90 research outputs found
Energy Gaps and Kohn Anomalies in Elemental Superconductors
The momentum and temperature dependence of the lifetimes of acoustic phonons
in the elemental superconductors Pb and Nb was determined by resonant spin-echo
spectroscopy with neutrons. In both elements, the superconducting energy gap
extracted from these measurements was found to converge with sharp anomalies
originating from Fermi-surface nesting (Kohn anomalies) at low temperatures.
The results indicate electron many-body correlations beyond the standard
theoretical framework for conventional superconductivity. A possible mechanism
is the interplay between superconductivity and spin- or charge-density-wave
fluctuations, which may induce dynamical nesting of the Fermi surface
Detection of electronic nematicity using scanning tunneling microscopy
Electronic nematic phases have been proposed to occur in various correlated
electron systems and were recently claimed to have been detected in scanning
tunneling microscopy (STM) conductance maps of the pseudogap states of the
cuprate high-temperature superconductor Bi2Sr2CaCu2O8+x (Bi-2212). We
investigate the influence of anisotropic STM tip structures on such
measurements and establish, with a model calculation, the presence of a
tunneling interference effect within an STM junction that induces
energy-dependent symmetry-breaking features in the conductance maps. We
experimentally confirm this phenomenon on different correlated electron
systems, including measurements in the pseudogap state of Bi-2212, showing that
the apparent nematic behavior of the imaged crystal lattice is likely not due
to nematic order but is related to how a realistic STM tip probes the band
structure of a material. We further establish that this interference effect can
be used as a sensitive probe of changes in the momentum structure of the
sample's quasiparticles as a function of energy.Comment: Accepted for publication (PRB - Rapid Communications). Main text (5
pages, 4 figures) + Supplemental Material (4 pages, 4 figures
Momentum-resolved electron-phonon interaction in lead determined by neutron resonance spin-echo spectroscopy
Neutron resonance spin-echo spectroscopy was used to monitor the temperature
evolution of the linewidths of transverse acoustic phonons in lead across the
superconducting transition temperature, , over an extended range of the
Brillouin zone. For phonons with energies below the superconducting energy gap,
a linewidth reduction of maximum amplitude eV was observed below
. The electron-phonon contribution to the phonon lifetime extracted from
these data is in satisfactory overall agreement with {\it ab-initio}
lattice-dynamical calculations, but significant deviations are found
Visualizing the Formation of the Kondo Lattice and the Hidden Order in URu2Si2
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 (STM) to examine the novel
electronic states that emerge from the uranium f states in URu2Si2. 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=17.5 K, URu2Si2 is known to undergo a 2nd
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
Visualizing Heavy Fermion Confinement and Pauli-Limited Superconductivity in Layered CeCoIn5
Layered material structures play a key role in enhancing electron-electron
interactions to create correlated metallic phases that can transform into
unconventional superconducting states. The quasi-two-dimensional electronic
properties of such compounds are often inferred indirectly through examination
of their bulk properties. Here we use scanning tunneling microscopy and
spectroscopy to directly probe in cross section the quasi-two-dimensional
correlated electronic states of the heavy fermion superconductor CeCoIn5. Our
measurements reveal the strong confined nature of heavy quasi-particles,
anisotropy of tunneling characteristics, and layer-by-layer modulated behavior
of the precursor pseudogap gap phase in this compound. Examining the interlayer
coupled superconducting state at low temperatures, we find that the orientation
of line defects relative to the d-wave order parameter determines whether
in-gap states form due to scattering. Spectroscopic imaging of the anisotropic
magnetic vortex cores directly characterizes the short interlayer
superconducting coherence length and shows an electronic phase separation near
the upper critical in-plane magnetic field, consistent with a Pauli-limited
first-order phase transition into a pseudogap phase
Coexisting Kondo hybridization and itinerant f-electron ferromagnetism in UGe2
Kondo hybridization in partially filled f-electron systems conveys significant amount of electronic states sharply near the Fermi energy leading to various instabilities from superconductivity to exotic electronic orders. UGe2 is a 5f heavy fermion system, where the Kondo hybridization is interrupted by the formation of two ferromagnetic phases below a 2nd order transition Tc ~ 52 K and a crossover transition Tx ~ 32 K. These two ferromagnetic phases are concomitantly related to a spin-triplet superconductivity that only emerges and persists inside the magnetically ordered phase at high pressure. The origin of the two ferromagnetic phases and how they form within a Kondo-lattice remain ambiguous. Using scanning tunneling microscopy and spectroscopy, we probe the spatial electronic states in the UGe2 as a function of temperature. We find a Kondo resonance and sharp 5f-electron states near the chemical potential that form at high temperatures above Tc in accordance with our density functional theory (DFT) + Gutzwiller calculations. As temperature is lowered below Tc, the resonance narrows and eventually splits below Tx dumping itinerant f-electron spectral weight right at the Fermi energy. Our findings suggest a Stoner mechanism forming the highly polarized ferromagnetic phase below Tx that itself sets the stage for the emergence of unconventional superconductivity at high pressure
Visualizing heavy fermions emerging in a quantum critical Kondo lattice
In solids containing elements with f orbitals, the interaction between
f-electron spins and those of itinerant electrons leads to the development of
low-energy fermionic excitations with a heavy effective mass. These excitations
are fundamental to the appearance of unconventional superconductivity and
non-Fermi-liquid behaviour observed in actinide- and lanthanide-based
compounds. Here we use spectroscopic mapping with the scanning tunnelling
microscope to detect the emergence of heavy excitations with lowering of
temperature in a prototypical family of cerium-based heavy-fermion compounds.
We demonstrate the sensitivity of the tunnelling process to the composite
nature of these heavy quasiparticles, which arises from quantum entanglement of
itinerant conduction and f electrons. Scattering and interference of the
composite quasiparticles is used to resolve their energy-momentum structure and
to extract their mass enhancement, which develops with decreasing temperature.
The lifetime of the emergent heavy quasiparticles reveals signatures of
enhanced scattering and their spectral lineshape shows evidence of
energy-temperature scaling. These findings demonstrate that proximity to a
quantum critical point results in critical damping of the emergent heavy
excitation of our Kondo lattice system.Comment: preprint version, 26 pages, 6 figures. Supplementary: 15 pages, 14
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