Phonon Dynamics Research Articles

Phonon Dynamics at an Oxide Layer in Silicon: Heat Flow and Kapitza Resistance

The interactions between heat flow and an oxide layer in Si are studied within two temperature windows using non‐equilibrium ab initio molecular‐dynamics (MD). The model system is a H‐saturated Si nanowire containing an amorphous SiOx layer. The nanowire is in a large 1‐D periodic box which prevents thermal contamination between image nanowires. The results show that the oxide acts as barrier to heat flow and substantially increases the time required for the system to reach thermal equilibrium. This effect is caused by the higher‐frequency vibrational modes in the oxide relative to Si, and is unrelated to the low thermal conductivity of SiOx. A new first‐principles method to calculate the Kapitza resistance of the interface directly from the MD data is proposed.

Wavefront shaping of acoustic waves in scattering media

As an essential necessity for fundamental study and a broad variety of technological applications, it is important to control the propagation of acoustic waves and select their particular wavefronts. This work is to achieve effective wavefront shaping of acoustic waves along specific paths in strongly scattering media including phononic periodic structures and random media. In the phononic crystal, a kind of artificial phononic structures, wavefront shaping, and super-resolution imaging have been achieved by designing the geometry and material parameters. Furthermore, the confocal technique of the optical beam and acoustic beam has been improved to achieve sub-wavelength imaging in random media with strongly scattering. The presented results of wavefront shaping have implications for tailoring phonon dynamics in scattering media, which offers further possibilities of controllable acoustic waves in complex materials and enlightens the interaction of multi-physics fields.

Compositional Effects and Electron Lone-pair Distortions in Doped Bournonites.

Bornite materials are naturally occurring systems composed of earth-abundant constituents. Bournonite, a representative of this class of materials, is of interest for thermoelectric applications due to its inherently low thermal conductivity, which has been attributed to the lattice distortions due to stereochemically active electron lone pair distributions. In this computational and experimental study, we present analyses of the lattice structure, electron and phonon dynamics, and charge localization and transfer properties for undoped and Ni and Zn doped bournonites. The results from our simulations reveal complex relations between bond length and bond angle characteristics, chemical bonding, and charge transfer upon doping. Analysis of the experimental results indicate that a microscopic description for bournonite and its doped compositions is necessary for a complete understanding of these materials, as well as for effective control of the transport properties for targeted applications.

Polarization rotation associated with phonon dynamics in monoclinic C phase near morphotropic phase boundary studied by diffuse and inelastic X-ray scattering from a Ti-composition-gradient Pb[(Mg1/3Nb2/3)1–xTix]O3 single crystal

ABSTRACTWe have investigated polarization rotation associated with phonon dynamics in the relaxor ferroelectrics Pb[(Mg1/3Nb2/3)1–xTix]O3. The intensity distributions of the diffuse scattering patterns show intensity- crossover behavior in the monoclinic-C(MC) phase over a wide Ti composition range (29 < x < 32 mol%). The transverse acoustic (TA) mode of the crossover region along the [001] direction softens and is damped, strongly correlating with the central peak intensity, which originates in the polar regions. These results indicate that the lattice can easily deform in the MC phase, and the polarization can rotate easily by associating with the TA mode.

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.

Atomistic Framework for Time-Dependent Thermal Transport

Phonons play a major role for the performance of nanoscale devices and consequently a detailed understanding of phonon dynamics is required. Using an auxiliary-mode approach, which has successfully been applied for the case of electrons, we develop a new method to numerically describe time-dependent phonon transport. This method allows one to gain insight into the behavior of local vibrations in molecular junctions, which are driven by time-dependent temperature differences between thermal baths. Exemplarily, we apply the method to study the nonequilibrium dynamics of quantum heat transport in an one-dimensional atomic chain as well as in realistic molecular junctions made of polyacetylene and polyethylene chains, in which the vibrational structure of the junction is described at the density functional theory level. We calculate the transient energies and heat currents and compare the latter to the standard Landauer approach in thermal equilibrium. We show that the auxiliary-mode representation is a powerful and versatile tool to study time-dependent thermal transport in nanoscale systems.

Coherent Spin and Quasiparticle Dynamics in Solution-Processed Layered 2D Lead Halide Perovskites.

Layered 2D halide perovskites with their alternating organic and inorganic atomic layers that form a self‐assembled quantum well system are analogues of the purely inorganic 2D transition metal dichalcogenides. Within their periodic structures lie a hotbed of photophysical phenomena such as dielectric confinement effect, optical Stark effect, strong exciton–photon coupling, etc. Detailed understanding into the strong light–matter interactions in these hybrid organic–inorganic semiconductor systems remains modest. Herein, the intricate coherent interplay of exciton, spin, and phonon dynamics in (C6H5C2H4NH3)2PbI4 thin films using transient optical spectroscopy is explicated. New insights into the hotly debated origins of transient spectral features, relaxation pathways, ultrafast spin relaxation via exchange interaction, and strong coherent exciton–phonon coupling are revealed from the detailed phenomenological modeling. Importantly, this work unravels the complex interplay of spin–quasiparticle interactions in these layered 2D halide perovskites with large spin–orbit coupling.

An Integrated Approach to Thermoelectrics: Combining Phonon Dynamics, Nanoengineering, Novel Materials Development, Module Fabrication, and Metrology

AbstractThis review discusses the longstanding efforts to develop advanced thermoelectrics through a multidisciplinary approach by combining condensed matter physics, nanotechnology, solid‐state chemistry, electrical engineering, mechanical engineering, and metrology. The phonon dynamics of skutterudites, clathrates, tetrahedrites, and layered LaOBiSSe are investigated through inelastic neutron scattering, allowing insights into their low lattice thermal conductivity due to rattling in a cage as well as under planar coordination. The electrical resistivity, Seebeck coefficient, and Hall coefficient of Bi‐nanowires are successfully measured with a home‐made system, demonstrating a size effect in thermoelectric and galvanomagnetic phenomena. For PbTe‐based bulk thermoelectrics, an exceptionally high figure of merit ZT (≈1.8 at 800 K) is achieved through nanostructuring. Moreover, correspondingly high conversion efficiency (≈11% for a temperature difference of 590 K) is demonstrated in nanostructured PbTe‐based modules. Sulfides (tetrahedrite, colusite, and CdI2‐type layered systems) and arsenides (LnFeAsO and BaZn2As2) are developed as environmentally friendly and emerging thermoelectric materials, respectively. The output power and efficiency of modules with novel materials, including nanostructured PbTe, Zn4Sb3, and clathrates, are measured with the highly accurate self‐made system. Future opportunities and challenges for the widespread use of thermoelectric waste heat recovery and energy harvesting are also discussed.

Photo-excited dynamics in the excitonic insulator Ta2NiSe5

The excitonic insulator is an intriguing correlated electron phase formed of condensed excitons. A promising candidate is the small band gap semiconductor Ta2NiSe5. Here we investigate the quasiparticle and coherent phonon dynamics in Ta2NiSe5 in a time resolved pump probe experiment. Using the models originally developed by Kabanov et al for superconductors (Kabanov et al 1999 Phys. Rev. B 59 1497), we show that the material’s intrinsic gap can be described as almost temperature independent for temperatures up to about 250 K to 275 K. This behavior supports the existence of the excitonic insulator state in Ta2NiSe5. The onset of an additional temperature dependent component to the gap above these temperatures suggests that the material is located in the BEC-BCS crossover regime. Furthermore, we show that this state is very stable against strong photoexcitation, which reveals that the free charge carriers are unable to effectively screen the attractive Coulomb interaction between electrons and holes, likely due to the quasi 1D structure of Ta2NiSe5.

A tunable time-resolved spontaneous Raman spectroscopy setup for probing ultrafast collective excitation and quasiparticle dynamics in quantum materials.

We present a flexible and efficient ultrafast time-resolved spontaneous Raman spectroscopy setup to study collective excitation and quasi-particle dynamics in quantum materials. The setup has a broad energy tuning range extending from the visible to near infrared spectral regions for both the pump excitation and Raman probe pulses. Additionally, the balance between energy and time-resolution can be controlled. A high light collecting efficiency is realized by high numerical aperture collection optics and a high-throughput flexible spectrometer. We demonstrate the functionality of the setup with a study of the zone-center longitudinal optical phonon and hole continuum dynamics in silicon and discuss the role of the Raman tensor in time-resolved Raman scattering. In addition, we show an evidence for unequal phonon softening rates at different high symmetry points in the Brillouin zone of silicon by means of detecting pump-induced changes in the two-phonon overtone spectrum. Demagnetization dynamics in the helimagnet Cu2OSeO3 is studied by observing softening and broadening of a magnon after photo-excitation, underlining the unique power of measuring transient dynamics in the frequency domain, and the feasibility to study phase transitions in quantum materials.

Topological nodal line semimetal in an orthorhombic graphene network structure

Topological semimetals are a fascinating class of quantum materials that possess extraordinary electronic and transport properties. These materials have attracted great interest in recent years for their fundamental significance and potential device applications. Currently a major focus in this research field is to theoretically explore and predict and experimentally verify and realize material systems that exhibit a rich variety of topological semimetallic behavior, which would allow a comprehensive characterization of the intriguing properties and a full understanding of the underlying mechanisms. In this paper, we report on ab initio calculations that identify a carbon allotrope with simple orthorhombic crystal structure in $Pbcm$ (${D}_{2h}^{11}$) symmetry. This carbon allotrope can be constructed by inserting zigzag carbon chains between the graphene layers in graphite or by a crystalline modification of a (3,3) carbon nanotube with a double cell reconstruction mechanism. Its dynamical stability has been confirmed by phonon and molecular dynamics simulations. Electronic band calculations indicate that it is a nodal-line semimetal comprising two nodal lines that go through the whole Brillouin zone in bulk and a projected surface flat band around the Fermi level. The present findings establish an additional topological semimetal system in the nanostructured carbon allotropes family and offer insights into its outstanding structural and electronic properties.

Electronic and mechanical properties of MgN compound: Prediction of stable half-metallic ferromagnet in NaCl and ZB phases

First-principles method within the generalized gradient approximation (GGA-PBEsol) and the modified Becke-Johnson approach (mBJ-GGA-PBEsol) for the exchange-correlation energy and potential were used to investigate the structural, elastic, mechanical and thermal properties of MgN compound in different phases. Various phases of MgN compound were considered, which are NaCl, CsCl, ZB, WZ, NiAs, PtH4I, and Pnma phases. We found that the NaCl phase is the lowest energy phase as a function of the volume. The ferromagnetic phase is energetically favored with respect to the non-magnetic phases, except for the CsCl and PtH4I phases. The calculated elastic properties for the NaCl, ZB and WB phases showed that they are elastically stable. Considering the phonon dynamics of MgN in the NaCl, ZB and WB phases, we observed that the MgN compound in the NaCl and ZB phases is dynamically stable. From electronic band structure and density of states, MgN compound shows half-metallic behavior in NaCl and ZB phases. The NaCl phase was found to have a robust half-metallic character with respect to the lattice compression and expansion. The half-metallic and magnetic character found in MgN compound is attributed to the presence of spin polarized 2p orbitals of the nitrogen atom. We found that the MgN compound in the NaCl and ZB phases is a half-metallic ferromagnet with magnetic moment of 1 μB per formula unit and half-metallic gaps of 0.01 eV (1.34 eV) and 0.57 eV (1.85 eV) within the GGA-PBEsol (mBJ-GGA-PBEsol) for the NaCl and ZB phases respectively.

Localization of disordered harmonic chain with long-range correlation

In the previous paper (Yamada, (2018)), we investigated localization properties of one-dimensional disordered electronic system with long-range correlation generated by modified Bernoulli (MB) map. In the present paper, we report localization properties of phonon in disordered harmonic chains generated by the MB map. Here we show that Lyapunov exponent becomes positive definite for almost all frequencies ω except ω=0, and the B−dependence changes to exponential decrease for B > 2, where B is a correlation parameter of the MB map. The distribution of the Lyapunov exponent of the phonon amplitude has a slow convergence, different from that of uncorrelated disordered systems obeying a normal central-limit theorem. Moreover, we calculate the phonon dynamics in the MB chains. We show that the time-dependence of spread in the phonon amplitude and energy wave packet changes from that in the disordered chain to that in the periodic one, as the correlation parameter B increases.

Two-Color Pump-Probe Measurement of Photonic Quantum Correlations Mediated by a Single Phonon.

We propose and demonstrate a versatile technique to measure the lifetime of the one-phonon Fock state using two-color pump-probe Raman scattering and spectrally resolved, time-correlated photon counting. Following pulsed laser excitation, the n=1 phonon Fock state is probabilistically prepared by projective measurement of a single Stokes photon. The detection of an anti-Stokes photon generated by a second, time-delayed laser pulse probes the phonon population with subpicosecond time resolution. We observe strongly nonclassical Stokes-anti-Stokes correlations, whose decay maps the single phonon dynamics. Our scheme can be applied to any Raman-active vibrational mode. It can be modified to measure the lifetime of n≥1 Fock states or the phonon quantum coherences through the preparation and detection of two-mode entangled vibrational states.

Simulation of Electron–Phonon Coupling and Heating Dynamics in Suspended Monolayer Graphene Including All the Phonon Branches

Thermal effects in monolayer graphene due to an electron flow are investigated with a direct simulation Monte Carlo (DSMC) analysis. The crystal heating is described by simulating the phonon dynamics of the several relevant branches, acoustic, optical, K and Z phonons. The contribution of each type of phonon is highlighted. In particular, it is shown that the Z phonons, although they do not enter the scattering with electrons, play a non-negligible role in the determination of the crystal temperature. The phonon distributions are evaluated by counting the emission and absorption processes during the MC simulation. The crystal temperature raise is obtained for several applied electric fields and for several positive Fermi energies. The latter produces the effect of a kind of n-doping in the graphene layer. Critical temperatures can be reached in a few tens of picoseconds posing remarkable issues regarding the cooling system in view of a possible application of graphene in semiconductor devices. Moreover, a significant influence of the lattice temperature on the characteristic curves is observed only for long times, confirming graphene rather robust as regards the electrical performance.

Comparison of Phonon Damping Behavior in Quantum Dots Capped with Organic and Inorganic Ligands.

Surface ligand modification of colloidal semiconductor nanocrystals has been widely used as a means of controlling photoexcited-state generation, relaxation, and coupling to the environment. While progress has been made in understanding how surface ligand modification affects the behavior of electronic states, less is known about the influence of surface ligand modification on phonon behavior, which impacts relaxation dynamics and transport phenomena. In this work, we compare the dynamics of optical and acoustic phonons in CdTe quantum dots (QDs), CdTe/CdSe core/shell QDs capped with octadecylphosphonic acid ligands, and CdTe QDs capped with Se2- to ascertain how ligand exchange from native aliphatic ligands to single-atom Se2- ligands affects phonon behavior. We use transient absorption spectroscopy and observe modulations in the kinetics of excited-state decay due to QD lattice vibrations from both optical and acoustic phonons, which we describe using the damped oscillator model. The longitudinal optical phonons have similar frequencies and damping behavior in all three samples. In contrast, the longitudinal acoustic phonon mode in the Se2--capped CdTe QDs is severely damped, much more so than in CdTe and CdTe/CdSe QDs capped with the native aliphatic ligands. We attribute these differences in the acoustic phonon behavior to the differences in how the QD dissipates vibrational energy to its surroundings as a function of ligand identity. Our results indicate that these inorganic surface-capping ligands enhance not only the electronic but also the mechanical coupling of nanocrystals with their environment.

Tetrahexcarbon: A two-dimensional allotrope of carbon

A two-dimensional structurally stable carbon allotrope is predicted using first-principles calculations. This unique network is composed of tetra and hexa-rings of carbon atoms known as Tetrahexcarbon. Tetrahexcarbon has a quasiparticle (QP) direct band gap of 3.70 eV at Γ, which is closer to the ZnS, a well-known direct bandgap semiconductor. Interestingly, it is dynamically stable and can withstand temperature up to 1000 K, which is confirmed by performing phonon and ab initio molecular dynamics (AIMD) simulations. The mobilities of both electrons and holes in this material are found to be anisotropic. It exhibits extraordinary room temperature in-plane electron mobility of order ∼104cm2V−1s−1, which is an order of magnitude higher than the black phosphorus monolayer (∼103cm2V−1s−1) and two orders of magnitude higher than MoS2 (∼200cm2V−1s−1) monolayer. The optical response of Tetrahexcarbon obtained by applying Bethe-Salpeter equation (BSE) to include excitonic effects on top of the partially self-consistent GW0 calculation. The absorption onsets strongly depend on the light polarization directions indicating Tetrahexcarbon an anisotropic material.

A study of electric transport in n- and p-type modulation-doped GaInNAs/GaAs quantum well structures under a high electric field

We present the results of longitudinal carrier transport under a high electrical field in n- and p-type modulation-doped Ga0.68In0.32NyAs1−y/GaAs (y = 0.009, 0.017) quantum well (QW) structures. Nitrogen composition-dependent drift velocities of electrons are observed to be saturated at and at 77 K for the samples with y = 0.009 and y = 0.017, respectively, while the drift velocities of holes do not saturate but slightly increase at the applied electric field in the range of interest. The hole drift velocity is observed to be higher than the electron drift velocity. The electron mobility exhibits an almost temperature-independent characteristic. On the other hand, the hole mobility exhibits a conventional temperature dependence of modulation-doped QW structures. As the temperature increases, the drift velocity of the electrons exhibits an almost an temperature-insensitive characteristic, but, on the other hand, for holes, drift velocity decreases approximately from 107–106 cm s−1. It is observed that the drift velocities of electrons and holes are N-dependent and suppressed at higher electric fields. Furthermore, experimental results show that there is no evidence of negative differential velocity (NDV) behaviour for both n- and p-type samples. To explore the observed electron and hole drift velocity characteristic at high electric fields, we use a simple theoretical model for carrier transport, which takes into account the effect of non-drifting hot phonons. The mobility mapping technique (comparison method) is used to extract hot hole temperature in order to employ it in the non-drifted phonon distribution and to obtain the drift velocity–electric field curves. Then hot electron temperatures are obtained from the drift velocity–electric field curves as a fit parameter using non-drifted hot phonon dynamics. The analytical model is well-matched to the experimental –E curves, indicating that carrier-hot phonon scattering is the main reason for suppressing the NDV mechanism in GaInNAs/GaAs QW structures with a carrier density higher than 1017 cm−3.

Effective holographic theory of charge density waves

We use Gauge/Gravity duality to write down an effective low energy holographic theory of charge density waves. We consider a simple gravity model which breaks translations spontaneously in the dual field theory in a homogeneous manner, capturing the low energy dynamics of phonons coupled to conserved currents. We first focus on the leading two-derivative action, which leads to excited states with non-zero strain. We show that including subleading quartic derivative terms leads to dynamical instabilities of AdS$_2$ translation invariant states and to stable phases breaking translations spontaneously. We compute analytically the real part of the electric conductivity. The model allows to construct Lifshitz-like hyperscaling violating quantum critical ground states breaking translations spontaneously. At these critical points, the real part of the dc conductivity can be metallic or insulating.

Effective thermal and mechanical properties of polycrystalline diamond films

Polycrystalline diamond films were demonstrated as good candidates for electron field emitters, and their mechanical/thermal properties should thus be considered for real devices. We utilized ultrafast optical techniques to investigate the phonon dynamics of several polycrystalline diamond films, prepared by microwave plasma enhanced chemical vapor deposition. The mechanical properties (longitudinal acoustic velocity) and thermal conductivities of diamond films were evaluated from the coherent and incoherent phonon dynamics, respectively. Ultrananocrystalline diamond films were grown using a CH4 (2%)/Ar plasma, while microcrystalline diamond films were grown using a CH4 (2%)/H2 plasma. The ultrananocrystalline diamond film (with a grain size of several nanometers) possesses low acoustic velocity (14.5 nm/ps) and low thermal conductivity (3.17 W/m K) compared with other kinds of diamond films. The acoustic velocity of diamond films increased abruptly to nearly the same as that of natural diamond and remained there when the rod-shaped diamond grains were induced due to the incorporation of H2 in the growth plasma (CH4/Ar). The thermal conductivities of the materials increased monotonously with increasing incorporation of H2 in the growth plasma (CH4/Ar). The thermal conductivity of 25.6 W/m K was attained for nanocrystalline diamond films containing spherical diamond grains (with a size of several tens of nanometers). Compared with single crystalline diamond, the low thermal conductivity of polycrystalline films results from phonon scattering at the interfaces of grains and amorphous carbon in the boundary phases.