996 research outputs found
Enhanced quantum coherence in exchange coupled spins via singlet-triplet transitions
Manipulation of spin states at the single-atom scale underlies spin-based
quantum information processing and spintronic devices. Such applications
require protection of the spin states against quantum decoherence due to
interactions with the environment. While a single spin is easily disrupted, a
coupled-spin system can resist decoherence by employing a subspace of states
that is immune to magnetic field fluctuations. Here, we engineered the magnetic
interactions between the electron spins of two spin-1/2 atoms to create a clock
transition and thus enhance their spin coherence. To construct and electrically
access the desired spin structures, we use atom manipulation combined with
electron spin resonance (ESR) in a scanning tunneling microscope (STM). We show
that a two-level system composed of a singlet state and a triplet state is
insensitive to local and global magnetic field noise, resulting in much longer
spin coherence times compared with individual atoms. Moreover, the spin
decoherence resulting from the interaction with tunneling electrons is markedly
reduced by a homodyne readout of ESR. These results demonstrate that
atomically-precise spin structures can be designed and assembled to yield
enhanced quantum coherence
Colloquium: Atomic spin chains on surfaces
In the present Colloquium, we focus on the properties of 1-D magnetic systems
on solid surfaces. From the emulation of 1-D quantum phases to the potential
realization of Majorana edge states, spin chains are unique systems to study.
The advent of scanning tunnelling microscope (STM) based techniques has
permitted us to engineer spin chains in an atom-by-atom fashion via atom
manipulation and to access their spin states on the ultimate atomic scale.
Here, we present the current state of research on spin correlations and
dynamics of atomic spin chains as studied by the STM. After a brief review of
the main properties of spin chains on solid surfaces, we classify spin chains
according to the coupling of their magnetic moments with the holding substrate.
This classification scheme takes into account that the nature and lifetimes of
the spin-chain excitation intrinsically depend on the holding substrate. We
first show the interest of using insulating layers on metals, which generally
results in an increase in the spin state's lifetimes such that their quantized
nature gets evident and they are individually accessible. Next, we show that
the use of semiconductor substrates promises additional control through the
tunable electron density via doping. When the coupling to the substrate is
increased for spin chains on metals, the substrate conduction electron mediated
interactions can lead to emergent exotic phases of the coupled spin
chain-substrate conduction electron system. A particularly interesting example
is furnished by superconductors. Magnetic impurities induce states in the
superconducting gap. Due to the extended nature of the spin chain, the in-gap
states develop into bands that can lead to the emergence of 1-D topological
superconductivity and, consequently to the appearance of Majorana edge states
Thermomechanical design rules for photovoltaic modules
We present a set of thermomechanical design rules to support and accelerate future (PV) module developments. The design rules are derived from a comprehensive parameter sensitivity study of different PV module layers and material properties by finite element method simulations. We develop a three dimensional finite element method (FEM) model, which models the PV module geometry in detail from busbar and ribbons up to the frame including the adhesive. The FEM simulation covers soldering, lamination, and mechanical load at various temperatures. The FEM model is validated by mechanical load tests on three 60-cell PV modules. Here, for the first time, stress within a solar cell is measured directly using stress sensors integrated in solar cells (SenSoCells®). The results show good accordance with the simulations. The parameter sensitivity study reveals that there are two critical interactions within a PV module: (1) between ribbon and solar cell and (2) between front/back cover and interconnected solar cells. Here, the encapsulant plays a crucial role in how the single layers interact with each other. Therefore, its mechanical properties are essential, and four design rules are derived regarding the encapsulant. Also four design rules concern front and back sides, and three address the solar cells. Finally, two design rules each deal with module size and frame, respectively. Altogether we derive a set of 15 thermomechanical design rules. As a rule of thumb of how well a bill of material will work from a thermomechanical point of view, we introduce the concept of specific thermal expansion stiffness as the product of Young\u27s modulus E, coefficient of thermal expansion , joint area A, and materials height h. The difference between two materials is a measure of how much thermal strain one material can induce in another. A strong difference means that the material with the larger value will induce thermal strain in the other
Sozialpolitik im Transformationsprozeß Mittel- und Osteuropas.
Sozialpolitik; Systemtransformation; Soziale Kosten; Polen; Tschechische Republik; Russland;
Anisotropic hyperfine interaction of surface-adsorbed single atoms
Hyperfine interactions between electron and nuclear spins have been widely
used in material science, organic chemistry, and structural biology as a
sensitive probe to the local chemical environment through spatial
identification of nuclear spins. With the nuclear spins identified, the
isotropic and anisotropic components of the hyperfine interactions in turn
offer unique insight into the electronic ground-state properties of the
paramagnetic centers. However, traditional ensemble measurements of hyperfine
interactions average over a macroscopic number of spins with different
geometrical locations and nuclear isotopes. Here, we use a scanning tunneling
microscope (STM) combined with electron spin resonance (ESR) to measure
hyperfine spectra of hydrogenated-titanium (Ti) atoms on MgO/Ag(100) and
thereby determine the isotropic and anisotropic hyperfine interactions at the
single-atom level. By combining vector-field ESR spectroscopy with STM-based
atom manipulation, we characterize the full hyperfine tensor of individual
Ti-47 and Ti-49 atoms and identify significant spatial anisotropy of hyperfine
interaction for both isotopes when they are adsorbed at low-symmetry binding
sites. Density functional theory calculations reveal that the large hyperfine
anisotropy arises from a highly anisotropic distribution of the ground-state
electron spin density. Our work highlights the power of ESR-STM-enabled
single-atom hyperfine spectroscopy as a powerful tool in revealing ground-state
electronic structures and atomic-scale chemical environments with
nano-electronvolt resolution.Comment: 17 pages, 4 figure
Electrically Driven Spin Resonance of 4f Electrons in a Single Atom on a Surface
A pivotal challenge in present quantum technologies lies in reconciling long
coherence times with efficient manipulation of the quantum states of a system.
Lanthanide atoms, with their well-localized 4f electrons, emerge as a promising
solution to this dilemma if provided with a rational design of the manipulation
and detection schemes. Here we utilize a scanning tunneling microscope to
construct tailored spin structures and perform electron spin resonance on a
single lanthanide atom in such a structure. A magnetically coupled structure
made of an Erbium and a Titanium atom at sub-nanometer distance enables us to
both drive Erbium's 4f electron spins and indirectly probe them through the
Titanium's 3d electrons. In this coupled configuration, the Erbium spin states
exhibit a four-fold increase in the spin relaxation time and a two-fold
increase in the driving efficiency compared to the 3d electron counterparts.
Our work provides a new approach to accessing highly protected spin states,
enabling us to control them in an all-electric fashion
Physical Fitness Training in Patients with Subacute Stroke (PHYS-STROKE): multicentre, randomised controlled, endpoint blinded trial
OBJECTIVE:
To determine the safety and efficacy of aerobic exercise on activities of daily living in the subacute phase after stroke.
DESIGN:
Multicentre, randomised controlled, endpoint blinded trial.
SETTING:
Seven inpatient rehabilitation sites in Germany (2013-17).
PARTICIPANTS:
200 adults with subacute stroke (days 5-45 after stroke) with a median National Institutes of Health stroke scale (NIHSS, range 0-42 points, higher values indicating more severe strokes) score of 8 (interquartile range 5-12) were randomly assigned (1:1) to aerobic physical fitness training (n=105) or relaxation sessions (n=95, control group) in addition to standard care.
INTERVENTION:
Participants received either aerobic, bodyweight supported, treadmill based physical fitness training or relaxation sessions, each for 25 minutes, five times weekly for four weeks, in addition to standard rehabilitation therapy. Investigators and endpoint assessors were masked to treatment assignment.
MAIN OUTCOME MEASURES:
The primary outcomes were change in maximal walking speed (m/s) in the 10 m walking test and change in Barthel index scores (range 0-100 points, higher scores indicating less disability) three months after stroke compared with baseline. Safety outcomes were recurrent cardiovascular events, including stroke, hospital readmissions, and death within three months after stroke. Efficacy was tested with analysis of covariance for each primary outcome in the full analysis set. Multiple imputation was used to account for missing values.
RESULTS:
Compared with relaxation, aerobic physical fitness training did not result in a significantly higher mean change in maximal walking speed (adjusted treatment effect 0.1 m/s (95% confidence interval 0.0 to 0.2 m/s), P=0.23) or mean change in Barthel index score (0 (-5 to 5), P=0.99) at three months after stroke. A higher rate of serious adverse events was observed in the aerobic group compared with relaxation group (incidence rate ratio 1.81, 95% confidence interval 0.97 to 3.36).
CONCLUSIONS:
Among moderately to severely affected adults with subacute stroke, aerobic bodyweight supported, treadmill based physical fitness training was not superior to relaxation sessions for maximal walking speed and Barthel index score but did suggest higher rates of adverse events. These results do not appear to support the use of aerobic bodyweight supported fitness training in people with subacute stroke to improve activities of daily living or maximal walking speed and should be considered in future guidelines.
TRIAL REGISTRATION:
ClinicalTrials.gov NCT01953549
Switching Magnetism and Superconductivity with Spin-Polarized Current in Iron-Based Superconductor
We have explored a new mechanism for switching magnetism and
superconductivity in a magnetically frustrated iron-based superconductor using
spin-polarized scanning tunneling microscopy (SPSTM). Our SPSTM study on single
crystal SrVOFeAs shows that a spin-polarized tunneling current can
switch the Fe-layer magnetism into a non-trivial (22) order, not
achievable by thermal excitation with unpolarized current. Our tunneling
spectroscopy study shows that the induced (22) order has
characteristics of plaquette antiferromagnetic order in Fe layer and strongly
suppressed superconductivity. Also, thermal agitation beyond the bulk Fe spin
ordering temperature erases the state. These results suggest a new
possibility of switching local superconductivity by changing the symmetry of
magnetic order with spin-polarized and unpolarized tunneling currents in
iron-based superconductors.Comment: 33 pages, 16 figure
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