arXiv subjects

Nico A. Hackner

Publications and source records attributed to Nico A. Hackner.

6 recordsLinked to original sources

arXiv2026

Reentrant superconductivity enabled by spin-orbit coupling: Application to UTe$_2$

Reentrant superconductivity has been understood primarily in terms of the Jaccarino-Peter field-compensation effect or from a change of the strength in the pairing interaction. However, neither mechanism appears able to entirely explain the remarkable phase diagram of UTe$_2$. Here we propose a generic theory of the field-enhancement of opposite-spin Cooper pairings which does not necessitate the coexistence of magnetism or the vicinity of a magnetic quantum critical point. Our analytical treatment shows that the reentrance has its origin in the interplay of the sublattice degrees of freedom and spin-orbit coupling, which can can strikingly enhance opposite-spin Cooper pairings at strong Zeeman fields. Based on these results, we show that a pairing state with B$_{3u}$ symmetry can reproduce the highly anisotropic phase diagram of the reentrant superconducting state of UTe$_2$.

cond-mat.supr-con↗

arXiv2025

Incommensuration in odd-parity antiferromagnets

Inversion-asymmetric antiferromagnets (AFMs) with odd-parity spin-polarization pattern have been proposed as a new venue for spintronics. These AFMs require commensurate ordering to ensure an effective time-reversal symmetry, which guarantees a strictly antisymmetric spin polarization of the electronic states. Recently, nonsymmorphic centrosymmetric crystals have been identified as a broad class of materials which could exhibit unit-cell doubling magnetism with odd-parity spin-polarization. Here we investigate the stability of these states against incommensuration. We first demonstrate that the symmetry conditions which permit a p-wave spin polarization pattern also permit the existence of a non-relativistic Lifshitz invariant in the phenomenological Ginzburg-Landau free energy. This implies magnetism with an incommensurate ordering vector, independent of its microscopic origin. AFMs with f- or h-wave spin-polarization are also prone to incommensurability, especially when they have an itinerant origin. Here the symmetry which ensures the odd-parity spin-polarization also guarantees the existence of van Hove saddle points off the time-reversal-invariant momenta, which promote incommensurate spin fluctuations in quasi-two-dimensional electronic systems. Finally, we study the effect of weak spin-orbit coupling in locally noncentrosymmetric materials and find that it favors antiferromagnetic phases with in-plane magnetic moments. However, the inclusion of the spin-orbit coupling also introduces a new mechanism for driving incommensuration. Our results imply that odd-parity AFMs are likely to be preceded by an incommensurate phase, or emerge directly from the normal state via a first order transition. These conclusions are consistent with the phase diagram of several candidate materials.

cond-mat.mes-hall↗

arXiv2025

Solving the SU($N$) Orbital Hatsugai-Kohmoto Model

We show that the physics of the SU($N$) Hubbard model can be realistically simulated with the recently developed orbital Hatsugai-Kohmoto model. In this approach, the momentum mixing absent from the band Hatsugai-Kohmoto model is included by grouping $n$-Hubbard atoms into a cluster. We take advantage of the rapid convergence of this scheme ($1/n^2$) and show that for $n\approx O(10)$, quantitative agreement with the determinantal quantum Monte Carlo method arises for the double occupancy as well as qualitative agreement for the filling and compressibility across the Mott transition. Additional features of this work are that we can obtain low-temperature physics, which could be qualitatively different from its high-temperature counterpart, and dynamical quantities without resorting to analytical continuation, thereby establishing the orbital Hatsugai-Kohmoto model as a useful alternative perspective for studying strong correlations in SU($N$) systems.

cond-mat.str-el↗

arXiv2024

Twisting the Hubbard model into the Momentum-Mixing Hatsugai-Kohmoto Model

The Hubbard model is a standard theoretical tool for studying materials with strong electron-electron interactions, such as the cuprate superconductors. Unfortunately, interaction-driven phenomena such as the transition into the strongly correlated Mott insulator phase are difficult to treat with established theoretical techniques. However, the exactly solvable Hatsugai-Kohmoto model displays similar Mott physics. Here we show how the Hatsugai-Kohmoto model can be deformed continuously into the Hubbard model. The trick is to systematically re-introduce all the momentum mixing the original Hatsugai-Kohmoto model omits. This can be accomplished by grouping $n$-momenta into a cell and hybridizing them resulting in the momentum-mixing Hatsugai-Kohmoto (MMHK) model. We recover the Bethe ansatz ground state energy of the one-dimensional Hubbard model to within 1$\%$ from only ten mixed momenta. Overall the convergence scales as $1/n^2$ as opposed to the inverse linear behaviour of standard finite-cluster techniques. Our results for a square lattice reproduce all known features from state-of-the-art simulations also with only a few mixed momenta. Consequently, we believe the MMHK model offers an alternative tool for strongly correlated quantum matter.

cond-mat.str-el↗

arXiv2023

Bardasis-Schrieffer-like phase mode in a superconducting bilayer

We theoretically study the low-lying collective modes of an even-parity spin-singlet superconducting bilayer, where strong spin-orbit coupling leads to a closely competing odd-parity pairing state. We develop a gauge-invariant theory for the coupling of phase fluctuations to an external electromagnetic field and show that the competing odd-parity pairing instability gives rise to a Bardasis-Schrieffer-like phase mode within the excitation gap. Accounting for the long-range Coulomb interaction, however, we find that this mode is converted into an antisymmetric plasmon and is likely pushed into the quasiparticle continuum.

cond-mat.supr-con↗

arXiv2023

Bound states around impurities in a superconducting bilayer

We theoretically study the appearance of bound states around impurities in a superconducting bilayer. We focus our attention on $s$-wave pairing, which includes unconventional odd-parity states permitted by the layer degree of freedom. Utilizing numerical mean-field and analytical $T$-matrix methods, we survey the bound state spectrum produced by momentum-independent impurity potentials in this model. For even-parity $s$-wave pairing bound states are only found for impurities which break time-reversal symmetry. For odd-parity $s$-wave states, in contrast, bound states are generically found for all impurity potentials, and fall into six distinct categories. This categorization remains valid for nodal gaps. Our results are conveniently understood in terms of the ``superconducting fitness'' concept, and show an interplay between the pair-breaking effects of the impurity and the normal-state band structure.

cond-mat.supr-con↗