Accessing the nonradiative near-field electromagnetic interactions with high in-plane momentum (q) is the key to achieve super resolution imaging far beyond the diffraction limit. At far-infrared and terahertz (THz) wavelengths (e.g., 300 μm = 1 terahertz = 4 meV), the study of high q response and nanoscale near-field imaging is still a nascent research field. In this work, we report on THz nanoimaging of exfoliated single and multilayer graphene flakes by using a state-of-the-art scattering-type near-field optical microscope (s-SNOM). We experimentally demonstrated that the single layer graphene is close to a perfect near-field reflector at ambient environment, comparable to that of the noble metal films at the same frequency range. Further modeling and analysis considering the nonlocal graphene conductivity indicate that the high near-field reflectivity of graphene is a rather universal behavior: graphene operates as a perfect high-q reflector at room temperature. Our work uncovers the unique high-q THz response of graphene, which is essential for future applications of graphene in nano-optics or tip-enhanced technologies.

Zirconium pentatelluride (ZrTe 5 ) has recently attracted renewed interest owing to many of its newly discovered extraordinary physical properties, such as 2D and 3D topological-insulator behavior, pressure-induced superconductivity, Weyl semimetal behavior, Zeeman splitting, and resistivity anomaly. The quasi-one-dimensional structure of single-crystal ZrTe 5 also promises large anisotropy in its thermal properties, which have not yet been studied. In this work, via timedomain thermoreflectance measurements, ZrTe 5 single crystals are discovered to possess a record-low thermal conductivity along the b-axis (through-plane), as small as 0.33 ± 0.03 W m −1 K −1 at room temperature. This ultralow b-axis thermal conductivity is 12 times smaller than its a-axis thermal conductivity (4 ± 1 W m −1 K −1 ) owing to the material's asymmetrical crystalline structure. First-principles calculations are further conducted to reveal the physical origins of the ultralow b-axis thermal conductivity, which can be attributed to: (1) the resonant bonding and strong lattice anharmonicity induced by electron lone pairs, (2) the weak interlayer van der Waals interactions, and (3) the heavy mass of Te atoms, which results in low phonon group velocity. This work sheds light on the design and engineering of high-efficiency thermal insulators for applications such as thermal barrier coatings, thermoelectrics, thermal energy storage, and thermal management.

Recently twisted bilayer graphene(t-BLG) 1-5 emerges as a new strongly correlated physical platform near a magic twist angle 4 , which hosts many exciting phenomena such as the Mott-like insulating and unconventional superconducting behavior [6][7][8][9][10] . Besides the apparent significance of band flatness 4 , band topology may be another critical element in strongly correlated twistronics yet receives much less attention [11][12][13][14] . Here we report the discovery of nontrivial high dimensional band topology in t-BLG moiré bands through a systematic nonlocal transport study 15,16 , in conjunction with an examination rooted in K-theory 17 . The moiré band topology of t-BLG manifests itself as two pronounced nonlocal responses in the electron and hole superlattice gaps.We further show that the nonlocal responses are robust to the interlayer electric field, twist angle, and edge termination, exhibiting a universal scaling law. While an unusual symmetry of t-BLG trivializes Berry curvature, we elucidate that two high dimensional Z2 invariants characterize the topology of the moiré Dirac bands, validating the topological origin of the observed nonlocal responses. Our findings not only provide a new perspective for understanding the emerging strongly correlated phenomena in twisted van der Waals heterostructures, but also suggest a potential strategy to achieve topologically nontrivial metamaterials from topologically trivial quantum materials based on twist engineering.

In this letter, we report optical pump terahertz (THz) near-field probe (n-OPTP) and optical pump THz near-field emission (n-OPTE) experiments of graphene/InAs heterostructures. Near-field imaging contrasts between graphene and InAs using these newly developed techniques as well as spectrally integrated THz nano-imaging (THz s-SNOM) are systematically studied. We demonstrate that in the near-field regime (/ 6000 λ), a single layer of graphene is transparent to near-IR (800 nm) optical excitation and completely "screens" the photo-induced far-infrared (THz) dynamics in its substrate (InAs). Our work reveals unique frequency-selective ultrafast dynamics probed at the near field. It also provides strong evidence that n-OPTE nanoscopy yields contrast that distinguishes single-layer graphene from its substrate.

The ability to localize and manipulate individual quasiparticles in mesoscopic structures is critical in experimental studies of quantum mechanics and thermodynamics, and in potential quantum information devices, e.g., for topological schemes of quantum computation. In strong magnetic field, the quantum Hall edge modes can be confined around the circumference of a small antidot, forming discrete energy levels that have a unique ability to localize fractionally charged quasiparticles. Here, we demonstrate a Dirac fermion quantum Hall antidot in a graphene, where charge transport characteristics can be adjusted through the coupling strength between the contacts and the antidot, from Coulomb blockade dominated tunneling under weak coupling to the effectively non-interacting resonant tunneling under strong coupling. Both regimes are characterized by single -flux and -charge oscillations in conductance persisting up to temperatures over 2 orders of magnitude higher than previous reports in other material systems. Such graphene quantum Hall antidots may serve as a promising platform for building and studying novel quantum circuits for quantum simulation and computation. *

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Studying the interplay between superconductivity and quantum magnetotransport in twodimensional materials has been a topic of interest in recent years. Towards such a goal it is important to understand the impact of magnetic field on the charge transport at the superconductornormal channel (SN) interface. Here we carried out a comprehensive study of Andreev conductance under weak magnetic fields using diffusive superconductor-graphene Josephson weak links. We observe that the Andreev conductance is suppressed even in magnetic fields far below the upper critical field of the superconductor. The suppression of Andreev conductance depends on and can be minimized by controlling the ramping of the magnetic field. We identify that the key factor behind this suppression is the reduction of the superconducting gap due to the piling of vortices on the superconducting contacts. In devices where superconducting gap at the superconductor-graphene interface is heavily reduced by proximity effect, the enlarged vortex cores overlap quickly with increasing magnetic field, resulting in a rapid decrease of the interfacial gap. However, in weak links with relatively large effective superconducting gap the AR conductance persists up to the upper critical field. Our results provide guidance to the study of quantum material-superconductor systems in presence of magnetic field, where 'survival' of induced superconductivity is critical.

Two-dimensional atomic crystals (2DACs) can be mechanically assembled with precision for the fabrication of heterostructures, allowing for the combination of material building blocks with great flexibility. In addition, while conventional nanolithography can be detrimental to most of the 2DACs which are not sufficiently inert, mechanical assembly potentially minimizes the nanofabrication processing and preserves the intrinsic physical properties of the 2DACs. In this work we study the interfacial charge transport between various 2DACs and electrical contacts, by fabricating and characterizing 2DAC-superconductor junctions through mechanical transfer. Compared to devices fabricated with conventional nanolithography, mechanically assembled devices show comparable or better interface transparency. Surface roughness at the electrical contacts is identified to be a major limitation to the interface quality.The study of two-dimensional (2D) atomic crystals [1] and their heterostructures spans over the past decade from graphene [2, 3] to a variety of semiconductors [4], insulators [5], superconductors [6], topological materials [7,8], etc. Through the widely practiced approach of mechanical co-lamination/transfer [9, 10], 2D atomic crystals can be directly stacked together with well-adjusted spatial and angular alignment relative to one another [10], even for material combinations which are not possible to synthesize through conventional methods. Such flexible combination of different 2D atomic crystals (2DACs) allows for the discovery of novel devices and artificial materials, which greatly broadens the horizon of low dimensional material research.An important concern when working with 2DACs and their heterostructures is to minimize the impact of the fabrication process on the intrinsic properties of these materials. Except for a few 2DACs (e.g. graphene, hBN), a majority of the recently explored 2D materials are air-sensitive [10][11][12]. Hence, the conventional nanofabrication process often causes degradation to these materials. On the other hand, following the basic approach of co-lamination, device fabrication can also be achieved through mechanical assembly, i.e. direct transfer of 2D materials/heterostructure onto predefined leads. It is therefore important to understand, compared to conventional lithography, how electrically transparent a contact/2DAC interface can be achieved through mechanical assembly, and what the limiting factors to the interface transparency are. In this work, we systematically studied the approach of creating transparent electrical contacts through mechanical assembly, hence minimizing the detrimental effects from conventional nanolithography on some of the air-sensitive 2DACs.Mechanical assembly of 2DAC devices is based on van der Waals interactions between the 2D crystals and the electric contacts, which may be sufficiently strong so that the interlayer distance is small and charge transfer OPEN ACCESS RECEIVED