20 research outputs found
Orbital character effects in the photon energy and polarization dependence of pure C60 photoemission
Recent direct experimental observation of multiple highly-dispersive C
valence bands has allowed for a detailed analysis of the unique photoemission
traits of these features through photon energy- and polarization-dependent
measurements. Previously obscured dispersions and strong photoemission traits
are now revealed by specific light polarizations. The observed intensity
effects prove the locking in place of the C molecules at low
temperatures and the existence of an orientational order imposed by the
substrate chosen. Most importantly, photon energy- and polarization-dependent
effects are shown to be intimately linked with the orbital character of the
C band manifolds which allows for a more precise determination of the
orbital character within the HOMO-2. Our observations and analysis provide
important considerations for the connection between molecular and crystalline
C electronic structure, past and future band structure studies, and for
increasingly popular C electronic device applications, especially those
making use of heterostructures
Linearly dispersive bands at the onset of correlations in KC films
Molecular crystals are a flexible platform to induce novel electronic phases.
Due to the weak forces between molecules, intermolecular distances can be
varied over relatively larger ranges than interatomic distances in atomic
crystals. On the other hand, the hopping terms are generally small, which
results in narrow bands, strong correlations and heavy electrons. Here, by
growing KC fullerides on hexagonal layered BiSe, we show
that upon doping the series undergoes a Mott transition from a molecular
insulator to a correlated metal, and an in-gap state evolves into highly
dispersive Dirac-like fermions at half filling, where superconductivity occurs.
This picture challenges the commonly accepted description of the low energy
quasiparticles as appearing from a gradual electron doping of the conduction
states, and suggests an intriguing parallel with the more famous family of the
cuprate superconductors. More in general, it indicates that molecular crystals
offer a viable route to engineer electron-electron interactions.Comment: 5 pages, 4 figures. Accepted at Physical Review Researc
Electronic transport mechanisms in a thin crystal of the Kitaev candidate -RuCl probed through guarded high impedance measurements
-RuCl is considered to be the top candidate material for the
experimental realization of the celebrated Kitaev model. It is however known
that additional interactions beyond the Kitaev model trigger in
-RuCl, a long-range zigzag antiferromagnetic ground state. In this
work, we investigate a nanoflake of -RuCl through guarded high
impedance measurements aimed at reaching through electronic transport, the
regime where the system turns into a zigzag antiferromagnet. We investigated a
variety of temperatures (\SI{1.45}{\kelvin} - \SI{175}{\kelvin}) and
out-of-plane magnetic fields ranging up to \SI{11}{\tesla}. We found a clear
signature of a structural phase transition at \,K as reported for
thin crystals of -RuCl, as well as a thermally activated behavior
at temperatures above \,K with a characteristic activation energy
significantly smaller than the energy gap that we observe for -RuCl
bulk crystals through our Angle Resolved Photoemission Spectroscopy (ARPES)
experiments. Additionally we found that below \,K, transport is
ruled by Efros-Shklovskii (ES) VRH. These observations point to the presence of
Coulomb impurities in our thin crystals. Most importantly, our data shows that
below the magnetic ordering transition known for bulk -RuCl
(\,K), there is a clear deviation from VRH or thermal activation
transport mechanisms. Our work demonstrates the possibility of reaching through
specialized high impedance measurements, the thrilling ground states predicted
for -RuCl at low temperatures in the frame of the Kitaev model, and
informs about the transport mechanisms in this material in a wide temperature
range as well as on important characteristic quantities such as the
localization length of the impurities in a thin -RuCl crystal.Comment: 8 pages, 6 figures, Supplementary Material
Characterization of collective ground states in single-layer NbSe2
Layered transition metal dichalcogenides (TMDs) are ideal systems for
exploring the effects of dimensionality on correlated electronic phases such as
charge density wave (CDW) order and superconductivity. In bulk NbSe2 a CDW sets
in at TCDW = 33 K and superconductivity sets in at Tc = 7.2 K. Below Tc these
electronic states coexist but their microscopic formation mechanisms remain
controversial. Here we present an electronic characterization study of a single
2D layer of NbSe2 by means of low temperature scanning tunneling
microscopy/spectroscopy (STM/STS), angle-resolved photoemission spectroscopy
(ARPES), and electrical transport measurements. We demonstrate that 3x3 CDW
order in NbSe2 remains intact in 2D. Superconductivity also still remains in
the 2D limit, but its onset temperature is depressed to 1.9 K. Our STS
measurements at 5 K reveal a CDW gap of {\Delta} = 4 meV at the Fermi energy,
which is accessible via STS due to the removal of bands crossing the Fermi
level for a single layer. Our observations are consistent with the simplified
(compared to bulk) electronic structure of single-layer NbSe2, thus providing
new insight into CDW formation and superconductivity in this model
strongly-correlated system.Comment: Nature Physics (2015), DOI:10.1038/nphys352