Sharp Raman Anomalies and Broken Adiabaticity at a Pressure Induced Transition from Band to Topological Insulator in Sb2Se3

The nontrivial electronic topology of a topological insulator is thus far known to display signatures in a robust metallic state at the surface. Here, we establish vibrational anomalies in Raman spectra of the bulk that signify changes in electronic topology: an E2 g phonon softens unusually and its linewidth exhibits an asymmetric peak at the pressure induced electronic topological transition (ETT) in Sb2Se3 crystal. Our first-principles calculations confirm the electronic transition from band to topological insulating state with reversal of parity of electronic bands passing through a metallic state at the ETT, but do not capture the phonon anomalies which involve breakdown of adiabatic approximation due to strongly coupled dynamics of phonons and electrons. Treating this within a four-band model of topological insulators, we elucidate how nonadiabatic renormalization of phonons constitutes readily measurable bulk signatures of an ETT, which will facilitate efforts to develop topological insulators by modifying a band insulator

Discovery of highly spin-polarized conducting surface states in the strong spin-orbit coupling semiconductor Sb2_2Se3_3

Majority of the A$_2$B$_3$ type chalcogenide systems with strong spin-orbit coupling, like Bi$_2$Se$_3$, Bi$_2$Te$_3$ and Sb$_2$Te$_3$ etc., are topological insulators. One important exception is Sb$_2$Se$_3$, where a topological non-trivial phase was argued to be possible under ambient conditions, but such a phase could be detected to exist only under pressure. In this Letter, we show that like Bi$_2$Se$_3$, Sb$_2$Se$_3$, displays generation of highly spin-polarized current under mesoscopic superconducting point contacts as measured by point contact Andreev reflection spectroscopy. In addition, we observe a large negative and anisotropic magnetoresistance in Sb$_2$Se$_3$, when the field is rotated in the basal plane. However, unlike in Bi$_2$Se$_3$, in case of Sb$_2$Se$_3$ a prominent quasiparticle interference (QPI) pattern around the defects could be obtained in STM conductance imaging. Thus, our experiments indicate that Sb$_2$Se$_3$ is a regular band insulator under ambient conditions, but due to it's high spin-orbit coupling, non-trivial spin-texture exists on the surface and the system could be on the verge of a topological insulator phase.Comment: 5 pages, 4 figures, supplemental material not include

Improved interfacial resistance and crystal-structure stability in a low-cobalt P2-type sodium-ion battery cathode material

We describe Na0.67Mn0.625Fe0.25Co0.125O2 (NMFCO), a P2-type sodium-ion battery cathode. Our composition, with significantly less Co than in an earlier study, shows discharge capacity close to 190 mAhg-1 and specific energy density exceeding 500 mWhg-1 in the 1.5 to 4.3 V range. The material also shows an improved structural stability over similar materials. Such changes, between the pristine phase (P63/mmc, P63 (OP4), or orthorhombic Cmcm) and the so-called Z phase, are endemic to other P2-type cathodes such as Na0.67Mn0.65Fe0.35O2 (NMFO). We propose two equivalent circuit models of impedance spectroscopy to understand electrochemical processes in our cells with a sodium metal anode. Our equivalent circuit modeling, combined with an analysis of the initial galvanostatic slope, reveals a significant reduction in the polarization and interfacial charge-transfer resistance at the solid electrolyte interface. We reveal that the combined effects of crystal structure stability, lower internal resistance, relatively high specific energy density, and improved battery health make this low-cobalt P2-type cathode composition a very promising candidate for new sodium-ion batteries.Comment: 49 pages, 9 figure

Charge Density Wave Order in the Topological Insulator Bi2Se3Bi_2Se_3

Hexagonally deformed Fermi surfaces and strong nesting, found in topological insulators (TIs) such as $Bi_2Se_3$ and $Bi_2Te_3$ over the past decade, have led to several predictions of possible Density Wave order in these systems. Recent evidence for strong Fermi nesting in superconducting $Cu-Bi_2Se_3$ and $Nb-Bi_2Se_3$ has led to further speculation about the importance of charge order in the context of unconventional superconductivity. Here, we report what we believe is the first direct observation of Charge Density Wave (CDW) order in $Bi_2Se_3$. Our results include the observation of a 140K metal-insulator-metal transition in resistivity as a function of temperature. We corroborate this with nuclear magnetic resonance (NMR) studies of the spin-lattice relaxation $\frac{1}{T_1}$ rate of the $^{209}Bi$ nucleus, which also displays a transition at 140K. Additionally, we use electron diffraction to reveal a periodic lattice distortion (PLD) in $Bi_2Se_3$, together with diffuse charge order between $\vec{k}$ and $\vec{k} \pm \Delta\vec{k}$. This diffuse scattering points toward the presence of an incommensurate charge density wave (I-CDW) above room temperature, which locks into a CDW upon cooling below $\sim140K$. We also observe two additional transitions in $\frac{1}{T_1}$ near 200K and 15K. The transition at 200K appears to display some anisotropy with the direction of applied magnetic field. In this report, we focus on the CDW transition at 140K. We include some speculation of the two other transitions observed at 15K and 200K by NMR, also revealed here for the first time.Comment: 16 pages, 5 figure

Two-dimensional Vortices in Superconductors

Superconductors have two key characteristics. They expel magnetic field and they conduct electrical current with zero resistance. However, both properties are compromised in high magnetic fields which can penetrate the material and create a mixed state of quantized vortices. The vortices move in response to an electrical current dissipating energy which destroys the zero resistance state\cite{And64}. One of the central problems for applications of high temperature superconductivity is the stabilization of vortices to ensure zero electrical resistance. We find that vortices in the anisotropic superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ (Bi-2212) have a phase transition from a liquid state, which is inherently unstable, to a two-dimensional vortex solid. We show that at high field the transition temperature is independent of magnetic field, as was predicted theoretically for the melting of an ideal two-dimensional vortex lattice\cite{Fis80,Gla91}. Our results indicate that the stable solid phase can be reached at any field as may be necessary for applications involving superconducting magnets\cite{Has04,Sca04,COHMAG}. The vortex solid is disordered, as suggested by previous studies at lower fields\cite{Lee93,Cub93}. But its evolution with increasing magnetic field displays unexpected threshold behavior that needs further investigation.Comment: 5 pages and 4 figures. submitted to Nature Physic

Floating Zone Growth of Bulk Single Crystals of Complex Oxides

ABSTRACTIn this MRS Proceedings paper, we present a review of the floating zone growth process, and describe growth techniques as relevant to four selected families of current or continued interest in the area of condensed matter physics of highly correlated electron systems: Cuprate oxides (Bi2Sr2CaCu2Oy), Ruthenate oxides (Sr2RuO4), Manganite oxides (La1-xSrxMnO3, La2–2xSr1+2xMn2O7, (RE) MnO3 (RE=Rare Earth)) and Ferrite oxides (BiFeO3). We also discuss our experiences with factors that tend to contribute to poor crystal growth, as relevant to experiments to probe correlated-electron physics in such crystals.</jats:p

Synthesis of YBa<SUB>2</SUB>Cu<SUB>4</SUB>O<SUB>8</SUB> in air through complex precursor formation and fine-particle techniques

We report the successful synthesis in air of the superconductor YBa<SUB>2</SUB>Cu<SUB>4</SUB>O<SUB>8</SUB> (1-2-4) without the need of high oxygen pressure or an alkali-carbonate based catalyst. The technique involves formation, in solution, of a fine-particle-complex based precursor. The 1-2-4 phase then forms in air with a single firing of 30 min at 900&#176;C and a T<SUB>c</SUB> near 80 K. We also report some results of attempts to prepare 1-2-4 using precursors from sol-gel and spray drying