Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userSuperlattice formation lifting degeneracy protected by non-symmorphic symmetry through a metal-insulator transition in RuAs
76
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
The single crystal of RuAs obtained by Bi-flux method shows obvious successive metal-insulator transitions at T_MI1~255 K and T_MI2~195$ K. The X-ray diffraction measurement reveals a formation of superlattice of 3x3x3 of the original unit cell below T_MI2, accompanied by a change of the crystal system from the orthorhombic structure to the monoclinic one. Simple dimerization of the Ru ions is nor seen in the ground state. The multiple As sites observed in nuclear quadrupole resonance (NQR) spectrum also demonstrate the formation of the superlattice in the ground state, which is clarified to be nonmagnetic. The divergence in 1/T_1 at T_MI1 shows that a symmetry lowering by the metal-insulator transition is accompanied by strong critical fluctuations of some degrees of freedom. Using the structural parameters in the insulating state, the first principle calculation reproduces successfully the reasonable size of nuclear quadrupole frequencies for the multiple As sites, ensuring the high validity of the structural parameters. The calculation also gives a remarkable suppression in the density of states (DOS) near the Fermi level, although the gap opening is insufficient. A coupled modulation of the calculated Ru d electron numbers and the crystal structure proposes a formation of charge density wave (CDW) in RuAs. Some lacking factors remain, but it shows that a lifting of degeneracy protected by the non-symmorphic symmetry through the superlattice formation is a key ingredient for the metal-insulator transition in RuAs.
rate research
Read More
Luttingers theorem is a fundamental result in the theory of interacting Fermi systems: it states that the volume inside the Fermi surface is left invariant by interactions, if the number of particles is held fixed. Although this is traditionally justified using perturbation theory, it can be viewed as arising from a momentum balance argument that examines the response of the ground state to the insertion of a single flux quantum [M. Oshikawa, Phys. Rev. Lett. 84, 3370 (2000)]. This reveals that the Fermi sea volume is a topologically protected quantity. Extending this approach, I show that spinless or spin-rotation-preserving fermionic systems in non-symmorphic crystals possess generalized topological Luttinger invariants that can be nonzero even in cases where the Fermi sea volume vanishes. A nonzero Luttinger invariant then forces energy bands to touch, leading to semimetals whose gaplessness is thus rooted in topology; opening a gap without symmetry breaking automatically triggers fractionalization. The existence of these invariants is linked to the inability of non-symmorphic crystals to host band insulating ground states except at special fillings. I exemplify the use of these new invariants by showing that they distinguish various classes of two- and three-dimensional semimetals.
The anisotropic changes in the electronic structure of a metal-to-insulator transition (MIT) material, RuAs, with two-step phase transition are reported by using polarized optical conductivity [$sigma(omega)$] spectra, angle-integrated photoelectron (PE) spectra, and band calculations based on local density approximation (LDA). Both the PE and $sigma(omega)$ spectra not only in the high-temperature (HT) phase but also in the low-temperature (LT) phase as well as the energy gap formation owing to the MIT were almost consistent with those derived from the LDA band calculations, so the fundamental electronic structure in the HT and LT phases can be explained without electron correlations. However, the electronic structure in the middle phase between the HT and LT phases has not been clarified. The polarized $sigma(omega)$ spectra revealed not only the anisotropic energy gap formation but also the anisotropic gap-opening temperature, i.e., the energy gap along the $c$ axis in the HT phase starts to open near the higher transition temperature, but that along the $b$ axis opens below the lower transition temperature. The finding suggests that the two-step MIT originates from the anisotropic energy gap formation.
We outline a general mechanism for Orbital-selective Mott transition (OSMT), the coexistence of both itinerant and localized conduction electrons, and show how it can take place in a wide range of realistic situations, even for bands of identical width and correlation, provided a crystal field splits the energy levels in manifolds with different degeneracies and the exchange coupling is large enough to reduce orbital fluctuations. The mechanism relies on the different kinetic energy in manifolds with different degeneracy. This phase has Curie-Weiss susceptibility and non Fermi-liquid behavior, which disappear at a critical doping, all of which is reminiscent of the physics of the pnictides.
Metal-insulator transition features as a transformation, from a highly charge conductive state to another state where charge conductivity is greatly suppressed when decreasing the temperature. Here we demonstrate two consecutive transitions in NdNiO3 films with CoFe2O4 capping, in which the metal-insulator transition occurs at 85 K, followed by an unprecedented insulator-metal transition below 40 K. The emerging insulator-metal transition associated with a weak antiferromagnetic behavior is observed in 20 unit cell-thick NdNiO3 with more than 5 unit cell CoFe2O4 capping. Differently, the NdNiO3 films with thinner CoFe2O4 capping only exhibit metal-insulator transition at 85 K, accompanied by a strong antiferromagnetic state below 40 K. Charge transfer from Co to Ni, instead of from Fe to Ni, formulates the ferromagnetic interaction between Ni-Ni and Ni-Co atoms, thus suppressing the antiferromagnetic feature and producing metallic conductive behavior. Furthermore, a phase diagram for the metal-insulator-metal transition in this system is drawn.
The local atomic and magnetic structures of the compounds $A$MnO$_2$ ($A$ = Na, Cu), which realize a geometrically frustrated, spatially anisotropic triangular lattice of Mn spins, have been investigated by atomic and magnetic pair distribution function analysis of neutron total scattering data. Relief of frustration in CuMnO$_2$ is accompanied by a conventional cooperative symmetry-lowering lattice distortion driven by Neel order. In NaMnO$_2$, however, the distortion has a short-range nature. A cooperative interaction between the locally broken symmetry and short-range magnetic correlations lifts the magnetic degeneracy on a nanometer length scale, enabling long-range magnetic order in the Na-derivative. The degree of frustration, mediated by residual disorder, contributes to the rather differing pathways to a single, stable magnetic ground state in these two related compounds. This study demonstrates how nanoscale structural distortions that cause local-scale perturbations can lift the ground state degeneracy and trigger macroscopic magnetic order.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
