58 research outputs found
Topological quantum materials from the viewpoint of chemistry
Topology, a mathematical concept, has recently become a popular and truly
transdisciplinary topic encompassing condensed matter physics, solid state
chemistry, and materials science. Since there is a direct connection between
real space, namely atoms, valence electrons, bonds and orbitals, and reciprocal
space, namely bands and Fermi surfaces, via symmetry and topology, classifying
topological materials within a single-particle picture is possible. Currently,
most materials are classified as trivial insulators, semimetals and metals, or
as topological insulators, Dirac and Weyl nodal-line semimetals, and
topological metals. The key ingredients for topology are: certain symmetries,
the inert pair effect of the outer electrons leading to inversion of the
conduction and valence bands, and spin-orbit coupling. This review presents the
topological concepts related to solids from the viewpoint of a solid-state
chemist, summarizes techniques for growing single crystals, and describes basic
physical property measurement techniques to characterize topological materials
beyond their structure and provide examples of such materials. Finally, a brief
outlook on the impact of topology in other areas of chemistry is provided at
the end of the article.Comment: Review Article, 28 Figure
Prediction of a magnetic Weyl semimetal without spin-orbit coupling and strong anomalous Hall effect in the Heusler compensated ferrimagnet Ti2MnAl
We predict a magnetic Weyl semimetal in the inverse Heusler Ti2MnAl, a
compensated ferrimagnet with a vanishing net magnetic moment and a Curie
temperature of over 650 K. Despite the vanishing net magnetic moment, we
calculate a large intrinsic anomalous Hall effect (AHE) of about 300 S/cm. It
derives from the Berry curvature distribution of the Weyl points, which are
only 14 meV away from the Fermi level and isolated from trivial bands.
Different from antiferromagnets Mn3X (X= Ge, Sn, Ga, Ir, Rh, and Pt), where the
AHE originates from the non-collinear magnetic structure, the AHE in Ti2MnAl
stems directly from the Weyl points and is topologically protected. The large
anomalous Hall conductivity (AHC) together with a low charge carrier
concentration should give rise to a large anomalous Hall angle. In contrast to
the Co-based ferromagnetic Heusler compounds, the Weyl nodes in Ti2MnAl do not
derive from nodal lines due to the lack of mirror symmetries in the inverse
Heusler structure. Since the magnetic structure breaks spin-rotation symmetry,
the Weyl nodes are stable without SOC. Moreover, because of the large
separation between Weyl points of opposite topological charge, the Fermi arcs
extent up to 75% of the reciprocal lattice vectors in length. This makes
Ti2MnAl an excellent candidate for the comprehensive study of magnetic Weyl
semimetals. It is the first example of a material with Weyl points, large
anomalous Hall effect and angle despite a vanishing net magnetic moment.Comment: 6 pages, 4 figure
Berry curvature unravelled by the Nernst effect in MnGe
The discovery of topological quantum materials represents a striking
innovation in modern condensed matter physics with remarkable fundamental and
technological implications. Their classification has been recently extended to
topological Weyl semimetals, i.e., solid state systems which exhibit the
elusive Weyl fermions as low-energy excitations. Here we show that the Nernst
effect can be exploited as a sensitive probe for determining key parameters of
the Weyl physics, applying it to the non-collinear antiferromagnet MnGe.
This compound exhibits anomalous thermoelectric transport due to enhanced Berry
curvature from Weyl points located extremely close to the Fermi level. We
establish from our data a direct measure of the Berry curvature at the Fermi
level and, using a minimal model of a Weyl semimetal, extract for the first
time the Weyl point energy and their distance in momentum-space
Improved infrared photoluminescence characteristics from circularly ordered self-assembled Ge islands
The formation of circularly ordered Ge-islands on Si(001) has been achieved because of nonuniform strain field around the periphery of the holes patterned by focused ion beam in combination with a self-assembled growth using molecular beam epitaxy. The photoluminescence (PL) spectra obtained from patterned areas (i.e., ordered islands) show a significant signal enhancement, which sustained till 200 K, without any vertical stacking of islands. The origin of two activation energies in temperature-dependent PL spectra of the ordered islands has been explained in detail
The iridium double perovskite Sr2YIrO6 revisited: A combined structural and specific heat study
Recently, the iridate double perovskite SrYIrO has attracted
considerable attention due to the report of unexpected magnetism in this
Ir (5d) material, in which according to the J model, a
non-magnetic ground state is expected. However, in recent works on
polycrystalline samples of the series BaSrYIrO no indication of
magnetic transitions have been found. We present a structural, magnetic and
thermodynamic characterization of SrYIrO single crystals, with emphasis
on the temperature and magnetic field dependence of the specific heat. Here, we
demonstrate the clue role of single crystal X-ray diffraction on the structural
characterization of the SrYIrO double perovskite crystals by reporting
the detection of a supercell, where ,
and are the unit cell dimensions of the reported monoclinic subcell. In
agreement with the expected non-magnetic ground state of Ir (5d) in
SrYIrO, no magnetic transition is observed down to 430~mK. Moreover,
our results suggest that the low temperature anomaly observed in the specific
heat is not related to the onset of long-range magnetic order. Instead, it is
identified as a Schottky anomaly caused by paramagnetic impurities present in
the sample, of the order of \%. These impurities lead to
non-negligible spin correlations, which nonetheless, are not associated with
long-range magnetic ordering.Comment: 20 pages, 10 figure
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Evidence for a percolative Mott insulator-metal transition in doped Sr2IrO4
Despite many efforts to rationalize the strongly correlated electronic ground states in doped Mott insulators, the nature of the doping-induced insulator-to-metal transition is still a subject under intensive investigation. Here, we probe the nanoscale electronic structure of the Mott insulator Sr2IrO4−δ with low-temperature scanning tunneling microscopy and find an enhanced local density of states (LDOS) inside the Mott gap at the location of individual defects which we interpret as defects at apical oxygen sites. A chiral behavior in the topography for those defects has been observed. We also visualize the local enhanced conductance arising from the overlapping of defect states which induces finite LDOS inside of the Mott gap. By combining these findings with the typical spatial extension of isolated defects of about 2 nm, our results indicate that the insulator-to-metal transition in Sr2IrO4−δ could be percolative in nature
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