1. Low temperature dissipation in coating materials S. Reid 1, I. Martin 1, E. Chalkley 1, H. Armandula 3, R. Bassiri 1, C. Comtet 4, M.M. Fejer 5, A. Gretarsson 6, G. Harry 7, J. Hough 1, P. Lu 5, I. McLaren 1, J-M.M. Mackowski 4, N. Morgado 4, R. Nawrodt 2, S. Penn 8, A. Remillieux 4, S. Rowan 1, R. Route 5, A. Schroeter 2, C. Schwarz 2, P. Seidel 2, K. Vijayraghavan 5, W. Vodel 2, A. Woodcraft 1 1 SUPA, University of Glasgow, Scotland. 2 Friedrich-Schiller University, Jena, Germany. 3 LIGO Laboratory, California Institute of Technology, USA. 4 LMA, Lyon, France. 5 Stanford University, USA. 6 Embry-Riddle Aeronautical University, USA. 7 LIGO Laboratory, Massachusetts Institute of Technology, USA. 8 Hobart and William Smith Colleges, USA.

2. Introduction Mechanical dissipation from dielectric mirror coatings is predicted to be a significant source of thermal noise for advanced detectors. Experiments suggest Ta 2 O 5 is the dominant source of dissipation in current SiO 2 /Ta 2 O 5 coatings Doping the Ta 2 O 5 with TiO 2 can reduce the mechanical dissipation Mechanism responsible for the observed mechanical loss in Ta 2 O 5 as yet not clearly identified GEO600 mirror suspension, with HR coating on front face. Studying dissipation as a function of temperature of interest to: Determine dissipation mechanisms in the coatings, possibly allowing dissipation to be reduced Evaluate coating for possible use in proposed cryogenic gravitational wave detectors

3. Single layer coating samples for low temperature studies Thin silicon substrates used for coating Loss of silicon decreases at low temperature Coating will dominate the loss 34mm 500  m 48  m Coating applied here Titania doped tantala coated silicon cantilever in clamp uncoated silicon cantilever in clamp ratio of energy stored in cantilever to energy stored in coating difference in loss between coated and un-coated cantilevers Typical amplitude ringdown

4. Mechanical loss measurements Comparison of the mechanical loss of the third bending mode (~1000 Hz) for a cantilever coated with Ta 2 O 5 with 14.5 % TiO 2, and an identical un- coated cantilever (in collaboration with Jena University)

5. Low temperature coating loss peak A dissipation peak at ~18-20 K observed in both TiO 2 -doped Ta 2 O 5 and pure Ta 2 O 5

6. Most internal friction mechanisms may be thought of as relaxation processes associated with transitions between equilibrium states, and typically: where  is the relaxation time  is a constant related to height of peak Thermally activated processes follow Arrhenius equation: where  0 -1 is the rate factor and E a is the activation energy for the process At the dissipation peak,  =1 and …hence: Interpretation and analysis

7. Fitting to Arrhenius equation This gives an activation energy associated with the dissipation peak in doped tantala of (42 ± 2) meV, and a rate factor of 3.3 ×10 14 Hz.

8. Glasgow (I. Martin et al) low T dissipation in single-layers Ta 2 O 5 and SiO 2 Dissipation peak observed at ~ 20 K in both pure and TiO 2 doped (14.5%) Ta 2 O 5 Activation energy of 42 meV for dissipation in Ta 2 O 5 is similar to that in bulk fused silica i.e. dissipation possibly arises from double well potential corresponding to stable Ta-O bond angles Evidence that TiO 2 doping reduces height of peak in addition to reducing loss at room temperature Loss of Ta 2 O 5 is higher than loss of SiO 2 between 10 and 290 K Scatter close to room temperature due to clamping loss, measurements to be repeated

9. Glasgow (I. Martin et al) Heat treatment of un-doped tantala films (c) (a) (b) Temperature (K) Loss of coated cantilever 0 50 100300 150200250 1E-5 1E-4 Large loss peak at ~ 80 to 90 K in un-doped Ta 2 O 5 coating heat treated at 800 °C, perhaps due to (expected) onset of polycrystalline structure Smaller, broader peak at with activation energy of 138 ± 4 meV at ~35 K in un-doped Ta 2 O 5 heat treated at 300°C Above: Electron diffraction measurement of Ta 2 O 5 heat treated at (i) 600 °C, showing amorphous structure and (ii) 800 °C, showing crystalline structure. Left: Loss at 1.9 kHz of 0.5 mm Ta 2 O 5 coatings heat treated at (a) 600, (b) 300 and (c) 800 °C

10. Glasgow (E. Chalkley et al) Alternative high index coating materials Investigation of possible alternative high index coating materials Initial studies of 0.5 mm thick HfO 2 coating, heat treated at 300°C show: a broad dissipation peak at ~ 50 K a plateau, or possibly a second peak, at ~200 K. Study of the effect of heat treatment on loss of hafnia planned. 500nm HfO 2 coating on a silicon cantilever uncoated silicon cantilever

11. Initial HfO 2 compared to undoped Ta 2 O 5 Below ~50 K, dissipation in HfO 2 and Ta 2 O 5 broadly equivalent (both heat treated 300°C) Note: the higher Young’s modulus of Hafnia should lead to lower thermal noise for the same .d (loss-thickness product) for silicon optics (not necessarily true for other materials e.g. fused silica) (using typical HfO 2 material property data). Initial room temperature studies on a multi-layer silica-hafnia coating on a fused silica substrate were found to be  hafnia =(5.7±0.3)×10 -4.

12. Conclusions - 1 Dissipation peak observed at ~ 20 K in both pure Ta 2 O 5 and in Ta 2 O 5 doped with 14.5 % TiO 2 Activation energy of dissipation process calculated to be: 42 ± 2 meV for doped coating (LMA). 24 ± 2 meV for undoped coating Possible dissipation mechanism is thermally activated transitions of the oxygen atoms, similar to that in fused silica Some evidence that TiO 2 doping reduces the height of the dissipation peak in Ta 2 O 5,in addition to reducing the loss at room temperature. A full understanding of the dissipation mechanism may allow Mechanical loss at room temperature to be further reduced Reduction of loss at particular temperatures of interest for future cryogenic detectors Ta 2 O 5 has consistently higher loss than SiO 2 between 10 and 300 K Collaboration ongoing with INFN lab at Legnaro to study the loss of coating materials at ultra-cryogenic temperatures – may yield more information on loss mechanisms.

13. Conclusions - 2 Material properties of coatings being investigated by ellipsometry, AFM and electron microscopy techniques. ADX Reduced Density Function analysis of TEM images to help quantify structure in amorphous thin-films. Preliminary results of the mechanical loss of Hafnia show similar levels of mechanical loss to the equivalent Tantala coating below ~50 K. However material properties for thin-film Hafnia are not well studied and any changes over bulk properties will change the results presented here. The optical properties also require further investigation, where initial recent absorption studies of a multilayer silica-hafnia coating by Markosyan et al. (Stanford University) lie in the range 60-80ppm, which is considerably higher than required.