By using transport and magnetic measurement,the upper critical field H_(c2)(T)and the irreversibility line Hirr(T)has been determined.A big separation between H_(c2)(0)and Hirr(0)has been found showing the existence o...By using transport and magnetic measurement,the upper critical field H_(c2)(T)and the irreversibility line Hirr(T)has been determined.A big separation between H_(c2)(0)and Hirr(0)has been found showing the existence of a quantum vortex liquid state induced by quantum fluctuation of vortices in the new superconductor MgB_(2).Further investigation on the magnetic relaxation shows that both the quantum tunneling and the thermally activated flux creep depends weakly on temperature.However,when the melting field Hirr is approached,a drastic rise of the relaxation rate is observed.This may indicate that the melting of the vortex matter at a finite temperature is also induced by the quantum fluctuation of vortices.展开更多
The new superconductor MgB_(2) has been prepared by two ways: high pressure and ambient pressure synthesis. Their superconducting properties were measured and compared. It is found that the sample prepared by high pre...The new superconductor MgB_(2) has been prepared by two ways: high pressure and ambient pressure synthesis. Their superconducting properties were measured and compared. It is found that the sample prepared by high pressure is much denser than that prepared under ambient pressure. Accordingly the high pressure sample has a very narrow transition width and a much higher bulk critical current density.展开更多
Resistive transitions of the new superconductor MgB_(2) are carried out under magnetic fields from 0 to 8 T.It is found that the irreversibility field Hirr(T)and the upper critical field Hc2(T)obtained are very close ...Resistive transitions of the new superconductor MgB_(2) are carried out under magnetic fields from 0 to 8 T.It is found that the irreversibility field Hirr(T)and the upper critical field Hc2(T)obtained are very close to those determined in the magnetic measurement.By using the Arrhenius activation lawρ=ρ0exp(-U/kBT),the activation barrier U has been determined by the low dissipation part of R(T)curves.It is found that U■kBT manifests a very weak thermal activation and thermal fluctuation effect.This may further indicate that the flux dynamics near Hirr(T)is induced by strong quantum fluctuation and tunneling of vortices.展开更多
The nearly single phase(R_(0.4)Pr_(0.6))_(0.5)Ca_(0.5)Ba_(2)Cu_(3)O_(7-δ)compounds(R=La,Pr,Nd,Sm,Eu,Gd and Y)with Tc about 100 K were prepared under high pressure and high temperature using the cubic anvil apparatus....The nearly single phase(R_(0.4)Pr_(0.6))_(0.5)Ca_(0.5)Ba_(2)Cu_(3)O_(7-δ)compounds(R=La,Pr,Nd,Sm,Eu,Gd and Y)with Tc about 100 K were prepared under high pressure and high temperature using the cubic anvil apparatus.The Tc is much higher than that of the traditional R-123 superconductors without doping Ca on the rare-earth site.For various rare earths with different ionic radius,the(R_(0.4)Pr_(0.6))_(0.5)Ca_(0.5)Ba_(2)Cu_(3)O_(7-δ)compounds have almost the same critical temperature.This is different from the result of the ion-size effect of rare-earth ions in R1-xPrxBa2Cu3O7-δcompounds.Our results show that Pr has the same properties as other rare earths in the 123-phase compounds due to the doping of Ca.展开更多
Measurement and characterization were carried out on Cu-doped and undoped La_(2)CuO_(4+δ) samples prepared under the same conditions.The magnetization measurement on the Cu-doped La_(2)CuO_(4+δ) sample shows coexist...Measurement and characterization were carried out on Cu-doped and undoped La_(2)CuO_(4+δ) samples prepared under the same conditions.The magnetization measurement on the Cu-doped La_(2)CuO_(4+δ) sample shows coexistence of long-range antiferromagnetic order and superconductivity.A stripe-like structure was observed at room temperature on the ab-plane of Cu-doped La_(2)CuO_(4+δ) sample by transmission electron microscope.While for the undoped La_(2)CuO_(4+δ) sample no such behavior or structure feature was found.In this letter,these phenomena were discussed in terms of phase separation.展开更多
CoCrFeO_(4) nanocrystals have been synthesized using a sol-gel method.The specimen has an average grain size of 8.1 nm.The zero-field-cooled and field-cooled curves show a superparamagnetic transition occurring at a b...CoCrFeO_(4) nanocrystals have been synthesized using a sol-gel method.The specimen has an average grain size of 8.1 nm.The zero-field-cooled and field-cooled curves show a superparamagnetic transition occurring at a blocking temperature of TB≈249 K.The increase in coercivity of CoCrFeO_(4) nanocrystals with the decreasing temperature has been detected below the blocking temperature and could be explained by the Stoner-Wohlfarth theory.The temperature dependence of saturation magnetization shows that Bloch's law holds but with the Bloch exponent lower than the bulk value.展开更多
基金Supported by the National Natural Science Foundation of China under Grant No.19825111the Ministry of Science and Technology of China(NKBRSF-G1999064602).
文摘By using transport and magnetic measurement,the upper critical field H_(c2)(T)and the irreversibility line Hirr(T)has been determined.A big separation between H_(c2)(0)and Hirr(0)has been found showing the existence of a quantum vortex liquid state induced by quantum fluctuation of vortices in the new superconductor MgB_(2).Further investigation on the magnetic relaxation shows that both the quantum tunneling and the thermally activated flux creep depends weakly on temperature.However,when the melting field Hirr is approached,a drastic rise of the relaxation rate is observed.This may indicate that the melting of the vortex matter at a finite temperature is also induced by the quantum fluctuation of vortices.
基金Supported by the Ministry of Science and Technology of China(NKBRSF-G19990646)the National Natural Science Foundation of China under Grant No.19825111。
文摘The new superconductor MgB_(2) has been prepared by two ways: high pressure and ambient pressure synthesis. Their superconducting properties were measured and compared. It is found that the sample prepared by high pressure is much denser than that prepared under ambient pressure. Accordingly the high pressure sample has a very narrow transition width and a much higher bulk critical current density.
基金Supported by the Ministry of Science and Technology of China(NKBRSF-G19990646)the National Natural Science Foundation of China under Grant No.19825111.
文摘Resistive transitions of the new superconductor MgB_(2) are carried out under magnetic fields from 0 to 8 T.It is found that the irreversibility field Hirr(T)and the upper critical field Hc2(T)obtained are very close to those determined in the magnetic measurement.By using the Arrhenius activation lawρ=ρ0exp(-U/kBT),the activation barrier U has been determined by the low dissipation part of R(T)curves.It is found that U■kBT manifests a very weak thermal activation and thermal fluctuation effect.This may further indicate that the flux dynamics near Hirr(T)is induced by strong quantum fluctuation and tunneling of vortices.
基金Supported by the Ministry of Science and Technology of China(NKBRSF-G19990646).
文摘The nearly single phase(R_(0.4)Pr_(0.6))_(0.5)Ca_(0.5)Ba_(2)Cu_(3)O_(7-δ)compounds(R=La,Pr,Nd,Sm,Eu,Gd and Y)with Tc about 100 K were prepared under high pressure and high temperature using the cubic anvil apparatus.The Tc is much higher than that of the traditional R-123 superconductors without doping Ca on the rare-earth site.For various rare earths with different ionic radius,the(R_(0.4)Pr_(0.6))_(0.5)Ca_(0.5)Ba_(2)Cu_(3)O_(7-δ)compounds have almost the same critical temperature.This is different from the result of the ion-size effect of rare-earth ions in R1-xPrxBa2Cu3O7-δcompounds.Our results show that Pr has the same properties as other rare earths in the 123-phase compounds due to the doping of Ca.
基金Supported by the National Center for Research and Developments on Superconductivity of China.
文摘Measurement and characterization were carried out on Cu-doped and undoped La_(2)CuO_(4+δ) samples prepared under the same conditions.The magnetization measurement on the Cu-doped La_(2)CuO_(4+δ) sample shows coexistence of long-range antiferromagnetic order and superconductivity.A stripe-like structure was observed at room temperature on the ab-plane of Cu-doped La_(2)CuO_(4+δ) sample by transmission electron microscope.While for the undoped La_(2)CuO_(4+δ) sample no such behavior or structure feature was found.In this letter,these phenomena were discussed in terms of phase separation.
文摘CoCrFeO_(4) nanocrystals have been synthesized using a sol-gel method.The specimen has an average grain size of 8.1 nm.The zero-field-cooled and field-cooled curves show a superparamagnetic transition occurring at a blocking temperature of TB≈249 K.The increase in coercivity of CoCrFeO_(4) nanocrystals with the decreasing temperature has been detected below the blocking temperature and could be explained by the Stoner-Wohlfarth theory.The temperature dependence of saturation magnetization shows that Bloch's law holds but with the Bloch exponent lower than the bulk value.
