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Interplay of charge density wave and multiband superconductivity in 2$H$-Pd$_x$TaSe$_2$
Authors:
D. Bhoi,
S. Khim,
W. Nam,
B. S. Lee,
Chanhee Kim,
B. -G. Jeon,
B. H. Min,
S. Park,
Kee Hoon Kim
Abstract:
2$H$-TaSe$_2$ has been one of unique transition metal dichalcogenides exhibiting several phase transitions due to a delicate balance among competing electronic ground states. An unusual metallic state at high-$T$ is sequentially followed by an incommensurate charge density wave (ICDW) state at $\approx$ 122 K and a commensurate charge density wave (CCDW) state at $\approx$ 90 K, and superconductiv…
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2$H$-TaSe$_2$ has been one of unique transition metal dichalcogenides exhibiting several phase transitions due to a delicate balance among competing electronic ground states. An unusual metallic state at high-$T$ is sequentially followed by an incommensurate charge density wave (ICDW) state at $\approx$ 122 K and a commensurate charge density wave (CCDW) state at $\approx$ 90 K, and superconductivity at $T_{\rm{C}}\sim$0.14 K. Upon systematic intercalation of Pd ions into TaSe$_2$, we find that CCDW order is destabilized more rapidly than ICDW to indicate a hidden quantum phase transition point at $x$$\sim$0.09-0.10. Moreover, $T_{\rm{C}}$ shows a dramatic enhancement up to 3.3 K at $x$ = 0.08, $\sim$24 times of $T_{\rm{C}}$ in 2$H$-TaSe$_2$, in proportional to the density of states $N(E_F)$. Investigations of upper critical fields $H_{c2}$ in single crystals reveal evidences of multiband superconductivity as temperature-dependent anisotropy factor $γ_H$ = $H_{c2}^{ab}$/$H_{c2}^{c}$, quasi-linear increase of $H_{c2}^{c}(T)$, and an upward, positive-curvature in $H_{c2}^{ab}(T)$ near $T_{\rm{C}}$. Furthermore, analysis of temperature-dependent electronic specific heat corroborates the presence of multiple superconducting gaps. Based on above findings and electronic phase diagram vs $x$, we propose that the increase of $N(E_F)$ and effective electron-phonon coupling in the vicinity of CDW quantum phase transition should be a key to the large enhancement of $T_{\rm{C}}$ in Pd$_x$TaSe$_2$.
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Submitted 18 March, 2016;
originally announced March 2016.
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Electron-boson spectral density of LiFeAs obtained from optical data
Authors:
J. Hwang,
J. P. Carbotte,
B. H. Min,
Y. S. Kwon,
T. Timusk
Abstract:
We analyze existing optical data in the superconducting state of LiFeAs at $T =$ 4 K, to recover its electron-boson spectral density. A maximum entropy technique is employed to extract the spectral density $I^2χ(ω)$ from the optical scattering rate. Care is taken to properly account for elastic impurity scattering which can importantly affect the optics in an $s$-wave superconductor, but does not…
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We analyze existing optical data in the superconducting state of LiFeAs at $T =$ 4 K, to recover its electron-boson spectral density. A maximum entropy technique is employed to extract the spectral density $I^2χ(ω)$ from the optical scattering rate. Care is taken to properly account for elastic impurity scattering which can importantly affect the optics in an $s$-wave superconductor, but does not eliminate the boson structure. We find a robust peak in $I^2χ(ω)$ centered about $Ω_R \cong$ 8.0 meV or 5.3 $k_B T_c$ (with $T_c =$ 17.6 K). Its position in energy agrees well with a similar structure seen in scanning tunneling spectroscopy (STS). There is also a peak in the inelastic neutron scattering (INS) data at this same energy. This peak is found to persist in the normal state at $T =$ 23 K. There is evidence that the superconducting gap is anisotropic as was also found in low temperature angular resolved photoemission (ARPES) data.
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Submitted 18 December, 2014;
originally announced December 2014.
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Anomalous Superconducting-Gap Structure of Slightly Overdoped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$
Authors:
T. Hajiri,
T. Ito,
M. Matsunami,
B. H. Min,
Y. S. Kwon,
S. Kimura
Abstract:
We observed the anisotropic superconducting-gap (SC-gap) structure of a slightly overdoped superconductor, Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ ($x=0.1$), using three-dimensional (3D) angle-resolved photoemission spectroscopy. Two hole Fermi surfaces (FSs) observed at the Brillouin zone center and an inner electron FS at the zone corner showed a nearly isotropic SC gap in 3D momentum space. Howeve…
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We observed the anisotropic superconducting-gap (SC-gap) structure of a slightly overdoped superconductor, Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ ($x=0.1$), using three-dimensional (3D) angle-resolved photoemission spectroscopy. Two hole Fermi surfaces (FSs) observed at the Brillouin zone center and an inner electron FS at the zone corner showed a nearly isotropic SC gap in 3D momentum space. However, the outer electron FS showed an anisotropic SC gap with nodes or gap minima around the M and A points. The different anisotropies obtained the SC gap between the outer and inner electron FSs cannot be expected from all theoretical predictions with spin fluctuation, orbital fluctuation, and both competition. Our results provide a new insight into the SC mechanisms of iron pnictide superconductors.
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Submitted 19 August, 2014;
originally announced August 2014.
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Superconductivity-induced Phonon Renormalization on NaFe$_{1-x}$Co$_{x}$As
Authors:
Y. J. Um,
Yunkyu Bang,
B. H. Min,
Y. S. Kwon,
M. Le Tacon
Abstract:
We report a study of the lattice dynamics in superconducting NaFeAs (Tc = 8 K) and doped NaFe0.97Co0.03As (Tc = 20 K) using Raman light scattering. Five of the six phonon modes expected from group theory are observed. In contrast with results obtained on iso-structural and iso-electronic LiFeAs, anomalous broadening of Eg(As) and A1g(Na) modes upon cooling is observed in both samples. In addition,…
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We report a study of the lattice dynamics in superconducting NaFeAs (Tc = 8 K) and doped NaFe0.97Co0.03As (Tc = 20 K) using Raman light scattering. Five of the six phonon modes expected from group theory are observed. In contrast with results obtained on iso-structural and iso-electronic LiFeAs, anomalous broadening of Eg(As) and A1g(Na) modes upon cooling is observed in both samples. In addition, in the Co-doped sample, a superconductivity-induced renormalization of the frequency and linewidth of the B1g(Fe) vibration is observed. This renormalization can not be understood within a single band and simple multi-band approaches. A theoretical model that includes the effects of SDW correlations along with sign-changing s-wave pairing state and interband scattering has been developed to explain the observed behavior of the B1g(Fe) mode.
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Submitted 6 May, 2014;
originally announced May 2014.
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Polarization-Dependent Three-Dimensional Angle-Resolved Photoemission Spectroscopy of BaFe$_{1.8}$Co$_{0.2}$As$_{2}$
Authors:
Tetsuya Hajiri,
Takahiro Ito,
Masaharu Matsunami,
Byeong Hun Min,
Yong Seung Kwon,
Shin-ichi Kimura
Abstract:
We performed polarization- and photon-energy-dependent angle-resolved photoemission spectroscopy of a slightly overdoped iron pnictide superconductor, BaFe$_{1.8}$Co$_{0.2}$As$_{2}$, to clarify the three-dimensional electronic structure including its orbital characters at the Brillouin zone center. Two hole Fermi surfaces (FSs) with $d_{xz/yz}$ and $d_{xy/x^2-y^2}$ orbitals were observed but…
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We performed polarization- and photon-energy-dependent angle-resolved photoemission spectroscopy of a slightly overdoped iron pnictide superconductor, BaFe$_{1.8}$Co$_{0.2}$As$_{2}$, to clarify the three-dimensional electronic structure including its orbital characters at the Brillouin zone center. Two hole Fermi surfaces (FSs) with $d_{xz/yz}$ and $d_{xy/x^2-y^2}$ orbitals were observed but $d_{z^2}$ hole FS, which has nodes according to a theory of the spin-fluctuation superconductivity mechanism, did not appear. These results suggest that no node will appear at hole FSs at the zone center.
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Submitted 20 November, 2013;
originally announced November 2013.
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Optical properties of iron-based superconductor LiFeAs single crystal
Authors:
Byeong Hun Min,
Jong Beom Hong,
Jae Hyun Yoon,
Takuya Iizuka,
Shin-ichi Kimura,
Yunkyu Bang,
Yong Seung Kwon
Abstract:
We have measured the reflectivity spectra of the iron based superconductor LiFeAs (Tc = 17.6 K) in the temperature range from 4 to 300 K. In the superconducting state (T < Tc), the clear opening of the optical absorption gap was observed below 25 cm-1, indicating an isotropic full gap formation. In the normal state (T > Tc), the optical conductivity spectra display a typical metallic behavior with…
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We have measured the reflectivity spectra of the iron based superconductor LiFeAs (Tc = 17.6 K) in the temperature range from 4 to 300 K. In the superconducting state (T < Tc), the clear opening of the optical absorption gap was observed below 25 cm-1, indicating an isotropic full gap formation. In the normal state (T > Tc), the optical conductivity spectra display a typical metallic behavior with the Drude type spectra at low frequencies, but we found that the introduction of the two Drude components best fits the data, indicating the multiband nature of this superconductor. A theoretical analysis of the low temperature data (T=4K < Tc) also suggests that two superconducting gaps best fit the data and their values were estimated as Δ1 = 1.59 meV and Δ2 = 3.15 meV, respectively. Using the Ferrell-Glover-Tinkham (FGT) sum rule and dielectric function ε1(ω), the superconducting plasma frequency (ωps) is consistently estimated to be 6,665 cm-1, implying that about 59 % of the free carriers in the normal state condenses into the SC condensate. To investigate the various interband transition processes (for ω > 200 cm-1), we have also performed the local-density approximation (LDA) band calculation and calculated the optical spectra of the interband transitions. The theoretical results provided a qualitative agreement with the experimental data below 4000 cm-1
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Submitted 1 July, 2013; v1 submitted 4 January, 2013;
originally announced January 2013.
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Magnetic origin of high-energy kink structure in heavily electron-doped Li$_{1+x}$FeAs
Authors:
T. Hajiri,
T. Ito,
R. Niwa,
S. Hirate,
M. Matsunami,
B. H. Min,
Y. S. Kwon,
S. Kimura
Abstract:
We report the origin of a high-energy kink structure of heavily electron-doped nonsuperconducting Li$_{1+x}$FeAs observed by three-dimensional angle-resolved photoemission spectroscopy. The $d_{xy}$ orbital at the center of the Brillouin zone is strongly renormalized, indicating stronger electron correlation, exceeding that of stoichiometric LiFeAs despite the fact that the orbital characters of a…
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We report the origin of a high-energy kink structure of heavily electron-doped nonsuperconducting Li$_{1+x}$FeAs observed by three-dimensional angle-resolved photoemission spectroscopy. The $d_{xy}$ orbital at the center of the Brillouin zone is strongly renormalized, indicating stronger electron correlation, exceeding that of stoichiometric LiFeAs despite the fact that the orbital characters of all bands remain unchanged. Two types of kink structure on the $d_{yz}$ band were identified: an isotropic kink at the binding energy of 20 meV, and another kink located at 100 meV observed only in the (110) direction. The higher-energy kink is considered to originate from a magnetic interaction, because the peak energy is consistent with that of a spin excitation.
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Submitted 30 January, 2013; v1 submitted 28 December, 2012;
originally announced December 2012.
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Similar zone-center gaps in the low-energy spin-wave spectra of NaFeAs and BaFe2As2
Authors:
J. T. Park,
G. Friemel,
T. Loew,
V. Hinkov,
Yuan Li,
B. H. Min,
D. L. Sun,
A. Ivanov,
A. Piovano,
C. T. Lin,
B. Keimer,
Y. S. Kwon,
D. S. Inosov
Abstract:
We report results of inelastic-neutron-scattering measurements of low-energy spin-wave excitations in two structurally distinct families of iron-pnictide parent compounds: Na(1-δ)FeAs and BaFe2As2. Despite their very different values of the ordered magnetic moment and Néel temperatures, T_N, in the antiferromagnetic state both compounds exhibit similar spin gaps of the order of 10 meV at the magne…
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We report results of inelastic-neutron-scattering measurements of low-energy spin-wave excitations in two structurally distinct families of iron-pnictide parent compounds: Na(1-δ)FeAs and BaFe2As2. Despite their very different values of the ordered magnetic moment and Néel temperatures, T_N, in the antiferromagnetic state both compounds exhibit similar spin gaps of the order of 10 meV at the magnetic Brillouin-zone center. The gap opens sharply below T_N, with no signatures of a precursor gap at temperatures between the orthorhombic and magnetic phase transitions in Na(1-δ)FeAs. We also find a relatively weak dispersion of the spin-wave gap in BaFe2As2 along the out-of-plane momentum component, q_z. At the magnetic zone boundary (q_z = 0), spin excitations in the ordered state persist down to 20 meV, which implies a much smaller value of the effective out-of-plane exchange interaction, J_c, as compared to previous estimates based on fitting the high-energy spin-wave dispersion to a Heisenberg-type model.
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Submitted 13 May, 2012; v1 submitted 4 April, 2012;
originally announced April 2012.
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Existence of Heavy Fermions in the Antiferromagnetic Phase of CeIn3
Authors:
Takuya Iizuka,
Takafumi Mizuno,
Byeong Hun Min,
Yong-Seung Kwon,
Shin-ichi Kimura
Abstract:
We report the pressure-dependent optical conductivity spectra of a heavy fermion (HF) compound CeIn3 below the Neel temperature of 10 K to investigate the existence of the HF state in the antiferromagnetic (AFM) phase. The peak due to the interband transition in the hybridization gap between the conduction band and nearly localized 4f states (c-f hybridization) appears at the photon energy of abou…
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We report the pressure-dependent optical conductivity spectra of a heavy fermion (HF) compound CeIn3 below the Neel temperature of 10 K to investigate the existence of the HF state in the antiferromagnetic (AFM) phase. The peak due to the interband transition in the hybridization gap between the conduction band and nearly localized 4f states (c-f hybridization) appears at the photon energy of about 20 meV not only in the HF regime but also in the AFM regime. Both the energy and intensity of the c-f hybridization peak continuously increase with the application of pressure from the AFM to the HF regime. This result suggests that the c-f hybridization, as well as the heavy fermions, exists even in the AFM phase of CeIn3.
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Submitted 22 March, 2012; v1 submitted 28 February, 2012;
originally announced February 2012.
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Raman Scattering Study of the Lattice Dynamics of Superconducting LiFeAs
Authors:
Y. J. Um,
J. T. Park,
B. H. Min,
Y. J. Song,
Y. S. Kwon,
B. Keimer,
M. Le Tacon
Abstract:
We report an investigation of the lattice dynamical properties of LiFeAs using inelastic light scattering. Five out of the six expected phonon modes are observed. The temperature evolution of their frequencies and linewidths is in good agreement with an anharmonic-decay model. We find no evidence for substantial electron-phonon coupling, and no superconductivity-induced phonon anomalies.
We report an investigation of the lattice dynamical properties of LiFeAs using inelastic light scattering. Five out of the six expected phonon modes are observed. The temperature evolution of their frequencies and linewidths is in good agreement with an anharmonic-decay model. We find no evidence for substantial electron-phonon coupling, and no superconductivity-induced phonon anomalies.
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Submitted 12 January, 2012; v1 submitted 27 September, 2011;
originally announced September 2011.
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Fermi Surface Reconstruction in CeTe2 Induced by Charge Density Wave: ARPES Study
Authors:
J. -S. Kang,
D. H. Kim,
H. J. Lee,
J. H. Hwang,
Han-Koo Lee,
H. -D. Kim,
B. H. Min,
K. E. Lee,
Y. S. Kwon,
J. W. Kim,
Kyoo Kim,
B. H. Kim,
B. I. Min
Abstract:
Electronic structures of a charge-density-wave CDW system CeTe_2-xSb_x (x=0, 0.05) have been investigated by employing angle-resolved photoemission spectroscopy (ARPES). The observed Fermi surface (FS) agrees very well with the calculated FS for the undistorted CeTe_2 both in shapes and sizes. The metallic states crossing the Fermi level(E_F) are observed in ARPES. The carriers near E_F have mainl…
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Electronic structures of a charge-density-wave CDW system CeTe_2-xSb_x (x=0, 0.05) have been investigated by employing angle-resolved photoemission spectroscopy (ARPES). The observed Fermi surface (FS) agrees very well with the calculated FS for the undistorted CeTe_2 both in shapes and sizes. The metallic states crossing the Fermi level(E_F) are observed in ARPES. The carriers near E_F have mainly the Te(1) 5p character, with the negligible contribution from Ce 4f states to the CDW formation. The supercell (shadow) bands and the corresponding very weak FS's are found to arise from band-folding due to the interaction with Ce-Te(2) layers. This work shows that the origin of the CDW formation in CeTe_2 is the FS nesting and that the CDW modulation vector is along Gamma-X (Q_CDW ~ X)
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Submitted 2 September, 2011;
originally announced September 2011.
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Pseudogap of Superconducting correlation origin in iron-pnictide superconductor Ba0.6+δK0.4-δFe2As2
Authors:
Yong Seung Kwon,
Jong Beom Hong,
Yu Ran Jang,
Hyun Jin Oh,
Yun Young Song,
Byeong Hun Min,
Takuya Iizuka,
Shin-ichi Kimura,
A. V. Balatsky,
Yunkyu Bang
Abstract:
Pseudogap (PG), a phenomenon of opening of gap like features above superconducting (SC) transition temperature (Tc), has been universally observed in the high-Tc cuprates (HTC) (1,2), some heavy fermion superconductors (3), and iron pnictides (4-7). Here, we report the observation of PG behavior in optical conductivity in an underdoped Ba0.6+δK0.4-δFe2As2 (Tconset =36 K) single crystal far above T…
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Pseudogap (PG), a phenomenon of opening of gap like features above superconducting (SC) transition temperature (Tc), has been universally observed in the high-Tc cuprates (HTC) (1,2), some heavy fermion superconductors (3), and iron pnictides (4-7). Here, we report the observation of PG behavior in optical conductivity in an underdoped Ba0.6+δK0.4-δFe2As2 (Tconset =36 K) single crystal far above Tc, up to 100 K (~ 3Tc). Unique scale separation in Ba0.6+δK0.4-δFe2As2 compound - magnetic and SC correlation energy scales being widely separated - enabled us to establish that the PG structures observed in the range of 50 cm-1 - 150 cm-1 in optical conductivity is caused by the SC correlation and the magnetic correlation origin is ruled out. Theoretical calculations, based on the preformed Cooper pair model (8), provided an excellent description to the temperature evolution of the optical conductivity data from below to above Tc.
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Submitted 21 July, 2010;
originally announced July 2010.
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Synthesis, anisotropy, and superconducting properties of LiFeAs single crystal
Authors:
Yoo Jang Song,
Jin Soo Ghim,
Byeong Hun Min,
Yong Seung Kwon,
Myung Hwa Jung,
Jong-Soo Rhyee
Abstract:
A LiFeAs single crystal with $T_c^{onset}$$\sim$19.7 K was grown successfully in a sealed tungsten crucible using the Bridgeman method. The electrical resistivity experiments revealed a ratio of room temperature to residual resistivity (RRR) of approximately 46 and 18 for the in-plane and out-of plane directions. The estimated anisotropic resistivity, $γ_ρ$=$ρ_c$ / $ρ_{ab}$, was approximately 3.…
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A LiFeAs single crystal with $T_c^{onset}$$\sim$19.7 K was grown successfully in a sealed tungsten crucible using the Bridgeman method. The electrical resistivity experiments revealed a ratio of room temperature to residual resistivity (RRR) of approximately 46 and 18 for the in-plane and out-of plane directions. The estimated anisotropic resistivity, $γ_ρ$=$ρ_c$ / $ρ_{ab}$, was approximately 3.3 at $T_c^{onset}$. The upper critical fields had large $H_{c2} ^{\shortparallel ab}$ and $H_{c2}^{\shortparallel c}$ values of 83.4 T and 72.5 T, respectively, and an anisotropy ratio is $γ_H$=$H_{c2}^{\shortparallel ab}$ / $H_{c2} ^{\shortparallel c}$$\sim$1.15. The high upper critical field value and small anisotropy highlight the potential use of LiFeAs in a variety of applications. The calculated critical current density $(J_c)$ from the $M$-$H$ loop is approximately 10$^3$ A/cm$^2$
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Submitted 10 February, 2010;
originally announced February 2010.
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The pseudogap behavior in the stoichiometric FeSe superconductor (Tc~9.4 K)
Authors:
Yoo Jang Song,
Jong Beom Hong,
Byeong Hun Min,
Kyu Jun Lee,
Myung Hwa Jung,
Jong-Soo Rhyee,
Yong Seung Kwon
Abstract:
This paper reports the synthesis and superconducting behaviors of the tetragonal iron-chalcogenide superconductor FeSe. The electrical resistivity and magnetic moment measurements confirmed its superconductivity with a $T_c^{zero}$ and $T_c^{mag}$ at 9.4 K under ambient pressure. EPMA indicated the sample to have a stoichiometric Fe:Se ratio of 1:1 ($\pm$0.02). The Seebeck coefficient which was…
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This paper reports the synthesis and superconducting behaviors of the tetragonal iron-chalcogenide superconductor FeSe. The electrical resistivity and magnetic moment measurements confirmed its superconductivity with a $T_c^{zero}$ and $T_c^{mag}$ at 9.4 K under ambient pressure. EPMA indicated the sample to have a stoichiometric Fe:Se ratio of 1:1 ($\pm$0.02). The Seebeck coefficient which was 12.3 $μ$V/K at room temperature, changed to a negative value near 200 K, indicating it to be a two carriers material. Above $T_c$, the $ρ(T)$ curve revealed an 'S' shape. Hence $dρ(T)/dT$, and $d^2ρ(T)/dT^2$ showed pseudogap-like behavior at $T^*$=110 K according to the resistivity curvature mapping (RCM) method for high $T_c$ cuprates. Moreover, the magnetoresistance $ρ_H(T)/ρ_{H=0}$ under a magnetic field and the Seebeck coefficient $S(T)$ revealed revealed pseudogap-like behavior near $T^*$. Interestingly, at the same temperature, 30 K, the sign of $S(T)$ and all signs of $d^2ρ(T)/dT^2$ changed from negative to positive above $T_c$.
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Submitted 10 February, 2010; v1 submitted 10 November, 2009;
originally announced November 2009.