
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)13218-5
10.1016/j.heliyon.2024.e37187
e37187
Research Article
Interfacial stability and electronic properties of YBCO/ABO3 heterostructures: A comparative DFT study
Laosiritaworn Yongyut a
Jaroenjittichai Atchara Punya atchara.punya@cmu.ac.th
ab⁎
a Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai, 50200, Thailand
b Center of Excellence in Quantum Technology, Faculty of Engineering, Chiang Mai University, Chiang Mai, 50200, Thailand
⁎ Corresponding author. Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai, 50200, Thailand. atchara.punya@cmu.ac.th
29 8 2024
15 9 2024
29 8 2024
10 17 e371874 3 2024
25 7 2024
28 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
In this study, we utilized density functional theory (DFT) to analyze the interfacial properties of YBa2Cu3O7 (YBCO) with perovskite metal oxides LaAlO3 (LAO), KTaO3 (KTO), and SrTiO3 (STO). We focused on surface energies, lattice mismatches, strain energies, and adhesion energies to gauge the stability and compatibility of these interfaces. Our findings indicate that KTO exhibits the highest surface preference due to its lowest surface energy (0.054 eV/Å 2), suggesting superior surface stability. Moreover, LAO and STO show minor lattice mismatches with YBCO, implying effective interface integration, while YBCO/KTO interfaces experience significant strain due to extensive lattice mismatches. STO is distinguished by the lowest strain energy (0.07 eV), indicating minimal energy requirement for lattice mismatch accommodation, unlike KTO, which demonstrates high strain energy (0.42 eV) and potential structural distortions. The strongest interfacial bond, as indicated by an adhesion energy of −2.16 eV, was observed at YBCO/LAO, while the weakest was found at the YBCO/KTO interface, with an adhesion energy of −0.56 eV. Additionally, charge density difference (CDD) analysis highlighted electron density redistribution at the interfaces, predominantly around interfacial oxygen atoms, indicating a mix of ionic and covalent bonding. This study provides comparative insights into the interfacial characteristics of YBCO/ABO3 heterostructures, suggesting pathways for optimizing their design and performance.

Graphical abstract

Image 1

Highlights

• DFT reveals stability of YBCO/ABO3 via lattice mismatch, strain, and adhesion metrics.

• YBCO/STO and YBCO/LAO show compatibility; YBCO/LAO slightly more stable.

• YBCO/KTO presents notable challenges due to mismatch, strain, and unique charge density differences.

Keywords

Adhesion energy
DFT
Interface
Oxide perovskites
YBCO
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pmc1 Introduction

The interfaces between superconductors and metal oxides [1] present a fruitful ground for discovering novel physical phenomena that challenge and extend the boundaries of condensed matter physics. Such interfaces, especially those involving Yttrium Barium Copper Oxide (YBCO) and various ABO3 perovskites, are not just academic curiosities but are important for the development of quantum computing and state-of-the-art electronic devices like Superconducting Tunnel Junctions (STJ) and Superconducting Quantum Interference Devices (SQUIDs). These interfaces offer the potential for advancements in energy-efficient electronic technologies and ultra-sensitive detection systems, paving the way for next-generation quantum technologies. Despite the technological and fundamental importance of superconductor-metal oxide interfaces, our understanding of their behavior remains incomplete. Critical questions about how interface structure, electronic reconstructions, and interaction effects determine the electronic properties and structural stability of materials remain unanswered. Addressing these gaps could unlock a wide array of applications in advanced computing and sensing technologies, enhancing properties of these interfaces. YBCO's interactions with oxides such as LaNiO3 [2], BiFeO3 [3], and La2/3Sr1/3MnO3 [4] have demonstrated its versatility. These studies reveal the potential for modifying and controlling material properties through interface engineering, exhibiting YBCO's adaptability in maintaining high-quality interfaces [5]. Among the explored interfaces, the YBCO/SrTiO3 (STO) junction, in particular, has gained significant attention due to its ability to modulate superconductivity. Studies have shown that YBCO layers on STO can elucidate the effects of interface disorder on superconductivity suppression [6] and allow for the manipulation of the superconducting critical temperature (Tc) [7,8]. Additionally, STO has played a crucial role in improving the transport properties of BiFeO3/YBCO heterostructures [9]. However, given the challenges associated with YBCO/STO interfaces, such as structural instability due to lattice mismatches [10,11], there is a compelling need to explore alternative oxides. LaAlO3 (LAO) [[12], [13], [14]] and KTaO3 (KTO) [15,16] emerge as promising candidates, offering potential improvements in structural stability and device performance. These oxides share structural similarities with STO but exhibit slight differences in lattice spacing that could discuss additional stability and functionality to the interfaces.

Consequently, this study employs Density Functional Theory (DFT) via Quantum Espresso, a renowned DFT package, to assess adhesion energy and examine the atomic structure of YBCO/LAO, YBCO/KTO and YBCO/STO interfaces. The main objective is to systematically analyze the structural and electronic properties of these interfaces, focusing on lattice relaxation effects, adhesion energies, and interfacial charge density differences. By considering LAO and KTO interfaces alongside STO, we aim to compare their advantages and disadvantages, evaluating compatibility and variations. The insights gained from this research will enhance our understanding of the interactions between different oxide insulators and YBCO, potentially guiding the design of superior high temperature superconducting devices.

2 Methodology

This work was performed using the DFT framework, as implemented in QUANTUM ESPRESSO package [17], and was divided into three main parts: bulk, slab and interface calculations. Firstly, for the bulk calculation, the modified Perdew–Burke-Ernzerhof (PBEsol) functional from SSSP library (precision version 1.2.1) [18], known to provide accurate structural properties, was employed as pseudopotentials to optimize the crystal structure of Pmmm YBCO and Pm 3‾ m ABO3. The k-point sampling grid of the Brillouin zone was defined as 8 × 8 × 3 for YBCO and 8 × 8 × 8 for ABO3, respectively. Additionally, the plane-wave cutoff energy of 50 Ry was tested to ensure total energy convergence within 0.1 eV for bulk, slab, and interface calculations. Secondly, the slab models of YBCO and ABO3 were created to represent the exposed surface of the materials. The 8-atomic layers (4-unit cells or supercell with a dimension of 1 × 1 × 4) and 12-atomic layers (2-unit cells or supercell with a dimension of 1 × 1 × 2), each with a vacuum region of 10 Å, were constructed for slabs of ABO3 and YBCO, respectively, as shown in Fig. 1a, b. The k-point mesh was reduced to 8 × 8 × 1 due to the expansion in the c-direction. Notably, this k-point mesh will be utilized in the interface calculations to ensure consistency. The convergence of the total energies within 0.1 eV for the slab was completed to confirm sufficient k-point mesh, atomic layers and length of the vacuum region. The surface energies of all slab models (Esurface) were calculated as [19](1) Esurface=12·Eslab−N·EunitcellA

where Eslab is the total energy of each slab model, Eunitcell is the total energy of each unit cell, N is the number of the unit cells in the slab model and A is the area of the slab surface. Thirdly, the interface models, combining the optimized bulk structures of YBCO and ABO3 were constructed as shown in Fig. 1c. The in-plane lattice constants of the heterostructures were assumed to be those of the substrate YBCO. This definitely causes stain at the surface. Thus, the strain energies (Ustrain) were carried out by(2) Ustrain=Estrain−Erelax,

where Estrain and Erelax represent the total energies of strained and relaxed structures respectively. Subsequently, relaxation calculations on the interface models were performed to optimize the atomic positions in the interface region. This step aids in obtaining a more accurate representation of the relaxed interface structure. The adhesion energies of the combined surfaces (Eadhesion) were calculated by(3) Eadhesion=EYBCO/ABO3−(EYBCO+EABO3),

where EYBCO/ABO3 is the total energy of the combined systems while EYBCO and EABO3 are the total energies of the isolated systems of YBCO and ABO3 respectively. Furthermore, the difference of electronic charge distribution between a combined and isolated systems was extracted. When two materials come into contact, the electronic charge near the interface redistributes. The charge density difference (CDD) before and after the connection highlights regions of charge accumulation or depletion at the interface, providing insights into the electronic structure and bonding characteristics.Fig. 1 The slab models, each with a vacuum region of 10 Å, consist of: (a) ABO3 with of 8-atomic layers (4-unit cells), (b) YBCO with 12-atomic layers (2-unit cells), (c) YBCO/ABO3 with 20-atomic layers. The elements A, B, Y, Ba, Cu and O are represented by brown, cyan, yellow, green, blue, and red atoms, respectively.

Fig. 1

3 Results and discussions

3.1 Surface energy

Surface energy represents the excess energy of surface atoms compared to those in the bulk. To obtain surface energy, we calculated the total energy of the relaxed slab model and subtracted it from that of the relaxed bulk structure. Then, the resulting energy difference was divided by the surface area of the slab to obtain the surface energy per unit area. Note that, the slab model represents two identical surfaces. The calculated surface energies of YBCO, LAO, KTO, and STO, as shown in Table 1, provide important information about the stability and reactivity of these materials at their surfaces. In this study, LAO has the highest surface energy, 0.105 eV/Å2, suggesting that its surface atoms prefer to reorganize themselves or interact with the environment to lower their energy. STO has a slightly lower surface energy of 0.071 eV/Å2, suggesting that its surface is more stable compared with LAO. KTO has the lowest surface energy of 0.054 eV/Å2, indicating high stability of its surface. Finally, YBCO's surface energy, 0.070 eV/Å2, is similar to STO's, implying comparable stabilities. Note that, these results are concerned with the stability of isolated surfaces. However, in the context of combined materials, it is crucial to evaluate the stability of interfaces, where distinct materials converge and interact.Table 1 Space group, relaxed lattice constants and surface energy of YBCO and ABO3.

Table 1Compounds	Space Group	Lattice constants (Å)	Surface Energy (eV/Å2)	
YBCO	Pmmm	a = 3.80, b = 3.85, c = 11.48	0.070	
LAO	Pm 3‾ m	a = b = c = 3.78	0.105	
STO	Pm 3‾ m	a = b = c = 3.89	0.071	
KTO	Pm 3‾ m	a = b = c = 3.99	0.054	

3.2 Lattice mismatch and strain energy

On considering YBCO/ABO3 interfaces, the lattice mismatches on the a-axis and b-axis between YBCO and ABO3 materials were investigated. Our findings, detailed in Table 2, reveal variations in compatibility. Specifically, lattice mismatches for LAO, STO, and KTO with YBCO are −0.53 %, 2.37 %, and 5.00 % along the a-axis, respectively. A positive value indicates that the ABO3 material has a larger lattice parameter compared to YBCO, while a negative value indicates the opposite. LAO's minimal negative mismatch of −0.53 % suggests a good match with YBCO along the a-axis, while STO and KTO display positive mismatches of 2.37 % and 5.00 %, indicating increasing strain. Along the b-axis, mismatches are −1.83 %, 1.04 %, and 3.54 % for LAO, STO, and KTO, respectively, with KTO showing the most significant strain. These results highlight KTO as having the largest positive lattice mismatch with YBCO on both axes, implying notable strain and potential interface instabilities. Thus, fabricating YBCO/KTO heterostructures necessitates careful attention to lessen the impacts of this significant mismatch for structural stability.Table 2 Lattice mismatch, strain, and adhesion energy of YBCO/ABO3 interfaces.

Table 2Interface	Lattice mismatch	Strain Energy
(eV)	Adhesion
Energy
(eV)	Adhesion
Strength	
on a
(%)	on b
(%)	
YBCO/LAO	−0.53	−1.82	0.21	−2.16	Strongest, energetically stable	
YBCO/STO	2.37	1.04	0.07	−1.85	Intermediate, relatively weaker	
YBCO/KTO	5.00	3.54	0.42	−0.56	Weakest, least energetically stable	

Next, we investigate the strain energy the at interface resulting from the lattice mismatch between the two materials. High strain energy reduces stability, while low strain energy suggests greater stability and preservation of intrinsic properties. In our study, we matched the surface area of ABO3 materials to that of YBCO, introducing surface strain in ABO3. Strain energy was calculated as the difference in total energy between strained and unstrained ABO3 structures. We found strain energies for LAO, STO, and KTO to be 0.21 eV, 0.07 eV, and 0.42 eV, respectively, indicating the energy cost of deforming ABO3 to align with YBCO's lattice parameters (see Table 2). STO shows the lowest strain energy, suggesting a lower cost to adapt to lattice mismatch. Our analysis indicates that the KTO interface exhibits the most significant lattice mismatches and the highest strain energy, presenting considerable challenges to stability and structural integrity. Conversely, the STO interface, despite experiencing substantial mismatch, demonstrates low strain energy, suggesting a greater capacity for deformation accommodation. The LAO interface occupies an intermediate position, characterized by minimal mismatch and moderate strain energy, thus offering a favorable balance between structural compatibility and energy expenditure. This highlights the necessity of considering both lattice mismatch and strain energy when optimizing YBCO/ABO3 interface fabrication.

3.3 Adhesion energy and charge density difference

Evaluating interface stability is crucial as it impacts the mechanical strength and durability of layered structures. Adhesion energy, which measures the energy needed to separate two adjoining surfaces, plays a key role in this assessment. According to our DFT calculations, the adhesion energies for YBCO/LAO, YBCO/STO, and YBCO/KTO interfaces are −2.16 eV, −1.85 eV, and −0.56 eV, respectively. These results indicate the strongest adhesion at the YBCO/LAO interface and the weakest at the YBCO/KTO interface. High adhesion energy signifies a strong bond that enhances structural stability and durability, while low adhesion energy indicates a weak bond, increasing the risk of interface failure under stress or in varying conditions. Negative adhesion energy values suggest favorable adhesion, implying energetic stability at the interface. These findings align with our previous analyses on lattice mismatch and strain energy, demonstrating optimal stability at the YBCO/LAO interface due to low strain and mismatch, whereas the YBCO/KTO interface shows weakness due to significant mismatch and high strain energy. This consistency across assessments highlights the reliability of our methods in evaluating interface characteristics and stability.

Closely related to adhesion energy, charge density at material interfaces significantly affects interfacial energies and properties. When surfaces come into contact, charge redistribution occurs, as investigated through the Charge Density Difference (CDD), revealing areas of charge depletion and accumulation. This phenomenon influences the bond strength between materials. Using DFT, we calculated and visualized the CDD at YBCO/ABO3 interfaces, revealing electron density redistribution and indicating charge transfer as well as electronic structure modifications. For consistent comparisons, we maintained the isosurface value at 0.02 e/Å3. It is clear that both charge depletion (cyan) and excess (yellow) near the interface of YBCO/LAO (Fig. 2a) and YBCO/STO (Fig. 2b) are comparable and larger than those at the YBCO/KTO interface (Fig. 2c). A higher CDD suggests stronger adhesion at the interface, consistent with the adhesion energy calculations in the previous section. For a more detailed analysis, as seen in Fig. 2, most of the CDD is localized on oxygen atoms (O1 and O25) at the interface. It is observed that oxygen ions throughout the interface experience an increase in charges while adjacent cations, which are La, Sr, K, Ta, Ti, Al and Cu, undergo a loss of charge. However, the majority of the charge accumulated on the interfacial oxygen originates from its neighboring atoms at A-site, which are La, Sr and K atoms. Thus, the A-O bonds have larger ionicity than that of Cu-O bond in these interfaces. Overall, the bonding at the interface exhibits a mixture of ionic and covalent characteristics. Note that, the charges dispersed across Cu-O at interface are interconnected (in YBCO/KTO observed at lower isosurface values, shown in Fig. 2d), resulting from the robust hybridization of Cu-d orbitals with O-p states. This mixing of orbitals leads to the formation of new electronic states. Some of these new bands may have higher energy levels compared to the original Cu-d and O-p states. Therefore, when electrons occupy these higher-energy states, it can easily lead to charge depletion.Fig. 2 Charge density difference (CDD) at the interfaces of (a) YBCO/LAO, (b) YBCO/STO and (c, d) YBCO/KTO. The depleted (cyan) and excess (yellow) electrons are shown.

Fig. 2

To further quantify the charge transfer at the interfaces, we used Bader charge analysis [20,21]. This method provides a detailed quantification of charge transfer and interfacial bonding by partitioning the charge density into regions associated with each atom, based on zero-flux surfaces of the charge density gradient. The analysis reveals the ionic charge states of interfacial atoms, highlighting the charge redistribution between Cu3 and O25 as well as between A4 (La, Sr, K) and O1, as presented in Fig. 2. Specifically, at the YBCO/LAO interface, Cu3 exhibits a charge state of +0.92 while O25 has −1.34, indicating significant charge transfer and strong bonding interaction. Similarly, La4 and O1 at the interface retain charge states of +2.03 and −1.12, reflecting their roles in stabilizing the interface through ionic interactions. At the YBCO/STO interface, Cu3 has a charge state of +0.90 and O25 of −1.20, with Sr4 and O1 showing charge states of +1.53 and −1.09, demonstrating a similar but slightly reduced charge transfer compared to the LAO interface. For YBCO/KTO, the charge state of Cu3 is +0.88 and O25 is −1.07, while K4 and O1 have charge states of +0.78 and −1.07, indicating a weaker ionic interaction at this interface. These ionic charges are less positive (or less negative) than the nominal charges, indicating a significant covalent character in the bonding. This is because the sharing of electrons between atoms reduces the effective charge on the ions.

We then compared lattice mismatches, strain energies, and adhesion energies across YBCO/ABO3 interfaces using a heatmap, as shown in Fig. 3. The heatmap includes annotated numbers for precise values and uses colors to indicate relative magnitudes, with ranks provided for each parameter. YBCO/LAO and YBCO/STO interfaces show the most compatible lattice structures, ranking first and second, respectively, aligning with studies that report high-quality interfaces [7,22]. Conversely, YBCO/KTO exhibits significant mismatches, indicating potential structural strain. Strain energy analysis highlights YBCO/STO as the most stable, with the lowest strain energy, while YBCO/KTO, experiences higher strain energy, indicating stress at the interface. Adhesion energy assessment shows YBCO/LAO with the strongest adhesive forces, suggesting superior structural integrity. YBCO/KTO, with the lowest adhesion energy, reveals weaker binding forces. Consequently, both YBCO/LAO and YBCO/STO show high ranks in terms of lattice mismatch, strain, and adhesion energy. However, the stability measures slightly favor YBCO/LAO. In contrast, the larger strain and weaker adhesion of YBCO/KTO highlight its challenges, which may explain the limited research on this interface. Finally, to quantitatively address the relationship between lattice mismatches, strain energies, and adhesion energies, we utilized scaling functions to fit the strain energy (Ustrain) and adhesion energy (Eadhesion), yielding the following formulas: Ustrain = −0.008Ma2 + 0.044Mb2 + 0.067 and Eadhesionc = 9.586Ustrain + 0.173/Ustrain − 5.00, where Ma and Mb are lattice mismatches in the a and b directions, respectively. The quadratic form of the first formula reflects that strain energy storage is proportional to the square of the deformation. The adhesion energy formula comprises terms directly and inversely proportional to strain energy. The direct term indicates stronger bonding with increased strain energy, whereas the inverse term represents bond weakening under excessive strain, balancing the overall energy contribution. These scaling functions offer a significant advantage by providing a simplified yet accurate representation of complex relationships between lattice mismatch, strain energy, and adhesion energy, aiding in the prediction and optimization of these material interfaces. However, it is important to note that while the predicted values closely match our DFT calculations, the precision of empirical models should be interpreted with caution. These formulas may not capture all physical phenomena and could be inaccurate when applied beyond their developed conditions.Fig. 3 Heatmap representing the lattice mismatch, strain energy, and adhesion energy at the YBCO/LAO, YBCO/STO, and YBCO/KTO interfaces. A diverging color scale illustrates values: red signifies higher and blue lower values for lattice mismatch (magnitude in %), strain energy (eV), and adhesion energy (eV). Specific values are annotated, with ranks in parenthesis (lower numbers indicate better stability) to facilitate quick comparison of interface stability and coherence. The combination of ranks and color intensities highlight YBCO/LAO as exhibiting optimal stability and coherence.

Fig. 3

4 Conclusion

In this work, we employed DFT calculations to investigate the interfacial properties of YBCO/ABO3 heterostructures, namely YBCO/LAO, YBCO/STO, and YBCO/KTO interfaces. By analyzing lattice mismatch, calculating strain energy, and evaluating adhesion energy, we extracted valuable stability information for these interfaces. Our investigations into lattice mismatches revealed that KTO exhibited the largest positive mismatches with YBCO along both the a-axis and b-axis, indicating significant interfacial strain. From a strain energy perspective, STO and LAO showed low strain energy, suggesting a better match with YBCO, whereas KTO faced potential challenges in interface preparation and stability due to higher strain energy. Adhesion energy assessments further confirmed the strong bonds between YBCO and LAO, as well as YBCO and STO, in contrast to the YBCO/KTO interface, which showed the weakest adhesion. These results indicate that interface stability favors LAO and STO, with LAO being slightly more favorable overall. Additionally, CDD and Bader charge analysis highlighted the mixture of covalent and ionic characters at the interfaces, with ionicity primarily due to interactions between A-site atoms and oxygen ions. The YBCO/KTO interface was notable not only for its lattice mismatches and strain energy but also for its relatively lower levels of charge accumulation and depletion, suggesting weaker bonding strength and relative instability. These findings suggest further opportunities to deepen our understanding of complex interface structures by detailed examination of various interface properties and the distribution of charge density differences.

CRediT authorship contribution statement

Yongyut Laosiritaworn: Writing – review & editing, Supervision, Investigation, Formal analysis. Atchara Punya Jaroenjittichai: Writing – original draft, Methodology, Investigation, Funding acquisition, Data curation, Conceptualization.

Declaration of generative AI and AI-assisted technologies in the writing process

During the preparation of this work the authors used ChatGPT in order to check grammar and improve readability. After using this tool/service, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research has received funding support from the NSRF via the Program Management Unit for Human Resources & Institutional Development, Research and Innovation [grant number B37G660011 and B39G670018 ], National Higher Education Science Research and Innovation Policy Council. Partial support from 10.13039/501100002842 Chiang Mai University is also acknowledged.
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