arXiv subjects

Thomas A. Maier

Publications and source records attributed to Thomas A. Maier.

At least 19 recordsLinked to original sources

arXiv2026

Tunable Superconductivity in 1313-La$_3$Ni$_2$O$_7$: Suppressed under Compression and Possible $s^{\pm}$ Pairing under Tension

Motivated by recent progress in the 1313-La$_3$Ni$_2$O$_7$ nickelate thin films (Nie et al., Nature {\bf 652}, 628 (2026)), we systematically investigate the effects of both compressive and tensile strain in this system. Self-doping effects between the single-layer (SL) and trilayer (TL) blocks are observed in our studies in both cases, but are most pronounced under tensile strain. We find that superconductivity is unlikely to emerge in the 1313 film on the LSAO substrate even with further hole doping. Remarkably, within the random-phase approximation, under {\it tensile} strain, a robust $s^{\pm}$-wave pairing state emerges in the TL subsystem with sign changes between the small electron-like $σ$ pocket at the $Γ$ point and the small hole-like $γ$ pocket at the M point. These pockets are connected by a wave vector close to $(π,π)$. Our calculations also suggest that superconductivity in 1313-LNO requires an optimally sized $γ$ pocket, because an oversized $γ$ pocket suppresses pairing formation. Overall, our results predict strain-dependent electronic reconstruction in 1313-La$_3$Ni$_2$O$_7$ and provide guiding principles for engineering superconductivity under ambient pressure conditions.

cond-mat.supr-con↗

arXiv2026

SmoQyElPhQMC.jl: An open-source Julia package for efficient and scalable quantum Monte Carlo simulations of electron-phonon coupled models

We introduce version 1.0 of the SmoQyElPhQMC package, an open-source Julia code for performing scalable quantum Monte Carlo simulations of electron-phonon coupled model Hamiltonians. SmoQyElPhQMC is built upon the SmoQyDQMC codebase and implements improved versions of the algorithms presented in [B. Cohen-Stead \textit{et al}., Phys. Rev. E {\bf 105}, 065302 (2022)] to enable linear-scaling simulations of a broad class of uncorrelated $e$-ph models both in system size and inverse temperature. By extending the functionality of the flexible scripting interface introduced in SmoQyDQMC, the SmoQyElPhQMC package continues to allow users to adapt it to different workflows and interface with other software packages in the Julia ecosystem. The code for this package can be downloaded from our GitHub repository at https://github.com/SmoQySuite/SmoQyElPhQMC.jl or installed using the Julia package manager. The online documentation, including examples, can be obtained from our documentation page at https://smoqysuite.github.io/SmoQyElPhQMC.jl/stable/.

cond-mat.str-el↗

arXiv2026

Intertwined charge, spin, and orbital degrees of freedom under electronic correlations in the one-dimensional Fe$^{3+}$ chalcogenide chain

Motivated by recent developments in the study of quasi-one-dimensional iron systems with Fe$^{2+}$, we comprehensively study the Fe$^{3+}$ chalcogenide chain system. Based on first-principles calculations, the Fe$^{3+}$ chain has a similar electronic structure as discussed before in the iron 2+ chain, due to similar Fe$X_4$ ($X$ = S or Se) tetrahedron chain geometry. Furthermore, a three-orbital electronic Hubbard model for this chain was constructed by using the density matrix renormalization group method. A robust antiferromagnetic coupling was unveiled in the chain direction. In addition, in the intermediate electronic correlation $U/W$ region, we found an interesting orbital-selective Mott phase with the coexistence of localized and itinerant electrons ($U$ is the on-site Hubbard repulsion, while $W$ is the electronic bandwidth) {\color{blue}based on the orbital-selective behavior observed in the charge fluctuations}. Furthermore, we do not observe any obvious pairing tendency in the Fe$^{3+}$ chain in the electronic correlation $U/W$ region, where superconducting pairing tendencies were reported before in iron ladders. This suggests that superconductivity is unlikely to emerge in the Fe$^{3+}$ systems. Our results establish with clarity the similarities and differences between Fe$^{2+}$and Fe$^{3+}$ iron chains, as well as iron ladders.

cond-mat.str-el↗

arXiv2026

Neural networks as low-cost surrogates for impurity solvers in quantum embedding methods

A promising application of machine learning is the creation of low-cost surrogate models to mitigate computational bottlenecks in quantum many-body simulations. Here, we explore whether a neural network (NN) can be trained in the low-data regime, with one to two orders of magnitude fewer training examples than previous works, as an efficient substitute for the impurity solver in dynamical mean-field theory simulations of correlated electron models. We show that the NN solver achieves accuracy comparable to popular continuous-time quantum Monte Carlo (CT-QMC) impurity solvers when interpolating between samples within the training set. While the NN's performance decreases notably when extrapolating to lower temperatures outside the training distribution, its output still provides an excellent initial guess for input to more accurate CT-QMC impurity solvers, thus accelerating the time to solution up to a factor of five. We discuss our results in the context of rapid phase-space exploration.

cond-mat.str-el↗

arXiv2026

Superconductivity in Ruddlesden-Popper nickelates: a review of recent progress, focusing on thin films

The discovery of superconductivity with Tc ~ 80 K in the nickelate Ruddlesden-Popper bilayer La3Ni2O7 at high pressure has opened a new platform for unconventional superconductivity, followed by the subsequent observation of superconductivity in trilayer La4Ni3O10, also at high pressure. Remarkably, ambient-pressure superconductivity was also observed recently in La3Ni2O7 ultra-thin films when grown on substrates that provide compressive strain. This discovery significantly extends the type of experimental techniques that can be used in nickelates, previously limited due to the high-pressure constraint. Discussing the similarities and differences among these nickel oxides will provide new insights into understanding the mechanism of high-Tc superconductivity in correlated electron systems. In this paper, we review the experimental and theoretical progress on RuddlesdenPopper nickelates, with emphasis on thin films, and discuss future perspectives and research directions.

cond-mat.supr-con↗

arXiv2025

Compressive Strain Turns $s^{\pm}$ into $d$-Wave Pairing in One-unit-cell La$_3$Ni$_2$O$_7$ Thin Film Via Substrate-Induced Hole Doping

Motivated by recent reports of ambient-pressure superconductivity in La$_3$Ni$_2$O$_7$ films grown on LaSrAlO$_4$, we investigate the superconducting instability in a one-unit cell thin film using {\it ab initio} and random-phase approximation techniques. Compared to the high-pressure bulk system, the ratio of inter-layer $d_{3z^2-r^2}$ hopping to intra-layer $d_{x^2-y^2}$ hopping is suppressed in the 1UC thin film, and the crystal-field splitting of the $e_g$ orbitals is increased. Our calculation indicates that spin-fluctuation-driven pairing correlations are weak for the stoichiometric case at ambient pressure, but increase significantly under hole doping. The leading pairing symmetry is also found to change by hole doping. Specifically, we obtain a leading $d_{x^2-y^2}$ pairing state at moderate hole doping, followed by a $d_{xy}$ state at higher doping. These states are driven by intra-band spin-fluctuation scattering {\it within} the $γ$ hole pocket centered around the M point, and arise primarily from states in the Ni layer {\it farther} from the substrate. These results strongly suggest that the thin-film superconducting samples are hole-doped and that pairing in this system predominantly arises in the layer, as opposed to the inter-layer pairing in the pressurized bulk system.

cond-mat.supr-con↗

arXiv2025

Quantum-Centric Algorithm for Sample-Based Krylov Diagonalization

Approximating the ground state of many-body systems is a key computational bottleneck underlying important applications in physics and chemistry. The most widely known quantum algorithm for ground state approximation, quantum phase estimation, is out of reach of current quantum processors due to its high circuit-depths. Subspace-based quantum diagonalization methods offer a viable alternative for pre- and early-fault-tolerant quantum computers. Here, we introduce a quantum diagonalization algorithm which combines two key ideas on quantum subspaces: a classical diagonalization based on quantum samples, and subspaces constructed with quantum Krylov states. We prove that our algorithm converges in polynomial time under the working assumptions of Krylov quantum diagonalization and sparseness of the ground state. We then demonstrate the scalability of our approach by performing the largest ground-state quantum simulation of impurity models using a Heron quantum processors and the Frontier supercomputer. We consider both the single-impurity Anderson model with 41 bath sites, and a system with 4 impurities and 7 bath sites per impurity. Our results are in excellent agreement with Density Matrix Renormalization Group calculations.

quant-ph↗

arXiv2025

Optimizing the Critical Temperature and Superfluid Density of a Metal-Superconductor Bilayer

A promising path to realizing higher superconducting transition temperatures $T_c$ is the strategic engineering of artificial heterostructures. For example, quantum materials could, in principle, be coupled with other materials to produce a more robust superconducting state. In this work, we add numerical support to the hypothesis that a strongly interacting superconductor weakened by phase fluctuations can boost its $T_c$ by hybridizing the system with a metal. Using determinant quantum Monte Carlo (DQMC), we simulate a two-dimensional bilayer composed of an attractive Hubbard model and a metallic layer in two regimes of the interaction strength $-|U|$. In the strongly interacting regime, we find that increasing the interlayer hybridization $t_\perp$ results in a nonmonotonic enhancement of $T_c$, with an optimal value comparable to the maximum $T_c$ observed in the single-layer attractive Hubbard model, confirming trends inferred from other approaches. In the intermediate coupling regime, when $-|U|$ is close to the value associated with the maximum $T_c$ of the single-layer model, increasing $t_\perp$ tends to decrease $T_c$, implying that the correlated layer was already optimally tuned. Importantly, we demonstrate that the mechanism behind these trends is related to enhancement in the superfluid stiffness, as was initially proposed by Kivelson [Physica B: Condensed Matter 318, 61 (2002)].

cond-mat.supr-con↗

arXiv2025

Interlayer Pairing in Bilayer Nickelates

The discovery of $T_c\sim 80$~K superconductivity in pressurized La$_3$Ni$_2$O$_7$ has launched a new platform to study high-temperature superconductivity. Using non-perturbative dynamic cluster approximation quantum Monte Carlo calculations, we characterize the magnetic and superconducting pairing behavior of a realistic bilayer two-orbital Hubbard-Hund model of this system that describes the relevant Ni $e_g$ states with physically relevant interaction strengths. We find a leading $s^\pm$ superconducting instability in this model and show that this state primarily arises from interlayer pairing in the $d_{3z^2-r^2}$ orbital that is driven by strong interlayer spin-fluctuations in that orbital. These results provide non-perturbative evidence supporting the picture that a simple single-orbital bilayer Hubbard model for the Ni $d_{3z^2-r^2}$ orbital provides an excellent low-energy effective description of the superconducting behavior of La$_3$Ni$_2$O$_7$.

cond-mat.str-el↗

arXiv2025

General trends of electronic structures, superconducting pairing, and magnetic correlations in the Ruddlesden-Popper nickelate $m$-layered superconductors La$_{m+1}$Ni$_{m}$O$_{3m+1}$

We report a comprehensive theoretical analysis of the Ruddlesden-Popper layered nickelates La$_{m+1}$Ni$_m$O$_{3m+1}$ ($m = 1$ to 6) under pressure. Our results suggest that, while these Ruddlesden-Popper layered nickelates display many similarities, they also show noticeable differences. The Ni $d_{3z^2-r^2}$ orbitals display bonding-antibonding, or bonding-antibonding-nonbonding, characteristic splittings, depending on the even or odd number of stacking layers $m$. In addition, the ratio of the in-plane interorbital hopping between $d_{3z^2-r^2}$ and $d_{x^2-y^2}$ orbitals and in-plane intraorbital hopping between $d_{x^2-y^2}$ orbitals was found to be large in La$_{m+1}$Ni$_m$O$_{3m+1}$ ($m = 1$ to 6), and this ratio increases from $m = 1$ to $m = 6$, suggesting that the in-plane hybridization will increase as the layer number $m$ increases. In contrast to the dominant $s^\pm$--wave state driven by spin fluctuations in the bilayer La$_3$Ni$_2$O$_7$ and trilayer La$_4$Ni$_3$O$_{10}$, two nearly degenerate $d_{x^2-y^2}$-wave and $s^\pm$-wave leading states were obtained in the four-layer stacking La$_5$Ni$_4$O$_{13}$ and five-layer stacking La$_6$Ni$_5$O$_{16}$. The leading $s^\pm$-wave state was recovered in the six-layer material La$_7$Ni$_6$O$_{19}$. In general, at the level of the random phase approximation treatment, the superconducting transition temperature $T_c$ decreases in stoichiometric bulk systems from the bilayer La$_3$Ni$_2$O$_7$ to the six-layer La$_7$Ni$_6$O$_{19}$, despite the $m$ dependent dominant pairing. Both in-plane and out-of-plane magnetic correlations are found to be quite complex. Within the in-plane direction, we obtained the peak of the magnetic susceptibility at ${\bf q} = (0.6 π, 0.6 π)$ for La$_5$Ni$_4$O$_{13}$ and La$_7$Ni$_6$O$_{19}$, and at ${\bf q} = (0.7 π, 0.7 π)$ for La$_6$Ni$_5$O$_{16}$.

cond-mat.supr-con↗

arXiv2025

Electronic Structure, Magnetic and Pairing Tendencies of Alternating Single-layer Bilayer Stacking Nickelate La$_5$Ni$_3$O$_{11}$ Under Pressure

Nickelates have continued to surprise since their unconventional superconductivity was discovered. Recently, the layered nickelate La$_5$Ni$_3$O$_{11}$ with hybrid single-layer and bilayer stacking showed superconductivity under high pressure. This compound combines features of La$_2$NiO$_4$ and La$_3$Ni$_2$O$_7$, but its pairing mechanism remains to be understood. Motivated by this finding, here we report a comprehensive theoretical study of this system. Our density functional theory calculations reveal that the undistorted P4/mmm phase without pressure is unstable due to three distortion modes. Increasing pressure suppresses these modes and causes ``charge transfer'' between the single-layer and bilayer sublattices, leading to hole-doping in the single-layer blocks. Our random-phase approximation calculations indicate a leading $d_{x^2-y^2}$-wave pairing state that arises from spin-fluctuation scattering between Fermi surface states mainly originating from the single-layer blocks and additional weaker contributions from the bilayer blocks. These spin-fluctuations could be detected by inelastic neutron scattering as a strong peak at ${\bf q}=(π, π)$. Our findings distinguish La$_5$Ni$_3$O$_{11}$ from other nickelate superconductors discovered so far and the high-$T_c$ cuprates. We also discuss both similarities and differences between La$_5$Ni$_3$O$_{11}$ and other hybrid stacking nickelates.

cond-mat.supr-con↗

arXiv2024

SmoQyDEAC.jl: A differential evolution package for the analytic continuation of imaginary time correlation functions

We introduce the SmoQyDEAC.jl package, a Julia implementation of the Differential Evolution Analytic Continuation (DEAC) algorithm [N. S. Nichols et al., Phys. Rev. E 106, 025312 (2022)] for analytically continuing noisy imaginary time correlation functions to the real frequency axis. Our implementation supports fermionic and bosonic correlation functions on either the imaginary time or Matsubara frequency axes, and treatment of the covariance error in the input data. This paper presents an overview of the DEAC algorithm and the features implemented in the SmoQyDEAC.jl. It also provides detailed benchmarks of the package's output against the popular maximum entropy and stochastic analytic continuation methods. The code for this package can be downloaded from our GitHub repository at https://github.com/SmoQySuite/SmoQyDEAC.jl or installed using the Julia package manager. The online documentation, including examples, can be accessed at https://smoqysuite.github.io/SmoQyDEAC.jl/stable/.

cond-mat.stat-mech↗

arXiv2024

Prediction of $s^\pm$-wave superconductivity enhanced by electronic doping in trilayer nickelates La$_4$Ni$_3$O$_{10}$ under pressure

Motivated by the recently reported signatures of superconductivity in trilayer La$_4$Ni$_3$O$_{10}$ under pressure, we comprehensively study this system using {\it ab initio} and random-phase approximation techniques. Without electronic interactions, the Ni $d_{3z^2-r^2}$ orbitals show a bonding-antibonding and nonbonding splitting behavior via the O $p_z$ orbitals inducing a ``trimer'' lattice in La$_4$Ni$_3$O$_{10}$, analogous to the dimers of La$_3$Ni$_2$O$_{7}$. The Fermi surface consists of three electron sheets with mixed $e_g$ orbitals, and a hole and an electron pocket made up of the $d_{3z^2-r^2}$ orbital, suggesting a Ni two-orbital minimum model. In addition, we find that superconducting pairing is induced in the $s^{\pm}$-wave channel due to partial nesting between the {\bf M}=$(π, π)$ centered pockets and portions of the Fermi surface centered at the {\bf $Γ$}=$(0, 0)$ point. With changing electronic density $n$, the $s^\pm$ instability remains leading and its pairing strength shows a dome-like behavior with a maximum around $n = 4.2$ ($\sim 6.7\%$ electron doping). The superconducting instability disappears at the same electronic density as that in the new 1313 stacking La$_3$Ni$_2$O$_7$, correlated with the vanishing of the hole pocket that arises from the trilayer sublattice, suggesting that the high-$T_c$ superconductivity of La$_3$Ni$_2$O$_7$ may $not$ originate from a trilayer- and single-layer structure. Furthermore, we predict an interesting spin-density-wave state in La$_4$Ni$_3$O$_{10}$ with an in-plane ($π$, $π$) order and antiferromagnetic coupling between the top and bottom Ni layers, while the middle layer has spin zero.

cond-mat.supr-con↗

arXiv2024

Electronic structure, self-doping, and superconducting instability in the alternating single-layer trilayer stacking nickelates La$_3$Ni$_2$O$_7$

Motivated by the recently proposed alternating single-layer trilayer stacking structure for the nickelate La$_3$Ni$_2$O$_7$, we comprehensively study this system using {\it ab initio} and random-phase approximation techniques. Our analysis unveils similarities between this novel La$_3$Ni$_2$O$_7$ structure and other Ruddlesden-Popper nickelate superconductors, such as a similar charge-transfer gap value and orbital-selective behavior of the $e_g$ orbitals. However, different from other Ruddlesden-Popper nickelate superconductors, we do not observe any obvious reconstruction of the Fermi surface from ambient conditions (Cmmm phase) to high pressures (P4/mmm phase). Pressure primarily increases the bandwidths of the Ni $e_g$ bands, suggesting an enhancement of the itinerant properties of those $e_g$ states. Furthermore, the $d_{3z^2-r^2}$ orbital also has a layer-selective behavior because the antibonding-bonding-nonbonding splitting can only be obtained in the trilayer. In addition, we observe a "self-doping" effect from the trilayer to the single-layer sublattices and this effect will be enhanced by overall electron doping. Moreover, we find a leading $d_{x^2-y^2}$-wave pairing state that is restricted to the single-layer. Because the effective coupling between the single layers is very weak -- due to the non-superconducting trilayer in between -- this suggests that the superconducting transition temperature $T_c$ in this structure should be much lower than in the bilayer structure.

cond-mat.supr-con↗

arXiv2024

Magnetic Correlations and Pairing Tendencies of the Hybrid Stacking Nickelate Superlattice La$_7$Ni$_5$O$_{17}$ (La$_3$Ni$_2$O$_7$/La$_4$Ni$_3$O$_{10}$) under Pressure

Motivated by the recent rapid progress in high-$T_c$ nickelate superconductors, we comprehensively study the physical properties of the alternating bilayer trilayer stacking nickelate La$_7$Ni$_5$O$_{17}$. The high-symmetry phase of this material, without the tilting of oxygen octahedra, is not stable at ambient conditions but becomes stable under high pressure, where a small hole pocket $γ_0$, composed of the $d_{3z^2-r^2}$ states in the trilayer sublattice, appears. This pocket was identified in our previous work for trilayer La$_4$Ni$_3$O$_{10}$ as important to develop superconductivity. Moreover, using random-phase approximation calculations, we find a leading $s^\pm$ pairing state for the high-symmetry phase under pressure with similar pairing strength as that obtained previously for the bilayer La$_3$Ni$_2$O$_7$ compound, suggesting a similar or higher superconducting transition temperature $T_c$. In addition, we find that the dominant magnetic fluctuations in the system driving this pairing state have antiferromagnetic structure both in-plane and between the planes of the top and bottom trilayer and bilayer sublattices, while the middle trilayer is magnetically decoupled.

cond-mat.supr-con↗

arXiv2024

Magnetic phase diagram of a two-orbital model for bilayer nickelates varying doping

Motivated by the recently discovered high-$T_c$ bilayer nickelate superconductor La$_3$Ni$_2$O$_7$, we comprehensively research a bilayer $2\times2\times2$ cluster for different electronic densities $n$ by using the Lanczos method. We also employ the random-phase approximation to quantify the first magnetic instability with increasing Hubbard coupling strength, also varying $n$. Based on the spin structure factor $S(q)$, we have obtained a rich magnetic phase diagram in the plane defined by $n$ and $U/W$, at fixed Hund coupling. We have observed numerous states, such as A-AFM, Stripes, G-AFM, and C-AFM. For half-filling $n=2$ (two electrons per Ni site, corresponding to $N$ = 16 electrons), the canonical superexchange interaction leads to a robust G-AFM state $(π,π,π)$ with antiferromagnetic couplings in plane and between layers. By increasing or decreasing electronic densities, ferromagnetic tendencies emerge from the ``half-empty'' and ``half-full'' mechanisms, leading to many other interesting magnetic tendencies. In addition, the spin-spin correlations become weaker both in the hole or electron doping regions compared with half-filling. At $n = 1.5$ (or $N=12$), density corresponding to La$_3$Ni$_2$O$_7$, we obtained the ``Stripe 2'' ground state (antiferromagnetic coupling in one in-plane direction, ferromagnetic coupling in the other, and antiferromagnetic coupling along the $z$-axis) in the $2\times2\times2$ cluster. In addition, we obtained a much stronger AFM coupling along the $z$-axis than the magnetic coupling in the $xy$ plane. The random-phase approximation calculations with varying $n$ give very similar results as Lanczos. Meanwhile, a state with $q/π= (0.6, 0.6, 1)$ close to the E-phase wavevector is found in our RPA calculations by slightly reducing the filling to $n=1.25$, possibly responsible for the E-phase SDW recently observed in experiments.

cond-mat.str-el↗

arXiv2024

Structural phase transition, $s_{\pm}$-wave pairing and magnetic stripe order in the bilayered nickelate superconductor La$_3$Ni$_2$O$_7$ under pressure

Motivated by the recently discovered high-$T_c$ superconductor La$_3$Ni$_2$O$_7$, we comprehensively study this system using density functional theory and random phase approximation calculations. At low pressures, the Amam phase is stable, containing the Y$^{2-}$ mode distortion from the Fmmm phase, while the Fmmm phase is unstable. Because of small differences in enthalpy and a considerable Y$^{2-}$ mode amplitude, the two phases may coexist in the range between 10.6 and 14 GPa, beyond which the Fmmm phase dominates. In addition, the magnetic stripe-type spin order with wavevector ($π$, 0) was stable at the intermediate region. Pairing is induced in the $s_{\pm}$-wave channel due to partial nesting between the {\bf M}=$(π, π)$ centered pockets and portions of the Fermi surface centered at the {\bf X}=$(π, 0)$ and {\bf Y}=$(0, π)$ points. This resembles results for iron-based superconductors but has a fundamental difference with iron pnictides and selenides. Moreover, our present efforts also suggest that La$_3$Ni$_2$O$_7$ is qualitatively different from infinite-layer nickelates and cuprate superconductors.

cond-mat.supr-con↗

arXiv2024

Topological Phase Transition without Single-Particle-Gap Closing in Strongly Correlated Systems

We show here that numerous examples abound where changing topology does not necessarily close the bulk insulating charge gap as demanded in the standard non-interacting picture. From extensive determinantal and dynamical cluster quantum Monte Carlo simulations of the half-filled and quarter-filled Kane-Mele-Hubbard model, we show that for sufficiently strong interactions at either half- or quarter-filling, a transition between topological and trivial insulators occurs without the closing of a charge gap. To shed light on this behavior, we illustrate that an exactly solvable model reveals that while the single-particle gap remains, the many-body gap does in fact close. These two gaps are the same in the non-interacting system but depart from each other as the interaction turns on. We purport that for interacting systems, the proper probe of topological phase transitions is the closing of the many-body rather than the single-particle gap.

cond-mat.str-el↗