arXiv subjects

Seoung-Hun Kang

Publications and source records attributed to Seoung-Hun Kang.

35 records · Page 2Linked to original sources

arXiv2023

Theoretical investigation of delafossite-Cu2ZnSnO4 as a promising photovoltaic absorber

In the quest for efficient and cost-effective photovoltaic absorber materials beyond silicon, considerable attention has been directed toward exploring alternatives. One such material, zincblende-derived Cu2ZnSnS4 (CZTS), has shown promise due to its ideal band-gap size and high absorption coefficient. However, challenges such as structural defects and secondary phase formation have hindered its development. In this study, we examine the potential of another compound Cu2ZnSnO4 (CZTO) with a similar composition to CZTS as a promising alternative. Employing ab initio density function theory (DFT) calculations in combination with an evolutionary structure prediction algorithm, we identify that the crystalline phase of the delafossite structure is the most stable among the 900 (meta)stable CZTO. Its thermodynamic stability at room temperature is also confirmed by the molecular dynamics study. Excitingly, this new phase of CZTO displays a direct band gap where the dipole-allowed transition occurs, making it a strong candidate for efficient light absorption. Furthermore, the estimation of spectroscopic limited maximum efficiency (SLME) directly demonstrates the high potential of delafossite-CZTO as a photovoltaic absorber. Our numerical results suggest that delafossite-CZTO holds another promise for future photovoltaic applications.

cond-mat.mtrl-sci↗

arXiv2023

Universal sublinear resistivity in vanadium kagome materials hosting charge density waves

The recent discovery of a charge density (CDW) state in ScV$_6$Sn$_6$ at $T_{\textrm{CDW}}$ = 91 K offers new opportunities to understand the origins of electronic instabilities in topological kagome systems. By comparing to the isostructural non-CDW compound LuV$_6$Sn$_6$, we unravel interesting electrical transport properties in ScV$_6$Sn$_6$, above and below the charge ordering temperature. We observed that by applying a magnetic field along the $a$ axis, the temperature behavior of the longitudinal resistivity in ScV$_6$Sn$_6$ changes from metal-like to insulator-like above the CDW transition. We show that in the charge ordered state ScV$_6$Sn$_6$ follows the Fermi liquid behavior while above that, it transforms into a non-Fermi liquid phase in which the resistivity varies sublinearly over a broad temperature range. The sublinear resistivity, which scales by $T^{3/5}$ is a common feature among other vanadium-containing kagome compounds exhibiting CDW states such as KV$_3$Sb$_5$, RbV$_3$Sb$_5$, and CsV$_3$Sb$_5$. By contrast, the non-Fermi liquid behavior does not occur in LuV$_6$Sn$_6$. We explain the $T^{3/5}$ universal scaling behavior from the Coulomb scattering between Dirac electrons and Van Hove singularities; common features in the electronic structure of kagome materials. Finally, we show anomalous Hall-like behavior in ScV$_6$Sn$_6$ below $T_{\textrm{CDW}}$, which is absent in the Lu compound. Comparing the transport properties of ScV$_6$Sn$_6$ and LuV$_6$Sn$_6$ is valuable to highlight the impacts of the unusual CDW in the Sc compound.

cond-mat.str-el↗

arXiv2023

Metastability and topology in the magnetic topological insulator MnBi$_{2}$Te$_{4}$

We study the effect of stacking faults on the topological properties of the magnetic topological insulator MnBi$_{2}$Te$_{4}$ (MBT) using density functional theory calculations and the Hubbard $U$ being tuned with many-body diffusion Monte Carlo techniques. We show that a modest deviation from the equilibrium interlayer distance leads to a topological phase transition from a non-trivial to a trivial topology, suggesting that tuning the interlayer coupling by adjusting the interlayer distance alone can lead to different topological phases. Interestingly, due to the locally increased interlayer distance of the top layer, a metastable stacking fault in MBT leads to a nearly gapless state at the topmost layer due to charge redistribution as the topmost layer recedes. We further find evidence of spin-momentum locking in the surface state along with a weak preservation of the band inversion in the near gapless state, which is indicative of the non-trivial topological surface states for the metastable stacking fault. Our findings provide a possible explanation for reconciling the long-standing puzzle of gapped and gapless states on MBT surfaces.

cond-mat.mtrl-sci↗

arXiv2023

Procedures for assessing the stability of proposed topological materials

We investigate the stability of MnPb$_{2}$Bi$_{2}$Te$_{6}$ (MPBT), which is predicted to be a magnetic topological insulator (TI), using density functional theory calculations. Our analysis includes various measures such as enthalpies of formation, Helmholtz free energies, defect formation energies, and dynamical stability. Our thermodynamic analysis shows that the phonon contribution to the energy gain from finite temperature is estimated to be less than 10~meV/atom, which may not be sufficient to stabilize MPBT at high temperatures, even with the most favorable reactions starting from binaries. While MPBT is generally robust against the formation of various defects, we find that anti-site defect formation of $\text{Mn}_{\text{Pb}}$ is the most likely to occur, with corresponding energy less than 60~meV. This can be attributed to the significant energy cost from compressive strain at the PbTe layer. Our findings suggest that MPBT is on the brink of stability in terms of thermodynamics and defect formation, underscoring the importance of conducting systematic analyses of the stability of proposed TIs, including MPBT, for their practical utilization. This study offers valuable insights into the design and synthesis of desirable magnetic TI materials with robust stabilities.

cond-mat.mtrl-sci↗

arXiv2023

A New Two-Dimensional Dirac Semimetal Based on the Alkaline Earth Metal, CaP$_3$

Using an evolutionary algorithm in combination with first-principles density functional theory calculations, we identify two-dimensional (2D) CaP$_3$ monolayer as a new Dirac semimetal due to inversion and nonsymmorphic spatial symmetries of the structure. This new topological material, composed of light elements, exhibits high structural stability (higher than the phase known in the literature), which is confirmed by thermodynamic and kinetic stability analysis. Moreover, it satisfies the electron filling criteria, so that its Dirac state is located near the Fermi level. The existence of the Dirac state predicted by the theoretical symmetry analysis is also confirmed by first-principles electronic band structure calculations. We find that the energy position of the Dirac state can be tuned by strain, while the Dirac state is unstable against an external electric field since it breaks the spatial inversion symmetry. Our findings should be instrumental in the development of 2D Dirac fermions based on light elements for their application in nanoelectronic devices and topological electronics.

cond-mat.mtrl-sci↗

arXiv2023

Emergence of a new band and the Lifshitz transition in kagome metal ScV$_6$Sn$_6$ with charge density wave

Topological kagome systems have been a topic of great interest in condensed matter physics due totheir unique electronic properties. The vanadium-based kagome materials are particularly intrigu-ing since they exhibit exotic phenomena such as charge density wave (CDW) and unconventionalsuperconductivity. The origin of these electronic instabilities is not fully understood, and the re-cent discovery of a charge density wave in ScV6Sn6provides a new avenue for investigation. In thiswork, we investigate the electronic structure of the novel kagome metal ScV6Sn6using angle resolvedphotoemission spectroscopy (ARPES), scanning tunneling microscopy (STM), and first-principlesdensity functional theory calculations. Our analysis reveals for the first time the temperature-dependent band changes of ScV6Sn6and identifies a new band that exhibits a strong signatureof a structure with CDW below the critical temperature. Further analysis revealed that this newband is due to the surface kagome layer of the CDW structure. In addition, a Lifshitz transition isidentified in the ARPES spectra that is related to the saddle point moving across the Fermi levelat the critical temperature for the CDW formation. This result shows the CDW behavior may alsobe related to nesting of the saddle point, similar to related materials. However, no energy gap is observed at the Fermi level and thus the CDW is not a typical Fermi surface nesting scenario. These results provide new insights into the underlying physics of the CDW in the kagome materials and could have implications for the development of materials with new functionality.

cond-mat.str-el↗

arXiv2021

Reshaped Weyl fermionic dispersions driven by Coulomb interactions in MoTe2

We report the direct evidence of impacts of the Coulomb interaction in a prototypical Weyl semimetal, MoTe2, that alter its bare bands in a wide range of energy and momentum. Our quasiparticle interference patterns measured using scanning tunneling microscopy are shown to match the joint density of states of quasiparticle energy bands including momentum-dependent self-energy corrections, while electronic energy bands based on the other simpler local approximations of the Coulomb interaction fail to explain neither the correct number of quasiparticle pockets nor shape of their dispersions observed in our spectrum. With this, we predict a transition between type-I and type-II Weyl fermions with doping and resolve its disparate quantum oscillation experiments, thus highlighting the critical roles of Coulomb interactions in layered Weyl semimetals.

cond-mat.mes-hall↗

arXiv2019

Symmetry-Protected Degeneracies in the Electronic Band Structure of Oxidized Black Phosphorous

We explore the oxidation of a single layer of black phosphorous using ab initio density functional theory calculation. We search for the equilibrium structures of phosphorene oxides, PO$_x$ with various oxygen concentrations $x$ ($0{\le}x{\le}1$). By evaluating the formation energies with diverse configurations and their vibrational properties for each of various $x$ values, we identify a series of stable oxidized structures with $x$ and confirm that the oxidation occurs naturally. We also find that oxidation makes some modes from the P-O bonds P-P bonds IR-active implying that the infrared spectra can be used to determine the degree of oxidation of phosphorene. Our electronic structure calculation reveals that the fully oxidized phosphorene (PO) has a direct band gap of 0.83 eV similar to the pristine phosphorene. Intriguingly, the PO possesses two nonsymmorphic symmetries with the inversion symmetry broken, guaranteeing a symmetry-protected band structures including the band degeneracy and four-fold degenerate Dirac points. Our results may provide a significant insight into the intriguing relations between symmetry of lattice and band topology of electrons.

cond-mat.mtrl-sci↗

arXiv2019

Symmetry dictated grain boundary state in a two-dimensional topological insulator

Structural imperfections such as grain boundaries (GBs) and dislocations are ubiquitous in solids and have been of central importance in understanding nature of polycrystals. In addition to their classical roles, advent of topological insulators (TIs) offers a chance to realize distinct topological states bound to them. Although dislocation inside three-dimensional TIs is one of the prime candidates to look for, its direct detection and characterization are challenging. Instead, in two-dimensional (2D) TIs, their creations and measurements are easier and, moreover, topological states at the GBs or dislocations intimately connect to their lattice symmetry. However, such roles of crystalline symmetries of GBs in 2D TIs have not been clearly measured yet. Here, we present the first direct evidence of a symmetry enforced Dirac type metallic state along a GB in 1T'-MoTe$_2$, a prototypical 2D TI. Using scanning tunneling microscope, we show a metallic state along a grain boundary with non-symmorphic lattice symmetry and its absence along the other boundary with symmorphic one. Our large scale atomistic simulations demonstrate hourglass like nodal-line semimetallic in-gap states for the former while the gap-opening for the latter, explaining our observation very well. The protected metallic state tightly linked to its crystal symmetry demonstrated here can be used to create stable metallic nanowire inside an insulator.

cond-mat.mes-hall↗

arXiv2019

Noncollinear magnetic sampling method for paramagnetic Mott insulator MnO

We present a new approach based on the static density functional theory (DFT) to describe paramagentic MnO, which is a representative paramagnetic Mott insulator. We appended the spin noncollinearity and the canonical ensemble to the magnetic sampling method (MSM), which is one of the supercell approaches based on disordered local moment model. The combination of the noncollinear MSM (NCMSM) with DFT$+U$ represents a highly favorable computational method called NCMSM$+U$ to accurately determine the paramagnetic properties of MnO with moderate numerical cost. The effects of electron correlations and spin noncollinearity on the properties of MnO were also investigated. We revealed that the spin noncollinearity plays an important role in determining the detailed electronic profile and precise energetics of paramagnetic MnO. Our results illustrate that the NCMSM$+U$ approach may be used as an alternative to the $\textit{ab initio}$ framework of dynamic mean field theory based on DFT in the simulation of the high-temperature properties of Mott insulators.

cond-mat.str-el↗

arXiv2019

Strain Effects on Phase Transitions in Transition Metal Dichalcogenides

We perform density functional theory calculation to investigate the structural and electronic properties of various two-dimensional transition metal dichalcogenides, MX$_2$ (M$=$Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, or W, and X$=$S or Se), and their strain-induced phase transitions. We evaluate the relative stability and the activation barrier between the octahedral-$T$ and the trigonal-$H$ phases of each MX$_2$. It is found that the equilibrium and phase transition characteristics of MX$_2$ can be classified by the group to which its metal element M belongs in the periodic table. MX$_2$ with M in the group 4 (Ti, Zr, or Hf), forms an octahedral-$T$ phase, while that with an M in the group 6 (Cr, Mo, or W) does a trigonal-\textit{H} phase. On the other hand, MX$_2$ with M in the group 5 (V, Nb, Ta), which is in-between the groups 4 and 6, may form either phase with a similar stability. It is also found that their electronic structures are strongly correlated to the structural configurations: mostly metallic in the $T$ phase, while semiconducting in the $H$ phase, although there are some exceptions. We also explore the effects of an applied stress and find for some MX$_2$ materials that the resultant strain, either tensile or compressive, may induce a structural phase transition by reducing the transition energy barrier, which is, in some cases, accompanied by its metal-insulator transition.

cond-mat.mtrl-sci↗

arXiv2019

Low Lattice Thermal Conductivity of a Two-Dimensional Phosphorene Oxide

A fundamental understanding of the phonon transport mechanism is important for optimizing the efficiency of thermoelectric devices. In this study, we investigate the thermal transport properties of the oxidized form of phosphorene called phosphorene oxide (PO) by solving phonon Boltzmann transport equation based on first-principles density functional theory. We reveal that PO exhibits a much lower thermal conductivity (2.42-7.08 W/mK, at 300 K) than its pristine counterpart as well as other two-dimensional materials. To comprehend the physical origin of such low thermal conductivity, we scrutinize the contribution of each phonon branch to the thermal conductivity by evaluating various mode-dependent quantities including Gruneisen parameters, anharmonic threep-honon scattering rate, phase space of three-phonon scattering processes. Our results show that its exible puckered structure of PO leads to smaller sound velocities; its broken-mirror symmetry allows more ZA phonon scattering; and the relatively-free vibration of dangling oxygen atoms in PO gives rise to additional scattering resulting in further reduction in the phonon lifetime. These results can be verified by the fact that PO has larger phase space for three-phonon processes than phosphorene. Furthermore we show that the thermal conductivity of PO can be optimized by controlling its size or its phonon mean free path, indicating that PO can be a promising candidate for low-dimensional thermoelectric devices.

cond-mat.mtrl-sci↗

arXiv2018

Enhanced Thermoelectric Properties in a New Silicon Crystal Si24 with Intrinsic Nanoscale Porous Structure

Thermoelectric device is a promising next-generation energy solution owing to its capability to transform waste heat into useful electric energy, which can be realized in materials with high elec- tric conductivities and low thermal conductivities. A recently synthesized silicon allotrope of Si$_{24}$ features highly anisotropic crystal structure with nanometre-sized regular pores. Here, based on first-principles study without any empirical parameter, we show that the slightly doped Si$_{24}$ can pro- vide an order-of-magnitude enhanced thermoelectric figure of merit at room temperature, compared with the cubic diamond phase of silicon. We ascribe the enhancement to the intrinsic nanostructure formed by the nanopore array, which effectively hinders heat conduction while electric conductivity is maintained. This can be a viable option to enhance the thermoelectric figure of merit without further forming an extrinsic nanostructure. In addition, we propose a practical strategy to further diminish the thermal conductivity without affecting electric conductivity by confining rattling guest atoms in the pores.

cond-mat.mtrl-sci↗

arXiv2017

Origins of the structural phase transitions in MoTe$_2$ and WTe$_2$

Layered transition metal dichalcogenides MoTe$_2$ and WTe$_2$ share almost similar lattice constants as well as topological electronic properties except their structural phase transitions. While the former shows a first-order phase transition between monoclinic and orthorhombic structures, the latter does not. Using a recently proposed van der Waals density functional method, we investigate structural stability of the two materials and uncover that the disparate phase transitions originate from delicate differences between their interlayer bonding states near the Fermi energy. By exploiting the relation between the structural phase transitions and the low energy electronic properties, we show that a charge doping can control the transition substantially, thereby suggesting a way to stabilize or to eliminate their topological electronic energy bands.

cond-mat.mes-hall↗

arXiv2015

Rigid Unit Modes in $sp$-$sp^2$ Hybridized Carbon Systems: Origin of Negative Thermal Expansion

Using density functional theory combined with quasi-harmonic approximation, we investigate the thermal expansion behaviors of three different types (α, β, and γ) of graphyne which is a two-dimensional carbon allotrope composed of sp and $sp^2$ bonds. For each type of graphyne, we obtain the temperature dependent area variation by minimizing its free energy as a function of temperature, which is calculated by considering all the phonon modes in the whole Brillouin zone. We find that all three types of graphyne exhibit negative in-plane thermal expansion up to $T\lesssim1000$ K. The observed in-plane thermal contraction can be attributed partially to the ripple effect, similarly in graphene. The ripple effect itself, however, is not sufficient to explain anomalously larger thermal contraction found in graphyne than in graphene. Our deliberate analysis on the phonon modes observed in graphyne enables us to discover another source causing such thermal expansion anomaly. We find that there are particular phonon modes with frequencies around a few hundreds of cm$^{-1}$ existing exclusively in graphyne that may fill empty spaces resulting in area reduction. These modes are identified as "rigid unit modes" corresponding to the libration of each rigid unit composed of $sp^2$ bonds.

cond-mat.mes-hall↗

arXiv2013

Carbon nanotubes with atomic impurities on boron nitride sheets under applied electric fields

We perform first-principles calculations to investigate the structural and electronic properties of metal-doped (10, 0) carbon nanotubes (CNTs) on a single hexagonal boron nitride (hBN) sheet in the presence of an external electric field. We consider K, Cl and Ni atoms as dopants to study the dependence of the electronic properties of the CNT on doping polarity and concentration. The electric field strength is varied from -0.2 V/Åto +0.2 V/Åto explore the effects of an external electric field on the electronic structures. Although the electronic energy bands of the hBN sheet are modified in accordance with the field strength, its electronic state in the valence or conduction band does not touch the Fermi level under the field strength considered. We conclude that the hBN as a substrate does not modify the electronic structure of the CNT, thereby leading to improvements in the device performance, compared with that of devices based on conventional substrate materials such as SiO$_2$.

cond-mat.mes-hall↗

arXiv2012

Adsorption properties of a nitrogen atom on the anionic golden fullerene Au$_{16}^{-}$

Using density functional theory, we examine a nitrogen-doped anionic golden cage (NAu$_{16}^-$). For the exohedral adsorption that is more stable than the endohedral doping, the bridge and hollow sites have larger binding energies than the atop sites by ~1 eV. When the N atom is adsorbed on the cage, electrons are transferred from Au$_{16}^-$ to the N atom. The transition between the exohedral and endohedral adsorption may occur thermally through a bridge site. In the infrared active vibrational spectra, exohedral doping causes greater intensities at higher frequencies than endohedral doping.

cond-mat.mes-hall↗