|:朱世林
1 吴伟
1,2,† 李政
1,2 雒建林
1,2,††
(1 中国科学院物理研究所)
(2 中国科学院大学物理科学学院)
本文选自《物理》2024年第2期
1
引 言
超导体具有零电阻和完全抗磁性(迈斯纳效应)两个特征,可用于电力传输、超导磁悬浮、核聚变等能源领域。此外,超导是一种宏观量子现象,可以用于量子计算、量子通讯等信息领域。自1911年第一个超导体被发现以来,超导体临界温度(
T
c)从最初的液氦温区提升到了液氮温区,已发现的常压下最高
T
c的超导体是HgBa
2Ca
2Cu
3O
8+
x,
T
c约134 K
[1]。然而目前已知的常压下超导体的
T
c都远低于室温,这极大限制了超导材料的应用。2023年7月22日韩国高丽大学Sukbae Lee等人声称在常压下铜掺杂的铅磷灰石中观测到
T
c为127℃(400 K)的超导转变
[2-4],其化学式为Pb
10-
xCu
x(PO
4)
6O (0.9<
x<1.1)。他们以两位主要发现者Sukbae Lee和Ji-Hoon Kim的姓氏缩写以及发现时间1999年,将这种材料命名为LK-99。该韩国团队的结论基于LK-99表现出的两种特性:室温下能在磁铁上以倾斜姿态悬浮以及电阻率陡降。室温超导体在电力传输、强磁场和信息等领域都具有巨大的应用潜能,可能引起能源领域的重大变革,甚至引起工业革命
[5-8]。因此LK-99的提出不仅受到科研工的关注,也吸引了众多社会人士的讨论,并且在社交媒体上被广泛报道。
随后,若干科研团队尝试重复LK-99的实验,有多个LK-99悬浮视频在社交媒体上流传,但没有任何科研团队观测到完全抗磁性和零电阻行为
[9-12]。有一些团队在几十微米的样品中观测到半磁悬浮现象,他们认为这可能与超导体中的迈斯纳效应有关,但由于样品过小,无法测量其电阻。而另外一些团队对半悬浮现象给出了理论分析,认为这些现象和超导电性无关。虽然不同研究组制备样品的物性各不相同,但是LK-99中电阻陡降的行为一直未被复现,而对LK-99电阻陡降行为的重复是判断其是否为超导体的关键。
2
数据猜测
韩国团队论文中提及的XRD数据表明LK-99中有Cu
2S杂质,因其含量不到10%而被忽视。然而超导探索中很多类似超导转变的行为都是杂质引起,因此对杂质的分析是排除假象的关键一步。根据文献报道,离子导体Cu
2S在400 K附近有一个从高温六角相转变为低温单斜相的相变
[13-15],Cu
2S的电导以及热膨胀系数在这个相变温度会发生明显的变化,因此LK-99的电阻陡变行为可能是杂质
Cu
2
S导致。之前的文献没有
Cu
2
S详细的升降温电阻率数据,因此我们团队将99.5%纯度的Cu
2S粉末冷压成片,用标准四电极法测量其电阻率。图1的测量结果表明,Cu
2S的电阻率在400 K附近确实存在一个陡峭的跳变,电阻率从高温到低温减小超过3个数量级。这个电阻陡降行为貌似超导转变,并且相变温度与LK-99的电阻率陡降温度接近
[16]。升温和降温的电阻率实验曲线显示,相变有约10 K的迟滞行为,说明这是一级相变。虽然Cu
2S电阻率下降超过3个数量级,但并未达到零电阻,并且在低温区,电阻随温度降低逐渐变大。Cu
2S的电阻行为使我们猜测,在LK-99中的类似超导转变可能源于其杂质Cu
2S的结构相变导致的电阻率骤降。
图1 对数坐标下Cu
2S电阻率随温度的变化关系。电阻率在400 K相变附近变化超过3个数量级。升降温曲线在相变温度不重合,有明显迟滞,说明此相变是一级相变。相变后电阻率不为零,而是保持一个有限值。插图为线性坐标下Cu
2S电阻随温度的变化关系,因为电阻变化几个数量级,所以画成线性坐标后电阻率变化和超导相变很像
[16]
3
实验设计
为了判断
Cu
2
S对LK-99的影响,我们制备了两种
Cu
2
S含量的LK-99:S1(含
Cu
2
S 5%)和S2(含
Cu
2
S 70%)。如图2所示,所有样品的电阻率都有一级结构相变特有的热迟滞行为,相变温度均在400 K附近。S1样品是绝缘体,在低温下,电阻率已经超过仪器的量程范围。在纯
Cu
2
S和S2中,400 K附近的电阻率行为和超导相变相似,转变温度区间很窄。S2样品在相变处电阻率下降达到70%,而且在100 K到相变温区,电阻率行为表现出金属行为,在100 K以下才慢慢表现出半导体行为。图2(b)和(d)为S2和S1样品在相变温度附近的放大图,从图中也能看出,这两个样品以及纯
Cu
2
S的相变温度并非完全重合,也说明
Cu
2
S在LK-99中的性质会受到一定的影响,可能来源于
Cu
2
S中S含量的差别。在S2样品中的电阻转变与韩国团队报道的LK-99的转变极其类似,完全重复了韩国团队关于电阻陡降的实验现象。
图2 (a)S2样品电阻率随升降温的变化关系,插图为对数坐标下电阻率和温度的对应关系;(b)S2样品相变附近放大图,此图和LK-99在400 K左右的行为基本一致;(c)S1样品电阻率随升降温的变化关系,整体表现出绝缘体行为,说明纯的LK-99应该更绝缘;(d)S1样品相变附近的放大图
[16]
磁化率数据也说明这个混合物是一个抗磁性材料
[15]。如图3(a)所示,在1 T下磁化率随温度(
MT)的升降温曲线显示S2在2—400 K都是抗磁行为,且在380 K能看到明显的相变,而且具有迟滞,再次确认这是一个一级相变,而且温度范围也对应于
Cu
2
S的结构相变温度
[15]。如图3(b)所示,磁化强度随磁场(
MH)的曲线也表明
M在第二和第四象限
,是标准的抗磁行为,磁场越大,抗磁性的行为越强。而第二类超导体在更高的磁场下,抗磁行为会减弱,最终完全变为零,S2样品的这些性质和超导体不同。
图3 (a)S2样品的
MT曲线,在380 K能看到明显的一级相变,右上角小图是相变附近的局部放大图;(b)S2样品
MH曲线,磁场越强,抗磁性的行为越明显。这些普通抗磁材料的性质和超导体的迈斯纳抗磁行为完全不同
[16]
综上所述,对比纯Cu
2S以及包含不同比例的Cu
2S的LK-99电阻率,发现韩国小组在LK-99中的类超导行为和Cu
2S在400 K附近的电阻行为极为相似。与超导体的不同之处在于在400 K附近有升降温的热迟滞行为。超导相变是二级相变,在相变温度不会出现热迟滞行为,而只有一级相变才会有热迟滞。通过电阻和磁化率精密测量结果判断:在LK-99中的类似超导行为起源于Cu
2S的一级结构相变引起的电阻率下降
[16]。
4
后 记
韩国团队合成LK-99的反应中产物不唯一:合成1份铜掺杂铅磷灰石(纯LK-99),会同时产生17份铜和5份硫。这些残留物会导致大量杂质,尤其是韩国团队报道的样品中存在Cu
2S。为了得到没有Cu
2S的LK-99,8月14日,德国Max-Planck固态研究所的一个团队报道合成了纯的单晶LK-99。与之前需要用到坩埚的合成方法不同,该团队采用浮区法晶体生长技术,不需要在反应中加入硫,可以避免Cu
2S杂质的产生。最终得到透明的纯LK-99紫色晶体,化学式为Pb
8.8Cu
1.2P
6O
25。分离了杂质的LK-99不是超导体,而是具有百万欧姆电阻的绝缘体。由于电阻过高,无法进行标准的电导率测量。纯LK-99晶体在较大的抗磁背景上表现出很小的铁磁性。该团队在结论中表示,“排除了存在超导性的可能。”另外该团队指出,LK-99中观察到的超导现象要归因于
Cu
2
S杂质,而他们的晶体中没有这种杂质
[17]。
判断一个材料是超导体的判据是这个材料具有零电阻和完全抗磁性。对电阻率急速下降和半悬浮的解释,以及纯的
Pb
8.8
Cu
1.2
P
6
O
25单晶的实验结果
[16-19],充分证明LK-99不具有零电阻和完全抗磁性,并非室温超导体。这个结论打破了对于LK-99是首个室温常压超导体的希望。此次事件的起因是
Cu
2
S的一级结构相变被误认为超导转变,之后众多团队合作使其在短时间内得到澄清,这为今后的科研工作提供了很好的范例。去伪才能存真,可以避免在错误的方向上浪费资源和精力,让科学研究保持在重要的
方向上。
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本
文2024年3月11日发表于微信公众号 中国物理学会期刊网(LK-99“室温超导”假象源自Cu2S一级结构相变
),风云之声获授权转载。
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#fff;--weui-BG-3: #f7f7f7;--weui-BG-4: #4c4c4c;--weui-BG-5: #fff;--weui-FG-0: #000;--weui-FG-HALF: #000;--weui-FG-1: rgba(0, 0, 0, .6);--weui-FG-2: rgba(0, 0, 0, .42);--weui-FG-3: rgba(0, 0, 0, .1);--weui-FG-4: rgba(0, 0, 0, .15);--weui-FG-5: rgba(0, 0, 0, .05);--weui-RED: #dc3636;--weui-ORANGERED: #d14730;--weui-ORANGE: #e17719;--weui-YELLOW: #bb8e00;--weui-GREEN: #4f8400;--weui-LIGHTGREEN: #2e8800;--weui-BRAND: #018942;--weui-BLUE: #007dbb;--weui-INDIGO: #0075e2;--weui-PURPLE: #6265f1;--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: #e17719;--weui-TAG-BACKGROUND-ORANGE: rgba(225, 119, 25, .1);--weui-TAG-TEXT-GREEN: #06ae56;--weui-TAG-BACKGROUND-GREEN: rgba(6, 174, 86, .1);--weui-TAG-TEXT-BLUE: #007dbb;--weui-TAG-BACKGROUND-BLUE: rgba(0, 125, 187, .1);--weui-TAG-TEXT-BLACK: rgba(0, 0, 0, .5);--weui-TAG-BACKGROUND-BLACK: 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(min-width:1024px){body:not(.pages_skin_pc) .wx-root[data-weui-theme=dark],body:not(.pages_skin_pc) body[data-weui-theme=dark]{--appmsgExtra-BG: #121212}}@media screen and (min-width:1024px){body:not(.pages_skin_pc)body:not(.pages_skin_pc){background:var(--weui-BG-2)}}@media screen and (min-width:1024px) and (prefers-color-scheme:dark){body:not(.pages_skin_pc) .wx-root[data-weui-mode=care]:not([data-weui-theme=light]),body:not(.pages_skin_pc) body[data-weui-mode=care]: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, .85);--weui-FG-HALF: rgba(255, 255, 255, .65);--weui-FG-1: rgba(255, 255, 255, .55);--weui-FG-2: rgba(255, 255, 255, .35);--weui-FG-3: rgba(255, 255, 255, .1);--weui-FG-4: rgba(255, 255, 255, .15);--weui-FG-5: 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) .wx-root:not([data-weui-theme=light]),body:not(.pages_skin_pc) body:not([data-weui-theme=light]){--weui-BG-0: #111;--weui-BG-1: #1e1e1e;--weui-BG-2: #191919;--weui-BG-3: 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