arXiv subjects

Seung Ryong Park

Publications and source records attributed to Seung Ryong Park.

11 recordsLinked to original sources

arXiv2025

OAM spatial demultiplexing by diffraction-based noiseless mode conversion with axicon

The physics of orbital angular momentum (OAM) carrying light has been well-defined since the 1990s. Leveraging its physical phenomena has become a significant focus in various areas of research. For instance, OAM is applied in hybrid free-space optical communication channels, like wavelength division multiplexing. Due to its orthogonality, OAM can be multiplexed and demultiplexed, enabling the use of multiple orthogonal OAM beams to achieve high-capacity optical communication systems. In this appliance, there will be instability of light because its phase distribution is quite complicated. This study focuses on spatial demultiplexing using Computer Generated Holography (CGH) based OAM diffraction. Earlier research on OAM diffraction primarily involved fork gratings. Spatial demultiplexing has phase distribution instability and noise problems in free space optics. Axicons have been studied extensively in the context of perfect vortex generation. Drawing inspiration from these two areas of physics, we propose a noiseless topological charge conversion through axicon properties in spatial demultiplexing using CGH. Furthermore, we estimate the axicon optical approximate solution that is appropriate for research purposes.

physics.optics↗

arXiv2024

Magnetic field control over the axialness of Higgs modes in charge-density wave compounds

Understanding how symmetry-breaking processes generate order out of disorder is among the most fundamental problems of nature. The scalar Higgs mode - a massive (quasi-) particle - is a key ingredient in these processes and emerges with the spontaneous breaking of a continuous symmetry. Its related exotic and elusive axial counterpart, a Boson with vector character, can be stabilized through the simultaneous breaking of multiple continuous symmetries. Here, we employ a magnetic field to tune the recently discovered axial Higgs-type charge-density wave amplitude modes in rare-earth tritellurides. We demonstrate a proportionality between the axial Higgs component and the applied field, and a 90$^{\circ}$ phase shift upon changing the direction of the B-field. This indicates that the axial character is directly related to magnetic degrees of freedom. Our approach opens up an in-situ control over the axialness of emergent Higgs modes.

cond-mat.str-el↗

arXiv2023

Linear scaling relationship of Néel temperature and dominant magnons in pyrochlore ruthenates

We present a systematic Raman spectroscopy study on a series of pyrochlore ruthenates, a system which is not yet clearly settled on its magnetic origin and structure. Apart from the Raman-active phonon modes, new peaks that appear in the energy range of 15 - 35 meV below the Néel temperature are assigned as one-magnon modes. The temperature evolution of one-magnon modes displays no significant thermal dependence in mode frequencies while the intensities decrease monotonically. Remarkably, one-magnons from all compounds show similar characteristics with a single dominant peak at lower energy and weaker side peaks at a couple of meV higher energy. Most importantly, we uncover a striking proportionality between the dominant magnon mode energies and the Néel temperature values. Our results suggest the Ru ions may have similar or the same magnetic phase in all pyrochlore ruthenates of our study. We have thus found an avenue for directly tuning the magnetic exchange interaction by the selection of the $A$-site ion.

cond-mat.str-el↗

arXiv2023

Collective magnetic Higgs excitation in a pyrochlore ruthenate

The emergence of scalar Higgs-type amplitude modes in systems where symmetry is spontaneously broken has been a highly successful, paradigmatic description of phase transitions, with implications ranging from high-energy particle physics to low-energy condensed matter systems. Here, we uncover two successive high temperature phase transitions in the pyrochlore magnet Nd$_2$Ru$_2$O$_7$ at $T_{\mathrm{N}} = 147$ K and $T^* = 97$ K, that lead to giant phonon instabilities and culminate in the emergence of a highly coherent excitation. This coherent excitation, distinct from other phonons and from conventional magnetic modes, stabilizes at a low energy of 3 meV. We assign it to a collective Higgs-type amplitude mode, that involves bond energy modulations of the Ru$_4$ tetrahedra. Its striking two-fold symmetry, incompatible with the underlying crystal structure, highlights the possibility of multiple entangled broken symmetries.

cond-mat.str-el↗

arXiv2021

$B_{\rm 1g}$ phonon anomaly driven by Fermi surface instability at intermediate temperature in YBa$_2$Cu$_3$O$_{7-δ}$

We performed temperature- and doping-dependent high-resolution Raman spectroscopy experiments on YBa$_2$Cu$_3$O$_{7-δ}$ to study $B$$_{\rm 1g}$ phonons. The temperature dependence of the real part of the phonon self-energy shows a distinct kink at $T=T_{\rm B1g}$ above $T$$_{\rm c}$ due to softening, in addition to the one due to the onset of the superconductivity. $T$$_{\rm B1g}$ is clearly different from the pseudogap temperature with a maximum in the underdoped region. The region between $T$$_{\rm B1g}$ and $T$$_{\rm c}$ resembles that of superconducting fluctuation or charge density wave order. While the true origin of the $B$$_{\rm 1g}$ phonon softening is not known, we can attribute it to a gap on the Fermi surface due to an electronic order. Our results may reveal the role of the $B$$_{\rm 1g}$ phonon not only in the superconducting state but also in the intertwined orders in multilayer copper oxide high-$T$$_{\rm c}$ superconductors.

cond-mat.supr-con↗

arXiv2017

Experimental observation of hidden Berry curvature in inversion-symmetric bulk 2H-WSe2

We investigate the hidden Berry curvature in bulk 2H-WSe2 by utilizing the surface sensitivity of angle resolved photoemission (ARPES). The symmetry in the electronic structure of transition metal dichalcogenides is used to uniquely determine the local orbital angular momentum (OAM) contribution to the circular dichroism (CD) in ARPES. The extracted CD signals for the K and K' valleys are almost identical but their signs, which should be determined by the valley index, are opposite. In addition, the sign is found to be the same for the two spin-split bands, indicating that it is independent of spin state. These observed CD behaviors are what are expected from Berry curvature of a monolayer of WSe2. In order to see if CD-ARPES is indeed representative of hidden Berry curvature within a layer, we use tight binding analysis as well as density functional calculation to calculate the Berry curvature and local OAM of a monolayer WSe2. We find that measured CD-ARPES is approximately proportional to the calculated Berry curvature as well as local OAM, further supporting our interpretation.

cond-mat.mtrl-sci↗

arXiv2017

Giant Rashba-type spin splitting through spin-dependent interatomic-hopping

We have performed density functional theory calculation and tight binging analysis in order to investigate the mechanism for the giant Rashba-type spin splitting (RSS) observed in Bi/Ag(111). We find that local orbital angular momentum induces momentum and spin dependent charge distribution which results in spin-dependent hopping. We show that the spin-dependent interatomic-hopping in Bi/Ag(111) works as a strong effective field and induces the giant RSS, indicating that the giant RSS is driven by hopping, not by a uniform electric field. The effective field from the hopping energy difference amounts to be ~18 V/Å. This new perspective on the RSS gives us a hint for the giant RSS mechanism in general and should provide a strategy for designing new RSS materials by controlling spin-dependence of hopping energy between the neighboring atomic layers.

cond-mat.mes-hall↗

arXiv2016

Experimental determination of the massive Dirac fermion model parameters for MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$

Monolayer MX$_2$ (M = Mo, W; X = S, Se) has drawn much attention recently for its possible application possibilities for optoelectronics, spintronics, and valleytronics. Its exotic optical and electronic properties include a direct band gap, circular polarization dependent optical transitions, and valence band (VB) spin band splitting at the $K$ and $-K$ points. These properties can be described within a minimal model, called the massive Dirac fermion model for which the parameters need to be experimentally determined. We propose that the parameters can be obtained from angle resolved photoemission (ARPES) data from bulk 2H-MX$_2$, instead of monolayer MX$_2$. Through tight binding calculations, we show how the electronic structure at high symmetry points evolves as the system changes from the monolayer to the three dimensional bulk 2H-MX$_2$ . We find vanishing $k_z$ dispersion and almost no change in the direct band gap at the $K$ and $-K$ points, in sharp contrast to the strong $k_z$ dispersion at the $Γ$ point. These facts allow us to extract the gap and spin band splitting at the $K$ point as well as the hopping energy from bulk ARPES data. We performed ARPES experiments on single crystals of MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$ at various photon energies and also with potassium evaporation. From the data, we determined the parameters for the massive Dirac fermion model for monolayer MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$.

cond-mat.mtrl-sci↗

arXiv2011

Orbital-Angular-Momentum Based Origin of Rashba-Type Surface Band Splitting

We propose that existence of local orbital angular momentum (OAM) on the surfaces of high-Z materials play a crucial role in the formation of Rashba-type surface band splitting. Local OAM state in a Bloch state produces asymmetric charge distribution (electric dipole). Presence of surface electric field aligns the electric dipole and results in chiral OAM states and the relevant Rashba- type splitting. Therefore, the band splitting originates from electric dipole interaction, not from the Zeeman splitting as proposed in the original Rashba picture. The characteristic spin chiral structure of Rashba states is formed through the spin-orbit coupling and thus is a secondary effect to the chiral OAM. Results from first principles calculations on a single Bi layer under an external electric field verify the key predictions of the new model, including the direction of the spin textures which is predicted to be in opposite direction in the Rashba picture.

cond-mat.mtrl-sci↗

arXiv2011

Chiral orbital-angular-momentum in the surface states of Bi2Se3

Locking of the spin of a quasi-particle to its momentum in split bands of on the surfaces of metals and topological insulators (TIs) is understood in terms of Rashba effect where a free electron in the surface states feels an effective magnetic field. On the other hand, the orbital part of the angular momentum (OAM) is usually neglected. We performed angle resolved photoemission experiments with circularly polarized lights and first principles density functional calculation with spin-orbit coupling on a TI, Bi2Se3, to study the local OAM of the surface states. We show from the results that OAM in the surface states of Bi2Se3 is significant and locked to the electron momentum in opposite direction to the spin, forming chiral OAM states. Our finding opens a new possibility to have strong light-induced spin-polarized current in the surface states.

cond-mat.mtrl-sci↗

arXiv2008

Angle-resolved photoemission spectroscopy of electron-doped cuprate superconductors: Isotropic electron-phonon coupling

We have performed high resolution angle resolved photoemission (ARPES) studies on electron doped cuprate superconductors Sm2-xCexCuO4 (x=0.10, 0.15, 0.18), Nd2-xCexCuO4 (x=0.15) and Eu2-xCexCuO4 (x=0.15). Imaginary parts of the electron removal self energy show step-like features due to an electron-bosonic mode coupling. The step-like feature is seen along both nodal and anti-nodal directions but at energies of 50 and 70 meV, respectively, independent of the doping and rare earth element. Such energy scales can be understood as being due to preferential coupling to half- and full-breathing mode phonons, revealing the phononic origin of the kink structures. Estimated electron-phonon coupling constant lambda from the self energy is roughly independent of the doping and momentum. The isotropic nature of lambda is discussed in comparison with the hole doped case where a strong anisotropy exists.

cond-mat.supr-con↗