Sc3CH@C80: selective 13C enrichment of the central carbon atom
K. Junghans, M. Rosenkranz, A. Popov, Chem. Commun., 2016,52, 6561-6564
Sc3CH@C80 is synthesized and characterized by 1H, 13C, and 45Sc NMR. A large negative chemical shift of the proton, −11.73 ppm in the Ih and −8.79 ppm in the D5h C80 cage isomers, is found. 13C satellites in the 1H NMR spectrum enabled indirect determination of the 13C chemical shift for the central carbon at 173 ± 1 ppm. Intensity of the satellites allowed determination of the 13C content for the central carbon atom. This unique possibility is applied to analyze the cluster/cage 13C distribution in mechanistic studies employing either 13CH4 or 13C powder to enrich Sc3CH@C80 with 13C.
Self-assembly of endohedral metallofullerenes: a decisive role of cooling gas and metal–carbon bonding
Q. Deng, T. Heine, S. Irle, A. Popov; Nanoscale 2016, 8, 3796-3808
The endohedral metallofullerene (EMF) self-assembly process in Sc/carbon vapor in the presence and absence of an inert cooling gas (helium) is systematically investigated using quantum chemical molecular dynamics simulations. It is revealed that the presence of He atoms accelerates the formation of pentagons and hexagons and reduces the size of the self-assembled carbon cages in comparison with analogous He-free simulations. As a result, the Sc/C/He system simulations produce a larger number of successful trajectories (i.e. leading to Sc-EMFs) with more realistic cage-size distribution than simulations of the Sc/C system. The main Sc encapsulation mechanism involves nucleation of several hexagons and pentagons with Sc atoms already at the early stages of carbon vapor condensation. In such proto-cages, both Sc–C σ-bonds and coordination bonds between Sc atoms and the π-system of the carbon network are present. Sc atoms are thus rather labile and can move along the carbon network, but the overall bonding is sufficiently strong to prevent dissociation even at temperatures around 2000 kelvin. Further growth of the fullerene cage results in the encapsulation of one or two Sc atoms within the fullerene. In agreement with experimental studies, an extension of the simulations to Fe and Ti as the metal component showed that Fe-EMFs are not formed at all, whereas Ti is prone to form Ti-EMFs with small cage sizes, including Ti@C28-Td and Ti@C30-C2v(3).
Direct observation of spin–orbit coupling in iron-based superconductors
S. V. Borisenko, D. V. Evtushinsky, Z.-H. Liu, I. Morozov, R. Kappenberger, S. Wurmehl, B. Büchner, A. N. Yaresko, T. K. Kim, M. Hoesch, T. Wolf & N. D. Zhigadlo; Nature Physics (2015)
Spin–orbit coupling is a fundamental interaction in solids that can induce a broad range of unusual physical properties, from topologically non-trivial insulating states to unconventional pairing in superconductors. In iron-based superconductors its role has, so far, not been considered of primary importance, with models based on spin- or orbital fluctuations pairing being used most widely. Using angle-resolved photoemission spectroscopy, we directly observe a sizeable spin–orbit splitting in all the main members of the iron-based superconductors. We demonstrate that its impact on the low-energy electronic structure and details of the Fermi surface topology is stronger than that of possible nematic ordering. The largest pairing gap is supported exactly by spin–orbit-coupling-induced Fermi surfaces, implying a direct relation between this interaction and the mechanism of high-temperature superconductivity.
Nesting-driven multipolar order in CeB6 from photoemission tomography
A. Koitzsch, N. Heming, M. Knupfer, B. Büchner, P. Y. Portnichenko, A. V Dukhnenko, N. Y. Shitsevalova, V. B. Filipov, L. L. Lev, V. N. Strocov, J. Ollivier, D. S. Inosov Nature Communications 7, Article Nr. 10876
Some heavy fermion materials show so-called hidden-order phases which are invisible to many characterization techniques and whose microscopic origin remained controversial for decades. Among such hidden-order compounds, CeB6 is of model character due to its simple electronic configuration and crystal structure. Apart from more conventional antiferromagnetism, it shows an elusive phase at low temperatures, which is commonly associated with multipolar order. Here we show that this phase roots in a Fermi surface instability. This conclusion is based on a full 3D tomographic sampling of the electronic structure by angle-resolved photoemission and comparison with inelastic neutron scattering data. The hidden order is mediated by itinerant electrons. Our measurements will serve as a paradigm for the investigation of hidden-order phases in f-electron systems, but also generally for situations where the itinerant electrons drive orbital or spin order.
Complex field-induced states in Linarite with a variety of high-order exotic SDWp states
B. Willenberg, M. Schäpers, A. U. B. Wolter, S.-L. Drechsler, M. Reehuis, J.-U. Hoffmann, B. Büchner, A. J. Studer, K. C. Rule, B. Ouladdiaf, S. Süllow, and S. Nishimotom; Physical Review Letter 116, 047202 (2016)
Low-temperature neutron diffraction and NMR studies of field-induced phases in linarite are presented for magnetic fields H || b axis. A two-step spin-flop transition is observed as well as a transition transforming a helical magnetic ground state into an unusual magnetic phase with sine-wave modulated moments || H. An effective single-chain model with a magnetization-dependent frustration ratio αeff is proposed. The latter is governed by skew interchain couplings and shifted to the vicinity of the ferromagnetic critical point. It explains qualitatively the observation of a rich variety of exotic longitudinal collinear spin-density wave SDWp states (9 ≥ p ≥ 2).
Synthesis and Structure of LaSc2N@Cs(hept)‐C80 with One Heptagon and Thirteen Pentagons
Y. Zhang, K. B. Ghiassi, Q. Deng, N. Samoylova, M. M. Olmstead, A. L. Balch, A. A. Popov; Angew. Chem. Int. Ed. 2015, 54 (2), 495-499
The synthesis and single-crystal X-ray structural characterization of the first endohedral metallofullerene to contain a heptagon in the carbon cage are reported. The carbon framework surrounding the planar LaSc2N unit in LaSc2N@Cs(hept)-C80 consists of one heptagon, 13 pentagons, and 28 hexagons. This cage is related to the most abundant Ih-C80 isomer by one Stone–Wales-like, heptagon/pentagon to hexagon/hexagon realignment. DFT computations predict that LaSc2N@Cs(hept)-C80 is more stable than LaSc2N@D5h-C80, and suggests that the low yield of the heptagon-containing endohedral fullerene may be caused by kinetic factors.
Magnetic Anisotropy of Endohedral Lanthanide Ions: Paramagnetic NMR Study of MSc2N@C80-Ih with M running through the Whole 4f Row
Y. Zhang, D. Krylov, M. Rosenkranz, S. Schiemenz, A. A. Popov; Chem. Sci. 2015, 6, 2328-2341
Paramagnetic and variable temperature 13C and 45Sc nuclear magnetic resonance studies are performed for nitride clusterfullerenes MSc2N@C80 with icosahedral Ih(7) carbon cage, where M runs through all lanthanides forming nitride clusters. The influence of the endohedral lanthanide ions on the NMR spectral pattern is carefully followed, and dramatic differences are found in peak positions and line widths. Thus, 13C lines broaden from 0.01–0.02 ppm in diamagnetic MSc2N@C80 molecules (M = La, Y, Lu) to several ppm in TbSc2N@C80 and DySc2N@C80. Direction of the paramagnetic shift depends on the shape of the 4f electron density in corresponding lanthanide ions. In TmSc2N@C80 and ErSc2N@C80 with prolate 4f-density of lanthanide ions, 13C signals are shifted down-field, whereas 45Sc peaks are shifted up-field versus diamagnetic values. In all other MSc2N@C80 molecules lanthanide ions have oblate-shaped 4f electron density, and the lanthanide-induced shift is negative for 13C and positive for 45Sc peaks. Analysis of the pseudocontact and contact contributions to chemical shifts revealed that the pseudocontact term dominates both in 13C and 45Sc NMR spectra, although contact shifts for 13C signals are also considerable. Point charge computations of the ligand field splitting are performed to explain experimental results, and showed reasonable agreement with experimental pseudocontact shifts. Nitrogen atom bearing large negative charge and located close to the lanthanide ion results in large magnetic anisotropy of lanthanide ions in nitride clusterfullerenes with quasi-uniaxial ligand field.
Surface Aligned Magnetic Moments and Hysteresis of an Endohedral Single-Molecule Magnet on a Metal
R. Westerström, A.-C. Uldry, R. Stania, J. Dreiser, C. Piamonteze, M. Muntwiler, F. Matsui, S. Rusponi, H. Brune, S. Yang, A. Popov, B. Büchner, B. Delley, T. Greber; Phys. Rev. Lett. 2015, 114, 087201
The interaction between the endohedral unit in the single-molecule magnet Dy2ScN@C80 and a rhodium (111) substrate leads to alignment of the Dy 4f orbitals. The resulting orientation of the Dy2ScN plane parallel to the surface is inferred from comparison of the angular anisotropy of x-ray absorption spectra and multiplet calculations in the corresponding ligand field. The x-ray magnetic circular dichroism is also angle dependent and signals strong magnetocrystalline anisotropy. This directly relates geometric and magnetic structure. Element specific magnetization curves from different coverages exhibit hysteresis at a sample temperature of ∼4 K. From the measured hysteresis curves, we estimate the zero field remanence lifetime during x-ray exposure of a submonolayer to be about 30 seconds.
Methane as a selectivity booster in the synthesis of endohedral fullerenes: towards selective synthesis of the single molecule magnet Dy2TiC@C80 and its congener Dy2TiC2@C80.
K. Junghans, C. Schlesier, A. Kostanyan, N. A. Samoylova, Q. Deng, M. Rosenkranz, S. Schiemenz, R. Westerström, T. Greber, B. Büchner, A. A. Popov; Angew. Chem. Int. Ed. 2015, 54 (45), 13411-13415
The use of methane as a reactive gas dramatically increases the selectivity of the arc-discharge synthesis of M-Ti-carbide clusterfullerenes (M=Y, Nd, Gd, Dy, Er, Lu). Optimization of the process parameters allows the synthesis of Dy2TiC@C80-I and its facile isolation in a single chromatographic step. A new type of cluster with an endohedral acetylide unit, M2TiC2@C80, is discovered along with the second isomer of M2TiC@C80. Dy2TiC@C80-(I,II) and Dy2TiC2@C80-I are shown to be single-molecule magnets (SMM), but the presence of the second carbon atom in the cluster Dy2TiC2@C80 leads to substantially poorer SMM properties.
Orbital reconstruction in nonpolar tetravalent transition-metal oxide layers
N. A. Bogdanov, V. M. Katukuri, J. Romhányi, V. Yushankhai, V. Kataev, B. Büchner, J. van den Brink, and L. Hozoi Nature Communications 6, Article number:7306
A promising route to tailoring the electronic properties of quantum materials and devices rests on the idea of orbital engineering in multilayered oxide heterostructures. Here we show that the interplay of interlayer charge imbalance and ligand distortions provides a knob for tuning the sequence of electronic levels even in intrinsically stacked oxides. We resolve in this regard the d-level structure of layered Sr2IrO4 by electron spin resonance. While canonical ligand-field theory predicts g||-factors less than 2 for positive tetragonal distortions as present in Sr2IrO4, the experiment indicates g|| is greater than 2. This implies that the iridium d levels are inverted with respect to their normal ordering. State-of-the-art electronic-structure calculations confirm the level switching in Sr2IrO4, whereas we find them in Ba2IrO4 to be instead normally ordered. Given the nonpolar character of the metal-oxygen layers, our findings highlight the tetravalent transition-metal 214 oxides as ideal platforms to explore d-orbital reconstruction in the context of oxide electronics.
Mutual Independence of Critical Temperature and Superfluid Density under Pressure in Optimally Electron-Doped Superconducting LaFeAsO1−xFx
G. Prando, Th. Hartmann, W. Schottenhamel, Z. Guguchia, S. Sanna, F. Ahn, I. Nekrasov, C.G.F. Blum, A.U.B. Wolter, S. Wurmehl, R. Khasanov, I. Eremin, B. Büchner
Phys. Rev. Lett. 114, 247004
The superconducting properties of LaFeAsO1−xFx under conditions of optimal electron doping are investigated upon the application of external pressure up to ∼23 kbar. Measurements of muon-spin spectroscopy and dc magnetometry evidence a clear mutual independence between the critical temperature Tc and the low-temperature saturation value for the ratio ns/m∗ (superfluid density over effective band mass of Cooper pairs). Remarkably, a dramatic increase of ∼30% is reported for ns/m∗ at the maximum pressure value while Tc is substantially unaffected in the whole accessed experimental window. We argue and demonstrate that the explanation for the observed results must take the effect of nonmagnetic impurities on multiband superconductivity into account. In particular, the unique possibility to modify the ratio between intraband and interband scattering rates by acting on structural parameters while keeping the amount of chemical disorder constant is a striking result of our proposed model.
Orbital-driven nematicity in FeSe
S-H. Baek, D. V. Efremov, J. M. Ok, J. S. Kim, J. van den Brink, B. Büchner
Nature Materials 14,210–214 (2015)
A fundamental and unconventional characteristic of superconductivity in iron-based materials is that it occurs in the vicinity of two other instabilities. In addition to a tendency towards magnetic order, these Fe-based systems have a propensity for nematic ordering: a lowering of the rotational symmetry while time-reversal invariance is preserved. Setting the stage for superconductivity, it is heavily debated whether the nematic symmetry breaking is driven by lattice, orbital or spin degrees of freedom. Here, we report a very clear splitting of NMR resonance lines in FeSe at Tnem = 91 K, far above the superconducting Tc of 9.3 K. The splitting occurs for magnetic fields perpendicular to the Fe planes and has the temperature dependence of a Landau-type order parameter. Spin–lattice relaxation rates are not affected at Tnem, which unequivocally establishes orbital degrees of freedom as driving the nematic order. We demonstrate that superconductivity competes with the emerging nematicity.
Femtosecond Dynamics of Momentum-Dependent Magnetic Excitations from Resonant Inelastic X-Ray Scattering in CaCu2O3
V. Bisogni, S. Kourtis, C. Monney, K. Zhou, R. Kraus, C. Sekar, V. Strocov, B. Büchner, J. van den Brink, L. Braicovich, T. Schmitt, M. Daghofer, J. Geck
Phys. Rev. Lett. 112, 147401 (2014)
Taking spinon excitations in the quantum antiferromagnet CaCu2O3 as an example, we could show that femtosecond dynamics of magnetic electronic excitations can be probed by direct resonant inelastic x-ray scattering (RIXS). To this end, we isolate the contributions of single and double spin-flip excitations in experimental RIXS spectra, identify the physical mechanisms that cause them, and determine their respective time scales. By comparing theory and experiment, we find that double spin flips need a finite amount of time to be generated, rendering them sensitive to the core-hole lifetime, whereas single spin flips are, to a very good approximation, independent of it. This shows that RIXS can grant access to time-domain dynamics of excitations and illustrates how RIXS experiments can distinguish between excitations in correlated electron systems based on their different time dependence.
Emergence of charge density wave domain walls above the superconducting dome in 1T-TiSe2Y
I. Joe, X. M. Chen, P. Ghaemi, K. D. Finkelstein, G. A. de la Peña, Y. Gan, J. C. T. Lee, S. Yuan, J. Geck, G. J. MacDougall, T. C. Chiang, S. L. Cooper, E. Fradkin, P. Abbamont
Nature Physics 10, 421–425 (2014)
Superconductivity in unconventional superconductors is nearly always found in the vicinity of another ordered state, such as antiferromagnetism, charge density wave (CDW), or stripe order. To better understand this connection, we studied this relation by means of high-pressure X-ray scattering in the prototypical CDW-material TiSe2, which was previously shown to exhibit
superconductivity when the CDW is destabilized by pressure or intercalation of Cu atoms. We succeeded in suppressing the CDW fully to zero temperature, establishing for the first time the existence of a quantum critical point (QCP) at Pc = 5.1 ± 0.2 GPa. The QCP is, however, more than 1 GPa beyond the end of the superconducting region. Unexpectedly, at P = 3 GPa we observed a reentrant incommensurate phase, suggesting that superconductivity in TiSe2 may not be connected to the QCP itself, but to the formation of CDW domain walls.
Size-dependent nanographene oxide as a platform for efficient carboplatin release
S. Makharza, G. Cirillo, A. Bachmatiuk, O. Vittorio, R. G. Mendes, S. Oswald, S. Hampel, M. H. Rümmeli
J. Mater. Chem. B, 1, 6107-6114 (2013)
Nanographene oxides (NGO) with well-defined sizes were produced from graphite via chemical exfoliation and separated into three different size distributions (300 nm, 200 nm, and 100 nm) using intense sonication and sucrose density gradient centrifugation. Prior to carboplatin (CP) loading, the NGO was functionalized with zero generation polyamidoamide (PAMAM) which renders improved dispersibility and stability of the nanocarrier platform in physiological media. Cell viability tests were conducted on pristine NGO samples with average widths of 200 nm and 300 nm that showed a cytotoxic effect on HeLa cancer cells and mesenchymal stem cells at low (50 μg ml−1) and high (100 μg ml−1) concentrations, while the pristine NGO sample with an average width of 100 nm revealed no significant cytotoxicity at 50 μg ml−1, and only recorded a 10% level at 100 μg ml−1. After functionalization with PAMAM, the carrier was found to be able to deliver carboplatin to the cancer cells, by enhancing the drug anticancer efficiency. Moreover, the carboplatin loaded NGO carrier shows no significant effect on the viability of mesenchymal stem cells (hMSCs) even at high concentration (100 μg ml−1).
Half-metallic ferromagnetism with unexpectedly small spin splitting in the Heusler compound Co2FeSi
D. Bombor, C. G. F. Blum, O. Volkonskiy, S. Rodan, S. Wurmehl, C. Hess, B. Büchner
Phys. Rev. Lett. 110, 066601 (2013)
A crucial ingredient of any kind of spintronics, i.e., the exploitation of spin-dependent electron transport phenomena is the realization of highly spin-polarized materials. Half-metallic ferromagnets (HMF) are considered ideal candidates because they are predicted to possess a 100% spin-polarization at the Fermi level EF. The complete absence of minority spin states at EF implies the existence of a gap ∆ which separates the occupied majority states from the unoccupied minority states. Since thus spin flip scattering of conduction electrons is exponentially suppressed, one expects also an exponential suppression of the corresponding characteristic T2 dependence in the electrical resistivity ρ. In a recent Physical Review Letter we presented the electronic transport properties, viz., resistivity, magnetoresistance and Hall effect, of high quality single-crystalline Co2FeSi which is a prime candidate HMF. We observed an exponentially suppressed ~T2 behavior of the resistivity at T≲100 K, and observed pronounced changes at ~100 K in the magnetotransport. Our results represent therefore a textbook example of the expected transport behavior for an HMF, however, with an unexpectedly small Δ~100 K.
Formation of the coherent heavy fermion liquid at the 'hidden order' transition in URu2Si2
S. Chatterjee, J. Trinckauf, T. Hanke, D. E. Shai, J. W. Harter, T. J. Williams, G. M. Luke, K. M. Shen, J. Geck
Phys. Rev. Lett., 110, 186401 (2013)
The interactions between localized and delocalized electrons in the so-called heavy fermion materials result in fascinating and unexpected quantum phenomena that continue to challenge condensed matter researchers. A most notorious example is the enigmatic ‘‘hidden order’’ state in URu2Si2. A large loss of entropy at THO=17.5 K clearly shows that some kind of ordering takes place, but the nature of the underlying microscopic order is still a hotly debated puzzle. We performed a high-resolution angle-resolved photoemission spectra of the heavy-fermion superconductor URu2Si2, which may help to solve it. Detailed measurements as a function of both photon energy and temperature allow us to disentangle a variety of spectral features, revealing the evolution of the low-energy electronic structure across the ‘‘hidden order’’ transition. Above the transition, our measurements already directly reveal the existence of weakly dispersive states below the Fermi level that exhibit a large scattering rate and do not appear to shift from above to below the Fermi level, as previously reported. Upon entering the hidden order phase, these states rapidly hybridize with light conduction band states and transform into a coherent heavy fermion liquid, coincident with a dramatic drop in the scattering rate. This evolution is in stark contrast with the gradual crossover expected in Kondo lattice systems, which we attribute to the coupling of the heavy fermion states to the hidden order parameter.
Facile Nanotube-Assisted Synthesis of Ternary Intermetallic Nanocrystals of the Ferromagnetic Heusler Phase Co2FeGa
M. Gellesch, M. Dimitrakopoulou, M. Scholz, C. G.F. Blum, M. Schulze, J. van den Brink, S. Hampel, S. Wurmehl, B. Büchner
Cryst. Growth & Design, 13, 2707 (2013)
A facile synthesis approach for the synthesis of ternary intermetallic nanocrystals was demonstrated exemplarily with the Heusler compound Co2FeGa. The method, involving prefabricated multiwalled carbon nanotubes which act as a template and protective shell, results in the formation of mainly spherical, chemically stable, and crystalline nanoparticles with a well-defined diameter distribution. As an example, for novel functionalities arising from downscaling a bulk material, we observe an enhancement of the coercive field of the Co2FeGa nanocrystals by a factor of ≈30. Our work can facilitate the exploration and eventually the tuning of physical properties of ternary and other intermetallic compounds at the nanoscale.
Interband Quasiparticle Scattering in Superconducting LiFeAs Reconciles Photoemission and Tunneling Measurements
C. Hess, S. Sykora, T. Hänke, R. Schlegel, D. Baumann, V. B. Zabolotnyy, L. Harnagea, S. Wurmehl, J. van den Brink, B. Büchner
Phys. Rev. Lett. 110, 017006 (2013)
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.
Observation of charge accumulation and onsite Coulomb repulsion at transition metal impurities in the iron pnictides
R. Kraus, V. Bisogni, L. Harnagea, S. Aswartham, S. Wurmehl, G. Levy, I. S. Elfimov, B. Büchner, G. A. Sawatzky, J. Geck
Phys. Rev. B 87, 134516 (2013)
High-temperature superconductivity (HTS) in the iron pnictides is one of the most intensively studied topics in current condensed matter science. Although research efforts established many properties of these materials, important questions still remain to be clarified. One of these questions regards the effect of replacing a few percent of Fe by other transition metals (TMs) like Co, Ni or Cu. The substitution of Fe by Co or Ni induces HTS and therefore it is very important to understand the effects of the TM-impurities on the electronic structure. Exactly these effects, however, remain controversial. We shed light on this issue by means of photoemission spectroscopy, Auger electron spectroscopy as well as model calculations. Specifically we report studies of Ca(Fe,Co)2As2 and Ba(Fe,TM)2As2 (TM=Ni, Cu). The valence band photoemission data show directly that the TM-states move to higher binding energies with increasing atomic number, contributing less and less to the states close to the Fermi level. Furthermore, the 3d8 final state of the LVV Auger decay, which is observed for Ni and Cu, unambiguously reveals the accumulation of charge at these impurities. We also show that the onsite Coulomb interaction on the impurity strongly increases when moving from Co over Ni to Cu. Our results quantify the impurity potentials and imply that the superconducting state is robust against impurity scattering.
A (3+3)-dimensional “hypercubic” oxide-ionic conductor: Type II Bi2O3 – Nb2O5
C. D. Ling, S. Schmid, P. E. R. Blanchard, V. Petricek, G .J. McIntyre, N. Sharma, A. Maljuk, A. A. Yaremchenko, V. V. Kharton, M. Gutmann, R. L. Withers
Journal of the American Chemical Society, vol.135 (2013) 6477-6484
The high-temperature cubic form of bismuth oxide, δ-Bi2O3, is the best intermediate-temperature oxide-ionic conductor known. The most elegant way of stabilizing δ-Bi2O3 to room temperature, while preserving a large part of its conductivity, is by doping with higher valent transition metals to create wide solid-solutions fields with exceedingly rare and complex (3 + 3)-dimensional incommensurately modulated “hypercubic” structures. These materials remain poorly understood because no such structure has ever been quantitatively solved and refined, due to both the complexity of the problem and a lack of adequate experimental data. We have addressed this by growing a large (centimeter scale) crystal using a novel refluxing floating-zone method, collecting high-quality single-crystal neutron diffraction data, and treating its structure together with X-ray diffraction data within the superspace symmetry formalism. The structure can be understood as an “inflated” pyrochlore, in which corner-connected NbO6 octahedral chains move smoothly apart to accommodate the solid solution. While some oxide vacancies are ordered into these chains, the rest are distributed throughout a continuous three-dimensional network of wide δ-Bi2O3-like channels, explaining the high oxide-ionic conductivity compared to commensurately modulated phases in the same pseudobinary system.
Phase dynamics and growth of Co2Cr1−xFexAl Heusler compounds: A key to understand their anomalous physical properties
A. Omar, M. Dimitrakopoulou, C. G. F. Blum, H. Wendrock, S. Rodan, S. Hampel, W. Löser, B. Büchner, S. Wurmehl
Cryst. Growth & Design, 13, 3925 (2013)
High quality samples are the basis to study e.g. materials with high spin polarization. Each specific material may requires a specific technique to yield high quality samples and crystals. The floating zone technique, as an example, has been proven as the method of choice for the growth of Heusler compounds. We recently used the floating zone technique to proof prove the existence of a phase transformation via spinodal decomposition in the intermetallic Cr-rich samples of the Co-Cr-Fe-Al system. This finding finally reconciles many controversial experimental results on thin films and bulk samples. As this example nicely illustrates, we can use the floating zone technique to reveal intrinsic materials properties by gaining knowledge about the respective phase dynamics during the growth process with further impact not only on bulk but also on thin film samples. Our study also enables us to predict a thermodynamically stable composition in the Co-Cr-Fe-Al system with the desired high spin polarization and high Curie temperature.
Role of in-plane and out-of-plane dilution in CeFeAsO: charge doping versus disorder
G. Prando, O. Vakaliuk, S. Sanna, G. Lamura, T. Shiroka, P. Bonfà, P. Carretta, R. De Renzi, H.-H. Klauss, C. G. F. Blum, S. Wurmehl, C. Hess, B. Büchner
Physical Review B 87 174519 (2013)
We provide direct experimental evidence for the identical effect of the in-plane Fe1−xCox and of the out-of-plane O1−xFx chemical dilution on the itinerant spin-density-wave (SDW) magnetic phase in CeFeAsO. Remarkably, the suppression of SDW is not sensitive at all to the different kinds of disorder introduced in the two cases. Still,it is clearly shown that the sizable in-plane disorder induced by the Fe1−xCox substitution is highly effectivein suppressing Tc. Differently from what is observed in CeFeAsO1−xFx, the ordered magnetic phase of the Cesublattice is preserved throughout the whole phase diagram in CeFe1−xCoxAsO (x≤0.2). An intriguing effectis encountered, whereby the magnetic coupling among Ce3+ions is enhanced by the superconducting phase.
Pressure-dependence of the charge density wave in 1T-TaS2 and its relation to superconductivity
T. Ritschel, J. Trinckauf, G. Garbarino, M. Hanfland, M. v. Zimmermann, H. Berger, B. Büchner, J. Geck
J. Phys. Rev. B, 87, 125135 (2013)
It is an intriguing fact that in many complex materials superconductivity - a state of matter where charge can move through a lattice without any resistance - often exists in close proximity to what appears to be exactly the opposite: the static spatial ordering of charge. But despite intense research efforts, the question if or under which circumstances electronic order competes with, coexists with or supports superconductivity remains mostly controversial. We studied this relationship in the prototypical material 1T-TaS2, where external pressure stabilizes a superconducting state at low temperatures. Our x-ray diffraction studies as a function of temperature and pressure prove that the charge density wave indeed exists in the superconducting region of the phase diagram. The data further imply that the ordered charge density wave structure as a whole becomes superconducting at low temperatures. In other words, our data provide strong experimental evidence that the entire electronic crystal forms a coherent macroscopic superconducting state. This astonishing conclusion is fundamentally different from a previously proposed phase-separation scenario.
NMR study of the impact of annealing on structural evolution of epitaxial Co2MnSi Heusler films
S. Rodan, A. Alfonsov, M. E. Belesi, F. Ferraro, J. T. Kohlhepp, H. J. M. Swagten, B. Koopmans, Y. Sakuraba, S. Bosu, K. Takanashi, B. Büchner, S. Wurmehl
Appl. Phys. Lett. 102, 242404 (2013)
The technological exploitation of high spin polarization in Heusler compounds in spintronics typically requires thin films. Such Co2MnSi Heusler films were recently reported to show a significant increase in current-perpendicular-to-plane-giant-magnetoresistance upon annealing. We used nuclear magnetic resonance (NMR) to study the impact of annealing as NMR is a powerful tool, sensitive to both magnetic and structural order of such films: In films which were annealed below 550 °C, no long-range L21-order is observed, while annealing above 550 °C leads to the formation of the ideal L21 Heusler structure, however, with a distinct degree of off-stoichiometry. Further evidence from restoring field measurements hints that interdiffusion may account for the drop in magnetoresistance observed for samples annealed above 600 °C. These results show that optimizing films for spintronics involves the identification of the best annealing temperature, high enough for long-range order to emerge, but low enough to maintain smooth interfaces.
Solitonic lattice and Yukawa forces in the rare earth orthoferrite TbFeO3
S. Artyukhin, M. Mostovoy, N. Paduraru Jensen, D. Le, K. Prokes, V. G.Paula, H. N. Bordallo, A. Maljuk, S. Landsgesell, H. Ryll, B. Klemke, S. Paeckel, K. Kiefer, K. Lefmann, L. Theil Kuhn, D. N. Argyriou
Nature Materials, vol.11 (2012) 694-699
The random fluctuations of spins give rise to many interesting physical phenomena, such as the ‘order-from-disorder’ arising in frustrated magnets and unconventional Cooper pairing in magnetic superconductors. Here we show that the exchange of spin waves between extended topological defects, such as domain walls, can result in novel magnetic states. We report the discovery of an unusual incommensurate phase in the orthoferrite TbFeO3 using neutron diffraction under an applied magnetic field. The magnetic modulation has a very long period of 340 Å at 3 K and exhibits an anomalously large number of higher-order harmonics. These domain walls are formed by Ising-like Tb spins. They interact by exchanging magnons propagating through the Fe magnetic sublattice. The resulting force between the domain walls has a rather long range that determines the period of the incommensurate state and is analogous to the pion-mediated Yukawa interaction between protons and neutrons in nuclei
Programmable Sub-nanometer Sculpting of Graphene with Electron Beams
F. Börrnert, L. Fu, S. Gorantla, M. Knupfer, B. Büchner, M. H. Rümmeli
ACS Nano 6, 10327 (2012)
Electron beams in transmission electron microscopes are very attractive to engineer and pattern graphene toward all-carbon device fabrication. The use of condensed beams typically used for sequential raster imaging is particularly exciting since they potentially provide high degrees of precision. However, technical difficulties, such as the formation of electron beam induced deposits on sample surfaces, have hindered the development of this technique. We demonstrate how one can successfully use a condensed electron beam, either with or without Cs correction, to structure graphene with sub-nanometer precision in a programmable manner. We further demonstrate the potential of the developed technique by combining it with an established route to engineer graphene nanoribbons to single-atom carbon chains.
Amorphous Carbon under 80 kV Electron Irradiation: A Means to Make or Break Graphene
F. Börrnert, S. M. Avdoshenko, A. Bachmatiuk, I. Ibrahim, B. Büchner, G. Cuniberti, M. H. Rümmeli
Advanced Matererials 24, 5630 (2012)
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.
Lattice Expansion in Seamless Bilayer Graphene Constrictions at High Bias
F. Börrnert, A. Barreiro, D. Wolf, M. I. Katsnelson, B. Büchner, L. M. K. Vandersypen, M. H. Rümmeli
Nano Letters 12, 4455 (2012)
Our understanding of sp2 carbon nanostructures is still emerging and is important for the development of high performance all carbon devices. For example, in terms of the structural behavior of graphene or bilayer graphene at high bias, little to nothing is known. To this end, we investigated bilayer graphene constrictions with closed edges (seamless) at high bias using in situ atomic resolution transmission electron microscopy. We directly observe a highly localized anomalously large lattice expansion inside the constriction. Both the current density and lattice expansion increase as the bilayer graphene constriction narrows. As the constriction width decreases below 10 nm, shortly before failure, the current density rises to 4 × 109 A cm−2 and the constriction exhibits a lattice expansion with a uniaxial component showing an expansion approaching 5% and an isotropic component showing an expansion exceeding 1%. The origin of the lattice expansion is hard to fully ascribe to thermal expansion. Impact ionization is a process in which charge carriers transfer from bonding states to antibonding states, thus weakening bonds. The altered character of C−C bonds by impact ionization could explain the anomalously large lattice expansion we observe in seamless bilayer graphene constrictions. Moreover, impact ionization might also contribute to the observed anisotropy in the lattice expansion, although strain is probably the predominant factor.
Probing the Unconventional Superconducting State of LiFeAs by Quasiparticle Interference
T. Hänke, S. Sykora, R. Schlegel, D. Baumann, L. Harnagea, S. Wurmehl, M. Daghofer, B. Büchner, J. van den Brink, C. Hess
Phys. Rev. Lett. 108, 127001 (2012)
A crucial step in revealing the nature of unconventional superconductivity is to investigate the symmetry of the superconducting order parameter. Scanning tunneling spectroscopy has proven a powerful technique to probe this symmetry by measuring the quasiparticle interference (QPI) which sensitively depends on the superconducting pairing mechanism. A particularly well-suited material to apply this technique is the stoichiometric superconductor LiFeAs as it features clean, charge neutral cleaved surfaces without surface states and a relatively high Tc ~ 18 K. Our data reveal that in LiFeAs the quasiparticle scattering is governed by a van Hove singularity at the center of the Brillouin zone which is in stark contrast to other pnictide superconductors where nesting is crucial for both scattering and s± superconductivity. Indeed, within a minimal model and using the most elementary order parameters, calculations of the QPI suggest a dominating role of the holelike bands for the quasiparticle scattering. Our theoretical findings do not support the elementary singlet pairing symmetries s++, s±, and d wave. This brings to mind that the superconducting pairing mechanism in LiFeAs is based on an unusual pairing symmetry such as an elementary p wave (which provides optimal agreement between the experimental data and QPI simulations) or a more complex order parameter (e.g., s + id wave symmetry).
Magnetic frustration, Phase Competition, and the Magnetoelectric Effect in NdFe3(BO3)4
J. E. Hamann-Borrero, S. Partzsch, S. Valencia, C. Mazzoli, J. Herrero-Martin, R. Feyerherm, E. Dudzik, C. Hess, A. Vasiliev, L. Bezmaternykh, B. Büchner, J. Geck
Phys. Rev. Lett., 109, 267202 (2012)
The coupling between magnetism and electric polarization in so-called multiferroic materials is a major topic of current condensed matter research, since it is of large interest for both basic science and technological applications. Unfortunately ferroelectric and magnetic order rarely coexist and even if they do, the coupling between them is usually very weak. It was therefore greeted with great excitement when frustrated magnetic materials were discovered, where the coupling of magnetic and ferroelectric orders is extraordinarily strong. The reasons of this strong magnetoelectric coupling in these new materials are, however, subject of intense research. Here we studied such a material, namely NdFe3(BO3)4, aiming at clarifying the micrsocopic origin for its magnetoelectric effect. To this end, we performed an element selective resonant magnetic x-ray scattering study of the as a function of temperature and applied magnetic field. Our measurements show that the magnetic order of the Nd sub-lattice is induced by the Fe spin order. When a magnetic field is applied parallel to the hexagonal basal plane, the helicoidal spin order is suppressed and a collinear ordering, where the moments are forced to align in a direction perpendicular to the applied magnetic field, is stabilized. This result excludes a non-collinear spin order as the origin of the magnetically induced electric polarization in this compound. Instead our data imply that magnetic frustration results in a phase competition, which is the origin of the magneto-electric response.
Observation of Electronic Ferroelectric Polarization in Multiferroic YMn2O5
S. Partzsch, S. B. Wilkins, J. P. Hill, E. Schierle, E. Weschke, D. Souptel, B. Büchner, J. Geck
Phys. Rev. Lett., 107, 057201 (2011)
Multiferroic materials with a coupled ordering of electric and magnetic moments are extremely interesting for technological applications. They could, for example, be used to build a magnetic computer memory that could be switched with an electrical field. However whether the microscopic origin is more of ionic or electronic origin, or even both is still hotly debated. Remarkably, we find that the temperature dependence of the integrated intensity in our resonant x-ray scattering experiment at the O K edge closely follows the macroscopic electric polarization, and hence is proportional to the ferroelectric order parameter. This is in contrast with the temperature dependence observed at the Mn L3 edge, which reflects the Mn magnetic order parameter. First-principles calculations provide a microscopic understanding of these results and show that a spin-dependent hybridization of O 2p- and Mn 3d states results in a purely electronic contribution to the ferroelectric polarization, which can exist even in the absence of lattice distortions.
Probing Local Hydrogen Impurities in Quasi-Free-Standing Graphene
M. Scheffler, D. Haberer, L. Petaccia, M. Farjam, R. Schlegel, D. Baumann, T. Hänke, A. Grüneis, M. Knupfer, C. Hess, B. Büchner
ACS Nano 6, 10590 (2012)
We report high-resolution STM/S of hydrogenated, quasi-free-standing graphene. Our measurements of the LDOS at hydrogenated patches of graphene reveal a hydrogen impurity state near the Fermi level, which was already predicted by theory and seen in the long-range electronic structure in ARPES. We further observe predominantly single hydrogenation sites as well as extended multiple C-H sites in parallel orientation to the lattice vectors, indicating an adsorption at the same graphene sublattice.
Phase diagram of the iron arsenide superconductors Ca(Fe1-xCox)2As2
L. Harnagea, S. Singh, G. Friemel, N. Leps, D. Bombor, M. Abdel-Hafiez, A.U.B. Wolter, C. Hess, R. Klingeler, G. Behr, S. Wurmehl, B. Büchner
Phys. Rev. B 83 94523 (2011)
CaFe2As2 has been intensively study due to the extreme pressure sensitivity of its structural and magnetic behavior. However, in contrast to BaFe2As2 and SrFe2As2 analogous series of compounds, systematic doping dependence studies in CaFe2As2 was missing until now. We report on the first systematic investigation of the electronic phase diagram and physical properties of the Ca(Fe1-xCox)2As2 series. Single crystals were obtained from the high temperature solution growth technique using Sn flux. Upon Co doping, the c-axis of the tetragonal unit cell decreases, while the a-axis shows a less significant trend. Pristine CaFe2As2 shows a combined spin density-wave and structural transition near 166 K which gradually shifts to lower temperatures and splits with increasing Co-substitution. Both transitions terminate abruptly at a critical Co-concentration of xC = 0.075 which is also the composition with the highest TC. Simultaneously, superconductivity appears at low temperatures with a maximum transition temperature TC of around 20 K. The electronic phase diagram of Ca(Fe1-xCox)2As2 (0 ≤ x ≤ 0.2) is mapped out from the magnetization and transport data. The superconducting volume fraction varies with Co concentration and diminishes rapidly for x > 0.10. These findings suggest that superconductivity appears only over a narrow composition range around xC unlike the case of the analogous Ba(Fe1-xCox)2As2.
Local formation of a Heusler structure in CoFeAl alloys
S. Wurmehl, P.J. Jacobs, J.T. Kohlhepp, H.J.M. Swagten, B. Koopmans, S. Maat, M.J. Carey, J.R. Childress
Appl. Phys. Lett. 98 12506 (2011)
The magnetotransport properties of current-perpendicular-to-the plane giant magnetoresistance (CPP-GMR) devices consisting of ferromagnetic Co-Fe alloys have recently been shown to be significantly improved by addition of up to 28 % Al. In order to further optimize CoFe-Al spin-valves, it is important to understand the impact of the Al alloying on the local and electronic structure. Nuclear magnetic resonance (NMR) is able to reveal the next neighbouring shells of the 59Co nuclei in the Co-Fe-Al magnetic films. Its sensitivity to small changes in the local (magnetic and electronic) environment makes NMR an ideal method to determine the local modifications upon addition of Al to the Co-Fe alloy. In our present NMR study, we demonstrate the local formation of a Heusler-like structure by addition of Al to the Co-Fe alloy in CPP-GMR multilayers. The observed local formation of a highly spin-polarized Heusler compound may be correlated to the observed enhancement of the GMR effect.
Single crystal growth and characterization of superconducting LiFeAs
I. Morozov, A. Boltalin, O. Volkova, A. Vasiliev, O. Kataeva, U. Stockert, M. Abdel-Hafiez, D. Bombor, A. Bachmann, L. Harnagea, M. Fuchs, H.-J. Grafe, G. Behr, R. Klingeler, S. Borisenko, C. Hess, S. Wurmehl, B. Büchner
Cryst. Growth and Design 10 4429 (2010)
Large and high quality single crystals of the new unconventional superconductor LiFeAs were grown by a new approach using the self-flux technique. Both energy dispersive X-ray spectroscopy and inductively coupled plasma mass spectroscopy revealed a stoichiometric Li/Fe/As composition. Measurements of the magnetic susceptibility reveal the superconducting transition at Tc =17Kwith a very sharp ΔTc and a 100% shielding fraction and, thus, bulk superconductivity. This sharp transition is also found by measurements of the specific heat and by measurements of the temperature dependence of the resistivity. Nuclear quadrupole resonance (NQR) spectroscopy reveals a very sharp resonance line, with a much smaller line width than reported for all other FeAs superconductors, confirming the high ordering of the LiFeAs single crystals also on a local scale.