arXiv subjects

Deung-Jang Choi

Publications and source records attributed to Deung-Jang Choi.

At least 19 recordsLinked to original sources

arXiv2025

Microscopic Scattering Approach to In-Gap States: Cr Adatoms on Superconducting β-Bi2Pd

We develop a microscopic scattering formalism to describe Yu-Shiba-Rusinov (YSR) states due to a single Cr adatom on the Bi-terminated surface of $β-Bi_2Pd$, by combining ab initio Wannier functions with a real-space Green's function approach in the Bogoliubov-de Gennes formalism. Our framework reproduces key scanning tunneling spectroscopy features, including a single particle-hole asymmetric YSR peak and isotropic dIdV maps around the impurity. Decomposing the YSR states reveals contributions from four nearly degenerate C4v representations, with energy broadening masking their individual signatures. Spin-orbit coupling induces partial spin polarization, while the spatial asymmetry between particle and hole components arises from Cr d-Bi p hybridization. These results highlight the importance of realistic band structures and microscopic modeling for interpreting STM data and provide a foundation for studying impurity chains hosting topological excitations.

cond-mat.supr-con↗

arXiv2025

Electron spin resonance with scanning tunneling microscopy: a tool for an on-surface quantum platform of identical qubits

Integration of electron spin resonance (ESR) in a scanning tunneling microscope (STM) has enabled an all-electrical control of atomic and molecular spins on solid surfaces with atomic-scale precision and energy resolution beyond thermal limitations. Further, coherent manipulation and detection of individual spins in an ESR-STM establishes a powerful quantum platform, allowing for the implementation of fundamental quantum logic operations to on-surface identical qubits. In this review, we introduce recent advances of ESR-STM, focusing on its application to atomic-scale qubits and extension to molecular qubit systems. We discuss the principles underlying ESR-STM, followed by single-spin addressability, coherent control via Rabi oscillations, and quantum state readout through frequency-resolved detection. We further demonstrate multi-qubit control architectures enabled by atom manipulation and local magnetic field engineering, culminating in the realization of multi-qubit logic gates such as the Controlled-NOT and Toffoli gates. These implementations highlight the specialty of ESR-STM towards atomic-scale quantum circuits. Indeed, ESR-STM can be an excellent tool to perform and evaluate quantum operations in molecular qubits. The results reviewed in this collection establish ESR-STM as a versatile tool for advancing quantum coherent science at the atomic and molecular level in solid-state environments.

cond-mat.mes-hall↗

arXiv2025

Unraveling spin entanglement using quantum gates with scanning tunneling microscopy-driven electron spin resonance

Quantum entanglement is a fundamental resource for quantum information processing, and its controlled generation and detection remain key challenges in scalable quantum architectures. Here, we numerically demonstrate the deterministic generation of entangled spin states in a solid-state platform by implementing quantum gates via electron spin resonance combined with scanning tunneling microscopy (ESR-STM). Using two titanium atoms on a MgO/Ag(100) substrate as a model, we construct a two-qubit system whose dynamics are coherently manipulated through tailored microwave pulse sequences. We generate Bell states by implementing a Hadamard gate followed by a controlled-NOT gate, and evaluate its fidelity and concurrence using the quantum-master equation-based code TimeESR. Our results demonstrate that ESR-STM can create entangled states with significant fidelity. This study paves the way for the realization of atom-based quantum circuits and highlights ESR-STM as a powerful tool for probing and engineering entangled states on surfaces.

quant-ph↗

arXiv2024

One dimensional chains of nickelocene fragments on Au(111)

We investigate the temperature-dependent deposition of nickelocene (NiCp$_2$) molecules on a single crystal Au(111) substrate, revealing distinct adsorption behaviors and structural formations. At low temperatures (4.2 K), individual NiCp$_2$ molecules adsorb on the herringbone elbows and step edges, forming ordered patterns as molecular coverage increases. However, at 77 K, the molecules dissociate, yielding two main fragments: NiCp fragments that are Ni atoms capped by cyclopentadienyl (Cp) rings, which preferentially adsorb at FCC hollow sites, and Cp radical fragments exhibiting strong substrate interactions. NiCp fragments self-assemble into one-dimensional (1-D) chains along the $\langle 1 1 \bar{2} \rangle$ directions, displaying higher protrusion in STM images. The strain and steric hindrance from the Cp protons induce chiral patterns within the chains, which are well-reproduced by our DFT simulations. In contrast, the Cp fragments maintain distances due to short-range repulsive forces and exhibit low diffusion barriers. Interestingly, the fragments are non-magnetic, as confirmed by both STM measurements and DFT calculations, in contrast to the magnetic signals from intact Nc molecules. In addition to linear chains, dimers of the Ni-Cp fragments form along the $\langle 1 \bar{1} 0\rangle$ directions, requiring gold adatoms for their creation. These results demonstrate the feasibility of constructing complex nanostructures based on metallocenes via on-surface synthesis, opening the possibility for realizing low-dimensional magnetic systems by selecting substrates that preserve the magnetic moment of the fragments.

cond-mat.mes-hall↗

arXiv2024

Multi-Orbital Interactions and Spin Polarization in Single Rare-Earth Adatoms

Surface-adsorbed rare-earth nanostructures are ideal platforms to investigate the interplay between intra-atomic interactions and multi-orbital spin configurations. However, addressing these properties has posed severe experimental and theoretical challenges. Here, we use the orbital selectivity offered by X-ray absorption spectroscopy to quantify the Coulomb integrals of Nd atoms on conductive surfaces, as well as the variation of individual orbital occupation upon cluster nucleation. Using X-ray magnetic circular dichroism we identify magnetic moments of the order of \MK{few tens of}~$μ_{\rm{B}}$ at the $5d$ orbitals and their magnetic coupling with the $4f$ spins. Our results validate orbital-resolved X-ray spectroscopy as a reliable method for quantifying complex multi-orbital interactions in surface-adsorbed lanthanides.

cond-mat.mes-hall↗

arXiv2024

In-gap states induced by magnetic impurities on wide-band s-wave superconductors: self-consistent calculations

The role of self-consistency in Bogoliubov-de Gennes equations is frequently underestimated in the investigation of in-gap states created by magnetic impurities in s-wave superconductors. Our research focuses on the impact of self-consistency on the in-gap states produced by magnetic stuctures on superconductors, specifically evaluating the density of states, the in-gap bands, and their topological attributes. Here, we show results ranging from single impurity to finite chains, and infinite ferromagnetic spin chains in wide-band s-wave superconductors. These results show that the order parameter contains important information regarding quantum phase transitions and their topological nature, underscoring the importance of self-consistency in such studies.

cond-mat.supr-con↗

arXiv2024

Two molecular devices for superconducting spintronics

We create two molecular devices with superconducting junctions, using nickelocene molecules, single Fe atoms, and Pb electrodes at low temperature. We find contrasting behavior based on the coordination of the Fe atom: one device shows low-bias features in its differential conductance due to the superposition of multiple Andreev reflections (MAR) and Fe-induced in-gap states. The other reveals interference between MAR and in-gap states, showcasing the diversity achievable in atomically engineered devices with identical components.

cond-mat.supr-con↗

arXiv2022

Moiré dispersion of edge states in spin chains on superconductors

Our calculations of ferromagnetic spin chains on s-wave superconductors show that the energy oscillations of edge states with the chain's length are due to a moiré pattern emerging from Friedel-like oscillations and the discreteness of the spin-chain lattice. By modifying the spin lattice, the moiré dispersion of edge states can be controlled. In particular, we can engineer non-dispersive edge states that remain at fixed energy regardless of the size distribution of the spin chains. This is an important step in the study of edge states of spin chains that can be fabricated with a certain size dispersion.

cond-mat.supr-con↗

arXiv2021

Atomic Manipulation of In-gap States on the $β$-Bi$_2$Pd Superconductor

Electronic states in the gap of a superconductor inherit intriguing many-body properties from the superconductor. Here, we create these in-gap states by manipulating Cr atomic chains on the $β$-Bi$_2$Pd superconductor. We find that the topological properties of the in-gap states can greatly vary depending on the crafted spin chain. These systems make an ideal platform for non-trivial topological phases because of the large atom-superconductor interactions and the existence of a large Rashba coupling at the Bi-terminated surface. We study two spin chains, one with atoms two-lattice-parameter apart and one with square-root-of-two lattice parameters. Of these, only the second one is in a topologically non-trivial phase, in correspondence with the spin interactions for this geometry.

cond-mat.supr-con↗

arXiv2020

Molecular templates of spin textures on superconducting surfaces

We create ordered islands of magnetically anisotropic nickelocene molecules on a Pb (111) substrate. By using inelastic electron tunneling spectra (IETS) and density functional theory, we characterize the magnetic response of these islands. This allows us to conclude that the islands present local and collective magnetic excitations. Furthermore, we show that nickelocene islands present complex non-collinear spin patterns on the superconducting Pb (111) surface, opening the possibility of using molecular arrays to engineer spin textures with important implications on topological superconductivity.

cond-mat.mes-hall↗

arXiv2020

Visualization of multifractal superconductivity in a two-dimensional transition metal dichalcogenide in the weak-disorder regime

Eigenstate multifractality is a distinctive feature of non-interacting disordered metals close to a metal-insulator transition, whose properties are expected to extend to superconductivity. While multifractality in three dimensions (3D) only develops near the critical point for specific strong-disorder strengths, multifractality in 2D systems is expected to be observable even for weak disorder. Here we provide evidence for multifractal features in the superconducting state of an intrinsic weakly disordered single-layer NbSe$_2$ by means of low-temperature scanning tunneling microscopy/spectroscopy. The superconducting gap, characterized by its width, depth and coherence peaks' amplitude, shows a characteristic spatial modulation coincident with the periodicity of the quasiparticle interference pattern. Spatial inhomogeneity of the superconducting gap width, proportional to the local order parameter in the weak-disorder regime, follows a log-normal statistical distribution as well as a power-law decay of the two-point correlation function, in agreement with our theoretical model. Furthermore, the experimental singularity spectrum f($α$) shows anomalous scaling behavior typical from 2D weakly disordered systems.

cond-mat.supr-con↗

arXiv2019

Colloquium: Atomic spin chains on surfaces

In the present Colloquium, we focus on the properties of 1-D magnetic systems on solid surfaces. From the emulation of 1-D quantum phases to the potential realization of Majorana edge states, spin chains are unique systems to study. The advent of scanning tunnelling microscope (STM) based techniques has permitted us to engineer spin chains in an atom-by-atom fashion via atom manipulation and to access their spin states on the ultimate atomic scale. Here, we present the current state of research on spin correlations and dynamics of atomic spin chains as studied by the STM. After a brief review of the main properties of spin chains on solid surfaces, we classify spin chains according to the coupling of their magnetic moments with the holding substrate. This classification scheme takes into account that the nature and lifetimes of the spin-chain excitation intrinsically depend on the holding substrate. We first show the interest of using insulating layers on metals, which generally results in an increase in the spin state's lifetimes such that their quantized nature gets evident and they are individually accessible. Next, we show that the use of semiconductor substrates promises additional control through the tunable electron density via doping. When the coupling to the substrate is increased for spin chains on metals, the substrate conduction electron mediated interactions can lead to emergent exotic phases of the coupled spin chain-substrate conduction electron system. A particularly interesting example is furnished by superconductors. Magnetic impurities induce states in the superconducting gap. Due to the extended nature of the spin chain, the in-gap states develop into bands that can lead to the emergence of 1-D topological superconductivity and, consequently to the appearance of Majorana edge states.

cond-mat.mes-hall↗

arXiv2019

Spin-current induced Kondo-resonance splitting of a single cobalt atom

We use a low-temperature scanning tunneling microscope to study the interplay between the Kondo effect of a single-atom contact and a spin current. To this end, a nickel tip is coated by a thick layer of copper and brought into contact with a single Co atom adsorbed on a Cu(100) surface. We show that upon contact the Kondo resonance of Co is spin split and attribute the splitting to the spin current produced by the nickel tip and flowing across the copper spacer. A quantitative line shape analysis indicates that the spin polarization of the junction amounts up to 18%, but decreases when a pristine nickel tip is directly contacted to the Co atom.

cond-mat.str-el↗

arXiv2018

Orbital-selective spin excitation of a magnetic porphyrin

Scattering of electrons by localized spins is the ultimate process enabling electrical detection and control of the magnetic state of a spin-doped material. At the molecular scale, this scattering is mediated by the electronic orbitals hosting the spin. Here we report the selective excitation of a molecular spin by electrons tunneling through different molecular orbitals. Spatially-resolved tunneling spectra on iron porphyrins on Au(111) reveal that the inelastic spin excitation extends beyond the iron site. The inelastic features also change shape and symmetry along the molecule. Combining DFT simulations with a phenomenological scattering model, we show that the extension and lineshape variations of the inelastic signal are due to excitation pathways assisted by different frontier orbitals, each of them with a different degree of hybridization with the surface. By selecting the intramolecular site for electron injection, the relative weight of iron and pyrrole orbitals in the tunneling process is modified. In this way, the spin excitation mechanism, reflected by its spectral lineshape, changes depending on the degree of localization and energy alignment of the chosen molecular orbital.

cond-mat.mes-hall↗

arXiv2017

Probing magnetic interactions between Cr adatoms on the $β$-Bi$_2$Pd superconductor

We show that the magnetic ordering of coupled atomic dimers on a superconductor is revealed by their intra-gap spectral features. Chromium atoms on the superconductor $β$-Bi$_2$Pd surface display Yu-Shiba-Rusinov bound states, detected as pairs of intra-gap excitations in the tunneling spectra. We formed Cr dimers by atomic manipulation and found that their intra-gap features appear either shifted or split with respect to single atoms. The spectral variations reveal that the magnetic coupling of the dimer changes between ferromagnetic and antiferromagnetic depending on its disposition on the surface, in good agreement with density functional theory simulations. These results prove that superconducting intra-gap state spectroscopy is an accurate tool to detect the magnetic ordering of atomic scale structures.

cond-mat.mes-hall↗

arXiv2016

From tunneling to contact in a magnetic atom: the non-equilibrium Kondo effect

A low-temperature scanning tunneling microscope was employed to study the differential conductance in an atomic junction formed by an adsorbed Co atom on a Cu(100) surface and a copper-covered tip. A zero-bias anomaly (ZBA) reveals spin scattering off the Co atom, which is assigned to a Kondo effect. The ZBA exhibits a characteristic asymmetric lineshape when electrons tunnel between tip and sample, while upon the tip-Co contact it symmetrizes and broadens. Through density functional theory calculations and the non-equilibrium non-crossing approximation we show that the lineshape broadening is mainly a consequence of the additional coupling to the tip, while non-equilibrium effects only modify the large-bias tails of the ZBA.

cond-mat.mes-hall↗

arXiv2016

Mapping the orbital structure of impurity bound states in a superconductor

A magnetic atomic impurity inside a superconductor locally distorts superconductivity. They scatter Cooper pairs as a potential with broken time-reversal symmetry, what leads to localized bound states with subgap excitation energies, named hereon Shiba states. Most conventional approaches to study Shiba states treat magnetic impurities as point scatterers with an isotropic exchange interaction, while the complex internal structure of magnetic impurities is usually neglected. Here, we show that the number and the shape of Shiba states are correlated to the spin-polarized atomic orbitals of the impurity, hybridized with the superconducting host, as supported by Density Functional Theory simulations. Using high-resolution scanning tunneling spectroscopy, we spatially map the five Shiba excitations found on sub-surface chromium atoms in Pb(111), resolving both their particle and hole components. While the maps of particle components resemble the \textit{d} orbitals of embedded Cr atoms, the hole components differ strongly from them. The orbital fingerprints of Shiba states thus unveil the magnetic ground state of the impurity, and identify scattering channels and interactions, all valuable tools for designing atomic-scale superconducting devices.

cond-mat.supr-con↗

arXiv2016

Structural and magnetic properties of FeMn$_x$ ($x=$1...6) chains supported on Cu$_2$N / Cu (100)

Heterogeneous atomic magnetic chains are built by atom manipulation on a Cu$_2$N/Cu (100) substrate. Their magnetic properties are studied and rationalized by a combined scanning tunneling microscopy (STM) and density functional theory (DFT) work completed by model Hamiltonian studies. The chains are built using Fe and Mn atoms ontop of the Cu atoms along the N rows of the Cu$_2$N surface. Here, we present results for FeMn$_x$ ($x$=1...6) chains emphasizing the evolution of the geometrical, electronic, and magnetic properties with chain size. By fitting our results to a Heisenberg Hamiltonian we have studied the exchange-coupling matrix elements $J$ for different chains. For the shorter chains, $x \leq 2$, we have included spin-orbit effects in the DFT calculations, extracting the magnetic anisotropy energy. Our results are also fitted to a simple anisotropic spin Hamiltonian and we have extracted values for the longitudinal-anisotropy $D$ and transversal-anisotropy $E$ constants. These parameters together with the values for $J$ allow us to compute the magnetic excitation energies of the system and to compare them with the experimental data.

cond-mat.mes-hall↗