arXiv subjects

J. P. Hague

Publications and source records attributed to J. P. Hague.

At least 19 recordsLinked to original sources

arXiv2026

Dynamical correlations and nonequilibrium sum rules in photodoped Hubbard ladders

Using matrix product state techniques we study the nonequilibrium dynamical response of the half-filled Hubbard ladder when subject to an optical pump. Optical pumping offers a way of producing and manipulating new strongly correlated phenomena by suppressing existing magnetic correlations. The ladder allows the effects of pump directionality to be investigated, and compared to a single chain it has strong spin-charge coupling and a fully gapped excitation spectrum, promising different nonequilibrium physics. We compute time-dependent correlations, including the nonequilibrium dynamical structure factors for spin and charge. By deriving a combined spin-charge sum rule that applies both in and out-of-equilibrium, we show that spectral weight is pumped directly from the antiferromagnetic spin response into a low energy $ω\sim 0$ charge response below the Mott gap. The transfer of weight is pump direction dependent: pumping directed along the legs disrupts magnetic correlations more than pumping in the rung direction, even if the post pump energy density is similar. The charge correlation length is dramatically enhanced by the pump, whilst the spin correlations are most strongly suppressed at nearest and next-nearest neighbour spacings. After the pump the system is in a nonthermal correlated metallic state, with gapless charge excitations and approximately equal spin and charge correlation lengths, emphasising the importance of treating these degrees of freedom on an equal footing in nonequilibrium systems.

cond-mat.str-el↗

arXiv2025

Quantum Monte Carlo Simulation of Bipolaron Superconductivity in Extended Hubbard--Holstein models on Face-Centered-Cubic and Body-Centered-Cubic Lattices

We investigate superlight pairing of bipolarons driven by electron-phonon interactions (EPIs) in face-center-cubic (FCC) and body-center-cubic (BCC) lattices using a continuous-time path-integral quantum Monte Carlo (QMC) algorithm. The EPIs are of the Holstein and extended Holstein types, and a Hubbard interaction is also included. Effects of adiabaticity are calculated. The number of phonons associated with the bipolaron, inverse mass, and radius are calculated and used to construct a phase diagram for bipolaron pairing (identifying the regions of pairing into intersite bipolarons and onsite bipolarons). From the inverse mass we determine that for the extended interaction, there is a region of light pairing associated with intersite bipolarons formed in both BCC and FCC lattices. Intersite bipolarons in the extended model at intermediate phonon frequency and large Coulomb repulsion become superlight due to first order hopping effects. We estimate the transition temperature, determining that intersite bipolarons are associated with regions of high transition temperatures.

cond-mat.str-el↗

arXiv2024

Ubiquitous light real-space pairing from long-range hopping and interactions

We systematically examine how long-range hopping and its synergy with extended interactions leads to light bound pairs. Pair properties are determined for a dilute extended Hubbard model with large on-site repulsion ($U$) and both near- and next-nearest neighbour hopping ($t$ and $t'$) and attraction ($V$ and $V'$), for cubic and tetragonal lattices. The presence of $t'$ and $V'$ promotes light pairs. For tetragonal lattices, $t'<0$ pairs can be lighter than non-interacting particles, and $d$-symmetric pairs form. Close packing transition temperatures, $T^{\ast}$ are estimated for the Bose-Einstein condensation (BEC) of pairs to be $k_{B}T^{\ast}\sim\overline{t} 0.1$, where $\overline{t}$ is the geometric mean of the hoppings on the Cartesian axes. When pairs have $d$-symmetry, the condensate has $d$-wave character. Thus, the presence of both $t'$ and $V'$ leads ubiquitously to small strongly bound pairs with an inverse mass that is linear in hopping, which could lead to high temperature BECs.

cond-mat.supr-con↗

arXiv2021

Fermion pairing in body-centered-cubic quantum simulators of extended Hubbard models

We investigate formation and condensation of fermion pairs in cold-atom quantum simulators for extended Hubbard models ($UV$ models) with body-centered-cubic (BCC) optical lattices in the dilute limit, predicting small and light pairs. Pair mass, radius, and binding conditions are calculated, and used to compute transition temperatures. We predict that: (a) local pairs form in BCC optical lattices and binding energies can be large; (b) for particular cases where onsite $U$ and intersite $V$ are attractive with similar size, pairs are both small and light; and (c) pairs of $^6$Li atoms Bose--Einstein condense at temperatures of around 10 nK.

cond-mat.str-el↗

arXiv2021

Hubbard models with arbitrary structures in programmable optical lattices

We investigate the use of programmable optical lattices for quantum simulation of Hubbard models, determining analytic expressions for the hopping and Hubbard U, finding that they are suitable for emulating strongly correlated systems with arbitrary structures, including those with multiple site basis and impurities. Programmable potentials are highly flexible, with the ability to control the depth and shape of individual sites in the optical lattice dynamically. Quantum simulators of Hubbard models with (1) arbitrary basis are required to represent many real materials of contemporary interest, (2) broken translational symmetry are needed to study impurity physics, and (3) dynamical lattices are needed to investigate strong correlation out of equilibrium. We derive analytic expressions for Hubbard Hamiltonians in programmable potential systems. We find experimental parameters for quantum simulation of Hubbard models with arbitrary basis, concluding that programmable optical lattices are suitable for this purpose. We discuss how programmable optical lattices can be used for quantum simulation of dynamical multi-band Hubbard models that represent complicated compounds, impurities, and non-equilibrium physics.

cond-mat.str-el↗

arXiv2020

Microscopic biophysical model of self-organization in tissue due to feedback between cell- and macroscopic-scale forces

We develop a microscopic biophysical model for self-organization and reshaping of artificial tissue, that is co-driven by microscopic active forces between cells and extracellular matrix (ECM), and macroscopic forces that develop within the tissue, finding close agreement with experiment. Microscopic active forces are stimulated by $μ$m scale interactions between cells and the ECM within which they exist, and when large numbers of cells act together these forces drive, and are affected by, macroscopic-scale self-organization and reshaping of tissues in a feedback loop. To understand this loop, there is a need to: (1) construct microscopic biophysical models that can simulate these processes for the very large number of cells found in tissues; (2) validate and calibrate those models against experimental data; and (3) understand the active feedback between cells and the extracellular matrix, and its relationship to macroscopic self-organization and reshaping of tissue. Our microscopic biophysical model consists of a contractile network representing the ECM, that interacts with a large number of cells via dipole forces, to describe macroscopic self-organization and reshaping of tissue. We solve the model using simulated annealing, finding close agreement with experiments on artificial neural tissue. We discuss calibration of model parameters. We conclude that feedback between microscopic cell-ECM dipole interactions and tissue-scale forces, is a key factor in driving macroscopic self-organization and reshaping of tissue. We discuss application of the biophysical model to simulation and rational design of artificial tissues.

physics.bio-ph↗

arXiv2020

A cold atom quantum simulator to explore pairing, condensation, and pseudogaps in extended Hubbard--Holstein models

We describe a quantum simulator for the Hubbard--Holstein model (HHM), comprising two dressed Rydberg atom species held in a monolayer by independent painted potentials, predicting that boson-mediated preformed pairing, and Berezinskii--Kosterlitz--Thouless (BKT) transition temperatures are experimentally accessible. The HHM is important for modeling the essential physics of unconventional superconductors. Experimentally realizable quantum simulators for HHMs are needed: (1) since HHMs are difficult to solve numerically and analytically, (2) to explore how competition between electron-phonon interactions and strong repulsion affects pairing in unconventional superconductors, (3) to understand the role of boson-mediated local pairing in pseudogaps and fermion condensates. We propose and study a quantum simulator for the HHM, using optical lattices, painted using zeros in the AC stark shift, to control two Rydberg atom species independently within a monolayer. We predict that interactions are sufficiently tunable to probe: (1) both HHMs and highly unconventional phonon-mediated repulsions, (2) the competition between intermediate-strength phonon and Coulomb mediated interactions, (3) BKT transitions, and preformed pairing that could be used to examine key hypotheses related to the pseudogap. We discuss how the quantum simulator can be used to investigate boson-mediated pairing and condensation of fermions in unconventional superconductors.

quant-ph↗

arXiv2017

Implementation strategies for multiband quantum simulators of real materials

The majority of quantum simulators treat simplified one-band strongly correlated models, whereas multiple bands are needed to describe materials with intermediate correlation. We investigate the sensitivity of multiband quantum simulators to: (1) the form of optical lattices (2) the interactions between electron analogues. Since the kinetic energy terms of electron analogues in a quantum simulator and electrons in a solid are identical, by examining both periodic potential and interaction we explore the full problem of many-band quantum simulators within the Born-Oppenheimer approximation. Density functional calculations show that bandstructure is highly sensitive to the form of optical lattice, and it is necessary to go beyond sinusoidal potentials to ensure that the bands closest to the Fermi surface are similar to those in real materials. Analysis of several electron analogue types finds that dressed Rydberg atoms (DRAs) have promising interactions for multi band quantum simulation. DRA properties can be chosen so that interaction matrices approximate those in real systems and decoherence effects are controlled, albeit with parameters at the edge of currently available technology. We conclude that multiband quantum simulators implemented using the principles established here could provide insight into the complex processes in real materials.

physics.atom-ph↗

arXiv2016

Role of substrate induced electron-phonon interactions in biased graphitic bilayers

Bilayers of graphitic materials have potential applications in field effect transistors (FETs). A potential difference applied between certain ionic bilayers made from insulating graphitic materials such as BN, ZnO and AlN could reduce gap sizes, turning them into useful semiconductors. On the other hand, opening of a small semiconducting gap occurs in graphene bilayers under applied field. The aim here is to investigate to what extent substrate induced electron-phonon interactions (EPIs) modify this gap change. We examine EPIs in several lattice configurations, using a perturbative approach. The typical effect of EPIs on the ionic bilayers is an undesirable gap widening. The size of this gap change varies considerably with lattice structure and the magnitude of the bias. When bias is larger than the non-interacting gap size, EPIs have the smallest effect on the bandgap, especially in configurations with $AA^{\prime}$ and $AB$ structures. Thus careful selection of substrate, lattice configuration and bias strength to minimise the effects of EPIs could be important for optimising the properties of electronic devices. We use parameters related to BN in this article. In practice, the results presented here are broadly applicable to other graphitic bilayers, and are likely to be qualitatively similar in metal dichalcogenide bilayers such as MoS$_2$, which are already of high interest for their use in FETs.

cond-mat.mes-hall↗

arXiv2015

Optimal interlayer hopping and high temperature Bose-Einstein condensation of local pairs in quasi 2D superconductors

Both FeSe and cuprate superconductors are quasi 2D materials with high transition temperatures and local fermion pairs. Motivated by such systems, we investigate real space pairing of fermions in an anisotropic lattice model with intersite attraction, V, and strong local Coulomb repulsion, U, leading to a determination of the optimal conditions for superconductivity from Bose-Einstein condensation. Our aim is to gain insight as to why high temperature superconductors tend to be quasi 2D. We make both analytically and numerically exact solutions for two body local pairing applicable to intermediate and strong V. We find that the Bose Einstein condensation temperature of such local pairs is maximal when hopping between layers is intermediate relative to in-plane hopping, indicating that the quasi 2D nature of unconventional superconductors has an important contribution to their high transition temperatures.

cond-mat.supr-con↗

arXiv2014

Enhancement of gaps in thin graphitic films for heterostructure formation

There are a large number of atomically thin graphitic films with similar structure to graphene. These films have a spread of bandgaps relating to their ionicity, and also to the substrate on which they are grown. Such films could have a range of applications in digital electronics where graphene is difficult to use. I use the dynamical cluster approximation to show how electron-phonon coupling between film and substrate can enhance these gaps in a way that depends on the range and strength of the coupling. One of the driving factors in this effect is the proximity to a charge density wave instability for electrons on a honeycomb lattice. The enhancement at intermediate coupling is sufficiently large that spatially varying substrates and superstrates could be used to create heterostructures in thin graphitic films with position dependent electron-phonon coupling and gaps, leading to advanced electronic components.

cond-mat.mes-hall↗

arXiv2014

Bias Free Gap Creation in Bilayer Graphene

For graphene to be utilized in the digital electronics industry the challenge is to create bandgaps of order 1eV as simply as possible. The most successful methods for the creation of gaps in graphene are (a) confining the electrons in nanoribbons, which is technically difficult or (b) placing a potential difference across bilayer graphene, which is limited to gaps of around 300meV for reasonably sized electric fields. Here we propose that electronic band gaps can be created without applying an external electric field, by using the electron-phonon interaction formed when bilayer graphene is sandwiched between highly polarisable ionic materials. We derive and solve self-consistent equations, finding that a large gap can be formed for intermediate electron-phonon coupling. The gap originates from the amplification of an intrinsic Coulomb interaction due to the proximity of carbon atoms in neighbouring planes.

cond-mat.mes-hall↗

arXiv2013

Cold Rydberg atoms for quantum simulation of exotic condensed matter interactions

Quantum simulators could provide an alternative to numerical simulations for understanding minimal models of condensed matter systems in a controlled way. Typically, cold atom systems are used to simulate e.g. Hubbard models. In this paper, we discuss a range of exotic interactions that can be formed when cold Rydberg atoms are loaded into optical lattices with unconventional geometries; such as long-range electron-phonon interactions and extended Coulomb like interactions. We show how these can lead to proposals for quantum simulators for complex condensed matter systems such as superconductors. Continuous time quantum Monte Carlo is used to compare the proposed schemes with the physics found in traditional condensed matter Hamiltonians for systems such as high temperature superconductors.

cond-mat.str-el↗

arXiv2013

Modelling of impaired cerebral blood flow due to gaseous emboli

Bubbles introduced to the arterial circulation during invasive medical procedures can have devastating consequences for brain function but their effects are currently difficult to quantify. Here we present a Monte-Carlo simulation investigating the impact of gas bubbles on cerebral blood flow. For the first time, this model includes realistic adhesion forces, bubble deformation, fluid dynamical considerations, and bubble dissolution. This allows investigation of the effects of buoyancy, solubility, and blood pressure on embolus clearance. Our results illustrate that blockages depend on several factors, including the number and size distribution of incident emboli, dissolution time and blood pressure. We found it essential to model the deformation of bubbles to avoid overestimation of arterial obstruction. Incorporation of buoyancy effects within our model slightly reduced the overall level of obstruction but did not decrease embolus clearance times. We found that higher blood pressures generate lower levels of obstruction and improve embolus clearance. Finally, we demonstrate the effects of gas solubility and discuss potential clinical applications of the model.

physics.bio-ph↗

arXiv2012

Gap modification of atomically thin boron nitride by phonon mediated interactions

A theory is presented for the modification of bandgaps in atomically thin boron nitride (BN) by attractive interactions mediated through phonons in a polarizable substrate, or in the BN plane. Gap equations are solved, and gap enhancements are found to range up to 70% for dimensionless electron-phonon coupling λ=1, indicating that a proportion of the measured BN bandgap may have a phonon origin.

cond-mat.mes-hall↗

arXiv2012

Polarons in highly doped atomically thin graphitic materials

Polaron spectral functions are computed for highly doped graphene-on-substrate and other atomically thin graphitic systems using the diagrammatic Monte Carlo technique. The specific aim is to investigate the effects of interaction on spectral functions when the symmetry between sub-lattices of a honeycomb lattice has been broken by the substrate or ionicity, inducing a band gap. Introduction of electron-phonon coupling leads to several polaronic features, such as band-flattening and changes in particle lifetimes. At the K point, differences between energies on each sub-lattice increase with electron-phonon coupling, indicating an augmented transport gap, while the spectral gap decreases slightly. Effects of phonon dispersion and long-range interactions are investigated, and found to lead to only quantitative changes in spectra.

cond-mat.str-el↗

arXiv2012

Bilayers of Rydberg atoms as a quantum simulator for unconventional superconductors

In condensed matter, it is often difficult to untangle the effects of competing interactions, and this is especially problematic for superconductors. Quantum simulators may help: here we show how exploiting the properties of highly excited Rydberg states of cold fermionic atoms in a bilayer lattice can simulate electron-phonon interactions in the presence of strong correlation - a scenario found in many unconventional superconductors. We discuss the core features of the simulator, and use numerics to compare with condensed matter analogues. Finally, we illustrate how to achieve a practical, tunable implementation of the simulation using painted spot potentials.

cond-mat.supr-con↗

arXiv2012

Mobile small bipolarons on a three-dimensional cubic lattice

We use numerically exact quantum Monte Carlo (QMC) to compute the properties of three-dimensional bipolarons for interaction strengths where perturbation theory fails. For intermediate electron-phonon coupling and Hubbard U, we find that bipolarons can be both small and light, a prerequisite for bipolaron superconductivity. We use the QMC results to make estimates of transition temperatures, which peak at between 90-120 K and are demonstrated to be insensitive to Coulomb repulsion and impurities.

cond-mat.str-el↗