Tailoring the nature and strength of electron–phonon interactions in the SrTiO3(001) 2D electron liquid

Abstract

Surfaces and interfaces o er new possibilities for tailoring the many-body interactions that dominate the electrical and thermal properties of transition metal oxides 1-4 . Here, we use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO 3 (001) surface 5-7 to reveal a remarkably complex evolution of electron-phonon coupling with the tunable carrier density of this system. At low density, where superconductivity is found in the analogous 2DEL at the LaAlO 3 /SrTiO 3 interface 8-13 , our angle-resolved photoemission data show replica bands separated by 100 meV from the main bands. This is a hallmark of a coherent polaronic liquid and implies long-range coupling to a single longitudinal optical phonon branch. In the overdoped regime the preferential coupling to this branch decreases and the 2DEL undergoes a crossover to a more conventional metallic state with weaker short-range electron-phonon interaction. These results place constraints on the theoretical description of superconductivity and allow a unified understanding of the transport properties in SrTiO 3 -based 2DELs.Carrier concentration is a key parameter defining the ground state of correlated electron systems. At the LaAlO 3 /SrTiO 3 interface, the 2DEL density can be tailored by field-effect gating. As the system is depleted of carriers, its ground state evolves from a high-mobility 2DEL 4 into a two-dimensional superconductor 8-10 with pseudogap behaviour 11 and possible pairing above T c (ref. 12). An analogous 2DEL can be induced by doping the (001) surface of SrTiO 3 . As for the interface, the surface 2DEL is confined by a band-bending potential in SrTiO 3 and consists of an orbitally polarized ladder of quantum confined Ti t 2g electrons that are highly mobile in the surface plane [5][6][7]14 . Thus far, the surface 2DEL has been studied only at carrier densities around 2 × 10 14 cm −2 , approximately a factor of five higher than typically observed at the LaAlO 3 /SrTiO 3 interface [5][6][7] . In the following, we present ARPES data extending to lower carrier densities that are directly comparable to the LaAlO 3 /SrTiO 3 interface. We achieve this by preparing SrTiO 3 (001) wafers in situ, which results in well-ordered clean surfaces that can be studied by ARPES over extended timescales, as they are less susceptible to the ultraviolet-induced formation of charged oxygen vacancies reported for cleaved SrTiO 3 5,7,15,16 . Details of the sample preparation are given in Methods. Figure 1a shows an energy-momentum intensity map for a 2DEL with a carrier density of n 2D ≈ 2.9 × 10 13 cm −2 estimated from the Luttinger volume of the first light subband and the two equivalent heavy subbands (see Supplementary Section 2). The most striking features of this data are replica bands at higher binding energy following the dispersion of the primary quasiparticle (QP) bands. The replica bands are all separated by approximately 100 meV and progressively lose intensity, but can be visualized up to the third replica in the curvature plot shown in Fi...

Absence of giant spin splitting in the two-dimensional electron liquid at the surface ofSrTiO3(001)

“…In conclusion, we have presented SARPES data from the STO (001) surface 2DEL prepared on in-situ fractured surfaces with a band dispersion in excellent agreement with all published HR-ARPES data [9][10][11]13,15,35,36 . Our SARPES measurements do not show any signifi-cant spin-polarization signal which is consistent with the predicted small, unconventional Rashba splitting of the 2DEL subbands.…”

“…The origin of this discrepancy remains unclear. It could be related to differences in the surfaces measured in each case, however this seems unlikely given that the subband structure observed by conventional ARPES is ubiquitous and observed consistently for very different surface preparations [9][10][11]13,15,35,36 . Alternatively, Altmeyer et al 22 proposed that an SARPES signal as was observed in Ref.…”

dxz/yz orbital subband structures and chiral orbital angular momentum in the (001) surface states of SrTiO3

“…Several theoretical and experimental studies of the surface states of STO have been performed so far [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] . Especially, exploiting its surface sensitivity, ARPES has been used to directly measure the band structures of STO metallic states.…”

Oxide Materials at the Two-Dimensional Limit

“…The EuO/SrTiO3 interface shows much smaller band filling compared to the LaAlO3/SrTiO3 case [207] that is manifested by small Fermi vector kF of the heavy dyz band (e). The waterfalls going from the band dispersions down in EB is a hallmark of all SrTiO3-based systems that signal polaronic nature of the charge carriers with their characteristic peak-dip-hump spectral response involving electron coupling to the LO3 phonon [207,208]. Furthermore, significantly smaller intensity of the EuO/SrTiO3…”