Pressure-Induced Topological Phase Transitions in CdGeSb2_2 and CdSnSb2_2

Topological quantum phase transitions (TQPTs) in a material induced by external perturbations are often characterized by band touching points in the Brillouin zone. The low-energy excitations near the degenerate band touching points host different types of fermions while preserving the topological protection of surface states. An interplay of different tunable topological phases offers an insight into the evolution of the topological character. In this paper, we study the occurrence of TQPTs as a function of hydrostatic pressure in CdGeSb$_2$ and CdSnSb$_2$ chalcopyrites, using the first-principles calculations. At ambient pressure, both materials are topological insulators having a finite band gap with inverted order of Sb-$s$ and Sb-$p_x$,$p_y$ orbitals of valence bands at the $\Gamma$ point. On the application of hydrostatic pressure the band gap reduces, and at the critical point of the phase transition, these materials turn into Dirac semimetals. On further increasing the pressure beyond the critical point, the band inversion is reverted making them trivial insulators. The pressure-induced change in band topology from non-trivial to trivial phase is also captured by L\"{u}ttinger model Hamiltonian calculations. Our model demonstrates the critical role played by a pressure-induced anisotropy in frontier bands in driving the phase transitions. These theoretical findings of peculiar coexistence of multiple topological phases in the same material provide a realistic and promising platform for the experimental realization of the TQPT.Comment: 7 pages, 7 figure

Stacking Order Driven Optical Properties and Carrier Dynamics in ReS2

Two distinct stacking orders in ReS2 are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one-unit cell displacement along the a axis. First-principle calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm-1 for AA stacking, and 20 cm-1 for AB stacking, making a simple tool for determining the stacking orders in ReS2. Polarized photoluminescence (PL) reveals that AB stacking possesses blue-shifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular to b axis. Our findings underscore the stacking-order driven optical properties and carrier dynamics of ReS2, mediate many seemingly contradictory results in literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order

Rattling-Induced Ultralow Thermal Conductivity Leading to Exceptional Thermoelectric Performance in AgIn5S8

Rattling has emerged as one of the most significant phenomenon for notably reducing the thermal conductivity in complex crystal systems. In this work, using first-principles density functional theory, we found that rattlers can be hosted in simpler crystal systems such as AgIn(5)S(8 )and CuIn5S8. Rattlers Ag and Cu exhibit weak and anisotropic bonding with the neighboring In and S and reside in a very shallow anharmonic potential well. The phonon spectra of these compounds have multiple avoided crossing of optical and acoustic modes, which are a signature of rattling motion. This leads to ultralow thermal conductivity, which is inversely proportional to mass and frequency span of rattling modes. Even though Ag atoms contribute to the valence band states, the rattler modes of Ag do not scatter carriers significantly, leaving the electronic transport virtually unaffected. Moreover, AgIn5S8 possesses a combination of heavy and light valence bands resulting in a very high power factor. A combination of favorable thermal and electronic transport results in a very high figure of merit of 2.2 in p-doped AgIn5S8 at 1000 K. The proposed idea of having rattlers in simpler systems can be extended to a wider class of materials, which would accelerate the development of thermoelectric modules for waste energy harvesting

Coupling the High-Throughput Property Map to Machine Learning for Predicting Lattice Thermal Conductivity

Low thermal conductivity materials are crucial for applications such as thermoelectric conversion of waste heat to useful energy and thermal barrier coatings. On the other hand, high thermal conductivity materials are necessary for cooling electronic devices. However, search for such materials via explicit evaluation of thermal conductivity either experimentally or computationally is very challenging. Here, we carried out high-throughput ab initio calculations, on a dataset containing 195 binary, ternary, and quaternary. compounds. The lattice thermal conductivity kappa(l) values of 120 dynamically stable and nonmetallic compounds are calculated, which span over 3 orders of magnitude. Among these, 11 ultrahigh and 15 ultralow kappa(l) materials are identified. An analysis of generated property map of this dataset reveals a strong dependence of kappa(l) on simple descriptors, namely, maximum phonon frequency, integrated Griineisen parameter up to 3 THz, average atomic mass, and volume of the unit cell. Using these descriptors, a Gaussian process regression-based machine learning (ML) model is developed. The model predicts log-scaled xi with a very small root mean square error of similar to 0.21. Comparatively, the Slack model, which uses more involved parameters, severely overestimates kappa(l). The superior performance of our ML model can ensure a reliable and accelerated search for multitude of low and high thermal conductivity materials

Pressure-Induced Topological Phase Transitions in CdGeSb₂ and CdSnSb₂

Using first-principles calculations, we study the occurrence of topological quantum phase transitions (TQPTs) as a function of hydrostatic pressure in CdGeSb₂ and CdSnSb₂ chalcopyrites. At ambient pressure, both materials are topological insulators, having a finite band gap with inverted order of Sb-s and Sb-p_<i>x</i>,p_<i>y</i> orbitals of valence bands at the Γ point. Under hydrostatic pressure, the band gap reduces, and at the critical point of the phase transition, these materials turn into Dirac semimetals. Upon further increasing the pressure beyond the critical point, the band inversion is reverted, making them trivial insulators. This transition is also captured by the Lüttinger model Hamiltonian, which demonstrates the critical role played by pressure-induced anisotropy in frontier bands in driving the phase transitions. These theoretical findings of peculiar coexistence of multiple topological phases provide a realistic and promising platform for experimental realization of the TQPTs

High Thermoelectric Performance in <i>n</i>‑Doped Silicon-Based Chalcogenide Si₂Te₃

Achieving large thermoelectric figure of merit in a low-cost material, having an appreciable degree of compatibility with the modern technology is required to convert waste heat into electrical energy efficiently. Using first-principles density functional theory and semiclassical Boltzmann transport theory, we report high thermoelectric performance of a silicon-based chalcogenide Si₂Te₃. Previously unknown ground state structure of Si₂Te₃ was obtained by finding out the 8 most energetically favorable sites for Si in a unit cell of 12 Te and 8 Si atoms. Out of total C­(28,8) combinations of structures, the search was narrowed down to 15 by using Wyckoff positions of space group <i>P</i>3̅<i>1c</i>. The minimum energy configuration having layered structure exhibits combination of desirable electronic and transport properties for an efficient thermoelectric material, including confinement of heavy and light bands near the band edges, large number of charge carrier pockets and low conductivity effective mass for <i>n</i>-type carriers. These features result into high thermopower and electrical conductivity leading to high power factor for <i>n</i>-type carriers. Furthermore, Si₂Te₃ possesses low frequency flat acoustical modes, which leads to low phonon group velocities and large negative Grüneisen parameters. These factors give rise to low lattice thermal conductivity below 2 W/mK at 1000 K. The combination of these excellent inherent electronic, transport and phononic properties renders an unprecedented <i>ZT</i> of 1.86 at 1000 K in <i>n</i>-doped Si₂Te₃, which is comparable to some of the best state-of-the-art thermoelectric materials. Our work presents an important advance in a long-standing search for the silicon-based thermoelectrics having exceptionally good energy conversion efficiency, and which could be integrated to the existing electronic devices

Origin of Ultralow Thermal Conductivity in n-Type Cubic Bulk AgBiS2: Soft Ag Vibrations and Local Structural Distortion Induced by the Bi 6s(2) Lone Pair

Crystalline materials with ultralow thermal conductivity are essential for thermal barrier coating and thermoelectric energy conversion. Nontoxic n-type bulk cubic AgBiS2 exhibits exceptionally low lattice thermal conductivity (kappa(lat)) of 0.68-0.48 W/m K in the temperature range of 298-820 K, which is near the theoretical minimum (kappa(min)). The low kappa(lat) is attributed to soft vibrations of predominantly Ag atoms and significant lattice anharmonicity because of local structural distortions along the 011] direction, arising because of the stereochemical activity of the 6s(2) lone pair of Bi, as suggested by pair distribution function analysis of the synchrotron X-ray scattering data. The low-temperature heat capacity of AgBiS2 shows a broad hump because of the Ag-induced low-energy Einstein modes as also suggested from phonon dispersion calculated by first-principle density functional theory. Low-energy optical phonons contributed by Ag and Bi strongly scatter heat-carrying acoustic phonons, thereby decreasing the kappa(lat) to a low value. A maximum thermoelectric figure of merit of similar to 0.7 is attained at 820 K for bulk spark plasma-sintered n-type AgBiS2