Electrically tunable transport in the antiferromagnetic Mott insulatorSr2IrO4

Abstract

The electronic band gap in conventional semiconductor materials, such as silicon, is fixed by the material's crystal structure and chemical composition. The gap defines the material's transport and optical properties and is of great importance for performance of semiconductor devices like diodes, transistors and lasers. The ability to tune its value would allow enhanced functionality and flexibility of future electronic and optical devices. Recently, an electrically tunable band gap was realized in a 2D material -electrically gated bilayer graphene [1][2][3] . Here we demonstrate the realization of an electrically tunable band gap in a 3D antiferromagnetic Mott insulator Sr 2 IrO 4 . Using nano-scale contacts between a sharpened Cu tip and a single crystal of Sr 2 IrO 4 , we apply a variable external electric field up to a few MV/m and demonstrate a continuous reduction in the band gap of Sr 2 IrO 4 by as much as 16%. We further demonstrate the feasibility of reversible resistive switching and electrically tunable anisotropic magnetoresistance, which provide evidence of correlations between electronic transport, magnetic order, and orbital states in this 5d oxide. Our findings suggest a promising path towards band gap engineering in 5d transition-metal oxides that could potentially lead to appealing technical solutions for next-generation electronic devices.Tuning material properties electrically is highly desirable for future developments of device physics and associated technologies. Transition metal oxides (TMOs) are promising candidates for such studies [4] . Of particular interest is the iridate Sr 2 IrO 4 (SIO), which is known to have comparable energy scales of spin-orbit coupling, crystal-field splitting, and electron correlations [5] [6] . Recently there has been a growing interest in the study of various physical phenomena in SIO, including magnetoelectricity in a canted antiferromagnetic phase [7][8] and non-Ohmic electron transport [9][10] . Nevertheless, little insight has been developed towards a clear understanding of the interconnections between spin-orbit coupling, electron transport, and lattice dynamics in SIO. In particular, transport mechanisms under external electric/magnetic fields in this Mott insulator remain to be addressed.Here we present a temperature-dependent study of magneto-transport in Sr 2 IrO 4 under dc electric fields up to a few MV/m achieved with the point-contact (PC) technique. Our 2 sample is a single crystal of Sr 2 IrO 4 (1.5 mm×1 mm×0.5 mm) synthesized via a self-flux technique [11] . The insert to Fig. 1a shows schematically a point contact between a sharpened Cu tip and the crystal. The tip is brought into contact with a (001) surface of the crystal with a standard mechanically controlled point-contact system described elsewhere [12] . The system provides a means to produce point contacts with sizes a (see insert to Fig. 1a) ranging from microns down to a few nanometers. An electrical current is injected through the contact into the crystal and flows (primarily) ...

The challenge of spin–orbit-tuned ground states in iridates: a key issues review

“…As discussed below, the rotation of IrO 6 octahedra, which corresponds to a distorted in-plane Ir1-O2-Ir1 bond angle, plays an extremely important role in determining the electronic and magnetic structures. The Ir1-O2-Ir1 bond angle can be tuned via magnetic field [50] , high pressure [ 95 ] , electric field [90] and epitaxial strain [39] . The lattice properties not only make the ground state readily tunable but also provide a new paradigm for development of functional materials and devices.…”

“…It is clear that r is current dependent and drastically decreases as I increases throughout the temperature range measured (see the inset of Fig.8) [4] . A recent study using nanoscale contacts reports a continuous reduction of the resistivity of Sr 2 IrO 4 with increasing bias, which is characterized by a reduction in the transport activation energy by as much as 16% [90] . This behavior, which is qualitatively consistent with that reported earlier [4] , is essentially attributed to changes in the Ir1-O2-Ir1 bond angle induced by the electric field [90] .…”

Electrically tunable transport and high-frequency dynamics in antiferromagnetic Sr3Ir2O7

“…In these systems, comparable energy scales of crystal-field splitting, electron correlations, and spin-orbit coupling (SOC) result in close interconnections between physical properties and crystal structure. For instance, a very large anisotropic magnetoresistance (AMR) [1] and reversible resistive switching driven by high-density currents/high electric fields [2] were both demonstrated in an AFM Mott insulator Sr 2 IrO 4 . These results provide an interesting insight into using such phenomena for writing and reading information in spintronic memory applications, but the relatively low Néel temperature of Sr 2 IrO 4 (T N =240 K) makes this specific material impractical.…”

“…In Sr 3 Ir 2 O 7 such an order may be initiated via the field effect: an applied dc bias leads to lattice distortions and associated electric polarization [2]; the lattice distortions, in turn, may alter the magnetic structure of Sr 3 Ir 2 O 7 via spinorbit coupling and promote a spiral-like AFM magnetic order at a certain level of applied dc bias (|I C |); finally, a sufficiently high ac current applied on top of the dc bias could excite the spin super currents. The absence of the switching in dc R(I) characteristics above this critical ac (microwave)…”

“…In summary, we have studied dc and high-frequency transport properties in antiferromagnetic [2]. The activation energy can be tuned by an applied dc electrical bias, which allows for a continuous variation of its resistivity by as much as 50% followed 8 by a reversible resistive switching at higher biases.…”