External Phonon Research Articles

Phonon Engineering in Solid Polymer Electrolyte toward High Safety for Solid-State Lithium Batteries.

Extensively-used rechargeable lithium-ion batteries (LIBs) face challenges in achieving high safety and long cycle life. To address such challenges, ultrathin solid polymer electrolyte (SPE) is fabricated with reduced phonon scattering by depositing the composites of ionic-liquid (1-ethyl-3-methylimidazolium dicyamide, EMIM:DCA), polyurethane (PU) and lithium salt on the polyethylene separator. The robust and flexible separator matrix not only reduces the electrolyte thickness and improves the mobility of Li+, but more importantly provides a relatively regular thermal diffusion channel for SPE and reduces the external phonon scattering. Moreover, the introduction of EMIM:DCA successfully breaks the random intermolecular attraction of the PU polymer chain and significantly decreases phonon scattering to enhance the internal thermal conductivity of the polymer. Thus, the thermal conductivity of the as-obtained SPE increases by approximately six times, and the thermal runaway (TR) of the battery is effectively inhibited. This work demonstrates that optimizing thermal safety of the battery by phonon engineering sheds a new light on the design principle for high-safety Li-ion batteries.

The second-order sideband enhancement in spinning resonators with an external phonon pump

The second-order sideband enhancement in spinning resonators with an external phonon pump

Tunable optical response and fast (slow) light in optomechanical system with phonon pump

We theoretically investigate the tunable optical response of a weak probe field in a single phonon mechanical driven cavity optomechanical system where an oscillating membrane acts as a mechanical mode. This membrane is also coupled to the optical cavity mode through both the linear optomechanical coupling (LOC) and the quadratic optomechanical coupling (QOC) simultaneously. An external single phonon mechanical driving field present in our scheme gives an additional coherence effect as well as its relative phase and intensity can control significantly the absorption (amplification) profile of the output probe field. We have also shown that the phase dispersion and group delay of the probe field can be controlled flexibly through both types of optomechanical coupling strengths as well as the mean number of thermal phonons. Our results provide a flexible and generalised route to control the light propagation in such kinds of complex cavity optomechanical systems with coherent mechanical excitation.

Vibrational Dynamics in crystalline 4-(dimethylamino) benzaldehyde: Inelastic Neutron Scattering and Periodic DFT Study.

The structure and dynamics of crystalline 4-(dimethylamino) benzaldehyde, 4DMAB, are assessed through INS spectroscopy combined with periodic DFT calculations. The excellent agreement between experimental and calculated spectra is the basis for a reliable assignment of INS bands. The external phonon modes of crystalline 4DMAB are quite well described by the simulated spectrum, as well as the modes involving low-frequency molecular vibrations. Crystal field splitting is predicted and observed for the modes assigned to the dimethylamino group. Concerning the torsional motion of methyl groups, four individual bands are identified and assigned to specific methyl groups in the asymmetric unit. The torsional frequencies of the four methyl groups in the asymmetric unit fall in a region of ca. 190 ± 20 cm−1, close to the range of values observed for methyl groups bonding to unsaturated carbon atoms. The hybridization state of the X atom in X-CH3 seems to play a key role in determining the methyl torsional frequency.

Enhancement of superconductivity with external phonon squeezing

Squeezing of phonons due to the non-linear coupling to electrons is a way to enhance superconductivity as theoretically studied in a recent work [Kennes et al. Nature Physics 13, 479 (2017)]. We study quadratic electron-phonon interaction in the presence of phonon pumping and an additional external squeezing. Interference between these two driving sources induces a phase-sensitive enhancement of electron-electron attraction, which we find as a generic mechanism to enhance any boson-mediated interactions. The strongest enhancement of superconductivity is shown to be on the boundary with the dynamical lattice instabilities caused by driving. We propose several experimental platforms to realize our scheme.

Rectangular quantum wire with an infinite potential GaAs/AlGaAs: Quantum theory of acoustomagnetoelectric effect in the presence of electromagnetic wave

The acoustomagnetoelectric (AME) effect in an GaAs/AlGaAs rectangular quantum wire with infinite potential (RQWIP) is investigated under the influence of electromagnetic wave (EMW) by using quantum kinetic equation (QKE) method. We obtain the electron distribution function in the interaction with internal and external phonon. By solving the inhomogeneous differential equation, the current density and the acoustomagnetoelectric field (E AME ) are obtained in the dependence on the EMW amplitude, external acoustic wave frequency, the system temperature and the RQWIP length. The results are numerically evaluated for GaAs/AlGaAs quantum wire. We compares it to the result obtained in case of the bulk semiconductors and others low-dimension system in order to show the difference and the novelty of the results. The E AME depends non-linearly on the wire length L z and exhibits an oscillatory behavior as the function of wire length, although the stability period in long wire length condition. Moreover, we survey the impact of EMW on EAME with the dependence on the external acoustic wave frequency The result also indicates that the current density amplifies exponentially as the temperature increases.

Stabilization of adiabatic population transfer by strong coupling to a phonon bath

We investigate the influence of the environment on the rapid adiabatic passage scheme for optimal population transfer in a two-level system. To cope with strong coupling to an external phonon bath with super-Ohmic spectral density, we are solving the time-dependent Schr\odinger equation of the extended system, including a finite number of bath modes using the multi-Davydov D2 ansatz. This allows for the treatment of the non-Markovian reduced dynamics of the two-level subsystem. Surprisingly, it is found that strong system-bath coupling stabilizes the transition probability from the lower to the upper level as a function of the area under the laser pulse. This dissipative engineering effect could only be uncovered by a non-Markovian treatment. For strong coupling, the transition probability then becomes a monotonically increasing function of the pulse area at zero temperature of the heat bath. Finite temperatures break the monotonicity in the range of pulse areas that we studied, but not the stability of the observed effect.

Polarized Raman spectra of hematite and assignment of external modes

AbstractNotwithstanding the significant practical importance of hematite, α‐Fe2O3, the complete assignment and understanding of the Raman spectrum acquired on this crystalline solid are uncertain. Above all, only one of the two external Eg phonons arising from the Γ point has been resolved and hence assigned. It is well known that the Eg mode at 294 cm−1 has been attributed to one of the two external phonons arising from the Γ point. To this end, we have undertaken studies to examine the polarized Raman scattering on a pure single crystal to gain a better understanding and assignment of phonons arising from the Γ point in the Raman spectrum of hematite. Here, we resolve and assign the previously unidentified external Eg phonon at 245 cm−1 and, additionally, confirm that the band at 294 cm−1 is an external Eg phonon, in the first‐order Raman spectrum of hematite. Further, our polarized Raman spectra display interesting polarization behavior of the 2LO mode at 1320 cm−1, where this band is only Raman active in ei||es polarization configurations.

Dielectric disorder in two-dimensional materials.

Understanding and controlling disorder is key to nanotechnology and materials science. Traditionally, disorder is attributed to local fluctuations of inherent material properties such as chemical and structural composition, doping or strain. Here, we present a fundamentally new source of disorder in nanoscale systems that is based entirely on the local changes of the Coulomb interaction due to fluctuations of the external dielectric environment. Using two-dimensional semiconductors as prototypes, we experimentally monitor dielectric disorder by probing the statistics and correlations of the exciton resonances, and theoretically analyse the influence of external screening and phonon scattering. Even moderate fluctuations of the dielectric environment are shown to induce large variations of the bandgap and exciton binding energies up to the 100 meV range, often making it a dominant source of inhomogeneities. As a consequence, dielectric disorder has strong implications for both the optical and transport properties of nanoscale materials and their heterostructures.

Analysis of germanium waveguide laser performance under external phonon injection.

We propose and design a germanium (Ge) waveguide laser under external phonon injection to reduce laser threshold. To take the phonon injection and the acousto-optic overlap into consideration, the theory of the photon-phonon laser action is further developed. The phononic crystal waveguide is introduced in the laser structure to intensify acousto-optic interaction, and characteristics of photon-phonon laser action in Ge waveguide are analyzed. With the external phonon injection, the two-quantum transition can be facilitated and the photon-phonon laser action is able to be established. The impacts of phononic crystal waveguide parameters, overlap of optical and acoustic fields, and phonon injection on the laser behavior are discussed. Optimal waveguide structural parameters are obtained to enhance acousto-optic interaction through the enlargement of the overlap of optical and acoustic fields. The results indicate that, for a Ge waveguide with the length of 200 μm, the threshold current is reduced to 0.2 μA and the slope efficiency reaches 0.7 W/A when the average phonon injection concentration is about 2.5 × 1021 cm-3. Our proposed scheme offers an effective approach to achieve laser oscillation in integrated Ge waveguide.

Interaction of External Acoustic Waves - Confined Electrons - Internal Phonons in a Cylindrical Quantum Wire with an Infinite Potential in the Presence of an External Magnetic Field

Interaction of external acoustic waves - confined electrons - internal phonons in a cylindrical quantum wire with an infinite potential (CQWIP) has been theoretically studied via the quantum kinetic equation for electrons in the presence of an external magnetic field (EMF). The quantum kinetic equation for the distribution function of electrons interacting with internal and external acoustic phonons is obtained from the Heisenberg equation of motion and the model of CQWIP. The density of acoustomagnetoelectric (AME) current and the analytical expressions for the AME field in the CQWIP in the presence of the EMF are obtained. The theoretical results are numerically evaluated for the specific CQWIP of GaAs/GaAsAl. It is shown that the AME field strongly depends on the system temperature in both cases of the strong magnetic field and the weak magnetic field. However, the graph showing the relationship between the AME field and the system temperature has the peaks in case of the strong magnetic field. The reason may be due to movement of the electrons between the mini-bands. The AME field obtained in this work are then compared with those of bulk semiconductors, superlattices, quantum wells (QW) and rectangular quantum wire (RQW).

Anisotropy of the proton kinetic energy in ice Ih

The partial vibrational density of states (pVDOS) of ice Ih, as simulated by first principle modeling based on density functional theory (DFT), is utilized for computing the Cartesian components of the proton and oxygen quantum kinetic energies, Ke(H) and Ke(O) respectively, along and perpendicular to the hydrogen bonds. The DFT method was found to yield better agreement with deep inelastic neutron scattering (DINS) measurements than the semi empirical (SE) calculations. The advantage of using the DFT method is to enable us to resolve the external and internal phonon bands of the Cartesian projections of the pVDOS, and hence those of the lattice and vibrational components of Ke(H). We show that a pVDOS analysis is a valuable tool in testing scattering results of complex systems and suggest its potential to explore competing quantum effects, e.g. on Ke(H) across phase transitions in water.

Ultrafast Energy Dissipation via Coupling with Internal and External Phonons in Two-Dimensional MoS2.

Atomically thin two-dimensional materials have emerged as a promising system for optoelectronic applications; however, the low quantum yield, mainly caused by nonradiative energy dissipation, has greatly limited practical applications. To reveal the details for nonradiative energy channels, femtosecond pump-probe spectroscopy with a detection wavelength ranging from visible to near-infrared to mid-infrared is performed on few-layer MoS2. With this method, the many-body effects, occupation effects, and phonon dynamics are clearly identified. In particular, thermalization of the MoS2 lattice via electron-phonon scattering is responsible for a redshift of the exciton resonance energy observed within tens to hundreds of picoseconds after photoexcitation, which provides a direct real-time sensor for measuring the change in lattice temperature. We find that the excess energy from the cooling of hot carriers and the formation of bound carriers is efficiently transferred to the internal phonon system within 2 ps, while that from Shockley-Read-Hall recombination (∼9 ps) is mainly dissipated from the MoS2 surfaces to external phonons.

Remote heat dissipation in atom-sized contacts

Understanding and control of heat dissipation is an important challenge in nanoelectronics wherein field-accelerated hot carriers in current-carrying ballistic systems release a large part of the kinetic energy into external bulk phonon baths. Here we report on a physical mechanism of this remote heat dissipation and its role on the stability of atomic contacts. We used a nano-fabricated thermocouple to directly characterize the self-heating in a mechanically-configurable Au junction. We found more pronounced heat dissipation at the current downstream that signifies the electron-hole asymmetry in Au nanocontacts. Meanwhile, the simultaneously measured single-atom chain lifetime revealed a minor influence of the heat dissipation on the contact stability by virtue of microleads serving as an effective heat spreader to moderate the temperature rise to several Kelvins from the ambient under microwatt input power. The present finding can be used for practical design of atomic and molecular electronic devices for heat dissipation managements.

Analog quantum simulation of(1+1)-dimensional lattice QED with trapped ions

The prospect of quantum simulating lattice gauge theories opens exciting possibilities for understanding fundamental forms of matter. Here, we show that trapped ions represent a promising platform in this context when simultaneously exploiting internal pseudo-spins and external phonon vibrations. We illustrate our ideas with two complementary proposals for simulating lattice-regularized quantum electrodynamics (QED) in (1+1) space-time dimensions. The first scheme replaces the gauge fields by local vibrations with a high occupation number. By numerical finite-size scaling, we demonstrate that this model recovers Wilson's lattice gauge theory in a controlled way. Its implementation can be scaled up to tens of ions in an array of micro-traps. The second scheme represents the gauge fields by spins 1/2, and thus simulates a quantum link model. As we show, this allows the fermionic matter to be replaced by bosonic degrees of freedom, permitting small-scale implementations in a linear Paul trap. Both schemes work on energy scales significantly larger than typical decoherence rates in experiments, thus enabling the investigation of phenomena such as string breaking, Coleman's quantum phase transition, and false-vacuum decay. The underlying ideas of the proposed analog simulation schemes may also be adapted to other platforms, such as superconducting qubits.

Luminescence characterization of sol-gel derived Pr3+ doped NaGd(WO4)2 phosphors for solid state lighting applications

In the present work, xPr3+:NaGd(WO4)2 (0.5 ≤ x ≤ 5.0 mol%) sub-micron phosphors were synthesised by sol-gel method. Low cost precursors of metal nitrates and low temperature thermal treatment was used compared to conventional solid state reaction. The formation of highly crystalline phosphors with tetragonal structure was confirmed by XRD and increase of Pr3+ ions content in host matrix leads to expansion of the unit cell volume. The surface morphology, size and particle distribution of the phosphors were observed by field emission scanning electron microscopy (FE-SEM). A rectangular shape particle with a size distribution ranging from 400 to 600 nm and tightly packed surface was seen in FE-SEM micrographs. The various internal and external phonon modes vibration corresponding to double tungstate structure was observed in Raman spectra. The optical properties of the synthesised phosphors were explored by ultraviolet visible (UV–Vis) absorption in diffuse reflectance and photoluminescence (PL) measurements. UV–Vis measurements distinguished the host and Pr3+ absorption and also reveal an increase in optical band gap values with an increase of Pr3+. The PL measurements show various emissions from green and red regions under 450 nm. The maximum intensity emission at 489 nm is due to 3P0 → 3H4 transition of Pr3+. From the maximum emission the critical doping concentration was calculated to be at 3.5 mol% and critical distance between two adjacent Pr3+ ions as 20.43 Å.

Interaction between light and highly confined hypersound in a silicon photonic nanowire

In the past decade, there has been a surge in research at the boundary between photonics and phononics. Most efforts centered on coupling light to motion in a high-quality optical cavity, typically geared towards observing the quantum state of a mechanical oscillator. It was recently predicted that the strength of the light-sound interaction would increase drastically in nanoscale silicon photonic wires. Here we demonstrate, for the first time, such a giant overlap between near-infrared light and gigahertz sound co-localized in a small-core silicon wire. The wire is supported by a tiny pillar to block the path for external phonon leakage, trapping $\mathbf{10} \; \textbf{GHz}$ phonons in an area below $\mathbf{0.1 \; \boldsymbol\mu}\textbf{m}^{\mathbf{2}}$. Since our geometry can be coiled up to form a ring cavity, it paves the way for complete fusion between the worlds of cavity optomechanics and Brillouin scattering. The result bodes well for the realization of low-footprint optically-pumped lasers/sasers and delay lines on a densely integrated silicon chip.

Indentation on two-dimensional hexagonal quasicrystals

This paper is concerned with contact problem for a half-space of two-dimensional hexagonal quasi-crystal punched by three common indenters (cylindrical flat-ended, conical and spherical punches). Based on the Green’s functions for the half-space subjected to an external phonon source exerted on the surface, the superposition principle is applied to constructing the boundary integral equation. Relations between the indentation force and the penetration depth, and the indentation stiffness constants are explicitly obtained for these three indenters. Complete and exact fields in the half-space are given in terms of elementary functions. The present theoretical solutions can not only serve as benchmark for computational contact mechanics, but also apply to guiding future indentation experiments.

The Phonon Confinement Effect on the Impurity States in Cylindrical Nanowire with a Finite Confining Potential in Presence of Electric and Magnetic Fields

In the framework of effective mass approximation, the LandauPekar variational procedure is adopted to study the ground-state binding energy of a hydrogenic impurity in a semiconductor nanowire with finite confining potential subjected to both external fields (electric and magnetic) and electronpolar optical phonon interaction taking into account the phonon confinement effect. The results for the binding energy as well as polaronic correction are obtained as a function of the wire radius, impurity position and applied fields. Calculated results reveal that the values of the polaronic shifts of impurity binding energy can be quite different for cases with finite and infinite confining potentials.

Electron–Phonon Coupling Dynamics at Oxygen Evolution Sites of Visible-Light-Driven Photocatalyst: Bismuth Vanadate

Bismuth vanadate (BiVO4) is an effective visible-light-driven photocatalyst for oxygen evolution from water. To understand the mechanism of photocatalytic oxidation of water, it is important to detect and characterize holes at the surfaces of powdered catalysts. Here, we report the transient absorption of BiVO4 in a wide time range from subpicosecond to 200 μs upon the excitation across the band gap with 400 nm femtosecond pulses. The effect of electron scavenger (Fe3+) on transient absorption decays indicates that the transitions at λ 700 nm rises almost instantaneously, the absorption λ < 700 nm has a slower rise component of τ ∼ 15 ps due to filling of surface traps with holes. Moreover, the rise component is modulated with strongly oscillating signals caused by coherent excitation of an external phonon mode between Bi3+ and VO43–. Thus, the transitions at λ < 700 nm associated with surface-trapped holes are...