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1.
The in-plane normal state resistance Rn(T, H) in an overdoped La2−xSrxCuO4 crystal (x = 0.24) has been measured in magnetic fields up to 20 T parallel to the c-axis. The Rn(T) curves in constant fields show a quadratic behavior in a wide range of temperature above Tc(H). Some characteristic features in Rn(H) are observed. In the low-field region Rn(H) increases with increasing H, reaches a maximum and then decreases with further increasing H. Possible origins for the observed unusual Rn(T, H) behavior are discussed.  相似文献   

2.
《Physica A》1995,220(3-4):585-598
An antiferromagnetic equivalent-neighbour Heisenberg interaction Hi between impurity spins is added to the reduced s-d Hamiltonian Hr previously introduced by simplifying the Kondo s-d exchange Hamiltonian HK. Asymptotic mean-field theory is developed for Hr + Hi, in the presence and absence of external magnetic field, and applied to (La1−xCex)Al2 alloys. Specific heat ci(T) and zero-field susceptibility χi(0,T) curves for (La1−xCex)Al2 are depicted. The coupling constants of Hr + Hi and conduction bandwidth are adjusted so that Tc temperatures for x = 0.2, 0.1 are equal to the experimental values. ci(T) exhibits a jump at Tc and is decreasing for T < Tc. χi(0,T) has a first order pole at Tc which corresponds to the maximum of experimental susceptibility and χi(0,0) > 0. These results improve those obtained earlier on the grounds of Hr theory.  相似文献   

3.
The structure of the vacuum is studied in the Yang-Mills theory as a function of the scale length R. As a candidate of the vacuum we analyze explicitly the Savvidy state. We obtain the following results: (i) there is no gluon condensation for R<Rc (perturbative phase); (ii) there is condensation of a uniform magnetic flux for Rc<R<Rc (Savvidy phase); (iii) the Savvidy state becomes unstable for R>Rc (Copenhagen phase). Here, Rc and Rc are calculable in terms of magnetic flux condensation H2.  相似文献   

4.
吴祖懿 《波谱学杂志》1986,3(2):147-157
本文提出了予测稠苯芳杂环及其烷基链上质子化学位移的计算方法。 将稠苯芳杂环化合物用凯库勒式表示,计算式为为需考虑的苯环内的乙烯基效应。σmi,ci为各苯环的环流效应。σ1,Hc为各芳杂环的屏蔽效应,对杂环上质子它就是该单独芳杂环上相应质子的δ值,对苯环上质子要将它分解为各结构因素的效应,即:σ1,He=(1/2)d-1δx=y(或σz)+σc-c·σm,H. σx-yσz为杂原子或其基团的屏蔽效应,σc=c为存在于芳杂环中的乙烯基的效应,σm,Hc为芳杂环的环流效应,d为对不同质子所考虑的键数。有取代基时需考虑取代基的效应。计算环上烷基质子的公式为:δ=σp,CH3+ασc,CH3+βσt,CH3+σl,G σl,G为稠苯芳杂环基的某级效应。  相似文献   

5.
The low temperature (1.3–20 K) heat capacity of the weak itinerant electron ferromagnet Sc3In was measured in magnetic fields up to 10 T. The measurements were made on three different samples containing 24.1, 24.3 and 24.4 at.% In with Curie temperatures at H = 0 of 5.5, 6.0 and 6.3 K, respectively. The heat capacity peak at Tc becomes smaller with increasing fields and at 9.98 T the magnetic entropies are only 11 to 19% of the zero field value. Above Tc the spin fluctuation contribution to heat capacity, which is enhanced by the magnetic field at low fields (<≈ 5 T), is quenched at high fields ( ≈ 5 T). Our results show that the spin fluctuations in Sc3In are almost completely quenched by the magnetic fields of ≈ 10 T, and that the characteristics spin fluctuation temperature, Ts, of this itinerant electron ferromagnet is about the same as Tc.  相似文献   

6.
Anomalous magnetotransport phenomena have been observed in θ-(BEDT-TTF)2I3 crystals at temperatures below 15 K. The magnetoresistance M : (1) is a linear function of the magnetic field H, (2) is not affected by the angle between the electric current and the magnetic field, (3) but depends on the magnetic field orientation with respect to the crystal axis. Magnetoresistance is expressed as M = (aH2a + bH2b + cH2c)0-3/2/H in terms of H = (Ha, Hb, Hc), the zero field resistivity 0, and parameters a, b, and c which are independent of temperature and magnetic field. We have found that b a > c. Magnetoresistance up to 40 is observed for H = 7T along the b-axis at T = 1.5K.  相似文献   

7.
Unsubstituted and lead-antimony substituted 2223 BiSrCaCu Oxygen compounds have been synthesized and compared by resistivity R(T) and susceptibility χ(T) measurements and chemical analysis. The R(T) and χ(T) measurements show that interface effects occur which are reflected in the R(T) characteristics. The chemical analysis of samples with a resistance zero Tc(0)=114 K and a diamagnetic onset at 115 K yields a severe loss of lead from nominally 40 at% down to about 3 at%, whereas the Tc(0)=110 K samples contain up to about 30 at% of lead. It is assumed that for the formation of the 115 K phase lead oxide acts as a catalyst not being incorporated to a considerable amount into the lattice.  相似文献   

8.
The full BCS Hamiltonian is given as a quasiparticle-conserving part Hc and a quasiparticle non-conserving one, Hnc. The moments of Hc can be calculated and propagated to different K-subspaces (subspaces of states with a well-defined number, K, of quasiparticles) by French's method. Here we give the propagation formulae for the second moment of Hac in terms of a small number of basic quantities. The spectral distribution method, applied so far to particle systems, can be therefore transcribed to quasiparticle ones. We derive also expressions for the partial widths, and study the extent to which the number of quasiparticles is conserved. Then we discuss a possible truncation of the quasiparticle basis and end up with a discussion on the behaviour of centroid energies and widths in different cases.  相似文献   

9.
Superconductivity of compressed, high-purity platinum powder (average grain size 2–3 μm) was found by measurements of resistivity, AC susceptibility and magnetization. The transition temperature into the superconducting state Tc and the critical magnetic field Bc strongly depend on the packing fraction f of the samples: we found 0.62Tc(0)1.38 mK and 6.6Bc(0)67 μT for 0.8f0.5, respectively. The temperature dependence of the critical magnetic fields can be described by Bc(T)=Bc(0)(1−(T/Tc)2). The discussion of these results includes possible explanations for the origin of superconductivity in this new superconducting material.  相似文献   

10.
We have measured the resistivity of textured Bi1.84Pb0.4Sr2Ca2Cu3Oy silver-clamped thick films as a function of temperature, current density ranging from 10 to 1×103 A/cm2 and magnetic field up to 0.3 T. We find that the effective activation energy Ue follows Ue(T,J,H)=U0(1−T/Tp)mln(Jc0/J)H with m=1.75 for Hab-plane and 2.5 for Hc-axis and =0.76 for Hab and 0.97 for Hc, for the current density regime above 100 A/cm2, where Tp is a function of applied magnetic field and current density. This result suggests the effective activation energy Ue be correlated with the temperature, current density and magnetic field. The possible dissipative mechanisms responsible for the temperature, current density and magnetic field dependence of the effective activation energy are discussed.  相似文献   

11.
We study the effects of Lorentz non-invariance on the physical pion velocity at the critical temperature Tc in an effective theory of hidden local symmetry (HLS) with the “vector manifestation” fixed point. We match at a “matching scale” ΛM the axial-vector current correlator in the HLS with the one in the operator product expansion for QCD, and present the matching condition to determine the bare pion velocity. We find that the physical pion velocity, which is found to be one at T=Tc when starting from the Lorentz invariant bare HLS, remains close to one with the Lorentz non-invariance, vπ(Tc)=0.83–0.99. This result is quite similar to the pion velocity in dense matter.  相似文献   

12.
A computer simulation model is used to study the density profile and flow of a miscible gaseous fluid mixture consisting of differing constituent masses (mA=mB/3) through an open matrix. The density profile is found to decay with the height ∝exp (−mA(B)h), consistent with the barometric height law. The flux density shows a power-law increase ∝(pcp)μ with μ2.3 at the porosity 1−p above the pore percolation threshold 1−pc.  相似文献   

13.
The susceptibilities of three Sn-cylinders have been measured at a temperature slightly below Tc and in an external magnetic field just below Hc(T). The results are compared with calculations for a flat plate. From this the d.c. conductivity, the mean free path and the reflectivity-factor, have been determined.  相似文献   

14.
Evaporated TbCu0.6Al0.4 thin films with different thicknesses, ranging from 26 to 100 nm, were prepared on a glass substrate. The complex refractive index of thin films has been determined for the first time from the measurements of reflectance, R, and transmittance, T, at almost normal incidence in the spectral range of 400–2200 nm. Some micro-characteristics of TbCu0.6Al0.4 alloy including free charge concentration, nc, the relaxation time, τ, the static conductivity, σ, and the electron velocity at the Fermi surface, VF, have been evaluated using the simplified Drude model.  相似文献   

15.
A spin one XY ferromagnet with uniaxial anisotropy has been investigated, using Green's function technique in random phase approximation (RPA). The Green functions associated with the anisotropy energy are treated without decoupling. A set of coupled equations have been obtained to find the critical temperature Tc and (SZ)2 at Tc as function of the uniaxial anisotropy parameter D. Tc and (SZ)2 at Tc are found to increase with D. The results are compared with the earlier results obtained in the Narath type of RPA.  相似文献   

16.
Pr substituted at constant Ca concentration for Y in (Y1−xyPrxCay)Ba2Cu3O7−δ superconductors have been prepared under identical conditions and the temperature dependence of the electrical resistivity of these samples are measured. The resistively determined values of Tc decrease linearly with increasing x (0 ≤ x ≤ 0.2) for constant y = 0.10 and 0.15 which provides convincing evidence that the suppression of superconductivity by Pr is mainly due to magnetic pair breaking. The suppression of superconductivity can also be correlated to the observed changes in oxygen content determined by iodometric analysis and to the average Cu-valences. However, it is found that the observed suppression of Tc cannot be compensated by appropriate hole doping with Ca.  相似文献   

17.
Magnetization and Mössbauer studies of TiCuxFeyBe2-x-y (x = 0, 0.03, 0.4, y = 0, 0.02) show that TiBe2 is an enhanced paramagnet, 0.02 Fe or 0.03 Cu reduce the susceptibility. On the other hand, TiCu0.4Be1.6 is ferromagnetic (Tc = 20 K) and 0.02 Fe reduces the magnetization and Curie point (Tc = 16 K). The magnetic properties of all samples are extremely sensitive to sample preparation and heat treatments.

The Mössbauer studies show that the itinerant magnetism resides on the Ti site, all Ti sites have the same local spin density irrespective of local environment.  相似文献   


18.
The magnetization σc has been observed to depend strongly on temperature and biasing field in the Γ4-phase. Two families of σc (T) occur corresponding to the poly-domain (H < 200 Oe), and mono-domain (H > 200 Oe) states of the samples. This was proved by observation of domain structures in these samples. In addition to the maximum due to the second-order phase transition Γ24, we have revealed a maximum in the susceptibility curves Xc(T) at the high-temperature point of phase transition. The second maximum strongly depends on the biasing field value. Domain structure behaviour in that temperature range, and in various external fields, allowed us to suggest that there exists a relation between this maximum and the shifting processes of domain boundaries, which create a significant susceptibility along the c-axis.  相似文献   

19.
In general, heavy elements contribute only to acoustic phonon modes, which are less important for the superconductivity of hydrides. However, it was revealed that the heavier elements could enhance the phonon-mediated superconductivity in ternary hydrides. In the H3S–Xe system, a novel H3SXe compound was discovered by first-principle calculations. The structural phase transitions of H3SXe under high pressures were studied. The R-3m phase of H3SXe was predicted to appear at pressures above 80 GPa, which transitions to C2/m, P-3m1, and Pm-3m phases at pressures of 90, 160, and 220 GPa, respectively. It has been anticipated that the Pm-3m-H3SXe phase with a similar structural feature as that of Im-3m-H3S is a potential high-temperature superconductor with a Tc of 89 K at 240 GPa. The Tc value of H3SXe is lower than that of H3S at high pressure. The “H3S” host lattice of Pm- 3m-H3SXe is a crucial factor influencing the Tc value. The Xe atoms could accelerate the hydrogen-bond symmetrization. With the increase of the atomic number, the Tc value linearly increases in the H3S–noble-gas-element system. This indicates that the superconductivity can be modulated by changing the relative atomic mass of the noble-gas element.  相似文献   

20.
Through the analyses on the irreversibility lines (ILLs) under H||c for various high-Tc superconducting (HTSC) single-crystalline samples, we found two universal functions representing ILLs. In these functions, the electromagnetic anisotropy factor γ2 plays a crucial role. The appropriate chemical substitution, such as Bi(Pb)2212, is an effective method to improve flux pinning properties via decreasing γ2.  相似文献   

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