In operando evidence of deoxygenation in ionic liquid gating of YBa2Cu3O7-X

Endometrial Cancer: A Pilot Study of the Tissue Microbiota

BACKGROUND

The endometrium remains a difficult tissue for the analysis of microbiota, mainly due to the low bacterial presence and the sampling procedures. Among its pathologies, endometrial cancer has not yet been completely investigated for its relationship with microbiota composition. In this work, we report on possible correlations between endometrial microbiota dysbiosis and endometrial cancer.

METHODS

Women with endometrial cancer at various stages of tumor progression were enrolled together with women with a benign polymyomatous uterus as the control. Analyses were performed using biopsies collected at two specific endometrial sites during the surgery. This study adopted two approaches: the absolute quantification of the bacterial load, using droplet digital PCR (ddPCR), and the analysis of the bacterial composition, using a deep metabarcoding NGS procedure.

RESULTS

ddPCR provided the first-ever assessment of the absolute quantification of bacterial DNA in the endometrium, confirming a generally low microbial abundance. Metabarcoding analysis revealed a different microbiota distribution in the two endometrial sites, regardless of pathology, accompanied by an overall higher prevalence of pathogenic bacterial genera in cancerous tissues.

CONCLUSIONS

These results pave the way for future studies aimed at identifying potential biomarkers and gaining a deeper understanding of the role of bacteria associated with tumors.

Reduction-Induced Magnetic Behavior in LaFeO3-δ Thin Films

The effect of oxygen reduction on the magnetic properties of LaFeO3-δ (LFO) thin films was studied to better understand the viability of LFO as a candidate for magnetoionic memory. Differences in the amount of oxygen lost by LFO and its magnetic behavior were observed in nominally identical LFO films grown on substrates prepared using different common methods. In an LFO film grown on as-received SrTiO3 (STO) substrate, the original perovskite film structure was preserved following reduction, and remnant magnetization was only seen at low temperatures. In a LFO film grown on annealed STO, the LFO lost significantly more oxygen and the microstructure decomposed into La- and Fe-rich regions with remnant magnetization that persisted up to room temperature. These results demonstrate an ability to access multiple, distinct magnetic states via oxygen reduction in the same starting material and suggest LFO may be a suitable materials platform for nonvolatile multistate memory.

Li iontronics in single-crystalline T-Nb2O5 thin films with vertical ionic transport channels

The niobium oxide polymorph T-Nb2O5 has been extensively investigated in its bulk form especially for applications in fast-charging batteries and electrochemical (pseudo)capacitors. Its crystal structure, which has two-dimensional (2D) layers with very low steric hindrance, allows for fast Li-ion migration. However, since its discovery in 1941, the growth of single-crystalline thin films and its electronic applications have not yet been realized, probably due to its large orthorhombic unit cell along with the existence of many polymorphs. Here we demonstrate the epitaxial growth of single-crystalline T-Nb2O5 thin films, critically with the ionic transport channels oriented perpendicular to the film's surface. These vertical 2D channels enable fast Li-ion migration, which we show gives rise to a colossal insulator-metal transition, where the resistivity drops by 11 orders of magnitude due to the population of the initially empty Nb 4d0 states by electrons. Moreover, we reveal multiple unexplored phase transitions with distinct crystal and electronic structures over a wide range of Li-ion concentrations by comprehensive in situ experiments and theoretical calculations, which allow for the reversible and repeatable manipulation of these phases and their distinct electronic properties. This work paves the way for the exploration of novel thin films with ionic channels and their potential applications.

First-principles calculation of the optical properties of the YBa2Cu3O7-δ oxygen vacancies model

We used first-principles methods to investigate how oxygen vacancy defects affect the optical properties of YBa₂Cu₃O_7-δ (0 < δ < 1), a high-temperature superconductor with potential applications in optical detectors. We calculated the electronic structure of YBa₂Cu₃O_7-δ with different amounts of oxygen vacancies at three different sites: Cu-O chains, CuO₂ planes, and apical oxygens. The formation energy calculations support the formation of oxygen vacancies in the Cu-O chain at higher concentrations of vacancy defects, with a preference for alignment in the same chain. The presence of oxygen vacancies affects the optical absorption peak of YBa₂Cu₃O_7-δ in different ways depending on their location and concentration. The optical absorption peaks in the visible range (1.6-3.2 eV) decrease in intensity and shift towards the infrared spectrum as oxygen vacancies increase. We demonstrate that oxygen vacancies can be used as a powerful tool to manipulate the optical response of YBa₂Cu₃O_7-δ to different wavelengths in optical detector devices.

Nonvolatile n-Type Doping and Metallic State in Multilayer-MoS2 Induced by Hydrogenation Using Ionic-Liquid Gating

Manipulation of the carrier density of layered transition-metal dichalcogenides (TMDs) is of fundamental significance for a wide range of electronic and optoelectronic applications. Herein, we applied the ionic-liquid-gating (ILG) method to inject the smallest ions, H⁺, into layered MoS₂ to manipulate its carrier concentration. The measurements demonstrate that the injection of H⁺ realizes a nonvolatile n-type doping and metallic state in multilayer-MoS₂ with a concentration of injection electron of ∼1.08 × 10¹³ cm^-2 but has no effect on monolayer-MoS₂, which clearly reveals that the H⁺ is injected into the interlayer of MoS₂, not in the crystal lattice. The H⁺-injected multilayer-MoS₂ was then used as the contact electrodes of a monolayer-MoS₂ field effect transistor to improve the contact quality, and its performance has been enhanced. Our work deepens the understanding of the ILG technology and extends its application in TMDs.

Oxide Spintronics as a Knot of Physics and Chemistry: Recent Progress and Opportunities

Transition-metal oxides (TMOs) constitute a key material family in spintronics because of mutually coupled degrees of freedom and tunable magneto-ionic properties. In this Perspective, we consider oxide spintronics as a knot of physics and chemistry and mainly discuss two current hot topics: spin-charge interconversion and magneto-ionics. First, spin-charge interconversion is focused on oxide films and heterostructures including 4d/5d heavy metal oxides (e.g., SrIrO₃) and two-dimensional electron gases. Based on spin-charge interconversion, charge currents can be transformed to spin currents and generate spin-orbit torque in oxide/metal and all-oxide heterostructures. Additionally, the voltage control of magnetism in TMOs by the magneto-ionic pathway has rapidly accelerated during the past few years due to the versatile advantages of effective control, nonvolatile nature, low power cost, etc. Typical magneto-ionic oxide systems and corresponding physicochemical mechanisms will be discussed. Finally, further developments of oxide spintronics are envisioned, including material discovery, physics exploration, device design, and manipulation methods.

Control of Oxygen Vacancy Ordering in Brownmillerite Thin Films via Ionic Liquid Gating

Oxygen defects and their atomic arrangements play a significant role in the physical properties of many transition metal oxides. The exemplary perovskite SrCoO_3-δ (P-SCO) is metallic and ferromagnetic. However, its daughter phase, the brownmillerite SrCoO_2.5 (BM-SCO), is insulating and an antiferromagnet. Moreover, BM-SCO exhibits oxygen vacancy channels (OVCs) that in thin films can be oriented either horizontally (H-SCO) or vertically (V-SCO) to the film's surface. To date, the orientation of these OVCs has been manipulated by control of the thin film deposition parameters or by using a substrate-induced strain. Here, we present a method to electrically control the OVC ordering in thin layers via ionic liquid gating (ILG). We show that H-SCO (antiferromagnetic insulator, AFI) can be converted to P-SCO (ferromagnetic metal, FM) and subsequently to V-SCO (AFI) by the insertion and subtraction of oxygen throughout thick films via ILG. Moreover, these processes are independent of substrate-induced strain which favors formation of H-SCO in the as-deposited film. The electric-field control of the OVC channels is a path toward the creation of oxitronic devices.

Extremely long-range, high-temperature Josephson coupling across a half-metallic ferromagnet

The Josephson effect results from the coupling of two superconductors across a spacer such as an insulator, a normal metal or a ferromagnet to yield a phase coherent quantum state. However, in junctions with ferromagnetic spacers, very long-range Josephson effects have remained elusive. Here we demonstrate extremely long-range (micrometric) high-temperature (tens of kelvins) Josephson coupling across the half-metallic manganite La_0.7Sr_0.3MnO₃ combined with the superconducting cuprate YBa₂Cu₃O₇. These planar junctions, in addition to large critical currents, display the hallmarks of Josephson physics, such as critical current oscillations driven by magnetic flux quantization and quantum phase locking effects under microwave excitation (Shapiro steps). The latter display an anomalous doubling of the Josephson frequency predicted by several theories. In addition to its fundamental interest, the marriage between high-temperature, dissipationless quantum coherent transport and full spin polarization brings opportunities for the practical realization of superconducting spintronics, and opens new perspectives for quantum computing.

Ionic-Liquid Gating in Two-Dimensional TMDs: The Operation Principles and Spectroscopic Capabilities

Ionic-liquid gating (ILG) is able to enhance carrier densities well above the achievable values in traditional field-effect transistors (FETs), revealing it to be a promising technique for exploring the electronic phases of materials in extreme doping regimes. Due to their chemical stability, transition metal dichalcogenides (TMDs) are ideal candidates to produce ionic-liquid-gated FETs. Furthermore, as recently discovered, ILG can be used to obtain the band gap of two-dimensional semiconductors directly from the simple transfer characteristics. In this work, we present an overview of the operation principles of ionic liquid gating in TMD-based transistors, establishing the importance of the reference voltage to obtain hysteresis-free transfer characteristics, and hence, precisely determine the band gap. We produced ILG-based bilayer WSe₂ FETs and demonstrated their ambipolar behavior. We estimated the band gap directly from the transfer characteristics, demonstrating the potential of ILG as a spectroscopy technique.

Solid-State Electrochemical Switch of Superconductor-Metal-Insulators

Controlling the oxygen content can manipulate the electrical conductivity of transition metal oxides (TMOs). Although the superconductor-metal-insulator transition is useful for functional devices, an electrical path must be developed to manipulate the oxygen deficiency (δ) while maintaining the solid state. YBa₂Cu₃O_7-δ (YBCO, 0 ≤ δ ≤ 1) is a high transition temperature (T_c) TMO that can be modulated from a superconductor (T_c ≈ 92 K when δ = 0) to an insulator (δ ≈ 1). Here, we show a simple and efficient way to manipulate δ in YBCO films using a solid-state electrochemical redox treatment. Applying a negative voltage injects oxide ions to the YBCO films, increasing T_c. Employing a positive voltage suppresses the superconducting transition and modulates the electrical conductivity. The present results demonstrate that the superconductor-metal-insulator transition of YBCO is modulated electrochemically in the solid state, opening possibilities of superconducting oxide-based device applications.

Electrically Enhanced Exchange Bias via Solid-State Magneto-ionics

Electrically induced ionic motion offers a new way to realize voltage-controlled magnetism, opening the door to a new generation of logic, sensor, and data storage technologies. Here, we demonstrate an effective approach to magneto-ionically and electrically tune the exchange bias in Gd/Ni_1-xCo_xO thin films (x = 0.50 and 0.67), where neither of the layers alone is ferromagnetic at room temperature. The Gd capping layer deposited onto antiferromagnetic Ni_1-xCo_xO initiates a solid-state redox reaction that reduces an interfacial region of the oxide to ferromagnetic NiCo. An exchange bias is established after field cooling (FC), which can be enhanced by up to 35% after a voltage conditioning and subsequently reset with a second FC. These effects are caused by the presence of an interfacial ferromagnetic NiCo layer, which further alloys with the Gd layer upon FC and voltage application, as confirmed by electron microscopy and polarized neutron reflectometry studies. These results highlight the viability of the solid-state magneto-ionic approach to achieve electric control of exchange bias, with potential for energy-efficient magneto-ionic devices.

Electric field-induced oxygen vacancies in YBa2Cu3O7

The microscopic doping mechanism behind the superconductor-to-insulator transition of a thin film of YBa₂Cu₃O₇ was recently identified as due to the migration of O atoms from the CuO chains of the film. Here, we employ density-functional theory calculations to study the evolution of the electronic structure of a slab of YBa₂Cu₃O₇ in the presence of oxygen vacancies under the influence of an external electric field. We find that, under massive electric fields, isolated O atoms are pulled out of the surface consisting of CuO chains. As vacancies accumulate at the surface, a configuration with vacancies located in the chains inside the slab becomes energetically preferred, thus providing a driving force for O migration toward the surface. Regardless of the defect configuration studied, the electric field is always fully screened near the surface, thus negligibly affecting diffusion barriers across the film.

Superconductor-insulator transition in space charge doped one unit cell Bi2.1Sr1.9CaCu2O8+x

The superconductor-insulator transition in two dimensions is a prototype continuous quantum phase transition at absolute zero, driven by a parameter other than temperature. Here we reveal this transition in one unit-cell Bi_2.1Sr_1.9CaCu₂O_8+x by space charge doping, a field effect electrostatic doping technique. We determine the related critical parameters and develop a reliable way to estimate doping in the nonsuperconducting region, a crucial and central problem in these materials. Finite-size scaling analysis yields a critical doping of 0.057 holes/Cu, a critical resistance of ~6.85 kΩ and a scaling exponent product νz ~ 1.57. These results, together with earlier work in other materials, provide a coherent picture of the superconductor-insulator transition and its bosonic nature in the underdoped regime of emerging superconductivity in high critical temperature superconductors.

Previous work on critical scaling at the superconductor-to-insulator transition has shown variations across different materials. Here, the authors use a space charge doping technique to tune the transition in a single layer cuprate sample and present evidence of the universal scaling behaviour.

A selective control of volatile and non-volatile superconductivity in an insulating copper oxide via ionic liquid gating

Manipulating the superconducting states of high transition temperature (high-T_c) cuprate superconductors in an efficient and reliable way is of great importance for their applications in next-generation electronics. Here, employing ionic liquid gating, a selective control of volatile and non-volatile superconductivity is achieved in pristine insulating Pr₂CuO_4±δ (PCO) films, based on two distinct mechanisms. Firstly, with positive electric fields, the film can be reversibly switched between superconducting and non-superconducting states, attributed to the carrier doping effect. Secondly, the film becomes more resistive by applying negative bias voltage up to - 4 V, but strikingly, a non-volatile superconductivity is achieved once the gate voltage is removed. Such phenomenon represents a distinctive route of manipulating superconductivity in PCO, resulting from the doping healing of oxygen vacancies in copper-oxygen planes as unravelled by high-resolution scanning transmission electron microscope and in situ X-ray diffraction experiments. The effective manipulation of volatile/non-volatile superconductivity in the same parent cuprate brings more functionalities to superconducting electronics, as well as supplies flexible samples for investigating the nature of quantum phase transitions in high-T_c superconductors.

Atomic Origin of Interface-Dependent Oxygen Migration by Electrochemical Gating at the LaAlO3-SrTiO3 Heterointerface

Electrical control of material properties based on ionic liquids (IL) has seen great development and emerging applications in the field of functional oxides, mainly understood by the electrostatic and electrochemical gating mechanisms. Compared to the fast, flexible, and reproducible electrostatic gating, electrochemical gating is less controllable owing to the complex behaviors of ion migration. Here, the interface-dependent oxygen migration by electrochemical gating is resolved at the atomic scale in the LaAlO3-SrTiO3 system through ex situ IL gating experiments and on-site atomic-resolution characterization. The difference between interface structures leads to the controllable electrochemical oxygen migration by filling oxygen vacancies. The findings not only provide an atomic-scale insight into the origin of interface-dependent electrochemical gating but also demonstrate an effective way of engineering interface structure to control the electrochemical gating.

Electric field control of ordered oxygen vacancy planes and antiferromagnetic structures in strontium cobaltite

The polarized ionic liquids (ILs) could generate intense electric fields on the surface of solid-state materials and create functional defects by ion migration within them, resulting in phase transitions of metal-insulator or paramagnet-ferromagnet, etc. Such a strong electric field even provides an opportunity for the control of spin ordering in antiferromagnetic (AFM) crystal which is difficult to be manipulated due to the strong exchange coupling between antiparallel spins in the whole bulk. Here we find that the ferromagnetic SrCoO₃of 40 nm could be transformed to AFM SrCoO_2.5with ordered oxygen vacancy planes either vertical (V-SrCoO_2.5) or parallel (P-SrCoO_2.5) to the surface by IL gating. The spin Hall magnetoresistances suggest that the AFM easy axes of V- and P-SrCoO_2.5are along [010] and11¯0, respectively. The orientations of gating induced oxygen vacancy planes are related to the oxygen framework rotation in the parent SrCoO₃and could be controlled by the strain engineering. Our results not only supply a novel way to manipulate the AFM spins by creating functional ordered defects, but also reveal the effect of oxygen framework rotation on the formation of oxygen vacancies under ionic liquid gating.

Emergent electric field control of phase transformation in oxide superlattices

Electric fields can transform materials with respect to their structure and properties, enabling various applications ranging from batteries to spintronics. Recently electrolytic gating, which can generate large electric fields and voltage-driven ion transfer, has been identified as a powerful means to achieve electric-field-controlled phase transformations. The class of transition metal oxides provide many potential candidates that present a strong response under electrolytic gating. However, very few show a reversible structural transformation at room-temperature. Here, we report the realization of a digitally synthesized transition metal oxide that shows a reversible, electric-field-controlled transformation between distinct crystalline phases at room-temperature. In superlattices comprised of alternating one-unit-cell of SrIrO3 and La0.2Sr0.8MnO3, we find a reversible phase transformation with a 7% lattice change and dramatic modulation in chemical, electronic, magnetic and optical properties, mediated by the reversible transfer of oxygen and hydrogen ions. Strikingly, this phase transformation is absent in the constituent oxides, solid solutions and larger period superlattices. Our findings open up this class of materials for voltage-controlled functionality.

Interfacial-Redox-Induced Tuning of Superconductivity in YBa2Cu3O7-δ

Solid-state ionic approaches for modifying ion distributions in getter/oxide heterostructures offer exciting potentials to control material properties. Here, we report a simple, scalable approach allowing for manipulation of the superconducting transition in optimally doped YBa₂Cu₃O_7-δ (YBCO) films via a chemically driven ionic migration mechanism. Using a thin Gd capping layer of up to 20 nm deposited onto 100 nm thick epitaxial YBCO films, oxygen is found to leach from deep within the YBCO. Progressive reduction of the superconducting transition is observed, with complete suppression possible for a sufficiently thick Gd layer. These effects arise from the combined impact of redox-driven electron doping and modification of the YBCO microstructure due to oxygen migration and depletion. This work demonstrates an effective step toward total ionic tuning of superconductivity in oxides, an interface-induced effect that goes well into the quasi-bulk regime, opening-up possibilities for electric field manipulation.

Ionic Liquid Gating Induced Protonation of Electron-Doped Cuprate Superconductors

Ion injection controlled by electric field has attracted growing attention due to its tunability over bulk-like materials. Here, we achieve protonation of an electron-doped high-temperature superconductor, La_2-xCe_xCuO₄, by gating in the electrochemical regime of the ionic liquid. Such a process induces a superconductor-insulator transition together with the crossing of the Fermi surface reconstruction point. Applying negative voltages not only can reverse the protonation process but also recovers superconductivity in samples deteriorated by moisture in the ambient. Our work extends the application of electric-field-induced protonation into high-temperature cuprate superconductors.

Nanofabricated tips for device-based scanning tunneling microscopy

We report on the fabrication and performance of a new kind of tip for scanning tunneling microscopy. By fully incorporating a metallic tip on a silicon chip using modern micromachining and nanofabrication techniques, we realize so-called smart tips and show the possibility of device-based STM tips. Contrary to conventional etched metal wire tips, these can be integrated into lithographically defined electrical circuits. We describe a new fabrication method to create a defined apex on a silicon chip and experimentally demonstrate the high performance of the smart tips, both in stability and resolution. In situ tip preparation methods are possible and we verify that they can resolve the herringbone reconstruction and Friedel oscillations on Au(111) surfaces. We further present an overview of possible applications.

Electrolyte-based ionic control of functional oxides

The use of electrolyte gating to electrically control electronic, magnetic and optical properties of materials has seen strong recent growth, driven by the potential of the many devices and applications that such control may enable. Contrary to initial expectations of a purely electrostatic response based on electron or hole doping, electrochemical mechanisms based on the motion of ions are now understood to be common, suggesting promising new electrical control concepts.

Oxygen Electromigration and Energy Band Reconstruction Induced by Electrolyte Field Effect at Oxide Interfaces

Electrolyte gating is a powerful means for tuning the carrier density and exploring the resultant modulation of novel properties on solid surfaces. However, the mechanism, especially its effect on the oxygen migration and electrostatic charging at the oxide heterostructures, is still unclear. Here we explore the electrolyte gating on oxygen-deficient interfaces between SrTiO_{3} (STO) crystals and LaAlO_{3} (LAO) overlayer through the measurements of electrical transport, x-ray absorption spectroscopy, and photoluminescence spectra. We found that oxygen vacancies (O_{vac}) were filled selectively and irreversibly after gating due to oxygen electromigration at the amorphous LAO/STO interface, resulting in a reconstruction of its interfacial band structure. Because of the filling of O_{vac}, the amorphous interface also showed an enhanced electron mobility and quantum oscillation of the conductance. Further, the filling effect could be controlled by the degree of the crystallinity of the LAO overlayer by varying the growth temperatures. Our results reveal the different effects induced by electrolyte gating, providing further clues to understand the mechanism of electrolyte gating on buried interfaces and also opening a new avenue for constructing high-mobility oxide interfaces.

Electrochemical Tuning of Metal Insulator Transition and Nonvolatile Resistive Switching in Superconducting Films

Modulation of carrier concentration in strongly correlated oxides offers the unique opportunity to induce different phases in the same material, which dramatically change their physical properties, providing novel concepts in oxide electronic devices with engineered functionalities. This work reports on the electric manipulation of the superconducting to insulator phase transition in YBa2Cu3O7-δ thin films by electrochemical oxygen doping. Both normal state resistance and the superconducting critical temperature can be reversibly manipulated in confined active volumes of the film by gate-tunable oxygen diffusion. Vertical and lateral oxygen mobility may be finely modulated, at the micro- and nano-scale, by tuning the applied bias voltage and operating temperature thus providing the basis for the design of homogeneous and flexible transistor-like devices with loss-less superconducting drain-source channels. We analyze the experimental results in light of a theoretical model, which incorporates thermally activated and electrically driven volume oxygen diffusion.

Direct imaging of structural changes induced by ionic liquid gating leading to engineered three-dimensional meso-structures

The controlled transformation of materials, both their structure and their physical properties, is key to many devices. Ionic liquid gating can induce the transformation of thin-film materials over long distances from the gated surface. Thus, the mechanism underlying this process is of considerable interest. Here we directly image, using in situ, real-time, high-resolution transmission electron microscopy, the reversible transformation between the oxygen vacancy ordered phase brownmillerite SrCoO_2.5 and the oxygen ordered phase perovskite SrCoO₃. We show that the phase transformation boundary moves at a velocity that is highly anisotropic, traveling at speeds ~30 times faster laterally than through the thickness of the film. Taking advantage of this anisotropy, we show that three-dimensional metallic structures such as cylinders and rings can be realized. Our results provide a roadmap to the construction of complex meso-structures from their exterior surfaces.

Local and reversible oxidation is used to exploit the very different properties of oxygen and vacancy ordered oxides. Here the authors directly image and make use of anisotropic migration velocities of oxygen in SrCoO_x to create 3D meso-structures of those two phases by ionic liquid gating.

ON the Nature of Ionic Liquid Gating of La2−xSrxCuO4

Ionic liquids have recently been used as means of modulating the charge carrier properties of cuprates. The mechanism behind it, however, is still a matter of debate. In this paper we report experiments on ionic liquid gated ultrathin La_2−xSr_xCuO₄ films. Our results show that the electrostatic part of gating has limited influence in the conductance of the cuprate in the gate voltage range of 0 to −2 V. A non-electrostatic mechanism takes over for gate voltages below −2 V. This mechanism most likely changes the oxygen concentration of the film. The results presented are in line with previous X-ray based studies on ionic liquid gating induced oxygenation of the cuprate materials YBa₂Cu₃O_7−x and La_2−xSr_xCuO₄.

The Mechanism of Electrolyte Gating on High-Tc Cuprates: The Role of Oxygen Migration and Electrostatics

Electrolyte gating is widely used to induce large carrier density modulation on solid surfaces to explore various properties. Most of past works have attributed the charge modulation to electrostatic field effect. However, some recent reports have argued that the electrolyte gating effect in VO₂, TiO₂, and SrTiO₃ originated from field-induced oxygen vacancy formation. This gives rise to a controversy about the gating mechanism, and it is therefore vital to reveal the relationship between the role of electrolyte gating and the intrinsic properties of materials. Here, we report entirely different mechanisms of electrolyte gating on two high-T_c cuprates, NdBa₂Cu₃O_7-δ (NBCO) and Pr_2-xCe_xCuO₄ (PCCO), with different crystal structures. We show that field-induced oxygen vacancy formation in CuO chains of NBCO plays the dominant role, while it is mainly an electrostatic field effect in the case of PCCO. The possible reason is that NBCO has mobile oxygen in CuO chains, while PCCO does not. Our study helps clarify the controversy relating to the mechanism of electrolyte gating, leading to a better understanding of the role of oxygen electro migration which is very material specific.