arXiv subjects

J. -Y. Lin

Publications and source records attributed to J. -Y. Lin.

34 records · Page 2Linked to original sources

arXiv2007

A Low-Temperature Specific Heat Study of the Giant Dielectric Constant Materials

Low-temperature specific-heat study has been performed on the insulating giant dielectric constant material CaCu3Ti4O12 and two related compounds, Bi2/3Cu3Ti4O12 and La0.5Na0.5Cu3Ti4O12, from 0.6 to 10 K. From analyzing the specific heat data at very low-temperature range, 0.6 to 1.5 K, and moderately low-temperature range, 1.5 to 5 K, in addition to the expected Debye terms, we noticed significant contributions originated from the linear and Einstein terms, which we attributed as the manifestation of low-lying elementary excitations due to lattice vibrations occurred 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 behaviors at both low and high frequency regions.

cond-mat.mtrl-sci↗

arXiv2007

Fabrication and Low Temperature Thermoelectric Properties of Na_xCoO_2 (x = 0.68 and 0.75) Epitaxial Films by the Reactive Solid-Phase Epitaxy

We have fabricated Na_xCoO_2 thin films via lateral diffusion of sodium into Co_3O_4 (111) epitaxial films (reactive solid-phase epitaxy: Ref. 4). The environment of thermal diffusion is key to the control of the sodium content in thin films. From the results of x-ray diffraction and in-plane resistivity, the epitaxial growth and the sodium contents of these films were identified. The thermoelectric measurements show a large thermoelectric power similar to that observed in single crystals. The quasiparticle scattering rate is found to approach zero at low temperatures, consistent with the small residual resistivity, indicating high quality of the Na_xCoO_2 thin films.

cond-mat.str-el↗

arXiv2006

Possible coexistence of s-wave and unconventional pairing in Na_xCoO_2-yH2O:a new insight from impurity effects

To shed light on the symmetry of the superconducting order parameter in Na_xCoO_2-yH_2O, 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 Tc suppression. This absence of the strong impurity effects on T_c is not consistent with the simple picture of a sign-changing order parameter. Coexistence of the s-wave and unconventional order parameters is proposed to reconcile all existing experiments and has been directly observed by the specific heat experiments.

cond-mat.supr-con↗

arXiv2005

Electronic structure and transport properties of La_0.7Ce_0.3MnO_3

X-ray absorption spectroscopy (XAS), optical reflectance spectroscopy, and the Hall effect measurements were used to investigate the electronic structure in La_0.7Ce_0.3MnO_3 thin films (LCeMO). The XAS results are consistent with those obtained from LDA+U calculations. In that the doping of Ce has shifted up the Fermi level and resulted in marked shrinkage of hole pockets originally existing in La_0.7Ca_0.3MnO_3 (LCaMO). The Hall measurements indicate that in LCeMO the carriers are still displaying the characteristics of holes as LDA+U calculations predict. Analyses of the optical reflectance spectra evidently disapprove the scenario that the present LCeMO might have been dominated by the La-deficient phases.

cond-mat.str-el↗

arXiv2005

Spatially-resolved relaxation dynamics of photoinduced quasiparticles in underdoped YBa$sub 2$Cu$sub 3$O$sub {7-delta}$

The spatially-resolved relaxation characteristics of photoinduced quasiparticles (QPs) in CuO$sub 2$ planes of underdoped YBCO are disclosed by polarized fs time-resolved spectroscopy. The relaxation time (tau) along b axis diverges at Tc, and appears to be governed by a temperature-dependent gap Delta(T) at T < Tc. Furthermore, for T > Tc, a monotonic increase of tau with decreasing T along the b axis and ab diagonal was observed and can be attributed to a temperature-independent gap Delta$sub p$. The results lend support to recombination dominant scenario of QP dynamics. However, the QP thermalization may take part along the nodal direction in the highly underdoped samples.

cond-mat.supr-con↗

arXiv2005

Evidence of nodal superconductivity in Na_0.35CoO2-1.3H_2O: a specific heat study

Comprehensive low-temperature specific heat data C(T,H) of Na_0.35CoO2-1.3H_2O with temperature T down to 0.6 K and the magnetic field H up to 8 T are presented. For the normal state, the values of gamma_n=13.94 mJ/mol K2, and Debye temperature 362 K are reported. At zero field, a very sharp superconducting anomaly was observed at Tc=4.5 K with DeltaC/gamma_nTc=1.45. The superconducting volume fraction is estimated to be 47.4 % based on the consideration of entropy balance at Tc for the second-order superconducting phase transition. In the superconducting state, the electronic contribution C_es at H=0 can be well described by the model of the line nodal order parameter. In low H, gamma(H) H^1/2 which is also a manifestation of the line nodes. The behaviors of both Tc(H) and gamma(H) suggest the anisotropy of Hc2 or possible crossovers or transitions occurring in the mixed state.

cond-mat.supr-con↗

arXiv2004

Magnetic field dependence of low temperature specific heat of spinel oxide superconductor LiTi_2O_4

Magnetic field dependence of low temperature specific heat of spinel oxide superconductor LiTi_2O_4 has been elaborately investigated. In the normal state, the obtained electronic coefficient of specific heat gamma_n = 19.15 mJ/mol K2, the Debye temperature is 657 K and some other parameters are compared with those reported earlier. The superconducting transition at Tc ~ 11.4 K is very sharp (DeltaTc ~ 0.3 K) and the estimated delataC/gamma_nTc is ~ 1.78. In the superconducting state, the best fit of data leads to the electronic specific heat C_es/Gamma_nTc = 9.87 exp (-1.58 Tc/T) without field and gamma(H) ~ H^0.95 with fields. In addition, Hc2(0) ~ 11.7 T, Hc(0) ~ 0.32 T, xi_GL(0) ~ 5.5 nm, lambda_GL(0) ~ 160 nm, and Hc1(0) ~ 26 mT are estimated from Werthamer-Helfand-Hohenberg (WHH) theory or other relevant relations. All results from the present study indicate that LiTi2O4 can be well described by a typical type-II, BCS-like, moderate coupling, and fully gapped superconductor in the dirty limit. It is further suggested that LiTi2O4 is a moderately electron-electron correlated system.

cond-mat.supr-con↗

arXiv2004

Specific heat of unconventional superconductivity in NaxCoO2-yH2O

Comprehensive studies of the low-temperature specific heat C(T,H) in NaxCoO2-yH2O are presented. At H=0, a very sharp anomaly was observed at T=4.7 K indicating the existence of bulk superconductivity. There exists the T^2 term in C(T,H=0) in the superconducting state manifesting the line nodal superconducting order parameter. The superconducting volume fraction is estimated to be 26.6 % based on the consideration of entropy conservation at Tc for the second-order superconducting phase transition. An abrupt change of the slope in Tc(H) was observed. Possible scenarios such as the multiple phase transitions in the mixed state are discussed.

cond-mat.supr-con↗

arXiv2003

Spatial Symmetry of Superconducting Gap in YBa2Cu3O7-δObtained from Femtosecond Spectroscopy

The polarized femtosecond spectroscopies obtained from well characterized (100) and (110) YBa2Cu3O7-δthin films are reported. This bulk-sensitive spectroscopy, combining with the well-textured samples, serves as an effective probe to quasiparticle relaxation dynamics in different crystalline orientations. The significant anisotropy in both the magnitude of the photoinduced transient reflectivity change and the characteristic relaxation time indicates that the nature of the relaxation channel is intrinsically different in various axes and planes. By the orientation-dependent analysis, d-wave symmetry of the bulk-superconducting gap in cuprate superconductors emerges naturally.

cond-mat.supr-con↗

arXiv2003

MgCNi3: a conventional and yet puzzling superconductor

The newly discovered superconductivity in MgCNi3, though with Tc<8 K lower than that of the celebrated MgB2, is probably even more interesting in its many puzzling physical properties. MgCNi3 has been theoretically speculated to be unstable towards ferromagnetism. However, there are numerous evidences from the specific heat C(T), Tunneling spectroscopy and NMR experiments indicating conventional s-wave superconductivity in MgCNi3. The Hall effect and the thermoelectric power experiments suggest that the carriers responsible for the transport properties are electrons, in obvious contrast to holes predicated by the band structure calculations. In this article, we report the results of C(T) experiments, upper critical field Hc2 measurements, and the pressure effects on MgCNi3. These experimental evidences clearly demonstrate that superconductivity in MgCNi3 is well explained within the conventional electron-phonon interaction scenario, at most with minor modifications from the magnetic interaction. The thermodynamic data C(T) is consistent with the conventional s-wave order parameter. Hc2 of all samples follows a universal WHH relation . Surprisingly, dTc/dP is positive which leaves room for further improvements in band structure calculations. There are other serious discrepancies between experiments and theory like in the transport properties and x-ray photoemission. The possible reconciliation within the two-band model and the consequent difficulties are discussed.

cond-mat.supr-con↗

arXiv2003

X-ray absorption and optical spectroscopy studies of (Mg$_{1-x}$Al$_x$)B$_2$

X-ray absorption spectroscopy and optical reflectance measurements have been carried out to elucidate the evolution of the electronic structure in (Mg$_{1-x}$Al$_{x}$)B$_{2}$ for $\emph{x}$ = 0.0,0.1, 0.2, 0.3, and 0.4. The important role of B 2$\emph{p}$ $σ$ hole states to superconductivity has been identified, and the decrease in the hole carrier number is $\emph{quantitatively}$ determined. The rate of the decrease in the hole concentration agree well with the theoretical calculations. On the other hand,while the evolution of the electronic structure is gradual through the doping range, $T_c$ suppression is most significant at $\emph{x}$ = 0.4. These results suggest that the superstructure in (Mg$_{1-x}$Al$_{x}$)B$_{2}$, in addition to the $σ$ holes, can affect the lattice dynamics and contributes to the $T_c$ suppression effect. Other possible explanations like the topological change of the $σ$ band Fermi surface are also discussed.

cond-mat.supr-con↗

arXiv2002

BCS-like Superconductivity in MgCNi3

The low-temperature specific heat C(T,H) of a new superconductor MgCNi3 has been measured in detail. (C/gammasub_nTc=1.97 is estimated from the anomaly at Tc. At low temperatures, the electronic contribution in the superconducting state follows Ces/gammasub_nTc=7.96exp(-1.46Tc/T). The magnetic field dependence of gamma(H) is found to be linear with respect to H. Tc estimated from the McMillan formula agrees well with the observed value. All the specific heat data appear to be consistent with each other within the moderate-coupling BCS context. It is amazing that such a superconductor unstable to ferromagnetism behaves so conventionally. The Debye temperature Thetasub_D=287 K and the normal state gammasub_n=33.6 mJ/mol K2 are determined for the present sample. Discussions on the existence of a small magnetic coupling and unconventional order parameter are also presented.

cond-mat.supr-con↗

arXiv2001

Order parameter of MgB_2: a fully gapped superconductor

We have measured the low-temperature specific heat C(T) for polycrystalline MgB_2 prepared by high pressure synthesis. C(T) below 10 K vanishes exponentially, which unambiguously indicates a fully opened superconducting energy gap. However, this gap is found to be too small to account for Tc of MgB_2. Together with the small specific heat jump DeltaC/gamma_nTc=1.13, scenarios like anisotropic s-wave or multi-component order parameter are called for. The magnetic field dependence of gamma(H) is neither linear for a fully gapped s-wave superconductor nor H^1/2 for nodal order parameter. It seems that this intriguing behavior of gamma(H) is associated with the intrinsic electronic properties other than flux pinning.

cond-mat.supr-con↗

arXiv2001

Possible evidence for the existence of the Fehrenbacher-Rice band: O K-edge XANES study on Pr1-xCaxBa2Cu3O7

X-ray absorption near edge structure (XANES), resistivity and thermoelectric power have been measured on Pr1-xCaxBa2Cu3O7. These data reveal an intriguing electronic structure in Pr-doped cuprates. The absorption peak in XANES associated with the Fehrenbacher-Rice (FR) band has been identified. The Ca-doped holes in Pr1-xCaxBa2Cu3O7 go to both the Zhang-Rice (ZR) and FR bands. Comparative studies on the related samples suggest that the FR band is partially filled and highly localized. Implications of these results on other recent experiments, such as the observation of superconductivity in PrBa2Cu3O7 single crystals, are discussed.

cond-mat.supr-con↗

arXiv2000

Impurity scattering effects on the low-temperature specific heat of d-wave superconductors

Very recently impurity scattering effects on quasiparticles in d-wave superconductors have attracted much attention. Especially, the thermodynamic properties in magnetic fields H are of interest. We have measured the low-temperature specific heat C(T,H) of La_1.78Sr_0.22Cu_1-xNi_xO4. For the first time, the impurity scattering effects on C(T,H) of cuprate superconductors were clearly observed, and are compared with theory of d-wave superconductivity. It is found that impurity scattering leads to gamma(H)=gamma(0)(1+D((H/H_c2)(ln(H_c2/H)) in small magnetic fields. Most amazingly, the scaling of C(T,H) breaks down due to impurity scattering.

cond-mat.supr-con↗

arXiv2000

Depression of quasiparticle density of states at zero energy in La1.9Sr0.1Cu1-xZnxO4

We have measured low-temperature specific heat C(T, H) of La1.9Sr0.1Cu1-xZnxO4 (x=0, 0.01, and 0.02) both in zero and applied magnetic fields. A pronounced dip of C/T below 2 K was first observed in Zn-doped samples, which is absent in the nominally clean one. If the origin of the dip in C/T is electronic, the quasiparticle density of states N(E) in Zn-doped samples may be depressed below a small energy scale E0. The present data can be well described by the model N(E)=N(0)+alphaE^1/2, with a non-zero N(0) and positive alpha. Magnetic fields depress N(0) and lead to an increase in E0, while leaving the energy dependence of N(E) unchanged. This novel depression of N(E) below E0 in impurity-doped cuprates can not be reconciled with the semi-classical self-consistent approximation model. Discussions in the framework based on the non-linear sigma model field theory and other possible explanations are presented in this Letter.

cond-mat.supr-con↗