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Science: KTaO3(111)界面的二维超导性和各向异性运输现象 精选

已有 3468 次阅读 2021-2-18 21:44 |个人分类:新观察|系统分类:海外观察

Science: KTaO3(111)界面的二维超导性和各向异性运输现象

诸平

据物理学家组织网(phys.org2021215日提供的消息,美国阿贡国家实验室(Argonne National Laboratory)、伊利诺伊大学香槟分校(University of Illinois at Urbana-Champaign);中国科学院物理研究所和北京凝聚态国家实验室(Beijing National Laboratory for Condensed Matter and Institute of Physics)、北京大学、济南大学的研究者合作,发现钽酸钾(KTaO3)的(111)界面的二维超导性和各向异性输运现象相关研究结果,已经在《科学》(Science)杂志网站发表——Changjiang Liu, Xi Yan, Dafei Jin, Yang Ma, Haw-Wen Hsiao, Yulin Lin, Terence M. Bretz-Sullivan, Xianjing Zhou, John Pearson, Brandon Fisher, J. Samuel Jiang, Wei Han, Jian-Min Zuo, Jianguo Wen, Dillon D. Fong, Jirong Sun, Hua Zhou, Anand Bhattacharya. Two-dimensional superconductivity and anisotropic transport at KTaO3 (111) interfaces. Science, 12 Feb 2021, Vol. 371, Issue 6530, pp. 716-721. DOI: 10.1126/science.aba5511

材料之间的界面通常来说可以展现不属于界面两边任一材料的量子态。特别是对于二维系统,界面上独特的电子结构能够允许非传统量子态的出现。如LaAlO3SrTiO3两个绝缘氧化物之间在200 mK的临界温度上出现的超导界面就是一个典型的例子。而2021212日发表于《科学》杂志网站的中美科学家的合作研究成果,报道了在一种电子气中发现了超导性。这种电子气形成于(111)取向KTaO3KTO)和EuO(或者LaAlO3)之间的界面,超导转变温度可达2.2 K,比LaAlO3/SrTiO3系统高出整整一个数量级。更值得一提的是,当温度下降至25 mK时,KTaO3 (001)界面上相似的电子气仍然能维持正常。临界场以及电流-电压测量表明,这一超导性是二维的。而在EuO/KTaO3 (111)样品中,研究人员还发现在超导性出现之前,在EuO/KTaO3 (111)样品能观察到面内输运各向异性的情况,表面条带状相的出现。基于这些实验结果,研究认为这项工作不仅有利于超导器件的开发,还为超导性的机理研究提供了崭新的机会。1KTO111)界面、XANESSTEM表征的示意图。

 image.png

Fig. 1 Schematics of KTO (111) surface, XANES and STEM characterizations. (A) KTaO3 lattice structure. The relative sizes of the ions are chosen to emphasize the Ta atoms. The three adjacent (111) planes containing Ta5+ ions are colored in light purple, blue and green, respectively. (B) Distribution of Ta5+ ions viewed along the [111] crystal axis. Ta5+ ions are shown with progressively smaller sizes in the three adjacent (111) planes, which are labeled as Ta – I, Ta – II and Ta – III, respectively. Solid lines between Ta5+ ions indicate the relative distance or coupling strength – with thicker lines representing stronger couplings, giving rise to a buckled honeycomb lattice for the first bilayer comprising Ta - I and Ta - II sites. (C) XANES data from sample EuO/KTO(111)_4 at the Ta Ledge. The KTO near the surface is only slightly reduced, with Ta valence close to its bulk value (5+). XANES data for pure Ta is also shown for comparison. STEM images of the (D) EuO/KTO (111) and (E) LAO/KTO (111) interfaces, looking down the [110] direction. Green box indicates the region near the interface. Credit: Science, doi: 10.1126/science.aba5511

在材料界面发现的独特电子结构可以使非常规量子态出现。美国伊利诺伊大学阿贡国家验室(Argonne National Laboratory, University of Illinois)和中国科学院(Chinese Academy of Sciences)研究人员组成的一个联合研究团队,详细研究了在钽酸钾(KTaO3)和绝缘覆盖层氧化铕(Europium-II oxide, EUO) 或铝酸镧(lanthanum aluminate, LaAlO3)之间的界面形成的电子气体中发现超导电性。在这项工作中观察到的超导转变温度接近2.2 K,是高于以前的铝酸镧/钛酸锶(lanthanum aluminate/strontium titanate)系统的一个数量级。临界场和电流-电压测量表明超导的二维(2-D)特性。研究小组注意到EUO/ KTaO3样品在超导性开始之前的自发面内传输各向异性,这表明在临界场附近出现了明显的“条纹”样相。

二维超导(Superconductivity in 2-D

刘长江(Changjiang Liu音译)等描述了氧化物-绝缘体/钽酸钾氧化物界面(oxide-insulator/potassium tantalate oxide interfaces)电子气体中的二维超导性。二维超导性是凝聚态物理和材料科学的中心主题。在二维表面上,电子-电子和电子-晶格的相互作用可以引起与超导相竞争的状态。因此,只有一小部分二维电子气(2-D electron gas, 2-DEG)和超薄金属薄膜是超导的。研究人员先前利用非晶薄膜进行了大部分二维超导的基础工作,以深入了解经典相变和量子相变的本质。

二维超导性可以在晶体材料和晶体材料之间的界面中实现,使科学家能够实现和打破对称性,以调整电子结构,而这在非晶态和无序薄膜中是迄今为止不可能的。例如,在具有强自旋-轨道耦合和破坏镜面对称性的二维超导体中,拉什巴相互作用(Rashba interaction)可以产生一个实现马约拉纳模式(Majorana modes)的候选平台。其中3个最突出的二维超导体的例子,在晶体界面涉及过渡金属氧化物与强电子-电子和电子-晶格相互作用来调整超导配对。

image.png

Fig. 2 Transport measurements of 2DEGs formed at different KTO interfaces. (A) Metallic temperature dependence of the sheet resistance of EuO/KTO (111) and (001) samples measured from 300 K to 4 K. (B) Measurement at lower temperatures shows superconducting transitions in EuO/KTO (111) samples (current along [11 2 ]) with varying carrier densities, which are determined from Hall measurement at T =10 K for samples EuO/KTO(111)_1, 2 and 3. The carrier density in EuO/KTO(111)_4 is estimated from growth condition. (C) Similar measurements on LAO/KTO (111) samples also show superconductivity. (D) No superconductivity is observed in samples with (001) oriented KTO interfaces with overlayers of either EuO or LAO down to 25 mK. The range of the carrier density is similar to those of the (111) oriented samples shown in (B) and (C). Credit: Science, doi: 10.1126/science.aba5511

用钽酸钾观察二维超导

钽酸钾(KTaO3KTO)单晶是一种新型的具有钙钛矿和烧绿石结构的晶体,在超导薄膜应用方面有着广阔的市场前景。KTO是一种具有立方钙钛矿型结构的绝缘体,由于低温下铁电跃迁过程中的量子涨落,KTO材料是一种“量子准电(quantum paraelectric)”衬底。研究人员可以使用离子液体门控技术,将KTO表面调整到弱超导状态。为了在KTO界面上实现二维电子气(2-D electron gas, 2-DEG),他们引入了真空劈裂(vacuum cleaving),然后暴露在紫外线或同步辐射下。使用角分辨光电子能谱学(angle-resolved photoemission spectroscopy, ARPES)对KTO表面进行研究,刘长江等人发现了一个从晶格结构中衍生出六重对称性的不同费米面(Fermi surface)。他们测量的转变温度高达2.2 K,在样品生长过程中,他们通过不同的载流子密度来调整温度。他们还注意到一个涌现的条纹相,此相打破了KTO表面的旋转对称性。

 image.png

Fig. 3 Critical field and current-voltage measurements on sample EuO/KTO(111)_3. (A,B) Sheet resistance measured at different temperatures as a function of the out-of-plane and in-plane magnetic fields, respectively. (C) Temperature dependence of critical fields, which are determined at half of RN (dotted horizontal line in (A) and (B)). (D) I-V curves measured at different temperatures. (E) I-V curves plotted on a logarithmic scale using same color codes as in (D). Black solid lines are linear fits to the data. Red dashed line is V ∝ I3, which is used to infer the BKT transition temperature. (F) Hysteresis of I-V curves near the critical current measured at temperatures below Tc0. In all the measurements (A)-(F), the current is applied along [112 ] direction. Credit: Science, doi: 10.1126/science.aba5511

实验

该团队接下来在钽酸钾(KTO)上制备了二维电子气(2-D electron gas, 2-DEG),方法是通过分子束外延或铝酸镧(lanthanum aluminate, LAO)脉冲激光沉积,他们证实了X射线衍射测量。使用像差校正高分辨率透射电子显微镜和扫描透射电子显微镜,他们检测到的氧空位附近的EUO/KTO界面。当他们降低温度时,界面显示超导性。刘长江等人在不同的温度和氧压下生长样品,以获得不同的载流子密度和迁移率。他们指出,在KTO界面观察到的晶体取向依赖的界面二维超导性与在钛酸锶(strontium titanate, STO)界面观察的2-DEGs形成鲜明对比,其中超导发生在所有方向。

电流电压行为与范德堡几何Van der Pauw geometry

EUO/KTO样品的超导性也表现出强大的临界电流行为。随着研究小组将温度提高到接近转变温度,他们注意到在低电流下电阻状态的逐渐开始。他们解释了二维超导体相对于一个BerezinskiiKosterlitzThoulessBKT)转型的超导性演变。因此,在有限温度下,由热波动产生的涡旋反涡旋对在电流驱动下解绑定,在超导状态下产生非线性的电流-电压(I-V)。结果进一步表明,二维超导是不均匀的(多样化的),其中薄弱环节加入了超导区域。

研究小组随后注意到在低载流子密度EUO/KTO样品中超导态附近出现了明显的相位,并在范德堡几何结构中进行了电阻测量;也就是说,一种测定电阻率和薄片电阻的简单分析技术。当温度降低到2.2 K以下时,沿晶轴方向的电流电阻增加了近50%,而沿不同晶体学方向的电流电阻则减少了50%。范德堡方法放大了高迁移率2-DEGs的输运各向异性,表明在宏观长度尺度上出现了一种打破旋转对称性的明显相位,这种相位在从2.2 K0.7 K的宽温度范围内持续存在。在更低的温度下,晶体学方向上的电阻迅速降为零,从而获得超导状态。

二维超导特性

在降低设置中的温度后,;刘长江等人注意到由于超导水坑(superconducting puddles)抑制弱耦合超导区域之间的传输,导致电阻增加。他们通过这些区域之间的约瑟夫森耦合(Josephson coupling)恢复了低温下的全局超导性。结果表明超导性是各向异性的,允许超导区域在宏观长度尺度上组织成相干排列的条纹。薄片电阻的磁场依赖性为各向异性的条状相提供了进一步的证据。随着磁场的增加,刘长江等人观察到电阻急剧增加,抑制了电流沿两个方向上的全局超导性。 

通过这种方式,当科学家们利用温度或磁场抑制全局超导性时,输运测量显示,在KTOSTO(钽酸钾和钛酸锶)界面中,条形相会产生沿类似晶体轴方向的大的各向异性输运。研究小组建议进行进一步的实验,包括探索超导的空间结构,以了解所观测到的超导性和电阻各向异性的本质。上述介绍仅供参考,更多信息请注意浏览原文或者相关报道。

A superconducting interface

Interfaces between materials can harbor quantum states that belong to neither of the materials. A classic example is the superconducting interface between two insulating oxides, LaAlO3 and SrTiO3, with a critical temperature of around 200 millikelvin. Liu et al. observed superconductivity at a different interface—formed between KTaO3 as a substrate and an overlayer of either EuO or LaAlO3—at a considerably higher temperature of about 2 kelvin. Transport measurements displayed anisotropy, which may indicate an unusual superconducting state.

Science, this issue p716

Abstract

The distinctive electronic structure found at interfaces between materials can allow unconventional quantum states to emerge. Here we report on the discovery of superconductivity in electron gases formed at interfaces between (111)-oriented KTaO3 and insulating overlayers of either EuO or LaAlO3. The superconducting transition temperature, as high as 2.2 kelvin, is about one order of magnitude higher than that of the LaAlO3/SrTiO3 system. Notably, similar electron gases at KTaO3 (001) interfaces remain normal down to 25 millikelvin. The critical field and current-voltage measurements indicate that the superconductivity is two-dimensional. In EuO/KTaO3 (111) samples, a spontaneous in-plane transport anisotropy is observed before the onset of superconductivity, suggesting the emergence of a distinct “stripe”-like phase, which is also revealed near the critical field.



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