arXiv subjects

Shik Shin

Publications and source records attributed to Shik Shin.

At least 19 recordsLinked to original sources

arXiv2025

Direct observation of the surface superconducting gap in the topological superconductor candidate \beta-PdBi2

\beta-PdBi2 is one of the candidates for topological superconductors with a superconducting (SC) transition temperature (Tc) of 5.3 K, in which parity mixing of spin singlet and spin triplet has been anticipated, being crucial for the further understanding of relationship with inversion symmetry and parity mixing in the superconductivity. In this work, we measured the SC gap in high-quality single crystal of \beta-PdBi2 by using high-resolution laser angle-resolved photoemission spectroscopy below Tc. We found the isotropic SC gaps in momentum space for multiple bands, and observed that the difference between the SC gap of the topological surface bands and the bulk bands is about 0.1 meV, consistent with other experimental results. These direct and quantitative experimental results support the possibility of \beta-PdBi2 as a topological superconductor, characterized by unique crystal and electronic band structures.

cond-mat.supr-con↗

arXiv2024

Unveiling van Hove singularity modulation and fluctuated charge order in kagome superconductor $\rm{CsV_3Sb_5}$ via time-resolved ARPES

Kagome superconductor CsV3Sb5, which exhibits intertwined unconventional charge density wave (CDW) and superconductivity, has garnered significant attention recently. Despite extensive static studies, the nature of these exotic electronic orders remains elusive. In this study, we investigate the non-equilibrium electronic structure of CsV3Sb5 via time- and angle-resolved photoemission spectroscopy. Our results reveal that upon laser excitation, the van Hove singularities immediately shift towards the Fermi level and subsequently oscillate in sync with a 1.3 THz coherent phonon mode. By analyzing the coherent intensity oscillations in the energy-momentum (E-k) map, we find that this coherent phonon is strongly coupled with electronic bands from both Sb and V orbitals. While typically observable only in the CDW state, remarkably, we find that the 1.3-THz coherent phonon mode can be persistently excited at temperatures above T_CDW, suggesting the potential existence of fluctuated CDW in CsV3Sb5. These findings enhance our understanding of the unconventional CDW control of kagome superconductivity.

cond-mat.mtrl-sci↗

arXiv2024

Anomalous Fermi pockets on Hund's metal surface of Sr2RuO4 induced by the correlation-enhanced spin-orbit coupling

The electronic structure of the topmost layer in Sr2RuO4 in the close vicinity of the Fermi level is investigated by angle-resolved photoemission spectroscopy (ARPES) with a 7-eV laser. We find that the spin-orbit coupling (SOC) predicted as 100 meV by the density functional theory (DFT) calculations is enormously enhanced in a real material up to 250 meV, even more than that of bulk state (200 meV), by the electron-correlation effect increased by the octahedral rotation in the crystal structure. This causes the formation of highly orbital-mixing small Fermi pockets and reasonably explains why the orbital-selective Mott transition (OSMT) is not realized in perovskite oxides with crystal distortion. Interestingly, Hund's metal feature allows the quasiparticle generation only near EF, restricting the spectral gap opening derived by band hybridization within an extremely small binding energy (< 10 meV). Furthermore, it causes coherent-incoherent crossover, making the Fermi pockets disappear at elevated temperatures. The anomalous Fermi pockets are characterized by the dichotomy of the orbital-isolating Hund's coupling and the orbital-mixing SOC, which is key to understanding the nature of Sr2RuO4.

cond-mat.str-el↗

arXiv2024

Coherence Length of Electronic Nematicity in Iron-Based Superconductors

Recent developments in laser-excited photoemission electron microscopy (laser-PEEM) advance the visualization of electronic nematicity and nematic domain structures in iron-based superconductors. In FeSe and BaFe$_2$(As$_{0.87}$P$_{0.13}$)$_2$ superconductors, it has been reported that the thickness of the electronic nematic domain walls is unexpectedly long, leading to the formation of mesoscopic nematicity wave [T. Shimojima $\textit{et al.}$, Science $\textbf{373}$ (2021) 1122]. This finding demonstrates that the nematic coherence length $\xi_{\rm nem}$ can be decoupled from the lattice domain wall. Here, we report that the electronic domain wall thickness shows a distinct variation in related materials: it is similarly long in FeSe$_{0.9}$S$_{0.1}$ whereas it is much shorter in undoped BaFe$_2$As$_2$. We find a correlation between the thick domain walls and the non-Fermi liquid properties of normal-state resistivity above the nematic transition temperature. This suggests that the nematic coherence length can be enhanced by underlying spin-orbital fluctuations responsible for the anomalous transport properties.

cond-mat.supr-con↗

arXiv2023

Non-Destructive Imaging of Breakdown Process in Ferroelectric Capacitors Using \textit{In-situ} Laser-Based Photoemission Electron Microscopy

HfO$_2$-based ferroelectrics are one of the most actively developed functional materials for memory devices. However, in HfO$_2$-based ferroelectric devices, dielectric breakdown is a main failure mechanism during repeated polarization switching. Elucidation of the breakdown process may broaden the scope of applications for the ferroelectric HfO$_2$. Here, we report direct observations of a breakdown process in HfO$_2$-based ferroelectric capacitors, by \textit{in-situ} laser-based photoemission electron microscopy (laser-PEEM). We have not only clearly visualized the hard dielectric breakdown (HDB) spot, but also observed the regions responsible for the soft dielectric breakdown (SDB) which is a precursor phenomenon to HDB. It was found that the low-resistance region formed after SDB is wider than the conduction path formed after HDB. Furthermore, our spectromicroscopic analysis revealed that the photoelectron spectrum after SDB shows an enhancement in intensity without spectral-shape modulation, interpreted that the initially existed defects are increased. In the HDB spot, however, an additional shoulder structure was observed. These results provide spectroscopic evidence that the electronic states responsible for the conduction path after SDB are different from those after HDB. Through this work, we propose this microscopic approach as a versatile tool for studying buried materials as they are, accelerating the development of material engineering for advanced electronic devices.

physics.app-ph↗

arXiv2023

Anomalously large spin-dependent electron correlation in nearly half-metallic ferromagnet CoS$_2$

The spin-dependent band structure of CoS$_2$ which is a candidate for a half-metallic ferromagnet was investigated by both spin- and angle-resolved photoemission spectroscopy and theoretical calculations, in order to reappraise the half-metallicity and electronic correlations. We determined the three-dimensional Fermi surface and the spin-dependent band structure. As a result, we found that a part of the minority spin bands is on the occupied side in the vicinity of the Fermi level, providing spectroscopic evidence that CoS$_2$ is not but very close to a half-metal. Band calculations using density functional theory with generalized gradient approximation showed a good agreement with the observed majority spin $e_g$ bands, while it could not explain the observed band width of the minority-spin eg bands. On the other hand, theoretical calculations using dynamical mean field theory could better reproduce the strong mass renormalization in the minority-spin $e_g$ bands. All those results strongly suggest the presence of anomalously enhanced spin-dependent electron correlation effects on the electronic structure in the vicinity of the half-metallic state. We also report the temperature dependence of the electronic structure across the Curie temperature and discuss the mechanism of the thermal demagnetization. Our discovery of the anomalously large spin-dependent electronic correlations not only demonstrates a key factor in understanding the electronic structure of half-metals but also provides a motivation to improve theoretical calculations on spin-polarized strongly correlated systems.

cond-mat.str-el↗

arXiv2023

Unveiling phase diagram of the lightly doped high-Tc cuprate superconductors with disorder removed

The currently established electronic phase diagram of cuprates is based on a study of single- and double-layered compounds. These CuO$_2$ planes, however, are directly contacted with dopant layers, thus inevitably disordered with an inhomogeneous electronic state. Here, we solve this issue by investigating a 6-layered Ba$_2$Ca$_5$Cu$_6$O$_{12}$(F,O)$_2$ with inner CuO$_2$ layers, which are clean with the extremely low disorder, by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillation measurements. We find a tiny Fermi pocket with a doping level less than 1% to exhibit well-defined quasiparticle peaks which surprisingly lack the polaronic feature. This provides the first evidence that the slightest amount of carriers is enough to turn a Mott insulating state into a metallic state with long-lived quasiparticles. By tuning hole carriers, we also find an unexpected phase transition from the superconducting to metallic states at 4%. Our results are distinct from the nodal liquid state with polaronic features proposed as an anomaly of the heavily underdoped cuprates.

cond-mat.supr-con↗

arXiv2023

Spontaneous breaking of mirror symmetry beyond critical doping in Pb-Bi2212

Identifying ordered phases and their underlying symmetries is the first and most important step toward understanding the mechanism of high-temperature superconductivity; critical behaviors of ordered phases are expected to be correlated with superconductivity. Efforts to find such ordered phases have been focused on symmetry breaking in the pseudogap region while the Fermi liquid-like metal region beyond the so-called critical doping $p_{c}$ has been regarded as a trivial disordered state. Here, we used rotational anisotropy second harmonic generation and uncovered a broken mirror symmetry in the Fermi liquid-like phase in (Bi,Pb)$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ with $p = 0.205 > p_{c}$. By tracking the temperature evolution of the symmetry-breaking response, we verify an order parameter-like behavior with the onset temperature $T_{up}$ at which the strange metal to Fermi liquid-like-metal crossover takes place. Complementary angle-resolved photoemission study showed that the quasiparticle coherence between $\mathrm{CuO_{2}}$ bilayers is enhanced in proportion to the symmetry-breaking response as a function of temperature, indicating that the change in metallicity and symmetry breaking are linked. These observations contradict the conventional quantum disordered scenario for over-critical-doped cuprates and provide new insight into the nature of the quantum critical point in cuprates.

cond-mat.supr-con↗

arXiv2023

Spin-polarized saddle points in the topological surface states of the elemental Bismuth revealed by a pump-probe spin-resolved ARPES

We use a pump-probe, spin-, and angle-resolved photoemission spectroscopy (ARPES) with a 10.7 eV laser accessible up to the Brillouin zone edge, and reveal for the first time the entire band structure, including the unoccupied side, for the elemental bismuth (Bi) with the spin-polarized surface states. Our data identify Bi as in a strong topological insulator phase ($Z_2$=1) against the prediction of most band calculations. We unveil that the unoccupied topological surface states possess spin-polarized saddle points yielding the van Hove singularity, providing an excellent platform for the future development of opto-spintronics.

cond-mat.mtrl-sci↗

arXiv2023

Nodeless electron pairing in CsV$_3$Sb$_5$-derived kagome superconductors

The newly discovered kagome superconductors represent a promising platform for investigating the interplay between band topology, electronic order, and lattice geometry. Despite extensive research efforts on this system, the nature of the superconducting ground state remains elusive. In particular, consensus on the electron pairing symmetry has not been achieved so far, in part owing to the lack of a momentum-resolved measurement of the superconducting gap structure. Here we report the direct observation of a nodeless, nearly isotropic, and orbital-independent superconducting gap in the momentum space of two exemplary CsV$_3$Sb$_5$-derived kagome superconductors -- Cs(V$_{0.93}$Nb$_{0.07}$)$_3$Sb$_5$ and Cs(V$_{0.86}$Ta$_{0.14}$)$_3$Sb$_5$, using ultrahigh resolution and low-temperature angle-resolved photoemission spectroscopy (ARPES). Remarkably, such a gap structure is robust to the appearance or absence of charge order in the normal state, tuned by isovalent Nb/Ta substitutions of V. Moreover, we observe a signature of the time-reversal symmetry (TRS) breaking inside the superconducting state, which extends the previous observation of TRS-breaking CDW in the kagome lattice. Our comprehensive characterizations of the superconducting state provide indispensable information on the electron pairing of kagome superconductors, and advance our understanding of unconventional superconductivity and intertwined electronic orders.

cond-mat.supr-con↗

arXiv2023

Coexistence of bulk-nodal and surface-nodeless Cooper pairings in a superconducting Dirac semimetal

The interplay of nontrivial topology and superconductivity in condensed matter physics gives rise to exotic phenomena. However, materials are extremely rare where it is possible to explore the full details of the superconducting pairing. Here, we investigate the momentum dependence of the superconducting gap distribution in a novel Dirac material PdTe. Using high resolution, low temperature photoemission spectroscopy, we establish it as a spin-orbit coupled Dirac semimetal with the topological Fermi arc crossing the Fermi level on the (010) surface. This spin-textured surface state exhibits a fully gapped superconducting Cooper pairing structure below Tc~4.5K. Moreover, we find a node in the bulk near the Brillouin zone boundary, away from the topological Fermi arc.These observations not only demonstrate the band resolved electronic correlation between topological Fermi arc states and the way it induces Cooper pairing in PdTe, but also provide a rare case where surface and bulk states host a coexistence of nodeless and nodal gap structures enforced by spin-orbit coupling.

cond-mat.supr-con↗

arXiv2022

Direct observation of topological surface state in the topological superconductor 2M-WS2

The quantum spin Hall (QSH) effect has attracted extensive research interest because of the potential applications in spintronics and quantum computing, which is attributable to two conducting edge channels with opposite spin polarization and the quantized electronic conductance of 2e2/h. Recently, 2M-WS2, a new stable phase of transition metal dichalcogenides with a 2M structure showing an identical layer configuration to that of the monolayer 1T' TMDs, was suggested to be a QSH insulator as well as a superconductor with critical transition temperature around 8 K. Here, high-resolution angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES are applied to investigate the electronic and spin structure of the topological surface states (TSS) in the superconducting 2M-WS2. The TSS exhibits characteristic spin-momentum-locking behavior, suggesting the existence of long-sought nontrivial Z2 topological states therein. We expect that 2M-WS2 with co-existing superconductivity and TSS might host the promising Majorana bound states.

cond-mat.supr-con↗

arXiv2022

Photo-induced nonlinear band shift and valence transition in SmS

The photo-induced band structure variation of a rare-earth-based semiconductor, samarium monosulfide (SmS), was investigated using high-harmonic-generation laser-based time-resolved photoelectron spectroscopy. A nonlinear photo-induced band shift of the Sm 4f multiplets was observed. The first one is a shift to the high-binding-energy side due to a large surface photovoltage (SPV) effect of approximately 93 meV, comparable to the size of the bulk band gap, with a much longer relaxation time than 0.1 ms. The second one is an ultrafast band shift to the low binding energy side, which is in the opposite direction to the SPV shift, suggesting an ultrafast valence transition from divalent to trivalent Sm ions due to photo-excitation. The latter energy shift was approximately 58 meV, which is consistent with the energy gap shift from ambient pressure to the boundary between the black insulator and golden metallic phase with the application of pressure. This suggests that the photo-induced valence transition can reach the phase boundary, but other effects are necessary to realize the golden metallic phase.

cond-mat.mtrl-sci↗

arXiv2022

Testing Electron-phonon Coupling for the Superconductivity in Kagome Metal $\rm{CsV_3Sb_5}$

In crystalline materials, electron-phonon coupling (EPC) is a ubiquitous many-body interaction that drives conventional Bardeen-Cooper-Schrieffer superconductivity. Recently, in a new kagome metal $\rm{CsV_3Sb_5}$, superconductivity that possibly intertwines with time-reversal and spatial symmetry-breaking orders is observed. Density functional theory calculations predicted weak EPC strength,$\lambda$, supporting an unconventional pairing mechanism in $\rm{CsV_3Sb_5}$. However, experimental determination of $\lambda$ is still missing, hindering a microscopic understanding of the intertwined ground state of $\rm{CsV_3Sb_5}$. Here, using 7-eV laser-based angle-resolved photoemission spectroscopy and Eliashberg function analysis, we determine an intermediate $\lambda$=0.45~0.6 at T=6 K for both Sb 5p and V 3d electronic bands, which can support a conventional superconducting transition temperature on the same magnitude of experimental value in $\rm{CsV_3Sb_5}$. Remarkably, the EPC on the V 3d-band enhances to $\lambda$~0.75 as the superconducting transition temperature elevated to 4.4 K in $\rm{Cs(V_{0.93}Nb_{0.07})_3Sb_5}$. Our results provide an important clue to understand the pairing mechanism in the Kagome superconductor $\rm{CsV_3Sb_5}$.

cond-mat.supr-con↗

arXiv2021

Direct observation of multiple conduction-band minima in high-performance thermoelectric SnSe

We report time- and angle-resolved photoemission spectroscopy on SnSe which currently attracts great interest due to its extremely high thermoelectric performance. Laser-assisted photoemission signals are observed within $\pm$20 fs of the pump pulse arrival. Around 30-50 fs after the photoexcitation, the conduction band minima are not populated by the photoexcited electrons while the valence bands are considerably broadened. In going from 90 fs to 550 fs after the photoexcitation, the photoexcited carriers are decayed into the multiple conduction band minima. The observed conduction bands are consistent with the band structure calculations. The multiple conduction minima suggest possibility of high and anisotropic thermoelectric performance of n-type SnSe single crystal if it is realized.

cond-mat.mtrl-sci↗

arXiv2021

Selective observation of surface and bulk bands in polar WTe2 by laser-based spin- and angle-resolved photoemission spectroscopy

The electronic state of WTe2, a candidate of type-II Weyl semimetal, is investigated by using laser-based spin- and angle-resolved photoemission spectroscopy (SARPES). We prepare the pair of WTe2 samples, one with (001) surface and the other with (00-1) surface, by "sandwich method", and measure the band structures of each surface separately. The Fermi arcs are observed on both surfaces. We identify that the Fermi arcs on the two surfaces are both originating from surface states. We further find a surface resonance band, which connects with the Fermi-arc band, forming a Dirac-cone-like band dispersion. Our results indicate that the bulk electron and hole bands are much closer in momentum space than band calculations.

cond-mat.mtrl-sci↗

arXiv2021

Optical study on electronic structure of the locally non-centrosymmetric CeRh$_2$As$_2$

The electronic structures of the heavy-fermion superconductor CeRh$_2$As$_2$ with the local inversion symmetry breaking and the reference material LaRh$_2$As$_2$ have been investigated by using experimental optical conductivity ($\sigma_1(\omega)$) spectra and first-principal DFT calculations. In the low-temperature $\sigma_1(\omega)$ spectra of CeRh$_2$As$_2$, a $4f$-conduction electron hybridization and heavy quasiparticles are clearly indicated by a mid-infrared peak and a narrow Drude peak. In LaRh$_2$As$_2$, these features are absent in the $\sigma_1(\omega)$ spectrum, however, it can nicely be reproduced by DFT calculations. For both compounds, the combination between a local inversion symmetry breaking and a large spin-orbit (SO) interaction plays an important role for the electronic structure, however, the SO splitting bands could not be resolved in the $\sigma_1(\omega)$ spectra due to the small SO splitting size.

cond-mat.str-el↗

arXiv2021

HHG-laser-based time- and angle-resolved photoemission spectroscopy of quantum materials

Time- and angle-resolved photoemission spectroscopy has played an important role in revealing the non-equilibrium electronic structures of solid-state materials. The implementation of high harmonic generation to obtain a higher photon energy also allows us to investigate the wide Brillouin zone on a time scale below 100 fs. In this article, we review our recent studies using high-harmonic-generation-laser-based time- and angle-resolved photoemission spectroscopy to study a variety of quantum materials. We reveal many unprecedented phenomena in each system and highlight some representative results.

cond-mat.str-el↗