arXiv subjects

Chao-Cheng Kaun

Publications and source records attributed to Chao-Cheng Kaun.

7 recordsLinked to original sources

arXiv2026

Electrically Controllable Flat Band in Two-Dimensional Electron Gases under Nonuniform Magnetic Fields

Flat bands underlie a diverse range of quantum phenomena, from strongly correlated phases to superconductivity. We theoretically establish that a two-dimensional electron gas under a linear magnetic-field gradient and a transverse electric field exhibits electrically tunable flat bands. When the electric field magnitude is tuned to a value within a discrete sequence, these bands become strictly dispersionless. By providing exact classical and quantum solutions, we demonstrate that these states are high-order Landau levels associated with drift-compensated cyclotron orbits of carriers arising from the synergy between the magnetic-field gradient and the electric field. These electrically controllable Landau levels exhibit quantized Hall conductance and a strongly enhanced density of states. Our results provide a new route for flat-band creation, magnetoelectric band engineering, and quantized Hall currents controlled via source-drain voltage.

cond-mat.mtrl-sci↗

arXiv2026

Anisotropic hot carrier relaxation mediated by electron phonon scattering in TiN thin films

Crystal orientations can shape the ultrafast energy relaxations of transition-metal nitride thin films. Here, we investigate the orientation-dependent electron-phonon (e-ph) mediated relaxation in titanium nitride (TiN) thin films along the [100], [110], and [111] directions by combining first-principles calculations with ultrafast pump-probe transient absorption spectroscopy. Using maximally localized Wannier functions, we evaluate e-ph quasiparticle scattering lifetimes near the Fermi level and identify a clear anisotropy: The TiN [111] orientation exhibits a longer e-ph scattering lifetime (15.96 fs) than [100] (13.69 fs) and [110] (11.12 fs), indicating reduced intrinsic e-ph scattering strength. Furthermore, we grew quasi-epitaxial, orientation-controlled TiN thin films on MgO substrates. Pump-probe measurements reveals that the population-level relaxation (hot-electron cooling) time also depends on orientations, with [111] films showing a significantly slower decay (110 fs) than [100] (90 fs) and [110] (80 fs). We emphasize that the calculated few-femtosecond scattering lifetimes and the measured few-hundred-femtosecond cooling time respectively represent single-event scattering and collective cooling, yet they exhibit consistent trends. These results demonstrate that crystallographic orientation provides a practical and powerful route to tune e-ph-governed relaxation in TiN thin films, offering essential design guidelines for refractory plasmonic and energy-conversion platforms.

cond-mat.mtrl-sci↗

arXiv2024

Driving noncollinear interlayer exchange coupling intrinsically in magnetic trilayers

Ferromagnetic side layers sandwiching a nonmagnetic spacer as a metallic trilayer has become a pivotal platform for achieving spintronic devices. Recent experiments demonstrate that manipulating the width or the nature of conducting spacer induces noncollinear magnetic alignment between the side layers. Our theoretical analysis reveals that altering the width of spacer significantly affects the interlayer exchange coupling (IEC), resulting in noncollinear alignment. Through analytic and first-principles methods, our study on the Fe/Ag/Fe trilayer shows that at a specific width of the Ag spacer, the magnetic moments of side layers tend to be perpendicular. This alignment is mediated by Ag quantum well states, exhibiting spin spirals across the trilayer. Our results reveal that the noncollinear IEC offers a degree of freedom to control magnetic devices and boot spintronic technology with improved transport capabilities.

cond-mat.mtrl-sci↗

arXiv2023

Determining Perpendicular Magnetic Anisotropy in Fe/MgO/Fe Magnetic Tunnel Junction: A DFT-Based Spin-Orbit Torque Method

In our JunPy package, we have combined the first-principles calculated self-consistent Hamiltonian with divide-and-conquer technique to successfully determine the magnetic anisotropy (MA) in an Fe/MgO/Fe magnetic tunnel junction (MTJ). We propose a comprehensive analytical derivation to clarify the crucial roles of spin-orbit coupling that mediates the exchange and spin-orbit components of spin torque, and the kinetic and spin-orbit components of spin current accumulation. The angular dependence of cumulative spin-orbit torque (SOT) indicates a uniaxial MA corresponding to the out-of-plane rotations of magnetic moments of the free Fe layers. Different from the conventional MA energy calculation and the phenomenological theory for a whole MTJ, our results provide insight into the orbital-resolved SOT for atomistic spin dynamics simulation in emergency complex magnetic heterojunctions.

cond-mat.mtrl-sci↗

arXiv2019

Unconventional order parameter induced by helical chiral molecules adsorbed on a metal proximity-coupled to a superconductor

Following our previous results, which provide evidence for the emergence of a chiral p-wave triplet-pairing component in superconducting Nb upon the adsorption of chiral molecules, we turned to investigate whether such an effect can take place in a proximal superconductor consisting of metal on superconductor bilayer. Note that in such proximity systems, correlated electron-hole (Andreev) pairs exist in the normal metal rather than genuine Cooper pairs. To that end, we used scanning tunneling spectroscopy (STS) on thin Au films grown in-situ on NbN (a conventional s-wave superconductor) before and after adsorbing helical chiral, alpha-helix polyalanine molecules. The tunneling spectra measured on the pristine Au surface showed conventional (s-wave like) proximity gaps. However, upon molecules adsorption the spectra significantly changed, all exhibiting a zero-bias conductance peak embedded inside a gap, indicating unconventional superconductivity. The peak reduced with magnetic field but did not split, consistent with equal-spin triplet-pairing p-wave symmetry. In contrast, adsorption of non-helical chiral cysteine molecules did not yield any apparent change in the order parameter, and the tunneling spectra exhibited only gaps free of in-gap structure.

cond-mat.supr-con↗

arXiv2018

Layer- and doping-tunable long range ferromagnetic order-ing in two-dimensional CrS2

Interlayer coupling is of vital importance for manipulating physical properties, e.g. electronic bandgap, in two-dimensional materials. However, tuning magnetic proper-ties in these materials is yet to be addressed. Here, we found a striped antiferromag-netic (sAFM) to ferromagnetic (FM) transition undergoing from monolayer to bilayer and thicker CrS2. This transition is attributed to charge sharing of interlayer S atoms and its resulting charge transfer from Cr d eg to t2g orbitals. The transferred charge reduces a portion of Cr4+ to Cr3+, which enhances the double-exchange mechanism favoring FM rather than sAFM ordering. Doping of electrons shares the same effect with stacking induced charge sharing. Therefore, charge doping could either manip-ulate the magnetic ordering between sAFM and FM or tune CrS2 between p- and n-doped magnetic semiconductors. We also proposed several prototype devices ena-bling to use external electric field for manipulating magnetic orderings of CrS2 layers. These results manifest the role of interlayer coupling in modifying magnetic proper-ties of layered materials

cond-mat.mtrl-sci↗

arXiv2015

Engineering the interlayer exchange coupling in magnetic trilayers

When the thickness of metal film approaches the nanoscale, itinerant carriers resonate between its boundaries and form quantum well states (QWSs), which are crucial to account for the film electrical, transport and magnetic properties. Besides the classic origin of particle-in-a-box, the QWSs are also susceptible to the crystal structures that affect the quantum resonance. Here we investigate the QWSs and the magnetic interlayer exchange coupling (IEC) in the Fe/Ag/Fe (001) trilayer from first-principles calculations. We find that the carriers at the Brillouin-zone center (belly) and edge (neck) separately form electron- and hole-like QWSs that give rise to an oscillatory feature for the IEC as a function of the Ag-layer thickness with long and short periods. Since the QWS formation sensitively depends on boundary conditions, one can switch between these two IEC periods by changing the Fe-layer thickness. These features, which also occur in the magnetic trilayers with other noble-metal spacers, open a new degree of freedom to engineer the IEC in magnetoresistance devices.

cond-mat.mtrl-sci↗