Nonvolatile and Reversible Ferroelectric Control of Electronic

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Nonvolatile and Reversible Ferroelectric Control of Electronic Properties of Bi2Te3 Topological Insulator Thin Films Grown on Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals Jian-Min Yan, Zhi-Xue Xu, Ting-Wei Chen, Meng Xu, Chao Zhang, Xu-Wen Zhao, Fei Liu, Lei Guo, Shu-Ying Yan, Guan-Yin Gao, Fei-Fei Wang, Jin-Xing Zhang, Sining Dong, Xiao-Guang Li, Hao-Su Luo, Weiyao Zhao, and Ren-Kui Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20406 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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ACS Applied Materials & Interfaces

Materials, ISEM Zheng, Ren-Kui; School of Materials Science and Engineering, Nanchang University, and Jiangxi Engineering Laboratory for Advanced Functional Thin Films; Shanghai Institute of Ceramics, Chinese Academy of Sciences

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Nonvolatile and Reversible Ferroelectric Control of Electronic Properties of Bi2Te3 Topological Insulator Thin Films Grown on Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals Jian-Min Yan, † Zhi-Xue Xu, † Ting-Wei Chen, ‡, Meng Xu, † Chao Zhang, § Xu-Wen Zhao, † Fei Liu, † Lei Guo,1 Shu-Ying Yan, # Guan-Yin Gao, § Fei-Fei Wang,  Jin-Xing Zhang, # Si-Ning Dong, *, Xiao-Guang Li, § Hao-Su Luo, † Weiyao Zhao, *,†,☩ and Ren-Kui Zheng*,‡, † State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China †

School of Materials Science and Engineering, Nanchang University, and Jiangxi Engineering Laboratory for Advanced Functional Thin Films, Nanchang 330031, China ‡

§

Hefei National Laboratory for Physical Sciences at the Microscale, Department of Physics,

and Collaborative Innovation Center of Advanced Microstructures, University of Science and Technology of China, Hefei 230026, China #



Department of Physics, Beijing Normal University, Beijing 100875, China

Key Laboratory of Optoelectronic Material and Device, Department of Physics, Shanghai Normal University, Shanghai 200234, China 

Department of Physics, University of Notre Dame, IN46556, USA

ISEM, Innovation Campus, University of Wollongong, Wollongong, NSW 2500, Australia

☩

Abstract: Single phase (00l)-oriented Bi2Te3 topological insulator (TI) thin films have been deposited on (111)-oriented ferroelectric 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-PT) single-crystal substrates. Taking advantage of the nonvolatile polarization charges induced by the polarization direction switching of PMN-PT substrates at room temperature, the carrier density, Fermi level, magnetoconductance, conductance channel, phase coherence length, and quantum corrections to the conductance can be in situ modulated in a reversible and nonvolatile manner. Specifically, upon the polarization switching from the positively poled 1


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Pr+ state (i.e., polarization direction points to the film) to the negatively poled

Pr― (i.e.,

polarization direction points to the bottom electrode) state, both the electron carrier density and the Fermi wave vector decreases significantly, reflecting a shift of the Fermi level toward the Dirac Point. The polarization switching from Pr+ to Pr― also results in significant increase of the conductance channel α from -0.15 to -0.3 and a decrease of the phase coherence length from 200 to 80 nm at T=2 K as well as a reduction of the electron-electron interaction. All these results demonstrate that electric-voltage control of physical properties using PMN-PT as both substrates and gating materials provides a simple and a straightforward approach to realize reversible and nonvolatile tuning of electronic properties of topological thin films and may be further extended to study carrier-density-related quantum transport properties of other quantum matter. KEYWORDS: Ferroelectric field effect, Ferroelectric single crystal, Electronic properties, Topological insulator thin films, Magnetoresistance, Surface state.

1. INTRODUCTION Topological insulators (TIs) are a new state of matter with a symmetry-protected non-trivial electronic band structure in which the surface state with linear dispersion forms Dirac cones in bulk gap. Due to this unique band structure the Dirac carriers’ dominate electronic transport properties, yielding an insulating bulk and conducting boundaries in TIs. One of the milestones in the development of TIs is the prediction1 and experimental verification2-4 of three dimensional TIs of Sb2Te3, Bi2Se3 and Bi2Te3, which are nontoxic, easy to fabricate and relatively stable. These materials have strongly potential applications, such as in data storage application,5 memory and logic devices,6 Sensors,7,8 flexible optoelectronic/plasmonic devices9 and so on. As of now, the Bi2Te3 family were intensively investigated via elemental substitution,10-12 magnetic doping,13-15 epitaxial strain16 superlattice fabrication17,18 and dielectric gating10,14 to realize quantum effects related to Dirac Fermions, 2



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e.g., quantum anomalous Hall effect in Cr:(Bi,Sb)2Te3.14 Although electric-voltage control of physical properties of TIs has been successfully demonstrated by using SiN,19 SiO2,10,20 BN,21 Al2O3,22 HfO2,23 and SrTiO324,25 dielectrics as gating layers, electric-voltage-induced achievable areal carrier densities using dielectric materials are relatively small (~1012-1013/cm2) and volatile, making it impossible to achieve nonvolatile modulation of carrier density and its related properties. From the viewpoint of fundamental physics study and practical applications, it is necessary to realize reversible and nonvolatile electric-voltage-control of carrier density and its related properties of TI films at ambient temperatures. In this context, we note that (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) single crystal with PT content near the morphotropic phase boundary (MPB) (0.29x0.33) exhibits excellent ferroelectric (Pr ~30-40 C/cm2) and piezoelectric (d33=2500 pC/N) properties.26 Polished PMN-xPT single crystals with particular crystallographic orientation [e.g., (001), (011), (111)] have been used as ferroelectrically and piezoelectrically active substrates to grow a variety of functional thin films such as magnetoresistive manganites,27-29 semiconductors,30-32 iron- and copper-based superconductors,33,34 ferromagnetic metals and alloys,35,36 ferrites,37 upconversion photoluminescent38 and two-dimentional materials,39 whose electronic, magnetic, and optical properties could be effectively modulated in either a volatile or nonvolatile manner by taking advantage of the electric-field-induced lattice strain or polarization charges of PMN-xPT. Therefore, electric-voltage control of physical properties using PMN-xPT as gating materials provides a great opportunity to achieve nonvolatile manipulation of carrier density and its related properties of TI films. In this paper, we constructed TI-based ferroelectric field effect transistors (FeFETs) by growing 6-nm Bi2Te3 thin film on one-side-polished PMN-PT(111) ferroelectric single-crystal substrates and successfully realized electric-voltage-control of the carrier density,

magnetoresistance,

electron-electron

interaction,

3


Fermi

wave

vector,

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electron-electron interaction, and phase coherence length of Bi2Te3 films in a reversible and nonvolatile manner through polarization switching of PMN-PT at room temperature, thus giving a better understanding of carrier-density-related quantum transport properties of 3D TI films. 2. EXPERIMENTAL SECTION Bi2Te3 thin films were deposited on one-side polished (111)-oriented PMN-PT single-crystal substrates by the pulsed laser deposition (PLD) using a KrF excimer laser (248 nm). A high purity Bi2Te3 target was prepared by spark plasma sintering for 8 min under 50 MPa at 400 oC. The target-to-substrate distance and laser energy density and repetition rate are 7 cm, 1 J/cm2, and 1 Hz, respectively. The base pressure of the deposition chamber was better than 8.0 × 10 -5 Pa. The working pressure was set at 40 Pa with 35 SCCM (standard-state cubic centimeter per minute) Ar flowing as working gas. Film deposition was carried out at a substrate temperature of 275 oC, followed by in situ annealing for 30 min and cooled to room temperature at a rate of 5 oC/min. The crystallographic structure, crystallinity, and phase purity of Bi2Te3 films were characterized by using a PANalytical X'Pert PRO X-ray diffractometer with CuKα1 radiation (λ=1.5406 Å). For in situ XRD measurements we firstly poled the PMN-PT positively (polarization direction points to the Bi2Te3 film, referred to 𝑃𝑟+ state), then the -2 scan was measured. After that, the PMN-PT was poled negatively (polarization direction points to the bottom Ag electrode, referred to 𝑃𝑟― state), then the -2 scan was measured again. Polarization-electric field hysteresis loops of PMN-PT were measured using a Precision Multiferroic ferroelectric analyzer (Radiant Technologies, Inc. USA). Piezoresponse force microscopy (PFM) images were measured using a MFP-3D Infinity atomic force microscope (Oxford Instruments Asylum Research Inc.). Electronic transport properties of Bi2Te3 films including resistance, carrier density, carrier type, and magnetoresistance were measured using a Physical Property Measurement 4



System (PPMS-9, Quantum Design). For Bi2Te3/PMN-PT structures the conducting Bi2Te3 film and the sputtered 100-nm Ag film act as top and bottom electrodes, respectively. The initial poling and followed 180o switching of polarization direction of PMN-PT were achieved by applying a positive or a negative dc electric voltage of 330 V (6.6 kV/cm) to the PMN-PT along the thickness direction (Figure S1, Supporting Information). We particularly note here that after PMN-PT substrates had been fully poled positively (see Figure 2b) or negatively (see Figure 2c), the dc electric voltage of 330 V was immediately turned off and then followed by electronic transport measurements. Therefore, the E values associated to the definition of states 𝑃𝑟+ and 𝑃𝑟― are zero. 3. RESULTS AND DISCUSSION Figure 1a shows XRD -2 scan pattern of a Bi2Te3 (6 nm)/PMN-PT(111) heterostructure. Only PMN-PT (lll) (l=1, 2) and Bi2Te3 (00l) (l=3, 6, 15, 18, 21) diffraction peaks appear, indicating that the Bi2Te3 film is single phase and c-axis oriented. The full width at half maximum of the XRD rocking curve taken on the Bi2Te3 (0015) diffraction peak is approximately 0.95o, [inset (I) of Figure 1a], implying good crystallinity of the Bi2Te3 film. Due to the large lattice mismatch between the Bi2Te3 (0001) plane and the PMN-PT (111) plane (-22.7%) in-plane epitaxial growth of the Bi2Te3 film on the PMN-PT(111) substrate is not realized for the Bi2Te3 (6 nm)/PMN-PT(111) structure. Atomic force microscopy (AFM) image reveals that the Bi2Te3 film has a flat surface with a root mean square (RMS) roughness of 0.6 nm [inset (II) of Figure 1a]. Figure 2a shows the relative resistance change (∆𝑅/𝑅) of the 6-nm Bi2Te3 film as a function of bipolar electric field (E) applied to the Bi2Te3/PMN-PT heterostructure. Here, ∆ 𝑅/𝑅 = [𝑅(𝐸) ― 𝑅(0)]/𝑅(0) where 𝑅(0) and 𝑅(𝐸) are the resistance of the Bi2Te3 film in the absence and presence of an electric field E, respectively. The variation of the electric fields follows 0  -6.6 kV/cm  0  +6.6 kV/cm  0. We note that the electric field was always applied to the PMN-PT substrate during scanning electric fields. One can find that ∆ 5


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𝑅/𝑅 increases rapidly and reaches its maximum of ~110 % at E= -4.8 kV/cm. With further increase in E from -4.8 to -6.6 kV/cm and decrease from -6.6 to 0 kV/cm ∆𝑅/𝑅 decreases significantly from ~110 % at E= -4.8 kV/cm to ~33 % at E= 0 kV/cm, exhibiting relaxation behaviors, which will be discussed in more detail in the following sections. Nevertheless, no obvious relaxation of ∆𝑅/𝑅 was observed upon decreasing electric field from +6.6 to 0 kV/cm. For E=0 kV/cm there are high and low resistance states which are marked as 𝑅(𝑃𝑟+ ) and 𝑅(𝑃𝑟― ) states in Figure 2a, implying nonvolatile resistance modulation by electric field. This hysteresis-like resistance versus electric field loop probably arises from changes in the carrier density of the Bi2Te3 film due to the 180o switching of the polarization direction of the PMN-PT. Hall measurements [inset (II) of Figure 2a] reveal that the slopes of Hall resistance versus magnetic field curves are negative whether the PMN-PT was in the 𝑃𝑟+ or 𝑃𝑟― state, indicating that the Bi2Te3 film is a n-type material whose majority carriers are electrons. According to the theory of ferroelectric field effect the charge carrier density of the Bi2Te3 film would be modified by the negative or positive polarization charges at the top surface of PMN-PT, as schematically illustrated in Figure 2b and 2c, respectively. As shown in the inset (I) of Figure 2a, the PMN-PT single-crystal substrate has a remnant polarization of 𝑃𝑟35 C/cm2 which corresponds to a surface charge density of 2.2×1014 charges/cm2. It has been reported that the volume carrier densities (n3D) of Bi2Te3 films are usually on the order of 1018-1019 charges/cm3.40 Supposing the volume carrier density of the 6-nm Bi2Te3 film has a relatively large n3D11020/cm3, an areal carrier density of n2D6×1013/cm2 could be approximately calculated using n2D=n3Dt where t is the thickness of the film. This value is much smaller than the aforementioned surface charge density of PMN-PT, suggesting that an effective modulation of carrier density of Bi2Te3 films is achievable by using the nonvolatile polarization charges of PMN-PT. For positive poling of PMN-PT negative polarization charges appear at the top surface of PMN-PT substrate [Figure 2b]. These negative polarization charges would extract hole carriers in the Bi2Te3 film to the interface region, 6



resulting in an increase in the electron carrier density of the Bi2Te3 film (i.e., electron accumulation state). As a result, the resistance of the Bi2Te3 film decreases upon positive poling of PMN-PT. On the contrary, negative poling of PMN-PT induces positive polarization charges at the top surface [Figure 2c]. These positive polarization charges would extract electrons from the Bi2Te3 film and thus reduces the electron carrier density of the Bi2Te3 film (i.e., electron depletion state). Consequently, the resistance of the Bi2Te3 film increases upon negative poling of PMN-PT. Hall measurements [inset (II) of Figure 2a] indeed show that the slope of Hall resistance versus magnetic field curves (i.e. Hall coefficient) changes upon the polarization switching of the PMN-PT, rigorously confirming that the electron carrier density of the Bi2Te3 film has been modified by the polarization charges of the PMN-PT. We measured the out-of-plane PFM images of the PMN-PT and XRD patterns of the Bi2Te3/PMN-PT heterostructure before and after the polarization switching from 𝑃𝑟+ to 𝑃𝑟― states. The PFM images for both polarization states are shown Figure 1b, where no variation in contrast could be observed within the dotted blue and red boxes, suggesting full switching of polarization vectors from the [111] crystallographic direction (i.e., 𝑃𝑟+ state) to the [-1-1-1] crystallographic direction (i.e., 𝑃𝑟― state) of the PMN-PT. Moreover, the XRD pattern for the 𝑃𝑟+ state almost fully overlaps with that for the 𝑃𝑟― state [Figure 2d], which strongly demonstrates that the polarization switching from [111] to [-1-1-1] didn’t induce lattice strain in the Bi2Te3 film and PMN-PT substrate. The lattice strain effects thus could be excluded for explaining the resistance changes upon polarization switching. This, together with the Hall effect results, implies that the ferroelectric field effect is the only driving force for the resistance and carrier density changes of the Bi2Te3 film. Taking advantage of the electric-field-controllable nonvolatile polarization charges, we successfully achieved in situ, reversible, and nonvolatile resistance switching by applying a steam of pulse electric fields to the PMN-PT at room temperature [Figure S2, Supporting Information]. Upon the application of a negative pulse electric field to the PMN-PT, the resistance increases sharply before 7


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exhibiting significant relaxation. Such a resistance relaxation is consistent with the aforementioned non-square ∆R R~E hysteresis loops [Figure 2a], where the resistance shows considerable decay when the electric field is decreased from -6.6 to 0 kV/cm. This resistance relaxation could be related to the defect states (e.g., antisite defects, vacancy defects) in the Bi2Te3 film, which is reported to reduce the effectiveness of the ferroelectric field effect.41,42 As discussed above, the application of a negative pulse electric field to the PMN-PT induces positive polarization charges on the top surface, which would extract some of the electrons trapped by vacancies to the interface region to screen the positive polarization charges, resulting in a sharp decrease in the electron carrier density and thus a sharp increase in the resistance.31,43 However, such an electron depletion state is metastable because some of the electrons trapped by the positive polarization charges at the interface would move back to defect sites in the film until an equilibrium is established.31 Namely, the delocalization of electrons from the bound state at the interface region results in the resistance relaxation behaviors. To further demonstrate the effects of polarization switching on the electronic transport properties of the Bi2Te3 film, we measured temperature dependence of the resistance and carrier density of the Bi2Te3 film for the Pr+ and Pr― states of the PMN-PT, respectively. As shown in Figure 3a, for the Pr+ state the resistance displays a metallic behavior (d/dT>0) for T>7 K and a weak increase with decreasing temperature from T7 K [inset (I) of Figure 3a]. Upon the polarization switching from Pr+ to Pr― [inset (II) and (III) of Figure 3a for schematic illustration], the resistance displays a semiconductor behavior (d/dT 𝐵𝜙, where 𝐵𝜙 is the coherence field 𝐵𝜙 = ℏ/4𝑒𝓁2𝜙), the destructive quantum interference is dramatically suppressed, resulting in 𝐾𝑞𝑖→0. Then, K is solely determined by Kee in the high field region (3 T  B  9 T), resulting in 𝐾 ≈ Kee. As can be seen in the inset of Figure 4a, in the field region from 3 to 9 T, K was modified by the polarization switching of the PMN-PT, which reflects that the polarization-switching-induced 10



changes in the areal carrier density has modified EEI and thus the conductance G. The polarization-switching-induced changes in the areal carrier density also have dramatic influences on the magnetoresistance (MR) of the Bi2Te3 film. Figure 5 shows the MR of the Bi2Te3 film at several fixed temperatures. Here, MR was defined as MR = [𝑅(𝐵) ― 𝑅(0)]/𝑅(0), where 𝑅(𝐵) and 𝑅(0) are the resistance of the Bi2Te3 film in the presence and absence of a magnetic field B, respectively. For 2 K  T  50 K, associated with the polarization switching from Pr+ to Pr― state (corresponding to a reduction in the carrier density), MR is significantly enhanced, for example, upon polarization switching MR was enhanced by 344% at T=2 K and 173% at T=50 K for B=9 T, which evidences the high effectiveness of in situ carrier density modulation on the magnetotransport properties. MR around B=0 shows a sharp dip at low temperatures (T8 K), particularly for the Pr― state, which is a sign of the weak antilocalization (WAL) effect due to the destructive quantum interference or the suppressed back scattering of carriers.53-55 We further measured MR of the Bi2Te3 film at T=2 K using two different magnetic-field geometries, i.e., the direction of the magnetic field perpendicular to both film plane and excitation current (𝐵 ⊥ ), and parallel to film plane and perpendicular to excitation current (𝐵//) [see Figure 6e and 6f for schematic illustrations]. MR for 𝐵 ⊥ and 𝐵// as well as other magnetic field orientations (=30o, 50o, 70o) and for both polarization states of the PMN-PT are shown in Figure 6a and 6b. It is obvious that MR shows strong anisotropy. Specifically, MR for 𝐵 ⊥ is much larger than that for 𝐵//, which is further corroborated by the angular dependence of the magnetoresistance measured at T=2 K and B=9 T for both polarization states of the PMN-PT [Figure 6c and 6d] and Moreover, for 𝐵//, MR dips are significantly suppressed. These features are also hints for topological surface state.53-55 Note that anisotropic magnetoresistance has also been observed at other low temperatures (see Figure S3, Supporting Information). To understand the effects of polarization switching on the weak antilocalization of the 11


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Bi2Te3 film, the low-field magnetoresistance for 𝐵 ⊥ (Figure 5) and for both Pr+ to Pr― states were presented in the form of magnetoconductance (ΔG). Here, ∆𝐺 = 𝐺(𝐵) ― 𝐺(0), where 𝐺(𝐵) and 𝐺(0) are the conductance of the Bi2Te3 film in the presence and absence of a magnetic field, respectively. The low-field (-0.3 T  B  0.3 T) ΔG are shown in Figure 7. The polarization switching affects the magnitude of ΔG appreciably, indicating that the carrier density has a significant effect on the surface states. It is known that for 2D systems the magnetoconductance related to the WAL can be described by the Hikami-Larkin-Nagaoka (HLN) equation54,55-59 𝑒2

[(+

∆𝐺 = 𝛼2𝜋2ℏ 𝜓

1

ℏ

2

4𝑒𝐵𝓁2𝜙

) ― ln( )] ℏ

4𝑒𝐵𝓁2𝜙

(2)

where e is the electric charge, ℏ is the reduced Planck constant, 𝓁𝜙 is the phase coherent length, and 𝜓(𝑥) is the digamma function. The value of 𝛼 should be equal to 1, 0, and -1/2 for the orthogonal, unitary, and symplectic cases, respectively.56 As shown in Figure 7, ΔG at fixed temperatures for both the Pr+ and Pr― states can be quite well fitted using Eq. (2), with the fitting parameters α and 𝓁𝜙 shown in Figure 8, where one may find that 𝛼 for the Pr― state is significantly larger than that for the Pr+ state. This is consistent with the aforementioned fact that the negative poling of the PMN-PT reduces n2D and shifts the Fermi level toward bulk band gap, resulting in more surface state electrons’ contribution to the conductance. Note that the 𝛼 value of 2D surface states of a 3D TI should be -1/2 for one topological surface or -1 for a film with top and bottom surfaces. The fitting parameter α is rather small ( -0.1) for the Pr+ state, which implies that the Fermi level has been shifted into the conduction band, significantly reducing the contribution of surface state electrons to the conductance. However, by switching the polarization state from Pr+ to Pr― , α increases to approximately -0.3 at low temperatures, which suggests a shift of the Fermi level toward bulk band gap [See the inset of Figure 8b for the schematic band diagram] and greater contribution of surface state electrons to the conductance. With increasing temperature α shows decreasing tendency, indicating a reduced contribution of the surface states to the 12



conductance. We note that the polarization switching from Pr+

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to Pr―

results in a

suppression of ΔG at temperatures for T < 20 K and its enhancement for T > 20 K. The electron phase coherence length 𝓁𝜙 decreases with increasing temperature from 2 to 50 K, regardless of the polarization states of the PMN-PT. For the Pr― state 𝓁𝜙 can be quite well fitted in the temperature region from 2 to 50 K using 𝓁𝜙 ∝ 𝑇 ―𝑝/2 with p=1, i.e., 𝓁𝜙 ∝ 𝑇 ―1/2, indicating that the surface state dominates the conductance of the Bi2Te3 film for the Pr― state. For the Pr+ state, 𝓁𝜙 can be quite well fitted within the same temperature region using 𝓁𝜙 ∝ 𝑇 ―𝑝/2 with p=1.5 (𝓁𝜙~𝑇 ―0.75), which agrees with the fact that a significant contribution of 3D bulk states to the conductance.

4. CONCLUSIONS In summary, we have fabricated Bi2Te3/PMN-PT ferroelectric field effect devices and studied the effects of ferroelectric polarization switching on the electronic properties of the Bi2Te3 film. The results demonstrate that the electron carrier density, Fermi wave vector, conductance, magnetoresistance, electron-electron interaction, number of conductance channel, and phase coherence length can be in situ modulated in a reversible and nonvolatile manner by controlling the polarization states of the PMN-PT substrate at room temperature. Upon polarization switching from the positively poled Pr+ state to the negatively poled Pr― state, both the carrier density and the Fermi wave vector decrease significantly, implying a shift of the Fermi level towards the bulk band gap. 2D weak antilocalization due to topological surface state was observed in the Bi2Te3 film and can be tuned by the polarization switching. At low temperatures (T

Nonvolatile and Reversible Ferroelectric Control of Electronic

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