arXiv subjects

Geng Li

Publications and source records attributed to Geng Li.

87 records · Page 5Linked to original sources

arXiv2020

Tunable vortex Majorana zero modes in LiFeAs superconductor

The recent realization of pristine Majorana zero modes (MZMs) in vortices of iron-based superconductors (FeSCs) provides a promising platform for long-sought-after fault-tolerant quantum computation. A large topological gap between the MZMs and the lowest excitations enabled detailed characterization of vortex MZMs in those materials. Despite those achievements, a practical implementation of topological quantum computation based on MZM braiding remains elusive in this new Majorana platform. Among the most pressing issues are the lack of controllable tuning methods for vortex MZMs and inhomogeneity of the FeSC Majorana compounds that destroys MZMs during the braiding process. Thus, the realization of tunable vortex MZMs in a truly homogeneous compound of stoichiometric composition and with a charge neutral cleavage surface is highly desirable. Here we demonstrate experimentally that the stoichiometric superconductor LiFeAs is a good candidate to overcome these two obstacles. Using scanning tunneling microscopy, we discover that the MZMs, which are absent on the natural surface, can appear in vortices influenced by native impurities. Our detailed analysis and model calculations clarify the mechanism of emergence of MZMs in this material, paving a way towards MZMs tunable by controllable methods such as electrostatic gating. The tunability of MZMs in this homogeneous material offers an unprecedented platform to manipulate and braid MZMs, the essential ingredients for topological quantum computation.

cond-mat.supr-con↗

arXiv2020

Relativistic calculations of $R(D^{()})$, $R(D^{()}_s)$, $R(η_c)$ and $R(J/ψ)$

Recently, the deviation of the ratios $R(D)$, $R(D^{*})$ and $R(J/ψ)$ have been found between experimental data and the Standard Model predictions, which may be the hint of New Physics. In this work, we calculate these ratios within the Standard Model by using the improved instantaneous Bethe-Salpeter method. The emphasis is pad to the relativistic correction of the form factors. The results are $R(D)=0.312 ^{+0.006}_{-0.007}$, $R(D^*)= 0.249^{+0.001}_{-0.002}$, $R(D_s)=0.320 ^{+0.009}_{-0.009}$, $R(D^*_s)=0.251 ^{+0.002}_{-0.003}$, $R(η_c)=0.384 ^{+0.032}_{-0.042}$, and $R(J/ψ)=0.267 ^{+0.009}_{-0.011}$, which are consistent with predictions of other models and the experimental data. The semileptonic decay rates and corresponding form factors at zero recoil are also given.

hep-ph↗

arXiv2020

How initial distribution affects symmetry breaking induced by panic in ants: experiment and flee-pheromone model

Collective escaping is a ubiquitous phenomenon in animal groups. Symmetry breaking caused by panic escape exhibits a shared feature across species that one exit is used more than the other when agents escaping from a closed space with two symmetrically located exists. Intuitively, one exit will be used more by more individuals close to it, namely there is an asymmetric distribution initially. We used ant groups to investigate how initial distribution of colonies would influence symmetry breaking in collective escaping. Surprisingly, there was no positive correlation between symmetry breaking and the asymmetrically initial distribution, which was quite counter-intuitive. In the experiments, a flee stage was observed and accordingly a flee-pheromone model was introduced to depict this special behavior in the early stage of escaping. Simulation results fitted well with the experiment. Furthermore, the flee stage duration was calibrated quantitatively and the model reproduced the observation demonstrated by our previous work. This paper explicitly distinguished two stages in ant panic escaping for the first time, thus enhancing the understanding in escaping behavior of ant colonies.

q-bio.QM↗

arXiv2020

Localized spin-orbit polaron in magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$

The kagome lattice Co$_3$Sn$_2$S$_2$ exhibits the quintessential topological phenomena of a magnetic Weyl semimetal such as the chiral anomaly and Fermi-arc surface states. Probing its magnetic properties is crucial for understanding this correlated topological state. Here, using spin-polarized scanning tunneling microscopy/spectroscopy (STM/S), we report the discovery of localized spin-orbit polarons (SOPs) with three-fold rotation symmetry nucleated around single-S vacancies in Co$_3$Sn$_2$S$_2$. The SOPs carry a spin-polarized magnetic moment and a large orbital magnetization of a topological origin associated with the Berry phase and the persistent circulating current. Appreciable magneto-elastic coupling of the SOP is detected by atomic force microscope and STM. Our findings suggest that the SOPs can enhance magnetism and stability of the magnetic Weyl nodes for more robust time-reversal-symmetry-breaking topological phenomena. Controlled engineering of the SOPs may pave the way toward practical applications in functional quantum devices.

cond-mat.mtrl-sci↗

arXiv2019

Study of the dilepton electromagnetic decays of $χ_{cJ}(1P)$

In this paper, the dilepton electromagnetic decays $χ_{cJ}(1P) \to J/ψe^+e^-$ and $χ_{cJ}(1P) \to Jψμ^+μ^-$, where $χ_{cJ}$ denotes $χ_{c0}$, $χ_{c1}$ and $χ_{c2}$, are calculated systematically in the improved Bethe-Salpeter method. The numerical results of decay widths and the invariant mass distributions of the final lepton pairs are given. The comparison is made with the recently measured experimental data of BESIII. It is shown that for the cases including $e^+e^-$, the gauge invariance is decisive and should be considered carefully. For the processes of $χ_{cJ}(1P) \to J/ψe^+e^-$, the branching fraction are: $\mathcal{B}[χ_{c0}(1P) \to J/ψe^+e^-]=1.06^{+0.16}_{-0.18} \times 10^{-4}$, $\mathcal{B}[χ_{c1}(1P) \to J/ψe^+e^-]=2.88^{+0.50}_{-0.53} \times 10^{-3}$, and $\mathcal{B}[χ_{c2}(1P) \to J/ψe^+e^-]=1.74^{+0.22}_{-0.21} \times 10^{-3}$. The calculated branching fractions of $χ_{cJ}(1P)\to J/ψμ^+μ^-$ channels are: $\mathcal{B}[χ_{c0}(1P) \to J/ψμ^+μ^-]=3.80^{+0.59}_{-0.64} \times 10^{-6}$, $\mathcal{B}[χ_{c1}(1P) \to J/ψμ^+μ^-]=2.04^{+0.36}_{-0.38} \times 10^{-4}$, and $\mathcal{B}[χ_{c2}(1P) \to J/ψμ^+μ^-]=1.66^{+0.19}_{-0.19} \times 10^{-4}$.

hep-ph↗

arXiv2019

Rates of $D^{}_{0}(2400)$, $ D_J^(3000) $ as the $D^{}_{0}(2P)$ and $D^{}_{0}(3P)$ in $B$ Decays

In this paper, we use the instantaneous Bethe-Salpeter method to calculate the semi-leptonic and non-leptonic production of the orbitally excited scalar $ D_0^* $ in $ B $ meson decays. When the final state is $ 1P $ state $ D_0^*(2400) $, our theoretical decay rate is consistent with experimental data. For $ D_J^*(3000) $ final state, which was observed by LHCb collaboration recently and here treated as the orbitally excited scalar $D^{*}_{0}(2P)$, its rate is in the order of $ 10^{-4} \sim 10^{-6}$. We find the special node structure of $D^{*}_{0}(2P)$ wave function possibly results in the suppression of its branching ratio and the abnormal uncertainty. The $3P$ states production rate is in the order of $ 10^{-5}$.

hep-ph↗

arXiv2019

Stochastic thermodynamics with odd controlling parameters

Stochastic thermodynamics extends the notions and relations of classical thermodynamics to small systems that experience strong fluctuations. The definitions of work and heat and the microscopically reversible condition are two key concepts in the current framework of stochastic thermodynamics. Herein, we apply stochastic thermodynamics to small systems with odd controlling parameters and find that the definition of heat and the microscopically reversible condition are incompatible. Such a contradiction also leads to a revision to the fluctuation theorems and nonequilibrium work relations. By introducing adjoint dynamics, we find that the total entropy production can be separated into three parts, with two of them satisfying the integral fluctuation theorem. Revising the definitions of work and heat and the microscopically reversible condition allows us to derive two sets of modified nonequilibrium work relations, including the Jarzynski equality, the detailed Crooks work relation, and the integral Crooks work relation. We consider the strategy of shortcuts to isothermality as an example and give a more sophisticated explanation for the Jarzynski-like equality derived from shortcuts to isothermality.

cond-mat.stat-mech↗

arXiv2019

A new Majorana platform in an Fe-As bilayer superconductor

Recently, iron-chalcogenide superconductors have emerged as a new and promising platform for studying and manipulating Majorana zero mode (MZM). By combining topological band structure and superconductivity in a multiband material, they provide significant advantages such as higher superconducting transition temperature (Tc) and isolated Majorana mode. However, iron-chalcogenide superconductors, especially Fe(Te,Se), suffer from strong inhomogeneity which may hamper their practical application. On the other hand, some iron-pnictide (Fe-As) superconductors, such as LiFeAs, have been demonstrated to have a similar topological band structure, yet no MZM has been observed in its vortex cores, raising a question of universality of MZM presence in iron-based superconductors. In this work, by using high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy, we identify the first Fe-As superconductor CaKFe4As4 (Tc = 35 K) which has both the superconducting Dirac surface states and the MZMs inside its vortex cores. The topological band inversion is largely due to the down-shift of the pz band caused by the bilayer band folding in this material. More strikingly, the energies and spatial line profiles of MZM and multiple quantized Caroli-de Gennes-Matricon bound states observed inside the topological vortex can be accurately reproduced by a simple theoretical model derived from a surface Dirac cone, firmly establishing Majorana nature of the zero mode.

cond-mat.supr-con↗

arXiv2019

The study of light invisible particles in $B_c$ decays

In this paper, we study the light scalar and pseudoscalar invisible particles in the flavor changing neutral current processes of the $B_c$ meson. Effective operators are introduced to describe the couplings between quarks and light invisible particles. The Wilson coefficients are extracted from the experimental results of the $B$ and $D$ mesons, which are used to predict the upper limits of the branching fractions of the similar decay processes for the $B_c$ meson. The hadronic transition matrix element is calculated with the instantaneously approximated Bethe-Salpeter method. The upper limits of the branching fractions when $m_χ$ taking different values are presented. It is found that at some region of $m_χ$, the channel $B_c\to D_s^{(\ast)}χχ$ has the largest upper limit which is of the order of $10^{-6}$, and for $B_c\to D_s^\astχχ^\dagger$, the largest value of the upper limits can achieve the order of $10^{-5}$. Other decay modes, such as $B_c\to D^{(*)}χχ^{(\dagger)}$ and $B_c\to B^{(*)}χχ^{(\dagger)}$, are also considered.

hep-ph↗

arXiv2018

G7.7-3.7: a young supernova remnant probably associated with the guest star in 386 CE (SN 386)

Although the Galactic supernova rate is about 2 per century, only few supernova remnants are associated with historical records. There are a few ancient Chinese records of "guest stars" that are probably sightings of supernovae for which the associated supernova remnant is not established. Here we present an X-ray study of the supernova remnant G7.7-3.7, as observed by XMM-Newton, and discuss its probable association with the guest star of 386 CE. This guest star occurred in the ancient Chinese asterism Nan-Dou, which is part of Sagittarius. The X-ray morphology of G7.7-3.7 shows an arc-like feature in the SNR south, which is characterized by an under-ionized plasma with sub-solar abundances, a temperature of 0.4--0.8 keV, and a density of $\sim 0.5(d/4 kpc)^{-0.5}$ cm$^{-3}$. A small shock age of $1.2\pm 0.6 (d/4 kpc)^{0.5}$ kyr is inferred from the low ionization timescale of $2.4^{+1.1}_{-1.3}\times 10^{10}$ cm$^{-3}$ s of the X-ray arc. The low foreground absorption ($N_H=3.5\pm0.5\times 10^{21}$ cm$^{-2}$) of G7.7-3.7 made the supernova explosion visible to the naked eyes on the Earth. The position of G7.7-3.7 is consistent with the event of 386 CE, and the X-ray properties suggest that also its age is consistent. Interestingly, the association between G7.7-3.7 and guest star 386 would suggest the supernova to be a low-luminosity supernova, in order to explain the not very long visibility (2--4 months) of the guest star.

astro-ph.HE↗

arXiv2018

Relativistic effects in the semileptonic $B_c$ decays to charmonium with the Bethe-Salpeter method

Relativistic effects are important in the rigorous study of heavy quarks. In this paper, we study the relativistic corrections of semileptonic $B_c$ decays to charmonium with the instantaneous Bethe-Salpeter method. Within the Bethe-Salpeter framework, we use two methods to study the relativistic effects. One of them is to expand the transition amplitude in powers of $\vec{q}$ which is the relative momentum between the quark and antiquark, and the other is to expand the amplitude base on the wave functions. In the level of decay width, the results show that, for the transition of $B_c\to η_c$, the relativistic correction is about $22\%$; for $B_c\to J/ψ$, it is about $19\%$; the relativistic effects of $1P$ final states are about $14\sim46\%$ larger than those of $1S$ final states; for $2S$ final states, they are about $19\sim 28\%$ larger than those of $1S$ final states; for $3S$ final states, they are about $12\sim13\%$ larger than those of $2S$ final states; for $2P$ final states, they are about $10\sim14\%$ larger than those of $1P$ final states; for $3P$ final states, they are about $7\sim12\%$ larger than those of $2P$ final states. We conclude that the relativistic corrections of the $B_c$ decays to the orbitally or radially excited charmonium (2S, 3S, 1P, 2P, 3P) are quite large.

hep-ph↗

arXiv2018

Doubly-charged scalar in four-body decays of neutral flavored mesons

In this paper, we study the four-body decay processes of neutral flavored mesons, including $\bar K^0$, $D^0$, $\bar B^0$, and $\bar B_s^0$. These processes, which induced by a hypothetical doubly-charged scalar particle, violate the lepton number. The quantity $Br\times\left(\frac{s_Δh_{ij}}{m_Δ^2}\right)^{-2}$ of different channels are calculated, where $s_Δ$, $h_{ij}$, and $M_Δ$ are parameters related to the doubly-charged scalar. For $\bar K^0\rightarrow h_1^+h_2^+l_1^-l_2^-$, $D^0\rightarrow h_1^-h_2^-l_1^+l_2^+$, and $\bar B_{d,s}^0\rightarrow h_1^+h_2^+l_1^-l_2^-$, it is of the order of $10^{-13}\sim 10^{-11}$ ${\rm GeV^4}$, $10^{-17}\sim 10^{-10}$ ${\rm GeV^4}$, and $10^{-17}\sim 10^{-10}$ ${\rm GeV^4}$, respectively. Based on the experimental results for the $D^0\rightarrow h_1^-h_2^-l_1^+l_2^+$ channels, we also set the upper limit for $\frac{s_Δh_{ij}}{M_Δ^2}$.

hep-ph↗

arXiv2017

Shortcuts to isothermality and nonequilibrium work relations

In conventional thermodynamics, it is widely acknowledged that the realization of an isothermal process for a system requires a quasi-static controlling protocol. Here we propose and design a strategy to realize a finite-rate isothermal transition from an equilibrium state to another one with same temperature, which is named shortcut to isothermality~(STI). By using STI, we unexpectedly derive three nonequilibrium work relations, including an identity between the free energy difference and the mean work due to the potential of the original system, a generalized Jarzynski equality, and the inverse relationship between the irreversible work and the total driving time. We numerically confirm these three relations by considering the motion of a Brownian particle trapped in a harmonic potential and dragged by a time-dependent force.

cond-mat.stat-mech↗

arXiv2016

Improved 2D Intelligent Driver Model simulating synchronized flow and evolution concavity in traffic flow

This paper firstly show that 2 Dimensional Intelligent Driver Model (Jiang et al., PloS one, 9(4), e94351, 2014) is not able to replicate the synchronized traffic flow. Then we propose an improved model by considering the difference between the driving behaviors at high speeds and that at low speeds. Simulations show that the improved model can reproduce the phase transition from synchronized flow to wide moving jams, the spatiotemporal patterns of traffic flow induced by traffic bottleneck, and the evolution concavity of traffic oscillations (i.e. the standard deviation of the velocities of vehicles increases in a concave/linear way along the platoon). Validating results show that the empirical time series of traffic speed obtained from Floating Car Data can be well simulated as well.

nlin.PS↗

arXiv2015

Retainability of canonical distributions for a Brownian particle controlled by a time-dependent harmonic potential

The retainability of canonical distributions for a Brownian particle controlled by a time-dependent harmonic potential is investigated in the overdamped and underdamped situations, respectively. Because of different time scales, the overdamped and underdamped Langevin equations (as well as the corresponding Fokker-Planck equations) lead to distinctive restrictions on protocols maintaining canonical distributions. Two special cases are analyzed in details: First, a Brownian particle is controlled by a time-dependent harmonic potential and embedded in medium with constant temperature; Second, a Brownian particle is controlled by a time-dependent harmonic potential and embedded in a medium whose temperature is tuned together with the potential stiffness to keep a constant effective temperature of the Brownian particle. We find that the canonical distributions are usually retainable for both the overdamped and underdamped situations in the former case. However, the canonical distributions are retainable merely for the overdamped situation in the latter case. We also investigate general time-dependent potentials beyond the harmonic form and find that the retainability of canonical distributions depends sensitively on the specific form of potentials.

cond-mat.stat-mech↗