Giovanni Vignale
Degree: PhD
Position: Visiting Faculty
Affiliation: University of Missouri
Research Type: Theory
Email: vignaleg@missouri.edu
Website: https://faculty.missouri.edu/~vignaleg/
CA2DM Publications:
2025 |
Li, Tianbo; Lin, Min; Dale, Stephen G; Shi, Zekun; Neto, Castro A H; Novoselov, Kostya S; Vignale, Giovanni Diagonalization without Diagonalization: A Direct Optimization Approach for Solid-State Density Functional Theory Journal Article JOURNAL OF CHEMICAL THEORY AND COMPUTATION, 21 (9), pp. 4730-4741, 2025, ISSN: 1549-9618. @article{ISI:001478048300001, title = {Diagonalization without Diagonalization: A Direct Optimization Approach for Solid-State Density Functional Theory}, author = {Tianbo Li and Min Lin and Stephen G Dale and Zekun Shi and Castro A H Neto and Kostya S Novoselov and Giovanni Vignale}, doi = {10.1021/acs.jctc.4c01551}, times_cited = {0}, issn = {1549-9618}, year = {2025}, date = {2025-04-28}, journal = {JOURNAL OF CHEMICAL THEORY AND COMPUTATION}, volume = {21}, number = {9}, pages = {4730-4741}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {We present a novel approach to address the challenges of variable occupation numbers in direct optimization of density functional theory (DFT). By parametrizing both the eigenfunctions and the occupation matrix, our method minimizes the free energy with respect to these parameters. As the stationary conditions require the occupation matrix and the Kohn-Sham Hamiltonian to be simultaneously diagonalizable, this leads to the concept of "self-diagonalization," where, by assuming a diagonal occupation matrix without loss of generality, the Hamiltonian matrix naturally becomes diagonal at stationary points. Our method incorporates physical constraints on both the eigenfunctions and the occupations into the parametrization, transforming the constrained optimization into an fully differentiable unconstrained problem, which is solvable via gradient descent. Implemented in JAX, our method was tested on aluminum and silicon, confirming that it achieves efficient self-diagonalization, produces the correct Fermi-Dirac distribution of the occupation numbers and yields band structures consistent with those obtained with SCF eigensolver methods in Quantum Espresso.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We present a novel approach to address the challenges of variable occupation numbers in direct optimization of density functional theory (DFT). By parametrizing both the eigenfunctions and the occupation matrix, our method minimizes the free energy with respect to these parameters. As the stationary conditions require the occupation matrix and the Kohn-Sham Hamiltonian to be simultaneously diagonalizable, this leads to the concept of "self-diagonalization," where, by assuming a diagonal occupation matrix without loss of generality, the Hamiltonian matrix naturally becomes diagonal at stationary points. Our method incorporates physical constraints on both the eigenfunctions and the occupations into the parametrization, transforming the constrained optimization into an fully differentiable unconstrained problem, which is solvable via gradient descent. Implemented in JAX, our method was tested on aluminum and silicon, confirming that it achieves efficient self-diagonalization, produces the correct Fermi-Dirac distribution of the occupation numbers and yields band structures consistent with those obtained with SCF eigensolver methods in Quantum Espresso. |
Chakraborty, Amarnath; Rodin, Aleksandr; Adam, Shaffique; Vignale, Giovanni Insulator-metal transition and magnetic crossover in bilayer graphene Journal Article PHYSICAL REVIEW B, 111 (12), 2025, ISSN: 2469-9950. @article{ISI:001458804200004, title = {Insulator-metal transition and magnetic crossover in bilayer graphene}, author = {Amarnath Chakraborty and Aleksandr Rodin and Shaffique Adam and Giovanni Vignale}, doi = {10.1103/PhysRevB.111.125130}, times_cited = {0}, issn = {2469-9950}, year = {2025}, date = {2025-03-11}, journal = {PHYSICAL REVIEW B}, volume = {111}, number = {12}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {In-plane magnetic fields offer a relatively unexplored opportunity to alter the band structure of stacks of two-dimensional (2D) materials so that they exhibit the desired physical properties. Here we show that an inplane magnetic field combined with a transverse electric field can induce an insulator-metal (IM) transition in bilayer graphene. Our study of the magnetic response reveals that the orbital magnetic susceptibility changes from diamagnetic to paramagnetic around the transition point. We discuss several strategies to observe the IM transition, switch the diamagnetism, and more generally control the band structure of stacked 2D materials at experimentally accessible magnetic fields.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In-plane magnetic fields offer a relatively unexplored opportunity to alter the band structure of stacks of two-dimensional (2D) materials so that they exhibit the desired physical properties. Here we show that an inplane magnetic field combined with a transverse electric field can induce an insulator-metal (IM) transition in bilayer graphene. Our study of the magnetic response reveals that the orbital magnetic susceptibility changes from diamagnetic to paramagnetic around the transition point. We discuss several strategies to observe the IM transition, switch the diamagnetism, and more generally control the band structure of stacked 2D materials at experimentally accessible magnetic fields. |
2024 |
Trushin, Maxim; Peng, Liangtao; Sharma, Gargee; Vignale, Giovanni; Adam, Shaffique High conductivity from cross-band electron pairing in flat-band systems Journal Article PHYSICAL REVIEW B, 109 (24), 2024, ISSN: 2469-9950. @article{ISI:001247474400001, title = {High conductivity from cross-band electron pairing in flat-band systems}, author = {Maxim Trushin and Liangtao Peng and Gargee Sharma and Giovanni Vignale and Shaffique Adam}, doi = {10.1103/PhysRevB.109.245118}, times_cited = {0}, issn = {2469-9950}, year = {2024}, date = {2024-06-13}, journal = {PHYSICAL REVIEW B}, volume = {109}, number = {24}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {Electrons in condensed matter may transition into a variety of broken-symmetry phase states due to electronelectron interactions. Applying diverse mean-field approximations to the interaction term is arguably the simplest way to identify the phase states theoretically possible in a given setting. Here, we explore electron-electron attraction in a two-band system comprising symmetric conduction and valence bands touching each other at a single point. We assume a mean-field pairing between the electrons having opposite spins, momenta, and in contrast to the conventional superconducting pairing, residing in opposite bands, i.e., having opposite energies. We show that electrons transition into a correlated ground state if and only if the bands are flat enough, i.e., the transition is impossible in the case of conventional parabolic bands. Although this state is not superconducting in the usual sense and does not exhibit a gap in its excitation spectrum, it is nevertheless immune to elastic scattering caused by any kind of disorder and is therefore expected to exhibit high electric conductivity at low temperature, mimicking the behavior of a real superconductor. Having in mind the recent experimental realizations of flat-band electronic systems in twisted multilayers, we foresee an exciting opportunity for observing a class of highly conductive materials.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Electrons in condensed matter may transition into a variety of broken-symmetry phase states due to electronelectron interactions. Applying diverse mean-field approximations to the interaction term is arguably the simplest way to identify the phase states theoretically possible in a given setting. Here, we explore electron-electron attraction in a two-band system comprising symmetric conduction and valence bands touching each other at a single point. We assume a mean-field pairing between the electrons having opposite spins, momenta, and in contrast to the conventional superconducting pairing, residing in opposite bands, i.e., having opposite energies. We show that electrons transition into a correlated ground state if and only if the bands are flat enough, i.e., the transition is impossible in the case of conventional parabolic bands. Although this state is not superconducting in the usual sense and does not exhibit a gap in its excitation spectrum, it is nevertheless immune to elastic scattering caused by any kind of disorder and is therefore expected to exhibit high electric conductivity at low temperature, mimicking the behavior of a real superconductor. Having in mind the recent experimental realizations of flat-band electronic systems in twisted multilayers, we foresee an exciting opportunity for observing a class of highly conductive materials. |
Peng, Liangtao; Yudhistira, Indra; Vignale, Giovanni; Adam, Shaffique Theoretical determination of the effect of a screening gate on plasmon-induced superconductivity in twisted bilayer graphene Journal Article PHYSICAL REVIEW B, 109 (4), 2024, ISSN: 2469-9950. @article{ISI:001173887400003, title = {Theoretical determination of the effect of a screening gate on plasmon-induced superconductivity in twisted bilayer graphene}, author = {Liangtao Peng and Indra Yudhistira and Giovanni Vignale and Shaffique Adam}, doi = {10.1103/PhysRevB.109.045404}, times_cited = {1}, issn = {2469-9950}, year = {2024}, date = {2024-01-05}, journal = {PHYSICAL REVIEW B}, volume = {109}, number = {4}, publisher = {AMER PHYSICAL SOC}, address = {ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA}, abstract = {The microscopic pairing mechanism for superconductivity in magic-angle twisted bilayer graphene remains an open question. Recent experimental studies seem to rule out a purely electronic mechanism due to the insensitivity of the critical superconducting temperature to either a highly doped screening layer or the proximity to a metallic screening gate. In this theoretical work, we explore the role of external screening layers on the superconducting properties of twisted bilayer graphene within a purely electronic mechanism. Consistent with the experimental observations, we find that the critical temperature is unaffected by screening unless the screening layer is closer than 3 nm from the superconductor. Thus, the available transport data are not in contradiction with a plasmon-mediated mechanism. We also investigate other properties of this plasmon-mediated superconductivity, including signatures in the tunneling density of states as probed in spectroscopy experiments.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The microscopic pairing mechanism for superconductivity in magic-angle twisted bilayer graphene remains an open question. Recent experimental studies seem to rule out a purely electronic mechanism due to the insensitivity of the critical superconducting temperature to either a highly doped screening layer or the proximity to a metallic screening gate. In this theoretical work, we explore the role of external screening layers on the superconducting properties of twisted bilayer graphene within a purely electronic mechanism. Consistent with the experimental observations, we find that the critical temperature is unaffected by screening unless the screening layer is closer than 3 nm from the superconductor. Thus, the available transport data are not in contradiction with a plasmon-mediated mechanism. We also investigate other properties of this plasmon-mediated superconductivity, including signatures in the tunneling density of states as probed in spectroscopy experiments. |