CAREER: The Extreme Quantum Limit of Weyl Semimetals

职业生涯：外尔半金属的极限量子极限

• 批准号: 1752784
• 负责人: Brad Ramshaw
• 金额: $ 97.41万
• 依托单位: Cornell University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-07-01 至 2023-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1752784&HistoricalAwards=false
• 关键词: CAREER; Extreme; Quantum; Limit; Weyl

Non-technical abstractElectrons behave differently in metals than they do in a vacuum: in a sense each metal has its own laws of physics that electrons must obey, turning each new material into its own universe where discoveries can be made. This work focuses on a new class of metals - Weyl semimetals - that have massless electrons similar to those in graphene but that are free to move in three dimensions. By applying extremely large magnetic fields this project seeks to force the Weyl electrons to interact with one another and form new states of matter. There is a large gap in our understanding of what happens to three dimensional metals in extreme magnetic fields, and this project fills that gap. The plan includes measurements under these extreme conditions, pushing the frontier for techniques such as ultrasound spectroscopy up to 100 tesla and eventually beyond. This broadens the range of parameters where discoveries can be made by all researchers investigating new materials. A resonant ultrasound spectroscopy lab module will be developed, bringing this modern experimental technique to the classroom. This module will be made available through the American Association of Physics Teachers Advanced Lab website, and the undergraduate will have the opportunity to present their work at the "Beyond First Year Labs" conference. Technical abstractThe quantum limit is the magnetic field beyond which all electrons in a metal are confined to the last Landau level. This state is highly degenerate and prone to the formation of density waves, excitonic phases, and possibly Wigner crystals. While the quantum limit has been studied extensively in gate-tunable two dimensional systems such as GaAs/AlGaAs heterostructures and graphene, very little is known about 3D metals in the quantum limit, and even less is known about 3D Dirac and Weyl semimetals in this limit. This project studies how Weyl fermions behave when magnetic fields provide the dominant energy scale in the system. Questions include: do Weyl fermions survive when the cyclotron and Zeeman energies are larger than the spin-orbit coupling; are Weyl fermions unstable to the formation of a new state of matter in high magnetic fields; and does the Weyl topology of the electronic structure play an important role in magnetic-field induced correlated states? This project develops experimental tools aligned along two complementary directions of ultrasound and electrical transport to answer these questions. This includes quantitative resistivity measurements using focused-ion-beam lithography for investigating phenomena such as the chiral anomaly. These transport measurements (including Hall effect) are used to develop a realistic tight-binding model for TaAs in the quantum limit, enabling theoretical calculations of possible high-field correlated states. The principle investigator's previous work on TaAs suggests the formation of a correlated electronic phase above 80 tesla. This project develops frequency-dependent pulse echo ultrasound, covering frequencies from the low MHz up to a few GHz, to probe quasiparticle dynamics near the field-induced phase transition, and to uncover which (if any) symmetry is broken at the transition. The synthesis of these two approaches will yield both a quantitative picture of how the electronic structure of a Weyl semimetal evolves with field into the quantum limit, and how that electronic structure becomes unstable to interactions and ultimately leads to a new field-induced state of matter.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

电子在金属中的表现与在真空中的表现不同：从某种意义上说，每种金属都有自己的物理定律，电子必须遵守这些定律，从而将每种新材料变成自己的宇宙，在那里可以进行发现。这项工作的重点是一类新的金属- Weyl半金属-具有类似于石墨烯中的无质量电子，但可以在三维空间中自由移动。通过施加极大的磁场，该项目试图迫使Weyl电子相互作用，形成新的物质状态。我们对三维金属在极端磁场中会发生什么有很大的了解，而这个项目填补了这一空白。该计划包括在这些极端条件下的测量，推动超声波光谱等技术的前沿达到100特斯拉，并最终超越。这扩大了所有研究人员在研究新材料时可以发现的参数范围。将开发共振超声光谱实验模块，将这一现代实验技术带入课堂。该模块将通过美国物理教师协会高级实验室网站提供，本科生将有机会在“超越第一年实验室”会议上展示他们的工作。技术摘要量子极限是指金属中所有电子都局限于最后朗道能级的磁场。这种状态是高度简并的，容易形成密度波、激子相和可能的维格纳晶体。虽然量子极限已经在栅极可调谐二维系统（如GaAs/AlGaAs异质结构和石墨烯）中得到了广泛的研究，但人们对量子极限下的3D金属知之甚少，对这种极限下的3D Dirac和Weyl半金属的了解就更少了。这个项目研究了当磁场在系统中提供主导能量尺度时Weyl费米子的行为。问题包括：当回旋加速器和塞曼能量大于自旋轨道耦合时，Weyl费米子是否存在？在强磁场下，Weyl费米子对物质新状态的形成是否不稳定？电子结构的Weyl拓扑是否在磁场诱导相关态中起重要作用？本项目开发了沿着超声和电传输两个互补方向对齐的实验工具来回答这些问题。这包括使用聚焦离子束光刻技术进行定量电阻率测量，用于研究手性异常等现象。这些输运测量（包括霍尔效应）用于开发量子极限下TaAs的现实紧密结合模型，从而实现可能的高场相关态的理论计算。首席研究员之前在TaAs上的工作表明，在80特斯拉以上形成了一个相关的电子相位。该项目开发频率相关的脉冲回波超声，覆盖从低MHz到几GHz的频率，以探测场诱导相变附近的准粒子动力学，并揭示哪些（如果有的话）对称在过渡中被破坏。这两种方法的综合将产生Weyl半金属的电子结构如何随场演化到量子极限的定量图像，以及该电子结构如何变得不稳定以相互作用并最终导致新的场诱导物质状态。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

项目成果

Elastic properties of hidden order in URu2Si2 are reproduced by a staggered nematic

URu2Si2 中隐藏序的弹性特性通过交错向列再现

• DOI: 10.1103/physrevb.102.075129
• 发表时间: 2020
• 期刊: Physical Review B
• 影响因子: 3.7
• 作者: Kent-Dobias, Jaron;Matty, Michael;Ramshaw, B. J.
• 通讯作者: Ramshaw, B. J.

Quantum oscillations and quasiparticle properties of thin film Sr2RuO4

• DOI: 10.1103/physrevb.104.045152
• 发表时间: 2021-03
• 期刊: Physical Review B
• 影响因子: 3.7
• 作者: Yawen Fang;H. Nair;L. Miao;B. Goodge;N. Schreiber;J. Ruf;L. Kourkoutis;K. Shen;D. Schlom;B. Ramshaw

Magnetotropic susceptibility

磁致磁化率

• DOI: 10.1103/physrevb.108.035111
• 发表时间: 2023
• 期刊: Physical Review B
• 影响因子: 3.7
• 作者: Shekhter, A.;McDonald, R. D.;Ramshaw, B. J.;Modic, K. A.
• 通讯作者: Modic, K. A.

Quantum Oscillations in Graphene Using Surface Acoustic Wave Resonators

使用表面声波谐振器研究石墨烯中的量子振荡

• DOI: 10.1103/physrevlett.130.246201
• 发表时间: 2023
• 期刊: Physical Review Letters
• 影响因子: 8.6
• 作者: Fang, Yawen;Xu, Yang;Kang, Kaifei;Davaji, Benyamin;Watanabe, Kenji;Taniguchi, Takashi;Lal, Amit;Mak, Kin Fai;Shan, Jie;Ramshaw, B. J.
• 通讯作者: Ramshaw, B. J.

Calorimetric measurement of nuclear spin-lattice relaxation rate in metals

金属核自旋晶格弛豫率的量热测量

• DOI: 10.1103/physrevb.107.195145
• 发表时间: 2023
• 期刊: Physical Review B
• 影响因子: 3.7
• 作者: Khansili, A.;Bangura, A.;McDonald, R. D.;Ramshaw, B. J.;Rydh, A.;Shekhter, A.
• 通讯作者: Shekhter, A.