73 research outputs found
Unconventional magnetism in multivalent charge-ordered YbPtGe probed by Pt- and Yb-NMR
Detailed Pt- and Yb nuclear magnetic resonance (NMR) studies
on the heterogeneous mixed valence system YbPtGe are reported. The
temperature dependence of the Pt-NMR shift indicates the
opening of an unusual magnetic gap below 200\,K. was analyzed by a
thermal activation model which yields an isotropic gap \,K. In contrast, the spin-lattice relaxation rate () does
not provide evidence for the gap. Therefore, an intermediate-valence picture is
proposed while a Kondo-insulator scenario can be excluded. Moreover,
() follows a simple metallic behavior, similar to the reference
compound YPtGe. A well resolved NMR line with small shift is assigned to
divalent Yb. This finding supports the proposed model with two sub-sets
of Yb species (di- and trivalent) located on the Yb2 and Yb1 site of the
YbPtGe lattice.Comment: Submitted in Physical Review B (Rapid Communication
Low field extension for magnetometers (TinyBee) used for investigations on low-dimensional superconductors with Bc1 < 5G
In this article a simple and easy to install low magnetic field extension of
the SQUID magnetometer Quantum Design MPMS-7 is described. This has been
accomplished by complementing the MPMS-7 magnet control system with a
laboratory current supply for the low magnetic field region (B \leq 200G). This
hard- and software upgrade provides a significant gain in the magnetic field
accuracy up to an order of magnitude compared with the standard instrument's
setup and is improving the resolution to better than 0.01G below 40G. The field
control has been integrated into the Quantum Design MultiVu software for a
transparent and user-friendly operation of this extension. The improvements
achieved are especially useful, when low magnetic field strengths (B < 1G) are
required at high precision. The specific advantages of this application are
illustrated by sophisticated magnetic characterisation of lowdimensional
superconductors like Sc3CoC4 and SnSe2{Co({\eta}-C5H5)2}x.Comment: 16 pages, 7 figure
Scaling Study and Thermodynamic Properties of the cubic Helimagnet FeGe
The critical behavior of the cubic helimagnet FeGe was obtained from
isothermal magnetization data in very close vicinity of the ordering
temperature. A thorough and consistent scaling analysis of these data revealed
the critical exponents , , and . The
anomaly in the specific heat associated with the magnetic ordering can be well
described by the critical exponent . The values of these
exponents corroborate that the magnetic phase transition in FeGe belongs to the
isotropic 3D-Heisenberg universality class. The specific heat data are well
described by ab initio phonon calculations and confirm the localized character
of the magnetic moments.Comment: 10 pages, 8 figure
Competing magnetic correlations across the ferromagnetic quantum critical point in the Kondo system CeTiVGe: V NMR as a local probe
V nuclear magnetic resonance (NMR) and magnetization studies on
CeTiVGe have been performed to explore the evolution from the
ferromagnetic () to the antiferromagnetic Kondo lattice state (), with focus on the emergence of a possible ferromagnetic quantum critical
point (FMQCP) at . From the temperature dependence of the
nuclear spin-lattice relaxation rate, , and the Knight shift,
\textit{K}, for and a considerable competition between ferro-
and antiferromagnetic correlations is found. Around the critical concentration
() quantum-critical spin fluctuations entail weak
antiferromagnetic spin fluctuations admixed with ferromagnetic spin
fluctuations. The FMQCP in CeTiVGe therefore is not purely
ferromagnetic in nature.Comment: 9 pages and 12 figures, accepted at PR
Resonant torsion magnetometry in anisotropic quantum materials
Unusual behavior of quantum materials commonly arises from their effective
low-dimensional physics, which reflects the underlying anisotropy in the spin
and charge degrees of freedom. Torque magnetometry is a highly sensitive
technique to directly quantify the anisotropy in quantum materials, such as the
layered high-T superconductors, anisotropic quantum spin-liquids, and the
surface states of topological insulators. Here we introduce the magnetotropic
coefficient , the second derivative of the
free energy F with respect to the angle between the sample and the
applied magnetic field, and report a simple and effective method to
experimentally detect it. A sub-g crystallite is placed at the tip of a
commercially available atomic force microscopy cantilever, and we show that
can be quantitatively inferred from a shift in the resonant frequency under
magnetic field. While related to the magnetic torque , takes the role of torque susceptibility, and thus provides
distinct insights into anisotropic materials akin to the difference between
magnetization and magnetic susceptibility. The thermodynamic coefficient is
discontinuous at second-order phase transitions and subject to Ehrenfest
relations with the specific heat and magnetic susceptibility. We apply this
simple yet quantitative method on the exemplary cases of the Weyl-semimetal NbP
and the spin-liquid candidate RuCl, yet it is broadly applicable in quantum
materials research.Comment: 7 pages including 6 figures and methods sectio
Superconductivity in Fullerides
Experimental studies of superconductivity properties of fullerides are
briefly reviewed. Theoretical calculations of the electron-phonon coupling, in
particular for the intramolecular phonons, are discussed extensively. The
calculations are compared with coupling constants deduced from a number of
different experimental techniques. It is discussed why the A_3 C_60 are not
Mott-Hubbard insulators, in spite of the large Coulomb interaction. Estimates
of the Coulomb pseudopotential , describing the effect of the Coulomb
repulsion on the superconductivity, as well as possible electronic mechanisms
for the superconductivity are reviewed. The calculation of various properties
within the Migdal-Eliashberg theory and attempts to go beyond this theory are
described.Comment: 33 pages, latex2e, revtex using rmp style, 15 figures, submitted to
Review of Modern Physics, more information at
http://radix2.mpi-stuttgart.mpg.de/fullerene/fullerene.htm
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