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Register a new userClassification of materials with divergent magnetic Gruneisen parameter
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Philipp Gegenwart
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At any quantum critical point (QCP) with a critical magnetic field $H_c$, the magnetic Gruneisen parameter $Gamma_{rm H}$, which equals the adiabatic magnetocaloric effect, is predicted to show characteristic signatures such as a divergence, sign change and $T/(H-H_c)^epsilon$ scaling. We categorize thirteen materials, ranging from heavy fermion metals to frustrated magnets, where such experimental signatures have been found. Remarkably, seven stoichiometric materials at ambient pressure show $H_c=0$. However, additional thermodynamic and magnetic experiments suggest that most of them do not show a zero-field QCP. While the existence of a pressure insensitive strange metal state is one possibility, for some of the materials $Gamma_{rm H}$ seems influenced by impurities or a fraction of moments which are not participating in a frozen state. To unambiguously prove zero-field and pressure sensitive quantum criticality, a $Gamma_{rm H}$ divergence is insufficient and also the Gruneisen ratio of thermal expansion to specific heat must diverge.
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The magneto-caloric effect (MCE), which is the refrigeration based on the variation of the magnetic entropy, is of great interest in both technological applications and fundamental research. The MCE is quantified by the magnetic Gruneisen parameter $Gamma_{textmd{mag}}$. We report on an analysis of $Gamma_{textmd{mag}}$ for the classical Brillouin-like paramagnet, for a modified Brillouin function taking into account a zero-field splitting originated from the spin-orbit (SO) interaction and for the one-dimensional Ising (1DI) model under longitudinal field. For both Brillouin-like model with SO interaction and the longitudinal 1DI model, for $ T rightarrow$ 0 and vanishing field a sign change of the MCE is observed, suggestive of a quantum phase transition. SO interaction leads to a narrowing of the critical fluctuations upon approaching the critical point. Our findings emphasize the relevance of $Gamma_{textmd{mag}}$ for exploring critical points. Also, we show that the Brillouin model with and without SO interaction can be recovered from the 1DI model in the regime of high-temperatures and vanishing coupling constant $J$.
The Gruneisen ratio ($Gamma$), i.e.,the ratio of the linear thermal expansivity to the specific heat at constant pressure, quantifies the degree of anharmonicity of the potential governing the physical properties of a system. While $Gamma$ has been intensively explored in solid state physics, very little is known about its behavior for gases. This is most likely due to the difficulties posed to carry out both thermal expansion and specific heat measurements in gases with high accuracy as a function of pressure and temperature. Furthermore, to the best of our knowledge a comprehensive discussion about the peculiarities of the Gruneisen ratio is still lacking in the literature. Here we report on a detailed and comprehensive overview of the Gruneisen ratio. Particular emphasis is placed on the analysis of $Gamma$ for gases. The main findings of this work are: emph{i)} for the Van der Waals gas $Gamma$ depends only on the co-volume $b$ due to interaction effects, it is smaller than that for the ideal gas ($Gamma$ = 2/3) and diverges upon approaching the critical volume; emph{ii)} for the Bose-Einstein condensation of an ideal boson gas, assuming the transition as first-order $Gamma$ diverges upon approaching a critical volume, similarly to the Van der Waals gas; emph{iii)} for $^4$He at the superfluid transition $Gamma$ shows a singular behavior. Our results reveal that $Gamma$ can be used as an appropriate experimental tool to explore pressure-induced critical points.
We report the magnetic phase diagram of single-crystalline LiFePO$_4$ in magnetic fields up to 58~T and present a detailed study of magneto-elastic coupling by means of high-resolution capacitance dilatometry. Large anomalies at tn in the thermal expansion coefficient $alpha$ imply pronounced magneto-elastic coupling. Quantitative analysis yields the magnetic Gruneisen parameter $gamma_{rm mag}=6.7(5)cdot 10^{-7}$~mol/J. The positive hydrostatic pressure dependence $dT_{rm N}/dp = 1.46(11)$~K/GPa is dominated by uniaxial effects along the $a$-axis. Failure of Gruneisen scaling below $approx 40$~K, i.e., below the peak temperature in the magneto-electric coupling coefficient [onlinecite{toft2015anomalous}], implies several competing degrees of freedom and indicates relevance of recently observed hybrid excitations~[onlinecite{yiu2017hybrid}]. A broad and strongly magnetic-field-dependent anomaly in $alpha$ in this temperature regime highlight the relevance of structure changes. Upon application of magnetic fields $B||b$-axis, a pronounced jump in the magnetisation implies spin-reorientation at $B_{rm SF} = 32$~T as well as a precursing phase at 29~T and $T=1.5$~K. In a two-sublattice mean-field model, the saturation field $B_{rm sat,b} = 64(2)$~T enables the determination of the effective antiferromagnetic exchange interaction $J_{rm af} = 2.68(5)$~meV as well as the anisotropies $D_{rm b} = -0.53(4)$~meV and $D_{rm c} = 0.44(8)$~meV.
We use the recently-proposed emph{compressible cell} Ising-like model [Phys. Rev. Lett. textbf{120}, 120603 (2018)] to estimate the ratio between thermal expansivity and specific heat (the Gruneisen parameter $Gamma$) in supercooled water. Near the critical pressure and temperature, $Gamma$ increases. The $Gamma$ value diverges near the pressure-induced finite-$T$ critical end-point [Phys. Rev. Lett. textbf{104}, 245701 (2010)] and quantum critical points [Phys. Rev. Lett. textbf{91}, 066404 (2003)], which indicates that two energy scales are governing the system. This enhanced behavior of $Gamma$ is caused by the coexistence of high- and low-density liquids [Science textbf{358}, 1543 (2017)]. Our findings support the proposed liquid-liquid critical point in supercooled water in the No-Mans Land regime, and indicates possible applications of this model to other systems.
High-quality single crystals of CoTiO$_3$ are grown and used to elucidate in detail structural and magnetostructural effects by means of high-resolution capacitance dilatometry studies in fields up to 15 T which are complemented by specific heat and magnetization measurements. In addition, we refine the single-crystal structure of the ilmenite ($Rbar{3}$) phase. At the antiferromagnetic ordering temperature $T_mathrm{N}$, pronounced $lambda$-shaped anomaly in the thermal expansion coefficients signals shrinking of both the $c$ and $b$ axes, indicating strong magnetoelastic coupling with uniaxial pressure along $c$ yielding six times larger effect on $T_mathrm{N}$ than the pressure applied in-plane. The hydrostatic pressure dependency derived by means of Gruneisen analysis amounts to $partial T_mathrm{N}/ partial papprox 2.7(4)$~K/GPa. The high-field magnetization studies in static and pulsed magnetic fields up to 60~T along with high-field thermal expansion measurements facilitate in constructing the complete anisotropic magnetic phase diagram of CoTiO$_3$. While the results confirm the presence of significant magnetodielectric coupling, our data show that magnetism drives the observed structural, dielectric, and magnetic changes both in the short-range ordered regime well-above $T_mathrm{N}$ as well as in the long-range magnetically ordered phase.
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