arXiv subjects

Chiung-Yuan Lin

Publications and source records attributed to Chiung-Yuan Lin.

7 recordsLinked to original sources

arXiv2018

Activated Layered Magnetism from Bulk TiN

The novel properties of a uniaxially-expanded TiN bulk arising from increasing the layer spacing from equilibrium are explored using a first-principles approach. We reveal a novel nonmagnetic-magnetic transition from a TiN bulk to its monolayer. We also investigate the electronic and magnetic structures of a few TiN atomic layers. We find that the bilayer and trilayer, like the TiN bulk, are nonmagnetic poor metals. On the other hand, the monolayer TiN is found to carry a magnetic moment on its Ti atoms, and likely be a semiconductor. The unpaired electron giving rise to magnetism on Ti is primarily in the orbital perpendicular to the layers, and we find it is freed to give rise to magnetism when the layers are slightly separated. We find two different antiferromagnetic states possible on the monolayer, as well as one ferromagnetic, with one of the antiferromagnetic being the lowest energy. The exchange couplings between Ti atoms in such a monolayer are calculated to be antiferromagnetic for both the nearest-neighbor and next-nearest-neighbor sites. We also analyze the binding nature of both the monolayer and bilayer TiN by searching for the predominant binding orbitals.

cond-mat.mes-hall↗

arXiv2012

Magnetic Interaction between Surface Engineered Rare-earth Atomic Spins

We report the ab initio study of rare-earth adatoms (Gd) on an insulating surface. This surface is of interest because of previous studies by scanning tunneling microscopy showing spin excitations of transition metal adatoms. The present work is the first study of rare-earth spin-coupled adatoms, as well as the geometry effect of spin coupling, and the underlying mechanism of ferromagnetic coupling. The exchange coupling between Gd atoms on the surface is calculated to be antiferromagnetic in a linear geometry and ferromagnetic in a diagonal geometry, by considering their collinear spins and using the PBE+U exchange correlation. We also find the Gd dimers in these two geometries are similar to the nearest-neighbor (NN) and the next-NN Gd atoms in GdN bulk. We analyze how much direct exchange, superexchange, and RKKY interactions contribute to the exchange coupling for both geometries by additional first-principles calculations of related model systems.

cond-mat.mes-hall↗

arXiv2012

Single Magnetic Atom on a Surface: Anisotropy Energy and Spin Density

Studying single-atom magnetic anisotropy on surfaces enables the exploration of the smallest magnetic storage bit that can be built. In this work, magnetic anisotropy of a single rare-earth atom on a surface is studied computationally for the first time. The single adatom and its substrate surface are chosen to be a Dysprosium (Dy) atom and a copper-nitrite surface, respectively, where single transition-metal magnetic atoms on the same surface were previously studied one atom at a time by scanning tunneling microscopes. We propose unconventional f and d subshell symmetries so that following the first-principles calculations, simple pictorial analyses of the spin-density distribution can be performed for the first time, independently for both a rare-earth atom Dy and a transition-metal Fe. The magnetic anisotropy energy of Dy on the surface is calculated to be a factor of five larger than the previous highest one, reaching a record-high value of 31 meV.

cond-mat.mes-hall↗

arXiv2010

First-principles Calculations of Engineered Surface Spin Structures

The engineered spin structures recently built and measured in scanning tunneling microscope experiments are calculated using density functional theory. By determining the precise local structure around the surface impurities, we find the Mn atoms can form molecular structures with the binding surface, behaving like surface molecular magnets. The spin structures are confirmed to be antiferromagnetic, and the exchange couplings are calculated within 8% of the experimental values simply by collinear-spin GGA+U calculations. We can also explain why the exchange couplings significantly change with different impurity binding sites from the determined local structure. The bond polarity is studied by calculating the atomic charges with and without the Mn adatoms.

cond-mat.mes-hall↗

arXiv2006

First-principles Calculation of the Single Impurity Surface Kondo Resonance

We have performed first-principles calculation of the surface and bulk wavefunctions of the Cu(111) surface and their hybridization energies to a Co adatom, including the potential scattering from the Co. By analyzing the calculated hybridization energies, we found the bulk states dominate the contribution to the Kondo temperature, in agreement with recent experiments. Furthermore, we also calculate the tunneling conductance of a scanning tunneling microscope (STM) and compare our results with recent experiments of Co impurities in the Cu(111) surface. Good quantitative agreement is found at short parallel impurity-tip distances (< 6 A). Our results indicate the need for a new formulation of the problem at larger distances.

cond-mat.str-el↗

arXiv2004

Microscopic Theory of the Single Impurity Surface Kondo Resonance

We develop a microscopic theory of the single impurity Kondo effect on a metallic surface. We calculate the hybridization energies for the Anderson Hamiltonian of a magnetic impurity interacting with surface and bulk states and show that, contrary to the Kondo effect of an impurity in the bulk, the hybridization matrix elements are strongly dependent on the momentum around the Fermi surface. Furthermore, by calculating the tunneling conductance of a scanning tunneling microscope (STM), we show that when the magnetic impurity is located at a surface the Kondo effect can occur with equal strength between bulk and surface states. We compare our results with recent experiments of Co impurities in Cu(111) and Cu(100) surfaces and find good quantitative agreement.

cond-mat.mtrl-sci↗

arXiv2000

Improved Precision Measurement of the Casimir Force

We report an improved precision measurement of the Casimir force. The force is measured between a large Al coated sphere and flat plate using an Atomic Force Microscope. The primary experimental improvements include the use of smoother metal coatings, reduced noise, lower systematic errors and independent measurement of surface separations. Also the complete dielectric spectrum of the metal is used in the theory. The average statistical precision remains at the same 1% of the forces measured at the closest separation.

quant-ph↗