界面载流子注入诱导非超导PrBa2Cu3O7薄膜超导转变
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更新于2024-08-26
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"这篇研究论文探讨了在非超导PrBa2Cu3O7薄膜中通过界面载流子注入触发的超导转变现象。实验在SrTiO3衬底上的PrBa2Cu3O7/Nb掺杂SrTiO3异质结构中进行，观察到随着注入电流的增加，原本非超导的薄膜逐渐转变为超导状态，同时超导转变温度也相应提升。这种现象揭示了界面载流子注入对氧化物超导性的影响，可能为提高高温超导体的临界温度提供新的策略。电流-电压特性分析显示多种传导机制共同作用，其中注入的电子可能被困在不稳定的准导带中，进而改变PrBa2Cu3O7薄膜的基本性质。这一发现对于理解和操控氧化物超导状态具有重要意义，并可能深化我们对高温超导机制的理解。"
本文的研究工作集中在高温超导材料领域，具体而言是铜酸盐超导体PrBa2Cu3O7。自从1986年在液氮温度以上的铜酸盐中发现高温超导性以来，科学家们一直在寻求提高其超导转变温度的方法。近年来，界面工程成为改善超导性能的新策略，比如在SrTiO3衬底上的FeSe薄膜中观察到的界面增强的高温超导性。
在这个实验中，研究者通过在PrBa2Cu3O7薄膜与Nb掺杂的SrTiO3之间引入界面载流子，观察到了一个非超导到超导的转变过程。随着注入电流的增加，这种转变不仅发生，而且转变温度也有所提高。这表明界面载流子注入可以有效调控材料的超导特性，可能为设计新型高温超导材料提供新的思路。
在电流-电压特性研究中，发现系统中存在多种传导机制，这暗示着复杂的电荷输运过程。作者提出，注入的电子可能被吸附在材料的不稳定准导带中，导致薄膜的电阻和超导性质发生变化。这一现象提示我们，界面载流子注入能够直接干预氧化物超导体的内在属性，为深入探索高温超导的物理机制提供了新的实验依据。
这项工作不仅揭示了界面载流子注入对PrBa2Cu3O7薄膜超导性的影响，也为未来的研究指出了一个方向，即通过操纵氧化物界面的载流子注入来控制和优化超导材料的性能。这对于推动高温超导技术的实际应用，如高效电力传输、磁悬浮列车以及量子计算等领域，具有深远的意义。
superconductivity above 90 K in YBa
2
Cu
3
O
7
[5],breaking
through the liquid n itrogen ‘thermal barrier’ (77 K),was
a great step towards app lica tion s. I n recent years, a family
of high-tem pera ture su per con du ctin g comp ound s of the
form RBa
2
Cu
3
O
7
(R =Y or a rare earth atom) ,suchas
GdBa
2
Cu
3
O
7
[6] and HoBa
2
Cu
3
O
7
[7] with transition
temperature (T
c
) 90 K, has attracted more and more
attention. However, PrBa
2
Cu
3
O
7
(PBCO) appears to be an
exception, and its supercondu c tivity remains controversial
[8–11].
Indeed, most PBCO samples have shown non-super-
conducting and non-metallic behaviors, in strong contrast to
the other RBa
2
Cu
3
O
7
samples. Chemically doped samples of
Y
1–x
Pr
x
Ba
2
Cu
3
O
7–δ
exhibit superconducting behavior while
the superconducting transition temperature T
c
decreases with
increasing Pr content and the superconductivity eventually
disappears when x>0.6 [12]. PBCO is neither metallic nor
superconducting, but its crystallographic structure is similar
to that of superconducting YBa
2
Cu
3
O
7
with almost the same
lattice parameters, so it is often used as a buffer film between
the superconducting film and substrate. It can provide a
potential barrier in superconductor–insulator–superconductor
tunneling Josephson junctions for applications in quantum
computing [13], superconductor–quantum dot devices [14]
and superconducting terahertz radiation devices [15].
It is well known that there are two characteristics that
define superconductors: zero resistance and the Meissner
effect [16]. According to the Bardeen, Cooper and Schrieffer
theory [17, 18], superconductivity in many metals and alloys
(such as Hg, Pb, NbTi, Nb
3
Sn and Nb
3
Ge) can be attributed
to electron–phonon coupling and the formation of Cooper
pairs. Many theoretical studies [13, 17, 19] have reported that
T
c
depends on the carrier (electrons and holes) concentration
in the superconductor. Over the past few decades a formid-
able number of different approaches for regulating T
c
in the
superconducting cuprates have been explored [20–23]. Most
of them focus on the structural distortions induced by spin
current injection [24] or chemical doping [12]. However, the
sample preparation processes in these methods are complex
and not easily controlled. Additionally, most of these studies
on superconducting films just focused on the superconducting
properties, such as regulating T
c
of an original super-
conductor, not on the conductivity mechanism (a non-super-
conductor turning into a superconductor).
On the other hand, it was recently reported that one-unit-
cell FeSe film is an electronic superconductor, and its physical
phase diagram is similar to that of the high-temperature
superconducting cuprates. T
c
of polycrystalline bulk FeSe is
∼8K [25], while T
c
of one-unit-cell FeSe film on SrTiO
3
could be able to break the 100 K barrier [26]. The interface
between FeSe and SrTiO
3
may contribute to the realization of
superconductivity in terms of carrier or electron–phonon
coupling [27–29]. Moreover, the interfacial effect has been
proved to play a crucial role in the enhancement of T
c
of
FeSe. However, the interfacial effects in high-temperature
superconducting cuprates have not yet been explored.
In this work, we present a promising approach for con-
trolling the injection of carriers through the interface of the
PBCO/Nb-doped SrTiO
3
heterostructure to realize a super-
conducting transition. More interestingly, T
c
of the PBCO
film can be tuned by the injection current. The mechanism of
resistance switching and the origin of the superconducting
transition are also discussed.
2. Experimental methods
A polycrystalline PBCO ceramic target was synthesized by
standard solid-state reaction techniques using appropriate
molar quantities of Pr
6
O
11
, BaCO
3
and CuO. These powders
were mixed and reacted in an alumina crucible at 900 °C for
24 h in an oxygen ambient. Samples were repeatedly heated
and analyzed by x-ray diffraction (XRD) until a single-phase
diffraction pattern was obtained. A 25.4 mm diameter disc
was made by pressing the PBCO powder. The sintering
process on the pressed ceramic disc was carried out by a final
heat treatment at 950 °C for 24 h. The c-axis-oriented epi-
taxial PBCO layer with a thickness of about 270 Å was
deposited on the Nb-doped SrTiO
3
(001)(NSTO, 0.7 wt%)
single-crystalline substrate by pulsed laser deposition ( PLD)
using a KrF excimer laser (λ=248 nm, repetition rate
∼3 Hz, energy density ∼2Jcm
−2
). The NSTO substrate
exhibits a conductivity three orders of magnitude larger than
that of undoped STO. As a deep work-function metal and an
n-type semiconductor, NSTO is usually used as a bottom
electrode [30], as in our work. A schematic diagram and
details of the preparation process of the PBCO layer are
shown in figure S1 in the supporting information (available
online at stacks.iop.org/SUST/33/025016/mmedia). The
deposition temperature and oxygen partial pressure during
film deposition are 800 °C and 150 mTorr, respectively. The
In top electrode was sputtered on the PBCO film to a thick-
ness of ∼7 nm using a mask, because the contact between In
and the PBCO film is ohmic. In order to protect the In top
electrode, Au was sputtered on top of the In with a thickness
of 150 nm; the transmission electron microscopy (TEM) cross
section of the Au/In/PBCO/NSTO structure is shown in
figure 2(c). The crystal structure of the PBCO films was
measured by XRD using a Bruker D8 diffractometer. The
microstructure of the PBCO/NSTO heterostructure was also
investigated by TEM (JEOL JEM-2100F; operated at 200 KV
with a lattice resolution of 0.23 nm). Image analysis was
carried out using Gatan Digital Micrograph software. Cross-
sectional samples for TEM were prepared using the focused
ion beam (FIB) technique parallel to the (001) plane of
NSTO. A FIB photo of a TEM sample of PBCO/NSTO can
be found in figure S2.
3. Results and discussion
The θ–2θ scan of the PBCO/NSTO heterostructure is shown
in figure 1. It can be seen from figure 1 that the only the
PBCO (001) and (002) peaks were observed, indicating that
the PBCO film shows a single phase (PDF=47–0164) and
good c-axis-oriented growth without any impurity phases.
2
Supercond. Sci. Technol. 33 (2020) 025016 LWeiet al
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