arXiv subjects

Zhihui Luo

Publications and source records attributed to Zhihui Luo.

12 recordsLinked to original sources

arXiv2025

Evolution of Correlated Electrons in ${\rm La_3Ni_2O_7}$ at Ambient Pressure: a Study of Double-Counting Effect

We employ cluster extension of dynamical mean-field theory (CDMFT) to systematically investigate the impact of double counting corrections on the correlated electronic structure of ${\rm La_3Ni_2O_7}$ under ambient pressure. By adjusting double-counting parameters, while maintaining a fixed Fermi surface, we observe a pronounced orbital-selective density of states change: the $d_{z^2}$ orbital undergoes significant variation near the Fermi level with increasing $E_{dc}^z$, while the $d_{x^2-y^2}$ orbital remains essentially unchanged throughout the entire range. Analysis of renormalization factor show the monotonic dependence with double counting in both $d_{z^2}$ and $d_{x^2-y^2}$ orbital, and it also identifies an optimal double counting window in $d_{z^2}$ orbital aligns with experimental values. We also find the interlayer Matsubara self energy exhibits non-monotonic dependence on $E_{dc}^z$, deviating from theoretical predictions. This anomaly is attributed to the metallization of oxygen-bridged pathways, which disrupts the prerequisite for charge transfer via apical oxygen. Our results establish $E_{dc}$ as a critical control parameter for correlated electronic structure in ${\rm La_3Ni_2O_7}$ and provide a computational framework for resolving orbital-dependent correlation effects in layered materials.

cond-mat.supr-con↗

arXiv2025

Electronic structures and superconductivity in Nd-doped La$_3$Ni$_2$O$_7$

The recent discovery of high-$T_c$ superconductivity in Ruddlesden-Popper (RP) nickelates has motivated extensive efforts to explore higher $T_c$ superconductors. Here, we systematically investigate Nd-doped La$_3$Ni$_2$O$_7$ using density functional theory (DFT) and renormalized mean-field theory (RMFT). DFT calculations reveal that both the lattice constants and interlayer spacing decrease upon Nd substitution, similar to the effect of physical pressure. However, the in-plane Ni-O-Ni bond angle evolves non-monotonically with doping, increasing to a maximum at 70% ($\sim$ 2/3) Nd doping level and then falling sharply at 80%, which leads to a reduction in orbital overlap. Moreover, Nd doping has a more pronounced effect on the Ni-$d{_{z^2}}$ orbital, demonstrating an orbital-dependent effect of rare-earth substitution. Through the bilayer two-orbital $t-J$ model, RMFT analysis further shows an $s\pm$-wave pairing symmetry, with $T_c$ rising to a maximum at about 70% Nd substitution before declining, in agreement with the transport measurements. The variation in $T_c$ can be traced to the competition between continuously enhanced interlayer superexchange coupling $J_\perp^z$ and a gradual decrease in particle density. These results highlight the delicate interplay among structural tuning, orbital hybridization, and superconductivity, providing important clues to design higher-$T_c$ RP nickelate superconductors.

cond-mat.supr-con↗

arXiv2025

Pairing symmetry and superconductivity in La$_3$Ni$_2$O$_7$ thin films

The recent discovery of superconductivity with a transition temperature $T_c$ over 40 K in La$_3$Ni$_2$O$_7$ and (La,Pr)$_{3}$Ni$_2$O$_7$ thin films at ambient pressure marks an important step in the field of nickelate superconductors. Here, we perform a renormalized mean-field theory study of the superconductivity in $\mathrm{La_3Ni_2O_7}$ thin films, using a bilayer two-orbital $t-J$ model. Our result reveals an $s_\pm$-wave pairing symmetry driven by the strong interlayer superexchange coupling of $d_{z^2}$ orbital, resembling the pressurized bulk case. Also, we roughly reproduce the experimentally observed nodeless shape of the superconducting gap at the $\beta$ pocket and the superconducting $T_c$. In addition, by analysing the orbital-resolved pairing configurations and their projections onto Fermi surface, we find that the nodeless feature of $\beta$ pocket is related to the interlayer pairing within both $d_{z^2}$ and $d_{x^2-y^2}$ orbitals. Moreover, we identify a formation of the inplane inter-orbital $d$-wave pairing between $d_{z^2}$ and $d_{x^2-y^2}$ orbitals, which can even enhance the dominated interlayer $s_\pm$-wave. Our study particularly highlights the diverse relations of different pairing channels in $\mathrm{La_3Ni_2O_7}$ that holds a complex Fermi surface.

cond-mat.supr-con↗

arXiv2025

Electronic structures and multi-orbital models of La$_3$Ni$_2$O$_7$ thin films at ambient pressure

The recent discovery of superconductivity with a transition temperature $T_c$ exceeding 40 K in La$_3$Ni$_2$O$_7$ and (La,Pr)$_{3}$Ni$_2$O$_7$ thin films at ambient pressure marks a significant breakthrough in the field of nickelate superconductors. Using density functional theory (DFT), we propose a double-stacked two-orbital effective model for La$_3$Ni$_2$O$_7$ thin film based on the Ni$-e_g$ orbitals. Our analysis of the Fermi surface reveals three electron pockets ($\alpha,\alpha^{\prime},\beta$) and two hole pockets ($\gamma,\gamma^{\prime}$), where the additional $\alpha^{\prime}$ and $\gamma^{\prime}$ pockets arise from inter-stack interactions. Furthermore, we introduce a high-energy model that incorporates O$-p$ orbitals to facilitate future studies. Calculations of spin susceptibility within the random phase approximation (RPA) indicate that magnetic correlations are enhanced by nesting of the $\gamma$ pocket, which is predominantly derived from the Ni$-d_{z^2}$ orbital. Our results provide a theoretical foundation for understanding the electronic and magnetic properties of La$_3$Ni$_2$O$_7$ thin films.

cond-mat.supr-con↗

arXiv2024

Modulation of the Octahedral Structure and Potential Superconductivity of La$_3$Ni$_2$O$_7$ through Strain Engineering

The recent transport measurement of La$_3$Ni$_2$O$_7$ uncover a "right-triangle" shape of the superconducting dome in the pressure-temperature (P-T) phase diagram. Motivated by this, we perform theoretical first-principles studies of La$_3$Ni$_2$O$_7$ with the pressure ranging from 0 to 100 GPa. Notably, we reveal a pressure dependence of the Ni-$d_{z^2}$ electron density at the Fermi energy ($n_z^{EF}$) that highly coincides with such shape. On this basis, we further explore the electronic structure under uniaxial stress. By tracking the stress response of $n_z^{EF}$, we propose that superconductivity can be achieved by applying only about 2 GPa of compression along the c axis. The idea is further exemplified from the perspectives of lattice distortion, band structure, Fermi surface and superconducting phase coherence. We also discuss the possible charge modulation under the stress and provide an insight to the relation between n_z^EF and the superconducting Tc in La$_3$Ni$_2$O$_7$ system. Our study provides a helpful guide to the future experiment.

cond-mat.supr-con↗

arXiv2024

Trilayer multi-orbital models of $\mathrm{La_{4}Ni_{3}O_{10}}$

Recently, the discovery of superconductivity in Ruddlesden-Popper (RP) $\mathrm{La_4Ni_3O_{10}}$ under pressure has further expanded the realm of nickelate-based superconductor family. In this paper, we performed a first-principle study of $\mathrm{La_4Ni_3O_{10}}$ for both $P2_1/a$ phase at ambient pressure and $I4/mmm$ phase at high pressure, with $U$=0, 3.5\ eV. Our results confirmed the characteristic upward shift of Ni-$d_{z^2}$ bonding band under pressure. Moreover, our analysis of electronic spectrum and orbital occupancy unveil the dynamic mechanism of electronic reconstructions under pressure, embedded in a critical dual effect. Based on our results, we further proposed a trilayer two-orbital model by performing Wannier downfolding on Ni-$e_g$ orbitals. Our model reveals four Fermi surface sheets with $\alpha,\beta,\beta^\prime,\gamma$ pockets, bearing resemblance to that of bilayer $\mathrm{La_3Ni_2O_7}$. According to the model, our calculated spin susceptibility under random phase approximation shows that $d_{x^2-y^2}$ orbital is also important for the magnetic fluctuation in RP series. Finally, a high energy sixteen-orbital model with direct $dp,pp$ hoppings is proposed, which implies that $\mathrm{La_4Ni_3O_{10}}$ also lies in charge-transfer picture within Zaanen-Sawatzky-Allen scheme. Our exposition of electronic reconstructions and multi-orbital models shed light on theoretical electronic correlation study and experimental exploration of lower pressure superconductor in RP series.

cond-mat.supr-con↗

arXiv2023

Superconductivity in the Two-Orbital Hubbard Model of Infinite-Layer Nickelates

The pairing symmetry in infinite-layer nickelate superconductors has been an intriguing problem under heated debates. In this work, we study a two-orbital Hubbard model with one strongly correlated $3d$ orbital and one more itinerant $5d$ orbital, by using an eight-site cellular dynamic mean field theory study. We establish a superconducting phase diagram with $d_{x^{2}-y^{2}}$, $s_{\pm}$ and $d+is$ wave pairing symmetries, based on which we clarify the roles of various relevant parameters including hybridization $V$, itinerant carrier density $\langle n_{c}\rangle$ and interaction $U_{c}$. We show that the inclusion of a less correlated $5d$ band in general suppresses the $d_{x^{2}-y^{2}}$ wave pairing. We demonstrate that the $d+is$ wave is maximized when the $5d$ orbital has a large Coulomb repulsion with intermediate hybridization parameter. We perform fluctuation diagnostics to show that the driving force behind the $d_{x^{2}-y^{2}}$ wave is the intraband antiferromagnetic fluctuations in the $3d$ orbital, while for the $s_{\pm}$ wave, the pairing is mainly from the antiferromagnetic correlations residing on the local $3d$-$5d$ bond in real space.

cond-mat.supr-con↗

arXiv2023

Charge Transfer and Zhang-Rice Singlet Bands in the Nickelate Superconductor $\mathrm{La_3Ni_2O_7}$ under Pressure

Recently, a bulk nickelate superconductor $\mathrm{La_3Ni_2O_7}$ is discovered at pressures with a remarkable high transition temperature $T_c \sim 80K$. Here, we study a Hubbard model with tight-binding parameters derived from \textit{ab initio} calculations of $\mathrm{La_3Ni_2O_7}$, by employing large scale determinant quantum Monte Carlo and cellular dynamical mean-field theory. Our result suggests that the superexchange couplings in this system are comparable to that of cuprates. The system is a charge transfer insulator as hole concentration becomes four per site at large Hubbard $U$. Upon hole doping, two low-energy spin-singlet bands emerge in the system exhibiting distinct correlation properties: while the one composed of the out-of-plane Ni-$d_{3z^2-r^2}$ and O-$p_z$ orbitals demonstrates strong antiferromagnetic correlations and narrow effective bandwidth, the in-plane singlet band consisting of the Ni-$d_{x^2-y^2}$ and O-$p_x / p_y$ orbitals is in general more itinerant. Over a broad range of hole doping, the doped holes occupy primarily the $d_{x^2-y^2}$ and $p_x / p_y$ orbitals, whereas the $d_{3z^2-r^2}$ and $p_z$ orbitals retain underdoped. We propose an effective $ t-J$ model to capture the relevant physics and discuss the implications of our result for comprehending the $\mathrm{La_3Ni_2O_7}$ superconductivity.

cond-mat.str-el↗

arXiv2023

High-T$_C$ superconductivity in $\mathrm{La_3Ni_2O_7}$ based on the bilayer two-orbital t-J model

The recently discovered high-T$_C$ superconductor La$_3$Ni$_2$O$_7$ has sparked renewed interest in the unconventional superconductivity. Here we study superconductivity in pressurized La$_3$Ni$_2$O$_7$ based on a bilayer two-orbital $t-J$ model, using the renormalized mean-field theory. Our results reveal a robust $s^\pm-$wave pairing driven by the inter-layer $d_{z^2}$ magnetic coupling, which exhibits a transition temperature within the same order of magnitude as the experimentally observed $T_c \sim 80$ K. We establish a comprehensive superconducting phase diagram in the doping plane. Notably, the La$_3$Ni$_2$O$_7$ under pressure is found situated roughly in the optimal doping regime of the phase diagram. When the $d_{x^2-y^2}$ orbital becomes close to half-filling, $d-$wave and $d+is$ pairing can emerge from the system. We discuss the interplay between Fermi surface topology and different pairing symmetries. The stability of the $s^\pm-$wave pairing against Hund's coupling and other magnetic exchange couplings is discussed.

cond-mat.supr-con↗

arXiv2023

Bilayer two-orbital model of La$_3$Ni$_2$O$_7$ under pressure

The newly discovered Ruddlesden-Popper bilayer La$_3$Ni$_2$O$_7$ reaches an remarkable superconducting transition temperature $T_c$ = 80 K under a pressure of above 14 GPa. Here we propose a minimal bilayer two-orbital model of the high-pressure phase of La$_3$Ni$_2$O$_7$. Our model is constructed with the Ni-3d$_{x^2-y^2}$, 3d$_{3z^2-r^2}$ orbitals by using Wannier downfolding of the density functional theory calculations, which captures the key ingredients of the material, such as band structure and Fermi surface topology. There are two electron pockets $α$, $β$ and one hole pocket $γ$ on the Fermi surface, in which the $α$, $β$ pockets show mixing of two orbitals, while the $γ$ pocket is associated with Ni-d$_{3z^2-r^2}$ orbital. The RPA spin susceptibility reveals a magnetic enhancement associating to the d$_{3z^2-r^2}$ state. A higher energy model with O-p orbitals is also provided for further study.

cond-mat.supr-con↗

arXiv2022

Single crystal growth and superconductivity in RbNi$_2$Se$_2$

We report the synthesis and characterization of RbNi$_2$Se$_2$, an analog of the iron chalcogenide superconductor Rb$_x$Fe$_2$Se$_2$, via transport, angle resolved photoemission spectroscopy, and density functional theory calculations. A superconducting transition at $T_{c}$ = 1.20 K is identified. In normal state, RbNi$_2$Se$_2$ shows paramagnetic and Fermi liquid behaviors. A large Sommerfeld coefficient yields a heavy effective electron mass of $m^{*}\approx6m_{e}$. In the superconducting state, zero-field electronic specific-heat data $C_{es}$ can be described by a two-gap BCS model, indicating that RbNi$_2$Se$_2$ is a multi-gap superconductor. Our density functional theory calculations and angle resolved photoemission spectroscopy measurements demonstrate that RbNi$_2$Se$_2$ exhibits relatively weak correlations and multi-band characteristics, consistent with the multi-gap superconductivity.

cond-mat.supr-con↗

arXiv2021

Inexorable Edge Kondo Breakdown in Topological Kondo Insulators

Kondo breakdown is one of the most intriguing problems in strongly correlated electron systems, as it is rooted in many anomalous electron behaviors found in heavy-fermion materials. In Kondo lattice systems, Kondo breakdown can arise from either strong magnetic frustrations or critical fluctuations of collective modes. Here, we reveal a new type of Kondo breakdown with a fully different origin in interacting topological Kondo insulators. By employing numerically exact quantum Monte Carlo simulations, we show that with open boundary conditions, Kondo screening is inexorably destroyed by interaction effects on edges or corners in these systems. We argue that the Kondo breakdown is enforced by the symmetries of the system, because the ground states are symmetry protected Haldane phases.

cond-mat.str-el↗