Browsing by Author "Lin, J-Y"

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3D visualizations of nanoscale phase separation and ultrafast dynamic correlation between phases in (Na0.32K0.68)(0.95)Fe1.75Se2
(2019-03-25)
Cheng, P. C.
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Tzeng, W. Y.
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Chu, Y. J.
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Luo, C. W.
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Zhukov, A. A.
Phase separation of metallic and antiferromagnetic (AFM) insulating phases in alkaline iron selenides (A(x)Fe(2)-ySe(2)) continues to attract intense interest because the relationship between two peculiar features probably is a key to clarifying the pairing mechanism of A(x)Fe(2-y)Se(2) superconductors. Here we report that the three-dimensional (3D) visualizations of nanoscale phase separation in (Na0.32K0.68)(0.95)Fe1.75Se2 single crystals are revealed by hybrid focused-ion-beam scanning electron microscopy and the superconducting paths are fully percolative in 3D. Moreover, the phase-related ultrafast dynamics in (Na0.32K0.68)(0.95)Fe1.75Se2 is studied by dual-color pump-probe spectroscopy. The anomalous changes in the electron and acoustic phonon components of transient reflectivity change (Delta R/R) identify two characteristic temperatures T* similar to 100K (the onset temperature of coupling between the nanometallic and AFM phases) and T-H similar to 230K (the onset temperature of the metallic-interface phase). An energy-transfer channel between the nanometallic and AFM phases is inferred. This proposed channel provides insight into the pairing mechanism of alkaline iron selenide superconductors.
A low-temperature specific heat study of giant dielectric constant materials
(2008-07-16)
Sun, C. P.
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Liu, Jianjun
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Lin, J-Y
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Duan, Chun-Gang
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Mei, W. N.
A low-temperature specific heat study has been performed on the insulating giant dielectric constant material CaCu(3)Ti(4)O(12) and two related compounds, Bi(2/3)Cu(3)Ti(4)O(12) and La(0.5)Na(0.5)Cu(3)Ti(4)O(12), from 0.6 to 10 K. From analyzing the specific heat data in a very low-temperature range, 0.6-1.5 K, and moderately low-temperature range, 1.5-5 K, in addition to the expected Debye terms, we observed significant contributions originating from the linear and Einstein terms, which we attributed as the manifestation of low-lying elementary excitations due to lattice vibrations occurring at the grain boundaries and induced by local defects. Together with the findings on electronic and mechanical properties, a phenomenological model is proposed to explain the high dielectric constant behavior in both low and high frequency regions.
Anisotropy in the upper critical field of FeSe and FeSe0.33Te0.67 single crystals
(2015-04-01)
Her, J. L.
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Kohama, Y.
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Matsuda, Y. H.
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Kindo, K.
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Yang, W-H
The electric resistivity of single crystalline Fe-based superconductors FeSe and FeSe0.33Te0.67 was measured in pulsed magnetic fields up to 55 T. Te-doped iron selenide, FeSe0.33Te0.67, shows rather isotropic behavior in both magnetoresistance (MR) and upper critical fields (H-c2(perpendicular to)(0)/H-c2(parallel to)(0) = 48 T/42 T similar to 1.14), when an external magnetic field is applied parallel and perpendicular to the crystallographic c-axis. These isotropic behaviors are frequently observed in an iron-based superconductor, and the isotropy in H-c2 can likely be described by the Pauli effect. In contrast, our measurements elucidate that the undoped iron selenide, FeSe, exhibits a clear anisotropy in both MR and H-c2 (H-c2(perpendicular to)(0)/H-c2(parallel to)(0) = 27 T/15 T similar to 1.8). This behavior occurs because the clean in-plane conduction (RRR = 35) allows to form a closed electron orbit within the two-dimensional Fermi sheets only for the magnetic field parallel to the c-axis, which results in an appearance of an intrinsic (orbital-effect limited) upper critical field for the undoped FeSe. Doping of Te breaks the clean in-plane conduction and leads to a crossover from an orbital limit dominating behavior to a Pauli limit dominating one.
Coherent acoustic phonons in hexagonal HoMnO(3) probed by femtosecond spectroscopy
(2010)
Luo, C. W.
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Shih, H. C.
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Wu, K. H.
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Lin, J-Y
;
Juang, J. Y.
We have observed the coherent acoustic phonons in the hexagonal manganite HoMnO(3) single crystals using wavelength-dependent femtosecond pump-probe spectroscopy. The oscillatory component of the photoinduced transient reflectivity change (Delta R/R) at various pump-probe wavelengths is consistent with the model of strain pulse propagation. Furthermore, the oscillation period of the oscillation is strong temperature- and wavelength-dependent. In particular, the period of all oscillations in Delta R/R for various wavelengths are very similar in the vicinity of the Neel temperature (T(N)), which associates with the appearance of the antiferromagnetic (AFM) long-range ordering.
Controllable subwavelength-ripple and -dot structures on YBa2Cu3O7 induced by ultrashort laser pulses
(2012-11-01)
Luo, C. W.
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Tang, W. T.
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Wang, H. I.
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Liao, L. W.
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Lo, H. P.
The evolution of periodic ripples and dots on an YBa2Cu3O7 surface irradiated by single-beam and dual-beam femtosecond laser pulses is investigated. The density and size of the dots can be controlled by the number of pulses. The spatial period of these ripples does not show a close relation to the pulse number. However, the orientation and geometrical period of the ripples are strongly dependent on the polarization and incident angle of the laser pulses, respectively. The formation processes are also discussed based on the dependence.
Effects of compressive epitaxial strain in the b-axis on the magnetization response of orthorhombic HoMnO(3) thin films
(2009)
Lin, T. H.
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Hsieh, C. C.
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Shih, H. C.
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Luo, C. W.
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Uen, T. M.
The orthorhombic phase HoMnO(3) (o-HMO in Pbnm symmetry setting) thin films were prepared on LaAlO(3)(110) (LAO(110)) substrates by pulsed laser deposition. While for films grown on LAO(110) substrates, the compressive strain along the b-axis was resulted from the tensile strain in the a-c plane. The films provide the opportunity of investigating the effects of strain, hence lattice elasticity, on the physical properties of this material. For o-HMO films an antiferromagnetic ordering with T(N)similar to 42 K, irrespective to the direction of applied field was clearly observed. However, an additional magnetic ordering occurring around 26.4 K was observed when the field was applied along the c-axis of o-HMO. This transition, however, was absent when the field was applied along a-and b-axis. These results indicate that the second magnetic ordering observed along the c-axis could be more relevant to the Mn moments lying along the partially strained b direction of the o-HMO which has been theoretically expected to result in incommensurate-commensurate lock-in transition.
Electronic Structure and Magnetic Anisotropies in Orthorhombic Multiferroic YMnO(3) Thin Films
(2009-01-01)
Hsieh, C. C.
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Lin, T. H.
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Shih, H. C.
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Lin, J-Y
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Hsu, C-H
Thermodynamically YMnO(3) (YMO) can only exist with hexagonal structure at ambient conditions. In this study, we demonstrate that the stain between film and substrate can play an important role in forming orthorhombic YMO (o-YMO). o-YMO films with nearly perfect crystallographic orientation alignment were obtained on LaAlO(3)(110) substrates. These films allow us to unambiguously disclose the intrinsic magnetic property and electronic structure anisotropies along different crystallographic orientations. The antiferromagnetic (AFM) phase transition was around 42 K. Moreover, we observed a signature of spin reordering at lower temperatures when the applied field was parallel to the a- or c-axis. On the other hand, only the AFM transition was observed when the field is parallel to the b-axis. By comparing the X-ray absorption spectroscopy (XAS) results to standard manganese oxides, YMnO(3) exhibits the dominant Mn(+3) characteristics as that obtained from the standard Mn(2)O(3) powder. The polarization dependent XAS also revealed anisotropic bonding behavior along each crystalline axis.
Electronic Structure and Magnetic Anisotropies in Orthorhombic Multiferroic YMnO3 Thin Films
(2009-01-01)
Hsieh, C. C.
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Lin, T. H.
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Shih, H. C.
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Lin, J-Y
;
Hsu, C-H
Thermodynamically YMnO3 (YMO) can only exist with hexagonal structure at ambient conditions. In this study, we demonstrate that the stain between film and substrate can play an important role in forming orthorhombic YMO (o-YMO). o-YMO films with nearly perfect crystallographic orientation alignment were obtained on LaAlO3(110) substrates. These films allow us to unambiguously disclose the intrinsic magnetic property and electronic structure anisotropies along different crystallographic orientations. The antiferromagnetic (AFM) phase transition was around 42 K. Moreover, we observed a signature of spin reordering at lower temperatures when the applied field was parallel to the a- or c-axis. On the other hand, only the AFM transition was observed when the field is parallel to the b-axis. By comparing the X-ray absorption spectroscopy (XAS) results to standard manganese oxides, YMnO3 exhibits the dominant Mn+3 characteristics as that obtained from the standard Mn2O3 powder. The polarization dependent XAS also revealed anisotropic bonding behavior along each crystalline axis.
Electronic structure of pyrochlore Cd(2)Re(2)O(7)
(2009-05-13)
Huang, Shih-Wen
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Jeng, Horng-Tay
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Lin, J-Y
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Chang, W. J.
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Chen, J. M.
Detailed band structure calculations have been performed for Cd(2)Re(2)O(7) in high-, middle- and low-temperature (T) phases. The calculations are based on the observed lattice structures from x-ray diffraction measurements. The spin-orbit interaction is incorporated self-consistently in both the generalized gradient approximation (GGA) and the GGA plus Hubbard U (GGA + U) approaches. It is found that the on-site U has negligible effects on the Re 5d band structures; therefore both the GGA and GGA + U Re 5d band energies agree well with the observed O K-edge x-ray absorption spectroscopy (XAS) spectrum, whereas the Cd 4d band energy observed from photoemission spectroscopy can only be correctly reproduced by GGA + U calculations, indicating the relatively itinerant Re 5d and localized Cd 4d electrons. On the other hand, the spin-orbit coupling gives rise to nontrivial spin and orbital magnetic moments for the middle-T phase. Most unexpectedly, we found that the low-T phase exhibits quasi-two-dimensional Fermi surfaces. The calculated carrier numbers for the three phases are, at least qualitatively, consistent with the measured Hall coefficient.
Electronic structure of pyrochlore Cd2Re2O7
(2009-05-13)
Huang, Shih-Wen
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Jeng, Horng-Tay
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Lin, J-Y
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Chang, W. J.
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Chen, J. M.
Detailed band structure calculations have been performed for Cd2Re2O7 in high-, middle- and low-temperature (T) phases. The calculations are based on the observed lattice structures from x-ray diffraction measurements. The spin-orbit interaction is incorporated self-consistently in both the generalized gradient approximation (GGA) and the GGA plus Hubbard U (GGA + U) approaches. It is found that the on-site U has negligible effects on the Re 5d band structures; therefore both the GGA and GGA + U Re 5d band energies agree well with the observed O K-edge x-ray absorption spectroscopy (XAS) spectrum, whereas the Cd 4d band energy observed from photoemission spectroscopy can only be correctly reproduced by GGA + U calculations, indicating the relatively itinerant Re 5d and localized Cd 4d electrons. On the other hand, the spin-orbit coupling gives rise to nontrivial spin and orbital magnetic moments for the middle-T phase. Most unexpectedly, we found that the low-T phase exhibits quasi-two-dimensional Fermi surfaces. The calculated carrier numbers for the three phases are, at least qualitatively, consistent with the measured Hall coefficient.
Investigation of normal and superconducting states in noncentrosymmetric Re24Ti5
(2013-05-01)
Lue, C. S.
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Liu, H. F.
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Kuo, C. N.
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Shih, P. S.
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Lin, J-Y
We report a detailed characterization of the noncentrosymmetric superconductor Re24Ti5 using powder x-ray diffraction (XRD), magnetic susceptibility, electrical resistivity, thermal conductivity, Seebeck coefficient, and specific heat measurements. Rietveld refinement of powder XRD data confirms that Re24Ti5 crystallizes in the alpha-Mn structure. All measured quantities demonstrate a bulk superconducting transition at T-c = 5.8 K. Our low-temperature specific heat data measured down to 0.5 K yield a Sommerfeld coefficient gamma = 111.8 mJ mol(-1) K-2, which implies a high density of states at the Fermi level. Moreover, the electronic specific heat in the superconducting state was found to obey a typical s-wave expression, revealing a single gap Delta/k(B) = 10.6 K. This value gives a ratio of 2D/k(B)T(c) = 3.68, higher than the value of 3.5 predicted from BCS theory. On this basis, we conclude that the noncentrosymmetric Re24Ti5 compound can be characterized as a moderately coupled BCS-type superconductor. Furthermore, the obtained parameters from the present study of Re24Ti5 were compared to those of the isostructural compound Re23.8Nb5.2, indicating the similarity between both systems.
Low-temperature specific heat of superconductors II: Progress in the new era
(2019-10-01)
Yang, H. D.
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Lin, J-Y
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Lin, J. G.
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Ho, J. C.
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交大名義發表
Beginning in the late 1980s, many unexpected superconductors have emerged. They raised not only the transition temperature limit but, just as importantly, the theoretical challenges beyond the BCS theory. Following the previous article on historical perspective of low-temperature specific heat of superconductors prior to 1986, we review briefly here some calorimetric studies on representative high-T-c and other novel superconductors discovered during the new era. Major emphasis is centered on special features of each set of temperature- and magnetic field-dependent specific heat data being fitted to select theoretical models. The models may involve multi-gap or a certain type of order-parameter symmetry.
Magnetic transition anisotropies in orthorhombic LuMnO(3) and HoMnO(3) multiferroic thin films
(2010)
Tsai, T. Y.
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Lin, T. H.
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Slowry, S.
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Luo, C. W.
;
Wu, K. H.
We have successfully prepared the b-axis-oriented orthorhombic LuMnO(3) (LuMO) and HoMnO(3) (HMO) thin films by pulsed laser deposition on (110)-LaAlO(3) substrates. The nearly perfect alignment between the film growth orientation and the substrate allows us to study the magnetic transitions along the respective crystal orientation, which has displayed marked anisotropic behaviours. In particular, with the largest ionic size difference between Lu and Ho for the family of RMnO(3) displaying the E-type AFM, the effects of lattice distortion on the magnetic transition are compared.
Marked enhancement of Neel temperature in strained YMnO(3) thin films probed by femtosecond spectroscopy
(2011-04-01)
Wu, K. H.
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Chen, H-J
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Chen, Y. T.
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Hsieh, C. C.
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Luo, C. W.
In this paper, the (001)-oriented hexagonal YMnO(3) (h-YMO) thin films with various nominal strain states were deposited on MgO(100), MgO(111) and YSZ(111) substrates by pulsed laser deposition. The ultrafast dynamics probed by wavelength-tunable femtosecond pump-probe spectroscopy was performed to disclose the effects of the epitaxial strain on the electronic structure and associated magnetism. Analyses based on the measured transient-reflectivity-change (Delta R/R) curves revealed that while the magnitude of the on-site Mn d-d transition energy E(dd)(T) changes only slightly with the variation of the lattice constant ratio c/a, a marked shift in the antiferromagnetic (AFM) Neel temperature T(N) was observed and the direction of shift was dependent on the type of strain. The possible mechanism for the observed strain effect on the AFM ordering is discussed. Copyright (C) EPLA, 2011
Marked enhancement of Neel temperature in strained YMnO3 thin films probed by femtosecond spectroscopy
(2011-04-01)
Wu, K. H.
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Chen, H-J
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Chen, Y. T.
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Hsieh, C. C.
;
Luo, C. W.
In this paper, the (001)-oriented hexagonal YMnO3 (h-YMO) thin films with various nominal strain states were deposited on MgO(100), MgO(111) and YSZ(111) substrates by pulsed laser deposition. The ultrafast dynamics probed by wavelength-tunable femtosecond pump-probe spectroscopy was performed to disclose the effects of the epitaxial strain on the electronic structure and associated magnetism. Analyses based on the measured transient-reflectivity-change (Delta R/R) curves revealed that while the magnitude of the on-site Mn d-d transition energy E-dd(T) changes only slightly with the variation of the lattice constant ratio c/a, a marked shift in the antiferromagnetic (AFM) Neel temperature T-N was observed and the direction of shift was dependent on the type of strain. The possible mechanism for the observed strain effect on the AFM ordering is discussed. Copyright (C) EPLA, 2011
Metamagnetic transitions and magnetoelectric coupling in acentric and nonpolar Pb2MnO4
(2019-05-16)
Kakarla, D. Chandrasekhar
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Wu, H. C.
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Hsieh, D. J.
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Sun, P. J.
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Dai, G. J.
Acentric and nonpolar Pb2MnO4 was predicted to exhibit unique multipiezo induced magnetoelectric (ME) phenomena. In this paper, we present the results of magnetization as well as dielectric properties as a function of temperature (T), magnetic field (H), pressure (P), and electric field (E) primarily to address the ME coupling and identify the underlying mechanism behind this phenomenon. Magnetization and specific-heat measurements reveal the antiferromagnetic ordering of Mn4+ spins at temperature T-N= 17 K. Metamagnetic transitions at three critical magnetic fields H-c1, H-c2 and H-c3) are observed for T < T-N and H > 3.5 T. Further, the influences of pressure and magnetic field on H-c1 and H-c2 are investigated. The T-N, H-c1, and H(c2)( )all decrease with increasing external pressure. The dielectric anomaly observed at T-N is influenced by applying a magnetic field of H > 3.5 T. However, the electric field has minimal influence on the metamagnetic transition. The scaling between dielectric constant and magnetization meaningfully resolves the existence of magnetic-field-induced higher-order ME coupling in Pb2MnO4 at T < T-N and H > 3.5 T.
Microstructure and magnetic properties of oxidized titanium nitride thin films in situ grown by pulsed laser deposition
(2013-02-20)
Chen, S. C.
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Sung, K. Y.
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Tzeng, W. Y.
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Wu, K. H.
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Juang, J. Y.
Different oxidation states of titanium nitride thin films, including pure TiN(h 0 0), TiN1-xOx(h 0 0), Ti2O3(0 0 l) and pure anatase TiO2(0 0 l), were prepared by pulsed laser deposition with various oxygen pressures (P-O2) using a TiN target. Elaborative evolutions of the crystal and electronic structures of the obtained films were examined systematically by x-ray diffraction and x-ray absorption spectroscopy. We found that the Ti2O3(0 0 l) film, which was prepared at oxygen pressures P-O2 = 10(-4) Torr, exhibited the maximum room temperature ferromagnetism (RTFM) behaviour. The bound magnetic polaron model is used to clarify the origin of RTFM in these films.
Mn vacancy defects, grain boundaries, and A-phase stability of helimagnet MnSi
(2016-01-20)
Ou-Yang, T. Y.
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Shu, G. J.
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Lin, J-Y
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Hu, C. D.
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Chou, F. C.
Mn vacancy defect and grain size are shown to modify the magnetic phase diagram of MnSi significantly, especially near the critical regime of A-phase (skyrmion lattice) formation and the helimagnetic phase transition. Crystals grown using controlled nonstoichiometric initial precursors creates both grain boundaries and intrinsic Mn vacancy defect of various levels in MnSi. The results of combined transport, specific heat, and AC spin susceptibility measurements are compared for MnSi single crystal samples of various manganese deficiency levels and grain sizes. The finite-size effect and Mn vacancy level dependent helical phase transition temperature T-c have been identified and verified. The stability of A-phase in H-T phase space has been examined through AC spin susceptibility data analysis.
Observation of oscillation like magnetocaloric effect in multiferroic Ni0.95Zn0.05Cr2O4
(2019-01-15)
Chandrasekhar, K. Devi
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Murthy, J. Krishna
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Lin, J-Y
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Wu, H. C.
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Yang, H. D.
In this study, the magnetic and magnetocaloric effects of Ni0.95Zn0.05Cr2O4 have been investigated systematically. The distinct magnetic phases in different temperature zones were identified using a combined analysis of Arrott plots and virgin magnetization curves. The magnetocaloric phenomenon is sensitive to the spin correlation of the nickel and chromium magnetic sublattices within each distinct temperature zone, as is reflected by the modulation of the magnetocaloric sign. This unique oscillatory like magnetocaloric effect within the multiferroic phase may indicate the possibility of a multicaloric effect in Ni0.95Zn0.05Cr2O4. (C) 2018 Elsevier B.V. All rights reserved.
Polarization-dependent x-ray absorption spectroscopy of hexagonal and orthorhombic TbMnO(3) thin films
(2010-01-01)
Wu, K. H.
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Gou, I. C.
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Luo, C. W.
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Uen, T. M.
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Lin, J-Y
Pure phase TbMnO(3) manganite thin films with hexagonal (h-TMO) and orthorhombic (o- TMO) crystal structures were prepared by pulsed laser deposition. The distinctive orientation alignments between film and substrate obtained here have allowed us to perform the x-ray absorption near edge spectroscopy (XANES) measurements with the electric field applied along the three major crystallographic directions. The XANES results, as expected, display significantly different spectral features for the h-TMO and o-TMO films. In addition, the XANES spectra also exhibit strong polarization dependence at O K and Mn L edges for both samples.