arXiv subjects

Niklas Witt

Publications and source records attributed to Niklas Witt.

16 recordsLinked to original sources

arXiv2026

Heterostructuring as Gateway to Electron Doping of Nickelate Superconductors

Despite enormous expenditures in the research field, the electron-doped side of nickelate superconductors remains uncharted territory. Substituting the trivalent rare-earth cations by a tetravalent one hitherto failed. Here, we demonstrate by first-principles calculations a disorder-free route to electron dope Ruddlesden-Popper nickelates. When intercalating wide-band-gap insulating layers such as La$X$O$_3$ ($X$=Al, Ga, Sc) into La$_2$NiO$_4$, the extra (LaO)$^+$ layers act as electron donors, releasing carriers into the Ni-3$d$ orbitals. This electron doping puts La$_2$NiO$_4$:La$_2$AlO$_4$ naturally in the optimal region for $d_{x^2-y^2}$-wave superconductivity with T$_c$ exceeding 50 K. The same concept also allows us to electron dope La$_3$Ni$_2$O$_7$, the superconductor in the limelight.

cond-mat.supr-con↗

arXiv2026

Altermagnons at the metal-insulator transition

By means of slave-boson theory for the Hubbard model on the checkerboard lattice, we calculate dynamical altermagnetic spin susceptibilities from the metallic to the Mott-insulating regime. We track magnon dispersion and lifetime renormalization, allowing us to uncover a crossover from a chirality-selective dissipation of magnon modes to coherent yet strongly deformed chiral magnon branches across the metal insulator transition. Our formalism lends itself to a quantitative description of collective spin dynamics in correlated altermagnets.

cond-mat.str-el↗

arXiv2026

First-principles calculation of coherence length and penetration depth based on density functional theory for superconductors

We develop a first-principles framework for evaluating the fundamental length scales of superconductivity, namely the coherence length $\xi_0$ and the magnetic penetration depth $\lambda_\mathrm{L}$, within superconducting density functional theory (SCDFT). By incorporating finite-momentum Cooper pairs, we formulate a microscopic scheme that enables a consistent and parameter-free determination of $\xi_0$, $\lambda_\mathrm{L}$, and the superconducting transition temperature $T_\mathrm{c}$ on the same theoretical footing. Applying the method to representative elemental superconductors, the A15 compound V$_3$Si, and H$_3$S under high pressure, we obtain results in good agreement with available experimental and reproduce the type-I/type-II classification across all materials studied. The unified access to $\xi_0$ and $\lambda_\mathrm{L}$ further allows us to construct the Uemura plot entirely from first principles, showing that higher-$T_\mathrm{c}$ systems are characterized by the simultaneous realization of strong pairing and large phase stiffness. Our results establish a predictive first-principles route to superconducting length scales and provide a microscopic interpretation of empirical correlations in superconductivity.

cond-mat.supr-con↗

arXiv2025

Tunable Electronic Correlations in 135-Kagome Metals

Kagome metals exhibit rich correlated-electron physics, yet a systematic understanding of the degree of correlation across transition-metal species remains elusive. Using density-functional theory plus multi-orbital slave-spin mean-field theory, we investigate electronic correlations in the Ti-, V-, and Cr-based 135 compounds with Sb and Bi pnictogens. We find that the significantly stronger degree of correlation of the Cr-based materials compared to Ti and V can only be explained through the synergy of two effects: the larger electron filling of the $d$-shell and the reduced characteristic kinetic energy. We put forward that the substitution of Sb with Bi strengthens correlations in all compounds and make the prediction that the-yet-to-be-synthesized CsCr$_3$Bi$_5$ must be the most strongly correlated member of the entire family. These findings provide a quantitative, band-structure-based framework for understanding and predicting correlation strength in Kagome metals.

cond-mat.str-el↗

arXiv2025

Intercalant-induced Kekule ordering and gap opening in quasi-free-standing graphene

We present a comprehensive investigation of the structural and electronic properties of Sn intercalated buffer layers on SiC(0001) using low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS), spot-profile analysis low-energy electron diffraction (SPA-LEED), and density functional theory (DFT) calculations. Sn intercalation effectively decouples the buffer layer, yielding quasi-free-standing monolayer graphene (QFMLG) while introducing local lattice distortions. Bias-dependent STM imaging revealed the coexistence of conventional and Kekule-ordered graphene domains, governed by the underlying Sn(1x1) reconstruction at the SiC interface. The measured STS spectra exhibit good agreement with DFT results. However, achieving homogeneous Sn(1x1) domains remains challenging, apparently, due to strain within the Sn monolayer, which drives the emergence of Kekule distortions and the associated electronic band-gap opening omogeneously in graphene. These findings highlight the crucial role of intercalant homogeneity and strain in tuning graphene`s structural and electronic properties.

cond-mat.mes-hall↗

arXiv2025

Local-moment magnetism in Mn-based pnictides

We report a comprehensive study of electronic-correlation effects in Manganese-based antiferromagnetic pnictides BaMn$_2$Pn$_2$ (Pn=P,As,Sb,Bi). Our density functional theory plus slave-spin mean-field simulations indicate that all the compounds lie on the strong-coupling side of an itinerant-to-localized moment crossover, corresponding to the critical interaction strength for the Mott transition in the high-temperature paramagnetic phase. We also show that the experimental N\'eel temperature of each compound scales with the distance from this crossover.

cond-mat.str-el↗

arXiv2025

Superconductivity governed by Janus-faced fermiology in strained bilayer nickelates

High-temperature superconductivity in pressurized and strained bilayer nickelates (La,Pr)$_3$Ni$_2$O$_7$ has emerged as a new frontier. One of the key unresolved issues concerns the fermiology that underlies superconductivity. On both theoretical and experimental sides, no general consensus has been reached, and conflicting results exist regarding whether the relevant Fermi surface involves a $\gamma$ pocket -- a hole pocket with $d_{z^2}$-orbital character centered at the Brillouin zone corner. Here, we address this issue by unveiling a Janus-faced role of the $\gamma$ pocket in spin-fluctuation-mediated superconductivity. We show that this pocket simultaneously induces dominant pair-breaking and pair-forming channels for the leading $s_\pm$-wave pairing. Consequently, an optimal superconducting transition temperature $T_\mathrm{c}$ is achieved when the $\gamma$ pocket surfaces at the Fermi level, placing the system near a Lifshitz transition. This suggests that superconductivity can emerge, provided the maximum energy level of the $\gamma$ pocket lies sufficiently close to the Fermi level, either from below or above. Our finding not only reconciles two opposing viewpoints on the fermiology, but also naturally explains recent experiments on (La,Pr)$_3$Ni$_2$O$_7$ thin films, including the superconductivity under compressive strain, two conflicting measurements on the Fermi surface, and the dome shape of $T_\mathrm{c}$ as a function of hole doping.

cond-mat.supr-con↗

arXiv2025

Real-Space Switching of Local Moments Driven by Quantum Geometry in Correlated Graphene Heterostructures

Graphene-based multilayer systems serve as versatile platforms for exploring the interplay between electron correlation and topology, thanks to distinctive low-energy bands marked by significant quantum metric and Berry curvature from graphene's Dirac bands. Here, we investigate Mott physics and local spin moments in Dirac bands hybridized with a flat band of localized orbitals in functionalized graphene. Via hybridization control, a topological transition is realized between two symmetry-distinct site-selective Mott states featuring local moments in different Wyckoff positions, with a geometrically enforced metallic state emerging in between. We find that this geometrically controlled real-space switching of local moments and associated metal-insulator physics may be realized through proximity coupling of epitaxial graphene on SiC(0001) with group IV intercalants, where the Mott state faces geometrical obstruction in the large-hybridization limit. Our work shows that chemically functionalized graphene provides a correlated electron platform, very similar to the topological heavy fermions in graphene moir\'e systems but at significantly enhanced characteristic energy scales.

cond-mat.str-el↗

arXiv2024

Electronic correlations in epitaxial graphene: Mott states proximitized to a relativistic electron gas

Graphene, renowned for its exceptional electronic and optical properties as a robust 2D material, traditionally lacks electronic correlation effects. Proximity coupling offers a promising method to endow quantum materials with novel properties. In this study, we achieve such a proximity coupling by intercalating Sn between the buffer layer of graphene on SiC(0001), allowing us to explore the coupling between a correlated 2D electron gas and a Dirac metal. This results in the stabilization of Sn-$\sqrt{3}$ superlattice structures at the interface, which reveal Mott-Hubbard bands, in excellent agreement with both experimental observations and theoretical predictions. Additionally, we found signatures of quasiparticle peaks close to the Fermi energy, in detail depending on the hybridization strength and doping level.

cond-mat.str-el↗

arXiv2024

Non-equilibrium carrier dynamics and band structure of graphene on 2D tin

Intercalation of epitaxial graphene on SiC(0001) with Sn results in a well-ordered Sn $(1\times1)$ structure on the SiC surface with quasi-freestanding graphene on top. While the electronic properties of the individual layers have been studied in the past, emerging phenomena arising from possible inter-layer interactions between the 2D\,Sn layer and graphene remain unexplored. We use time- and angle-resolved photoemission spectroscopy to reveal a surprisingly short-lived non-equilibrium carrier distribution inside the Dirac cone of Sn-intercalated graphene. Further, we find that the graphene $\pi$-band exhibits a transient increase in binding energy that we attribute to charging of the graphene layer with holes. We interpret our results with support from density functional theory calculations of the graphene - 2D\,Sn heterostructure that reveal a substantial hybridization between the graphene $\pi$-bands and Sn states, providing a channel for efficient ultrafast charge transfer between the layers. Our results on the graphene - 2D\,Sn model system are expected to trigger similar investigations on related heterostructures obtained by intercalation of epitaxial graphene. Regarding the huge choice of materials that have been successfully intercalated in the past, we believe that the interlayer interactions revealed in the present work only represent the tip of the iceberg with many fascinating emerging phenomena to be discovered in the near future.

cond-mat.mes-hall↗

arXiv2023

Quenched pair breaking by interlayer correlations as a key to superconductivity in La$_3$Ni$_2$O$_7$

The recent discovery of superconductivity in La$_3$Ni$_2$O$_7$ with $T_\mathrm{c} \simeq 80~\mathrm{K}$ under high pressure opens up a new route to high-$T_\mathrm{c}$ superconductivity. This material realizes a bilayer square lattice model featuring a strong interlayer hybridization unlike many unconventional superconductors. A key question in this regard concerns how electronic correlations driven by the interlayer hybridization affect the low-energy electronic structure and the concomitant superconductivity. Here, we demonstrate using a cluster dynamical mean-field theory that the interlayer electronic correlations (IECs) induce a Lifshitz transition resulting in a change of Fermi surface topology. By solving an appropriate gap equation, we further show that the leading pairing instability, $s \pm$-wave, is enhanced by the IECs. The underlying mechanism is the quenching of a strong ferromagnetic channel, resulting from the Lifshitz transition driven by the IECs. Based on this picture, we provide a possible reason of why superconductivity emerges only under high pressure.

cond-mat.supr-con↗

arXiv2023

Bypassing the lattice BCS-BEC crossover in strongly correlated superconductors: resilient coherence from multiorbital physics

Superconductivity emerges from the spatial coherence of a macroscopic condensate of Cooper pairs. Increasingly strong binding and localization of electrons into these pairs compromises the condensate's phase stiffness, thereby limiting critical temperatures -- a phenomenon known as the BCS-BEC crossover in lattice systems. In this study, we demonstrate enhanced superconductivity in a multiorbital model of alkali-doped fullerides (A$_3$C$_{60}$) that goes beyond the limits of the lattice BCS-BEC crossover. We identify that the interplay of strong correlations and multiorbital effects results in a localized superconducting state characterized by a short coherence length but robust stiffness and a domeless rise in critical temperature with increasing pairing interaction. To derive these insights, we introduce a new theoretical framework allowing us to calculate the fundamental length scales of superconductors, namely the coherence length ($\xi_0$) and the London penetration depth ($\lambda_{\mathrm{L}}$), even in presence of strong electron correlations.

cond-mat.str-el↗

arXiv2023

No superconductivity in Pb$_9$Cu$_1$(PO$_4$)$_6$O found in orbital and spin fluctuation exchange calculations

Finding a material that turns superconducting under ambient conditions has been the goal of over a century of research, and recently Pb$_{10-x}$Cu$_x$(PO$_4$)$_6$O aka LK-99 has been put forward as a possible contestant. In this work, we study the possibility of electronically driven superconductivity in LK-99 also allowing for electron or hole doping. We use an $\textit{ab initio}$ derived two-band model of the Cu $e_g$ orbitals for which we determine interaction values from the constrained random phase approximation (cRPA). For this two-band model we perform calculations in the fluctuation exchange (FLEX) approach to assess the strength of orbital and spin fluctuations. We scan over a broad range of parameters and enforce no magnetic or orbital symmetry breaking. Even under optimized conditions for superconductivity, spin and orbital fluctuations turn out to be too weak for superconductivity anywhere near to room-temperature. We contrast this finding to non-self-consistent RPA, where it is possible to induce spin-singlet $d$-wave superconductivity at $T_{\mathrm{c}}\geq300$ K if the system is put close enough to a magnetic instability.

cond-mat.supr-con↗

arXiv2022

sparse-ir: optimal compression and sparse sampling of many-body propagators

We introduce sparse-ir, a collection of libraries to efficiently handle imaginary-time propagators, a central object in finite-temperature quantum many-body calculations. We leverage two concepts: firstly, the intermediate representation (IR), an optimal compression of the propagator with robust a-priori error estimates, and secondly, sparse sampling, near-optimal grids in imaginary time and imaginary frequency from which the propagator can be reconstructed and on which diagrammatic equations can be solved. IR and sparse sampling are packaged into stand-alone, easy-to-use Python, Julia and Fortran libraries, which can readily be included into existing software. We also include an extensive set of sample codes showcasing the library for typical many-body and ab initio methods.

physics.comp-ph↗

arXiv2022

Doping fingerprints of spin and lattice fluctuations in moiré superlattice systems

Twisted Van der Waals systems offer the unprecedented possibility to tune different states of correlated quantum matter with an external non-invasive electrostatic doping. The nature of the superconducting order presents a recurring open question in this context. In this work, we quantitatively assess the case of spin-fluctuation-mediated pairing for $Γ$-valley twisted transition metal dichalcogenide homobilayers. We self-consistently and dynamically calculate the doping dependent superconducting transition temperature $T_{\mathrm{c}}$ revealing a superconducting dome with a maximal $T_{\mathrm{c}}\approx 0.1-1$ K depending on twist angle. We compare our results with conventional phonon-mediated superconductivity and identify clear fingerprints in the doping dependence of $T_{\mathrm{c}}$, which allow experiments to distinguish between different pairing mechanisms.

cond-mat.supr-con↗

arXiv2021

Efficient fluctuation exchange approach to low-temperature spin fluctuations and superconductivity: from the Hubbard model to Na$_x$CoO$_2\cdot y$H$_2$O

Superconductivity arises mostly at energy and temperature scales that are much smaller than the typical bare electronic energies. Since the computational effort of diagrammatic many-body techniques increases with the number of required Matsubara frequencies and thus with the inverse temperature, phase transitions that occur at low temperatures are typically hard to address numerically. In this work, we implement a fluctuation exchange (FLEX) approach to spin fluctuations and superconductivity using the "intermediate representation basis" (IR) [Shinaoka et al., PRB 96, 2017] for Matsubara Green functions. This FLEX+IR approach is numerically very efficient and enables us to reach temperatures on the order of $10^{-4}$ in units of the electronic band width in multi-orbital systems. After benchmarking the method in the doped repulsive Hubbard model on the square lattice, we study the possibility of spin-fluctuation-mediated superconductivity in the hydrated sodium cobalt material Na$_x$CoO$_2\cdot y$H$_2$O reaching the scale of the experimental transition temperature $T_{\mathrm{c}}=4.5$ K and below.

cond-mat.str-el↗