The ultrafast dynamics of photoexcited carriers and coherent phonons in ultrathin Bi;Te;thermoelectric films are studied through transient differential transmission spectroscopy.An ultralow frequency coherent optical ...The ultrafast dynamics of photoexcited carriers and coherent phonons in ultrathin Bi;Te;thermoelectric films are studied through transient differential transmission spectroscopy.An ultralow frequency coherent optical phonon at 0.16 THz emerges,especially in ultrathin films,and it is ascribed to interlayer breathing modes.It can divide the ultrathin films into two groups which have outof-phase vibration along the normal of a film plane,causing a destructive interference between in-plane propagating thermal waves in the two groups of quintuple layers,and thus possibly reducing the thermal conductivity of the ultrathin films.The excitation power dependence of ultrafast dynamics reveals carrier-carrier scattering dominating thermalization,which provides a microscopic understanding of the reported high electrical conductivity and anomalously high power factor of ultrathin Bi;Te;films at room temperature.展开更多
In this work, pronounced oscillations in the time-resolved reflectivity of Heusler alloy Co2MnAl films which are epitaxially grown on Ga As substrates are observed and investigated as a function of film thickness, pro...In this work, pronounced oscillations in the time-resolved reflectivity of Heusler alloy Co2MnAl films which are epitaxially grown on Ga As substrates are observed and investigated as a function of film thickness, probe wavelength,external magnetic field and temperature. Our results suggest that the oscillation response at 24.5 GHz results from the coherent phonon generation in Co2MnAl film and can be explained by a propagating strain pulse model. From the probe wavelength dependent oscillation frequency, a sound velocity of（3.85±0.1）×10-3m/s at 800 nm for the epitaxial Co2MnAl film is determined at room temperature. The detected coherent acoustic phonon generation in Co2MnAl reported in this work provides a valuable reference for exploring the high-speed magnetization manipulation via magnetoelastic coupling for future spintronic devices based on Heusler alloy films.展开更多
The magnetic dynamics of a thin Co_(2)FeAl film epitaxially grown on GaAs substrate was investigated using the timeresolved magneto-optical Kerr measurement under an out-of-plane external field.The intrinsic magnetic ...The magnetic dynamics of a thin Co_(2)FeAl film epitaxially grown on GaAs substrate was investigated using the timeresolved magneto-optical Kerr measurement under an out-of-plane external field.The intrinsic magnetic damping constant,which should do not vary with the external magnetic field,exhibits an abnormal huge increase when the precession frequency is tuned to be resonant with that of the coherent longitudinal acoustic phonon in the Co_(2)FeAl/GaAs heterostructure.The experimental finding is suggested to result from the strong coherent energy transfer from spins to acoustic phonons via magnetoelastic effect under a resonant coupling condition,which leads to a huge energy dissipation of spins and a greatly enhanced magnetic damping in Co_(2)FeAl.Our experimental findings provide an experimental evidence of spin pumping-like effect driven by propagating acoustic phonons via magnetoelastic effect,suggesting an alternative approach to the possible long-range spin manipulation via coherent acoustic waves.展开更多
Phonon coherence can reflect electron‒phonon coupling information and has been proven to modulate electronic states and charge transport.The manipulation of phonon coherence through spacer cation engineering in organi...Phonon coherence can reflect electron‒phonon coupling information and has been proven to modulate electronic states and charge transport.The manipulation of phonon coherence through spacer cation engineering in organic‒inorganic hybrid perovskites(OIHPs)has been extensively demonstrated;however,the underlying structural origin remains elusive at the molecular level.Herein,we present molecular structure and temperature-dependent coherent phonon studies via a combination of sum frequency generation vibrational spectroscopy(SFG-VS)and transient absorption spectroscopy(TAS).The conformational order of spacer cations dictates the coherent phonon oscillations in 2D OIHPs.Our study further analyzes the static order and dynamic disorder in 2D perovskites.This work provides molecular-level insights into the role of spacer cations in tuning structural order and may provide valuable guidance for advancing emergent optoelecltronics development.展开更多
Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd_3As_（2 ）is carried out.An acoustic phonon（AP）mode with central frequency f=0.037 THz（i.e.,1.23 cm^（-1）or 0.153 meV）is unambiguously ge...Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd_3As_（2 ）is carried out.An acoustic phonon（AP）mode with central frequency f=0.037 THz（i.e.,1.23 cm^（-1）or 0.153 meV）is unambiguously generated and detected,which we attribute to laser-induced thermal strain.An AP chirping（i.e.,variation of the phonon frequency）is clearly detected,which is ascribed to heat capacity variation with time.By comparing our experimental results and the theoretical model,we obtain a chirping time constant,which is 31.2 ps at 6 K and 19.8 ps at 300 K,respectively.Significantly,we identify an asymmetry in the AP frequency domain peak and find that it is caused by the chirping,instead of a Fano resonance.Moreover,we experimentally demonstrate that the central frequency of AP is extremely stable with varying laser fluence,as well as temperature,which endows Cd_3As_2application potentials in thermoelectric devices.展开更多
We report on the theoretical and the experimental investigations of the coherent phonon dynamics in sapphire crystal using the femtosecond time-resolved coherent anti-Stokes Raman scattering （fs-CARS） technique. The...We report on the theoretical and the experimental investigations of the coherent phonon dynamics in sapphire crystal using the femtosecond time-resolved coherent anti-Stokes Raman scattering （fs-CARS） technique. The temporal chirped white-light continuum （WLC） is used for the Stokes pulse, therefore we can perform the selective excitation of the phonon modes without using a complicated laser system. The expected quantum beat phenomenon is clearly observed. The theoretical formulas consist very well with the experimental results. The dephasing times of the excited phonon modes, the wavenumber difference, and the phase shift between the simultaneously excited modes are obtained and discussed. This work opens up a way to study directly high-frequency coherent phonon dynamics in bulk crystals on a femtosecond time scale and is especially helpful for understanding the nature of coherent phonons.展开更多
Besides equilibrium behavior,exploring the spin–phonon coupling in multiferroic materials under non-equilibrium conditions is crucial for a deep understanding of the mechanisms as well as their high-frequency applica...Besides equilibrium behavior,exploring the spin–phonon coupling in multiferroic materials under non-equilibrium conditions is crucial for a deep understanding of the mechanisms as well as their high-frequency applications.Here,by utilizing time-resolved refectance spectroscopy,we demonstrate ultrafast spin–phonon coupling dynamics in multiferroic 0.58BiFeO_(3)-0.42Bi_(0.5)K_(0.5)TiO_(3)(BF-BKT)single crystals.With ultrafast laser pumping,coherent acoustic phonons with low damping are created in BF-BKT.Temperature-dependent results indicate that both the frequency and amplitude of laser-induced coherent phonons are sensitive to the emergence of antiferromagnetic order.Moreover,the spin state change driven by external magnetic felds can enhance the oscillation amplitude of the coherent acoustic phonons even above the magnetic Néel temperature.These fndings experimentally confrm that spin–phonon coupling in multiferroic materials exists not only in the spin-ordered state but also in the spin-disordered state,and not only in the equilibrium state but also in the non-equilibrium state excited by ultrafast lasers,suggesting their promising applications in high-frequency devices.展开更多
Lead-free halide perovskites provide a promising solution for efficient thermoelectric materials due to their ultralow lattice thermal conductivity(κ_(L)).However,disadvantages such as the electrically resistive natu...Lead-free halide perovskites provide a promising solution for efficient thermoelectric materials due to their ultralow lattice thermal conductivity(κ_(L)).However,disadvantages such as the electrically resistive nature strongly affect their power factor.In this work,we introduced the Te-based halide perovskites thermoelectric material Cs_(2)TeI_(6),which is already known as a promising candidate for photovoltaic applications due to its moderate band gap.Our findings reveal that Cs_(2)TeI_(6)has an exceptionally ultralow κ_(L)at room temperature,reaching as low as 0.17 W·m^(-1)·K^(-1).We found that the four-phonon scattering processes play a dominant role in suppressing the thermal transport,leading to an approximate 50% reduction in its particle-like thermal conductivity κ_(p) at 300 K.The ultralow κ_(L)can be mainly attributed to the strong discrepancy in bonding strength,which induces large anharmonicity.The flat and dense phonon dispersions result in a strong phonon scattering rate,making it easy to generate wavelike phonon tunneling.After accounting for the wavelike thermal conductivity κ_(c),a nonstandard T^(-0.30)temperature dependence was observed.Benefiting from the ultralow κ_(L),n-type Cs_(2)TeI_(6)is predicted to achieve an extraordinary ZT of 2.26 at 700 K.This work highlights a pathway for searching high-performance and low-cost thermoelectrics based on lead-free halide perovskites.展开更多
We investigate the carrier, phonon, and spin dynamics in the ferromagnetic semiconductor(In,Fe)Sb using ultrafast optical pump-probe spectroscopy. We discover two anomalies near T^(*)(~40 K) and T^(†)(~200 K) in the p...We investigate the carrier, phonon, and spin dynamics in the ferromagnetic semiconductor(In,Fe)Sb using ultrafast optical pump-probe spectroscopy. We discover two anomalies near T^(*)(~40 K) and T^(†)(~200 K) in the photoexcited carrier dynamics, which can be attributed to the electron-spin and spin-lattice scattering processes influenced by the magnetic phase transition and modifications in magnetic anisotropy. The magnetization change can be revealed by the dynamics of coherent acoustic phonon. We also observe abrupt changes in the photoinduced spin dynamics near T^(*)and T^(†), which not only illustrate the spin-related scatterings closely related to the long-range magnetic order, but also reveal the D'yakonov–Perel and Elliott–Yafet mechanisms dominating at temperatures below and above T^(†), respectively. Our findings provide important insights into the nonequilibrium properties of the photoexcited(In,Fe)Sb.展开更多
Two thin-film 2 D organic-inorganic hybrid perovskites,i.e.,2-phenylethylammonium lead iodide(PEPI)and 4-phenyl-1-butylammonium lead iodide(PBPI)were synthesized and investigated by steady-state absorption,temperature...Two thin-film 2 D organic-inorganic hybrid perovskites,i.e.,2-phenylethylammonium lead iodide(PEPI)and 4-phenyl-1-butylammonium lead iodide(PBPI)were synthesized and investigated by steady-state absorption,temperature-dependent photoluminescence,and temperature-dependent ultrafast transient absorption spectroscopy.PBPI has a longer organic chain(via introducing extra ethyl groups)than PEPI,thus its inorganic skeleton can be distorted,bringing on structural disorder.The comparative analyses of spectral profiles and temporal dynamics revealed that the greater structural disorder in PBPI results in more defect states serving as trap states to promote exciton dynamics.In addition,the fine-structuring of excitonic resonances was unveiled by temperature-dependent ultrafast spectroscopy,suggesting its correlation with inorganic skeleton rather than organic chain.Moreover,the photoexcited coherent phonons were observed in both PEPI and PBPI,pointing to a subtle impact of structural disorder on the low-frequency Raman-active vibrations of inorganic skeleton.This work provides valuable insights into the optical properties,excitonic behaviors and dynamics,as well as coherent phonon effects in 2 D hybrid perovskites.展开更多
Two-dimensional Dion–Jacobson(D-J)phase perovskites are prospective photovoltaic and optoelectronic materials.To study their mechanical properties and carrier-lattice interactions,we conduct femtosecond spectroscopic...Two-dimensional Dion–Jacobson(D-J)phase perovskites are prospective photovoltaic and optoelectronic materials.To study their mechanical properties and carrier-lattice interactions,we conduct femtosecond spectroscopic experiments on the films of a D-J perovskite.After optical excitation,a∼33 meV bandgap oscillation is observed in the film by transient absorption spectroscopy.With the help of transient reflection methods,we reveal that the oscillation originates from the transport of coherent longitudinal acoustic phonons through the film.Large bandgap oscillation indicates a strong coupling between carriers and lattice,and significant bandgap modulation by strains in D-J perovskites.展开更多
We use molecular dynamics simulation to calculate the thermal conductivities of（5, 5） carbon nanotube superlattices（CNTSLs） and defective carbon nanotubes（DCNTs）, where CNTSLs and DCNTs have the same size. It is...We use molecular dynamics simulation to calculate the thermal conductivities of（5, 5） carbon nanotube superlattices（CNTSLs） and defective carbon nanotubes（DCNTs）, where CNTSLs and DCNTs have the same size. It is found that the thermal conductivity of DCNT is lower than that of CNTSL at the same concentration of Stone–Wales（SW） defects. We perform the analysis of heat current autocorrelation functions and observe the phonon coherent resonance in CNTSLs, but do not observe the same effect in DCNTs. The phonon vibrational eigen-mode analysis reveals that all modes of phonons are strongly localized by SW defects. The degree of localization of CNTSLs is lower than that of DCNTs, because the phonon coherent resonance results in the phonon tunneling effect in the longitudinal phonon mode. The results are helpful in understanding and tuning the thermal conductivity of carbon nanotubes by defect engineering.展开更多
Based on Lee-Low-Pines(LLP) unitary transformation, this article adopts the variational method of the Pekar type and gets the energy and wave functions of the ground state and the first excited state of strong-couplin...Based on Lee-Low-Pines(LLP) unitary transformation, this article adopts the variational method of the Pekar type and gets the energy and wave functions of the ground state and the first excited state of strong-coupling bipolaron in two-dimensional quantum dot in electric field, thus constructs a bipolaron qubit. The numerical results represent that the time oscillation period T0 of probability density of the two electrons in qubit decreases with the increasing electric field intensity F and dielectric constant ratio of the medium η; the probability density Q of the two electrons in qubit oscillates periodically with the increasing time t; the probability of electron appearing near the center of the quantum dot is larger, while that appearing away from the center of the quantum dot is much smaller.展开更多
We report a theoretical investigation of coherent-to-incoherent heat conduction in multilayer nanostructures.In the coherent regime where the phonon motion is quasi-harmonic,the elastic continuum model gives accurate ...We report a theoretical investigation of coherent-to-incoherent heat conduction in multilayer nanostructures.In the coherent regime where the phonon motion is quasi-harmonic,the elastic continuum model gives accurate cross-plane thermal conductivity predictions of upper limits and demonstrates that the coherent transport is the result of the interplay between intrinsic wave effects.As the temperature or system size increases,the phonon dephasing scattering results in the deviation of thermal conductivity from the coherent-limit calculation.By further introducing the incoherence of phonons,we reproduce the classical minimum thermal conductivity,indicating the feasibility of extending the pure wave model into the wave-particle crossing regime.展开更多
Layered materials exhibit different electronic and phonon properties along in-plane and out-of-plane directions;existing studies focus on their in-plane behaviors,and the influence of such anisotropies on the dynamics...Layered materials exhibit different electronic and phonon properties along in-plane and out-of-plane directions;existing studies focus on their in-plane behaviors,and the influence of such anisotropies on the dynamics of photocarriers and phonons is unknown.Here,we fabricate layered PdSe_(2)crystals with flat edge surfaces and compare the time-resolved ultrafast spectroscopies on their basal and edge surfaces.Pronounced differences in the transient reflection spectroscopies reveal the inconsistent photocarrier and phonon dynamics behaviors on the two surfaces:the slow hot carrier relaxation process is accelerated and the thermoelasticity-induced longitudinal coherent acoustic phonon oscillation completely vanishes on the edge surface,as compared with the basal surface.Theoretical analysis reveals that the inconsistent hot carrier dynamics originate from the anisotropic properties of low-energy phonons in PdSe_(2),and the absence of phonon oscillation on the edge surface results from the wavevector-limited sensitivity of acoustic B_(1u)mode.Moreover,polarization-dependent spectroscopies indicate the diverse optical anisotropies beyond the in-plane of PdSe_(2).This work provides a new method to explore unique physical properties and modulate the optical anisotropy of layered materials.展开更多
Measuring fluctuations in matter’s low-energy excitations is the key to unveiling the nature of the non-equilibrium response of materials.A promising outlook in this respect is offered by spectroscopic methods that a...Measuring fluctuations in matter’s low-energy excitations is the key to unveiling the nature of the non-equilibrium response of materials.A promising outlook in this respect is offered by spectroscopic methods that address matter fluctuations by exploiting the statistical nature of light-matter interactions with weak few-photon probes.Here we report the first implementation of ultrafast phase randomized tomography,combining pump-probe experiments with quantum optical state tomography,to measure the ultrafast non-equilibrium dynamics in complex materials.Our approach utilizes a time-resolved multimode heterodyne detection scheme with phase-randomized coherent ultrashort laser pulses,overcoming the limitations of phase-stable configurations and enabling a robust reconstruction of the statistical distribution of phase-averaged optical observables.This methodology is validated by measuring the coherent phonon response inα-quartz.By tracking the dynamics of the shot-noise limited photon number distribution of fewphoton probes with ultrafast resolution,our results set an upper limit to the non-classical features of phononic state inα-quartz and provide a pathway to access non-equilibrium quantum fluctuations in more complex quantum materials.展开更多
In addition to the pressurized high-temperature superconductivity,bilayer and trilayer nickelate superconductors La_(n+1)Ni_(n)O_(3n+1)(n=2 and 3)exhibit many intriguing properties at ambient pressure,such as orbital-...In addition to the pressurized high-temperature superconductivity,bilayer and trilayer nickelate superconductors La_(n+1)Ni_(n)O_(3n+1)(n=2 and 3)exhibit many intriguing properties at ambient pressure,such as orbital-dependent electronic correlation,non-Fermi liquid behavior,and density-wave transitions.Here,using ultrafast reflectivity measurement,we observe a drastic difference between the ultrafast dynamics of the bilayer and trilayer nickelates at ambient pressure.We observe a coherent phonon mode in La_(4)Ni_(3)O_(10)involving the collective vibration of La,Ni,and O atoms,which is absent in La_(3)Ni_(2)O_(7).Temperature-dependent relaxation time diverges near the density-wave transition temperature of La_(4)Ni_(3)O_(10),while it is inversely proportional to the temperature in La_(3)Ni_(2)O_(7)above~150 K,suggesting a non-Fermi liquid behavior of La_(3)Ni_(2)O_(7).Moreover,we estimate the electron–phonon coupling constants to be 0.05–0.07 and 0.12–0.16 for La_(3)Ni_(2)O_(7)and La_(4)Ni_(3)O_(10),respectively,suggesting a relatively minor role of electron–phonon coupling in the electronic properties of La_(n+1)Ni_(n)O_(3n+1)at ambient pressure.The relevant microscopic interaction and dynamic information are essential for further studying the interplay between superconductivity and density-wave transitions in nickelate superconductors.展开更多
基金partially supported by the National Natural Science Foundation of China(Grant Nos.11774438,and 12074441)National Basic Research Program of China(Grant No.2013CB922403)Guangdong Basic and Applied Basic Research Foundation,China(Grant No.2019A1515011572)。
文摘The ultrafast dynamics of photoexcited carriers and coherent phonons in ultrathin Bi;Te;thermoelectric films are studied through transient differential transmission spectroscopy.An ultralow frequency coherent optical phonon at 0.16 THz emerges,especially in ultrathin films,and it is ascribed to interlayer breathing modes.It can divide the ultrathin films into two groups which have outof-phase vibration along the normal of a film plane,causing a destructive interference between in-plane propagating thermal waves in the two groups of quintuple layers,and thus possibly reducing the thermal conductivity of the ultrathin films.The excitation power dependence of ultrafast dynamics reveals carrier-carrier scattering dominating thermalization,which provides a microscopic understanding of the reported high electrical conductivity and anomalously high power factor of ultrathin Bi;Te;films at room temperature.
基金supported by the National Basic Research Program of China(Grant No.2013CB922303)the National Natural Science Foundation of China(Grant No.61334006)
文摘In this work, pronounced oscillations in the time-resolved reflectivity of Heusler alloy Co2MnAl films which are epitaxially grown on Ga As substrates are observed and investigated as a function of film thickness, probe wavelength,external magnetic field and temperature. Our results suggest that the oscillation response at 24.5 GHz results from the coherent phonon generation in Co2MnAl film and can be explained by a propagating strain pulse model. From the probe wavelength dependent oscillation frequency, a sound velocity of（3.85±0.1）×10-3m/s at 800 nm for the epitaxial Co2MnAl film is determined at room temperature. The detected coherent acoustic phonon generation in Co2MnAl reported in this work provides a valuable reference for exploring the high-speed magnetization manipulation via magnetoelastic coupling for future spintronic devices based on Heusler alloy films.
基金This work was supported by the National Key R&D Program of China(No.2017YFB0405700)National Natural Science Foundation of China(No.12074370).
文摘The magnetic dynamics of a thin Co_(2)FeAl film epitaxially grown on GaAs substrate was investigated using the timeresolved magneto-optical Kerr measurement under an out-of-plane external field.The intrinsic magnetic damping constant,which should do not vary with the external magnetic field,exhibits an abnormal huge increase when the precession frequency is tuned to be resonant with that of the coherent longitudinal acoustic phonon in the Co_(2)FeAl/GaAs heterostructure.The experimental finding is suggested to result from the strong coherent energy transfer from spins to acoustic phonons via magnetoelastic effect under a resonant coupling condition,which leads to a huge energy dissipation of spins and a greatly enhanced magnetic damping in Co_(2)FeAl.Our experimental findings provide an experimental evidence of spin pumping-like effect driven by propagating acoustic phonons via magnetoelastic effect,suggesting an alternative approach to the possible long-range spin manipulation via coherent acoustic waves.
基金supported by the National Natural Science Foundation of China(Nos.21925302,92250306)the Strategic Priority Research Program of the Chinese Academy of Sciences(XDB0450202)+1 种基金the Innovation Program for Quantum Science and Technology(2021ZD0303303)the National Key Research and Development Program of China(2018YFA0208702).
文摘Phonon coherence can reflect electron‒phonon coupling information and has been proven to modulate electronic states and charge transport.The manipulation of phonon coherence through spacer cation engineering in organic‒inorganic hybrid perovskites(OIHPs)has been extensively demonstrated;however,the underlying structural origin remains elusive at the molecular level.Herein,we present molecular structure and temperature-dependent coherent phonon studies via a combination of sum frequency generation vibrational spectroscopy(SFG-VS)and transient absorption spectroscopy(TAS).The conformational order of spacer cations dictates the coherent phonon oscillations in 2D OIHPs.Our study further analyzes the static order and dynamic disorder in 2D perovskites.This work provides molecular-level insights into the role of spacer cations in tuning structural order and may provide valuable guidance for advancing emergent optoelecltronics development.
基金Supported by the National Key Research and Development Program of China under Grant Nos 2017YFA0303603,2016YFA0300303,2017YFA0302901 and 2016YFA0300604the National Natural Science Foundation of China under Grant Nos11774408,11574383 and 11774399+1 种基金the External Cooperation Program of Chinese Academy of Sciences under Grant No GJHZ1826the Interdisciplinary Innovation Team of Chinese Academy of Sciences
文摘Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd_3As_（2 ）is carried out.An acoustic phonon（AP）mode with central frequency f=0.037 THz（i.e.,1.23 cm^（-1）or 0.153 meV）is unambiguously generated and detected,which we attribute to laser-induced thermal strain.An AP chirping（i.e.,variation of the phonon frequency）is clearly detected,which is ascribed to heat capacity variation with time.By comparing our experimental results and the theoretical model,we obtain a chirping time constant,which is 31.2 ps at 6 K and 19.8 ps at 300 K,respectively.Significantly,we identify an asymmetry in the AP frequency domain peak and find that it is caused by the chirping,instead of a Fano resonance.Moreover,we experimentally demonstrate that the central frequency of AP is extremely stable with varying laser fluence,as well as temperature,which endows Cd_3As_2application potentials in thermoelectric devices.
基金supported by the National Natural Science Foundation of China (Grant No. 20973050)
文摘We report on the theoretical and the experimental investigations of the coherent phonon dynamics in sapphire crystal using the femtosecond time-resolved coherent anti-Stokes Raman scattering （fs-CARS） technique. The temporal chirped white-light continuum （WLC） is used for the Stokes pulse, therefore we can perform the selective excitation of the phonon modes without using a complicated laser system. The expected quantum beat phenomenon is clearly observed. The theoretical formulas consist very well with the experimental results. The dephasing times of the excited phonon modes, the wavenumber difference, and the phase shift between the simultaneously excited modes are obtained and discussed. This work opens up a way to study directly high-frequency coherent phonon dynamics in bulk crystals on a femtosecond time scale and is especially helpful for understanding the nature of coherent phonons.
基金supported by the National Key R&D Program of China(Grant No.2021YFA1600200)the National Natural Science Foundation of China(Grant Nos.U2032218 and 12111530283)。
文摘Besides equilibrium behavior,exploring the spin–phonon coupling in multiferroic materials under non-equilibrium conditions is crucial for a deep understanding of the mechanisms as well as their high-frequency applications.Here,by utilizing time-resolved refectance spectroscopy,we demonstrate ultrafast spin–phonon coupling dynamics in multiferroic 0.58BiFeO_(3)-0.42Bi_(0.5)K_(0.5)TiO_(3)(BF-BKT)single crystals.With ultrafast laser pumping,coherent acoustic phonons with low damping are created in BF-BKT.Temperature-dependent results indicate that both the frequency and amplitude of laser-induced coherent phonons are sensitive to the emergence of antiferromagnetic order.Moreover,the spin state change driven by external magnetic felds can enhance the oscillation amplitude of the coherent acoustic phonons even above the magnetic Néel temperature.These fndings experimentally confrm that spin–phonon coupling in multiferroic materials exists not only in the spin-ordered state but also in the spin-disordered state,and not only in the equilibrium state but also in the non-equilibrium state excited by ultrafast lasers,suggesting their promising applications in high-frequency devices.
基金supported by the National Natural Science Foundation of China(Grant Nos.12204482 and U2330104)the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi(Grant No.2020L0537)+2 种基金the Natural Science Foundation of Shanxi Province(Grant No.202403021221164)Higher Education Teaching Reform and Innovation Project of Shanxi Province(Grant No.J20220480)Graduate Curriculum Ideological and Political Education Project of Shanxi Normal University(Grant No.010520233013)。
文摘Lead-free halide perovskites provide a promising solution for efficient thermoelectric materials due to their ultralow lattice thermal conductivity(κ_(L)).However,disadvantages such as the electrically resistive nature strongly affect their power factor.In this work,we introduced the Te-based halide perovskites thermoelectric material Cs_(2)TeI_(6),which is already known as a promising candidate for photovoltaic applications due to its moderate band gap.Our findings reveal that Cs_(2)TeI_(6)has an exceptionally ultralow κ_(L)at room temperature,reaching as low as 0.17 W·m^(-1)·K^(-1).We found that the four-phonon scattering processes play a dominant role in suppressing the thermal transport,leading to an approximate 50% reduction in its particle-like thermal conductivity κ_(p) at 300 K.The ultralow κ_(L)can be mainly attributed to the strong discrepancy in bonding strength,which induces large anharmonicity.The flat and dense phonon dispersions result in a strong phonon scattering rate,making it easy to generate wavelike phonon tunneling.After accounting for the wavelike thermal conductivity κ_(c),a nonstandard T^(-0.30)temperature dependence was observed.Benefiting from the ultralow κ_(L),n-type Cs_(2)TeI_(6)is predicted to achieve an extraordinary ZT of 2.26 at 700 K.This work highlights a pathway for searching high-performance and low-cost thermoelectrics based on lead-free halide perovskites.
基金supported by the National Key R&D Program of China (Grant No. 2024YFA1408502)the National Natural Science Foundation of China (Grant Nos. 92365102, 62027807, 12474107, and 12174383)+1 种基金the Chinese Academy of Sciences project for Yong Scientists in Basic Research (Grant No. YSBR-030)the Guangdong Basic and Applied Basic Research Foundation (Grant No. 2024A1515011600)。
文摘We investigate the carrier, phonon, and spin dynamics in the ferromagnetic semiconductor(In,Fe)Sb using ultrafast optical pump-probe spectroscopy. We discover two anomalies near T^(*)(~40 K) and T^(†)(~200 K) in the photoexcited carrier dynamics, which can be attributed to the electron-spin and spin-lattice scattering processes influenced by the magnetic phase transition and modifications in magnetic anisotropy. The magnetization change can be revealed by the dynamics of coherent acoustic phonon. We also observe abrupt changes in the photoinduced spin dynamics near T^(*)and T^(†), which not only illustrate the spin-related scatterings closely related to the long-range magnetic order, but also reveal the D'yakonov–Perel and Elliott–Yafet mechanisms dominating at temperatures below and above T^(†), respectively. Our findings provide important insights into the nonequilibrium properties of the photoexcited(In,Fe)Sb.
基金supported by the National Key Research and Development Program on Nano Science and Technology of the Ministry of Science and Technology of China(No.2016YFA0200602 and No.2018YFA0208702)the National Natural Science Foundation of China(No.21573211 and No.21633007)the Anhui Initiative in Quantum Information Technologies(No.AHY090200)。
文摘Two thin-film 2 D organic-inorganic hybrid perovskites,i.e.,2-phenylethylammonium lead iodide(PEPI)and 4-phenyl-1-butylammonium lead iodide(PBPI)were synthesized and investigated by steady-state absorption,temperature-dependent photoluminescence,and temperature-dependent ultrafast transient absorption spectroscopy.PBPI has a longer organic chain(via introducing extra ethyl groups)than PEPI,thus its inorganic skeleton can be distorted,bringing on structural disorder.The comparative analyses of spectral profiles and temporal dynamics revealed that the greater structural disorder in PBPI results in more defect states serving as trap states to promote exciton dynamics.In addition,the fine-structuring of excitonic resonances was unveiled by temperature-dependent ultrafast spectroscopy,suggesting its correlation with inorganic skeleton rather than organic chain.Moreover,the photoexcited coherent phonons were observed in both PEPI and PBPI,pointing to a subtle impact of structural disorder on the low-frequency Raman-active vibrations of inorganic skeleton.This work provides valuable insights into the optical properties,excitonic behaviors and dynamics,as well as coherent phonon effects in 2 D hybrid perovskites.
基金supported by the Ministry of Science and Technology(No.2018YFA0208704)the National Natural Science Foundation of China(No.22173096).
文摘Two-dimensional Dion–Jacobson(D-J)phase perovskites are prospective photovoltaic and optoelectronic materials.To study their mechanical properties and carrier-lattice interactions,we conduct femtosecond spectroscopic experiments on the films of a D-J perovskite.After optical excitation,a∼33 meV bandgap oscillation is observed in the film by transient absorption spectroscopy.With the help of transient reflection methods,we reveal that the oscillation originates from the transport of coherent longitudinal acoustic phonons through the film.Large bandgap oscillation indicates a strong coupling between carriers and lattice,and significant bandgap modulation by strains in D-J perovskites.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11404278 and 11275163)the Science Foundation of Hunan Province,China(Grant No.2016JJ2131)
文摘We use molecular dynamics simulation to calculate the thermal conductivities of（5, 5） carbon nanotube superlattices（CNTSLs） and defective carbon nanotubes（DCNTs）, where CNTSLs and DCNTs have the same size. It is found that the thermal conductivity of DCNT is lower than that of CNTSL at the same concentration of Stone–Wales（SW） defects. We perform the analysis of heat current autocorrelation functions and observe the phonon coherent resonance in CNTSLs, but do not observe the same effect in DCNTs. The phonon vibrational eigen-mode analysis reveals that all modes of phonons are strongly localized by SW defects. The degree of localization of CNTSLs is lower than that of DCNTs, because the phonon coherent resonance results in the phonon tunneling effect in the longitudinal phonon mode. The results are helpful in understanding and tuning the thermal conductivity of carbon nanotubes by defect engineering.
基金supported by the Natural Science Foundation of Hebei Province(No.E2013407119)the Items of Institution of Higher Education Scientific Research of Hebei Province(Nos.ZD20131008 and Z2015149)
文摘Based on Lee-Low-Pines(LLP) unitary transformation, this article adopts the variational method of the Pekar type and gets the energy and wave functions of the ground state and the first excited state of strong-coupling bipolaron in two-dimensional quantum dot in electric field, thus constructs a bipolaron qubit. The numerical results represent that the time oscillation period T0 of probability density of the two electrons in qubit decreases with the increasing electric field intensity F and dielectric constant ratio of the medium η; the probability density Q of the two electrons in qubit oscillates periodically with the increasing time t; the probability of electron appearing near the center of the quantum dot is larger, while that appearing away from the center of the quantum dot is much smaller.
文摘We report a theoretical investigation of coherent-to-incoherent heat conduction in multilayer nanostructures.In the coherent regime where the phonon motion is quasi-harmonic,the elastic continuum model gives accurate cross-plane thermal conductivity predictions of upper limits and demonstrates that the coherent transport is the result of the interplay between intrinsic wave effects.As the temperature or system size increases,the phonon dephasing scattering results in the deviation of thermal conductivity from the coherent-limit calculation.By further introducing the incoherence of phonons,we reproduce the classical minimum thermal conductivity,indicating the feasibility of extending the pure wave model into the wave-particle crossing regime.
基金supported by the National Natural Science Foundation of China(Grant Nos.12074202,and 12174207)the Natural Science Foundation of Tianjin City(Grant No.22JCYBJC00390)the Fundamental Research Funds for the Central Universities-Nankai University(Grant No.63231065)。
文摘Layered materials exhibit different electronic and phonon properties along in-plane and out-of-plane directions;existing studies focus on their in-plane behaviors,and the influence of such anisotropies on the dynamics of photocarriers and phonons is unknown.Here,we fabricate layered PdSe_(2)crystals with flat edge surfaces and compare the time-resolved ultrafast spectroscopies on their basal and edge surfaces.Pronounced differences in the transient reflection spectroscopies reveal the inconsistent photocarrier and phonon dynamics behaviors on the two surfaces:the slow hot carrier relaxation process is accelerated and the thermoelasticity-induced longitudinal coherent acoustic phonon oscillation completely vanishes on the edge surface,as compared with the basal surface.Theoretical analysis reveals that the inconsistent hot carrier dynamics originate from the anisotropic properties of low-energy phonons in PdSe_(2),and the absence of phonon oscillation on the edge surface results from the wavevector-limited sensitivity of acoustic B_(1u)mode.Moreover,polarization-dependent spectroscopies indicate the diverse optical anisotropies beyond the in-plane of PdSe_(2).This work provides a new method to explore unique physical properties and modulate the optical anisotropy of layered materials.
基金supported by the European Research Council through the project INCEPT(grant agreement no.677488)D.F.,E.M.R.,A.M.,and G.J.acknowledge the support of the Gordon and Betty Moore Foundation through the grant(CENTQC).
文摘Measuring fluctuations in matter’s low-energy excitations is the key to unveiling the nature of the non-equilibrium response of materials.A promising outlook in this respect is offered by spectroscopic methods that address matter fluctuations by exploiting the statistical nature of light-matter interactions with weak few-photon probes.Here we report the first implementation of ultrafast phase randomized tomography,combining pump-probe experiments with quantum optical state tomography,to measure the ultrafast non-equilibrium dynamics in complex materials.Our approach utilizes a time-resolved multimode heterodyne detection scheme with phase-randomized coherent ultrashort laser pulses,overcoming the limitations of phase-stable configurations and enabling a robust reconstruction of the statistical distribution of phase-averaged optical observables.This methodology is validated by measuring the coherent phonon response inα-quartz.By tracking the dynamics of the shot-noise limited photon number distribution of fewphoton probes with ultrafast resolution,our results set an upper limit to the non-classical features of phononic state inα-quartz and provide a pathway to access non-equilibrium quantum fluctuations in more complex quantum materials.
基金funded by the National Key R&D Program of China(2022YFA1403100,2022YFA1403201,and 2022YFA1402703)the National Natural Science Foundation of China(12274251,92365204,12004270,52272265,12361141826,and 12074212)+3 种基金the Guangdong Basic and Applied Basic Research Foundation(2022B1515120020)Lexian Yang acknowledges support from the Tsinghua University Initiative Scientific Research Program and the Fund of Science and Technology on Surface Physics and Chemistry Laboratory(XKFZ202102)Gang Li acknowledges the support of 2021-Fundamental Research Area 21JC1404700,and Sino-German Mobility program(M-0006)Luyi Yang acknowledges the support of the Beijing Natural Science Foundation(Z240006)。
文摘In addition to the pressurized high-temperature superconductivity,bilayer and trilayer nickelate superconductors La_(n+1)Ni_(n)O_(3n+1)(n=2 and 3)exhibit many intriguing properties at ambient pressure,such as orbital-dependent electronic correlation,non-Fermi liquid behavior,and density-wave transitions.Here,using ultrafast reflectivity measurement,we observe a drastic difference between the ultrafast dynamics of the bilayer and trilayer nickelates at ambient pressure.We observe a coherent phonon mode in La_(4)Ni_(3)O_(10)involving the collective vibration of La,Ni,and O atoms,which is absent in La_(3)Ni_(2)O_(7).Temperature-dependent relaxation time diverges near the density-wave transition temperature of La_(4)Ni_(3)O_(10),while it is inversely proportional to the temperature in La_(3)Ni_(2)O_(7)above~150 K,suggesting a non-Fermi liquid behavior of La_(3)Ni_(2)O_(7).Moreover,we estimate the electron–phonon coupling constants to be 0.05–0.07 and 0.12–0.16 for La_(3)Ni_(2)O_(7)and La_(4)Ni_(3)O_(10),respectively,suggesting a relatively minor role of electron–phonon coupling in the electronic properties of La_(n+1)Ni_(n)O_(3n+1)at ambient pressure.The relevant microscopic interaction and dynamic information are essential for further studying the interplay between superconductivity and density-wave transitions in nickelate superconductors.
