Two-dimensional electron liquids (2DELs) have increasing technological relevance for ultrafast electronics and spintronics, yet significant gaps in their fundamental understanding are exemplified on the prototypical SrTiO3. We correlate the exact SrTiO3(001) surface structure with distinct 2DELs through combined microscopic angle-resolved photoemission spectroscopy and non-contact atomic force microscopy on truly bulk-terminated surfaces that alleviate structural uncertainties inherent to this long-studied system. The SrO termination is shown to develop a 2DEL following the creation of oxygen vacancies, unlike the intrinsically metallic TiO2 termination. Differences in degeneracy of the 2DELs, with nearly the same band filling and identical band bending, are assigned to polar distortions of the Ti atoms in combination with spin order, supported with the extraction of fundamental electron-phonon coupling strength. These results not only resolve the ambiguities regarding 2DELs on SrTiO3 thus far, but also pave the way to manipulating band filling and spin order in oxide 2DELs in general.

Center for Photon Science Paul Scherrer Institut Villigen Switzerland

Department of Surface and Plasma Science Faculty of Mathematics and Physics Charles University Prague Czech Republic

European Theoretical Spectroscopy Facility Institute of Condensed Matter and Nanosciences Université catholique de Louvain Louvain la Neuve Belgium

Institut de Physique École Polytechnique Fédérale de Lausanne Lausanne Switzerland

Institute of Applied Physics TU Wien Vienna Austria

MAX 4 Laboratory Lund University Lund Sweden

WEL Research Institute Wavre Belgium

Zobrazit více v PubMed

Müller, K. A. & Burkard, H. SrTiO3: An intrinsic quantum paraelectric below 4 K. Phys. Rev. B19, 3593 (1979).

Yang, Z. et al. Epitaxial SrTiO3 films with dielectric constants exceeding 25,000. Proc. Natl. Acad. Sci. USA119, e2202189119 (2022). PubMed PMC

Santander-Syro, A. F. et al. Two-dimensional electron gas with universal subbands at the surface of SrTiO3. Nature469, 189 (2011). PubMed

Meevasana, W. et al. Creation and control of a two-dimensional electron liquid at the bare SrTiO3 surface. Nat. Mater.10, 114 (2011). PubMed

Wang, Z. et al. Anisotropic two-dimensional electron gas at SrTiO3(110). Proc. Natl. Acad. Sci. USA111, 3933–3937 (2014). PubMed PMC

King, P. D. C. et al. Quasiparticle dynamics and spin–orbital texture of the SrTiO3 two-dimensional electron gas. Nat. Commun.5, 3414 (2014). PubMed

Guedes, E. B. et al. Single spin-polarized fermi surface in SrTiO3 thin films. Phys. Rev. Res.2, 033173 (2020).

Guedes, E. B. et al. Universal structural influence on the 2D electron gas at SrTiO3 surfaces. Adv. Sci.8, 2100602 (2021). PubMed PMC

Rebec, S. N. et al. Dichotomy of the photo-induced 2-dimensional electron gas on SrTiO3 surface terminations. Proc. Natl. Acad. Sci. USA116, 16687–16691 (2019). PubMed PMC

Yan, X. et al. Origin of the 2D electron gas at the SrTiO3 surface. Adv. Mater.34, 2200866 (2022). PubMed

Thiel, S., Hammerl, G., Schmehl, A., Schneider, C. W. & Mannhart, J. Tunable quasi-two-dimensional electron gases in oxide heterostructures. Science313, 1942–1945 (2006). PubMed

Okamoto, S., Millis, A. J. & Spaldin, N. A. Lattice relaxation in oxide heterostructures: LaTiO3/SrTiO3 superlattices. Phys. Rev. Lett.97, 056802 (2006). PubMed

Chikina, A. et al. Orbital ordering of the mobile and localized electrons at oxygen-deficient LaAlO3/SrTiO3 interfaces. ACS Nano12, 7927–7935 (2018). PubMed

Ohtomo, A. & Hwang, H. Y. A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface. Nature427, 423 (2004). PubMed

Seo, J. et al. Feld-induced modulation of two-dimensional electron gas at LaAlO3/SrTiO3 interface by polar distortion of LaAlO3. Nat. Communs.15, 5268 (2024). PubMed PMC

Takagi, H. & Hwang, H. Y. An emergent change of phase for electronics. Science327, 1601–1602 (2010). PubMed

Sokolović, I., Schmid, M., Diebold, U. & Setvín, M. Incipient ferroelectricity: A route towards bulk-terminated SrTiO3. Phys. Rev. Mater.3, 034407 (2019).

Sokolović, I. et al. Quest for a pristine unreconstructed SrTiO3(001) surface: An atomically resolved study via noncontact atomic force microscopy. Phys. Rev. B103, L241406 (2021).

Noguera, C. Polar oxide surfaces. J. Phys. Condens. Matter12, R367 (2000).

Goniakowski, J., Finocchi, F. & Noguera, C. Polarity of oxide surfaces and nanostructures. Rep. Prog. Phys.71, 016501 (2007).

Sokolović, I. et al. Resolving the adsorption of molecular O2 on the rutile TiO2(110) surface by noncontact atomic force microscopy. Proc. Natl. Acad. Sci. USA117, 14827–14837 (2020). PubMed PMC

Sokolović, I., Schmid, M., Diebold, U. & Setvín, M. How to cleave cubic perovskite oxides. Rev. Sci. Instr.96, 035113 (2025). PubMed

Kato, T. et al. Polarity-dependent charge density wave in the kagome superconductor CsV3Sb5. Phys. Rev. B106, L121112 (2022).

To, D. Q. et al. Spin to charge conversion at Rashba-split SrTiO3 interfaces from resonant tunneling. Phys. Rev. Res.3, 043170 (2021).

Chen, C., Avila, J., Frantzeskakis, E., Levy, A. & Asensio, M. C. Observation of a two-dimensional liquid of fröhlich polarons at the bare SrTiO3 surface. Nature Commun.6, 8585 (2015). PubMed PMC

Wang, Z. et al. Tailoring the nature and strength of electron–phonon interactions in the SrTiO3(001) 2D electron liquid. Nat. Mater.15, 835 (2016). PubMed

Plumb, N. C. et al. Mixed dimensionality of confined conducting electrons in the surface region of SrTiO3. Phys. Rev. Lett.113, 086801 (2014). PubMed

Chikina, A. et al. X-ray writing of metallic conductivity and oxygen vacancies at silicon/SrTiO3 interfaces. Adv. Func. Mater.29, 1900645 (2019).

McKeown Walker, S. et al. A laser-ARPES view of the 2D electron systems at LaAlO3/SrTiO3 and Al/SrTiO3 interfaces. Adv. Electron. Mater.8, 2101376 (2022).

Alonso, M., Cimino, R. & Horn, K. Surface photovoltage effects in photoemission from metal-GaP(110) interfaces: Importance for band bending evaluation. Phys. Rev. Lett.64, 1947 (1990). PubMed

Knotek, M. L. & Feibelman, P. J. Ion desorption by core-hole Auger decay. Phys. Rev. Lett.40, 964 (1978).

Dulub, O. et al. Electron-induced oxygen desorption from the TiO2(011)-2×1 surface leads to self-organized vacancies. Science317, 1052–1056 (2007). PubMed

Setvín, M. et al. Polarity compensation mechanisms on the perovskite surface KTaO3(001). Science359, 572–575 (2018). PubMed

Queiroz, R. et al. Sputtering-induced reemergence of the topological surface state in Bi2Se3. Phys. Rev. B93, 165409 (2016).

Song, K. et al. Electronic and structural transitions of LaAlO3/SrTiO3 heterostructure driven by polar field-assisted oxygen vacancy formation at the surface. Adv. Sci.8, 2002073 (2021). PubMed PMC

Salluzzo, M. et al. Structural and electronic reconstructions at the LaAlO3/SrTiO3 interface. Adv. Mater.25, 2333 (2013). PubMed

Bahramy, M. S. et al. Emergent quantum confinement at topological insulator surfaces. Nat. Commun.3, 1159 (2012). PubMed

Kramers, H. A. Théorie générale de la rotation paramagnétique dans les cristaux. Proc. Acad. Amst. 33, 959 (1930).

Santander-Syro, A. F. et al. Giant spin splitting of the two-dimensional electron gas at the surface of SrTiO3. Nat. Mat.13, 1085–1090 (2014). PubMed

Krempaský, J. et al. Altermagnetic lifting of Kramers spin degeneracy. Nature626, 517–522 (2024). PubMed PMC

Ast, C. R. et al. Giant spin splitting through surface alloying. Phys. Rev. Lett.98, 186807 (2007). PubMed

Hsieh, D. et al. Observation of unconventional quantum spin textures in topological insulators. Science323, 919 (2009). PubMed

Krempaský, J. et al. Fully spin-polarized bulk states in ferroelectric GeTe. Phys. Rev. Res. 2, 013107 (2020).

Krempaský, J. et al. Operando imaging of all-electric spin texture manipulation in ferroelectric and multiferroic Rashba semiconductors. Phys. Rev. X8, 021067 (2018).

Rinaldi, C. et al. Ferroelectric control of the spin texture in GeTe. Nano Lett.18, 2751 (2018). PubMed PMC

Djani, H. et al. Rationalizing and engineering Rashba spin-splitting in ferroelectric oxides. npj Quantum Mater.4, 1–6 (2019).

Noël, P. et al. Non-volatile electric control of spin–charge conversion in a SrTiO3 Rashba system. Nature580, 483–486 (2020). PubMed

Franchini, C., Reticcioli, M., Setvin, M. & Diebold, U. Polarons in materials. Nat. Rev. Mater.6, 560 (2021).

Reticcioli, M. et al. Interplay between adsorbates and polarons: CO on rutile TiO2(110). Phys. Rev. Lett.122, 016805 (2019). PubMed

Yim, C. M. et al. Engineering polarons at a metal oxide surface. Phys. Rev. Lett.117, 116402 (2016). PubMed

Kaviani, M., Strand, J., Afanas’ev, V. V. & Shluger, A. L. Deep electron and hole polarons and bipolarons in amorphous oxide. Phys. Rev. B94, 020103 (2016).

Di Valentin, C. & Selloni, A. Bulk and surface polarons in photoexcited anatase TiO2. J. Phys. Chem. Lett.2, 2223 (2011).

Wang, Z. et al. Surface chemistry on a polarizable surface: Coupling of CO with KTaO3(001). Sci. Adv.8, eabq1433 (2022). PubMed PMC

Reticcioli, M. et al. Competing electronic states emerging on polar surfaces. Nat. Commun.13, 1 (2022). PubMed PMC

Ellinger, F., Shafiq, M., Ahmad, I., Reticcioli, M. & Franchini, C. Small polaron formation on the Nb-doped SrTiO3(001) surface. Phys. Rev. Mater.7, 064602 (2023).

Rashba, E. Properties of semiconductors with an extremum loop. I. Cyclotron and combinational resonance in a magnetic field perpendicular to the plane of the loop. Sov. phys., Solid state2, 1109 (1960).

Varotto, S. et al. Direct visualization of Rashba-split bands and spin/orbital-charge interconversion at KTaO3 interfaces. Nat. Commun.13, 6165 (2022). PubMed PMC

Šmejkal, L., Sinova, J. & Jungwirth, T. Emerging research landscape of altermagnetism. Phys. Rev. X12, 040501 (2022).

Hellenes, A. B., Jungwirth, T., Sinova, J. & Šmejkal, L. Exchange spin-orbit coupling and unconventional p-wave magnetism. arXiv preprint arXiv:2309.01607 (2023).

Vogt, H. Hyper-Raman tensors of the zone-center optical phonons in SrTiO3 and KTaO3. Phys. Rev. B38, 5699–5708 (1988). PubMed

Johnson, P. D. et al. Doping and temperature tependence of the mass enhancement observed in the cuprate Bi2Sr2CaCu2O8+δ. Phys. Rev. Lett.87, 177007 (2001). PubMed

Bryan, R. K. Maximum entropy analysis of oversampled data problems. Eur. Biophys. J.18, 165–174 (1990).

Grimvall, G.The Electron–Phonon Interaction in Metals (North-Holland, Amsterdam, 1981).

Moser, S. An experimentalist’s guide to the matrix element in angle resolved photoemission. J. Electron Spectrosc. Relat. Phenom.214, 29–52 (2017).

Jungwirth, T. & MacDonald, A. H. Electron-electron interactions and two-dimensional–two-dimensional tunneling. Phys. Rev. B53, 7403–7412 (1996). PubMed

Jiang, J. et al. High-Resolution Angle-Resolved Photoemission Study of the Al(100) Single Crystal. e-J. Surf. Sci. Nanotechnol.7, 57–60 (2009).

Damascelli, A. Probing the electronic structure of complex systems by ARPES. Phys. Scr.T109, 61 (2004).

Verga, S., Knigavko, A. & Marsiglio, F. Inversion of angle-resolved photoemission measurements in high-Tc cuprates. Phys. Rev. B67, 054503 (2003).

Zhou, J.-J., Hellman, O. & Bernardi, M. Electron-Phonon Scattering in the Presence of Soft Modes and Electron Mobility in SrTiO3 Perovskite from First Principles. Phys. Rev. Lett.121, 226603 (2018). PubMed

Xie, Y. et al. Oxygen Vacancy Induced Flat Phonon Mode at FeSe /SrTiO3 interface. Sci Rep5, 10011 (2015). PubMed PMC

Cancellieri, C. et al. Polaronic metal state at the LaAlO3/SrTiO3 interface. Nat. Commun.7, 10386 (2016). PubMed PMC

Setvín, M. et al. Ultrasharp tungsten tips—characterization and nondestructive cleaning. Ultramicroscopy113, 152–157 (2012).

Giessibl, F. J. Sensor for noncontact profiling of a surface. US patent 8,393,009 (2013).

Huber, F. & Giessibl, F. J. Low noise current preamplifier for qPlus sensor deflection signal detection in atomic force microscopy at room and low temperatures. Rev. Sci. Instrum.88, 073702 (2017). PubMed

Schmid, M., Setvín, M. & Diebold, U. Device for suspending a load in a vibration-insulated manner. US patent WO/2018/037102 (2018).