Koushik Pal

The Origin of Ultralow Thermal Conductivity in InTe: Lone-Pair-Induced Anharmonic Rattling.

Understanding the origin of intrinsically low thermal conductivity is fundamentally important to the development of high-performance thermoelectric materials, which can convert waste-heat into electricity. Herein, we report an ultralow lattice thermal conductivity (ca. 0.4 W m(-1)  K(-1) ) in mixed valent InTe (that is, In(+) In(3+) Te2 ), which exhibits an intrinsic bonding asymmetry with coexistent covalent and ionic substructures. The phonon dispersion of InTe exhibits, along with low-energy flat branches, weak instabilities associated with the rattling vibrations of In(+) atoms along the columnar ionic substructure. These weakly unstable phonons originate from the 5s(2) lone pair of the In(+) atom and are strongly anharmonic, which scatter the heat-carrying acoustic phonons through strong anharmonic phonon-phonon interactions, as evident in anomalously high mode Grüneisen parameters. A maximum thermoelectric figure of merit (z T) of about 0.9 is achieved at 600 K for the 0.3 mol % In-deficient sample, making InTe a promising material for mid-temperature thermoelectric applications.

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 E(g)(2) phonon softens unusually and its linewidth exhibits an asymmetric peak at the pressure induced electronic topological transition (ETT) in Sb(2)Se(3) 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.

Intrinsic Rattler-Induced Low Thermal Conductivity in Zintl Type TlInTe2

Understanding the nature of chemical bonding and lattice dynamics together with their influence on phonon-transport is essential to explore and design crystalline solids with ultralow thermal conductivity for various applications including thermoelectrics. TlInTe2, with interlocked rigid and weakly bound substructures, exhibits lattice thermal conductivity as low as ca. 0.5 W/mK near room temperature, owing to rattling dynamics of weakly bound Tl cations. Large displacements of Tl cations along the c-axis, driven by electrostatic repulsion between localized electron clouds on Tl and Te ions, are akin to those of rattling guests in caged-systems. Heat capacity of TlInTe2 exhibits a broad peak at low-temperatures due to contribution from Tl-induced low-frequency Einstein modes as also evidenced from THz time domain spectroscopy. First-principles calculations reveal a strong coupling between large-amplitude coherent optic vibrations of Tl-rattlers along the c-axis, and acoustic phonons that likely causes the low lattice thermal conductivity in TlInTe2.

Thermoelectric properties of materials with nontrivial electronic topology

Materials with good thermoelectric (TE) properties such as Bi2Te3 and SnTe have recently come to be known as topological insulators (TIs). It is fundamentally interesting to explore if other materials with non-trivial electronic topology may also exhibit good TE properties. In this work, we use first-principles density functional theoretical calculations to determine and analyze the electronic thermoelectric properties (Seebeck coefficient (S), electrical conductivity (σ) and thermoelectric power factor (P)) of topological insulators (β-As2Te3, BiTeCl, and PbTe), a topological Dirac semimetal (Na3Bi) and a Weyl semimetal (TaAs) and semimetallic YPtBi, belonging to different symmetry and topological classes. We find that the multiple sub-band structure, the small band gap of topological insulators, and their vicinity to a metallic state associated with an electronic topological transition (ETT) are responsible for their superior TE performance. In addition, sensitivity of their electronic structure to strain makes their thermoelectric properties highly tunable. We predict that TaAs is a promising TE for experimental exploration, and propose that the thermoelectric modulators based on TIs such as SnTe and PbTe will be more efficient under mechanical load.

Strain induced Z2 topological insulating state of β-As2Te3

Topological insulators are non-trivial quantum states of matter which exhibit a gap in the electronic structure of their bulk form, but a gapless metallic electronic spectrum at the surface. Here, we predict a uniaxial strain induced electronic topological transition (ETT) from a band to topological insulating state in the rhombohedral phase (space group: R3¯m) of As2Te3 (β-As2Te3) through first-principles calculations including spin-orbit coupling within density functional theory. The ETT in β-As2Te3 is shown to occur at the uniaxial strain ϵzz = −0.05 (σzz = 1.77 GPa), passing through a Weyl metallic state with a single Dirac cone in its electronic structure at the Γ point. We demonstrate the ETT through band inversion and reversal of parity of the top of the valence and bottom of the conduction bands leading to change in the ℤ2 topological invariant ν0 from 0 to 1 across the transition. Based on its electronic structure and phonon dispersion, we propose ultra-thin films of As2Te3 to be promising for us...

Raman anomalies as signatures of pressure induced electronic topological and structural transitions in black phosphorus: Experiments and theory

We report high-pressure Raman experiments of black phosphorus up to 24 GPa. The linewidths of first-order Raman modes A(g)(1), B-2g, and A(g)(2) of the orthorhombic phase show a minimum at 1.1 GPa. Our first-principles density functional analysis reveals that this is associated with the anomalies in electron-phonon coupling at the semiconductor to topological insulator transition through inversion of valence and conduction bands marking a change from trivial to nontrivial electronic topology. The frequencies of B-2g and A(g)(2) modes become anomalous in the rhombohedral phase at 7.4 GPa, and new modes appearing in the rhombohedral phase show anomalous softening with pressure. This is shown to originate from unusual structural evolution of black phosphorous with pressure, based on first-principles theoretical analysis.

Pressure-induced phase transition in Bi2Se3 at 3 GPa: electronic topological transition or not?

In recent years, a low pressure transition around P3 GPa exhibited by the A2B3-type 3D topological insulators is attributed to an electronic topological transition (ETT) for which there is no direct evidence either from theory or experiments. We address this phase transition and other transitions at higher pressure in bismuth selenide (Bi2Se3) using Raman spectroscopy at pressure up to 26.2 GPa. We see clear Raman signatures of an isostructural phase transition at P2.4 GPa followed by structural transitions at ∼ 10 GPa and 16 GPa. First-principles calculations reveal anomalously sharp changes in the structural parameters like the internal angle of the rhombohedral unit cell with a minimum in the c/a ratio near P3 GPa. While our calculations reveal the associated anomalies in vibrational frequencies and electronic bandgap, the calculated Z2 invariant and Dirac conical surface electronic structure remain unchanged, showing that there is no change in the electronic topology at the lowest pressure transition.

Photochemical water splitting by bismuth chalcogenide topological insulators

As one of the major areas of interest in catalysis revolves around 2D materials based on molybdenum sulfide, we have examined the catalytic properties of bismuth selenides and tellurides, which are among the first chalcogenides to be proven as topological insulators (TIs). We find significant photochemical H2 evolution activity with these TIs as catalysts. H2 evolution increases drastically in nanosheets of Bi2 Te3 compared to single crystals. First-principles calculations show that due to the topology, surface states participate and promote the hydrogen evolution.

Emergence of a weak topological insulator from the BixSey family

We report the emergence of a weak topological insulator (WTI), BiSe, of the Bi-chalcogenide family with an indirect bandgap of 42 meV. Its structural unit consists of a bismuth bilayer (Bi-2), a known quantum spin hall insulator sandwiched between two units of Bi2Se3 which are three dimensional strong topological insulators. Our density functional theory calculations confirm the WTI phase and angle resolved photo-emission spectroscopy measurements carried out on cleaved single crystal flakes show Rashba states that closely agree with our theoretical predictions. Finally, we present a comparison between electronic and magneto-transport properties measured on single crystal flakes and thin films of BiSe. Published by AIP Publishing.

Localized Vibrations of Bi Bilayer Leading to Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in Weak Topological Insulator n-Type BiSe

Realization of high thermoelectric performance in n-type semiconductors is of imperative need on account of the dearth of efficient n-type thermoelectric materials compared to the p-type counterpart. Moreover, development of efficient thermoelectric materials based on Te-free compounds is desirable because of the scarcity of Te in the Earths crust. Herein, we report the intrinsic ultralow thermal conductivity and high thermoelectric performance near room temperature in n-type BiSe, a Te-free solid, which recently has emerged as a weak topological insulator. BiSe possesses a layered structure consisting of a bismuth bilayer (Bi2) sandwiched between two Bi2Se3 quintuple layers [Se-Bi-Se-Bi-Se], resembling natural heterostructure. High thermoelectric performance of BiSe is realized through the ultralow lattice thermal conductivity (κlat of ∼0.6 W/mK at 300 K), which is significantly lower than that of Bi2Se3 (κlat of ∼1.8 W/mK at 300 K), although both of them belong to the same layered homologous family (Bi2) m(Bi2Se3) n. Phonon dispersion calculated from first-principles and the experimental low-temperature specific heat data indicate that soft localized vibrations of bismuth bilayer in BiSe are responsible for its ultralow κlat. These low energy optical phonon branches couple strongly with the heat carrying acoustic phonons, and consequently suppress the phonon mean free path leading to low κlat. Further optimization of thermoelectric properties of BiSe through Sb substitution and spark plasma sintering (SPS) results in high ZT ∼ 0.8 at 425 K along the pressing direction, which is indeed remarkable among Te-free n-type thermoelectric materials near room temperature.