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Non-universal Scaling of Thermoelectric Efficiency in 3D and 2D Thermoelectric Semiconductors ; Hasdeo, Eddwi Hesky in Advances in Natural Sciences: Nanoscience and Nanotechnology (2021), 12 We performed the first-principles calculation on common thermoelectric semiconductors Bi2Te3, Bi2Se3, SiGe, and PbTe in bulk three-dimension (3D) and two-dimension (2D). We found that miniaturization of ... [more ▼] We performed the first-principles calculation on common thermoelectric semiconductors Bi2Te3, Bi2Se3, SiGe, and PbTe in bulk three-dimension (3D) and two-dimension (2D). We found that miniaturization of materials does not generally increase the thermoelectric figure of merit (ZT) according to the Hicks and Dresselhaus (HD) theory. For example, ZT values of 2D PbTe (0.32) and 2D SiGe (0.04) are smaller than their 3D counterparts (0.49 and 0.09, respectively). Meanwhile, the ZT values of 2D Bi2Te3 (0.57) and 2D Bi2Se3 (0.43) are larger than the bulks (0.54 and 0.18, respectively), which agree with HD theory. The HD theory breakdown occurs because the band gap and band flatness of the materials change upon dimensional reduction. We found that flat bands give a larger electrical conductivity (σ) and electronic thermal conductivity (κel) in 3D materials, and smaller values in 2D materials. In all cases, maximum ZT values increase proportionally with the band gap and saturate for the band gap above 10 kBT. The 2D Bi2Te3 and Bi2Se3 obtain a higher ZT due to the flat corrugated bands and narrow peaks in their DOS. Meanwhile, the 2D PbTe violates HD theory due to the flatter bands it exhibits, while 2D SiGe possesses a small gap Dirac-cone band. [less ▲] Detailed reference viewed: 15 (3 UL)Electron hydrodynamics of anomalous Hall materials Hasdeo, Eddwi Hesky ; Ekström, Carl Johan Ingvar ; Idrisov, Edvin et al in Physical Review. B (2021), 103(12), 125106 We study two-dimensional electron systems in the hydrodynamic regime. We show that a geometrical Berry curvature modifies the effective Navier-Stokes equation for viscous electron flow in topological ... [more ▼] We study two-dimensional electron systems in the hydrodynamic regime. We show that a geometrical Berry curvature modifies the effective Navier-Stokes equation for viscous electron flow in topological materials. For small electric fields, the Hall current becomes negligible compared to the viscous longitudinal current. In this regime, we highlight an unconventional Poiseuille flow with an asymmetric profile and a deviation of the maximum of the current from the center of the system. In a two-dimensional infinite geometry, the Berry curvature leads to current whirlpools and an asymmetry of potential profile. This phenomenon can be probed by measuring the asymmetric non-local resistance profile. [less ▲] Detailed reference viewed: 36 (8 UL)Strain effects on band structure and Dirac nodal-line morphology of ZrSiSe ; Hasdeo, Eddwi Hesky in Journal of Applied Physics (2021), 129(1), 014306 The Dirac nodal-line semimetals are new promising materials for technological applications due to their exotic properties, which originate from band structure dispersion and nodal-line behavior. We report ... [more ▼] The Dirac nodal-line semimetals are new promising materials for technological applications due to their exotic properties, which originate from band structure dispersion and nodal-line behavior. We report strain effects on the band structure of ZrSiSe Dirac nodal-line semimetal through the density functional theory calculations. We found that the kz=0 Dirac nodal-line of ZrSiSe is robust to all strains under reasonable magnitude although there are significant changes in the band oscillation amplitude, bandgap, and band occupancy due to orbital interactions and the Fermi energy shift upon strains. We also found that the effective strains to tune the nodal-line and band structure are equi-biaxial tensile, uniaxial (100) tensile, and xz-plane shear strains. [less ▲] Detailed reference viewed: 51 (0 UL)Optical properties of topological flat and dispersive bands Habibi, Alireza ; ; et al E-print/Working paper (2021) We study the optical properties of topological flat and dispersive bands. Due to their topological nature, there exists an anomalous Hall response which gives rise to a transverse current without applied ... [more ▼] We study the optical properties of topological flat and dispersive bands. Due to their topological nature, there exists an anomalous Hall response which gives rise to a transverse current without applied magnetic field. The dynamical Hall conductivity of systems with flat bands exhibits a sign change when the excitation energy is on resonance with the band gap, similar to the magnetotransport Hall conductivity profile. The sign change of the Hall conductivity is located at the frequency corresponding to the singularity of the joint density of states, i.e., the van Hove singularity (VHS). For perfectly flat bands, this VHS energy matches the band gap. On the other hand, in the case of dispersive bands, the VHS energy is located above the band gap. As a result, the two features of the Hall conductivity, i.e., the resonant feature at the band gap and the sign change at the VHS energy, become separated. This anomalous Hall response rotates the polarization of an electric field and can be detected in the reflected and transmitted waves, as Kerr and Faraday rotations, respectively, thus allowing a simple optical characterization of topological flat bands. [less ▲] Detailed reference viewed: 18 (4 UL)Kerr effect in tilted nodal loop semimetals Ekström, Carl Johan Ingvar ; Hasdeo, Eddwi Hesky ; Farias, Maria Belen et al E-print/Working paper (2021) We investigate the optical activity of tilted nodal loop semimetals. We calculate the full conductivity matrix for a band structure containing a nodal loop with possible tilt in the x−y plane, which ... [more ▼] We investigate the optical activity of tilted nodal loop semimetals. We calculate the full conductivity matrix for a band structure containing a nodal loop with possible tilt in the x−y plane, which allows us to study the Kerr rotation and ellipticity both for a thin film and a bulk material. We find signatures in the Kerr signal that give direct information about the tilt velocity and direction, the radius of the nodal loop and the internal chemical potential of the system. These findings should serve as guide to understanding optical measurements of nodal loop semimetals and as an additional tool to characterize them. [less ▲] Detailed reference viewed: 15 (1 UL)Formation of ultra-thin Ge1−xSnx/Ge1−x−ySixSny quantum heterostructures and their electrical properties for realizing resonant tunneling diode ; ; et al in Applied Physics Letters (2020), 117(23), 232104 Huge thermal noise owing to the narrow energy bandgap is one of the critical issues for group IV-based photonics in the mid-infrared regime. With this motivation, we examined to form Ge1−xSnx ... [more ▼] Huge thermal noise owing to the narrow energy bandgap is one of the critical issues for group IV-based photonics in the mid-infrared regime. With this motivation, we examined to form Ge1−xSnx/Ge1−x−ySixSny quantum heterostructures (QHs) by molecular beam epitaxy for realizing resonant tunneling diodes composed of group-IV materials. We confirmed the formation of approximately 2 nm-thick Ge1−xSnx/Ge1−x−ySixSny QHs with atomically flat interfaces by x-ray diffraction and transmission electron microscopy methods. Moreover, by the current density–voltage (J–V) measurement at 10 K, we observed the occurrence of a non-linear distinct hump in the J–V characteristic, which is possibly originated from quantum transport of heavy holes. According to the tunneling transmission spectra simulation result, the hump property would be due to two possible scenarios: a resonant tunneling of heavy holes in the QH and/or a resonance phenomenon that heavy holes pass just above a potential barrier. [less ▲] Detailed reference viewed: 58 (0 UL)Thermoelectric properties of two-dimensional hydrogenated borophene: A first-principles study ; Hasdeo, Eddwi Hesky ; et al in AIP Conference Proceedings (2020), 2256(1), 030017 We theoretically study electronic and thermoelectric properties of two-dimensional hydrogenated borophene (”boro-phane”). We show that, according to the first-principles calculation, hydrogenated ... [more ▼] We theoretically study electronic and thermoelectric properties of two-dimensional hydrogenated borophene (”boro-phane”). We show that, according to the first-principles calculation, hydrogenated borophene is semimetallic, with two bands meeting at a single point at the Fermi level. The thermoelectric properties evaluated by using the Boltzmann equation with a constant relaxation time approximation (CRTA). At room temperature, we obtain large power factor for electron doping regime. Therefore, appropriate doping to this material can enhance its thermoelectric efficiency. [less ▲] Detailed reference viewed: 28 (0 UL)Thermoelectric properties of two-dimensional Dirac materials Hasdeo, Eddwi Hesky ; ; in AIP Conference Proceedings (2020), 2256(1), 030010 We performed Boltzmann transport calculation to obtain the Seebeck coefficient, electrical conductivity, electronic thermal conductivity, and thermoelectric figure of merit (ZT) for Dirac systems. We ... [more ▼] We performed Boltzmann transport calculation to obtain the Seebeck coefficient, electrical conductivity, electronic thermal conductivity, and thermoelectric figure of merit (ZT) for Dirac systems. We found an enhancement of ZT due to the gap opening. When the phonon thermal conductivity is small enough, the optimum ZT in gapped Dirac system can be larger than 1, which is preferable for thermoelectric applications. [less ▲] Detailed reference viewed: 17 (0 UL)Ab-initio calculation of muon spin polarization function to study lithium-ion diffusion in LiTi2O4 battery material ; ; et al in AIP Conference Proceedings (2020), 2256(1), 030015 We report the study of lithium-ion diffusion in LiTi2O4 battery material by the analysis of muon spin polarization function at the muon site by DFT calculation. The important parameters which explain the ... [more ▼] We report the study of lithium-ion diffusion in LiTi2O4 battery material by the analysis of muon spin polarization function at the muon site by DFT calculation. The important parameters which explain the lithium-ion diffusion will be derived from the function, including the field fluctuation rate and the local field distribution. The calculated results are shown in good agreement with the previously measured field distribution and the field fluctuation rate in LiTi2O4 at the ground state temperature. This method, therefore, may apply to the study of other battery materials. [less ▲] Detailed reference viewed: 57 (1 UL)Thermoelectric properties of Mexican-hat band structures ; ; et al in Advances in Natural Sciences: Nanoscience and Nanotechnology (2020), 11(1), 015012 Materials with Mexican-hat electronic energy dispersions emerging from heterostructures, substrate effects, or spin–orbit couplings are believed to exhibit excellent thermoelectric properties due to its ... [more ▼] Materials with Mexican-hat electronic energy dispersions emerging from heterostructures, substrate effects, or spin–orbit couplings are believed to exhibit excellent thermoelectric properties due to its van Hove singularity of density of states in two-dimension. However, within a constant relaxation time approximation, we disprove this belief and we find that the singularity effect is cancelled down by the group velocity contribution in the thermoelectric transport distribution. Nevertheless, the band parameters can still be optimised to reach thermoelectric figure of merit larger than 2 in a wide bandwidth, thus keeping the potential of materials with Mexican-hat bands for thermoelectric applications. [less ▲] Detailed reference viewed: 16 (0 UL)Thermoelectric properties of Mexican-hat band structures ; ; et al in Advances in Natural Sciences: Nanoscience and Nanotechnology (2020), 11(1), 015012 Materials with Mexican-hat electronic energy dispersions emerging from heterostructures, substrate effects, or spin–orbit couplings are believed to exhibit excellent thermoelectric properties due to its ... [more ▼] Materials with Mexican-hat electronic energy dispersions emerging from heterostructures, substrate effects, or spin–orbit couplings are believed to exhibit excellent thermoelectric properties due to its van Hove singularity of density of states in two-dimension. However, within a constant relaxation time approximation, we disprove this belief and we find that the singularity effect is cancelled down by the group velocity contribution in the thermoelectric transport distribution. Nevertheless, the band parameters can still be optimised to reach thermoelectric figure of merit larger than 2 in a wide bandwidth, thus keeping the potential of materials with Mexican-hat bands for thermoelectric applications. [less ▲] Detailed reference viewed: 16 (0 UL)Modulation of coherent phonon amplitudes in low-dimensional materials by ultrafast laser pulse trains ; ; et al in AIP Conference Proceedings (2020), 2256(1), 020005 We theoretically investigate how coherent phonon amplitudes in low dimensional materials can be modulated by ultrafast laser pulse trains. Important laser parameters for the modulation of coherent phonon ... [more ▼] We theoretically investigate how coherent phonon amplitudes in low dimensional materials can be modulated by ultrafast laser pulse trains. Important laser parameters for the modulation of coherent phonon amplitudes are the pulse width, repetition period, and number of pulses in the pulse train. We find that it is possible to switch on or switch off the radial breathing mode (RBM) and the G-band phonons in a single wall carbon nanotube (SWNT), as a typical model of low-dimensional materials. In particular, if the repetition period matches with integer multiple of the RBM phonon period, the RBM phonon could be switched on, while the other modes are switched off. On the other hand, for the G-band, which has a higher frequency (shorter period) than the RBM, the number of pulses in the pulse train also affects the switching process. The ratios of the G-band and RBM amplitudes are found to be significantly enhanced at certain integer multiples of pulse number as a function of SWNT diameter. Such a “magic number” phenomenon in ultrafast spectroscopy can be extended to other low-dimensional materials, in which we may realize a phonon switch in the future. [less ▲] Detailed reference viewed: 50 (2 UL)Origin of the Flat Band in Heavily Cs-Doped Graphene ; ; et al in ACS Nano (2020), 14(1), 1055 A flat energy dispersion of electrons at the Fermi level of a material leads to instabilities in the electronic system and can drive phase transitions. Here we show that the flat band in graphene can be ... [more ▼] A flat energy dispersion of electrons at the Fermi level of a material leads to instabilities in the electronic system and can drive phase transitions. Here we show that the flat band in graphene can be achieved by sandwiching a graphene monolayer by two cesium (Cs) layers. We investigate the flat band by a combination of angle-resolved photoemission spectroscopy experiment and the calculations. Our work highlights that charge transfer, zone folding of graphene bands, and the covalent bonding between C and Cs atoms are the origin of the flat energy band formation. Analysis of the Stoner criterion for the flat band suggests the presence of a ferromagnetic instability. The presented approach is an alternative route for obtaining flat band materials to twisting bilayer graphene which yields thermodynamically stable flat band materials in large areas. [less ▲] Detailed reference viewed: 24 (1 UL)The Quest and Hope of Majorana Zero Modes in Topological Superconductor for Fault Tolerant Quantum Computing: An Introductory Overview ; ; Hasdeo, Eddwi Hesky E-print/Working paper (2020) Ettore Majorana, in his short life, unintendedly has uncovered the most profound problem in quantum computation by his discovery of Majorana fermion, a particle which is its own anti-particle. Owing to ... [more ▼] Ettore Majorana, in his short life, unintendedly has uncovered the most profound problem in quantum computation by his discovery of Majorana fermion, a particle which is its own anti-particle. Owing to its non-Abelian exchange statistics, Majorana fermions may act as a qubit for a universal quantum computer which is fault-tolerant. The existence of such particle is predicted in mid-gap states (zero modes) of a topological superconductor as bound states that have a highly entangled degenerate ground state. This introductory overview will focus on the simplest theoretical proposals of Majorana fermions for topological quantum computing in superconducting systems, emphasizing the quest from the scalability problem of quantum computer to its possible solution with topological quantum computer employing non-Abelian anyons on various platforms of certain Majorana fermion signature encountered. [less ▲] Detailed reference viewed: 18 (1 UL)Characterization of electron and phonon transports in Bi-doped CaMnO3 for thermoelectric applications ; ; et al in arXiv (2020) Electron and phonon transports in CaMnO3 and its Bi-doped counterpart, Bi0. 03Ca0. 97MnO3, are investigated by thermoelectric transport measurements, Raman spectroscopy, and first-principles calculations ... [more ▼] Electron and phonon transports in CaMnO3 and its Bi-doped counterpart, Bi0. 03Ca0. 97MnO3, are investigated by thermoelectric transport measurements, Raman spectroscopy, and first-principles calculations. In particular, we focus on CaMnO3 and Bi0. 03Ca0. 97MnO3's electronic structures, temperature-dependent electron and phonon lifetimes, and their sound velocities. We find that the anti-ferromagnetic insulator CaMnO3 breaks the Wiedemann-Franz (WF) law with the Lorenz number reaching four times that of ordinary metals at room temperature. Bismuth doping reduces both the electrical resistivity and the Seebeck coefficient of CaMnO3, thus it recovers the WF law behavior. Raman spectroscopy confirms that Bi0. 03Ca0. 97MnO3 has a lower Debye frequency as well as a shorter phonon lifetime. As a result, Bi0. 03Ca0. 97MnO3 exhibits superior thermoelectric properties over the pristine CaMnO3 due to the lower thermal conductivity and electronic resistivity. [less ▲] Detailed reference viewed: 14 (2 UL) |
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