Detection of two -dimensional small polarons at oxide interfaces by optical spectroscopy Chi Sin Tang123 Shengwei Zeng4 Jing Wu3 Shunfeng Chen1 Muhammad A. Naradipa4_2

2025-05-06 0 0 1.47MB 19 页 10玖币

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Detection of two-dimensional small polarons at oxide interfaces by optical

spectroscopy

Chi Sin Tang,1,2,3 Shengwei Zeng,4 Jing Wu,3 Shunfeng Chen,1 Muhammad A. Naradipa,4

Dongsheng Song,5 M. V. Milošević6, Ping Yang,2 Caozheng Diao,2 Jun Zhou,3 Stephen J.

Pennycook,7,9 Mark B.H. Breese,2,4 Chuanbing Cai,1 Thirumalai Venkatesan,10 Ariando

Ariando,4 Ming Yang,8,* Andrew T.S. Wee,4,9,* Xinmao Yin 1,*

Affiliations:

1Shanghai Key Laboratory of High Temperature Superconductors, Department of Physics,

Shanghai University, Shanghai 200444, China

2Singapore Synchrotron Light Source (SSLS), National University of Singapore, Singapore

117603

3Institute of Materials Research and Engineering, A∗STAR (Agency for Science, Technology

and Research), 2 Fusionopolis Way, Singapore, 138634 Singapore

4Department of Physics, Faculty of Science, National University of Singapore, Singapore

117542

5Institute of Physical Science and Information Technology, Anhui University, Hefei 230601,

China

6Departement Fysica, Universiteit Antwerpen, Groenenborgerlaan 171, B-2020 Antwerpen,

Belgium

7Department of Materials Science and Engineering, National University of Singapore,

Singapore, 117575, Singapore

8Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong

Kong, China

9Centre for Advanced 2D Materials and Graphene Research, National University of Singapore,

Singapore 117546

10Center for Quantum Research and Technology, University of Oklahoma, Norman, Oklahoma

73019, USA

*Correspondence to: kevin.m.yang@polyu.edu.hk (M.Y.), phyweets@nus.edu.sg

(A.T.S.W)， yinxinmao@shu.edu.cn (X.Y.)

Two-dimensional (2D) perovskite oxide interfaces are ideal systems to uncover diverse

emergent properties such as the arising polaronic properties from short-range charge-

lattice interactions. Thus, a technique to detect this quasiparticle phenomena at the

buried interface is highly coveted. Here, we report the observation of 2D small-polarons

at the LaAlO3/SrTiO3 (LAO/STO) conducting interface using high-resolution

spectroscopic ellipsometry. First-principles investigations shows that interfacial electron-

lattice coupling mediated by the longitudinal phonon mode facilitates the formation of

these polarons. This study resolves the longstanding question by attributing the formation

of interfacial 2D small polarons to the significant mismatch between experimentally

measured interfacial carrier density and theoretical values. Our study sheds light on the

complexity of broken periodic lattice-induced quasi-particle effects and its relationship

with exotic phenomena at complex oxide interfaces. Meanwhile, this work establishes

spectroscopic ellipsometry as a useful technique to detect and locate optical evidence of

polaronic states and other emerging quantum properties at the buried interface.

Introduction

The emergence of quasiparticles due to the interplay between electronic and lattice degrees of

freedom in strongly-correlated systems is the cornerstone of multiple fundamental phenomena

including transport processes, colossal magnetoresistance1 and high-temperature

superconductivity2. Polarons are an ideal example where motion of mobile charges are retarded

due to strong charge-lattice interaction 3. Though well-established in 3D systems4, polaronic

formation and modifications to their properties by reduced dimensionality in 2D systems

remains challenging due to the incomplete understanding of polaron self-trapping in

anisotropic structures5. Nevertheless, the key role of strongly-bound electronic polarons and

bipolarons which possess lower-temperature coherence could hold the key in understanding

many-body phenomena such as itinerant ferromagnetism6 and unconventional Cooper pairing

in superconductivity7, 8. Such prospects are tantalizingly attractive especially with reports of

2D polaronic behaviour4, 5 alongside their causality with 2D superconductivity9. The effective

identification of polaronic activities and the underlying mechanisms leading to their formation

play a critical role in assessing the distinguish the short- and long-range transport

characteristics and sheds new light on the nature of quasiparticle interactions in diverse material

systems4. Such knowledge holds immediate relevance especially in the functionalization of

novel quantum systems in applications over a wide range of domains related to electronic

transport10, 11, photocatalysis12 and energy storage devices13.

Recent reports of 2D perovskite oxide superconductivity at the LaAlO3/KTaO3 interface have

renewed fresh interests in the emergence of anomalous quantum metallic states14, 15. The

quintessential LaAlO3/SrTiO3 (LAO/STO) conducting interface remains the ideal candidate

for such investigation especially with the versatility and capacity to effectively manipulate its

charge, spin, lattice, and orbital degrees-of-freedom at the nanometer-thickness interface16.

Furthermore, insights to the interfacial many-body charge dynamics in the region ~10 meV

within the Fermi level is pivotal to explain the severe discrepancy between the theoretically

predicted 0.5 electrons per unit cell (e-/uc) charge transfer by multiple models10, 17, 18, as

compared to a meagre 0.05 e-/uc elucidated by experiments19-21.

Here, we make the observation of small-polarons at the LAO/STO interface using high-

resolution spectroscopic ellipsometry, a highly sensitive non-destructive photon-in photon-out

optical technique with a spectral resolution of 0.02 eV which, with its full polarization and

symmetry features, can explore the role of electron-phonon coupling and effectively resolve

the anisotropic properties of the LAO/STO system and extract the optical properties of its

interfacial 2D electron gas. This is in light of the progress in the experimental techniques to

characterize polaronic responses, that while there were initially no evidence of sufficiently

large electron-phonon coupling to induce small polaron formation in Nb-doped STO22, a

subsequent experimental study then suggested polaronic correlations in the LAO/STO system

which possesses strongly frequency-dependent mobility similar to that of Nb-doped STO23.

Even though significant breakthrough was then made with the observation of large polaron at

the LAO/STO interface using angle resolved photoemission spectroscopy (ARPES)10, it can

only study the interfacial properties in the vicinity of the fermi level. The unique properties of

spectroscopic ellipsometry allows one to study the electronic properties and quasiparticle

dynamics of systems into the near infrared, visible and ultraviolet regime24-27 that provide

evidence in identifying the optical response of the small polarons presented at the 2D

LAO/STO interface. First-principles calculations further suggest the strong coupling between

the interfacial electrons and the Ti-lattice as the key mechanism leading to the formation of the

localized small polarons which is 2D in nature. Based on an integrated analysis of our

experimental results and findings from the first-principle studies, the hard longitudinal optical

phonon mode, LO3, which has previously been identified as the key mediator in the formation

of large polarons10, 13, has also been determined to play a critical role in the formation of the

2D small-polarons. Importantly, by showing that ~50% of the interfacial charges couple

strongly with the Ti lattice sites to form highly localized 2D small polarons [Fig. 1(a)], this

study effectively resolved the longstanding question why the experimentally-measured

interfacial carrier density is significantly lower than theoretically predicted values10, 17, 18. In

light of how quasiparticle dynamics governs superconductivity at perovskite oxide interfaces,

our study further highlights the strong interactions between excess interfacial electrons and Ti

lattices that results in the polaronic states where lattice distortion invariably breaks the periodic

lattice symmetry4. This study is therefore potentially analogous to other heterostructure

systems such as that of magic-angle twisted bilayer graphene where their superconductive

states could be attributed to the many-body correlations induced by broken periodic lattice

symmetry28-32. With the escalating challenge to characterize emergent quantum orders in

complex low-dimensional systems, this study further highlights spectroscopic ellipsometry as

the premier experimental methodology to unambiguously identify and analyse anisotropic

quasiparticle dynamics and other emergent order-parameter nanostructures at the buried

complex quantum interfaces.

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Detectionoftwo-dimensionalsmallpolaronsatoxideinterfacesbyopticalspectroscopyChiSinTang,1,2,3ShengweiZeng,4JingWu,3ShunfengChen,1MuhammadA.Naradipa,4DongshengSong,5M.V.Milošević6,PingYang,2CaozhengDiao,2JunZhou,3StephenJ.Pennycook,7,9MarkB.H.Breese,2,4ChuanbingCai,1ThirumalaiVenkatesan,10AriandoAria...

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Detection of two -dimensional small polarons at oxide interfaces by optical spectroscopy Chi Sin Tang123 Shengwei Zeng4 Jing Wu3 Shunfeng Chen1 Muhammad A. Naradipa4_2

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