arXiv subjects

Vu Hung Dao

Publications and source records attributed to Vu Hung Dao.

8 recordsLinked to original sources

arXiv2024

Charge Collective Modes in Correlated Electron Systems: Plasmons Beyond the Random Phase Approximation

Elucidating the impact of strong electronic interactions on the collective excitations of metallic systems has been of longstanding interest, mainly due to the inadequacy of the random phase approximation (RPA) in the strongly correlated regime. Here, we adopt our newly developed radial Kotliar and Ruckenstein slave boson representation to analyze the charge excitation spectrum of a Hubbard model, extended with long range interactions. Working on the face centered cubic lattice, at half filling, and in different coupling regimes ranging from uncorrelated to the metal-to-insulator transition, we compare our results to conventional RPA as a benchmark. We focus on the influence of the local and long range couplings on the particle-hole excitation continuum and the plasmon and upper Hubbard band collective modes. Beyond the weak coupling regime, we find numerous quantitative and even qualitative discrepancies between our method and standard RPA. Our work thus deepens the understanding of charge collective modes in correlated systems, and lays the foundations for future studies of a broad series of materials.

cond-mat.str-el↗

arXiv2023

Spin and charge modulations of a half-filled extended Hubbard model

We introduce and analyze an extended Hubbard model, in which intersite Coulomb interaction as well as a staggered local potential (SLP) are considered, on the square lattice at half band filling, in the thermodynamic limit. Using both Hartree-Fock approximation and Kotliar and Ruckenstein slave boson formalism, we show that the model harbors charge order (CO) as well as joint spin and charge modulations (SCO) at finite values of the SLP, while the spin density wave (SDW) is stabilized for vanishing SLP, only. We determine their phase boundaries and the variations of the order parameters in dependence on the SLP, as well as on the on-site and nearest-neighbor interactions. Domains of coexistence of CO and SCO phases, suitable for resistive switching experiments, are unraveled. We show that the novel SCO systematically turns into the more conventional SDW phase when the zero-SLP limit is taken. We also discuss the nature of the different phase transitions, both at zero and finite temperature. In the former case, no continuous CO to SDW (or SCO) phase transition occurs. In contrast, a paramagnetic phase (PM), which is accompanied with continuous phase transitions towards both spin or charge ordered phases, sets in at finite temperature. A good quantitative agreement with numerical simulations is demonstrated, and a comparison between the two used approaches is performed.

cond-mat.str-el↗

arXiv2022

Effect of the next-nearest-neighbor hopping on the charge collective modes in the paramagnetic phase of the Hubbard model

The charge dynamical response function of the $t-t'-U$ Hubbard model is investigated on the square lattice in the thermodynamical limit. The correlation function is calculated from Gaussian fluctuations around the paramagnetic saddle-point within the Kotliar and Ruckenstein slave-boson representation. The next-nearest-neighbor hopping only slightly affects the renormalization of the quasiparticle mass. In contrast a negative $t'/t$ notably decreases (increases) their velocity, and hence the zero-sound velocity, at positive (negative) doping. For low (high) density $n \lesssim 0.5$ ($n \gtrsim 1.5$) we find that it enhances (reduces) the damping of the zero-sound mode. Furthermore it softens (hardens) the upper-Hubbard-band collective mode at positive (negative) doping. It is also shown that our results differ markedly from the random phase approximation in the strong-coupling limit, even at high doping, while they compare favorably with existing quantum Monte Carlo numerical simulations.

cond-mat.str-el↗

arXiv2019

Combining complex and radial slave boson fields within the Kotliar-Ruckenstein representation of correlated impurities

The gauge symmetry group of any slave boson representation allows to gauge away the phase of bosonic fields. One benefit of this radial field formulation is the elimination of spurious Bose condensations when saddle-point approximation is performed. Within the Kotliar-Ruckenstein representation, three of the four bosonic fields can be radial while the last one has to remain complex. In this work, we present the procedure to carry out the functional integration involving constrained fermionic fields, complex bosonic fields, and radial bosonic fields. The correctness of the representation is verified by exactly evaluating the partition function and the Green's function of the Hubbard model in the atomic limit.

cond-mat.str-el↗

arXiv2017

Particle-hole symmetry of charge excitation spectra in the paramagnetic phase of the Hubbard model

The Kotliar and Ruckenstein slave-boson representation of the Hubbard model allows to obtain an approximation of the charge dynamical response function resulting from the Gaussian fluctuations around the paramagnetic saddle-point in analytical form. Numerical evaluation in the thermodynamical limit yields charge excitation spectra consisting of a continuum, a gapless collective mode with anisotropic zero-sound velocity, and a correlation induced high-frequency mode at $ω\approx U$. In this work we show that this analytical expression obeys the particle-hole symmetry of the model on any bipartite lattice with one atom in the unit cell. Other formal aspects of the approach are also addressed.

cond-mat.str-el↗

arXiv2017

Collective modes in the paramagnetic phase of the Hubbard model

The charge dynamical response function of the Hubbard model is investigated on the square lattice in the thermodynamical limit. The obtained charge excitation spectra consist of a continuum, a gapless collective mode with anisotropic zero-sound velocity, and a correlation induced high-frequency mode at $ω\approx U$. The correlation function is calculated from Gaussian fluctuations around the paramagnetic saddle-point within the Kotliar and Ruckenstein slave-boson representation. Its dependence on the on-site Coulomb repulsion $U$ and density is studied in detail. An approximate analytical expression of the high frequency mode, that holds for any lattice with one atom in the unit cell, is derived. Comparison with numerical simulations, perturbation theory and the polarization potential theory is carried out. We also show that magnetic instabilities tend to vanish for $T\gtrsim t/6$, and finite temperature phase diagrams are established.

cond-mat.str-el↗

arXiv2011

Size of stripe domains in a superconducting ferromagnet

In a superconducting ferromagnet, the superconducting state appears in the ferromagnetic phase where usually a domain structure has already developed. We study the influence of the superconducting screening currents on a stripe structure with out-of-plane magnetization, in a film of arbitrary thickness. We find that superconductivity always induces a shrinkage of the domains, and there is a critical value of penetration depth below which a mono-domain structure is more stable than the periodic one. Furthermore we investigate the possible different effects of singlet and triplet superconductivity on the domain width, as well as the conditions for the existence of vortices in the domains. The obtained results are then discussed in light of the experimental data of superconducting ferromagnets URhGe, UGe2, and UCoGe.

cond-mat.supr-con↗

arXiv2010

Scanning SQUID microscopy of vortex clusters in multiband superconductors

In type-1.5 superconductors, vortices emerge in clusters, which grow in size with increasing magnetic field. These novel vortex clusters and their field dependence are directly visualized by scanning SQUID microscopy at very low vortex densities in MgB2 single crystals. Our observations are elucidated by simulations based on a two-gap Ginzburg-Landau theory in the type-1.5 regime.

cond-mat.supr-con↗