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1. Colossal magnetoresistance in the multiple wave vector charge density wave regime of an antiferromagnetic Dirac semimetal

   https://doi.org/10.1126/sciadv.adh0145  

   Singha, Ratnadwip ; Dalgaard, Kirstine J. ; Marchenko, Dmitry ; Krivenkov, Maxim ; Rienks, Emile D. ; Jovanovic, Milena ; Teicher, Samuel M. ; Hu, Jiayi ; Salters, Tyger H. ; Lin, Jingjing ; et al ( October 2023 , Science Advances)

   Colossal negative magnetoresistance is a well-known phenomenon, notably observed in hole-doped ferromagnetic manganites. It remains a major research topic due to its potential in technological applications. In contrast, topological semimetals show large but positive magnetoresistance, originated from the high-mobility charge carriers. Here, we show that in the highly electron-doped region, the Dirac semimetal CeSbTe demonstrates similar properties as the manganites. CeSb_0.11Te_1.90hosts multiple charge density wave modulation vectors and has a complex magnetic phase diagram. We confirm that this compound is an antiferromagnetic Dirac semimetal. Despite having a metallic Fermi surface, the electronic transport properties are semiconductor-like and deviate from known theoretical models. An external magnetic field induces a semiconductor metal–like transition, which results in a colossal negative magnetoresistance. Moreover, signatures of the coupling between the charge density wave and a spin modulation are observed in resistivity. This spin modulation also produces a giant anomalous Hall response.

    

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2. Magnetic memory driven by topological insulators

   https://doi.org/10.1038/s41467-021-26478-3  

   Wu, Hao ; Chen, Aitian ; Zhang, Peng ; He, Haoran ; Nance, John ; Guo, Chenyang ; Sasaki, Julian ; Shirokura, Takanori ; Hai, Pham Nam ; Fang, Bin ; et al ( October 2021 , Nature Communications)

   Giant spin-orbit torque (SOT) from topological insulators (TIs) provides an energy efficient writing method for magnetic memory, which, however, is still premature for practical applications due to the challenge of the integration with magnetic tunnel junctions (MTJs). Here, we demonstrate a functional TI-MTJ device that could become the core element of the future energy-efficient spintronic devices, such as SOT-based magnetic random-access memory (SOT-MRAM). The state-of-the-art tunneling magnetoresistance (TMR) ratio of 102% and the ultralow switching current density of 1.2 × 10⁵ A cm⁻²have been simultaneously achieved in the TI-MTJ device at room temperature, laying down the foundation for TI-driven SOT-MRAM. The charge-spin conversion efficiencyθ_SHin TIs is quantified by both the SOT-induced shift of the magnetic switching field (θ_SH = 1.59) and the SOT-induced ferromagnetic resonance (ST-FMR) (θ_SH = 1.02), which is one order of magnitude larger than that in conventional heavy metals. These results inspire a revolution of SOT-MRAM from classical to quantum materials, with great potential to further reduce the energy consumption.

    

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3. Spin–orbit torque nano-oscillator with giant magnetoresistance readout

   https://doi.org/10.1038/s42005-020-00454-7  

   Chen, Jen-Ru ; Smith, Andrew ; Montoya, Eric A. ; Lu, Jia G. ; Krivorotov, Ilya N. ( October 2020 , Communications Physics)

   Spin-orbit torque nano-oscillators based on bilayers of ferromagnetic and nonmagnetic metals are ultra-compact current-controlled microwave signal sources. They are attractive for practical applications such as microwave assisted magnetic recording, neuromorphic computing, and chip-to-chip wireless communications. However, a major drawback of these devices is low output microwave power arising from the relatively small anisotropic magnetoresistance of the ferromagnetic layer. Here we experimentally show that the output power of a spin-orbit torque nano-oscillator can be significantly enhanced without compromising its structural simplicity. Addition of a ferromagnetic reference layer to the oscillator allows us to employ current-in-plane giant magnetoresistance to boost the output power of the device. This enhancement of the output power is a result of both large magnitude of giant magnetoresistance compared to that of anisotropic magnetoresistance and their different angular dependencies. Our results hold promise for practical applications of spin-orbit torque nano-oscillators.

    

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4. Two-fold symmetric superconductivity in few-layer NbSe2

   https://doi.org/10.1038/s41567-021-01219-x  

   Hamill, Alex ; Heischmidt, Brett ; Sohn, Egon ; Shaffer, Daniel ; Tsai, Kan-Ting ; Zhang, Xi ; Xi, Xiaoxiang ; Suslov, Alexey ; Berger, Helmuth ; Forró, László ; et al ( April 2021 , Nature Physics)

   null (Ed.)

   The strong Ising spin–orbit coupling in certain two-dimensional transition metal dichalcogenides can profoundly affect the superconducting state in few-layer samples. For example, in NbSe2, this effect combines with the reduced dimensionality to stabilize the superconducting state against magnetic fields up to ~35 T, and could lead to topological superconductivity. Here we report a two-fold rotational symmetry of the superconducting state in few-layer NbSe2 under in-plane external magnetic fields, in contrast to the three-fold symmetry of the lattice. Both the magnetoresistance and critical field exhibit this two-fold symmetry, and it also manifests deep inside the superconducting state in NbSe2/CrBr3 superconductor-magnet tunnel junctions. In both cases, the anisotropy vanishes in the normal state, demonstrating that it is an intrinsic property of the superconducting phase. We attribute the behaviour to the mixing between two closely competing pairing instabilities, namely the conventional s-wave instability typical of bulk NbSe2 and an unconventional d- or p-wave channel that emerges in few-layer NbSe2. Our results demonstrate the unconventional character of the pairing interaction in few-layer transition metal dichalcogenides and highlight the exotic superconductivity in this family of two-dimensional materials. 

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5. Reconfigurable Physical Reservoir in GaN/α-In2Se3 HEMTs Enabled by Out-of-Plane Local Polarization of Ferroelectric 2D Layer

   https://doi.org/10.1021/acsnano.3c00187  

   Yang, Jeong Yong ; Park, Minseong ; Yeom, Min Jae ; Baek, Yongmin ; Yoon, Seok Chan ; Jeong, Yeong Je ; Oh, Seung Yoon ; Lee, Kyusang ; Yoo, Geonwook ( April 2023 , ACS Nano)

   Significant effort for demonstrating a gallium nitride (GaN)-based ferroelectric metal–oxide–semiconductor (MOS)-high-electron-mobility transistor (HEMT) for reconfigurable operation via simple pulse operation has been hindered by the lack of suitable materials, gate structures, and intrinsic depolarization effects. In this study, we have demonstrated artificial synapses using a GaN-based MOS-HEMT integrated with an α-In2Se3 ferroelectric semiconductor. The van der Waals heterostructure of GaN/α-In2Se3 provides a potential to achieve high-frequency operation driven by a ferroelectrically coupled two-dimensional electron gas (2DEG). Moreover, the semiconducting α-In2Se3 features a steep subthreshold slope with a high ON/OFF ratio (∼1010). The self-aligned α-In2Se3 layer with the gate electrode suppresses the in-plane polarization while promoting the out-of-plane (OOP) polarization of α-In2Se3, resulting in a steep subthreshold slope (10 mV/dec) and creating a large hysteresis (2 V). Furthermore, based on the short-term plasticity (STP) characteristics of the fabricated ferroelectric HEMT, we demonstrated reservoir computing (RC) for image classification. We believe that the ferroelectric GaN/α-In2Se3 HEMT can provide a viable pathway toward ultrafast neuromorphic computing. 

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