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Register a new userAnisotropic heavy Fermi-liquid formation in the valence fluctuating $alpha$-YbAlB$_4$
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$alpha$-YbAlB$_4$ is the locally isostructural polymorph of $beta$-YbAlB$_4$, the first example of an Yb-based heavy fermion superconductor which exhibits pronounced non-Fermi-liquid behavior above $T_{rm c}$. Interestingly, both $alpha$-YbAlB$_4$ and $beta$-YbAlB$_4$ have strongly intermediate valence. Our single crystal study of the specific heat, magnetization and resistivity has confirmed the Fermi liquid ground state of $alpha$-YbAlB$_4$ ~in contrast with the quantum criticality observed in $beta$-YbAlB$_4$. Both systems exhibit Kondo lattice behavior with the characteristic temperature scale $T^* sim$ 8 K in addition to a valence fluctuation scale $sim 200$ K. Below $T^*$, $alpha$-YbAlB$_4$ a heavy Fermi liquid state with an electronic specific heat coefficient $gammasim$ 130 mJ/mol K$^2$ and a large Wilson ratio more than 7, which indicates ferromagnetic correlation between Yb moments. A large anisotropy in the resistivity suggests that the hybridization between 4$f$ and conduction electrons is much stronger in the $ab$-plane than along the c-axis. The strongly anisotropic hybridization as well as the large Wilson ratio is the key to understand the unusual Kondo lattice behavior and heavy fermion formation in mixed valent compounds.
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Density functional theory methods are applied to investigate the properties of the new superconductor $beta$-YbAlB$_4$ and its polymorph $alpha$-YbAlB$_4$. We utilize the generalized gradient approximation + Hubbard U (GGA+U) approach with spin-orbit(SO) coupling to approximate the effects of the strong correlations due to the open $4f$ shell of Yb. We examine closely the differences in crystal bonding and symmetry of $beta$-YbAlB$_4$ and $alpha$-YbAlB$_4$. The in-plane bonding structure amongst the dominant itinerant electrons in the boron sheets is shown to differ significantly. Our calculations indicate that, in both polymorphs, the localized 4$f$ electrons hybridize strongly with the conduction sea when compared to the related materials YbRh$_{2}$Si$_{2}$ and YbB$_{2}$. Comparing $beta$-YbAlB$_4$ to the electronic structure of related crystal structures indicates a key role of the 7-member boron coordination of the Yb ion in $beta$-YbAlB$_4$ in producing its enhanced Kondo scale and superconductivity. The Kondo scale is shown to depend strongly on the angle between the B neighbors and the Yb ion, relative to the $x-y$ plane, which relates some of the physical behavior to structural characteristics.
Electrical resistivity $rho$, specific heat C, and magnetic susceptibility $chi$ measurements made on the filled skutterudite CeRu_4As_{12} reveal non-Fermi liquid (NFL) T - dependences at low T, i.e., $rho$(T) $sim$ T^{1.4} and weak power law or logarithmic divergences in C(T)/T and $chi$(T). Measurements also show that the T - dependence of the thermoelectric power S(T) deviates from that seen in other Ce systems. The NFL behavior appears to be associated with fluctuations of the Ce valence between 3^+ and 4^+ rather than a typical Kondo lattice scenario that would be appropriate for an integral Ce valence of 3^+.
We analyze the key origin of quantum valence criticality in the heavy electron metal $beta$-YbAlB$_4$ evidenced in the sister compound $alpha$-YbAl$_{0.986}$Fe$_{0.014}$B$_4$. By constructing a realistic canonical model for $beta$-YbAlB$_4$, we evaluate Coulomb repulsion between the 4f and 5d electrons at Yb $U_{rm fd}approx 6.2$ eV realizing the quantum critical point (QCP) of the Yb-valence transition. To reveal the Yb 5d contribution to the quantum critical state, we propose ultrasound measurement. We find that softening of elastic constants of not only the bulk modulus but also the shear moduli is caused by electric quadrupole fluctuations enhanced by critical 4f and 5d charge fluctuations for low temperatures at the valence QCP. Possible relevance of these results to $beta$-YbAlB$_4$ and also $alpha$-YbAl$_{1-x}$Fe$_x$B$_4$ is discussed.
$beta$-YbAlB$_4$ is the first Yb-based heavy fermion superconductor with $T_{rm c} = 80$ mK. We measured low temperature magnetization of high-purity single crystals down to $T=$ 25 mK. The measurements have revealed a considerable amount of volume fractions of the superconductivity and the upper critical field $B_{c2}$ curve under field along the c axis, consistent with the previous results. In the normal state, the previously observed divergent behavior in the temperature dependence of the magnetization has been confirmed using higher quality samples and under a low field of 22 mT. In addition, the measurements have revealed a power law behavior, namely, $dM/dTpropto T^{3/2}$, which has a slightly higher exponent than the previous results.
The perovskite ruthenate has attracted considerable interest due to reports of possible non-Fermi-liquid behavior and its proximity to a magnetic quantum critical point, yet its ground state and electronic structure remain enigmatic. Here we report the first measurements of the Fermi surface and quasiparticle dispersion in CaRuO3 through a combination of oxide molecular beam epitaxy and in situ angle-resolved photoemission spectroscopy. Our results reveal a complex and anisotropic Fermi surface consisting of small electron pockets and straight segments, consistent with the bulk orthorhombic crystal structure with large octahedral rotations. We observe a strongly band-dependent mass renormalization, with prominent heavy quasiparticle bands which lie close to the Fermi energy and exhibit strong temperature dependence. These results are consistent with a heavy Fermi liquid with a complex Fermiology and small hybridization gaps near the Fermi energy. Our results provide a unified framework for explaining previous experimental results on CaRuO3, such as its unusual optical conductivity, and demonstrate the importance of octahedral rotations in determining the quasiparticle band structure, and electron correlations in complex transition metal oxides.
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