Jenh-YIh Juang - Academia.edu (original) (raw)

Papers by Jenh-YIh Juang

Physical Review Letters, Jun 20, 2012

Quasiparticle dynamics of FeSe single crystals revealed by dual-color transient reflectivity meas... more Quasiparticle dynamics of FeSe single crystals revealed by dual-color transient reflectivity measurements (∆R/R) provides unprecedented information on Fe-based superconductors. The amplitude of the fast component in ∆R/R clearly gives a competing scenario between spin fluctuations and superconductivity. Together with the transport measurements, the relaxation time analysis further exhibits anomalous changes at 90 and 230 K. The former manifests a structure phase transition as well as the associated phonon softening. The latter suggests a previously overlooked phase transition or crossover in FeSe. The electron-phonon coupling constant λ is found to be 0.16, identical to the value of theoretical calculations. Such a small λ demonstrates an unconventional origin of superconductivity in FeSe.

Journal of Applied Physics, Nov 15, 2008

Orthorhombic YMnO3 thin films with (200), (020), and (001) orientations were, respectively, obtai... more Orthorhombic YMnO3 thin films with (200), (020), and (001) orientations were, respectively, obtained by pulsed laser deposition on SrTiO3(110), LaAlO3(110), and SrTiO3(001) substrates. The results demonstrate that the strain between film and substrate can serve as an alternative in transforming the thermodynamically stable hexagonal YMnO3 into the orthorhombic phase, which previously could be obtained with high-pressure high temperature syntheses and epitaxy-stabilized thin film processes. More importantly, these films allow us, for the first time, to unambiguously disclose the intrinsic magnetic property along different crystallographic orientations. Our results show that, although the antiferromagnetic (AFM) ordering remains the same, there is an additional spin reordering transition which is very much dependent on the crystallographic orientation along which the measuring field was applied and on the in-plane crystallographic alignment of the films. Detailed analyses indicate that the origin of the observed second reordering may be due to some strain-induced canted AFM state instead of the incommensurate lock-in transitions conceived previously.

Journal of physics, Mar 1, 2009

In this work, we use ultrafast optical pump-optical probe (OPOP) spectroscopy to probe the polaro... more In this work, we use ultrafast optical pump-optical probe (OPOP) spectroscopy to probe the polaron dynamics in La0.7Ca0.3MnO3 (LCMO) thin films. The temporal evolution in transient reflectivity change ∆R/R exhibits two relaxing components: a fast component with a time constant of subpicosecond and a slow component with time constant ranging from tens of ps to hundreds of ps. The amplitude of the fast component exhibits the similar temperature depence with that of the resistivity, and the neutron scattering intensity due to nanoscale correlated polarons. The results strongly suggest that the fast photoinduced reflectivity change may have been due to the photoexcitation and trapping process of correlated Jahn-Teller (T-J) polarons in the paramagnetic (PM) and ferromagnetic (FM) phases.

Proceedings of SPIE, Mar 14, 2013

Ultrafast dynamics of carriers and phonons in topological insulator Bi 2 Se 3, Cu x Bi 2 Se 3 (x ... more Ultrafast dynamics of carriers and phonons in topological insulator Bi 2 Se 3, Cu x Bi 2 Se 3 (x = 0, 0.1, 0.125) single crystals were studied by time-resolved pump-probe spectroscopy. The coherent optical phonon (A 1g 1) is found via the damped oscillation in the transient reflectivity changes (∆R/R) for Cu x Bi 2 Se 3. The observed red shift of A 1g 1 phonon frequency suggests the intercalation of Cu atoms between a pair of the quintuple layers of Bi 2 Se 3 crystals. Moreover, the relaxation processes of Dirac fermion near the Dirac point of Bi 2 Se 3 are studied by optical pump and mid-infrared probe spectroscopy through analyzing the negative peak of the ∆R/R. The Dirac fermion-phonon coupling strength was found in the range of 0.08-0.19 and the strength is reduced as it gets closer to the Dirac point. The ultrafast dynamics and fundamental parameters revealed by time-resolved pump-probe spectroscopy are important for designing the optoelectronics in the mid-IR and THz ranges.

npj quantum materials, Jun 28, 2017

The nematic order (nematicity) is considered as one of the essential ingredients to understand th... more The nematic order (nematicity) is considered as one of the essential ingredients to understand the mechanism of Fe-based superconductivity. In most Fe-based superconductors (pnictides), nematic order is reasonably close to the antiferromagnetic order. In FeSe, in contrast, a nematic order emerges below the structure phase transition at T s = 90 K with no magnetic order. The case of FeSe is of paramount importance to a universal picture of Fe-based superconductors. The polarized ultrafast spectroscopy provides a tool to probe simultaneously the electronic structure and the magnetic interactions through quasiparticle dynamics. Here we show that this approach reveals both the electronic and magnetic nematicity below and, surprisingly, its fluctuations far above T s to at least 200 K. The quantitative pump-probe data clearly identify a correlation between the topology of the Fermi surface and the magnetism in all temperature regimes, thus providing profound insight into the driving factors of nematicity in FeSe and the origin of its uniqueness.

arXiv (Cornell University), Mar 13, 2019

Phase separation of metallic and antiferromagnetic (AFM) insulating phases in alkaline iron selen... more Phase separation of metallic and antiferromagnetic (AFM) insulating phases in alkaline iron selenides (AxFe2-ySe2) continues to attract intense interest because the relationship between two peculiar features probably is a key to clarifying the pairing mechanism of AxFe2-ySe2 superconductors. Here we report that the 3D visualizations of nano-scale phase separation in (Na0.32K0.68)0.95Fe1.75Se2 single crystals are revealed by hybrid focused-ion-beam scanning electron microscopy and the superconducting paths are fully percolative in 3D. Moreover, the phaserelated ultrafast dynamics in (Na0.32K0.68)0.95Fe1.75Se2 is studied by dual-color pump-probe spectroscopy. The anomalous changes in the electron and acoustic phonon components of transient reflectivity change (ΔR/R) identify two characteristic temperatures T *~1 00 K (the onset temperature of coupling between nano-metallic and AFM phases) and TH~230 K (the onset temperature of metallicinterface-phase). An energy-transfer channel between the nano-metallic and AFM

IEEE Transactions on Applied Superconductivity, Mar 1, 2003

The characteristics of photogenerated terahertz radiation from a current-biased superconducting Y... more The characteristics of photogenerated terahertz radiation from a current-biased superconducting YBa 2 Cu 3 O 7 (YBCO) bow-tie antenna were investigated using a free-space electrooptic sampling technique. Picosecond electromagnetic pulses about 450 fs wide were obtained. The frequency spectrum derived by Fourier transforming the picosecond pulses spans over 0.1-4 THz. The dynamics of the quasi-particles optically induced by the ultrafast laser pulse primarily determines the performances of the transient terahertz radiation generated under different operating parameters. The results indicate a characteristic quasi-particle relaxation time of about 2.5 ps close to the critical temperature , and a faster time at lower temperatures.

Chemcatchem, Nov 12, 2020

Direct ethanol fuel cells (DEFCs) promise the use of ethanol as a bio‐renewable and non‐toxic fue... more Direct ethanol fuel cells (DEFCs) promise the use of ethanol as a bio‐renewable and non‐toxic fuel for energy conversion through the ethanol oxidation reaction (EOR). Well dispersed Pt3Sn and Pt nanoparticles on graphene (denoted Pt3Sn/G and Pt/G) support electrocatalysts made with alcohol reduction were tested towards EOR. The HRTEM and XRD characterizations provide the morphology and crystal phase of the Pt3Sn alloy nanoparticles, which has a uniform particle size of 2.8±0.08 nm, as is consistent with a Pt/G catalyst as reference. According to the in‐situ quick X‐ray‐absorption near‐edge structure (QXANES) spectra during an anodic scan of CV for the EOR test to explain clearly the potential‐dependent electronic state of the prepared electrocatalysts, the white‐line intensities of the Pt L3‐edge QXANES spectra and their spectral profiles vary appreciably with the electrode voltage. Moreover, Pt3Sn/G shows a better EOR performance than Pt/G because SnO2 can improve adsorption and dissociation during the oxidation by an appropriate expansion of the lattice parameters in the PtSn alloy. This work provides insight into the reaction mechanism of dissociative adsorption of ethanol on alloyed Pt surface, which has an important role in enhancing the EOR activity for a complete ethanol oxidation.

ACS applied electronic materials, May 31, 2022

Superconductor Science and Technology, Sep 12, 2013

The comprehensive ρ(T) measurements and the consequent resistivity curvature mapping (RCM) on Y 0... more The comprehensive ρ(T) measurements and the consequent resistivity curvature mapping (RCM) on Y 0.7 Ca 0.3 Ba 2 Cu 3 O 7−δ thin films (doping levels p = 0.08-0.21) elucidate a phase diagram for the whole doping range. This phase diagram further strengthens a view that the 'normal' phase in hole-doped cuprates should be divided into a strong superconducting (SC) fluctuation phase and the 'real' normal phase in which there is no significant influence of SC. The temperature of superconducting fluctuations T f as a function of p was calculated using the Ginzburg-Landau model for layered superconductors. Comparisons between T f and the Nernst temperature establish the origin of the Nernst effect as SC fluctuations. Some of the details in ρ(T) cannot be fully understood by the existing models and call for a more sophisticated theory of carrier dynamics in cuprates.

Coatings, Oct 15, 2022

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Journal of Materials Chemistry C, 2023

This work reports molecular beam epitaxy of two-dimensional GaSe1−xTex ternary alloys that have r... more This work reports molecular beam epitaxy of two-dimensional GaSe1−xTex ternary alloys that have recently attracted a lot of interest in physics and material sciences even though facing crucial challenges in their epitaxial technology.

$Research paper thumbnail of \textbf{Nematicity in FeSe single crystals probed by pump-probe spectroscopy}$

Bulletin of the American Physical Society, Mar 18, 2016

The anisotropic quasiparticle dynamics in FeSe single crystals have been studied by polarized pum... more The anisotropic quasiparticle dynamics in FeSe single crystals have been studied by polarized pump-probe spectroscopy. Two distinguishable relaxation components were unambiguously observed in transient reflectivity changes (∆R/R). The orientation-dependent fast component with the timescale of 0.1-1.5 ps associated with the electronic structure clearly shows twofold symmetry, which further reveals the gap opening along k y below the temperature of structure phase transition (T s) and the electronic nematicity can persist up to 200 K. For the slow component with the timescale of 8-25 ps, it is assigned to the energy relaxation through spin subsystem and also shows a twofold symmetry below T s. However, this twofold symmetry is dramatically weakened above T s and surprisingly persists up to at least 200 K. Consequently, the high-temperature nematic fluctuations in FeSe may be driven by the order parameters which associated with both charge (orbital) and spin subsystems .

$Research paper thumbnail of Fabrication and Control of Sodium Concentration in Cobaltate Na$_{x}$CoO$_{2}$ thin films ($x $= 0.68, 0.75) by Thermal Diffusion$

Bulletin of the American Physical Society, Mar 6, 2007

We have fabricated Na x CoO 2 (x ∼0.68 & 0.75) thin films on sapphire (0001) substrates via later... more We have fabricated Na x CoO 2 (x ∼0.68 & 0.75) thin films on sapphire (0001) substrates via lateral diffusion of sodium into Co 3 O 4 (111) epitaxial films. The environment of thermal diffusion is key to the control of the sodium concent in thin films. From the results of x-ray diffraction and in-plane resistivity ρ ab , the epitaxial growth and the sodium contents of these Na x CoO 2 thin films were identified. The thermoelectric measurements show a large thermoelectric power in our films, similar to that of single crystals. The quasiparticle scattering rate is found to approach zero, consistent with the small residual resistivity, indicating high quality of the Na x CoO 2 thin films.

Journal of Superconductivity and Novel Magnetism, Dec 19, 2012

The ultrafast quasiparticle dynamics in FeSe single crystals were measured by using dual-color tr... more The ultrafast quasiparticle dynamics in FeSe single crystals were measured by using dual-color transient reflectivity measurements (R/R) from 4.4 to 290 K. In general, the typical R/R of FeSe includes two significant components. One is the relaxation of photoinduced quasiparticles, which has been used to estimate the electronphonon coupling strength (λ = 0.16). The other is the oscillation component due to the acoustic phonon. Moreover, the acoustic phonon's energy estimated from the period of oscillation in R/R markedly shrinks around 90 K, which is the so-called phonon softening.

Journal of Alloys and Compounds, Aug 1, 2010

This article reports a nanomechanical response study of the contact-induced deformation behavior ... more This article reports a nanomechanical response study of the contact-induced deformation behavior in AlN thin film by using a combination of nanoindentation and the cross-sectional transmission electron microscopy (XTEM) techniques. AlN thin films are deposited on Si(1 1 1) substrates by using the helicon sputtering system. The hardness and Young's modulus of the AlN thin film are measured by a Berkovich nanoindenter operated with the continuous contact stiffness measurements (CSM) mode. The obtained values of hardness and Young's modulus are 24.79 ± 0.33 GPa and 223.92 ± 5.34 GPa, respectively. XTEM samples are prepared by using focused ion beam (FIB) milling to accurately position the cross-section of the indented area. Although the film's surface was free from fracture, AlN thin film showed evidence of radial crack along the columnar grain boundary underneath the center of Berkovich indentation for an applied load of 100 mN. XTEM results also indicated that the slip bands on {1 1 1} planes and an indentation-induced phase transformation zone of silicon substrate, containing the metastable phases of Si-III and Si-XII, and, the amorphous phases are observed.

We report on the formation of organized sub-micron YBa 2 Cu 3 O 7 (YBCO) dots induced by irradiat... more We report on the formation of organized sub-micron YBa 2 Cu 3 O 7 (YBCO) dots induced by irradiating femtosecond laser pulses on YBCO films prepared by pulse laser deposition with fluence in the range of 0.21∼0.53 J/cm 2. The morphology of the YBCO film surface depends strongly on the laser fluences irradiated. At lower laser fluence (∼0.21 J/cm 2) the morphology was pattern of periodic ripples with sub-micrometer spacing. Slightly increasing the laser fluence to 0.26 J/cm 2 changes the pattern into organized sub-micron dots with diameters ranging from 100 nm to 800 nm and height of 150 nm. Further increase of the laser fluence to over 0.32 J/cm 2 , however, appeared to result in massive melting and led to irregular morphology. The mechanism and the implications of the current findings will be discussed. Arrays of YBCO sub-micron dots with T c = 89.7 K were obtained.

Physical Review Materials, Mar 25, 2019

Science of Advanced Materials, Nov 1, 2017

Physica B: Condensed Matter, 2009

The temperature-dependent X-ray absorption near-edge spectroscopy (XANES) of LaMnO 3 thin films d... more The temperature-dependent X-ray absorption near-edge spectroscopy (XANES) of LaMnO 3 thin films doped with various divalent ions was systematically carried out to investigate the effects of the ion size and associated Jahn-Teller distortions (JTDs) on the electronic structure modifications. Doped LaMnO 3 thin films were grown on (1 0 0) SrTiO 3 and (11 0) NdGaO 3 substrates by pulsed-laser deposition with typical thickness of 250-300 nm. Temperature-dependent resistivity r(T) and magnetization M(T) were measured. The results indicate that the drastic changes in magneto-transport properties induced by the substitution of A-site ion and the splitting of the O K-edge pre-edge peak associated with the e g (m) and t 2g (k) energy levels at low temperatures in La 1Àx Ca x MnO 3 and La 1Àx Sr x MnO 3 (x ¼ 0.3, 0.375) may both relate to the lattice disorder and the associated JTD effects on modifying the electronic structure of doped-LaMnO 3 .