Predicted Janus monolayer ZrSSe with enhanced n-type thermoelectric properties compared with monolayer ZrS2\mathrm{ZrS_2}

In analogy to transition-metal dichalcogenide (TMD) monolayers, which have wide applications in photoelectricity, piezoelectricity and thermoelectricity, Janus MoSSe monolayer has been successfully synthesized by substituting the top Se atomic layer in $\mathrm{MoSe_2}$ by S atoms. In this work, Janus monolayer ZrSSe is proposed by ab initio calculations. For the electron part, the generalized gradient approximation (GGA) plus spin-orbit coupling (SOC) is used as exchange-correlation potential, while GGA for lattice part. Calculated results show that the ZrSSe monolayer is dynamically and mechanically stable, which exhibits mechanical flexibility due to small Young's modulus. It is found that ZrSSe monolayer is an indirect-gap semiconductors with band gap of 0.60 eV. The electronic and phonon transports of ZrSSe monolayer are investigated by semiclassical Boltzmann transport theory. In n-type doping, the $ZT_e$ between ZrSSe and $\mathrm{ZrS_2}$ monolayers is almost the same due to similar outlines of conduction bands. The p-type $ZT_e$ of ZrSSe monolayer is lower than that of $\mathrm{ZrS_2}$ monolayer, which is due to larger spin-orbit splitting for ZrSSe than $\mathrm{ZrS_2}$ monolayer. The room-temperature sheet thermal conductance is 33.6 $\mathrm{W K^{-1}}$ for ZrSSe monolayer, which is lower than 47.8 $\mathrm{W K^{-1}}$ of $\mathrm{ZrS_2}$ monolayer. Compared to $\mathrm{ZrS_2}$ monolayer, the low sheet thermal conductance of ZrSSe monolayer is mainly due to small group velocities and short phonon lifetimes of ZA mode. Considering their $ZT_e$ and lattice thermal conductivities, the ZrSSe monolayer may have better n-type thermoelectric performance than $\mathrm{ZrS_2}$ monolayer. These results can stimulate further experimental works to synthesize ZrSSe monolayer.Comment: 9 pages, 12 figure

Strain effect on power factor in monolayer MoS2\mathrm{MoS_2}

Biaxial strain dependence of electronic structures and thermoelectric properties of monolayer $\mathrm{MoS_2}$, including compressive and tensile strain, are investigated by using local-density approximation (LDA) plus spin-orbit coupling (SOC). Both LDA and LDA+SOC results show that $\mathrm{MoS_2}$ is a direct gap semiconductor with optimized lattice constants. It is found that SOC has important effect on power factor, which can enhance one in n-type doping, but has a obvious detrimental influence for p-type. Both compressive and tensile strain can induce direct-indirect gap transition, which produce remarkable influence on power factor. Calculated results show that strain can induce significantly enhanced power factor in n-type doping by compressive strain and in p-type doping by tensile strain at the critical strain of direct-indirect gap transition. These can be explained by strain-induced accidental degeneracies, which leads to improved Seebeck coefficient. Calculated results show that n-type doping can provide better power factor than p-type doping. These results make us believe that thermoelectric properties of monolayer $\mathrm{MoS_2}$ can be improved in n-type doping by compressive strain.Comment: 6 pages, 6 figure

Potential thermoelectric material Cs2[PdCl4]I2\mathrm{Cs_2[PdCl_4]I_2}: a first-principles study

The electronic structures and thermoelectric properties of $\mathrm{Cs_2[PdCl_4]I_2}$ are investigated by the first-principles calculations and semiclassical Boltzmann transport theory. Both electron and phonon transport are considered to attain the figure of merit $ZT$. A modified Becke and Johnson (mBJ) exchange potential, including spin-orbit coupling (SOC), is employed to investigate electronic part of $\mathrm{Cs_2[PdCl_4]I_2}$. It is found that SOC has obvious effect on valence bands, producing huge spin-orbital splitting, which leads to remarkable detrimental effect on p-type power factor. However, SOC has a negligible influence on conduction bands, so the n-type power factor hardly change. The temperature dependence of lattice thermal conductivity by assuming an inverse temperature dependence is attained from reported ultralow lattice thermal conductivity of 0.31 $\mathrm{W m^{-1} K^{-1}}$ at room temperature. Calculating scattering time $\tau$ is challenging, but a hypothetical $\tau$ can be adopted to estimate thermoelectric conversion efficiency. The maximal figure of merit $ZT$ is up to about 0.70 and 0.60 with scattering time $\tau$=$10^{-14}$ s and $\tau$=$10^{-15}$ s, respectively. These results make us believe that $\mathrm{Cs_2[PdCl_4]I_2}$ may be a potential thermoelectric material.Comment: 5 pages, 6 figures. arXiv admin note: text overlap with arXiv:1605.0888

Biaxial tensile strain tuned up-and-down behavior on lattice thermal conductivity in β\beta-AsP monolayer

Various two-dimensional (2D) materials with graphene-like buckled structure emerge, and the $\beta$-phase AsP monolayer has been recently proposed to be thermodynamically stable from first-principles calculations. The studies of thermal transport are very useful for these 2D materials-based nano-electronics devices. Motivated by this, a comparative study of strain-dependent phonon transport of AsP monolayer is performed by solving the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). It is found that the lattice thermal conductivity ($\kappa_L$) of AsP monolayer is very close to one of As monolayer with similar buckled structure, which is due to neutralization between the reduce of phonon lifetimes and group velocity enhancement from As to AsP monolayer. The corresponding room-temperature sheet thermal conductance of AsP monolayer is 152.5 $\mathrm{W K^{-1}}$. It is noted that the increasing tensile strain can harden long wavelength out-of-plane (ZA) acoustic mode, and soften the in-plane longitudinal acoustic (LA) and transversal acoustic (TA) modes. Calculated results show that $\kappa_L$ of AsP monolayer presents a nonmonotonic up-and-down behavior with increased strain. The unusual strain dependence is due to the competition among reduce of phonon group velocities, improved phonon lifetimes of ZA mode and nonmonotonic up-and-down phonon lifetimes of TA/LA mode. It is found that acoustic branches dominate the $\kappa_L$ in considered strain range, and the contribution from ZA branch increases with increased strain, while it is opposite for TA/LA branch. By analyzing cumulative $\kappa_L$ with respect to phonon mean free path (MFP), tensile strain can modulate effectively size effects on $\kappa_L$ in AsP monolayer.Comment: 8 pages, 13 figure

Thermal Resonance Fusion

We first show a possible mechanism to create a new type of nuclear fusion, thermal resonance fusion, i.e. low energy nuclear fusion with thermal resonance of light nuclei or atoms, such as deuterium or tritium. The fusion of two light nuclei has to overcome the Coulomb barrier between these two nuclei to reach up to the interacting region of nuclear force. We found nuclear fusion could be realized with thermal vibrations of crystal lattice atoms coupling with light atoms at low energy by resonance to overcome this Coulomb barrier. Thermal resonances combining with tunnel effects can greatly enhance the probability of the deuterium fusion to the detectable level. Our low energy nuclear fusion mechanism research - thermal resonance fusion mechanism results demonstrate how these light nuclei or atoms, such as deuterium, can be fused in the crystal of metal, such as Ni or alloy, with synthetic thermal vibrations and resonances at different modes and energies experimentally. The probability of tunnel effect at different resonance energy given by the WKB method is shown that indicates the thermal resonance fusion mode, especially combined with the tunnel effect, is possible and feasible. But the penetrating probability decreases very sharply when the input resonance energy decreases less than 3 keV, so for thermal resonance fusion, the key point is to increase the resonance peak or make the resonance sharp enough to the acceptable energy level by the suitable compound catalysts, and it is better to reach up more than 3 keV to make the penetrating probability larger than 10^{-10}.Comment: 4 pages, 3 figure

Isolated highly localized bands in YbI2\mathrm{YbI_2} monolayer caused by 4f4f orbitals

The novel electronic structures can induce unique physical properties in two-dimensional (2D) materials. In this work, we report isolated highly localized bands in $\mathrm{YbI_2}$ monolayer by the first-principle calculations within generalized gradient approximation (GGA) plus spin-orbit coupling (SOC). It is found that $\mathrm{YbI_2}$ monolayer is an indirect-gap semiconductor using both GGA and GGA+SOC. The calculations reveal that Yb-$4f$ orbitals constitute isolated highly localized bands below the Fermi level at the absence of SOC, and the bands are split into the $j = 7/2$ and $j = 5/2$ states with SOC. The isolated highly localized bands can lead to very large Seebeck coefficient and very low electrical conductivity in p-type doping by producing very large effective mass of the carrier. It is proved that isolated highly localized bands have very strong stability again strain, which is very important for practical application. When the onsite Coulomb interaction is added to the Yb-$4f$ orbitals, isolated highly localized bands persist, and only their relative positions in the gap change. These findings open a new window to search for novel electronic structures in 2D materials.Comment: 5 pages, 7 figure

Born effective charge removed anomalous temperature dependence of lattice thermal conductivity in monolayer GeC

Due to potential applications in nano- and opto-electronics, two-dimensional (2D) materials have attracted tremendous interest. Their thermal transport properties are closely related to the performance of 2D materials-based devices. Here, the phonon transports of monolayer GeC with a perfect planar hexagonal honeycomb structure are investigated by solving the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). Without inclusion of Born effective charges ($Z^*$) and dielectric constants ($\varepsilon$), the lattice thermal conductivity ($\kappa_L$) almost decreases linearly above 350 K, deviating from the usual $\kappa_L$$\sim$$1/T$ law. The underlying mechanism is because the contribution to $\kappa_L$ from high-frequency optical phonon modes increases with increasing temperature, and the contribution exceeds one from acoustic branches at high temperature. These can be understood by huge phonon band gap caused by large difference in atom mass between Ge and C atoms, which produces important effects on scattering process involving high-frequency optical phonon. When considering $Z^*$ and $\varepsilon$, the phonon group velocities and phonon lifetimes of high-frequency optical phonon modes are obviously reduced with respect to ones without $Z^*$ and $\varepsilon$. The reduced group velocities and phonon lifetimes give rise to small contribution to $\kappa_L$ from high-frequency optical phonon modes, which produces the the traditional $\kappa_L$$\sim$$1/T$ relation in monolayer GeC. Calculated results show that the isotope scattering can also reduce anomalous temperature dependence of $\kappa_L$ in monolayer GeC. Our works highlight the importance of $Z^*$ and $\varepsilon$ to investigate phonon transports of monolayer GeC.Comment: 7 pages, 9 figure

Lower lattice thermal conductivity in SbAs than As or Sb monolayer: a first-principles study

Phonon transports of group-VA elements (As, Sb, Bi) monolayer semiconductors have been widely investigated in theory, and Sb monolayer (antimonene) of them has recently been synthesized. In this work, phonon transport of SbAs monolayer is investigated from a combination of first-principles calculations and the linearized phonon Boltzmann equation. It is found that the lattice thermal conductivity of SbAs monolayer is lower than ones of both As and Sb monolayers, and the corresponding sheet thermal conductance is 28.8 $\mathrm{W K^{-1}}$ at room temperature. Calculated results show that group velocities of SbAs monolayer are between ones of As and Sb onolayers, but phonon lifetimes of SbAs are smaller than ones of both As and Sb monolayers. Hence, low lattice thermal conductivity in SbAs monolayer is attributed to very small phonon lifetimes. Unexpectedly, the ZA branch has very little contribution to the total thermal conductivity, only 2.4\%, which is obviously different from ones of As and Sb monolayers with very large contribution. This can be explained by very small phonon lifetimes for ZA branch of SbAs monolayer. The large charge transfer from Sb to As atoms leads strongly polarized covalent bond, being different from As or Sb monolayer. The strongly polarized covalent bond of SbAs monolayer can induce stronger phonon anharmonicity than As or Sb monolayer, leading to lower lattice thermal conductivity. It is found that isotope scattering produces neglectful effect, and the lattice thermal conductivity with the characteristic length smaller than 30 nm can reach a decrease of about 47\%. These results may offer perspectives on tuning lattice thermal conductivity by mixture of multi-elements for applications of thermal management and thermoelectricity, and motivate further experimental efforts to synthesize monolayer SbAs.Comment: 7 pages, 8 figure

Phonon transport of three-fold degeneracy topological semimetal MoP

Recently, three-component new fermions in topological semimetal MoP are experimentally observed, which may have potential applications like topological qubits, low-power electronics and spintronics. These are closely related to thermal transport properties of MoP. In this work, the phonon transport of MoP is investigated by solving the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). The calculated room-temperature lattice thermal conductivity is 18.41 $\mathrm{W m^{-1} K^{-1}}$ and 34.71 $\mathrm{W m^{-1} K^{-1}}$ along the in- and cross-plane directions, exhibiting very strong anisotropy. The isotope and size effects on the lattice thermal conductivity are also considered. It is found that isotope scattering produces little effect, and phonon has little contribution to the lattice thermal conductivity, when phonon mean free path(MFP) is larger than 0.15 $\mathrm{\mu m}$ at 300 K. It is noted that average room-temperature lattice thermal conductivity of MoP is lower than that of representative Weyl semimetal TaAs, which is due to smaller group velocities and larger Gr$\mathrm{\ddot{u}}$neisen parameters. Our works provide valuable informations for the thermal management of MoP-based nano-electronics devices, and motivate further experimental works to study thermal transport of MoP.Comment: 5 pages, 6 figure

Importance of spin-orbit coupling in power factor calculations for half-Heusler ANiB (A=Ti, Hf, Sc, Y; B=Sn, Sb, Bi)

We investigate the spin-orbit coupling (SOC) effects on the electronic structures and semi-classic transport coefficients of half-Heusler $\mathrm{ANiB}$ (A=Ti, Hf, Sc, Y; B=Sn, Sb, Bi) by using generalized gradient approximation (GGA). Calculated results show that SOC splits the valence bands at high symmetry $\Gamma$ point, and modifies the outline of $\Gamma$-centered valence bands, which has remarkable effects on the electron transport properties. Thermoelectric properties are performed through solving Boltzmann transport equations within the constant scattering time approximation. It is found that the compounds containing Sn atom have larger power factor in p-type doping than ones in n-type doping, and it is just the opposite for compounds containing Sb and Bi elements. The SOC has obvious detrimental influence on power factor in p-type doping, while has a negligible effect in n-type doping. These can be understood by considering the effects of SOC on the valence bands and conduction bands. The maximum power factors (MPF) are extracted in n-type and p-type doping with GGA and GGA+SOC, and the MPF at 300 K with SOC is predicted to be about 4.25\%$\sim$44.13\% smaller than that without SOC in the case of p-type doping for $\mathrm{ANiB}$ (A=Ti, Hf, Sc, Y; B=Sn, Sb, Bi). Therefore, it is crucial to consider SOC effects for theoretical analysis in the case of p-type doping in half-Heusler compounds composed of heavy elements.Comment: 5 pages, 5 figures in Journal of Alloys and Compounds (2016