Table S1.

Results for fitting constants in Eq. S9

Sample/direction	${\alpha }_e$	${\alpha }_{ph}$
YBCO6.60
Plane	0.14 ± 0.01	2.0 ± 0.1
Chain	0.413 ± 0.007	2.58 ± 0.06
YBCO6.67
Plane	0.13 ± 0.02	2.1 ± 0.3
Chain	0.34 ± 0.06	2.6 ± 0.5
YBCO6.75
Plane	0.25 ± 0.02	1.64 ± 0.09
Chain	0.53 ± 0.02	2.82 ± 0.2