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Register a new userSignature of band inversion in the perovskite thin-film alloys BaSn$_{1-x}$Pb$_x$O$_3$
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Perovskite oxides ABO$_3$ containing heavy B-site elements are a class of candidate materials to host topological metals with a large spin-orbit interaction. In contrast to the band insulator BaSnO$_3$, the semimetal BaPbO$_3$ is proposed to be a typical example with an inverted band structure, the conduction band of which is composed of mainly the O-2p orbital. In this study, we exemplify a band-gap modification by systematic structural, optical, and transport measurements in BaSn$_{1-x}$Pb$_x$O$_3$ films. A sudden suppression of the conductivity and an enhancement of the weak antilocalization effect at $x$ = 0.9 indicate the presence of a singular point in the electronic structure as a signature of the band inversion. Our findings provide an intriguing platform for combining topological aspects and electron correlation in perovskite oxides based on band-gap engineering.
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For disordered Heisenberg systems with small single ion anisotropy, two spin glass transitions below the long range ordered phase transition temperature has been predicted theoretically for compositions close to the percolation threshold. Experimental verification of these predictions is still controversial for conventional spin glasses. We show that multiferroic spin glass systems can provide a unique platform for verifying these theoretical predictions via a study of change in magnetoelastic and magnetoelectric couplings, obtained from an analysis of diffraction data, at the spin glass transition temperatures. Results of macroscopic and microscopic (x-ray and neutron scattering) measurements are presented on disordered BiFeO3, a canonical Heisenberg system with small single ion anisotropy, which reveal appearance of two spin glass phases SG1 and SG2 in coexistence with the LRO phase below the A-T and G-T lines. It is shown that the temperature dependence of the integrated intensity of the antiferromagnetic peak shows dips with respect to the Brillouin function behaviour around the SG1 and SG2 transition temperatures. The ferroelectric polarisation changes significantly at the two spin glass transition temperatures. These results, obtained using microscopic techniques, clearly demonstrate that the SG1 and SG2 transitions occur on the same magnetic sublattice and are intrinsic to the system. We also construct a phase diagram showing all the magnetic phases in BF-xBT system. While our results on the two spin glass transitions support the theoretical predictions, it also raises several open questions which need to be addressed by revisiting the existing theories of spin glass transitions by taking into account the effect of magnetoelastic and magnetoelectric couplings as well as electromagnons.
The characteristics of topological insulators are manifested in both their surface and bulk properties, but the latter remain to be explored. Here we report bulk signatures of pressure-induced band inversion and topological phase transitions in Pb$_{1-x}$Sn$_x$Se ($x=$0.00, 0.15, and 0.23). The results of infrared measurements as a function of pressure indicate the closing and the reopening of the band gap as well as a maximum in the free carrier spectral weight. The enhanced density of states near the band gap in the topological phase give rise to a steep interband absorption edge. The change of density of states also yields a maximum in the pressure dependence of the Fermi level. Thus our conclusive results provide a consistent picture of pressure-induced topological phase transitions and highlight the bulk origin of the novel properties in topological insulators.
In the metallic pyrochlore Nd$_2$Mo$_2$O$_7$, the conducting Molybdenum sublattice adopts canted, yet nearly collinear ferromagnetic order with nonzero scalar spin chirality. The chemical potential may be controlled by replacing Nd$^{3+}$ with Ca$^{2+}$, while introducing only minimal additional disorder to the conducting states. Here, we demonstrate the stability of the canted ferromagnetic state, including the tilting angle of Molybdenum spins, in (Nd$_{1-x}$Ca$_{x}$)$_2$Mo$_2$O$_7$ (NCMO) with $xle 0.15$ using magnetic susceptibility measurements. Mo-Mo and Mo-Nd magnetic couplings both change sign above $x=0.22$, where the canted ferromagnetic state gives way to a spin-glass metallic region. Contributions to the Curie-Weiss law from two magnetic sublattices are separated systematically.
Based on first-principles calculations, we show that the maximum reachable concentration $x$ in the (Ga$_{1-x}$In$_x$)$_2$O$_3$ alloy in the low-$x$ regime (i.e. In solubility in $beta$-Ga$_2$O$_3$) is around 10%. We then calculate the band alignment at the (100) interface between $beta$-Ga$_2$O$_3$ and (Ga$_{1-x}$In$_x$)$_2$O$_3$ at 12%, the nearest computationally treatable concentration. The alignment is strongly strain-dependent: it is of type-B staggered when the alloy is epitaxial on Ga$_2$O$_3$, and type-A straddling in a free-standing superlattice. Our results suggest a limited range of applicability of low-In-content GaInO alloys.
We report the observation that thermoelectric thin-films of La-doped SrTiO3 grown on SrTiO3 substrates yield anomalously high values of thermopower to give extraordinary values of power factor at 300K. Thin-films of Sr0.98La0.02TiO3, grown via pulsed laser deposition at low temperature and low pressure (450C, 10-7Torr), do not yield similarly high values when grown on other substrates. The thin-film growth induces oxygen reduction in the SrTiO3 crystals, doping the substrate n-type. It is found that the backside resistance of the SrTiO3 substrates is as low (~12ohm/square) as it is on the film-side after film growth.
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