Quantum-well states at the surface of a heavy-fermion superconductor

Abstract

Two-dimensional electronic states at surfaces are often observed in simple wide-band metals such as Cu or Ag (refs. 1–4). Confinement by closed geometries at the nanometre scale, such as surface terraces, leads to quantized energy levels formed from the surface band, in stark contrast to the continuous energy dependence of bulk electron bands2,5–10. Their energy-level separation is typically hundreds of meV (refs. 3,6,11). In a distinct class of materials, strong electronic correlations lead to so-called heavy fermions with a strongly reduced bandwidth and exotic bulk ground states12,13. Quantum-well states in two-dimensional heavy fermions (2DHFs) remain, however, notoriously difficult to observe because of their tiny energy separation. Here we use millikelvin scanning tunnelling microscopy (STM) to study atomically flat terraces on U-terminated surfaces of the heavy-fermion superconductor URu2Si2, which exhibits a mysterious hidden-order (HO) state below 17.5 K (ref. 14). We observe 2DHFs made of 5f electrons with an effective mass 17 times the free electron mass. The 2DHFs form quantized states separated by a fraction of a meV and their level width is set by the interaction with correlated bulk states. Edge states on steps between terraces appear along one of the two in-plane directions, suggesting electronic symmetry breaking at the surface. Our results propose a new route to realize quantum-well states in strongly correlated quantum materials and to explore how these connect to the electronic environment.

Interfacial Distortion of Sb2Te3–Sb2Se3 Multilayers via Atomic Layer Deposition for Enhanced Thermoelectric Properties

“…5 As an alternative, Gao et al fabricated PbSe/SnSe multiple quantum well structures, achieving a high power factor of 25.7 μW cm −1 K −2 at 300 K, which is 4 times larger than that of a PbSe single layer. 6 This article is licensed under CC-BY 4 Seebeck coefficient from 61 to 320 μV K −1 . 7 However, as the insulator barrier does not contribute to electronic transportation, there was no improvement in the carrier concentration and electrical conductivity.…”

“…One of the effective strategies to enhance thermoelectric performance is to reduce the dimensions, thereby introducing the quantum confinement effect in nanostructured thermoelectric materials . The electronic states are confined by closed geometries at the nanometer scale, leading to quantized energy levels and enhancing the density of electron states per unit volume occurring for a small well within . In this case, the carrier effective mass and Seebeck coefficient can be elevated rapidly.…”

“… 3 The electronic states are confined by closed geometries at the nanometer scale, leading to quantized energy levels and enhancing the density of electron states per unit volume occurring for a small well within. 4 In this case, the carrier effective mass and Seebeck coefficient can be elevated rapidly. Hicks et al demonstrated a 3-fold increase in the Seebeck coefficient value of PbTe/Pb 0.927 Eu 0.073 Te compared to bulk PbTe, attributed to the quantum confinement effect in the nanostructured system.…”

Two-dimensional heavy fermion in a monoatomic-layer Kondo lattice YbCu2

“…The ground state of two-dimensional (2D) HF can be easily controlled to the vicinity of a quantum critical point, which is the host to realize unconventional physical properties such as HF superconductivity, by simple external fields such as gate-tuning 11 , 12 , and surface doping 13 in addition to traditional external perturbations; temperature, pressure, and magnetic field. Fabricating artificial low-dimensional strongly correlated electron systems and quantizing a three-dimensional HF state by quantum confinement 14 are suitable methods to investigate the novel electronic phase. In the Ce-based artificial superlattice, the suppression of antiferromagnetic (AFM) ordering as well as the increase of the effective electron mass with decreasing of the thickness of the Ce-layer 15 and the emergence of the strong-coupling superconductivity 16 have been reported.…”

“…To understand the fundamental properties of 2D HF systems, it is necessary to clarify the electronic band structure and the formation mechanism of the HF. However, the details have remained unclear due to the lack of promising materials and the extremely low transition temperatures of less than a few kelvin to HF even in known materials 14 , 15 , 17 , 18 .…”