The interface between the two band insulators SrTiO3 and LaAlO3 has the unexpected properties of a two-dimensional electron gas. It is even superconducting with a transition temperature, Tc, that can be tuned using gate bias Vg, which controls the number of electrons added or removed from the interface. The gate bias–temperature (Vg, T) phase diagram is characterized by a dome-shaped region where superconductivity occurs, that is, Tc has a non-monotonic dependence on Vg, similar to many unconventional superconductors. Here, we report, the frequency of the quantum resistance-oscillations versus inverse magnetic field for various Vg. This frequency follows the same non-monotonic behaviour as Tc; a similar trend is seen in the low field limit of the Hall coefficient. We theoretically show that electronic correlations result in a non-monotonic population of the mobile band, which can account for the experimental behaviour of the normal transport properties and the superconducting dome.

Layered van der Waals (vdW) materials are emerging as one of the most versatile directions in the field of quantum condensed matter physics. They allow an unprecedented control of electronic properties via stacking of different types of two-dimensional (2D) materials [1,2] and, moreover, by tuning the relative angle between them [3,4]. A fascinating frontier, largely unexplored, is the stacking of strongly-correlated phases of matter in vdW materials. Here, we study 4Hb-TaS 2 , which naturally realizes an alternating stacking of a Mott insulator, recently reported as a gapless spin-liquid candidate(1T-TaS 2 ) [5,6], and a 2D superconductor competing with charge-density wave order (1H-TaS 2 ) [7]. This raises the question of how these two components affect each other. We find a superconducting ground state with a transition temperature of 2.7K, which is significantly elevated compared to the 2H polytype (Tc=0.7K). Strikingly, the superconducting state exhibits signatures of timereversal-symmetry breaking abruptly appearing at the superconducting transition, which can be naturally explained by a chiral superconducting state.Chiral superconductors (SCs) have received much attention in recent years as a promising platform for hosting Majorana bound states in the vortex cores or at sample edges due to the topological nature of their ground states [8,9]. In 2D, chiral superconductors are characterized by a Chern number. The Majorana bound states are predicted to possess non-Abelian statistics, which makes them candidates for performing fault-tolerant quantum computations [10]. The order parameter of these chiral states break time-reversal symmetry (TRS), which manifests itself at edges and defects [11] and can be detected with probes such as muon spin relaxation and polar Kerr effect.Of all the known superconductors, only few exhibit signatures of TRS breaking, and even fewer are candidates for this elusive chiral phase. The best known among them are the spin-triplet superconductors Sr 2 RuO 4 , believed to be of p + ip symmetry [12] and UPt 3 [13], a potential f + if superconductor, as well as the spin-singlet heavy-fermion superconductors URu 2 Si 2 [14] and SrPtAs [15,16], which were suggested to be of d + id symmetry. Open questions remain, however, in all cases [17,18].In this work, we show evidence for chiral superconductivity in the transition-metal dichalcogenide (TMD) 4Hb-TaS 2 . We show that this polymorph of TaS 2 is a superconductor with a relatively high T c , anomalous transport properties and that it exhibits a spontaneous appearance of magnetic moments with the onset of superconductivity.4Hb-TaS 2 belongs to the P 63/mmc hexagonal space group, with a unit cell that consists of alternating layers of 1H-TaS 2 (half of 2H-TaS 2 ) and 1T-TaS 2 , see Fig. 1(a). The overall crystal is inversion symmetric, with the inversion point lying in the center of the 1T layer. The weak interlayer coupling allows to describe 4Hb-TaS 2 as a stack of 2D monolayers: 1H-TaS S with a locally broken inversion symmetry giving r...

The interface between the two insulating oxides SrTiO 3 and LaAlO 3 gives rise to a two-dimensional electron system with intriguing transport phenomena, including superconductivity, which are controllable by a gate. Previous measurements on the (001) interface have shown that the superconducting critical temperature, the Hall density, and the frequency of quantum oscillations, vary nonmonotonically and in a correlated fashion with the gate voltage. In this Letter we experimentally demonstrate that the (111) interface features a qualitatively distinct behavior, in which the frequency of Shubnikov-de Haas oscillations changes monotonically, while the variation of other properties is nonmonotonic albeit uncorrelated. We develop a theoretical model, incorporating the different symmetries of these interfaces as well as electronic-correlation-induced band competition. We show that the latter dominates at (001), leading to similar nonmonotonicity in all observables, while the former is more important at (111), giving rise to highly curved Fermi contours, and accounting for all its anomalous transport measurements.

In two-dimensional (2D) superconductors an insulating state can be induced either by applying a magnetic field, H, or by increasing disorder. Many scenarios have been put forth to explain the superconductor to insulator transition (SIT): dominating fermionic physics after the breaking of Cooper pairs, loss of phase coherence between superconducting islands embedded in a metallic or insulating matrix and localization of Cooper pairs with concomitant condensation of vortex-type excitations. The difficulty in characterizing the insulating state and its origin stems from the lack of a continuous mapping of the superconducting to insulating phase diagram in a single sample. Here we use the two-dimensional (2D) electron liquid formed at the interface between the two insulators (111) SrTiO3 and LaAlO3 to study the superconductor to insulator transition. This crystalline interface surprisingly exhibits very strong features previously observed only in amorphous systems. By use of electrostatic gating and magnetic fields, the sample is tuned from the metallic region, where supeconductivity is fully manifested, deep into the insulating state. Through examination of the field dependence of the sheet resistance and comparison of the response to fields in different orientations we identify a new magnetic field scale, Hpairing, where superconducting fluctuations are muted. Our findings show that vortex fluctuations excitations and Cooper pair localization are responsible for the observed SIT and that these excitations surprisingly persist deep into the insulating state.The superconductor to insulator transition is a prototypical quantum phase transition where the ground state of a 2D system transitions from a superconductor into an insulator upon changing a control parameter such as film thickness, disorder or magnetic field. This transition has been demonstrated in a variety of thin films such as bismuth [1], InO x [2-6], MoGe [7], TiN [8,9], cuprate superconductors [10,11] and more [12][13][14][15][16].The many scenarios put forth to explain the SIT can be divided into two main categories. The fermionic scenarios suggest the insulating behaviour is the result of fermionic physics dominating after the breaking of Cooper pairs [17][18][19]. In contrast, in the bosonic scenarios the insualting state coincided with and is related to the existence of Cooper pairs. The two main theoretical ideas for the bosonic insulator are loss of phase coherence between superconducting islands embedded in an insulating matrix [20][21][22] and localization of Cooper pairs with concomitant condensation of vortex excitations [23,24].Many intriguing phenomena are observed in the SIT, such as scaling near a quantum critical point [2,4,5,7,10,11,13], large magnetoresistance peaks [3,5,6,8,9] and thermally activated insulating behaviour [3,6,9,15]. However, some of these effects are not observed in all materials that exhibit a SIT, and a continuous tuning from the superconductor to the insulator state (where the sheet resistance becomes greater than h/...

Epitaxial growth of atomically-sharp interfaces serves as one of the main building blocks of nanofabrication. Such interfaces are crucial for the operation of various devices including transistors, photo-voltaic cells, and memory components. In order to avoid charge traps that may hamper the operation of such devices, it is critical for the layers to be atomically-sharp. Fabrication of atomically sharp interfaces normally requires ultra-high vacuum techniques and high substrate temperatures. We present here a new self-limiting wet chemical process for deposition of epitaxial layers from alkoxide precursors. This method is fast, cheap, and yields perfect interfaces as we validate by various analysis techniques. It allows the design of heterostructures with half-unit cell resolution. We demonstrate our method by designing hole-type oxide interfaces SrTiO3/BaO/LaAlO3. We show that transport through this interface exhibits properties of mixed electron-hole contributions with hole mobility exceeding that of electrons. Our method and results are an important step forward towards a controllable design of a p-type oxide interface.Growth methods of epitaxial thin films can be roughly categorized as physical and chemical. While physical methods (i.e. molecular beam epitaxy, pulsed laser deposition (PLD) [1]) are based on creating a beam of the film material and transporting it in vacuum onto the substrate. Chemical methods such as: chemical vapor deposition and atomic layer deposition (ALD) use a chemical precursor, a compound containing the film growth material. The precursor is transferred in vacuum onto the substrate, where the surface catalyzes the precursor dissociation reaction, and the deposition of the film. While giving very good results for film growth, these methods lack the versatility and become increasingly complex [2] when a wide variety of surface monolayers is required. In Solution monolayer epitaxy (SoME) the substrate of choice, in this case a (100) TiO 2 terminated SrTiO 3 , is submersed in a solution of a dissolved precursor of choice, at a temperature slightly lower than its decomposition temperature. Under these conditions the precursor molecules do not decompose in the solution unless they are in close proximity to the surface of the substrate, which catalyzes the decomposition of the precursor and the required material resulting in a monolayer.In our case SoME is used to grow a BaO monolayer (termination) on a SrTiO 3 substrate as described in Figure 1. We then grow an additional epitaxial layer of LaAlO 3 using traditional pulsed laser deposition, givarXiv:1710.04216v1 [cond-mat.str-el]