Session T57: Metallic and Superconducting Two-Dimensional Electronic States in Complex Oxides

Understanding and Controlling Electronic Structures in Transition Metal Oxides: Insights from Photoemission Spectroscopy on SrVO3 and LaTiO3 Films

Due to electronic correlations, transition metal oxides (TMOs) exhibit a rich phase diagram associated with specific functionalities. To utilize them in devices, the understanding and control of the microscopic electronic structures has yet to be improved, especially for thin films and heterostrcutures. High-energy photoemission spectroscopy has emerged as a powerful tool, being able to differentiate between surface and bulk electronic structure and give access to both core-level and valence band information. We present two case studies: (1) Using the prototypical correlated metal SrVO₃ as an example, we demonstrate that the usual description of TMO films as ideally terminated with stoichiometric composition overlooks an essential ingredient: oxygen adsorbing at the surface apical sites. The oxygen adatoms, which are present even if the films are kept in an ultrahigh vacuum environment and not exposed to air, are shown to severely affect the intrinsic electronic structure of a TMO film. Their presence leads to the formation of an electronically dead surface layer but also alters the band filling and the electron correlations in thin films. These findings highlight that it is important to consider the specific oxygen configuration imposed by a capping layer to predict the behavior of ultrathin films of TMOs near the 2D limit.¹ (2) In LaTiO₃ films, we examine the electronic structure across the band-filling induced metal-insulator transition, which could be of use in future Motttronic devices.² While LaTiO₃ films tend to grow oxygen-rich, the oxygen content can be controlled (reduced) by X-ray irradiation, thus changing the electronic configuration towards d¹ (=Mott insulator). This allows us to observe mass renormalization as in a Brinkman- Rice scenario by angle-resolved photoemission, consistent with thermodynamic data of (La,Sr)TiO₃. Additionally, we infer different correlation strengths for the surface and bulk of the films from our photoemission data, while Brinkman-Rice-like behavior is observed in both cases.

¹Adv. Electron. Mater. 2022, 0, 2101006

² Adv. Mater. 2018, 30, 1706708   

Effect of uniaxial strain on KTaO3 superconductivity

The pairing mechanism of superconductivity in oxide-based heterostructures has been a topic of enduring debate. Among the numerous proposed mechanisms, those connected to structural transitions have gained prominence. Oxides like undoped SrTiO3 and KTaO3 exhibit quantum paraelectric behavior, which can be manipulated towards ferroelectricity through various means such as electric fields, oxygen substitution, hydrostatic pressure, and strain. Several theoretical studies [1][2][3] propose a potential link between superconductivity and ferroelectricity in these emerging ferroelectrics [1]. To explore the potential correlation, we conduct transport experiments at millikelvin temperatures under applied uniaxial stress at the LaAlO3/KTaO3 interface. We compare our findings with strain-dependent studies of SrTiO3-based devices.

 

[1]  S. E. Rowley, C. Enderlein, J. F. de Oliveira, D. A. Tompsett, E. Baggio Saitovitch, S. S. Saxena, and G. G. Lonzarich, Superconductivity in the vicinity of a ferroelectric quantum phase transition, (2018), arXiv:1801.08121 [cond-mat.supr-con].

[2] J. M. Edge, Y. Kedem, U. Aschauer, N. A. Spaldin, and A. V. Balatsky, Quantum critical origin of the superconducting dome in SrTiO3, Phys. Rev. Lett. 115, 247002 (2015).

[3] S. E. Rowley, L. J. Spalek, R. P. Smith, M. P. M. Dean, M. Itoh, J. F. Scott, G. G. Lonzarich, and S. S. Saxena, Ferroelectric quantum criticality, Nat. Phys. 10, 367 (2014).   

Coexistence of superconductivity and magnetism in AlOx/KTaO3 two-dimensional electron gases

We report on measurements of the magnetoresistance of two-dimensional electron gases (2DEGs) that form at the surface of KTaO₃ (KTO) with three different surface crystal orientations: (001),  (110) and (111).  The 2DEGs are fabricated by depositing a thin layer of AlO_x on the KTO surface.  Low temperature magnetic field measurements of the longitudinal and transverse (Hall) resistance show hysteresis in fields both perpendicular to and parallel with the plane of the 2DEGs, a signature of underlying long range magnetic order.  This hysteretic behavior persists as the temperature is lowered and the (110) and (111) oriented samples become superconducting, showing a coexistence of superconductivity and magnetism, similar to what has been observed earlier in LAO/STO devices.  We report on the behavior of the samples in the superconducting state in both perpendicular and parallel magnetic fields.   

Strain Tunable Quantum Oscillations at the LaAlO3/SrTiO3 Interface

The correlated oxide interface LaAlO₃/SrTiO₃ (LAO/STO) manifests distinctive emergent phenomena, including quantum oscillations^[1], strong Rashaba spin-orbit coupling^[2] (SOC), and electron pairing without superconductivity^[3]. These properties can usually be tuned by electric and magnetic fields and temperature. Here we realize a continuous strain tunning of the LAO/STO interface at 1.5 K through a homemade amplified strain cell. Consequently, an ultra-low-field quantum oscillation gradually emerges as carrier density and mobility are greatly tuned by the compressive strain. Further combined with a milli-Kelvin atomic force microscope, our platform is promising to explore strain tunable behaviors in real space and by quantum transport at oxide interfaces.   

Enhancing the reproducibility of high-mobility γ-Al2O3/SrTiO3 interfaces

For almost two decades, oxide-based two-dimensional electron gasses (2DEGs) have been intensively studied. While most studies focus on the LaAlO₃/SrTiO₃ interface, the oxide interface with the highest achieved charge carrier mobility, μ = 140 000 cm2V-1s-1 at 2K, is γ-Al₂O₃/SrTiO₃.[1] This is an attractive property as it allows for mean free path dependent quantum and spin-charge inter-conversion measurements[2], and has potential in applications such as high-frequency devices, transparent conductors, and thermoelectric materials.[3] Here, I will present our attempt to increase the reproducibility of high-mobility γ-Al₂O₃/SrTiO₃ 2DEGs using pulsed laser deposition.

References

[1] Y. Z. Chen, N. Bovet, F. Trier, D. V. Christensen, F. M. Qu, N. H. Andersen, T. Kasama, W. Zhang, R. Giraud, J. Dufouleur, T. S. Jespersen, J. R. Sun, A. Smith, J. Nygård, L. Lu, B. B¨uchner, B. G. Shen, S. Linderoth, and N. Pryds, Nature Communications 4, 1371 (2013).

[2] M. Oltscher, M. Ciorga, M. Utz, D. Schuh, D. Bougeard, and D. Weiss, Phys. Rev. Lett. 113, 236602 (2014).

[3] F. Trier, D. V. Christensen, and N. Pryds, Journal of Physics D: Applied Physics 51, 293002 (2018).   

Imaging Ferroelastic Domains in LAO/STO Nanostructures using Cryogenic PFM

LaAlO3 (LAO) and SrTiO3 (STO) nanostructures exhibit a wide range of interesting quantum phenomena, such as superconductivity and a tunable metal-insulator transition, which offers great promise for application toward quantum computation and quantum devices. The pairing "glue" responsible for superconductivity in STO is still debated. STO undergoes a structural phase transition at T=105 K, and there is evidence that ferroelastic domains strongly influence superconductivity in the LAO/STO heterostructures and nanostructures. We use piezoresponse force microscopy (PFM), a non-destructive measurement technique, to spatially map the ferroelastic domain structure in conductive nanostructures created by conductive atomic force microscopy lithography. The experiments are performed below the structural phase transition, enabling a unique identification of ferroelastic domain structures with conductive patterns   

Metal-insulator transition in Pd1-xCuxCrO2 delafossite thin films

ABO₂ delafossite oxides possess a layered crystal structure, providing a highly conducting 2D channel for electronic conduction with a large mean free path. The overall conductivity is governed by the choice of A-site elements. For instance, Cu- and Ag-based delafossites are insulating or semiconducting, while Pd- and Pt-based delafossites are metallic. In addition, the B-site cation can be chosen to make the material either magnetic (Cr, Fe) or non-magnetic (Co, Rh). However, the research on delafossite thin films has been limited due to the difficulty in synthesizing high-quality epitaxial thin films. We recently discovered the use of CuCrO₂ as a buffer layer critically enables the epitaxy of PdCrO₂ thin films. In this study, we further attempted to synthesize Pd_1-xCu_xCrO₂ thin films using pulsed laser epitaxy (PLE) to understand the electronic structure evolution, yielding a metal-insulator transition. We conducted a systematic analysis of the structure and electronic structure as well as transport property of A-site-doped delafossite thin films across various Cu doping levels. Our observations of Cu concentration-dependent electronic structures in transport and optical conductivity measurements suggest a clear transition of the band structure in the Pd_1-xCu_xCrO₂ system. We also examined the materials to check the viability of inducing cation-ordered structure that may lead to a topological band structure, and the overall result will be systematically compared to explain the challenges associated with the precision control of cation ordering. Our findings underscore the potential and challenges for manipulating the band structure of delafossite oxide thin films.   

Surface acoustic waves studies on STO and KTO based heterostructures

LAO/STO attracts interest due to its properties like gate tuneable 2DEG, ferroelectricity, ferroelasticity , superconductivity, etc. KTO, despite being structurally similar to STO, does not show a bulk ferroelastic transition.The superconducting Tc of KTO is dependent on its crystal orientation with Tc ~ 2K along [111] and 50mK along [001].  We use surface acoustic waves (SAW) to probe possible structural phase transitions in these heterostructures. SAW are generated using interdigitated transducers and are sensitive to surface phonon modes. We also discuss the possibility of integrating SAWs with nanostructures to coherently shuttle electrons or electron pairs.   

Unconventional Shubnikov-de Haas oscillations at the EuO/KTaO3 interface

The coexistence of electric-field controlled superconductivity and spin-orbit interaction in two-dimensional electron gas (2DEG) based on complex oxides (e.g., SrTiO3 and KTaO3) hold great promise for advancement in spintronics and quantum computing. However, a comprehensive understanding of the electronic structure that gives rise to the multifunctional character of these 2DEG remains elusive.

To address this, we recently investigated Shubnikov-de Haas oscillations in EuO/KTaO3 in high magnetic fields (60 T) and at low temperatures (0.5 K). Remarkably, we observed a progressive increase in cyclotron mass and oscillation frequency with the magnetic field, indicating the presence of non-trivial electronic bands induced by spin-orbit interaction [1]. Besides providing experimental evidence for non-trivial electronic states in oxides-2DEG, these findings shed light on the recent predictions of topological states in the 2DEG based on similar perovskite transition metal oxides.   

Capacitance enhancement from charge-structure coupling in LaAlO3-SrTiO3

The LaAlO₃-SrTiO₃ (LAO-STO) heterostructure is known to host an electron gas of two-dimensional character (2DEG) at its interface. There is evidence of an enhancement in the measured differential capacitance of top-gated LAO-STO [1,2], in excess of the geometric capacitance. In this work, we present a phenomenological model that features a coupling between charge (in 2DEG) and structural distortion (in surface layers of STO), and calculate the modification of differential capacitance due to this term. We classify the phases of single-domain STO coupled to 2DEG charge, account for the contribution from ferroelastic domain boundaries, and make connections with data available from experiments.

[1] L. Li, C. Richter, S. Paetel, T. Kopp, J. Mannhart, and R. C. Ashoori, Science 332(6031), 825 (2011).

[2] F. Bi, M. Huang, C.-W. Bark, S. Ryu, S. Lee, C.-B. Eom, P. Irvin, and J. Levy, J. Appl. Phys. 119, 025309 (2016).   

Hidden anisotropy for electrons at a 2D oxide interface

The recent discovery of 2D superconductivity at interfaces of  the strongly spin-orbit coupled cubic perovskite KTaO₃ (KTO) offers a new playground for exploring emergent quantum phenomenon. Here we report on observation of anisotropic transport properties within KTO (110) two-dimensional electron gases (2DEG). When the KTO (110) 2DEG  is realized using non-magnetic overlayers, we observe anisotropy similar to that found in 2DEGs at SrTiO₃ (110) interfaces. However,  when the KTO (110) 2DEG is realized using a magnetic overlayer, we find anisotropic behavior that is consistent with an anisotropic spin susceptibility, which suggests a peculiar spin texture at the Fermi surface. In particular, the critical fields for 2D superconductivity in heterostructures with magnetic overlayer along different crystal axis shows a competition between orbital and spin susceptibilities. In addition, we observed an anisotropic enhancement of quantum interference of the 2D electrons within the heterostructure with magnetic overlayer at low temperature.