High resolution angle-resolved photoemission measurements have been carried out to study the superconducting gap in the （Ba0.6K0.4）Fe2As2 superconductor with Tc=35 K. Two hole-like Fermi surface sheets around the F ...High resolution angle-resolved photoemission measurements have been carried out to study the superconducting gap in the （Ba0.6K0.4）Fe2As2 superconductor with Tc=35 K. Two hole-like Fermi surface sheets around the F point exhibit different superconducting gaps. The inner Fermi surface sheet shows larger （10 - 12 meV） and slightly momentum-dependent gap while the outer one has smaller （7 - 8 meV） and nearly isotropic gap. The lack of gap node in both Fermi surface sheets favours s-wave superconducting gap symmetry. Superconducting gap opening is also observed at the M（π, π） point. The two Fermi surface spots near the M point are gapped below Tc but the gap persists above Tc. The rich and detailed superconducting gap information will provide key insights and constraints in understanding pairing mechanism in the iron-based superconductors.展开更多
Different element substitution effects in transition metal oxypnictide Re（O1-xFx）TAs, with Re=La, Ce, Nd, Eu, Od, Tm, T=Fe, Ni, Ru, are studied. Similar to the La- or Ce-based systems, we find that the pure NdOFeAs ...Different element substitution effects in transition metal oxypnictide Re（O1-xFx）TAs, with Re=La, Ce, Nd, Eu, Od, Tm, T=Fe, Ni, Ru, are studied. Similar to the La- or Ce-based systems, we find that the pure NdOFeAs shows a strong resistivity anomaly near 145K, which is ascribed to the spin-density-wave instability. Electron doping by F increases Tc to about 50K. While in the case of Gd, Tc is reduced below 10K. The tetragonal ZrCuSiAs-type structure could not be formed for Eu or Tm substitution in our preparing process. For the Ni-based case, although both pure and F-doped LaONiAs are superconducting, no superconductivity is found when La is replaced by Ce in both the cases, instead a ferromagnetic ordering transition is likely to form at low temperature in the undoped sample. We also synthesize LaO1-xFxRuAs and CeO1-xFxRuAs compounds. The metallic behaviour is observed down to 4 K.展开更多
A series of layered （Sr1-xKx）Fe2As2 compounds with nominal x = 0-0.40 are synthesized by solid state reaction method. Similar to other parent compounds of iron-based pnictide superconductors, pure SrFe2As2 shows a s...A series of layered （Sr1-xKx）Fe2As2 compounds with nominal x = 0-0.40 are synthesized by solid state reaction method. Similar to other parent compounds of iron-based pnictide superconductors, pure SrFe2As2 shows a strong resistivity anomaly near 210 K, which was ascribed to the spin-density-wave instability. The anomaly temperature is much higher than those observed in LaOFeAs and BaFe2As2, the two prototype parent compounds with ZrCuSiAs- and ThCr2Si2-type structures. K-doping strongly suppresses this anomaly and induces superconductivity. Like in the case of K-doped BaFe2As2, sharp superconducting transitions at Tc ～ 38 K is observed. We perform the Hall coefficient measurement, and confirm that the dominant carriers are hole-type. The carrier density is enhanced by a factor of 3 in comparison to F-doped LaOFeAs superconductor.展开更多
Angle-resolved photoemission spectroscopy is performed to study the bulk and surface electronic structures of non- superconducting IrTe2 and superconducting Pto.oblro.95 Te2. In addition to the bulk electronic bands p...Angle-resolved photoemission spectroscopy is performed to study the bulk and surface electronic structures of non- superconducting IrTe2 and superconducting Pto.oblro.95 Te2. In addition to the bulk electronic bands predicted by the local density approximation calculations, we observe two Dirac cone-like bands at the Brillouin zone center, which are non-dispersive along kz, suggesting that the extra bands are surface state bands. As the experimental results are well consistent with the ab initio calculations of surface states, the parity analysis proves that these surface state bands are topologically trivial and thus exclude （PtxIr1-x）Te2 as a possible topological superconductor candidate.展开更多
High resolution photoemission measurements are carried out on non-superconducting SmOFeAs paxent compound and superconducting SmFeAs（O1-xFx） （x = 0.12, and 0.15） compounds. The momentum-integrated spectra exhibit ...High resolution photoemission measurements are carried out on non-superconducting SmOFeAs paxent compound and superconducting SmFeAs（O1-xFx） （x = 0.12, and 0.15） compounds. The momentum-integrated spectra exhibit a clear Fermi cutoff that shows little leading-edge shift in the superconducting state. A robust feature at 13 meV is identified in all these samples. Spectral weight suppression near IF, F with decreasing temperature is observed in both undoped and doped samples that points to a possible existence of a pseudogap in these Fe-based compounds.展开更多
High resolution photoemission measurements are carried out on non-superconducting LaFeAsO parent com- pound and various superconducting RFeAs（O1-ZFx） （R=La, Ce and Pr） compounds. It is found that the parent LaFeAs...High resolution photoemission measurements are carried out on non-superconducting LaFeAsO parent com- pound and various superconducting RFeAs（O1-ZFx） （R=La, Ce and Pr） compounds. It is found that the parent LaFeAsO compound shows a metallic character. By extensive measurements, several common features are identified in the electronic structure of these Fe-based compounds： （1） 0.2 eV feature in the valence band, （2） a universal 13-16 meV feature, （3） near EF spectral weight suppression with decreasing temperature. These uni- versal features can provide important information about band structure, superconducting gap and pseudogap in these Fe-based materials.展开更多
We report an optical spectroscopy study on intermediate valence system Ybl-xLuxA13 with x = 0, 0.25, 0.5, 0.75, and 1. The Kondo temperature in the system is known to increase with increasing Lu concentration. Therefo...We report an optical spectroscopy study on intermediate valence system Ybl-xLuxA13 with x = 0, 0.25, 0.5, 0.75, and 1. The Kondo temperature in the system is known to increase with increasing Lu concentration. Therefore, it is expected that the energy scale of the hybridization gap should increase with increasing Lu concentration based on the periodic Anderson model. On the contrary, we find that the spectral structure associated with the hybridization effect shifts monotonically to lower energy. Furthermore, the Lu substitution results in a substantial increase of the free carrier spectral weight and less pronounced plasma frequency reduction upon lowering temperature. We attribute the effect to the disruption of the Kondo lattice periodicity by the random substitution of Yb by Lu. The work highlights the importance of the lattice periodicity of the rare earth element for understanding the Kondo lattice phenomena.展开更多
We perform 31p nuclear magnetic resonance （NMR） measurements on a single crystal of RuP. The anomalies in resistivity at about TA = 270 K and TB = 330 K indicate that two phase transitions occur. The line shape of a...We perform 31p nuclear magnetic resonance （NMR） measurements on a single crystal of RuP. The anomalies in resistivity at about TA = 270 K and TB = 330 K indicate that two phase transitions occur. The line shape of alp NMR spectra in different temperature ranges is attributed to the charge density distribution. The Knight shift and spin-lattice relaxation rate 1/T1T are measured from 1OK to 30OK. At about TA = 270K, they both decrease abruptly with the temperature reduction, which reveals the gap-opening behavior. Well below TA, they act like the case of normal metal. Charge-density-wave phase transition is proposed to interpret the transition occurring at about TA.展开更多
Transmission electron microscopy （TEM） study of SrPt2As2 reveals two incommensurate modulations appearing in the charge-density-wave （CDW） state below TCDW ≈ 470 K. These two structural modulations can be well ex...Transmission electron microscopy （TEM） study of SrPt2As2 reveals two incommensurate modulations appearing in the charge-density-wave （CDW） state below TCDW ≈ 470 K. These two structural modulations can be well explained in terms of condensations of two-coupled phonon modes with wave vectors of q1=0.62a＊ on the a＊-b＊ plane and q2 = 0.23a＊ on the a＊-c＊ plane. The atomic displacements occur along the b-axis direction for q1 and along the c-axis direction for q2, respectively. Moreover, the correlation between ql and q2 can be generally written as q1 = （q2 ＋ a＊）/2 in the CDW state, suggesting the presence of essential coupling between q1 and q2. A small fraction of Ir doping on the Pt site in Sr（Pt1-xIrx）2As2 （x ≤ 0.06） could moderately change these CDW modulations and also affect their superconductivities.展开更多
In this short overview, we summarize the optical spectroscopy FeTezxSex and AxFe2-ySe2. We elaborate that optical spectroscopy band structure evolution across the AFM phase transition temperature, studies on iron sele...In this short overview, we summarize the optical spectroscopy FeTezxSex and AxFe2-ySe2. We elaborate that optical spectroscopy band structure evolution across the AFM phase transition temperature, studies on iron selenide superconducting systems measurements yield fruitful information about the the electronic correlation effect, the superconduct- ing pairing energy gap, the condensed carrier density or penetration depth, the inhomogeneity and the nanoscale phase separation between superconductivity and antiferromagnetism in those systems.展开更多
Utilizing infrared spectroscopy,we study the charge dynamics of the topological superconductor candidate Sr_(x)Bi_(2)Se_(3).The frequency-dependent reflectivity R(ω)demonstrates metallic feature and the scattering ra...Utilizing infrared spectroscopy,we study the charge dynamics of the topological superconductor candidate Sr_(x)Bi_(2)Se_(3).The frequency-dependent reflectivity R(ω)demonstrates metallic feature and the scattering rate of the free carriers decreases with temperature decreasing.The plasma edge shows a slight blue shift upon cooling,similar to the behavior of Cu_(x)Bi_(2)Se_(3).As the carrier concentration n obtained by Hall resistivity increases slightly with the decreasing temperature,the effective mass is proved to increase as well,which is in contrast with that of Cu_(x)Bi_(2)Se_(3).We also perform the ultrafast pump-probe study on the Sr_(0.2)Bi_(2)Se_(3)compounds.Resembling its parent compound Bi_(2)Se_(3),three distinct relaxation processes are found to contribute to the transient reflectivity.However,the deduced relaxation times are quite different.In addition,the electron-optical-phonon coupling constant is identified to beλ=0.88.展开更多
High quality single crystal CaFe4As3 was grown by using the Sn flux method. Unlike layered CaFe2As2, CaFe4As3 crystallizes in an orthorhombic three-dimensional structure. Two magnetic ordering transitions are observed...High quality single crystal CaFe4As3 was grown by using the Sn flux method. Unlike layered CaFe2As2, CaFe4As3 crystallizes in an orthorhombic three-dimensional structure. Two magnetic ordering transitions are observed at - 90 K and - 27 K, respectively. The high temperature transition is an antiferromagnetic（AF） ordering transition. However, the low temperature transition shows complex properties. It shows a ferromagnetic-like transition when a field is applied along b-axis, while antiferromagnetism-like transition when a field is applied perpendicular to b-axis. These results suggest that the low temperature transition at 27 K is a first-order transition from an AF state to a canted AF state. In addition, the low temperature electron specific heat coefficient reaches as high as 143 mJ/mol.K2, showing a heavy fermion behavior.展开更多
Magnetoresistances of SrFe2As2 and BaFe2As2 in the magnetic ordered state are studied. Positive magnetoresistance is observed in the magnetic fields H applied in the azimuthes of θ = 0° and 30° with respect...Magnetoresistances of SrFe2As2 and BaFe2As2 in the magnetic ordered state are studied. Positive magnetoresistance is observed in the magnetic fields H applied in the azimuthes of θ = 0° and 30° with respect to the c-axis. The magnetoresistance can reach 20% for SrFe2As2 and 12% for BaFe2As2 at H = 9 T with θ = 0°（H ‖ c）. Above the magnetic transition temperature, the magnetoresistance becomes negligible. The data in the magnetic ordered state could be described by a modified two-band galvanomagnetic model including the enhancement effect of the applied magnetic field on the spin-density-wave gap. The field enhanced spin-density-wave gaps for different types of carriers are different. Temperature dependencies of the fitting parameters are discussed.展开更多
We report an optical spectroscopy study on LaSb, a compound recently identified to exhibit extremely large magnetoresistance. Our optieal measurement indicates that the material has a low carrier density. More inter- ...We report an optical spectroscopy study on LaSb, a compound recently identified to exhibit extremely large magnetoresistance. Our optieal measurement indicates that the material has a low carrier density. More inter- estingly, the study reveals that the plasma frequency increases with decreasing temperature. This phenomenon suggests either an increase of the conducting carrier density or/and a decrease of the effective mass of carriers with decreasing temperature. We attribute it primarily to the latter effect. Two possible scenarios on its physical origin are examined and discussed. The study offers new insight into the electronic structure of this compound.展开更多
基金Supported by the National Natural Science Foundation of China, the National High-Tech Research and Development Programme of China under Grant Nos 2006CB601002 and 2006CB921302, and Chinese Academy of Sciences （ITSNEM）.We thank T. Xiang for useful discussion.
文摘High resolution angle-resolved photoemission measurements have been carried out to study the superconducting gap in the （Ba0.6K0.4）Fe2As2 superconductor with Tc=35 K. Two hole-like Fermi surface sheets around the F point exhibit different superconducting gaps. The inner Fermi surface sheet shows larger （10 - 12 meV） and slightly momentum-dependent gap while the outer one has smaller （7 - 8 meV） and nearly isotropic gap. The lack of gap node in both Fermi surface sheets favours s-wave superconducting gap symmetry. Superconducting gap opening is also observed at the M（π, π） point. The two Fermi surface spots near the M point are gapped below Tc but the gap persists above Tc. The rich and detailed superconducting gap information will provide key insights and constraints in understanding pairing mechanism in the iron-based superconductors.
基金Supported by the National Natural Science Foundation of China, the Knowledge Innovation Project of the Chinese Academy of Sciences, and the National Basic Research Programme of China under Grant No 2006CB9213001.
文摘Different element substitution effects in transition metal oxypnictide Re（O1-xFx）TAs, with Re=La, Ce, Nd, Eu, Od, Tm, T=Fe, Ni, Ru, are studied. Similar to the La- or Ce-based systems, we find that the pure NdOFeAs shows a strong resistivity anomaly near 145K, which is ascribed to the spin-density-wave instability. Electron doping by F increases Tc to about 50K. While in the case of Gd, Tc is reduced below 10K. The tetragonal ZrCuSiAs-type structure could not be formed for Eu or Tm substitution in our preparing process. For the Ni-based case, although both pure and F-doped LaONiAs are superconducting, no superconductivity is found when La is replaced by Ce in both the cases, instead a ferromagnetic ordering transition is likely to form at low temperature in the undoped sample. We also synthesize LaO1-xFxRuAs and CeO1-xFxRuAs compounds. The metallic behaviour is observed down to 4 K.
基金Supported by the National Natural Science Foundation of China, the Knowledge Innovation Project of the Chinese Academy of Sciences, and the National Basic Research Programme of China.
文摘A series of layered （Sr1-xKx）Fe2As2 compounds with nominal x = 0-0.40 are synthesized by solid state reaction method. Similar to other parent compounds of iron-based pnictide superconductors, pure SrFe2As2 shows a strong resistivity anomaly near 210 K, which was ascribed to the spin-density-wave instability. The anomaly temperature is much higher than those observed in LaOFeAs and BaFe2As2, the two prototype parent compounds with ZrCuSiAs- and ThCr2Si2-type structures. K-doping strongly suppresses this anomaly and induces superconductivity. Like in the case of K-doped BaFe2As2, sharp superconducting transitions at Tc ～ 38 K is observed. We perform the Hall coefficient measurement, and confirm that the dominant carriers are hole-type. The carrier density is enhanced by a factor of 3 in comparison to F-doped LaOFeAs superconductor.
基金Supported by the National Basic Research Program of China under Grant No 2013CB921700the Strategic Priority Research Program of Chinese Academy of Sciences under Grant No XDB07000000the National Natural Science Foundation of China under Grant Nos 11204359 and 11121063
文摘Angle-resolved photoemission spectroscopy is performed to study the bulk and surface electronic structures of non- superconducting IrTe2 and superconducting Pto.oblro.95 Te2. In addition to the bulk electronic bands predicted by the local density approximation calculations, we observe two Dirac cone-like bands at the Brillouin zone center, which are non-dispersive along kz, suggesting that the extra bands are surface state bands. As the experimental results are well consistent with the ab initio calculations of surface states, the parity analysis proves that these surface state bands are topologically trivial and thus exclude （PtxIr1-x）Te2 as a possible topological superconductor candidate.
基金Supported by the National Natural Science Foundation of China, the National Basic Research Programme of China under Grant Nos 2006CB601002 and 2006CB921302, and the ITSNEM Project Chinese Academy of Sciences.
文摘High resolution photoemission measurements are carried out on non-superconducting SmOFeAs paxent compound and superconducting SmFeAs（O1-xFx） （x = 0.12, and 0.15） compounds. The momentum-integrated spectra exhibit a clear Fermi cutoff that shows little leading-edge shift in the superconducting state. A robust feature at 13 meV is identified in all these samples. Spectral weight suppression near IF, F with decreasing temperature is observed in both undoped and doped samples that points to a possible existence of a pseudogap in these Fe-based compounds.
基金Supported by the National Natural Science Foundation of China, the National Basic Research Programme of China under Grant Nos 2006CB601002 and 2006CB921302, and the ITSNEM Project Chinese Academy of Sciences.
文摘High resolution photoemission measurements are carried out on non-superconducting LaFeAsO parent com- pound and various superconducting RFeAs（O1-ZFx） （R=La, Ce and Pr） compounds. It is found that the parent LaFeAsO compound shows a metallic character. By extensive measurements, several common features are identified in the electronic structure of these Fe-based compounds： （1） 0.2 eV feature in the valence band, （2） a universal 13-16 meV feature, （3） near EF spectral weight suppression with decreasing temperature. These uni- versal features can provide important information about band structure, superconducting gap and pseudogap in these Fe-based materials.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11327806 and GZ1123)the National Key Research and Development Program of China(Grant Nos.2016YFA0300902 and 2017YFA0302904)
文摘We report an optical spectroscopy study on intermediate valence system Ybl-xLuxA13 with x = 0, 0.25, 0.5, 0.75, and 1. The Kondo temperature in the system is known to increase with increasing Lu concentration. Therefore, it is expected that the energy scale of the hybridization gap should increase with increasing Lu concentration based on the periodic Anderson model. On the contrary, we find that the spectral structure associated with the hybridization effect shifts monotonically to lower energy. Furthermore, the Lu substitution results in a substantial increase of the free carrier spectral weight and less pronounced plasma frequency reduction upon lowering temperature. We attribute the effect to the disruption of the Kondo lattice periodicity by the random substitution of Yb by Lu. The work highlights the importance of the lattice periodicity of the rare earth element for understanding the Kondo lattice phenomena.
基金Supported by the National Natural Science Foundation of China under Grant No 11025422the National Basic Research Program of China under Grant Nos 2011CB921700 and 2015CB921300the Strategic Priority Research Program of the Chinese Academy of Sciences under Grant No XDB07020200
文摘We perform 31p nuclear magnetic resonance （NMR） measurements on a single crystal of RuP. The anomalies in resistivity at about TA = 270 K and TB = 330 K indicate that two phase transitions occur. The line shape of alp NMR spectra in different temperature ranges is attributed to the charge density distribution. The Knight shift and spin-lattice relaxation rate 1/T1T are measured from 1OK to 30OK. At about TA = 270K, they both decrease abruptly with the temperature reduction, which reveals the gap-opening behavior. Well below TA, they act like the case of normal metal. Charge-density-wave phase transition is proposed to interpret the transition occurring at about TA.
基金Project supported by the National Basic Research Program of China(Grant Nos.2011CBA00101,2010CB923002,2012CB821404,and 2011CB921703)the National Natural Science Foundation of China(Grant Nos.11190022,11274368,and 51272277)the Funds from the Chinese Academy of Sciences
文摘Transmission electron microscopy （TEM） study of SrPt2As2 reveals two incommensurate modulations appearing in the charge-density-wave （CDW） state below TCDW ≈ 470 K. These two structural modulations can be well explained in terms of condensations of two-coupled phonon modes with wave vectors of q1=0.62a＊ on the a＊-b＊ plane and q2 = 0.23a＊ on the a＊-c＊ plane. The atomic displacements occur along the b-axis direction for q1 and along the c-axis direction for q2, respectively. Moreover, the correlation between ql and q2 can be generally written as q1 = （q2 ＋ a＊）/2 in the CDW state, suggesting the presence of essential coupling between q1 and q2. A small fraction of Ir doping on the Pt site in Sr（Pt1-xIrx）2As2 （x ≤ 0.06） could moderately change these CDW modulations and also affect their superconductivities.
文摘In this short overview, we summarize the optical spectroscopy FeTezxSex and AxFe2-ySe2. We elaborate that optical spectroscopy band structure evolution across the AFM phase transition temperature, studies on iron selenide superconducting systems measurements yield fruitful information about the the electronic correlation effect, the superconduct- ing pairing energy gap, the condensed carrier density or penetration depth, the inhomogeneity and the nanoscale phase separation between superconductivity and antiferromagnetism in those systems.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.12074042 and 11704033)the National Key Research and Development Program of China(Grant Nos.2021YFA1400400 and 2016YFA0302300)the Fundamental Research Funds for the Central Universities。
文摘Utilizing infrared spectroscopy,we study the charge dynamics of the topological superconductor candidate Sr_(x)Bi_(2)Se_(3).The frequency-dependent reflectivity R(ω)demonstrates metallic feature and the scattering rate of the free carriers decreases with temperature decreasing.The plasma edge shows a slight blue shift upon cooling,similar to the behavior of Cu_(x)Bi_(2)Se_(3).As the carrier concentration n obtained by Hall resistivity increases slightly with the decreasing temperature,the effective mass is proved to increase as well,which is in contrast with that of Cu_(x)Bi_(2)Se_(3).We also perform the ultrafast pump-probe study on the Sr_(0.2)Bi_(2)Se_(3)compounds.Resembling its parent compound Bi_(2)Se_(3),three distinct relaxation processes are found to contribute to the transient reflectivity.However,the deduced relaxation times are quite different.In addition,the electron-optical-phonon coupling constant is identified to beλ=0.88.
基金Project supported by the National Natural Science Foundation of China(Grant No.Y1JA011x11)
文摘High quality single crystal CaFe4As3 was grown by using the Sn flux method. Unlike layered CaFe2As2, CaFe4As3 crystallizes in an orthorhombic three-dimensional structure. Two magnetic ordering transitions are observed at - 90 K and - 27 K, respectively. The high temperature transition is an antiferromagnetic（AF） ordering transition. However, the low temperature transition shows complex properties. It shows a ferromagnetic-like transition when a field is applied along b-axis, while antiferromagnetism-like transition when a field is applied perpendicular to b-axis. These results suggest that the low temperature transition at 27 K is a first-order transition from an AF state to a canted AF state. In addition, the low temperature electron specific heat coefficient reaches as high as 143 mJ/mol.K2, showing a heavy fermion behavior.
基金Supported by the National Natural Science Foundation of China under Grant No 10874213, the Knowledge Innovation Project of Chinese Academy of Sciences, and the National Basic Research Program of China.
文摘Magnetoresistances of SrFe2As2 and BaFe2As2 in the magnetic ordered state are studied. Positive magnetoresistance is observed in the magnetic fields H applied in the azimuthes of θ = 0° and 30° with respect to the c-axis. The magnetoresistance can reach 20% for SrFe2As2 and 12% for BaFe2As2 at H = 9 T with θ = 0°（H ‖ c）. Above the magnetic transition temperature, the magnetoresistance becomes negligible. The data in the magnetic ordered state could be described by a modified two-band galvanomagnetic model including the enhancement effect of the applied magnetic field on the spin-density-wave gap. The field enhanced spin-density-wave gaps for different types of carriers are different. Temperature dependencies of the fitting parameters are discussed.
基金Supported by the National Natural Science Foundation of China under Grant Nos 11327806 and GZ1123the National Key Research and Development Program of China under Grant No 2016YFA0300902
文摘We report an optical spectroscopy study on LaSb, a compound recently identified to exhibit extremely large magnetoresistance. Our optieal measurement indicates that the material has a low carrier density. More inter- estingly, the study reveals that the plasma frequency increases with decreasing temperature. This phenomenon suggests either an increase of the conducting carrier density or/and a decrease of the effective mass of carriers with decreasing temperature. We attribute it primarily to the latter effect. Two possible scenarios on its physical origin are examined and discussed. The study offers new insight into the electronic structure of this compound.
