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ABSTRACT: Inelastic electron scattering is applied to investigate the impact of
potassium intercalation on the charge carrier plasmon energy and dispersion in
the charge-density wave (CDW) bearing compound 2H-tantalum-diselenide. We
observe an unususal doping dependence of the plasmon dispersion, which even
changes sign upon alkali addition. In contrast to the continous energy shift of
the plasmon position upon doping at lowest momentum transfer, its dispersion
changes in a rather discontinuous manner. We argue that the observed dynamics
can only be explained in a picture, where complex phenomena are taken into
account including the suppression of the CDW upon doping as well as the
interplay of the CDW and the plasma resonance.
04/2013;
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ABSTRACT: We present a detailed $^7$Li- and $^{31}$P-NMR study on single crystalline
LiMnPO$_4$ in the paramagnetic and antiferromagnetic phase (AFM, $T_N \sim$ 34
K). This allows us to determine the spin directions in the field-induced
spin-flop phase. In addition, the anisotropic dipolar hyperfine coupling tensor
of the $^7$Li- and $^{31}$P-nuclei is also fully determined by orientation and
temperature dependent NMR experiments and compared to the calculated values
from crystal structure data. Deviations of the experimental values from the
theoretical ones are discussed in terms of Mn disorder which is induced by
Li-disorder. In fact, the disorder in the Mn-sublattice is directly revealed by
our diffuse x-ray scattering data. The present results provide experimental
evidence for the Li-diffusion strongly coupling to structural distortions
within the MnPO$_4$ host, which is expected to significantly affect the
Li-mobility as well as the performance of batteries based on this material.
04/2013;
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Matthias Golecki,
Jochen Lach,
Alexander Jeremies,
Frank Lungwitz,
Michael Fronk,
Georgeta Salvan,
Dietrich R T Zahn,
Jaena Park,
Yulia Krupskaya,
Vladislav Kataev, [......],
Benjamin Mahns,
Martin Knupfer,
Pablo F Siles,
Daniel Grimm,
Oliver G Schmidt,
Andreas Reis,
Werner R Thiel,
Daniel Breite,
Bernd Abel,
Berthold Kersting
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ABSTRACT: A new strategy for the fixation of redox-active dinickel(II) complexes with high-spin ground states to gold surfaces was developed. The dinickel(II) complex [Ni2 L(Cl)]ClO4 (1ClO4 ), in which L(2-) represents a 24-membered macrocyclic hexaaza-dithiophenolate ligand, reacts with ambidentate 4-(diphenylphosphino)benzoate (dppba) to form the carboxylato-bridged complex [Ni2 L(dppba)](+) , which can be isolated as an air-stable perchlorate [Ni2 L(dppba)]ClO4 (2ClO4 ) or tetraphenylborate [Ni2 L(dppba)]BPh4 (2BPh4 ) salt. The auration of 2ClO4 was probed on a molecular level, by reaction with AuCl, which leads to the monoaurated Ni(II) 2 Au(I) complex [Ni(II) 2 L(dppba)Au(I) Cl]ClO4 (3ClO4 ). Metathesis of 3ClO4 with NaBPh4 produces [Ni(II) 2 L(dppba)Au(I) Ph]BPh4 (4BPh4 ), in which the Cl(-) is replaced by a Ph(-) group. The complexes were fully characterized by ESI mass spectrometry, IR and UV/Vis spectroscopy, X-ray crystallography (2BPh4 and 4BPh4 ), cyclic voltammetry, SQUID magnetometry and HF-ESR spectroscopy. Temperature-dependent magnetic susceptibility measurements reveal a ferromagnetic coupling J=+15.9 and +17.9 cm(-1) between the two Ni(II) ions in 2ClO4 and 4BPh4 (H=-2 JS1 S2 ). HF-ESR measurements yield a negative axial magnetic anisotropy (D<0), which implies a bistable (easy axis) magnetic ground state. The binding of the [Ni2 L(dppba)]ClO4 complex to gold was ascertained by four complementary surface analytical methods: contact angle measurements, atomic-force microscopy, X-ray photoelectron spectroscopy, and spectroscopic ellipsometry. The results indicate that the complexes are attached to the Au surface through coordinative AuP bonds in a monolayer.
Chemistry 04/2013; · 5.93 Impact Factor
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ABSTRACT: The nature of low energy plasmon excitations plays an important role in
understanding the low energy electronic properties and coupling mechanism of
different superconducting compounds such as CaC$_6$. Recent ab-initio studies
predict a charge carrier intraband plasmon in keeping with a low energy
acoustic plasmon. Here, we have studied the low-energy electronic excitations
of CaC$_6$ using high-resolution electron energy-loss spectroscopy in
transmission at low temperatures. The analysis of the core-level excitations
leads to the conclusion that hybridization between graphite and calcium states
plays an essential role in this graphite intercalated compound. Regarding the
low energy plasmon excitation, we observe the formation of an intraband (charge
carrier) plasmon with a negative dispersion at about 3.5 eV in sound agreement
with the theory. Finally, a weak excitation around 1.2 eV with an almost linear
dispersion relation can be observed as predicted for an acoustic plasmon that
may mediate the superconducting coupling in CaC$_6$. However its optical limit
at ~1 eV challenges the theoretical predictions and safely rules out an
electronic superconducting coupling mechanism in CaC$_6$.
03/2013;
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Claude Monney,
Valentina Bisogni,
Ke-Jin Zhou,
Roberto Kraus,
Vladimir N Strocov,
Günter Behr,
Jiři Málek,
Roman Kuzian,
Stefan-Ludwig Drechsler,
Steve Johnston,
Alexandre Revcolevschi, Bernd Büchner,
Henrik M Rønnow,
Jeroen van den Brink,
Jochen Geck,
Thorsten Schmitt
[show abstract]
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ABSTRACT: We report a high-resolution resonant inelastic soft x-ray scattering study of the quantum magnetic spin-chain materials Li_{2}CuO_{2} and CuGeO_{3}. By tuning the incoming photon energy to the oxygen K edge, a strong excitation around 3.5 eV energy loss is clearly resolved for both materials. Comparing the experimental data to many-body calculations, we identify this excitation as a Zhang-Rice singlet exciton on neighboring CuO_{4} plaquettes. We demonstrate that the strong temperature dependence of the inelastic scattering related to this high-energy exciton enables us to probe short-range spin correlations on the 1 meV scale with outstanding sensitivity.
Physical Review Letters 02/2013; 110(8):087403. · 7.37 Impact Factor
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ABSTRACT: Half-metallic ferromagnetism stands for the technologically sought-after metallicity with 100% spin polarization. Electrical transport should, in principle, sensitively probe half-metallic ferromagnetism, since electron-magnon scattering processes are expected to be absent, with clear-cut consequences for the resistivity and the magnetoresistance. Here we present electrical transport data for single-crystalline Co_{2}FeSi, a candidate half-metallic ferromagnet Heusler compound. The data reveal a textbooklike exponential suppression of the electron-magnon scattering rate with decreasing temperature which provides strong evidence that this material indeed possesses perfect spin polarization at low temperature. However, the energy scale for thermally activated spin-flip scattering is relatively low (activation gap Δ≈100 K) which has decisive influence on the magnetoresistance and the anomalous Hall effect, which exhibit strong qualitative changes when crossing T≈100 K.
Physical Review Letters 02/2013; 110(6):066601. · 7.37 Impact Factor
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Ronny Schönfelder,
Alicja Bachmatiuk,
Franziska Schäffel,
Jürgen Eckert,
Jürgen Thomas,
Ewa Borowiak-Palen,
Sandeep Gorantla,
Bernd Büchner,
Boris I Yakobson,
Felix Börrnert,
Mark H Rümmeli,
Maria Dimitrakopoulou, Jamie H Warner,
Thomas Gemming
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ABSTRACT: We have examined the singlet excitons in two representatives of acene-type
(tetracene and pentacene) and phenacene-type (chrysene and picene) molecular
crystals, respectively, using electron energy-loss spectroscopy at low
temperatures. We show that the excitation spectra of the two hydrocarbon
families significantly differ. Moreover, close inspection of the data indicates
that there is an increasing importance of charge-transfer excitons at lowest
excitation energy with increasing length of the molecules.
01/2013;
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ABSTRACT: Several angle-resolved photoemission spectroscopy (ARPES) studies reveal a poorly nested Fermi surface of LiFeAs, far away from a spin density wave instability, and clear-cut superconducting gap anisotropies. On the other hand a very different, more nested Fermi surface and dissimilar gap anisotropies have been obtained from quasiparticle interference (QPI) data, which were interpreted as arising from intraband scattering within holelike bands. Here we show that this ARPES-QPI paradox is completely resolved by interband scattering between the holelike bands. The resolution follows from an excellent agreement between experimental quasiparticle scattering data and T-matrix QPI calculations (based on experimental band structure data), which allows disentangling interband and intraband scattering processes.
Physical Review Letters 01/2013; 110(1):017006. · 7.37 Impact Factor
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ABSTRACT: Detailed 57Fe Mossbauer spectroscopy measurements on superconducting NaFeAs
powder samples have been performed in the temperature range 13 K < T < 300 K.
The 57Fe spectra recorded in the paramagnetic range (T > TN ~ 46 K) were
discussed supposing that most of the Fe2+ ions are located in distorted (FeAs4)
tetrahedral of NaFeAs phase, while additional minor (< 10%) component of the
spectra corresponds to impurity or intergrowth NaFe2As2 phase with a nominal
composition near "NaFe2As2". Our results reveal that the structural transition
(TS ~ 55K) has a weak effect on the electronic structure of iron ions, while at
T < TN the spectra show a continuous distribution of hyperfine fields HFe .
Shape of these spectra was analyzed in term of two models: (i) an
incommensurate spin density wave modulation of iron magnetic structure, (ii) a
formation of a microdomain structure or phase separation. It was shown that the
hyperfine parameters obtained using these two methods have very similar values
over the whole temperature range. The analysis of the temperature dependence
HFe(T) with the Bean-Rodbell model leads to ksi = 1.16 +/- 0.05, suggesting
that the magnetic phase transition is first-order in nature. A sharp evolution
of the VZZ(T) and etha(T) parameters of the full Hamiltonian of hyperfine
interactions near T ~ (TN, TS), were interpreted as a manifestation of the
anisotropic electron redistribution between the dxz, dyz- and dxy-orbitals of
the iron ions.
11/2012;
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Hemke Maeter,
Gwendolyne Pascua,
Hubertus Luetkens,
Johannes Knolle,
Saicharan Aswartham,
Sabine Wurmehl,
Günter Behr, Bernd Büchner,
Zurab Shermadini,
Kamil Sedlak,
Alex Amato,
Roderich Moessner,
Ilya Eremin,
Hans-Henning Klauss
[show abstract]
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ABSTRACT: We study the interplay of magnetic and superconducting order in single
crystalline hole doped Ba1-xNaxFe2As2 using muon spin relaxation. We find
microscopic coexistence of magnetic order and superconductivity. In a strongly
underdoped specimen the two forms of order coexist without any measurable
reduction of the ordered magnetic moment by superconductivity, while in a
nearly optimally doped sample the ordered magnetic moment is strongly
suppressed below the superconducting transition temperature. This coupling can
be well described within the framework of an effective two-band model
incorporating inter- and intra-band interactions. In optimally doped
Ba1-xNaxFe2As2 we observe no traces of static or dynamic magnetism and the
temperature dependence of the superfluid density is consistent with two s-wave
gaps without nodes.
10/2012;
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Hemke Maeter,
Jorge Enrique Hamann Borrero,
Til Goltz,
Johannes Spehling,
Andrej Kwadrin,
Agnieszka Kondrat,
Louis Veyrat,
Guillaume Lang,
Hans-Joachim Grafe,
Christian Hess,
Günter Behr, Bernd Büchner,
Hubertus Luetkens,
Chris Baines,
Alex Amato,
Norman Leps,
Rüdiger Klingeler,
Ralf Feyerherm,
Dimitri Argyriou,
Hans-Henning Klauss
[show abstract]
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ABSTRACT: We have studied the structural and electronic phase diagrams of CeFeAsO1-xFx
and SmFeAsO1-xFx by a detailed analysis of muon spin relaxation experiments,
synchrotron X-ray diffraction, M\"ossbauer spectroscopy, electrical
resistivity, specific heat, and magnetic susceptibility measurements (Full
abstract in the main document).
10/2012;
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ABSTRACT: Treatment of the diethyl ester of o-phenylenebis(oxamic acid) (opbaH(2)Et(2), ) with 5/6 equivalent of MeNH(2) in abs. EtOH results in the exclusive formation of the ethyl ester of o-phenylene(N'-methyl oxamide)(oxamic acid) (opooH(3)EtMe, ) in ca. 50% yield. Treatment of with four equivalents of [Me(4)N]OH followed by the addition of Cu(ClO(4))(2)·6H(2)O gave [Me(4)N](2)[Cu(opooMe)]·H(2)O () in ca. 80% yield. As appears to be a hygroscopic solid, the related [(n)Bu(4)N](+) salts [(n)Bu(4)N](2)[M(opooMe)]·H(2)O (M = Cu (), Ni ()) have been synthesized. By addition of two equivalents of [Cu(pmdta)(NO(3))(2)] to a MeCN solution of the novel asymmetric trinuclear complex [Cu(3)(opooMe)(pmdta)(2)](NO(3))(2) () could be obtained in ca. 90% yield. Compounds , , , and have been characterized by elemental analysis and NMR/IR spectroscopy. Furthermore, the solid state structures of in the form of [Me(4)N](2)[Cu(opooMe)]·MeOH (), in the form of [(n)Bu(4)N](2)[Cu(opooMe)] (), in the form of [(n)Bu(4)N](2)[Ni(opooMe)]·1.25H(2)O () and in the form of [Cu(3)(opooMe)(pmdta)(2)] (NO(3))(2)·3MeCN (), respectively, have been determined by single-crystal X-ray diffraction studies. By controlled cocrystallization, diamagnetically diluted (1%) in the host lattice of (99%) in the form of single crystals have been made available, allowing single crystal EPR studies to extract all components of the g-factor and the tensors of onsite (Cu)A and transferred (N)A hyperfine interaction. Out of these studies the spin density distribution of the [Cu(opooMe)](2-) complex fragment could be determined. The magnetic properties of were studied by susceptibility measurements versus temperature. An intramolecular J parameter of -65 cm(-1) has been obtained, unexpectedly, as should possess two different J values due to its two different spacers between the adjacent Cu(II) ions, namely an oxamate (C(2)NO(3)) and an oxamidate (C(2)N(2)O(2)) fragment. This unexpected result is explained by a summarizing discussion of the experimentally obtained EPR results (spin density distribution) of , the geometries of the terminal [Cu(pmdta)](2+) fragments of determined by X-ray crystallographic studies and accompanying quantum chemical calculations of the spin density distribution of the mononuclear [Cu(opooMe)](2-) and of the magnetic exchange interactions of trinuclear [Cu(3)(opooMe)(pmdta)(2)](2+) complex fragments.
Dalton Transactions 10/2012; · 3.84 Impact Factor
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ABSTRACT: Amorphous carbon irradiated by electrons at acceleration voltages of 80 kV is studied in high-resolution transmission electron microscopy. Amorphous carbon deposited on graphene or h-BN membranes forms graphene layers parallel to the support due to van der Waals interactions. One can use deposited amorphous carbon to engineer graphene either for its catalyst-free fabrication or its destruction.
Advanced Materials 08/2012; · 13.88 Impact Factor
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ABSTRACT: We present electrical transport data for single-crystalline Co$_2$FeSi which
provide clear-cut evidence that this Heusler compound is truly a half-metallic
ferromagnet, i.e. it possesses perfect spin-polarization. More specifically,
the temperature dependence of $\rho$ is governed by electron scattering off
magnons which are thermally excited over a sizeable gap $\Delta\approx 100 K$
($\sim 9 meV$) separating the electronic majority states at the Fermi level
from the unoccupied minority states. As a consequence, electron-magnon
scattering is only relevant at $T\gtrsim\Delta$ but freezes out at lower
temperatures, i.e., the spin-polarization of the electrons at the Fermi level
remains practically perfect for $T\lesssim\Delta$. The gapped magnon population
has a decisive influence on the magnetoresistance and the anomalous Hall effect
(AHE): i) The magnetoresistance changes its sign at $T\sim 100 K$, ii) the
anomalous Hall coefficient is strongly temperature dependent at $T\gtrsim 100
K$ and compatible with Berry phase related and/or side-jump electronic
deflection, whereas it is practically temperature-independent at lower
temperatures.
07/2012;
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ABSTRACT: Many physical and chemical properties of semiconducting nanocrystals strongly depend on their spatial dimensions and crystallographic structure. For these reasons, achieving a high degree of size and shape control plays an important role with respect to their application potential. In this report we present a facile route for the direct colloidal synthesis of copper(I) sulfide nanorods. A high reactivity of the starting materials is essential to obtain nanorods. We achieve this by using a thiol that thermally decomposes easily and serves as the sulfur source. The thiol is mixed in a noncoordinating solvent, which acts as the reaction medium. Adjustment of the nucleation temperature makes it possible to tailor uniform nanorods with lengths from 10 to 100 nm. The nanorods are single crystalline, and the growth direction is shown to occur along the a-axis of djurleite. The growth process and character of the nanorods were investigated through UV-vis and NIR absorption spectroscopy, transmission electron microscopy, and powder X-ray diffraction measurements.
ACS Nano 06/2012; 6(7):5889-96. · 10.77 Impact Factor
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ABSTRACT: We have prepared potassium intercalated 1,2;8,9-dibenzopentacene films under
vacuum conditions. The evolution of the electronic excitation spectra upon
potassium addition as measured using electron energy-loss spectroscopy clearly
indicate the formation of particular doped phases with compositions
K$_x$dibenzopentacene ($x$ = 1,2,3). Moreover, the stability of these phases as
a function of temperature has been explored. Finally, the electronic excitation
spectra also give insight into the electronic ground state of the potassium
doped 1,2;8,9-dibenzopentacene films.
06/2012;
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ABSTRACT: Single-walled carbon nanotubes (SWCNTs) have attractive electrical and physical properties, which make them very promising for use in various applications. For some applications however, in particular those involving electronics, SWCNTs need to be synthesized with a high degree of control with respect to yield, length, alignment, diameter, and chirality. With this in mind, a great deal of effort is being directed to the precision control of vertically and horizontally aligned nanotubes. In this review the focus is on the latter, horizontally aligned tubes grown by chemical vapor deposition (CVD). The reader is provided with an in-depth review of the established vapor deposition orientation techniques. Detailed discussions on the characterization routes, growth parameters, and growth mechanisms are also provided.
Small 05/2012; 8(13):1973-92. · 8.35 Impact Factor
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Sandeep Gorantla,
Stanislav Avdoshenko,
Felix Börrnert,
Alicja Bachmatiuk,
Maria Dimitrakopoulou,
Franziska Schäffel,
Ronny Schönfelder,
Jürgen Thomas,
Thomas Gemming,
Jamie H. Warner,
Gianaurelio Cuniberti,
Jürgen Eckert, Bernd Büchner,
Mark H. Rümmeli
[show abstract]
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ABSTRACT: In situ low-voltage aberration corrected transmission electron microscopy (TEM) observations of the dynamic entrapment of a C60 molecule in the saddle of a bent double-walled carbon nanotube is presented. The fullerene interaction is non-covalent, suggesting
that enhanced π-π interactions (van der Waals forces) are responsible. Classical molecular dynamics calculations confirm that
the increased interaction area associated with a buckle is sufficient to trap a fullerene. Moreover, they show hopping behavior
in agreement with our experimental observations. Our findings further our understanding of carbon nanostructure interactions,
which are important in the rapidly developing field of low-voltage aberration corrected TEM and nano-carbon device fabrication.
KeywordsCarbon nanotubes-fullerenes-low-voltage transmission electron microscopy-molecule trap
Nano Research 04/2012; 3(2):92-97. · 6.97 Impact Factor
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ABSTRACT: Carbon-encapsulated magnetic nanoparticles are a new class of materials where the core magnetic nanoparticle is protected
from reactions with its environment by graphite shells. Having a structure similar to carbon nanotubes, these nanoparticles
could be potentially functionalized using methods which are already applied to those structures. We present the effects of
acidic treatments based on HCl, HNO3, and H2SO4 on these nanoparticles highlighting the impact on their magnetic and surface properties. We show that acidic treatments based
on HNO3 can be successfully applied for the generation of carboxylic groups on the surface of the nanoparticles. Using methylamine
as a model, we demonstrate that these functional groups can be used for further functionalization with amino-containing biomolecules
via diimide-activated amidation.
Journal of Nanoparticle Research 04/2012; 12(2):513-519. · 3.29 Impact Factor
