Browsing by Author "Yang, H. D."

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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.
Anisotropic spin-flip-induced multiferroic behavior in kagome Cu3Bi(SeO3)(2)O2Cl
(2017-03-15)
Wu, H. C.
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Chandrasekhar, K. Devi
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Yuan, J. K.
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Huang, J. R.
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Lin, J. -Y.
Compared to the previous report on Cu3Bi(SeO3)(2)O2X (X = Cl, Br) [V. Gnezdilov et al., (arXiv: 1604.04249)], where no dielectric anomaly was observed without magnetic field near T-N similar to 25.6K, we further present the dielectric behaviors without and with magnetic field in Cu3Bi(SeO3)(2)O2Cl. At zero field H = 0, an antiferromagnetic transition from magnetization and specific heat measurements is clearly established at T-N, while no dielectric anomaly was observed. Those results are similar to the previous report [V. Gnezdilov et al., (arXiv: 1604.04249)]. Above the critical field H-c similar to 0.8 T, a metamagnetic spin-flip transition from antiferromagnetic to ferrimagnetic order at T similar to T-N is induced anisotropically only for H broken vertical bar broken vertical bar c. Meanwhile, a ferroelectric behavior from dielectric and pyroelectric current measurements is observed below T similar to T-N; then a corresponding type-II multiferroics emerges above H-c. The key mechanism of the anisotropic spin-flip-induced multiferroicity in Cu3Bi(SeO3)(2)O2Cl can be ascribed to the breaking of magnetic twofold symmetry in the bc plane above H-c.
Antiferroelectric antiferromagnetic type-I multiferroic Cu9O2(SeO3)(4)Cl-6
(2019-12-11)
Wu, H. C.
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Denisova, K. N.
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Menzel, D.
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Kakarla, D. Chandrasekhar
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Maximova, O. V.
We report that Cu9O2(SeO3)(4)Cl-6 is a new multiferroic compound. Comprehensive studies of this compound have been carried out on single crystals as well as polycrystalline samples. Magnetic susceptibility chi and specific heat C with H parallel to c axis both reveal an anomaly at T-N = 37 K associated with the long-range antiferromagnetic order. However, no signature of this phase transition is seen with the H parallel to b axis, providing evidence of the anisotropic nature of its magnetic properties. The results of chi measurements are consistent with the temperature evolution of the intensity of magnetic reflections from the neutron diffraction experiments. The magnetic structure derived from neutron scattering measurements shows that half of the Cu(5) ions carry no moments. Furthermore, an anomaly in the dielectric constant near T-N and observable magnetoelectric coupling below T-N are found. At high temperatures, a dielectric peak at T-E = 267 K is identified with no measurable spontaneous polarization below T-E, thereby suggesting an antiferroelectric order. A steplike anomaly seen in chi(T) at T-E hints the weak spin-lattice coupling. Concomitantly, high-resolution temperature-dependent synchrotron x-ray diffraction experiments elucidate a local structural distortion at T-E.
Antisite disorder driven spontaneous exchange bias effect in La2-xSrxCoMnO6 (0 <= x <= 1)
(2016-03-02)
Murthy, J. Krishna
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Chandrasekhar, K. D.
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Wu, H. C.
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Yang, H. D.
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Lin, J. Y.
Doping at the rare-earth site by divalent alkaline-earth ions in perovskite lattice has witnessed a variety of magnetic and electronic orders with spatially correlated charge, spin and orbital degrees of freedom. Here, we report an antisite disorder driven spontaneous exchange bias effect as a result of hole carrier (Sr2+) doping in La2-xSrxCoMnO6 (0 < x < 1) double perovskites. X-ray diffraction and Raman spectroscopy have evidenced an increase in disorder with the increase of Sr content up to x = 0.5 and thereby a decrease from x = 0.5 to 1. X-ray absorption spectroscopy has revealed that only Co is present in the mixed valence of Co2+ and Co3+ states with Sr doping to compensate the charge neutrality. Magnetotransport is strongly correlated with the increase of antisite disorder. The antisite disorder at the B-site interrupts the long-range ferromagnetic order by introducing various magnetic interactions and instigates reentrant glassy dynamics, phase separation and canted type antiferromagnetic behavior with the decrease of temperature. This leads to a novel magnetic microstructure with unidirectional anisotropy that causes a spontaneous exchange bias effect that can be tuned with the amount of antisite disorder.
Coexistence of isotropic and extended s-wave order parameters in FeSe as revealed by low-temperature specific heat
(2011-12-27)
Lin, J. -Y.
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Hsieh, Y. S.
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Chareev, D. A.
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Vasiliev, A. N.
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Parsons, Y.
The comprehensive low-temperature specific-heat C(T) data identify both an isotropic s-wave and extended s-wave order parameters coexisting in a superconducting single-crystal FeSe with T-c = 8.11 K. The isotropic gap Delta(0) = 1.33 meV on the hole Fermi sheets and the extended s-wave gap Delta = Delta(e)(1+ alpha cos2 phi) with Delta(e) = 1.13 meV and alpha = 0.78 on the electron Fermi sheets. The extended s wave is rather anisotropic, but the low-energy quasiparticle excitations demonstrate no sign of the accidental nodes. The coefficient gamma(H) manifesting the quasiparticle contribution to C is a nonlinear function of the applied magnetic field H in the mixed state in accord with the anisotropic multiorder parameters.
Colossal electroresistance and colossal magnetoresistance in spinel multiferroic CdCr(2)S(4)
(2010-03-22)
Sun, C. P.
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Huang, C. L.
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Lin, C. C.
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Her, J. L.
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Ho, C. J.
Colossal magnetoresistance (CMR) and colossal electroresistance (CER) induced by the electric field in spinel multiferroic CdCr(2)S(4) are reported. It is found that a metal-insulator transition in CdCr(2)S(4) is triggered by the electrical field. In magnetic fields, the resistivity rho of CdCr(2)S(4) responds similarly to that of CMR manganites. Combing previous reports, these findings make CdCr(2)S(4) the unique compound to possess all four properties of the colossal magnetocapacitive, colossal electrocapacitive, CER, and CMR. The present results open a new venue for searching new materials to show CMR by tuning electric and magnetic fields.
Colossal electroresistance and colossal magnetoresistance in spinel multiferroic CdCr2S4
(2010-03-22)
Sun, C. P.
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Huang, C. L.
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Lin, C. C.
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Her, J. L.
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Ho, C. J.
Colossal magnetoresistance (CMR) and colossal electroresistance (CER) induced by the electric field in spinel multiferroic CdCr2S4 are reported. It is found that a metal-insulator transition in CdCr2S4 is triggered by the electrical field. In magnetic fields, the resistivity rho of CdCr2S4 responds similarly to that of CMR manganites. Combing previous reports, these findings make CdCr2S4 the unique compound to possess all four properties of the colossal magnetocapacitive, colossal electrocapacitive, CER, and CMR. The present results open a new venue for searching new materials to show CMR by tuning electric and magnetic fields.
Effect of Mn impurities on the superconductivity in NaxCoO2 center dot yH(2)O
(2007-09-01)
Chen, Y.-J.
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Liu, C.-J.
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Wang, J.-S.
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Lin, J.-Y.
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Sun, C. P.
To shed light on the symmetry of the superconducting order parameter in NaxCoO2 center dot yH(2)O, the Mn doping effects are studied. X-ray absorption spectroscopy verifies that the doped Mn impurities occupy the Co sites and are with a valance close to +4. Impurity scattering by Mn is in the unitary limit that, however, does not lead to strong T-c suppression. This absence of the strong impurity effects on T-c is inconsistent with the simple picture of a sign-changing order parameter. Coexistence of the s-wave superconducting order parameter and the nodal one is proposed to reconcile all the existing experiments and has been implied by the specific heat experiments.
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.
Experimental evidence for a two-gap structure of superconducting NbSe(2): A specific-heat study in external magnetic fields
(2007-12-01)
Huang, C. L.
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Lin, J. -Y.
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Chang, Y. T.
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Sun, C. P.
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Shen, H. Y.
To resolve the discrepancies of the superconducting order parameter in quasi-two-dimensional NbSe(2), comprehensive specific-heat measurements have been carried out. By analyzing both the zero-field and mixed-state data with magnetic fields perpendicular (H perpendicular to c) to and parallel (H parallel to c) to the c axis of the crystal and using the two-gap model, we conclude that (1) more than one energy scale of the order parameter is required for superconducting NbSe(2) due to the thermodynamic consistency; (2) Delta(L)=1.26 meV and Delta(S)=0.73 meV are obtained; (3) N(S)(0)/N(0)=11%-20%; (4) the observation of the kink in gamma(H) curve suggests that the two-gap scenario is more favorable than the anisotropic s-wave model to describe the gap structure of NbSe(2); and (5) Delta(S) is more isotropic and has a three-dimensional-like feature and is located either on the Se or the bonding Nb Fermi sheets.
Experimental evidence for a two-gap structure of superconducting NbSe2: A specific-heat study in external magnetic fields
(2007-12-01)
Huang, C. L.
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Lin, J. -Y.
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Chang, Y. T.
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Sun, C. P.
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Shen, H. Y.
To resolve the discrepancies of the superconducting order parameter in quasi-two-dimensional NbSe2, comprehensive specific-heat measurements have been carried out. By analyzing both the zero-field and mixed-state data with magnetic fields perpendicular (H perpendicular to c) to and parallel (H parallel to c) to the c axis of the crystal and using the two-gap model, we conclude that (1) more than one energy scale of the order parameter is required for superconducting NbSe2 due to the thermodynamic consistency; (2) Delta(L)=1.26 meV and Delta(S)=0.73 meV are obtained; (3) N-S(0)/N(0)=11%-20%; (4) the observation of the kink in gamma(H) curve suggests that the two-gap scenario is more favorable than the anisotropic s-wave model to describe the gap structure of NbSe2; and (5) Delta(S) is more isotropic and has a three-dimensional-like feature and is located either on the Se or the bonding Nb Fermi sheets.
Induced magnetic moment by Mn doping in NaxCo1-zMnzO2 center dot yH(2)O
(2007-03-01)
Lin, J.-Y.
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Chen, Y.-J.
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Liu, C.-J.
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Wang, J.-S.
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Sun, C. P.
Impurity effects on NaxCoO2 (.) yH(2)O are studied by Mn doping. Spectroscopic evidence reveals that the doped Mn ions occupy the Co sites and the valence is +4. The magnetic local moment induced by Mn doping was observed and mu(eff) 3: 20 mB. The induced local moment could lead to magnetic pair-breaking and is likely to be responsible for the Tc suppression by Mn impurities in Na(x)Co(1-z)MnzO(2) (.) yH(2)O. (c) 2006 Elsevier B. V. All rights reserved.
Low-temperature specific heat of superconductors - I. Historical perspective prior to 1986
(2019-10-01)
Ho, J. C.
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Lin, J. G.
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Lin, J. Y.
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Yang, H. D.
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交大名義發表
Specific heat is a fundamental thermodynamic quantity closely related to internal energy and entropy of a system. In superconductivity research, it has been playing important roles ever since the intriguing critical phenomenon was discovered. Looking back, though, calorimetric measurements, along with the relevant low-temperature techniques, improved slowly over decades. Erroneous conclusions were reached often in the beginning, but eventually several calorimetrically-derived key features formed part of the basis in the microscopic BCS theory and its extended versions. While unable to predict whether or when a given material would become superconducting, theories helped data analysis and guided new superconductors search. Many elements, alloys and compounds joined the long list of known superconductors over the 75-year period between 1911 and 1986. Specific heat data yielded characteristic parameters, and ascertained whether an observed transition was a bulk effect. In the later years, theoretical models or experimental results also revealed a few classes of unconventional superconductors, mostly involving the interplay between superconductivity and magnetism. This article is purposely focused, as historical perspective, on how such activities progressed. Only initial findings and some related reports are cited, leaving out much more detailed and better executed work published later. For the same reason, only select figures and discussions of cited articles, but not their full extent, are included.
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.
Magnetism-induced ferroelectric polarization in the c-axis-oriented orthorhombic HoMnO(3) thin films
(2009-11-15)
Lin, T. H.
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Hsieh, C. C.
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Luo, C. W.
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Lin, J. -Y.
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Sun, C. P.
The magnetism-induced ferroelectricity in the E-type orthorhombic HoMnO(3) (o-HMO) films grown on Nb-doped and undoped SrTiO(3)(001) substrates was studied. The temperature dependent magnetization in these films evidently exhibits an antiferromagnetic ordering around 44 K and an anomalous second magnetic ordering along the c-axis near 35 K. Interestingly, at this anomalous transition temperature, an associated ferroelectric polarization was probed by the P-E hysteresis measurements, indicating the intimated coupling between the ferroelectric response and the reordering of Mn spins, which has never been disclosed previously by dielectric constant measurements. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3260241]
Magnetism-induced ferroelectric polarization in the c-axis-oriented orthorhombic HoMnO3 thin films
(2009-11-15)
Lin, T. H.
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Hsieh, C. C.
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Luo, C. W.
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Lin, J. -Y.
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Sun, C. P.
The magnetism-induced ferroelectricity in the E-type orthorhombic HoMnO3 (o-HMO) films grown on Nb-doped and undoped SrTiO3(001) substrates was studied. The temperature dependent magnetization in these films evidently exhibits an antiferromagnetic ordering around 44 K and an anomalous second magnetic ordering along the c-axis near 35 K. Interestingly, at this anomalous transition temperature, an associated ferroelectric polarization was probed by the P-E hysteresis measurements, indicating the intimated coupling between the ferroelectric response and the reordering of Mn spins, which has never been disclosed previously by dielectric constant measurements. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3260241]
Magnetostructural coupling and multiferroic properties in the spin-frustrated system Ni1-xZnxCr2O4
(2016-11-28)
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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Tseng, W. J.
The magnetic phase diagram of Ni1-xZnxCr2O4 demonstrates complex magnetic transitions for x = 0.05 with two long-range magnetic- order transitions at T-C1 = 41K and T-C2 = 17 K. A glasslike feature emerges between TC1 and TC2 with the application of a direct current (dc) magnetic field in alternating current (ac) susceptibility measurements. Dielectric measurements show a change from the conventional dielectric to a quantum-paraelectric-like behavior without anomalies near the magnetic ordering temperature in the range 0.05 <= x <= 1. The strong correlation among magnetic, dielectric, and ferroelectric properties near the structural relaxation temperatures 41 and 17 K confirms the emergence of multiferroic behavior in Ni0.95Zn0.05Cr2O4. A significant x-ray magnetic circular dichroism signal at the Ni site and ac susceptibility measurements indicate a spin-canting behavior between TC1 and TC2. The dilute doping of the non-Jahn-Teller Zn ion in the Jahn-Teller Ni site modulate the different magnetic exchange interactions of Ni and Cr, which could invoke spin-canting and multiferroic behaviors.
Metamagnetic behaviour and effect of field cooling on sharp magnetization jumps in multiferroic Y2CoMnO6
(2014-10-01)
Murthy, J. Krishna
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Chandrasekhar, K. D.
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Wu, H. C.
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Yang, H. D.
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Lin, J. Y.
We present sharp magnetization jumps and field-induced irreversibility in magnetization in multiferroic Y2CoMnO6. The appearance of magnetic relaxation and field sweep rate dependence of magnetization jumps resembles the martensite-like scenario and suggests the coexistence of E*-type antiferromagnetic and ferromagnetic phases at low temperatures. In Y2CoMnO6, the critical field required for the sharp jump can be increased or decreased depending on the magnitude and direction of the cooling field; this is remarkably different from manganites or other metamagnetic materials where the critical field increases irrespective of the direction of the applied field cooling. The cooling field dependence on the sharp magnetization jumps has been described by considering exchange pinning mechanism at the interface, like in the exchange bias model. Copyright (C) EPLA, 2014
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.