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We report observation of electric field driven conductivity with negative differential conductance and resistive switching in insulating ${\mathrm{SrTiO}}_{3}$ samples over a wide range of applied voltages at low temperatures. The observed current follows $I={I}_{0}\mathrm{exp}[\ensuremath{-}{({E}^{*}/E)}^{1/2}]$ at large applied electric field, corresponding to variable range hopping conduction with a Coulomb gap in domain walls. Our data are sufficient to discriminate unambiguously between Shklovskii and Mott hopping via their different electric field exponent. Under some conditions space-charge-limited currents are observed, and the charge mobility limit is determined to be in the range of 17 and $210\text{ }\text{ }{\mathrm{cm}}^{2}/\mathrm{Vs}$.

We report the control of the interplane magnetic exchange coupling in CaIrO3 perovskite thin films and superlattices with SrTiO3. By analyzing the anisotropic magneto-transport data, we demonstrate...

Zero-energy states in tunnel spectra, an important manifestation of topological superconductivity, have been the focus of the field of topological phases of matter. However, experimentally creating and controlling the zero-energy states is difficult, e.g., it requires a large magnetic field in the case of topological superconductivity. Here, we report on an effective approach to generate controllable zero-energy states in the superconducting Rashba oxide heterostructure by in-plane magnetic field cooling. A pronounced zero-bias conductance peak emerging below a threshold magnetic field down to zero after in-plane magnetic field cooling of a Rashba perovskite oxide heterostructure connecting to an s-wave superconductor. We further demonstrate the high tunability of the zero-energy states between a zero-bias conductance peak and a zero-bias conductance valley by electrical gating at a zero magnetic field. Our theoretical analysis indicates that the in-plane magnetic field or magnetization could lead to finite center-of-mass momentum pairing that supports topological zero-energy states in the Rashba system and provides an opportunity to manipulate topological zero-energy states through gating.

Electric field effect in electronic double layer transistor (EDLT) configuration with ionic liquids as the dielectric materials is a powerful means of exploring various properties in different materials. Here we demonstrate the modulation of electrical transport properties and extremely high mobility of two-dimensional electron gas at LaAlO$_3$/SrTiO$_3$ (LAO/STO) interface through ionic liquid-assisted electric field effect. By changing the gate voltages, the depletion of charge carrier and the resultant enhancement of electron mobility up to 19380 cm$^2$/Vs are realized, leading to quantum oscillations of the conductivity at the LAO/STO interface. The present results suggest that high-mobility oxide interfaces which exhibit quantum phenomena could be obtained by ionic liquid-assisted field effect., 22 pages, 4 figures, ACS Nano, March 09, 2016

The Mott transition and the vicinity of a Mott insulating phase have constantly been a fertile ground for exploring exotic quantum states, most notably the high-${T}_{c}$ cuprates. The layered iridate represents another intriguing example. The nature of the Mott insulator phase and transition mechanism to a metallic state is still under debate. Much of the challenge originates from a series of energy scales involved in the electronics phases. Here, we report synthetization, characterization, and transport measurements on doped and undoped $\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x}\mathrm{Ir}{\mathrm{O}}_{3}$ films grown on $\mathrm{SrTi}{\mathrm{O}}_{3}$. The films are fully strained up to 70 nm thick and have a tetragonal lattice structure. For a doping level of $x=0.1$, a bad-metal state with linear temperature dependence of the resistivity beyond the Mott-Ioffe-Regel limit emerges in a wide temperature range down to a critical temperature ${T}_{c}\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, below which the system shows Mott variable range hopping (Mott VRH) conduction behavior. The ground state is confirmed to be insulating by the Mobius criterion. A strong correlation between the bad-metal state and Mott VRH localized state is found as the slope of the linear resistivity is inversely proportional to film thickness and the size of the Mott VRH activation energy $\mathrm{\ensuremath{\Delta}}$ is linearly proportional to ${T}_{c}$. We further show that upon doping the spin-orbit Mott insulator, itinerant metallic regions coexist with localized regions within a nanoscale phase-separated ground state with a small activation energy. Our results shed light on the nature of the metallic state and a crossover to a bad-metal phase for doping the spin-orbit Mott insulator.

Anisotropic magnetoresistance (AMR) was observed by Lord Kelvin one-and-half centuries ago in iron and nickel. The resistance of these ferromagnetic conductors showed a few percent change when a magnetic field was applied along or across the current. Subsequently, a 20% AMR was demonstrated in alloys of nickel and iron (permalloys). Efforts have then been devoted to extend this effect in multifunctional materials. The oxide heterostructure exhibiting two-dimensional electron liquid is one of the potential candidates as it has shown to exhibit emergent magnetic ordering, strong spin-orbit interactions, and anisotropic magnetoresistance. Here we show a giant crystalline AMR as large as 57% to 104% in anisotropic quantum wells based on nonmagnetic perovskite oxides $\mathrm{LaAl}{\mathrm{O}}_{3}$ and $\mathrm{SrTi}{\mathrm{O}}_{3}$, providing an alternative way in tailoring AMR with an extremely large effect. The AMR maximum appears when the magnetic field points along the in-plane $[1\overline{1}0]$ direction, irrespective of the direction of current flow, which is consistent with the idea of crystalline AMR. Data analysis and density functional theory calculation show that the observed giant crystalline AMR mainly originates from the strong anisotropic spin-orbit field at the interface due to its unique elliptical Fermi surface related to its orbital configuration and reconstruction. This work demonstrates that perovskite oxide interface is a unique platform for orbital physics.

$\mathrm{SrTi}{\mathrm{O}}_{3}$ undergoes a cubic to tetragonal phase transition at ${T}_{c}=105--110\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which can be described by a Brillouin zone corner ${\ensuremath{\Gamma}}_{25}$ (111) soft phonon. Even though clear anomalies in specific heat, thermal expansion coefficient, and sound velocity have been observed, the correlation between phase transition and electronic transport properties in $n$-type doped $\mathrm{SrTi}{\mathrm{O}}_{3}$ is still controversial. Here, we report phase transition induced electronic transport anomaly in temperature dependence of the temperature coefficient of resistance (TCR) consistently observed in a wide variety of $\mathrm{SrTi}{\mathrm{O}}_{3}$-based systems by detailed transport measurements and first-principles calculations. The observed TCR anomaly, which can be well fitted with the electron--${\ensuremath{\Gamma}}_{25}$ soft phonon scattering around ${T}_{c}$, is found to be caused by anomalies in both mobility and carrier density, with the former taking the dominant role. Moreover, the magnitude of the anomaly is found to decrease with increasing carrier density. These findings demonstrate the role of the electron--${\ensuremath{\Gamma}}_{25}$ soft phonon scattering in the conduction mechanism in $\mathrm{SrTi}{\mathrm{O}}_{3}$-based systems.

This work is supported by the National University of Singapore (NUS) Academic Research Fund (AcRF Tier 1 Grants No. R-144-000-346-112 and No. R-144-000-364-112) and the Singapore National Research Foundation (NRF) under the Competitive Research Programs (CRP Awards No. NRF-CRP 8-2011-06 and No. NRF-CRP10-2012-02) by the Institutional Strategy of the University of Tübingen (Deutsche Forschungsgemeinschaft, Grant No. ZUK 63), and by the EU-FP6-COST Grant No. MP1308. We demonstrate electrical mapping of tetragonal domains and electric field-induced twin walls in SrTiO3 as a function of temperature and gate bias utilizing the conducting LaAlO3/SrTiO3 interface and low-temperature scanning electron microscopy. Conducting twin walls appear below 105 K, and new twin patterns are observed after thermal cycling through the transition or on electric field gating. The nature of the twin walls is confirmed by calculating their intersection angles for different substrate orientations. Numerous walls formed when a large side- or back-gate voltage is applied are identified as field-induced ferroelectric twin walls in the paraelectric tetragonal matrix. The walls persist after switching off the electric field and on thermal cycling below 105 K. These observations point to a new type of ferroelectric functionality in SrTiO3, which could be exploited together with magnetism and superconductivity in a multifunctional context. Publisher PDF

We use ionic liquid-assisted electric field effect to tune the carrier density in an electron-doped cuprate ultrathin film and cause a two-dimensional superconductor-insulator transition (SIT). The low upper critical field in this system allows us to perform magnetic field (B)-induced SIT in the liquid-gated superconducting film. Finite-size scaling analysis indicates that SITs induced both by electric and magnetic field are quantum phase transitions and the transitions are governed by percolation effects - quantum mechanical in the former and classical in the latter case. Compared to the hole-doped cuprates, the SITs in electron-doped system occur at critical sheet resistances (Rc) much lower than the pair quantum resistance RQ=h/(2e)2=6.45 k{\Omega}, suggesting the possible existence of fermionic excitations at finite temperature at the insulating phase near SITs., Comment: Manuscript including supplementary materials

The two-dimensional electron gas (2DEG) formed at the perovskite oxides heterostructures is of great interest because of its potential applications in oxides electronics and nanoscale multifunctional devices. A canonical example is the 2DEG at the interface between a polar oxide LaAlO3 (LAO) and non-polar SrTiO3 (STO). Here, the LAO polar oxide can be regarded as the modulating or doping layer and is expected to define the electronic properties of 2DEG at the LAO/STO interface. However, to practically implement the 2DEG in electronics and device design, desired properties such as tunable 2D carrier density are necessary. Here, we report the tuning of conductivity threshold, carrier density and electronic properties of 2DEG in LAO/STO heterostructures by insertion of a La0.5Sr0.5TiO3 (LSTO) layer of varying thicknesses, and thus modulating the amount of polarization of the oxide over layers. Our experimental result shows an enhancement of carrier density up to a value of about five times higher than that observed at the LAO/STO interface. A complete thickness dependent metal-insulator phase diagram is obtained by varying the thickness of LAO and LSTO providing an estimate for the critical thickness needed for the metallic phase. The observations are discussed in terms of electronic reconstruction induced by polar oxides.

We report magnetotransport properties of double heterointerfaces in LaAlO3/SrTiO3/LaAlO3/SrTiO3(001) (LAO/STO/LAO/STO). A strong nonlinearity in the Hall resistivity is found when the temperature is below 80 K. This effect is attributed to multichannel conduction of interfacial charges generated in double heterostructures of LAO/STO where two-dimensional electron gas (2DEG) is produced. The multichannel conduction is confirmed by back gating modulation of Hall effect. Our result suggests the possibility to achieve coupled bilayer 2DEG layers in LAO/STO superlattices.