In condensed matter systems, higher temperatures typically disfavors ordered phases leading to an upper critical temperature for magnetism, superconductivity, and other phenomena. A notable exception is the Pomeranchuk effect in 3He, in which the liquid ground state freezes upon increasing the temperature due to the large entropy of the paramagnetic solid phase. Here we show that a similar mechanism describes the finite temperature dynamics of spin and valley-isospins in magic-angle twisted bilayer graphene. Most strikingly a resistivity peak appears at high temperatures near superlattice filling factor nu = -1, despite no signs of a commensurate correlated phase appearing in the low-temperature limit. Tilted field magnetotransport and thermodynamic measurements of the inplane magnetic moment show that the resistivity peak is adiabatically connected to a finite-field magnetic phase transition at which the system develops finite isospin polarization. These data are suggestive of a Pomeranchuk-type mechanism, in which the entropy of disordered isospin moments in the ferromagnetic phase stabilizes it relative to an isospin unpolarized Fermi liquid phase at elevated temperatures. Measurements of the entropy, S/kB indeed find it to be of order unity per unit cell area, with a measurable fraction that is suppressed by an in-plane magnetic field consistent with a contribution from disordered physical spins. In contrast to 3He, however, no discontinuities are observed in the thermodynamic quantities across this transition. Our findings imply a small isospin stiffness, with implications for the nature of finite temperature transport as well as the mechanisms underlying isospin ordering and superconductivity in twisted bilayer graphene and related systems.
Pomeranchuk-like electronic localization above 100 K in twisted MoS$_{2}$
State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Optoelectronics, Shanxi University（山西大学光电研究所量子光学与量子光学器件国家重点实验室）；Collaborative Innovation Center of Extreme Optics, Shanxi University（山西大学极端光学协同创新中心）；Liaoning Academy of Materials（辽宁省材料研究院）；School of Physical Science and Technology, ShanghaiTech University（上海科技大学物理科学与技术学院）；ShanghaiTech Laboratory for Topological Physics, ShanghaiTech University（上海科技大学拓扑物理实验室）；Hefei National Laboratory（合肥国家实验室）
Contrasting $Γ$- and K-Valley Moiré Physics in Twisted Monolayer/Bilayer WSe$_2$
University of California, Los Angeles（加州大学洛杉矶分校）；Max Planck Institute for Chemical Physics of Solids（马克斯·普朗克固体化学物理研究所）；University of Wisconsin-Madison（威斯康星大学麦迪逊分校）；National Institute for Materials Science（国立材料研究所）
Lifshitz transitions and isospin polarization in twist-decoupled monolayer-bilayer graphene
Istituto Italiano di Tecnologia（意大利理工学院）；Istituto Nanoscienze – CNR, NEST - Scuola Normale Superiore（纳米科学研究所-国家研究委员会，NEST-比萨高等师范学院）；The University of Manchester（曼彻斯特大学）；National Institute for Materials Science（国立材料研究所）；Henry Royce Institute for Advanced Materials（亨利·罗伊斯先进材料研究所）；QuTech and Kavli Institute of Nanoscience, Delft University of Technology（代尔夫特理工大学奎克技术与卡弗里纳米科学研究所）
Feasibility study of continuous electronic Pomeranchuk cooling with a flavor-degenerate Wigner crystal
RWTH Aachen University（亚琛工业大学）；JARA Fundamentals of Future Information Technology（JARA未来信息技术基础）；nd Institute of Physics, RWTH Aachen University（亚琛工业大学第二物理研究所）；HFML-FELIX（荷兰强磁场实验室FELIX中心）；Radboud University（拉德堡德大学）；Max Planck Institute for the Structure and Dynamics of Matter（马普物质结构与动力学研究所）；Center for Free Electron Laser Science（自由电子激光科学中心）；University of Hamburg（汉堡大学）；I. Institute for Theoretical Physics, University of Hamburg（汉堡大学第一理论物理研究所）；Institute for Theoretical Physics and Astrophysics, University of Würzburg（维尔茨堡大学理论与天体物理研究所）；Peter Grünberg Institute (PGI-9), Forschungszentrum Jülich（于利希研究中心彼得·格林贝格研究所（PGI-9））
Simultaneous nanoscale imaging of local conductivity and chemical potential in a quantum Hall isospin ferromagnet
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所凝聚态物理国家实验室）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学物理科学学院）；School of Physics and Technology, Wuhan University（武汉大学物理科学与技术学院）；Key Laboratory for Quantum Materials of Zhejiang Province, Department of Physics, School of Science, Westlake University（西湖大学理学院物理学系浙江省量子材料重点实验室）；Key Laboratory of Quantum State Construction and Manipulation (Ministry of Education), School of Physics, Renmin University of China（中国人民大学物理学院量子态构建与操控教育部重点实验室）；Department of Physics and Astronomy, University of Manchester（曼彻斯特大学物理与天文学系）
Orientation-tunable correlated Chern insulating states in chiral twisted double bilayer graphene proximitized by WSe2
Nanjing University（南京大学）；Hefei National Laboratory（合肥国家实验室）；Yangtze Delta Region Academy of Beijing Institute of Technology (Jiaxing)（北京理工大学长三角研究院（嘉兴））；National Institute for Materials Science（国立材料研究所）；Beijing Institute of Technology（北京理工大学）；Nanjing University of Science and Technology（南京理工大学）；Jiangsu Physical Science Research Center（江苏物理科学研究中心）
