16,773 research outputs found
Experimental evidence for a stable GaAs surface near (113)
GaAs surfaces vicinal to (113) with a continuous range of misorientation angles up to 11.5° in all azimuthal directions were created by grinding a spherical depression into (113) oriented samples. Thin homoepitaxial layers were grown onto these samples by molecular beam epitaxy (MBE), and the surfaces were in situ studied by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The surface quality in the depression was verified by reproducing LEED patterns of the (113) and (114) surfaces. A stable GaAs surface was found that is oriented from (113) by 9°±2° towards [11̅0̅]. STM and LEED images of this surface are presented
Probing the superconducting condensate on a nanometer scale
Superconductivity is a rare example of a quantum system in which the
wavefunction has a macroscopic quantum effect, due to the unique condensate of
electron pairs. The amplitude of the wavefunction is directly related to the
pair density, but both amplitude and phase enter the Josephson current : the
coherent tunneling of pairs between superconductors. Very sensitive devices
exploit the superconducting state, however properties of the {\it condensate}
on the {\it local scale} are largely unknown, for instance, in unconventional
high-T cuprate, multiple gap, and gapless superconductors.
The technique of choice would be Josephson STS, based on Scanning Tunneling
Spectroscopy (STS), where the condensate is {\it directly} probed by measuring
the local Josephson current (JC) between a superconducting tip and sample.
However, Josephson STS is an experimental challenge since it requires stable
superconducting tips, and tunneling conditions close to atomic contact. We
demonstrate how these difficulties can be overcome and present the first
spatial mapping of the JC on the nanometer scale. The case of an MgB film,
subject to a normal magnetic field, is considered.Comment: 7 pages, 6 figure
The graphene sheet versus the 2DEG: a relativistic Fano spin-filter via STM and AFM tips
We explore theoretically the density of states (LDOS) probed by an STM tip of
2D systems hosting an adatom and a subsurface impurity,both capacitively
coupled to AFM tips and traversed by antiparallel magnetic fields. Two kinds of
setups are analyzed, a monolayer of graphene and a two-dimensional electron gas
(2DEG). The AFM tips set the impurity levels at the Fermi energy, where two
contrasting behaviors emerge: the Fano factor for the graphene diverges, while
in the 2DEG it approaches zero. As result, the spin-degeneracy of the LDOS is
lifted exclusively in the graphene system, in particular for the asymmetric
regime of Fano interference. The aftermath of this limit is a counterintuitive
phenomenon, which consists of a dominant Fano factor due to the subsurface
impurity even with a stronger STM-adatom coupling. Thus we find a full
polarized conductance, achievable just by displacing vertically the position of
the STM tip. To the best knowledge, our work is the first to propose the Fano
effect as the mechanism to filter spins in graphene. This feature arises from
the massless Dirac electrons within the band structure and allows us to employ
the graphene host as a relativistic Fano spin-filter
Electronic Phase Separation Transition as the Origin of the Superconductivity and the Pseudogap Phase of Cuprates
We propose a new phase of matter, an electronic phase separation transition
that starts near the upper pseudogap and segregates the holes into high and low
density domains. The Cahn-Hilliard approach is used to follow quantitatively
this second order transition. The resulting grain boundary potential confines
the charge in domains and favors the development of intragrain superconducting
amplitudes. The zero resistivity transition arises only when the intergrain
Josephson coupling is of the order of the thermal energy and phase
locking among the superconducting grains takes place. We show that this
approach explains the pseudogap and superconducting phases in a natural way and
reproduces some recent scanning tunneling microscopy dataComment: 4 pages and 5 eps fig
Scanning Tunneling Microscopy studies on CeCoIn and CeIrIn
High--quality single crystals of the heavy fermion superconductors CeCoIn
and CeIrIn have been studied by means of low--temperature Scanning
Tunneling Microscopy. Methods were established to facilitate \textit{in-situ}
sample cleaving. Spectroscopy in CeCoIn reveals a gap which persists to
above , possibly evidencing a precursor state to SC. Atomically resolved
topographs show a rearrangement of the atoms at the crystal surface. This
modification at the surface might influence the surface properties as detected
by tunneling spectroscopy
Robust procedure for creating and characterizing the atomic structure of scanning tunneling microscope tips
Scanning tunneling microscopes (STM) are used extensively for studying and
manipulating matter at the atomic scale. In spite of the critical role of the
STM tip, the control of the atomic-scale shape of STM tips remains a poorly
solved problem. Here, we present a method for preparing tips {\it in-situ} and
for ensuring the crystalline structure and reproducibly preparing tip structure
up to the second atomic layer. We demonstrate a controlled evolution of such
tips starting from undefined tip shapes.Comment: 12 pages preprint-style; 5 figure
The fabrication of reproducible superconducting scanning tunneling microscope tips
Superconducting scanning tunneling microscope tips have been fabricated with
a high degree of reproducibility. The fabrication process relies on sequential
deposition of superconducting Pb and a proximity-coupled Ag capping layer onto
a Pt/Ir tip. The tips were characterized by tunneling into both normal-metal
and superconducting films. The simplicity of the fabrication process, along
with the stability and reproducibility of the tips, clear the way for tunneling
studies with a well-characterized, scannable superconducting electrode.Comment: 4 pages, 3 figures, REVTeX. Submitted to Rev. Sci. Instru
Spin-polarized tunneling spectroscopic studies of the intrinsic heterogeneity and pseudogap phenomena in colossal magnetoresistive manganite La_{0.7}Ca_{0.3}MnO_{3}
Spatially resolved tunneling spectroscopic studies of colossal
magnetoresistive (CMR) manganite (LCMO) epitaxial
films on substrate are investigated as
functions of temperature, magnetic field and spin polarization by means of
scanning tunneling spectroscopy. Systematic surveys of the tunneling spectra
taken with Pt/Ir tips reveal spatial variations on the length scale of a few
hundred nanometers in the ferromagnetic state, which may be attributed to the
intrinsic heterogeneity of the manganites due to their tendency towards phase
separation. The electronic heterogeneity is found to decrease either with
increasing field at low temperatures or at temperatures above all magnetic
ordering temperatures. On the other hand, spectra taken with Cr-coated tips are
consistent with convoluted electronic properties of both LCMO and Cr. In
particular, for temperatures below the magnetic ordering temperatures of both
Cr and LCMO, the magnetic-field dependent tunneling spectra may be
quantitatively explained by the scenario of spin-polarized tunneling in a
spin-valve configuration. Moreover, a low-energy insulating energy gap eV commonly found in the tunneling conductance spectra of bulk metallic
LCMO at may be attributed to a surface ferromagnetic insulating (FI)
phase, as evidenced by its spin filtering effect at low temperatures and
vanishing gap value above the Curie temperature. Additionally, temperature
independent pseudogap (PG) phenomena existing primarily along the boundaries of
magnetic domains are observed in the zero-field tunneling spectra. The PG
becomes strongly suppressed by applied magnetic fields at low temperatures when
the tunneling spectra of LCMO become highly homogeneous. These findings suggest
that the occurrence PG is associated with the electronic heterogeneity of the
manganites.Comment: 15 pages, 15 figures. Published in Physical Review B. Corresponding
author: Nai-Chang Yeh (E-mail: [email protected]
Tunneling spectroscopy in the magnetic superconductor TmNi2B2C
We present new measurements about the tunneling conductance in the
borocarbide superconductor TmNiBC. The results show a very good
agreement with weak coupling BCS theory, without any lifetime broadening
parameter, over the whole sample surface. We detect no particular change of the
tunneling spectroscopy below 1.5K, when both the antiferromagnetic (AF) phase
and the superconducting order coexist.Comment: Submitted to Phys. Rev. B, Rapid Communication
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