arXiv subjects

Sung-Po Chao

Publications and source records attributed to Sung-Po Chao.

15 recordsLinked to original sources

arXiv2024

Pursuing equitable access to vaccines for the next epidemic

To mitigate the pandemic stemming from COVID-19, numerous nations have initiated extensive vaccination campaigns for their citizens since late 2020. While affluent countries have predominantly received vaccine allocations, fewer doses have been dispatched to nations with lower average incomes. This unequal distribution not only widens the disparity between wealthy and impoverished regions but also prolongs the pandemic, evident in the emergence of new viral variants. Our research delves into the correlation between the duration of the pandemic and the timing of vaccine distribution between two countries with migratory ties. By using a pair of coupled Susceptible- Infected-Recovered-Deceased (SIRD) models incorporating vaccination data, we demonstrate that timely sharing of vaccines benefits both nations, regardless of the presence of viral variants. This underscores that in the realm of vaccine distribution, self-interest and altruism are not mutually exclusive.

physics.soc-ph↗

arXiv2020

Simplified model on the timing of easing the lockdown

Lockdown procedures have been proven successful in mitigating the spread of the viruses in this COVID-19 pandemic, but they also have devastating impact on the economy. We use a modified Susceptible-Infectious-Recovered-Deceased model with time dependent infection rate to simulate how the infection is spread under lockdown. The economic cost due to the loss of workforce and incurred medical expenses is evaluated with a simple model. We find the best strategy, meaning the smallest economic cost for the entire course of the pandemic, is to keep the strict lockdown as long as possible.

physics.soc-ph↗

arXiv2019

Superconductivity in the Bi/Ni bilayer

Motivated by the recent observations of possible p-wave superconductivity in the bismuth-nickel (Bi/Ni) bilayer, we explore theoretically the possibilities of realizing p-wave superconductivity in this bilayer. We begin with a literature survey on this system and related materials which have similar superconducting transition temperature. From the survey, the superconducting mechanism in this bilayer system is suggested to be phonon mediated type II superconductivity. A simple model is proposed to explain why the p-wave like Andreev reflection signals are likely to be observed in the surface probe, assuming the strong spin-orbit coupled surface state of Bi thin film is not completely destroyed by the formation of alloys.

cond-mat.str-el↗

arXiv2016

Controversy over large proximity induced s wave like pairing from a d wave superconductor

Proximity effect is used to generate effective topological superconductor by placing strong spin orbit interacting metals with superconducting materials, aiming to produce Majorana zero modes useful for topologically protected quantum computations. In recent experiments high $T_c$ material $\text{Bi}_2\text{Sr}_2\text{CaCu}_2\text{O}_{8+δ}$ is put in contact with a few quintuple layer thick $\text{Bi}_2\text{Se}_3$, and conflicting experimental results are reported. We use standard mean field approach to study this heterostructure, and find it is unlikely to have large proximity induced superconducting gap. The claimed observed s-wave gap might not be purely superconducting coherence gap despite its seemingly correct temperature dependence. Further study on the proximity induced bulk coherence gap and the band structure at the interface should shed light on this issue.

cond-mat.str-el↗

arXiv2015

Tunneling between helical Majorana modes and helical Luttinger liquids

We propose and study the charge transport through single and double quantum point contacts setup between helical Majorana modes and an interacting helical Luttinger liquid. We show that the differential conductance decreases for stronger repulsive interactions and that the point contacts become insulating above a critical interaction strength. For a single point contact, the differential conductance as a function of bias voltage shows a series of peaks due to Andreev reflection of electrons in the Majorana modes. In the case of two point contacts, interference phenomena make the structure of the individual resonance peaks less universal and show modulations with different separation distance between the contacts. For small separation distance the overall features remain similar to the case of a single point contact.

cond-mat.str-el↗

arXiv2014

Decoherence Patterns of Topological Qubits from Majorana Modes

We investigate the decoherence patterns of topological qubits in contact with the environment by a novel way of deriving the open system dynamics other than the Feynman-Vernon. Each topological qubit is made of two Majorana modes of a 1D Kitaev's chain. These two Majorana modes interact with the environment in an incoherent way which yields peculiar decoherence patterns of the topological qubit. More specifically, we consider the open system dynamics of the topological qubits which are weakly coupled to the fermionic/bosonic Ohmic-like environments. We find atypical patterns of quantum decoherence. In contrast to the cases of non-topological qubits for which they always decohere completely in all Ohmic-like environments, the topological qubits decohere completely in the Ohmic and sub-Ohmic environments but not in the super-Ohmic ones. Moreover, we find that the fermion parities of the topological qubits though cannot prevent the qubit states from decoherence in the sub-Ohmic environments, can prevent from thermalization turning into Gibbs state. We also study the cases in which each Majorana mode can couple to different Ohmic-like environments and the time dependence of concurrence for two topological qubits.

cond-mat.str-el↗

arXiv2014

Long-range interaction induced phases in Weyl semimetals

The interplay of spin orbit coupling and electron electron interaction condensing new phases of matter is an important new phenomena in solid state physics. In this paper we explore the nature of excitonic phases induced in Weyl semimetals by long range Coulomb repulsion. Its has been previously shown that short range repulsion leads to a ferromagnetic insulator while short range attraction results in a charge density wave state. Here we show the the charge density wave is the energetically favored state in the presence of long range repulsion.

cond-mat.str-el↗

arXiv2014

Kane-Mele Hubbard model on a zigzag ribbon: stability of the topological edge states and quantum phase transitions

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 non-interacting limit, the 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 inter-edge hopping terms are generated due to finite-size effect. In the presence of an on-site Coulomb repulsive interaction and the inter-edge 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 inter-edge 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.

cond-mat.str-el↗

arXiv2013

Odd parity superconductivity in Weyl semimetals

Unconventional superconducting states of matter are realized in the presence of strong spin orbit coupling. In particular, non degenerate bands can support odd parity superconductivity with rich topological content. Here we study whether this is the case for Weyl semimetals. These are systems whose low energy sector, in the absence of interactions, is described by linearly dispersing chiral fermions in three dimensions. The energy spectrum has nodes at an even number of points in the Brillouin zone. Consequently both intranodal finite momentum pairing and internodal BCS superconductivity are allowed. For local attractive interaction the finite momentum pairing state with chiral p-wave symmetry is found to be most favorable at finite chemical potential. The state is an analog of the superfluid $^{3}$He A phase, with cooper pairs having finite center of mass momentum. For chemical potential at the node the state is preempted by a fully gapped charge density wave. For long range attraction the BCS state wins out for all values of the chemical potential.

cond-mat.supr-con↗

arXiv2013

Nonequilibrium transport of helical Luttinger liquids through a quantum dot

We study a steady state non-equilibrium transport between two interacting helical edge states of a two dimensional topological insulator, described by helical Luttinger liquids, through a quantum dot. For non-interacting dot the current is obtained analytically by including the self-energy correction to the dot Green's function. For interacting dot we use 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 hoping strength between dot and the leads as well as the energy level of the dot with respect to the Fermi levels of the leads, ranging from weak interaction regime for dot level off resonance to strong interaction regime for dot in resonance with the equilibrium Fermi level. Nearby the transition various singular behaviors of current noise, dot density of state, and the decoherence rate (inverse of lifetime) of the dot are briefly discussed.

cond-mat.str-el↗

arXiv2012

Excitonic Phases from Weyl Semi-Metals

Systems with strong spin-orbit coupling, which competes with other interactions and energy scales, offer a fertile playground to explore new correlated phases of matter. Weyl semimetals are an example where the phenomenon leads to a low energy effective theory in terms of massless linearly dispersing fermions in three dimensions. In the absence of interactions chirality is a conserved quantum number, protecting the semi-metallic physics against perturbations that are translationally invariant. In this letter we show that the interplay between interaction and topology yields a novel chiral excitonic insulator. The state is characterized by a complex vectorial order parameter leading to a gapping out of the Weyl nodes. A striking feature is that it is ferromagnetic, with the phase of the order parameter determining the direction of the induced magnetic moment.

cond-mat.str-el↗

arXiv2012

Effect of inplane electric field on magnetotransport in helical metal

The existence of helical surface states in a bulk insulator, with anomalous magneto-electric properties, is a remarkable new development in solid state physics. The linear dispersion of the the fermions leads to a form of Lorentz invariance, with the Fermi velocity playing the role of the velocity of light ($c$). In a crossed electric and magnetic field the single particle states form Landau levels whose energies can be changed by varying the applied \emph{in-plane electric} field. The degeneracy remains constant and is determined by the magnetic field. In the letter we study the nature of the mangeto-oscillation in conductivity and thermopower as a function of the \emph{in-plane electric} field.

cond-mat.mtrl-sci↗

arXiv2011

Kondo and charge fluctuation resistivity due to Anderson impurities in graphene

Motivated by experiments on ion irradiated graphene, we compute the resistivity of graphene with dilute impurities. In the local moment regime we employ the perturbation theory up to third order in the exchange coupling to determine the behavior at high temperatures within the Kondo model. Resistivity due to charge fluctuations is obtained within the mean field approach on the Anderson impurity model. Due to the linear spectrum of the graphene the Kondo behavior is shown to depend on the gate voltage applied. The location of the impurity on the graphene sheet is an important variable determining its effect on the Kondo scale and resistivity. Our results show that for chemical potential nearby the node the charge fluctuations is responsible for the observed temperature dependence of resistivity while away from the node the spin fluctuations take over. Quantitative agreement with experimental data is achieved if the energy of the impurity level varies linearly with the chemical potential.

cond-mat.mtrl-sci↗

arXiv2011

Electric and thermoelectric transport in graphene and helical metal in finite magnetic fields

We study electrical and thermoelectric transport properties of the surface state of the topological insulator and graphene in the presence of randomly distributed impurities. For finite impurity strength, the dependence of the transport coefficients as a function of gate voltage, magnetic field and impurity potential, are obtained numerically. In the limit of zero impurities (clean limit), analytic results for the peak values of the magneto-oscillations in thermopower are derived. Analogous with the conventional two dimensional electron gas, the peak values are universal in the clean limit. Unlike graphene, in topological insulators the coupling of the electron spin to its momentum leads to a dependence of the transport coefficients on the gyromagnetic ratio ($g$). We compare our results with data on graphene and identify unique signatures expected in topological insulators due to the magnetoelectric coupling.

cond-mat.mtrl-sci↗

arXiv2011

Non-equilibrium Transport in the Anderson model of a biased Quantum Dot: Scattering Bethe Ansatz Phenomenology

We derive the transport properties of a quantum dot subject to a source-drain bias voltage at zero temperature and magnetic field. Using the Scattering Bethe Anstaz, a generalization of the traditional Thermodynamic Bethe Ansatz to open systems out of equilibrium, we derive exact results for the quantum dot occupation out of equilibrium and, by introducing phenomenological spin- and charge-fluctuation distribution functions in the computation of the current, obtain the differential conductance for large U/Γ. The Hamiltonian to describe the quantum dot system is the Anderson impurity Hamiltonian and the current and dot occupation as a function of voltage are obtained numerically. We also vary the gate voltage and study the transition from the mixed valence to the Kondo regime in the presence of a non-equilibrium current. We conclude with the difficulty we encounter in this model and possible way to solve them without resorting to a phenomenological method.

cond-mat.str-el↗