arXiv subjects

Yong Seung Kwon

Publications and source records attributed to Yong Seung Kwon.

17 recordsLinked to original sources

arXiv2021

Optical evidence of local and itinerant states in Ce- and Yb-heavy-fermion compounds

The electronic properties of Cerium (Ce) and ytterbium (Yb) intermetallic compounds may display a more local or more itinerant character depending on the interplay of the exchange interactions among the $4f$ electrons and the Kondo coupling between $4f$ and conduction electrons. For the more itinerant case, the materials form heavy-fermions once the Kondo effect is developed at low temperatures. Hence, a temperature variation occurs in the electronic structure that can be traced by investigating the optical conductivity ($σ(ω)$) spectra. Remarkably, the temperature variation in the $σ(ω)$ spectrum is still present in the more localized case, even though the Kondo effect is strongly suppressed. Here, we clarify the local and itinerant character in the electronic structure by investigating the temperature dependence in the $σ(ω)$ spectra of various Ce and Yb compounds with a tetragonal ThCr$_2$Si$_2$-type crystal structure. We explain the temperature change in a unified manner. Above temperatures of about 100 K, the temperature dependence of the $σ(ω)$ spectra is mainly due to the electron-phonon interaction, while the temperature dependence below is due to the Kondo effect.

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arXiv2021

Effect of the sample work function on alkali metal dosing induced electronic structure change

Alkali metal dosing (AMD) has been widely used as a way to control doping without chemical substitution. This technique, in combination with angle resolved photoemission spectroscopy (ARPES), often provides an opportunity to observe unexpected phenomena. However, the amount of transferred charge and the corresponding change in the electronic structure vary significantly depending on the material. Here, we report study on the correlation between the sample work function and alkali metal induced electronic structure change for three iron-based superconductors: FeSe, Ba(Fe$_{0.94}$Co$_{0.06}$)$_{2}$As$_{2}$ and NaFeAs which share a similar Fermi surface topology. Electronic structure change upon monolayer of alkali metal dosing and the sample work function were measured by ARPES. Our results show that the degree of electronic structure change is proportional to the difference between the work function of the sample and Mulliken's absolute electronegativity of the dosed alkali metal. This finding provides a possible way to estimate the AMD induced electronic structure change.

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arXiv2018

Evidence for a preformed Cooper pair model in the pseudogap spectra of a Ca10(Pt4As8)(Fe2As2)5 single crystal with a nodal superconducting gap

For high-Tc superconductors, clarifying the role and origin of the pseudogap is essential for understanding the pairing mechanism. Among the various models describing the pseudogap, the preformed Cooper pair model is a potential candidate. Therefore, we present experimental evidence for the preformed Cooper pair model by studying the pseudogap spectrum observed in the optical conductivity of a Ca10(Pt4As8)(Fe2As2)5 (Tc = 34.6 K) single crystal. We observed a clear pseudogap structure in the optical conductivity and observed its temperature dependence. In the superconducting (SC) state, one SC gap with a gap size of Δ = 26 cm-1, a scattering rate of 1/τ = 360 cm-1 and a low-frequency extra Drude component were observed. Spectral weight analysis revealed that the SC gap and pseudogap are formed from the same Drude band. This means that the pseudogap is a gap structure observed as a result of a continuous temperature evolution of the SC gap observed below Tc. This provides clear experimental evidence for the preformed Cooper pair model.

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arXiv2017

Thermal activation energy of 3D vortex matter in NaFe1-xCoxAs (x=0.01, 0.03 and 0.07) single crystals

We report on the thermally activated flux flow dependency on the doping dependent mixed state in NaFe1-xCoxAs (x=0.01, 0.03, and 0.07) crystals using the magnetoresistivity in the case of B//c-axis and B//ab-plane. It was found clearly that irrespective of the doping ratio, magnetoresistivity showed a distinct tail just above the Tc, offset associated with the thermally activated flux flow (TAFF) in our crystals. Furthermore, in TAFF region the temperature dependence of the activation energy follows the relation U(T, B)=U_0 (B) (1-T/T_c )^q with q=1.5 in all studied crystals. The magnetic field dependence of the activation energy follows a power law of U_0 (B)~B^(-α) where the exponent α is changed from a low value to a high value at a crossover field of B=~2T, indicating the transition from collective to plastic pinning in the crystals. Finally, it is suggested that the 3D vortex phase is the dominant phase in the low-temperature region as compared to the TAFF region in our series samples.

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arXiv2017

Incarnation of Majorana Fermions in Kitaev Quantum Spin Lattice

Kitaev quantum spin liquid is a topological magnetic quantum state characterized by Majorana fermions of fractionalized spin excitations, which are identical to their own antiparticles. Here, we demonstrate emergence of Majorana fermions thermally fractionalized in the Kitaev honeycomb spin lattice α-RuCl3. The specific heat data unveil the characteristic two-stage release of magnetic entropy involving localized and itinerant Majorana fermions. The inelastic neutron scattering results further corroborate these two distinct fermions by exhibiting quasielastic excitations at low energies around the Brillouin zone center and Y-shaped magnetic continuum at high energies, which are evident for the ferromagnetic Kitaev model. Our results provide an opportunity to build a unified conceptual framework of fractionalized excitations, applicable also for the quantum Hall states, superconductors, and frustrated magnets.

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arXiv2017

Optical properties of the optimally doped Ca$_{8.5}$La$_{1.5}$(Pt$_3$As$_8$)(Fe$_{2}$As$_{2}$)$_5$ single crystal

We have measured the reflectivity of the optimally doped Ca$_{8.5}$La$_{1.5}$(Pt$_3$As$_8$)(Fe$_{10}$As$_{10}$) single crystal ($T_c$ = 32.8K) over the broad frequency range from 40 to 12000 $cm^{-1}$ and for temperatures from 8 to 300 K. The optical conductivity spectra of the low frequency region ($< 1,000$ $cm^{-1}$) in the normal state (80 K $< T \leq$ 300 K) is well fitted with two Drude forms, which indicates the presence of multiple bands at the Fermi level. Decreasing temperature below 80 K, this low frequency Drude spectra develops pseudogap (PG) hump structure at around $\approx 100$ $cm^{-1}$ and continuously evolves into the fully opened superconducting (SC) gap structure below $T_c$. Theoretical calculations of the optical conductivity with the preformed Cooper pair model provide an excellent description of the temperature evolution of the PG structure above $T_c$ into the SC gap structure below $T_c$. The extracted two SC gap sizes are $Δ_S$ = 4.9 $meV$ and $Δ_L$ = 14.2 $meV$, suggesting Ca$_{8.5}$La$_{1.5}$(Pt$_3$As$_8$)(Fe$_{10}$As$_{10}$) as a multiple gap superconductor with a mixed character of the weak coupling and strong coupling superconductivity.

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arXiv2016

Fluctuation magnetoconductivity in pristine and protonirradiated Ca8.5La1.5(Pt3As8)(Fe2As2)5 single crystals

The influence of the proton irradiation on the thermally fluctuation-induced conductivity in Ca8.5La1.5(pt3As8)(Fe2As2)5 single crystal was investigatyed. The in-plane magnetoconductivity was measured up to H=13T. It is observed that the Tc was suppressed up to 30.3 from 32.5 K when the proton is irradiated whereas the amplitude of the fluctuation effect is the almost same in both samples. The results were analyzed by the Ullah and Dorsey scaling functions based on the Hartree approximation to the Ginzburg-Landau free energy, including the critical-fluctuation regime. In the pristine sample, the 3D-2D crossover in the fluctuation effect was observed near the Tc. When the 3D-2D occurs, it is newly found that there is a regime simultaneously described by 2D and 3D fluctuation behaviors. Meanwhile, the proton-irradiated sample showed the 3D fluctuation.

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arXiv2016

Optical Evidence of Itinerant-Localized Crossover of $4f$ Electrons in Cerium Compounds

Cerium (Ce)-based heavy-fermion materials have a characteristic double-peak structure (mid-IR peak) in the optical conductivity [$σ(ω)$] spectra originating from the strong conduction ($c$)--$f$ electron hybridization. To clarify the behavior of the mid-IR peak at a low $c$-$f$ hybridization strength, we compared the $σ(ω)$ spectra of the isostructural antiferromagnetic and heavy-fermion Ce compounds with the calculated unoccupied density of states and the spectra obtained from the impurity Anderson model. With decreasing $c$-$f$ hybridization intensity, the mid-IR peak shifts to the low-energy side owing to the renormalization of the unoccupied $4f$ state, but suddenly shifts to the high-energy side owing to the $f$-$f$ on-site Coulomb interaction at a slight localized side from the quantum critical point (QCP). This finding gives us information on the change in the electronic structure across QCP.

cond-mat.str-el↗

arXiv2013

Polarization-Dependent Three-Dimensional Angle-Resolved Photoemission Spectroscopy of BaFe$_{1.8}$Co$_{0.2}$As$_{2}$

We performed polarization- and photon-energy-dependent angle-resolved photoemission spectroscopy of a slightly overdoped iron pnictide superconductor, BaFe$_{1.8}$Co$_{0.2}$As$_{2}$, to clarify the three-dimensional electronic structure including its orbital characters at the Brillouin zone center. Two hole Fermi surfaces (FSs) with $d_{xz/yz}$ and $d_{xy/x^2-y^2}$ orbitals were observed but $d_{z^2}$ hole FS, which has nodes according to a theory of the spin-fluctuation superconductivity mechanism, did not appear. These results suggest that no node will appear at hole FSs at the zone center.

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arXiv2013

Optical properties of iron-based superconductor LiFeAs single crystal

We have measured the reflectivity spectra of the iron based superconductor LiFeAs (Tc = 17.6 K) in the temperature range from 4 to 300 K. In the superconducting state (T < Tc), the clear opening of the optical absorption gap was observed below 25 cm-1, indicating an isotropic full gap formation. In the normal state (T > Tc), the optical conductivity spectra display a typical metallic behavior with the Drude type spectra at low frequencies, but we found that the introduction of the two Drude components best fits the data, indicating the multiband nature of this superconductor. A theoretical analysis of the low temperature data (T=4K < Tc) also suggests that two superconducting gaps best fit the data and their values were estimated as Δ1 = 1.59 meV and Δ2 = 3.15 meV, respectively. Using the Ferrell-Glover-Tinkham (FGT) sum rule and dielectric function ε1(ω), the superconducting plasma frequency (ωps) is consistently estimated to be 6,665 cm-1, implying that about 59 % of the free carriers in the normal state condenses into the SC condensate. To investigate the various interband transition processes (for ω > 200 cm-1), we have also performed the local-density approximation (LDA) band calculation and calculated the optical spectra of the interband transitions. The theoretical results provided a qualitative agreement with the experimental data below 4000 cm-1

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arXiv2011

Anisotropy of the coherence length from critical currents in the stoichiometric superconductor LiFeAs

Miniature Hall-probe arrays were used to measure the critical current densities for the three main directions of vortex motion in the stoichiometric LiFeAs superconductor. These correspond to vortices oriented along the c-axis moving parallel to the ab-plane, and to vortices in the ab-plane moving perpendicular to, and within the plane, respectively. The measurements were carried out in the low-field regime of strong vortex pinning, in which the critical current anisotropy is solely determined by the coherence length anisotropy parameter, ε_ξ. This allows extraction of ε_ξ at magnetic fields far below the upper critical field B_c2. We find that increasing magnetic field decreases the anisotropy of the coherence length.

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arXiv2011

Isotropic three-dimensional gap in the iron-arsenide superconductor LiFeAs from directional heat transport measurements

The thermal conductivity k of the iron-arsenide superconductor LiFeAs (Tc ~ 18K) was measured in single crystals at temperatures down to T~50mK and in magnetic fields up to H=17T, very close to the upper critical field Hc2~18T. For both directions of the heat current, parallel and perpendicular to the tetragonal c-axis, a negligible residual linear term k/T is found as T ->0, revealing that there are no zero-energy quasiparticles in the superconducting state. The increase in k with magnetic field is the same for both current directions and it follows closely the dependence expected for an isotropic superconducting gap. There is no evidence of multi-band character, whereby the gap would be different on different Fermi-surface sheets. These findings show that the superconducting gap in LiFeAs is isotropic in 3D, without nodes or deep minima anywhere on the Fermi surface. Comparison with other iron-pnictide superconductors suggests that a nodeless isotropic gap is a common feature at optimal doping (maximal Tc).

cond-mat.supr-con↗

arXiv2011

Small anisotropy of the lower critical field and $s_\pm$-wave two-gap feature in single crystal LiFeAs

The in- and out-of-plane lower critical fields and magnetic penetration depths for LiFeAs were examined. The anisotropy ratio $γ_{H_{c1}}(0)$ is smaller than the expected theoretical value, and increased slightly with increasing temperature from 0.6$T_c$ to $T_c$. This small degree of anisotropy was numerically confirmed by considering electron correlation effect. The temperature dependence of the penetration depths followed a power law($\sim$$T^n$) below 0.3$T_c$, with $n$$>$3.5 for both $λ_{ab}$ and $λ_c$. Based on theoretical studies of iron-based superconductors, these results suggest that the superconductivity of LiFeAs can be represented by an extended $s_\pm$-wave due to weak impurity scattering effect. And the magnitudes of the two gaps were also evaluted by fitting the superfluid density for both the in- and out-of-plane to the two-gap model. The estimated values for the two gaps are consistent with the results of angle resolved photoemission spectroscopy and specific heat experiments.

cond-mat.supr-con↗

arXiv2010

Nodeless two-gap superconductivity in stoichiometric iron pnictide LiFeAs

The variations of in- and inter- plane London penetration depths, $Δλ(T)$, were measured using a tunnel diode resonator in single crystals of the intrinsic pnictide superconductor LiFeAs. This compound appears to be in the clean limit with a residual resistivity of 4 ($T\to0$) to 8 ($T_c$) $μΩ\cdot$cm and $RRR$ of 65 to 35, respectively. The superfluid density, $ρ_s(T)=λ^2(0)/λ^2(T)$, is well described by the self-consistent two-gap $γ-$model. Together with the previous data, our results support the universal evolution of the superconducting gap from nodeless to nodal upon departure from optimal doping. We also conclude that pairbreaking scattering plays an important role in the deviation of the low-temperature behavior of $λ(T)$ from exponential in Fe-based compounds.

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arXiv2010

Pseudogap of Superconducting correlation origin in iron-pnictide superconductor Ba0.6+δK0.4-δFe2As2

Pseudogap (PG), a phenomenon of opening of gap like features above superconducting (SC) transition temperature (Tc), has been universally observed in the high-Tc cuprates (HTC) (1,2), some heavy fermion superconductors (3), and iron pnictides (4-7). Here, we report the observation of PG behavior in optical conductivity in an underdoped Ba0.6+δK0.4-δFe2As2 (Tconset =36 K) single crystal far above Tc, up to 100 K (~ 3Tc). Unique scale separation in Ba0.6+δK0.4-δFe2As2 compound - magnetic and SC correlation energy scales being widely separated - enabled us to establish that the PG structures observed in the range of 50 cm-1 - 150 cm-1 in optical conductivity is caused by the SC correlation and the magnetic correlation origin is ruled out. Theoretical calculations, based on the preformed Cooper pair model (8), provided an excellent description to the temperature evolution of the optical conductivity data from below to above Tc.

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arXiv2010

Synthesis, anisotropy, and superconducting properties of LiFeAs single crystal

A LiFeAs single crystal with $T_c^{onset}$$\sim$19.7 K was grown successfully in a sealed tungsten crucible using the Bridgeman method. The electrical resistivity experiments revealed a ratio of room temperature to residual resistivity (RRR) of approximately 46 and 18 for the in-plane and out-of plane directions. The estimated anisotropic resistivity, $γ_ρ$=$ρ_c$ / $ρ_{ab}$, was approximately 3.3 at $T_c^{onset}$. The upper critical fields had large $H_{c2} ^{\shortparallel ab}$ and $H_{c2}^{\shortparallel c}$ values of 83.4 T and 72.5 T, respectively, and an anisotropy ratio is $γ_H$=$H_{c2}^{\shortparallel ab}$ / $H_{c2} ^{\shortparallel c}$$\sim$1.15. The high upper critical field value and small anisotropy highlight the potential use of LiFeAs in a variety of applications. The calculated critical current density $(J_c)$ from the $M$-$H$ loop is approximately 10$^3$ A/cm$^2$

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arXiv2010

The pseudogap behavior in the stoichiometric FeSe superconductor (Tc~9.4 K)

This paper reports the synthesis and superconducting behaviors of the tetragonal iron-chalcogenide superconductor FeSe. The electrical resistivity and magnetic moment measurements confirmed its superconductivity with a $T_c^{zero}$ and $T_c^{mag}$ at 9.4 K under ambient pressure. EPMA indicated the sample to have a stoichiometric Fe:Se ratio of 1:1 ($\pm$0.02). The Seebeck coefficient which was 12.3 $μ$V/K at room temperature, changed to a negative value near 200 K, indicating it to be a two carriers material. Above $T_c$, the $ρ(T)$ curve revealed an 'S' shape. Hence $dρ(T)/dT$, and $d^2ρ(T)/dT^2$ showed pseudogap-like behavior at $T^*$=110 K according to the resistivity curvature mapping (RCM) method for high $T_c$ cuprates. Moreover, the magnetoresistance $ρ_H(T)/ρ_{H=0}$ under a magnetic field and the Seebeck coefficient $S(T)$ revealed revealed pseudogap-like behavior near $T^*$. Interestingly, at the same temperature, 30 K, the sign of $S(T)$ and all signs of $d^2ρ(T)/dT^2$ changed from negative to positive above $T_c$.

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