By considering the nonmonotonic d-wave gap effect, the energy and momentum dependence of quasiparticle scattering interference is studied in the presence of a single impurity. It is shown that the pattern of the quasi...By considering the nonmonotonic d-wave gap effect, the energy and momentum dependence of quasiparticle scattering interference is studied in the presence of a single impurity. It is shown that the pattern of the quasiparticle scattering peaks in the full Brillouin zone of electron-doped cuprate superconductors is very different from that in the hole-doped case described by the Octet model. This difference is the result of the nonmonotonic d-wave superconducting gap in the electron-doped case. As the energy increases, the position of the local peaks in the Brillouin zone moves rapidly. In particular, the characteristic peaks of the electron-doped cuprate superconductors appear between the antinodal and nodal directions, unlike in the hole-doped case.展开更多
In the last few years, charge order and its entanglement with superconductivity are under hot debate in high-Tc community due to the new progress on charge order in high-Tc cuprate superconductors YBa2Cu3O6+x. Here, w...In the last few years, charge order and its entanglement with superconductivity are under hot debate in high-Tc community due to the new progress on charge order in high-Tc cuprate superconductors YBa2Cu3O6+x. Here, we will briefly introduce the experimental status of this field and mainly focus on the experimental progress of high-field nuclear magnetic resonance(NMR) study on charge order in YBa2Cu3O6+x. The pioneering high-field NMR work in YBa2Cu3O6+x sets a new stage for studying charge order which has become a ubiquitous phenomenon in high-Tc cuprate superconductors.展开更多
Model of hole-pairs in electrical transport along ab plane in cuprate superconductors has already been proposed. It has been found to be in the shape of 3dx2–y2 orbital of an electron in an atom. This time, model of ...Model of hole-pairs in electrical transport along ab plane in cuprate superconductors has already been proposed. It has been found to be in the shape of 3dx2–y2 orbital of an electron in an atom. This time, model of hole-pairs in transport along c-axis in cuprate superconductors is proposed. In ab-plane, hole-pairs are formed along CuO2 plane;one hole-pair covering 9 - 10 two dimensional CuO2 unit cells in 3dx2–y2 configuration. In the investigation of c-axis hole-pairs, cuprate superconductors have been sub-divided into three categories depending on the number of CuO2 planes/formula unit. There is a little different treatment for finding out the order parameter in each category. Coherence lengths along ab-planes are of the order of a few tens of Angstroms, whereas along c-axis, they are less than even their a-, b-lattice constants. In cuprates with 2 or 3 CuO2 planes, the order parameter is of 3dz2–x2 type in zx-plane with lobes along both the axes much constrained. For cuprates with a single CuO2 layer, the order parameter is of 3dx2–y2 type, but its dimensions are less than a-, b-lattice constants.展开更多
Electron-phonon coupling （EPC） in cuprate and iron-based superconducting systems, as revealed by Raman scat- tering, is briefly reviewed. We introduce how to extract the coupling information through phonon lineshape...Electron-phonon coupling （EPC） in cuprate and iron-based superconducting systems, as revealed by Raman scat- tering, is briefly reviewed. We introduce how to extract the coupling information through phonon lineshape. Then we discuss the strength of EPC in different high-temperature superconductor （HTSC） systems and possible factors affecting the strength. A comparative study between Raman phonon theories and experiments allows us to gain insight into some crucial electronic properties, especially superconductivity. Finally, we summarize and compare EPC in the two existing HTSC systems, and discuss what role it may play in the HTSC.展开更多
The recent discovery of superconductivity in doped rare-earth infinite-layer nickelates RNiO_(2),R=Nd,Pr as a new family of unconventional superconductors has inspired extensive research on their intriguing properties...The recent discovery of superconductivity in doped rare-earth infinite-layer nickelates RNiO_(2),R=Nd,Pr as a new family of unconventional superconductors has inspired extensive research on their intriguing properties.One of the major motivation to explore the nickelate superconductors originated from their similarities with and differences from the cuprate superconductors,which have been extensively studied over the last decades but are still lack of the thorough understanding.In this short review,we summarized our recent investigation of the relevance of Ni/Cu-3d multiplet structure on the hole doped spin states in cuprate and recently discovered nickelate superconductors via an impurity model incorporating all the 3d orbitals.Further plausible explorations to be conducted are outlined as well.Our presented work provides an insightful framework for the investigation of the strongly correlated electronic systems in terms of the multiplet structure of transition metal compounds.展开更多
Tt-phase electron-doped superconductor Pr1-xLaCexCuO4-δ（PLCCO） thin films are successfully prepared on SrTiOs （100） substrates by using the dc magnetron sputtering method. It is found that the films each have a h...Tt-phase electron-doped superconductor Pr1-xLaCexCuO4-δ（PLCCO） thin films are successfully prepared on SrTiOs （100） substrates by using the dc magnetron sputtering method. It is found that the films each have a highly oriented structure along the c-axis. For optimally doped films with x ≈ 0.10, the superconducting transition temperature Tc is 25.5 K, which is similar to that of a single crystal. The quadratic temperature dependence of the resistivity is observed when T 〉 To, which can be attributed to the two-dimensional Fermi liquid behaviour. Besides, the optimal conditions for preparing the T1-phase PLCCO thin films are also discussed in detail.展开更多
We propose two possible new compounds, Ba2CuO2Fe2As2and K2CuO2Fe2Se2, which hybridize the building blocks of two high temperature superconductors, cuprates and iron-based superconductors. These compounds consist of sq...We propose two possible new compounds, Ba2CuO2Fe2As2and K2CuO2Fe2Se2, which hybridize the building blocks of two high temperature superconductors, cuprates and iron-based superconductors. These compounds consist of square CuO2 layers and antifluorite-type Fe2X2（X = As, Se） layers separated by Ba/K. The calculations of binding energies and phonon spectra indicate that they are dynamically stable, which ensures that they may be experimentally synthesized. The Fermi surfaces and electronic structures of the two compounds inherit the characteristics of both cuprates and iron-based superconductors. These compounds can be superconductors with intriguing physical properties to help to determine the pairing mechanisms of high Tc superconductivity.展开更多
We have earlier proposed models of preformed hole pairs based on the results of our electron paramagnetic resonance experiments. A hole doped in a cuprate superconductor causes ferromagnetic alignment of the spins of ...We have earlier proposed models of preformed hole pairs based on the results of our electron paramagnetic resonance experiments. A hole doped in a cuprate superconductor causes ferromagnetic alignment of the spins of the holes of 4 Cu2+ ions of the plaquette (CuO)4 in which it enters. Spin alignments undergo oscillations from vertically upward to vertically downward of the CuO2 plane. Vertical projections of spins go on changing when they pass through different plaquettes going to zero when they pass through the CuO2 plane. Ferromagnetic alignments of spins produce magnetic fields on the plane proportional to their vertical projections. When two holes travelling in CuO2 plane come across each other at a certain distance between them, they are attracted towards each other by Heisenberg exchange interaction and their path is decided by the magnetic field produced due to spin alignments. Their path is similar to 3dx2 - y2 atomic orbital. Y-123 has been chosen as an example. Due to plethora of evidence of antiferromagnetic fluctuations in cuprates, hole-pair formation has been tried in Y-123 assuming antiferromagnetic fluctuations in it. It has been found that hole-pair formation in spite of AFM fluctuations can be explained on the same lines as done earlier. Hole-pair formation was tried in Tl-2201 to test whether the same rules apply in cuprates with very high coherence lengths. Coherence length in Tl-2201 = 52 Å, whereas in Y-123 = 15 20 Å in CuO2 plane. It has been reported that in Tl-2201 the CuO2 plane is very flat and smooth. From this it was concluded that high coherence length is the result of the smoothness of the plane. Further it was concluded that the smoothness of the CuO2 plane depends upon the nature of the near neighbors of the CuO2 plane. Near neighbors of Y-123 and Tl-2201 have been compared.展开更多
As a typical hole-doped cuprate superconductor,Bi_(2)Sr_(2)CaCu_(2)O_(8+δ)(Bi2212)carrier doping is mostly determined by its oxygen content.Traditional doping methods can regulate its doping level within the range of...As a typical hole-doped cuprate superconductor,Bi_(2)Sr_(2)CaCu_(2)O_(8+δ)(Bi2212)carrier doping is mostly determined by its oxygen content.Traditional doping methods can regulate its doping level within the range of hole doping.Here we report the first application of CaH_(2)annealing method in regulating the doping level of Bi2212.By continuously controlling the anneal time,a series of differently doped samples can be obtained.The combined experimental results of x-ray diffraction,scanning transmission electron microscopy,resistance and Hall measurements demonstrate that the CaH_(2)induced topochemical reaction can effectively change the oxygen content of Bi2212 within a very wide range,even switching from hole doping to electron doping.We also found evidence of a low-T_c superconducting phase in the electron doping side.展开更多
To decipher the mechanism of high temperature superconductivity(SC),it is important to know how the superconducting pairing emerges from the unusual normal states of cuprate superconductors,including pseudogap,anomalo...To decipher the mechanism of high temperature superconductivity(SC),it is important to know how the superconducting pairing emerges from the unusual normal states of cuprate superconductors,including pseudogap,anomalous Fermi liquid and strange metal(SM).A long-standing issue under debate is how the superconducting pairing is formed and condensed in the SM phase because the superconducting transition temperature is the highest in this phase.展开更多
A mean-field spin-density wave (SDW) analysis of pseudogap in the underdoped cuprates is proposed on the dependence of the energy gap which mimics the pseudogap near (π, 0) point at least. It implies that the pseudog...A mean-field spin-density wave (SDW) analysis of pseudogap in the underdoped cuprates is proposed on the dependence of the energy gap which mimics the pseudogap near (π, 0) point at least. It implies that the pseudogap structure near (π, 0) is not sensitive to the long-range order and will survive leading to the pseudogap phenomenon in the underdoped metallic phase. On the other hand, in the long-range ordering antiferromagnetic region, the mean-field SDW theory holds and the pseudogap structure predicated by the theory should be observable experimentallyv. Then one prediction is that the pseudogap would smoothly extrapolate between itinerant antiferromagnetic phase and underdoped metallic phase.展开更多
Dynamic charge transfers,or charge flux oscillations,generated by anharmonic phonon coupling,have attracted increasing interest in cuprate superconductors.In this article,a new computational method is developed to ana...Dynamic charge transfers,or charge flux oscillations,generated by anharmonic phonon coupling,have attracted increasing interest in cuprate superconductors.In this article,a new computational method is developed to analyze such charge fluxes along all atomic bonds for a given material,which unveils a surprising fact that cupratematerials with high superconducting transition temperature show a strong tendency to support global charge flux flows beyond local charge oscillations.Such fluxes further show a strong correlation with both the maximum superconducting transition temperature of different cuprate families and the strong magnetic fluctuations as well.Motivated by these findings,we construct a charge flux model derived from quantum field theory to evaluate the effective interactions mediated by these flux flows.Finally,we discuss the implications of this flux-driven pairing mechanism for the design of new high-T_(c) superconductors,offering a potential strategy for discovering higher T_(c) superconductive materials.展开更多
We report systematic in-plane magnetoresistance measurements on the electron-doped cuprate La2-x,.CexCuO4±δ thin films as a function of Ce doping and oxygen content in the magnetic field up to 14 T. A crossover ...We report systematic in-plane magnetoresistance measurements on the electron-doped cuprate La2-x,.CexCuO4±δ thin films as a function of Ce doping and oxygen content in the magnetic field up to 14 T. A crossover from negative to positive magnetoresistance occurs between the doping level x = 0.07 and 0.08. Above x = 0.08, the positive magnetoresistance effect appears, and is almost indiscernible at x = 0.15. By tuning the oxygen content, the as-grown samples show negative magnetoresistance effect, whereas the optimally annealed ones display positive magnetoresistance effect at the doping level x = 0.15. Intriguingly, a linear-field dependence of in-plane magnetoresistanee is observed at the underdoping level x = 0.06, the optimal doping level x = 0. i and slightly overdoping level x = 0.11. These anomalies of in-plane magnetoresistance may be related to the intrinsic inhomogeneity in the cuprates, which is discussed in the framework of network model.展开更多
Specific heat is a powerful tool to investigate the physical properties of condensed materials.Superconducting state is achieved through the condensation of paired electrons,namely,the Cooper pairs.The condensed Coope...Specific heat is a powerful tool to investigate the physical properties of condensed materials.Superconducting state is achieved through the condensation of paired electrons,namely,the Cooper pairs.The condensed Cooper pairs have lower entropy compared with that of electrons in normal metal,thus specific heat is very useful in detecting the low lying quasiparticle excitations of the superconducting condensate and the pairing symmetry of the superconducting gap.In this brief overview,we will give an introduction to the specific heat investigation of the physical properties of superconductors.We show the data obtained in cuprate and iron based superconductors to reveal the pairing symmetry of the order parameter.展开更多
We trace the conceptual basis of the Multi-Band Approach (MBA) and recall the reasons for its wide following for composite superconductors (SCs). Attention is then drawn to a feature that MBA ignores: the possibility ...We trace the conceptual basis of the Multi-Band Approach (MBA) and recall the reasons for its wide following for composite superconductors (SCs). Attention is then drawn to a feature that MBA ignores: the possibility that electrons in such an SC may also be bound via simultaneous exchanges of quanta with more than one ion-species—a lacuna which is addressed by the Generalized BCS Equations (GBCSEs). Based on several papers, we give a concise account of how this approach: 1) despite employing a single band, meets the criteria satisfied by MBA because a) GBCSEs are derived from a temperature-incorporated Bethe-Salpeter Equation the kernel of which is taken to be a “superpropagator” for a composite SC-each ion-species of which is distinguished by its own Debye temperature and interaction parameter and b) the band overlapping the Fermi surface is allowed to be of variable width. GBCSEs so-obtained reduce to the usual equations for the Tc and Δ of an elemental SC in the limit superpropagator → 1-phonon propagator;2) accommodates moving Cooper pairs and thereby extends the scope of the original BCS theory which restricts the Hamiltonian at the outset to terms that correspond to pairs having zero centre-of-mass momentum. One can now derive an equation for the critical current density (j0) of a composite SC at T = 0 in terms of the Debye temperatures of its ions and their interaction parameters— parameters that also determine its Tc and Δs;3) transforms the problem of optimizing j0 of a composite SC, and hence its Tc, into a problem of chemical engineering;4) provides a common canopy for most composite SCs, including those that are usually regarded as outside the purview of the BCS theory and have therefore been called “exceptional”, e.g., the heavy-fermion SCs;5) incorporates s±-wave superconductivity as an in-built feature and can therefore deal with the iron-based SCs, and 6) leads to presumably verifiable predictions for the values of some relevant parameters, e.g., the effective mass of electrons, for the SCs for which it has been employed.展开更多
The electron-doped cuprate superconductor exhibits a unique electronic structure,where both electron and hole Fermi surface(FS)pockets coexist in the optimally doped(OP)region,while in the overdoped(OD)region there ex...The electron-doped cuprate superconductor exhibits a unique electronic structure,where both electron and hole Fermi surface(FS)pockets coexist in the optimally doped(OP)region,while in the overdoped(OD)region there exists only a large hole FS pocket.It is therefore an intriguing question whether or not a p-n junction arises if the OD electron-doped cuprate interfaces with the OP compound.Here,we construct such an in-plane junction by selectively modulating the doping levels in thin films of La_(2-x)Ce_(x)CuO_(4)(LCCO)—a typical electron-doped cuprate.We find that the junction exhibits non-linear,asymmetricⅠ-Ⅴcharacteristics,which are consistent with those of a p-n semiconductor junction,across a wide temperature range from 250 K to 10 K,regardless of the Hall coefficient sign change or the superconducting transition.We attribute these features to a potential barrier formed at the junction,which is set by the band bending in both OD and OP LCCO.展开更多
基金Acknowledgements This work was supported by the funds from the National Natural Science Foundation of China under Grant Nos. 10774082 and 11447144.
文摘By considering the nonmonotonic d-wave gap effect, the energy and momentum dependence of quasiparticle scattering interference is studied in the presence of a single impurity. It is shown that the pattern of the quasiparticle scattering peaks in the full Brillouin zone of electron-doped cuprate superconductors is very different from that in the hole-doped case described by the Octet model. This difference is the result of the nonmonotonic d-wave superconducting gap in the electron-doped case. As the energy increases, the position of the local peaks in the Brillouin zone moves rapidly. In particular, the characteristic peaks of the electron-doped cuprate superconductors appear between the antinodal and nodal directions, unlike in the hole-doped case.
基金Project partially supported by the National Natural Science Foundation of China(Grant Nos.11522434 and U1532145)the Recruitment Program of Global Experts,Chinathe Chinese Academy of Sciences Hundred Talent Program
文摘In the last few years, charge order and its entanglement with superconductivity are under hot debate in high-Tc community due to the new progress on charge order in high-Tc cuprate superconductors YBa2Cu3O6+x. Here, we will briefly introduce the experimental status of this field and mainly focus on the experimental progress of high-field nuclear magnetic resonance(NMR) study on charge order in YBa2Cu3O6+x. The pioneering high-field NMR work in YBa2Cu3O6+x sets a new stage for studying charge order which has become a ubiquitous phenomenon in high-Tc cuprate superconductors.
文摘Model of hole-pairs in electrical transport along ab plane in cuprate superconductors has already been proposed. It has been found to be in the shape of 3dx2–y2 orbital of an electron in an atom. This time, model of hole-pairs in transport along c-axis in cuprate superconductors is proposed. In ab-plane, hole-pairs are formed along CuO2 plane;one hole-pair covering 9 - 10 two dimensional CuO2 unit cells in 3dx2–y2 configuration. In the investigation of c-axis hole-pairs, cuprate superconductors have been sub-divided into three categories depending on the number of CuO2 planes/formula unit. There is a little different treatment for finding out the order parameter in each category. Coherence lengths along ab-planes are of the order of a few tens of Angstroms, whereas along c-axis, they are less than even their a-, b-lattice constants. In cuprates with 2 or 3 CuO2 planes, the order parameter is of 3dz2–x2 type in zx-plane with lobes along both the axes much constrained. For cuprates with a single CuO2 layer, the order parameter is of 3dx2–y2 type, but its dimensions are less than a-, b-lattice constants.
基金supported by the National Basic Research Program of China(Grant Nos.2011CBA00112 and 2012CB921701)
文摘Electron-phonon coupling （EPC） in cuprate and iron-based superconducting systems, as revealed by Raman scat- tering, is briefly reviewed. We introduce how to extract the coupling information through phonon lineshape. Then we discuss the strength of EPC in different high-temperature superconductor （HTSC） systems and possible factors affecting the strength. A comparative study between Raman phonon theories and experiments allows us to gain insight into some crucial electronic properties, especially superconductivity. Finally, we summarize and compare EPC in the two existing HTSC systems, and discuss what role it may play in the HTSC.
基金supported by the startup fund from Soochow Universitythe Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions
文摘The recent discovery of superconductivity in doped rare-earth infinite-layer nickelates RNiO_(2),R=Nd,Pr as a new family of unconventional superconductors has inspired extensive research on their intriguing properties.One of the major motivation to explore the nickelate superconductors originated from their similarities with and differences from the cuprate superconductors,which have been extensively studied over the last decades but are still lack of the thorough understanding.In this short review,we summarized our recent investigation of the relevance of Ni/Cu-3d multiplet structure on the hole doped spin states in cuprate and recently discovered nickelate superconductors via an impurity model incorporating all the 3d orbitals.Further plausible explorations to be conducted are outlined as well.Our presented work provides an insightful framework for the investigation of the strongly correlated electronic systems in terms of the multiplet structure of transition metal compounds.
基金supported by the State Key Program for Basic Research of China(Grant No 2004CB619004-1)the National Natural Science Foundation of China(Grant No 10474121)
文摘Tt-phase electron-doped superconductor Pr1-xLaCexCuO4-δ（PLCCO） thin films are successfully prepared on SrTiOs （100） substrates by using the dc magnetron sputtering method. It is found that the films each have a highly oriented structure along the c-axis. For optimally doped films with x ≈ 0.10, the superconducting transition temperature Tc is 25.5 K, which is similar to that of a single crystal. The quadratic temperature dependence of the resistivity is observed when T 〉 To, which can be attributed to the two-dimensional Fermi liquid behaviour. Besides, the optimal conditions for preparing the T1-phase PLCCO thin films are also discussed in detail.
基金Project supported by the National Basic Research Program of China(Grant No.2015CB921300)the National Natural Science Foundation of China(Grant Nos.1190020 and 11334012)the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB07000000)
文摘We propose two possible new compounds, Ba2CuO2Fe2As2and K2CuO2Fe2Se2, which hybridize the building blocks of two high temperature superconductors, cuprates and iron-based superconductors. These compounds consist of square CuO2 layers and antifluorite-type Fe2X2（X = As, Se） layers separated by Ba/K. The calculations of binding energies and phonon spectra indicate that they are dynamically stable, which ensures that they may be experimentally synthesized. The Fermi surfaces and electronic structures of the two compounds inherit the characteristics of both cuprates and iron-based superconductors. These compounds can be superconductors with intriguing physical properties to help to determine the pairing mechanisms of high Tc superconductivity.
文摘We have earlier proposed models of preformed hole pairs based on the results of our electron paramagnetic resonance experiments. A hole doped in a cuprate superconductor causes ferromagnetic alignment of the spins of the holes of 4 Cu2+ ions of the plaquette (CuO)4 in which it enters. Spin alignments undergo oscillations from vertically upward to vertically downward of the CuO2 plane. Vertical projections of spins go on changing when they pass through different plaquettes going to zero when they pass through the CuO2 plane. Ferromagnetic alignments of spins produce magnetic fields on the plane proportional to their vertical projections. When two holes travelling in CuO2 plane come across each other at a certain distance between them, they are attracted towards each other by Heisenberg exchange interaction and their path is decided by the magnetic field produced due to spin alignments. Their path is similar to 3dx2 - y2 atomic orbital. Y-123 has been chosen as an example. Due to plethora of evidence of antiferromagnetic fluctuations in cuprates, hole-pair formation has been tried in Y-123 assuming antiferromagnetic fluctuations in it. It has been found that hole-pair formation in spite of AFM fluctuations can be explained on the same lines as done earlier. Hole-pair formation was tried in Tl-2201 to test whether the same rules apply in cuprates with very high coherence lengths. Coherence length in Tl-2201 = 52 Å, whereas in Y-123 = 15 20 Å in CuO2 plane. It has been reported that in Tl-2201 the CuO2 plane is very flat and smooth. From this it was concluded that high coherence length is the result of the smoothness of the plane. Further it was concluded that the smoothness of the CuO2 plane depends upon the nature of the near neighbors of the CuO2 plane. Near neighbors of Y-123 and Tl-2201 have been compared.
基金supported by the National Natural Science Foundation of China(Grant Nos.11888101 and U1832202)the Chinese Academy of Sciences(Grant Nos.QYZDB-SSWSLH043 and XDB33000000)+4 种基金the K.C.Wong Education Foundation(Grant No.GJTD-2018-01)the Informatization Plan of Chinese Academy of Sciences(Grant No.CAS-WX2021SF-0102)supported by the Synergetic Extreme Condition User Facility(SECUF)supported by China Postdoctoral Science Foundation(Grant Nos.2020M680726 and YJ20200325)supported by US DOE(Grant Nos.DE-SC0010526 and DE-SC0012704)。
文摘As a typical hole-doped cuprate superconductor,Bi_(2)Sr_(2)CaCu_(2)O_(8+δ)(Bi2212)carrier doping is mostly determined by its oxygen content.Traditional doping methods can regulate its doping level within the range of hole doping.Here we report the first application of CaH_(2)annealing method in regulating the doping level of Bi2212.By continuously controlling the anneal time,a series of differently doped samples can be obtained.The combined experimental results of x-ray diffraction,scanning transmission electron microscopy,resistance and Hall measurements demonstrate that the CaH_(2)induced topochemical reaction can effectively change the oxygen content of Bi2212 within a very wide range,even switching from hole doping to electron doping.We also found evidence of a low-T_c superconducting phase in the electron doping side.
文摘To decipher the mechanism of high temperature superconductivity(SC),it is important to know how the superconducting pairing emerges from the unusual normal states of cuprate superconductors,including pseudogap,anomalous Fermi liquid and strange metal(SM).A long-standing issue under debate is how the superconducting pairing is formed and condensed in the SM phase because the superconducting transition temperature is the highest in this phase.
文摘A mean-field spin-density wave (SDW) analysis of pseudogap in the underdoped cuprates is proposed on the dependence of the energy gap which mimics the pseudogap near (π, 0) point at least. It implies that the pseudogap structure near (π, 0) is not sensitive to the long-range order and will survive leading to the pseudogap phenomenon in the underdoped metallic phase. On the other hand, in the long-range ordering antiferromagnetic region, the mean-field SDW theory holds and the pseudogap structure predicated by the theory should be observable experimentallyv. Then one prediction is that the pseudogap would smoothly extrapolate between itinerant antiferromagnetic phase and underdoped metallic phase.
文摘Dynamic charge transfers,or charge flux oscillations,generated by anharmonic phonon coupling,have attracted increasing interest in cuprate superconductors.In this article,a new computational method is developed to analyze such charge fluxes along all atomic bonds for a given material,which unveils a surprising fact that cupratematerials with high superconducting transition temperature show a strong tendency to support global charge flux flows beyond local charge oscillations.Such fluxes further show a strong correlation with both the maximum superconducting transition temperature of different cuprate families and the strong magnetic fluctuations as well.Motivated by these findings,we construct a charge flux model derived from quantum field theory to evaluate the effective interactions mediated by these flux flows.Finally,we discuss the implications of this flux-driven pairing mechanism for the design of new high-T_(c) superconductors,offering a potential strategy for discovering higher T_(c) superconductive materials.
基金supported by the National Key Basic Research Program of China (Grant Nos. 2015CB921000, and 2016YFA0300301)the National Natural Science Foundation of China (Grant Nos. 11674374, and 11474338)the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (Grant No. QYZDB-SSW-SLH008)
文摘We report systematic in-plane magnetoresistance measurements on the electron-doped cuprate La2-x,.CexCuO4±δ thin films as a function of Ce doping and oxygen content in the magnetic field up to 14 T. A crossover from negative to positive magnetoresistance occurs between the doping level x = 0.07 and 0.08. Above x = 0.08, the positive magnetoresistance effect appears, and is almost indiscernible at x = 0.15. By tuning the oxygen content, the as-grown samples show negative magnetoresistance effect, whereas the optimally annealed ones display positive magnetoresistance effect at the doping level x = 0.15. Intriguingly, a linear-field dependence of in-plane magnetoresistanee is observed at the underdoping level x = 0.06, the optimal doping level x = 0. i and slightly overdoping level x = 0.11. These anomalies of in-plane magnetoresistance may be related to the intrinsic inhomogeneity in the cuprates, which is discussed in the framework of network model.
文摘Specific heat is a powerful tool to investigate the physical properties of condensed materials.Superconducting state is achieved through the condensation of paired electrons,namely,the Cooper pairs.The condensed Cooper pairs have lower entropy compared with that of electrons in normal metal,thus specific heat is very useful in detecting the low lying quasiparticle excitations of the superconducting condensate and the pairing symmetry of the superconducting gap.In this brief overview,we will give an introduction to the specific heat investigation of the physical properties of superconductors.We show the data obtained in cuprate and iron based superconductors to reveal the pairing symmetry of the order parameter.
文摘We trace the conceptual basis of the Multi-Band Approach (MBA) and recall the reasons for its wide following for composite superconductors (SCs). Attention is then drawn to a feature that MBA ignores: the possibility that electrons in such an SC may also be bound via simultaneous exchanges of quanta with more than one ion-species—a lacuna which is addressed by the Generalized BCS Equations (GBCSEs). Based on several papers, we give a concise account of how this approach: 1) despite employing a single band, meets the criteria satisfied by MBA because a) GBCSEs are derived from a temperature-incorporated Bethe-Salpeter Equation the kernel of which is taken to be a “superpropagator” for a composite SC-each ion-species of which is distinguished by its own Debye temperature and interaction parameter and b) the band overlapping the Fermi surface is allowed to be of variable width. GBCSEs so-obtained reduce to the usual equations for the Tc and Δ of an elemental SC in the limit superpropagator → 1-phonon propagator;2) accommodates moving Cooper pairs and thereby extends the scope of the original BCS theory which restricts the Hamiltonian at the outset to terms that correspond to pairs having zero centre-of-mass momentum. One can now derive an equation for the critical current density (j0) of a composite SC at T = 0 in terms of the Debye temperatures of its ions and their interaction parameters— parameters that also determine its Tc and Δs;3) transforms the problem of optimizing j0 of a composite SC, and hence its Tc, into a problem of chemical engineering;4) provides a common canopy for most composite SCs, including those that are usually regarded as outside the purview of the BCS theory and have therefore been called “exceptional”, e.g., the heavy-fermion SCs;5) incorporates s±-wave superconductivity as an in-built feature and can therefore deal with the iron-based SCs, and 6) leads to presumably verifiable predictions for the values of some relevant parameters, e.g., the effective mass of electrons, for the SCs for which it has been employed.
基金Project supported by the National Key Research and Development Program of China(Grant No.2022YFA1403100)the National Natural Science Foundation of China(Grant Nos.52388201,12361141820,and 12274249)the Innovation Program for Quantum Science and Technology(Grant No.2021ZD0302400)。
文摘The electron-doped cuprate superconductor exhibits a unique electronic structure,where both electron and hole Fermi surface(FS)pockets coexist in the optimally doped(OP)region,while in the overdoped(OD)region there exists only a large hole FS pocket.It is therefore an intriguing question whether or not a p-n junction arises if the OD electron-doped cuprate interfaces with the OP compound.Here,we construct such an in-plane junction by selectively modulating the doping levels in thin films of La_(2-x)Ce_(x)CuO_(4)(LCCO)—a typical electron-doped cuprate.We find that the junction exhibits non-linear,asymmetricⅠ-Ⅴcharacteristics,which are consistent with those of a p-n semiconductor junction,across a wide temperature range from 250 K to 10 K,regardless of the Hall coefficient sign change or the superconducting transition.We attribute these features to a potential barrier formed at the junction,which is set by the band bending in both OD and OP LCCO.
