Orientational Tuning of the Fermi Sea of Confined Electrons at theSrTiO3(110) and (111) Surfaces

Abstract

We report the existence of confined electronic states at the (110) and (111) surfaces of SrTiO 3 . Using angle-resolved photoemission spectroscopy, we find that the corresponding Fermi surfaces, subband masses, and orbital ordering are different from the ones at the (001) surface of SrTiO 3 . This occurs because the crystallographic symmetries of the surface and subsurface planes and the effective electron masses along the confinement direction influence the symmetry of the electronic structure and the orbital ordering of the t 2g manifold. Remarkably, our analysis of the data also reveals that the carrier concentration and thickness are similar for all three surface orientations, despite their different polarities. The orientational tuning of the microscopic properties of two-dimensional electron states at the surface of SrTiO 3 echoes the tailoring of macroscopic (e.g., transport) properties reported recently in LaAlO 3 =SrTiO 3 (110) and (111) interfaces, and is promising for searching new types of two-dimensional electronic states in correlated-electron oxides.Two-dimensional electron gases (2DEGs) in transitionmetal oxides (TMOs) present remarkable phenomena that make them unique from a fundamental viewpoint and promising for applications [1,2]. For instance, heterostructures grown on the (001) surface of SrTiO 3 , a TMO insulator with a large band gap of approximately 3.5 eV, can develop 2DEGs showing metal-to-insulator transitions [3], superconductivity [4], or magnetism [5,6]. Recently, 2DEGs at the (111) and (110) interfaces of LaAlO 3 =SrTiO 3 were also reported [7]. The latter showed a highly anisotropic conductivity [8] and a superconducting state spatially more extended than the one at the (001) interface [9]. Interestingly, theoretical works have also predicted that exotic, possibly topological, electronic states might occur at interfaces composed of (111) bilayers of cubic TMOs [10-13], as two (111) planes of transition-metal ions form a honeycomb lattice, similar to the one found in graphene. In this context, the discoveries that 2DEGs can also be created at the bare (001) surfaces of SrTiO 3 [14-16] and KTaO 3 [17,18], and more recently at the (111) surface of KTaO 3 [19], open new roads in the fabrication and study of different types of 2DEGs in TMOs-in particular, using surface-sensitive spectroscopic techniques, which give direct information about the Fermi surface and subband structure of the confined states. The origin of the confinement is attributed to a local doping of the surface region due to oxygen vacancies and/or lattice distortions.Here we show that new types of 2DEGs can be directly tailored at the bare (110) and (111) surfaces of SrTiO 3 . Imaging their electronic structure via angle-resolved photoemission spectroscopy (ARPES), we find that their Fermi surfaces, subband masses, and orbital ordering are different from the ones of the 2DEG at the SrTiO 3 ð001Þ surface [14,15] and the ones predicted for the bulk, being thus uniquely sensitive to the confining crystallographic directi...

Two-dimensional superconductivity at (110) LaAlO3/SrTiO3 interfaces

“…Relatively, the study of (110) LAO/STO interfaces is in its infancy, much more attention is needed to verify the other expected phenomena similar to those of the (001) LAO/STO interfaces, such as ferromagnetic order, electric field tunable superconductor-insulator quantum phase transition and so on, and as well as to explore the stimulated new novel physics different to those of (001) LAO/STO interfaces due to the unique Ti 3d subbands [20][21][22] when added one more degree freedom of crystal anisotropy at (110) LAO/STO interfaces. The similarities of the superconductivity of (001) and (110) LAO/STO interfaces also imply that the condensed quasi 2DEG at both (001) and (110) LAO/STO interfaces may have a unified origin.…”

“…The Pauli paramagnetic limiting field is √ , where is the Bohr magneton (with a g factor of 2), is Boltzmann's constant, and 0.184 K. So the mT, while the H c2||,0K 1187 mT. Given the difference between the Ti 3d subbands of (110) LAO/STO interfaces and those of (001) LAO/STO ones, [20][21][22] e.g., the Ti 3d xz /d yz orbitals lie at the lowest energy rather than the Ti 3d xy for (110) LAO/STO, 21,22,45 while which is the opposite for (001) LAO/STO, 20,[46][47][48] it is beyond expectation that these characteristics of 2D superconductivity at the (110) LAO/STO interfaces, i.e., the values of T c , T BKT , the in-plane coherence length and the superconductive thickness, and the violations of the Pauli paramagnetic limit, are all comparable to those of (001) LAO/STO interfaces. In spite of the origin of the superconductivity of (001) LAO/STO interfaces, also that of (110) LAO/STO interfaces, is not fully understood, the unexpected similarity suggests that the ground state and the mechanism of the superconductivity at both (001) and (110) LAO/STO interfaces should be same.…”

“…What is more, because there are two different crystal orientations, i.e., (010) and (1)(2)(3)(4)(5)(6)(7)(8)(9)(10), in the plan of (110) LAO/STO interfaces, the added degree freedom of crystal anisotropy would play a role and may produce newfangled physics. [20][21][22] Therefore, the discovery of the quasi 2DEG at (110) LAO/STO interfaces injects new vigor and presents new opportunities into the research of heterojunctions of TMO. While the deep study on (110) LAO/STO interfaces is lack, particularly, the ground state, whether the electrons condensed into a ferromagnetic state or a superconducting state when the temperature approaches absolute zero, has not been reported by now.…”

Engineering two-dimensional electron gases at the (001) and (101) surfaces ofTiO2anatase using light

“…The archetypal example, SrTiO 3 -based heterostructures, display many fundamentally interesting properties [10][11][12], such as field-effect induced insulator-to-superconductor transitions [13], magnetism [14] and the coexistence of magnetism and superconductivity [15]. More recently, the discoveries of 2DEGs at the bare (001), (110) and (111) surfaces of SrTiO 3 [16][17][18][19][20][21] and KTaO 3 [22][23][24], triggered new avenues of research. Additionally, very recent spin-resolved measurements of the 2DEG on SrTiO 3 (001) revealed a giant spin splitting of its electronic structure [25], epitomizing the richness of physical properties that can be found in these 2DEGs.…”

Magnetoresistance in the superconducting state at the (111) LaAlO3/SrTiO3 interface

“…[3][4][5][6][7][8] In the past few years, interest has grown in LAO/STO interfaces in the (111) crystal orientation. The Ti atoms at the (111) LAO/STO interface have hexagonal symmetry, 11,12 and consequently have been predicted to have potential topological properties, 13 and to possibly host time-reversal symmetry breaking superconducting pairing states.…”