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近年来, 拓扑量子材料因其独特的能带结构及非平庸拓扑性质, 成为凝聚态物理研究的重要前沿方向. 其中, 拓扑半金属由于在费米能级附近存在狄拉克或外尔节点[1], 并伴随高迁移率载流子及拓扑保护表面态, 在量子输运及磁响应等方面展现出丰富的物理现象[2–6]. 与超导体形成异质结后, 拓扑半金属中的电子态可通过超导邻近效应参与相干输运, 从而为研究约瑟夫森效应及其相关量子干涉行为提供了重要平台[7–10].
NiTe2作为一种典型的第二类狄拉克半金属, 属于CdI2型1T层状结构, 空间群为
$ P\bar{3}m1 $ , 由Te-Ni-Te层沿$ c $ 轴堆叠形成, 其能带结构具有明显倾斜的狄拉克锥, 并伴随较强的自旋-轨道耦合效应[11–15]. 对于NiTe2本征超导性的研究, 理论计算表明NiTe2体态因自旋涨落抑制而不具备超导性, 其临界温度$ {T}_{\text{c}} $ 低于0.1 K, 但其范霍夫奇点(Van Hove singularity, VHs)靠近费米能级$ {E}_{\text{F}} $ , 磁场诱导的利夫希兹相变(Lifshitz phase transition)可导致约瑟夫森二极管效应(Josephson diode effect, JDE)符号反转[16]; 实验发现NiTe2体态在低至30 mK的温度下仍保持有限电阻而不具备超导性, 但其与Au界面处可因应变诱导平带形成界面超导, 临界温度$ {T}_{\text{c}} $ 为150—200 mK[17]; 但另外的实验发现, 化学计量比NiTe2单晶在约260 mK以下具有体态超导性, 且拓扑表面态在费米能级处与超导共存[18]. 其本征超导电性仍需进一步的研究加以澄清. 在NiTe2超导邻近效应研究方面, 实验发现NiTe2拓扑表面态在面内磁场下动量偏移导致约瑟夫森二极管效应[17,19], 以及观测到体态超流被面内磁场抑制过滤后仍然稳定存在的棱态超流[20]. 尽管如此, 针对基于NiTe2的约瑟夫森结的系统实验研究仍相对有限, 尤其是在同一器件中对直流约瑟夫森效应、宏观量子衍射现象及交流约瑟夫森效应等进行综合分析的工作仍有待进一步开展.基于上述背景, 本文构建了基于NiTe2纳米片的Al/NiTe2/Al约瑟夫森结, 并在毫开尔文温区系统研究其超导输运行为. 实验结果表明, 在该体系中不仅观测到稳定的约瑟夫森超流及典型的类夫琅禾费衍射图样, 还观测到一定的约瑟夫森二极管效应和在微波辐照下呈现出的清晰夏皮罗台阶结构. 本工作为进一步探索拓扑材料中的约瑟夫森效应及其在超导介观器件中的应用提供了实验基础.
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通过机械剥离法从NiTe2单晶上得到高质量、表面干净的薄层纳米片. 首先, 使用蓝膜胶带反复解理NiTe2单晶, 获得薄层样品, 随后将胶带贴合至聚二甲基硅氧烷(polydimethylsiloxane, PDMS), 通过光学显微镜选择厚度适中、表面平整且无裂纹的薄片. 然后, 利用干法转移技术将PDMS上的NiTe2薄层纳米片转移至预清洗的SiO2(300 nm)/Si++衬底上, 衬底在转移前经过丙酮(acetone)、异丙醇(isopropanol, IPA)和去离子水超声清洗, 并用氮气枪吹干.
结合电子束曝光(electron beam lithography, EBL)与电子束蒸发沉积制备电极. 首先, 在转移有NiTe2薄层纳米片的衬底上旋涂聚甲基丙烯酸甲酯(polymethyl methacrylate, PMMA)光刻胶, 前烘温度为120 ℃, 时间为3 min. 然后, 利用电子束曝光系统(加速电压20 kV)定义电极, 并用甲基异丁基酮(methyl isobutyl ketone, MIBK)与异丙醇的混合液(MIBK∶IPA = 1∶3)显影1 min. 接着, 利用电子束蒸发沉积系统沉积超导金属. 在正式沉积前, 对样品进行原位Ar离子束刻蚀, 刻蚀电压为400 V, 刻蚀时间为100 s, 去除表面氧化层及显影后可能残留的有机污染物, 以提高界面质量. 刻蚀完成后, 沉积超导Al, 沉积厚度为80 nm, 沉积速率为1 Å/s. 最后, 在丙酮中进行剥离(lift-off)后得到可测量器件.
使用扫描电子显微镜(scanning electron microscope, SEM)对器件进行形貌表征与结构尺寸测量. 约瑟夫森结器件如图1(a)所示, 结区宽度W = 2.7 μm, 结区长度L = 300 nm, 电极宽度d = 600 nm. 初步电学表征在室温下利用探针台(probe station)进行. 通过探针与器件电极接触, 采用伪四端法测量器件电阻, 初步判断电极接触质量与NiTe2薄层纳米片的导电性.
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器件制备与初步表征完成后, 使用引线键合仪将器件电极与样品座建立电学连接, 确保低温下电接触稳定可靠, 为后续低温电输运测量做好准备. 测量在稀释制冷机中进行, 其基础温度为47 mK, 系统配备有可施加最大14 T垂直方向磁场的超导磁体. 为抑制外部电磁干扰, 所有测量线缆均经过低温滤波器处理.
低温电输运测量采用伪四端法, 利用源表(Keithley 2612B)施加直流偏置电压
$ {V}_{\text{dc}} $ , 同时利用锁相放大器(LI5650, NF Corporation)提供交流激励电压$ {V}_{\text{ac}} $ 和测量器件的交流电压信号$ V_{\text{device}} $ , 以此得到器件的微分电阻$ \text{d}V/\text{d}I=V_{\text{device}}/I_{\text{ac}} $ , 测量配置如图1(b)所示. 其中在源表输出端串联一个1 MΩ电阻将输出的直流电压$ {V}_{\text{dc}} $ 转换为直流电流$ {I}_{\text{dc}} $ , 同样在锁相放大器输出端串联一个1 MΩ电阻将输出的交流电压$ {V}_{\text{ac}} $ 转换为交流电流$ {I}_{\text{ac}} $ , 并在回路中串联一个1 kΩ的取样电阻$ {R}_{\text{s}} $ , 通过测量其交流对地电压$ {V}_{\text{s}} $ 而得到准确的$ {I}_{\text{ac}} $ . 交流约瑟夫森效应测量中, 使用微波源(MG3692C)施加微波辐照. 所有测量数据通过编程控制仪器自动采集, 并采用Python脚本进行后续数据处理与分析. -
首先, 测量器件微分电阻
$ \text{d}V/\text{d}I $ 随温度$ T $ 降低的变化曲线. 图2(a)展示器件微分电阻$ \text{d}V/\text{d}I $ 的温度依赖性, 显示了两个转变. 第1个转变发生在$ {T}_{\text{SC}}=1.21\text{ K} $ , 此时电极开始进入超导态. 第2个转变发生在更低的温度$ {T}_{\text{JJ}}=1.15\text{ K} $ , 此时整个约瑟夫森结开始进入超导态, 使得超导电流流经NiTe2层. 当温度进一步降低至基础温度$ {T}_{\min }=47\text{ mK} $ 时, 器件完全进入超导态, 证明Al/NiTe2/Al结区形成了良好的超导邻近耦合[21]. 然后, 在最低温$ {T}_{\min }=47\text{ mK} $ 下, 测量器件微分电阻$ \text{d}V/\text{d}I $ 随直流偏置电流$ {I}_{\text{dc}} $ 的变化曲线. 图2(b)表明在正向和负向偏置电流扫描中, 临界超流分别为$ {I}_{\text{c}^+}=290.3\text{ μA} $ 和$ {I}_{\text{c}^-}=-316.0\text{ μA} $ , 展示出约瑟夫森二极管效应. 但由于剩余磁场(0.65 mT)的存在, 此时并非处于真正的零磁场, 在后续加垂直磁场的测量中会进行修正. 通过对图2(b)曲线进行数值积分, 可以获得器件的电流-电压(I -V )特性曲线. 图2(c)中的I -V特性曲线呈现零电压平台, 表明存在无耗散的约瑟夫森超流, 即存在直流约瑟夫森效应[22]. 当直流偏置电流$ {I}_{\text{dc}} $ 超过临界超流$ {I}_{\text{c}} $ 后, 器件两端出现有限电压, 且电压随电流增大而单调上升, 符合电阻电容分路结(RCSJ)模型的预期行为[23]. -
为了研究约瑟夫森超流的宏观相干性, 在最低温
$ {T}_{\min }=47\text{ mK} $ 下测量器件微分电阻$ \text{d}V/\text{d}I $ 关于垂直磁场$ {B}_{z} $ 和直流偏置电流$ {I}_{\text{dc}} $ 的二维图, 如图3(a), (b)所示. 图3对剩余磁场进行修正, 平移了0.65 mT. 从图3(a), (b)中可以提取并拟合约瑟夫森临界超流$ {I}_{\text{c}} $ 随$ {B}_{z} $ 的演化, 如图3(c)上图和图3(d)所示. 图3(c)上图和图3(d)显示$ {I}_{\text{c}} $ 受垂直磁场$ {B}_{z} $ 调制而出现类夫琅禾费衍射图样[24], 这说明了器件超流具有良好的相干性. 夫琅禾费衍射函数拟合与实验数据有一定的偏差, 这可能是由于超流沿结宽方向分布不均匀导致的. 图3(c)下图是约瑟夫森二极管效率$ \eta = \dfrac{{I}_{\text{c}^+} - | {I}_{\text{c}^-}|}{{I}_{\text{c}^+} + | {I}_{\text{c}^-}|}$ 关于$ {B}_{z} $ 的演化图, 虽然数据涨落较大, 但可在零磁场附近临界超流中心包络处(±1.3 mT以内)看出存在一定的约瑟夫森二极管效应(紫色数据点). 在中心振荡包络以外, 受到垂直磁场Bz的调制, 正向临界超流${I}_{\text{c}^+} $ 与负向临界超流绝对值$|{I}_{\text{c}^-} |$ 均较小, 且在振荡包络节点处接近零. 由$ \eta $ 的定义可知, 此时分母$ \left({I}_{\text{c}^+}+\left| {I}_{\text{c}^-}\right| \right) $ 相对较小, 分子$ \left({I}_{\text{c}^+}-\left| {I}_{\text{c}^-}\right| \right) $ 细微的变化可能会引起产生较大的涨落. 因此, 在中心振荡包络以外, 测量和数据提取导致的${I}_{\text{c}^+} $ 和$|{I}_{\text{c}^-}| $ 涨落, 会使得$ \eta $ 有较大涨落. 从图3(c)可获得$ {I}_{\text{c}} $ 随$ {B}_{z} $ 演化的周期$ \Delta{B}_{z}=1.30\text{ mT} $ . 理论上如果超流沿结均匀分布, 则其与磁通$ {{\varPhi }}_{\text{J}} $ 的关系为$ {I}_{\text{c}}\left({{\varPhi }}_{\text{J}}\right)= {I}_{\text{c}}\left(0\right)\times \left| \sin \left(\text{π}\dfrac{{{\varPhi }}_{\text{J}}}{{{\varPhi }}_{0}}\right) \Big/ \left(\text{π}\dfrac{{{\varPhi }}_{\text{J}}}{{{\varPhi }}_{0}}\right)\right| $ , 其中磁通量子$ {{\varPhi }}_{0} = {h}/({2e})\;\approx\; 2.06\times{10}^{-15}\text{ Wb} $ ,$ {{\varPhi }}_{\text{J}}\;=\;{B}_{z}{S}_{\text{eff}} $ ,$ {S}_{\text{eff}}\;= \left(L+2{\lambda }_{\text{L}}\right)W $ 为结的有效面积,$ L $ 为结长,$ W $ 为结宽,$ {\lambda }_{\text{L}} $ 为超导电极的伦敦穿透深度[25]. 实际器件对应的有效面积$ {S}_{\text{eff}}=1.4{\text{ μm}}^{2} $ , 磁场周期$ \Delta B_{z}'= 1.47\text{ mT} $ , 与实验测得磁场周期数值基本一致. 整体干涉行为清晰表明该器件中的约瑟夫森超流具备良好的宏观量子相干性. -
在验证直流约瑟夫森效应的基础上, 进一步研究了器件在最低温
$ T_{\min}=47\text{ mK} $ 时微波辐照下的交流约瑟夫森效应. 测量在不同频率$ f $ 微波辐照下器件微分电阻$ \text{d}V/\text{d}I $ 关于微波功率$ P $ 和直流偏置电流$ {I}_{\text{dc}} $ 的二维图, 如图4(a), (b)所示, 其对应的频率$ f $ 分别为0.62 GHz和1.95 GHz. 在固定微波频率$ f $ 的条件下, 当微波功率$ P $ 较低时, 以零电压超流平台为主, 而随着微波功率$ P $ 逐渐增大, 出现了若干个电压平台, 即夏皮罗台阶[26].对图4(a), (b)进行积分处理, 得到对应频率下不同微波功率
$ P $ 的器件电压$ V $ 关于直流偏置电流$ {I}_{\text{dc}} $ 的单线图, 如图4(c), (d)所示, 在电压$ V_n= nhf/(2\text{e})(n=0,\pm1,\pm2,\cdots) $ 附近观察到了清晰的夏皮罗台阶. 改变微波频率$ f $ , 所观测到的台阶电压位置也相应呈线性比例变化, 进一步确认了交流约瑟夫森效应的存在[27]. 该实验结果表明, Al/NiTe2/Al约瑟夫森结能够与外部的微波场实现耦合, 展现出良好的相干性与频率响应. -
本文围绕第二类狄拉克半金属NiTe2, 构建了Al/NiTe2/Al约瑟夫森结器件, 并在毫开尔文温区系统研究了其超导输运性质. 实验结果表明, 在超导邻近效应作用下, NiTe2中能够形成稳定的约瑟夫森超流, 其I -V特性呈现典型的零电压平台, 体现了直流约瑟夫森效应[22,23]. 在外加垂直磁场作用下, 器件临界超流呈现出规则的类夫琅禾费衍射图样, 表明超流在结区具有良好的宏观相干性[24]. 通过对磁场周期的定量分析, 其结果与器件几何尺寸及超导电极的伦敦穿透深度基本一致[25]. 进一步, 实验发现了一定的约瑟夫森二极管效应, 表现为正负向临界超流随垂直磁场演化不对称, 其成因仍需进一步深入研究. 在微波辐照条件下, 实验观测到清晰的夏皮罗台阶, 验证了交流约瑟夫森效应[27].
本工作观测到的约瑟夫森超流、宏观量子干涉、约瑟夫森二极管效应及微波响应, 可在约瑟夫森理论框架下统一理解[28]. NiTe2不仅是高质量邻近诱导超流的理想平台, 也为研究拓扑表面态参与的相干输运及二极管效应提供了途径. 在强自旋轨道耦合与拓扑能带背景下, 拓扑表面态可能对超流输运产生重要贡献[29]. 本研究为基于拓扑半金属的约瑟夫森器件提供了系统实验依据, 并为探索拓扑超导及马约拉纳束缚态等新奇量子态提供参考. 未来可通过界面工程、多端结构及外场调控进一步优化器件性能, 揭示拓扑态在超导输运中的具体作用机制.
基于NiTe2的约瑟夫森结低温输运研究
Low-temperature transport study of NiTe2-based Josephson junctions
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摘要: 拓扑半金属因其独特的能带结构和非平庸拓扑边界态, 在超导邻近效应作用下可实现非常规超导输运行为, 而构建约瑟夫森结器件是重要的研究手段. 本文基于第二类狄拉克半金属NiTe2, 构建了Al/NiTe2/Al约瑟夫森结, 并在毫开尔文温区系统研究其超导输运特性. 实验中观测到显著的约瑟夫森超流, 证实其直流约瑟夫森效应. 在外加垂直磁场作用下, 临界超流呈现类夫琅禾费宏观量子衍射图样, 说明约瑟夫森超流具有良好的相干性. 此外, 发现正负向临界超流随磁场演化呈现差异, 表明器件中存在一定的约瑟夫森二极管效应. 进一步, 在微波辐照条件下, 观测到了清晰的夏皮罗台阶, 验证了其交流约瑟夫森效应. 上述结果表明, Al/NiTe2/Al约瑟夫森结是研究拓扑材料超导输运行为的理想候选平台.Abstract:
Topological semimetals are a class of quantum materials characterized by band crossings near the Fermi level, which give rise to massless quasiparticles exhibiting linear energy-momentum dispersion. Based on the degeneracy and dimensionality of these crossings, they are classified into Dirac semimetals (fourfold degenerate points), Weyl semimetals (twofold degenerate points with chirality), and nodal-line semimetals (closed loops). Among them, type-II Dirac semimetals, such as NiTe2, feature strongly tilted Dirac cones that break Lorentz invariance, leading to anisotropic transport properties and exotic surface states. These unique electronic properties make topological semimetals promising candidates for exploring exotic superconducting phenomena when coupled with a superconductor. When a topological semimetal is placed in intimate contact with a superconductor, the superconducting proximity effect comes into play. This effect occurs when a non-superconducting material acquires superconducting properties over a characteristic length scale. Specifically, Cooper pairs from the superconductor diffuse into the adjacent semimetal, inducing a finite superconducting order parameter and enabling supercurrent to flow through the otherwise non-superconducting region. Leveraging this proximity effect, we fabricate Al/NiTe2/Al Josephson junctions using the type-II Dirac semimetal NiTe2 and systematically investigate their superconducting transport properties at millikelvin temperatures. The fabrication of high-quality devices relies on advanced micro/nano-fabrication techniques. In particular, electron-beam lithography combined with high-precision alignment is employed to define the junction regions with precisely controllable spacing on mechanically exfoliated NiTe2 flakes. Subsequently, Al electrodes with a thickness of 80 nm are deposited on both sides of the junction regions using electron-beam evaporation, forming SNS Josephson junctions. To ensure clean and high-quality interfaces, an in-situ argon ion milling step is performed prior to Al deposition to remove surface oxides and contaminants, thereby promoting efficient superconducting proximity coupling between the Al electrodes and the NiTe2 flakes. At millikelvin temperatures, a clear Josephson supercurrent is observed, as evidenced by the direct current (DC) Josephson effect. The measured current-voltage characteristics exhibit well-defined zero-resistance supercurrent branches. When an external perpendicular magnetic field is applied, the critical supercurrent evolves into a Fraunhofer-like diffraction pattern, indicating excellent spatial coherence of the supercurrent across the junction. Notably, the positive and negative branches of the critical supercurrent display asymmetric evolution with the magnetic field, suggesting the emergence of a Josephson diode effect. The origin of this diode effect requires further investigation and may be related to the topological surface states of NiTe2. Under microwave irradiation, well-defined Shapiro steps are resolved in the current-voltage characteristics. The voltage steps occur at $ V_{\mathrm{n}}=nhf/(2e)$, confirming the alternating current (AC) Josephson effect and demonstrating robust phase coherence under non-equilibrium conditions. The persistence of these Shapiro steps over a range of microwave powers further attests to the high quality of the Al/NiTe2 interfaces. Collectively, these results establish the Al/NiTe2/Al Josephson junction as an ideal platform for exploring superconducting transport phenomena in topological materials. Specifically, this device provides a promising avenue for verifying the existence of supercurrents carried by topological surface states and higher-order hinge states. Through carefully designed junction geometries and phase-sensitive transport measurements, it may become possible to distinguish bulk contributions from boundary contributions to the Josephson supercurrent. Furthermore, this platform enables detailed investigations into whether the current-phase relationship deviates from the conventional sinusoidal form expected for ordinary SNS junctions, as any such deviation could arise from the nontrivial band topology. Thus, beyond the observations reported in this work, the Al/NiTe2/Al Josephson junction may serve as a versatile testbed for addressing key unresolved questions regarding topological superconductivity, including the role of boundary states and the nature of the current-phase relation, potentially offering implications for future applications in topological quantum computing and superconducting electronics. -
Key words:
- topological semimetal /
- Josephson junction /
- Josephson effect /
- Josephson diode .
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图 1 器件与测量配置 (a) Al/NiTe2/Al约瑟夫森结伪彩SEM图; (b)电输运测量配置示意图
Figure 1. Device and measurement configuration: (a) False-color scanning electron microscopic image of the Al/NiTe2/Al Josephson junction; (b) schematic of the electrical transport measurement circuit.
图 2 器件降温曲线与I-V特性曲线 (a)器件微分电阻
$ \text{d}V/\text{d}I $ -温度$ T $ 曲线; (b)器件微分电阻$ \text{d}V/\text{d}I $ -直流偏置电流$ {I}_{\text{dc}} $ 曲线; (c)器件I -V特性曲线Figure 2. Cooling curve and the I-V characteristic curve of the device: (a) The differential resistance
$ \text{d}V/\text{d}I $ versus temperature$ T $ curve of the device; (b) the differential resistance$ \text{d}V/\text{d}I $ versus direct bias current$ {I}_{\text{dc}} $ curve of the device; (c) the I -V characteristic curve of the device obtained after integration.
图 3 器件宏观量子衍射图案 (a)较小磁场范围内器件微分电阻
$ \text{d}V/\text{d}I $ 关于垂直磁场$ {B}_{z} $ 和直流偏置电流$ {I}_{\text{dc}} $ 的二维图; (b)较大磁场范围内$ \text{d}V/\text{d}I $ 关于$ {B}_{z} $ 和$ {I}_{\text{dc}} $ 的二维图; (c)上图为从图(a)中提取出的约瑟夫森临界超流$ {I}_{\text{c}} $ 随$ {B}_{z} $ 的演化图, 并用夫琅禾费衍射函数拟合, 下图为约瑟夫森二极管效率$ \eta $ 关于$ {B}_{z} $ 的演化图; (d)从图(b)中提取出的$ {I}_{\text{c}} $ 随$ {B}_{z} $ 的演化图, 并用夫琅禾费衍射函数拟合Figure 3. Macroscopic quantum diffraction patterns of the device: (a) The differential resistance
$ \text{d}V/\text{d}I $ versus the perpendicular magnetic field$ {B}_{z} $ and the bias current$ {I}_{\text{dc}} $ in a small magnetic field range; (b) the$ \text{d}V/\text{d}I $ versus$ {B}_{z} $ and$ {I}_{\text{dc}} $ in a large magnetic field range; (c) the upper panel shows the Josephson critical supercurrent$ {I}_{\text{c}} $ as a function of$ {B}_{z} $ extracted from panel (a) and fitted with a Fraunhofer diffraction function; the lower panel displays the Josephson diode efficiency$ \eta $ as a function of$ {B}_{z} $ ; (d) the$ {I}_{\text{c}} $ extracted from panel (b) as a function of$ {B}_{z} $ , fitted with a Fraunhofer diffraction function.
图 4 温度为
$ T_{\min}=47\text{ mK} $ 时微波辐照下器件的夏皮罗台阶图案 (a), (b)当微波频率$ f $ 分别为0.62 GHz和1.95 GHz时, 器件微分电阻$ \text{d}V/\text{d}I $ 关于微波功率$ P $ 和直流偏置电流$ {I}_{\text{dc}} $ 的二维图; (c), (d)从图(a), (b)积分得到的不同微波功率$ P $ 下器件电压$ V $ 关于偏置电流$ {I}_{\text{dc}} $ 的单线图Figure 4. The Shapiro step patterns of the device under microwave irradiation at a temperature of
$ T_{\min}=47\text{ mK} $ : (a), (b) The differential resistance$ \text{d}V/\text{d}I $ versus the microwave power$ P $ and the bias current$ {I}_{\text{dc}} $ at microwave frequency$ f=\text{0.62 GHz, 1.95 GHz} $ , respectively; (c), (d) the voltage$ V $ versus the$ {I}_{\text{dc}} $ at different microwave powers$ P $ of the device obtained by integrating from panels (a), (b). -
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