Triplet Pairing Superconductivity Research Articles

Light-induced switching between singlet and triplet superconducting states

While the search for topological triplet-pairing superconductivity has remained a challenge, recent developments in optically stabilizing metastable superconducting states suggest a new route to realizing this elusive phase. Here, we devise a testable theory of competing superconducting orders that permits ultrafast switching to an opposite-parity superconducting phase in centrosymmetric crystals with strong spin-orbit coupling. Using both microscopic and phenomenological models, we show that dynamical inversion symmetry breaking with a tailored light pulse can induce odd-parity (spin triplet) order parameter oscillations in a conventional even-parity (spin singlet) superconductor, which when driven strongly can send the system to a competing minimum in its free energy landscape. Our results provide new guiding principles for engineering unconventional electronic phases using light, suggesting a fundamentally non-equilibrium route toward realizing topological superconductivity.

Evidence for nonunitary triplet-pairing superconductivity in noncentrosymmetric TaRuSi and comparison with isostructural TaReSi

Evidence for nonunitary triplet-pairing superconductivity in noncentrosymmetric TaRuSi and comparison with isostructural TaReSi

Signatures of superconducting triplet pairing in Ni–Ga-bilayer junctions

Ni–Ga bilayers are a versatile platform for exploring the competition between strongly antagonistic ferromagnetic and superconducting phases. We characterize the impact of this competition on the transport properties of highly-ballistic Al/Al2O3(/EuS)/Ni–Ga tunnel junctions from both experimental and theoretical points of view. While the conductance spectra of junctions comprising Ni (3 nm)–Ga (60 nm) bilayers can be well understood within the framework of earlier results, which associate the emerging main conductance maxima with the junction films’ superconducting gaps, thinner Ni (1.6 nm)–Ga (30 nm) bilayers entail completely different physics, and give rise to novel large-bias (when compared to the superconducting gap of the thin Al film as a reference) conductance-peak subseries that we term conductance shoulders. These conductance shoulders might attract considerable attention also in similar magnetic superconducting bilayer junctions, as we predict them to offer an experimentally well-accessible transport signature of superconducting triplet pairings that are induced around the interface of the Ni–Ga bilayer. We further substantiate this claim performing complementary polarized neutron reflectometry measurements on the bilayers, from which we deduce (1) a nonuniform magnetization structure in Ga in a several nanometer-thick area around the Ni–Ga boundary and can simultaneously (2) satisfactorily fit the obtained data only considering the paramagnetic Meissner response scenario. While the latter provides independent experimental evidence of induced triplet superconductivity inside the Ni–Ga bilayer, the former might serve as the first experimental hint of its potential microscopic physical origin. Finally, we introduce a simple phenomenological toy model to confirm also from the theoretical standpoint that superconducting triplet pairings around the Ni–Ga interface can indeed lead to the experimentally observed conductance shoulders, which convinces that our claims are robust and physically justified. Arranging our work in a broader context, we expect that Ni–Ga-bilayer junctions could have a strong potential for future superconducting-spintronics applications whenever an efficient engineering of triplet-pairing superconductivity is required.

Correlation-Induced Triplet Pairing Superconductivity in Graphene-Based Moiré Systems.

Motivated by the possible non-spin-singlet superconductivity in the magic-angle twisted trilayer graphene experiment, we investigate the triplet-pairing superconductivity arising from a correlation-induced spin-fermion model of Dirac fermions with spin, valley, and sublattice degrees of freedom. We find that the f-wave pairing is favored due to the valley-sublattice structure, and the superconducting state is time-reversal symmetric, fully gapped, and nontopological. With a small in-plane magnetic field, the superconducting state becomes partially polarized, and the transition temperature can be slightly enhanced. Our results apply qualitatively to Dirac fermions for the triplet-pairing superconductivity in graphene-based moiré systems, which is fundamentally distinct from triplet superconductivity in ^{3}He and ferromagnetic superconductors.

Effect of pressure on structural and electronic properties of the noncentrosymmetric superconductor Rh2Mo3N

Noncentrosymmetric molybdenum nitride $\mathrm{R}{\mathrm{h}}_{2}\mathrm{M}{\mathrm{o}}_{3}\mathrm{N}$ is a conventional $s$-wave superconductor at ambient pressure. Here we report the evolution of structural and electronic properties of $\mathrm{R}{\mathrm{h}}_{2}\mathrm{M}{\mathrm{o}}_{3}\mathrm{N}$ under high pressure. X-ray diffraction measurements reveal the stability of pristine \ensuremath{\beta}-Mn-type cubic structure up to 47.9 GPa, accompanied by the continuously enhanced distortion of $\mathrm{M}{\mathrm{o}}_{6}\mathrm{N}$ octahedra. Electronic transport experiments show the robustness of superconductivity under pressure, i.e., the superconducting transition temperature ${T}_{\mathrm{c}}$ varies only \ensuremath{\sim}0.7 K upon compression up to 38.5 GPa. Meanwhile, detailed analyses of both structural and transport results consistently reveal a critical pressure of \ensuremath{\sim}14.0 GPa. In agreement with the enhancement of octahedral distortion, the temperature dependence of upper critical magnetic field accords with a $p$-wave model at 22.0 GPa and an $s$-wave model at 6.0 GPa. These results suggest the emergence of novel superconductivity in the distorted $\mathrm{R}{\mathrm{h}}_{2}\mathrm{M}{\mathrm{o}}_{3}\mathrm{N}$ cubic phase under pressure, which could be attributed to the enhanced strength of antisymmetric spin-orbit coupling (ASOC) via pressure-modified local octahedral distortion. Our findings may shed light on the searching and understanding of unique triplet-pairing superconductivity in other inversion-broken superconductors with strong ASOC.

What can Andreev bound states tell us about superconductors?

Zero-energy Andreev bound states, which manifest themselves in the tunnelling spectra as zero-bias conductance peaks (ZBCPs), are abundant at interfaces between superconductors and other materials and on the nodal surface of high-temperature superconductors. In this review, we focus on the information such excitations can provide on the properties of superconductor systems. First, a general introduction to the physics of Andreev bound states in superconductor/normal metal interfaces is given with a particular emphasis on why they appear at zero energy in d-wave superconductors. Then, specific spectroscopic tunnelling studies of thin films, bilayers and junctions are described, focusing on the corresponding ZBCP features. Scanning tunnelling spectroscopy (STS) studies show that the ZBCPs on the c-axis YBa2Cu3O7-δ (YBCO) films are correlated with the surface morphology and appear only in proximity to (110) facets. STS on c-axis La1.88Sr0.12CuO4 (LSCO) films exhibiting the 1/8 anomaly shows spatially modulated peaks near zero bias associated with the anti-phase ordering of the d-wave order parameter predicted at this doping level. ZBCPs were also found in micrometre-size edge junctions of YBCO/SrRuO3/YBCO, where SrRuO3 is ferromagnetic. Here, the results are consistent with a crossed Andreev reflection effect (CARE) at the narrow domain walls of the SrRuO3 ZBCPs measured in STS studies of manganite/cuprate bilayers could not be attributed to CARE because the manganite's domain wall is much larger than the coherence length in YBCO, and instead are attributed to proximity-induced triplet-pairing superconductivity with non-conventional symmetry. And finally, ZBCPs found in junctions of non-intentionally doped topological insulator films of Bi2Se3 and the s-wave superconductor NbN are attributed to proximity-induced px + ipy triplet order parameter in the topological material.This article is part of the theme issue 'Andreev bound states'.

Ferromagnetism and superconductivity with possiblep+ippairing symmetry in partially hydrogenated graphene

By means of first-principles calculations, we predict two new types of partially hydrogenated graphene systems: C$_{6}$H$_{1}$ and C$_{6}$H$_{5}$, which are shown to be ferromagnetic (FM) semimetal and FM narrow-gap semiconductor, respectively. When properly doped, the Fermi surface of the two systems consists of an electron pocket or six hole patches in the first Brillouin zone with completely spin-polarized charge carries. If superconductivity exists in these systems, the stable pairing symmetries are shown to be $p+ip$ for electron doped case, and anisotropic $p+ip$ for hole doped case. The predicted systems may provide fascinating platforms for studying the novel properties of ferromagnetism and triplet-pairing superconductivity as well as two-dimensional spintronics.

Coexistence of Ferromagnetism with Spin Triplet Superconductivity

The experimental results for ZrZn2, URhGe, and in some pressure ranges also for UGe2, showed that the ferromagnetic superconductors are weak itinerant ferromagnets. Guided by these results we describe the phenomenon of coexistence between equal spin triplet pairing superconductivity and ferromagnetism using the extended Stoner model, which includes in Hamiltonian the on-site Coulomb interaction, U , and occupation dependent hopping integral. We use the Hartree–Fock approximation and the Green functions technique. In the Hartree–Fock approximation the on-site Coulomb interaction plays the role of the on-site exchange (Hund’s) field. All inter-site

Study on the competition between density waves, singlet, and triplet pairing superconductivity in organic conductors (TMTSF) 2X

We study the effect of dimerization of TMTSF molecules and the effect of magnetic field (Zeeman splitting) on the phase competition in quasi one-dimensional organic superconductors (TMTSF) 2X by applying the random phase approximation method. As for the dimerization effect, we conclude that due to the decrease of the dimerization, which corresponds to applying the pressure and cooling, spin and charge density wave states are suppressed and give way to a superconducting state. As for the magnetic field effect, we find generally that spin-triplet pairing mediated by a coexistence of 2 k F spin and 2 k F charge fluctuations can be strongly enhanced by applying magnetic field rather than triplet pairing due to a ferromagnetic spin fluctuations. Applying the above idea to (TMTSF) 2X compounds, a magnetic field induced singlet–triplet transition is consistent with above mechanism in (TMTSF) 2ClO 4.

Study on the competition between density waves, singlet, and triplet pairing superconductivity in an organic conductor [formula omitted

We study the effect of the dimerization of molecules and the effect of magnetic field on the phase competition in quasi-one-dimensional organic superconductors ( TMTSF ) 2 X by applying the random phase approximation method. We conclude the following: (i) Due to the decrease of the dimerization, which corresponds to applying the pressure and cooling, spin and charge density wave states are suppressed and give way to a superconducting state. (ii) Applying the magnetic field favors the spin triplet f-wave pairing with spins parallel to the magnetic field due to the cooperation between the magnetic field and the long range off-site interactions.

NMR study of novel heavy fermion superconductor [formula omitted

Si 29 NMR experiments were performed to study microscopically the normal and superconducting states in Si 29 enriched CePt 3 Si . A Si 29 Knight shift parallel to the c-axis did not decrease below the T c . The 1 / T 1 result taken with a field cycling method showed no distinct coherence peak just below T c and a steep decrease below T c on cooling. The estimated value of the superconducting energy gap was about 2 Δ = 3.6 k B T c . These results may be an evidence for triplet pairing superconductivity.

Magnon Exchange Mechanism of Superconductivity in a Generalized Kondo Lattice Model

Magnon exchange mechanism of superconductivity is studied on the basis of the spin fermion model or a generalized Kondo lattice model, where the antiferromagnetic long range or short range order exists in the localized spins. We obtain the effective interaction and the gap equation taking into account the two magnon exchange interaction. The gap equation is solved just below the transition temperature in the quasi-two-dimensional cases. As a result, it is found that the triplet pairing superconductivity is favored for dilute fermion densities, while the singlet pairing is favored for high densities. The transition temperature is found to be very low for realistic parameters. The possibility that the superconductivity by the magnon exchange interaction is observed experimentally is discussed.

Weak-localization, near-magnetism, and triplet-pairing superconductivity in three dimensions.

In three-dimensional nearly magnetic Fermi liquids, disorder due to impurity scattering is shown to enhance the paramagnon strength and to weaken the triplet superconductivity pair-breaking parameter (through weak-localization quantum effects). As a result (a) ``heavy fermion'' superconductors are good candidates to exhibit triplet pairing and (b) normal liquid $^{3}\mathrm{He}$ can be considered as both nearly magnetic and nearly localized.

Possible diminution of impurity pair breaking for triplet pairing superconductivity in two-dimensional or quasi-two-dimensional, weakly localized, nearly magnetic systems

We propose a conjecture according to which, as a consequence of weak localization in two-dimensional, nearly magnetic itinerant paramagnets, the pair-breaking parameter due to normal impurity scattering, in triplet pairing superconductivity, may be reduced at low enough temperature. It might then, in principle, become easier to observe triplet pairing superconductivity in dirty two-dimensional or quasi-two-dimensional metals, than in three-dimensional ones; thus some recently observed puzzling superconductive behaviors should be reexamined.

Spin fluctuations effects on the low temperature properties of paramagnetic intermediate valence and Kondo compounds

A generalization of the paramagnon theory to arbitrary band structures was previously shown to give a good phenomenologic description of the low temperature dependence of the paramagnetic susceptibility of CeIn3- xSnx. A discussion is presented here to complemente that study, including examination of the specific heat and applications to some other compounds like CeAl3. The nature of the low temperature transition to an ordered state, when it occurs, is also discussed as possibly be of ferro, antiferro, helimagnetic, excitonic or superconducting character. Possible applications to catalysis or triplet pairing superconductivity are examined.