Browsing by Author "Chareev, D. A."

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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.
;
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.
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.
;
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.
Quasiparticle Dynamics and Phonon Softening in FeSe Superconductors
(2012-06-20)
Luo, C. W.
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Wu, I. H.
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Cheng, P. C.
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Lin, J. -Y.
;
Wu, K. H.
Quasiparticle dynamics of FeSe single crystals revealed by dual-color transient reflectivity measurements (Delta R/R) provides unprecedented information on Fe-based superconductors. The amplitude of the fast component in Delta 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 lambda is found to be 0.16, identical to the value of theoretical calculations. Such a small lambda demonstrates an unconventional origin of superconductivity in FeSe.
Quasiparticle Dynamics in FeSe Superconductors Studied by Femtosecond Spectroscopy
(2013-04-01)
Luo, C. W.
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Wu, I. H.
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Cheng, P. C.
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Lin, J. -Y.
;
Wu, K. H.
The ultrafast quasiparticle dynamics in FeSe single crystals were measured by using dual-color transient reflectivity measurements (Delta R/R) from 4.4 to 290 K. In general, the typical Delta R/R of FeSe includes two significant components. One is the relaxation of photoinduced quasiparticles, which has been used to estimate the electron-phonon coupling strength (lambda=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 Delta R/R markedly shrinks around 90 K, which is the so-called phonon softening.
Ultrafast dynamics and phonon softening in Fe1+ySe1-xTex single crystals
(2012-10-31)
Luo, C. W.
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Wu, I. H.
;
Cheng, P. C.
;
Lin, J-Y
;
Wu, K. H.
The ultrafast quasiparticle dynamics of Fe1+ySe1-xTex single crystals were investigated by dual-color transient reflectivity measurements (Delta R/R) from 4.3 to 290 K. The electron-phonon coupling strength lambda (= 0.16-0.01) and the temperature-dependent energy of longitudinal-acoustic phonons were, respectively, obtained from the relaxation time of a fast component and the period of an oscillation component in Delta R/R. Such a small lambda demonstrates the unconventional origin of superconductivity in FeSe. Moreover, the temperature-dependent Delta R/R exhibits anomalous changes at T-c, 90 K and 230 K, unambiguously revealing the phase transition as well as the phonon softening via the magnetoelastic effect.