Structural Phase Transition and Superconductivity in 2H-BaGaGe with Buckled Honeycomb Layers

We report on the structural and superconducting properties of the intermetallic compound BaGaGe. We find that this material undergoes a structural second-order phase transition from the distorted AlB$_2$-type structure (1H, $a$ = 4.3254(2) \r{A}, $c$ = 5.1078(3) \r{A}, P6/mmm) into the CaIn$_2$-type structure (2H, $a$ = 4.3087(3) \r{A}, $c$ = 10.2117(6) \r{A}, P6$_3$/mmc) at a transition temperature of $T_{\rm S}$ = 253 K. We find that the structural phase-transition corresponds to a coherent buckling of the honeycomb layers, which we can interpret as a disorder-to-order transition of the atoms located within this layer. We show that the 2H-BaGaGe phase becomes superconducting at a critical temperature of $T_{\rm c}$ = 2.1 K. The bulk nature of the superconductivity in 2H-BaGaGe is confirmed by means of specific heat measurements, where we determine a value of $\Delta C$/$\gamma T_{\rm c}$ = 1.59, which is close to the expected BCS value in the weak coupling limit

A ferroelectric quantum phase transition inside the superconducting dome of Sr1-xCaxTiO3-delta

SrTiO3, a quantum paraelectric(1), becomes a metal with a super-conducting instability after removal of an extremely small number of oxygen atoms(2). It turns into a ferroelectric upon substitution of a tiny fraction of strontium atomswith calcium(3). The two orders may be accidental neighbours or intimately connected, as in the picture of quantum critical ferroelectricity(4). Here, we show that in Sr1-xCaxTiO3-delta (0.002 < x < 0.009, delta < 0.001) the ferroelectric order coexists with dilute metallicity and its superconducting instability in a finite window of doping. At a critical carrier density, which scales with the Ca content, a quantum phase transition destroys the ferroelectric order. We detect an upturn in the normal-state scattering and a significant modification of the superconducting dome in the vicinity of this quantum phase transition. The enhancement of the superconducting transition temperature with calcium substitution documents the role played by ferroelectric vicinity in the precocious emergence of superconductivity in this system, restricting possible theoretical scenarios for pairing