arXiv subjects

Sung-Kwan Mo

Publications and source records attributed to Sung-Kwan Mo.

At least 55 records · Page 3Linked to original sources

arXiv2022

Evidences for the exciton gas phase and its condensation in monolayer 1T-ZrTe2

The excitonic insulator (EI) is a Bose-Einstein condensation (BEC) of excitons bound by electron-hole interaction in a solid, which could support high-temperature BEC transition. The material realization of EI has been elusive, which is further challenged by the difficulty of distinguishing it from a conventional charge density wave (CDW) state. In the BEC limit, the pre-condensation exciton gas phase is a hallmark to distinguish EI from conventional CDW, yet direct experimental evidence has been lacking. Here we report a distinct correlated phase beyond the $2\times2$ CDW ground state emerging in epitaxially grown monolayer 1T-ZrTe2 and its investigation by angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). The results show novel band- and energy-dependent folding behavior in a two-step process, evidenced by an exciton gas phase prior to its condensation into the final CDW state. The excellent agreement between experiments and theoretical predictions on the recovery of the pristine band structure by carrier-density-dependent suppression of the CDW state further corroborates the monolayer 1T-ZrTe2 as an EI. Our findings provide a versatile two-dimensional platform that allows tuning of the excitonic effect.

cond-mat.mtrl-sci↗

arXiv2022

Nonsymmorphic Symmetry-Protected Band Crossings in a Square-Net Metal PtPb$_4$

Topological semimetals with symmetry-protected band crossings have emerged as a rich landscape to explore intriguing electronic phenomena. Nonsymmorphic symmetries in particular have been shown to play an important role in protecting the crossings along a line (rather than a point) in momentum space. Here we report experimental and theoretical evidence for Dirac nodal line crossings along the Brillouin zone boundaries in PtPb$_4$, arising from the nonsymmorphic symmetry of its crystal structure. Interestingly, while the nodal lines would remain gapless in the absence of spin-orbit coupling (SOC), the SOC in this case plays a detrimental role to topology by lifting the band degeneracy everywhere except at a set of isolated points. Nevertheless, the nodal line is observed to have a bandwidth much smaller than that found in density functional theory (DFT). Our findings reveal PtPb$_4$ to be a material system with narrow crossings approximately protected by non-symmorhpic crystalline symmetries.

cond-mat.mtrl-sci↗

arXiv2022

Electronic structure of superconducting nickelates probed by resonant photoemission spectroscopy

The discovery of infinite-layer nickelate superconductors has spurred enormous interest. While the Ni$^{1+}$ cations possess nominally the same 3$d^9$ configuration as Cu$^{2+}$ in cuprates, the electronic structure variances remain elusive. Here, we present a soft x-ray photoemission spectroscopy study on parent and doped infinite-layer Pr-nickelate thin films with a doped perovskite reference. By identifying the Ni character with resonant photoemission and comparison to density functional theory + U (on-site Coulomb repulsion energy) calculations, we estimate U ~5 eV, smaller than the charge transfer energy $Δ$ ~8 eV, confirming the Mott-Hubbard electronic structure in contrast to charge-transfer cuprates. Near the Fermi level ($E_F$), we observe a signature of occupied rare-earth states in the parent compound, which is consistent with a self-doping picture. Our results demonstrate a correlation between the superconducting transition temperature and the oxygen 2$p$ hybridization near $E_F$ when comparing hole-doped nickelates and cuprates.

cond-mat.supr-con↗

arXiv2022

Evidence for a spinon Kondo effect in cobalt atoms on single-layer 1T-TaSe$_2$

Quantum spin liquids (QSLs) are highly entangled, disordered magnetic states that arise in frustrated Mott insulators and host exotic fractional excitations such as spinons and chargons. Despite being charge insulators some QSLs are predicted to exhibit gapless itinerant spinons that yield metallic behavior in the spin channel. We have deposited isolated magnetic atoms onto single-layer (SL) 1T-TaSe$_2$, a gapless QSL candidate, to experimentally probe how itinerant spinons couple to impurity spin centers. Using scanning tunneling spectroscopy we observe the emergence of new, impurity-induced resonance peaks at the 1T-TaSe$_2$ Hubbard band edges when cobalt adatoms are positioned to have maximal spatial overlap with the Hubbard band charge distribution. These resonance peaks disappear when the spatial overlap is reduced or when the magnetic impurities are replaced with non-magnetic impurities. Theoretical simulations using a modified Anderson impurity model integrated with a gapless quantum spin liquid show that these resonance peaks are consistent with a Kondo resonance induced by spinons combined with spinon-chargon binding effects that arise due to QSL gauge-field fluctuations.

cond-mat.str-el↗

arXiv2021

Observation of a Smoothly Tunable Dirac Point in $Ge(Bi_{x}Sb_{1-x})_{2}Te_{4}$

State-of-the-art topological devices require the use topological surface states to drive electronic transport. In this study, we examine a tunable topological system, $Ge(Bi_{x}Sb_{1-x})_{2}Te_{4}$, for a range of 'x' values from 0 to 1, using a combination of Fourier Transform Scanning Tunneling Spectroscopy (FT-STS) and Angle-Resolved Photoemission Spectroscopy (ARPES). Our results show that the Dirac point shifts linearly with 'x', crossing the Fermi energy near x = 0.7. This novel observation of a smoothly tunable, isolated Dirac point crossing through the topological transport regime and having strong linear dependence with substitution can be critical for future topological spintronics applications.

cond-mat.mtrl-sci↗

arXiv2021

Kramers-Weyl fermions in the chiral charge density wave material (TaSe$_4$)$_2$I

The quasi-one-dimensional chiral charge density wave (CDW) material (TaSe$_4$)$_2$I has been recently predicted to host Kramers-Weyl (KW) fermions which should exist in the vicinity of high symmetry points in the Brillouin zone in chiral materials with strong spin-orbit coupling. However, direct spectroscopic evidence of KW fermions is limited. Here we use helicity-dependent laser-based angle resolved photoemission spectroscopy (ARPES) in conjunction with tight-binding and first-principles calculations to identify KW fermions in (TaSe$_4$)$_2$I. We find that topological and symmetry considerations place distinct constraints on the (pseudo-) spin texture and the observed spectra around a KW node. We further reveal an interplay between the spin texture around the chiral KW node and the onset of CDW order in (TaSe$_4$)$_2$I. Our findings highlight the unique topological nature of (TaSe$_4$)$_2$I and provide a pathway for identifying KW fermions in other chiral materials.

cond-mat.str-el↗

arXiv2021

Large bandgap quantum anomalous hall insulator in a designer ferromagnet-topological insulator-ferromagnet heterostructure

Combining magnetism and nontrivial band topology gives rise to quantum anomalous Hall (QAH) insulators and exotic quantum phases such as the QAH effect where current flows without dissipation along quantized edge states. Inducing magnetic order in topological insulators via proximity to a magnetic material offers a promising pathway towards achieving QAH effect at high temperature for lossless transport applications. One promising architecture involves a sandwich structure comprising two single layers of MnBi2Te4 (a 2D ferromagnetic insulator) with ultra-thin Bi2Te3 in the middle, and is predicted to yield a robust QAH insulator phase with a bandgap well above thermal energy at room temperature (25 meV). Here we demonstrate the growth of a 1SL MnBi2Te4 / 4QL Bi2Te3 /1SL MnBi2Te4 heterostructure via molecular beam epitaxy, and probe the electronic structure using angle resolved photoelectron spectroscopy. We observe strong hexagonally warped massive Dirac Fermions and a bandgap of 75 meV. The magnetic origin of the gap is confirmed by the observation of broken time reversal symmetry and the exchange-Rashba effect, in excellent agreement with density functional theory calculations. These findings provide insights into magnetic proximity effects in topological insulators, that will move lossless transport in topological insulators towards higher temperature.

cond-mat.mes-hall↗

arXiv2021

Flatband-Induced Itinerant Ferromagnetism in RbCo$_2$Se$_2$

$A$Co$_2$Se$_2$ ($A$=K,Rb,Cs) is a homologue of the iron-based superconductor, $A$Fe$_2$Se$_2$. From a comprehensive study of RbCo$_2$Se$_2$ via measurements of magnetization, transport, neutron diffraction, angle-resolved photoemission spectroscopy, and first-principle calculations, we identify a ferromagnetic order accompanied by an orbital-dependent spin-splitting of the electronic dispersions. Furthermore, we identify the ordered moment to be dominated by a $d_{x^2-y^2}$ flatband near the Fermi level, which exhibits the largest spin splitting across the ferromagnetic transition, suggesting an itinerant origin of the ferromagnetism. In the broader context of the iron-based superconductors, we find this $d_{x^2-y^2}$ flatband to be a common feature in the band structures of both iron-chalcogenides and iron-pnictides, accessible via heavy electron doping.

cond-mat.supr-con↗

arXiv2021

Non-Thermal Emergence of an Orbital-Selective Mott Phase in FeTe$_{1-x}$Se$_x$

Electronic correlation is of fundamental importance to high temperature superconductivity. Iron-based superconductors are believed to possess moderate correlation strength, which combined with their multi-orbital nature makes them a fascinating platform for the emergence of exotic phenomena. A particularly striking form is the emergence of an orbital selective Mott phase, where the localization of a subset of orbitals leads to a drastically reconstructed Fermi surface. Here, we report spectroscopic evidence of the reorganization of the Fermi surface from FeSe to FeTe as Se is substituted by Te. We uncover a particularly transparent way to visualize the localization of the $d_{xy}$ electron orbital through the suppression of its hybridization with the more coherent $d$ electron orbitals, which leads to a redistribution of the orbital-dependent spectral weight near the Fermi level. These noteworthy features of the Fermi surface are accompanied by a divergent behavior of a band renormalization in the $d_{xy}$ orbital. All of our observations are further supported by our theoretical calculations to be salient spectroscopic signatures of such a non-thermal evolution from a strongly correlated metallic phase towards an orbital-selective Mott phase in FeTe$_{1-x}$Se$_x$ as Se concentration is reduced.

cond-mat.supr-con↗

arXiv2020

Emergence of Quasiparticles in a Doped Mott Insulator

How a Mott insulator develops into a weakly coupled metal upon doping is a central question to understanding various emergent correlated phenomena. To analyze this evolution and its connection to the high-$T_c$ cuprates, we study the single-particle spectrum for the doped Hubbard model using cluster perturbation theory on superclusters. Starting from extremely low doping, we identify a heavily renormalized quasiparticle dispersion that immediately develops across the Fermi level, and a weakening polaronic side band at higher binding energy. The quasiparticle spectral weight roughly grows at twice the rate of doping in the low doping regime, but this rate is halved at optimal doping. In the heavily doped regime, we find both strong electron-hole asymmetry and a persistent presence of Mott spectral features. Finally, we discuss the applicability of the single-band Hubbard model to describe the evolution of nodal spectra measured by angle-resolved photoemission spectroscopy (ARPES) on the single-layer cuprate La$_{2-x}$Sr$_x$CuO$_4$ ($0 \le x \le 0.15$). This work benchmarks the predictive power of the Hubbard model for electronic properties of high-$T_c$ cuprates.

cond-mat.str-el↗

arXiv2020

Imaging spinon density modulations in a 2D quantum spin liquid

Two-dimensional triangular-lattice antiferromagnets are predicted under some conditions to exhibit a quantum spin liquid ground state whose low-energy behavior is described by a spinon Fermi surface. Directly imaging the resulting spinons, however, is difficult due to their fractional, chargeless nature. Here we use scanning tunneling spectroscopy to image spinon density modulations arising from a spinon Fermi surface instability in single-layer 1T-TaSe$_2$, a two-dimensional Mott insulator. We first demonstrate the existence of localized spins arranged on a triangular lattice in single-layer 1T-TaSe$_2$ by contacting it to a metallic 1H-TaSe$_2$ layer and measuring the Kondo effect. Subsequent spectroscopic imaging of isolated, single-layer 1T-TaSe$_2$ reveals long-wavelength modulations at Hubbard band energies that reflect spinon density modulations. This allows direct experimental measurement of the spinon Fermi wavevector, in good agreement with theoretical predictions for a 2D quantum spin liquid. These results establish single-layer 1T-TaSe$_2$ as a new platform for studying novel two-dimensional quantum-spin-liquid phenomena.

cond-mat.str-el↗

arXiv2020

A plausible method of preparing the ideal p-n junction interface of a thermoelectric material by surface doping

Recent advances in two-dimensional (2D) crystals make it possible to realize an ideal interface structure that is required for device applications. Specifically, a p-n junction made of 2D crystals is predicted to exhibit an atomically well-defined interface that will lead to high device performance. Using angle-resolved photoemission spectroscopy, a simple surface treatment was shown to allow the possible formation of such an interface. Ta adsorption on the surface of a p-doped SnSe shifts the valence band maximum towards higher binding energy due to the charge transfer from Ta to SnSe that is highly localized at the surface due to the layered structure of SnSe. As a result, the charge carriers of the surface are changed from holes of its bulk characteristics to electrons, while the bulk remains as a p-type semiconductor. This observation suggests that the well-defined interface of a p-n junction with an atomically thin {\it n}-region is formed between Ta-adsorbed surface and bulk.

cond-mat.mtrl-sci↗

arXiv2020

Metallic surface states in a correlated d-electron topological Kondo insulator candidate FeSb2

The resistance of a conventional insulator diverges as temperature approaches zero. The peculiar low temperature resistivity saturation in the 4f Kondo insulator (KI) SmB6 has spurred proposals of a correlation-driven topological Kondo insulator (TKI) with exotic ground states. However, the scarcity of model TKI material families leaves difficulties in disentangling key ingredients from irrelevant details. Here we use angle-resolved photoemission spectroscopy (ARPES) to study FeSb2, a correlated d-electron KI candidate that also exhibits a low temperature resistivity saturation. On the (010) surface, we find a rich assemblage of metallic states with two-dimensional dispersion. Measurements of the bulk band structure reveal band renormalization, a large temperature-dependent band shift, and flat spectral features along certain high symmetry directions, providing spectroscopic evidence for strong correlations. Our observations suggest that exotic insulating states resembling those in SmB6 and YbB12 may also exist in systems with d instead of f electrons.

cond-mat.str-el↗

arXiv2020

Visualization of multifractal superconductivity in a two-dimensional transition metal dichalcogenide in the weak-disorder regime

Eigenstate multifractality is a distinctive feature of non-interacting disordered metals close to a metal-insulator transition, whose properties are expected to extend to superconductivity. While multifractality in three dimensions (3D) only develops near the critical point for specific strong-disorder strengths, multifractality in 2D systems is expected to be observable even for weak disorder. Here we provide evidence for multifractal features in the superconducting state of an intrinsic weakly disordered single-layer NbSe$_2$ by means of low-temperature scanning tunneling microscopy/spectroscopy. The superconducting gap, characterized by its width, depth and coherence peaks' amplitude, shows a characteristic spatial modulation coincident with the periodicity of the quasiparticle interference pattern. Spatial inhomogeneity of the superconducting gap width, proportional to the local order parameter in the weak-disorder regime, follows a log-normal statistical distribution as well as a power-law decay of the two-point correlation function, in agreement with our theoretical model. Furthermore, the experimental singularity spectrum f($α$) shows anomalous scaling behavior typical from 2D weakly disordered systems.

cond-mat.supr-con↗

arXiv2020

Identifying substitutional oxygen as a prolific point defect in monolayer transition metal dichalcogenides with experiment and theory

Chalcogen vacancies are considered to be the most abundant point defects in two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors, and predicted to result in deep in-gap states (IGS). As a result, important features in the optical response of 2D-TMDs have typically been attributed to chalcogen vacancies, with indirect support from Transmission Electron Microscopy (TEM) and Scanning Tunneling Microscopy (STM) images. However, TEM imaging measurements do not provide direct access to the electronic structure of individual defects; and while Scanning Tunneling Spectroscopy (STS) is a direct probe of local electronic structure, the interpretation of the chemical nature of atomically-resolved STM images of point defects in 2D-TMDs can be ambiguous. As a result, the assignment of point defects as vacancies or substitutional atoms of different kinds in 2D-TMDs, and their influence on their electronic properties, has been inconsistent and lacks consensus. Here, we combine low-temperature non-contact atomic force microscopy (nc-AFM), STS, and state-of-the-art ab initio density functional theory (DFT) and GW calculations to determine both the structure and electronic properties of the most abundant individual chalcogen-site defects common to 2D-TMDs. Surprisingly, we observe no IGS for any of the chalcogen defects probed. Our results and analysis strongly suggest that the common chalcogen defects in our 2D-TMDs, prepared and measured in standard environments, are substitutional oxygen rather than vacancies.

cond-mat.mtrl-sci↗

arXiv2020

Electronic bandstructure of in-plane ferroelectric van der Waals $β'-In_{2}Se_{3}$

Layered indium selenides ($In_{2}Se_{3}$) have recently been discovered to host robust out-of-plane and in-plane ferroelectricity in the $α$ and $β$' phases, respectively. In this work, we utilise angle-resolved photoelectron spectroscopy to directly measure the electronic bandstructure of $β'-In_{2}Se_{3}$, and compare to hybrid density functional theory (DFT) calculations. In agreement with DFT, we find the band structure is highly two-dimensional, with negligible dispersion along the c-axis. Due to n-type doping we are able to observe the conduction band minima, and directly measure the minimum indirect (0.97 eV) and direct (1.46 eV) bandgaps. We find the Fermi surface in the conduction band is characterized by anisotropic electron pockets with sharp in-plane dispersion about the $\overline{M}$ points, yielding effective masses of 0.21 $m_{0}$ along $\overline{KM}$ and 0.33 $m_{0}$ along $\overline{ΓM}$. The measured band structure is well supported by hybrid density functional theory calculations. The highly two-dimensional (2D) bandstructure with moderate bandgap and small effective mass suggest that $β'-In_{2}Se_{3}$ is a potentially useful new van der Waals semiconductor. This together with its ferroelectricity makes it a viable material for high-mobility ferroelectric-photovoltaic devices, with applications in non-volatile memory switching and renewable energy technologies.

cond-mat.mtrl-sci↗

arXiv2019

Visualizing Exotic Orbital Texture in the Single-Layer Mott Insulator 1T-TaSe2

Mott insulating behavior is induced by strong electron correlation and can lead to exotic states of matter such as unconventional superconductivity and quantum spin liquids. Recent advances in van der Waals material synthesis enable the exploration of novel Mott systems in the two-dimensional limit. Here we report characterization of the local electronic properties of single- and few-layer 1T-TaSe2 via spatial- and momentum-resolved spectroscopy involving scanning tunneling microscopy and angle-resolved photoemission. Our combined experimental and theoretical study indicates that electron correlation induces a robust Mott insulator state in single-layer 1T-TaSe2 that is accompanied by novel orbital texture. Inclusion of interlayer coupling weakens the insulating phase in 1T-TaSe2, as seen by strong reduction of its energy gap and quenching of its correlation-driven orbital texture in bilayer and trilayer 1T-TaSe2. Our results establish single-layer 1T-TaSe2 as a useful new platform for investigating strong correlation physics in two dimensions.

cond-mat.str-el↗

arXiv2019

Electric Field-Tuned Topological Phase Transition in Ultra-Thin Na3Bi - Towards a Topological Transistor

The electric field induced quantum phase transition from topological to conventional insulator has been proposed as the basis of a topological field effect transistor [1-4]. In this scheme an electric field can switch 'on' the ballistic flow of charge and spin along dissipationless edges of the two-dimensional (2D) quantum spin Hall insulator [5-9], and when 'off' is a conventional insulator with no conductive channels. Such as topological transistor is promising for low-energy logic circuits [4], which would necessitate electric field-switched materials with conventional and topological bandgaps much greater than room temperature, significantly greater than proposed to date [6-8]. Topological Dirac semimetals(TDS) are promising systems in which to look for topological field-effect switching, as they lie at the boundary between conventional and topological phases [3,10-16]. Here we use scanning probe microscopy/spectroscopy (STM/STS) and angle-resolved photoelectron spectroscopy (ARPES) to show that mono- and bilayer films of TDS Na3Bi [3,17] are 2D topological insulators with bulk bandgaps >400 meV in the absence of electric field. Upon application of electric field by doping with potassium or by close approach of the STM tip, the bandgap can be completely closed then re-opened with conventional gap greater than 100 meV. The large bandgaps in both the conventional and quantum spin Hall phases, much greater than the thermal energy kT = 25 meV at room temperature, suggest that ultrathin Na3Bi is suitable for room temperature topological transistor operation.

cond-mat.mtrl-sci↗