arXiv subjects

Chung-Yu Mou

Publications and source records attributed to Chung-Yu Mou.

At least 37 records · Page 2Linked to original sources

arXiv2016

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

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.

cond-mat.str-el↗

arXiv2016

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

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.

cond-mat.supr-con↗

arXiv2015

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

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.

cond-mat.quant-gas↗

arXiv2015

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

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.

cond-mat.supr-con↗

arXiv2014

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

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.

cond-mat.mtrl-sci↗

arXiv2014

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

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.

cond-mat.mtrl-sci↗

arXiv2014

A Memory of Majorana Fermions through Quantum Quench

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.

cond-mat.mes-hall↗

arXiv2013

Intrinsic high-temperature superconductivity in ternary iron selenides

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.

cond-mat.supr-con↗

arXiv2013

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

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.

cond-mat.supr-con↗

arXiv2013

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

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.

cond-mat.supr-con↗

arXiv2012

Quench Dynamics of Topological Maximally-Entangled States

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.

cond-mat.str-el↗

arXiv2012

Persistent currents in a graphene ring with armchair edges

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.

cond-mat.mes-hall↗

arXiv2012

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

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 $J_{H}$.

cond-mat.supr-con↗

arXiv2011

Folding a protein with equal probability of being helix or hairpin

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.

cond-mat.soft↗

arXiv2011

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

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.

cond-mat.str-el↗

arXiv2011

Graphene-based modulation-doped superlattice structures

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.

cond-mat.mes-hall↗

arXiv2010

Dynamical Decoupling of Qubits in Spin Bath under Periodic Quantum Control

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.

quant-ph↗

arXiv2010

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

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.

cond-mat.str-el↗