The design of new complex mixed metal tellurides (containing low toxicity cations) with intrinsic ultralow thermal conductivity is of paramount importance in the field of thermoelectrics. Herein, we ...report the synthesis and characterization of polycrystalline and single crystals of a new mixed-metal quaternary telluride Mn
1.8(1)
In
0.8(1)
Si
2
Te
6
. The structural aspects and chemical formula of this phase at room temperature have been established using single crystal X-ray diffraction and EDX studies. The trigonal centrosymmetric (space group:
P
3&cmb.macr;1
m
) structure of the title phase has cell constants of
a
=
b
= 7.0483(7) Å and
c
= 7.1277(8) Å. The structure has three independent cationic sites, one mixed (In1/Mn1), one partially filled Mn2, and one Si1, which are bonded with Te1 atoms. Each metal atom (In and Mn) in the structure is octahedrally coordinated with six neighboring Te1 atoms. The structure also features dimers of Si atoms, and each Si atom is bonded to three Te1 atoms to form ethane-like Si
2
Te
6
units. The optical absorption study of a polycrystalline Mn
1.8
In
0.8
Si
2
Te
6
sample shows a narrow optical bandgap of 0.6(2) eV. Temperature-dependent resistivity and Seebeck coefficient studies confirmed the
p
-type semiconducting nature of the sample with high values of
S
(301 μV K
−1
to 444 μV K
−1
). The total thermal conductivity (
k
tot
) study of the polycrystalline sample shows a decreasing trend on heating with an extremely low value of 0.28 W m
−1
K
−1
at 773 K. Magnetic measurements indicate a glassy magnetic behavior for the sample below 8 K.
The first quaternary telluride, Mn
1.8(1)
In
0.8(1)
Si
2
Te
6
, of the Mn-In-Si-Te system has been synthesized and characterized. The phase shows ultralow thermal conductivity (0.28 W m
−1
K
−1
at 773 K) values.
Despite the fundamental nature of the edge state in topological physics, direct measurement of electronic and optical properties of the Fermi arcs of topological semimetals has posed a significant ...experimental challenge, as their response is often overwhelmed by the metallic bulk. However, laser-driven currents carried by surface and bulk states can propagate in different directions in nonsymmorphic crystals, allowing for the two components to be easily separated. Motivated by a recent theoretical prediction G. Chang et al., Phys. Rev. Lett. 124, 166404 (2020), we have measured the linear and circular photogalvanic effect currents deriving from the Fermi arcs of the nonsymmorphic, chiral Weyl semimetal RhSi over the 0.45–1.1 eV incident photon energy range. Our data are in good agreement with the predicted spectral shape of the circular photogalvanic effect as a function of photon energy, although the direction of the surface photocurrent departed from the theoretical expectation over the energy range studied. Surface currents arising from the linear photogalvanic effect were observed as well, with the unexpected result that only two of the six allowed tensor element were required to describe the measurements, suggesting an approximate emergent mirror symmetry inconsistent with the space group of the crystal.
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Heusler compounds have potential in electrocatalysis because of their mechanical robustness, metallic conductivity, and wide tunability in the electronic structure and element compositions. This ...study reports the first application of Co
YZ-type Heusler compounds as electrocatalysts for the oxygen evolution reaction (OER). A range of Co
YZ crystals was synthesized through the arc-melting method and the e
orbital filling of Co was precisely regulated by varying Y and Z sites of the compound. A correlation between the e
orbital filling of reactive Co sites and OER activity was found for Co
MnZ compounds (Z=Ti, Al, V, and Ga), whereby higher catalytic current was achieved for e
orbital filling approaching unity. A similar trend of e
orbital filling on the reactivity of cobalt sites was also observed for other Heusler compounds (Co
VZ, Z=Sn and Ga). This work demonstrates proof of concept in the application of Heusler compounds as a new class of OER electrocatalysts, and the influence of the manipulation of the spin orbitals on their catalytic performance.
The crystal symmetry of a material dictates the type of topological band structure it may host, and therefore, symmetry is the guiding principle to find topological materials. Here we introduce an ...alternative guiding principle, which we call ‘quasi-symmetry’. This is the situation where a Hamiltonian has exact symmetry at a lower order that is broken by higher-order perturbation terms. This enforces finite but parametrically small gaps at some low-symmetry points in momentum space. Untethered from the restraints of symmetry, quasi-symmetries eliminate the need for fine tuning as they enforce that sources of large Berry curvature occur at arbitrary chemical potentials. We demonstrate that quasi-symmetry in the semi-metal CoSi stabilizes gaps below 2 meV over a large near-degenerate plane that can be measured in the quantum oscillation spectrum. The application of in-plane strain breaks the crystal symmetry and gaps the degenerate point, observable by new magnetic breakdown orbits. The quasi-symmetry, however, does not depend on spatial symmetries and hence transmission remains fully coherent. These results demonstrate a class of topological materials with increased resilience to perturbations such as strain-induced crystalline symmetry breaking, which may lead to robust topological applications as well as unexpected topology beyond the usual space group classifications.The concept of quasi-symmetry—a perturbatively small deviation from exact symmetry—is introduced and leads to topological materials with strong resilience to perturbations.
Abstract
The Mn
3
X family of compounds was the first in which a large anomalous Hall effect (AHE) was predicted to arise from a purely antiferromagnetic structure, due to the Berry curvature in ...momentum space. Nearly simultaneously with this prediction, a large AHE was observed experimentally in one of the hexagonal members of this family, Mn
3
Sn. Aligning antiferromagnetic domains, a necessary step for observation of the AHE, is more challenging for the cubic members of the Mn
3
X family, due to a combination of smaller spontaneous ferromagnetic moments and much stronger magnetic anisotropy. Here, we use a combination of uniaxial stress and applied magnetic field to align domains of bulk single-crystal Mn
3
Pt, and demonstrate for the first time a substantial AHE in a bulk sample of a cubic member of the Mn
3
X family. The AHE remains locked in with essentially no quantitative variation when the stress is ramped back to zero, which shows that it is not a consequence of any stress-induced ferromagnetic moment.
Quantum phases can be classified by topological invariants, which take on discrete values capturing global information about the quantum state
. Over the past decades, these invariants have come to ...play a central role in describing matter, providing the foundation for understanding superfluids
, magnets
, the quantum Hall effect
, topological insulators
, Weyl semimetals
and other phenomena. Here we report an unusual linking-number (knot theory) invariant associated with loops of electronic band crossings in a mirror-symmetric ferromagnet
. Using state-of-the-art spectroscopic methods, we directly observe three intertwined degeneracy loops in the material's three-torus, T
, bulk Brillouin zone. We find that each loop links each other loop twice. Through systematic spectroscopic investigation of this linked-loop quantum state, we explicitly draw its link diagram and conclude, in analogy with knot theory, that it exhibits the linking number (2, 2, 2), providing a direct determination of the invariant structure from the experimental data. We further predict and observe, on the surface of our samples, Seifert boundary states protected by the bulk linked loops, suggestive of a remarkable Seifert bulk-boundary correspondence. Our observation of a quantum loop link motivates the application of knot theory to the exploration of magnetic and superconducting quantum matter.
The classification scheme of electronic phases uses two prominent paradigms: correlations and topology. Electron correlations give rise to superconductivity and charge density waves, while the ...quantum geometric Berry phase gives rise to electronic topology. The intersection of these two paradigms has initiated an effort to discover electronic instabilities at or near the Fermi level of topological materials. Here we identify the electronic topology of chiral fermions as the driving mechanism for creating van Hove singularities that host electronic instabilities in the surface band structure. We observe that the chiral fermion conductors RhSi and CoSi possess two types of helicoid arc van Hove singularities that we call type I and type II. In RhSi, the type I variety drives a switching of the connectivity of the helicoid arcs at different energies. In CoSi, we measure a type II intra-helicoid arc van Hove singularity near the Fermi level. Chemical engineering methods are able to tune the energy of these singularities. Finally, electronic susceptibility calculations allow us to visualize the dominant Fermi surface nesting vectors of the helicoid arc singularities, consistent with recent observations of surface charge density wave ordering in CoSi. This suggests a connection between helicoid arc singularities and surface charge density waves.Strong correlations between electrons in topological surface states drive the formation of surface van Hove singularities. These may be linked to charge density waves in the surface states.
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