Browsing by Author "Mou, Chung-Yu"

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Andreev reflection in two-dimensional relativistic materials with realistic tunneling transparency in normal-metal/superconductor junctions
(2017-08-17)
Chang, Yung-Yeh
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Mou, Chung-Yu
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Chung, Chung-Hou
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電子物理學系
;
Department of Electrophysics
The Andreev conductance across realistic two-dimensional (2D) normal-metal (N)/superconductor (SC) junctions with a relativistic Dirac spectrum is theoretically investigated within the Blonder-Tinkham-Klapwijk formalism with tunable tunneling transparency. It is known that due to the effect of Klein tunneling, impurity potentials at the interface of 2D relativistic materials will enhance (not suppress) the tunneling and therefore are not suitable to model a realistic tunnel junction of these materials. Here, we propose a way to construct a more realistic tunnel junction by adding a narrow, homogeneous local strain, which effectively generates a delta-gauge potential and variations of electron hopping at the interface, to adjust the transparency of the N/SC junction. Remarkable suppression of the Andreev conductance is indeed observed in the graphene N/SC junction as the strength of the local strain increases. We also explore the Andreev conductance in a topological N/SC junction at the two inequivalent Dirac points and predict the distinctive behaviors for the conductance across the chiral-to-helical topological phase transition. The relevance of our results for the adatom-doped graphene is discussed.
Doping evolution of Zhang-Rice singlet spectral weight: A comprehensive examination by x-ray absorption spectroscopy
(2013-10-31)
Chen, Y. -J.
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Jiang, M. G.
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Luo, C. W.
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Lin, J. -Y.
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Wu, K. H.
The total spectral weight S of the emergent low-energy quasiparticles in high-temperature superconductors (HTSCs) is explored by x-ray absorption spectroscopy. In order to examine the applicability of the Hubbard model, regimes that cover from zero doping to overdoping are investigated. In contrast to results of mean-field theory, we found that S deviates from linear dependence on the doping level p. The slope of S versus p changes continuously throughout the whole doping range with no sign of saturation up to p = 0.23. Therefore, the picture of the Zhang-Rice singlet remains intact within the most prominent doping regimes of HTSCs.
Duality in topological superconductors and topological ferromagnetic insulators in a honeycomb lattice
(2016-02-19)
Huang, Shin-Ming
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Tsai, Wei-Feng
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Chung, Chung-Hou
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Mou, Chung-Yu
;
電子物理學系
The ground state of the large Hubbard U limit of a honeycomb lattice near half filling is known to be a singlet d + id-wave superconductor. It is also known that this d + id superconductor exhibits a chiral p + ip pairing locally at the Dirac cone, characterized by a 2Z topological invariant. By constructing a dual transformation, we demonstrate that this 2Z topological superconductor is equivalent to a collection of two topological ferromagnetic insulators. As a result of the duality, the topology of the electronic structures for a d + id superconductor is controllable via the change of the chemical potential by tuning the gate voltage. In particular, instead of always being a chiral superconductor, we find that the d + id superconductor undergoes a topological phase transition from a chiral superconductor to a quasihelical superconductor as the gap amplitude or the chemical potential decreases. The quasihelical superconducting phase is found to be characterized by a topological invariant in the pseudospin charge sector with vanishing both the Chern number and the spin Chern number. We further elucidate the topological phase transition by analyzing the relationship between the topological invariant and the rotation symmetry. Due to the angular momentum carried by the gap function and spin-orbit interactions, we show that by placing d + id superconductors in proximity to ferromagnets, varieties of chiral superconducting phases characterized by higher Chern numbers can be accessed, providing a platform for hosting large numbers of Majorana modes at edges.
Emergence of a Fermionic Finite-Temperature Critical Point in a Kondo Lattice
(2016-04-27)
Chou, Po-Hao
;
Zhai, Liang-Jun
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Chung, Chung-Hou
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Mou, Chung-Yu
;
Lee, Ting-Kuo
The underlying Dirac point is central to the profound physics manifested in a wide class of materials. However, it is often difficult to drive a system with Dirac points across the massless fermionic critical point. Here by exploiting screening of local moments under spin-orbit interactions in a Kondo lattice, we show that below the Kondo temperature, the Kondo lattice undergoes a topological transition from a strong topological insulator to a weak topological insulator at a finite temperature T-D. At T-D, massless Dirac points emerge and the Kondo lattice becomes a Dirac semimetal. Our analysis indicates that the emergent relativistic symmetry dictates nontrivial thermal responses over large parameter and temperature regimes. In particular, it yields critical scaling behaviors both in magnetic and transport responses near T-D.
Enhanced quantum oscillations in Kondo insulators
(2020-03-02)
Lu, Yen-Wen
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Chou, Po-Hao
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Chung, Chung-Hou
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Lee, Ting-Kuo
;
Mou, Chung-Yu
Quantum oscillations have long been regarded as the manifestation of the Fermi surface in metals. However, they were recently observed in Kondo insulators. We examine the Kondo screening due to Landau levels in Kondo insulators. It is shown that, even for large Kondo insulating gaps, appreciable amplitudes of quantum oscillations that are consistent with experimental observations are present both in magnetization and resistivity. Specifically, we show that, due to the periodic alignment between the Landau levels in the conduction and the f-orbit electrons, the Kondo screening itself undergoes oscillations so that the electronic structure oscillates with the magnetic field. Our results explain main features of quantum oscillations observed in experiments. They indicate that the nonrigidity of the electronic structure results in observable quantum oscillation Kondo insulators. This effect provides a way to probe the Fermi surface geometry of insulators.
Magnetic Transitions of Multiferroic Frustrated Magnets Revealed by Resonant Soft X-ray Magnetic Scattering
(2010-01-01)
Huang, Di-Jing
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Okamoto, Jun
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Huang, Shih-Wen
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Mou, Chung-Yu
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電子物理學系
The coexistence of magnetism and ferroelectricity with cross coupling, termed multiferroicity, rarely occurs. The discovery of gigantic magnetoelectric coupling in frustrated magnets has revived interest in their multiferroic behavior. Here, we review the measurements of resonant soft-X-ray magnetic scattering in the multiferroic frustrated magnets TbMn(2)O(5), LiCu(2)O(2), and CoCr(2)O(4). In addition to the experimental technique used, the evolution of the wave vector of magnetic ordering about the temperature of multiferroic transitions is discussed. We proffer scattering evidence of multiferroicity and a pathway for understanding the intricate coupling between magnetism and ferroelectricity in magnets with spin spirals. Our results also reveal the low-dimension nature of a quantum spin-chain multiferroics and the evolution of the interrelation between the polarization P, the magnetization M, and the spiral wave vector Q.
Pairing symmetry and upward curvature of the upper critical field in superconducting Na0.35CoO2 center dot yH(2)O
(2007-01-01)
Kao, Jun-Ting
;
Lin, Jiunn-Yuan
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Mou, Chung-Yu
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物理研究所
;
Institute of Physics
The origin for the upward curvature of the upper critical field (H-c2) observed in hydrate cobaltate Na0.35CoO2 center dot yH(2)O is investigated based on the microscopic gap equation. It is shown that the observed upward curvature results from the transition between two different pairing symmetries that occur on different energy bands. Furthermore, different pairing symmetries involved in the transition results in different upward curvatures. By considering transitions among all lowest possible pairing symmetries, it is found that the transition of the pairing symmetry from s-wave at low fields to d(x)(2)-y(2)+id(xy) at high fields is the best fit to the experimental data. Our results provide an important clue to the understanding of the superconductivity in hydrate cobaltate.
Quantum phase transition in a two-dimensional Kondo-Heisenberg model: A dynamical Schwinger-boson large-N approach
(2020-09-15)
Wang, Jiangfan
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Chang, Yung-Yeh
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Mou, Chung-Yu
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Kirchner, Stefan
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Chung, Chung-Hou
We generalize the dynamical large-N multichannel Schwinger-boson approach to study the quantum phase transition of a two-dimensional Kondo-Heisenberg model, relevant for heavy electron systems, close to an antiferromagnetic Kondo destruction quantum critical point. By breaking up a Kondo singlet into a charge fermion (holon) and a bosonic spinon, we identify and characterize the quantum phase transition from an antiferromagnetically ordered state to a Kondo-dominated paramagnetic state, and attribute a jump in the holon phase shift to Kondo breakdown. We calculate transport and thermodynamic quantities. The global finite-temperature phase diagram and the critical behavior of various physical observables therein show a close resemblance to experimental observations.
Strain-induced superconducting pair density wave states in graphene
(2018-11-02)
Xu, Feng
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Chou, Po-Hao
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Chung, Chung-Hou
;
Lee, Ting-Kuo
;
Mou, Chung-Yu
Graphene is known to be nonsuperconducting. However, surprising superconductivity is recently discovered in a flat band in a twisted bilayer graphene. Here, we show that superconductivity can be more easily realized in topological flat bands induced by strain in graphene through periodic ripples. Specifically, it is shown that by including correlation effects, the chiral d-wave superconductivity can be stabilized under strain even for slightly doped graphene. The chiral d-wave superconductivity generally coexists with charge density waves (CDW) and pair density waves (PDW) of the same period. Remarkably, a pure PDW state with doubled period that coexists with the CDW state is found to emerge at a finite-temperature region under reasonable strain strength. The emergent PDW state is shown to be superconducting with nonvanishing superfluid density, and it realizes the long-sought-after superconducting states with nonvanishing center-of-mass momentum for Cooper pairs.
Tunable topological semimetallic phases in Kondo lattice systems
(2019-01-22)
Lu, Yen-Wen
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Chou, Po-Hao
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Chung, Chung-Hou
;
Mou, Chung-Yu
;
電子物理學系
We exploit topological semimetallic phases resulting from the Kondo screening in Anderson lattice models. It is shown that by including spin-orbit interactions both in the bulk electrons and in the hybridization between the conduction electrons and electrons in the f orbit, all types of topological semimetallic phases can be realized in Anderson lattice models. Specifically, upon either broken time-reversal symmetry or broken inversion symmetry, we find that either the Weyl semimetallic phase, Dirac semimetallic phase, or nodal-ring semimetallic phases always emerge between insulating phases and can be accessed by tuning either temperature or spin-orbit interaction. For Anderson lattice models with general three-dimensional spin-orbit hybridization between the conduction electrons and electrons in the f orbit, we find that Weyl nodal-ring semimetallic phases emerge between strong and weak topological insulating phases. Furthermore, in the presence of an exchange field, Weyl semimetallic phases form after two Weyl points of charge +/- 1 split off from a Dirac point at time-reversal momenta. On the other hand, when the spin-orbit interaction is included in the conduction electron, we find that upon the rotation symmetry being broken with anisotropic hopping amplitudes, a Weyl semimetallic phase emerges with a double Weyl node with charges of +/- 2. Furthermore, the Weyl semimetallic phases with charges of +/- 2 can be tuned into Weyl semimetallic phases with charges of +/- 1 through the inclusion of the Rashba spin-orbit interaction. Our analyses indicate that Anderson lattices with appropriate spin-orbit interactions provide a platform for realizing all types of topological semimetallic phases.