The Fermi surface (FS) is essential for understanding the properties of metals. It can change under both conventional symmetry-breaking phase transitions and Lifshitz transitions (LTs), where the FS, but not the crystal symmetry, changes abruptly. Magnetic phase transitions involving uniformly rotating spin textures are conventional in nature, requiring strong spin-orbit coupling (SOC) to influence the FS topology and generate measurable properties. LTs driven by a continuously varying magnetization are rarely discussed. Here we present two such manifestations in the magnetotransport of the kagome magnet YMn6Sn6: one caused by changes in the magnetic structure and another by a magnetization-driven LT. The former yields a 10% magnetoresistance enhancement without a strong SOC, while the latter a 45% reduction in the resistivity. These phenomena offer a unique view into the interplay of magnetism and electronic topology, and for understanding the rare-earth counterparts, such as TbMn6Sn6, recently shown to harbor correlated topological physics.
Quantum critical points separating weak ferromagnetic and paramagnetic phases trigger many novel phenomena. Dynamical spin fluctuations not only suppress the long‐range order, but can also lead to unusual transport and even superconductivity. Combining quantum criticality with topological electronic properties presents a rare and unique opportunity. Here, by means of ab initio calculations and magnetic, thermal, and transport measurements, it is shown that the orthorhombic CoTe2 is close to ferromagnetism, which appears suppressed by spin fluctuations. Calculations and transport measurements reveal nodal Dirac lines, making it a rare combination of proximity to quantum criticality and Dirac topology.
Cu2TSiS4 (T = Mn and Fe) polycrystalline
and single-crystal materials were prepared with high-temperature solid-state
and chemical vapor transport methods, respectively. The polar crystal
structure (space group Pmn21) consists
of chains of corner-sharing and distorted CuS4, Mn/FeS4, and SiS4 tetrahedra, which is confirmed by Rietveld
refinement using neutron powder diffraction data, X-ray single-crystal
refinement, electron diffraction, energy-dispersive X-ray spectroscopy,
and second harmonic generation (SHG) techniques. Magnetic measurements
indicate that both compounds order antiferromagnetically at 8 and
14 K, respectively, which is supported by the temperature-dependent
(100–2 K) neutron powder diffraction data. Additional magnetic
reflections observed at 2 K can be modeled by magnetic propagation
vectors k = (1/2,0,1/2) and k =
(1/2,1/2,1/2) for Cu2MnSiS4 and Cu2FeSiS4, respectively. The refined antiferromagnetic structure
reveals that the Mn/Fe spins are canted away from the ac plane by about 14°, with the total magnetic moments of Mn and
Fe being 4.1(1) and 2.9(1) μB, respectively. Both
compounds exhibit an SHG response with relatively modest second-order
nonlinear susceptibilities. Density functional theory calculations
are used to describe the electronic band structures.
Polycrystalline LiMo8O10 was prepared
in
a sealed Mo crucible at 1380 °C for 48 h using the conventional
high-temperature solid-state method. The polar tetragonal crystal
structure (space group I41
md) is confirmed based on the Rietveld refinement of powder neutron
diffraction and 7Li/6Li solid-state NMR. The
crystal structure features infinite chains of Mo4O5 (i.e., Mo2Mo4/2O6/2O6/3) as a repeat unit containing edge-sharing Mo6 octahedra with strong Mo–Mo metal bonding along the chain.
X-ray absorption near-edge spectroscopy of the Mo-L3 edge
is consistent with the formal Mo valence/configuration. Magnetic measurements
reveal that LiMo8O10 is paramagnetic down to
1.8 K. Temperature-dependent resistivity [ρ(T)] measurement
indicates a semiconducting behavior that can be fitted with Mott’s
variable range hopping conduction mechanism in the temperature range
of 215 and 45 K. The ρ(T) curve exhibits an exponential increase
below 5 K with a large ratio of ρ1.8/ρ300 = 435. LiMo8O10 shows a negative
field-dependent magnetoresistance between 2 and 25 K. Heat capacity
measurement fitted with the modified Debye model yields the Debye
temperature of 365 K.
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