In Situ STM Observation of Nonmagnetic Impurity Effect in MBE-grown CeCoIn₅ Films

Abstract

Local electronic effects in the vicinity of an impurity provide pivotal insight into the origin of unconventional superconductivity, especially when the materials are located on the edge of magnetic instability. In high-temperature cuprate superconductors, a strong suppression of superconductivity and appearance of low-energy bound states are clearly observed near nonmagnetic impurities. However, whether these features are common to other strongly correlated superconductors has not been established experimentally. Here, we report the in situ scanning tunneling microscopy observation of electronic structure around a nonmagnetic Zn impurity in heavy-fermion CeCo(In1−xZnx)5 films, which are epitaxially grown by the state-of-the-art molecular beam epitaxy technique in ultrahigh vacuum. The films have very wide atomically flat terraces and Zn atoms residing on two different In sites are clearly resolved. Remarkably, no discernible change is observed for the superconducting gap at and around the Zn atoms. Moreover, the local density of states around Zn atoms shows little change inside the hybridization gap between f -and conduction electrons, which is consistent with calculations for a periodic Anderson model without local magnetic order. These results indicate that no nonsuperconducting region is induced around a Zn impurity and do not support the scenario of antiferromagnetic droplet formation suggested by indirect measurements in Cd-doped CeCoIn5. These results also highlight a significant difference of the impurity effect between cuprates and CeCoIn5, in both of which d-wave superconductivity arises from the non-Fermi liquid normal state near antiferromagnetic instabilities.

Colloquium : Heavy-electron quantum criticality and single-particle spectroscopy

“…Epitaxial thin films of members of the 115 family and CeIn 3 have been available for some time (Shishido et al, 2010) but STM measurements on these films of CeCoIn 5 and CeRhIn 5 have only been reported very recently (Haze et al, 2019(Haze et al, , 2018. The observed onset of the hybridization gap in the STM spectrum demonstrates the high-energy T 0 scale which, as we have emphasized, is consistent with a Kondo-destruction ground state in CeRhIn 5 .…”

“…(Data taken from (Aynajian et al, 2012)). STM studies on CeCoIn 5 (and to a much lesser extent, on CeRhIn 5 and CeIrIn 5 ) have been performed by several groups (Allan et al, 2013;Aynajian et al, 2012Aynajian et al, , 2014Ernst et al, 2010;Haze et al, 2018;Zhou et al, 2013). In Fig.…”

Quantum well states in fractured crystals of the heavy fermion material CeCoIn$_5$

“…Previous STM results showed flat terraces of only tens of nanometers in CeCoIn 5 samples cleaved at room temperature [41]. Thin films grown by molecular beam epitaxy (MBE) also exhibit terraces significantly smaller than 1 µm [42]. In contrast, the observation of QWSs requires the presence of two parallel, atomically flat interfaces much larger than those seen in STM on cleaved crystals or MBE-grown films.…”

Structural and electrical characterization of the monolayer Kondo-lattice compound CePt$_6$/Pt(111)

“…While ARPES directly detects the sharp state, STS and point contact spectroscopy (PCS) capture the resonance state as a Fano-Kondo resonance, where a quantum interference between a transfer into the Kondo resonance state and that into a conduction band leads to an asymmetric single dip or peak depending on the transfer ratio 51,52 . By further decreasing temperature, the hybridization gap is formed by band renormalization between the conduction band and the Kondo resonance state below T * (<< T K ) [1][2][3][7][8][9][10][11][12][13][14][15][16][17][18] . The detailed temper- A solid white curve is a fitting curve for an energy dispersion reflecting the surface state shown in Fig.…”

“…With decreasing temperature further, an interplay between the f electrons develops via hybridization, and causes the coherent Kondo state with a hybridization gap, an ordered state with the Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling, or heavy-fermion superconductivity, depending on the strength of the hybridization. In the last decade, the surface-science techniques with high energy and spatial resolutions in real and reciprocal spaces have provided new insight into the novel electronic states 5,6 using bulk single crystals [7][8][9][10][11][12][13][14][15][16][17] and epitaxially grown films 18 .…”