SUPERCONDUCTIVITY
Atomic manipulation of the gap in Bi 2 Sr 2 CaCu 2 O8+x
F. Massee^1 *, Y. K. Huang^2 , M. Aprili^1
Single-atom manipulation within doped correlated electron systems could help disentangle the
influence of dopants, structural defects, and crystallographic characteristics on local electronic
states. Unfortunately, the high diffusion barrier in these materials prevents conventional
manipulation techniques. Here, we demonstrate thepossibility to reversibly manipulate select sites
in the optimally doped high-temperature superconductor Bi 2 Sr 2 CaCu 2 O8+xusing the local electric
field of the tip of a scanning tunneling microscope.We show that upon shifting individual Bi atoms at
the surface, the spectral gap associated with superconductivity is seen to reversibly change by
as much as 15 milli–electron volts (on average ~5% of the total gap size). Our toy model, which
captures all observed characteristics, suggests that the electric field induces lateral movement of
local pairing potentials in the CuO 2 plane.
O
ne of the challenges in the study of high-
temperature superconductivity in the
cuprates ( 1 ) is their intrinsic inhomo-
geneous nature. This is exemplified by
the archetypal system Bi 2 Sr 2 CaCu 2 O8+x
(Bi2212), whose complicated crystal structure
includes an incommensurate structural super-
modulation ( 2 ) and interstitial oxygen dopant
atoms. Particularly notable is the large varia-
tion in spectral gap size over nanometer-scale
distances ( 3 – 5 ), which has been shown to
reflect local variations in the superconducting
transition temperatureTc( 6 , 7 ) and has been
correlated to both oxygen dopants ( 8 , 9 ) and
structural inhomogeneity ( 10 ). These correla-
tions, however, typically involve averages over
a large number of sites. To directly probe the
influence of dopants, structural defects, and
crystallographic characteristics on the local
electronic states at the atomic scale, direct non-
invasive control of the dopant positions and the
inhomogeneous crystal structure they inhabit
is therefore desirable. Unfortunately, common
techniques for single-atom manipulation in-
volving short-range forces between the tip and
the atom ( 11 – 13 ) and/or vibrational excita-
tion using the tunneling current ( 14 – 16 ) are
not well suited for controllable manipulation
of atoms in single-crystal cuprate materials.
This is because the dopant atoms are buried
under the surface and the diffusion barrier
of the surface atoms themselves is too high.
Alternatively, the electric field can be used to
manipulate a surface ( 17 – 19 ); however, be-
cause the field profile depends on the size
of the tip apex, this process is, in practice,
difficult to control unless specific atoms are
more sensitive to the field than others owing
to their charge or local environment. We dis-
covered that this is exactly the case in Bi2212,
wherewefindtwolocalenvironmentsthatare
more readily influenced by the electric field
than the rest of the system.
Figure 1A shows a schematic of the two
environments that allow for field-induced
atom manipulation: surface Bi atoms on the
crest of the periodic modulation of the bulk
crystal structure ( 2 ), henceforth referred to as
the supermodulation, and the recently dis-
covered weakly coupled oxygen dopants ( 20 ).
To manipulate the atoms, we position our tip
above the surface and slowly increase the sam-
ple bias voltage,Vs. Upon reaching ~800 mV
at a tunnel current of ~100 pA, we start to
observe jumps in the current, where each
jump corresponds to the manipulation of an
atom below the tip. For manipulation volt-
agesVs≥1.2 V, the current typically showstwo types of jumps (Fig. 1B): small ones cor-
responding to the manipulation of a near-
surface oxygen dopant, which we can detect
through their signature in the differential con-
ductance ( 8 , 9 , 20 ), and big jumps correspond-
ing to the manipulation of a surface Bi atom
on the crest of the supermodulation that are
observed directly inthe constant current
images (Fig. 1C) [section 1 of ( 21 )]. We can
freeze-in any new surface-and-dopant config-
uration by switching back to low bias voltage.
Because the spatial extent of the highest electric
field is determined by the size of the tip apex,
which is typically a few tens of nanometers
in diameter, in principle, several hundred Bi
and oxygen dopants can be affected. However,
unlike in a homogeneous system where the
threshold field for manipulation is identical
for all field-affected atoms or molecules ( 19 ),
the existence of intrinsic lattice distortions, the
supermodulation, and numerous local dopant
environments in Bi2212 ( 20 )leadstoarange
of threshold fields, enabling selective and re-
versible manipulation by carefully tuning the
tip position and the manipulation voltage.
As expected for electric field–induced manip-
ulations, all manipulations we observe are
confined to a roughly circular area with a
diameter of a few tens of nanometers around
the location of the tip during the application
of the manipulation voltage (fig. S5). Owing to
the aforementioned intrinsic inhomogeneous
nature of Bi2212, studying the influence of
single atoms on the local electronic structure
is normally impossible, and averaging over a
large number of sites is required instead. With
theabilitytolocallyrearrangeaselectnumber
of atoms, however, we can effectively remove
the inhomogeneous background information
by considering the difference in local electronicRESEARCH
Masseeet al.,Science 367 ,68–71 (2020) 3 January 2020 1of4
(^1) LaboratoiredePhysiquedesSolides,CNRSUMR8502,
Bâtiment 510, Université Paris-Sud, Université
Paris-Saclay, 91405 Orsay, France.^2 Institute of Physics,
University of Amsterdam, 1098XH Amsterdam,
Netherlands.
*Corresponding author. Email: [email protected]
BC
150
140
130
120
Current (pA)
0 5 10 15 20 25 Time (s)
Vs = 1.3 V
E Bi
A
Bi
O O
2- 2-
high
z(
r)
low
Fig. 1. Electric field–induced atom manipulation.(A) Illustration of the field-induced manipulation of surface Bi atoms and near-surface oxygen
dopants (Bi, blue; O, black or orange; Cu, red). (B) Current as a function of time atVs= 1.3 V. Small jumps signal oxygen dopant manipulation (O^2 −),
large jumps signal surface Bi rearrangement (Bi). (C) Constant current images, z(r), (Eset=−100 meV;Iset= 100 pA) of reversible surface Bi atom
manipulation upon treatment withVs> 1.2 V. The atoms in the dashed box are manipulated. Scale bars, 1 nm. All measurements throughout this
work were performed at temperatureT≤4.2 K.