@inproceedings {713,
title = {External electromagnetic effects on the spin transport of the open one-dimensional single-band Fermi-Hubbard lattice},
booktitle = {Proceedings of the 43rd Samahang Pisika ng Pilipinas Physics Conference},
year = {2025},
month = {25{\textendash}28 Jun 2025},
pages = {SPP-2025-2E-03},
address = {Quezon City},
abstract = {Experimentally, conductivities help probe phases of matter among strongly correlated systems. In this study, the effect of external electromagnetic field on the spin conductivity of an open one-dimensional single-band Fermi-Hubbard lattice is calculated with the assumption of linear response. It is shown that for repulsive two-particle interactions, the spin conductivity can be optimized at finite low-temperature when the lattice is subject to a strong magnetic field and a more positive electric field. On the other hand, external electromagnetic fields drive the spin conductivity to an asymptotic behavior when the two-particle interactions are attractive.},
url = {https://proceedings.spp-online.org/article/view/SPP-2025-2E-03},
author = {Vladimir P. Villegas and Cristine D. Villagonzalo}
}
@article {700,
title = {Optimal polarization and susceptibility profiles of a half-filled open magnetoelectric Fermi-Hubbard optical dimer},
journal = {Phys. Lett. A},
volume = {525},
year = {2024},
pages = {129898},
abstract = {Magnetoelectric materials, such as thin films and magnetoelectric antennae, exhibit tunable magnetic polarization using external electric field and electric polarization using external magnetic field at room temperature by introducing strain, for instance. A more straightforward method to optimize magnetoelectric effect is by adjusting the applied electromagnetic fields in systems modeled by the Fermi-Hubbard dimer. In this study, direct and converse magnetoelectric effects are theoretically observed in an open Fermi-Hubbard dimer by tuning the external electromagnetic fields. Results show that an external magnetic field can enhance the electric polarization and susceptibility of the dimer particles without the necessity of mechanical strain. Conversely, gradients in an external electric field modulate the magnetic polarization and susceptibility. This striking behavior in the Fermi-Hubbard dimer can serve as a physical model for developing quantum control of complex materials such as in biosensors, biomedical processes, and energy harvesting.},
doi = {10.1016/j.physleta.2024.129898},
author = {Vladimir P. Villegas and Cristine D. Villagonzalo}
}
@conference {644,
title = {Caloric and spin transport profile of Fermi-Hubbard systems},
year = {2023},
month = {6{\textendash}10 Nov 2023},
pages = {P.16},
publisher = {Asian Network School and Workshop on Complex Condensed Matter Systems 2023},
address = {IOP VAST, Vietnam},
abstract = {An open one-dimensional Fermi-Hubbard optical lattice exposed to external electromagnetic fields is investigated using the grand canonical ensemble formalism. As the system was reduced to a dimer, the eigenstates were determined using the exact diagonalization method while tuning the two-particle interactions of the lattice, a varying chemical potential, and applied external electromagnetic fields. Entropy and heat capacity gradients exist due to the interaction of the system with the external magnetic and electric fields, also known as magnetocaloric and electrocaloric effects, respectively. At particular temperatures, the isothermal entropic change was calculated to determine the necessary conditions for direct or inverse caloric effects to occur. The effects of more lattice sites and the elastic two-particle interaction can be incorporated through analytical methods. It was shown that at low temperatures, spins tend to scatter under the influence of repulsive two-particle interactions, while backflow occurs in the attractive regime. Furthermore, suitable temperature ranges for spin conductivity to occur can be found by adjusting the electromagnetic fields.},
url = {https://iop.vast.vn/asian/2023/program.php?t=1\&n=16},
author = {Vladimir P. Villegas and Cristine D. Villagonzalo}
}
@inproceedings {624,
title = {Effects of interparticle interaction on the spin transport in a half-filled open elastic Fermi-Hubbard optical lattice},
booktitle = {Proceedings of the 41st Samahang Pisika ng Pilipinas Physics Conference},
year = {2023},
month = {19{\textendash}21 July 2023},
pages = {SPP-2023-1F-05},
address = {Siargao},
abstract = {Spin is a particle property whose study of transport receives attention due to lossless transfer of information. In this study, spin transport is dependent on interparticle interactions in a one-dimensional half-filled open elastic Fermi-Hubbard optical lattice. Near absolute zero temperature temperature, spins flow in the repulsive regime and are not transported in the attractive regime. In addition, as the temperature approaches zero, the abrupt increase of the conductivity for the repulsive regime and the decrease to zero of the conductivity for the attractive regime take place at relatively higher temperatures.},
url = {https://proceedings.spp-online.org/article/view/SPP-2023-1F-05},
author = {Vladimir P. Villegas and Cristine D. Villagonzalo}
}
@article {649,
title = {Magnetocaloric and electrocaloric heat engine and refrigeration cycles in the open Fermi-Hubbard optical dimer with attractive interactions},
journal = {Phys. Lett. A},
volume = {481},
year = {2023},
pages = {129011},
abstract = {Quantum heat engines and refrigerators can be realized in a Fermi-Hubbard optical dimer through immersion in a mean-field Fermi gas bath using the magnetocaloric and electrocaloric effects. It is demonstrated that in the presence of onsite and heat bath-induced attractive interactions, singlet-to-triplet transitions are achieved with a finite external magnetic field. Similarly, triplet-to-singlet transitions occur in the presence of an applied finite electric field. Results show that to lift the degeneracies, considerable electric potential magnitudes are needed. These magnitudes are greater than the magnetic potential strengths where the degeneracies occur. Furthermore, in the attractive regime, combinations of direct and inverse caloric effects can be combined to operate quantum heat engines and refrigerators, like what happens in the repulsive regime.},
doi = {10.1016/j.physleta.2023.129011},
url = {https://www.sciencedirect.com/science/article/abs/pii/S0375960123003912},
author = {Vladimir P. Villegas and Cristine D. Villagonzalo}
}
@article {577,
title = {Caloric effects in an open Fermi{\textendash}Hubbard optical dimer due to onsite and heat bath-induced two-particle interactions},
journal = {J. Magn. Magn. Mater.},
volume = {564},
year = {2022},
pages = {170094},
abstract = {The immersion of the Fermi{\textendash}Hubbard optical dimer in a mean field Fermi gas, acting as a heat bath, modifies the behavior of the statistical mechanical properties of the particles trapped in it. This is done by varying the two-particle interactions induced by the heat bath and the onsite two-particle interaction U. It is shown that in the presence of the heat bath-induced interaction, the partition function Z drops to a minimum finite value in the repulsive regime of U at the normalized temperature region kT ≲ t/2, where t is the tunneling parameter. In this temperature region, the convergence points of the respective isolines of the entropy S and the heat capacity Cᵥ move towards each other. Such shift in their features finetune S and Cᵥ relative to their behavior in the absence of two-particle interactions with the bath.},
doi = {10.1016/j.jmmm.2022.170094},
author = {Vladimir P. Villegas and Cristine D. Villagonzalo}
}
@inproceedings {583,
title = {Chemical potential-induced effects on caloric properties of a Fermi-Hubbard dimer},
booktitle = {Proceedings of the 40th Samahang Pisika ng Pilipinas Physics Conference},
year = {2022},
month = {19{\textendash}21 Oct 2022},
pages = {SPP-2022-1G-04},
address = {Legazpi City},
abstract = {Chemical potential is one of the tunable properties of an optical lattice system when its temperature is varied. In this study, the effects of the chemical potential on the entropy and the heat capacity of a Fermi-Hubbard dimer immersed in a mean-field Fermi gas bath are calculated. Repulsive two-particle interactions are found to require lesser chemical potentials to observe a rapid increase in the entropy at low temperatures. Such phenomena signify quantum level crossings. Furthermore, half-filling of the optical dimer is unnecessary for such phenomena to occur.},
url = {https://proceedings.spp-online.org/article/view/SPP-2022-1G-04},
author = {Vladimir P. Villegas and Cristine D. Villagonzalo}
}
@article {574,
title = {Refrigeration using magnetocaloric and electrocaloric effects in a Fermi-Hubbard optical dimer exposed to a heat bath},
journal = {Physica A},
volume = {600},
year = {2022},
pages = {127540},
abstract = {We consider a Fermi{\textendash}Hubbard dimer exposed to external electromagnetic fields and a Fermi gas bath under the assumptions of a repulsive two-particle interaction and the half-filling condition. Using the exact diagonalization method, it was found that the entropy and heat capacity rapidly rise within distinct localized regions of the magnetic and electric fields at low temperatures. The behavior of the magnetic (electric) Gr{\"u}neisen ratio denotes that the direction of the external field determines the magnetization or demagnetization (polarization or depolarization) due to the magnetocaloric (electrocaloric) effect among the lattice particles at a given temperature. At the same time, the isothermal entropic change as a function of the temperature indicates that refrigeration is optimized when the electric field along the line joining the two sites of the optical lattice is assisted by a transverse magnetic field.},
doi = {10.1016/j.physa.2022.127540},
author = {Vladimir P. Villegas and Cristine D. Villagonzalo}
}
@conference {593,
title = {Spin transport in a Fermi-Hubbard optical lattice interacting with a Fermi gas bath},
year = {2022},
month = {18{\textendash}24 Aug 2022},
pages = {P23-SF4-07},
publisher = {29th International Conference on Low Temperature Physics},
address = {Presented online},
url = {https://www.lt29.jp/},
author = {Vladimir P. Villegas and Cristine D. Villagonzalo}
}
@inproceedings {villegas2021,
title = {Canonical ensemble profile of a Fermi-Hubbard model treated as a double qubit chain},
booktitle = {Proceedings of the 39th Samahang Pisika ng Pilipinas Physics Conference},
year = {2021},
month = {20{\textendash}22 Oct 2021},
pages = {SPP-2021-2B-03},
address = {Presented online},
abstract = {Through the Jordan-Wigner transformation, a Fermi-Hubbard (FH) optical lattice can be mapped onto a double qubit chain system (QS). In this study, the partition function and the Helmholtz free energy were calculated for a FH dimer and a double QS double well. The differences of the values of the two properties as functions of temperature were taken and are shown as saturating to a constant as the temperature increases. This implies that electrons can be emulated as qubits.},
url = {https://proceedings.spp-online.org/article/view/SPP-2021-2B-03},
author = {Vladimir P. Villegas and Cristine Villagonzalo}
}
@inproceedings {549,
title = {Free energy in a spin transport among particles in a Fermi-Hubbard optical lattice},
booktitle = {Proceedings of the 38th Samahang Pisika ng Pilipinas Physics Conference},
year = {2020},
month = {19{\textendash}23 Oct 2020},
pages = {SPP-2020-5C-03},
address = {Presented online},
abstract = {The Fermi-Hubbard model investigates the dynamics of fermions confined in a one-dimensional (1D) optical lattice. In turn, the lattice is confined to a heat bath consisting of a fermionic quantum gas. Here, we use the path integral method to investigate the effects of the coupling at zero temperature and determine the five processes among bath particles which facilitate spin transport among optical lattice particles. Furthermore, we compute for the free energy per lattice site during such process.},
url = {https://paperview.spp-online.org/proceedings/article/view/SPP-2020-5C-03},
author = {Vladimir P. Villegas and Cristine Villagonzalo}
}