38 research outputs found
Scanning tunneling spectroscopy of a dilute two-dimensional electron system exhibiting Rashba spin splitting
Using scanning tunneling spectroscopy (STS) at 5 K in B-fields up to 7 T, we
investigate the local density of states of a two-dimensional electron system
(2DES) created by Cs adsorption on p-type InSb(110). The 2DES, which in
contrast to previous STS studies exhibits a 2D Fermi level, shows standing
waves at B = 0 T with corrugations decreasing with energy and with wave numbers
in accordance with theory. In magnetic field percolating drift states are
observed within the disorder broadened Landau levels. Due to the large electric
field perpendicular to the surface, a beating pattern of the Landau levels is
found and explained quantitatively by Rashba spin splitting within the lowest
2DES subband. The Rashba splitting does not contribute significantly to the
standing wave patterns in accordance with theory.Comment: 9 pages, 9 figures, submitted to Phys. Rev.
Probing electron-electron interaction in quantum Hall systems with scanning tunneling spectroscopy
Using low-temperature scanning tunneling spectroscopy applied to the
Cs-induced two-dimensional electron system (2DES) on p-type InSb(110), we probe
electron-electron interaction effects in the quantum Hall regime. The 2DES is
decoupled from p-doped bulk states and exhibits spreading resistance within the
insulating quantum Hall phases. In quantitative agreement with calculations we
find an exchange enhancement of the spin splitting. Moreover, we observe that
both the spatially averaged as well as the local density of states feature a
characteristic Coulomb gap at the Fermi level. These results show that
electron-electron interaction effects can be probed down to a resolution below
all relevant length scales.Comment: supplementary movie in ancillary file
Bistability and oscillatory motion of natural nano-membranes appearing within monolayer graphene on silicon dioxide
The recently found material graphene is a truly two-dimensional crystal and
exhibits, in addition, an extreme mechanical strength. This in combination with
the high electron mobility favours graphene for electromechanical
investigations down to the quantum limit. Here, we show that a monolayer of
graphene on SiO2 provides natural, ultra-small membranes of diameters down to 3
nm, which are caused by the intrinsic rippling of the material. Some of these
nano-membranes can be switched hysteretically between two vertical positions
using the electric field of the tip of a scanning tunnelling microscope (STM).
They can also be forced to oscillatory motion by a low frequency ac-field.
Using the mechanical constants determined previously, we estimate a high
resonance frequency up to 0.4 THz. This might be favorable for
quantum-electromechanics and is prospective for single atom mass spectrometers.Comment: 9 pages, 4 figure
Electrical transport and low-temperature scanning tunneling microscopy of microsoldered graphene
Using the recently developed technique of microsoldering, we perform a
systematic transport study of the influence of PMMA on graphene flakes
revealing a doping effect of up to 3.8x10^12 1/cm^2, but a negligible influence
on mobility and gate voltage induced hysteresis. Moreover, we show that the
microsoldered graphene is free of contamination and exhibits a very similar
intrinsic rippling as has been found for lithographically contacted flakes.
Finally, we demonstrate a current induced closing of the previously found
phonon gap appearing in scanning tunneling spectroscopy experiments, strongly
non-linear features at higher bias probably caused by vibrations of the flake
and a B-field induced double peak attributed to the 0.Landau level of graphene.Comment: 8 pages, 3 figure
Wave function mapping in graphene quantum dots with soft confinement
Using low-temperature scanning tunneling spectroscopy, we map the local
density of states (LDOS) of graphene quantum dots supported on Ir(111). Due to
a band gap in the projected Ir band structure around the graphene K point, the
electronic properties of the QDs are dominantly graphene-like. Indeed, we
compare the results favorably with tight binding calculations on the honeycomb
lattice based on parameters derived from density functional theory. We find
that the interaction with the substrate near the edge of the island gradually
opens a gap in the Dirac cone, which implies soft-wall confinement.
Interestingly, this confinement results in highly symmetric wave functions.
Further influences of the substrate are given by the known moir{\'e} potential
and a 10% penetration of an Ir surface resonanceComment: 7 pages, 11 figures, DFT calculations directly showing the origin of
soft confinment, correct identification of the state penetrating from Ir(111)
into graphen
Increasing the active surface of titanium islands on graphene by nitrogen sputtering
Titanium-island formation on graphene as a function of defect density is
investigated. When depositing titanium on pristine graphene, titanium atoms
cluster and form islands with an average diameter of about 10nm and an average
height of a few atomic layers. We show that if defects are introduced in the
graphene by ion bombardment, the mobility of the deposited titanium atoms is
reduced and the average diameter of the islands decreases to 5nm with
monoatomic height. This results in an optimized coverage for hydrogen storage
applications since the actual titanium surface available per unit graphene area
is significantly increased
Large tunable valley splitting in edge-free graphene quantum dots on boron nitride
Coherent manipulation of binary degrees of freedom is at the heart of modern
quantum technologies. Graphene offers two binary degrees: the electron spin and
the valley. Efficient spin control has been demonstrated in many solid state
systems, while exploitation of the valley has only recently been started, yet
without control on the single electron level. Here, we show that van-der Waals
stacking of graphene onto hexagonal boron nitride offers a natural platform for
valley control. We use a graphene quantum dot induced by the tip of a scanning
tunneling microscope and demonstrate valley splitting that is tunable from -5
to +10 meV (including valley inversion) by sub-10-nm displacements of the
quantum dot position. This boosts the range of controlled valley splitting by
about one order of magnitude. The tunable inversion of spin and valley states
should enable coherent superposition of these degrees of freedom as a first
step towards graphene-based qubits
Readout of a antiferromagnetic spintronics systems by strong exchange coupling of Mn2Au and Permalloy
In antiferromagnetic spintronics, the read-out of the staggered magnetization
or Neel vector is the key obstacle to harnessing the ultra-fast dynamics and
stability of antiferromagnets for novel devices. Here, we demonstrate strong
exchange coupling of Mn2Au, a unique metallic antiferromagnet that exhibits
Neel spin-orbit torques, with thin ferromagnetic Permalloy layers. This allows
us to benefit from the well-estabished read-out methods of ferromagnets, while
the essential advantages of antiferromagnetic spintronics are retained. We show
one-to-one imprinting of the antiferromagnetic on the ferromagnetic domain
pattern. Conversely, alignment of the Permalloy magnetization reorients the
Mn2Au Neel vector, an effect, which can be restricted to large magnetic fields
by tuning the ferromagnetic layer thickness. To understand the origin of the
strong coupling, we carry out high resolution electron microscopy imaging and
we find that our growth yields an interface with a well-defined morphology that
leads to the strong exchange coupling.Comment: 9 pages, 5 figure
Room temperature magnetic order on zigzag edges of narrow graphene nanoribbons
Magnetic order emerging in otherwise non-magnetic materials as carbon is a
paradigmatic example of a novel type of s-p electron magnetism predicted to be
of exceptional high-temperature stability. It has been demonstrated that atomic
scale structural defects of graphene can host unpaired spins. However, it is
still unclear under which conditions long-range magnetic order can emerge from
such defect-bound magnetic moments. Here we propose that in contrast to random
defect distributions, atomic scale engineering of graphene edges with specific
crystallographic orientation, comprising edge atoms only from one sub-lattice
of the bipartite graphene lattice, can give rise to a robust magnetic order. We
employ a nanofabrication technique based on Scanning Tunneling Microscopy to
define graphene nanoribbons with nanometer precision and well-defined
crystallographic edge orientations. While armchair ribbons display quantum
confinement gap, zigzag ribbons narrower than 7 nm reveal a bandgap of about
0.2 - 0.3 eV, which can be identified as a signature of interaction induced
spin ordering along their edges. Moreover, a semiconductor to metal transition
is revealed upon increasing the ribbon width, indicating the switching of the
magnetic coupling between opposite ribbon edges from antiferromagnetic to
ferromagnetic configuration. We found that the magnetic order on graphene edges
of controlled zigzag orientation can be stable even at room temperature,
raising hope for graphene-based spintronic devices operating under ambient
conditions
Anisotropy of 4f states in 3d-4f single-molecule magnets
We have measured angular-dependent fluorescence-yield x-ray magnetic circular dichroism spectra on single crystals of the heterometallic 3d-4f 12-metallacrown-4 TbMn4 and DyMn4 complexes. Simulated spectra using crystal-field multiplet calculations reproduce the experimentally observed spectra. The orientation of the molecules causes linear dichroism spectra of the 4f absorption spectra. This natural linear dichroism shows the anisotropic charge distribution of the rare-earth 4f state in the tetragonal crystal field despite the small 4f crystal-field splitting. The magnetic moment of the molecule is dominated by the rare-earth moment revealing a considerably large contribution of orbital moment. From a sum-rule analysis of experimental and simulated x-ray magnetic circular dichroism, we determined corrected spin and orbital Dy moments at low temperature (14 K) within a magnetic field of 7 T. We find a significant angular dependence of the Dy magnetic moments, indicating the presence of fourth-order magnetic anisotropy