Flat bands in the
2D kagome lattice
in complex oxides
Topological Dirac fermions in the 2D kagome network
X-ray and microwave spectroscopy of superconducting qubits
As the quantum revolution blossoms, the search for a platform to realize pristine, tunable, coupled two-level systems (quantum bits, or qubits) has become a race. This race has recently motivated rapid progress in the field of quantum control for a number of different physical systems ranging from the photonic to the solid-state. Each platform comes with benefits and disadvantages, many of which derive from how the physical qubits couple to their environment. This coupling is a double-edged sword. On one hand, it can lead to uncontrolled dissipative evolution of the system and the uninvited loss of phase information, in other words, decoherence. On the other hand, quantum systems can be utilized as sensitive probes to gain insights about the external degrees of freedom.
Superconducting circuits offer one of the most promising platforms for exploring quantum information as their large size (microns) enables fabrication with well-established lithography techniques, and the nature of the platform allows for easy control of coupling between elements. However, the mesoscopic scale of these devices is accompanied by the presence of material defects which often hinder their performance as quantum systems. In collaboration with the group of Will Oliver, we aim to explore material defects that plague superconducting circuits with a highly interdisciplinary approach. We will use the notion of quantum systems as a probe combined with the well-established physics of magnetic resonance, and complementary approaches involving X-ray absorption spectroscopy. With these tools, we hope to better understand the microscopic origin of electric and magnetic defects, as well as develop approaches to mitigate their effects on superconducting devices.
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)
Strong correlation phenomena in Ni-based oxides
Rare earth nickelates, with chemical formula RENiO3, exhibit a rich variety of collective electron behavior including charge and spin ordering, and metal-to-insulator transitions. The ground state properties of RENiO3 are highly sensitive to different physical parameters such as lattice distortion, strain, carrier doping, and dimensional confinement. We are interested in understanding the electronic and magnetic ground state across the electronic phase diagram of these materials. Supported by collaborations with many scientists all over the world, we have studied the unique properties of these materials using X-ray spectroscopy and resonant soft X-ray scattering on nanometer-thick thin films of RENiO3. Recent topics of interests include: electronic inhomogeneity and phase separation phenomena in the spin (antiferromagnetic domain structure) and charge (metal/insulator phase coexistence) degrees of freedom; study of the electronic and magnetic properties in doped RENiO3 via oxygen vacancies and hydrogen intercalation; and characterization and manipulation of the non-collinear magnetic structure.
Electronic structure of the 2D kagome network
In a periodic solid, the symmetries of the atomic lattice are imprinted onto the electronic wave function, often engendering new electronic phenomena. A celebrated example is the emergence of (relativistic) Dirac electrons protected by the hexagonal symmetry of graphene. The kagome lattice is a two-dimensional network of corner-sharing triangles resembling ‘David star’ or ‘Japanese basket weaving’ patterns (top-left). The symmetry of the kagome lattice is special, in the sense that it simultaneously protects both infinitely light massless Dirac fermions and infinitely heavy dispersionless flat bands (top-right).
Despite the long-standing theoretical interest on the peculiar band structure of the 2D kagome lattice, its experimental realization in real materials has been challenging. Only very recently, a series of 3d-transition metal kagome metals has been identified as prime candidates to exhibit Dirac fermions and flat bands arising from the kagome lattice symmetry. The kagome network in these compounds is based on 3d-transition metal atoms (top-left), and depending on the chemistry (V, Mn, Fe, Co, Ni) and interlayer stacking order (AA, AB, etc), this series of compounds offers a remarkable variety of phenomena at the intersection of magnetism, spin-orbit physics, electronic topology, and dimensionality. Since 2017, our group has been studying the electronic band structure of kagome metals and its nontrivial topology. Examples of our recent findings using angle-resolved photoemission spectroscopy include massive double Dirac cones in ferromagnetic kagome metal Fe3Sn2, coexisting surface and bulk Dirac electrons in antiferromagnetic FeSn, topological flat bands in paramagnetic CoSn (collaboration with J. Checkelsky), and van Hove singularity-derived Fermi surface nesting in CsV3Sb5 (collaboration with S. Wilson).