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Growth of delafossite CuAlO2 single crystals in a reactive crucible
Authors:
Du hyung Kim,
Minsik Kong,
Myeongjun Kang,
Minjae Kim,
Seohee Kim,
Youngwook Kim,
Sangmoon Yoon,
Jong Mok Ok
Abstract:
Delafossite oxide CuAlO2 has engaged great attention as a promising p-type conducting oxide. In this work, high-quality CuAlO2 single crystals with a size of several millimeters (mm) are successfully achieved with a reactive crucible melting method. The crystals are characterized by X-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, transport measurement, and magn…
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Delafossite oxide CuAlO2 has engaged great attention as a promising p-type conducting oxide. In this work, high-quality CuAlO2 single crystals with a size of several millimeters (mm) are successfully achieved with a reactive crucible melting method. The crystals are characterized by X-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, transport measurement, and magnetic susceptibility measurement. The grown single crystals are free of contamination from a copper oxide flux. This work provides a new approach to growing high-quality delafossite oxide CuAlO2 with a few mm size.
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Submitted 22 May, 2022;
originally announced May 2022.
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Direct evidence of electronic interaction at the atomic-layer-deposited MoS2 monolayer/SiO2 interface
Authors:
Minji Lee,
Yejin Kim,
Ahmed Yousef Mohamed,
Han-Koo Lee,
Kyuwook Ihm,
Dae Hyun Kim,
Tae Joo Park,
Deok-Yong Cho
Abstract:
The electronic structure of an atomic-layer-deposited MoS2 monolayer on SiO2 was investigated using X-ray absorption spectroscopy (XAS) and synchrotron X-ray photoelectron spectroscopy (XPS). The angle-dependent evolution of the XAS spectra and the photon-energy-dependent evolution of the XPS spectra were analyzed in detail using an ab-initio electronic structure simulation. Although similar to th…
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The electronic structure of an atomic-layer-deposited MoS2 monolayer on SiO2 was investigated using X-ray absorption spectroscopy (XAS) and synchrotron X-ray photoelectron spectroscopy (XPS). The angle-dependent evolution of the XAS spectra and the photon-energy-dependent evolution of the XPS spectra were analyzed in detail using an ab-initio electronic structure simulation. Although similar to the theoretical spectra of an ideal free-standing MoS2 ML, the experimental spectra exhibit features that are distinct from those of an ideal ML, which can be interpreted as a consequence of S-O van der Waals (vdW) interactions. The strong consensus among the experimental and theoretical spectra suggests that the vdW interactions between MoS2 and adjacent SiO2 layers can influence the electronic structure of the system, manifesting a substantial electronic interaction at the MoS2-SiO2 interface.
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Submitted 27 April, 2021;
originally announced April 2021.
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Super-Gaussian, super-diffusive transport of multi-mode active matter
Authors:
Seungsoo Hahn,
Sanggeun Song,
Dae Hyun Kim,
Gil-Suk Yang,
Kang Taek Lee,
Jaeyoung Sung
Abstract:
Living cells exhibit multi-mode transport that switches between an active, self-propelled motion and a seemingly passive, random motion. Cellular decision-making over transport mode switching is a stochastic process that depends on the dynamics of the intracellular chemical network regulating the cell migration process. Here, we propose a theory and an exactly solvable model of multi-mode active m…
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Living cells exhibit multi-mode transport that switches between an active, self-propelled motion and a seemingly passive, random motion. Cellular decision-making over transport mode switching is a stochastic process that depends on the dynamics of the intracellular chemical network regulating the cell migration process. Here, we propose a theory and an exactly solvable model of multi-mode active matter. Our exact model study shows that the reversible transition between a passive mode and an active mode is the origin of the anomalous, super-Gaussian transport dynamics, which has been observed in various experiments for multi-mode active matter. We also present the generalization of our model to encompass complex multi-mode matter with arbitrary internal state chemical dynamics and internal state dependent transport dynamics.
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Submitted 1 August, 2017;
originally announced August 2017.
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Beyond Bean's critical state model: On the origin of paramagnetic Meissner effect
Authors:
Sangjun Oh,
Dong Keun Oh,
Won Nam Kang,
Jung Ho Kim,
Shi Xue Dou,
Dojun Youm,
Dong Ho Kim
Abstract:
Solving phenomenological macroscopic equations instead of microscopic Ginzburg-Landau equations for superconductors is much easier and can be advantageous in a variety of applications. However, till now, only Bean's critical state model is available for the description of irreversible properties. Here we propose a plausible overall macroscopic model for both reversible and irreversible properties,…
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Solving phenomenological macroscopic equations instead of microscopic Ginzburg-Landau equations for superconductors is much easier and can be advantageous in a variety of applications. However, till now, only Bean's critical state model is available for the description of irreversible properties. Here we propose a plausible overall macroscopic model for both reversible and irreversible properties, combining London theory and Bean's model together based on superposition principle. First, a simple case where there is no pinning is discussed, from which a microscopic basis for Bean's model is explored. It is shown that a new concept of 'flux share' is needed when the field is increased above the lower critical field. A portion of magnetic flux is completely shielded, named as 'Meissner share' and the rest penetrates through vortices, named as 'vortices share'. We argue that the flux shares are irreversible if there is pinning. It is shown that the irreversible flux shares can be the reason for observed peculiar reversible magnetization behavior near zero field. The overall macroscopic model seems to be valuable for the analysis of fundamental physical properties as well. As an example, it is shown the origin of paramagnetic Meissner effect can be explained by the phenomenological macroscopic model.
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Submitted 14 December, 2016;
originally announced December 2016.
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Soft X-ray Absorption Spectroscopy Study of Multiferroic Bi-substituted Ba(1-x)Bi(x)Ti(0.9)Fe(0.1)O(3)
Authors:
Hyun Woo Kim,
D. H. Kim,
Eunsook Lee,
Seungho Seong,
Deok Hyeon Kim,
B. W. Lee,
Y. Ko,
J. -Y. Kim,
J. -S. Kang
Abstract:
The electronic structures of multiferroic oxides of Ba(1-x)Bi(x)Ti(0.9)Fe(0.1)O(3) (0 < x < 0.12) have been investigated by employing photoemission spectroscopy and soft x-ray absorption spectroscopy (XAS). The measured Fe and Ti 2p XAS spectra show that Ti ions are in the Ti4+ states for all x and that Fe ions are Fe2+-Fe3+ mixed-valent for x > 0. The valence states of Fe ions are found to be nea…
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The electronic structures of multiferroic oxides of Ba(1-x)Bi(x)Ti(0.9)Fe(0.1)O(3) (0 < x < 0.12) have been investigated by employing photoemission spectroscopy and soft x-ray absorption spectroscopy (XAS). The measured Fe and Ti 2p XAS spectra show that Ti ions are in the Ti4+ states for all x and that Fe ions are Fe2+-Fe3+ mixed-valent for x > 0. The valence states of Fe ions are found to be nearly trivalent for x=0, and decreases with increasing x from being nearly trivalent (v(Fe)~ 3) for x=0 to v(Fe)~ 2.6 for x=0.12. The valence states of both Ti and Ba ions do not change for all x < 0.12. Based on the obtained valence states of Fe ions, the electronic and magnetic properties of Ba(1-x)Bi(x)Ti(0.9)Fe(0.1)O(3) are explored.
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Submitted 22 May, 2016;
originally announced May 2016.
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High-frequency behavior of FeN thin films fabricated by reactive sputtering
Authors:
Tae-Jong Hwang,
Joonsik Lee,
Ki Hyeon Kim,
Dong Ho Kim
Abstract:
We investigated high-frequency behavior of FeN thin films prepared by reactive sputtering through ferromagnetic resonance (FMR) and its relationship with the static magnetic properties. The FMR was observed in the frequency range from 2 to 18 GHz in the FeN films fabricated at proper nitrogen flow rate (NFR). In those FeN thin films, a decrease of the saturation magnetization and the corresponding…
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We investigated high-frequency behavior of FeN thin films prepared by reactive sputtering through ferromagnetic resonance (FMR) and its relationship with the static magnetic properties. The FMR was observed in the frequency range from 2 to 18 GHz in the FeN films fabricated at proper nitrogen flow rate (NFR). In those FeN thin films, a decrease of the saturation magnetization and the corresponding decrease of the FMR frequency were observed as NFR was increased during the deposition. The external field dependences of the FMR frequencies were well fit to the Kittel formula and the Landé g-factors determined from the fit were found to be very close to the free electron value. The high-field damping parameters were almost insensitive to the growth condition of NFR. However, the low-field damping parameters exhibited high sensitivity to NFR very similar to the dependence of the hard-axis coercivity on NFR, suggesting that extrinsic material properties such as impurities and defect structures could be important in deciding the low-field damping behavior.
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Submitted 19 May, 2016;
originally announced May 2016.
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Photoluminescence Saturation and Exciton Decay Dynamics in Transition Metal Dichalcogenide Monolayers
Authors:
Min Ju Shin,
Dong Hak Kim,
D. Lim
Abstract:
We report a photoluminescence (PL) and transient reflection spectroscopy study of exciton dynamics in monolayer transition transition-metal dichalcogenides (TMDs). PL saturation in monolayer MoSe2 occurs an excitation intensity more than two orders of magnitude lower than in monolayer MoS2. Transient reflection shows that the nonlinear exciton-exciton annihilation is the dominant exciton decay pro…
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We report a photoluminescence (PL) and transient reflection spectroscopy study of exciton dynamics in monolayer transition transition-metal dichalcogenides (TMDs). PL saturation in monolayer MoSe2 occurs an excitation intensity more than two orders of magnitude lower than in monolayer MoS2. Transient reflection shows that the nonlinear exciton-exciton annihilation is the dominant exciton decay process in monolayer MoSe2 in contrast to the previously reported linear exciton decay in monolayer MoS2. In addition, the exciton lifetime in MoSe2, > 125 ps, is more than an order of magnitude longer than the several-ps exciton lifetime in MoS2. We find that the dramatically different exciton decay mechanism and PL saturation behavior of MoSe2 and MoS2 monolayers can be explained by the difference in their exciton lifetime.
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Submitted 24 September, 2014; v1 submitted 16 September, 2014;
originally announced September 2014.
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Thickness and power dependence of the spin-pumping effect in Y3Fe5O12/Pt heterostructures measured by the inverse spin Hall effect
Authors:
M. B. Jungfleisch,
A. V. Chumak,
A. Kehlberger,
V. Lauer,
D. H. Kim,
M. C. Onbasli,
C. A. Ross,
M. Kläui,
B. Hillebrands
Abstract:
The dependence of the spin-pumping effect on the yttrium iron garnet (Y3Fe5O12, YIG) thickness detected by the inverse spin Hall effect (ISHE) has been investigated quantitatively. Due to the spin-pumping effect driven by the magnetization precession in the ferrimagnetic insulator YIG film a spin-polarized electron current is injected into the Pt layer. This spin current is transformed into electr…
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The dependence of the spin-pumping effect on the yttrium iron garnet (Y3Fe5O12, YIG) thickness detected by the inverse spin Hall effect (ISHE) has been investigated quantitatively. Due to the spin-pumping effect driven by the magnetization precession in the ferrimagnetic insulator YIG film a spin-polarized electron current is injected into the Pt layer. This spin current is transformed into electrical charge current by means of the ISHE. An increase of the ISHE-voltage with increasing film thickness is observed and compared to the theoretically expected behavior. The effective damping parameter of the YIG/Pt samples is found to be enhanced with decreasing YIG film thickness. The investigated samples exhibit a spin mixing conductance of g=(7.43 \pm 0.36) \times 10^{18} m^{-2} and a spin Hall angle of theta_{ISHE} = 0.009 \pm 0.0008. Furthermore, the influence of nonlinear effects on the generated voltage and on the Gilbert damping parameter at high excitation powers are revealed. It is shown that for small YIG film thicknesses a broadening of the linewidth due to nonlinear effects at high excitation powers is suppressed because of a lack of nonlinear multi-magnon scattering channels. We have found that the variation of the spin-pumping efficiency for thick YIG samples exhibiting pronounced nonlinear effects is much smaller than the nonlinear enhancement of the damping.
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Submitted 17 August, 2013;
originally announced August 2013.
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Determination of the origin of the spin Seebeck effect - bulk vs. interface effects
Authors:
Andreas Kehlberger,
René Röser,
Gerhard Jakob,
Ulrike Ritzmann,
Denise Hinzke,
Ulrich Nowak,
Mehmet C. Onbasli,
Dong Hun Kim,
Caroline A. Ross,
Matthias B. Jungfleisch,
Burkard Hillebrands,
Mathias Kläui
Abstract:
The observation of the spin Seebeck effect in insulators has meant a breakthrough for spin caloritronics due to the unique ability to generate pure spin currents by thermal excitations in insulating systems without moving charge carriers. Since the recent first observation, the underlying mechanism and the origin of the observed signals have been discussed highly controversially. Here we present a…
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The observation of the spin Seebeck effect in insulators has meant a breakthrough for spin caloritronics due to the unique ability to generate pure spin currents by thermal excitations in insulating systems without moving charge carriers. Since the recent first observation, the underlying mechanism and the origin of the observed signals have been discussed highly controversially. Here we present a characteristic dependence of the longitudinal spin Seebeck effect amplitude on the thickness of the insulating ferromagnet (YIG). Our measurements show that the observed behavior cannot be explained by any effects originating from the interface, such as magnetic proximity effects in the spin detector (Pt). Comparison to theoretical calculations of thermal magnonic spin currents yields qualitative agreement for the thickness dependence resulting from the finite effective magnon propagation length so that the origin of the effect can be traced to genuine bulk magnonic spin currents ruling out parasitic interface effects.
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Submitted 4 June, 2013;
originally announced June 2013.
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Instability of Amorphous Oxide Semiconductors via Carrier-Mediated Structural Transition between Disorder and Peroxide State
Authors:
Ho-Hyun Nahm,
Yong-Sung Kim,
Dae Hwan Kim
Abstract:
The excited holes occupying the valence band tail states in amorphous oxide semiconductors are found to induce formation of meta-stable O$_2^{2-}$ peroxide defects. The valence band tail states are at least partly characterized by the O-O ppσ* molecular orbital, and the localized-hole-mediated lattice instability results in the formation of the peroxide defects. Along with the O-O bond formation,…
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The excited holes occupying the valence band tail states in amorphous oxide semiconductors are found to induce formation of meta-stable O$_2^{2-}$ peroxide defects. The valence band tail states are at least partly characterized by the O-O ppσ* molecular orbital, and the localized-hole-mediated lattice instability results in the formation of the peroxide defects. Along with the O-O bond formation, the ppσ* state is heightened up into the conduction bands, and two electrons are accordingly doped in the electronic ground state. The energy barrier from the O$_2^{2-}$ peroxide state to the normal disorder state is found to be 0.97 eV in hybrid density functional theory. The hole-mediated formation of the meta-stable peroxide defects and their meta-stability is suggested as an origin of the negative bias and/or illumination stress instability in amorphous oxide semiconductors.
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Submitted 20 December, 2011;
originally announced December 2011.
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Fermi Surface Reconstruction in CeTe2 Induced by Charge Density Wave: ARPES Study
Authors:
J. -S. Kang,
D. H. Kim,
H. J. Lee,
J. H. Hwang,
Han-Koo Lee,
H. -D. Kim,
B. H. Min,
K. E. Lee,
Y. S. Kwon,
J. W. Kim,
Kyoo Kim,
B. H. Kim,
B. I. Min
Abstract:
Electronic structures of a charge-density-wave CDW system CeTe_2-xSb_x (x=0, 0.05) have been investigated by employing angle-resolved photoemission spectroscopy (ARPES). The observed Fermi surface (FS) agrees very well with the calculated FS for the undistorted CeTe_2 both in shapes and sizes. The metallic states crossing the Fermi level(E_F) are observed in ARPES. The carriers near E_F have mainl…
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Electronic structures of a charge-density-wave CDW system CeTe_2-xSb_x (x=0, 0.05) have been investigated by employing angle-resolved photoemission spectroscopy (ARPES). The observed Fermi surface (FS) agrees very well with the calculated FS for the undistorted CeTe_2 both in shapes and sizes. The metallic states crossing the Fermi level(E_F) are observed in ARPES. The carriers near E_F have mainly the Te(1) 5p character, with the negligible contribution from Ce 4f states to the CDW formation. The supercell (shadow) bands and the corresponding very weak FS's are found to arise from band-folding due to the interaction with Ce-Te(2) layers. This work shows that the origin of the CDW formation in CeTe_2 is the FS nesting and that the CDW modulation vector is along Gamma-X (Q_CDW ~ X)
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Submitted 2 September, 2011;
originally announced September 2011.
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Fabrication of FeSe1-x superconducting films with bulk properties
Authors:
Soon-Gil Jung,
Nam Hoon Lee,
Eun-Mi Choi,
Won Nam Kang,
Sung-Ik Lee,
Tae-Jong Hwang,
Dong Ho Kim
Abstract:
We have fabricated high-quality FeSe1-x superconducting films with a bulk Tc of 11-12 K on different substrates, Al2O3(0001), SrTiO3(100), MgO(100), and LaAlO3(100), by using a pulsed laser deposition technique. All the films were grown at a high substrate temperature of 610 oC, and were preferentially oriented along the (101) direction, the latter being to be a key to fabricating of FeSe1-x super…
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We have fabricated high-quality FeSe1-x superconducting films with a bulk Tc of 11-12 K on different substrates, Al2O3(0001), SrTiO3(100), MgO(100), and LaAlO3(100), by using a pulsed laser deposition technique. All the films were grown at a high substrate temperature of 610 oC, and were preferentially oriented along the (101) direction, the latter being to be a key to fabricating of FeSe1-x superconducting thin films with high Tc. According to the energy dispersive spectroscopy data, the Fe:Se composition ratio was 1:0.90+-0.02. The FeSe1-x film grown on a SrTiO3 substrate showed the best quality with a high upper critical magnetic field [Hc2(0)] of 56 T.
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Submitted 3 May, 2010;
originally announced May 2010.
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Potassium-doped BaFe2As2 superconducting thin films with a transition temperature of 40 K
Authors:
Nam Hoon Lee,
Soon-Gil Jung,
Dong Ho Kim,
Won Nam Kang
Abstract:
We report the growth of potassium-doped BaFe2As2 thin films, where the major charge carriers are holes, on Al2O3 (0001) and LaAlO3 (001) substrates by using an ex-situ pulsed laser deposition technique. The measured Tc's are 40 and 39 K for the films grown on Al2O3 and LaAlO3, respectively and diamagnetism indicates that the films have good bulk superconducting properties below 36 and 30 K, respec…
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We report the growth of potassium-doped BaFe2As2 thin films, where the major charge carriers are holes, on Al2O3 (0001) and LaAlO3 (001) substrates by using an ex-situ pulsed laser deposition technique. The measured Tc's are 40 and 39 K for the films grown on Al2O3 and LaAlO3, respectively and diamagnetism indicates that the films have good bulk superconducting properties below 36 and 30 K, respectively. The X-ray diffraction patterns for both films indicated a preferred c-axis orientation, regardless of the substrate structures of LaAlO3 and Al2O3. The upper critical field at zero temperature was estimated to be about 155 T.
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Submitted 27 April, 2010;
originally announced April 2010.
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In-situ fabrication of cobalt-doped SrFe2As2 thin films by using pulsed laser deposition with excimer laser
Authors:
Eun-Mi Choi,
Soon-Gil Jung,
Nam Hoon Lee,
Young-Seung Kwon,
Won Nam Kang,
Dong Ho Kim,
Myung-Hwa Jung,
Sung-Ik Lee,
Liling Sun
Abstract:
The remarkably high superconducting transition temperature and upper critical field of iron(Fe)-based layered superconductors, despite ferromagnetic material base, open the prospect for superconducting electronics. However, success in superconducting electronics has been limited because of difficulties in fabricating high-quality thin films. We report the growth of high-quality c-axis-oriented c…
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The remarkably high superconducting transition temperature and upper critical field of iron(Fe)-based layered superconductors, despite ferromagnetic material base, open the prospect for superconducting electronics. However, success in superconducting electronics has been limited because of difficulties in fabricating high-quality thin films. We report the growth of high-quality c-axis-oriented cobalt(Co)-doped SrFe2As2 thin films with bulk superconductivity by using an in-situ pulsed laser deposition technique with a 248-nm-wavelength KrF excimer laser and an arsenic(As)-rich phase target. The temperature and field dependences of the magnetization showing strong diamagnetism and transport critical current density with superior Jc-H performance are reported. These results provide necessary information for practical applications of Fe-based superconductors.
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Submitted 24 July, 2009;
originally announced July 2009.
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Effect of epitaxial strain on ferroelectric polarization in multiferroic BiFeO3 films
Authors:
Dae Ho Kim,
Ho Nyung Lee,
Michael D. Biegalski,
Hans M. Christen
Abstract:
Multiferroic BiFeO3 epitaxial films with thickness ranging from 40 nm to 960 nm were grown by pulsed laser deposition on SrTiO3 (001) substrates with SrRuO3 bottom electrodes. X-ray characterization shows that the structure evolves from angularly-distorted tetragonal with c/a ~ 1.04 to more bulk-like distorted rhombohedral (c/a ~ 1.01) as the strain relaxes with increasing thickness. Despite thi…
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Multiferroic BiFeO3 epitaxial films with thickness ranging from 40 nm to 960 nm were grown by pulsed laser deposition on SrTiO3 (001) substrates with SrRuO3 bottom electrodes. X-ray characterization shows that the structure evolves from angularly-distorted tetragonal with c/a ~ 1.04 to more bulk-like distorted rhombohedral (c/a ~ 1.01) as the strain relaxes with increasing thickness. Despite this significant structural evolution, the ferroelectric polarization along the body diagonal of the distorted pseudo-cubic unit cells, as calculated from measurements along the normal direction, barely changes.
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Submitted 11 December, 2007; v1 submitted 27 October, 2007;
originally announced October 2007.
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Surface versus bulk characterization of the electronic inhomogeneity in a VO_{2} film
Authors:
Y. J. Chang,
J. S. Yang,
Y. S. Kim,
D. H. Kim,
T. W. Noh,
D. -W. Kim,
E. OH,
B. Kahng,
J. -S. Chung
Abstract:
We investigated the inhomogeneous electronic properties at the surface and interior of VO_{2} thin films that exhibit a strong first-order metal-insulator transition (MIT). Using the crystal structural change that accompanies a VO_{2} MIT, we used bulk-sensitive X-ray diffraction (XRD) measurements to estimate the fraction of metallic volume p^{XRD} in our VO_{2} film. The temperature dependence…
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We investigated the inhomogeneous electronic properties at the surface and interior of VO_{2} thin films that exhibit a strong first-order metal-insulator transition (MIT). Using the crystal structural change that accompanies a VO_{2} MIT, we used bulk-sensitive X-ray diffraction (XRD) measurements to estimate the fraction of metallic volume p^{XRD} in our VO_{2} film. The temperature dependence of the p$^{XRD}$ was very closely correlated with the dc conductivity near the MIT temperature, and fit the percolation theory predictions quite well: $σ$ $\sim$ (p - p_{c})^{t} with t = 2.0$\pm$0.1 and p_{c} = 0.16$\pm$0.01. This agreement demonstrates that in our VO$_{2}$ thin film, the MIT should occur during the percolation process. We also used surface-sensitive scanning tunneling spectroscopy (STS) to investigate the microscopic evolution of the MIT near the surface. Similar to the XRD results, STS maps revealed a systematic decrease in the metallic phase as temperature decreased. However, this rate of change was much slower than the rate observed with XRD, indicating that the electronic inhomogeneity near the surface differs greatly from that inside the film. We investigated several possible origins of this discrepancy, and postulated that the variety in the strain states near the surface plays an important role in the broad MIT observed using STS. We also explored the possible involvement of such strain effects in other correlated electron oxide systems with strong electron-lattice interactions.
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Submitted 30 July, 2007;
originally announced July 2007.
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Kinetic Roughening of Ion-Sputtered Pd(001) Surface: Beyond the Kuramoto-Sivashinsky Model
Authors:
T. C. Kim,
C. -M. Ghim,
H. J. Kim,
D. H. Kim,
D. Y. Noh,
N. D. Kim,
J. W. Chung,
J. S. Yang,
Y. J. Chang,
T. W. Noh,
B. Kahng,
J. -S. Kim
Abstract:
We investigate the kinetic roughening of Ar$^+$ ion-sputtered Pd(001) surface both experimentally and theoretically. \textit{In situ} real-time x-ray reflectivity and \textit{in situ} scanning tunneling microscopy show that nanoscale adatom islands form and grow with increasing sputter time $t$. Surface roughness, $W(t)$, and lateral correlation length, $ξ(t)$, follows the scaling laws,…
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We investigate the kinetic roughening of Ar$^+$ ion-sputtered Pd(001) surface both experimentally and theoretically. \textit{In situ} real-time x-ray reflectivity and \textit{in situ} scanning tunneling microscopy show that nanoscale adatom islands form and grow with increasing sputter time $t$. Surface roughness, $W(t)$, and lateral correlation length, $ξ(t)$, follows the scaling laws, $W(t)\sim t^β$ and $ξ(t)\sim t^{1/z}$ with the exponents $β\simeq 0.20$ and $1/z\simeq 0.20$, for ion beam energy $ε=0.5$ keV, which is inconsistent with the prediction of the Kuramoto-Sivashinsky (KS) model. We thereby extend the KS model by applying the Sigmund theory of sputter erosion to the higher order, ${\cal O}(\nabla^4, h^2)$, where $h$ is surface height, and derive a new term of the form $\nabla^2(\nabla h)^2$ which plays an indispensable role in describing the observed morphological evolution of the sputtered surface.
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Submitted 16 September, 2003;
originally announced September 2003.
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Formation of Co nanoclusters in epitaxial Ti_{0.96} Co_{0.04} O_2 thin films and their ferromagnetism
Authors:
D. H. Kim,
J. S. Yang,
K. W. Lee,
S. D. Bu,
T. W. Noh,
S. -J. Oh,
Y. -W. Kim,
J. -S. Chung,
H. Tanaka,
H. Y. Lee,
T. Kawai
Abstract:
Anatase Ti0.96Co0.04O2 films were grown epitaxially on SrTiO3 (001) substrates by using plused laser deposition with in-situ reflection high-energy electron diffraction. The oxygen partial pressure, PO2, during the growth was systematically varied. As PO2 decreased, the growth behavior was changed from a 2-dimensional layer-by-layer-like growth to a 3-dimensional island-like one, which resulted…
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Anatase Ti0.96Co0.04O2 films were grown epitaxially on SrTiO3 (001) substrates by using plused laser deposition with in-situ reflection high-energy electron diffraction. The oxygen partial pressure, PO2, during the growth was systematically varied. As PO2 decreased, the growth behavior was changed from a 2-dimensional layer-by-layer-like growth to a 3-dimensional island-like one, which resulted in an increase in the saturation magnetization. These structural and magnetic changes were explained in terms of the formation of cobalt clusters whose existence was proved by transmission-electron-microscopie studies. Our work clearly indicates that the cobalt clustering will cause room-temperature ferromagnetism in the Co-doped TiO2 films.
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Submitted 3 August, 2002;
originally announced August 2002.
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Universal scaling of Hall resistivity in clean and moderately clean limits for Hg- and Tl-based superconductors
Authors:
W. N. Kang,
Wan-Seon Kim,
S. J. Oh,
Sung-Ik Lee,
D. H. Kim,
C. H. Choi,
H. -C. Ri,
C. W. Chu
Abstract:
The mixed-state Hall resistivity and the longitudinal resistivity in HgBa_{2}CaCu_{2}O_{6}, HgBa_{2}Ca_{2}Cu_{3}O_{8}, and Tl_{2}Ba_{2}CaCu_{2}O_{8} thin films have been investigated as functions of the magnetic field up to 18 T. We observe the universal scaling behavior between ρ_{xy} and ρ_{xx} in the regions of the clean and the moderately clean limit. The scaling exponent βis 1.9 in the clea…
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The mixed-state Hall resistivity and the longitudinal resistivity in HgBa_{2}CaCu_{2}O_{6}, HgBa_{2}Ca_{2}Cu_{3}O_{8}, and Tl_{2}Ba_{2}CaCu_{2}O_{8} thin films have been investigated as functions of the magnetic field up to 18 T. We observe the universal scaling behavior between ρ_{xy} and ρ_{xx} in the regions of the clean and the moderately clean limit. The scaling exponent βis 1.9 in the clean limit at high field and low temperature whereas βis 1.0 in the moderately clean limit at low field and high temperature, consistent with a theory based on the midgap states in the vortex cores. This finding implies that the Hall conductivity is also universal in Hg- and Tl-based superconductors.
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Submitted 14 May, 1999;
originally announced May 1999.
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Theory of spin excitations in undoped and underdoped cuprates
Authors:
Don H. Kim,
Patrick A. Lee
Abstract:
We consider the magnetic properties of high Tc cuprates from a gauge theory point of view, with emphasis on the underdoped regime. Underdoped cuprates possess certain antiferromagnetic correlations, as evidenced, for example, by different temperature dependence of the Cu and O site NMR relaxation rates, that are not captured well by slave boson mean field theories of the t-J model. We show that…
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We consider the magnetic properties of high Tc cuprates from a gauge theory point of view, with emphasis on the underdoped regime. Underdoped cuprates possess certain antiferromagnetic correlations, as evidenced, for example, by different temperature dependence of the Cu and O site NMR relaxation rates, that are not captured well by slave boson mean field theories of the t-J model. We show that the inclusion of gauge fluctuations will remedy the deficiencies of the mean field theories. As a concrete illustration of the gauge-fluctuation restoration of the antiferromangetic correlation and the feasibility of the 1/N perturbation theory, the Heisenberg spin chain is analyzed in terms of a 1+1D U(1) gauge theory with massless Dirac fermions. The 1/N-perturbative treatment of the same gauge theory in 2+1D (which can be motivated from the mean field pi-flux phase of the Heisenberg model) leads to a dynamical mass generation corresponding to an antiferromagnetic ordering. On the other hand, it is argued that in a similar gauge theory with an additional coupling to a Bose (holon) field, symmetry breaking does not occur, but antiferromagnetic correlations are enhanced, which is the situation in the underdoped cuprates.
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Submitted 24 March, 1999; v1 submitted 12 October, 1998;
originally announced October 1998.
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Spin-Landau Orbit Coupling in Units of the Flux Quantum Observed from Zeeman Splitting of a Quantum Wire Array
Authors:
J. C. Woo,
W. S. Kim,
H. S. Ko,
Y. A. Leem,
Y. S. Kim,
D. S. Kim,
M. Y. Choi,
D. H. Kim,
T. Schmiedel
Abstract:
In Zeeman spectra of a GaAs-AlGaAs quantum wire array with superlattice period lp, the zero-field shift is found to increase in steps of (2 pi hbar/ e)lp^{-2} at the cyclotron radius Rc ~lp, lp/2, and lp/3. This shift, caused by spin-Landau orbit coupling and quantized in units of the flux quantum 2 pi hbar /e,is a manifestation of the gauge invariance, and the first observation of the flux quan…
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In Zeeman spectra of a GaAs-AlGaAs quantum wire array with superlattice period lp, the zero-field shift is found to increase in steps of (2 pi hbar/ e)lp^{-2} at the cyclotron radius Rc ~lp, lp/2, and lp/3. This shift, caused by spin-Landau orbit coupling and quantized in units of the flux quantum 2 pi hbar /e,is a manifestation of the gauge invariance, and the first observation of the flux quantization in a non-superconducting material.The quantum interference associated with the phase difference in the quantum barrier scattering is also reported.
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Submitted 14 April, 1998;
originally announced April 1998.
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Massless Dirac Fermions, Gauge Fields, and Underdoped Cuprates
Authors:
Don H. Kim,
Patrick A. Lee,
Xiao-Gang Wen
Abstract:
We study 2+1 dimensional massless Dirac fermions and bosons coupled to a U(1) gauge field as a model for underdoped cuprates. We find that the uniform susceptibility and the specific heat coefficient are logarithmically enhanced (compared to linear-in-T behavior) due to the fluctuation of transverse gauge field which is the only massless mode at finite boson density. We analyze existing data, an…
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We study 2+1 dimensional massless Dirac fermions and bosons coupled to a U(1) gauge field as a model for underdoped cuprates. We find that the uniform susceptibility and the specific heat coefficient are logarithmically enhanced (compared to linear-in-T behavior) due to the fluctuation of transverse gauge field which is the only massless mode at finite boson density. We analyze existing data, and find good agreement in the spin gap phase. Within our picture, the drop of the susceptibility below the superconducting T_c arises from the suppression of gauge fluctuations.
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Submitted 25 February, 1997;
originally announced February 1997.
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Bosons, gauge fields, and high-Tc cuprates
Authors:
Don H. Kim,
Derek K. K. Lee,
Patrick A. Lee
Abstract:
A simple model of a degenerate two-dimensional Bose liquid interacting with a fluctuating gauge field is investigated as a possible candidate to describe the charge degree of freedom in the normal state of the cuprate superconductors. We show that the fluctuating gauge field destroys superfluidity even in the Bose degenerate regime. We discuss the nature of the resulting normal state in terms of…
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A simple model of a degenerate two-dimensional Bose liquid interacting with a fluctuating gauge field is investigated as a possible candidate to describe the charge degree of freedom in the normal state of the cuprate superconductors. We show that the fluctuating gauge field destroys superfluidity even in the Bose degenerate regime. We discuss the nature of the resulting normal state in terms of the geometric properties of the imaginary-time paths of the bosons. We will also present numerical results on the transport properties and the density correlations in the system. We find a transport scattering rate of the order of 2T, consistent with experiments on the optimally-doped cuprates. We also find that the density correlations of our model resemble the charge correlations of the t-J model.
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Submitted 21 July, 1996;
originally announced July 1996.
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Degenerate Bose liquid in a fluctuating gauge field
Authors:
Derek K. K. Lee,
Don H. Kim,
Patrick A. Lee
Abstract:
We study the effect of a strongly fluctuating gauge field on a degenerate Bose liquid, relevant to the charge degrees of freedom in doped Mott insulators. We find that the superfluidity is destroyed. The resulting metallic phase is studied using quantum Monte Carlo methods. Gauge fluctuations cause the boson world lines to retrace themselves. We examine how this world-line geometry affects the p…
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We study the effect of a strongly fluctuating gauge field on a degenerate Bose liquid, relevant to the charge degrees of freedom in doped Mott insulators. We find that the superfluidity is destroyed. The resulting metallic phase is studied using quantum Monte Carlo methods. Gauge fluctuations cause the boson world lines to retrace themselves. We examine how this world-line geometry affects the physical properties of the system. In particular, we find a transport relaxation rate of the order of 2kT, consistent with the normal state of the cuprate superconductors. We also find that the density excitations of this model resemble that of the full tJ model.
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Submitted 2 January, 1996;
originally announced January 1996.