arXiv subjects

Bingnan Zhang

Publications and source records attributed to Bingnan Zhang.

9 recordsLinked to original sources

arXiv2025

Optimization of the Woodcock Particle Tracking Method Using Neural Network

The acceptance rate in Woodcock tracking algorithm is generalized to an arbitrary position-dependent variable $q(x)$. A neural network is used to optimize $q(x)$, and the FOM value is used as the loss function. This idea comes from physics informed neural network(PINN), where a neural network is used to represent the solution of differential equations. Here the neural network $q(x)$ should solve the functional equations that optimize FOM. For a 1d transmission problem with Gaussian absorption cross section, we observe a significant improvement of the FOM value compared to the constant $q$ case and the original Woodcock method. Generalizations of the neural network Woodcock(NNW) method to 3d voxel models are waiting to be explored.

physics.comp-ph↗

arXiv2022

Evidences of the Generalizations of BKT Transition in Quantum Clock Model

We calculate the ground state energy density $ε(g)$ for the one dimensional N-state quantum clock model up to order 18, where $g$ is the coupling and $N=3,4,5,...,10,20$. Using methods based on Padé approximation, we extract the singular structure of $ε''(g)$ or $ε(g)$. They correspond to the specific heat and free energy of the classical 2D clock model. We find that, for $N=3,4$, there is a single critical point at $g_c=1$.The heat capacity exponent of the corresponding 2D classical model is $α=0.34\pm0.01$ for $N=3$, and $α=-0.01\pm 0.01$ for $N=4$. For $N>4$, There are two exponential singularities related by $g_{c1}=1/g_{c2}$, and $ε(g)$ behaves as $Ae^{-\frac{c}{|g_c-g|^σ}}+analytic\ terms$ near $g_c$. The exponent $σ$ gradually grows from $0.2$ to $0.5$ as N increases from 5 to 9, and it stabilizes at 0.5 when $N>9$. These phase transitions should be generalizations of Kosterlitz-Thouless transition, which has $σ=0.5$. The physical pictures of these phase transitions are still unclear.

cond-mat.stat-mech↗

arXiv2022

JT Gravity Coupled to Fermions

We argue that two-dimensional dilaton gravity models can all be derived from an analog of Jacobson's covariant version of the first law of thermodynamics. We then specialize to the JT gravity model and couple it to massless fermions. This model is exactly soluble in quantum field theory, and we present a new derivation of that result. The field theory model violates two principles one might want to impose on a quantum theory of gravity describing the near horizon region of an extremal charged black hole in four dimensions: finiteness of the entropy for finite causal diamonds, and the absence of global conservation laws. It preserves an infinite number of conservation laws that one would have expected to be violated, since the fermion state on each side of the $AdS_2$ wormhole is unavoidably thermal. We describe a cutoff version of the model, with extra interactions, which cures these difficulties. Our UV completion of the model depends on the AKK map of non-relativistic fermions in an inverted oscillator potential to Weyl fermions in Minkowski space. We argue that gauging the $Z_2$ symmetry of the oscillator model, using a density matrix with temperature that depends on the oscillator coordinates, and inserting chaotic interactions at (almost) infinite oscillator coordinate, we obtain a model with properties expected of quantum gravity in the near horizon region of an extremal charged black hole in four dimensions.

hep-th↗

arXiv2022

Abelian Chern-Simons Gauge Theory on The Lattice

The Abelian Chern-Simons gauge theory is constructed on the three-dimensional spacetime lattice. This proposal introduces both lattice and dual lattice, and the gauge field on the dual lattice is expressed in terms of the gauge field on the original lattice. This treatment circumvents the issue of forward/backward difference, which is the common problem that many previous proposals have, and also avoids the duplication problem, which prevents people from introducing the dual lattice. The form of the lattice action is very simple, and is symmetric with respect to the three spacetime dimensions. These features make it straightforward to calculate the expectation values of Wilson loops, and the results agree with the topological field theory in continuous spacetime. Generalizations to multiple types of lattices are also discussed.

hep-th↗

arXiv2021

Lattice BF Theory, Dumbbells, and Composite Fermions

We formulate $U(1)$ $bda$ Chern-Simons theory, which is also called BF theory, on a lattice, adapting a method proposed by Kantor and Susskind for the groups $\mathbb{R}$ and $\mathbb{Z}_N$. Our method applies to any finite or infinite abelian group. We study the discrete symmetries and use the model to provide a rigorous treatment of the composite fermion theory of the fractional quantum Hall effect (FQHE), with no ambiguities relating to intersecting Wilson/'t Hooft lines. We derive Jain's fractions, and one can also calculate corrections to the mean field solution within this framework. We also generalize the formalism to higher form gauge models in arbitrary dimension, and suggest a possible non-Abelian extension.

hep-th↗

arXiv2021

Comment on Coleman-DeLuccia Instantons

We complete an old argument that causal diamonds in the crunching region of the Lorentzian continuation of a Coleman-Deluccia instanton for transitions out of de Sitter space have finite area, and provide quantum models consistent with the principle of detailed balance, which can mimic the instanton transition probabilities for the cases where this diamond is larger or smaller than the causal patch of de Sitter space. We review arguments that potentials which do not have a positive energy theorem when the lowest de Sitter minimum is shifted to zero, may not correspond to real models of quantum gravity.

hep-th↗

arXiv2021

On the Colloidal Phase of the Homogeneous Electron Fluid

We provide semi-rigorous arguments that the Homogeneous Electron Fluid (HEF) has a colloidal phase separating the Wigner Crystal from the high density fluid phase. Near the crossover between crystal and fluid ground state energies, the argument is quite general and valid for practically any quantum transition between a crystal and a more amorphous phase. In this regime, the colloid is a gel and its "Goldstone" modes are flows of irregular fluid droplets separated by crystalline walls. A metal insulator transition occurs when a single bubble of fluid spans the entire system. Beyond this transition the colloid is a sol and its properties depend on the existence of meta-stable finite crystallites with negative surface tension. If these exist, the sol phase has lower energy than the homogeneous fluid. In the two dimensional HEF, Kivelson and Spivak argued that such negative surface tension objects always exist, at least in the form of stripes. We argue that the existence of stripes also implies the existence of finite negative tension elliptical crystallites and the detailed competition between the finite crystallite phase and striped phases is difficult to calculate. We also provide weaker arguments that finite crystallites exist in three dimensions. This provides evidence for our claim that the gapless excitations at non-zero wavenumber, observed in the numerical calculations of\cite{haule}\cite{gapless} are quantum remnants of these crystallites (Bosonic quasi-particles) in the limit of vanishing surface tension. Finally, we suggest a Landau mean field theory for the second order quantum phase transition between the fluid and sol phases.

cond-mat.str-el↗

arXiv2019

On the Low Density Regime of Homogeneous Electron Gas

We investigate the low density limit of the Homogeneous Electron system, often called the {\it Strictly Correlated} regime. We begin with a systematic presentation of the expansion around infinite $r_S$, based on the first quantized treatments suggested in the existing literature. We show that the expansion is asymptotic in the parameter $r_S^{1/4}$ and that the leading order result contains exponential corrections that are significant even for $r_S \sim 100$. Thus, the systematic expansion is of limited utility. As a byproduct of this analysis, we find that there is no Wigner Crystal (WC) in one spatial dimension. This is an example of the Mermin-Wagner theorem, but was not appreciated in some earlier literature. More modern work has come to conclusions identical to ours. Note that the long range Coulomb potential modifies the dispersion relation of phonons in one dimension, but still leads to the instability of the crystal, due to a very weak infrared divergence. We then propose a new approximation scheme based on renormalization group ideas. We show that the Wegner-Houghton-Wilson-Polchinski exact renormalization group equation reduces, in the low density limit, to a classical equation for scale dependent electron and plasmon fields. In principle, this should allow us to lower the wave number cutoff of the model to a point where Wigner's intuitive argument for dominance of the classical Coulomb forces becomes rigorously correct.

cond-mat.str-el↗

arXiv2019

Instantons, Colloids and Convergence of the 1/N Expansion for the Homogeneous Electron Gas

We investigate non-perturbative corrections to the large $N$ expansion of the homogeneous electron gas. These are associated with instanton solutions to the effective action of the plasmon field. We show that, although the large field behavior of that action dominates the quadratic bare Coulomb term, there are no solutions at large field, and consequently none at large density. We argue that solutions would exist at low density if the large $N$ theory had a Wigner crystal (WC) phase. However, we argue that this is not the case. Together with the implied convergence of the large $N$ expansion, this implies that the homogeneous electron gas with $N$ component spins and a Coulomb interaction scaling like $1/N$ can only have a WC phase below a curve in the plane of $N$ and density, which asymptotes to zero density at infinite $N$. We argue that for systems with a semi-classical expansion for order parameter dynamics, and a first order quantum transition between fluid and crystal phases, there are instantons associated with the decays of meta-stable fluid and crystal phases in the appropriate regions of the phase diagram. We argue that the crystal will decay into one or more colloidal or bubble phases\cite{kivspiv} rather than directly into the fluid. The transition to a translationally invariant phase is likely to be second order. Unfortunately, the HEG does not have a crystal phase at large $N$, where these semi-classical ideas could be examined in detail. We suggest that the evidence for negative dielectric function at intermediate densities for $N = 2$ is an indicator of this second order transition. It is possible that the closed large $N$ equation for the plasmon two point function, derived in\cite{ergheg} might capture at least the qualitative features of the second order transition.

cond-mat.str-el↗