arXiv subjects

Y. S. Kwon

Publications and source records attributed to Y. S. Kwon.

32 records · Page 2Linked to original sources

arXiv2011

Electronic Structure Studies of Detwinned BaFe$_2$As$_2$ by Photoemission

We performed angle resolved photoelectron spectroscopy (ARPES) studies on mechanically detwinned BaFe2As2. We observe clear band dispersions and the shapes and characters of the Fermi surfaces are identified. Shapes of the two hole pockets around the Γ-point are found to be consistent with the Fermi surface topology predicted in the orbital ordered states. Dirac-cone like band dispersions near the Γ-point are clearly identified as theoretically predicted. At the X-point, split bands remain intact in spite of detwinning, barring twinning origin of the bands. The observed band dispersions are compared with calculated band structures. With a magnetic moment of 0.2 ?B per iron atom, there is a good agreement between the calculation and experiment.

cond-mat.str-el↗

arXiv2010

Anisotropic upper critical field and a possible Fulde-Ferrel-Larkin-Ovchinnikov state in a stoichiometric pnictide superconductor LiFeAs

Measurements of the temperature and angular dependencies of the upper critical fields $H_{c2}$ of a stoichiometric single crystal LiFeAs in pulsed magnetic fields up to 50 T were performed using a tunnel diode resonator. Full $H^{\parallel c}_{c2}(T)$ and $H^{\bot c}_{c2}(T)$ curves with $H^{\parallel c}_{c2}(0)=17 \pm 1$ T, $H^{\bot c}_{c2}(0)=26 \pm 1$ T, and the anisotropy parameter ${γ_H}(T) \equiv H_{c2}^{\bot c}/H_{c2}^{\parallel c}$ decreasing from $\approx 2.5$ at $T_c$ to 1.5 at $T\ll T_c$ were observed. The results for both orientations are in excellent agreement with a theory of $H_{c2}$ for two-band $s^\pm$ pairing in the clean limit. We show that $H^{\parallel c}_{c2}(T)$ is mostly limited by the orbital pairbreaking, whereas the shape of $H^{\bot c}_{c2}(T)$ indicates strong paramagnetic Pauli limiting and the inhomogeneous Fulde-Ferrel-Larkin-Ovchinnikov (FFLO)state below $T_F \sim 5$ K.

cond-mat.supr-con↗

arXiv2010

Calorimetric Evidence for Nodes in the Overdoped Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$

We present low-temperature specific heat of the electron-doped Ba(Fe$_{0.9}$Co$_{0.1}$)$_{2}$As$_{2}$, which does not show any indication of an upturn down to 400 mK, the lowest measuring temperature. The lack of a Schottky-like feature at low temperatures or in magnetic fields up to 9 Tesla enables us to identify enhanced low-temperature quasiparticle excitations and to study anisotropy in the linear term of the specific heat. Our results can not be explained by a single or multiple isotropic superconducting gap, but are consistent with multi-gap superconductivity with nodes on at least one Fermi surface sheet.

cond-mat.supr-con↗

arXiv2009

Kondo-like behaviors in magnetic and thermal properties of single crystal Tm5Si2Ge2

We grew the single crystal of stoichiometric Tm5Si2.0Ge2.0 using a Bridgeman method and performed XRD, EDS, magnetization, ac and dc magnetic susceptibilities, specific heat, electrical resistivity and XPS experiments. It crystallizes in orthorhombic Sm5Ge4-type structure. The mean valence of Tm ions in Tm5Si2.0Ge2.0 is almost trivalent. The 4f states is split by the crystalline electric field. The ground state exhibits the long range antiferromagnetic order with the ferromagnetically coupled magnetic moments in the ac plane below 8.01 K, while the exited states exhibit the reduction of magnetic moment and magnetic entropy and -log T-behaviors observed in Kondo materials.

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↗

arXiv2007

Probing the Band Structure of LaTe_2 Using Angle Resolved Photoemission Spectroscopy

With the current interest in the rare-earth tellurides as high temperature charge density wave materials, a greater understanding of the physics of these systems is needed, particularly in the case of the ditellurides. We report a detailed study of the band structure of LaTe_2 in the charge density wave state using high-resolution angle resolved photoemission spectroscopy (ARPES). From this work we hope to provide insights into the successes and weaknesses of past theoretical study as well as helping to clear up prior ambiguities by providing an experimental basis for future work in the tellurides.

cond-mat.str-el↗

arXiv2007

Revealing Charge Density Wave Formation in the LaTe_2 System by Angle Resolved Photoemission Spectroscopy

We present the first direct study of charge density wave (CDW) formation in quasi-2D single layer LaTe_2 using high-resolution angle resolved photoemission spectroscopy (ARPES) and low energy electron diffraction (LEED). CDW formation is driven by Fermi surface (FS) nesting, however characterized by a surprisingly smaller gap (50meV) than seen in the double layer RTe_3 compounds, extending over the entire FS. This establishes LaTe_2 as the first reported semiconducting 2D CDW system where the CDW phase is FS nesting driven. In addition, the layer dependence of this phase in the tellurides and the possible transition from a stripe to a checkerboard phase is discussed.

cond-mat.str-el↗

arXiv2005

Infrared spectroscopy under multi-extreme conditions: Direct observation of pseudo gap formation and collapse in CeSb

Infrared reflectivity measurements of CeSb under multi-extreme conditions (low temperatures, high pressures and high magnetic fields) were performed. A pseudo gap structure, which originates from the magnetic band folding effect, responsible for the large enhancement in the electrical resistivity in the single-layered antiferromagnetic structure (AF-1 phase) was found at a pressure of 4 GPa and at temperatures of 35 - 50 K. The optical spectrum of the pseudo gap changes to that of a metallic structure with increasing magnetic field strength and increasing temperature. This change is the result of the magnetic phase transition from the AF-1 phase to other phases as a function of the magnetic field strength and temperature. This result is the first optical observation of the formation and collapse of a pseudo gap under multi-extreme conditions.

cond-mat.str-el↗

arXiv2005

The origin of the phase separation in partially deuterated $κ$-(ET)$_2$Cu[N(CN)$_2$]Br studied by infrared magneto-optical imaging spectroscopy

The direct observation of the phase separation between the metallic and insulating states of 75 %-deuterated $κ$-(ET)$_2$Cu[N(CN)$_2$]Br ($d33$) using infrared magneto-optical imaging spectroscopy is reported, as well as the associated temperature, cooling rate, and magnetic field dependencies of the separation. The distribution of the center of spectral weight ($< ω>$) of $d33$ did not change under any of the conditions in which data were taken and was wider than that of the non-deuterated material. This result indicates that the inhomogenity of the sample itself is important as part of the origin of the metal - insulator phase separation.

cond-mat.str-el↗

arXiv2003

First Investiation of magnetic ground states in the rare-earth intermetallic compounds RAl$_{0.9}$Si$_{1.1}$ (R = Ce, Pr, Gd)

We report the magnetic properties strongly varying with the rare-earth elements in the newly found ternary compounds $R$Al$_{0.9}$Si$_{1.1}$, which crystallize in the tetragonal $α$-ThSi$_2$-type structure. For $R$ = Ce the alloy has a weak ferromagnetism below 11 K and for $R$ = Pr it orders ferromagnetically at 17 K, while for $R$ = Gd it is antiferromagnetic with $T_{\rm N}$ = 30.5 K. In addition, we find no field effect on $T_{\rm N}$ of $R$ = Gd because of the large internal mean field, but significant changes in the magnetic properties of $R$ = Ce and Pr.

cond-mat.str-el↗

arXiv2003

Auger Effect in the High-Resolution Ce 3d-edge Resonant Photoemission

The bulk-sensitive Ce 4$f$ spectral weights of various Ce compounds including CeFe$_2$, CeNi$_2$, and CeSi$_2$ were obtained with the resonant photoemission technique at the Ce 3d-edge. We found the lineshapes change significantly with the small change of the incident photon energy. Detailed analysis showed that this phenomenon results primarily from the Auger transition between different multiplet states of the Ce $\underline{3d_{5/2}}4f^2$ (bar denotes a hole) electronic configuration in the intermediate state of the resonant process. This tells us that extra care should be taken for the choice of the resonant photon energy when extracting Ce 4$f$ spectral weights from the Ce 3$d$-edge resonant photoemission spectra. The absorption energy corresponding to the lowest multiplet structure of the Ce $\underline{3d_{5/2}}4f^2$ configuration seems to be the logical choice.

cond-mat.str-el↗

arXiv2002

Anderson lattice in CeNiSi$_2$ intermediate valence compound

We present experimental results of electrical resistivity, Hall coefficient, magnetic susceptibility, and specific heat for single crystals of Ce-based intervalent compound CeNiSi$_2$. The results show similar behaviors observed in Yb-based intervalent compounds and support recent thoery of the Anderson lattice, in which the Fermi-liquid coherence is gloval over the whole lattice. There is a low-temperature scale $T_{coh} \sim$ 50 K for the onset of Fermi-liquid coherence, in addition to a high-temperature scale $T_K^* \sim$ 150 K for the Kondo-lattice condensation. Therefore, we conclude that two energy scales are generic in intermediate valence compounds based on Ce where the orbital degeneracy is smaller and where the size of the $4f$ orbital is larger than those based on Yb.

cond-mat.str-el↗

arXiv2002

Electrical and magnetic properties of the new Kondo-lattice compound Ce3Pd4Ge4

We have measured the electric resistivity, magnetoresistance, magnetic susceptibility and magnetization of the new Kondo-lattice compound Ce3Pd4Ge4. The electrical resistivity exhibits a rapid drop at temperatures below 6 K, while the magnetic susceptibility does not show any corresponding anomaly at that temperature. This phenomenon is similar to that of Ce3Pd20Ge6 which shows quadrupolar interation. We suggest that there is the possibility of quadrupolar interaction in the orthorhombic 4f-electron system Ce3Pd4Ge4. In addition, it is realized that the spin-dependent scattering effect is responsible for the magnetotransport.

cond-mat.str-el↗

arXiv1999

Photoemission Study of Rare-Earth Ditelluride Compounds (ReTe_2 : Re = La, Pr, Sm, and Gd)

We studied the electronic structure of rare-earth ditelluride (ReTe_2 : Re = La, Pr, Sm, and Gd) using photoemission spectroscopy. From the x-ray photoelectron spectroscopy (XPS) study of the 3d core levels of rare-earth elements, we found that all the rare earth elements are trivalent. We have also made theoretical calculations using the Gunnarsson and Schoenhammer approximation and multiplet calculations for the rare earth elements to find that the La and Gd~3d peaks are well explained using our calculations. There is no considerable change in the line-shape of the Te~3d peaks depending on different rare earth elements. On ther other hand, valence band spectra studied with the ultraviolet photoelectron spectroscopy (UPS) show a small change in the Te p band depending on rare-earth elements. According to the UPS data, LaTe_2 has very low carrier density at the Fermi level while SmTe_2 and PrTe_2 show strongly metallic band structure effects near the Fermi level.

cond-mat.str-el↗