Browsing by Author "Chung, Chung-Hou"

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Andreev reflection in two-dimensional relativistic materials with realistic tunneling transparency in normal-metal/superconductor junctions
(2017-08-17)
Chang, Yung-Yeh
;
Mou, Chung-Yu
;
Chung, Chung-Hou
;
電子物理學系
;
Department of Electrophysics
The Andreev conductance across realistic two-dimensional (2D) normal-metal (N)/superconductor (SC) junctions with a relativistic Dirac spectrum is theoretically investigated within the Blonder-Tinkham-Klapwijk formalism with tunable tunneling transparency. It is known that due to the effect of Klein tunneling, impurity potentials at the interface of 2D relativistic materials will enhance (not suppress) the tunneling and therefore are not suitable to model a realistic tunnel junction of these materials. Here, we propose a way to construct a more realistic tunnel junction by adding a narrow, homogeneous local strain, which effectively generates a delta-gauge potential and variations of electron hopping at the interface, to adjust the transparency of the N/SC junction. Remarkable suppression of the Andreev conductance is indeed observed in the graphene N/SC junction as the strength of the local strain increases. We also explore the Andreev conductance in a topological N/SC junction at the two inequivalent Dirac points and predict the distinctive behaviors for the conductance across the chiral-to-helical topological phase transition. The relevance of our results for the adatom-doped graphene is discussed.
Commensurate lock-in and incommensurate supersolid phases of hard-core bosons on anisotropic triangular lattices
(2009-08-01)
Isakov, Sergei V.
;
Chien, Hong-Chi
;
Wu, Jian-Jheng
;
Chen, Yung-Chung
;
Chung, Chung-Hou
We investigate the interplay between commensurate lock-in and incommensurate supersolid phases of the hard-core bosons at half-filling with anisotropic nearest-neighbor hopping and repulsive interactions on triangular lattice. We use numerical quantum and variational Monte Carlo as well as analytical Schwinger boson mean-field analysis to establish the ground states and phase diagram. It is shown that, for finite-size systems, there exist a series of jumps between different supersolid phases as the anisotropy parameter is changed. The density ordering wave vectors are locked to commensurate values and jump between adjacent supersolids. In the thermodynamic limit, however, the magnitude of these jumps vanishes leading to a continuous set of novel incommensurate supersolid phases. Copyright (C) EPLA, 2009
Competing Topological and Kondo Insulator Phases on a Honeycomb Lattice
(2013-07-03)
Feng, Xiao-Yong
;
Dai, Jianhui
;
Chung, Chung-Hou
;
Si, Qimiao
;
電子物理學系
We investigate the competition between the spin-orbit interaction of itinerant electrons and their Kondo coupling with local moments densely distributed on the honeycomb lattice. We find that the model at half-filling displays a quantum phase transition between topological and Kondo insulators at a nonzero Kondo coupling. In the Kondo-screened case, tuning the electron concentration can lead to a new topological insulator phase. The results suggest that the heavy-fermion phase diagram contains a new regime with a competition among topological, Kondo-coherent and magnetic states, and that the regime may be especially relevant to Kondo lattice systems with 5d-conduction electrons. Finally, we discuss the implications of our results in the context of the recent experiments on SmB6 implicating the surface states of a topological insulator, as well as the existing experiments on the phase transitions in SmB6 under pressure and in CeNiSn under chemical pressure.
Designing heterostructures with higher-temperature superconductivity
(2011-07-22)
Le Hur, Karyn
;
Chung, Chung-Hou
;
Paul, I.
;
電子物理學系
;
Department of Electrophysics
We propose to increase the superconducting transition temperature T-c of strongly correlated materials by designing heterostructures which exhibit a high pairing energy as a result of magnetic fluctuations. More precisely, applying an effective theory of the doped Mott insulator, we envisage a bilayer Hubbard system where both layers exhibit intrinsic intralayer (intraband) d-wave superconducting correlations. Introducing a finite asymmetry between the hole densities of the two layers such that one layer becomes slightly more underdoped and the other more overdoped, we show a visible enhancement of T-c compared to the optimally doped isolated layer. Using the bonding and antibonding band basis, we show that the mechanism behind this enhancement of T-c is the interband pairing correlation mediated by the hole asymmetry which strives to decrease the paramagnetic nodal contribution to the superfluid stiffness. For two identical layers, T-c remains comparable to that of the isolated layer until moderate values of the interlayer single-particle tunneling term. These heterostructures shed new light on fundamental questions related to superconductivity.
Duality in topological superconductors and topological ferromagnetic insulators in a honeycomb lattice
(2016-02-19)
Huang, Shin-Ming
;
Tsai, Wei-Feng
;
Chung, Chung-Hou
;
Mou, Chung-Yu
;
電子物理學系
The ground state of the large Hubbard U limit of a honeycomb lattice near half filling is known to be a singlet d + id-wave superconductor. It is also known that this d + id superconductor exhibits a chiral p + ip pairing locally at the Dirac cone, characterized by a 2Z topological invariant. By constructing a dual transformation, we demonstrate that this 2Z topological superconductor is equivalent to a collection of two topological ferromagnetic insulators. As a result of the duality, the topology of the electronic structures for a d + id superconductor is controllable via the change of the chemical potential by tuning the gate voltage. In particular, instead of always being a chiral superconductor, we find that the d + id superconductor undergoes a topological phase transition from a chiral superconductor to a quasihelical superconductor as the gap amplitude or the chemical potential decreases. The quasihelical superconducting phase is found to be characterized by a topological invariant in the pseudospin charge sector with vanishing both the Chern number and the spin Chern number. We further elucidate the topological phase transition by analyzing the relationship between the topological invariant and the rotation symmetry. Due to the angular momentum carried by the gap function and spin-orbit interactions, we show that by placing d + id superconductors in proximity to ferromagnets, varieties of chiral superconducting phases characterized by higher Chern numbers can be accessed, providing a platform for hosting large numbers of Majorana modes at edges.
Dynamical properties of a nonequilibrium quantum dot close to a dissipative quantum phase transition
(2011-03-07)
Chung, Chung-Hou
;
電子物理學系
;
Department of Electrophysics
The dynamical decoherence rate and charge susceptibility of a nonequilibrium quantum dot close to a dissipative quantum phase transition are calculated. The setup concerns a resonance-level quantum dot coupled to two spinless fermionic baths with a finite bias voltage and an ohmic bosonic bath representing a dissipative environment. The system is equivalent to an anisotropic Kondo model. As dissipation strength increases, the system at zero temperature and zero bias exhibits a quantum phase transition of the Kosterlitz-Thouless (KT) type between a conducting delocalized phase and an insulating localized phase. Within the nonequilibrium frequency-dependent renormalization group (RG) approach, the finite bias crossover in dynamical decoherence rate and charge susceptibility close to the transition are addressed. The dynamical decoherence rate is found to increase with increasing frequency. In the delocalized phase, it shows a singularity at frequencies equal to positive or negative bias voltage. As the system cross overs to the localized phase, the decoherence rate at low frequencies gets progressively smaller and the singular feature is gradually smeared out, leading to a single linear frequency dependence. The dynamical charge susceptibility at low frequencies shows a dip-to-peak crossover across the transition. Relevance of these results to the experiments is discussed.
Emergence of a Fermionic Finite-Temperature Critical Point in a Kondo Lattice
(2016-04-27)
Chou, Po-Hao
;
Zhai, Liang-Jun
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Chung, Chung-Hou
;
Mou, Chung-Yu
;
Lee, Ting-Kuo
The underlying Dirac point is central to the profound physics manifested in a wide class of materials. However, it is often difficult to drive a system with Dirac points across the massless fermionic critical point. Here by exploiting screening of local moments under spin-orbit interactions in a Kondo lattice, we show that below the Kondo temperature, the Kondo lattice undergoes a topological transition from a strong topological insulator to a weak topological insulator at a finite temperature T-D. At T-D, massless Dirac points emerge and the Kondo lattice becomes a Dirac semimetal. Our analysis indicates that the emergent relativistic symmetry dictates nontrivial thermal responses over large parameter and temperature regimes. In particular, it yields critical scaling behaviors both in magnetic and transport responses near T-D.
Enhanced quantum oscillations in Kondo insulators
(2020-03-02)
Lu, Yen-Wen
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Chou, Po-Hao
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Chung, Chung-Hou
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Lee, Ting-Kuo
;
Mou, Chung-Yu
Quantum oscillations have long been regarded as the manifestation of the Fermi surface in metals. However, they were recently observed in Kondo insulators. We examine the Kondo screening due to Landau levels in Kondo insulators. It is shown that, even for large Kondo insulating gaps, appreciable amplitudes of quantum oscillations that are consistent with experimental observations are present both in magnetization and resistivity. Specifically, we show that, due to the periodic alignment between the Landau levels in the conduction and the f-orbit electrons, the Kondo screening itself undergoes oscillations so that the electronic structure oscillates with the magnetic field. Our results explain main features of quantum oscillations observed in experiments. They indicate that the nonrigidity of the electronic structure results in observable quantum oscillation Kondo insulators. This effect provides a way to probe the Fermi surface geometry of insulators.
Finite-frequency-dependent noise of a quantum dot in a magnetic field
(2014-04-29)
Moca, C. P.
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Simon, P.
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Chung, Chung-Hou
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Zarand, G.
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電子物理學系
We present a detailed study for the finite- frequency current noise of a Kondo quantum dot in the presence of a magnetic field by using a recently developed real- time functional renormalization group approach [C. P. Moca, P. Simon, C. H. Chung, and G. Zarand, Phys. Rev. B 83, 201303(R) (2011)]. The scaling equations are modified in an external magnetic field; the couplings and nonlocal current vertices become strongly anisotropic, and develop new singularities. Consequently, in addition to the natural emission threshold frequency, (sis) omega = | eV |, a corresponding singular behavior is found to emerge in the noise spectrum at frequencies (sis)omega approximate to | eV +/- B|. The predicted singularities are measurable with present- day experimental techniques.
Helical Majorana fermions in d(x2-y2) + id(xy)-wave topological superconductivity of doped correlated quantum spin Hall insulators
(2016-04-11)
Sun, Shih-Jye
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Chung, Chung-Hou
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Chang, Yung-Yeh
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Tsai, Wei-Feng
;
Zhang, Fu-Chun
There has been growing interest in searching for exotic self-conjugate, charge-neutral low-energy fermionic quasi-particles, known as Majorana fermions (MFs) in solid state systems. Their signatures have been proposed and potentially observed at edges of topological superconcuctors with non-trivial topological invariant in the bulk electronic band structure. Much effort have been focused on realizing MFs in odd-parity superconductors made of strong spin-orbit coupled materials in proximity to conventional superconductors. In this paper, we propose a novel mechanism for realizing MFs in 2D spin-singlet topological superconducting state induced by doping a correlated quantum spin Hall (Kane-Mele) insulator. Via a renormalized mean-field approach, the system is found to exhibits time-reversal symmetry (TRS) breaking d(x2-y2) + id(xy)-wave (chiral d-wave) superconductivity near half-filling in the limit of large on-site repulsion. Surprisingly, however, at large spin-orbit coupling, the system undergoes a topological phase transition and enter into a new topological phase protected by a pseudo-spin Chern number, which can be viewed as a persistent extension of the quantum spin Hall phase upon doping. From bulk-edge correspondence, this phase is featured by the presence of two pairs of counter-propagating helical Majorana modes per edge, instead of two chiral propagating edge modes in the d + id' superconductors.
Kane-Mele Hubbard model on a zigzag ribbon: Stability of the topological edge states and quantum phase transitions
(2014-07-14)
Chung, Chung-Hou
;
Lee, Der-Hau
;
Chao, Sung-Po
;
電子物理學系
;
Department of Electrophysics
We study the quantum phases and phase transitions of the Kane-Mele Hubbard (KMH) model on a zigzag ribbon of honeycomb lattice at a finite size via the weak-coupling renormalization group (RG) approach. In the noninteracting limit, the Kane-Mele (KM) model is known to support topological edge states where electrons show helical property with orientations of the spin and momentum being locked. The effective interedge hopping terms are generated due to finite-size effect. In the presence of an on-site Coulomb (Hubbard) interaction and the interedge hoppings, special focus is put on the stability of the topological edge states (TI phase) in the KMH model against (i) the charge and spin gaped (II) phase, (ii) the charge gaped but spin gapless (IC) phase, and (iii) the spin gaped but charge gapless (CI) phase depending on the number (even/odd) of the zigzag ribbons, doping level (electron filling factor) and the ratio of the Coulomb interaction to the interedge tunneling. We discuss different phase diagrams for even and odd numbers of zigzag ribbons. We find the TI-CI, II-IC, and II-CI quantum phase transitions are of the Kosterlitz-Thouless (KT) type. By computing various correlation functions, we further analyze the nature and leading instabilities of these phases. The relevance of our results for graphene is discussed.
Kondo effect in a side-coupled double quantum dot system embedded in a mesoscopic ring
(2010-05-15)
Tsai, I-Ling
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Chung, Chung-Hou
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電子物理學系
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Department of Electrophysics
We study finite-size effect of the Kondo screening cloud in a double quantum dot setup via a large-N slave-boson mean-field theory. In this setup, one of the dots is embedded in a close metallic ring with a finite size L and the other dot is side coupled to the embedded dot via an antiferromagnetic spin-spin-exchange coupling with a strength K. The antiferromagnetic coupling favors local spin-singlet state and suppresses the Kondo screening. The mean-field phase diagram as a function of 1/L and K shows a crossover to the local spin-singlet ground state from the Kondo phase to occur at K < K(c) (L = 4n, 4n + 1, 4n + 3) and at K > K(c) (L = 4n + 2). The effective Kondo temperature T(k) (proptotional to inverse of the Kondo screening cloud size) shows the Kosterlitz-Thouless (KT) scaling at finite sizes for L = 4n, 4n + 2, indicating quantum transition of the KT type between the Kondo screened phase for K <= K(c) and a local spin-singlet phase for K >= K(c) in the thermodynamic limit with Kc being the critical value. The local density of states on the embedded quantum dot and the persistent current at finite sizes with different values of K are calculated in the crossover region.
Kondo effect in coupled quantum dots with RKKY interaction: Effects of finite temperature and magnetic field
(2007-07-01)
Chung, Chung-Hou
;
Hofstetter, Walter
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電子物理學系
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Department of Electrophysics
We study transport through two quantum dots coupled by a Ruderman-Kittel-Kasuya-Yoshida interaction as a function of temperature and magnetic field. By applying the numerical renormalization group method, we obtain the transmission and the linear conductance. At zero temperature and magnetic field, we observe a quantum phase transition between the Kondo screened state and a local spin singlet as the RKKY interaction is tuned. Above the critical RKKY coupling, the Kondo peak is split. However, we find that both finite temperature and magnetic field restore the Kondo resonance. Our results agree well with recent transport experiments on gold grain quantum dots in the presence of magnetic impurities [H. B. Heersche , Phys. Rev. Lett. 96, 017205 (2006)].
Mechanism of a strange metal state near a heavy-fermion quantum critical point
(2018-01-26)
Chang, Yung-Yeh
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Paschen, Silke
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Chung, Chung-Hou
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電子物理學系
;
電機學院
Unconventional metallic or strange metal (SM) behavior with non-Fermi liquid (NFL) properties, generic features of heavy-fermion systems near quantum phase transitions, are yet to be understood microscopically. A paradigmatic example is the magnetic field-tuned quantum critical heavy-fermion metal YbRh2Si2, revealing a possible SM state over a finite range of fields at low temperatures when substituted with Ge. Above a critical field, the SM state gives way to a heavy Fermi liquid with Kondo correlation. The NFL behavior, most notably a linear-in-temperature electrical resistivity and a logarithmic-in-temperature followed by a power-law singularity in the specific heat coefficient at low temperatures, still lacks a definite understanding. We propose the following mechanism as origin of the experimentally observed behavior: a quasi-2d fluctuating short-ranged resonating-valence-bond spin liquid competing with the Kondo correlation. Applying a field-theoretical renormalization group analysis on an effective field theory beyond a large-N approach to an antiferromagnetic Kondo-Heisenberg model, we identify the critical point and explain remarkably well the SM behavior. Our theory goes beyond the well-established framework of quantum phase transitions and serves as a basis to address open issues in quantum critical heavy-fermion systems.
Non-Centrosymmetric Superconductors on Honeycomb Lattice
(2018-09-01)
Lee, Der-Hau
;
Chung, Chung-Hou
;
電子物理學系
;
Department of Electrophysics
Non-centrosymmetric topological superconductivity is studied in correlated doped quantum spin-Hall insulators (QSHI) on honeycomb lattice without inversion symmetry where the intrinsic (Kane-Mele) and Rashba spin-orbit couplings can in general exist. The generic topologically non-trivial superconducting phase diagram of the model system is explored. Over a certain parameter range, the parity-mixing superconducting state with co-existing spin-singlet d+id and spin-triplet p+ip-wave pairing is found. On a zigzag nanoribbon, the parity-mixing superconducting state shows co-existing helical and chiral Majorana fermions at edges. The relevance of our results for experiments is discussed.
Nonequilibrium noise correlations in a point contact of helical edge states
(2012-12-17)
Lee, Yu-Wen
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Lee, Yu-Li
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Chung, Chung-Hou
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電子物理學系
;
Department of Electrophysics
We investigate theoretically the nonequilibrium finite-frequency current noise in a four-terminal quantum point contact of interacting helical edge states at a finite bias voltage. Special focus is put on the effects of the single-particle and two-particle scattering between the two helical edge states on the fractional charge quasiparticle excitations shown in the nonequilibrium current noise spectra. Via the Keldysh perturbative approach, we find that the effects of the single-particle and the two-particle scattering processes on the current noise depend sensitively on the Luttinger liquid parameter. Moreover, the Fano factors for the auto-and cross correlations of the currents in the terminals are distinct from the ones for tunneling between the chiral edge states in the quantum Hall liquid. The current noise spectra in the single-particle-scattering-dominated and the two-particle-scattering-dominated regime are shown. Experimental implications of our results on the transport through the helical edges in two-dimensional topological insulators are discussed. DOI: 10.1103/PhysRevB.86.235121
Nonequilibrium occupation number and charge susceptibility of a resonance level close to a dissipative quantum phase transition
(2010-08-30)
Chung, Chung-Hou
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Latha, K. V. P.
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電子物理學系
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Department of Electrophysics
Based on the recent paper [Phys. Rev. Lett. 102, 216803 (2009)], we study the nonequilibrium occupation number n(d) and charge susceptibility chi of a resonance level close to dissipative quantum phase transition of the Kosterlitz-Thouless (KT) type between a delocalized phase for weak dissipation and a localized phase for strong dissipation. The resonance level is coupled to two spinless fermionic baths with a finite bias voltage and an Ohmic bosonic bath representing the dissipative environment. The system is equivalent to an effective anisotropic Kondo model out of equilibrium. Within the nonequilibrium renormalization-group approach, we calculate nonequilibrium magnetization M and spin susceptibility chi in the effective Kondo model, corresponding to 2n(d)-1 and chi of a resonance level, respectively. We demonstrate the smearing of the KT transition in the nonequilibrium magnetization M as a function of the effective anisotropic Kondo couplings, in contrast to a perfect jump in M at the transition in equilibrium. In the limit of large bias voltages, we find M and chi at the KT transition and in the localized phase show deviations from the equilibrium Curie-law behavior. As the system gets deeper in the localized phase, both n(d)-1/2 and chi decrease more rapidly to zero with increasing bias voltages.
Nonequilibrium quantum transport through a dissipative resonant level
(2013-06-21)
Chung, Chung-Hou
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Le Hur, Karyn
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Finkelstein, Gleb
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Vojta, Matthias
;
Woelfle, Peter
The resonant-level model represents a paradigmatic quantum system which serves as a basis for many other quantum impurity models. We provide a comprehensive analysis of the nonequilibrium transport near a quantum phase transition in a spinless dissipative resonant-level model, extending earlier work [Phys. Rev. Lett 102, 216803 (2009)]. A detailed derivation of a rigorous mapping of our system onto an effective Kondo model is presented. A controlled energy-dependent renormalization-group approach is applied to compute the nonequilibrium current in the presence of a finite bias voltage V. In the linear-response regime V -> 0, the system exhibits as a function of the dissipative strength a localized-delocalized quantum transition of the Kosterlitz-Thouless (KT) type. We address fundamental issues of the nonequilibrium transport near the quantum phase transition: Does the bias voltage play the same role as temperature to smear out the transition? What is the scaling of the nonequilibrium conductance near the transition? At finite temperatures, we show that the conductance follows the equilibrium scaling for V < T, while it obeys a distinct nonequilibrium profile for V > T. We furthermore provide different signatures of the transition in the finite-frequency current noise and ac conductance via a recently developed functional renormalization group (FRG) approach. The generalization of our analysis to nonequilibrium transport through a resonant level coupled to two chiral Luttinger liquid leads, generated by fractional quantum Hall edge states, is discussed. Our work on the dissipative resonant level has direct relevance to experiments on a quantum dot coupled to a resistive environment, such as H. Mebrahtu et al., [Nature (London) 488, 61 (2012)].
Nonequilibrium Transport at a Dissipative Quantum Phase Transition
(2009-05-29)
Chung, Chung-Hou
;
Le Hur, Karyn
;
Vojta, Matthias
;
Woelfle, Peter
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電子物理學系
We investigate the nonequilibrium transport near a quantum phase transition in a generic and relatively simple model, the dissipative resonant level model, that has many applications for nanosystems. We formulate a rigorous mapping and apply a controlled frequency-dependent renormalization group approach to compute the nonequilibrium current in the presence of a finite bias voltage V and a finite temperature T. For V -> 0, we find that the conductance has its well-known equilibrium form, while it displays a distinct nonequilibrium profile at finite voltage.
Nonequilibrium transport of helical Luttinger liquids through a quantum dot
(2013-08-08)
Chao, Sung-Po
;
Silotri, Salman A.
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Chung, Chung-Hou
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電子物理學系
;
Department of Electrophysics
We study a steady-state nonequilibrium transport between two interacting helical edge states of a two-dimensional topological insulator, described by helical Luttinger liquids, through a quantum dot. For a noninteracting dot, the current is obtained analytically by including the self-energy correction to the dot's Green function. For an interacting dot, we use the equation-of-motion method to study the influence of weak on-site Coulomb interaction on the transport. We find the metal-to-insulator quantum phase transition for attractive or repulsive interactions in the leads when the magnitude of the interaction strength characterized by a charge sector Luttinger parameter K goes beyond a critical value. The critical Luttinger parameter K-cr depends on the hopping strengths between the dot and the leads, as well as the energy level of the dot with respect to the Fermi levels of the leads, ranging from the weak-interaction regime for the dot level off-resonance to the strong-interaction regime for the dot in resonance with the equilibrium Fermi level. Near the transition, there are various singular behaviors of current noise, dot density of state, and the decoherence rate (inverse of lifetime) of the dot, which are briefly discussed.