Strong electron–phonon coupling influences carrier transport and thermoelectric performances in group-IV/V elemental monolayers

The interactions between electrons and phonons play the key role in determining the carrier transport properties in semiconductors. In this work, comprehensive investigations on full electron–phonon (el–ph) couplings and their influences on carrier mobility and thermoelectric (TE) performances of 2D group IV and V elemental monolayers are performed, and we also analyze the selection rules on el–ph couplings using group theory. For shallow n/p-dopings in Si, Ge, and Sn, ZA/TA/LO phonon modes dominate the intervalley scatterings. Similarly strong intervalley scatterings via ZA/TO phonon modes can be identified for CBM electrons in P, As, and Sb, and for VBM holes, ZA/TA phonon modes dominate intervalley scatterings in P while LA phonons dominate intravalley scatterings in As and Sb. By considering full el–ph couplings, the TE performance for these two series of monolayers are predicted, which seriously downgrades the thermoelectric figures of merits compared with those predicted by the constant relaxation time approximation.

Approximate Models for the Lattice Thermal Conductivity of Alloy Thermoelectrics

Thermoelectric generators (TEGs) convert waste heat to electricity and are a leading contender for improving energy efficiency at a range of scales. Ideal TE materials show a large Seebeck effect, high electrical conductivity, and low thermal conductivity. Alloying is a widely-used approach to engineering the heat transport in TEs, but despite many successes the underlying mechanisms are poorly understood. In previous work, first-principles modelling has successfully been used to study the thermodynamics of alloy formation and to investigate its effect on the electronic structure and phonon spectrum. However, it has so far only been possible to examine qualitatively the impact of alloying on the lattice thermal conductivity. In this work, we develop and test two new approaches to addressing this. The constant relaxation-time approximation (CRTA) assumes the primary effect of alloying is on the phonon group velocities, and allows the thermal conductivity to be calculated assuming a suitable constant lifetime. Alternatively, setting the three-phonon interaction strengths to a constant further enables an assessment of how changes to the phonon frequency spectrum influence the lifetimes. We test both approaches for the <i>Pnma</i> Sn(S_1-<i>x</i>Se_<i>x</i>) alloy system and are able to account for the substantially-reduced thermal conductivity measured in experiments.

Efficient calculation of carrier scattering rates from first principles

The electronic transport behaviour of materials determines their suitability for technological applications. We develop a computationally efficient method for calculating carrier scattering rates of solid-state semiconductors and insulators from first principles inputs. The present method extends existing polar and non-polar electron-phonon coupling, ionized impurity, and piezoelectric scattering mechanisms formulated for isotropic band structures to support highly anisotropic materials. We test the formalism by calculating the electronic transport properties of 23 semiconductors, including the large 48 atom CH3NH3PbI3 hybrid perovskite, and comparing the results against experimental measurements and more detailed scattering simulations. The Spearman rank coefficient of mobility against experiment (rs = 0.93) improves significantly on results obtained using a constant relaxation time approximation (rs = 0.52). We find our approach offers similar accuracy to state-of-the art methods at approximately 1/500th the computational cost, thus enabling its use in high-throughput computational workflows for the accurate screening of carrier mobilities, lifetimes, and thermoelectric power.

Intrinsic defect formation and effect of transition metals doping on transport property in ductile thermoelectric material α-Ag2S: First-principles study

In this paper, the electronic structure and transport property of ductile thermoelectric material α-Ag2S are examined by the first-principles calculations combining with Boltzmann transport equation within the constant relaxation-time approximation....

Electronic Thermal Conductivity and Thermoelectric Performance of PbBi4Te7

Bismuth telluride and its related compounds are the state-of-the-art thermoelectric materials operating at room temperature. Bismuth telluride with Pb substituted, PbBi4Te7, has been found to be a new quasi-binary compound with an impressive high power factor. In this work, in the framework of density functional theory, we study the electronic thermal conductivity of the compound by employing the solution of Boltzmann Transport Equation in a constant relaxation-time approximation. The results show that the electronic thermal conductivity drastically increases with the increase of temperature and carrier concentration which have a detrimental effect on the thermoelectric performance. At a particular temperature, the competition between the thermal conductivity, the Seebeck coefficient and the electrical conductivity limits the thermoelectric figure of merit, ZT. The maximum ZT value of about 0.47 occurs at 520 K and at the carrier concentration of 5.0×1019cm-3 for n-type doping. This suggests that to maximize the thermoelectric performance of the compound, the carrier concentration must be carefully controlled and optimized whereas the best operating temperature is around 500 K.

Enhancement of the Physical Stability of Amorphous Sildenafil in a Binary Mixture, with either a Plasticizing or Antiplasticizing Compound

The main purpose of this paper was to evaluate the impact of both high- and low-Tg polymer additives on the physical stability of an amorphous drug, sildenafil (SIL). The molecular mobility of neat amorphous SIL was strongly affected by the polymeric excipients used (Kollidon VA64 (KVA) and poly(vinylacetate) (PVAc)). The addition of KVA slowed down the molecular dynamics of amorphous SIL (antiplasticizing effect), however, the addition of PVAc accelerated the molecular motions of the neat drug (plasticizing effect). Therefore, in order to properly assess the effect of the polymer on the physical stability of SIL, the amorphous samples at both: isothermal (at constant temperature—353 K) and isochronal (at constant relaxation time—τα = 1.5 ms) conditions were compared. Our studies showed that KVA suppressed the recrystallization of amorphous SIL more efficiently than PVAc. KVA improved the physical stability of the amorphous drug, regardless of the chosen concentration. On the other hand, in the case of PVAc, a low polymer content (i.e., 25 wt.%) destabilized amorphous SIL, when stored at 353 K. Nevertheless, at high concentrations of this excipient (i.e., 75 wt.%), its effect on the amorphous pharmaceutical seemed to be the opposite. Therefore, above a certain concentration, the PVAc presence no longer accelerates the SIL recrystallization process, but inhibits it.

Thermoelectric Properties of ALiF3 (A= Ca, Sr and Ba): First-Principles Calculation

The energy band structure obtained from WIEN2k calculations is used to calculate the transport coefficients via the semi-classical Boltzmann transport theory with constant relaxation time (t) as employed in the BoltzTraP package for ALiF3(A= Ca, Sr and Ba) using mBJ-GGA potential. The thermoelectric properties of the above compounds are investigated through the calculation of the main transport properties: Seebeck coefficient (S), electrical (s) and electronic thermal (ke) conductivity, figure of merit (ZT) and power factor. All compounds show insulating behavior.

Computational study on unconventional superconductivity and mechanical properties of novel antiferrromagnetic (Ca,Sr,Ba)Fe2Bi2 compounds

The ab initio calculation is performed to investigate about the structural and the electron transport properties of the experimentally reported (parent) compounds viz., BaFe2As2, SrFe2As2, CaFe2As2 and the novel compounds which are anticipated from our computational work namely BaFe2Bi2, SrFe2Bi2, CaFe2Bi2 with different magnetic order. The space group of the reported compounds is I4/mmm (139) and belong to ThCr2Si2 type. The formation energies of the reported compounds are compared in the anti-ferromagnetic (AFM), nonmagnetic (NM) and ferromagnetic (FM) orders. From the comparison, it reveals that the anti-ferro magnetism is the stabled state for the reported compounds. At ambient temperature with constant relaxation time, the resistivity, power factor, Seebeck coefficient and electrical conductivity are computed by using BoltzTraP transport theory code. To explain the superconducting nature of the novel compounds the transition temperature (T[Formula: see text]), electron–phonon coupling factor and Debye temperature are calculated and presented. The mechanical stability of the compounds is examined by using Young’s, bulk and shear modulus, anisotropy constant and Poisson’s ratio which are calculated by using Tetra-elastic code. The Mechanical Temperament of these compounds is analyzed by using Pugh’s ratio. The ELATE tool is used to visualize the elastic properties of these compounds. The thermodynamical stability of the compounds is examined by using Gibbs free energy, vibrational Helmholtz free energy and entropy which are calculated by using Gibbs2 code. All the properties of the theoretically predicted (novel) compounds are analyzed and compared with their parent (experimentally reported) compounds.

Hamiltonian dynamics of the SIS epidemic model with stochastic fluctuations

Empirical records of epidemics reveal that fluctuations are important factors for the spread and prevalence of infectious diseases. The exact manner in which fluctuations affect spreading dynamics remains poorly known. Recent analytical and numerical studies have demonstrated that improved differential equations for mean and variance of infected individuals reproduce certain regimes of the SIS epidemic model. Here, we show they form a dynamical system that follows Hamilton’s equations, which allow us to understand the role of fluctuations and their effects on epidemics. Our findings show the Hamiltonian is a constant of motion for large population sizes. For small populations, finite size effects break the temporal symmetry and induce a power-law decay of the Hamiltonian near the outbreak onset, with a parameter-free exponent. Away from the onset, the Hamiltonian decays exponentially according to a constant relaxation time, which we propose as a metric when fluctuations cannot be neglected.