Flat bands in the
2D kagome lattice
in complex oxides
Topological Dirac fermions in the 2D kagome network
High pressure synthesis of 2D diamond
The search for 2D diamond has recently become a trending topic in materials science and condensed matter physics due to the promise of combining bulk diamond’s remarkable properties to new properties that may arise from the reduced dimensionality. Its existence was first proposed over a decade ago and efforts to experimentally obtain 2D diamond are in initial stage. In this brand new and fast-moving field of 2D diamond research, there are two main routes being explored for synthesis: high-pressure compression and chemical functionalization of few-layer graphene. Recent high-pressure experiments provided significant advancements in the field, however, expected properties of a 2D-like diamond such as sp3 content, transparency, and hardness, have not been observed together in a compressed graphene system. In our recent work, we showed for the first time the formation of a hard, transparent sp3-containing 2D phase from few-layer graphene compressed in a water pressure transmitting medium, from several changes in the Raman spectra and optical images upon compression as well as from indentation marks on the SiO2 substrate- a material considerably harder than graphene systems-, as evidenced by atomic force microscopy (AFM) measurements of the samples post-compression. Our theoretical calculations and experimental data indicate a novel, surface-to- bulk phase transition mechanism that gives hint of the formation of diamondene: a 2D ferromagnetic semiconductor with spin-polarized bands.
See also L. G. Pimenta Martins et al. Hard, transparent, sp3-containing 2D phase formed from few-layer graphene under compression, Carbon 173, 744 (2020)
An advance in x-ray imaging unlocked by randomized light
Because many materials – particularly strongly interacting electron systems – are intrinsically heterogeneous at the nanoscale, imaging their properties rather than just measuring an average is increasingly important. A major advance in x-ray imaging has been the introduction of coherent ‘lensless’ microscopy, which has unlocked the ability for researchers to take high-resolution snapshots with x-rays. However, these methods typically require special sample processing steps which are incompatible with many materials, limiting the use of lensless imaging. In our recent work, we show how the use of “band-limited random” light (see figure) enables the fast visualization of extended, unprocessed samples at the nanoscale. This method, termed Randomized Probe Imaging (RPI), can be readily implemented at existing instruments for X-ray microscopy and scanning nanodiffraction experiments. Because of its robustness and ease of implementation, RPI is poised to become a standard part of the x-ray imaging scientist’s toolbox.
See also A. Levitan et al. Single-frame far-field diffractive imaging with randomized illumination, Optics Express 28, 37103 (2020)
Electronic bands in an ideal 2D kagome lattice
The symmetries of the atomic lattice shape the quantum motion of electrons in solids: the best known example is the emergence of massless Dirac fermions protected by the hexagonal symmetry of graphene. The kagome lattice is a two-dimensional network of corner-sharing triangles resembling the ‘David star’ or ‘Japanese basket weaving’ patterns. The symmetry of the kagome lattice is special, in the sense that it simultaneously harbors infinitely light, massless Dirac fermions and infinitely heavy, dispersionless flat bands. This prototypical electronic structure of the kagome lattice could not be observed for a long time, but we recently found these signatures in single-layer kagome metals FeSn and CoSn. We combined two complementary electronic structure probes: angle-resolved photoemission spectroscopy (ARPES) and de-Haas van Alphen quantum oscillation experiments. In FeSn, our termination-resolved ARPES experiments with a micro-focused beam reveal unexpected surface Dirac electrons on the specific lattice terminations. Such unique and rich two-dimensional electronic states further coexist with room-temperature magnetism in FeSn. In FeSn and CoSn, we further discovered the signatures of the long-sought flat bands, and demonstrated their topological nature in CoSn, as support by a large (80 meV) spin-orbit gap between the Dirac and flat bands. Harnessing the observed electronic and magnetic properties of transition metal-based kagome compounds is an exciting could provide a new basis for novel correlated topological phases and new spintronic devices.
See also M. Kang et al. Dirac fermions and flat bands in the ideal kagome metal FeSn, Nature Materials 19, 163 (2020)
And M. Kang et al. Topological flat bands in frustrated kagome lattice CoSn, Nature Communications 11, 4004 (2020)
X-ray imaging of magnetic fractals in complex oxides
Fractals are mathematical patterns that lack a sense of scale – they look identical at all levels of detail, a characteristic known as self-similarity. As a result, a fractal contour has fundamentally different properties than a line or a curve, and its dimensionality is not a simple integer (1, 2, or 3) as is otherwise prescribed by Euclidean geometry. We report the discovery of nanoscale magnetic fractals in a quantum material that is uniquely proximate to a critical point. To visualize this state, we have used the scanning X-ray nanoscope that can detect (antiferro-)magnetic domain with nanometric precision. By applying this technique to a thin film of the magnetic oxide NdNiO3 (neodymium nickelate), we have observed textures of unique richness spanning multiple spatial scales. Most strikingly, we have found that these magnetic patterns have a fractal nature and are characterized a non-integer dimensionality. Our research suggests that novel functional oxide materials can display a very rich nanoscale structure that is key to understand their unique phenomenology and unlock their potential for future applications in spintronics and neuromorphic computing.
See also J. Li et al. Scale-invariant magnetic textures in the strongly correlated oxide NdNiO3, Nature Communications 10, 4568 (2019)
In the news: MIT News
An electronic glass in lightly electron-doped copper-oxide superconductors
Charge order is a collective ground state of quantum material in which electrons self-organize into a periodic pattern breaking the translational symmetry of lattice. Recent experiments have revealed the charge order phase in cuprate high-temperature superconductors, raising questions on its origin and its relation with superconductivity. Until now, charge order in cuprate has been detected only along Cu-O bond direction (or a, b axes of unit cell) with C4 rotational symmetry in momentum space. In stark contrast, our recent resonant X-ray scattering study on electron-doped (Nd,Pr)2CuO4 has uncovered an unprecedented form of charge order with Cinf rotational symmetry in momentum space. Viewed in real-space, this observation corresponds to a ‘glassy’ short-range order of electrons with well-defined periodicity but without any orientational preference. Our charge susceptibility calculation reveals that this glassy electronic order originates from defect-induced Friedel oscillation under strong antiferromagnetic correlation, highlighting the interplay of spin and charge degrees of freedom in superconducting cuprates.
See also M. Kang et al. Evolution of charge order topology across a magnetic phase transition in cuprate superconductors, Nature Physics 15, 335-340 (2019)
Anomalous room temperature antiferromagnetism in ruthenium oxide
Ruthenium oxide (RuO2) has been extensively used in electrocatalysis owing to its remarkable efficiency and stability. For decades, it has been considered as a Pauli paramagnet, until a recent neutron study on a bulk crystal of RuO2 revealed the presence of itinerant, above-room-temperature antiferromagnetism. We studied the magnetic ordering of thin films and bulk crystals of rutile RuO2 using resonant magnetic x-ray scattering across the Ru-L2 absorption edge. Combining polarization analysis and azimuthal angle dependence of the magnetic Bragg signal, we have established the presence and texture of the collinear antiferromagnetic order in RuO2 with TNeel > 300 K. In addition to revealing a robust antiferromagnetic ground state, the persistence of collinear spin order even in nanometer-thick films paves the way for potential applications of RuO2 in antiferromagnetic spintronic devices.
See also Z. Zhu et al. Anomalous Antiferromagnetism in Metallic RuO2 Determined by Resonant X-ray Scattering, Physical Review Letters 122, 017202 (2019)
Anomalous Hall physics and Berry curvature in a kagome metal
Recent theoretical studies have stimulated interests on kagome lattice as a platform for novel topological physics. In these proposals, the major role is played by topological Dirac and flat bands imposed by the symmetry of kagome plane. Despite of these predictions, the characteristic band structure has avoided experimental observations. Recently, we have successfully observed a pair of two-dimensional Dirac bands in bulk kagome compounds Fe3Sn2 using angle-resolved photoemission spectroscopy. Combined with intrinsic spin-orbit coupling and ferromagnetism, the Dirac bands generates sizable intrinsic anomalous Hall conductivity, observed by our transport measurements over a wide temperature range (0.6 K ~ 400 K). Our results establish the direct link between the band structure and emergent topological transport in the correlated kagome metal.
See also L. Ye, M. Kang et al., Massive Dirac fermions in a ferromagnetic kagome metal, Nature 555, 638 (2018)