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Sharp superconducting transition anomalies observed in a new generation of single crystals establish that bulk superconductivity is intrinsic to high purity ${\mathrm{YFe}}_{2}{\mathrm{Ge}}_{2}$. Low temperature heat capacity measurements suggest a disorder and field dependent residual Sommerfeld coefficient, consistent with disorder-induced in-gap states as expected for a sign-changing order parameter. The sevenfold reduction in disorder scattering in these new crystals to residual resistivities $\ensuremath{\simeq}0.45\text{ }\text{ }\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\text{ }\mathrm{cm}$ was achieved using a new liquid transport growth technique, paving the way for multiprobe experiments investigating the normal and superconducting states of ${\mathrm{YFe}}_{2}{\mathrm{Ge}}_{2}$.

We have synthesized ceramic specimens of the tetragonal tungsten bronze ${\mathrm{K}}_{3}\mathrm{L}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{a}}_{5}{\mathrm{O}}_{15}$ (KLT) and characterized its phase transition via x-ray diffraction, dielectric permittivity, resonant ultrasonic spectroscopy, and heat capacity measurements. The space group of KLT is reported as both $P4/mbm$ and $Cmmm$ with the orthorhombic distortion occurring when there are higher partial pressures of volatile K and Li used inside the closed crucibles for the solid state synthesis. The data show strong relaxor behavior, with the temperature at which the two dielectric relative permittivity peaks decreasing, with $104\ensuremath{\ge}{T}_{m1}\ensuremath{\ge}69\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and $69\ensuremath{\ge}{T}_{m2}\ensuremath{\ge}46\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ as probe frequency $f$ is reduced from 1 MHz to 316 Hz. $F$ tests show that the data satisfies a Vogel-Fulcher model better than Arrhenius with an extrapolated freezing temperature for ${\ensuremath{\varepsilon}}^{\ensuremath{'}}$ and ${\ensuremath{\varepsilon}}^{\ensuremath{''}}$ of ${T}_{f1}=+15.8$ and $\ensuremath{-}11.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{f2}=\ensuremath{-}5.0$ and $\ensuremath{-}15.0\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for $\phantom{\rule{4pt}{0ex}}f\ensuremath{\rightarrow}0$ (tending to dc). This difference between ${T}_{f}$ from real and imaginary values, albeit counterintuitive, is mandatory, according to the theory of Tagantsev. Therefore, by tuning frequency, the transition could be shifted to absolute zero, suggesting KLT has a relaxor-type quantum critical point. In addition, we have reanalyzed the conflicting literature for $\mathrm{P}{\mathrm{b}}_{2}\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{7}$ pyrochlore which suggests that this also has a relaxor-type quantum critical point since the freezing temperature from the Vogel-Fulcher fitting is below absolute zero. Since the transition temperature evidenced in the dielectric data at approximately 100 kHz shifts below 0 K for very low frequencies, this transition would not be seen with heat capacity data collected in the zero-frequency (dc) limit. Both of these materials show promise for possible new relaxor-type quantum critical points with nonperovskite based structures.

The iron-based intermetallic YFe_{2}Ge_{2} stands out among transition metal compounds for its high Sommerfeld coefficient of the order of 100 mJ/(mol K^{2}), which signals strong electronic correlations. A new generation of high quality samples of YFe_{2}Ge_{2} show superconducting transition anomalies below 1.8 K in thermodynamic, magnetic, and transport measurements, establishing that superconductivity is intrinsic in this layered iron compound outside the known superconducting iron pnictide or chalcogenide families. The Fermi surface geometry of YFe_{2}Ge_{2} resembles that of KFe_{2}As_{2} in the high pressure collapsed tetragonal phase, in which superconductivity at temperatures as high as 10 K has recently been reported, suggesting an underlying connection between the two systems.