The two-dimensional electron system at the interface between LaAlO 3 and SrTiO 3 has several unique properties that can be tuned by an externally applied gate voltage. In this work, we show that this gate tunability extends to the effective band structure of the system. We combine a magnetotransport study on top-gated Hall bars with self-consistent Schrödinger-Poisson calculations and observe a Lifshitz transition at a density of 2.9 × 10 13 cm −2 . Above the transition, the carrier density of one of the conducting bands decreases with increasing gate voltage. This surprising decrease is accurately reproduced in the calculations if electronic correlations are included. These results provide a clear, intuitive picture of the physics governing the electronic structure at complex-oxide interfaces. DOI: 10.1103/PhysRevLett.118.106401 The two-dimensional electron system (2DES) at the interface between the band insulators LaAlO 3 (LAO) and SrTiO 3 (STO) displays many intriguing phenomena, which may be harnessed for novel electronic devices [1][2][3][4][5]. The discovery of superconductivity [6], magnetic signatures [7][8][9][10], and their apparent coexistence [11] sparked growing interest in this material system. These properties can be tuned by varying parameters during growth [4,7], as well as by an externally applied electric field after growth [12]. Using this field effect, control of superconductivity [13][14][15][16][17], of spin-orbit coupling [17][18][19][20], and of carrier mobility [21,22] has been reported. Recent progress on local control of superconductivity [16] opened a route towards electrically controlled oxide Josephson junctions [23,24], providing new opportunities for superconducting electronic devices. Because these phenomena are related to the interfacial band structure, a fundamental understanding of the band structure is vital for the understanding of these phenomena and their exploitation in electronic devices.The interface band structure is formed by the conduction band of STO, which is bent down at the interface and crosses the Fermi level [25]. The origin of the band bending is still an open question [26][27][28]. This band bending creates a potential well, confining the carriers to a few nanometers in the out-of-plane direction [29][30][31][32]. In the well, the effective band structure is formed by the Ti t 2g orbitals. For interfaces grown along the [001] direction, the in-plane oriented d xy bands lie below the out-of-plane oriented d yz;xz bands in energy due to the confinement, as measured using x-ray absorption [33].By backgating the interface through the STO substrate, an additional conduction channel was observed to emerge above a carrier density of ð1.7 AE 0.1Þ × 10 13 cm −2 [34]. This observation was linked to tuning the Fermi level across the bottom of the d yz;xz bands, making additional electron pockets available for conduction. As a result, the Fermi surface topology changes, which is the characteristic feature of a Lifshitz transition [35].The model proposed by Joshua et...

In situ spectroscopy of intrinsic Bi2Te3 topological insulator thin films and impact of extrinsic defects Ngabonziza, P.; Heimbuch, R.; de Jong, N.; Klaassen, R.A.; Stehno, M.P.; Snelder, M.; Solmaz, A.; Ramankutty, S.V.; Frantzeskakis, E.; van Heumen, E.; Koster, G.; Golden, M.S.; Zandvliet, H.J.W.; Brinkman, A. Published in:Physical Review B DOI:10.1103/PhysRevB.92.035405 Link to publicationCitation for published version (APA): Ngabonziza, P., Heimbuch, R., de Jong, N., Klaassen, R. A., Stehno, M. P., Snelder, M., ... Brinkman, A. (2015). In situ spectroscopy of intrinsic Bi2Te3 topological insulator thin films and impact of extrinsic defects. Physical Review B, 92(3), [035405]. https://doi.org/10.1103/PhysRevB.92.035405 General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Combined in situ x-ray photoemission spectroscopy, scanning tunneling spectroscopy, and angle resolved photoemission spectroscopy of molecular beam epitaxy grown Bi 2 Te 3 on lattice mismatched substrates reveal high quality stoichiometric thin films with topological surface states without a contribution from the bulk bands at the Fermi energy. The absence of bulk states at the Fermi energy is achieved without counterdoping. We observe that the surface morphology and electronic band structure of Bi 2 Te 3 are not affected by in vacuo storage and exposure to oxygen, whereas major changes are observed when exposed to ambient conditions. These films help define a pathway towards intrinsic topological devices.

Combining the ability to prepare high-quality, intrinsic Bi2Te3 topological insulator thin films of low carrier density with in-situ protective capping, we demonstrate a pronounced, gate-tunable change in transport properties of Bi2Te3 thin films. Using a back-gate, the carrier density is tuned by a factor of ∼ 7 in Al2O3 capped Bi2Te3 sample and by a factor of ∼ 2 in Te capped Bi2Te3 films. We achieve full depletion of bulk carriers, which allows us to access the topological transport regime dominated by surface state conduction. When the Fermi level is placed in the bulk band gap, we observe the presence of two coherent conduction channels associated with the two decoupled surfaces. Our magnetotransport results show that the combination of capping layers and electrostatic tuning of the Fermi level provide a technological platform to investigate the topological properties of surface states in transport experiments and pave the way towards the implementation of a variety of topological quantum devices. PACS numbers: 73.50.-h, 73.20.-r, 75.47.-m arXiv:1604.03722v1 [cond-mat.mes-hall]

Topological insulators (TIs) hold great promise for topological quantum computation in solidstate systems. Recently, several groups reported experimental data suggesting that signatures of Majorana modes have been observed in topological insulator Josephson junctions (TIJJs). A prerequisite for the exploration of Majorana physics is to obtain a good understanding of the properties of low-energy Andreev bound states (ABS) in a material with topologically non-trivial band structure. Here, we present experimental data and a theoretical analysis demonstrating that the band structure inversion close to the surface of a TI has observable consequences for supercurrent transport in TIJJs prepared on surface-doped Bi 2 Se 3 thin films. Electrostatic carrier depletion of the film surface leads to an abrupt drop in the critical current of such devices. The effect can be understood as a relocation of low-energy ABS from a region deeper in the bulk of the material to the more strongly disordered surface which is driven by the topology of the effective band structure in the presence of surface dopants.

We investigate transport and shot noise in lateral N-TI-S contacts, where N is a normal metal, TI is a Bi-based three dimensional topological insulator (3D TI), and S is an s-type superconductor. In the normal state, the devices are in the elastic diffusive transport regime, as demonstrated by a nearly universal value of the shot noise Fano factor FN ≈ 1/3 in magnetic field and in a reference normal metal contact. In the absence of magnetic field, we identify the Andreev reflection (AR) regime, which gives rise to the effective charge doubling in shot noise measurements. Surprisingly, the Fano factor FAR ≈ 0.22 ± 0.02 is considerably reduced in the AR regime compared to FN, in contrast to previous AR experiments in normal metals and semiconductors. We suggest that this effect is related to a finite thermal conduction of the proximized, superconducting TI owing to a residual density of states at low energies.A surface state of a three-dimensional topological insulator (3D TI) is a unique example of a spin-orbit coupled and symmetry protected conductor 1 . Similar to graphene, in 3D TI the surface electronic states are massless Dirac fermions. Unlike in graphene, however, a single Dirac cone is lacking spin and valley degeneracies. That makes a 3D TI an intriguing candidate for the realization of a solid state two-dimensional topological superconductor 2 . As originally proposed by Fu and Kane 3 , p-wave like superconducting correlations are expected to emerge via proximity coupling the 3D TI to a conventional stype superconductor (S). This gives rise to symmetry protected Majorana zero modes bound at vortices or at the boundaries of various hybrid structures 4,5 . Emerging zero modes are predicted to have a strong impact on the low energy physics, modifying Andreev reflection (AR) at the interface with a normal metal 6 , affecting the edge conductance distribution 7 , noise 8 and thermal transport 9,10 .Proximity induced superconductivity has been demonstrated in 3D TIs based on Bi 11-17 and HgTe 18,19 with the reported values of the induced gap on the order of a few 100 µeV. Similar gap values were recently observed via Andreev spectroscopy 20 in quaternary BiSbTeSe compound, established as a 3D TI with negligible contribution of the bulk conduction 21,22 . In spite of these advances, the microscopic nature of the proximity induced gap in Bi-based 3D TIs remains largely unexplored. This particularly concerns the statistics of the transmission eigenvalue distribution in short junctions and the role of possible in-gap states. On this route, valuable information, often hidden in transport, can be obtained via measurements of the non-equilibrium current fluctuationsthe shot noise 23 . Relevant for the AR, prominent examples include unusual shot noise behavior at the interface between a normal metal and superconductors with other than s-type order parameter symmetry 24,25 and, more recently, shot noise detection of the thermal and charge transport via Majorana zero modes 9,10,26 .Here, we investigate the AR in latera...