Bulletin of the American Physical Society
Annual Meeting of the Four Corners Section of the APS
Volume 58, Number 12
Friday–Saturday, October 18–19, 2013; Denver, Colorado
Session M4: Condensed Matter V: Thermal Properties |
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Chair: Mingzhong Wu, Colorado State University Room: 281 |
Saturday, October 19, 2013 2:15PM - 2:39PM |
M4.00001: Probing Maxwell's Demon with a Nanoscale Thermometer Invited Speaker: Charles Stafford Recent advances in thermal microscopy, where spatial and thermal resolutions of 10nm and 15mK, respectively, have been achieved, raise a fundamental question, ``On how short a length scale can a statistical quantity like temperature be meaningfully defined?'' We tackle this question theoretically by developing a realistic model\footnote{J.P. Bergfield, S.M. Story, R.C. Stafford, C.A. Stafford, ACS Nano 7, 4429-4440 (2013)} of a scanning thermal microscope with atomic resolution, operating in the tunneling regime in ultrahigh vacuum. The thermometer acts as an open third terminal in a thermoelectric circuit. We investigate the temperature distributions in molecular junctions and graphene nanoribbons\footnote{J.P. Bergfield, M.A. Ratner, C.A. Stafford, M. Di Ventra, arXiv:1305.6602.} under thermal bias, and find that the local temperature in these systems exhibits quantum oscillations; quantum interference mimics the actions of a Maxwell Demon, allowing electrons from the hot electrode to tunnel onto the temperature probe when it is at certain locations near the system, and blocking electrons from the cold electrode, or vice versa. A crossover to a classical temperature distribution consistent with Fourier's law of heat conduction is predicted as the spatial resolution of the temperature probe is reduced. [Preview Abstract] |
Saturday, October 19, 2013 2:39PM - 3:03PM |
M4.00002: Diffusive, Ballistic, and Quantum Thermal Transport in Thin Films and Nanostructures Invited Speaker: Barry Zink The understanding and manipulation of heat flow in novel materials and small structures plays an essential role in current and future technologies, such as heat removal from integrated circuits and thermoelectric energy generation. This talk will discuss our recent work that shows there are still surprises in the fundamental physics of heat flow in such systems. One example is the role of phonons with long mean free path in heat conduction near room temperature. By adding scattering centers to the surfaces of suspended Si-N bridges via deposition of a series of (initially discontinuous) gold films, we have shown that phonons of surprisingly long mean free path and wavelength carry up to $40$\% of the heat even in this highly disordered material. This echoes recent results in crystalline materials, where other researchers are also finding large contributions to thermal conductivity from long mean free path via ultra-fast pump-probe measurements. A second example comes in a very different temperature range, where from $50$ mK to 1 K, the thermal conductance in suspended Si-N structures very similar to the transition-edge sensors (TESs) used in state-of-the-art photon detectors shows unexpected contributions not well-explained by any model. The data, somewhat surprisingly, has a temperature dependence similar to that of a quantum-limited thermal conductance. Finally, we present evidence of a previously unknown mechanism for thermal transport in ferromagnets that is revealed by apparent violation of the Wiedemann-Franz law in very thin iron films. [Preview Abstract] |
Saturday, October 19, 2013 3:03PM - 3:15PM |
M4.00003: Nanoscale thermal transport measurements: Bridging ultrafast and steady-state Brian G. Green, Mark E. Siemens Macroscale thermal transport is explained by classical thermal diffusion, but as nanostructure length scales are reduced towards the order of the phonon mean free path, transport of thermal energy takes on a fundamentally different character and manifests ballistic effects. We investigate nanoscale thermal transport by comparing results from two different techniques applied to a thermally isolated suspended bridge structure. One technique uses the transient thermoreflectance method to measure sub-nanosecond cooling dynamics following ultrafast laser heating in a micron-sized region of a metallic film deposited on the bridge; the second is a DC technique measuring transport driven by a thermal gradient across the bridge, through the full, far larger volume of the film. These very different methods give similar results of reduced thermal conductivity relative to macroscale values, and in combination they are a powerful tool for investigating and understanding thermal transport at the nanoscale. [Preview Abstract] |
Saturday, October 19, 2013 3:15PM - 3:27PM |
M4.00004: Nanoscale Absolute Thermopower Measurements Sarah Mason Significant advancements in thermoelectric device efficiencies have been due to size reduction to the nanoscale. With reduced dimensions come complications in measuring thermoelectric material properties. Quantities needed to characterize thermoelectric material efficiency, such as the thermopower, or Seebeck coefficient, $S$, are primarily contingent upon the measurement apparatus, so that measuring a thermally generated voltage gives, $\frac{dV}{dT}= S_{sample}-S_{lead}$. If accurate values of, $S_{lead}$, are available, simple subtraction provides the film's absolute thermopower value. This is rarely the case in nanoscale measurement devices, with leads exclusively made from thin film materials that do not have well known bulk-like thermopower values. We have developed a technique to directly measure $S$ as a function of $T$ using a micromachined thermal isolation platform consisting of a suspended, patterned SiN membrane. By measuring a series of thicknesses of metallic films up to the infinite thin film limit, in which the thermopower is no longer increasing with thickness, but still not at bulk values, along with the effective electron mean free path, we are able to show the contribution of the leads needed to measure this property. Having a comprehensive understanding of the background contribution we are able to determine the absolute thermopower of a wide variety of thin films. [Preview Abstract] |
Saturday, October 19, 2013 3:27PM - 3:39PM |
M4.00005: Optical thickness determination of hexagonal boron nitride flakes Dheeraj Golla, Kanokporn Chattrakun, Kenji Watanabe, Takashi Taniguchi, Brian J. LeRoy, Arvinder Sandhu Optical reflectivity contrast provides a simple, fast, and noninvasive method for characterization of few monolayer samples of two-dimensional materials. Here, we apply this technique to measure the thickness of thin flakes of hexagonal Boron Nitride (hBN), which is a material of increasing interest in nanodevice fabrication. The optical contrast shows a strong negative peak at short wavelengths and zero contrast at a thickness dependent wavelength. The optical contrast varies linearly for 1-80 layers of hBN, which permits easy calibration of thickness. We demonstrate the applicability of this quick characterization method by comparing atomic force microscopy and optical contrast results. [Preview Abstract] |
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