Research

Research Plan

(i) Research group on the basic physics of topological quantum matter and quantum computer.

Topological related research on quantum matter has been one of the hottest topics in condensed matter physics for the past decade. Topological quantum materials usually host "Majorana bound states". The topological stability of the bound state, which has been demonstrated in the literature, can be practically used in quantum computers thereby reducing the quantum decoherence. One of the topics in our center is to study the topology of heterostructures formed by superconductors and ferromagnets. This heterostructure has a wide range of applications in spintronics. The main reason is that the superconductor itself has zero energy loss, so the superconductor-based heterostructure can become the next generation of low-loss electronic devices. Our goal is to show that this electronic device can not only be used in spintronics, but also have further applications in quantum computing. In general, novel quantum states of topological materials often exist at edges or interfaces of the materials. Their spin-polarized properties, which are immune to perturbation, can be used in next-generation components such as quantum computers. In addition to using theory to predict materials or structures with topological quantum states, we also use scanning tunneling microscopy with atomic-level spatial resolution to carefully verify the theoretical predictions step by step.

Our research in this area will not only help to increase the understanding of this novel quantum state for researchers in Taiwan, but also establish a good theoretical framework for future practical applications. Additionally, recent technologies have enabled direct manipulation of quantum states, opening up new possibilities for "quantum technologies" such as quantum computers, quantum simulators, quantum information, quantum sensing, and cutting-edge photonics. Technology that exploits the properties of quantum mechanics is regarded as one of the most important technologies of this century. Theoretical studies and precise simulations are crucial to its development. For example, the controllability of quantum states is one of the interesting theoretical questions. The center will carry out this novel research direction in the future.



(ii) Theoretical and computational simulation research group of novel quantum material devices.

It is divided into the following 3 research sub-topics:

a.The challenge for shrinking the sized of semiconductor devices to nanometer scale is manifold. Transition metal dichalcogenide (TMD) monolayer is only one layer of atom thick with a suitable bandgap. Therefore, heterojunctions fabricated from transition metal dichalcogenide (TMD) monolayer become a superior candidate. However, the challenges are manifold. It is required to cope with short channel effect raised from quantum transport and low dimensionality of TMD materials. To compete with traditional silicon-based technology, reducing the large contact resistance due to Schottky barrier caused by charge transfer between the TMD and the metal electrode is critical. For this reason, we study quantum transport of electrons in the Metal-TMD-Metal contact from first principles approaches. We investigate the current-voltage characteristics in gated M-TMD-M nanojunctions and seeking ways to reduce the contact resistance. In addition, we investigate thermoelectric properties of the M-TMD-M nanojuctions associate to the low-dimensional channel materials.

b.  Since graphene was for the first time discovered at the beginning of this century, atomically thin two-dimensional (2D) materials have emerged as a new subfield of condensed matters receiving vast and still persisyently increasing interest from the  both communities of  fundamental research and front-end applications. Because of the low-dimensional structures, 2D materials  inherently possess exceptional physical and quantum properties drastically different from those of the bulk counterparts. With the advancement of material technology, the family of atomically thin 2D materials nowadays has been expanded to comprise metals, semiconductors, insulators, ferromagnetics and topologically nontrivial materials, forming an interesting correspondence and complementary relationship with the bulk counterparts. Two-dimensional materials can be arbitrarily stacked to form verstile functional heterostructures by van der Waals forces, setting up the fascinating prospect of non-silicon-based cutting-edge applications. On the other hand, the low-dimensionilty of 2D materials essentially changes the fundmenal physics underlying in the band syructures , many-body interactions, and the optical, electrical, acoustic, magnetic and quantum properties,  all of which are advantageous in the emergent non-silicon-based microelectronic as well as new quantum techology but rely solid theoretical and computational studies to grip the true underlying physics. So far, the theoretical and computational research on this unique class of quantum material are extensively undergoing but still remains plenty of room to explore.  

c. Two-dimensional van der Waals systems consist of a single layer of atoms, which can be stacked like Lego blocks of the same or different materials. They usually obtain excellent flexibility and novel electronic properties. The most common materials include graphene and transition-metal dichalcogenides. Taking graphene as an example, when it is a single layer, it shows the Dirac energy band that is only visible in high-energy systems. When stacking two or more layers, multi-electron physics becomes important. In particular, if the two layers of graphene are stacked at a small angle, the system will generate a Moiré lattice and the corresponding Moiré energy band. When adjusting the angle to the magic angle (~1 degree), the Moiré band becomes quite flat, making the multibody physics more embodied. It has been observed in this system that the superconducting state and the anomalous Hall effect can be generated when the electron fill factor is some multiple of one-fourth. In the future, the center will theoretically study the formation mechanism of the novel phenomena in the two-dimensional van der Waals systems, and predict other novel quantum phenomena that may appear and possibly apply to the construction of novel quantum devices.


(iii) Research group of novel quantum states of strongly correlated electron systems.

It is divided into the following 3 research sub-topics:

a. Over the past two decades, there have been growing experimental phenomena on thermodynamics and electron transport in strongly correlated electronic systems that violate Landau's Fermi liquid theory for ordinary metals. These "non-Fermi liquid" phenomena have been widely observed in various strongly correlated systems. They generally exist near magnetic quantum phase transitions. The most puzzling aspect of these non-Fermi liquids is their "strange metal" properties, including: quasi-T-linear resistivity and T-logarithmic divergent specific heat coefficient, T-power-law divergent magnetic susceptibility, and violation of the Wiedemann-Franz Law (thermal conductivity and electrical conductivity are not equal at zero temperature). These exotic metal properties exist in copper-based high-temperature superconductors (high-Tc cuprate superconductors), iron-based superconductors (Fe-based superconductors), organic superconductors, heavy-fermion metals and superconductors (heavy-fermion metals and superconductors), and the magic angle twisted bi-layer graphene. When the system is close to the quantum critical point, due to the increase of quantum fluctuations, the quasi-particle weight vanishes and the electrons effective mass diverges. Therefore, the coherent quasi-particles picture in Landau's Fermi liquid framework breakdowns here. The microscopic mechanism of strange metals and their relationship with quantum critical points have become an outstanding open problem in condensed matter physics. A new paradigmatic theoretical framework is urgently needed to address this problem. The breakthroughs we have achieved on this topic over the recent years further facilitate us to construct new theoretical foundation for this open problem. The following are the specific research topics along this line for the coming years: 1. Theory of strange metal phases (ground states) in heavy fermion quantum critical systems, 2. Strange metal near ferromagnetic-Kondo quantum critical points in heavy fermion systems 3. The mechanism for strange metal in high temperature superconductors. The main theoretical methods we use here include: perturbative renormalization group, dynamical large-N theory, and the effective field theory. Our focus will be on establishing a theoretical framework to explain the strange metal states and comparing the theoretical results with the experimental phenomena observed in correlated materials to achieve a comprehensive understanding of these intriguing phenomena. We also make theoretical predictions, which are useful for experimentalists to confirm and to realize in future experiments.

b.Recently, some theoretical condensed matter physicists have proposed that the dominant pairing mechanism for high-Tc superconductors comes from the apical lateral longitudinal phonons (ALLP) along with minor AF fluctuations. It has been observed experimentally that the oxygen atoms at the apical side of cuprates exhibit a vibrational mode of 40 mev, and the coupling matrix element of this vibration agrees with the Born-Meyer approximation. To verify this pairing mechanism, we need to propose a sufficiently simple microscopic model to comprehensively describe the properties of this material in all aspects of the entire doping-temperature map, including the normal and d-wave superconducting states. To gain more physical insight to the two-dimensional electrons in the pseudo-gap phase of cuprate superconductors, we consider the nearest and next nearest neighbor (t-t') electron hoping and on-site Coulomb repulsion with fourfold symmetry within the one-band Hubbard model. Our preliminary results indicate that in order to properly describe the pseudo-gap features, the parameters we need to consider are roughly limited to the range of t'~-0.2t, U~6t. In the next three years, we plan to further study the d-wave pairing mechanism of high-temperature superconductors within this t-t' Hubbard model.

c. There are many exotic features in unconventional superconductors, including high phase transition temperature, pseudo-gap states, and non-Fermi liquid. The physics of pseudo-gap state is among the most intriguing phenomena. This phenomenon cannot be explained in terms of the traditional BCS mean field theory. We apply the BCS-BEC crossover theory to reveal the possible mechanism for it. This theory is based on the strong correlation between electrons. We hope to use this theory to explain the pseudo-gap states recently observed in FeSe superconductors. It was reported that superconductivity in FeSe might come from electrons in two energy bands. Experimentally, the size of pseudo-gap in FeSe was found to be much smaller than that of other unconventional superconductors. This interesting finding will allow us to check how accurate the BCS-BEC crossover theory is.


(iv) Quantum Field Theory and Computational Physics Research Group.

It is divided into the following 2 research sub-topics:

a. Research in quantum field theory is at the front line of various disciplines in theoretical physics theory. It was originally obtained by combining quantum mechanics and special relativity. In high-energy physics and cosmology, currently tt is considered to be the most important theoretical tool for understanding properties of elementary particles and the universe. There are many important questions in quantum field theory that cannot be solved by traditional methods such as perturbation theory. The field theory research group at National Yang Ming Chiao Tung University pay special attention to such challenging and important issues. Our research methods are mainly based on the use of large-scale numerical computation. This research group has access to high-performance computational resources. These include the Fugaku supercomputer (the fastest computer around the world as for 2022) in Kobe, the facilities at National High-performance Computing, as well as clusters on campus. Computational methods suitable for quantum computers will also be developed in the future.

b. Quantum entanglement is a characteristic in a quantum system. The quantum algorithm based on the entanglement is expected to be much faster than the traditional classical algorithm. In the recent years, the research on the quantum entanglement has greatly deepened our understanding on the quantum information theory and quantum gravity theory. We have systematically studied entanglement in quantum field theory by using holographic correspondence. Now we are working on the entanglement inequalities and their physical significance in quantum field theory and quantum information theory. In addition, we are using entanglement wedge reconstruction method to resolve the information paradox of the black hole Hawking radiation. This study will shed light on understanding the quantum space-time and help us to construct the quantum gravity theory.


(v) Astronomy and Particle Physics Research Group.

In recent years, the research on gravitational wave is one of the most popular topics in the world. National Yang Ming Chiao Tung University has joined the Kamioka Gravitational Wave Detector (KAGRA) collaboration in Japan in April, 2020. By the international collaboration, we hope to enhance study level in the field of gravitational wave in Taiwan, and make significant contribution in the future. Currently, we have three topics: 1, Using the Deepclean pipeline based on machine learning to instantaneously remove noise in the gravitational wave data, and make early alert for gravitational wave events. 2, Produce waveforms for large mass-different binary black holes merging, and use the waveform to make parameter estimation. 3, Study the power spectrum of the gravitational wave produced from the inflation in the early universe evolution stage.

In particle astrophysics research, our school has participated in the Jiangmen Neutrino International Research Collaboration (JUNO) for many years. In addition to participating in laboratory construction, we have also proposed two major research themes: dark matter detection and new physics related to beryllium solar neutrinos. At present, simulation data is used for physical analysis, and analysis methods are developed so that realistic physics analysis can be carried out when the data taking begins in 2023. We have made considerable progress in the detection of dark matter, and we are about to publish a collaboration paper; as for the beryllium solar neutrino, a new research topic, in conjunction with our ongoing research on the cooling mechanism of supernovae, we are expected to obtain crucial information on new physics beyond the standard model.


(vi) Biophysical Research Group.

Biological physics is a multidisciplinary science that studies biological systems by methods of Physics. We collaborate experimentalists and develop new types of electronic devices based on DNA molecule, a natural made nanowire. We observed that DNA chelated with nickel behaves like a multi-functioning memristor and memcapacitor (resistor and capacitor with memory effect). Memristor, memcapacitor, and meminductor are three basic components of memcomputer that can process and store data in the same unit beyond the von Neumann architecture of traditional computer. Moreover, Ni-DNA reveals unique dynamic Seebeck effect. We construct model and predict that Ni-DNA can generate enormous Seebeck coefficient. It provides us an unexplored biological-inspired thermopower mechanism learned from life for green energy application.


研究計劃

(i) 拓樸量子物質與量子電腦之物理基礎研究群。

量子物質的拓樸性相關研究可說是近十年來凝態物理裡面最熱門的一個主題。具有拓樸性的量子物質通常會具有“馬約拉納束縛態”。因此束縛態的拓樸穩定性,已有研究證實此一束縛態可以實際應用在量子電腦裡並減低量子退相干的效應。在本研究群裡的其中一個課題是去研究超導體以及鐵磁體形成異質結構中的拓樸性。此一異質結構在自旋電子學裡有廣泛的應用,最主要的原因是因為超導體本身零能量損耗的特性,因此以超導體為基礎的異質結構可以成為下一代低損耗的電子元件。我們的目標即是希望此一電子元件除了能應用在自旋電子學上,還能夠更進一步具有量子計算上的應用。拓撲材料的新奇量子態往往存在於邊緣或界面、並具有能抵抗微擾的自旋極化特性、有極高的可能性被運用於量子電腦等次世代元件中。吾人除了運用理論預測具有拓撲量子態存在的材料或結構以外,並藉由具有原子級空間解析的探針顯微術、逐步驗證理論的推測。此方面的研究除了有助於增加國內對此新穎量子態的瞭解,並為未來實際應用建立良好理論基礎。另外,最近的技術已經實現了對量子態的直接操縱,從而為量子計算機,量子模擬器,量子信息,量子感測和尖端的光子學等“量子技術”開闢了新的可能性。利用量子力學性質的技術有望成為本世紀最重要的技術之一。 理論研究和精確模擬對其發展至關重要。例如,量子態的可控性是有趣的理論問題之一。本中心未來將朝此一新穎之研究方向開展。


(ii) 新穎量子材料元件之理論與計算模擬研究群。

分為以下3個研究子題:

a.當半導體元件縮小化至奈米尺度時,除了需要面對先進製程所產生的技術難度挑戰外,也需要解決傳統半導體的材料性質限制,與電子的量子輸運所產生的物理極限與短通道效應。以上問題是發展下一世代半導體元件技術需要面對的大問題。因為過渡金屬硫族化物(TMD)的半導體二維特性與能隙,使 TMD 材料成為次世代半導體元件最具潛力的材料。但是 TMD 材料與金屬電極接觸時,因凡德瓦力造成巨大的蕭特基能障,大幅降低電流。因此降低蕭特基能障與量子短通道效應是二維 TMD 材料能否取代矽基材料成為次世代半導體材料的關鍵。我們以原子級第一原理計算方式,研究三維電極連接二維 TMD 所產生的異質接面電荷轉移、蕭特基能障、量子化電流、與短通道效應的影響。藉由電荷轉移物理機制的物理機制,尋找有效率調控蕭特基能障的方式,控制量子電流與伴隨而生的量子物理現象。


b.自從本世紀初石墨烯的問世以來,二維材料成為基礎研究及元件應用上熱門的研究主題。因為其獨特的低維度結構,二維材料具備與塊材截然不同的物理與量子特性。至今, 不僅僅半金屬的石墨烯本身,從金屬、半導體、絕緣體、鐵磁到拓樸的二維材料均已陸續發現,與塊材的材料世界形成有趣的對應與互補的關係,加上二維材料間僅需透過凡德瓦力即可任意堆疊形成元件所需的異質結構,二維材料成為開啟未來非矽基尖端應用的關鍵性物質。然而,二維材料的低維本質具體的改變了材料的能帶結構,更凸顯了電子間的多體效應,進而根本影響了材料光、電、聲、磁及量子性質。至今,對此獨特的量子材料理論及計算研究尚未完整,元件的設計與應用確有賴相關理論與計算技術的建立。


c.二維的凡德瓦系統由單層原子組成, 可以相同或不同材料,如樂高積木般堆疊 。通常具有極佳可繞性與特殊電子特性。最常見的材料包括石墨烯與過渡金屬二硫化物。以石墨烯為例,單一層時,他會展現出原來需在高能系統才可見的狄拉克能帶。當堆疊兩層或數層,多電子物理變得重要。特別是若將兩層石墨烯間轉個微小角度堆疊後,系統會產生莫列晶格與對應的莫列能帶。在調整角度至神奇角度(~1 度)時,莫列能帶會變得相當平緩,使多體物理更加體現。目前在此系統已經觀察到,在電子填滿係數是四分之一的某些倍數,可以觀察到超導態,或是反常霍爾效應。本中心未來將從理論上研究這些二維凡德瓦系統出現之新穎現象之形成機制,並預測可能出現之其他新穎量子現象與其可能於建構新穎量子元件上之應用。


(iii) 強關聯電子系統之新穎量子態研究群。

分為以下3個研究子題:

a.近二三十年來,在強關聯電子系統中有愈來愈多關於熱力學及電子傳輸的實驗現象違反Landau 用以解釋一般金屬所建立之費米液體典範架構(Landau’s Fermi liquid theory)。這些“非費米液體”(non-Fermi liquid)現象已廣泛地在各種強關聯系統中被觀察到。它們通常存在於發生磁性量子相變(magnetic quantum phase transition)之處。這些非費米液體最令人費解之處在於其“奇異金屬”(strange metal)性質,包括:電阻率與溫度呈現近似線性之關係(quasi-T-linear resistivity)(一般金屬則為溫度平方之關係)及比熱係數與溫度呈現對數發散(T-logarithmic divergent specific heat coefficient)之關係(一般金屬之比熱係數維與溫度無關之常數)。磁化率隨溫度呈現冪次方之發散,及 Wiedemann-Franz Law之違反(導熱度與導電度在零溫時不相等)。這些奇異金屬性質廣泛地存在於銅基高溫超導體(high-Tc cuprate superconductors),鐵基超導體(Fe-based supeconductors),有機超導體,重費米子金屬與超導體(heavy-fermion metals and superconductors),及魔術轉角雙層石墨烯(magic angle twisted bi-layer graphene)之中。當系統靠近量子臨界點時,因量子擾動增加之故,系統之準粒子機率權重消失(vanish of quasi-particle weight)而電子之等效質量發散(divergence of electron effective mass)。因此,Landau 費米液體典範架構中同調性準粒子之圖像遭崩解破壞(breakdown of coherent quasi-particle picture within Landau’s Fermi liquid theory)而無法適用。奇異金屬之微觀形成機制及與量子臨界點之關聯即成為凝態物理界一重要之有待解決之“懸案”。為解決此一難題,物理學家急需建立一新的典範理論架構。因著本人近年來在此一研究課題已獲得之許多重要成果,此研究計畫將更深入地從全新的角度試圖在解開此一難題之研究上獲突破性的進展。以下為本研究計劃未來 5 年之具體研究課題:1. 重費米子量子臨界系統中之奇異金屬量子相(基態)之理論,2. 重費米子系統中鐵磁-近藤量子臨界點附近之奇異金屬形成機制, 3. 銅基高溫超導體之奇異金屬形成機制.此計畫所採取之主要理論方法包括:微擾重整化群(perturbative renormalization group),大 N 動力學(dynamical large-N theory)及等效場論(effective field theory)等。研究重點將放在建立解釋奇異金屬態之理論架構並將理論結果與相關材料之實驗現象互相比較以期能完整而一致的解釋相關之實驗現象,進而解開此一謎團。同時,也將理論預測提供給實驗學家參考,以期在未來之實驗中能獲得實現。

b.近來有理論凝態研究者提出銅酸鹽(cuprate)中頂點側邊縱向聲子優勢配對及少許的反鐵磁漲落是造成高溫超導體的主要機制。實驗中觀察到位於頂點氧原子展現 40mev 的振動模態,且這個振動的耦合矩陣元符合 Born-Meyer 近似。若要驗證這個配對圖像,我們必須提出一個夠簡單的微觀模型來全面的描述這個材料在整個掺雜-溫度像圖上各面向的特性,包括正常態及 d 波超導態。為了瞭解銅氧平面上二維電子氣偽能隙(pseudo-gap)的相關物理,我們只考慮具有四倍(fourfold)對稱的最近臨及次近鄰(t-t')與 on-site 排斥能 U 的單能帶哈伯模型。初步的研究發現在我們所採用的模型中,為了能適切的描述偽能隙相關物理,我們所需考慮的參數大致局限在 t'~-0.2t,U~6t 的範圍內。未來三年內,我們計畫以上述的 t- t'哈伯模型來深入研究高溫超導體的 d 波配對機制。

c.非傳統超導材料裡有很多有趣的物理性質,包括了高相變溫度,虛能隙,非費米液體等等。其中關於虛能隙的物理因為無法用傳統的 BCS 平均場理論解釋。我們利用 BCS-BEC 渡越理論來闡述其背後可能的物理機制。此一理論利用了電子之間的強關聯性。我們希望利用此一理論來解釋近年來關於 FeSe 超導體中虛能隙的相關發現。目前已知 FeSe 超導是來自於兩個能帶的貢獻,實驗上發現 FeSe 的虛能隙比起其他非傳統超導要小的很多,這個有趣的發現也可以讓我們驗證 BCS-BEC 渡越理論的正確性。


(iv) 量子場論與計算物理研究群。

分為以下2個研究子題:

a.量子場論是結合量子力學與相對論而發展出來的當代尖端物理學理論,也被認為是了解基本粒子與宇宙之起源最重要的理論。量子場論之中有很多重要問題是沒有辦法用傳統數學推導可以精確計算得到解答的。交大的相關研究群特別注重在研究這些深具挑戰性的重要問題。研究的方法主要是使用大規模的數值運算。近年本研究群在科三館機房建製了一套 64 個結點的 Intel Xeon-Phi Knights Landing 叢集,結點之間以傳輸速率每秒 100GB 的高速網路連結。未來也將開發適用於量子電腦的計算方式。

b.量子糾纏是量子系統特有的性質。利用量子糾纏,量子算法的計算速度可以大幅超越古典算法。近年來,在量子力學和量子場論中關於量子糾纏熵的研究,極大的促進了我們對於量子信息理論以及量子重力理論的理解。利用全像對偶原理,我們系統的研究了量子場論中的量子糾纏熵。我們目前計劃進一步研究在量子場論中糾纏熵滿足的不等式,以及這些不等式在量子場論與量子信息理論中的物理意義。此外,我們還計劃運用重建量子糾纏熵光錐時空區域的方法研究黑洞蒸發中的霍金輻射悖論。這個研究可以幫助我們理解量子時空,進一步構建量子重力理論。


(v) 天文與粒子物理研究群。

重力波研究是近幾年國際上最重要的物理課題之一,本校於 101 年 4 月加入了日本神崗重力波探測器(KAGRA)的合作研究,希望可以透過參與國際大型合作,使台灣在重力波研究領域達到國際水準,並作出重要貢獻。我們目前的研究計劃包括兩個方向。1,在數據分析中應用機器學習方法來優化重力波參數估計的統計模型算法。2,研究宇宙早期暴漲產生的重力波及其時間演化過程,並給出目前觀測能譜的理論預測。在粒子天文物理研究方面,本校多年來參加江門微中子國際研究團隊(JUNO),除了參與實驗室建造,我們也擬定暗物質探測與鈹太陽微中子相關新物理兩大研究主題。目前運用模擬數據進行物理分析,發展分析方法,等 2022 年底取數據時可以從事實際研究。在暗物質探測方面我們已經獲得相當進展,即將發表團隊論文; 至於鈹太陽微中子則是新的研究主題,配合我們正進行中的超新星冷卻機制研究,可望獲得更多新物理資訊。


(vi) 生物物理研究群。

傳統電腦的運算架構是 von Neumann 建立的。在 von Neumann 架構下,中央處理器(CPU)和記憶體(memory)是不同的區塊,運算時資料在處理器與記憶體間頻繁的交換資料,大部分的電腦運算資源都耗費在處理器與記憶體之間的資料交換上。憶電腦(memcomputing)是近幾年所提出的新電腦架構,憶電腦的基本的運算單元是憶阻器(memristor)、憶容器(memcapacitor)與憶感器(meminductor)。與傳統電腦運算架構不同的地方是該系統的記憶與計算是在同一單元上進行,可以避免無效的資料交換,其運算的處理方式和人腦比較接近。近幾年也因為環保意識崛起,綠能成為能源開發的優先新選項。我們以 DNA 生物材料為基礎,理論配合實驗證明 Ni-DNA 仿生材料系統具有憶阻器與憶容器的特性適合模擬人腦計算。模擬類人腦人工智慧的仿生元件與綠色能源的仿生熱電元件與其應用。我們從理論物理的角度建構 DNA 的物理與數學模型,在此模型下以數值計算方式模擬憶阻器、憶容器特性與其在人工智慧上的應用。並探討其記憶特性所導致巨大且特殊的動態熱電效應,與其在環保綠能上的應用。

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