Dear Dr. Zhang,

We note that you have submitted Supplemental Material that contains references.
Please ensure all such references also appear in the Reference section of the
main paper to benefit readers and help ensure they receive the credit they
deserve. Note that references are not included in any length limit.

One way to achieve this is via the following example:

[20] See Supplemental Material [url] for [brief description], which includes
Refs. [21-24].
[21] [first reference in Supplemental Material not already in paper] ...
[24] [last reference in Supplemental Material not already in paper]

We await your revised file(s) addressing this issue.

Yours sincerely,

Journal Services
Physical Review Letters