arXiv subjects

Chul-Hee Min

Publications and source records attributed to Chul-Hee Min.

14 recordsLinked to original sources

arXiv2026

Disentangling bulk and surface states in the electronic structure of PtBi$_2$(0001)

Recent reports of surface-localized topological superconductivity in trigonal PtBi$_2$ highlight the importance of understanding its surface electronic structure. We investigate the bulk and surface band structure of PtBi$_2$ using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. Through photon-energy- and polarization-dependent measurements, we disentangle bulk dispersions from surface states on the two distinct surface terminations of PtBi$_2$(0001). For both terminations, we assign several different surface states and find good agreement between experiment and calculations. Based on our calculations, we analyze the orbital composition in the surface and bulk bands and compare the results to polarization-dependent ARPES measurements. Together, our results provide a coherent picture of the surface electronic structure of PtBi$_2$ across both surface terminations.

cond-mat.str-el↗

arXiv2023

Real-time observation of phonon-electron energy and angular momentum flow in laser-heated nickel

Identifying the microscopic nature of non-equilibrium energy transfer mechanisms among electronic, spin and lattice degrees of freedom is central for understanding ultrafast phenomena such as manipulating magnetism on the femtosecond timescale. Here we use time and angle-resolved photoemission spectroscopy to go beyond the often-employed ensemble-averaged view of non-equilibrium dynamics in terms of quasiparticle temperature evolutions. We show for ferromagnetic Ni that the non-equilibrium electron and spin dynamics display pronounced variations with electron momentum whereas the magnetic exchange interaction remains isotropic. This highlights the influence of lattice-mediated scattering processes and opens a pathway towards unraveling the still elusive microscopic mechanism of spin-lattice angular momentum transfer.

cond-mat.mtrl-sci↗

arXiv2023

Femtosecond concerted rotation of molecules on a 2D material interface

Interfaces between molecules and 2D materials exhibit energy-driven functionalities, wherein charge transfer directs molecular motion. Unlike equilibrium systems, where molecular assemblies settle into static configurations, continuous energy input can drive transient, collective molecular rearrangements. Here, we reveal ultrafast spectroscopic fingerprints of a collective rotational response of molecules on a 2D material following photoexcitation. Our results suggest that photoinduced charge transfer reshapes the interfacial energy potential, giving rise to macroscopic, unidirectional molecular rotation and the formation of a homochiral molecular arrangement. Using a multiplexed ultrafast photoemission spectroscopy approach, we simultaneously track, electronic states, atomic positions, and orbital wavefunctions with femtosecond and sub-{\aa}ngstr\"om resolution. Multimodal valence and core electron emission analysis disentangles the intertwined electronic-structural dynamics of the molecule and the 2D material, revealing the dynamic modulation of charge distribution and intermolecular forces that drive collective molecular motion. Our findings open a pathway for designing energy-driven molecular systems with tunable interfacial dynamics, with potential applications in chiral engineering and active matter systems.

cond-mat.mtrl-sci↗

arXiv2021

Semimetallic Nature and Magnetic Polarons in EuB6 Studied by Angle-Resolved Photoemission Spectroscopy

Temperature-dependent angle-resolved photoemission spectroscopy (ARPES) was carried out on single-crystalline EuB6 samples. By measuring ARPES spectra in an extended Brillouin zone, a B 2p hole pocket centered at the X point is clearly observed, thus proving the semimetallic nature of EuB6. Below the Curie temperature TC, ARPES spectra show two B 2p bands of which separation is due to an exchange interaction between local Eu 4f and itinerant B 2p electrons. The exchange splitting becomes smaller as the temperature increases and disappears well above TC. Additionally, a diffuse structure near the Fermi level survives just above TC. Such behavior is well described by Monte Carlo simulations of a Kondo lattice model, thus supporting the formation of magnetic polarons in EuB6, which accounts for the resistivity upturn near above TC when lowering the temperature.

cond-mat.str-el↗

arXiv2021

Robust surface states and coherence phenomena in magnetically alloyed SmB6

Samarium hexaboride is a candidate for the topological Kondo insulator state, in which Kondo coherence is predicted to give rise to an insulating gap spanned by topological surface states. Here we investigate the surface and bulk electronic properties of magnetically alloyed Sm1-xMxB6 (M=Ce, Eu), using angle-resolved photoemission spectroscopy (ARPES) and complementary characterization techniques. Remarkably, topologically nontrivial bulk and surface band structures are found to persist in highly modified samples with up to 30% Sm substitution, and to coexist with antiferromagnetism in the case of Eu doping. The results are interpreted in terms of a hierarchy of energy scales, in which surface state emergence is linked to the formation of a direct Kondo gap, while low temperature transport trends depend on the indirect gap.

cond-mat.str-el↗

arXiv2020

Orbital Fingerprint of Topological Fermi Arcs in a Weyl Semimetal

The monopnictides TaAs and TaP are well-established Weyl semimetals. Yet, a precise assignment of Fermi arcs, accomodating the predicted chiral charge of the bulk Weyl points, has been difficult in these systems, and the topological character of different surface features in the Fermi surface is not fully understood. Here, employing a joint analysis from linear dichroism in angle-resolved photoemission and first-principles calculations, we unveil the orbital texture on the full Fermi surface of TaP(001). We observe pronounced switches in the orbital texture at the projectedWeyl nodes, and show how they facilitate a topological classification of the surface band structure. Our findings establish a critical role of the orbital degrees of freedom in mediating the surface-bulk connectivity in Weyl semimetals.

cond-mat.str-el↗

arXiv2018

Prediction and observation of the first antiferromagnetic topological insulator

Magnetic topological insulators (MTIs) are narrow gap semiconductor materials that combine non-trivial band topology and magnetic order. Unlike their nonmagnetic counterparts, MTIs may have some of the surfaces gapped due to breaking the time-reversal symmetry, which enables a number of exotic phenomena having potential applications in spintronics. So far, MTIs have only been created by means of doping nonmagnetic TIs with 3d transition metal elements, however, such an approach leads to strongly inhomogeneous magnetic and electronic properties of these materials, restricting the observation of important effects to very low temperatures. Finding intrinsic MTI, i.e. a stoichiometric well-ordered magnetic compound, could be an ideal solution to these problems, but no such material was observed to date. Here, using density functional theory we predict and further confirm by means of structural, transport, magnetic, angle- and spin-resolved photoemission spectroscopy measurements the realization of the antiferromagnetic (AFM) TI phase, that is hosted by the van der Waals layered compound MnBi$_2$Te$_4$. An interlayer AFM ordering makes MnBi$_2$Te$_4$ invariant with respect to the combination of the time-reversal ($\Theta$) and primitive-lattice translation ($T_{1/2}$) symmetries, $S = \Theta T_{1/2}$, giving rise to the $Z_2$ topological classification of AFM insulators. We find $Z_2 = 1$ for MnBi$_2$Te$_4$, which confirms its topologically nontrivial nature. The $S$-breaking (0001) surface of MnBi$_2$Te$_4$ exhibits a giant bandgap in the topological surface state as evidenced by ab initio calculations and photoemission measurements. These results culminate almost a decade-long search of an AFMTI, predicted in 2010. Furthermore, MnBi$_2$Te$_4$ is the first intrinsic magnetic TI realized experimentally.

cond-mat.mtrl-sci↗

arXiv2018

Chemical and valence reconstruction at the surface of SmB6 revealed with resonant soft x-ray reflectometry

Samarium hexaboride (SmB$_6$), a Kondo insulator with mixed valence, has recently attracted much attention as a possible host for correlated topological surface states. Here, we use a combination of x-ray absorption and reflectometry techniques, backed up with a theoretical model for the resonant $M_{4,5}$ absorption edge of Sm and photoemission data, to establish laterally averaged chemical and valence depth profiles at the surface of SmB$_6$. We show that upon cleaving, the highly polar (001) surface of SmB$_6$ undergoes substantial chemical and valence reconstruction, resulting in boron termination and a Sm$^{3+}$ dominated sub-surface region. Whereas at room temperature, the reconstruction occurs on a time scale of less than two hours, it takes about 24 hours below 50 K. The boron termination is eventually established, irrespective of the initial termination. Our findings reconcile earlier depth resolved photoemission and scanning tunneling spectroscopy studies performed at different temperatures and are important for better control of polarity and, as a consequence, surface states in this system.

cond-mat.str-el↗

arXiv2017

Irreversible proliferation of magnetic moments at cleaved surfaces of the topological Kondo insulator SmB6

The compound SmB$_6$ is the best established realization of a topological Kondo insulator, in which a topological insulator state is obtained through Kondo coherence. Recent studies have found evidence that the surface of SmB$_6$ hosts ferromagnetic domains, creating an intrinsic platform for unidirectional ballistic transport at the domain boundaries. Here, surface-sensitive X-ray absorption (XAS) and bulk-sensitive resonant inelastic X-ray scattering (RIXS) spectra are measured at the Sm N$_{4,5}$-edge, and used to evaluate electronic symmetries, excitations and temperature dependence near the surface of cleaved samples. The XAS data show that the density of large-moment atomic multiplet states on a cleaved surface grows irreversibly over time, to a degree that likely exceeds a related change that has recently been observed in the surface 4f orbital occupation.

cond-mat.str-el↗

arXiv2016

Valence Characterization of Surface and Subsurface Region in SmB6

Samarium hexaboride (SmB6), which lies in the mixed valence regime in the Anderson model, has been predicted to possess topologically protected surface states. The intensive investigations on SmB6 have brought up the long standing questions about the discrepancy between the surface and bulk electronic properties in rare-earth compounds in general. Here, we investigate and eventually clarify this discrepancy in the particular case of SmB6 by the photoemission core-level spectra. We focus on the change in both Sm and B states depending on time, temperature, probing depth, and surface termination on the cleaved (100) surface. Our spectra show that the unusual time-dependent change in the Sm valence occurs within a period of hours, which is not related to the adsorption of residual gases. Moreover, we observe a reduction of the surface feature in the B and Sm states on the same timescale accompanied by the formation of a subsurface region. Thus, it indicates the relatively slow charge redistribution between the surface and subsurface regions. Our findings demonstrate that the f states is strongly involved in the surface relaxation.

cond-mat.str-el↗

arXiv2016

Electronic structure of YbB$_{6}$: Is it a Topological Insulator or not?

To resolve the controversial issue of the topological nature of the electronic structure of YbB$_{6}$, we have made a combined study using density functional theory (DFT) and angle resolved photoemission spectroscopy (ARPES). Accurate determination of the low energy band topology in DFT requires the use of modified Becke-Johnson exchange potential incorporating the spin-orbit coupling and the on-site Coulomb interaction $U$ of Yb $4f$ electrons as large as 7 eV. We have double-checked the DFT result with the more precise GW band calculation. ARPES is done with the non-polar (110) surface termination to avoid band bending and quantum well confinement that have confused ARPES spectra taken on the polar (001) surface termination. Thereby we show definitively that YbB$_{6}$ has a topologically trivial B 2$p$-Yb 5$d$ semiconductor band gap, and hence is a non-Kondo non-topological insulator (TI). In agreement with theory, ARPES shows pure divalency for Yb and a $p$-$d$ band gap of 0.3 eV, which clearly rules out both of the previous scenarios of $f$-$d$ band inversion Kondo TI and $p$-$d$ band inversion non-Kondo TI. We have also examined the pressure-dependent electronic structure of YbB$_{6}$, and found that the high pressure phase is not a Kondo TI but a \emph{p}-\emph{d} overlap semimetal.

cond-mat.str-el↗

arXiv2015

Universal properties of the near-gap spectra of SmB6: dynamical mean-field calculations and photoemission experiments

Samarium hexaboride (SmB6) has been presumed to show a topological Kondo insulating state consisting of fully occupied quasiparticle bands in the concept of a Fermi liquid. This gap emerging below a small coherence temperature is the ultimate sign of coherence for a many-body system, which in addition induces a non-trivial topology. Here, we demonstrate that just one energy scale governs the gap formation in SmB6, which supports the Fermi liquid description. The temperature dependence of the gap formation in the mixed valence regime is captured within the dynamical mean field (DMFT) approximation to the periodic Anderson model (PAM). The scaling property of the model with the topological coherence temperature provides a strong connection to the photoemission spectra of SmB6. Our results suggest a simple way to compare a model study and an experiment result for heavy fermion insulators.

cond-mat.str-el↗

arXiv2013

The importance of Charge Fluctuations for the Topological Phase in SmB$_6$

The discovery of topologically non-trivial states in band insulators has induced an extensive search for topological phase in strongly correlated electron systems. In particular, samarium hexaboride (SmB$_6$) has drawn much attention as it might represent a new class of condensed matter called topological Kondo insulator. Kondo insulators (KI) can have non-trivial Z$_2$ topology because the energy gap opens at the Fermi energy ($E_F$) by hybridization between an odd- parity renormalized $f$ band and an even- parity conduction $d$ band. However, the characteristics of SmB$_6$ deviate from the conventional KI because its gap is insensitive to doping and pressure. Thus, it is unclear what makes the gap of SmB$_6$ different from that of KI, and how the band inversion features occur. In this manuscript, we demonstrate the importance of charge fluctuations in SmB$_6$. Our angle-resolved photoemission spectroscopy (ARPES) results reveal that with decreasing temperature the bottom of the $d$-$f$ hybridized band at the $\bar{\text{X}}$ point gradually shifts from below to above $E_F$. This shift accompanies a redistribution of spectral weight from the upper to the lower quasiparticle band, and reflects the change in both carrier density and the number of localized $f$ electron. Moreover, because this hybridized band is predicted to have odd parity and to induce a non-trivial $Z_{2}$ topology, we compare our ARPES spectra with theoretical surface band structures and find signatures of topological surface states. Therefore, our results indicate that SmB$_6$ is a charge fluctuating topological insulator having the odd-parity hybridized band above $E_F$ at the $X$ point. This demonstrates that the charge fluctuations do not only drive the insulating bulk phase but also the non-trivial topological phase.

cond-mat.str-el↗

arXiv2012

Single Dirac-cone on the Cs-covered topological insulator surface Sb2Te3(0001)

Using angle-resolved photoelectron spectroscopy we investigate the surface electronic structure of the three-dimensional topological insulator (TI) Sb2Te3(0001). Our data show the presence of a topological surface state in the bulk energy gap with the Dirac-point located above the Fermi level. The adsorption of Cs-atoms on Sb2Te3(0001) gives rise to a downward energy shift of the electronic valence band states which saturates at a value of ~200 meV. For the saturation coverage the Dirac-point of the linearly dispersive surface state resides in close proximity to the Fermi level. The electronic structure of the Cs/Sb2Te3 interface therefore considerably deviates from previously studied metal-TI interfaces based on the isostructural compound Bi2Se3 which points to the importance of atomic composition in these hetero systems.

cond-mat.mes-hall↗