近二十年来,拓扑材料在凝聚态物理研究中占有举足轻重的地位,而磁性与拓扑物态的结合则为探索各种奇异输运现象提供了极佳的平台。因此,作为首个本征反铁磁拓扑绝缘体,近年来MnBi
2
Te
4
受到了广泛的关注与研究。
在2013年,MnBi
2Te
4首次被发现并合成,但当时研究者仅仅探索了其热电性质。随后在拓扑材料研究的浪潮中,MnBi
2Te
4因其Mn原子贡献的磁性而得到了许多关注。最初,研究者通过计算发现MnBi
2Te
4覆盖的拓扑绝缘体具有较大的表面磁能隙,因此推断MnBi
2Te
4因为和拓扑绝缘体Bi
2Te
3具有相似的晶体结构和成分,从而使得Mn引起的表面磁化更加高效。随后的实验研究发现,Mn掺杂到Bi
2Te
3里后并非是无序的杂质,而是整个系统形成了MnBi
2Te
4七元和Bi
2Te
3五元层的异质结构,且这种自组织异质结构具有较大的磁性表面能隙。尽管这些最初的研究并未发现MnBi
2Te
4本身的拓扑性质,但在这些线索的指引下,2019年几个理论研究小组同时揭示了MnBi
2Te
4的拓扑性。图1展示了理论预测中MnBi
2Te
4中可以实现的各种磁和拓扑相。与此同时,对MnBi
2Te
4的磁性拓扑相的实验研究也迅速展开。
图1. MnBi
2Te
4中可实现的各种磁和拓扑相(红色表明反铁磁相,蓝色表明铁磁相)
对反铁磁拓扑绝缘体MnBi
2Te
4的广泛研究主要源于两个动机:实现高温量子反常霍尔效应和探索其他拓扑和磁序引发的新奇物相,如轴子绝缘体。此外,MnBi
2Te
4层状结构对于调节其磁性和拓扑性,以及进行输运实验的测量都非常有利。目前,研究者已经在反铁磁MnBi
2Te
4偶数层薄膜中观察到了零霍尔平台和层霍尔效应,以及在奇数层薄膜中观察到了量子反常霍尔效应,同时在铁磁MnBi
2Te
4薄膜中发现了高温陈绝缘体相和高陈数相。然而,在此研究方向仍然存在许多机遇与挑战。例如,对于磁性打开的表面能隙,仍然缺乏对高于奈尔温度下观察到表面能隙未闭合的解释;为了控制杂质效应合成高质量样品,需要更先进的晶体生长技术,并且深入研究磁性杂质的行为是必要的;对于薄膜材料的输运研究,需要更高精度的器件制备技术,以进行至关重要的层分辨测量。图2给出了MnBi
2Te
4研究中的一些未来探索方向。对MnBi
2Te
4的探索为磁性和拓扑结合研究的进一步发展打下了重要基石。
图2. MnBi
2Te
4中边界态输运和磁结构设计的未来探索方向
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rgba(255, 255, 255, .1);--weui-RED: #fa5151;--weui-ORANGERED: #ff6146;--weui-ORANGE: #c87d2f;--weui-YELLOW: #cc9c00;--weui-GREEN: #74a800;--weui-LIGHTGREEN: #3eb575;--weui-BRAND: #07c160;--weui-BLUE: #10aeff;--weui-INDIGO: #1196ff;--weui-PURPLE: #8183ff;--weui-WHITE: rgba(255, 255, 255, .8);--weui-LINK: #7d90a9;--weui-TEXTGREEN: #259c5c;--weui-FG: #fff;--weui-BG: #000;--weui-TAG-TEXT-RED: rgba(250, 81, 81, .6);--weui-TAG-BACKGROUND-RED: rgba(250, 81, 81, .1);--weui-TAG-TEXT-ORANGE: rgba(250, 157, 59, .6);--weui-TAG-BACKGROUND-ORANGE: rgba(250, 157, 59, .1);--weui-TAG-TEXT-GREEN: rgba(6, 174, 86, .6);--weui-TAG-BACKGROUND-GREEN: rgba(6, 174, 86, .1);--weui-TAG-TEXT-BLUE: rgba(16, 174, 255, .6);--weui-TAG-BACKGROUND-BLUE: rgba(16, 174, 255, .1);--weui-TAG-TEXT-BLACK: rgba(255, 255, 255, .5);--weui-TAG-BACKGROUND-BLACK: rgba(255, 255, 255, .05)}}@media screen and (min-width:1024px) and (prefers-color-scheme:dark){body:not(.pages_skin_pc) 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#10aeff;--weui-INDIGO: #1485ee;--weui-PURPLE: #6467f0;--weui-WHITE: #fff;--weui-LINK: #576b95;--weui-TEXTGREEN: #06ae56;--weui-FG: #000;--weui-BG: #fff;--weui-TAG-TEXT-RED: rgba(250, 81, 81, .6);--weui-TAG-BACKGROUND-RED: rgba(250, 81, 81, .1);--weui-TAG-TEXT-ORANGE: #fa9d3b;--weui-TAG-BACKGROUND-ORANGE: rgba(250, 157, 59, .1);--weui-TAG-TEXT-GREEN: #06ae56;--weui-TAG-BACKGROUND-GREEN: rgba(6, 174, 86, .1);--weui-TAG-TEXT-BLUE: #10aeff;--weui-TAG-BACKGROUND-BLUE: rgba(16, 174, 255, .1);--weui-TAG-TEXT-BLACK: rgba(0, 0, 0, .5);--weui-TAG-BACKGROUND-BLACK: rgba(0, 0, 0, .05)}.wx-root,body{--weui-BTN-ACTIVE-MASK: rgba(0, 0, 0, .1)}.wx-root,body{--weui-BTN-DEFAULT-ACTIVE-BG: #e6e6e6}.wx-root,body{--weui-DIALOG-LINE-COLOR: rgba(0, 0, 0, .1)}.wx-root,body{--weui-BG-COLOR-ACTIVE: #ececec}@media(prefers-color-scheme:dark){.wx-root:not([data-weui-theme=light]),body:not([data-weui-theme=light]){--weui-BTN-ACTIVE-MASK: rgba(255, 255, 255, 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59, .6);--weui-TAG-BACKGROUND-ORANGE: rgba(250, 157, 59, .1);--weui-TAG-TEXT-GREEN: rgba(6, 174, 86, .6);--weui-TAG-BACKGROUND-GREEN: rgba(6, 174, 86, .1);--weui-TAG-TEXT-BLUE: rgba(16, 174, 255, .6);--weui-TAG-BACKGROUND-BLUE: rgba(16, 174, 255, .1);--weui-TAG-TEXT-BLACK: rgba(255, 255, 255, .5);--weui-TAG-BACKGROUND-BLACK: rgba(255, 255, 255, .05)}}@media(prefers-color-scheme:dark){.wx-root:not([data-weui-theme=light]),body:not([data-weui-theme=light]){--discussInput-BG: rgba(255, 255, 255, .03)}}@media(prefers-color-scheme:dark){.wx-root:not([data-weui-theme=light]),body:not([data-weui-theme=light]){--nickName-FG: #959595}}@media(prefers-color-scheme:dark){.wx-root:not([data-weui-theme=light]),body:not([data-weui-theme=light]){--weui-BG-0: #111;--weui-BG-1: #1e1e1e;--weui-BG-2: #191919;--weui-BG-3: #202020;--weui-BG-4: #404040;--weui-BG-5: #2c2c2c;--weui-FG-0: rgba(255, 255, 255, .8);--weui-FG-HALF: rgba(255, 255, 255, .6);--weui-FG-1: rgba(255, 255, 255, .5);--weui-FG-2: rgba(255, 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