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J. P. F. LeBlanc

Publications and source records attributed to J. P. F. LeBlanc.

At least 19 recordsLinked to original sources
arXiv2026

The ALPS project release 3.0: open source software for strongly correlated systems

We present release 3.0 of the ALPS (Algorithms and Libraries for Physics Simulations) project, an open-source software project to develop libraries and application programs for the simulation of strongly correlated quantum lattice models such as quantum magnets, lattice bosons, and strongly correlated fermion systems. As in previous releases, development is centered on common data formats, on libraries to simplify and speed up code development, and on full-featured simulation programs that let non-experts carry out serial or parallel numerical simulations using the important algorithms for quantum lattice models: classical and quantum Monte Carlo (QMC) using non-local updates, extended-ensemble simulations, exact and full diagonalization (ED), the density matrix renormalization group (DMRG), and continuous-time QMC solvers for dynamical mean-field theory (DMFT). Major changes in release 3.0 include distribution of the pyalps binary through the Python Package Index (pip install pyalps) and through Spack for HPC systems; migration of development to GitHub with continuous integration and automated testing; relicensing of the package under the permissive MIT license; a rebuilt documentation and tutorial website, including a set of Jupyter-notebook tutorials and localized content; a broad modernization of the C++ codebase (C++17 compliance, Boost and NumPy 2.0 compatibility, and warning and dead-code cleanup) together with a major DMRG update and associated reliability and build-compatibility fixes; the removal of legacy components (the VisTrails provenance integration and the TEBD, MPS, and directed-worm-algorithm application codes); archival of release 3.0.0 with a Zenodo DOI; and a formal governance and sustainability model developed under the US National Science Foundation (NSF) POSE program. The software is available at https://alps.comp-phys.org.

cond-mat.str-el↗
arXiv2026

Variationally Optimized Imaginary-time Polynomial Filters for Ground State Projection

In this work, we develop a variational imaginary-time evolution (ITE) framework based on polynomial filtering, derived from an operator-level action principle, which yields an optimized non-unitary projector expressed as a polynomial in the Hamiltonian. Starting from a single-ancilla, first-order imaginary-time update defined by a Taylor expansion and Trotter-Suzuki (TS) decompositions, we show that replacing these approximations with alternative variational formulas substantially improves both accuracy and stability at larger time steps, leading to up to an order-of-magnitude enhancement in the final success probability. We further derive rigorous error bounds that depend only on static properties of the Hamiltonian, providing practical guidance for selecting the simulation time step. Benchmarks on the transverse-field Ising model demonstrate faster convergence to the ground-state energy and improved robustness compared to standard TS--Taylor ITE, highlighting variational polynomial filtering as a practical route to higher-fidelity ground-state preparation on near-term quantum devices.

quant-ph↗
arXiv2026

A Variational Framework for Time-Dependent Quantum Systems with Applications to Floquet Hamiltonians

We introduce a variational framework for approximating the time-evolution operator $\hat{U}(t)$ within a physically motivated operator manifold, reformulating quantum dynamics as a tractable problem in operator space using stationary action principle. For periodically driven systems, the resulting approximate evolution operator directly yields an effective Floquet Hamiltonian, offering a non-perturbative alternative to conventional expansion-based methods. The framework is systematically improvable by enlarging the operator pool and naturally incorporates symmetries and physical constraints. When the operator manifold is chosen from the terms of a truncated Magnus expansion, the variational procedure effectively resums the Magnus series within the restricted space, significantly enhancing accuracy. We benchmark the approach on the driven Rabi model, the driven Lipkin-Meshkov-Glick model, and the one-dimensional driven Ising chain, yielding effective Floquet Hamiltonians that are systematically more accurate than low-order Magnus expansions, particularly in regimes where the latter converge poorly, and illustrating applicability to systems with exponentially large Hilbert spaces. Although we focus here on Floquet systems, the formalism applies equally to generic time-dependent Hamiltonians, providing a versatile tool for non-equilibrium quantum dynamics.

quant-ph↗
arXiv2026

Beyond Trotterization: Variational Product Formulas for Quantum Simulation

We propose a variational alternative to the Trotter-Suzuki decomposition that provides greater control over errors while preserving the unitary structure of time evolution. The variational parameters in our ansatz are derived from a global action principle, where Euler-Lagrange equations govern their optimal dynamics. Unlike conventional wavefunction-based variational methods, our approach specifically targets the time evolution operation and this allows a single set of optimized parameters to be applied to any initial state for a fixed Hamiltonian avoiding costly optimization procedures. Our method outperforms the standard Trotter-Suzuki formulas, typically achieving higher accuracy than higher-order Suzuki schemes. This translates directly to quantum computing applications, where it enables the design of quantum circuits with fewer gates which reduces noise and improves precision. Although we focus on quantum dynamics, the method is broadly applicable to problems involving general time-evolution operators. Applied to various model Hamiltonians, our approach reduces errors by factors of 2 to 5 compared to Trotter-Suzuki decompositions, demonstrating its promise for accurate quantum simulation with improved efficiency. In certain cases, the variational ansatz achieves higher accuracy than more complex higher-order Suzuki formulas while reducing the gate count by nearly half within a single circuit layer. Furthermore, we derive approximate analytical expressions for the variational parameters up to cubic order in time, valid for generic Hamiltonians. These approximations enable long-time quantum simulations with improved accuracy over equivalent Suzuki decompositions, providing ready-to-use evolution formulas that match Suzuki's gate complexity while delivering better performance.

quant-ph↗
arXiv2026

NonHermitian Topological Phases in a Hermitian Modified Bosonic Kitaev Chain

We present a modification to the bosonic Kitaev chain that, despite being Hermitian, supports both nonHermitian skin effect and nontrivial topological edge modes in its excitation Hamiltonian. We establish an exact mapping between the excitation Hamiltonian of our system and a nonHermitian Su-Schrieffer-Heeger (SSH) model, which allows for a completely analytical characterization of its topology. In particular, topological phase transition points separating a topologically trivial and nontrivial regime were identified analytically by the appropriate winding number invariant and the presence of zero energy modes. Similarly to the regular bosonic Kitaev chain, the nonHermitian skin effect and some (but not all) topological edge modes are quickly destroyed at nonzero bosonic onsite potential (harmonic oscillator frequency). Remarkably, however, disorder partially recovers some of these features. This work thus demonstrates the potential of a modified bosonic Kitaev chain as a platform to generate rich nonHermitian topological phenomena from a completely Hermitian system's perspective. Lastly, we suggest a possible experimental realization of the model, which could allow for total control over the parameter space.

quant-ph↗
arXiv2025

Computation Kernel for Feynman Diagrams

We present a general representation for solving problems in many-body perturbation theory. By projecting the single-particle Green's function to an auxiliary space we show how one can convert an arbitrary Feynman graph to a universal kernel representation. Once constructed, the computation kernel contains no problem specific information yet contains all explicit temperature and frequency dependence of the diagram. This computation kernel is problem agnostic, and valid for any physical problem that would normally leverage the Matsubara formalism of many-body perturbation theory. The result of any diagram can be written as a linear combination of these computation kernel elements with coefficients given by a sum over products of known tensor elements that are themselves problem specific and represent spatial degrees of freedom. We probe the efficacy of this approach by generating the computation kernel for a low order self-energy diagram which we then use to construct solutions to distinct problems.

cond-mat.str-el↗
arXiv2025

Graphene/hBN heterostructure based Valley transistor: Dynamic Control of valley current in synchronized nonzero voltages, within the time-dependent regime

Graphene/hexagonal boron nitride (hBN) heterostructures represent a promising class of metal-insulator-semiconductor systems widely explored for multifunctional digital device applications. In this work, we demonstrate that graphene, when influenced by carrier-dependent trapping in the hBN spacer triggered by a localized potential from Kelvin probe force microscopy (KPFM), can exhibit valley transistor behavior under specific conditions. We employ a tight-binding model that self-consistently incorporates a Gaussian-shaped potential to represent the effect of the tip gate. Crucially, we show that the heterostructure functions as a field-effect transistor (FET), with its operation governed by the bias gate (shifting the Fermi level) and the tip-induced potential (breaking electron-hole symmetry via selective trapping of electron or hole quasiparticles). Our results reveal that, under specific lattice geometry, pulse frequency, and gate voltage conditions, the device exhibits valley transistor functionality. The valley current (e.g., I_K1=-K or I_K2=+K) can be selectively controlled by synchronizing the frequencies and polarities of the tip and bias gate voltages. Notably, when both gates are driven with the same polarity, the graphene channel outputs a periodically modulated, pure valley-polarized current. This enables switching between distinct ON/OFF valley current states even at finite bias. Remarkably, when the I_K1=-K current is ON (forward current), the I_K2=+K current is OFF. Reversing the gate polarity inverts this behavior: I_K1=-K turns OFF, while I_K2=+K turns ON (reverse current). These findings pave the way toward low-voltage valley transistors in metal-insulator-semiconductor architectures, offering new avenues for valleytronics and advanced gating technologies.

cond-mat.mes-hall↗
arXiv2025

Lifshitz transition and triplet $p$-wave pairing from the induced ferromagnetic plaquette via spin differentiated nonlocal interaction

We study the two-dimensional extended Hubbard model on a square lattice and incorporate spin-differentiated nearest neighbor (NN) interactions where the equal-spin ($V_{uu}$) and unequal-spin ($V_{ud}$) terms are independently tuned parameters. We compute single-particle excitations as well as static spin and pairing susceptibilities perturbatively up to the fourth order within the thermodynamic limit and at a finite fixed temperature. By explicitly encoding a ferromagnetic-like NN interaction ($V_{uu} < V_{ud}$), we induce a competition among the uniform $q = (0,0)$, collinear $q = (π,0)$, and staggered $q = (π,π)$ spin excitations. This results in the formation of short-ranged $2\times 2$ ferromagnetic plaquettes arranged in staggered or striped patterns. Kinetic frustration in hopping, both within and between these plaquettes, manifests in single-particle properties, resulting in a reduction of bandwidth and ultimately triggering a Lifshitz transition to quasi-one-dimensional bands. Furthermore, an attractive effective interaction within the localized ferromagnetic plaquette results in the emergence of equal-spin triplet $p$-wave pairing. We demonstrate that sufficiently strong magnetic fluctuations, even at finite length scales, can significantly influence single-particle and pairing properties without breaking translational symmetry. Our approach provides a novel pathway to realize a variety of rich magnetic phases and Fermi surface reconstruction driven by interactions in the absence of explicit geometric frustration.

cond-mat.str-el↗
arXiv2024

Pairing susceptibility in the weakly interacting multilayer Hubbard model evaluated by direct perturbative expansion

We present a systematic study of the interaction, doping, and layer dependence of the $d_{x^2-y^2}$-wave pairing susceptibility of the Hubbard model for a stacked 2D square lattice. We perform a multi-index perturbative expansion up to fourth-order to obtain coefficients in powers of the Hubbard $U$, the inter-layer $V$, and the pair-hopping $J$ interactions. We evaluate the vertex diagrams that contribute to the pairing susceptibility for $\ell= 2,3, 4$ layered models in the $U$-$V$-$J$ interaction space. This provides unprecedented access to the pairing amplitudes, allowing us to identify the processes that enhance or reduce pairing. We distinguish pairing within the diagonal channel, $P^{\parallel}_{d}$, and off-diagonal channel, $P^{\perp}_{d}$, and find that, in the absence of $J$, the qualitative behavior of the layered system is equivalent to the single-layer model. In the presence of $J$, we show that pairing is enhanced sublinearly with increasing $\ell$ and is primarily mediated by the $P^{\perp}_{d}$ component and find which coefficients and diagram sets are responsible. Finally, we construct a generalized $\ell$-dependent equation for $ P^{\perp}_{d}$ to speculate pairing beyond $\ell=4$.

cond-mat.supr-con↗
arXiv2024

Feynman diagrammatics based on discrete pole representations: A path to renormalized perturbation theories

By merging algorithmic Matsubara integration with discrete pole representations we present a procedure to generate fully analytic closed form results for impurity problems at fixed perturbation order. To demonstrate the utility of this approach we study the Bethe lattice and evaluate the second order self-energy for which reliable benchmarks exist. We show that, when evaluating diagrams on the Matsubara axis, the analytic sums of pole representations are extremely precise. We point out the absence of a numerical sign problem in the evaluation, and explore the application of the same procedure for real-frequency evaluation of diagrams. We find that real-frequency results are subject to noise that is controlled at low temperatures and can be mitigated at additional computational expense. We further demonstrate the utility of this approach by evaluating dynamical mean-field and bold diagrammatic self-consistency schemes at both second and fourth order and compare to benchmarks where available.

cond-mat.str-el↗
arXiv2023

TorchAmi: Generalized CPU/GPU Implementation of Algorithmic Matsubara Integration

We present torchami, an advanced implementation of algorithmic Matsubara integration (AMI) that utilizes pytorch as a backend to provide easy parallelization and GPU support. AMI is a tool for analytically resolving the sequence of nested Matsubara integrals that arise in virtually all Feynman perturbative expansions. In this implementation we present a new AMI algorithm that creates a more natural symbolic representation of the Feynman integrands. In addition, we include peripheral tools that allow for import and labelling of simple graph structures and conversion to torchami input. The code is written in c++ with python bindings provided.

hep-ph↗
arXiv2023

Emergent nearest-neighbor attraction in the fully renormalized interactions of the single-band repulsive Hubbard model at weak coupling

We compute the perturbative expansion for the effective interaction $W$ of the half-filled 2-dimensional Hubbard model. We derive extensions of standard RPA resummations that include arbitrarily high order contributions in the $W_{\uparrow\uparrow}$ and $W_{\uparrow\downarrow}$ basis. Using algorithmic tools we explore the static $Q$-dependent interaction as well as the same-time quantity both in momentum- and real-space. We emphasize the absence of screening in the Hubbard interaction where we find an enhanced repulsive local $W_{\uparrow\downarrow}$ with a non-zero attractive $W_{\uparrow\uparrow}$. Finally, starting from only a locally repulsive bare interaction find an emergent non-local nearest-neighbor attraction for low temperatures at sufficiently large values of $U/t$ which may be key to understanding pairing processes in the model.

cond-mat.str-el↗
arXiv2023

Pairing susceptibility of the two-dimensional Hubbard model in the thermodynamic limit

We compute the diagrammatic expansion of the particle-particle susceptibility via algorithmic Matsubara integration and compute the correlated pairing susceptibility in the thermodynamic limit of the 2D Hubbard Model. We study the static susceptibility and its dependence on the pair momentum $\mathbf{q}$ for a range of temperature, interaction strength, and chemical potential. We show that $d_{x^2-y^2}$-wave pairing is expected in the model in the $U/t\to 0^+ $ limit from direct perturbation theory. From this, we identify key second and third-order diagrams that support pairing processes and note that the diagrams responsible are not a part of charge or spin susceptibility expansions. We find two key components for pairing at momenta $(0,0)$ and $(π,π)$ that can be well fit as separate bosonic modes. We extract amplitudes and correlation length scales where we find a predominantly local $(π,π)$ pairing and non-local $\mathbf{q}=(0,0)$ pairs and present the relative weights of these modes for variation in temperature, doping, and interaction strength.

cond-mat.str-el↗
arXiv2023

Symbolic determinant construction of perturbative expansions

We present a symbolic algorithm for treating perturbative expansions of Hamiltonians with general two-body interactions. The method, formally equivalent to determinant Monte Carlo methods, merges well-known analytics with the recently developed symbolic integration tool, algorithmic Matsubara integration (AMI) that allows for the evaluation of the imaginary frequency/time integrals. By explicitly performing Wick contractions at each order of the perturbative expansion we order-by-order construct the fully analytic solution of the Green's function and self energy expansions. A key component of this process is the assignment of momentum/frequency conserving labels for each contraction that motivates us to present a fully symbolic Fourier transform procedure which accomplishes this feat. These solutions can be applied to a broad class of quantum chemistry problems and are valid at arbitrary temperatures and on both the real- and Matsubara-frequency axis. To demonstrate the utility of this approach, we present results for simple molecular systems as well as model lattice Hamiltonians. We highlight the case of molecular problems where our results at each order are numerically exact with no stochastic uncertainty.

cond-mat.str-el↗
arXiv2023

Planckian behaviour in the optical conductivity of the weakly coupled Hubbard model

We study the frequency and temperature dependence of the optical conductivity in the weakly coupled two-dimensional Hubbard model using a renormalized perturbative expansion. The perturbative expansion is based on the skeleton series for the current-current correlation function with a dressed Green`s function and the results are obtained directly on the real frequency axis using Algorithmic Matsubara Integration (AMI). The resulting conductivity shows a temperature-independent power law behaviour in the intermediate frequency regime. Moreover, the associated transport scattering time and renormalized mass exhibit a Planckian behaviour. We show that the self-energy of the Hubbard model, however, is distinct from existing Planckian models. The Planckian behaviour of the conductivity, observed in optimally doped cuprates for example, can thus be obtained from a different form of self-energy than the Planckian model, such as the weakly coupled Hubbard model at half-filling.

cond-mat.str-el↗
arXiv2022

Renormalized Perturbation Theory for Fast Evaluation of Feynman Diagrams on the Real Frequency Axis

We present a method to accelerate the numerical evaluation of spatial integrals of Feynman diagrams when expressed on the real frequency axis. This can be realized through use of a renormalized perturbation expansion with a constant but complex renormalization shift. The complex shift acts as a regularization parameter for the numerical integration of otherwise sharp functions. This results in an exponential speed up of stochastic numerical integration at the expense of evaluating additional counter-term diagrams. We provide proof of concept calculations within a difficult limit of the half-filled 2D Hubbard model on a square lattice.

cond-mat.str-el↗
arXiv2022

Influence of incommensurate structure on the elastic constants of crystalline Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ by Brillouin light scattering spectroscopy

Brillouin light scattering spectroscopy was used to probe the room-temperature elasticity of crystalline high-$T_c$ superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. A complete set of best-estimate elastic constants was obtained using established relationships between acoustic phonon velocities and elastic constants along with a simple expression relating crystal elastic constant $C_{22}$ to the corresponding constants of the constituent incommensurate sublattices. This latter relationship, which was derived and validated in the present work, has important implications for those studying incommensurate systems as it appears that it may be applied in its general form to any composite incommensurate crystal. The results obtained in this work are also consistent with sublattice assignments of Bi$_2$Sr$_2$O$_4$ and CaCu$_2$O$_4$ reported in a previous Brillouin scattering study.

cond-mat.mtrl-sci↗
arXiv2022

One- and two-particle properties of the weakly interacting two-dimensional Hubbard model in proximity to the van Hove singularity

We study the weak-coupling limit of the $t-t^\prime-U$ Hubbard model on a two-dimensional square lattice using a direct perturbative approach. Aided by symbolic computational tools, we compute the longitudinal density-density correlation functions in the $χ_{\uparrow \uparrow}$ and $χ_{\uparrow \downarrow}$ basis from which we can obtain the dynamical spin and charge susceptibilities at arbitrary doping and temperature. We find that for non-zero $t^\prime$, the zero frequency commensurate $\mathbf{q} = (π, π)$ spin and charge excitations are each strongest at different densities and we observe a clear behavioral change that appears tied to the van Hove singularity of the non-interacting dispersion upon which the perturbative expansion is built. We find a strongly reduced compressibility in the vicinity of the van Hove singularity as well as a behavioral change in the double occupancy. For finite $t^\prime$, the observed van Hove singularity occurs away from half-filling leading us to conclude that that this reduction in compressibility is distinct from Mott insulating physics that one expects in the strong-coupling regime. We compute the full dynamical spin and charge excitations and observe distinct structure for electron and hole doped scenarios in agreement with experiments on cuprate materials. Finally, we observe a peculiar splitting in spin and charge excitations in the vicinity of the van Hove singularity, the origin of which is traced to a splitting near the bottom of the band.

cond-mat.str-el↗