Quasiparticle dynamics and spin-orbital texture of the SrTiO3 two-dimensional electron gas

Two-dimensional electron gases (2DEGs) in SrTiO$_3$ have become model systems for engineering emergent behaviour in complex transition metal oxides. Understanding the collective interactions that enable this, however, has thus far proved elusive. Here we demonstrate that angle-resolved photoemission can directly image the quasiparticle dynamics of the $d$-electron subband ladder of this complex-oxide 2DEG. Combined with realistic tight-binding supercell calculations, we uncover how quantum confinement and inversion symmetry breaking collectively tune the delicate interplay of charge, spin, orbital, and lattice degrees of freedom in this system. We reveal how they lead to pronounced orbital ordering, mediate an orbitally-enhanced Rashba splitting with complex subband-dependent spin-orbital textures and markedly change the character of electron-phonon coupling, co-operatively shaping the low-energy electronic structure of the 2DEG. Our results allow for a unified understanding of spectroscopic and transport measurements across different classes of SrTiO$_3$-based 2DEGs, and yield new microscopic insights on their functional properties.Comment: 10 pages including supplementary information, 4+4 figure

The dynamics of ultraviolet-induced oxygen vacancy at the surface of insulating SrTiO3(001)

The effect of ultra-violet (UV) irradiation on the electronic structure and the surface resistance of an insulating SrTiO3(001) crystal is studied in this work. Upon UV irradiation, we shows that the two-dimensional electron gas (2DEG) emerges at the insulating SrTiO3 surface and there is a pronounced change in the surface resistance. By combining the observations of the change in valance band and the resistance change under different environments of gas pressure and gas species, we find that UV-induced oxygen vacancies at the surface plays a major role in the resistance change. The dynamic of the resistance change at different oxygen pressures also suggests an alternative method of low-pressure sensing.PostprintPeer reviewe

Negative electronic compressibility and tunable spin splitting in WSe2

This work was supported by the Engineering and Physical Sciences Research Council, UK (Grant Nos. EP/I031014/1, EP/M023427/1, EP/L505079/1, and EP/G03673X/1), TRF-SUT Grant RSA5680052 and NANOTEC, Thailand through the CoE Network. PDCK acknowledges support from the Royal Society through a University Research Fellowship. MSB was supported by the Grant-in-Aid for Scientific Research (S) (No. 24224009) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.Tunable bandgaps1, extraordinarily large exciton-binding energies2, 3, strong light–matter coupling4 and a locking of the electron spin with layer and valley pseudospins5, 6, 7, 8 have established transition-metal dichalcogenides (TMDs) as a unique class of two-dimensional (2D) semiconductors with wide-ranging practical applications9, 10. Using angle-resolved photoemission (ARPES), we show here that doping electrons at the surface of the prototypical strong spin–orbit TMD WSe2, akin to applying a gate voltage in a transistor-type device, induces a counterintuitive lowering of the surface chemical potential concomitant with the formation of a multivalley 2D electron gas (2DEG). These measurements provide a direct spectroscopic signature of negative electronic compressibility (NEC), a result of electron–electron interactions, which we find persists to carrier densities approximately three orders of magnitude higher than in typical semiconductor 2DEGs that exhibit this effect11, 12. An accompanying tunable spin splitting of the valence bands further reveals a complex interplay between single-particle band-structure evolution and many-body interactions in electrostatically doped TMDs. Understanding and exploiting this will open up new opportunities for advanced electronic and quantum-logic devices.PostprintPeer reviewe

Nearly-free-electron system of monolayer Na on the surface of single-crystal HfSe2

The electronic structure of a single Na monolayer on the surface of single-crystal HfSe2 is investigate dusing angle-resolved photoemission spectroscopy. We find that this system exhibits analmost perfect “nearly-free-electron” behavior with an extracted effective mass of ∼1me, in contrast to heavier masses found previously for alkali metal monolayers on other substrates. Our density functional-theory calculations indicate that this is due to the large lattice constant, causing both exchange and correlation interactions to be suppressed, and to the weak hybridization between the overlayer and the substrate. This is therefore an ideal model system for understanding the properties of two-dimensional materials.PostprintPeer reviewe

Josephson coupled Ising pairing induced in suspended MoS2 bilayers by double-side ionic gating

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