arXiv subjects

Jean-Baptiste Morée

Publications and source records attributed to Jean-Baptiste Morée.

9 recordsLinked to original sources

arXiv2026

Bridging ambient- and high-pressure superconductivity in La$_2$LnNi$_2$O$_7$ films

The discovery of high critical-temperature $T_{\mathrm{c}}$ superconductivity near 80 K in bilayer nickelates under high pressure has sparked extensive studies. While superconductivity exceeding 40 K was subsequently discovered at ambient pressure in compressively strained films, the relationship between ambient- and high-pressure regimes remains an open question. Here we present a systematic investigation of superconductivity in compressively strained La$_2$LnNi$_2$O$_7$ films (Ln = lanthanides) at ambient and high pressures. The normal-state resistivity at ambient pressure, revealed by suppressing superconductivity with magnetic fields of 59 T, tends toward $T^2$ behaviour. Under high pressure in a cubic-anvil cell, $T_{\mathrm{c}}$ was enhanced from 41-42 K at ambient pressure to 67-73 K at 16 GPa. On the other hand, lattice compression induced by Ln substitution, which may mimic effects of pressure, lowers $T_{\mathrm{c}}$. In both cases, $T_{\mathrm{c}}$ correlates with the evolution of normal-state transport between $T^2$ and $T$-linear behaviour, offering insight into the interplay between lattice structure and superconductivity in bilayer nickelates.

cond-mat.supr-con↗

arXiv2026

Interpretable machine learning of magnetic transition temperature in Heusler magnets via hierarchical dependence extraction

We employ interpretable machine learning to analyze the material dependence of the magnetic transition temperature $T_c$ in ferromagnetic and ferrimagnetic Heusler compounds. For over 200 candidate materials with the same $F\overline{4}3m$ crystal structure but different chemical formulae and lattice constants, we consider both experimental $T_c$ and those computed via classical Monte Carlo simulations using magnetic interactions derived from ab initio calculations. We use the hierarchical dependence extraction (HDE) procedure [Morée and Arita, Phys. Rev. B 110, 014502 (2024)] to determine how $T_c$ depends on chemical composition and magnetic moments, from leading to higher-order effects, and use these dependencies to construct an explicit expression for $T_c$. Our results show that the HDE framework predicts $T_c$ with accuracy comparable to other machine-learning approaches such as neural network and random forest algorithms while remaining fully interpretable. $T_c$ is primarily governed by the proportions of Fe, Co, and Mn, increasing systematically with their concentration. These findings clarify how chemical composition and magnetic moments influence $T_c$ in collinear Heusler alloys and support the use of the HDE for computationally guided discovery of new functional materials with tailored $T_c$ values.

cond-mat.mtrl-sci↗

arXiv2026

Pressure-induced superconductivity in epitaxially-stabilized Pr$_3$Ni$_2$O$_7$ films

The discovery of high critical-temperature $T_{\mathrm{c}}$ superconductivity in La$_3$Ni$_2$O$_7$ under high pressure has led to a rapid expansion of the $T_{\mathrm{c}}$ range through lanthanide $Ln$ substitution, and to ambient-pressure superconductivity in strained thin films, yet the exploration of new bilayer nickelates remains strongly constrained by thermodynamic stability. Beyond the difficulty of synthesis of bulk single-crystals, here we report on the pressure-induced high-$T_{\mathrm{c}}$ superconductivity in epitaxially-stabilized Pr$_3$Ni$_2$O$_7$ thin films. While the Pr$_3$Ni$_2$O$_7$ films exhibit insulating behaviour at ambient pressure regardless of ozone-annealing treatment, they show $T$-linear metallic transport and superconductivity reaching an onset $T_{\mathrm{c}}$ of 66 K and zero-resistance at nearly 40 K at 22 GPa. Furthermore, Nd$_3$Ni$_2$O$_7$, with the smaller rare-earth ion Nd, can also be stabilized, however, superconductivity is not observed in the measured pressure range. Epitaxial stabilization enables us to examine the dependence of $T_{\mathrm{c}}$ and the critical pressure $P_{\mathrm{c}}$ for superconductivity on the $Ln$ ion in $Ln_3$Ni$_2$O$_7$ ($Ln$ = La, Pr, Nd). These results suggest that a higher $P_{\mathrm{c}}$ is required for smaller $Ln$ ions, consistent with trends observed in bulk studies of $Ln$ substitution. This study demonstrates that epitaxial stabilization is a powerful technique to further expand the family of superconducting bilayer nickelates.

cond-mat.supr-con↗

arXiv2025

Charge and magnetic orders in a two-band model with long-range interactions for infinite-layer nickelates NdNiO$_2$

We present an effective two-band model for infinite-layer nickelates NdNiO$_2$ that consisting of a $d$ band centered at Ni site and an interstitial $s$-like band centered at Nd site. To the large extent of the wave functions, we find intersite Coulomb interactions to be substantial. We then use the variational cluster approach together with mean-field theory to investigate magnetic and charge ordering. While tendencies towards charge modulation are found, they are weak and might be due to finite-size effects. Magnetic order is determined mostly by the filling of the $d$ band and hardly affected by including longer-ranged interactions. For a $d$-band density consistent with density-functional theory, magnetic ordering vanishes once quantum fluctuations are included to a sufficient spatial extent. Apart from self-doping, $d$ and $s$ bands remain largely uncoupled despite the presence of inter-orbital Coulomb interaction and (small) inter-orbital hopping.

cond-mat.str-el↗

arXiv2024

Universal Chemical Formula Dependence of $Ab$ $Initio$ Low-Energy Effective Hamiltonian in Single-Layer Carrier Doped Cuprate Superconductors -- Study by Hierarchical Dependence Extraction Algorithm

We explore the possibility to control the superconducting (SC) transition temperature at optimal hole doping $T_{c}^{\rm opt}$ in cuprates by tuning the chemical formula (CF). $T_{c}^{\rm opt}$ can be theoretically predicted from the parameters of the \textit{ab initio} low-energy effective Hamiltonian (LEH) with one antibonding (AB) Cu$3d_{x^2-y^2}$/O$2p_σ$ orbital per Cu atom in the CuO$_2$ plane, notably the nearest neighbor hopping amplitude $|t_1|$ and the ratio $u=U/|t_1|$, where $U$ is the onsite effective Coulomb repulsion. However, the CF dependence of $|t_1|$ and $u$ is a highly nontrivial question. In this paper, we propose the universal dependence of $|t_1|$ and $u$ on the CF and structural features in hole doped cuprates with a single CuO$_2$ layer sandwiched between block layers. To do so, we perform extensive \textit{ab initio} calculations of $|t_1|$ and $u$ and analyze the results by employing a machine learning method called Hierarchical Dependence Extraction (HDE). The main results are the following: (a) $|t_1|$ has a main-order dependence on the radii $R_{\rm X}$ and $R_{\rm A}$ of the apical anion X and cation A in the block layer. ($|t_1|$ increases when $R_{\rm X}$ or $R_{\rm A}$ decreases.) (b) $u$ has a main-order dependence on the negative ionic charge $Z_{\rm X}$ of X and the hole doping $δ$ of the AB orbital. ($u$ decreases when $|Z_{\rm X}|$ increases or $δ$ increases.) We elucidate and discuss the microscopic mechanism of (a,b). We demonstrate the predictive power of the HDE by showing the consistency between (a,b) and results from previous works. The present results provide a basis for optimizing SC properties in cuprates and possibly akin materials. Also, the HDE method offers a general platform to identify dependencies between physical quantities.

cond-mat.supr-con↗

arXiv2023

Dome structure in pressure dependence of superconducting transition temperature for HgBa$_2$Ca$_2$Cu$_3$O$_8$ -- Studies by $ab$ $initio$ low-energy effective Hamiltonian

The superconducting (SC) cuprate HgBa$_2$Ca$_2$Cu$_3$O$_8$ (Hg1223) has the highest $T_{c}^{\rm opt}\simeq 138$ K (the experimental SC transition temperature at optimal hole doping) among cuprates at ambient pressure $P_{\rm amb}$. $T_{c}^{\rm opt}$ increases under pressure $P$ and reaches $164$ K at $P_{\rm opt}\simeq 30$ GPa, then decreases with increasing $P>P_{\rm opt}$. To understand the microscopic origin of this dome-like $P$ dependence of $T_{c}^{\rm opt}$, we consider the $ab$ $initio$ low-energy effective Hamiltonian (LEH) for the antibonding (AB) Cu$3d_{x^2-y^2}$/O$2p_σ$ band. In the AB LEH for cuprates with $N_\ell \leq 2$ laminated CuO$_2$ planes between block layers, it was proposed that $T_{c}^{\rm opt}\simeq 0.16|t_1|F_{\rm SC}$, where $t_1$ is the nearest neighbor hopping and the SC order parameter $F_{\rm SC}$ at optimal hole doping mainly depends on $u=U/|t_1|$ ($U$ is the onsite effective Coulomb repulsion): $F_{\rm SC}$ is maximal at $u_{\rm opt}\simeq 8.5$ and decreases sharply with decreasing $u$ for $u P_{\rm opt}$, the decrease in $T_{c}^{\rm opt}$ is accounted for by (II): The rapid decrease in $F_{\rm SC}$ with decreasing $u$ for $u<u_{\rm opt}$ dominates over (I). We support (A,B) based on studies of other cuprates.

cond-mat.supr-con↗

arXiv2023

Superconductivity studied by solving ab initio low-energy effective Hamiltonians for carrier doped CaCuO$_2$, Bi$_2$Sr$_2$CuO$_6$, Bi$_2$Sr$_2$CaCu$_2$O$_8$, and HgBa$_2$CuO$_4$

We numerically analyze superconductivity (SC) in the cuprate superconductors by using ab initio effective Hamiltonians consisting of the antibonding combination of Cu $3d_{x^2-y^2}$ and O $2p_σ$ orbitals. We perform variational Monte Carlo calculations for the four carrier doped cuprates with diverse experimental optimal SC critical temperature $T_{c}^{\rm opt}$: CaCuO$_2$ ($T_{c}^{\rm opt} \sim 110$ K), Bi$_2$Sr$_2$CuO$_6$ ($T_{c}^{\rm opt} \sim 10$-$40$ K), Bi$_2$Sr$_2$CaCu$_2$O$_8$ ($T_{c}^{\rm opt} \sim 85$-$100$ K), and HgBa$_2$CuO$_4$ ($T_{c}^{\rm opt} \sim 90$ K). Materials and hole doping concentration ($δ$) dependencies of the SC order parameter $F_{\rm SC}$ and the competition with spin/charge order show essential and quantitative agreements with the available experiments in the following points: (1) The ground state is commonly the SC state, which is severely competing with the charge/spin stripe and antiferromagnetic states. (2) $F_{\rm SC}$ shows amplitude consistent with the superfluid density measured in the muon spin resonance and its dome structure found in $δ$ dependence shows consistency with that of the SC gap in the tunneling and photoemission measurements. We further find insights into the universal SC mechanism: (I) $F_{\rm SC}$ increases with the ratio $U/|t_1|$, indicating that $U/|t_1|$ is the principal component controlling the SC. Here, $U$ and $t_1$ are the onsite Coulomb repulsion and the nearest neighbor hopping, respectively, in the Hamiltonians. (II) A universal scaling $T_{c}^{\rm opt}\sim 0.16 \lvert t_1 \rvert F_{\rm SC}$ holds. (III) SC is enhanced and optimized if $U$ is increased beyond the real available materials. It is further enhanced by decreasing the offsite interaction. The present findings provide useful clues for the design of new SC materials with even higher $T_{c}^{\rm opt}$.

cond-mat.supr-con↗

arXiv2022

$Ab$ $initio$ low-energy effective Hamiltonians for high-temperature superconducting cuprates Bi$_2$Sr$_2$CuO$_6$, Bi$_2$Sr$_2$CaCu$_2$O$_8$, HgBa$_2$CuO$_4$ and CaCuO$_2$

We derive $ab$ $initio$ low-energy effective Hamiltonians (LEH) for high-temperature superconducting (SC) copper oxides Bi$_2$Sr$_2$CuO$_6$ (Bi2201, $N_{\ell}=1$, $T_c^{\rm exp} \sim 10$ K), Bi$_2$Sr$_2$CaCu$_2$O$_8$ (Bi2212, $N_{\ell}=2$, $T_c^{\rm exp} \sim 84$ K), HgBa$_2$CuO$_4$ (Hg1201, $N_{\ell}=1$, $T_c^{\rm exp} \sim 90$ K) and CaCuO$_2$ (Ca11, $N_{\ell}=\infty$, $T_c^{\rm exp} \sim 110$ K), with different experimental optimal SC transition temperature $T_c^{\rm exp}$ and number $N_{\ell}$ of laminated CuO$_2$ planes between the two neighboring block layers. We apply the latest methodology of the multiscale $ab$ $initio$ scheme for correlated electron systems (MACE), and focus on the LEH consisting of one antibonding (AB) Cu$3d_{x^2-y^2}$/O$2p_σ$ orbital centered on each Cu atom. We discuss prominent features of this LEH: (1) The ratio $U/|t_1|$ between the onsite effective Coulomb repulsion (ECR) $U$ and amplitude of nearest neighbour hopping $t_1$ increases with $T^{\rm exp}_c$ and $N_{\ell}$, consistently with the expected increase in $d$-wave SC correlation function $P_{dd}$ with $U/|t_1|$. One possible cause of the increase of $U/|t_1|$ is the replacement of apical O atoms by Cu atoms from neighbouring CuO$_2$ planes when $N_{\ell}$ increases. Furthermore, we show that the increase in distance between Cu and apical O atoms decreases the effective screening (ES) by electrons outside of the LEH and increases $U/|t_1|$. (2) For Hg1201 and Ca11, we show that $U/|t_1|$ decreases when hole doping per AB orbital $δ$ increases, which may partly account for the disappearance of SC when $δ$ exceeds the optimal value in experiment. (3) For $N_{\ell} \geq 2$, off-site inter-CuO$_2$ plane ECR is comparable to off-site intra-CuO$_2$ plane ECR. We discuss contributions of inter-CuO$_2$ plane ECR to both $P_{dd}$ and the stability of the SC state.

cond-mat.str-el↗

arXiv2021

Ab initio Derivation of Low-Energy Hamiltonians for Systems with Strong Spin-Orbit Interaction and Its Application to Ca5Ir3O12

We present an ab initio derivation method for effective low-energy Hamiltonians of material with strong spin-orbit interactions. The effective Hamiltonian is described in terms of the Wannier function in the spinor form, and effective interactions are derived with the constrained random phase approximation (cRPA) method. Based on this formalism and the developed code, we derive an effective Hamiltonian of a strong spin-orbit interaction material Ca5Ir3O12. This system consists of three edge-shared IrO6 octahedral chains arranged along the c axis, and the three Ir atoms in the ab plane compose a triangular lattice. For such a complicated structure, we need to set up the Wannier spinor function under the local coordinate system. We found that a density-functional band structure near the Fermi level is formed by local dxy and dyz orbitals. Then, we constructed the ab initio dxy/dyz model. The estimated nearest neighbor transfer t is close to 0.2 eV, and the cRPA onsite U and neighboring V electronic interactions are found to be 2.4-2.5 eV and 1 eV, respectively. The resulting characteristic correlation strength defined by (U-V)/t is above 7, and thus this material is classified as a strongly correlated electron system. The onsite transfer integral involved in the spin-orbit interaction is 0.2 eV, which is comparable to the onsite exchange integrals near 0.2 eV, indicating that the spin-orbit-interaction physics would compete with the Hund physics. Based on these calculated results, we discuss possible rich ground-state low-energy electronic structures of spin, charge and orbitals with competing Hund, spin-orbit and strong correlation physics.

cond-mat.str-el↗