Hantao Lu

Enhanced charge order in a photoexcited one-dimensional strongly correlated system.

We present a compelling response of a low-dimensional strongly correlated system to an external perturbation. Using the time-dependent Lanczos method we investigate a nonequilibrium evolution of the half-filled one-dimensional extended Hubbard model, driven by a transient laser pulse. When the system is close to the phase boundary, by tuning the laser frequency and strength, a sustainable charge order enhancement is found that is absent in the Mott insulating phase. We analyze the conditions and investigate possible mechanisms of emerging charge order enhancement. Feasible experimental realizations are proposed.

Optical properties of monoclinic HfO2 studied by first-principles local density approximation + U approach

The band structures and optical properties of monoclinic HfO2 are investigated by the local density approximation (LDA)+U approach. With the on-site Coulomb interaction being introduced to 5d orbitals of Hf atom and 2p orbitals of O atom, the experimental band gap is reproduced. The imaginary part of the complex dielectric function shows a small shoulder at the edge of the band gap, coinciding with the experiments. This intrinsic property of crystallized monoclinic HfO2, which is absent in both the tetragonal phase and cubic phase, can be understood as a consequence of the reconstruction of the electronic states near the band edge following the adjustment of the crystal structure. The existence of a similar shoulder-like-structure in the monoclinic phase of ZrO2 is predicted.

Photoinduced in-gap excitations in the one-dimensional extended Hubbard model

We investigate the time evolution of optical conductivity in the half-filled one-dimensional extended Hubbard model driven by a transient laser pulse, by using the time-dependent Lanczos method. Photoinduced in-gap excitations exhibit a qualitatively different structure in the spin-density wave (SDW) in comparison to the charge-density-wave (CDW) phase. In the SDW, the origin of a low-energy in-gap excitation is attributed to the even-odd parity of the photoexcited states, while in the CDW an in-gap state is due to confined photogenerated carriers. The signature of the in-gap excitations can be identified as a characteristic oscillation in the time evolution of physical quantities.

Numerical method to compute optical conductivity based on pump-probe simulations

A numerical method to calculate optical conductivity based on a pump-probe setup is presented. Its validity and limits are tested and demonstrated via concrete numerical simulations on the half-filled one-dimensional extended Hubbard model both in and out of equilibrium. By employing either a steplike or a Gaussian-like probing vector potential, it is found that in nonequilibrium, the method in the narrow-probe-pulse limit can be identified with variant types of linear-response theory, which, in equilibrium, produce identical results. The observation reveals the underlying probe-pulse dependence of the optical conductivity calculations in nonequilibrium, which may have applications in the theoretical analysis of ultrafast spectroscopy measurements.

Electronic structures and optical properties of HfO2–TiO2 alloys studied by first-principles GGA + U approach

The phase diagram of HfO2-TiO2 system shows that when Ti content is less than 33.0 mol%, HfO2-TiO2 system is monoclinic; when Ti content increases from 33.0 mol% to 52.0 mol%, it is orthorhombic; when Ti content reaches more than 52.0 mol%, it presents rutile phase. So, we choose the three phases of HfO2-TiO2 alloys with different Ti content values. The electronic structures and optical properties of monoclinic, orthorhombic and rutile phases of HfO2-TiO2 alloys are obtained by the first-principles generalized gradient approximation (GGA) + U approach, and the effects of Ti content and crystal structure on the electronic structures and optical properties of HfO2-TiO2 alloys are investigated. By introducing the Coulomb interactions of 5d orbitals on Hf atom (U-1(d)), those of 3d orbitals on Ti atom (U-2(d)), and those of 2p orbitals on O atom (U-p) simultaneously, we can improve the calculation values of the band gaps, where U-1(d), U-2(d), and U-p values are 8.0 eV, 7.0 eV, and 6.0 eV for both the monoclinic phase and orthorhombic phase, and 8.0 eV, 7.0 eV, and 3.5 eV for the rutile phase. The electronic structures and optical properties of the HfO2-TiO2 alloys calculated by GGA + U-1(d) (U-1(d) = 8.0 eV) + U-2(d) (U-2(d) = 7.0 eV) + U-p (U-p = 6.0 eV or 3.5 eV) are compared with available experimental results.

Optical properties of anatase and rutile TiO

The optical properties of thermally annealed TiO2 samples depend on their preparation process, and the TiO2 thin films usually exist in the form of anatase or rutile or the mixture of the two phases. The electronic structures and optical properties of anatase and rutile TiO2 are calculated by means of First-principles generalized gradient approximation (GGA) +U approach. By Introducing the Coulomb interactions on 3d orbitals of Ti atom (Ud) and 2p orbitals of O atom (Up), we can reproduce the experimental values of the band gap. The optical properties of anatase and rutile TiO2 are obtained by means of GGA+U method, well agreeing with experimental results and other theoretical data. Further we present the comparison of the electronic structure, birefringence and anisotropy between the two phases of TiO2.

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Abstract ZrW2O8 exhibits isotropic negative thermal expansions over its entire temperature range of stability, yet so far its physical properties and mechanism have not been fully addressed. In this article, the electronic structure, elastic, thermal, optical and phonon properties of α-ZrW2O8 are systematically investigated from first principles. The agreements between the generalized gradient approximation (GGA) calculation and experiments are found to be quite satisfactory. The calculation results can be useful in relevant material designs, e.g., when ZrW2O8 is employed to adjust the thermal expansion coefficient of ceramic matrix composites.

studied by GGA + U*

By employing the time-dependent Lanczos method, the nonequilibrium process of the half-filled one-dimensional extended Hubbard model under the irradiation of a transient laser pulse is investigated. We show that in the spin-density-wave (SDW) phase, the antiferromagnetic spin correlations are impaired by the photoinduced charge carriers. Near the phase boundary between the SDW and charge-density-wave (CDW) phases, a local enhancement of charge (spin) order that is absent in the original SDW (CDW) phase can be realized with proper laser frequency and strength. The possibility of restoration of spin orders from the CDW phase by optical means is discussed.