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Kee Hoon Kim

Publications and source records attributed to Kee Hoon Kim.

At least 37 records · Page 2Linked to original sources
arXiv2017

Transparent perovskite barium stannate with high electron mobility and thermal stability

Transparent conducting oxides (TCOs) and transparent oxide semiconductors (TOSs) have become necessary materials for a variety of applications in the information and energy technologies, ranging from transparent electrodes to active electronics components. Perovskite barium stannate (BaSnO3), a new TCO or TOS system, is a potential platform for realizing optoelectronic devices and observing novel electronic quantum states due to its high electron mobility, excellent thermal stability, high transparency, structural versatility, and flexible doping controllability at room temperature. This article reviews recent progress in the doped BaSnO3 system, discussing the wide physical properties, electron-scattering mechanism, and demonstration of key semiconducting devices such as pn diodes and field-effect transistors. Moreover, we discuss the pathways to achieving two-dimensional electron gases at the interface between BaSnO3 and other perovskite oxides and describe remaining challenges for observing novel quantum phenomena at the heterointerface.

cond-mat.mtrl-sci↗
arXiv2017

Microscopic Observation of Entangled Multi-Magnetoelectric Coupling Phenomenon

Searching for new functionality in next generation electronic devices is a principal driver of material physics research. Multiferroics simultaneously exhibit electric and magnetic order parameters that may be coupled through magnetoelectric (ME) effects. In single-phase materials the ME effect arises from one of three known mechanisms: inverse Dzyaloshinskii-Moriya (IDM) interaction, spin dependent ligand-metal (p-d) orbital hybridization, and exchange striction. However, the coupling among these mechanisms remains largely unexplored despite envisioned potential capabilities. Here, we present cooperative tuning between both IDM interaction and p-d hybridization that leads to discrete ME states in Ba0.5Sr2.5Co2Fe24O41. In-situ x-ray diffraction exposes the microscopic interplay between these two mechanisms, marked by a unique ME susceptibility upon electric and magnetic fields. The entangled multi-ME coupling phenomenon observed in this room-temperature ME hexaferrite offers a pathway to novel functional control for ME device applications.

cond-mat.mtrl-sci↗
arXiv2016

Interplay of charge density wave and multiband superconductivity in 2$H$-Pd$_x$TaSe$_2$

2$H$-TaSe$_2$ has been one of unique transition metal dichalcogenides exhibiting several phase transitions due to a delicate balance among competing electronic ground states. An unusual metallic state at high-$T$ is sequentially followed by an incommensurate charge density wave (ICDW) state at $\approx$ 122 K and a commensurate charge density wave (CCDW) state at $\approx$ 90 K, and superconductivity at $T_{\rm{C}}\sim$0.14 K. Upon systematic intercalation of Pd ions into TaSe$_2$, we find that CCDW order is destabilized more rapidly than ICDW to indicate a hidden quantum phase transition point at $x$$\sim$0.09-0.10. Moreover, $T_{\rm{C}}$ shows a dramatic enhancement up to 3.3 K at $x$ = 0.08, $\sim$24 times of $T_{\rm{C}}$ in 2$H$-TaSe$_2$, in proportional to the density of states $N(E_F)$. Investigations of upper critical fields $H_{c2}$ in single crystals reveal evidences of multiband superconductivity as temperature-dependent anisotropy factor $γ_H$ = $H_{c2}^{ab}$/$H_{c2}^{c}$, quasi-linear increase of $H_{c2}^{c}(T)$, and an upward, positive-curvature in $H_{c2}^{ab}(T)$ near $T_{\rm{C}}$. Furthermore, analysis of temperature-dependent electronic specific heat corroborates the presence of multiple superconducting gaps. Based on above findings and electronic phase diagram vs $x$, we propose that the increase of $N(E_F)$ and effective electron-phonon coupling in the vicinity of CDW quantum phase transition should be a key to the large enhancement of $T_{\rm{C}}$ in Pd$_x$TaSe$_2$.

cond-mat.supr-con↗
arXiv2016

Observation of magnetoelectric effects in a S=1/2 frustrated spin chain magnet SrCuTe2O6

The magnetoelectric (ME) effects are investigated in a cubic compound SrCuTe2O6, in which uniform Cu2+ (S=1/2) spin chains with considerable spin frustration exhibit a concomitant antiferromagnetic transition and dielectric constant peak at TN=5.5 K. Pyroelectric Jp(T) and magnetoelectric current JME(H) measurements in the presence of a bias electric field are used to reveal that SrCuTe2O6 shows clear variations of Jp(T) across TN at constant magnetic fields. Furthermore, isothermal measurements of JME(H) also develop clear peaks at finite magnetic fields, of which traces are consistent with the spin-flop transitions observed in the magnetization studies. As a result, the anomalies observed in Jp(T) and JME(H) curves well match with the field-temperature phase diagram constructed from magnetization and dielectric constant measurements, demonstrating that SrCuTe2O6 is a new magnetoelectric compound with S=1/2 spin chains.

cond-mat.mtrl-sci↗
arXiv2015

X-ray scattering study of pyrochlore iridates: crystal structure, electronic and magnetic excitations

We have investigated the structural, electronic, and magnetic properties of the pyrochlore iridates Eu2Ir2O7 and Pr2Ir2O7 using a combination of resonant elastic x-ray scattering, x-ray powder diffraction, and resonant inelastic x-ray scattering (RIXS). The structural parameters of Eu2Ir2O7 have been examined as a function of temperature and applied pressure, with a particular emphasis on regions of the phase diagram where electronic and magnetic phase transitions have been reported. We find no evidence of crystal symmetry change over the range of temperatures (~6 to 300 K) and pressures (~0.1 to 17 GPa) studied. We have also investigated the electronic and magnetic excitations in single crystal samples of Eu2Ir2O7 and Pr2Ir2O7 using high resolution Ir L3-edge RIXS. In spite of very different ground state properties, we find these materials exhibit qualitatively similar excitation spectra, with crystal field excitations at ~3-5 eV, spin-orbit excitations at ~0.5-1 eV, and broad low-lying excitations below ~0.15 eV. In Eu2Ir2O7 we observe highly damped magnetic excitations at ~45 meV, which display significant momentum dependence. We compare these results with recent dynamical structure factor calculations.

cond-mat.str-el↗
arXiv2015

Observation of S = 1/2 quasi-one-dimensional magnetic and magneto-dielectric behavior in a cubic SrCuTe2O6

We investigate magnetic, thermal, and dielectric properties of SrCuTe2O6, which is isostructural to PbCuTe2O6, a recently found, Cu-based 3D frustrated magnet with a corner sharing triangular spin network having dominant first and second nearest neighbor (nn) couplings [B. Koteswararao, et al. Phys. Rev. B 90, 035141 (2014)]. Although SrCuTe2O6 has a structurally similar spin network, but the magnetic data exhibit the characteristic features of a typical quasi -one-dimensional magnet, which mainly resulted from the magnetically dominant third nn coupling, uniform chains. The magnetic properties of this system are studied via magnetization (M), heat capacity (Cp), dielectric constant, measurements along with ab-initio band structure calculations. Magnetic susceptibility chi(T) data show a broad maximum at 32 K and the system orders at low temperatures TN1=5.5 K and TN2=4.5 K, respectively. The analysis of chi(T) data gives an intra-chain coupling, J3/kB, to be about - 42 K with non-negligible frustrated inter-chain couplings (J1/kB and J2/kB). The hopping parameters obtained from LDA band structure calculations also suggest the presence of coupled uniform chains. The observation of simultaneous anomalies in dielectric constant at TN1 and TN2 suggests the presence of magneto-dielectric effect in SrCuTe2O6. A magnetic phase diagram is also built based on M, C p, and dielectric constant results.

cond-mat.str-el↗
arXiv2014

Diluted magnetism in Mn doped SrZnO2 single crystals

We have investigated the magnetic properties of Mn and Cu substituted SrZnO2 single crystals (SrZn0.99Mn0.01O2 and SrZn0.99Cu0.01O2). We observed signatures of weak ferromagnetism as a sharp increase of magnetic susceptibility below 5 K even in the low percentage (x= 0.01) of Mn substituted single crystals. Magnetic susceptibility data measured parallel or perpendicular to the ab plane yield anisotropic behavior with Curie Weiss temperature of about -320 K and -410 K, respectively, suggesting the presence of strong antiferromagnetic couplings among Mn atoms at high temperatures, similar to the Mn doped ZnO and Fe doped BaTiO3 samples. In contrast, the SrZn0.99Cu0.01O2 crystal shows paramagnetic behavior down to 2 K.

cond-mat.mtrl-sci↗
arXiv2014

Magnetic properties and heat capacity of the three-dimensional frustrated S=1/2 antiferromagnet PbCuTe2O6

We report magnetic susceptibility (chi) and heat capacity Cp measurements along with ab-initio electronic structure calculations on PbCuTe2O6, a compound made up of a three dimensional 3D network of corner-shared triangular units. The presence of antiferromagnetic interactions is inferred from a Curie-Weiss temperature (theta_CW) of about -22 K from the chi(T) data. The magnetic heat capacity (Cm) data show a broad maximum at T^max ~ 1.15 K (i.e. T^max/theta_CW ~ 0.05), which is analogous to the the observed broad maximum in the Cm/T data of a hyper-Kagome system, Na4Ir3O8. In addition, Cm data exhibit a weak kink at T^* ~ 0.87 K. While the T^max is nearly unchanged, the T^* is systematically suppressed in an increasing magnetic field (H) up to 80 kOe. For H > 80 kOe, the Cm data at low temperatures exhibit a characteristic power-law (T^α) behavior with an exponent α slightly less than 2. Hopping integrals obtained from the electronic structure calculations show the presence of strongly frustrated 3D spin interactions along with non-negligible unfrustrated couplings. Our results suggest that PbCuTe2O6 is a candidate material for realizing a 3D quantum spin liquid state at high magnetic fields.

cond-mat.str-el↗
arXiv2014

Electrical control of large magnetization reversal in a helimagnet

In spite of both technical and fundamental importance, reversal of a macroscopic magnetization by an electric field (E) has been limitedly realized and remains as one of great challenges. Here, we report the realization of modulation and reversal of large magnetization (M) by E in a multiferroic crystal Ba0.5Sr1.5Zn2(Fe0.92Al0.08)12O22, in which a transverse conical spin state exhibits a remanent M and electric polarization below ~150 K. Upon sweeping E between +- 2 MV/m, M is quasi-linearly varied between +- 2 μB/f.u., resulting in the M reversal. Moreover, the remanent M shows non-volatile changes of ΔM = +- 0.15 μB/f.u., depending on the history of the applied electric fields. The large modulation and the non-volatile two-states of M at zero magnetic field are observable up to ~150 K where the transverse conical spin state is stabilized. Nuclear magnetic resonance measurements provide microscopic evidences that the electric field and the magnetic field play an equivalent role, rendering the volume of magnetic domains change accompanied by the domain wall motion. The present findings point to a new pathway for realizing the large magnetization reversal by electric fields at fairly high temperatures.

cond-mat.mtrl-sci↗
arXiv2013

Enhanced Upper Critical Fields in a New Quasi-one-dimensional Superconductor Nb2PdxSe5

We report a discovery of superconductivity with Tc = 5.5 K in Nb2PdxSe5, in which one-dimensional (1D) Nb-Se chains existing along the b-direction hybridize each other to form the conducting b-c* plane. The magnetic susceptibility and specific heat data in both single- and poly-crystals show evidence of bulk superconductivity. The resistivity, Hall coefficient, and magneto-resistance data all indicate the presence of an energy scale T* = ~ 50 K, which becomes systematically lowered under hydrostatic pressure and competes with the stabilization of superconductivity. Combined with the band calculation results showing the Fermi surfaces with 1D character, we postulate that the energy scale T* is related to a formation of a density wave or a stabilization of low dimensional electronic structure. The zero temperature upper critical field, Hc2(0), of the single crystal is found to be 10.5, 35 and 22 T in the a', b and c*-directions, respectively. While the linearly increasing Hc2(T) for H // c* indicates the multi-band effect, Hc2(0) for H // b and c* are found to be much bigger than the BCS Pauli limiting field, 1.84 Tc ~ 9 T. The suppressed Pauli paramagnetic effect points to a possibility of the enhanced spin-orbit scattering related to the low dimensional electronic structure or the presence of heavy elements such as Pd.

cond-mat.supr-con↗
arXiv2013

PbCu3TeO7: An S=1/2 staircase Kagome lattice with significant intra- and inter-plane couplings

We have synthesized polycrystalline and single crystal samples of PbCu3TeO7 and studied its properties via magnetic susceptibility chi(T) and heat-capacity Cp(T) measurements and also electronic structure calculations. Whereas the crystal structure is suggestive of the presence of a quasi-2D network of Cu2+ (S = 1/2) buckled staircase Kagome layers, the chi(T) data show magnetic anisotropy and three magnetic anomalies at temperatures, TN1 ~ 36 K, TN2 ~ 25 K, TN3 ~ 17 K, respectively. The chi(T) data follow the Curie-Weiss law above 200 K and a Curie-Weiss temperature Theta_CW ~ -150 K is obtained. The data deviate from the simple Curie-Weiss law below 200 K, which is well above TN1, suggesting the presence of competing magnetic interactions. The magnetic anomaly at TN3 appears to be of first-order from magnetization measurements, although our heat-capacity Cp(T) results do not display any anomaly at TN3. The hopping integrals obtained from our electronic structure calculations suggest the presence of significant intra-Kagome (next-nearest neighbor and diagonal) and inter-Kagome couplings. These couplings take the PbCu3TeO7 system away from a disordered ground state and lead to long-range order, in contrast to what might be expected for an ideal (isotropic) 2D Kagome system.

cond-mat.str-el↗
arXiv2012

Theoretical Modeling of ME effect at Low frequency and Resonance Frequency for Magnetoelectric Laminates with Anisotropic Piezoelectric Properties

A new theory is developed for the magnetoelectric (ME) coupling in a symmetric 2-2 ME laminate having a representative piezoelectric crystal (PMN-PT) particularly with anisotropic piezoelectric properties. Considering the average field method, the theoretical expressions for the transverse ME voltage coefficients at low and resonance frequencies were derived. The theory takes into account the anisotropic properties of the piezoelectric materials providing two different expressions of transverse ME voltage coefficients for different in-plane magnetic fields both at low and resonance frequencies. The numerical simulations show multiple resonance frequencies and phase differences between transverse ME voltage coefficients showing good agreement with the experimental results. Our theory should be generally applicable to other ME laminates with any piezoelectric with anisotropic piezoelectric coefficients.

cond-mat.mtrl-sci↗
arXiv2012

Physical properties of transparent perovskite oxides (Ba,La)SnO3 with high electrical mobility at room temperature

Transparent electronic materials are increasingly in demand for a variety of optoelectronic applications. BaSnO3 is a semiconducting oxide with a large band gap of more than 3.1 eV. Recently, we discovered that La doped BaSnO3 exhibits unusually high electrical mobility of 320 cm^2(Vs)^-1 at room temperature and superior thermal stability at high temperatures [H. J. Kim et al. Appl. Phys. Express. 5, 061102 (2012)]. Following that work, we report various physical properties of (Ba,La)SnO3 single crystals and films including temperature-dependent transport and phonon properties, optical properties and first-principles calculations. We find that almost doping-independent mobility of 200-300 cm^2(Vs)^-1 is realized in the single crystals in a broad doping range from 1.0x10^19 to 4.0x10^20 cm^-3. Moreover, the conductivity of ~10^4 ohm^-1cm^-1 reached at the latter carrier density is comparable to the highest value. We attribute the high mobility to several physical properties of (Ba,La)SnO3: a small effective mass coming from the ideal Sn-O-Sn bonding, small disorder effects due to the doping away from the SnO2 conduction channel, and reduced carrier scattering due to the high dielectric constant. The observation of a reduced mobility of ~70 cm^2(Vs)^-1 in the film is mainly attributed to additional carrier-scatterings which are presumably created by the lattice mismatch between the substrate SrTiO3 and (Ba,La)SnO3. The main optical gap of (Ba,La)SnO3 single crystals remained at about 3.33 eV and the in-gap states only slightly increased, thus maintaining optical transparency in the visible region. Based on these, we suggest that the doped BaSnO3 system holds great potential for realizing all perovskite-based, transparent high-frequency high-power functional devices as well as highly mobile two-dimensional electron gas via interface control of heterostructured films.

cond-mat.mtrl-sci↗
arXiv2012

High Mobility in a Stable Transparent Perovskite Oxide

We discovered that La-doped BaSnO3 with the perovskite structure has an unprecedentedly high mobility at room temperature while retaining its optical transparency. In single crystals, the mobility reached 320 cm^2(Vs)^-1 at a doping level of 8x10^19 cm^-3, constituting the highest value among wide-band-gap semiconductors. In epitaxial films, the maximum mobility was 70 cm^2(Vs)^-1 at a doping level of 4.4x10^20 cm^-3. We also show that resistance of (Ba,La)SnO3 changes little even after a thermal cycle to 530 Deg. C in air, pointing to an unusual stability of oxygen atoms and great potential for realizing transparent high-frequency, high-power functional devices.

cond-mat.mtrl-sci↗
arXiv2012

Orbital selective Fermi surface shifts and mechanism of high T$_c$ superconductivity in correlated AFeAs (A=Li,Na)

Based on the dynamical mean field theory (DMFT) and angle resolved photoemission spectroscopy (ARPES), we have investigated the mechanism of high $T_c$ superconductivity in stoichiometric LiFeAs. The calculated spectrum is in excellent agreement with the observed ARPES measurement. The Fermi surface (FS) nesting, which is predicted in the conventional density functional theory method, is suppressed due to the orbital-dependent correlation effect with the DMFT method. We have shown that such marginal breakdown of the FS nesting is an essential condition to the spin-fluctuation mediated superconductivity, while the good FS nesting in NaFeAs induces a spin density wave ground state. Our results indicate that fully charge self-consistent description of the correlation effect is crucial in the description of the FS nesting-driven instabilities.

cond-mat.supr-con↗
arXiv2012

Probing the Order Parameter of Superconducting LiFeAs using Pb/LiFeAs and Au/LiFeAs Point-Contact Spectroscopy

We have fabricated c-axis point contact junctions between high-quality LiFeAs single crystals and Pb or Au tips in order to study the nature of the superconducting order parameter of LiFeAs, one of the few stoichiometric iron-based superconductors. The observation of the Josephson current in c-axis junctions with a conventional s-wave superconductor as the counterelectrode indicates that the pairing symmetry in LiFeAs is not pure d-wave or pure spin-triplet p-wave. A superconducting gap is clearly observed in point contact Andreev reflection measurements performed on both Pb/LiFeAs and Au/LiFeAs junctions. The conductance spectra can be well described by the Blonder-Tinkham-Klapwijk model with a lifetime broadening term, resulting in a gap value of \approx 1.6 meV (2Δ/kBTC \approx 2.2).

cond-mat.supr-con↗
arXiv2012

Electric field control of nonvolatile four-state magnetization at room temperature

We find the realization of large converse magnetoelectric (ME) effects at room temperature in a multiferroic hexaferrite Ba$_{0.52}$Sr$_{2.48}$Co$_{2}$Fe$_{24}$O$_{41}$ single crystal, in which rapid change of electric polarization in low magnetic fields (about 5 mT) is coined to a large ME susceptibility of 3200 ps/m. The modulation of magnetization then reaches up to 0.62 $μ$$_{B}$/f.u. in an electric field of 1.14 MV/m. We find further that four ME states induced by different ME poling exhibit unique, nonvolatile magnetization versus electric field curves, which can be approximately described by an effective free energy with a distinct set of ME coefficients.

cond-mat.mtrl-sci↗