High-temperature superconductivity in monolayer Bi2Sr2CaCu2O8+δ

Electronic inhomogeneity and charge-ordered state
Next, we focus on the electronic structure of monolayer Bi-2212 beyond 
the superconducting energy gap 
Δ
0
. In particular, the energy scale 
Δ
1
is associated with the anti-nodal pseudogap and other correlated states 
that are intricately linked to superconductivity
21
. In contrast to the 
relatively homogeneous superconducting gap 
Δ
0
, the pseudogap 
Δ
1
varies widely at the nanometre length scale on bulk copper oxides
18
. 
To study the inhomogeneity in monolayer Bi-2212, we extract 
Δ
1
from 
the local differential conductance spectra collected on a dense array 
of locations on samples at various doping levels, and construct the gap 
map 
r
Δ ( )
1
as shown in Fig. 5a–c. Similar to previous measurements on 
bulk Bi-2212, we find that wide, nanometre-scale variations in 
Δ
1
dimin-
ish as the doping level increases in the monolayer; meanwhile 
Δ
1
averaged over the entire field of view, 
Δ¯
1
, shifts to lower energies.
Close examination of 
Δ
1
histograms reveals that 
Δ¯
1
in monolayers is in 
general larger than that in bulk source crystals from which the monolay-
ers are cleaved (Extended Data Fig. 9), and the deviation varies from 
q
CO
10
200 meV
1
g (E = 20 meV)
Low
High
a
b
c
e
f
g
h
j
d
i
1
= 120 meV
UD50 1L
OP88 1L
OD55 1L
0
1
2
0
100
200
UD501L
OP881L
OD551L
UD50 Bulk
OP88 Bulk
OD55 Bulk
Δ
1
= 39 meV
Δ
1
= 38 meV
Δ
Δ
Ar
ea (%)
1
/
Δ Δ
1