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Register a new userManipulation of electronic and magnetic properties of M$_2$C (M=Hf, Nb, Sc, Ta, Ti, V, Zr) monolayer by applying mechanical strains
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Tuning the electronic and magnetic properties of a material through strain engineering is an effective strategy to enhance the performance of electronic and spintronic devices. Recently synthesized two-dimensional transition metal carbides M$_2$C (M=Hf, Nb, Sc, Ta, Ti, V, Zr), known as MXenes, has aroused increasingly attentions in nanoelectronic technology due to their unusual properties. In this paper, first-principles calculations based on density functional theory are carried out to investigate the electronic and magnetic properties of M$_2$C subjected to biaxial symmetric mechanical strains. At the strain-free state, all these MXenes exhibit no spontaneous magnetism except for Ti$_2$C and Zr$_2$C which show a magnetic moment of 1.92 and 1.25 $mu_B$/unit, respectively. As the tensile strain increases, the magnetic moments of MXenes are greatly enhanced and a transition from nonmagnetism to ferromagnetism is observed for those nonmagnetic MXenes at zero strains. The most distinct transition is found in Hf$_2$C, in which the magnetic moment is elevated to 1.5 $mu_B$/unit at a strain of 15%. We further show that the magnetic properties of Hf$_2$C are attributed to the band shift mainly composed of Hf(5$d$) states. This strain-tunable magnetism can be utilized to design future spintronics based on MXenes.
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The family of two-dimensional transition metal carbides, so called MXenes, has recently found new members with ordered double transition metals M$_2$M$$C$_2$, where M$$ and M$$ stand for transition metals. Here, using a set of first-principles calculations, we demonstrate that some of the newly added members, oxide M$_2$M$$C$_2$ (M$$= Mo, W; M$$= Ti, Zr, Hf) MXenes, are topological insulators. The nontrivial topological states of the predicted MXenes are revealed by the $Z_2$ index, which is evaluated from the parities of the occupied bands below the Fermi energy at time reversal invariant momenta, and also by the presence of the edge states. The predicted M$_2$M$$C$_2$O$_2$ MXenes show nontrivial gaps in the range of 0.041 -- 0.285 eV within the generalized gradient approximation and 0.119 -- 0.409 eV within the hybrid functional. The band gaps are induced by the spin-orbit coupling within the degenerate states with $d_{x^2-y^2}$ and $d_{xy}$ characters of M$$ and M$$, while the band inversion occurs at the $Gamma$ point among the degenerate $d_{x^2-y^2}$/$d_{xy}$ orbitals and a non-degenerate $d_{3z^2-r^2}$ orbital, which is driven by the hybridization of the neighboring orbitals. The phonon dispersion calculations find that the predicted topological insulators are structurally stable. The predicted W-based MXenes with large band gaps might be suitable candidates for many topological applications at room temperature. In addition, we study the electronic structures of thicker ordered double transition metals M$_2$M$_2$C$_3$O$_2$ (M$$= Mo, W; M$$= Ti, Zr, Hf) and find that they are nontrivial topological semimetals.
We have investigated the plastic deformation properties of non-equiatomic single phase Zr-Nb-Ti-Ta-Hf high-entropy alloys from room temperature up to 300 {deg}C. Uniaxial deformation tests at a constant strain rate of 10$^{-4}$ s$^{-1}$ were performed including incremental tests such as stress-relaxations, strain-rate- and temperature changes in order to determine the thermodynamic activation parameters of the deformation process. The microstructure of deformed samples was characterized by transmission electron microscopy. The strength of the investigated Zr-Nb-Ti-Ta-Hf phase is not as high as the values frequently reported for high-entropy alloys in other systems. We find an activation enthalpy of about 1 eV and a stress dependent activation volume between 0.5 and 2 nm$^3$. The measurement of the activation parameters at higher temperatures is affected by structural changes evolving in the material during plastic deformation.
The helimagnets Cr$_{1/3}M$S$_2$ ($M$ = Nb or Ta) have attracted renewed attention due to the discovery of a chiral soltion lattice (CSL) stabilized in Cr$_{1/3}$NbS$_2$ in an applied magnetic field, but reports of unusual low-temperature transport and magnetic properties in this system lack a unifying explanation. Here we present electronic structure calculations demonstrating that Cr$_{1/3}M$S$_2$ ($M$ = Nb or Ta) are half-metals whose low-temperature electronic and magnetic behavior can be explained by the presence of a gap-like feature (width in range 40-100 meV) in the density of states of one spin channel. Our magnetometry measurements confirm the existence of this gap. Dynamic spin fluctuations driven by excitations across this gap are seen over a wide range of frequencies (0.1 Hz to MHz) with AC susceptibility and muon-spin relaxation ($mu^+$SR) measurements. We show further how effects due to the CSL in Cr$_{1/3}$NbS$_2$, as detected with $mu^+$SR, dominate over the gap-driven magnetism when the CSL is stabilized as the majority phase.
We report a strategy to induce superconductivity in the BiS$_2$-based compound LaOBiS$_2$. Instead of substituting F for O, we increase the charge-carrier density (electron dope) via substitution of tetravalent Th$^{+4}$, Hf$^{+4}$, Zr$^{+4}$, and Ti$^{+4}$ for trivalent La$^{+3}$. It is found that both the LaOBiS$_2$ and ThOBiS$_2$ parent compounds are bad metals and that superconductivity is induced by electron doping with emph{T$_c$} values of up to 2.85 K. The superconducting and normal states were characterized by electrical resistivity, magnetic susceptibility, and heat capacity measurements. We also demonstrate that reducing the charge-carrier density (hole doping) via substitution of divalent Sr$^{+2}$ for La$^{+3}$ does not induce superconductivity.
In this review, we present a comprehensive overview of superconductivity in electron-doped metal nitride halides $M$N$X$ ($M$ = Ti, Zr, Hf; $X$ = Cl, Br, I) with layered crystal structure and two-dimensional electronic states. The parent compounds are band insulators with no discernible long-range ordered state. Upon doping tiny amount of electrons, superconductivity emerges with several anomalous features beyond the conventional electron-phonon mechanism, which stimulate theoretical investigations. We will discuss experimental and theoretical results reported thus far and compare the electron-doped layered nitride superconductors with other superconductors.
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