Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K45: Helium 4 |
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Sponsoring Units: DCMP Chair: Gary Williams, Univ of California - Los Angeles Room: LACC 505 |
Wednesday, March 7, 2018 8:00AM - 8:12AM |
K45.00001: AC Response of a Field Effect Transistor Composed of Electrons on Helium Kostyantyn Nasyedkin, Heejun Byeon, Liangji Zhang, Stephen Hemmerle, Joshua Milem, Reza Loloee, Johannes Pollanen We report on the AC response of a unique field effect transistor composed of electrons on helium in the frequency range between 10 kHz and 100 kHz. The impedance of the device can be modeled as a two-dimensional transmission line formed between the source and drain electrodes. At low temperature the coupling between the source and drain is predominantly capacitive when the channel above them is fully populated. In this regime the current that flows from source to drain has a phase angle φ = 90° relative to the excitation voltage. However, at sufficiently high temperature φ decreases and can become negative in the vicinity of depletion. In this case the imaginary component of the source-drain current reverses sign. We have analyzed this behavior and find that it can be understood by considering the propagation of strongly damped voltage oscillations along the transmission line formed by the system of two-dimensional electrons above, and between, the source and drain electrodes. |
Wednesday, March 7, 2018 8:12AM - 8:24AM |
K45.00002: Electrons on Helium Surface: Commensurate-Incommensurate Transitions Kirill Moskovtsev, Mark Dykman Electrons on the surface of liquid helium are strongly correlated. This system enables studying the interplay between strong correlations and periodic spatial modulation in a free from defects environment. The modulation can be electrostatic or magnetic, coming from structures submerged into helium at a micron depth, which is the typical interelectron distance. Its strength and spatial structure can be controlled, and the temperature can be varied across the liquid to Wigner crystal transition. We have performed molecular dynamics simulations to study the structure and the ac and dc transport of the electron system in a one-dimensional periodic electrostatic potential. We show that the potential strongly affects the temperature of the electron freezing and can significantly smear the freezing transition. Because the Coulomb interaction is isotropic, many-electron “accommodation” to the external potential can display features different from those in solid-on-solid structures. We find that the electron transport sensitively depends on the emerging electron structure. We discuss how a periodic structure can be revealed using resonant ac conductivity and the spectrum of the inter-subband microwave absorption. |
Wednesday, March 7, 2018 8:24AM - 8:36AM |
K45.00003: Piezoacoustic Measurements of Electrons on Helium Heejun Byeon, Kostyantyn Nasyedkin, Reza Loloee, Stephen Hemmerle, Johannes Pollanen We report on simultaneous low-frequency transport and high-frequency piezoacoustic measurements of electrons on the surface of superfluid helium. By coupling a piezoacoustic surface wave to a nearby two-dimensional electron system we measure the high frequency conductivity of the electron system in a wave-vector selective manner. Additionally, a dynamical piezoelectric standing wave creates a modulated and tunable potential that can be used to simulate a two-dimensional system exhibiting commensurate-incommensurate transitions which can be detected via quasi-dc transport measurements. |
Wednesday, March 7, 2018 8:36AM - 8:48AM |
K45.00004: Transport of hot electrons on helium over an energy barrier in a micro-channel device. Ethan Kleinbaum, Stephen Lyon Electronic surface states above superfluid helium exhibit unprecedented isolation. This isolation is expected to result in remarkably long coherence times for both spin and orbital quantum states making this system attractive for the manipulation of quantum information. Nonetheless, the lack of interactions with the surrounding environment can prevent effective electron thermalization allowing small electric fields to heat electrons significantly above the bath temperature. Poor electron thermalization can lead to a reduction in coherence times and gate fidelity. In order to gain a greater understanding of physics governing hot electrons and energy relaxation processes, we study transport of hot electrons above an energy barrier in a micro-channel device. Maintaining temperatures greater than 1.5K, above the Wigner crystal regime we find that the current across the barrier varies with barrier height as expected for different bath temperatures. Additionally, we have evidence that externally heating the electron system increases the current across the barrier. |
Wednesday, March 7, 2018 8:48AM - 9:00AM |
K45.00005: A Novel Mean Field Model for Quantum Supersolids Vili Heinonen, Keaton Burns, Jörn Dunkel Recent experiments have seen characteristics of the elusive quantum supersolid phase – periodic spatial ordering of a quantum liquid. This calls for theoretical tools to explain and design the experiments. [J.-R. Li et al. Nature, 543, 91–94. (2017); |
Wednesday, March 7, 2018 9:00AM - 9:12AM |
K45.00006: Laser control and readout of superfluid flow Stefan Forstner, Yauhen Sachkou, Xin He, Gian-Marco Schnueringer, Andreas Sawadsky, Christopher Baker, Warwick Bowen We report on progress in laser control and readout of excitations in two-dimensional |
Wednesday, March 7, 2018 9:12AM - 9:24AM |
K45.00007: Nucleation of Bubbles by Electrons in Liquid Helium-4 Yiming Yang, Stephen Sirisky, Wanchun Wei, George Seidel, Humphrey Maris We report on experiments in which we study cavitation resulting from electrons in liquid helium. Electrons are introduced into the liquid by a radioactive source. After electron comes to rest, it forms an electron bubble. To study cavitation a sound pulse is generated by a hemispherical piezoelectric transducer producing a large pressure oscillation at the acoustic focus. If an electron is near the focus and the negative going pressure swing exceeds a critical value, a cavitation bubble is produced which can be detected by light scattering. Three distinct critical pressures have been measured as a function of temperature. The first, also the largest, corresponds to cavitation resulting from the application of a reduced pressure to liquid where an electron bubble is already present. The second is the critical pressure needed to lead to cavitation when an electron enters the liquid at a time and place where there is already a reduced pressure. The third, also the smallest, is surprisingly close to SVP, which indicates that the object resulting in cavitation must have high energy concentration. We have no explanation for its origin. It is only detected when the radioactive source is present. |
Wednesday, March 7, 2018 9:24AM - 9:36AM |
K45.00008: Mechanism for Depinning a Macroscopic Vortex in Superfluid Helium Rena Zieve, Daniel Eilbott, Nathan McLaughlin We measure a superfluid vortex, pinned to a thin wire along part or all of its length. After ceasing rotation, this vortex is metastable and heating the fluid can lead to depinning. Several observations show that the key to the depinning is the counterflow velocity through the inlet hole for the helium fill line: the importance of the rate of change of temperature rather than temperature itself, the behavior for different size inlet holes, the location where the vortex depins, and the relative stability of fully or partially attached vortices. All of these observations point to the role of the thermally induced velocity. Although we cannot prove how this velocity leads to depinning, one possibility is that the velocity field enlarges vortices trapped nearby on the cell wall until vortex loops break free and subsequently interact with the trapped central vortex. We comment on the level of reproducibility of our measurements and what it implies for such wall vortices. |
Wednesday, March 7, 2018 9:36AM - 9:48AM |
K45.00009: Enstrophy Cascade in Two-Dimensional Quantum Turbulence Andrew Forrester, Han-Ching Chu, Gary Williams The forward enstrophy cascade in 2D turbulence in an incompressible superfluid film is investigated using the dynamic renormalization group. Out of equilibrium vortex pairs are injected into the film, which is connected to a thermal bath at low temperature. The constant-current cascade has a logarithmic two-point correlation function, and resulting k-3 energy spectrum. The spectrum differs from the classical fluid cascade, varying linearly with the enstrophy injection rate instead of a 2/3 power, since there is an additional factor of the vortex diffusion time. The dynamics of the constant-current cascade is studied by switching on and off the input forcing, and by following the decay from an initially non-uniform state. A fundamental connection to temperature-quenched 2D superfluids is made, where the phase-ordering decay of the vortex density is the result of the turbulent cascade to small scales. |
(Author Not Attending)
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K45.00010: Critical exponents z and ν for superfluid helium near absolute zero Vladimir Udodov We propose a new interpolation formula for the dynamic critical exponent z for the mixture of liquid He4 and He3 at low temperatures: |
Wednesday, March 7, 2018 10:00AM - 10:12AM |
K45.00011: Thermal Expansion and Superfluid Onset for 4He Films Adsorbed on Carbon Nanotubes Emin Menachekanian, Vito Iaia, Mingyu Fan, Jingjing Chen, Chaowei Hu, Ved Mittal, Gengming Liu, Raul Reyes, Fufang Wen, Gary Williams Third sound measurements of superfluid 4He thin films adsorbed on 10 nm diameter multiwall carbon nanotubes are used to make the first measurement of the thermal expansion coefficient normal to the film surface, and to probe the superfluid onset temperature as a function of the film thickness. The nanotubes provide a highly ordered carbon surface, with layer-by-layer growth of the adsorbed film as shown by oscillation peaks in the third sound velocity at the completion of the third, fourth, and fifth atomic layers, arising from oscillations in the film compressibility. The high sensitivity of the third sound to film thickness allows a measurement of the thermal expansion coefficient, which is found to display a large-amplitude oscillation between positive and negative values, precisely out of phase with the compressibility oscillation. The linear slope of the superfluid onset temperature with thickness is found to be anomalous, a factor of three smaller than the universal value predicted by the Kosterlitz-Thouless theory. |
Wednesday, March 7, 2018 10:12AM - 10:24AM |
K45.00012: Observation of Superfluidity in 2nd Layer Helium-4 Films adsorbed on Grafoil Investigated by a Double-resonant Frequency Torsional Oscillator Jaewon Choi, Oleksiy Zadorozhko, JeaKyung Choi, Eunseong Kim Helium film adsorbed on graphite substrates provides rich phase diagram due to the interplay of interactions between helium atoms and substrate. A torsional oscillator experiment of Crowell and Reppy [1] reported first reentrant superfluidity in the second layer of 4He films adsorbed on Grafoam., which was reproduced recently [2, 3]. Both experiments suggest the existence of a possible 2D supersolid phase, although an ab initio Monte Carlo simulation disagree with the interpretation [4]. Here, we investigated superfluidity and elasticity dependent response in the second layer of 4He film adsorbed on Grafoil with a TO with two different resonant frequency. The measurements were performed in the atomic coverages ranging from 17 to 20 atoms/nm2. We found no frequency dependent TO responses that can be attributed to the rigidity change. Instead, frequency independent response appeared with a dissipation peak, demonstrating the evidence of superfludity in the 2nd layer helium films on Grafoil. |
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