arXiv subjects

Steffen Bötzel

Publications and source records attributed to Steffen Bötzel.

15 recordsLinked to original sources

arXiv2026

Nodal Orbital-Anti-Phase Superconducting State in Bilayer Nickelates

The recent discovery of high-$T_c$ superconductivity in the bilayer nickelate La$_3$Ni$_2$O$_7$ (La-327) under applied pressure and compressive strain opened a new avenue to elucidate the interplay between multiorbital intralayer and interlayer electronically driven Cooper-pairing in bilayer systems. Depending on the details of the electronic structure in the normal state, the superconducting gap in bilayer nickelates is predicted to have either bonding-antibonding $s_{\pm}$-wave symmetry, driven by dominant interlayer Cooper-pairing, or $d$-wave symmetry with substantial intralayer Cooper-pairing. Despite this general picture, the orbital structure of the superconducting gap in these multiorbital systems has been less explored. Here, we analyze the consequences of an orbital-anti-phase structure of the superconducting gap and discuss its possible experimental signatures. We demonstrate that additional pairs of nodes may appear on the $α$ and/or $β$ Fermi surface sheets due to the sign change of the superconducting gap between the involved orbitals. Apart from this additional nodal structure, which is not enforced by the symmetries of the gap function and can be probed in ARPES experiments, the orbital-anti-phase gap modifies the temperature dependence of the superfluid stiffness at low temperatures, providing a concrete experimental prediction to test its realization in bilayer nickelates and related multiorbital systems.

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arXiv2026

Density waves in low-pressure bilayer nickelates

The low-pressure phase diagram of La$_3$Ni$_2$O$_7$ provides an important reference for understanding its pressure-induced high-temperature superconductivity. While the spin-density-wave transition at $T_{\text{SDW}}\approx150$ K is increasingly well established, the origin of the second density-wave transition at $T_{\text{DW}}\approx130$ K has remained unresolved. Here, we perform unrestricted Hartree-Fock calculations to investigate the potential origin of the second transition. {Within the orthorhombic phase, the degeneracy between possible ordering wavevectors at $\boldsymbol{Q}_{Y}=(0,π)$ and at $\boldsymbol{Q}_{X}=(π,0)$ is lifted and the electronic system} develops a double-stripe spin-density wave with ordering vector $\boldsymbol{Q}_{Y}=(0,π)$. We identify that the pure double stripe spin state is unstable in La$_3$Ni$_2$O$_7$ towards a commensurate charge-density wave instability, which favors a spin-modulated double stripe order with intertwined charge and spin instabilities and establish the hierarchy of ordered states in La$_3$Ni$_2$O$_7$. We further discuss our results in the context of available experimental literature and propose further experimental tests to elucidate the origin of the SDW/DW states in this system.

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arXiv2026

Manifest charge-transfer physics in T$^\prime$-La$_2$NiO$_4$

The La$_2$NiO$_4$ compound stabilizes in nature in the so-called T structure with octahedral oxygen coordination of the Ni site. Motivated by a similarly existing polymorph for La$_2$CuO$_4$ in the cuprate system, we here study La$_2$NiO$_4$ in the T$^\prime$-structure with square-planar oxygen coordination of Ni by means of first-principles many-body theory. The hypothetical T$^\prime$-La$_2$NiO$_4$ compound turns out to be a manifest charge-transfer insulator with a cuprate-like charge gap of $\sim 1.8$ eV. Upon doping, a strong carrier asymmetry is revealed, i.e. while holes dominantly enter O$(2p)$-derived states, electrons may become itinerant majorly within effective Ni-$d_{x^2-y^2}$ states. Nearest-neighbor antiferromagnetic ordering at stoichiometry appears absent in the T$^\prime$ structure, in stark contrast to the strong antiferromagnetism in the T structure. Our theoretical study opens up new pathways for correlation physics in layered nickelates with challenging charge-transfer signatures, awaiting experimental inquiries.

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arXiv2026

Electronic theory for scanning tunneling microscopy spectra in bilayer nickelate thin films

Recent Scanning Tunneling Microscopy (STM) experiments measuring the superconducting gap features in thin films of superconducting bilayer nickelates La2PrNi2O7 at ambient pressure and compressive strain paved the way to study the Cooper-pairing models and the band-selective identification of the gap features in these systems. Here, using the realistic two-orbital bilayer model and the continuum Green's function formalism, we theoretically analyze orbital and band-selective local density of states as well as the corresponding STM spectra. We find that the multiorbital character and the spatial dependence of the Wannier functions leads to the spectra developing characteristic features depending on the position of the scanning tunneling microscope's tip. This allows for a band-resolved analysis of the superconducting coherence peaks and scattering momenta. We identify a clear path for experimental measurements to not only identify the debated incipiency of the gamma-band, but also identification of the coherence peaks' band origins via distance dependent measurements of the local density of states and its corrections through impurity scattering.

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arXiv2026

Structural symmetry effects on the competition of density waves and superconductivity in bilayer nickelates

We investigate the interplay between spin-density-wave (SDW) order and superconductivity in the bilayer nickelate La$_3$Ni$_2$O$_7$ using the functional renormalization group~(fRG) applied to multiorbital weak-coupling models of both the ambient- and high-pressure crystal structures. As Hund's coupling increases, the leading instability evolves from superconductivity to an SDW state with ordering vector $\mathbf{Q}_1 \approx (π/2,π/2)$ (equivalently $\mathbf{Q}_Y \approx (0,π)$ in the orthorhombic $Amam$ unit cell), in agreement with experimental observations. Surprisingly, the ambient- and high-pressure structures exhibit nearly identical non-interacting susceptibilities and leading fRG instabilities, indicating that the emergence of superconductivity under pressure cannot be explained solely by changes in the low-energy electronic structure. Instead, our results identify the suppression of orthorhombicity as a key ingredient for superconductivity. As the system approaches the tetragonal limit, symmetry-related SDW fluctuations become nearly degenerate, frustrating long-range magnetic order while enhancing pairing interactions. These findings highlight lattice symmetry as a central tuning parameter of the competing ordered states in bilayer nickelates and suggest that reducing orthorhombicity through uniaxial strain could stabilize bulk superconductivity already at ambient pressure.

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arXiv2026

Co-operating multiorbital and nonlocal correlations in bilayer nickelate

The interplay of multiorbital physics and nonlocal self-energy effects is studied within an effective three-orbital model for the high-pressure normal state of superconducting bilayer nickelate La$_3$Ni$_2$O$_7$. The model is solved within an advanced many-body framework capturing $k$-dependent correlations beyond dynamical mean-field theory. Different low-energy scenarios subtly depend on the strength of the interorbital interaction, either placing the notorious flat $γ$ quasiparticle band in the occupied part of the spectrum, or letting it cross the Fermi level. In the latter case, intriguing spin-polaron formation due to the scattering of electrons with paramagnon excitations takes place. This leads to bound states appearing as a shadow band with incoherent low-energy spectral weight below the Fermi level. Our results uncover additional competing states that exist in bilayer nickelates and could explain the controversy of recent angle-resolved photoemission experiments.

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arXiv2025

Interplay of orbital-selective Mott criticality and flat-band physics in La$_3$Ni$_2$O$_6$

Superconductivity in nickelates apparently takes place in two different Ni oxidation regimes, namely either for infinite-layer-type compounds close to Ni$^{+}$, or for Ruddlesden-Popper materials close to Ni$^{2+}$. The reduced La$_3$Ni$_2$O$_6$ bilayer with a nominal Ni$^{1.5+}$ oxidation state may therefore serve as a normal-state mediator between the two known families of $3d^8$-like and $3d^9$-like superconducting nickelates. Using first-principles many-body theory, we explain its experimental 50\,meV charge gap as originating from a new type of correlated (quasi-)insulator. Flat-band electrons of Ni-$d_{z^2}$ character become localized from scattering with orbital-selective Mott-localized Ni-$d_{x^2-y^2}$ electrons, by trading in residual hopping energy for a gain in local exchange energy in a ferromagnetic Kondo-lattice scenario. Most importantly, the flat-band electrons offer another route to unconventional superconductivity in nickelates at ambient pressure.

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arXiv2025

Low-energy perspective of interacting electrons in the normal state of superconducting bilayer nickelate

Developing a low-energy model is essential for understanding unconventional superconductivity in bilayer nickelate La$_3$Ni$_2$O$_7$. Here, we analyze distinct low-energy scenarios of the normal state by downfolding the ab-initio determined band structure and applying the mean-field regime of rotational-invariant slave-boson theory. We compare models based on the single-site two-orbital, the two-site four-orbital and a proposed minimal cluster (MC) picture. The latter builds up on three adapted orbitals located on the sites of the basic Ni-O-Ni cluster across the bilayer. Intriguing interplay between the Hund coupling $J_{\rm H}$ and the interlayer exchange $J_{\perp}$ is encountered in the multiorbital multi-site problem. While the tendency for interlayer Ni-$d_{z^2}$ singlet formation is pronounced, a complete localization remains hindered by the coupling to the Ni-$d_{x^2-y^2}$ orbitals. The correlation physics in the MC picture is peculiar with respect to the effective interorbital/site exchange.

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arXiv2025

Pressure-enhanced splitting of density wave transitions in La$_3$Ni$_2$O$_{7-δ}$

The observation of superconductivity in La$_3$Ni$_2$O$_{7-δ}$ under pressure, following the suppression of a high-temperature density wave state, has attracted considerable attention. The nature of this density wave order was not clearly identified. Here, we probe the magnetic response of the zero-pressure phase of La$_3$Ni$_2$O$_{7-δ}$ as hydrostatic pressure is applied and find that the apparent single density wave transition at zero applied pressure splits into two. The comparison of our muon-spin rotation and relaxation experiments with dipole-field numerical analysis reveals the magnetic structure's compatibility with a stripe-type arrangement of Ni moments, characterized by alternating lines of magnetic moments and nonmagnetic stripes at ambient pressure. When pressure is applied, the magnetic ordering temperature increases, while the unidentified density wave transition temperature falls. Our findings reveal that the ground state of the La$_3$Ni$_2$O$_{7-δ}$ system is characterized by the coexistence of two distinct orders -- a magnetically ordered spin density wave and a lower-temperature ordering that is most likely a charge density wave -- with a notable pressure-enhanced separation between them.

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arXiv2024

Theory of potential impurity scattering in pressurized superconducting La$_3$Ni$_2$O$_7$

Recently discovered high-T$_c$ superconductivity in pressurized bilayer nickelate La$_3$Ni$_2$O$_7$ (La-327) is believed to be driven by the non-phononic repulsive interaction. Depending on the strength of the interlayer repulsion, the symmetry of the superconducting order parameter is expected to be either $d$-wave or sign-changing bonding-antibonding $s_{\pm}$-wave. Unfortunately, due to the need of high pressure to reach superconducting phase, conventional spectroscopic probes to validate the symmetry of the order parameter are hard to use. Here, we study the effect of the point-like non-magnetic impurities on the superconducting state of La-327 and show that $s_{\pm}$-wave and $d$-wave symmetries show a very different behavior as a function of impurity concentration, which can be studied experimentally by irradiating the La-327 samples by electrons prior applying the pressure. While $d-$wave superconducting state will be conventionally suppressed, the $s_{\pm}$-wave state shows more subtle behavior, depending on the asymmetry between bonding and antibonding subspaces. For the electronic structure, predicted to realize in La-327, the $s_{\pm}-$wave state will be robust against complete suppression and the transition temperature, $T_c$ demonstrates a transition from convex to concave behavior, indicating a crossover from $s_{\pm}$-wave to $s_{++}$-wave symmetry as a function of impurity concentration. We further analyze the sensitivity of the obtained results with respect to the potential electronic structure modification.

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arXiv2024

Electronic instability, layer selectivity and Fermi arcs in La$_3$Ni$_2$O$_7$

Using advanced dynamical mean-field theory on a realistic level we study the normal-state correlated electronic structure of the high-pressure superconductor La$_3$Ni$_2$O$_7$ and compare the features of the conventional bilayer (2222) Ruddelsden-Popper crystal structure with those of a newly-identified monolayer-trilayer (1313) alternation. Both structural cases display Ni-$d_{z^2}$ flat-band character at low-energy, which drives an electronic instability with a wave vector ${\bf q_{\rm I}}=(0.25,0.25,q_z)$ at ambient pressure, in line with recent experimental findings. The 1313 electronic structure exhibits significant layer selectivity, rendering especially the monolayer part to be Mott-critical. At high pressure, this layer selectivity weakens and the 1313 fermiology displays arcs reminiscent to those of high-$T_c$ cuprates. In contrast to dominant inter-site self-energy effects in the latter systems, here the Fermi arcs are the result of the multiorbital and multilayer interplay within a correlated flat-band scenario.

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arXiv2024

Theory of magnetic excitations in multilayer nickelate superconductor La$_{3}$Ni$_{2}$O$_{7}$

Motivated by the recent reports of high-$T_c$ superconductivity in La$_3$Ni$_2$O$_7$ under pressure, we analyzed theoretically the magnetic excitations in the normal and the superconducting state in this compound, which can be measured by inelastic neutron scattering or RIXS. We show that the bilayer structure of the spin response allows to elucidate the role of the interlayer interaction and the nature of the Cooper-pairing in a very efficient way. In particular, we demonstrate the key difference between the potential $s_\pm$ and $d$-wave gaps, proposed recently, by comparing the corresponding response in the even and odd channels of the spin susceptibility. We show that the mostly interlayer driven bonding-antibonding $s_\pm$ Cooper-pairing produces a single large spin resonance peak in the odd channel only near the $X$ point whereas spin resonances in both the odd and the even channel are predicted for the $d$-wave scenario.

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arXiv2023

Electronic correlations and superconducting instability in La$_3$Ni$_2$O$_7$ under high pressure

Motivated by the report of superconductivity in bilayer La$_3$Ni$_2$O$_7$ at high pressure, we examine the interacting electrons in this system. First-principles many-body theory is utilized to study the normal-state electronic properties. Below 100\,K, a multi-orbital non-Fermi liquid state resulting from loss of Ni-ligand coherence within a flat-band dominated low-energy landscape is uncovered. The incoherent low-temperature Fermi surface displays strong mixing between Ni-$d_{z^2}$ and Ni-$d_{x^2-y^2}$ orbital character. In a model-Hamiltonian picture, spin fluctuations originating mostly from the Ni-$d_{z^2}$ orbital give rise to strong tendencies towards a superconducting instability with $B_{1g}$ or $B_{2g}$ order parameter. The dramatic enhancement of $T_{\rm c}$ in pressurized La$_3$Ni$_2$O$_7$ is due to stronger Ni-$d_{z^2}$ correlations compared to those in the infinite-layer nickelates.

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arXiv2023

Quasiparticle approach to the transport in infinite-layer nickelates

The normal-state transport properties of superconducting infinite-layer nickelates are investigated within an interacting three-orbital model. It includes effective Ni-$d_{z^2}$, Ni-$d_{x^2-y^2}$ bands as well as the self-doping band degree of freedom. Thermopower, Hall coefficient and optical conductivity are modelled within a quasiparticle approximation to the electronic states. Qualitative agreement in comparison to experimentally available Hall data is achieved, with notably a temperature-dependent sign change of the Hall coefficient for larger hole doping $x$. The Seebeck coefficient changes from negative to positive in a non-trivial way with $x$, but generally shows only modest temperature dependence. The optical conductivity shows a pronounced Drude response and a prominent peak structure at higher frequencies due to interband transitions. While the quasiparticle picture is surely approximative to low-valence nickelates, it provides enlightening insights into the multiorbital nature of these challenging systems.

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arXiv2022

Feedback of non-local $d_{xy}$ nematicity on the magnetic anisotropy in FeSe

We analyze theoretically the magnetic anisotropy in the nematic phase of FeSe by computing the spin and the orbital susceptibilities from the microscopic multiorbital model. In particular, we take into account both the $xz/yz$ and the recently proposed non-local $xy$ nematic ordering and show that the latter one could play a crucial role in reproducing the experimentally-measured temperature dependence of the magnetic anisotropy. This provides a direct fingerprint of the different nematic scenarios on the magnetic properties of FeSe.

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