Clayton A. Jackson

Toward an artificial Mott insulator: Correlations in confined high-density electron liquids in SrTiO3

We investigate correlation physics in high-density, two-dimensional electron liquids that reside in narrow SrTiO3 quantum wells. The quantum wells are remotely doped via an interfacial polar discontinuity and the three-dimensional (3D) carrier density is modulated by changing the width of the quantum well. It is shown that even at 3D densities well below one electron per site, short-range Coulomb interactions become apparent in transport, and an insulating state emerges at a critical density. We also discuss the role of disorder in the insulating state.

Quantum critical behaviour in confined SrTiO3 quantum wells embedded in antiferromagnetic SmTiO3

Quantum phase transitions are driven by quantum fluctuations that alter the nature of the electronic quasiparticles, resulting in phenomena such as non-Fermi liquid behaviour. Oxide heterostructures offer fundamentally new ways of manipulating quantum criticality. Here, we report on non-Fermi liquid behaviour in thin SrTiO3 quantum wells that are embedded in insulating, antiferromagnetic SmTiO3, as a function of temperature, quantum well thickness and SmTiO3 layer thickness in superlattices. Such quantum wells contain very high sheet carrier densities on the order of one electron per pseudocubic planar unit cell. We show that the quantum well thickness is a tuning parameter for non-Fermi liquid behaviour. Increasing the thickness by a single atomic layer and coupling in superlattices recover the Fermi liquid behaviour. The critical exponents, the symmetry of the order parameter, the role of carrier densities and symmetry-lowering distortions are discussed, and the results are compared with those of quantum wells embedded in ferrimagnetic GdTiO3.

Energetic, spatial, and momentum character of the electronic structure at a buried interface : The two-dimensional electron gas between two metal oxides

The interfaces between two condensed phases often exhibit emergent physical properties that can lead to new physics and novel device applications and are the subject of intense study in many disciplines. We here apply experimental and theoretical techniques to the characterization of one such interesting interface system: the two-dimensional electron gas (2DEG) formed in multilayers consisting of SrTiO3 (STO) and GdTiO3 (GTO). This system has been the subject of multiple studies recently and shown to exhibit very high carrier charge densities and ferromagnetic effects, among other intriguing properties. We have studied a 2DEG-forming multilayer of the form [6 unit cells (u.c.) STO/3 u.c. of GTO](20) using a unique array of photoemission techniques including soft and hard x-ray excitation, soft x-ray angle-resolved photoemission, core-level spectroscopy, resonant excitation, and standing-wave effects, as well as theoretical calculations of the electronic structure at several levels and of the actual photoemission process. Standing-wave measurements below and above a strong resonance have been exploited as a powerful method for studying the 2DEG depth distribution. We have thus characterized the spatial and momentum properties of this 2DEG in detail, determining via depth-distribution measurements that it is spread throughout the 6 u.c. layer of STO and measuring the momentum dispersion of its states. The experimental results are supported in several ways by theory, leading to a much more complete picture of the nature of this 2DEG and suggesting that oxygen vacancies are not the origin of it. Similar multitechnique photoemission studies of such states at buried interfaces, combined with comparable theory, will be a very fruitful future approach for exploring and modifying the fascinating world of buried-interface physics and chemistry.

Interface-induced magnetism in perovskite quantum wells

RAPID COMMUNICATIONS PHYSICAL REVIEW B 88, 180403(R) (2013) Interface-induced magnetism in perovskite quantum wells Clayton A. Jackson and Susanne Stemmer Materials Department, University of California, Santa Barbara, California 93106-5050, USA (Received 9 July 2013; published 19 November 2013) We investigate the angular dependence of the magnetoresistance of thin (<1 nm), metallic SrTiO 3 quantum wells epitaxially embedded in insulating, ferrimagnetic GdTiO 3 and insulating, antiferromagnetic SmTiO 3 , respectively. The SrTiO 3 quantum wells contain a high density of mobile electrons (∼7 × 10 14 cm −2 ). We show that the longitudinal and transverse magnetoresistance in the structures with GdTiO 3 are consistent with anisotropic magnetoresistance, and thus indicative of induced ferromagnetism in the SrTiO 3 , rather than a nonequilibrium proximity effect. Comparison with the structures with antiferromagnetic SmTiO 3 shows that the properties of thin SrTiO 3 quantum wells can be tuned to obtain magnetic states that do not exist in the bulk material. DOI: 10.1103/PhysRevB.88.180403 PACS number(s): 75.75.−c, 75.30.Gw, 75.70.−i, 77.84.Bw Proximity effects at interfaces between ferromagnetic insulators and conductors with strong spin-orbit coupling have attracted attention as components of hybrid structures for spintronics, quantum computing, and as a route to Majorana fermions. 1–3 Oxide heterostructures are particularly attrac- tive for inducing phenomena through interfacial proximity, because the relevant phenomena, such as superconductivity, spin-orbit coupling, two-dimensional electron gases, and magnetism can all be found in a single materials class, the perovskites, allowing for high-quality epitaxial structures. A prototypical perovskite heterostructure is that between the ferrimagnetic Mott insulator GdTiO 3 , and the band insulator SrTiO 3 . Such interfaces exhibit a high-density, two-dimensional electron gas (2DEG) with ∼3 × 10 14 cm −2 mobile carriers in the SrTiO 3 . 4 Thin (<2 nm) quantum wells of SrTiO 3 embedded in GdTiO 3 show magnetoresistance hysteresis at low temperatures. 5 It was suggested that the proximity to the ferrimagnetic GdTiO 3 likely plays a role, but the magnetic state of the SrTiO 3 was not resolved. For example, it is possible that the SrTiO 3 quantum well has become magnetic due to exchange coupling. Theoretical calculations suggest a tendency towards ferromagnetism for thin quantum wells. 6,7 In this case, the SrTiO 3 quantum well may exhibit magnetotransport properties typical of ferromagnets, such as anisotropic magnetoresistance (AMR). An alternative explanation is that the resistance hysteresis reflects the orientation of the magnetization in the GdTiO 3 , without the SrTiO 3 quantum well itself being ferromagnetic. The latter is a nonequilibrium proximity effect that has become known as spin Hall magnetoresistance (SMR), and results from a combination of direct and inverse spin Hall effects. 8–10 Spin-orbit coupling is at the core of both SMR and AMR. As spin-related effects are important in both bulk SrTiO 3 (Ref. 11) and SrTiO 3 2DEGs, 12,13 either AMR (for a ferromagnetic quantum well) or SMR (for a nonmagnetic quantum well) may occur. The two phenomena are distinguishable by the dependence of the magnetoresistance on the orientation of the magnetic field. 8 To engineer novel states at interfaces between conducting, nonmagnetic perovskites and insulating, magnetic perovskites, it is essential that the magnetic state of these interfaces be understood. In this Rapid Communication, we report angular- dependent magnetoresistance studies of narrow SrTiO 3 quan- tum wells, embedded in ferrimagnetic GdTiO 3 and antiferro- magnetic SmTiO 3 , respectively. The results are consistent with exchange coupling–induced ferromagnetism in the SrTiO 3 in case of interfaces with GdTiO 3 and, more indirectly, with induced antiferromagnetism in the case of SmTiO 3 . GdTiO 3 /SrTiO 3 /GdTiO 3 quantum well structures were grown by hybrid molecular beam epitaxy on (001) (LaAlO 3 ) 0.3 (Sr 2 AlTaO 6 ) 0.7 (LSAT) crystals. The data reported here are for a sample with 4-nm top and bottom GdTiO 3 layers, and 0.8-nm SrTiO 3 (about three SrO layers 14 ). Samples of SmTiO 3 /SrTiO 3 /SmTiO 3 with varying SrTiO 3 thicknesses were also investigated. In both samples, the SrTiO 3 quantum wells are metallic and contain a high, mobile carrier den- sity (∼7 × 10 14 cm −2 ), due to interface doping from each interface. 4 In contrast to GdTiO 3 , which is ferrimagnetic with a Curie temperature of ∼30 K in bulk 15 and ∼20 K in the samples with 4-nm GdTiO 3 , 5 SmTiO 3 is antiferromagnetic with a N´eel temperature of ∼50 K. 16 Electrical and structural characterization, as well as growth details have been described elsewhere. 5,14,17,18 The carrier mobility was an order of mag- nitude higher than in conducting, doped GdTiO 3 or SmTiO 3 , showing that electrical transport occurs only in the SrTiO 3 quantum well [further evidence comes from magnetotransport; Seebeck measurements, which are SrTiO 3 -like; 19 and the band offsets, which favor charge transfer into the SrTiO 3 (Refs. 4 and 20)]. Electrical contacts were deposited by electron beam evap- oration in van der Pauw geometry with a shadow mask and consisted of 40-nm Ti/400-nm Au, with the Au contact being the topmost layer. Magnetoresistance and Hall data were collected using a Physical Property Measurement System (Quantum Design PPMS Dynacool). The system’s resistivity option was used for Hall and sheet resistance and the electrical transport option for magnetoresistance. The latter utilizes an internal lock-in technique with a frequency of 70.1 Hz and an averaging time of 20 s per measurement. An external junction box allows assigning any function to any contact pad using external cables, and was used to confirm that the same behavior was measured within each possible contact geometry for a specific measurement geometry. The magnetoresistance was measured between ± 1 T, pausing every 0.016 T to ©2013 American Physical Society

Modulation of over 1014 cm−2 electrons in SrTiO3/GdTiO3 heterostructures

We demonstrate charge modulation of over 1014 cm−2 electrons in a two-dimensional electron gas formed in SrTiO3/GdTiO3 inverted heterostructure field-effect transistors. Increased charge modulation was achieved by reducing the effect of interfacial region capacitances through thick SrTiO3 cap layers. Transport and device characteristics of the heterostructure field-effect transistors were found to match a long channel field effect transistor model. SrTiO3 impurity doped metal–semiconductor field effect transistors were also demonstrated with excellent pinch-off and current density exceeding prior reports. The work reported here provides a path towards oxide-based electronics with extreme charge modulation exceeding 1014 cm−2.

Extreme charge density SrTiO3/GdTiO3 heterostructure field effect transistors

We report on the fabrication and electrical characteristics of the first SrTiO3/GdTiO3 (STO/GTO) heterostructure field-effect transistors (HFETs). The high two-dimensional electron gas (2DEG) density of 3 × 1014 cm−2 formed due to the polar discontinuity at the STO/GTO interface was used as a channel to create inverted HFETs. Plasma O2 treatment was found to reduce current leakage by 3 orders of magnitude at reverse bias, leading to rectifying Schottky behavior. A charge modulation of 0.6 × 1014 cm−2 is reported here, which represents the highest sheet charge modulated in any planar field effect transistor to date.

Correlation between metal-insulator transitions and structural distortions in high-electron-density SrTiO3 quantum wells

PHYSICAL REVIEW B 89, 075140 (2014) Correlation between metal-insulator transitions and structural distortions in high-electron-density SrTiO 3 quantum wells Jack Y. Zhang, 1 Clayton A. Jackson, 1 Ru Chen, 2 Santosh Raghavan, 1 Pouya Moetakef, 1,* Leon Balents, 3 and Susanne Stemmer 1 Materials Department, University of California, Santa Barbara, California 93106, USA Department of Physics, University of California, Santa Barbara, California 93106, USA Kavli Institute of Theoretical Physics, University of California, Santa Barbara, Santa Barbara, California 93106, USA (Received 10 January 2014; revised manuscript received 13 February 2014; published 28 February 2014) The electrical and structural characteristics of SmTiO 3 /SrTiO 3 /SmTiO 3 and GdTiO 3 /SrTiO 3 /GdTiO 3 het- erostructures are compared. Both types of structures contain narrow SrTiO 3 quantum wells, which accommodate a confined, high-density electron gas. As shown previously [Phys. Rev. B 86, 201102(R) (2012)] SrTiO 3 quantum wells embedded in GdTiO 3 show a metal-to-insulator transition when their thickness is reduced so that they contain only two SrO layers. In contrast, quantum wells embedded in SmTiO 3 remain metallic down to a single SrO layer thickness. Symmetry-lowering structural distortions, measured by quantifying the Sr-column displacements, are present in the insulating quantum wells, but are either absent or very weak in all metallic quantum wells, independent of whether they are embedded in SmTiO 3 or in GdTiO 3 . We discuss the role of orthorhombic distortions, orbital ordering, and strong electron correlations in the transition to the insulating state. DOI: 10.1103/PhysRevB.89.075140 PACS number(s): 71.27.+a, 71.30.+h, 81.07.St I. INTRODUCTION Quantum-confined transition-metal oxides allow for cre- ating new states of matter through manipulation of spin and orbital order, interfacial proximity effects, and reduced dimensionality, and can thus serve to elucidate the physics of two-dimensional, strongly correlated electron systems [1]. For example, narrow, high-electron-density quantum wells of a nonmagnetic band insulator, SrTiO 3 , which are embedded in a Mott insulating ferrimagnet, GdTiO 3 , show ferromagnetism and mass enhancement due to strong electron correlations [2–4]. At the smallest dimensions, when the quantum wells contain just two SrO layers, the electron system abruptly localizes and the resistivity increases by several orders of magnitude [2]. The transition to the insulating state is accompanied by structural distortions of the Ti-O octahedra, which can be experimentally detected by measuring concurrent displacements of the Sr cations [5]. Metal-insulator transitions at reduced thicknesses have also been observed in narrow quantum wells and thin films of many other perovskite materials, such as SrVO 3 [6], LaNiO 3 [7–9], and NdNiO 3 [10]. In general, in many d-electron systems, symmetry breaking of spin and orbital degrees of freedom plays a crucial role in promoting an insulating state in materials that undergo a metal-insulator transition [11]. Transition-metal–oxygen octahedral tilts that reduce the symmetry relative to the parent cubic perovskite structure are modified in quantum wells due to film strain [12,13] and interfacial coherency [3,14–16]. To understand the underlying physics of Mott transitions in confined quantum wells, such as the relative roles of disorder, the interactions among the electrons themselves (strong correlations), and interactions of the carriers with the lattice, it is useful to explore if the localization can be systematically tuned by changing the external parameters of Present address: Department of Chemistry and Biochemistry, University of Maryland, College Park, MD. the system. Towards this goal, we compare the electrical and structural properties of thin SrTiO 3 quantum wells embedded in GdTiO 3 and SmTiO 3 , respectively. We have previously reported on the electrical and structural properties of the structures with GdTiO 3 [2,5], and they are included here for comparison. In both cases, the quantum wells contain a two-dimensional electron gas with sheet carrier densities of close to one electron per (pseudo-)cubic planar unit cell, which is introduced by the charge discontinuity at the interface [2,17]. This sheet carrier density is independent of the film thicknesses. It is important to emphasize that SrTiO 3 is a band insulator in bulk, and has the ideal cubic perovskite structure at room temperature. Therefore, and in contrast to the aforementioned confined correlated metals, such as the nickelates, correlated properties—including magnetism, mass enhancement, and metal-insulator transitions—are induced in a material that does not exhibit Mott physics in the bulk. Both GdTiO 3 and SmTiO 3 are prototypical Mott insulators, with a d 1 electron configuration. SmTiO 3 has the same orthorhombic crystal structure as GdTiO 3 , albeit with slightly smaller octahedral distortions [18]. The two compounds also differ in their low-temperature magnetic properties—GdTiO 3 is ferrimagnetic, while SmTiO 3 is antiferromagnetic [19]. These properties couple with the electron system in the quantum well [4]. Furthermore, they exhibit different orbital ordering, which is antiferro-orbital in GdTiO 3 and ferro-orbital in SmTiO 3 , respectively [20–22]. II. EXPERIMENT All films were grown by hybrid molecular beam epitaxy (MBE) [23,24] on (001) (La 0.3 Sr 0.7 )(Al 0.65 Ta 0.35 )O 3 (LSAT, ˚ substrates. Electrical measurements were carried a = 3.86 A) out on GdTiO 3 /SrTiO 3 /GdTiO 3 and SmTiO 3 /SrTiO 3 /SmTiO 3 quantum well structures that contained a single SrTiO 3 quan- tum well. The GdTiO 3 and SmTiO 3 layers were 10 nm thick. The thicknesses of the SrTiO 3 quantum wells are specified in ©2014 American Physical Society

Magnetism and local structure in low-dimensional Mott insulating GdTiO3

Author(s): Zhang, Jack Y.; Jackson, Clayton A.; Raghavan, Santosh; Hwang, Jinwoo; Stemmer, Susanne | Abstract: Cation displacements, oxygen octahedral tilts, and magnetism of epitaxial, ferrimagnetic, insulating GdTiO3 films sandwiched between cubic SrTiO3 layers are studied using scanning transmission electron microscopy and magnetization measurements. With decreasing GdTiO3 film thickness, structural (GdFeO3-type) distortions are reduced, concomitant with a reduction in the Curie temperature. Ferromagnetism persists to smaller deviations from the cubic perovskite structure than is the case for the bulk rare-earth titanates. The results indicate that the ferromagnetic ground state is controlled by the narrow bandwidth, exchange and orbital ordering, and only to second order depends on the amount of the GdFeO3-type distortion.

Modulation doping to control the high-density electron gas at a polar/non-polar oxide interface

A modulation-doping approach to control the carrier density of the high-density electron gas at a prototype polar/non-polar oxide interface is presented. It is shown that the carrier density of the electron gas at a GdTiO3/SrTiO3 interface can be reduced by up to 20% from its maximum value (∼3 × 1014 cm−2) by alloying the GdTiO3 layer with Sr. The Seebeck coefficient of the two-dimensional electron gas increases concurrently with the decrease in its carrier density. The experimental results provide insight into the origin of charge carriers at oxide interfaces exhibiting a polar discontinuity.

Capacitance-voltage analysis of high-carrier-density SrTiO3/GdTiO3 heterostructures

We report on capacitance-voltage (C-V) analysis of SrTiO3/GdTiO3 heterostructures that contain a high-density, two-dimensional electron gas (2DEG) at the interface. The complex admittance was measured as a function of frequency for different gate biases applied to a Schottky contact on the SrTiO3. A one-dimensional, complex impedance transmission line model was used to extract C-V characteristics from the frequency dependent admittance. The extracted capacitance was nearly independent of the gate voltage (up to −0.5 V), indicating a fully depleted SrTiO3 layer. The results are used to estimate the dielectric constant of the SrTiO3, the degree of modulation of the 2DEG by the maximum applied voltage (∼2.5%), and to establish an upper limit of the residual carrier density in the bulk of the SrTiO3 film (∼9 × 1018 cm−3).