Session A13: Novel Phases in Complex Oxide Heterostructures

Unconventional superconductivity in SrTiO3/LaAlO3/SrTiO3 trilayer system

A trilayer structure, composed of 7 unit cell (uc) of SrTiO₃ (STO) and one uc of LaAlO₃ (LAO) deposited on an STO substrate, was fabricated and its electrical transport properties at low temperatures were studied. We have found that the trilayer become superconducting at temperature below 180 mK. We have measured the magnetic field dependence of the critical current of the trilayer. The critical current exhibits a hysteresis with the magnetic field. Surprisingly, the direction of the hysteresis is opposite to that of the conventional magnetoresistance hysteresis. The superconductivity and magnetism coexist at this trilayer system and the superconducting order parameter of the system can have a helical ordering. We have also fabricated vertical Josephson junction by depositing Al on top of the trilayer. Superconducting energy gap is estimated from the Andreev reflection peak of the Josephson junction. Measured gap ratio, Δ/k_BT_C = 1.31, suggests that this system is not a conventional BCS-type superconductor.   

Temperature Dependent Giant Resistance Anomaly in LaAlO3/SrTiO3 Nanostructures

The electron gas at the interface between LaAlO₃ and SrTiO₃ has been investigated since more than a decade. Its resistance typically drops monotonically with temperature and R/T curves during cooling and warm-up look identical for large area structures. We have been able to fabricate nanostructures which show a surprising temperature dependence. If the LAO/STO is laterally restricted by nanopatterning the resistance exhibits a temperature anomaly. Warming up nanostructures from low temperatures leads to one or two pronounced resistance peaks between 50 and 100 K not observed for larger dimensions. This result can be explained by the structural phase transitions which take place in the STO substrate during cool-down and warm-up.During cool-down current filaments emerge at the domain walls that form during the phase transition. This effect had already been observed by scanning SQUID microscopy. During warm-up, however, the reverse phase transition can interrupt filaments before the sheet conductivity which dominates at higher temperature is reestablished. It appears that the area between the domain walls becomes completely insulating and due to the limited number of filaments in a nanostructure this process can result in a complete loss of conductance.   

Optical and Electrical Control of Confined Electrons in SrTiO3/LaAlO3 Quantum Well Heterostructures

While the SrTiO₃/LaAlO₃ (STO/LAO) system has been heavily studied in hopes of utilizing the interfacial 2DEG in all-oxide electronics, the 2.4 eV conduction band offset between STO and LAO also offers the opportunity to engineer quantum well (QW) heterostructures for use in electronic and optoelectronic devices. The operation of many such devices relies on the precise optical and/or electronic control of confined electrons through the stimulation of intersubband transitions or resonant tunneling. Here, we present optical spectroscopy measurements demonstrating room-temperature intersubband absorption in STO/LAO QW heterostructures on the order of hundreds of meV as well as tunneling measurements showing resonant tunneling through confined states. The presence of these phenomena indicates confinement of electrons in the STO conduction band and shows the ability to precisely control confined states in these structures. As a necessary prerequisite for the observation of these phenomena is the growth of high-quality samples, we also present a thorough structural characterization of our heterostructures indicating unprecedentedly low interfacial roughness of 0.3 unit cells/interface. We also discuss the feasibility of integrating these devices on Si (001) for use in Si photonics.   

Breaking electron pairs in pseudogap state in SmTiO3/SrTiO3/SmTiO3 quantum wells

Pseudogap behavior has recently been found in strongly confined SmTiO₃/SrTiO₃/SmTiO₃ quantum wells (Marshall et al, PRL 117, 046402 (2016)). We employ low-temperature transport measurements on SmTiO₃/SrTiO₃/SmTiO₃ Hall bar structures to determine if this phase can be associated with the formation of a paired electron liquid phase. We drive a portion of the device out of equilibrium with voltages that exceed the pseudogap energy and measure the resulting longitudinal and Hall conductance. A unidirectional change in conductance is observed when the pair-breaking drive amplitude exceeds a critical value, providing evidence for the existence of long-lived quasiparticle excitations.   

Band offset and built-in potential at LaMnO3 / SrTiO3 polar/non-polar heterojunctions

The deposition of polar perovskites on non-polar substrates has been predicted to lead to a increasing electric field in the film; after a critical thickness is reached, an electronic reconstruction alleviates this polar catastrophe by removing the built-in potential. In epitaxial heterojunctions of polar LaMnO₃(001) (LMO) on non-polar SrTiO₃(001) (STO), it has been observed that antiferromagnetic LMO films become ferromagnetic at a critical thickness of 5 - 6 u.c.; this is attributed to the electronic reconstruction. Recent DFT calculations (Chen et al., Phys. Rev. Lett. 119, 156801 (2017)) of LMO/STO heterojunctions predict the magnitude of the built-in potential to be 0.177 V/Å, but the presence of this potential has not yet been directly probed. With in situ XPS, we have directly probed both the band alignment and the presence of a built-in potential in epitaxial LMO/STO heterojunctions as a function of thickness. The valence state of Mn is evaluated with Mn L-edge XAS, and the local structural features, valence state, and extent of intermixing across the interface are evaluated with high resolution STEM imaging with EDS and EELS measurements. In light of our results, we propose a model for the electronic structure of stoichiometric LMO/STO heterojunctions.   

Resonant X-Ray Reflectometry Study of Orbital Polarization in Quantum Critical SmTiO3 Heterostructures

Strongly correlated electron systems continue to shed new light on the fundamental interactions of electrons in solids. With hallmark behaviors like metal-to-insulator transitions and high temperature superconductivity, Mott insulating transition metal oxides are a canonical example of strong electron-electron correlations. Here we investigate epitaxial heterostructures of the Mott insulating rare earth titanate SmTiO₃ (SmTO) embedded with quantum wells of the band insulator SrTiO₃ (STO). This system exhibits both quantum critical transport and pseudogap behavior suggesting strong parallels with bulk Mott materials. In this work, we use resonant X-ray reflectometry at the Ti L-edge to probe the orbital polarization depth profiles of a SmTO film and SmTO-STO superlattices with varying STO quantum well thickness. Our results suggest that the d¹ electron in SmTO preferentially occupies the in-plane d_xy orbital despite nearly 1% compressive strain from the underlying substrate. Similar in-plane polarization is observed for both SmTO and STO layers in the superlattices. These results are analyzed in terms of competition between compressive strain, quantum confinement, and Jahn-Teller distortions to the crystal field.   

Phase control and hidden order in oxide superlattices: dimensional and interfacial effects

In electronically correlated systems, including many complex oxides, atomic layer heterostructuring can expose new phenomena – transition temperatures may be modified, order parameters may be strengthened or weakened, and new phases may emerge. Understanding the origin of these effects is vital for designing tailored materials as well as for improving theories of correlated physics. However, many overlapping and competing processes influence the emergent phases. In this talk, I discuss the distinct roles of dimensional confinement and interfacial coupling in correlated oxide heterostructures in the context of experiments on atomically layered NdNiO₃ superlattices. In our work, we systematically track the magnetic, charge, and orbital order parameters of NdNiO₃ and uncover “hidden” phases that arise as the thickness is reduced down to the single atomic layer limit. The layer-dependent phase diagram shows distinct behavior for single- and multilayer structures. In particular, although magnetic order persists down to two atomic layers of NdNiO₃, it becomes 2D in nature; furthermore, the transition to the magnetic and charge-ordered state decouples from the transition to the insulating state at these thicknesses, whereas they coincide in the bulk. In the single atomic layer case, all forms of order are found to vanish. Through first-principles and model calculations, we establish that reduced dimensionality plays the prominent role in the multilayer structures, while the phase behavior in the single-layer case is dominated by interfacial effects. We identify a unique interfacial reconstruction based on ionicity mismatch that leads to the observed unordered ground state.   

Exploring the phase diagram of LaxNd1-xNiO3 thin films

In this study we present a series of results on thin films of La_xNd_1-xNiO₃. The phase diagram spanned by this mixed nickelate compound extends from metallic, paramagnetic and rhombohedral LaNiO₃ for x = 1 to NdNiO₃, a orthorhombic perovskite with a metal-charge transfer insulator transition for x = 0.
The evolution of the structural and electronic properties are traced across this phase diagram in an effort to understand the onset of the metal-insulator transition for which the nickelate family has become so famous.   

SmNiO3/NdNiO3 superlattices

Rare earth nickelates RNiO₃ (R≠La) stand out for their unique metal to insulator transition (MIT), accompanied by a symmetry lowering from an orthorhombic to a monoclinic structure. Moreover, they display an unusual antiferromagnetic ordering at T_Néel≤T_MI. Notably, the critical temperatures are a function of the Ni-O-Ni bond angle characterizing each member of the family and subtle changes in this angle can strongly modify the electronic behavior of the system. Bulk NdNiO₃ (NNO) shows T_Néel=T_MI, whereas for SmNiO₃ (SNO) T_MI>T_Néel. Strain allows these transitions to be tuned, but given the same substrate, it is always found that T_{MI_NNO}<T_{MI_SNO}. Here, we adopt a new strategy to inspect the evolution of structural distortions in RNiO₃-based heterostructures and their impact on the electronic properties by growing [n(SNO)/n(NNO)]_m superlattices (SLs). We have found that the T_MI and T_Néel of the whole system is controlled by the thickness of the individual SNO and NNO layers. For thicker SLs periods two separate MITs corresponding to those of the individual SNO and NNO layers are observed. However, the two separate MITs merge into a single one as the SL period is reduced. The relation between T_MI and T_Néel will also be investigated.   

Orbital Ordering and Correlation Effects in a NdNiO3/NdAlO3 Superlattice

Nickel oxide heterostructures in the RNiO₃ family show a complex interplay of structure and electronic correlations that depends on the rare earth atom R, strain and number of layers of the perovskite nickelate. These lead to a metal-insulator transition, magnetic and charge order as well as structural breathing distortions. Recently there has been an effort to engineer the orbital order of nickelates through heterostructuring in such a way as to reproduce the High Tc Cuprate band structure. Experiments on (NdNiO₃)_n/(NdAlO₃)_m superlattices [1] have shown that the magnetic order characteristic of the insulating phase for NdNiO₃ appears for a number of nickelate layers n>1; at n=1 it can no longer be detected. X-Ray measurements have measured orbital polarization in the eg d-shell for n>1 despite a lack of bond symmetry breaking, which we showed can be understood as an effect of anisotropic ionicity [1]. Here we explain the competing roles of quantum confinement, electron correlations and heterostructuring on the NdNiO₃ monolayer through DFT, slave-boson and DMFT methods and model calculations.
[1] Physical Review Materials 1, 024410, 2017   

Design of n- and p-type thermoelectrics in oxide superlattices exploiting interface polarity

Transition metal oxides and in particular their heterostructures are attractive for thermoelectric applications. We explore the implications of interface polarity in LaNiO₃/SrTiO₃(001) superlattices on structural, electronic, and thermoelectric properties by combining DFT+U calculations and Boltzmann transport theory. While superlattices containing (LaO)⁺/(TiO₂)⁰ interfaces result in an n-type material, a (NiO₂)^-/(SrO)⁰ stacking leads to p-type doping. We find that significant octahedral tilts are induced in the SrTiO₃ region and that the La-Sr distances act as a fingerprint of the interface type. In contrast to LaAlO₃/SrTiO₃(001), the electrostatic doping is accommodated in the metallic nickelate layers. The electronic structure displays an orbital-selective quantization of Ni-3d-derived quantum well states. Complex cylindrical Fermi surfaces emerge, which show a tendency towards nesting that depends on the interface polarity. Finally, we demonstrate that the thermoelectric response of the superlattice can be selectively controlled by a targeted interface design. This opens a route for constructing oxide-based thermoelectric generators [1].

[1] B. Geisler et al., PRB 95, 125301 (2017); patent pending   

Prediction of a Mobile Two-Dimensional Electron Gas at the LaScO3/BaSnO3 (001) Interface

Two-dimensional electron gases (2DEG) at oxide interfaces, such as LaAlO₃/SrTiO₃ (001), have aroused significant interest due to their high carrier density (~10¹⁴ cm^-2) and strong lateral confinement (~1 nm). However, these 2DEGs are normally hosted by the weakly dispersive d-bands (e. g., Ti-3d bands), which are strongly coupled to the lattice, causing mobility of such 2DEGs to be relatively low at room temperature (~1 cm²/Vs). Here, we propose using oxide host with the conduction bands formed from s-obitals to increase carrier mobility and soften its temperature dependence. Using the density functional theory calculations, we investigate LaScO₃/BaSnO₃ (001) heterostructure and as a model system, where the conduction band hosts the s-like carriers. We find that the polar discontinuity at this interface leads to electronic reconstruction resulting in the formation of the 2DEG. The conduction electrons reside in the highly dispersive Sn-5s bands, which have a large band width and a low effective mass. We expect the predicted 2DEG to be highly mobile, even at room temperature and bring about more pronounced quantum phenomena. A qualitatively similar behavior is predicted for a doped BaSnO₃, where a monolayer of BaO is replaced with LaO.   

In-situ Investigations for Possible Two-Dimensional Electron Gases on BiO2-Terminated BaBiO3 Thin Films

We analyzed the atomic configuration and the electronic structures of BaBiO₃ (BBO) thin films adopting in-situ photoemission. Two-dimensional electron gases (2DEGs) was theoretically predicted by density-functional-theory on BiO₂-terminated BBO surfaces. The so-called self-doping effect was suggested as the origin for 2DEG on BBO, which can be a generic mechanism to form 2DEGs on surfaces of charge-skipping oxides. However, direct visualization for surface atomic configuration using transmission electron microscopy has been restricted because BBO is too vulnerable to electron beam. In this work, we inferred depth-profile information for our BBO films using angle-dependent core-level spectra measurement, which is dramatically affected by finite escape depths of photoelectrons. We found that our BBO films were terminated by BiO₂ layers, rather than BaO or other possible mixtures of BaO and BiO₂. By measuring angle-resolved photoemission, we distinguished surface states and bulk states, and none of them showed any signatures from 2DEGs. Surface states were observed 4 eV below Fermi level, not near the Fermi level, which suggests that electronic structure description for BBO should accompany with sophisticated approaches.