Date: Sep. 15, 1:30 PM, Location: General Building 4th floor, Lecture Room A
Speaker: Ching-Kai Chiu (RIKEN)
Topic: KNOT THEORY AND TOPOLOGICAL SEMIMETALS,
Abstract: Hunting topological nodal line semimetals with nontrivial links and knots attracts great interests and remains a difficult task in the condensed matter community. Encouraged by the recent discovery of the topological nodal chain semimetals, we propose the realization of the exotic link and knot semimetals by considering the evolution of the nodal chain semimetals. Borrowing the idea of the knot theory, we use the Jones polynomial as a general topological invariant to capture the oriented nodal lines in the semimetals. Every possible change in Jones polynomial describes the local evolutions around every point where two nodal lines touch. As an application of our theory, we show that nodal chain semimetals with four touching points can evolve to a Hopf link. We extend our theory to 3D non-Hermitian multiband exceptional line semimetals and provide a recipe to understand the knot topology transition for protected nodal lines.
Reference: Zhesen Yang, Ching-Kai Chiu, Chen Fang, and Jiangping Hu, Physical Review Letters 124 (18), 186402 (2020) (Editors’ Suggestion)
Date: Sep. 22, 1:30 PM, Location: Physics Building R019
Speaker: Hsiu-Chuan Hsu (NCCU)
Topic: The effects of disorder on topological insulators
Abstract: In this talk, I would like to introduce the effects of disorder on three topological insulator (TI) systems. In the one-dimensional model, it is shown that disorder drives topological phase transitions between different topological indices. The mechanism is explained by band gap renormalization and Anderson localization, as a consequence of disorder scattering. The second system is a TI nanowire threaded by magnetic flux. The theoretical results based on the Landauer-Buttikerformalism show that Aharonov–Bohm oscillation appears in the disordered limit. By analyzing the localization length, it is shown that the bulk states are localized by disorder scattering and fail to contribute to transport. Lastly, in the presence of a rotating magnetic field, a reentrant quantum spin Hall effect is predicted. The numerical simulation shows that the reentrant quantum spin Hall effect survives the disorder strength up to eight times the energy gap, confirming its robustness.
Date: Sep. 29, 1:30 PM, Location: Physics Building R019
Speaker: Chang-Tse Hsieh (RIKEN)
Topic: Maxwell’s other Demons
Abstract: Maxwell's demon, created by James Clerk Maxwell to demonstrate how the second law of thermodynamics might hypothetically be violated, has sharpened our understanding of the asymmetry of time (in thermodynamics). There also exist demons revealing the breakdown of other symmetries in our nature. In this seminar, I will talk about the demon appearing in Maxwell his own theory of electromagnetism, by pointing out the asymmetry of electricity and magnetism, i.e., the breakdown of electric-magnetic duality, in the full quantum theory of electromagnetism.
Date: Oct. 6, 1:30 PM, Location: Physics Building R019
Speaker: Yu-Chin Tzeng (NCKU)
Title: Hunting for the non-Hermitian exceptional points with fidelity susceptibility
Abstract: Recently, more and more non-Hermitian systems are experimentally simulated in many optical set up with gain and loss. Besides the optics, the condensed matter theorists have been recently studying on the non-Hermitian effect in the PT-symmetric systems, e.g. the non-Hermitian Su-Schriffer-Heeger (SSH) model. The exceptional point (EP) of a non-Hermitian Hamiltonian is particularly interesting to be analysed before playing with the non-Hermitian systems. However, due to the non-Hermitian quantum mechanics is still new for many researchers, the lack of theoretical tools to detect EP has become a top priority. In our work, the fidelity susceptibility is generalized to the non-Hermitian system by including the geometry of the Hilbert space. The fidelity susceptibility is originally used for detecting quantum phase transition. Now we find that it is also a good tool for the EP.
Reference arXiv:2009.07070
Date: Oct. 13, 1:30 PM, Location: Physics Building R019
Speaker: Wei-Ting Hsu (NTHU)
Topic: Engineering Valley Excitons in Two-Dimensional Materials through Interlayer Electronic Coupling
Abstract: Van der Waals (vdW) bilayer assembled by two-dimensional (2D) layered materials, such as graphene, hBN and transition metal dichalcogenide (TMD), has become a fast evolving field for designing novel quantum materials. Many fascinating properties that have never been possessed by constituent monolayers have been reported, such as the famous unconventional superconductivity in twisted bilayer graphene, in which the flat band is formed at the magic angle due to electronic coupling. Specifically, the key control knob used to manipulate the vdW bilayer is through the stacking configuration, interlayer spacing and moiré pattern controlled by lattice mismatch and/or twist angle. In this talk, I will highlight recent advances in this field and discuss our work on tailoring the interlayer electronic coupling of valley excitons in vdW bilayers. By applying gigapascals high pressure, the greatly enhanced coupling strength further points to a new frontier in designing novel 2D electronic systems.
Date: Oct. 20, 1:30 PM, Location: Physics Building R019
Speaker: Cheng-Tien Chiang (IAMS)
Topic: Double photoemission with megahertz high-order harmonics
Abstract: Studying the interaction between electrons in solids has been a major research topic since the beginning of solid state physics. Despite the central role of electron-electron interaction in magnetism, superconductivity, as well as heavy Fermion systems, a direct spectroscopy on interacting electron pairs is still missing. Aiming at an ultimate tool to quantify the strength of electron-electron interaction in solids, double photoemission (DPE) spectroscopy has been developed since decades. In this talk, laser-based laboratory DPE experiments will be introduced [1]. As a laboratory light source operating at megahertz repetition rates and having a widely tunable photon energy range, high-order harmonic generation using a high power femtosecond laser has been established [2]. Based on this unique light source, band-resolved DPE experiments were performed for the first time [3].
In the DPE experiments, a pair of photoelectrons is excited upon the absorption of one single photon. The photoelectron pair is analyzed by a pair of time-of-flight electron spectrometers with angular and energy resolution [4]. On the paradigmatic metal surfaces Ag(001) and Cu(111), the characteristic binding energy of the d bands can be observed in the two-dimensional DPE spectra. As a consequence, the band character of the photoelectron pairs can be directly assigned [3]. In contrast, the intensity of photoelectron pairs from the strongly correlated material NiO increases rapidly towards lower kinetic energies [1,4]. These results will be discussed in terms of electron electron interaction in these materials.
[1] C.-T. Chiang et al., Prog. Surf. Sci. 95, 100572 (2020).
[2] C.-T. Chiang et al., New J. Phys. 17, 013035 (2015).
[3] A. Türtzschler, M. Huth, and C.-T. Chiang et al., Phys. Rev. Lett. 118, 136401 (2017). [4] M. Huth, C.-T. Chiang et al., Appl. Phys. Lett. 104, 061602 (2014).
Date: Oct. 27, 1:30 PM, Location: Physics Building R019
Speaker: Chun-Lian Lin (NCTU)
Topic: Temperature-Dependent Electronic Structures of TMD Weyl Semimetals
Abstract: Transition metal dichalcogenides (TMDs) are layered materials with chemical compositions described as MX 2 . Here, M represents an element of transition metals such as Nb, Mo, and W, and X a chalcogen atom such as S, Se, and Te. Most TMDs are semiconducting with valley degrees of freedom to generate an application in information processing. On the other hand, MoTe 2 and WTe 2 are TMDs and have been proposed as candidates for Weyl semimetals [1, 2]. Both have gathered a great deal of attention because of the quasiparticles inside them behave as massless chiral fermions−Weyl fermions. One of the unique characteristics of Weyl semimetals is the emergence of a topologically protected surface state called Fermi arc, which can be observed by STM and ARPES. Recently temperature-dependent transport properties of MoTe 2 are reported [3]. Therefore, it is urgent to reveal the temperature-dependent electronic structures of these TMD Weyl Semimetals. By using STM and STM-QPI, both structure and electronic structures of MoTe 2 and WTe 2 are clearly revealed. Surprisingly, huge variations are found in between the results measured at 5K and 77K. Our results may provide information to explain the temperature-dependent transport properties.
Reference: [1] C. L. Lin et al., J. Phys.: Condens. Matter 32, 243001(2020). [2] C. L. Lin et al., ACS Nano 11, 11459 (2017). [3] Q. L. Pei et al., Phys. Rev. B 96, 075132 (2017).
Date: Nov. 3, 1:30 PM, Location: Physics Building R019
Speaker: Ching-Hao Chang (NCKU)
Topic: Curve bilayer graphene to create new Hall effects in zero magnetic field
Abstract: The ability to engineer the electronic band structure and, more strikingly, to access new exotic phase of matter has been the cornerstone of the advance of science and technology. Here, we present (both theory and measurement) a new route to create non-trivial band structure that can realize an artificially corrugated bilayer graphene wherein the real-space and momentum-space pseudo-magnetic fields (Berry curvatures) coexist and have nontrivial properties, namely, the Berry curvature dipole. This new class of condensed-matter systems enables us to observe the so-called nonlinear anomalous Hall effect and a new type of Hall effect without breaking the time-reversal symmetry.
Date: Nov. 10, 1:30 PM, Location: Physics Building R019
Speaker: Yu-Miin Sheu (NCTU)
Topic: Antiferromagnets can be more promising for spintronics
Abstract: On-demand spin orientation with a long polarized lifetime and an easily detectable signal is the ultimate goal for spintronics. However, there still exists a trade-off between controllability and stability of spin polarization, awaiting a significant breakthrough. Here, I will demonstrate switchable optomagnet effects in (Fe1−xZnx)2Mo3O8, from which we can obtain tunable magnetization (spanning from −40% to 40% of a saturated magnetization) that is created from zero magnetization in the antiferromagnetic state without magnetic fields. It is accomplishable by utilizing circularly polarized laser pulses to excite spin-flip transitions in polar antiferromagnets that have no spin canting, traditionally hard to control without very strong magnetic fields. The spin controllability in (Fe1−xZnx)2Mo3O8 originates from its polar structure that breaks the crystal inversion symmetry, allowing distinct on-site d-d transitions for selective spin flip. By chemical doping, we exploit the phase competition between antiferromagnetic and ferrimagnetic states to enhance and stabilize the optomagnet effects, which result in long-lived photoinduced Kerr rotations.[1] Moreover, we discovered that if the ferromagnetic state becomes the ground state by field cooling or by chemical doping, the huge optomagnet effect disappears. This implies that antiferromagnets can be more promising than ferromagnet/ferrimagnet for on-demand spintronics.
Reference:
[1] Y. M. Sheu, Y. M. Chang, C. P. Chang, Y. H. Li, K. R. Babu, G. Y. Guo, T. Kurumaji, and Y. Tokura, Phys. Rev. X, 9, 031038 (2019)
Date: Nov. 17, 1:30 PM, Location: Physics Building R019
Speaker: Chung-Hou Chung (NCTU)
Topic: Quantum critical strange metal phase in paramagnetic heavy-fermion Kondo lattice
Abstract: Over the recent decades, there has been growing experimental evidences in correlated electron systems of which thermodynamic and transport properties violate the Landau’s Fermi liquid paradigm for metals. These non-Fermi liquid behaviors, ranging from unconventional superconductors, heavy-fermion metals and superconductors to magic-angle twisted bi-layered graphene, often exist near a magnetic quantum phase transition and exhibit so-called “strange metal (SM)” phenomena: with (quasi-)linear-in-temperature resistivity and singular logarithmic-in-temperature specific heat coefficient. In spite of the ubiquitous presence of SM features, the microscopic origin of them is largely un-explained, and it has become an outstanding open problem in correlated electron systems. Recently, an even more exotic quantum critical SM phase was observed in paramagnetic frustrated heavy-fermion materials near Kondo breakdown (KB) [1].
In this talk, I first take an overview of the SM phenomena. I further offer a microscopic mechanism to uncover the mystery of SM seen in Ref. [1]. This mechanism is based on competition between the Kondo correlation and the quasi-2d short-ranged antiferromagnetic resonating-valence-bond spin-liquid near the antiferromagnetic Kondo breakdown quantum critical point [2][3].We establish a controlled theoretical framework to this issue via a dynamical large-N fermionic multichannel approach to the two-dimensional Kondo-Heisenberg lattice model, where KB transition separates a heavy-Fermi liquid from fermionic spin-liquid state [4]. With Kondo fluctuations being fully considered, we find a distinct SM behavior with quasi-linear-in-temperature scattering rate associated with KB. When particle-hole symmetry is present, signatures of a critical spin-liquid SM phase as T à0 are revealed with w/T scaling extended to a wide range. We attribute these features to the interplay of critical bosonic charge (Kondo) fluctuations and gapless fermionic spinons. The implications of our results for the experiments are discussed.
Date: Nov. 24, 1:30 PM, Location: Physics Building R019
Speaker: Tay-Rong Chang (NCKU)
Topic: Topological material: Intrinsic ferromagnetic axion insulator
Abstract: In the past decade, the correlation between symmetry and topology have taken the central stage of modern physics. It has attracted intensive research interests in condensed matter physics and materials science since the discovery of various topological materials such as quantum spin Hall insulators, 3D topological insulators, and inversion-symmetry breaking Weyl semimetals. Despite tremendous progress, the majority of known topological materials are nonmagnetic while the novel magnetic topological materials have remains elusive. In this work, we predict the existence of an intrinsic ferromagnetic axion state in the new magnetic topological material MnBi8Te13 for the first time [1]. Our finding provides a superior material realization to explore zero-field QAH effect, quantized topological magnetoelectric effect, and associated phenomena.
[1] Chaowei Hu et al., Realization of an intrinsic ferromagnetic topological state in MnBi8Te13, Science Advances 6, eaba4275 (2020).
Date: Dec. 8, 1:30 PM, Location: Physics Building R019
Speaker: Yu-Ping Lin (University of Colorado Boulder)
Topic: Novel phases at Van Hove singularities in graphene moiré systems
Abstract: I will present weak-coupling renormalization group analyses and explain how various novel correlated phases can develop at the Van Hove singularities in graphene moiré systems. Motivated by the structures of low-energy flat bands on the moiré superlattices, I will focus on the two-orbital hexagonal lattice models at the Van Hove doping. First, I will show that a chiral 'high-Tc'-like phase diagram is hosted by an SU(4) symmetric model with conventional Van Hove singularity. Here a d-wave staggered-flux Chern insulator is flanked by the d-wave chiral superconducting domes on both sides of doping. Upon valley splitting which breaks the SU(4) symmetry, the phase diagram turns into a competition between chiral superconductivity and various spin and/or valley density waves. Second, I will demonstrate how various polarized ordered phases may occur when the Van Hove singularities become 'high-order'. These include the p-wave chiral and helical and d-wave chiral superconductivities, s-wave ferromagnetism, as well as f-wave and p-wave polar valley-polarized orders.
Reference:
[1] Y.-P. Lin and R. M. Nandkishore, Phys. Rev. B 100, 085136 (2019).
[2] Y.-P. Lin and R. M. Nandkishore, arXiv:2008.05485.
Date: Dec. 15, 1:30 PM, Location: Physics Building R019
Speaker: Ro-Ya Liu (NSRRC)
Topic: Introduction to Nano-ARPES and recent studies in hourglass fermion semimetal Nb3SiTe6
Abstract: Angle-resolved photoemission spectroscopy (ARPES) is one of the most direct tools to reveal the momentum-resolved electronic structure at the surface of crystalline materials. Using the wavelength-continuous VUV to soft xray high brilliant light source generated from beamlines at synchrotron radiation facilities, we are able to study electronic structures from fermi edge, valence bands, to core level electrons. However, traditional ARPES are limited to certain high-quality samples due to the macroscopic beam size, sample grounding issues and ultra-high vacuum environment.
With recent progress in beamline optical design, we are able to focus VUV beams to sub-micron scale. Materials in micron-scale, or heterostructure made by stacking layered materials, are able to be studied by Nano-ARPES, such as heterojunction TMD structure. Moreover, versatile spatial arrangements of samples are allowed, for ex. Back-gated 2D field effect devices. The electronic structure information hide in many deices samples are now can be validated by Nano-ARPES. For ex, band gap renormalization, trion state, and band diagram near sample surfaces can be directly studied by Nano-ARPES. This field opens a new era for realizing electronic structures in 2D materials, gated devices. In this talk, I will give an introduction of ARPES, synchrotron facility, my recent studies of an hourglass fermion semimetal Nb3SiTe6 as an example for ARPES, and the most frontier studies within 5 years in Nano-ARPES fields as examples.
Ref: [1] [2] [3] [4]
[1] P. V. Nguyen, N. C. Teutsch, N. P. Wilson, J. Kahn, X. Xia, A. J. Graham, V. Kandyba, A. Giampietri, A. Barinov, G. C. Constantinescu, N. Yeung, N. D. M. Hine, X. Xu, D. H. Cobden, and N. R. Wilson, Visualizing Electrostatic Gating Effects in Two-Dimensional Heterostructures, Nature 572, 220 (2019).
[2] J. Katoch, S. Ulstrup, R. J. Koch, S. Moser, K. M. McCreary, S. Singh, J. Xu, B. T. Jonker, R. K. Kawakami, A. Bostwick, E. Rotenberg, and C. Jozwiak, Giant Spin-Splitting and Gap Renormalization Driven by Trions in Single-Layer WS 2 /h-BN Heterostructures, Nat. Phys. 14, 355 (2018).
[3] J. Avila & María and C. C. Asensio, First NanoARPES User Facility Available at SOLEIL: An Innovative and Powerful Tool for Studying Advanced Materials, Synchrotron Radiat. News 27, 24 (2014).
[4] J. Avila, I. Razado-Colambo, and S. Lorcy, ANTARES, a Scanning Photoemission Microscopy Beamline at SOLEIL Related Content Interferometer-Controlled Soft X-Ray Scanning Photoemission Microscope at SOLEIL, J. Phys. Conf. Ser. OPEN ACCESS 425, 192023 (2013).
Date: Dec. 22, 1:30 PM, Location: Physics Building R019
Speaker: Tzu-Kan Hsiao (Delft, Netherlands, remote talk)
Topic: TBA
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Date: Dec. 29, 1:30 PM, Location: Physics Building R019
Speaker: Cheng-Maw Cheng (NSRRC)
Topic: TBA
Abstract:TBA
Date: Jan. 05, 1:30 PM, Location: Physics Building R019
Speaker: Hong-Yah Shih (Acdemia Sinica)
Topic: TBA
Abstract:TBA