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Register a new userThreefold Reduction in Thermal Conductivity of Vycor Glass Due to Adsorption of Liquid 4He
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We report thermal conductivity measurements of porous Vycor glass when it is empty and when the pores are filled with helium between 0.06 and 0.5 K. The filling of liquid 3He and liquid 4He inside the Vycor pores brings about respectively two and three fold reduction of the thermal conductivity as compared with empty Vycor. This dramatic reduction of thermal conductivity, not seen with solid 3He and 4He in the pores, is the consequence of hydrodynamic sound modes in liquid helium that greatly facilitate the quantum tunneling of the two-level systems (TLS) in Vycor and enhance the scattering of the thermal phonons in the silica network.
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In these torsional oscillator experiments the samples of solid $^4$He were characterized by measuring their thermal conducitvity. Polycrystalline samples of helium of either high isotopic purity or natural concentration of $^3$He were grown in an annular container by the blocked-capillary method and investigated before and after annealing. No correlation has been found between the magnitude of the low-temperature shift of the torsional oscillator frequency and the amount of crystalline defects as measured by the thermal conductivity. In samples with the natural $^3$He concentration a substantial excess thermal conductivity over the usual $T^3$ dependence was observed below 120 mK.
The Anderson localization of thermal phonons has been shown only in few nano-structures with strong random disorder by the exponential decay of transmission to zero and a thermal conductivity maximum when increasing system length. In this work, we present a path to demonstrate the phonon localization with distinctive features in graded superlattices with short-range order and long-range disorder. A thermal conductivity minimum with system length appears due to the exponential decay of transmission to a non-zero constant, which is a feature of partial phonon localization caused by the moderate disorder. We provide clear evidence of localization through the combined analysis of the participation ratio, transmission, and real-space phonon number density distribution based on our quantum transport simulation. The present work would promote heat conduction engineering by localization via the wave nature of phonons.
We have studied the liquid - solid (L-S) phase transition of ^4He confined in nanoporous glass, which has interconnected nanopores of 2.5 nm in diameter. The L-S boundary is determined by the measurements of pressure and thermal response during slow cooling and warming. Below 1 K, the freezing pressure is elevated to 1.2 MPa from the bulk freezing pressure, and appears to be independent of temperature. The T-independent L-S boundary implies the existence of a localized Bose-Einstein condensation state, in which long-range superfluid coherence is destroyed by narrowness of the nanopores and random potential.
We present neutron scattering measurements of the phonon-roton (P-R) mode of superfluid 4He confined in 47 AA MCM-41 at T = 0.5 K at wave vectors, Q, beyond the roton wave vector (Q_R =1.92 AA$^{-1}$). Measurements beyond the roton require access to high wave vectors (up to Q = 4 AA$^{-1}$) with excellent energy resolution and high statistical precision. The present results show for the first time that at T = 0.5 K the P-R mode in MCM-41 extends out to wave-vector Q$simeq$3.6 AA$^-1$ with the same energy and zero width (within precision) as observed in bulk superfluid 4He. Layer modes in the roton region are also observed. Specifically, the P-R mode energy, $omega_Q$, increases with Q for Q > QR and reaches a plateau at a maximum energy Q = 2{Delta} where {Delta} is the roton energy, {Delta} = 0.74 $pm$ 0.01 meV in MCM-41. This upper limit means the P-R mode decays to two rotons when its energy exceeds 2{Delta}. It also means that the P-R mode does not decay to two layers modes. If the P-R could decay to two layer modes, $omega_Q$ would plateau at a lower energy, $omega_Q$ = 2{Delta}L where {Delta}L = 0.60 meV is the energy of the roton like minimum of the layer mode. The observation of the P-R mode with energy up to 2{Delta} shows that the P-R mode and the layer modes are independent modes with apparently little interaction between them.
We have carried out a study of the momentum distribution and of the spectrum of elementary excitations of liquid $^4$He across the normal-superfluid transition temperature, using the path integral Monte Carlo method. Our results for the momentum distribution in the superfluid regime show that a kink is present in the range of momenta corresponding to the roton excitation. This effect disappears when crossing the transition temperature to the normal fluid, in a behavior currently unexplained by theory.
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