15 research outputs found
Tuning fulleride electronic structure and molecular ordering via variable layer index
C60 fullerides are uniquely flexible molecular materials that exhibit a rich
variety of behavior, including superconductivity and magnetism in bulk
compounds, novel electronic and orientational phases in thin films, and quantum
transport in a single-C60 transistor. The complexity of fulleride properties
stems from the existence of many competing interactions, such as
electron-electron correlations, electron-vibration coupling, and intermolecular
hopping. The exact role of each interaction is controversial due to the
difficulty of experimentally isolating the effects of a single interaction in
the intricate fulleride materials. Here we report a unique level of control of
the material properties of KxC60 ultra-thin films through well-controlled
atomic layer indexing and accurate doping concentrations. Using STM techniques,
we observe a series of electronic and structural phase transitions as the
fullerides evolve from two-dimensional monolayers to quasi-threedimensional
multilayers in the early stages of layer-by-layer growth. These results
demonstrate the systematic evolution of fulleride electronic structure and
molecular ordering with variable KxC60 film layer index, and shed new light on
creating novel molecular structures and devices.Comment: 16 pages, 4 figures, to appear in Nature Material
Revealing the high-energy electronic excitations underlying the onset of high-temperature superconductivity in cuprates
In strongly-correlated systems the electronic properties at the Fermi energy (EF) are intertwined with those at high energy scales. One of the pivotal challenges in the field of high-temperature superconductivity (HTSC) is to understand whether and how the high energy scale physics associated with Mott-like excitations (|E-EF|>1 eV) is involved in the condensate formation. Here we show the interplay between the many-body high-energy CuO2 excitations at 1.5 and 2 eV and the onset of HTSC. This is revealed by a novel optical pump supercontinuum-probe technique, which provides access to the dynamics of the dielectric function in Y-Bi2212 over an extended energy range, after the photoinduced suppression of the superconducting pairing. These results unveil an unconventional mechanism at the base of HTSC both below and above the optimal hole concentration required to attain the maximum critical temperature (Tc)
RELATIONSHIP BETWEEN ATOMIC AND ELECTRONIC-STRUCTURE OF CLEAN AND OXYGEN COVERED COPPER (110) SURFACE
We described the electronic band-structure of the clean and oxygen covered Cu(110) surface using a tight-binding method in a recursive layer by layer scheme. These are compared to angle-resolved ultraviolet photoelectron and angle-resolved inverse photoemission data. Good agreement for all (except image potential) surface states for the clean surface is obtained and also for the oxygen covered surface if the “ buckled-row” reconstruction is assumed, where alternate 〈001〉 copper rows are displaced outwardly by 0.6± 0.2 Å from the first layer and the oxygen atoms are located 0.1 ± 0.2 Å below these copper atoms. From this we conclude that the room temperature Cu(110)-p2 × 1-O superstructure is the result of a “buckled-row” reconstruction. The possibility of a “missing-row” reconstruction for the high temperature Cu(110)-p2 × 1-O superstructure is discussed
SPECTRAL-WEIGHT TRANSFER - BREAKDOWN OF LOW-ENERGY-SCALE SUM-RULES IN CORRELATED SYSTEMS
In this paper we study the spectral-weight transfer from the high- to the low-energy scale by means of exact diagonalization of finite clusters for the Mott-Hubbard and charge-transfer model. We find that the spectral-weight transfer is very sensitive to the hybridization strength as well as to the amount of doping. This implies that the effective number of low-energy degrees of freedom is a function of the hybridization and therefore of the volume and temperature. In this sense it is not possible to define a Hamiltonian which describes the low-energy-scale physics unless one accepts an effective nonparticle conservation