Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userStrain-induced Dirac cone-like electronic structures and semiconductor-semimetal transition in graphdiyne
170
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
By means of the first-principles calculations combined with the tight-binding approximation, the strain-induced semiconductor-semimetal transition in graphdiyne is discovered. It is shown that the band gap of graphdiyne increases from 0.47 eV to 1.39 eV with increasing the biaxial tensile strain, while the band gap decreases from 0.47 eV to nearly zero with increasing the uniaxial tensile strain, and Dirac cone-like electronic structures are observed. The uniaxial strain-induced changes of the electronic structures of graphdiyne come from the breaking of geometrical symmetry that lifts the degeneracy of energy bands. The properties of graphdiyne under strains are disclosed different remarkably from that of graphene.
rate research
Read More
Very recently, a new type of two-dimensional layered material MoSi2N4 has been fabricated, which is semiconducting with weak interlayer interaction, high strength, and excellent stability. We systematically investigate theoretically the effect of vertical strain on the electronic structure of MA2Z4 (M=Ti/Cr/Mo, A=Si, Z=N/P) bilayers. Taking bilayer MoSi2N4 as an example, our first principle calculations show that its indirect band gap decreases monotonically as the vertical compressive strain increases. Under a critical strain around 22%, it undergoes a transition from semiconductor to metal. We attribute this to the opposite energy shift of states in different layers, which originates from the built-in electric field induced by the asymmetric charge transfer between two inner sublayers near the interface. Similar semiconductor to metal transitions are observed in other strained MA2Z4 bilayers, and the estimated critical pressures to realize such transitions are within the same order as semiconducting transition metal dichalcogenides. The semiconductor to metal transitions observed in the family of MA2Z4 bilayers present interesting possibilities for strain-induced engineering of their electronic properties.
Topological electrons in semimetals are usually vulnerable to chemical doping and environment change, which restricts their potential application in future electronic devices. In this paper we report that the type-II Dirac semimetal $mathbf{VAl_3}$ hosts exceptional, robust topological electrons which can tolerate extreme change of chemical composition. The Dirac electrons remain intact even after a substantial part of V atoms have been replaced in the $mathbf{V_{1-x}Ti_xAl_3}$ solid solutions. This Dirac semimetal state ends at $x=0.35$ where a Lifshitz transition to $p$-type trivial metal occurs. The V-Al bond is completely broken in this transition as long as the bonding orbitals are fully depopulated by the holes donated from Ti substitution. In other words, the Dirac electrons in $mathbf{VAl_3}$ are protected by the V-Al bond whose molecular orbital is their bonding gravity center. Our understanding on the interrelations among electron count, chemical bond and electronic properties in topological semimetals suggests a rational approach to search robust, chemical-bond-protected topological materials.
Using angle resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations we studied the low-energy electronic structure of bulk ZrTe$_2$. ARPES studies on ZrTe$_2$ demonstrate free charge carriers at the Fermi level, which is further confirmed by the DFT calculations. An equal number of hole and electron carrier density estimated from the ARPES data, points ZrTe$_2$ to a semimetal. The DFT calculations further suggest a band inversion between Te $p$ and Zr $d$ states at the $Gamma$ point, hinting at the non-trivial band topology in ZrTe$_2$. Thus, our studies for the first time unambiguously demonstrate that ZrTe$_2$ is a topological semimetal. Also, a comparative band structure study is done on ZrSe$_2$ which shows a semiconducting nature of the electronic structure with an indirect band gap of 0.9 eV between $Gamma (A) $ and $M (L)$ high symmetry points. In the below we show that the metal-chalcogen bond-length plays a critical role in the electronic phase transition from semiconductor to a topological semimetal ingoing from ZrSe$_2$ to ZrTe$_2$.
Band-crossings occurring on a mirror plane are compelled to form a nodal loop in the momentum space without spin-orbit coupling (SOC). In the presence of other equivalent mirror planes, multiple such nodal loops can combine to form interesting nodal-link structures. Here, based on first-principles calculations and an effective $mathbf{k.p}$ model analysis, we show that CaAuAs hosts a unique starfruit-like nodal-link structure in the bulk electronic dispersion in the absence of SOC. This nodal-link is comprised of three nodal loops, which cross each other at the time-reversal-invariant momentum point $A$. When the SOC is turned on, the nodal loops are gapped out, resulting in a stable Dirac semimetal state with a pair of Dirac points along the $mathrm{Gamma-A}$ direction in the Brillouin zone. The Dirac points are protected by the combination of time reversal, inversion, and $C_3$ rotation symmetries. We show how a systematic elimination of the symmetry constraints yields a Weyl semimetal and eventually a topological insulator state.
Raman scattering in the three-dimensional Dirac semimetal Cd3As2 shows an intricate interplay of electronic and phonon degrees of freedom. We observe resonant phonon scattering due to interband transitions, an anomalous anharmonicity of phonon frequency and intensity, as well as quasielastic (E~0) electronic scattering. The latter two effects are governed by a characteristic temperature scale T* ~ 100 K that is related to mutual fluctuations of lattice and electronic degrees of freedom. A refined analysis shows that this characteristic temperature corresponds to the energy of optical phonons which couple to interband transitions in the Dirac states of Cd3As2. As electron-phonon coupling in a topological semimetal is primarily related to phonons with finite momenta, the back action on the optical phonons in only observed as anharmonicities via multi-phonon processes involving a broad range of momenta. The resulting energy density fluctuations of the coupled system have previously only been observed in low dimensional or frustrated spin systems with suppressed long range ordering.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
