We study the acoustomagnetoelectric (AME) effect in two-dimensional graphene with an energy bandgap using the semiclassical Boltzmann transport equation within the hypersound regime, (where represents the acoustic wav...We study the acoustomagnetoelectric (AME) effect in two-dimensional graphene with an energy bandgap using the semiclassical Boltzmann transport equation within the hypersound regime, (where represents the acoustic wavenumber and is the mean free path of the electron). The Boltzmann transport equation and other relevant equations were solved analytically to obtain an expression for the AME current density, consisting of longitudinal and Hall components. Our numerical results indicate that both components of the AME current densities display oscillatory behaviour. Furthermore, geometric resonances and Weiss oscillations were each defined using the relationship between the current density and Surface Acoustic Wave (SAW) frequency and the inverse of the applied magnetic field, respectively. Our results show that the AME current density of bandgap graphene, which can be controlled to suit a particular electronic device application, is smaller than that of (gapless) graphene and is therefore, more suited for nanophotonic device applications.展开更多
The van der Waals(vdW)heterostructures of bilayer transition metal dichalcogenide obtained by vertically stacking have drawn increasing attention for their enormous potential applications in semiconductors and insulat...The van der Waals(vdW)heterostructures of bilayer transition metal dichalcogenide obtained by vertically stacking have drawn increasing attention for their enormous potential applications in semiconductors and insulators.Here,by using the first-principles calculations and the phonon Boltzmann transport equation(BTE),we studied the phonon transport properties of WS2/WSe2 bilayer heterostructures(WS2/WSe2-BHs).The lattice thermal conductivity of the ideal WS2/WSe2-BHs crystals at room temperature(RT)was 62.98 W/mK,which was clearly lower than the average lattice thermal conductivity of WS2 and WSe2 single layers.Another interesting finding is that the optical branches below 4.73 THz and acoustic branches have powerful coupling,mainly dominating the lattice thermal conductivity.Further,we also noticed that the phonon mean free path(MFP)of the WS2/WSe2-BHs(233 nm)was remarkably attenuated by the free-standing monolayer WS2(526 nm)and WSe2(1720 nm),leading to a small significant size effect of the WS2/WSe2-BHs.Our results systematically demonstrate the low optical and acoustic phonon modes-dominated phonon thermal transport in heterostructures and give a few important guidelines for the synthesis of van der Waals heterostructures with excellent phonon transport properties.展开更多
We show that by integrating out the electric field and incorporating proper boundary conditions,a Boltzmann equation can describe electron transport properties,continuously from the diffusive to ballistic regimes.Gene...We show that by integrating out the electric field and incorporating proper boundary conditions,a Boltzmann equation can describe electron transport properties,continuously from the diffusive to ballistic regimes.General analytical formulas of the conductance in D = 1,2,3 dimensions are obtained,which recover the Boltzmann–Drude formula and Landauer–B ¨uttiker formula in the diffusive and ballistic limits,respectively.This intuitive and efficient approach can be applied to investigate the interplay of system size and impurity scattering in various charge and spin transport phenomena,when the quantum interference effect is not important.展开更多
We propose an optimized scheme to determine the smearing parameter in the Gaussian function that is used to replace the Dirac δ function in the first Brillouin zone sampling. The broadening width is derived by analyz...We propose an optimized scheme to determine the smearing parameter in the Gaussian function that is used to replace the Dirac δ function in the first Brillouin zone sampling. The broadening width is derived by analyzing the difference of the results from the phase-space method and Gaussian broadening method. As a demonstration, using the present approach,we investigate the phonon transport in a typical layered material, graphite. Our scheme is benchmarked by comparing with other zone sampling methods. Both the three-phonon phonon scattering rates and thermal conductivity are consistent with the prediction from the widely used tetrahedron method and adaptive broadening method. The computational efficiency of our scheme is more than one order of magnitude higher than the two other methods. Furthermore, the effect of fourphonon scattering in phonon transport in graphite is also investigated. It is found that four-phonon scattering reduces the through-plane thermal conductivity by 10%. Our methods could be a reference for the prediction of thermal conductivity of anisotropic material in the future.展开更多
The performance of double gate GaSb nMOSFETs with surface orientations of(100) and(111) are compared by deterministically solving the time-dependent Boltzmann transport equation(BTE).Results show that the on-sta...The performance of double gate GaSb nMOSFETs with surface orientations of(100) and(111) are compared by deterministically solving the time-dependent Boltzmann transport equation(BTE).Results show that the on-state current of the device with(111) surface orientation is almost three times larger than the(100) case due to the higher injection velocity.Moreover,the scattering rate of the(111) device is slightly lower than that of the(100) device.展开更多
Gallium nitride(GaN), the notable representative of third generation semiconductors, has been widely applied to optoelectronic and microelectronic devices due to its excellent physical and chemical properties. In th...Gallium nitride(GaN), the notable representative of third generation semiconductors, has been widely applied to optoelectronic and microelectronic devices due to its excellent physical and chemical properties. In this paper, we investigate the surface scattering effect on the thermal properties of GaN nanofilms. The contribution of surface scattering to phonon transport is involved in solving a Boltzmann transport equation(BTE). The confined phonon properties of GaN nanofilms are calculated based on the elastic model. The theoretical results show that the surface scattering effect can modify the cross-plane phonon thermal conductivity of GaN nanostructures completely, resulting in the significant change of size effect on the conductivity in GaN nanofilm. Compared with the quantum confinement effect, the surface scattering leads to the order-of-magnitude reduction of the cross-plane thermal conductivity in GaN nanofilm. This work could be helpful for controlling the thermal properties of Ga N nanostructures in nanoelectronic devices through surface engineering.展开更多
First-principles approaches have recently been developed to replace the phenomenological modeling approaches with adjustable parameters for calculating carrier mobilities in semiconductors.However,in addition to the h...First-principles approaches have recently been developed to replace the phenomenological modeling approaches with adjustable parameters for calculating carrier mobilities in semiconductors.However,in addition to the high computational cost,it is still a challenge to obtain accurate mobility for carriers with a complex band structure,e.g.,hole mobility in common semiconductors.Here,we present a computationally efficient approach using isotropic and parabolic bands to approximate the anisotropy valence bands for evaluating group velocities in the first-principles calculations.This treatment greatly reduces the computational cost in two ways:relieves the requirement of an extremely denseκmesh to obtain a smooth change in group velocity,and reduces the 5-dimensional integral to 3-dimensional integral.Taking Si and SiC as two examples,we find that this simplified approach reproduces the full first-principles calculation for mobility.If we use experimental effective masses to evaluate the group velocity,we can obtain hole mobility in excellent agreement with experimental data over a wide temperature range.These findings shed light on how to improve the first-principles calculations towards predictive carrier mobility in high accuracy.展开更多
We propose a deterministic solver for the time-dependent multi-subband Boltzmann transport equation(MSBTE)for the two dimensional(2D)electron gas in double gate metal oxide semiconductor field effect transistors(MOSFE...We propose a deterministic solver for the time-dependent multi-subband Boltzmann transport equation(MSBTE)for the two dimensional(2D)electron gas in double gate metal oxide semiconductor field effect transistors(MOSFETs)with flared out source/drain contacts.A realistic model with six-valleys of the conduction band of silicon and both intra-valley and inter-valley phonon-electron scattering is solved.We propose a second order finite volume method based on the positive and flux conservative(PFC)method to discretize the Boltzmann transport equations(BTEs).The transport part of the BTEs is split into two problems.One is a 1D transport problem in the position space,and the other is a 2D transport problem in the wavevector space.In order to reduce the splitting error,the 2D transport problem in the wavevector space is solved directly by using the PFC method instead of splitting into two 1D problems.The solver is applied to a nanoscale double gate MOSFET and the current-voltage characteristic is investigated.Comparison of the numerical results with ballistic solutions show that the scattering influence is not ignorable even when the size of a nanoscale semiconductor device goes to the scale of the electron mean free path.展开更多
We have studied the axial resistivity of chiral single-walled carbon nanotubes (SWCNTs) in the presence of a combined direct current and high frequency alternating fields. We employed semiclassical Boltzmann equations...We have studied the axial resistivity of chiral single-walled carbon nanotubes (SWCNTs) in the presence of a combined direct current and high frequency alternating fields. We employed semiclassical Boltzmann equations approach and compared our results with a similar study that examined the circumferential resistivity of these unique materials. Our work shows that these materials display similar resistivity for both axial and circumferential directions and this largely depends on temperature, intensities of the applied fields and material parameters such as chiral angle. Based on these low-temperature bidirectional conductivity responses, we propose chiral SWCNTs for design of efficient optoelectronic devices.展开更多
Hafnium diboride(HfB_(2))is an important metallic ceramic that works in harsh environments,due to its high strength and thermal conductivity.Although the thermal conductivity of HfB_(2) has been measured,the experimen...Hafnium diboride(HfB_(2))is an important metallic ceramic that works in harsh environments,due to its high strength and thermal conductivity.Although the thermal conductivity of HfB_(2) has been measured,the experimental results are scattered.Also,the thermal transport mechanism of HfB_(2) is not well understood.In this work,we study the thermal transport in both pristine and defective HfB_(2) from first-principles calculations.For the pristine HfB_(2),the room-temperature thermal conductivities are 175.0 and 157.7 W·m^(-1)·K^(-1)on a-and c-axes,respectively,where the contributions from electron and phonon are comparable.The Lorenz number is significantly smaller than the Sommerfeld value and shows a temperature dependence,which demonstrates that the Wiedemann-Franz law cannot be used to estimate electronic thermal conductivity.The phonon-isotope and the phonon-electron scattering are non-negligible compared to the phonon-phonon scattering.For the defective HfB_(2),the grain size effects are negligible with length scales larger than 1μm.The pore can limit thermal conductivity when its occupancy is larger than 10%.The vacancy is found to induce scattered results in experiments.The phonon thermal conductivity significantly reduces even with only 1%vacancy,while the electronic thermal conductivity is not sensitive to the vacancy.Our study provides an in-depth understanding of the thermal transport in HfB_(2),and the revealed mechanisms provide important guidance on the design of HfB_(2)-based materials.展开更多
This article presents an extensive review of the status of the search of the dark matter. The first eight sections are devoted to topics in dark matter and its experimental searches, and the rest to selected topics in...This article presents an extensive review of the status of the search of the dark matter. The first eight sections are devoted to topics in dark matter and its experimental searches, and the rest to selected topics in astrophysics and cosmology, which are intended to supply some of the needed background for students in particle physics. Sections 9 and 13 are introductory cosmology. The three astrophysical topics, Big Bang nucleosynthesis Section 10, Boltzmann transport equation and freeze out of massive particles Section 11, and CMB anisotropy Section 12 can all be studied in analytical approaches when reasonable approximations are made. Their original analytically forms, to which this article follows very closely, were given by particle physicists. Dark matter is an evolving subject requiring timely update to stay current. Hence a review of such a subject matter would undoubtedly have something wanting when it appears in print. It is hoped that this review can form a humble basis for those graduate students who would like to pursue the subject of dark matter. The reader can use the extensive table of contents to see in some details the materials covered in the article.展开更多
文摘We study the acoustomagnetoelectric (AME) effect in two-dimensional graphene with an energy bandgap using the semiclassical Boltzmann transport equation within the hypersound regime, (where represents the acoustic wavenumber and is the mean free path of the electron). The Boltzmann transport equation and other relevant equations were solved analytically to obtain an expression for the AME current density, consisting of longitudinal and Hall components. Our numerical results indicate that both components of the AME current densities display oscillatory behaviour. Furthermore, geometric resonances and Weiss oscillations were each defined using the relationship between the current density and Surface Acoustic Wave (SAW) frequency and the inverse of the applied magnetic field, respectively. Our results show that the AME current density of bandgap graphene, which can be controlled to suit a particular electronic device application, is smaller than that of (gapless) graphene and is therefore, more suited for nanophotonic device applications.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.51720105007,51806031,11602149,and GZ1257)the Fundamental Research Funds for the Central Universities,China(Grant Nos.DUT16RC(3)116 and DUT19RC(3)006)The computing resources from Supercomputer Center of Dalian University of Technology and ScGrid are greatly acknowledged。
文摘The van der Waals(vdW)heterostructures of bilayer transition metal dichalcogenide obtained by vertically stacking have drawn increasing attention for their enormous potential applications in semiconductors and insulators.Here,by using the first-principles calculations and the phonon Boltzmann transport equation(BTE),we studied the phonon transport properties of WS2/WSe2 bilayer heterostructures(WS2/WSe2-BHs).The lattice thermal conductivity of the ideal WS2/WSe2-BHs crystals at room temperature(RT)was 62.98 W/mK,which was clearly lower than the average lattice thermal conductivity of WS2 and WSe2 single layers.Another interesting finding is that the optical branches below 4.73 THz and acoustic branches have powerful coupling,mainly dominating the lattice thermal conductivity.Further,we also noticed that the phonon mean free path(MFP)of the WS2/WSe2-BHs(233 nm)was remarkably attenuated by the free-standing monolayer WS2(526 nm)and WSe2(1720 nm),leading to a small significant size effect of the WS2/WSe2-BHs.Our results systematically demonstrate the low optical and acoustic phonon modes-dominated phonon thermal transport in heterostructures and give a few important guidelines for the synthesis of van der Waals heterostructures with excellent phonon transport properties.
基金Project supported by the National Basic Research Program of China(Grant Nos.2015CB921202 and 2014CB921103)the National Natural Science Foundation of China(Grant No.11225420)
文摘We show that by integrating out the electric field and incorporating proper boundary conditions,a Boltzmann equation can describe electron transport properties,continuously from the diffusive to ballistic regimes.General analytical formulas of the conductance in D = 1,2,3 dimensions are obtained,which recover the Boltzmann–Drude formula and Landauer–B ¨uttiker formula in the diffusive and ballistic limits,respectively.This intuitive and efficient approach can be applied to investigate the interplay of system size and impurity scattering in various charge and spin transport phenomena,when the quantum interference effect is not important.
基金support from the National Natural Science Foundation of China (Grant No. 51706134)。
文摘We propose an optimized scheme to determine the smearing parameter in the Gaussian function that is used to replace the Dirac δ function in the first Brillouin zone sampling. The broadening width is derived by analyzing the difference of the results from the phase-space method and Gaussian broadening method. As a demonstration, using the present approach,we investigate the phonon transport in a typical layered material, graphite. Our scheme is benchmarked by comparing with other zone sampling methods. Both the three-phonon phonon scattering rates and thermal conductivity are consistent with the prediction from the widely used tetrahedron method and adaptive broadening method. The computational efficiency of our scheme is more than one order of magnitude higher than the two other methods. Furthermore, the effect of fourphonon scattering in phonon transport in graphite is also investigated. It is found that four-phonon scattering reduces the through-plane thermal conductivity by 10%. Our methods could be a reference for the prediction of thermal conductivity of anisotropic material in the future.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.61674008,61421005,and 61404005)
文摘The performance of double gate GaSb nMOSFETs with surface orientations of(100) and(111) are compared by deterministically solving the time-dependent Boltzmann transport equation(BTE).Results show that the on-state current of the device with(111) surface orientation is almost three times larger than the(100) case due to the higher injection velocity.Moreover,the scattering rate of the(111) device is slightly lower than that of the(100) device.
基金supported by the National Natural Science Foundation of China(Grant Nos.11302189 and 11321202)the Doctoral Fund of Ministry of Education of China(Grant No.20130101120175)
文摘Gallium nitride(GaN), the notable representative of third generation semiconductors, has been widely applied to optoelectronic and microelectronic devices due to its excellent physical and chemical properties. In this paper, we investigate the surface scattering effect on the thermal properties of GaN nanofilms. The contribution of surface scattering to phonon transport is involved in solving a Boltzmann transport equation(BTE). The confined phonon properties of GaN nanofilms are calculated based on the elastic model. The theoretical results show that the surface scattering effect can modify the cross-plane phonon thermal conductivity of GaN nanostructures completely, resulting in the significant change of size effect on the conductivity in GaN nanofilm. Compared with the quantum confinement effect, the surface scattering leads to the order-of-magnitude reduction of the cross-plane thermal conductivity in GaN nanofilm. This work could be helpful for controlling the thermal properties of Ga N nanostructures in nanoelectronic devices through surface engineering.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11925407 and 61927901)the Key Research Program of Frontier Sciences,Chinese Academy of Sciences(Grant No.ZDBS-LY-JSC019).
文摘First-principles approaches have recently been developed to replace the phenomenological modeling approaches with adjustable parameters for calculating carrier mobilities in semiconductors.However,in addition to the high computational cost,it is still a challenge to obtain accurate mobility for carriers with a complex band structure,e.g.,hole mobility in common semiconductors.Here,we present a computationally efficient approach using isotropic and parabolic bands to approximate the anisotropy valence bands for evaluating group velocities in the first-principles calculations.This treatment greatly reduces the computational cost in two ways:relieves the requirement of an extremely denseκmesh to obtain a smooth change in group velocity,and reduces the 5-dimensional integral to 3-dimensional integral.Taking Si and SiC as two examples,we find that this simplified approach reproduces the full first-principles calculation for mobility.If we use experimental effective masses to evaluate the group velocity,we can obtain hole mobility in excellent agreement with experimental data over a wide temperature range.These findings shed light on how to improve the first-principles calculations towards predictive carrier mobility in high accuracy.
基金supported by the NKBRP(Grants 2006CB302705,2005CB321704)the NSFC(Grants 10701005,11011130029)sponsored by SRF for ROCS,SEM.
文摘We propose a deterministic solver for the time-dependent multi-subband Boltzmann transport equation(MSBTE)for the two dimensional(2D)electron gas in double gate metal oxide semiconductor field effect transistors(MOSFETs)with flared out source/drain contacts.A realistic model with six-valleys of the conduction band of silicon and both intra-valley and inter-valley phonon-electron scattering is solved.We propose a second order finite volume method based on the positive and flux conservative(PFC)method to discretize the Boltzmann transport equations(BTEs).The transport part of the BTEs is split into two problems.One is a 1D transport problem in the position space,and the other is a 2D transport problem in the wavevector space.In order to reduce the splitting error,the 2D transport problem in the wavevector space is solved directly by using the PFC method instead of splitting into two 1D problems.The solver is applied to a nanoscale double gate MOSFET and the current-voltage characteristic is investigated.Comparison of the numerical results with ballistic solutions show that the scattering influence is not ignorable even when the size of a nanoscale semiconductor device goes to the scale of the electron mean free path.
文摘We have studied the axial resistivity of chiral single-walled carbon nanotubes (SWCNTs) in the presence of a combined direct current and high frequency alternating fields. We employed semiclassical Boltzmann equations approach and compared our results with a similar study that examined the circumferential resistivity of these unique materials. Our work shows that these materials display similar resistivity for both axial and circumferential directions and this largely depends on temperature, intensities of the applied fields and material parameters such as chiral angle. Based on these low-temperature bidirectional conductivity responses, we propose chiral SWCNTs for design of efficient optoelectronic devices.
基金financially supported by the National Natural Science Foundation of China (NSFC) (No.52122606)support by Shanghai Municipal Natural Science Foundation (No.22YF1400100)the Fundamental Research Funds for the Central Universities (No.2232022D-22)。
文摘Hafnium diboride(HfB_(2))is an important metallic ceramic that works in harsh environments,due to its high strength and thermal conductivity.Although the thermal conductivity of HfB_(2) has been measured,the experimental results are scattered.Also,the thermal transport mechanism of HfB_(2) is not well understood.In this work,we study the thermal transport in both pristine and defective HfB_(2) from first-principles calculations.For the pristine HfB_(2),the room-temperature thermal conductivities are 175.0 and 157.7 W·m^(-1)·K^(-1)on a-and c-axes,respectively,where the contributions from electron and phonon are comparable.The Lorenz number is significantly smaller than the Sommerfeld value and shows a temperature dependence,which demonstrates that the Wiedemann-Franz law cannot be used to estimate electronic thermal conductivity.The phonon-isotope and the phonon-electron scattering are non-negligible compared to the phonon-phonon scattering.For the defective HfB_(2),the grain size effects are negligible with length scales larger than 1μm.The pore can limit thermal conductivity when its occupancy is larger than 10%.The vacancy is found to induce scattered results in experiments.The phonon thermal conductivity significantly reduces even with only 1%vacancy,while the electronic thermal conductivity is not sensitive to the vacancy.Our study provides an in-depth understanding of the thermal transport in HfB_(2),and the revealed mechanisms provide important guidance on the design of HfB_(2)-based materials.
文摘This article presents an extensive review of the status of the search of the dark matter. The first eight sections are devoted to topics in dark matter and its experimental searches, and the rest to selected topics in astrophysics and cosmology, which are intended to supply some of the needed background for students in particle physics. Sections 9 and 13 are introductory cosmology. The three astrophysical topics, Big Bang nucleosynthesis Section 10, Boltzmann transport equation and freeze out of massive particles Section 11, and CMB anisotropy Section 12 can all be studied in analytical approaches when reasonable approximations are made. Their original analytically forms, to which this article follows very closely, were given by particle physicists. Dark matter is an evolving subject requiring timely update to stay current. Hence a review of such a subject matter would undoubtedly have something wanting when it appears in print. It is hoped that this review can form a humble basis for those graduate students who would like to pursue the subject of dark matter. The reader can use the extensive table of contents to see in some details the materials covered in the article.