In the quest for exotic superconducting pairing states, the Rashba effect, which lifts the electronspin degeneracy as a consequence of strong spin-orbit interaction (SOI) under broken inversion symmetry, has attracted considerable interest. Here, to introduce the Rashba effect into two-dimensional (2D) strongly correlated electron systems, we fabricate non-centrosymmetric (tricolor) superlattices composed of three kinds of f -electron compounds with atomic thickness; d-wave heavy fermion superconductor CeCoIn5 sandwiched by two different nonmagnetic metals, YbCoIn5 and YbRhIn5. We find that the Rashba SOI induced global inversion symmetry breaking in these tricolor Kondo superlattices leads to profound changes in the superconducting properties of CeCoIn5, which are revealed by unusual temperature and angular dependences of upper critical fields that are in marked contrast with the bulk CeCoIn5 single crystals. We demonstrate that the Rashba effect incorporated into 2D CeCoIn5 block layers is largely tunable by changing the layer thickness. Moreover, the temperature dependence of in-plane upper critical field exhibits an anomalous upturn at low temperatures, which is attributed to a possible emergence of a helical or stripe superconducting phase. Our results demonstrate that the tricolor Kondo superlattices provide a new playground for exploring exotic superconducting states in the strongly correlated 2D electron systems with the Rashba effect.
Unconventional superconductivity and magnetism are intertwined on a microscopic level in a wide class of materials. A new approach to this most fundamental and hotly debated issue focuses on the role of interactions between superconducting electrons and bosonic fluctuations at the interface between adjacent layers in heterostructures. Here we fabricate hybrid superlattices consisting of alternating atomic layers of the heavy-fermion superconductor CeCoIn_{5} and antiferromagnetic (AFM) metal CeRhIn_{5}, in which the AFM order can be suppressed by applying pressure. We find that the superconducting and AFM states coexist in spatially separated layers, but their mutual coupling via the interface significantly modifies the superconducting properties. An analysis of upper critical fields reveals that, upon suppressing the AFM order by applied pressure, the force binding superconducting electron pairs acquires an extreme strong-coupling nature. This demonstrates that superconducting pairing can be tuned nontrivially by magnetic fluctuations (paramagnons) injected through the interface.
Unconventional superconductivity and magnetism are intertwined on a microscopic level in a wide class of materials, including high-T c cuprates, iron pnictides, and heavy-fermion compounds. Interactions between superconducting electrons and bosonic fluctuations at the interface between adjacent layers in heterostructures provide a new approach to this most fundamental and hotly debated subject. We have been able to use a recent state-of-the-art molecular-beam-epitaxy technique to fabricate superlattices consisting of different heavy-fermion compounds with atomic thickness. These Kondo superlattices provide a unique opportunity to study the mutual interaction between unconventional superconductivity and magnetic order through the atomic interface. Here, we design and fabricate hybrid Kondo superlattices consisting of alternating layers of superconducting CeCoIn5 with d-wave pairing symmetry and nonmagnetic metal YbCoIn5 or antiferromagnetic heavy fermion metals such as CeRhIn5 and CeIn3. In these Kondo superlattices, superconducting heavy electrons are confined within the two-dimensional CeCoIn5 block layers and interact with neighboring nonmagnetic or magnetic layers through the interface. Superconductivity is strongly influenced by local inversion symmetry breaking at the interface in CeCoIn5/YbCoIn5 superlattices. The superconducting and antiferromagnetic states coexist in spatially separated layers in CeCoIn5/CeRhIn5 and CeCoIn5/CeIn3 superlattices, but their mutual coupling via the interface significantly modifies the superconducting and magnetic properties. The fabrication of a wide variety of hybrid superlattices paves a new way to study the relationship between unconventional superconductivity and magnetism in strongly correlated materials.
To study the mutual interaction between unconventional superconductivity and magnetic order through an interface, we fabricate hybrid Kondo superlattices consisting of alternating layers of the heavy-fermion superconductor CeCoIn5 and the antiferromagnetic (AFM) heavy-fermion metal CeIn3. The strength of the AFM fluctuations is tuned by applying hydrostatic pressure to the CeCoIn5(m)/CeIn3(n) superlattices with m and n unit-cell-thick layers of CeCoIn5 and CeIn3, respectively. The superconductivity in CeCoIn5 and the AFM order in CeIn3 coexist in spatially separated layers in the whole thickness and pressure ranges. At ambient pressure, the Néel temperature TN of the CeIn3 block layers (BLs) of CeCoIn5 (7)/CeIn3(n) shows little dependence on the thickness n, in sharp contrast to CeIn3(n)/LaIn3(4) superlattices where TN is strongly suppressed with decreasing n. This suggests that each CeIn3 BL is magnetically coupled by the RKKY interaction through the adjacent CeCoIn5 BL and a three-dimensional magnetic state is formed. With applying pressure to CeCoIn5(7)/CeIn3(13), TN of the CeIn3 BLs is suppressed up to 2.4 GPa, showing a similar pressure dependence as bulk CeIn3 single crystals. An analysis of the upper critical field reveals that the superconductivity in the CeCoIn5 BLs is barely influenced by the AFM fluctuations in the CeIn3 BLs, even when the CeIn3 BLs are in the vicinity of the AFM quantum critical point. This is in stark contrast to CeCoIn5/CeRhIn5 superlattices, in which the superconductivity in the CeCoIn5 BLs is profoundly affected by AFM fluctuations in the CeRhIn5 BLs. The present results show that although AFM fluctuations are injected into the CeCoIn5 BLs from the CeIn3 BLs through the interface, they barely affect the force which binds superconducting electron pairs. These results demonstrate that two-dimensional AFM fluctuations are essentially important for the pairing interactions in CeCoIn5.
It is a long-standing important issue in heavy fermion physics whether f -electrons are itinerant or localized when the magnetic order occurs. Here we report the in situ scanning tunneling microscopy observation of the electronic structure in epitaxial thin films of CeRhIn5, a prototypical heavy fermion compound with antiferromagnetic ground state. The conductance spectra above the Néel temperature TN clearly resolve the energy gap due to the hybridization between local 4f electrons and conduction bands as well as the crystal electric field excitations. These structures persist even below TN . Moreover, an additional dip in the conductance spectra develops due to the antiferromagnetic order. These results provide direct evidence for the presence of itinerant heavy f -electrons participating in the Fermi surface even in the magnetically ordered state of CeRhIn5.
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