arXiv subjects

Bong-Jun Kim

Publications and source records attributed to Bong-Jun Kim.

15 recordsLinked to original sources

arXiv2016

Highly repeatable nanoscale phase coexistence in vanadium dioxide films

The metal-insulator transition (MIT) in vanadium dioxide (VO2) has the potential to lead to a number of disruptive technologies, including ultra-fast data storage, optical switches, and transistors which move beyond the limitations of silicon. For applications, VO2 films are deposited on crystalline substrates to prevent cracking observed in bulk VO2 crystals across the thermally driven MIT. Near the MIT, VO2 films exhibit nanoscale coexistence between metallic and insulating phases, which opens up further potential applications such as memristors, tunable capacitors, and optically engineered devices such as perfect absorbers. It is generally believed that the formation of phase domains must be affected to some extent by random processes which lead to unreliable performance in nanoscale MIT based devices. Here we show that nanoscale randomness is suppressed in the thermally driven MIT in sputtered VO2 films; the observed domain patterns of metallic and insulating phases in the vicinity of the MIT in these films behave in a strikingly reproducible way. This result opens the door for realizing reliable nanoscale VO2 devices.

cond-mat.str-el↗

arXiv2012

Terahertz nano antenna enabled early transition in VO2

We study terahertz transmission through nano-patterned vanadium dioxide thin film. It is found that the patterning allows the lowering of the apparent transition temperature. For the case of the smallest width nano antennas, the transition temperature is lower by as many as ten degrees relative to the bare film, so that the nano patterned hysteresis curves completely separate themselves from their bare film counterparts. This early transition comes from the one order of magnitude enhanced effective dielectric constants by nano antennas. This phenomenon opens up the possibility of transition temperature engineering.

physics.optics↗

arXiv2011

Nanoscale imaging of the electronic and structural transitions in vanadium dioxide

We investigate the electronic and structural changes at the nanoscale in vanadium dioxide (VO2) in the vicinity of its thermally driven phase transition. Both electronic and structural changes exhibit phase coexistence leading to percolation. In addition, we observe a dichotomy between the local electronic and structural transitions. Nanoscale x-ray diffraction reveals local, non-monotonic switching of the lattice structure, a phenomenon that is not seen in the electronic insulator-to-metal transition mapped by near-field infrared microscopy.

cond-mat.str-el↗

arXiv2011

Reconfigurable Gradient Index using VO2 Memory Metamaterials

We demonstrate tuning of a metamaterial device that incorporates a form of spatial gradient control. Electrical tuning of the metamaterial is achieved through a vanadium dioxide layer which interacts with an array of split ring resonators. We achieved a spatial gradient in the magnitude of permittivity, writeable using a single transient electrical pulse. This induced gradient in our device is observed on spatial sc ales on the order of one wavelength at 1 THz. Thus, we show the viability of elements for use in future devices with potential applications in beamforming and communications

cond-mat.mes-hall↗

arXiv2010

Memory Metamaterials

The resonant elements that grant metamaterials their unique properties have the fundamental limitation of restricting their useable frequency bandwidth. The development of frequency-agile metamaterials has helped to alleviate these bandwidth restrictions by allowing real-time tuning of the metamaterial frequency response. We demonstrate electrically-controlled persistent frequency tuning of a metamaterial, allowing lasting modification of its response using a transient stimulus. This work demonstrates a form of memory capacitance which interfaces metamaterials with a class of devices known collectively as memory devices.

cond-mat.mes-hall↗

arXiv2009

Infrared spectroscopy and nano-imaging of the insulator-to-metal transition in vanadium dioxide

We present a detailed infrared study of the insulator-to-metal transition (IMT) in vanadium dioxide (VO2) thin films. Conventional infrared spectroscopy was employed to investigate the IMT in the far-field. Scanning near-field infrared microscopy directly revealed the percolative IMT with increasing temperature. We confirmed that the phase transition is also percolative with cooling across the IMT. We present extensive near-field infrared images of phase coexistence in the IMT regime in VO2. We find that the coexisting insulating and metallic regions at a fixed temperature are static on the time scale of our measurements. A novel approach for analyzing the far-field and near-field infrared data within the Bruggeman effective medium theory was employed to extract the optical constants of the incipient metallic puddles at the onset of the IMT. We found divergent effective carrier mass in the metallic puddles that demonstrates the importance of electronic correlations to the IMT in VO2. We employ the extended dipole model for a quantitative analysis of the observed near-field infrared amplitude contrast and compare the results with those obtained with the basic dipole model.

cond-mat.str-el↗

arXiv2008

Switching of the Mott transition based on the hole-driven MIT theory

Switching voltage of first-order metal-insulator transition (MIT) in VO_2, an inhomogeneous strongly correlated system, is changed by irradiating an infrared light with wavelength, 1.5 micrometer, and applying the electric field (photo-induced switching). This was predicted in the hole-driven MIT theory in which hole doping of a low concentration below 0.01% into conduction band (Fermi surface) induces the abrupt MIT as correlation effect. The switching is explained by the Mott transition not the Peierls transition.

cond-mat.str-el↗

arXiv2008

Mott transition in VO2 revealed by infrared spectroscopy and nano-imaging

Electrons in correlated insulators are prevented from conducting by Coulomb repulsion between them. When an insulator-to-metal transition is induced in a correlated insulator by doping or heating, the resulting conducting state can be radically different from that characterized by free electrons in conventional metals. We report on the electronic properties of a prototypical correlated insulator vanadium dioxide (VO2) in which the metallic state can be induced by increasing temperature. Scanning near-field infrared microscopy allows us to directly image nano-scale metallic puddles that appear at the onset of the insulator-to-metal transition. In combination with far-field infrared spectroscopy, the data reveal the Mott transition with divergent quasiparticle mass in the metallic puddles. The experimental approach employed here sets the stage for investigations of charge dynamics on the nanoscale in other inhomogeneous correlated electron systems.

cond-mat.str-el↗

arXiv2006

Temperature dependence of Mott transition in VO_2 and programmable critical temperature sensor

The temperature dependence of the Mott metal-insulator transition (MIT) is studied with a VO_2-based two-terminal device. When a constant voltage is applied to the device, an abrupt current jump is observed with temperature. With increasing applied voltages, the transition temperature of the MIT current jump decreases. We find a monoclinic and electronically correlated metal (MCM) phase between the abrupt current jump and the structural phase transition (SPT). After the transition from insulator to metal, a linear increase in current (or conductivity) is shown with temperature until the current becomes a constant maximum value above T_{SPT}=68^oC. The SPT is confirmed by micro-Raman spectroscopy measurements. Optical microscopy analysis reveals the absence of the local current path in micro scale in the VO_2 device. The current uniformly flows throughout the surface of the VO_2 film when the MIT occurs. This device can be used as a programmable critical temperature sensor.

cond-mat.str-el↗

arXiv2006

Monoclinic and Correlated Metal Phase in VO_2 as Evidence of the Mott Transition: Coherent Phonon Analysis

In femtosecond pump-probe measurements, the appearance of coherent phonon oscillations at 4.5 THz and 6.0 THz indicating the rutile metal phase of VO_2 does not occur simultaneously with the first-order metal-insulator transition (MIT) near 68^oC. The monoclinic and correlated metal(MCM) phase between the MIT and the structural phase transition (SPT) is generated by a photo-assisted hole excitation which is evidence of the Mott transition. The SPT between the MCM phase and the rutile metal phase occurs due to subsequent Joule heating. The MCM phase can be regarded as an intermediate non-equilibrium state.

cond-mat.str-el↗

arXiv2006

Hole-driven MIT theory, Mott transition in VO_2, MoBRiK

For inhomogeneous high-T_c superconductors, hole-driven metal-insulator transition (MIT) theory explains that the gradual increase of conductivity with increasing hole doping is due to inhomogeneity with the local Mott system undergoing the first-order MIT and the local non-Mott system. For VO_2, a monoclinic and correlated metal (MCM) phase showing the linear characteristic as evidence of the Mott MIT is newly observed by applying electric field and temperature. The structural phase transition occurs between MCM and Rutile metal phases. Devices using the MIT are named MoBRiK.

cond-mat.str-el↗

arXiv2006

Synthesis of VO_2 Nanowire and Observation of the Metal-Insulator Transition

We have fabricated crystalline nanowires of VO_2 using a new synthetic method. A nanowire synthesized at 650^oC shows the semiconducting behavior and a nanowire at 670^oC exhibits the first-order metal-insulator transition which is not the one-dimensional property. The temperature coefficient of resistance in the semiconducting nanowire is 7.06 %/K at 300 K, which is higher than that of commercial bolometer.

cond-mat.str-el↗

arXiv2006

Mott transition observed by micro-Raman scattering in VO_2

A strongly correlated Mott first-order metal-insulator transition (MIT) (or Jump) not accompanied by the structural phase transition (SPT) was clearly revealed in VO_2, (New J. Phys. 6 (1004) 52, Appl. Phys. Lett. 86 (2005) 242101, Physica B 369 (2005) 76). In order to re-confirm the MIT for a VO_2-based device with a narrow width of 3 micrmeter and a length of 20 micrometer such as a rod (Fig. A), both phonon peaks (Fig. B) by a micro-Raman scattering with a laser beam of about 5 micrometer and the MIT with jump in I-V curve (Fig. C) were simultaneously measured. A device like a rod has less inhomogeneity. The current was restricted for measurements. The phonon peaks of monoclinic exist even after the abrupt jump, and disappear in over 10 mA. The jump was changed to negative differential resistance type during Raman measurement after the jump. The high current causes a Joule heat which arises from the SPT near 68^oC from monoclinic to tetragonal. The clean film surface without a breakdown damage after several measurements was taken by a micro-photograph camera (Fig. A). The MIT (jump) occurs prior to the SPT and not affected by the SPT as evidence of electron-phonon interaction. Thus VO_2 is a Mott insulator not Peierls insulator.

cond-mat.str-el↗

arXiv2005

Highly oriented VO2 thin films prepared by sol-gel deposition method

Highly oriented VO2 thin films were grown on sapphire substrates by the sol-gel method that includes a low pressure annealing in an oxygen atmosphere. This reduction process effectively promotes the formation of the VO2 phase over a relatively wide range of pressures below 100 mTorr and temperatures above 400oC. X-ray diffraction analysis showed that as-deposited films crystallize directly to the VO2 phase without passing through intermediate phases. VO2 films have been found to be with [100]- and [010]-preferred orientations on Al2O3(1012) and Al2O3(1010) substrates, respectively. Both films undergo a metal-insulator transition with an abrupt change in resistance, with different transition behaviors observed for the differently oriented films. For the [010]-oriented VO2 films a larger change in resistance of 1.2x10^4 and a lower transition temperature are found compared to the values obtained for the [100]-oriented films.

cond-mat.mtrl-sci↗

arXiv2000

Observation of a linear temperature dependence of the critical current density in a Ba_{0.63}K_{0.37}BiO_3 single crystal

For a Ba_{0.63}K_{0.37}BiO_3 single crystal with T_c=31 K, H_{c1}=750 Oe at 5 K, and dimensions 3x3x1 mm^3, the temperature and field dependences of magnetic hysteresis loops have been measured within 5-25 K in magnetic fields up to 6 Tesla. The critical current density is J_c(0)=1.5 x 10^5 A/cm^2 at zero field and 1 x 10^5 A/cm^2 at 1 kOe at 5 K. J_c decreases exponentially with increasing field up to 10 kOe. A linear temperature dependence of J_c is observed below 25 K, which differs from the exponential and the power-law temperature dependences in high-Tc superconductors including the BKBO. The linear temperature dependence can be regarded as an intrinsic effect in superconductors.

cond-mat.supr-con↗