arXiv subjects

H. -D. Kim

Publications and source records attributed to H. -D. Kim.

At least 19 recordsLinked to original sources

arXiv2022

Characterization of photoinduced normal state through charge density wave in superconducting YBa$_2$Cu$_3$O$_{6.67}$

The normal state of high-Tc cuprates has been considered one of the essential topics in high-temperature superconductivity research. However, compared to the high magnetic fields study of it, understanding a photoinduced normal state remains elusive. Here, we explore a photoinduced normal state of YBa$_2$Cu$_3$O$_{6.67}$ (YBCO) through a charge density wave (CDW) with time-resolved resonant soft x-ray scattering, as well as a high-magnetic field x-ray scattering. In the non-equilibrium state in which people predict a quenched superconducting state based on the previous optical spectroscopies, we experimentally observed a similar analogy to the competition between superconductivity and CDW shown in the equilibrium state. We further observe that the broken pairing states in the superconducting CuO$_2$ plane via the optical pump lead to nucleation of three-dimensional CDW precursor correlation, revealing that the photoinduced CDW is similar to phenomena shown under magnetic fields. Ultimately, these findings provide a critical clue that the characteristics of the photoinduced normal state show a solid resemblance to those under magnetic fields in equilibrium conditions.

cond-mat.supr-con↗

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↗

arXiv2011

Fermi Surface Reconstruction in CeTe2 Induced by Charge Density Wave: ARPES Study

Electronic structures of a charge-density-wave CDW system CeTe_2-xSb_x (x=0, 0.05) have been investigated by employing angle-resolved photoemission spectroscopy (ARPES). The observed Fermi surface (FS) agrees very well with the calculated FS for the undistorted CeTe_2 both in shapes and sizes. The metallic states crossing the Fermi level(E_F) are observed in ARPES. The carriers near E_F have mainly the Te(1) 5p character, with the negligible contribution from Ce 4f states to the CDW formation. The supercell (shadow) bands and the corresponding very weak FS's are found to arise from band-folding due to the interaction with Ce-Te(2) layers. This work shows that the origin of the CDW formation in CeTe_2 is the FS nesting and that the CDW modulation vector is along Gamma-X (Q_CDW ~ X)

cond-mat.str-el↗

arXiv2011

Reduction of charge fluctuation energies in ultrathin NiO films on Ag(001)

We investigate on-site Coulomb interaction energy between two 3p holes U(Ni 3p) of ultrathin NiO films on Ag(001) by both x-ray photoelectron spectroscopy and Auger electron spectroscopy. As the film becomes thin, U(Ni 3p) monotonically decreases, and the difference of U(Ni 3p) for 1 monolayer (ML) film from that of bulk-like thick film δU(Ni 3p) reaches ~ -2.2 eV. The observed δU(Ni 3p) for 1 ML film is well reproduced by the differences of both the image potential and polarization energies between 1 ML film and the bulk-like thick film. Hence, the present results provide an evidence for the picture originally proposed by Duffy et al. [J. Phys. C: Solid State Phys., 16, 4087 (1983)] and Altieri et al. [Phys. Rev. B 59, R2517 (1999)]

cond-mat.str-el↗

arXiv2011

Metallic characteristics in superlattices composed of insulators, NdMnO3/SrMnO3/LaMnO3

We report on the electronic properties of superlattices composed of three different antiferromagnetic insulators, NdMnO3/SrMnO3/LaMnO3 grown on SrTiO3 substrates. Photoemission spectra obtained by tuning the x-ray energy at the Mn 2p -> 3d edge show a Fermi cut-off, indicating metallic behavior mainly originating from Mn e_g electrons. Furthermore, the density of states near the Fermi energy and the magnetization obey a similar temperature dependence, suggesting a correlation between the spin and charge degrees of freedom at the interfaces of these oxides.

cond-mat.str-el↗

arXiv2009

Crystalline electric field effects in Ce 3$d$ core-level spectra of heavy-fermion systems: Hard X-ray photoemission spectroscopy on CeNi$_{1-x}$Co$_x$Ge$_2$

High-resolution hard X-ray photoemission measurements have been performed to clarify the electronic structure originating from the strong correlation between electrons in bulk Ce 3$d$ core-level spectra of CeNi$_{1-x}$Co$_x$Ge$_2$ (0 $\leq$ $x$ $\leq$ 1). In the Ce 3$d_{5/2}$ core-level spectra, the variation of satellite structures ($f^2$ peaks) shows that the hybridization strength between Ce 4$f$- and conduction electrons gradually increases with Co concentration in good agreement with the results of Ce 3$d-4f$ and 4$d-4f$ resonant photoemission spectroscopies. Particularly, in Ce 3$d_{3/2}$ core-level spectra, the multiplet structures of $f^1$ peaks systematically change with the degeneracy of $f$-states which originates from crystalline electric field effects.

cond-mat.str-el↗

arXiv2008

Spin-orbit interaction effect in the electronic structure of \BiTe \ observed by angle-resolved photoemission spectroscopy

The electronic structure of $p$-type doped \BiTe is studied by angle resolved photoemission spectroscopy (ARPES) to experimentally confirm the mechanism responsible for the high thermoelectric figure of merit. Our ARPES study shows that the band edges are located off the $Γ$-Z line in the Brillouin zone, which provides direct observation that the spin-orbit interaction is a key factor to understand the electronic structure and the corresponding thermoelectric properties of \BiTe. Successive time dependent ARPES measurement also reveals that the electron-like bands crossing E$_F$ near the $\underlineΓ$ point are formed in an hour after cleaving the crystals. We interpret these as surface states induced by surface band bending, possibly due to quintuple inter-layer distance change of \BiTe.

cond-mat.mtrl-sci↗

arXiv2007

Effect of linear density of states on the quasi-particle dynamics and small electron-phonon coupling in graphite

We obtained the spectral function of very high quality natural graphite single crystals using angle resolved photoelectron spectroscopy (ARPES). A clear separation of non-bonding and bonding bands and asymmetric lineshape are observed. The asymmetric lineshapes are well accounted for by the finite photoelectron escape depth and the band structure. The extracted width of the spectral function (inverse of the photohole life time) near the K point is, beyond the maximum phonon energy, approximately proportional to the energy as expected from the linear density of states near the Fermi energy. The upper bound for the electron-phonon coupling constant is about 0.2, a much smaller value than the previously reported one.

cond-mat.str-el↗

arXiv2007

Electron Removal Self Energy and its application to Ca2CuO2Cl2

We propose using the self energy defined for the electron removal Green's function. Starting from the electron removal Green's function, we obtained expressions for the removal self energy Sigma^ER (k,omega) that are applicable for non-quasiparticle photoemission spectral functions from a single band system. Our method does not assume momentum independence and produces the self energy in the full k-omega space. The method is applied to the angle resolved photoemission from Ca_2CuO_2Cl_2 and the result is found to be compatible with the self energy value from the peak width of sharp features. The self energy is found to be only weakly k-dependent. In addition, the Im Sigma shows a maximum at around 1 eV where the high energy kink is located.

cond-mat.supr-con↗

arXiv2006

The case for the bulk nature of the spectroscopic Luttinger liquid signatures observed in angle resolved photoemission of Li0.9Mo6O17

Angle resolved photoemission spectroscopy (ARPES) has been performed on quasi-one dimensional Li0.9Mo6O17 using photon energy 500 eV. Measured band dispersions are in agreement with those from both low photon energy measurements and band structure calculations. The momentum integrated ARPES spectrum is well fit by the finite temperature Luttinger liquid (LL)spectral function, with an anomalous exponent 0.6 that is the same within experimental uncertainty as the value found with photon energy 30 eV. These identical findings at both low and high photon energies are entirely consistent with reasoning based on the crystal structure, that the quasi-one dimensional chains lie two layers below the cleavage plane so that the observed spectroscopic LL behavior of Li0.9Mo6O17 is a bulk property.

cond-mat.str-el↗

arXiv2006

Photoemission study of (V$_{1-x}$M$_x$)$_2$O$_3$ (M=Cr, Ti)

We present high-resolution bulk-sensitive photoemission spectra of (V$_{1-x}$M$_x$)$_2$O$_3$ (M=Cr, Ti). The measurements were made for the paramagnetic metal (PM), paramagnetic insulator (PI), and antiferromagnetic insulator (AFI) phases of (V$_{1-x}$M$_x$)$_2$O$_3$ with the samples of $x$ = 0, 0.012, and 0.028 for Cr-doping and $x$ = 0.01 for Ti-doping. In the PM phase, we observe a prominent quasiparticle peak in general agreement with theory, which combines dynamical mean-field theory with the local density approximation (LDA+DMFT). The quasiparticle peak shows a significantly larger peak width and weight than in the theory. For both the PI and AFI phases, the vanadium 3d parts of the valence spectra are not simple one peak structures. For the PI phase, there is not yet a good theoretical understanding of these structures. The size of the electron removal gap increases, and spectral weight accumulates in the energy range closer to the chemical potential, when the PI to AFI transition occurs. Spectra taken in the same phases with different compositions show interesting monotonic changes as the dopant concentration increases, regardless of the dopant species. With increased Cr-doping, the AFI phase gap decreases and the PI phase gap increases.

cond-mat.str-el↗

arXiv2006

Static vs. dynamical mean field theory of Mott antiferromagnets

Studying the antiferromagnetic phase of the Hubbard model by dynamical mean field theory, we observe striking differences with static (Hartree-Fock) mean field: The Slater band is strongly renormalized and spectral weight is transferred to spin-polaron side bands. Already for intermediate values of the interaction $U$ the overall bandwidth is larger than in Hartree-Fock, and the gap is considerably smaller. Such differences survive any renormalization of $U$. Our photoemission experiments for Cr-doped V$_2$O$_3$ show spectra qualitatively well described by dynamical mean field theory.

cond-mat.str-el↗

arXiv2005

Magnetism and Transport in YbMn2Sb2

A new ternary intermetallic compound, namely, YbMn2Sb2, has been synthesized and its magnetic and electrical transport properties have been studied in the temperature range of 2 to 300 K. This compound crystallizes in a trigonal, La2O2S type structure (space group P3bm1, No. 164) and is found to be ferromagnetically ordered at room temperature. The magnetism is attributed to the ordering of Mn sublattice. M5 xray absorption spectrum of YbMn2Sb2 obtained at room temperature suggests that the valency of Yb in this compound is close to 2. Electrical resistivity of this compound is metal like and a positive magnetoresistance of 13 percent is observed at 5 K in an applied field of 9T. Key words Rare earth intermetallics and alloys, Magnetic properties, Xray absorption spectroscopy, Electrical transport.

cond-mat.mtrl-sci↗

arXiv2004

Filling of the Mott-Hubbard gap in the high temperature photoemission spectrum of (V_0.972Cr_0.028)_2O_3

Photoemission spectra of the paramagnetic insulating (PI) phase of (V_0.972Cr_0.028)_2O_3, taken in ultra high vacuum up to the unusually high temperature (T) of 800 K, reveal a property unique to the Mott-Hubbard (MH) insulator and not observed previously. With increasing T the MH gap is filled by spectral weight transfer, in qualitative agreement with high-T theoretical calculations combining dynamical mean field theory and band theory in the local density approximation.

cond-mat.str-el↗

arXiv2004

Two Aspects of the Mott-Hubbard Transition in Cr-doped V_2O_3

The combination of bandstructure theory in the local density approximation with dynamical mean field theory was recently successfully applied to V$_2$O$_3$ -- a material which undergoes the f amous Mott-Hubbard metal-insulator transition upon Cr doping. The aim of this sh ort paper is to emphasize two aspects of our recent results: (i) the filling of the Mott-Hubbard gap with increasing temperature, and (ii) the peculiarities of the Mott-Hubbard transition in this system which is not characterized by a diver gence of the effective mass for the $a_{1g}$-orbital.

cond-mat.str-el↗

arXiv2004

Full orbital calculation scheme for materials with strongly correlated electrons

We propose a computational scheme for the ab initio calculation of Wannier functions (WFs) for correlated electronic materials. The full-orbital Hamiltonian H is projected into the WF subspace defined by the physically most relevant partially filled bands. The Hamiltonian H^{WF} obtained in this way, with interaction parameters calculated by constrained LDA for the Wannier orbitals, is used as an ab initio setup of the correlation problem, which can then be solved by many-body techniques, e.g., dynamical mean-field theory (DMFT). In such calculations the self-energy operator Σ(e) is defined in WF basis which then can be converted back into the full-orbital Hilbert space to compute the full-orbital interacting Green function G(r,r',e). Using G(r,r',e) one can evaluate the charge density, modified by correlations, together with a new set of WFs, thus defining a fully self-consistent scheme. The Green function can also be used for the calculation of spectral, magnetic and electronic properties of the system. Here we report the results obtained with this method for SrVO3 and V2O3. Comparisons are made with previous results obtained by the LDA+DMFT approach where the LDA DOS was used as input, and with new bulk-sensitive experimental spectra.

cond-mat.str-el↗

arXiv2003

Prominent quasi-particle peak in the photoemission spectrum of the metallic phase of V_2O_3

We present the first observation of a prominent quasi-particle peak in the photoemission spectrum of the metallic phase of V_2O_3 and report new spectral calculations that combine the local density approximation with the dynamical mean-field theory (using quantum Monte Carlo simulations) to show the development of such a distinct peak with decreasing temperature. The experimental peak width and weight are significantly larger than in the theory.

cond-mat.str-el↗

arXiv2002

Photoemission Line shape Study on La$_{0.7}$Sr$_{0.3}$Mn$_{1-δ}$Al$_δ$O$_{3}$ ($δ$ = 0, 0.03, 0.06)

We have studied the line shapes of ultraviolet photoemission spectra of La$_{0.7}$Sr$_{0.3}$Mn$_{1-δ}$Al$_δ$O$_3$ ($δ$ = 0, 0.03, 0.06) systems to test the extrinsic image charge screening effect on photoemission spectra recently suggested by R. Joynt (Science {\bf 284}, 777 (1999)), who argued that the photoemission spectrum near the Fermi energy, specially for poorly conducting system, can be very different from the intrinsic density of states because the outgoing electron has probability of losing its kinetic energy due to the image force. We tested this argument in real materials experimentally by measuring the photoemission line shapes of La$_{0.7}$Sr$_{0.3}$Mn$_{1-δ}$Al$_δ$O$_3$ systems, for which all the requirements of this theory are satisfied and sample resistivities change systematically. We found that experimental photoemission spectra do not show the change of line shapes expected from the extrinsic image force effect and we conclude that the influence of this long range interaction is not so large as suggested in that paper.

cond-mat.str-el↗