Session F64: Transport, Superconductivity in SrTiO3, and Related Thin Films and Heterostructures

Advances in computing charge carrier dynamics in oxides from first principles

Advances in first-principles calculations enable accurate predictions of charge carrier mobility and scattering mechanisms in materials. However, many oxides remain an open problem for these methods since they exhibit soft phonon modes due to structural phase transitions and electron-phonon interactions strong enough to form polarons. The presence of polarons and soft modes makes first-principles calculations of charge transport highly challenging even in non-correlated oxides.
This talk will discuss new approaches for treating electron-phonon coupling due to soft phonon modes, as well as a cumulant diagram-resummation approach for rigorously computing the carrier mobility in the large polaron regime. We apply these approaches to cubic SrTiO3 perovskite as a paradigmatic case and analyze in detail soft mode and polaron contributions to charge transport.
Our advances allow us to accurately predict the temperature dependence [1] and absolute value [2] of the mobility in SrTiO₃, while providing long-sought microscopic details about the scattering mechanisms, bandstructure renormalization and beyond-quasiparticle features in the spectral function due to the strong electron-phonon coupling. Efforts to extend these calculations to a range of oxides and future application of these approaches to interfaces and heterostructures will be discussed

[1] J.-J. Zhou, O. Hellman, M. Bernardi. “Electron-Phonon Scattering in the Presence of Soft Modes and Electron Mobility in SrTiO3 Perovskite from First Principles.” Phys. Rev. Lett. 121, 226603 (2018)
[2] J.-J. Zhou, M. Bernardi. "Predicting Charge Transport in the Presence of Polarons: The Beyond-Quasiparticle Regime in SrTiO3." Preprint: Arxiv 1905.03414.   

Superconducting split-gate quantum point contacts in ionic liquid-gated SrTiO3

SrTiO₃ is one of few materials to exhibit superconductivity at carrier densities low enough to be manipulated electrostatically. In this work, we demonstrate electrostatically defined superconducting devices that combine global ionic liquid gating of the leads with local dielectric split gates. This design enables patterning of a 2D electron system with sub-100 nm features without the need for mesa etching.
When tuned into a normal state, these devices behave as ballistic quantum point contacts, with conductance quantization near pinch-off. Quantization is in increments of e²/h, even at zero magnetic field, suggesting spontaneous spin-polarization.
When the device leads are tuned to be superconducting, the weak link can be gated through a sequence of transport regimes: pinch-off, coulomb blockade, superconducting tunnel junction, and SNS or SS’S Josephson junction. We demonstrate that the local constriction can be tuned into a regime where its critical field exceeds that of its superconducting leads. This offers promise of exploring the microscopic physics of underdoped SrTiO₃, where two-dimensional superconductivity is usually suppressed by the lack of macroscopic coherence.   

Probing the superconductivity of ferroelectric SrTiO3 films

We report on the superconducting characteristics of doped 180-nm-thick SrTiO₃ films, grown by molecular beam epitaxy on (001) LSAT substrates. The epitaxial compressive strain induced by the substrate stabilizes a ferroelectric phase in the SrTiO₃ for carrier concentrations ranging from 5×10¹⁹ – 1.5×10²⁰ cm^-3 [1,2]. A systematic study of the critical temperature and critical field as a function of the carrier density and type of doping atom reveal deviations from those expected for a single-band BCS superconductor. In particular, the critical temperature and critical field show qualitative different evolutions, with the latter exceeding the Pauli limit for certain values. We discuss the role of the band structure and inversion symmetry breaking.

[1] K. Ahadi, et al., Sci. Adv. 5, eaaw0120 (2019).
[2] R. Russell, et al., Phys. Rev. Mater. 3, 091401(R) (2019).   

Superconductivity and Antiferrodistortive Phase Transition in Sr1-xNdxTiO3 Films

Despite over 60 years of extensive research, the study of superconductivity, ferroelectricity and antiferrodistortive phase transition in SrTiO₃ remains puzzling yet intriguing. Through intrinsic defect management, we achieved for the first time electron density as low as 3 × 10¹⁷ cm^-3 in Sr_1-xNd_xTiO₃ films with the corresponding mobility exceeding 42000 cm²/vs at 1.8 K. By systematically varying the carrier density over three orders of magnitude, we present an important role of intra- vs. inter-band scattering on the superconducting transition temperature T_c across the Lifshitz transition. Significantly, we show a sizable effect of the antiferrodistortive phase transition on the normal state transport properties around 110 K. Finally, we discuss the role of dopant concentration, strain and dimensionality on the interplay between antiferrodistortive phase transition, incipient ferroelectricity, and normal and superconducting transport behavior in epitaxial Sr_1-xNd_xTiO₃ thin films grown by hybrid molecular beam epitaxy approach.
  

Ferroelectric Control of Normal and Superconducting States in Oxide Interface by Intrinsic and Extrinsic Bias

We deposit the polar oxide LaAlO₃ on Ca doped SrTiO₃ with various Ca concentrations. The latter becomes ferroelectric below 30K. The resulting interface is conducting with a critical thickness of 3 unit cells of LaAlO₃. A large increase in the interface resistance is observed as the temperature is decreased below the ferroelectric transition with a strong hysteretic behavior as a function of gate voltage. Below T_c 300mK the sample becomes superconducting with a clear hysteresis in T_c with respect to the applied gate voltage. We use scanning SQUID to image the current flow patterns and follow them as a function of gate voltage. We find that the polar LaAlO₃ induces an effective gate bias even when cooling down at zero applied electric field. This effective gate bias has a three-fold effect: it pins the ferroelectric domains near the interface, it reduces the carrier density and confines the current flow. We suggest this effective gate bias as a way to control the initial state of a ferroelectric material. The hysteresis observed both in the normal state and in the superconducting one can lead to a multifaceted controllable memory device.   

Nonlinear transport in gate-induced 2D Rashba superconductor SrTiO3

A polar conductor with Rashba spin-orbit coupling is a potential material platform for exotic quantum transport and spintronic functionalities [1,2]. One of their inherent properties is the nonreciprocal transport, where the magnetoresistance becomes inequivalent between the rightward and leftward current directions, because of the breaking of both spatial inversion and time reversal symmetries. Such a rectification effect reflecting polar symmetry has been studied at the interface or bulk polar semiconductor [3,4]. However, the mechanism of the nonreciprocity in a polar superconductor remains elusive.
To elucidate the noncentrosymmetric nature of the Rashba superconductor, we have investigated the nonreciprocal transport in gated SrTiO₃, which is known as a 2D Rashba superconductor. We found the gigantic enhancement in the nonlinear resistance in the amplitude and phase fluctuation regions. Also, we discuss the possible origins of nonreciprocity in the 2D Rashba superconductor, such as paraconductivity with a parity mixing in the Cooper pairs and rectified vortex-antivortex motions.
[1] E. Lesne et al., Nat. Mater. 15, 1261 (2016).
[2] R. Ohshima et al., Nat. Mater. 16, 609 (2017).
[3] P. He et al., Phys. Rev. Lett. 120, 266802 (2018).
[4] T. Ideue et al., Nat. Phys. 13, 578 (2017).   

Fingerprints of interface superconductivity in ferroelectric tunnel junctions

Ferroelectric tunnel junctions open the possibility of controlling the tunneling current with the ferroelectric state of the barrier. We have studied ferroelectric tunnel junctions with La_0.7Sr_0.3MnO₃ (LSMO) bottom electrode, a ferroelectric (FE) BaTiO₃ barrier and La_0.84Sr_0.16CuO_3-δ (LSCO) top electrode. LSCO is a cuprate capable of hosting large densities of oxygen vacancies (OV), yet superconductivity has never been observed. Coupled switching of oxygen vacancies and the ferroelectric polarization may produce drastic changes of the doping of the interface yielding unexpected electronic phases. We have found that the removal of OV from the cuprate interface has a strong hole doping effect of the material. We have observed the emergence of a new gap in the tunneling differential conductance curves which scales with temperature as a superconducting gap, strongly suggesting the nucleation of a superconducting phase at the interface. Switching the ferroelectric polarization of the barrier with an external electric field enables turning the superconducting gap on and off suggesting the possibility of the field effect control of the interface superconductivity.   

In Situ, Monolayer Control over the 2D Electron Gas on SrTiO3

SrTiO₃ (001) is the principal material in the field of oxide electronics, due to both its properties and the fact that the composition of its surface can be controlled, leading to TiO₂ surface termination. Employing in situ synchrotron X-ray methods during growth by molecular beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES), we discover the existence of a 2D electron gas (2DEG) at the as-prepared, TiO₂-terminated SrTiO₃ (001) surface. Interestingly, the 2DEG appears correlated with the presence of a TiO₂ double layer [S. Cook et al., Sci. Adv. 5, eaav0764 (2019).], as deposition of single layer of SrO leads to island formation and complete elimination of the 2DEG. The subsequent growth of a TiO₂ monolayer leads to smoothing and return of the 2DEG. We will provide an atomic-level description of the evolving surface and how it relates to the surface electronic structure.   

A combined experimental and first principles study of small hole polarons in YTiO3 thin films

Rare-earth titanate YTiO₃ is a Mott insulator whose electronic structure description has been quite challenging to density functional theory (DFT) calculations within standard (semi)local approximations to the exchange-correlation term. The reported onset of optical absorption in YTiO₃ is ~0.6 eV while the calculated Mott-Hubbard gap using hybrid DFT is ~2 eV. Combining experiments on hybrid molecular beam epitaxy (MBE) grown YTiO₃ heterostructures with DFT calculations we investigated the electronic and transport properties of YTiO₃. We test the performance of the meta-GGA SCAN functional in the description of the electronic structure of YTiO₃, comparing the results of hybrid functional and photoemission measurements. We also study the formation, stability and migration of small hole polarons in YTiO₃, and compare our findings with the measurements of the Seebeck coefficient and temperature-dependent resistivity. We also carry out a detailed analysis of the effects of the TiO₆ octahedral tilt and rotations, and lattice distortions on the Mott-Hubbard gap.   

Controlling spin-polarized carriers at the SrTiO3 /EuO interface via the ferroelectric field effect

Using first principle calculations, we investigate the magneto-electric field effect at the SrTiO₃/EuO interface in the BaTiO₃/SrTiO₃/EuO heterostructure. The spontaneous polarization in tetragonal BaTiO₃ is demonstrated to control the movement of the two-dimensional electron gas in SrTiO₃ across the interface with ferromagnetic EuO as well as induce spin polarized two-dimensional hole gas and influence the electronic structure at this interface. We also investigate the effect of oxygen vacancy in SrTiO₃ near the SrTiO₃/EuO interface. To investigate the ferroelectric polarization morphology, we use the phenomenological Landau-Khalatnikov model and compare it with the results of first principle calculations.   

Anomalous Transverse Resistance at the Interface of (111) (La0.3Sr0.7)(Al0.65Ta0.35 )O3 /SrTiO3 Heterostructures

A variety of interesting physical phenomenon have been found to live at the interface between complex oxide layers. Significant work has been done to characterize the electrical transport properties of the two dimensional conducting gases present in systems like LaAlO₃ (LAO)/ SrTiO₃ (STO). Recently, (La_0.3Sr_0.7)(Al_0.65Ta_0.35)O₃ (LSAT) /STO has been investigated due to improved lattice matching when compared to LAO/STO, and interesting effects due to strong spin orbit interactions^1,2. Here we report an anomalous transverse resistance observed in Hall bars at the interface of (111) oriented LSAT/STO. This effect does not track with low field Hall coefficient, nor longitudinal resistance over temperatures from 4 K to 70 K. We discuss these findings in the context of similar effects seen in La_2-xSr_xCuO₄ (LSCO)/STO and LAO/STO systems^3,4.

[1] Bal et al., Phys Rev B. 98, 085416 (2018)
[2] Bal et al., Phys Rev B. 99, 035408 (2019)
[3] Wu et al., J. of Supercon Novel Magn. 32:1623-1628 (2019)
[4] Davis et al., Phys Rev B. 98, 024504 (2018)   

Synthesis of epitaxial ferrimagnetic NiCo2O4 thin films on SrTiO3 and Sr3Al2O6 buffered SrTiO3 substrates

The inverse spinel NiCo₂O₄ (NCO) is ferrimagnetic with high Curie temperature (T_C) and spin polarization. It is of high research interest to integrate it with other functional oxides, while many of them have the perovskite structure. We investigated the effects of a perovskite substrate on the magnetotransport properties of NCO by depositing 30 nm NCO thin films on SrTiO₃ (STO) and Sr₃Al₂O₆ (SAO) buffered STO substrates using off-axis RF magnetron sputtering. Both types of samples exhibit insulating behavior below 300 K, and the films on SAO buffered STO shows more than 10 times higher resistivity at 300 K. We estimated the T_C from the magnetoresistance hysteresis. These samples show T_C of up to 150 K, well below that for films deposited on MgAl₂O₄ substrates (above 300 K). The reduced T_C can be attributed to a high density of anti-phase boundaries due to the structural mismatch between NCO and STO. We also discuss the possibility of suspending the NCO thin films by water etching the SAO buffer layer.   

Deal-Grove-like thermal oxidation of Si (001) buried under a thin layer of SrTiO3

Dry oxidation of Si (001) beneath a thin epitaxial SrTiO₃ layer has been studied using furnace annealing in flowing oxygen. A 10-nm layer of SrTiO₃ is epitaxially grown on Si with no SiO₂ interlayer. For such a structure, an annealing temperature of 800C was found to be the limiting temperature to prevent silicate formation and disruption of the interface structure. The effect of annealing time on thickness of the SiO₂ layer was investigated. In situ x-ray photoelectron spectroscopy(XPS) and reflection-high-energy electron diffraction(RHEED) were used to ensure that the quality of SrTiO₃ is unchanged after the annealing process. The experimental annealing data is compared with a theoretical oxygen diffusion model based on those due to Deal, Grove and Massoud. The model fits the experimental data well, indicating that oxygen diffusion through the SrTiO₃ layer is not the limiting factor. One can therefore readily control the thickness of the SiO₂ interlayer by simply controlling the annealing time in flowing oxygen.