摘要:

It has been established as a result of successful direct experimental studies of the symmetry of the superconducting order parameter that the pairing symmetry in the compounds YBCO, GdBCO, Tl2201, and Bi2212 is of thed-wave type. In this paper, experimental and theoretical results concerning thed-wave superconducting state and the peculiarities of magnetic properties of thed-type superconductor such as pseudogap, charge ordering, and unusual structure of vortex excitations differing from that fors-states are analyzed. Several mechanisms ofd-pairing and related microscopic theories are discussed and possible experiments for their verification are proposed. ©1998 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593677

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A phenomenological model is proposed for the transformation of interband conductivity caused by electron transitions through the optical (dielectric) gapℏ&ohgr;ginto the intraband conductivity of hole carriers in high-temperature superconductors under chemical doping. The interrelation between the interband and intraband conductivity components is analyzed in terms of the spectral functionN(&ohgr;)∼∫&sgr;(&ohgr;)d&ohgr;for integral conductivity of the normal phase. Two groups of coexisting charge carries of thep- andd-types with different relations with interband transitions are singled out. The integral conductivity of narrow-bandd-carriers is determined by interband excitations with the gap attenuation&Ggr;∝&ohgr;g.The integral conductivity of wide-bandp-carriers is not connected with interband excitations and is determined by the standard Drude spectrum. The obtained spectral functions are compared with the available data forLa2−xSrxCuO4andYBa2Cu3O6+xin the doping range from the beginning of metallization up to loss of superconductivity. The good agreement with the experimental data leads to the following conclusions: (i) the integral interband conductivity at the doping stage with increasing temperature of superconducting transition is mainly determined by thed-component to which interband excitations are “pumped;” (ii) as soon as one of the planesCuO2orCuOxgoes over to a predominantlyp-metal state, a noncorrelated metal with loss of superconductivity is formed. ©1998 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593678

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

It is shown that arrays of edge dislocations with parallel Burgers vectors in the slip plane can lead to significant local changes in the transition temperatureTcof the deformed crystals of high-temperature superconductors. These changes are due to the redistribution of free charge carriers in the elastic strain fields of the ionic crystal lattice and a strong nonmonotonic dependence ofTcon the concentration of charge carriers if the characteristic length of dislocation arrays is much larger than the coherence length and screening radius. ©1998 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593679

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The isotherms&sgr;T(H)and the high-field magnetization polytherms&sgr;H(T)of polycrystals of the dilute spin-glass systemLi0.5Fe2.5−xGaxO4are studied in the temperature range 4.2–380 K in fields up to 25 kOe. The nonmagneticGa3+ions in the crystals have concentrationsx=1.4and 1.6 in the vicinity of the multicritical pointx0=1.5on thex-Tphase diagram, near which all types of magnetic states have cluster-type spatially inhomogeneous structures. It is found that the dependences&sgr;H(T)obey Bloch’sT3/2law in the paramagnet temperature rangeT>Tf=18 K,x=1.6andT>TC=160 K,x=1.4forH⩾10 kOe.This is attributed to spin wave excitations in individual noninteracting clusters. TheT3/2law is violated atT<Tfforx=1.6,but is obeyed atT<TCforx=1.4.The clearly manifested phase transition at the Curie point in the form of a kink on the linear dependence&sgr;H(T3/2)suggests that the spin wave excitations are cooperative: their spectrum is formed atT⩽TCby the entire crystal, i.e., by the clusters and the matrix which is responsible for the long-range exchange coupling between clusters. At low temperatures, theT3/2law is violated atT∼50 K,where the sample withx=1.4previously displayed a first-order phase-transition. ©1998 American Institute of Physics. 

ISSN:1063-777X    

DOI:10.1063/1.593680

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The magnetoresistive&rgr;(T,H)and thermoelectric (the Seebeck coefficient)S(T,H)properties ofLa0.7□0.3MnO3−&dgr;thin films (□ is a cation vacancy) grown by the magnetron deposition technique are investigated. The magnetic polaron origin of the conductivity of such systems is established in the temperature range77 K⩽T⩽350 Kin magnetic fields0⩽H⩽10 kOe.The experimental dependences&rgr;(T,H)andS(T,H=0)are approximated by a universal phenomenological expression. Thermopower measurements indicate a considerable change of the mobility as well as the density of states of charge carriers in the region of magnetic phase transition. ©1998 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593681

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

Spin dynamics of soliton-like localized excitations in a discrete ferromagnetic chain with an easy-axis anisotropy and weak exchange interaction is studied. The relation of these excitations and dynamic magnetic solitons in the long-wave approximation is determined, and the dependence of frequency of localized excitations on the exchange interaction parameter for a fixed value of the total number of spin deviations is constructed. It is shown that this dependence is modified significantly for values of exchange interaction of the order of the one-ion anisotropy value. ©1998 American Institute of Physics. 

ISSN:1063-777X    

DOI:10.1063/1.593682

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The unusual non-linear effects in hopping conduction of single-crystalLa2CuO4+&dgr;with excess oxygen has been observed. The resistance is measured as a function of the applied voltageU(voltage controlled regime) in the temperature range5 K⩽T⩽300 Kand voltage range10−3−25 V.At relatively high voltage (approximately atU>0.1 V)the conduction of sample investigated corresponds well to variable-range hopping (VRH). That is, in the range0.1 V<U⩽1 Vthe conductivity does not depend onU(Ohmic behavior) and the temperature dependence of resistanceR(T)follows closely Mott’s law of VRH[R∝exp(T0/T)1/4].In the range of highest applied voltage the conduction has been non-Ohmic: the resistance decreases with increasingU. This non-linear effect is quite expected in the frame of VRH mechanism, since the applied electric field increases the hopping probability. A completely different and unusual conduction behavior is found, however, in the low voltage range (approximately below 0.1 V), where the influence of electric field and (or) electron heating effect on VRH ought to be neglected. Here we have observed strong increase in resistance at increasingUatT⩽20 K,whereas atT>20 Kthe resistance decreases with increasingU. The magnetoresistance of the sample below 20 K has been positive at low voltage and negative at high voltage. The observed unusual non-Ohmic behavior at low voltage range is attributable to inhomogeneity of the sample, namely, to the enrichment of sample surface with oxygen during the course of the heat treatment of the sample in helium and air atmosphere before measurements. At low enough temperature (below ≈20 K) the surface layer with increased oxygen concentration is presumed to consist of disconnected superconducting regions in a poorly conducting (dielectric) matrix. This allows us to explain the observed unusual non-linear effects in the conduction of sample studied. The results obtained demonstrate that in some cases the measured transport properties of cuprate oxides cannot be attributed to the intrinsic bulk properties. ©1998 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593683

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

A consistent phenomenological approach is used to show that a true long-range order can exist in two-sublattice two-dimensional antiferromagnets (AFM) and ferrites closed to the compensation point. The effect is due to the long-range component of dipole forces. A similar result was obtained earlier for ferromagnets by Maleev [Sov. Phys. JETP43, 1240 (1976)], who suggested that the Mermin–Wagner theorem may not be valid for interactions decreasing in proportion to1/R3or more slowly. It is found that the effect exists in the case of magnets with completely identical sublattices (AFM) only due to some types of the Dzyaloshinskii–Moriya interaction. For example, it is observed for AFM with an even (in Turov’s sense) principal axis and is absent otherwise. For a magnet with nonidentical sublattices, the effect can take place only for ferrites, i.e., for sublattices that are not compensated in the exchange approximation. The effect of stabilization of long-range order disappears at the point of compensation of magnetic moment. If this point does not coincide with the point of compensation of spin angular momentum, the intensities of fluctuations are nonmonotonic functions of temperature. The obtained estimates for the phase transition temperature are compared with experimental results. ©1998 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593684

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

The data on adsorption isotherms of4Hein synthetic adsorbents are obtained at 4.2 K in the pressure range1–10−2 torr.The dependence of pressure on the amount of adsorbed gas in adsorption pumps is studied under conditions simulating the pump operation in dilution refrigerators. It is found that the pressure in the pump is practically independent of the amount of adsorbed helium for a constant pumping rate in the interval10−6−10−4 mole/s,over a wide pressure range. The obtained characteristics can be used for designing and developing pumps for dilution refrigerators. ©1998 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593685

出版商:AIP    

年代:1998

数据来源: AIP

摘要:

An empirical analysis of the results of time- and temperature dependences of the electrical resistanceRof metal-ammonia solutionsNH3:Nawith different Na concentrations quenched in liquid nitrogen point towards an anomalously high but finite conductivity rather than superconductivity. In particular, the time dependenceR(t)of solutionsNH3:Naquenched rapidly atT<80 Kcan be described correctly by a “logistic curve” with a nonzero initial valueR(0)≠0(att=0).©1998 American Institute of Physics.

ISSN:1063-777X    

DOI:10.1063/1.593686

出版商:AIP    

年代:1998

数据来源: AIP