- Tailoring spin waves in 2D transition metal phosphorus trichalcogenides via atomic-layer substitutionAlberto M Ruiz, Dorye L Esteras, Andrey Rybakov, and José J Baldovı́Dalton Transactions, 2022
The family of two-dimensional (2D) van der Waals transition metal phosphorus trichalcogenides has received renewed interest due to their intrinsic 2D antiferromagnetism, which proves them as unprecedented and highly tunable building blocks for spintronics and magnonics at the single-layer limit. Herein, motivated by the exciting potential of atomic-substitution demonstrated by Janus transition metal dichalcogenides, we investigated the crystal, electronic and magnetic structures of selenized Janus monolayers based on MnPS3 and NiPS3 from first-principles. In addition, we calculated the magnon dispersion and performed real-time real-space atomistic dynamic simulations to explore the propagation of spin waves in MnPS3, NiPS3, MnPS1.5Se1.5 and NiPS1.5Se1.5. Our calculations predict a drastic enhancement of magnetic anisotropy and the emergence of large Dzyaloshinskii-Moriya interactions, which arise from the induced broken inversion symmetry in the 2D Janus layers. These results pave the way for the development of Janus 2D transition metal phosphorus trichalcogenides and highlight their potential for magnonic applications.
- Probing the Spin Dimensionality in Single-Layer CrSBr Van Der Waals Heterostructures by Magneto-Transport MeasurementsCarla Boix-Constant, Samuel Mañas-Valero, Alberto M Ruiz, Andrey Rybakov, Krzysztof Aleksander Konieczny, Sébastien Pillet, José J Baldovı́, and Eugenio CoronadoAdvanced Materials, 2022
2D magnetic materials offer unprecedented opportunities for fundamental and applied research in spintronics and magnonics. Beyond the pioneering studies on 2D CrI3 and Cr2Ge2Te6, the field has expanded to 2D antiferromagnets exhibiting different spin anisotropies and textures. Of particular interest is the layered metamagnet CrSBr, a relatively air-stable semiconductor formed by antiferromagnetically-coupled ferromagnetic layers (Tc 150 K) that can be exfoliated down to the single-layer. It presents a complex magnetic behavior with a dynamic magnetic crossover, exhibiting a low-temperature hidden-order below T* 40 K. Here, the magneto-transport properties of CrSBr vertical heterostructures in the 2D limit are inspected. The results demonstrate the marked low-dimensional character of the ferromagnetic monolayer, with short-range correlations above Tc and an Ising-type in-plane anisotropy, being the spins spontaneously aligned along the easy axis b below Tc. By applying moderate magnetic fields along a and c axes, a spin-reorientation occurs, leading to a magnetoresistance enhancement below T*. In multilayers, a spin-valve behavior is observed, with negative magnetoresistance strongly enhanced along the three directions below T*. These results show that CrSBr monolayer/bilayer provides an ideal platform for studying and controlling field-induced phenomena in two-dimensions, offering new insights regarding 2D magnets and their integration into vertical spintronic devices.
- Magnon straintronics in the 2D van der Waals ferromagnet CrSBr from first-principlesDorye L Esteras, Andrey Rybakov, Alberto M Ruiz, and José J Baldovı́Nano Letters, 2022
The recent isolation of two-dimensional (2D) magnets offers tantalizing opportunities for spintronics and magnonics at the limit of miniaturization. One of the key advantages of atomically thin materials is their outstanding deformation capacity, which provides an exciting avenue to control their properties by strain engineering. Herein, we investigate the magnetic properties, magnon dispersion, and spin dynamics of the air-stable 2D magnetic semiconductor CrSBr (TC = 146 K) under mechanical strain using first-principles calculations. Our results provide a deep microscopic analysis of the competing interactions that stabilize the long-range ferromagnetic order in the monolayer. We showcase that the magnon dynamics of CrSBr can be modified selectively along the two main crystallographic directions as a function of applied strain, probing the potential of this quasi-1D electronic system for magnon straintronics applications. Moreover, we predict a strain-driven enhancement of TC by 30%, allowing the propagation of spin waves at higher temperatures.
- Toward multifunctional molecular cells for quantum cellular automata: exploitation of interconnected charge and spin degrees of freedomPhysical Chemistry Chemical Physics, 2021
We discuss the possibility of using mixed-valence (MV) dimers comprising paramagnetic metal ions as molecular cells for quantum cellular automata (QCA). Thus, we propose to combine the underlying idea behind the functionality of QCA of using the charge distributions to encode binary information with the additional functional options provided by the spin degrees of freedom. The multifunctional (“smart”) cell is supposed to consist of multielectron MV dn-dn+1-type (1 ≤ n ≤ 8) dimers of transition metal ions as building blocks for composing bi-dimeric square planar cells for QCA. The theoretical model of such a cell involves the double exchange (DE), Heisenberg-Dirac-Van Vleck (HDVV) exchange, Coulomb repulsion between the two excess electrons belonging to different dimeric half-cells and also the vibronic coupling. Consideration is focused on the topical case in which the difference in Coulomb energies of the two excess electrons occupying nearest neighboring and distant positions significantly exceeds both the electron transfer integral and the vibronic energy. In this case the ground spin-state of the isolated square cell is shown to be the result of competition of the second-order DE producing a ferromagnetic effect and the HDVV exchange that is assumed to be antiferromagnetic. In order to reveal the functionality of the magnetic cells, the cell-cell response function is studied within the developed model. The interaction of the working cell with the polarized driver-cell is shown to produce an antiferromagnetic effect tending to suppress the ferromagnetic second-order DE. As a result, under some conditions the electric field of the driver cell is shown to force the working cell to exhibit spin-switching from the state with maximum dimeric spin values to that having minimal spin values.
- Exploration of the double exchange in quantum cellular automata: proposal for a new class of cellsChemical Communications, 2020
In this communication we propose to considerably extend the class of systems suitable as cells for quantum cellular automata by including magnetic quantum dots and molecular mixed valence dimers exhibiting double exchange. As distinguished from the previous works we propose to use not only charges as the information carriers but also spin degrees of freedom. In this context we focus on the two key points: (1) properties of the magnetic cell as reservoir for charges carrying binary information, and (2) identification of conditions under which spin degrees of freedom can be employed.
- Mixed-valence magnetic molecular cell for quantum cellular automata: Prospects of designing multifunctional devices through exploration of double exchangeAndrew Palii, Juan Modesto Clemente-Juan, Sergey Aldoshin, Denis Korchagin, Andrey Rybakov, Shmuel Zilberg, and Boris TsukerblatThe Journal of Physical Chemistry C, 2020
In this article, we propose to use multielectron square-planar mixed-valence (MV) molecules as molecular cells for quantum cellular automata (QCA) devices. As distinguished from previous proposals in this area, in multielectron cells, the electronic pair encoding binary information is shared among localized spins (“spin cores”). Hopefully, this will allow exploring not only charge degrees of freedom encoding binary information in the antipodal electronic distributions but also spin degrees of freedom through the magnetic interactions such as double exchange and Heisenberg-Dirac-Van Vleck (HDVV) exchange. To develop the proposed route, the square-planar tetrameric cells for QCA have been theoretically modeled. The considered cells comprise a pair of excess electrons shared among four spin cores and hence they exhibit double exchange and HDVV exchange. Such cells can be based either on the square-planar mixed-valence molecular clusters or on the similar square arrays of multielectron quantum dots. The detailed case study of the cell representing the transition-metal tetramer of the type of d2-d2-d1-d1 shows that depending on the relative strength of the second-order double exchange and HDVV exchange, the isolated cell has either ground localized spin-triplet or one of the two delocalized spin-singlets, exhibiting different extents of electron delocalization. Due to different sensitivities of these states to the quadrupole electrostatic field induced by the polarized neighboring driver cell, the latter is shown to be able to produce switching between different ground states (including spin-switching between spin-singlet and spin-triplet), which leads to the nonmonotonic behavior of the cell-cell response function. This opens new perspectives for the designing of multifunctional devices combining the QCA functionality with spin-switching function.
- Semiclassical versus quantum-mechanical vibronic approach in the analysis of the functional characteristics of molecular quantum cellular automataPhysical Chemistry Chemical Physics, 2019
In the context of the decisive role that vibronic interactions play in the functioning of molecular quantum cellular automata, in this article we give a comparative analysis of the two alternative vibronic approaches to the evaluation of the key functional characteristics of molecular cells. Semiclassical Born-Oppenheimer approximation and quantum mechanical evaluations of the vibronic energy pattern, electronic density distributions and cell-cell response function are performed for two-electron square-planar mixed valence molecular cells subjected to the action of a molecular driver. Special emphasis is put on the description of the cell-cell response function, which describes strong non-linearity as a prerequisite for the effective action of quantum cellular automata. Comparison of results obtained within the semiclassical and quantum-mechanical approaches has revealed a drastic difference between the shapes of the cell-cell response functions evaluated within these two approaches in the case of moderate vibronic coupling when the energy levels of the square cell interacting with a weakly polarized driver undergo large tunneling splitting in shallow adiabatic potential minima. In contrast, in the limits of strong vibronic coupling (a double-well adiabatic potential with deep minima) and weak vibronic coupling (a single well adiabatic potential) the adiabatic approximation is shown to describe the cell-cell response function with rather good accuracy.
- Double-dimeric versus tetrameric cells for quantum cellular automata: A semiempirical approach to evaluation of cell–cell responses combined with quantum-chemical modeling of molecular structuresThe Journal of Physical Chemistry C, 2019
Quantum dot cellular automata is a computing paradigm based on transistor-free logic, which in turn relies on the idea of encoding binary information in bistable charge configurations of quantum dots and process information via Coulomb interactions. In the context of molecular implementation of quantum dot cellular automata, we have compared the properties of two possible kinds of molecular square cells, namely, cells tailored from two one-electron mixed valence dimers (double-dimeric cells) and a two-electron mixed valence tetramer. The physical model (based on the Hubbard-type Hamiltonian) of the cells involves the Coulomb interelectronic interaction, electron transfer, and vibronic coupling. We have demonstrated that the difference in the transfer pathways in the two types of cells gives rise to a considerable difference in their functional characteristics. Thus, the double-dimeric cell exhibits a more abrupt nonlinear cell-cell response, which is a prerequisite for the efficient functioning of quantum cellular automata. The difference in the cell-cell responses for the two kinds of cells is shown to be smaller for a weak electron transfer and/or strong vibronic coupling when the mobility of the electronic pair is strongly constrained. The dimeric and tetrameric systems, 1,4-dithia-hexane and crown ether 1,4,7,10-tetrathiacyclododecane, were selected as the molecular systems for the implementation of the proposed Hubbard-type analysis. This choice is prompted by the positive charge localization on the S-atoms, which are not connected covalently. We have performed the quantum-chemical calculations of the 1,4-dithia-compound with two S-atoms connected by a saturated carbon bridge CH2CH2 (proposed as a dimeric subunit) and the corresponding tetrameric structures of the crown ethers 1,4,7,10-tetrathiacyclododecane: parent neutral molecule, cation, and dication. The quantum-chemical estimations allowed us to quantitatively unveil the key parameters of the dimeric and tetrameric systems and to conclude that the proposed compounds can serve as cells with predominantly antipodal charge separation, which are potentially able to encode binary information.