High-temperature superconductivity in monolayer Bi2Sr2CaCu2O8+δ

Tunable high-temperature superconductivity
The reduction in dimensionality produces a key advantage: the HTS in 
monolayer Bi-2212 becomes extremely tunable. The tunability stems 
from the fact that both sides of the monolayer are exposed, making it 
easy for interstitial oxygen to escape from or enter the crystal. Specifi-
cally, we find that mild vacuum annealing at temperatures between 
300 K and 380 K drives oxygen out of the monolayer. Meanwhile, anneal-
ing at about 200 K in ozone (partial pressure approximately 0.5 mbar) 
increases the oxygen concentration (Extended Data Fig. 2). These
findings enable us to continuously vary the doping level and track the 
evolution of various phases, including superconductivity, from an
over-doped to deeply under-doped regime (and vice versa) in a single 
monolayer sample. Figure 2a displays a set of measurements of
temperature-dependent resistivity, 
□
R T
( ), of a monolayer Bi-2212
(sample A), acquired between annealing treatments at 300–380 K in 
vacuum (base pressure <10
−4
mbar). The annealing treatments progres-
sively lower the hole doping level in the monolayer and induce a tran-
sition from superconducting to insulating behaviour. Meanwhile, the 
room-temperature resistivity increases by one order of magnitude 
from about 1 kΩ to about 30 kΩ. Details of the transition become more 
apparent when the resistivity of the same sample (normalized to its 
value at T = 200 K) is plotted as a function of temperature and hole 
doping level p, as shown in Fig. 2b. (Here the hole doping level is deter-
mined from 
□
p
R T
= const./
( = 200 K); the value of the constant (const.) 
is chosen so that p = 0.16 at optimal doping
48,49
, and the precise value 
of p does not affect our conclusions.) As p decreases, T
c
(defined as the 
temperature at which 
□
R
T
d
/d
= 0
2
2
; see Extended Data Fig. 3) rises at 
first, then falls continuously, giving rise to a superconducting dome 
that ends at p ≈ 0.022. An insulating phase appears next to the
superconducting dome. In addition, we observe at 
T
T
* >
c
the onset of 
the pseudogap phase that is marked by deviation from a linear
□
R T
( ) in the normal state of a high-temperature superconductor under 
various doping levels (open black circles in Fig. 2b; see Extended Data 
Fig. 3 for detailed analysis). Figure 2b, therefore, maps out a phase 
0
100
200
300
10
–2
10
0
10
2
10
4
10
6
84
85
86
87
88
89
90
0.0
0.2
0.4
0.6
0.8
0
100
200
300
1/R
Ƒ
(200 K) (mS)
T 
(K
)
0
1
2
3
R
Ƒ
/R
Ƒ
(2
00
K)