10 research outputs found
Universality of pseudogap and emergent order in lightly doped Mott insulators
It is widely believed that high-temperature superconductivity in the cuprates
emerges from doped Mott insulators. The physics of the parent state seems
deceivingly simple: The hopping of the electrons from site to site is
prohibited because their on-site Coulomb repulsion U is larger than the kinetic
energy gain t. When doping these materials by inserting a small percentage of
extra carriers, the electrons become mobile but the strong correlations from
the Mott state are thought to survive; inhomogeneous electronic order, a
mysterious pseudogap and, eventually, superconductivity appear. How the
insertion of dopant atoms drives this evolution is not known, nor whether these
phenomena are mere distractions specific to hole-doped cuprates or represent
the genuine physics of doped Mott insulators. Here, we visualize the evolution
of the electronic states of (Sr1-xLax)2IrO4, which is an effective spin-1/2
Mott insulator like the cuprates, but is chemically radically different. Using
spectroscopic-imaging STM, we find that for doping concentration of x=5%, an
inhomogeneous, phase separated state emerges, with the nucleation of pseudogap
puddles around clusters of dopant atoms. Within these puddles, we observe the
same glassy electronic order that is so iconic for the underdoped cuprates.
Further, we illuminate the genesis of this state using the unique possibility
to localize dopant atoms on topographs in these samples. At low doping, we find
evidence for much deeper trapping of carriers compared to the cuprates. This
leads to fully gapped spectra with the chemical potential at mid-gap, which
abruptly collapse at a threshold of around 4%. Our results clarify the melting
of the Mott state, and establish phase separation and electronic order as
generic features of doped Mott insulators.Comment: This version contains the supplementary information and small updates
on figures and tex
Single Si dopants in GaAs studied by scanning tunneling microscopy and spectroscopy
\u3cp\u3eWe present a comprehensive scanning tunneling microscopy and spectroscopy study of individual Si dopants in GaAs. We explain all the spectroscopic peaks and their voltage dependence in the band gap and in the conduction band. We observe both the filled and empty donor state. Donors close to the surface, which have an enhanced binding energy, show a second ionization ring, corresponding to the negatively charged donor D\u3csup\u3e-\u3c/sup\u3e. The observation of all predicted features at the expected spectral position and with the expected voltage-distance dependence confirms their correct identification and the semiquantitative analyses of their energetic positions.\u3c/p\u3
Scanning tunneling microscopy reveals LiMnAs is a room temperature anti-ferromagnetic semiconductor
We performed scanning tunneling microscopy and spectroscopy on a LiMnAs(001) thin film
epitaxially grown on an InAs(001) substrate by molecular beam epitaxy. While the in situ cleavage
exposed only the InAs(110) non-polar planes, the cleavage continued into the LiMnAs thin layer
across several facets. We combined both topography and current mappings to confirm that the
facets correspond to LiMnAs. By spectroscopy we show that LiMnAs has a band gap. The band
gap evidenced in this study, combined with the known Ne´el temperature well above room
temperature, confirms that LiMnAs is a promising candidate for exploring the concepts of high
temperature semiconductor spintronics based on antiferromagnets
Enhanced binding energy of manganese acceptors close to the GaAs(110) surface
Scanning tunneling spectroscopy was performed at low temperature on buried manganese (Mn) acceptors below the (110) surface of gallium arsenide. The main Mn-induced features consisted of a number of dI/dV peaks in the band gap of the host material. The peaks in the band gap are followed by negative differential conductivity, which can be understood in terms of an energy-filter mechanism. The spectroscopic features detected on the Mn atoms clearly depend on the depth of the addressed acceptor below the surface. Combining the depth dependence of the positions of the Mn-induced peaks and using the energy-filter model to explain the negative resistance qualitatively proves that the binding energy of the hole bound to the Mn atom increases for Mn acceptors closer to the surfac
Tetragonal phase of epitaxial room-temperature antiferromagnet CuMnAs
Recent studies have demonstrated the potential of antiferromagnets as the
active component in spintronic devices. This is in contrast to their current
passive role as pinning layers in hard disk read heads and magnetic memories.
Here we report the epitaxial growth of a new high-temperature antiferromagnetic
material, tetragonal CuMnAs, which exhibits excellent crystal quality, chemical
order and compatibility with existing semiconductor technologies. We
demonstrate its growth on the III-V semiconductors GaAs and GaP, and show that
the structure is also lattice matched to Si. Neutron diffraction shows
collinear antiferromagnetic order with a high Ne\'el temperature. Combined with
our demonstration of room-temperature exchange coupling in a CuMnAs/Fe bilayer,
we conclude that tetragonal CuMnAs films are suitable candidate materials for
antiferromagnetic spintronics.Comment: 16 pages, 5 figures, Published in Nature Communications (2013