Unconventional Cooper Pairing Research Articles

Pair density wave state in a monolayer high-Tc iron-based superconductor.

The pair density wave (PDW) is an extraordinary superconducting state in which Cooper pairs carry non-zero momentum1,2. Evidence for the existence of intrinsic PDW order in high-temperature (high-Tc) cuprate superconductors3,4 and kagome superconductors5 has emerged recently. However, the PDW order in iron-based high-Tc superconductors has not been observed experimentally. Here, using scanning tunnelling microscopy and spectroscopy, we report the discovery of the PDW state in monolayer iron-based high-Tc Fe(Te,Se) films grown on SrTiO3(001) substrates. The PDW state with a period of λ ≈ 3.6aFe (aFe is the distance between neighbouring Fe atoms) is observed at the domain walls by the spatial electronic modulations of the local density of states, the superconducting gap and the π-phase shift boundaries of the PDW around the vortices of the intertwined charge density wave order. The discovery of the PDW state in the monolayer Fe(Te,Se) film provides a low-dimensional platform to study the interplay between the correlated electronic states and unconventional Cooper pairing in high-Tc superconductors.

Quantum-critical scale invariance in a transition metal alloy

Quantum-mechanical fluctuations between competing phases induce exotic collective excitations that exhibit anomalous behavior in transport and thermodynamic properties, and are often intimately linked to the appearance of unconventional Cooper pairing. High-temperature superconductivity, however, makes it difficult to assess the role of quantum-critical fluctuations in shaping anomalous finite-temperature physical properties. Here we report temperature-field scale invariance of non-Fermi liquid thermodynamic, transport, and Hall quantities in a non-superconducting iron-pnictide, Ba(Fe1/3Co1/3Ni1/3)2As2, indicative of quantum criticality at zero temperature and applied magnetic field. Beyond a linear-in-temperature resistivity, the hallmark signature of strong quasiparticle scattering, we find a scattering rate that obeys a universal scaling relation between temperature and applied magnetic fields down to the lowest energy scales. Together with the dominance of hole-like carriers close to the zero-temperature and zero-field limits, the scale invariance, isotropic field response, and lack of applied pressure sensitivity suggests a unique quantum critical system unhindered by a pairing instability.

Polar Nanodomains in a Ferroelectric Superconductor.

The mechanisms by which itinerant carriers compete with polar crystal distortions are a key unresolved issue for polar superconductors, which offer new routes to unconventional Cooper pairing. Strained, doped SrTiO3 films undergo successive ferroelectric and superconducting transitions, making them ideal candidates to elucidate the nature of this competition. Here, we reveal these interactions using scanning transmission electron microscopy studies of the evolution of polar nanodomains as a function of doping. These nanodomains are a precursor to the ferroelectric phase and a measure of long-range Coulomb interactions. With increasing doping, the magnitude of the polar displacements, the nanodomain size, and the Curie temperature are systematically suppressed. In addition, we show that disorder caused by the dopant atoms themselves presents a second contribution to the destabilization of the ferroelectric state. The results provide evidence for two distinct mechanisms that suppress the polar transition with doping in a ferroelectric superconductor.

Superconducting Interfaces between Weyl Semimetal and Normal Metal

AbstractInterfaces between materials with different crystal structures and electronic properties offer an approach for creating and controlling novel quantum states of matter, in which inversion symmetry breaking and other effects at the boundaries may introduce additional electronic states. Here the observation of superconductivity at ≈3 K at the interface of the Weyl semimetal TaAs and the normal metal Ag is reported, as evidenced by soft‐point‐contact spectroscopy and the standard four‐probe resistance measurements. This is different from the previous reports on interface superconductivity based on the materials that are known to be superconducting in bulk; here both TaAs and Ag are not superconducting even via chemical doping or application of high pressure. The topologically nontrivial band structure of TaAs is considered to play a crucial role in this system and an unconventional Cooper pairing would be expected. This finding opens up a new route for the realization and study of the possible topological superconductivity.

Unusual Superconducting Proximity Effect in Magnetically Doped Topological Josephson Junctions

The transport properties of a topological Josephson junction fabricated from a magnetically doped topological insulator (TI) were investigated. The conductance spectra of the Nb/Fe-Bi$_2$Te$_2$Se/Nb junction below 1 K showed an unusual trident-shaped zero-bias conductance peak with a tiny peak width of $\sim$ 6 $\mu$V. The central peak of the trident peak presents the dc-Josephson current, and the side peaks may reflect an induced unconventional Cooper pairing. Additionally, the critical currents followed inverse to temperature, which may also reflect the presence of an unconventional proximity effect. Furthermore, microwave irradiation derived a drastic change in the conductance spectra from the peak structure into oscillatory ones, a hallmark of the ac-Josephson supercurrent. The current-phase relation of the ac-Josephson effect under high power radiofrequency-irradiation was found to be 4$\pi$-periodic. The results suggest that the junction based on magnetically doped 3D TIs may realize an unconventional Cooper pairing, thus enabling access to the basic physics of Majorana bound states and unconventional superconductivity.

Quantum Phase Transition of Correlated Iron-Based Superconductivity in LiFe1−xCoxAs

The interplay between unconventional Cooper pairing and quantum states associated with atomic scale defects is a frontier of research with many open questions. So far, only a few of the high-temperature superconductors allow this intricate physics to be studied in a widely tunable way. We use scanning tunneling microscopy to image the electronic impact of Co atoms on the ground state of the LiFe_{1-x}Co_{x}As system. We observe that impurities progressively suppress the global superconducting gap and introduce low energy states near the gap edge, with the superconductivity remaining in the strong-coupling limit. Unexpectedly, the fully opened gap evolves into a nodal state before the Cooper pair coherence is fully destroyed. Our systematic theoretical analysis shows that these new observations can be quantitatively understood by the nonmagnetic Born-limit scattering effect in an s±-wave superconductor, unveiling the driving force of the superconductor to metal quantum phase transition.

Nematic and superconducting order in a three‐band model for the iron superconductors

AbstractWe investigate collective ordering tendencies in a three‐band model for iron superconductors, driven by intra‐ and inter‐orbital local interactions, by means of a renormalization group (RG) approach. Our study extends earlier fundamental RG studies by Chubukov et al. [Phys. Rev. B 78, 134512 (2008)] by allowing the flowing interactions to be orbital‐dependent. While for small inter‐orbital repulsion , we retrieve the previous picture of a constructive interplay between antiferromagnetic spin‐density wave ordering and sign‐changing s‐wave pairing, for larger , ’“nematic” orbital ordering becomes strong and changes the groundstate of the undoped or slightly doped system qualitatively. At the transition between the two regimes, both sign‐changing pairing and sign‐changing ‐nematic are most pronounced. Our results suggest a much richer range of possibilities for the ground states of iron superconductors and related materials and shows how nematic order can act as an additional source for unconventional Cooper pairing. For the strongly electron‐doped case without hole pockets we also find a novel pairing that has no sign‐change as function of the wavevector but changes sign between the and the orbitals.

Polar Phase of Superfluid (3)He in Anisotropic Aerogel.

We report the first observation of the polar phase of superfluid (3)He. This phase appears in (3)He confined in a new type of aerogel with a nearly parallel arrangement of strands which play the role of ordered impurities. Our experiments qualitatively agree with theoretical predictions and suggest that in other systems with unconventional Cooper pairing (e.g., in unconventional superconductors) similar phenomena may be found in the presence of anisotropic impurities.

Unconventional Cooper pairing results in a pseudogap-like phase in s-wave superconductors

The impact of disorder on the superconducting (SC) pairing mechanism is the centre of much debate. Some evidence suggests a loss of phase coherence of pairs while others point towards the formation of a competing phase. In our work we show that the two perspectives may be different sides of the same coin. Using an extension of the perturbative renormalization group approach we compare the impact of different disorder-induced interactions on a SC ground state. We find that in the strongly disordered regime an interaction between paired fermions and their respective disordered environment replaces conventional Cooper pairing. For these unconventional Cooper pairs the phase coherence condition, required for the formation of a SC condensate, is not satisfied.

Evidence for spin-triplet odd-parity superconductivity close to type-II van Hove singularities

Searching for unconventional Cooper pairing states has been at the heart of superconductivity research since the discovery of BCS superconductors. In particular, spin-triplet odd-parity pairing states were recently revisited due to the possibility of tuning towards topological superconductors. In this context, it is interesting to note a recent proposal that such a spin-triplet pairing instability occurs when the band filling is near van Hove singularities (vHS) associated with momenta away from time-reversal invariant momenta named type-II vHS. However, this result was obtained within a weak coupling renormalization group with Fermi surface patch approximation. To explore superconducting instabilities beyond this weak coupling Fermi surface patch approximation, we perform systematic study on Hubbard model in a two-dimensional square lattice using three different methods: random phase approximation, large-scale dynamical mean field theory simulations with continuous time quantum Monte Carlo (CTQMC) impurity solver, and large-scale dynamical cluster simulations with CTQMC cluster solver. We find, in a wide doping range centered around the type-II van Hove filling, a two-fold degenerate, spin-triplet, odd-parity $p$-wave pairing state emerges due to repulsive interaction, when the Fermi surface is not sufficiently nested. Possible relevance of our findings to the recently discovered superconductors LaO$_{1-x}$F$_{x}$BiS$_{2}$, Ir$_{1-x}$Pt$_{x}$Te$_{2}$ and proposed doped BC$_{3}$ are also discussed.

Field-angle resolved flux-flow resistivity as a phase-sensitive probe of unconventional Cooper pairing

We theoretically investigate the applied magnetic field-angle dependence of the flux-flow resistivity $\rho_{\rm f}(\alpha_{\rm M})$ for an uniaxially anisotropic Fermi surface. $\rho_{\rm f}$ is related to the quasiparticle scattering rate $\varGamma$ inside a vortex core, which reflects the sign change in the superconducting pair potential. We find that $\rho_{\rm f}(\alpha_{\rm M})$ is sensitive to the sign-change in the pair potential and has its maximum when the magnetic field is parallel to the gap-node direction. We propose the measurement of the field-angle dependent oscillation of $\rho_{\rm f}(\alpha_{\rm M})$ as a phase-sensitive field-angle resolved experiment.

Effect of correlated disorder on the temperature of unconventional cooper pairing: 3He in aerogel

Using liquid 3He in aerogel as an example, it is shown that correlations in positions of impurities affect the temperature $T_c$ of transition of Fermi liquid in an unconventional superfluid or superconductive state. The effect is significant if the correlation length for impurities is greater than the coherence length in the superfluid or superconductive state $\xi_0$. For 3He in aerogel the suppression of $T_c$ is expressed in terms of the structure factor of aerogel. With the account of the fractal structure of aerogel a simple expression is obtained for the decrease of $T_c$ from its clean value. This expression is in a satisfactory agreement with experimental data.

Unconventional Cooper pairing in the superconducting state ofUPd2Al3

A possible type of Cooper pairing in an anisotropic superconducting state of ${\mathrm{UPd}}_{2}{\mathrm{Al}}_{3}$ is discussed on the basis of the recent measurement of magnetic exciton modes which are expected to mediate the pairing interaction. The most likely gap is one with ${A}_{\mathrm{g}}$ symmetry with the line node on the plane very close to the zone boundary of the folded Brillouin zone in the antiferromagnetically ordered state.

Electron-hole system in one dimension

A high-density electron-hole system in one dimension is studied theoretically. Competition between the exciton Bose condensation and other Fermi-surface instabilities is clarified within a framework of the two-band g-ology. There is a parameter region where unconventional interband Cooper pairing with nonzero total momentum occurs. Optical spectra yield the Fermi-edge singularity resulting from the exciton condensation. Effects of backward scattering and random potential are also discussed.