arXiv subjects

Pin-Jui Hsu

Publications and source records attributed to Pin-Jui Hsu.

15 recordsLinked to original sources

arXiv2025

Magnetic Triple-q State in Antiferromagnetic Monolayer Interfaced with Bismuthene

We have successfully fabricated the bismuthene covered Mn monolayer on Ag(111) by evaporating Mn atoms onto (p x root3)-Bi/Ag(111) at room temperature. By using spin-polarized scanning tunneling microscopy (SP-STM), we have resolved the magnetic triple-q (3Q) state. In combination with density-functional theory (DFT) calculations, the 3Q3-like spin texture is the magnetic ground state for the bismuthene covered Mn monolayer/Ag(111). Interestingly, the uniaxial magnetic anisotropy of 3Q3 state triggered by the bismuthene on top of Mn monolayer/Ag(111) has been revealed, which is consistent with the switching of 3Q3up and 3Q3down domains observed by SP-STM measurements with external magnetic fields.

cond-mat.mtrl-sci↗

arXiv2023

Atomistic Control in Molecular Beam Epitaxy Growth of Intrinsic Magnetic Topological Insulator MnBi2Te4

Intrinsic magnetic topological insulators have emerged as a promising platform to study the interplay between topological surface states and ferromagnetism. This unique interplay can give rise to a variety of exotic quantum phenomena, including the quantum anomalous Hall effect and axion insulating states. Here, utilizing molecular beam epitaxy (MBE), we present a comprehensive study of the growth of high-quality MnBi2Te4 thin films on Si (111), epitaxial graphene, and highly ordered pyrolytic graphite substrates. By combining a suite of in-situ characterization techniques, we obtain critical insights into the atomic-level control of MnBi2Te4 epitaxial growth. First, we extract the free energy landscape for the epitaxial relationship as a function of the in-plane angular distribution. Then, by employing an optimized layer-by-layer growth, we determine the chemical potential and Dirac point of the thin film at different thicknesses. Overall, these results establish a foundation for understanding the growth dynamics of MnBi2Te4 and pave the way for the future applications of MBE in emerging topological quantum materials.

cond-mat.mtrl-sci↗

arXiv2022

Self-Assembly of Magnetic Co Atoms on Stanene

We have investigated the magnetic Co atoms self-assembled on the ultraflat stanene on Cu(111) substrate by utilizing scanning tunneling microscopy/spectroscopy (STM/STS) in conjunction with density functional theory (DFT). By means of depositing Co onto the stanene/Cu(111) held at 80 K, Co atoms have developed into the monomer, dimer, and trimer structures containing one, two, and three Co atoms respectively. As per atomically resolved topographic images and bias-dependent apparent heights, the atomic structure models based on Sn atoms substituted by Co atoms have been deduced, which are in agreement with both self-consistent DFT calculations and STM simulations. Apart from that, the projected density of states (PDOS) has revealed a minimum at around -0.5 eV from the Co-3d3z2-r2 minority band, which contributes predominately to the peak feature at about -0.3 eV in tunneling conductance (dI/dU) spectra taken at the Co atomic sites. As a result of the exchange splitting between the Co-3d majority and minority bands, there are non-zero magnetic moments, including about 0.60 uB in monomer, 0.56 uB in dimer, and 0.29 uB in trimer of the Co atom assembly on the stanene. Such magnetic Co atom assembly therefore could provide the vital building blocks in stabilizing the local magnetism on the two-dimensional (2D) stanene with non-trivial topological properties.

cond-mat.mtrl-sci↗

arXiv2021

Superconducting Proximity Effect in R7xR7R19.1o Ni Nanoislands on Pb(111)

We have studied the proximity_induced superconductivity in R7xR7R19.1o Ni nanoislands by combing scanning tunnelling microscopy_spectroscopy (STM_STS) with density functional theory (DFT) calculation. Through depositing Ni onto Pb(111) substrate at 80 K, the monolayer Ni nanoislands with the R7xR7R19.1o surface structure have been fabricated, where the termination of Ni atoms at hexagonal close packed (hcp) site is energetically preferred and the electron filling of 3d orbitals from the charge transfer leads to the vanishing magnetic moment of Ni atoms. The topographic R7xR7R19.1o lattice as well as the asymmetric height contrast in atomic unit cell are further corroborated by the STM simulations. With high spatial and energy resolution, tunneling conductance spectra have resolved an isotropic superconducting gap with Delta_Ni_(R7xR7R19.1o)_1.29 meV, which is slightly larger than Delta_Pb_1.25 meV. The temperature dependence of Delta_Ni_(R7xR7R19.1o) supports the substrate_induced superconducting proximity effect according to the same transition temperature Tc_7.14 K with the Pb(111). The line spectroscopy has spatially mapped out the small increase of Delta_Ni_(R7xR7R19.1o), which could be explained by an enhanced electron_phonon interaction under the framework of Bardeen_Cooper_Schrieffer (BCS) theory as a manifestation of the hole doping of Pb(111) from the surface Ni atoms.

cond-mat.supr-con↗

arXiv2021

Proximity-Effect-Induced Anisotropic Superconductivity in Monolayer Ni-Pb Binary Alloy

Proximity effect facilitates the penetration of Cooper pairs that permits superconductivity in normal metal, offerring a promising approach to turn heterogeneous materials into superconducting and develop exceptional quantum phenomena. Here, we have systematically investigated proximity-induced anisotropic superconductivity in monolayer Ni-Pb binary alloy by combining scanning tunneling microscopy/ spectroscopy(STM/STS) with theoretical calculations. By means of high temperature growth, the(3root3by3root3)R30o Ni-Pb surface alloy has been fabricated on the Pb(111), where the appearance of domain boundary as well as lattice transformation are further corroborated by the STM simulations. Given the high spatial and energy resolution, tunnelling conductance (dI/dU) spectra have resolved a reduced but anisotropic superconducting gap NiPb about 1.0 meV, in stark contrast to the isotropic Pb about 1.3 meV on the conventional Pb(111). In addition, the higher density of states at Fermi energy (D(EF)) of Ni-Pb surface alloy results in an enhancement of coherence peak height. According to the same Tc about 7.1 K with Pb(111) from the temperature dependent NiPb and a short decay length Ld about 3.55 nm from the spatially monotonic decrease of NiPb, both results are supportive for the proximity-induced superconductivity. Despite a lack of bulk counterpart, the atomic-thick Ni-Pb bimetallic compound opens a new pathway to engineer superconducting properties down to the low-dimensional limit, giving rise to the emergence of anisotropic superconductivity via proximity effect.

cond-mat.supr-con↗

arXiv2020

Quantum Well Electronic States in Spatially Decoupled 2D Pb Nanoislands on Nb-doped SrTiO3(001)

Two-dimensional (2D) Pb nanoisland has established an ideal platform for studying the quantum size effects on growth mechanism, electronic structures as well as high-temperature superconductivity. Here, we investigate the growth and quantum well electronic states of the 2D Pb nanoisland on Nb-doped SrTiO3(001) by scanning tunneling microscopy and spectroscopy. In contrast to Pb/Si(111), Pb/Cu(111) and Pb/Ag(111), there is no wetting layer of Pb formed on Nb-doped SrTiO3(001) surface, resulting in isolated Pb nanoislands with an apparent height of 4 atomic layers as the building blocks for the island growth. According to the thickness-dependent quantum well states resolved in both occupied and unoccupied energy regions, the constant group velocity vg = 1.804+-0.106 m/s and Fermi wavevector kF = 1.575 Å-1, have been extracted from a linear fit of the Pb(111) band dipersion along the Γ-L direction. In addition, the energy-dependent scattering phase shift ϕ(E) obtained by means of phase accumulation model shows a metallic-like scattering interface analogous to Pb/Ag(111). These spatially decoupled 2D Pb nanoislands thus realize an opportunity to explore the intrinic quantum confinement phenomena in nanoscale superconductors on the doped titanium-oxide-type substrate.

cond-mat.mtrl-sci↗

arXiv2020

Orbital-enhanced Warping Effect in P\textsubscript{x},P\textsubscript{y}-derived Rashba Spin Splitting of Monatomic Bismuth Surface Alloy Surface Alloy

Spin-split Rashba bands have been exploited to efficiently control the spin degree of freedom of moving electrons, which possesses a great potential in frontier applications of designing spintronic devices and processing spin-based information. Given that intrinsic breaking of inversion symmetry and sizeable spin-orbit interaction, two-dimensional (2D) surface alloys formed by heavy metal elements exhibit a pronounced Rashba-type spin splitting of the surface states. Here, we have revealed the essential role of atomic orbital symmetry in the hexagonally warped Rashba spin-split surface state of $\sqrt{3}\times\sqrt{3} R30^{\circ}$ BiCu$_{2}$ monatomic alloy by scanning tunneling spectroscopy (STS) and density functional theory (DFT). From $\mathrm{d}I/\mathrm{d}U$ spectra and calculated band structures, three hole-like Rashba-split bands hybridized from distinct orbital symmetries have been identified in the unoccupied energy region. Because of the hexagonally deformed Fermi surface, quasi-particle interference (QPI) mappings have resolved scattering channels opened from interband transitions of \textit{p$_{x},$p$_{y}$}($m_{j}=1/2$) band. In contrast to the \textit{s,p$_{z}$}-derived band, the hexagonal warping predominately is accompanied by substantial out-of-plane spin polarization $S_{z}$ up to 24\% in the dispersion of \textit{p$_{x}$,p$_{y}$}($m_{j}=1/2$) band with an in-plane orbital symmetry.

cond-mat.mtrl-sci↗

arXiv2018

Tuning non-collinear magnetic states by hydrogenation

Two different superstructures form when atomic H is incorporated in the Fe monolayer on Ir(111). Depending on the amount of H provided, either a highly ordered p(2x2) hexagonal superstructure or an irregular roughly square structure is created. We present here spin-polarized scanning tunneling microscopy (SP-STM) measurements which reveal that in both cases the magnetic nanoskyrmion lattice state of the pristine Fe monolayer is modified. Our measurements of the magnetic states in these hydrogenated films are in agreement with superpositions of cycloidal spin spirals which follow the pattern and the symmetry dictated by the H superstructures. We thus demonstrate here the possibility to vary the symmetry of a non-collinear magnetic state in an ultrathin film without changing its substrate.

cond-mat.mes-hall↗

arXiv2017

Inducing skyrmions in ultrathin Fe films by hydrogen exposure

Magnetic skyrmions are localized nanometer-sized spin configurations with particle_like properties, which are envisioned to be used as bits in next_generation information technology. An essential step towards future skyrmion-based applications is to engineer key magnetic parameters for developing and stabilizing individual magnetic skyrmions. Here we demonstrate the tuning of the non_collinear magnetic state of an Fe double layer on an Ir111 substrate by loading the sample with atomic hydrogen. By using spin_polarized scanning tunneling microscopy, we discover that the hydrogenated system supports the formation of skyrmions in external magnetic fields, while the pristine Fe double layer does not. Based on ab initio calculations, we attribute this effect to the tuning of the Heisenberg exchange and the Dzyaloshinsky_Moriya interactions due to hydrogenation. In addition to interface engineering, hydrogenation of thin magnetic films offers a unique pathway to design and optimize the skyrmionic states in low_dimensional magnetic materials.

cond-mat.mtrl-sci↗

arXiv2017

Temperature-induced increase of spin spiral periods

Spin-polarized scanning tunneling microscopy investigations reveal a significant increase of the magnetic period of spin spirals in three-atomic-layer-thick Fe films on Ir(111), from about 4nm at 8K to about 65nm at room temperature. We attribute this considerable influence of temperature on the magnetic length scale of noncollinear spin states to different exchange interaction coefficients in the different Fe layers. We thus propose a classical spin model which reproduces the experimental observations and in which the crucial feature is the presence of magnetically coupled atomic layers with different interaction strengths. This model might also apply for many other systems, especially magnetic multilayers.

cond-mat.mes-hall↗

arXiv2016

Tailoring non-collinear magnetism by misfit dislocation lines

The large epitaxial stress induced by the misfit between a triple atomic layer Fe film and an Ir(111) substrate is relieved by the formation of a dense dislocation line network. Spin-polarized scanning tunneling microscopy (SP-STM) investigations show that the strain is locally varying within the Fe film and that this variation affects the magnetic state of the system. Two types of dislocation line regions can be distinguished and both exhibit spin spirals with strain-dependent periods (ranging from 3 nm to 10 nm). Using a simple micromagnetic model, we attribute the changes of the period of the spin spirals to variations of the effective exchange coupling in the magnetic film. This assumption is supported by the observed dependence of the saturation magnetic field on the spin spiral period. Moreover, magnetic skyrmions appear in an external magnetic field only in one type of dislocation line area, which we impute to the different pinning properties of the dislocation lines.

cond-mat.mes-hall↗

arXiv2016

Coexistence of charge and ferromagnetic order in fcc Fe

Phase coexistence phenomena have been intensively studied in strongly correlated materials where several ordered states simultaneously occur or compete. Material properties critically depend on external parameters and boundary conditions, where tiny changes result in qualitatively different ground states. However, up to date, phase coexistence phenomena have exclusively been reported for complex compounds composed of multiple elements. Here we show that charge- and magnetically ordered states coexist in double-layer Fe on Rh(001). Scanning tunneling microscopy and spectroscopy measurements reveal periodic charge order stripes below a temperature of 130 K. Close to liquid helium temperature, they are superimposed by ferromagnetic domains as observed by spin-polarized scanning tunneling microscopy. Temperature-dependent measurements reveal a pronounced cross-talk between charge and spin order at the ferromagnetic ordering temperature about 70 K, which is successfully modeled within an effective Landau theory including sixth-order terms. Our results show that subtle balance between structural modifications can lead to competing ordering phenomena.

cond-mat.mtrl-sci↗

arXiv2016

Symmetry breaking in spin spirals and skyrmions by in-plane and canted magnetic fields

The influence of in-plane and canted magnetic fields on spin spirals and skyrmions in atomic bilayer islands of palladium and iron on an Ir(111) substrate is investigated by scanning tunnelling microscopy at low temperatures. It is shown that the spin spiral propagation direction is determined by the island's border which can be explained by equilibrium state calculations on a triangular lattice. By application of in-plane fields, the spin spiral reorientates its propagation direction and becomes distorted, thereby allowing a proof for its cycloidal nature. Furthermore, it is demonstrated that the skyrmions' shape is distorted in canted fields which allows to determine the sense of magnetisation rotation as enforced by the interfacial Dzyaloshinskii-Moriya interaction.

cond-mat.mes-hall↗

arXiv2016

Electric field driven switching of individual magnetic skyrmions

Controlling magnetism with electric fields is a key challenge to develop future energy-efficient devices, however, the switching between inversion symmetric states, e.g. magnetization up and down as used in current technology, is not straightforward, since the electric field does not break time-reversal symmetry. Here, we demonstrate that local electric fields can be used to reversibly switch between a magnetic skyrmion and the ferromagnetic state. These two states are topologically inequivalent, and we find that the direction of an electric field directly determines the final state. This observation establishes the possibility to combine energy-efficient electric field writing with the recently envisaged skyrmion racetrack-type memories.

cond-mat.mes-hall↗

arXiv2013

Hysteretic melting transition of a soliton lattice in a commensurate charge modulation

We report on the observation of the hysteretic transition of a commensurate charge modulation in IrTe$_2$ from transport and scanning tunneling microscopy (STM) studies. Below the transition ($T_{\rm C} \approx 275$ K on cooling) a $q = 1/5$ charge modulation was observed, which is consistent with previous studies. Additional modulations [$q_n = (3n+2)^{-1}$] appear below a second transition at $T_{\rm S}\approx 180$ K on cooling. The coexistence of various modulations persist up to $T_{\rm C}$ on warming. The atomic structures of charge modulations and the temperature dependent STM studies suggest that 1/5 modulation is a periodic soliton lattice which partially melts below $T_{\rm S}$ on cooling. Our results provide compelling evidence that the ground state of IrTe$_2$ is a commensurate 1/6 charge modulation, which originates from periodic dimerization of Te atoms visualized by atomically resolved STM images.

cond-mat.str-el↗