Browsing by Author "Vasiliev, A. N."

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3D visualizations of nanoscale phase separation and ultrafast dynamic correlation between phases in (Na0.32K0.68)(0.95)Fe1.75Se2
(2019-03-25)
Cheng, P. C.
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Tzeng, W. Y.
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Chu, Y. J.
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Luo, C. W.
;
Zhukov, A. A.
Phase separation of metallic and antiferromagnetic (AFM) insulating phases in alkaline iron selenides (A(x)Fe(2)-ySe(2)) continues to attract intense interest because the relationship between two peculiar features probably is a key to clarifying the pairing mechanism of A(x)Fe(2-y)Se(2) superconductors. Here we report that the three-dimensional (3D) visualizations of nanoscale phase separation in (Na0.32K0.68)(0.95)Fe1.75Se2 single crystals are revealed by hybrid focused-ion-beam scanning electron microscopy and the superconducting paths are fully percolative in 3D. Moreover, the phase-related ultrafast dynamics in (Na0.32K0.68)(0.95)Fe1.75Se2 is studied by dual-color pump-probe spectroscopy. The anomalous changes in the electron and acoustic phonon components of transient reflectivity change (Delta R/R) identify two characteristic temperatures T* similar to 100K (the onset temperature of coupling between the nanometallic and AFM phases) and T-H similar to 230K (the onset temperature of the metallic-interface phase). An energy-transfer channel between the nanometallic and AFM phases is inferred. This proposed channel provides insight into the pairing mechanism of alkaline iron selenide superconductors.
A new layered triangular antiferromagnet Li4FeSbO6: spin order, field-induced transitions and anomalous critical behavior
(2013)
Zvereva, E. A.
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Savelieva, O. A.
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Titov, Ya D.
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Evstigneeva, M. A.
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Nalbandyan, V. B.
Structure, electrochemical, magnetic and resonance properties of new layered antimonate Li4FeSbO6 were comprehensively studied using powder X-ray diffraction, cyclic voltammetry, magnetic susceptibility, heat capacity, electron spin resonance and Mossbauer spectroscopy. In the crystal structure the iron ions form the triangular network within (LiFeSbO6)(3-) layers alternating with nonmagnetic lithium layers. The electrochemical activity studied implies an Fe3+/Fe4+ redox couple at 4.3 V (ox.) and 3.9 V (red.) thereby revealing that Li can be reversibly extracted. The long-range antiferromagnetic order was found to occur at the Neel temperature, T-N approximate to 3.6 K, confirmed both by the magnetic susceptibility data and specific heat ones. The effective magnetic moment is estimated to be 5.93 mu(B)/f.u. and satisfactorily agrees with theoretical estimations assuming high-spin configuration of Fe3+ (S = 5/2). In the magnetically ordered state, though, the magnetization demonstrates rather peculiar behavior. An additional anomaly on the M(T) curves appears at T-2 < T-N in moderate magnetic field. The positions of transitions at T-N and T-2 separate increasingly with increasing external field. Multiple measurements consistently demonstrated field-sensitive moving of magnetic phase boundaries constituting a unique phase diagram for the compound under study. The complex low-dimensional (2D) nature of magnetic coupling was confirmed by the dynamic magnetic properties study. Electron spin resonance from Fe3+ ions in paramagnetic phase is characterized by a temperature independent effective g-factor of 1.99 +/- 0.01. However, the distortion and broadening of the ESR line were found to take place upon approaching the magnetically ordered state from above. The divergence of the temperature-dependent linewidth is analyzed in terms of both critical behavior close to long-range magnetic order and the Berezinskii-Kosterlitz-Thouless (BKT)-type transition. Heat capacity measurements even at zero field manifested an appearance of the additional anomaly at temperatures below the Neel temperature. The temperature dependence of ESR intensity, linewidth and shift of the resonant field imply an extended region of short-range order correlations in the compound studied. The rich variety of the anomalies in magnetic and resonance properties makes this new antimonate a very interesting system to investigate the multiple phase transitions and competing exchange interaction due to the critical role of the layered structure organization accompanied by the frustration effects in triangular antiferromagnets.
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.
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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.
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.
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.
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.
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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.
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Lin, J-Y
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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.
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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(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.
Zigzag antiferromagnetic quantum ground state in monoclinic honeycomb lattice antimonates A(3)Ni(2)SbO(6) (A = Li, Na)
(2015-10-01)
Zvereva, E. A.
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Stratan, M. I.
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Ovchenkov, Y. A.
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Nalbandyan, V. B.
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Lin, J. -Y.
We present a comprehensive experimental and theoretical study of the electronic and magnetic properties of two quasi-two-dimensional (2D) honeycomb lattice monoclinic compounds A(3)Ni(2)SbO(6) (A = Li, Na). Magnetic susceptibility and specific heat data are consistent with the onset of antiferromagnetic long-range order with Neel temperatures of similar to 14 and 16 K for Li3Ni2SbO6 and Na3Ni2SbO6, respectively. The effective magnetic moments of 4.3 mu(B)/f.u. (Li3Ni2SbO6) and 4.4 mu(B)/f.u. (Na3Ni2SbO6), where f.u. is formula units, indicate that Ni2+ is in a high-spin configuration (S = 1). The temperature dependence of the inverse magnetic susceptibility follows the Curie-Weiss law in the high-temperature region and shows positive values of the Weiss temperature, similar to 8 K (Li3Ni2SbO6) and similar to 12 K (Na3Ni2SbO6), pointing to the presence of nonnegligible ferromagnetic interactions, although the system orders antiferromagnetically at lowtemperatures. In addition, themagnetization curves reveal a field-induced (spin-flop type) transition below T-N that can be related to the magnetocrystalline anisotropy in these systems. These observations are in agreement with density functional theory calculations, which show that both antiferromagnetic and ferromagnetic intralayer spin exchange couplings between Ni2+ ions are present in the honeycomb planes, supporting a zigzag antiferromagnetic ground state. Based on our experimental measurements and theoretical calculations, we propose magnetic phase diagrams for the two compounds.