Unconventional Type Of Superconductivity Research Articles

Ce2Ir3Ga5 : A new locally noncentrosymmetric heavy fermion system

Recently, a new type of unconventional superconductivity with a field-induced transition between two different superconducting (SC) states was discovered in the heavy fermion system CeRh2As2. This unusual SC state was proposed to be based on specific symmetries of the underlying structure, i.e., a globally centrosymmetric layered structure, but where the Ce layers themselves lack inversion symmetry. This new type of SC state has attracted strong interest, prompting the search for further heavy fermion systems crystallizing in structures with appropriate symmetries. We report the discovery and the study of a new Ce-based heavy fermion system with a globally centrosymmetric structure but without inversion symmetry on the Ce-site, Ce2Ir3Ga5. A single crystal x-ray diffraction study revealed an orthorhombic U2Co3Si5 type structure. Resistivity, specific heat, and magnetization measurements indicate a moderate-heavy fermion behavior with a Kondo energy scale of the order of 40 K. Most experimental results suggest the absence of magnetic order, but a tiny anomaly in the specific heat opens the possibility for a very weak, itinerant type of ordering. Published by the American Physical Society 2024

Model of an Exotic Chiral Superconducting Phase in a Graphene Bilayer

We theoretically demonstrate the formation of a new type of unconventional superconductivity in graphene materials, which exhibits a gapless property. The studied superconductivity is based on an interlayer pairing of chiral electrons in bilayer graphene, which results in an exotic s-wave spin-triplet condensate order with anomalous thermodynamic properties. These include the possibility of a temperature-induced condensation causing an increase of the pairing gap with increasing temperature and an entropy of the stable superconducting state which can be higher than its value in the normal state. Our study reveals the analogy of the interlayer superconductivity in graphene materials to the color superconductivity in dense quark matter and the gapless pairing states in nuclear matter and ultracold atomic gases.

Exotic superconducting state embedded in the hidden order state of URu2Si2

The heavy-fermion compound URu2Si2 has mystified researchers since the superconducting state (Tc=1.45K) is embedded within the enigmatic ‘‘hidden order” phase (Th=17.5K). Here, we report charge and thermal transport measurements on ultraclean single crystals of URu2Si2 with very large residual-resistivity-ratio down to 30mK (∼Tc/50), which reveal a number of unprecedented superconducting properties. The results provide strong evidence for a new type of unconventional superconductivity with two distinct gaps having different nodal topology. We propose a gap function with chiral d-wave form Δ(k)=Δ0kz(kx+iky). We also demonstrate that a distinct flux line lattice melting transition with outstanding characters occurs well below the upper critical fields even at sub-Kelvin temperature. The intriguing superconducting state of URu2Si2 adds a unique and exciting example to the list of unconventional superconductors.

Superconducting gap structure of heavy-Fermion compound URu2Si2 determined by angle-resolved thermal conductivity

In heavy-Fermion compound URu2Si2, unusual superconductivity is embedded in an enigmatic ‘hidden order’ phase. Recently, it has been shown that URu2Si2 is essentially a multiband superconductor associated with the semimetallic compensated electronic structure. Here, to pin down the detailed superconducting gap structure, we have performed thermal transport measurements on ultraclean URu2Si2 single crystals in magnetic fields rotating various directions relative to the crystal axes. By changing the amplitude of magnetic fields, we determined the nodal topology of electron and hole band separately. The results indicate a new type of unconventional superconductivity with two distinct gaps, in which horizontal line nodes lie within the basal ab plane of the light-hole band with small gap and point nodes along the c-axis in the heavy electron band with large gap. This gap structure is consistent with ‘chiral’ d-wave symmetry with a form .

Exotic Superconducting Properties in the Electron-Hole-Compensated Heavy-Fermion “Semimetal”URu2Si2

We show that the charge and thermal transport measurements on ultraclean crystals of URu2Si2 reveal a number of unprecedented superconducting properties. The uniqueness is best highlighted by the peculiar field dependence of thermal conductivity including the first-order transition at Hc2 with a reduction of entropy flow. This is a consequence of multiband superconductivity with compensated electronic structure in the hidden order state of this system. We provide strong evidence for a new type of unconventional superconductivity with two distinct gaps having different nodal topology.

Josephson effect with heavy-fermion superconductors

Metallic junctions of the heavy-fermion superconductors strongly suppress the Josephson effect, possibly indicating an unconventional type of superconductivity. Junctions with classical superconductors like cadmium can also have a reduced critical current I c, a result of quantum fluctuations of the Josephson plasma due to a small dynamic capacitance. Applying this scenario to heavy-fermion superconductors reveals an intrinsic I c underestimated by an order of magnitude.