arXiv subjects

Eirik Erlandsen

Publications and source records attributed to Eirik Erlandsen.

8 recordsLinked to original sources

arXiv2022

Going beyond the Chandrasekhar-Clogston limit in a flat-band superconductor

The Chandrasekhar-Clogston limit normally places stringent conditions on the magnitude of the magnetic field that can coexist with spin-singlet superconductivity, restricting the critical induced Zeeman shift to a fraction of the superconducting gap. Here, we consider a model system where the spin-singlet Cooper pairing in a dispersive band crossing the Fermi level is boosted by an additional flat-band located away from the Fermi level. The boosting of the pairing in the dispersive band allows for nontrivial solutions to the coupled gap equations for spin-splitting fields considerably larger than the superconducting gaps at zero field. Further, the additional Cooper pairing in the flat-band, away from the Fermi level, increases the superconducting condensation energy without affecting the paramagnetic susceptibility of the system, making the free energy favor the superconducting state. This opens up the possibility for spin-singlet superconductivity beyond the standard Chandrasekhar-Clogston limit.

cond-mat.supr-con↗

arXiv2022

Magnon drag in a metal-insulating antiferromagnet bilayer

We study a bilayer structure consisting of an antiferromagnetic insulator and a normal metal. An electron current is driven in the normal metal with direction parallel to the interface between the materials. Due to interfacial exchange coupling between the localized spins in the antiferromagnet and the itinerant electrons in the normal metal, a magnon current can then be induced in the antiferromagnet. Using an uncompensated antiferromagnetic interface, creating an asymmetry in the interfacial coupling to the two degenerate magnon modes, we find that it is possible to generate a magnon spin-current. The magnon spin-current can be enhanced by increasing the temperature or by spin-splitting the magnon modes.

cond-mat.mes-hall↗

arXiv2021

The effect of midgap states on the magnetic exchange interaction mediated by a $d$-wave superconductor

We theoretically study the indirect interaction between two ferromagnetic contacts located on the surface of a $d$-wave superconductor. When the magnets are connected to a $\{010\}$ edge of the superconductor we find an oscillating RKKY interaction that varies in sign as the distance between the magnetic contacts is varied. However, when coupling the magnets to an $\{110\}$ edge of the superconductor, we find that the presence of midgap states qualitatively changes the results. The ground state of the system is then found to always favor alignment of the magnets as this configuration most strongly suppresses the midgap states, leading to a larger condensation energy which dominates over the intrinsic RKKY interaction.

cond-mat.supr-con↗

arXiv2021

Eliashberg study of superconductivity induced by interfacial coupling to antiferromagnets

We perform Eliashberg calculations for magnon-mediated superconductivity in a normal metal, where the electron-magnon interaction arises from interfacial coupling to antiferromagnetic insulators. In agreement with previous studies, we find $p$-wave pairing for large doping when the antiferromagnetic interfaces are uncompensated, and $d$-wave pairing close to half-filling when the antiferromagnetic interfaces are compensated. However, for the $p$-wave phase, we find a considerable reduction in the critical temperature compared to previous weak-coupling results, as the effective frequency cutoff on the magnon propagator in this case is found to be much smaller than the cutoff on the magnon spectrum. The $d$-wave phase, on the other hand, relies less on long-wavelength magnons, leading to a larger effective cutoff on the magnon propagator. Combined with a large density of states close to half-filling, this might allow the $d$-wave phase to survive up to higher critical temperatures. Based on our findings, we provide new insight into how to realize interfacially induced magnon-mediated superconductivity in experiments.

cond-mat.supr-con↗

arXiv2020

Schwinger boson study of superconductivity mediated by antiferromagnetic spin-fluctuations

We study superconductivity in a normal metal, arising from effective electron-electron interactions mediated by spin-fluctuations in a neighboring antiferromagnetic insulator. Introducing a frustrating next-nearest neighbor interaction in a Néel antiferromagnet with an uncompensated interface, the superconducting critical temperature is found to be enhanced as the frustration is increased. Further, for sufficiently large next-nearest neighbor interaction, the antiferromagnet is driven into a stripe phase, which can also give rise to attractive electron-electron interactions. For the stripe phase, as previously reported for the Néel phase, the superconducting critical temperature is found to be amplified for an uncompensated interface where the normal metal conduction electrons are coupled to only one of the two sublattices of the magnet. The superconducting critical temperature arising from fluctuations in the stripe phase antiferromagnet can be further enhanced by approaching the transition back to the Néel phase.

cond-mat.supr-con↗

arXiv2020

Magnon-mediated superconductivity on the surface of a topological insulator

We study superconductivity on the surface of a topological insulator, mediated by magnetic fluctuations in an adjacent ferromagnetic or antiferromagnetic insulator. Superconductivity can arise from effective interactions between helical fermions induced by interfacial fermion-magnon interactions. For both ferromagnetic and antiferromagnetic insulators, these fermion-fermion interactions have the correct structure to facilitate pairing between particles located on the same side of the Fermi surface, also known as Amperean pairing. In antiferromagnets, the strength of the induced interactions can be enhanced by coupling the topological insulator asymmetrically to the two sublattices of the antiferromagnet. This effect is further amplified by next nearest neighbor frustration in the antiferromagnetic insulator. The enhancement makes the induced interactions significantly stronger in the antiferromagnetic case, as compared to the ferromagnetic case. These results indicate that an uncompensated antiferromagnetic interface might be a better candidate than a ferromagnetic interface for proximity-induced magnon-mediated superconductivity on the surface of a topological insulator.

cond-mat.supr-con↗

arXiv2019

Enhancement of superconductivity mediated by antiferromagnetic squeezed magnons

We investigate theoretically magnon-mediated superconductivity in a heterostructure consisting of a normal metal and a two-sublattice antiferromagnetic insulator. The attractive electron-electron pairing interaction is caused by an interfacial exchange coupling with magnons residing in the antiferromagnet, resulting in p-wave, spin-triplet superconductivity in the normal metal. Our main finding is that one may significantly enhance the superconducting critical temperature by coupling the normal metal to only one of the two antiferromagnetic sublattices employing, for example, an uncompensated interface. Employing realistic material parameters, the critical temperature increases from vanishingly small values to values significantly larger than 1 K as the interfacial coupling becomes strongly sublattice-asymmetric. We provide a general physical picture of this enhancement mechanism based on the notion of squeezed bosonic eigenmodes.

cond-mat.supr-con↗

arXiv2018

Superfluid Drag in Multicomponent Bose-Einstein Condensates on a Square Optical Lattice

The superfluid drag-coefficient of a weakly interacting three-component Bose-Einstein condensate is computed deep into the superfluid phase, starting from a Bose-Hubbard model with component-conserving, on-site interactions and nearest-neighbor hopping. Rayleigh-Schrödinger perturbation theory is employed to provide an analytic expression for the drag density. In addition, the Hamiltonian is diagonalized numerically to compute the drag within mean-field theory at both zero and finite temperatures to all orders in inter-component interactions. Moreover, path integral Monte Carlo simulations have been performed to support the mean-field results. In the two-component case the drag increases monotonically with the magnitude of the inter-component interaction $γ_{AB}$ between the two components A and B. This no longer holds when an additional third component C is included. Instead of increasing monotonically, the drag can either be strengthened or weakened depending on the details of the interaction strengths, for weak and moderately strong interactions. The general picture is that the drag-coefficient between component A and B is a non-monotonic function of the inter-component interaction strength $γ_{AC}$ between A and a third component C. For weak $γ_{AC}$ compared to the direct interaction $γ_{AB}$ between A and B, the drag-coefficient between A and B can {\it decrease}, contrary to what one naively would expect. When $γ_{AC}$ is strong compared to $γ_{AB}$, the drag between A and B increases with increasing $γ_{AC}$, as one would naively expect. We attribute the subtle reduction of $ρ_{d,AB}$ with increasing $γ_{AC}$, which has no counterpart in the two-component case, to a renormalization of the inter-component scattering vertex $γ_{AB}$ via intermediate excited states of the third condensate $C$.

cond-mat.quant-gas↗