The two-dimensional electron system at the interface between the insulating oxides LaAlo 3 and srTio 3 exhibits ferromagnetism, superconductivity and a range of unique magnetotransportproperties. An open experimental challenge is to identify, out of the multitudinous energy bands predicted to exist at the interface, the key ingredients underlying its emergent transport phenomena. Here we show, using magnetotransport measurements, that a universal Lifshitz transition between d orbitals of different symmetries lies at the core of the observed phenomena. We find that LaAlo 3 /srTio 3 systems generically switch from one-to two-carrier transport at a universal carrier density, which is independent of the LaAlo 3 thickness and electron mobility. Interestingly, the maximum superconducting critical temperature occurs also at the Lifshitz density, indicating a possible connection between the two phenomena. A simple band model, allowing for spin-orbit coupling at the atomic level, connects the observed transition to a variety of previously reported properties. our results demonstrate that the fascinating behaviour observed so far in these oxides follows from a small but fundamental set of bands.

The emerging field of complex oxide interfaces is generically built on one of the most celebrated substrates--strontium titanate (SrTiO3). This material hosts a range of phenomena, including ferroelasticity, incipient ferroelectricity, and most puzzlingly, contested giant piezoelectricity. Although these properties may markedly influence the oxide interfaces, especially on microscopic length scales, the lack of local probes capable of studying such buried systems has left their effects largely unexplored. Here we use a scanning charge detector--a nanotube single-electron transistor--to non-invasively image the electrostatic landscape and local mechanical response in the prototypical LaAlO3/SrTiO3 system with unprecedented sensitivity. Our measurements reveal that on microscopic scales SrTiO3 exhibits large anomalous piezoelectricity with curious spatial dependence. Through electrostatic imaging we unravel the microscopic origin for this extrinsic piezoelectricity, demonstrating its direct, quantitative connection to the motion of locally ordered tetragonal domains under applied gate voltage. These domains create striped potential modulations that can markedly influence the two-dimensional electron system at the conducting interface. Our results have broad implications to all complex oxide interfaces built on SrTiO3 and demonstrate the importance of microscopic structure to the physics of electrons at the LaAlO3/SrTiO3 interface.

Controlling the coupling between localized spins and itinerant electrons can lead to exotic magnetic states. A novel system featuring local magnetic moments and extended 2D electrons is the interface between LaAlO 3 and SrTiO 3 . The magnetism of the interface, however, was observed to be insensitive to the presence of these electrons and is believed to arise solely from extrinsic sources like oxygen vacancies and strain. Here we show the existence of unconventional electronic phases in the LaAlO 3 /SrTiO 3 system pointing to an underlying tunable coupling between itinerant electrons and localized moments. Using anisotropic magnetoresistance and anomalous Hall effect measurements in a unique in-plane configuration, we identify two distinct phases in the space of carrier density and magnetic field. At high densities and fields, the electronic system is strongly polarized and shows a response, which is highly anisotropic along the crystalline directions. Surprisingly, below a density-dependent critical field, the polarization and anisotropy vanish whereas the resistivity sharply rises. The unprecedented vanishing of the easy axes below a critical field is in sharp contrast with other coupled magnetic systems and indicates strong coupling with the moments that depends on the symmetry of the itinerant electrons. The observed interplay between the two phases indicates the nature of magnetism at the LaAlO 3 /SrTiO 3 interface as both having an intrinsic origin and being tunable. The electronic system at the LaAlO 3 /SrTiO 3 (LAO/STO) interface (1) has shown an intriguing combination of superconductivity (2, 3), spin-orbit coupling (4, 5), and most recently, magnetism (6-13). An especially fascinating feature of this system is the existence of localized magnetic moments (14, 15) in proximity with itinerant d electrons (16-21) resulting in interesting coexistence phenomena (7-10). An unresolved issue central to a microscopic understanding of these properties is whether the electrons and moments interact with each other. It was shown that the itinerant electrons can be gate-tuned through a Lifshitz transition (22), where they change from populating light d XY bands with a circular Fermi surface to occupying also heavy d XZ =d YZ bands with highly elongated elliptical Fermi surfaces oriented along crystalline axes. The latter bands can have preferred axes for anisotropy along crystalline directions (21). Preferred crystalline directionality may also arise due to the localized magnetic moments, because they too originate from d orbitals localized on individual Ti atoms. Therefore, signatures of if and how the moments couple to the electrons will be embedded in the spatial character of the ground states of the LAO/STO system.Measurements of anisotropic magnetoresistance (23) (AMR) in a rotating in-plane magnetic field are a powerful tool to determine these symmetries. Previous AMR measurements in this system have addressed the effects of surface terraces (24), possible magnetic ordering (25), and prominent Rashba spin-orbi...

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We study the exciton gas-liquid transition in GaAs/AlGaAs coupled quantum wells. Dipolar excitons in coupled quantum wells (CQW) offer an interesting test bed for studying collective effects of an interacting quantum degenerate system [1, 2]. Their relatively light mass, which is smaller than that of a free electron, implies that the necessary conditions for achieving quantum degeneracy can occur already at cryogenic temperatures, and their strong dipole-dipole interaction may give rise to formation of ordered phases. Extensive attempts have been made over the past two decades to observe these phases and to determine the phase diagram of this system [3][4][5][6][7][8][9][10][11][12].In recent years there are mounting evidences for a phase transition that occurs at low temperatures in this system, yet its nature and thermodynamics remain open questions.Many of the studies are performed in a trap geometry, which confines the excitons to a narrow region around the illuminated spot [13,14], and evidences for condensation are found through photoluminescence (PL) anomalies: The appearance of spontaneous coherence [7], onset of non-radiative recombination ("PL darkening") [9] and large blueshift of the PL energy [10,11]. An alternative approach to study this phase transition uses an open geometry, where photo-excited carriers are free to move away from the illumination spot, and their diffusion is limited only by the mesa boundary. We have recently studied the behavior of indirect excitons in such an open geometry, and found an abrupt phase transition at a critical temperature and excitation power density [12]. The PL separates into two spatial regions, one which consists of electron-hole plasma and another that has a set of properties of a high density liquid.In this work we investigate this phase transition using spatially resolved PL and resonant Rayleigh scattering (RRS). Measuring the threshold power density as a function of temperature we determine the phase diagram of the system over the temperature range 0.1 -4.8K. Pump probe measurements, in which the liquid is created by a focused pump beam and studied by a much weaker probe, reveal that the liquid is dark and diffuses to large distances away from the illuminated spot, filling the entire area of the mesa at low temperatures. We find that the RRS spectrum narrows significantly and becomes uniform over macroscopic distances at the liquid phase, indicating that the disorder in the sample is effectively screened.The sample structure is identical to that used in [12] and consists of two GaAs quantum wells with well widths of 12 and 18 nm, separated by a 3-nm Al 0.28 Ga 0.72 As barrier. Top and bottom n-doped layers allow the application of voltage that shifts the energy levels of the wide well (WW) relative to the narrow well (NW) [15]. To create the liquid we apply voltages exceeding -2.4V and excite the system with a laser diode at energy of 1.590eV, focused to a Gaussian spot with 22µm half width at half maximum (HWHM) [16]. Figure 1 shows the evolution...