Weyl semimetals are a class of materials that can be regarded as three-dimensional analogs of graphene upon breaking time-reversal or inversion symmetry. Electrons in a Weyl semimetal behave as Weyl ...fermions, which have many exotic properties, such as chiral anomaly and magnetic monopoles in the crystal momentum space. The surface state of a Weyl semimetal displays pairs of entangled Fermi arcs at two opposite surfaces. However, the existence of Weyl semimetals has not yet been proved experimentally. Here, we report the experimental realization of a Weyl semimetal in TaAs by observing Fermi arcs formed by its surface states using angle-resolved photoemission spectroscopy. Our first-principles calculations, which match remarkably well with the experimental results, further confirm that TaAs is a Weyl semimetal.
Surface magnetism and its correlation with the electronic structure are critical to understanding the topological surface state in the intrinsic magnetic topological insulator MnBi2Te4. Here, using ...static and time resolved angle-resolved photoemission spectroscopy (ARPES), we find a significant ARPES intensity change together with a gap opening on a Rashba-like conduction band. Comparison with a model simulation strongly indicates that the surface magnetism on cleaved MnBi2Te4 is the same as its bulk state. The inability of surface ferromagnetism to open a gap in the topological surface state uncovers the novel complexity of MnBi2Te4 that may be responsible for the low quantum anomalous Hall temperature of exfoliated MnBi2Te4.
We study the normal state electronic structure of the recently discovered infinite-layer nickelate superconductor, Nd1−x Srx NiO2, using density functional theory plus dynamical mean-field theory ...calculations. Starting with the multiorbital compound SrNiO2, our calculations show that despite large charge-carrier doping from SrNiO2 to LaNiO2, the Ni − 3d total occupancy is barely changed due to the decreased hybridization with the occupied oxygen-2p states and increased hybridization with the unoccupied La − 5d states. Thus, using SrNiO2 as a reference, La1−x Srx NiO2 is naturally and conclusively found to be a multiorbital electronic system with characteristic Hund's metal behaviors, such as metallicity, the importance of high-spin configurations, tendency towards orbital differentiation, and the absence of magnetism in regimes which are ordered according to static mean-field theories. Our results are in good agreement with the existing spectroscopic studies and make an essential step towards an understanding of the electronic structures of Nd 1−x Srx NiO2.
The kagome lattice
, which is the most prominent structural motif in quantum physics, benefits from inherent non-trivial geometry so that it can host diverse quantum phases, ranging from spin-liquid ...phases, to topological matter, to intertwined orders
and, most rarely, to unconventional superconductivity
. Recently, charge sensitive probes have indicated that the kagome superconductors AV
Sb
(A = K, Rb, Cs)
exhibit unconventional chiral charge order
, which is analogous to the long-sought-after quantum order in the Haldane model
or Varma model
. However, direct evidence for the time-reversal symmetry breaking of the charge order remains elusive. Here we use muon spin relaxation to probe the kagome charge order and superconductivity in KV
Sb
. We observe a noticeable enhancement of the internal field width sensed by the muon ensemble, which takes place just below the charge ordering temperature and persists into the superconducting state. Notably, the muon spin relaxation rate below the charge ordering temperature is substantially enhanced by applying an external magnetic field. We further show the multigap nature of superconductivity in KV
Sb
and that the Formula: see text ratio (where T
is the superconducting transition temperature and λ
is the magnetic penetration depth in the kagome plane) is comparable to those of unconventional high-temperature superconductors. Our results point to time-reversal symmetry-breaking charge order intertwining with unconventional superconductivity in the correlated kagome lattice.
Condensed matter systems have now become a fertile ground to discover emerging topological quasiparticles with symmetry protected modes. While many studies have focused on fermionic excitations, the ...same conceptual framework can also be applied to bosons yielding new types of topological states. Motivated by Zhang et al.'s recent theoretical prediction of double Weyl phonons in transition metal monosilicides Phys. Rev. Lett. 120, 016401 (2018)PRLTAO0031-900710.1103/PhysRevLett.120.016401, we directly measure the phonon dispersion in parity-breaking FeSi using inelastic x-ray scattering. By comparing the experimental data with theoretical calculations, we make the first observation of double Weyl points in FeSi, which will be an ideal material to explore emerging bosonic excitations and its topologically nontrivial properties.
Abstract
In crystalline materials, electron-phonon coupling (EPC) is a ubiquitous many-body interaction that drives conventional Bardeen-Cooper-Schrieffer superconductivity. Recently, in a new kagome ...metal CsV
3
Sb
5
, superconductivity that possibly intertwines with time-reversal and spatial symmetry-breaking orders is observed. Density functional theory calculations predicted weak EPC strength, λ, supporting an unconventional pairing mechanism in CsV
3
Sb
5
. However, experimental determination of λ is still missing, hindering a microscopic understanding of the intertwined ground state of CsV
3
Sb
5
. Here, using 7-eV laser-based angle-resolved photoemission spectroscopy and Eliashberg function analysis, we determine an intermediate λ=0.45–0.6 at
T
= 6 K for both Sb 5
p
and V 3
d
electronic bands, which can support a conventional superconducting transition temperature on the same magnitude of experimental value in CsV
3
Sb
5
. Remarkably, the EPC on the V 3
d
-band enhances to λ~0.75 as the superconducting transition temperature elevated to 4.4 K in Cs(V
0.93
Nb
0.07
)
3
Sb
5
. Our results provide an important clue to understand the pairing mechanism in the kagome superconductor CsV
3
Sb
5
.
Abstract
The intertwining between spin, charge, and lattice degrees of freedom can give rise to unusual macroscopic quantum states, including high-temperature superconductivity and quantum anomalous ...Hall effects. Recently, a charge density wave (CDW) has been observed in the kagome antiferromagnet FeGe, indicative of possible intertwining physics. An outstanding question is that whether magnetic correlation is fundamental for the spontaneous spatial symmetry breaking orders. Here, utilizing elastic and high-resolution inelastic x-ray scattering, we observe a c-axis superlattice vector that coexists with the 2
$$\times$$
×
2
$$\times$$
×
1 CDW vectors in the kagome plane. Most interestingly, between the magnetic and CDW transition temperatures, the phonon dynamical structure factor shows a giant phonon-energy hardening and a substantial phonon linewidth broadening near the c-axis wavevectors, both signaling the spin-phonon coupling. By first principles and model calculations, we show that both the static spin polarization and dynamic spin excitations intertwine with the phonon to drive the spatial symmetry breaking in FeGe.
Abstract
The Kitaev quantum spin liquid epitomizes an entangled topological state, for which two flavors of fractionalized low-energy excitations are predicted: the itinerant Majorana fermion and the ...Z
2
gauge flux. It was proposed recently that fingerprints of fractional excitations are encoded in the phonon spectra of Kitaev quantum spin liquids through a novel fractional-excitation-phonon coupling. Here, we detect anomalous phonon effects in α-RuCl
3
using inelastic X-ray scattering with meV resolution. At high temperature, we discover interlaced optical phonons intercepting a transverse acoustic phonon between 3 and 7 meV. Upon decreasing temperature, the optical phonons display a large intensity enhancement near the Kitaev energy,
J
K
~8 meV, that coincides with a giant acoustic phonon softening near the Z
2
gauge flux energy scale. These phonon anomalies signify the coupling of phonon and Kitaev magnetic excitations in α-RuCl
3
and demonstrates a proof-of-principle method to detect anomalous excitations in topological quantum materials.
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