This study presents experimental evidence of the dependence of non-radiative recombination processes on the electron-phonon coupling of perovskite in perovskite solar cells(PSCs).Via A-site cation engineering,a weaker...This study presents experimental evidence of the dependence of non-radiative recombination processes on the electron-phonon coupling of perovskite in perovskite solar cells(PSCs).Via A-site cation engineering,a weaker electron-phonon coupling in perovskite has been achieved by introducing the structurally soft cyclohexane methylamine(CMA^(+))cation,which could serve as a damper to alleviate the mechanical stress caused by lattice oscillations,compared to the rigid phenethyl methylamine(PEA^(+))analog.It demonstrates a significantly lower non-radiative recombination rate,even though the two types of bulky cations have similar chemical passivation effects on perovskite,which might be explained by the suppressed carrier capture process and improved lattice geometry relaxation.The resulting PSCs achieve an exceptional power conversion efficiency(PCE)of 25.5%with a record-high opencircuit voltage(V_(OC))of 1.20 V for narrow bandgap perovskite(FAPbI_(3)).The established correlations between electron-phonon coupling and non-radiative decay provide design and screening criteria for more effective passivators for highly efficient PSCs approaching the Shockley-Queisser limit.展开更多
The electron-phonon interaction can reveal the microscopic mechanism of heat transfer in metals.The two-step heat conduction considering electron-phonon interaction has become an effective theoretical model for extrem...The electron-phonon interaction can reveal the microscopic mechanism of heat transfer in metals.The two-step heat conduction considering electron-phonon interaction has become an effective theoretical model for extreme environments,such as micro-scale and ultrafast processes.In this work,the two-step heat transfer model is further extended by considering the Burgers heat conduction model with the secondorder heat flux rate for electrons.Then,a novel generalized electron-phonon coupling thermoelasticity is proposed with the Burgers electronic heat transfer.Then,the problem of one-dimensional semi-infinite copper strip subject to a thermal shock at one side is studied by the Burgers two-step(BTS)model.The thermoelastic analytical solutions are systematically derived in the Laplace domain,and the numerical Laplace inversion method is adopted to obtain the transient responses.The new model is compared with the parabolic two-step(PTS)model and the hyperbolic two-step(HTS)model.The results show that in ultrafast heating,the BTS model has the same wave front jump as the HTS model.The present model has the faster wave speed,and predicts the bigger disturbed regions than the HTS model.More deeply,all two-step models also have the faster wave speeds than one-step models.This work may benefit the theoretical modeling of ultrafast heating of metals.展开更多
Using a transient thermoreflectance （TTR） technique, several Au films with different thicknesses on glass and SiC substrates are measured for thermal characterization of metMlic nano-films, including the electron ph...Using a transient thermoreflectance （TTR） technique, several Au films with different thicknesses on glass and SiC substrates are measured for thermal characterization of metMlic nano-films, including the electron phonon coupling factor G, interfazial thermal resistance R, and thermal conductivity Ks of the substrate. The rear heating-front detecting （RF） method is used to ensure the femtosecond temporal resolution. An intense laser beam is focused on the rear surface to heat the film, and another weak laser beam is focused on the very spot of the front surface to detect the change in the electron temperature. By varying the optical path delay between the two beams, a complete electron temperature profile can be scanned. Different from the normally used single-layer model, the double-layer model involving interfaciM thermal resistance is studied here. The electron temperature cooling profile can be affected by the electron energy transfer into the substrate or the electron-phonon interactions in the metallic films. For multiple-target optimization, the genetic algorithm （GA） is used to obtain both G and R. The experimental result gives a deep understanding of the mechanism of ultra-fast heat transfer in metals.展开更多
A new non-perturbative method is used to discuss the persistent current in a one-dimensional mesoscopic ring threaded by a flux φ with electron-phonon interaction in the lattice model. The current is periodic in φ w...A new non-perturbative method is used to discuss the persistent current in a one-dimensional mesoscopic ring threaded by a flux φ with electron-phonon interaction in the lattice model. The current is periodic in φ with a flux quantum φ 0 and the electron-phonon interaction suppresses the persistent current. By considering the contributions of many-phonon correlations, we could obtain more accurate results.展开更多
Electron-phonon coupling （EPC） in cuprate and iron-based superconducting systems, as revealed by Raman scat- tering, is briefly reviewed. We introduce how to extract the coupling information through phonon lineshape...Electron-phonon coupling （EPC） in cuprate and iron-based superconducting systems, as revealed by Raman scat- tering, is briefly reviewed. We introduce how to extract the coupling information through phonon lineshape. Then we discuss the strength of EPC in different high-temperature superconductor （HTSC） systems and possible factors affecting the strength. A comparative study between Raman phonon theories and experiments allows us to gain insight into some crucial electronic properties, especially superconductivity. Finally, we summarize and compare EPC in the two existing HTSC systems, and discuss what role it may play in the HTSC.展开更多
We present our theoretical investigations on the effects of disorder on the electron-phonon interaction in semiconducting GaAs system. Both the temperature (T) and disorder (electron mean free path l) dependences of t...We present our theoretical investigations on the effects of disorder on the electron-phonon interaction in semiconducting GaAs system. Both the temperature (T) and disorder (electron mean free path l) dependences of the electron-phonon scattering rate have been determined. On consideration of the dynamic screening, we find a significant change in the temperature exponent as well as the pre factor from the earlier reported temperature power law dependence result ?T6 obtained under static screening. Also the dynamic screening makes a noticeable change in the character of the dependence of scattering rate on the mean free path from the static screening result.展开更多
Traditional ligand-field theory has to be improved by taking into account both 'pure electronic' contribution and electron-phonon interaction one (including lattice-vibrational relaxation energy). By means of ...Traditional ligand-field theory has to be improved by taking into account both 'pure electronic' contribution and electron-phonon interaction one (including lattice-vibrational relaxation energy). By means of improved ligand-field theory, R1, R2, R'3, R'2, and R'1 lines, U band, ground-state zero-field-splitting (GSZFS), and ground-state g factors of ruby and/or GSGG: Cr3+ as well as thermal shifts of GSZFS, R1 line and R2 line of ruby have been calculated.The results are in very good agreement with the experimental data. Moreover, it is found that the value of cubic-field parameter given by traditional ligand-field theory is inappropriately large. For thermal shifts of GSZFS, R1 line and R2 line of ruby, several conclusions have also been obtained.展开更多
We present a study of electrical and thermal transport in Weyl semimetal WTe_(2)down to 0.3 K.The Wiedemann-Franz law holds below 2 K and a downward deviation starts above.The deviation is more pronounced in cleaner s...We present a study of electrical and thermal transport in Weyl semimetal WTe_(2)down to 0.3 K.The Wiedemann-Franz law holds below 2 K and a downward deviation starts above.The deviation is more pronounced in cleaner samples,as expected in the hydrodynamic picture of electronic transport,where a fraction of electron-electron collisions conserve momentum.Phonons are the dominant heat carriers and their mean-free-path does not display a Knudsen minimum.This is presumably a consequence of weak anharmonicity,as indicated by the temperature dependence of the specific heat.Frequent momentum exchange between phonons and electrons leads to quantum oscillations of the phononic thermal conductivity.Bloch-Grüneisen picture of electron-phonon scattering breaks down at low temperature when Umklapp ph-ph collisions cease to be a sink for electronic flow of momentum.Comparison with semi-metallic Sb shows that normal ph-ph collisions are amplified by anharmonicity.In both semimetals,at cryogenic temperature,e-ph collisions degrade the phononic flow of energy but not the electronic flow of momentum.展开更多
Electron-phonon coupling(EPC) in bulk materials is an important effect in multifarious physical and chemical phenomena. It is the key to explaining the mechanisms for superconductivity, electronic transport, etc. The ...Electron-phonon coupling(EPC) in bulk materials is an important effect in multifarious physical and chemical phenomena. It is the key to explaining the mechanisms for superconductivity, electronic transport, etc. The EPC matrix describes the coupling of the electronic eigenstates of the studied system under the perturbation of phonons. Although the EPC matrix is closely relevant to many fundamental physicochemical properties, it remains a challenge to calculate the EPC matrix precisely due to the high computational cost. In recent years, Giustino et al. developed the EPW method on open-source ab-initio software Quantum Espresso, which uses Wannier functions(WFs) to calculate EPC matrix. However, due to the limitation of their implementation,it is not possible yet to calculate the EPC matrix under some important computational conditions, e.g., for DFT+U and HSE calculation. Given the importance of these computational conditions(e.g., for transition metal oxides), we have developed our own implementation of EPC matrix calculation based on the domestic ab-initio software PWmat. Our code allows the DFT+U and HSE correction, so we can get a more accurate EPC matrix in the related problems. In this article, we will first review the formulae and elucidate how to calculate the EPC matrix by constructing WFs. Then we will introduce our code along with its workflow on PWmat and present our test results of two classical semiconductor systems Al As and Si, showing consistency with EPW. Next, the EPC matrix of Li Co O_(2), a classical cathode material for lithium-ion batteries, is calculated using different exchange correlation(XC) functionals including LDA, PBE, DFT+U and HSE. A comparison is provided for the related EPC matrix. It shows there could be a significant difference for the EPC matrix elements due to the use of different XC functionals.Our implementation thus opens the way for fast calculation of EPC for the important class of materials, like the transition metal oxides.展开更多
It has now become recognized that the electron-phonon coupling(EPC)may play an important role in governing the phonon transport,especially for metallic and semiconducting systems at high carrier concentration.Here we ...It has now become recognized that the electron-phonon coupling(EPC)may play an important role in governing the phonon transport,especially for metallic and semiconducting systems at high carrier concentration.Here we focus on the Weyl semimetals TaAs and NbAs and give a comparative study on their phonon transport properties by explicitly including the EPC in first-principles calculations.It is found that the lattice thermal conductivities of both systems are significantly reduced by the EPC,which is more pronounced for the TaAs compared with the NbAs at the same carrier concentration.Detailed analysis indicates that the TaAs exhibits smaller EPC phonon relaxation time,as characterized by stronger EPC strength which is associated with larger deformation potential constant and Born effective charge.Moreover,we see that the TaAs exhibits obviously larger overlap between the EPC relaxation time and that from intrinsic phonon-phonon scattering,which could further reduce the lattice thermal conduc-tivity.Our work not only highlights the vital importance of EPC in accurately predicting the phonon transport behaviors,but also offers a simple alternative to evaluate the EPC strength of various material systems.展开更多
XB_(2)(X=Mg and Al)compounds have drawn great attention for their superior electronic characteristics and potential applications in semiconductors and superconductors.The study of phonon thermal transport properties o...XB_(2)(X=Mg and Al)compounds have drawn great attention for their superior electronic characteristics and potential applications in semiconductors and superconductors.The study of phonon thermal transport properties of XB_(2)is significant to their application and mechanism behind research.In this work,the phonon thermal transport properties of three-dimensional(3D)and two-dimensional(2D)XB_(2)were studied by first-principles calculations.After considering the electron-phonon interaction(EPI),the thermal conductivities(TCs)of 3D Mg B_(2)and 3D Al B_(2)decrease by 29%and 16%which is consistent with experimental values.Moreover,the underlying mechanisms of reduction on lattice TCs are the decrease in phonon lifetime and heat capacity when considering quantum confinement effect.More importantly,we are surprised to find that there is a correlation between quantum confinement effect and EPI.The quantum confinement will change the phonon and electron characteristics which has an impact on EPI.Overall,our work is expected to provide insights into the phonon thermal transport properties of XB_(2)compounds considering EPI and quantum confinement effect.展开更多
It has been widely recognized that,based on standard density functional theory calculations of the electron-phonon coupling,the superconducting transition temperature(T_(c))in bulk FeSe is exceptionally low(almost 0 K...It has been widely recognized that,based on standard density functional theory calculations of the electron-phonon coupling,the superconducting transition temperature(T_(c))in bulk FeSe is exceptionally low(almost 0 K)within the Bardeen-Cooper-Schrieffer formalism.Yet the experimentally observed T_(c)is much higher(∼10 K),and the underlying physical origin remains to be fully explored,especially at the quantitative level.Here we present the first accurate determination of T_(c)in FeSe where the correlation-enhanced electron-phonon coupling is treated within first-principles dynamical mean-field theory.Our studies treat both the multiple electronic bands across the Fermi level and phononic bands,and reveal that all the optical phonon modes are effectively coupled with the conduction electrons,including the important contributions of a single breathing mode as established by previ-ous experiments.Accordingly,each of those phonon modes contributes pronouncedly to the electron pairing,and the resultant T_(c)is drastically enhanced to the experimentally observed range.The approach developed here should be broadly applicable to other superconducting systems where correlation-enhanced electron-phonon coupling plays an important role.展开更多
A new variational-ansatz of states of electrons and phonons was proposed on the basis of the Holstein model in strongly coupled electron-phonon systems for studying the influence of nonadiabatic phonon fluctuation,ari...A new variational-ansatz of states of electrons and phonons was proposed on the basis of the Holstein model in strongly coupled electron-phonon systems for studying the influence of nonadiabatic phonon fluctuation,arising from the motion and density fluctuation of electrons,on the properties of ground state,uncertainty relation,stability of polarons,charge density wave (CDW) and phonon staggered ordering. The new ansatz represents the correlation among the displacement and squeezing states of phonons and polaron’s state of electrons as well as the squeezing-antisqueezing effect. The correlation and squeezing-antisqueezing ef-fect result in the decrease of ground state energy,enhancement of stability of the systems,increase of binding energy of the polarons,weakening of the growing speed of polaron narrowing of the electron band,increase of the charge density wave order and suppression of the increased tendency of anomalous quantum fluctuation of the phonons in such a case,when compared with the uncorrelated case in the systems. The results obtained show that the ground state determined by the new state ansatz is most stable,thus the new ansatz describing the properties of the coupled electron phonon systems is very relevant and available,especially in strongly coupled and largely squeezed cases.展开更多
基金supported by the National Natural Science Foundation of China(U21A20331,81903743,22005322,22279151,and 22275004)National Science Fund for Distinguished Young Scholars(21925506).
文摘This study presents experimental evidence of the dependence of non-radiative recombination processes on the electron-phonon coupling of perovskite in perovskite solar cells(PSCs).Via A-site cation engineering,a weaker electron-phonon coupling in perovskite has been achieved by introducing the structurally soft cyclohexane methylamine(CMA^(+))cation,which could serve as a damper to alleviate the mechanical stress caused by lattice oscillations,compared to the rigid phenethyl methylamine(PEA^(+))analog.It demonstrates a significantly lower non-radiative recombination rate,even though the two types of bulky cations have similar chemical passivation effects on perovskite,which might be explained by the suppressed carrier capture process and improved lattice geometry relaxation.The resulting PSCs achieve an exceptional power conversion efficiency(PCE)of 25.5%with a record-high opencircuit voltage(V_(OC))of 1.20 V for narrow bandgap perovskite(FAPbI_(3)).The established correlations between electron-phonon coupling and non-radiative decay provide design and screening criteria for more effective passivators for highly efficient PSCs approaching the Shockley-Queisser limit.
基金Project supported by the Fundamental Research Funds for the Central Universities of China(Nos.D5000230066 and D5000210117)the Natural Science Basic Research Plan in Shaanxi Province of China(No.2022JQ-358)。
文摘The electron-phonon interaction can reveal the microscopic mechanism of heat transfer in metals.The two-step heat conduction considering electron-phonon interaction has become an effective theoretical model for extreme environments,such as micro-scale and ultrafast processes.In this work,the two-step heat transfer model is further extended by considering the Burgers heat conduction model with the secondorder heat flux rate for electrons.Then,a novel generalized electron-phonon coupling thermoelasticity is proposed with the Burgers electronic heat transfer.Then,the problem of one-dimensional semi-infinite copper strip subject to a thermal shock at one side is studied by the Burgers two-step(BTS)model.The thermoelastic analytical solutions are systematically derived in the Laplace domain,and the numerical Laplace inversion method is adopted to obtain the transient responses.The new model is compared with the parabolic two-step(PTS)model and the hyperbolic two-step(HTS)model.The results show that in ultrafast heating,the BTS model has the same wave front jump as the HTS model.The present model has the faster wave speed,and predicts the bigger disturbed regions than the HTS model.More deeply,all two-step models also have the faster wave speeds than one-step models.This work may benefit the theoretical modeling of ultrafast heating of metals.
基金supported by the National Natural Science Foundation of China (Grant Nos. 50730006,50976053,and 50906042)
文摘Using a transient thermoreflectance （TTR） technique, several Au films with different thicknesses on glass and SiC substrates are measured for thermal characterization of metMlic nano-films, including the electron phonon coupling factor G, interfazial thermal resistance R, and thermal conductivity Ks of the substrate. The rear heating-front detecting （RF） method is used to ensure the femtosecond temporal resolution. An intense laser beam is focused on the rear surface to heat the film, and another weak laser beam is focused on the very spot of the front surface to detect the change in the electron temperature. By varying the optical path delay between the two beams, a complete electron temperature profile can be scanned. Different from the normally used single-layer model, the double-layer model involving interfaciM thermal resistance is studied here. The electron temperature cooling profile can be affected by the electron energy transfer into the substrate or the electron-phonon interactions in the metallic films. For multiple-target optimization, the genetic algorithm （GA） is used to obtain both G and R. The experimental result gives a deep understanding of the mechanism of ultra-fast heat transfer in metals.
文摘A new non-perturbative method is used to discuss the persistent current in a one-dimensional mesoscopic ring threaded by a flux φ with electron-phonon interaction in the lattice model. The current is periodic in φ with a flux quantum φ 0 and the electron-phonon interaction suppresses the persistent current. By considering the contributions of many-phonon correlations, we could obtain more accurate results.
基金supported by the National Basic Research Program of China(Grant Nos.2011CBA00112 and 2012CB921701)
文摘Electron-phonon coupling （EPC） in cuprate and iron-based superconducting systems, as revealed by Raman scat- tering, is briefly reviewed. We introduce how to extract the coupling information through phonon lineshape. Then we discuss the strength of EPC in different high-temperature superconductor （HTSC） systems and possible factors affecting the strength. A comparative study between Raman phonon theories and experiments allows us to gain insight into some crucial electronic properties, especially superconductivity. Finally, we summarize and compare EPC in the two existing HTSC systems, and discuss what role it may play in the HTSC.
文摘We present our theoretical investigations on the effects of disorder on the electron-phonon interaction in semiconducting GaAs system. Both the temperature (T) and disorder (electron mean free path l) dependences of the electron-phonon scattering rate have been determined. On consideration of the dynamic screening, we find a significant change in the temperature exponent as well as the pre factor from the earlier reported temperature power law dependence result ?T6 obtained under static screening. Also the dynamic screening makes a noticeable change in the character of the dependence of scattering rate on the mean free path from the static screening result.
基金The project supported by National Natural Science Foundation of China under Grant No. 60478010, the Science and Technology Committee of Guangdong Province of China under Grant Nos. 2004B10301014 and 04105406, Science and Technology Bureau of Guangzhou under Grant Nos. 200J1-C0031 and 2004J1-C0226, and Education Bureau of Guangzhou under Grant No. 2024
文摘Traditional ligand-field theory has to be improved by taking into account both 'pure electronic' contribution and electron-phonon interaction one (including lattice-vibrational relaxation energy). By means of improved ligand-field theory, R1, R2, R'3, R'2, and R'1 lines, U band, ground-state zero-field-splitting (GSZFS), and ground-state g factors of ruby and/or GSGG: Cr3+ as well as thermal shifts of GSZFS, R1 line and R2 line of ruby have been calculated.The results are in very good agreement with the experimental data. Moreover, it is found that the value of cubic-field parameter given by traditional ligand-field theory is inappropriately large. For thermal shifts of GSZFS, R1 line and R2 line of ruby, several conclusions have also been obtained.
基金The project supported by the Science and Technology Project of Adwnced Academy of Guangzhou City under Grant No. 2060, National Natural Science Foundation of China under Grant Nos. 60276004 and 60390073, and the Scientific Research Foundation for the Returned 0verseas Chinese Scholars of the Ministry of Education of China
基金supported by the National Key Research and Development Program of China(Grant No.2022YFA1403500)the National Science Foundation of China(Grant Nos.12004123,51861135104,and 11574097)+3 种基金the Fundamental Research Funds for the Central Universities(Grant No.2019kfy XMBZ071)supported by the Agence Nationale de la Recherche(Grant No.ANR-19-CE30-0014-04)the China National Postdoctoral Program for Innovative Talents(Grant No.BX20200143)the China Postdoctoral Science Foundation(Grant No.2020M682386)。
文摘We present a study of electrical and thermal transport in Weyl semimetal WTe_(2)down to 0.3 K.The Wiedemann-Franz law holds below 2 K and a downward deviation starts above.The deviation is more pronounced in cleaner samples,as expected in the hydrodynamic picture of electronic transport,where a fraction of electron-electron collisions conserve momentum.Phonons are the dominant heat carriers and their mean-free-path does not display a Knudsen minimum.This is presumably a consequence of weak anharmonicity,as indicated by the temperature dependence of the specific heat.Frequent momentum exchange between phonons and electrons leads to quantum oscillations of the phononic thermal conductivity.Bloch-Grüneisen picture of electron-phonon scattering breaks down at low temperature when Umklapp ph-ph collisions cease to be a sink for electronic flow of momentum.Comparison with semi-metallic Sb shows that normal ph-ph collisions are amplified by anharmonicity.In both semimetals,at cryogenic temperature,e-ph collisions degrade the phononic flow of energy but not the electronic flow of momentum.
基金supported by the starting fund of Peking University Shenzhen Graduate SchoolFujian Science&Technology Innovation Laboratory for Energy Devices of China (Grant No. 1C-LAB)+2 种基金the Chemistry and Chemical Engineering Guangdong Laboratory (Grant No. 1922018)the Soft Science Research Project of Guangdong Province (Grant No. 2017B030301013)the Major Science and Technology Infrastructure Project of Material Genome Big-Science Facilities Platform supported by Municipal Development and Reform Commission of Shenzhen。
文摘Electron-phonon coupling(EPC) in bulk materials is an important effect in multifarious physical and chemical phenomena. It is the key to explaining the mechanisms for superconductivity, electronic transport, etc. The EPC matrix describes the coupling of the electronic eigenstates of the studied system under the perturbation of phonons. Although the EPC matrix is closely relevant to many fundamental physicochemical properties, it remains a challenge to calculate the EPC matrix precisely due to the high computational cost. In recent years, Giustino et al. developed the EPW method on open-source ab-initio software Quantum Espresso, which uses Wannier functions(WFs) to calculate EPC matrix. However, due to the limitation of their implementation,it is not possible yet to calculate the EPC matrix under some important computational conditions, e.g., for DFT+U and HSE calculation. Given the importance of these computational conditions(e.g., for transition metal oxides), we have developed our own implementation of EPC matrix calculation based on the domestic ab-initio software PWmat. Our code allows the DFT+U and HSE correction, so we can get a more accurate EPC matrix in the related problems. In this article, we will first review the formulae and elucidate how to calculate the EPC matrix by constructing WFs. Then we will introduce our code along with its workflow on PWmat and present our test results of two classical semiconductor systems Al As and Si, showing consistency with EPW. Next, the EPC matrix of Li Co O_(2), a classical cathode material for lithium-ion batteries, is calculated using different exchange correlation(XC) functionals including LDA, PBE, DFT+U and HSE. A comparison is provided for the related EPC matrix. It shows there could be a significant difference for the EPC matrix elements due to the use of different XC functionals.Our implementation thus opens the way for fast calculation of EPC for the important class of materials, like the transition metal oxides.
基金support from the National Natural Science Foundation(Grant Nos.62074114,51772220).
文摘It has now become recognized that the electron-phonon coupling(EPC)may play an important role in governing the phonon transport,especially for metallic and semiconducting systems at high carrier concentration.Here we focus on the Weyl semimetals TaAs and NbAs and give a comparative study on their phonon transport properties by explicitly including the EPC in first-principles calculations.It is found that the lattice thermal conductivities of both systems are significantly reduced by the EPC,which is more pronounced for the TaAs compared with the NbAs at the same carrier concentration.Detailed analysis indicates that the TaAs exhibits smaller EPC phonon relaxation time,as characterized by stronger EPC strength which is associated with larger deformation potential constant and Born effective charge.Moreover,we see that the TaAs exhibits obviously larger overlap between the EPC relaxation time and that from intrinsic phonon-phonon scattering,which could further reduce the lattice thermal conduc-tivity.Our work not only highlights the vital importance of EPC in accurately predicting the phonon transport behaviors,but also offers a simple alternative to evaluate the EPC strength of various material systems.
基金financially supported by the National Natural Science Foundation of China(Nos.51720105007,52076031 and 51806031)the Fundamental Research Funds for the Central Universities(No.DUT19RC(3)006)the computing resources from Super-computing Center of Dalian University of Technology。
文摘XB_(2)(X=Mg and Al)compounds have drawn great attention for their superior electronic characteristics and potential applications in semiconductors and superconductors.The study of phonon thermal transport properties of XB_(2)is significant to their application and mechanism behind research.In this work,the phonon thermal transport properties of three-dimensional(3D)and two-dimensional(2D)XB_(2)were studied by first-principles calculations.After considering the electron-phonon interaction(EPI),the thermal conductivities(TCs)of 3D Mg B_(2)and 3D Al B_(2)decrease by 29%and 16%which is consistent with experimental values.Moreover,the underlying mechanisms of reduction on lattice TCs are the decrease in phonon lifetime and heat capacity when considering quantum confinement effect.More importantly,we are surprised to find that there is a correlation between quantum confinement effect and EPI.The quantum confinement will change the phonon and electron characteristics which has an impact on EPI.Overall,our work is expected to provide insights into the phonon thermal transport properties of XB_(2)compounds considering EPI and quantum confinement effect.
基金supported by the National Key R&D Program of China(Grant No.2017YFA0303500)the National Natural Science Foundation of China(Grant Nos.11634011,11974323,and 12004364)+2 种基金the Anhui Initiative in Quantum Information Technologies(Grant No.AHY170000)the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB30000000)the China Postdoctoral Science Foundation(Grant No.2019TQ0314)。
文摘It has been widely recognized that,based on standard density functional theory calculations of the electron-phonon coupling,the superconducting transition temperature(T_(c))in bulk FeSe is exceptionally low(almost 0 K)within the Bardeen-Cooper-Schrieffer formalism.Yet the experimentally observed T_(c)is much higher(∼10 K),and the underlying physical origin remains to be fully explored,especially at the quantitative level.Here we present the first accurate determination of T_(c)in FeSe where the correlation-enhanced electron-phonon coupling is treated within first-principles dynamical mean-field theory.Our studies treat both the multiple electronic bands across the Fermi level and phononic bands,and reveal that all the optical phonon modes are effectively coupled with the conduction electrons,including the important contributions of a single breathing mode as established by previ-ous experiments.Accordingly,each of those phonon modes contributes pronouncedly to the electron pairing,and the resultant T_(c)is drastically enhanced to the experimentally observed range.The approach developed here should be broadly applicable to other superconducting systems where correlation-enhanced electron-phonon coupling plays an important role.
基金the National "973" Project of China (Grant No. 2007CB6103)
文摘A new variational-ansatz of states of electrons and phonons was proposed on the basis of the Holstein model in strongly coupled electron-phonon systems for studying the influence of nonadiabatic phonon fluctuation,arising from the motion and density fluctuation of electrons,on the properties of ground state,uncertainty relation,stability of polarons,charge density wave (CDW) and phonon staggered ordering. The new ansatz represents the correlation among the displacement and squeezing states of phonons and polaron’s state of electrons as well as the squeezing-antisqueezing effect. The correlation and squeezing-antisqueezing ef-fect result in the decrease of ground state energy,enhancement of stability of the systems,increase of binding energy of the polarons,weakening of the growing speed of polaron narrowing of the electron band,increase of the charge density wave order and suppression of the increased tendency of anomalous quantum fluctuation of the phonons in such a case,when compared with the uncorrelated case in the systems. The results obtained show that the ground state determined by the new state ansatz is most stable,thus the new ansatz describing the properties of the coupled electron phonon systems is very relevant and available,especially in strongly coupled and largely squeezed cases.
