therefore likely that $\ensuremath{\kappa}$ and $\frac{\ensuremath{\Delta}v}{v}$ are dominated by the localized excitations responsible for the excess specific heat of vitreous silica. The tunneling-states model with a single energy-dependent density of states adequately accounts for the measured ${C}_{\mathrm{ex}}$, $\ensuremath{\kappa}$, and $\frac{\ensuremath{\Delta}v}{v}$ of unirradiated vitreous silica. Changes in these properties with neutron irradiation can be explained using this model by altering only the density of tunneling states.