Showing 1–50 of 82 results for author: Mou, C

Quantum critical behavior of cuprate superconductors observed by inelastic X-ray scattering

Authors: H. Y. Huang, C. Y. Mou, A. Singh, J. S. Su, J. Okamoto, S. Komiya, C. T. Chen, T. K. Lee, A. Fujimori, D. J. Huang

Abstract: Progress toward a complete understanding of cuprate superconductors has been hindered by their intricate phase diagram, potentially linked to a quantum critical point (QCP). However, conclusive evidence for the QCP is lacking, as the presumed QCP is buried under the superconducting dome, disguising its presence. Here, we use high-resolution resonant inelastic X-ray scattering to examine the dynami… ▽ More Progress toward a complete understanding of cuprate superconductors has been hindered by their intricate phase diagram, potentially linked to a quantum critical point (QCP). However, conclusive evidence for the QCP is lacking, as the presumed QCP is buried under the superconducting dome, disguising its presence. Here, we use high-resolution resonant inelastic X-ray scattering to examine the dynamical charge-charge correlation in La$_{2-x}$Sr$_x$CuO$_4$ and uncover the quantum critical scaling, a key feature required for a QCP. Specifically, \djh{we observed that the inverse correlation lengths for various dopings and temperatures collapsed onto a universal scaling curve, yielding a critical exponent $ν$ of $0.74 \pm 0.08$. The non-negativity of this exponent confirms the presence of a QCP. Remarkably, the value of $ν$ suggests that while the QCP is manifested through the charge-density wave, other orders also participate, such that the QCP appears to belong to the universality class characterized by the O(4) symmetry, reminiscent of the microscopic SO(4) symmetry in the Hubbard model at half-filling. Further analysis indicates that the QCP is highly dissipative with a short quasi-particle lifetime, reflecting the intertwined quantum fluctuations due to its being buried inside the superconducting state. △ Less

Submitted 25 February, 2026; v1 submitted 11 February, 2026; originally announced February 2026.

Non-reciprocal circular dichroism of axial phonons coupled to ferro-rotational order

Authors: H. Y. Huang, G. Channagowdra, D. Banerjee, E. V. Komleva, J. Okamoto, C. T. Chen, M. Guennou, S. Johnston, S. V. Streltsov, C. Y. Mou, A. Fujimori, S-W. Cheong, D. J. Huang

Abstract: Circular dichroism (CD) in X-ray absorption, defined as the difference in absorption between opposite circular polarizations, is fundamentally enabled by the breaking of time-reversal symmetry or inversion symmetry. It is therefore sensitive to magnetism, chirality, and their interplay. In contrast, the sample symmetry alone is insufficient to determine whether CD in resonant inelastic X-ray scatt… ▽ More Circular dichroism (CD) in X-ray absorption, defined as the difference in absorption between opposite circular polarizations, is fundamentally enabled by the breaking of time-reversal symmetry or inversion symmetry. It is therefore sensitive to magnetism, chirality, and their interplay. In contrast, the sample symmetry alone is insufficient to determine whether CD in resonant inelastic X-ray scattering (RIXS) is allowed. Rather, RIXS-CD is governed by both the sample symmetry and the scattering geometry. Here, using RIXS, we identify circularly polarized phonons coupled to ferro-rotational order in MnTiO$_3$, which we refer to as ferro-axial phonons. Their excitations provide a direct demonstration of non-reciprocal RIXS-CD, in which the dichroic response changes upon reversing the propagation direction of the incident X-rays, although the system globally preserves both inversion and time-reversal symmetries. We propose that a condensate of these phonons, manifested as standing waves, underlies the ferro-rotational order in MnTiO$_3$. The observed non-reciprocal CD reflects the interplay among photon helicity, phonon polarization, and ferro-rotational order. △ Less

Submitted 7 August, 2026; v1 submitted 26 December, 2025; originally announced December 2025.

Altermagnetic boosting of chiral phonons

Authors: J. Okamoto, C. Y. Mou, H. Y. Huang, G. Channagowdra, C. Won, K. Du, X. Fang, E. V. Komleva, C. T. Chen, S. V. Streltsov, A. Fujimori, S-W. Cheong, D. J. Huang

Abstract: Chirality characterizes the asymmetry between a structure and its mirror image and underlies a wide range of chiral functionalities. In crystallographically chiral materials, phonons with non-zero linear momentum $\textbf{k}$ can acquire a $k$-induced longitudinal magnetization, giving rise to chiral phonons. Helical spin order, with its proper screw-type configuration, breaks all mirror symmetrie… ▽ More Chirality characterizes the asymmetry between a structure and its mirror image and underlies a wide range of chiral functionalities. In crystallographically chiral materials, phonons with non-zero linear momentum $\textbf{k}$ can acquire a $k$-induced longitudinal magnetization, giving rise to chiral phonons. Helical spin order, with its proper screw-type configuration, breaks all mirror symmetries and therefore carries magnetic chirality. Such helical spins also generate non-relativistic spin splitting for any quasiparticle excitations propagating along the screw axis. To explore the possible connection between chiral phonons and magnetic chirality, we investigated the crystallographically polar and chiral compound (Mn,Ni)$_3$TeO$_6$, which hosts three distinct states: a paramagnetic state, a helical spin state with magnetic chirality, and a collinear spin state without magnetic chirality. We find an approximately tenfold enhancement of chiral-phonon coupling in the helical spin state along the screw axis, compared with both the paramagnetic and collinear spin states. These results identify a new route to amplify chiral phonons through an altermagnetic effect arising from the broken parity-time symmetry in helical spins. %from non-relativistic spin splitting. △ Less

Submitted 29 November, 2025; originally announced December 2025.

Comments: 8 pages, 5 figures. (Supplement: 9 pages, 1 fiture)

Topological electronic structures of non-collinear magnetic phases in a multi-orbital Hubbard model with spin-orbit interactions

Authors: Ying-Lin Li, Po-Hao Chou, Chung-Yu Mou

Abstract: We explore topological electronic structure of magnetic phases in a multi-orbital Hubbard model with spin-orbit interactions. To account for more general antiferromagnetic orders that go beyond the collinear Néel order, two different spin-orbit interactions, Dresselhaus and Rashba spin-orbit interactions, are considered. By performing the canonical transformation, we derive the corresponding gener… ▽ More We explore topological electronic structure of magnetic phases in a multi-orbital Hubbard model with spin-orbit interactions. To account for more general antiferromagnetic orders that go beyond the collinear Néel order, two different spin-orbit interactions, Dresselhaus and Rashba spin-orbit interactions, are considered. By performing the canonical transformation, we derive the corresponding generalized t-J model. At half filling, employing self-consistent magnetic order calculations, we find distinctive spin arrangements under Dresselhaus or Rashba spin-orbit interactions. For the Dresselhaus spin-orbit interaction, the spin configuration exhibits collinear antiferromagnetic order. On the other hand, Rashba interaction results in spins antiferromagnetically aligning in xy-plane and a small interaction controlled by hopping parameter induces spin tilting, causing antiferromagnetic alignment in xy-plane but ferromagnetic alignment in z-direction. We categorize topological properties of these phases for low doping in the generalized t-J model.: for 3D collinear antiferromagnetic order, the system possesses a modified time-reversal symmetry, characterized by the Z2 index. In contrast, for systems with tilted antiferromagnetic orders, it is protected by inversion symmetry and characterized by the Z4 index. We further examine the bulk-edge correspondence for non-collinear magnetic phases, revealing that the surface state becomes gapless when the surface is parallel to the ferromagnetic component of tilted antiferromagnetic order; otherwise, the surface state exhibits a gap. Our findings offer a comprehensive topological characterization for doped and canted antiferromagnetic insulators with spin-orbit interactions, providing valuable insights into the interplay between spin arrangements, symmetries, and topological properties in systems governed by the multi-orbital Hubbard model. △ Less

Submitted 10 July, 2025; originally announced July 2025.

Comments: 11 pages, 17 figures, submitted to Phys. Rev. B

Journal ref: Phys.Rev. B 112, 115151 (2025)

Quantum Spin Liquid phases in Kitaev Materials

Authors: Po-Hao Chou, Chung-Yu Mou, Chung-Hou Chung, Sungkit Yip

Abstract: We develop a gauge-invariant renormalized mean-field theory (RMFT) to reliably find the quantum spin liquid (QSL) states and their field response for realistic Kitaev materials under strong magnetic fields and described by the generalized Kitaev $J$-$K$-$Γ$-$Γ'$ model. Remarkably, while our RMFT reproduces previous results based on using more complicated numerical methods, it also predicts several… ▽ More We develop a gauge-invariant renormalized mean-field theory (RMFT) to reliably find the quantum spin liquid (QSL) states and their field response for realistic Kitaev materials under strong magnetic fields and described by the generalized Kitaev $J$-$K$-$Γ$-$Γ'$ model. Remarkably, while our RMFT reproduces previous results based on using more complicated numerical methods, it also predicts several new stable QSL states. In particular, since Kitaev spin liquid (KSL) is no longer a saddle point solution, a new exotic 2-cone state distinct from the KSL, is found to describe experimental observations well, and hence should be the candidate state realized in the Kitaev material, $α$-RuCl$_3$. We further explore the mechanism for the suppression of the observed thermal Hall conductivity at low temperatures within the fermionic framework, and show that the polar-angle dependence of the fermionic gap can distinguish the found 2-cone state from the KSL state in further experiments. △ Less

Submitted 4 January, 2026; v1 submitted 13 March, 2025; originally announced March 2025.

Journal ref: npj Quantum Materials, 10, Article number: 90 (2025)

Proximity-Induced Nodal Metal in an Extremely Underdoped CuO$_2$ Plane in Triple-Layer Cuprates

Authors: Shin-ichiro Ideta, Shintaro Adachi, Takashi Noji, Shunpei Yamaguchi, Nae Sasaki, Shigeyuki Ishida, Shin-ichi Uchida, Takenori Fujii, Takao Watanabe, Wen O. Wang, Brian Moritz, Thomas P. Devereaux, Masashi Arita, Chung-Yu Mou, Teppei Yoshida, Kiyohisa Tanaka, Ting-Kuo Lee, Atsushi Fujimori

Abstract: ARPES studies have established that the high-$T_c$ cuprates with single and double CuO$_2$ layers evolve from the Mott insulator to the pseudogap state with a Fermi arc, on which the superconducting (SC) gap opens. In four- to six-layer cuprates, on the other hand, small hole Fermi pockets are formed in the innermost CuO$_2$ planes, indicating antiferromagnetism. Here, we performed ARPES studies o… ▽ More ARPES studies have established that the high-$T_c$ cuprates with single and double CuO$_2$ layers evolve from the Mott insulator to the pseudogap state with a Fermi arc, on which the superconducting (SC) gap opens. In four- to six-layer cuprates, on the other hand, small hole Fermi pockets are formed in the innermost CuO$_2$ planes, indicating antiferromagnetism. Here, we performed ARPES studies on the triple-layer Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+δ}$ over a wide doping range, and found that, although the doping level of the inner CuO$_2$ plane was extremely low in underdoped samples, the $d$-wave SC gap was enhanced to the unprecedentedly large value of $Δ_0\sim$100 meV at the antinode and persisted well above $T_{c}$ without the appearance of a Fermi arc, indicating a robust ``nodal metal''. We attribute the nodal metallic behavior to the unique local environment of the inner clean CuO$_2$ plane in the triple-layer cuprates, sandwiched by nearly optimally-doped two outer CuO$_2$ planes and hence subject to strong proximity effect from both sides. In the nodal metal, quasiparticle peaks showed electron-hole symmetry, suggesting $d$-wave pairing fluctuations. Thus the proximity effect on the innermost CuO${_2}$ plane is the strongest in the triple-layer cuprates, which explains why the $T_c$ reaches the maximum at the layer number of three in every multi-layer cuprate family. △ Less

Submitted 21 February, 2025; originally announced February 2025.

Journal ref: Nature Communications 16, 9470 (2025)

Giant X-ray circular dichroism in a time-reversal invariant altermagnet

Authors: Jun Okamoto, Ru-Pan Wang, Yen-Yi Chu, Hung-Wei Shiu, Amol Singh, Hsiao-Yu Huang, Chung-Yu Mou, Sucitto Teh, Horng-Tay Jeng, Kai Du, Xianghan Xu, Sang-Wook Cheong, Chao-Hung Du, Chien-Te Chen, Atsushi Fujimori, Di-Jing Huang

Abstract: X-ray circular dichroism, arising from the contrast in X-ray absorption between opposite photon helicities, serves as a spectroscopic tool to measure the magnetization of ferromagnetic materials and identify the handedness of chiral crystals. Antiferromagnets with crystallographic chirality typically lack X-ray magnetic circular dichroism because of time-reversal symmetry, yet exhibit weak X-ray n… ▽ More X-ray circular dichroism, arising from the contrast in X-ray absorption between opposite photon helicities, serves as a spectroscopic tool to measure the magnetization of ferromagnetic materials and identify the handedness of chiral crystals. Antiferromagnets with crystallographic chirality typically lack X-ray magnetic circular dichroism because of time-reversal symmetry, yet exhibit weak X-ray natural circular dichroism. Here, we report the observation of giant natural circular dichroism in the Ni $L_3$-edge X-ray absorption of Ni$_3$TeO$_6$, a polar and chiral antiferromagnet with effective time-reversal symmetry. To unravel this intriguing phenomenon, we propose a phenomenological model that classifies the movement of photons in a chiral crystal within the same symmetry class as that of a magnetic field. The coupling of X-ray polarization with the induced magnetization yields giant X-ray natural circular dichroism, revealing the altermagnetism of Ni$_3$TeO$_6$. Our findings provide evidence for the interplay between magnetism and crystal chirality in natural optical activity. Additionally, we establish the first example of a new class of magnetic materials exhibiting circular dichroism with time-reversal symmetry. △ Less

Submitted 23 February, 2024; v1 submitted 12 December, 2023; originally announced December 2023.

Comments: Accepted by Advanced Materials (2024.2.16) Revised title: Giant X-ray circular dichroism in a time-reversal invariant altermagnet Revised drafts: Main 14 pages, 4 figures, and SI 20 pages, 8 figures

Journal ref: adv. mater. 36, 2309172 (2024)

Manifestation of chiral magnetic current in Floquet-Weyl semimetals

Authors: Tsung-Yu Chen, Po-Hao Chou, Chung-Yu Mou

Abstract: Materials that can host macroscopic persistent current are important because they are useful for energy storage. However, there are very few examples of such materials in nature. Superconductors are known as an example in which flow of supercurrent can persist up to 100,000 years. The chiral magnetic current is possibly the second example predicted by the chiral magnetic effect. It was proposed to… ▽ More Materials that can host macroscopic persistent current are important because they are useful for energy storage. However, there are very few examples of such materials in nature. Superconductors are known as an example in which flow of supercurrent can persist up to 100,000 years. The chiral magnetic current is possibly the second example predicted by the chiral magnetic effect. It was proposed to be realized in recently discovered Weyl semimetals. However, a no-go theorem negates the chiral magnetic effect and shows that the chiral magnetic current is generally absent in any equilibrium condensed-matter system. Here we show how to break the no-go theorem by resorting to dynamical transitions in time-frequency space. By driving an insulator using a time-periodic potential and coupling it to a phonon heat bath that provides suitable dissipation, we show that a Floquet-Weyl semi-metallic phase with Fermi-Dirac-like distribution emerges. Furthermore, we show that even in the presence of a static magnetic field, the resulting steady Floquet-Weyl semimetal supports non-vanishing chiral magnetic current. Our dynamical model provides a systematic way to fully realize the chiral magnetic effect in condensed matter systems. △ Less

Submitted 13 July, 2023; originally announced July 2023.

Comments: 7 page, 4 figures

Journal ref: Phys. Rev. B 108, 035120 (2023)

Precise dd excitations and commensurate intersite Coulomb interactions in the dissimilar cuprate YBa_2Cu_3O_(7-x) and La_(2-x)Sr_xCuO_4

Authors: Shih-Wen Huang, L. Andrew Wray, Yu-Cheng Shao, Cheng-Yau Wu, Shun-Hung Wang, Jenn-Min Lee, Y-J. Chen, R. W. Schoenlein, C. Y. Mou, Yi-De Chuang, J. -Y. Lin

Abstract: Using high-resolution extreme ultraviolet resonant inelastic X-ray scattering (EUVRIXS) spectroscopy at Cu M-edge, we observed the doping dependent spectral shifts of inter-orbital (dd) excitations of YBa_2Cu_3O_(7-x) and La_(2-x)Sr_xCuO_4. With increasing hole doping level from undoped to optimally doped superconducting compositions, the leading edge of dd excitations is found to shift towards lo… ▽ More Using high-resolution extreme ultraviolet resonant inelastic X-ray scattering (EUVRIXS) spectroscopy at Cu M-edge, we observed the doping dependent spectral shifts of inter-orbital (dd) excitations of YBa_2Cu_3O_(7-x) and La_(2-x)Sr_xCuO_4. With increasing hole doping level from undoped to optimally doped superconducting compositions, the leading edge of dd excitations is found to shift towards lower energy loss in a roughly linear trend that is irrespective to the cuprate species. The magnitude of energy shift can be explained by including a 0.15 eV Coulomb attraction between Cu 3d_(x^2-y^2) electrons and the doped holes on the surrounding oxygens in the atomic multiplet calculations. The consistent energy shift between distinct cuprate families suggests that this inter-site Coulomb interaction energy scale is relatively material-independent, and provides an important reference point for understanding charge density wave phenomena in the cuprate phase diagram. △ Less

Submitted 18 April, 2023; originally announced April 2023.

Comments: 29 pages; 8 figures. Physical Review B, in press. This paper reveals a Cu 3d-O 2p intersite interaction energy for the first time experimentally. It also explains why Tc of YBCO is higher than that of LSCO

Topological Kondo Superconductors

Authors: Yung-Yeh Chang, Khoe Van Nguyen, Kuang-Lung Chen, Yen-Wen Lu, Chung-Yu Mou, Chung-Hou Chung

Abstract: Spin-triplet $p$-wave superconductors are promising candidates for topological superconductors. They have been proposed in various heterostructures where a material with strong spin-orbit interaction is coupled to a conventional $s$-wave superconductor by proximity effect. However, topological superconductors existing in nature and driven purely by strong electron correlations are yet to be studie… ▽ More Spin-triplet $p$-wave superconductors are promising candidates for topological superconductors. They have been proposed in various heterostructures where a material with strong spin-orbit interaction is coupled to a conventional $s$-wave superconductor by proximity effect. However, topological superconductors existing in nature and driven purely by strong electron correlations are yet to be studied. Here we propose a realization of such a system in a class of Kondo lattice materials in the absence of spin-orbit coupling and proximity effect. Therein, the odd-parity Kondo hybridization mediates ferromagnetic spin-spin coupling and leads to spin-triplet resonant-valence-bond ($t$-RVB) pairing between local moments. Spin-triplet $p\pm i p^\prime$-wave topological superconductivity is reached when Kondo effect co-exists with $t$-RVB. We identify the topological nature by the non-trivial topological invariant and the Majorana fermions at edges. Our results offer a comprehensive understanding of experimental observations on UTe$_2$, a U-based ferromagnetic heavy-electron superconductor. △ Less

Submitted 2 January, 2023; originally announced January 2023.

Comments: 13 pages, 7 figures

Electrically Sign-Reversible Topological Hall Effect in a Top-Gated Topological Insulator (Bi,Sb)2Te3 on a Ferrimagnetic Insulator Europium Iron Garnet

Authors: Jyun-Fong Wong, Ko-Hsuan Mandy Chen, Jui-Min Chia, Zih-Ping Huang, Sheng-Xin Wang, Pei-Tze Chen, Lawrence Boyu Young, Yen-Hsun Glen Lin, Shang-Fan Lee, Chung-Yu Mou, Minghwei Hong, Jueinai Kwo

Abstract: Topological Hall effect (THE), an electrical transport signature of systems with chiral spin textures like skyrmions, has been observed recently in topological insulator (TI)-based magnetic heterostructures. However, the intriguing interplay between the topological surface state and THE is yet to be fully understood. In this work, we report a large THE of ~10 ohm (~4 micro-ohm*cm) at 2 K with an e… ▽ More Topological Hall effect (THE), an electrical transport signature of systems with chiral spin textures like skyrmions, has been observed recently in topological insulator (TI)-based magnetic heterostructures. However, the intriguing interplay between the topological surface state and THE is yet to be fully understood. In this work, we report a large THE of ~10 ohm (~4 micro-ohm*cm) at 2 K with an electrically reversible sign in a top-gated 4 nm TI (Bi0.3Sb0.7)2Te3 (BST) grown on a ferrimagnetic insulator (FI) europium iron garnet (EuIG). Temperature, external magnetic field angle, and top gate bias dependences of magnetotransport properties were investigated and consistent with a skyrmion-driven THE. Most importantly, a sign change in THE was discovered as the Fermi level was tuned from the upper to the lower parts of the gapped Dirac cone and vice versa. This discovery is anticipated to impact technological applications in ultralow power skyrmion-based spintronics. △ Less

Submitted 13 April, 2023; v1 submitted 31 December, 2022; originally announced January 2023.

Quantum oscillations of Kondo screening phases in strong magnetic fields

Authors: Po-Hao Chou, Chung-Hou Chung, Chung-Yu Mou

Abstract: We generalize the iterative diagonalization procedure adopted in method of numerical renormalization group to analyze the Kondo effect in strong magnetic fields, where the density of states for itinerary electrons at the chemical potential varies discontinuously as the magnetic field changes. We first examine phases of many-body ground states in the presence of single impurity. By investigating ch… ▽ More We generalize the iterative diagonalization procedure adopted in method of numerical renormalization group to analyze the Kondo effect in strong magnetic fields, where the density of states for itinerary electrons at the chemical potential varies discontinuously as the magnetic field changes. We first examine phases of many-body ground states in the presence of single impurity. By investigating change of $z$-component of total spin, $ΔS_z$, and spin-spin correlation between the impurity and conduction electrons, we find that there are three states competing for the ground state when Zeeman splitting is present. One of the states is doublet in which the impurity spin is unscreened. The other two states are Kondo screening states with $ΔS_z=1/2$ and $ΔS_z=1$, in which the impurity spin is partially screened and completely screened respectively. For Kondo systems with two-impurities in strong magnetic fields, we find that the interplay between the Kondo screening effect, RKKY interaction, and quantum oscillations due to Landau levels determines the ground state of the system. Combination of these three factors results in different screening scenarios for different phases in which spins of two impurities can form spin-0 or spin-1 states, while impurity spins in these phases can be either screened, partially screened, or unscreened by conduction electrons. The emergence of the ground state from these competing states oscillates with the change of magnetic field, chemical potential or inter-impurity distance. This leads to quantum oscillations in magnetization and conductivity. In particular, we find extra peak structures in longitudinal conductivity that reflect changes of Kondo screening phases and are important features to be observed in experiments. Our results provide a complete characterization of phases for Kondo effect in strong magnetic fields. △ Less

Submitted 4 November, 2022; v1 submitted 1 November, 2022; originally announced November 2022.

Comments: 14 pages, 9 figures

Journal ref: Phys. Rev. B, 106, 195107 (2022)

Quantum fluctuations of charge order induce phonon softening in a superconducting cuprate

Authors: H. Y. Huang, A. Singh, C. Y. Mou, S. Johnston, A. F. Kemper, J. van den Brink, P. J. Chen, T. K. Lee, J. Okamoto, Y. Y. Chu, J. H. Li, S. Komiya, A. C. Komarek, A. Fujimori, C. T. Chen, D. J. Huang

Abstract: Quantum phase transitions play an important role in shaping the phase diagram of high-temperature cuprate superconductors. These cuprates possess intertwined orders which interact strongly with superconductivity. However, the evidence for the quantum critical point associated with the charge order in the superconducting phase remains elusive. Here we show the short-range charge orders and the spec… ▽ More Quantum phase transitions play an important role in shaping the phase diagram of high-temperature cuprate superconductors. These cuprates possess intertwined orders which interact strongly with superconductivity. However, the evidence for the quantum critical point associated with the charge order in the superconducting phase remains elusive. Here we show the short-range charge orders and the spectral signature of the quantum fluctuations in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) near the optimal doping using high-resolution resonant inelastic X-ray scattering. On performing calculations through a diagrammatic framework, we discovered that the charge correlations significantly soften several branches of phonons. These results elucidate the role of charge order in the LSCO compound, providing evidence for quantum critical scaling and discommensurations associated with charge order. △ Less

Submitted 26 September, 2021; v1 submitted 25 August, 2021; originally announced August 2021.

Comments: 9 pages, 4 figures

Journal ref: Phys. Rev. X 11, 041038 (2021)

Geometry-induced topological superconductivity

Authors: Po-Hao Chou, Chia-Hsin Chen, Shih-Wei Liu, Chung-Hou Chung, Chung-Yu Mou

Abstract: Intrinsic topological superconductors with p-wave pairing are rare in nature. Its underlying reason is due to the fact that it is usually difficult to change the relative strength between the singlet and triplet channels for the electron-electron interaction in material. Here we show that by considering superconductivity occurring on surfaces of topological insulators (TIs), the relative strength… ▽ More Intrinsic topological superconductors with p-wave pairing are rare in nature. Its underlying reason is due to the fact that it is usually difficult to change the relative strength between the singlet and triplet channels for the electron-electron interaction in material. Here we show that by considering superconductivity occurring on surfaces of topological insulators (TIs), the relative strength between the singlet and triplet channels can be changed by geometry and sizes of TIs. Specifically, we show that pairing of electrons at different locations on the surface of a topological insulator generally tends to favor the triplet pairing and can induce topological superconductivity by controlling the surface curvature and size of the topological insulator. We illustrate the effects in two configurations, thin film geometry and the spherical geometry with a sphere or a hemisphere, and find that topological superconductivity arises with the $p \pm ip$ pairing symmetry dominated in nanoscale size of the TI. As a consequence, vortices can spontaneously form on surfaces of topological insulators with roughness of appropriate curvature. These vortices support a Majorana zero mode inside each core and can be used as a platform to host Majorana zero modes without invoking real magnetic fields. Our theoretical discovery opens a new route to realize topological superconductivity in material. △ Less

Submitted 7 January, 2021; originally announced January 2021.

Comments: 8 pages, 6 figures, to appear in Phys Rev B

Journal ref: Phys. Rev. B 103, 014508, 2021

Superfluid weight and Berezinskii-Kosterlitz-Thouless transition temperature of strained graphene

Authors: Feng Xu, Lei Zhang, Liyun Jiang, Wendeng Huang, Chung-Yu Mou

Abstract: We obtain the superfluid weight and Berezinskii-Kosterlitz-Thouless (BKT) transition temperature for highly unconventional superconducting states with the coexistence of chiral d-wave superconductivity, charge density waves and pair density waves in the strained graphene. Our results show that the strain-induced flat bands can promote the superconducting transition temperature approximately… ▽ More We obtain the superfluid weight and Berezinskii-Kosterlitz-Thouless (BKT) transition temperature for highly unconventional superconducting states with the coexistence of chiral d-wave superconductivity, charge density waves and pair density waves in the strained graphene. Our results show that the strain-induced flat bands can promote the superconducting transition temperature approximately $50\%$ compared to that of the original doped graphene, which suggests that the flat-band superconductivity is a potential route to get superconductivity with higher critical temperatures. In particular, we obtain the superfluid weight for the pure superconducting pair-density-wave states from which the deduced superconducting transition temperature is shown to be much lower than the gap-opening temperature of the pair density wave, which is helpful to understand the phenomenon of the pseudogap state in high-$T_c$ cuprate superconductors. Finally, we show that the BKT transition temperature versus doping for strained graphene exhibits a dome-like shape and it depends linearly on the spin-spin interaction strength. △ Less

Submitted 1 January, 2021; originally announced January 2021.

Enhanced quantum oscillations in Kondo insulators

Authors: Yen-Wen Lu, Po-Hao Chou, Chung-Hou Chung, Ting-Kuo Lee, Chung-Yu Mou

Abstract: 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 i… ▽ More 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 non-rigidity of the electronic structure results in observable quantum oscillations Kondo insulators. This new effect provides a new way to probe the Fermi surface geometry of insulators. △ Less

Submitted 27 February, 2020; originally announced February 2020.

Comments: 7 pages, 6 figures, to appear in Phys Rev B

Journal ref: Phys Rev B 101, 115102 (2020)

Quantum phase transition in a two-dimensional Kondo-Heisenberg model: a Schwinger-boson large-N approach

Authors: Jiangfan Wang, Yung-Yeh Chang, Chung-Yu Mou, Stefan Kirchner, Chung-Hou Chung

Abstract: Strange metal behavior arises in heavy fermion metals close to antiferromagnetic transitions. An increasing amount of experiments indicates a link of such behavior to a Kondo breakdown quantum critical point. To shed light on this intriguing problem, we study the 2D Kondo-Heisenberg model using a dynamical large-N multichannel Schwinger boson approach. We identify and characterize the quantum phas… ▽ More Strange metal behavior arises in heavy fermion metals close to antiferromagnetic transitions. An increasing amount of experiments indicates a link of such behavior to a Kondo breakdown quantum critical point. To shed light on this intriguing problem, we study the 2D Kondo-Heisenberg model using a dynamical large-N multichannel Schwinger boson approach. We identify and characterize the quantum phase transition from an antiferromagnetically ordered ground state to a Kondo-dominated paramagnetic state, and attribute a jump in certain phase shift to Kondo breakdown. In addition, we calculate transport and thermodynamic quantities and discuss them in the context of the experimental observations in quantum critical heavy fermion systems. △ Less

Submitted 12 February, 2019; v1 submitted 29 January, 2019; originally announced January 2019.

Comments: 5 pages, 4 figures. Title, abstract, author list, introduction, Fig. 3 revised. Typos corrected

Journal ref: Phys. Rev. B 102, 115133 (2020)

Tunable topological semi-metallic phases in Kondo lattice systems

Authors: Yen-Wen Lu, Po-Hao Chou, Chung-Hou Chung, Chung-Yu Mou

Abstract: We exploit topological semi-metallic 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 $f$ orbit, all types of topological semi-metallic phases can be realized in Anderson lattice models. Specifically, upon either time-rever… ▽ More We exploit topological semi-metallic 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 $f$ orbit, all types of topological semi-metallic phases can be realized in Anderson lattice models. Specifically, upon either time-reversal symmetry broken or inversion symmetry broken, we find that either Weyl semi-metallic phase, Dirac semi-metallic phase or nodal-ring semi-metallic phases always emerge between insulating phases and can be accessed by tuning either temperature or spin-orbit interaction. For Anderson lattice models with general 3D spin-orbit hybridization between the conduction electrons and electrons in $f$ orbit, we find that Weyl nodal-ring semi-metallic phases emerges between strong and weak topological insulating phases. Furthermore, in the presence of an exchange field, Weyl semi-metallic phases forms after two Weyl points of charge $\pm1$ 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, Weyl semi-metallic phase emerges with double Weyl node of charges of $\pm2$. Furthermore, the Weyl semi-metallic phases with charges of $\pm2$ can be tuned into Weyl semi-metallic phases with charges of $\pm1$ 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 semi-metallic phase △ Less

Submitted 11 January, 2019; v1 submitted 10 January, 2019; originally announced January 2019.

Comments: 19 pages, 7 figures, to appear in Phys. Rev. B

Journal ref: Phys. Rev. B 99, 035141 (2019)

Strain induced superconducting pair-density-wave states in graphene

Authors: Feng Xu, Po-Hao Chou, Chung-Hou Chung, Ting-Kuo Lee, Chung-Yu Mou

Abstract: Graphene is known to be non-superconducting. However, surprising superconductivity is recently discovered in a flat-band in a twisted bi-layer 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… ▽ More Graphene is known to be non-superconducting. However, surprising superconductivity is recently discovered in a flat-band in a twisted bi-layer 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 non-vanishing superfluid density, and it realizes the long searched superconducting states with non-vanishing center of mass momentum for Cooper pairs. △ Less

Submitted 17 October, 2018; originally announced October 2018.

Comments: 7 pages, 5 figures, to appear in Phys. Rev. B

Journal ref: Phys. Rev. B 98, 205103 (2018)

Strange superconductivity near an antiferromagnetic heavy fermion quantum critical point

Authors: Y. Y. Chang, F. Hsu, S. Kirchner, C. Y. Mou, T. K. Lee, C. H. Chung

Abstract: The heavy fermion CeMIn5 family with M = Co, Rh, Ir provide a prototypical example of strange superconductors with unconventional d-wave pairing and strange metal normal state, emerged near an antiferromagnetic quantum critical point. The microscopic origin of strange superconductor and its link to antiferromagnetic quantum criticality and strange metal state are still open issues. We propose a mi… ▽ More The heavy fermion CeMIn5 family with M = Co, Rh, Ir provide a prototypical example of strange superconductors with unconventional d-wave pairing and strange metal normal state, emerged near an antiferromagnetic quantum critical point. The microscopic origin of strange superconductor and its link to antiferromagnetic quantum criticality and strange metal state are still open issues. We propose a microscopic mechanism for strange superconductor, based on the coexistence and competition between the Kondo correlation and the quasi-2d short-ranged antiferromagnetic resonating-valence-bond spin-liquid near the antiferromagnetic quantum critical point via a large-N Kondo-Heisenberg model and renormalization group analysis beyond the mean-field level. We find the coexistence (competition) between the two types of correlations well explains the overall features of superconducting and strange metal state. The interplay of these two effects provides a qualitative understanding on how superconductivity emerges from the SM state and the observed superconducting phase diagrams for CeMIn5 near the anti-ferromagnetic quantum critical point. △ Less

Submitted 11 June, 2018; originally announced June 2018.

Comments: 6 pages, 2 figures

Journal ref: Phys. Rev. B 99, 094513 (2019)

Scaling of the chiral magnetic effect in quantum diffusive Weyl semimetals

Authors: Yen-Ting Lin, Liang-Jun Zhai, Chung-Yu Mou

Abstract: We investigate the effect of short-range spin-independent disorder on the chiral magnetic effect (CME) in Weyl semimetals. Based on a minimum two-band model, the disorder effect is examined in the quantum diffusion limit by including the Drude correction and the correction due to the Cooperon channel. It is shown that the Drude correction renormalizes the CME coefficient by a factor to a finite va… ▽ More We investigate the effect of short-range spin-independent disorder on the chiral magnetic effect (CME) in Weyl semimetals. Based on a minimum two-band model, the disorder effect is examined in the quantum diffusion limit by including the Drude correction and the correction due to the Cooperon channel. It is shown that the Drude correction renormalizes the CME coefficient by a factor to a finite value that is independent of the system size. Furthemore, due to an additional momentum expansion involved in deriving the CME coefficient, the contribution of Cooperon to the CME coefficient is governed by the quartic momentum term. As a result, in contrast to the weak localization and weak anti-localization effects observed in the measurement of conductivity of Dirac fermions, we find that in the limit of zero magnetic field, the CME coefficients of finite systems manifest the same scaling of localization even in three dimension. Our results indicate that while the chiral magnetic current due to slowly oscillating magnetic fields can exist in clean systems, its observability will be limited by suppression due to short-range disorder in condensed matters. △ Less

Submitted 5 June, 2018; originally announced June 2018.

Comments: 13 pages, 4 figure, to appear in Phys. Rev. B

Journal ref: Phys. Rev. B 97, 245121 (2018)

Andreev reflection in 2D relativistic materials with realistic tunneling transparency in normal-metal-superconductor junctions

Authors: Yung-Yeh Chang, Chung-Yu Mou, Chung-Hou Chung

Abstract: The Andreev conductance across 2d normal metal (N)/superconductor (SC) junctions with relativistic Dirac spectrum is investigated theoretically in the Blonder-Tinkham-Klapwijk formalism. It is shown that for relativistic materials, due to the Klein tunneling instead of impurity potentials, the local strain in the junction is the key factor that determines the transparency of the junction. The loca… ▽ More The Andreev conductance across 2d normal metal (N)/superconductor (SC) junctions with relativistic Dirac spectrum is investigated theoretically in the Blonder-Tinkham-Klapwijk formalism. It is shown that for relativistic materials, due to the Klein tunneling instead of impurity potentials, the local strain in the junction is the key factor that determines the transparency of the junction. The local strain is shown to generate an effective Dirac $δ$-gauge field. A remarkable suppression of the conductance are observed as the strength of the gauge field increases. The behaviors of the conductance are in well agreement with the results obtained in the case of 1d N/SC junction. We also study the Andreev reflection in a topological material near the chiral-to-helical phase transition in the presence of a local strain. The N side of the N/SC junction is modeled by the doped Kane-Mele (KM) model. The SC region is a doped correlated KM t-J (KMtJ) model, which has been shown to feature d+id'-wave spin-singlet pairing. With increasing intrinsic spin-orbit (SO) coupling, the doped KMtJ system undergoes a topological phase transition from the chiral d-wave superconductivity to the spin-Chern superconducting phase with helical Majorana fermions at edges. We explore the Andreev conductance at the two inequivalent Dirac points, respectively and predict the distinctive behaviors for the Andreev conductance across the topological phase transition. Relevance of our results for the adatom-doped graphene is discussed. △ Less

Submitted 3 January, 2017; originally announced January 2017.

Journal ref: Phys. Rev. B 96, 054514 (2017)

Magnetic phases and unusual topological electronic structures of Weyl semimetals in strong interaction limit

Authors: Liang-Jun Zhai, Po-Hao Chou, Chung-Yu Mou

Abstract: The interplay of electronic band structures and electron-electron interactions is known to brew new phases in condensed matter. In this paper, we investigate thermodynamic phases and corresponding electronic structures of the Weyl semimetal in the strong onsite Coulomb interaction limit. Based on a minimum model of the Weyl semimetal with two linear Weyl nodes, it is shown that generically the Wey… ▽ More The interplay of electronic band structures and electron-electron interactions is known to brew new phases in condensed matter. In this paper, we investigate thermodynamic phases and corresponding electronic structures of the Weyl semimetal in the strong onsite Coulomb interaction limit. Based on a minimum model of the Weyl semimetal with two linear Weyl nodes, it is shown that generically the Weyl semimetal becomes magnetic in the presence of interactions. In particular, it is shown that the Dzyaloshinskii-Moriya exchange interaction is generally induced so that the A-type antiferromagnetic (A-AFM) phase and the spiral spin density wave (SSDW) states are two generic phases. Furthermore, we find that Weyl nodes proliferate and it is possible to doubly enhance the unusual properties of non-interacting Weyl semimetals through the realization of double-Weyl nodes in strong correlation limit. Specifically, it is shown that in the SSDW phase, linear Weyl nodes are tuned into double-Weyl nodes with the corresponding charges being $\pm 2$. As the spin-orbit coupling increases, a quantum phase transition occurs with the SSDW phase being turned into an A-AFM phase and at the same time, double-Weyl nodes are disintegrated into two pairs of linear Weyl nodes. Our results reveal the unusual interplay between the topology of electronic structures and magnetism in strongly correlated phases of Weyl semimetals. △ Less

Submitted 13 September, 2016; originally announced September 2016.

Comments: 19 pages, 7 figure, to appear in Phys. Rev. B

Journal ref: Phys. Rev. B94, 125135 (2016)

Emergence of fermionic finite-temperature critical point in a Kondo lattice

Authors: Po-Hao Chou, Liang-Jun Zhai, Chung-Hou Chung, Chung-Yu Mou, Ting-Kuo Lee

Abstract: 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 tran… ▽ More 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 non-trivial thermal responses over large parameter and temperature regimes. In particular, it yields critical scaling behaviors both in magnetic and transport responses near $T_D$. △ Less

Submitted 13 April, 2016; originally announced April 2016.

Comments: 5 pages, 3 figures, to appear in Phys. Rev. Lett

Journal ref: Phys. Rev. Lett. 116, 177002 (2016)

Observation of Collective Coulomb Blockade in a Gate-controlled Linear Quantum-dot Array

Authors: Wen-Yao Wei, Tung-Sheng Lo, Chiu-Chun Tang, Kuan-Ting Lin, Markus Brink, Dah-Chin Ling, Cheng-Chung Chi, Chung-Yu Mou, Jeng-Chung Chen, Dennis M. Newns, Chang C. Tsuei

Abstract: The quantum transport of electrons in an artificial atom, such as a quantum dot (QD), is governed by the Coulomb blockade (CB) effects, revealing the ground-state charge configuration of the electronic system under interplays of the on-site strong Coulomb interactions. In a coherently-coupled QD array, i.e. artificial molecules, the phenomenon of collective CB (CCB) was predicted by theoretical st… ▽ More The quantum transport of electrons in an artificial atom, such as a quantum dot (QD), is governed by the Coulomb blockade (CB) effects, revealing the ground-state charge configuration of the electronic system under interplays of the on-site strong Coulomb interactions. In a coherently-coupled QD array, i.e. artificial molecules, the phenomenon of collective CB (CCB) was predicted by theoretical studies circa two decades ago but its evidence remains controversial. Here, we present direct evidence for the observation of CCB in a six-quantum-dot array (QDA) under high magnetic fields at 20 mK. The coherent inter-dot coupling is enhanced and mediated via the Quantum Hall edge states of the GaAs sample substrate. Two continuously fine-tuned gate voltages enable the quantum dot conductance spectrum to undergo a localization to delocalization transition process which manifests as an emergence and a collapse of CCB. The transition between these two distinct quantum phases is analogous to the Mott-Hubbard metal-insulator transition. The gate-voltage-dependent conductance map thus obtained makes it possible to utilize QDA as an on-chip laboratory for studying CCB and Mott physics. Our QDA device provides a platform for developing engineered QD materials, qubit systems and artificial molecular devices in the future. △ Less

Submitted 15 March, 2016; originally announced March 2016.

Journal ref: Phys. Rev. B 95, 155445 (2017)

Duality in topological superconductors and topological ferromagnetic insulators in a honeycomb lattice

Authors: Shin-Ming Huang, Wei-Feng Tsai, Chung-Hou Chung, Chung-Yu Mou

Abstract: The ground state of 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 $2\mathbb{Z}$ topological invariant. By constructing a dual transformation, we demonstrate that this $2\mathbb{Z}$ topological superco… ▽ More The ground state of 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 $2\mathbb{Z}$ topological invariant. By constructing a dual transformation, we demonstrate that this $2\mathbb{Z}$ 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 being always a chiral superconductor, we find that the $d+id$ superconductor undergoes a topological phase transition from a chiral superconductor to a quasi-helical superconductor as the gap amplitude or the chemical potential decreases. The quasi-helical superconducting phase is found to be characterized by a topological invariant in the pseudo-spin 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 new platform for hosting large numbers of Majorana modes at edges. △ Less

Submitted 6 January, 2016; originally announced January 2016.

Comments: 12 pages, 6 figures

Journal ref: Phys. Rev. B 93, 054518 (2016)

Emergence of Metallic Quantum Solid Phase in a Rydberg-Dressed Fermi Gases

Authors: Wei-Han Li, Tzu-Chi Hsieh, Chung-Yu Mou, Daw-Wei Wang

Abstract: We examine possible low-temperature phases of a repulsively Rydberg-dressed Fermi gas in a three-dimensional free space. It is shown that the collective density excitations develop a roton minimum, which is softened at a wavevector smaller than the Fermi wavevector when the particle density is above a critical value. The mean field calculation shows that unlike the insulating charge density waves… ▽ More We examine possible low-temperature phases of a repulsively Rydberg-dressed Fermi gas in a three-dimensional free space. It is shown that the collective density excitations develop a roton minimum, which is softened at a wavevector smaller than the Fermi wavevector when the particle density is above a critical value. The mean field calculation shows that unlike the insulating charge density waves states often observed in conventional condensed matters, a self-assembled metallic density wave state emerges at low temperatures. In particular, the density wave state supports a Fermi surface and a body-center-cubic crystal order at the same time with the estimated critical temperature being about one-tenth of the non-interacting Fermi energy. Our results suggest the emergency of a fermionic quantum solid that should be observable in current experimental setup. △ Less

Submitted 18 December, 2015; v1 submitted 13 December, 2015; originally announced December 2015.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. Lett. 117, 035301 (2016)

Raman and fluorescence characteristics of resonant inelastic X-ray scattering from doped superconducting cuprates

Authors: H. Y. Huang, C. J. Jia, Z. Y. Chen, K. Wohlfeld, B. Moritz, T. P. Devereaux, W. B. Wu, J. Okamoto, W. S. Lee, M. Hashimoto, Y. He, Z. X. Shen, Y. Yoshida, H. Eisaki, C. Y. Mou, C. T. Chen, D. J. Huang

Abstract: Measurements of spin excitations are essential for an understanding of spin-mediated pairing for superconductivity; and resonant inelastic X-ray scattering (RIXS) provides a considerable opportunity to probe high-energy spin excitations. However, whether RIXS correctly measures the collective spin excitations of doped superconducting cuprates remains under debate. Here we demonstrate distinct Rama… ▽ More Measurements of spin excitations are essential for an understanding of spin-mediated pairing for superconductivity; and resonant inelastic X-ray scattering (RIXS) provides a considerable opportunity to probe high-energy spin excitations. However, whether RIXS correctly measures the collective spin excitations of doped superconducting cuprates remains under debate. Here we demonstrate distinct Raman- and fluorescence-like RIXS excitations of Bi$_{1.5}$Pb$_{0.6}$Sr$_{1.54}$CaCu$_{2}$O$_{8+δ}$ in the mid-infrared energy region. Combining photon-energy and momentum dependent RIXS measurements with theoretical calculations using exact diagonalization provides conclusive evidence that the Raman-like RIXS excitations correspond to collective spin excitations, which are magnons in the undoped Mott insulators and evolve into paramagnons in doped superconducting compounds. In contrast, the fluorescence-like shifts are due primarily to the continuum of particle-hole excitations in the charge channel. Our results show that under the proper experimental conditions RIXS indeed can be used to probe paramagnons in doped high-$T_c$ cuprate superconductors. △ Less

Submitted 28 October, 2015; originally announced October 2015.

Journal ref: Scientific Reports 6, 19657 (2016)

Tunneling spectroscopy and Majorana modes emergent from topological gapless phases in high-Tc cuprate superconductors

Authors: Jun-Ting Kao, Shin-Ming Huang, Chung-Yu Mou, C. C. Tsuei

Abstract: We explore possible signatures for observing Majorana Fermions in the tunneling spectroscopy of high-Tc cuprate superconductors. We find that as long as the Rashba spin orbit interaction is in presence either through proximity effect due to an electrode made by heavy metal or by the intrinsic nature of cuprates, in addition to the Heisenberg spin exchange interaction, the Dzyaloshinskii-Moriya and… ▽ More We explore possible signatures for observing Majorana Fermions in the tunneling spectroscopy of high-Tc cuprate superconductors. We find that as long as the Rashba spin orbit interaction is in presence either through proximity effect due to an electrode made by heavy metal or by the intrinsic nature of cuprates, in addition to the Heisenberg spin exchange interaction, the Dzyaloshinskii-Moriya and spin dipole-dipole interactions are induced. As a result, $p$-wave superconductivity is induced with the gap function $d$-vector being not aligned with the internal magnetic field of the spin-orbit interaction. Most importantly, the ground state goes through transitions into gapless phases with split nodal points. The split nodal structure always results in Majorana modes for any interfaces that are not exactly in (100) or (010) directions. Hence for general interfaces, existence of Majorana bound edge states is a robust feature. Our results indicate that these Majorana modes would result in a small plateau in tunneling spectrum near zero bias peak and in 4 $π$ periodicity in typical SIS$'$ junctions . As a result, it is easy for a $π$-ring in tricrystal experiments to hold Majorana Ferimions and exhibit periods of two flux quanta in external magnetic fields. These phenomena may have been already observed in experiments and their connections to experimental results are discussed. △ Less

Submitted 27 March, 2015; v1 submitted 19 March, 2015; originally announced March 2015.

Comments: 15 pages, 16 figures, to appear in Phys. Rev. B

Journal ref: Phys. Rev. B 91, 134501 (2015)

Ferromagnetism and quantum anomalous Hall effect in one-side-saturated buckled honeycomb lattices

Authors: Shin-Ming Huang, Shi-Ting Lee, Chung-Yu Mou

Abstract: The recently synthesized silicene as well as theoretically discussed germanene are examples of buckled honeycomb structures. The buckled structures allow one to manipulate asymmetry between two underlying sublattices of honeycomb structures. Here by taking germanene as a prototype of buckled honeycomb lattices, we explore magnetism induced by breaking sublattice symmetry through saturating chemica… ▽ More The recently synthesized silicene as well as theoretically discussed germanene are examples of buckled honeycomb structures. The buckled structures allow one to manipulate asymmetry between two underlying sublattices of honeycomb structures. Here by taking germanene as a prototype of buckled honeycomb lattices, we explore magnetism induced by breaking sublattice symmetry through saturating chemical bonds on one-side of the buckled honeycomb lattice. It is shown that when fractions of chemical bonds on one-side are saturated, two narrow bands always exist at half filling. Furthermore, the narrow bands generally support flat band ferromagnetism in the presence of the Hubbard $U$ interaction. The induced magnetization is directly related to the saturation fraction and is thus controllable in magnitude through the saturation fraction. Most importantly, we find that depending on the saturation fraction, the ground state of an one-side saturated germanene may become a quantum anomalous Hall (QAH) insulator characterized by a Chern number that vanishes for larger magnetization. The non-vanishing Chern number for smaller magnetization implies that the associated quantum Hall effect tends to survive at high temperatures. Our findings provide a potential method to engineer buckled honeycomb structures into high-temperature QAH insulators. △ Less

Submitted 13 May, 2014; originally announced May 2014.

Comments: 13 pages, 11 figures. Accepted for publication as a Regular Article in Physical Review B

Journal ref: Phys. Rev. B 89, 195444 (2014)

A Memory of Majorana Fermions through Quantum Quench

Authors: Ming-Chiang Chung, Yi-Hao Jhu, Pochung Chen, Chung-Yu Mou, Xin Wan

Abstract: We study the sudden quench of a one-dimensional p-wave superconductor through its topological signature in the entanglement spectrum. The long-time evolution of the system and its topological characterization depend on a pseudomagnetic field ${\bs R}_{\mbox{\text eff}}(k)$, which connects both the initial and the final Hamiltonians, hence exhibiting a memory effect. In particular, we explore the r… ▽ More We study the sudden quench of a one-dimensional p-wave superconductor through its topological signature in the entanglement spectrum. The long-time evolution of the system and its topological characterization depend on a pseudomagnetic field ${\bs R}_{\mbox{\text eff}}(k)$, which connects both the initial and the final Hamiltonians, hence exhibiting a memory effect. In particular, we explore the robustness of the Majorana zero-mode associated with the entanglement cut in the topologically nontrivial phase and identify the parameter space in which the mode can survive in the infinite-time limit. △ Less

Submitted 19 January, 2014; v1 submitted 2 January, 2014; originally announced January 2014.

Comments: 5 pages, 4 figures

Stability of Z$_2$ topological order in the presence of vacancy-induced impurity band

Authors: Shi-Ting Lee, Shin-Ming Huang, Chung-Yu Mou

Abstract: Although topological insulators (TIs) are known to be robust against non-magnetic perturbations and exhibit edge or surface states as their distinct feature, experimentally it is known that vacancies often occur in these materials and impose strong perturbations. Here we investigate effects of vacancies on the stability of Z$_{2}$ topological order using the Kane-Mele (KM) model as a prototype of… ▽ More Although topological insulators (TIs) are known to be robust against non-magnetic perturbations and exhibit edge or surface states as their distinct feature, experimentally it is known that vacancies often occur in these materials and impose strong perturbations. Here we investigate effects of vacancies on the stability of Z$_{2}$ topological order using the Kane-Mele (KM) model as a prototype of topological insulator. It is shown that even though a vacancy is not classified as a topological defect in KM model, it generally induces a pair of degenerate midgap states only in the TI phase. We show that these midgap states results from edge states that fit into vacancies and are characterized by the same Z$_{2}$ topological order. Furthermore, in the presence of many vacancies, an impurity band that is degenerate with edge states in energy is induced and mixes directly with edge states. However, the Z$_{2}$ topological order persists and edge states exist between the impurity band and perturbed bulk bands until a phase transition occurs when Dirac cones near Dirac points are depleted. Our analyses indicate that the same scenario holds for point vacancies or line of vacancies in 3D TIs as well. △ Less

Submitted 1 June, 2014; v1 submitted 19 August, 2013; originally announced August 2013.

Comments: 8 pages, 6 figures

Journal ref: Journal of Physics: Condensed Matter 26, 255502 (2014)

Doping evolution of Zhang-Rice singlet spectral weight: a comprehensive examination by x-ray absorption spectroscopy

Authors: Y. -J. Chen, M. G. Jiang, C. W. Luo, J. -Y. Lin, K. H. Wu, J. M. Lee, J. M. Chen, Y. K. Kuo, J. Y. Juang, Chung-Yu Mou

Abstract: The total spectral weight \textit{S} of the emergent low-energy quasipaticles in high-temperature superconductors 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 mean field theory, we found that \textit{S} deviates from linear dependence on the doping level \… ▽ More The total spectral weight \textit{S} of the emergent low-energy quasipaticles in high-temperature superconductors 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 mean field theory, we found that \textit{S} deviates from linear dependence on the doping level \textit{p}. The slope of \textit{S} versus \textit{p} changes continuously throughout the whole doping range with no sign of saturation up to \textit{p} = 0.23. Therefore, the picture of Zhang-Rice singlet remains intact within the most prominent doping regimes of HTSC's. △ Less

Submitted 4 August, 2013; originally announced August 2013.

Comments: 5 pages, 3 figures

Intrinsic high-temperature superconductivity in ternary iron selenides

Authors: Shin-Ming Huang, Chung-Yu Mou, Ting-Kuo Lee

Abstract: We examine superconductivity in the mesoscopically mixed antiferromagnetic(AF) and superconducting (SC) phases of ternary iron selenides K$_{y}$Fe$_{2-x}$Se$_{2}$. It is shown that the interlayer hopping and AF order are key factors to determine $T_{c}$ of the SC phase. In general, the hopping will produce deformed Fermi surfaces (FS's) that tend to suppress superconductivity. However, contrary to… ▽ More We examine superconductivity in the mesoscopically mixed antiferromagnetic(AF) and superconducting (SC) phases of ternary iron selenides K$_{y}$Fe$_{2-x}$Se$_{2}$. It is shown that the interlayer hopping and AF order are key factors to determine $T_{c}$ of the SC phase. In general, the hopping will produce deformed Fermi surfaces (FS's) that tend to suppress superconductivity. However, contrary to the common expectation, we find that larger AF order actually results in larger SC order, which explains the observed relatively high $T_{c}$ in these phases. Furthermore our results indicate that by reducing the interlayer hopping appropriately, phase-separated K$_{y}$Fe$_{2-x}$Se$_{2}$ may exhibit its intrinsic SC phase in the two dimensional limit with a much higher $T_{c}$ ($\sim 65K$) than what has been observed. △ Less

Submitted 1 November, 2013; v1 submitted 13 April, 2013; originally announced April 2013.

Comments: 7 pages, 7 figures. Accepted for publication as a Regular Article in Physical Review B

Journal ref: Phys. Rev. B 88, 174510 (2013)

Quench Dynamics of Topological Maximally-Entangled States

Authors: Ming-Chiang Chung, Yi-Hao Jhu, Pochung Chen, Chung-Yu Mou

Abstract: We investigate the quench dynamics of the one-particle entanglement spectra (OPES) for systems with topologically nontrivial phases. By using dimerized chains as an example, it is demonstrated that the evolution of OPES for the quenched bi-partite systems is governed by an effective Hamiltonian which is characterized by a pseudo spin in a time-dependent pseudo magnetic field $\vec{S}(k,t)$. The ex… ▽ More We investigate the quench dynamics of the one-particle entanglement spectra (OPES) for systems with topologically nontrivial phases. By using dimerized chains as an example, it is demonstrated that the evolution of OPES for the quenched bi-partite systems is governed by an effective Hamiltonian which is characterized by a pseudo spin in a time-dependent pseudo magnetic field $\vec{S}(k,t)$. The existence and evolution of the topological maximally-entangled edge states are determined by the winding number of $\vec{S}(k,t)$ in the $k$-space. In particular, the maximally-entangled edge states survive only if nontrivial Berry phases are induced by the winding of $\vec{S}(k,t)$. In the infinite time limit the equilibrium OPES can be determined by an effective time-independent pseudo magnetic field $\vec{S}_{\mb{eff}}(k)$. Furthermore, when maximally-entangled edge states are unstable, they are destroyed by quasiparticles within a characteristic timescale in proportional to the system size. △ Less

Submitted 7 June, 2012; v1 submitted 31 May, 2012; originally announced May 2012.

Comments: 5 pages, 3 figures

Unconventional superconducting gap via spin fluctuations in iron-vacancy ordered A$_{y}$Fe$_{2-x}$Se$_{2}$

Authors: Shin-Ming Huang, Chung-Yu Mou

Abstract: Based on an effective 12-orbital tight-binding model, we examine the superconducting states induced by the antiferromagnetic fluctuations for iron-vacancy-ordered A$_{y}$Fe$_{2-x}$Se$_{2}$. It is shown that due to the broken reflection symmetry induced by the iron vacancies, new superconducting states with $C_{4h}$ symmetry emerge. In particular, we show that in the $C_{4h}$ symmetry, symmetric ax… ▽ More Based on an effective 12-orbital tight-binding model, we examine the superconducting states induced by the antiferromagnetic fluctuations for iron-vacancy-ordered A$_{y}$Fe$_{2-x}$Se$_{2}$. It is shown that due to the broken reflection symmetry induced by the iron vacancies, new superconducting states with $C_{4h}$ symmetry emerge. In particular, we show that in the $C_{4h}$ symmetry, symmetric axes of the pairing momenta do not need to coincide with axes of the unit cell. As a result, in addition to the magnitude of the pairing gap, the relative orientation of the pairing wave function to the lattice forms another degree of freedom for characterizing the superconducting gap and can further help in gaining the condensation energy. Nonetheless, similar to other iron-based superconductors, the singlet ground state is still dominated by s-wave or d-wave, which are nearly degenerate with anisotropic gaps. Furthermore, s-wave and d-wave superconducting states are separated by a quantum critical point controlled by the Hund's rule coupling $J_{H}$. △ Less

Submitted 30 April, 2012; v1 submitted 6 February, 2012; originally announced February 2012.

Comments: 9 pages, 8 figures

Journal ref: Phys. Rev. B 85, 184519 (2012)

Persistent currents in a graphene ring with armchair edges

Authors: Bor-Luen Huang, Ming-Che Chang, Chung-Yu Mou

Abstract: A graphene nano-ribbon with armchair edges is known to have no edge state. However, if the nano-ribbon is in the quantum spin Hall (QSH) state, then there must be helical edge states. By folding a graphene ribbon to a ring and threading it by a magnetic flux, we study the persistent charge and spin currents in the tight-binding limit. It is found that, for a broad ribbon, the edge spin current app… ▽ More A graphene nano-ribbon with armchair edges is known to have no edge state. However, if the nano-ribbon is in the quantum spin Hall (QSH) state, then there must be helical edge states. By folding a graphene ribbon to a ring and threading it by a magnetic flux, we study the persistent charge and spin currents in the tight-binding limit. It is found that, for a broad ribbon, the edge spin current approaches a finite value independent of the radius of the ring. For a narrow ribbon, inter-edge coupling between the edge states could open the Dirac gap and reduce the overall persistent currents. Furthermore, by enhancing the Rashba coupling, we find that the persistent spin current gradually reduces to zero at a critical value, beyond which the graphene is no longer a QSH insulator. △ Less

Submitted 24 May, 2012; v1 submitted 18 January, 2012; originally announced January 2012.

Journal ref: J. Phys: Condens. Matter 24, 245304 (2012)

Folding a protein with equal probability of being helix or hairpin

Authors: Chun-Yu Lin, Nan-Yow Chen, Chung-Yu Mou

Abstract: We explore the possibility for the native state of a protein being inherently a multi-conformation state in an ab initio coarse-grained model. Based on the Wang-Landau algorithm, the complete free energy landscape for the designed sequence 2D4X: INYWLAHAKAGYIVHWTA is constructed. It is shown that 2DX4 possesses two nearly degenerate native states: one has a helix structure, while the other has a h… ▽ More We explore the possibility for the native state of a protein being inherently a multi-conformation state in an ab initio coarse-grained model. Based on the Wang-Landau algorithm, the complete free energy landscape for the designed sequence 2D4X: INYWLAHAKAGYIVHWTA is constructed. It is shown that 2DX4 possesses two nearly degenerate native states: one has a helix structure, while the other has a hairpin structure and their energy difference is less than 2% of that of local minimums. Two degenerate native states are stabilized by an energy barrier of the order 10kcal/mol. Furthermore, the hydrogen-bond and dipole-dipole interactions are found to be two major competing interactions in transforming one conformation into the other. Our results indicate that degenerate native states are stabilized by subtle balance between different interactions in proteins; furthermore, for small proteins, degeneracy only happens for proteins of sizes being around 18 amino acids or 40 amino acids. These results provide important clues to the study of native structures of proteins. △ Less

Submitted 30 November, 2011; originally announced December 2011.

Comments: 15 pages, 7 figures, submitted to Biophysical Journal

Journal ref: Biophysical Journal 103(1), 99-108 (2012)

Effective tight-binding model for the iron vacancy ordered A$_{y}$Fe$%_{1.6}$Se$_{2}$

Authors: Shin-Ming Huang, Chung-Yu Mou

Abstract: We investigate the electronic structure of the ternary iron selenide K$_{y}$% Fe$_{1.6}$Se$_{2}$ by considering the spatial symmetry of the $\sqrt{5}% \times \sqrt{5}$ vacancy ordered structure. Based on three orbitals of $% t_{2g}$, which are believed to play major physics in iron-based superconductors, an effective two-dimensional tight binding Hamiltonian is constructed with the vacancy ordered… ▽ More We investigate the electronic structure of the ternary iron selenide K$_{y}$% Fe$_{1.6}$Se$_{2}$ by considering the spatial symmetry of the $\sqrt{5}% \times \sqrt{5}$ vacancy ordered structure. Based on three orbitals of $% t_{2g}$, which are believed to play major physics in iron-based superconductors, an effective two-dimensional tight binding Hamiltonian is constructed with the vacancy ordered structure being explicitly included. It is shown that the constructed band model, when combined with generalized Hubbard interactions, yields a spin susceptibility which exhibits both the block-checkerboard antiferromagnetism instability and the stripe antiferromagnetism instability. In particular, for large Hund's rule couplings, the block-checkerboard antiferromagnetism wins over the stripe antiferromagnetism, in agreement with the observation in experiments. We argue that such a model with correct symmetry and Fermi surface structures should be the starting point to model K$_{y}$Fe$_{1.6}$Se$_{2}$. The spin fluctuations at $\mathbf{q}$=($π,π$) suggest that interblock fluctuations of spins might play an important role in the mechanism of superconductivity occurring in this system. △ Less

Submitted 24 November, 2011; v1 submitted 24 August, 2011; originally announced August 2011.

Comments: 9 pages, 4 figures

Journal ref: Physical Review B 84, 184521 (2011)

Possible $s^{\pm}$-wave pairing evidenced by midgap surface bound states in Fe-pnictide superconductors

Authors: C. S. Liu, J. Y. Chang, W. C. Wu, Chung-Yu Mou

Abstract: A phenomenological theory of tunneling spectroscopy for Fe-pnictide superconductors is developed by taking into consideration of asymmetric interface scattering between particle and holes. It is shown that in consistent with anti-phase $s^{\pm}$-wave pairing, appreciable zero-energy surface bound states exist on the [100] surface of Fe-pnictide superconductors. However, in contrast to the [110] bo… ▽ More A phenomenological theory of tunneling spectroscopy for Fe-pnictide superconductors is developed by taking into consideration of asymmetric interface scattering between particle and holes. It is shown that in consistent with anti-phase $s^{\pm}$-wave pairing, appreciable zero-energy surface bound states exist on the [100] surface of Fe-pnictide superconductors. However, in contrast to the [110] bound states in $d$-wave cuprate superconductors, these bound states arise as a result of non-conservation of momentum perpendicular to the interface for tunneling electrons and the $s^{\pm}$ pairing, and hence they can only exist in a small window ($\sim \pm 6^{\circ}$) in the orientation of edges near [100] direction. Our results explain why zero-bias conductance peak is often observed in tunneling spectroscopy and when it disappears, two coherent peaks show up. These results provide unambiguous signals to test the possible $s^{\pm}$-wave pairing in Fe-pnictide superconductors. △ Less

Submitted 8 August, 2013; v1 submitted 27 April, 2011; originally announced April 2011.

Comments: 10 pages, 4 figures

Journal ref: J. Phys: Condens. Matter 25, 365701 (2013)

Dynamical Decoupling of Qubits in Spin Bath under Periodic Quantum Control

Authors: Jun-Ting Kao, Jo-Tzu Hung, Pochung Chen, Chung-Yu Mou

Abstract: We investigate the feasibility for the preservation of coherence and entanglement of one and two spin qubits coupled to an interacting quantum spin-1/2 chain within the dynamical decoupling (DD) scheme. The performance is examined by counting number of computing pulses that can be applied periodically with period of $T$ before qubits become decoherent, while identical decoupling pulse sequence is… ▽ More We investigate the feasibility for the preservation of coherence and entanglement of one and two spin qubits coupled to an interacting quantum spin-1/2 chain within the dynamical decoupling (DD) scheme. The performance is examined by counting number of computing pulses that can be applied periodically with period of $T$ before qubits become decoherent, while identical decoupling pulse sequence is applied within each cycle. By considering pulses with mixed directions and finite width controlled by magnetic fields, it is shown that pulse-width accumulation degrades the performance of sequences with larger number of pulses and feasible magnetic fields in practice restrict the consideration to sequences with number of decoupling pulses being less than 10 within each cycle. Furthermore, within each cycle $T$, exact nontrivial pulse sequences are found for the first time to suppress the qubit-bath coupling to $O(T^{N+1})$ progressively with minimum number of pulses being $4,7,12$ for $N=1,2,3$. These sequences, when applied to all qubits, are shown to preserve both the entanglement and coherence. Based on time-dependent density matrix renormalization, our numerical results show that for modest magnetic fields (10-40 Tesla) available in laboratories, the overall performance is optimized when number of pulses in each cycle is 4 or 7 with pulse directions be alternating between x and z. Our results provide useful guides for the preservation of coherence and entanglement of spin qubits in solid state. △ Less

Submitted 29 December, 2010; originally announced January 2011.

Comments: 11 pages, 9 figures

Journal ref: Phys. Rev. A 82, 062120 (2010)

Graphene-based modulation-doped superlattice structures

Authors: Dima Bolmatov, Chung-Yu Mou

Abstract: The electronic transport properties of graphene-based superlattice structures are investigated. A graphene-based modulation-doped superlattice structure geometry is proposed and consist of periodically arranged alternate layers: InAs/graphene/GaAs/graphene/GaSb. Undoped graphene/GaAs/graphene structure displays relatively high conductance and enhanced mobilities at elevated temperatures unlike mod… ▽ More The electronic transport properties of graphene-based superlattice structures are investigated. A graphene-based modulation-doped superlattice structure geometry is proposed and consist of periodically arranged alternate layers: InAs/graphene/GaAs/graphene/GaSb. Undoped graphene/GaAs/graphene structure displays relatively high conductance and enhanced mobilities at elevated temperatures unlike modulation-doped superlattice structure more steady and less sensitive to temperature and robust electrical tunable control on the screening length scale. Thermionic current density exhibits enhanced behaviour due to presence of metallic (graphene) mono-layers in superlattice structure. The proposed superlattice structure might become of great use for new types of wide-band energy gap quantum devices. △ Less

Submitted 24 February, 2011; v1 submitted 12 November, 2010; originally announced November 2010.

Comments: 5 figures

Journal ref: JETP 112, 102 (2011)

Density of states of a graphene in the presence of strong point defects

Authors: Bor-Luen Huang, Ming-Che Chang, Chung-Yu Mou

Abstract: The density of states near zero energy in a graphene due to strong point defects with random positions are computed. Instead of focusing on density of states directly, we analyze eigenfunctions of inverse T-matrix in the unitary limit. Based on numerical simulations, we find that the squared magnitudes of eigenfunctions for the inverse T-matrix show random-walk behavior on defect positions. As a r… ▽ More The density of states near zero energy in a graphene due to strong point defects with random positions are computed. Instead of focusing on density of states directly, we analyze eigenfunctions of inverse T-matrix in the unitary limit. Based on numerical simulations, we find that the squared magnitudes of eigenfunctions for the inverse T-matrix show random-walk behavior on defect positions. As a result, squared magnitudes of eigenfunctions have equal {\it a priori} probabilities, which further implies that the density of states is characterized by the well-known Thomas-Porter type distribution. The numerical findings of Thomas-Porter type distribution is further derived in the saddle-point limit of the corresponding replica field theory of inverse T-matrix. Furthermore, the influences of the Thomas-Porter distribution on magnetic and transport properties of a graphene, due to its divergence near zero energy, are also examined. △ Less

Submitted 8 November, 2010; originally announced November 2010.

Comments: 6 figures

Journal ref: Phys. Rev. B 82, 155462 (2010)

Dynamical spectral weight in YBa$_2$Cu$_3$O$_y$ probed by x-ray absorption spectroscopy

Authors: J. -Y. Lin, P. R. Lee, Y. T. Liu, Chung-Yu Mou, Y. -J. Chen, K. H. Wu, C. W. Luo, J. Y. Juang, T. M. Uen, J. M. Lee, J. M. Chen

Abstract: The comprehensive study of the temperature dependent x-ray absorption spectroscopy (XAS) reveals a dynamical spectral weight $α$ in YBa$_2$Cu$_3$O$_y$ (YBCO). Large spectral weight changes for both the Upper Hubbard band and the Zhang-Rice band due to dynamics of holes are experimentally found in the underdoped regime. A large value of $α\geq 0.3$ is indispensable to describing XAS of YBCO with th… ▽ More The comprehensive study of the temperature dependent x-ray absorption spectroscopy (XAS) reveals a dynamical spectral weight $α$ in YBa$_2$Cu$_3$O$_y$ (YBCO). Large spectral weight changes for both the Upper Hubbard band and the Zhang-Rice band due to dynamics of holes are experimentally found in the underdoped regime. A large value of $α\geq 0.3$ is indispensable to describing XAS of YBCO with the conservation of states. The value of $α$ is linearly proportional to the pseudogap temperature in the underdoped regime, but becomes smaller as the doping level goes to the undoped limit. Our results clearly indicate that the pseudogap is related to the double occupancy and originates from bands in higher energies. △ Less

Submitted 19 September, 2010; v1 submitted 13 September, 2010; originally announced September 2010.

Comments: 10 pages; 3 figures; references updated

Josephson effect in Graphene SNS Junction with a Single Localized Defect

Authors: Dima Bolmatov, Chung-Yu Mou

Abstract: Imperfections change essentially the electronic transport properties of graphene. Motivated by a recent experiment reporting on the possible application of graphene as junctions, we study transport properties in graphene-based junctions with single localized defect. We solve the Dirac-Bogoliubov-de-Gennes equation with a single localized defect superconductor-normal(graphene)-superconductor (SNS)… ▽ More Imperfections change essentially the electronic transport properties of graphene. Motivated by a recent experiment reporting on the possible application of graphene as junctions, we study transport properties in graphene-based junctions with single localized defect. We solve the Dirac-Bogoliubov-de-Gennes equation with a single localized defect superconductor-normal(graphene)-superconductor (SNS) junction. We consider the properties of tunneling conductance and Josephson current through an undoped strip of graphene with heavily doped s-wave superconducting electrodes in the dirty limit. We find that spectrum of Andreev bound states are modified in the presence of single localized defect in the bulk and the minimum tunneling conductance remains the same. The Josephson junction exhibits sign oscillations. △ Less

Submitted 7 June, 2010; originally announced June 2010.

Comments: 5 pages, 4 figures

Journal ref: Physica B: Condensed Matter 405, 2896 (2010)

Tunneling Conductance of The Graphene SNS Junction with a Single Localized Defect

Authors: Dima Bolmatov, Chung-Yu Mou

Abstract: We study the electronic transport in a graphene-based superconductor-normal(graphene)-superconductor (SNS) junction by use of the Dirac-Bogoliubov-de Gennes equation. We consider the properties of tunneling conductance through an undoped strip of graphene with heavily doped superconducting electrodes in the dirty limit. We find that spectrum of Andreev bound states are modified in the presence of… ▽ More We study the electronic transport in a graphene-based superconductor-normal(graphene)-superconductor (SNS) junction by use of the Dirac-Bogoliubov-de Gennes equation. We consider the properties of tunneling conductance through an undoped strip of graphene with heavily doped superconducting electrodes in the dirty limit. We find that spectrum of Andreev bound states are modified in the presence of single localized defect in the bulk. The minimum tunneling conductance remains the same and this result doesn't depend on the actual location of the imperfection. △ Less

Submitted 7 June, 2010; originally announced June 2010.

Comments: 10 pages, 4 figures

Journal ref: JETP 110, 612 (2010)

Dispersive resonance bands within the space charge layer of metal- semiconductor junction

Authors: S. -J. Tang, Tay- Rong Chang, Chien-Chung Huang, Chang-Yeh Lee, Cheng-Maw Cheng, Ku-Ding Tsuei, H. -T. Jeng, Chung-Yu Mou

Abstract: Based on measurements of angle resolved photoemission, we report that in the Pb/Ge(111)- \sqrt{3}x\sqrt{3} R30^\circ structure, in addition to three bands resembling Ge heavy hole (HH), light hole (LH), and split off (SO) bulk band edges, a fourth dispersive band resembling the non split off (NSO) band is found near the surface zone center. While three Ge bulk-like bands get distorted due to stron… ▽ More Based on measurements of angle resolved photoemission, we report that in the Pb/Ge(111)- \sqrt{3}x\sqrt{3} R30^\circ structure, in addition to three bands resembling Ge heavy hole (HH), light hole (LH), and split off (SO) bulk band edges, a fourth dispersive band resembling the non split off (NSO) band is found near the surface zone center. While three Ge bulk-like bands get distorted due to strong coupling between Pb and Ge, the NSO-like band gets weaker and disappears for larger thickness of Pb, which, when combined with ab initio calculations, indicates its localized nature within space charge layer. Our results are clearly important for designing electronics involved with metal-semiconductor contacts. △ Less

Submitted 2 June, 2010; originally announced June 2010.

Comments: 21 pages, 4 figures, Phys. Rev. B 81, 245406 (2010)

Journal ref: Phys. Rev. B 81, 245406 (2010)

Unusual Phase Behavior of Confined Heavy Water

Authors: Yang Zhang, Antonio Faraone, William A. Kamitakahara, Kao-Hsiang Liu, Chung-Yuan Mou, Juscelino B. Leão, Sung Chang, Sow-Hsin Chen

Abstract: Many of the anomalous properties of water are amplified in the deeply supercooled region. Here we present neutron scattering measurements of the density of heavy water confined in a nanoporous silica matrix MCM-41-S (\approx15 Å pore diameter), namely, the equation of state ρ(T,P), in a temperature-pressure range, from 300 K to 130 K and from 1 bar to 2900 bar, where bulk water will crystalize. A… ▽ More Many of the anomalous properties of water are amplified in the deeply supercooled region. Here we present neutron scattering measurements of the density of heavy water confined in a nanoporous silica matrix MCM-41-S (\approx15 Å pore diameter), namely, the equation of state ρ(T,P), in a temperature-pressure range, from 300 K to 130 K and from 1 bar to 2900 bar, where bulk water will crystalize. A sudden change of slope in the otherwise continuous density profile (a "kink") is observed below a certain pressure Pc; however, this feature is absent above Pc. Instead, a hysteresis phenomenon in the density profiles between the warming and cooling scans becomes prominent above Pc. Hence, the data can be interpreted as a line of apparent 2nd-order phase transition at low pressures evolving into a line of 1st-order phase transition at high pressures. If so, the existence of a "tricritical point" at Pc \approx 1500 bar, Tc \approx 210 K becomes another possible scenario to explain the exceptionally rich phase behavior of low-temperature confined water. △ Less

Submitted 21 June, 2011; v1 submitted 28 May, 2010; originally announced May 2010.

Comments: 15 pages, 4 figures

Impurity band induced by point defects in graphene

Authors: Bor-Luen Huang, Chung-Yu Mou

Abstract: It is pointed out that point defects on graphene are strongly correlated and can not be treated as independent scatters. In particular, for large on-site defect potential, it is shown that defects induce an impurity band with density of state characterized by the Wigner semi-circle law. We find that the impurity band enhances conductivity to the order of $4 e^2 /h $ and explains the absence of s… ▽ More It is pointed out that point defects on graphene are strongly correlated and can not be treated as independent scatters. In particular, for large on-site defect potential, it is shown that defects induce an impurity band with density of state characterized by the Wigner semi-circle law. We find that the impurity band enhances conductivity to the order of $4 e^2 /h $ and explains the absence of strong localization. Furthermore,the impurity band supports ferromagnetism with the induced magnetic moment approaching 1$μ_B$ per defect in the limit of infinite quasi-particle lifetime. △ Less

Submitted 3 May, 2009; originally announced May 2009.

Comments: 4, pages, 2 figures

Journal ref: Europhys. Lett. 88, 68005 (2009)

Ferromagnetism in armchair graphene nanoribbon

Authors: Hsiu-Hau Lin, Toshiya Hikihara, Horng-Tay Jeng, Bor-Luen Huang, Chung-Yu Mou, Xiao Hu

Abstract: Due to the weak spin-orbit interaction and the peculiar relativistic dispersion in graphene, there are exciting proposals to build spin qubits in graphene nanoribbons with armchair boundaries. However, the mutual interactions between electrons are neglected in most studies so far and thus motivate us to investigate the role of electronic correlations in armchair graphene nanoribbon by both analy… ▽ More Due to the weak spin-orbit interaction and the peculiar relativistic dispersion in graphene, there are exciting proposals to build spin qubits in graphene nanoribbons with armchair boundaries. However, the mutual interactions between electrons are neglected in most studies so far and thus motivate us to investigate the role of electronic correlations in armchair graphene nanoribbon by both analytical and numerical methods. Here we show that the inclusion of mutual repulsions leads to drastic changes and the ground state turns ferromagnetic in a range of carrier concentrations. Our findings highlight the crucial importance of the electron-electron interaction and its subtle interplay with boundary topology in graphene nanoribbons. Furthermore, since the ferromagnetic properties sensitively depends on the carrier concentration, it can be manipulated at ease by electric gates. The resultant ferromagnetic state with metallic conductivity is not only surprising from an academic viewpoint, but also has potential applications in spintronics at nanoscale. △ Less

Submitted 27 January, 2009; originally announced January 2009.

Comments: 10 pages, 5 figures

Journal ref: Phys. Rev. B 79, 035405 (2009)