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Low temperature dissipative behavior in uncoated fused silica slabs Flavio Travasso Dip. Fisica – Università di Perugia and INFN Perugia Virgo - Perugia

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Cryogenic Activity in Perugia 1.Cryogenic Coating Measurements: –Changes in the coating Ф coat (T): to find a change in the coating changing the temperature –Different coatings: to measure the different loss angles of different coatings Measured Slabs (3 samples – provided and coated by LMA-Virgo Lion): –Uncoated Slab –Titania doped tantala coated slab (slab A) –Cobalt doped tantala coated slab (slab B) Dimension: –A & B: 41mm x 5 mm x 104 μm –Uncoated: 45mm x 5 mm x 104 μm Different frequencies Coatings: –A: 520 nm TiO 2 doped Ta 2 O 5 mono-layer coating –B: 500 nm Co doped Ta 2 O 5 mono-layer coating 2.Fused silica Substrate cryogenic behavior –Experimental activity: about 20 modes studied for 3 uncoated slabs –Theoretical activity: For the amorphous material the classical laws used for the crystalline materials are not so easy to use or to support that’s why is usefull and hard find a: theory to explain the Ф frequency trend of the modes at each temperature above 140K theory to explain the Ф temperature peak around 20K

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Sample holds

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Labview Interface Ni He Ni He Cu Laser HV Amplifier Clamp tightened using a spring Cooling down rate: 1-2 K/h …to avoid particular thermal/mechanical stress Measurement Apparatus

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Summury on coating activity Work in progress: To improve the measurements of coating at low temperature we plan on testing the same coating on new substrates To design a new clamping system and/or new geometry for the samples New materials for the coating Coating Results: The coating Ф Coat is almost costant in the temperature range of 300K-90K The Cobalt doped tantala coating shows a Ф Coat better than the titania doped tantala coating: Ф Coat Mean Value = 3.4E-3 ± 1E-3 Ф Coat Mean Value = 7E-4 ± 2E-4 The measurements are limited by the substrates losses… (see Work in progress)

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Fused Silica Substrates data In the following we focus our attention on the low temperature properties of the fused silica material

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Introduction 1.Φ vs temp: choosing a mode of the slab, how change the loss angle of this mode changing the temperature We found 2 peaks 2. Φ vs freq: selecting a temperature, what is the loss angle of the first 20 modes of the slabs (or rather how the loss angle changes with the frequency) We found 3 different scenarious in 3 temperature ranges => 3 different dissipative processes What we are going to see:

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Ф vs. Temp (for a fixed mode)

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Ф vs. Temp Quite costant loss angle A new dissipative mechanism comes into play: Frequency dependent trend (see next slides) A different process… ( see next slides) All the modes have the same behavior

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Ф vs. Freq (for a fixed temperature)

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Losses vs freq 3 Scenarios 1.290K-140K: The samples show a quite costante loss angle 3. 70K- 4K: the slabs still have a frequency dependent trend but with a different slope...see next slide 2. 140K-70K: There’s a frequency depended loss angle This plot is not in the same scale of the other ones because there’s a factor 100 of difference Frequency [Hz]

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Φ for T < 140K ( Freq. dep. process) The frequency dependent trend is clear… …but it’s also clear that the data for T<30K have a slope smaller than the data between 140K-40K.

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Power law We used the following 2 power laws: What we can do? We can use the first simpler law to fit the data and to check the second law in order to understand where the double well potential model is valid …that comes from a double well potential model

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Exponent of power law: B Above 140K the loss angle appears to be NOT frequency dependent The freq. Dependent process is becoming more active These results are interesting because 1.In literature the explored frequency range is 500Hz-MHz (there are no infos on our frequency range) 2.In literature K = 0 …we have to consider another process to improve the actual physical models A different dissipative mechanism comes into play: dissipative quantum tunnelling, that is quantum tunnelling assisted by thermal fluctuations Sharp transition? System instability? Work in progress…

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Linear Fit of B: 110K-40K …as you remind the BWP forseen a liner law for B(T)

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Amplitude of power law: A The trend is very similar to the Ф(T) one… infact A α Ф(T)

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Fit A Using for B the value evaluated in the previous slide The losses are higher than what forseen by the double well potential model: 2 competive dissipative processes

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Comments SiO2 Results: The measurements show a clear behaviour with temperature: - an almost constant loss angle above 140K - between 140K and 30K the loss angle has a significant increase that can be interpreted by calling for thermally activated relaxation dynamics (in multi- stable potentials) - below 30K the loss angle starts to decrease: the thermally activated dissipation is less effective and a different dissipative mechanism starts to drive the dynamics (quantum tunnelling effects become active at very low temperature… that is quantum tunnelling assisted by thermal fluctuations ) Work in progress: A new refined dynamical model for the interpretation of the losses in the low frequency region is in preparation (See F. Marchesoni)

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