• 한국진공학회:학술대회논문집
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• 한국진공학회 2013년도 제45회 하계 정기학술대회 초록집
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• pp.143.2-143.2
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• 2013

Nanomechanical and electrical properties of vanadium dioxide (VO2) thin films across thermal-driven phase transition are investigated with ultra-high vacuum atomic force microscopy. VO2 thin films have been deposited on the n-type heavily doped silicon wafer by pulsed laser deposition. X-ray diffraction reveals that it is textured polycrystalline with preferential orientation of (100) and (120) planes in monoclinic phase. As the temperature increases, the friction decreased at the temperature below the transition temperature, and then the friction increased as increasing temperature above the transition temperature. We attribute this observation to the combined effect of the thermal lubricity and electronic contribution in friction. Furthermore, the dependence of nanoscale conductance on the local pressure was indicated at the various temperatures, and the result was discussed in the view of pressure-induced metal-insulator transition.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2000년도 추계학술대회 논문집
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• pp.555-558
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• 2000

Layered perovskite La$_{2-x}$Ca$_{l-x}$Mn$_2$O$_{7}$ phases were synthesized by solid state reaction. Single phase R-P could be obtained in the range of 0.4$_{2-x}$Ca$_{l-x}$Mn$_2$O$_{7}$. About 30% of MR ratio was obtained at 270K when 5 T of magnetic field was applied.ied.ied.ied.

• 한국표면공학회지
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• 제50권3호
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• pp.141-146
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• 2017

We investigated configuration of metallic and insulating domains in $VO_2$ thin films, while spanning metal-insulator phase transition. Kelvin probe force microscopy, of which spatial resolution is less than 100 nm, enables us to measure local work function (WF) at the sample surface. The WF of $VO_2$ thin films decreased (increased) as increasing (decreasing) the sample temperature, during the phase transition. The higher and lower WF regions corresponded to the insulating and metallic domains, respectively. The metallic fraction, estimated from the WF maps, well explained the temperature-dependent resistivity based on the percolation model. The WF mapping also showed us how the structural defects affected the phase transition behaviors.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2015년도 제49회 하계 정기학술대회 초록집
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• pp.169.2-169.2
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• 2015

$VO_2$ is a well-known a metal-to-insulator-transition (MIT) material, accompanied with a first order structural phase transition near room temperature. Because of the structural phase transition and the MIT occur near a same temperature, there is an ongoing argument whether the MIT is induced by the structural phase transition. $VO_2$ exhibits a relatively weak anti-oxidization ability and can be oxidized to higher-valence oxides (e.g., $V_4$ $O_7$ or $V_2$ $O_5$) when annealed at a high temperature in an oxygen-rich atmosphere. We fabricated $VO_2$ films on $Al_2$ $O_3$ (0001) substrates using a DC magnetron sputtering deposition process with carefully control the $O_2$ percentage in an atmosphere. X-ray diffraction measurements from the films showed only (0l0) peaks with no extra peaks, indicating b-oriented films. The temperature-dependent local structural properties of $VO_2$ films were investigated by using in-situ X-ray absorption fine structure (XAFS) measurements at the V K edge. XAFS revealed that the structural phase transition was occurred nearly $70^{\circ}C$ for heating process and reproducible. Resistance measurements as a function of temperature (R-T) demonstrated that the resistance of $VO_2$ films was changed by a factor of 4 near $75^{\circ}C$ which was higher than $68^{\circ}C$ reported from a $VO_2$ bulk. We will discuss the MIT of $VO_2$ films, comparing with the local structural properties determined by XAFS measurements.

• Progress in Superconductivity
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• 제14권1호
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• pp.24-29
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• 2012

$(TMTSF)_2BF_4$ containing non-centrosymmetric anions is well known to exhibit a metal insulator transition around 37 K by ordering of the anions with a $q_2$=(1/2, 1/2, 1/2) wave vector. We established pressure-temperature phase diagram of the $(TMTSF)_2BF_4$ compound and showed that it can belong to the general phase diagram of the $(TMTSF)_2X$ family. Application of hydrostatic pressure decreases the anion ordering transition temperature and the superconducting state is finally stabilized below 3.77 K under 7.7 kbar. Magnetoresistance measurement on the $(TMTSF)_2BF_4$ under 7.8 kbar is performed but neither the field-induced spin-density-wave state nor the rapid oscillation is observed up to 9 T.

• 한국세라믹학회지
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• 제52권5호
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• pp.361-365
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• 2015

Vanadium dioxide ($VO_2$) of monoclinic phase exhibits Metal Insulator Phase Transition (MIPT) phenomenon involving a sharp change in electrical and optical properties at $68^{\circ}C$. Solution-based process is applied to form uniform $VO_2$ coating layer on ceramic tiles. This can selectively block the near-infrared light to possibly reduce the energy loss and prevent dew condensation caused by the temperature difference. Heat treatment conditions including temperature and dwell time were examined to obtain a monoclinic $VO_2$ single phase. Both rutile and monoclinic $VO_2$ phases were observed from in the tiles post-annealed below $700^{\circ}C$. Desired monoclinic $VO_2$ single phase was grown in the tiles heat treated at $750^{\circ}C$. Nano facets of irregular size were observed in the monoclinic $VO_2$ phase involving the phase-transition. Grain growth of monoclinic $VO_2$ phase was observed as a function of dwell time at $750^{\circ}C$.

• 한국세라믹학회지
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• 제36권8호
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• pp.843-847
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• 1999

The dc resistivity dc magnetization and thermopower of layered perovskite La1.6Ca1.4Mn2O7.07 have been studied. The ceramic sample of La1.6Ca1.4Mn2O7.07 undergoes the metal-insulator transition at 120K while a first-order phase transition from a ferromagnetic phase to a paramagnetic phae is observed at 260 K=TC This behavior is quite different from that of the well-known double exchange ferromagnets such as La1-xCaxMnO3 This phenomenon could be understood by considering the effects of the anisotropic double exchange interaction caused by two dimensional Mn-O-Mn networks in this materials. The dc magnetization between 120K and 250K is nearly constant and decreases rapidly with increasing temperature above 250K The measurements of dc resistivity and thermopower indicate that Zener polaron hopping conduction takes place above 260 K.

• 한국진공학회지
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• 제12권S1호
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• pp.67-69
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• 2003

The magnetic properties of single- and poly-crystalline $La_{0.7}Sr_{0.3}MnO_3/Pr_{0.65}Ca_{0.35}MnO_3$ multilayered (ML) films, and composite (CP) $(La_{0.7}Sr_{0.3})_{0.5}(Pr_{0.65}Ca_{0.35}_{0.5}MnO_3$ films, prepared by laser ablation, have been investigated in a wide temperature range. It was shown that the transformation from an incoherent to a coherent interface in the ML films leads to an enhancement of the ferromagnetic coupling between layers and to a single-phase magnetic transition. The amorphous CP films demonstrate a paramagnetic behavior of the magnetization with a sharp peak at $T_{G}\approx$45 K, which was interpreted as the formation of Griffiths phase. A short-term annealing at $750^{\circ}C$ induced the complete crystallization of film, and a recovery of the ferromagnetic and the metal-insulator transitions.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2016년도 제50회 동계 정기학술대회 초록집
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• pp.125.1-125.1
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• 2016

The excitonic insulator (EI), which is one of fundamental insulators, was theoretically proposed in 1967 but its material realization has not been established well. Only a few materials were proposed as EIs but their experimental evidences were indirect such as the renormalization of band dispersions or an anomaly in electrical resistivity. We conducted scanning tunneling microscopy / spectroscopy measurements and found out that $Ta_2$ $NiSe_5$, which was the most recently proposed as an EI, had a metal-insulator phase transition with the energy gap of 700 meV at 78 K. Moreover, the spatially delocalized excitonic energy level was observed within the energy gap, which could be the direct evidence of the EI ground state. Our theoretical model calculation with the order parameter of 150 meV reproduces the spectral function and the excitonic energy gap very well. In addition, experimental data shows that the band character is inverted at the valence and conduction band edges by the exciton formation, indicating that the mechanism of exciton condensation is similar to the Bardeen-Cooper-Schrieffer (BCS) mechanism of cooper pairs in superconductors.

• Journal of Magnetics
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• 제12권3호
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• pp.103-107
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• 2007

Magnetic field induced structural transition has been systematically investigated for $La_{1-x}Ba_xMnO_3$ with the fine control of carrier doping $(0.15{\leq}x{\leq}0.20)$. Application of a magnetic field results in the suppression of the rhombohedral-orthorhombic transition temperature $(T_s)$ and the increase of insulator-metal transition temperature $(T_{MI})$. Near x = 0.17, where $T_S$ is similar to $T_{MI}$ at zero magnetic field, we found that the $T_S$ smoothly decreased with magnetic field even though it intersected the $T_{MI}$ near 3 T. Also, the magnetostructural phase diagram obtained from the temperature sweep and from the magnetic field sweep is not significantly modified. By comparing the magnetostructural transition in $La_{1-x}Sr_xMnO_3$, we have suggested that the large disorder originated from ionic size differences between La and Ba may weaken the sensitivity of the kinetic energy of $e_g$ electrons on the degree of lattice distortion in $La_{1-x}Ba_xMnO_3$.