1.Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan 430074, China 2.State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, Wuhan 430074, China
Fund Project:Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0401703)
Received Date:19 July 2019
Accepted Date:24 August 2019
Available Online:27 November 2019
Published Online:05 December 2019
Abstract:In recent years, measuring the electrical transport properties of materials in different directions of applied magnetic field has become an important experimental study of topological quantum materials. With the development of condensed matter physics, scientific research has shown that under the ultra-high intensity pulsed magnetic field, the electrical transport study of materials may extend to the quantum limit region, and more abundant physical phenomena will be observed. However, in the existing electric transport measurement system, the rotation sample rod under the action of steady-state field presents a large size and significant eddy current effect, which makes it difficult to meet the requirements for pulsed field measurement, and the current commercial physical property measurement system (PPMS) can only operate under ±16 T steady magnetic field. In addition, the conventional rotation sample rod encounters the problems of insufficient angular resolution and space utilization when used in pulsed high magnetic environment. So there is an urgent need to develop a higher performance rotation measurement system. In view of the above background, in this paper we present a kind of electrical transport measurement system designed by Wuhan National High Magnetic Field Center (WHMFC), which consists of five modules: pulse power supply, pulse magnet, control center, cryogenic system, and signal measurement. The key component is the sample measuring rod with rotation function, which restricts the movement of the drawbar through a double-groove structure to achieve an angular change in a range from –5° to 185°. An angle calibration coil is mounted on the back of the sample stage. Based on the double-calibration method, the angle control accuracy of 0.1° is achieved. The temperature, magnetoresistance and Hall resistance signal are collected by the integrated circuit on sample stage and extracted by compensation circuit and virtual digital lock-in amplifier, and the accuracy of electric transport measurement is better than 0.1 mΩ. Furthermore, the effect of eddycurrent and material deformation at low temperatures are completely eliminated by using polyetheretherketone material, which effectively improves the stability and reliability of the rotation sample rod. Using this measuring rod, we complete a series of experiments in the 8 mm sample cavity in the center of the pulse magnet: the minimum ambient temperature reaches 1.3 K, the maximum magnetic field strength arrives at 65 T, and the direction angle of the magnetic field is able to change in a 190° range. Thus the universally applicable measurement system of electric transport experiment in pulsed high magnetic field is successfully established. In this paper, we elaborate the principle and device components of the measurement system, the design and fabrication of the angle measuring rod, and the calibration principle and measurement process. Relevant experimental results show that the system has important application value in the research of 3D Fermi surface, topological insulator surface state, quantum limit transport, superconductivity analysis, etc. Based on this system, the electrical transport experimental system at WHMFC provides an effective means for the relevant research teams (home and abroad) engaged in the exploration of the intrinsic physical characteristics of quantum materials in extremely pulsed high magnetic field and low temperature environment. Keywords:pulsed high magnetic field/ electrical transport measurement/ level rotation sample rod
表1不同样品杆技术参数比较 Table1.Technical parameters of different kind of measurement rod.
23.5.基于转角样品杆的电输运测量过程 -->
3.5.基于转角样品杆的电输运测量过程
以半金属样品WTe2为例, 介绍本文转角样品杆在脉冲强磁场电输运测量系统中的实际操作步骤. 第一步, 确定旋转角. 将WTe2样品焊接在样品台上, 根据研究目的确定所需角度范围为0°—90°, 选取0°, 35°, 45°, 60°, 75°, 90°为一组实验角度值, 转动旋钮控制样品台旋转量到估值附近, 操作脉冲电源对脉冲磁体放电产生脉冲磁场, 样品台背部角度标定线圈感应出实时转角并反馈至控制中心, 此时角度测量线圈与磁场测量线圈测得的信号如图5(a)所示, 由(1)式和(2)式可以计算得出此时样品的精确转角值为33°. 图 5 60 T脉冲强磁场下转角电输运实验相关数据处理过程 Figure5. Signal processing of angular-dependent electrical transport experiment in 60 T pulsed high magnetic field.
第二步, 测量样品输运信号. 对样品通入高频电流, 通过四线法测量得到样品电压, 测量结果如图5(b)所示, 可以看出, 这一信号信噪比(signal-noise ratio, SNR)极小, 必须进行降噪信号处理. 第三步, 获得测试结果. 基于虚拟数字锁相放大(virtual digital lock-in amplify, VDLIA)的方法来计算并提取出微弱的样品磁电阻信号, 其相关计算原理可参考文献[24], 其结果分辨率可达0.1 mΩ. 处理完样品的B-T与Amp-T信号后, 将其拟合并得到这一角度下样品磁阻随磁场的变化曲线, 如图5(c)所示, 重复这一过程, 直到获得整组角度信息, 如图5(d)所示, 此时已经可以看出各个角度下清晰的SdH量子振荡现象. 实际测量过程中, 高速采集卡得到的所有信号经补偿、放大、数字处理单元后进入上位机总线, 通过光纤返回控制中心, 对脉冲磁体、脉冲电源、样品转角等参数进行反馈调节, 从而构成完整的脉冲强磁场下的转角电输运测量系统. 4.脉冲场转角电输运实验近年来, 通过使用转角样品杆作为工具, 国内外科学工作者开展了一系列转角电输运测量实验, 揭示了各类新型材料在强磁场、极低温环境下的电输运行为, 有力推动了凝聚态物理学探索拓扑材料微观结构的进程, 在以下几个热点方向都取得了重要的研究成果. 1)构造量子材料三维费米面结构. 费米面是动量空间中占据最高能级的等能面, 只有费米面附近的电子决定材料中的各类物理性质, 因此费米面结构的研究对探索材料的电学行为具有重要意义. 电输运测量实验中, 科研工作者通过旋转样品角度至不同的晶轴方向, 在强磁场下进行低温Shubnikov-de Haas效应测量SdH频率的角度依赖性, 根据昂色格(Onsager)关系推出不同角度下的费米面截面积, 从而构造三维费米面信息. 例如北京大学量子材料中心王健课题组[25]在武汉强磁场中心4.2 K, 60 T脉冲强磁场下, 观察到三维狄拉克半金属Cd3As2沿不同晶轴方向上的SdH振荡表现出显著的各向异性, 如图6所示. 通过分析各个角度下的磁阻振荡数据, 揭示出单晶Cd3As2的费米面是由两个嵌套在一起的椭球结构. 类似方法在研究重费米子超导体[26]、外尔半金属[27,28]、拓扑半金属[29]、过渡金属二硫化物[30]等体系中都获得了应用. 图 6 Cd3As2三维嵌套各向异性费米面构造图 (a) 不同晶轴方向的费米面最大横截面; (b) B[112]方向上的量子振荡 Figure6. 3D nested anisotropic Fermi surface construction of Cd3As2: (a) Largest cross section of Fermi surface versus the magnetic field orientation; (b) quantum oscillation for B[112] direction.
2)研究拓扑绝缘体二维表面态特征. 这种无能隙的表面态完全由体电子态的拓扑结构所决定, 在时间反演对称性的保护下, 不会受到杂质和无序的影响, 在未来的自旋电子学和量子计算中有着巨大的应用潜力. 理论上, 三维拓扑绝缘体的体态是绝缘性的, 边界上存在着导电的二维表面态. 而实际中由于样品缺陷等原因, 三维拓扑绝缘体的体态并不完全绝缘, 因而对验证其无能隙的表面态带来了极大的干扰. 斯坦福大学材料与能源科学研究所Analytis等[31]在研究Bi2Se3晶体时将磁场提高到50 T左右, 使样品体态电子坍塌至朗道能级为零的量子极限状态, 此时在磁场中旋转样品, 发现如图7(a)所示的量子振荡现象仅仅依赖于磁场B的垂直分量, 这一奇特的SdH振荡信息为其二维表面态特征提供了明确的证据. 两年后, Tel Aviv University物理与天文学院Petrushevsky[32]也在强磁场环境下进行了类似实验, 结果如图7(b)所示, 表明SdH振荡频率与旋转角度呈明显的cosθ函数关系, 拓扑绝缘体Bi2Se3的二维表面态得到了进一步的证明. 图 7 拓扑绝缘体Bi2Se3二维表面态特征 (a) 量子振荡现象[31]; (b) 振荡频率随转角的变化曲线[32] Figure7. 2D surface state of a topological insulator Bi2Se3: (a) Quantum oscillation[31]; (b) frequency of the oscillations as a function of θ[32].
3)量子极限输运性质研究. 量子极限态指的是磁场到达一定强度时电子高度简并在最低朗道能带, 成为一种强关联体系, 此时样品会表现出许多新奇的物理性质. 武汉强磁场中心王俊峰与合作者利用65 T脉冲磁体, 通过一系列的变温和转角电输运测量, 对新型Weyl半金属TaP开展了强磁场下的转角电输运研究[33]. 如图7(a)所示, 在34.4 T的临界磁场下, TaP的霍尔信号发生明显反转, 由此证实了TaP的一对手性相反的Weyl点在强磁场下发生湮灭. 在量子极限范围下, 具有相反手性的Weyl点将被移动到同一动量坐标上, 电子的磁长度倒数与Weyl点的动量距离相比拟而发生磁隧穿效应时, 有可能实现能隙的打开以及Weyl费米子的湮灭. 该研究在国际上首次观察并证实了Weyl点的湮灭现象, 通过强磁场下的量子极限输运研究深刻揭示了Weyl费米子在强磁场下的非平庸拓扑性质. 文献[34]研究了元素Bi的输运性质, 通过测量SdH振荡周期的角度依赖性, 发现当进入量子极限状态时, 在某些特定角度下霍尔电阻率与磁场无关. 文献[35]则描述了Bi在量子极限状态外的极大不稳定性, 通过–10°—10°的转角测量, 发现在磁场强度40 T时, 超量子体系的输运特性表现出了显著不与角相关的电子不稳定性, 增强了导电率的绝对值和金属温度的依赖性, 见图8(b). 图 8 脉冲强磁场下的半金属材料量子极限输运研究 (a) TaP; (b) Bi Figure8. Quantum limit electrical transport of semi-metal materials in pulsed high magnetic fields: (a) TaP; (b) Bi.
4)探索超导材料的超导电性. 超导是凝聚态物质中电子的一种宏观多体量子态, 同时满足零电阻和迈斯纳效应两个物理性质, 其中发现的种种奇异量子现象是现代基础科学研究的重要源泉, 在能源、医疗、通信等领域都有着广泛的应用前景. 武汉强磁场中心左华坤及其合作者进行了准一维铬基砷化物超导材料K2Cr3As3在强磁场下的转角电输运实验, 得到如图9所示的实验结果[36], 可以看出上方临界场中的各向异性随温度的降低成反比关系, 在θ = 0°和θ = 90°时取最大值(远超泡利顺磁极限), 其正常态表现出具有线性电阻率的非费米液体行为, 并且在低温下显示出独特的三重调制, 表明这类准一维铬基材料可能具有非常规的自旋-三重态超导电性. 图 9 超导材料K2Cr3As3的转角电输运实验结果 (a) π/2处出现上方临界场(Hc2)的最大值表明了泡利极限的缺失; (b) 极坐标图则体现了Hc2的三重调制性 Figure9. Electrical transport of the superconducting material K2Cr3As3: (a) The maximum value of the upper critical field (Hc2) at π/2 indicates the absence the Pauli paramagnetic effect; (b) the polar map of Hc2 shows a unique three fold modulation.