Seminars 2020

Dimensionality and carrier concentration have been two most important control knobs for engineering the electronic properties of layered materials and inducing novel properties, e.g. superconductivity. So far, control of the interlayer interaction is mainly achieved by reducing the sample thickness to atomic scale, however, such atomically thin samples are usually difficult to obtain, unstable in air and with extremely low superconducting transition temperature Tc. In this talk, I will report our recent experimental approach to control both the interlayer coupling and carrier concentration to obtain organic-inorganic hybrid materials with tailored properties through intercalation of organic cations from ionic liquids to transition metal dichalcogenides. Using topological semimetals MoTe2 and WTe2 as examples, I will discuss how such intercalation leads to tailored topological properties and enhanced superconductivity with good sample stability. Such intercalation method can be extended to other layered materials, for example, intercalation of semiconducting SnSe2 crystals leads to superconductivity in the intercalated compound.

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Two-dimensional (2D) atomic crystals, best exemplified by graphene, have emerged as a new class of material that may impact future science and technology. From a material physicist's point of view, 2D materials provides vast opportunities on two fronts. First, the reduced dimensionality in these 2D crystals often leads to novel material properties that are different from those in the bulk. Second, the entire 2D crystal is a surface, so it is possible to have better control of their material properties with external perturbations. In this talk I will illustrate these two points with examples. In particular, few-layer MnBi2Te4 is an intrinsic magnetic topological insulator, and its superior material quality has recently enabled us to observe the quantum anomalous Hall effect; we are also able to exfoliate high temperature superconductor Bi2Sr2CaCu2O8+d down to monolayer. We explore their electronic properties while the doping and dimensionality of the 2D systems are modulated.

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Two-dimensional triangular-lattice antiferromagnets are predicted under some conditions to exhibit a quantum spin liquid ground state whose low-energy behavior is described by a spinon Fermi surface. This “ghost” Fermi surface (in an otherwise insulating material) is a key concept for understanding spin liquids and their relationship to other quantum phases. Directly imaging the spinon Fermi surface, however, is difficult due to the fractional and chargeless nature of spinons. I will discuss how we have used scanning tunneling microscopy (STM) to image density fluctuations arising from a spin liquid Fermi surface in single-layer 1T-TaSe2, a two-dimensional Mott insulator . Quantum spin liquid behavior was observed in isolated single layers of 1T-TaSe2 through long-wavelength modulations of the local density of states at Hubbard band energies. These modulations reflect a spinon Fermi surface instability in singlelayer 1T-TaSe2 and allow direct experimental measurement of the spinon Fermi wavevector, in agreement with theoretical predictions for a 2D quantum spin liquid. Our results suggest that single-layer 1T-TaSe2 is a useful new platform for studying novel two-dimensional quantum spin liquid phenomena.

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Quantum computing with few to tens of qubits is now available, but the scale-up to a universally programmable quantum computer for real world application remains significant challenge. Amongst various priorities, increasing the number of qubits whilst maintaining a manageable error rate is paramount. This a multidisciplinary problem requiring scientific and engineering breakthroughs in materials, processes, multi-qubit architectures, and quantum measurement techniques in the least. In this talk, I will introduce our recent efforts to establish the capabilities for building spin-valley qubits based on monolayer 2D semiconductors. The unique spin-valley coupling in such materials is expected to suppress decoherence since a spin flip requires the concomitant change of valley. In addition, the inherent spin-orbit interaction provides for fast gate operations, and the compatibility with electrostatically gated planar qubit architectures can be advantageous for reducing system complexity and hence scalability. I shall present our recent results in materials engineering and device results toward this goal.

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I will discuss our work on thin epitaxial films of Na3Bi (a topological Dirac semimetal) which, when thinned to a few atomic layers, becomes a large bandgap (>300 meV) quantum spin Hall insulator, with an electric-field tuned topological-to-conventional transition, making it a promising platform for topological electronics. We use STM to demonstrate that thick (3D) epitaxial films of Na3Bi have potential fluctuations comparable to high-quality graphene on hBN. Quasi-particle interference mapping via STM allows us to measure the valence and conduction band structure, where we find only two Dirac cones over a large energy range, and a surprisingly high electron velocity which suggests interactions are important in determining the bandstructure. When thinned to a few atomic layers Na3Bi is a large gap (>300 meV) 2D topological insulator with topologically protected edge modes observable in STM[3]. Electric field applied perpendicular to the Na3Bi film, by potassium doping or by proximity of an STM tip, closes the bandgap completely and reopens it as a conventional insulator. Electrical transport experiments in the edge conduction regime show a novel giant negative magnetoresistance due to suppression of spin-flip scattering by magnetic field. Comparison to a simple theoretical model indicates >98% of the resistance is due to spin-flip scattering, well beyond the limit of 2/3 for a generic non-helical metal with exchange-mediated scattering, providing an unambiguous signature of helical transport.

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After decades of explorations, suffering from the subtle nature and sample quality, whether a metallic ground state exists in a two-dimensional (2D) system beyond Anderson localization is still a mystery. Our work reveals how quantum phase coherence evolves across bosonic superconductor-metal-insulator transitions via magneto-conductance quantum oscillations in high-Tc superconducting films with patterned nanopores. A robust intervening anomalous metallic state characterized by both resistance and oscillation amplitude saturations in the low temperature regime is detected, which suggests that the saturation of phase coherence plays a prominent role in the formation of the anomalous metallic state. Furthermore, we carried out a systematic transport study on the macro-size ambient-stable ultrathin crystalline PdTe2 films grown by molecular beam epitaxy (MBE). Remarkably, at ultralow temperatures, the film undergoes superconducting state and anomalous metallic state with increasing perpendicular magnetic field. The high quality filters are used to exclude the influence from external high frequency noise. Our findings offer the reliable evidences on the existence of anomalous quantum metallic ground states in 2D systems, which could be of fundamental importance for the understanding of quantum materials. Ising superconductor is a kind of superconducting system with strong spin-orbit coupling (SOC). It is reported that the broken in-plane inversion symmetry gives rise to Zeeman-type SOC, which polarizes the spins of the electrons to the outof-plane direction and leads to a huge in-plane critical magnetic field much larger than Pauli limit. The Pauli limit is defined as the magnetic field required to destroy the Cooper pairs via the spin pair breaking effect in conventional superconductors. This special superconductivity with strong Zeeman-type SOC is called Ising superconductivity. Because of Zeeman-type SOC and spin polarizations, Ising superconductors exhibit large in-plane critical field up to several times of the Pauli limit. For the first time, we reported the observation of Ising superconductivity in macro-size monolayer NbSe2 films grown by MBE [3] and the interface induced Ising superconductivity in ultrathin crystalline Pb films [4]. Furthermore, the 6-monolayer (ML) (around 3 nm) PdTe2 film exhibits a large in-plane critical field more than 7 times of the Pauli limit, which is the characteristic of Ising superconductivity. Different from the previously reported Ising superconductors, the PdTe2 film keeps the in-plane inversion symmetry, which indicates that there exists a new mechanism of Ising superconductivity, so-called type-II Ising superconductivity.

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In 1954, Schafroth proposed a mechanism for superconductivity that is physically possible, but ended up not being the explanation of the well known BCS supercon- ductors. The proposal argued correctly that a Bose condensate of charged bosons should also be a superconductor. In 1996, V.I. Yudson proposed a way to produce a charged boson by attaching two free charges to an exciton in a semiconductor, to make a “quaternion.” While that state was never seen in III-V semiconductors, it may be possible in structures made with monolayers of transition metal dichalcogenide (TMD) materials. I will present the theory of this system and experimental spectroscopic measurements that agree with the theory, which indicate that we may have observed this charged boson state.

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Topological superconductors attract lots of attentions recently, since they are predicted to host Majorana zero mode (MZM), who behaves like Majorana fermion and can be used in fault-tolerant quantum computation relying on their non-Abelian braiding statistics. Currently, most topological superconductors are artificially engineered based on a normal superconductor and the exotic properties of the electronic surface states of a topological insulator. Signatures of the MZMs have been reported as zero energy modes in various systems. As predicted, MZM in the vortex of topological superconductor appears as a zero energy mode with a cone like spatial distribution. Also, MZM can induce spin selective Andreev reflection (SSAR), a novel magnetic property which can be used to detect the MZMs. Here, I will show you that the Bi2Te3/NbSe2 hetero-structure is an ideal artificial topological superconductor and all the three features are observed for the MZMs inside the vortices on the Bi2Te3/NbSe2. Especially, by using spin-polarized scanning tunneling microscopy/spectroscopy (STM/STS), we observed the spin dependent tunneling effect, which is a direct evidence for the SSAR from MZMs, and fully supported by theoretical analyses. More importantly, all evidences are self-consistent. Our work provides definitive evidences of MFs and will stimulate the MZMs research on their novel physical properties, hence a step towards their statistics and application in quantum computing. Finally, the possible application in topological quantum computing is discussed.

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Since the discovery of two-dimensional van der Waals magnets, the field of studying 2D magnetic phenomena has been rapidly developed. In this talk, I will present our recent progress along this direction. I will firstly discuss the observation of antiferromagnetic exciton and multiple exciton phonon bound states in zigzag antiferromagnet NiPS3. I will then discuss the emergent orbital ferromagnetism in twisted monolayer-bilayer graphene. The ferromagnetism occurs at one-quarter filling of the conduction band, with a large associated anomalous Hall effect. Uniquely, the magnetization direction can be switched purely with electrostatic doping at zero magnetic field.

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