All-optical spin switching on an ultrafast time scale.

Information technology revolution demands bigger and faster magnetic storage. All-optical spin switching (AOS) may offer a solution, where an ultrafast laser pulse alone can switch magnetization from one direction to another faithfully within 1-10 ps, free of a magnetic field. There are two types of switching: One is the helicity-dependent all-optical spin switching (HD-AOS) and the other the helicity-independent all-optical spin switching (HID-AOS). In a few alloys, one single laser pulse, with sufficient fluence, can switch spin, but the majority of magnetic materials requires multiple pulses. Both material-specific and laser-specific properties strongly affect the switching process. However, the underlying mechanism is still under debate. As the entire research field moves toward applications, it is very appropriate to review what has been achieved in the last decade. This review covers some of the major experimental and theoretical developments within the last decade, and serves as an introduction to the uninitiated reader in this field and a summary for the seasoned researchers.

Investigation of the exact spin channels in laser-induced spin dynamics in two mononuclear Cu(II) complexes.

In the quest to harness the potential of nanospintronic applications, we analyze and investigate the spin channels for the ultrafast spin dynamics in mononuclear Cu2+(tdp)Cl2 (Cutdp) and Cu2+(tdp)Cl2·MeCN (Cutdp·MeCN) using a high-level ab initio many-body theory. In that spirit, we select two slightly different polymerizations arising from one parent complex. We establish the difference in magnetic behavior between the two complexes which arises solely from the geometrical differences. We calculate the static magnetic properties, such as the magnetic anisotropy of the complexes, which is analyzed by means of the magnetic moment of the ground state. The asymmetry of the core Cu-Cl-Cu-Cl axial plane unit is also reflected in the ground state absorption spectra of the two complexes. Comparisons with the experimental data are in good agreement with the exception of one peak in the theoretical calculations for each of the complexes, confirming the reliability of theoretical methods employed. A major finding in this work is the distinction between classical and coherent superpositions of Λ processes. We employ the selective blocking and retention (SBR) technique to find the unique path or paths for spin dynamic scenarios like spin flip and spin transfer. Additionally, we also present two different scenarios in which intermediate states are involved in spin dynamic processes, (i) classical superposition of Λ processes (i.e., there are many unique paths for transition, even with just one intermediate state the transition completes successfully), and (ii) collective coherent superposition of Λ processes (i.e., there is only one path for the transition, which requires more than one intermediate state to be in a specific coherent superposition). As a consequence, we gain insight into the type of correlations (static or dynamic) involved in a particular spin dynamic scenario.

Using single and double laser pulses on the molecular Ni4@C48H36 system to design integrated nanospintronic units.

The accomplishment of long-distance spin transfer scenarios between several magnetic centers is a big challenge for building and supporting spin-logic units for developing future all-optical magnetic unit operations. Using high-level quantum chemistry theory CCSD and EOM-CCSD, we systematically study the ultrafast laser-induced spin-dynamics process on a carbon-based material, to which four magnetic centers are attached. We show that the CCSD method with the 6-31G basis set calculation is sensitive to the C-Ni bond length. The spin density distribution, which is computed using EOM-CCSD with LanL2DZ+ECP calculations, Mulliken population analysis, including spin-orbit-coupling (SOC) and a magnetic field, fulfills the requirements for achieving spin dynamics processes. Different local spin-flip and spin-transfer processes are accomplished within the subpicosecond regime. The impact of the propagation direction of the laser pulse by switching their polar and the azimuthal angles in spherical coordinates on the spin dynamics processes is analyzed. Double laser pulses with time delay δt ≥ 200 × FWHM yield in a realistic magnetic field gradient selectively a lateral resolution, which corresponds to distances smaller than the CMOS scale (2 nm in 2024) while our system size is comparable to the CMOS scale. Here Λ and V processes with two quasi-degenerate intermediate levels are used. We propose a model of an integrated spin-logic processor created from an array of individual spin-logic blocks, which are realized by four magnetic centers Ni. The findings of this study demonstrate the enormous potential of using laser-induced spin dynamics as the fundamental mechanism for future molecular magnetic technology.

Unlocking Ultrafast Spin Transfer in Single-Magnetic-Center-Decorated Triangulene Systems.

Triangulene, as a typical open-shell graphene fragment, has attracted widespread attention for nanospintronics, promising to serve as building blocks in spin-logic units. Here, using ab initio calculations, we systematically study the laser-induced ultrafast spin-dynamic processes on triangulene nanoflakes, decorated with a transition-metal atom. The results reveal a competition between the induced magnetic center and the carbon edge of the triangulene, resulting in the coexistence of dual spin-density-distribution patterns on such single-magnetic-center systems, thus opening up possibilities of complex spin-dynamic scenarios beyond the spin flip. Interestingly, no matter what direction the spin points to, it is possible to achieve reversible spin-transfer processes using the same laser pulse. Increasing the pool of elementary processes to contain not only spin-direction-dependent but also spin-direction-independent scenarios allows for more versatile spin-logic operations, including classical handling of information and quantum computing. In the present work, we suggest downscaling nanospintronic devices by integrating triangulene-based nanostructures.

Optically Driven Both Classical and Quantum Unary, Binary, and Ternary Logic Gates on Co-Decorated Graphene Nanoflakes.

Nanospintronics holds great potential for providing high-speed, low-power, and high-density logic and memory elements in future computational devices. Here, using ab initio many-body theory, we suggest a nanoscale framework for building quantum computation elements, based on individual magnetic atoms deposited on graphene nanoflakes. We show the great possibilities of this proposal by exemplarily presenting four quantum gates, namely, the unary Pauli-X, Pauli-Y, Pauli-Z, and Hadamard gates, as well as the universal classical ternary Toffoli gate, which preserves information entropy and is therefore fully reversible and minimally energy consuming. All our gates operate within the subpicosecond time scale and reach fidelities well above 90%. We demonstrate the ability to control the spin direction and localization, as well as to create superposition states and to control the quantum phase of states, which are indispensable ingredients of quantum computers. Additionally, being optically driven, their predicted operating speed by far beats that of modern quantum computers.

Laser-induced ultrafast spin-transfer processes in non-linear zigzag carbon chain systems.

We combine the high-level quantum chemistry theory CCSD and EOM-CCSD together with local and global Λ processes to investigate the details of the laser-induced ultrafast spin manipulation scenarios in non-linear zigzag carbon chain systems Ni2@C32H32 and Ni2@C36H36. The spin density distribution, which is calculated on each many-body state using a Mulliken population analysis, fulfills the requirements to accomplish the spin dynamics processes. Various spin-flip and spin-transfer scenarios are accomplished. All the spin-dynamics processes can be achieved within subpicosecond times. Under the influence of a magnetic field, we find that the spin-transfer scenarios are preserved, while the local spin-flip scenario on a Ni atom can be significantly inhibited depending on the strength of the magnetic field. The impact of the propagation direction of the laser pulse on the spin dynamics processes by varying their polar and azimuthal angles in spherical coordinates is investigated. Additionally, we find that double laser pulses successfully induce the spin-transfer processes. Our outcomes underline the significant potential of carbon chain systems as building blocks for developing future all-optical integrated logic processing units.

Ultrafast control of laser-induced spin-dynamics scenarios on two-dimensional Ni3@C63H54 magnetic system.

The concept of building logically functional networks employing spintronics or magnetic heterostructures is becoming more and more popular today. Incorporating logical segments into a circuit needs physical bonds between the magnetic molecules or clusters involved. In this framework, we systematically study ultrafast laser-induced spin-manipulation scenarios on a closed system of three carbon chains to which three Ni atoms are attached. After the inclusion of spin-orbit coupling and an external magnetic field, different ultrafast spin dynamics scenarios involving spin-flip and long-distance spin-transfer processes are achieved by various appropriately well-tailored time-resolved laser pulses within subpicosecond timescales. We additionally study the various effects of an external magnetic field on spin-flip and spin-transfer processes. Moreover, we obtain spin-dynamics processes induced by a double laser pulse, rather than a single one. We suggest enhancing the spatial addressability of spin-flip and spin-transfer processes. The findings presented in this article will improve our knowledge of the magnetic properties of carbon-based magnetic molecular structures. They also support the relevant experimental realization of spin dynamics and their potential applications in future molecular spintronics devices.

A theoretical study of laser-induced ultrafast spin dynamics in trigonal monopyramidal iron and nickel complexes.

We present a first-principles study of the geometries, electronic structures, and laser-induced ultrafast spin dynamics in four trigonal monopyramidal complexes [tpat-BuFe]-, [tcmat-BuFe]-, [tpat-BuNi]-, and [tcmat-BuNi]- [tpa: tris-(pyrrolylmethyl)amine; tcma: tris(carbamoyl-methyl)amine; t-Bu: tert-butyl]. It is found that the low-lying level distribution of the four structures is similar, however, their spin and charge localization differs substantially. Detailed analysis demonstrates that the iron complexes have much more singly spin localized states located in the low energy region, while the nickel complexes have more charge-transfer (CT) states and more states with spin equally distributed between the Ni and the ligands. Affected by these features, more ultrafast spin-crossover (SCO) scenarios are achieved in the two iron complexes, and better CT dynamics is obtained in nickel complexes. In particular, for the CT scenarios combined with spin bifurcation, the charge is transferred from the tpa/tcma ligand to the Fe/Ni atoms, while spin-density transfer occurs in the opposite direction. Among the scenarios illustrated in the paper, the SCO processes turn out to be more complicated since they involve many more intermediate states and exhibit relatively low fidelity. In addition, the transferability of each scenario is analyzed from the absorption spectra of the initial and final states. All these results can provide significant insights into the electronic and magnetic natures of the four complexes, guide the experimental realization of the relevant scenarios, and thus promote their applications in molecular spintronics.

Experimental and Theoretical Study of the Ultrafast Dynamics of a Ni2 Dy2 -Compound in DMF After UV/Vis Photoexcitation.

Invited for this month's cover picture are the groups of Wolfgang Hübner (TU Kaiserslautern, Germany), Annie Powell (Karlsruhe Institut of Technology, Germany), and Andreas-Neil Unterreiner (Karlsruhe Institut of Technology, Germany). The cover picture shows the Dy2 Ni2 -molecular magnet being excited with a UV/Vis laser pulse, together with its time-resolved spectrum after the pulse. The comparison of the theoretical and the experimental spectra together with both the observed and the calculated relaxation times reveal, among others, three key points: the intermediate states participating in the laser-induced dynamics, the partial metal-to-oxygen charge-transfer excitations, and the order of magnitude of the coupling of the molecular magnet to the thermal bath of the environment. Read the full text of their Full Paper at 10.1002/open.202100153.

Laser-Controlled Implementation of Controlled-NOT, Hadamard, SWAP, and Pauli Gates as Well as Generation of Bell States in a 3d-4f Molecular Magnet.

Using high-level ab initio many-body theory, we theoretically propose that the Dy and the Ni atoms in the [Dy2Ni2(L)4(NO3)2(DMF)2] real molecular magnet as well as in its core, that is, the [Dy2Ni2O6] system, act as two-level qubit systems. Despite their spatial proximity we can individually control each qubit in this highly correlated real magnetic system through specially designed laser-pulse combinations. This allows us to prepare any desired two-qubit state and to build several classical and quantum logic gates, such as the two-qubit (binary) CNOT gate with three distinct laser pulses. Other quantum logic gates include the single-qubit (unary) quantum X, Y, and Z Pauli gates; the Hadamard gate (which necessitates the coherent quantum superposition of two many-body electronic states); and the SWAP gate (which plays an important role in Shor's algorithm for integer factorization). Finally, by sequentially using the achieved CNOT and Hadamard gates we are able to obtain the maximally entangled Bell states, for example, (12)(|00⟩ + |11⟩).

Strain manipulation of the local spin flip on Ni@B80 endohedral fullerene.

Using first principles, we theoretically investigate the strain manipulation of the ultrafast spin-flip processes on the Ni@B80 endohedral fullerene by using highly correlated quantum chemical calculations. It is shown that the ultrafast local spin flip on Ni@B80 can be achieved via Λ processes with high fidelities in both the equilibrium and distorted structures. Moreover, the applied strain on Ni@B80 can significantly lead to the redistribution of spin density, and therefore dominate the spin-flip processes. It is interesting that the strain effects on the spin-flip processes of Ni@B80 are not identical. Specifically, when a strain is applied along the direction across the Ni atom, the influence is exactly opposite to the case when the strain direction goes without crossing the Ni atom. This orientation-dependent strain effect is also demonstrated by analyzing the modulated energy gaps between the singly occupied molecular orbital (SOMO) and the lowest unoccupied molecular orbital (LUMO) of the system. The present results shed some light on the mechanical control of the magneto-optic dynamics behavior of the endohedral fullerenes, and further provide the idea that strain engineering and spin engineering can be combined for the design of nanoscale magnetic storage units and spintronic devices.

Long-Distance Ultrafast Spin Transfer over a Zigzag Carbon Chain Structure.

Using high-level ab initio quantum theory we suggest an optically induced subpicosecond spin-transfer scenario over 4.428 nm, a distance which is directly comparable to the actual CMOS scale. The spin-density transfer takes place between two Ni atoms and over a 40-atom-long zigzag carbon chain. The suitable combination of the local symmetries of the participating carbon atoms and the global symmetry of the whole molecule gives rise to what we term the dynamical Goodenough-Kanamori rules, allowing the long-range coupling of the two Ni atoms. We also present local spin-flip scenarios, and compare spin flip and spin transfer with respect to their sensitivity against an external static magnetic gradient. Finally, we use two identical laser pulses, rather than a single one, which allows us to accurately control local (intrasite) vs global (intersite) processes, and we thus solve the problem of embedding individually addressable molecular nanologic elements in an integrated nanospintronic circuit. Our results underline the great potential of carbon chain systems as building and supporting blocks for designing future all-optical magnetic processing units.

Reversible ultrafast spin switching on Ni@B80 endohedral fullerene.

We present the configurations and stability of the endohedral metallofullerene Ni@B80 by using strict and elaborate geometric modeling. The ultrafast spin switching on Ni@B80 is explored through ab initio calculations. It is shown that there are three stable configurations of Ni@B80 endohedral fullerene with the encaged Ni atom located at different sites. The ultrafast spin switching on Ni@B80via Λ processes can be achieved through at least eight paths with different laser pulses. Among them, the fastest one can be accomplished within 100 fs. In particular, it is found that all the spin-switching processes achieved on the H-type structure are reversible with the use of the same or different laser pulses. Considering the obtained high fidelities of these switching processes, the present theoretical prediction could lead to promising applications in the design of integrated spin-logic devices through appropriate spin manipulation in endohedral boron fullerenes.

Optical response of small closed-shell sodium clusters.

Absorption spectra of closed-shell Na(2), Na(3) (+), Na(4), Na(5) (+), Na(6), Na(7) (+), and Na(8) clusters are calculated using a complex Bethe-Salpeter equation derived using a conserving linear response method. In the framework of a quasiparticle approach, we determine electron-hole correlations in the presence of an external field. The calculated results are in excellent agreement with experimental spectra, and some possible cluster geometries that occur in experiments are analyzed. The position and the broadening of the resonances in the spectra arise from a consistent treatment of the scattering and dephasing contributions in the linear response calculation. Comparison between the experimental and the theoretical results yields information about the cluster geometry, which is not accessible experimentally.

Ab initio investigation of Pt dimers on Cu(001) surface.

Using ab initio methods, we theoretically investigate the adsorption of one and two Pt dimers on Cu(001). Treating the interaction of adsorbates on surfaces as a local phenomenon, a representation of the substrate by a large cluster of at least 62 Cu atoms allows one to treat the electronic structures of both systems, that is, the adsorbate and the surface, on equal footing. Theoretical results concerning the adsorbate energetics, structure, and density of states are presented. We find that the Pt atoms of the adsorbates prefer to locate on top of the hollow sites of the Cu lattice. A symmetry-adapted-cluster expansion configuration-interaction method is used to calculate the excited states and the optical absorption spectra of the systems. The most intense peaks of the absorption spectra are around 1 eV and result from excited states of B2 symmetry. Last but not least, we identify surface-mediated interdimer interactions.

Angular momentum conservation for coherently manipulated spin polarization in photoexcited NiO: an ab initio calculation.

In an ultrafast laser-induced magnetization-dynamics scenario we demonstrate for the first time an exact microscopic spin-switch mechanism. Combining ab initio electronic many-body theory and quantum optics analysis we show in detail how the coherently induced material polarization for every elementary process leads to angular-momentum exchange between the light and the irradiated antiferromagnetic NiO. Thus we answer the long-standing question where the angular momentum goes. The calculation also predicts a dynamic Kerr effect, which provides a signature for monitoring spin dynamics, by simply measuring the transient rotation and ellipticity of the reflected light.