Princeton physicists have discovered an abrupt change in quantum behavior while experimenting with a three-atom-thin insulator that can be easily switched into a superconductor.

美國普林斯頓大學的物理學家們，在使用三原子的薄絕緣體實驗時，已經發現量子行為中一個突然改變化，能輕易被轉變成一種超導體。

The research promises to enhance our understanding of quantum physics in solids in general and also propel the study of quantum condensed matter physics and superconductivity in potentially new directions. The results were recently published in the scientific journal Nature Physics.

該項研究可望提升，我們有關固體量子物理方面的總體瞭解，而且於潛在的新方向上，推動量子凝聚態物理學及超導性的研究。此些研究結果，最近(2024年1月)發表於《自然•物理學》科學期刊。

The researchers, led by Sanfeng Wu, assistant professor of physics at Princeton University, found that the sudden cessation (or “death”) of quantum mechanical fluctuations exhibits a series of unique quantum behaviors and properties that appear to lie outside the purview of established theories.

這些由美國普林斯頓大學物理學助理教授，Sanfeng Wu領導的研究人員們發現，量子力學漲落(不斷變動)的突然中斷(也就是“終止”)展現出一系列，顯然超出經認定之理論範圍的獨特量子行為及屬性。

Fluctuations are temporary random changes in the thermodynamic state of a material that is on the verge of undergoing a phase transition. A familiar example of a phase transition is the melting of ice to water. The Princeton experiment investigated fluctuations that occur in a superconductor at temperatures close to absolute zero.

漲落是在處於，經歷相變邊緣之材料熱力學狀態的暫時胡亂改變。https://www.google.com.tw/url?sa=i&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FQuantum_fluctuation&psig=AOvVaw0uY_CH9ixGWL--Y1zEzfgi&ust=1714095289959000&source=images&cd=vfe&opi=89978449&ved=0CBEQjRxqFwoTCPiO7fKg3IUDFQAAAAAdAAAAABAE

相變為人熟知的一個例子是，冰融化成水。該普林斯頓大學的實驗調查研究了，在接近絕對零度之溫度下，發生於超導體中的漲落。

“What we found, by directly looking at quantum fluctuations near the transition, was clear evidence of a new quantum phase transition that disobeys the standard theoretical descriptions known in the field,” said Wu. “Once we understand this phenomenon, we think there is a real possibility for an exciting, new theory to emerge.”

Wu宣稱：「藉由直接觀察，接近轉變期的量子漲落，我們所發現的是一種，違反該領域中，已知標準理論描述之新量子相變的明確證據。一旦瞭解此現象，我們認為，真的有可能出現一種，令人振興奮的新理論。」

In the physical world, phase transitions occur when a material such as a liquid, gas or solid changes from one state or form to another. But phase transitions occur on the quantum level as well. These occur at temperatures approaching absolute zero (-273.15 degrees Celsius), and involve the continuous tuning of some external parameter, such as pressure or magnetic field, without raising the temperature.

在物理世界中，當諸如液體、氣體或固體等材料，從一種狀態或形式轉變成另一種狀態或形式時，發生相變。不過，相變也發生在量子層級。此些發生在，接近絕對零度(-273.15℃)的溫度下，且涉及諸如壓力或磁場等，不升高溫度之一些外部參數的不斷調變。

Researchers are particularly interested in how quantum phase transitions occur in superconductors, materials that conduct electricity without resistance. Superconductors can speed up the process of information and form the basis of powerful magnets used in healthcare and transportation.

研究人員們特別感興趣於，在超導體(無電阻的導電材料)中，量子相變如何發生。超導體能加速資訊處理且構成，被使用於醫療保健及交通運輸中的強力磁鐵基礎。

“How a superconducting phase can be changed to another phase is an intriguing area of study,” said Wu. “And we have been interested in this problem in atomically thin, clean, and single crystalline materials for a while.”

Wu宣稱：「超導相如何能被轉變成另一種相，是個引人感興趣的研究領域。因此，我們一直感興趣於，該在原子薄度、潔淨且單晶材料中的問題，達一段時間。」

Superconductivity occurs when electrons pair up and flow in unison without resistance and without dissipating energy. Normally, electrons travel through circuits and wires in an erratic manner, jostling each other in a manner that is ultimately inefficient and wastes energy. But in the superconducting state, electrons act in concert in a way that is energy efficient.

當電子配成對且無阻力及不耗散能量，一致流動時，產生超導性。通常，電子以一種最終低效率且浪費能量之相互推擠的不穩定方式，流經電路及電線。不過，在超導狀態，電子以一種具能源效率的方式，一致行動。

Superconductivity has been known since 1911, although how and why it worked remained largely a mystery until 1956, when quantum mechanics began to shed light on the phenomenon. But it has only been in the last decade or so that superconductivity has been studied in clean, atomically thin two-dimensional materials. Indeed, for a long time, it was believed that superconductivity was impossible in a two-dimensional world.

打從1911年以來，超導性已經被知曉。雖然，直到1956年，當量子力學開始闡明此現象時，其為何及如何發生作用，大部分仍然是一個謎。不過，直到最近十年左右，超導性已經在潔淨、原子薄度的二維材料中，被研究。事實上，一段長時間以來，被認為在二維世界中，超導性是不可能的。

“This came about because, as you go to lower dimensions, fluctuations become so strong that they ‘kill’ any possibility of superconductivity,” said Nai Phuan Ong, the Eugene Higgins Professor of Physics at Princeton University and an author of the paper. 

該項論文的一名撰文人，普林斯頓大學物理學尤金希金斯教授，Nai Phuan Ong宣稱：「這是因為，當進入較低的維度時，漲落變得很強烈，以至於它們‘扼殺’了超導性的任何可能性。」

The main way fluctuations destroy two-dimensional superconductivity is by the spontaneous emergence of what is called a quantum vortex. Each vortex resembles a tiny whirlpool composed of a microscopic strand of magnetic field trapped inside a swirling electron current. When the sample is raised above a certain temperature, vortices spontaneously appear in pairs: vortices and anti-vortices. Their rapid motion destroys the superconducting state.

漲落摧毀二維超導性的主要方式是，所謂量子渦旋的自發出現。每個渦旋類似，由被陷於旋轉之電子流內，一串細微磁場組成的小旋渦。當樣本被提升超過一定溫度時，渦旋會自發地成對出現：渦旋與反渦旋。它們的快速移動摧毀了超導狀態。

“A vortex is like a whirlpool,” said Ong. “They are quantum versions of the eddy seen when you drain a bathtub.”

Ong宣稱：「渦旋就像旋渦。它們是當浴缸排水時，被看到之漩渦的量子版。」

Physicists now know that superconductivity in ultrathin films does exist below a certain critical temperature known as the BKT transition, named after the condensed matter physicists Vadim Berezinskii, John Kosterlitz and David Thouless. The latter two shared the Nobel Prize in physics in 2016 with Princeton physicist F. Duncan Haldane, the Sherman Fairchild University Professor of Physics.

目前，物理學家們知曉，超薄之薄膜中的超導性，確實存在低於，以凝聚態物理學家Vadim Berezinskii、John Kosterlitz及David Thouless命名，被通稱為BKT轉變之固定臨界溫度的超導性。後兩者與普林斯頓大學物理學家、物理學的Sherman Fairchild大學教授，F. Duncan Haldane共同獲得了， 2016年諾貝爾物理學獎。

The BKT theory is widely regarded as a successful description of how quantum vortices proliferate in two-dimensional superconductors and destroy the superconductivity. The theory applies when the superconducting transition is induced by warming up the sample. 

BKT理論廣被視為，量子渦旋於二維超導體中，如何擴散及摧毀超導性的一種成功描述。該理論適用於，當藉由加熱樣本引起超導轉變時。

The question of how two-dimensional superconductivity can be destroyed without raising the temperature is an active area of research in the fields of superconductivity and phase transitions. At temperatures close to absolute zero, a quantum transition is induced by quantum fluctuations. In this scenario the transition is distinct from the temperature-driven BKT transition.

沒有升高溫度，如何能摧毀二維超導性的問題，是在超導性及相變領域中，一個進展中的研究領域。在接近絕對零度的溫度下，量子轉變是由量子漲落引起。在此腳本中，轉變不同於溫度驅動的BKT轉變。

The researchers began with a bulk crystal of tungsten ditelluride (WTe₂), which is classified as a layered semi-metal. The researchers began by converting the tungsten ditelluride into a two-dimensional material by increasingly exfoliating, or peeling, the material down to a single, atom-thin layer.

此些研究人員以一塊被歸類為，一種層狀之半金屬的二碲化鎢(WTe₂)晶體開始。此些研究人員開始是，藉由逐漸使該種材料剝離成為單一、原子的薄層，而將此二碲化鎢轉變成一種二維材料。

At this level of thinness, the material behaves as a very strong insulator, which means its electrons have limited motion and hence cannot conduct electricity. Amazingly, the researchers found that the material exhibits a host of novel quantum behaviors, such as switching between insulating and superconducting phases.

在此薄度水平下，該種材料表現，如同一種非常強的絕緣體。這意味著，其電子具有限的移動力，因此無法導電。令人驚訝的是，此些研究人員發現，該材料展現出諸多新穎的量子行為。諸如，在絕緣相與超導相之間切換。

They were able to control this switching behavior by building a device that functions just like an “on and off” switch.

藉由建構一種功能類似“開及關”的開關裝置，他們能夠控制此切換行為。

But this was only the first step. The researchers next subjected the material to two important conditions. The first thing they did was cool the tungsten ditelluride down to exceptionally low temperatures, roughly 50 milliKelvin (mK).

不過，這只是第一步。此些研究人員接著，使該種材料曝露於兩個重要條件下。他們做的第一件事，是使二碲化鎢冷卻到，大約50毫開爾文(溫度的計量單位)(mK)的極低溫度。

Fifty millikelvins is -273.10 degrees Celsius (or -459.58 degrees Fahrenheit), an incredibly low temperature at which quantum mechanical effects are dominant.

50毫開爾文是-273.10℃(也就是-459.58℉)，這是於量子力學效應中，最具優勢之令人難以置信的低溫。

The researchers then converted the material from an insulator into a superconductor by introducing some extra electrons to the material. It did not take much voltage to achieve the superconducting state.

然後，此些研究人員藉由，將一些額外電子引進此種材料中，使之從絕緣體轉變成超導體。這無需太多電壓，來獲得超導狀態。

“Just a tiny amount of gate voltage can change the material from an insulator to a superconductor,” said Tiancheng Song, a postdoctoral researcher in physics and the lead author of the paper. “This is really a remarkable effect.”

該項論文首要撰文人，物理學方面的博士後研究員，Tiancheng Song宣稱：「僅需少量的柵極電壓，就能使該種材料從絕緣體轉變成超導體。這確實是個，非常顯著的效能。」

The researchers found that they could precisely control the properties of superconductivity by adjusting the density of electrons in the material via the gate voltage. At a critical electron density, the quantum vortices rapidly proliferate and destroy the superconductivity, prompting the quantum phase transition to occur.

此些研究人員發現，藉由經由閘極電壓調整，於此材料中的電子密度。他們能精確地控制超導性的諸多屬性。在臨界的電子密度下，量子渦旋迅速激增並摧毀此超導性，這促使發生了量子相變。

To detect the presence of these quantum vortices, the researchers created a tiny temperature gradient on the sample, making one side of the tungsten ditelluride slightly warmer than the other.

為了檢測此些量子渦旋的存在，此些研究人員，在這種樣本上，創建了一個微小的溫度梯度，使二碲化鎢的一側稍微比另一側暖和。

“Vortices seek the cooler edge,” said Ong. “In the temperature gradient, all vortices in the sample drift to the cooler part, so what you have created is a river of vortices flowing from the warmer to the cooler part.”

Ong宣稱：「渦旋尋找較冷的邊緣。在上述溫度梯度中，於這種樣本中的所有渦旋，漂移到較冷的部分。因此，所創建的是一條，從較暖和部分流向較冷部分的渦旋河。」

The flow of vortices generates a detectable voltage signal in a superconductor. This is due to an effect named after Nobel Prize-winning physicist Brian Josephson, whose theory predicts that whenever a stream of vortices crosses a line drawn between two electrical contacts, they generate a weak transverse voltage, which can be detected by a nano-volt meter.

在超導體中，這種渦旋河產生可偵測的電壓訊號。這是由於一種，以獲得諾貝爾獎之物理學家，Brian Josephson命名的效應。其理論預測，每當一連串渦旋穿過，在兩個電觸點之間被描繪的線時，會產生能藉由奈米伏特表，檢測的微弱橫向電壓。

“We can verify that is the Josephson effect; if you reverse the magnetic field, the detected voltage reverses,” said Ong. 

Ong宣稱：「我們能證實，那是約瑟夫森效應(電流在由一層薄絕緣材料隔開之兩片超導材料間的流動)；倘若反轉磁場，偵測到的電壓也反轉。」

“This is a very specific signature of a vortex current,” added Wu. “The direct detection of these moving vortices gives us an experimental tool to measure quantum fluctuations in the sample, which is otherwise difficult to achieve.”

Wu附言：「這是渦旋流非常獨特的特徵。此些移動之渦旋的直接檢測，賦予了我們測量，於樣品中，以其他方式難達成之量子漲落的實驗工具。」

Once the authors were able to measure these quantum fluctuations, they discovered a series of unexpected phenomena. The first surprise was the remarkable robustness of the vortices.

一旦此些撰文者們能測量這些量子漲落。他們發現了，一系列意想不到的現象。 第一個驚奇是，此些渦旋顯著穩健。

The experiment demonstrated that these vortices persist to much higher temperatures and magnetic fields than expected. They survive at temperatures and fields well above the superconducting phase, in the resistive phase of the material.

該實驗證實，此些渦旋在比預期高得多的溫度及磁場中持續存在。在此材料的電阻相中，它們持續存在於，遠高於超導相的溫度及磁場中。

A second major surprise is that the vortex signal abruptly disappeared when the electron density was tuned just below the critical value at which the quantum phase transition of the superconducting state occurs. At this critical value of electron density, which the researchers call the quantum critical point (QCP) that represents a point at zero temperature in a phase diagram, quantum fluctuations drive the phase transition.

第二個主要驚奇是，當電子密度被調變到，剛好低於超導態發生量子相變的臨界值時，渦旋訊號突然消失。在此些研究人員稱為，代表相圖中，零溫度點之量子臨界點(QCP)的電子密度臨界值處，量子漲落驅動了此相變。

“We expected to see strong fluctuations persist below the critical electron density on the non-superconducting side, just like the strong fluctuations seen well above the BKT transition temperature,” said Wu. “Yet, what we found was that the vortex signals ‘suddenly’ vanish the moment the critical electron density is crossed. And this was a shock. We can’t explain at all this observation — the ‘sudden death’ of the fluctuations.”

Wu宣稱：「我們預期，在非超導側的臨界電子密度下看到，就像遠高於BKT轉變溫度時，被看到的強烈漲落，持續存在。然而，我們發現的是，當跨過臨界電子密度時，渦旋訊號‘突然’消失。因此，這真是一件令人震驚的事。我們完全無法解釋，此些漲落之“突然終止”的此觀察結果。

Ong added, “In other words, we’ve discovered a new type of quantum critical point, but we don’t understand it.”

Ong附言：「換句話說，我們已經發現一種，新型的量子臨界點。不過，我們並不瞭解它。」

In the field of condensed matter physics, there are currently two established theories that explain phase transitions of a superconductor, the Ginzburg-Landau theory and the BKT theory. However, the researchers found that neither of these theories explain the observed phenomena.

在凝聚態物理領域中，目前有Ginzburg-Landau理論及BKT理論，這兩種解釋超導體相變，經認定的理論。不過，此些研究人員發現，無一此些理論能解釋，該被觀察到的現象。

“We need a new theory to describe what is going on in this case,” said Wu, “and that’s something we hope to address in future works, both theoretically and experimentally.” 

Wu宣稱：「我們需要一種新理論，來描述在此情況下，發生了什麼事。因此，那是於未來之研究中，理論上及實驗上，我們希望解決的重點事情。」

網址：https://www.princeton.edu/news/2024/01/19/researchers-discover-abrupt-change-quantum-behavior-defies-current-theories

翻譯：許東榮