Jun Song - UBC

Jun Song
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Jun Song

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Pub Categories

High Energy Physics - Phenomenology (14)
Nuclear Theory (13)
Physics - Materials Science (7)
Physics - Other (7)
High Energy Physics - Theory (6)
Mathematics - Algebraic Geometry (3)
Quantum Physics (2)
Physics - Strongly Correlated Electrons (2)
Physics - Mesoscopic Systems and Quantum Hall Effect (2)
Statistics - Machine Learning (2)
Quantitative Biology - Quantitative Methods (1)
Statistics - Computation (1)
Physics - Disordered Systems and Neural Networks (1)
Nuclear Experiment (1)
Physics - Atomic Physics (1)
Mathematics - Numerical Analysis (1)

Publications Authored By Jun Song

Yield ratios of identified hadrons observed in high multiplicity p+p and p+Pb collisions at LHC show remarkable similarity with those in Pb+Pb collisions, indicating some important and universal underlying dynamics in hadron production for different quark gluon final states. We use the quark combination model to explain the data of yield ratios in these three collision systems. The observed $p/\pi$ and $\Lambda/K_s^0$ can be reproduced simultaneously by quark combination, and these two ratios reflect the rate of baryon production at hadronization which is the same in light sector and strange sector and is roughly constant in p+p, p+Pb and Pb+Pb collision systems over three orders of magnitude in charged particle multiplicity. Read More

Many contemporary statistical learning methods assume a Euclidean feature space, however, the "curse of dimensionality" associated with high feature dimensions is particularly severe for the Euclidean distance. This paper presents a method for defining similarity based on hyperspherical geometry and shows that it often improves the performance of support vector machine compared to other competing similarity measures. Specifically, the idea of using heat diffusion on a hypersphere to measure similarity has been proposed and tested by \citet{Lafferty:2015uy}, demonstrating promising results based on an approximate heat kernel, however, the exact hyperspherical heat kernel hitherto remains unknown. Read More

We introduce a novel approach for parallelizing MCMC inference in models with spatially determined conditional independence relationships, for which existing techniques exploiting graphical model structure are not applicable. Our approach is motivated by a model of seismic events and signals, where events detected in distant regions are approximately independent given those in intermediate regions. We perform parallel inference by coloring a factor graph defined over regions of latent space, rather than individual model variables. Read More

The dynamical fluctuation and correlation of multiplicity distributions of identified baryons and antibaryons produced by the hadronization of the bulk quark system are systematically studied in quark combination model. Starting from the most basic dynamics of quark combination which are necessary for multiplicity study, we analyze moments (variance, skewness and kurtosis) of inclusive multiplicity distribution of identified baryons, two-baryon multiplicity correlations, and baryon-antibaryon multiplicity correlations after the hadronization of quark system with given quark number and antiquark number. We obtain a series of interesting findings, e. Read More

The production of hadronic resonances $K^{*}(892)$, $\phi(1020)$, $\Sigma^{*}(1385)$, and $\Xi^{*}(1530)$ in central AA collisions at $\sqrt{s_{NN}}=$ 17.3, 200, and 2760 GeV are systematically studied. The direct production of these resonances at system hadronization are described by the quark combination model and the effects of hadron multiple-scattering stage are dealt with by a ultra-relativistic quantum molecular dynamics model (UrQMD). Read More

In the framework of the quark combination, we derive the yield formulas and study the yield ratios of the hidden-charm pentaquark states in ultra-relativistic heavy ion collisions. We propose some interesting yield ratios which clearly exhibit the production relationships between different hidden-charm pentaquark states. We show how to employ a specific quark combination model to evaluate the yields of exotic $P_c^+(4380)$, $P_c^+(4450)$ and their partners on the basis of reproducing the yields of normal identified hadrons, and execute the calculations in central Pb+Pb collisions at $\sqrt{s_{NN}}= 2. Read More

Hydrogen embrittlement of metals is widely observed, but its atomistic origins remain little understood and much debated. Combining a unique identification of interstitial sites through polyhedral tessellation and first-principles calculations, we study hydrogen adsorption at grain boundaries in a variety of face-centered cubic metals of Ni, Cu, gamma-Fe and Pd. We discover the chemomechanical origin of variation of adsorption energetics for interstitial hydrogen at grain boundaries. Read More

The quantum anomalous Hall system with Chern number 2 can be destroyed by sufficiently strong disorder. During its process towards localization, it was found that the electronic states will be directly localized to an Anderson insulator (with Chern number 0), without an intermediate Hall plateau with Chern number 1. Here we investigate the topological origin of this phenomenon, by calculating the band structures and Chern numbers for disordered supercells. Read More

Geometric frustrations and quantum mechanical fluctuations may prohibit the formation of long-range ordering even at the lowest temperature, and therefore liquid-like ground states could be expected. A good example is the quantum spin liquid in frustrated magnets that represents an exotic phase of matter and is attracting enormous interests. Geometric frustrations and quantum fluctuations can happen beyond magnetic systems. Read More

The baryon-strangeness correlation in the hadronization of the quark matter is studied within the quark combination mechanism. We calculate the correlation coefficient $C_{BS} = -3\big(\langle B S \rangle -\langle B\rangle \langle S\rangle\big)/\big( \langle S^2 \rangle-\langle S \rangle^2 \big)$ of initial hadrons produced from the deconfined free quark system with $C^{(q)}_{BS}=1$. The competition of the production of baryons against that of mesons is the key dynamics that is most relevant to the change of baryon-strangeness correlation during system hadronization. Read More

The occurrence of the proton bubble-like structure has been studied within the relativistic Hartree-Fock-Bogoliubov (RHFB) and relativistic Hartree-Bogoliubov (RHB) theories by exploring the bulk properties, the charge density profiles and single proton spectra of argon isotopes and $N = 28$ isotones. It is found that the RHFB calculations with PKA1 effective interaction, which can properly reproduce the charge radii of argon isotopes and the $Z=16$ proton shell nearby, do not support the occurrence of the proton bubble-like structure in argon isotopes due to the prediction of deeper bound proton orbit $\pi2s_{1/2}$ than $\pi1d_{3/2}$. For $N = 28$ isotones, $^{42}$Si and $^{40}$Mg are predicted by both RHFB and RHB models to have the proton bubble-like structure, owing to the large gap between the proton $\pi2s_{1/2}$ and $\pi1d_{5/2}$ orbits, namely the $Z=14$ proton shell. Read More

Rare-earth pyrochlores, commonly exhibiting anomalously low lattice thermal conductivities, are considered as promising topcoat materials for thermal barrier coatings. However the structural origin underlying their low thermal conductivities remain unclear. In the present study, we investigated the phonon properties of two groups of RE pyrochlores, Ln2Zr2O7 (Ln = La, Nd, Sm, Gd) and Gd2T2O7 (T = Zr, Hf, Sn, Pb) employing density functional theory and quasi harmonic approximation. Read More

Graphene and boron nitride (GPBN) heterostructures provide a viable way to realize tunable bandgap, promising new opportunities in graphene-based nanoelectronic and optoelectronic devices. In the present study, we investigated the interplay between vacancies and graphene/h-BN interfaces in monolayer GPBN heterostructures. The energetics and kinetics of monovacancies and divacancies in monolayer GPBN heterostructures were examined using first-principle calculations. Read More

Employing density-functional theory (DFT) calculations, the generalized-stacking-fault energy (GSFE) curves along two crystallographic slips, glide and shuffle, for both pristine graphene and impurity of boron (B) or nitrogen (N) doped graphene were examined. The effects of B and N doping on the GSFE were clarified and correlated with local electron interactions and bonding configurations. The GSFE data were then used to analyze dislocation dipole and core structure, and subsequently combined with the Peierls-Nabarro (P-N) model to examine the role of doping on several key characteristics of dislocations in graphene. Read More

Cross-planar di-vacancies (CPDVs) within stacked graphene hexagonal boron nitride (h-BN) heterostructures provide stabilized covalent links to bridge adjacent graphene and h-BN sheets. It was shown that the CPDVs serve as focal points for cross-planar atom transport between graphene and h-BN, and the chemical nature of interlayer links along with associated cross-planar migration pathways at these defects can be predictively manipulated through modulation of the chemical environment and charge engineering, to achieve consistent B or N doping and simultaneous healing of graphene. The present study proposed a viable approach integrating irradiation, chemical and charge engineering, to produce high-quality graphene with tunable electronic and electrochemical properties, using the h-BN substrate. Read More

The purpose of this paper is to present three new methods for finding all simple zeros of polynomials simultaneously. First, we give a new method for finding simultaneously all simple zeros of polynomials constructed by applying the Weierstrass method to the zero in the trapezoidal Newton's method, and prove the convergence of the method. We also present two modified Newton's methods combined with the derivative-free method, which are constructed by applying the derivative-free method to the zero in the trapezoidal Newton's method and the midpoint Newton's method, respectively. Read More

Half-life of proton radioactivity of spherical proton emitters is studied within the scheme of covariant density functional (CDF) theory, and for the first time the potential barrier that prevents the emitted proton is extracted with the similarity renormalization group (SRG) method, in which the spin-orbit potential along with the others that turn out to be non-negligible can be derived automatically. The spectroscopic factor that is significant is also extracted from the CDF calculations. The estimated half-lives are found in good agreement with the experimental values, which not only confirms the validity of the CDF theory in describing the proton-rich nuclei, but also indicates the prediction power of present approach to calculate the half-lives and in turn to extract the structural information of proton emitters. Read More

We propose a new kind of two-particle correlation of identified hadrons in longitudinal rapidity space, called $G_{\alpha\beta}(y_{\alpha},y_{\beta})$, which can reflect clearly the charge correlations of hot quark system produced in AA collisions at LHC energies. It is derived from the basic scenario of quark combination mechanism of hadron production. Like the elliptic flow of identified hadrons at intermediate transverse momentum, this correlation is independent of the absolute hadronic yields but dependent only on the flavor compositions of hadrons, and thus exhibits interesting properties for different kinds of hadron species. Read More

Recently the ALICE Collaboration has published a few data of charm hadrons in Pb + Pb collisions at the CERN Large Hadron Collider (LHC). We extend the quark combination to the charm sector and point out that the measurement of charm hadron yields can provide important insights into charm quark hadronization mechanism and properties of the hot and dense matter produced in high energy reactions. We propose several types of yield ratios, e. Read More

Novel materials with tunable magnetic states play a significant role in the development of next-generation spintronic devices. In this paper, we examine the role of biaxial strain on the electronic properties of vacancy-decorated hexagonal boron nitride (h-BN) monolayers using density functional theory calculations. We found that the strain can lead to switching of the magnetic state for h-BN monolayers with boron vacancy or divacancy. Read More

Experimental data at RHIC and SPS energies suggest a systematic correlation between yield ratio $\bar{p}/p$ and $K^{-}/K^{+}$, which reveal some universality of hadron production in relativistic heavy ion collisions. We propose an explanation based on the quark combination mechanism in which the production asymmetry between baryons and antibaryons is focused on especially. We start from the basic ideas of quark combination to carry out a general analysis of the properties of the global production of baryons and mesons and obtain the yields of baryons, antibaryons and mesons as functions of the number of constituent quarks and antiquarks just before hadronization. Read More

We report in this Letter the results of our investigation of 2D Bose gases beyond the dilute limit emphasizing the role played by three-body scattering events. We demonstrate that a competition between three-body attractive interactions and two-body repulsive forces results in the chemical potential of 2D Bose gases to exhibit a maximum at a critical scattering length beyond which these quantum gases possess a negative compressibility. For larger scattering lengths, the increasingly prominent role played by three-body attractive interactions leads to an onset instability at a second critical value. Read More

We study the hadron yield correlation in the combination models in high energy heavy ion collisions. We derive the relationship between the average yields of different hadrons produced in the combination of a system consisting of equal number of quarks and antiquarks. We present the results for the directly produced hadrons as well as those for the final hadrons including the strong and electromagnetic decay contributions. Read More

We calculate the charge balance function of the bulk quark system before hadronization and those for the directly produced and the final hadron system in high energy heavy ion collisions. We use the covariance coefficient to describe the strength of the correlation between the momentum of the quark and that of the anti-quark if they are produced in a pair and fix the parameter by comparing the results for hadrons with the available data. We study the hadronization effects and decay contributions by comparing the results for hadrons with those for the bulk quark system. Read More

Applying a quark combination model for the hadronization of Quark Gluon Plasma (QGP) and A Relativistic Transport (ART) model for the subsequent hadronic rescattering process, we investigate the production of $K^{*0}$ and $\Sigma^*$ resonances in central Au+Au collisions at $\sqrt{s_{NN}}=$ 200 GeV and 62.4 GeV. The initial $K^{*0}$ produced via hadronization is higher than the experimental data in the low $p_T$ region and is close to the data at 2-3 GeV/c. Read More

In this paper, we explore the nature of three-dimensional Bose gases at large positive scattering lengths via resummation of dominating processes involving a minimum number of virtual atoms. We focus on the energetics of the nearly fermionized Bose gases beyond the usual dilute limit. We also find that an onset instability sets in at a critical scattering length, beyond which the near-resonance Bose gases become strongly coupled to molecules and lose the metastability. Read More

The quark combination mechanism of hadron production is applied to nucleus-nucleus collisions at the CERN Super Proton Synchrotron (SPS) and BNL Alternating Gradient Synchrotron (AGS). The rapidity spectra of identified hadrons and their spectrum widths are studied. The data of $\pi^{-}$, $K^{\pm}$, $\phi$, $\Lambda$, $\bar{\Lambda}$, $\Xi^{-}$, and $\bar{\Xi}^{+}$ at 80 and 40 AGeV, in particular at 30 and 20 AGeV where the onset of deconfinement is suggested to happen, are consistently described by the quark combination model. Read More

We study the energetics and dispersion of anomalous dimers that are induced by the Pauli blocking effect in a quantum Fermi gas of majority atoms near interspecies resonances. Unlike in vacuum, we find that both the sign and magnitude of the dimer masses are tunable via Feshbach resonances. We also investigate the effects of particle-hole fluctuations on the dispersion of dimers and demonstrate that the particle-hole fluctuations near a Fermi surface (with Fermi momentum $\hbar k_F$) generally reduce the effective two-body interactions and the binding energy of dimers. Read More

Based on the assumption of the production of deconfined quark matter, we use a quark combination model to systematically investigate hadron yields in heavy ion collisions from RHIC $\sqrt{s_{NN}}=200, 130, 62.4$ GeV to SPS $E_{beam}=158, 80, 40, 30, 20 $ AGeV. We find that as the collision energy is greater than or equal to 30 AGeV the yields of various hadrons, their correlations, in particular, the observables $A=\frac{\bar{\Lambda} k^{-} p}{\Lambda k^{+} \bar{p}}$ and $B=\frac{\Lambda k^{-}\bar{\Xi}^{+}}{\bar{\Lambda} k^{+}\Xi^{-}}$, are all reproduced; however, as the collision energy drops to 20 AGeV quark combination fails. Read More

In this Letter we have studied dressed bound states in Fermi-Bose mixtures near broad interspecies resonance, and implications on many-body correlations. We present the evidence for a first order phase transition between a mixture of Fermi gas and condensate, and a fully paired mixture where extended fermionic molecules occupy a single pairing channel instead of forming a molecular Fermi surface. We have further investigated the effect of Fermi surface dynamics, pair fluctuations and discussed the validity of our results. Read More

We use two available methods, the Duhem-Gibbs relation and the entropy formula in terms of particle phase space distributions, to calculate the entropy in a quark combination model. The entropy of the system extracted from the Duhem-Gibbs relation is found to increase in hadronization if the average temperature of the hadronic phase is lower than that of the quark phase. The increase of the entropy can also be confirmed from the entropy formula if the volume of the hadronic phase is larger than 2. Read More

We consider ultracold matter of spin-2 atoms in optical lattices. We derive an effective Hamiltonian for the studies of spin ordering in Mott states and investigate hyperfine spin correlations. Particularly, we diagonalize the Hamiltonian in an on-site Hilbert space taking into account spin-dependent interactions and exchange between different sites. Read More

The quark combination mechanism of QGP hadronization is applied to nucleus-nucleus collisions at top SPS energy. The yields, rapidity and transverse momentum distributions of identified hadrons in most central Pb+Pb collisions at $\sqrt{s_{NN}}= 17.3$ GeV are systematically studied. Read More

In this letter, we study bosonic atoms at large scattering lengths using a variational method where the condensation amplitude is a variational parameter. We further examine momentum distribution functions, chemical potentials and speed of sound, and spatial density profiles of cold bosonic atoms in a trap in this limit. The later two properties turn out to bear similarities of those of Fermi gases. Read More

We have studied spin structures of fluctuation-driven fractionalized vortices and topological spin order in 2D nematic superfluids of cold sodium atoms. Our Monte Carlo simulations suggest a softened pi-spin disclination structure in a half-quantum vortex when spin correlations are short ranged; in addition, calculations indicate that a unique non-local topological spin order emerges simultaneously as cold atoms become a superfluid below a critical temperature. We have also estimated fluctuation-dependent critical frequencies for half-quantum vortex nucleation in rotating optical traps and discussed probing these excitations in experiments. Read More

We investigate dynamic creation of fractionalized half-quantum vortices in Bose-Einstein condensates of sodium atoms. Our simulations show that both individual half-quantum vortices and vortex lattices can be created in rotating optical traps when additional pulsed magnetic trapping potentials are applied. We also find that a distinct periodically modulated spin-density-wave spatial structure is always embedded in square half-quantum vortex lattices; this structure can be conveniently probed by taking absorption images of ballistically expanding cold atoms in a Stern-Gerlach field. Read More

Utilizing the three-fireball picture within the quark combination model, we study systematically the charged particle pseudorapidity distributions in both Au+Au and Cu+Cu collision systems as a function of collision centrality and energy, $\sqrt{s_{NN}}=$ 19.6, 62.4, 130 and 200 GeV, in full pseudorapidity range. Read More

The centrality dependence of transverse momentum spectra for identified hadrons at midrapidity in Au+Au collisions at $\sqrt{s_{NN}}= 200$ GeV is systematically studied in a quark combination model. The $\mathrm{{p}_{T}}$ spectra of $\pi^{\pm}$, $K^{\pm}$, $p(\bar{p})$ and $\Lambda(\bar{\Lambda})$ in different centrality bins and the nuclear modification factors ($R_{CP}$) for these hadrons are calculated. The centrality dependence of the average collective transverse velocity $<\beta (r)>$ for the hot and dense quark matter is obtained in Au+Au collisions, and it is applied to a relative smaller Cu+Cu collision system. Read More

Within a quark combination model, we study systematically the yields and rapidity spectra of various hadrons in central Au+Au collisions at $\sqrt{s_{NN}}= 200$ GeV. We find that considering the difference in rapidity between net-quarks and newborn quarks, the data of multiplicities, rapidity distributions for $\pi^{\pm}$, $K^{\pm}$, $p(\bar{p})$ and, in particular the ratios of charged antihadron to hadron as a function of rapidity, can be well described. The effect of net-quarks on various hadrons is analysed, and the rapidity distributions for $K^{0}_{s}$, $\Lambda(\bar{\Lambda})$, $\Sigma^{+}(\bar{\Sigma}^{_-})$, $\mathrm{\Xi^{-}}$ ($\mathrm{\bar{\Xi}^{_+}}$) and $\mathrm{\Omega^{-}}(\mathrm{\bar{\Omega}}^{_+})$ are predicted. Read More

Temporal evolution of a macroscopic condensate of ultra cold atoms is usually driven by mean field potentials, either due to scattering between atoms or due to coupling to external fields; and coherent quantum dynamics have been observed in various cold-atom experiments. In this article, we report results of studies of a class of quantum spin dynamics which are purely driven by zero point quantum fluctuations of spin collective coordinates. Unlike the usual mean-field coherent dynamics, quantum fluctuation-controlled spin dynamics or QFCSD studied here are very sensitive to variation of quantum fluctuations and can be tuned by four to five order of magnitude using optical lattices. Read More

The rapidity and transverse momentum spectra for identified hadrons in central Au+Au collisions at $\sqrt{s_{NN}}= 200$ GeV are computed in a quark combination model. The data of rapidity distributions for $\pi^{\pm}$, $K^{\pm}$, $p(\bar{p})$ and net protons $(p-\bar{p})$ are well described. We also predict rapidity distributions for $K^{0}_{s}$, $\Lambda(\bar{\Lambda})$, $\mathrm{\Xi^{-}}$ ($\mathrm{\bar{\Xi}^{_+}}$) and $\mathrm{\Omega^{-}}+\mathrm{\bar{\Omega}}^{_+}$. Read More

It is shown that zero point quantum fluctuations (ZPQFs) completely lift the accidental continuous degeneracy that is found in mean field analysis of quantum spin nematic phases of hyperfine spin 2 cold atoms. The result is two distinct ground states which have higher symmetries: a uniaxial spin nematic and a biaxial spin nematic with dihedral symmetry ${Dih}_4$. There is a novel first order quantum phase transition between the two phases as atomic scattering lengths are varied. Read More

In this paper we study the concurrence and the block-block entanglement in the $S=1/2$ spin ladder with four-spin ring exchange by the exact diagonalization method of finite cluster of spins. The relationship between the global phase diagram and the ground-state entanglement is investigated. It is shown that the block-block entanglement of different block size and geometry manifests richer information of the system. Read More

In this paper, we study the entanglement between two-neighboring sites and the rest of the system in a simple quantum phase transition of 1D transverse field Ising model. We find that the entanglement shows interesting scaling and singular behavior around the critical point, and then can be use as a convenient marker for the transition point. Read More

These brief notes record our puzzles and findings surrounding Givental's recent conjecture which expresses higher genus Gromov-Witten invariants in terms of the genus-0 data. We limit our considerations to the case of a projective line, whose Gromov-Witten invariants are well-known and easy to compute. We make some simple checks supporting his conjecture. Read More

We use algebraic methods to compute the simple Hurwitz numbers for arbitrary source and target Riemann surfaces. For an elliptic curve target, we reproduce the results previously obtained by string theorists. Motivated by the Gromov-Witten potentials, we find a general generating function for the simple Hurwitz numbers in terms of the representation theory of the symmetric group S_n. Read More

In this ``experimental'' research, we use known topological recursion relations in genera-zero, -one, and -two to compute the n-point descendant Gromov-Witten invariants of P^1 for arbitrary degrees and low values of n. The results are consistent with the Virasoro conjecture and also lead to explicit computations of all Hodge integrals in these genera. We also derive new recursion relations for simple Hurwitz numbers similar to those of Graber and Pandharipande. Read More

Using N=2 Landau-Ginzburg theories, we examine the recent conjectures relating the SU(3) WZW modular invariants, finite subgroups of SU(3) and Gorenstein singularities. All isolated three-dimensional Gorenstein singularities do not appear to be related to any known Landau-Ginzburg theories, but we present some curious observations which suggest that the SU(3)_n/SU(2)xU(1) Kazama-Suzuki model may be related to a deformed geometry of C^3/Z_{n+3}xZ_{n+3}. The toric resolution diagrams of those particular singularities are also seen to be classifying the diagonal modular invariants of the SU(3)_n as well as the SU(2)_{n+1} WZW models. Read More

The ubiquitous ADE classification has induced many proposals of often mysterious correspondences both in mathematics and physics. The mathematics side includes quiver theory and the McKay Correspondence which relates finite group representation theory to Lie algebras as well as crepant resolutions of Gorenstein singularities. On the physics side, we have the graph-theoretic classification of the modular invariants of WZW models, as well as the relation between the string theory nonlinear $\sigma$-models and Landau-Ginzburg orbifolds. Read More