We synthesize high-quality single crystal of CeGaSi by a Ga self-flux method and investigate its physical properties through magnetic susceptibility,specific heat and electrical resistivity measurements as well as hig...We synthesize high-quality single crystal of CeGaSi by a Ga self-flux method and investigate its physical properties through magnetic susceptibility,specific heat and electrical resistivity measurements as well as high pressure effect.Magnetic measurements reveal that an antiferromagnetic order develops below T_(m)~10.4 K with magnetic moments orientated in the ab plane.The enhanced electronic specific heat coefficient and the negative logarithmic slope in the resistivity of CeGaSi indicate that the title compound belongs to the family of Kondo system with heavy fermion ground states.The max magnetic entropy change-ΔS_(M)^(max)(μ_(0)H⊥c,μ_(0)H=7 T) around T_(m) is found to reach up to 11.85 J·kg^(-1)·K^(-1).Remarkably,both the antiferromagnetic transition temperature and-ln T behavior increase monotonically with pressure applied to 20 kbar(1 bar=10~5 Pa),indicating that much higher pressure will be needed to reach its quantum critical point.展开更多
We revisited the vortex states of 2H-NbSe_(2) towards zero fields by a low-temperature scanning tunneling microscope.Fine structures of the anisotropic vortex states were distinguished, one is a spatially non-splittin...We revisited the vortex states of 2H-NbSe_(2) towards zero fields by a low-temperature scanning tunneling microscope.Fine structures of the anisotropic vortex states were distinguished, one is a spatially non-splitting zero bias peak, and the other is an in-gap conductance anomaly resembling evolved crossing features around the center of the three nearest vortices.Both of them distribute solely along the next nearest neighboring direction of the vortex lattice and become unresolved in much higher magnetic fields, implying an important role played by the vortex–vortex interactions. To clarify these issues,we have studied the intrinsic vortex states of the isolated trapped vortex in zero fields at 0.45 K. It is concluded that the anisotropic zero bias peak is attributed to the superconducting gap anisotropy, and the spatially evolved crossing features are related to the vortex–vortex interaction. The vortex core size under the zero-field limit is determined. These results provide a paradigm for studying the inherent vortex states of type-II superconductors especially based on an isolated vortex.展开更多
We report on soft c-axis point-contact Andreev reflection(PCAR)spectroscopy combining with resistivity measurements on BaFe_(2)(As_(0.7)P_(0.3))_(2),to elucidate the superconducting gap structure in the vicinity of th...We report on soft c-axis point-contact Andreev reflection(PCAR)spectroscopy combining with resistivity measurements on BaFe_(2)(As_(0.7)P_(0.3))_(2),to elucidate the superconducting gap structure in the vicinity of the quantum critical point.A double peak at the gap edge plus a dip feature at zero-bias has been observed on the PCAR spectra,indicative of the presence of a nodeless gap in BaFe_(2)(As_(0.7)P_(0.3))_(2).Detailed analysis within a sophisticated theoretical model reveals an anisotropic gap with deep gap minima.The PCARs also feature additional structures related to the electron-bosonic coupling mode.Using the extracted superconducting energy gap value,a characteristic bosonic energy Ω_(b) and its temperature dependence are obtained,comparable with the spin-resonance energy observed in neutron scattering experiment.These results indicate a magnetism-driven quantum critical point in the BaFe_(2)(As_(1-x)P_(x))_(2) system.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
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
We report on the single crystal growth and superconducting properties of PbTaSe2 with the non-centrosymmetric crystal structure. By using the chemicM vapor transport technique, centimeter-size single crystals are succ...We report on the single crystal growth and superconducting properties of PbTaSe2 with the non-centrosymmetric crystal structure. By using the chemicM vapor transport technique, centimeter-size single crystals are success- fully obtained. The measurement of temperature dependence of electricaJ resistivity p（T） in both normal and superconducting states indicates a quasi-two-dimensional electronic state in contrast to that of polycrystalline samples. Specific heat C（T） measurement reveals a bulk superconductivity with Tc ≈ 3.75K and a specific heat jump ratio of 1.42. All these results are in agreement with a moderately electron-phonon coupled, type-g Bardeen-Cooper-Schrieffer superconductor.展开更多
The discovery of cuprate high TC superconductors has inspired the search for unconventional superconductors in magnetic materials.A successful recipe has been to suppress long−range order in a magnetic parent compound...The discovery of cuprate high TC superconductors has inspired the search for unconventional superconductors in magnetic materials.A successful recipe has been to suppress long−range order in a magnetic parent compound by doping or high pressure to drive the material towards a quantum critical point.We report an exception to this rule in the recently discovered potassium iron selenide.The superconducting composition is identified as the iron vacancy ordered K_(0.83)(2)Fe_(1.64)(1)Se_(2) with T_(C) above 30 K.A novel large moment 3.31μB/Fe antiferromagnetic order that conforms to the tetragonal crystal symmetry has an unprecedentedly high ordering temperature TN≈559 K for a bulk superconductor.Staggeringly polarized electronic density of states is thus suspected,which would stimulate further investigation into superconductivity in a strong spin-exchange field under new circumstances.展开更多
We develop an experimental tool to investigate the order parameter of superconductors by combining pointcontact spectroscopy measurement with high-pressure technique.It is demonstrated for the first time that planar p...We develop an experimental tool to investigate the order parameter of superconductors by combining pointcontact spectroscopy measurement with high-pressure technique.It is demonstrated for the first time that planar point-contact spectroscopy measurement on noncentrosymmetric superconducting PbTaSe2 single crystals is systematically subjected to hydrostatic pressures up to 12.1 kbar.Under such a high pressure,the normal-state contact resistance is sensitive to the applied pressure,reflecting the underlying variation of contact transparency upon pressures.In a superconducting state,the pressure dependence of the energy gap△0 and the critical temperature Tc for gap opening/closing are extracted based on a generalized Blond-Tinkham-Klapwijk model.The gap ratio 2△0/kB Tc indicates a crossover from weak coupling to strong coupling in electron pairing strength upon pressure for PbTaSe2.Our experimental results show the accessibility and validity of high-pressure pointcontact spectroscopy,offering rich information about high-pressure superconductivity.展开更多
We report a comprehensive high-pressure study on the monoclinic TIFeSe2 single crystal,which is an antiferromagnetic jnsulator with quasi-one-dimensional crystal structure at ambient pressure.It is found that TIFeSe2 ...We report a comprehensive high-pressure study on the monoclinic TIFeSe2 single crystal,which is an antiferromagnetic jnsulator with quasi-one-dimensional crystal structure at ambient pressure.It is found that TIFeSe2 undergoes a pressure-induced structural transformation from the monoclinic phase to an orthorhombic structure above Pc≈13 GPa,accompanied with a large volume collapse of ΔV/V0=8.3%.In the low-pressure monoclinic phase,the insulating state is easily metallized at pressures above 2 GPa;while possible superconductivity with Tconset^2 K is found to emerge above 30 GPa in the high-pressure phase.Such a great tunability of TIFeSe2 under pressure indicates that the ternary AFeSe2 system(A=Tl,K,Cs,Rb) should be taken as an important platform for explorations of interesting phenomena such as insulator-metal transition,dimensionality crossover,and superconductivity.展开更多
We report superconductivity in a new ternary molybdenum pnictide Rb2 Mo3 As3,synthesized via the solid state reaction method.Powder x-ray diffraction analysis reveals a hexagonal crystal structure with space group P6 ...We report superconductivity in a new ternary molybdenum pnictide Rb2 Mo3 As3,synthesized via the solid state reaction method.Powder x-ray diffraction analysis reveals a hexagonal crystal structure with space group P6 m2(No.187),and the refined lattice parameters are a=10.431(5)A,c=4.460(4)A.SEM images show rod-like grains with good ductility,confirming a quasi-one-dimensional(Q1 D)structure.Electrical resistivity and dc magnetic susceptibility characterizations exhibit superconductivity with an onset of Tc=10.5 K.The upper critical field of Rb2 Mo3 As3 is estimated to be 28.2 T at zero temperature,providing an evidence of possible unconventional superconductivity.Our recent discovery of MoAs-based superconductors above 10 K provides a unique platform for the study of exotic superconductivity in 4 d electron systems with Q1 D crystal structures.展开更多
We report comprehensive angle-resolved photoemission investigations on the electronic structures and nematicity of the parent compounds of the iron-based superconductors including CeFeAsO, BaFe2As2, NaFeAs, FeSe and u...We report comprehensive angle-resolved photoemission investigations on the electronic structures and nematicity of the parent compounds of the iron-based superconductors including CeFeAsO, BaFe2As2, NaFeAs, FeSe and undoped FeSe/SrTiO3 films with 1, 2 and 20 layers. While the electronic structure near tile Brillouin zone center F varies dramatically among different materials, the electronic structure near the Brillouin zone corners （M points）, as well as their temperature dependence, are rather similar. The electronic structure near the zone corners is dominated by the electronic nematicity that gives rise to a band splitting of the dxz and dyz bands below the nematie transition temperature. A clear relation is observed between the band splitting magnitude arid the onset temperature of nematicity. Our results may shed light on the origin of nematicity, its effect on the electronic structures, and its relation with superconductivity in the iron-based superconductors.展开更多
We report a direct microwave synthesis method for the preparation of 11-type high quality Fe(Te,Se) polycrystalline superconductors. The bulk samples are rapidly synthesized under the microwave irradiation during seve...We report a direct microwave synthesis method for the preparation of 11-type high quality Fe(Te,Se) polycrystalline superconductors. The bulk samples are rapidly synthesized under the microwave irradiation during several minutes, with a subsequent annealing process at 400℃. The samples exhibit a nearly single phase of the tetragonal PbO-type crystal structure with minor impurities. Morphology characterization shows high density, tight grain connectivity and large grain sizes around 100 μm with small cavities inside the sample. Resistivity and magnetization measurements both show similar superconducting transitions above 14 K. The magnetic hysteresis measurements display broad and symmetric loops without magnetic background, and a high critical current density J_c about 1.2 × 10~4 A/cm^2 at 2 K and 7 T is estimated by the Bean model. Compared with the solidstate reaction synthesized samples, these superconducting bulks from microwave-assisted synthesis are possibly free of the interstitial Fe due to smaller c-axis, higher T_c in magnetic transitions, better M–H loops without magnetic background and greatly enhanced J_c, and are promising as raw materials for the non-toxic Fe-based superconducting wires for large currents and high field applications.展开更多
In the iron-based high-To bulk superconductors, Tc above 50 K was only observed in the electron-doped 1111-type compounds. Here we revisit the electron-doped SmFeAsO polycrystals to make a further investigation for th...In the iron-based high-To bulk superconductors, Tc above 50 K was only observed in the electron-doped 1111-type compounds. Here we revisit the electron-doped SmFeAsO polycrystals to make a further investigation for the highest Tc in these materials. To introduce more electron carriers and less crystal lattice distortions, we study the Th and F eodoping effects into the Sm-O layers with heavy electron doping. Dozens of Sm1-xThx FeAsO1-yFy samples are synthesized through the solid state reaction method, and these samples are carefully characterized by the structural, resistive, and magnetic measurements. We find that the codoping of Th and F clearly enhances the superconducting Tc more than the Th or F single-doped samples, with the highest record Tc up to 58.6K when x = 0.2 and y=0.225. Further element doping causes more impurities and lattice distortions in the samples with a weakened superconductivity.展开更多
We report the tip-induced superconductivity on the topological semimetal NbSb_(2), similar to the observation on TaAs_(2) and NbAs_(2). Belonging to the same family of MPn_(2), all these materials possess similar band...We report the tip-induced superconductivity on the topological semimetal NbSb_(2), similar to the observation on TaAs_(2) and NbAs_(2). Belonging to the same family of MPn_(2), all these materials possess similar band structures, indicating that the tip-induced superconductivity may be closely related to their topological nature and share a common mechanism. Further analysis suggests that a bulk band should play the dominant role in such local superconductivity most likely through interface coupling. In addition, the compatibility between the induced superconductivity and tips’ ferromagnetism gives an evidence for its unconventional nature. These results provide further clues to elucidate the mechanism of the tip-induced superconductivity observed in topological materials.展开更多
We report the discovery of superconductivity on the border of antiferromagnetic order in a quasi-one-dimensional material RbMn_(6)Bi_(5)via measurements of resistivity and magnetic susceptibility under high pressures....We report the discovery of superconductivity on the border of antiferromagnetic order in a quasi-one-dimensional material RbMn_(6)Bi_(5)via measurements of resistivity and magnetic susceptibility under high pressures.Its phase diagram of temperature versus pressure resembles those of many magnetism-mediated superconducting systems.With increasing pressure,its antiferromagnetic ordering transition with T_(N)=83K at ambient pressure is first enhanced moderately and then suppressed completely at a critical pressure of P_(c)≈13GPa,around which bulk superconductivity emerges and exhibits a dome-like T_(c)(P)with a maximal T_(c)^(onset)≈9.5K at about 15GPa.In addition,the superconducting state around P_(c) is characterized by a large upper critical fieldμ_(0)H_(c2)(0)exceeding the Pauli paramagnetic limit,implying a possible unconventional paring mechanism.The present study,together with our recent work on KMn_(6)Bi_(5)(the maximum T_(c)^(onset)≈9.3 K),makes AMn_(6)Bi_(5)(A=alkali metal)a new family of Mn-based superconductors with relatively high T_(c).展开更多
There is a long-standing confusion concerning the physical origin of the anomalous resistivity peak in transition metal pentatelluride HfTe5. Several mechanisms, such as the formation of charge density wave or polaron...There is a long-standing confusion concerning the physical origin of the anomalous resistivity peak in transition metal pentatelluride HfTe5. Several mechanisms, such as the formation of charge density wave or polaron, have been proposed, but so far no conclusive evidence has been presented. In this work, we investigate the unusual temperature dependence of magneto-transport properties in HfTe5. It is found that a three-dimensional topological Dirac semimetal state emerges only at around Tp （at which the resistivity shows a pronounced peak）, as manifested by a large negative magnetoresistance. This accidental Dirac semimetal state mediates the topological quantum phase transition between the two distinct weak and strong topological insulator phases in HfTe5. Our work not only provides the first evidence of a temperature-induced critical topological phase transition inHfTe5 but also gives a reasonable explanation on the long-lasting question.展开更多
基金Project supported by the National Natural Science Foundation of China (Grant No. 12274440)the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB33010100)+1 种基金the Fund from the Ministry of Science and Technology of China (Grant No. 2022YFA1403903)the Fund of the Synergetic Extreme Condition User Facility (SECUF)。
文摘We synthesize high-quality single crystal of CeGaSi by a Ga self-flux method and investigate its physical properties through magnetic susceptibility,specific heat and electrical resistivity measurements as well as high pressure effect.Magnetic measurements reveal that an antiferromagnetic order develops below T_(m)~10.4 K with magnetic moments orientated in the ab plane.The enhanced electronic specific heat coefficient and the negative logarithmic slope in the resistivity of CeGaSi indicate that the title compound belongs to the family of Kondo system with heavy fermion ground states.The max magnetic entropy change-ΔS_(M)^(max)(μ_(0)H⊥c,μ_(0)H=7 T) around T_(m) is found to reach up to 11.85 J·kg^(-1)·K^(-1).Remarkably,both the antiferromagnetic transition temperature and-ln T behavior increase monotonically with pressure applied to 20 kbar(1 bar=10~5 Pa),indicating that much higher pressure will be needed to reach its quantum critical point.
基金Project supported by the National Key R&D Program of China (Grant No. 2022YFA1403203)the Innovation Program for Quantum Science and Technology (Grant No. 2021ZD0302802)+3 种基金the National Natural Science Foundation of China (Grant Nos. 12074002, 12374133, and 11804379)the Major Basic Program of Natural Science Foundation of Shandong Province (Grant No. ZR2021ZD01)the supports of the National Natural Science Foundation of China (Grant No. 12274001)the Natural Science Foundation of Anhui Province (Grant No. 2208085MA09)。
文摘We revisited the vortex states of 2H-NbSe_(2) towards zero fields by a low-temperature scanning tunneling microscope.Fine structures of the anisotropic vortex states were distinguished, one is a spatially non-splitting zero bias peak, and the other is an in-gap conductance anomaly resembling evolved crossing features around the center of the three nearest vortices.Both of them distribute solely along the next nearest neighboring direction of the vortex lattice and become unresolved in much higher magnetic fields, implying an important role played by the vortex–vortex interactions. To clarify these issues,we have studied the intrinsic vortex states of the isolated trapped vortex in zero fields at 0.45 K. It is concluded that the anisotropic zero bias peak is attributed to the superconducting gap anisotropy, and the spatially evolved crossing features are related to the vortex–vortex interaction. The vortex core size under the zero-field limit is determined. These results provide a paradigm for studying the inherent vortex states of type-II superconductors especially based on an isolated vortex.
基金supported by the National Natural Science Foundation of China(Grant Nos.11774303 and 11574373)the National Key Research and Development Program of China(Grant Nos.2022YFA1403402,2021YFA1400401,and 2020YFA0406003)+1 种基金the Chinese Academy of Sciences(Grant Nos.XDB33000000 and GJTD-2020-01)financial support from the Joint Fund of Yunnan Provincial Science and Technology Department(Grant No.2019FY003008)。
文摘We report on soft c-axis point-contact Andreev reflection(PCAR)spectroscopy combining with resistivity measurements on BaFe_(2)(As_(0.7)P_(0.3))_(2),to elucidate the superconducting gap structure in the vicinity of the quantum critical point.A double peak at the gap edge plus a dip feature at zero-bias has been observed on the PCAR spectra,indicative of the presence of a nodeless gap in BaFe_(2)(As_(0.7)P_(0.3))_(2).Detailed analysis within a sophisticated theoretical model reveals an anisotropic gap with deep gap minima.The PCARs also feature additional structures related to the electron-bosonic coupling mode.Using the extracted superconducting energy gap value,a characteristic bosonic energy Ω_(b) and its temperature dependence are obtained,comparable with the spin-resonance energy observed in neutron scattering experiment.These results indicate a magnetism-driven quantum critical point in the BaFe_(2)(As_(1-x)P_(x))_(2) system.
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 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 Basic Research Program of China under Grant Nos 2015CB921303,2011CBA00100 and 2012CB821404the Strategic Priority Research Program(B) of Chinese Academy of Sciences under Grant Nos XDB07020100and XDB07020200the National Natural Science Foundation of China under Grant No 11174350
文摘We report on the single crystal growth and superconducting properties of PbTaSe2 with the non-centrosymmetric crystal structure. By using the chemicM vapor transport technique, centimeter-size single crystals are success- fully obtained. The measurement of temperature dependence of electricaJ resistivity p（T） in both normal and superconducting states indicates a quasi-two-dimensional electronic state in contrast to that of polycrystalline samples. Specific heat C（T） measurement reveals a bulk superconductivity with Tc ≈ 3.75K and a specific heat jump ratio of 1.42. All these results are in agreement with a moderately electron-phonon coupled, type-g Bardeen-Cooper-Schrieffer superconductor.
基金by the National Natural Science Foundation of China under Grant Nos 11034012,10834013 and 10974254the National Basic Research Program of China under Grant Nos 2011CBA00112,2010CB923000 and 2009CB009100.
文摘The discovery of cuprate high TC superconductors has inspired the search for unconventional superconductors in magnetic materials.A successful recipe has been to suppress long−range order in a magnetic parent compound by doping or high pressure to drive the material towards a quantum critical point.We report an exception to this rule in the recently discovered potassium iron selenide.The superconducting composition is identified as the iron vacancy ordered K_(0.83)(2)Fe_(1.64)(1)Se_(2) with T_(C) above 30 K.A novel large moment 3.31μB/Fe antiferromagnetic order that conforms to the tetragonal crystal symmetry has an unprecedentedly high ordering temperature TN≈559 K for a bulk superconductor.Staggeringly polarized electronic density of states is thus suspected,which would stimulate further investigation into superconductivity in a strong spin-exchange field under new circumstances.
基金Supported by the National Science Foundation of China(Grant Nos.11574373 and 11774303)the Joint Fund of Yunnan Provincial Science and Technology Department(Grant No.2019FY003008)。
文摘We develop an experimental tool to investigate the order parameter of superconductors by combining pointcontact spectroscopy measurement with high-pressure technique.It is demonstrated for the first time that planar point-contact spectroscopy measurement on noncentrosymmetric superconducting PbTaSe2 single crystals is systematically subjected to hydrostatic pressures up to 12.1 kbar.Under such a high pressure,the normal-state contact resistance is sensitive to the applied pressure,reflecting the underlying variation of contact transparency upon pressures.In a superconducting state,the pressure dependence of the energy gap△0 and the critical temperature Tc for gap opening/closing are extracted based on a generalized Blond-Tinkham-Klapwijk model.The gap ratio 2△0/kB Tc indicates a crossover from weak coupling to strong coupling in electron pairing strength upon pressure for PbTaSe2.Our experimental results show the accessibility and validity of high-pressure pointcontact spectroscopy,offering rich information about high-pressure superconductivity.
基金Supported by the National Key R&D Program of China(2018YFA0305700)the National Natural Science Foundation of China(11904391,11834016,11874400,11888101,11921004)+3 种基金the Beijing Natural Science Foundation(Z190008)the Strategic Priority Research Program and the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences(XDB25000000,QYZDB-SSW-SLH013)the CAS Interdisciplinary Innovation Team(JCTD-2019-01)JPS acknowledges the support from the China Postdoctoral Science Foundation and the Postdoctoral Innovative Talent Program.
文摘We report a comprehensive high-pressure study on the monoclinic TIFeSe2 single crystal,which is an antiferromagnetic jnsulator with quasi-one-dimensional crystal structure at ambient pressure.It is found that TIFeSe2 undergoes a pressure-induced structural transformation from the monoclinic phase to an orthorhombic structure above Pc≈13 GPa,accompanied with a large volume collapse of ΔV/V0=8.3%.In the low-pressure monoclinic phase,the insulating state is easily metallized at pressures above 2 GPa;while possible superconductivity with Tconset^2 K is found to emerge above 30 GPa in the high-pressure phase.Such a great tunability of TIFeSe2 under pressure indicates that the ternary AFeSe2 system(A=Tl,K,Cs,Rb) should be taken as an important platform for explorations of interesting phenomena such as insulator-metal transition,dimensionality crossover,and superconductivity.
基金Supported by the National Key Research and Development of China(Grant Nos.2018YFA0704200 and 2016YFA0300301)the National Natural Science Foundation of China(Grant No.11774402)the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB25000000)。
文摘We report superconductivity in a new ternary molybdenum pnictide Rb2 Mo3 As3,synthesized via the solid state reaction method.Powder x-ray diffraction analysis reveals a hexagonal crystal structure with space group P6 m2(No.187),and the refined lattice parameters are a=10.431(5)A,c=4.460(4)A.SEM images show rod-like grains with good ductility,confirming a quasi-one-dimensional(Q1 D)structure.Electrical resistivity and dc magnetic susceptibility characterizations exhibit superconductivity with an onset of Tc=10.5 K.The upper critical field of Rb2 Mo3 As3 is estimated to be 28.2 T at zero temperature,providing an evidence of possible unconventional superconductivity.Our recent discovery of MoAs-based superconductors above 10 K provides a unique platform for the study of exotic superconductivity in 4 d electron systems with Q1 D crystal structures.
基金Supported by the National Natural Science Foundation of China under Grant Nos 11190022,11334010 and 11534007the National Basic Research Program of China under Grant No 2015CB921000the Strategic Priority Research Program(B)of Chinese Academy of Sciences under Grant No XDB07020300
文摘We report comprehensive angle-resolved photoemission investigations on the electronic structures and nematicity of the parent compounds of the iron-based superconductors including CeFeAsO, BaFe2As2, NaFeAs, FeSe and undoped FeSe/SrTiO3 films with 1, 2 and 20 layers. While the electronic structure near tile Brillouin zone center F varies dramatically among different materials, the electronic structure near the Brillouin zone corners （M points）, as well as their temperature dependence, are rather similar. The electronic structure near the zone corners is dominated by the electronic nematicity that gives rise to a band splitting of the dxz and dyz bands below the nematie transition temperature. A clear relation is observed between the band splitting magnitude arid the onset temperature of nematicity. Our results may shed light on the origin of nematicity, its effect on the electronic structures, and its relation with superconductivity in the iron-based superconductors.
基金Supported by the National Natural Science Foundation of China under Grant Nos 11474339 and 11774402the National Basic Research Program of China under Grant No 2016YFA0300301+1 种基金the Strategic Priority Research Program of Chinese Academy of Sciences under Grant No XDB25000000the Youth Innovation Promotion Association of Chinese Academy of Sciences
文摘We report a direct microwave synthesis method for the preparation of 11-type high quality Fe(Te,Se) polycrystalline superconductors. The bulk samples are rapidly synthesized under the microwave irradiation during several minutes, with a subsequent annealing process at 400℃. The samples exhibit a nearly single phase of the tetragonal PbO-type crystal structure with minor impurities. Morphology characterization shows high density, tight grain connectivity and large grain sizes around 100 μm with small cavities inside the sample. Resistivity and magnetization measurements both show similar superconducting transitions above 14 K. The magnetic hysteresis measurements display broad and symmetric loops without magnetic background, and a high critical current density J_c about 1.2 × 10~4 A/cm^2 at 2 K and 7 T is estimated by the Bean model. Compared with the solidstate reaction synthesized samples, these superconducting bulks from microwave-assisted synthesis are possibly free of the interstitial Fe due to smaller c-axis, higher T_c in magnetic transitions, better M–H loops without magnetic background and greatly enhanced J_c, and are promising as raw materials for the non-toxic Fe-based superconducting wires for large currents and high field applications.
基金Supported by the National Natural Science Foundation of China under Grant No 11474339the National Basic Research Program of China under Grant No 2016YFA0300301the Youth Innovation Promotion Association of the Chinese Academy of Sciences
文摘In the iron-based high-To bulk superconductors, Tc above 50 K was only observed in the electron-doped 1111-type compounds. Here we revisit the electron-doped SmFeAsO polycrystals to make a further investigation for the highest Tc in these materials. To introduce more electron carriers and less crystal lattice distortions, we study the Th and F eodoping effects into the Sm-O layers with heavy electron doping. Dozens of Sm1-xThx FeAsO1-yFy samples are synthesized through the solid state reaction method, and these samples are carefully characterized by the structural, resistive, and magnetic measurements. We find that the codoping of Th and F clearly enhances the superconducting Tc more than the Th or F single-doped samples, with the highest record Tc up to 58.6K when x = 0.2 and y=0.225. Further element doping causes more impurities and lattice distortions in the samples with a weakened superconductivity.
基金Project supported by the National Key R&D Program of China(Grant Nos.2017YFA0302904,2017YFA0303201,2018YFA0305602,and 2016YFA0300604)the National Natural Science Foundation of China(Grants Nos.12074002,11574372,11674331,11804379,and 11874417)+4 种基金the National Basic Research Program of China(Grant No.2015CB921303)the Strategic Priority Research Program(B)of the Chinese Academy of Sciences(Grants Nos.XDB07020300,XDB07020100,and XDB33030100)the Collaborative Innovation Program of Hefei Science Center,the Chinese Academy of Sciences(Grant No.2020HSCCIP002)CASHIPS Director’s Fund,China(Grant No.BJPY2019B03)the Recruitment Program for Leading Talent Team of Anhui Province,China(2019-16)。
文摘We report the tip-induced superconductivity on the topological semimetal NbSb_(2), similar to the observation on TaAs_(2) and NbAs_(2). Belonging to the same family of MPn_(2), all these materials possess similar band structures, indicating that the tip-induced superconductivity may be closely related to their topological nature and share a common mechanism. Further analysis suggests that a bulk band should play the dominant role in such local superconductivity most likely through interface coupling. In addition, the compatibility between the induced superconductivity and tips’ ferromagnetism gives an evidence for its unconventional nature. These results provide further clues to elucidate the mechanism of the tip-induced superconductivity observed in topological materials.
基金supported by the Beijing Natural Science Foundation (Grant No. Z190008)the National Key R&D Program of China (Grant Nos. 2018YFA0305700 and 2021YFA1400200)+6 种基金the National Natural Science Foundation of China (Grant Nos. 12025408, 11874400, 11834016, 11921004, 11904391, 12174424, and 11888101)the Strategic Priority Research Program and Key Research Program of Frontier Sciences of CAS (Grant Nos. XDB25000000, XDB33000000 and QYZDB-SSW-SLH013)the CAS Interdisciplinary Innovation Team (Grant No. JCTD-201-01)K.C.Wong Education Foundation (GJTD-2020-01)the Users with Excellence Program of Hefei Science Center CAS (Grant No. 2021HSC-UE008)Youth Promotion Association of CAS (Grant No. 2018010)support from JSPS KAKENHI (Grant No. JP19H00648)
文摘We report the discovery of superconductivity on the border of antiferromagnetic order in a quasi-one-dimensional material RbMn_(6)Bi_(5)via measurements of resistivity and magnetic susceptibility under high pressures.Its phase diagram of temperature versus pressure resembles those of many magnetism-mediated superconducting systems.With increasing pressure,its antiferromagnetic ordering transition with T_(N)=83K at ambient pressure is first enhanced moderately and then suppressed completely at a critical pressure of P_(c)≈13GPa,around which bulk superconductivity emerges and exhibits a dome-like T_(c)(P)with a maximal T_(c)^(onset)≈9.5K at about 15GPa.In addition,the superconducting state around P_(c) is characterized by a large upper critical fieldμ_(0)H_(c2)(0)exceeding the Pauli paramagnetic limit,implying a possible unconventional paring mechanism.The present study,together with our recent work on KMn_(6)Bi_(5)(the maximum T_(c)^(onset)≈9.3 K),makes AMn_(6)Bi_(5)(A=alkali metal)a new family of Mn-based superconductors with relatively high T_(c).
基金Supported by the National Basic Research Program of China under Grant Nos 2015CB921303 and 2013CB921700the National Key Research Program of China under Grant No 2016YFA0300604the'Strategic Priority Research Program(B)'of the Chinese Academy of Sciences under Grant No XDB07020100
文摘There is a long-standing confusion concerning the physical origin of the anomalous resistivity peak in transition metal pentatelluride HfTe5. Several mechanisms, such as the formation of charge density wave or polaron, have been proposed, but so far no conclusive evidence has been presented. In this work, we investigate the unusual temperature dependence of magneto-transport properties in HfTe5. It is found that a three-dimensional topological Dirac semimetal state emerges only at around Tp （at which the resistivity shows a pronounced peak）, as manifested by a large negative magnetoresistance. This accidental Dirac semimetal state mediates the topological quantum phase transition between the two distinct weak and strong topological insulator phases in HfTe5. Our work not only provides the first evidence of a temperature-induced critical topological phase transition inHfTe5 but also gives a reasonable explanation on the long-lasting question.
