Study probes link between magnetism, superconductivity
磁力和超導性的聯結
Monday, December 13, 2010
European and U.S. physicists this week are offering up the strongest evidence yet that magnetism is the driving force behind unconventional superconductivity. The findings by researchers from Rice University, the Max Planck Institute for Chemical Physics of Solids (MPI-CPfS) in Dresden, Germany, and other institutions were published online today in Nature Physics.
本週歐洲和美國物理學家提出自今為止最有力的證據來說明磁場是不尋常的超導性的主要成因。這一發現來自萊斯大學(Rice University)的研究人員、馬克斯普朗克(Max Planck Institute) 固體化學物理研究所 (MPI-CPfS)，和其他機構，並在今天公佈在自然物理學(Nature Physics)上。
The findings follow more than three decades of research by the team that discovered unconventional superconductivity in 1979. That breakthrough, which was led by MPI-CPfS Director Frank Steglich, preceded by seven years the more widely publicized discovery of unconventional superconductivity at high temperatures. In the latest study, the team revisited the same heavy-fermion material -- a mix of cerium, copper and silicon -- that was used in 1979, applying new experimental techniques and theoretical knowledge unavailable 30 years ago.
本研究的發現是延續該團隊自1979年來，發現非常規超導性(unconventional superconductivity)至今超過三十年的研究。這一突破由MPI-CPfS研究員 Frank Steglich領導，承續七年來更廣為人知的、非常規超導體的高臨界溫度特性。在最新研究中，該團隊應用過去所沒有的實驗技術和理論知識來研究1979年發現的重費米子(heavy-fermion)材料 –其材料混合鈰，銅和矽。
"In 1979, there was not much understanding of quantum criticality or of the collective way that electrons behave at the border of magnetism," said Rice physicist Qimiao Si, the lead theorist and co-author of the new paper. "Today, we know a great deal about such collective behavior in the regime where materials transition to a superconducting state. The question we examined in this study is, How does all of that new knowledge translate into an understanding of the superconducting state itself?"
該團隊主要理論物理學家Qimiao Si同時也是本篇論文共同作者說：“在1979年，我們對於量子臨界性(quantum criticality)，電子在磁場邊界的集體的行為並不了解”。 然而 “今天，我們知道了很多關於這種集體行為在材料過渡到超導態狀況下的知識。我們研究的問題是：如何將我們所擁有的新知識轉化為對超導態更清楚的認識？”
Magnetism -- the phenomenon that drives compass needles and keeps notes stuck to refrigerators the world over -- arises when the electrons in a material are oriented in a particular way. Every electron is imbued with a property called spin, and electron spins are oriented either up or down. In most materials, the arrangement of electron spins is haphazard, but in everyday refrigerator magnets -- which scientists call ferromagnets -- electron spins are oriented collectively, in the same direction.
磁力存在於我們熟知的現象：如日常生活中羅盤指針指向南北極，或是冰箱上使便條紙穩固吸附的磁鐵；然而磁力只在材料中電子排列整齊時出現。每個電子都有一種稱為自旋(Spin)的特性，電子自旋只有向上或向下兩種。在大多數材料，電子自旋的安排是散亂的，但在一些材料中－例如冰箱磁鐵內－電子的自旋卻是朝向同一方向，這個現象被科學家稱為鐵磁性(ferromagnets)。
Classical superconductors, which were discovered almost a century ago, were the first materials known to conduct electrons without losing energy due to resistance. Electrons typically bump and ricochet from atom to atom as they travel down a wire, and this jostling leads to a loss of energy in the form of electrical resistance. Resistance costs the energy industry billions of dollars per year in lost power, so scientists have been keen to put superconducting wires to widespread use, but it hasn't been easy.
近一個世紀前發現的古典超導體(Classical superconductors)是第一個已知的零電阻材料。通常導電時，電子會和導體中原子碰撞而反彈，這就是導致能量損失的電阻。電阻造成能源相關產業每年損失數十億美元在失去的能量上，所以科學家們一直熱衷於實現超導體導線的廣泛應用，但它並不容易。
It took physicists almost 50 years to explain classical superconductivity: At extremely low temperatures, electrons pair up and move in unison, thus avoiding the jostling they experience by themselves. These electron twosomes are called Cooper pairs, and physicists began trying to explain how they form in unconventional superconductors as soon as Steglich's findings were published in 1979. Si said theorists studying the question have increasingly been drawn to the collective behavior of electrons, particularly at the border of magnetism -- the critical point where a material changes from one magnetic state to another.
物理學家花了近50年來解釋古典超導電性(classical superconductivity)：在極低的溫度下，電子在導體忠誠對的移動以避免和導體中的元子碰撞。這些電子對被稱為庫柏對(Cooper pairs)。在1979年，Steglich關於非常規超導性的研究報告發表後，物理學家開始試圖解釋庫柏對如何在非常規超導體中形成。Si說，理論家的研究已經漸漸將此問題指向電子的集體行為，特別是在磁場邊緣附近，物質的狀態(State)在臨界點時，從一個狀態轉變為到另一個狀態的集體電子效應。
In the new experiments, Steglich, the lead experimentalist co-author, and his group collaborated with physicists at the Jülich Centre for Neutron Science at the Institut Laue-Langevin in Grenoble, France, to bombard heavy fermion samples with neutrons. Because neutrons also have spin, those experiments allowed the team to probe the spin states of the electrons in the heavy fermions.
在新的實驗中，Steglich和論文合著者(同時也是該團隊中主要的實驗學家)以及他的研究團隊與法國Grenoble的Laue-Langevin研究所的物理學家合作，以中子轟擊重費米子(heavy fermion)樣品。由於中子也有自旋，這些實驗允許他們探測在重費米子中電子的自旋態。
"Our neutron-scattering data provide convincing evidence that the cerium-based heavy fermion compound is located near a quantum critical point," said Oliver Stockert, a study co-author and a neutron-scattering specialist from MPI-CPfS. "Moreover, the data revealed how the magnetic spectrum changes as the material turns into a superconductor."
同為論文共同作者，MPI-CPfS的中子散射專家奧利弗(Oliver Stockert)說，“我們的中子散射數據提供了令人信服的證據表明，鈰基重費米子化合物是位於量子臨界點的附近。 ” “此外，數據也顯示出磁譜如何在物質變成超導體時變化。”
From the data, Si and co-author Stefan Kirchner, a theorist from the Max Planck Institute for the Physics of Complex Systems and a former postdoctoral fellow at Rice, determined the amount of magnetic energy that was saved when the system entered the superconducting state.
從數據來看，Si和論文合著者Stefan Kirchner (Max Planck Institute複雜物理系統研究所的理論學家同時也是Rice前博士後研究員)，共同測量了當系統進入超導狀態時，磁場被保存的能量。
"We have calculated that the saved magnetic energy is more than 10 times what is needed for the formation of the Cooper pairs," Kirchner said.
Kirchner 表示：”根據我們的計算，儲存的磁能遠超過形成庫珀對所需能量的10倍”。
"Why the magnetic exchange in the superconductor yields such a large energy saving is a new and intriguing question," said Si, Rice's Harry C. and Olga K. Wiess Professor of Physics and Astronomy. He said one possible origin is the electronic phenomenon known as the "Kondo effect," which is involved in a class of unconventional quantum critical points advanced by Si and colleagues in a theoretical paper published in Nature in 2001. Regardless of the final answer, Si said the present study already constitutes a definitive proof that "collective fluctuations of the electrons at the border of magnetism are capable of driving superconductivity."
Rice大學的天文與物理研究所的教授Si說：“為什麼在磁狀態的改變需要儲存如此巨大的能量是一個新奇而有趣的問題”。他說，一個可能的來源是稱為“近藤效應”(Kondo effect) 的電子效應，這是涉及非傳統量子臨界點的現象，其結果在2001年由Si和同事發表在Nature期刊上。不管最後的答案，Si表示，目前的研究已構成了明確的證據，即“在磁場邊緣的電子集體擾動有能力導致超導電性 。”
Si and Steglich found it remarkable that the notion of quantum criticality is providing fresh insights into the workings of the very first unconventional superconductor ever discovered. At the same time, both said more studies are needed to determine the precise way that quantum-critical fluctuations give rise to heavy-fermion superconductivity. And thanks to key differences between the heavy-fermion materials and high-temperature superconductors, additional work must be done to determine whether the same findings apply to both.
Si和Steglich發現量子臨界的概念在非常規超導體的形成機制上提供了全新而突破性的見解。在同一時間，他們都表示接下來需要更多研究，以確定量子臨界擾動引起的重費米子超導性的詳細機制。幸好重費米子材料和高溫超導體有著關鍵性的差異，接下來我們必須做更多的工作，以確定這套解釋方法是否同時適用於高溫超導體。
"We are certain that we are on the right track with our investigations, however," Steglich said.
Steglich說：“我們確信，我們的研究正步入正確的軌道上”。
