Low temperature dissipative behavior in uncoated fused silica slabs

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

2. Cryogenic Activity in Perugia • Cryogenic Coating Measurements: • Changes in the coating Фcoat(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 TiO2 doped Ta2O5 mono-layer coating • B: 500 nm Co doped Ta2O5 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

3. Sample holds

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

5. Summury on coating activity Coating Results: • The coating ФCoat is almost costant in the temperature range of 300K-90K • The Cobalt doped tantala coating shows a ФCoat better than the titania doped tantala coating: ФCoat Mean Value = 3.4E-3 ± 1E-3 ФCoat Mean Value = 7E-4 ± 2E-4 • The measurements are limited by the substrates losses… (see Work in progress) 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

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

7. Introduction What we are going to see: • Φ vs temp:choosing a mode of the slab, how change the loss angle of this mode changing the temperature • We found 2 peaks • 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

8. Ф vs. Temp(for a fixed mode)

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

10. Ф vs. Freq (for a fixed temperature)

11. Losses vs freq 3 Scenarios • 290K-140K: The samples show a quite costante loss angle This plot is not in the same scale of the other ones because there’s a factor 100 of difference 2. 140K-70K: There’s a frequency depended loss angle Frequency [Hz] Frequency [Hz] 3. 70K- 4K: the slabs still have a frequency dependent trend but with a different slope...see next slide Frequency [Hz]

12. 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.

13. Power law We used the following 2 power laws: …that comes from a double well potential model 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

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

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

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

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

18. 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)