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The influence of spin-phonon coupling on antiferromagnetic spin fluctuations in FeSe under pressure: the First-principles calculations with van der Waals corrections  [PDF]
Qian-Qian Ye,Kai Liu,Zhong-Yi Lu
Physics , 2013, DOI: 10.1103/PhysRevB.88.205130
Abstract: The electronic structures, lattice dynamics, and magnetic properties of crystal {\beta}-FeSe under hydrostatic pressure have been studied by using the first-principles electronic structure calculations with van der Waals (vdW) corrections. With applied pressures, the energy bands around the Fermi energy level consisting mainly of Fe-3d orbitals show obvious energy shifts and occupation variations, and meanwhile the frequencies of all optical phonon modes increase. Among these phonon modes, the A1g mode, which relates to the Se height from the Fe-Fe plane, shows a clear frequency jump in the range between 5 and 6 GPa. This is also the pressure range within which the highest superconducting transition temperature Tc of FeSe is reached in experiments. In comparison with the other phonon modes, the zero-point vibration of the A1g mode induces the strongest fluctuation of local magnetic moment on Fe under a pressure from 0 to 9 GPa, and the induced fluctuation reaches the maximum around 5 GPa. These results suggest that the effect of phonon via spin-phonon coupling need to be considered when exploring the superconducting mechanism in iron-based superconductors.
Why Does Undoped FeSe Become A High Tc Superconductor Under Pressure?  [PDF]
T. Imai,K. Ahilan,F. L. Ning,T. M. McQueen,R. J. Cava
Physics , 2009, DOI: 10.1103/PhysRevLett.102.177005
Abstract: Unlike the parent phases of the iron-arsenide high Tc superconductors, undoped FeSe is not magnetically ordered and exhibits superconductivity with Tc~9K. Equally surprising is the fact that applied pressure dramatically enhances the modest Tc to ~37K. We investigate the electronic properties of FeSe using 77Se NMR to search for the key to the superconducting mechanism. We demonstrate that the electronic properties of FeSe are very similar to those of electron-doped FeAs superconductors, and that antiferromagnetic spin fluctuations are strongly enhanced near Tc. Furthermore, applied pressure enhances spin fluctuations. Our findings suggest a link between spin fluctuations and the superconducting mechanism in FeSe.
Pressure-induced antiferromagnetic transition and phase diagram in FeSe  [PDF]
Taichi Terashima,Naoki Kikugawa,Shigeru Kasahara,Tatsuya Watashige,Takasada Shibauchi,Yuji Matsuda,Thomas Wolf,Anna E. B?hmer,Frédéric Hardy,Christoph Meingast,Hilbert v. L?hneysen,Shinya Uji
Physics , 2015, DOI: 10.7566/JPSJ.84.063701
Abstract: We report measurements of resistance and ac magnetic susceptibility on FeSe single crystals under high pressure up to 27.2 kbar. The structural phase transition is quickly suppressed with pressure, and the associated anomaly is not seen above $\sim$18 kbar. The superconducting transition temperature evolves nonmonotonically with pressure, showing a minimum at $\sim12$ kbar. We find another anomaly at 21.2 K at 11.6 kbar. This anomaly most likely corresponds to the antiferromagnetic phase transition found in $\mu$SR measurements [M. Bendele \textit{et al.}, Phys. Rev. Lett. \textbf{104}, 087003 (2010)]. The antiferromagnetic and superconducting transition temperatures both increase with pressure up to $\sim25$ kbar and then level off. The width of the superconducting transition anomalously broadens in the pressure range where the antiferromagnetism coexists.
Precise Pressure Dependence of the Superconducting Transition Temperature of FeSe: Resistivity and ^77Se--NMR Study  [PDF]
S. Masaki,H. Kotegawa,Y. Hara,H. Tou,K. Murata,Y. Mizuguchi,Y. Takano
Physics , 2009, DOI: 10.1143/JPSJ.78.063704
Abstract: We report the precise pressure dependence of FeSe from a resistivity measurement up to 4.15 GPa. Superconducting transition temperature (T_c) increases sensitively under pressure, but shows a plateau between 0.5-1.5 GPa. The maximum T_c, which is determined by zero resistance, is 21 K at approximately 3.5 GPa. The onset value reaches ~37 K at 4.15 GPa. We also measure the nuclear spin-lattice relaxation rate 1/T_1 under pressure using 77Se--NMR measurement. 1/T_1 shows that bulk superconductivity is realized in the zero-resistance state. The pressure dependence of 1/T_1T just above T_c shows a plateau as well as the pressure dependence of T_c, which gives clear evidence of the close relationship between 1/T_1T and T_c. Spin fluctuations are suggested to contribute to the mechanism of superconductivity.
Anomalous Pressure Dependence of the Superconducting Transition Temperature in FeSe1-x Studied by DC Magnetic Measurements  [PDF]
K. Miyoshi,Y. Takaichi,E. Mutou,K. Fujiwara,J. Takeuchi
Physics , 2009, DOI: 10.1143/JPSJ.78.093703
Abstract: The pressure dependence of superconducting transition temperature $T_{\rm c}$ has been investigated through the DC magnetic measurements for FeSe$_{0.8}$ and FeSe$_{1.0}$. For both samples, with increasing pressure $P$, the $T_{\rm c}$$-$$P$ curve exhibits a two-step increase, showing a local maximum of $\sim$11 K at $P$$\sim$1.0 GPa and a rapid increase with an extremely large pressure coefficient for $P$$>$1.5 GPa. $T_{\rm c}$ saturates at $\sim$25 K (21 K) in FeSe$_{1.0}$ (FeSe$_{0.8}$) for $P$$>$3 GPa. A rapid decrease in superconducting volume fraction is observed with an increase in $T_{\rm c}$ above 1.5 GPa, suggesting the presence of electronic inhomogeneity.
The influence of spin and charge fluctuations on the pressure dependence of the Neel temperature near a quantum phase transition in rare-earth intermetallic compounds  [PDF]
Valery V. Val'kov,Anton O. Zlotnikov
Physics , 2015,
Abstract: In the one-loop approximation for the periodic Anderson model the contributions of spin and charge fluctuations to the renormalization of the antiferromagnetic order parameter are calculated. It is shown that taking into account the fluctuation corrections allows to quantitatively describe the pressure dependence of the Neel temperature observed in quasi-two-dimensional intermetallic antiferromagnet with heavy fermions CeRhIn$_5$.
Pressure effects on FeSe family superconductors  [PDF]
Yoshikazu Mizuguchi,Fumiaki Tomioka,Keita Deguchi,Shunsuke Tsuda,Takahide Yamaguchi,Yoshihiko Takano
Physics , 2009, DOI: 10.1016/j.physc.2009.10.098
Abstract: We investigated the pressure effects on the FeSe superconductor and the related compounds. Pressure dependence of superconducting transition temperature (Tc) for FeSe0.8S0.2 exhibits a dome-shaped behavior below 0.76 GPa. On the other hand, the Tc of FeSe0.25Te0.75 linearly increases up to 0.99 GPa. Here we discuss the relation between the physical pressure effects and the chemical pressure effects on the FeSe system.
Enhancement of Superconducting Transition Temperature due to the strong Antiferromagnetic Spin Fluctuations in Non-centrosymmetric Heavy-fermion Superconductor CeIrSi3 :A 29Si-NMR Study under Pressure  [PDF]
H. Mukuda,T. Fujii,T. Ohara,A. Harada,M. Yashima,Y. Kitaoka,Y. Okuda,R. Settai,Y. Onuki
Physics , 2008, DOI: 10.1103/PhysRevLett.100.107003
Abstract: We report a 29Si-NMR study on the pressure-induced superconductivity (SC) in an antiferromagnetic (AFM) heavy-fermion compound CeIrSi3 without inversion symmetry. In the SC state at P=2.7-2.8 GPa, the temperature dependence of the nuclear-spin lattice relaxation rate 1/T_1 below Tc exhibits a T^3 behavior without any coherence peak just below Tc, revealing the presence of line nodes in the SC gap. In the normal state, 1/T_1 follows a \sqrt{T}-like behavior, suggesting that the SC emerges under the non-Fermi liquid state dominated by AFM spin fluctuations enhanced around quantum critical point (QCP). The reason why the maximum Tc in CeIrSi3 is relatively high among the Ce-based heavy-fermion superconductors may be the existence of the strong AFM spin fluctuations. We discuss the comparison with the other Ce-based heavy-fermion superconductors.
The origin of nematic order in FeSe  [PDF]
Andrey V. Chubukov,Rafael M. Fernandes,Joerg Schmalian
Physics , 2015, DOI: 10.1103/PhysRevB.91.201105
Abstract: The origin of the 90 K nematic transition in the chalcogenide FeSe, which displays no magnetic order down to T=0, remains a major puzzle for a unifying theory for the iron-based superconductors. We analyze this problem in light of recent experimental data which reveal very small Fermi pockets in this material. We show that the smallness of the Fermi energy leads to a near-degeneracy between magnetic fluctuations and fluctuations in the charge-current density-wave channel. While the two fluctuation modes cooperate to promote the same preemptive Ising-nematic order, they compete for primary order. We argue that this explains why in FeSe the nematic order emerges when the magnetic correlation length is smaller than in other Fe-based materials, and why no magnetism is observed. We discuss how pressure lifts this near-degeneracy, resulting in a non-monotonic dependence of the nematic transition with pressure, in agreement with experiments.
Antiferromagnetic fluctuations and d-wave superconductivity in electron-doped high-temperature superconductors  [PDF]
B. Kyung,J. S. Landry,A. -M. S. Tremblay
Physics , 2002, DOI: 10.1103/PhysRevB.68.174502
Abstract: We show that, at weak to intermediate coupling, antiferromagnetic fluctuations enhance d-wave pairing correlations until, as one moves closer to half-filling, the antiferromagnetically-induced pseudogap begins to suppress the tendency to superconductivity. The accuracy of our approach is gauged by detailed comparisons with Quantum Monte Carlo simulations. The negative pressure dependence of Tc and the existence of photoemission hot spots in electron-doped cuprate superconductors find their natural explanation within this approach.
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