Visualizing the formation of the Kondo lattice and the hidden order in URu ₂ Si ₂

Abstract

Heavy electronic states originating from the f atomic orbitals underlie a rich variety of quantum phases of matter. We use atomic scale imaging and spectroscopy with the scanning tunneling microscope to examine the novel electronic states that emerge from the uranium f states in URu 2 Si 2 . We find that, as the temperature is lowered, partial screening of the f electrons' spins gives rise to a spatially modulated Kondo-Fano resonance that is maximal between the surface U atoms. At T ¼ 17.5 K, URu 2 Si 2 is known to undergo a second-order phase transition from the Kondo lattice state into a phase with a hidden order parameter. From tunneling spectroscopy, we identify a spatially modulated, bias-asymmetric energy gap with a mean-field temperature dependence that develops in the hidden order state. Spectroscopic imaging further reveals a spatial correlation between the hidden order gap and the Kondo resonance, suggesting that the two phenomena involve the same electronic states.heavy fermion | scanning tunneling spectroscopy A remarkable variety of collective electronic phenomena have been discovered in compounds with partially filled f orbitals, where electronic excitations act as heavy fermions (1, 2). Like other correlated electronic systems, such as the high temperature superconducting cuprates, several of the heavy fermion compounds display an interplay between magnetism and superconductivity and have a propensity toward superconducting pairing with unconventional symmetry (1-5). However, unlike cuprates, or the newly discovered ferropnictides, the heavy fermion systems do not suffer from inherent dopant-induced disorder and offer a clean material system for the study of correlated electrons. The local f electrons interact both with the itinerant spd electrons as well as with each other, resulting in a rich variety of electronic phases. In many of these materials, screening of the local moments by the Kondo effect begins at relatively high temperatures resulting in a heavy fermion state at low temperatures. Exchange interactions between the local moments become more important at lower temperatures and can result in the formation of magnetic phases as well as superconductivity at even lower temperatures. Among the heavy fermion compounds perhaps the most enigmatic is the URu 2 Si 2 system, which undergoes a second-order phase transition with a rather large change in entropy (6-8) at 17.5 K from a paramagnetic phase with Kondo screening to a phase with an unknown order parameter (9). This material possesses low-energy commensurate and incommensurate spin excitations, which are gapped below the hidden order (HO) transition temperature (10-13). These features are believed to be signatures of a more complex order parameter, the identification of which has so far not been possible despite numerous investigations (12-18). Moreover, analogous to other correlated systems, this unusual conducting phase is transformed into an unconventional superconducting state at 1.5 K (6,8,19), the understanding of which hinges on fo...

Formation of the Coherent Heavy Fermion Liquid at the Hidden Order Transition inURu2Si2

“…The onset of hybridization at T HO was also observed in recent scanning tunneling spectroscopy measurements [28,29]. However, the ARPES data at hand yield essential new experimental insights, which were not known previously: (i) Already above T HO , "incoherent heavy fermion" states exist below the Fermi level.…”

Unusual Magnetic Field-Dependence of the Electronic Dispersion Relations in a Hidden Ordered Phase.

“…(1)] reveals the existence of an almost temperature-independent hybridization gap shifted from the Fermi-energy [19] and a pseudo-gap at the Fermi-energy which rapidly appears below the transition temperature [20]. …”

Tunneling in Heavy-Fermion Junctions

“…3 Experimentally, one of the main challenges in probing the emergence of the heavy quasiparticles is in the lack of high resolution spectroscopic measurements. Remarkably, this challenge has been overcome in scanning tunneling microscopy measurements [4][5][6][7][8] as well as in the point contact spectroscopy. 9,10,14 Recent tunneling experiments have been convincingly able to trace the formation of the heavy quasiparticles.…”

“…What is more, momentum and energy resolved tunneling spectra visualized not only the formation of the heavy quasiparticles, but also the formation of unconventional superconductivity in a prototypical Kondo lattice heavy-fermion superconductor CeCoIn 5 . 4,7 Formation of the heavy-particles has also been successfully resolved in more itinerant systems, such as 'hidden order' compound URu 2 Si 2 5 and in a best candidate for correlated topological insulator SmB 6 . 8 Asymmetric or Fano lineshape of the differential tunneling conductance is the basic feature observed in tunneling experiments into Kondo lattice systems.…”

“…(10,11), provides more than adequate description of the available experimental data. [4][5][6][7][8] At the same time, for the analysis of tunneling experiments into a superconducting Kondo lattice 7 these terms may actually be important, especially for probing unconventional Cooper pairing mechanisms.…”