Strong Anharmonicity-Induced Low Thermal Conductivity and High n-type Mobility in the Topological Insulator Bi1.1 Sb0.9 Te2 S.

Intrinsically low lattice thermal conductivity (κlat ) while maintaining the high carrier mobility (µ) is of the utmost importance for thermoelectrics. Topological insulators (TI) can possess high µ due to the metallic surface states. TIs with heavy constituents and layered structure can give rise to high anharmonicity and are expected to show low κlat . Here, we demonstrate that Bi1.1 Sb0.9 Te2 S (BSTS), which is a 3D bulk TI, exhibits ultra-low κlat of 0.46 Wm-1 K-1 along with high µ of ≈401 cm2  V-1 s-1 . Sound velocity measurements and theoretical calculations suggest that chemical bonding hierarchy and high anharmonicity play a crucial role behind such ultra-low κlat . BSTS possesses low energy optical phonons which strongly couple with the heat carrying acoustic phonons leading to ultra-low κlat . Further, Cl has been doped at the S site of BSTS which increases the electron concentration and reduces the κlat resulting in a promising n-type thermoelectric figure of merit (zT) of ≈0.6 at 573 K.

Topological delocalization and tuning of surface channel separation in Bi2Se2Te Topological Insulator Thin films.

The surface states of a 3D topological insulator (TI) exhibit topological protection against backscattering. However, the contribution of bulk electrons to the transport data is an impediment to the topological protection of surface states. We report the tuning of the chemical potential in the bulk in Bi2Se2Te TI thin films, pinning it near the center of the bulk band gap, thereby suppressing the bulk carriers. The temperature dependent resistance of these films show activated behavior down to 50 K, followed by a metallic transition at lower temperatures, a hallmark of robustness of TI surface states. Manifestation of topological protection and surface dominated transport is explained by 2D weak antilocalization phenomenon. We further explore the effect of surface to bulk coupling in TI in this work, which is captured by the number of effective conducting surface channels that participate in the transport. The presence of a single conducting channel indicates a strong surface to bulk coupling which is detrimental to purely topological transport. We demonstrate the decoupling of topological surface states on opposite surfaces of thin films, thereby suppressing the bulk transport. Our findings provide a deeper understanding of surface to bulk coupling along with topological transport behavior and their respective tunability.

Probing quantum discord in a Heisenberg dimer compound.

A quantitative estimation of quantum discord is performed for a Heisenberg spin 1/2 dimer compound (NH4CuPO4, H2O) by means of experimental magnetic and thermal measurements. Magnetic susceptibility and specific heat data were collected for NH4CuPO4, H2O and analyzed within the framework of the Heisenberg isolated dimer model. Internal energy as a function of temperature is obtained by integrating the specific heat versus temperature data. Subsequently, quantum discord, total correlations and spin-spin correlation function are quantified from susceptibility and internal energy and plotted as a function of temperature. Violation of Bell's inequality is also tested for NH4CuPO4, H2O via both experimental susceptibility and specific heat data signifying the presence of entanglement.