“ Nonlinear and nonreciprocal transport effects in untwinned thin films of ferromagnetic Weyl metal SrRuO3”, Phys. Rev. X 14, 011022(2024).

Revealing topological surface states via nonlinear and nonreciprocal transport effects: One of the challenging subjects in topological systems is to identify the charge transport signatures associated with the unusual surface states due to nontrivial band topology. In a recent international collaborating work led by Dr. Wei-Li Lee and Prof. Guang-Yu Guo, an unusual large nonreciprocal and nonlinear charge transport effects (NRTE) in both longitudinal and transverse channels were observed in thin films of topological ferromagnetic Weyl metal SrRuO3 (SRO). These behaviors align with a proposed scenario of an effective Berry curvature dipole originating from Fermi-arc surface states accompanied by 1D chiral edge modes, which is supported by electronic band structure calculations. Our findings not only highlight the significance of NRTE as a charge transport probe for topological surface states with a broken inversion symmetry but also feature potential applications in nonreciprocal electronics and nonlinear optics using topological materials. The complete data and analyses have been recently accepted for publication at Physical Review X. Uddipta kar, Elisha Lu, Akhilesh Singh and P.V. Sreenivasa Reddy are the co-first authors. The SRO thin film growth, device fabrication and transport measurements were carried out by Dr. Wei-Li Lee’s group at IoPAS and Prof. I-Chun Cheng’s group at NTU. The electronic band structure calculations were performed by Prof. Guan-Yu Guo’s group at NTU and Prof. Wei-Cheng Lee’s group at SUNY Binghamton. The optical SHG measurements were performed by Prof. David Hsieh’s group at Caltech. The SRO thin film structural characterizations were performed by Director Chia-Hung Hsu’s group at NSRRC.

“The thickness dependence of quantum oscillations in ferromagnetic Weyl metal SrRuO3”, npj Quantum Materials 8, 8(2023).

A glimpse of Weyl-orbit quantum oscillations in Weyl metal SrRuO3: In a topological Weyl semimetal, the correspondence between bulk Weyl nodes and surface Fermi-arc states gives rise to a unique nonlocal cyclotron motion via the electron tunnelings between the top and bottom Fermi-arc surface states, which was referred to as the Weyl-orbit effect exhibiting an unusual thickness dependent quantum oscillations. In an international collaboration led by Dr. Wei-Li Lee’s group, such an unusual thickness dependent Weyl-orbit quantum oscillation was demonstrated in untwinned Weyl metal thin films of SrRuO3 grown by oxide molecular beam epitaxy facility at IoPAS. The quantum oscillations measurements with field up to 35 T were carried out at EMFL-Nijmegen in Netherlands in collaboration with Prof. Steffen Wiedmann’s group, and the rigorous band calculations were done by Prof. Wei-Cheng Lee’s group at SUNY Binghamton in USA. The high precision X-ray characterizations were performed at the NSRRC, Hsinchu in Taiwan in collaboration with Director Chia-Hung Hsu‘s group. The complete data and analyses have been recently published online at npj Quantum Materials (https://rdcu.be/c31ub). Uddipta Kar and Dr. Akhilesh Singh are the first two authors with equal contributions and responsible for the quantum oscillation data analyses and also the growths and structural characterizations of the SrRuO3 thin films.

“Field-induced resistance peak in a superconducting niobium thin film proximity coupled to a surface reconstructed SrTiO3”, npj Quantum Materials 5, 45(2020).

Revelation of the exotic electronic states in surface-reconstructed SrTiO3: Oxygen vacancies (OVs) on the surface of a wide band-gap SrTiO3 (STO) were known to induce surface reconstructions (SR), which not only resulted in electron dopings but also was theoretically predicted to show magnetism with unusual spin-polarized in-gap states. Experimental confirmations for those exotic states are hampered by the difficulty of maintaining such surface structures under ambient conditions. In a recent collaborating work led by Dr. Wei-Li Lee at IoPAS and Prof. Wei-Cheng Lee at Binghamton University SUNY, such exotic electronic states in SR-STO were revealed by proximity coupling a superconductor (SC), serving as a probe and also a protection layer, to a SR-STO. Dr. Akhilesh Singh and Uddipta Kar, who are the first and the second authors of the paper, utilized the interconnected multi-UHV chambers facility at IoPAS. The SR-STO was prepared and characterized via electron diffraction under UHV followed by UHV deposition of superconducting niobium (Nb) thin films to ensure a clean interface between SC and SR-STO. An unusual and generic field-induced peak resistance effect was uncovered for the first time in such a hybrid SC/SR-STO system. Combining magnetization measurements and also detailed DFT calculations, we proposed a model based on the interface resistance between a SC and a highly spin-polarized metal that gives a self-consistent explanation to the observed field-induced peak resistance effect. Our results strongly support for the existence of spin-polarized in-gap states in SR-STO.

“Determination of spin-orbit scattering lifetime at the interface of LaAlO3/SrTiO3 from the superconducting upper critical fields”, Phys. Rev. Research 2, 013311 (2020).

Electronic phase separation for the interface superconductivity in LaAlO3/SrTiO3: The formation of Cooper pairs can be more favorable for electrons from certain orbitals, giving rise to an unusual state with the coexistence of normal fluid and superfluid. Such a phase is likely to happen for the interface superconductivity in LaAlO3/SrTiO3. The orbital nature of an electron largely affects its spin-orbit interaction, which can be determined independently either from the weak-localization model in normal state or from the upper critical fields in superconducting state. We have developed a practical scheme to extract spin-orbit parameters from upper critical fields for low transition temperature superconducting systems with strong spinorbit coupling. A striking discrepancy in the extracted spin-orbit coupling parameters was uncovered for the first time in high quality LaAlO3/SrTiO3 samples grown by oxide molecular beam epitaxy technique, indicating a difference in the spin-relaxation mechanism for the charge transport above and below the superconducting transition temperature. Such a finding supports for a possible orbital selectivity for the Cooper pairing and thus an electronic phase separation at the interface of LaAlO3/SrTiO3 that may shed some lights toward understanding the exotic low-density superconductivity.

"Scale-Invariant Quantum Anomalous Hall Effect in Magnetic Topological Insulators beyond the Two-Dimensional Limit", Phys. Rev. Lett. 113, 137201 (2014).

Demonstration of robust quantum anomalous Hall effect (QAHE) in macroscopic and 3D ferromagnetic topological insulator system: Anomalous Hall effect refers to the phenomenon of linear proportionality between the anomalous Hall resistance and its magnetic moment in a magnetic system. In normal magnetic metal and semiconductor,  there is no universal value for the anomalous Hall resistance. Nevertheless, in a ferromagnetic topological insulator (FM TI) under zero external magnetic field, the Dirac surface states, originating from the non-trivial bulk band topology,  attain different effective masses on different surfaces. This results in a dissipationless chiral edge channel forming at the Dirac fermion mass domain-wall and gives rise to the quantization of the anomalous Hall resistance.This effect was now further demonstrated in a macroscopic and 3D FM TI. This may provide a new direction for resolving the heat dissipation problem in electronics using dissipationless chiral edge channel in a topological material.

“Full electric field tuning of the nonreciprocal and nonlinear charge transport in massive chiral Fermions with trigonal warping”, Physical Review Research 3, 033160 (2021).

Full electric field tuning of the nonreciprocal and nonlinear charge transport in massive chiral Fermions with trigonal warping: Recently, there are a number of interests for the non-reciprocal transport effect (NRTE) in non-centrosymmetric systems that may realize an intriguing possibility of current rectification via a single-phase material. Most of the reported single-phase systems with NRTE require a finite external magnetic field owing to the magneto-electric anisotropy term in the form of k∙E×B. Here, a full electric-field tuning of NRTE in dual-gated bilayer graphene (BLG) device was demonstrated in the absence of magnetic field, which originates from the unique massive chiral fermions with trigonal warping in a gapped BLG. The magnitude of NRTE is at maximum when the Fermi surface undergoes a Lifshitz transition near the band edges. Our findings may open up a new direction for the emerging fields of “valleytronics” and “twistronics”.

"Evidence for enhanced phase fluctuations in nanostructured niobium thin films", Phys. Rev. B 96, 020506(R) (2018).

Enhanced superconducting phase fluctuation via artificial nanostructure engineering: In a conventional superconductor, a large Cooper pair density gives rise to a extremely stiff phase rigidity, thus obscuring the rich phenomena associated with phase fluctuations. The phase rigidity in superconducting niobium (Nb) films can be effectively reduced by imposing well-ordered anti-dots, forming a interconnected Nb honeycomb lattice (INHL). By using a special technique we developed previously, the first author, Ms. Ting-Hui Chen, succeeded in fabricating a remarkable series of INHLs with sub-100nm nanostructures extending over 108 cells and up to few millimeters in size, where an unusual superconducting transition width narrowing in fields was observed for the first time in superconducting systems. The broadened transition width in zero field is a direct consequence of the enhanced phase fluctuations due to the intriguing interplay between the phase slips and flux-line lattice. Our works demonstrate the feasibility of reducing the phase rigidity in a superconductor via artificial nanostructure engineering.