arXiv subjects

T. D. Kühne

Publications and source records attributed to T. D. Kühne.

9 recordsLinked to original sources

arXiv2026

Emergence of Resonating Valence-Bond Correlations in Stretched Graphene

Electronic correlations in graphene are generally considered weak due to the large bandwidth of its $π$ electrons. Here we show that tensile expansion of the honeycomb lattice provides a direct route to enhancing correlation effects. Using variational and diffusion quantum Monte Carlo, we compare a conventional Jastrow-Slater determinant wave function with a resonating-valence-bond (RVB) Jastrow-antisymmetrized geminal product ansatz for a series of stretched graphene lattices. We find that the energy gain of the RVB state relative to the single-determinant description increases with bond expansion up to a critical strain $δ_{\mathrm{cr}}$, and decreases beyond it, revealing a nonmonotonic evolution of electronic correlations. The crossover is found to occur in the range $15\% < δ_{\mathrm{cr}} < 20\%$, in agreement with mechanical stability limits. This behavior indicates a transition from a weakly correlated Dirac semimetal to a regime with enhanced non-dynamic correlation and short-range singlet pairing. Our results provide direct many-body evidence that lattice expansion drives graphene into a regime where RVB-like correlations become energetically favorable, offering a simple route to tuning correlation effects in Dirac materials.

cond-mat.str-el↗

arXiv2025

Resonating valence bond pairing energy in graphene by quantum Monte Carlo

We determine the resonating-valence-bond (RVB) state in graphene using real-space quantum Monte Carlo with correlated variational wave functions. Variational and diffusion quantum Monte Carlo (DMC) calculations with Jastrow-Slater-determinant and Jastrow-antisymmetrized-geminal-power ansatze are employed to evaluate the RVB pairing energy. Using a rectangular graphene sample that lacks $π/3$ rotational symmetry, we found that the single-particle energy gap near the Fermi level depends on the system size along the $x$-direction. The gap vanishes when the length satisfies $L_x=3n\sqrt{3}d$, where $n$ is an integer and $d$ is the carbon-carbon bond length, otherwise, the system, exhibits a finite gap. Our DMC results show no stable RVB pairing in the zero-gap case, whereas the opening of a finite gap near the Fermi level stabilizes the electron pairing. The DMC predicted absolute value of pairing energy at the thermodynamic limit for a finite-gap system is $\sim 0.48(1)$ mHa/atom. Our results reveal a feometry-driven electron pairing mechanism in the confined graphene nanostructure.

cond-mat.str-el↗

arXiv2025

Quantum Monte Carlo description of correlated electrons in two-dimensional FeSe

An interesting question in physics is how the correlation energy of atoms evolves upon forming a solid. Here, we address this problem for a specific case of double-layer FeSe. We used many-body wavefunction-based quantum Monte Carlo (QMC) techniques to compute the correlation energies of double-layer FeSe with different geometrical configurations and compared them with those of isolated Fe and Se atoms. Variational and diffusion QMC calculations were carried out with Slater Jastrow trial wavefunctions employing two alternative forms for the homogeneous two-body pair correlation term. The ground-state energy was obtained in the thermodynamic limit using two types of trial wave functions of JDFT, in which only the Jastrow factor is optimized while the Slater determinant is derived from the local density approximation, and JSD, where both the Jastrow factor and the Slater determinant are optimized simultaneously. Our results indicate that the correlation energy of double layer FeSe at the thermodynamic limit is mainly determined by the atomic contributions, with the bonding between atoms playing a comparatively minor role in it. After optimizing the interlayer separation of double-layer FeSe under tensile strain, we analyze the correlation energy as a function of strain and separation. We found that with increasing tensile stretch and interlayer spacing, the correlation energy of double-layer FeSe stochastically approaches that of its constituent atomic fragments.

cond-mat.str-el↗

arXiv2025

Elastic properties of transition metal dichalcogenides

We present a comprehensive first-principles study of the structural and elastic properties of 2H-MX$_2$ transition metal dichalcogenides (TMDs) (M = W, Mo, Ta, Nb; X = S, Se). Using density functional theory with various van der Waals exchange-correlation functionals, we systematically investigate the influence of nonlocal interactions on lattice parameters, elastic constants, and mechanical moduli. Our results reveal a fundamental distinction between semiconducting and metallic TMDs: metallic compounds exhibit larger in-plane lattice parameters and reduced interlayer spacing, consistent with their bonding characteristics. We find that metallic TMDs display significantly lower in-plane stiffness and shear modulus compared to their semiconducting counterparts. We discuss this behaviour in the context of the observed charge density waves. In addition, we establish clear trends in the bulk, Young's, and shear moduli, demonstrating the role of atomic number and chemical composition in determining mechanical stability.

cond-mat.mtrl-sci↗

arXiv2023

On the transport of CO$_2$ through humidified facilitated transport membranes

Membrane-based CO$_2$ removal from exhaust streams has recently gained much attention as a means of reducing emissions and limiting climate change. Novel membranes for CO$_2$ removal include so called facilitated transport membranes (FTMs), which offer very high selectivities for CO$_2$ while maintaining decent permeabilities. Recently, these FTMs have been scaled up from laboratory level to plant-sized pilot modules with promising results. However, the molecular details of CO$_2$ transport in these has not yet been fully unraveled. In this work, experimental studies were combined with quantum-mechanical ab initio molecular dynamics simulations to gain insight into the underlying molecular mechanism of CO$_2$ permeation through FTMs. Various compositions of polyvinyl alcohol (PVA) as the membrane matrix with polyvinyl amine (PVAm), monoethanolamine (MEA), or 4-amino-1-butanol (BA) as carrier molecules were experimentally tested. Our experiments revealed that water was essential for the CO$_2$ transport and a transport superposition was achieved with a mixed composition of PVAm and MEA in PVA. Furthermore, sorption measurements with PVA were conducted with humidified N$_2$ and CO$_2$ to quantify water sorption-induced swelling and its contribution to the gas uptake. As the carbonic acid--amine interaction is assumed to cause transport facilitation, electronic structure-based ab initio molecular dynamics simulations were conducted to study the transport of CO$_2$ in the form of carbonic acid along PVAm polymer chains. In particular, the necessity of local water for transport facilitation was studied at different water contents. The simulations show that transport is fastest in the system with low water content and does not happen in the absence of water.

cond-mat.soft↗

arXiv2018

Quantum Monte Carlo calculations of van der Waals interactions between aromatic benzene rings

The magnitude of finite-size effects and Coulomb interactions in quantum Monte Carlo simulations of van der Waals interactions between weakly bonded benzene molecules are investigated. To that extent, two trial wave functions of the Slater-Jastrow and Backflow-Slater-Jastrow types are employed to calculate the energy-volume equation of state. We assess the impact of the backflow coordinate transformation on the non-local correlation energy. We found that the effect of finite-size errors in quantum Monte Carlo calculations on energy differences is particularly large and may even be more important than the employed trial wave function. Beside the cohesive energy, the singlet excitonic energy gap and the energy gap renormalization of crystalline benzene at different densities are computed.

cond-mat.soft↗

arXiv2017

Nuclear Quantum Effects Induce Metallization of Dense Solid Molecular Hydrogen

We present an accurate computational study of the electronic structure and lattice dynamics of solid molecular hydrogen at high pressure. The band-gap energies of the $C2/c$, $Pc$, and $P6_3/m$ structures at pressures of 250, 300, and 350 GPa are calculated using the diffusion quantum Monte Carlo (DMC) method. The atomic configurations are obtained from ab-initio path-integral molecular dynamics (PIMD) simulations at 300 K and 300 GPa to investigate the impact of zero-point energy and temperature-induced motion of the protons including anharmonic effects. We find that finite temperature and nuclear quantum effects reduce the band-gaps substantially, leading to metallization of the $C2/c$ and $Pc$ phases via band overlap; the effect on the band-gap of the $P6_3/m$ structure is less pronounced. Our combined DMC-PIMD simulations predict that there are no excitonic or quasiparticle energy gaps for the $C2/c$ and $Pc$ phases at 300 GPa and 300 K. Our results also indicate a strong correlation between the band-gap energy and vibron modes. This strong coupling induces a band-gap reduction of more than 2.46 eV in high-pressure solid molecular hydrogen. Comparing our DMC-PIMD with experimental results available, we conclude that none of the structures proposed is a good candidate for phases III and IV of solid hydrogen.

cond-mat.mtrl-sci↗

arXiv2015

Van der Waals forces from first principles for periodic systems: Application to graphene-water interactions

We extend the method of Silvestrelli [P. L. Silvestrelli, J. Chem. Phys. 139, 054106 (2013)] to approximate long-range van der Waals interactions at the density functional theory level based on maximally localized Wannier functions combined with the quantum harmonic oscillator model, to periodic systems. Applying this scheme to study London dispersion forces between graphene and water layers, we demonstrate that collective many-body effects beyond simple additive pair-wise interactions are essential to accurately describe van der Waals forces.

cond-mat.mtrl-sci↗

arXiv2015

On Fermionic Shadow Wave Functions for strongly-correlated multi-reference systems based on a single Slater determinant

We demonstrate that extending the Shadow Wave Function to fermionic systems facilitates to accurately calculate strongly-correlated multi-reference systems such as the stretched H2 molecule. This development considerably extends the scope of electronic structure calculations and enables to efficiently recover the static correlation energy using just a single Slater determinant.

physics.comp-ph↗