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We report point-contact measurements of anisotropic magnetoresistance (AMR) in a single crystal of antiferromagnetic Mott insulator Sr 2 IrO 4 . The point-contact technique is used here as a local probe of magnetotransport properties on the nanoscale. The measurements at liquid nitrogen temperature reveal negative magnetoresistances (up to 28%) for modest magnetic fields (250 mT) applied within the IrO 2 a-b plane and electric currents flowing perpendicular to the plane. The angular dependence of magnetoresistance shows a crossover from fourfold to twofold symmetry in response to an increasing magnetic field with angular variations in resistance from 1% to 14%. We tentatively attribute the fourfold symmetry to the crystalline component of AMR and the field-induced transition to the effects of applied field on the canting of antiferromagnetic-coupled moments in Sr 2 IrO 4 . The observed AMR is very large compared to the crystalline AMRs in 3d transition metal alloys or oxides (0.1%-0.5%) and can be associated with the large spin-orbit interactions in this 5d oxide while the transition provides evidence of correlations between electronic transport, magnetic order, and orbital states. The finding of this work opens an entirely new avenue to not only gain a new insight into physics associated with spin-orbit coupling but also to better harness the power of spintronics in a more technically favorable fashion. [13] behaviors, which makes them an attractive playground for studying physics driven by spin-orbit interactions. As AMR is known to be closely associated with spin-orbit interaction, the strong spin-orbit interaction in this 5d transition metal oxide may favor stronger AMR compared to 3d transition metal alloys and oxides. The recent magnetotransport studies in Sr 2 IrO 4 single crystals [14,15] and thin films [6] revealed largely unexplored correlations between electronic transport, magnetic order, and orbital states.Here, we present the first observation of the pointcontact AMR in single crystals of the AFM Mott insulator Sr 2 IrO 4 [14-16], which can potentially be used to sense the AFM order parameter in spintronic nanodevices. The pointcontact technique allows us to probe very small volumes and, therefore, measures electronic transport on a microscopic scale. Point-contact measurements with single crystals of Sr 2 IrO 4 are intended to examine whether the additional local resistance associated with a small contact area between a sharpened Cu tip and the antiferromagnet shows a magnetoresistance (MR) like that seen in bulk crystals. The measurements at liquid nitrogen temperature reveal large MRs (up to 28%) for modest magnetic fields (250 mT) applied within the IrO 2 a-b plane. The angular dependence of MR reveals an AMR with an intriguing transition from fourfold to twofold symmetry in response to

Here we present experimental results on laser-driven ion acceleration from relativistically transparent, overdense plasmas in the break-out afterburner (BOA) regime. Experiments were preformed at the Trident ultra-high contrast laser facility at Los Alamos National Laboratory, and at the Texas Petawatt laser facility, located in the University of Texas at Austin. It is shown that when the target becomes relativistically transparent to the laser, an epoch of dramatic acceleration of ions occurs that lasts until the electron density in the expanding target reduces to the critical density in the non-relativistic limit. For given laser parameters, the optimal target thickness yielding the highest maximum ion energy is one in which this time window for ion acceleration overlaps with the intensity peak of the laser pulse. A simple analytic model of relativistically induced transparency is presented for plasma expansion at the

We report point-contact measurements of anisotropic magnetoresistance (AMR) in a single crystal of antiferromagnetic (AFM) Mott insulator Sr2IrO4. The point-contact technique is used here as a local probe of magnetotransport properties on the nanoscale. The measurements at liquid nitrogen temperature revealed negative magnetoresistances (MRs) (up to 28%) for modest magnetic fields (250 mT) applied within the IrO2 a-b plane and electric currents flowing perpendicular to the plane. The angular dependence of MR shows a crossover from four-fold to two-fold symmetry in response to an increasing magnetic field with angular variations in resistance from 1-14%. We tentatively attribute the four-fold symmetry to the crystalline component of AMR and the field-induced transition to the effects of applied field on the canting of AFM-coupled moments in Sr2IrO4. The observed AMR is very large compared to the crystalline AMRs in 3d transition metal alloys/oxides (0.1-0.5%) and can be associated with the large spin-orbit interactions in this 5d oxide while the transition provides evidence of correlations between electronic transport, magnetic order and orbital states. The finding of this work opens an entirely new avenue to not only gain a new insight into physics associated with spin-orbit coupling but also better harness the power of spintronics in a more technically favorable fashion.

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) ...