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封面文章 Switching plasticity in compensated ferrimagnetic multilayers for neuromorphic computing Weihao Li(李伟浩), Xiukai Lan(兰修凯), Xionghua Liu(刘雄华), Enze Zhang(张恩泽), Yongcheng Deng(邓永城), and Kaiyou Wang(王开友) Chin. Phys. B, 2022, 31 (11): 117106 文章亮点介绍 神经形态计算机不仅具有低功耗的优点,而且能大大提高执行复杂任务(包括识别、控制和决策)的效率,因此备受关注。自旋器件凭借着其超快的动力学特性和良好的耐久度,在神经形态计算领域展现出很大的竞争潜力。其中,补偿的亚铁磁综合了铁磁和反铁磁的优势,即丰富的磁化操作(写/读)、超快和节能的自旋轨道力矩翻转,这些优势使亚铁磁有希望成为神经形态计算硬件的候选者之一。然而,关于亚铁磁神经形态计算功能的研究很少,开展相关研究有利于推动基于自旋电子学的神经形态计算。 本工作通过磁控溅射生长具有垂直各向异性的Co/Gd亚铁磁多层膜,研究了不同面内和垂直各向异性场对Co/Gd亚铁磁翻转可塑性的影响。证明了当磁畴成核在Co/Gd磁化翻转占主导时,二态翻转特性能够被谐调成多态翻转。这种优异的翻转可塑性使得Co/Gd亚铁磁有望应用于神经形态计算。本工作的创新性在于:一方面,利用Co/Gd亚铁磁的多态磁化翻转实现了生物突触特有的脉冲时间依赖可塑性;另一方面,将Co/Gd亚铁磁用作神经元构建了一个三层人工神经网络,该网络能够很好地识别MNIST的手写体数字,准确率超过97%。因此,本文不仅为设计多态自旋电子器件提供了一种思路,而且演示了自旋器件用于高性能神经形态计算的可能。 原文链接 PDF Fig. 1. (a) The illustration of CoGd-based artificial synapse. Spikes (blue) propagate between the neurons via synapses. (b) Two engineered spikes with different shapes named as spike 1 and spike 2, arriving from the pre-neuron and post-neuron. (c) Single-pulse Rhall–I SOT switching loop displays clear current threshold (Ith) for Ta/Pt/[Co/Gd]6/Pt/AlOx(2) under Hx = −500 Oe. (d) Measurement of STDP generated with spike 1 and spike 2. The device was demagnetized before each spike applied. 亮点文章 Yueshui Zhang(张越水) and Lei Wang(王磊) Chin. Phys. B, 2022, 31 (11): 110205 文章亮点介绍 连续矩阵乘积算符(cMPO)是新近提出的,用以计算低维强关联格点模型有限温热力学性质的张量网络结构。与过去的张量方法比较,cMPO完全消去了温度方向的离散误差。以连续矩阵乘积态(cMPS)近似cMPO的最大本征态,cMPO方法可以计算局域的有限温热力学量、虚时关联函数和谱函数。 本文研究了一维横场伊辛模型的cMPO的结构。解析上,作者证明了该cMPO具有演化算符的形式,对应的哈密顿量是生活在长度为β 环上的量子场论。在横场伊辛临界点,该哈密顿量具有伊辛共形场论的结构。本文还分析了cMPO基态的关联函数和纠缠熵,为cMPO方法的合理性提供了一定的理论依据。受解析结果启发,数值上,作者给出了计算cMPO激发态的方法,并和横场伊辛的解析结果验证一致。该方法可以计算一维格点模型的临界指数。本文的结果为理解连续矩阵乘积算符这一全新的张量网络结构提供了一个具体的视角和切入点,同时也为一维格点模型临界指数的计算提供了一种新方案。 原文链接 PDF Fig. 3. Structure of conformal tower for TFIM’s cMPO at β=28. The variational cMPS has bond dimension bondD=24. 亮点文章 Junqin Cao(曹君勤), Zhixin Chen(陈志歆), Yaxin Wang(王亚新), Tianfeng Feng(冯田峰), Zhihao Li(李志浩), Zeyu Xing(邢泽宇), Huashan Li(李华山), and Xiaoqi Zhou(周晓祺) Chin. Phys. B, 2022, 31 (11): 114204 文章亮点介绍 近年来,随着光量子芯片尺寸的增加,对芯片上数量众多的移相器进行校准变得越来越困难,尤其是对多级级联干涉仪中那些难以解耦的移相器的校准成为一个亟待解决的问题。传统校准级联干涉仪的方法比较低效,所需的采样点数量随着移相器数量的增加而指数增长,十分耗时且难以扩展。为了解决这个问题,2021年中山大学周晓祺课题组在理论上提出了一种高效的校准方法,所需的采样点数量只随级联移相器的数量线性增长,相比之前的方法具有指数加速的优势。 本文首次报告了这种高效校准方法在光量子芯片上的实验验证。具体来说,本文实验制备了一个九移相器级联的硅基光量子芯片,并使用这种新方法对全部九个移相器进行了高效率高精度的校准,平均保真度达到99.26%。与传统的校准方法相比,新方法所需的采样点数量只随级联移相器的数量线性增加,大大节省了所耗费的资源,该方法有望用于将来的大规模集成光量子芯片的实验应用。 原文链接 PDF Fig. 1. Schematic of the experimental setup and the measured results for the calibration of the phase shifters. (a) A silicon quantum photonic chip cascaded with nine phase shifters. (b)-(j) The calibration experimental data of the nine phase shifters in reverse order. The calibration of the second to the ninth phase shifters is done by fitting the range of the transmittance ΔT with the square of the current applied on the corresponding phase shifter. The calibration of the first phase shifter is done by fitting the transmittance T with the square of the current applied on the first phase shifter. 亮点文章 Yinlu Gao(高寅露), Kai Cheng(程开), Xue Jiang(蒋雪), and Jijun Zhao(赵纪军) Chin. Phys. B, 2022, 31 (11): 117304 文章亮点介绍 GaN基异质结广泛应用于光电器件,但复杂的表面重构和晶格失配极大地限制了其应用。二维材料具有单原子层厚度、表面自钝化等优点,很好地突破了传统三维半导体的一些限制。过渡金属硫化物(TMD)材料做为典型的二维材料已经被广泛制备和研究。在重构GaN表面上堆叠TMD单层(TMD = MoS2, MoSSe, MoSe2)不仅能有效地克服较大的失配,而且可带来新的电子、光学特性。 本文报道了通过GaN和二维TMD的异质结界面工程,实现对GaN基光电器件应用范围的有效调控。理论计算表明:形成异质结后,界面处电荷重新分布, 引起电子波函数极化,形成界面偶极,对异质结的带阶产生重要影响。由于更激烈的电荷重排,TMD单层和吸附Ga原子钝化GaN表面之间的价带阶值比吸附N原子钝化的表面更大。对特定的异质结,如MoSe2/Ga-ter GaN和MoSSe/N-ter GaN,对红外光有着很好的吸收,可以将其应用范围从原本的短波紫外区域扩展到红外区域。本文的研究揭示了复杂的表面重构对异质结性能的影响,为TMD/重构GaN异质结作为高性能宽波段光电器件提供了可能。 原文链接 PDF Fig. 5. Calculated optical absorption spectra for TMD/reconstructed GaN heterostructures (up) and the intrinsic GaN and TMD monolayers. The shaded area indicates the visible light range. The insert is the atom structures proposed for the MoS2/Ga-ter GaN heterostructure. 亮点文章 Hao Wang(王浩), Liang Qiao(乔亮), Zu-Ying Zheng(郑祖应), Hong-Bo Hao(郝宏波), Tao Wang(王涛), Zheng Yang(杨正), and Fa-Shen Li(李发伸) Chin. Phys. B, 2022, 31 (11): 114206 文章亮点介绍 随着对于高频微波吸收机制及稀土合金磁特性研究的不断深入,稀土合金复合材料由于其优异的阻抗匹配特性受到越来越多科研人员的关注。不同于以往简单地从材料电磁参数出发表征微波吸收性能的研究思路,本文立足于阻抗匹配及界面反射模型,详细阐述微波吸收及性能调控机理。 本文具有几大创新点。其一,创新性地利用还原扩散法制备稀土合金,充分利用丰富的稀土氧化物资源,相对于熔炼法达到了降本增效的效果;其二,出于阻抗匹配及厚度匹配对于性能的影响,将阻抗匹配、匹配厚度及反射损耗峰之间的数据联系以图的形式进行直观展示;其三,提出了基于实际需求的微波吸收性能评估方法。基于“厚度薄”、“质量轻”、“有效吸收带宽宽”及“反射损耗峰强”的传统要求,从厚度设计要求及实际频段微波吸收需求,对样品性能进行评估;其四,基于反射损耗峰产生原理,从微波幅值出发,对有效吸收带宽公式进行了推导,计算结果与实验结果符合良好。此公式对于有效吸收带宽的预估及调控具有重要参考意义。 原文链接 PDF Fig. 10. (a) The RL–f, (b) tm–f and |dtm/d f |–f curves. (c) Γ, Π, and Γ + Π vary with frequency. (d) The vector pole figure of Γ and Π at 5.8726 GHz for YFN@Si/PU (25 vol%) absorber with thickness of 2.50 mm. 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