arXiv subjects

Chang-Jong Kang

Publications and source records attributed to Chang-Jong Kang.

At least 37 records · Page 2Linked to original sources

arXiv2022

An optical investigation of the heavy fermion normal state in superconducting UTe$_2$

The recently discovered superconductor, UTe$_2$, has attracted immense scientific interest due to the experimental observations that suggest odd-parity superconductivity. It is believed that the material becomes a heavy-fermion metal at low temperatures although details of the normal state are unclear. Using Fourier transform infrared spectroscopy (FTIR), the normal state electronic structure of UTe$_2$ was investigated at zero applied magnetic field. Combining the measured reflectivity with the dc resistivity, the complex optical conductivity was obtained over a large frequency range. The frequency dependence of the real part of the optical conductivity exhibits a MIR peak around 4000 cm$^{-1}$ and a narrow Drude peak that develops below 40 K. A combination of density functional and dynamic mean field theory (DFT + DMFT) gives spectra in close correspondence to the experiment. Via this comparison we attribute the prominent MIR peak to inter-band transitions involving a narrow U 5$f$ feature that develops near the Fermi level. In this regard, our data gives spectroscopic evidence for the existence of a low energy Kondo resonance at temperatures just above the onset of superconductivity and implicates heavy electrons in the formation of the superconducting state. We find that the coherent Kondo resonance is primarily associated with a collapse of scattering and less with a transfer of spectral weight.

cond-mat.supr-con↗

arXiv2021

Optical properties of the infinite-layer La$_{1-x}$Sr$_{x}$NiO$_{2}$ and hidden Hund's physics

We investigate the optical properties of the normal state of the infinite-layer La$_{1-x}$Sr$_x$NiO$_2$ using DFT+DMFT. We find a correlated metal which exhibits substantial transfer of spectral weight to high energies relative to the density functional theory. The correlations are not due to Mott physics, which would suppress the charge fluctuations and integrated optical spectral weight as we approach a putative insulating state. Instead we find the unusual situation, that the integrated optical spectral weight {\it decreases} with doping and {\it increases } with increasing temperature. We contrast this with the coherent component of the optical conductivity, which {\it decreases} with increasing temperature as a result of a coherence$-$incoherence crossover. Our optical studies support a picture of a Hund's metallic state, where dynamical orbital fluctuations are visible at intermediate energies, even if at low energies the Fermi surface has primarily $d_{x^2 - y^2}$ character and we propose a low-energy two-band model with atom centered $e_g$ states.

cond-mat.str-el↗

arXiv2020

Vacancy defect control of colossal thermopower in FeSb2

Iron diantimonide is a material with the highest known thermoelectric power. By combining scanning transmission electron microscope study with electronic transport neutron, X-ray scattering and first principle calculation we identify atomic defects that control colossal thermopower magnitude and nanoprecipitate clusters with Sb vacancy ordering which induce additional phonon scattering and substantially reduce thermal conductivity. Defects are found to cause rather weak but important monoclinic distortion of the unit cell Pnnm to Pm. The absence of Sb along [010] for high defect concentration forms conducting path due to Fe d orbital overlap. The connection between atomic defect anisotropy and colossal thermopower in FeSb2 paves the way for the understanding and tailoring of giant thermopower in related materials.

cond-mat.mtrl-sci↗

arXiv2020

Ultrafast triggering of insulator-metal transition in two-dimensional VSe$_2$

Assembling transition metal dichalcogenides (TMDCs) at the two-dimensional (2D) limit is a promising approach for tailoring emerging states of matter such as superconductivity or charge density waves (CDWs). Single-layer (SL) VSe$_2$ stands out in this regard because it exhibits a strongly enhanced CDW transition with a higher transition temperature compared to the bulk in addition to an insulating phase with an anisotropic gap at the Fermi level, causing a suppression of anticipated 2D ferromagnetism in the material. Here, we investigate the interplay of electronic and lattice degrees of freedom that underpin these electronic phases in SL VSe$_2$ using ultrafast pump-probe photoemission spectroscopy. In the insulating state, we observe a light-induced closure of the energy gap on a timescale of 480 fs, which we disentangle from the ensuing hot carrier dynamics. Our work thereby reveals that the phase transition in SL VSe$_2$ is driven by electron-lattice coupling and demonstrates the potential for controlling electronic phases in 2D materials with light.

cond-mat.mtrl-sci↗

arXiv2020

Pressure-induced inverse order-disorder transition in double perovskites

Given the consensus that pressure improves cation order in most of known materials, a discovery of pressure-induced disorder could require reconsideration of order-disorder transition in solid state physics/chemistry and geophysics. Double perovskites Y2CoIrO6 and Y2CoRuO6 synthesized at ambient pressure show B-site order, while the polymorphs synthesized at 6 and 15 GPa are partially-ordered and disordered respectively. With the decrease of ordering degrees, the lattices are shrunken and the crystal structures alter from monoclinic to orthorhombic symmetry. Correspondingly, long-range ferrimagnetic order in the B-site ordered phases are gradually overwhelmed by B-site disorder. Theoretical calculations suggest that unusual unit cell compressions under external pressures unexpectedly stabilize the disordered phases of Y2CoIrO6 and Y2CoRuO6.

cond-mat.mtrl-sci↗

arXiv2019

Low energy band structure and symmetries of UTe2 from angle resolved photoemission spectroscopy

The compound UTe2 has recently been shown to realize spin triplet superconductivity from a non-magnetic normal state. This has sparked intense research activity, including theoretical analyses that suggest the superconducting order parameter to be topologically nontrivial. However, the underlying electronic band structure is a critical factor for these analyses, and remains poorly understood. Here, we present high resolution angle resolved photoemission (ARPES) measurements covering multiple planes in the 3D Brillouin zone of UTe2, revealing distinct Fermi-level features from two orthogonal quasi-one dimensional light electron bands and one heavy band. The electronic symmetries are evaluated in comparison with numerical simulations, and the resulting picture is discussed as a platform for unconventional many-body order.

cond-mat.str-el↗

arXiv2019

Orbital-selective Kondo-lattice and enigmatic f-electrons emerging from inside the antiferromagnetic phase of a heavy fermion

Novel electronic phenomena frequently form in heavy fermions as a consequence of the mutual nature of localization and itineracy of f electrons. On the magnetically ordered side of the heavy fermion phase diagram, f moments are expected to be localized and decoupled from the Fermi surface. It remains ambiguous whether a Kondo lattice can develop inside the magnetically ordered phase. Using spectroscopic imaging with the scanning tunneling microscope, complemented by neutron scattering, x ray absorption spectroscopy, and dynamical mean field theory, we probe the electronic states in the antiferromagnetic USb2 as a function of temperature. We visualize a large gap in the antiferromagnetic phase at high temperatures, T lower than TN 200 K, within which Kondo hybridization gradually develops below Tcoh 80 K. Our dynamical mean field theory calculations indicate the antiferromagnetism and Kondo lattice to reside predominantly on different f orbitals, promoting orbital selectivity as a new conception into how these two phenomena coexist in heavy fermions. Finally, at Tstar 45 K we discover a novel 1st order like electronic transition through the abrupt emergence of non trivial 5f electronic states that may share some resemblance to the hidden order phase of URu2Si2.

cond-mat.str-el↗

arXiv2019

Multiple topological Dirac cones in a mixed-valent Kondo semimetal: g-SmS

We demonstrate theoretically that the golden phase of SmS ($g$-SmS), a correlated mixed-valent system, exhibits nontrivial surface states with diverse topology. It turns out that this material is an ideal playground to investigate different band topologies in different surface terminations. We have explored surface states on three different (001), (111), and (110) surface terminations. Topological signature in the (001) surface is not apparent due to a hidden Dirac cone inside the bulk-projected bands. In contrast, the (111) surface shows a clear gapless Dirac cone in the gap region, demonstrating the unambiguous topological Kondo nature of $g$-SmS. Most interestingly, the (110) surface exhibits both topological-insulator-type and topological-crystalline-insulator (TCI)-type surface states simultaneously. Two different types of double Dirac cones, Rashba-type and TCI-type, realized on the (001) and (110) surfaces, respectively, are analyzed with the mirror eigenvalues and mirror Chern numbers obtained from the model-independent \emph{ab initio} band calculations.

cond-mat.str-el↗

arXiv2019

High temperature singlet-based magnetism from Hund's rule correlations

Uranium compounds can manifest a wide range of fascinating many-body phenomena, and are often thought to be poised at a crossover between localized and itinerant regimes for 5f electrons. The antiferromagnetic dipnictide USb2 has been of recent interest due to the discovery of rich proximate phase diagrams and unusual quantum coherence phenomena. Here, linear-dichroic X-ray absorption and elastic neutron scattering are used to characterize electronic symmetries on uranium in USb2 and isostructural UBi2. Of these two materials, only USb2 is found to enable strong Hund's rule alignment of local magnetic degrees of freedom, and to undergo distinctive changes in local atomic multiplet symmetry across the magnetic phase transition. Theoretical analysis reveals that these and other anomalous properties of the material may be understood by attributing it as the first known high temperature realization of a singlet ground state magnet, in which magnetism occurs through a process that resembles exciton condensation.

cond-mat.str-el↗

arXiv2019

Topological Phase Transition in an Archetypal f-electron Correlated System: Ce

A typical f-electron Kondo lattice system Ce exhibits the well-known isostructural transition, the so-called gamma-alpha transition, accompanied by an enormous volume collapse. Most interestingly, we have discovered that a topological-phase transition also takes place in elemental Ce, concurrently with the gamma-alpha transition. Based on the dynamical mean-field theory approach combined with density functional theory, we have unravelled that the non-trivial topology in alpha-Ce is driven by the f-d band inversion, which arises from the formation of coherent 4f band around the Fermi level. We captured the formation of the 4f quasi-particle band that is responsible for the Lifshitz transition and the non-trivial Z2 topology establishment across the phase boundary. This discovery provides a concept of 'topology switch' for topological Kondo systems. The 'on' and 'off' switching knob in Ce is versatile in a sense that it is controlled by available pressure (around 1 GPa) at room temperature.

cond-mat.str-el↗

arXiv2019

Correlated materials design: prospects and challenges

The design of correlated materials challenges researchers to combine the maturing, high throughput framework of DFT-based materials design with the rapidly-developing first-principles theory for correlated electron systems. We review the field of correlated materials, distinguishing two broad classes of correlation effects, static and dynamics, and describe methodologies to take them into account. We introduce a material design workflow, and illustrate it via examples in several materials classes, including superconductors, charge ordering materials and systems near an electronically driven metal to insulator transition, highlighting the interplay between theory and experiment with a view towards finding new materials. We review the statistical formulation of the errors of currently available methods to estimate formation energies. Correlation effects have to be considered in all the material design steps. These include bridging between structure and property, obtaining the correct structure and predicting material stability. We introduce a post-processing strategy to take them into account.

cond-mat.str-el↗

arXiv2019

Material design of indium based compounds: possible candidates for charge, valence, and bond disproportionation and superconductivity

We design and investigate the physical properties of new indium compounds AInX$_{3}$ (A = alkali metals, X = F or Cl). We find nine new In based materials in their ground state and are thermodynamically stable but are not reported in ICSD (Inorganic Crystal Structure Database). We also discuss several metastable structures. This new series of materials display multiple valences, charge and bond disproportionation, and dimerization. The most common valence of In is 3+. We also find two rare alternatives, one has In$^{2+}$ with In-In dimerization and the other shows valence disproportionation to In$^{1+}$ and In$^{3+}$ with bond disproportionation. We study the possibility of superconductivity in these new In compounds and find that CsInF$_{3}$ has a transition temperature about 24 K with sufficient hole doping and pressure.

cond-mat.mtrl-sci↗

arXiv2018

Strong interband interaction in the excitonic insulator phase of Ta$_2$NiSe$_5$

Excitonic insulator (EI) was proposed in 60's as a distinct insulating state originating from pure electronic interaction, but its material realization has been elusive with extremely few material candidates and with only limited evidence such as anomalies in transport properties, band dispersions, or optical transitions. We investigate the real-space electronic states of the low temperature phase in Ta$_2$NiSe$_5$ with an atomic resolution to clearly identify the quasiparticle energy gap together with the strong electron-hole band renormalization using scanning tunneling microscopy (STM) and spectroscopy (STS). These results are in good agreement with the EI transition scenario in Ta$_2$NiSe$_5$. Our spatially-resolved STS data and theoretical calculations reveal further the orbital inversion at band edges, which indicates the exciton condensation close to the Bardeen-Cooper-Schrieffer regime.

cond-mat.mtrl-sci↗

arXiv2018

Polaronic transport and thermoelectricity in Fe$_{1-x}$Co$_x$Sb$_2$S$_4$ ($x$ = 0, 0.1, and 0.2)

We report a study of Co-doped berthierite Fe$_{1-x}$Co$_x$Sb$_2$S$_4$ ($x$ = 0, 0.1, and 0.2). The alloy series of Fe$_{1-x}$Co$_x$Sb$_2$S$_4$ crystallize in an orthorhombic structure with the Pnma space group, similar to FeSb$_2$, and show semiconducting behavior. The large discrepancy between activation energy for conductivity, $E_ρ$ (146 $\sim$ 270 meV), and thermopower, $E_S$ (47 $\sim$ 108 meV), indicates the polaronic transport mechanism. Bulk magnetization and heat capacity measurements of pure FeSb$_2$S$_4$ ($x$ = 0) exhibit a broad antiferromagnetic (AFM) transition ($T_N$ = 46 K) followed by an additional weak transition ($T^*$ = 50 K). Transition temperatures ($T_N$ and $T^*$) slightly decrease with increasing Co content $x$. This is also reflected in the thermal conductivity measurement, indicating strong spin-lattice coupling. Fe$_{1-x}$Co$_x$Sb$_2$S$_4$ shows relatively high value of thermopower (up to $\sim$ 624 $μ$V K$^{-1}$ at 300 K) and thermal conductivity much lower when compared to FeSb$_{2}$, a feature desired for potential applications based on FeSb$_{2}$ materials.

cond-mat.str-el↗

arXiv2018

Study for material analogs of FeSb$_{2}$: material design for thermoelectric materials

Using the \emph{ab initio} evolutionary algorithm (implemented in USPEX) and electronic structure calculations we investigate the properties of a new thermoelectric material FeSbAs, which is a material analog of the enigmatic thermoelectric FeSb$_{2}$. We utilize the density functional theory and the Gutzwiller method to check the energetics. We find that FeSbAs can be made thermodynamically stable above $\sim$30 GPa. We investigate the electronic structure and thermoelectric properties of FeSbAs based on the density functional theory and compare with those of FeSb$_{2}$. Above 50 K, FeSbAs has higher Seebeck coefficients than FeSb$_{2}$. Upon doping, the figure of merit becomes larger for FeSbAs than for FeSb$_{2}$. Another material analog FeSbP, was also investigated, and found thermodynamically unstable even at very high pressure. Regarding FeSb$_{2}$ as a member of a family of compounds (FeSb$_{2}$, FeSbAs, and FeSbP) we elucidate what are the chemical handles that control the gaps in this series. We also investigate solubility (As or P for Sb in FeSb$_{2}$) we found As to be more soluble. Finally, we study a two-band model for thermoelectric properties and find that the temperature dependent chemical potential and the presence of the ionized impurities are important to explain the extremum in the Seebeck coefficient exhibited in experiments for FeSb$_{2}$.

cond-mat.mtrl-sci↗

arXiv2017

Temperature-dependent evolutions of excitonic superfluid plasma frequency in a srong excitonic insulator candidate, Ta$_2$NiSe$_5$

We investigate an interesting anisotropic van der Waals material, Ta$_{2}$NiSe$_{5}$, using optical spectroscopy. Ta$_{2}$NiSe$_{5}$ has been known as one of the few excitonic insulators proposed over 50 years ago. Ta$_{2}$NiSe$_{5}$ has quasi-one dimensional chains along the $a$-axis. We have obtained anisotropic optical properties of a single crystal Ta$_{2}$NiSe$_{5}$ along the $a$- and $c$-axes. The measured $a$- and $c$-axis optical conductivities exhibit large anisotropic electronic and phononic properties. With regard to the $a$-axis optical conductivity, a sharp peak near 3050 cm$^{-1}$ at 9 K, with a well-defined optical gap ($Δ^{EI} \simeq$ 1800 cm$^{-1}$) and a strong temperature-dependence, is observed. With an increase in temperature, this peak broadens and the optical energy gap closes around $\sim$325 K($T_c^{EI}$). The spectral weight redistribution with respect to the frequency and temperature indicates that the normalized optical energy gap ($Δ^{EI}(T)/Δ^{EI}(0)$) is $1-(T/T_c^{EI})^2$. The temperature-dependent superfluid plasma frequency of the excitonic condensation in Ta$_{2}$NiSe$_{5}$ has been determined from measured optical data. Our findings may be useful for future research on excitonic insulators.

cond-mat.mtrl-sci↗

arXiv2016

Phase stability and large in-plane resistivity in the 112-type iron-based superconductor Ca$_{1-x}$La$_{x}$FeAs$_{2}$

The recently discovered high-T$_c$ superconductor Ca$_{1-x}$La$_{x}$FeAs$_{2}$ is a unique compound not only because of its low symmetry crystal structure, but also because of its electronic structure which hosts Dirac-like metallic bands resulting from (spacer) zig-zag As chains. We present a comprehensive first principles theoretical study of the electronic and crystal structures of Ca$_{1-x}$La$_{x}$FeAs$_{2}$. After discussing the connection between the crystal structure of the 112 family, which Ca$_{1-x}$La$_{x}$FeAs$_{2}$ is a member of, with the other known structures of Fe pnictide superconductors, we check the thermodynamic phase stability of CaFeAs$_{2}$, and similar hyphothetical compounds SrFeAs$_{2}$ and BaFeAs$_{2}$ which, we find, are slightly higher in energy. We calculate the optical conductivity of Ca$_{1-x}$La$_{x}$FeAs$_{2}$ using the DFT + DMFT method, and predict a large in-plane resistivity anisotropy in the normal phase, which does not originate from electronic nematicity, but is enhanced by the electronic correlations. In particular, we predict a 0.34 eV peak in the $yy$ component of the optical conductivity of the 30\% La doped compound, which correponds to coherent interband transitions within a fast-dispersing band arising from the zig-zag As-chains which are unique to this compound. We also study the Landau free energy for Ca$_{1-x}$La$_{x}$FeAs$_{2}$ including the order parameter relevant for the nematic transition and find that the free energy does not have any extra terms that could induce ferro-orbital order. This explains why the presence of As chains does not broaden the nematic transition in Ca$_{1-x}$La$_{x}$FeAs$_{2}$.

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↗