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Velkos, Georgios,Krylov, Denis S.,Kirkpatrick, Kyle,Spree, Lukas,Dubrovin, Vasilii,Bü,chner, Bernd,Avdoshenko, Stanislav M.,Bezmelnitsyn, Valeriy,Davis, Sean,Faust, Paul,Duchamp, James,Dorn, Harry John Wiley and Sons Inc. 2019 Angewandte Chemie. international edition Vol.58 No.18
<P><B>Abstract</B></P><P>The azafullerene Tb<SUB>2</SUB>@C<SUB>79</SUB>N is found to be a single‐molecule magnet with a high 100‐s blocking temperature of magnetization of 24 K and large coercivity. Tb magnetic moments with an easy‐axis single‐ion magnetic anisotropy are strongly coupled by the unpaired spin of the single‐electron Tb−Tb bond. Relaxation of magnetization in Tb<SUB>2</SUB>@C<SUB>79</SUB>N below 15 K proceeds via quantum tunneling of magnetization with the characteristic time <I>τ</I><SUB>QTM</SUB>=16 462±1230 s. At higher temperature, relaxation follows the Orbach mechanism with a barrier of 757±4 K, corresponding to the excited states, in which one of the Tb spins is flipped.</P>
Orbital-driven nematicity in FeSe
Baek, S-H.,Efremov, D. V.,Ok, J. M.,Kim, J. S.,van den Brink, Jeroen,Bü,chner, B. Nature Publishing Group, a division of Macmillan P 2015 NATURE MATERIALS Vol.14 No.2
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 T<SUB>nem</SUB> = 91 K, far above the superconducting T<SUB>c</SUB> 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 T<SUB>nem</SUB>, which unequivocally establishes orbital degrees of freedom as driving the nematic order. We demonstrate that superconductivity competes with the emerging nematicity.
New insights in the formation processes of Pu(IV) colloids
Walther, Clemens,Rothe, Jö,rg,Brendebach, Boris,Fuss, Markus,Altmaier, Marcus,Marquardt, Christian M.,Bü,chner, Sebastian,Cho, Hye-Ryun,Yun, J.-I.,Seibert, Alice De Gruyter Oldenbourg 2009 RADIOCHIMICA ACTA Vol.97 No.4
<P><B>Abstract</B></P><P>The high tendency of tetravalent plutonium to form polymeric complexes and colloids is well known but the exact processes underlying their formation are still controversially discussed. In the present work, the nucleation of small polynuclear hydroxide complexes,<I>i.e.</I>, ionic species containing more than one Pu ion, their aggregation and formation of larger colloids (polymers exceeding some 5 nm in size) and finally ripening processes of freshly formed amorphous Pu(IV) colloids towards more crystalline particles are investigated by use of a combination of various spectroscopic techniques. By electrospray mass-spectrometry small polymers such as dimers, trimers and tetramers containing mixed oxidation states of Pu were observed. These polymers might be responsible for the equilibration between the Pu(III)/Pu(IV) and the plutonyl species Pu(V)/Pu(VI) even in dilute solutions in the absence of colloids or precipitates.</P>
Evidence for a Field-Induced Quantum Spin Liquid in α - RuCl3
Baek, S.-H.,Do, S.-H.,Choi, K.-Y.,Kwon, Y. S.,Wolter, A. U. B.,Nishimoto, S.,van den Brink, Jeroen,Bü,chner, B. American Physical Society 2017 Physical Review Letters Vol.119 No.3
<P>We report a Cl-35 nuclear magnetic resonance study in the honeycomb lattice alpha-RuCl3, a material that has been suggested to potentially realize a Kitaev quantum spin liquid (QSL) ground state. Our results provide direct evidence that alpha-RuCl3 exhibits a magnetic-field-induced QSL. For fields larger than similar to 10 T, a spin gap opens up while resonance lines remain sharp, evidencing that spins are quantum disordered and locally fluctuating. The spin gap increases linearly with an increasing magnetic field, reaching similar to 50 K at 15 T, and is nearly isotropic with respect to the field direction. The unusual rapid increase of the spin gap with increasing field and its isotropic nature are incompatible with conventional magnetic ordering and, in particular, exclude that the ground state is a fully polarized ferromagnet. The presence of such a field-induced gapped QSL phase has indeed been predicted in the Kitaev model.</P>
The catalytic potential of high-κ dielectrics for graphene formation
Scott, Andrew,Dianat, Arezoo,Bö,rrnert, Felix,Bachmatiuk, Alicja,Warner, Jamie H.,Borowiak-Paleń,, Ewa,Knupfer, Martin,Bü,chner, Bernd,Cuniberti, Gianaurelio,Rü,mmeli, Mark H. American Institute of Physics 2011 APPLIED PHYSICS LETTERS Vol.98 No.7
Nematicity and magnetism in LaFeAsO single crystals probed by As75 nuclear magnetic resonance
Ok, J. M.,Baek, S.-H.,Efremov, D. V.,Kappenberger, R.,Aswartham, S.,Kim, J. S.,van den Brink, Jeroen,Bü,chner, B. American Physical Society 2018 Physical review. B Vol.97 No.18
<P>We report a As-75 nuclear magnetic resonance study in LaFeAsO single crystals, which undergoes nematic and antiferromagnetic transitions at T-nem similar to 156 K and T-N similar to 138 K, respectively. Below T-nem, the As-75 spectrum splits sharply into two for an external magnetic field parallel to the orthorhombic a or b axis in the FeAs planes. Our analysis of the data demonstrates that the NMR line splitting arises from an electronically driven rotational symmetry breaking. The As-75 spin-lattice relaxation rate as a function of temperature shows that spin fluctuations are strongly enhanced just below T-nem. These NMR findings indicate that nematic order promotes spin fluctuations in magnetically ordered LaFeAsO, as observed in nonmagnetic and superconducting FeSe. We conclude that the origin of nematicity is identical in both FeSe and LaFeAsO regardless of whether or not a long-range magnetic order develops in the nematic state.</P>