Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F15: Phonon dynamics and thermal conductivity at the nanoscaleFocus
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Sponsoring Units: DMP Chair: Sridhar Sadasivam, Argonne Natl Lab Room: LACC 304C |
Tuesday, March 6, 2018 11:15AM - 11:51AM |
F15.00001: Lower and upper limits to the vibrational thermal conductivity of amorphous polymers and polymer salts Invited Speaker: David Cahill The low thermal conductivity of glassy polymers is advantageous for thermal insulation and problematic for the thermal management of systems that use polymeric materials as dielectrics, adhesives, and for environmental protection. Our work experimentally explores the lowest and highest possible thermal conductivities that can be achieved in amorphous polymers. The pressure dependence (P < 10 GPa) of the thermal conductivity of PMMA provides insights on how the thermal conductivity is controlled by the strength of molecular bonds. Using the high throughput capability of time-domain thermoreflectance (TDTR), we studied the thermal conductivity of eight amorphous polymers and ten polymer salts to advance fundamental understanding of the lower and upper limits to heat conduction in this class of materials. We prepare film of polymers with a thickness on the order of 100 nm on Si substrates and measure their thermal conductivities and heat capacities using TDTR. Varying the modulation frequency allows us to span from thermally thick to thermally thin and change the relative sensitivities of the TDTR measurement to thermal conductivity and heat capacity. The thermal conductivities vary by an order of magnitude, from 0.06 W m-1 K-1 for functionalized fullerenes to 0.7 W m-1 K-1 for poly(vinylphosphonic calcium salt). We measure the longitudinal modulus by picosecond acoustics and the shear modulus using an elastomeric phase-shift mask that enables us to use pump-probe methods to determine surface acoustic wave velocities for acoustic wavelengths of 700 nm and frequencies on the order of a few GHz. Overall, the thermal conductivities are well correlated with the scaling of the model of the minimum thermal conductivity with heat capacity and sound velocities. |
Tuesday, March 6, 2018 11:51AM - 12:03PM |
F15.00002: Thermal conductivity switching of side-chain azobenzene polymer films by photo-triggered reversible crystalline lamellar formation Jungwoo Shin, Jaeuk Sung, Minjee Kang, Byeongdu Lee, Cecilia Leal, Nancy Sottos, Paul Braun, David Cahill We study the structural and thermal properties of side-chain azobenzene polymer films via in-situ synchrotron X-ray scattering and in-situ time-domain thermoreflectance (TDTR). The spin-coated side-chain azobenzene polymer films show vertically aligned azobenzene substituents on Al surface. We found that the increase in number of repeating units of side-chain azobenzene polymers results in transition from disordered 2-D structure to highly ordered crystalline lamellar structure with increasing out-of-plane thermal conductivity from 0.2 to 0.35 W m-1 K-1. In addition, we observed that ultraviolet (UV) light drives trans-to-cis photoisomerization of azobenzene substituents, resulting in isotropic liquid states with thermal conductivity of 0.1-0.15 W m-1K-1 while visible light completely restores the crystalline lamellar structure. We attribute the out-of-plane thermal conductivity switching of azobenzene side-chain polymer film to the tunable molecular thermal pathwaycontrolled by number of repeating units and conformation changes driven by light. |
Tuesday, March 6, 2018 12:03PM - 12:15PM |
F15.00003: High Temperature Magnon Thermal Conductivity in Cuprates ELLA PEK, Xi Chen, Sean Sullivan, Jianshi Zhou, Li Shi, David Cahill Some cuprates have high thermal conductivities due to the strong exchange interaction, which leads to high magnon group velocities. However, little is known about the behavior of the magnons in the cuprates above room temperature. In our study, we use the time-domain thermoreflectance (TDTR) to investigate the high magnon thermal conductivities of different cuprates at temperatures from 75 K to 600 K. Each of the cuprates studied has a different arrangement of spin-spin coupling: SrCuO2 (a spin-chain), CaCu2O3 (a buckled spin ladder/pseudo-two-leg-ladder), and La2CuO4 (a two-dimensional square lattice). CaCu2O3 and La2CuO4 have peaks in their thermal conductivities near room temperature (~300K), but the temperature dependence of thermal conductivity differs at higher temperatures. While SrCuO2 has a monotonically decreasing thermal conductivity with temperature, its thermal conductivity at higher temperatures are still at least two times higher than the other two cuprates. Our work seeks to explain how different arrangements of spin-spin coupling, the spin-phonon interaction, and magnetic dispersion lead to the different behaviors of thermal conductivity in these cuprates. |
Tuesday, March 6, 2018 12:15PM - 12:27PM |
F15.00004: Surface Acoustic Wave Generation and Detection on LAO/STO Dengyu Yang, Yun-Yi Pai, Yuhe Tang, Yang Hu, Hyungwoo Lee, Jung-Woo Lee, Chang-Beom Eom, Patrick Irvin, Jeremy Levy We aim to observe Surface Acoustic Wave (SAW) in LAO/STO heterostructures. Using well-developed conductive-AFM lithography, one can switch between the conducting and insulating phases of the material, thus interdigited transducers (IDT) can be created in the structure, which can convert electronic signals into acoustic signals and vice versa. Two IDTs are thus written on the structure as a generator and a detector. Due to material and dimensional constraint, a radio frequency electronic signal is used to maximize the effect. SAW can generate potential minima relying on the piezoelectric property of LAO/STO, and by changing position of these potential minima electrons can be transported along the nanowire, which is an emerging option for quantum information transfer. |
Tuesday, March 6, 2018 12:27PM - 12:39PM |
F15.00005: The Role of Interfaces in the Thermal Transport in InAlAs/InGaAs Superlattices Gabriel Jaffe, Song Mei, Colin Boyle, Jeremy Kirch, Irena Knezevic, Dan Botez, Luke Mawst, Max Lagally, Mark Eriksson Superlattices (SLs) of ternary alloys InxAl1-xAs and InxGa1-xAs are used in many emerging technologies, such as light conversion devices and quantum cascade lasers. Proper thermal management in these devices is difficult because the relative importance of phonon scattering in the bulk alloys versus phonon scattering at SL interfaces is not well understood. Using the three omega method, we measure the thermal resistivity of bulk films of InAlAs, InGaAs, as well as SLs of the two materials. These measurements enable us to quantify how alloy concentration of the bulk materials, phonon mode mismatch, and interface roughness between layers affect the SL resistivity. We find that interface effects account for 20% of the resistance of our lattice-matched SLs and 40% of the resistance of In.3Al.7As/In.75Ga.25As SLs. This result demonstrates a counterintuitive property for these SLs, despite the fact that the resistivity of the bulk alloys in the SL may decrease as the alloy concentrations move away from lattice matched (x~.5 for both materials), the total SL thermal resistance actually increases due to worsening phonon mode mismatch and roughness at the interfaces. |
Tuesday, March 6, 2018 12:39PM - 12:51PM |
F15.00006: Nanoscale Thermal Transport Across a GaAs/AlGaAs Interface Matthew Gorfien, Jianming Cao, Xuan Wang As the size of transistors reaches the nanoscale, thermal transport is drastically altered due to the presence of boundaries, which act as a barrier and can decrease the thermal conductivity by orders of magnitude. The possibility of controlling the thermal transport at the nanometer scale requires measurement techniques which can provide adaquate spatial and temporal resolution to map the ultrafast dynamics of phonons. Here, we report the results of measuring phonon-mediated nanoscale thermal transport across a GaAs/AlGaAs quantum well interface and the dependence of the GaAs nanofilm cooling rate on temperature, using a direct and ultrafast lattice probe of ultrafast electron diffraction. The thermal boundary conductance temperature dependence is also extracted using a diffusive model and possible interpretations of these experimental results will be discussed. |
Tuesday, March 6, 2018 12:51PM - 1:03PM |
F15.00007: Thermal transport across epitaxial NiAl1-xGax/GaAs interfaces Ning Ye, Joseph Feser While the topic of thermal interface conductance is widely studied, there are still just a few studies of transport across epitaxial metal-semiconductor interfaces, and thus reliable benchmark data for atomistic computational tools remains scarce. NiAl and NiGa are good metals with simple cubic lattice structures of nearly identical lattice constant (a=2.88 A), nearly half of that of GaAs. We report the epitaxially growth of NiAl1-xGax on GaAs (a=5.65 A) across the entire compositional range, x, for 12 different alloy compositions as well as measurements of their thermal interface conductance over a wide range of temperatures (77-700K). The results are compared to several theoretical models of electron/phonon transport coupled to first-principles calculations. In particular, we find that phonon-only models of transport dramatically overestimate the observed conductance. Calculating the electron-phonon coupling parameters and lattice thermal conductivity of the metals using first-principles calculations, we show that these effects must be accurately included to capture the observed transport observations. |
Tuesday, March 6, 2018 1:03PM - 1:15PM |
F15.00008: Role of Phonon Coherence in 1-D Superlattices Studied Using the Spectral Boltzmann Transport Equation with Ab-Initio Inputs Erika Ye, Austin Minnich The effect of phonon coherence on the thermal conductivity of 1-D superlattices has been a topic of debate. Recent measurements on superlattices show a non-monotonic trend of thermal conductivity with respect to period. The Simkin-Mahan model suggests that this behavior is explained by coherent phonon propagation and the folded phonon band-structure of superlattices. However, the model assumes that all phonons have a constant mean free path, an assumption that prior works have shown to be poor. Here, we use numerical solutions of the spectral Boltzmann Transport Equation (BTE) with ab-initio inputs to calculate the thermal conductivity of superlattices assuming a variety of transmission profiles at each interface. We find that the non-monotonic behavior of thermal conductivity versus period cannot be explained with static transmission coefficients, supporting the role of coherence and wave interference. Our work provides further insights into how phonon coherence can alter thermal transport in heterogeneous solids. |
Tuesday, March 6, 2018 1:15PM - 1:27PM |
F15.00009: Modification of the Phonon Spectrum and Transport Properties of Materials via Substitutional Doping Observed with the Brillouin-Mandelstam Spectroscopy Fariborz Kargar, Elias Penilla, Ece Aytan, Jacob Lewis, Ruben Salgado, Javier Garay, Alexander Balandin The ability to modify the propagation of acoustic phonons has important implications for thermal management of electronics. The thermal transport in nanostructured or doped materials can be affected via the changed phonon–boundary and phonon–point defect scattering rates. However, the thermal conductivity can also be altered via the changes in the phonon group velocity. In this presentation, we show on the example of neodymium (Nd) doped sapphire (Al2O3), that substitution of Al atoms with much heavier Nd atoms results in a noticeable decrease in the acoustic phonon group velocity. The acoustic phonon spectra for each sample were measured directly using the Brillouin-Mandelstam spectroscopy (BMS) at room temperature. Our BMS results clearly show that with the increase in the Nd doping, the frequency of both longitudinal acoustic and transverse acoustic phonon modes, at fixed phonon wave-vector, decreases, indicating the change in the phonon group velocities. |
Tuesday, March 6, 2018 1:27PM - 1:39PM |
F15.00010: Tuning of thermoelectric properties using coherent phonon Masato Ohnishi, Junichiro Shiomi Recently, improvement of thermoelectric properties, decrease in thermal conductivity and increase in power factor, of single-walled carbon nanotubes (SWNTs) due to fullerene encapsulation at room temperature has been observed [Nat. Mater. 295, 828 (2017)]. We reveal how the fullerene encapsulation affects phonon properties of SWNTs. The fullerene encapsulation causes expansion in radial direction and wavy deformation in outer shell, SWNT. The structural deformation lead to variations of phonon dispersion: softening of longitudinal modes, zone-folding, and hybridization between vibrations of SWNT and encapsulated fullerenes. The modulation of phonon dispersion decreases both group velocity and relaxation time. The contributions of change of group velocity and relaxation time to the decrease in thermal conductivity are comparable regardless phonon modes and frequency. Our detailed analysis paves the new way for thermal management using coherent phonons. |
Tuesday, March 6, 2018 1:39PM - 1:51PM |
F15.00011: Engineering Phonon Wave Properties for Thermally Efficient LEDs: Tracing and Tailoring Phase of Acoustic Phonons Young-Dahl Jho, Hoonil Jeong, Austin Minnich, Christopher Stanton The phonon wave manipulations in thermal engineering are still scarce, where the ability to trace and tailor its phase could enable a wide range of applications. Even though the relationship between generation mechanisms and phase has been known for decades, the acoustic (AC) phonon phase information was obscured by the dynamic interference behavior of propagating wavepackets. Here, we present a new type of phase detection scheme of AC phonons where the tensile and compressive strains are distinguished and the entire phononic spectrum is imposed in nonlinear photoelastic regime. The vertical electric field along the symmetry axis of LEDs, in this way, could tune the initial phase of AC phonons. From the time-domain analysis, we further clarify the relationship between the phase and anharmonic phononic scatterings. This work in combination with our previous works on modal manipulations [1] and nanoscale thermal characterization [2,3] could be useful in understanding detailed pictures of AC heat carriers toward the wavemechanical thermal managements. |
Tuesday, March 6, 2018 1:51PM - 2:03PM |
F15.00012: Tailoring near-field heat transfer through inverse design and nonlinear media Chinmay Khandekar Non-contact near-field energy transfer is an important process with ramifications to emerging applications in energy conversion. In this talk, we describe recent efforts aimed at enhancing and modifying radiative near field heat transfer (RHT) in nanostructured media. We present large-scale optimization techniques that can discover aperiodic multi-layered structures outperforming RHT in uniform media and which we also exploit to demonstrate complex 2d metallic structures (periodic gratings with complex surface topology) exhibiting some of the largest heat-transfer rates reported thus far. Finally, we describe a scheme which exploits the externally driven, intrinsic Kerr χ(3) nonlinearity of III-V semiconductors and chalcogenides to upconvert thermal radiation from mid-infrared to near-infrared wavelengths. We employ this scheme in planar materials supporting surface polaritons to demonstrate significant near field heat transfer between materials of dissimilar resonance wavelengths and show that it can be exploited to achieve thermal refrigeration and tunable heat exchange (under arbitrary temperature differentials). |
Tuesday, March 6, 2018 2:03PM - 2:15PM |
F15.00013: Extreme near field heat transfer modulation with graphene plasmon resonators Ognjen Ilic, Nathan Thomas, Thomas Christensen, Michelle Sherrott, Marin Soljacic, Austin Minnich, Owen Miller, Harry Atwater
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