Session T18: Focus Session: Interfaces in Complex Oxides - Superconductivity and Magnetism at Interfaces

Local imaging of the superfluid density at the LAO/STO interface as a function of gate voltage

The interface between two insulating oxides, LaAlO$_{3}$ and SrTiO$_{3}$, exhibits a two-dimensional electron system with high mobility, magnetism, superconductivity at low temperatures, and an electric-field-tuned superconductor-insulator transition. This interface has been studied extensively using transport and magnetization, which do not directly probe potential variation on a local length scale. We use a scanning SQUID microscope to locally probe superconductivity and magnetism in LAO/STO heterostructures. We measure the local diamagnetic susceptibility and critical temperature of as a function of position and gate voltage. Our local susceptibility measurement is related to the density of superconducting carriers which gives us a map of superfluid density. We find that the superfluid density is inhomogeneous, showing regions of susceptibility that varies over a large fraction of the total response while the critical temperature remains relatively uniform across the sample. Tracking the evolution of both of these parameters as a function of gate voltage and position enables investigation of the local onset of the superconductor-insulator transitions on both sides of the dome.   

Magnetism and superconductivity at LAO/STO-interfaces: the role of Ti 3d interface electrons

Ferromagnetism and superconductivity are in most cases adverse. However, recent experiments reveal that they coexist at interfaces of LaAlO$_{3}$ and SrTiO$_{3}$ [1]. We analyze the ferromagnetic state within density functional theory and provide evidence that it is also generated by Ti 3d interface electrons, as is the two-dimensional electron liquid at the interface which gives rise to superconductivity [2]. We demonstrate that oxygen vacancies in the TiO$_{2}$ interface layer enhance the tendency for ferromagnetism considerably. This allows for the notion that areas with increased density of oxygen vacancies produce ferromagnetic puddles and account for the previous observation of a superparamagnetic behavior in the superconducting state [3].\\[4pt] [1] Lu Li, C.Richter, J.Mannhart, and R.C.Ashoori, Nature Physics 7, 762 (2011).\\[0pt] [2] N. Reyren et al., Science 317, 1196 (2007).\\[0pt] [3] N.Pavlenko, T.Kopp, E.Y.Tsymbal, G.A.Sawatzky, and J.Mannhart, cond-mat/arXiv:1105.1163 (2011)   

Superconductivity and magnetism in the presence of interface-induced Rashba spin-orbit coupling

Two dimensional electron systems at oxide interfaces are often influenced by a Rashba type spin-orbit coupling (SOC), which is tunable by a transverse electric field. Ferromagnetism at the interface can simultaneously induce strong local magnetic fields. This combination of SOC and magnetism leads to anisotropic two-sheeted Fermi surfaces, on which superconductivity with finite-momentum pairing is favored. The superconducting order parameter is derived within a generalized pairing model realizing both, the FFLO superconductor in the limit of vanishing SOC and a mixed-parity pairing state with zero pair momentum if the magnetism vanishes. The nature of the pairing state is discussed in the context of interface superconductivity and ferromagnetism at LAO-STO interfaces [1,2]. \\[4pt] [1] Lu Li, C. Richter, J. Mannhart, and R. C. Ashoori, Nature Physics \textbf{7}, 762 (2011) \\[0pt] [2] J. A. Bert, B. Kallisky, C. Bell, M. Kim, Y. Hikita, H. Y. Hwang, and K. A. Moler, Nature Physics \textbf{7}, 767 (2011)   

Superconductivity and magnetism in a three-band model of the LAO/STO interface from a weak-coupling perspective

Recent experiments on LAO/STO interfaces have shown the occurrence of both superconductivity and magnetism in this system. Motivated by these experiments, we analyze various Fermi-liquid instabilities in a three-band model of the LAO/STO interface with purely repulsive interactions by calculating susceptibilities for superconducting, magnetic and nematic orders. In light of recent transport experiments proposing a Lifshitz transition between d orbitals, we particularly study how the susceptibilities depend on the chemical potential. Further, we investigate the interplay of these different orders.   

Magnetism of LaAlO$_3$/SrTiO$_3$ Heterostructure Interface

The LaAlO$_3$/SrTiO$_3$ heterostructure is a potential candidate for a high mobility two-dimensional electron system with novel electronic and magnetic properties. Magnetic ordering has been proposed to arise from d-shell electrons transferred by the polarization discontinuity at the interface. However the magnetization of this system cannot easily be detected with standard techniques due to the small volume of the interfacial region. Using torque magnetometry, we measure the magnetic moment of the interface system directly. Our results indicate the existence of a magnetic ordering at the two-dimensional conductive interface. The ferromagnetic-like ordering state persists up to 200 K. Such a state is hardly explained by ion-exchange at the interface, since LaTiO$_3$ is antiferromagnetic. Moreover, the same magnetic behavior persists even when the sample is superconducting, which suggests an unconventional two-dimensional superconducting phase.   

Signatures of local superconducting islands at elevated temperatures in LaAlO$_3$/SrTiO$_3$ Heterostructure Interface

Superconductivity has been observed in LaAlO$_3$ /SrTiO$_3$ heterostructures below 300 mK in resistivity measurements and later confirmed with Meissner effect. Our detailed magnetization measurements in several LaAlO$_3$/SrTiO$_3$ samples indicate a quasi superconducting state persisting up to 4 K. In these samples, we discovered large nonreversible magnetization vs. field curves. Moreover, the magnitude of the nonreversible loop is proportional to sweep rates, suggesting that it is caused by local magnetic moments generated by eddy currents. Based on this eddy current picture, the inferred conductance is found to be 7 orders of magnitude larger than the conductance across the whole interface. This contrast suggests the existence of small local quasi superconducting regions at a temperature of 4 K, well above the long range superconducting transition temperature.   

Superconductivity and Ferromagnetism in Oxide Interface Structures: Possibility of Finite Momentum Pairing

We present a model that captures the physical properties of the interface between two oxides, LaAlO$_3$ and SrTiO$_3$. Despite extensive experimental studies of these systems, no clear theoretical picture has emerged so far. The model that we suggest for the interface electrons explains the main experimental observations. In particular, we address one of the most intriguing phenomena observed in these system: the coexistence of ferromagnetism and superconductivity. Ordinarily this ferromagnetism would destroy superconductivity, but due to strong spin-orbit coupling near the interface, the magnetism and superconductivity can coexist by forming an FFLO-type condensate of Cooper pairs at finite momentum. Surprisingly, this unconventional superconducting state survives even at strong disorder.   

Tuning the ferromagnetism in LaAlO$_{3}$/SrTiO$_{3}$

The interface of LaAlO$_{3}$/SrTiO$_{3}$ heterostructures exhibits both conductivity and magnetism. Significant effort has been invested researching the details and characteristics of the conductivity, such as conductance only above a critical LaAlO$_{3}$ thickness. However, reports of ferromagnetism differ in a variety of details requiring further investigation into the material properties that control magnetism. In this study we use a scanning SQUID microscope to locally image the landscape of ferromagnetism as a function of several tuning parameters including the LaAlO$_{3}$ thickness, back gate voltage, local strain and surface treatment. We find that the ferromagnetism is inhomogeneous within each sample, varies considerably between samples, and appears only in samples whose LaAlO$_{3}$ layer is thicker than a threshold value, similar to the conductance critical thickness. The ferromagnetism changes with local strain, surface treatment with polar solvents and applied field. However it is not affected by changing the gate voltage or cycling the temperature up to 300K. These results provide experimental input for determining and controlling the mechanisms of magnetism in engineered complex oxide interfaces.   

Strong magnetic interaction in localized two-dimensional electron gas in LaAlO$_3$/SrTiO$_3$/NdGaO$_3$ heterostructures

Via reducing the thickness of SrTiO$_3$ layer, strong localization of two-dimensional electron gas is artificially introduced at LaAlO$_3$/SrTiO$_3$ interface grown on NdGaO$_3$ (110) substrates. This localization is characterized by the low carrier density, robust insulating ground state and variable range hopping behavior at low temperature. Given the spatially-limited conducting channel in the thin SrTiO$_3$ layer, the degeneration of Ti 3d orbitals at the interface should be responsible for this strong localization. Moreover, the typical butterfly-like hysteresis loop and unusual anisotropic features in magnetoresistance observed in the thinner SrTiO$_3$ samples indicate the enhanced magnetic interaction in this strongly localized two-dimensional electron gas.   

Investigation of electric field induction of superconductivity at complex oxide interfaces

We examine the modified electronic states and change in carrier density at the interfaces of complex oxide films produced by an external electric field. Using a Ginzburg-Landau formalism and ab-initio calculations, we show that linear coupling of an electric potential can influence the superconducting order parameter and induce a transition to a superconducting phase. Further, we examine the correlation between carrier density and the superconducting critical temperature $T_c$ by investigating capacitance and density of states with changing electric potential. We will discuss implications of this work in the context of interfaces formed by LaAlO3 and SrTiO3 thin films. This approach points to an alternative path to superconducting devices with tunable transition temperature. Work was carried out under the help and support of the National Nuclear Security Administration of the U.S. Department of Energy at Los Alamos National Laboratory under Contract No. DE-AC52-06NA25396.   

Evidence for charge-vortex duality at the LaAlO$_3$-SrTiO$_3$ interface

The conducting gas formed at the inteface between LaAlO$_3$ and SrTiO$_3$ undergoes a superconductor to insulator transition (SIT) on the application of a back gate voltage, $V_g$. The system also shows evidence of ferromagnetic order coexisting with superconductivity.\footnote{Dikin \textit{et al}, PRL \textbf{107}, 056802 (2011)} The juxtaposition of the ferromagnet with the conducting gas allows for the observation of a novel manifestation of charge-vortex duality. The field due to the magnetization dynamics in the ferromagnet causes a sharp \textit{increase} in resistance on the superconducting side of the transition, in the magnetoresistance measurements, and a sharp \textit{decrease} in resistance on the insulating side. The system can be modeled as an array of Josephson junctions, with two characteristic energy scales: a Josephson coupling energy $E_J$, and a Coulomb charging energy $E_c$. $V_g$ then tunes the ratio, $E_J/E_c$, to cause the transition. We will present external field sweep-rate dependent magnetoresistance data on both sides of the transition to elucidate the nature of the superconducting and insulating states.   

Single-Mode Cooper Pair Channel in LaAlO$_3$/SrTiO$_3$ Nanowires

The conducting LaAlO$_{3}$/SrTiO$_{3}$ interface becomes superconducting\footnote{N. Reyren, \textit{et al.}, Science \textbf{317}, 1196-9 (2007).} below a critical temperature T$_{c}\sim$100-400~mK. Here, we investigate the transport characteristics of LaAlO$_{3}$/SrTiO$_{3}$ structures formed from $\sim$10 nm-wide nanowire segments produced by a conductive atomic force microscope lithography technique\footnote{C. Cen, S. Thiel, K. E. Andersen, C. S. Hellberg, J. Mannhart, and J. Levy, Nature Materials \textbf{7}, 2136 (2008).}. Above T$_{c}\sim$200~mK we find a characteristic four-terminal conductance $G\sim e^{2}/h$ that is independent of the channel length. Below T$_{c}$ we find that the conductance increases to $G\sim 4e^{2}/h$. This increase is attributed to the formation of Cooper pairs that propagate in a single mode. We also discuss the interactions between Cooper pairs and spin-polarized transport in these structures. This work is supported by AFOSR (FA9550-10-1-0524).   

Two doped holes with different distributions in $\rm Sr_{2}CuO_{4-\delta}-La_{2}CuO_{4}$ superlattices

X-ray absorption in $\rm Sr_{2}CuO_{4-\delta}-La_{2}CuO_{4}$ (SCO-LCO) superlattices shows a variable occupation with doping of two hole states. In addition to the holes doped for $x [Preview Abstract]

Highly Spin-Polarized Conducting State at the Interface between Nonmagnetic Band Insulators: LaAlO3/FeS2 (001)

Interface engineering of complex oxide heterostructures allows creating interfaces with properties and functionalities distinct from those typical for the respective bulk constituents. In the spirit of the well known conducting LaAlO3/SrTiO3 interface we study a similar interface with the added functionality of being unambiguously ferromagnetic. Our first-principles density functional calculations demonstrate that such a spin-polarized two-dimensional conducting state can be realized at the (001) interface between the two non-magnetic band insulators FeS2 and LaAlO3. The (001) surface of FeS2(pyrite), a diamagnetic insulator, supports a localized surface state deriving from the Fe d-orbitals near the conduction band minimum. We find that, similar to the LaAlO3/SrTiO3 system, the deposition of a few unit cells of the polar perovskite oxide LaAlO3 leads to electron transfer into these surface bands, thereby creating a conducting interface. The occupation of these narrow bands leads to an exchange splitting between the spin sub-bands, yielding a highly spin-polarized conducting state quite distinct from the rest of the non-magnetic, insulating bulk. [Ref: J. D. Burton and E. Y. Tsymbal, Phys. Rev. Lett. 107, 166601 (2011).]