Session M68: Phonons, Electron-Phonon Coupling, and Temperature at the Nanoscale

Engineering nanostructures to control phonon transport

Of particular interest for thermal management of electronic and photonic devices is the ability to tune and direct heat transfer within systems as they continue to shrink. For materials where phonons are the dominant thermal energy carriers, nanostructuring enables tuning of the thermal properties during fabrication. For example, strategically-engineered periodically porous materials can create bandgaps in the phonon dispersion relationship. Furthermore, strain can impact the atomic vibrations and the interaction of the nanostructuring and the applied strain could be used to control thermal transport performance. Strategic design of the nanostructure relative to the applied strain can actively enhance or reduce the anisotropy in in-plane thermal conductivity of such structures. This talk will focus on methods to tune the thermal conductivity of nanostructured materials by understanding and controlling the phonon transport at the nanoscale including both tuning of properties during fabrication and active tuning of properties with strain.   

Nanoscale Interface Phonon Dynamics Imaged by Electron Microscopy

Thermal transport mechanisms are drastically altered by the inclusion of alloys, nanostructures, and superlattice interfaces in materials. To understand the dynamics at these nanoscale structures, a high-resolution technique capable of probing phonons at the nanoscale is essential. Recent developments in electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) have made it possible to study phonons at nanometer resolution¹. Additionally, vibrational EELS allows for nanoscale temperature imaging using principle of detailed balance². Here we demonstrate the two-dimensional nanoscale vibrational mapping of a single SiGe quantum dot (QD) using an atom-sized probe in the electron microscope. We develop a novel technique to map propagating phonon modes that allows for the imaging of real space interface specularity and use principle of detailed balance to investigate ultra-high temperature gradients in the presence of nanostructures and interfaces.

 

Experimental work was supported by the Department of Energy (DOE), Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Grant No. DESC0014430, and partially by National Science Foundation (NSF) under Grant No. DMR-1506535 and DMR-1629270.   

Revealing Local Phonon Modes at Stacking faults and Interfaces

Imperfections such as stacking faults and interfaces are recognized as controlling factors in modifying thermal properties and heat transport by scattering phonon and changing vibrational structure [1]. However, the effect of crystal defects on thermal conductivity is being theoretically treated by perturbation methods without considering the local change of phonon dispersion relation. Direct detection of local defect-induced phonon modes has not been realized until very recently [2,3]. Here we demonstrate that space- and angle-resolved vibrational spectroscopy in a transmission electron microscope enables the study of the vibrational structure of individual crystal defects. At a single stacking fault in a cubic silicon carbide, the acoustic vibration modes at X point undergo a red shift of several millielectronvolts, become enhanced, and are confined to within a few nanometers of the stacking fault [3]. The interfacial phonon modes localized at interfaces were also revealed in both Si-Ge epitaxial heterojunctions [4] and monolayer MoS₂-WSe₂ heterostructure [5]. Our work paves an avenue to investigating phonon propagation around crystal defects and provides guidance to the engineering of desired thermal performance for semiconductor and power electronic devices.   

Quantifying Temperature Susceptivity of Electron Scattering in Scanning Transmission Electron Microscopy

Phonons can influence the trajectory of incoming radiations and affect the outcome of any scattering experiment. Thus, proper quantification of the influence may provide the temperature of the material at the length scale equivalent to the size of the probe used in the scattering experiment. In this work, we report the sensitivity of electron scattering to sample temperature as a function of the scattering angle in scanning transmission electron microscopy (STEM). Unlike the scattering intensities at higher angle ranges that are known to be dominated by thermal diffuse scattering, the temperature sensitivity of the intensities at lower scattering angles has been less understood. We show that the scattering intensities at low-to-intermediate angles increase at higher temperatures, and the amplitude of the change is larger than that in the high angle range. This trend was observed in position averaged convergent beam electron diffraction as well as the atomic-scale images acquired from 4-dimensional STEM. Based on the finding, we quantified the trend as a function of both temperature and the scattering vector, which provides important guidance toward realizing the temperature measurement of materials with high-temperature precision and spatial resolution close to the atomic scale.   

Temperature-Dependent Excited State Lifetimes of Nitrogen Vacancy Centers in Individual Nanodiamonds

Modern electronic and data storage technologies combine shrinking feature sizes with ever-increasing operating speeds, leading to transient nanoscale hotspots that can limit device performance. Quantifying hotspots in operating devices thus requires a non-invasive thermometer with high spatial and temporal resolution. Nitrogen vacancy (NV) centers are luminescent defects widely employed for thermometry, most commonly via temperature-dependent shifts of their optically detected magnetic resonance. Recently, alternative all-optical approaches have also gained traction. Excited state lifetime thermometry is an all-optical technique that has been implemented using other fluorophores but has not previously been demonstrated for NV centers in individual nanodiamonds (NDs). We measured the excited state lifetime of NV centers in individual NDs between 300 K and 500 K and recorded a 32 ± 7.0% and 35 ± 8.3% average decrease in the lifetimes of individual NDs on silicon and glass substrates, respectively, over this temperature range. A linear approximation applicable to nearly all measured NDs yields temperature coefficients of -2000 ± 240 ppm/K and -2600 ± 280 ppm/K for NDs on silicon and glass, respectively. Beyond all-optical operation, single-ND lifetime thermometry offers ~100 ns temporal resolution and utilizes time-correlated single photon counting measurements ideally suited to low emission intensities, a limiting factor for other NV center thermometry techniques above 700 K. We also demonstrate that atomic force microscope nanomanipulation can position individual NDs at critical locations on a sample, enabling single-point temperature measurements that combine ~100 ns temporal resolution and ~100 nm spatial resolution. Finally, our results have implications for other single-ND excited state lifetime sensing applications, where care is required to avoid conflating changes in temperature and other parameters.   

Rational design of phononic flat-band materials

Electronic Flat band (FB) is of great interest in the field of condensed matter physics. Here, using spring-mass model and symmetry analysis, we prescribe a generic recipe to realize FB in both line-graph (LG) lattices and non-LG phononic lattices. We first demonstrate the design principle in several two-dimensional (2D) LG and non-LG model lattices. Then we carry out first-principles calculations, guided by the newly developed recipe, to search for phononic FB materials and discover that 2D Kagome BN is a perfect candidate to host phononic FB. Our work will not only offer a comprehensive understanding of phononic FB and enrich the FB physics, but also pave a path for exploring phononic FB materials.   

Signature of Many-Body Localization of Phonons in Strongly Disordered Superlattices

The experimental realization of many-body localization (MBL) in solid-state systems has remained challenging. In parallel, open questions remain on the localization of phonons as the strong interactions and disorders in phononic superlattice systems would call for an MBL description. The elucidation of phonon localization through the lens of MBL would have major implications for connecting the MBL phenomenology to a mesoscopic solid-state system. In this talk, I will report evidence of a possible phonon MBL phase in disordered GaAs/AlAs superlattices. Using grazing-incidence inelastic X-ray scattering, we observe a strong deviation of the phonon population in samples doped with ErAs nanodots at low temperature, signaling a departure from thermalization. This behavior occurs within finite phonon energy and wavevector windows, suggesting a localization-thermalization crossover. We support our observation by proposing a theoretical model for the effective phonon Hamiltonian in disordered superlattices, and showing that it can be mapped exactly to a disordered 1D Bose–Hubbard model with a known MBL phase. Our study opens up new opportunities as a new experimental solid-state platform for realizing possible MBL phenomena, and as a novel phonon phase far away from equilibrium.   

Phonons as Quantum Transducer Utilizing the Phonon Drag Effect

The emergence of novel nanostructured materials in recent decades presents rich opportunities for the realization of phonon-based quantum computing. One of the challenges is the sensitive detection of a single phonon without destroying it. While how a densely populated phonon bath destroys quantum coherence has long been understood, only recently has it come to be appreciated that phonons may instead provide an effective means to transmit quantum information when excited in sufficiently small numbers coherently. The strong coupling of phonons to other quasiparticles (electrons, photons, or spins) makes them well suited for this task. Exploration of phonon detection requires suitable techniques to source and detect phonon fluxes.  Here, we demonstrate the idea of exploiting the electron-phonon coupling in a van der Waals heterostructure via the phonon drag effect as a sensitive phonon detector.   We consider a single layer or AB-stacked bilayer graphene on top of a multi-layer hexagonal boron nitride (hBN) in this heterostructure. We assume that a surface polar phonon (SPP) in the hBN layer is excited and propagates across the hBN layer, which plays the role of a phononic waveguide.  We calculate electron-phonon coupling for electrons in graphene with this non-equilibrium SPP in the hBN layer and determine phonon drag voltage by solving the Boltzmann transport equation.  Our numerical results demonstrate that the drag voltage is on the scale of a few microvolts, which is well above the detection limit in the typical experimental setups.   

Maximizing and minimizing the boundary scattering mean free path in diameter-modulated coaxial cylindrical nanowires

The thermal conductivity (k) of semiconducting nanomaterials is controlled by the geometry-dependent phonon boundary scattering mean free path (Λ_Bdy). Here, we use phonon ray tracing simulations to study phonon backscattering and geometry-dependent Λ_Bdy in recently fabricated coaxial cylindrical nanowires. We use simulated average transmissivity t to calculate Λ_Bdy via a Landauer-Bttiker formalism. For a fixed smaller cylinder diameter (D₁) and cylinder length ratio, we find that Λ_Bdy of periodic nanowires can be maximized or minimized via geometric control of the pitch (p) and larger cylinder diameter (D₂). Saturated phonon backscattering for small pitch ratio (p_r) nanowires causes a minimum in Λ_Bdy/D₁ at p_r near unity, while the maximum in Λ_Bdy/D₁ for large p_r nanowires can be understood by a simple thermal resistor model for two individual nanowires in series. Combining our Λ_Bdy calculations with analytical phonon dispersion and bulk scattering models, we predict that k of periodic silicon nanowires with fixed D₁ can be tuned by up to 34% in the boundary scattering dominated regime by modifying D₂ and p. These results provide insight into phonon backscattering mechanisms in periodic nanomaterials, and can be used to model future experiments on coaxial cylindrical nanowires.   

Thermoelectricity in single-molecule quantum dots

Thermoelectricity is the conversion between voltage and temperature differences in a system.  While it has been a technologically important concept to integrate in electronics; fundamentally, it is a study of the relationship between electronic and thermal properties of a material. Theoretical studies suggest that single-molecule devices can host strong thermoelectric effects. This advantage is achieved via different molecular designs for an optimized tunnel coupling or a sharp transmission close to the Fermi energy, etc. Recently, we have created single-molecule thermoelectric quantum dot devices, where valuable information such as Seebeck coefficient and the thermoelectric power factor are directly obtained from their strong thermoelectric responses. We further show that fundamental physical quantities, including entropy changes, excited states and the universality of the Kondo effect, can be determined in the thermoelectric quantum dot devices. The rich physics in thermoelectric quantum dot devices and their technological implications open another research direction for nanoscale devices.   

High-temperature transport and polaron speciation in the anharmonic Holstein model

We study the finite-temperature transport of electrons coupled to anharmonic local phonons. Our focus is on the high-temperature incoherent regime, where controlled calculations are possible both for weak and strong electron-phonon coupling. At strong coupling, the dynamics is described in terms of a multiple-species gas of small polarons: this emergent "speciation" is driven by energy conservation. We explicitly compute the dc and ac response in this regime. We discuss the breakdown of the polaron picture, and the onset of localization, in the limit where phonons become quasistatic. Our work exemplifies unusual  transport phenomena that can be uncovered even in well-studied models of condensed matter by exploring poorly-explored regimes of parameters, using ideas and techniques sharpened by recent forays into even more exotic settings, such as MBL. Our findings may be realised in the setting of narrow bands engineered in moiré superlattices, where the role of electron-phonon interactions remains debated.