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Phononic Casimir corrections for Graphene resonator
摘要： By calculating a Casimir energy for the acoustic phonons of Graphene, we find some temperature-dependent corrections for the pretension of a Graphene sheet suspended on a trench. We obtain values of the order of few mN/m for these corrections in fully as well as doubly clamped Graphene on a narrow trench with one nanometer width, at room temperature. These values are considerable compared to the experimental values, and can increase the fundamental resonance frequency of the Graphene. The values of these corrections increase by increasing the temperature, and so they can be utilized for tuning the Graphene pretension.
关键词： Resonance frequency,Graphene sheet,Casimir energy,Acoustic phonons,Pretension,Nanoelectromechanical systems
更新于2025-09-23 15:23:52
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Giant-Shell CdSe/CdS Nanocrystals: Exciton Coupling to Shell Phonons Investigated by Resonant Raman Spectroscopy
摘要： The interaction between excitons and phonons in semiconductor nanocrystals plays a crucial role in the exciton energy spectrum and dynamics, and thus in their optical properties. We investigate the exciton-phonon coupling in giant-shell CdSe/CdS core-shell nanocrystals via resonant Raman spectroscopy. The Huang-Rhys parameter is evaluated by the intensity ratio of the longitudinal-optical (LO) phonon of CdS with its first multiscattering (2LO) replica. We used four different excitation wavelengths in the range from the onset of the CdS shell absorption to well above the CdS shell band edge to get insight into resonance effects of the CdS LO phonon with high energy excitonic transitions. The isotropic spherical giant-shell nanocrystals show consistently stronger exciton-phonon coupling as compared to the anisotropic rod-shaped dot-in-rod (DiR) architecture, and the 2LO/LO intensity ratio decreases for excitation wavelengths approaching the CdS band edge. The strong exciton-phonon coupling in the spherical giant-shell nanocrystals can be related to the delocalization of the electronic wave functions. Furthermore, we observe the radial breathing modes of the GS nanocrystals and their overtones by ultra-low frequency Raman spectroscopy with nonresonant excitation, using laser energies well below the band gap of the heteronanocrystals, and highlight the differences between higher order optical and acoustic phonon modes.
关键词： Giant-shell nanocrystals,acoustic phonons,Dot-in-rods,Raman spectroscopy,Core-shell heterostructures,exciton-phonon coupling
更新于2025-09-23 15:23:52
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Coherent photo-induced phonon emission in the charge-density-wave state of K<sub>0.3</sub>MoO<sub>3</sub>
摘要： We report on the observation of coherent terahertz (THz) emission from the quasi-one-dimensional charge-density wave (CDW) system, blue bronze (K0.3MoO3), upon photo-excitation with ultrashort near-infrared optical pulses. The emission contains a broadband, low-frequency component due to the photo-Dember effect, which is present over the whole temperature range studied (30-300 K), as well as a narrow-band doublet centered at 1.5 THz, which is only observed in the CDW state and results from the generation of coherent transverse-optical phonons polarized perpendicular to the incommensurate CDW b-axis. As K0.3MoO3 is centrosymmetric, the lowest-order generation mechanism which can account for the polarization dependence of the phonon emission involves either a static surface field or quadrupolar terms due to the optical field gradients at the surface. This phonon signature is also present in the ground-state conductivity, and decays in strength with increasing temperature to vanish above T ~100 K, i.e. significantly below the CDW transition temperature. The temporal behavior of the phonon emission can be well described by a simple model with two coupled modes, which initially oscillate with opposite polarity.
关键词： charge-density wave,terahertz emission,coherent phonons,K0.3MoO3,photo-Dember effect
更新于2025-09-23 15:23:52
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Transverse phonons and intermediate-range order in Sr-Mg fluorophosphate glasses
摘要： Vibrational and ultrasonic spectroscopies have been employed to study the composition and polarization dependence of the vibrational and elastic properties of the pseudo-binary mixed fluoride-phosphate glass-forming system xSr(PO3)2-(1-x)(0.62MgF2-0.38AlF3) with x: 0, 0.04, 0.06, 0.1, 0.15, 0.2, 0.3, 0.4, 0.8, 0.9 and 1. Composition-induced alterations in the short-range order have been quantitatively followed by means of Raman and IR spectroscopies. The analysis has shown that the incorporation of the phosphate groups in the fluoride network results in a less interconnected network with suppressed rigidity. The specific amorphous materials have been used as a model system in terms of wide glass-forming ability to elucidate the effect of variation in connectivity between the fluoride and phosphate sub-networks on the Boson peak nature. Emphasis has been given in the low-frequency Raman phenomenology, which in conjunction with the elastic properties allowed us to establish a possible link between the Boson peak and the transverse phonons in these glasses.
关键词： phonons,amorphous materials,oxide glasses,Boson peak,elastic properties,Raman spectroscopy,ATR spectroscopy
更新于2025-09-23 15:22:29
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Room temperature multi-phonon upconversion photoluminescence in monolayer semiconductor WS2
摘要： Photon upconversion is an anti-Stokes process in which an absorption of a photon leads to a reemission of a photon at an energy higher than the excitation energy. The upconversion photoemission has been already demonstrated in rare earth atoms in glasses, semiconductor quantum wells, nanobelts, carbon nanotubes and atomically thin semiconductors. Here, we demonstrate a room temperature upconversion photoluminescence process in a monolayer semiconductor WS2, with energy gain up to 150 meV. We attribute this process to transitions involving trions and many phonons and free exciton complexes. These results are very promising for energy harvesting, laser refrigeration and optoelectronics at the nanoscale.
关键词： photoluminescence,monolayer semiconductor,phonons,energy gain,room temperature,WS2,excitons,trions,upconversion
更新于2025-09-23 15:22:29
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Recipe for Dirac Phonon States with a Quantized Valley Berry Phase in Two-Dimensional Hexagonal Lattices
摘要： The topological quantum states in two-dimensional (2D) materials are fascinating subjects of research, which usually highlight electron-related systems. In this work, we present a recipe that leads to Dirac phonon states with quantized valley Berry phase in 2D hexagonal lattices by first-principles calculations. We show that candidates possessing the three-fold rotational symmetry at the corners of the hexagonal Brillouin zone host valley Dirac phonons, which are guaranteed to remain intact with respect to perturbations. We identify that such special topological features populated by Dirac phonons can be realized in various 2D materials. In particular, the monolayer CrI3, an attractive 2D magnetic semiconductor with exotic applications in spintronics, is an ideal platform to investigate nontrivial phonons in experiments. We further confirm that the phonon Berry phase is quantized to ±π at two inequivalent valleys. The phonon edge states terminated at the projection of phonon Dirac cones are clearly visible. This work demonstrates that 2D hexagonal lattices with attractive valley Dirac phonons will extend the knowledge of valley physics, providing wide applications of topological phonons.
关键词： Dirac phonons,first-principles calculations,valley Berry phase,phonon edge states,2D hexagonal lattices
更新于2025-09-23 15:21:21
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Long Exciton Dephasing Time and Coherent Phonon Coupling in CsPbBr <sub/>2</sub> Cl Perovskite Nanocrystals
摘要： Fully-inorganic cesium lead halide perovskite nanocrystals (NCs) have shown to exhibit outstanding optical properties such as wide spectral tunability, high quantum yield, high oscillator strength as well as blinking-free single photon emission and low spectral diffusion. Here, we report measurements of the coherent and incoherent exciton dynamics on the 100 fs to 10 ns timescale, determining dephasing and density decay rates in these NCs. The experiments are performed on CsPbBr2Cl NCs using transient resonant three-pulse four-wave mixing (FWM) in heterodyne detection at temperatures ranging from 5 K to 50 K. We found a low-temperature exciton dephasing time of 24.5±1.0 ps, inferred from the decay of the photon-echo amplitude at 5 K, corresponding to a homogeneous linewidth (FWHM) of 54±5 μeV. Furthermore, oscillations in the photon-echo signal on a picosecond timescale are observed and attributed to coherent coupling of the exciton to a quantized phonon mode with 3.45 meV energy.
关键词： quantum dots,coherence,four wave mixing,nanocrystals,heterodyne detection,Perovskite,T2,lead halide,photon echo,phonons
更新于2025-09-23 15:21:21
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Charge qubit under optical control in a thin semiconductor slab
摘要： The dynamics of a single-electron double quantum dot (charge qubit) in an optical microcavity has been theoretically analysed taking into account the influence of optical and acoustic phonons. The Lamb modes of a two-dimensional mechanical resonator (thin slab) are considered as an example of an acoustic phonon subsystem. It is found that an optical phonon mode can be used as a qubit control tool, similar to the microcavity photon one. The probability of the quantum operation ‘NOT’ has been calculated for two qubit control scenarios: in the microcavity photon field and in the combined photon – phonon field of a microcavity and slab. It is shown that the coherent energy exchange between a qubit and a set of acoustic phonon modes reduces this probability, which depends on the number of modes, the initial state of the phonon field, and the decay rate of modes.
关键词： acoustic phonons,quantum dot,microcavity,quantum computer,qubit
更新于2025-09-23 15:21:21
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Generation of coherent phonons by coherent extreme ultraviolet radiation in a transient grating experiment
摘要： We investigate the excitation of coherent acoustic and optical phonons by ultrashort extreme ultraviolet (EUV) pulses produced by a free electron laser. Two crossed femtosecond EUV (wavelength 12.7 nm) pulses are used to excite coherent phonons at a wavelength of 280 nm, which are detected via diffraction of an optical probe beam. Longitudinal and surface acoustic waves are measured in BK-7 glass, diamond, and Bi4Ge3O12; in the latter material, the excitation of a coherent optical phonon mode is also observed. We discuss probing different acoustic modes in reflection and transmission geometries and frequency mixing of surface and bulk acoustic waves in the signal. The use of extreme ultraviolet radiation will allow the creation of tunable GHz to THz acoustic sources in any material without the need to fabricate transducer structures.
关键词： transient grating experiment,optical phonons,acoustic waves,extreme ultraviolet radiation,coherent phonons
更新于2025-09-23 15:21:01
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Quantum Chip with the Optimized Tunnel Structure for Measuring a Charge Qubit Based on a Double Quantum Dot
摘要： A circuit of a measuring chip for determining an arbitrary pure state of a charge qubit is proposed. The strength of the current flowing through a single-electron transistor in the steady-state mode depends on the state of the qubit. To enhance the sensitivity of the transistor, its working part is built from three quantum dots (QDs) with the energy levels forming a symmetric configuration. The parameters of the system are calculated using a microscopic model of two-dimensional QDs. The time dependences of the population of the structure’s states are obtained and the current strength, sensitivity, and measuring contrast as functions of the geometric parameters of the system are determined. The effect of dissipative processes related to the acoustic phonons on the measurements is investigated and the rates of electron relaxation and dephasing in a two-level system are calculated.
关键词： quantum dot,charge qubit,acoustic phonons,single-electron transistor,quantum detector,electron tunneling
更新于2025-09-23 15:21:01