1 Engineered Kondo screening and nonzero Berry phase in SrTiO 3LaTiO 3SrTiO 3 heterostructures

2025-04-30 0 0 606.15KB 13 页 10玖币

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Engineered Kondo screening and nonzero Berry phase in

SrTiO3/LaTiO3/SrTiO3 heterostructures

Fang Yang1,*, Zhenzhen Wang1,2, Yonghe Liu1,2, Shuai Yang1, Ze Yu1,2, Qichang An1, Zhaoqing

Ding1,2, Fanqi Meng1, Yanwei Cao3, Qinghua Zhang1, Lin Gu4, Miao Liu1, Yongqing Li1, Jiandong

Guo1,2,5,*, Xiaoran Liu1,*

1. Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of

Sciences, Beijing 100190, P. R. China

2. School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China

3. Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201,

China

4. Beijing National Center for Electron Microscopy and Laboratory of Advanced Materials, Department of

Materials Science and Engineering, Tsinghua University, Beijing 100084, China

5. Songshan Lake Materials Laboratory, Dongguan 523808, China

Abstract

Controlling the interplay between localized spins and itinerant electrons at the oxide

interfaces can lead to exotic magnetic states. Here we devise SrTiO3/LaTiO3/SrTiO3

heterostructures with varied thickness of the LaTiO3 layer (n monolayers) to investigate the

magnetic interactions in the two-dimensional electron gas system. The heterostructures exhibit

significant Kondo effect when the LaTiO3 layer is rather thin (n = 2, 10), manifesting the strong

interaction between the itinerant electrons and the localized magnetic moments at the interfaces,

while the Kondo effect is greatly inhibited when n = 20. Notably, distinct Shubnikov-de Haas

oscillations are observed and a nonzero Berry phase of π is extracted when the LaTiO3 layer is

rather thin (n = 2, 10), which is absent in the heterostructure with thicker LaTiO3 layer (n = 20).

The observed phenomena are consistently interpreted as a result of sub-band splitting and

symmetry breaking due to the interplay between the interfacial Rashba spin-orbit coupling and

the magnetic orderings in the heterostructures. Our findings provide a route for exploring and

manipulating nontrivial electronic band structures at complex oxide interfaces.

Introduction

The complex oxide interface has been revealed as a remarkable playground for the

exploration of intriguing collective electronic and magnetic phenomena [

]. In addition to the

lattice, charge, orbital, and spin degrees of freedom inherited from constituent materials, unique

opportunities can be provided at the coherently formed interfaces, triggering emergent or

hidden properties inaccessible in bulk compounds [

]. In particular, at an oxide interface with

intrinsic inversion symmetry breaking, the Rashba spin-orbit coupling (SOC) is naturally

induced due to the potential drop across the interface, lifting the spin degeneracy of the d

electrons and locking the spins to the linear momentum. Such a “Rashba spin splitting” scenario

has been recognized as playing key roles in a vast variety of developing fields in spintronics,

orbitronics, and topological quantum states of matter [

As one of the most representative phenomena, the two-dimensional electron gases (2DEGs)

at complex oxide interfaces have attracted tremendous attention over the past 20 years [

Unlike the situations in conventional II-VI or III-V semiconductor quantum wells with s or p

mobile electrons, 2DEGs with the correlated d electrons have exhibited fascinating emergent

features including superconductivity [

], ferromagnetism [

], Kondo effect [

], significant

charge transfer [

], spin and orbital polarizations [

]. Recently, more interest has been focused

on investigating and manipulating the effect of the Rashba SOC in these systems [

] .

Specifically, Caviglia et al. reported the discovery of a large Rashba SOC at the interface of

LaAlO3/SrTiO3, whose magnitude can be effectively modulated by an external gate voltage [20].

Herranz et al. revealed that the Rashba SOC and the corresponding band splitting at

LaAlO3/SrTiO3 interface is tunable via the selection of the crystal orientations [21]. Lin et al.

fabricated asymmetric LaAlO3/SrTiO3/LaAlO3 quantum wells with opposite polar

discontinuities at the top and bottom interfaces, and found a transition from the cubic Rashba

effect to the coexistence of linear and cubic Rashba effects [24]. Very recently, Li et al. studied

systematically the effect of Rashba SOC at RAlO3/SrTiO3 (R = La, Pr, Nd, Sm, and Gd)

interfaces [25].

Compared to LaAlO3, LaTiO3 exhibits more complicated features from several aspects.

First, LaTiO3 is a Mott insulator (Ti3+, 3d1) with a small gap of only ~0.1 eV [

]. Second, it

undergoes a G-type antiferromagnetic (AFM) phase transition below ~146 K [

] . Third,

unlike the termination-dependent doping of SrTiO3 (p- or n-type) at the LaAlO3/SrTiO3

interface, the 3d electrons are intrinsically transferred from LaTiO3 to SrTiO3 at their interfaces,

leading to remarkably enhanced amounts of localized magnetic moments [

] . Thereby, the

LaTiO3-based 2DEG system is an ideal platform to understand the role of interfacial Rashba

SOC in the magnetic interactions, as well as how it affects the topology of the low-dimensional

electronic band structures.

In this work, we synthesized a set of (001)-oriented SrTiO3/LaTiO3/SrTiO3

heterostructures with the thickness of LaTiO3 layers (n monolayers) varying from 2 to 20

monolayer (ML) and investigated their electronic and magnetic properties via various transport

measurements. Distinct Kondo screening of the localized moments by the mobile carriers is

observed in samples with ultrathin LaTiO3 layer (n = 2 and 10), whereas the effect is much

suppressed when the LaTiO3 layer is sufficiently thick (n = 20). The latter indicates the

formation of the bulk-like AFM ordering by which the localized moments are pinned or

polarized, resulting in the suppression of scattering. More intriguingly, samples with ultrathin

LaTiO3 layer exhibit a nonzero Berry phase of π of the conductive electrons as deduced from

the Shubnikov-de Haas (SdH) oscillations, which becomes zero in thick one. These phenomena

can be consistently interpreted as a result of the interplay of the Rashba spin splitting scenario

plus the breaking of the time-reversal symmetry in the heterostructures. These findings open

new pathways for engineering the Rashba SOC and manipulating the nontrivial band topology

at low-dimensional systems.

Results

The SrTiO3/LaTiO3/SrTiO3 heterostructures [see Fig. 1(a)] were fabricated on SrTiO3 (001)

substrates by pulsed laser deposition equipped with in-situ reflection high energy electron

diffraction (RHEED). The thickness of the LaTiO3 layers n was varied from 2 to 20 MLs, while

the thickness of the SrTiO3 quantum wells on both sides are fixed at 10 MLs. Before the

deposition, TiO2-terminated SrTiO3 (001) substrates were prepared by etching in buffered

hydrofluoric acid for 45 s, followed by annealing in an oxygen atmosphere at 950 ℃ for 2 hours.

The RHEED pattern of the treated SrTiO3 substrate is displayed in Fig. 1(b) inset (left panel).

To prevent from oxidizing LaTiO3 to La2Ti2O7 or any other oxygen-rich phases, we grew the

LaTiO3 layers under a high vacuum condition with the pressure of ~10-7 mbar at a substrate

temperature of ~720℃ measured by a pyrometer. The heterostructures were established in the

layer-by-layer growth mode, as evidenced by the distinct oscillations of RHEED intensity

shown in Fig. 1b. This allows us to precisely control the thickness of each compound in the

heterostructures by counting the number of oscillations during the deposition process. The

RHEED pattern after the deposition [see right panel of the inset in Fig. 1(b)], which exhibits

practically the same pattern as the substrate, indicates the establishment of flat and well-

crystallized surfaces of the films. The morphology of the heterostructures is further revealed by

atomic force microscopy imaging [Fig. 1(c)], which demonstrates the formation of high-quality

films with well-defined steps and terraces on the surface.

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1EngineeredKondoscreeningandnonzeroBerryphaseinSrTiO3/LaTiO3/SrTiO3heterostructuresFangYang1,*,ZhenzhenWang1,2,YongheLiu1,2,ShuaiYang1,ZeYu1,2,QichangAn1,ZhaoqingDing1,2,FanqiMeng1,YanweiCao3,QinghuaZhang1,LinGu4,MiaoLiu1,YongqingLi1,JiandongGuo1,2,5,*,XiaoranLiu1,*1.BeijingNationalLaboratoryforCond...

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1 Engineered Kondo screening and nonzero Berry phase in SrTiO 3LaTiO 3SrTiO 3 heterostructures

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