arXiv subjects

Chengliang Xia

Publications and source records attributed to Chengliang Xia.

9 recordsLinked to original sources

arXiv2026

Hubbard-$U$-corrected electron-phonon interactions in strongly correlated materials via the finite-displacement method

Although the density functional theory plus Hubbard $U$ correction method (DFT+$U$) is broadly used to study electronic structure of strongly correlated materials, the extension of this method to electron-phonon $g$ matrices has received limited attention. Here, we implement an algorithm that integrates DFT+$U$ method with the finite-displacement method for the calculations of phonons and electron-phonon $g$ matrices. The Hubbard $U$ corrections are applied not only to electronic and phonon structures, but, more importantly, also to electron-phonon $g$ matrices. We demonstrate our algorithm in two prototypical correlated materials: infinite-layer nickelates LaNiO$_2$ and ruthenium dioxide RuO$_2$. We find that: i) While the Hubbard $U$ corrections weakly increase the electron-phonon interaction of 20% hole-doped LaNiO$_2$, its total electron-phonon coupling remains small and is insufficient to account for the observed superconducting transition temperature of about 10-30 K. Our results contrast with the recent work showing that the full GW corrections yield an elevated electron-phonon coupling of 20% hole-doped LaNiO$_2$ five times larger than its DFT value. We attribute this discrepancy to the differences in the Fermi surface topology between DFT+$U$ and GW methods. ii) The inclusion of Hubbard $U$ corrections eliminates the imaginary phonon modes of RuO$_2$ under strain on the TiO$_2$ substrate and substantially reduces the electron-phonon coupling. Our results alleviate the discrepancy between the reported large theoretical electron-phonon coupling and the low superconducting transition temperature observed experimentally. Our work provides an algorithm that fully includes the Hubbard $U$ corrections on electron-phonon properties of correlated materials, and highlights the importance of Fermi surface shape and correlation effects on phonon spectrum and electron-phonon $g$ matrices.

cond-mat.str-el↗

arXiv2026

Nearly perfect Fermi surface nesting in hole-doped La$_3$Ni$_2$O$_7$ enables bulk superconductivity without pressure or strain

The discovery of high-temperature superconductivity in Ruddlesden-Popper nickelates has drawn great attention. However, unlike cuprates and iron-based superconductors, Ruddlesden-Popper nickelates exhibit superconductivity either under high pressure in bulk samples or under compressive strain in thin films. Genuine bulk superconductivity under ambient pressure has remained elusive in these materials, precluding key measurements such as specific heat and superfluid density. In this work, we combine density-functional-theory, dynamical-mean-field-theory, and random-phase-approximation to solve the superconducting gap equation for bulk hole-doped bilayer nickelate La$_{3-x}$Sr$_x$Ni$_2$O$_7$ at ambient pressure. We find that hole doping induces a Ni-$d_{3z^2-r^2}$-derived $\gamma$ pocket on the Fermi surface, and serves as a tuning parameter for both its size and \textit{shape}. As $x$ approaches 0.4, the $\gamma$ pocket evolves from circular to diamond-shaped and expands to span half of the Brillouin zone, resulting in nearly perfect Fermi surface nesting with the optimal nesting vector $\textbf{Q} = (\pi, \pi)$. This, in turn, strongly enhances antiferromagnetic spin fluctuations and substantially increases the leading superconducting eigenvalue to a level at which superconductivity becomes experimentally observable. Our work provides both a robust mechanism and an experimentally feasible route to inducing the long-sought bulk superconductivity in La$_3$Ni$_2$O$_7$ without pressure or strain.

cond-mat.supr-con↗

arXiv2025

Origin of local magnetic exchange interaction in infiite-layer nickelates

Significant magnetic exchange interactions have been observed in infinite-layer nickelates RNiO2 (R = La, Pr, Nd), which exhibit unconventional superconductivity upon hole doping. Despite their structural and Fermi surface similarities to cuprates, infinite-layer nickelates possess a larger charge transfer gap, which influences their magnetic exchange interactions via oxygen. In this work, we performed 17O nuclear magnetic resonance (NMR) measurements on LaNiO2 and Sr-doped LaNiO2, revealing glassy spin dynamics originating from Ni-O planes. This indicates that infinite-layer nickelates are in proximity to magnetic ordering and that magnetic correlations play a crucial role in their physics. More importantly, our analysis of the Knight shift and hyperfine coupling of 17O nuclei revealed that the Ni-Ni superexchange interaction, mediated by the {\sigma} bond between the Ni-dx2-y2 and O-p orbitals, is one order of magnitude weaker than that in cuprates. This alone cannot account for the total magnetic exchange interaction observed in nickelates. First-principles many-body calculations indicate that an interstitial s orbital near the Fermi level, coupled with the Ni-d3z2-r2 orbital, significantly enhances the superexchange interaction. This contrasts with cuprates, where magnetic interactions are predominantly governed by Cu-dx2-y2 superexchange via oxygen. Our findings provide new insights into the distinct magnetic interactions in infinite-layer nickelates and their potential role in unconventional superconductivity.

cond-mat.supr-con↗

arXiv2025

Three-Dimensional Fermi Surface, Van Hove Singularity and Enhancement of Superconductivity in Infinite-Layer Nickelates

Recent experiments reveal a three-dimensional (3D) Fermi surface with a clear $k_z$ dispersion in infinite-layer nickelates, distinguishing them from their cuprate superconductor counterparts. However, the impact of this difference on the superconducting properties of nickelates remains unclear. Here, we employ a combined random-phase-approximation and dynamical-mean-field-theory (RPA+DMFT) approach to solve the linearized gap equation for superconductivity. We find that, compared to the cuprate-like two-dimensional (2D) single-orbital Fermi surface, the van Hove singularities on the 3D Fermi surface of infinite-layer nickelates strengthen spin fluctuations by driving the system closer to antiferromagnetic instabilities, thereby significantly enhancing superconductivity. Our findings underscore the critical role of the van Hove singularities in shaping the superconducting properties of infinite-layer nickelates and, more broadly, highlight the importance of subtle Fermi surface features in modeling material-specific unconventional superconductors.

cond-mat.supr-con↗

arXiv2024

Electronic Structure of Superconducting Infinite-Layer Lanthanum Nickelates

Revealing the momentum-resolved electronic structure of infinite-layer nickelates is essential for understanding this new class of unconventional superconductors, but has been hindered by the formidable challenges in improving the sample quality. In this work, we report for the first time the angle-resolved photoemission spectroscopy of superconducting La$_{0.8}$Sr$_{0.2}$NiO$_{2}$ films prepared by molecular beam epitaxy and ${\mathrm{\textit{in situ}}}$ atomic-hydrogen reduction. The measured Fermi topology closely matches theoretical calculations, showing a large Ni-$d_{x^2-y^2}$ derived Fermi sheet that evolves from hole-like to electron-like along $k_{z}$, and a three-dimensional (3D) electron pocket centered at Brillouin zone corner. The Ni-$d_{x^2-y^2}$ derived bands show a mass enhancement ($m^*/m_{\rm{DFT}}$) of 2-3,while the 3D electron band shows negligible band renormalization. Moreover, the Ni-$d_{x^2-y^2}$ derived states also display a band dispersion anomaly at higher binding energy, reminiscent of the waterfall feature and kinks observed in cuprates.

cond-mat.supr-con↗

arXiv2023

Sensitive dependence of pairing symmetry on Ni-$e_g$ crystal field splitting in the nickelate superconductor La$_3$Ni$_2$O$_7$

The discovery of high-temperature superconductivity in La$_3$Ni$_2$O$_7$ under pressure has drawn great attention. However, consensus has not been reached on its pairing symmetry in theory. By combining density-functional-theory (DFT), maximally-localized-Wannier-function, and linearized gap equation with random-phase-approximation, we find that the pairing symmetry of La$_3$Ni$_2$O$_7$ is $d_{xy}$, if its DFT band structure is accurately reproduced by a downfolded bilayer two-orbital model. More importantly, we reveal that the pairing symmetry of La$_3$Ni$_2$O$_7$ sensitively depends on the crystal field splitting between two Ni-$e_g$ orbitals. A slight increase in Ni-$e_g$ crystal field splitting alters the pairing symmetry from $d_{xy}$ to $s_{\pm}$. Such a transition is associated with the change in inverse Fermi velocity and susceptibility, while the shape of Fermi surface remains almost unchanged. Our work highlights the sensitive dependence of pairing symmetry on low-energy electronic structure in multi-orbital superconductors, which calls for care in the downfolding procedure when one calculates their pairing symmetry.

cond-mat.supr-con↗

arXiv2022

Dynamical structural instability and its implication on the physical properties of infinite-layer nickelates

We use first-principles calculations to find that in infinite-layer nickelates $R$NiO$_2$, the widely studied tetragonal $P4/mmm$ structure is only dynamically stable for early lanthanide elements $R$ = La-Sm. For late lanthanide elements $R$ = Eu-Lu, an imaginary phonon frequency appears at $A=(π,π,π)$ point. For those infinite-layer nickelates, condensation of this phonon mode into the $P4/mmm$ structure leads to a more energetically favorable $I4/mcm$ structure that is characterized by an out-of-phase rotation of "NiO$_4$ square". Special attention is given to two borderline cases: PmNiO$_2$ and SmNiO$_2$, in which both the $P4/mmm$ structure and the $I4/mcm$ structure are local minima and the energy difference between the two structures can be fine-tuned by epitaxial strain. Compared to the $P4/mmm$ structure, $R$NiO$_2$ in the $I4/mcm$ structure has a substantially reduced Ni $d_{x^2-y^2}$ bandwidth, a smaller Ni $d$ occupancy, a "cleaner" Fermi surface with a lanthanide-$d$-derived electron pocket suppressed at $Γ$ point, and a decreased critical $U_{\textrm{Ni}}$ to stabilize long-range antiferromagnetic ordering. All these features imply enhanced correlation effects and favor Mott physics. Our work reveals the importance of structure-property relation in infinite-layer nickelates, in particular, the spontaneous "NiO$_4$ square" rotation provides a tuning knob to render $R$NiO$_2$ in the $I4/mcm$ structure a closer analogy to superconducting infinite-layer cuprates.

cond-mat.mtrl-sci↗

arXiv2021

Pressure-induced metal-insulator transition in oxygen-deficient LiNbO$_3$-type ferroelectrics

Hydrostatic pressure and oxygen vacancies usually have deleterious effects on ferroelectric materials because both tend to reduce their polarization. In this work we use first-principles calculations to study an important class of ferroelectric materials - LiNbO$_3$-type ferroelectrics (LiNbO$_3$ as the prototype), and find that in oxygen-deficient LiNbO$_{3-δ}$, hydrostatic pressure induces an unexpected metal-insulator transition between 8 and 9 GPa. Our calculations also find that strong polar displacements persist in both metallic and insulating oxygen-deficient LiNbO$_{3-δ}$ and the size of polar displacements is comparable to pristine LiNbO$_3$ under the same pressure. These properties are distinct from widely used perovskite ferroelectric oxide BaTiO$_3$, whose polarization is quickly suppressed by hydrostatic pressure and/or oxygen vacancies. The anomalous pressure-driven metal-insulator transition in oxygen-deficient LiNbO$_{3-δ}$ arises from the change of an oxygen vacancy defect state. Hydrostatic pressure increases the polar displacements of oxygen-deficient LiNbO$_{3-δ}$, which reduces the band width of the defect state and eventually turns it into an in-gap state. In the insulating phase, the in-gap state is further pushed away from the conduction band edge under hydrostatic pressure, which increases the fundamental gap. Our work shows that for LiNbO$_3$-type strong ferroelectrics, oxygen vacancies and hydrostatic pressure combined can lead to new phenomena and potential functions, in contrast to the harmful effects occurring to perovskite ferroelectric oxides such as BaTiO$_3$.

cond-mat.mtrl-sci↗

arXiv2019

Coexistence of polar displacements and conduction in doped ferroelectrics: an ab initio comparative study

Polar metals are rare because free carriers in metals screen electrostatic potential and eliminate internal dipoles. Degenerate doped ferroelectrics may create an approximate polar metallic phase. We use first-principles calculations to investigate $n$-doped LiNbO$_3$-type oxides (LiNbO$_3$ as the prototype) and compare to widely studied perovskite oxides (BaTiO$_3$ as the prototype). In the rigid-band approximation, substantial polar displacements in $n$-doped LiNbO$_3$ persist even at 0.3 $e$/f.u. ($\simeq$ 10$^{21}$ cm$^{-3}$), while polar displacements in $n$-doped BaTiO$_3$ quickly get suppressed and completely vanish at 0.1 $e$/f.u. Furthermore, in $n$-doped LiNbO$_3$, Li-O displacements decay more slowly than Nb-O displacements, while in $n$-doped BaTiO$_3$, Ba-O and Ti-O displacements decay approximately at the same rate. Supercell calculations that use oxygen vacancies as electron donors support the main results from the rigid-band approximation and provide more detailed charge distributions. Substantial cation displacements are observed throughout LiNbO$_{3-δ}$($δ= 4.2\%$), while cation displacements in BaTiO$_{3-δ}$($δ= 4.2\%$) are almost completely suppressed. We find that conduction electrons in LiNbO$_{3-δ}$ are not as uniformly distributed as in BaTiO$_{3-δ}$, implying that the rigid-band approximation should be used with caution in simulating electron doped LiNbO$_3$-type oxides. Our work shows that polar distortions and conduction can coexist in a wide range of electron concentration in $n$-doped LiNbO$_3$, which is a practical approach to generating an approximate polar metallic phase. Combining doped ferroelectrics and doped semiconductors may create new functions for devices.

physics.comp-ph↗