(THz) applications14"16. However, the electrical anisotropy properties of the grating gated 2DEG systems are largely overlooked. Herein, we conducted experiments of the electrical current angle dependence of the magnetoresistance (MR) in a grating gated 2DEG system. We achieved an extremely large anisotropic magnetoresistance (AMR) effect, which is continuously tunable by the applied gating voltage and magnetic field.

Our findings provide an economical and practical way to design large in-plane electrical anisotropy device.

3. The AlGaN/GaN heterostructure films [Fig. 1(a)] used in this work were purchased from Suzhou Enkris Semiconductor Inc. The AlGaN/GaN films with a 14.5 nm capping layer were patterned into a cross bar geometry [Fig. 1(b)] by using photolithography and inductively coupled plasma (ICP) etching. The currents I can be applied in any desired in-plane orientations, which allows rapid and convenient measurement of the in-plane conductance anisotropy17,18. The bridge width of the cross bar is w = 60 ,«m.

The 2DEG is at 20 nm below the sample surface. Electrodes of Ti(30 nm)/Al(150 nm)/Ni(50 nm)/Au(100 nm) with Ohmic contacts were made by electron beam evaporating, followed by a high temperature annealing at 850 °C for 30 seconds. At last, grating gates made of 30nm Ti/lOOnm Au, were fabricated on top of the sample as shown in Fig. 1(a). The width of each grating stripe line is 5 ,wm and the period is 10 ,wm. The gate stripe lines are alternatively separated in to two groups to produce periodically modulated potential in the 2DEG, as shown in Figs. 1(a) and 1(b). One group of the gating electrodes are grounded, while a gate voltage Eg is applied to the other group of the gating electrodes to regulate the potential amplitude. We define that L e tte rs The magnetic field for magnetoresistance measurements was applied along the normal direction of the film plane. We conducted the experimental measurements in a Cryogen Free Superconducting Magnet Systems (CRYOMAGNETICS, INC.).

Figure 1(c) presents the polar coordinate profile of the longitudinal resistance as a function of the current orientation 0 under various gate voltages under zero magnetic field and at 1.8 K. When the negative grating gating voltage Vg is applied from 0 V to -4 V, the resistances for all the current directions increase gradually due to the decrease of carrier density. At Vg = -4 V, the resistance for 6 = 0° is 52.5 H which is slightly larger than R = 48.4 H for 6 = 90°. Here we define the electrical anisotropy factor a = -Rmax/ftmin, where Rmax and Rmin are the maximum and minimum of the measured resistances, respectively. It can be obtained o = 1.085 at Vg = -4 V, indicating a very tiny electrical anisotropy induced by grating gating under zero magnetic field. Surprisingly, this uniaxial anisotropy becomes dramatically enhanced when an out-of-plane magnetic field is applied. Figure 1(d) shows R vs. 0 curves at Vg = -4 V in several selected magnetic fields. A giant electrical anisotropy is observed at a high magnetic field. The longitudinal resistances for the current directions near 0 = 0° (180°) exhibit quadratic increases with increasing the magnetic field from zero to 9 T. On the contrary, the shows the angular 0 dependence of longitudinal resistance under various applied gate voltages and at a magnetic field of 9 T. One can see that the gating effect can tremendously influence the anisotropy of the electrical properties for this system under a high out-of-plane magnetic field. Thus, the large electrical anisotropy is in-situ tunable by both the grating gate voltage and the out-of-plane magnetic field.

5. The resistances change with the magnetic and electrical fields in Fig. 1(d) and 1 (e), respectively, except for several certain current directions (e.g. 6-15°, 105°, 255°, 285°), which separate the positive magnetoresistance (PMR) region and the negative magnetoresistance (NMR) region. To directly demonstrate the magnetoresistance effects, we measured the field dependent magnetoresistance at various current orientations [Fig. 2(a)]. The magneto resistance is defined as MR = [R(B) -R(0)]ZR(0)

x 100%, where R(B) and R(0) are the resistance in a magnetic field B and that in a zero field, respectively. The MR curve shows a nearly zero magnetoresistance for 0 = 75° at ±9 Tesla. The MR for 9 = 0° has a largest PMR without any sign of saturation at magnetic fields as high as 9 T, while the MR for 9 = 90° has a large NMR up to -82% at 9 T. The MRs of our grating gated 2DEG sample present positive and negative extremes at the two orthogonal directions, respectively, leading to a huge electrical anisotropy factor a at high magnetic fields. 2(c), it shows a monotonic decrease in u, whose value finally becomes to be around 1 at Fg = 0 V or further applying a positive Fg. Hence, these results demonstrate again that the periodic gating potential formed in the 2DEG channel leads to a dramatic and tunable electrical anisotropy in magnetic fields. 6. To investigate the thermal evolution of the electrical anisotropy of the system, we carried out magnetoresistance measurements at a series of selected temperatures. Figure 3 7. Previous investigation shows that the same 2DEG sample covered by a uniform gate pad displays a PMR of 10% under a flat gating voltage of -4 V20. Therefore, the giant PMR and NMR observed in the sample with the grating gate are ascribed to the artificial periodic potential in the 2DEG channel, which has notable difference on the transport properties for the currents with different directions. For 0 = 0° (the current is perpendicular to the periodic gate stripes) the PMR is approximately quadratic as a function of magnetic field. This behavior is similar as that found in GaAs/AlGaAs 2DEG systems with a periodic potential at the low magnetic field range21'22, which can be explained by considering the classical trajectories of electrons in a periodic potential21. The suppression of the SdH oscillations was also observed in the GaAs/AlGaAs 2DEG system, which results from the broadening of the Landau levels by the periodic potential21. The PMR for 6 = 0° in the AlGaN/GaN 2DEG system probably originates from a similar mechanism. However, differing from the quadratic PMR effect in the GaAs/AlGaAs, which only exists in the weak magnetic field (below a few hundred milli-Tesla22), the quadratic PMR phenomenon is non-saturating up to 9 T in our AlGaN/GaN sample. A non-saturating extremely large PMR in the multi band topological quantum materials, such as in a Weyl semimetal WTe2 crystal, was attributed to the balanced electron-hole populations23, which is clearly not the case in our system. Furthermore, the periodicity of the ID grating potential in this work is 20 ,«m which much larger than that of sub micrometers in the GaAs 2DEG21,22. Aside from L e tte rs that, the electron mobility of GaAs/AlGaAs 2DEG is over 106 cm2 V"1 s"1 which is 100 times larger than that of the AlGaN/GaN system used in this work (see Fig. 3(d)).

8. The uniform AlGaN/GaN 2DEG without a grating gating shows a notable NMR effect, which originates from the memory effect induced by interface roughness scattering20. This NMR effect is suppressed by applying a uniform gating voltage20. In contrast, the grating gate voltage significantly enhances the NMR effect for 0 = 90° in this work [Figs. 3(a) and 3(c)]. The MR vs. B curves show a steep drop at low fields [Fig. 3(c)], which is similar to the colossal NMR discovered in a GaAs/AlGaAs quantum well24. The giant NMR in the GaAs quantum well was attributed to a hydrodynamic mechanism that magnetic field decreases the diagonal viscosity in ultrahigh-mobility 2D electrons25. Farge NMR was also observed in the width limited channels of ultrapure 2D metal PdCoCk26 and high mobility graphene27 based on electron viscous flow. Electrical gating alters both the carrier density and the mobility in AlGaN/GaN 2DEG system20. Hence, the grating gate produces stripe channels alternately containing two different carriers. However, the mean free path of electrons in this AlGaN/GaN 2DEG system is about 0.4 ,wm20 that is much smaller than the period 20 ,wm of the grating gate, which is incompatible with the requirement of electron viscous flow in the high purity 2D samples25. Nevertheless, the PMR at 6 = 0° and the NMR at 0 = 90° in our grating gated 2DEG sample lead to incredible in-plane uniaxial electrical anisotropy in a high magnetic field. Future investigations, such as the period universal phenomenon in an electrical anisotropic system, which could benefit applications, such as artificial biological neural network synapse devices1,4"6. 10. In conclusion, the grating gate induces a periodic potential in the 2DEG of AlGaN/GaN, which creates in-situ tunable artificial electronic nematicity with tailored electrical anisotropy. Extremely large PMR and NMR behaviors have been obtained when current is rotated along different in-plane directions in a cross-bar sample. This results in a dramatically large in-plane electrical anisotropy as revealed by both the longitudinal resistances and the transverse resistances. Our findings could be advantageous for designing programmable devices, such as multi-terminal memtransistors, for complex neuromorphic applications. Room temperature high mobility materials, such as graphene, may also be a good candidate, whose electrical anisotropy can be artificially manipulated at much higher temperatures.

See the supplementary materials for details of the measurement method and the conductance tensor calculations. dimensional electron gas in a strong periodic potential," Phys. Rev. B 42, 9229 (1990).

This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.