Atomically crafted quantum magnets and their anomalous excitations

Atomically crafted quantum magnets and their anomalous excitations
Experimental and theoretical inelastic scanning tunneling spectroscopy (STS) fingerprint of a Cr adatom on the Nb(110) surface with varying external magnetic fields. (A) Experimental topography image of the adatom. (B and C) Experimental dI/dU and d2I/dU2 spectra, I = 3 nA, and T = 600 mK. (D) Density of spin-flip excitations obtained from TD-DFT. (E) Theoretical inelastic STS spectrum corresponding to the renormalized local density of states from MBPT calculated at a height of 4.7 Å above the adatom. (F) Similar to (E) with the spin-resolved theoretical spectra. The different field-dependent spectra are shifted to improve visibility. Credit: Science Advances, DOI: 10.1126/sciadv.abi7291

Quantum magnets can be studied using high-resolution spectroscopic studies to access magnetodynamic quantities including energy barriers, magnetic interactions, and lifetime of excited states. In a new report now published in Science Advances, Sascha Brinker and a team of scientists in advanced simulation and microstructure physics in Germany studied a previously unexplored flavor of low-energy spin excitation for quantum spins coupled to an electron bath. The team combined time-dependent and many-body perturbation theories and magnetic field-dependent tunneling spectra to identify magnetic states of the nanostructures and rationalized the results relative to ferromagnetic and antiferromagnetic interactions. The atomically crafted nanomagnets are appealing to explore electrically pumped spin systems.

Anomalous magnetodynamics

Magnetodynamics at the atomic scale form the cornerstone of spin-based nanoscale devices with applications in future information technologies. Interactions of local spin states also play a crucial role with the local environment to determine their properties. Researchers have described the impact of orbital hybridization effects, charge transfer, and the presence of nearby impurities as strong influencers on the magnetic ground state, to determine a range of magnetodynamic qualities, including magnetic anisotropy, spin lifetime and spin-relaxation mechanisms. Experimental methods can be developed to directly capture these properties and analyze the magnetic phenomena of classical and semiclassical descriptions at sub-nanometer scales to reveal the emergence of exquisite quantum mechanical effects. These achievements can facilitate the understanding of classical computational schemes to set the pace to test ideas and concepts with direct impact on innovative spin-based quantum computational schemes. In this work, Brinker et al. discussed the emergence of anomalous spectroscopic signals centered around the Fermi energy of chromium coupled to metallic niobium. The team then also described the Kondo effect, arising from the quantum mechanical interplay between electrons of a host metal and the magnetic impurity, to yield local charge and spin variations around the magnetic impurity; a phenomenon of significance in many-body physics, yet of limited technical importance.

Atomically crafted quantum magnets and their anomalous excitations
Theoretical and experimental inelastic STS fingerprint of the Cr dimers along the [001], [11¯¯¯0] , and [11¯¯¯1] directions on the Nb(110) surface. (A) Illustration of the dimers (STM topography) and its ground-state magnetic structures. The magnetic structures are obtained from self-consistent noncollinear DFT calculations. (B) Experimental inelastic STS spectra of the dimers, I = 3 nA and T = 600 mK (colored lines). The measurements on the single Cr adatom are shown as reference (thin gray lines; see also Fig. 1). The different spectra are vertically shifted to improve visibility. (C) Magnetic susceptibility obtained from TD-DFT. The spectra of the [001] and [11¯¯¯1] dimer are scaled by a factor of 50 and 2, respectively. Credit: Science Advances, DOI: 10.1126/sciadv.abi7291

Adatoms

The team described the magnetic field of the sample atoms alongside their evolution when adatoms were coupled by creating atomically crafted ferromagnetic and antiferromagnetic nanostructures. Adatoms or adsorbed atoms are atoms that lay on a crystal surface and act as the opposite of a surface vacancy. Using first-principles simulations, Brinker et al. next identified paradigmatic spectroscopic manifestations of spin excitations. While researchers had previously explored thin insulating substrates, the team here explored metallic substrates to design antiferromagnetic or ferromagnetic magnets made of a few exchange-coupled chromium atoms. As a prototypical platform, they focused on chromium (Cr) atoms coupled to the surface of niobium (Nb) and prepared the Nb crystal according to previous studies and deposited the single Cr adatoms directly on a cold substrate. They then regulated the atoms to create a nanostructure with an isolated Cr atom, well separated from the surrounding . This setup allowed them to analyze the effect of local magnetic moments coupled to an electron bath, while excluding the influence of nearby adatoms. The team obtained by positioning the tip directly above the Cr adatom and using the experimental process, they measured the evolution of spectroscopic data and the magnetic origin of excitations, while exploring the origin of the anomalous spectral features. In contrast to adatoms deposited on an insulator, the magnetic moments on a metal experienced additional effects including magnetic anisotropy, Zeeman energy and magnetic-exchange interactions. The model accounted for all experimental trends relative to energy shift and broadening and allowed the scientists to reproduce the anomalies using ab initio simulations. The outcomes highlighted the experimental limits by explaining the difficulty of identifying similar spin excitation spectra, for instance, with cobalt on copper and silver surfaces, when compared to chromium on niobium, as seen in this work.

Atomically crafted quantum magnets and their anomalous excitations
Spin-polarized STS acquired on a seven-adatom chain along [001]. (A) Spectroscopic data collected by positioning the tip on top of each adatom along the chain (see numbers). The data reveal the existence of antiferromagnetic coupling. (B) Comparison between the STS spectra acquired on neighboring adatom [3 and 4 in (A)], highlighting the opposite spin contrast. Measurements have been acquired at 5 T with a Cr-terminated tip. Stabilization parameters: V = −10 meV, I = 3 nA, and modulation Vrms = 200 μV. Credit: Science Advances, DOI: 10.1126/sciadv.abi7291

Dimers and trimers to long chains

Brinker et al. further investigated the consistency of the experimental data by exploring the spin-excitation paradigm by using atomic manipulation methods to artificially create magnetic dimers oriented along different crystallographic directions on the Niobium surface. To confirm the outcome of the distinct spin excitation spectra that were identified for the dimers triggered by their spin moments, the scientists artificially created a trimer along all distinct directions. They then noted that unlike the dimers, the trimers could respond to an external magnetic field, while preserving their collinear antiferromagnetic configuration. Thereafter, the team experimentally scrutinized the spin-resolved spectroscopic signatures for long chains.

Atomically crafted quantum magnets and their anomalous excitations
Chromium (Cr) trimers along the [001], [11¯¯¯0] , and [11¯¯¯1] directions on the niobium Nb(110) surface. (A) Illustration of the trimers (STM topography). (B) Experimental inelastic STS spectra of the trimers at 1 T (colored lines). The measurements on the single Cr adatom are shown as reference (thin gray lines, see also Fig. 1). (C) Magnetic susceptibility obtained from TD-DFT at 5 T. The spectra of atom B in the [11¯¯¯0] direction and in the [001] direction are scaled by a factor 10 and 30, respectively. All trimer spectra are vertically shifted to improve visibility. The insets show the magnetic structures obtained from self-consistent noncollinear DFT calculations. Credit: Science Advances, DOI: 10.1126/sciadv.abi7291

Outlook

In this way, Sascha Brinker and colleagues systematically revealed the anomalies in chromium (Cr) atom nanostructures deposited on the metallic niobium (Nb) surface. Using inelastic scanning tunneling spectroscopy (STS) data, they revealed an asymmetric, single-step-like zero-bias feature, which was preserved yet could be manipulated depending on several factors including the strength of an external magnetic field, the size of the nanostructure and its spatial orientation on the substrate. The spatial orientation was useful to engineer inter-atom magnetic coupling to be ferromagnetic or antiferromagnetic. The team tracked these complex results using ab-initio simulations to combine time-dependent density functional and many-body perturbation theories, as seen by constructing the nano-object with the desired magnetic state. The work provides new light to interpret zero-bias anomalies and form a new path to engineer low-energy features by regulating the spin excitation spectra based on an underlying magnetic structure.

More information: Sascha Brinker et al, Anomalous excitations of atomically crafted quantum magnets, Science Advances (2022). DOI: 10.1126/sciadv.abi7291

A. Spinelli et al, Imaging of spin waves in atomically designed nanomagnets, Nature Materials (2014). DOI: 10.1038/nmat4018

Journal information: Science Advances , Nature Materials

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