Session N71: Superconductivity and Magnetism in Oxide Thin Films and Heterostructures

Pseudogap Behavior and Interfacial Electron-Phonon Coupling in Single-Layer FeSe/SrTiO3

Single-layer FeSe grown on SrTiO₃ (FeSe/SrTiO₃) has attracted interest due to its unique characteristics as an atomically thin, interfacially enhanced high-T_c superconductor, exhibiting a spectroscopic gap-opening temperature nearly one order of magnitude higher than that of its bulk counterpart.  A better understanding of this enhanced superconducting state holds great potential for opening up a new frontier for high-temperature superconductivity through engineering atomic interfaces.  In this talk, I will discuss a series of experiments aimed at unveiling the nature and origin of the enhanced superconducting state of FeSe/SrTiO₃.  Using a first-ever combination of ARPES and in situ resistivity measurements, we reveal a striking dichotomy between the spectroscopic and transport properties of monolayer FeSe/SrTiO₃ - while spectroscopic measurements indicate the initial formation of a superconducting gap at temperatures as high as 70 K, a true zero-resistance state is not achieved until below 30 K. We show that this discrepancy is due to an unprecedentedly large pseudogap regime not previously observed in iron-based superconductors, but arising here from the intrinsic 2D nature of the system [1].  Additionally, we consider the potential influence of interfacial coupling between FeSe electrons and substrate phonons, widely believed to contribute to the enhanced T_c effect.  We perform a detailed analysis of the energy splittings and photon energy-dependent intensities of replica bands observed by ARPES in comparison to existing theoretical calculations, allowing us to confirm the origin of the observed replica bands as signatures of intrinsic electron-boson coupling and to directly quantify the strength of the coupling constant [2].    

Muon spin rotation studies of magnetism in superconducting infinite layer nickelates

The recent discovery and ongoing expansion of a new family of superconductors, the hole-doped infinite layer rare earth nickelates (La,Pr,Nd)_1-xSr_xNiO₂ ^1–4, has provoked much research activity with the goal of understanding their fundamental nature as well as their similarities to cuprates. One of the central questions has been their magnetic state with no long-range order observed in the bulk parent compounds ⁵ but magnetic excitations detected on the nickel sites ⁶.

 

Muon spin rotation offers a unique opportunity to probe intrinsic magnetism with extremely local sensitivity. With superconductivity so far only observed in thin film samples, low energy muons with a minimum implantation depth of just a few nanometers are utilized. Muon decay asymmetry spectra were recorded for various members of the superconducting infinite layer nickelate family. The effect of the rare earth ion and hole-doping through strontium substitution have been investigated.   

Hydrogen incorporation during reduction to the infinite-layer phase of superconducting nickelates

A recently discovered and expanding field, superconductivity in thin-film nickelates such as Nd_1-xSr_xNiO₂ (0.125 < x < 0.25) relies on the unusual Ni+ valence state stabilized by the infinite-layer structure. This structural phase and therefore superconductivity are, for complex reasons, difficult to synthesize and reproduce. Precursor films of the perovskite ANiO₃ undergo chemical reduction through reaction with CaH₂, but reduction may not be uniform through the depth of the film, and hydrogen may be incorporated into the film through the surface; either may from theory suppress superconductivity. We used the complementary depth-profiling techniques of neutron reflectometry and secondary ion mass spectroscopy to explore the role of cation doping, film thickness, sample preparation, and strain on uniformity of the infinite-layer phase in reduced ANiO₃ (A = Nd, Pr, Sr) nickelate thin films. We find that depending on these details, films can be composed of multiple regions of varying oxygen content, and hydrogen species are observed throughout the films in decreasing concentration away from the surface. These non-ideal infinite-layer phases correlate with lower crystalline quality and higher resistance. Thus, these variables are crucial to designing superconducting nickelate films.   

In Situ Synchrotron X-ray Studies of Nickelate Growth and Reduction

Long-sought non-cuprate superconductivity has recently been discovered in epitaxial thin films of alloyed neodymium nickelate (Nd_0.8Sr_0.2NiO₂) [1]. Interestingly, the superconducting  behavior is only observed in the infinite layer A₁B₁O₂ (112) phase and not the more typical A₁B₁O₃ (113) perovskite phase. Reduction of the 113 to the 112 phase generally requires use of a strong reducing agent such as CaH₂. However, the topotactic phase transition is non-trivial and difficult to control as it is correlated with the significant generation of defects (oxygen vacancies) throughout the entirety of the film. Here we present results from in situ synchrotron X-ray studies of the reduction reaction in nickelate heterostructures. The films are grown by pulsed laser deposition from a Nd_0.8Sr_0.2NiO₃ target on SrTiO₃ (001). We will discuss the evolution of the Nd_0.8Sr_0.2NiO₃ structure as a function of reduction temperature and time. We find that the gas-solid reaction with CaH₂ leaves both the infinite-layer Nd_0.8Sr_0.2NiO₂ and an intermediate Nd_0.8Sr_0.2NiO_2+δ phase, accompanied with changes to the oxygen octahedral rotation pattern from a^-a^-c^- to a⁰a⁰c^-. Our results provide much needed structural insight into the 113 to 112 phase transition in Nd_0.8Sr_0.2NiO₃ and other complex oxide heterostructures.   

Superconductivity in a quintuple-layer square-planar nickelate

Since the discovery of high-temperature superconductivity in the copper oxide materials, there have been sustained efforts to both understand the origins of this phase and discover new cuprate-like superconductors.  One prime materials candidate has been the rare-earth nickelates and indeed superconductivity was recently discovered in the doped compound Nd_0.8Sr_0.2NiO₂.  Undoped NdNiO₂ belongs to a series of layered square-planar nickelates with chemical formula Nd_n+1Ni_nO_2n+2 and is known as the ‘infinite-layer’ (n = ∞) nickelate. Here, using reactive oxide molecular beam epitaxy, which provides atomic level layer-by-layer control of thin film synthesis, we design and synthesize the quintuple-layer (n = 5) member of this series, Nd₆Ni₅O₁₂, which achieves optimal cuprate-like electron filling (3d^8.8) without chemical doping.  We observe a superconducting transition beginning at ~13 K.  Electronic structure calculations, in tandem with magnetoresistive and spectroscopic measurements, suggest that Nd₆Ni₅O₁₂ interpolates between cuprate-like and infinite-layer nickelate-like behavior.  In engineering a distinct superconducting nickelate, we identify the square-planar nickelates as a new family of superconductors which can be tuned via both doping and dimensionality.   

Anomalous Transport in High-Mobility Superconducting SrTiO3 Thin Films

The study of subtle effects on transport in semiconductors requires high-quality epitaxial structures with low defect density. Using hybrid molecular beam epitaxy (MBE), SrTiO₃ films with low-temperature mobility exceeding 42,000 cm²V^-1s^-1 at a low carrier density of 3 × 10¹⁷ cm^-3 were achieved. A sudden and sharp decrease in residual resistivity accompanied by an enhancement in the superconducting transition temperature was observed across the second Lifshitz transition (LT) where the third band becomes occupied, revealing dominant intra-band scattering. These films further revealed an anomalous behavior in the Hall carrier density as a consequence of the antiferrodistortive (AFD) transition and the temperature dependence of the Hall scattering factor. Using hybrid MBE growth, phenomenological modeling, temperature-dependent transport measurements, and scanning superconducting quantum interference device imaging, we provide critical insights into the important role of inter- vs. intra-band scattering and of AFD domain walls on normal-state and superconducting properties of SrTiO₃.   

Atomic-Scale Tuning of the Charge Distribution by Strain Engineering in Oxide Heterostructures

Strain engineering of complex oxide heterostructures has provided routes to explore the influence of local perturbations to the materials’ physical properties. Due to the challenge of disentangling intrinsic and extrinsic effects at oxide interfaces, the combined effects of epitaxial strain and charge transfer mechanisms have been rarely studied. Here, we reveal the local charge distribution in manganite slabs by means of high-resolution scanning transmission electron microscopy and spectroscopy via investigating how the strain locally alters the electronic and magnetic properties of La₂CuO₄–La_0.5Sr_0.5MnO₃ heterostructures. The charge rearrangement results in two different magnetic phases, an interfacial ferromagnetically reduced layer and an enhanced ferromagnetic metallic region away from the interfaces. In addition, the magnitude of the charge redistribution can be controlled via epitaxial strain, which further influences the macroscopic physical properties in a way opposed to strain effects reported on single-phase films. [1]

[1] Y.-M. Wu, Y. E. Suyolcu et al., ACS Nano 2021, 15, 16228-16235.

*Y.-M. Wu and Y. E. Suyolcu contributed equally to this work.   

Interfacial ferromagnetism in an atomically ordered three-component manganite superlattice

Lattice distortions play a key role in the ferromagnetic-metallic ground state of the mixed valent manganite, La_2/3Sr_1/3MnO₃. To explore the effect of such distortions on the electronic and magnetic properties, we substitute 50% of the La atoms with Y to grow La_1/3Y_1/3Sr_1/3MnO₃ thin films where the La, Y and Sr atoms all randomly occupy the A-site. But the random occupancy also leads to disorder. We decouple this disorder by constructing an ordered superlattice of three manganite phases, LaMnO₃, YMnO_3, and SrMnO₃, stacked at the atomic layer limit. Our electronic transport measurements show an insulating ground state in both the ordered superlattice and the random alloy samples although a crossover to a metallic state around 120K is observed in the ordered superlattice. We find a quasiparticle peak due to interfacial reconstruction at the oxygen K-edge in the ordered superlattice when probed with resonant soft x-rays. Furthermore, a more than four times stronger dichroic signal is observed in the ordered superlattice compared to the random alloy sample. These measurements point to ferromagnetic metallic tendencies at the Mn³⁺/Mn⁴⁺ interfaces in the ordered superlattice.   

Multiple magnetic transitions in Sr3Ru2O7 films grown by solid-source metal-organic MBE

The family of layered ruthenates (Sr_n+1Ru_nO_3n+1) hosts many interesting electrical and magnetic properties, ranging from superconductivity in Sr₂RuO₄ (n=1) to ferromagnetism in SrRuO₃ (n=∞). Sr₃Ru₂O₇ is the n = 2 member of this Ruddlesden-Popper family. Bulk single-crystals of Sr₃Ru₂O₇ are paramagnetic metals at room-temperature, with the presence of several magnetic transitions as a function of temperature. In this talk, we present the effect of epitaxial strain on these magnetic transitions in addition to showing the stabilization of electronic nematic phase at low temperatures. We have grown phase-pure, single crystalline, epitaxial Sr₃Ru₂O₇ films on LSAT (001) substrates using our novel solid-source metal-organic molecular beam epitaxy technique. Using LSAT (001) substrate produces ~ 0.5% in-plane compressive strain. These films showed the residual resistivity ratio of 22, which is highest reported value to-date in thin films of this material. Using temperature-dependent magnetotransport and magnetometry measurements, we reveal ferromagnetic transition at ~ 170 K, followed by several other magnetic transitions at lower temperatures. We also discuss the role of thickness on electronic and magnetic ordering in epitaxial Sr₃Ru₂O₇ films.   

Epitaxial stabilization of (111)-oriented frustrated quantum pyrochlore thin films

Frustrated rare-earth pyrochlore titanates, Yb₂Ti₂O₇ and Tb₂Ti₂O₇, have been introduced as promising candidates to realize quantum spin ice (QSI). Multiple unusual quantum phases, including Coulombic ferromagnet, quantum valence bond solid, and quadrupolar ordering, have been predicted to emerge in the QSI state upon applying a (111)-oriented external magnetic field. Here, we report on the successful layer-by-layer growth of high-quality  thin films of the frustrated quantum pyrochlores, R₂Ti₂O₇ (R = Er, Yb, and Tb), along the (111) direction. We confirm their high crystallinity and proper chemical composition by in-situ RHEED, x-ray diffraction, reciprocal space mapping, transmission electron microscopy, and x-ray photoelectron spectroscopy. We discovered a robust mode of growth for Er₂Ti₂O₇ and Yb₂Ti₂O₇, and observed  a pressure-sensitive growth for Tb₂Ti₂O₇. The availability of large area (111)-oriented QSI structures with planar geometry offers a new complementary to the bulk platform to explore the strain and the magnetic field-dependent properties in the quasi-2D limit.   

Epitaxial growth and structural and electronic properties of oriented thin films of GeNi2O4 and GeCu2O4 spinels

Frustrated magnets can host exotic many-body quantum and topological phenomena. GeNi₂O₄ is a three-dimensional S = 1 frustrated magnet with an unusual two-stage transition to the two-dimensional antiferromagnetic ground state, while GeCu₂O₄ is a high-pressure phase with a strongly tetragonally distorted spinel structure and magnetic lattice formed by S = 1/2 CuO₂ linear chains with frustrated interchain exchange interactions and exotic magnetic behavior. Here, we report on the first thin-film epitaxial stabilization of these two compounds in (100) and (111) directions. The developed growth mode, surface morphology, crystal structure, and valence state were characterized by in situ reflection high-energy electron diffraction, atomic force microscopy, x-ray reflectivity, x-ray diffraction, x-ray photoelectron spectroscopy, and resonant x-ray absorption spectroscopy. Our results pave an alternative route to the investigation of the puzzling magnetic properties of these compounds and the exploration of emergent features driven by strain. Furthermore, the availability of large-area high-quality GeCu₂O₄ thin films opens a road for future experimentation to reveal the controversial nature of its ground state magnetism and elucidate the origin of multiferroicity in this compound.   

Probing Oxygen Content and Oxygen Isotope Effect in Manganite/Cuprate Heterostructures

It is believed that a long-range ferromagnet/superconductor proximity effect, involving odd-frequency pairing of spin triplets [1], exists in La_2/3Ca_1/3MnO₃/YBa₂Cu₃O_7-δ (LCMO/YBCO) heterostructures [2].  However, recent studies have shown that intergrowths and deoxygenation in the YBCO layer, induced by epitaxial strain from the LCMO layers, can also explain the anomalously long length scale of the critical-temperature (T_c) attenuation observed [3]. To shed crucial light on this problem, we carry out both x-ray absorption spectroscopy (XAS) and oxygen isotope effect (OIE) studies on LCMO/YBCO/LCMO trilayers grown by pulsed laser deposition. XAS is used to measure the YBCO oxygen content vs. layer thickness, thus determining the extent epitaxial strain deoxygenates YBCO. OIE is studied by measuring the T_c shift between ¹⁶O- and ¹⁸O- annealed samples of varying YBCO thickness, to asses whether the attenuated superconductivity in this system involves appreciable electron-phonon coupling [4]. These measurements allow us to rule out magnetism as playing a significant role in the observed proximity effect of LCMO/YBCO heterostructures.

[1]P. Lee et. al, PRB103 (2021)

[2]C.Visani et. al, Nat.Phys.8 (2012)

[3]H.Zhang et. al, APL103 (2013); ibid, arXiv:1710.10668v1

[4]G.Zhao et. al, PRB51 (1995)   

Enhanced Superconductivity of La2-xSrxCuO4 Thin Films by Atomic-Scale Interface Engineering

Ruddlesden-Popper (RP) oxides (A_n+1B_nO_3n+1, n = 1, 2, ...) have been widely studied with tunable properties such as high-T_c superconductivity and colossal magnetoresistance.^[1,2] However, high-quality RP film growth has been disturbed by extended defects, so-called out-of-phase boundaries (OPBs).^[3] Since OPB formation hampers the functionalities of films, OPB suppression is highly required to carry out high-performance RP-based functional devices. ^[4]

Here, we suppressed OPBs in RP thin films using single-terminated LaSrAlO₄ (LSAO) substrates. We developed a surface treatment recipe of LSAO substrates, which exhibited the uniform surface reconstruction having LaO-terminated surfaces. As a model system, the high-T_c cuprates La_1.85Sr_0.15CuO₄ (LSCO) films were employed. On mixed-terminated LSAO surfaces, LSCO films exhibited huge OPB formations. In contrast, on single-terminated LSAO surfaces, the OPBs were suppressed. These OPB-free LSCO films exhibited significantly enhanced superconductivity (T_c^zero ~ 30 K) than the film with huge OPBs (T_c^zero ~ 5 K), under given thickness (~ 6.5 nm). By reflection high-energy electron diffraction (RHEED) and density functional theory (DFT) calculation, we observed that the LSCO stacking sequence can be controlled by LSAO surface termination.