394 research outputs found
Direct evidence for efficient ultrafast charge separation in epitaxial WS/graphene heterostructure
We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to
investigate ultrafast charge transfer in an epitaxial heterostructure made of
monolayer WS and graphene. This heterostructure combines the benefits of a
direct gap semiconductor with strong spin-orbit coupling and strong
light-matter interaction with those of a semimetal hosting massless carriers
with extremely high mobility and long spin lifetimes. We find that, after
photoexcitation at resonance to the A-exciton in WS, the photoexcited holes
rapidly transfer into the graphene layer while the photoexcited electrons
remain in the WS layer. The resulting charge transfer state is found to
have a lifetime of \,ps. We attribute our findings to differences in
scattering phase space caused by the relative alignment of WS and graphene
bands as revealed by high resolution ARPES. In combination with spin-selective
excitation using circularly polarized light the investigated WS/graphene
heterostructure might provide a new platform for efficient optical spin
injection into graphene.Comment: 28 pages, 14 figure
Direct evidence for efficient ultrafast charge separation in epitaxial WS<sub>2</sub>/graphene heterostructures
We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast charge transfer in an epitaxial heterostructure made of monolayer WS2 and graphene. This heterostructure combines the benefits of a direct-gap semiconductor with strong spin-orbit coupling and strong light-matter interaction with those of a semimetal hosting massless carriers with extremely high mobility and long spin lifetimes. We find that, after photoexcitation at resonance to the A-exciton in WS2, the photoexcited holes rapidly transfer into the graphene layer while the photoexcited electrons remain in the WS2 layer. The resulting charge-separated transient state is found to have a lifetime of ∼1 ps. We attribute our findings to differences in scattering phase space caused by the relative alignment of WS2 and graphene bands as revealed by high-resolution ARPES. In combination with spin-selective optical excitation, the investigated WS2/graphene heterostructure might provide a platform for efficient optical spin injection into graphene
Experimental time-resolved photoemission and ab initio study of lifetimes of excited electrons in Mo and Rh
We have studied the relaxation dynamics of optically excited electrons in
molybdenum and rhodium by means of time resolved two-photon photoemission
spectroscopy (TR-2PPE) and ab initio electron self-energy calculations
performed within the GW and GW+T approximations. Both theoretical approaches
reproduce qualitatively the experimentally observed trends and differences in
the lifetimes of excited electrons in molybdenum and rhodium. For excitation
energies exceeding the Fermi energy by more than 1 eV, the GW+T theory yields
lifetimes in quantitative agreement with the experimental results. As one of
the relevant mechanisms causing different excited state lifetime in Mo and Rh
we identify the occupation of the 4d bands. An increasing occupation of the 4d
bands results in an efficient decrease of the lifetime even for rather small
excitation energies of a few 100 meV.Comment: 8 pages, 10 figure
Tracking primary thermalization events in graphene with photoemission at extreme timescales
Direct and inverse Auger scattering are amongst the primary processes that
mediate the thermalization of hot carriers in semiconductors. These two
processes involve the annihilation or generation of an electron-hole pair by
exchanging energy with a third carrier, which is either accelerated or
decelerated. Inverse Auger scattering is generally suppressed, as the
decelerated carriers must have excess energies higher than the band gap itself.
In graphene, which is gapless, inverse Auger scattering is instead predicted to
be dominant at the earliest time delays. Here, femtosecond
extreme-ultraviolet pulses are used to detect this imbalance, tracking both the
number of excited electrons and their kinetic energy with time- and
angle-resolved photoemission spectroscopy. Over a time window of approximately
25 fs after absorption of the pump pulse, we observe an increase in conduction
band carrier density and a simultaneous decrease of the average carrier kinetic
energy, revealing that relaxation is in fact dominated by inverse Auger
scattering. Measurements of carrier scattering at extreme timescales by
photoemission will serve as a guide to ultrafast control of electronic
properties in solids for PetaHertz electronics.Comment: 16 pages, 8 figure
Theory for the reduction of products of spin operators
In this study we show that the sum of the powers of arbitrary products of
quantum spin operators such as can be reduced by one
unit, if this sum is equal to 2S+1, S being the spin quantum number. We
emphasize that by a repeated application of this procedure \em all \em
arbitrary spin operator products with a sum of powers larger than 2S can be
replaced by a combination of spin operators with a maximum sum of powers not
larger than 2S. This transformation is exact. All spin operators must belong to
the same lattice site. By use of this procedure the consideration of single-ion
anisotropies and the investigation of the magnetic reorientation within a
Green's function theory are facilitated. Furthermore, it may be useful for the
study of time dependent magnetic properties within the ultrashort (fsec) time
domain.Comment: 11 pages, 1 table, uses rotatin
Dynamics of Excited Electrons in Copper and Ferromagnetic Transition Metals: Theory and Experiment
Both theoretical and experimental results for the dynamics of photoexcited
electrons at surfaces of Cu and the ferromagnetic transition metals Fe, Co, and
Ni are presented. A model for the dynamics of excited electrons is developed,
which is based on the Boltzmann equation and includes effects of
photoexcitation, electron-electron scattering, secondary electrons (cascade and
Auger electrons), and transport of excited carriers out of the detection
region. From this we determine the time-resolved two-photon photoemission
(TR-2PPE). Thus a direct comparison of calculated relaxation times with
experimental results by means of TR-2PPE becomes possible. The comparison
indicates that the magnitudes of the spin-averaged relaxation time \tau and of
the ratio \tau_\uparrow/\tau_\downarrow of majority and minority relaxation
times for the different ferromagnetic transition metals result not only from
density-of-states effects, but also from different Coulomb matrix elements M.
Taking M_Fe > M_Cu > M_Ni = M_Co we get reasonable agreement with experiments.Comment: 23 pages, 11 figures, added a figure and an appendix, updated
reference
Reliability of the Multidimensional Pain Inventory and stability of the MPI classification system in chronic back pain
Contains fulltext :
109346.pdf (publisher's version ) (Open Access)ABSTRACT: BACKGROUND: This cross validation study examined the reliability of the Multidimensional Pain Inventory (MPI) and the stability of the Multidimensional Pain Inventory Classification System of the empirically derived subgroup classification obtained by cluster analysis in chronic musculoskeletal pain. Reliability of the German Multidimensional Pain Inventory was only examined once in the past in a small sample. Previous international studies mainly involving fibromyalgia patients showed that retest resulted in 33-38% of patients being assigned to a different Multidimensional Pain Inventory subgroup classification. METHODS: Participants were 204 persons with chronic musculoskeletal pain (82% chronic non-specific back pain). Subgroup classification was conducted by cluster analysis at 4 weeks before entry (=test) and at entry into the pain management program (=retest) using Multidimensional Pain Inventory scale scores. No therapeutic interventions in this period were conducted. Reliability was quantified by intraclass correlation coefficients (ICC) and stability by kappa coefficients (kappa). RESULTS: Reliability of the Multidimensional Pain Inventory scales was least with ICC = 0.57 for the scale life control and further ranged from ICC = 0.72 (negative mood) to 0.87 (solicitous responses) in the other scales. At retest, 82% of the patients in the Multidimensional Pain Inventory cluster interpersonally distressed (kappa = 0.69), 80% of the adaptive copers (kappa = 0.58), and 75% of the dysfunctional patients (kappa = 0.70) did not change classification. In total, 22% of the patients changed Multidimensional Pain Inventory cluster group, mainly into the adaptive copers subgroup. CONCLUSION: Test-retest reliability of the German Multidimensional Pain Inventory was moderate to good and comparable to other language versions. Multidimensional Pain Inventory subgroup classification is substantially stable in chronic back pain patients when compared to other diagnostic groups and other examiner-based subgroup Classification Systems. The MPI Classification System can be recommended for reliable and stable specification of subgroups in observational and interventional studies in patients with chronic musculoskeletal pain
Epitaxial film growth and magnetic properties of Co_2FeSi
We have grown thin films of the Heusler compound Co_2FeSi by RF magnetron
sputtering. On (100)-oriented MgO substrates we find fully epitaxial
(100)-oriented and L2_1 ordered growth. On Al_2O_3 (11-20) substrates, the film
growth is (110)-oriented, and several in-plane epitaxial domains are observed.
The temperature dependence of the electrical resistivity shows a power law with
an exponent of 7/2 at low temperatures. Investigation of the bulk magnetic
properties reveals an extrapolated saturation magnetization of 5.0 mu_B/fu at 0
K. The films on Al_2O_3 show an in-plane uniaxial anisotropy, while the
epitaxial films are magnetically isotropic in the plane. Measurements of the
X-ray magnetic circular dichroism of the films allowed us to determine element
specific magnetic moments. Finally we have measured the spin polarization at
the surface region by spin-resolved near-threshold photoemission and found it
strongly reduced in contrast to the expected bulk value of 100%. Possible
reasons for the reduced magnetization are discussed.Comment: 9 pages, 12 figure
Etude spectrale de l'échantillonnage par traversée de niveaux
La tendance actuelle en microélectronique est d'intégrer des systèmes complexes sur une seule puce (« SoC » pour Sytems on chip): capteur, convertisseur analogique-numérique (CAN), traitement numérique du signal, transmission de données. Ces systèmes doivent être peu coûteux, de taille réduite, faible bruit et surtout peu consommateur d'énergie parce qu'il sont souvent alimentés par batteries. Parmi les étages cités, les CAN sont les plus difficiles à concevoir dans le respect du cahier des charges. Dans cette perspective, une nouvelle classe de convertisseur a été développée à partir de l'association de la technologie asynchrone et de l'échantillonnage non uniforme par traversée de niveaux. Le schéma d'échantillonnage est analysé pour les signaux périodiques mettant en évidence un repliement de spectre. Cependant l'étude de la reconstruction montre qu'un signal peut être reconstruit grâce à l'information portée par les intervalles de temps et ce malgré le repliement
Magnetostatic coupling of 90° domain walls in Fe19Ni81/Cu/Co trilayers
The magnetic interlayer coupling of Fe19Ni81/Cu/Co trilayered microstructures
has been studied by means of x-ray magnetic circular dichroism in combination
with photoelectron emission microscopy (XMCD-PEEM). We find that a parallel
coupling between magnetic domains coexists with a non-parallel coupling
between magnetic domain walls (DWs) of each ferromagnetic layer. We attribute
the non-parallel coupling of the two magnetic layers to local magnetic stray
fields arising at DWs in the magnetically harder Co layer. In the magnetically
softer FeNi layer, non-ordinary DWs, such as 270° and 90° DWs with overshoot
of the magnetization either inwards or outwards relative to the turning
direction of the Co magnetization, are identified. Micromagnetic simulations
reveal that in the absence of magnetic anisotropy, both types of overshooting
DWs are energetically equivalent. However, if a uniaxial in-plane anisotropy
is present, the relative orientation of the DWs with respect to the anisotropy
axis determines which of these DWs is energetically favorable
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