Browsing by Author "Volkova, O. S."

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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.
Noncollinear ferrimagnetic ground state in Ni(NO3)(2)
(2014-10-10)
Volkova, O. S.
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Mazurenko, V. V.
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Solovyev, I. V.
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Deeva, E. B.
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Morozov, I. V.
Both spin-liquid and magnetically ordered phases of both half-integer and integer low-spin quantum magnets are of interest, since the magnetic structures found in the latter case usually have no classical counterparts. Such a magnetic structure was found in a combined experimental and theoretical study of the integer spin system Ni(NO3)(2). Our thermodynamic measurements have revealed a magnetically ordered phase with small spontaneous magnetization at T-C = 5.5 K. The magnetization saturation of about 2 mu(B) at low temperatures corresponds to the high-spin state (S = 1) of Ni2+ ions evidenced in L-2,L-3 edges in x-ray absorption spectroscopy spectra. We show that a consistent description of the available data is possible within a noncollinear umbrella-type ferrimagnetic ground state model for which both intra-and interlayer magnetic interactions should be antiferromagnetic. Such a scenario is suggested by the first-principles and model calculations.
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.
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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.
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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.
Weak ferrimagnetism and multiple magnetization reversal in alpha-Cr(3)(PO(4))(2)
(2012-01-18)
Vasiliev, A. N.
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Volkova, O. S.
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Hammer, E.
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Glaum, R.
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Broto, J. -M.
The chromium(II) orthophosphate alpha-Cr(3)(PO(4))(2) is a weak ferrimagnet with the Curie temperature T(C) = 29 K confirmed by a lambda-type peak in specific heat. Dominant antiferromagnetic interactions in this system are characterized by the Weiss temperature Theta = 96 K, indicating an intermediate frustration ratio vertical bar Theta vertical bar/T(C) similar to 3. In its magnetically ordered states alpha-Cr(3)(PO(4))(2) exhibits a remarkable sequence of temperature-induced magnetization reversals sensitive to the protocol of measurements, i.e., either field-cooled or zero-field-cooled regimes. The reduction of the effective magnetic moment 4.5 mu(B)/Cr(2+), as compared to the spin-only moment 4.9 mu(B)/Cr(2+), cannot be ascribed to the occurrence of the low-spin state in any crystallographic site of the Jahn-Teller active 3d(4) Cr(2+) ions. X-ray absorption spectra at the K edge indicate divalent chromium and unravel the high-spin state of these ions at the L(2,3) edges. Weak ferrimagnetism and multiple magnetization reversal phenomena seen in this compound could be ascribed to incomplete cancellation and distortion of partial spontaneous magnetization functions of Cr(2+) in its six crystallographically inequivalent positions.
Weak ferrimagnetism and multiple magnetization reversal in alpha-Cr-3(PO4)(2)
(2012-01-18)
Vasiliev, A. N.
;
Volkova, O. S.
;
Hammer, E.
;
Glaum, R.
;
Broto, J. -M.
The chromium(II) orthophosphate alpha-Cr-3(PO4)(2) is a weak ferrimagnet with the Curie temperature T-C = 29 K confirmed by a lambda-type peak in specific heat. Dominant antiferromagnetic interactions in this system are characterized by the Weiss temperature Theta = 96 K, indicating an intermediate frustration ratio vertical bar Theta vertical bar/T-C similar to 3. In its magnetically ordered states alpha-Cr-3(PO4)(2) exhibits a remarkable sequence of temperature-induced magnetization reversals sensitive to the protocol of measurements, i.e., either field-cooled or zero-field-cooled regimes. The reduction of the effective magnetic moment 4.5 mu(B)/Cr2+, as compared to the spin-only moment 4.9 mu(B)/Cr2+, cannot be ascribed to the occurrence of the low-spin state in any crystallographic site of the Jahn-Teller active 3d(4) Cr2+ ions. X-ray absorption spectra at the K edge indicate divalent chromium and unravel the high-spin state of these ions at the L-2,L-3 edges. Weak ferrimagnetism and multiple magnetization reversal phenomena seen in this compound could be ascribed to incomplete cancellation and distortion of partial spontaneous magnetization functions of Cr2+ in its six crystallographically inequivalent positions.