Phonon Tunneling Research Articles

Optimally Suppressed Phonon Tunneling in van der Waals Graphene-WS2 Heterostructure with Ultralow Thermal Conductivity.

Van der Waals heterostructures have great potential for realizing ultimately low thermal conductivity because defectless interfaces can be constructed at a length scale smaller than the phonon wavelength, allowing modulation of coherent phonon transport. In this Letter, we demonstrate the mechanism for thermal conductivity reduction at a mode-resolved level. The graphene-WS2 heterostructure with the lowest cross-plane thermal conductivity of 0.048 W/(m·K) is identified from 16,384 candidates by combining Bayesian optimization and molecular dynamics simulations. Then, the angle-resolved phonon transmission is calculated using the mode-resolved atomistic Green's function. The results reveal that the optimal heterostructure nearly completely terminates phonon transport with finite incident angles, owing to the reduced critical incident angle and suppression of phonon tunneling.

Photon‐Assisted Quantum Phase Transitions in a Cavity Optomechanical System with Two Nonlinear Mechanical Modes

AbstractPhoton‐assisted quantum phase transitions (QPTs) are studied in a cavity optomechanical system that consists of one optical cavity and two nonlinear mechanical resonators and explore ground‐state properties of quantum phases. It is indicated that the cavity mode can induce effective two‐phonon tunneling interaction between two mechanical resonators. It is found that QPTs between the localization phase (LP) and delocalization phase (DP) of phonons can happen through tuning the phonon tunneling interaction. It is shown that the photon‐assisted QPT is the second order QPT. Ground‐state properties of the quantum phases are also investigated. It is indicated that the ground state of the LP is a separable squeezed state while the ground state of the DP is an entangled state. The results open a new way to engineer quantum phases and QPTs in macroscopic mechanical systems and can benefit a wide range of criticality‐enhanced quantum sensing applications.

Imaging Nanomechanical Vibrations and Manipulating Parametric Mode Coupling via Scanning Microwave Microscopy.

In this study, we present a novel platform based on scanning microwave microscopy for manipulating and detecting tiny vibrations of nanoelectromechanical resonators using a single metallic tip. The tip is placed on the top of a grounded silicon nitride membrane, acting as a movable top gate of the coupled resonator. We demonstrate its ability to map mechanical modes and investigate mechanical damping effects in a capacitive coupling scheme, based on its spatial resolution. We also manipulate the energy transfer coherently between the mode of the scanning tip and the underlying silicon nitride membrane, via parametric coupling. Typical features of optomechanics, such as anti-damping and electromechanically induced transparency, have been observed. Since the microwave optomechanical technology is fully compatible with quantum electronics and very low temperature conditions, it should provide a powerful tool for studying phonon tunnelling between two spatially separated vibrating elements, which could potentially be applied to quantum sensing.

Enhanced Thermoelectric Performance of PbTe Nanocomposites with Ag Nanoinclusions

Nanocomposites have already been widely used to enhance thermoelectric performance since they exhibit exceptionally low lattice thermal conductivities. Further improvement is expected to arise from boosting the power factor and charge carrier relevant figure of merit ZT with nanoinclusions. Here we present that Ag nanoscale inclusions embedding in the n-PbTe can meet this requirement. The quantitative relationships between such nanostructures in PbTe and thermoelectric properties, using a versatile model including the details of geometry, electron–phonon interactions, quantization, and tunneling, showed that the Seebeck coefficient can be significantly enhanced due to interface potential barrier induced by Ag nanoinclusions. Additionally, it was found that high-concentration Ag nanoinclusions with 1–2 nm radius can effectively enhance the thermoelectric performance of PbTe through energy-selective carrier scattering, especially around room temperature. The power factor and charge carrier relevant figure of merit ZT can be elevated by at least 80% and 260%, respectively. These findings highlight effective strategies to nanostructuring for overcoming the high threshold to commercial application for cooling and power generation.

Extreme near-field heat transfer between silica surfaces

Despite recent experiments exhibiting an impressive enhancement in radiative heat flux between parallel planar silica surfaces with gap sizes of about 10 nm, the exploration of sub-nanometric gap distances remains unexplored. In this work, by employing non-equilibrium molecular dynamics (NEMD) simulations, we study the heat transfer between two SiO2 plates in both their amorphous and crystalline forms. When the gap size is 2 nm, we find that the heat transfer coefficient experiences a substantial ∼30-fold increase compared to the experimental value at the gap size of 10 nm confirming the dependence on the distance inversely quadratic as predicted by the fluctuational electrodynamics (FE) theory. Comparative analysis between NEMD and FE reveals a generally good agreement, particularly for amorphous silica. Spectral heat transfer analysis demonstrates the profound influence of gap size on heat transfer, with peaks corresponding to the resonances of dielectric function. Deviations from the fluctuational electrodynamics theory at smaller gap sizes are interpreted in the context of acoustic phonon tunneling and the effects of a gradient of permittivity close to the surfaces.

Ultralow Lattice Thermal Transport and Considerable Wave-like Phonon Tunneling in Chalcogenide Perovskite BaZrS3.

Chalcogenide perovskites provide a promising avenue for nontoxic, stable thermoelectric materials. Here, the thermal transport and thermoelectric properties of BaZrS3 as a typical orthorhombic perovskite are investigated. An extremely low lattice thermal conductivity κL of 1.84 W/mK at 300 K is revealed for BaZrS3, due to the softening effect of Ba atoms on the lattice and the strong anharmonicity caused by the twisted structure. We demonstrate that coherence contributions to κL, arising from wave-like phonon tunneling, lead to an 18% thermal transport contribution at 300 K. The increasing temperature softens the phonons, thus reducing the group velocity of materials and increasing the scattering phase space. However, it simultaneously reduces the anharmonicity, which is dominant in BaZrS3 and ultimately improves the particle-like thermal transport. In addition, via replacement of the S atom with Se- and Ti-alloying strategy, the ZT value of BaZrS3 is significantly increased from 0.58 to 0.91 at 500 K, making it an important candidate for thermoelectric applications.

Constructing of amorphous/PbSe/amorphous multiple quantum wells with record high thermoelectric properties

Quantum well (QW) is one of the proposals to improve the thermoelectric properties. However, previous studies have shown that the large electron and phonon tunneling across the layers in semiconductor-based QWs and oxide-based QWs, respectively, are the two main challenges that need to be addressed. In this work, PbSe/amorphous-SrTiO3 (α-STO) supperlattices with a multiple quantum-well (MQW) structure can overcome both limitations. We demonstrate that the PbSe/α-STO MQW shows a high power factor of ∼ 24.5 μW cm−1K−2 at room temperature due to the strong quantum confine effect. In addition, the amorphous-SrTiO3 layer can engender intrinsically low thermal conductivity (0.27± 0.05 W m−1 K−1). The maximum estimated zT value is 2.7 for PbSe/α-STO MQW at room temperature, which is more than 1800 % higher than that of pure PbSe films (zT = 0.15).

Overdamped Phonon Diffusion and Nontrivial Electronic Structure Leading to a High Thermoelectric Figure of Merit in KCu5Se3.

Thermoelectric copper selenides are highly attractive owing to not only their constituent nontoxic, abundant elements but also their ultralow liquid-like lattice thermal conductivity (κlat). For the first time, the promising thermoelectric properties of the new KCu5Se3 are reported herein, showing a high power factor (PF = 9.0 μWcm-1 K-2) and an intrinsically ultralow κlat = 0.48 Wm-1 K-1. The doped K1-xBaxCu5Se3 (x = 0.03) realizes a figure-of-merit ZT = 1.3 at 950 K. The crystallographic structure of KCu5Se3 allows complex lattice dynamics that obey a rare dual-phonon transport model well describing a high scattering rate and an extremely short phonon lifetime that are attributed to interband phonon tunneling, confinement of the transverse acoustic branches, and temperature-dependent anharmonic renormalization, all of which generate an unprecedently high contribution of the diffusive phonons (70% at 300 K). The overall weak chemical bonding feature of KCu5Se3 gives K+ cations a quiescence behavior that further blocks the heat flux transfer. In addition, the valence band edge energy dispersion of KCu5Se3 is quasilinear that allows a large Seebeck coefficient even at high hole concentrations. These in-depth understandings of the ultralow lattice thermal conductivity provide new insights into the property-oriented design and synthesis of advanced complex chalcogenide materials.

Anisotropic Phonon Scattering and Thermal Transport Property Induced by the Liquid-like Behavior of AgCrSe2.

Superionic conductors exhibiting a periodic crystalline lattice and liquid-like ionic conductivity have emerged as promising materials in energy-conversion devices. Herein, we have investigated the interplay among anharmonic lattice dynamics, thermal conduction, and ultrafast atomic diffusion across the superionic transition of AgCrSe2. We show that the thermal conductivity (κ) contributions from convection and conduction-convection interactions increase simultaneously due to the gradual fluidization of Ag atoms, leading to a temperature-independent κ in the superionic state. We demonstrate a non-Peierls type thermal transport behavior induced by the strong lattice anharmonicity of Ag atoms, which promotes a nontrivial wave-like phonon tunneling in the normal state of AgCrSe2. Our current fluctuation analysis demonstrates an anisotropic phonon-liquid scattering behavior that the in-plane nondispersive transverse acoustic (TA) phonons near the zone boundary collapse, while the zone center and boundary TA phonons in the direction perpendicular to the liquid-like flow of Ag atoms survive.

Role of the Nottingham effect in heat transfer in the extreme near-field regime

We analyze the heat transfer between two metals separated by a vacuum gap in the extreme near-field regime. In this cross-over regime between conduction and radiation, heat exchanges are mediated by photon, phonon and electron tunneling. We quantify the relative contribution of these carriers with respect to both the separation distance between the two bodies and the applied bias voltage. In the presence of a weak bias ($V_{\rm b}<100$~mV), electrons and phonons can contribute equally to the heat transfer near contact, while the contribution of photons becomes negligible. On the other hand, for larger bias voltages, electrons play a dominant role. Moreover, we demonstrate that depending on the magnitude of this bias, electrons can either cool down or heat up the hot body by the Nottingham effect. Our results emphasize some inconsistencies in recent experimental results about heat exchanges in the extreme near-field regime and set a road map for future experiments.

Phonon transport in Cu2GeSe3 : Effects of spin-orbit coupling and higher-order phonon-phonon scattering

Much attention has been focused on understanding the mechanism of low lattice thermal conductivity in Cu-based diamondlike thermoelectric compounds. For ${\mathrm{Cu}}_{2}\mathrm{Ge}{\mathrm{Se}}_{3}$, the underlying origin of low lattice thermal conductivity remains to be clarified. In this work, the first-principles calculations are employed to systematically investigate the effects of spin-orbit coupling, higher-order phonon-phonon scattering, phonon wavelike tunneling, temperature-induced renormalization of phonon and phonon-phonon interaction, and volumetric expanse on the phonon transport of ${\mathrm{Cu}}_{2}\mathrm{Ge}{\mathrm{Se}}_{3}$. We show that the spin-orbit coupling results in no detectable change on the phonon frequencies, compared with those obtained without spin-orbit coupling, while it induces slight increase in both Gr\"uneisen parameter and lattice thermal conductivity of ${\mathrm{Cu}}_{2}\mathrm{Ge}{\mathrm{Se}}_{3}$, and the underlying mechanism is thoroughly analyzed. With the fourth-order phonon scattering and temperature-induced renormalization, the calculated lattice thermal conductivities are well consistent with the experimental results. Due to the enhanced four-phonon scattering process, there are remarkable reductions for the thermal conductivity. In ${\mathrm{Cu}}_{2}\mathrm{Ge}{\mathrm{Se}}_{3}$, the coherences term ${\ensuremath{\kappa}}_{c}$ contributes increasingly to the total lattice thermal conductivity when temperature arises. And after considering the effects of temperature-induced renormalization and higher-order phonon-phonon interactions, the ${\ensuremath{\kappa}}_{c}$ would provide $\ensuremath{\sim}5%$ and above 25% of total conductivity at 300 and 800 K, respectively. Our finding clarifies the mechanism of low thermal conductivity in ${\mathrm{Cu}}_{2}\mathrm{Ge}{\mathrm{Se}}_{3}$, and benefits the design of similar Cu-based diamondlike materials in thermoelectric applications.

Predicting the Lattice Thermal Conductivity in Nitride Perovskite LaWN3 from ab initio Lattice Dynamics.

Using a density functional theory-based thermal transport model, which includes the effects of temperature (T)-dependent potential energy surface, lattice thermal expansion, force constant renormalization, and higher-order quartic phonon scattering processes, it is found that the recently synthesized nitride perovskite LaWN3 displays strong anharmonic lattice dynamics manifested into a low lattice thermal conductivity (κL ) and a non-standard κL ∝T-0.491 dependence. At high T, the departure from the standard κL ∝T-1 law originates in the dual particle-wave behavior of the heat carrying phonons, which includes vibrations tied to the N atoms. While the room temperature κL =2.98 W mK-1 arises mainly from the conventional particle-like propagation of phonons, there is also asignificant atypical wave-like phonon tunneling effect, leading to a 20% glass-like heat transport contribution. The phonon broadening effect lowers the particle-like contribution but increases the glass-like one. Upon T increase, the glass-like contribution increases and dominates above T = 850 K. Overall, the low κL with a weak T-dependence points to a new utility for LaWN3 in energy technology applications, and motivates synthesis and exploration of nitrideperovskites.

Origin of ultralow phonon transport and strong anharmonicity in lead-free halide perovskites

All-inorganic lead-free halide double perovskites offer a promising avenue toward non-toxic, stable optoelectronic materials, properties that are missing in their prominent lead-containing counterparts. Their large thermopowers and high carrier mobilities also make them promising for thermoelectric applications. Here, we present a first-principles study of the lattice vibrations and thermal transport behaviors of Cs2SnI6 and γ-CsSnI3, two prototypical compounds in this materials class. We show that conventional static zero temperature density functional theory (DFT) calculations severely underestimate the lattice thermal conductivities (κl) of these compounds, indicating the importance of dynamical effects. By calculating anharmonic renormalized phonon dispersions, we show that some optic phonons significantly harden with increasing temperature (T), which reduces the scattering of heat carrying phonons and enhances calculated κl values when compared with standard zero temperature DFT. Furthermore, we demonstrate that coherence contributions to κl, arising from wave like phonon tunneling, are important in both compounds. Overall, calculated κl with temperature-dependent interatomic force constants, built from particle and coherence contributions, are in good agreement with available measured data, for both magnitude and temperature dependence. Large anharmonicity combined with low phonon group velocities yield ultralow κl values, with room temperature values of 0.26 W/m-K and 0.72 W/m-K predicted for Cs2SnI6 and γ-CsSnI3, respectively. We further show that the lattice dynamics of these compounds are highly anharmonic, largely mediated by rotation of the SnI6 octahedra and localized modes originating from Cs rattling motion. These thermal characteristics combined with their previously computed excellent electronic properties make these perovskites promising candidates for optoelectronic and room temperature thermoelectric applications.

Atomistic simulation of phonon heat transport across metallic vacuum nanogaps

The understanding and modeling of the heat transport across nanometer and sub-nanometer gaps where the distinction between thermal radiation and conduction become blurred remains an open question. In this work, we present a three-dimensional atomistic simulation framework by combining the molecular dynamics (MD) and phonon non-equilibrium Green's function (NEGF) methods. The relaxed atomic configuration and interaction force constants of metallic nanogaps are generated from MD as inputs into harmonic phonon NEGF. Phonon tunneling across gold-gold and copper-copper nanogaps is quantified, and is shown to be a significant heat transport channel below gap size of 1nm. We demonstrate that lattice anharmonicity contributes to within 20-30% of phonon tunneling depending on gap size, whereas electrostatic interactions turn out to have negligible effect for the small bias voltage typically used in experimental measurements. This work provides detailed information of the heat current spectrum and interprets the recent experimental determination of thermal conductance across gold-gold nanogaps. Our study contributes to deeper insight into heat transport in the extremely near-field regime, as well as hints for the future experimental investigation.

Probing Quantum Correlations in a Hybrid Optomechanical System

In quantum simulations and experiments on optomechanical cavities, coherence control is a challenging issue. We propose a scheme of two coupled optomechanical cavities to enhance the intracavity entanglement. Photon hopping is employed to establish couplings between optical modes, while phonon tunneling is utilized to establish couplings between mechanical resonators. Both cavities are driven by classical light. We explore the influences of coupling strengths of the quantum correlations generated inside each cavity using two types of quantum measures: logarithmic negativity and quantum steering. This analysis will reveal the significance of these quantum metrics as well as their various aspects in the Doppler regime. We also investigate stability conditions based on coupling strengths. Therefore, it is possible to quantify the degree of intracavity entanglement. The generated entanglement can be enhanced by choosing the appropriate photon and phonon hopping strengths. A set of parameters based on the currently available experimental data was used in the calculations.

Strong slowing down of the thermalization of solids interacting in the extreme near field

When two solids at different temperatures are separated by a vacuum gap they relax toward their equilibrium state by exchanging heat either by radiation, phonon or electron tunneling, depending on their separation distance and on the nature of materials. The interplay between this exchange of energy and its spreading through each solid entirely drives the relaxation dynamics. Here we highlight a significant slowing down of this process in the extreme near-field regime at distances where the heat flux exchanged between the two solids is comparable or even dominates over the flux carried by conduction inside each solid. This mechanism, leading to a strong effective increase of the system thermal inertia, should play an important role in the temporal evolution of thermal state of interacting solids systems at nanometric and subnanometric scales.

Phonon thermal conductivity reduction in silicene nanotubes with isotope substitution.

We used molecular dynamics simulations to study the isotopic doping effects on phonon thermal conductivity in armchair silicene nanotubes (SNTs). The phonon thermal conductivity of armchair SNTs can be effectively tuned with isotope substitution. Randomly and superlattice-structured isotopic doping can significantly reduce thermal conductivity. By analyzing the phonon vibrational spectrum, we reveal the underlying physical insights into the relationship between randomly isotopic doping concentration and thermal conductivity. Given the same doping concentration, the superlattice-structured doping method can reduce thermal conductivity more significantly than the disordered doping. For the isotopic superlattice doping method, the completion between the phonon interfacial scattering and phonon tunneling may cause minimum thermal conductivity at the critical period length. This study provides a possible means to effectively reduce the thermal conductivity of thermoelectric SNTs through isotopic doping engineering.

Theory of tuning ‘tunnelling-probability’ through potential barrier by acoustically augmented phonons (AAP)

This paper attempts to theoretically establish the technical possibility of manually controlling the tunnelling probability of phonons through potential barrier by superimposing Ultra High Frequency (UHF) acoustic wave over the source of phonons transmission, which results into Acoustically Augmented Phonons (AAP). Tuneable high fidelity acoustic piezoelectric transducer emitters may be used to tune the tunnelling. If the probability of reflectivity of phonon (P) during tunnelling through the potential barrier is given by P(R)p and that of Acoustically Augmented Phonons(AAP) is P(R)AAP then it is analytically obtained that P(R)AAP = ∝ P(R)P. Where ∝ is dimensionless number and is defined as the augmentation factor which is dependent on structure based damping coefficient, wave number of superimposed UHF acoustic wave and phonon’s kinetic energy. Enhancement of tunnelling probability by mechanically superimposing UHF acoustic wave over the in situ phonons is independent of initial UHF acoustic wave’s amplitude. It is noted that the range of ‘tunnelling enhancement AAP’s wave numbers’ (or frequencies) gradually broadens with increase in the kinetic energy of phonons and also gradually tends to a limit. Thus, existence of almost zero reflectivity of tunnelling probability through the potential barrier by Acoustically Augmented Phonons (AAP) is theoretically obtained. A general observation is that to achieve very low reflectivity of phonon tunnelling probability ‘structural damping factor’ must be small enough than superimposed UHF of acoustic wave number.

Thermal bridging of graphene nanosheets via covalent molecular junctions: A non-equilibrium Green’s functions–density functional tight-binding study

Despite the uniquely high thermal conductivity of graphene is well known, the exploitation of graphene into thermally conductive nanomaterials and devices is limited by the inefficiency of thermal contacts between the individual nanosheets. A fascinating yet experimentally challenging route to enhance thermal conductance at contacts between graphene nanosheets is through molecular junctions, allowing covalently connecting nanosheets otherwise interacting only via weak Van der Waals forces. Beside the bare existence of covalent connections, the choice of molecular structures to be used as thermal junctions should be guided by their vibrational properties, in terms of phonon transfer through the molecular junction. In this paper, density functional tight-binding combined with Green functions formalism was applied for the calculation of thermal conductance and phonon spectra of several different aliphatic and aromatic molecular junctions between graphene nanosheets. Effect of molecular junction length, conformation, and aromaticity were studied in detail and correlated with phonon tunnelling spectra. The theoretical insight provided by this work can guide future experimental studies to select suitable molecular junctions in order to enhance the thermal transport by suppressing the interfacial thermal resistances, particularly attractive for various systems, including graphene nanopapers and graphene polymer nanocomposites, as well as related devices. In a broader view, the possibility to design molecular junctions to control phonon transport currently appears as an efficient way to produce phononic devices and controlling heat management in nanostructures.

Quantum synchronization of two mechanical oscillators in coupled optomechanical systems with Kerr nonlinearity

We investigate the quantum synchronization phenomena of two mechanical oscillators of different frequencies in two optomechanical systems under periodically modulating cavity detunings or driving amplitudes, which can interact mutually through an optical fiber or a phonon tunneling. The cavities are filled with Kerr-type nonlinear medium. It is found that, no matter which the coupling and periodically modulation we choose, both of the quantum synchronization of nonlinear optomechanical system are more appealing than the linear optomechanical system. It is easier to observe greatly enhanced quantum synchronization with Kerr nonlinearity. In addition, the different influences on the quantum synchronization between the two coupling ways and the two modulating ways are compared and discussed.