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Register a new userType-II multiferroic Hf$_{2}$VC$_{2}$F$_{2}$ MXene monolayer with high transition temperature
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Achieving multiferroic two-dimensional (2D) materials should enable numerous functionalities in nanoscale devices. Until now, however, predicted 2D multiferroics are very few and with coexisting yet only loosely coupled (type-I) ferroelectricity and magnetism. Here, a type-II multiferroic MXene Hf$_{2}$VC$_{2}$F$_{2}$ monolayer is identified, where ferroelectricity originates directly from its magnetism. The noncollinear Y-type spin order generates a polarization perpendicular to the spin helical plane. Remarkably, the multiferroic transition is estimated to occur above room temperature. Our investigation should open the door to a new branch of 2D materials for pursuit of intrinsically strong magnetoelectricity.
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Two dimensional multiferroics inherit prominent physical properties from both low dimensional materials and magnetoelectric materials, and can go beyond their three dimensional counterparts for their unique structures. Here, based on density functional theory calculations, a MXene derivative, i.e., i-MXene (Ta$_{2/3}$Fe$_{1/3}$)$_2$CO$_2$, is predicted to be a type-I multiferroic material. Originated from the reliable $5d^0$ rule, its ferroelectricity is robust, with a moderate polarization up to $sim12.33$ $mu$C/cm$^2$ along the a-axis, which can be easily switched and may persist above room temperature. Its magnetic ground state is layered antiferromagnetism. Although it is a type-I multiferroic material, its Neel temperature can be significantly tuned by the paraelectric-ferroelectric transition, manifesting a kind of intrinsic magnetoelectric coupling. Such magnetoelectric effect is originated from the conventional magnetostriction, but unexpectedly magnified by the exchange frustration. Our work not only reveals a nontrivial magnetoelectric mechanism, but also provides a strategy to search for more multiferroics in the two dimensional limit.
Single layers of transition metal dichalcogenides (TMDs) are direct gap semiconductors with nondegenerate valley indices. An intriguing possibility for these materials is the use of their valley index as an alternate state variable. Several limitations to such a utility include strong, phonon-enabled intervalley scattering, as well as multiparticle interactions leading to multiple emission channels. We prepare single-layer WS$_{2}$ such that the photoluminescence is from either the neutral or charged exciton (trion). After excitation with circularly polarized light, the neutral exciton emission has zero polarization, however, the trion emission has a large polarization (28%) at room temperature. The trion emission also has a unique, non-monotonic temperature dependence that we show is a consequence of the multiparticle nature of the trion. This temperature dependence enables us to determine that coulomb assisted intervalley scattering, electron-hole radiative recombination, and a 3-particle Auger process are the dominant mechanisms at work in this system. Because this dependence involves trion systems, one can use gate voltages to modulate the polarization (or intensity) emitted from TMD structures.
We have studied the longitudinal spin Seebeck effect in a polar antiferromagnet $alpha$-Cu$_{2}$V$_{2}$O$_{7}$ in contact with a Pt film. Below the antiferromagnetic transition temperature of $alpha$-Cu$_{2}$V$_{2}$O$_{7}$, spin Seebeck voltages whose magnetic field dependence is similar to that reported in antiferromagnetic MnF$_{2}$$mid$Pt bilayers are observed. Though a small weak-ferromagnetic moment appears owing to the Dzyaloshinskii-Moriya interaction in $alpha$-Cu$_{2}$V$_{2}$O$_{7}$, the magnetic field dependence of spin Seebeck voltages is found to be irrelevant to the weak ferromagnetic moments. The dependences of the spin Seebeck voltages on magnetic fields and temperature are analyzed by a magnon spin current theory. The numerical calculation of spin Seebeck voltages using magnetic parameters of $alpha$-Cu$_{2}$V$_{2}$O$_{7}$ determined by previous neutron scattering studies reveals that the magnetic-field and temperature dependences of the spin Seebeck voltages for $alpha$-Cu$_{2}$V$_{2}$O$_{7}$$mid$Pt are governed by the changes in magnon lifetimes with magnetic fields and temperature.
Electric current has been experimentally demonstrated to be able to drive the insulator-to-metal transition (IMT) in VO$_2$. The main mechanisms involved are believed to be the Joule heating effect and the strong electron-correlation effect. These effects are often entangled with each other in experiments, which complicates the understanding of the essential nature of the observations. We formulate a phase-field model to investigate theoretically in mesoscale the pure correlation effect brought by the current on the IMT in VO$_2$, i.e., the isothermal process under the current. We find that a current with a large density ($sim 10^1$ nA/nm$^2$) induces a few-nanosecond ultrafast switch in VO$_2$, in agreement with the experiment. The temperature-current phase diagram is further calculated, which reveals that the current may induce the M2 phase at low temperatures. The current is also shown capable of driving domain walls to move. Our work may assist related experiments and provide guidance to the engineering of VO$_2$-based electric switching devices.
Valleytronics targets the exploitation of the additional degrees of freedom in materials where the energy of the carriers may assume several equal minimum values (valleys) at non-equivalent points of the reciprocal space. In single layers of transition metal dichalcogenides (TMDs) the lack of inversion symmetry, combined with a large spin-orbit interaction, leads to a conduction (valence) band with different spin-polarized minima (maxima) having equal energies. This offers the opportunity to manipulate information at the level of the charge (electrons or holes), spin (up or down) and crystal momentum (valley). Any implementation of these concepts, however, needs to consider the robustness of such degrees of freedom, which are deeply intertwined. Here we address the spin and valley relaxation dynamics of both electrons and holes with a combination of ultrafast optical spectroscopy techniques, and determine the individual characteristic relaxation times of charge, spin and valley in a MoS$_{2}$ monolayer. These results lay the foundations for understanding the mechanisms of spin and valley polarization loss in two-dimensional TMDs: spin/valley polarizations survive almost two-orders of magnitude longer for holes, where spin and valley dynamics are interlocked, than for electrons, where these degrees of freedom are decoupled. This may lead to novel approaches for the integration of materials with large spin-orbit in robust spintronic/valleytronic platforms.
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