1 Rotational symmetry breaking in superconducting nickelate Nd 0.8Sr0.2NiO 2 films Haoran Ji1 Yanan Li1 Yi Liu2 Xiang Ding3 Zheyuan Xie1 Shichao Qi1 Liang

2025-04-28 0 0 3.26MB 36 页 10玖币

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Rotational symmetry breaking in superconducting nickelate Nd0.8Sr0.2NiO2 films

Haoran Ji1#, Yanan Li1#, Yi Liu2#, Xiang Ding3#, Zheyuan Xie1, Shichao Qi1, Liang

Qiao3*, Yi-feng Yang4,5,6, Guang-Ming Zhang7,8 and Jian Wang1,9,10,11*

1International Center for Quantum Materials, School of Physics, Peking University,

Beijing 100871, China

2Department of Physics, Renmin University of China, Beijing 100872, China

3School of Physics, University of Electronic Science and Technology of China,

Chengdu 610054, China

4Beijing National Lab for Condensed Matter Physics and Institute of Physics, Chinese

Academy of Sciences, Beijing 100190, China

5School of Physical Sciences, University of Chinese Academy of Sciences, Beijing

100049, China

6Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China

7State Key Laboratory of Low-Dimensional Quantum Physics and Department of

Physics, Tsinghua University, Beijing 100084, China.

8Frontier Science Center for Quantum Information, Beijing 100084, China

9Collaborative Innovation Center of Quantum Matter, Beijing 100871, China

10CAS Center for Excellence in Topological Quantum Computation, University of

Chinese Academy of Sciences, Beijing 100190, China

11Beijing Academy of Quantum Information Sciences, Beijing 100193, China

#These authors contribute equally: Haoran Ji, Yanan Li, Yi Liu and Xiang Ding.

*Corresponding author: jianwangphysics@pku.edu.cn (J.W.), liang.qiao@uestc.edu.cn

(L.Q.)

The infinite-layer nickelates, isostructural to the high-Tc superconductor cuprates,

have risen as a promising platform to host unconventional superconductivity and

stimulated growing interests in the condensed matter community. Despite

numerous researches, the superconducting pairing symmetry of the nickelate

superconductors, the fundamental characteristic of a superconducting state, is still

under debate. Moreover, the strong electronic correlation in the nickelates may

give rise to a rich phase diagram, where the underlying interplay between the

superconductivity and other emerging quantum states with broken symmetry is

awaiting exploration. Here, we study the angular dependence of the transport

properties on the infinite-layer nickelate Nd0.8Sr0.2NiO2 superconducting films

with Corbino-disk configuration. The azimuthal angular dependence of the

magnetoresistance (R(φ)) manifests the rotational symmetry breaking from

isotropy to four-fold (C4) anisotropy with increasing magnetic field, revealing a

symmetry breaking phase transition. Approaching the low temperature and large

magnetic field regime, an additional two-fold (C2) symmetric component in the

R(φ) curves and an anomalous upturn of the temperature-dependent critical field

are observed simultaneously, suggesting the emergence of an exotic electronic

phase. Our work uncovers the evolution of the quantum states with different

rotational symmetries and provides deep insight into the global phase diagram of

the nickelate superconductors.

The conventional superconductivity with transition temperature (Tc) lower than 40 K

was successfully explained by the Bardeen-Cooper-Schrieffer (BCS) theory, in which

the electrons with anti-parallel spins and time-reversed momenta form Cooper pairs,

and the superconducting order parameter is of isotropic s-wave symmetry 1,2. However,

the discovery of high-temperature superconductivity (Tc > 40 K) in cuprates is beyond

the expectation of the BCS theory, and the superconducting order parameters of

cuprates are believed to be of nodal d-wave symmetry3,4. Thereafter, the mechanism of

unconventional high-Tc superconductivity has become one of the most important

puzzles in physical sciences. Recently, the observation of superconductivity in infinite-

layer nickelates with a maximal Tc of 15 K in Nd1-xSrxNiO2 has motivated extensive

3 
 
researches in this emerging new superconducting family5-9. Mimicking the d9 electronic 
configuration  and  the  layered  structure  including  CuO2  planes  of  the  cuprates,  the 
isostructural  infinite-layer  nickelates  are  promising  candidates  for  high-Tc 
unconventional  superconductivity5-9.  Discerning  the  similarities  and  the  differences 
between  the  nickelates  and  the  cuprates,  especially  in  the  symmetry  of  the 
superconducting order parameters, should be of great significance for understanding 
the mechanism of unconventional high-Tc superconductivity.   
Theoretical calculations have suggested that the nickelates are likely to give rise to a 
d-wave superconducting pairing, analogous to the cuprate superconductors. However, 
the consensus has not been reached and there are several proposals, including dominant 
dx2-y2-wave10,11, multi-band d-wave12,13, and even a transition from s-wave to (d+is)-
wave  and  then  to  d-wave  depending  on  the  doping  level  and  the  electrons  hoping 
amplitude14.  Experimentally,  through  the  single-particle  tunneling  spectroscopy, 
different spectroscopic features showing s-wave, d-wave, and even a mixture of them 
are observed on different locations of the nickelate film surface, which complicates the 
determination  of  the  pairing  symmetry  in  the  nickelates15. The  London  penetration 
depth of the nickelates family are also measured, and the results on La-based and Pr-
based nickelate compounds support the existence of a d-wave component16,17. However, 
the Nd-based nickelate, Nd0.8Sr0.2NiO2, exhibits more complex behaviors that may be 
captured by a predominantly isotropic nodeless pairing16,17. The pairing symmetry of 
the superconducting order parameter in the nickelate superconductors, the fundamental 
characteristic  of  the  superconducting  state,  is  still  an  open  question,  thus  further 
explorations with diverse experimental techniques are highly desired. 
In addition to the mystery  of  the superconducting pairing symmetry,  the strong 
electronic correlation in nickelates is another element that makes the nickelate systems 
intriguing. The strong correlation is theoretically believed to play an important role in 
the  nickelates  systems8,9,18,19  and  the  strong  antiferromagnetic  (AFM)  exchange 
interaction between Ni spins has been experimentally detected20. Generally, the strong 
correlated electronic systems are anticipated to host a rich phase diagram and multiple 

competing states including superconductivity, magnetic order, charge order, pair

density wave (PDW), etc21,22. In the nickelate thin films, the charge order, a spatially

periodic modulation of the electronic structure that breaks the translational symmetry,

has been experimentally observed by the resonant inelastic X-ray scattering (RIXS)23-

25. However, the charge order is only observable in the lower doping regime where the

nickelates are non-superconducting. The interplay between the superconductivity and

the charge order as well as other underlying symmetry-broken states is still awaiting

explorations.

Fig. 1 | Structure and the quasi-two-dimensional superconductivity in

Nd0.8Sr0.2NiO2. a, Crystal structure of the infinite-layer nickelate Nd0.8Sr0.2NiO2. b,

Temperature dependence of the resistance R(T) at zero magnetic field from 2 K to 300

K. The inset shows the R(T) curves below 20 K at 0 T (black circles), B∥ = 16 T (red

circles), and B⊥ = 16 T (purple circles). Here, the B∥ is applied along the a/b-axis and

B⊥ is along the c-axis. The blue solid line represents the BKT transition fitting using

the Halperin-Nelson equation. c, Schematic image and optical photo (inset) of the

Corbino-disk configuration for polar (θ) angular dependent magnetoresistance R(θ)

measurements on the Nd0.8Sr0.2NiO2 thin film. Here, θ represents the angle between the

magnetic field and the c-axis of the Nd0.8Sr0.2NiO2. d, Temperature dependence of the

critical magnetic field Bc(T) for the magnetic fields along the c-axis (denoted as ⊥),

the a/b-axis (∥, 0⁰), and the ab diagonal direction (∥, 45⁰). Here, the Bc is defined as

the magnetic field corresponding to 50% normal resistance. The blue and the orange

solid lines are the 2D G-L fittings of the Bc(T) data near Tc. e, Polar angular dependence

of the critical magnetic field Bc(θ) at T = 6 K. The inset shows a close-up of the Bc(θ)

around θ = 90⁰. The red solid line and the blue solid line are the fittings with the 2D

Tinkham model (Bc(θ)sin(θ)/Bc∥)2+|Bc(θ)cos(θ)/Bc⊥| = 1 and the 3D anisotropic

mass model B𝑐(θ) = Bc∥/(sin2(θ)+γ2cos2(θ))1/2 with γ = Bc∥/Bc⊥, respectively. f, g,

Representative polar angular dependence of the magnetoresistance R(θ) at temperature

T = 5 K under B = 12 T (f) and B = 8 T (g).

With these motivations, we investigate the polar (θ) and azimuthal (φ) angular

dependence of the critical magnetic field and the magnetoresistance of the infinite-layer

nickelate Nd0.8Sr0.2NiO2 superconducting films. The perovskite precursor

Nd0.8Sr0.2NiO3 thin films are firstly deposited on the SrTiO3 (001) substrates by pulsed

laser deposition (PLD). The apical oxygen is then removed by the soft-chemistry

topotactic reduction method using CaH2 power. Through this procedure, the nickelate

thin films undergo a topotactic transition from the perovskite phase to the infinite-layer

phase, and thus the superconducting Nd0.8Sr0.2NiO2 thin films are obtained5. Figure 1a

presents the schematic crystal structure of Nd0.8Sr0.2NiO2. In agreement with the

previous reports5, the temperature-dependence of the resistance R(T) exhibits metallic

behavior from room temperature to low temperature followed by a superconducting

transition beginning at Tc

onset of 14.7 K (Fig. 1b). Here, Tc

onset is determined at the

point where R(T) deviates from the extrapolation of the normal state resistance (RN).

Note that the R(T) curve shows a considerably broad superconducting transition with a

smooth tail, which can be described by the Berezinskii-Kosterlitz-Thouless (BKT)

transition in two-dimensional (2D) superconductors26-29 . As shown in the inset of Fig.

1b, the R(T) curve under 0 T can be reproduced by the BKT transition using the

Halperin-Nelson equation30, 𝑅 = R0exp [-2b(Tc

'-TBKT

T-TBKT )1/2] (R0 and b are material-

dependent parameters, and Tc

' is the superconducting critical temperature), yielding

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1RotationalsymmetrybreakinginsuperconductingnickelateNd0.8Sr0.2NiO2filmsHaoranJi1#,YananLi1#,YiLiu2#,XiangDing3#,ZheyuanXie1,ShichaoQi1,LiangQiao3*,Yi-fengYang4,5,6,Guang-MingZhang7,8andJianWang1,9,10,11*1InternationalCenterforQuantumMaterials,SchoolofPhysics,PekingUniversity,Beijing100871,China2Dep...

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1 Rotational symmetry breaking in superconducting nickelate Nd 0.8Sr0.2NiO 2 films Haoran Ji1 Yanan Li1 Yi Liu2 Xiang Ding3 Zheyuan Xie1 Shichao Qi1 Liang

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