We study the Drude weight D and optical conductivity of the two-dimensional (2D) Hubbard model athalf filling with staggered magnetic flux (SMF).When SMF being introduced,the hopping integrals are modulated bythe magn...We study the Drude weight D and optical conductivity of the two-dimensional (2D) Hubbard model athalf filling with staggered magnetic flux (SMF).When SMF being introduced,the hopping integrals are modulated bythe magnetic flux.The optical sum rule,which is related to the mean kinetic energy of band electrons,is evaluatedfor this 2D Hubbard Hamiltonian.Our present result gives the dependence of the kinetic energy,D and the opticalconductivity on SMF and U.At half filling D vanishes exponentially with system size.We also find in the frequencydependence of the optical conductivity,there is 6-function peak at ω≈2|m|U and the incoherent excitations begin topresent themselves extended to a higher energy region.展开更多
As opposed to the prototypical MoS2 with centroasymmetry,Janus ferrovalley materials such as H-VSSe are less symmetric with the mirror symmetry and time reversal symmetry broken,and hence possess spontaneous valley po...As opposed to the prototypical MoS2 with centroasymmetry,Janus ferrovalley materials such as H-VSSe are less symmetric with the mirror symmetry and time reversal symmetry broken,and hence possess spontaneous valley polarization and strong ferroelasticity.The optical transition is an important means to excite the valley carriers.We investigate the optical spectrum of H-VSSe by using the many-body perturbation-based GW approach and solving the Bethe–Salpeter equation(BSE)to include the electron–hole interactions.It is found that after the GW correction,the band gaps of the quasiparticle bands are much larger than those obtained by the normal density functional theory.The system is ferromagnetic and the valley gaps become non-degenerate due to spin–orbit coupling(SOC).The position of the lowest BSE peak is much lower than the quasiparticle band gap,indicating that the excitonic effect is large.The peak is split into two peaks by the SOC.The binding energy difference between these two BSE peaks is about the same as the difference between the inequivalent valley gaps.Our results show that in Janus H-VSSe the two lowest exciton peaks are from the two inequivalent valleys with different gaps,in contrast to the A and B exciton peaks of MoS2 which are from the same valley.展开更多
We present an infrared spectroscopy study of the magnetic topological insulator MnBi_(4)Te_7 with antiferromagnetic(AFM) order below the Neel temperature TN= 13 K. Our investigation reveals that the low-frequency opti...We present an infrared spectroscopy study of the magnetic topological insulator MnBi_(4)Te_7 with antiferromagnetic(AFM) order below the Neel temperature TN= 13 K. Our investigation reveals that the low-frequency optical conductivity consists of two Drude peaks, indicating a response of free carriers involving multiple bands. Interestingly, the narrow Drude peak grows strongly as the temperature decreases, while the broad Drude peak remains relatively unchanged. The onset of interband transitions starts around 2000 cm^(-1), followed by two prominent absorption peaks around 10000 cm^(-1) and 20000 cm^(-1). Upon cooling, there is a notable transfer of spectral weight from the interband transitions to the Drude response. Below TN, the AFM transition gives rise to small anomalies of the charge response due to a band reconstruction.These findings provide valuable insights into the interplay between magnetism and the electronic properties in MnBi_(4)Te_7.展开更多
In order to predict the levels of corona noise from high-voltage alternating current (AC) transmission lines,the mechanism of corona noise and the corresponding theoretical prediction model are investigated. On the ba...In order to predict the levels of corona noise from high-voltage alternating current (AC) transmission lines,the mechanism of corona noise and the corresponding theoretical prediction model are investigated. On the basis of Drude model,the motion of positive and negative ions produced by high-voltage corona is analyzed,and the mechanism of corona noise is discovered. The theoretical prediction model is put forward by using Kirchhoff formula,which is verified by the well agreement between our result and others',considering the case of three-phase single lines. Moreover,the calculation results show that for both single and bundled lines,the sound pressure level of the typical frequency,i.e. twice the power frequency,attenuates slowly and leads to an obviously interferential phenomenon near the transmission lines,but the level of the bundled lines is smaller than that of the single ones under the same transmission voltage. Based on the mechanism of corona noise and the prediction model,it is obvious that bundled lines and/or increased line radius can be adopted to reduce corona noise in the practical engineering applications effectively. This model can also provide a theoretical guidance for the high-voltage AC transmission line design.展开更多
An in-depth understanding of the photoconductivity and photocarrier density at the interface is of great significance for improving the performance of optoelectronic devices. However, extraction of the photoconductivi...An in-depth understanding of the photoconductivity and photocarrier density at the interface is of great significance for improving the performance of optoelectronic devices. However, extraction of the photoconductivity and photocarrier density at the heterojunction interface remains elusive. Herein, we have obtained the photoconductivity and photocarrier density of 173 nm Sb2Se3/Si(type-Ⅰ heterojunction) and 90 nm Sb2Se3/Si(type-Ⅱ heterojunction) utilizing terahertz(THz) time-domain spectroscopy(THz-TDS) and a theoretical Drude model. Since type-Ⅰ heterojunctions accelerate carrier recombination and type-Ⅱ heterojunctions accelerate carrier separation, the photoconductivity and photocarrier density of the type-Ⅱ heterojunction(21.8×10^(4)S·m^(-1),1.5 × 10^(15)cm^(-3)) are higher than those of the type-Ⅰ heterojunction(11.8×10^(4)S·m^(-1),0.8×10^(15)cm^(-3)). These results demonstrate that a type-Ⅱ heterojunction is superior to a type-Ⅰ heterojunction for THz wave modulation. This work highlights THz-TDS as an effective tool for studying photoconductivity and photocarrier density at the heterojunction interface. In turn, the intriguing interfacial photoconductivity effect provides a way to improve the THz wave modulation performance.展开更多
基金National Natural Science Foundation of China under Grant No.10247010the Foundation for Key Program of Ministry of Education of China under Grant No.205181
文摘We study the Drude weight D and optical conductivity of the two-dimensional (2D) Hubbard model athalf filling with staggered magnetic flux (SMF).When SMF being introduced,the hopping integrals are modulated bythe magnetic flux.The optical sum rule,which is related to the mean kinetic energy of band electrons,is evaluatedfor this 2D Hubbard Hamiltonian.Our present result gives the dependence of the kinetic energy,D and the opticalconductivity on SMF and U.At half filling D vanishes exponentially with system size.We also find in the frequencydependence of the optical conductivity,there is 6-function peak at ω≈2|m|U and the incoherent excitations begin topresent themselves extended to a higher energy region.
基金Project supported by the National Natural Science Foundation of China (Grant No.11874315)the Postgraduate Scientific Research Innovation Project of Hunan Province of China (Grant No.CX20220663)。
文摘As opposed to the prototypical MoS2 with centroasymmetry,Janus ferrovalley materials such as H-VSSe are less symmetric with the mirror symmetry and time reversal symmetry broken,and hence possess spontaneous valley polarization and strong ferroelasticity.The optical transition is an important means to excite the valley carriers.We investigate the optical spectrum of H-VSSe by using the many-body perturbation-based GW approach and solving the Bethe–Salpeter equation(BSE)to include the electron–hole interactions.It is found that after the GW correction,the band gaps of the quasiparticle bands are much larger than those obtained by the normal density functional theory.The system is ferromagnetic and the valley gaps become non-degenerate due to spin–orbit coupling(SOC).The position of the lowest BSE peak is much lower than the quasiparticle band gap,indicating that the excitonic effect is large.The peak is split into two peaks by the SOC.The binding energy difference between these two BSE peaks is about the same as the difference between the inequivalent valley gaps.Our results show that in Janus H-VSSe the two lowest exciton peaks are from the two inequivalent valleys with different gaps,in contrast to the A and B exciton peaks of MoS2 which are from the same valley.
基金Project supported by the the National Natural Science Foundation of China (Grant No.12274442)the National Key R&D Program of China (Grant No.2022YFA1403901)。
文摘We present an infrared spectroscopy study of the magnetic topological insulator MnBi_(4)Te_7 with antiferromagnetic(AFM) order below the Neel temperature TN= 13 K. Our investigation reveals that the low-frequency optical conductivity consists of two Drude peaks, indicating a response of free carriers involving multiple bands. Interestingly, the narrow Drude peak grows strongly as the temperature decreases, while the broad Drude peak remains relatively unchanged. The onset of interband transitions starts around 2000 cm^(-1), followed by two prominent absorption peaks around 10000 cm^(-1) and 20000 cm^(-1). Upon cooling, there is a notable transfer of spectral weight from the interband transitions to the Drude response. Below TN, the AFM transition gives rise to small anomalies of the charge response due to a band reconstruction.These findings provide valuable insights into the interplay between magnetism and the electronic properties in MnBi_(4)Te_7.
文摘In order to predict the levels of corona noise from high-voltage alternating current (AC) transmission lines,the mechanism of corona noise and the corresponding theoretical prediction model are investigated. On the basis of Drude model,the motion of positive and negative ions produced by high-voltage corona is analyzed,and the mechanism of corona noise is discovered. The theoretical prediction model is put forward by using Kirchhoff formula,which is verified by the well agreement between our result and others',considering the case of three-phase single lines. Moreover,the calculation results show that for both single and bundled lines,the sound pressure level of the typical frequency,i.e. twice the power frequency,attenuates slowly and leads to an obviously interferential phenomenon near the transmission lines,but the level of the bundled lines is smaller than that of the single ones under the same transmission voltage. Based on the mechanism of corona noise and the prediction model,it is obvious that bundled lines and/or increased line radius can be adopted to reduce corona noise in the practical engineering applications effectively. This model can also provide a theoretical guidance for the high-voltage AC transmission line design.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 12261141662, 12074311, and 12004310)。
文摘An in-depth understanding of the photoconductivity and photocarrier density at the interface is of great significance for improving the performance of optoelectronic devices. However, extraction of the photoconductivity and photocarrier density at the heterojunction interface remains elusive. Herein, we have obtained the photoconductivity and photocarrier density of 173 nm Sb2Se3/Si(type-Ⅰ heterojunction) and 90 nm Sb2Se3/Si(type-Ⅱ heterojunction) utilizing terahertz(THz) time-domain spectroscopy(THz-TDS) and a theoretical Drude model. Since type-Ⅰ heterojunctions accelerate carrier recombination and type-Ⅱ heterojunctions accelerate carrier separation, the photoconductivity and photocarrier density of the type-Ⅱ heterojunction(21.8×10^(4)S·m^(-1),1.5 × 10^(15)cm^(-3)) are higher than those of the type-Ⅰ heterojunction(11.8×10^(4)S·m^(-1),0.8×10^(15)cm^(-3)). These results demonstrate that a type-Ⅱ heterojunction is superior to a type-Ⅰ heterojunction for THz wave modulation. This work highlights THz-TDS as an effective tool for studying photoconductivity and photocarrier density at the heterojunction interface. In turn, the intriguing interfacial photoconductivity effect provides a way to improve the THz wave modulation performance.
