100,724 research outputs found
Kapitza Resistance of Si/SiO₂ Interface
No full textA phonon wave packet dynamics method is used to characterize the Kapitza resistance of a Si/SiO2 interface in a Si/SiO2/Si heterostructure. By varying the thickness of SiO2 layer sandwiched between two Si layers, we determine the Kapitza resistance for the Si/SiO 2 interface from both wave packet dynamics and a direct, non-equilibrium molecular dynamics approach. The good agreement between the two methods indicates that they have each captured the anharmonic phonon scatterings at the interface. Moreover, detailed analysis provides insights as to how individual phonon mode scatters at the interface and their contribution to the Kapitza resistance
Ultrafast Kapitza-Dirac effect
No full textSimilar to the optical diffraction of light passing through a material
grating, the Kapitza-Dirac effect occurs when an electron is diffracted by a
standing light wave. In its original description the effect is
time-independent. In the present work, we extend the Kapitza-Dirac concept to
the time domain. By tracking the spatiotemporal evolution of a pulsed electron
wave packet diffracted by a femtosecond (10 15 second) standing wave pulse in a
pump-probe scheme, we observe so far unseen time-dependent diffraction
patterns. The fringe spacing in the observed pattern differs from that
generated by the conventional Kapitza-Dirac effect, moreover it decreases as
the pump-probe delay time increases. By exploiting this time-resolved
diffraction scheme, we gather access to the time evolution of the previously
inaccessible phase properties of a free electron
Kapitza resistance at the H↓/liquid He interface
No full textIn spin-polarized atomic hydrogen, the way to low T and high density is restricted by the Kapitza resistance. This is calculated, together with the corresponding characteristic time, for one channel : from H gas to liquid helium ripplons.Dans l'hydrogène atomique polarisé, l'accès aux basses températures et fortes densités est bloqué par la résistance thermique de Kapitza. Cette résistance est calculée, ainsi que le temps caractéristique associé, pour un canal significatif : du gaz hydrogène aux modes de surface de l'hélium liquide
Liquid-solid 4He interface: Kapitza resistance
No full textWe have measured the Kapitza resistance RK between liquid and solid 4He, from 100 mK to 0.7 K. We show that R K T3 varies linearly with T-2 for a rough interface and the crystalline anisotropy. The surface tension alone cannot explain the results which are interpreted as indicating an interface inertia. For the facets, the classical result is found : RK T3 = constant.Nous avons mesuré la résistance de Kapitza RK entre les phases liquide et solide de 4He, de 100 mK à 0,7 K. Nous mettons en évidence la variation linéaire de RK T3 avec T-2 pour les interfaces rugueuses et l'anisotropie cristalline. L'influence de la tension de surface ne peut seule expliquer les résultats qui sont interprétés comme étant dus à l'inertie de l'interface. Pour les facettes on retrouve le résultat classique RK T3 = Cte
Systematic Investigation of the Misorientation- and Temperature-Dependent Kapitza Resistance in CeO₂
No full textThe misorientation- and temperature-dependent grain boundary thermal (Kapitza) resistance in CeO2 is investigated using molecular dynamics simulations. A few empirical potentials for molecular dynamics simulations are evaluated for their predicted properties such as the phonon dispersion curves, bulk thermal conductivity, and grain boundary structures. Through the comparison of these properties with experimental results, the most reasonable potential (Gotte2007) is selected. The Kapitza resistances of tilt and twist grain boundaries with misorientation angles ranging from 3°to 87°are calculated and a clear transition angle at about 16°is observed. The Kapitza resistance is found to increase almost linearly with misorientation angle in the low-angle regime but remain nearly constant at the high-angle regime, a behavior very similar to the grain boundary energy. A nearly linear correlation between Kapitza resistance and grain boundary energy is thus obtained. Similar to the grain boundary energy, the Read-Shockley model can well describe the misorientation-dependent Kapitza resistance at low-angle regime. The Kapitza conductance (the inverse of Kapitza resistance) is found to increase almost linearly with temperature in our simulations
TUNNELLING AS A MECHANISM FOR THE ANOMALOUS KAPITZA RESISTANCE
No full textNous essayons d'expliquer la résistance de Kapitza anormale en introduisant des états tunnel liés aux atomes d'hélium proches de l'interface entre un solide et l'hélium (liquide ou solide). Nous attirons l'attention sur le fait que les états tunnel des "atomes quantiques" des quelques premières couches qui peuvent jouer un rôle important dans les anomalies de conduction thermique. Nous discutons des aspects expérimentaux susceptibles de vérifier la théorie.We try to explain the anomalous Kapitza resistance introducing tunnelling states of He atoms close to the boundary between a solid and liquid or solidified He. It is emphasized that tunnelling states of "quantum atoms" in the first few layers may play an important role in the anomalous heat conduction. We suggest further experimental aspects to verify the theory
Thermal design studies in superconducting rf cavities: Phonon peak and Kapitza conductance
No full textThermal design studies of superconducting radio frequency (SRF) cavities involve two thermal parameters, namely the temperature dependent thermal conductivity of Nb at low temperatures and the heat transfer coefficient at the Nb-He II interface, commonly known as the Kapitza conductance. During the fabrication process of the SRF cavities, Nb sheet is plastically deformed through a deep drawing process to obtain the desired shape. The effect of plastic deformation on low temperature thermal conductivity as well as Kapitza conductance has been studied experimentally. Strain induced during the plastic deformation process reduces the thermal conductivity in its phonon transmission regime (disappearance of phonon peak) by 80%, which may explain the performance limitations of the defect-free SRF cavities during their high field operations. Low temperature annealing of the deformed Nb sample could not recover the phonon peak. However, moderate temperature annealing during the titanification process recovered the phonon peak in the thermal conductivity curve. Kapitza conductance measurements for the Nb-He II interface for various surface topologies have also been carried out before and after the annealing. These measurements reveal consistently increased Kapitza conductance after the annealing process was carried out in the two temperature regimes
Letter, [Author unclear] to Paulina T. Merritt
No full textHandwritten letter to Paulina Merritt from an unknown author, October 1, 1876.
Measuring gravitational acceleration by cold atom multimode interference with three Kapitza–Dirac pulses
No full textWe theoretically investigate atom interferometric schemes based on three Kapitza–Dirac pulses in a one-dimensional harmonic trap. Kapitza–Dirac pulses as beam splitters create several spatially addressable modes with different momentum. The one-dimensional harmonic potential well acts as a mirror in an optical interferometer, which causes different modes to interfere and numerous high-contrast fringes appear in each mode at the time of measurement. The sensitivity of this atom interferometer in measuring gravitational acceleration can reach , which is further improved with increasing the number of modes
Theory for normal state critical Kapitza resistance of ⁴He
No full textWe study a steady state bearing a heat current, Q₀, in the normal state of liquid ⁴He with
constraining geometry, near the lambda point, at temperatures T greater than the lambda temperature, Тλ. Critical order parameter fluctuations near the boundary are incorporated in the expression for the
non-local thermal resistivity. The Kapitza resistance is manifested by the additional temperature rise at
the boundary, as compared with the bulk-extrapolated value. Sensitivity of the calculations to the
application of the Dirichlet boundary condition is discussed.The work has been supported by NASA Grant
NAG3-1867
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