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Type-II thin film superconductors studied by terahertz radiation
Tesař, Roman ; Skrbek, Ladislav (advisor) ; Lipavský, Pavel (referee) ; Němec, Hynek (referee)
Title: Type-II thin film superconductors studied by terahertz radiation Author: RNDr. Roman Tesař Department: Department of Low Temperature Physics Supervisor: prof. RNDr. Ladislav Skrbek, DrSc. Consultant: RNDr. Jan Koláček, CSc. Abstract: Utilization of type-II superconductors for future practical applications such as fluxonics requires detailed knowledge of their physical properties, espe- cially at high frequencies within the THz spectral region. We have investigated interactions of thin-film NbN samples deposited on Si substrate and of a high quality epitaxial film of the NbN superconductor grown on a birefringent R-cut sapphire substrate with monochromatic linearly polarized laser beam both below and above the critical temperature Tc. For photon energies lower than the optical gap, detailed measurements of transmission in zero field provide BCS-like tem- perature curves with a pronounced peak below Tc which disappears as the energy of incident radiation is increased above the gap. In externally applied magnetic fields up to 10 T oriented perpendicularly to the sample, i.e., in the Faraday exper- imental geometry, the temperature behavior of transmission is modified because the gap is suppressed and vanishes at the upper critical field and, additionally, the presence of quantized vortices changes the shape...
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Visualization of selected flows of superfluid helium using solid hydrogen tracer particles
Duda, Daniel ; Skrbek, Ladislav (advisor) ; Babuin, Simone (referee)
Daniel Duda Visualization of selected flows of superfluid helium using solid hydrogen tracer particles 5 Thesis title: Visualization of selected flows of superfluid helium using solid hydrogen tracer particles Author: Bc. Daniel Duda Department: Department of Low Temperature Physics, Supervisor: prof. RNDr. Ladislav Skrbek, DrSc, Department of Low Temperature Physics, Faculty of Mathematics and Physics, Charles University in Prague. Consultant: Dr. Marco La Mantia, PhD. Abstract: Quantum turbulence generated in thermal counterflow of He II is studied experimentally by visualization. The statistical properties of the motion of micron size solid deuterium particles are studied by using the particle tracking velocimetry technique at length scales comparable to the mean distance between quantized vorti- ces. The probability density function (PDF) of the longitudinal velocity displays two peaks that correspond to two velocity fields of the two-fluid description of He II. The PDF of the transversal velocity displays a classical-like Gaussian core with non- classical power-law tails, confirming the quantum nature of turbulence in counter- flowing He II. The distribution of the particle acceleration is found to be similar in shape to the classical one, in the range of investigated parameters. The observed de-...
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Steady state and decay of quantum turbulence generated in channel flows and detected by second sound attenuation
Varga, Emil ; Skrbek, Ladislav (advisor) ; Vinen, William Frank (referee)
Steady state and decay of quantum turbulence generated in channel flows and detected by second sound attenuation Bc. Emil Varga Abstract Quantum turbulence is studied in superfluid 4 He under classical flow condi- tions. Turbulence is generated by a flow through a 7 × 7 mm square channel with a flow conditioner either with an additional grid or without it. The flow is generated mechanically by squeezing a stainless steel bellows. Vortex line den- sity is measured by attenuation of second sound in both steady state and decay for a range of temperatures 1.17 - 2.16 K. In the steady state, temperature- independent scaling of the vortex line density with flow velocity of the form L ∝ V 3/2 is observed. In the decay the expected late-time behaviour L ∝ t−3/2 is observed. Explanation for both of these observations is based on a quasi- classical model of quantum turbulence, that allows the extraction of the effective kinematic viscosity νeff, which approximately agree with the values available in the literature. Two models based on counterflow theory are also explored and the effect of inhomogeneous vortex line distribution on the measurement tech- nique is studied.
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Visualization of selected flows of water and cryogenic helium using tracer particles
Pilcová, Veronika ; Skrbek, Ladislav (advisor) ; Kohout, Jaroslav (referee)
Flow visualization techniques have recently been applied for the investigation of various cryogenic flows of liquid helium. Particle image velocimetry and particle tracking velocimetry techniques, proven in the past as very fruitful in many scientific and industrial areas of research, are being used for the analysis of cryogenic flows. The Joint Low Temperature Laboratory at the Charles University in Prague is the first in Europe to employ flow visualization techniques to investigate liquid helium flows. The approach had to be optimized due to a number of technical and fundamental dificulties, i.e., the optical access to the helium bath and choice of suitable tracer particles. Water experiments at room temperature were performed to prove that the experimental apparatus is well-suited for the low-temperature experiments performed as the main part of the work. The latter focused on thermal counter flow. The results from both, room-temperature experiments and low-temperature experiments are discussed and positively compared with well-known theoretical results.
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Quantum turbulence in superfluid helium studied by particle tracking velocimetry visualization technique
Duda, Daniel ; Skrbek, Ladislav (advisor) ; Chára, Zdeněk (referee) ; Skyba, Peter (referee)
❚✐t❧❡✿ ◗✉❛♥t✉♠ t✉r❜✉❧❡♥❝❡ ✐♥ s✉♣❡r✢✉✐❞ ❤❡❧✐✉♠ st✉❞✐❡❞ ❜② ♣❛rt✐❝❧❡ tr❛❝❦✐♥❣ ✈❡❧♦❝✐♠❡tr② ✈✐s✉❛❧✐③❛t✐♦♥ t❡❝❤♥✐q✉❡ ❆✉t❤♦r✿ ❘◆❉r✳ ❉❛♥✐❡❧ ❉✉❞❛ ❉❡♣❛rt♠❡♥t✿ ❉❡♣❛rt♠❡♥t ♦❢ ▲♦✇ ❚❡♠♣❡r❛t✉r❡ P❤②s✐❝s ❙✉♣❡r✈✐s♦r✿ ♣r♦❢✳ ❘◆❉r✳ ▲❛❞✐s❧❛✈ ❙❦r❜❡❦✱ ❉r❙❝✳ ❆❜str❛❝t✿ ❚❤❡ P❛rt✐❝❧❡ ❚r❛❝❦✐♥❣ ❱❡❧♦❝✐♠❡tr② ✈✐s✉❛❧✐③❛t✐♦♥ t❡❝❤♥✐q✉❡ ✉s✐♥❣ ♠✐❝✲ r♦♠❡t❡r s✐③❡ s♦❧✐❞ ❞❡✉t❡r✐✉♠ ♣❛rt✐❝❧❡s ❛s tr❛❝❡rs ❤❛s ❜❡❡♥ ❛♣♣❧✐❡❞ t♦ st✉❞② ♦s❝✐❧✲ ❧❛t♦r② ✢♦✇s ♦❢ ❍❡ ■■✱ ✇❤✐❝❤ ✐s ❛ q✉❛♥t✉♠ ✢✉✐❞ ✇✐t❤ q✉❛♥t✐③❡❞ ✈♦rt✐❝✐t②✱ ❛s ✇❡❧❧ ❛s ✢♦✇s ♦❢ ❍❡ ■✱ ✇❤✐❝❤ ✐s ❛ ❝❧❛ss✐❝❛❧ ✈✐s❝♦✉s ❧✐q✉✐❞✱ ❢♦❝✉s✐♥❣ ♦♥ t❤❡ s✐♠✐❧❛r✐t✐❡s ❛♥❞ ❞✐✛❡r❡♥❝❡s ❜❡t✇❡❡♥ t❤❡ q✉❛♥t✉♠ ❛♥❞ ❝❧❛ss✐❝❛❧ ✢♦✇s✳ ❚❤r❡❡ ❡①♣❡r✐♠❡♥ts ❛r❡ ❞❡s❝r✐❜❡❞✿ t❤❡ ✢♦✇ ♣❛st ❛ ❧❛r❣❡✲❛♠♣❧✐t✉❞❡ ❧♦✇✲❢r❡q✉❡♥❝② ♦s❝✐❧❧❛t✐♥❣ ♦❜st❛❝❧❡ ✐♥ t❤❡ ❢♦r♠ ♦❢ ❛ ♣r✐s♠❀ t❤❡ st❡❛❞② str❡❛♠✐♥❣ ✢♦✇ ❞✉❡ t♦ ❛ s♠❛❧❧✲❛♠♣❧✐t✉❞❡ ❧❛r❣❡✲ ❢r❡q✉❡♥❝② ♦s❝✐❧❧❛t✐♥❣ q✉❛rt③ t✉♥✐♥❣ ❢♦r❦ ✲ ❛ ✇✐❞❡❧② ✉s❡❞ t♦♦❧ t♦ st✉❞② q✉❛♥t✉♠ t✉r❜✉❧❡♥❝❡❀ ❛♥❞ t❤❡ ♣r♦❞✉❝t✐♦♥ ♦❢ ❝❛✈✐t❛t✐♦♥ ✐♥ t❤❡ ✈✐❝✐♥✐t② ♦❢ ❛ ❢❛st✲♦s❝✐❧❧❛t✐♥❣ t✉♥✐♥❣ ❢♦r❦✳ ❚❤❡ ♠❛✐♥ ♦✉t❝♦♠❡ ✐s t❤❡ ♦❜s❡r✈❛t✐♦♥ t❤❛t t❤❡s❡ ✢♦✇s ❛r❡ s✐♠✐❧❛r ✐♥ ❍❡ ■ ❛♥❞ ✐♥ ❍❡ ■■ ❛t ❧❛r❣❡ ❧❡♥❣t❤✲s❝❛❧❡s✱ ✇❤❡r❡❛s ❛t s♠❛❧❧ s❝❛❧❡s✱ t❤❡② ❡①❤✐❜✐t t♦t❛❧❧② ❞✐✛❡r❡♥t st❛t✐st✐❝❛❧ ♣r♦♣❡rt✐❡s✳ ▼♦r❡♦✈❡r✱ ✐♥ ❍❡ ■■✱ t❤❡s❡ s♠❛❧❧ s❝❛❧❡ st❛✲ t✐st✐❝❛❧ ♣r♦♣❡rt✐❡s ❛r❡ ✉♥✐✈❡rs❛❧ ✐♥ t❤❛t t❤❡② ❞♦ ♥♦t ❞❡♣❡♥❞ ♦♥ t❤❡ t②♣❡ ♦❢ t❤❡ ✐♠♣♦s❡❞ ♠❡❛♥ ✢♦✇ ♦❢ t❤❡ s✉♣❡r✢✉✐❞ ❛♥❞ ♥♦r♠❛❧...
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