278 research outputs found
Solitonic spin-liquid state due to the violation of the Lifshitz condition in FeTe
A combination of phenomenological analysis and M\"ossbauer spectroscopy
experiments on the tetragonal FeTe system indicates that the magnetic
ordering transition in compounds with higher Fe-excess, 0.11, is
unconventional. Experimentally, a liquid-like magnetic precursor with
quasi-static spin-order is found from significantly broadened M\"ossbauer
spectra at temperatures above the antiferromagnetic transition. The
incommensurate spin-density wave (SDW) order in FeTe is described by a
magnetic free energy that violates the weak Lifshitz condition in the Landau
theory of second-order transitions. The presence of multiple Lifshitz
invariants provides the mechanism to create multidimensional, twisted, and
modulated solitonic phases.Comment: 5 pages, 2 figure
Pressure-induced ferromagnetism due to an anisotropic electronic topological transition in Fe1.08Te
A rapid and anisotropic modification of the Fermi-surface shape can be
associated with abrupt changes in crystalline lattice geometry or in the
magnetic state of a material. In this study we show that such an electronic
topological transition is at the basis of the formation of an unusual
pressure-induced tetragonal ferromagnetic phase in FeTe. Around 2 GPa,
the orthorhombic and incommensurate antiferromagnetic ground-state of
FeTe is transformed upon increasing pressure into a tetragonal
ferromagnetic state via a conventional first-order transition. On the other
hand, an isostructural transition takes place from the paramagnetic
high-temperature state into the ferromagnetic phase as a rare case of a `type
0' transformation with anisotropic properties. Electronic-structure
calculations in combination with electrical resistivity, magnetization, and
x-ray diffraction experiments show that the electronic system of FeTe
is instable with respect to profound topological transitions that can drive
fundamental changes of the lattice anisotropy and the associated magnetic
order.Comment: 7 pages, 4 figur
Hybridization gap and Fano resonance in SmB
We present results of Scanning Tunneling Microscopy and Spectroscopy (STS)
measurements on the "Kondo insulator" SmB. The vast majority of surface
areas investigated was reconstructed but, infrequently, also patches of varying
size of non-reconstructed, Sm- or B-terminated surfaces were found. On the
smallest patches, clear indications for the hybridization gap and
inter-multiplet transitions were observed. On non-reconstructed surface areas
large enough for coherent co-tunneling we were able to observe clear-cut Fano
resonances. Our locally resolved STS indicated considerable finite conductance
on all surfaces independent of their structure.Comment: 5 pages, 4 figure
Disorder-driven electronic localization and phase separation in superconducting Fe1+yTe0.5Se0.5 single crystals
We have investigated the influence of Fe-excess on the electrical transport
and magnetism of Fe1+yTe0.5Se0.5 (y=0.04 and 0.09) single crystals. Both
compositions exhibit resistively determined superconducting transitions (Tc)
with an onset temperature of about 15 K. From the width of the superconducting
transition and the magnitude of the lower critical field Hc1, it is inferred
that excess of Fe suppresses superconductivity. The linear and non-linear
responses of the ac-susceptibility show that the superconducting state for
these compositions is inhomogeneous. A possible origin of this phase separation
is a magnetic coupling between Fe-excess occupying interstitial sites in the
chalcogen planes and those in the Fe-square lattice. The temperature derivative
of the resistivity drho/dT in the temperature range Tc < T < Ta with Ta being
the temperature of a magnetic anomaly, changes from positive to negative with
increasing Fe. A log 1/T divergence of the resistivity above Tc in the sample
with higher amount of Fe suggests a disorder driven electronic localization.Comment: 7 page
Pressure-induced phase transitions and high-pressure tetragonal phase of Fe1.08Te
We report the effects of hydrostatic pressure on the temperature-induced
phase transitions in Fe1.08Te in the pressure range 0-3 GPa using synchrotron
powder x-ray diffraction (XRD). The results reveal a plethora of phase
transitions. At ambient pressure, Fe1.08Te undergoes simultaneous first-order
structural symmetry-breaking and magnetic phase transitions, namely from the
paramagnetic tetragonal (P4/nmm) to the antiferromagnetic monoclinic (P2_1/m)
phase. We show that, at a pressure of 1.33 GPa, the low temperature structure
adopts an orthorhombic symmetry. More importantly, for pressures of 2.29 GPa
and higher, a symmetry-conserving tetragonal-tetragonal phase transition has
been identified from a change in the c/a ratio of the lattice parameters. The
succession of different pressure and temperature-induced structural and
magnetic phases indicates the presence of strong magneto-elastic coupling
effects in this material.Comment: 11 page
Precursor phenomena at the magnetic ordering of the cubic helimagnet FeGe
We report on detailed magnetic measurements on the cubic helimagnet FeGe in
external magnetic fields and temperatures near the onset of long-range magnetic
order at K. Precursor phenomena display a complex succession of
temperature-driven crossovers and phase transitions in the vicinity of .
The A-phase region, present below and fields kOe, is split in
several pockets. Relying on a modified phenomenological theory for chiral
magnets, the main part of the A-phase could indicate the existence of a
Skyrmion lattice, the adjacent A pocket, however, appears to be related to
helicoids propagating in directions perpendicular to the applied field.Comment: 5 pages, 4 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
Polaronic state and nanometer-scale phase separation in colossal magnetoresistive manganites
High resolution topographic images obtained by scanning tunneling microscope
in the insulating state of Pr0.68Pb0.32MnO3 single crystals showed regular
stripe-like or zigzag patterns on a width scale of 0.4 - 0.5 nm confirming a
high temperature polaronic state. Spectroscopic studies revealed inhomogeneous
maps of zero-bias conductance with small patches of metallic clusters on length
scale of 2 - 3 nm only within a narrow temperature range close to the
metal-insulator transition. The results give a direct observation of polarons
in the insulating state, phase separation of nanometer-scale metallic clusters
in the paramagnetic metallic state, and a homogeneous ferromagnetic state
Recommended from our members
Calcite incorporated in silica/collagen xerogels mediates calcium release and enhances osteoblast proliferation and differentiation
Multiphasic silica/collagen xerogels are biomaterials designed for bone regeneration. Biphasic silica/collagen xerogels (B30) and triphasic xerogels (B30H20 or B30CK20) additionally containing hydroxyapatite or calcite were demonstrated to exhibit several structural levels. On the first level, low fibrillar collagen serves as template for silica nanoparticle agglomerates. On second level, this silica-enriched matrix phase is fiber-reinforced by collagen fibrils. In case of hydroxyapatite incorporation in B30H20, resulting xerogels exhibit a hydroxyapatite-enriched phase consisting of hydroxyapatite particle agglomerates next to silica and low fibrillar collagen. Calcite in B30CK20 is incorporated as single non-agglomerated crystal into the silica/collagen matrix phase with embedded collagen fibrils. Both the structure of multiphasic xerogels and the manner of hydroxyapatite or calcite incorporation have an influence on the release of calcium from the xerogels. B30CK20 released a significantly higher amount of calcium into a calcium-free solution over a three-week period than B30H20. In calcium containing incubation media, all xerogels caused a decrease in calcium concentration as a result of their bioactivity, which was superimposed by the calcium release for B30CK20 and B30H20. Proliferation of human bone marrow stromal cells in direct contact to the materials was enhanced on B30CK20 compared to cells on both plain B30 and B30H20
- …